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Xinzhi Wu, [Longquan Wang](https://orcid.org/0009-0009-9910-9770), [Airan Li](https://orcid.org/0009-0004-7318-4821), Gang Wu, [Zhao Hu](https://orcid.org/0000-0003-4252-6572), Fei Frank Yun, [Takao Mori](https://orcid.org/0000-0003-2682-1846)

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[High Conversion Efficiency in Intrinsic High Power‐Density Mg                    <sub>2</sub>                    Sn‐GeTe Thermoelectric Generator](https://mdr.nims.go.jp/datasets/c23c274d-0969-494e-9cc2-633be59c7016)

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High Conversion Efficiency in Intrinsic High Power‐Density Mg2Sn‐GeTe Thermoelectric GeneratorRESEARCH ARTICLEwww.advancedscience.comHigh Conversion Efficiency in Intrinsic High Power-DensityMg2Sn-GeTe Thermoelectric GeneratorXinzhi Wu, Longquan Wang, Airan Li, Gang Wu, Zhao Hu, Fei Frank Yun,and Takao Mori*Thermoelectric generators (TEGs) offer a sustainable solution for thermalenergy harvesting, where maximizing energy output necessitates achievingboth high power density and high conversion efficiency. However, TEGs withintrinsically high power density by employing high power factor materialsoften face efficiency limitations due to their usually high thermal conductivity.Here, intrinsically high power density TEGs based on Mg2Sn-GeTe for the firsttime is presented, simultaneously delivering a remarkable conversionefficiency of 9% under a temperature gradient of 418 K, thereby setting a newbenchmark in the field. This exceptional performance is attributed to thesignificant balance between the moderating carrier and phonon transport inMg2Sn, enabled by a stepwise aliovalent Sb and Bi solid solution, withoutover-compromising its outstanding power factor. Consequently, a highthermoelectric figure of merit of 1.4 is achieved in Mg2Sn0.8(Sb0.5Bi0.5)0.2. Thehigh-performance Mg2Sn–GeTe TEGs introduced here represent a significantadvancement in thermoelectric technology, offering an innovative andefficient solution for off-grid energy supply in waste-heat-rich environments.1. IntroductionIndustrial processes are a major source of waste heat, with upto 60% of energy often lost as residual heat, exacerbating en-vironmental concerns.[1] Coupled with the ongoing global en-ergy crisis, the increasing environmental challenges, and the ur-gent push toward carbon neutrality and peak emissions targets,there is an escalating demand for sustainable energy conver-sion technologies.[2,3] Thermoelectric generators (TEGs) presentX. Wu, L. Wang, A. Li, G. Wu, Z. Hu, F. F. Yun, T. MoriResearch Center for Materials Nanoarchitectonics (MANA)National Institute for Materials Science (NIMS)Tsukuba 305–0044, JapanE-mail: mori.takao@nims.go.jpZ.Hu, T.MoriGraduate School of Pure andAppliedScienceUniversity of Tsukuba1-1-1 Tennodai, Tsukuba, Ibaraki 305–8671, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/advs.202506997© 2025 The Author(s). Advanced Science published by Wiley-VCHGmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/advs.202506997a promising solution, as they directly con-vert heat into electricity without producingemissions or noise, offering a clean and effi-cient method for energy recovery.[4,5] This isespecially relevant in scenarios with abun-dant heat sources, where TEGs can promis-ingly address energy and environmentalchallenges.[4]To expand their applicability across di-verse heat-rich environments, enhancingthe energy output of TEGs is crucial, asit directly determines their capability tomeet varied power demands.[6] Maximiz-ing the energy output of TEGs requiresboth high power density and high con-version efficiency. Power density ensuresthe amount of energy generated per unitarea, while conversion efficiency dictateshow effectively heat is converted into elec-trical energy and minimizes energy losses.Power density is primarily influenced bythe power factor of thermoelectric (TE)materials, while conversion efficiency isdetermined by the thermoelectric figure of merit (ZT) withoutconsidering device size and interfaces.