Theoretical Evidence for Low Charging Overpotentials of Superoxide Discharge Products in Metal–Oxygen Batteries

Abstract

Li–oxygen and Na–oxygen batteries are some of the most promising next-generation battery systems because of their high energy densities. Despite the chemical similarity of Li and Na, the two systems exhibit distinct characteristics, especially the typically higher charging overpotential observed in Li–oxygen batteries. In previous theoretical and experimental studies, this higher charging overpotential was attributed to factors such as the sluggish oxygen evolution or poor transport property of the discharge product of the Li–oxygen cell; however, a general understanding of the interplay between the discharge products and overpotential remains elusive. Here, we investigated the charging mechanisms with respect to the oxygen evolution reaction (OER) kinetics, charge-carrier conductivity, and dissolution property of various discharge products reported in Li–oxygen and Na–oxygen cells. The OER kinetics were generally faster for superoxides (i.e., LiO2 and NaO2) than for peroxides (i.e., Li2O2 and Na2O2). The electronic and ionic conductivities were also predicted to be significantly higher in superoxide phases than in peroxide phases. Moreover, systematic calculations of the dissolution energy of the discharge products in the electrolyte, which mediate a solution-based OER reaction, revealed that the superoxide phases, particularly NaO2, exhibited markedly low dissolution energy compared with the peroxide phases. These results imply that the formation of superoxides instead of peroxides during discharge may be the key to improving the energy efficiency of metal–oxygen batteries in general.

Understanding Rechargeable Li−O2 Batteries via First‐Principles Computations

“…However, computations suggest that the energy barrier of the disproportionation of large LiO 2 clusters is significantly higher than that of the LiO 2 dimer, indicating that the disproportionation rate of large clusters is much slower than that of the dimer. [75] Besides, the solution mediated mechanism of charging was also investigated and the dissolution energy of LiO 2 is lower than that of Li 2 O 2 . Then, in 2016, Lu et al [74] provided evidence that it is possible to have a one-electron discharge process with forming only LiO 2 in a LiÀO 2 battery.…”

High capacity surface route discharge at the potassium-O2 electrode

“…[1] In addition, similar studies of the Na-O2 system also suggest that conductivity of NaO2 is limited and that discharge by a surface route is not a dominant discharge pathway, [14,22] although calculation shows that NaO2 has a higher conductivity than LiO2. [23] Since the K-O2 battery was reported in 2013, [8] only a few studies of the system have been reported and the discharge mechanism in the K-O2 battery remains poorly understood. [24][25][26][27] Here we explore discharge in the K-O2 battery using solvents that are expected to induce a surface route.…”

Strategies Toward Stable Nonaqueous Alkali Metal–O2 Batteries

“…Hence, very few reports focused on developing catalysts for Na-O 2 and K-O 2 superoxide batteries. [16] They found that the OER barriers at major surfaces of the superoxides were substantially lower than those of the peroxides. [15] Theoretical calculations were conducted to address the discrepancy in the charge overpotential between peroxides and superoxides.…”