High nickel content layered oxide positive electrodes for Li-ion batteries suffer from capacity fade due to, among other factors, particle cracking induced by lithiation/delithiation, transition metal dissolution into the electrolyte, and attack from HF. Surface modification has been shown to reduce the severity of these degradation mechanisms by blocking transition metal ions from dissolving, and protecting the surface from the electrolyte decomposition products, as well as physically restraining the particles to reduce cracking. Atomic layer deposition (ALD) is a coating method that promises good uniformity and a high level of thickness control with a wide range of possible coating materials. ALD is a derivation of chemical vapor deposition in which chemical precursors are injected separately into a low pressure heated reaction vessel. Precursors are selected to create a self-limiting reaction at each step, allowing monolayers of precursor to be deposited on each cycle.¹ With a large chemical space to explore, the challenge remains to find a coating material that offers substantial protection and lifetime extension, while allowing facile Li⁺ diffusion that does not substantially impede the capacity and rate capability of the cell.

Alumina, Al₂O₃, being the archetypal ALD coating material has been frequently explored to improve the stability of known Li-ion positive electrode materials.² Recently Wang et al. found marked stability improvement of Al₂O₃ coated LiNi_0.6Mn_0.2Co_­0.2O₂ over even as few as 40 charge/discharge cycles.³

In this work, we present a comparison study of alumina coatings on LiNi_0.6Mn_0.2Co_­0.2O₂ deposited by ALD versus coatings prepared through a sol-gel method tested to 4.5 V vs Li⁺/Li (Figure 1). Depositing a range of Al₂O₃ thicknesses by ALD we see a general trend of increased capacity retention with increased coating thickness. Multiple Al-containing precursors have been investigated for sol-gel coatings and we have found varying results, some giving comparable performances to the ALD coated samples, whereas other sol-gel precursors resulted in poorer performance than the uncoated material. Both types of films have been characterised by low energy ion scattering and x-ray photoelectron spectroscopy to detect the presence of the coatings, as well as electron microscopy with energy dispersive x-ray spectroscopy to characterise the uniformity and thickness of the coatings.

1. R. L. Puurunen, J. Appl. Phys., 97, 121301 (2005).
2. L. A. Riley, S. V. Atta, A. S. Cavanagh, Y. Yan, S. M. George, P. Liu, A. C. Dillon, and S. Lee, J. Power Sources, 196, 3317–3324 (2011).
3. X. Wang J. Cai, Y. Liu, X. Han, Y. Ren, J. Li, Y. Liu, and X. Meng, Nanotechnology, 32, 115401 (2020).

Figure 1: Capacity retention after 3 formation cycles of LiNi_0.6Mn_0.2Co_0.2O₂ positive electrodes uncoated (black), Al₂O₃ coating by 50 ALD cycles (blue) and 2 wt% Aluminium tri sec-butoxide sol-gel (red). Cells were cycled at 2.5-4.5 V vs Li⁺/L at 25 °C at 1C after 3 formation cycles at C/10. The negative electrode was Li metal and the electrolyte was 1.0 M LiPF6 in ethylene carbonate : ethyl methyl carbonate 3:7 v/v with 1 wt% vinyl carbonate.