The biggest challenge in the non-aqueous Li-O₂ system is the extremely large overpotential.  The total polarization between discharging and charging process reaches >0.5 V, even in the case of best one ever reported¹.  An approach for the improvement in the power density of the Li-O₂ battery is utilization of oxygen reduction process in aqueous media.  In the aqueous system, the electrochemical reaction at the gas-diffusion electrode is based on oxygen-water couple described in the following equations. 

                O₂ + 2H₂O + 4e^-  4OH^-                                         (1)

                O₂ + 6H₂O + 4Li⁺ + 4e^-  4LiOH·H₂O                   (1)’

This reaction shows much lower overpotential than the non-aqueous system, however the aqueous system still needs catalysts both for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).  Therefore, a lot of bi-functional catalyst materials have been investigated for a long time, regarding rechargeable zinc-air battery applications.  Another problem of the aqueous Li-O₂ battery is water consumption during the oxygen reduction process.  Since the excess electrolyte solution needs to be confined in the cell, the energy density of the aqueous system needs to be sacrificed. 

Here we introduce a novel electrochemistry for the aqueous Li-O₂ battery based on oxygen-peroxide couple using a catalyst-free carbon-based electrode, showing extremely low over potential for OER. 

Experimental

Since the redox reaction of the oxygen-peroxide couple does not require any catalyst except carbon material, a catalyst-free gas-diffusion electrode was prepared by mixing a carbon black and polytetrafluoroethylene (PTFE) binder to form a sheet.  A conventional three-electrode cell was fabricated to investigate the reaction mechanism in the present system.  Also a pouch cell equipped with a protected lithium negative electrode was fabricated and confined in a chamber filled with pure oxygen gas. A saturated LiCl aqueous solution with additional LiOH was prepared as the electrolyte solution. 

Result and discussion

Characterization of the discharging product in the present system was carried out after 50 hours of constant current discharging process at 1.0 mAcm^-2.  An XRD pattern and an SEM image of the gas-diffusion electrode after the discharging process is shown in Fig. 1.  Surprisingly the discharging product in the present cell was NOT LiOH·H₂O but Li₂O₂.  Also flake-shaped particles were observed at the surface of the electrode.  Since the gas-diffusion electrode does not have any catalyst materials, only the two-electron transfer process in the following equation (2) should occur during the discharging process. 

                O₂ + 2H⁺ + 2e^-  H₂O₂ (in acid solutions)                                              (2) 

                O₂ + H₂O + 2e^-  HO₂^- + OH^- (in alkaline solutions)                              (2)’

Thus we think the formation of the Li₂O₂ is due to the high concentration of lithium in the electrolyte solution and poor solubility of the Li₂O₂.  Furthermore the charging overpotential in the present system is extremely low: <0.1 V at 1.0 mAcm^-2.  We think utilization of the oxygen-peroxide couple has advantage in reversibility and power density, compared with the conventional oxygen water couple². 

 References 

  (1)  Elia, G. A.; Hassoun, J.; Kwak, W. J.; Sun, Y. K.; Scrosati, B.; Mueller, F.; Bresser, D.; Passerini, S.; Oberhumer, P.; Tsiouvaras, N.; Reiter, J. Nano letters 2014, 14, 6572.

  (2)  Yeager, E.; Krouse, P.; Rao, K. V. Electrochimica Acta 1964, 9, 1057.