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For applications in mobile and remote sensing platforms, microsupercapacitors are attractive energy storage devices due to their robust lifetimes and high specific power capacity. Utilization of green electrolytes in these devices reduces environmental impact and simplifies packaging by avoiding the stringent oxygen and moisture free conditions required for organic and ionic liquid based electrolytes. Porous silicon nanowire based microsupercapacitor electrode materials are promising for on chip applications using an environmentally benign aqueous electrolyte, 1 M KCl, however they are prone to oxidation. A silicon carbide coating was found to mitigate this issue. The fabrication techniques, involving low-temperature electroless etching of silicon, are compatible with current integrated circuit processing methods and may be readily integrated at the micro device level. The electrode materials are in good electrical contact with the underlying substrate and require no additional current collector. The base porous silicon nanowires are coated with a thin silicon carbide passivation layer by low pressure chemical vapor deposition. The demonstrated capacitance of the electrode materials, ~1700 μF/cm2 projected area, is comparable to other carbon based microsupercapacitor electrodes, remains stable over many charge/discharge cycles, and maintains capacitive behavior over a wide range of charge/discharge rates. An improved passivation method for the porous silicon nanowires has also been developed. The selective coating procedure deposits an ultra-thin (~ 1-3 nm) carbon sheath over the nanowires and passivates them. The ultra-thin nature of the coating enables solvent access to the pore area and hence a large improvement of active specific surface over the SiC coated PSiNWs discussed above. The electrochemical performance of these coated nanowires is characterized in both an aqueous electrolyte and an ionic liquid electrolyte. Specific capacitance values reaching 325 mF cm 2 are achieved in ionic liquid, and calculations indicate that the theoretical maximum capacitance of the pristine wires is reached. TEM studies confirm the coating thickness and its conformality. Raman spectroscopy indicates that the carbon in the coating is mainly sp2 hybridized, with corresponding high conductivity. At the time of writing, these materials represent the largest specific energy microsupercapacitor electrode published. A test device is prepared and demonstrated powering an LED. The testing results of silicon carbide (SiC) nanowires (NW) as an electrode material for micro-supercapacitors is described. SiC NWs are grown on a SiC thin film coated with a thin Ni catalyst layer via chemical vapor deposition. A specific capacitance of ~240 µF cm-2 is demonstrated. Charge-discharge studies demonstrate the SiC nanowires exhibit exceptional stability, with 95% capacitance retention after 2×105 charge/discharge cycles in an environmentally benign, aqueous electrolyte. Doping of the nanowires with nitrogen through the addition of 5 at% ammonia to the precursor gas flow rate improves the conductivity of the nanowire films by over an order of magnitude leading to increased power capabilities. A method to transfer silicon and silicon carbide nanowire arrays to arbitrary substrates while maintaining electrical contact through the entire array is elucidated. The nanowires are grown on graphene sheets on SiO2 coupons. The graphene acts as both the flexible material for maintaining structural continuity and electrical contact through the array during transfer. The SiO2 acts as the sacrificial growth substrate which is etched after growth in order to release the nanowire/graphene hybrid. The nanowire/graphene hybrids are structurally characterized by XRD and electron microscopy. Good electrical contact is confirmed through testing of the SiCNW/graphene hybrids as supercapacitor electrode materials in an aqueous electrolyte. The specific capacitance, ~340 mF cm-2, is similar to SiCNW arrays grown on oxide while the electrical conductivity is improved and cycling stability tests show less than a 1% decrease in capacitance after 10,000 cycles.

International audience; Due to its high capacity compared to graphite, silicon has attracted attention for li-ion battery technology as a promising material for negative electrodes. It is abundant and non-toxic. However: this material is weil known to undergo large volume changes upon lithation and delithation (up to 320 %). This phenomenon causes particle cracking,jnstability of the solid electrolyte interphase (SEI) at the interface between solid Si and liquid electrolyte, and finally leads to electrode delamination and battery performance loss. To overcome these problems, several strategies have been proposed such as using submicronic particles « 150 nm) to mitigate the volume changes and protection of the silicon material with a carbon layer to stabilize the active surface in contact with electrolyte. Combining both strategies, Si@C. core-shell nanoparticles synthesized in one step process have recently been proposed as a promising anode material [1]. Specifically, the Si-based nanoparticles are synthesized in a two stage laser pyrolysis reactor, which yields carbon coated silicon nanoparticles in a si~gle step. This approach mitigates material oxidation because there is no air exposure between the synthesis of the core and shell. Additionally, the nanometric size of the particles prevents material pulverization upon cycling. The synthesis technique also allows control of the core crystallinity, and both highly crystalline and amorphous silicon cores have been synthesized. Moreover, the shell thickness can be tuned by changing the flow of carbon precursor. To optimize the design of composite electrodes based on such active materials, an in-depth understanding of their performance and chemical/mechanical degradation processes remains critical. In this work, the analysis of the electrochemical performances of su ch Si-based electrodes was performed using several electrochemical techniques to compare crystalline or amorphous Si nanoparticles without shell as weil as crystalline Si nanoparticles coated by a thin or a thick layer of carbon (Si@C). The EIS study, carried out at various states of lithiation of Si material, allows tracking the evolution of several critical parameters (for example SEI resistance and charge transfer resistance) of the equivalent electrical circuit describing the elec.trode electrical behavior. Figure la shows typical impedance spectra while Figure lb shows the evolution of the of charge transfer resistance for the different coated and non-coated materials. A very different behavior is observed as a function of the interface material and carbon thickness. The modificatfon of the interface between e1ectrolyte and Si or Si@C materials is also visible on measured equilibrium potentials and power capabilities. This presentation will be devoted to the analysis of the impact of interfaces between electrolyte and Si or Si@C materials on electrochemical performances.

