Concentrated Electrolytes Unlock the Full Energy Potential of Potassium-Sulfur Battery Chemistry

Rechargeable K-S batteries provide a theoretical energy density almost double that of the current Li-ion batteries. However, in practice, this energy potential is largely compromised predominantly because of the dissolution and shuttle reactions of the redox intermediates in the commonly used electrolytes. Herein, we demonstrate for the first time that a 5 M concentrated electrolyte can effectively mitigate this parasitic effect and enables a full energy utilization of the K-S battery chemistry. The prototype K-S batteries operating at an average discharging voltage of 2.1 V deliver a discharging energy density of ~1270 Wh/kgS or ~700 Wh/kg(K2S3), which reaches the theoretical limit and is ~40%–100% higher than those of the current Li-ion batteries. X-ray powder diffraction analysis provides the first unambiguous evidence that the K-S battery chemistry involves reversible stepwise phase transformations of S8⇔K2S6⇔K2S5⇔K2S4⇔K2S3. This work shines a light on the full utilization and in-depth mechanistic understanding of high-energy-density K-S batteries.