An ingredient in toothpaste could propel electric cars

One of the ingredients in many toothpastes is sodium fluoride, which is a compound of fluorine. It is added to protect teeth from decay. But there are other practical uses for fluorine-containing compounds 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 performance degradation.

“An exciting new generation of battery types for electric vehicles beyond lithium ion is on the horizon,” said group leader Zhengcheng (John) Zhang of Argonne’s Chemical Sciences and Engineering Division.

The chemistry of non-lithium-ion batteries stores twice or more energy in a given volume or weight than lithium-ion. They could power cars over much longer distances and could one day power long-haul trucks and planes. The hope is that the widespread use of such batteries will help address the problem of climate change. The main problem is that their high energy density degrades rapidly upon repeated charge and discharge.

One of the main contenders has an anode (negative electrode) made of lithium metal in place of the graphite typically used in lithium-ion batteries. That’s why it’s called a “lithium metal” battery. The cathode (positive electrode) is a metal oxide consisting of nickel, manganese and cobalt (NMC). Although this can provide more than double the energy density possible with lithium-ion batteries, the excellent performance rapidly vanishes within less than a hundred charge-discharge cycles.

The team’s solution involved replacing the electrolyte, a liquid through which lithium ions move between the cathode and anode, to effect a charge and discharge. In lithium metal batteries, the electrolyte is a liquid containing lithium-containing salts 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 the solid-electrolyte-interphase (SEI), acts like a protector, allowing lithium ions to move freely in and out of the anode, charging and discharging the battery, respectively. Is.

Their research has been published in the journal nature communication,

The team discovered a new fluoride solvent that maintains a strong protective layer for hundreds of cycles. It combines a fluorinated component that is positively charged (cation) with a different fluorinated component that is negatively charged (anion). Scientists call this combination an ionic liquid – a liquid that contains 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 cation portion of the ionic liquid,” said Zhang. “This made a huge difference in maintaining high performance for hundreds of cycles in a test lithium metal cell.”

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

As Zhang explained, simulations of the ALCF on the Theta supercomputer showed that fluorine cations stick to and accumulate on the anode and cathode surfaces before any charge-discharge cycling occurs. Then, during the early stages of cycling, a flexible SEI layer is formed that is superior to previous electrolytes.

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

The team was able to adjust the ratio of fluoride solvent and lithium salt to create a layer with optimal properties, including a SEI thickness that was not too thick or thin. Because of this layer, lithium ions can efficiently flow in and out of the electrode during charge and discharge for hundreds of cycles.

The team’s new electrolyte offers several other benefits as well. It costs less because it can be made with extremely high purity and yield in one simple step instead of multiple steps. It is eco-friendly as it uses very little solvent, which is volatile and can release contaminants into the environment. And it is more safe because it is not flammable.

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