Aqueous ion batteries represent a promising technology for large-scale stationary energy storage, offering inherent safety, material sustainability, and low maintenance requirements. However, their practical implementation faces significant challenges, primarily related to the compatibility of electrode materials with water-based electrolytes. Beyond the restricted electrochemical stability window, electrode degradation arises from the strong solvating power and complex acid–base chemistry of aqueous systems.

Framework materials such as Na SuperIonic CONductors (NASICON) and Prussian Blue Analogues (PBAs) have attracted considerable attention due to their structural diversity, chemical stability, and low cost. A deep understanding of degradation mechanisms and strategies for their mitigation is essential to unlock the full potential of these materials and enable efficient aqueous battery systems.

Among NASICON-type compounds, NaTi₂(PO₄)₃ (NTP) stands out as one of the most suitable negative electrodes for aqueous electrolytes, owing to its favorable redox potential of -0.62 V vs SHE. Nevertheless, parasitic reactions significantly limit its Coulombic efficiency, induce self-discharge, and reduce charge capacity. To elucidate these degradation pathways, we performed operando monitoring of local pH, complemented by solid-state NMR, XRD, and EDX analyses. Our findings reveal that both hydrogen evolution (HER) and oxygen reduction (ORR) occur during NTP cycling, contributing to pH increase. While HER remains negligible under mildly alkaline conditions, chemical ORR driven by O₂ reacting with Ti^(III) species plays a dominant role. Interestingly, NTP exhibits minimal capacity loss at pH 7, but degradation becomes pronounced at pH 10, even in oxygen-free environments. We demonstrate that the time NTP spends in alkaline solution largely determines its stability at low cycling rates. Furthermore, only a fraction of the observed capacity fade is attributable to NTP decomposition; most losses stem from electronic contact failure within the electrode structure, caused by the growth of aqueous interphase layers composed primarily of amorphous TiO(OH)(H₂PO₄)·nH₂O phases. These insights suggest that careful control of local oxygen concentration and pH can enable highly stable NTP operation in simple, low-concentration aqueous electrolytes.

Prussian Blue Analogues, with their open framework and large cavities, are particularly appealing for multivalent ion storage, such as Zn²⁺ insertion. Zn-rich phases like KₓZn₂[Fe(CN)₆] (x → 0) show promise as positive electrodes for aqueous Zn-ion batteries. We investigated the synthesis–structure–property relationships of KₓZn₂[Fe(CN)₆], focusing on phase formation, Fe^(II/III) oxidation states, and hydration levels. Electrochemical performance was assessed via operando XRD in ZnSO₄ aqueous electrolytes, revealing previously unreported aspects of phase evolution during synthesis and cycling. These findings provide critical insights for optimizing PBA-based electrodes in next-generation aqueous Zn-ion batteries.