Research progress of two-dimensional covalent bond substructure Zintl phase thermoelectric materials

Thermoelectric materials can realize the direct conversion between thermal energy and electrical energy, and thus having important applications in semiconductor refrigeration and heat recovery. Zintl phase is composed of highly electronegative cations and anions, which accords with the concept of "phonon glass, electron crystal" (PGEC). Thermoelectric properties of Zintl phase have attracted extensive interest, among which the two-dimensional (2D) covalent bond structure featured Zintl phases have received more attention for their outstanding electrical properties. In this review, Zintl phase materials with two-dimensional covalent bond substructures are reviewed, including 1-2-2 type, 9-4+x-9-type, 2-1-2 type and 1-1-1 type Zintl phase. The 1-2-2-type Zintl phase is currently the most widely studied and best-performing Zintl material. It is worth mentioning that the maximum ZT value for the Mg3Sb2-based n-type Zintl material with the CaAl2Si2 structure has been reported to reach 1.85, and the average ZT value near room temperature area also reaches 1.4. The 9-4+x-9-type Zintl material with a mass of atoms in unit cell contributes to lower thermal conductivity thus relatively high ZT value. The 2-1-2-type Zintl material has extremely low thermal conductivity due to the intrinsic vacancies, which has been developing in recent years. The 1-1-1-type Zintl material with the same ZrBeSi structure as the 2-1-2-type Zintl material, shows better electrical transport performance. In sum, this review summarizes the recent progress and optimization methods of those typical Zintl phases above. Meanwhile, the future optimization and development of Zintl phase with two-dimensional covalent bond substructures are also prospected.