Discovery of Calcium-Metal Alloy Anodes for Reversible Ca-Ion Batteries

Ca-ion batteries (CIBs) show promise to achieve the high energy density required by emerging applications like electric vehicles because of their potentially improved capacities and high operating voltages. The development of CIBs has been hindered by the failure of traditional graphite and calcium metal anodes due to the intercalation difficulty and lacking efficient electrolyte. Recently a high voltage (4.45 V) CIB cell using Sn as the anode was reported achieving a remarkable cyclability (> 300 cycles). The calciation of Sn was observed to end at Ca7Sn6, which is surprising, since higher Ca-content compounds are known (e.g. Ca2Sn). Here, we investigate computationally the Sn electrochemical calciation reaction process and explore the reaction driving force as a function of Ca content using density functional theory (DFT) calculations. This exploration allows us to identify threshold voltages which govern the limits of the calciation process. We then use this information to design a four-step screening strategy and use high-throughput DFT to search for anode materials with higher properties. We predict that many metalloids (Si, Sb, Ge), (post-)transition metals (Al, Pb, Cu, Cd, CdCu2) are promising inexpensive anode candidates and warrant further experimental investigations.