High capacity lithium (Li) ion batteries are highly desirable now for applications in consumer electronics, electrical vehicles, and grid storage. For these purposes, many high-capacity cathode (including Li−sulfur and Li−air) and anode materials (Li metal, silicon, tin, and metal oxide) have recently re-emerged. Among these materials, Li metal is the ideal anode material for rechargeable Li batteries due to its various advantages including low redox potential (−3.04 V vs standard hydrogen electrode), high specific capacity (3860 mAh g⁻¹), and low density (0.534 g cm⁻³). More importantly, the stable operation of Li metal is the enabling technology for Li−sulfur and Li−air batteries.

The earliest non-rechargeable Li batteries using Li metal as anode appeared in the 1970s. However, despite over 40 years of research, Li metal anodes are still not possible for practical application due to the uncontrollable growth of Li dendrites and low Coulombic efficiency (CE). Over the cycling process, the repeated stripping/plating of a lithium layer will form dendritic and mossy metal deposits, which will eventually penetrate through the separators and cause safety hazards. In addition, the growth of lithium dendrites associated with larger volumetric changes will cause cracks in the solid−electrolyte interphase (SEI) that lead to exposure of the fresh lithium metal to the electrolyte. Therefore, electrolyte will continue to decompose and result in low CE (for carbonate solvents CE is generally 80−90% and for ether solvents 90− 95%).

… Here, we describe a new strategy of in situ formation of interfacial coating on the lithium metal anode by employing viologen, a chemically reducible material into the electrolyte, to realize stable cycling of lithium metal anode.

—Wu et al.

Illustrations of the design principles of using MV to form a stable interfacial coating to allow the stable cycling of lithium metal. Wu et al. Click to enlarge.

Using this approach, they obtained a lifetime of 300 cycles with Columbic efficiency of 99.1% and 400 cycles with Columbic efficiency of 98.2% at a current density of 1 mA cm^–2 in ether-based electrolyte can be obtained. This lifetime is more than three times higher than the control ether electrolyte.

In addition, this approach can enhance the performances of lithium metal anode in a carbonate-based electrolyte. Compared with the previous approaches, the new strategy has many advantages such as low cost, easy manipulation, and compatibility with current lithium ion batteries, the research team said.

The methyl viologen molecule used by the researchers can be dissolved in the electrolytes in the charged states. Once the molecules meet the lithium metal, they are immediately reduced to form a stable coating on top of the metal electrode.

The UCR Office of Technology Commercialization has filed a patent application for the technology.

Resources

• Haiping Wu, Yue Cao, Linxiao Geng and Chao Wang (2017) “In Situ Formation of Stable Interfacial Coating for High Performance Lithium Metal Anodes” Chemistry of Materials doi: 10.1021/acs.chemmater.6b05475