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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