Preparation of carbon/silicon hybrid nanostructured anodes for lithium ion batteries by CVD and liquid phase deposition

Carbon/Silicon hybrid systems for lithium ion batteries anodes have been studied for many years.[1] The beneficial properties of carbon, like softness, conductivity and the capability of intercalating lithium ions, are ideal to prevent the dramatic volumetric expansion of silicon crystalline lattice due to its alloying with lithium. The combination of these two materials allows to exploit the high theoretical capacitance of SiLix alloys (up to 4200mAh/g for x=4.4), which is far superior to the performance offered by traditional anode materials such as graphite (372mAh/g), keeping at the same time high safety standards. [2] However, these hybrid materials cannot always offer the expected performance levels in terms of durability and integrity. Even with an outstanding initial capacitance, values tend to fade over time.[3] It is recognized that silicon must be well incorporated and delocalized inside the carbon matrix, with a Si/C ratio as high as possible in order to maximize the capacitance gain. Additionally the carbon material must offer a well-developed porous structure with large surface area and optimized pore dimension to shorten Li+ ions pathways.[4] The aim of this work is to present a method to obtain a highly stable composite material made of a nanostructured silicon enclosed into an interconnected carbon nanostructure. The carbon matrix is added to the composite by successive chemical vapour deposition (CVD) process steps, thus gaining an exceptional control over the final product in terms of morphology and structure, while the silicon component is added by liquid phase deposition over and inside the carbon matrix after each CVD step. 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. An example of such materials is a hybrid structure made by growing carbon nanowalls with a peculiar hydrogen-free CVD process[5], and through the deposition of silicon by dip coating using a stable suspension of 4g/l of Si nanoparticles in ethanol. This material was built over a high conductive carbon paper sheet in order to obtain a monolithic anode which was tested in T cell lithium ion devices in the range 0.04-1.2V vs Li/Li+ with metallic Lithium as counter electrode. The material could offer a far superior retention, maintaining stability over several cycles, while an uncovered Si electrode capacitance fades and the device stops working after few cycles. The facile route developed, offering high control of the synthetic conditions, paves the way to the design and fabrication of a library of customized nanomaterials with optimized morphology for a variety of applications in the field of energy production and storage devices.