Scalable Manufacturing of Selenide Solid-state Electrolytes for Sodium-ion Batteries

Sodium-ion batteries are garnering enormous attention for large-scale energy storage applications due to their low cost, enhanced sustainability, and decent performance. Moreover, their energy density and innate safety could be further improved by substitution of solid-state superionic conductors for conventional liquid electrolytes. Among the various classes of electrolytes, inorganic Na-based solid-state electrolytes, and specifically ternary chalcogenides and their derivatives are promising candidates that can achieve high ionic conductivity and low mechanical stiffness. Replacement of sulfide with selenide anion can provide larger channel size for ionic transport and lower binding energy between the anionic lattice framework and Na ions, leading to higher ionic conductivity. Conventional synthesis routes of selenides include mechanochemistry and high-temperature solid-state reactions which are energy-intensive and not amenable to scale-up. Herein, we employed solution chemistry through precursor and solvent engineering to design a scalable and cost-efficient solution route for the production of air-stable Na₃SbSe₄. Specifically, we showed a simple pseudo metathesis route toward the production of binary Sb₂Se₃ as a precursor that could further be extended to the formation of ternary Na₃SbSe₄ in two different synthetic routes: alcohol-mediated redox and alkahest amine-thiol approaches.<br/>For Sb₂Se₃ preparation, elemental Se was reduced using NaBH₄, a common reducing agent, and further reacted with SbCl₃ via metathesis reaction to form Sb₂Se₃ and NaCl as a byproduct in an aqueous solution. Recovered amorphous Sb₂Se₃ was crystalized through mild heat treatment and used as is with no apparent impurities in the electrolyte reactions. In the alcohol-mediated route, the electrolyte was successfully synthesized in ethanol using a similar redox solution coupled with Sb₂Se₃ and NaOH as a basic agent. In the other approach, an amine-thiol solvent mixture is utilized as an alkahest system for the dissolution of elemental Se and Na and further reaction with the binary precursor to obtain Na₃SbSe₄. The electrolyte recovered from the alcohol solution without any post-reaction treatment has remarkable ionic conductivity (0.19 mS cm^-1) and low electronic conductivity (1.6*10^-6 mS cm^-1) at room temperature on par with reported performance from other conventional routes. Electrolytes obtained from the amine-thiol route produced a similar performance (σ_Na+ = 0.26 mS cm^-1) via heat -treatment at 300 °C. These novel solution-phase approaches showcase the diversity and application of wet chemistry in producing selenide-based electrolytes for all-solid-state sodium batteries.