In situ polymerization-driven quasi-solid electrolytes for Li-metal batteries

Abstract

Li-metal batteries (LMBs) have emerged as the most promising energy storage technology to achieve high energy density exceeding 400 W h kg⁻¹. However, the conventional liquid electrolytes in LMBs present significant challenges, including severe reactivity with Li metal anodes and substantial safety risks. Quasi-solid electrolytes (QSEs), which combine the advantages of both solid and liquid electrolytes, have been identified as an ideal alternative for practical LMB applications. In particular, QSEs fabricated through in situ polymerization strategies offer enhanced interfacial compatibility, simplified manufacturing processes, reduced production costs, and represent one of the most commercially viable pathways for QSE development. This review provides a comprehensive analysis of in situ polymerization-driven QSEs and their application in LMBs. QSEs are systematically categorized based on the polymerization mechanisms of the polymer matrix, including free radical, cationic, anionic, and electrochemical polymerizations. Furthermore, the strategies for improving the performance of QSE-based LMBs are thoroughly discussed, with a focus on enhancing flame-retardant properties, high-voltage compatibility, and low-temperature operation. Finally, the review concludes with a critical assessment of the current state and future prospects of QSE-based LMBs, providing valuable insights into the future development of safer and more efficient energy storage systems.

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Jinxuan Liu

Jinxuan Liu obtained a Bachelor's degree in Materials Chemistry from Jingdezhen Ceramic University in 2023. He is currently pursuing a Master's degree at Shanghai University of Engineering Science and working as an exchange student in Professor Zhuang's team. His project is to develop quasi-solid polymer electrolytes.

Yinglei Wu

Yinglei Wu obtained her Master's degree from Shanghai Jiao Tong University and then carried out doctoral research at the University of Twente in the Netherlands. She is now working at Shanghai University of Engineering Science. Her current research interests are focused on materials and chemicals, including lithium-ion batteries.

Jinhui Zhu

Jinhui Zhu obtained his PhD from Shanghai Jiao Tong University in 2018. He then did his postdoctoral research at Shanghai Jiao Tong University. He is currently an associate professor at Shanghai Jiao Tong University. His current research interests are focused on the development of key materials and technologies for solid-state Li batteries.

Xiaodong Zhuang

Xiaodong Zhuang, a synthetic material chemist, is a full professor at Shanghai Jiao Tong University and the head of The Meso-Entropy Matter Lab. His scientific interest is focused on meso-entropy matter, including special aromatics, polymers, carbons and two-dimensional materials, for energy storage and conversion.

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1. Introduction

The global electrified energy revolution has spurred an unprecedented demand for advanced rechargeable batteries. While commercial Li-ion batteries (LIBs) based on graphite anodes and liquid electrolytes (LEs) have dominated the market, their limited energy density and inherent safety concerns hinder their ability to meet the growing requirements of high-energy-density and safe energy storage systems, particularly for applications such as electric vehicles and electric aircrafts.^1,2 To address these challenges, the development of next-generation anode materials and electrolytes with enhanced safety, higher energy density, longer cycle life, and cost-effectiveness has become imperative. Among potential anode materials, Li metal anodes (LMAs) stand out as the “holy grail” due to their exceptional theoretical specific capacity (3860 mA h g⁻¹) and the lowest electrochemical potential (−3.04 V vs. SHE), enabling Li-metal batteries (LMBs) to achieve energy densities exceeding 400 W h kg⁻¹.^3,4 However, the practical application of LMBs remains hindered by several critical issues, including dendrite formation, significant volume changes, low coulombic efficiency (CE), and severe interfacial side reactions between LMAs and LEs. These side reactions often lead to heat generation and safety hazards such as LE volatilization, combustion, or even explosion.⁵

To mitigate these challenges, replacing LEs with solid electrolytes (SEs) has emerged as a promising strategy. SEs not only address safety concerns but also alleviate some intrinsic issues of LMAs through tailored electrolyte design.^6,7 SEs can be broadly classified into two categories: quasi-solid electrolytes (QSEs) and all-solid electrolytes (ASEs). While ASE-based LMBs offer enhanced safety by eliminating liquid components entirely, they face significant hurdles, including severe side reactions between LMAs and inorganic ASEs (e.g., sulfides^8,9 and halides¹⁰), poor ionic conductivity in polymer-type ASEs,¹¹ and inadequate solid–solid interfacial contact.¹² In contrast, QSEs, which consist of a polymer matrix infused with minimal LEs, combine the advantages of both LEs and SEs. The polymer matrix provides mechanical stability and confines the LE, reducing the risk of flammable solvent leakage, while the residual liquid ensures effective electrode wetting and facilitates ionic conduction.^13,14 This unique combination enables QSEs to achieve a balance between mechanical robustness and electrochemical performance, making them a compelling alternative for next-generation LMBs.¹⁵

QSEs can be fabricated via two primary approaches: ex situ and in situ methods.¹⁶ The ex situ approach involves preparing QSE membranes by dissolving polymer matrices, Li salts, and plasticizers into solvents to form a homogeneous solution, which is then cast into membranes using conventional techniques. However, this method often results in significant interfacial gaps and reduced contact between the QSE membrane and electrodes, leading to high interfacial impedance and suboptimal cell performance.¹⁷ In contrast, the in situ approach involves assembling the cell with a precursor solution containing polymer monomers, Li salts, and other components, followed by triggering polymerization under specific conditions (e.g., heat or chemical initiation). This method eliminates the need for laborious out-of-cell membrane preparation, enabling the direct formation of QSEs within the cell. The in situ approach not only enhances interfacial compatibility but also simplifies the manufacturing process, reduces production costs, and represents one of the most commercially viable pathways for QSE development.

In situ polymerization typically involves impregnating a separator with a precursor solution composed of monomers, Li salts, initiators, and optional additives (e.g., plasticizers or fillers), followed by polymerization under controlled conditions.¹⁸ Depending on the monomer and initiation mechanism, in situ polymerization patterns can be categorized into free radical polymerization,¹⁹ cationic polymerization,²⁰ anionic polymerization,²¹ and electrochemical polymerization.²² These methods utilize monomers with unsaturated functional groups (e.g., vinyl, allyl, epoxide) to form either linear or cross-linked polymer structures.

Currently, significant research efforts are focused on elucidating the mechanisms of in situ polymerization for QSE fabrication and advancing the electrochemical performance of QSE-based LMBs. This review aims to provide a comprehensive analysis of in situ polymerization techniques for quasi-solid-state (QSS) LMBs, systematically evaluating the advantages and limitations of different polymerization strategies, including free radical, cationic, anionic, and electrochemical polymerizations. Additionally, this review explores strategies to enhance specific performance metrics of in situ polymerization-based QSSLMBs, such as flame retardancy, high-voltage stability, and low-temperature operation. Finally, we offer a forward-looking perspective on the future of QSEs and their role in the development of high-performance LMBs, providing insights to guide further research and innovation in this rapidly evolving field.

2. Mechanisms of polymerizations

The polymer matrix is a critical component of QSEs, and its design must satisfy several key requirements: (1) incorporation of polar functional groups to enhance compatibility with Li salts and improve ionic conduction efficiency;²³ (2) low glass transition temperature and low crystallinity to facilitate LE absorption and promote ion mobility; and (3) high mechanical strength, thermal stability, and chemical stability to broaden the electrochemical stability window (ESW).²⁴ Consequently, the selection of polymer monomers for in situ polymerization is pivotal, as the resulting polymers must adhere to these principles. Furthermore, a deep understanding of the polymerization mechanisms is essential for monomer functionalization and the optimization of polymerization conditions. Based on the types of monomers, four primary polymerization mechanisms are employed: free radical polymerization, cationic polymerization, anionic polymerization, and electrochemical polymerization. Each mechanism is discussed in detail below.

2.1 Free radical polymerization mechanism

Free radical polymerization is one of the most widely used methods for constructing polymer matrices in QSEs. This process involves three key stages: initiation, propagation, and termination of free radicals. Olefinic monomers containing unsaturated C=C bonds, such as vinyl carbonates and acrylates, are commonly used as raw materials. The polymerization begins with the decomposition of an initiator (e.g., peroxides, azo compounds, or redox systems) to generate free radicals (R˙). These radicals, possessing unpaired electrons, attack the π-electron cloud of the monomer's unsaturated bonds, forming reactive radical intermediates. The double bond breaks, allowing the radicals to react with monomers, initiating chain growth. The polymer chain continues to propagate by reacting with additional monomers until termination occurs, typically through radical collision or reaction with a chemical agent, halting further growth. Free radicals can be generated via thermal decomposition of initiators, UV irradiation, high-energy radiation, electrochemical methods, or plasma techniques. However, thermal initiation remains the most cost-effective, safe, and commercially viable approach for constructing QSE matrices in LMBs. For instance, as illustrated in Fig. 1a, the initiator azobisisobutyronitrile (AIBN) undergoes thermal decomposition to generate free radicals. These radicals then react with ethylene glycol diacrylate (EGDA) monomers, forming EGDA radical intermediates. These intermediates undergo further cross-linking and polycondensation, ultimately yielding a PEGDA polymer matrix.²⁵

Fig. 1 Representative polymerization mechanisms: (a) free radical polymerization, (b) cationic polymerization, (c) anionic polymerization, and (d) electrochemical polymerization.

The ring size of monomers plays a significant role in the kinetics of free radical polymerization. For vinyl monomers bearing cyclic substituents (e.g., methyl methacrylate derivatives), the ring structure can influence the stability of propagating radicals. For example, cyclohexyl methacrylate exhibits a slower polymerization rate than its linear analogs due to steric hindrance imposed by the cyclic group, which restricts radical rotation and chain propagation. This steric effect reduces the monomer's reactivity during radical addition steps, impacting both molecular weight distribution and overall polymerization efficiency.²⁶

The limitations of the free radical polymerization method include the following: (1) the high-temperature reaction conditions can lead to heat accumulation, gas evolution, and degradation of plasticizers. (2) Compared to other polymerization techniques, free radical polymerization typically exhibits lower monomer conversion rates, which may result in an inhomogeneous polymer matrix. (3) The use of radical initiators can introduce non-electrolyte components, which may react with the LMA and adversely affect battery cycling performance.

