Preparation And Improvement Of Electrochemical Performance Of LiCoPO₄ As High Voltage Cathode Material

Olivine-structured LiCoPO4 cathode has attracted a great deal of attention because of its high theoretical energy density(800 Wh kg-1),higher operating potential(approximately 4.8 V vs.Li/Li+),and high safety(for the strong P-O covalent bond).In theory,LiCoPO4 is a good candidate for use as a high-voltage cathode material in lithium-ion batteries that can meet the critical demand for a higher energy density in EV/HEV.Unfortunately,LiCoPO4 is rarely used in practical applications due to its poor electronic,low ionic conductivities,and the severe capacity fading upon cycling that caused by the decomposition of electrolyte at high oxidation potential,the HF corrosion(generated from the reaction between the trace water and LiPF6 in electrolyte),and the antisite defects(caused by the exchange between lithium and cobalt atoms during the cycling process).To solve these critical problems and improve the performance of LiCoPO4 battery,this dissertation mainly focuses on the synthesis of LiCoPO4@C material,the optimization of electrolyte,the modification of LiCoPO4 with AIF3,the substitution of Cr3+ for Co2+ in LiCoPO4 and the synthesis of LiCo0.9Mn0.1PO4@Li3PO4 core-shell material.The following is the detail.(1)The synthesis of LiCoPO4@C material.A LiCoPO4@C composite material has been successfully prepared by sol-gel methode.The gel was first moved into a rotary furnace,calcined at 400 ? for 3 h with air atmosphere to eliminate the excess carbon and then calcined at 700 ? for 2 h under Ar flow to form LiCoP04@C composite material.On the basis,the carbon was coated on the LiCoPO4 particles homogeneously,which could further improve the electronic conductivity and the electrochemical performances of the composite.The LiCoPO4@C cathode delivered an initial discharge capacity of 130.2 mA h g-1 at 0.1 C and showed a capacity retention of 75%after 50 cycles between 3.0 and 5.0 V with electrolyte of 1 M LiPF6 EC:DMC(v/v 1/1).(2)The optimization of electrolyte.In order to relieve the severe decomposition of EC-based electrolyte caused by Co2+/Co3+ redox reaction at 5 V high voltage,TMSB was used as electrolyte additive.It is found out that TMSB oxidized preferentially to normal electrolyte and formed a stable SEI film on LiCoPO4 surface at the first cycle which effectively restrained the continuous decomposition of electrolyte at the following cycles.The electrolyte with 1wt.%TMSB showed the best cycling performance.The LiCoPO4 cathode delivers an initial discharge capacity of 144 mA h g-1 at 0.1 C and with a capacity retention of 76%after 50 cycles.(3)The substitution of Cr3+ for Co2+ in LiCoPO4.Electrical and ionic conductivity were two major limiting factors for LiCoPO4 cathode material.To overcome these shortcomings,a Cr-substituted LiCoPO4 core with a conductive carbon layer cathode material(LiCo1-1.5xCrxPO4@C)was synthesized with the sol-gel method.Cr substitution lead no changes in the olivine structure and basic morphology,but exhibited a decrease in the unit cell volume and weakened the Li-O interaction.The Cr-substituted LiCoPO4/C cathode material exhibited a lower antisite defect concentration,smaller polarization and improved ionic and electrical conductivities.Therefore,the LiCo0.94Cr0.04PO4/C cathode delivered an initial discharge capacity of 144 mAh g-1 at 0.1 C and showed a capacity retention of 71%after 100 cycles.(4)The modification of LiCoPO4 with AIF3.In order to revise the LiCoPO4 cathode with desirable properties,uniform AlF3-modified LiCoPO4 cathode materials in nano-sized distribution were synthesized.The AIF3 layer stabilized the interface between the cathode and electrolyte,formed steady SEI film and suppressed the electrolyte continuous decomposition at 5 V high voltages,thus significantly improved the cycling performances of LiCoPO4 cathode within the voltage range of 3.0?5.0 V.After a series of optimization,4 mol%AlF3-coated LiCoPO4 material exhibited the best properties including an initial discharge capacity of 159 mA h g-1 at 0.1 C with 91%capacity retention after 50 cycles.(5)The synthesis of LiCo0.9Mn0.1PO4@Li3PO4 core-shell material.To integrate the advantages of AIF3 modification and Cr3+ substitution,the cathode material of LiCo0.9Mn0.1PO4@Li3PO4 with core-shell structure was synthesized with good electrochemical performances,such as:delivered an initial discharge capacity of 142.5 mA h g-1 at 0.1 C with 71%capacity retention after 100 cycles;showed an initial discharge capacity of 112 mA h g-1 at 1 C with 71%capacity retention after 220 cycles and at displayed an initial discharge capacity of 96.3 mA h g-1 at 2 C with 64.6%capacity retention after 500 cycles.Hence,this kind of LiCo0.9Mn0.1PO4@Li3PO4 cathode material has a great application prospect.