Influence of vanadium compound coating on lithium-rich layered oxide cathode for lithium-ion batteries

Abstract

A vanadium compound is applied as a coating material to improve the electrochemical performance of the lithium-rich layered oxide Li_1.2Mn_0.6Ni_0.2O₂. The physicochemical properties of the material before and after coating are characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (FT-IR). Results reveal that Li_δV₂O₅ (δ is very small) is successfully coated on the as-prepared material, and the crystal properties of the powder have been modified after coating. The formation of the Li_δV₂O₅ coating layer is a result of some Li-ions diffusing from the Li_1.2Mn_0.6Ni_0.2O₂ particle to the coating layer at the interface. The material before and after coating serve as the cathode for lithium-ion batteries and were investigated by galvanostatic measurements within a voltage range of 2.0–4.8 V (vs. Li/Li⁺). The initial coulombic efficiency (CE1) of Li_1.2Mn_0.6Ni_0.2O₂ is improved from 71.8% to 87.7% due to the Li_δV₂O₅ coating layer, which can act as an insertion host to accept the lithium ions that could not be inserted back into the bulk lattice during the first discharge process. Additionally, the electrochemical performances (cycling performance and rate capability) of the modified Li_1.2Mn_0.6Ni_0.2O₂ are very superior to the pristine one. The significantly improved electrochemical performances are attributed primarily to: (i) the modified crystal properties after coating; (ii) the amelioration of the charge-transfer resistance after coating; (iii) the coating layer which can contribute to stabilizing the electrode surface by suppressing the side reactions between electrode and electrolyte.