| Lithium ion battery has become the hot research topic of new electrochemical energy storage devices due to its high energy density,high cell-voltage,excellent cycling behavior and environmental friendliness.Among a series of electrode materials,the monoclinic Li3V2?PO4?3stands out due to its stable structure,high theoretical capacity,good cycling performance and low price.However,the low electronic and Li+ion conductivity of Li3V2?PO4?3 affects its rate performance and cycling performance.In this paper,carbon coated Li3V2?PO4?3 samples were prepared by a simple solid-state reaction.In order to fullfill the industrialization requirements,through the study of the influence of the order and the amount of citric acid added before and after the first ball milling on Li3V2?PO4?3,it is concluded that when the molar ratio of V and citric acid added before the first ball milling is 1:1,the diffusion coefficient of lithium ion is the highest,the internal resistance is the smallest,and Li3V2?PO4?3/C shows the best rate performance and excellent cycling behavior.When the cell is cycled at a rate of 0.5 C and the voltage is between 3.0-4.3 V,the initial discharge capacity reaches 128 mAh g-1.And when the cell is cycled at a rate of 10 C,even 20 C,it still reached 105 mAh g-1 and 98 mAh g-1,respectively,with capacity retention of87%.The shortage of lithium resources on the earth and the increasing demand for lithium ion batteries lead to the high price of lithium resources.As a new type of energy storage battery in the future,sodium ion battery can be used as an important substitute for lithium ion battery.The Na3V2?PO4?3 with the three-dimensional frame NASICON structure has attracted the attention of researchers for its stable structure,high discharge specific capacity and lower price.Na3V2?PO4?3 is considered to be a very promising cathode material.In this paper,off-stoichiometric Na3-3x-3x V2+x?PO4?3/C?x=0–0.10?nanocomposites is synthesized by high energy ball milling,which has excellent high-rate performance and cyclic performance.We synthesize nonstoichiometric Na3-3xV2+x?PO4?3/C by adjusting the different molar ratios of Na and V.Rietveld refinement results revealed that with the increase of V content,the lattice volume of NVP samples slightly expanded.which may make the Na ion diffusion into and out of Na3-3xV2+x?PO4?3 particles easier,increase ionic mobility and diffusion coefficient,thus increase the charge-discharge performance at high rates.However,excessive V ions will occupy the voids,hinder the migration of sodium ions,and reduce the charge and discharge performance at high rates.When x=0.05,Na2.85V2.05?PO4?3/C exhibits the most excellent performance.The discharge capacity of 112.4 mAh g-1 is obtained at 1 C,reaching 96%of the theoretical specific capacity.The discharge capacity of 100.7 mAh g-11 is obtained at 0.5 C after 500 cycles,the capacity retention of 95%is retained. |
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