A team of scientists from Rice University may have solved a problem encountered while creating rechargeable lithium-ion (Li-ion) batteries to be used in harsh environments—using clay.
The team discovered that the Li-ion chemistry-based battery it created was strong enough to supply stable electrochemical power in temperatures up to 248° Fahrenheit (120°C).
This discovery hinged on the flexible qualities of bentonite clay and room-temperature ionic liquids that serve as both a separator and an electrolyte system and provide a conductive path between a 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,” says Kaushik Kalaga, a graduate student 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,” he adds.
Kalaga and his team started by baking commercial clay particles at 1200°F (649°C) for an hour to dry them out. Then they combined a room-temperature ionic liquid and lithium salt and mixed them into the clay in an oxygen-free glove box. The liquefied salt acts as a source of lithium ions that conduct through the electrolyte to the electrodes.
The researchers spread the resulting product, which had a bit of peanut butter-like texture, between lithium metal electrodes and encapsulated them in coin-shaped batteries for testing at various temperatures.
Typically, organic electrolytes cannot be used in batteries over 140°F (60°C) because they have a low boiling temperature and the vapors that form beyond 176°F (80°C) can result in an explosion, according to the team.
There are existing batteries that have solid-state electrolytes for work in high temperatures, but the electrolytes do not connect as well with electrodes, and this hinders performance.
The researchers built their composite electrolyte to be tough and conductive and still present the maximum surface area to electrodes to provide a solid path for current.
The units were able to deliver current at high temperatures with a stable voltage window of 3 volts over 120 charge-discharge cycles and featured both the thermal stability of solid-state electrolytes and the wetting properties of liquid electrolytes, which indicated there was good contact between the electrodes. The voltage window is the range between which the electrolyte is stable and is not chemically degraded.
The clay-based batteries with electrolyte/separators were tested at up to 250°F (121°C) and showed strong performance over 120 charge-discharge cycles, according to scientists.
The battery actually got better in higher temperatures. These batteries could find a place in space, defense and oil and gas applications.
“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,” says Pulickel Ajayan, co-author of the research. “Our lab is at the forefront of discoveries in this area.”