Electrolytic V 2 O 5 : Synthesis, Characterization and Lithium Insertion Behavior
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With the aim of understanding the origin of this strange phenomenon, measurements of the chemical diffusion coefficient of Li÷ ions were undertaken by several electrochemical techniques. Figure 8 shows that it increases during the first process and then decreases over the second one. This decrease becomes more important for a Li content larger than 1.5, probably because of some steric hindrance in the interlayer space. However, in this composition range, it is much larger in charge. The internal resistance of the cell was inferred by the
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Figure 9 :Variations of the internal resistance of the cell vs. the intercalation rate.
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Figure 8 Variations of the Li+ diffusion
shows that it increases with the Li content which indicates a decrease of the material electronic conductivity with x. Therefore, one can believe that the increase in polarization for x>1.5 in discharge has a kinetic origin mainly, due to a strong lowering of the Li diffusion coefficient. However it is difficult to understand why the diffusion coefficient is not the same in charge and in discharge. The cycling behavior has been studied in non-optimized conditions at C/5 and C/3 rates. Compared to y- and to W-V20 5, prepared by Li insertion into cc-V 20 5 , one can see (Figure 10) that e-V 20 5 exhibits a much better capacity retention, in the 2 voltage ranges, down to 2V and 1.5V. But the reversible capacity down to 2V is only -100Ah/kg, due to kinetics limitations.
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voltage response to a current pulse. Figure 9
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Figure 10 : Comparison of the cycling behaviors of an e-V 20 5 compound and w-V 20 5 , in the 4V2V/Li (a) and 4V-1.5V/Li (b) voltage ranges.
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CONCLUSION It has been shown that the electrodeposition of a vanadium oxide is possible at an inert electrode by oxidation of a vanadyl sulfate solution at 1.8
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