The energy efficiency and cycle life of electrochemical cells with dissolved active materials are inextricably tied to the stability, conductivity, and transport selectivity of the cell's membrane. Membrane design rules have been lacking for such cells operating under harsh conditions, such as high alkalinity, due to the lack of selective, stable membranes. Here, we examined several classes of membranes for three aqueous Zn-based cell chemistries. In doing so, we uncovered a simple relationship between the membrane selectivity and the cell's cycle life, such that it is now possible to predict the lifetime of the cell on the basis of its membrane properties, thus avoiding time- or resource-intensive experimentation in large-format cells. Our work should greatly accelerate the identification of membranes for long-lasting, MW-scale redox-flow, and other low-cost grid batteries, which are required to last 10–20 years.