Insertion Reaction of Lithium into Iron-Substituted V₆O₁₃

Abstract

Among the members of a series of vanadium oxides between V₂O₄ and V₂O₅, which have promising properties as an active material for secondary lithium batteries, hexavanadium tridecaoxide V₆O₁₃ is known to exhibit an excellent cycling performance.
In this study, Fe_yV_6-yO₁₃ (0<y<1.0) samples, in which Fe³⁺ substitutes for V⁴⁺, forming V⁵⁺ in proportional to the y value, have been synthesized as a modification of V₆O₁₃ by heating mixtures of Fe₂O₃, V₂O₅ and V with appropriate ratios in evacuated quartz ampoules. The chemical lithium insertion reaction into these oxides has been investigated using n-BuLi/hexane solutions.
The interplanar spacings of Li_xFe_yV_(6-y)O₁₃ expand along either the b or c axis in a discontinuous manner with increasing x values. The upper limit of lithium insertion, x_max, increases linearly with the iron contenty. These results suggest that V⁵⁺ is responsible for the reduction through lithium insertion. The electrode potential vs. x relation indicates that insertion of lithium up to x_max proceeds stepwise; in the first step up to [Li_(1+y)Fe³⁺_yV⁴⁺_(5-y)V⁵⁺O₁₃], and further to [Li_xmaxFe³⁺_yV³⁺_(2+y)V⁴⁺_(4-2y)O₁₃] in the second step where V⁵⁺ ions in the oxides are reduced to V³⁺ completely.
It was shown that x_max can be extended by forming extra V⁵⁺ ions through the substitution of iron and structural changes accompanied by valence change of vanadium in the host lattice is responsible for determining x_max.