Topic of research: Silicon nanowires for electrochemical energy storage and thermoelectric
power generation Description of research work:
Growing Silicon Nanowire on Copper Substrates: Applications in High Energy Density Battery and Thermoelectric power generation
Silicon Nanowires (SiNWs) are being considered for various applications like efficient electrodes for batteries and supercapacitors. Silicon is the most sought after material for battery anode due to its highest reported theoretical capacity of ~4200 mAh/g, which is ten times higher than that of currently used graphite anode in commercial batteries. The alloying reaction of Li with Si during the charge cycle delivers a high capacity, but with a huge structural change of Si. Nanostructuring of Si minimizes the stress developed in the anode during the charge-discharge cycle, giving longer life to the battery. Of late there are reports of their potential use in thermoelectric devices as well as electron emitters. The geometric aspects of the SiNWs such as length, tapered core-shell structure and the diameter are seen to be important from these application points of view.
Figure 1: A cross sectional schematic illustration of the steps involved in growth of SiNWs by vapor liquid solid (VLS) mechanism of growth using the HWCVP (i) Sn deposited on the substrate (ii) atomic hydrogen treatment to remove the oxide layer on the Sn surface making the catalytic templates ready for growth (iii) continuous flux of Si forming Sn-Si liquid alloy droplet (iv) super saturation of liquid alloy droplet and nucleation of SiNWs (v) Sn particle rides to the top and growth of SiNWs continues (vi) The continuous flux of Si causes a-Si deposition in the axial direction, forming an amorphous shell around the crystalline core.
Our growth method (as outlined in figure 1) aims for SiNW growth on battery grade Cu in a facile way at low temperature (400˚C), so as to facilitate for scale up to industrial production. The hot wire chemical vapor process (HWCVP) parameters such as gas pressure and flow rate, deposition time and the temperature of the filament can be varied individually for tuning the morphology of SiNWs. The size of Sn defines the diameter of crystalline core, thus can be varied according to need. The crystalline Si core provides high electrical conductivity and the relatively open amorphous Si shell accommodates the stress generated during the charge discharge cycles. The adhesion between the SiNWs and Cu foils is good enough to avoid the use of binders and additives, further enhancing the energy density of the batteries.
Figure 2: (i) Cluster tool used for growth of SiNWs (ii) SEM micrograph of SiNWs on Cu substrate (iii) Reversible capacity of SiNW (y-axis) anode against Li at various rates of charge discharge.
The electrochemical testing of nanowires against Li in “half-cell” configuration exhibit promising performance at various charge discharge rates. Further efforts are ongoing in the direction of increasing the cycle life of the cell (no. of charge discharge cycles undergone before the battery degrades). These results are a stepping stone towards development of high energy density batteries for applications in electric vehicles and grid energy storage.