Controllable Synthesis And Modification Of LiFePO₄ Cathode Materials For Lithium-ion Batteries

As the energy crisis and environmental pollution problems become increasingly prominent,developing new energy sources and solving storage problems have become the current main energy strategy.High-performance energy storage equipment is an important carrier for new energy utilization.Green lithium-ion batteries are popular because of their advantages such as suitable working voltage,long cycle life,high energy density,and low self-discharge.Compared with other electrode materials,LiFePO₄ with olivine structure has been favored by people for its advantages of good thermal stability,high safety performance,low price and friendly environment,which also has become one of the cathode materials with the greatest development potential.However,its shortcomings such as poor electrical conductivity and slow lithium ion diffusion rate seriously affect the electrochemical performance of the material and limit the application space of the material.Based on consulting a large number of related literatures and in order to improve the conductivity and lithium ion diffusion coefficient of LiFePO₄ materials,this work mainly focuses on the ion doping,carbon layer modification,particle nanometerization and interface design to solve these shortcomings.The main contents of the paper are as follows:Firstly,in order to improve the electrical conductivity of LiFePO₄ material from within the grain,the polyanion P-site B doping was explored by solvothermal method to prepare LiFeP_1-xB_xO_4-δmaterial.The effects of doping amount on phase structure,microstructure and electrochemical properties of the materials were systematically studied.The results show that the appropriate amount of B doping does not change the phase structure of the material,but reduces the cell volume,improves the dispersion of particles and enhances the electron conduction between Fe O₆ octahedral layers,thus improving the electrochemical performance of the material.When the doping amount of B is 2%,the specific capacity of LiFeP_0.98B_0.02O_4-δis 144.8 m Ah g^-1 at 1C,which is 5.4%higher than that of pure LiFePO₄.However,when the doping amount of B increases to 4%,the electrochemical performance decreases,which is mainly caused by the generation of oxygen defects.In order to avoid the generation of oxygen defects in the preparation process of the above materials,Li Fe Mg_xP_1-xB_xO₄ material was prepared by using Mg balance charge on the basis of the preparation of LiFeP_1-xB_xO_4-δby p-doped B.The results show that the introduction of Mg can not only shorten the diffusion path of lithium ions,but also effectively eliminate oxygen defects,so as to improve the electrochemical performance of the materials.The specific discharge capacities of Li Fe Mg_0.02P_0.98B_0.02O₄at 0.1,0.2,0.5,1,2,5 and 10C are 153.4,151.3,148.6,146.3,140.6,123.3 and 114.9m Ah g^-1,respectively,when the doping amount of B and Mg is 2%.However,when the doping amount of B and Mg is 4%,the electrochemical performance of the material decreases,which is mainly due to the introduction of excessive Mg,which may enter the Li site and block the transmission of lithium ions.Secondly,the surface carbon coating of LiFePO₄ material was modified to improve its electronic conductivity,so as to further improve the high rate charge and discharge performance of the material.A new LiFePO₄/C cathode material with phosphorus modified carbon layer was successfully prepared by calcination using triphenyl phosphine as doping agent.The results show that the crystal structure,morphology and dispersion of LiFePO₄ particles and the thickness of the surface carbon layer are not changed by appropriate amount of phosphorus modified carbon layer.However,the doped phosphorus atoms create more electron transport channels and increase the electron transport rate,thus improving the large-magnification performance of the material.For the first time,this work proposed that egg yolk was used as the carbon,nitrogen and phosphorus sources to realize the carbon coating of LiFePO₄ and complete the co-doping of nitrogen and phosphorus in the carbon layer.Meanwhile,further explored and analyzed the influence of the amount of egg yolk on the material.When the mass ratio of LiFePO₄ to egg yolk is 1:1,the composite material shows excellent cycle performance and large rate performance.This is mainly because both N and P atoms are electron donors,and their electrons are more easily ionized,and the generated electrons will become redundant carriers,which will accelerate the electron transfer rate,thereby improving the conductivity of carbon materials.The synergistic effect of the two electron donors causes this double-doped composite material to exhibit excellent electrochemical performance.Thirdly,LiFePO₄ was synthesized by solvothermal method with glycol as solvent and phytic acid as phosphorus source.The effects of reaction temperature,reaction time,molar ratio and concentration on the morphology and electrochemical properties of LiFePO₄ were studied in detail,and the mechanism of the synthesis process was discussed.It is found that the LiFePO₄ material prepared at the temperature of 180℃,time of 4 h,molar ratio of Li:Fe:P of 3:1:1.5 and concentration of 0.5 M has a special layered structure and relatively good crystallinity and uniformity.The study on the mechanism of synthesis of LiFePO₄ shows that due to the strong chelation of phytic acid,the intermediate Fe₃（PO₄）₂ is first generated,which then combines with Li⁺to form LiFePO₄ gradually.Finally,using phytic acid as phosphorus source,a kind of in situ self-assembled LFP@MWCNTs composite with 3D nano-conductive network and good interface structure was prepared,and the formation mechanism was clarified.Phy A can not only complexate with Fe²⁺,but also form chemical bonds with oxidizing groups such as-C=O or-COOH on the surface of functionalized MWCNTs.After solvothermal and high temperature calcination,LFP@MWCNTs composites with good interface characteristics can be generated,thus having excellent electrochemical performance.