Vanadium-oxide-based electrode materials for Li-ion batteries

Abstract

Vanadium pentoxide (V2O5) with a layered crystalline structure is a promising cathode material for lithium-ion batteries (LIBs). V2O5 possesses theoretical capacities of 442 mAh g−1 for three Li+ intercalations, or 294 mAh g−1 for two Li+ intercalations per formula. These values are much higher than those of traditional cathode materials, such as LiFePO4. However, several problems largely restrict the battery performance of V2O5, such as small Li+ diffusion coefficient, low electrical conductivity, irreversible phase transitions, and dissolution of vanadium into the electrolyte. Therefore, V2O5 exhibits poor rate capability and cycling stability. This thesis aims to improve the performance of V2O5 in regard to its electrical conductivity, lithium diffusion coefficient, and structural stability of V2O5 by using strategies, such as nanostructuring, element doping, adding carbon additives, and conductive polymer coating. The effects of electrode compositions and voltage windows on the electrochemical properties of V2O5 were investigated. The electrode compositions were varied by changing the ratio among V2O5, carbon black (CB), and poly(vinylidene fluoride) (PVDF). Two electrodes were prepared with the different V2O5:CB:PVDF ratios of 7:2:1 and 8:1:1. The V2O5 electrode with the 7:2:1 composition exhibited better cycling and rate performance in the voltage range of 1.5–4.0 V, due to higher electric conductivities. The electrochemical properties of this electrode were further improved by changing the voltage windows of charge/discharge. The narrow voltage window of 2–4 V only allows V2O5 to have a maximum of two lithium intercalations, which excludes the formation of irreversible ω-Li3V2O5 phase. In the voltage range of 2–4 V, the electrochemical reversibility of the 7:2:1 V2O5 electrode was greatly improved, leading to better cycling performance. Poly(3,4-ethylenedioxythiophene) (PEDOT) and multi-walled carbon nanotubes (MWCNTs) were employed to modify commercial V2O5 to prepare composite electrode materials for LIBs. It was found that MWCNTs improved the electric conductivity of the MWCNT-modified V2O5. In comparison, PEDOT not only performed better in enhancing the electric conductivity of the PEDOT-modified V2O5 but also enhanced its stability against electrolyte. The V2O5 modified with 20% PEDOT exhibited the current density of 574 mA g−1 for charging at 2.6 V, which is much larger than the 293 mA g−1 of the sample with 20% MWCNT. When applied as a cathode for lithium ion batteries, the sample modified with PEDOT performed the best, followed by the sample modified with both MWCNTs and PEDOT, and the sample modified with MWCNTs only. Cu-doped V2O5 nanobelts were synthesized by a facile hydrothermal treatment method as cathode materials for LIBs. The single phase Cu-doped V2O5 nanobelts were obtained with up to 4 mol% of copper. Both the V2O5 and Cu0.04V2O5 nanobelts were highly interconnected to form web networks. The width of the Cu0.04V2O5 nanobelts was smaller than that of the V2O5 nanobelts. The electric conductivity of the Cu0.04V2O5 was enhanced, due to the mixed valences of Cu and V ions. When applied as the cathode material for LIBs, the Cu0.04V2O5 nanobelts showed better cycling and rate performance than that of V2O5 nanobelts.