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U Maryland and US Army Research Lab furthering “water-in-salt” electrolyte Li-ion battery; targeting EV use

University of Maryland (UMD) and US Army Research Lab (ARL) researchers are spearheading a public-private sector collaboration to further develop a lithium-ion battery that would be safer to operate and less costly to dispose of than those currently available on the market. Their approach involves using a high concentration of salt in aqueous lithium batteries to boost energy density and viability. (Earlier post.) The UMD-ARL team won the Invention of Year of UMD in 2016 for the technology.

This water-in-salt electrolyte lithium battery recently achieved an energy density of 200 Wh/kg, said Dr. Chunsheng Wang at UMD, adding that the DOE had sought 150 Wh/kg, or double that of a traditional aqueous Li-ion battery. “The researchers in the field and program managers alike are very excited,” Dr. Kang Xu at ARL said, adding the next target is 300 Wh/kg.

Historically, lithium batteries with aqueous electrolytes have been limited in their energy density. The UMD-ARL project began as a research collaboration between UMD and the Electrochemistry Branch, Sensor and Electron Devices Directorate of the Power and Energy Division of the US Army Research Laboratory in nearby Adelphi, Maryland.

Lithium-ion batteries raise safety, environmental, and cost concerns, which mostly arise from their nonaqueous electrolytes. The use of aqueous alternatives is limited by their narrow electrochemical stability window (1.23 volts), which sets an intrinsic limit on the practical voltage and energy output. We report a highly concentrated aqueous electrolyte whose window was expanded to ~3.0 volts with the formation of an electrode-electrolyte interphase. A full lithium-ion battery of 2.3 volts using such an aqueous electrolyte was demonstrated to cycle up to 1000 times, with nearly 100% coulombic efficiency at both low (0.15 coulomb) and high (4.5 coulombs) discharge and charge rates.

—Suo et al.

Prof. Wang and Dr. Liumin Suo at UMD and Drs. Xu, Oleg Borodin and Arthur Von Cresce at ARL achieved a higher energy density by forming a solid-electrolyte interphase (SEI) with a salt with a high solubility in water—specifically lithium bis(trifluoromethane sulfonyl)imide (LiTFSI). The establishment of a SEI during initial charging expands the electrochemical stability window of aqueous electrolytes to 3.0 V.

The water-in-salt electrolyte idea was derived from the high concentration salt use in organic electrolytes for traditional lithium batteries. Lithium batteries with organic electrolytes already use a “very high concentration salt to stabilize the anode and the cathode, but nobody tried this idea in the water. We are the first to borrow this idea from organic electrolytes but apply it in the hydrous lithium battery, Wang said.”

Their initial findings, published in Science last November (Suo et al.), won the backing of the Department of Energy for a second phase of research with matching funding from UMD and private companies.

The second phase is anticipated to start soon with a total of $4 million in funding for three years. The research teams are providing $1 million in matching funds and requested $3 million from DOE. This second phase will comprise teams from UMD and the Army Research Lab as well as Saft and Liox (earlier post).

One of the main objectives of the second phase of the project is to lower the cost of the electrolytes with alternative salts or mixed salts. The goals are also to enhance the energy density and cycle life of the batteries, and bring out a commercial demonstration cell.

Liox and Saft will cooperate on commercializing high energy density aqueous batteries for use in electric vehicles, according to the proposal for phase 2.

One goal is to deliver a 500 Wh battery storage unit to DOE for evaluation. Another application in mind is an aqueous Li-ion batteries to be delivered to the Army’s Tank Automotive Research Development and Engineering Center (TARDEC).

A provisional patent for Aqueous and Aqueous/Non-Aqueous Hybrid Electrolytes with Wide Electrochemical Stability Windows was filed with the US Patent Office in 2015.

Suo
The performance of aqueous Li-ion chemistries benefits from the expanded electrochemical stability window of aqueous electrolytes. (A) Performance of aqueous Li-ion batteries based on various electrochemical couples. Color code for cycling stability: red, 1000 cycles. (B) Illustration of expanded electrochemical stability window for water-in-salt electrolytes together with the modulated redox couples of LiMn2O4 cathode and Mo6S8 anode caused by high salt concentration. Suo et al. Click to enlarge.

Resources

  • Liumin Suo, Oleg Borodin, Tao Gao, Marco Olguin, Janet Ho, Xiulin Fan, Chao Luo, Chunsheng Wang, Kang Xu (2015) ““Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries” Science  Vol. 350, Issue 6263, pp. 938-943 doi: 10.1126/science.aab1595

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