(181q) Electrolyte Chemistries with Responsive Polymers for Thermal Safety in Li-Ion Batteries

In recent years, remarkable progress in the development of Li-ion battery technology has led to devices that deliver records in power and energy density. Thermal instabilities, however, creates a major roadblock to large-format implementation for applications in transportation and intermittent energy storage. Efforts to mitigate safety issues pertaining to flammability, reactivity, and thermal runaway focus on either low conductivity solid-state materials or irreversible and destructive safety devices, which prove problematic for high-power, large-format systems. In this presentation, we will discuss a unique approach to thermal stability that uses an solution-based electrolyte mixture with a polymer that phase separates from solution above a target temperature. The phase separation leads to a gel-like polymer coating on the electrode, which increases the internal resistance and prevents the flow of current. This approach is advantageous over existing methods because the phase separation is a localized process that occurs where hot spots develop to prevent catastrophic events at the source of the failure. Some chemistries may exhibit reversibility such that battery operation can be stored.

The concept of thermally reversible electrolyte behavior was initially reported using aqueous systems, where a commonly known responsive polymer, poly(N-isopropylacrylamide), was functionalized with ionic groups to provide ionic conductivity. At low temperatures, the polymer dissolved in solution and ions could transfer through the solution. At higher temperatures, the polymer precipitated from the water, thus removing the ions from solution and significantly reducing the conductivity. Next, two different responsive electrolyte systems were described in ionic liquids (ILs): poly(ethylene oxide) (PEO) and poly(benzyl methacrylate) (PBMA). Both systems exhibited Li-ion concentration dependent phase behavior, where ion concentration affects the temperature at which the phase transition occurs and the reversibility of the transition. While the PEO-IL system exhibits a change in solution conductivity and charge transfer resistance above the transition temperature, the PBMA-IL system only exhibits an increase in charge transfer resistance, but to a greater extent. Solution and ion transport resistances are measured using electrochemical impedance spectroscopy (EIS) to correlate resistance with temperature and to identify various resistances. More recently and more relevant to conventional Li-ion batteries, we demonstrated that a copolymer, poly (2-chloroethyl vinyl ether-alt-maleic anhydride) (PCVEMA), exhibits a temperature-activated phase transition in carbonate-based solvents; and therefore, can be utilized to inhibit Li-ion migration/intercalation at the electrode/electrolyte interface. A large voltage drop and capacity decrease were observed above 80 °C due to interfacial hindrances imposed by the precipitation of PCVEMA. The influence of temperature on the polymer and solution properties will be discussed for the various systems, along with how these physical changes affect electrochemical processes.