Cryogenic-TEM image of high-voltage cathode LNMO (LiNi0.5Mn1.5O4) particle cycled in carbonate baseline electrolyte. (Image credit: Minghao Zhang/Joanna Tsai.)
Cryogenic-TEM image of high-voltage cathode LNMO (LiNi0.5Mn1.5O4) particle cycled in carbonate baseline electrolyte. (Image credit: Minghao Zhang/Joanna Tsai.)

Conventional, carbonate-based electrolytes enable the flow of charge between the anode and cathode in lithium-ion batteries. But while this approach has been successful for the past three decades, conventional electrolytes are limited at higher voltages and temperatures. Carbonate-based electrolytes are also highly flammable and too intrinsically unstable to be used with more aggressive chemistries. Now, however, researchers from the US Army Research Laboratory, University of California, San Diego, and City University of New York have come up with a carbonate-free alternative electrolyte that is cheap, safe, and works better at high voltages and temperatures [Alvarado et al., Materials Today (2018),].

“To address the issue on a chemical level instead of using additives, we developed a new carbonate-free electrolyte system that exhibits superior cycling performance compared to the current state of the art,” explains Kang Xu, who led the effort.

Unlike carbonate electrolytes, which release carbon dioxide under high voltage, temperature, or acidic conditions, the new electrolyte based on a simple two-component system of a solvent, sulfolane, and a salt, lithium bis(fluorosulfonyl)imide (LiFSI), does not release gas even upon oxidation. The highly conductive lithium salt forms unique interphases on both positive graphitic anodes and high-voltage negative cathodes. At the anode, a LiF-rich interphase suppresses solvent co-intercalation and graphite exfoliation.

“Researchers have been aware of the attractive properties of sulfolane as an electrolyte solvent, such as excellent oxidative and high temperature stability, low cost, and high dielectric constant, for many years,” points out Xu.

But they have also been aware of its obvious disadvantage – its inability to function with graphite anodes. By using sulfolane in combination with LiFSI, however, Xu and his colleagues demonstrated that the electrolyte is stable with a graphitic anode and high-voltage cathode, even over many cycles of charging and discharging. Sulfolane, moreover, is cheap and readily available because it is used in the purification of natural gas and other petrochemicals. There are challenges ahead, nevertheless, admits Xu.

“The issues that need to be addressed are the electrolyte’s viscosity, poor wetting behavior, and low temperature performance,” he told Materials Today.

A combination of co-solvents and additives should be able to address these shortcomings successfully, he believes.

“The next major barrier is industrial scale-up of the salt production, which has already started and resulted in a significant reduction in the cost at the lab scale,” says Xu.

The researchers are now actively working on optimizing the new electrolyte with co-solvents and additives, while exploring how it could be used with lithium metal anodes, which is already showing promise.