Y-Element Doping Improves Electrochemical Performance and Single-Crystal Structural Stability of Cathode LiNi0.8Co0.1Mn0.1O2

As a typical Ni-rich cathode material for lithium-ion batteries, LiNi0.8Co0.1Mn0.1O2 (NCM811) shows exceptional performance in terms of large capacity and high energy density and has attracted great attention. Compared with polycrystalline NCM811 materials, single-crystalline NCM811 materials effectively eliminated secondary particles, which caused capacity decay and poor stability. However, due to large grain sizes of single crystals, the ion transport path alongside the cathode is long, resulting in a slow ion diffusion rate during the charge–discharge cycles, leading to lower capacity than that of polycrystals. It is an efficient method for raising the capacity of single-crystal materials by altering the charge/discharge voltages to enhance the diffusion rate of Li ions. However, high cutoff voltages lead to excessive Li+ detachment on the grain boundary of the crystal, which in turn can easily cause layer collapse and impede the transportation of Li+, eventually causing irreparable and permanent losses. In this study, Y-element doping was adapted to improve the stability of NCM811 single-crystal cathode materials at high cutoff voltage. Y–O bonds with high binding energy were formed, and the location of the transition metal changed. By this means, the stability of the crystal structure could be improved, but excessive Y doping caused the reduction of lattice spacing, which hindered the diffusion of Li+. Through a series of characterization analyses, it was found that 0.5% Y doping could retain a large lattice spacing and maintain the stability of the lattice Li layer. The capacity retention of the optimized 0.5% Y sample remained at 94.53% after 100 cycles at the voltage window of 2.7–4.5 V, while the capacity retention of the original sample was only 81.25%. The experimental results show that the Y-doped single-crystal NCM811 materials have better stability and higher capacity retention under long cycles. The crystal structure was regulated by Y doping in this work, which gives a notion of modifying the performance of Ni-rich single-crystal materials. This research presents a method for identifying electrode materials and increasing the performance of Li-ion batteries.