(360q) Explaining Improvements in Li-Ion Battery Performance By Atomic Layer Deposition of Alumina Using Molecular Dynamics Simulation

Atomic layer deposition (ALD) on layered cathode oxides like LiCoO₂ and LiNi_1-y-zMn_yCo_zO₂ (NMC) has been shown to stabilize the surface and extend the cycling lifetime of these cathode active materials in lithium-ion batteries. While low cycle numbers of ALD have been experimentally shown to be beneficial to the lithium-ion cathode stability, the causes and mechanism for this stability are still elusive. We have utilized all-atom molecular dynamics simulations with accurate models for lithium Ni-Mn-Co oxides, as part of the INTERFACE force field (IFF) to study the alumina film morphology and the distribution of surface lithium through the film. We derive correlations between the number of alumina layers, the film crystallinity, the film thermodynamic stability, the lithium availability for charge transport at the film-cathode oxide interface, and the experimentally observed battery performance in terms of discharge capacity and number of ALD cycles .

Specifically, we studied alumina (Al₂O_3­) deposition in contact with a layered cathode oxide (e.g., LiCoO₂ or other NMCs) without specific chemical modifications (such as hydration or carbonation products), and we modeled the trimethylaluminum (TMA)/water ALD chemistry with the cathode oxide surface after complete reaction. IFF in the 12-6 Lennard Jones form was used to model both the alumina and the layered cathode oxide. Using molecular dynamics simulations, we studied the structure, energy, and lithium distribution for zero to six monolayers of Al₂O₃: including multiple initial structures and annealing protocols to sample the configuration space and various degrees of simulated X-Ray crystallinity of the ALD films.

We find alumina films containing two to three monolayers exhibit favorable binding energies to the layered cathode oxide surface and optimum lithium availability on the exterior of the alumina film. Alumina layer numbers outside of two to three layers are thermodynamically less favorable. Generally, the more thermodynamically stable thin film-cathode interfaces possess lower alumina film crystallinities with the exception of two layers, which is highly crystalline, while thicker films are more crystalline. The lithium density profiles through the films show that the lithium exists on either side of thin Al₂O₃ films while the lithium becomes increasingly incorporated in the bulk of thicker films.

These atomic scale findings help explain experimental data showing two to four alumina ALD cycles yield a maximum discharge capacity and longer cycling stability compared to the uncoated NMC oxide surfaces.