Lydia Laffont

Toward Understanding of Electrical Limitations (Electronic, Ionic) in LiMPO4 (M = Fe , Mn) Electrode Materials

To better understand the factors responsible for the poor electrochemical performances of the olivine-type LiMnPO 4 , various experiments such as chemical delithiation, galvanostatic charge and discharge, cyclic voltamperometry, and impedance conductivity, were carried out on both LiFePO 4 and LiMnPO 4 . Chemical delithiation experiments confirmed a topotactic two-phase electrochemical mechanism between LiMnPO 4 and the fully delithiated phase MnPO 4 (a = 5.909(5) A, b = 9.64(1) A, and c = 4.768(6) A). We conclude that the limiting factor in the MnPO 4 /LiMnPO 4 electrochemical reaction is nested mostly in the ionic and/or electronic transport within the LiMnPO 4 particles themselves rather than in charge transfer kinetics or structural instability of the MnPO 4 phase. For instance, the electrical conductivity of LiMnPO 4 (σ ∼ 3.10 - 9 S cm - 1 at 573 K, ΔE ∼ 1.1 eV) was found to be several orders of magnitude lower than that of LiFePO 4 (σ ∼ 10 - 9 S cm - 1 at 298 K, ΔE ∼ 0.6 eV).

Reactivity of Antimony Oxides and MSb2O6 ( M = Cu , Ni , Co ) , Trirutile-type Phases with Metallic Lithium

Isostrutural MSb 2 O 6 (M = Cu,Ni,Co) trirutile-type phases were prepared by solid-state synthesis and their electrochemical activity towards lithium was investigated. NiSb 2 O 6 and CoSb 2 O 6 showed similar electrochemical behavior, with an uptake of about 18-19 Li per formula unit along the first reduction and only 6-7 Li reversibly removed upon subsequent cycling. This totally differs from the behavior of CuSb 2 O 6 that was found to react with only about 7 Li during the first reduction, without any capacity recovered on subsequent charge. From X-ray diffraction data, both Cu and Co phases lead to an amorphous composite down to 0 V. From high-resolution transmission electron microscopy observations, reduced CoSb 2 O 6 consists of 10-50 nm Co particles dispersed in a Li 3 Sb + Li 2 O matrix. For CuSb 2 O 6 , our results and observations enlightened a two-step reduction: First, the formation of Cu clusters through an electrochemically-driven exchange reaction leading to Li 2 Sb v 2 O 6 , and then reduction of this matrix into 5 nm Sb domains dispersed in an insulating amorphous Li-rich Li-Sb v -O matrix, preventing any further alloying reaction and any charge reaction. The complete charge irreversibility observed on Li/Sb 2 O 5 half-cells confirmed this point, while the Li reduction of Sb 2 O 3 emphasized a close similarity with the reactivity of the (Ni/Co)Sb 2 O 6 phases, suggesting a first reduction step of Sb 5+ into Sb 3+ , assuring conduction and subsequent cycling capacity. Despite the composite nature of the as-formed electrode and the very fine Sb particles, the reversibility of the alloying reaction was not found to be satisfactory for Li-ion cells.

Plasmon as a tool for in situ evaluation of physical properties for carbon materials

Various carbon materials were studied using electron energy loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The aim was to relate the information that can be extracted from EELS (plasmon energy values) to physical properties in order to obtain a new, simple and in situ tool that would allow the properties of carbon materials to be deduced routinely from EELS. The principle of the method is expected to be valid for any carbon material whatever the morphology (fibre, film), texture (anisotropic, isotropic), nanotexture, structure (turbostratic, graphitised) and also whatever the way the materials are processed. Provided the EELS spectroscopy is performed using appropriate acquisition parameters, the only limitation revealed so far is about the carbon material type, which has to derive from thermoplastic precursors. Reasons for both relations and limitations are discussed.

Eco-Efficient Synthesis of LiFePO4 with Different Morphologies for Li-Ion Batteries

Formula is presently the most studied electrode material for battery applications. It can be prepared via solution, although it requires well-controlled pH conditions to master the iron valence state in the newly created material. Here we report its synthesis via the use of “latent bases” capable of releasing a nitrogen base upon heating. This way of controlling the reaction pH enables, in the absence of excess Li, the preparation of Formula -free Formula powders having various morphologies and showing good electrochemical performance. This approach is shown to offer great opportunities for the low-temperature synthesis of various electrode materials.

