A Functionally Graded Cathode Architecture for Extending the Cycle-Life of Potassium-Oxygen Batteries

Abstract

Potassium-Oxygen (K−O2) batteries have a high theoretical energy density of 935 Wh kg−1 due to a single-electron redox process in the reversible formation of potassium superoxide. Despite this advantage, standard K−O2 batteries have limited cycle-life (5-10) due to molecular oxygen crossover from cathode to anode, resulting in side reactions forming undesired superoxide on the anode. In this article, a K−O2 battery fabricated with a functionally-graded cathode (FGC) architecture is presented to address oxygen crossover at the cathode. This K−O2 battery lasts >125 cycles with minimal loss in coulombic efficiency when charged/discharged to 300 μAh (238 μAh cm−2). The FGC is comprised of a carbon fiber layer, microporous carbon and polypyrrole doped with hexafluorophosphate. It provides a scalable architecture for regulating K+ ion and oxygen transport at the cathode trilayer (liquid-solid-air) interface. The PPy(PF6) formed on the cathode is observed to have an ORR rate two orders greater than that of carbon-based electrodes, as it promotes the reversible formation of potassium superoxide at the cathode, minimizes the transport of molecular oxygen into the electrolyte, and subsequently improves anode stability and cycle-life of the K−O2 battery. These improvements in performance with FGC come at a marginal increase in K−O2 battery material cost, which is estimated to be $44 kWh−1. The combination of performance and cost kWh−1 makes this architecture the most cost-effective electrochemical energy storage device for stationary applications.