I have observed that the rapid development of renewable energy and efficient energy storage technologies has been driven by the global pursuit of carbon neutrality. Among electrochemical energy storage systems, lithium-ion batteries have dominated portable electronics and electric vehicles due to their high energy density, long cycle life, and environmental friendliness. However, the limited capacity of graphite anodes and the safety hazards of conventional liquid electrolytes have motivated the exploration of lithium metal anodes paired with solid-state electrolytes. In my view, the solid state cell architecture, especially those employing polymer electrolytes, represents one of the most promising pathways to achieve high-energy-density batteries with improved safety. The solid state cell eliminates flammable organic solvents, suppresses lithium dendrite growth, and enables the use of high-voltage cathodes. Nevertheless, solid polymer electrolytes still suffer from low ionic conductivity at room temperature, a trade-off between conductivity and mechanical strength, and low lithium-ion transference number. In this review, I summarize the progress and challenges of solid polymer electrolytes for lithium metal batteries. I discuss ion conduction mechanisms, various polymer matrices, modification strategies, and preparation methods, with extensive tables and equations to systematize the knowledge. My aim is to provide a comprehensive perspective on how to design high-performance solid polymer electrolytes for next-generation solid state cells.
The development of solid state cells dates back to the 19th century, when fast ion conduction in silver sulfide and lead fluoride was discovered. Later, sodium-beta-alumina was shown to exhibit excellent sodium-ion conductivity, enabling high-temperature sodium-sulfur batteries. In the 1970s, the first polymer electrolyte based on poly(ethylene oxide) (PEO) and alkali metal salts was proposed, and since then, solid polymer electrolytes have attracted continuous attention. The solid state cell concept has evolved from inorganic solid electrolytes to organic polymer electrolytes and composite polymer electrolytes. Inorganic electrolytes offer high ionic conductivity and mechanical strength but suffer from brittleness, poor processability, and high interfacial resistance. In contrast, polymer electrolytes provide flexibility, easy processing, and good interfacial contact, making them attractive for solid state cells. However, their intrinsic low ionic conductivity remains a major bottleneck. I believe that understanding the ion transport mechanism is crucial for rational design. Therefore, I first analyze the conduction mechanisms in polymer electrolytes.
Ion conduction in polymer electrolytes is generally believed to occur in the amorphous regions above the glass transition temperature, where polymer chain segments can move and facilitate lithium-ion transport. The widely accepted mechanism involves the coordination of lithium ions with polar groups on the polymer chains, followed by ion hopping between coordination sites and coupled motion with segmental relaxation. The number of free lithium ions and the mobility of polymer chains determine the ionic conductivity. However, some studies have suggested that crystalline phases can also conduct ions, particularly in PEO6:LiXF6 complexes, where lithium ions migrate along cylindrical channels without segmental motion. This controversy indicates that the conduction mechanism is still not fully resolved. To describe the temperature dependence of ionic conductivity, several models have been proposed. I summarize these models in Table 1 and present their mathematical forms.
| Model | Equation | Parameters | Applicability |
|---|---|---|---|
| Arrhenius | $$\sigma = \sigma_0 \exp\left(-\frac{E_a}{k_B T}\right)$$ | $\sigma_0$: pre-exponential factor; $E_a$: activation energy; $k_B$: Boltzmann constant; $T$: temperature | Simple thermally activated processes, often for crystalline or low-temperature regions |
| Vogel-Tammann-Fulcher (VTF) | $$\sigma = \sigma_0 T^{-1/2} \exp\left(-\frac{B}{T – T_0}\right)$$ | $B$: pseudo-activation energy; $T_0$: Vogel temperature (often $T_g – 50$ K) | Amorphous polymer electrolytes above $T_g$, coupling with segmental relaxation |
| Williams-Landel-Ferry (WLF) | $$\log \frac{\sigma(T)}{\sigma(T_g)} = \frac{C_1 (T – T_g)}{C_2 + (T – T_g)}$$ | $C_1, C_2$: empirical constants; $T_g$: glass transition temperature | Relaxation behavior near $T_g$, often equivalent to VTF |
| Free volume | $$\sigma = A \exp\left(-\frac{B}{f}\right)$$ | $f$: free volume fraction; $A, B$: constants | Describes ion transport as a function of free volume in amorphous polymers |
From these models, I infer that reducing crystallinity, decreasing glass transition temperature, and promoting salt dissociation are key strategies to enhance ionic conductivity. In the following sections, I discuss different polymer matrices and their modification strategies.
