Synergistic Effect of F^- Doping and LiF Coating on Improving the High-Voltage Cycling Stability and Rate Capacity of LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Materials for Lithium-Ion Batteries

A commercial LiNi_0.5Co_0.2Mn_0.3O₂ (LNCM) cathode material is purposefully modified using a small account of LiPF₆ as one precursor via a simple means at low calcination temperature in air. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy images reveal that this modification process keeps the layered bulk structure of LNCM even though the surface components have obviously been modified. Electron energy loss spectroscopy and X-ray photoelectron spectroscopy with different etching depths further prove the formation of LiF and F^- doping on the LNCM surface, which simultaneously triggers partial Ni³⁺ reduction to Ni²⁺ and the metal-oxygen bond is partially replaced by a higher energy metal-fluorine bond. The modified material (LNCM-2) retains 93.7% of its initial capacity and delivers 179.4 mAh g^-1 at a current density of 0.5 C after 100 stable cycles at 3.0-4.5 V. Meanwhile, LNCM-2 is able to maintain capacity retention up to 81.1% after 300 cycles at 5 C, much better than the original LNCM (35.1%) in the commercial electrolyte. Remarkably, 90% of initial capacity is retained for LNCM-2 with considerably improved Coulombic efficiency (>99.5%) at 5 C after 300 cycles within a voltage range of 3-4.5 V compared with the primary LNCM using succinonitrile-based electrolyte. Consequently, these results fully demonstrate the advantages of synergistic effect between F^- doping and LiF coating.