High-Efficient Nanocomposite Ion Exchange Membranes for Lithium Polysulfide (Li-PS) Redox Flow Battery (RFB)
Authors: Wang Tongshuai , Fifield Natalie , Gao Yuechen , Wang Xiaofeng , Bae Chulsung , Kim Sangil 
Published: December 2020

Redox flow batteries (RFBs) have attracted extensive research interest in recent years due to their low cost, safe operation condition, design flexibility, and easy scalability for large scale energy storage applications. Among all the RFBs, the lithium polysulfide redox flow battery (Li-PS RFB) has been considered as one of the most promising new energy storage systems because of their high theoretical energy density (~2600 Wh/kg and 2199 Wh/L for elemental Li and S) and low material cost.
However, despite their great promise, the practical application of Li-PS RFBs has been hindered mainly by the dissolution and shuttling of intermediate polysulfides species that cause rapid capacity decay, low coulombic efficiency, and undesired electrode fouling. This shuttling effect is more detrimental in Li-PS RFBs which doesn’t have carbon matrix to immobilize the solid-state sulfur. To address these issues, an effective strategy is to employ a highly selective membrane separator which can suppress polysulfide crossover while allowing fast lithium-ion conductance. Unfortunately, commercial porous battery separators (e.g. Celgard) cannot be adopted due to their low rejection for polysulfide (PSn-) active species and fast capacity decay. Recent few research works revealed the feasibility of using ion exchange membranes (IEMs) as a barrier layer to selectively transport Li+ ions and block PSn- ions. However, most commercial IEMs are unstable in the organic polysulfide electrolyte because of its high swelling ratio which can cause the electrolytes crossover. Hence, there is a critical need to develop high-performance membrane materials for Li-PS RFB applications.
In this work, we will present our recent progress on developing high-performance nanoengineered IEM materials for the Li-PS RFBs which can greatly suppress the polysulfide shuttling while maintaining high Li+ conductivity and mechanical stability. Our biphenyl polymer membrane (BPSA) presents excellent stability for the DOL/DME organic electrolyte solution, exhibiting potential for Li-PS RFBs application. To further improve the selectivity of Li-ion over PS, we developed nanocomposite membranes consisting of carbon nanotubes (CNT), boron nitride nanotubes (BNNT), and BPSA. The CNT layer can act as a PS shielding layer as well as reducing the interfacial resistance of membranes, while the BNNT layer can facilitate heat dissipation and suppressing the lithium dendrite formation. Our composite membranes greatly reduce PS crossover which is lower than commercial Celgard 2325 by 4-order of magnitude (3.5×10-7 cm2/sec vs. 2.2×10-7 cm2/sec), while areal resistance is only 3 times higher than Celgard 2325 (15.12 Ω cm2 vs. 4.64 Ω cm2). As a result, the performance of the Li-PS RFB single cell with our composite membranes is superior to that of Celgard and other commercial IEMs (e.g. Nafion 215) under the same condition. At the 100th cycle, the LiPS RFB capacity with Celgard 2325 loses all initial capacity at 40 cycles, while our composite membrane keeps around 95% of the initial capacity after 100 cycles. More detailed descriptions of ion transport properties, electrochemical properties, and battery performance of the membrane separators will be presented.