The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries.

Tao Liu,Nuria Garcia-Araez,Rachel N Kerber,Clare P Grey,Nicolas Dubouis,Pieter C M M Magusin,James T Frith,Michael T L Casford,Yuanlong Shao,Gunwoo Kim,Zigeng Liu

doi:10.1021/jacs.7b11007

Abstract

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H2O on the oxygen chemistry in a nonaqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li+). When water and the quinone are used together in a (largely) nonaqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li2O2 crystals can grow up to 30 μm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O2 battery is obtained.

Highlights

Quinones represent an important class of organic redox molecules that are involved in energy transduction and storage in biological systems.[1−4] For example, they play a pivotal role in proton-coupled electron transfer for natural respiratory and photosynthetic processes.[5]
Quinones have been explored as redox mediators for the oxygen reduction reaction (ORR) in Li-O2 batteries
The nonaqueous Li-O2 battery is considered the ultimate battery as it possesses a theoretical energy density close to gasoline, 10 times higher than the state-of-the-art lithium ion battery.[25−28] Its operation typically involves O2 reduction during discharge, the first step involving a oneelectron electrochemical step to form LiO2, which chemically disproportionates to form Li2O2; a solid phase precipitates out of the liquid electrolyte and deposits on the porous electrode

Summary

Introduction

Quinones represent an important class of organic redox molecules that are involved in energy transduction and storage in biological systems.[1−4] For example, they play a pivotal role in proton-coupled electron transfer for natural respiratory and photosynthetic processes.[5]. The nonaqueous Li-O2 battery is considered the ultimate battery as it possesses a theoretical energy density close to gasoline, 10 times higher than the state-of-the-art lithium ion battery.[25−28] Its operation typically involves O2 reduction during discharge, the first step involving a oneelectron electrochemical step to form LiO2, which chemically disproportionates to form Li2O2; a solid phase precipitates out of the liquid electrolyte and deposits on the porous electrode. The solid discharge product is decomposed releasing O2. Realizing the theoretical capacity is, associated with significant challenges, in part because an electronically insulating discharge product tends to form as small particles or conformal films that quickly passivate electrode surfaces,[29−31] impeding further interfacial electron.

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Conclusion