Performance Limitationsof CaCoSO as a Positive ElectrodeMaterial for Calcium Storage

Calcium metal batteries (CMBs) are promising candidates for next-generation electrochemical energy storage systems due to their high volumetric capacity, abundance, sustainability, and safety. Recent DFT predictions suggested that the layered CaCoSO phase can enable sequential Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ redox activity at an average potential of 2.8 V vs Ca²⁺/Ca, making it a promising candidate for high-energy-density CMBs [Torres, A. Chem. Mater. 2021, 33(7), 2488–2497]. Inspired by these metrics, in this work, we present the synthesis and electrochemical analysis of the CaCoSO phase. Theoretical capacity can be extracted through galvanostatic cycling, albeit accompanied by high polarization. <i>In situ</i> XRD and DEMS analyses, however, reveal that the capacity arises primarily from a combination of material decomposition and electrolyte degradation rather than reversible Ca²⁺ ion storage. The apparent discharge capacity is attributed to the cathodic decomposition of generated water during the subsequent anodic step, making the overall electrochemical process appear as reversible. This work underscores the complexity of achieving stable calcium-ion storage and aligns with similar challenges reported for other systems, highlighting the need for realistic testing conditions and providing critical insights to guide the development of advanced electrode materials and electrolytes for CMBs.