(Nanowerk News) The ingredient in many toothpastes is sodium fluoride, a compound of fluorine. It is added to protect teeth from decay. But fluorine-containing compounds have other practical uses that may surprise you. Scientists at the US Department of Energy’s (DOE) Argonne National Laboratory have discovered a fluoride electrolyte that could protect the next generation of batteries from degrading performance.
“An exciting new generation of battery types for electric vehicles beyond lithium ion is on the horizon,” said Zhengcheng (John) Zhang, group leader in Argonne’s Chemical Science and Engineering division.
A paper about this research appears in Nature Communications (“Fluorinated cations introduce a new interfacial chemistry to activate high-voltage lithium metal batteries”).
Non-lithium-ion battery chemistry offers twice or more the energy stored in a given volume or weight compared to lithium ion. They could propel cars for greater distances and may even propel trucks and planes over long distances one day. The hope is that the widespread use of such batteries will help tackle the problem of climate change. The main problem is that its high energy density decreases rapidly with repeated charging and discharging.
One of the main competitors has an anode (negative electrode) made of lithium metal instead of the graphite normally used in lithium ion batteries. Thus called a “lithium metal” battery. The cathode (positive electrode) is a metal oxide containing nickel, manganese and cobalt (NMC). While it can provide more than double the energy density with lithium-ion batteries, that excellent performance quickly dissipates in less than a hundred charge cycles.
The team’s solution involves changing the electrolyte, a liquid in which lithium ions move between the cathode and anode to apply charge and discharge. In lithium metal batteries, the electrolyte is a liquid consisting of a lithium containing salt dissolved in a solvent. The source of the short cycle-life problem is that the electrolyte does not form an adequate protective layer on the anode surface during the first few cycles. This layer, also called solid-electrolyte-interphase (SEI), acts like a shield, allowing lithium ions to freely enter and leave the anode to charge and discharge the battery.
The team discovered a new fluoride solvent that maintains a tough protective coating for hundreds of cycles. It pairs a positively charged fluorinated component (cation) with a different negatively charged fluorinated component (anion). It is this combination that scientists call an ionic liquid — a liquid made up of positive and negative ions.
“The main difference in our new electrolyte is the substitution of fluorine for hydrogen atoms in the ring-like structure of the cationic portion of the ionic liquid,” said Zhang. “This makes all the difference in maintaining high performance over hundreds of cycles in lithium metal test cells.”
To better understand the mechanisms behind these differences at the atomic scale, the team leveraged the high-performance computing resources of the Argonne Leadership Computing Facility (ALCF), a user facility of the DOE Office of Science.
As Zhang explains, simulations on the Theta ALCF supercomputer reveal that fluorine cations attach to and accumulate on the anode and cathode surfaces prior to the discharge cycle. Then, during the early stages of cycling, a tough SEI layer is formed that is superior to anything possible in the previous electrolyte.
High-resolution electron microscopy at the Argonne National Laboratory and the Pacific Northwest revealed that the highly protective SEI coatings on the anode and cathode lead to a stable cycle.
The team was able to adjust the proportions of fluoride solvent to lithium salts to create a layer with optimal properties, including an SEI thickness that was neither too thick nor too thin. Because of this layer, lithium ions can efficiently flow in and out of the electrode during charging and discharging for hundreds of cycles.
The team’s new electrolyte also offers many other advantages. It is low cost because it can be prepared with very high purity and yields in one simple step rather than a few steps. Environmentally friendly as it uses less solvents, which are volatile and can release contaminants into the environment. And safer because it is not flammable.
“Our lithium metal battery with fluorinated cation electrolyte can greatly improve the electric vehicle industry,” said Zhang. “And the usefulness of this electrolyte undoubtedly extends to other types of advanced battery systems beyond lithium ion.”