Mixed bonds in SiOCs lead to a more disordered/defect-free carbon phase with high capacity for reversible storage of Li ions.

We demonstrate a cost-effective synthesis route that provides Si-based anode materials with capacities between 2000 and 3000 mAh·g (400 and 600 mAh·g), Coulombic efficiencies above 99.5%, and almost 100% capacity retention over more than 100 cycles. The Si-based composite is prepared from highly porous silicon (obtained by reduction of silica) by encapsulation in an organic carbon and polymer-derived silicon oxycarbide (C/SiOC) matrix. Molecular dynamics simulations show that the highly porous silicon morphology delivers free volume for the accommodation of strain leading to no macroscopic changes during initial Li-Si alloying. In addition, a carbon layer provides an electrical contact, whereas the SiOC matrix significantly diminishes the interface between the electrolyte and the electrode material and thus suppresses the formation of a solid-electrolyte interphase on Si. Electrochemical tests of the micrometer-sized, glass-fiber-derived silicon demonstrate the up-scaling potential of the presented approach.

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Silicon is one of the most promising materials when it comes to lithium-ion battery anodes because of its high theoretical capacity and the low working potential versus Li/Li + . However, the drastic volume change during lithiation and delithiation leads to a rapid failure of the electrode. In order to accommodate the large volume change, Si@C core−shell nanocomposites have been investigated, as they efficiently protect the Si surface from being exposed to the electrolyte and thus limit side reactions and improve the cycling stability through a stable solid electrolyte interface layer. In recent years, phenolic resins have been investigated as the carbon source due to their facile synthesis and the possibility of scale-up. Here, the influence of the chemical structure of the Si−C interface on electrochemical performance has been analyzed by comparing pristine, silanol-rich and epoxide-functionalized Si/ phenolic resin-derived nanocomposites. Whereas pristine Si@C exhibits the highest initial specific capacity of around 2000 mA h/g Si , introduction of silanol groups to the native surface leads to a more homogeneous carbon shell around the Si and thus to an overall higher Coulombic efficiency and a more stable cycling behavior. Additional epoxide functionalization, however, leads to a drastic decrease in initial capacity due to an overall increased resistance and prolongs the activation process. Nevertheless, in the long term, the additional layer leads to more stable cycling, especially at high current rates. For all nanocomposites, the electrochemical performance, characterized by cyclic voltammetry, cycling experiments, and electrochemical impedance spectroscopy, is correlated with the structure of the Si−C interface, determined by transition electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman, scanning electron microscopy, and IR-spectroscopy. To the best of our knowledge, the influence of the Si−C interface of a core−shell nanocomposite on structure and electrochemistry by chemically modifying the silicon surface is analyzed and reported for the first time.