To address these challenges, recent research has proposed several strategies: (1) initiator development: the use of novel initiators, such as low-temperature initiators (e.g., azo compounds) and photoinitiators with high selectivity and conversion efficiency (e.g., 2,2-dimethoxy-2-phenylacetophenone), helps lower the polymerization temperature, minimizing thermal degradation and improving matrix uniformity. (2) Monomer design: the development of multifunctional monomers compatible with existing initiation systems can enhance both the monomer conversion rate and the homogeneity of the resulting polymer matrix. (3) Initiator optimization: selecting initiators that are chemically compatible with LMA or modifying existing initiators to reduce side reactions can improve the overall stability and performance of the QSEs.

2.2 Cationic polymerization mechanism

Cationic polymerization differs from free radical polymerization in that the growing polymer chain carries a positive charge, balanced by a counter-anion.²⁷ This mechanism enables the preparation of QSE matrices under mild conditions without introducing impurities, making it suitable for conventional electrolyte materials. The process begins with the generation of cations from strong acids or Lewis acid initiators (e.g., H⁺, PF₅, AlCl₃, or BF₃). These cations react with the double bonds of monomers or epoxide structures, forming positively charged intermediates. The intermediates propagate the polymerization chain by reacting with additional monomers. Termination occurs when the cation combines with a solvent molecule, impurity, or counter-anion.²⁸

A prominent example is the cationic polymerization of 1,3-dioxolane (DOL), an ether solvent widely used in electrolyte preparation. DOL exhibits a low dielectric constant (ε ≈ 7), good ionic conductivity (≈1.0 mS cm⁻¹), and low interfacial resistance post-polymerization.²⁹ As shown in Fig. 1b, the initiator LiPF₆ decomposes into LiF and PF₅, with PF₅ reacting with trace water to form the strong Lewis acid H⁺(PF₅OH)⁻. This acid initiates the ring-opening polymerization (ROP) of DOL, resulting in a linear long-chain polymer.

Similar to free radical polymerization, monomer ring size has a pronounced effect on the kinetics of cationic polymerization. This is primarily due to the influence of ring strain on the ease of ring-opening. Monomers with smaller rings exhibit higher ring strain, resulting in lower energy barriers for ring-opening and faster polymerization rates. Five-membered rings such as DOL are commonly used in cationic polymerization due to their moderate ring strain and ease of activation by Lewis acids (e.g., AlCl₃, BF₃) or Li salts (e.g., LiPF₆). In contrast, six-membered rings like 1,3-dioxane (DOX) require stronger initiators or elevated temperatures due to reduced ring strain. Nevertheless, polymers derived from larger rings often exhibit improved ionic conduction anisotropy due to their more rigid polymer backbones.

Therefore, small-ring monomers like DOL offer advantages in terms of rapid polymerization and high ionic conductivity but may require optimization to ensure oxidative stability. Conversely, macrocyclic monomers such as DOX are better suited for high-voltage applications but demand tailored polymerization conditions. Future studies may explore the tuning of monomer properties—through fluorination, silanization, copolymerization, or nano-confinement—to further balance ring size effects and optimize battery performance.

The cationic polymerization approach also presents several limitations: (1) it is highly sensitive to moisture and impurities, which may cause premature termination or unwanted side reactions. (2) The oxidative stability of the resulting QSEs is typically poor, making them susceptible to degradation under high-voltage conditions. (3) The ionic conductivity of cationically polymerized QSEs is often lower than that of those produced via free radical polymerization.

To mitigate these issues, the following solutions have been explored: (1) purity control: employing high-purity raw materials can minimize the negative impact of moisture and contaminants. (2) Chemical modification: introducing fluorinated or halogenated groups into the polymer matrix can significantly enhance oxidative stability. (3) Composite design: incorporating high-conductivity additives such as ionic liquids or nano-fillers can improve the overall ionic conductivity of the QSEs.

2.3 Anionic polymerization mechanism

Anionic polymerization is another chain-growth polymerization mechanism, where the growing polymer chain carries a negative charge balanced by counter-cations.³⁰ This method is particularly effective for monomers with electron-withdrawing substituents, which render the double bonds electrophilic. Initiators such as alkali metals, their hydrides, amides, and metal–organic compounds generate nucleophilic species that initiate polymerization. Anionic polymerization is often referred to as “living polymerization” because the propagation reaction continues without termination until all monomers are consumed. This characteristic allows for precise control over polymer architecture, enabling the synthesis of well-defined block copolymers and other complex structures.³¹ Additionally, anionic polymerization produces polymers with narrow molecular weight distributions due to the high stability of the anionic active centers, which do not terminate spontaneously.

For example, ethyl cyanoacrylate (ECA), a highly electrophilic monomer, undergoes anionic polymerization initiated by Li⁺ from Li salt decomposition. As illustrated in Fig. 1c, the generated anions react with downstream monomers, achieving controlled chain growth.^21,32 Polymers containing C=O and –C≡N functional groups exhibit strong hydrogen bonding interactions with the aluminum-plastic film in pouch cells, effectively immobilizing the electrolyte and enhancing leakage prevention, thereby improving battery safety.

In anionic polymerization, the impact of monomer ring size on polymerization kinetics mirrors that observed in cationic systems. Specifically, under comparable conditions, small-ring monomers (e.g., ethylene oxide) exhibit polymerization rates that are one to two orders of magnitude faster than those of larger-ring counterparts (e.g., propylene oxide), due to the higher ring strain that facilitates faster ring-opening reactions.³³

Anionic polymerization is associated with a complex process, stringent reaction conditions, and QSEs with generally low ionic conductivity. To overcome these drawbacks, researchers have proposed the following strategies: (1) conducting polymerization in an inert atmosphere to stabilize reactive species. (2) Designing monomers with functional groups that promote high ionic conductivity and oxidative stability. (3) Enhancing the mechanical and electrochemical properties of the resulting QSEs through heat treatment or chemical crosslinking.

2.4 Electrochemical polymerization mechanism

Electrochemical polymerization, depicted in Fig. 1d, involves the application of an electric field (constant potential or current) to trigger redox reactions of monomers at the electrode surface. This process generates highly reactive intermediates, such as free radicals or cations, which combine with other monomers to form cross-linked networks or linear polymers. Unlike the other polymerization methods, electrochemical polymerization does not require an initiator. Instead, it relies on the oxidation of monomers to form reactive intermediates, which propagate the polymerization chain. Termination occurs when the reactive intermediates are exhausted.³⁴ This method enables the rapid and controllable synthesis of conductive polymers, making it a promising approach for QSE fabrication in LMBs.

Although electrochemical polymerization offers precise control over QSE formation, it suffers from several drawbacks, including non-uniform film formation, high production costs, limited material compatibility, and inferior mechanical strength. These limitations currently hinder its scalability and practical application compared to other polymerization methods.

2.5 Summary and comparison

As discussed in the preceding sections, the polymerization mechanism critically determines the structural and compositional properties of QSEs, thereby governing the performance and safety of QSSLMBs. To provide readers with a clear and systematic overview, we have summarized the key advantages, disadvantages, and potential applications of each polymerization approach in Table 1. This comparative analysis offers an intuitive reference for selecting the most suitable synthesis strategy based on specific battery requirements.

Table 1 Summary of QSEs prepared using different polymerization mechanisms

Mechanisms	Core advantages	Major limitations	Typical application scenario
Free radical	(1) Wide range of monomers (styrene, acrylates, etc.)	(1) Wide molecular weight distribution (dispersity D > 1.5)	Composite QSEs, interfacial modification
	(2) Simple operation, low cost (heat initiation)	(2) Large amounts of initiators (e.g., peroxides, azo compounds) are required and harmful by-products may remain
	(3) The triggering method is more flexible
Cationic	(1) Suitable for low polarity monomers (e.g., ethers, epoxy resins)	(1) Harsh reaction conditions, sensitive to impurities (H2O, O2, etc., terminate the reaction)	High ionic conductivity QSEs (e.g., PDOL)
	(2) High reactivity (can be performed at low temperatures)	(2) High toxicity of initiators (e.g., PF5, BF3)
	(3) Non-terminating “active” features
Anionic	(1) Narrow molecular weight distribution (D ≈ 1.05–1.2)	(1) Strict anaerobic conditions (complex operation)	Wide ESW QSEs (e.g., PAN-based)
	(2) Controllable “activity” (designable block copolymers)	(2) High catalyst cost (e.g., alkyl Li)
Electro-chemical	(1) No chemical initiators required (green)	(1) High requirements for conductive substrate	Interface protection layer, flexible batteries
	(2) High spatiotemporal control (electrode surface-oriented polymerization)	(2) Demanding monomers (need redox active groups)
	(3) Fast film formation (suitable for battery interface modification)	(3) High equipment complexity (requires precise potential control)

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3. Categories of QSEs

The QSE matrices fabricated via the in situ polymerization method exhibit significant diversity, primarily influenced by the choice of monomers, initiators, and reaction conditions. In this section, the various types of QSEs and their corresponding LMBs are systematically categorized and discussed based on the polymerization mechanisms employed for the QSE substrates.

3.1 Free radical polymerization pattern

Free radical polymerization is a pivotal technique for the in situ preparation of QSE matrices, offering a versatile approach to address critical challenges in LMBs, such as Li dendrite growth and interfacial instability. QSEs synthesized via this method exhibit excellent mechanical properties, high ionic conductivity, and robust stability, making them highly suitable for advanced battery applications. The primary monomers used in free radical polymerization include acrylates (e.g., methacrylate, ethylene glycol acrylates, and pentaerythritol acrylates) and carbonates (e.g., vinyl ethylene carbonate (VEC) and vinyl carbonate (VC)). Common initiators for this process include azo compounds, peroxides, and redox systems.³⁵ Free radicals are typically generated through thermal decomposition of initiators, but alternative methods such as UV irradiation, high-energy radiation, electrolysis, and plasma technology are also employed, providing flexibility for diverse application scenarios and process requirements.

