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Figure 4. Capacitive behaviors and both electrochemical and morphological analysis after 1000 cycles of β-MnO₂ and R-Fe_xMn_1-xO₂ cathodes: (A) Diffusion-controlled and surface-controlled contributions to capacity at different scan rates; (B) Mn 2p and O 1s spectra; (C) Nyquist plots at the open circuit voltage state (Supplementary Figure 15 illustrates the electrical equivalent circuit used for fitting EIS spectra); (D and E) EIS spectra of β-MnO₂ and R-Fe_xMn_1-xO₂ cathodes during charge process; (F) SEM images of β-MnO₂ and R-Fe_xMn_1-xO₂ cathodes. EIS: Electrochemical impedance spectroscopy; SEM: scanning electron microscopy.