More than forty years after the first report of a rechargeable lithium battery, electrochemical cells that utilize metallic lithium anodes are again under active study for their potential to provide more energy dense storage in batteries. Electrolytes based on small-molecule ethers and their polymeric counterparts are known to form stable interfaces with alkali metal electrodes and for this reason are among the most promising choices for rechargeable lithium batteries. Uncontrolled anionic polymerization of the electrolyte at the low anode potentials and oxidative degradation at the working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. In this work, we show that cationic chain transfer agents in an ether electrolyte provide a fundamental strategy for limiting polymer growth at the anode, enabling long term (at least 2000) cycles of high-efficiency operation of asymmetric lithium cells. Building on these ideas, we also report that cathode electrolyte interphases composed of anionic polymers and the superstructures they form spontaneously at high electrode potentials provide as fundamental a strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits. Through computational chemistry, we discuss the mechanistic processes responsible for the extended high voltage stability and on this basis report Li||NCM cells based on a simple diglyme electrolyte that offer unprecedented stability in extended galvanostatic cycling studies.