Polymer electrolytes consist of a polymer matrix and a lithium salt. The polymer matrix acts as a host for ion transport, while the lithium salt provides charge carriers. The dissociation of lithium salts and the flexibility of polymer chains directly affect ionic conductivity. Therefore, selecting an appropriate polymer matrix is essential. Common matrices include PEO, polyacrylonitrile (PAN), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and poly(methyl methacrylate) (PMMA). Each has distinct advantages and drawbacks, as summarized in Table 2.
| Polymer Matrix | Structure | Ionic Conductivity (S·cm-1) | Temperature (°C) | Advantages | Disadvantages |
|---|---|---|---|---|---|
| PEO | $$-\left[CH_2CH_2O\right]_n-$$ | 2.3×10-4 | 25 | Good compatibility, flexibility, low interfacial impedance | Low ionic conductivity at low temperature, poor high-voltage stability, poor mechanical properties |
| PAN | $$-\left[CH_2CH(CN)\right]_n-$$ | 2.96×10-4 | 25 | Easy film processing, good electrochemical stability, wide window | Unstable against lithium metal, poor ionic conductivity |
| PVDF | $$-\left[CH_2CF_2\right]_n-$$ | 3×10-4 | 25 | Good elasticity, adhesion, wide electrochemical window | High crystallinity |
| PMMA | $$-\left[CH_2C(CH_3)(COOCH_3)\right]_n-$$ | 2.7×10-3 | 30 | Good interfacial contact, high room-temperature conductivity | Poor mechanical strength and flexibility |
PEO is one of the most extensively studied polymer matrices due to its excellent compatibility with lithium metal and low interfacial impedance. However, its high crystallinity at room temperature results in low ionic conductivity (around 10-8 to 10-6 S·cm-1). To address this, many researchers have focused on reducing PEO crystallinity. For example, blending PEO with polyimide (PI) nanofibers increases chain disorder and enhances ionic conductivity. Adding porous boron nitride nanofibers (BNNFs) provides abundant active sites that lower crystallinity. Copolymerizing PEO with poly(p-phenylene benzobisoxazole) (PBO) reduces crystallinity while improving mechanical strength. Incorporating nano-LiF and succinonitrile into glass fiber-reinforced PEO can achieve zero crystallinity and a room-temperature ionic conductivity of 2.6×10-4 S·cm-1. These strategies demonstrate that modifying PEO can significantly improve its performance in solid state cells.
PAN offers good mechanical strength, high dielectric constant, and a wide electrochemical window. However, its nitrile groups react with lithium metal, causing poor interfacial stability. To mitigate this, researchers have designed multilayer electrolytes where a PAN layer faces the cathode and a PVDF-based layer faces the lithium anode. Such a configuration suppresses dendrite growth and improves compatibility. For instance, a laminated electrolyte comprising PAN and a PVDF/Pyr14TFSI/LiTFSI/LLZTO layer achieved an ionic conductivity of 1.5×10-4 S·cm-1 at room temperature. Symmetric cells with this electrolyte cycled stably for over 900 h, whereas pure PAN cells short-circuited after 40 h. In situ hydrolysis of tetraethyl orthosilicate in PAN creates an interconnected network that promotes lithium salt dissociation, yielding an ionic conductivity of 3.5×10-4 S·cm-1. These modifications make PAN more viable for solid state cells.
PVDF-based electrolytes possess polar C-F groups that dissolve lithium salts and facilitate ion transport. PVDF has a wide electrochemical window (>4.6 V) and forms a LiF interface layer that inhibits dendrite growth. However, PVDF is semi-crystalline, limiting ionic conductivity. Introducing hexafluoropropylene (HFP) into the PVDF chain reduces crystallinity and increases amorphous regions. Nevertheless, PVDF-HFP electrolytes still face issues such as gaps between spherical particles and side reactions with Li-DMF solvation molecules. To overcome these, researchers have introduced fluorinated graphene (FG) to promote nucleation and suppress particle growth, improving lithium-ion transport. Adding ethylenediaminetetraacetic acid (EDTA) induces conformational transitions in PVDF-HFP, enhancing mechanical strength and shortening ion transport paths, achieving an ionic conductivity of 2.47×10-4 S·cm-1. A dual-salt solid polymer electrolyte with crosslinked NPGDA-VEC copolymer and LiTFSI/LiBOB reached 2.64×10-4 S·cm-1 at room temperature. These modifications enhance the performance of PVDF-based solid state cells.
