Fig. 4. Characterization of the Ca_0.4MnO₂ cathode.

a Schematic illustration of the atomic structure change during the in situ electrochemical conversion. The HADDF–STEM images of b the Mn₃O₄ precursor and c The Ca_0.4MnO₂ cathode material. Blue, orange, green, and red balls represent Mn³⁺, Mn²⁺, Ca, and O atoms, respectively. Scale bars are 2 nm in Fig. 4b and c. d Synchrotron powder diffraction patterns of the Mn₃O₄, birnessite MnO₂, and Ca_0.4MnO₂. The peaks marked with star symbol correspond to the stainless-steel mesh current collector. e CV curves of the Ca_0.4MnO₂ cathode collected at a scan rate of 0.3 mV s^–1.