Effect of Al₂O₃ Coating on Stabilizing LiNi_0.4Mn_0.4Co_0.2O₂ Cathodes

Using atomic layer deposition of Al₂O₃ coating, improved high-voltage cycling stability has been demonstrated for the layered nickel–manganese–cobalt pseudoternary oxide, LiNi_0.4Mn_0.4Co_0.2O₂. To understand the effect of the Al₂O₃ coating, we have utilized electrochemical impedance spectroscopy, operando synchrotron-based X-ray diffraction, and operando X-ray absorption near edge fine structure spectroscopy to characterize the structure and chemistry evolution of the LiNi_0.4Mn_0.4Co_0.2O₂ cathode during cycling. Using this combination of techniques, we show that the Al₂O₃ coating successfully mitigates the strong side reactions of the active material with the electrolyte at higher voltages (>4.4 V), without restricting the uptake and release of Li ions. The impact of the Al₂O₃ coating is also revealed at beginning of lithium deintercalation, with an observed delay in the evolution of oxidation and coordination environment for the Co and Mn ions in the coated electrode due to protection of the surface. This protection prevents the competing side reactions of the electrolyte with the highly active Ni oxide sites, promoting charge compensation via the oxidation of Ni and enabling high-voltage cycling stability

Electrochemical Performance of Nanosized Disordered LiVOPO₄

ε-LiVOPO₄ is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO₄, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by ⁷Li and ³¹P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO₄ at C/50 (1 C = 153 mA h g^–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO₄/Li₃VO₄/acetylene black composite yields the high cycling capacity of 250 mA h g^–1 at C/5 for over 70 cycles