Synthesis And Electrochemical Performance Of P2 Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ Cathode Material For Sodium Ion Battery

As a secondary battery,sodium ion battery has wide raw material resources and relatively stable electrochemical performance,and is considered as one of the most promising energy storage systems.However,due to the large ionic radius of sodium ions and their slow diffusion,the development of cathode materials for sodium ion batteries still faces huge challenges.In this paper,P2-type Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ has been prepared by a co-precipitation method for sodium ion batteries.Taking it as the research object,a series of studies were conducted on its electrochemical performance,the effect of phase transformation on the material under high voltage,and the effect of different additives on the performance of the full cell assembled with the hard carbon.In this paper,the structure and morphology of the material have been characterized by X-ray diffraction?XRD?,and scanning electron microscopy?SEM?,respectively.Through the introduction of Co element,the material's rate capability has been improved,and it delivers a specific discharge capacity of 38 m Ah/g at a rate of 20 C?1 C = 100 m A/g?.Through the GITT test and calculation,it is found that the sodium ion diffusion coefficient of the material has been improved.In addition,the material also exhibits good cycling performance,and the capacity retention rate after 800 cycles at 1C is 73%.In addition,the electrochemical performance of P2-type Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ under different cut-off voltages is explored,and it is found that the material capacity decays rapidly under high-voltages.To explore the mechanisms,in-situ XRD and transmission electron microscope?TEM?characterizations have been performed.Through in-situ XRD investigation,it is concluded that the transition metal layered oxide cathode material will undergo an irreversible phase change under high voltages.The TEM was used to observe the distribution and depth of the micro-cracks generated after the phase transition of the material,and the micro-cracks were further elaborated to explain the capacity decay mechanism of such materials under high voltage during the cyclic charging process.The repetitive phase transitions in the following lead to the existence of residual stresses in the crystal lattice,and the occurrence of intracrystalline cracks in the material is the main reason for the attenuation of the material capacity.In order to make this research have a reference value and a guiding role for sodium ion batteries in actual industrial production,it is also of great significance to study the adaptation of the electrolyte to the positive and negative electrode materials.In this paper,Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ is used as the positive electrode material,and hard carbon is used as the negative electrode material to assemble the full battery.By changing the electrolyte additive components?such as fluorinated vinyl ester?FEC?,1,3-propane sultone?PS?and PS / FEC?to explore the impact of different additives on the electrochemical performance of the whole battery.And finally found that when both the additives FEC and PS coexist,the sodium ion full-cell device has higher energy density,large rate performance and good cycle stability?79% capacity can be maintained after 100 cycles?.