Janus Highly Fluorinated Ether: K+-Channel Activator or Anode Corrosion Trigger in High-Concentration Electrolytes for K–O2 Batteries?

The capacity and lifespan of superoxide-based potassium–oxygen batteries (POBs) are limited by the failure of the potassium (K) anode due to oxygen crossover. The high-concentration potassium bis(trifluoromethanesulfonyl)imide (KTFSI)/dimethoxyethane recipe enables efficient anode passivation, but its high viscosity and elevated cost necessitate the use of a highly fluorinated ether co-solvent. In this study, we elucidate the indeterminate role of fluoroether in constructing the anode passivation layer. The fluoroether participates with TFSI − anions to form a robust solid electrolyte interphase (SEI) that effectively mitigates oxygen crossover. Moreover, the fluoroether shows a Janus effect that excessive addition triggers severe K corrosion, while an optimal amount leads to a desirable SEI on the K surface. This SEI acts both as an oxygen permeation barrier and as an efficient K + -conductive channel. With a more stable environment for the K anode, a POB utilizing the KTFSI-based electrolyte delivers over 200 cycles (above 800 h) with low round-trip overpotentials and high average coulombic efficiency (98.1%) as opposed to one using the potassium hexafluorophosphate-based electrolyte that fails early. Spectroscopic analyses and theoretical simulations were utilized to elucidate the molecular-scale interactions in the electrolyte components. This study provides fundamental insights and practical strategies for the development of long-life POBs.