Liquid flow batteries have potential to achieve high energy efficiency as a large-scale energy storage system. However, the ion exchange membranes (IEMs) currently used in flow batteries do not have...
Redox flow batteries (RFBs) have attracted extensive attention in recent years due to their low cost, safe operation, and design flexibility for large scale energy storage applications. Among various types of high-energy-density candidates, lithium-polysulfide (LiPS) and the NaPS equivalent RFBs are especially attractive due to their high energy density and natural abundance of sulfur, which can significantly decrease the capital cost of the RFB system to metrics suitable for deep market penetration. Owing to the lower cost of Na than Li, NaPS RFBs will be able to offer more advantageous for large grid-scale energy storage system applications. Although a molten electrode-based NaS RFB has been successfully developed to support stationary energy storage systems, its high operation temperatures (> 250 °C) and the use of ceramic ion conductors (e.g., beta-alumina solid electrolytes) cause safety and high-cost concerns. To overcome the limitations of high-temperature NaS battery, room temperature NaPS (RT-NaPS) batteries emerged as alternatives. However, the current status of RT-NaPS RFB have to overcome several issues associated with deleterious self-discharge and high rate of capacity-fade. These two macroscopic challenges are fundamentally underpinned by the crossover of polysulfide (PS), so-called shuttle effect, during cycling. Although ceramic conductors (e.g., NASICON) show promising ion selectivity, their fragile mechanical properties and high cost pose major drawbacks for broader application in RFBs. Celgard®, the most widely adopted porous membrane for the LIBs, are highly permeable to PS ions, thus it is not suitable as a separator for NaPS RFBs. The use of Nafion, an ion selective membrane (ISM), in NaPS RFB improved discharge capacity and cycle stability compared to Celgard (350 mAh/g vs. 200 mAh/g at 20 cycles). However, Nafion cannot achieve good performance in organic phase of Na-PS RFBs due to the persistently high crossover of redox couples caused by swelling of the perfluorinated membrane in organic electrolytes.Here we present a multifunctional electrochemical nanocomposite membrane (mECM) with excellent polysulfide blocking properties and chemical stability in organic electrolytes. A biphenyl backbone-based aromatic hydrocarbon membrane reinforced by porous carbon nanotubes layer and boron nitride nanotube layer shows high sodium conductivity as well as sodium selectivity over PS. Our mECM greatly reduce PS crossover which is lower than commercial Celgard 2325 by 4-order of magnitude, while areal resistance is only 3 times higher than Celgard 2325 (25.10 Ω cm2 vs. 7.6 Ω cm2). As a result, the performance of the Na-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. The capacity decay rate of the Na-PS RFB cell with the mECM is significantly lower than that of Celgard 2325, which is largely attributed to the excellent capability of mECM to suppress PS crossover. After 200 charge/discharge cycle, the ...
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.
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