MANIOBRAS ESPECIALES

The large first cycle irreversible loss and rapid capacity fade can be minimized by preventing the material pulverization during repeated cycling. The mechanical stresses resulting from the large volumetric changes during the alloying and de-alloying reactions can be greatly minimized by controlling the microstructure, morphology, and particle size combined with use of active/inactive matrices etc. The hypothesis for the approaches that were implemented in this dissertation are discussed in the following:

 Crystallite size reduction: Decreasing the particle or crystallite size to less than 100 to 200 nm alleviates the mechanical stresses responsible for propagation of microcracks leading to fracture of material. Smaller particle size materials undergo superplastic deformation, thereby delaying the advent of fracture. Additionally, decreasing the particle size results in short Li+ ion diffusion distances, improving the rate characteristics of the battery.

 Use of amorphous phase: Amorphous silicon has a completely disordered structure containing defects and large amount of free volume. The volume expansion of silicon occurs in an isotropic fashion compared to that of crystalline silicon, resulting in a homogenous distribution of mechanical stresses.

 Use of appropriate support matrix: Development and use of a strong, mechanically robust and electronically conducting matrix to accommodate the mechanical strain

developed in silicon will help alleviate the capacity fade contributing to improved charge transfer kinetics.

 Develop strain engineered morphology composed that is able to withstand the strain without fracture upon repeated cycling.

The overall objectives of this dissertation are to develop cost effective strategies to synthesize silicon based nanostructures such that they deliver:

 High specific capacity greater (>1000 mAh/g)  Low first cycle irreversible loss (FIR) (<15%)

 Good capacity retention, cyclability and rate capability  High coulombic efficiency (>99%)

 Obtain a fundamental scientific understanding of the ensuing electrochemical and material related phenomena at the structural, microstructural and compositional end

Accordingly, this dissertation is aimed at achieving the above mentioned objectives by synthesizing nanocrystalline and amorphous structures of silicon using cost effective methods. The specific research goals are thus outlined below:

1. Research goal 1: To generate high surface area silicon nanoparticles by high energy mechanical milling (HEMM) and study its potential for use as Li-ion anodes

A simple and facile approach utilizing inexpensive precursors via a mechanochemical reduction approach will be explored to generate silicon nanoparticles. An acid etching process will be used to generate the desired nanocrystalline silicon exhibiting with very high specific

surface area (SSA), small particle size and high porosity. The obtained silicon nanoparticles will be mixed with binder and conductive additives and tested for their electrochemical response using conventional slurry based electrodes.

2. Research goal 2: To generate and study amorphous silicon by electrodeposition of halide salts of Si from organic solvents

Electrochemical reduction of SiCl4 from an organic solvent will be conducted to obtain thin films of amorphous silicon (a-Si). These films will directly be deposited on copper foil and will be directly assembled in the battery serving as a binder-less approach capable of producing thin films of amorphous silicon and easily scaled up to produce a-Si potentially at large industrial scales.

3. Research goal 3: To synthesize and investigate CNT-Si heterostructures by chemical vapor deposition (CVD)

Vertically aligned carbon nanotubes (VACNTs) will be grown on quartz substrates from a liquid based carbon precursor and a floating iron based catalyst. Silicon will be deposited on these CNTs by thermal cracking of silane (SiH4) gas. The deposition parameters will be selected to obtain two different sets of morphologies of silicon (nanoscale droplets and thin films) on the CNTs. The CNT-Si heterostructures will be removed from the quartz substrate and mixed with conductive additives and binders to prepare conventional slurry based electrodes.

4. Research goal 4: To identify and conduct a fundamental study of a scalable synthesis approach for the generation of hollow of Silicon Nanotubes as high capacity Li-ion anodes

To obtain large quantities of silicon nanotubes, high aspect ratio nanowires of MgO will be synthesized by a simple hydrothermal route which will be used as a template to uniformly coat amorphous silicon on the surface of the nanowires. The MgO nanowires will then be removed by leaching in a mineral acid to obtain copious amounts of silicon nanotubes (SiNTs) that can potentially be extended for large scale production. A fundamental understanding of the synthesis parameters and their influence on the structure and microstructure of hollow (h-SiNTs) and their corresponding contribution to the electrochemical response will be systematically studied.

Characterization of the synthesized Si based nanocomposites

 X-Ray diffraction and Raman spectroscopy will be used to investigate the presence and nature of various phases present and also study the structure of silicon obtained by the various synthesis techniques discussed earlier.

 High resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) will be used to study the morphology of the interface structure of the nanocomposites developed.

 The electrochemical characteristics of the nanocomposites will be investigated in detail and the structure-property correlation relating the morphology and microstructure of the nanocomposites; the interface between the electrochemically active and the electrolyte interphase and their electrochemical performance will be established using a variety of

galvanostatic , potentiostatic and impedance techniques with the ultimate goal being to develop strategies for optimizing the synthesis parameters to develop nanoscale structures and architectures that will exhibit the desired optimal performance characteristics of high capacity, reduced FIR and good capacity retention at low and high current rates.