Na₂PO₃F对LiNi_0.83Co_0.11Mn_0.06O₂材料的复合改性及机理分析

Since the commercialization of lithium-ion battery in 1991, it has promoted human society development for nearly three decades. Due to its higher energy density, Ni-rich layered oxides currently stand out as the most promising cathode materials to build power batteries for portable electronic devices and new energy electric vehicles. However, the severe side effects on the electrode/electrolyte interface and the structural instability of the material hinder its development, and a lot of research focus on improving the cycling stability and rate capability of nickel-rich cathode material. Here, a facile treatment with Na₂PO₃F was carried out to modify the Ni-rich LiNi_0.83Co_0.11Mn_0.06O₂ material at surface and bulk regions by using wet methods. By virtue of the fact that Na₂PO₃F dissolves in water and releases fluorine ions, a F^- doped and LiF coated Ni-rich LiNi_0.83Co_0.11Mn_0.06O₂ material was obtained. The X-ray diffraction (XRD) results showed that (003) peak shifted to a higher angle due to the replacement of O^2- by smaller F^-. Besides, the XRD Rietveld refinement and X-ray photoelectron spectroscopy (XPS) sputtering data determined that part of F^- ions had been successfully doped into the lattice, and the basic layer structure of the material was still well preserved in the process of modification. Scanning electron microscope (SEM), energy disperse spectroscopy (EDS), and transmission electron microscope (TEM) combined with XPS proved the existence of surface LiF coating layer. The 2~10 nm LiF layer was uniformly covered onto the surface of the cathode material, acting as a physical barrier against direct corrosion from the electrolyte, thus enhancing the cycling stability. In addition, the lithium ion diffusion coefficients (D_Li⁺) for bare and modified samples were calculated from cyclic voltammetry test results, which reveals a better rate capability from the synergistic effect of surface LiF coating and lattice F^- doping. The electrochemical tests also showed a better cycling stability and enhanced rate capability of the modified samples: within 2.75~4.3 V, the capacity retention after 200 cycles at 1 C-rate had been elevated from 32.2% to 65.2%; the discharge capacity under 10 C-rate was also improved from 145.7 to 161.5 mAh/g. To elaborate the improved effect, post-cycling characterizations were conducted. The morphology of modified particles after 200 cycles at 1 C still maintained intact grains, in contrast with the bare samples, exhibiting many micro-cracks. XPS spectra on decorated cathodes after cycling showed weaker signals of by-products including LiF, Li_xPO_yF_z, NiF₂ in the CEI layer, indicating side reaction on the interface was effectively suppressed, thus contributing to enhancing the cycling stability. The applicable method herein demonstrates a promising solution for improvement of Ni-rich cathode materials and their coming commercial application.