Unveiling olivine cathodes for high energy-density lithium-ion batteries: a comprehensive review from the atomic level to the electrode scale

Abstract

The development of cathode materials for lithium-ion batteries (LIBs) aims to achieve high energy density, cost-effectiveness, and thermal as well as mechanical stability. It generally proceeds through multidimensional design rules at the atomic, phase, particle, and electrode levels. Recently, new strategies have been proposed to achieve high energy density even in olivine-structured cathodes, which have a relatively lower theoretical capacity compared to layered cathode materials. These strategies involve substituting iron (Fe) with manganese (Mn), controlling particle morphology, and fabricating thick electrodes to simultaneously achieve high energy density and cost-effectiveness. However, comprehensive design guidelines for the material design, particle morphology, and fabrication of thick electrodes for olivine cathodes are still lacking. Herein, we provide detailed insights from well-established prior knowledge on olivine cathodes and share the latest findings related to their particle morphology control and electrode thickening. Additionally, our review closely investigates further considerations related to kinetics, such as low electronic and ionic conductivity, and mechanical instability issues that arise while thickening electrodes. It is emphasized that these issues can be resolved through a comprehensive understanding and strategies from the atomic level to the electrode level. Therefore, this review aims to contribute to achieving high energy density in olivine-based cathodes by understanding their kinetic limitations and mechanical instabilities.

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Wonchan Hwang

Wonchan Hwang received his PhD in 2022 from the School of Chemical and Biological Engineering at Seoul National University. He worked as a researcher at the Center for Nanoparticle Research at the Institute for Basic Science (IBS) in 2022. Currently, he is a postdoctoral researcher in the Energy Storage Research Center, Korea Institute of Science and Technology (KIST). His current research focuses on developing advanced electrode designs to enhance the energy density of energy storage devices and investigating reaction mechanisms of materials used in cathode electrodes.

Jaehwan Kim

Jaehwan Kim received his PhD (2023) from the Department of Biosystems and Biomaterials Science and Engineering, Seoul National University. He is currently a researcher in the Energy Storage Research Center, Korea Institute of Science and Technology (KIST). His current research interests include fabricating advanced electrodes and designing binders with functional biopolymers.

Shin-Yeong Kim

Shin-Yeong Kim obtained his BS degree from the School of Chemical and Biological Engineering, Seoul National University in 2019. Currently, he is pursuing his PhD at the School of Chemical and Biological Engineering, Seoul National University, under the supervision of Prof. Yung-Eun Sung. His current research includes designing electrode materials and understanding the reaction mechanism in Li–S batteries and all-solid-state batteries.

Eunseo Ko

Eunseo Ko is conducting an integrated PhD course at the Department of Chemical and Biological Engineering, Korea University and the Energy Storage Research Center, Korea Institute of Science and Technology (KIST), under the supervision of Dr Jungjin Park. Her current research is focused on the analysis of cathode materials, especially olivine materials.

Seojin Lee

Seojin Lee obtained her BS degree from the Department of Energy Engineering as major and minor in Social Innovation, Hanyang University, in 2023. Currently, she is conducting an integrated PhD course at the Department of Chemical and Biological Engineering, Korea University, and the Energy Storage Research Center, Korea Institute of Science and Technology (KIST), under the supervision of Dr Jungjin Park. Her current research includes designing high energy density lithium ion batteries and controlling dendritic growth in the Li metal anode.

Minseo Kim

Minseo Kim is currently pursuing her PhD at the School of Chemical and Biological Engineering, Seoul National University, under the supervision of Prof. Yung-Eun Sung. Her current research interests include devising electrode materials for Li–S batteries as well as modeling and simulation of lithium-ion batteries.

Seung-Ho Yu

Seung-Ho Yu is an associate professor in the Department of Chemical and Biological Engineering at Korea University, which he joined in 2019. He received his BS degree (2008) and PhD (2013) from the School of Chemical and Biological Engineering at Seoul National University. He worked as a researcher at the Research Institute of Advanced Materials (RIAM) and as a senior researcher at the Center for Nanoparticle Research in the Institute for Basic Science (IBS) at Seoul National University. From 2015 to 2019, he worked as a postdoctoral associate in the Department of Chemistry and Chemical Biology at Cornell University. His current research focuses on designing electrode materials and revealing the reaction mechanisms of electrode materials for Li-ion and post-Li-ion batteries.

Yung-Eun Sung

Yung-Eun Sung received his PhD in chemistry from the University of Illinois at Urbana-Champaign. He completed his postdoctoral studies in Prof. Allen Bard's laboratory at the University of Texas at Austin and started working as Assistant Professor at Gwangju Institute of Science and Technology (GIST) in 1998. Subsequently, he moved to Seoul National University in 2004 and became a professor of chemical and biological engineering in 2008. Currently, he is also Associate Director at the Center for Nanoparticle Research at Institute of Basic Science. His research focuses on electrochemistry in the area of batteries, hydrogen, fuel cells, and other energy conversion and storage.

Hyung-Seok Kim

Hyung-Seok Kim received his PhD in Materials Science and Engineering from the University of California, Los Angeles, and worked as a PTD engineer at Intel Corp. He is currently a principal research scientist at the Energy Storage Research Center at Korea Institute of Science and Technology (KIST) and associate professor in the Division of Energy & Environment Technology at Korea University of Science and Technology (UST). His current research interests include developing cathode materials for lithium/sodium ion batteries and zinc anodes for aqueous zinc-ion secondary batteries.

Chunjoong Kim

Chunjoong Kim received his PhD in Materials Science and Engineering from Seoul National University, Korea. He worked as a postdoctoral researcher at Lawrence Berkeley National Laboratory and as a researcher at Samsung Fine Chemicals. He joined the Department of Chemistry at the University of Illinois at Chicago as Research Assistant Professor. Since 2015, he has been working as Associate Professor in the Department of Materials Science and Engineering at Chungnam National University. His current research interests include the understanding of the reaction mechanism in electrode materials for energy storage and conversion devices.

Jungjin Park

Jungjin Park is a senior research scientist in the Energy Storage Research Center at Korea Institute of Science and Technology (KIST) and associate professor in the Division of Energy & Environment Technology at Korea University of Science and Technology (UST). He received his PhD (2015) from the School of Chemical and Biological Engineering at Seoul National University. He was a postdoctoral researcher in the Department of Chemical and Biomolecular Engineering at the University of California, Berkeley (UCB), and the Department of Materials Science and Engineering at Stanford University. He is currently working on developing anode and cathode materials for lithium/sodium ion batteries and understanding their reaction mechanism through electrochemical and synchrotron X-ray-based spectroscopic/microscopic analyses.

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1. Demands of high energy-density olivine cathodes

The rapidly increasing diversity in the applications of lithium-ion batteries (LIBs), spanning from powering compact portable devices to emerging fields such as electric vehicles (EVs) and energy storage systems (ESSs), underscores the paramount significance of their energy density.^1,2 Gravimetric and volumetric energy density, the amount of energy a battery can store per unit weight and volume, respectively, represent the most important parameters directly impacting their portability and operational longevity in various applications. Therefore, attaining high energy-density has been considered a focal point of research and development in the battery industry.

The pursuit of high energy-density has spurred significant interest in layered cathodes such as lithium nickel cobalt manganese oxide (NCM) and lithium nickel cobalt aluminium oxide (NCA), which are renowned for their excellent energy storage capabilities.^3,4 NCM has been the material of choice in the market. NCM not only exhibits prolonged battery life but also high energy-density under moderately high cut-off voltage, ensuring electrochemical stability during battery operation due to the substitution of cobalt (Co) with nickel (Ni) and manganese (Mn).⁴ However, despite these advantages, Ni and Co-based layered cathodes exhibit significant thermal instability, resulting in thermal runaway in the case of short circuit, high temperature exposure, and other circumstances (Fig. 1).^6,7 Moreover, the reliance of NCM on costly and scarce metals such as Ni and Co raises concerns regarding its sustainability and cost-effectiveness.⁸ These limitations highlight the ongoing quest for alternative cathode materials capable of matching the high energy density of NCM while addressing safety, economic, and environmental concerns.

Fig. 1 (a) Olivine and layered active material crystal structure. (b) Radar chart comparing the parameters of olivine cathodes and layered oxide cathodes.⁵ Copyright 2023, John Wiley and Sons.

Recently, lithium iron phosphate (LFP) has emerged as an alternative because of its enhanced structural and thermal stability, despite the absence of expensive metals such as Co and Ni.^8–10 However, LFP encounters significant challenges, notably its inherently low redox potential and capacity (energy density), which undermine its competitiveness in electric device applications such as EVs and ESSs. In this case, recent advancements in cell-to-pack (CTP) technology have narrowed the energy density gap between LFP (with an olivine structure) and NCM (with a layered structure) cathodes.^11,12 However, to make LFP even more competitive, innovative approaches are required to enhance its energy density beyond CTP.

In this review, we present the strategies to increase the energy density of lithium batteries by improving the voltage of their active materials and/or controlling their electrode composition.^13–23 We comprehensively address the challenges that arise in this process and propose strategies to overcome them at the atomic, particle, and electrode levels. In particular, we discuss the essential factors for redesigning the chemical composition of olivine active materials, optimizing the ion and electron transport capabilities in electrodes, and preventing mechanical failure (Fig. 2). Furthermore, we review the necessary conditions for enhancing the mass loading based on the recent research and development, thereby providing insight into future research directions for achieving high energy-density olivine cathodes.

Fig. 2 Schematic of the main issues in the fabrication of thick electrodes for high energy density LIBs. Mechanical problems (microcracks and delamination) and conductivity problems (ion electronic non-uniformity).

2. Multiscale strategies to fabricate high energy-density olivine cathodes

Achieving a high energy density in olivine cathodes necessitates a rationalized redesign of existing architectures from the atomic level to the electrode level. It has been noticed that new strategies such as increasing the working voltage through fine tuning of the active materials and/or using thick electrodes to augment the ratio of active materials per unit area can make a breakthrough in elevating the energy density within the existing battery volume dimensions.

Atomic-level approaches are effective in increasing the operating voltage. Controlling the chemical composition such as Mn,^16–18 Co,^19–21 and Ni can elevate the redox voltage, leading to a higher energy density albeit with an isostructure. At the particle level, controlling the particle size and shape influences the lithium-ion (Li-ion) path length and electrode packing density, which are crucial for achieving a high energy density. Smaller or specially shaped particles can improve the electrochemical performance of the material by shortening the ion paths, but increase the electrode porosity, thus reducing the bulk density. Additionally, nanoparticle aggregates can create pores within the electrode, improving its volumetric density. At the electrode level, designing thick electrodes for olivine cathodes is considered a pivotal strategy in the battery industry to enhance the energy density.^13–15,24 Thick electrodes achieve this by reducing the proportion of current collectors and separators per unit volume and increasing the ratio of active electrode material. Additionally, given that the energy stored in batteries primarily originates from electrochemically active materials, increasing the loading amount of active material per unit area, while reducing the proportion of inactive components such as conductive agents, binders, and separators is imperative.^25–28

3. Simultaneous control of reaction kinetics in materials with high energy density

3.1. Trends in the development of high-energy olivine cathodes

LFP reveals the operating voltage of 3.4 V (vs. Li/Li⁺) and the practical capacity of ∼160 mA h g⁻¹.²⁹ Considering the carbon content in LFP, its practical capacity closely matches its theoretical capacity. Hence, an improvement in its energy density can only be achieved by increasing its operating voltage.²⁹ Lithium manganese phosphate (LMP) shows an isostructure with LFP, which can offer a similar theoretical capacity. However, its operating voltage is higher, i.e., ∼4.1 V (vs. Li/Li⁺), enabling an energy density exceeding 690 W h kg⁻¹, which is over 20% higher than that of LFP (578 W h kg⁻¹).²⁹ Meanwhile, LMP has poor electronic and ionic conductivity, together with structural instability.³⁰ Consequently, lithium manganese iron phosphate (LiMn_xFe_(1−x)PO₄ with 0.5 < x < 1, LMFP) has emerged as a promising alternative candidate that combines the beneficial properties of both LFP and LMP. Nonetheless, olivine cathodes still exhibit sluggish kinetics stemming from the inherent properties of the olivine structure, leading to a reduced energy density.

