Deik
Petersen
a,
Monja
Gronenberg
a,
German
Lener
b,
Ezequiel P. M.
Leiva
b,
Guillermina L.
Luque
*b,
Sasan
Rostami
c,
Andrea
Paolella
d,
Bing Joe
Hwang
e,
Rainer
Adelung
a and
Mozaffar
Abdollahifar
*a
aChair for Functional Nanomaterials, Department of Materials Science, Faculty of Engineering, Kiel University, Kaiserstr. 2, 24143, Kiel, Germany. E-mail: moza@tf.uni-kiel.de
bDepartamento de Química Teórica y Computacional, INFIQC, Av Medina Allende y Haya de la Torre, Ciudad Universitaria, CP X5000HUA Córdoba, Argentina. E-mail: guillerminaluque@unc.edu.ar
cDepartment of Physics and Energy Engineering, Amirkabir University of Technology (Tehran Polytechnique), Tehran, Iran
dDipartimento di Scienze Chimiche e Geologich eUniversità degli Studi di Modena e Reggio EmiliaVia Campi 103, Modena 41125, Italy
eSustainable Electrochemical Energy Development Center, National Taiwan University of Science and Technology, Taipei 10617, Taiwan
First published on 9th September 2024
Anode-free metal batteries (AFMBs) are a new architecture of battery technology that relies solely on current collectors (CCs) at the anode side, eliminating the need for traditional metal anodes. This approach can pave the way for higher energy densities, lower manufacturing costs, and lower environmental footprints associated with metal batteries. This comprehensive review provides an in-depth exploration of AFMB technology, extending its scope beyond lithium and into a broader range of metals (sodium Na, potassium K, magnesium Mg, zinc Zn and aluminum Al). The concept of “metal-philicity” is discussed, which plays a pivotal role in understanding and controlling metal plating behavior within AFMBs, and also computational studies that employ first-principles calculations. This novel notion offers valuable insights into the interactions between metals and CC surfaces, which are essential for designing efficient battery systems. Moreover, the review explores various materials and experimental methods to enhance metal plating efficiency while mitigating issues such as dendrite formation through the realm of surface modifications and coatings on CCs. By providing a deeper understanding of strategies for optimizing anode-free post-Li metal battery technologies, this review aims to contribute to developing more efficient, sustainable, and cost-effective energy storage for the near future.
Wider impactThe global demand for more efficient and sustainable energy storage solutions has increased significantly in recent years. Traditional battery technologies face limitations regarding energy density, manufacturing costs, and environmental impact. In response, researchers and industries have been exploring new materials and architectures to overcome these challenges. Anode-free metal batteries (AFMBs) have emerged as a promising solution that can eliminate the need for traditional metal anodes, relying solely on current collectors (CCs) placed at the anode side and use of a cathode as the sole active material, paving the way for higher energy densities and potentially lower manufacturing costs and environmental footprints associated with metal batteries. Therefore, innovative solutions for CC chemistries via surface modifications and coatings while mitigating issues such as dendrite formation are aimed at optimizing AFMBs. Central to the discussion is the “metal-philicity” concept, which plays a pivotal role in understanding and controlling metal-plating behavior within AFMBs beyond lithium metal batteries. Through computational studies employing first-principles calculations, researchers gain valuable insights into the interactions between metals and CC surfaces, essential for designing efficient battery systems. Ultimately, this review serves as a guiding beacon for researchers, offering a deeper understanding of the strategies necessary to optimize the CC of AFMBs. |
Fig. 1 (a) A compression of post-Li-ions (Li+, Na+, K+, Mg2+, Zn2+, Al3+): (1) relative atomic mass, (2) standard potential (V) vs. standard hydrogen electrode (SHE), and the values are negative. (3) theoretical specific capacity, (4) volumetric energy density, (5) crustal abundance, (6) data costs obtained from.8 (b) Schematic illustration of cell volume for (left) a conventional metal-ion battery with an anode, (middle) a metal battery (MB) with a metallic anode like Li, Na, K, etc., often denoted as ‘half-cell’ for cathode testing, and (right) an anode-free metal battery (AFMB) using just a current collector (CC) as a nucleation site for the metal anode. The cathode is fixed, and the electrolyte could be a solid electrolyte or a liquid electrolyte with a separator. The AFMB clearly enables the highest volumetric energy density compared to a conventional metal-ion battery and a MB. (c) Schematic representation of how a functionalization of the metal-CC-interface can guide the metal growth from a dendritic and non-uniform plating to a planar growth by changing the surface energy of the CC. |
Anode-free metal batteries (AFMBs) represent a ground-breaking approach to energy storage by eliminating the conventional anode component found in metal batteries (MBs). Having a look at Fig. 1b, it becomes clear that AFMBs have the perspective of achieving significantly higher energy densities compared to traditional metal-ion batteries. This increased energy density can translate to longer-lasting and more powerful energy storage solutions for applications ranging from consumer electronics to electric vehicles. In addition, eliminating the anode simplifies the battery structure, potentially reducing manufacturing costs and making these batteries more economically viable on a larger scale. By reducing the components needed for battery assembly, not only the costs but also the energy consumption during manufacturing can eventually be decreased, resulting in a reduced environmental impact.
Ongoing research and development efforts are actively addressing these hurdles, paving the way for the widespread adoption of this transformative technology in the near future.17 The main challenge is to enable a homogeneous plating/stripping behavior over hundreds of cycles. Therefore, a homogeneous nucleation of the metal layer during the first cycle is decisive. If the first metal layer that forms on the CC is not flat and homogeneous, the same holds for the SEI that will form and consequently, the local current density will change due to locally enhanced electric field strengths and enhanced transport kinetics. Over the course of several cycles, this can lead to an amplification of the surface roughness and the formation of dead metal or hazardous dendrites, decreasing the structural integrity of the cell.
To ensure homogeneous metal plating conditions – especially during the first cycle – several measures can be taken. For the homogenous nucleation of the metal on the CC, the so-called “metal-philicity” plays a decisive role, i.e. the surface of the CC should be functionalized in a way that the metal atoms will preferably bond to the CC surface rather than to form metal–metal bonds. The homogeneity of the metal layer is decisive in enabling a homogeneous current density distribution and long-term cycling stability. The metal-philicity as an important concept that will be explained in more detail in the next section. Another way to homogenize the plating process and to prevent dendrite formation is to introduce a mechanically stable and ionically conductive SEI that cannot be pierced easily by growing dendrites and that ensures the growth of an equally smooth metal surface.
In this review paper, the advances in AFMB technology, particularly on optimizing CCs for a variety of metals such as Na, potassium (K), magnesium (Mg), zinc (Zn), and Al other than Li, or PLMBs, will be presented and discussed. Fig. 1c gives an overview of the main strategies to increase the number of nucleation sites on the CC that can help to increase the philicity towards the metal to be deposited.21,22 Generally speaking, the surface can be functionalized by a physical treatment changing the topology and the surface area of the interface or by a chemical treatment changing the surface energy and hence the binding energy towards the metal to be deposited (or by a combination of both). Following the introduction of metal-philicity, computational studies of AFMBs are reviewed and discussed, and challenges and solutions of CC chemistries for each metal in anode-free PLMBs are then addressed in separate sections.
For anode-free Li-metal batteries (AFLMB), talking about the lithiophilicity factor, and similarly in the case of anode-free Na-, K-, Mg-, Zn-, and Al-metal batteries, it would be sodiophilicity (for AFSMB), potassiophilicity (for AFKMB), magnesiophilicity (for AFMMB), zincophilicity (for AFZMB), and aluminophilicity (for AFAMB). In each case, a material's innate attraction and interaction to M-ions or M-metal is referred, significantly impacting the efficiency of M-based battery technologies. Materials demonstrating elevated “metal-philicity” are highly sought after for AFMBs, as they play a critical role in accelerating the plating/stripping of M-ions during (dis)charging processes. This property is vital for maintaining the stability and effectiveness of the AFMB playing an important role to ensure a controlled and uniform growth of reduced metal. For each of the metals, three main steps need to be considered in the nucleation and growth of the metal on the CC to achieve a high-performance AFMB, as discussed for Li-metal by Wang et al.27 The discussion is for AFLMBs, but could be considered for the other metals as well. Firstly, the M-ions have to migrate to the CC surface under the driving forces of gradient concentration and the electric field force within the battery. Secondly, M-ions near the CC surface electrically contact the surface and reduce to the metallic form, driven by the electrostatic interaction of the nucleation sites and the ions. Finally, the M atoms diffuse over the surface and nucleate. The metal plating is a competitive process of M–M bonding and M–CC bonding. Nevertheless, there is no singular formula to calculate the metal-philicity, since it is related to different parameters such as the binding energy of the M atoms, the nucleation overpotential, the wettability, the electric field and the morphology of the deposited surface as will be further discussed. This clearly goes beyond a purely chemical picture and must also take into account aspects of solid-state physics and crystallography, emphasizing the importance of interdisciplinary research on that topic.
