Preparation And Modification Of P2-Na_2/3[Ni_1/3Mn_2/3]O₂ As Cathode Material Of Sodium Ion Batteries

In this study,P2-Na_2/3[Ni_1/3Mn_2/3]O₂ cathode material for sodium ion batteries had been synthesized via acetate combustion method.Firstly,the effects of different diaphragms on the electrochemical properties of the material were investigated.Secondly,the effects of calcination temperature,calcination time and presintering temperature on the composites were discussed.Subsequently,the P2-Na_2/3[Ni_1/3Mn_2/3]O₂ material was modified by doping metal element.The effects of different membranes on the discharge capacity were investigated.Found that the use of glass fiber membrane can play the maximum capacity of the battery.Further studies demonstrated that optimum technologic parameters were as followed:the presintering temperature was at 450?,the presinteringtime was 5 h,the calcination temperature was at 900?,the calcination time was 15 h.The P2-Na_2/3[Ni_1/3Mn_2/3]O₂material synthesized by acetate combustion method had excellent electrochemical performance in the voltage range of 2.0-4.0 V.The material delivered the initial discharge capacities at 0.1,0.2,0.5 and 1.0 C were 84.1 mAh/g,84.7 mAh/g,78.4 mAh/g,72.6 mAh/g,and had a capacity retention of 103.2%,98.9%,98.5%and 98.6%after 10 cycles,respectively.To further lift the discharge capacity of the material,it delivered a discharge capacity of 168.5 mAh/g at 0.1 C between 1.5 and 4.5 V,and a capacity retention of only82.6%after 10 cycles.Even it delivered a discharge capacity of only 79.5 mAh/g at 1.0 C.The poor cycle performance was due to P2-O2 irreversible phase transformation when charging to high cut-off voltage.Therefore,it was necessary to modify the Na_2/3[Ni_1/3Mn_2/3]O₂ material to improve its electrochemical performance under high voltage.P2-Na_2/3[Ni_1/3Mn_2/3]O₂ material was modified by Fe,Cu and Zr transition metal elements.The influence of different doping elements and amount on the properties of composites were investigated.2%Fe,6%Cu and 6%Zr-doped sample had the best electrochemical performance.In the voltage range of 2.0-4.0 V,they delivered an initial discharge capacity of 83.7 mAh/g,80.2 mAh/g and 79.3 mAh/g,and a capacity retention of104.8%,100.4%and 109.0%after 10 cycles at 0.1 C,respectively.Even they showed the discharge capacity of 77.4 mAh/g,69.1 mAh/g and 70.4 mAh/g,and a capacity retention of99.6%,99.3%,and 99.6%,respectively.The optimal sample was charged-discharged and tested at 1.5-4.5 V,2%Fe,6%Cu and 6%Zr-doped samples could deliver the initial discharge capacity of 156.3 mAh/g,211.6 mAh/g,and 196.5 mAh/g at 0.1 C,and the capacity retention of 90.6%,93.9%,and89.6%after 10 cycles,respectively.Even they showed the high discharge capacity of 99.6mAh/g,115.2 mAh/g,and 101.3 mAh/g at 1.0 C,and a capacity retention of 94.3%93.5%and 98.2%after 10 cycles,respectively.The results demonstrated that the electrochemical properties of the materials were improved at high cut-off voltage.In order to realize the industrial application,the synthesis process of P2-Na_2/3[Ni_1/3Mn_2/3]O₂ was explored.Results showed that the best layered structure,the uniform particle size and the optimal electrochemical performance were achieved for the sample synthesized with ahydroxide co-precipitation method followed by a solid state reaction.The material deliveredan initial discharge capacity of 87.9 mAh/g at 0.1 C.It also provided the initial discharge capacity of 72.3 mAh/g and maintain 108.0%of its initial capacity after 50 cycles between 2.0 and 4.0 V.