G. Ouvrard

An Electrochemical Cell for Operando Study of Lithium Batteries Using Synchrotron Radiation

A new electrochemical cell has been specially designed for operando experiments at synchrotron facilities both for X-ray diffraction and X-ray absorption. It allows the investigation of insertion materials under high current densities (up to 5C rate) and hence to study complex phenomena of structural and electronic changes out of equilibrium. The LiFePO 4 -FePO 4 system has been chosen as a case study to validate this cell, and tricky phenomena, with apparent delays in phase formation compared with the number of electrons exchanged, have been spotted.

Redox processes in the LixFeS2/Li electrochemical system studied through crystal, Mössbauer, and EXAFS analyses

Li2FeS2 is a compound that can be de-intercalated, leading to a new FeS2 cathodic material. The structure of the lithiated phase is developed from hexagonal-close-packing of S2− anions with tetrahedral iron ions, constituting (2D) infinite sheets. Lithium fills some of the tetrahedral and octahedral voids of the host in equivalent proportion. The first lithium removal corresponds to the oxidation of iron and to an important shift from its tetrahedral sites to others in the structure, as evidenced by Mossbauer and EXAFS measurements. De-intercalation of the second lithium results in the oxidation of sulfur, the final structure of the completely oxidized phase being Fe3+(Td)S2−(S2)2−12. As shown earlier, this charged material can be re-intercalated up to its original composition.

X-ray absorption study of lithium intercalated thiophosphate NiPS3

Abstract An EXAFS study of lithium-intercalated NiPS3 has been performed at the nickel K edge for various lithium contents. The large modifications of EXAFS spectra in the intercalates can be related to the displacement of reduced nickel atoms from their initial octahedral sulfur sites to tetrahedral ones within the slab. The results are discussed in comparison with previous physical measurements. Hypotheses for the reduction process and the induced structural modifications are proposed.

Transition metal displacement in cathodic host structures upon lithium intercalation

Abstract Reversible intercalation of lithium leads generally to reduction of the transition metal. An instability of the new electronic configuration acquired upon reduction maylead also to its shift to more stable coordination. A transition metal displacement may also take place in order to balance the ionic charge distribution which was disturbed by the reduction (or oxidation) state. These behaviours can explain either the forming (and/or ageing) of cathodic materials or their progressive amorphization.

A DFT study of lithium battery materials: application to the β-VOXO4 systems (X = P, As, S)

Abstract VO X O 4 systems have been considered as potential lithium battery electrodes. They mainly present two distinct structural types: the tetragonal “ α ” type with a two-dimensional framework, and the three-dimensional orthorhombic “ β ”. DFT calculations were performed on this latter system for several β -Li x VO X O 4 compounds ( x =0, 1; X =P, As, S). They allowed to propose structural models for VOAsO 4 and LiVOSO 4 , not fully crystallographically well described yet. Based on an experimental model of two-phase processes, these calculations led also to a good simulation of electrochemical potential values. A density of states analysis put in evidence the “inductive effect” and the role played by ( X O 4 ) n − groups inside the host frameworks on these potentials.

General behavior upon cycling of LiNiVO4 as battery electrode

Abstract X-ray absorption spectroscopy (XAS) study on the nickel and vanadium oxide LiNiVO 4 gives a lot of information on the local structural changes and the redox processes involved upon lithium uptake. This paper is mainly devoted to the comparison between the behaviors of two pristine materials which differ only by the annealing temperature. The general behavior at the atomic level is identical: in a first step, a reduction and a coordination change of vanadium from tetrahedral to octahedral, then a reduction and pairing of nickel, and few changes for the main part of the lithium uptake. The reversibility is insured by vanadium which move from octahedra to tetrahedra and is reoxidized to its pristine oxidation state. From the XAS results, we may conclude that the differences in the electrochemical curves is due to side reaction with the electrolyte.

Properties of amorphous nickel hexathiodiphosphate, Ni2P2S6

Abstract Transition metal thiophosphates can be prepared through “soft chemistry” reactions leading to amorphous phases. From nickel nitrate and lithium thiophosphate in aqueous solutions, a-NiPS3 can be obtained. Because of its highly divided structure, this amorphous material readily reacts with atmospheric moisture, and it must be handled in neutral atmosphere. Open circuit voltage curves of the Li/a- and c-NiPS3 have been studied. The amorphous phase shows a higher voltage and a lower polarization. A continuous decrease of the potential in the case of a-NiPS3 is consistent with the absence of microdomains of reduced and unreduced nickel, as was shown to be the case for c-NiPS3.

Nature of the oxidizing centres in transition metal phosphorus trisulfides MPS3 (M = Fe, Ni)

Abstract On the basis of FePS3 and NiPS3 band structure, it has been demonstrated that electron donation corresponding to lithium intercalation in these layered structures has to take place on low-lying, partially-filled, 3d-block bands. Past 31P NMR studies on the LixNiPS3 have been reassessed and are shown to be quite consistent with nickel reduction. Mossbauer spectroscopy studies suggest the occurrence of highly reduced iron sites with unusual characteristics.

Synergetic theoretical and experimental structure determination of nanocrystalline materials: study of LiMoS2

A combined approach is proposed to solve the structure of badly crystallized materials. It couples poor quality powder X-ray diagram (XRD) and XRD simulation deduced from first-principle geometry optimization. It is used to completely solve the LiMoS2 structure.

Olivine-spinel transitions in Mn2SiSe4 at high pressure

Abstract High pressure behavior of Mn 2 SiSe 4 with the olivine structure is investigated using in situ Raman spectroscopy and X-ray diffraction. The evidence for the occurrence of an olivine-spinel phase transition in the 20–40 kbar range at room temperature is presented, most probably accompanied by formation of spinelloid compounds at intermediate pressures.