Bernard Lestriez

A low-cost and high performance ball-milled Si-based negative electrode for high-energy Li-ion batteries

A Si-based anode with improved performance can be achieved using high-energy ball-milling as a cheap and easy process to produce Si powders prepared from a coarse-grained material. Ball-milled powders present all the advantages of nanometric Si powders, but not the drawbacks. Milled powders are nanostructured with micrometric agglomerates (median size ∼10 μm), made of submicrometric cold-welded particles with a crystallite size of ∼10 nm. The micrometric particle size provides handling and non-toxicity advantages compared to nanometric powders, as well as four times higher tap density. The nanostructuration is assumed to provide a shortened Li+ diffusion path, a fast Li+ diffusion path along grain boundaries and a smoother phase transition upon cycling. Compared to non-milled 1–5 μm powders, the improved performance of nanostructured milled Si powders is linked to a strong lowering of particle disconnection at each charge, while the irreversibility due to SEI formation remains unchanged. An electrode prepared in acidic conditions with the CMC binder achieves 600 cycles at more than 1170 mA h per gram of the milled Si-based electrode, in an electrolyte containing FEC/VC SEI-forming additives, with a coulombic efficiency above 99%, compared to less than 100 cycles at the same capacity for an electrode containing nanometric Si powder.

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.

Design of Aqueous Processed Thick LiFePO4 Composite Electrodes for High-Energy Lithium Battery

Small-amplitude oscillatory rheology is used to probe the microstructure present in aqueous composite electrode slurries for lithium batteries. The materials prepared with carboxymethyl cellulose as the thickener displays a solidlike behavior due to the buildup of a three-dimensional network of colloidal carbon black (CB) particles bridged by the polymeric chains. This network is able to support and inhibit the settling of the larger LiFePO 4 particles. Thus, a homogeneous morphology is achieved in the dried composite electrode and good electrochemical performance is displayed both at low and high rates. Contrarily, hydroxypropylmethyl cellulose is observed to create weaker bonds between the CB particles and the materials prepared with this thickener display a liquidlike behavior. Then, the settling of the LiFePO 4 particles results in a concentration gradient, and thus poor electronic wiring and electrochemical performance, unless drying is accelerated by heating.

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.

Non-aqueous carbon black suspensions for lithium-based redox flow batteries: rheology and simultaneous rheo-electrical behavior

We report on the rheological and electrical properties of non-aqueous carbon black (CB) suspensions at equilibrium and under steady shear flow. The smaller the primary particle size of carbon black is, the higher the magnitude of rheological parameters and the conductivity are. The electrical percolation threshold ranges seem to coincide with the strong gel rather than the weak gel rheological threshold ones. The simultaneous measurements of electrical properties under shear flow reveal the well-known breaking-and-reforming mechanism that characterises such complex fluids. The small shear rate breaks up the network into smaller agglomerates, which in turn transform into anisometric eroded ones at very high shear rates, recovering the network conductivity. The type of carbon black, its concentration range and the flow rate range are now precisely identified for optimizing the performance of a redox flow battery. A preliminary electrochemical study for a composite anolyte (CB/Li4Ti5O12) at different charge-discharge rates and thicknesses is shown.

Is LiFePO4 Stable in Water? Toward Greener Li–Ion Batteries

The stability of LiFePO 4 in water was investigated. From high-resolution transmission electron microscopy observation, electron energy-loss spectroscopy analyses, Mossbauer experiments, and chemical analyses, we showed that a Li 3 PO 4 layer is present at the LiFePO 4 grains surface after immersion in water. This layer, which is a few nanometers thick, is accompanied by an increase of the Fe III percentage in the grains and a continuous dissolution of the layer. If experimental conditions such as immersion time, pH, and LiFePO 4 concentration are set to optimal values, the transition to an aqueous route for preparing LiFePO 4 -based positive electrodes could, in fact, be achieved.

Hierarchical and Resilient Conductive Network of Bridged Carbon Nanotubes and Nanofibers for High-Energy Si Negative Electrodes

A design of thick composite electrode for lithium batteries, based on a hierarchical and resilient conductive network of bridged carbon nanotubes [multiwall carbon nanotubes (MWNTs)] and nanofibers [vapor-grown carbon nanofibers (VGCFs)], is presented. This thick silicon/MWNT/VGCF electrode shows a greatly improved cyclability. The combination of VGCF and MWNT easily adapts to large volume changes and favors electronic percolation from nano- to micrometer levels. The whole procedure is simple and could be extended to other anode and cathode materials.

Electronic and Ionic Wirings Versus the Insertion Reaction Contributions to the Polarization in LiFePO4 Composite Electrodes

electrode are experimentally discriminated in this work. The electrodetotal resistance is dominated at high rate by the contribution of the electronic and the ionic wires, the former being more importantin the case of electrodes with low compaction, while the latter being more important in the case of electrodes with highcompaction. A porosity in the 35%–40% range allows to minimize the electrode polarization. At low rate, the electrode resistanceis dominated by the resistance to lithium insertion into the active mass and follows the predictions of M. Gaberscek and J. Jamnik

Multiscale electronic transport mechanism and true conductivities in amorphous carbon–LiFePO4 nanocomposites

Composite and nanostructured materials have hierarchical architecture with different levels: (a) macroscopic (substructure of porous clusters); (b) mesostructural (particles constituting the clusters); and (c) microscopic and nanometric (coatings, bulk of the particles). The identification of the key parameters that affect the electronic transport across all observed size scales is required, but is not possible using conventional dc-conductivity measurements. In this paper, the powerful broadband dielectric spectroscopy (BDS) from low-frequencies (few Hz) to microwaves (few GHz) is applied to one of the most important composite materials for lithium batteries. LiFePO4 is wrapped in a carbon coating whose electrical properties, although critical for battery performance, have never been measured due to its nanometre-size and the powdery nature of the material. We provide a description of the electronic transport mechanism from the nanoscale (sp2 crystallites) up to the sample macroscopic scale for this material. Moreover, the true conductivities and their respective drop when going from one scale to another are given, for the very first time, in the case of a composite powdery material for lithium batteries.

Tailoring the Binder of Composite Electrode for Battery Performance Optimization

To increase electrode cycling performance in lithium batteries, most researchers generally play on the active material optimization. In this paper, it is shown that there is also a need for fundamental studies in the field of nonactive components of the composite electrode. Optimization of the environment of Li 1 . 2 V 3 O 8 active material within the composite electrode leads to a roomtemperature cycling capacity of 280 mAh/g instead of 180 mAh/g with Bellcore-type electrode. Well performing composite electrode was achieved with efficient electronic conduction network, good carbon black/Li 1 . 2 V 3 O 8 interface, and total collection of active material grains. The key role of the homogeneous and efficient carbon black (CB) distribution, due to good interactions between pre-plastifed polyethylene oxide binder and CB, and optimized PEO/CB ratio, has been determined.