High Formability and Fast Lithium Diffusivity in Metastable Spinel Chloride for Rechargeable All‐Solid‐State Lithium‐Ion Batteries

Materials with high formability and Li‐ion diffusivity are desired to realize safe bulk‐type all‐solid‐state Li‐ion batteries with high energy density. Spinel‐type Li 2 FeCl 4 , which is expected as a Li‐ion‐conductive electrode, adopts the high‐temperature cubic phase (space group: Fd‐3m ) by a mechanochemical synthesis method. The powder‐compressed pellet shows a high relative density of 92% and large neck formation between the particles. An analysis of the distribution of relaxation times from the AC impedance results indicates that the contribution of the grain boundary impedance is almost negligible (≈3%) in the powder‐compressed pellet. Even the Li diffusion coefficient, which is underestimated by the Nernst–Einstein equation, is several orders of magnitude higher than those of conventional oxide and sulfide electrode materials. Analyzing the diffusion pathways using a classical force field calculation suggests that stabilizing the high‐temperature phase at room temperature delocalizes Li ions in the diffusion pathway, thus realizing high Li‐ion diffusivity. The Li 2 FeCl 4 green compact containing 10 wt% carbon as an electron‐conductive additive shows a one‐electron charge reaction with a capacity of 126 mAh g −1 , with no large overpotential at a high operating voltage of 3.6 V versus Li/Li + at 30 °C.