Chemical modification by doping of graphitic carbon and silicon based anode materials

Abstract

This body of work investigates the structural, chemical, and electrochemical effects of substitutional doping on three distinct materials systems: phosphorus-doped graphitic carbon, aluminum-doped graphitic carbon, and phosphorus-doped silicon. Success of doping is found to depend on the synthetic route employed (bottom-up or top-down) as well as the selection of dopant with respect to the host material. Al- and P-doped graphitic carbons were prepared by bottom-up pyrolysis of liquid precursors at low to modest temperatures (800-1100 °C). In both cases, doping was found to be challenging to achieve without the formation of byproducts and phase segregation. Efforts to synthesize phosphorus-doped graphitic carbons led to an interesting discovery that the phosphorus allotrope side-product could be controlled by altering the precursor. This work quantifies the relative phosphorus allotropes present (white, red, or a combination thereof) in each composite using a combination of materials characterization techniques. Such materials are interesting lithium-ion anode materials that exhibit the first evidence of the reversible lithiation of white phosphorus, enabled by stabilization of P 4 domains between graphitic sheets. Aluminum- doped graphitic carbon, on the other hand, was found to be extremely difficult to obtain without forming mullite (3Al 2O 3 x 2SiO 2) as a byproduct. Nevertheless, we report the first signature of trigonal planar or puckered AlC 3 type doping sites. Lastly, we explored phosphorus-doped silicon materials prepared via top-down solid-state synthesis strategies. Homogenously and heterogeneously doped silicon nanoparticles were obtained by exploring a wide range of synthetic parameters and successful doping was confirmed by X-ray diffraction. Phosphorus doping in dilute quantities (1000 ppm) is found to unambiguously and positively impact the electrochemical performance of silicon as a lithium-ion anode. Ultimately, the work presented in this thesis illuminates the challenges associated with doping by both bottom-up and top-down synthesis strategies while also exploring the electrochemical relevance of chemically-modified graphitic and silicon-based materials.