Gradient doping of phosphate in Ni-rich LiNi0.6Co0.2Mn0.2O2 (NCM) and P0.02-nrNCM: A Li-ion battery cathode with enhanced electrochemical performance at high cut-off voltage

In this study, Ni-rich LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) with phosphate (PO₄^3 −) gradient doping (P_0.02-nrNCM) was introduced as a cathode material for high-performance Li-ion batteries (LIBs). The P_0.02-nrNCM modified via gradient doping with PO₄³⁻ exhibited reduced primary particle growth owing to controlled particle growth, leading to the formation of a Li₃PO₄ layer on P_0.02-nrNCM particles. The morphological, chemical, and physical properties of P_0.02-nrNCM at various preparation stages were characterized via scanning electron microscopy (SEM), cross-sectional SEM, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. The P_0.02-nrNCM exhibited enhanced EC performance (reversible capacity: 166.07 mAh/g, capacity retention: 92.4 % after 100 cycles) at 1 C (180 mAh/g) under 3.0–4.5 V, superior to that of the pristine material (reversible capacity: 156.65 mAh/g, capacity retention: 79.1 %). The improved EC performance of P_0.02-nrNCM, as confirmed by the results of electrochemical impedance spectroscopy and galvanostatic intermittent titration technique, was attributed to the enhanced Li-ion mobility resulting from the reduced primary particle size, unified growth direction of particle, and formation of Li₃PO₄ layer on the P_0.02-nrNCM particles. The enhanced ion mobility of P_0.02-nrNCM due to gradient doping with PO₄³⁻ ensured long-term EC stability and capacity retention of the Ni-rich NCM cathodes (P_0.02-nrNCM). These results suggest that the method proposed in this study is a promising approach for enhancing the EC performance of P_0.02-nrNCM in high-voltage LIB applications.