Anne-Claire Gaillot

Ionic vs Electronic Power Limitations and Analysis of the Fraction of Wired Grains in LiFePO4 Composite Electrodes

This study, realized within the framework of the optimization of aqueous LiFePO 4 composite electrodes, relies on Prosinis approach [J. Electrochem. Soc. 152, A1925 (2005)] that characterizes the LiFeP0 4 /Li discharge behavior through simple equations. Two key parameters extracted from the LiFeP0 4 discharge curves are analyzed to determine the optimal electrode engineering and to interpret the origins of the electrode performance limitations. In particular, the calendaring step plays a critical role. Low packing results in electronic limitation, while the ionic contribution dominates for dense electrodes. The best compromise is achieved for an optimal porosity in the 30-35% volume range. A simple equation is proposed to predict the ionic limitations of rate performance from the electrode thickness and porosity, and the liquid electrolyte diffusion constant.

Short-range and long-range order of phyllomanganate nanoparticles determined using high-energy X-ray scattering

High-energy X-ray scattering (HEXS) is used to explore the pH-dependent structure of randomly stacked manganese oxide nanosheets of nominal formula [delta]-MnO2. Data are simulated in real space by pair distribution function (PDF) analysis and in reciprocal space by both the Bragg-rod method and the Debye equation in order to maximize the information gained from the total scattering measurements. The essential new features of this triple-analysis approach are (1) the use of a two-dimensional supercell in PDF modeling to describe local distortions around Mn layer vacancies, (2) the implementation in Bragg-rod calculations of a lognormal crystal size distribution in the layer plane and an empirical function for the effect of strain, and (3) the incorporation into the model used with the Debye equation of an explicit elastic deformation of the two-dimensional nanocrystals. The PDF analysis reveals steady migration at acidic pH of the Mn atoms from layer to interlayer sites, either above or below the Mn layer vacancies, and important displacement of the remaining in-layer Mn atoms toward vacancies. The increased density of the vacancy-interlayer Mn pairs at low pH causes their mutual repulsion and results in short-range ordering. The layer microstructure, responsible for the long-range lateral disorder, is modeled with spherically and cylindrically bent crystallites having volume-averaged radii of 20-40 A. The b unit-cell parameter from the hexagonal layer has different values in PDF, Bragg-rod and Debye equation modeling, because of the use of different weighting contributions from long-range and short-range distances in each method. The PDF b parameter is in effect a measure of the average inlayer Mn...Mn distance and consistently deviates from the average structure value determined by the Bragg-rod method by 0.02 A at low pH, as a result of the local relaxation induced by vacancies. The layer curvature increases the Bragg-rod value by 0.01-0.02 A with the cylindrical model and as much as 0.04-0.05 A with the spherical model. Therefore, in principle, the diffraction alone can unambiguously determine with good accuracy only a volume-averaged apparent layer dimension of the manganese oxide nanosheets. The b parameter is model dependent and has no single straightforward interpretation, so comparison of b between different samples only makes sense if done in the context of a single specified model.

Determination of manganese valence states in (Mn3+, Mn4+) minerals by electron energy-loss spectroscopy

Abstract Various manganese valence quantification methods using manganese L2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn3+, Mn4+) valence system. Multiple linear least-squares (MLLS) fitting of Mn L2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K-edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn3+, Mn4+) minerals due to the small separation between the primary L3 peaks from Mn4+ and Mn3+ valence. As to the methods based on calibration curves, the energy difference between Mn L3 and oxygen K, i.e., ΔE (L3-O K) vs. valence, is the most valence-sensitive method in the range of Mn3+ and Mn4+ and yields good agreement with the actual values.

Lowering interfacial chemical reactivity of oxide materials for lithium batteries. A molecular grafting approach

This paper proposes a molecular grafting approach as a way to modify the interfacial chemical reactivity of oxide materials, which is detrimental to their long-term energy storage properties. The present study demonstrates that diazonium chemistry offers an efficient path to graft molecules at the surface of a powder oxide material. Indeed, surface derivatization of Li1.1V3O8 nanograins was accomplished by in situ electrografting of a diazonium salt during Li-ion intercalation. The results show that aryl molecules are strongly bonded to the surface forming an organic multilayer the thickness of which can be modulated. Based on TEM, XPS and electrochemical probing of the surface reactivity, the results demonstrate the interest of the proposed surface modifications as a way to tailor both electrochemical and chemical reactivities of oxide electrode materials. Interestingly, charge transfer at the surface of the material is not impeded, while electrolyte decomposition is inhibited. It is anticipated that molecular derivatization of electrode surfaces is a new research direction which will be developed in the field of battery science in the near future, in order to prevent targeted side reactions occurring at different steps of battery manufacturing and use, such as storage, electrode processing, simple contact with electrolyte, and cycling.

Raising the redox potential in carboxyphenolate-based positive organic materials via cation substitution

Meeting the ever-growing demand for electrical storage devices requires both superior and “greener” battery technologies. Nearly 40 years after the discovery of conductive polymers, long cycling stability in lithium organic batteries has now been achieved. However, the synthesis of high-voltage lithiated organic cathode materials is rather challenging, so very few examples of all-organic lithium-ion cells currently exist. Herein, we present an inventive chemical approach leading to a significant increase of the redox potential of lithiated organic electrode materials. This is achieved by tuning the electronic effects in the redox-active organic skeleton thanks to the permanent presence of a spectator cation in the host structure exhibiting a high ionic potential (or electronegativity). Thus, substituting magnesium (2,5-dilithium-oxy)-terephthalate for lithium (2,5-dilithium-oxy)-terephthalate enables a voltage gain of nearly +800 mV. This compound being also able to act as negative electrode via the carboxylate functional groups, an all-organic symmetric lithium-ion cell exhibiting an output voltage of 2.5 V is demonstrated.Organic electrode materials could enable novelty chemistry required by the new generation of batteries. Here the authors show the synthesis and electrochemical performance of Mg(Li2)-p-DHT as a lithiated cathode material that cycles at 3.4 V due to the presence of a spectator cation in the host structure.