The development of multivalent metal (such as Mg and Ca) based battery systems is hindered by lack of suitable cathode chemistry that shows reversible multi‐electron redox reactions. Cationic redox centres in the classical cathodes can only afford stepwise single‐electron transfer, which are not ideal for multivalent‐ion storage. The charge imbalance during multivalent ion insertion might lead to an additional kinetic barrier for ion mobility. Therefore, multivalent battery cathodes only exhibit slope‐like voltage profiles with insertion/extraction redox of less than one electron. Taking VS 4 as a model material, reversible two‐electron redox with cationic–anionic contributions is verified in both rechargeable Mg batteries (RMBs) and rechargeable Ca batteries (RCBs). The corresponding cells exhibit high capacities of >300 mAh g −1 at a current density of 100 mA g −1 in both RMBs and RCBs, resulting in a high energy density of >300 Wh kg −1 for RMBs and >500 Wh kg −1 for RCBs. Mechanistic studies reveal a unique redox activity mainly at anionic sulfides moieties and fast Mg 2+ ion diffusion kinetics enabled by the soft structure and flexible electron configuration of VS 4 .

The high-theoretical-capacity (∼170 mAh/g) Prussian white (PW), Na x Fe[Fe(CN) 6 ] y · n H 2 O, is one of the most promising candidates for Na-ion batteries on the cusp of commercialization. However, it has limitations such as high variability of reported stable practical capacity and cycling stability. A key factor that has been identified to affect the performance of PW is water content in the structure. However, the impact of airborne moisture exposure on the electrochemical performance of PW and the chemical mechanisms leading to performance decay have not yet been explored. Herein, we for the first time systematically studied the influence of humidity on the structural and electrochemical properties of monoclinic hydrated (M-PW) and rhombohedral dehydrated (R-PW) Prussian white. It is identified that moisture-driven capacity fading proceeds via two steps, first by sodium from the bulk material reacting with moisture at the surface to form sodium hydroxide and partial oxidation of Fe 2+ to Fe 3+ . The sodium hydroxide creates a basic environment at the surface of the PW particles, leading to decomposition to Na 4 [Fe(CN) 6 ] and iron oxides. Although the first process leads to loss of capacity, which can be reversed, the second stage of degradation is irreversible. Over time, both processes lead to the formation of a passivating surface layer, which prevents both reversible and irreversible capacity losses. This study thus presents a significant step toward understanding the large performance variations presented in the literature for PW. From this study, strategies aimed at limiting moisture-driven degradation can be designed and their efficacy assessed.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202103090.