Investigating Surface Reactivity of a Ni-Rich Cathode Material toward CO₂, H₂O, and O₂ Using Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract

High Resolution Image Download MS PowerPoint Slide Layered Ni-rich transition metal oxide materials are considered the most promising cathodes for use in commercial Li-ion batteries. Due to their instability in air, an impurity layer forms during storage under ambient conditions, and this layer increases electrochemical polarization during charging and discharging, which ultimately leads to a lower cycling capacity. In this work, we found that storage of the LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC 811) material in ultrahigh vacuum (UHV) can restore the surface by reducing the amount of native carbonate species in the impurity layer. In this work, in situ soft X-ray ambient pressure photoelectron spectroscopy is used to directly follow the interaction between common gases found in air and the NMC 811 surface. During gas exposure of the NMC 811 surface to pure CO 2, O 2, and a mixture of both pure gases, surface-adsorbed CO 2 or/and O 2 were detected; however, permanent changes could not be identified under UHV after the gas exposure. In contrast, a permanent increase in metal hydroxide species was observed on the sample surface following H 2 O vapor exposure, and an increased intensity in the carboxylate peak was observed after exposure to a mixture of CO 2 /O 2 /H 2 O. Thus, the irreversible degradation reaction with CO 2 is triggered in the presence of H 2 O (on relevant time scales defined by the experiment). Additional measurements revealed that X-ray irradiation induces the formation of metal carbonate species on the NMC 811 surface under CO 2 and H 2 O vapor pressure.