DFT is an atomistic first-principles modelling method used in physics, chemistry, and materials science to investigate the electronic structure of atoms, molecules, and condensed phases. It allows to calculate the energies of atomic structures and in order to gain a fundamental understanding of ion transport mechanisms, the electronic structure and its dynamics, thermodynamics, electron transfer, and various other properties in materials. In addition, first-principles calculations can be applied to predict new materials with little empirical input, as has been demonstrated for a wide range of battery, photovoltaic, thermoelectric, and catalyst materials.29 Meanwhile, AIMD simulations have several advantages over other computational techniques. Compared to classical molecular dynamics (MD) simulations that model the real-time Newtonian dynamics of all atoms30 in the materials using force fields that may not be readily available for the candidate material, AIMD simulations are chemically agnostic and are more suitable for studying new materials. In addition, the real-time ion dynamics of AIMD simulations allow direct observation of diffusion mechanisms without the need for a priori assumption about diffusion pathways. Therefore, AIMD simulations can be used to reveal diffusion modes within materials and to predict new fast ion-conducting materials.31
In general terms, the nucleation process that takes place in AFMBs is controlled by thermodynamic and kinetic factors. From the thermodynamic point of view, nucleation is governed by the decrease in free energy due to the phase transition and the increase in surface energy because of the creation of a new interface. A higher value of binding energy can lead to a lower metal nucleation barrier and therefore to a lower nucleation overpotential. So, the binding energy values of the metal to the surface can be taken as a descriptor of the metal-philicity of the CC surface.27,32 In general terms, the binding energies Eb of the atoms to the surface are calculated as:
Eb = Esubs+M − Esubs − EM | (1) |
For AFLMBs, lithiophilicity is one of the most important indicators to evaluate the quality of host materials when they are used as substrates for Li metal plating. Even though, there is not a direct way or formula to calculate the lithiophilicity, there are indirect ways used to evaluate it. A surface is called to be lithiophilic if it shows a good wetting towards Li-metal, i.e., if it has a low contact angle with molten Li. For a homogeneous nucleation, ideally the interaction strength between CC and Li metal should be strong, i.e., stronger than the interaction between Li and electrolyte and between individual Li atoms. A lithiophilic surface also requires a lower nucleation overpotential for Li metal plating compared to a lithiophobic surface.32 Yan et al.33 performed studies of Li plating on different metal surfaces. These authors deposited Li on two groups of metals. The first group consisted of the metals Pt, Al, Mg, Zn, Ag, and Au, which exhibit some solubility in Li. In the second group of metals, they considered Li plating on Si, Sn, C, Ni, and copper (Cu), which show negligible solubility in Li. They found that while the substrates of the former group yield a negligible overpotential for Li nucleation, this was not the case for the second group. This behavior was explained by the definite solubility of the substrate into the Li metal, which produces a solid solution buffering layer, previous to the formation of Li metal. Wang et al.34 studied the electrodeposition behavior of Li plating on a Li2Te–Cu surface and compared it to the plating on a fcc Cu surface using DFT and electrochemical studies. They found that the monolayer plating of Li atoms on Li2Te–Cu is more stable than the formation of Li clusters, concluding that the electro-deposited Li film will be uniform. On Cu, however, the surface plating of three-dimensional (3D) Li islands is as stable as the Li monolayer plating, favoring in this way dendrite formation. Other surfaces like Li alloys were shown by Pande et al.26 to present ideal characteristics for Li nucleation and surface diffusion. In this work, the authors found that the best-performing CC surfaces should possess Li adsorption energy close to zero to present a low activation energy of Li diffusion on the surface. The nucleation overpotential is determined by the adsorption free energy of Li on the CC surface and requires that the CC binds Li strongly. Shin et al.35 demonstrated by DFT studies that Li adsorbs preferentially on Ag and AgLi than on Cu since Ag forms an alloy with Li creating a homogeneous and smoother surface morphology. Cho et al.36 also used Ag as a highly lithiophilic material using Ag nanoparticles in polyethyleneimine (PEI) as a stabilizing agent in the presence of LiNO3. They analyzed the system by DFT and AIMD and experimental studies finding that the Cu surface modification by Ag and PEI in the presence of LiNO3 allowed for achieving a stable SEI surface controlling dendrite formation through a good nucleation process.
The nucleation rate J is a probabilistic process that is related to the free energy of formation of a critical cluster size ΔGcrit according to:
(2) |
For a cluster growing on a foreign surface the function ΔG(N) can be written as:
(3) |
In this sense, the metal-philicity of the different surfaces for different metals has been studied using first-principles calculation and will be discussed within each of them in the following parts. Broadly speaking, a review of the literature across various metals suggests that the efficacy of specific surface modifications or functionalization tends to be generalizable. In other words, a technique that proves beneficial for one metal is likely to demonstrate similar effectiveness when applied to other metals. As an example, the work of Chen et al.37 can be considered that have studied the effect of doped carbon materials with the finding that O and boron (B) doping are the ones that will perform better as materials for AFSMBs and AFKMBs. They observed that Na and K present the same binding energy tendency on the doped carbon surfaces as observed in Fig. 2a and b, respectively. They showed that apart from the presence of a carboxylic group (aO-doping); doping (bgB-, egB-), pyridinic N (pN-), epoxy (eO-), and ketone (kO-) doping are promising doping alternatives for practical design of three-dimensional carbon hosts taking in consideration the binding energy of Na and K on these surfaces.
Fig. 2 The summary of binding energy on carbon materials: (a) the binding energy between Na and carbon, (b) the binding energy between K and carbon. Reprinted with permission,37 Elsevier, Copyright 2020. |
Theoretical computational studies not only help to screen materials that potentially could be useful in the construction of anode-free CC, but also these tools play an important role in the understanding of the experimental results. These valuable tools allow us to gain an atomistic understanding of the interactions that take place between the surfaces of the CC and the metal-ions. Through these studies, it is possible to explore the energy barriers, the diffusion of atoms on the surface, and the reaction pathways helping in the understanding and designing possible new metalphilicity favorable surfaces. This last thing helps in the screening process of a wide range of materials, accelerating the discovery of the more promising ones. Computational studies such as DFT and AIMD are efficient and cost-effective tools that help to explore and understand the metalphilicity properties of different materials.
Fig. 3 (a) Schematic representation of the Cu@Bi preparation process and Na plating on Cu@Bi surface. Reprinted with permission,47 MDPI, Copyright 2023. (b) Schematic Illustration of uniform Na plating pattern on a Cu foil coated with PC-CFe. Reprinted with permission,48 John Wiley and Sons, Copyright 2022. (c) Schematic Illustration of the Na plating mechanisms on both a planar Cu foil and a Cu NW-Cu substrate. Reprinted with permission,49 Elsevier, Copyright 2018. (d) Illustration of Cu CC modified with the HCOONa. Reprinted with permission,50 John Wiley and Sons, Copyright 2023. (e) Diagram depicting the plating of Na on untreated Cu or Al foils, contrasted with Cu@C or Al@C substrates. Reprinted with permission,51 John Wiley and Sons, Copyright 2022. |
The uniform plating of Na on the CC is essential for optimizing the performance, safety, and efficiency of anode-free Na metal batteries. To achieve this, Lee et al.48 investigated the development of novel 3D nanostructured porous carbon particles with carbon shell-coated iron nanoparticles (PC-CFe) as a host for controlled Na metal growth in AFSMBs. The PC-CFe host was synthesized by carbonization of colloidal particles of nanostructured block copolymers, resulting in the formation of Fe nanoparticles coated with an ultrathin carbon layer (Fig. 3c). This hierarchical structure, which included sub-micrometer-sized carbon particles with ordered open channels and uniformly dispersed carbon-shell-coated Fe nanoparticles, enhanced cycling performance and demonstrated reversibility in Na plating/stripping operations. The PC-CFe's high efficiency is attributed to its 3D conductive network, which reduces energy loss, and its ultrathin carbon coating, which improves sodiophilicity and promotes homogeneous Na nucleation while limiting dendrite formation and successfully preventing oxidation. The hierarchical porous structure promotes efficient ion transport and accommodates volume fluctuations throughout cycle, increasing stability and lifespan.
Another notable contribution in this field was made by Wang et al.,49 who worked on the development of a CC that allows for uniform plating of Na-metal. Based on this, they introduced an approach by using a three-dimensional Cu-foam CC (CuNW-Cu) reinforced with Cu nanowires to enable reversible Na storage (Fig. 3d). Incorporating Cu nanowires onto a Cu foam CCs enhances its surface area, offering several benefits for Na plating. These nanowires act as abundant nucleation sites, promoting uniform Na deposition and mitigating dendritic growth. The increased surface area also reduces local current density, further suppressing dendrite formation. Experimental results demonstrate the CuNW-Cu CC's stability during prolonged cycling, ensuring the durability of sodium metal anodes. While direct sodium plating offers energy advantages, the CuNW-Cu design provided a practical solution by controlling nucleation and minimizing dendrite growth, enhancing the overall performance and safety of sodium metal anodes. In addition, the CC exhibited stability throughout prolonged cycling, showing minimal thickness variation. Furthermore, the EIS results indicated a significant reduction in charge transfer resistance for the Na@CuNW-Cu anode compared to the unmodified Cu CC. The lower charge transfer resistance of the Na@CuNW-Cu anode, compared to the unmodified Cu CC, is due to the increased surface area and uniform distribution of active sites provided by the in situ formed Cu nanowires. This suggested that the modified CC promoted efficient ion transport and electron transfer during the charge–discharge process. The improved kinetics contributed to higher CE and cycling stability, which are crucial for the long-term performance of Na-metal batteries.
