Lithium-sulfur (Li-S) batteries have garnered attention as a possible successor to the state-of-the-art Li-ion batteries owing to its high theoretical specific energy and the abundance of sulfur. The stated promises have been hampered by inherent problems in the system such as poor reversibility of sulfur cathode, polysulfide shuttle effect, and safety issues associated with using Li metal anode.¹Our work seeks to primarily explore the possibility of utilizing a graphitic anode that successfully intercalates and de-intercalates Li ions in an electrolyte using only dimethoxyethane (DME) as the solvent which can also work with polysulfide cathode, enabling Li metal-free rechargeable Li-S batteries.

Experimental

The anode was prepared by slurry casting MCMB graphite into carbon paper. MCMB particles are fully embedded in the carbon fiber matrix of the carbon paper. Here, the graphitic carbon paper functions as an anode as well as a current collector. 0.25 M Li₂S₆ was prepared in methanol by stirring in appropriate quantities of lithium sulfide (Li₂S) and sulfur (S₈) powder in an Argon-filled glove box. This polysulfide solution was added into multi-walled carbon nanotube (MWCNT) paper and dried for 24 h to remove the methanol. Thus, the polysulfide in a binder-free MWCNT paper functioned as the cathode. The electrolyte consists of high concentration of 3 M lithium bis(fluorosulfonyl)imide (LiFSI) and 1 M lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) salts in DME solvent.

Results and Discussion

In comparison with the common electrolyte used in Li-S batteries containing mixture solvent of DME and 1,3-dioxolane (DOL), the DME-based electrolyte supports higher reversible capacity of the graphitic anode as shown in Fig. 1a. It also shows comparable performance as the conventional carbonate-based electrolyte (1 M LiPF₆ in EC/DEC). This electrolyte enables reversible Li-ion intercalation and de-intercalation into graphite without solvent co-intercalation. The high salt concentration in DME is also beneficial for suppressing the polysulfide shuttle without any additives rendering it appropriate for use in Li-S systems.²

A binder-free polysulfide cathode is coupled with pre-lithiated graphite to obtain a full cell. The voltage profile of the full cell shows the typical voltage profile for Li-S batteries with the voltage gradually lowered as the cell is cycled as shown in Fig. 1b. The cell exhibits a high initial capacity of over 1,400 mAh g^-1 and a reversible capacity of nearly 900 mAh g^-1based on sulfur in the cathode at C/10 after 50 cycles while maintaining an efficiency of over 97% as shown in Fig. 1c. The cell also shows excellent rate performance in Fig. 1d, demonstrating good kinetics even at high currents.

In summary, a DME-based electrolyte with the capability to enable reversible intercalation of Li ions into graphitic carbon has been developed. This anode-electrolyte system can be coupled with a polysulfide cathode to develop a metal-free Li-S battery. The cell exhibits excellent capacity retention and high rate performance. This work provides insights on further developing Li-S systems with non Li metal anode, which could lead to the successful implementation of Li-S batteries for practical application.

References

1. A. Manthiram, Y.-Z. Fu, S.-H. Chung, C. Zu and Y.-S. Su, Chem. Rev. 114, 11751 (2014).

2. L. Suo, Y.-S. Hu, H. Li, M. Armand and L. Chen, Nat. Commun. 4, 1481 (2013).