Synthesis,Modification And Properties Of Ni-rich LiNi_0.85Mn_0.15O₂ Cathode Material

Ni-rich cathode materials have attracted much attention due to their high specific capacity,good cost performance and environmental friendliness.However,there are still problems such as rapid capacity decay and poor cycle performance at high voltages,which limit their commercial applications.In recent years,due to the scarcity and high price of Co resources,low-cobalt and cobalt-free Ni-rich cathode materials have become the frontier and hotspot of cathode material research.How to maintain the stable layered structure of such cathode materials under cobalt-free conditions and improve.Its lithium storage performance is a problem that must be solved in the future.In this paper,through the research on the controllable synthesis of high nickel binary cathode material LiNi_0.85Mn_0.15O₂,regulation of p H,lithium excess ratio,and sintering temperature,good performance conductive graphite（KS-6）with Zr-doped and co-modified KS-6,Mg,Zr co-modified non-cobalt and high-nickel binary cathode materials were prepared.Such cathode materials were characterized by XRD,SEM,EDS,TEM,XPS,ICP,etc.and the corresponding electrochemical test analysis.The main contents are as follows:（1）Synthesis regulation and lithium storage performance of high nickel LiNi_0.85Mn_0.15O₂cathode material.Firstly,the synthesis method was discussed,and the hydrothermal method,microwave-assisted synthesis and reaction kettle synthesis were compared,and the advantages of microwave method and reaction kettle method were determined,and the synthesis p H,lithium excess ratio and sintering temperature in the synthesis process of the reactor were also studied.The results show that The p H value was controlled at 11.3±0.02during the synthesis in the atmospheric pressure reactor,and the Li excess was controlled by10%when the precursor was mixed with lithium.The c/a value of the synthesized cathode material after sintering at 780℃is greater than 4.9.It has a complete hexagonal layered structure,the peak intensity ratio I_（003）/I_（104）is up to 1.38,higher than 1.2,and the degree of cation mixing is low.The capacity retention rate after 100 cycles of charge and discharge at 1C is as high as 74.97%,and the discharge specific capacity at 5 C is as high as 141.24m Ah·g^–1,with low polarization resistance and stable level.（2）Preparation and properties of conductive graphite and Zr-doped LiNi_0.85Mn_0.15O₂cathode materials.Microwave-assisted liquid phase synthesis of Zr-doped LiNi_0.85Mn_0.15O₂cathode materials were prepared by microwave-assisted liquid phase synthesis,and KS-6 and Zr-doped LiNi_0.85Mn_0.15O₂cathode materials were prepared by doping with conductive graphite KS-6.Zr doping is beneficial to stabilize the crystal structure of LiNi_0.85Mn_0.15O₂cathode material,and the doping of KS-6 can improve the conductivity of LiNi_0.85Mn_0.15O₂cathode material.The results show that the c/a value of both unmodified and modified materials is higher than 4.9,the layered structure of the material is complete,the peak intensity ratio increases from 1.195 to 1.296,and the degree of cation mixing decreases.Zr doping and conductive graphite doping did not change the structure and morphology of the material,and KS-6 was uniformly distributed in the Zr-doped cathode material and constructed a bridged conductive network,which enhanced the conductivity of the material.The doping of Zr increases the interplanar spacing,which is beneficial to the diffusion of Li⁺.After doping,the content of Ni³⁺in the cathode material increases significantly,which reduces the mixing of Li⁺/Ni²⁺cations,which can effectively improve the structural stability and charge-discharge performance of the material.For example,the Zr-doped LiNi_0.85Mn_0.15O₂cathode material containing 2 wt.%and 3 wt.%KS-6 has the higher first discharge specific capacity after charging to 4.35 V,which are 203.32 and 203.88 m Ah·g^-1respectively.The Zr-doped performance cathode material of 2 wt.%KS-6 has a capacity retention rate of 81.09%after 100 cycles at a charge-discharge rate of 1 C.The discharge specific capacity at 5 C is 152.5 m Ah·g^-1,the potential difference is significantly reduced,and the cycle and rate performance is significantly enhanced.（3）Preparation and properties of conductive graphite reinforced Mg,Zr co-modified LiNi_0.85Mn_0.15O₂cathode material.The synthesized spherical precursor Ni_0.85Mn_0.15（OH）₂was co-doped with Mg and Zr to obtain Mg and Zr co-modified LiNi_0.85Mn_0.15O₂cathode material.The conductive graphite reinforced Mg,Zr co-modified LiNi_0.85Mn_0.15O₂positive electrode material was prepared by adding conductive graphite KS-6 in a certain proportion.The results show that the c/a of both unmodified and modified materials is greater than 4.9,and the layered structure is complete;The LiNi_0.85Mn_0.15O₂cathode material containing 3 mol%Mg and 3 mol%Zr has weak Li₂Zr O₃diffraction peaks,and the（003）and（104）peaks are also shifted,indicating that Mg and Zr are doped into the lattice of the material.At the same time,it is accompanied by the formation of Li₂Zr O₃.The doping of Mg increases the interlayer spacing of Li,which is beneficial to the diffusion of Li⁺.The incorporation of Zr and the Li₂Zr O₃coating layer are beneficial to the stability of the layered structure of the LiNi_0.85Mn_0.15O₂cathode material,reducing the degree of cation mixing and inhibiting surface side reactions.The specific discharge capacities of the cathode material containing 3 mol%Mg and 3 mol%Zr and the cathode material containing 2 wt.%KS-6,3 mol%Mg and 3mol%Zr are 187.22 and 188.78 m Ah g^-1,respectively.Under the 1 C charge-discharge conditions of 25℃,the capacity retention rates after 100 cycles are 87.34%and 87.56%.The specific discharge capacity of the cathode material containing 2 wt.%KS-6,3 mol%Mg,and3 mol%Zr at 5 C is 158.55 m Ah g^-1,and its rate performance is significantly enhanced.The conductive graphite is uniformly distributed in the cathode material and builds a bridging conductive network,which reduces the polarization resistance.