PMMA is a lightweight, transparent polymer with about 96% amorphous content at 25 °C. Its carbonyl groups interact strongly with plasticizers, enabling low interfacial impedance and good interfacial stability. However, PMMA-based electrolytes are brittle and have low room-temperature ionic conductivity. Copolymerization, blending, and crosslinking can improve both mechanical and electrical properties. For example, blending PMMA with PVDF-HFP and PEO produced an electrolyte with an electrochemical window of 4.7 V and an ionic conductivity of 1.87×10-4 S·cm-1 at 60 °C. UV-curing a mixture of PMMA, PVDF-HFP, and LLZTO yielded room-temperature ionic conductivity of 2.6×10-4 S·cm-1 and tensile strength of 17.8 MPa. Other matrices such as polycarbonate and polysiloxane also offer unique properties but require further optimization. Polycarbonate has strong polar carbonate groups that reduce ion aggregation but poor lithium metal compatibility. Polysiloxane has high oxidative stability but low salt solubility. Overall, each matrix has strengths and weaknesses, and modification strategies are essential for their application in solid state cells.
To systematically compare the effects of different modification strategies, I have compiled data from various studies in Table 3. This table summarizes the electrolyte composition, ionic conductivity, temperature, electrochemical window, and battery performance.
| Electrolyte Composition | Ionic Conductivity (S·cm-1) | Temperature (°C) | Electrochemical Window (V) | Cell Configuration | Cycle Number | Capacity Retention (%) |
|---|---|---|---|---|---|---|
| PEO/LiTFSI/PI | 2.91×10-4 | 60 | 4.87 | LFP-Li | 418 | 89.5 |
| 1% BNNFs/PEO/LiTFSI | 5.05×10-4 | 60 | 4.75 | LFP-Li | 100 | 92.11 |
| GP-LiF@3-SN@120 | 2.6×10-4 | 25 | 4.9 | LFP-Li | 150 | ~100 |
| p(TFPO-PEE)/LiTFSI | 3.98×10-3 | 25 | 5.1 | NMC622-Li | 1000 | 78 |
| p(F4EO-EOEC)/LiTFSI | 1.9×10-5 | 25 | 5.2 | NCM811-Li | 500 | 98 |
| DLSE (PAN+PPL) | 1.5×10-4 | 25 | 5.0 | NCM523-Li | 200 | 94.5 |
| PAN-insitu | 3.5×10-4 | 20 | 5.2 | NCM622-Li | 200 | 93.7 |
| P-PPL GPE | 8×10-4 | 80 | >4.5 | LFP-Li | 1570 | 92.5 |
| EDTA-PVDF-HFP/LiTFSI | 2.47×10-4 | 25 | 4.7 | NCM811-Li | 400 | 73 |
| N2V8L1-0.1SPE | 2.64×10-4 | 25 | 5.0 | LFP-Li | 1400 | 98.42 |
| p(VDF-VEPFSIS-Li)/EC | 5×10-4 | 25 | 4.3 | NMC-Li | 200 | 80.16 |
| PVDF-PPOA lithiation | 4.84×10-4 | 25 | 4.8 | NCM811-Li | 300 | 84.1 |
| PVDF-HFP/PDA@LLZTO/SN/LiTFSI | 2.49×10-4 | 30 | 4.8 | LFP-Li | 687 | 93.45 |
| PEO/PMMA/PVDF-HFP/LiTFSI | 1.87×10-4 | 60 | 4.7 | LFP-Li | 200 | 95.7 |
| PVC/SN-LLZTO/LiTFSI | 6.5×10-4 | 25 | 4.96 | NCM811-Li | 150 | 82.3 |
Beyond matrix selection, modification strategies play a pivotal role in enhancing the performance of solid polymer electrolytes. I categorize these strategies into interface optimization, framework construction, and filler addition. Interface optimization aims to improve the contact between the electrolyte and electrodes, reducing interfacial resistance and suppressing side reactions. For instance, in situ polymerization can form intimate contacts with electrodes, while multilayer structures can stabilize both anode and cathode interfaces. Framework construction involves creating a three-dimensional network that provides mechanical support and continuous ion transport pathways. Examples include PI nanofiber frameworks, porous BNNF networks, and crosslinked polymer matrices. Filler addition incorporates inorganic or organic fillers to disrupt crystallinity, provide Lewis acid sites, and enhance lithium salt dissociation. Common fillers include SiO2, LLZTO, BN, and fluorinated graphene. Table 4 summarizes these strategies and their effects.
