Janus Highly Fluorinated Ether: K ⁺ ‐Channel Activator or Anode Corrosion Trigger in High‐Concentration Electrolytes for K–O ₂ Batteries?

ABSTRACT 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.