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Abstract
Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. The significant problem of oxidizing the solid insulating lithium peroxide can greatly be facilitated by incorporating redox mediators that shuttle electron-holes between the porous substrate and lithium peroxide. Redox mediator stability is thus key for energy efficiency, reversibility, and cycle life. However, the gradual deactivation of redox mediators during repeated cycling has not conclusively been explained. Here, we show that organic redox mediators are predominantly decomposed by singlet oxygen that forms during cycling. Their reaction with superoxide, previously assumed to mainly trigger their degradation, peroxide, and dioxygen, is orders of magnitude slower in comparison. The reduced form of the mediator is markedly more reactive towards singlet oxygen than the oxidized form, from which we derive reaction mechanisms supported by density functional theory calculations. Redox mediators must thus be designed for stability against singlet oxygen.
Highlights
Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge
The catalytic effect of redox mediators (RMs) deteriorates with repeated cycling. Reported reasons for this deterioration include side reactions with the anode when unprotected lithium metal is used[29,30,31,32] and reaction with the electrolyte[18,21,22]. Even when both of these effects are excluded by protecting the anode with, e.g., a solid electrolyte and choosing mediators that are inert towards the electrolyte, the RMs still gradually degrade and the energy efficiency decreases
Redox mediators may be deactivated by any of the potentially reactive species that appear during cycling of the cell, including O2, O2, Li2O2, and, as recently revealed, the highly reactive 1O2
Summary
Non-aqueous lithium-oxygen batteries cycle by forming lithium peroxide during discharge and oxidizing it during recharge. Solid catalysts act only near their surface and may accelerate the decomposition of Li2O2 and the undesired side reactions involving the electrode and electrolyte[12,14,17] The second type, redox mediators (RMs), are soluble catalysts in the electrolyte to chemically decompose Li2O2 They are oxidized at the porous electrode substrate and diffuse to Li2O2, which decomposes to Li+ and O2 by reforming the original reduced state[18,19,20,21,22]. Reported reasons for this deterioration include side reactions with the anode when unprotected lithium metal is used[29,30,31,32] and reaction with the electrolyte[18,21,22] Even when both of these effects are excluded by protecting the anode with, e.g., a solid electrolyte and choosing mediators that are inert towards the electrolyte, the RMs still gradually degrade and the energy efficiency decreases. Their side reactions with cell components caused by 1O2 need to be considered comprehensively
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