Study On Synthesis And Performance Of Lithium Iron Silicate As A Cathode Material For Lithium-ion Batteries

Li₂FeSiO₄ material with polyanion structure has been regarded as the best candidate for the cathode material of the new generation of lithium ion batteries because of its high theoretical specific capacity and superior safety performance.This material can achieve two Li⁺insertion and extraction and reach the theoretical value of 342 mA·h·g^-1.However,its inherent low conductivity and Li⁺diffusion coefficient make its electrochemical performance poor,which hinders its practical application.The surface carbon coating can effectively prevent the aggregation of the pure phase crystallites of the material,establish a more efficient electronic conductive network,improve the electronic conductivity,thereby reducing the electrode polarization and improving the electrochemical performance.Adding a complexing agent and modifying the precursor solution can improve the dispersibility of the raw materials and affect the formation of the crystal structure of the target product.Foreign metal ion doping will produce much more lattice defects,and change the chemical environment around lithium ion and redox-active ion.It can affect the charge/discharge plateau potential,and enhance the electrical conductivity and the diffusion coefficient of Li⁺in the material.Based on the above discussions for the material,the thesis explores the synthesis conditions of Li₂FeSiO₄ as a cathode material for lithium ion batteries.By adjusting the pH value of the precursor solution,optimizing the structure of the surface carbon coating and doping foreign ions at the Si sites,the capacity and conductivity of the material are improved,which provides an effective and feasible design idea and solution for the research and development of high-performance Li₂FeSiO₄ cathode materials.The main research contents are as follows:?1?Li₂FeSiO₄ cathode material was synthesized by high temperature solid-state reaction with anhydrous lithium acetate,ferric nitrate nonahydrate and hydrophilic nano-Si O₂ as the lithium source,iron source and silicon source respectively.The particle size is 29.2 nm,and the specific charge capacity is maintained at around 40 mA·h·g^-1.Subsequently,Li₂FeSiO₄/C composites with different sitering temperatures were synthesized by sol-gel method,and the influence of sintering temperature on properties of the materials was studied.Citric acid acts as the carbon source and precursor complexing agent of the material.Through TG-DTA and XRD tests,it was found that the particles of the material gradually became larger as the sintering temperature increased.When the molar ratio of citric acid:Fe=2:1,the sintering conditions are 300?-3 h,700?-10 h,the Li₂FeSiO₄/C lithium ion battery cathode composite material can be synthesized and it has good electrochemical performance.The capacity of Li₂FeSiO₄/C can be kept above 133 m A·h·g^-1 after 20 cycles.With citric acid as the pyrolytic carbon source,a three-dimensional conductive network is formed,which increases the conductivity of the whole electrode and also forms a rich pore structure.It is conducive to the rapid penetration of the electrolyte and the rapid conduction of Li⁺,while keeping the material structure stable during charging and discharging.?2?Li₂FeSiO₄/C composite materials with different pH values of the precursor solution are prepared by sol-gel method.Anhydrous lithium acetate,ferric nitrate nonahydrate,hydrophilic nano-SiO₂ and citric acid were used as lithium source,iron source,silicon source and carbon source as well precursor complexing agent,respectively.The pH of the precursor solution was adjusted to 1-11 with ammonia.The effect of sol morphology on the electrochemical performance of materials was discussed.The study found that the electrochemical performance of the material is the best when the precursor solution is alkaline.Under alkaline conditions,the hydrolysis reaction of the system is caused by the nucleophilic substitution of OH^-.This nucleophilic reaction is less aggressive,and the hydrolysis rate is lower than the polycondensation rate.Therefore,the polycondensate has a high crosslinking degree,and can form a macromolecular polymer with a high degree of crosslinking.The stable network skeleton structure of the material improves the dispersion of various elements ions in precursor sol and xerogel,reduces the agglomeration between crystallites,and is conductive to the formation of nano particle phase.Moreover,the pyrolytic carbon coating formed after pyrolysis is also convenient for the transmission of Li⁺,which helps to improves the electrochemical properties of the materials.After testing,it was found that the sample with precursor solution pH=11 had the highest charging capacity and the best cycling stability.The first discharge capacity is 169.82 mA·h·g^-1,and the capacity can still reach above 140 mA·h·g^-1after 30 cycles.?3?Li₂FeSi_1-xCr_xO₄/C?x=0,0.03,0.06,0.10?was synthesized as cathode material for lithium ion batteries by refluxing-assisted sol-gel method.The effect of Cr doping at Si site on the structure and electrochemical performance of the material was studied.Among them,the best doping amount of Cr is 10 at.%of LFS/C-Cr10%.The material not only has the common characteristics of nano scale,uniformly dispersed,large specific surface area,which is conducive to the penetration of electrolyte,but also has special structural modification effect due to the Cr doping into Si site.Rietveld refinement results show that the 10 at.%Cr doping increases the cell parameters of material structure in a,b,c directions and widens the transport path of Li⁺.The increase of Li-O bond length indicates the decrease of the force and binding energy between Li⁺and O^2-.The above two points contribute to the rapid migration of Li⁺to the surface of electrode material and have a high mass transfer kinetics.Higher charge and discharge capacity,Li⁺diffusion coefficient and electronic conductivity provide the most favorable support for the above results.In the voltage range of 2.0-4.5 V,the capacity is149.36 mA·h·g^-1 at 0.1 C and reaches 148.65 m A·h·g^-1 with a high capacity retention rate after 20 cycles.?4?In order to obtain better coating effect,we developed and designed a simple coating device based on chemical vapor deposition?CVD?method to coat the surface of material particles.The device consists of three parts:sealing system,reaction chamber system and gas supply system.Based on the principle of chemical vapor deposition,specially designed reaction chamber tank,bearing plate and intermittent electromagnetic pulse valve are used to make the materials in the reaction chamber mix evenly and coat uniformly through the three-dimensional movement of up and down,left and right,front and back,as well as the self-rotation movement.The dispersion of the material in the reaction chamber is strengthened,increasing the contact area and residence time with the carrier gas of the coating precursor.The agglomeration of particles and fusion of coating layer are avoided,and the coating quality and electrochemical performance of the material are improved.In addition,the optimal design of the pore distribution of the bearing body has been discussed using the pore distribution analysis method based on the cone-shaped conduction of gas-solid material flow.