3.1.1 Acrylate monomers. Acrylate monomers are particularly suitable for in situ QSE matrix fabrication due to their compatibility with common LEs and ease of initiation.³⁶ Sun et al.³⁷ pioneered the use of acrylic monomers for thermally initiated QSE preparation, achieving a high ionic conductivity of 1.0 mS cm⁻¹ and excellent electrode interface stability. Subsequent studies have expanded the scope of acrylate-based QSEs by designing multifunctional monomers. For instance, a QSE with crowded ion channels was developed using EGDA and 1,2-dimethoxyethane (DME) as raw materials (Fig. 2a).³⁸ This QSE demonstrated remarkable properties, including an ionic conductivity of 3.6 mS cm⁻¹, a Li⁺ transference number of 0.55, and an ESW of up to 4.7 V vs. Li/Li⁺ at room temperature (RT). The corresponding Li|Li symmetric cell exhibited stable cycling for over 3000 h at 0.5 mA cm⁻² without dendrite formation. Furthermore, the Li|LiFePO₄ (LFP) full cell delivered a specific capacity of 159.2 mA h g⁻¹ at 0.1C and 97 mA h g⁻¹ at 2C, retaining 77.4% capacity after 600 cycles at 0.5C (Fig. 2b–d). These results highlight the potential of acrylate-based QSEs for scalable battery production and their role in advancing QSSLMBs.

Fig. 2 In situ preparation of QSE from the EGDA monomer (a) and the electrochemical performance of the corresponding Li|LFP full cells (b)–(d). In situ preparation of QSE from the TPMA monomer with the TEP flame retardant molecule (e), along with the cycling stability of the corresponding Li|LFP (f) and Li|LCO full cells (g).

Functionalized acrylate monomers further enhance the properties of QSEs. For example, trifluoromethyl methacrylate (TFMA), a F-rich monomer, was used to prepare a QSE with a dual-salt system (Li bis(trifluoromethane)sulfonimide (LiTFSI) and Li difluoro-oxalate-boronate (LiDFOB)) (Fig. 2e).³⁹ The F-rich groups facilitated the formation of an inorganic-rich solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI), improving electrochemical performance. Additionally, the incorporation of triethyl phosphonate (TEP) enhanced flame retardancy through polar interactions with TFMA. The resulting Li|Li cell achieved stable cycling for over 1000 h at 0.1 mA cm⁻², while the Li|LFP full cell retained a specific capacity of 110.4 mA h g⁻¹ after 500 cycles at 0.5C (Fig. 2f). Notably, this QSE demonstrated compatibility with high-voltage LiCoO₂ (LCO) cathodes, maintaining a specific capacity of 120 mA h g⁻¹ after 200 cycles at 4.2 V (Fig. 2g).

3.1.2 Carbonate monomers. Carbonate monomers, widely used as solvents and additives in LEs, are also ideal for in situ QSE preparation due to their high oxidative stability. Among these, VEC and VC have garnered significant attention. VEC, with its five-membered ring structure containing C=O and C=C functional groups, exhibits weak coordination with Li⁺ and facile polymerization, making it a safe and stable choice for LMBs.⁴⁰ Similarly, VC, an unsaturated additive, features a (–O(C=O)–O–) structure that strongly interacts with Li⁺, enhancing SEI formation.^41,42

VEC-based QSEs have demonstrated high ionic conductivity (2.1 mS cm⁻¹ at RT), a wide ESW (up to 4.5 V vs. Li/Li⁺), and excellent interfacial stability.⁴³ The Li⁺ transport mechanism in these QSEs involves complex coupling/decoupling interactions between Li⁺ and O atoms in C=O and C–O groups, as well as segmental polymer motions. Consequently, Li|LFP full cells achieved a specific capacity of ∼165 mA h g⁻¹ at RT and ∼104 mA h g⁻¹ at −15 °C at 0.1C.

To further enhance their performance, copolymer matrices have been developed by combining VEC with linear diallyl carbonate (DAC) and N,N′-methylenebis-acrylamide (MBA) (Fig. 3a).⁴⁴ The weakly solvating DAC weakened Li⁺ coordination, while MBA cross-linking improved mechanical strength and restricted anion migration through hydrogen bonding with DFOB⁻. This design, based on an enthalpy–entropy manipulation strategy (Fig. 3b), resulted in a QSE with an ionic conductivity of 0.66 mS cm⁻¹, a Li⁺ transference number of 0.76, and high oxidation stability. The Li|LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) full cell retained 82% capacity after 800 cycles, attributed to the robust cross-linked network that mitigated intergranular cracking (Fig. 3c and d).

Fig. 3 (a) Design principle of QSE based on enthalpy–entropy manipulation. (b) Mechanism of the enthalpy–entropy manipulation strategy. (c) Cycling stability of the Li|NCM811 cell using the in situ prepared QSE. (d) Illustration of cathode degradation suppression by the prepared QSE.

VC-based QSEs, inspired by their role in SEI stabilization, have also been explored.^45,46 These QSEs exhibited an ESW of up to 4.5 V vs. Li/Li⁺ and compatibility with high-voltage LCO cathodes. However, their low ionic conductivity (9.82 × 10⁻⁵ S cm⁻¹ at 50 °C) remains a limitation.

3.1.3 Acrylate and carbonate monomers. Combining the advantages of acrylates and carbonates, copolymer-based QSEs have been developed through in situ polymerization. For example, a double-salt QSE was synthesized using VEC, pentaerythritol tetraacrylate (PETEA), LiTFSI, and Li bis(oxalate)borate within a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) porous skeleton (Fig. 4a and b).⁴⁷ This QSE achieved high ionic conductivity (6.9 mS cm⁻¹ at RT), a Li⁺ transference number of 0.51, and an ESW of 5.3 V vs. Li/Li⁺. The Li|LCO full cell retained 83.4% capacity after 100 cycles with an average CE of 99.86%, attributed to a stable interfacial layer rich in B⁻ and F⁻ components.

Fig. 4 (a) Schematic of the in situ fabrication of VEC- and PETEA-based QSE, and (b) HOMO–LUMO energy levels of Li salts and solvents. (c) Schematic of the in situ fabrication of VC- and EGDA-based QSE, and (d) HOMO–LUMO energy levels of Li salts and solvents.

Another innovative QSE was prepared by incorporating 2,2,3,4,4,4-hexafluorobutyl methacrylate and EGDA into VC-based polymer chains (Fig. 4c and d).⁴⁸ This QSE demonstrated an ionic conductivity of 0.63 mS cm⁻¹, a Li⁺ transference number of 0.82, and an ESW of up to 5.1 V vs. Li/Li⁺. The loose coordination of polymer chains and efficient ion transport through oligomers facilitated rapid Li⁺ migration, ensuring compatibility with both LMA and high-voltage cathodes.

Despite its widespread use, free radical polymerization faces several challenges. First, the monomer conversion rate is relatively low compared to other polymerization methods, such as cationic ROP. The observed differences in monomer conversion efficiency arise primarily from the inherent distinctions between free radical polymerization and ionic ROP, particularly in their propagation and termination mechanisms. Free radical polymerization proceeds via a three-step process: chain initiation → chain propagation → chain termination. Due to the high reactivity of free radicals, termination reactions (such as radical coupling or disproportionation) readily occur, often leading to premature chain termination and thus lower monomer conversion. For instance, Kwok et al. reported that when EGDA was polymerized using AIBN as the initiator, the monomer conversion reached only about 70% in the absence of inhibitors. Moreover, residual free radicals were found to cause interfacial side reaction.⁴⁹ In contrast, ionic ROP involves the nucleophilic or electrophilic opening of cyclic ether rings by an initiator, resulting in the formation of a stable active center that supports continuous chain growth without an inherent termination step (unless a chain transfer agent is present). This stability typically leads to higher monomer conversion rates. For example, in the case of LiPF₆-initiated polymerization of DOL, a monomer conversion of 85.2% was achieved, and the resultant linear polymer exhibited a dense structure with ESW up to 4.6 V vs. Li/Li⁺.⁵⁰ Similarly, Li et al. demonstrated that ROP-derived copolymers achieved approximately 90% monomer conversion, compared to 50–80% for those produced via free radical polymerization under similar conditions.⁵¹

Initiator selection also significantly influences the polymerization efficiency in free radical systems. The two primary classes of initiators are azo compounds and peroxides. Azo compounds, such as AIBN, typically decompose at 40–60 °C, while peroxides require higher temperatures (>80 °C) to generate radicals, which may not be compatible with the thermal constraints of LMB environments. Consequently, peroxide-initiated systems often exhibit slower reaction rates and lower monomer conversions. For instance, the AIBN-initiated polymerization of EGDA in an ethylene carbonate (EC)/DMC solvent system yielded monomer conversion rates of approximately 45–55%.⁵² Meanwhile, benzoyl peroxide-initiated DOL polymerization in liquid electrolyte systems resulted in lower conversions of 30–40.⁵³

Furthermore, in LMB systems, conventional initiators may exhibit reduced activity due to potential side reactions with electrolyte components or electrode materials. This challenge underscores the need for continued innovation. Future research could explore co-initiator strategies, polymer architecture design, and the use of tailored catalysts or additives to enhance radical generation and promote higher conversion rates in free radical polymerization processes.

Second, the introduction of non-electrolyte components (e.g., initiators) may adversely affect QSE performance, including potential reactions with LMA. Third, the requirement for high-temperature thermal initiation raises concerns about heat dissipation, gas formation, and Li salt solubility in large-capacity pouch cells. Addressing these issues through further research and optimization is essential for advancing QSSLMB technology.

3.2 Cationic polymerization pattern

Cationic polymerization is a highly efficient strategy for the in situ synthesis of QSE matrices. This method typically relies on protonic acids or Lewis acids as initiators, targeting O atoms in O-containing heterocyclic rings (e.g., cyclic ethers, cyclic carbonates) or N-containing heterocyclic monomers. These monomers are susceptible to cationic attack, enabling ROP.^54,55 This approach is particularly suitable for epoxy alkane electrolyte materials, as it can initiate polymerization at double bonds or ring structures. A key advantage of cationic polymerization is its rapid chain growth; however, the high reactivity of carbocations can lead to side reactions or reduced product stability, necessitating careful control of the polymerization process. Among the monomers used in cationic polymerization, cyclic ethers have garnered significant attention due to their excellent compatibility with LMA, low viscosity, and high ionic conductivity. Representative cyclic ether monomers include DOL,⁵⁶ DOX,⁵⁷ and 1,3,5-trioxane (TXE).⁵⁸ This section also explores the in situ polymerization of DOL in the presence of mechanical supports to enhance QSE performance.

