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Using redox mediators for Li-O2 and Li-S batteries and Li recycling

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Using redox mediators for Li-O2 and Li-S batteries and Li recycling

N. Garcia-Araez, L. Yang, J.T Frith J. W. Dibden, N. Intaranont, and J.R. Owen University of Southampton Highfield Campus, SO17 1BJ , Southampton, United Kingdom

Lithium-oxygen and lithium-sulfur batteries have the potential to revolutionize the energy storage market, since they can in principle store >5 times more energy than the current lithium-ion battery technologies. However, the performance of Li-O2 and Li-S batteries is currently limited by several fundamental issues.

We have recently shown that redox mediators can solve the two more important issues associated to Li O2 batteries, i.e. electrode passivation and degradation reactions.1 Degradation reactions are initiated by superoxide radical species that are formed as intermediates in the pathway of the reduction of oxygen: O2 e- Li+ → LiO2 LiO2 e- Li+ → Li2O2 On the example of ethyl viologen, EtV2+, we demonstrated that redox mediators can be used to decrease the lifetime of superoxide species by promoting the fast reduction of superoxide to peroxide: EtV2+ e- → EtV LiO2 EtV Li → Li2O2 EtV2 where EtV2+ acts as homogeneous catalyst and it is not consumed in the reaction. Furthermore, EtV2+ also acts as an electron shuttle, preventing electrode passivation by displacing the location of the reduction of oxygen from the electrode surface to the solution. Indeed, a major increase in capacity is obtained upon addition of EtV2+ to a Li-O2 cell.

In Li-S batteries, a variety of polysulfides, Li2Sn, are formed in the pathway of the reduction of S to Li2S: n S 2e- 2Li+ → Li2Sn Li2Sn (2n-2) e- (2n-2) Li+ → n Li2S We hypothesize that polysulfides can act as redox mediators and shuttles in Li-S batteries, and their action can be optimized via a selection of suitable electrolytes. We have recently developed a new method that allows predicting the equilibrium concentration of polysulfides in Li-S cells, as well as the theoretical electrochemical response of the cell as a function of the amount of electrolyte.2 This new approach, based on the construction of an experimental phase diagram, will be used for the rational screening of electrolytes for Li-S batteries.

Redox mediators are also a promising tool for Li recycling. We have shown that the reaction of battery materials (e.g. LiFePO4) with redox agents is a fast, selective and efficient new method to extract lithium from natural brines and for recycling spent batteries.3 References: 1. L. Yang, J.T. Frith, N. Garcia-Araez, J.R. Owen. Chem. Commun. 2015, 51,1705-1708. 2. J. W. Dibden, J. W. Smith, N. Zhou, N. Garcia-Araez, J. R. Owen. Chem. Commun. 2016, DOI : 10.1039/C6CC05881G 3. N. Intaranont, N. Garcia-Araez, A. Hector, J.A. Milton, J.R. Owen. J. Mater. Chem. A, 2014, 2, 6374-6377.

This talk is part of the Physical Chemistry Research Interest Group series.

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