Structural Refinement of Ni doped LiFePO4 Materials used in Energy Storage Devices

New materials with advanced functional properties are needed to improve the performances of energy storage devices like rechargeable lithium ion (Li-ion) batteries, currently used in a variety of applications. In this work\ud we have concentrated our efforts in the synthesis and detailed characterization of promising cathode materials such as metal-doped LiFePO4 Olivines.\ud We have succesfully synthesized Ni-doped solid solutions LiFe1−xNixPO4 (0≥x≤0.15,0.9,1.0) and the novel materials were characterized by elemental analysis and scanning electron microscopy. Some materials were electrochemically\ud tested by cyclic voltammetry, showing that Ni doping may play a role in reducing electrode polarization increasing slightly the performances in Liion cells at low Ni concentrations x. The solid solutions were deeply studied\ud by X-ray diffraction (XRD) and X-ray Absorption Spectroscopy (XAS), in order to check modifications of the lattice and of the local structure as well as possible correlations with the functional properties. Rietveld refinement of XRD data showed that Ni doping induces an anisotropic contraction of the unit cell which mainly concerns the volume of the M2 octahedral sites.\ud Results of both XRD and XAS techniques are consistent and indicates the formation of homogeneous impurity-free solid solutions for x ≤ 0.15, ordering of Li in the (octahedral) M1 site, and of Fe and Ni in the M2 site of the olivine structure. Ni doping is found to induce an anisotropic shrinking of the unit cell with both Fe and Ni six-coordinated with oxygens, occupying distorted octahedral sites. The local structure measured by XAS shows that\ud average Fe-O and Ni-O distances do not change appreciably with Ni doping indicating that the reduction of cell size is mainly associated with the presence of shorter Ni-O distances at M2 sites. Possible connections among the\ud presence of a distribution of distorted octahedra of different size in the structure and different electrochemical performances of the material as a function\ud of doping are briefly discussed.\ud Most results of this work, obtained in the framework of an extended collaboration, were published on an international scientific journal (Journal of Power Source, vol. 213, 287-295 (2012

Chemical vapor deposition of molybdenum disulphide on platinum foil

Transition metal dichalcogenides (TMDCs) of group -VIB (MX2, M = Mo, W; X = S, Se, Te, etc.) with sizeable bandgap exhibit different interesting physical and chemical properties which open up new avenues to their technological applications. However, refinement and expansion of growth of TMDCs are required to realize commercializable products and to bring these technological applications to consumer's market. Here, we present growth of MoS2 catalyst by ambient pressure chemical vapor deposition (CVD). A Pt foil substrate was used to grow MoO2 TMDC from the reaction at high temperature (similar to 800 degrees C) between molybdenum trioxide (MoO3) and sulfur (S) in the presence of argon flow. The area coverage of 4 cm(2) of MoS2 layers was achieved on the Pt substrate, which is promising for application requiring large area coverage. The Pt foil was reusable in subsequent growth experiments although there was signature of surface sulphurization that occurred due to elevated temperature needed in the growth process. To confirm the growth of MoS2, Raman spectroscopy was performed. The frequency difference between the featured modes (E2g(1) at 384.7 cm(-1) and A(1g) at 409.3 cm(-1)) of MoS2 was 24.6 cm(-1), which confirmed the multilayer growth of MoS2 on Pt surface. The chemical composition of the asgrown MoS2 film was obtained by X-ray photoelectron spectroscopy (XPS): Mo 3d and S 2s doublets were observed which supported MoS2 formation

Double-edge X-ray absorption study of LiFe 1−x_1-x 1 - x Ni x_x x PO 4_4 4 cathode materials

X-ray absorption spectroscopy was used to study the evolution of the local structure induced by nickel doping in LiFe1−xNixPO4 solid solutions, functional materials for Li-ion cells. Samples with increasing Ni content (x = 0, 0.03, 0.05, 0.10, 0.15) were measured and analyzed by extended X-ray absorption fine structure (EXAFS) at both Ni and Fe K-edges. Experiments were carried out at room temperature and at various temperatures in the 23–300 K range. Fe–O and Ni–O nearest neighbor distances were accurately measured by EXAFS double-edge data-analysis. The Ni and Fe octahedral sites have been found to be largely distorted in the solid solutions, with two atoms at longer distances. Mean square relative displacements of neighboring atoms (O and P) are found to be larger indicating the presence of an increased level of configurational disorder. EXAFS shows that the basic connectivity structure between Fe/Ni- and P-based polyhedra is preserved, but differences in local geometry and distance distributions are found. Possible connections among the presence of a distribution of distorted octahedra of different size in the structure and different electrochemical performances of the material as a function of doping are briefly discussed

Structural study of LiFePO4-LiNiPO4 solid solutions

Modifications of the local structure and lattice parameters in LiFe1x NixPO4 (0 < x < 0.15,0.9,1.0) olivine-type solid solutions have been studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Samples have been synthesized and characterized in our laboratory, and a preliminary electrochemical characterization shows that Ni doping increases slightly the performances in Li-ion cells at low Ni concentrations x. Results of both XRD and XAS techniques are consistent and indicate ordering of Li in the M1 site, and of Fe and Ni in the M2 site of the olivine structure. Ni doping is found to induce an anisotropic shrinking of the unit cell with both Fe and Ni six-coordinated with oxygens, occupying distorted octahedral sites. The local structure measured by XAS shows that average Fe-O and Ni-O distances do not change appreciably with Ni doping indicating that the reduction of cell size is mainly associated with the presence of shorter NieO distances at M2 sites. Possible connections among the presence of a distribution of distorted octahedra of different size in the structure and different electrochemical performances of the material as a function of doping are briefly discussed