Study on Enhancing the Thermoelectric Stability of the β-Cu₂Se Phase by Mn Doping

Cu₂Se is a promising thermoelectric (TE) material due to its low cost, Earth abundance, and high thermoelectric properties. However, the biggest problem of Cu₂Se is its unstable chemical properties. In particular, under the action of an electric field or gradient temperature field, the chemical potential of copper ions inside the material increases. When the external field is strong enough, the chemical potential of copper ions at the negative end of the material reaches the chemical potential of elemental copper. Under these conditions, copper ions must precipitate out, causing Cu₂Se to be unstable, and making it unsuitable for use in applications. In this study, we prepared Cu_2-xMn_xSe (x = 0, 0.02, 0.04 and 0.06) series bulk materials by vacuum melting-annealing and sintered by spark plasma sintering (SPS). We investigated the effects of Mn doping on the composition, microstructure, band structure, scattering mechanism, thermoelectric properties, and stability of Cu₂Se. The results show that Mn doping can adjust the carrier concentration, promote the stabilization of the β-phase structure and improve the electrical properties of Cu₂Se. When x = 0.06, the highest power factor (PF) value of Cu_1.94Mn_0.06Se at 873 K was 1.62 mW m^-1 K^-2. The results of carrier scattering mechanism analysis based on the conductivity ratio method show that the sample doped with Mn and pure Cu₂Se had the characteristics of ionization impurity scattering, and the scattering factor was 3/2. However, the deterioration in thermal conductivity was large, and a superior zT value needs to be obtained. The cyclic test results of high-temperature thermoelectric properties show that Mn doping can hinder Cu⁺ migration and improve its thermoelectric stability, which preliminarily verifies the feasibility of using the stable zirconia mechanism to improve the thermoelectric stability of Cu₂Se.