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-K_0.03MnO₂ 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-K_0.03MnO₂ 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.