Research On The Synthesis And Electrochemical Properties Of LiNi_0.8Co_0.2O₂Cathode Material For Lithium-ion Batteries

LiNi_0.8Co_0.2O₂has the advantages of low cost, environment-friendly, high specificcapacity, high energy density, high operating voltage and excellent cycling performance,and is considered as an ideal substitute of successfully commercialized LiCoO2cathodematerial.Aiming at industrial production, environmental protection and cost reduction, thispaper is to improve the specific discharge capacity and electrochemical cyclingperformance, especially the cycling performance in the process of charge-discharge testin high temperature environment, of LiNi_0.8Co_0.2O₂cathode material. In this study, bothof the liquid phase co-precipitation method and high-temperature solid-phase methodwere used to prepare LiNi_0.8Co_0.2O₂cathode material, then doping modification wasmade to the noumenon, and the phase structure, mophorogy and electrochemicalproperties were investigated with TG, XRD, SEM, battery tester, CV and other means.（1） The optimum conditions when preparing the precursor Ni_0.8Co_0.2（OH）₂wereselected as follows: a mixed metal ion concentration of nickel salt and cobalt salt is1mol/L, concentration of the precipitating agent NaOH2mol/L, coprecipitation reactionPH value11, the coprecipitation reaction temperature50℃, the coprecipitation reactiontime5h, and then high-density and high-activity hydroxide precursor Ni_0.8Co_0.2（OH）₂was sythesized. The analysis by XRD, SEM, EDS, laser particle size demonstrated thatthe precursor Ni_0.8Co_0.2（OH）₂, synthesized with liquid phase co-precipitation method,had regulal crystal structure, uniformly distributed particles, spherical morphology, andvery high tap density （up to1.91g/cm3）. TG analysis of the mixure of LiOH·H2O andthe precursor Ni_0.8Co_0.2（OH）₂showed that it was appropriate to adopt the two-stagesintering condition, and the temperature range was divided into two sections,600℃⁷00℃and725℃⁸50℃, respectively. The optimized experimental synthesisconditions were achieved as follows:700℃9h750°C12h, in air atmosphere, lithiummolar ratio n（Li）: n（Ni+Co）=1.13:1, the desired sample had a perfect layered structure,uniform distributed particles, low ions disorder degree, the initial discharge capacitywas up to153.0mAh/g, and20-time cycle capacity retention was98.50%, which demonstrated that the cathode material had most excellent electrochemical performance.Cyclic voltammetry tests proved that the cathode material had good reversibility andstronge cycle stability. The charge-discharge rates experiments showed that0.5C wasthe most ideal rate for the charge-discharge tests of LiNi_0.8Co_0.2O₂cathode material. Theelectrochemical properties of this material prepared under optimal experimentalconditions in the process of charge-discharge tests under50±3℃were inferior to thoseof the sample under25±3°C, and the reason was that in the process of charge-dischargetests in a high temperature environment, the electrolyte would be oxidized anddecomposed in high temperature environment, and the layered structure of the materialwould change, thereby deteriorating the electrochemical properties of the Lithium-ionBattery. This led to a new problem: how to improve its electrochemical properties in theprocess of charge-discharge tests in high temperature environment?（2） The effects of Al, Cr, V-doping on the properties of LiNi_0.8Co_0.2O₂cathodematerial were studied, it was proved that when the Al-doping amount was0.05, theCr-doping amount0.05, the V-doping amount0.02, LiNi_0.8Co_0.2O₂cathode material hadhigher initial discharge capacity and multiple cycle capacity retention, which showedthe better electrochemical performance in the process of charge-discharge tests in hightemperature environment than that of the sample in room temperature environment,which meant that certain doping amount can improve the electrochemical properties inthe process of charge-discharge tests in high temperature environment.