Design of the lithium-metal battery, with an electrolyte containing a fluorinated cation (atomic structure at center). The ‘interface’ area represents the layer with fluorine that forms on the anode surface, as well as on the cathode surface. Image: Argonne National Laboratory.
Design of the lithium-metal battery, with an electrolyte containing a fluorinated cation (atomic structure at center). The ‘interface’ area represents the layer with fluorine that forms on the anode surface, as well as on the cathode surface. Image: Argonne National Laboratory.

Sodium fluoride, a compound of fluorine, is an ingredient in many toothpastes, added to protect teeth against decay. But compounds containing fluorine have other practical uses. Scientists at the US Department of Energy (DOE)’s Argonne National Laboratory have now developed a fluorine-based electrolyte that could protect a next-generation battery against performance decline.

“An exciting new generation of battery types for electric vehicles beyond lithium ion is on the horizon,” said Zhengcheng (John) Zhang, a group leader in Argonne’s Chemical Sciences and Engineering division. The scientists report this novel electrolyte in a paper in Nature Communications.

The chemistries of some next-generation batteries allow them to store more than twice as much energy for a given volume or weight compared to conventional lithium-ion batteries. These batteries could power cars for much longer distances and could even power long-haul trucks and aircraft one day. The expectation is that widespread use of such batteries will help address the problem of climate change. The main problem is that their high energy density declines rapidly with repeated charge and discharge.

One of the main contenders has an anode (negative electrode) made of lithium metal in place of the graphite normally used in lithium-ion batteries. It is thus called a ‘lithium-metal’ battery. The cathode (positive electrode) is a metal oxide that contains nickel, manganese and cobalt (NMC). While a lithium-metal battery can deliver more than double the energy density possible with a lithium-ion battery, its outstanding performance rapidly vanishes, within less than 100 charge-discharge cycles.

The team’s solution involved changing the electrolyte, a liquid through which lithium ions move between the cathode and anode to implement charge and discharge. In lithium-metal batteries, the electrolyte is a liquid consisting of a lithium-containing salt dissolved in a solvent. The short cycle-life problem is caused by this electrolyte not forming an adequate protective layer on the anode surface during the first few cycles. This layer, termed a solid-electrolyte-interphase (SEI), acts like a guardian, allowing lithium ions to pass freely in and out of the anode to charge and discharge the battery, respectively.

The team has now discovered a new fluoride solvent that can maintain a robust protective layer for hundreds of cycles in a lithium-metal battery. It couples a fluorinated component that is positively charged (cation) with a different fluorinated component that is negatively charged (anion). This combination is what scientists call an ionic liquid – a liquid consisting of positive and negative ions.

“The key difference in our new electrolyte is the substitution of fluorine for hydrogen atoms in the ring-like structure of the cation part of the ionic liquid,” Zhang said. “This made all the difference in maintaining high performance for hundreds of cycles in a test lithium-metal cell.”

To better understand the mechanism behind this difference at the atomic scale, the team drew upon the high-performance computing resources of the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science user facility.

As Zhang explained, simulations on the ALCF’s Theta supercomputer revealed that the fluorine cations stick to and accumulate on the anode and cathode surfaces before any charge-discharge cycling. Then, during the early stages of cycling, a resilient SEI layer forms that is superior to what has been possible with previous electrolytes.

High-resolution electron microscopy at Argonne and Pacific Northwest National Laboratory revealed that the highly protective SEI layer on the anode and cathode led to stable cycling.

The team was able to tune the proportion of fluoride solvent to lithium salt to create a layer with optimal properties, including just the right thickness. Because of this layer, lithium ions could efficiently flow in and out of the electrodes during charge and discharge for hundreds of cycles.

The team’s new electrolyte offers many other advantages as well. It is low cost because it can be made with an extremely high purity and yield in one simple step rather than multiple steps. It is environmentally friendly because it uses much less solvent, which is volatile and can release contaminants into the environment. And it is safer because it is not flammable.

“Lithium-metal batteries with our fluorinated cation electrolyte could considerably boost the electric vehicle industry,” Zhang said. “And the usefulness of this electrolyte undoubtedly extends to other types of advanced battery systems beyond lithium ion.”

This story is adapted from material from Argonne National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.