In the past 20 years, silicon has been offered a prime position among the promising materials for increasing lithium-ion batteries performance. Indeed, it shows a ten-time higher specific capacity than graphite, the leading active material in the negative electrode. This comes however with huge mechanical constraints during lithiation and delithiation, that can only be leveraged by downsizing the silicon into the nano range. Aiming at developing active materials for negative electrodes of lithium-ion batteries with a high energy density, we thus design novel synthesis methods for silicon nanowires and their composites. As compared to nanowire growth for other applications (electronics or sensors), the growth method here has to address demanding requirements in terms of silicon mass production to allow for electrochemical tests in batteries. We designed a CVD-like process in a high-pressure vessel to attain gram-scale synthesis of silicon nanowire-graphite composites at the laboratory level, with manageable chemical risk. Remarkable results in energy density and stability in long-term cycling of batteries were obtained. In parallel, we could also design silicon nanowire-based hydrogen-evolving electrodes for photoinduced water-splitting. This system takes advantage of an easy silicon growth inside the porosity of the carbon-cloth electrode, and of the mechanically strong contact between nanowire and carbon substrate. We now try to tune the silicon nanowire chemistry, size and shape (average diameter, diameter distribution, straight or curly shape, crystallinity, doping, native surface layer, growth homogeneity). The aim is to increase the performance of these systems, while reducing cost and environmental impact. On one hand, we replace the standard gold seeds for silicon nanowire growth by other metal and metallic compounds. Metal sulfide nanoparticles in particular appear as interesting and highly tunable novel growth seeds. On the other hand, we study the reaction mechanism of the decomposition of the silicon source in the gas phase to get a better control on the rate and lower the temperature of growth, targeting <350°C.