Synthesis And The Relation Between Structural, Physical Properties And Electrochemical Performance Of LiNi_xCo（1-2x）Mn_xO₂Cathode Materials For Li-ion Batteries

The energy is the most important thing as the cornerstone of humansociety. As we all know, fossil energy has become a kind of conventionalpower source since the industrial revolution. However, with thedevelopment of industrialization, people realized that the oil and coalcombustion would cause serious environmental pollution. Therefore, thedevelopment of new energy has become the most important topic ofscientific research. As the conversion device between chemical andelectrical energy, the battery is an essential part of new energy utilization.Because of environmental benefit, high efficiency of energy utilizationand rechargeable, the lithium ion battery become the most popular batterysystem. The cathode, anode and electrolyte are the main parts of battery system. During charge-discharge process of lithium ion battery, the redoxreaction occurs in the electrodes. And lithium ion would transfer betweencathode and anode as a positive charge carrier. The reversible capacity,cycle life, toxicity, safety performance of lithium battery is mainlyaffected by cathode material. Layered, spinel and polyanions oxide arethree kinds of cathode materials. The manganese oxide cathode withspinel structure would undergo phase transition during electrochemistryprocess. The reversible capacity would loss with charge-discharge cyclesbecause of the phase transition. Due to low conductivity of polyanionoxides, the rate performance of this kind of cathode would not so well.Therefore, the layered LiCoO₂material is the most popular cathodewhich use in commercial lithium ion batteries. But because ofenvironment harmful, poor thermal stability, high cost and other reasons,the application of LiCoO₂cathode is limited. This limitation has led tointensive studies being conducted to find alternative cathode materials.One of the most promising candidates is mixed Li-Ni-Co-Mn-O layered oxides. Although the electrochemical, structural properties have beeninvestigated widely since LiNi^1/3Co^1/3Mn^1/3O₂synthesized by Ohzukuand Makimura, the electrochemical performance in high voltage window,structure evolution as charge-discharge process and magnetism propertiesshould still be further research.At the first, different preparation methods have been comparedbecause it would influence electrochemical performance greatly. Thesolid-state, sol-gel and co-precipitation methods are adopted to synthesisLiNi^1/3Co^1/3Mn^1/3O₂. Because it is very hard to mix the raw materialthoroughly, the pure LiNi^1/3Co^1/3Mn^1/3O₂has not prepared by solid-statemethod. The soft chemical method could achieve uniform mixingprecursor compound, however, the particle size, morphology are hardlyto control. So the electrochemical performance of cathode synthesized bysol-gel method is not satisfied. Although the strict preparation processes,the complex of synthesis steps, co-precipitation method is a reliablemethod to obtain LiNi^1/3Co^1/3Mn^1/3O₂with excellent electrochemical performance. Beside this, the component has also been optimizedbecause nickel, cobalt and manganese plays different role during theelectrochemical process. LiNi_xCo_1-2xMn_xO₂（0.1<x<0.5） has beenprepared and test by XRD, Raman and FTIR. It could found MO6octahedral would shrink as the increasing of cobalt content.The magnetic properties of material would be affected by crystalstructure, just like electrochemical properties. So, the relationshipbetween magnetic and electrochemical property might be establishedbase on Li-Ni-Co-Mn-O layered oxides. The magnetic properties ofLiNi^1/3Co^1/3Mn^1/3O₂, LiNi_0.4Co_0.2Mn_0.4O₂synthesized by co-precipitationmethod and LiNi_0.4Co_0.2Mn_0.4O₂prepared by sol-gel method have beeninvestigated by zero field/field cold susceptibility, ac susceptibility underdifferent magnetic field and magnetization curves. Although the spinglass behaviors have been found in all samples, the reasons of suchmagnetic behavior are not the same. The geometry frustration anddisorder of ions are the main reason for LiNi^1/3Co^1/3Mn^1/3O₂. But the geometry frustration is a less important factor for LiNi_0.4Co_0.2Mn_0.4O₂synthesized by co-precipitation method. Because of the large ionsdisorder degree, the magnetic transition behavior of LiNi_0.4Co_0.2Mn_0.4O₂synthesized by sol-gel method is cluster spin glass. From the reason formagnetic behavior transition, it could suggest the local structure ofsamples. So, the relationship between magnetic and electrochemicalproperty could be established.In the first charge, LiNi^1/3Co^1/3Mn^1/3O₂has been test by ex-situ XRD,Raman and FTIR. The material keeps hexagonal structure with Li+ionsextracting until cutoff voltage to4.6V. And the space symmetry is notchange significantly. The capacity retention of LiNi^1/3Co^1/3Mn^1/3O₂in2.5-4.6V is much worse than2.5-4.4V voltage window. The EIS datashows that film resistance at4.6V is much larger than4.4V. Thus, itcould conclude that the degeneration of capacity retention is mostlybecause of solid electrolyte interphase （SEI） layer onLiNi^1/3Co^1/3Mn^1/3O₂. The ex-situ synchrotron, XPS, HRTEM, EIS have been used tofurther research LiNi^1/3Co^1/3Mn^1/3O₂performance in2.5-4.6V voltagewindow. The synchrotron patterns of sample after10,20and30cyclesindicate the structural stability of LiNi^1/3Co^1/3Mn^1/3O₂is excellent. Evenlattice parameters are not change significantly. In contrary, XPS datashows the chemical composition on electrode particles surface haschange greatly as the charge-discharge cycles. The Li₂CO₃on the surfaceof LiNi^1/3Co^1/3Mn^1/3O₂is dissolved by HF attacking in LiPF6electrolyteand replaced by SEI compositions. From HRTEM pictures, it could find40nm coating layer on electrode surface clearly. Meanwhile, the filmresistance of LiNi^1/3Co^1/3Mn^1/3O₂increase continuously with the cyclenumber according to EIS data. So, it could conclude that theelectrochemical performance of LiNi^1/3Co^1/3Mn^1/3O₂in high voltagewindow would be influenced by SEI film continued accumulation on theelectrode surface.In this paper, the electrochemical property in high voltage window, evolution of structure as charge-discharge cycle and magnetic property ofLi-Ni-Co-Mn-O have been investigated thoroughly.