Remaining Li-Content Dependent Structural Evolution during High Temperature Re-Heat Treatment of Quantitatively Delithiated Li-Rich Cathode Materials with Surface Defect-Spinel Phase

Pre-extracting Li+ from Li-rich layered oxides by chemical method is considered to be a targeted strategy for improving this class of cathode material. Understanding the structural evolution of the delithiated material is very important because this is directly related to the preparation of electrochemical performance enhanced Li-rich material. Herein, we perform a high temperature reheat treatment on the quantitatively delithiated Li-rich materials with different amounts of surface defect-spinel phase and carefully investigate the structural evolution of these delithiated materials. It is found that the high temperature reheat treatment could cause the decomposition of the unstable surface defect-spinel structure, followed by the rearrangement of transition metal ions to form the thermodynamically stable phases, More importantly, we find that this process has high correlation with the remaining Li-content in the delithiated material. When the amount of extracted Li+ is relatively small (corresponding to the higher remaining Li-content), the surface defect-spinel phase could be dominantly decomposed into the LiMO2 (M = Ni, Co, and Mn) layered phase along with the significant improvement of electrochemical performance, and continuing to decrease remaining Li-content could lead to the emergence of M3O4-type spinel impurity embedding in the final product. However, when the extracted Li+ further achieves a certain amount, after the high temperature heat-treatment the Mn-rich Li2MnO3 phase (C2/m) could be separated from Ni-rich phases (including R3m, Fd3m, and Fm3m), thus resulting in a sharp deterioration of initial capacity and voltage. These findings suggest that reheating the delithiated Li-rich material to high temperature may be a simple and effective way to improve the predelithiation modification method, but first the amount of extracted Li+ should be carefully optimized during the delithiation process.