Preparations And Electrochemical Properties Of Li₂FeSiO₄/C And Li₂FeSiO₄/LiFePO₄/C Composites

Lithium iron silicate(Li₂FeSiO₄)cathode materials have received extensive attention in the lithium-ion battery industry due to their structural stability,wide range of materials,environmental friendliness,high energy density and environmental friendliness,while cathode materials are lithium-ion battery ratios.The impact of the upper limit of the capacity,its performance determines the value of the battery.In addition,silicon and iron are abundant in nature,which makes mass production of lithium iron silicate possible.Lithium iron phosphate(LiFePO₄)has many advantages in the positive electrode material,but there are also disadvantages.Lithium iron phosphate has a vertical relationship between the discharge curve and the working platform curve in the late discharge stage,so the voltage and specific capacity become nonlinear.Relationship,and this problem causes the open circuit voltage method to accurately measure the state of the battery's load voltage,which further has a greater impact on the battery management system test,and it can be seen from the charge and discharge curve of the Li₂FeSiO₄cathode material.The degree of ramping of the discharge curve is very high,especially below 3.0 V.Therefore,the charge and discharge curve of the lithium iron phosphate electrode material is ramped by optimizing the lithium iron silicate lithium cathode material in the composition of the lithium iron phosphate electrode material.Finally,the lithium iron phosphate battery can be linearly related to the specific capacity in the late stage of discharge,and it is difficult to solve the common problem of the residual electricity(SOC)by simply measuring the battery voltage.In view of the existing problems,the research work on the synthesis of Li₂FeSiO₄,carbon coating and construction of the dual active material Li₂FeSiO₄/Li Fe PO₄/C composites:(1)In the third chapter,lithium hydroxide(LiOH·H₂O),ferrous sulfate(FeSO₄·7H₂O)and silica(Si O₂)are used as raw materials,glucose is used as carbon source,deionized water is used as solvent,the shuttle morphology of Li₂FeSiO₄and Li₂FeSiO₄/C cathode materials was synthesized by hydrothermal method.XRD analysis showed that carbon exists in amorphous form,and both positive electrode materials are isomeric with Li₃PO₄,belonging to Pmn2₁space group,and the carbon-coated composite material is more evenly distributed.By coating different carbon contents of Li₂FeSiO₄,it was found that the electrochemical performance of the sample with carbon content of 10%was better than that of other structural materials,and at different rates of 0.1 C,0.5 C,1 C,2 C,3 C and 0.1 C,the specific discharge capacities were 102.2 m Ah g^-1,93.5 m Ah g^-1,88.9 m Ah g^-1,81.6 m Ah g^-1,78.4 m Ah g^-1and 113.8 m Ah g^-1,respectively,when the discharge rate of 3 C returns to 0.1 C,the specific capacity can be recovered well,It shows that the composite has good reversibility.(2)In the fourth chapter,Li₂FeSiO₄/C composites were synthesized by solid phase method using lithium carbonate(Li₂CO₃),silica(SiO₂),ferrous oxalate(FeC₂O₄·2H₂O)as raw materials and glucose as carbon source,the Li₂FeSiO₄/C composites were synthesized by solid phase method,and the electrochemical properties of different carbon contents were compared.The carbon-coated sample by XRD analysis showed no impurity peaks,and the particle size distribution was uniform when the carbon content was 10%.Electrochemical performance analysis showed that the electrochemical properties of the carbon-coated materials were greatly improved.The samples with carbon content of 10%had the highest charge-discharge ratio capacity and cycle performance.The composite materials were respectively 0.1.C,0.5 C,1 C,2 C,3 C and 0.1 C rate charge and discharge cycle curves,respectively,corresponding to the reversible specific capacity of 120.3 m Ah g^-1,106.2 m Ah g^-1,102.1 m Ah g^-1,96.4 m Ah g^-1,93.3 m Ah g^-1and 119.7 m Ah g^-1.When the current changes from 3 C to 0.1 C,its capacity immediately returns to a value comparable to the initial cycle,indicating that the composite has good reversibility.(3)The Li₂FeSiO₄/Li Fe PO₄/C composites were synthesized by spray drying method using the Li₂FeSiO₄/C cathode material prepared by the solid phase method in Chapter 4 and the industrialized Li FePO₄/C as raw materials.After SEM analysis,the composite still maintains a spherical morphology after high temperature treatment.The button cell made of the composite material was tested for electrochemical performance,and its specific capacities correspond to 170.1 m Ah g^-1and 165.6 m Ah g^-1,161.3 m Ah g-1,154.4 m Ah g-1,149.5 m Ah g-1,170.4 m Ah g-1 at 0.1 C,0.5 C,1 C,2 C,3 C and 0.1 C rates,respectively,and most importantly,the composite exhibits a ramped curve after 3.3 V voltage platform,with Li Fe PO₄/C,and linearly fitting its R value to 0.98272 is very close to the correlation coefficient 1,which indicates that the discharge curve after the voltage of 3.3 V can be represented by a simple formula,which cannot simply pass the test voltage for Li Fe PO₄/C.The problem of obtaining the SOC estimate has been effectively improved,further indicating that the Li Fe PO₄/C discharge curve can be ramped by doping Li₂FeSiO₄/C material is correct and effective.