The explosion and fire hazards associated with thermal runaway in lithium-ion (Li-ion) cells require monitoring internal temperature changes in seconds in order to ensure battery safety. Among the various techniques for internal temperature cell determination, only impedance-based temperature monitors have the characteristics necessary for rapid response. Here we discuss a novel method for prediction and prevention of thermal runaway in Li-ion batteries based on rapid (on the order of seconds) monitoring of cell impedance. The electrical impedance - a complex quantity - is represented by two real numbers, e.g., impedance amplitude and phase shift (phi). The phase shift phi at a specified frequency has a higher measurement resolution and is much less dependent on the size of Li-ion cells (Ah capacity). We demonstrate in this work emergence of characteristic changes in phi before and during a rapidly evolving thermal runaway event. We highlight the capability to predict and possibly prevent thermal runaway in a Li-ion cell by incorporating a phase-shift monitor in a battery management system.