High Areal Capacity Sulfur Cathodes for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have practical energy densities of >500 Wh/kg, which are three times higher than the current Li-ion technology based on lithium metal oxide cathode and graphite anode (~150-200 Wh/kg). As such, they have the potential as a next generation energy storage for electric vehicles, portable devices and grid storage. Moreover, the abundance of sulfur and its high theoretical capacity of 1672 mAh/g suggest that costs of <$200 kWh can be met to significantly increase the rate of electric vehicle adoption.  

However, to realize the potential of Li-S batteries, the utilization of active material at cell level must be significantly improved. Today, most of the high specific capacity (900-1100 mAh/g) and/or long cycle life data in the literature have been recorded with low sulfur loading (~0.5-2 mg/cm²), translating to an areal capacity of <2 mAh/cm² (Figure 1a). Another often-overlooked factor is the electrolyte-to-sulfur (E/S in ml/g) ratio. Usually, a high E/S ratio of 10-60 is used to compensate for the depletion of electrochemically active Li-ions and electrolyte solvent during Li stripping and plating due to the formation of solid-electrolyte-interface (SEI). The excessive passive weight accompanying high E/S ratio significantly reduces the energy density of the cell. Ideally, the E/S ratio of <3 is desired¹.

Analogous to techniques that were developed for thick transitional metal oxide electrodes by our group², electrodes with low tortuosity pathways have been developed. Whereas a 250 µm thick sulfur/carbon nanotube (CNT) electrodes with density ~1.2 g/cm³ used as a reference electrode gives an initial capacity of ~300 mAh/g at C-rate of 1/30 h^-1 and a corresponding current density of 0.67 mA/cm² (Figure 1b), tuning of porosity and surface chemistry through a grafting technique produces in the same thickness and sulfur density an electrode with threefold higher specific capacity (867 mAh/g) and a high areal capacity of 10.5 mAh/cm² (Figure 1c). A Coulombic efficiency of >93% was achieved for the developed high-areal-capacity Li-S battery, which retained about half of the initial capacity after 50 cycles at a C-rate of 1/15 h^-1. Importantly, these results were achieved at a relatively low E/S ratio of 3.

Acknowledgment

This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. X. Chen acknowledges the financial support from the Agency for Science, Technology and Research (A*STAR), Singapore.

Reference

1         M. Hagen, P. Fanz and J. Tübke, J. Power Sources, 2014, 264, 30–34.

2         C.-J. Bae, C. K. Erdonmez, J. W. Halloran and Y.-M. Chiang, Adv. Mater., 2013, 25, 1254–8.

Figure 1: (a) The dependency of energy density of a Li-S battery on the areal capacity of the electrode. The calculated energy densities are on based on a cell level with a structure of “current collector-cathode-electrolyte-separator-protective layer-anode-current collector”. The pink area highlights the current status of Li-S and the red dot marks the highest achieved areal capacity in this work. An inset shows SEM micrograph of the cathode composite with a thickness of 250 µm. The S loading is ~12.1 mg/cm². (b) Galvanostatic charge-discharge profiles of the Li-S showing the 2^nd cycle of Li-S batteries comparing between our cathode vs a S/CNT cathode (electrode thickness of 250 µm) at a current density of 0.67 mA/cm² (theoretical C-rate of 1/30 h^-1).  (c) Comparison of the energy density and areal capacity for different electrode thickness to highlight the improvement of our Li-S batteries over conventional Li-S batteries.