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

Layered Ni-rich transition metal oxide materials have been considered as the most promising cathode utilized in Li-ion batteries, e.g., LiNi 0.8 Mn 0.1 Co 0.1 O 2 (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 CO 2 and H 2 O 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 CO 2 . The carbonate compounds start to appear already within 2 minutes in CO 2 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