Ionic Shield for Polysulfides Toward High-Stable Lithium Sulfur Battery

Lithium sulfur batteries attract great attention due to their high energy density, while the real applications are still hindered by the rapid capacity degradation. Despite great effort devoted to solving the polysulfide shuttle between the cathode and anode electrodes, it remains as a serious challenge to build high-stable lithium sulfur battery, which related to the diffusion and reaction between polysulfides and the metal lithium anode.

We demonstrate a strategy of introducing an ion selective membrane to improve the stability and coulombic efficiency of lithium sulfur battery. Generally, porous polymer membrane serves as membranes in batteries to avoid the short circuit between anode and cathode electrodes. As indicated in Figure 1a, such routine membrane cannot stop the shuttle effect. Herein, we replace the routine membrane (Figure 1b) with an ion selective membrane, in which the sulfonate-ended perfluoroalkyl ether groups on the ionic separators are connected by pores or channels that are around several nanometers in sizes. These -SO3^-- groups coated channels allow ion hopping of positive charge species (Li⁺) but reject hopping of negative ions, such as polysulfide anions (S_n^2-) in this specific case due to the coulombic interactions. Consequently, this cation permselective membrane acts as an electrostatic shield for polysulfide anions, and confines the polysulfides in the cathode side. An ultra-low decay rate of 0.08% per cycle is achieved within the initial 500 cycles for membrane developed in this work, which is less than half that of the routine membranes.

Such ion selective membrane is versatile for various electrodes and working conditions, which is promising for the construction of high performance batteries.

Reference

 [1] Huang JQ, Zhang Q, et al. Ionic shield for polysulfides toward high-stable lithium sulfur battery. Energy & Environ Sci. doi: 10.1039/c3ee42223b.

Figure 1. Schematic of the different lithium sulfur battery configurations. (a) Cell with routine membranes, in which polysulfides shuttle between cathode and anode side. (b) Cell with ion selective membranes, in which polysulfide anions are limited in the cathode side. (c) Enlarged schematic showing the cation permselective membrane allows the transport of lithium cations and blocks the polysulfide anions.