High thermoelectric performance enabled by convergence of nested conduction bands in Pb7Bi4Se13 with low thermal conductivity.

Thermoelectrics enable waste heat recovery, holding promises in relieving energy and environmental crisis. Lillianite materials have been long-term ignored due to low thermoelectric efficiency. Herein we report the discovery of superior thermoelectric performance in Pb7Bi4Se13 based lillianites, with a peak figure of merit, zT of 1.35 at 800 K and a high average zT of 0.92 (450-800 K). A unique quality factor is established to predict and evaluate thermoelectric performances. It considers both band nonparabolicity and band gaps, commonly negligible in conventional quality factors. Such appealing performance is attributed to the convergence of effectively nested conduction bands, providing a high number of valley degeneracy, and a low thermal conductivity, stemming from large lattice anharmonicity, low-frequency localized Einstein modes and the coexistence of high-density moiré fringes and nanoscale defects. This work rekindles the vision that Pb7Bi4Se13 based lillianites are promising candidates for highly efficient thermoelectric energy conversion.

Thermal Conductive 2D Boron Nitride for High-Performance All-Solid-State Lithium-Sulfur Batteries.

Polymer-based solid-state electrolytes are shown to be highly promising for realizing low-cost, high-capacity, and safe Li batteries. One major challenge for polymer solid-state batteries is the relatively high operating temperature (60-80 °C), which means operating such batteries will require significant ramp up time due to heating. On the other hand, as polymer electrolytes are poor thermal conductors, thermal variation across the polymer electrolyte can lead to nonuniformity in ionic conductivity. This can be highly detrimental to lithium deposition and may result in dendrite formation. Here, a polyethylene oxide-based electrolyte with improved thermal responses is developed by incorporating 2D boron nitride (BN) nanoflakes. The results show that the BN additive also enhances ionic and mechanical properties of the electrolyte. More uniform Li stripping/deposition and reversible cathode reactions are achieved, which in turn enable all-solid-state lithium-sulfur cells with superior performances.

Origin of High Thermoelectric Performance in Earth-Abundant Phosphide-Tetrahedrite.

Phosphide-based thermoelectrics are a relatively less studied class of compounds, primarily due to the presence of light elements, which result in high thermal conductivity and inherent stability problems. In this work, we present a stable phosphide-tetrahedrite, Ag6Ge10P12, which possesses the highest zT (∼0.7) among all known phosphides at intermediate temperatures (750 K). We examine the intrinsic electronic and thermal transport properties of this compound by expressing the transport properties in terms of weighted mobility (µW), transport coefficient (σE0), and material quality factor (B), from which we are able to elucidate that the origin of its high zT can be attributed to the platelike Fermi surface and high level of band multiplicity related to its complex band structure. Finally, we discuss the origin of the low lattice thermal conductivity observed in this compound using experimental sound velocity, elastic properties, and Debye-Callaway model, thus laying the foundation for similar stable phosphides as potentially earth-abundant and nontoxic intermediate-temperature thermoelectric materials.