[7] Interfacial resistancehas long posed a bottleneck for device advancement,[8] and en-couragingly, recent advances have significantly mitigated this is-sue, reducing interfacial resistivity to as low as 10 μΩ·cm2,[9–12]which limits ZT losses below 5%.[13] TEGs with high power den-sity hold unique advantages in practical application for waste heatrecovery, such as in industrial environments or automotive ex-haust heat recovery.[14] Choosing TE materials with a high powerfactor can facilitate the development of high-power-density TEGs.However, the usually high thermal conductivity of such TEmate-rials often leads to a reduced ZT, ultimately lowering the conver-sion efficiency of these TEGs. For instance, some typical materi-als NiAu, Fe2VAl, NbFeSb,Mg2Sn, etc.[15–17] Therefore, balancingthe trade-off between the electrical power factor and thermal con-ductivity via moderating carrier and phono transport remains asignificant challenge in developing TEGs with high power den-sity and conversion efficiency.The n-type Mg2Sn-based[18–27] and p-type GeTe-based[28–31]compounds stand out as promising candidates for power-oriented TEGs due to their high power factor, which grantsthem intrinsic advantages in achieving high power density inmid-temperature applications. Additionally, their environmen-tally friendly composition makes them sustainable options. Thep-type GeTe has garnered significant attention for its high powerAdv. Sci. 2025, 12, e06997 e06997 (1 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbHhttp://www.advancedscience.commailto:mori.takao@nims.go.jphttps://doi.org/10.1002/advs.202506997http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.202506997&domain=pdf&date_stamp=2025-07-30www.advancedsciencenews.com www.advancedscience.comFigure 1. Stepwise Optimized performance of TE materials and device via aliovalent solid solution Strategy. The temperature-dependent power factorvariations for n-type Mg2Sn a) and p-type GeTe b), compared to several benchmark TE materials.[41] c) ZT values as a function of temperature, and theinset shows the ZTave values over the temperature range from room temperature to 773 K. d) A 2D map correlating the maximum power density (𝜔max)with the maximum conversion efficiency (𝜂max) for Mg2Sn-based TE devices, along with comparative data from the literature.[42–47] e) Photograph ofthe device setup in the commercial instrument Mini-PEM, with a close-up view of the device configuration.factor and further reduced low thermal conductivity by en-tropy, defect or charge transfer engineering, with numerous re-ports demonstrating a ZT value exceeding 2.[32–36] However, theprogress in enhancing theZT ofMg2Sn has lagged behind that ofGeTe. Zaitsev et al.[18] first reported a ZT of 1.1 at 723 K in 2006,with subsequent enhancements to ZT 1.4 and a power factor of5 Wm−1 K−2.[20,37] Since then, interest in Mg2Sn has surged, cul-minating in recent breakthroughs,[19,20,24,38,39] achieving ZT 1.6for Mg2(Si0.4Sn0.6)0.985Sb0.015 at 823 K.[40] However, experimen-tal validation of these high-ZT materials at the device level re-mains unreported. Despite its inherently high power factor, theMg2Sn exhibits a thermal conductivity of over 5 Wm−1 K−1 ≈300K, which imposes a fundamental constraint on the conversion ef-ficiency of fabricated TEGs. Nevertheless, addressing the intrin-sic high thermal conductivity of n-typeMg2Sn while maintaininghigh power density remains an overlooked challenge at the devicelevel. To date, no practical implementation of p-n TEGs integrat-ing these two high-power-factor TE materials has been reported.In this study, we present a novel Mg2Sn─GeTe TEGs with theintrinsic high-power-density, achieving an impressive conver-sion efficiency of 9% under a temperature gradient of 418 K. Thisperformance is realized through a stepwise aliovalent Sb and Bisolid solution strategy on Mg2Sn, which effectively balances thepower factor