An alternative method to improve the stability of Na plating/stripping processes involves the use of 3D Cu nanowires with diameters of less than 40 nm, which was investigated by Lu et al.52 They aimed to show how the unique structure of these nanowires can efficiently distribute the electric field, make a stable plating of Na-metal, and consequently suppress dendrite growth, which is a typical challenge in other planar CCs surfaces. The success of the study lies in the increased surface area of the 3D Cu structure through thinner nanowires, which stabilizes the Na plating by creating more nucleation sites. Importantly, testing has shown that thin-diameter 3D Cu nanowires are better at preventing dendrite growth than thicker wires (about 300 nm). Charge centers on electrode surfaces are crucial for uniform Na plating and dendrite prevention in MBs. Thin-diameter 3D Cu nanowires offer abundant charge centers due to their large surface area and inherent morphological features, promoting uniform Na plating by evenly distributing the electric field. While defects on flat Cu surfaces could also act as nucleation sites, the inherent roughness of these surfaces often leads to uneven plating. In contrast, the controlled, structured surface of 3D Cu nanowires enhances plating uniformity, mitigating the potential drawbacks of increased SEI formation associated with larger surface areas. The stable plating behavior and good capacity retention observed with 3D Cu nanowires further underscore their efficacy in improving Na-metal battery performance. To avoid side reactions with electrolytes and the growth of dendritic Na deposits during (dis)charging cycles, a composite material derived from a Cu-based metal–organic framework (Cu@C), consisting of Cu nanoparticles integrated within a carbon framework (Fig. 3e) was proposed.51 When applied to conventional Cu and Al CCs, Cu@C serves as a layer that promotes uniform Na plating by acting as a nucleus buffer. The Cu@C exhibited a high graphitic carbon content and reduced the nucleation barrier for Na plating on both Cu and Al CCs. In situ dilatometric measurements showed that the Cu@C nucleating film promoted the formation of dense metallic Na plating and suppressed the thickness growth caused by the accumulation of dead Na, which resulted in good cycling stability.
Al-based AFSMBs show significant promise due to their high energy density and cost-effectiveness. While Al is an attractive choice as a CC thanks to its abundance, light weight, and excellent conductivity, modifications are necessary to address challenges like metal-philicity, dendrite growth and cycling stability. Ongoing research focuses on enhancing aluminum foil's sodiophilicity and ensuring stable cycling performance, bringing these promising Al-based AFSMBs closer to practical applications in energy storage systems. Several research has revealed that carbon coatings can serve as nucleation sites for Na plating, considerably enhancing the performance of Na metal anodes. for example, Cohn et al.53 demonstrated that a carbon film on an Al CC (C/Al) improved uniform Na plating, reduced nucleation overpotential, and increased mechanical stability. This resulted in a high CE of 99.8% over 1000 cycles and an energy density of approximately 400 W h kg−1 (Fig. 4). Similarly, Dahunsi et al.54 created nanosized carbon films on Al (C@Al), which resulted in a more homogenous Na plating procedure, lower overpotential, and better cycling stability, with over 99.5% CE and 93.0% capacity retention after 100 cycles. Furthermore, Cohn et al.55 evaluated different nucleation layer compositions and discovered that carbon-coated CCs, whether hard carbon or carbon black, had lower nucleation overpotentials and higher electrochemical performance. These findings are applicable equally to Cu, highlighting the adaptability of carbon coatings in improving the sodiophilic characteristics and stability of Na metal anodes. Another work that investigated obstacles related to undesirable dendrite growth was conducted by Lee et al.56 by developing a core–shell structure of silver nanofibers and nitrogen-rich carbon thin layers on Al CCs (SNF@NCL). The SNF@NCL nanohybrid structure can act as a catalytic template and improves Na ion nucleation by providing a high concentration of sodiophilic sites, promoting uniform metal deposition while reducing dendrite development. This leads to excellent cycling stability, with CE values exceeding 99% over long cycles. The combination of the conductive silver nanofiber core and the nitrogen-rich carbon layer promotes efficient electron transport and optimum Na ion interactions, increasing the performance of the battery.
Fig. 4 (a) Photographs and micrographs of metallic Na on C/Al electrodes at 0.5 mA cm−2, and also SEM and micrograph images of hexagon-shaped Na metal islands. (b) Schematic Illustration of the charge/discharge process of the C/Al CC (anode) with sodiated pyrite cathode in a AFSMB cell, and some electrochemical results. Reprinted with permission,53 American Chemical Society, Copyright 2017. |
To address the common challenges in Na plating/stripping, such as non-uniform deposition leading to dendrite formation, and low CE, researchers have investigated alternative CCs beyond conventional Cu or Al in AFSMBs. For instance, Wang et al.57 created a 3D Ag@C cloth CCs as an efficient host for alkali metals by a simple thermal evaporation method to enable uniform Na plating. By pairing the Ag@C with a prussian white cathode, they showed a high initial capacity of 133 mA h g−1, and the cells exhibited a capacity retention of around 56% after undergoing 800 cycles. Besides the sodiophilicity and nucleation sites of Ag@C, the good results can be also due to the smaller resistance created during Na plating/stripping cycles.
Liu et al.58 performed DFT calculations with the finding that the simultaneous presence of functional groups containing O and N on a carbon matrix could regulate the Na adsorption on the surface in AFSMBs. The DFT calculation results helped to design 3D carbon-based nanofibers with sodiophilic O and N functional groups, which simultaneously facilitates homogeneous Na+ distribution, a dendrite-free construction, and reduces the plating overpotential. The sodiophilicity of 3D carbon fibers derived from polyacrylontrile@zeolite imidazolate (PZC)59 was experimentally and theoretically studied by means of DFT that observed that the presence of pyridinic and pyrrolic N and Zn sites within the carbon fiber host helped in the homogenous plating of Na decreasing the nucleation barrier. The study of electrolyte additives60 has also been performed to attain a homogeneous Na plating by AIMD, helping in the design of a new electrolyte additive 1,2-dibromobenzene, which has the advantage of regulating the Na plating since it can decompose yielding sodium bromide increasing the Na diffusion across SEI layer.
Computational studies, particularly DFT simulations, have provided valuable insights into the mechanisms behind Na plating and the interactions between Na and different CC materials. These simulations have aided in the design and optimization of CCs with improved sodiophilicity and reduced nucleation energy barriers.
There is a summary of CCs reported for AFSMBs in Table 1. So far, NVP family cathodes and electrolytes with NaPF6 are commonly used in AFSMBs. As AFSMBs continue to develop, these advancements could offer reliable and sustainable energy storage solutions. The reported achievements in terms of cycling stability, energy density, and capacity retention, coupled with the exploration of novel materials and fabrication methods, highlight the potential of AFSMBs in revolutionizing the landscape of Na-battery technology. As these innovations continue to evolve, AFSMBs hold great promise for addressing the growing demand for high-energy-density, cost-effective, and environmentally friendly energy storage systems in the future. Upcoming researches may focus on further refining CC modifications, exploring novel electrolyte materials (solid, sem-isolid and liquids), and integrating computational modelling to accelerate the development of next-generation AFSMBs.
CC | Cathode | Electrolyte | CE | Capacity retention | Capacity | Ref. | |
---|---|---|---|---|---|---|---|
Cu-based | HCOONa | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.7% | 88.2% after 400 cycles | 101.2 mA h g−1 after 800 cycles at 2C | 50 |
3D Cu nanowires | Na3V2(PO4)3/C | 1 M NaPF6 in DME | 98% | 99% after 200 cycles | 92.5 mA h g−1 at 0.5C | 52 | |
Cu@C(5Na) | Na3V2(PO4)3/C | 1 M NaPF6 in diglyme | 99.9% | 98.7% after 80 cycles | 81 mA h g−1 1900 cycle at 5C | 51 | |
CuNW-Cu | FeS2 | 1 M NaPF6 in DME | 97.5% | — | 320 mA h g−1 for 50 cycles at a current density of 0.2 A g−1 | 49 | |
PC-CFe | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | — | 97% after 100 cycles | 114 mA h g−1 at 2C | 48 | |
Na3V2(PO4)2F2 | — | 85% after 500 cycles | 104 mA h g−1 at 2C | ||||
Cu@Bi | Na3V2(PO4)3 | 1 M NaPF6 in DME | 99.2% | — | 95.6 mA h g−1 for 80 cycles at 1C | 47 | |
Al-based | SNF@NCL | Na1.5VPO4.8F0.7 | 1 M NaPF6 in diglyme | >99% | 95% after 100 cycles | 108 mA h g−1 at 0.1 A g−1 | 56 |
Hard carbon | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.78% | 82.5% after 100 cycles | 300 mA h g−1 after 3 cycles | 55 | |
Carbon black | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.90% | — | >100 mA h g−1 after 3 cycles | ||
Bi | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.85% | — | 320 mA h g−1 after 3 cycles | ||
Sn | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.63% | — | >800 mA h g−1 after 3 cycles | ||
Carbon | pre-sodiated FeS2 | 1 M NaPF6 in diglyme | 99.8% | — | >300 mA h g−1 after 5 cycles | 53 | |
Cu@C | Na3V2(PO4)3/C | 1 M NaPF6 in diglyme | 99.9% | 96% after 30 cycles | 87 mA h g−1 after 1300 cycle at 5C | 51 | |
C@Al | Na3V2(PO4)3 | 1 M NaPF6 in diglyme | 99.9% | ∼93% after 100 cycles | — | 54 | |
Ag@C cloth | Prussian white | 1 M NaClO4 in EC/DEC/FEC | 94.1% | 56% after 800 cycles | 136 mA h g−1 after 3 cycles at 0.1C | 57 | |
Na2(Sb2/66Te3/6Vac1/6) | Na3V2(PO4)3 | 1 M NaPF6 in G2 | 99.9% | 91% after 1000 cycles | 105 mA h g−1 after at 1C | 61 |
On the other hand, metallic K is inherently more reactive than metallic Li and must therefore be handled with greater care. Because K is so demanding in storage and handling, there is currently no K ribbon of defined thickness on the market, making it difficult to design reproducible experiments.63 As K reacts violently with water, cell damage could have severe consequences and should be avoided at all costs. Additionally, K has a low melting point of ∼63.5 °C, which is much lower than that of metallic Li (180.5 °C) and Na (97.8 °C). In the event of a thermal runaway of a K-metal battery, this can lead to the melting of solid K, further increasing safety issues.63 The low melting temperature could on the other hand be useful to flatten the metal surface by annealing at elevated temperatures as a self-healing strategy. Summing up, the existing challenges of K-metal anodes appear to be more severe than those of Li-metal and Na-metal anodes. This is mainly attributed to the high reactivity of K, the large volume expansion effect, the unstable SEI, and the rapid growth of dendrites.63
AFKMBs avoid some of the disadvantages of K-metal by eliminating the need to handle pure K-metal during manufacture. For an AFKMB where each time the K is plated/stripped completely, it is of outmost importance to provide potassiophilic nucleation sites to guarantee a homogeneous nucleation and growth of metallic K. This can, for example, be achieved by providing a mesoporous host-material64 or a tissue with good wetting properties as compared with K as a CC.65 To further homogenize the plating process Wang et al.66 and Si et al.67 introduced a carbon coating on the separator while Li et al.64 and Tang et al.68 tailored the SEI such, that dendrite growth was suppressed.
