People involved in the research topic :
Guy OUVRARD (Pr), Philippe MOREAU (MC), Florent BOUCHER (CR)

• Description of the research topic
• Some results
• Collaborations
• Publications

Figure 1 : tin oxides can be used as negative electrodes in lithium ion batteries. The irreversible production of Li2O was first supposed to occur before the formation of an Li-Sn alloy. The Radial Distribution Function around the Sn atom shows that, for the composition Li2SnO, the tin atom still retains a oxygen environment, in contradiction with the previous hypothesis. The complete study led us to propose the existence of a composite Li, Sn, O phase. (Collaboration L. Nazar group, Univ. of Waterloo (Ontario).

Figures 2 & 3 : LiNiVO₄ can reversibly react, in a lithium battery, with 7 lithium atoms per formula unit. Such a behavior is unusual and led us to study the redox centres (V and/or Ni) in this compound. Are V and Ni atoms in their elemental state at the end of the discharge?

X-ray absorption edges reflect the oxydation states of the considered atoms. In performing experiments at the Ni and V K edges, for different lithium compositions, we showed that the V atoms are reversibly reduced from the initial oxydation +V state to the state +II, but never down to the state 0. The Nickel atoms are reduced to an almost element state from their initial +II state. This process is not fully reversible.

This last result confirms the hypothesis of an initial formation of an amorphous matrix containing Nickel agregates. (EXAFS results).

Figures 4 & 5 :

Absorption edges contain a lot of information on the electronic structures and the site symmetry of the considered atom. It is often difficult to extract quantitative results from the experimental data.

In the case of Li_1,2V₃O₈, considered as one of the best material to be used in future positive electrodes for lithium-polymer batteries, it is both important to understand the redox processes and the structural modifications occuring during the reaction with lithium.

We have extracted the decomposition of the spectra using simple mathematical functions for the first 20 eV of the vanadium K edge.

From the positions and intensities of all the fitted functions, we could obtain information on the degree of distorsion of the octahedra (pre-peak intensity) as well as a precise measurement of the evolution of the oxidation state of the vanadium atom (arc-tangent position).