A clay-based compound invented at Rice University is an electrolyte and a separator for lithium-ion batteries for use in high-temperature environments. Photo: Jeff Fitlow/Rice University.
A clay-based compound invented at Rice University is an electrolyte and a separator for lithium-ion batteries for use in high-temperature environments. Photo: Jeff Fitlow/Rice University.

A unique combination of materials developed at Rice University, including a clay-based electrolyte, can produce a lithium-ion battery that is robust enough to supply stable electrochemical power at temperatures up to 120°C (248°F). Such batteries could find use in space, defense, and oil and gas applications, among others.

Chemist Pulickel Ajayan and his colleagues at Rice University and at Wayne State University in Detroit describe the material in a paper in ACS Applied Materials and Interfaces.

This discovery, like earlier work on supercapacitors by the lab, depends on the malleable qualities of bentonite clay and room-temperature ionic liquids, which serve as both a separator and an electrolyte system and provide a conductive path between the battery's anode and cathode.

"Clay naturally has a lot of moisture in it, and that's not a problem when you're doing supercapacitors," said Kaushik Kalaga, a graduate student in Ajayan's lab and lead author of the new study. "But a battery has to have a lithium-ion conductive species in the electrolyte to conduct lithium ions from the cathode or anode, or vice versa, when you charge and discharge. Lithium is very reactive with water, so our first challenge was to eliminate water from the clay while keeping its structure intact."

Kalaga and his team started by baking commercial clay particles at 650°C for an hour to dry them out. They then combined a room-temperature ionic liquid with a lithium salt and mixed them into the clay in an oxygen-free glove box. The liquefied salt acts as the source of lithium ions that conduct through the clay electrolyte to the electrodes.

The researchers spread the resulting peanut butter-like slurry between lithium metal electrodes and encapsulated them in coin-shaped batteries for testing at various temperatures. Conventional organic electrolytes cannot be used in batteries over 60°C, due to their low boiling temperature; the vapors that form beyond 80°C can lead to an explosion, Kalaga said. Batteries with solid-state electrolytes can work at high temperatures, but the electrolytes don't connect as well with the electrodes, hampering performance.

The researchers designed their composite electrolyte to be tough and conductive while still presenting the maximum surface area to the electrodes, thereby providing a solid path for current. The resultant batteries proved able to deliver current at high temperatures with a stable voltage window, meaning the range between which the electrolyte is stable and not chemically degraded, of 3V over 120 charge-discharge cycles. Furthermore, they combined the thermal stability of solid-state electrolytes with the wetting properties of liquid electrolytes, ensuring good contact with the electrodes.

"It's able to produce pretty good performance at room temperature, but it gets better at higher temperatures," Kalaga said. "The clay-based electrolyte gets less viscous but still retains its consistency at least to 150°C. The next step is to push the limits further."

According to the researchers, the nature of the material makes it suitable for forming into many types of batteries, from thin films to commercial-scale units. "There are many applications that need energy storage devices to work in extreme environments, and there needs to be innovation in the materials systems, particularly electrolytes, to expand the window of operation conditions," Ajayan said. "Our lab is at the forefront of discoveries in this area."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.