Synergistic role of Ni-doping in electrical and phonon transport properties of Cu₂Sn_1-xNi_xS₃

Cu₂SnS₃ (copper tin sulfide, CTS) is regarded as a promising eco-friendly TE material due to its phonon-glass-electron-crystal nature. In our work, ceramics of p-type pristine and Ni-doped Cu₂SnS₃ bulk materials were prepared by solid-state reaction and spark plasma sintering process. Ni-doping effectively optimized the electrical transport performance, with a very high PF about 0.9 mWm⁻¹K⁻² due to the unique electron-crystal advantages of CTS. Induced by Ni-substitution, the crystal structure of CTS transformed from original monoclinic to cubic symmetry with a softening of M-S bonds and a disordered metal atom arrangement which gave rise to a reduction of sound velocity and a strongly intensified phonon scattering. Low lattice thermal conductivity, ranging from 0.95 Wm⁻¹K⁻¹ near 300 K to 0.41 Wm⁻¹K⁻¹ at 723 K, was achieved, with the latter approaching the theoretical minimum. As a result, a relatively high ZT up to 0.56 was obtained at 723 K in the Cu₂Sn_0.9Ni_0.1S₃ sample.