Kinetically asymmetric charge and discharge behavior of LiNi _0.5Mn_1.5O₄ at low temperature observed by in situ X-ray diffraction

Capacity decrease at low temperatures is one of the issues to be solved for secondary batteries especially for automobile applications and it is thus important to clarify the reaction kinetics in operating batteries and identify the rate determining step that governs the performance at low temperatures. Phase transitions in electrode active materials are important factors that affect the reaction kinetics particularly for thin electrodes used in high power applications. In this study, the phase transition dynamics of thin LiNi _0.5Mn_1.5O₄ electrodes at various temperatures is examined using electrochemical methods combined with temperature-controlled in situ X-ray diffraction analysis to directly capture the reacting species and elucidate the reaction mechanism. The analysis shows that there occur consecutive phase transitions of LiNi_0.5Mn_1.5O₄ (Li1 phase) ↔ Li_0.5Ni_0.5Mn_1.5O ₄ (Li0.5 phase) and the Li0.5 phase ↔ Ni_0.5Mn _1.5O₄ (Li0 phase) at room temperature and above. At lower temperatures the transition of Li1 → Li0.5 proceeds during the charging process but further delithiation to form the Li0 phase is restricted, leading to the capacity decrease. On the other hand, on discharging at low temperatures the amount of the Li0 phase to be lithiated is limited and this causes the capacity decrease. There is no Li0.5 phase formation on discharging at low temperatures, revealing remarkable kinetic asymmetry of the reaction processes for charging and discharging. It is suggested that the Li0.5 phase formed on discharging is instantly lithiated to form the Li1 phase, due to the small potential gap between the two transitions. These results indicate that the phase transition kinetics of Li0.5 ↔ Li0 is slower than that of Li1 ↔ Li0.5 and the former transition is the rate determining step at low temperatures.