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.
Li–O2 batteries are plagued by side reactions
that cause poor rechargeability and efficiency. These reactions were
recently revealed to be predominantly caused by singlet oxygen, which
can be neutralized by chemical traps or physical quenchers. However,
traps are irreversibly consumed and thus only active for a limited
time, and so far identified quenchers lack oxidative stability to
be suitable for typically required recharge potentials. Thus, reducing
the charge potential within the stability limit of the quencher and/or
finding more stable quenchers is required. Here, we show that dimethylphenazine
as a redox mediator decreases the charge potential well within the
stability limit of the quencher 1,4-diazabicyclo[2.2.2]octane. The
quencher can thus mitigate the parasitic reactions without being oxidatively
decomposed. At the same time the quencher protects the redox mediator
from singlet oxygen attack. The mutual conservation of the redox mediator
and the quencher is rational for stable and effective Li–O2 batteries.
The effect of a single pulse of electric current with short duration on the quasi-static tensile behavior of a magnesium AZ31 alloy is experimentally investigated. A single pulse of electric current with duration less than 1 second is applied to the specimen, while the specimen is being deformed in the plastic region under quasi-static tensile loads. After a nearly instant decrease of flow stress at the pulse of electric current, the flow stress shows strain hardening until the failure of the specimen. The experimental result shows that the strain-hardening parameters (the strength coefficient and the strain-hardening exponent) of the hardening curve after the electric current strongly depend on the applied electric energy density (electric energy per unit volume). Empirical expressions are suggested to describe the hardening behavior after the pulse as a function of the electric energy density and are compared with the empirical expressions suggested for advanced high-strength steels.
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