LFP crystallizes into an orthorhombic phase with the Pnma space group and belongs to the polyanion family, where the phosphate (PO₄) groups create a strong covalent bonding network (Fig. 1a).³¹ This network contributes to the thermal and chemical stability of the cathode but also influences its electronic structure and Li-ion path (Fig. 1). In its crystal structure, oxygen forms corner-shared FeO₆ and LiO₆ octahedra, while the PO₄ tetrahedra share an edge with the FeO₆ and LiO₆ octahedra. The LiO₆ octahedra arrange into linear chains of edge-shared octahedra aligned parallel to the b-axis in alternating a–b planes. Consequently, Li diffuses along the [010] direction, corresponding to the b-axis, and this one-dimensional Li diffusion pathway makes the material susceptible to blockages by defects, resulting in a significant loss in the Li diffusivity.³¹ Electronic transport in LFP occurs through polaron hopping.³² This process involves the movement of electrons via the reduction–oxidation (redox) of Fe²⁺/Fe³⁺ during the battery operation. Electrons move across the lattice by the hopping of these small polarons from one iron site to another. A closer look at the structural aspect reveals that the FeO₆ octahedra are connected through corner-sharing in LFP. This configuration hinders the formation of a continuous FeO₆ network, leading to a comparatively lower electrical conductivity than that observed in LiCoO₂, where the CoO₆ octahedra are edge shared.

Meanwhile, substituting iron (Fe) with other transition metals (TM) such as Mn, Co, and Ni aims to increase the operating voltage and achieve a higher energy density, but it also causes differences in ionic and electronic conductivity.^33–35 The voltage changes can arise from a variety of factors related to morphology and electronic and ionic properties. In the case of LFP, electron transport is governed by the hopping mechanism facilitated by the Fe²⁺/Fe³⁺ redox reaction, as described previously, therefore substituting Fe with another TM modifies the characteristics of the redox pair, affecting the mobility of polarons. If the substituted metal shows lower polaron mobility compared to Fe, substitution lowers the electronic conductivity. Specifically, LMP, with its Mn²⁺/Mn³⁺ redox pair, exhibits lower polaron mobility, and consequently lower conductivity than the Fe redox pair in LFP. Li conduction in the olivine structure is closely linked with electron movement, indicating that changes in polaron mobility influence the Li conductivity. Furthermore, the ionic radius of Fe and other TMs is different, resulting in alterations to the lattice parameters in lithium transition metal phosphate (LiTMPO₄). These changes can lead to tuning of the lattice parameters and potentially impede the diffusion of Li. Mn-containing olivine cathodes demonstrated that LMFP experiences lattice distortion during charging due to the Jahn–Teller distortion induced by the oxidized Mn³⁺ ions, which is known to reduce the Li conductivity in the bulk.³⁴

Consequently, in both LFP and LMP, as well as their solid solution, LMFP, where Fe is replaced by other TMs, the issues related to the sluggish electronic and ionic conductivity are more pronounced. Therefore, we extensively review the research on enhancing the ionic diffusivity and electronic conductivity of olivine cathodes at the material level.

3.2. Doping strategies

Enhancing the inherent ion/electron diffusion rate of olivine-structured active materials is pivotal for improving their electrochemical performance because slow lithium diffusion reduces the practical capacity of materials. Currently, ion doping in olivine cathodes is under investigation as a means to boost the ion/electron diffusion rate in the active materials. Doping is conducted in the TM and oxygen (O) sites of the olivine active material, facilitating the diffusion of Li by modifying its lattice structure and electronic properties. Currently, numerous studies are exploring dopants for olivine cathodes; however, finding a suitable dopant that matches the ionic radius of the original ion and improves the conductivity without negatively affecting the electrochemical properties of LFP is challenging. In this chapter, we present the studies that have improved the inherent electron/ion diffusion in the active materials through doping in the TM and O sites.

3.2.1. TM-site doping. Introducing a different metal into the Fe site can modify the bond energy by adjusting the bond length and angle of the crystal lattice.^36–39 Doping LFP with magnesium (Mg) resulted in a decrease in cell volume, a shortening of the Fe–O and P–O bonds, and elongation of the Li–O bonds. These changes directly enhance the mobility of Li by reducing the energy barrier necessary for Li diffusion. Thus, Mg²⁺-doping improved the electronic conductivity of LFP, while preventing the formation of Li–Fe antisite defects and promoting the formation of Fe₂P.³⁷ Consequently, the electronic and ionic conductivity significantly increased by 275 times and 3.6 times, respectively (Fig. 3a).³⁷ Recently, it was reported that Mg and titanium (Ti) co-doping can also contribute to significantly improving the electrical conductivity by adjusting the lattice structure.³⁹ In the manganese-containing olivine structure (LMFP or LMP), the formation of Mn³⁺ during the charging process leads to significant distortion due to the Jahn–Teller effect, which significantly degrades its capacity due to the structural mismatch during the charge and discharge processes. Doping Mg^38,40,41 or Ti⁴² in the Mn-site can also increase the structural stability and alleviate the distortion and maintain high capacity. Doping with high-valence state metals is also a viable consideration.^43–46 Pagot et al. explored the doping of olivine cathodes with high-valence transition metals (HVTMs) such as V⁴⁺, Nb⁵⁺, and Ta⁵⁺ to enhance the fast cycling performance of LFP cathodes.⁴⁷ When HVTM was doped, additional positive charges were introduced in its crystal lattice, which increased the internal mobility of Li and expanded the lithium diffusion pathway, thereby increasing the Li conductivity of LFP. In particular, Nb-doped LFP exhibited a high voltage and specific capacity and greatly improved electrical conductivity (Fig. 3b).

Fig. 3 (a) Electrochemical properties and long cycling stability of LFP and Mg-LFP at 0.1C.³⁷ Copyright 2022, the American Chemical Society. (b) Rate capability of olivines doped with different HVTMs (V⁴⁺, Nb⁵⁺, and Ta⁵⁺) and the undoped reference. A specific capacity of 100% corresponds to the discharge capacity of the first cycle at 0.5.⁴⁷ Copyright 2020, RSC Publishing.

In a view the TM site doping strategy, different metals serve unique purposes, each producing distinct elemental doping effects. TM site doping offers diverse approaches, resulting in favorable outcomes. Moreover, research into complex doping elements can be pursued based on theoretical studies of the underlying mechanism.

3.2.2. O-site doping. Although O-site doping has received comparatively less attention, it holds promise for enhancing the intrinsic properties of materials. Doping sulfur into LFP results in lattice expansion due to the larger anion doping (S²⁻ is larger than O²⁻) and the lower bond dissociation energy.⁴⁸ Additionally, sulfur doping contributes to the suppression of Li–Fe antisite defects, facilitating the unhindered migration of Li through the diffusion channels. In the case of chlorine,⁴⁹ the doped material demonstrates enhanced crystallinity, reduced antisite defects, and expanded lithium diffusion channels, all contributing to the improved kinetics for Li diffusion. However, relatively little doping is performed at the O-sites. To conduct various future studies, the complex effects can be confirmed by applying TM doping and co-doping at the O-sites.

3.3. Surface modification and material architectures

The mobility of electrons and ions within olivine cathodes is influenced not only by their bulk properties but also by the interface reactions between the electrode and the electrolyte, or among particles. Surface resistance impedes the transfer of electrons and Li-ions, potentially decreasing the polaron mobility and overall ionic conductivity of the active materials. Therefore, controlling the interfacial properties through electron-conductive coatings on the surface or modifying the architecture of materials is crucial for enhancing the electronic conductivity, addressing the inherent challenges of low electronic conductivity in LFP.

3.3.1. Surface modification. Carbon is the most commonly employed conductive agent, which is capable of enhancing the electronic conductivity. The formed coating layer on the surface serves as an electron transport channel, thereby improving the electronic conductivity across the particle-to-particle and particle-to-electrolyte interfaces. Furthermore, the coating layer also acts as a protective shield, preventing both chemical degradation caused by the reaction between LFP and HF in the electrolyte and Mn²⁺ dissolution into the electrolyte, thereby retarding the capacity deterioration and ensuring chemical stability, which is critically governed by the thickness and uniformity of the carbon layer (1–3 nm).⁵⁰ The coating layer with excessive thickness hinders Li conduction and lowers the ratio of active materials. Conversely, a coating layer with an insufficient thickness poses challenges in uniform coating. It is well known that the optimized thickness of the carbon layer is around 1–3 nm.⁵⁰ Thus, extensive research has been dedicated to developing uniform coating methodologies and screening coating materials. Generally, chemical vapor deposition (CVD) is widely used for surface coating, offering an advantage for the formation of uniform and conformal carbon layers on materials. Tian et al. proposed a CVD method using solid glucose as the carbon source to coat carbon nanorods on an LFP material.^51,52 Glucose was placed next to LFP and heated for its deposition on the LFP surface through vapor deposition.⁵¹ Compared to direct physical mixing methods, the CVD method demonstrates high purity and uniform coating layer of carbons on the surface. However, the CVD process has the disadvantages of slow coating speed and difficulties in scaling up at the industrial level. Meanwhile, the sol–gel method has a short calcination time, easy control of the amount of carbon source, mass production, and low cost; however, it also has the drawback of non-uniform coating layers and/or undesirable agglomeration of the active materials during the wet process.⁵³ Therefore, the integrated consideration of enhancing the electrochemical kinetics and manufacturing processes is crucial.

Additionally, the higher the degree of graphitization of the carbon layer, the better the conductivity. Therefore, the degree of graphitization is also a crucial factor in enhancing the conductivity. Consequently, carbon sources with a high degree of graphitization and excellent conductivity, such as graphene^54–58 and carbon nanotubes (CNT),⁵⁹ are utilized. Chien et al. enhanced the electrochemical performance of LFP/C composite cathode materials by modifying pristine graphene oxide (GO) to achieve a wrinkled three-dimensional (3D) morphology with an interconnected pore structure.⁵⁴ However, it is difficult to achieve a uniform dispersion and coating using graphene and LFP, prompting a shift towards employing or growing graphene, a highly graphitized form of carbon, as a carbon source during the synthesis process. Hu et al. prepared graphene-modified LiMn_0.8Fe_0.2PO₄ composites via in situ pyrolysis and catalytic graphitization using trace ferrocene as the catalyst precursor (Fig. 4a and b).⁵⁵ The thickness of the graphene carbon layer was approximately 2–3 nm, providing a uniform carbon layer. Therefore, prepared LMFP demonstrated a high discharge capacity of 155.9 mA h g⁻¹ at a C-rate of 0.5, with a capacity retention of 93.8% after 300 charge–discharge cycles.

Fig. 4 (a) Scheme of graphene-modified LiMn_0.8Fe_0.2PO₄ (LMFP/C–Fe) composites prepared via the solid-phase reaction. (b) High-resolution transmission electron microscopy (HRTEM) image of LMFP/C–Fe.⁵⁵ Copyright 2024, Elsevier Ltd.

