Enhancement of the Li+ Conductivity of Li3AlF6 for Stable All-Solid-State Lithium-Ion Batteries

All-solid-state lithium-ion batteries (LIBs) are promising energy storage devices with a high energy density and safety. Their high performances are attributed to solid electrolytes with high Li+ conductivity. Recently, lithium metal bromides and chlorides have been developed as high Li+ conductors, offering excellent battery performance. Among the halide solid electrolytes, lithium metal fluorides have seldom been considered as candidates for solid electrolytes, despite their exceptional electrochemical stability. This is because of their insufficient Li+ conductivity compared with the chlorides and bromides with similar chemical compositions. Enhancement of the conductivity of fluorides can lead to the development of all-solid-state LIBs with high chemical stability and tolerance for high voltage. This study reports improvement in the Li+ conductivity of Li3AlF6 via Si4+ doping. The Li3AlF6–Li2SiF6 solid solution in which the host Al3+ is substituted with Si4+ was confirmed. The Li+ vacancy was considered as the most likely defect for charge compensation. Ball-milled Li3AlF6–Li2SiF6 exhibited an orthorhombic phase. The conductivity of 4Li3AlF6·Li2SiF6 pellets was 3 × 10–5 S/cm at room temperature, which is the highest reported value among the known lithium metal fluorides. Furthermore, the conductivity did not decline under ambient conditions (25 °C and a relative humidity of 70%), indicating the excellent moisture stability of 4Li3AlF6·Li2SiF6. Graphite/Li(Ni0.3Co0.6Mn0.1)O2 cell cycles at 50 °C revealed a gradual decrease in the capacities during cycling. The Coulombic efficiencies were <99%, which are further deteriorated at 105 °C. Scanning electron microscopy/energy dispersive X-ray spectrometry analysis of the cell after cycling indicated a Si-rich region at the graphite interphase, possibly because of the reductive decomposition of 4Li3AlF6·Li2SiF6. Li4Ti5O12/Li(Ni0.3Co0.6Mn0.1)O2 exhibited enhanced cycle performances; even after 35 cycles no capacity fading was observed, and the Coulombic efficiency was >99%. This study is the first to report on stable all-solid-state LIBs with lithium metal fluoride electrolytes.