Preparation And Modification Of NaMnCoNiO Cathode Material For Sodium-ion Batteries

As a cost-effective and sustainable alternative to lithium-ion batteries for large-scale energy storage systems,sodium-ion batteries are attracting increasing numbers of researchers due to the great availability of sodium resources and their similar electrochemical properties.Manganese based transition metal electrode has the characteristics of environmental protection and large discharge capacity.It is a potential electrode material for sodium ion batteries.However,due to the large radius of sodium ion,the low diffusion coefficient of sodium ion still exists in the charging and discharging process,and a series of phase transitions will occur in the material during the charging and discharging process.The above problems lead to poor material cycle performance,serious capacity attenuation and low capacity at high multiple times.The positive electrode material has great influence on the performance of sodium ion battery.Therefore,the preparation of cathode materials with stable structure,high specific capacity and long cycle life is an urgent problem in the field of sodium ion batteries.The main research contents of this paper are as follows:（1）The Na_0.67Mn_0.66Co_0.17Ni_0.17O₂ electrode materials is prepared by coprecipitation method combined with piecewise calcination.The microstructure analysis shows that the cathode material is 2μm in length.When the electrode material is applied to the battery,the specific capacity of 218 m Ah g^-1 is reached at 0.1 C,and the capacity is only maintained at 36 m Ah g^-1 after 500 cycles at 10 C.This is mainly because the charge and discharge process of sodium-ion batteries depends on the repeated removal of sodium ions from the cathode material interlayer,and the spacing between material layers increases,resulting in P2-O2 phase transition.The cathode material has a considerable initial discharge capacity and there is still a lot of room for optimizing its performance.（2）Aiming at the existing problem of ternary cathode material,Zn,Mg,Li element doping in Na_0.67Mn_0.66Co_0.17Ni_0.17O₂ have been carried on by piecewise calcination.The results show that the initial specific capacities of Zn,Mg and Li doped electrodes are229,237,212 m Ah g^-1 at 0.1 C.According to the calculated ion mobility coefficient,the ion diffusion coefficient of Mg doping is the largest.Mg doping can effectively increase the layer spacing,and reduce the effect of harmful phase transformation,thus improving the electrochemical characteristics of electrode capacity.（3）Na_0.67Mn_0.66Co_0.17Ni_0.17O₂cathode material with the structure of hierarchical microsphere is prepared by controlling preparation parameters.P2-Na_0.67Mn_0.66Co_0.17Ni_0.17O₂ show excellent discharge specific capacity(266 m Ah g^-1 at0.1 C,89 m Ah g^-1 at 5 C).The core and shell formed by the convergence of micro and nano sheets provide excellent stability,and the void between the core and shell effectively alleviates the volume expansion caused by sodium ion delamination.Transition metal doping effectively ameliorates the structural distortion caused by the Jahn-Teller effect and provides a higher ion mobility coefficient(1.5865×10^-11).