Fig. 5 Demonstration of the potassiophilicity of a functionalized CC in contact with molten K metal. (a) A reduced graphene oxide (rGO) coating on a Cu CC leads to a complete wetting with molten K metal within 6 s, while the untreated Cu CC shows no K infusion at all. Reprinted with permission,69 John Wiley and Sons, Copyright 2019. (b) An ammonia-treated carbon cloth CC shows complete K metal infusion within 4 s. Reprinted with permission,65 American Chemical Society, Copyright 2022. (c) Wetting behavior of molten K on different CCs. (d) Top-view SEM images after 2.0 mA h cm−2 of K plating, Scale bar: 10 μm. (e) First cycle overpotentials at 50 μA cm−2 for the three CCs. (f) Schematic representation of film growth modes with respect to surface energy differences of film and substrate. (g) EIS Nyquist plots in the course of several cycles for the CCs Al@G, Al@C, and bare Al. Reprinted with permission,71 John Wiley and Sons, Copyright 2019. |
Zhao et al.71 showed that a 150 nm thin coating of a defect-rich graphene-derived coating (Al@G) on an Al–CC showed a low plating overpotential and a superior wetting and plating behavior when compared to a graphite coating (Al@C) and to the bare Al–CC (Fig. 5c–e). As a reason for the homogeneous film growth on Al@G Zhao et al. state that the surface energy of the Al@G substrate is much higher (66.6 mJ m−2) than the surface energy of the K metal resulting in the Frank-van der Merwe (F–M) mode (Fig. 5f). In contrast, if the surface energy of the substrate is low, like for the bare Al CC, the surface energy of the K metal determines the total energy of the system and the growth of isolated islands (i.e. dendrites) becomes energetically more favorable (Volmer–Weber (V–W) mode). The Stranski–Krastanov (S–K) growth mode describes the transition region and shows both characteristics, i.e. island growth on top of a closed film. Electrochemical impedance spectroscopy (EIS) analysis revealed the evolution of the charge-transfer resistance (Rct) during cycling of the differently functionalized CCs (Fig. 5g). The Rct value for the Al@G interface was not only the lowest, it even showed a decrease during cycling (from 239 to 192 Ω) while the Rrct of Al@C and of the Al–CC showed a gradual increase. Zhao et al.71 concluded that at the interface with the Al@G electrode a stable SEI was able to form that would not suffer from repeated cycling. This underlines that a smooth growth of K-metal is the key for a highly reversible plating/stripping and a high CE.
Due to the unstable nature of bare K metal and the large volume changes during plating/stripping, it is advisable to introduce a 3D host material that serves as a robust matrix for a homogeneous nucleation and growth of the K metal. An ideal host material should provide a robust structure that allows for good accessibility of all surfaces by the electrolyte. Hence, the porosity of the host material must not be too small. A large surface area would also lead to a huge consumption of electrolyte to build up the SEI. A macroporous structure, however, is often not able to withstand the electrode's volume changes.64 As an answer to this, Li et al.64 proposed a 3D mesoporous, carbonaceous fibrous film (called MCNF) that has a high mechanical strength. The MCNF is produced by pyrolysis of polyacrylonitrile (PAN) fibers embedded into a MOF. Its mesoporous structure of interconnected narrowly distributed pores allows for good ionic conductivity while having an overall surface area as low as 91.5 m2 g−1.64 The surface of the carbonaceous host possesses nitrogen-doped defects and is potassiophilic. In combination with their optimized electrolyte Li et al. achieved an average CE of 99.3% for their MCNF||K half cells with an area loading of 3 mA h cm−2 and a very good cycling stability under galvanostatic cycling with 3 mA cm−2.64 A Full cell with a stable Prussian blue (PB) cathode (K1.81Fe[Fe-(CN)6]0.82·0.47H2O) against a MCNF matrix CC (as anode) was also demonstrated. The MCNF||PB cell achieved a high energy density of 362 W h kg−1 at 20 mA gPB−1 for 100 cycles with a capacity retention of 86%.64
Potassiophilic Pd/Cu CC was studied by Wang et al.73 based on the selection of some potassiophilic materials using first principles studies. Although the high cost of Pd, the choice was due to the binding energy of K on this material. A comparison with Cu showed that the Pd coating has a better potassiophilicity than the original Cu foam. This leads to the reduction of the overpotential of K+ plating on Pd by effectively guiding the uniform K+ plating and inhibiting the growth of K dendrites, finding a great correlation with the experimental results.
In general, AFKMBs can be built in combination with a variety of different pre-potassiated cathode materials such as potassiated prussian blue (KPB),64,74 K0.51V2O5,67 K0.7Mn0.7Ni0.3O2 (KMNO),65 3,4,9,10-perylenetetracarboxylicacid-dianhydride (KPTCDA),68 potassiated FeS271 or cyanoperovskite KxMnFe(CN)6.75
Fig. 6 Comparison between K metal plating with a standard glass fiber (GF) separator (a), with a GF separator functionalized with rGO (b). The functionalization with rGO leads to a homogeneous flux of K+ ions and a smooth K metal surface, while dendrites are formed without the rGO-functionalization. The rGO separator facilitates the reduction of K+ ions by improving the interface between the electrolyte and the metal surface, leading to increased peak currents during cyclic voltammetry (CV) (d) compared to the CV of an untreated GF separator (c). Reprinted with permission,67 American Chemical Society, Copyright 2023. |
To our understanding, the physics behind the positive effect on plating/stripping that was observed by Si et al.67 is indeed different. The coating on top of the separator is unlikely to improve the ionic conductivity. In contrast, the rGO rather introduces a diffusion limitation that homogenizes local concentration gradients and hence the electric field. At the same time, the transport of the K+-ions through the KF-rich SEI is dramatically improved, leading to an overall increase in the peak currents during CV. In other words, besides the effect of homogenizing the ionic flux by building up a transport limitation through the rGO coating, the rGO could act as an artificial SEI that facilitates the removal of the solvation shell of the K+ ions and hence improves the cycling rate. rGO was also used by Liu et al.69 for coating their 3D-Cu CC. As already mentioned in Section 6.1 (Fig. 5b) they found a good wetting of rGO with molten K after 6s, indicating that rGO is potassiophilic.
CC | Functionalization | Cathode | Electrolyte | CE | Capacity retention | Capacity | Ref. |
---|---|---|---|---|---|---|---|
Cu-foil | PDMS interlayer for low temperature use | KPTCDA | 0.4 M KPF6-DME with 2 vol% PDMS | ∼98.9% at −40 °C | 82% after 50 cycles | 98.6 mA h g−1, (152 W h kg−1) at 0.2C at −40 °C | 19 |
Amine functionalized carbon cloth | K0.7Mn0.7Ni0.3O2 | 0.8 M KFSI in EC:DMC (1:1) | 99.996% | 68.5% after 8000 cyclces | 55 mA h g−1 at 1 A g−1 after 8000 cycles | 65 | |
Functionalized separator by rGO | K0.51V2O5 | 3 M KTFSI in DME | 95.2% after 70 cycles | 65 mA h g−1 after 70 cycles at 0.5 A g−1 | 67 | ||
Mesoporous carbo-naceous nitrogen-doped film (MCNF) | KPB | KFSI/DME/TTE (1/3/2 by mol) | 99% | 95% after 500 cycles | 5.72 W h cm−2, 362 W h kg−1 at 100 mA gPB−1 | 67 | |
Titanium-deficient nitrogen-containing MXene/carbon nanotube freestanding scaffold | Sulfurized polyacrylo-nitrile (SPAN) | 0.8 M KPF6 in EC/DEC (1:1, v/v) or 0.8 M KFSI in EC/DEC (1:1) | 99.2% | 69.5% after 500 cycles | ∼ 312 mA h g−1 at 0.5 C | 67 | |
3D-Cu mesh | rGO | Half-cell tests (symmetric) | 0.8 M KPF6 in EC/DEC | 69 | |||
Al-foil | Defect-rich Al@G graphene-coating (high surface energy) | (K-FeS2) | 4 M KFSI in DME | 99% | K||Al@G: 4 mA h cm−2 at 0.5 mA cm−2 for 1000 h | 71 |
– A stiff and stable SEI that is mainly anion-derived and of an inorganic nature helps to keep the metal surface flat and suppress dendrites.
– Tuning the wetting properties, i.e. increasing the surface energy of the CC helps to achieve a homogenous nucleation. CCs with surface terminations like –OH, –O, –NH, and –F demonstrated a high potassiophilicity, such as rGO (with an increased defect density compared to graphene), ammonia-treated carbons, and MXenes. Also, oxide materials such as NiO nanoparticles on the CC surface could serve as nucleation sites.
