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

Abstract

γ/β-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 γ/β-MnO2, its approximate entropy (ΔS) at various depths of discharge (DOD) in a quasi-equilibrium state is studied. The ΔS analysis reveals that the reduction process of γ/β-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 γ/β-MnO2 has a low charge transfer impedance (Rct) and solid phase diffusion impedance (Zw). 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 γ/β-MnO2 can be determined. Furthermore, the thermodynamics of γ/β-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.