Measuring Oxygen Release from Delithiated LiNi_xMn_yCo_1-x-yO₂and Its Effects on the Performance of High Voltage Li-Ion Cells

There can be a trade-off between the lifetime and energy density of LiNi x Mn y Co 1-x-y O 2 (NMC)-containing cells that depends on their upper cutoff voltage. This work applies thermogravimetric analysis coupled with mass spectrometry (TGA-MS) to measure the release of oxygen from delithiated NMC electrode materials at high electrode potentials, i.e., low lithium content. This release is observed at relatively mild temperatures as low as 40°C. The amount of oxygen released is greatly limited using single crystal NMC particles. Electrochemical measurements demonstrate a correlation between TGA-MS results and the cycling performance of NMC/graphite cells. X-ray photoelectron spectroscopy complements the TGA-MS results and provides evidence of the solid electrolyte interphase decomposition. The results in this work offer strong support that the release of oxygen from NMC can cause oxidative decomposition of the electrolyte and is a major reason why high voltage cells can generate gas and can have poor capacity retention.