Enlisting Potential Cathode Materials for Rechargeable Ca Batteries

The development of rechargeable batteries based on a Ca metal anode demands the identification of suitable cathode materials. This work investigates the potential application of a variety of compounds, which are selected from the inorganic crystal structural database, considering 3d-transition metal oxysulfides, pyrophosphates, silicates, nitrides, and phosphates with a maximum of four different chemical elements in their composition. Cathode performances of CaFeSO, CaCoSO, CaNiN, Ca3MnN3, Ca2Fe(Si2O7), CaM(P2O7) (M = V, Cr, Mn, Fe, and Co), CaV2(P2O7)2, Ca(VO)2(PO4)2, and α-VOPO4 are evaluated through the calculation of operation voltages, volume changes associated with the redox reaction, and mobility of Ca2+ ions. Some materials exhibit attractive specific capacities and intercalation voltages combined with energy barriers for Ca migration around 1 eV (CaFeSO, Ca2FeSi2O7, and CaV2(P2O7)2). Based on the Density Functional Theory results, αI-VOPO4 is identified as a potential Ca cathode with a maximum theoretical specific capacity of 312 mAh/g, an average intercalation voltage of 2.8 V, and calculated energy barriers for Ca migration below 0.65 eV.