Enhanced n-type thermopower in distortion-free LiMn2O4

Thermoelectric devices for power generation and solid-state refrigeration represent an important technology that could potentially offer long term solid-state solutions for increased energy efficiency and address environmental concerns. However, the efficiency of existing thermoelectric materials, limited temperature stability and potential scarcity of key elements, such as Te, have motivated researchers to consider oxides as thermoelectric materials. Traditional thermoelectric materials such as Si, Bi2Te3 and Sb2Te3 achieve large ZT values by adjusting carrier density to compromise between electrical conductivity and thermopower. Typically the carrier density is 10–10 cm 3 with electrical conductivity of 10 S m 1 and thermopower of 200–300 mV K . Some of the most recent advances in the field of thermopower generation rely on such conventional materials. The most promising oxide thermoelectrics are layered cobaltates, NaxCo2O4. These exhibit metallic conductivity as well as an unusually large thermopower considering the high carrier density, 10 cm 3 (Seebeck coefficient, Q300 K 1⁄4 100 mV K 1 increasing to over 200 mVK 1 at 800 K). In fact, at high temperatures ZT > 1 has been achieved in layered cobaltates making them comparable to the best conventional materials. The uncommonly large thermopower for a material with large carrier concentration has been attributed to the additional entropy arising from the spin and orbital degrees of freedom in Co and Co ions. For strongly correlated systems, such as NaxCo2O4, the Hubbard model expression for the Seebeck coefficient Q reduces to the Heikes formula: