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 proposed 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, which cannot stop the shuttle effect. By the introduction of permselective functional layer, the polysulfide anions can be confined in the cathode side, which would favor the cyclic stability and lower self-discharge performance.

We replace the routine membrane 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.

We also proposed a unique lithium-sulfur battery configuration with ultrathin graphene oxide (GO) membrane for high stability. The oxygen electronegative atoms modified GO into a polar plane and the carboxyl groups acted as ion hopping sites of positively charged species (Li⁺) while rejected the transportation of negatively charged species (S_n^2-) due to the electrostatic interactions. Such electrostatic repulsion and physical inhibition largely decreased the transference of polysulfides across the GO membrane in lithium-sulfur system. By the incorporation of permselective GO membrane, cyclic capacity decay rate is also reduced from 0.49 to 0.23 %/cycle.

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

References:

Huang JQ, Zhang Q, Peng HJ, Liu XY, Qian WZ, Wei F, 2014. Ionic shield for polysulfides towards highly-stable lithium–sulfur batteries. Energy & Environmental Science2014, 7, 347.

Huang JQ, Zhuang TZ, Zhang Q, Peng HJ, Chen CM, Wei F. Permselective graphene oxide membrane for high-stable and anti-self-discharge lithium-sulfur batteries. ACS Nano 2015, 9, 3002.