Insight into the Carbonate Formation of Ni Rich NMC Cathode Materials with CO2

The state-of-the-art high Ni LiNi_xCo_yMn_zO₂ (NMC) cathode materials suffer from a high surface reactivity issue in ambient environments and their sensitivity to CO₂ and H₂O remains an open question. We combine the experimental technique of transient kinetics with density functional theory calculations to study the reaction of CO₂ with NMC cathode materials with different ratios of nickel content (x:y:z =8:1:1(NMC811), 6:2:2 (NMC622), 5:3:2 (NMC532), 4:3:3 (NMC433), 1:1:1 (NMC111)) to investigate the reactivities. At 25℃, significant CO₂ conversion was observed without any C containing gas phase product formation on all materials excluding NMC111 in a temporal analysis products (TAP) reactor indicating that CO₂ adsorption is an irreversible process. The amount of CO₂ uptake follows the order: NMC532 > NMC433 > NMC622 > NMC811 > 111. Temperature programmed desorption experiments after CO₂ adsorption showed the CO₂ desorption temperature follow a different order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111.

CO₂ adsorption was studied theoretically with the PBE+U and HSE06 functionals. Carbonate formation is an exothermic process leading to ion pair interactions with two O atoms bonded to a Li and a transition metal and the C atom bonded to a surface O. The overall reactivity of CO₂ characterized by the binding strength follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111, and is dependent on the charge of oxygen with the oxygen with the less negative charge being more reactive. The surface capacity for CO₂ uptake is dependent on both transition metal and oxygen as the most stable adsorption sites follow the order of Mn > Co > Ni. Although it is generally thought that the more Ni content, the more sensitive it is to the ambient environment, our results show that the sensitivity is complicated and is dependent on other factors in addition to CO₂ adsorption.