Renaud Bouchet

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).

Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries

Electrochemical energy storage is one of the main societal challenges of this century. The performances of classical lithium-ion technology based on liquid electrolytes have made great advances in the past two decades, but the intrinsic instability of liquid electrolytes results in safety issues. Solid polymer electrolytes would be a perfect solution to those safety issues, miniaturization and enhancement of energy density. However, as in liquids, the fraction of charge carried by lithium ions is small (<20%), limiting the power performances. Solid polymer electrolytes operate at 80 °C, resulting in poor mechanical properties and a limited electrochemical stability window. Here we describe a multifunctional single-ion polymer electrolyte based on polyanionic block copolymers comprising polystyrene segments. It overcomes most of the above limitations, with a lithium-ion transport number close to unity, excellent mechanical properties and an electrochemical stability window spanning 5 V versus Li(+)/Li. A prototype battery using this polyelectrolyte outperforms a conventional battery based on a polymer electrolyte.

Critical Role of Polymeric Binders on the Electronic Transport Properties of Composites Electrode

The role of the polymeric binder nature and composition on the electronic transport properties of composite electrodes based on Li 1.2 V 3 O 8 , carbon black (CB), and poly(ethylene oxide) (PEO)/poly(vinylidene difluoride)-co-hexafluoropropylene/ethylene carbonate-propylene carbonate (EC-PC)/Li bis(trifluoromethansulfon)imide binders were examined. The variation of the electrical conductivity vs CB volume fraction is typical of tunneling-percolation systems. Lower percolation threshold ,; found for preplasticized binders is related to a more efficient CB dispersion due to the presence of EC-PC in the liquid suspension at the time of the composite processing. Above Φ c the conductivity is a unique function of the PEO to CB concentration ratio in the suspension, log σ = log(σ CB )-aΦ PEO /Φ CB . This ratio controls the amount of polymer that adsorbs at the surface of the CB particles before the CB conducting network forms. The Li + /e - insertion behavior was studied at low current rate, for which ionic conductivity is not a limiting factor. The electrochemical capacity sharply increases at Φ c . However, for CB content typical of practical composite electrodes, the electronic conductivity of the CB network is not the only parameter that governs the electrode performance. It depends also on the electronic wiring at the CB/Li 1.2 V 3 O 8 interface, which is improved when adding EC-PC and lithium salt in the formulation.

An EIS Study of the Anode Li/PEO-LiTFSI of a Li Polymer Battery

In this work, an industrially produced polymer electrolyte based on the poly(ethylene oxide) PEO-LiTFSI is studied. The evolution of the impedance spectra of symmetric cells Li/PEO-LiTFSI/Li with the aging time at 90°C is presented. The variation of impedance spectra as a function of temperature and electrolyte geometry is also shown, especially the low frequency part (up to 0.5 mHz). An equivalent electrical circuit is proposed to describe the whole spectra. The major contributions to the impedance of this interface are identified. From the low frequency contribution, the salt diffusion coefficient is determined. Finally, we use a simple model of the surface layer to gain some insight into its properties.

Influence of molecule size on its transport properties through a porous medium.

The mass transfer kinetics of toluene and polystyrenes (of which the M(w) varies from 162 to 1.85 x 10(6) g mol(-1)) through columns filled with silica porous spheres were studied by inverse size exclusion chromatography. The mass transfer parameters were measured by modeling the band broadening of the chromatograms. The experimental height equivalent to a theoretical plate (HETP) data were analyzed using the general rate model in order to determine the effective diffusion coefficient in porous particles as a function of molecular size. The bulk molecular diffusion coefficients were experimentally determined by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA). The topological tortuosity of the porous particles was determined by electrical measurements. The effective molecular diffusion coefficient through porous particles was modeled taking into account exclusion, friction, and at last tortuosity effects. A phenomenological law is proposed to model the evolution of the tortuosity experienced by a molecule in a porous particle as a function of its size. It gives a good prediction of the evolution of effective diffusion coefficient with the molecule/pore size ratio.

Lithium Metal Batteries Operating at Room Temperature Based on Different PEO-PVdF Separator Configurations

Gel polymer electrolyte ~GPE! membranes based on two polymers, the polyethylene oxide ~PEO! and a copolymer of polyvinylidene fluoride-hexafluoropropylene ~PVdF-HFP!, and a plasticizer, the dibutylphthalate ~DBP!, were elaborated in two ways. First, the polymers and the plasticizer were mixed together to obtain a single membrane. Second, a bilayer separator membrane was made by adjunction, through lamination, of a DBP plasticized PVdF-HFP film and a homemade DBP-PEO thin film. The physicochemical properties of the gels were analyzed. AC impedance spectroscopy was carried out on symmetric Li/GPE/Li cells using either the single layer or bilayer membrane as a function of aging ~isothermal at 20 and 70°C!, temperature ~240 to 70°C!, and finally, galvanostatic cell polarization. Both GPE membranes exhibit high ionic conductivities, but the most spectacular result was the measured decrease in the interface resistance, indicative of a deep modification of the interface Li/GPE when the cells were polarized. Aside from having a good interface with the Li metal electrode, such membranes were also shown to form good interfaces with the cathode because assembled Li/GPE/Li4Ti5O12 flat cells were able to sustain, at room temperature, more than