Metals or metal oxides can act as supporters to enhance the electrical conductivity of active materials due to their high electronic conductivity and scavengers to mitigate the surface reaction with the electrolyte. Ultimately, this increases the cycle reversibility of LIBs and reduces the electrochemical impedance. However, it is challenging to achieve a uniform coating on the surface of the active material due to experimental difficulties, and thus industry-viable coating methods should be developed. Furthermore, excessive coating layers can decrease the ionic conductivity, which can degrade the electrical conductivity. Also, chemical side reactions with the electrolyte and cost competitiveness should be considered. The utilization of various metal and metal oxides such as Au,⁶⁰ Ga,⁶¹ Al₂O₃,^62,63 ZnO,⁶⁴ MoS_x,⁶⁵ LaFeO₃,⁶⁶ and TiO₂ ^67,68 can increase the electrochemical performance by enhancing the conductivity and mitigating side reactions. These materials can synergistically interact with the carbon layer, further increasing the surface conductivity albeit with the minimal carbon content. Additionally, for industrial applications, it is necessary to consider the use of low-cost materials and the development of new synthesis methods that enable uniform surface coating. Moreover, research that utilizes various materials cooperatively to result in simultaneous improvements in conductivity, capacity, and durability should be considered.

3.3.2. Architecture. Controlling the size and shape of particles has also been proven to be beneficial. The extraction and insertion of Li-ions in LFP primarily occur at the interface between the electrolyte and the active material. Therefore, small and uniformly sized particles with a large surface area can provide short Li diffusion paths and large reaction surface, allowing fast charge and discharge cycles by significantly enhancing the rate capability.^69–71 Additionally, limiting the b-axis growth in the crystal structure, where Li movement occurs in LFP, can further improve the Li diffusion by shortening the diffusion path. By controlling the particle shape such as the form of a nanorod coated with a carbon layer,⁷² nanosheets,⁷³ or in the form of a sea urchin⁷⁴ or flower shape,⁷⁵ the electrochemical performance can be greatly enhanced by increasing the surface area of the particle and reducing the diffusion length of lithium in the particle. However, polygonal particles have a higher specific surface area compared to spherical particles, which results in an improved electrochemical rate performance. Nevertheless, these polygonal particles have a relatively low trap density due to their high specific surface area. In addition, in the case of electrode designs requiring a high energy density, active materials with a spherical shape lead to an increase in the fractional density, thereby enhancing the electrode density (Fig. 5a).⁷⁶ Nano-sized particles with a high specific surface area also reveal a lower packing density compared with micron-sized particles (Fig. 5b).⁷⁷ Recently, the blending engineering of spherical particles with various sizes has been introduced in industry to further increase the electrode density (Fig. 5c).⁷⁸ This approach has shown results that simultaneously increase the density within the particles and the electrode.

Fig. 5 (a) Packing density according to the particle shape (degree of roundness).⁷⁶ Copyright 2015, Southern African Institute for Industrial Engineering. (b) Packing fraction of different spherical materials as a function of their size.⁷⁷ Copyright 2020, Elsevier Ltd. (c) Packing density depending on the particle size and bimodal particle distribution loading.⁷⁸ Copyright 2022, Springer.

Core shell^42,79 or concentration gradient structures⁸⁰ can serve as alternative material designs to enhance the conductivity. Yang et al. synthesized an LiMn_0.8Fe_0.2PO₄ cathode with a concentration gradient (CG-LMFP), which was characterized by a linear increase in Mn concentration from the particle edge to the center and the reverse trend for Fe (Fig. 6a–d).⁸⁰ This structure enhanced the particle conductivity by placing Mn, which has low ionic/electronic conductivity, in the core, and Fe, known for its high conductivity, in the shell direction. This structure effectively suppressed the Mn dissolution, particularly at high temperatures, thereby improving both the rate performance and cycling stability of the cathode. In addition to improving the ionic and electronic conductivity of the particle, modification of the material structure is also necessary to achieve a high energy density. Simple nanoparticles create numerous micropores within the electrode, thus extending the diffusion distance for charge carriers. Alternatively, constructing porous spherical aggregates can reduce the tortuosity and generate relatively larger pores within the electrode. Also, synthesizing porous spherical nanoparticle aggregates can achieve a higher trap density compared to powders composed solely of nanoparticles and increase the yield during the production process. Additionally, these spherical aggregates can contribute to increasing the packing density in the electrode than nanoparticles.^81,82 Recently, research has also been conducted on constructing electrodes by mixing two different types of spherical aggregates to achieve a higher packing density (Fig. 5c).

Fig. 6 (a) Schematic of the CG-LMFP synthesis process. (b) SEM image of cross-sections for CG-LMFP and line scan of the atomic ratio of Mn and Fe. (c) Initial charge–discharge curves of LMFP and CG-LMFP at 0.05C and (d) charged at 0.2C and discharged at different rates. Rate capability of LMFP and CG-LMFP.⁸⁰ Copyright 2016, Elsevier Ltd.

Overall, various approaches for enhancing the conductivity at the atomic level have been discussed. By adopting comprehensive strategies such as doping, applying conductive material coatings on surfaces, and modifying material structures in the bulk, it is possible to increase the electrical conductivity, facilitate Li diffusion, and stabilize materials. We anticipate that these adjustments can improve the electrochemical performance of olivine cathodes with high stability.

4. Enhancement of electronic conductivity in thick electrodes

4.1. New design of electronic network

The utilization of Fe, which possesses a high electron transition energy, as the active site in LFP inherently results in low electron conductivity.^83–85 Therefore, the surface coating of the active material with carbon is commonly chosen as a method to effectively facilitate the transfer of electrons from the current collector to the active material.^86–89 However, as the electrode thickens, the active materials located farther from the current collector experience significant depletion of electrons during fast cycling.^90,91 Hence, the introduction of new designs of conductive materials and dispersal methods to establish a well-developed electron network within the electrode has become indispensable. However, to maximize the loading of active materials in the electrode, the loading level of conductive materials should be minimized. Typically, the electrical conductivity (σ_c) of cathode composites follows the percolation theory, which can be described as follows:
where σ_f and ϕ are the electrical conductivity and weight fraction of conductive filler, respectively. ϕ_c is the percolation threshold, while t is the percolation exponent. Therefore, improving the electrical percolation threshold of the electrode should be considered an important criterion in determining the dimensionality and dispersive characters of conductive materials. Historically, zero-dimensional (0D) conductive materials such as carbon black have been utilized in cathodes; however, they are not suitable as the electrode thickens because the critical concentration of 0D carbon required to exceed the electrical percolation threshold within the electrode is higher than that for one-dimensional (1D) or two-dimensional (2D) conductive materials due to their insufficient spatial configuration and point-to-point contact.⁹² Also, 1D and 2D conductive materials easily agglomerate due to their high surface area, requiring additional consideration of effective shear force during the manufacturing of electrode slurries.^92–94 Many strategies have been introduced to address these challenges, and several studies have reported high discharge capacities despite the increased electrode loading amounts (Table 1). In this chapter, we review the recent strategies for selecting the dimensionality of carbons, conductive binders and dispersion methods in the slurry process to lower the electrical percolation threshold of the electrode (Fig. 7).

Table 1 Comparison of the conductive agents, dispersion methods and cell performances

Conductive agent	Dispersion methods	Discharge capacity (mA h g^−1)	Cycle retention	Loading amount (mg cm^−2)	Energy density	Reference
Double-walled carbon nanotubes	Polyvinylpyrrolidone (PVP) as a dispersing agent and binder	∼150 (0.1C)		0.5		95
Ti3C2Tx MXene nanosheets	MXene aqueous dispersion	159 (0.1C)	70 (0.3C)	3.8–4.2		96
SWCNT, Ti3C2Tx MXene	1 h probe sonication to disperse SWCNT in DIW using PVP (Mw = 10 000) as surfactant, and use of think mixer to disperse MXene	∼160 (0.2C)	200 (2C)	∼10		97
S, N-doped CNT	Enhancing hydrophilicity for dispersion in NMP or water	∼165 (0.2C)	100 (1C)	∼3.4		98
SWCNT	Use of PVP and sodium cholate (SC) mixed surfactant to disperse SWCNT	169.2 (0.2C)	500 (5C)	∼1.5		99
		162.9 (0.2C)	200 (1C)	∼8.1
Carbon black, CPC binder	LFP + carbon dispersion in NMP, and CPC binder dispersion in THF/water	∼165 (0.2C)	200 (0.5C)	∼2	∼530 W h kg^−1 (0.1C)	100
CPC binder	LFP dispersion in NMP, and CPC binder dispersion in THF/water	∼150 (0.2C)		∼7
Poly(furfuryl alcohol) (PFA) + acetylene black	Use of furfuryl alcohol and oxalic acid as monomers, and mixing monomers with LFP to fabricate LFP–PFA electrodes	160.3 (0.1C)	500 (2C)	1.69	1350.5 W h L^−1 (0.2C)	101
		∼140 (0.5C)	500 (0.5C)	5.19

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Fig. 7 Strategic approach for forming electrically conductive network for thick electrodes in high energy-density LIBs.

4.2. Dimensionality of carbon

Carbon black is widely recognized for its excellent compatibility with polyvinylidene fluoride (PVDF) and 1-methyl-2-pyrrolidone (NMP), making it a common choice in commercial applications.^102–104 However, its 0D morphology requires a significant quantity to establish an efficient electrical network within the electrode, given its point contact with active materials.⁹² In contrast, 1D and 2D conductive materials demonstrate superior specific surface areas compared to carbon black.¹⁰² These structures, based on line and areal contacts, can construct a dense 3D network, effectively reducing the electrical percolation threshold.¹⁰² 1D conductive materials such as CNTs or carbon nanofibers (CNFs) offer the advantage of a large aspect ratio (>1000),¹⁰⁵ providing a high contact area to effectively encapsulate active materials, while minimizing the hindrance to Li diffusion.^97,102 CNTs exist in two main forms, i.e., single-walled carbon nanotubes (SWCNTs), which consist of a single graphene sheet, and multi-walled carbon nanotubes (MWCNTs), involving additional graphene sheets wrapped around SWCNTs through van der Waals interactions. Due to their high specific surface area, CNTs exhibit not only high electrical conductivity but also excellent thermal and mechanical properties.^106,107 Seo et al. compared the performance enhancement effects of single-wall carbon nanotubes (SWCNTs) and double-wall carbon nanotubes (DWCNTs) as conductive additives for LFP electrodes.⁹⁵ DWCNTs exhibit superior mechanical strength, enhanced crystallinity, and a higher aspect ratio compared to SWCNTs. Both DWCNTs (Fig. 8a) and SWCNTs (Fig. 8b) wrapped around the surface of the LFP particles, forming dense nanotube networks. The LFP/DWCNT electrode (sheet resistance (R_s) = 5.4) revealed lower resistance compared to the LFP/SWCNT electrode (R_s = 8.7). Meanwhile, the LFP/carbon black (LFP/CB) electrode exhibited higher resistance (R_s = 16.7) than the CNT-based electrodes. This is attributed to its higher aspect ratio. DWCNTs with a higher aspect ratio (>5000) than that of SWCNT (>3000) and carbon black (∼1) effectively established an electrical network within the electrode. All three electrodes showed similar initial capacities at a 0.1C charging rate, but the LFP/SWCNT and LFP/DWCNT electrodes exhibited reduced polarization resistance compared to LFP/CB (Fig. 8c). Additionally, the LFP/DWCNT electrode demonstrated superior capacity at accelerated rates (0.2C, 0.5C, and 1C) compared to the other electrodes, owing to the high electrical conductivity of DWCNTs. In the case of 2D conductive materials such as MXene or graphene, they encapsulate the active material based on planar contacts, enabling the formation of an electrical network similar to 1D conductive materials. Zhang et al. reported the formation of an interconnected metal nanosheet network utilizing MXene (Ti₃C₂T_x) as a conductive additive in LFP electrodes, leveraging excellent electrical conductivity and mechanical integrity/strength (Fig. 8d).⁹⁶ LFP nanoparticles were thoroughly encapsulated by MXene, forming an interconnected sandwich structure. The electrical conductivity of LFP/MXene was significantly enhanced compared to LFP/CB. LFP/MXene exhibited very low polarization at various C-rates and delivered a capacity of 159 mA h g⁻¹ at 0.1C (Fig. 8e).

