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International audienceUnderstanding the failure mechanism of silicon based negative electrodes for lithium ion batteries is essential for solving the problem of low coulombic efficiency and capacity fading on cycling and to further implement this new very energetic material in commercial cells. To reach this goal, several techniques are used here: post mortem7Li MAS NMR and SEM, electrochemical impedance spectroscopy (EIS) and three-electrode-based electrochemical analysis. 7Li MAS NMR analyses of the charged batteries demonstrate that the major part of the lithium lost during the charge of batteries is not trapped in LixSi alloys but instead at the surface of the Si particles, likely as a degradation product of the liquid electrolyte. Observed by SEM, a dead electrode has a thick "SEI" layer at its surface. EIS and incremental capacity analyses demonstrate that the growth of this layer is responsible for the failure of the electrode through a continuous decrease of its active surface area associated with a rise of the electrode polarization. It is demonstrated that the main cause of capacity fade of Si-based negative electrodes is the liquid electrolyte degradation in the case of nano Si-particles formulated with the carboxymethyl cellulose (CMC) binder. This degradation results in the formation of a blocking layer on the active mass, which further inhibits lithium diffusion through the composite electrode

International audienceA 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 [similar]10 μm), made of submicrometric cold-welded particles with a crystallite size of [similar]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

International audienceA nanosilicon-based composite electrode that can achieve more than 700 cycles at a high capacity of 960 mAh/g of electrode was prepared using aqueous processing in an acidic medium. The buffering of the aqueous solution is mandatory to promote covalent bonding between Si particles and the carboxymethyl cellulose (CMC) binder. The latter is claimed to allow the formation of mechanically stronger contacts within the composite electrode in addition to the CMC bridging of the Si and carbon black particles

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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.

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This study, realized within the framework of the optimization of aqueous LiFePO4 composite electrodes, relies on Prosini’s approach [ J. Electrochem. Soc. 152 , A1925 (2005) ] that characterizes the LiFePO4/Li discharge behavior through simple equations. Two key parameters extracted from the LiFePO4 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.