High Efficient Ion Selective Membrane Separator for Room Temperature Sodium Polysulfide Redox Flow Batteries

Abstract

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 Na-PS cell with Celgard 2325 lost 70% of its initial capacity (initial 1000 mAh/g to 300 mAh/g); in contrast, the Na-PS cell with the mECM achieved 81% capacity retention (initial 900 mAh/g to 750 mAh/g), demonstrating the possibility of our membrane to use in long-term operation of RT-NaPS RFB (Fig. a). Our results show that the performance of our RT-NaPS cell (Fig. b) outperforms other RT-NaS cells reported so far. More detailed descriptions of ion transport properties, electrochemical properties, and battery performance of the membrane separators will be presented. Figure 1