Thermoelectric properties and transport mechanisms of sustainable TiS₂-based materials

Thermoelectrics can directly convert a temperature difference or heat flow into electricity and vice versa, on the basis of the Peltier and Seebeck effect, thereby showing great potential in alleviating energy stress, mitigating environmental impact and building a sustainable society. Thermoelectric materials have been primarily used in niche applications such as spacecrafts or specialized industrial processes. However, with ongoing research and development, advanced materials and design strategies enable the production of more-efficient, cost-effective and eco-friendly thermoelectric devices. Advancing sustainable thermoelectric materials is crucial for expanding their applications beyond niche uses such as spacecraft to more everyday scenarios like portable and medical cooling applications, wearables, outdoor PowerPot, etc. In recent years, a member of the transition-metal dichalcogenides (TMDCs) family, TiS₂, has been reported to be great potential thermoelectric material for medium temperature applications. In addition to its large Seebeck coefficient (close to –300 μVK⁻¹) at room temperature, TiS₂ also offers significant sustainability advantages. It is known for being environmentally friendly, cost-effective, composed of non-critical elements, and lightweight. With this motivation, the main work of my PhD thesis is to fabricate sustainable TiS₂-based thermoelectric materials with good thermoelectric properties via different strategies. The first strategy is based on the relatively wide van der Waals gaps of layered TiS₂ material, which can accept a variety of species as intercalants. In this work, iron intercalated TiS₂ (FeₓTiS₂) compounds with x varying from 0 to 0.05 were prepared using a solid-liquid-vapor reaction and spark plasma sintering. The intercalated iron cations are served as the electron donor, leading to a substantial decrease in electrical resistivity. In the meantime, structural disorder caused by iron intercalation significantly contributes to the reduction of lattice thermal conductivity in the direction parallel to the pressing direction. However, its contribution is limited in the direction perpendicular to the pressing direction. The second approach is to minimize its lattice thermal conductivity in the perpendicular direction by designing entropy-engineered transition metal sulfides (Ti/Nb/Ta/Zr)S₂ to introduce strong in-plane point defects. The composition with TiS₂ as the primary phase, supplemented by minor doping of Zr, Nb, and Ta, holds promise for reducing lattice thermal conductivity while preserving its high power factor. The third work continues with the entropy engineering strategy but introduces more elements on both Ti and S sites. In this work, multi-element doped TiS₂ materials, with Y and Nb, La and Ta on Ti sites, and Se on S sites, were investigated to further explore the potential for reducing the lattice thermal conductivity in the perpendicular direction. The lattice thermal conductivity was observed to decrease progressively with the increased diversity of elements in multi-element doped TiS₂. Importantly, the multi-element doping strategy enables the reduction of lattice thermal conductivity independently, without significantly compromising its large power factor. In summary, my PhD work focuses on designing sustainable TiS₂-based thermoelectric materials. The findings indicate that intercalation effectively reduces lattice thermal conductivity in the parallel direction, thereby notably enhancing the figure of merit in this direction. Conversely, for the perpendicular direction, entropy engineering emerges as a promising strategy for minimizing lattice thermal conductivity while maintaining the integrity of the electrical transport framework.