and thermal conductivity via moderating carrierand phono transport, achieving a low lattice thermal conductivityof 1.8 W·m−1·K−1 and a peak ZT of 1.4. Notably, this optimiza-tion entails only a ≈20% reduction in power factor, yet yields a∼47% decrease in lattice thermal conductivity—an outcome thatsubstantially improves ZT and offers a promising route towardan intrinsic high-power-density system. These advancementsaddress the inherent trade-off between electrical propertiesand thermal conductivity, culminating in a high-performanceMg2Sn0.8(Sb0.5Bi0.5)0.2–Ge0.9Sb0.1Te TEG. Our optimized TEGsprovide a sustainable and efficient solution for energy harvestingin waste-heat-rich, mid-temperature environments, offering apractical reference for other intrinsic high-power density TEdevices.2. Main Text2.1. Stepwise Optimized Materials and Device PerformanceDrawing inspiration from the intrinsic advantages of high power,we demonstrate a strategy that effectively balances the powerfactor and thermal conductivity through aliovalent solid solu-tions Sb and Bi in Mg2Sn-based TE materials, leading to sig-nificantly improved ZT. As shown in Figure 1a, the power fac-tor of Mg2Sn0.9Sb0.1 reaches ≈7 mW m−1 K−2 at 773 K. Undersimilar Sb doping concentration, the power factor of Ge0.9Sb0.1Tewith high ZT exceeds ≈3 mWm−1 K−2 at 773 K, also surpassingmost conventional p-type TE materials[41] such as PbTe, ZrCoSb,Zn4Sb3, BiCuSeO, and MgAgSb[48] (Figure 1b), thereby provid-ing a fundamental requirement for high-power-density devices.However, the ZT of Mg2Sn-based TE materials with an averageZTave of 0.56 for Mg2Sn0.9Sb0.1 remains lower than that of GeTe(Figure S1, Supporting Information), As shown in Figure 1c, in-creasing the Sb doping concentration raises the ZTave from 0.56for Mg2Sn0.9Sb0.1 to 0.67 for Mg2Sn0.8Sb0.2. Similarly, the peakpower factor decreases from over 7 mWm−1 K−2 to ≈5 mWm−1K−2 as Sb doping concentration increases from 0.1 to 0.2. Never-theless, at Sb = 0.2, the peak power factor remains higher thanthat of other n-type TE materials[41] such as TiNiSn, SiGe, PbTe,Adv. Sci. 2025, 12, e06997 e06997 (2 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 2. TE properties of Mg2Sn-based materials. a–c) Temperature dependence of electrical resistivity (𝜌), Seebeck coefficient (S), and weightedmobility (µw) for various compositions. (e) The temperature-dependent thermal conductivity 𝜅, and lattice 𝜅L. f) The relationship between 𝜅L andconfiguration entropy (ΔS), along with comparative data from the literature.[23]La3Te4, and Mg3Sb2, making it an ideal baseline for further opti-mizing its thermal conductivity (Figure 1a).The addition of Bi raises the average ZTave to 0.84 inMg2Sn0.8(Sb0.5Bi0.5)0.2 (Figure 1c), even with a slight improve-ment in the power factor compared with the Mg2Sn0.8Sb0.2(Figure 1a), particularly between room temperature to 773 Ktemperature range. The ZT of the repeated batches exhibitsfluctuations within 5%, confirming the reliability of our find-ings (Figure S2, Supporting Information). The assembled 2 pairMg2Sn0.8(Sb0.5Bi0.5)0.2-Ge0.9Sb0.1Te device and the testing setupare shown in Figure 1d,e, demonstrating a power density of0.7 W cm−2 and conversion efficiency of 9%. It is worth not-ing that the power density is also affected by the device’s phys-ical dimensions, suggesting that further improvements couldbe achieved through structural optimization of the TE device.In addition, the integration with p-type Ge0.9Sb0.1Te results insignificantly enhanced conversion efficiency, which is higherthan that of other Mg2Sn-based TEGs such as Mg2Si0.3Sn0.7-Mg2Si0.3Sn0.7,[43,46] Mg2Si-MnSi,[42] and Mg2Si0.3Sn0.7-MgAgSbTEGs.