A novel non-porous separator that possesses an outstanding lithium transference number (0.92) and effectively suppresses lithium dendrite formation upon lithium deposition is developed. The separator is made of triblock copolymer poly(styrene-block-isoprene-block-sulfonated isoprene) bearing pendant lithium sulfonates and, while taking up only 30 wt% liquid electrolyte (1 M Lithium hexafluorophosphate in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (v/v/v = 4/3/3)), it allows a high ion conductivity of 10−4 S cm−1. With good mechanical strength and processing feasibility, the new non-porous separator is shown to enable dendrite-free lithium electro-deposition and stripping with flat overpotential profiles upon prolonged galvanostatic cycling of the lithium symmetrical cells in the carbonate-based electrolyte.