Thesis presented March 22, 2022

Abstract:
Silicon nanowires have already demonstrated their excellent performances as electrode materials for lithium-ion batteries. Silicon allows to drastically increase the energy density, and its nanostructuration as wires solves part of the associated problems. During lithiation, silicon swells up to four times its volume, which drives to irreversible losses in the electrochemical cell. Nanostructuration limits the cracking of the particles, and the unidimensional form guarantees cohesiveness and a good connection of the material.
Synthesis methods associated to these materials remain costly, and a commercialization in Li-ion batteries seems complicated. A recent innovative synthesis method developed and patented in SyMMES laboratory in 2015 is the starting point of this PhD project. By growing silicon nanowires as a powder, it allows to use them directly in the electrode ink of the electrochemical cell. The growth takes place in a closed reactor, containing powder as a support (sometimes sacrificial), a catalyst and the silicon source (often an oil of diphenylsilane, easy to handle at the lab scale).
This method is worked and optimized in order to answer the challenges of lithium-ion batteries: electrodes stability, energy density increase, and power issues. The synthesis is worked to vary the nanowires diameter, and the implications in electrochemistry are discussed. Likewise, two different catalysts are used for the growth of silicon nanowires. Here again, the consequences in electrochemical cells are discussed. Improvement of the electrochemical performances was achieved thanks to the fabrication of silicon-graphite composite, by introducing graphite directly in the synthesis reactor, allowing to grow the composite one-pot. The material is finely characterized, and the implications on the nano- and micro-structures of these composites are studied by small angle and wide-angle X-ray scattering (SAXS-WAXS), operando under the synchrotron beamline.

Keywords:
silicon nanowires, lithium-ion batteries, energy storage, nanomaterials, nanochemistry, anodes

On-line thesis.