Liquefied Gas Electrolytes Allow Lithium Batteries to Operate at Very Low Temperatures

It is well known that prevalent lithium-ion batteries do not operate at temperatures of -20 °C and lower. At present, the Engineers of University of California San Diego have made an advancement in the field of electrolyte chemistry for enabling lithium batteries to operate at lower temperatures of -60 °C with exceptional performance.

The innovative electrolytes also allow electrochemical capacitors to operate at temperatures of -80 °C, which at present operate at low temperatures of -40 °C. Apart from ensuring operation at very low temperatures, the technology also maintains greater performance at room temperature. The new electrolyte chemistry can enhance not only the energy density but also the safety of electrochemical capacitors and lithium batteries.

The research was published online in the Science journal on 15^th June 2017.

The technology will enable electric vehicles in cold countries to cover greater distances on a single charge, thus eliminating range anxiety in winter months in cities such as Boston. The technology can also be applied to power crafts such as satellites, high atmosphere WiFi drones, interplanetary rovers, weather balloons and other aerospace applications under severe cold conditions.

The electrochemical capacitors and batteries created by the research team are specifically cold hardy as the electrolytes in them are formed of liquefied gas solvents (i.e. gases liquefied under moderate pressures) that are more resistant to freezing when compared to standard liquid electrolytes. Liquefied fluoromethane gas was used to synthesize electrolyte for the lithium battery. Liquefied difluoromethane gas was used to synthesize electrolyte for the electrochemical capacitor.

Deep de-carbonization hinges on the breakthroughs in energy storage technologies. Better batteries are needed to make electric cars with improved performance-to-cost ratios. And once the temperature range for batteries, ultra-capacitors and their hybrids is widened, these electrochemical energy storage technologies can be adopted in many more emerging markets. This work shows a promising pathway and I think the success of this unconventional approach can inspire more scientists and researchers to explore the unknown territories in this research area.

Shirley Meng, Senior Author and Nanoengineering Professor, Jacobs School of Engineering, UC San Diego.

Meng also heads the Laboratory for Energy Storage and Conversion and is the director of the Sustainable Power and Energy Center, both located at UC San Diego.

“It is generally agreed upon that the electrolyte is the primary bottleneck to improve performance for next generation energy storage devices,” stated Cyrus Rustomji, and first Author of the study and a Postdoctoral Researcher in Meng’s group. “Liquid-based electrolytes have been thoroughly researched and many are now turning their focus to solid state electrolytes. We have taken the opposite, albeit risky, approach and explored the use of gas based electrolytes.”

The Researchers from UC San Diego are the pioneers in analyzing gas-based electrolytes for use in electrochemical energy storage devices. The futuristic application of this technology might be to power spacecraft for interplanetary exploration.

Mars rovers have a low temperature specification that most existing batteries cannot meet. Our new battery technology can meet these specs without adding expensive and heavy heating elements.

Cyrus Rustomji, first Author of the study and Postdoctoral Researcher in Meng’s group

During the research, the Researchers found out that gases possess a characteristic ‒ namely, low viscosity. Enabling them to operate effectively at temperatures in which traditional liquid electrolytes get frozen, “Low viscosity leads to high ion mobility, which means high conductivity for the battery or capacitor, even in the extreme cold,” explained Rustomji.

Although the Researchers analyzed a wide array of prospective gas samples, they were interested in two particular new electrolytes: one made of liquefied difluoromethane, used for electrochemical capacitors and the other made of liquefied fluoromethane, used for lithium batteries.

Apart from the excellent performance at low temperature, the new electrolytes are highly safe to use. They eliminate the difficulty of thermal runaway, that is, a point at which the battery gets heated to a temperature that leads to a hazardous chain of chemical reactions that causes further heating of the battery. The new electrolytes restrict the ability of the battery to self-heat at temperatures considerably greater than ambient temperature because at higher temperatures, the ability of the electrolytes to dissolve salts is lost, resulting in the loss of conductivity of the battery and ultimately failure of the battery.

This is a natural shutdown mechanism that prevents the battery from overheating. As soon as the battery gets too hot, it shuts down. But as it cools back down, it starts working again. That’s uncommon in conventional batteries.

Cyrus Rustomji, first Author of the study and Postdoctoral Researcher in Meng’s group

Rustomji further added that during more extreme situations, for example, an automobile accident, when the battery is damaged and gets shorted, the electrolyte gas escapes from the cell and ‒ as there is no electrolyte conductivity ‒ avoids the thermal runaway reaction which cannot be avoided when traditional liquid electrolytes are used.

Compatible electrolyte for lithium metal anodes

Meng, Rustomji, and their collaborators have come very close to achieving another long-time ambition of becoming battery researchers: synthesizing an electrolyte that operates well with the lithium metal anode. Lithium is perceived to be the best anode material due to its light weight and its ability to store more charge than prevalent anodes. However, one specific difficulty is that lithium reacts with traditional liquid electrolytes, resulting in the low Coulombic efficiency of the lithium metal, that is, it can go through only a lesser number of charge and discharge cycles before the operation of the battery stops.

Another difficulty encountered when using traditional liquid electrolytes with the lithium metal anode is that after repeated charge and discharge cycles, lithium can get accumulated at specific places on the electrode. Consequently, needle-like structures, or dendrites, are formed and can puncture portions of the battery, leading to short-circuit.

Applying high mechanical pressure on the electrode, using electrolytes with low viscosity, and using the so-called fluorinated electrolyte additives to produce an optimal chemical composition on the surface of the lithium metal electrode are the techniques employed earlier to overcome these difficulties. The innovative liquefied gas electrolytes synthesized by the UC San Diego Researchers integrate all the significant characteristics mentioned above into a single electrolyte system. The ensuing interphase formed on the electrode is an exceptionally uniform and dendrite-free surface that ensures enhanced battery conductivity and a high Coulombic efficiency of more than 97%. The Researchers have demonstrated for the first time that an electrolyte can exhibit high performance on lithium metal as well as classical cathode materials, thus considerably increasing the overall energy density of batteries.

Next steps

In the future, the goal of the research team is to enhance the cyclability and energy density of electrochemical capacitors as well as batteries and to ensure operations at even lower temperatures of less than -100 °C. This research can open the door for developing innovative technology to power spacecraft used to investigate outer planets (e.g. Jupiter and Saturn).

Rustomji is the head of a UC San Diego-based team of Researchers working to commercialize the technology through a startup called South 8 Technologies.