All-Solid-State Lithium-Ion Batteries Using the Li3AlF6-Based Composite as the Solid Electrolyte

All-solid-state lithium-ion batteries (LIBs) are considered promising energy storage devices owing to their high energy density and safety. The development of solid electrolytes with high Li+ conductivity, wide electrochemical stability window, air stability, and favorable mechanical properties directly leads to the realization of high-performance all-solid-state LIBs. Fluoride-based materials are potential candidates that meet these requirements. In the present study, Li+ conductivity and the electrochemical performance of β-Li3AlF6-based composites have been reported. The amorphous Li3AlF6–Li2SO4 composites were fabricated via planetary ball milling. The conductivity of Li3AlF6–Li2SO4 (Li2SO4: 50 mol %), when ball-milled for 70 h, was 6 × 10–4 S/cm at 150 °C. The conductivity did not degrade when Li3AlF6–Li2SO4 was kept at 150 °C for 12 h. In the cyclic voltammetry measurements, the Li plating/stripping current was clearly observed at 0 V (vs Li+/Li), and the continuous anodic decomposition of Li3AlF6–Li2SO4 was not confirmed up to 5 V (vs Li+/Li). Using Li3AlF6–Li2SO4 as the solid electrolyte, the all-solid-state LIB of graphite/Li (Ni0.3Co0.6Mn0.1)O2 was fabricated by uniaxial pressing without sintering. The battery was fabricated and its performance was evaluated at 120 °C in a dry room, where the dew point was maintained at −40 °C. A discharge capacity of 99 mAh/g was obtained at 0.1 C, which was maintained at 88 mAh/g at 1 C. The charge–discharge cycles were stable, and drastic capacity fading was not observed for 47 cycles at 1 C. The results indicate that fluoride-based materials are promising candidates for solid electrolytes in all-solid-state LIBs.