Metal Ions versus Protons: Tracking of Charge‐Carrier Insertion into a Cathode Oxide in Aqueous Rechargeable Batteries

Abstract Protons in aqueous electrolytes can perform as an additional type of charge carrier for insertion/extraction in addition to the primary carrier cations in aqueous rechargeable batteries. Despite many diverse claims regarding the effect of protons, mutually conflicting experimental results and their interpretations without direct evidence have been reported over the last decade. Systematic examinations and analyses are thus imperative to clarify the conditions of proton insertion in aqueous rechargeable batteries. Utilizing V 2 O 5 as a model cathode and beaker‐type cells with a sufficient amount of ZnSO 4 aqueous electrolytes in this work, it is demonstrated that protons are inserted into the cathode prior to Zn‐ions in low‐pH conditions (pH ≤ 3.0). In stark contrast, the influence of protons on the discharge voltage and capacity is insignificant, when either the pH becomes high…