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Copper ion liquid-like thermoelectrics


Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling1,2. Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity2. Here we report on Cu2−xSe, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu2−xSe form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary ‘liquid-like’ behaviour of copper ions around a crystalline sublattice of Se in Cu2−xSe results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.

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Figure 1: Crystal structure of Cu2Se at high temperatures (β-phase) with a cubic anti-fluorite structure.
Figure 2: Thermoelectric properties of the low-temperature (α) and high-temperature (β) phases in Cu2−xSe.
Figure 3: In situ HRTEM observation of the phase transformation in Cu2Se.
Figure 4: The high-temperature specific heat capacity of Cu2Se.

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This work is in part supported by National Natural Science Foundation of China (NSFC) Grants (51121064 and 50825205), Shanghai Science and Technology Commission (Pujiang Program with No. 11PJ1410200 and Program of Shanghai Subject Chief Scientist with No. 09XD1404400), and CAS/SAFEA International Partnership Program for Creative Research Teams. F.X. wishes to acknowledge the support of the National Basic Research Program of China (973 program) under Project 2009CB939904 and NSFC Grants (60936001). C.U. wishes to acknowledge the support of the Center for Solar and Thermal Energy Conversion Research Center funded by the Department of Energy (DOE) under No. DE-SC00000957. G.J.S. acknowledges support from AFOSR-MURI. Q.L. acknowledges support from the US DOE, Office of Basic Energy Science, Materials Sciences and Engineering Division, under contract no. DEAC0298CH10886.

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H.L. and X.S. prepared the samples and measured the thermoelectric properties. F.X., W.Z., L.C., Q.L., G.J.S. and C.U. provided discussion on the experimental data. F.X. and L.Z. performed TEM measurements and analysis. T.D. and G.J.S. performed room-temperature speed of sound measurements. H.L., X.S., L.C., Q.L., C.U. and G.J.S. wrote and edited the manuscript.

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Correspondence to Xun Shi or Lidong Chen.

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Liu, H., Shi, X., Xu, F. et al. Copper ion liquid-like thermoelectrics. Nature Mater 11, 422–425 (2012).

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