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

Abstract 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 Ca 2+ storage. This study investigates the Ca 2+ storage mechanism in tunneled α‐K 0.03 MnO 2 cathodes, revealing distinct electrochemical behaviors in two cell configurations using either activated carbon (AC) anodes or Ca metal anodes. While Ca 2+ insertion/extraction dominates the charge storage process in Ca metal anode systems, negligible Ca 2+ insertion occurs with AC anodes. Despite that, detailed mechanistic investigations of the Ca metal anode systems indicate sluggish Ca 2+ diffusion within the α‐K 0.03 MnO 2 framework, leading to irreversible cation trapping. Moreover, progressive Ca 2+ accumulation causes deep calciation that triggers irreversible phase transition into CaMn 2 O 4 and eventually complete structural degradation. To address these issues, a composite cathode combining ultrasmall, low‐crystallinity MnO 2 nanoparticles with graphene oxide (u‐MnO 2 @GO) is developed, demonstrating improved Ca 2+ diffusion kinetics, enhanced cycling stability over 60 cycles, and superior rate capability up to 50 mA g −1 . This work provides critical insights into Ca 2+ storage mechanisms in oxide cathodes and offers effective strategies for designing high‐performance cathodes for RCBs.