| Energy and environment are the focus of attention from all walks of life.Li-ion batteries have been widely used in power and energy storage systems because of their longer cycle life and better safety performance.However,due to the limited storage capacity and high price of lithium resources,the application of lithium ion batteries in large-scale energy storage faces new problems.Sodium and lithium have similar physical and chemical properties,and have abundant reserves and low prices,making sodium ion batteries one of the research hotspots.P2 type Mn-Ni-Fe ternary active material is considered to be the new generation of sodium ion battery cathode material with the most commercial potential due to its high specific capacity(The theoretical capacity is 240 mAh·g-1,1.5-4.3 V),economy and environmental protection.However,the rate performance and cycle performance of the P2 Mn-Ni-Fe ternary active material are not good,and it needs to be modified to optimize its electrochemical performance.In this paper,P2-Na0.67Mn0.5Ni0.3Fe0.2O2 cathode material was synthesized by sol-gel method.The metal Zr was used to modify the P2-Na0.67Mn0.5Ni0.3Fe0.2O2 cathode material.Using MgO to coat and modify P2-Na0.67Mn0.5Ni0.3Fe0.2O2 cathode material.The effects of reaction pH,calcination temperature,Zr doping amount and MgO coating amount on the physical and electrochemical properties of the cathode materials were investigated.The best process parameters for synthesis and modification of P2-Na0.67Mn0.5Ni0.3Fe0.2O2 cathode materials were obtained.The main conclusions are as follows:(1)Through thermogravimetric analysis and controlled variable method,a better preparation process of P2-Na0.67Mn0.5Ni0.3Fe0.2O2 cathode material was determined:The gel preparation process had a pH of 2.56;the pre-baking temperature was 500℃ and the holding time was 6 hours;the baking temperature was 900℃ and the holding time was 12 hours.The results show that the particle size of the prepared P2-Na0.67Mn0.5Ni0.3Fe0.2O2 material is relatively uniform,the boundary is clear,and the structure is a regular hexagonal prism,and the particles are uniformly and densely packed.In the voltage range of 1.5-4.3 V and the current density of 0.5C(1C=140 mA·g-1,the specific discharge capacity of the first cycle of the cathode material is 235.3 mAh·g-1,and the capacity retention rate after 100 cycles is 40.5%.It has a discharge specific capacity of 46.7 mAh·g-1 at a high rate of 5C,and a reversible capacity recovery rate of 67.3%when it returns to a rate of 0.1 C again.(2)The Zr doping modification of P2-Na0.67Mn0.5Ni0.3Fe0.2O2 material was studied by sol-gel method.The results show that small particles will appear on the surface of the crystal after Zr doping.When the Zr doping amount x=0.05,Na0.67[Mn0.5Ni0.3Fe0.2]1-xZrxO2 cathode material has better electrochemical performance.At a current density of 0.5C,the specific discharge capacity at the first turn was 207.7 mAh·g-1,and after 100 cycles,the capacity retention rate was 56.2%.It can still provide 78.6 mAh·g-1 high discharge specific capacity under 5C(700 mA·g-1)high rate,which is obviously larger than 46.7 mAh·g-1 before undoped,and the reversible capacity recovery rate is 75.7%.Zr doping improves the ion mobility of sodium ions.(3)The solid-phase method was used to study the modification of P2-Na0.67Mn0.5Ni0.3Fe0.2O2 materials with magnesium oxide(MgO)coating.The results show that the characteristic peaks of P2 type Na0.67Mn0.5Ni0.3Fe0.2O2-MgO cathode materials coated with MgO do not change significantly.With the increase of MgO coating ratio,the agglomeration of P2-Nao.67Mn0.5Ni0.3Fe0.2O2 particles gradually increased,but the secondary crystal grains of the material were not damaged.When the MgO coating ratio is 3%,the cathode material exhibits better electrochemical performance.The specific discharge capacity at the first cycle of 0.5C is 179.2 mAh·g-1,and the capacity retention rate is still 61.8%after 100 cycles.The material has a specific discharge capacity of 64.8 mAh·g-1 at a high rate of 5.0C,and the reversible capacity recovery rate is increased to 87.9%.The current exchange density i0 of the material is 7.46×10-5 mA·cm-2,which is much larger than that before uncoated,which is beneficial to the extraction and insertion of sodium ions. |
PDF Full Text Request