Mass transport, including ionic migration, is crucial in various applications like batteries and fuel cells. However, ionic migration can also cause phase instability. β-Zn₄Sb₃, a material with mixed electronic and ionic conduction and promising thermoelectric (TE) performance, presents a unique opportunity to investigate the use of ionic migration for fast materials synthesis and phase stabilization. The mobile Zn²⁺ ions in β-Zn₄Sb₃ contribute significantly to its low thermal conductivity and TE performance. However, its commercialization is hindered by challenges in efficient synthesis and maintaining phase stability during operation (400–700 K). This research addresses these challenges by exploring the use of fast Zn²⁺ ion migration under an electric field to achieve ultrafast synthesis and self-adaptive phase stabilization of β-Zn₄Sb₃.

Previous studies have highlighted the promising thermoelectric properties of β-Zn₄Sb₃, but its synthesis and phase stability have been significant hurdles. Research has shown that the mobile Zn ions are key to its unique properties, but also contribute to its instability. Methods like melt-quench followed by spark plasma sintering (MQ+SPS) have been used, but they are time-consuming and the resultant material is prone to instability. Various doping strategies have been attempted to improve stability, but without complete success. This paper aims to address these limitations through a novel synthesis method that leverages the inherent ionic conductivity of the material.

The researchers employed an electric field-assisted synthesis (EFAS) process. A stoichiometric mixture of Zn and Sb powders (and dopants Cd or Ge) was loaded into a graphite die coated with BN to ensure current flow through the mixture. A sawtooth waveform electric field (0–9 A, 1.8–3.6 V, 0.4 Hz) was applied, resulting in a maximum temperature of 598 K—significantly lower than conventional methods. The process was monitored using various parameters (pressure, temperature, displacement, current, voltage). For comparison, a control sample was prepared using the MQ+SPS method. The synthesized samples were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy-dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and in situ TEM biasing experiments. Thermoelectric properties (electrical conductivity, Seebeck coefficient, thermal conductivity, and ZT) were measured. Ionic conductivity of Zn²⁺ was determined using AC electrochemical impedance spectroscopy (EIS) and DC polarization with electron-blocking electrodes. Electromigration tests were conducted to assess phase stability under high current density and elevated temperatures.

The EFAS method enabled the synthesis of single-phase β-Zn₄Sb₃ within 60 seconds, significantly faster than conventional methods. The EFAS process resulted in a core-shell microstructure with crystalline β-Zn₄Sb₃ cores and amorphous, non-stoichiometric Zn₄Sb₃ shells. This core-shell structure, along with Cd or Ge doping, effectively suppressed Zn²⁺ ion migration and prevented Zn precipitation, leading to significantly improved phase stability. The room-temperature ionic conductivity was reduced by over an order of magnitude in EFAS samples compared to the MQ+SPS sample (5.9 × 10⁻⁵ S m⁻¹ to 4.2 × 10⁻⁶ S m⁻¹ for Ge-doped). The Ge-doped sample (Zn₃.₉₇Ge₀.₀₃Sb₃) showed exceptional stability, exhibiting almost no change under high current density (20 A cm⁻²) at 473 K for 24 h. The Cd-doped sample (Zn₃.₉₆Cd₀.₀₄Sb₃) achieved a maximum ZT of 1.2 at 693 K, while the Ge-doped sample reached a maximum ZT of 1.12 at 700 K, showcasing superior thermoelectric performance compared to the MQ+SPS sample. In situ TEM observations revealed that the amorphous grain boundaries acted as barriers to Zn²⁺ migration, preventing the degradation observed in the MQ+SPS sample. The formation of the 3D β-Zn₄Sb₃ network accelerated the synthesis process, while the amorphous shell acted as a self-adaptive stabilizer. Beyond a critical voltage, the amorphous grain boundary crystallized, leading to instability; this was not observed in the MQ+SPS sample.

The findings demonstrate that utilizing fast Zn²⁺ ion migration under an electric field is a highly effective strategy for achieving both ultrafast synthesis and enhanced phase stability in β-Zn₄Sb₃. The core-shell microstructure and doping significantly suppressed Zn²⁺ migration, improving the long-term stability and reproducibility of the thermoelectric properties. The improved ZT values confirm the efficacy of this approach. These results have significant implications for the development of high-performance thermoelectric materials and broader applications of mixed-ionic electronic conductors.

This study successfully demonstrated a novel electric field-assisted synthesis (EFAS) method for the rapid and stable production of β-Zn₄Sb₃. The resulting core-shell microstructure and dopant incorporation significantly enhanced phase stability and thermoelectric performance. This approach offers a promising pathway for developing other mixed-ionic electronic conductors for diverse applications. Future research could explore the optimization of EFAS parameters and the application of this technique to other materials systems.

The study focused on β-Zn₄Sb₃ and its Cd/Ge-doped variants. The generalizability of the EFAS method to other material systems needs further investigation. The long-term stability under real-world operating conditions requires further evaluation. While the in situ TEM study provided valuable insights, more detailed microscopic analyses could further elucidate the phase transformation mechanisms.