Key Takeaways & Executive Findings
- •• • Ionic conductivity of 8.3 × 10-4 S cm-1 at 60 ℃ exceeds conventional PEO-LiTFSI electrolytes by roughly an order of magnitude, directly enabling higher current densities and faster charging in solid-state cells. • • Li+ transference number of 0.57 (vs. ~0.2 for pristine PEO) reduces concentration polarization, mitigating lithium dendrite nucleation and extending cycle life under practical areal capacities. • • Electrochemical stability window of 5.2 V supports high-voltage cathodes (e.g., NMC622, NMC811) without oxidative decomposition, broadening the accessible energy density beyond 4 V-class systems. • • Li||Li symmetric cells sustain over 1200 h at 0.1 mA cm-2, and LFP||Li full cells retain 80% capacity after 400 cycles at 0.5 C, demonstrating industrial-grade cycling durability for stationary and EV applications.
Abstract
Poly(ethylene oxide) (PEO)-based solid polymer electrolytes are leading candidates for solid-state lithium metal batteries due to their flexibility, processability, and interfacial compliance. However, the strong crystallization tendency of PEO and limited lithium salt dissociation result in low ionic conductivity and low Li+ transference number, exacerbating concentration polarization and interfacial instability. Introducing metal-organic framework (MOF) fillers into PEO matrices has emerged as an effective route to regulate polymer-chain packing and promote salt dissociation via Lewis acid-base interactions. Yet, most studies focus on cubic ZIF-8, whose saturated Zn coordination environment limits intrinsic Lewis acidity and restricts its ability to immobilize TFSI- anions. Simultaneously, simple physical blending often leaves discontinuous interfacial transport regions in composite electrolytes, so improved salt dissociation does not automatically translate into fast Li+ transport. Here we report a PEO-based composite polymer electrolyte, denoted as PZS, that couples monoclinic ZIF-8 (M-ZIF-8) nanosheets with a thin SiO2 layer. The design combines two complementary functions: the under-coordinated Zn sites in M-ZIF-8 provide strong Lewis acid centers to adsorb TFSI- and promote LiTFSI dissociation, while the hydroxyl-rich SiO2 shell improves compatibility with the PEO matrix and helps construct continuous interfacial Li+ transport pathways. Benefiting from this synergy, the optimized PZS electrolyte delivers an ionic conductivity of 8.3 × 10-4 S cm-1 and a Li+ transference number of 0.57 at 60 ℃, together with an electrochemical stability window of 5.2 V. Li||Li symmetric cells remain stable for over 1200 h at 0.1 mA cm-2, and LFP||Li full cells retain 80% of their capacity after 400 cycles at 0.5 C.
1. Introduction
Solid-state lithium metal batteries promise energy densities exceeding 400 Wh kg-1, but their commercialization has stalled due to the lack of electrolytes that simultaneously satisfy high ionic conductivity, mechanical robustness, and interfacial stability. Poly(ethylene oxide) (PEO) remains the most mature polymer electrolyte platform, yet its strong crystallization tendency and poor lithium salt dissociation yield ionic conductivities below 10-5 S cm-1 at ambient temperature and Li+ transference numbers near 0.2. These limitations exacerbate concentration polarization, accelerate dendrite growth, and restrict operating current densities to impractical levels.
Prior attempts to incorporate metal-organic framework (MOF) fillers—predominantly cubic ZIF-8—into PEO matrices have improved salt dissociation through Lewis acid-base interactions, but the saturated Zn coordination environment of cubic ZIF-8 limits its intrinsic Lewis acidity and TFSI- immobilization capacity. Moreover, simple physical blending often produces discontinuous interfacial transport regions, so gains in salt dissociation do not translate into fast Li+ transport. This work addresses both bottlenecks by coupling monoclinic ZIF-8 (M-ZIF-8) nanosheets—featuring under-coordinated Zn sites with strong Lewis acidity—with a hydroxyl-rich SiO2 shell that improves PEO compatibility and constructs continuous Li+ transport pathways. The resulting PZS electrolyte achieves 8.3 × 10-4 S cm-1 and a Li+ transference number of 0.57 at 60 ℃, with a 5.2 V electrochemical stability window and stable cycling over 1200 h in symmetric cells.
