Rechargeable battery technologies such as Li-ion have revolutionised personal electronics over the last 25 years. However, Beyond Li-ion technologies are becoming increasingly important towards meeting society’s future energy storage needs. We must explore these alternatives, such as the Li-air (O₂) battery, which can offer higher practical specific energy densities than Li-ion batteries; however there remain many challenges to be overcome before Li-O₂ batteries are commercialised.^1-5 One spin-off from the recent interest in rechargeable Li-O₂ batteries, based on aprotic electrolytes, is the significant advances made in understanding the fundamental processes occurring on O₂ reduction (discharge) at the cathode.^6-12

Based on these studies, it is generally accepted that a solution growth mechanism will be required to achieve high rates and capacities. One way to achieve discharge in solution is use of high donor or acceptor number (DN/AN) electrolytes,^13,14 but these are typically less stable towards LiO₂ and Li₂O₂ than their low DN/AN counterparts.¹⁵ To solve this dilemma, additives, such as redox mediators, have been introduced into low DN/AN electrolytes to encourage the discharge in solution.^13,16-18 Here, we discuss our recent studies into the electrochemistry at the cathode, in the presence of additives and utilise a range of electrochemical and spectroscopic techniques to determine the precise nature in which additives influence O₂ reduction.

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