Calcium‐Ion Insertion Chemistry in Tunneled α‐MnO2 Cathodes for Calcium Metal Batteries

Rechargeable calcium batteries (RCBs) are a promising sustainable energy storage technology with high theoretical energy density. However, their development is hindered by the lack of suitable cathodes that enable facile and reversible Ca2+ storage. This study investigates the Ca2+ storage mechanism in tunneled α-K0.03MnO2 cathodes, revealing distinct electrochemical behaviors in two cell configurations using either activated carbon (AC) anodes or Ca metal anodes. While Ca2+ insertion/extraction dominates the charge storage process in Ca metal anode systems, negligible Ca2+ insertion occurs with AC anodes. Despite that, detailed mechanistic investigations of the Ca metal anode systems indicate sluggish Ca2+ diffusion within the α-K0.03MnO2 framework, leading to irreversible cation trapping. Moreover, progressive Ca2+ accumulation causes deep calciation that triggers irreversible phase transition into CaMn2O4 and eventually complete structural degradation. To address these issues, a composite cathode combining ultrasmall, low-crystallinity MnO2 nanoparticles with graphene oxide (u-MnO2@GO) is developed, demonstrating improved Ca2+ diffusion kinetics, enhanced cycling stability over 60 cycles, and superior rate capability up to 50 mA g-1. This work provides critical insights into Ca2+ storage mechanisms in oxide cathodes and offers effective strategies for designing high-performance cathodes for RCBs.