Modification And Electrochemical Properties Of P2-type Layered Oxide Na_0.67Ni_0.15Mn_0.85O₂

The scarcity of lithium resources has shifted attention to sodium ion batteries,which have more abundant reserves and lower prices.The large radius of Na⁺（1.02（?））will cause problems such as complex structural evolution,slow ion diffusion kinetics and large volume expansion of the material,resulting in poor electrochemical performance.P2-type Na_xNi_yMn_1-yO₂ cathode materials are favored by researchers because of their simple preparation,high specific capacity and fast ion transport.In this paper,P2-Na_0.67Ni_0.15Mn_0.85O₂ material was selected as the research object to investigate the conformational relationship between the crystal structure and electrochemical properties of the material through the strategies of temperature regulation and elemental doping to prepare a highly stable P2-type cathode material.The main contents are as follows:（1）The P2-Na_0.67Ni_0.15Mn_0.85O₂ cathode material was prepared by the sol-gel method,and the effects of different calcination temperatures on its electrochemical properties were investigated,and the structural evolution and kinetic behavior of the material during the charging and discharging process were analyzed.The results show that the crystallinity of the material becomes better and the grain size becomes larger as the calcination temperature increases.The material has optimal electrochemical properties at the calcination temperature of 900℃.The P2-Na_0.67Ni_0.15Mn_0.85O₂material has optimal electrochemical properties.The first discharge specific capacity is145.5 m Ah g^-1 at 0.1 C;it has 71.7%capacity retention after 200 cycles at 1 C.（2）To improve the cycling performance of P2-Na_0.67Ni_0.15Mn_0.85O₂ cathode materials,a series of P2-Na_0.67Ni_0.15Mn_0.85-xTi_xO₂（x=0,0.05,0.10,0.15）materials were synthesized by cation(Ti⁴⁺)doping.The effects of different doping amounts on the morphology,structure and electrochemical properties of the materials were investigated.The effects of different doping amounts on the morphology,structure,and electrochemical properties of the materials were investigated.The non-in situ XRD and CV tests showed that the reversibility of Na⁺exfoliation/embedding increased and the crystal stability was enhanced after Ti⁴⁺doping.The material has the best electrochemical performance when the doping amount is 5%mol,and the capacity retention of the material is 85.4%after 200 cycles at 1 C.It is demonstrated that Ti⁴⁺substitution can stabilize the crystal structure of the material and increase the sodium ion diffusion layer spacing as well as enhance the electronic conductivity.（3）To further enhance the electrochemical properties of the materials,anion（F^-）doping was performed on top of Ti⁴⁺doping.The results show that the substitution of the more electronegative F^-can enhance the relative content of Ni³⁺and Mn⁴⁺,which is beneficial to the redox reaction between Ni²⁺/Ni³⁺/Ni⁴⁺.In addition,F^-can broaden the diffusion layer of the material and suppress the oxygen loss from the O^2-/O^-reaction in the high-voltage region by reducing the content of lattice oxygen,enhancing the structural stability.The P2-Na_0.67Ni_0.15Mn_0.80Ti_0.05O_1.8F_0.2 material has a first discharge specific capacity of 137.1 m Ah g^-1 at 0.1 C and has a capacity retention of 94.9%after50 cycles at 0.2 C.The above results indicate that the F^-doped materials can optimize the cycling stability of the materials by changing the redox center activity.