Nonuniform Growth of Sub-2 Nanometer Atomic Layer Deposited Alumina Films on Lithium Nickel Manganese Cobalt Oxide Cathode Battery Materials

The deposition of alumina ALD films on Li ion battery cathode particles is known to enhance the cycling stability of lithium ion batteries fabricated from those coated particles. It is commonly assumed that the film on the particles is of uniform thickness and is optimally thin enough to facilitate lithium diffusion while blocking side reactions of the electrolyte with the cathode substrate. Here, we elucidate the nature of thin alumina films deposited with between 2 and 15 ALD cycles on lithium nickel manganese cobalt oxide cathode precursor particles. Low energy ion scattering (LEIS) and secondary ion mass spectroscopy (SIMS) methods were used to characterize thin (<2 nm) films that were deposited by ALD. Surface analysis showed that low-cycle number ALD films were not uniform nor uniformly thick over the surface of the cathode particles and that alumina ALD preferentially deposited on transition metal bound sites on the cathode particle surface and coated Li on the surface to a lesser extent. Lithium was found to still be present on the cathode powder surface, even after 10 ALD cycles. Contrary to current supposition, low-cycle ALD appeared to improve the cycling stability of battery cathode active materials through this preferential growth that stabilized the transition metal oxides in the presence of electrolyte without blocking lithium intercalation pathways. This is the first study to determine that Li remains exposed on the as-synthesized surface of ALD coated cathode particles and that the ALD film is nonuniform and nonuniformly thick when less than 10 ALD cycles are used.