[47]2.2. TE PerformanceSb and Bi play a pivotal role in optimizing the ZT of Mg2Sn, ne-cessitating a detailed investigation of their individual effects. Inthis section, we elucidate the underlying mechanisms of Sb andBi aliovalent solid solution, specifically the Bi-induced retentionof power factor and the Sb/Bi-driven reduction in thermal con-ductivity. This analysis highlights the delicate balance betweenmoderating carrier transport to mitigate power factor degra-dation and enhancing phonon scattering to suppress thermalconductivity.XRD and SEM analyses confirm the phase purity and com-positional uniformity of the Sb- and Bi-doped samples (FiguresS3–S6, Supporting Information), consistent with the high solu-bility of these elements in the Mg2Sn matrix.[23,49,50] Figure 2a–dillustrates the electrical transport properties of the Mg2Sn aliova-lent element solid solution system. As shown in Figure 2a, thetemperature-dependent electrical resistivity (𝜌) curves reveal adistinct transition in conduction behavior. At Sn= 0.9, the systemexhibits degenerate semiconductor behavior, characterized by adecreasing 𝜌 with increasing temperature, indicative of metal-lic conduction. In contrast, as the Sn content decreases to 0.7,the material transitions to non-degenerate semiconductor behav-ior, with 𝜌 showing a stronger temperature dependence, charac-teristic of thermally activated transport. Figure 2b presents theSeebeck coefficient (S), which increases monotonically with tem-perature for all compositions. Notably, the S also shows a gen-eral increase with higher Sb content. However, the power factordecreases systematically with decreasing Sn content, indicatingthat lower Sn concentrations adversely affect the overall electri-cal performance (Figure 1a). This trend suggests that electricalproperties are strongly influenced by the Sn and Sb/Bi content.Adv. Sci. 2025, 12, e06997 e06997 (3 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comSb doping induces a non-monotonic evolution in carrier effectivemass, extracted fromPisarenko analysis: effectivemass increasesfrom ≈2.9 me to ≈3.4 me at Sb = 0.2, then slightly decreasesto ≈3.1 me at Sb = 0.3 (Figures S7, Supporting Information).This mirrors trends in Bi-doped Mg2Sn, likely attributed to bandstructure changes driven by heavy and light band convergence.[49]To elucidate the mechanisms underlying these observations, theweighted mobility (µw) was analyzed using Snyder’s modifiedmodel:[51]𝜇w = 3h3𝜎8𝜋e(2mekBT)3∕2⎡⎢⎢⎢⎣exp[ |S|kB∕e− 2]1 + exp[−5( |S|kB∕e− 1)]+3𝜋2|S|kB∕e1 + exp[5( |S|kB∕e− 1)]⎤⎥⎥⎥⎦(1)Where 𝜎 is experimental measurements of electrical conductiv-ity, kB is the Boltzmann constant, h is the Planck constant, e isthe electron charge, and me is the free electron mass. The µw ex-hibits significant variations, particularly around room tempera-ture, where it decreases as the Sn content decreases (Figure 2c).This trend suggests that µw could be a key factor influencing 𝜌,aligning with grain boundary scattering mechanisms commonlyobserved in Mg2Si, Mg3Sb2, and half-Heusler TE materials.[52,53]The µw is related to the µH and the m* by the relation µw ∝ µH/(m*/me)3/2.[51] Across the Mg2Sn1-xSbx series, the m* exhibits aslight variation (2.9-3.4 me), whereas µH decreases sharply from43.7 to 11.1 4 cm2 V−1 s−1 as Sb content increases from x = 0.1to 0.3, consistent with the corresponding drop in µw from 324.7to 79.4 4 cm2 V−1 s−1 (FiguresS 8). It should be noted that µH isalso influenced by the band structure of the material. Accordingto the relationship µH ∝ 𝜏/m*, the reduction in µH arises fromthe combined effects of changed effective mass and carrier scat-tering. In other words, the varied band structure with increasingSb content affects the µH. In addition, enhanced carrier scatteringat higher Sb concentrations also contributes to decreased carriermobility in Mg2Sn as the Sb content increases.Figure 2d-f presents the systematic variations in thermal prop-erties. The thermal conductivity (𝜅) exhibits a decreasing trendwith Sb increases (Figure 2d). A monotonic reduction in 𝜅is observed with decreasing Sn content from Mg2Sn0.9Sb0.1 toMg2Sn0.8Sb0.7. For example, the 𝜅 decreases from 5.1 Wm−1 K−1for Mg2Sn0.9Sb0.1 to 2.1 W m−1 K−1 for Mg2Sn0.7Sb0.3 at roomtemperature. In the Mg2Sn0.8(Sb0.5Bi0.5)0.2, room-temperature 𝜅is ≈2.4 W