Lithium-sulfur batteries
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uction Today’s world has been unalterably changed by rechargeable lithium-ion (Li-ion) batteries that power a multitude of indispensable portable electronic devices owing to their relatively high energy density. This was recognized this year with the award of the Draper Prize for Engineering to the founders of the technology.1 These batteries have found their way into the hybrid and electric vehicle market with ever-increasing popularity and are also gradually starting to make inroads into larger-scale energy storage for renewable energy sources such as solar power. Over 200 MWs of Li-ion batteries are installed globally for grid applications at present,2 and this number is predicted to increase as the need to manage the intermittency of renewable generation rises. At the same time, the inherent limitations of Li-ion batteries based on Li+ insertion/extraction chemistry are becoming apparent, which include both cost and performance factors.3,4 Although much effort is being expended to create nextgeneration Li-ion systems, it is also appreciated that they are approaching their energy density boundaries, anticipated to have a ceiling about double that of today’s cells. Their price remains an impediment to achieving the 3–5-fold decrease in cost regarded as necessary for widespread penetration of electric vehicles in the automotive market, with even lower cost per kWh being essential for grid storage. New directions are
needed to inspire change for energy storage systems that differ from conventional Li-ion systems. Lithium-sulfur (Li-S) batteries provide a promising option that could theoretically achieve the necessary step up, considering both cost and specific energy. Elemental sulfur—abundant and inexpensive—has become one of the most actively researched cathode materials in the last few years, with 445 papers published since 2012 alone at the time of writing. Its high theoretical specific capacity is based on a redox reaction that reversibly interconverts sulfur and Li2S via intermediate lithium polysulfide species (Li2Sn, n = 2–8).5,6 The chemistry is fundamentally different from the intercalation process that governs Li-ion cells and has higher energy density, but it comes with several challenges. Li-S cells have suffered from low sulfur utilization and poor long-term cycling in the past; they experience a large volumetric expansion upon formation of Li2S; and the dissolution of lithium polysulfide intermediate species in commonly used liquid electrolytes triggers a parasitic shuttle-type diffusion process of Li2Sn polysulfides that can result in low Coulombic efficiency.7 Although many significant achievements have been made recently to overcome these shortcomings, additional inroads in materials and understanding of the dissolution-precipitation cell chemistry are still required in order to realize the promise of the technology. Because several recent excellent reviews are available
Linda F. Nazar, University of Waterloo, Ontario, Canada; [email protected] Marine Cuisinier, University of Waterloo, Ontario, Canada
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