Lithium-Ion Conduction Pathways in LLZO-PEO Composite Solid Electrolytes

Solid-state lithium–metal batteries utilizing composite solid electrolytes show great potential for overcoming the safety and energy density issues associated with conventional Li-ion batteries. Nevertheless, the fundamental mechanism of Li-ion conduction in composite electrolytes is still unclear. In this study, Li6.25Ga0.25La3Zr2O12-poly(ethylene oxide) (LLZO-PEO) composite electrolytes were fabricated by dispersing LLZO into a PEO matrix in different weight ratios to uncover the Li-ion conduction in both polymer- and ceramic-rich systems. The Li-ion transport in the LLZO-PEO composite was investigated by 2-probe AC impedance measurements at different temperatures. The in-depth impedance analysis based on conduction models confirms that Li-ions take different routes in composites depending on the LLZO ceramic content. For polymer-rich composites (up to ∼85 wt % LLZO), Li-ion conduction primarily occurred at the interfaces between PEO and LLZO, whereas above this threshold, conduction predominantly occurred through LLZO. The mechanistic insights into conduction behavior could be determinant in further optimizing the composite electrolytes for good cycling performance of solid-state lithium batteries.