As soluble catalysts, redox-mediators (RMs) endow mobility to catalysts for unconstrained access to tethered solid discharge products, lowering the energy barrier for Li2 O2 formation/decomposition; however, this desired mobility is accompanied by the undesirable side effect of RM migration to the Li metal anode. The reaction between RMs and Li metal degrades both the Li metal and the RMs, leading to cell deterioration within a few cycles. To extend the cycle life of redox-mediated Li-O2 batteries, herein graphene oxide (GO) membranes are reported as RM-blocking separators. It is revealed that the size of GO nanochannels is narrow enough to reject 5,10-dihydro-5,10-dimethylphenazine (DMPZ) while selectively allowing the transport of smaller Li+ ions. The negative surface charges of GO further repel negative ions via Donnan exclusion, greatly improving the lithium ion transference number. The Li-O2 cells with GO membranes efficiently harness the redox-mediation activity of DMPZ for improved performance, achieving energy efficiency of above 80% for more than 25 cycles, and 90% for 78 cycles when the capacity limits were 0.75 and 0.5 mAh cm-2 , respectively. Considering the facile preparation of GO membranes, RM-sieving GO membranes can be cost-effective and processable functional separators in Li-O2 batteries.
Keywords: 5,10-dihydro-5,10-dimethylphenazine (DMPZ); graphene oxide; lithium-oxygen batteries; redox mediator; separator.
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