As we approach the limits of lithium-ion batteries, new alternative chemistries are required^[1]. Those that involve the sulfur conversion reaction at the positive electrode are among the most promising to move beyond Li-ion^[2]. The conversion reaction of sulfur to form lithium sulfide is characterized by a high theoretical specific capacity of 1672mAh g^-1, translating in a theoretical energy density of c.a. 2500 Wh kg^-1, a value 2.5 times higher than that of commercial state-of-the-art Li-ion cells, using transition metal oxide based cathodes^[3].

There are well-documented challenges associated with the traditional Li-S system which have been demonstrated to restrict the practical energy density and the cycle life^[4]. Soluble polysulfides are generally formed, resulting in the shuttling mechanism, a major issue causing low coulombic efficiency, high self-discharge rates and negative electrode corrosion^[4]. This reaction is also associated with large volume changes. This can lead to cell failure, due to the alteration of the cathode and anode structure and the favoured formation of lithium dendrites^[5].

Strategies to enhance cycle life, while maximizing practical energy density, require a holistic approach. This includes optimization of the positive electrode (e.g., selection of conversion reaction catalyst, use of polysulfide adsorbents, binder selection); the negative electrode (e.g., lithium coating, functionalization and protection); the separator (e.g. functional barriers and polymeric membranes); the electrolyte (e.g. use of alternative solvents and salts, additives).

This work will focus on the recent negative electrode advances at OXIS Energy Ltd, the pioneer in the research and development of large-scale, high-energy, longer cycle life lithium-sulfur batteries.

[1] D. A. J. R. Ronald M. Dell, Lithium Batteries, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2013.

[2] B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci. 2011, 4, 3287.

[3] M. S. Whittingham, Proc. IEEE 2012, 100, 1518.

[4] A. Manthiram, Y. Fu, S. H. Chung, C. Zu, Y. S. Su, Chem. Rev. 2014, 114, 11751.

[5] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Adv. Mater. 2017, 29, 1601759.