Rechargeable potassium-ion batteries enabled by potassium-iodine conversion chemistry

Potassium-ion batteries (KIBs) have emerged as attractive alternatives to dominative lithium-ion batteries for grid-level electricity storage due to lower potential of K+/K redox couple in non-aqueous electrolyte and abundant potassium resource. The current intercalation cathodes used in KIBs provide limited active sites in their frameworks while conversion chemistry offers the feasibility for much improved potassium-ion storage capability. We for the first time demonstrate a rechargeable potassium-iodine (K-I2) battery enabled by a two-electron K-I2 conversion reaction. With the help of in-situ Raman and ex-situ X-ray photoelectron spectroscopy, we confirm that the highly reversible conversion among I2, KI3 and KI involves solid-liquid-solid phase transitions and the formation of soluble KI3 intermediate enables the fast reaction kinetics. The K-I2 battery delivers a reversible capacity of 156 mA h/g and a small capacity decay of 0.058% per cycle over 500 cycles.