An integral part of promoting the large-scale use of new renewable energy sources is the development of cost- effective energy storage technologies.^1,2 Recently, potassium-ion batteries (KIBs) have attracted widespread interest as a low-cost, alternative energy storage medium to replace the widely used lithium-ion batteries (LIBs) because potassium has higher natural abundance (the reserve of potassium is close to that of Na in the Earth’s crust) and a lower standard redox potential than other metallic elements (E°(Li/Li⁺): -3.04 V; E°(K/K⁺): -2.93 V; E°(Ca/Ca²⁺): -2.87 V; E°(Na/Na⁺): -2.71 V; E°(Mg/Mg²⁺): -2.27 V versus the standard hydrogen redox potential).³

Room-temperature metal–sulfur batteries have attracted extensive interest due to their advantages, which include high theoretical capacities, high elemental abundances, low costs, and environmental friendliness.³ We proposed a different type of room-temperature K–S battery composed of a solution-phase potassium polysulfide (K₂S_x) catholyte and a 3D freestanding carbon-nanotube-film (3D-FCN-flm) electrode.⁴ Based on the reversible conversion reactions, K₂S_x (5 ≤ x ≤ 6) → K₂S₃ (discharge) → K₂S₅ (charge), the proposed K-S battery delivered a high discharge capacity of ∼400 mAh/g 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 K₂S_x (5 ≤ x ≤ 6) catholyte was constructed to demonstrate the feasibility of a safe K-S battery system free of metallic potassium.

References

1. J.-Y. Hwang, S.-T. Myung and Y.-K. Sun, Chem. Soc. Rev., 2017, 46, 3529–3614.
2. J.-Y. Hwang, J. Kim, T.-Y. Yu, S.-T. Myung, and Y.-K. Sun, Energy Environ. Sci., 2018,11, 2821-2827.
3. J.-Y. Hwang, S.-T. Myung, and Y.-K. Sun, Adv. Funct. Mater. 2018, 28, 1802938.
4. J.-Y. Hwang, H. M. Kim, C. S. Yoon, and Y.-K. Sun, ACS Energy Lett. 2018, 3, 540-541.