2, State University of New York, Binghamton, New York, United States
Lithium metal batteries and sodium metal batteries (LMBs and NMBs) are an emerging alternative to conventional lithium ion batteries (LIBs) due to their greatly improved energy. Despite the heavy focus on specific energy (energy per weight) in scientific literature, it is the energy density (energy per volume) that is the primary consideration for most portable, stationary and even automotive applications. Researchers are pursuing LMB and NMB cathodes such as S and Se, which have the potential to provide 2-5 times capacity of LIBs. Selenium possesses similar chemical and electrochemical properties to sulfur, but orders of magnitude higher electrical conductivity, giving it a performance edge. Our approach here is fundamentally different from that of previous studies. Rather than seeking to create a nanostructured high – surface area electrode based on selenium and a nano – carbon, we achieve the opposite by creating a low surface area monolithic electrode-grade film. On a per volume basis, a dense electrode is twice as energetic as the same electrode when it is fabricated into a micro or a nano powder. Because our dense low surface area Se-carbon electrodes remain nanostructured on the inside, the kinetics and cyclability remain very good. For Li storage, the cathode delivered reversible capacity of 1028 mAh cm-3 (578 mAh g-1) and 82% retention over 300 cycles. The electrodes yield superb volumetric energy densities, being 1727 Wh L-1 for Li-Se and 980 Wh L-1 for Na-Se normalized by total composite volume. Such an approach also brings the selenium system closer to commercial electrode formulations, where the surface areas are purposely kept relatively low as to minimize parasitic surface reactions with the electrolyte.