Theoretical Analysis of IS of Polycrystalline Materials with Blocking or Conducting Grain Boundaries: From Microcrystals to Nanocrystals

Impedance spectra (IS) of a polycrystalline sample with macroscopic dimensions are analyzed based on the brick layer model with parallel and series grain boundaries (gb). The gb width is taken as 0.5 nm, in accordance with the classical Fisher model of gb diffusion. Three mean grain size values of 500, 50, and 5 nm are simulated, corresponding to a change from microcrystalline to nanocrystalline ceramics. A constant conductivity of the grain interior (10 - 3 S/m) and of the series gb (10 - 6 S/m) is assumed. Impedance spectra are simulated for parallel gb conductivity values between 10 - 6 (blocking) and 10 - 1 S/m (conductingl. Two impedance arcs are observed in all cases, but the conventional attribution of the high-frequency arc to the grain interior and the low-frequency semicircle to the series gb is valid only when the parallel gb contribution is insignificant, i.e., for low parallel gb conductivity and/or large mean grain size.

Multiscale characterization of a lithium/sulfur battery by coupling operando X-ray tomography and spatially-resolved diffraction

Due to its high theoretical specific capacity, the lithium/sulfur battery is one of the most promising candidates for replacing current lithium-ion batteries. In this work, we investigate both chemical and morphological changes in the electrodes during cycling, by coupling operando spatially resolved X-ray diffraction and absorption tomography to characterize Li/S cells under real working conditions. By combining these tools, the state of the active material in the entire cell was correlated with its electrochemical behavior, leading to a deeper understanding of the performance limiting degradation phenomena in Li/S batteries. Highly heterogeneous behavior of lithium stripping/plating was observed in the anode, while the evolution of sulfur distribution in the cathode depth was followed during cycling.

Non-trivial network driven modifications of ion transport in an ionic liquid confined inside a polymer system

We discuss here the polymer specificities which completely govern in a non-trivial manner the effective ion transport in polymer gel electrolytes, an important class of soft matter electrolytes. Confinement of a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)–pyrrolidinium-cation based ionic liquid solution inside a polymer physical network composed of two different polymers with different functionalities is achieved. A physical network of two polymers with different functional groups, viz., one with acrylate and another with acrylonitrile (PAN) leads to multiple interesting consequences. Due to chemical differences between the acrylate-based polymer (formed from (3-trimethoxysilyl)propyl methacrylate (MSMA) monomers) and PAN, the physical knots in the PAN network unlock, leading to a decrease in the elastic modulus with improved mechanical compliance and chain flexibility in the gel. Additionally, ion–polymer interactions increase, resulting in higher free charge carrier density in the gel compared to the unconfined ionic liquid solution. Thus, ion transport is no longer assisted by the ionic liquid and polymer relaxations, as it would be in conventional polymer electrolytes, but fully driven by the chemical characteristics of the polymer physical network. Notably, the polymer matrix significantly influences the anion mobility and transference number. Contrary to the unconfined ionic liquid electrolyte where cations (predominantly due to the pyrrolidinium cation) and anions contribute to the ionic conductivity, the ionic conductivity in PN gels is predominantly due to anions. The gel ionic conductivity is nearly half an order of magnitude higher than that of the unconfined ionic liquid electrolyte and displays good dimensional stability and electrochemical performance in a separator-free battery configuration. Most importantly, this work may revitalize research on single-ion conductors and stimulate new and simple chemical designs of polymer electrolytes displaying high single-ion conductivity.

Morphology and reactivity of aluminium nanocrystalline powders

Aluminium powders obtained by mechanical milling are characterised in terms of morphology, nanostructure and thermal properties. Platelets like particles with a thickness in the micrometre range are obtained. It is shown that these platelets are constituted of many nanocrystallites whose size governs the melting behaviour of the powders. Their reactivity towards oxygen is compared with that of spherical particles. Samples obtained by mechanical milling start to be oxidised at a higher temperature the amount of oxidised material is higher than that in the case of spherical particles having the same surface area. Both the thermal behaviour and the reactivity could be explained by the presence of alumina at the grain boundaries. This work shows that the milling process appears as a good alternative to gas condensation methods or wire electro–explosion processes to prepare highly reactive particles.