New protective coating can improve battery performance
A universal solution with indirect climate protection
A research team at the Paul Scherrer Institute PSI has developed a new sustainable process that can be used to improve the electrochemical performance of lithium-ion batteries. Initial tests of high-voltage batteries modified in this way have been successful. This method could be used to make lithium-ion batteries, for example those for electric vehicles, significantly more efficient.
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.
Original publication
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The topic world Battery Technology combines relevant knowledge in a unique way. Here you will find everything about suppliers and their products, webinars, white papers, catalogs and brochures.