Chemists Boost Grid-Scale Energy Storage

Researchers at the Department of Energy's Oak Ridge National Laboratory have advanced the knowledge required to improve large-scale energy storage. In doing so, they have revealed distinct chemical reactions that improve the stability and efficiency of a promising energy-storage system. Their findings suggest positive implications for U.S. energy security, including increased national power grid reliability and affordability.

Energy sources continue to proliferate, contributing more power to the grid every year. An increasing challenge is how to store this energy at large scales for later use. Meeting these needs at grid scale will require the combination of reliable, affordable, accessible materials in a scalable platform that can be easily manufactured and maintained.

This study - part of ORNL's ongoing effort to advance new energy technologies designed to strengthen the grid while unleashing affordable power - will also help drive U.S. energy independence and fuel economic growth.

"We need to build grid-scale energy storage that is reliable and robust," said ORNL's Guang Yang, who led a study published in ACS Energy Letters with ORNL's Wenda Wu. "This is especially important for addressing power shortages during extreme weather events, such as hurricanes, floods and winter storms. The goal is to create a stronger grid that can restore power across communities as soon as possible after natural disasters."

Limitations of lithium-ion batteries for grid-scale storage

Although lithium-ion batteries are what many of us think of when we consider energy storage, they are not capable of this task on a much larger scale. Rather than liquid solutions, they rely on solid materials. They dominate the market for phones, electronics and electric vehicles, but cost and safety concerns limit their role in energy storage, particularly at larger scales.

Enter electrolytes - liquids that allow ions to flow from a battery's positive electrode to its negative electrode. Ions carry electrical charges in "flow batteries," which charge and discharge throughout a battery's lifecycle. A key factor in improving cycling stability - a battery's ability to retain a high percentage of its original charge throughout many charge-and-discharge cycles - is electrolyte chemistry.

"In the lithium-ion battery, a 'central sandwich' contains a central, power-generating station, with electrodes, a separator and solid materials that can store and release the energy," Wu said. "In flow batteries, however, liquids instead of solids power the system."

The liquid electrolytes contain specific materials - in this study, sulfur - which dissolve in certain solvents, with supporting salts, to provide electrical conductivity. Electrolytes are contained in tanks outside the power-generating central sandwich.

"The unique architecture of the flow battery isolates the tank from the central sandwich, which enables us to tune one parameter without affecting another," Wu said.

In this study, enhanced electrolyte solutions were prepared with various glymes, or liquid-ether solvents. Glymes belong to a versatile class of organic compounds known for high stability, low volatility and the ability to dissolve a range of substances, especially salts.

Yang, Wu and colleagues are studying different glymes to determine which can best improve the performance of flow batteries over hundreds of charge/discharge cycles. Solvation effects, or the impacts of each solvent on the properties and behaviors of the molecules, help reveal the feasibility of various glyme configurations.

The glymes in this study contained varying numbers of oxygen atoms. According to Yang and Wu's findings, the size of glyme molecules directly affects the likelihood that sulfur materials will penetrate the separator in the central sandwich. This penetration causes batteries to lose stored energy, as the barrier between positive and negative sides is lost.

Comparing three different glyme solvents, the researchers found that the glyme solvent with the greatest number of oxygen atoms most effectively reduced the undesirable penetration by sulfur materials.

The team also found that the glyme solvents participate in the ionic movement inside batteries. A better-designed solvent might control penetration by sulfur materials and speed the movement of electric charge carriers within the system.

Yang and Wu have demonstrated that some electrolyte flow batteries function well with the help of inexpensive sodium metal and plentiful sulfur materials. Together, they present an increasingly attractive energy storage alternative to today's flow-battery models, which drive massive electrical grids and power plants but rely on vanadium, a rare metallic element.

"Vanadium is on the U.S. government's list of critical minerals," Yang said. "It's expensive, and DOE has set a cost-related goal. By 2035, we must reduce the cost of electricity to 5 cents per kilowatt-hour, per cycle. Vanadium is somewhere around 18 cents per kilowatt-hour, per cycle. Obviously, by using only the traditional, already commercialized vanadium-based flow battery system, we can never reach that goal."

We need to build grid-scale energy storage that is reliable and robust. This is especially important for addressing power shortages during extreme weather events, such as hurricanes, floods and winter storms. The goal is to create a stronger grid that can restore power across communities as soon as possible after natural disasters.