The invention provides a method of improving the cycle-life of a rechargeable alkali metal-sulfur cell. The method comprises implementing an anode-protecting layer between an anode active material layer and a porous separator/electrolyte, and/or implementing a cathode-protecting layer between a cathode active material and the porous separator/electrolyte, wherein the anode-protecting layer or cathode-protecting layer comprises a conductive sulfonated elastomer composite having from 0.01% to 50% by weight of a conductive reinforcement material dispersed in a sulfonated elastomeric matrix material and the protecting layer has a thickness from 1 nm to 100 μm, a fully recoverable tensile strain from 2% to 500%, a lithium ion conductivity from 10−7 S/cm to 5×10−2 S/cm, and an electrical conductivity from 10−7 S/cm to 100 S/cm when measured at room temperature. This battery exhibits an excellent combination of high sulfur content, high sulfur utilization efficiency, high energy density, and long cycle life.