Jannis Küpper1,Simon Jakobi1,Ulrich Simon1
RWTH Aachen University1
Jannis Küpper1,Simon Jakobi1,Ulrich Simon1
RWTH Aachen University1
The potassium-oxygen battery (KOB) is a promising energy storage technology with high theoretical specific energy of 935 Wh/kg and long cycle life. In recent years, the KOB has distinguished itself from other alkali metal-oxygen batteries by its facile cathodic cell chemistry that enables thousands of cycles with negligible performance loss. Capacities relevant for practical applications are achieved using affordable carbon paper as cathode material without need for precious metal catalysts. In addition, operation in dry ambient air has been demonstrated.<br/>The cathode chemistry of KOB is based on the highly reversible interconversion of oxygen and potassium superoxide (KO<sub>2</sub>). Formation of KO<sub>2</sub> is thermodynamically and kinetically favored over potassium peroxide (K<sub>2</sub>O<sub>2</sub>) and potassium oxide (K<sub>2</sub>O). A distinctive chemical stability renders KO<sub>2</sub> unreactive towards cell components and is the driver for the remarkable cycling stability of KOB. Recently, it was found that a further reduction of KO<sub>2</sub> towards K<sub>2</sub>O<sub>2</sub> and K<sub>2</sub>O occurs in the absence of oxygen, which induces currently unknown parasitic side reactions. In consequence, the coulombic and round-trip efficiencies are diminished. These findings raise several questions: How does oxygen partial pressure p(O<sub>2</sub>) influence the cell chemistry? Can K<sub>2</sub>O<sub>2</sub> formation during discharge be prevented? How does p(O<sub>2</sub>) affect the performance and what role does the cathode structure play?<br/>We tackled these questions in an experimental approach. The influence of p(O<sub>2</sub>) on the discharge performance was investigated by extensive battery testing over a wide range of p(O<sub>2</sub>). Analytical techniques were used in situ and post-mortem to study the effects of p(O<sub>2</sub>) on product formation, parasitic side reactions and cell failure mechanisms.<br/>We find that higher p(O<sub>2</sub>) is all around beneficial for the discharge performance of KOB and substantially improves the maximum discharge capacity and rate capability. Furthermore, we show that proper design of the cathode structure allows for fast, gaseous transport of oxygen throughout the entire cathode, which drastically improves the rate capability by enabling the homogeneous deposition of KO<sub>2</sub> and high degrees of pore volume filling. Our findings emphasize the critical importance of efficient mass transport for high performance KOB.