Fig. 8 SEM images LFP electrodes containing (a) DWCNTs and (b) SWCNTs. (c) Charge–discharge curves for LFP/SWCNTs, LFP/DWCNTs and LFP/CB electrodes.⁹⁵ Copyright 2024, the American Chemical Society. (d) Scheme of MXene as conductive binders for both high-voltage cathode and high-rate anode. (e) GCD curves of LFP/MX at different C-rates (top) with its cycling performance shown in the bottom panel.⁹⁶ Copyright 2023, John Wiley and Sons. (f and g) Electrode geometries used in simulation with overlaid mapping of Li-ion concentration in the electrolyte during charging at 5C. (h) Rate performance of the LFP/CNT/Ti₃C₂T_x, LFP/Ti₃C₂T_x, and reference LFP/SP/PVDF cathodes operated with a CC charge–discharge protocol.⁹⁷ Copyright 2024, the American Chemical Society.

Conductive materials in a 2D form, which possess larger surface areas than the active material (e.g., graphene sheets), can hinder Li-ion diffusion.^108,109 Thus, to tackle these challenges, creating a composite conductive network by combining one-dimensional (1D) conductive materials with a high aspect ratio and minimal hindrance to Li-ion diffusion can offer an effective solution. Checko et al. utilized a composite electrode material, combining 1D-SWNT and 2D-MXene (Ti₃C₂T_x) nanosheets, to prepare binder-free LFP electrodes.⁹⁷ The CNTs were coated with porosity on the surface of LFP to enhance the local electron transport, while MXene allowed particle connectivity, thereby improving the electrode integrity, mechanical stability, and electrical pathways. Numerical simulations were performed to predict the distribution of Li concentration within the electrode. Specifically, two types of electrodes were considered, electrodes composed solely of 2D conductive materials and electrodes containing multidimensional conductive fillers. Upon 5C charging, the electrode made solely of 2D conductive materials exhibited sluggish Li-ion diffusion from the active material. This behaviour was attributed to the limited electron mobility, leading to a low ion concentration near the separator. In contrast, the multidimensional electrode showed a uniform concentration gradient due to the efficient electron network, facilitating localized ion extraction near the separator (Fig. 8f and g). Based on the comparison of the electrodes composed of 2D and 0D conductive materials, the multidimensional electrode exhibited superior capacity retention at elevated charging rates (Fig. 8h). Additionally, owing to the excellent ion diffusion and electron conductivity, the multidimensional electrode exhibited a low overpotential, maintaining high capacity even after 200 cycles.⁹⁷

4.3. Dispersiveness of conductive agents

1D or 2D conductive materials such as CNTs and graphene can undergo strong π–π interactions and entanglement, which arise from their substantial surface area and unique topological properties, ultimately resulting in particle aggregation.¹¹⁰ This aggregation impedes the effective dispersion of conductive materials within the electrode. In addition, the aggregation process interferes with the establishment of a well-constructed network connecting the active materials and conductive components.⁹⁴ Consequently, the aggregation process results in an inhomogeneous distribution of active materials among particles within the electrode, weakening its mechanical strength and inducing polarization effects.¹¹¹ It has been reported that various dispersion methods, i.e., physical and chemical dispersion with additives, can contribute to the deagglomeration of the conductive agents.

To disperse a conductive additive throughout the electrode slurry, a physical dispersion process should be employed.^94,110 This process involves applying shear forces stronger than the interactions between the conductive additives, thereby increasing the distance between them. In terms of the physical dispersion of conductive materials, the physical dispersion methods include ultrasonication,^110,112,113 ball milling,^114–116 grinding^117,118 and friction.¹¹⁹ In addition, physical dispersion methods can reduce the degree of aggregation by widening the diameter distribution (Fig. 9a).¹²⁰ However, mechanical dispersion methods such as ball milling and grinding may destroy the structure of the conductive additives,^113,121 potentially altering their electrical and mechanical properties. Moreover, relying solely on physical dispersion methods may not effectively prevent the reaggregation of the conductive additives within the electrode slurry,¹²² necessitating additional post-treatment processes.

Fig. 9 (a) Influence of particle size on the particle fraction response for ground sample using four rotation speeds in a planetary ball mill.¹²⁰ Copyright 2013, Elsevier Ltd. (b) Precipitation of different CNT aqueous dispersion afters standing for 1 h. (c) TEM images of SCNTs. (d) Rate performances of CNTs, SCNTs, and SNCNTs.⁹⁸ Copyright 2021, Elsevier Ltd. (e) Schematic diagram of the process of disassembling bundled CNTs into individually dispersed nanotubes using dispersants. (f) Rate performance.⁹⁹ Copyright 2023, John Wiley and Sons.

To mitigate the agglomeration of the conductive materials, enhancing the interaction between the conductive materials and the solvent or weakening the interaction among the conductive materials should be employed. Among these strategies, introducing additional functional groups on the surface of the conductive materials is a viable approach. Through thermochemical oxidation processes, functional groups containing oxygen atoms such as –C=O, –COOH, and –OH can be decorated on the surface of the carbon additives. This results in the formation of charged oxygen atoms, leading to the generation of negative charges and inducing electrostatic repulsion. Consequently, this facilitates minimal precipitation and yields stable dispersion solutions. However, this method poses a risk of structural degradation during the oxidation process, potentially degrading their inherent characteristics.

Zhao et al. synthesized hydrophilic S-doped carbon nanotubes (SCNTs) by doping sulfur functional groups in the CNT skeleton.⁹⁸ SCNTs exhibited a stable dispersion in an aqueous slurry (Fig. 9b), and their tube morphology remained unchanged even after sulfur doping (Fig. 9c). Compared to the pristine CNTs, SCNTs showed a higher Raman D-band to G-band ratio (I_D/I_G), indicating the introduction of more defects due to sulfur doping. Nonetheless, SCNTs demonstrated a superior discharge capacity at 5C (132 mA h g⁻¹) compared to pristine CNTs (124 mA h g⁻¹) (Fig. 9d). This improvement can be attributed to the excellent dispersion of SCNTs within the electrode, which facilitates the formation of a conductive network.

In addition to physical and chemical methods, dispersion studies utilizing physical adsorption techniques such as surfactants and polymer wrapping have been reported.^123–125 These dispersion methods through surface coating induce electrostatic repulsion, thereby reducing the van der Waals forces.¹²³ One of the advantages of these dispersion methods via surface coating is the ability to preserve the electrical conductivity without damaging the surface of the conductive additives.¹²⁶ However, the use of dispersants or polymers often necessitates an additional removal process in the final stage due to their low electrical conductivity, resulting in additional costs.

Guo et al. utilized a poly(vinylpyrrolidone) (PVP) and sodium cholate (SC) mixed dispersant, which could be adsorbed onto SWCNTs (Fig. 9e), and confirmed that re-agglomeration could be prevented by electrostatic and steric hindrance effects through molecular dynamics simulation (MD).⁹⁹ The MD simulation demonstrated that under the dipole–dipole electrostatic interactions of PVP/SC, serving as agents to disperse the SWCNT bundles, the efficient dispersion of SWCNTs could be achieved in NMP. The electrodes fabricated with SWCNTs effectively distributed LFP and PVP/SC, therefore showing the discharge capacity of 161.5, 156.1, 146.8, 130.2, and 111.9 mA h g⁻¹ at 0.5, 1, 2, 5C, respectively (Fig. 9f). Furthermore, the retention rates were 87.4% and 71.3% after 200 cycles at 2C and after 500 cycles at 5C, respectively.

4.4. Conductive polymers

Binders are essential for strengthening the physical cohesion among the components of the electrode. Nevertheless, the utilization of electrically non-conductive binders impedes electron migration within the active materials, resulting in detrimental effects on the electronic conductivity of the electrode.^127,128

When considering an olivine cell excluding electrochemically inert elements, it theoretically attains a volumetric energy density of 2107 W h L⁻¹ (intrinsic material density of 3.6 g cm⁻³).¹²⁹ However, including electrochemically inactive components, the volumetric energy density of commercial olivine batteries is estimated to be around 1100 W h L⁻¹.¹⁰¹ This indicates that only half of the theoretical value can be utilized in the actual cell, highlighting the importance of not only increasing the electrode thickness but also enhancing the functions and optimizing the ratio of conductive additives and binders to achieve a higher volumetric energy density.

Conductive polymers, which are capable of fulfilling the dual roles of conductive additives and binders, have attracted attention as promising functional materials for the utilization of the theoretical volumetric energy density in olivine batteries. These conductive binders not only sustain the physical architecture of the electrode but also facilitate electron conductivity, thereby amplifying the critical mass loading of the active materials for thick olivine electrodes. Typically, conductive polymers exhibit a conjugate structure, an alternating structure between single and double bonds, which lower the energy difference between the bonding and non-bonding orbitals compared to non-conjugate structures.^130–132 This unique structure enables delocalized π electrons to conduct electricity directionally under an applied electric field.¹³¹

Pace et al. reported the utilization of an anionic conjugated polyelectrolyte (CPE) and cationic polymeric ionic liquid (PIL), which were electrostatically complexed into a CPE-PIL composite (CPC) as a conductive binder for LFP electrodes (Fig. 10a).¹⁰⁰ Poly[6-(thiophen-3-yl)hexane-1-sulfonate-co-3-(hexylthiophene)] (PTHS : P3HT) (90 : 10) showing insulation properties up to approximately 3.2 V was used as the CPE. Consequently, it revealed insulating property in the discharge voltage range (3.4–3.2 V) of LFP, providing protection against over-discharge. Poly[(3-methyl-1-propylimidazolylacrylamide)-co-3-methyl-1-(propylacrylamide)] (90 : 10) was used as the PIL. The acrylate functionalized with an imidazolium side chain in PIL exhibited a broad electrochemical stability window. The LFP cathodes containing the CPC binder showed higher capacities compared to that containing PVDF binder, ranging from 0.1 to 6C rates. Furthermore, at each rate, the LFP cathodes with CPC binder showed a superior performance in terms of capacity retention compared to that with PVDF binder. This improvement was attributed to enhanced charge transport kinetics of the electrode due to the CPC conductive polymer. The galvanostatic intermittent titration technique (GITT) proved that the CPC binder decreased the kinetic barrier by increasing the transfer rate of the active material during the charge and discharge processes. Although the thermodynamic overpotential was similar for both binders, a reduced kinetic overpotential was observed for the LFP cathodes containing CPC compared to that containing PVDF, particularly at the potential plateau (3.4–3.5 V). This contribution enabled the LFP cathodes with CPC to provide a stable electrochemical performance over multiple cycles, as evidenced by their higher cycle retention rate of 72% compared to 61% for the PVDF cells after 300 cycles (Fig. 10b).

