Abstract
This Tutorial Review describes how the development of dissolved redox-active molecules is beginning to unlock the potential of three of the most promising 'next-generation' battery technologies - lithium-air, lithium-sulfur and redox-flow batteries. Redox-active molecules act as mediators in lithium-air and lithium-sulfur batteries, shuttling charge between electrodes and substrate systems and improving cell performance. In contrast, they act as the charge-storing components in flow batteries. However, in each case the performance of the molecular species is strongly linked to their solubility, electrochemical and chemical stability, and redox potentials. Herein we describe key examples of the use of redox-active molecules in each of these battery technologies and discuss the challenges and opportunities presented by the development and use of redox-active molecules in these applications. We conclude by issuing a "call to arms" to our colleagues within the wider chemical community, whose synthetic, computational, and analytical skills can potentially make invaluable contributions to the development of next-generation batteries and help to unlock of world of potential energy-storage applications.
Highlights
(2) The function that soluble redox-active molecules (RAMs) can play in overcoming many of the challenges faced by next-generation battery technologies, and the mechanisms they employ to improve battery performance
The gradual decarbonization of the transportation sector towards hybrid and all-electric vehicles has been facilitated by advances in lithium-ion batteries (LIBs), though cost, range and lifetime all remain limiting factors in the widespread uptake of both technologies
Numerous optimisation strategies have found success in recent years,[5] and as a result LIB technology is beginning to approach its limits.[6]. Whilst this is sufficient for applications in smaller consumer electronics and some electric vehicles, even nextgeneration LIBs will struggle to displace cheap and highly energy dense fossil fuels in applications which are sensitive to cost and/or specific energy
Summary
Among EES technologies, the LIB has long been regarded as the benchmark thanks to its high energy density, cycling stability and low self-discharge relative to other competing battery technologies.[3]. Tsao has demonstrated that, whilst metallocenes have been shown to activate Li2S at the discharged positive electrode, more stringent redox targeting enabled by organic RMs could be applied to further reduce the overpotentials for Li2S activation.[43] Quinones were selected as a suitable class of RMs due to their reversible and highly tuneable pair of redox potentials, and a new anthraquinone with triethylene glycol monomethyl ether substituents (AQT) was synthesised and shown to exhibit two reversible redox processes at B2.1 and B2.45 V vs Li+/Li. When tested in cells using lithium metal foil and Li2S on carbon paper as the negative and positive electrodes, and a RM : Li2S ratio of 10 : 1 in 1 : 1 DOL/DME (2 wt% LiNO3), the AQT-mediated cell showed a remarkably high discharge capacity (1402 mA h gsulfurÀ1), corresponding to 85% sulfur utilization, a low average charge potential (2.45 V). EPT shows two reversible one-electron redox processes separated by
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