| The increasing price of lithium and the increasing shortage of lithium resources have limited the application and development of lithium-ion batteries in large-scale energy storage.Sodium and lithium have similar physical and chemical properties,and sodium has abundant reserves and low prices,so low-cost sodium ion batteries have shown great potential in future large-scale energy storage systems.The electrochemical performance of sodium ion batteries depends to a large extent on the performance of the electrode materials.The positive electrode material directly affects the capacity,life,cycle stability,and high-current fast charge and discharge capabilities of sodium ion batteries.Therefore,the study of sodium ions Battery anode materials have important theoretical and practical significance.In this paper,a low-cost,environment-friendly high-capacity sodium ion O3 type NaNi0.4Mn0.4Fe0.2O2 is used as the research object.A high-capacity NaNi0.4Mn0.4Fe0.2O2 material was prepared by the sol-gel method,and the cathode material was modified by Ti doping.Combined with SEM,XRD and electrochemical testing techniques,the electrochemical performance of the cathode material and the phase transition mechanism during charge and discharge were studied.(1)NaNi0.4Mn0.4Fe0.2O2 materials were prepared by solid-phase method,NaOH-sulfate coprecipitation method,Na2CO3-acetate co-precipitation method,and citrate sol-gel method,and their electrochemical properties were evaluated.The comparison shows that the NaNi0.4Mn0.4Fe0.2O2 material prepared by the sol-gel method has good electrochemical performance.The first cycle discharge capacity of the material at 1C rate was 120.7 mAh·g-1.The capacity retention rate after 200 cycles was 69%,and the Coulomb efficiency during charging and discharging was basically close to 100%.(2)The electrochemical properties of NaNi0.4Mn0.4Fe0.2O2 materials at different voltages were studied.The results show that over-discharge at 1.5-2 V and over-charge at 4-4.3 V are not good for the cycling performance of NaNi0.4Mn0.4Fe0.2O2 materials,and a reasonable charge-discharge voltage range of 2-4 V is determined.(3)The effects of roasting temperature,pH of the liquid to be evaporated and the amount of citric acid on the electrochemical performance of NaNi0.4Mn0.4Fe0.2O2 materials were studied.The results show that the material prepared under the conditions of a mass ratio of citric acid to total metal cations of 1.5:1,a pH value of the liquid to be evaporated of 2.63,and a roasting temperature of 900℃ has a higher charge-discharge specific capacity.The specific discharge capacity at a rate of 0.1C was 161.5 mAh·g-1,and the specific discharge capacity at a high rate of 5C was 100.2 mAh·g-1.(4)The NaNi0.4Mn0.4-xFe0.2TixO2 material in which Ti is substituted for Mn has been studied.The results show that the proper amount of Ti doping improves the cycling performance of the cathode material;but when the Ti doping amount is too high,the material capacity is sharp Slip and cycle performance deteriorate.(5)The Na[Ni0.4Mn0.4Fe0.2]1-xTixO2 material with Ti substituted for all transition metal elements was studied.The results show that when the Ni/Mn/Fe ratio in the material is unchanged,the cycle performance of the material is improved with the increase of the Ti doping amount.When x=0.05,the capacity retention rate of 200 cycles at 0.5C rate of the cathode material was increased from 63.2%to 80.3%,and the capacity retention rate of 200 cycles at 1C rate was increased from 69.9%to 84.1%.(6)Ex-situ XRD tests were performed on NaNi0.4Mn0.4Fe0.2O2 materials and Na[Ni0.4Mn0.4Fe0.2]0.95Ti0.05O2 materials,and the results showed that O3?P3 phase transition occurred in both materials during charge and discharge. |
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