Effects of Cell Construction Parameters on the Performance of Lithium/Sulfur Cells

AIChE Journal 61 (9), 2749-2756 (2015) Effects of Cell Construction Parameters on the Performance of Lithium/Sulfur Cells Min-Kyu Song The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Dept. of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720 Yuegang Zhang The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Elton J. Cairns Dept. of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 DOI 10.1002/aic.14947 Published online August 7, 2015 in Wiley Online Library (wileyonlinelibrary.com) Current lithium-ion batteries are predicted to be unable to provide the specific energy required to meet the ever- increasing demands of rapidly emerging technologies. Due to a high theoretical specific capacity of 1675 mAh/g, sulfur has gained much attention as a promising positive electrode material for high specific energy rechargeable batteries. Although the lithium/sulfur cell has been studied for many years and continues to receive much attention today as an alternative power source for zero-emission vehicles and advanced electronic devices, the realization of this novel cell’s promise as a commercial product has yet to be successful. The major problems with sulfur electrodes involve: (1) the dissolution of sulfur (as polysulfides) and the resulting diffusion of dissolved polysulfides and (2) the deposition of insu- lating products (including Li 2 S) on both the negative and the positive electrodes. These solid deposits can physically block the electrode reaction sites, thus passivating the electrode surfaces. Another important problem is the large vol- ume change that occurs with the conversion of S to Li 2 S. It is important to understand that the performance of Li/S cells is hampered by linked chemical and mechanical degradations and both degradation mechanisms must be correctly alle- viated in order to markedly improve current-technology Li/S cells. In this study, improved cycling performance via the reactive functional groups on graphene oxide to successfully immobilize sulfur and lithium polysulfides during operation has been demonstrated. The use of a new electrolyte and binder leads to improved cell performance in terms of high- rate capability (up to at least 2 C) and good reversibility (S $ Li 2 S), yielding at least 800 cycles have also been dem- C 2015 American Institute of Chemical Engineers AIChE J, 61: 2749–2756, 2015 onstrated. V Keywords: sulfur, graphene oxide, nanocomposites, electrolyte, binder, lithium batteries Introduction Among electrochemical energy storage devices, recharge- able lithium-ion batteries still embody the state-of-the-art technology available in the present market. 1–3 However, there are several challenges that continue to require attention. 4 Above all, it is believed that current oxide-based cathode materials have insufficient capacity to meet the important needs of emerging technologies such as electric vehicles or plug-in hybrid electric vehicles. 5,6 Advanced batteries with a capability of more than 400 Wh/kg are desired because current Correspondence concerning this article should be addressed to E. J. Cairns at ejcairns@lbl.gov. Current address of Y. Z.: i-LAB, Suzhou Institute of Nano-Tech and Nano- Bionics, Chinese Academy of Sciences, Suzhou, 215123, China. C 2015 American Institute of Chemical Engineers V AIChE Journal lithium-ion batteries are reaching their maximum specific energy (225 Wh/kg). 7,8 Recently, lithium/sulfur cells, with a theoretical specific energy of 2680 Wh/kg, have emerged as a promising candidate for a new power source for electric vehicles and advanced electronic devices. 9–21 The very high theoretical specific capacity (1675 mAh/g) of sulfur electro- des, based on a two-electron reaction, is notably larger than that of current cathode materials. Sulfur cathodes, however, have shown a short cycle life and poor rate capability that has previously restricted further devel- opment of this attractive technology into a practical prod- uct. 22–26 When sulfur atoms electrochemically react with lithium ions to form Li 2 S, intermediate lithium polysulfide spe- cies are produced and they are readily soluble in most organic electrolytes. This indicates that active materials are lost from the positive electrode during cell operation, which is a major reason for the poor cycling performance. In addition to the loss of sulfur, the dissolved lithium polysulfides can diffuse to the September 2015 Vol. 61, No. 9