(644a) Improved Understanding of the Role of Ultra-Thin ALD Films on Lithium Metal Oxide Cathode Materials

The surface engineering of Li-ion battery cathode materials has been of interest for over a decade. Large capacity fading, voltage decay and low rate capability are observed upon cycling of many otherwise promising cathode materials. These failures stem from dissolution of metals into liquid electrolytes and structural instability of cathode materials caused by lattice strain induced by Li+ intercalation upon each discharge. In general, surface coatings act as a protective layer to prevent direct contact of the electrochemically active cathode material with the electrolyte and thereby decrease the dissolution of transition metal ions. Additionally, these coatings suppress phase transitions, improve structural stability and decrease disorder of cations in crystalline lattices. As a result, the surface modification improves the structural stability of cathode materials with minimal loss of available specific capacity compared to other coating methods that deposit significantly more material. Consequently, marked improvement of the electrochemical performance of coated electrode materials includes their coulomb efficiency in the first cycle, cycling behavior, rate capability, and overcharge tolerance.

The deposition of < 6 alumina ALD nanofilms on Li ion battery cathodes enhances the cycling stability of lithium ion batteries. It is commonly theorized that ALD results in a uniform film that optimally is thin enough to facilitate lithium diffusion while blocking side reactions of the electrolyte with the cathode material. Here, we elucidate the true nature of low-cycle number ALD films on lithium nickel manganese cobalt oxide (NMC) cathode materials. Cathode particles were coated with alumina ALD films of 2,4,6,8,10, 12, and 15 cycle numbers, and then studied using EDS mapping, HRTEM/FIB milling, XPS, low energy ion scattering (LEIS) and secondary ion mass spectroscopy (SIMS). Surface analysis shows that low-cycle ALD films are not uniform over the surface of the NMC particles and that alumina ALD preferentially deposits on transition metal sites on the cathode surface and coats Li on the surface to a lesser degree, leaving some Li exposed. Contrary to current supposition, low-cycle ALD improves the cycling stability of battery cathodes through this preferential growth that stabilizes the transition metal oxides in the presence of electrolyte without blocking lithium intercalation pathways. The ALD film coating on the surface for < 6 ALD cycles is not uniform, but better represents a film that is nucleating on substrate sites prior to forming a uniform film. This is expected. The unexpected result is that the film preferentially coats transition metal sites to a greater degree than the coating of Li sites. An environment has been created that facilitates the preferential ALD growth of Al2O3 on Mn, Co, and Ni.