3.2.1 Cyclic ether monomers.
DOL monomer. DOL, a five-membered cyclic ether, is widely used as an electrolyte solvent due to its low viscosity and high ionic conductivity. However, its low ESW (4 V vs. Li/Li⁺) limits its application. Cationic ROP of DOL offers an effective strategy to improve both the electrochemical stability of the electrolyte and the cycling performance of corresponding LMBs. This polymerization can be initiated by positively charged intermediates, such as LiPF₆, Al(OTf)₃, and BF₃.

Early studies explored the ROP of DOL,^59,60 but recent advancements have focused on LiPF₆ as an initiator due to its dual role as an electrolyte component, eliminating the need for additional additives.²⁹ For example, a novel QSE was synthesized by in situ ROP of DOL initiated by LiPF₆ in the presence of DME at ambient temperature (Fig. 5a and b). During the reaction, LiPF₆ decomposes into LiF and PF₅, with PF₅ reacting with trace water to form the strong Lewis acid H⁺(PF₅OH)⁻, which triggers DOL polymerization into a linear long-chain polymer. The resulting QSE exhibited an ionic conductivity of 3.8 mS cm⁻¹ and excellent compatibility with LMA. The Li|Li symmetric cell demonstrated stable cycling for over 800 h at 0.5 mA cm⁻² with minimal overpotential. Additionally, the QSE was compatible with various cathodes, including S, LFP, and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622), delivering excellent cycling performance (1000, 700, and 100 cycles for Li|S, Li|LFP, and Li|NCM622 cells, respectively). This simple, efficient, and impurity-free strategy represents a promising direction for advanced QSE systems in LMBs.

Fig. 5 (a) Schematic of the LiPF₆-induced polymerization mechanism of DOL. (b) Optical images of the LE and the prepared QSE. (c) Schematic comparison of ex situ and in situ QSE synthesis. (d) Reaction mechanism illustrating the Al(OTf)₃-initiated polymerization of DOL. Inset: Digital photograph of the liquid DOL electrolyte (left) and the prepared QSE (right).

To further enhance DOL-derived QSEs, plasticizers such as EC,⁶¹ fluoroethylene carbonate (FEC),⁶² and methyl ethyl carbonate (MEC)⁶³ have been incorporated. These carbonate plasticizers improve thermal stability, dielectric properties, and reduce volatility. For instance, Archer et al.⁶¹ developed a QSE by blending DOL with EC, achieving a solid-like QSE even at low DOL concentrations (<40%). The resulting QSE exhibited superior electrochemical performance and oxidative stability, enabling compatibility with high-voltage NCM622 cathodes. Similarly, a QSE prepared from DOL, FEC, and hexamethylene diisocyanate (HDI) demonstrated enhanced compatibility with LCO cathodes, attributed to the synergistic effects of FEC and HDI in forming stable interfacial layers.⁶²

Al(OTf)₃ has also been employed as an initiator for DOL polymerization (Fig. 5c and d).⁶⁴ The Al-based cation attaches to the O atom in DOL, initiating ROP. The resulting QSE exhibited moderate ionic conductivity (>1 mS cm⁻¹ at RT) and excellent interfacial compatibility. The Li|Cu cell achieved a Li plating/stripping efficiency of >98% over 300 cycles, while full cells with S, LFP, and NCM622 cathodes demonstrated stable cycling for over 700 cycles with >99% CE. This work highlights the potential of cationic polymerization to meet both bulk and interfacial conductivity requirements for practical QSSLMBs. Despite their advantages, DOL-derived QSEs suffer from poor oxidative stability, limiting their compatibility with high-voltage cathodes.

DOX monomer. To address the oxidative stability limitations of DOL, DOX, a six-membered cyclic ether, has been explored as a monomer for QSE synthesis.⁵⁷ Like DOL, DOX undergoes cationic ROP initiated by Lewis acids such as Al(OTf)₃ (Fig. 6a). The lower highest occupied molecular orbital (HOMO) energy level of DOX-based polymers compared to DOL-based polymers (Fig. 6b) results in a wider ESW (up to 4.7 V vs. Li/Li⁺) (Fig. 6c). Additionally, DOX-derived QSEs facilitate the formation of robust, inorganic-rich SEI layers, enhancing Li metal compatibility. The Li|Li symmetric cell exhibited a high Li⁺ transference number (0.75) and stable cycling (over 1300 h). The DOX-derived QSE also demonstrated excellent compatibility with high-voltage LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) cathodes, retaining 84% capacity after 100 cycles at 1C and 4.5 V vs. Li/Li⁺ (Fig. 6d). This study underscores the importance of monomer molecular design in improving QSE performance for high-energy-density QSSLMBs.

Fig. 6 (a) Polymerization mechanism of DOX, (b) HOMO energy levels, and (c) ESW of DOX-derived QSE, along with (d) cycling performance of the Li|NCM111 full cell using DOX-derived QSE. (e) Polymerization mechanism of TXE, (f) voltage profiles, and (g) cycling performance of the Li|LCO full cell using TXE-derived QSE.

TXE monomer. TXE has also been investigated as a monomer for QSE synthesis. For example, a eutectic QSE was prepared by ROP of TXE initiated by LiDFOB in the presence of succinonitrile (SN) (Fig. 6e).⁵⁸ The interaction between the O atoms in TXE and the cyano groups in SN formed a deep eutectic solution, which polymerized at 80 °C to yield a QSE with an ionic conductivity of 0.114 mS cm⁻¹ and an ESW of 4.5 V vs. Li/Li⁺. The QSE formed protective layers on both the LCO cathode and LMA, enabling the 4.3 V Li|LCO cell to retain 88% capacity after 200 cycles (Fig. 6f and g).

3.2.2 DOL monomer with supports. The spontaneous polymerization of DOL upon mixing with Lewis acid initiators can lead to uneven QSE distribution, negatively impacting electrochemical performance and scalability. To address this, functionalized supports, such as porous polymer films, have been employed. For instance, a cellulose paper support functionalized with alumina and Al(OTf)₃ was used to enhance DOL-derived QSEs.⁶⁵ The alumina improved mechanical strength, while Al(OTf)₃ initiated DOL polymerization. The resulting QSE exhibited a wide ESW (4.4 V vs. Li/Li⁺), a high Li⁺ transference number (0.72), and excellent cycling stability, retaining 83.5% capacity after 1000 cycles at 2C in Li|LFP cells.

Similarly, a heat-resistant porous polyimide nanofiber (PI NF) film was developed via sol–gel electrospinning and heat treatment (Fig. 7a–d).⁶⁶ The PI NF-supported QSE, prepared by LiPF₆-initiated DOL polymerization, demonstrated high ionic conductivity (2.9 mS cm⁻¹), a Li⁺ transference number of 0.61, and an ESW of 4.8 V vs. Li/Li⁺. The Li|LFP full cell achieved a specific capacity of 151.8 mA h g⁻¹ at 0.5C, retaining 91.8% capacity after 200 cycles. This QSE is also suitable for large-capacity pouch cells, highlighting its practical potential.

Fig. 7 (a) In situ polymerization of DOL within a porous PI NF film at RT to fabricate nanofibrous QSE films. (b)–(d) Surface morphology and digital photographs of the PAA NF film, PI NF film, and nanofibrous QSE film. (e) In situ polymerization of DOL within a 3D PDA/PVDF-HFP support to prepare QSE, with SEM images of the support film and the prepared QSE film. (f) and (g) Surface morphology and (h) cross-sectional view. Inset: Optical image of the prepared QSE film.

Another innovative approach involved modifying PVDF-HFP with polydopamine (PDA) to create a composite nanofibrous skeleton (Fig. 7e–h).⁶⁷ The PDA-enhanced QSE exhibited improved mechanical strength, ionic conductivity, and Li⁺ transference number due to hydrogen bonding with the polymer matrix and TFSI anions. The Li|LFP full cell demonstrated stable cycling for over 800 cycles at 2C, with 83.2% capacity retention, effectively suppressing Li dendrite growth. This strategy offers a promising pathway for high-performance, safe QSSLMBs compatible with existing battery production processes.

Cationic ROP offers several advantages, including rapid reaction rates, high molecular weight polymers, and compatibility with epoxy monomers. However, challenges such as sensitivity to moisture and impurities, difficulty in termination, and suboptimal electrochemical stability remain. Ether-based QSEs, in particular, exhibit low oxidative stability, limiting their compatibility with high-voltage cathodes and the energy density of QSSLMBs. Future research should focus on enhancing the oxidative stability of cyclic ether-based QSEs through solvent/salt ratio optimization, cross-linking copolymerization, plasticizer incorporation, and high-voltage additives. These advancements will be critical for developing QSEs capable of meeting the demands of next-generation high-energy-density batteries.

3.3 Anionic polymerization pattern

Anionic polymerization is another effective strategy for the in situ synthesis of QSE matrices. This method is initiated by nucleophilic reagents, such as alkali metals, their hydrides, amides, and metal–organic compounds.^28,30 Often referred to as “living polymerization,” anionic polymerization can proceed without termination under appropriate conditions, enabling precise control over polymer architecture. This characteristic allows for the synthesis of well-defined copolymers, such as block copolymers, by sequentially adding different monomers until complete conversion to macromolecules is achieved. Additionally, anionic polymerization produces polymers with narrow molecular weight distributions and high reproducibility, as the active anionic centers do not terminate spontaneously. However, the process is highly sensitive to moisture and O₂, requiring stringent reaction conditions to ensure reproducibility. While QSEs prepared via anionic polymerization are less extensively studied compared to other methods, they have shown promise, particularly with monomers containing cyano and acrylate groups, such as ECA. Furthermore, cyclic ethers (e.g., DOL) and carbonates can also be polymerized through anionic ring-opening strategies.

