PTFE Enhances Discharge Performance of Carbon Cathodes in Potassium‐Oxygen Batteries**

Abstract

Potassium‐oxygen batteries (KOB) are a promising energy storage technology with high theoretical energy of 935 Wh/kg and long cycle life. Potential applications require the development of affordable cathode materials with high practical capacity. In this article, we show that low‐cost polytetrafluoroethylene (PTFE) treatment increases the discharge performance of a commercial carbon paper cathode. Cross‐sectional scanning electron microscopy reveals that PTFE alleviates mass transport limitations and facilitates homogeneous deposition of the discharge product potassium superoxide (KO2) within the cathode pore structure. Using electrochemical impedance spectroscopy, we found that PTFE, in combination with the appropriate electrolyte volume, can prevent pore flooding by the electrolyte. Free volume permits fast, gaseous oxygen transport throughout the cathode, which lowers mass transfer resistances and improves the rate capability. Moreover, PTFE enables high pore volume filling by KO2 and, in turn, high discharge capacity. Our results demonstrate that controlling the mass transport is essential for high‐performance cathodes for KOB.

The effects of oxygen pressure on the discharge performance of potassium–oxygen batteries

“…In addition, it is in accordance with recent findings that increasing O 2 transport via cathode modification is especially beneficial for high rates. 8…”

“…In addition, it is in accordance with recent ndings that increasing O 2 transport via cathode modication is especially benecial for high rates. 8 Comparing proles at the same p(O 2 ) and different J shows that the plateau voltage for any given p(O 2 ) is considerably lower for the higher J. The voltage gap between the rates decreases with higher p(O 2 ) and for the highest p(O 2 ) ¼ 11 atm it is <100 mV.…”

“…6,7 Furthermore, practically relevant capacities can be achieved with a commercial carbon paper cathode without need for precious metal catalysts. 8 This remarkable performance is enabled by the facile cell chemistry of KOB, where potassium superoxide (KO 2 ) is the sole discharge product. 9 The formation of KO 2 (eqn (1)) is energetically and kinetically favored and a subsequent disproportionation towards potassium peroxide K 2 O 2 (eqn (2)) is not evident.…”

“…In our recent study, we found that KO 2 growth induces several rate-dependent failure mechanisms. 8 At low discharge rates, large amounts of deposited KO 2 cause pore clogging and cathode surface passivation. At high rates, the available O 2 is rapidly consumed and the rate of O 2 transport cannot support the applied discharge rate.…”

“…Based on these findings we demonstrated that modifying the cathode wettability enhances transport of gaseous O 2 throughout the entire cathode. 8 Homogeneous KO 2 growth even at high rates enables a Q improvement up to 300%. Nonetheless, the achieved performance is still below the requirements of practical applications.…”