Elucidating fundamental mechanisms in energy generation and energy storage materials - A Raman spectroscopic study

Fundamental mechanisms that enhance the performance of thermoelectric (TE) materials, for example, charged grain boundaries in nano-structured n-type Bi2Te2:7Se0:3 and strong anharmonic phonon scattering effects in single crystalline SnSe, were investigated via micro-Raman spectroscopy. In n-type Bi2Te2:7Se0:3, I observed the presence of a forbidden IR-active mode in the Raman spectra of chemically and mechanically exfoliated (C/ME) n-type Bi2Te2:7Se0:3 flakes, due to the creation of sub-quintuples and consequent lowering of crystal symmetry. Furthermore, a previous study hypothesized that charged grain boundaries are formed upon restacking and densifying the C/ME processed flakes via spark-plasma-sintering, and I directly observed this by Kelvin Probe Force Microscopy as discussed in chapter 4. A record high TE performance was reported in single crystalline SnSe which is primarily attributed to its low thermal conductivity, originating from strong anharmonicity. I undertook a temperature dependent Raman study of fully dense SnSe, which revealed a relatively higher softening of the phonon modes in the b-c plane (by a factor of six) compared to the phonon mode softening along the a axis, as well as ultrashort phonon lifetimes (~0.1 ps). Analysis of the Raman peak frequencies and linewidths showed phonon decay to be dominated by a three-phonon scattering process. The anharmonic coeffcients αR and αC calculated from the Raman and heat capacity measurements, respectively, are in excellent agreement with each other as discussed in chapter 5. This study provides a deeper understanding of phonon-phonon interaction and anharmonicity in SnSe leading to outstanding thermal transport properties. In addition to using Raman spectroscopy to study thermoelectric materials, I also used in situ micro-Raman spectroscopy to elucidate the charge/discharge mechanism in sulfurized polyacrylonitrile (SPAN) based cathodes for lithium sulfur batteries (LSBs). From the irreversible Raman intensity of the 475 cm – 1 peak which I observed during the first discharge cycle, it can be inferred that the S-S bond linkages cleaved, resulting in the formation of radicals in SPAN with negative S sites. The reversible Raman peak intensities of the electrolyte in LSBs allowed me to monitor Li+ ion concentration and diffusion in the diffusion layer near the surface of SPAN-graphene foam (GF) cathode. Unlike cathodes containing elemental sulfur, the radicals in SPAN react reversibly with Li+ ions instead of forming polysulfide intermediates and Li2S/Li2S2 discharge products. Owing to the lightweight and porous structure of 3-dimensional GF, the LSBs with SPAN-GF cathodes exhibited a rate capacity of 900-1000 mAh/g at 0.1C over a large range of sulfur loadings (1.1-10.6 mg cm – 2). An areal capacity of 17.1 mAh cm – 2 was achieved with a sulfur loading of 19.7 mg cm – 2 The LSBs prepared in this study delivered simultaneously a gravimetric energy density of ~366 Wh Kg – 1 and a power density of ~580 W Kg – 1 at the electrode level as discussed in chapter 6