An Investigation On Synthesis And Modification Of Li₃V₂?PO₄?₃/C Cathode Material For Lithium-Ion Battery

Monoclinic Li₃V₂?PO₄?₃ has attracted lots of attentions due to its relatively high specific capacity, high voltage, fast Li+ diffusion and stable cycling performance. However, the relatively low electron conductivity is adverse to its charge-discharge capability at large current, and thus restricts the application in power lithium-ion batteries. This article uses different synthesis methods and different carbon sources to prepare LVP/C composite, in order to solve this problem and improve the rate capability. The main parameters of the synthesis process have been optimized, and the optimal LVP/C composite has been paired with commercial Li4Ti5O12?LTO? material to assemble coin-type full cells which have also been tested and analyzed in detail.Chitosan?CHI? and alginic acid?AA?, for the first time, are used as carbon sources in preparing LVP/C composite, and the corresponding solid-state method and rheology-phase method are applied respectively. The effects of different carbon sources during synthesis process have been discussed and the enhancement of electrochemical performance has also been explained reasonably. X-ray diffraction, Raman spectrum analysis, thermogravimetric analysis, particle size analysis, scanning electron microscope and transmission electron microscope are applied to investigate physical properties of LVP/C. The electrochemical properties are fully investigated by galvanostatic charge-discharge test, cyclic voltametry and AC impedance test.When chitosan assisted solid-state method is applied to synthesized the LVP/C composite, the additive amount of chitosan is optimized to be 25wt% and sintering temperature is optimized to be 800?. The optimal LVP/C performs outstanding rate and cycling performances, it can deliver 106.9 m Ah g^-1 at 20 C charge-discharge rate. When charged at 10 C rate and then discharged at 30 C and 50 C rates, it can deliver 108 mAh g^-1 and 91.1 mAh g^-1, respectively, and there is no evident capacity loss for both after 400 cycles.In alginic acid assisted rheology-phase method, when ethylene glycol is used as progress medium, 25wt% amount of AA is added and sincering temperature is set at 800?, the corresponding LVP/C product shows the excellent performance. During the process, the synergistic effects of EG and AA are not only good to the refinement and distribution of raw materials, but also good to the stability of rheological system. The LVP/C particles present a loose pseudo-hierarchical structure, and this structure owns electron-ion di-continuous transporting paths, which benefits the rapid charge transfer reaction and high rate capability. Between 3.0 and 4.3 V, it can cycle at 40 C rate for 600 times and release 45.8 mAh g^-1 at last. When charged at 5 C rate and then discharged at 90 C rate, 61.4 mAh g^-1 can be achieved after 600 cycles. In the range of 3.0-4.8 V, it can deliver an initial capacity of 123.2 mAh g^-1 at 20 C rate, and there is no evident capacity fading after 400 cycles.For the LVP ? LTO full cell, different capacity matching ratios of positive electrode to negative electrode have been studied and the preferred CR ratio turn out to be 1.2. These full cells show relatively good capacity property, voltage property and cycling performance, which indicates that LVP is a promising cathode material in applying in power lithium-ion batteries.