Structural and electrical properties of gold nanowires/P(VDF-TrFE) nanocomposites

High aspect ratio gold nanowires were uniformly dispersed into a poly(vinylidene difluoride–trifluoroethylene) [P(VDF-TrFE)] matrix. The nanowires were synthesized by electrodeposition using nanoporous anodic alumina oxide templates. The intrinsic optical conductivity of the gold nanowires was determined by valence electron energy loss spectroscopy. The effect of increasing volume fraction of Au nanowires on the morphology and crystallization of P(VDF-TrFE) matrix was investigated by differential scanning calorimetry. The crystallinity of P(VDF-TrFE) is strongly depressed by the randomly dispersed nanowires. Above 30 vol% the crystallization of P(VDF-TrFE) is suppressed. The bulk electrical conductivity of nanocomposite films, at room temperature, obeys a percolation behaviour at a low threshold of 2.2 vol% and this was confirmed using the surface resistivity value. An electrical conductivity of 100 S m−1 is achieved for a 3 vol% filler content.

Microstructural Characterization of Graphite Spheroids in Ductile Iron

The present work brings new insights by transmission electron microscopy allowing disregarding or supporting some of the models proposed for spheroidal growth of graphite in cast irons. Nodules consist of sectors made of graphite plates elongated along a 〈a〉 direction and stack on each other with their c axis aligned with the radial direction. These plates are the elementary units for spheroidal growth and a calculation supports the idea that new units continuously nucleate at the ledge between sectors.

The reaction of lithium with CuCr2S4—lithium intercalation and copper displacement/extrusion

The electrochemical reduction of CuCr2S4 by Li was shown to occur in two steps, involving first a combined Cu extrusion/Li insertion leading to a Li1.75Cu0.25Cr2S4 phase that, upon further lithiation, undergoes a conversion reaction. Analytical transmission electron microscopy, high resolution electron energy loss spectroscopy and neutron diffraction experiments have been used to elucidate the copper, sulphur and chromium oxidation states during the first electrochemical step. HRTEM shows the growth of copper dendrites during the discharge of the compound and their disappearance upon subsequent re-oxidation, while EDS analysis indicates that all of the copper cannot be extruded from the pristine phase upon discharge, nor be fully reinserted on charging. HR-EELS confirms the following oxidation state Cu+Cr23+(S2−)3S− for the pristine material and demonstrates that the insertion of lithium in the sample only corresponds to a change of the sulphur oxidation state whereas the valence of chromium was not modified. This observation was directly confirmed by neutron data that show an increase in the S–S bond length upon lithium insertion.

Piezoelectric properties of polyamide 11/NaNbO3nanowire composites

Polyamide 11(PA 11)/sodium niobate nanowire (NW) 0–3 composites with different volume fractions of NWs were synthesized. The electric polarization (P) was measured as a function of the applied electric field (E). The P–E hysteresis loop was used to work out the remanent polarization Pr of these materials. The dielectric permittivity and the piezoelectric strain constant were determined. Good impedance matching between inorganic and organic phases leads to higher electroactivity than conventional lead-free 0–3 composites. The piezoelectric voltage of the PA 11/NaNbO3 NW composites is of the same order as those obtained for fluorinated piezoelectric polymers. These composites could have some applications in flexible, low-cost, environmentally friendly piezoelectric sensors and actuators.

Misorientations in spheroidal graphite: some new insights about spheroidal graphite growth in cast irons

Local diffraction patterning, orientation mapping and high resolution transmission electron microscopy imaging have been used to characterize misorientations in graphite spheroids of cast irons. Emphasis is put here on bulk graphite, away from the nucleus as well as from the outer surface of the spheroids in order to get information on their growth during solidification. The results show that spheroidal graphite consists in conical sectors made of elementary blocks piled up on each other. These blocks are elongated along the prismatic a direction of graphite with the c axes roughly parallel to the radius of the spheroids. This implies that the orientation of the blocks rotates around the spheroid centre giving low angle tilting misorientations along tangential direction within each sector. Misorientations between neighbouring sectors are of higher values and their interfaces show rippled layers which are characteristic of defects in graphene. Along a radius of the spheroid, clockwise and anticlockwise twisting between blocks is observed. These observations help challenging some of the models proposed to explain spheroidal growth in cast ions.

HRTEM, TSC and broadband dielectric spectroscopy of a discotic liquid crystal

Due to its molecular configuration, Pyrene 1,3,6,8-tetracarboxylic rac-2-ethyl hexyl ester exhibits a mesomorphic phase at room temperature. High resolution transmission electron microscopy (HRTEM) of samples with various thermal history yield important results on discotic liquid crystal nanostructure. The evolution of the molecular organisation with the temperature has been determined by differential scanning calorimetry (DSC). The physical structure of this columnar liquid crystal is shown by dynamic dielectric spectroscopy (DDS) along a broad temperature and frequency range. As this liquid crystal is used as film electronic devices, the thermostimulated micro-technique (μ-TSC) has been improved and developed. The behavior and molecular mobility in this columnar liquid crystal are studied. The correlation with thermal studies allows us to understand dielectric properties as a function of physical state.