Fig. 10 (a) Molecular structure of CPC-PIL composite. (b) Discharge capacity and (b) cycling stability for LFP : carbon : binder (85 : 6 : 9 wt%) composite cathodes using the CPC binder or PVDF.¹⁰⁰ Copyright 2023, the American Chemical Society. Capacity retention of composite cathodes shown as specific capacity vs. cycle number. (c) Chemical structure of FA and PFA and the oligomer containing a conjugated diene structure. (d) Rate performance of the LFP–PFA electrode from 0.2 to 5C. (e) Comparison of the practical volumetric performance of the different samples.¹⁰¹ Copyright 2019, John Wiley and Sons.

Liu et al. fabricated poly(furfuryl alcohol) (PFA) as a conductive polymer, which played a key role in the superior mechanical strength and electrochemical properties of LFP cells.¹⁰¹ PFA, synthesized via an in situ polymerization mechanism with furfuryl alcohol (FA) monomers, exhibited a long-range order structure of conjugated dienes, which efficiently facilitated electron transfer, while oxygen heteroatoms reduced the diffusion barrier of Li-ions, thereby reducing the pore/volume/electrolyte content at the electrode level (Fig. 10c). During polymerization, a dehydration reactions occurred at the hydroxymethyl groups of FA, coupling with the cations at the C5 position of the other furan rings. Despite the absence of conductive additives, the LFP–PFA electrodes exhibited a comparable rate performance and cycling stability to the LFP–PVDF electrodes (Fig. 10d). Furthermore, they reported that the LFP–PFA electrodes showed a better specific energy density and specific power compared to the LFP–PVDF electrodes. This was attributed to the increase in the electrode content without the need for additional conductive additives (Fig. 10e).

5. Enhancement of ionic conductivity in thick electrode

The electrodes in LIBs are composites composed of solid particles of various shapes, inherently resulting in a porous structure. These porous channels represent the pathways that the Li-ions in the electrolyte navigate to reach the active material, which are expressed as tortuosity (α). As the electrode thickness increases, the lithium-ion pathway lengthens, resulting in increased tortuosity near the current collector. During the high-rate charge and discharge process, high tortuosity in the electrode leads to a progressively lower lithium-ion concentration environment, thereby causing reaction non-uniformity among the active materials. This non-uniformity can degrade the performance and efficiency of the battery. According to Newman's model, the effective electrolyte diffusion in a porous electrode (D_l,eff) is related to the porosity in the liquid electrolyte-filled region (ε_l), as follows:¹³³

D_l,eff = D_lε_l^α

To realize fast ion transport in the active material of lithium-ion battery electrodes, two key factors are important. Firstly, the design of new anions or the optimization of lithium salt concentration can significantly impact the lithium-ion conductivity in the electrolyte. In addition, the development of lithium-ion transport channels in the electrode can shorten the lithium-ion pathway, thereby reducing the tortuosity of the electrode (Fig. 11).

Fig. 11 Strategic approach for constructing a conductive network of Li-ions in the electrolyte of thick olivine cathodes.

5.1. Electrolyte engineering

The rational design of electrolytes becomes more important in thick electrodes. The facile transport of electrons and Li-ions should be achieved throughout the electrode structure. However, during fast cycling, thick electrodes suffer from more pronounced Li-ion and electron depletion near the current collector and the electrolyte, respectively.¹³⁴ This unbalanced charge transport deteriorates the material utilization at the electrode level. Furthermore, it accelerates the material degradation given that only the localized active materials react under high current conditions. Therefore, it is of great importance to rationally design new salt anions or optimize the salt concentration for electrodes with an increased thickness. In addition, the implementation of structural engineering to develop electrolyte percolation in thick electrodes can be beneficial for facile Li-ion access. Although it is widely known that Li-ion diffusion is optimized in carbonate-based organic electrolytes at a lithium salt concentration of around 1 M, the overall electrolyte system should be revalidated to maximize the utilization of thick cathodes.

5.1.1. Addition of solvent species. A common method to improve the electrolyte conductivity is to change the solvent species. In the case of conventional LIBs, electrolytes consisting of lithium hexafluorophosphate (LiPF₆) as the lithium salt and solvent mixture based on organic carbonates such as linear carbonates and cyclic carbonates are used. Linear carbonates show low viscosity, which can enhance the diffusion of ions, but have a low dielectric constant, limiting the dissociation of lithium salts such as LiPF₆. Alternatively, cyclic carbonates have relatively high viscosity and high dielectric constant, and thus although they facilitate the dissociation of the lithium salt, the ionic conductivity is limited. Thus, an approach is required to enhance the ion transport kinetics by lowering the electrolyte viscosity through control of its solvent species, considering the dielectric constant and viscosity characteristics of each solvent species.

One of the solvents commonly used together with carbonate electrolytes is aliphatic esters, such as methyl, ethyl, and propyl acetate (MA, EA, and PA, respectively).^135,136 These ester solvents exhibit low viscosity and moderate dielectric constant, which significantly improve the mobility of Li-ions. Cai et al. conducted research to improve the rate performance of thick electrodes using MA and acetonitrile (ACN) as solvent additive species.¹³⁵ These additives were added to conventional electrolytes based on ethylene carbonate (EC) and ethyl methyl carbonate (EMC). MA with a moderate dielectric constant and viscosity enabled the fast diffusion of Li-ions owing to the lower viscosity of the electrolyte. ACN with low viscosity and high dielectric constant promotes the dissociation of Li salt and migration of Li-ions. Accordingly, ACN also provides high ionic conductivity, which contributes to improving the performance of Li-ion batteries. The performance at a high current density, capacity retention at increased cycling rates and ionic conductivity differences were also evaluated in cells using ester-based solvents (MA, EA, PA) (Fig. 12a and b).¹³⁶ These studies showed that the combination of thick electrodes and ester-based electrolytes is a promising approach to improve the rate performance of Li-ion batteries. However, ester-based solvents are known to be less stable than carbonate solvents given that they form SEI components with poor passivating properties. Other additives may also have similar disadvantages, and thus it is necessary to apply a solvent species for thick electrodes considering this.

Fig. 12 (a) Ionic conductivity of MA (pink), EA (blue), PA (gray), and carbonate (black) electrolytes. (b) Charge/discharge cycling evaluation of thick-electrode LTO/LCO cells using MA (pink), EA (blue), PA (gray), and carbonate (black) electrolytes.¹³⁶ Copyright 2023, the American Chemical Society. (c) Specific discharge capacity with five different LiPF₆ concentrations and (d) relative discharge capacity with five different LiPF₆ concentrations compared to specific capacity at the same current density using a 1 M electrolyte.¹³⁷ Copyright 2020, Wiley-VCH. (e) Conductivity of LiFSI and LiPF₆ in (EC : EMC) (30 : 70 wt%) as a function of concentration and temperature. (f) Charge (different C rate) and discharge voltage curves (all at 0.5C) of NCM811/graphite cells with LiPF₆ and LiFSI electrolyte.¹³⁸ Copyright 2019, Elsevier Ltd.

5.1.2. Salt concentration. Another strategy to solve the above-mentioned issue is increasing the concentration of Li-ions in the electrolyte. A high-concentration electrolyte (HCE) with a concentration higher than 1 M allows more Li-ions to be transferred at the cathode-electrolyte interface, lowering the charge transfer resistance, and Li-ion depletion in the electrolyte can be alleviated, which occurs when thick electrode batteries operate at high current densities. Through modelling, Du et al. proposed a method of increasing the lithium salt concentration in the electrolyte from 1 to 1.5 M to solve the problem of ion inhomogeneities occurring in thick electrodes.¹³⁹ According to the simulation, the active materials close to the aluminium (Al) current collector react less during the discharge process compared to that close to the separator. The concentration of Li-ions in the electrolyte was greatly reduced near the Al current collector, which became more severe during the high-rate discharging process. As a strategy to solve this issue, Du et al. proposed a method of increasing the lithium salt concentration in the electrolyte from 1 to 1.5 M.¹³⁹ As the concentration increased, depletion of Li-ions was prevented and the mass transfer limitation was resolved, resulting in an increase in energy density. Kremer et al. experimentally investigated the effect of increasing the LiPF₆ salt concentration from 1 to 2.3 M on the performance of ultra-thick NCM 622 electrodes in Li-ion batteries (Fig. 12c and d).¹³⁷ According to this study, when the electrolyte salt concentration was more than 1 M, the discharge capacity of the ultra-thick electrodes was significantly improved by more than 50% at a current density of 3 mA cm⁻². This improvement originated from the relief of Li-ion depletion in the electrolyte-filled pore space of the electrodes, which was resolved by a high salt concentration. However, when the concentration exceeded the optimal concentration of 1.9 M, the cell performance deteriorated due to the too high viscosity of the electrolyte, followed by a decrease in the ionic conductivity. These studies emphasized the importance of controlling the lithium salt concentration in the electrolyte to optimize the performance of Li-ion batteries with thick electrodes and suggested that a balance between charge carrier density and electrolyte viscosity is necessary to improve the energy density. Although these studies are based on NCM-based cells, olivine-based thick electrodes seem to show similar behaviour. Therefore, the Li-ion transport should be carefully considered for the stable operation of LFP-based thick electrodes.

5.1.3. Selection of salt species. The chemical structure, solubility, ionization energy, and ionization tendency of the lithium salt directly influence the dissolution and formation of Li-ions in the electrolyte, serving as the primary determinants of the ionic conductivity of the electrolyte. Therefore, lithium salts that facilitate the structural dissolution of Li-ions exhibit higher ionic conductivity. Furthermore, the transference number, representing the ratio of positive and negative ions contributing to the overall current flow, is an important factor in ion transfer.^140,141 For example, as the transference number of Li-ion approaches 1,¹⁴⁰ the charge movement is predominantly due to Li-ions. This is preferable for Li to transfer a large amount of ionic current. However, in common electrolytes, the transference number is typically less than 0.5.¹⁴¹ Even with electrolytes exhibiting relatively low ion conductivity, an increase in the transference number can significantly enhance the battery performance. Consequently, an electrolyte with both high Li-ion conductivity and transference number is considered ideal. In general, Li salts with a large-size anion such as lithium bis(fluorosulfonyl)imide (LiFSI) exhibit a higher transference number compared with common salts such as LiPF₆ because the movement of anions is limited.^138,142,143 Du et al. demonstrated that LiFSI exhibits higher ionic conductivity and an Li-ion transference number than the commonly used LiPF₆ salt. As a result, it significantly enhances the fast-charging performance of high-energy-density Li-ion batteries (Fig. 12e and f).¹³⁸ Furthermore, a new high-performance electrolyte composed of LiFSI, LiPF₆ and carbonates demonstrated both high ionic conductivity and superior electrochemical stability, even at a 6C charging rate.¹⁴³ These studies confirmed that using LiFSI with a bulky anion enhances the cell performance by facilitating Li-ion transfer through an electrolyte with a higher transference number. However, it should be noted that the stability of lithium salts is a highly important factor in achieving a long battery life. A high ionic conductivity in the electrolyte can only be maintained when the lithium salts show thermal and chemical stability. Also, the Li salt should prevent decomposition and corrosion of the electrolyte or corrosion of the Al current collector. Lastly, lithium salts can react with the active materials, affecting the properties at the electrolyte–cathode interface, followed by forming layers such as cathode–electrolyte-interphase (CEI) layer. A CEI layer has a strong effect on the ionic conductivity. Therefore, the selection of an appropriate lithium salt can be an important factor in the battery performance including not only the ionic conductivity of the electrolyte, but also the stability of the electrolyte, formation of an interface between the electrode and electrolyte, and efficient lithium conduction at the interface.

