Cécile Tessier

Stacking faults in the structure of nickel hydroxide: a rationale of its high electrochemical activity

The usually observed broadening of some lines in the X-ray diffraction pattern of Ni(OH) 2 is shown to result from the presence of stacking faults leading to the existence of some fcc domains within the hexagonal compact oxygen packing. Simulation of the X-ray diffraction pattern with the DIFFaX program allowed us to propose a structural model and to estimate the amount of defects. The existence of these stacking faults explains in a very convenient way the relation between the line broadening and both the electrochemical behaviour and the presence of unexpected bands in the Raman spectra.

Optimized Chemical Stability and Electrochemical Performance of LiFePO4 Composite Materials Obtained by ZnO Coating

An improved electrode performance is achieved in LiFePO 4 materials of different origin by a simple coating procedure. LiFePO 4 samples were prepared by a conventional ceramic synthesis. 57 Fe Mossbauer data evidenced that heating above 750°C with carbon should be avoided to limit the amount of iron phosphide impurities. The ceramic sample was compared to a commercial sample differing in particle size and porous system. ZnO precipitation of a ca. 2 nm films on the particles surface of both phosphate samples resulted in a protection vs electrolyte hydrolysis product that limits iron dissolution during lithium battery operation. As a consequence, capacity retention improves significantly at different rates.

Viscosity and Carbon Dioxide Solubility for LiPF6, LiTFSI, and LiFAP in Alkyl Carbonates: Lithium Salt Nature and Concentration Effect

In this paper, we have reported the CO2 solubility in different pure alkyl carbonate solvents (EC, DMC, EMC, DEC) and their binary mixtures as EC/DMC, EC/EMC, and EC/DEC and for electrolytes [solvent + lithium salt] LiX (X = LiPF6, LiTFSI, or LiFAP) as a function of the temperature and salt concentration. To understand the parameters that influence the structure of the solvents and their ability to dissolve CO2, through the addition of a salt, we first analyzed the viscosities of EC/DMC + LiX mixtures by means of a modified Jones-Dole equation. The results were discussed considering the order or disorder introduced by the salt into the solvent organization and ion solvation sphere by calculating the effective solute ion radius, rs. On the basis of these results, the analysis of the CO2 solubility variations with the salt addition was then evaluated and discussed by determining specific ion parameters Hi by using the Setchenov coefficients in solution. This study showed that the CO2 solubility has been affected by the shape, charge density, and size of the ions, which influence the structuring of the solvents through the addition of a salt and the type of solvation of the ions.

Role of propane sultone as an additive to improve the performance of a lithium-rich cathode material at a high potential

This study presents the use of 1,3-propane sultone (PS) in the [EC–DMC + 1 mol L−1 LiPF6] electrolyte as a protective additive for the Li-rich-NMC xLi2MnO3–(1 − x)LiMO2 (x ≫ 1; M = Ni, Co, Mn) cathode–electrolyte interface during cathode material activation and cycling at a high potential (5 V vs. Li). The results showed that the presence of 1% PS (w/w) ensured complete and better electrode activation during the first cycle than EC–DMC + 1 mol L−1 LiPF6. Thus, Li//Li-rich-NMC half-cell and Gr//Li-rich-NMC full-cell provided capacities as high as C = 330 mA h g−1 during charge and C = 275 mA h g−1 during discharge with a higher cut-off voltage of 5 V. Measurements by cyclic voltammetry demonstrated that activating at such a voltage enhanced the redox activity from Li2MnO3 activation. At same time, the contribution of nickel and cobalt electroactivity is decreased at their regular voltage. This feature was attributed to structural modifications occurring on the surface to the bulk of the material. Long-cycling tests of Li//Li-rich-NMC half-cells with PS provided a higher reversible capacity and superior capacity retention (245 mA h g−1 after 240 cycles) with good coulombic efficiency (99 ± 1%) and better high-discharge rate capability (above 180 mA h g−1 at 1 C regime) than those obtained using conventional electrolytes without additive.

