Synthesis,Doping And Electrochemical Performance Of LiFePo₄ And LiNi_0.8Co_0.1Mn_0.1O₂ Positive Electrode Materials

With the increasingly serious energy crisis and environmental pollution,China attaches great importance to the development of energy storage devices and electric vehicles,and lithium-ion batteries with longer cycle life and higher energy density have become the focus of research and development.Cathode materials are an important part of lithium-ion batteries.High nickel ternary Li Ni_xCo_yMn_zO₂（0.6≤x<1,x+y+z=1）and lithium iron phosphate positive electrode materials are of great potential for development.However,conventional high nickel ternary cathodes in polycrystalline form suffer from problems such as electrode/electrolyte side reactions and poor cycling stability.High nickel ternary materials with single crystal morphology can help alleviate problems such as electrode/electrolyte side reactions during cycling.Lithium iron phosphate positive electrodes possess good cycling stability,but suffer from problems such as low electronic conductivity and low lithium-ion diffusion rate.Carbon coating modification of lithium iron phosphate can help to improve its low electrical conductivity.In this paper,the synthesis and doping modification studies of high nickel ternary and lithium iron phosphate positive electrode materials are considered both from the preparation process and electrochemical performance point of view,with the main research contents as follows.（1）Small particle size spherical Ni_0.8Co_0.1Mn_0.1（OH）₂ precursors were prepared by co-precipitation method.The obtained precursors were mixed with Li OH in a certain ratio and then calcined at 750 ^oC under oxygen atmosphere to obtain single crystal Li Ni_0.8Co_0.1Mn_0.1O₂ positive electrode material.Li F was used as a dopant to modify the bulk phase of the high nickel single crystal positive electrode material.The doped material was able to maintain good structural stability,inhibit the side reactions between the material interface and electrolyte,and widen the diffusion channels of Li⁺in the material,thus improving the rate performance and cycling stability of the material.The first discharge specific capacity of the doped sample at 0.1 C rate was 240.00 m Ah g^-1（2.8-4.3 V）,and the capacity retention rate was 81.30%after 100 cycles at 1 C rate.（2）Single-crystal Li Ni_0.8Co_0.1Mn_0.1O₂ cathode materials doped with P element were successfully prepared by H₃PO₄ solution added co-precipitation process in combination with subsequent high-temperature calcination process.Compared with the conventional doping method,doping during the co-precipitation reaction can reduce the material preparation process.The expanded crystal plane spacing improves the structural stability of the material during long cycles and enhances the diffusion rate of lithium ions.94.05 m Ah g^-1 of discharge specific capacity at 10 C rate and 83.53%of capacity retention after 100 cycles at 1 C rate were achieved.（3）The Fe PO₄ precursor was prepared by co-precipitation method,and the Fe PO₄precursor was mixed with Li₂CO₃ and glucose in a certain ratio to obtain the nano-scale Li Fe PO₄ positive electrode material by high-temperature solid-phase method.The Li Fe PO₄ positive electrode material was modified by doping with sulfur monomers as dopants by means of sulfur on the gas phase.The doping optimizes the crystal structure of LFP and facilitates the enhancement of its electrical conductivity and lithium-ion diffusion coefficient,which results in superior electrochemical performance of LFP.The doped sample has a high first discharge specific capacity of 168.74 m Ah g^-1（2.5-4.0 V）at 0.1 C rate and a capacity retention rate of 84.13%after 450 cycles at 1 C rate.