Optimization And Electrochemical Performance Of P2-Na_0.71Co_0.96O₂ Sodium Ion Battery Cathode Material

At present,lithium-ion batteries have been widely used in people’s production and life,but the lack of lithium resources and uneven distribution make it impossible to better meet people’s needs in the future.Therefore,the sodium-ion battery,which has similar charging and discharging mechanism of the lithium-ion battery,has attracted the attention of researchers.As we all know,the cathode material is the key material that determines the performance of sodium ion batteries.More researched cathode materials include transition metal oxides,polyanionic compounds,and Prussian blues.Among them,transition metal oxides are the most promising cathode materials with high energy density,various types,and simple synthesis process.This type of material still has an irreversible phase transition process during the sodium insertion/de-sodium removal process,which leads to the collapse of the crystal structure of the electrode material.In order to solve the above problems,this thesis uses different methods to improve the electrochemical performance of the P2Na_0.71Co_0.96O₂（NCO）electrode material,and conducts a series of correlation studies.The specific research content is as follows:（1）This chapter uses a simple sucrose-assisted combustion method,Co（NO₃）₂?6H₂O as Co source,and Na₂CO₃as Na source to synthesize P2-Na_0.71Co_0.96O₂cathode material.The material has a large unit cell volume and interplanar spacing,and can provide high and stable reversible capacity as a sodium ion battery electrode.The sucrose content can adjust the degree of combustion exotherm,so as to obtain Na_0.71Co_0.96O₂with different interlayer spacing.The crystal structure and electrochemical performance analysis proved that when the proportion of sucrose in the raw material is about 10%,the best electrochemical performance is shown.The highest discharge capacity of Na_0.71Co_0.96O₂is 113.40mAhg^-1.After 300 cycles,the discharge capacity stabilizes at 89.20mAhg^-1,and the coulombic efficiency is close to 100%（capacity retention rate reaches 78.60%）.The excellent electrochemical performance of Na_0.71Co_0.96O₂is attributable to the fact that sucrose assisted combustion can expand the interplanar spacing of Na_0.71Co_0.96O₂,thereby broadening the sodium ion transport channel.（2）Using high temperature solid phase method,Na_0.71Co_0.96-xTi_xO₂（x=0,0.057,0.073,0.09）cathode material was prepared by replacing part of Co with Ti in Na_0.71Co_0.96O₂.Through XRD Rietveld refinement,as the doped Ti content increases,the unit cell data becomes larger and the crystal volume becomes larger.When it is used as a cathode material for electrochemical testing,it shows good performance.Ti doping can obtain a smoother cyclic voltammetry curve,indicating that Ti effectively alleviates the complex phase transitions during charge and discharge,and improves electrochemical performance.When x=0.073,Na_0.71Co_0.887Ti_0.073O₂shows the best electrochemical performance.At a high current density of 1000mAg^-1,the first-cycle discharge specific capacity is 105.60mAhg^-1,which decays to 71.70mAhg^-1after 1000 cycles,and the capacity retention rate is 67.90%.（3）Using the Co₃O₄obtained by the oxidation treatment of ZIF-67 as the cobalt source and the anhydrous sodium carbonate as the molten salt,The Na_0.71Co_0.96O₂cathode material was prepared by high-temperature solid-phase method.XRD and SEM were used to characterize the phase and morphology of the material.Through the electrochemical performance test,the Na_0.71Co_0.96O₂cathode material shows a higher initial capacity(at a current density of 100mAg^-1,the initial discharge specific capacity is as high as 126.30m Ahg^-1).Electrochemical kinetics shows that the cathode material Na_0.71Co_0.96O₂has a large Na⁺diffusion coefficient,but the cycle performance of the electrode material is poor due to the uneven particle size.