Innovative CNW/Si nanoparticles composites architectures as electrodes for lithium-ion batteries

The possibility of designing a hybrid Carbon/Silicon anode for lithium-ion batteries has been studied for many years [1]. Carbon beneficial properties like softness, conductivity, and the capability of intercalating lithium ions, are ideal to prevent the volumetric expansion of silicon crystalline lattice due to its alloying with lithium (up to 300% of the initial volume). Therefore, the design and fabrication of C/Si hybrid materials allows to combine the high theoretical capacitance of Silicon and Lithium alloys (up to 4200mAh/g for Si5Li22), far superior to the capacitance offered by traditional graphitic anodes (372mAh/g), with carbon properties, thus keeping at the same time high safety standards [2]. However, hybrid materials cannot always offer the expected performances in terms of durability and integrity and initial capacitance values tend to fade over time [3]. It is recognized that silicon must be well embedded and delocalized inside the carbon matrix, with the highest possible Si/C ratio to maximize the capacitance gain. Additionally, the carbon material must have a well-developed porous structure with large surface area and optimized pore dimension to shorten Li+ ions pathways [4]. In such a framework, we propose here an innovative nanostructured architecture based on a highly stable composite material made of successive and alternate deposition of silicon nanoparticles (average diameter 50nm) embedded into an interconnected matrix of carbon nanowalls (CNW), a peculiar graphitic carbon nanostructures where walls of graphitic carbon are grown perpendicularly to the substrate surface. Silicon nanoparticles are added inside and over the carbon matrix after each CVD step by dip coating using a stable suspension of 4g/l of Si nanoparticles in ethanol, while CNW are obtained by a hydrogen-free chemical vapor deposition (CVD) process in a hot filament plasma enhanced CVD reactor [5]. The possibility to tune the conditions for each step allows the construction of a customized compact porous material, where the active component is well embedded inside a light support whose pores offer large gaps to accommodate silicon dimensional variations without losing electrical contact. Morphology, elemental analysis and distribution have been studied with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), while Raman spectroscopy has been used to evaluate the graphitization degree of the carbonaceous component and the crystalline quality of Si nanoparticles in samples. Electrochemical properties have been tested in T-cell lithium ion devices in the range 0.04-1.2V vs Li/Li+ with metallic lithium as counter electrode and LiPF6 in EC:DMC 1:1 electrochemical solution. The material proposed offers a high distribution of Si nanoparticles along the CNW which surround the Si nanoparticles with ordered graphitic domains and as an electrode it offers superior retention and stability over several cycles compared to an uncovered Si electrode whose capacitance fades after few cycles.