E. Prouzet

Structure determination of ZnPS3

Abstract ZnPS 3 structure determination was performed from single crystal diffraction data. The structure refinement was conducted from 634 reflexions and 29 variables in C2/m space group to R = 4.0%. The phase exhibits the expected MPS 3 layered structure (CdCl 2 type). The particular behavior of the MPS 3 atomic species respective to the cation size and their electronic configuration as pointed out in the other MPS 3 s (M  Mn, Fe, Co, Ni, Cd) is confirmed by the present study. Within error margin, the phase was found to be stoichiometric with empty Van der Waals gap.

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.

Structural, physical and electrochemical characteristics of a vanadium oxysulfide, a cathode material for lithium batteries

A vanadium oxysulfide is obtained by a reaction between water solutions of a vanadyl salt and sodium sulfide at room temperature. After drying under mild conditions, the formulation of this phase is V2O3S·3H2O. Thermogravimetric analyses show that it is not possible to remove completely water without losing sulfur. This is in agreement with proton nuclear magnetic resonance experiments which prove that water molecules are tightly bonded to vanadium. Magnetic susceptibility and X-ray absorption spectroscopy measurements allow to define the oxidation states of vanadium and sulfur, (IV) and (−II) respectively. From extended X-ray absorption fine structure spectroscopy at the vanadium K edge and infrared spectroscopy, the local structure around vanadium can be defined as a distorted octahedron, with a vanadyl bond and an opposite sulfur atom. Magnetic susceptibility and X-ray absorption spectroscopy measurements on chemically lithiated compounds show a complex charge transfer from lithium to the host structure upon lithium intercalation. If it appears that vanadium atoms are reduced, a possible role of sulfur atoms in the redox process has to be considered. Cycling tests of lithium batteries whose positive consists of oxysulfide are promising with 70 cycles under a regime of C8, without noticeable loss in capacity of 120 Ah/kg.

Room temperature synthesis study of highly disordered a-Ni2P2S6

Abstract The metathesis reaction in aqueous media between Ni 2+ and (P 2 S 6 ) 4− ions, leading after drying to the Ni 2 P 2 S 6 phase, was followed by in situ X-ray absorption. This reaction proceeds regularly to the final sulfide phase. The complexes formed in water during the reaction as deduced from the EXAFS study correspond to single sheet crystallites with a diameter of ∼0.5 nm and a composition close to (H or Li) 1.6 Ni 2 (P 2 S 6 ) 1.4 . After drying, this phase does present a local structure corresponding to crystallized Ni 2 P 2 S 6 and a transmission electron microscopy study confirms the occurrence of these crystallites, scattered within an amorphous matrix. The temperature dependence of the magnetic susceptibility has been measured from 5 to 310 K. The magnetic behavior, which shows magnetic ordering of only the crystalline portion of the phase, is well explained by the structure of the end-product a-NiPS 3 .

Reassessing of the lithium intercalation mechanism in layered nickel

The study of the powder diffraction diagrams of the LixNiPS3 intercalates has been reassessed using the highly sensitive INEL curve detector. This detector used in the Debye-Scherrer geometry and with a monochromated radiation allowed the accurate recording of the air sensitive nickel thiophosphate intercalates powder diffraction patterns and their subsequent processings, an experiment never done before. The study showed that the intercalation proceeds according to a macroscopic biphased mechanism, which was not found in previous works. The two phases implied are the starting phase NiPS3 and the fully reduced end-compound Li2NiPS3. For this latter material, monoclinic and hexagonal cell parameters are proposed. The study of the pristine phases Full Width at Half Maximum diffraction line variation during lithium intercalation shows that the non-intercalated grains are sensitive, through microstrain broadening or/and size decrease, to the formation of the Li2NiPS3 intercalate. These results answer earlier conflicting observations, in particular about the quasi-equilibrium discharge curves.

EXAFS study of lithium-intercalated iron thiophosphate

The lithium intercalates LixFePS3 (0 ≤ x < 1.5) were studied using EXAFS at the iron K edge. The important modifications of the spectra were interpreted as originating in displacements of reduced iron atoms from their original octahedral sites to tetrahedral ones. Contrary to parent LixNiPS3 phases where the same cationic shift led to nickel pairs in (Ni0S4)2 groups, Fe0 is found to be located at the same time within the van der Waals gap and within the slab. Although reoxydation of the LixFePS3 phases shift the iron ions to octahedral sites, that of the gap remained partially occupied, the phase undergoing a 2D → 3D structural modification.

EXAFS and EELS evidence for the synthesis of a new amorphous nickel oxythiophosphate NiPS2O

Abstract The room temperature synthesis of NiPS3 by mixing a nickel salt and [Li4+(P2S6)4−] in water from which oxygen has not been removed leads to an oxygenated phase. Nickel K edge X-ray absorption data and nickel L2,3, phosphorus and sulfur K edges electron energy loss spectroscopy experiments show that a pure oxythiophosphate phase with a formulation close to NiPS2O is obtained. This composition is explained by a partial oxidation of the pristine (P2S6)4− anion, before reaction to form (P2S4O2)4−. The higher oxidized anions are destroyed by the breaking of the phosphorus-phosphorus bond.