3.3.1 Cyanoacrylate monomers. Cyanoacrylate monomers, such as ECA, are ideal candidates for anionic polymerization due to the presence of strong electron-withdrawing cyano groups (C≡N) attached to the C=C bond. The cyano group's low-energy lowest unoccupied molecular orbital (LUMO) enhances thermal stability, while its strong interaction with transition metal ions prevents their diffusion from the cathode to the anode, mitigating capacity loss—a critical advantage for high-voltage cathodes (>5 V vs. Li/Li⁺).²¹ These properties make cyanoacrylate-based polymers highly suitable for QSEs paired with high-voltage cathodes, offering both performance and safety benefits.

For example, the ECA monomer has been used to in situ prepare QSEs through anionic polymerization (Fig. 8a).⁶⁸ The resulting QSE, composed of an ECA polymer matrix and LiClO₄/carbonate electrolyte, achieved an ionic conductivity of 2.7 mS cm⁻¹ at RT and an ESW of up to 4.8 V vs. Li/Li⁺. When paired with LFP and LiNi_1.5Mn_0.5O₄ cathodes, the QSE demonstrated stable charge–discharge cycling and excellent rate capability (Fig. 8b and c). Additionally, a flexible thin-film Li|LFP cell incorporating this QSE successfully powered an LED and delivered a specific capacity of ∼130 mA h g⁻¹ after 20 cycles with high CE (Fig. 8d). Beyond QSEs, ECA polymer nanocoatings have also been employed to protect high-voltage LiNi_0.5Mn_1.5O₄ cathodes⁶⁹ and LMA,⁷⁰ further highlighting their versatility.

Fig. 8 (a) Anionic polymerization mechanism of ECA. Cycling performance of the assembled full cells with (b) LFP and (c) LiNi_1.5Mn_0.5O₄ cathodes. (d) Schematic illustration and cycling performance of an assembled flexible cell. (e) Interaction mechanism between HFiP and DOL. Cycling performance of assembled (f) Li|Li and (g) Li|S cells.

3.3.2 DOL monomer. DOL, a widely studied cyclic ether, can also undergo anionic ROP. In the second stage of electrophilic-initiated DOL polymerization, the process exhibits nucleophilic characteristics: an O atom on the free DOL ring, carrying a lone pair of electrons, attacks the carbon atoms in the –O–C–O– portion of the DOL ring already bonded to the initiator, facilitating chain propagation.⁶⁰ In the presence of Li⁺, certain initiators, such as tris(1,1,1,3,3,3-hexafluoroisopropanol) (HFiP), can nucleophilically initiate DOL polymerization during the first stage.⁷¹ The P–O and C–O bonds in HFiP readily break in solution, generating charged groups that attack the O atoms in the DOL ring, triggering ROP (Fig. 8e).⁷² The resulting QSE exhibited an ionic conductivity of 1.6 mS cm⁻¹ at RT, a high Li⁺ transference number (>0.6), and an ESW of up to 4.5 V vs. Li/Li⁺. The Li|Li symmetric cell demonstrated stable Li plating/stripping for 800 h at 1.65 mA cm⁻² (Fig. 8f), while the Li–S battery achieved over 500 cycles with a CE exceeding 99.2% (Fig. 8g).

3.3.3 Carbonate monomers. Carbonate monomers can also be polymerized via anionic ring-opening reactions to form QSEs. For instance, a polymer matrix with a polycarbonate main chain and poly(ethylene oxide) side chains was synthesized by in situ ROP of functionalized cyclic carbonates, initiated by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/thiourea (TU).⁷³ The resulting QSE, incorporating LiTFSI, achieved an ionic conductivity of 2 × 10⁻⁵ S cm⁻¹ at 30 °C and a Li⁺ transference number of 0.67. Both the main chain and side chains facilitated Li⁺ transport, enabling the Li|LFP cell to deliver a specific capacity of 141 mA h g⁻¹ and retain 89% capacity after 100 cycles at 0.5C and 60 °C.

Anionic polymerization has significant potential for QSSLMBs, not only for in situ QSE preparation but also for fabricating protective polymer layers for LMA and cathode materials. By precisely controlling polymerization conditions, polymers with excellent ionic conductivity, mechanical strength, and electrochemical stability can be synthesized. Future research should focus on developing new functional monomers with enhanced properties, such as high electrochemical stability and high-temperature resistance, to further improve the performance of QSEs in QSSLMBs. Additionally, addressing the sensitivity of anionic polymerization to moisture and O₂ will be critical for scaling up production and ensuring reproducibility. These advancements will pave the way for next-generation high-performance and safe energy storage systems.

3.4 Electrochemical polymerization pattern

Electrochemical polymerization is an effective technique for the in situ construction of QSE matrices. This method can be initiated by an electrochemical reaction without the need for additional thermal or chemical initiators, offering advantages such as simplified processing, reduced solvent consumption, and enhanced material properties.^74,75 However, electropolymerization faces challenges including the formation of high-molecular-weight polymers and thick films due to the limited diffusion of monomers to the substrate electrode surface. Additionally, if the resulting polymer layer exhibits low electronic conductivity, the rate and continuity of subsequent polymerization reactions may be hindered.

A well-studied monomer, DOL, can undergo ring-opening chain growth through electrochemical polymerization (Fig. 9a).⁷⁶ It had been observed that after several cycles in a Li|LCO cell using a DOL-based electrolyte at a relatively high current density (100 mA g⁻¹), the LE (1 M LiTFSI in DOL/DME with a 2 : 1 weight ratio) undergoes irreversible polymerization, resulting in improved cycling stability. Cyclic voltammetry (CV) tests confirm the irreversibility of this polymerization process, which does not negatively impact the electrochemical performance of LCO. The proposed mechanism involves an electronic attack on the O atom of the DOL ring, leading to C–O bond cleavage and the formation of a negatively charged O species along with a –CH₂ radical. This radical then interacts with another DOL monomer, triggering successive polymerization reactions.

Fig. 9 Electrochemical polymerization process of DOL (a) and DPGDME (b) monomers.

Another ether-based monomer, dipropylene glycol dimethyl ether (DPGDME), has also been explored for in situ QSE fabrication via electrochemical polymerization (Fig. 9b).³⁴ The polymerization of DPGDME was confirmed through cycling tests in a Li|Li cell, where the initial voltage–capacity curve exhibited a characteristic crescent shape but gradually evolved into a profile resembling that of other polymer electrolytes. The proposed polymerization mechanism involves Li⁺ attacking the DPGDME molecule, leading to the cleavage of a single C–O bond. The resulting DPGDME radicals then attack the hypomethyl group, where the active α-H is abstracted, leading to the formation of di-DPGDME. Concurrently, the dimer loses a –CH₃ group, ultimately forming the DPGDME polymer. As a result, the QSE-based Li|Li cell demonstrated exceptional stability for over 3000 h, as well as robust performance under high-temperature conditions (50 °C) and high areal capacities (10 mA h cm⁻²). Furthermore, the assembled Li|SPAN cell achieved high sulfur utilization (98.4%) and rapid charging capability (10C). Notably, this system exhibited remarkable cycling stability, with 99.5% capacity retention over 400 cycles and an average CE exceeding 99.9995%, indicating negligible irreversible Li consumption.

Despite its potential, electrochemical polymerization faces challenges such as the insufficient ionic conductivity of the resulting polymers and the complexity of the underlying chemical reactions, making precise control difficult. Future research directions should focus on: (1) developing novel functional monomers to enhance ionic conductivity and stability, and (2) optimizing polymerization conditions to achieve efficient and uniform film formation.

4. Characters of QSSLMBs

QSE represents an intermediate state between LE and ASE, combining the advantages of both. It exhibits relatively high ionic conductivity, low liquid content, and good mechanical strength. These properties enable QSSLMBs to achieve high energy density while maintaining enhanced safety. However, to broaden their potential applications, certain aspects of QSSLMBs, such as safety performance and cell performance under extreme conditions (e.g., high voltage and low temperature), still require further improvement.

Compared to traditional LMBs that use highly flammable LEs, QSE-based QSSLMBs significantly reduce the risk of severe accidents, such as fires or explosions, in cases of thermal runaway, short circuits, or external impacts. Nevertheless, since QSEs still contain trace amounts of flammable solvents, these safety risks cannot be entirely eliminated. Therefore, further enhancing the safety of QSSLMBs remains a critical area of research. Additionally, to increase the energy density of QSSLMBs, cathode active materials with high voltage plateaus are desirable. However, these materials are often incompatible with organic QSEs. Furthermore, the ionic conductivity of organic QSEs decreases significantly at low temperatures due to the freezing of their liquid components. As a result, modifying organic QSEs is essential to improve cell performance under extreme conditions, including high voltage and low temperature environments. The mechanical properties of in situ polymerized QSEs, along with the energy density and safety performance of the corresponding QSSLMBs, are summarized in Table 2. Detailed characterizations and discussions are presented in the following subsections.

Table 2 The mechanical properties of in situ polymerized QSEs, along with the energy density and safety performance of the corresponding QSSLMBs

QSEs	Monomer	Polymerization method	Mechanical strength	Pouch cell	Safety thresholds	Ref.
F-GPE	PEGDA	Free radical	—	1.35 A h	−40 to 60 °C	25
					ESW 4.7 V vs. Li/Li^+
DOL–EC	DOL/EC	Cationic	15.31 MPa	1.0 A h	ESW 5.0 V vs. Li/Li^+	29
VEC–DAC–MBA	VEC/DAC/MBA	Free radical	1.0 MPa	—	ESW 4.5 V vs. Li/Li^+	44
PFVS	VC/PEGDA	Free radical	—	2.6 A h	ESW 5.1 V vs. Li/Li^+	77
SGPE	TFMA/TEP	Cationic	Good mechanical strength and flexibility	—	0–80 °C	39
					ESW 5.1 V vs. Li/Li^+
P-ECA	ECA	Anionic	Good flexibility and strength	—	ESW 4.8 V vs. Li/Li^+	68
P-DPGDME	DPGDME	Electrochemical	—	—	Cycling at 50 °C for 400 cycles	34
FRSE	VC/TEP	Cationic	12.4 GPa	2.4 A h	Stable operation at 50 °C passed the pinprick test	78
FGPE	HFBA/PETEA	Free radical	—	402.5 W h kg^−1	High temperature	79
					Self-extinguishing
					ESW 5.0 V vs. Li/Li^+

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4.1 Flame-retardant properties

In LIBs, thermal runaway typically progresses through three critical stages: heat generation, heat accumulation, and thermal runaway. However, LMBs face even more severe thermal runaway challenges due to the exceptionally high chemical reactivity of LMA with LEs.⁸⁰ Studies have demonstrated that electrolytes play a pivotal role in the thermal runaway process. Therefore, developing electrolytes with high thermal stability and flame retardancy is essential for enhancing battery safety. QSEs have shown significant potential in improving the safety performance of LMBs, primarily due to their excellent leakage prevention properties and enhanced interfacial compatibility with electrode materials through the incorporation of plasticizers. Nevertheless, the flammability of residual LE components in QSEs remains a safety concern that requires further attention. To address this issue, researchers are exploring strategies such as incorporating highly effective flame retardants into electrolytes or chemically grafting flame-retardant functional groups onto the polymer backbone to create molecular-level fire barriers.

