(70j) Ionic Liquids: Platforms for Thermally-Responsive Polymer Electrolytes

Authors: 
Kelly, J. C., Clemson University
Roberts, M. E., Clemson University

In recent years, remarkable progress in the development of batteries, supercapacitors and Li-ion technology has led to devices that deliver new records in power and energy density. The thermal instability in batteries, particularly lithium ion, creates a major roadblock to implementing these devices into large-format systems to support emerging, intermittent energy sources such as wind and solar. Efforts to mitigate safety issues such as flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible safety devices, which prove problematic for efficient, large-scale energy systems. Before high performance devices can be applied to large format energy storage applications, thermal safety must be improved.

“Smart materials” or stimuli-responsive materials are those with properties responding in a desired manner to an external stimulus or environmental change. While several classes of responsive materials exist, soft materials, particularly synthetic polymers, represent an opportunity to design systems with unique functionality to achieve a wide variety of chemical and physical properties. Here we present a system utilizing thermally-responsive polymers capable of reducing lithium conductivity or lithium charge transfer ability in non-aqueous electrolytes with increasing temperature. These proposed systems would create self-limiting lithium reactions within a battery should the temperature increase beyond a target, thus inhibiting thermal runaway.

 Li-ion systems utilizing polymer gel electrolytes are commercially available and ionic liquids (ILs) have gained attention as potential replacements for traditional electrolytes (low volatility, non-flammability, high conductivity, and a wide potential window). Poly(ethylene oxide) (PEO) gels have long been combined with lithium salts to create polymeric electrolytes, which while safe, possess diminished conductivity. These PEO systems have recently been combined with ILs to boost performance while retaining the safety and mechanical stability of conventional polymer gel electrolytes.  In our proposed system, we combine current research efforts utilizing polymer gel electrolytes and ILs, both of which provide improved thermal stability, while making use of the recently discovered lower critical solution temperature (LCST) between PEO and certain ILs, to modulate electrolyte conductivity with temperature [1,2]. We design the polymer electrolyte to decrease conductivity during a phase transition at elevated temperatures. As the temperature cools, the initial conductivity is reversibly restored as the electrolyte returns to its initial state. Additionally, we show how the PEO/IL composition and the addition of lithium salts affect both the magnitude of change in conductivity and the LCST in these thermally-responsive polymer electrolyte systems.

 We then demonstrate how these thermally-responsive polymer electrolytes can be used to control electrochemical properties in both model supercapacitor systems, consisting of activated and mesoporous carbon supercapacitors, and lithium ion battery systems, consisting of graphitic anodes and lithium cobalt oxide (LiCoO2) cathodes.  Through the thermal phase transition above the normal operation temperatures of these electrochemical devices, we demonstrate how PEO/IL systems can be utilized to reduce the conductivity in electrolyte systems and reduce both the double layer charging ability in supercapacitor and redox charge transfer ability in lithium ion batteries (through a decrease in power and energy density).

 The experiments performed to date show tremendous promise for controlling battery and supercapacitor operation using temperature. Further development of responsive electrolytes will lead to numerous opportunities associated with thermal safety and “smart” electrochemical systems that will ultimately lead to large-format battery applications will built-in thermal control that is both reversible and non-destructive. 

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