| Cathode materials play a decisive role in the energy density and cycle life of lithium ion batteries,while the application of high nickel ternary materials in the field of batteries is accelerated mainly due to high capacity and low cost.However,high nickel materials face much greater difficulty in preparation and storage than ordinary ternary materials.Among them,LiNi0.8Co0.1Mn0.1O2 cathode material is prone to structural changes during charging and discharging due to its high nickel content,especially in the state of deep lithium removal.As a result,the material itself has some inherent defects,such as poor cycling performance,serious Li/Ni mixing,low conductivity and poor safety.Therefore,it is one of the better methods to improve its electrochemical properties and thermal stability by means of coating and doping.In this paper,the coating modification of LiNi0.8Co0.1Mn0.1O2 cathode material with Li3VO4 and PANI was used to explore its physicochemical properties and electrochemical properties,and the electrochemical-calorimetry was used to study the thermoelectrochemical properties of Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode materials.1.Using ethanol as solvent,V2O5 and LiOH·H2O as coating materials,coating different contents of Li3VO4(1 wt.%,3 wt.%,5 wt.%)on the surface of LiNio.8Co0.1Mn0.1O2 cathode material does not change the α-NaFeO2 layered structure of material,but also can effectively reduce cation mixing.The comprehensive analysis of SEM,TEM and EDS shows that Li3VO4 is successfully coated on LiNi0.8Co0.1Mn0.1O2 particles,and the coating thickness of 3 wt.%Li3VO4 coating on LiNi0.8Co0.1Mn0.1O2 cathode material is about 2-12 nm.Particle size analysis showed that LiNi0.8Co0.1Mn0.1O2 cathode material was a mixture of large and small particles,and D50 before and after coating were 12.17 and 12.32 μm,respectively.Particle size distribution after modification was more concentrated.Powder resistance test shows that the volume resistivity of positive electrode material decreases with the increase of pressure,and the conductivity increases with the increase of pressure.The cathode material coated with Li3VO4 has lower resistance and higher conductivity.Electrochemical tests showed that,compared with pure LiNi0.8Co0.1Mn0.1O2,the initial discharge specific capacity of 3 wt.%Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode material was 195.9 mAh·g-1,and the capacity retention rate was 94.13%,with excellent discharge specific capacity,cycle performance and rate performance.In addition,cyclic voltammetry and AC impedance tests showed that 3 wt.%Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode material can enhance the lithium ion diffusion between the electrode/electrolyte interface,reduce the impedance,inhibit the side reaction between the active material and electrolyte,and improve the material performance.2.The thermal stability analysis of LiNi0.8CO0.1Mn0.1O2 and 3 wt.%Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode materials showed that the exothermic temperature,heat flow and exothermic heat of the cathode materials before and after Li3VO4 modification were respectively 213.6℃,3096.0 mW·g-1,319.6 J·g-1 and 218.0℃,2390.6 mW·g-1,283.5 J·g-1.After Li3VO4 coating,the exothermic peak temperature of LiNi0.8Co0.1Mn0.1O2 cathode material increases and the heat release decreases,which effectively improves the thermal stability of the material.In addition,the electrochemical performance tests of lithium ion batteries at different temperatures showed that the specific discharge capacities of 3 wt.%Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode materials at 1 C were 194.2(30℃),175.5(40℃)and 168.1(50℃)mAh·g-1,after 50 charge and discharge cycles the capacity retention rates were 96.7%,95.9%and 91.4%,respectively,the electrochemical performance is better than that of pure LiNi0.8Co0.1Mn0.1O2 cathode material.As the temperature increases,the activation reaction speeds up and at the same time accelerates the formation of the passivation film,resulting in an increase in the interface impedance of the cathode/solution and a decrease in the cycling ability of pure LiNi0.8Co0.1Mn0.1O2 cathode material at high temperature.However,after the modification of Li3VO4 coating,the diffusion of lithium ions is accelerated,side reactions are reduced,and the impedance in the charging and discharging process is effectively reduced.Thermoelectrochemical studies have shown that during the charge and discharge process of lithium ion battery cathode materials at the same temperature,the magnification increases,and the heat,helium and entropy increase.At 30,40 and 50℃,the heat production,enthalpy change and entropy change of Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode material in the charge-discharge reaction system are lower than that of pure LiNi0.8Co0.1Mn0.1O2.This study shows that Li3VO4 coating significantly improves the thermal stability of the cathode material.This is because the Li3VO4 coating protects the active material from electrolyte corrosion,promotes lithium ion transport,reduces side reactions and electrode polarization,and enables Li3VO4 coated LiNi0.8Co0.1Mn0.1O2 cathode material to have excellent thermochemical properties and thermal stability.3.Preparation of PANI-coated LiNi0.8Co0.1Mn0.1O2 cathode material effectively solves the hygroscopic problem of LiNi0.8Co0.1Mn0.1O2 active material.The morphology and structure analysis of cathode materials by instruments such as XRD,FTIR,SEM,TEM and EDS shows that a moderate amount of PANI coating does not change the main structure of LiNi0.8Co0.1Mn0.1O2 cathode material,which is conducive to reducing the lithium nickel mixture,reducing the residual lithium content on the surface and improving the storage performance of active substances.Electrochemical performance studies show that PANI coating can reduce discharge specific capacity,but it will form a favorable coating layer to protect the active material and improve the first discharge efficiency,cycle performance and rate performance of lithium.Among them,the initial discharge specific capacities of 3 wt.%PANI-coated LiNi0.8Co0.1Mn0.1O2 cathode material at 1 C and 5 C were 193.8 mAh·g-1 and 151.7 mAh·g-1,with the capacity retention rate of 96.24%and 76.20%,and excellent cycling performance.The excellent electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode material is mainly attributed to the formation of favorable coating layer,which protects the active material,reduces the side reaction and improve the stability of the electrode-electrolyte interface.At the same time,PANI provides an effective conductive path,reducing the resistance and capacity loss during charge and discharge.In addition,in the PANI coated LiNi0.8Co0.1Mn0.1O2 cathode material,the H+/Li+exchange reaction optimizes the diffusion of Li+ during the cycle test and promotes the rapid charge transfer reaction,thus improving the electrochemical performance of the positive electrode material.Cyclic voltammetry and EIS impedance analysis showed that PANI-coated LiNi0.8Co0.1 Mn0.1O2 cathode material effectively reduced irreversible phase transition and polarization during charge and discharge,and improved electrochemical performance. |
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