4.1.1 Incorporation of flame retardants. Organic phosphate esters,^78,81 fluorinated solvents,⁸² and nitrile solvents⁸³ have been widely used as flame retardants in traditional LMBs and can also be applied to QSSLMBs. In LMBs, organic phosphate esters exhibit excellent flame-retardant properties but often show poor compatibility with LMA. This incompatibility arises from their tendency to undergo reduction reactions on the LMA surface, disrupting the formation of a stable SEI layer and negatively impacting battery cycling performance.⁸⁴ In contrast, the detrimental effects of organic phosphate esters can be mitigated in QSE-based LMAs, as these additives can be embedded within the polymer matrix.

For instance, a non-flammable QSE was developed by encapsulating the flame-retardant liquid TEP into an in situ solidified polycarbonate (VC polymer) matrix (Fig. 10a–c).⁷⁸ This optimized QSE demonstrated a high ionic conductivity of 4.4 mS cm⁻¹ (Fig. 10d), a large Li⁺ transference number of 0.76, a Young's modulus of 12.4 GPa, and a wide ESW of 4.9 V vs. Li/Li⁺. These properties enabled the QSE to effectively suppress Li dendrite growth and undesirable side reactions at the LMA while maintaining compatibility with high-voltage cathodes. The assembled Li|NCM811 prototype cell achieved 87.7% capacity retention after 200 cycles (Fig. 10e), and an A h-level pouch cell exhibited impressive safety by passing the nail penetration test. This study provides valuable insights into the design of non-flammable QSEs for the practical realization of safe and stable QSSLMBs (Fig. 10f).

Fig. 10 (a) Schematic of the in situ polymerization of VC-based QSE. (b) Advantages of the tailor-made QSE. (c) Polymerization mechanism of VC. (d) Ionic conductivity of the prepared QSE. (e) Cycling performance of the assembled Li|NCM811 full cell. (f) Radar chart comparing the prepared QSE with other electrolytes.

4.1.2 Grafting of flame-retardant functional groups. In addition to physically mixing flame retardants into QSEs, chemically grafting flame-retardant groups (typically containing P, F, N, or Si) onto the polymer backbone of QSEs represents another effective approach to enhance flame-retardant performance. Compared to physical mixing, chemical grafting offers several advantages: (1) the grafted flame retardants exhibit better compatibility with the polymer matrix and more uniform distribution, reducing the required flame-retardant content while enhancing mechanical strength and minimizing adverse effects on electrochemical performance. (2) The grafted flame retardants have limited contact with the LMA, thereby reducing their decomposition and improving electrochemical stability.

To address the drawbacks of physically mixed flame retardants, Cao and Yang et al.⁸⁵ developed a P-containing flame-retardant QSE through the in situ polymerization of methyl methacrylate (MMA), allyl diethylene glycol carbonate (ADC), and the flame-retardant monomer diethyl vinyl phosphate (DEVP) (Fig. 11a). The QSE with chemically grafted flame retardants exhibited more durable non-flammability compared to QSEs with physically mixed flame retardants (Fig. 11b). Additionally, the grafted QSE demonstrated a high ionic conductivity of 0.18 mS cm⁻¹ at RT, a wide ESW of 4.4 V vs. Li/Li⁺, excellent thermal stability (up to 190 °C), and superior compatibility with LMAs. As a result, the assembled Li|LFP cell delivered remarkable cycling stability, retaining 70.9% of its capacity after 1000 cycles at 25 °C and 69.1% after 200 cycles at 80 °C.

Fig. 11 (a) Schematic comparison and (b) burning tests of QSEs with flame retardants introduced via addition and grafting. (c) Schematic illustration and optical images of the in situ prepared QSE. (d) Combustion tests comparing the LE and the prepared QSE.

Another innovative approach involved the fabrication of a F-containing flame-retardant QSE through the in situ radical polymerization of hexafluorobutyl acrylate (HFBA) with a PETEA crosslinker (Fig. 11c).⁸⁶ This method integrates fluorinated solvents into the polymer backbone of the QSE, eliminating the need for additional flame retardants. The HFBA-based QSE leverages the gas-phase radical scavenging effect of F radicals, which trap hydrogen radicals generated during combustion, thereby interrupting the chain reaction and extinguishing flames. In ignition tests, this QSE demonstrated exceptional non-flammability (Fig. 11d). When overcharged to 4.5 V, pouch cells with LEs experienced severe thermal runaway, while those with the QSE showed only a slight temperature increase. Furthermore, the QSE-based pouch cells exhibited minimal temperature changes during overheating and open flame exposure tests, highlighting their superior safety performance under abusive conditions. These results underscore the potential of this innovative QSE for real-world applications, particularly in electric vehicles, power tools, and energy storage systems.

The incorporation of flame retardants into QSEs significantly enhances the safety of QSSLMBs, reducing the risks of fire or explosion during overcharging, over-discharging, thermal runaway, or external impacts. While physical mixing of flame retardants offers a straightforward approach, chemical grafting is more advantageous and represents the future direction for QSE development, despite its complexity and higher cost. Additionally, flame retardants must be precisely designed to minimize adverse effects on battery performance. As safety requirements for batteries continue to rise, the development of non-flammable QSSLMBs will play a critical role in meeting global safety standards and environmental regulations, particularly in high-demand applications such as electric vehicles and grid-scale energy storage systems.

4.2 High-voltage compatibility

To maximize the energy density of QSSLMBs, cathode active materials with high voltage plateaus are essential. However, high-voltage QSSLMBs face challenges due to the instability of QSEs. The organic components of QSEs are prone to oxidative decomposition under high voltage, leading to poor electrochemical stability and safety concerns. To ensure compatibility with commercial high-voltage cathode materials, QSEs must exhibit an ESW exceeding 4.3 V (vs. Li/Li⁺). Therefore, developing QSEs with high-voltage stability is crucial. This can be achieved through chemical modifications of the polymer matrix, such as copolymerization, cross-linking, and halogenation, or by incorporating functional fillers, including Li⁺-insulating and Li⁺-conductive materials.

4.2.1 Modification of the polymer matrix. Chemical modification of the QSE polymer matrix, including halogenation, copolymerization, cross-linking, and their combinations, can alter the main chain, side chain, and end-group structures of the matrix. These modifications regulate the energy levels and kinetic properties at the molecular level, improving the compatibility between QSEs and high-voltage cathode materials.

Fluorination of the polymer matrix is a widely used method to enhance oxidation resistance. The introduction of F atoms reduces the regularity of the polymer structure and lowers the energy levels due to F's high electronegativity.⁸⁷ For example, a novel QSE with a fluorinated polymer matrix was developed using PETEA and hexafluoroacetone (HFA) monomers (Fig. 12a).⁷⁹ The HFA monomer exhibits a high HOMO energy level (−7.0241 eV), enabling it to participate in the formation of a CEI layer, significantly enhancing the QSE's compatibility with high-voltage cathodes (Fig. 12b). Additionally, the strong interaction between the F-containing structure in the QSE matrix and the PF₆⁻ anion suppresses anodic currents, resulting in a wide ESW of 5.0 V (vs. Li/Li⁺) (Fig. 12c). The assembled Li|NCM811 full cell with this fluorinated QSE delivered a specific capacity of 224 mA h g⁻¹ at 0.1C (Fig. 12d) and retained 58.6% of its capacity after 200 cycles at 1C (Fig. 12e). The fluorinated QSE not only benefits the cathode side but also facilitates the formation of a LiF-rich interfacial layer on the LMA, suppressing side reactions and Li dendrite growth due to the low LUMO energy level (−2.8938 eV) of the HFA segment.

Fig. 12 (a) Schematic diagram of the fluorinated QSE. (b) LUMO and HOMO energy levels of the employed monomers. (c) ESW of the fluorinated QSE. Voltage profiles (d) and cycling performance (e) of the Li|NCM811 cell with fluorinated QSE. (f) Schematic diagram of the cross-linked QSE. (g) ESW of the cross-linked QSE. Cycling performance of Li|NCM622 cells with cut-off voltages of (h) 4.3 V and (i) 4.7 V.

Copolymerization and cross-linking are also effective strategies to enhance the high-voltage stability of QSEs. Monomers containing polar groups are particularly suitable for copolymerization and cross-linking, as polar groups readily form covalent bonds. For instance, a QSE with a fluorinated and cross-linked polymer matrix was prepared via the in situ polymerization of 3,3,3-trifluoropropene oxide (TFPO) and pentaerythritol glycidyl ether (PEE) monomers (Fig. 12f).⁸⁸ This QSE exhibited a high oxidation potential of 5.1 V (vs. Li/Li⁺) (Fig. 12g), attributed to the fluorinated and cross-linked polyether matrix. The QSSLMB with an NCM622 cathode maintained 78% capacity retention after 1000 cycles at 0.5C and a cutoff voltage of 4.3 V (Fig. 12h). Even at an ultrahigh cutoff voltage of 4.7 V, the cell achieved 74.2% capacity retention after 100 cycles at 0.5C (Fig. 12i). The electron-withdrawing –CF₃ group in the QSE matrix contributed to the formation of a LiF-rich SEI/CEI layer, enhancing compatibility with the LMA.

