When Li-ion batteries and supercapacitors quickly charge and discharge, the internal temperature of the devices can spike and possibly cause them to overheat and catch fire. Typically, electronic circuits protect batteries by stopping current flow to the battery when the temperature is too high. However, temperature-sensitive internal components could also be designed to shut down the devices.

A new polymer electrolyte solution that becomes a gel over a wide range of temperatures has been used to almost completely shut down a coin cell supercapacitor. This is a crucial mechanism to prevent thermal runaway in the devices.

Temperature-sensitive polymer electrolytes for energy storage devices become a gel when the temperature increases. The polymer gel traps ions in the electrolyte, preventing current from flowing between the electrodes and shutting down a supercapacitor. Polymers that reversibly transition between a gel and liquid state, such as a previous smart electrolyte made from poly(N-isopropylacrylamide-co-acrylamide) (PNIPAM/AM), could enable an overheated battery to restart itself when the temperature cools.

Guihua Yu, at The University of Texas at Austin, and his colleagues wanted to use a reversible temperature-sensitive polymer electrolyte that transitioned to a gel over a wider temperature range, and was easier to access, than PNIPAM. As reported in a recent issue of Advanced Materials, they started with an aqueous solution of Pluronic, a commercially available polymer of polypropylene oxide(PPO) sandwiched between two blocks of polyethylene oxide(PEO). When liquid, this polymer forms micelles with hydrophobic PPO on the inside and chains of PEO on the outside. As the temperature increases, the PEO chains stretch, become tangled, and form a gel. As the temperature drops, the chains shrink and polymer becomes a liquid again.

By changing the concentration or the average molecular weight of the polymer in the solution, the researchers tuned the temperature at which the polymer became a gel from 20°C to 90°C. The previous PNIPAM polymer formed a gel between 40–50°C (read the abstract in Advanced Materials).

Next, Yu and colleagues built a coin cell supercapacitor using an aqueous electrolyte containing Pluronic with an average molecular weight of about 4400 Da. At 60°C, the cell lost almost all of the capacitance it had at 20°C. The Pluronic electrolyte is also compatible with activated carbon or polypyrrole electrodes, as well as hydrogen and lithium ions commonly found in electrolytes in lithium-ion batteries.

Xiaodong Chen, at Nanyang Technological University, thought it was novel that the gel transition of this polymer could reduce the capacity of a device by almost 100 percent. This polymer could be used in commercial electrolyte solutions, although a version soluble in organic solvents needs to be developed for use in batteries, he said.

The researchers are now working on an organic-soluble polymer electrolyte that could be used in Li-ion batteries, Ye Shi, lead author of the paper, said.

Read the abstract in Advanced Materials.