Stanford University scientists have proposed a design strategy for electrolytes that enable anode-free Li metal batteries with single-solvent single-salt formations at standard concentrations. Based on the results reported in a paper in the journal Nature Energy, the design concept provides a promising path to high-energy, long-cycling Li metal batteries, the researchers said.

A lithium metal battery can hold about twice as much electricity per kilogram as today’s conventional lithium-ion battery. Lithium metal batteries do this by replacing the graphite anode with lithium metal, which can store significantly more energy.

Lithium metal batteries are very promising for electric vehicles, where weight and volume are a big concern. But during operation, the lithium metal anode reacts with the liquid electrolyte. This causes the growth of lithium microstructures called dendrites on the surface of the anode, which can cause the battery to catch fire and fail.

—co-author Zhenan Bao, the K.K. Lee Professor in the School of Engineering

Researchers have spent decades trying to address the dendrite problem.

The electrolyte has been the Achilles’ heel of lithium metal batteries. In our study, we use organic chemistry to rationally design and create new, stable electrolytes for these batteries.

—co-lead author Zhiao Yu

For the study, Yu and his colleagues explored whether they could address the stability issues with a common, commercially available liquid electrolyte.

We hypothesized that adding fluorine atoms onto the electrolyte molecule would make the liquid more stable. Fluorine is a widely used element in electrolytes for lithium batteries. We used its ability to attract electrons to create a new molecule that allows the lithium metal anode to function well in the electrolyte.

—Zhiao Yu

The result was a novel synthetic compound—fluorinated 1,4-dimethoxylbutane (FDMB)—that can be readily produced in bulk.

Design scheme and molecular structures of three liquids studied in this work: DME (a), DMB (b) and FDMB (c). Yu et al.

Our design enables the FDMB molecule to solvate Li⁺ ions with a unique Li–F interaction that is beneficial to both Li metal anode compatibility and high-voltage tolerance. Paired with 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in a single-salt, single-solvent formulation, this 1 M LiFSI/FDMB electrolyte not only endows Li metal with an ultrathin SEI (~6nm) observed by cryogenic transmission electron microscopy (cryo-TEM) and high CE (~99.52%) along with a fast activation process (CE > 99% within five cycles), but also achieves >6 V oxidative stability.

The limited-excess Li|NMC (lithium nickel manganese cobalt oxide) full cells retain 90% capacity after 420 cycles with an average CE of 99.98%. Industrial anode-free Cu|NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂) pouch cells achieve ~325 Wh kg⁻¹ single-cell energy density, while Cu|NMC532 (LiNi_0.5Mn_0.3Co_0.2O₂) ones show a record-high 80% capacity retention after 100 cycles.

Furthermore, the 1 M LiFSI/FDMB electrolyte is less flammable than commercial electrolytes and can be synthesized at large scale with low cost. Our electrolyte formulation satisfies the stringent requirements for a practical Li metal battery…

—Yu et al.

Battery500. The US Department of Energy (DOE) is funding a large research consortium called Battery500 to make lithium metal batteries viable, which would allow car manufacturers to build lighter electric vehicles that can drive much longer distances between charges. This study was supported in part by a grant from the consortium, which includes Stanford and SLAC.

By improving anodes, electrolytes and other components, Battery500 aims to nearly triple the amount of electricity that a lithium metal battery can deliver, from about 180 watt-hours per kilogram when the program started in 2016 to 500 watt-hours per kilogram.

The anode-free battery in our lab achieved about 325 watt-hours per kilogram specific energy, a respectable number. Our next step could be to work collaboratively with other researchers in Battery500 to build cells that approach the consortium’s goal of 500 watt-hours per kilogram.

—Yi Cui, co-corresponding author

In addition to longer cycle life and better stability, the FDMB electrolyte is also far less flammable than conventional electrolytes, as the researchers demonstrated.

This work was also supported by the Battery Materials Research Program in the DOE Office of Vehicular Technologies. The facility used at Stanford is supported by the National Science Foundation.

Resources

• Yu, Z., Wang, H., Kong, X. et al. (2020) “Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries.” Nat Energy doi: 10.1038/s41560-020-0634-5