Article pubs.acs.org/cm
Significantly Enhanced Thermoelectric Performance of γ‑In2Se3 through Lithiation via Chemical Diffusion Jiaolin Cui,*,† Hua Peng,‡ Zhiliang Song,† Zhengliang Du,† Yimin Chao,*,§ and Gang Chen‡,* †
School of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China School of Physics and Technology, University of Jinan, Jinan 250022, China § School of Chemistry, University of East Anglia, Norwich NR4 7TJ, United Kingdom ‡
S Supporting Information *
ABSTRACT: γ-In 2 Se 3 is selected as a thermoelectric candidate because it has a unique crystal structure and thermal stability at relatively high temperatures. In this work, we have prepared lithiated γ-In2Se3 through chemical diffusion and investigated its band structures and thermoelectric performance. After lithiation of γ-In2Se3 in a lithium acetate (CH3COOLi) solution at 50 °C for 30 h, we have observed a high Hall carrier concentration (nH) of ≤1.71 × 1018 cm−3 at room temperature, which is ∼4 orders of magnitude higher than that of pristine γ-In2Se3. The enhancement of nH is directly responsible for the remarkable improvement in electrical conductivity and can be elucidated as the Fermi level (Fr) unpinning and moving toward the conduction band through the dominant interstitial occupation of Li+ in the γ-In2Se3 lattice. Combined with the minimum lattice thermal conductivity (κL = 0.30−0.34 W K−1 m−1) at ∼923 K, the highest ZT value of 0.62−0.67 is attained, which is approximately 9−10 times that of pristine γ-In2Se3, proving that the lithiation in γ-In2Se3 is an effective approach to the improvement in thermoelectric performance. structural vacancies existing along the c-axis in γ-In2Se3, which accommodates cations of different sizes. It has been reported that the diffusion of small cations, such as Li, into the crystal lattice of In2Se3 forms metallic phase Li0.1In2Se3, enhancing the free carrier concentration by >3 orders of magnitude (from 1016 to 1.5 × 1019 cm−3).20 In addition, the impurity occupation in the cation sites could induce the shift of the Fermi level (Fr), thus engineering the band structure.21 Therefore, impurity doping in γ-In2Se3 has a profound impact on the structure and TE performance of the host materials. In this work, we have prepared lithiated γ-In2Se3 powders via chemical diffusion and examined transport and TE properties from room temperature (RT) to ∼930 K. The experiments reveal that doping of Li ion in γ-In2Se3 enhances the Hall carrier concentration (nH) by ∼4 orders of magnitude and thereby significantly improves the TE performance with the highest ZT value of 0.62−0.67 being found at ∼923 K. This value is 9−10 times that of pristine γ-In2Se3, proving that lithiation in γ-In2Se3 is playing a sizable role in improving TE performance.
1. INTRODUCTION Thermoelectric (TE) materials have attracted a great deal of attention in recent years because they are capable of harvesting huge amounts of waste heat by converting heat into electricity. However, the conversion efficiency is still low, and highperformance TE materials remain limited. Although many compounds, such as PbTe,1,2 SnSe,3,4 Mg2Si,5 and some other tellurides,6 exhibit potential TE performance, developing highperformance and new environmentally benign TE materials is still urgently needed for midtemperature power generation applications. Indium selenide (In2Se3) could be used as a phase-change random access memory device and thermoelectric material, because of its large bandgap, 7 intrinsic low thermal conductivity, and high Seebeck coefficient.8−10 However, there are different coexisting phases and crystal structures, such as rhombohedral/hexagonal α/β phases and hexagonal γ and δ phases, some of which, for example, α and β phases, exist in a metastable state and are inclined to mutual transformation upon being heated or cooled.11 Therefore, it is difficult to synthesize single α- or β-In2Se3-based solid solutions.12,13 Accordingly, the γ phase, which is stable above 520 °C,12 625 °C,14 or 650 °C,15 in terms of different experiments, might be an alternative indium selenide used for TE applications in the region of intermediate to high temperatures. γ-In2Se3 behaves like an insulator with a bandgap of 1.9 eV.16 Unlike α-In2Se3, it has intrinsic screwlike ordering vacancies,17−19 instead of layerlike ones. However, there are 1/3 © 2017 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Sample Preparations. Two elemental powders of In and Se with a purity of >99.999% were loaded into the vacuum silica tube, Received: June 14, 2017 Revised: August 8, 2017 Published: August 9, 2017 7467
DOI: 10.1021/acs.chemmater.7b02467 Chem. Mater. 2017, 29, 7467−7474
Article
Chemistry of Materials
measured by the TC-1200RH instrument from RT to 930 K with an uncertainty of
