Low-cost synthesis of high-purity Li₂S for sulfide solid state electrolytes enabled by polyvinyl alcohol

Sulfide solid electrolytes (S-SEs) are widely preferred for their high ionic conductivity and processability. However, the further development of S-SEs is hindered by the excessive price of its critical raw materials of Li₂S. Herein, a low-cost and environmentally friendly method is proposed to synthesize Li₂S by the carbothermal reduction reaction of Li₂SO₄ in one step, and the effects of various factors are also discussed. As a result, a purity of 99.67% is obtained over the self-prepared Li₂S. More importantly, the cost of the self-prepared Li₂S is only about 50 $/kg, which is significantly lower than that of the commercial counterpart (10000–15000 dollar/kg). Moreover, the ionic conductivity of Li_5.5PS_4.5Cl_1.5 prepared using self-prepared Li₂S as raw materials is 4.19 mS/cm at room temperature, which is a little higher than that of Li_5.5PS_4.5Cl_1.5 using commercial Li₂S (4.05 mS/cm). And the all-solid-state lithium batteries (ASSLBs) with the as-prepared electrolytes could maintain a discharge capacity of 109.9 mA·h/g with an average coulombic efficiency (CE) of 98% after 100 cycles at 0.2C, which is equivalent to that using commercial Li₂S, demonstrating that the preparation strategy of Li₂S proposed in this work is feasible.