Electrochemical Activation of Li(2)MnO(3) 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 Li(2)MnO(3) 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 Li(2)MnO(3), oxygen loss, formation of per-oxo like species (O(2)(2−)) and the surface chemistry. It was established that performing a few initial charge/discharge (activation) cycles of Li(2)MnO(3) 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 Li(2)MnO(3) at these higher temperatures, Li(2)MnO(3) 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 0.35Li(2)MnO(3)·0.65Li[Mn(0.45)Ni(0.35)Co(0.20)]O(2) 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 Li(2)MnO(3) with solvents in ethylene carbonate–methyl-ethyl carbonate/LiPF(6) solutions. Our TEM studies revealed that typically, upon initial cycling both at 0 °C and 30 °C, Li(2)MnO(3) underwent partial structural layered-to-spinel (Li(2)Mn(2)O(4)) transition.