Dual functions of gradient phosphate polyanion doping on improving the electrochemical performance of Ni-rich LiNi0.6Co0.2Mn0.2O2 cathode at high cut-off voltage and high temperature

摘要

In this paper, a gradient phosphate polyanion doping strategy is applied to enhance the electrochemical properties of LiNi0.6Co0.2Mn0.2O2 cathode. This strategy synergistically achieves the gradient doping of phosphate polyanion and in-situ coating of Li3PO4 layer. The gradient doping improves the cycling stability and rate performance of LiNi0.6Co0.2Mn0.2O2. Especially, the capacity retention of P0.02-NCM sample is 92.9% at high cut-off voltage (4.5 V) and high temperature (55 degrees C), whereas the LiNi0.6Co0.2Mn0.2O2 sample only shows lower capacity retention of 55.7%. The X-ray powder diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR) results confirm that the large tetrahedral PO43- polyanions are doped into oxygen layer for stabilizing the lattice structure. In addition, scanning transmission electron microscopy (STEM) and inductively coupled plasma (ICP-MS) analysis show that the Li3PO4 layer availably inhibits the dissolution of transition metal ions (Ni, Co, Mn). Interestingly, the Li3PO4 coating layer as a fast ion conductor also optimizes the Li+ thorn diffusion coefficient. Such excellent results indicate that the surface gradient phosphate polyanion doping strategy is very valuable and useful to remarkably enhance the electrochemical properties of LiNi0.6Co0.2Mn0.2O2 cathode at high cut-off voltage (4.5 V) and high temperature (55 degrees C). (C) 2019 Elsevier Ltd. All rights reserved.