– Functionalizing a separator with rGO can limit the transport of the K+ ions and homogenize the ionic flux for a homogeneous plating of metallic K.
Due to the large volume changes (by molecular weight) compared to Li and Na, it is advisable to introduce a matrix as a nucleation site and as a space holder for the plating of K. Owing to its large atomic weight, K metal cannot compete with Li metal in terms of volumetric or gravimetric capacity. K ion batteries, however, can provide high energy and power densities and have the potential to become cheaper than LIBs. Hence, it is likely that KIBs will play an important role in stationary energy conversion systems. This is especially true for AFKMBs as during the manufacturing of the AFKMBs no handling of reactive K metal is needed. This will drop the costs and prevent any accidents with burning K metal during cell assembly. In addition, AFKMBs per definition can achieve greater energy densities than comparable KMBs.
The concept of anode-free batteries applied to Mg-metal has emerged relatively recently, to the point that the academic search on this topic with the keywords “anode-free Mg battery” yields references from the year 2021 onwards,81–87 Similar to other metal-free arrangements, in an anode-free configuration, Mg2+ ions are extracted from a fully pre-magnesiated cathode during an initial charging process. These ions are electrodeposited as metallic Mg on the CC, made of another material, which serves as a negative electrode for subsequent battery cycling. In the following sections, we discuss Mg electrodeposition on different CCs, which may be of great relevance to use as anodes in AFMMBs.
Fig. 7 (a) Schematic illustration concepts of the magnesiophilic bare Cu and Au–Cu CCs. SEM images of Mg deposit on (b) bare Cu and (c) Au–Cu CCs with a capacity of 2 mA h cm−2. (d) Cycling performances of the CE of Mg||Cu and Mg||Au–Cu asymmetric cells with a capacity of 1 mA h cm−2 under 1 mA cm−2. Reprinted with permission,88 American Chemical Society, Copyright 2022. (e) Comparison of galvanostatic profiles of Mg plating on different CCs, and also operando optical observations after Mg plating at 10 mA cm−2: stainless steel (SS), Cu and the same CCs treated with Au seeds. Reprinted with permission,80 American Chemical Society, Copyright 2022. |
The superiority of the Cu–Au system is also reflected in electrochemical experiments with asymmetric cells with Mg||bareCu and Mg||Au–Cu electrodes (Fig. 7d). The cell with a bare Cu electrode presented sharp fluctuations in the CE curves, demonstrating a low CE at the first 25 cycles. In contrast, the Au–Cu electrode presented an initial CE of 89.42%, increasing to 99.88% in the second cycle and holding excellent behavior in the following 400 Mg plating/stripping cycles. As shown in Table 3, Au presents a larger magnesiophilicity than Cu, resulting in a stronger energy reduction during bonding. This contributes to homogenizing the Mg plating and suppressing “dead” Mg formation. The benefits of using magnesiophilic gold sites on different CCs were also demonstrated by Kwak et al.80 They deposited diamond-shaped Au nanoseeds on different CCs (stainless steel (SS), Au treated SS, and Au treated Cu) using a sputtering procedure. While Mg plating on a Mg CC showed a nucleation overpotential of ca. 0.57 V at 10 μA cm−2, the presence of the Au nanoseeds on the Mg CC reduced this value to 0.43 V. Even at a high current of 10 mA cm−2, the Au nanoseeds were found to be effective in reducing the Mg nucleation overpotential on SS and Cu CCs. Under these high current conditions, in operando optical and X-ray observations showed that the Au nanoseeds prevented the occurrence of dendrites (Fig. 7e). In this way, the Au particles also delayed the short-circuit upon Mg plating compared with plating on a bare Mg surface. The compatibility of Mg and Au seems to be surprising when one starts to look for the physical reason. In principle, the bonding of Mg towards Cu should be energetically more favorable from the crystallographic point of view. Cu has a higher theoretical surface energy compared to gold and it can form alloys and interstitial phases with Mg. So, most likely the native oxide on the Cu CC which is not-existent on the more noble Au is changing the surface energy in a way that binding/wetting with Mg is less favorable. For the case of the Au nanoseeds, the mechanism might in fact rather be a physical functionalization. By introducing many tiny, highly conductive tips on the surface, the electric field is enhanced around these particles converting them into nucleation sites. Also, compared to a bulk material, nanoparticles do always have a high surface energy due to their large surface to volume ratio.
System: Mg atom on | Binding energy/eV | Ref. |
---|---|---|
a Values refer to bulk Mg metal. | ||
Au(111) | −0.83a | 79 |
Cu(111) | −0.22a | 79 |
Au(111) | −2.01 | 79 |
Cu(111) | −1.61 | 79 |
Mg(0001) | −0.57 | 79 |
In(101) | −1.36 | 89 |
MgIn(001) | −1.56 | 89 |
In(101) | −0.81 | 89 |
Mg(002) | −0.75 | 89 |
Mg(001) | −1.12 | 90 |
MOF | −2.73 | 90 |
MgO(001) surface | −0.51 | 90 |
MgO(001) step | −1.26 | |
Cu(111) | −0.28a | 91 |
Mo(110) | −0.46a | 91 |
Mg(0001) | 0.67a | 91 |
O-terminated graphene | −1.06a | 91 |
Graphitic, pyridinic, pyrrolic N at graphene | −0.03, −1.44, −3.23 | 91 |
CO and –OH at graphene | −1.88, −1.33 | 91 |
M-Xene Ti3C2O2, | −2.82 | 91 |
Ti3C2(OH)2, | −0.73 | |
and Ti3C2F2 | −0.33 | |
Ni(OH)2 (110) | −1.89 | 92 |
Ni(OH)2 (002) | −0.963 | |
Graphite | −0.57 | 92 |
Cu | −0.36 | 92 |
In principle, there may be several factors affecting the Mg growth mechanism on a foreign surface, like the gradients in electrolyte concentration, mechanical properties of the SEI, relative surface energies of the metal involved, etc. The authors concentrated on two of them: the diffusion barrier of Mg atoms on the metallic substrate, and the binding energy of Mg atoms to the surface. A quantitative assessment can be made in terms of energetic considerations using DFT as a powerful tool. In Table 3, different binding energies for the interaction of a Mg atom with different substrates are shown. Depending on the references, Emg may be the reference of the isolated Mg atom or the Mg atom in the bulk Mg material. The latter is indicated with an asterisk on the Table 3. To relate both references, it may be considered that the cohesive energy of Mg is −1.50 eV.79 According to this table, for example, an Au(111) surface presents a considerably larger magnesiophilicity than a Cu(111) surface, since it presents a larger binding energy.
Kwak et al.80 calculated by DFT the diffusion barriers for the motion of a Mg-ion to an adjacent site on Au(111), Cu(111) and Mg(0001). They pointed out that these values (0.033, 0.018 and 0.018 eV) were considerably lower than the diffusion barrier for a Li atom (0.14 eV), suggesting that this may be the reason why Mg is less prone to dendritic growth than Li metal.
Concerning the first of these factors, the discussion given in Section 3 can be addressed, where it was found that the barriers for Mg diffusion are considerably lower than those for Li diffusion, thus justifying why Mg is less prone to dendrite formation. As shown in Table 3, indium-containing surfaces should also show magnesiophilicity. For this reason, Yang et al.89 used InCl3 as an additive in magnesium triflate ((Mg(OTf)2))in combination with 1,2 dimethoxyethane as a solvent. The test was performed in Mg‖Al asymmetric cells. While the cells prepared using pure Mg(OTf)2 electrolyte only lasted for 60 cycles, with a CE of 71.6%, the cells prepared with the InCl3 additive endured 250 cycles, with an average CE of 98.7%. Besides the specific magnesiophilic effect, the presence of InCl3 also resulted in improved reaction kinetics, as inferred from exchange current density measurements. The SEM measurements of deposits with and without the additive and finite elements simulations showed that the flat deposits obtained with InCl3 yield a more homogeneous distribution of the electric field, in contrast with the surfaces without any additive, which yields a large electric field around sharp edges that lead to preferential plating on them.
Another example of the study of Mg plating on inorganic CCs can be found in the work of Bae et al.83 They prepared a MgO-wrapped Zn-skeleton, which was used as a CC for Mg electrodeposition. To prepare these CCs, the surface of a Mg CC was scratched and immersed in a 1 M ZnCl2 in tetrahydrofuran solution. Under these conditions, a chemical conversion reaction deposits Zn, while Mg+2 ions are dissolved into the solution. As a consequence of this reaction, a 5 μm Zn layer appears on the Mg collector. Simultaneously, an ultra-thin layer (2 nm) of amorphous MgO appears on the Zn-skeleton. The Magnesium plating reaction occurring on these structures leads to an interphase which is mainly composed of a MgO/MgxOy/Mg mixture, subtended to a 6 nm region. Symmetric Zn-skeleton cells were assembled and their behavior was compared with that of Mg CCs. Upon cycling, the former presented an interfacial resistance reduced by a factor of 20.
While the bare symmetric Mg cells showed a rapidly increasing overpotential (1.58 V) due to the formation of a passivation layer on the Mg surface, the symmetric Zn-skeleton cell presents a very stable cycling performance with a considerably smaller overpotential (0.03 V). The authors attributed the good performance of the Zn Skeleton to the fact that the lattice mismatch between the Zn-skeleton and the MgO layer results in a very defective MgO layer, where the Mg+2 ions may be rapidly transported, facilitating Mg plating. However, the authors did not appeal to the concept of magnesiophilicity in this work, for the sake of completion the binding energy of a Mg atom to Mg (001) surfaces and steps in Table 3 is added, which were found to be of −0.51 eV and −1.26 eV respectively. Since the authors point towards the occurrence of defective nanocrystalline MgO layers, the latter value should be rather considered for a measure of the magnesiophilicity of the surface. A Mg-Li hybrid battery prepared based on the Zn-Skeleton anode and a NMC (layered lithium-nickel-manganese-cobalt oxide) cathode endured 250 cycles at 0.5C and resulted in an energy density of 412.5 W h kg−1 at 0.1C, which compares with the energy density of Graphite‖NMC LIB.
