Rechargeable lithium (Li) metal batteries have received increasing attentions in recent years because of their high theoretical energy densities from the contribution of Li metal anode that has an ultrahigh theoretical specific capacity (3860 mAh g^-1), the lowest redox potential (-3.040 V vs. the standard hydrogen electrode) and a low gravimetric density (0.534 g cm^-3). However, significant challenges including Li dendrite growth and low Coulombic efficiency still impede the successful deployment of rechargeable Li metal batteries. A lot of efforts have been made in the past 45 years to address these problems, including the application of polymer electrolytes, solid-state electrolytes, ionic liquids, highly concentrated liquid electrolytes, additives, protecting layers, interlayers between Li and separator, nanoscale design, selective deposition, Li-reduced graphene oxide composites, and so on. Although some progresses have been achieved the challenges still remain.

It has been reported that ether-based electrolytes are relatively stable with Li metal anode and can allow Li metal anodes to be cycled at reasonably high charging current densities, but these electrolytes are not stable at voltages above 4 V vs. Li/Li⁺. The conventional Li-ion battery electrolytes based on LiPF₆/carbonate-solvents are stable at least up to 4.3 V but they suffer from the low Coulombic efficiency and poor charging current densities due to the reactivity of Li metal with LiPF₆ salt and carbonate solvents.

In the talk, we present the development of electrolytes based on LiTFSI-LiBOB dual-salt in carbonate solvent mixtures for rechargeable Li metal batteries with conventional Li-ion battery cathodes. It has been found that the dual-salt electrolytes greatly enhance the performances of Li metal batteries in terms of long-term cycling stability and fast charging capability over the conventional LiPF₆/carbonate electrolytes. The dual-salt electrolytes also protect Li metal anodes by suppressing the corrosions of Li metal caused by the reactions of Li metal and electrolytes. Certain additives at a small amount level can even further significantly protect the Li metal anode and improve the battery performances. The possible mechanisms have been proposed based on the comprehensive characterizations. The details of the results will be reported at the meeting.

Acknowledgements

The research has been supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program and the Battery500 Consortium. The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.