4.2.2 Composite QSEs with functional fillers. Similar to solid polymer electrolytes, QSEs can be enhanced by incorporating functional fillers, which can be either Li⁺-insulating (e.g., Al₂O₃, SiO₂, ZrO₂, BaTiO₃, CeO₂) or Li⁺-conductive (e.g., Li₇La₃Zr₂O₁₂ (LLZO), Li_0.33La_0.56TiO₃ (LLTO), Li_6.28La₃Al_0.24Zr₂O₁₂ (LLAZO) Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO), and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP)).⁸⁹ These fillers improve QSE performance in two ways: (1) they form hydrogen bonds with the polymer matrix or Li salts, promoting Li⁺ dissociation and mobility, thereby reducing interfacial impedance. (2) They enhance the polymer matrix's ability to adsorb anions, reducing oxidative decomposition at high voltages.^90,91

For example, yttria-stabilized zirconia (YSZ), a Li⁺-insulating filler, was introduced into a QSE prepared via the in situ polymerization of DOL (Fig. 13a).⁹² The Lewis acidic properties of YSZ (Zr⁴⁺, Y³⁺, and O vacancies) increased the polymerization efficiency of DOL to 98.5%, minimized by-product formation, and enhanced high-voltage stability, resulting in an ESW exceeding 4.9 V (vs. Li/Li⁺). The QSE-based Li|NCM622 cell exhibited stable cycling for over 800 cycles, with the formation of a Li₂ZrO₃-rich SEI layer that promoted uniform Li deposition and suppressed dendrite growth.

Fig. 13 (a) Schematic of DOL-derived QSE with YSZ filler. In situ preparation of PEGDA-based QSEs with LLAZO filler (b) and LLZTO filler (c).

Li⁺-conductive fillers, such as garnet-type materials, are particularly advantageous due to their high ionic conductivity, wide ESWs (>5 V vs. Li/Li⁺), and compatibility with LMAs.¹¹ For instance, a QSE was prepared by in situ polymerization of EGDA with one-dimensional LLAZO nanofibers (Fig. 13b).⁹³ The LLAZO nanofibers were chemically functionalized with silane and covalently grafted to the polymer matrix, preventing filler aggregation and enhancing interfacial compatibility. The strong Lewis acid–base interactions between LLAZO and Li salts facilitated Li⁺ migration, reducing interfacial impedance and forming a stable interfacial layer. This QSE achieved an impressive ESW of 5.3 V (vs. Li/Li⁺) and enabled the Li|NCM111 cell to deliver an excellent rate performance up to 10C.

Another QSE, fabricated via in situ polymerization of EGDA with asymmetric LLZTO fillers (Fig. 13c),⁹⁴ demonstrated enhanced high-voltage stability. The LLZTO fillers, enriched on the QSE surface, promoted uniform Li deposition, inhibited dendrite formation, and improved the ESW to 5.13 V (vs. Li/Li⁺) due to the filler's high oxidative stability.

QSEs are inherently unstable under high-voltage conditions due to the high HOMO energy levels of the polymer matrix (susceptible to reactive O species) and various reactive functional groups (susceptible to free electrons). High-voltage stability can be improved through chemical modifications of the polymer matrix to enhance oxidation resistance or by incorporating high-voltage-resistant inorganic fillers. While chemical modification offers superior performance, it is more complex and costly. In contrast, filler incorporation is simpler and more cost-effective, though less impactful. Therefore, the ideal approach may involve combining both strategies to develop QSEs with excellent high-voltage stability, paving the way for their application in high-energy-density batteries for electric vehicles, power tools, and grid-scale energy storage systems.

4.3 Low-temperature operation

The performance of LIBs at low temperatures (below −20 °C) is a critical factor in evaluating their practicality, especially given the wide range of applications in cold environments, such as winter conditions, polar regions, and outer space. Conventional LIBs often fail to operate effectively at low temperatures due to the sharp decline in the ionic conductivity of LEs. Similarly, QSEs suffer from significantly reduced ionic conductivity at low temperatures, primarily due to the “freezing” of the polymer matrix and the limited mobility of residual LEs.⁹⁵ At low temperatures, the mobility of polymer chain segments decreases, and the viscosity of the solvent increases, both of which hinder ionic migration. Additionally, the polymer matrix becomes glassy, increasing its rigidity and potentially leading to brittleness. This reduced mechanical strength can cause cracks or fractures in the polymer film, compromising the integrity of the QSE and ultimately resulting in battery failure or shortened lifespan. To address these challenges, researchers have explored various strategies to enhance the low-temperature performance of QSEs, including the introduction of liquid plasticizers (e.g., ionic liquids, carbonate solvents, or oligomer molecules), interfacial regulation (e.g., optimizing the SEI and CEI), and chemical modifications of the polymer matrix (e.g., copolymerization, cross-linking, or the introduction of ionic side groups).

4.3.1 Introduction of plasticizers. The addition of liquid plasticizers with low freezing points is an effective strategy to improve the ionic conductivity of traditional LEs at low temperatures.⁹⁶ This approach has also been successfully applied to QSEs. For instance, methyl propionate (MP), which has a freezing point of −88 °C, was incorporated into an in situ fabricated QSE to enhance its low-temperature ionic conductivity (Fig. 14a).⁹⁷ This flexible QSE demonstrated excellent ionic conductivity (1.0 mS cm⁻¹ at −30 °C, Fig. 14b) and good compatibility with LMA, as evidenced by the stable performance in Li|Li cells for over 2000 h. The assembled Li|NCM811 cell exhibited remarkable electrochemical performance at low temperatures, delivering a specific capacity of 109 mA h g⁻¹ at 0.2C over 100 cycles at −20 °C (Fig. 14c). Additionally, the cell achieved specific capacities of 201, 191, 172, 148, 114, and 64 mA h g⁻¹ at 20, 10, 0, −10, −20, and −30 °C, respectively, at 0.1C (Fig. 14d).

Fig. 14 (a) Illustration of the integrated QSE and cell production via in situ polymerization. (b) Ternary phase diagram of ionic conductivity for the developed QSE. (c) Cycling performance of the assembled Li|NCM811 cell at −20 °C. (d) Voltage profiles of the Li|NCM811 cell from −30 to 20 °C. (e) Melting points and viscosity (at 25 °C) of various solvents. (f) HOMO and LUMO energy levels of commonly used Li salts and solvents. (g) Cycling performance of the assembled Li|NCM811 cell at −20 °C. (h) Proposed SEI formation on the LMA. (i) Schematic of Li⁺ transport pathways in the as-prepared QSEs. (j) Cycling stability of the assembled Li|LFP cell at −35 °C.

4.3.2 Regulation of interfaces. The performance of batteries at low temperatures is also influenced by the insufficient dynamics of the interfacial layers. Therefore, optimizing the SEI and CEI is crucial to improve thermodynamic stability and charge-transfer kinetics. According to molecular orbital theory, the properties of the interphase are closely related to the physical properties of solvents and the energy levels of Li salts, solvents, and additives (Fig. 14e and f). Based on this principle, a rationally designed QSE was developed through the in situ polymerization of TXE, combined with 2,2,2-trifluoro-N,N-dimethylacetamide (FDMA), FEC, and LiDFOB.⁹⁸ This QSE exhibited an ionic conductivity of 2.2 × 10⁻⁴ S cm⁻¹ at −20 °C. The corresponding Li|NCM811 coin cell delivered a specific capacity of nearly 150 mA h g⁻¹ and retained 99.1% of its capacity after 200 cycles at 20 mA g⁻¹ and −20 °C (Fig. 14g). Furthermore, the Li|NCM811 pouch cell demonstrated stable operation at −20 °C. This QSE facilitated the formation of a dual-layered SEI on the LMA and stabilized the cathode, thereby enhancing interfacial ionic conduction and suppressing Li dendrite growth (Fig. 14h).

In another study, a novel QSE was prepared via the in situ polymerization of DOL with a single-solute LiBF₄ and a high-dielectric-constant solvent, FEC.⁹⁶ The synergistic effect of the DOL polymer, BF₄⁻ anions, and FEC enabled the formation of a robust and dynamically stable SEI layer containing LiF and Li⁺-conducting Li_xBO_yF_z. This resulted in a Li|LCO cell that exhibited stable cycling performance across a wide temperature range, from −60 to −20 °C.

4.3.3 Modification of the polymer matrix. The ionic conductivity of the polymer matrix can be enhanced through structural and compositional design, which also improves the low-temperature performance of QSEs. For example, an innovative multi-structured QSE framework was developed by copolymerizing a liquid crystal monomer (containing rigid units and flexible ion-conductive segments) with lithiophilic ethylene dimethacrylate (EGDMA) and a pentaerythritol tetrathio(3-mercaptopropionic acid) (PETMP) cross-linker, followed by coating with a PVDF-HFP polymer network (Fig. 14i).⁹⁹ This QSE featured a homogeneous micro-nano porous structure, which enhanced the distribution of functional groups on the polymer surface, facilitating efficient Li⁺ solvation and fast Li⁺ transport kinetics. The QSE achieved an ionic conductivity of 1.91 mS cm⁻¹ at RT. The assembled Li|Li cell maintained uniform Li deposition for up to 2400 h at 3.0 mA cm⁻², while the Li|LFP cell achieved 3300 cycles with a 93.5% capacity retention and stable operation at −35 °C (Fig. 14j).

Improving the low-temperature performance of QSSLMBs is critical for their practical application in cold environments. This can be achieved through the optimization of the polymer matrix, contained LEs, and additives. Nevertheless, achieving reliable performance at extremely low temperatures remains a major challenge, particularly in maintaining both conductivity and mechanical integrity. Future research should focus on advancing existing methods and developing new strategies to enhance the low-temperature performance of QSEs. Such advancements will drive the widespread adoption of electric vehicles and portable electronic devices in cold regions and provide innovative solutions for grid energy storage. With continued research, we anticipate the development of more efficient, stable, and safe low-temperature QSSLMBs, contributing to the realization of green energy and sustainable development goals.

4.4 Impact of optimization strategies

To meet the growing demands for high energy density, enhanced safety, and broad temperature adaptability in QSSLMBs, the optimization of QSEs must consider material innovation, interfacial regulation, and process engineering. In this section, we systematically review key strategies used to enhance QSE performance, clarify their synergistic effects on Li⁺ transport, ESW, and mechanical properties, and highlight promising future research directions.

