Synthesis And Modification Of Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials For Lithium-ion Batteries

Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ is one of the most promising cathode materials for lithium-ion batteries,which has the advantages of high energy density,low toxicity and low cost.High nickel content gives it a great advantage in energy density,but it also brings some problems that seriously affect its structure and electrochemical properties,such as serious mixing of Li⁺/Ni²⁺,poor stability of the interface between cathode and electrolyte,and unstable structure during circulation,serious safety problems and other risks greatly hinder its practical large-scale application.In view of the above problems,this paper selects the Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material as the research object,and carries out a series of modification researches.It mainly includes the optimization of precursor synthesis conditions,surface coating of Li Ni_0.8Co_0.1Mn_0.1O₂cathode material,ion doping and composite with high voltage cathode material,which provides technical exploration and theoretical guidance for improving its cycle stability and magnification performance,and the main conclusions are as follows.The influence of p H and ammonia concentration on the reaction system of Ni_0.8Co_0.1Mn_0.1（OH）₂ precursor are investigated by the control crystallization.The optimum synthesis conditions are obtained,that is,the p H is 11.8 and the ammonia concentration is 0.8 mol/L.The effects of different amount（0,0.5‰,1‰）of surfactant FC-4430 on the crystal structure,morphology and partical size distribution of synthesized Ni_0.8Co_0.1Mn_0.1（OH）₂ precursor are investigated.The results show that the Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material has good layered structure and comprehensive electrochemical properties when the adding amount of surfactant FC-4430 is 0.5‰.Three important factors affecting the electrochemical performance of Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material（excess lithium content,sintering temperature and holding time）are investigated comprehensively.The optimal sintering conditions are obtained by optimizing the technical parameters,that is,the excess lithium amount is 5%,the sintering temperature is 780℃,and the holding time is12 h.Under these conditions,after 100 cycles of 1 C rate,the discharge specific capacity retention rate is 97.9%.Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material coated with WO₃ in different proportions（0,0.25 and 0.5 wt.%）are synthesized by the combination of liquid phase and solid phase.It is found that an appropriate amount of WO₃ coating modification can reduce the polarization of Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material and improve its electrochemical performance.The NCM811@0.25wt.%WO₃sample coating with 0.25 wt.%WO₃ showed the best cyclic stability and rate performance.With the help of surfactant,nickel boride,who with high conductivity and stable performance,are successfully coated on the Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂cathode materials with different proportions（0,0.25,0.5,0.75 and 1 wt.%）.It is found that an appropriate amount of nickel boride coating not only reduces the diffusion resistance and AC impedance of lithium ion,but also decreases the polarization of the material.It is found that the NCM811@0.75wt.%Ni₂B sample coating with 0.75 wt.%nickel boride had better rate performance and discharge specific capacity of 159.5 m Ah g^-1 at 15 C.After 100 cycles at 1C and voltage range of 2.8-4.3 V,the specific capacity retention rate of the NCM811@0.75wt.%Ni₂B sample coating with 0.75 wt.%nickel boride is 92.4%,which is much higher than79.2%of the original Li Ni_0.8Co_0.1Mn_0.1O₂ sample.Combined with theoretical calculation,it is found that the density of electronic states in the crystal of the material increases significantly after the bulk phase co-doping of fluorine/strontium.Electrons will more easily transition from the valence band to the conduction band,and electron conductivity will increase.Through a series of tests,it is proved that fluorine/strontium is successfully and uniformly co-doped into the internal lattice of Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material.The sample with fluorine and strontium doping ratios of 0.5 mol%have the best rate performance,at the rate of 0.1,0.5,1,3,5 and 10 C,the discharge specific capacities are 190.9,180.4,175.8,167.6,163.4 and 155.5 m Ah g^-1,respectively.Through kinetic mechanism analysis,it is found that the positive electrode material modified by proper amount of fluorine/strontium co-doping has smaller voltage gap（0.1365V）and less resistance in the electrochemical reaction process by impedance test analysis.The high-voltage Li VPO₄F and Ni-rich Li Ni_0.8Co_0.1Mn_0.1O₂composite cathode material is successfully prepared by simple and easy method.The discharge specific capacity of Li Ni_0.8Co_0.1Mn_0.1O₂-Li VPO₄F composite cathode material increases significantly at high discharge rate.After 150 cycles at 1 C,the discharge specific capacity retention rates of the composite cathode material are 95.93 and 83.38%at the cut-off voltage of 4.3 and 4.5V,respectively,which are higher than those of the original Li Ni_0.8Co_0.1Mn_0.1O₂ cathode material（91.68 and 79.34%）.The improvement of the electrochemical performance of the composite cathode material proves that the composite of high-voltage Li VPO₄F cathode material with other cathode materials is effective and can be further applied to the improvement of other high-energy density lithium-ion battery cathode materials.