(126e) Ultrathin: Understanding the Nanostructure of Alumina Atomic Layer Deposited Films on Layered Cathode Oxides

Atomic layer deposition (ALD) is a surface deposition technique that has been shown on layered transition metal oxides, such as LiCoO2 and LiNixMnyCo1-x-yO2, to improve the cycling stability and reduce the capacity fade of the resulting Li-ion batteries. In particular, thin (< 2 nm thick) alumina ALD films have been experimentally shown to be beneficial, with evidence that a Li-Al-O film is formed. Since the cathode surface forms an interface with electrolyte in a battery, it is important for this surface to promote a stable interface. ALD modifies the transition metal oxide surface. Our present work investigates alumina ALD on layered transition metal oxides and seeks to address why a Li-Al-O film is formed and how it is beneficial to the cathode oxide. In this work, we employed molecular dynamics (MD) simulations to explore the nanostructure of a Li-Al-O film in contact with a layered cathode oxide.

The length scale of MD simulation is on the order of nanometers, which matches the length scale of the ALD films investigated. The Li-Al-O film in contact with a layered cathode oxide was simulated by bringing an alumina surface (1-10 monolayers thick) into contact with a LiCoO2 surface and performing a sequence of annealing steps in the NVT ensemble (where the number of moles, the volume, and temperature are held constant) to generate disordered Li-Al-O films. System properties such as film density, energy of Li inclusion, element mass densities in the z direction, and atom coordination number were obtained from subsequent NVT production runs. The Interface Force Field in the 12-6 Lennard Jones form was used to model both the alumina and the lithium cobalt oxide.

The simulated film density agreed with the reported experimental density of alumina ALD films. Element mass density profiles in the z direction revealed layering of Al and O close to the LiCoO2 surface. This layering eventually transitioned to an isotropic disordered structure that agreed with conventional understanding that ALD alumina films are amorphous. Aluminum atoms with coordination number of four were the predominant species in the film bulk.

Our present work provides atomic insight into the experimentally observed Li-Al-O film. This work helps inform the understanding o f the protective mechanism of alumina ALD films on layered transition metal oxides for Li-ion battery applications.