Layered thermoelectric materials: Structure, bonding, and performance mechanisms

The ever-increasing world-wide energy consumption and crisis of environmental pollution have aroused enthusiasm on developing high-efficiency and green-clean energy conversion technology. Thermoelectric materials enable an environmentally friendly conversion between heat and electricity, and therefore serve as an optimum candidate for solving the current dilemma and contribute to the carbon-neutral target. Among the thermoelectric family, layered materials have shared a great portion with impressive thermoelectric performance originating from their (quasi-)two-dimensional crystal structure with hierarchical bonding, i.e., strong intralayer and weak interlayer bonds. This structure and bonding feature is believed to be propitious to low lattice thermal conductivity, low-dimensional electrical features, and anisotropic electron and phonon transport behaviors, which offer great opportunity to disentangle the inter-coupled thermoelectric parameters. For those benefits, layered materials emerge endlessly in the field of thermoelectricity and have achieved extensive attention. In this review, we highlight the recent progress in the field of layered thermoelectric materials. The structure and bonding peculiarities of layered thermoelectric materials are outlined. Then, following the classification of single-unit, quasi-double-unit, and double-unit layered thermoelectric materials, the crystal and bonding features in some typical layered thermoelectric materials are discussed, with focus on their current research interest and progresses. The possible mechanisms behind the performance optimization will be analyzed. Finally, some personal views on the prospect of this field, including chemical bond perspective and interlayer electronic transport enhancement are also presented.