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For a successful integration of silicon in high-capacity anodes of Li-ion batteries, its intrinsic capacity decay on cycling due to severe volume swelling should be minimized. In this work, Ni-Sn...

Varying the amounts of silicon and carbon, different composites have been prepared by ball milling of Si, Ni 3.4 Sn 4 , Al and C. Silicon and carbon contents are varied from 10 to 30 wt.% Si, and 0 to 20 wt.% C. The microstructural and electrochemical properties of the composites have been investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and electrochemical galvanostatic cycling up to 1000 cycles. Impact of silicon and carbon contents on the phase occurrence, electrochemical capacity and cycle-life are compared and discussed. For C-content comprised between 9 and 13 wt.% and Si-content  20 wt.%, Si nanoparticles are embedded in a Ni 3.4 Sn 4 -Al-C matrix which is chemically homogeneous at the micrometric scale. For other carbon contents and low Si-amount (10 wt.%), no homogeneous matrix is formed around Si nanoparticles. When homogenous matrix is formed, both Ni 3 Sn 4 and Si participate to the reversible lithiation mechanism, whereas no reaction between Ni 3 Sn 4 and Li is observed for no homogenous matrix. Moreover, best cycle-life performances are obtained when Si nanoparticles are embedded in a homogenous matrix. Composites with carbon in the 9-13 wt.% range and 20 wt.% silicon lead to the best balance between capacity and life duration upon cycling. This work experimentally demonstrates that embedding Si in an intermetallic/carbon matrix allows to efficiently accommodate Si volume changes on cycling to ensure long cycle-life.

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Embedding silicon nanoparticles in an intermetallic matrix is a promising strategy to produce remarkable bulk anode materials for lithium-ion (Li-ion) batteries with low potential, high electrochemical capacity and good cycling stability. These composite materials can be synthetized at a large scale using mechanical milling. However, for Si-Ni3Sn4 composites, milling also induces a chemical reaction between the two components leading to the formation of free Sn and NiSi2, which is detrimental to the performance of the electrode. To prevent this reaction, a modification of the surface chemistry of the silicon has been undertaken. Si nanoparticles coated with a surface layer of either carbon or oxide were used instead of pure silicon. The influence of the coating on the composition, (micro)structure and electrochemical properties of Si-Ni3Sn4 composites is studied and compared with that of pure Si. Si coating strongly reduces the reaction between Si and Ni3Sn4 during milling. Moreover, contrary to pure silicon, Si-coated composites have a plate-like morphology in which the surface-modified silicon particles are surrounded by a nanostructured, Ni3Sn4-based matrix leading to smooth potential profiles during electrochemical cycling. The chemical homogeneity of the matrix is more uniform for carbon-coated than for oxygen-coated silicon. As a consequence, different electrochemical behaviors are obtained depending on the surface chemistry, with better lithiation properties for the carbon-covered silicon able to deliver over 500 mAh/g for at least 400 cycles.

The increasing miniaturization of electronic devices and the demand for stand-alone systems require adapted power sources in terms of dimensions and energy. All-solid-state microbatteries are an interesting solution for new applications (secured smart cards, medical implants, sensors, etc.). They are obtained by successive deposition of thin films of electrode active materials and solid electrolyte. However, these batteries have a rather low capacity (50-200 µAh/cm²) due to the difficulty to use electrodes thicker than 5µm (electronic conductivity, lithium diffusion and mechanical stress issues). Most of commercially available microbatteries use a LiCoO2 positive electrode due to its interesting properties: a 4V/Li+/Li nominal voltage, an excellent cycle life and a high electronic conductivity [1] . Thin films of LiCoO2 are mainly obtained by sputtering [2, 3] , followed by an annealing (500-700°C) necessary to obtain the electrochemically active phase of LiCoO2 (R-3m). This latter step then limits the possible substrates. Moreover the low deposition rate (10nm/mn) and the required equipments and facilities make this process quite costly. To circumvent these drawbacks, thin films of LiCoO2 were synthesized by a novel single-step route: the electrochemical-hydrothermal deposition. This technique combines the effects of the hydrothermal conditions (temperature and pressure) with a galvanostatic electrodeposition. Using ethanol as co-solvent in the aqueous media, we succeded in obtaining a pure and highly-crystalline LiCoO2 films, without any Co3O4 impurity. Moreover, the surface roughness is greatly decreased (fig.1) compared with films obtained in pure aqueous media. The obtained thickness can be increased up to 30µm, equal to a deposition rate of 200nm/mn. Galvanostatic cycling and linear sweep voltammetry experiments were carried out on these electrodes using CR2032 coin cells. The electrochemical properties of these films are very promising and clearly evidence the influence of each deposition parameter (temperature, current density, water/ethanol ratio, etc.) (fig2). [1] Reimers, J.N. and J.R. Dahn, Journal of the Electrochemical Society, 1992. 139(8): p. 2091-2097. [2] Wei, G., T.E. Haas, and R.B. Goldner, Solid State Ionics, 1992. 58(1-2): p. 115-122. [3] Wang, B., et al., Journal of the Electrochemical Society, 1996. 143(10): p. 3203-3213.