Fast ion transport for synthesis and stabilization of β-Zn4Sb3

Mobile ion-enabled phenomena make β-Zn4Sb3 a promising material in terms of the re-entry phase instability behavior, mixed electronic ionic conduction, and thermoelectric performance. Here, we utilize the fast Zn2+ migration under a sawtooth waveform electric field and a dynamical growth of 3-dimensional ionic conduction network to achieve ultra-fast synthesis of β-Zn4Sb3. Moreover, the interplay between the mobile ions, electric field, and temperature field gives rise to exquisite core-shell crystalline-amorphous microstructures that self-adaptively stabilize β-Zn4Sb3. Doping Cd or Ge on the Zn site as steric hindrance further stabilizes β-Zn4Sb3 by restricting long-range Zn2+ migration and extends the operation temperature range of high thermoelectric performance. These results provide insight into the development of mixed-conduction thermoelectric materials, batteries, and other functional materials.

the admixture to measure the reaction temperature. In a typical process, a vacuum level  20 Pa was maintained. Next, a pulsed sawtooth-shaped current is applied to the mold for 60 s. The resulting pellet with the diameter of 16 mm and the height of 3 mm had a relative density of more than 98%. The ingot was cut into appropriate shapes for the thermoelectric property measurements and electromigration tests.
Moreover, the electric field-assisted synthesis (EFAS) processes for Bi2Te3, ZnSb, Cu2Se, and Cu2S compounds are really similar. disorder among Zn atoms, which improves the kinetic stability of the samples. Furthermore, we have tested the low temperature electrical transport properties on PPMS, the data displayed in Fig. S6, S9 and S12. There is a sharp inflection point in the low temperature transport curve of the Zn4Sb3 (MQ + SPS) sample near 230 K ( Fig. S6), while the curve of the Zn4Sb3 sample prepared by the EFAS method is much smoother (Fig. S9). The low temperature transport curve of Zn3.96Cd0.04Sb3 is very similar to that of Zn4Sb3 (EFAS) (Fig. S9), and a small inflection point appears near 240 K. Of course, the carrier concentration is reduced after doping with Cd. The curve for the Zn3.97Ge0.03Sb3 sample is very flat before 240 K (Fig. S12), in contrast to the curves of Zn4Sb3 and Zn3.96Cd0.04Sb3 EFAS samples. This may be due to changes in the band structure of Zn4Sb3 upon doping with Ge 2 . With the increasing temperature, the power exponent p of the temperature dependence of the carrier mobility ( ~ T -p ) changes gradually from 0 to 0.5, and then to 1.5, which means that alloy scattering gradually transforms into acoustic phonon scattering.
Moreover, the lattice thermal conductivity L of -Zn4Sb3 (EFAS) is lower than that of -Zn4Sb3 (MQ+SPS) over the entire temperature range, even though the latter is more dynamically unstable ( Fig. S4 and S7). This is due to the special composite structure of the sample prepared by the EFAS method. The amorphous grain boundary and its large number of disordered regions inside the grains, as well as the dynamic process of decomposition and re-formation of the -Zn4Sb3 compound under the temperature difference, would lead to strong scattering of phonons. Furthermore, the L of the Zn3.97Ge0.03Sb3 EFAS sample is lower than that of the -Zn4Sb3 EFAS compound (Fig. S10), which is caused by additional alloy scattering. Interestingly, the lattice thermal conductivity of the Zn3.96Cd0.04Sb3 EFAS sample is lower than that of the Zn3.97Ge0.03Sb3 EFAS sample ( Fig. S7 and S10). This is likely the result of a greater content of Cd impurity, as well as its much heavier mass compared to an atom of Ge, leading to a greater mass contrast and thus stronger mass and stress field fluctuation scattering of phonons.
In addition, the thermoelectric properties of the samples were tested repeatedly, and the thermoelectric properties of new samples were re-prepared by the same EFAS process. All TE properties of different samples are shown in Fig. S14 -S20. The results of two test cycles of TE properties of the Zn4Sb3 (MQ + SPS) sample are very different (Fig. S14), which indicates that the sample has experienced changes after high temperature treatment. In contrast, TE properties of the samples prepared by the EFAS method are stable even after 10 cycles of testing (Fig. S15, S17 and S19). In the meantime, TE properties of the re-prepared new samples are also very close to each other (Fig. S16, S18 and S20). All these results show that the reproducibility of the samples is really good.

