Li-Fe-Cl Families as Novel Solid Electrolytes for All-Solid-State Batteries

The halides have attracted much attention as novel solid electrolytes because of their easy synthesis, high electrochemical stability, and high ionic conductivities. However, the reported halides for solid electrolytes are still understudied compared with the oxides and sulfides. Here, we studied the Li-Fe-Cl phases that include Li₂FeCl₄ and Li₆FeCl₈. Using the self-doping approach, a maximum ionic conductivity of 2.0 × 10^-4 S cm^-1 at 50 °C was achieved for Li_1.8Fe_1.1Cl₄. It was improved by 3 orders of magnitude compared with that of Li₂FeCl₄ (8.27 × 10^-7 S cm^-1 at 50 °C). For the Li|Li_1.8Fe_1.1Cl₄|Li half-cell, it cycled for 2000 h at 50 °C under a current density of 0.01 mA cm^-2, indicating an acceptable compatibility between Li₂FeCl₄ and Li. Finally, an all-solid-state battery was successfully assembled with Li_1.8Fe_1.1Cl₄@LFP as the cathode, Li_1.8Fe_1.1Cl₄ as the electrolyte, and a Li sheet as the anode. The initial specific charge capacity of the battery was 76.36 mAh g^-1 at 0.1C and 50 °C. The initial Coulombic efficiency was 73.06%. This study suggests Li₂FeCl₄ as a new solid electrolyte, and the introduction of Li vacancies into the Li site is an efficient way to improve the electrochemical properties of halides.