m−1 K−1, which is slight lower than the value ofMg2Sn0.8Bi0.2 (2.7 W·m−1 K−1). The lattice conductivity (𝜅L) isplotted after subtracting the electronic contribution. With in-creasing Sb content, 𝜅L deviates from the T−1 temperature depen-dence typical of phonon-phonon scattering, indicating the dom-inance of strong phonon scattering from point defects and grainboundaries introduced by Sb/Bi solid solution (Figure 2e). Soundvelocity measurements support these observations, showing aclear reduction with increasing Sb/Bi content. The sound veloc-ity decreases from 3005 m s−1 for Mg2Sn0.9Sb0.1 to 2852 m s−1for Mg2Sn0.8(Sb0.5Bi0.5)0.2, accompanied by a progressive reduc-tion in 𝜅L, which declines from 3.4 Wm−1 K−1 for Mg2Sn0.8Sb0.2to 1.8 Wm−1 K−1 for Mg2Sn0.8(Sb0.5Bi0.5)0.2 (Figures S9, Support-ing Information). Debye–Callaway analysis reveals that the dom-inant contribution to the reduced 𝜅L stems from point defectscattering, consistent with behaviors observed in Bi-containingMg2Sn-based TE materials. The 𝜅L of our samples is slightlyhigher than that of the Bi-containing counterparts, likely dueto weaker point-defect scattering. This is supported by the cal-culated mass fluctuation parameters, where Γ = 0.0445, forMg2Sn0.8(Sb0.5Bi0.5)0.2—lower than Γ = 0.0697 for Mg2Sn0.8Bi0.2(Figures S10, Supporting Information). In addition, the reduc-tion in 𝜅L can be attributed to the disorder induced by increasedentropy, which serves as a promising strategy for optimizing TEperformance.[54–56] Figure 2f illustrates the significant reductionin 𝜅L with increasing configurational entropy (ΔS). As ΔS in-creases from 5.3 to 7.0 J mol−1 K−1, the 𝜅L decreases from 5.8 Wm−1 K−1 in Mg2Sn to 1.8 W m−1 K−1 in Mg2Sn0.8(Sb0.5Bi0.5)0.2.To further enhance the entropy could effectively reduce the 𝜅L to≈0.8 W m−1 K−1.[23] However, this study lies in achieving the op-timum compromise of electrical and thermal transport, and theoptimized ZT achieved in this work is 1.4, which is higher thanthe 1.3 reported for high-entropy systems in previous studies.[23]In addition, this trend also aligns with lattice softening and en-hanced phonon scattering due to Bi doping,[57] which is morepronounced than that observed with Sb doping[40] in Mg2Sn-based TE materials. Introducing chemical complexity and disor-der by Bi can significantly influence lattice dynamics, electronicstructure, and thermal conductivity. Specifically, Bi has a muchlarger atomic mass (208.98 u) compared to Sb (121.76 u), leadingto a significant increase in the mass fluctuation parameter.[24,58]Moreover, the atomic radius of Bi (160 pm) is notably largerthan that of Sb (140 pm), which increases the strain fluctuationparameter through greater lattice mismatch, further amplifyingphonon scattering.[57]These combined effects result in a pronounced reduction in𝜅L while preserving favorable electrical properties, ensuring a fa-vorable trade-off between thermal and electrical transport. Thequality factor B is widely used to assess the combined influenceof electron and phonon transport, highlighting the compositionaloptimization for peak thermoelectric performance:[51]B =(kBe)2 8𝜋e(2mekBT)3∕23h3𝜇w𝜅LT (2)Here, we also use this parameter B to quantify variations in ma-terial performance. The highest B value of 0.9 is observed inMg2Sn0.8(Sb0.5Bi0.5)0.2 which is higher than the value of 0.7 inMg2Sn0.8Sb0.2 (Figure S11, Supporting Information), resulting ina maximum ZT of 1.4 for Mg2Sn0.8(Sb0.5Bi0.5)0.2 (Figure 1c).2.3. Interface PerformanceTo validate the exceptional performance of theMg2Sn0.8(Sb0.5Bi0.5)0.2 TE material, the TE device was fabri-cated using Cu as the electrode material. Note that Cu may notbe optimal in terms of interfacial robustness; it provides a practi-cal adopted platform for evaluating the optimized Mg2Sn-basedTE materials.