Lithium-ion batteries are considered a key technology for decarbonisation. Therefore, researchers around the world are working to continuously improve their performance, for example by increasing their energy density. “One way to achieve this is to increase the operating voltage,” says Mario El Kazzi from the Center for Energy and Environmental Sciences at Paul Scherrer Institute PSI. "If the voltage increases, the energy density also increases.”

However, there is a problem: At operating voltages above 4.3 volts, strong chemical and electrochemical degradation processes take place at the transition between the cathode, the positive pole, and the electrolyte, the conductive medium. The surface of the cathode materials gets severely damaged by the release of oxygen, dissolution of transition metals, and structural reconstruction – which in turn results in a continuous increase in cell resistance and a decrease in capacity. This is why commercial battery cells, such as those used in electric cars, have so far only run at a maximum of 4.3 volts.

To solve this problem, El Kazzi and his team have developed a new method to stabilise the surface of the cathode by coating it with a thin, uniform protective layer. The researchers report on their discovery in a study published in the scientific journal ChemSusChem (Wiley).

Operating voltages up to 4.8 volts

The process centres on a gas that is produced as a by-product during the manufacture of plastics such as PTFE, PVDF, and foam: trifluoromethane, with the chemical formula CHF3. In the laboratory, El Kazzi and his team initiated a reaction at 300 degrees Celsius between the CHF3 and the thin layer of lithium carbonate that covers the surface of the cathodes. This converts the lithium at the interface into lithium fluoride (LiF). It is important to note that the lithium atoms of the cathode material remain as ions, that is, as positively charged particles. These lithium ions must be able to move back and forth between the cathode and the anode, the negative pole, during charging and discharging so that the battery capacity is not impaired during subsequent operation.

In a further step, the researchers tested the effectiveness of the protective coating by carrying out electrochemical tests at high operating voltages. The gratifying result: The protective coating remained stable even at high voltages. It protects the cathode material so well that it is possible to operate at voltages of 4.5 and even 4.8 volts.

Compared to batteries with unprotected cathodes, the coated batteries performed significantly better in all important parameters. For example the impedance, that is, the resistance for the lithium ions at the cathode interface, was around 30 percent lower after one hundred charging and discharging cycles than in the batteries with untreated cathodes. “This is a clear sign that our protective layer minimises the increase in resistance caused by the interfacial reactions that would otherwise occur,” explains El Kazzi.

The capacity retention was also compared. This represents the amount of lithium ions that can still migrate from the cathode to the anode after a certain number of charging and discharging cycles. The closer this value is to 100 percent, the lower the drop in capacity. Here too, the battery with a coated cathode proved to be superior in the tests: The capacity retention was more than 94 per cent after 100 charging and discharging cycles without a decrease in charging speed, while the untreated battery only achieved 80 per cent.

A universal solution with indirect climate protection

The coating process developed at PSI opens up new ways to increase the energy density of different types of batteries: “We can assume that our lithium fluoride protective coating is universal and can be used with most cathode materials,” El Kazzi emphasises. “For example, it also works with nickel- and lithium-rich high-voltage batteries.”

Another important aspect of the new process is that trifluoromethane is a highly potent greenhouse gas and more than 10,000 times more harmful to the climate than carbon dioxide, which is why it should never be released into the atmosphere. For El Kazzi, converting it into a uniform thin LiF protective layer on the surface of cathode materials is an efficient solution to monetise the gas by making it part of a circular economy. With the new coating process, CHF3 can be recycled and bound long-term as a protective layer in high-voltage cathodes.

Text: Andreas Lorenz-Meyer

Small battery guide

They can be found in mobile phones, laptops, power tools, electric cars, and stationary energy storage devices: lithium-ion batteries. These all-rounders of our electrified everyday lives are so called because it is lithium ions that move back and forth between the two electrodes, cathode and anode, during charging and discharging. They move through the liquid electrolyte and pass through the separator between the electrodes. While the anode, the negative pole, is usually made of graphite or silicon, the cathode, the positive pole, has very different chemical compositions. If, for example, it consists of nickel, cobalt, and manganese in addition to lithium, it is referred to as NCM battery. A cathode made of lithium iron phosphate is referred to as LFP battery.

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of future technologies, energy and climate, health innovation and fundamentals of nature. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2300 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 460 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).

Original publication

Converting the CHF3 Greenhouse Gas into Nanometer-Thick LiF Coating for High-Voltage Cathode Li-ion Batteries Materials
Aleš Štefančič, Carlos Antonio Fernandes Vaz, Dominika Baster, Elisabeth Müller, Mario El Kazzi
ChemSusChem (Wiley), 03.01.2025
DOI: 10.1002/cssc.202402057

New protective coating can improve battery performance