Critical Indicator Determining the Discharge Capacity of Li-Oxygen Batteries
Authors: Hase Yoko , Komori Yasuhiro , Seki Juntaro , Shiga Tohru , Kamiya Kazuhide , Nakanishi Shuji 
Published: February 2020

Li-O2 battery is one of the possible candidates for future energy devices and has attracted much attention in the beyond lithium ion battery research field.1-3 Despite its specific high energy capacity, many obstructions, including lower capacity, impede their use for practical applications. In typical Li-O2 batteries, Li metal and carbon were applied as the anode and the cathode materials, respectively, and the Li2O2 is formed as the discharge product and deposited on the cathode. As Li2O2 shows inherent low electron conductivity,4,5 once the positive electrode was fully covered with the deposition, it inhibits the continuous discharging and then the discharge potential is suddenly dropped. Several strategies such as control of the solubility of LiO2, which is an intermediate in the discharge reaction, have been studied with interest for sustainable discharge. It is known that Donor Number (DN), the indicator of Lewis basicity of solvents, highly influences on the solubility of LiO2.6 The electrolytes with higher DN solvents enhance the discharge capacity of Li-O2 batteries, because solubility of LiO2 is increased and Li2O2 is formed in the solution (solution pathway). In contrast, Li2O2 is formed on the cathode surface in solvents of lower DN (surface pathway). The effect of additives in solvents on enhancing the solubility of LiO2 was also reported,7-9 however, the understanding of the underlying principles that determine the discharge capacity is still limited.
Herein we present an alternative indicator to DN that controls the discharge capacity in the Li-O2 batteries. We focused on the effect of the range of combinations of cathode substrates and electrolytes on the discharge capacity. When we used Au-mesh as cathode, higher discharge capacity was obtained in a higher DN of dimethyl sulfoxide (DMSO) electrolytes (LiTFSI in DMSO) than that obtained in a lower DN of acetonitrile (MeCN) electrolyte, as reported previously.6 However, interestingly, higher capacity was obtained in the MeCN electrolytes when a carbon material was applied as the cathode. Through these experiments, we also found that the negative differential resistance (NDR), which appears when the coverage of the inhibitor on the electrode depends on the potential of the electrode, is a critical indicator determining the discharge capacity of Li-O2 batteries. The NDR in the oxygen reduction reaction (ORR) potential region implies that LiO2 works as an inhibitor of ORR. The higher discharge capacity can be obtained when the ORR proceeds at higher potential than NDR region, because LiO2 is desorbed from the electrode and the solution pathway is promoted. Our results indicate that this correlation between the NDR and the discharge capacity is generally applied to Li-O2 battery system.
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