5.1.4. Electrolyte additive. Generally, electrolyte additives play a role in enhancing the cycle performance and interface stability of cells due to the stabilization of the electrode surface. Meanwhile, several studies have focused on the improvement of electrical and ionic conductivity by electrolyte additives. Recently, the conductivity of thick electrodes could be improved by adding electrolyte additives, which led to the formation of a stable cathode electrolyte interphase (CEI) layer on the surface of olivine cathodes. Moon et al. proposed a triazole-based electrolyte additive, 1-(trimethylsilyl)-1H-benzotriazole (TMSBTA), which helped mitigate the dissolution of iron and construct a balanced CEI layer for HF scavenging.¹⁴⁴ This additive not only mitigated iron dissolution and reinforced the CEI layer but also supported the creation of a conductive CEI layer, which maintained a balance between the ionic and electronic conduction. This conductive CEI layer enhanced the electrochemical reaction rate and stability of olivine cathode materials. Therefore, electrolyte additive engineering that can enable the formation of an electrochemically active CEI layer can be one of the critical research themes for achieving high energy density in thick electrodes.

Overall, the various approaches to improve the conductivity of Li-ions in the electrolyte were discussed. The conductivity can be increased and stabilized by optimizing all the components of the electrolyte, such as solvent, Li salt species, salt concentration, and additive. The optimal electrolyte system may vary depending on the thickness, porosity, etc. of the electrode, and thus optimization and in-depth analysis of all the parameters that can affect the conductivity are necessary according to the electrode structure.

5.2. Electrode architecture engineering

Many researchers have conducted studies aimed at enhancing the energy storage performance of batteries by designing high operating voltages and specific capacities through the manipulation of intrinsic properties such as crystal structure and chemical composition of the active material. However, despite the collective efforts of many researchers, the actual energy storage performance of batteries often falls significantly short of their theoretical values. The disparity between theoretical and practical performance is typically attributed to the incomplete activity of the active material. Specifically, the scale of ion transfer paths within the electrode is much smaller than that of the active material itself. Also, the uneven distribution of these paths can adversely affect the activity of the active material. Within the electrode, the ion transfer paths of the active material can become pores. As the electrode becomes thicker, it introduces complex pathways and extended distances for Li-ion diffusion, leading to deficiencies in the physical ion transfer capability within the electrode.¹⁴⁵ Therefore, research on electrode pore engineering for enhancing the ion transportation is considered crucial. Numerous three-dimensional porous fabrication strategies have been introduced, employing various types of conductive additives and binders (Table 2).

Table 2 Comparison of the conductive agents, binders, electrode structure architecting methods and cell performances

Conductive agent, binder	Electrode structure architecting methods	Discharge capacity (mA h g^−1)	Cycle retention	Loading amount (mg cm^−2)	Energy density	Reference
Super P/polyethersulfone (PESF) and polyvinyl pyrrolidone (PVP)	Phase inversion	∼160 (0.1C)		60		146
		∼160 (0.1C)		45
		∼160 (0.1C)	120 (100 mA g^−1)	30
Super P/polyethersulfone (PESF) and polyvinyl pyrrolidone (PVP)	Phase inversion	∼150 (0.1C)	120 (80 mA g^−1)	70		147
		∼150 (0.1C)		55
		∼150 (0.1C)		30
Cellulose nanofiber (CNF) instead of CNT/binder	Template methods – directional freezing methods	∼150 (0.2C)	100 (1C)	20		148
		∼160 (0.2C)		30
		∼150 (0.2C)		50
		∼150 (0.2C)	40 (1C)	60	15.2 mW h cm^−2
Carbon black (CB)/sodium carboxymethyl cellulose (CMC)	Template methods	155 (0.5 mA cm^−2)	150 (150 mA g^−1)	128		149
Ketjen black (KB)/poly(vinyl pyrrolidone) (PVP)	Additive manufacturing – 3D printing	133 (0.2 mA cm^−2)	∼50 (0.2 mA cm^−2)	108	20 mW h cm^−2	150
Acetylene black (AB), and MWCNTs/PVDF-HFP	Additive manufacturing – 3D printing	∼150 (0.1C)	150 (0.1C)	10		151
		7.5 mA h cm^−2 (∼0.1C)	∼60 (∼0.1C)	∼50	69.41 J cm^−2 (2.78 × 10^−4 W h cm^−2)

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5.2.1. Phase inversion methods. The phase inversion method involves immersing the electrode slurry into a nonsolvent bath, leading to mutual exchange between the solvent and nonsolvent through diffusion.^152–154 This process results in the precipitation of the electrode, while concurrently forming finger-like pores, where solvent and nonsolvent regions coexist.¹⁵⁵ Finger-like pores reduce the tortuosity of the ion transport pathways within the electrode, thereby shortening the pathway for Li-ions from the separator to the electrode.^152,153 Wu et al. utilized an electrode slurry in the phase inversion process during electrode manufacturing, applying an external magnetic field to design electrodes with vertically aligned pores together with a gradient of the active material (Fig. 13a).¹⁴⁶ The concentration of Li-ions in the electrolyte at the bottom of the electrode was only 18.6% compared to that at the top, gradually decreasing the concentration of the active material from the top to the bottom to enhance its activity (Fig. 13b). The vertically aligned pores conferred a low tortuosity effect, reducing the electrolyte penetration and ion transfer distances. Therefore, even with a high loading of active material, the rate performances were well maintained, and the discharge capacity was reported to be the highest when the concentration gradient of the active material was downward (discharge capacities of 100.8 and 67.3 mA h g⁻¹ at discharge rates of 1 and 1.5C, respectively) (Fig. 13c).

Fig. 13 (a) Fabrication process of the LFP gradient in low-tortuosity electrodes. (b) COMSOL simulation of Li⁺ distribution in low-tortuosity electrodes. (c) Rate performance of upward, homogeneous, and downward electrodes at LFP loading of 60 mg cm⁻².¹⁴⁶ Copyright 2022, the American Chemical Society. (d) Schematic of enhanced Li⁺ concentration distribution in gradient porous electrodes compared to uniform architecture. Weight distribution maps of NMF components corresponding to carbon and LFP down to an electrode depth of 55 μm on (e) the small pore side (125 × 125 μm) and (f) the large pore side (225 × 225 μm). Rate performance comparison of invert (g) and upright (h) oriented electrodes under different mass loadings, with corresponding semiempirical fitting curves.¹⁴⁷ Copyright 2022, American Chemical Society.

The ion flux can vary depending on the gradient of the pore space diameter, which can impact the electrochemical performance of the cell. Zhang et al. developed gradient porous electrodes with not only a concentration gradient of the active material but also a pore size gradient using the phase inversion method (Fig. 11d).¹⁴⁷ In the inverted gradient porous electrode, larger pores (Fig. 13f) were positioned near the separator to accommodate a higher ion flux, while smaller pores (Fig. 13e) were located towards the current collector to maximize the active material loading. These inverted porous electrodes exhibited a significantly improved rate performance compared to the upright-oriented electrodes. This effect intensified as the mass loading per unit area increased. At a mass loading of 70 mg cm⁻², the inverted porous electrode showed a discharge capacity of 122.8 mA h g⁻¹ at 0.5C (Fig. 13g), whereas that of the upright electrode was only 53.5 mA h g⁻¹ (Fig. 13h).

5.2.2. Template methods. The template method involves utilizing a scaffold within the electrode to either autonomously create ion channels or to be sacrificed to form ion channels. As the template structure becomes more aligned, ion channels with low tortuosity are formed.^156–158

The solvent in the aqueous electrode slurry, water, exhibits the characteristic of ice crystals aggregating when the temperature falls below the ice point at a certain rate. If freezing begins in one direction, it can form ice crystal channels within the electrode.^159–161 Li et al. employed the directional freezing method to create vertical ion channels within the electrodes of 3D electrode slurry structures fabricated using 3D printing (Fig. 14a).¹⁴⁸ The directional freezing method involves forming ice templates vertically within the electrode, and subsequently sacrificing the ice crystals through additional freeze drying to create aligned vertical ion channels within the electrode (Fig. 14b and c). Electrodes with directional channels exhibit superior ion diffusion kinetics compared to that with unidirectional channels, resulting in better performance outcomes in terms of speed.

Fig. 14 (a) Schematic of the fabrication process of the 3D-printed flexible LIB electrode. (b) Side view and (c) enlarged SEM images of the conventional freezing printed LFP/CNT/CNF electrode.¹⁴⁸ Copyright 2022, John Wiley and Sons. (d) Schematic of the FAT electrode. (e) Image showing the rolled-up cylindrical electrode and the FAT electrode after cutting. (f) 3D reconstructed X-ray μ-CT images of the interior morphology of the FAT electrode, showing a uniform and highly aligned carbon fiber arrangement in the thickness direction of the electrode.¹⁴⁹ Copyright 2020, the American Chemical Society.

Aligned conductive materials can serve as templates, facilitating the design of electrode structures that promote not only the movement of Li-ions but also efficient electron transfer. Shi et al. fabricated thick electrodes with low tortuosity by embedding the LFP active material within an aligned carbon fiber membrane. The fiber-aligned thick (FAT) electrode, leveraging the unidirectional alignment of carbon fibers, enhanced both the electrical and thermal conductivity of the electrode, enabling rapid ion transfer in low tortuosity channels within the electrolyte (Fig. 14d).¹⁴⁹ FAT electrodes could be manufactured in cylindrical shapes with a diameter of ∼18 mm (Fig. 14f) and height of ∼55 mm using the roll and cut method (Fig. 14e). Compared to the slurry-cast electrodes, the FAT electrodes demonstrated a superior rate capability at the current densities of 0.5, 2, and 5 mA cm⁻². At a current density of 2 mA cm⁻², the FAT electrodes demonstrated 68% capacity retention after 150 cycles, whereas the slurry-cast electrodes retained only 45% of their initial capacity.

5.2.3. Additive manufacturing. Three-dimensional porous electrodes can provide shorter Li-ion diffusion pathways and larger surface areas than conventionally prepared electrodes, thereby enabling fast Li-ion transport to the surface of LFP particles. This fast Li⁺ transport can alleviate the reaction inhomogeneity among the particles throughout the electrode. Complex and hierarchical nanopores have been extensively utilized to construct 3D electrode structures.

Additive manufacturing, which digitally controls the stacking of multiple 2D layers, is utilized to architect complex 3D matrices, providing precise control and enabling rapid prototyping capabilities.¹⁶² This method requires software-assisted equipment to achieve precise shape control of the electrode slurry. Thus, the additive manufacturing technique is beneficial to introduce physical gradients within the 3D electrode.

Recently, a printing method called aerosol jet printing (AJP) has been proposed, which involves the deposition of extremely fine electrode slurry droplets onto a substrate through pneumatic collision atomization (Fig. 15a). Wilkinson et al. evaluate AJP as a new hybrid process suitable for micro-manufacturing due to its faster deposition rate compared to the traditional 3D printing method, confirming that it is suitable for large-scale production.¹⁶³

Fig. 15 (a) Overview of AJP.¹⁶³ Copyright 2019, Springer. Cross-sectional scanning electron microscopy images of the LFP cathode prepared through aerosol jet printing and imaged at magnifications of (b) 100× and (c) 500× (d). Voltage versus discharge capacity for an aerosol jet-printed LFP cathode tested at rates of C/15, C/10, C/5, C/2, 1C. (e) Average discharge capacities recorded for aerosol jet-printed cathodes. Print A average corresponds to the average discharge capacity for two battery samples made from one printed LFP tape. Print B average corresponds to the average discharge capacity for three battery samples made from another printed LFP tape. Tapes A and B were made on different days from different batches of ink.¹⁶⁴ Copyright 2019, John Wiley and Sons.

