Na_(2)PO_(3)F对LiNi_(0.83)Co_(0.11)Mn0.06O_(2)材料的复合改性及机理分析

Dual-Decoration and Mechanism Analysis of Ni-rich LiNi_(0.83)Co_(0.11)Mn_(0.06)O_(2) Cathodes by Na_(2)PO_(3)F

采用湿化学法使用Na_(2)PO_(3)F对LiNi_(0.83)Co_(0.11)Mn_(0.06)O_(2)进行表面改性,得到F-掺杂和LiF包覆的正极材料.X射线衍射谱(XRD)结果显示(003)衍射峰向高角度偏移,结合X射线光电子能谱(XPS)及透射电子显微镜(TEM)证明F^(-)进入到材料晶格内部;扫描电镜(SEM)、TEM及XPS结果显示,改性后材料表面存在均匀LiF包覆层,可提高电极/电解液界面稳定性,改善循环稳定性;通过计算锂离子扩散系数,证明Li^(+)传输速率得到提升,倍率性能改善.电化学性能测试结果显示,材料的循环稳定性和倍率性能均得到显著提高:在2.75~4.3 V电压窗口下,材料1 C循环200周后容量保持率由32.2%提高到65.2%,10 C条件下放电比容量由145.7 mAh/g提高到161.5 mAh/g.对循环后极片进行XPS分析,正极-电解质界面(CEI层)层中的LiF,Li_(x)PO_(y)F_(z),NiF_(2)减少,有利于提高材料稳定性及循环性能.

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 ma terials 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 Na_(2)PO_(3)F was carried out to modify the Ni-rich LiNi_(0.83)Co_(0.11)Mn_(0.06)O_(2) material at surface and bulk regions by using wet meth-ods.By virtue of the fact that Na_(2)PO_(3)F dissolves in water and releases fluorine ions,a F^(-)doped and LiF coated Ni-rich LiNi_(0.83)Co_(0.11)Mn_(0.06)O_(2) 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 spectros-copy(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_(x)PO_(y)F_(z),NiF_(2) 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.