International audience; Performant electrochemical storage devices appear as one of the solution to face the challenge of energy transition. In this context, lithium-ion batteries are a well-developed technology 1. This work is focused on increasing the negative electrode's capacity by understanding the degradation mechanism occurring in the material. Graphitic carbon is commonly used as a negative electrode in commercial battery systems because of its stability, electronic conductivity, and its natural abundance. However, its maximum energy density remains too low to meet the requirements of demanding applications such as electric vehicles. Silicon is a promising alternative anode material to increase its capacity up to 3579 mAh/g, ten times higher than the 350 mAh/g of graphite 2. However, fractures issues occur in the material, due the high volumetric change over cycling. Using nanoparticles has been shown to alleviate the problem 3 but at this size, the formation of an interphase between the electrolyte and the solid (named SEI) becomes predominant. This SEI stability is fundamental to obtain stable performance of silicon electrodes. The coating of the silicon surface by carbon has proved to protect the bare silicon surface and obtain a more stable SEI 4. The integration of such nanoparticles in Li-ion anodes improves the electrodes' specific capacities. Those particles are synthetized by laser pyrolysis, a one step process. Impedance spectroscopy, a powerful and non-destructive technique, is used to probe the electrode's interfaces. In this paper, using this technique, an improved stability of carbon coated silicon particles will be demonstrated by comparison to pure silicon.

International audience; Among energy storage devices, Lithium ion batteries (LlBs) are efficient power sources used for many applications inc1uding mobile microelectronics. However, ageing phenomena are not yet fully understood. These 8henomena are a crucial issue to pro vide safe and stable batteries!. LIBs are usually compbsed of a negative electrode where the active material is graphite, a positive electrode usualli a lithium metal oxide and an organic liquid electrolyte. Ortiz et al. have shown that radiolysis is a powerful tool to simulate the degradation of the latter one in short time: minutes/hours instead of weeks/months by electrolysis (Fig. 1). Moreover, radiolysis allows performing experiments at the picosecond time scale thus giving access to reaction mechanisms. During the first cycles of the battery, the reduced surface of the negative electrode reacts with the electrolyte producing a solid interphase (solid electrolyte interphase, SEI) which is responsible for the capacity loss of the battery. In this work, we investigated the SEI formation by radiolysis at the surface of various carbonaceous materials inc1uding crystalline graphite (lithiated or not) and carbon nanoparticles (amorphous as weIl as organized) prepared by laser pyrolysis. Materials were dispersed in a mixture of carbonate solvents containing LiPF. Composition and morphology of SEI were invesigated by XPS and TEM while the composition of gas and liquid phases was studied by gas chromatography and high resolution mass spectrometry, respectively. We show that an artificial SEI can be produced by radiolysis. We observe always the same degradation mechanisms of the electrolyte but interestingly the SEI composition depends on the carbonaceous material. The artificial SEI formed at the surface of graphite is composed of Li carbonate, oxalate and oligomers of poly(ethylene oxide) while the SEI formed at the surface of carbon nanoparticles contains Li salts as Li$_2$CO$_3$. Radiolysis allows producing materials with modified surface that will be tested as new materials for negative electrode.

International audience; CUITent LiB technology relies on graphitic carbon as the anode material, with a theoretical capacity of 372 mAh/g. In order to increase the energy density of LiBs, anode materials with a greater capacity for lithium storage are under intense investigation. Materials which form alloys with lithium such as antimony, germanium, silicon, and tin, all have theoretical capacities which far surpass graphite. However silicon, as the most naturally abundant e1ement and possessing a theoretical capacity of 3579 mAh/g in the Li$_{15}$Si$_4$ alloy, is the most promising for global adoption in next generation LiBs.

International audience; Laser Pyrolysis Derived Silicon-Carbon Core-Shell Nanomaterials for Lithium Ion Batteries As the world moves away from distributed fossil fuel use in order to mitigate the climatic effects of carbon pollution, the need for high energy density storage devices continues to grow. Secondary lithium ion batteries (LiB) are one such attractive energy storage device. Current LiB technology relies on graphitic carbon as the anode material, with a theoretical capacity of 372 mAh/g. In order to increase the energy density of LiBs, anode materials with a greater capacity for lithium storage are under intense investigation. Materials which form alloys with lithium such as antimony, germanium, silicon, and tin, all have theoretical capacities which far surpass graphite. However silicon, as the most naturally abundant element and possessing a theoretical capacity of 3579 mAh/g in the Li$_{15}$Si$_4$ alloy, is the most promising for global adoption in next generation LiBs.

