Dynamic Ag⁺-intercalation with AgSnSe₂ nano-precipitates in Cl-doped polycrystalline SnSe₂ toward ultra-high thermoelectric performance

Recently, thermoelectric lead-free selenides have attracted great attention due to their earth-abundant, low-cost and environment-friendly characteristics. Here we report a new strategy to simultaneously enhance the electronic transport properties and reduce the thermal conductivity of polycrystalline SnSe₂. By combining weak van der Waals bonding with the mobile behavior of Ag⁺ ions, the carrier concentration is optimized over a wide temperature range, which can be attributed to the dynamic Ag⁺-intercalation into the van der Waals gap from the Ag⁺ ion reservoir AgSnSe₂. On account of additional electrical bridges between interlayers contributed by the intercalated Ag⁺ ions and weak anisotropy, an exciting high power factor of up to ∼7.46 μW cm⁻¹ K⁻² at 789 K is achieved along the pressing direction. In addition, the thermal conductivity is simultaneously reduced to ∼0.57 W m⁻¹ K⁻¹ at 789 K, owing to numerous line defects, phase interfaces, twin boundaries, dislocations and intercalated atomic layers generated after Ag introduction, as well as the anharmonic vibration of Ag⁺ ions. As a result, a record peak ZT of ∼1.03 at 789 K is realized along the pressing direction, which is ∼1.6 times larger than the highest reported value (0.63) of polycrystalline SnSe₂ and even comparable to that of p-type polycrystalline SnSe. This study opens a new way to achieve ultra-high thermoelectric performance, especially in layered materials.