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Probing Mechanisms for Inverse Correlation between Rate Performance and Capacity in K−O2 Batteries Neng Xiao, Xiaodi Ren, Mingfu He, William D. McCulloch, and Yiying Wu* Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Ohio 43210, United States S Supporting Information *
ABSTRACT: Owing to the formation of potassium superoxide (K+ + O2 + e− = KO2), K−O2 batteries exhibit superior round-trip efficiency and considerable energy density in the absence of any electrocatalysts. For further improving the practical performance of K−O2 batteries, it is important to carry out a systematic study on parameters that control rate performance and capacity to comprehensively understand the limiting factors in superoxide-based metal−oxygen batteries. Herein, we investigate the influence of current density and oxygen diffusion on the nucleation, growth, and distribution of potassium superoxide (KO2) during the discharge process. It is observed that higher current results in smaller average sizes of KO2 crystals but a larger surface coverage on the carbon fiber electrode. As KO2 grows and covers the cathode surface, the discharge will eventually end due to depletion of the oxygen-approachable electrode surface. Additionally, higher current also induces a greater gradient of oxygen concentration in the porous carbon electrode, resulting in less efficient loading of the discharge product. These two factors explain the observed inverse correlation between current and capacity of K−O2 batteries. Lastly, we demonstrate a reduced graphene oxide-based K−O2 battery with a large specific capacity (up to 8400 mAh/gcarbon at a discharge rate of 1000 mA/gcarbon) and a long cycle life (over 200 cycles). KEYWORDS: K−O2 batteries, KO2 growth mechanism, capacity, rate performance, oxygen diffusion, electrode passivation
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INTRODUCTION After years of intense research, lithium ion batteries are reaching specific energy densities of 250 Wh/kg.1 With the increasing desire for superior electrochemical energy storage devices, rechargeable Li−O2 batteries have received significant interest as one of the most promising substitutes of lithium ion batteries due to their high theoretical energy density (3505 Wh/kg).1−4 Nevertheless, significant challenges remain unsolved and hinder the commercialization of Li−O2 batteries, such as its low round-trip efficiency.5−7 A high polarization upon charging, resulting from a large band gap and low electronic conductivity of Li2O2, is one of the most challenging problems in a Li−O2 system.6,8 Recently, alternative metal−oxygen batteries, including Na− O2 and K−O2, have attracted increasing attention from researchers owing to their favorable round-trip efficiencies and energy densities.9,10 For lower charging overpotentials to be achieved, lithium ion has been replaced by larger cations (Na+, K+) to stabilize the superoxide (O2−) discharge product.11 Previously, we demonstrated the idea of a K−O2 battery with the lowest overpotential (
