Quantitative Analysis of Large Voltage Hysteresis of Lithium Excess Materials by Backstitch Charge and Discharge Method

Lithium-excess (LEX) materials (LiMO2·Li2MnO3; M = Co, Ni, etc.) are attractive as potential positive electrodes of high-capacity lithium-ion batteries, however, large voltage hysteresis of LEX materials in charge and discharge disturbs real application. Thus, the large voltage hysteresis was investigated by a novel electrochemical method termed as a "backstitch charge and discharge" (backstitch CD) method, in which unsymmetrical charge and discharge with a small capacity were continuously repeated. The backstitch CD method was applied to Li[Li1/5Co2/5Mn2/5]O2 (0.5LCoO2·0.5Li2MnO3) known as an LEX material for the first time. Li[Li1/5Co2/5Mn2/5]O2 showed different reversible potentials during the backstitch CD processes in spite of symmetrical polarization behavior at each process. The voltage hysteresis in Li[Li1/5Co2/5Mn2/5]O2 resulted from electrochemically reversible but different reactions occurring during charging and discharging processes. Contributions of overpotential and difference in reversible potential to the voltage hysteresis can be evaluated quantitatively by the backstitch CD method.