Where is the charge stored? Mechanism validation in aqueous multivalent-ion batteries

Aqueous multivalent-ion batteries are widely regarded as promising systems for safe and low-cost energy storage. However, a central assumption underlying this field remains insufficiently validated: that the named multivalent cation is the primary charge carrier. This review critically examines charge-storage mechanisms in aqueous batteries based on Zn²⁺, Mg²⁺, Ca²⁺, Al³⁺, and Mn-based chemistries, with emphasis on identifying the actual charge carrier, the operative reaction pathway, and the level of evidence required for mechanistic assignment. Across these systems, proton insertion, co-insertion, dissolution-deposition, and conversion reactions can dominate or coexist with nominal multivalent-ion intercalation, depending on electrode and electrolyte chemistry. Electrochemical signatures alone are shown to be insufficient for mechanism identification, as distinct processes can produce similar voltage profiles and cycling behavior. A critical assessment of experimental approaches reveals that reliable validation requires quantitative ion tracking, operando characterization, and correlation across multiple techniques. On this basis, a framework is proposed to distinguish competing mechanisms and define minimum standards for mechanistic claims. Establishing clear validation criteria is essential for directing material design and avoiding misinterpretation in aqueous battery research.