Systematic study on anisotropic lattice volume changes in Ni-rich layered oxides: Role of doping and gradient coating

Abstract

The Ni-rich layered oxides in lithium -ion batteries undergo structural breakdown due to the anisotropic lattice volume changes during charge/discharge, which is one of the major factors that degrade electrochemical performance. However, a thorough understanding of the lattice anisotropic volume change caused by stability issues, poor cycle performance and thermal instability is lacking, and research to improve this phenomenon is still ongoing. To address this problem, experiments are conducted to suppress the anisotropic lattice volume change by gradient-Al2O3 coating and triple-doped (Na+, Br-, Al3+) Ni-rich cathode (NBA). Furthermore, the mechanism of anisotropic lattice volume change suppression is systematically and thoroughly studied by calculating the c and a lattice difference, the volume change, and the Li slab change during delithiation using DFT calculation. As a result of the experiment, the gradient coating and doping are confirmed through Rietveld refinement analysis based on XRD and SEM-EDS analysis. NBA not only showed that the volume change of the anisotropic lattice is suppressed by ex-situ XRD analysis according to the charging voltage, but also that cracks inside the particles are less generated after cycles. Therefore, the NBA demonstrates higher capacity retention (84.09 %) than Pristina (76.4 %) after 50 cycles at 1C. The systematic study of gradient coating and triple doping provides insight into advanced cathode design strategies for LIBs that mitigate structural degradation caused by long-term cycling.