Storage of Nickel-Rich Layered Oxide Cathode Active Materials under Humid Air with or without CO2: Effect of H+ Intercalation and Surface Contaminants on Cycle Life

Abstract

Exposure of cathode active materials (CAMs) to ambient air is particularly detrimental for Ni-rich CAMs, leading to accelerated performance degradation. Upon storage, H2O and CO2 from ambient air react with the CAM, leading to a build-up of contaminants and the formation of a protonated phase (HMO2). Here, their formation mechanism is elucidated for the room-temperature storage of a Ni-rich NCM (LiNi0.94Co0.03Mn0.03O2) and LNO (LiNiO2) under humidified air (50% relative humidity at 25 °C), with and without CO2 (0, 500 and 1500 ppm). X-ray photoelectron spectroscopy reveals that without CO2, surface reactions are self-limiting, whereas with CO2, a continuous contaminant build-up alongside a reduction in nickel oxidation state is observed. Quantification of the HMO2 phase and surface contaminants via thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and acid–base titration reveals distinct pH-driven mechanisms, highlighting the criticality of CO2. Electrochemical charge/discharge cycling of stored LNO|graphite full cells with a Li-RE at 25 °C reveals that a humid and CO2-free atmosphere does not negatively affect cycle life, while the presence of CO2 results in loss of cyclable CAM and inferior cycling performance. These results reveal the detrimental impact of CO2 in combination with H2O as the driving force of ambient-air degradation in Ni-rich CAMs.