Revealing the Role of Active Fillers in Li-ion Conduction of Composite Solid Electrolytes

Composite solid electrolytes (CSEs) consisting of polyethylene oxide (PEO) matrix and active inorganic fillers have shown great potential for practical applications. However, mechanisms of how different active fillers enhance ion transport in CSEs still remain inconclusive. In this work, the component dependencies of ionic conductivity of PEO-based CSEs are investigated by comparing two widely investigated active fillers: NASICON-type (LATP) and garnet-type (LLZTO). In terms of ionic conductivity, the optimum ratios are strikingly different for LLZTO (10 wt%) and LATP (50 wt%). Through experimental and computational studies, it is demonstrated that the high affinity between LATP and PEO facilitates unhindered interfacial Li⁺ transfer so that LATP functions as a bulk-active filler to provide additional inorganic ion pathways. By contrast, Li⁺ transfer between LLZTO and PEO is found to be sluggish. Instead, LLZTO mainly improves ionic conductivity by dissociating lithium salt, making it a surface-active filler. Through categorizing active fillers based on their Li⁺ conductive mechanisms, this work provides new understanding and guidelines for componential design and optimization of solid composite electrolytes.