Lithium-ion (Li-ion) batteries power much of our digital and mobile lifestyle. However, their adoption in more strategically important applications such as vehicle electrification and grid storage has been slower, mainly because of concerns raised over their safety, cost, and environmental impact. Most of these concerns come from the nonaqueous electrolytes needed to withstand the high voltages (>3.0 V) of the chemistries, because the ester-based solvents are highly flammable and reactive with the charged electrodes, and the lithium salt (LiPF₆) is thermally unstable and extremely toxic. Substantial costs are incurred not only directly by these electrolyte components but also to a larger degree by the stringent moisture-free process and safety management required for the dangerous combination of flammable electrolytes and energy-intensive electrodes.

Aqueous electrolytes could resolve these concerns, but their electrochemical stability window (1.23 V) is too narrow to support most of the electrochemical couples used in Li-ion batteries. Hydrogen evolution at the anode presents the most severe challenge, as it occurs at a potential (2.21 to 3.04 V versus Li, depending on pH value) far above where most Li-ion battery anode materials operate. Even in trace amounts, hydrogen severely deteriorates the electrode structure during cycling. A common practice to suppress hydrogen evolution in aqueous electrochemistry is to adjust the alkalinity, so that the water reduction potential shifts downward to allow the use of anode materials otherwise prohibited under neutral or acidic conditions. However, as the overall electrochemical stability window of aqueous electrolytes remains constant, anodic stability against oxygen evolution suffers a corresponding compromise …

In contrast to nonaqueous electrolyte systems, where cathode and anode materials often operate far beyond the thermodynamic stability limits of electrolyte components, kinetic protection from a solid-electrolyte interphase (SEI) in aqueous media has never been considered possible. Such interphases, situating between electrode surfaces and electrolyte, are formed by sacrificial electrolyte decomposition during the initial charging, and they constitute a barrier allowing ionic conduction but forbidding electronic conduction. Their presence substantially expands the usable electrochemical stability window of electrolytes. In conventional aqueous electrolytes, a protective interphase is absent because none of the decomposition products from water (H₂, O₂, or OH^–) can deposit in a dense solid state. In the absence of interphases, aqueous Li-ion batteries are typically limited to low voltage (<1.5 V) and low energy density (<70 Wh/kg), often with rapid fading of capacity and low coulombic efficiency. … We report the formation of such interphases in an aqueous electrolyte by manipulating the source of electrolyte decomposition during the initial charging processes.

—Suo et al.

The team formulated a “water-in-salt” electrolyte by dissolving lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) at extremely high concentrations (molality >20 m) in water. This led to an anion-containing Li⁺ solvation sheath, which results in the formation of a dense solid interphase on the anode surface, mainly arising from anion reduction.

When combined with the substantially reduced electrochemical activity of water at such a high concentration, the water-in-salt electrolyte provided an expanded electrochemical stability window of ~3.0 V.

The team constructed a full aqueous Li-ion battery using a model electrochemical couple (LiMn₂O₄ and Mo₆S₈) and demonstrated an open circuit voltage (OCV) of 2.3 V. The cell was was cycled at nearly 100% coulombic efficiency for up to 1000 cycles at both low (0.15 C) and high (4.5 C) rates.

Through this work we were able to increase the electrochemical window of aqueous electrolyte from less than 1.5 Volts to ~ 3.0 Volts and demonstrated high voltage aqueous full Lithium-ion cell with 2.3 Volts, showing for the first time that aqueous batteries could seriously compete in terms of power and energy density with the non-aqueous lithium-ion batteries that power our mobile, digital lifestyle.

—Chunsheng Wang, UMD

According to Lt. Col. (Retired) Edward Shaffer, who heads the Army Research Laboratory’s Energy and Power Division, the significant potential advantages this new approach has over current batteries “could lead to thermally, chemically and environmentally safer batteries carried and worn by soldiers; safe, reduced-footprint energy storage for confined spaces, particularly submarines; and novel hybrid power solutions for military platforms and systems.”

What’s most important about our work is the breakthrough made at the fundamental level. Prior to this work no one thought it possible to form SEI in water-based [batteries], but we demonstrated that it can happen.

—UMD Postdoctoral Research Associate Liumin Suo, first author

The UMD & ARL team compared the performance of their new “Water-in-Salt” battery with that of other aqueous battery systems. They showed that high stability of other aqueous batteries was achieved only at the expense of voltage and energy density and vice versa. However, the formation of the anode/electrolyte interphase in their “Water-in-Salt” electrolyte allowed them to break this inverse relationship between cycling stability and high voltage and to achieve both simultaneously.

(A) Performance of aqueous Li-ion batteries based on various electrochemical couples. Color code for cycling stability: red, <100 cycles; blue, 100 to 200 cycles; green, >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. Credit: Suo et al. Click to enlarge.

Researchers in the Li-ion battery field have recently found that previously ‘useless’ solvents could be made functional in Li-ion cells through the addition of high concentrations of salts. The work by Suo et al., extends this idea to the case of the solvent, water. By extending the operational voltage window to approximately 3 Volts, it is possible that a new generation of safer and possibly less expensive Li-ion cells could result. Only further R&D efforts will be able to verify the practicality of this discovery, so prudence is needed in assessing the potential of this, or any basic research advance.

—Dalhousie University Professor Jeff Dahn (not involved in the study)

This research received funding from the Department of Energy, Advanced Research Projects Agency-Energy (ARPA-E) (DEAR0000389) and support from the Maryland NanoCenter and its Nanoscale Imaging Spectroscopy & Properties Laboratory. This UMD lab is supported in part by the National Science Foundation. Modeling efforts were supported by the ARL Enterprise for Multiscale Research of Materials.

Resources

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