| In recent years,with the widespread use of lithium-ion batteries,the shortage of lithium resources has become increasingly serious,and there is an urgent need to find effective alternatives.Sodium in the same main group as lithiumat has abundant resources,wide distribution,low price,and similar properties to lithium.These advantages make sodium-ion battery a very promising secondary battery.Among many sodium-ion battery cathode materials,the transition metal oxide P2-type Na0.67Fe0.5Mn0.5O2 has the advantages of high theoretical specific capacity,volumetric/mass energy density and low cost.In recent years,it has become a key research object of sodium-ion battery cathode materials.However,this material has poor structural stability and cycle performance,which needs to be improved.In this study,Na0.67Fe0.5Mn0.5O2 materials with high crystallinity and good morphology were prepared by spray-drying method and sol-gel method,then modified by V-doping and preparing composite electrodes to improve the electrochemical performance.(1)Na0.67Fe0.5Mn0.5O2 was prepared by spray drying method with organic manganese salt and iron salt as raw materials.When polyethylene glycol was selected as the additive and calcined at 850 ?,the sample shows a hollow spherical appearance composed of many Na0.67Fe0.5Mn0.5O2 primary particles.Electrochemical analysis shows that the sample with both a hexagonal sheet architecture and a hollow sphere architecture exhibits the best electrochemical performance,which initial discharge specific capacities of the sample at 0.1 C rate is 202.3 mAh·g-1 after 10 cycles at 0.1 C?0.2 C?0.5 C?1 C?2 C?0.1 C rates(0.1 C for 5 times cycles),the capacity retention rate is 88.2%.(2)Na0.67Fe0.5Mn01.5O2 cathode material was synthesized by the sol-gel method with ethylenediaminetetraacetic acid as the chelating agent.The sample P2-Na0.67Fe0.5Mn0.5O2 calcined at 850? shows the best electrochemical performance.Compared with the sample synthesized by spray drying method,the samples prepared under this condition exhibits higher purity and crystallinity,and the hexagonal sheets are more evenly distributed.The initial discharge specific capacities of the sample at 0.1 C rate is 240.0 mAh·g-1,which is very close to the theoretical value(260 mAh·g-1),respectively,after 10 cycles at 0.1 C?0.2 C?0.5 C?1 C?2 C?4 C?0.1 C rates(0.1 C for 5 times cycles),the capacity retention rate is 88.4%,and the sodium ion diffusion coefficient is 1.94×10-10 cm2·s-1.(3)Based on the preparation of P2-Na0.67Fe0.5Mn0.5O2 by the sol-gel method,the material was modified by doping with high-valent vanadium ions.The presence of V5+ can increase the grain size,refine the particles,effectively improve the material stability and electrochemical performance during the sodium-ions de-intercalation/intercalation process.Na0.67Fe0.5Mn0.5O2 with a doping amount of i=0.02 has discharge specific capacity 238.9 mAh·g-1,close to theoretical value.The initial discharge specific capacities of the samples at 1 C,2 C,4 C and 6 C rate are 114.2,80.0,48.4 and 20.4 mAh·g-1.The discharge specific capacity at large magnification is greatly improved compared to the undoped sample.The sodium ion diffusion coefficient of V-doped sample(i=0.02)is 3.92×10-10 cm2·s-1,which is better than the undoped one.(4)Na3V2(PO4)2F3/C and Na0.67Fe0.5Mn0.5O2 cathode materials were mixed evenly in an inert atmosphere to prepare the xNa0.67Fe0.5Mn0.5O2-Na3V2(PO4)2F3/C electrode.The best electrochemical performance is obtained with x=3.The initial discharge specific capacity of the samples at 0.1 C rate is 216.1 mAh·g-1,which is very close to the theoretical value(227 mAh·g-1).The composite electrode shows better rate performance and cycle performance than the Na0.67Fe0.5Mn0.5O2 electrode.The initial discharge specific capacities of the sample at 4 C,6 C and 8 C rates are 51.3,30.1 and 14.5 mAh·g-1.Compared with the V-doped sample,the discharge specific capacity of the composite electrode is larger at 4 C and 6 C rates,and it still has charge/discharge performance at 8 C rates.After 300 cycles at 1 C,the discharge curve still has an initial discharge voltage of 3.8 V,and the capacity retention rate is 16.1%higher than that of the uncompounded sample.The cathode materials prepared by the above four methods all show good electrochemical performance.Among them,the best specific discharge capacity of the optimal sample synthesized by the sol-gel method is closest to the theoretical specific capacity;The modification of V-doping can improve the specific discharge capacity and cycle stability at a large rate while ensuring that the specific discharge capacity of the sample is not attenuated at small rate;The composite electrode solves the problem of low initial discharge voltage,improves the discharge specific capacity and cycle stability at a large rate,thus it has the best comprehensive electrochemical performance. |
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