[43,59,60] Figure 3a shows the characterization of theinterface resistance of the n-type single-leg. After polishing theAdv. Sci. 2025, 12, e06997 e06997 (4 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 3. Electrical and microstructural characteristics of the Mg2Sn0.8(Sb0.5Bi0.5)0.2-based material and electrode interface. a) Resistance R versusposition curve, with the inset showing an optical photograph of the single-leg device. b) Magnified view of the interface region in (a), highlighting theinterface resistance determined from the resistance jump across the probe displacement. c) Scanning electron microscopy (SEM) image and corre-sponding energy dispersive spectroscopy (EDS) elemental maps of the interface region depicted in (a).interface region, the resistance-displacement curve was usedto calculate the interface resistivity (𝜌c). Notably, the transitionregion of the interface shows no significant resistance jumps,indicating acceptable electrical contact. A closer examination ofthe interface region in Figure 3b reveals a low 𝜌c of 4.6 μΩ cm2.Additionally, the 𝜌 of the Mg2Sn0.8(Sb0.5Bi0.5)0.2 was determinedto be 9.3 μΩ m using a scanning probe method, which closelymatches the value of 8.9 μΩ m obtained from ZEM measure-ments. This consistency demonstrates the reliability of themeasurement system and confirms that the TE material retainsits optimized electrical properties even after contact fabrication.Microstructural analysis of the interface region was performedto further elucidate the underlying mechanisms. As shown inFigure 3c, the SEM image highlights a well-bonded interfacewithout noticeable voids or cracks, confirming the high-qualityfabrication process. However, EDS elemental mapping revealsan obvious interdiffusion of elements, with a diffusion lengthof ≈100 μm. This level of interdiffusion is consistent with priorreports using Cu as a thermoelectric interface material.[61] TheCu is considered beneficial to its beneficial to n-type transportbehavior,[62] which would not excessively deteriorate the n-typeTE performance shortly. Thus, this diffusion has limited effectson both the interface and the TE performance of thematerial overshort durations.According to Xiong et al.[13] The device ZTD and material ZTcan be quantitatively correlated by considering the 𝜌 of TE mate-rial, 𝜌c, and device height L. They report that an 𝜌c below 5 μΩcm2 maintains more than 95% of the material ZT, for a 2 mmBi2Te3 device.ZTD = L(L + 2𝜌c𝜎)ZT (3)Therefore, the 𝜌c reported in this work is well within this idealrange. Besides, the interface of p-type Ti/Ge0.9Sb0.1Te also ex-hibits a low 𝜌c of < 5 μΩ cm2 (Figures S12 and S13, SupportingInformation). Note that this work aims to establish the consis-Adv. Sci. 2025, 12, e06997 e06997 (5 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 4. The power generation performance of the Mg2Sn0.8(Sb0.5Bi0.5)0.2-Ge0.9Sb0.1Te module. Panels a–d) illustrate the Voltage 𝑉, Output power 𝑃,Heat flow at the cold side 𝑄𝑐, and Conversion efficiency 𝜂 as a function of current 𝐼 under various temperature differences Δ𝑇, respectively. e) Maximumconversion efficiency 𝜂max as a function of temperature difference ΔT, compared to other reported Mg2Sn-based TE devices,[42–47,64] f) 𝜂 with the cyclenumber, Th cycling at 473–673 K.tency between the material and device and does not yet includethe design or optimization of diffusion barriers for long-term in-terfacial stability. Future studies could explore electrode interfacestability, with further optimization through alloying strategies,[61]phase diagram-guided material selection,[8] or self-optimizingcontact design.