Deiner et al. reported the fabrication of thick micro-structured LFP cathodes with thicknesses ranging from approximately 100 to 150 μm using the AJP method by depositing submicron electrode slurry droplets (Fig. 15b).¹⁶⁴ The AJP-fabricated LFP cathode contained over three times higher pore volume (1.7 nm to 300 nm) than the conventionally cast LFP cathodes. Although the AJP-fabricated electrode density was 0.9–1.0 g cm⁻³, which is around half of the commercial LFP cathode density (1.8 g cm⁻³), an excellent rate performance could be achieved (151 mA h g⁻¹, 145 mA h g⁻¹, 130 mA h g⁻¹, 119 mA h g⁻¹, and 105 mA h g⁻¹, at discharge rates at C/15, C/10, C/5, C/2, and 1C, respectively (Fig. 15d)). At a C/5 rate, the AJP-fabricated LFP cathode exhibited 89% cycle retention over 50 cycles, demonstrating enhanced electrochemical stability (Fig. 15e).

The 3D printing method enables the direct, rapid, and precise control of the 3D structure of electrode slurries with viscoelastic and rheological properties, allowing the fabrication of customized ion channel architectures.^165,166 The rheological properties of the electrode slurry used in 3D printing significantly affect the printing quality such as printability and resolution.^165,167,168

Wei et al. employed the direct ink writing (DIW) 3D printing method to create architecture-customized LFP electrodes (Fig. 16a).¹⁵⁰ The LFP-based ink consisted of LFP and Ketjen black (KB) carbon particles, with PVP serving as a dispersant. This ink exhibited the necessary viscoelastic properties for direct ink writing (Fig. 16b and c). Consequently, the cells fabricated using LFP electrodes with a thickness of 1 mm demonstrated a capacity of 133 mA h g⁻¹ at a current density of 14.5 mA cm⁻² (Fig. 16d). In another study, Wang et al. fabricated LFP electrodes with various geometric structures using LFP/acetylene black (AB)/MWCNT/PVDF-HFP ink and a 3D printer modified with a paste-extrusion-type tool head (Fig. 16e).¹⁵¹ These LFP electrodes featured grid, ring, and line patterns (Fig. 16f). The spaces between the filaments allowed the fast diffusion of the electrolyte into the active material. The LFP electrodes with three different patterns exhibited a capacity of approximately 150 mA h g⁻¹ at a current density of 0.1C. However, at higher speeds, the line pattern showed a higher capacity (Fig. 16g), again highlighting that the porous morphology of the electrodes can influence the electrochemical behaviour of the cell.

Fig. 16 (a) Schematic representation (expanded view) of a fully 3D printed Li-ion square cell battery with outer dimensions of 1 cm × 1 cm × 2.5 mm and inner hole dimensions of 6 mm × 6 mm. (b) Apparent viscosity as a function of the shear rate and (c) elastic (G′) and loss (G′′) moduli of the four functional (cathode, separator, anode, and packaging) inks. (d) Voltage as a function of areal capacity data for LFP/lithium titanium oxide Swagelok cells of varying electrode thicknesses at 0.2 mA cm⁻².¹⁵⁰ Copyright 2018, John Wiley and Sons. (e) Schematic of 3D-printed self-supported LFP electrodes. (f) Schematic and SEM images of three types of 3D-printed LFP electrodes. (g) Comparison of the rate performance of the three types of LFP electrodes.¹⁵¹ Copyright 2018, the American Chemical Society.

6. Enhancement of mechanical stability in thick electrodes

6.1. New design of electronic network

In the manufacturing process of thick electrodes, conventional slurry coating methods are associated with several challenges (Fig. 17), as follows: (1) during the drying process, solvent evaporation can induce contraction stress in the electrode.^13,169,170 (2) Calendaring generates high shear stress, thus increasing the risk of mechanical failure such as electrode delamination or breakage.¹³ (3) As the thickness of the electrode increases, the gradient of Li-ion concentration in the electrolyte becomes more pronounced, thus leading to the inhomogeneity of mechanical stress along the vertical direction of the electrode due to localized volume changes.^171,172 (4) Surface adhesion mismatches between the active material layer and the current collector also generate uneven shear stress, leading to the risk of mechanical failure of the electrode upon cycling.^173–177 These issues arise when insufficient binding force exists between the components of the electrode or weak interaction between the electrode slurry and the current collector, compounded by the volume expansion of the active material.¹²⁷ Thus, to address these challenges, we briefly review the recent strategies to enhance the mechanical stability of electrodes by optimizing the binders and current collectors.

Fig. 17 Schematic summarizing the factors leading to mechanical failure of the electrode.

6.2. High adhesion binder

The primary role of the binder is to maintain the integrity of the active material and conductive materials, preserving the structure of the electrode. However, insufficient binding ability against external stresses can lead to electrode delamination and cracking, impairing the efficient exchange of ions and electrons between the components within the electrode, and finally degradation of the electrochemical performance.^{127,178–180} Also, breakage of the electrode can lead to short-circuiting in batteries, posing safety concerns. The deficiency in binding ability is mainly attributed to a lack of interfacial adhesion between the binder and adherent, and binder breakage itself.

6.2.1. Enhancing interfacial adhesion of binder. The phenomenon of interfacial adhesion deficiency occurs due to inadequate physical bonding between the binder and the adherend. As the mass loading of active material increases, more adherends such as active materials, conductive materials and current collector need to be attached, and stronger adhesion is required to maintain the structural stability. However, the commonly used PVDF binder falls short of satisfying these requirements. The C–F bonds in PVDF interact with the adherent through weak van der Waals forces.¹⁸¹ However, as the mass loading of active material increases, the adhesion becomes insufficient to maintain the integrity of the electrode. Additionally, PVDF easily absorbs the electrolyte, leading to swelling due to the absorption of the electrolyte, which weakens the adhesive strength and mechanical strength of the binder and may cause potential instability in the integrity of the electrode.^182,183 One method to address this issue is introducing functional groups that interact more strongly with the adherent, such as hydrogen bonds.^184–188 In addition, coulombic forces, π–π stacking, etc., rather than van der Waals forces, can enhance the adhesion.

However, excessive interaction among functional groups can induce the self-cohesion of the binders, leading to a deterioration in their adhesion and elasticity. Accordingly, introducing a crosslinking agent can serve as a solution to address these issues. Zhang et al. introduced 6-amino-1-hexanol (AH) into poly(acrylic acid) (PAA) binders rich in carboxyl groups (–COOH), inducing the deprotonation of the carboxyl groups in PAA.¹⁸⁹ The deprotonated carboxyl groups (–COO–) reduced the hydrogen bonding due to electrostatic repulsion, causing the PAA chains to elongate, thereby reducing the self-cohesion and resolving the adhesion and elasticity degradation (Fig. 18a). The AH–PAA complex (APA) exhibited a higher adhesive strength of APA/CNT (5.26 N) compared to PAA (3.58 N) due to the abundant hydrogen bonding and covalent bonding interactions, as well as an efficient sheet-to-point bonding mode (Fig. 18b). This superior adhesion was attributed to the higher wettability of the APA/CNT slurry compared to the PAA slurry, as determined by contact angle measurements. Upon folding the APA/CNT and PAA electrodes three times, surface cracking occurred in the PAA electrode. In contrast, the APA/CNT electrode showed minimal changes, indicating excellent adhesion (Fig. 18c). The APA/CNT electrode demonstrated lower charge transfer resistance compared to the PAA electrode. Furthermore, the APA/CNT electrode exhibited a high capacity of 158.2 mA h g⁻¹ at 0.5C, revealing a superior performance over the PAA electrode. Su et al. introduced lithium into cellulose sulfate, an inorganic ester derivative of cellulose, to manufacture cellulose sulfate lithium (CSL),¹⁹⁰ which facilitated the diffusion of Li-ions, while demonstrating excellent adhesive properties as a binder for LiFePO₄ (LFP). The CSL binder was synthesized through the esterification reaction between refined cellulose (RC) and sulfur trioxide pyridine complex (SO₃-Py) (Fig. 18d). The peel test results demonstrated that the CSL binder exhibited stronger adhesion compared to the PVDF binder (Fig. 18e). Upon evaluating the electrochemical performance of the two cells, LFP–CSL exhibited a higher average voltage and discharge capacity compared to LFP–PVDF as the cycling proceeded.

Fig. 18 (a) Graphical representation of the APA/CNT binder. (b) Force–displacement curves. (c) Folding tests.¹⁸⁹ Copyright 2024, RSC Publishing. (d) Synthesis route for CSL. (e) Peeling test profiles of different binders.¹⁹⁰ Copyright 2023, Elsevier Ltd. (f) Schematic of the dry electrode processes. Normalized F_H at 30 μm depth from the surfaces of (g) wet- and (h) dry-processed LFP electrode.¹³ Copyright 2024, John Wiley and Sons.

If the dispersion of the binder within the electrode is poor, a lack of interfacial adhesion can occur due to insufficient contact between the adherend and the binder. In the case of PVDF electrode slurries, non-uniformity within the electrode arises during solvent evaporation, leading to binder migration near the electrode surface and potentially hindering adequate adherend-binder contact. Thus, to address this issue, Kwon et al. introduced fiber-shaped polytetrafluoroethylene binders in the electrode through a dry process, enhancing the dispersion of the binder within the electrode (Fig. 18f).¹³ This method mitigated the reduction in Li-ion diffusion efficiency caused by the binder compared to PVDF slurries, while simultaneously improving the adhesion force.¹³ Surface and interfacial cutting analysis system (SAICAS) measurements were conducted to assess the adhesion strength of the wet-processed LFP and dry-processed LFP electrodes (Fig. 18g and h). Although the dry-processed LFP exhibited nearly uniform adhesion strength within the electrode, the wet-processed LFP showed a decrease in adhesion strength by approximately 12% from the top to the bottom of the electrode. This indicates that the dry-processed LFP showed a better dispersion of the components within the electrode compared to the wet-processed LFP. These findings may be associated with wet-processed LFP exhibiting a higher overpotential during the charging and discharging processes compared to the dry-processed LFP. Furthermore, the rate performance test results indicated sluggish Li transport in the wet-processed LFP compared with the dry-processed LFP.

6.2.2. Binder breakage. LFP exhibits minimal volume expansion during both lithiation and delithiation.¹⁹¹ Consequently, the binders used in thick LFP electrodes experience minimal binder breakage. However, practical LFP batteries can still encounter binder breakage due to external forces operating the cell.¹⁹² Therefore, it is essential to enhance the mechanical strength of the binder to improve its mechanical stability. To prevent mechanical failure resulting from binder breakage, it is crucial to fabricate binders with high toughness. To achieve high toughness in the binder, its structure should be designed to effectively dissipate energy when subjected to external forces.^188,192

Xi et al. developed binders using three commercially available thermally stable polymers containing oxygen polar groups, including poly(caprolactone) (PCL), PPC, and poly(lactic-co-propylene carbonate) (PPCLA), via a dry process (Fig. 19a).¹⁹³ The appropriate glass transition temperature (T_g) of these binders enabled a phase transition at specific temperatures, thereby facilitating Li-ion transfer and movement. This ensured the excellent electrochemical properties of the binders. Additionally, PPC and PPCLA exhibited the tensile strength of 5.9 MPa and 11.4 MPa, respectively, with PPC demonstrating the maximum elongation of 474.6%, indicating its flexibility. The electrode based on PPCLA (62.6 Ω) exhibited a lower charge-transfer resistance compared to the electrodes based on PCL (164.1 Ω) and PPC (79.5 Ω). Consequently, the PPCLA-based electrode demonstrated a higher rate capability than the PPC-based electrode, showing an initial discharge capacity of approximately 160.4 mA h g⁻¹ at 0.2C (Fig. 19b).