A Conductive Additive for Positive Electrodes of Alkaline Batteries Structural, Physicochemical, and Electrochemical Evaluation of Li Co O 2

For the first time, LiCoO 2 materials were synthesized at a very low temperature, below 100°C. The influence of synthesis parameters, such as temperature and time of reaction, on the structure and conductivity of the material was investigated. It was shown that these materials contain structural defects, such as stacking faults and lithium vacancies, which lead to a good electronic conductivity of these materials as compared to classical high-temperature materials. The time and temperature of reaction decrease the amounts of stacking faults in the materials. However, lithium intercalation can occur during a long-time synthesis, leading to a decrease in conductivity of the material. If process parameters are conveniently chosen, a LiCoO 2 material with a conductivity of around 10 - 1 S cm - 1 can be obtained and used with success as a conductive additive in positive electrodes of alkaline batteries. The major advantage of this new additive is that LiCoO 2 prevents the loss of capacity of alkaline cells at low voltage, in the whole potential window of the positive electrode in alkaline batteries.

Crystal structure evolution via operando neutron diffraction during long-term cycling of a customized 5 V full Li-ion cylindrical cell LiNi0.5Mn1.5O4vs. graphite

Disordered spinel LiNi0.5Mn1.5O4 (d-LNMO) is the cathode material of choice for next generation batteries based on 5 V systems. Unfortunately, once cycled under real conditions i.e. in a full-cell configuration (versus graphite), it displays a quite pronounced fading of the electrochemical performance, even under optimized cycling conditions, and about a half of the specific charge is ‘lost’ after 500 cycles. Thus, we intensively investigated the crystal structure evolution of a full-cell d-LNMO vs. graphite by means of operando neutron diffraction. For this purpose, a new cylindrical electrochemical cell was designed, suitable for operando neutron diffraction studies and allowing for precise Rietveld refinement analyses. During the first cycle, lithium content in the electrode materials (graphite and d-LNMO) could be determined, thus, allowing an estimation of the lithium consumption in side reactions. The neutron diffraction data obtained after long-term cycling (100 cycles) show that the fading of the electrochemical performance can be attributed to an insufficient amount of lithium in the system, which presumably is consumed by side reactions since no structural damage was observed in the positive and negative electrodes.

New insights into the characterization of the electrode/electrolyte interfaces within LiMn2O4/Li4Ti5O12 cells, by X-ray photoelectron spectroscopy, scanning Auger microscopy and time-of-flight secondary ion mass spectrometry

This work aims to study the electrode/electrolyte interfaces in a Li4Ti5O12 (LTO)/LiMn2O4 (LMO) cell assembled with a VC-containing electrolyte and operating at 60 °C. LMO and LTO electrodes were mainly analyzed by X-ray Photoelectron Spectroscopy (XPS) after the first and tenth galvanostatic cycles. The XPS results indicate that both electrodes are covered by surface layers during the first charge, coming from the degradation of electrolyte species, inducing irreversible capacity losses. Although the compositions of both layers are similar, the one formed on LTO electrodes is thicker than the one formed on LMO electrodes and contains small amounts of MnF2, homogeneously spread over the surface, as revealed by the fluorine elemental mapping obtained by a complementary scanning Auger microscopy experiment. An additional measurement by time-of-flight secondary ion mass spectrometry indicates that the MnF2 is located on top of the surface layer. XPS analysis also indicates that during the first discharge, the thickness of the LTO electrode surface layer slightly decreases, due to a partial dissolution, while no changes are observed on the LMO electrode. After the tenth charge, the layers do not present any noticeable changes compared to the first charge. Interfacial layers in the LMO/LTO cell are mainly formed during the first charge, inducing an irreversible capacity loss. During the following cycles, the surface layer on LMO electrodes is stable, while it is slightly dissolved and reformed in each cycle on LTO electrodes, as suggested by the electrochemical data showing smaller and decreasing capacity losses, characteristic of the gradual passivation of these electrodes.