Cathode‐Electrolyte‐Interphase Film Formation on a LiNiO2 Surface in Conventional Aqueous Electrolytes: Simple Method to Improve the Electrochemical Performance of LiNiO2 Electrodes for Use in Aqueous Li‐Ion Batteries

Abstract Passivation films, the so‐called solid–electrolyte interphase and cathode–electrolyte interphase (CEI), are considered to be essential for the operation of rechargeable batteries because they have a positive impact on electrochemical performance including cyclability. In the field of aqueous Li‐ion batteries (ALIBs) to date, it is generally accepted that these films can only be formed in super‐concentrated electrolytes containing fluorine‐based organic anions such as N(SO 2 CF 3 ) 2 and SO 3 CF 3 . This study demonstrates for the first time that a CEI film can be created on a LiNiO 2 (LNO) electrode in conventional aqueous electrolytes and improve electrochemical performance such as reversible capacities and cyclability. The results reveal that a CEI film, mainly composed of Li 2 CO 3 and LiOH, is formed on the LNO surface in a saturated LiNO 3 aqueous electrolyte containing LiOH beginning at the first de‐intercalation of Li + ions, suppressing side reactions between the surface and aqueous electrolyte, and improving the recovery rate of structural changes of LNO during the charge/discharge process. As a result, this study suggests that LNO has great potential for application as a cathode material for high energy density ALIBs.