a b s t r a c tThe effect of the Si electrode morphology (amorphous hydrogenated silicon thin films -a-Si:H as a model electrode and Si nanowires -SiNWs electrode) on the interphase chemistry was thoroughly investigated by the surface science techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS analysis shows a strong attenuation and positive shift of the Si 2p peaks after a complete charge/discharge performed in PC-and EC:DMC-based electrolytes for both electrodes (a-Si:H and SiNW), confirming a formation of a passive film (called solid electrolyte interphase -SEI layer). As evidenced from the XPS analysis performed on the model electrode, the thicker SEI layer was formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. XPS and ToF-SIMS investigations reveal the presence of organic carbonate species on the outer surface and inorganic salt decomposition species in the inner part of the SEI layer. Significant modification of the surface morphology for the both electrodes and a full surface coverage by the SEI layer was confirmed by the scanning electron microscopy (SEM) analysis.

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International audienceThe concept of a hybrid nanostructured collector made of thin vertically aligned carbon nanotubes (CNTs) decorated with Si nanoparticles provides high power density anodes in lithium-ion batteries. An impressive rate capability is achieved due to the efficient electronic conduction of CNTs combined with well defined electroactive Si nanoparticles: capacities of 3000 mAh g−1 at 1.3C and 800 mAh g−1 at 15C are achieved

Iron is a widely used catalyst for the growth of carbon nanotubes (CNTs) or carbon nanofibers (CNFs) by catalytic chemical vapor deposition. However, both Fe and Fe−C compounds (generally, Fe3C) have been found to catalyze the growth of CNTs/CNFs, and a comparison study of their respective catalytic activities is still missing. Furthermore, the control of the crystal structure of iron-based catalysts, that is α-Fe or Fe3C, is still a challenge, which not only obscures our understanding of the growth mechanisms of CNTs/CNFs, but also complicates subsequent procedures, such as the removal of catalysts for better industrial applications. Here, we show a partial control of the phase of iron catalysts (α-Fe or Fe3C), obtained by varying the growth temperatures during the synthesis of carbon-based nanofibers/nanotubes in a plasma-enhanced chemical vapor deposition reactor. We also show that the structure of CNFs originating from Fe3C is bamboo-type, while that of CNFs originating from Fe is not. Moreover, we directly compare the growth rates of carbon-based nanofibers/nanotubes during the same experiments and find that CNFs/ CNTs grown by α-Fe nanoparticles are longer than CNFs grown from Fe3C nanoparticles. The influence of the type of catalyst on the growth of CNFs is analyzed and the corresponding possible growth mechanisms, based on the different phases of the catalysts, are discussed.

The effect of lithiation−delithiation rate and of the number of cycles on the properties of Si nanowires (SiNWs) and electrolyte interface is presented in this paper. The surface and bulk modifications of SiNW electrode induced by electrochemical process of lithiation−delithiation were investigated by combined electrochemical tests (galvanostatic cycling), field emission gun scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. Low lithiation−delithiation rate improves electrochemical performance due to a better penetration depth of lithium into the SiNW electrode and the formation of a homogeneous solid electrolyte interphase (SEI) layer on the SiNWs after the first cycle. However, after repeated cycling, SiNWs suffered strong mechanical stress leading to a rough or porous SiNW structure covered by a porous SEI layer. This study highlights the SEI modifications caused by the lithiation−delithiation rate and the modifications of the Si electrode upon cycling.

Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Fick's second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10(-15) cm(2).s(-1). This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.

International audienceFor the first time, the effect of the nanowires diameter in terms of size and distribution on electrochemical properties of SiNWs grown by VLS was investigated. The diameter size was tuned by using three different gold catalyst film thicknesses. The crucial influence of this parameter is evidenced through comparison of the charge-discharge behavior and a study of the rate capability for the three samples. The rechargeable capacity as well as the rate capability is shown to be the best when the smallest diameters (<65 nm) are used compared to larger one (<210 nm and <490 nm). High capacity values of 3500 mAh g−1 are obtained for the smallest diameters at C/5 rate but still 2500, 1500 and 500 mAh g− are recovered at C, 2.5 C and 5 C. An excellent cycle life over 50 cycles is achieved at 1.3 C with a capacity of 2500 mAh g−1 . This shows that by tailoring the diameter size and distribution, SiNWs can provide high power density anodes in lithium ion batterie

International audienceThe electrochemical reactivity of a carbon nanowalls electrode was highlighted. The carbon nanowalls were synthesized at 520 degrees C in an acetylene/ammonia electron cyclotronic resonance plasma without any metal catalyst. The electrode surface was characterized by scanning and transmission electron microscopy. Its electrochemical reactivity was studied by both cyclic voltammetry and electrochemical impedance spectroscopy. After the carbon nanowalls deposition, the electronic transfer rate constant and the electroactive surface area were found to be increased by a factor of 7 and 3, respectively