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