The concept of magnesiophilicity also led Kwak et al.93 to prepare Ag-decorated Cu foams (ACF) using a straightforward galvanic replacement procedure, by immersing a Cu foam (CF) in a 1.0 mM AgNO3 solution. This led to the formation of Ag nanoparticles on the CF, resulting in a magnesiophilic surface with excellent behavior for Mg plating/stripping. The ACF CC reduced considerably the Mg nucleation overpotential at two different current densities.
Fig. 8 Overpotential of three asymmetric cells (Mg‖Ni(OH)2@CC, Mg‖CC, and Mg‖Cu) at different current densities of (a) 1 mA cm−2 and (b) 4 mA cm−2/10 mA cm−2. (c) Onset reduction potentials of cell CV curves at 1 mV s−1. (d) Electrochemical Mg plating/stripping profiles at 4 mA cm−2. Reprinted with permission,92 American Chemical Society, Copyright 2022. (e) The schematic illustration of Mg electrodepositing on carbon cloth and VNCA@C (nitrogen- and oxygen-doped carbon nanofiber arrays on carbon cloth) CCs. (f) Cyclic voltammograms for Mg stripping/plating on the CCs. (g) Overpotential as a function of plating time for the galvanostatic plating on the CCs at 10 mA cm−2. Reprinted with permission,94 John Wiley and Sons, Copyright 2021. (h) and (i) Schematic representation of Mg plating behaviors on Cu and Ti3C2Tx MXene CCs. Reprinted with permission,87 John Wiley and Sons, Copyright 2023. (j) and (k) Schematic illustrations of three-stacked Cu mesh (3D-CM) and the 3D Mg affinity-controlled architecture (3D-MACA) CCs and their Mg plating behaviors. Reprinted with permission,95 Elsevier, Copyright 2023. |
Lim et al.91 have explored the use of a three-dimensional macroporous graphitic carbon nanosubstrate (GC-NS) for Mg plating. This GC-NS was prepared through thermal treatment of cellulose pellicles of bacterial origin, yielding a complex network structure, with many macropores. They compared Mg plating on GC-NS with the same phenomenon on Mo, Cu, stainless steel, and Mg as a CC. They found that the overpotential for GC-NS, (0.20 V) was considerably smaller than for Mo (0.27 V), Cu (0.53 V) and SS (0.43 V). Besides a better performance towards Mg nucleation, the GC-NS anode presented a larger affinity for the Mg deposit. The latter feature was derived from observations of the separator of the cell. While the system with the GC-NS anode presented a clean surface of the separator after the plating of 5 mA h cm−2 of Mg, the separators used with the other CCs were at least partially stained by the Mg metal. The calculations presented in Table 3 suggest that oxygen species terminating the CG-CS, which provide a binding energy of −1.06 eV relative to bulky Mg, may be responsible for the improved affinity of GC-NS for Mg.
Structures based on carbon nanofibers have also been used by Song et al.94 in Mg anode-free cells for Mg plating, with the additional doping of N-species via polypyrrole nanofiber arrays on carbon cloth plating, which were carbonized at 800 °C in a nitrogen atmosphere to yield a cloth containing C (graphitic), N (pyridinic, pyrrolic, graphitic) and O(CO, OH, O2−). This cloth was denoted with VNCA@C, to describe a Vertically aligned Nitrogen and oxygen-doped Carbon nanofiber array (Fig. 8e). Mg stripping/plating kinetics was investigated on these VNCA@C structures by cyclic voltammetry and galvanostatic transients uniformly electrodepositing metallic Mg, in comparison with the same process on carbon cloth and Cu CCs (Fig. 8f and g). All experiments were conducted in all-phenyl-complex electrolytes.96 While the voltametric profiles show that the VNCA@C electrode presents higher current responses during both cathodic and anodic sweeps as compared with those of carbon cloth and Cu electrodes (Fig. 8f) the galvanostatic transients for the former also show much lower overpotentials than for the other electrodes (Fig. 8g). The galvanostatic cycling performance of asymmetrical cells with Mg was tested at a current density of 10.0 mA cm−2, and the Mg‖VNCA@C could run for 40 hours with overpotentials of less than 0.5 V, but the other alternatives endured less than 5 hours with overpotentials over 1.0 V.
Besides the magnesiophilicity, a further factor that influences the Mg plating was the concavity of the surface on which this metal is deposited. A surface with periodically arranged concave structures would facilitate Mg nucleation on these sites by periodically enhancing the electric field strength and thus yield a smooth top surface. When many nuclei form and grow equally distributed on the surface, they will finally form a homogeneous film. In this respect, protrusions of Mg electrodeposits trigger the occurrence of large aggregates, as observed with carbon cloth and Cu foil (tip effect).
At the time of writing this review, the most recent work related to AFMMBs was that of Li et al.87 who prepared MXene-based AFMMBs. They prepared 3D MXene (Ti3C2Tx) films as a CC, where Tx represents the fact that the surface contains –O and –F containing terminations. A high magnesiophilicity, could be driven particularly by the oxygen terminations (Fig. 8h and i). This is mirrored in the large binding energy of Mg to Ti3C2O2 of (Table 3). This strong interaction and the small lattice missmatch of MXene with Mg leads to dendrite-free Mg plating. The estimated lattice mismatches of Mg (001) with Ti3C2O2, Ti3C2(OH)2, and Ti3C2F2 are 4.7%, 3.8%, and 3.8%, respectively. Fluorine terminations lead to the formation of a homogeneous and resistant MgF2 solid electrolyte interface layer. This is very important for inhibiting the consumption of Mg via a reaction with liquid electrolytes. Furthermore, the (Ti3C2Tx) films provide a reasonable conductivity of 4.69 × 104 S m−1 making it suited as a CC (Fig. 8i). At a large current density of 5.0 mA cm−2, these electrodes run over 350 cycles with an average CE of 99.7%. In contrast, under analogous conditions a Cu electrode shows severe voltage fluctuations, collapsing after about 120 cycles.
The intermediate figures of Fig. 8j and k illustrate the nucleation and growth of Mg in the two types of architectures. While in the 3D-CM Mg is electrochemically deposited mainly on the top layer, since ions in the electrolyte find the shortest pathway, in the case of the 3D-MACA, the initial nucleation and subsequent growth take place in the bottom layer, which has the largest magnesiophilicity. This translates into the voltage-capacity behavior: While 3D-CM presents an overvoltage of 0.214 V, 3D-MACA this overvoltage is reduced to 0.064 V.
The different anodes were pre-deposited with Mg in a half-cell to 1 mA h cm−2, then re-assembled into a full-cell in combination with the Chevrel phase as a cathode and cycled galvanostatically. The results showed that the 3D-MACA electrode presents an efficiency close to 100% up to 300 cycles, with capacity retention of about 80%. However, the 3D-CM electrodes showed a drop in capacity below 60%.
There are excellent reviews discussing the choice of electrolytes for secondary Mg batteries, which in principle could also apply to anode-free Mg systems. For the work previous to 2019 the review of Deivanayagam et al.,97 Ma et al.99 and Attia et al.100 can be addressed. A more recent review also addressing interphase issues has been given by Sun et al.101
In general, a plethora of electrolytes are viable for Mg metal batteries, however, the requirements of a large voltage window limits the practicality of many for battery applications. Among the articles revised for the present topic, by far the most used choice of electrolyte is the so-called all-phenyl complex (APC) in tetrahydrofuran (THF).
The first generation of electrolyte solutions was based on Grignard-type compounds.100 They resulted from the reaction of organo-magnesium R2Mg (R = alkyl of aryl group) with organic halo-aluminum compounds of the type AlCl3−nRn. In a suitable combination, this reaction yielded compounds with the formal composition Mg(AlCl4−nRn)2, which were dissolved in THF or glymes.102 While exhibiting an excellent electrochemical performance for Mg dissolution plating, it was found that they presented a relatively reduced electrochemical window of 2.4 V, which precludes its practical application in combination with some Mg-cathode oxides like V2O5 and MoO3.103 The reason for this reduced electrochemical window was found to be the relatively weak Al–C bond of the electrolyte, that breaks via a β-H elimination in an electrochemical reaction.104 Thus, the solution found by Mizrahi et al.96 was to replace the R- aliphatic ligands with phenyl Ph-, which does not have a β hydrogen.
This proposal led to the so-called all phenyl complex (APC)-type electrolytes, which exhibit high anodic stability (>3.3 V), low overpotential for Mg plating/stripping, and high CE. The second most used electrolyte was the magnesium trifluoromethane sulfonate (Mg(OTf)2)-based electrolyte, where OTf represents SO3CF3, also denominated triflate. This compound was used with the addition of MgCl2 or InCl3. Mg(OTf)2 has the advantages of being thermally stable, insensitive to ambient moisture, non-toxic, and available commercially with high purities.105 The addition of Cl− salts allowed the complete dissolution of the triflate into the 1,2-dimethoxyethane solvent, and the resulting solution outperformed the behavior of Mg(TFSI)2 (TFSI = bis(trifluoromethane)sulfonilimide).
