(47b) Investigating Li-Ion Behavior in ALD Coated NMC Cathode Materials Via Molecular Dynamics

High nickel content LiNi_1-y-zMn_yCo_zO₂ (NMC) has emerged as a cathode capable of delivering high capacities in Li-ion batteries. In particular, high-nickel NMCs are being used commercially in electric vehicles (EVs) and are projected to comprise over a third of EV cathode chemistries by 2026.¹ However, high-nickel cathodes experience structural instability with the charge/discharge cycling of the battery, which leads to large capacity fading, voltage decay, and low rate capability of the overall battery. Atomic layer deposited (ALD) lithium-containing nano-coatings of NMC cathodes have been shown to help stabilize the transition metal sites while still allowing the movement of Li-ions across the coating, leading to greater capacity retention of the Li-ion battery.²

Our prior experimental work has demonstrated that alumina particle ALD produces a less than two nanometer, mixed Al₂O₃ – LiAlO₂ film that preferentially covers the transition metals of the cathode powder while leaving lithium relatively uncovered and presumably available for transport. Expanding upon this work, molecular dynamics simulations were used to investigate the behavior of surface lithium of uncoated cathodes and ALD coated cathodes. The Interface Force Field in the 12-6 Lennard Jones form was used to model the pairwise interactions.³ Both amorphous and crystalline films were simulated at 393 K, with the crystalline film being used as a comparison to the amorphous, ALD-like one. The thickness and uniformity of the ALD coating affects the surface lithium movement. For example, lithium is able to completely travel through one simulated Al₂O₃ layer, while it cannot for two or more simulated Al₂O₃ layers. Further, the energy of the interface between the cathode and coating continues to decrease (i.e., become more favorable) as the simulated coating thickness increases until the energy plateaus off and is about the same for all later coating thicknesses. At this energy plateau, the representative energy of deposition for Al₂O₃ on LiCoO₂ is -1760 mJ/m² for the amorphous film and -2180 mJ/m² for the crystalline film.

By 2030, electric vehicles are projected to comprise almost 30% of new car sales globally. A protective nano-coating on the cathode that preserves battery capacity should help hasten the global transition from combustion vehicles to EVs.

References:

(1) Frith, James. Battery Prices and Manufactured Costs (Highlights from Bloomberg NEF EVO Report 2020), 2021.

(2) Hoskins, A. L.; McNeary, W. W.; Millican, S. L.; Gossett, T. A.; Lai, A.; Gao, Y.; Liang, X.; Musgrave, C. B.; Weimer, A. W. Nonuniform Growth of Sub-2 Nanometer Atomic Layer Deposited Alumina Films on Lithium Nickel Manganese Cobalt Oxide Cathode Battery Materials. ACS Appl. Nano Mater. 2019, 2 (11), 6989–6997. https://doi.org/10.1021/acsanm.9b01490.

(3) Dhakane, A.; Varshney, V.; Liu, J.; Heinz, H.; Jain, A. Molecular Dynamics Simulations of Separator-Cathode Interfacial Thermal Transport in a Li-Ion Cell. Surf. Interfaces 2020, 21, 100674. https://doi.org/10.1016/j.surfin.2020.100674.