Microphase separated block copolymers consisting of an amorphous poly(ethylene oxide) (PEO)-based polymer covalently bound to a second polymer offer a highly attractive avenue to achieving both dimensional stability and high ionic conductivity in polymer electrolytes for solid-state rechargeable lithium batteries. However, due to the strong thermodynamic incompatibility typically found for most polymer pairs, the disordered, liquid state of the copolymer can rarely be achieved without the incorporation of a solvent, which complicates processing. Herein, we report the design of new block copolymer electrolytes based on poly(methyl methacrylate), PMMA, and poly(oligo oxyethylene methacrylate), POEM, which are segmentally mixed at elevated temperatures appropriate for melt processing, while exhibiting a microphase separated (ordered) morphology at ambient temperature. Although pure PMMA-b-POEM is segmentally mixed at all temperatures, it is shown that microphase separation in these materials can be induced in a controlled manner by the incorporation of even limited amounts of lithium trifluoromethane sulfonate (LiCF3SO3), a salt commonly employed to render PEO ionically conductive. Such "salt-induced'' microphase separation suggests a simple method for designing new solid polymer electrolytes combining high ionic conductivities with excellent dimensional stability and improved processing flexibility.