Regulation of Multiphase Ion Migration in Polyoxometalate-Based Composite Solid-State Electrolytes

Polyethylene oxide (PEO)-based composite solid polymer electrolytes (CPEs) are emerging materials for all-solid-state lithium batteries (ASSLBs). However, the low ionic conductivity and slow multiphase ion transport have stymied their development. To address these limitations, we report a class of multifunctional active fillers based on Keggin-type polyoxometalates (POMs), Li3[PM12O40] (M = Mo, W). Unlike conventional inert fillers, these molecularly precise metal-oxygen clusters provide abundant Lewis basic surface sites, as well as intrinsic Li+ conduction pathways within their crystalline lattice. Comprehensive solid-state nuclear magnetic resonance, density functional theory calculations, and electrochemical analyses revealed that the [PMo12O40]3- cluster exhibited a weaker Li+ coordination affinity and a lower ion migration barrier than its tungsten analog. This resulted in a higher concentration of free Li+ alongside efficient interfacial and bulk transport channels. Notably, the PEO-based CPE with 10 wt % loading exhibited an ionic conductivity of 7.8 × 10-4 S/cm at 60 °C, a high Li+ transference number of 0.80, and superior long-term stability over 1500 h in symmetric Li cells. A high capacity retention of 87% was achieved after 800 cycles at 1 C for ASSLBs assembled with LiFePO4 cathodes, which outperformed conventional PEO-based electrolytes. This study advances the fundamental understanding of multiphase ion migration in solid-state systems and establishes POM clusters as a versatile component of high-performance composite electrolytes.