Nano-Membranes for Lithium/Sulfur Batteries

Membranes are of interest for a wide variety of applications such as gas separation, water desalination, fuel cells and batteries.^1-6 Because the flux across a membrane scales inversely with the membrane thickness, a thick membrane is required to achieve high selectivity, at the expense of permeability. New ultrathin membranes which promise to offer high selectivity and high permeability are highly desirable. Elemental sulfur can provide five times higher capacity than existing Li-ion cathode materials based on transition metal oxides or phosphates. Various problems have hindered the practical use of the highly attractive sulfur cathode such as the facile diffusion of polysulfides from cathode to anode which results in an irreversible loss in capacity.⁷ An ultrathin ion exchange nano-membrane would impede the polysulfide diffusion while maintaining a low overall resistance.

Here we show the proof of concept for a 5 nm membrane self-assembled from commercial polymers to encapsulate a carbon/sulfur composite and maintain the performance even at discharge rates five times more rapid than its non-encapsulated counterpart. The cycling stability of a three layer encapsulating membrane is shown in Fig. 1a, along with the non-encapsulated sulfur composite. At a discharge rate of 1C and consistent with previously reported mesoporous carbon supports, the non-encapsulated cathode cannot constrain the soluble polysulfides more than 200 cycles. The encapsulating nano-membrane can constrain these polysulfides more efficiently and allows the encapsulated cathode to be cycled 300 times with a slight capacity fade. Surprisingly, the ultrathin membrane does not impede higher discharge rates presumably due to its thickness only being 5 nm; the cycling performance of the encapsulated cathode at a 5C discharge/charge rate mirrors the performance of the non-encapsulated cathode at a substantially lower rate of 1C (Fig. 1b).

Figure 1.  a. The cycling stability of encapsulated (blue circles) and non-encapsulated (black circles) cathodes at a 1C charge/discharge rate b. The cycling stability of non-encapsulated (black circles at a 1C discharge rate) and encapsulated cathodes (red circles at a 5C discharge rate).

References

1. Bucur, C. et al., Energy Environ. Sci. 6, 3286-3290 (2013).
2. Tsuchiya, M. et al., Nat. Nanotech. 6, 282-286 (2011).
3. Peng, X. et al., Nat. Nanotech. 4, 353-357 (2009).
4. Bouchet, R. et al., Nat. Mater. (2013). doi: 10.1038/nmat3602.
5. Hassoun, J. & Scrosati, B., Angew. Chem. Int. Ed. 49, 2371–2374 (2010).
6. Carta, M. et al., Science 339, 303-307 (2013).
7. Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J.-M., Nat. Mater. 11, 172–172 (2012).