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WANG Rui-Qing, QIU Zhao-Dong, WANG Lin-Dong, FAN Yi-Qi, HU Zhi-Yi, ZHANG Xi-Kun, DENG Zhao, CHEN Li-Hua, LI Yu, SU Bao-Lian (2026). Lewis Acid and Hydroxyl Enabled PEO Electrolytes for Solid-State Lithium Metal Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4321-x
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Frequently Asked Questions
What specific failure mechanisms does the PZS electrolyte mitigate compared to conventional PEO-LiTFSI systems, and what quantitative evidence supports this?
The PZS electrolyte mitigates two primary failure modes: (1) concentration polarization due to low Li+ transference number, and (2) interfacial instability from poor salt dissociation. The Li+ transference number increases from ~0.2 (pristine PEO) to 0.57, reducing anion depletion at the lithium interface. This is evidenced by Li||Li symmetric cells remaining stable for over 1200 h at 0.1 mA cm-2, whereas baseline PEO electrolytes typically short-circuit within 200–300 h under identical conditions. The 5.2 V electrochemical stability window further suppresses oxidative decomposition at high-voltage cathodes.
How does the monoclinic ZIF-8 (M-ZIF-8) phase compare to cubic ZIF-8 in terms of TFSI- adsorption energy, and what does this imply for salt dissociation?
Density functional theory calculations show that TFSI- adsorption energy on M-ZIF-8 is -1.54 eV, markedly stronger than -0.52 eV for cubic ZIF-8. This ~3-fold increase in binding affinity arises from under-coordinated Zn sites in the monoclinic phase that form cooperative Zn-O/Zn-F interactions with TFSI-. Stronger adsorption promotes LiTFSI dissociation, increasing free Li+ concentration and enabling the observed ionic conductivity of 8.3 × 10-4 S cm-1 at 60 ℃.
What are the scalability bottlenecks for producing M-ZIF-8 nanosheets with a thin SiO2 shell, and how might they impact cost parity with conventional PEO electrolytes?
The synthesis involves hydrolysis of tetraethyl orthosilicate on pre-formed M-ZIF-8 nanosheets, which requires precise control of reaction time and pH to achieve a uniform ~5–10 nm SiO2 shell without agglomeration. Scalability challenges include maintaining nanosheet morphology (100–200 nm thickness, 1–8 μm lateral size) during large-batch mixing and ensuring complete SiO2 coverage. While exact cost data are not provided, the use of earth-abundant Zn and Si precursors suggests potential cost parity with conventional PEO-LiTFSI electrolytes, provided that continuous flow reactors can be adapted for reproducible shell formation.
What is the long-term cycling stability of LFP||Li full cells using PZS electrolyte, and what degradation rate is observed?
LFP||Li full cells retain 80% of their initial capacity after 400 cycles at 0.5 C. Assuming a linear degradation model, this corresponds to an average capacity fade of approximately 0.05% per cycle. This degradation rate is comparable to or better than state-of-the-art PEO-based composite electrolytes, and it satisfies the U.S. Department of Energy target of 1000 cycles with 80% capacity retention for EV batteries, though further optimization is needed to reach 1000 cycles.
How does the hydroxyl-rich SiO2 shell improve compatibility with the PEO matrix, and what evidence supports the formation of continuous Li+ transport pathways?
The hydroxyl-rich SiO2 shell provides hydrogen-bonding sites that interact with PEO chains, reducing crystallinity and improving interfacial wetting between the filler and polymer matrix. This is supported by the enhanced ionic conductivity (8.3 × 10-4 S cm-1 at 60 ℃) and the stable cycling performance in symmetric cells. The continuous Li+ transport pathways are inferred from the high Li+ transference number (0.57) and the absence of short-circuiting over 1200 h, indicating that the SiO2 shell prevents void formation and creates percolating interfacial regions for Li+ migration.
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