In his laboratory, Mario El Kazzi has developed a method that converts trifluoromethane, a climate-damaging gas, into a protective layer for high-voltage batteries and thus increases their performance.
Paul Scherrer Institute PSI/Mahir Dzambegovic
Lithium-ion batteries are considered a key technology for decarbonization. Around the world, scientists are therefore working to continually improve their performance, among other things by increasing their energy density. “One way to achieve this is to increase the operating voltage,” explains Mario El Kazzi from the Center for Energy and Environmental Sciences at the Paul Scherrer Institute PSI in Aargau. If the voltage increases, the energy density also increases.”
But there is a problem: operating voltages of more than 4.3 volts induce significant chemical and electrochemical degradation processes at the junction of the cathode (positive pole) and the electrolyte (conducting medium). The surface of cathode materials is heavily damaged by the release of oxygen, dissolution of transition metals and structural reconstruction, resulting in a continuous increase in cell resistance and a decline in capacitance. This is why commercial battery cells, such as those in electric cars, currently only operate with a maximum voltage of 4.3 volts.
A layer to protect the cathode
To solve this problem, Mario El Kazzi and his team developed a new method, which stabilizes the surface of the cathode by covering it with a thin, uniform protective layer. The scientists report their discovery in a study published in the specialist journal “ChemSusChem”.
The process centers around a gas that forms as a byproduct during the production of plastics such as PTFE, PVDF and plastic foam: trifluoromethane, whose chemical formula is CHF3. In their laboratory, Mario El Kazzi and his team induced, at a temperature of 300°C, a reaction between CHF3 and the thin layer of lithium carbonate which covers the cathode. The lithium then transforms into lithium fluoride (LiF) at the boundary layer. Important fact: the lithium atoms in the cathode material remain in the form of ions, i.e. positively charged particles. These lithium ions must in fact be able to continue to move between the cathode and the anode (negative pole) during charging and discharging, so that the capacity of the accumulator is not reduced during its subsequent use.
Voltages of 4.5 and 4.8 volts
In the next step, the scientists checked the effectiveness of the protective layer by carrying out electrochemical tests at high operating voltages. The result was pleasing: the protective layer remained stable even at high operating voltages. It protected the cathode material so well that operating voltages of 4.5 volts and even 4.8 volts were possible.
Compared to batteries whose cathodes were not protected, those equipped with the coating did much better across all parameters. Thus, after 100 charge and discharge cycles, the impedance, that is to say the resistance of the lithium ions at the cathode interface, was approximately 30% lower than that of batteries whose cathode n had not been processed. “This clearly indicates that our protective layer mitigates the increase in resistance due to reactions that normally occur at interfaces,” explains Mario El Kazzi
Capacity maintenance was also compared. This expression designates the quantity of lithium ions which can still migrate from the cathode to the anode after a certain number of charge and discharge cycles. The closer this value is to 100%, the lower the capacity drop. However, here too, the battery with cathode coating proved superior during tests: capacity retention was 94% after 100 charge and discharge cycles, without slowing down the charging speed, whereas the untreated battery only reached 80%.
Many applications
The coating process developed at PSI opens up new possibilities for increasing the energy density of different types of batteries: “We can assume that our protective layer with lithium fluoride (LiF) is universally applicable to the majority of cathode materials, underlines Mario El Kazzi. It also works with high-voltage batteries rich in nickel and lithium, for example.”
Lithium-ion batteries are found in mobile phones, laptops, power tools, electric cars and stationary energy storage. These versatile accumulators of our daily life are so called because they are lithium ions which migrate from one electrode to another during charging and discharging. In doing so, these ions pass through the liquid electrolyte and the separator that separates the electrodes. While the anode is most often composed of graphite or silicon, the cathode has very diverse chemical compositions. If it is composed, for example, of nickel, cobalt and manganese in addition to lithium, it is called an NCM battery. If the cathode is lithium iron phosphate, it is an LFP battery.
A harmful gas converted into a useful material
Another important aspect of this new method: trifluoromethane is a powerful greenhouse gas and more than 10,000 times more harmful to the climate than carbon dioxide. It must therefore under no circumstances be found in the atmosphere. For Mario El Kazzi, its conversion into a uniform thin layer of LiF affixed to the surface of cathode materials represents an effective solution for monetizing this gas by integrating it into a circular economy. The new coating process allows CHF3 to be recycled and permanently fixed as a protective layer in high-voltage cathodes.
