Silicon is one of the most promising anode materials for Lithium-ion batteries. Silicon endures volume changes upon cycling, which leads to subsequent pulverization and capacity fading. These drawbacks lead to a poor lifespan and hamper the commercialization of silicon anodes. In this work, a hybrid nanostructured anode based on silicon nanoparticles (SiNPs) anchored on vertically aligned carbon nanotubes (VACNTs) with defined spacing to accommodate volumetric changes is synthesized on commercial macroscopic current collector. Achieving electrodes with good stability and excellent electrochemical properties remain a challenge. Therefore, we herein tune the active silicon areal loading either through the modulation of the SiNPs volume by changing the silicon deposition time at a fixed VACNTs carpet length or through the variation of the VACNT length at a fixed SiNPs volume. The low areal loading of SiNPs improves capacity stability during cycling but triggers large irreversible capacity losses due to the formation of the solid electrolyte interphase (SEI) layer. By contrast, higher areal loading electrode reduces the quantity of the SEI formed, but negatively impacts the capacity stability of the electrode during the subsequent cycles. A higher gravimetric capacity and higher areal loading mass of silicon is achieved via an increase of VACNTs carpet length without compromising cycling stability. This hybrid nanostructured electrode shows an excellent stability with reversible capacity of 1330 mAh g -1 after 2000 cycles.

Silicon-based anode fabrication with nano-scale structuration improves the energy density and life cycle of Li-ion batteries. As-synthesized silicon (Si) nanowires (NWs) or nanoparticles (NPs) directly on the current collector represent a credible alternative to conventional graphite anode. However, the operating potentials of these electrodes are below the electrochemical stability window of all electrolytes used in commercial Li-ion systems. During the first charging phase of the cell, partial decomposition of the electrolyte takes place, which leads to the formation of a layer at the surface of the electrode, called Solid Electrolyte Interphase (SEI). A stable and continuous SEI layer formation is a critical factor to achieve reliable life-time stability of the battery. Once formed, the SEI acts as a passivation layer that minimizes further degradation of the electrolyte during cycling, while allowing lithium ions diffusion with their subsequent insertion into the active material and ensures reversible operation of the electrode. However, one of the major issues requiring deeper investigation is the assessment of the morphological extension of the SEI layer into the active material which is one of the main parameters affecting the anode performances. In the present study, we use electron tomography (ET) with low electron dose to retrieve three-dimensional information on the SEI layer formation and its stability around SiNWs and SiNPs. The possible mechanisms of the SEI evolution could be inferred from the interpretation and analysis of the reconstructed volumes. Significant volume variations in the SiNW and an inhomogeneous distribution of the SEI layer around the NWs are observed during cycling and provide insights of the potential mechanism leading to the generally reported SiNWs anodes capacity fading. By contrast, analysis of the reconstructed SiNPs' volume for a sample undergoing one lithiation-delithiation cycle evidence that the SEI remains homogeneously distributed around the NPs that retain their spherical morphology and points to the potential benefit of such nanoscale Si anode materials to improve their cycling lifetime.