OAK RIDGE, TN—Engineers at Oak Ridge National Laboratory (ORNL) have developed a thin, flexible, solid-state electrolyte that may double energy storage for next-generation electric vehicles and portable electronic devices. The durable sheets of solid-state electrolytes could enable scalable production of batteries with higher energy density electrodes.

By separating negative and positive electrodes, they would prevent dangerous electrical shorts while providing high-conduction paths for ion movement. This could lead to greater safety, performance and energy density compared to today’s batteries that use liquid electrolytes, which are flammable, chemically reactive, thermally unstable and prone to leakage.

“Our achievement could at least double energy storage to 500 watt-hours per kilogram,” claims Guang Yang, Ph.D., a chemical engineer at ORNL working on the project. “The major motivation to develop solid-state electrolyte membranes that are 30 micrometers or thinner was to pack more energy into lithium-ion batteries so electric vehicles, laptops and cell phones can run much longer before needing to recharge.”

According to Yang, the goal of the R&D project is to find the “Goldilocks” spot—a film thickness just right for supporting both ion conduction and structural strength.

Current solid-state electrolytes use a plastic polymer that conducts ions, but their conductivity is much lower than that of liquid electrolytes. Sometimes, polymer electrolytes incorporate liquid electrolytes to improve performance.

Sulfide solid-state electrolyte has ionic conductivity comparable to that of the liquid electrolyte currently used in lithium-ion batteries.

“It’s very appealing,” says Yang. “The sulfide compounds create a conducting path that allows lithium to move back and forth during the charge-discharge process.”

Yang and his colleagues discovered that the polymer binder’s molecular weight is crucial for creating durable solid-state-electrolyte films. Films made with lightweight binders, which have shorter polymer chains, lack the strength to stay in contact with the electrolytic material.

However, films made with heavier binders, which have longer polymer chains, have greater structural integrity. Additionally, it takes less long-chain binder to make a good ion-conducting film.

“We want to minimize the polymer binder, because it does not conduct ions,” explains Yang. “The binder’s only function is to lock the electrolyte particles into the film. Using more binder improves the film’s quality, but reduces ion conduction. Conversely, using less binder enhances ion conduction but compromises film quality.”