title: The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li-O2 batteries.
abstract: When lithium-oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain O2 reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences O2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2(*) ⇌ Li(sol)(+) + O2(-)(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium-oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.
authors.affiliation: 1] EastChem, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK [2] Departments of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.
authors.forename: Lee
authors.initials: L
authors.lastname: Johnson
authors.affiliation: 1] EastChem, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK [2] Laboratoire de Réactivité et Chimie des Solides - UMR CNRS 6007, 33 rue Saint-Leu, 80039 Amiens Cedex, France.
authors.forename: Chunmei
authors.initials: C
authors.lastname: Li
authors.affiliation: 1] EastChem, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK [2] Departments of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Liu
authors.affiliation: 1] EastChem, School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK [2] Departments of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.
authors.forename: Yuhui
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Christian Doppler Laboratory for Lithium Batteries, and Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria.
authors.forename: Stefan A
authors.initials: SA
authors.lastname: Freunberger
authors.affiliation: SUPA, School of Physics &Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Praveen C
authors.initials: PC
authors.lastname: Ashok
authors.affiliation: SUPA, School of Physics &Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Bavishna B
authors.initials: BB
authors.lastname: Praveen
authors.affiliation: SUPA, School of Physics &Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK.
authors.forename: Kishan
authors.initials: K
authors.lastname: Dholakia
authors.affiliation: Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex, France.
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Tarascon
authors.affiliation: 1] Departments of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK [2] Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR.
authors.forename: Peter G
authors.initials: PG
authors.lastname: Bruce