Investigating Surface Sensitivity of Ni-Rich Cathode Material Towards CO2 and H2O

Abstract

Layered Ni-rich transition metal oxide materials have been considered as the most promising cathode utilized in Li-ion batteries, e.g., LiNi0.8 Mn0.1 Co0.1 O2 (NMC 811). However, one of the drawbacks of NMC 811 is its high air sensitivity, leading to a degradation layer forming on the surface, and a lower cycling performance. Since the degradation mechanism is not fully understood, in this work, we use ambient pressure photoelectron spectroscopy (APPES)1 to investigate the surface sensitivity of NMC 811 towards CO2 and H2O in situ, aiming to determine the factor triggering the degradation. Before gas exposure, NMC 811 surface was Ar+ sputtered to remove the pre-existing boron oxide coating. The changes in surface chemical composition were monitored as a function of time and gas pressure. Results show that carbonate compounds are formed on the surface when NMC 811 is exposed to CO2. The carbonate compounds start to appear already within 2 minutes in CO2 at around 10-5 mbar. More interestingly, this reaction is reversed at around 3x10-6 mbar. Changes induced by water are slower, however, the water vapour effect on NMC 811 surface is irreversible. Results indicate that lithium hydroxide is formed, where Li+/H+ exchange on the surface is a possible route.2 Kokkonen, E.; Lopes Da Silva, F.; Mikkelã, M.-H.; Johansson, N.; Huang, S.-W.; Lee, J.-M.; Andersson, M.; Bartalesi, A.; Reinecke, B. N.; Handrup, K.; Tarawneh, H.; Sankari, R.; Knudsen, J.; Schnadt, J.; Såthe, C.; Urpelainen, S., Upgrade of the SPECIES beamline at the MAX IV Laboratory. Journal of Synchrotron Radiation 2021, 28 (2), 588-601.Hartmann, L.; Pritzl, D.; Beyer, H.; Gasteiger, H. A., Evidence for Li+/H+ Exchange during Ambient Storage of Ni-Rich Cathode Active Materials. Journal of The Electrochemical Society 2021, 168 (7), 070507. Figure 1