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Lithium-air battery
by Patrick Schwager

 

Since the gradual replacement of conventional, fossil energy sources by renewable energies, which strongly depends on environmental conditions, there have increasingly been calls for high-power energy storage devices. Compared to lead batteries, redox-flow batteries, Nickel-based systems or state of the art Lithium-ion batteries, Lithium-air batteries are of particular interest due to its outstanding energy density. The functionality is based on the oxygen reduction reaction (ORR) within a gas diffusion electrode (GDE). Oxygen is reduced to O2- which coordinates with Li+ to form LiO2- [1]. On the one hand this intermediate can adsorb at the electrode surface and disproportionate or undergoes a second reduction step to form the final solid ORR product Li2O2 which is suspect to clog the GDE thus reduces the devices capacity. On the other hand LiO2- is preferably dissolved up to a certain extent in the electrolyte. This strongly depends on the solvents properties and can increase the performance of the battery [2].

The focus of interest of our research group lies on electrochemical processes within the GDE. The  mechanisms  of  charging  and  discharging  in  organic electrolytes  are  investigated  using  positionable  microelectrodes.  Short-lived intermediates as well as the mass transport in porous gas diffusion electrodes are characterized by means of SECM (scanning electrochemical microscopy). In our approach we developed a setup to monitor the oxygen diffusivity and permeability of the gas diffusion electrode. A microelectrode is positioned in a micrometer range distance above the gas diffusion electrode to detect oxygen. Since no steady-state current for oxygen reduction is reachable in Li-containing electrolytes a pulsed  potential is applied to the microelectrode to eliminate the effect of surface passivation [3]. Furthermore this technique is used to monitor the clogging of the gas diffusion electrode while discharging (ORR). 

(*) To calculate the charge and energy density only active material of anode and cathode were considered. Furthermore a porosity of 70% and a maximum of pore filling by the discharge product of 50% were assumed for the GDE.

[1]C.O. Laoire, S. Mukerjee, K.M. Abraham, E.J. Plichta, M.A. Hendrickson, J. Phys. Chem. C. 2009, 113, 20127-34.
[2]L. Johnson, C. Li, Z. Liu, Y. Chen, S.A. Freunberger, P.C. Ashok, B.B. Praveen, K. Dholakia, J.-M. Tarascon, P.G. Bruce, Nat Chem 2014, 6, 1091-9.
[3]P. Schwager, D. Fenske, G. Wittstock, J. Electroanal. Chem. 2015, 750, 82-7.

 

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