Temperature dependence of oxygen release from LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) cathode materials for Li-ion batteries

Promising cathode materials for Li-ion batteries are layered transition metal oxides (LiNi_xMn_yCo_zO₂, NMC). Here, we will investigate the temperature dependence of oxygen release from NMC622, caused by the transformation of the near-surface structure from the layered to spinel and/or rock-salt structure. We will demonstrate that oxygen release is not a potential driven process but occurs once ~81% of the lithium ions are removed from the NMC structure. Consequently, the onset potential for oxygen release in NMC-graphite cells decreases only by ~60 mV from 4.42 V at 25°C to 4.36 V at 50°C, which is simply due to lower overpotentials at higher temperature. The amount of evolved oxygen increases significantly with increasing temperature, indicating the formation of thicker spinel/rock-salt surface layers. As the released oxygen causes chemical oxidation of the electrolyte, the amounts of CO₂ and CO occurring simultaneously with O₂ release also increase with temperature. Further experiments in NMC-Li cells as well as with ¹³C-labelled ethylene carbonate (EC) electrolyte show that CO₂ evolved prior to O₂ release results from i) EC hydrolysis and ii) electrolyte impurity oxidation. In agreement with the onset potentials for oxygen release, we will show that stable cycling of NMC622-graphite full-cells is possible at the different temperatures up to ~81% state-of-charge.