Study On The Structure And Cycle Stability Of Lithium Vanadium Phosphate Based Cathode Material

In recent years,monoclinic Li3V2(PO4)3/C cathode material has become a research hot spot of Li ion batteries,due to its high oxidation-reduction potential,structural stability,and high theoretical specific capacity.However,the cycling stability of Li3V2(PO4)3/C is poor under multi electron exchange conditions,which limits the commercialization and application of Li3V2(PO4)3/C in the field of power batteries.Therefore,the structure and cycling stability of Li3V2(PO4)3/C cathode material were studied in this paper.Hydrogen peroxide was added in ball milling process by using carbothermal reduction method.The influence of the structure and performance of synthetic Li3V2(PO4)3/C cathode material was investigated by adding hydrogen peroxide.The result shows synthetic Li3V2(PO4)3/C is 8.2 g,hydrogen peroxide is added 15 m L and the sintering time is 6 h.The electrochemical performance of the synthetic Li3V2(PO4)3/C is not decreased,but also increased slightly.The first discharge capacity is 127 m Ah?g-1 when the charge cut-off voltage is 4.3 V.This is because vanadium pentoxide was formed to wet gel by adding hydrogen peroxide in ball milling process,and the raw materials were reached the molecular level and mixed evenly.Therefore this can significantly be improved the electrochemical performance of Li3V2(PO4)3/C material.In addition,the influence of cycling stability of Li3V2(PO4)3/C cathode material was also investigated with the increase of charge cut-off voltage.When the voltage test range was 3.0~4.3 V,the initial discharge capacity of Li3V2(PO4)3/C was 127.1 m Ah?g-1at 0.2 C,the initial discharge capacity was 122.9 m Ah?g-1 at 1 C,and the capacity retention rate was 98.6% after 50 cycles.When the voltage test range was 3.0~4.6 V,the initial discharge capacity of Li3V2(PO4)3/C was 146.2 m Ah?g-1at 0.2 C,the initial discharge capacity was 142.0 m Ah?g-1 at 1 C,and the capacity retention rate was 92.8% after 50 cycles.When the voltage test range was 3.0~4.8 V,the initial discharge capacity of Li3V2(PO4)3/C was 172.6 m Ah?g-1at 0.2 C,the initial discharge capacity was 165.5 m Ah?g-1 at 1 C,and the capacity retention rate was 86.8% after 50 cycles.When the charge cut-off vol t age was increased from 4.3 V to 4.8 V and returned to the discharge state after charge and discharge cycles,the accumulation of V5+ on the cathode material surface was increased,the lattice volume of Li3V2(PO4)3/C material has greater irreversible expansion,and an longer bond length of Li(3)-O.The bond lengths of Li(2)-O and Li(1)-O become shorter after cycling,which is the reason for the deterioration of the material's cycling stability.In order to improve the cycling stability of Li3V2(PO4)3/C cathode material under the multi electron exchange conditions,the influence of Mg2+ doping on the structure and cycling stability of Li3V2(PO4)3/C material was investigated.The result shows that the cycling stability of Li3V2(PO4)3/C cathode material is enhanced by doping Mg2+ in V position.In Li3(V1-x Mgx)2(PO4)3/C system,Li3(V0.09Mg0.01)2(PO4)3/C shows the best cycling stability.When the voltage test range was 3.0~4.3 V,the initial discharge capacity of Li3(V0.9Mg0.1)2(PO4)3/C was 129.1 m Ah?g-1at 0.2 C,the initial discharge capacity was 125.1 m Ah?g-1 at 1 C,and the capacity retention rate was 99.0% after 50 cycles.When the voltage test range was 3.0~4.6 V,the initial discharge capacity of Li3(V0.9Mg0.1)2(PO4)3/C was 147.2 m Ah?g-1at 0.2 C,the initial discharge capacity was 142.8 m Ah?g-1 at 1 C,and the capacity retention rate was 93.1% after 50 cycles.When the voltage test range was 3.0~4.8 V,the initial discharge capacity of Li3(V0.9Mg0.1)2(PO4)3/C was 175.6 m Ah?g-1at 0.2 C,the initial discharge capacity was 167.2 m Ah?g-1 at 1 C,and the capacity retention rate was 87.8% after 50 cycles.By comparing with Li3V2(PO4)3/C,Li3(V0.9Mg0.1)2(PO4)3/C shows an higher cycling stability in multi electron exchange process.In addition,the reason of lower cycle stability is discussed under multi electron exchange.The bond lengths of Li(1)–O and Li(2)–O become shorter with the increased charge cut-off voltage,it is unfavorable for the intercalation and removal of the two lithium ions in the lattice structure.while the bond length of Li(3)–O becomes longer with the increase of charge cut-off voltage.This facilitates the intercalation and removal of Li(3)ion in the lattice structure.Therefore,the cycling stability of Li3(V0.9Mg0.1)2(PO4)3/C sample decreased with the increase of charging cut-off voltage.The effect of Al2O3 coated Li3V2(PO4)3/C cathode material on the cycling ability was investigated.Compared with the XRD spectra of Al2O3 uncoated Li3V2(PO4)3/C material,Al2O3 coated Li3V2(PO4)3/C sample shows almost the same diffraction peaks and no impurity peaks are found.In Al2O3 coated Li3V2(PO4)3/C samples,Al2O3(4.5 mol%)coated Li3V2(PO4)3/C has the best cycling stability.When the voltage test range was 3.0~4.3 V,3.0~4.6 V and 3.0~4.8 V respectively,the initial discharge capacity of Li3V2(PO4)3/C was 131.5 m Ah?g-1,148.0 m Ah?g-1 and 176.1 m Ah?g-1at 0.2 C respectively.The initial discharge capacity of Li3V2(PO4)3/C was 126.2 m Ah?g-1,143.4 m Ah?g-1 ? 168.6 m Ah?g-1 respectively,and the capacity retention rate was 99.5%,93.4% and 91.6% after 50 cycles.Compared with Al2O3(4.5 mol%)coated Li3V2(PO4)3/C sintered for 6 h by adding hydrogen peroxide during the ball milling process and Mg2+ doped Li3V2(PO4)3/C,Al2O3(4.5 mol%)coated Li3V2(PO4)3/C material shows a higher cycling stability.