Electrochemical activation of Li₂MnO₃ electrodes at 0°c and its impact on the subsequent performance at higher temperatures

This work continues our systematic study of Li-and Mn-rich cathodes for lithium-ion batteries. We chose Li₂MnO₃ as a model electrode material with the aim of correlating the improved electrochemical characteristics of these cathodes initially activated at 0°C with the structural evolution of Li₂MnO₃, oxygen loss, formation of per-oxo like species (O₂²⁻) and the surface chemistry. It was established that performing a few initial charge/discharge (activation) cycles of Li₂MnO₃ at 0°C resulted in increased discharge capacity and higher capacity retention, and decreased and substantially stabilized the voltage hysteresis upon subsequent cycling at 30°C or at 45°C. In contrast to the activation of Li₂MnO₃ at these higher temperatures, Li₂MnO₃ underwent step-by-step activation at 0°C, providing a stepwise traversing of the voltage plateau at >4.5 V during initial cycling. Importantly, these findings agree well with our previous studies on the activation at 0°C of 0.35Li₂MnO₃·0.65Li[Mn_0.45Ni_0.35Co_0.20]O₂ materials. The stability of the interface developed at 0°C can be ascribed to the reduced interactions of the per-oxo-like species formed and the oxygen released from Li₂MnO₃ with solvents in ethylene carbonate–methyl-ethyl carbonate/LiPF₆ solutions. Our TEM studies revealed that typically, upon initial cycling both at 0°C and 30°C, Li₂MnO₃ underwent partial structural layered-to-spinel (Li₂Mn₂O₄) transition.