Redox flow secondary battery and electrolyte membrane for redox flow secondary battery

An electrolyte membrane for a redox flow secondary battery, comprising

• an ion-exchange resin composition comprising
• a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1):
• —[CF2—CX1X2]a—[CF2—CF((—O—CF2—CF(CF2X3))b—Oc—(CFR1)d—(CFR2)e—(CF2)f—X4)]g— (1)
• wherein
• X1, X2, and X3 each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms;
• X4 represents SO3Z, wherein Z represents a hydrogen atom;
• R1 and R2 each independently represents one or more selected from the group consisting of halogen atoms, perfluoroalkyl groups having 1 to 10 carbon atoms, and fluorochloroalkyl groups having 1 to 10 carbon atoms;
• and
• a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1,
• b represents an integer of 0 to 8,
• c represents 0 or 1, and
• d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time);
• and
• wherein
• the fluorine-based polyelectrolyte polymer has a terminal group of a main molecular chain of CF3;
• in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in a solution is 0.03% or smaller of a whole amount of fluorine in an immersed polymer.