Electrochemical oxidation of Ca₂MnO_3.5 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.