Rough electrodeposition and dendrite-induced short-circuits have hindered development of advanced energy storage technologies based on metallic lithium, sodium and aluminum electrodes. Electroconvection and associated over-limiting conductance plays an important role in producing rough, dendritic deposition of metals at planar electrodes. Solid polymer electrolytes have shown great potential to suppress lithium dendrite growth, but the dual challenges of maintaining good mechanical properties and high ionic conductivity at room temperature have hindered progress towards commercial systems. In this thesis, we designed a solid-state polymer electrolyte composed of cross-linked polymer networks containing dangling ionic liquid moieties. We show that a simple UV synthesis can be used to create free- standing membranes with controlled structure. The membranes preserve these traits when soaked with a liquid electrolyte, but also exhibit good ionic conductivity at room temperature. Application of the materials as separators in lithium metal batteries show that they are able to completely eliminate over-limiting conductance up to potentials as high as 5V, where the liquid electrolyte component itself becomes electrochemically unstable.