4.4.1 Chemical modification. Chemical modification of the polymer matrix is a fundamental strategy to improve the ESW of QSEs. For example, fluorination of the polymer backbone effectively reduces the HOMO energy level, thereby suppressing oxidative degradation under high-voltage operation. Studies have shown that incorporating fluorinated monomers (e.g., HFA) expands the ESW beyond 5.0 V vs. Li/Li⁺, promotes the formation of LiF-rich interfaces, and mitigates Li dendrite formation. Moreover, block copolymerization and crosslinking strategies help balance ionic conductivity and mechanical strength by fine-tuning the density and distribution of polar groups within polymer chains. A representative example is the copolymerization of VEC with DAC, which significantly enhances both the Li⁺ transference number (up to 0.76) and oxidative stability (4.7 V vs. Li/Li⁺).

4.4.2 Composite design. The integration of functional fillers—either Li⁺-conductive (e.g., LLZO, LLZTO) or Li⁺-insulating (e.g., YSZ, Al₂O₃)—into QSEs has demonstrated significant performance improvements. Conductive fillers facilitate Li⁺ dissociation through Lewis acid–base interactions and enhance interfacial stability. For instance, the incorporation of LLZO nanofibers expands the ESW to 5.3 V and enables high-rate cycling of Li|NCM111 cells at 10C. Insulating fillers can suppress side reactions by adsorbing anions. The inclusion of YSZ, for example, raises the ESW of DOL-based QSEs to 4.9 V vs. Li/Li⁺. Asymmetric filler distribution, such as surface-enriched LLZTO layers, synergistically suppresses dendrite formation and improves high-voltage compatibility. Furthermore, introducing porous support structures (e.g., PI nanofiber membranes) addresses the uniformity challenges of in situ polymerized QSEs, enabling the combination of high ionic conductivity and mechanical resilience—key requirements for large-capacity pouch cells.

4.4.3 Interfacial engineering. Optimizing the SEI and CEI is crucial for enhancing the thermodynamic stability and charge-transfer kinetics. Based on molecular orbital theory, interfacial properties are governed by the energy levels of Li salts, solvents, and additives, as well as their physical interactions. Strategic use of additives enables the formation of stable, ion-conductive interphases. For example, a QSE containing MP maintains an ionic conductivity of 1.0 mS cm⁻¹ at −30 °C and supports stable cycling of Li|NCM811 cells at −20 °C. Additionally, in situ polymerization of DOL with LiBF₄ forms a LiF/Li_xBO_yF_z-rich interphase, which sustains robust performance of Li|LCO cells even at −60 °C.

4.4.4 Polymerization process. The polymerization process plays a decisive role in determining the microstructure and performance of QSEs. By selecting appropriate monomers and initiation mechanisms, various properties can be optimized: cationic polymerization allows rapid chain propagation and high monomer conversion. Electrochemical polymerization simplifies fabrication by eliminating the need for chemical initiators. Hybrid strategies, combining electrochemical and free radical polymerization, enable in situ formation and interfacial reinforcement of QSEs. For instance, a QSE formed by electrochemical polymerization of DPGDME exhibits long-term cycling stability (>3000 h at 50 °C) and a high sulfur utilization rate (98.4%) at 10C, demonstrating how process innovation can improve interfacial contact and overall cell performance.

5. Summary and outlook

5.1 Summary

QSEs, which combine the advantages of ASEs and LEs, represent one of the most promising candidates for practical LMB applications. Current research on in situ polymerization-derived QSEs primarily focuses on two key directions: (1) developing novel polymer matrices through innovative monomers, initiators, and polymerization reactions to achieve enhanced mechanical strength and electrochemical stability. (2) Tailoring QSE properties by optimizing the polymer matrix, Li salts, plasticizers, and additives to enable specific functionalities such as flame retardancy, high-voltage compatibility, and low-temperature performance.

Despite these advancements, several critical challenges remain: (1) interfacial instability with LMAs: the significant volume expansion/contraction of LMAs during cycling leads to poor interfacial contact, promoting Li dendrite formation. QSEs must therefore balance high mechanical strength (to suppress dendrites) with sufficient ionic conductivity. (2) Limited ESW: high-voltage cathodes are essential for high-energy-density QSSLMBs, yet most QSEs exhibit insufficient oxidative stability due to residual monomers and plasticizers. (3) Complex optimization of preparation conditions: QSE fabrication involves numerous interdependent parameters (e.g., component selection, concentrations, polymerization temperature/time, infiltration duration), requiring precise control to ensure electrolyte homogeneity and process reproducibility. (4) Reduced energy density: the incorporation of additives (e.g., flame retardants, plasticizers) often lowers the energy density of QSSLMBs compared to LE-based systems.

Notable progress has been made in addressing these challenges. For instance: a fluorinated polymer matrix (derived from PETEA and VEC monomers) achieved an ultra-wide ESW (up to 5.3 V vs. Li/Li⁺).⁴⁷ A non-flammable QSE was developed by encapsulating liquid TEP within an in situ-formed polycarbonate (VC polymer) matrix.⁷⁸ A fluorinated polyether-based QSE enabled a QSSLMB with an energy density exceeding 400 W h kg⁻¹.⁶ Future research should prioritize: (1) streamlining QSE preparation processes. (2) Enhancing mechanical robustness and electrochemical stability. (3) Minimizing trade-offs between functionality and energy density. These efforts will be critical to advancing the practical deployment of QSEs in next-generation LMBs.

In this review, the in situ fabricated QSEs were classified from the aspects of polymerization patterns of the polymer matrix, as follows:

Free radical polymerization is one of the most widely used methods for synthesizing the QSE matrix. This method involves the initiation of polymerization through free radicals, which can be generated thermally, photochemically, or via redox reactions. QSEs produced via free radical polymerization typically exhibit good ionic conductivity and mechanical strength. However, they often suffer from limited electrochemical stability at high voltages, which restricts their use in high-energy-density batteries.

Cationic polymerization is another approach to fabricate the QSE matrix, where the polymerization is initiated by cationic species. This method is particularly useful for producing QSEs with high molecular weights and good thermal stability. However, the ionic conductivity of QSEs obtained through cationic polymerization is generally lower compared to those produced by free radical polymerization. Additionally, the process is sensitive to moisture and other impurities, which can hinder the polymerization process.

Anionic polymerization involves the initiation of polymerization by anionic species. This method allows for precise control over the molecular weight and architecture of the polymer, leading to a QSE matrix with well-defined structures and properties. QSEs produced via anionic polymerization often exhibit excellent mechanical properties and thermal stability. However, similar to cationic polymerization, the ionic conductivity of these QSEs is relatively low, and the process is highly sensitive to impurities.

Electrochemical polymerization is a unique method where the polymerization is initiated by an applied electric field. This technique allows for the direct formation of QSEs on the electrode surface, leading to excellent interfacial contact between the electrolyte and the electrode. QSEs produced via electrochemical polymerization typically exhibit high ionic conductivity and good electrochemical stability. However, the process is complex and requires precise control over the polymerization conditions.

The characteristics of QSSLMBs were also introduced, as follows:

Flame retardancy is a critical safety feature for QSEs in LMBs. Several strategies have been explored to enhance the flame retardancy of QSEs, including the incorporation of flame-retardant additives and the use of inherently flame-retardant polymers. While significant progress has been made, achieving a balance between flame retardancy and other performance metrics, such as ionic conductivity and mechanical strength, remains a challenge.

High-voltage performance is essential for the development of high-energy-density LMBs. QSEs with high electrochemical stability are required to withstand the high voltages encountered in these batteries. While some QSEs have demonstrated good high-voltage performance, further improvements are needed to enhance their stability and prevent degradation at high voltages.

Low-temperature performance is another critical aspect for the practical application of LMBs. QSEs with high ionic conductivity at low temperatures are essential to ensure the efficient operation of batteries in cold environments. Several approaches, such as the use of low-temperature plasticizers and the optimization of polymer structures, have been explored to improve the low-temperature performance of QSEs. However, achieving high ionic conductivity at low temperatures without compromising other properties remains a significant challenge.

5.2 Outlook

Despite the substantial progress achieved in the development of QSEs for LMBs, several key challenges remain. These include the need to further enhance ionic conductivity—particularly under low-temperature conditions—improve electrochemical stability at high voltages, and boost flame-retardant performance. Additionally, issues related to the scalability, cost-efficiency, and practicality of polymerization methods must be addressed to enable the widespread commercial application of QSEs.

Future research should focus on the following directions:

Development of novel functional monomers: emphasis should be placed on designing new monomers with properties such as self-healing capabilities, high oxidative stability, and inherent flame retardancy. This may be achieved through chemical modifications such as fluorination, phosphorization, or sulfurization of existing monomers. Such developments aim to produce QSEs with superior Li dendrite suppression, high-voltage compatibility, and improved safety profiles.

Optimization of polymerization strategies: the polymerization process plays a critical role in determining the performance of the polymer matrix. Combining theoretical modeling with experimental data can help in precisely controlling the molecular weight and segment distribution of polymer chains. Additionally, hybrid polymerization techniques—such as the integration of photoinitiated and thermally initiated processes—can reduce energy consumption while enhancing control. Innovative in situ polymerization methods based on microfluidic technology also show promise for achieving uniform electrolyte thickness and improved interfacial compatibility.

Incorporation of advanced functional materials: introducing nanostructured fillers (e.g., MXenes, MOF-derived materials) and ionic liquids can significantly enhance ion transport efficiency and flame retardancy. These materials offer multifunctional benefits and open new possibilities for tuning the mechanical and electrochemical properties of QSEs.

Interdisciplinary and technological integration: cross-disciplinary approaches are essential for accelerating innovation in this field. The application of machine learning algorithms can assist in optimizing molecular design and predicting polymer properties. Coupling these insights with advances in battery system integration—such as the development of compatible electrode materials and optimized cell architectures—can facilitate the practical deployment of QSEs. Furthermore, expanding research into emerging applications such as flexible electronics, wearable devices, and grid-level energy storage systems will broaden the impact of QSE technology.

In summary, while considerable advancements have been made in the field of QSEs for LMBs, further innovation in materials science, polymer chemistry, and system integration is essential. The future success of QSEs hinges on the development of advanced materials and scalable polymerization methods that can meet the performance demands of next-generation QSSLMBs.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC: 22209107 and 52173205).

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