The Research On Modification Of P2-Phase Layered Oxide Na_0.67Ni_0.33Mn_0.67O₂ As Cathode Material For Sodium Ion Battery

With the development of electric vehicles and smart grid,people have put forward higher requirements for energy storage and conversion devices.A variety of secondary battery systems with high specific energy and low cost characteristics have been widely studied.Sodium ion battery（SIBs）is considered as a new type of energy storage device that is likely to replace lithium ion battery（LIBs）in some applications for the reason that sodium have abundant resources,low price and similar physical and chemical properties to lithium.The electrochemical properties of batteries are greatly influenced by the properties of cathode materials.P2-phase layer oxides have high discharge voltage and capacity and are considered as a promising cathode material.But the biggest problem of these materials is the capacity degrade rapidly during the cycle which limiting their use in sodium ion batteries.In this paper,F-ion doping and Al₂O₃ coating were used to improve the electrochemical properties of P2-Na_0.67Ni_0.33Mn_0.67O₂ cathode material.The specific research content is as follows:Different content F-ion were introduced to Na_0.67Ni_0.33Mn_0.67O_2-xF_x by solid-state reaction method.The physical properties of the materials were studied by means of XRD,SEM,TEM,Raman spectra,XPS and other characterization techniques.The electrochemical properties were analyzed by means of electrochemical cycle,ratio,impedance and CV.The results show that O doping of F ion can significantly improve the cycling and rating performance of P2-phase layered oxide cathode material.When X=0.15,the cycling and rating performance of the material is the best.The capacity of the first cycle is 91 mAh g^-1,and the capacity retention rate can still reach 70.9%after 450cycles at the current density of 120 mA g^-1.Using this method of mixing isopropanol aluminum and P2-Na_0.67Ni_0.33Mn_0.67O₂material and then calcinated at 500℃to obtain Na_0.67Ni_0.33Mn_0.67O₂ material with different proportion of Al₂O₃ coated.XRD and TEM results show that no impurity phase are obtained after Al₂O₃ coated.The charge and discharge curves show that the material had the best cycle retention rate under the condition of coating 1 wt%Al₂O₃.The electrochemical performance of pure phase material and 1 wt%Al₂O₃ coating show that the cycle and rate performance of the material were improved after coated 1 wt%Al₂O₃.The capacity of P2-Na_0.67Ni_0.33Mn_0.67O₂ material was 41.5 mAh g^-1after 200 cycles under the current density of 120 mA g^-1,with a 61.9%capacity retention.However,the material after coated 1 wt%Al₂O₃,the capacity can still reach 69.7 mAh g^-1and with a88.7%capacity retention after 200 cycles;EIS show that the material of coated Al₂O₃will have lower charge transfer resistance between materials and electrolyte.The reason why electrochemical properties improved of the material after coated Al₂O₃ was that the Al₂O₃ coating layer can reduce the side effects between the electrolyte and material effectively,and can stable structure of the material to a certain extent by inhibiting the dissolution of cathode material.The two works in this paper have effectively improved the electrochemical properties of P2-Na_0.67Ni_0.33Mn_0.67O₂ by doping and surface coating modification methods.The synthesis method is simple and easy to scale up,which is of great reference significance for the industrialized production of positive electrode materials for sodium ion batteries.