Thermodynamics, kinetics and crystal structure of γ/β-MnO2 in Li/MnO2 primary batteries

gamma/beta-MnO2 as a cathode material is a key factor for the performance of Li/MnO2 primary batteries. To clarify the thermodynamic and kinetic characteristics of gamma/beta-MnO2, its approximate entropy (Delta S) at various depths of discharge (DOD) in a quasi-equilibrium state is studied. The AS analysis reveals that the reduction process of gamma/beta-MnO2 with Li+ intercalation is divided into three stages (single-phase, two-phase and single-phase reaction) but these three stages are hard to distinguish by X-ray diffraction (XRD). This result is different from an earlier conclusion of only a single-phase reaction. The two-phase reaction of gamma/beta-MnO2 has a low charge transfer impedance (R-ct) and solid phase diffusion impedance (Z w ). This is advantageous for providing a high electrochemical reduction reaction rate of MnO2 and a high power output of the battery. The functional relationship between x in LixMnO2 and its corresponding open circuit voltage (OCV, the stable potential of MnO2 vs Li/Li+) is a mathematical formula similar to the Nernst equation. According to the formula parameters (m and n), the state of charge (SOC) of gamma/beta-MnO2 can be determined. Furthermore, the thermodynamics of gamma/beta-MnO2 during discharge demonstrates that the rate controlling steps (RCSs) are ion solid diffusion and electron transport, rather than the sole solid phase diffusion as previously reported. (C) 2020 Elsevier Ltd. All rights reserved.