Role of Magnetism and Disorder in Tuning the Electrochemical Properties of the Discharge Product in K-O2 Batteries
Authors: Kim Yoolhee , Pande Vikram , Viswanathan Venkatasubramanian 
Published: February 2020

The energy density of batteries can be increased by utilizing metal-air batteries, allowing longer driving ranges in electric vehicles. Li-air and Na-air batteries have been investigated, but they come with many issues such as low efficiency, limited capacity, and poor cycle life. Potassium-air batteries have not been investigated nearly as much, but they have a high theoretical energy density of 935 Wh/kg, around three times that of Li-ion batteries. It has been shown that K-air batteries have the lowest reported overpotential of 20-50mV in metal-oxygen batteries, which gives a very high-energy efficiency of 98% [1]. This battery comprises of a potassium metal anode, carbon cathode, and an electrolyte, with potassium superoxide (KO2) as the discharge product growing on the cathode [1]. This discharge product is thermodynamically stable and commercially available, unlike the discharge products of Li-air and Na-air batteries. However, there is very limited knowledge on the discharge product. Given a better understanding on the various phases and properties of the discharge product, we could tune the properties of KO2 to have desirable electrochemical characteristics. For example, we could use a magnetic substrate to alter the properties of the formed discharge product, such as the electronic conductivity of KO2. Electronic conductivity of the discharge product is key to increasing the capacity of the discharge cycle, as seen in Li-air and Na-air batteries [5].

In this work, we will use density functional theory (DFT) to explore the role of magnetism and disorder of the discharge product (KO2) in K-air batteries. We have calculated the formation energy of three different structures of potassium superoxide using the BEEF-vdW functional, in order to determine the most stable phase for both the antiferromagnetic and ferromagnetic configurations. The results of the calculations can be seen in Table I, and agree with previous work that using a metal-chloride reference for calculating the formation energy significantly reduces the mean absolute error (MAE), as well as the variation in calculated energy across various functionals [3]. The BEEF ensemble results as shown in Table II illustrates how the scheme using the metal chloride reference gives the least standard deviation across the various functionals. Table III reveals that the ferromagnetic state is the more stable structure, which disagrees with experimental results that KO2 is paramagnetic at room temperature, and antiferromagnetic at temperatures below 7K [4]. Thus, we will explore the more disordered phases of KO2 under various temperatures, including the tetragonal, monoclinic, and triclinic structures with different orientations of the oxygen dimers [4]. We will report on energetics of these phases within the random phase approximation (RPA). We will discuss the band structures and the band gaps for these various phases and its implications for K-O2 batteries. Lastly, we will explore the possibility of magnetic substrates preferentially nucleating and growing KO2 phases with high electronic conductivity to maximize the battery capacity.

[1] X. Ren, Y. Wu, JACS, (2013).

[2] X. Ren et al., Adv. Energy Mater., (2016).

[3] R. Christensen, J. S. Hummelshoj, H. A. Hansen, T. Vegge, J. Phys. Chem. C, (2015).

[4] M. Labhart, D. Raoux, and W. Kanzig, PRB, Volume 20, Number 1, (1979).

[5] V. Viswanathan, K. S. Thygesen, J. S. Hummelshoj, J. K., Norskov, G. Girishkumar, B. D. McCloskey, A. C. Luntz, J. Chem. Phys., (2011).

Figure 1