| Modification Strategy | Example | Mechanism | Ionic Conductivity (S·cm-1) | Reference System |
|---|---|---|---|---|
| Interface optimization | In situ polymerization | Forms conformal contact, reduces interfacial resistance | Up to 5.54×10-3 | PDOL-based electrolyte |
| Interface optimization | Multilayer electrolyte | Stabilizes anode and cathode interfaces separately | 1.5×10-4 | PAN/PPL laminate |
| Framework construction | PI nanofiber framework | Increases chain disorder, provides mechanical support | 2.91×10-4 | PEO/LiTFSI/PI |
| Framework construction | Crosslinked network | Balances conductivity and mechanical strength | 2.64×10-4 | NPGDA-VEC copolymer |
| Filler addition | BN nanofibers | Lowers crystallinity, provides active sites | 5.05×10-4 | BNNFs/PEO/LiTFSI |
| Filler addition | LLZTO | Promotes lithium salt dissociation, creates fast ion pathways | 2.6×10-4 | PMMA/PVDF-HFP/LLZTO |
| Filler addition | Fluorinated graphene | Suppresses side reactions, promotes nucleation | ~10-4 | PVDF-HFP/FG |
Preparation methods also significantly influence the ionic conductivity and mechanical properties of solid polymer electrolytes. I have evaluated five common methods: solution casting, phase inversion, in situ polymerization, UV curing, and electrospinning. Solution casting is simple and scalable but often suffers from residual solvent and poor interfacial contact. Phase inversion produces porous structures that can absorb liquid electrolytes, enhancing conductivity but often increasing interfacial resistance. In situ polymerization injects a liquid precursor into the battery and polymerizes it, forming excellent electrode-electrolyte contacts and reducing interfacial resistance. However, residual monomers and initiators may degrade performance. UV curing rapidly solidifies liquid precursors under UV light, enabling high conductivity and dendrite suppression. Electrospinning produces nanofibrous membranes with high porosity and flexibility, improving ionic conductivity and mechanical strength. Table 5 compares these methods.
| Preparation Method | Advantages | Disadvantages | Typical Ionic Conductivity (S·cm-1) |
|---|---|---|---|
| Solution casting | Simple, scalable, good film uniformity | Residual solvent, poor interfacial contact | 3.21×10-3 (MOF-808/PVDF-HFP) |
| Phase inversion | Porous structure, high electrolyte uptake | High interfacial resistance, complex process | 1.33×10-3 (EC/PVDF) |
| In situ polymerization | Excellent interfacial contact, simple injection | Residual monomers/initiators, requires specific conditions | 5.54×10-3 (PDOL-based) |
| UV curing | Fast, low energy consumption, good mechanical properties | Limited penetration depth, requires photoinitiators | 2.6×10-4 (DLSCE) |
| Electrospinning | High porosity, large surface area, flexible | Complex parameter control, low production rate | 2.68×10-3 (cellulose acetate) |
Despite significant progress, several challenges remain for the practical application of solid polymer electrolytes in solid state cells. First, the ion conduction mechanism is not fully understood, especially the role of crystalline phases and interfacial ion transport. Advanced characterization techniques and multiscale modeling are needed to elucidate these processes. Second, ionic conductivity at room temperature is still far below that of liquid electrolytes. While modification strategies have improved conductivity, a trade-off between conductivity and mechanical strength persists. Flexible polymer chains are necessary for high conductivity but often compromise mechanical integrity, making dendrite suppression difficult. Third, interfacial engineering is critical. Poor electrode-electrolyte contact leads to high interfacial resistance and side reactions. In situ polymerization shows promise, but initiator compatibility and polymerization conditions must be optimized. Fourth, industrialization requires matching polymer electrolytes with high-voltage cathodes. Most polymer matrices oxidize above 4 V, and interfacial side reactions and volume changes during cycling degrade performance. Material optimization, interface stabilization, and cost-effective manufacturing are essential.
In my assessment, future research should focus on the following directions. (1) Deepening the understanding of ion conduction mechanisms through in situ and operando techniques, combined with theoretical simulations, to guide the design of high-performance electrolytes. (2) Developing novel polymer matrices and modification strategies to simultaneously achieve high ionic conductivity and robust mechanical properties. This includes exploring topological designs, crosslinked networks, and composite structures. (3) Advancing interface engineering by developing self-healing electrolytes, atomic layer deposition interlayers, and multifunctional separators to stabilize both anode and cathode interfaces. (4) Promoting the industrialization of solid state cells by optimizing scalable fabrication processes, reducing costs, and integrating with high-voltage cathodes. The solid state cell with polymer electrolytes holds great promise for next-generation energy storage, but interdisciplinary efforts are required to overcome the remaining obstacles.
In conclusion, I have reviewed the progress and challenges of solid polymer electrolytes for lithium metal batteries. The solid state cell offers a safer and higher-energy-density alternative to conventional lithium-ion batteries. Polymer electrolytes provide flexibility, easy processing, and good interfacial contact, but their low ionic conductivity and mechanical limitations hinder commercialization. Through a systematic analysis of ion conduction mechanisms, polymer matrices, modification strategies, and preparation methods, I have highlighted the key factors that influence performance. The extensive tables and equations presented here serve as a reference for researchers. I am optimistic that continued innovation in materials design and interface engineering will enable solid state cells to achieve their full potential in the near future.