[63]2.4. Device PerformanceWe paired n-type Mg2Sn0.8(Sb0.5Bi0.5)0.2 with p-type Ge0.9Sb0.1Tein this work for the first time and conducted power generationtests under various temperature gradients to validate the device’sperformance. The V–I characteristics exhibit linear dischargecurves at various temperature differences (ΔT), with a negativeslope indicative of power generation behavior (Figure 4a). Thislinearity confirms good ohmic contact across the device underall operating conditions. The open-circuit voltage increases from0.05 V at ΔT = 78 K to 0.36 V at ΔT = 418 K. At the same ΔT, theoutput power initially increases with current, reaching a maxi-mum when internal resistance equals the load resistance, andthen decreases with further increases in current. As shown inFigure 4b, themaximumoutput power rises with increasing tem-perature difference, reaching 0.67W atΔT= 418 K. Similarly, theheat flow increases with current (Figure 4c), with contributionsfrom both Joule heat and Fourier heat. The rise in open-circuitheat flow across different ΔT is mainly attributed to Fourier heattransfer. Figure 4d illustrates the conversion efficiency (𝜂) as afunction of current, displaying a parabolic trend where 𝜂 initiallyincreases with current before declining. The 𝜂max improves withincreasing ΔT, reaching 9% under the ΔT of 418 K (Figure 4e).We also fabricated Mg2Sn0.8(Sb0.5Bi0.5)0.2 single-leg devices forvalidation, and its 𝜂max is 7.3% under the ΔT of 420 K (FigureS14, Supporting Information), slightly lower than the 7.5% re-ported by Chen et al.[45] However, compared to other reportedTEmodules involving the n-typeMg2Sn-based with alternative p-type TE materials,[42–47,64] this combination achieves a high 𝜂max,underscoring its potential for practical TE applications. In addi-tion, the single-leg GeTe device demonstrated a conversion effi-ciency of 8.8% under a temperature gradient of 421 K (FigureS15, Supporting Information). Measured performance from thePEM mini system deviates from theoretical predictions (FigureS16, Supporting Information), likely due to parasitic line resis-tance during assembly and overestimated heat flux from radiativelosses.[65] Moreover, the device exhibits acceptable cycling stabil-ity, with no noticeable 𝜂 degradation observed after five thermalcycles between 473 and 673 K (Figure 4f), as well as the outputpower (Figures S17, Supporting Information). Note that deviceAdv. Sci. 2025, 12, e06997 e06997 (6 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comperformance is intrinsically governed by material TE properties,including power factor, ZT, as well as external factors such as de-vice sized, 𝜌c, and temperature gradient. Herein, we focus onma-terials properties and demonstrate that balancing power factorsand thermal conductivity via a stepwise Sb-Bi aliovalent solid so-lution strategy enables high efficiency in intrinsically high powerdensity thermoelectric modules.3. ConclusionThis work presents a novel combination of Mg2Sn and GeTe, in-trinsically high-power-density devices that have not been previ-ously explored. Our design approach addresses the challenge ofefficiency limitations due to their usually high thermal conductiv-ity by incorporating aliovalent Sb andBi solid solutions inMg2Sn,which effectively reduces thermal conductivity while retainingthe power factor. This results in a lattice thermal conductivity of1.8 W m−1 K−1 and a peak ZT of 1.4 in Mg2Sn0.8(Sb0.5Bi0.5)0.2.The Mg2Sn-GeTe module achieves a 𝜂max of 9% at a high powerdensity of 0.7 W·cm−2 under a 418 K temperature gradient, offer-ing a promising strategy for efficient thermal energy conversionand advancing the development of sustainable energy solutionsin intrinsically high-power-density TEG.4. Experimental SectionMaterials Synthesis: The Mg2.06Sn0.9Sb0.1, Mg2.06Sn0.8Sb0.2,Mg2.06Sn0.8(Sb0.5Bi0.5)0.2, and Mg2.06Sn0.7Sb0.3 compounds weresynthesized using high-purity magnesium turnings (99.95%), tin shots(99.99%), antimony shots (99.999%), and bismuth shots (99.999%).Excess magnesium is denoted as Mg2 in the text, with similar abbrevia-tions for other compositions. For example, Mg2.06Sn0.9Sb0.1 is denotedas Mg2Sn0.9Sb0.1. The raw materials were weighed according to their sto-ichiometric ratios and mechanically alloyed for 7 h using an SPEX-8000Dhigh-energy mill under an argon atmosphere. The resulting powders wereconsolidated into bulk samples via vacuum spark plasma sintering at873 K and 60 MPa for 5 min using the SPS-1080 System (SPS SYNTEXINC). The Ge0.9Sb0.1Te samples, bulk Ge (99.99%), Te (99.99%), and Sb(99.99%) were weighed stoichiometrically and sealed in evacuated quartztubes. The tubes were gradually heated to 1323 K, held at this temperaturefor 20 h to ensure complete mixing and reaction, and then cooled to roomtemperature. The alloys obtained were ground into fine powders using anagate mortar and consolidated by SPS under an axial pressure of 60 MPaat 873 K for 10 min (SPS-322Lx, Dr. Sintering).Characterization and Measurements: The phase and phase transitionswere investigated using a X-ray diffractometer (Rigaku SmartLab 9 kW).The electrical transport properties, including S and the 𝜎, were measuredby ZEM-3 (Advance Riko, ± 5% uncertainty), and the thermal transportproperty 𝜅 was calculated by the formula: 𝜅 = D𝜌Cp, where the D repre-sents thermal diffusivity and was measured by LFA467 (Netzsch, ±3% un-certainty). The sample density 𝜌 and Cp were obtained by the Archimedesmethod andDulong-Petit law. TheHall measurements were performed us-ing a Quantum Design Physical Property Measurement System. The mag-netic field was swept from −4 T to +4 T at room temperature. The Hallcoefficient RH was extracted from the slope of the linear fit of Hall resis-tance versus magnetic field. The carrier concentration n and Hall mobilityµH were calculated using the relation n = 1/(eRH) and µ = 𝜎RH, where eis the elementary charge. The contact resistance of the TE junctions wasmeasured by a two-axis resistance distribution measurement instrument(S1331, Mottainai Energy). The thermoelectric modules were tested by thecommercial instrument Mini-PEM (Advance Riko).TE Device Fabrication and Simulation: The Mg2Sn0.8(Sb0.5Bi0.5)0.2 TEleg was fabricated by sandwiching two layers of Cu powders as interfacematerials, followed by SPS (SPS-322Lx, Dr. Sintering) at 873 K and 60 MPafor 5 min. The sintered TE legs were then cut into dice with dimensions of≈3.8 × 3.8 × 6.5 mm3. Two-pair TE modules were constructed using thesep-type Ge0.9Sb0.1Te legs and n-type Mg2Sn0.8(Sb0.5Bi0.5)0.2 TE legs. A Tidiffusion barrier layer was introduced at the GeTe side to suppress inter-facial reactions. The dimensions of the two-pair TE module are ≈10 × 10× 10 mm3, with the AlN ceramic plate and copper electrode being 0.635and 0.2 mm, respectively. Finite-element simulations were performed us-ing COMSOL Multiphysics to model both the single-leg and two-pair TEdevices.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsX.W., L.W., and A.L. contributed equally to this work. The authors acknowl-edge support from JST Mirai Large-Scale Program JPMJMI19A1. Institu-tional support from the JSPSWPI Academy Program is also acknowledged.Conflict of InterestThe authors declare no conflict of interest.Author ContributionsXinzhi Wu wrote the original manuscript. TakaoMori designed the project.XinzhiWu, LongquanWang, and Airan Li prepared the samples and carriedout the measurements. Xinzhi Wu, Gang Wu, Zhao Hu, and Fei Frank Yunanalyzed the data. TakaoMori supervised the whole project. All the authorsdiscussed, reviewed, and edited the manuscript.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywordsconversion efficiency, Mg2Sn-GeTe device, power density, thermoelectricgeneratorsReceived: April 19, 2025Revised: July 4, 2025Published online: July 30, 2025Adv. Sci. 2025, 12, e06997 e06997 (7 of 9) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. 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Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 40, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202506997 by National Institute For, Wiley Online Library on [08/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.com High Conversion Efficiency in Intrinsic High Power-Density Mg2Sn-GeTe Thermoelectric Generator 1. Introduction 2. Main Text 2.1. Stepwise Optimized Materials and Device Performance 2.2. TE Performance 2.3. Interface Performance 2.4. Device Performance 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Author Contributions Data Availability Statement Keywords