Residual alkali effect in LLZO-modified composite solid polymer electrolyte

Solid polymer electrolytes (SPEs) are regarded as ideal solid electrolytes for next-generation lithium-ion batteries due to their high safety, but their low ionic conductivity limits practical applications. The incorporation of garnet-type Lithium Lanthanum Zirconium Oxide (LLZO)-based electrolytes with high Li+ conductivity is an effective strategy to enhance the performance of polyethylene oxide (PEO)-based SPEs. In this study, highly Li+-conductive Ga-doped LLZO (Li6.4Ga0.2La3Zr2O12) was prepared by wet and dry methods, Ga-doped (Ga = 0.2) LLZO is referred to as LLZO for brevity throughout this manuscript, yielding samples denoted as wet-LLZO and dry-LLZO, with abundant and minimal surface residual alkali, respectively. These LLZO nanoparticles with different residual alkali were incorporated as fillers in composite solid polymer electrolytes (CSPEs). Studies demonstrate that the residual alkali, such as Li2CO3, on the LLZO surface effectively disrupts the ordered arrangement of PEO segments, significantly reducing their crystallinity and thus facilitating Li+ transport. Based on this, CSPE with 15 wt% wet-LLZO achieves a high ionic conductivity of 0.80 mS cm-1 and a lithium-ion transference number of 0.36 at 60 °C. The assembled Li/LiFePO4 all-solid-state battery maintains a discharge specific capacity of 142.5 mAh g-1 and a Coulombic efficiency exceeding 99% after 300 cycles at a 1.0 C rate. This work reveals the role of residual surface alkali in the LLZO composite solid electrolyte, providing novel insights for designing high-performance solid-state batteries through filler surface engineering.