Cellular structured Cu₂Sn_0.8Co_0.2S₃ with enhanced thermoelectric performance realized by liquid-phase sintering

As an eco-friendly and earth-abundant thermoelectric material, P-type Cu₂SnS₃ has been studied intensively, revealing a maximal ZT of ∼0.85 in Cu₂Sn_0.8Co_0.2S₃. Preliminarily in this work, increasing sintering temperature realized a high power factor of ∼14 μW cm⁻¹ K⁻² with a detrimental increase of electronic thermal conductivity and resulted in a lowered ZT of ∼0.6 at 773 K. Accordingly, a Sn-assisted liquid-phase sintering was adopted, obtaining a series of cellular structured Cu₂Sn_0.8Co_0.2S₃-xSn (x = 0-0.06) bulks with Cu₂Sn_0.8Co_0.2S₃ grains mainly isolated by monoclinic-phase-rich boundaries with dense dislocation arrays, which functioned as barriers for both low-energy carriers and phonons at the same time. This caused a dramatic reduction of electrical conductivity and electronic thermal conductivity (maximally by 90% for both), with a simultaneous enhancement of Seebeck coefficient and a modest sacrifice of power factor, while more importantly, a strongly suppressed lattice thermal conductivity was observed. Finally, a maximum ZT of ∼1.03 at 773 K was achieved in the Cu₂Sn_0.8Co_0.2S₃-0.05Sn composite.