Abstract
Although redox mediator (RM) strategy can decrease the overpotential in Li-O2 batteries by tuning the electrochemical formation/degradation of Li2O2 from the circumscribed surface pathway to the solution one, the redox shuttling causes an unexpected RM degradation and a continuous deterioration of Li anode, finally leading to a poor cyclability. This work presents the first report detailing the development of a novel MnxCo3-xO4-decorated separator for Li-LiI-O2 batteries. Benefiting from the promotion effect of MnxCo3-xO4 nanocages on I−/I3− and I3−/I2 redox coupling, the cell with as-prepared separator maintains a low charge potential of ~3.3 V till the death of cell cycling. In addition, as-prepared separator can efficiently restrain the redox shuttling, leading to an obvious improvement on cycling stability for the cell. Moreover, the contributions of LiI to the battery performance and the operation mechanism are systematically investigated. These results present a promising progress in the development of multi-functional separator materials for RM-involved Li-O2 batteries and the new design of hybrid energy storage device.
Highlights
Li-O2 batteries have been attracting much attentions as a promising energy device for electric vehicles (EVs) due to their much higher theoretical energy density than that of conventional Li-ion batteries [1,2]
The charge transfer is initiated via a reversible redox reaction of redox mediator (RM) + e− ⇄ RM− or RM ⇄ RM+ + e−, in which RM is electrochemically reduced to RM− or oxidized to RM+ at the electrode, and RM− or RM+ diffuses via the liquid electrolyte and chemically reduces or oxidizes Li2O2 [17,18,19]
One big obstacle for a RM-based Li-O2 system is the redox shuttling between the diffusible RM−, RM or RM+ and Li anode [30,31,32,33], leading to an unexpected RM degradation and a continuous deterioration of Li anode
Summary
Li-O2 batteries have been attracting much attentions as a promising energy device for electric vehicles (EVs) due to their much higher theoretical energy density than that of conventional Li-ion batteries [1,2]. To develop a novel material and/or to explore a facile way to inhibit the redox shuttling is still a big challenge. In this respect, using a modified separator to restrain the shuttling effect could be considered as an innovative strategy. The results confirm that MnxCo3-xO4 nanocages promote both I− /I3− and I3− /I2 redox conversion, facilitating the reaction kinetics linking to Li2O2 oxidation (i.e., 2I3− + Li2O2 → 2Li+ + 6I− + O2 and 3I2 + Li2O2 → 2Li+ + 2I3− + O2), which substantially lowers the over potential and improves the energy efficiency for the Li-O2 battery. To the best of our knowledge, this is the first report detailing the design of a dual functional separator, which can work as the redox shuttling inhibitor and the promoter for redox conversions in Li-O2 batteries
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