Fig. 19 (a) Structure and synthetic scheme of PCL, PPC and PPCLA. (b) Rate capability (0.1–2 C) of two batteries with the PPC and PPCLA cathode.¹⁹³ Copyright 2024, Elsevier Ltd. (c) Fabrication process of the UCFR-LFP electrode and its unique fast electron and ion transporting properties, ultrahigh capacity, excellent thermal stability, and fire resistance. (d) Digital images of the UCFR-LFP-18 mg cm⁻² electrode under different physical deformations.¹⁹⁴ Copyright 2019, John Wiley and Sons.

Organic binder-based electrodes, which are susceptible to heat, may rupture due to their low thermal stability in high-temperature environments. In contrast, inorganic binder-based electrodes, with higher thermal stability compared to organic binders, contribute to the design of fire-resistant electrodes. Li et al. reported the fabrication of ultrahigh-capacity, fire-resistant LFP (UCFR-LFP) electrodes, which maintained flexibility even at a high active material mass loading of 18 mg cm⁻². They achieved this by adopting mechanically reinforced building blocks composed of highly thermally stable hydroxyapatite nanowires (HAP NWs) with a network structure, together with 1D-shaped inorganic binders (Fig. 19c).¹⁹⁴ Following the principle of reinforcing steel bars in concrete, HAP NWs and carbon fibers (CFs) effectively enhanced the mechanical strength of the LFP electrodes through energy dissipation during the fiber pulling process. The UCFR-LFP electrode exhibited noticeable damage-free characteristics under various physical deformations (bending, folding, and twisting) and demonstrated excellent mechanical strength and high flexibility (Fig. 19d). At a current density of 0.5C, the UCFR-LFP electrode exhibited a higher initial discharge capacity (149.5 mA h g⁻¹) than the conventional LFP (130.4 mA h g⁻¹) electrode. Furthermore, as the current density increased, the capacity difference between the two electrodes became more pronounced. This enhanced performance was attributed to their robust mechanical stability even under a high mass loading.

6.3. Modifying current collector for high adhesion

In electrochemical systems, current collectors often receive less attention compared to other battery components due to their inherent inertness. However, the current collectors play dual roles, serving as mechanical supports for the electrodes and acting as bridges connecting the external electrical circuit to the battery system. The intimate contact between the current collector and the electrode can significantly impact the utilization of the electrode. Specifically, as the adhesion force between the active material layer and the current collector strengthens, the bonding force is enhanced, leading to more effective charge transfer.^173,195 Consequently, a decrease in the charge transfer resistance allows the preservation of electrochemical performance. Meanwhile, the size of the LFP active material is typically falls in the micron scale. Therefore, surfaces such as smooth Al current collectors exhibit limited interface adhesion. This poor adhesion makes them susceptible to delamination from the current collector, leading to increased interfacial resistance due to the restricted electron transfer channels.¹⁹⁶

6.3.1. Carbon coating. On the smooth surface of the current collector, coating sub-micrometer scale carbon particles can fill the voids between the active material and the current collector, allowing electrons to flow and enhancing the integrity of the binder. Wang et al. synthesized hierarchical graphene on an Al current collector (hG-Al) to address the poor contact between the smooth Al current collector and the active material (Fig. 20a).¹⁷³ The quantitative measurement of the electrochemically active surface area (ESCA) revealed that hG-Al exhibited a geometrically ten-fold higher contact area between the current collector and the active material compared to bare Al (b-Al). Furthermore, hG-Al demonstrated significantly increased adhesion strength (20.50 N m⁻¹) compared to b-Al (4.21 N m⁻¹) (Fig. 20b). This enhancement was attributed to the increased area of van der Waals interaction between the vertical graphene branches of hG-Al and the electrode-current collector interface. In addition, the electrode on b-Al exhibited a sharp performance decline with an increase in current density, whereas hG-Al maintained a satisfactory performance. Busson et al. developed primed current collectors (PCCs) by coating pristine Al foil (RCC) (Fig. 20c) with a primer mixture of carbon particles and polymer to aid the formation of an interface with the LFP active material.¹⁷⁴ The PCCs featured a primer layer with a thickness of 2 μm. The rugosity profile analysis revealed an increase in surface roughness compared to RCC (Fig. 20d). PCCs exhibited a surface energy of 54 mN m⁻¹ and a contact angle close to 0° with NMP. In contrast, RCC showed a relatively lower surface energy of 36 mN m⁻¹ and a contact angle of 36° with NMP, suggesting its improved wettability in NMP and enhanced adhesion. The Nyquist plot from the electrochemical impedance spectroscopy (EIS) analysis revealed an increase in the contact area at the interface, leading to effective electron transfer and positive effects on electrochemical reactions. At a high current density, the PCCs demonstrated an excellent rate capability (80 mA h g⁻¹ at 5C rate), cyclability (60% capacity retention after 200 cycles at 2C rate), and improved energy density, while maintaining a good power density compared to RCC. This improvement was attributed to the increased electrical contact points provided by PCCs.

Fig. 20 (a) Schematic showing the model interface between bare Al foil current collector and hierarchical-graphene-engineered Al foil current collector with electrode materials. (b) Force–displacement curves of the mechanical peeling-off tests of LFP electrodes on different current collectors.¹⁷³ Copyright 2020, the American Chemical Society. (c) Schematics of C-LFP/CB and C-LFP coated with primer electrodes. (d) Typical rugosity profile for RCC (black) and PCC (red).¹⁷⁴ Copyright 2018, Elsevier Ltd.

6.3.2. Surface roughness treating. During the drying process of the electrode slurry on the current collector, the phenomenon of delamination can occur, which is closely related to the surface tension of the electrode slurry on the current collector.¹⁸⁹ Generally, low surface tension indicates good affinity between the electrode slurry and the current collector, alleviating issues such as electrode delamination during the drying process. In this case, engineering the surface morphology of the current collector can play a critical role in controlling the surface tension.

Ravesio et al. improved the adhesion by enhancing the wettability between the electrode slurry and the current collector through texturing the surface morphology of the current collector using a laser. The laser-treated current collectors exhibited a uniform structure resembling crater-like features (Fig. 21a and b).¹⁷⁵ Observation of the wettability of the pristine and laser-textured current collectors in water revealed that both samples were hydrophilic (contact angle < 90°), but the contact angle of the laser-textured current collector (<5°) (Fig. 21d) was lower than that of the pristine current collector (61.9°) (Fig. 21c), indicating the better wettability of the aqueous slurry on the laser-textured current collector. The decrease in surface tension corresponded to a decrease in the contact angle, ultimately leading to the improved adhesion of the electrode due to the lower surface tension. As a result, when the electrodes (thickness = 300 μm) were loaded onto the current collectors, delamination occurred with the pristine current collector but not with the laser-textured current collector. Furthermore, during electrochemical testing, as the cycling rate increased, the capacity of the pristine current collector decreased significantly, whereas that of the laser-textured current collector remained stable. Dai et al. fabricated porous 3D Al foil using electrochemical etching with Fe(NO₃)₃ solution (Fig. 21e).¹⁷⁶ In the electrochemical reaction between Fe(NO₃)₃ solution and Al foil, NO₃⁻ regulated the etching pattern, while Fe³⁺ contributed to the generation of uniformly sized pores. Additionally, an external electric field was applied to control the etching rate. The resulting Al foil showed that the pore size decreased as the etching time increased, while the porosity increased. Due to the enhanced surface roughness resulting from the 3D porous structure of the porous Al foil, its hydrophilicity and wetting properties toward NMP were superior to that of the pristine Al foil (Fig. 21f). Consequently, the mechanical bonding between the current collector and electrode surface increased, and the electron mobility at the interface improved, leading to better cycling stability. Multiphysics simulations analysing the Li-ion flux on the electrode surface confirmed that the porous Al foil had a higher current density than the pristine Al foil, corresponding to an enhanced charge transfer (Fig. 21g).

Fig. 21 SEM micrographs of (a) pristine current collector and (b) laser-textured current collector. Contact angle measurement of (c) pristine current collector and (d) laser-textured current collector.¹⁷⁵ Copyright 2023, Elsevier Ltd. (e) Schematic diagram of the preparation of porous Al foil via electrochemical etching. (f) Water and NMP wettability of pristine Al, Al-DC 0.5 h, Al-DC 1.0 h, Al-DC 1.5 h, and Al-DC 2.0 h. (g) Simulation diagram of the wetting states of the pristine and porous Al foils.¹⁷⁶ Copyright 2023, Elsevier Ltd.

7. Summary and perspective

Given that LIBs are increasingly used as energy storage devices, improving their energy density has attracted significant attention. Many researchers suggest mainly three strategies to enhance the energy density of olivine cathodes, as follows: (1) increasing their redox potential through substitutional doping with elements such as Mn, Co, and Ni, (2) achieving a high trap density through the particle aggregation of nanoparticles, and (3) increasing the loading of active material per unit area by thickening the electrode. However, these strategies exhibit trade-offs in terms of electrochemical kinetics. The main origins include the decrease in polaron mobility due to transition metal substitutional doping and the formation of complex ionic and electronic pathways in the electrode matrix, leading to uneven concentrations of ions and electrons. The electrode components (active materials, binders, and conductive materials) are engaged in the transport paths of Li-ions and electrons throughout the electrode structure. Therefore, careful consideration must be given not to deteriorate the Li-ion and electron transport processes from the atomic level to the electrode level. Given that both Li-ion and electron transport determine the electrochemical kinetics, it is crucial to improve all the contributing factors simultaneously. Additionally, mechanical stability to dissipate the volume changes in the active material during repeating charge and discharge cycles should not be overlooked. In this regard, it is also essential to understand the physical nature of the electrode from a comprehensive perspective of inorganic–organic composites, thereby achieving the gradual optimization of the energy density.

Overall, to achieve high energy density in olivine electrodes, it is crucial to improve the bulk electronic and ionic conductivity through doping of the active material, lowering the electrical percolation threshold of the electrode, enhancing the Li-ion mobility throughout the electrode, and constructing shorter ion pathways. The mechanical stability of the electrode must be ensured using an appropriate binder system or preparation method to preserve continuous physical contact among the components during the charge and discharge process (Fig. 22). These optimized processes will enhance the cost-efficiency in industry and prevent the misuse of materials. Our review provides a comprehensive and detailed understanding from the atomic level to the electrode level, offering universal guidelines for the kinetic optimization of high-energy-density olivine electrodes and contributions to both experimental and theoretical applications.

Fig. 22 Schematic illustration of unifying strategies for the preparation of high-energy, low-cost, long-lasting olivine cathodes.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Author contributions

W. Hwang, J. Kim: investigation, conceptualization, draft writing, review, and editing. S.-Y. Kim, E. Ko, S. Lee, M. Kim: investigation, and draft review. S.-H. Yu, Y.-E. Sung, H.-S. Kim: draft review, and editing. C. Kim, J. Park: conceptualization, supervision. review, editing, and funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was funded by National Research Foundation of Korea, RS-2024-00453139, RS-2023-00238374, 2022R1C1C101145613 and Korea Institute of Science and Technology, 2E33271 and 2V10180. This research was supported by ‘National Research Council of Science & Technology (NST)’-‘Korea Institute of Science and Technology (KIST)’ Postdoctoral Fellowship Program for Young Scientists at KIST in South Korea.

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Footnote

† Both authors contributed equally.

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