Enhanced thermoelectric performance of p-type Mg₂Sn single crystals via multi-scale defect engineering

Mg₂Sn is a promising mid-temperature thermoelectric (TE) material consisting of earth-abundant, low-cost, and non-toxic elements. Currently, the TE performance of p-type Mg₂Sn is still poor due to a lower power factor (PF) and a higher lattice thermal conductivity (κ_lat) than those of n-type Mg₂Sn. To overcome these disadvantages, we synthesized Li-doped Mg₂Sn single crystals (SCs) by the melting method. Li-doping successfully changed the conduction of the Mg₂Sn SC from an n-type to a p-type. The Li-doped Mg₂Sn SCs contain Mg vacancies, dislocation cores, and Sn-rich precipitates. These multi-scale defects in the Li-doped Mg₂Sn SCs did not deteriorate carrier mobility and they effectively scattered phonons with a wide range of frequencies. Since grain boundaries did not exist in the Li-doped Mg₂Sn SCs, higher carrier mobility and PF were achieved compared with other p-type Mg₂Sn polycrystals (PCs) and SCs. Moreover, the κ_lat of the Li-doped Mg₂Sn SC was lower than that of p-type Mg₂Sn PCs and SCs. Owing to the enhanced PF and reduced κ_lat, a maximum dimensionless figure-of-merit zT of ∼0.38 at 700 K was achieved for the p-type Li-doped Mg₂Sn SC with a Li content of 2.5%, the highest value for a p-type Mg₂Sn ever reported.