Low-cost And Durable Vanadium Flow Batteries With Non-fluorine Enhanced Membrane And Asymmetric Electrolyte

Vanadium flow batteries(VFB)have become one of the most promising energy storage technologies due to high safety,deep charge/discharge capability and flexible design.At present,the problems in actual production and application of VFB are mainly concentrated on two aspects.As a core material,the cost of ion exchange membranes(IEMs)is unaffordbale,which is not suitable for industrial promotion.Besides,rapid capacity decay caused by comprehensive reasons such as penetration of vanadium ion and electrolyte imbalance is another obstacle in VFB applications.To date,perfluorosulfonic Nafion series membranes are the most widely used IEM in the VFB.However,their high price and fast vanadium ion penetration are major problems when used in VFB system.Non-fluorine sulfonic acid membranes,such as sulfonated poly(ether ether ketone)(SPEEK),is widely used in VFB systems due to its advantages of low cost,ease synthesis,and low vanadium permeability.Nevertheless,the SPEEK membranes are poor in chemical stability and mechanical properties.It easily suffers from bulk swelling,micro-dissolution,and even macro-rupture during VFB cycling.Therefore,two kinds of SPEEK reinforced membranes have been designed to improve their durability:(a)the hydrogen bonds formed between the hydroxyl groups in the cellulose of paper and the sulfonic acid groups in SPEEK greatly enhance the physicochemical properties of SPEEK.By testing the physical and chemical properties of three kinds of paper(copy paper,filter paper,and rice paper)reinforced membranes,we optimized the rice paper reinforced SPEEK membrane(S@RP)for further study.The results show that the heat-crosslinked S@RP membrane(S@RP-C6)significantly improves the long-term cycling performance.(b)fabricating a reinforced sulfonated poly(ether ether ketone)(SPEEK)membrane using a lithium-ion battery separator,ceramic-coated porous polyethylene(CCP),as a robust scaffold.The ceramic reinforced SPEEK membrane(S@CCP)is prepared by solution casting method,S@CCP significantly improves the stability and durability of SPEEK in VFB.In addition,the S@CCP membranes exhibits stable performance over 1500 cycles and high resistant to temperature fluctuations over a wide range of-20~60 °C.Both of the IEMs are durable,wide-temperature adaptable and affordable,which are ideal membrans for VFB application.Rapid capacity decay is one of the most important reasons that restrict the development of VFB.Herein,the concept of an asymmetric vanadium flow battery(aVFB)is introduced,in which the asymmetric design of a catholyte and an anolyte is used to suppress the capacity decay of the VFB.Based on the comprehensive analysis of the capacity decay and electrolyte imbalance process of the traditional symmetric VFB,it was found that the capacity fading is mainly owing to the loss of the anolyte in the longterm cycling test.Therefore,this work attempts to use excess anolyte(i.e.10%,20% and 30%)to mitigate the capacity decay during the long-term operation of the VFB.To gain deeper insights into the capacity retention mechanism of these novel anolyte overhang aVFBs,long-term cycle performance of the corresponding symmetric overhang VFBs and catholyte overhang aVFBs is investigated for comparison.The optimal excess ratio of anolyte and how to add the excess anolyte are also suggested for future study.We believe that both the enhanced non-fluorine sulfonic acid membranes and the asymmetric design would further promote the application of VFB.