Effect of Operating Temperature in Li-Air Cells

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Due to its very high value of energy density,[1] the lithium-oxygen battery has received considerable attention worldwide. Recent results have demonstrated that the battery, if operated under proper conditions, may provide acceptable values of cycle life and capacity.[2] In its most classical configuration, the Li/O2 battery is formed by a lithium metal anode, a liquid organic electrolyte and a carbon-supported (with or without catalyst) air electrode. Recently, a new configuration, where the lithium metal is replaced by a lithium alloy silicon anode, has also been reported. Key parameters in assuring proper Li/O2 battery behavior are (i) the optimization of the positive electrode structure, in terms of use of an adequate gas diffusion layer and of effective catalysts, and (ii) the choice of an electrolyte stable to superoxide attack.[2,3] It is in fact well-known that the basic electrochemical cell process, leading to the reversible formation and dissolution of lithium peroxide, involves an intermediate oxygen anion radical O2 ·-[3,4] namely, a highly reactive base that readily attacks and decomposes conventional electrolytes, such as organic carbonate solutions. Dimethoxyethane (DME)-based and ionic liquid-based solutions have been proposed as alternative electrolyte media, however with little success.Recent works demonstrated that the best results in terms of Li/O2 battery stability and cycling may be obtained with the use of long chain, ether-based glymes, such as tetraethylene glycol dimethyl ether (TEGDME) electrolyte solutions.[2] In a previous paper we have reported a detailed transmission electron microscopy (TEM) study showing that Li/O2 batteries based on the TEGDME-LiCF3SO3 electrolyte indeed show a very promising behavior at room temperature.[5] In this paper we report an electrochemical and morphological study of the response of lithium−oxygen cells cycled at various temperatures, that is, ranging from -10 to 70 ℃. The results show that the electrochemical process of the cells is thermally influenced in an opposite way, that is, by a rate decrease, due to a reduced diffusion of the lithium ions from the electrolyte to the electrode interface, at low temperature and a rate enhancement, due to the decreased electrolyte viscosity and consequent increased oxygen mobility, at high temperature. In addition, we show that the temperature also influences the crystallinity of lithium peroxide, namely of the product formed during cell discharge by measuring the TEM.