Approaches toward Silicon-Based High-Capacity Anodes for Lithium-Ion Batteries
The Kumta Lab is developing low-cost methods for producing nanoscale silicon composites as lithium-ion anodes. These heterostructures comprise nanocrystalline or amorphous Si and may contain a variety of carbon precursors. The Si provides high capacity while the graphitic and disordered carbon acts as an electrically-conductive matrix as well as a mechanically compliant phase. The Kumta Lab’s multi-pronged approach towards anode fabrication includes chemical vapor deposition (CVD), high-energy mechanical milling (HEMM), and electro-reduction.
In the CVD process, a carbon nanotube forest is grown on a quartz substrate and silicon nanoclusters are deposited on the surface of the nanotubes (Fig. 1). The nanostructured Si accommodates the large volume changes associated with lithium-ion insertion and extraction, leading to improved cyclability, while the carbon nanotube functions as a flexible mechanical support for additional strain release and provides an efficient conducting channel. An interfacial layer of amorphous carbon binds the Si to the carbon nanotube, leading to good stability and 99.9% coulombic efficiency. This composite anode has exhibited a specific capacity of 1000 mAh/g at a 2.5C rate.
In the HEMM process, Si and graphite are mixed with polyacrylonitrile (PAN), followed by pyrolysis at 800-900 °C. Additionally, nanocrystalline/amorphous Si is obtained via novel mechano-chemical reduction of a variety of Si precursors and solid-phase reductants; this synthetic route is a high-yield, low-cost alternative to CVD. The polymer additive, PAN, preserves the graphitic carbon and prevents SiC formation, thereby lowering the irreversible capacity loss. An anode capacity of ~1000 mAh/g has been achieved at a C/6 rate, and improvement of the rate capability through Si doping is being investigated.
The promising performance of nanocrystalline/amorphous-Si is due to free volume, the presence of defects, and the lack of long range order — all of which accommodate expansion upon Li insertion and result in increased cyclability. To design toward these characteristics, amorphous Si films are synthesized directly on copper foil by electro-reduction of a silicon halide-based electrolyte. Raman spectroscopy of the resultant film indicates that it is indeed composed of amorphous-Si. Electrochemical tests indicate a large first cycle inefficiency due to impurities in the preliminary samples. The Kumta Lab is working on reducing this first cycle loss to the desired 15% observed in the Si nanocomposites described above. After the first cycle, however, a stable reversible capacity of ~1300 mAh/g has been obtained. This approach of developing thin amorphous-Si films directly on the current collector eliminates the use of binders and conducting agents, thereby simplifying the process and making it amenable to large-scale manufacturing.