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.