Structural Study of Lithium Insertion in Vanadium Oxides Used as Cathode Materials
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and LiVO 3 and the vanadium oxide V6 0 11 . For the highest inserted product (Li 3V 2O5 ) (Fig. ld) two large diffraction peaks at 4.95A and 4.85kA belonging to the reported (oLixV 20 5 phase, confirm the validity of these syntheses and compare well with electrochemical ones. However, Xray analysis requiring good micro-crystalline samples and as the products obtained for xexp = 1.2 and 1.6 are characteristic of high temperature compounds, we decided to continue via a solid state process. First we verified that for x=1.2 and 1.6, this route was pertinent and we prepared several samples for x values increasing up to 3. An example of XRD patterns is shown in Fig. 2.
b) Li 3VO4
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90 60
c) LiVO2 (X
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II , ead 2(a p15 • pt i Fig. 2 : Comparative XRD patterns. (a) experimental , (b) (c) computed I
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Clearly, the yLixV 2 0 5 homogeneity range is limited to x=l. Excess lithium induces the formation of vanadates in which all the vanadium is in V5+ state and vanadium oxides containing V 3+and Vn1owing the disproportionation of the supplementary V4+, created by the excess. Several phases are detected ; out-of-equilibrium mechanisms occur during the synthesis, in agreement with previous electrochemical experiments [14]. Based on the main products corresponding to each theoretical composition, one can separate domains related to different kind of de-mixing mechanisms. First, LiVO 3 is formed. Then Li 3VO 4 is observed with successively vanadium oxide exhibiting increasing amounts of V31. The formation of LiVO 3 and Li3 VO 4 is linked to the necessary higher capability to withstand the Li÷ excess while the different oxides take into account the electron excess. The highest lithium yield leads to the formation of the Li VO 0 2 phase which supports both excess in V + and Li'. All these data can be summarized by use of a schematic diagram (Fig. 3). Then, the different domains can be easily identified by the successive 115
compositional triangles intercepted when lithium amount increases (xLiV205 straight line). The typical discharge curve of Li//V2 0 5 cell (Fig. 4), exhibits for x>1 four plateaus. Each plateau is characteristic of a de-mixing mechanism. Their limits are close to that of the high temperature domains described above and so, validate the correlation between the electrochemical behavior and solid state diagram.
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Moreover, the formation of the socalled "nLixV 2O5 " phase can be accounted for. This "phase" is formed at the highest lithium yield (around x=3). Insertion mechanisms are claimed reversible throughout the 1.5
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