Electrode Design for Lithium-Sulfur Batteries Featuring High Sulfur Loading and Low Electrolyte
Authors: Chung Sheng-Heng 
Published: December 2020

The limitations encountered by insertion-compound cathodes for offering lithium batteries with an enhanced energy density at an affordable cost allow the conversion-reaction cathodes to garner significant attention. With no restrictions in maintaining their initial structure during discharge and charge, a high-capacity sulfur cathode in the lithium-sulfur battery enables a complete two-electron conversion reaction per atom and therefore provides an order of magnitude higher theoretical capacity (1,672 mA h g-1) than the currently used insertion-compound cathodes. Additionally, sulfur is inexpensive and environmentally benign. However, as a promising post lithium-ion battery technology, lithium-sulfur battery cathodes suffer from the insulating nature of sulfur and the irreversible migration of polysulfides within the cells. These challenges restrict the development of a lithium-sulfur battery cathode featuring high electrochemical efficiency and stability with a sufficient sulfur loading and content of above 4.0 mg cm-2 and 65 wt.%, respectively, in a cell with a reasonable low electrolyte-to-sulfur ratio of less than 10.0 uL mg-1.
Here, we present innovations on electrode design enabling lithium-sulfur batteries to operate excellently with a high amount of sulfur (i.e., 14.4 mg cm-2 and 70 wt.%) and a low electrolyte-to-sulfur ratio of 4.0 uL mg-1. The cathode substrate is prepared by carbonized electrospinning nanofibers having 20 wt.%, 40 wt.%, and 100wt.% polyacrylonitrile mixing with Nafion as the sacrificial polymer. With a high content of the sacrificial Nafion, the carbonized nanofibers (e.g., 020CNF resulted from 20 wt.% polyacrylonitrile and 80 wt.% sacrificial polymer) generate a higher specific surface area of up to 640 m2 g-1 with 150 m2 g-1 contributed from micropores. Although a high surface area and micropores of the electrode substrate benefit the polysulfide retention, our experimental and analytical results prove that the majority of the electrolyte added in the cell is initially absorbed by this highly porous cathode substrate. This explains why a high amount of electrolyte is needed in the lithium-sulfur research, which causes a low energy density. This also expounds why our high-loading cathodes with a low amount of electrolyte either dry out in a short cycle life nor cannot get sufficient charges transferred via the electrolyte, resulting in a poor cell performance. On the other hand, the carbonized electrospinning nanofiber with no sacrificial polymer (i.e., 100CNF) possesses a relatively low specific surface area of 130 m2 g-1 and no micropores. Benefiting from a unique electrode-design criterion, the limited surface area ameliorates the fast consumption of electrolyte, while still blocking the fast polysulfide diffusion.
As a result of the electrode design, the cells with the 100CNF substrate output a stable discharge capacity and Coulombic efficiency of above 650 mA h g-1 and 98%, respectively, for 100 cycles. Such excellent electrochemical efficiency and stability demonstrate a high capacity retention of 98% with a high areal capacity and energy density of 10 mA h cm-2 and 20 mW h cm-2, respectively, in a cell with a low electrolyte-to-sulfur ratio of only 4 uL mg-1. In conclusion, the electrochemical enhancements and engineering designs of the cathode substrates with limited nanopores make them advanced cathode designs for the development of high-loading/content sulfur cathodes in high-energy-density lithium-sulfur batteries.