The Synthesis And Modification Of Layered LiNi_xCo_1-2xMn_xO₂ For Lithium Ion Batteries

Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electode materials.Multi-element transition metal oxides of LiNi_xCo_1-2xMn_xO₂ is considered as a potential and active one of the new cathode materials of Li-ion batteries to replace LiCoO₂,for its inexpensive,which also has higher specific capacity,better thermal stability and better cyclic performance compared with LiCoO₂.Recently,this material begins to be commercially employed and has been applied in more and more fields.The multiple complex oxides of LiNi_xCo_1-2xMn_xO₂ have more complicated preparation than LiCoO₂.In this dissertation,the research aims at synthesizing LiNi_xCo_1-2xMn_xO₂ that can reach a level of commercial application.The layer-structured material of LiNi_0.4Co_0.2Mn_0.4O₂ with low Co content is selected to be emphases of the research works.To make a breakthrough in prepared method, the materials had been synthesized by two different routes as spray pyrolysis and co-precipitated metal hydroxide by controlling crystallization.In addition,the effect of different transition-metal content on the structural and electrochemical characteristics had been studied.The influence of doping anion and cation on the cyclic performance at higher cut-off voltage were also investigated.The main results are as follows:(1) The mechanism model of spray pyrolysis was established and the experimental conditions was researched.To overcome low tap density caused by the hollow and crushing phenomenon,in this work,on the basis of studying the process stages of the particle formation in spray pyrolysis,the mathematical model of the key controlling stages was established and the mechanism model of forming the filled dense particle was proposed.Mechanism model is described as follows:during the solvent evaporation from a free liquid surface,at the moments of the solute concentration on the droplet surface reaching its critical supersaturation(t=(?)₁) and full crust forming(t=(?)),if the droplet center concentration reached equilibrium saturation of solute,the liquid droplet would form the filled dense particle.The effects on the particle morphology were also studied by small experimental with different synthesis conditions.Moreover,the experimental results showed that addition of polymeric precursor solutions containing urea,which decomposed and produces OH^- and NH₄⁺ as the precipitation during spray pyrolysis process,was advantageous to formation of filled dense particle.(2) The engineering practice of spray pyrolysis in the field of preparing lithium-ion battery cathode material were implemented firstly.To explorate the feasiblitity of realizing industrial applications of spray pyrolysis,based of the small experiment,the pilot experiment was implemented on pilot experimental platform designed by ourself and the effect of synthesis conditions on the performance of LiNi_0.4Co_0.2Mn_0.4O₂ was studied.The test result indicated,when the precursor prepared by spray pyrolysis was used as primal materials,LiNi_0.4Co_0.2Mn_0.4O₂ with the perfect crystal and excellent electrochemical performance could be synthesized by low temperature(900℃) and short sintering time(6h).(3) The pilot technique of LiNi_0.4Co_0.2Mn_0.4O₂ was also synthesized by co-precipitated metal hydroxide by controlling crystallization.The effect of operating conditions on precursor morphology,particle size,tap density and specific surface area was investigated.PH and ammonia content significantly influenced micro-morphology of precursor.With increasing pH,preparing temperature and ammonia content in reaction solution,tap density of precursor was increased and specific surface area was decreased. Meanwhile,influence of sintering temperature and sintering atmosphere of lithiation sinter on material performance was investigated.The optimized sintering conditions was obtained as follows:At the elevating rate of 100℃/h,the material was sintered at 750℃for 10h,then,raise to 950℃for 10h.On this basis,a pilot-production technique of co-precipitated metal hydroxide by controlling crystallization was developed to prepare LiNi_0.4Co_0.2Mn_0.4O₂.The technique includes:the physical & chemical index,technology technique parameter and main equipment.(4) A systematic study of the performance difference on LiNi_xCo_1-2xMn_xO₂ is conducted.The series composites of LiNi_xCo_1-2xMn_xO₂(x=0.25,0.33,0.4,0.5) were synthesized by spray pyrolysis.The influence of different transition-metal content on the structural and electrochemical characteristics were studied.The increase of Co content improved characteristics of two-dimensional layered structure and structure stablitity,reduced the degree of the cation mixing and improved the cyclic performance and rate capability.The capacity of LiNi_xCo_1-2xMn_xO₂ increases with higher cut-off voltage,but the capacity retention in the cyclic tests become worse.(5) The effects of doping F and Ti on the structure and electrochemical performance of LiNi_0.4Co_0.2Mn_0.4O₂ were studied. The results indicated that the material with doping F and Ti could help to improve the cyclic performance at higher cut-off voltage. LiNi_0.4Co_0.2Mn_0.38Ti_0.02O_1.94F_0.06 exhibited the specific capacity of 193.5 mAh·g^-1,and its capacity retained 91.5%after cycling 40 times in the voltage range of 2.5～4.6V.