As usual in metal-based batteries, the formation of dendrites is one of the main bottlenecks to be solved to ensure safety and reliability. Zn surfaces with (002) preferential orientation offer a series of advantages, mainly because they provoke a planar and dendritic-free plating of Zn.107 There are different strategies to achieve this, such as interfacial layer, heterostructures, electrolyte composition, and metal surface modification with zincophilic structures, among others.106–108
Another zincophilic strategy to regulate Zn plating over a modified Cu-CC using Sb (Fig. 9a and b) has been proposed by Zheng et al.110 The authors designed a Sb/Sb2Zn3@Cu-heterostructured interface as a CC for an anode-free Zn–Br2 battery (Fig. 9c). They first studied the nucleation and growth of Zn over the modified Sb/Sb2Zn3@Cu surface using Zn foil as a counter electrode in a half-cell configuration, exhibiting a high stability over 700 h with an average CE of 97.8% and low overpotential of −20 mV. The improved performance in comparison to the Cu||Zn asymmetric cell is attributed to the suppression of Zn dendrites by uniform, homogeneous, and compact Zn electrodeposition without any discernible dendrites. After the excellent results in the half-cell, the full cell was built using Sb@Cu as an anode, carbon felt (CF) as cathode, ZnBr2 and tetrapropylammonium bromide (TPABr) as the salts of an aqueous electrolyte. The authors first explored the rate capability and long-term cyclability. After cycling, the CC surface exhibited a dendrite-free morphology, indicating a homogeneous Zn nucleation given by the zincophilicity of the surface. Finally, this technology was scaled up (Fig. 9c) and connected to a photovoltaic cell panel as a battery module, resulting in an energy of 9 W h (6 V, 1.5 Ah) and 400 mA h g−1 of specific discharge capacity over 400 cycles (Fig. 9d).
Fig. 9 (a) Scheme for the Zn nucleation and growth using Sb/Sb2Zn3@Cu modified and Zn foil. (b) Scheme for the full cell operation. (c) Scheme for the scaled-up cell applied to photovoltaic panel. (d) Capacity stability for the scaled-up cell. Reprinted with permission,110 Springer Nature, Copyright 2023. (e) Scheme for the Cu@AOF synthesis and coating over Cu CC. (f)–(i) electrochemical results. Reprinted with permission,111 John Wiley and Sons, Copyright 2023. |
An additional Cu CC modification for an anode-free aqueous Zn battery has been proposed by Wang et al.112 for room and low temperature operation. They synthesized and deposited Al2((OH)0.46F0.54)6·H2O (AOF) on Cu foil (Cu@AOF) to guide the Zn nucleation and suppress dendrite formation. The deposited AOF shows a high crystallinity, a strong binding with H2O molecules and a low diffusion energy for Zn adatoms which leads to good desolvation and fast diffusion processes on the surface, which drastically enhances layer-by-layer growth. The full cell setup was completed using pre-zincified VO2 as a cathode, ZnSO4 and Zn(OTF)2 as the electrolyte, and Cu@AOF as an anodic CC. The asymmetric Zn||Cu@AOF half-cell showed a high cyclability of 6000 cycles at room temperature and 500 cycles at −20 °C. That is caused by a homogeneous, thin, and dense electrodeposition, as well as highly controlled plating/stripping processes which drastically inhibit the dendritic growth and side reactions such as ZnSO4 formation on the CC surface and hydrogen evolution. The full cell ensemble displayed an initial specific capacity of 200 mA h g−1 at 1 A g−1 with 60% capacity retention after 2000 cycles at room temperature, whereas 140 mA h g−1 at 1 A g−1 with 80% capacity retention after 500 cycles at −20 °C (Fig. 9e–i).
To improve the Zn plating on Cu CCs, Wang et al. proposed an anode-free Zn-graphite (ZGBs) dual ion battery with Cu protected with a thin layer (40 nm) of Ag as a CC and Zn(TFSI)2/EMC as electrolyte.107 Thus, the Zn2+ deposits on the Cu@Ag CC by electroplating and the FSI− intercalates into graphite cathode. The authors evaluated the system using half-cell and full-cell tests. Thus, they found an excellent behavior of the Zn plating/stripping process leading to a uniform Zn plating on Cu@Ag. They identified AgZn/AgZn3 alloys as zincophilic, which guides the subsequent dendrite-free Zn metal plating, and delivered a specific capacity of 117 mA h g−1. In terms of full cell, the cycling was done under 0.1–2.85 V at 0.1 A g−1. As a result, the full-cell ensemble presented a specific capacity of 114 mA h g−1 (based on the graphite mass in the cathode) with 82% capacity retention after 1000 cycles at 0.5 A g−1.
Duan et al. developed an approach by controlling the reaction kinetics of the electrolyte in AFZMB using a Cu CC.113 The authors proposed LiFePO4 (LFP) as cathode, Zn(CF3SO3)2 and LiCF3SO3 as aqueous/Ethylene glycol (EG) electrolyte, and Cu foil as a CC. The presence of EG in the electrolyte provokes the uniform Zn plating and prevents the Zn dendrite formation thorough steric hindrance of [Zn(H2O)m(EG)n]2+ complex, which retards the Zn2+ deposition.114 The full cell LFP||Cu delivered an initial specific capacity of ∼120 mA h g−1 at 1 mA cm−2 with 75.2% capacity retention after 100 cycles.
In another recent study, An et al. designed a zincophilic robust heterointerface composed of 0D and 2D metal carbide nanosheets to adjust the Zn nucleation and growth for stable and homogenous Zn plating/stripping.115 Thus, they synthesized an interface based on Na intercalated MXene Ti3C2Tx/Sn composite as a coating of the Cu CC (Cu@Na-MX@Sn). Herein, the rich O and F surface terminations of MXene likely anchor the Sn nanoparticles. The final surface has zincophilic sites distributed on it, favoring the (002) hexagonal Zn nucleation, avoiding the dendrite formation. The homogeneous Zn plating is probed at different rates and capacities. At current density of 1.0 mA h cm−2, the Cu@Na-MX@Sn delivers stable CEs and long cycle life, whereas Cu@Na-MX shows a short-circut after 129 cycles. The full cell was assembled with a LiMn2O4 (LMO) cathode. The LMO||Cu@Na-MX@Sn cell showed a higher initial CE (71.2%) than LMO||Cu (42.1%). At a charging rate of 100 mA g−1 the specific capacity of LMO||Cu@Na-MX@Sn was 50.09 mA h g−1, with a capacity retention of 73.97%, whereas LMO||Cu delivered 11.96 mA h g−1. The improvement is not only due to the zincophilic sites of Sn but also by the SEI stability given by ZnF2 and homogeneous electrical field provided by MXene.115
A similar approach was conducted by Chen et. Al who also used Ti3C2Tx as a robust heterostructure interface to control the Zn nucleation and growth.116 The authors designed Ti3C2Tx/nanocellulose hybrid films with different weight percentages of Ti3C2Tx. The aqueous hybrid Zn–Li batteries were assembled with Li2MnO4 as the cathode, a mixture of Li2SO4, ZnSO4, and ZnF2 as the electrolyte, and Cu coated by Ti3C2Tx/nanocellulose as a CC. In addition, they developed quasi-solid-state batteries using polyvinyl alcohol (PVA)/ZnSO4/Li2SO4/ZnF2 hydrogel electrolyte. A specific capacity of about 100 mA h g−1 during 100 cycles at 0.2 A g−1 and 60 mA h g−1 at 1 A g−1 were obtained for the liquid electrolyte. Moreover, the full cell showed a lower nucleation overpotential with respect to the comparative hemi-cells. In the case of quasi-solid-state batteries, the full cell delivered a specific capacity of 42.4 mA h g−1 over 2000 cycles with a capacity retention of 81.5% at 1 A g−1, corresponding to a gravimetric energy density of 187.7 W h kg−1 at a power density of 181.2 W kg−1 (based on the LMO mass) and a volumetric energy density of 24.1 mWh cm−3 at 23.2 mW cm−3 (based on the total volumes of two electrodes and hydrogel electrolyte). The demonstrated performance is associated with the synergist effect of the nanocellulose and Ti3C2Tx to improve the mechanical properties, electrical conductivity, and electrolyte wettability.
Yan et al.118 studied the preferential growth of Zn (002) on Cu (100) obtaining a compact and planar-free dendrite anode. To demonstrate the preferential crystal orientation of Zn on Cu (100) they performed AIMD simulations and observed that the first Zn layer (underpotential plating of Zn on Cu) transforms from a hcp to a fcc crystal structure (close to the interatomic distance of Cu (100)). The first Zn layers that adopt the Cu (100) structure serve as a template for bulk Zn plating in a (002) direction inhibiting the growth of dendrites. Wang et al.119 identified AgZn/AgZn3 alloys as a zincophilic phase that guides the subsequent dendrite-free Zn metal plating, and the cell delivered a specific capacity of 117 mA h g−1. DFT calculations suggest that the trans-bis(trifluoromethanesulfonyl)imide anion (TFSI)−-intercalated graphite is more stable than the cis-TFSI− intercalation compound by 0.435 eV. Also, by DFT calculations An et al.115 indicate that Sn (101) interacts effectively with Zn with a strong interfacial charge density. The electric field distribution modeled by COMSOL reveals that Na-MXene@Cu can homogenize the electric field and improve the wettability with ZnSO4 electrolyte. MXenes are a kind of two-dimensional materials that consist of transition metal (M) atoms sandwiched between layers of carbon/nitrogen atoms. They are similar to graphene, but with the addition of transition metal atoms, which give them unique properties and possible applications in various fields of technology.120 The improved electrochemical performance observed by Wang et al.112 for aluminum hydroxide fluoride (AOF) coated on Cu foil was explained by DFT. The adsorption energies were obtained, giving as a result that AOF promotes the desolvation process of Zn2+. Binding energies showed that AOF favored a uniform Zn plating on the surface AOF.
