Elsevier

Journal of Power Sources

Volume 195, Issue 4, 15 February 2010, Pages 1235-1240
Journal of Power Sources

Short communication
Discharge characteristic of a non-aqueous electrolyte Li/O2 battery

https://doi.org/10.1016/j.jpowsour.2009.08.088Get rights and content

Abstract

Discharge characteristic of Li/O2 cells was studied using galvanostatic discharge, polarization, and ac-impedance techniques. Results show that the discharge performance of Li/O2 cells is determined mainly by the carbon air electrode, instead by the Li anode. A consecutive polarization experiment shows that impedance of the air electrode is progressively increased with polarization cycle number since the surfaces of the air electrode are gradually covered by discharge products, which prevents oxygen from diffusing to the reaction sites of carbon. Based on this observation, we proposed an electrolyte-catalyst “two-phase reaction zone” model for the catalytic reduction of oxygen in carbon air electrode. According to this model, the best case for electrolyte-filling is that the air electrode is completely wetted while still remaining sufficient pores for fast diffusion of gaseous oxygen. It is shown that an electrolyte-flooded cell suffers low specific capacity and poor power performance due to slow diffusion of the dissolved oxygen in liquid electrolyte. Therefore, the status of electrolyte-filling plays an essential role in determining the specific capacity and power capability of a Li/O2 cell. In addition, we found that at low discharge currents the Li/O2 cell showed two discharge voltage plateaus. The second voltage plateau is attributed to a continuous discharge of Li2O2 into Li2O, and this discharge shows high polarization due to the electrically isolating property of Li2O2.

Introduction

Non-aqueous electrolyte Li/air battery was first introduced in 1996 by Abraham and Jiang [1], and further developed by many scientists over the world [2], [3], [4], [5], [6], [7], [8], [9], [10]. This battery uses a metal lithium anode, a carbon or catalyst-loaded carbon air electrode, and a non-aqueous organic electrolyte, in which the active cathode material is oxygen coming from the environment and the air electrode provides a site for catalytic reduction of oxygen. In organic electrolytes, the catalytic reduction of oxygen is mostly through a two-electron process [1], [8], therefore, the overall reaction of a Li/air battery can be written as:2Li + O2  Li2O2  3.10 V

Depending on discharge current and electrolyte composition, part of the resulting Li2O2 can be further discharged to form Li2O:2Li + Li2O2  2Li2O  2.72 V

Combining Eqs. (1), (2) leads to Eq. (3) and a 2.91 V overall voltage:4Li + O2  2Li2O  2.91 V

Therefore, Li2O2 and Li2O coexist in the final discharge products of a Li/air battery [2]. No matter what the final discharge products are, both Eqs. (1), (3) give a theoretical specific capacity of 3862 mAh g−1 vs. metal Li. Due to its high theoretical capacity, the Li/air battery has recently been proposed by U.S. Army as an electrochemical power source for the field charger of Li-ion batteries.

Real capacity of a Li/air battery does not correspond to the theoretical capacity of metal Li due to the insolubility of discharge products (Li2O2 and Li2O) in non-aqueous organic electrolyte. The discharge products are deposited on the surfaces of carbon or catalyst in the air electrode, which blocks oxygen from diffusing to the reaction sites. Therefore, the real capacity that a Li/air battery can achieve is determined by the carbon air electrode, especially by the pore volume available for the deposition of discharge products, instead by the Li anode. In previous works [2], [3], [5], we have reported the effect of electrolyte formulation and air electrode morphology on the specific capacity of electrolyte-flooded Li/O2 cells in terms of the oxygen solubility and diffusion in liquid electrolyte and the specific surface area and porosity of carbon and air electrode. In this work, we further study the discharge characteristic of Li/air battery and discuss the air electrode model that favors achieving high specific capacity and power capability. To avoid the effects of moisture and carbon dioxide in air, all cells in this work are sealed in a small oxygen bag, and we hereafter call such cells as “Li/O2 cell”.

Section snippets

Experimental

Activated carbon, M-30 (having a specific surface area of 2500–3200 m2 g−1 and a mean diameter of 25–30 μm, Osaka Gas Chemicals Co. Ltd.), was used as the active material of air electrode without the addition of other catalysts. Using polytetrafluoroethylene (PTFE) emulsion (Teflon®, solid content = 61.5%, DuPont Co.) as the binder, two free-standing carbon air electrodes were prepared as the procedure described previously [2], [3], [5]. One was composed by weight of 92% M-30 and 8% PTFE and the

Li/air battery vs. traditional metal/air batteries

Fig. 2 shows a typical structure of metal/air batteries. The traditional batteries, such as Zn/air battery, use an aqueous alkaline solution as the electrolyte and a catalyst-loaded carbon as the air electrode. In such batteries, oxygen is reduced to form OH anions on the surfaces of catalyst and the resulting OH anions are subsequently dissolved into electrolyte. Therefore, the air electrode only serves as a reaction site for the catalytic reduction of oxygen. The specific capacities of such

Conclusions

Li/O2 cell is unique in that the discharge products are insoluble in organic liquid electrolyte. This feature requires an electrolyte-catalyst “two-phase reaction zone” model to achieve high specific capacity and good power capability. According to this model, the air electrode should be completely wetted with the liquid electrolyte to supply the maximum reaction area for high specific capacity of carbon. Meanwhile, the air electrode should not be flooded by the liquid electrolyte so that the

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