International audience; Although the Li-ion battery (LIB) currently offers the most suitable balance between power and autonomy for consumer electronics and electric vehicle applications, there continues to be a demand for increased energy capacity. One strategy to increase LiB’s energy density is to replace graphite (372 mAh.g-1) as the anode active material by higher specific capacity materials. Silicon appears as an attractive alternative material thanks to its high theoretical specific capacity (3579 mAh.g-1 for the Li3,75Si phase) and its low discharge potential. Despite being the focus of scientific activity for over 10 years, the use of silicon based anodes have not yet been realized because the performance of these materials degrades rapidly during cycling. Silicon nanostructuration together with association of carbon to Si greatly enhance the performances in terms of both cyclability and capacity. In particular, core-shell silicon-carbon Si@C nanoparticles are attractive candidates as active material to increase the capacity of Li-ion batteries while mitigating the detrimental effects of volume expansion upon lithiation processes.The innovative solution proposed here is to use at the anode nanoparticles of Si@C synthesized in a single step by a scalable continuous gas phase method particularly interesting for industrial production, i.e. the laser pyrolysis method. Moreover, thanks to the control of experimental parameters, this method allows producing an amorphous core of silicon (a-Si) as well as a crystalline one (c-Si); Indeed using a-Si as core material, instead of c-Si, is an considered option not often considered but it appears promising to enhance cyclability because a-Si is not subject to the drastic crystalline state alteration upon its first lithiation. In order to cumulate all the benefits cited above, active material should be a composite of an a-Si core covered with a carbon shellWe report the synthesis, in a single-step process, of amorphous silicon nanoparticles coated with a carbon shell (a-Si@C), via a two-stage laser pyrolysis where decomposition of silane and ethylene are conducted in two successive reaction zones. Auger electron spectroscopy and scanning transmission electron microscopy show a carbon shell about 1 nm in thickness which prevents detrimental oxidation of the a-Si cores. The advantages of the a-Si@C material will be emphasized by comparison with c-Si@C material used as active materials. In particular, cyclic voltammetry demonstrates that the amorphous core-shell composite reaches its maximal lithiation during the first sweep, which is attributed to the amorphous core. After 500 charge/discharge cycles, it retains a capacity of 1250 mAh.g-1 at a C/5 rate and 800 mAh.g-1 at 2C, with an outstanding coulombic efficiency of 99.95 %. Moreover, postmortem observations show an electrode expansion of less than 20% in volume where the nanostructuration is preserved

International audience; As the world moves away from distributed fossil fuel use in order to mitigate the climatic effects of carbon pollution, the need for high energy density storage devices continues to grow. Secondary lithium ion batteries (LiB) are one such attractive energy storage device. Current LiB technology relies on graphitic carbon as the anode material, with a theoretical capacity of 372 mAh/g. In order to increase the energy density of LiBs, anode materials with a greater capacity for lithium storage are under intense investigation. Materials which form alloys with lithium such as antimony, germanium, silicon, and tin, all have theoretical capacities which far surpass graphite. However silicon, as the most naturally abundant element and possessing a theoretical capacity of 3579 mAh/g in the Li$_{15}$Si$_4$ alloy, is the most promising for global adoption in next generation LiBs. There are issues which require resolution before silicon can be implemented. Large volumetric changes associated with the lithiation-delithiathion process ($\sim$300%) result in material pulverization and loss of electrical contact. Also unstable solid-electrolyte-interphase (SEI) formation during cycling results in the consumption of lithium during operation and capacity fade [2]. Previous studies have conclusively shown that the former issue may be mitigated by utilizing nano-scale silicon materials, with particles under 150 nm in diameter remaining intact during the swelling and contraction associated with cycling. It has also been demonstrated that by encapsulating the silicon materials in carbon shells shows promise in stabilizing the SEI. Here we present a scalable process to achieve this core-shell morphology via laser mediated pyrolysis. The technique, which has been used to produce various ceramic, oxide, and metallic particles, has already been utilized on the industrial scale for silicon nanoparticle production. Previously our group demonstrated the capacity of crystalline silicon core-carbon shell materials, synthesized in a two stage pyrolysis reactor, reaches ~500 mAh/g and retains over 70% capacity at a fast 2C rate over 500 cycles. Amorphous silicon, with isotropic expansion upon lithiation, holds promise in forming a more stable SEI than crystalline silicon, and hence increased capacity retention. We have tuned pyrolysis reaction parameters in order to obtain consistent production of amorphous silicon nanoparticle cores. In this talk, a comparison of the battery testing results for amorphous vs. crystalline silicon cores will be presented. Steps to overcome present challenges with the cyclability and irreversible capacity loss due to SEI formation will also be discussed.