Toward High-Safety Potassium–Sulfur Batteries Using a Potassium Polysulfide Catholyte and Metal-Free Anode

Based on the reversible conversion reactions, K2Sx (5 ≤ x ≤ 6) →discharge K2S3 →charge K2S5, the proposed K–S battery delivered a high discharge capacity of ∼400 mAh g–1 at 0.1 C-rate with stable cycle retention (94% after 20 cycles) and good rate capability up to 2 C-rate. In addition, instead of an explosive and highly reactive potassium metal electrode, a full cell consisting of an electrochemically potassium-impregnated hard carbon and the K2Sx (5 ≤ x ≤ 6) catholyte was constructed to demonstrate the feasibility of a safe K–S battery system free of metallic potassium.