Novel lithium-salt-based oxalate filler enabling interfacial Li plus redistribution and segmental dynamics enhancement in PEO solid electrolytes

초록

Solid polymer electrolytes are promising candidates for next-generation all-solid-state batteries (ASSBs), as they can simultaneously enhance safety and energy density. However, the practical application of poly(ethylene oxide) (PEO) is restricted by its high crystallinity and low ionic conductivity at ambient temperatures. Here, we introduce lithium oxalate (Li2C2O4) as a lithium-salt-based inorganic filler to improve structural stability and ion transport in PEO. The incorporation of Li2C2O4 suppresses PEO crystallinity and creates additional Li+ conduction pathways along the filler surface and polymer-filler interfaces. Although Li2C2O4 is not fully soluble in the polymer matrix, its surface undergoes partial Li+ dissociation and exchange with the surrounding PEO/LiTFSI environment, increasing the population of mobile Li+ through interfacial Li+ activation. When the filler loading is insufficient, dispersed Li2C2O4 grains induce localized polymer-filler interactions that transiently restrict EO segmental mobility before continuous interfacial networks are established. At an optimal filler content, balanced EO-Li+ and TFSI- -Li+ interactions promote salt dissociation and increase the concentration of free Li+. As a result, the composite electrolyte exhibits enhanced ionic conductivity, a larger Li+ transference number, and a wider electrochemical stability window. These improvements align with molecular dynamics simulations, which reveal the highest Li+ diffusion coefficients at the optimal composition. These results indicate that the dominant enhancement mechanism is interfacial Li+ redistribution driven by C2O42- -Li+ coordination rather than bulk Li2C2O4 dissolution or purely physical effects. Overall, lithium oxalate operates as a multifunctional salt-type filler that strengthens Li+ transport in PEO-based electrolytes and offers strong potential for high-safety, highperformance ASSBs.

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