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 x 10(-4) S cm(-1) at 50 degrees C was achieved for Li1.8Fe1.1Cl4. It was improved by 3 orders of magnitude compared with that of Li2FeCl4 (8.27 x 10(-7) S cm(-1) at 50 degrees C). For the Li|Li1.8Fe1.1Cl4|Li half-cell, it cycled for 2000 h at 50 degrees 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 Li1.8Fe1.1Cl4@LFP as the cathode, Li1.8Fe1.1Cl4 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 degrees 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.