Y-Element Doping Improves the Electrochemical Performance and Structural Stability of Single-Crystal Lini0.8co0.1mn0.1o2 Cathode

LiNi0.8Co0.1Mn0.1O2 (NMC811), a common cathode material for lithium-ion batteries, has received widespread attention for its superior performance in terms of high capacity and high energy density. However, polycrystalline NMC811 materials are prone to secondary particle rupture during long-term cycling, leading to the occurrence of problems such as rapid capacity decay and poor stability. Single-crystal NCM811 has eliminated the occurrence of secondary particle crushing problems. However, since single-crystal materials have larger grain sizes, the ion transport path inside the crystal is longer, resulting in a slower ion diffusion rate inside the material, which is one of the main reasons why the capacity of single-crystal is always smaller than that of polycrystal at the conventional cut-off voltage (4.3V). It is an effective way to improve the capacity of single-crystal materials by increasing the voltage of charge/discharge to increase the diffusion rate of li ions. However, high cut-off voltages can lead to excessive Li+ detachment at the edge of the crystal, which in turn can easily cause Li layer collapse and impede the internal Li entry and exit, eventually causing irreparable and permanent losses. In this paper, we propose to improve the stability of NMC811 single crystal cathode material under high cut-off voltage by Y doping, which occupies the transition metal sites of the material and forms high bonding energy Y-O bonds, improving the stability of the overall structure of the crystal, so that the Li layer can remain stable and not collapse when highly delithiated. However, excessive doping will cause the reduction of lattice spacing due to too many Y-O bonds, which will hinder 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 after extensive delithiation. The capacity retention of the optimized 0.5% Y-doped sample reached 94.53% after 100 cycles at 2.7 ~4.5 V voltage window, while the capacity retention of the original sample was only 81.25%. The experimental results show that this modified material has better stability and higher capacity retention under long cycles. In this study, the crystal structure of NMC811 was modulated by Y-doping to improve its stability at high cut-off voltage, which provides a new idea for the performance tuning of high nickel single-crystal materials. This work provides a new strategy and method for finding better electrode materials and improving the performance of Li-ion batteries.