Studies on the Kinetic Properties of Mn-Rich LiFe_xMn_1-xPO₄Olivine Phases as Cathode Materials for Li-Ion Rechargeable Batteries

Abstract

The kinetic properties of electrons and Li-ions in Mn rich olivine structure cathode materials were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV tests revealed that electronic conductivity in Mn rich olivine phase is similar to that in LiFePO4 during Fe2+/Fe3+ redox reaction, but much poorer than that in Mn2+/Mn3+ redox reaction. The chemical diffusion coefficient of Li ions was much poorer in Mn rich olivine phases by nearly two orders of magnitude compared to that in LiFePO4; chemical diffusion was also severely affected by the type of redox reaction. The EIS tests showed that charge transfer resistance of the Mn rich olivine electrode is almost 5∼6 times higher than that of LiFePO4, especially for low SOC levels. The chemical diffusion coefficient of Li ions in Mn rich olivine phase was very changeable depending on SOC level, showing two peak values at two different SOC levels, while LiFePO4 showed a linear increment of that value with increasing SOC level.