Studies of cycling behavior, ageing, and interfacial reactions of LiNi _0.5Mn_1.5O₄ and carbon electrodes for lithium-ion 5-volt cells

The cycling and storage behavior of LiNi_0.5Mn _1.5O₄ and MCMB electrodes for 5-volt Li-ion batteries was investigated at elevated temperatures using a variety of electrochemical (CV, EIS) and spectroscopic (XPS, micro-Raman) tools. It was established that LiNi_0.5Mn_1.5O₄ electrodes could be cycled highly reversibly, demonstrating sufficient capacity retention at 60 °C by a constant current/constant voltage mode in DMC-EC/1.5 M LiPF₆ solutions. By studying the influence of temperature on the impedance of LiNi_0.5Mn_1.5O₄ electrodes, we conclude that when the initial electrode's surface chemistry is developed at a high temperature (60 °C) it becomes nearly invariant, and hence, their impedance remains steady upon cycling and storage. Prolonged storage of these electrodes at 60 °C may result in local Mn and Ni dissolution and transformation of the active material to λ-MnO₂. We have found that the surface chemistry of aged LiNi_0.5Mn_1.5O₄ electrodes (free of carbon black and PVdF) involves the formation of LiF, C-F and P-F _x species. Storage of MCMB electrodes in LiPF₆ containing solutions at open circuit conditions (before their first lithiation) leads to significant morphological changes and the formation of lithium fluoride on the electrode surface, as determined by the XRD studies. LiF is probably a product of a catalytic thermal decomposition of LiPF₆. These initial changes further influence the impedance and kinetics of the lithiated electrodes.