Supplementary Note 4
Test of ionic conductivity of Zn 2+ : The migration rate of Zn 2+ ions in the commercial zinc-loaded montmorillonite is 3.3  10 -3 Sm -1 , as tested through AC electrochemical impedance spectroscopy (EIS) 3-6 ( Fig. S21).  In contrast, we applied a current density of 20 A/cm 2 through a Zn4Sb3 (MQ+SPS) sample. Zn precipitates formed at the downstream end of the sample after a day at ambient conditions ( Fig. S13). Obviously, -Zn4Sb3 is susceptible to a kinetic instability as Zn 2+ ions diffuse rapidly under an electric field, even at room temperature. For the Zn4Sb3 sample made with EFAS method, the surface color has become yellowish after the electromigration test (Fig. S30). For the Zn3.96Cd0.04Sb3 sample, its surface color has turned light yellow after the electromigration experiment, and the color is much lighter than that of the Zn4Sb3 EFAS sample (Fig. S31). The grain surface of the Zn3.96Cd0.04Sb3 sample is very clean, although a small number of micro-cracks is still observed.
In the case of the Zn3.97Ge0.03Sb3 sample, the surface color shows almost no change after the electromigration experiment (Fig. S32). Moreover, no cracking is observed in the Zn3.97Ge0.03Sb3 sample, and the grain surfaces are really clean.  (Fig. S33). When the die with the sample is inverted (turned upside down) and charging continues for additional 18 s, single-phase Zn4Sb3 re-emerges everywhere again (Fig. S34). Obviously, there is a temporal window for the phase formation of -Zn4Sb3, beyond which the decomposition occurs in the presence of an electric field.
Due to the fast ion migration behavior of Zn 2+ , We cannot prepare single-phase Zn4Sb3 in precisely 60 s every time, so we placed a Zn plate at the upstream side of the sample (Fig. S35), which would extend the single-phase time window to a range of 55-64 s, and greatly improve the reproducibility of the sample.
If the starting material, rather than elemental powders of Zn and Sb, is made of "Zn+3ZnSb", the single-phase -Zn4Sb3 compound can still be prepared under the pulsed electric current field within 30s (Fig. S36). The maximum reaction temperature detected in this case is only 420 K (Fig. S37), much lower than the growth-from-the-melt temperature of 1023 K 1,2 , and the temperature of about 700 K used in the direct SPS synthesis 7,8 . Thus, it can be seen that the special phase formation process is more closely related to the electric field. If only the heat treatment is used, a nearly single-phase Zn4Sb3 compound is obtained by thermal explosion at 773 K for 10 min (Fig. S38). The experiment indirectly indicates that the electric field-assisted synthesis technique of the Zn4Sb3 compound in a short period of time has a special formation mechanism, which is closely related to the DC current.
Interestingly, the Bi2Te3 single phased compound cannot be obtained in 60 s by using the EFAS method, along with residual Bi and Te in the product (Fig. S39). In the process, the peak temperature reached 541 K, only 3 K lower than the melting point of Bi (Fig. S40). If we continue to extend the reaction time or increase the current, bismuth will become liquid, flow out of the mold, and we will not get a single-phase compound any more. However, as shown previously, Bi2Te3 can be prepared rapidly by the self-propagating high-temperature synthesis technique in several seconds 9,10 . This indirectly proves that the EFAS method is related to the ion migration behavior.

