Li₂CO₃-Derived Low-Cost Li₂S for Sulfide Solid Electrolytes Exceeding 11 mS cm^-1

Sulfide solid electrolytes (SSEs) hold great promise for all-solid-state batteries (ASSBs), owing to their high ionic conductivity and excellent deformability. However, their practical application is severely hindered by high cost, primarily originating from lithium sulfide (Li₂S), which accounts for ∼90% of the total SSE cost. Here, we report a novel strategy to produce low-cost Li₂S from lithium carbonate (Li₂CO₃) via its reaction with ammonium thiocyanate (NH₄SCN). This reaction generates only gaseous by-products, eliminating purification procedures and enabling scalable production of high-quality Li₂S. The resulting Li₂S enables the synthesis of representative SSEs, Li_5.4PS_4.6Cl_0.8Br_0.8 (LPSCB) and Li_5.4PS_4.6Cl_1.6 (LPSC), with room-temperature ionic conductivities of 11.33 and 7.94 mS cm^-1, respectively. When coupled with LiNbO₃-coated LiNi_0.895Co_0.077Mn_0.028O₂ cathode, ASSBs deliver discharge capacities of 192.2 and 198.8 mAh g^-1 at 0.1C, and retain 94.85% and 94.89% of initial capacities after 800 cycles at 1C, respectively. Cost analysis reveal that the total cost of SSEs synthesized from this Li₂CO₃-derived Li₂S is reduced by 86.6% and 88.5%, highlighting its significant techno-economic advantages for commercializing SSEs toward ASSBs.