Lithium- and Sodium-Sulfur Cells with High Sulfur Loading
Authors: Manthiram Arumugam 
Published: February 2020

Development of advanced energy-storage systems needs to consider a balance among cost, cycle life, safety, energy, power, and environmental benignity. With these requirements, lithium-sulfur (Li-S) and sodium-sulfur (Na-S) batteries are promising candidates as next-generation energy-storage systems because of the high charge-storage capacity, natural abundance, and environmental friendliness of sulfur. However, the practical utility of Li-S and Na-S cells is hampered by the low electrochemical utilization of sulfur and severe polysulfide diffusion from the cathode to the anode. These drawbacks result in low discharge capacity and poor cycle life.

The Li-S and Na-S battery chemistries have several scientific challenges in common. The degradation of both the anode and cathode active materials, as well as the decomposition of the liquid electrolyte, causes a fast capacity fade and low electrochemical utilization of the active material. Unfortunately, the efforts to overcome the persistent problems often result in the incorporation of excessive conductive carbon into the electrode and low sulfur loading per unit area. In fact, it is often easy in the literature to obtain greatly improved performance by using either low sulfur content, low sulfur loading, or low sulfur mass in a cathode. The use of low-sulfur-loading cathodes can defeat the energy-density advantage of sulfur cathodes and the purpose of Li-S and Na-S cells replacing the current lithium-ion technology. Moreover, the traditional cathode configuration borrowed from the commercial insertion-compound cathodes may not allow the pure sulfur cathode to put its unique materials chemistry to good use due to the very different battery chemistries between the solid insertion-compound oxide cathodes and the electrochemical conversion-reaction sulfur cathodes. Recognizing the importance of operating the Li-S and Na-S cells with high-sulfur-loading cathodes for being competitive with the existing lithium-ion technology, this presentation will focus on unique approaches in engineering the sulfur cathodes with high-sulfur loadings and employing solid electrolytes. The cathode engineering presented can lead the way towards employing high-loading cathodes with promising cell performance while the easily-prepared pure sulfur powders are utilized as the active material.

The first high-capacity cathode engineering involves a layer-by-layer strategy. This method confines sulfur powders between porous carbon nanofiber (PCN) multi-layers. The layer-by-layer cathode facilitates fast ion and electron transport and traps the soluble polysulfide intermediates within the multilayered electrode configuration. A single-sulfur-layer cathode coupled with two-PCN layers offers a high discharge capacity of 1265 mAh g-1 at C/5 rate with good cyclability. The cathode engineering further facilitates high areal sulfur loading by increasing the number of sulfur layers. For example, a six-sulfur-layer cathode attains a high areal sulfur loading of 11.4 mg cm-2. The high-sulfur-loading cathode delivers a high initial discharge capacity of 995 mA h g-1 at C/10 rate with a corresponding areal capacity of 11.3 mA h cm-2.

The second high-capacity cathode engineering involves an edge-encapsulation method. An omurice-type cathode consists of a light-weight multiwall carbon nanotube (MWCNT) thin-film as the omelette cover and the active-material filling as the fried rice. The active-material paste is first dropped onto an aluminum-foil current collector and then covered by the MWCNT thin-film, followed by pressing the edge of the electrode for the favorable edge-encapsulation of the sulfur core. As a result, the polysulfide migration is stabilized within the structural cathode due to the strong tortuosity of MWCNT layer and the edge-encapsulated cathode configuration, benefiting the capacity retention. The omurice-type cathodes exhibit a high discharge capacity (1190 mAh g-1), a long lifespan (500 cycles), and low capacity fade (0.08 % cycle-1). In addition, the structural cathodes with a good balance among high sulfur loading (4.2 – 10.0 mg cm-2), high sulfur content (50 – 60 wt. %), and high sulfur mass (4.2 – 10.0 mg cathode-1) are able to attain a high areal capacity (5.1 – 7.8 mAh cm-2).

In addition, polysulfide-trapping interlayers, multi-functional separators, and solid electrolyte membranes developed in our group are integrated with the above advanced structural cathodes for suppressing polysulfide migration and alkali-metal dendrite growth. The integrated cells with high active-material loading exhibit good cyclability.