Applying the HSAB design principle to the 3.5 V-class all-solid-state Li-ion batteries with a chloride electrolyte†
All-solid-state Li-ion batteries are expected to be the next generation of batteries with a high energy density and safety. However, for Li-ion batteries to endure high-voltage operations, the decomposition of solid electrolytes must be suppressed first. A high potential at the cathode tends to promote battery degradation because of the oxidation of the cathode electrolyte. This study aims to achieve the high-potential operation of all-solid-state batteries using LiAlCl4 as a chloride electrolyte with a high oxidation resistance. However, batteries with commonly used oxide electrodes (e.g., LiFePO4) exhibit low capacity (∼0.5 mA h g−1), despite having working potentials less than the oxidation potential of LiAlCl4. First-principles calculations and 27Al MAS-NMR measurements suggest that acid–base reactions based on the hard and soft acid–base (HSAB) rule occur between the electrode and the electrolyte. In contrast, a high voltage of ∼3.65 V (vs. Li+/Li) and high-capacity utilisation (reversible capacity ∼100 mA h g−1) are observed at room temperature by combining the same chloride electrode (Li2FeCl4) without side reactions between these chlorides. These results indicate that material design based on the HSAB rule is also instructive when considering electrode/electrolyte material combinations, which realizes a 3.5 V-class all-solid-state Li-ion battery.