Ambient-pressure synthesis of potassium sulfides via potassium polysulfide melts and discovery of a K3SbS4 polymorph

Sulfide solid electrolytes combine high ionic conductivity with good formability and are leading candidates for all-solid-state batteries, yet their synthesis has long depended on vacuum-sealed ampoules to suppress sulfur volatilization, which has been an obstacle to both materials exploration and scale-up. Here we extend the polysulfide self-flux route, previously established for sodium systems, to potassium. Molten potassium polysulfide (K 2 S x ) serves simultaneously as a stoichiometric reactant and as a low-volatility flux, allowing four K–M–S sulfides (K 4 GeS 4 , K 4 SnS 4 , K 3 BS 3 , and K 3 SbS 4 ) to be synthesized under flowing argon at ambient pressure, without sealed tubes. K 2 S reacts with S to form polysulfides, thereby immobilizing the sulfur in ionic forms. Upon heating, the resulting K 2 S x melt acts as both a potassium–sulfur source and a reactive flux, promoting its reaction with the Ge, Sn, B, or Sb starting materials to form the corresponding ternary sulfides. X-ray diffraction confirms that K 4 GeS 4 , K 4 SnS 4 and K 3 BS 3 are obtained as nearly single phases. Notably, at 450 °C, within the low-temperature flux regime, K 3 SbS 4 crystallizes as a previously unreported polymorph, identified from a primitive orthorhombic unit cell ( a = 8.100, b = 11.199, c = 10.590 Å) that is incompatible with the four known phases. AC impedance measurements show that all four compounds exhibit room-temperature conductivities of 10 −11 –10 −9 S cm −1 and activation energies for conduction of 52–69 kJ mol −1 . The method using potassium polysulfide melts offers a mild, scalable route to potassium sulfide solid electrolytes and provides selective access to metastable phases crystallizing in the low-temperature flux regime.