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Page 18 of 35 Martin-Gonzalez et al. Energy Mater. 2025, 5, 500121 https://dx.doi.org/10.20517/energymater.2025.32

sensitivity to degradation, particularly due to the oxidation of Sn, which necessitates extreme purity during
synthesis. This sensitivity poses substantial challenges for the scalability and long-term stability of SnSe-
based TE devices, which must be, for example, encapsulated. Again, bipolar conduction, prevalent in
narrow-bandgap semiconductors, further limits their effectiveness at elevated temperatures, highlighting the
need for ongoing research into alternative materials and strategies to mitigate these issues.

High-temperature applications
High-temperature TE materials are essential for applications in remote power stations, deep-space
spacecraft exploration powered by radioisotope TEG, etc. Oxides and silicides are predominant in this
domain, i.e., metal oxides, including ZnO [250-252] doped with Al [253,254] , have been widely studied for such
[198]
applications. Other oxides such as MgO, Cr/Mo/Ru/MnFe O 4 [197] , Ba CoO 2 [255] , cobalt-oxide , and SrTiO -
3
1/3
2
based have also garnered attention. There are non-oxidic materials like Cu Se (which has demonstrated
[256]
2
[239]
zT of ~2 around 1,000 K , while Cu Se-BiCuSeO-graphene composites have shown exceptional potential,
2
[257]
achieving a zT = 2.82 at 1,000 K , and Si-Ge alloys have shown promise at high temperatures. However,
[258]
these materials must endure significant thermal stress, which can induce mechanical failure and maintain
chemical instability, which poses a challenge for their long-term performance. Si-Ge alloys have been shown
to have long stability and have been used by the National Aeronautics and Space Administration (NASA) in
the spacecraft Voyager, Galileo, Ulysses, Cassini, and New Horizons [258,259] . Although theoretical predictions
suggest a thermoelectric zT exceeding 2 for Si-Ge systems , experimental studies have reached
[260]
[28]
approximately 1.84 at 1,073 K .
It is also important to note that at very high temperatures, the strategy of nanostructuring may be
compromised, as materials must endure extreme temperatures for extended periods. Table 3 provides a
detailed list of the state-of-the-art TE materials and their performance according to their operational
temperature.
Challenges in thermoelectric device integration
The landscape of TE materials has evolved significantly over the past few decades due to advancements in
phonon and electron transport engineering. While materials with zT > 2 are now commonplace in
laboratory settings, their translation into commercially viable TEGs requires addressing critical challenges at
the device integration level. This section gathers critical insights to highlight the most significant challenges
and considerations in this transition and future priorities.

Scalability and processing challenges: The first major bottleneck is scalability and manufacturing. Current
synthesis methods for hierarchical nanostructures (e.g., ball milling, SPS) face limitations in producing
homogeneous bulk materials at industrial scales. For instance, while PbTe-SrTe nanocomposites achieve
κ ≈ 0.5 W·m ·K in lab settings, batch-to-batch variability exceeding 15% persists due to incomplete SrTe
-1
-1
p
nanoparticle dispersion during mechanical alloying [32,126] . Such variation in overall TE performance can also
be noticed for other high perforce materials such as Ag/Cu or Sn based selenides [239,270,271] . For example, SPS
introduces anisotropic transport properties in materials such as BiSbTe alloys, resulting in conductivity
-1
-1
variations by a factor of 2 between the perpendicular (σ⊥ ≈ 900 S·cm ) and parallel (σ|| ≈ 450 S·cm )
directions relative to the pressing axis . This anisotropy stems from grain alignment during uniaxial
[272]
compression, where (001)-plane textures develop preferentially [272,273] . Fine-tuning and optimizing the
processing parameters, such as temperature profile and pressure can help minimize the directional
performance and potentially alleviate batch to batch variabilities.

Materials selection: Another significant factor is the selection of TE materials, which must exhibit optimal