Building Bulk and Interface Dual Fast Li+ Conducting Pathway in Composite Solid Polymer Electrolyte Membrane for All‐Solid‐State Lithium‐Metal Batteries

Solid polymer electrolytes (SPEs) are considered a promising solution to the safety problems of lithium-ion batteries (LIBs) using liquid electrolytes. However, the high crystallinity and low ionic conductivity hinder the practical application of SPEs. Herein, we design a composite solid polymer electrolyte with a dual fast Li+ conducting pathway in bulk and interface by incorporating highly Li+ conductive ceramic Li6.4Ga0.2La3Zr2O12 (LGLZO) in polyethylene oxide (PEO)/Li-bis (trifluoromethanesulfonyl) imide (LiTFSI) system. Compared to Li6.5La3Zr1.5Ta0.5O12 (LLZTO), LGLZO provides better Li+ conductivity; therefore, a fast Li+ conducting pathway will form in the bulk of LGLZO nanofillers. Besides, LGLZO nanofiller accelerates the dissociation of LiTFSI and benefits the transfer of free Li+ through the SPEs near the LGLZO surface, forming another interface fast Li+ conducting pathway in the SPEs. Benefits from the dual fast Li+ pathway design, the composite electrolyte membrane with 15wt% LGLZO nanoparticles presents a high ionic conductivity of 8.0 × 10−4 S cm−1 at 60 °C. The Li-Li symmetric cells with optimized content LGLZO show good cycling stability (no short circuit even after 1000 h), and the all-solid Li/LiFePO4 batteries exhibit excellent cycling performance (remained 154 mAh g−1 after 500 cycles at 0.2 C under 60 °C).