Functional, Water-Soluble Binders for Improved Capacity, Rate Capability and Stability of Lithium-Sulfur Batteries

In recent years, great strides forward have been made in increasing the practical capacity and cycle life of the lithium-sulfur system. Many of the strategies currently pursued involve optimisation of the conductive host structure, in order to maximise sulfur utilisation; coupled with some form of encapsulation of the active material, to mitigate capacity fade due to the well-explored reactions of polysulfide intermediates at the anode[1,2].

Recently, our group has investigated the use of functional polymers as binders in this system, particularly poly(ethylene oxide) (PEO)[3] and mixtures of PEO and poly(vinylpyrrolidone) (PVP)[4]. PVP has recently been similarly investigated elsewhere[5]. We have identified that a binder of 4:1 PEO:PVP can significantly improve the capacity, rate capability and stability of Li-S cells compared to conventional alternatives, e.g., PVdF. We have used galvanostatic cycling and impedance spectroscopy (IS) to determine that these improvements are most likely due to the influence of the polymer functionality on the chemistry of polysulfides in the electrolyte near the electrode surface.

These commodity polymers are inexpensive, water soluble, environmentally friendly and widely available, making them particularly interesting for mass production of Li-S cells. Through this simple approach we can demonstrate reversible capacities in excess of 1000 mAh g^-1 for cathodes with total sulfur content of at least 58%.

Figure: comparison of different water-soluble binders: left) cycling of simple Li-S cells based on Super P host with 50% total sulfur loading at C/5; right) first discharge of Ketjen Black-based composites with 58% sulfur loading at C/50.

References:

[1] X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500.

[2] X. L. Li, Y. L. Cao, W. Qi, L. V Saraf, J. Xiao, Z. M. Nie, J. Mietek, J. G. Zhang, B. Schwenzer, J. Liu, J. Mater. Chem. 2011, 21, 16603.

[3] M. J. Lacey, F. Jeschull, K. Edström, D. Brandell, Chem. Commun. 2013, 49, 8531.

[4] M. J. Lacey, F. Jeschull, K. Edström, D. Brandell, submitted.

[5] Z. W. Seh, Q. Zhang, W. Li, G. Zheng, H. Yao, Y. Cui, Chem. Sci. 2013, 4, 3673.