Supplementary Note 9
Heat flow tests: Fig. S43 and S44 present the low and high temperature heat flow of the Zn4Sb3-based samples, respectively, detected by Q2000 (TA, USA).
For the Zn4Sb3 (MQ + SPS) compound in the low temperature region, there are two endothermic peaks on the heating stage (Fig. S43), corresponding to the temperatures of 236.6 K and 252.8 K, which correspond to phase transitions of  '- and -, respectively. Because the phase transition process is reversible, there are two exothermic peaks on the cooling stage, corresponding to temperatures of 234.4 K and 250.1 K. This may explain why the carrier concentration fluctuates violently near 230K (Fig. S6c). In the high temperature region, the Zn4Sb3 (MQ + SPS) compound exhibits a broad exothermic peak around 428 K (at 417.7 K and 438.3 K) and an endothermic peak at 561.5 K (Fig. S44a), indicating that the sample is unstable. This corresponds to the sharp fluctuation of the carrier concentration near 423 K and 560 K (Fig. S5a). This phenomenon was also observed by other authors. Lin et al. 11 found that Zn4Sb3 compound becomes metastable and gradually decomposes into Zn(hcp) and ZnSb around 425 K, while it recovers its stability above 565 K.
More interestingly, in the low temperature region, there is only one endothermic peak on the heating stage and one exothermic peak on the cooling stage of the Zn4Sb3 sample synthesized by the EFAS method, corresponding to the temperature of 235.4 K and 232.9 K (Fig. S43). With the increasing content of Cd doping at the site of Zn, the latent heat gradually decreases (Fig. S43a). In the high temperature region, there is no obvious heat absorption and the release peak in the heat flow curve of Cd-doped samples, only a slight fluctuation near 428 K and 560 K (Fig. S44c). The above observed phenomena show that Cd doping, combined with the EFAS method has a significant impact on the low temperature phase transition process, and can, to a certain degree, stabilize the high temperature structure.
In lightly Ge-doped samples, the position of the endothermic peak is significantly lower than that in the undoped sample in the low temperature range (Fig.   S43b). The endothermic peak is reduced to 222 K and 215 K for 0.25at% and 0.5at% Ge-doping samples, respectively, and the latent heat of phase transition is greatly reduced. When the doping content of Ge is 0.75at%, there is almost no phase transition peak observed in the range of 203 K-313 K. The high temperature heat flow curve of the 0.25at% Ge-doped sample still fluctuates slightly near 428 K and 560 K (Fig. S44e), and the high temperature heat flow curves become very flat when the doping content of Ge increases further. The above observed experimental results show that Ge doping can greatly inhibit the low temperature phase transition and stabilize the high temperature structure of the material.

Supplementary Note 10
Other compounds obtained through EFAS technique: More interestingly, the high TE performance ZnSb compound could also be synthesized within 60 s by the EFAS process. Fig. S46 shows the reaction parameters, phase composition, and the corresponding TE performance. At the same time, we subdivided the synthesis process, and the intermediate stages in the phase transformation process of "Zn + Sb" mixed powder under the pulsed current are displayed in Fig. S47.
In the process, the highest temperature of the stoichiometric "Zn + Sb" mixed powder is only 514 K. As is well known, the ZnSb compound is not a fast ion conductor, so how come that it can also be synthesized so quickly? It is precisely because of the formation of the Zn4Sb3 compound during the intermediate reaction process that the ion transport channels of Zn formed. This leads to rapid chemical reactions and the synthesis of ZnSb in such mild conditions. The ZnSb compound synthesized by this technique has a relatively high TE performance (Fig. S46). In the range of 300K -723 K, the electrical conductivity  decreases at first and then increases with the temperature, while the Seebeck coefficient  increases at first and then decreases correspondingly. There is a turning point near 450 K, which may be the result of intrinsic excitations. The power factor PF can reach 1.35 mWm -1 K -2 at 723K. With the rising temperature, the overall thermal conductivity  decreases at first and then increases, and the lowest value of 1.16 Wm -1 K -1 is reached at 550 K. Due to the low  of the material, the lattice thermal conductivity L accounts for the vast majority of the total thermal conductivity.
Finally, the calculated ZT value reaches its maximum of 0.71 at 727 K.
Moreover, we also tried to prepare Cu2Se and Cu2S compounds using the EFAS process. Fig. S48 displays the reaction parameters and phase composition of stoichiometric "2Cu + Se" and "2Cu + S" mixed powders under the pulsed electric field. It only takes 30 s to synthesize Cu2Se and Cu2S compounds, and the highest detectable temperatures are only 370 K and 354 K, respectively.
The above experiments show that the electric field-assisted synthesis method, by relying on the ion transport channels to achieve rapid mass transfer, may be suitable for the synthesis of solid fast ionic conductor materials or compounds.