Study On Preparation And Modification Of LiFePO₄ Cathode Material

Sustainable development strategies are driving the arrival of the new energy era.Lithium iron phosphate has become a new generation of highly promising lithium-ion battery cathode material due to its low price,environmental friendliness,high platform voltage and high theoretical capacity,but its poor conductivity,which leads to unsatisfactory multiplier performance,limits its application in high-power devices.For Li-ion batteries,the longer cycle life essentially depends on the structural stability of the electrode material,and the multiplicative performance is usually determined by the transport process of lithium ions at the interface between the electrode material and the electrolyte.Surface modification of the cathode material can improve the electrical conductivity of the material,and the coating material,as a protective passivation film,can prevent direct contact between the cathode material and the electrolyte,reduce the erosive effect of the electrolyte on the cathode material,and improve the long-cycle performance of the material.However,it is difficult for existing coating materials to meet multiple requirements at the same time,so we are committed to identify surface coating agents that can synergistically improve the electrochemical performance of LiFePO₄ with each other.The main research contents and conclusions are as follows.1.carbon as a good electron conductive agent for lithium iron phosphate has an irreplaceable role.Firstly,FePO₄,Li₂CO₃ were used as iron source and lithium source,asphalt,glucose,citric acid,and a composite carbon of glucose and citric acid were used as four groups of carbon sources,high temperature solid phase method to explore the effect of different carbon coating on the electrochemical performance of lithium iron phosphate.Raman test and thermogravimetric results showed that the highest degree of graphitization of surface residual carbon was formed by the composite carbon of glucose and citric acid,and the LiFePO₄/C prepared with it had a discharge specific capacity of 154 m Ah g^-1 at 0.2C current density.2.Metal oxides become important coating materials due to their stability under electrochemical windows.On the basis of the above obtained LiFePO₄/C sample,the LiFePO₄/C@Al₂O₃ was prepared by adding Al₂O₃ coating agent and adjusting the preparation process to find the suitable capping amount（1%,3%,5%mol）for high performance long cycle stability.The results showed that the capacity of the prepared sample could reach 145 m Ah g^-1 at 1C when the coating amount was 1%,and the capacity retention rate was 98%after 200 cycles,indicating the excellent stability of the prepared material.3.Based on the above study found that although ordinary metal oxides have excellent stability,can prevent electrolyte erosion and improve the capacity retention of the material,but have poor electrochemical activity,which is not conducive to the improvement of large multiplicity performance.The rare earth oxide La₂O₃ has both the stability of metal oxides and a low lithium ion diffusion potential,and is expected to be used as a capping agent to prepare large multiplicity long cycle stable lithium iron phosphate.Experimental data show that the prepared LiFePO₄/C@La₂O₃,with an excellent lithium ion diffusion rate of 1.4982×10^-13,has an electric capacity close to the theoretical value of 0.1C multiplicity devolved 165 m Ah g^-1,126 m Ah g^-1 at a large magnification of 10C,and a capacity retention of 96.5%for 500 cycles at 1C magnification.The above conclusions indicate that lithium iron phosphate with long-term high-rate cycling stability can be prepared by modification with rare earth oxides.