Studying the Diffusion of Al3+ ions from Atomic Layer Deposited Al2O3 Surface Coatings into Positive Electrode Materials and the Effects Thereof Using Full Coin Cells

Abstract

Increasing the energy density of lithium-ion batteries (LIBs) is an important goal for battery research for electric vehicle and grid storage applications. One method of doing this is to use a protective coating on the positive electrode material to protect against electrode/electrolyte reactions that occur as the charging voltage of the cells is increased, which increases energy density. This work develops methods to prepare full coin cells and uses these methods to examine the effectiveness of Al2O3 coatings prepared by atomic layer deposition (ALD). Two coating thicknesses on two positive electrode materials, LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Co0.15Al0.05O2 (NCA801505), were examined. Samples annealed to various temperatures were characterized using scanning electron microscopy (SEM), 27Al magic angle spinning solid-state nuclear magnetic resonance spectroscopy (ssNMR), and X-ray photoelectron spectroscopy (XPS). ssNMR on the thick-coated NMC series suggests Al3+ diffusion into the NMC material occurs at annealing temperature ≥ 400 ̊C. However, XPS results only show signs of Al3+ diffusion at ≥ 600 ̊C for the thin-coated material and ≥ 700 ̊C for the thick-coated material. This apparent disagreement occurs because the Al2O3 coatings are thicker than the XPS measurement depth and so the initial diffusion from the coating is not visible to XPS. As the annealing temperature increases, so does the diffusion rate of the Al2O3 layer into the NMC. Thus, the coating layer thickness is reduced below the XPS measurement depth at higher temperatures. Full coin cells prepared from the ALD- coated materials show that for both NMC and NCA materials with the thicker Al2O3 coating, annealing to 600°C provides the best cycling results.