Another interesting strategy is the design of co-solvent electrolytes. In this sense, Ming et al.122 have proposed propylene carbonate (PC) and triflate anions in Zn(OTf)2 aqueous electrolyte to form hydrophobic domains resulting in a solid electrolyte interface that avoids Zn dendrite formation and side reactions in a wide range of cathode materials. The salt effect is due to the amphipathic effect of the OTf- anion; the hydrophobic -CF3 group and the hydrophilic SO3- group.123 Using FT-IR spectroscopy, it was found that the presence of propylene carbonate (PC)/water mixture can regulate the coordination environment of OTf- anions by forming a [PC-Otf—H2O] complex. In terms of electrochemistry, the authors studied the compatibility of the hybrid electrolyte with different cathodes in the full cell set-up at a low rate of 50 mA g−1, finally choosing the Zn-rich ZnMn2O4 cathode to avoid complicated and time-consuming pre-zincification reactions. The anode-free Cu (CC)|| ZnMn2O4 with Zn(OTf)2 in propylene carbonate/water as electrolyte showed a high capacity retention of 80% after 300 cycles.
In other study, Duan et al. developed an approach by controlling the reaction kinetics of the electrolyte in AFZMB using a Cu CC.113 The authors proposed LiFePO4 (LFP) as cathode, Zn(CF3SO3)2 and LiCF3SO3 as aqueous/Ethylene glycol (EG) electrolyte, and Cu foil as a CC. The presence of EG in the electrolyte provokes the uniform Zn plating and prevents the Zn dendrite formation through steric hindrance of [Zn(H2O)m(EG)n]2+ complex, which retards the Zn2+ deposition.114 The full cell LFP||Cu delivered an initial specific capacity of ∼120 mA h g−1 at 1 mA cm−2 with 75.2% capacity retention after 100 cycles.
Cu coating CC | Electrolyte | Cathode | VW/V | SC/mA h g−1 | CR/cycles | Current Density | Ref. |
---|---|---|---|---|---|---|---|
@Ag | Organic: Zn(TFSI)2/EMC | Graphite-Zn | 0.1–2.8 | 114 | 82%/1000 | 0.5 A g−1 | 107 |
@Polyvinil Pyrrolidone | Aqueous: ZnI2, I2, ZnSO4 | ZnI2 | 0.6–1.6 | 150 | 64%/200 | 1 A g−1 | 124 |
@Carbon nanodisc | Aqueous: Zn(CF3SO3)2, Mn(CF3SO3)2 | MnO2 | 0.8–1.8 | 200 | 69%/80 | 1 A cm−2 | 125 |
— | Aqueous/EG: Zn(CF3SO3)2, LiCF3SO3 | LiFePO4 | 0.6–1.6 | 120 | 75%/100 cycles | 1 mA cm−2 | 113 |
@Mxene-Ti3C2Tx/Sn | Aqueous/ZnSO4 | LiMn2O4 | 1.4–2.1 | 50 | 50%/ | 100 mA g−1 | 115 |
@Ti3C2Tx/nanocellulose | Aqueous/Li2SO4, ZnSO4, ZnF2 | LiMn2O4 | 1.4–2 | 60 | 86%/2000 | 1 A g−1 | 116 |
@Ti3C2Tx/nanocellulose | Aqueous-PVA/Solid State Li2SO4, ZnSO4, ZnF2 | LiMn2O4 | 1.4–2 | 42 | 82%/2000 | 1 A g−1 | 116 |
@Cu 3D-nanostructured | Aqueous/ZnBr2, ZnSO4 | MnO2/Br2 | 0.6–1.6 | 450 | 89%/1000 | 10 mA cm−2 | 109 |
@Sb/Sb2Zn3 | Aqueous/ZnBr2, TPABr | Br2 | 0.5–1.6 | 220 | 95%/40 | 20 mA cm−2 | 110 |
@Al2((OH)0.46F0.54)6·H2O | Aqueous/ZnSO4 Zn(OTF)2 | Zn0.5VO2 | 0.3–1.6 | 200 | 60%/2000 | 1 A g−1 | 111 |
The scalability and application in real systems are still at an early stage in terms of engineering, design and development. At present, the processes such as the synthesis/deposition of the nano-heterostructures and the pre-zincification of the cathodes, are not easily transferable to an industrial scale. Nevertheless, these problems can be overcome in the near future.
Fig. 10 Electrochemical measurements and ex situ characterization of different anodic CCs. (a) Cycling performance of different anodic CCs at a fixed rate of 50 mA g−1 (b) SEM and EDX measurements of fully discharged Mo anodic CC after 100 cycles. (c) AES image and corresponding AES elemental mapping of fully charged GP anodic CC etched for 20 min by argon ion bombardment. Reprinted with permission,128 Elsevier, Copyright 2022. |
Meng et al. improved this concept by using titanium (Ti) as the CC with a 10 nm Au lattice-matching layer, resulting in an average CE of 99.92% for more than 4500 h in a cell versus Al-Metal.131 In a dual-ion battery configuration with Ti/Au as an anode and graphite as the cathode, a CE >98% was achieved with a capacity retention of more than 78% after 2000 cycles. Furthermore, a reduction of the Au layer thickness to 2 nm did not significantly decrease the stability of the plating/stripping.
CC | Electrolyte | Cathode | VW/V | SC/mA h g−1 | CR/cycles | Current Density | Ref. |
---|---|---|---|---|---|---|---|
Al | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | 58 | 98.6%/1000 | 0.2 A g−1 | 128 |
GP | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | 54.6 | 98.1%/1000 | 0.2 A g−1 | 128 |
Mo | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | 53.2 | 95.8%/1000 | 0.2 A g−1 | 128 |
Ag | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | ∼28 | 0%/300 | 0.2 A g−1 | 128 |
Ni | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | ∼25 | 0%/∼150 | 0.2 A g−1 | 128 |
SS304 | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | ∼20 | 0%/∼670 | 0.2 A g−1 | 128 |
Mg | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | ∼10 | 0%/∼400 | 0.2 A g−1 | 128 |
Cu | AlCl3/[EMIm]Cl | Graphite Paper | 1.5–2.5 | ∼35 | 0%/∼140 | 0.2 A g−1 | 128 |
Au–SS | Aqueous/ZnBr2, TPABr | Graphene | 1.5–2.5 | ∼45 | 74%/2000 | 1.4 mA cm−2 | 127 |
Au@Ti | Aqueous/ZnSO4 Zn(OTF)2 | EG | 0.5–2.3 | 107.4 | 80%/900 | 0.2 A g−1 | 131 |
Promising electrochemical performances have been obtained in full-cell configurations, but in all cases the CE and specific capacity are still decreasing during cycling. In this sense, the modification and coating of the CC surface, which induces the desired metal philicity, plays a fundamental role in the ordered plating of the metals, diminishing the overpotentials and the dendrite formation. There are some common materials for AFMBs, such as M-Xenes, carbon coatings, metal alloys, nanoparticles, etc., which could allow the consideration of these strategies as a possible general solution for the different AFMBs.
All these coatings have in common that they exhibit a high surface energy compared to the surface energy of the metal to be deposited. A high surface energy is generally achieved by introducing a functionalization with a high defect density, e.g. Nitrogen defects, vacancies or polar –F or –NH sites.
Furthermore, a functionalization of the separator side facing the metal can be beneficial. As for the CC, a good wettability between separator and metal enable a homogeneous nucleation which is especially important for anode free batteries. To ensure a planar growth of the metal, the electrolyte should be tuned such, that a smooth and strong anion-derived SEI can build up. Ideally, such a strong SEI that does not break during cycling can guide the metal to grow as a planar film. One further requirement for the SEI is that its ion channels have to be evenly distributed. As is well known by now, a SEI is rarely comprised of one single compound but rather agglomerates of different decomposition products, which can have totally different ionic conductivities. Therefore, an even distribution is necessary to avoid uneven electroplating and thereby continuous rupture and reformation of the SEI.
• The authors suggest to enable a homogenous metal nucleation by an adjustment of the cycling routine. A pulsed deposition in the first formation cycle can result in a much more homogeneous plating. In this context, the series resistance of all individual components should be taken into focus of research as the network of all series resistances is what determines the electric field distribution.
• The study of Fang and al.134 indicates that also the cell pressure affects the homogeneity of the plated metal morphology, i.e. a high cell pressure promotes planar growth. However, this parameter is not yet in the focus of AFMB-research. Therefore, for future works the authors suggest to provide detailed information about the stacking height and the spring chosen for coin cells. Advanced in situ imaging techniques such as in situ optical microscopy, transmission X-ray microscopy, and similar need to be further developed to learn more about the nucleation and growth mechanisms of different metals. Meanwhile, in situ spectroscopic techniques such as in situ FTIR, in situ Raman, in situ XPS, in situ NMR etc. should be used to explore the electrolyte decomposition, SEI and dead-metal formation on various metals.
• More studies on binding energies between the individual battery components - CC, metal anode, SEI and separator – will help to effectively tackle the main difficulties.
• Recycling and life-cycle analysis: It's crucial to assess the recyclability and overall environmental impact of AFMBs throughout their lifecycle to ensure they truly represent a sustainable alternative.
• Scalability: While lab-scale demonstrations are encouraging, the scalability of AFMB production processes needs careful evaluation to meet the demands of large-scale energy storage – especially with respect to safety concerns during fabrication and operation.
Overall: post-lithium AFMBs seek to improve cost-effectiveness through the use of abundant and inexpensive materials, streamlined manufacturing processes, and an increased energy density, making energy storage more accessible and economically viable. These advances have the potential to drive innovation in various industries, leading to more sustainable, efficient, and affordable energy in the future.
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