Tailoring nanomaterials for energy applications

Silicon-based materials present high theoretical capacities (4200 mAh/g), making them among the most promising materials for Li-ion battery anodes. However, the major problem with them is massive volume change (∼ 300%) that occurs during cycling, resulting in pulverisation and significant capacity drop of these anode materials. Besides, the use of additives and metallic current collectors to make the anodes reduces the overall battery energy density as these components could not contribute any capacities to the battery. In this work, free-standing silicon-coated multi-wall carbon nanotube (MWCNT) anodes were developed without any polymeric binders and carbonaceous additives being added. These materials were achieved through exploiting atmospheric pressure chemical vapour deposition as well as advanced nanomaterials fabrication and processing methods. Electrodes were prepared utilising MWCNT papers (so-called buckypapers) as robust, electrically conductive flexible (22.2 S/cm) and free-standing scaffolds since the capacity of these buckypapers is too low to use as a new active material for the battery (0.3 mAh/cm2 in the 300th cycle). Hence, these buckypapers were functionalised with silicon nanoparticles attached through a strong silicon carbide interface and coated with a protective amorphous carbon layer. The SiC interface was partially developed between Si NPs and MWCNTs as excessive SiC could reduce electrochemical capability on the material due to its low electroactivity. The coin cell battery test demonstrated a high areal capacity of up to 2.01 mAh/cm2. These free-standing Si-CNT-SiC-C materials showed good rate capability and stability performance (0.55 mAh/cm2 after 300 cycles with coulombic efficiency of 99.7%). As a result, the material interface design, which is the key of this work, enhances the connection of Si NPs to the MWCNT walls and addresses the issue of Si detachment. A post-mortem result also revealed the structural stabilisation of the electrode after the repeated cycling test. In addition, the pouch-cell battery was fabricated to confirm the use of this free-standing Si-CNT-SiC-C electrode without the influence of the battery component. This new anode architecture performs better than the buckypaper, thereby increasing the areal capacity by 83.3%, allowing better functionality for this material to be implemented in a full-cell battery system.