Elucidation of the redox activity of Ca2MnO3.5 and CaV2O4 in calcium batteries using operando XRD: charge compensation mechanism and reversibility

Ca2MnO3.5 and CaV2O4 were found to be potentially interesting as positive electrode materials for calcium metal-based high-energy density batteries with DFT-predicted average voltages of 3.7 V and 2.5 V and energy barriers for Ca migration of 1.1 eV and 0.6 eV, respectively. Both compounds were prepared by solid state reaction under reducing atmospheres. Optimum conditions to achieve Ca2MnO3.5 comprised the reduction of Ca2MnO4 under NH3 gas at 420⁰C with a flow rate of 1200 ml/min while CaV2O4 was achieved by reduction of CaV2O6 at 700 ⁰C under H2 flow. Electrochemical oxidation of Ca2MnO3.5 in lithium or calcium cells resulted in the formation of an orthorhombic phase with cell parameter (a= 5.2891(1), b= 10.551(2), c= 12.1422(1)). Operando synchrotron radiation diffraction experiments indicate that the charge compensation mechanism is not related to Ca2+ extraction but to intercalation of F− (originated from electrolyte salt decomposition) into the anion vacancy position, as confirmed by EELS and EDS. This process was found to be irreversible. In the case of CaV2O4, oxidation induces the electrochemical extraction of calcium with the formation of an orthorhombic phase (space group Pbnm) with cell parameters a = 10.72008(9) Å, b = 9.20213(2) Å and c = 2.89418(3) Å. The process was also investigated via operando synchrotron radiation diffraction, with the oxidized phase being found to reintercalate Ca2+ ions upon reduction, with the formation of a solid solution. Preliminary cycling tests reveals a decrease in the polarization after the first cycle and call for further investigation of this system.