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Solution synthesis of sodium-based multinary chalcogenides for next-generation battery applications
Ahmadi Vaselabadi, Saeed
Ahmadi Vaselabadi, Saeed
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2024
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The development of efficient and sustainable storage systems is essential for the integration of intermittent renewable energy such as wind and solar. Sodium-ion batteries have emerged as a sustainable solution for large-scale energy storage. Sodium all-solid-state batteries, in particular, are appealing as they replace liquid electrolytes with solid-state electrolytes (SSEs), that improve energy density and reduce safety concerns. In this context, Na-based multinary chalcogenides with superionic properties are promising SSE candidates. However, conventional high-temperature sintering methods for their synthesis are costly and not easily scaled for industrial use. Liquid-phase synthesis, in contrast, offers a scalable and cost-effective approach with minimal thermal budget requirements. To date there have been limited reports on the liquid-phase synthesis of multinary chalcogenides, particularly selenides, so there is a critical need to better understand the underlying reaction mechanisms. This work aims to develop new synthetic approaches and address these knowledge gaps.
A key enabling technology was the development of metathesis reactions in polar solvents to synthesize binary chalcogenides such as Sb2Ch3 and SnCh2 (Ch = S, Se). These compounds are key precursors and major cost drivers for the synthesis of ternary and quaternary sodium chalcogenides. The metathesis reactions were carried out using Na chalcogenide solutions (NaHCh) with SbCl3 and SnCl4.5H2O, producing amorphous SbCh2 and SnCh3 compounds that crystallized upon heat treatments at 250-300 °C. These binary precursors were then applied to solution synthesis of air-stable Na3SbCh4 in both ethanol and water. For Na3SbS4, ethanol was the superior solvent – improving yield, reducing H2S side products, and resulting in a higher ionic conductivity of 0.52 mS cm-1. The synthesis of the Se analog, Na3SbSe4, proved more complex due to the unreactive nature of Se. Two novel routes for Na3SbeS4 were developed. First, in an alcohol-mediated route, NaBH4 was used to reduce Se to create the highly reactive NaHSe intermediate that, in the presence of other precursors (Sb2Se3, Se) with appropriate control of the redox environment (NaOH) resulted in Na3SbSe4. Alternatively, mixtures of ethylenediamine (EDA) and ethanethiol (ET) facilitated the dissolution of elemental Se and Na, enabling further reaction with Sb2Se3 to form Na3SbSe4. Both methods produced electrolytes with ionic conductivity of ‎~0.2 mS cm-1, comparable to other conventional thermo-mechanical routes. However, the ethanolic reaction was more advantageous due to its use of a benign, environmentally friendly solvent and easier sample recovery through precipitation at room temperature.
Next, we aimed to simplify the synthesis of Na3SbCh4 (Ch = S, Se) by eliminating the intermediate binary synthesis steps and developing a direct one-step precipitation process. In these reactions, ethanolic NaHCh solutions were reacted with SbBr3 in the presence of excess chalcogens. The sulfide reaction produced highly pure compounds (~99 wt% purity) with a yield of 92-95% while the selenide reaction required optimization of precursor concentration, resulting in Na3SbSe4 with a lower yield of 74-79% but with a high purity of 97.5-99.6 wt%. We also observed that the solution color of activated Se in ethanol varied, which appeared to correlate with different concentrations of Se species, significantly influencing the reaction products. Moreover, we found that NaOH and activated Se concentration were critical in facilitating the redox mechanism of Sb(III) and Sb(V) during the formation of Na3SbSe4. The as-synthesized Na3SbCh4 exhibited high ionic conductivity (0.18-0.35 mS/cm) and low activation energy (0.19-0.21 eV).
Finally, we focused on quaternary chalcogenides, Na11Sn2PnCh12 (Pn = P, Sb; Ch = S, Se), and investigated their synthesis in various solvents, including aprotic, protic, and alkahest solvents. The synthesis of Na11Sn2PS12 is more complex due to the high reactivity of the P2S5 precursor in polar solvents. To address this, we used acetonitrile (ACN) which facilitated the formation of amorphous thiophosphate moieties. Short annealing at T > 300 °C was required to obtain crystalline Na11Sn2PS12. In contrast, water enabled the reaction and crystallization of Na11Sn2SbS12 at the low temperature of 150 ºC, with additional annealing at 550 °C required to enhance transport properties (σNa+ = 0.42 mS/cm). Additionally, we achieved the first liquid-phase synthesis of Na11Sn2SbSe12 using amine-thiol solvents, although traces of Na3SbSe4 and carbonaceous species were present, which negatively impacted its ionic and electronic conductivity.
This thesis provided new fundamental insights into the chemistry of chalcogen precursors and low-cost metal halides that have wide applicability to a broad class of compounds. Synthesis of sulfide compounds was relatively straightforward, whereas selenide analogs required careful control of the redox environment. This knowledge was used to shift the one-pot strategy discussed above from Na3SbCh4 formation to the synthesis of NaSbCh2 (Ch = S, Se), semiconductors with tunable band gaps appropriate for harvesting solar energy. We also demonstrated the generality of this approach by expanding pnictides to include bismuth as well as the potential for Li compounds.
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