Lithium-air and lithium-sulfur batteries
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Introduction The high energy density (energy stored per unit mass and volume) of rechargeable lithium-ion (Li-ion) batteries has transformed portable electronics over the last two decades. Such batteries are the technology of choice for the electrification of transport and are expected to find application in static electricity storage, especially in grid distribution networks. Intensive research efforts worldwide are being devoted to the realization of new generations of Li-ion batteries to address these markets. While this work is important, the best that can be achieved is a doubling of energy density compared with the Li-ion batteries of today. A twofold increase in energy density is insufficient for the long-term demands of transport and electricity storage; for example, it is unlikely to deliver a +300 mile driving range between chargings, which would transform electric vehicles. Achieving a step-change in energy storage requires an investigation of technologies that differs from Li-ion based systems. In this article, we consider two lithium battery technologies, lithium-air (Li-air) and lithium-sulfur (Li-S), that could theoretically deliver the transformation in performance required in the long-term. The reactions at the cathode (positive electrode) in Li-air and Li-S cells involve the reversible reduction of O2 and S, respectively, and are fundamentally different from those in Li-ion cells. Neither of these technologies is an established commercial product, and both
present significant challenges, including, in particular, materials challenges in order to realize practical devices. In this article, we will begin by considering the energy that can be stored in Li-air and Li-S batteries, and then examine each technology, in turn, highlighting the challenges that must be addressed in transforming these batteries from theory to practice. Several recent reviews on Li-air and Li-S are available.1–4 A third battery technology, zinc-air, will not be considered here but has been reviewed recently.4
Energy storage of lithium-air and lithium-sulfur batteries The theoretical energy density of a battery is based on the overall cell reaction. The values for Li-air and Li-S are given in Table I, along with the corresponding values for a typical Li-ion cell and rechargeable Zn-air. Schematic representations of the Li-ion, Li-air (based on aqueous and non-aqueous electrolytes), and Li-S cells are shown in Figure 1. In the Li-ion cell, Li is removed from an intercalation cathode, for example LiCoO2, on charging and is inserted between sheets of carbon atoms in the graphite negative electrode; discharge reverses this process. In the Li-air cell, the intercalation cathode of the Li-ion cell is replaced by a porous, electronically conducting substrate, usually carbon, flooded with the electrolyte. Upon discharge, Li-ions form by oxidation of the lithium
Peter G. Bruce, School of Chemistry, University of St. Andrews, UK; [email protected] Laurence J. Hardwick, School of Chemistry, University of St. Andrews, UK; ljh21@st-andrew
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