Conventional processes to fabricate cathodes in lithium batteries require a glue that binds the active material to the substrate, which both takes up space and lowers the battery performance. A research team from Najing University, Xerion Advanced Battery Corporation and the University of Illinois have now found a way to bypass the glue by electroplating the active material onto the substrate. Electroplating allowed for higher design flexibility, quality and performance of the resulting cathodes.
Lithium (Li) ion batteries are essential for phones, laptops and other portable electronics. The flow of electric energy during charge and discharge of the battery is created by Li ions that move between two electrodes called anode and cathode. Whereas the anode is typically made of a carbon material, the cathode is often composed of a Li transition metal oxide.
The Li transition metal oxides can be synthesised as a powder at high temperatures. For constructing the electrode, one first mixes the powder into a slurry with a glue and other additives and spreads the slurry onto a thin sheet of aluminium foil, the current collector. After drying, the cathode can then be assembled in the battery.
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The glue gets in the way in conventional cathode fabrication
The problem with this cathode fabrication, however, is the glue. It limits how much energy the battery can store because it has to be thin. And it also limits the battery’s performance.
“The glue is not active. It doesn’t contribute anything to the battery, and it gets in the way of electricity flowing in the battery”, says Hailong Ning, co-author of the study from the Xerion Advanced Battery Corporation. “You have all this inactive material taking up space inside the battery, while the whole world is trying to get more energy and power from the battery.”
The research team, a collaboration between Najing University in China, Xerion Advanced Battery Corporation and the University of Illinois at Urbana-Champaign in the USA, discovered a procedure to electroplate the Li material on the aluminium foil bypassing the slurry and glue process altogether.
The findings allow for the production of cathode materials of high quality and high performance, and open the door to batteries composed of a solid material that can store a lot energy. The research was recently published in the journal Science Advances.
Electroplated cathodes show better performance, quality and flexibility
The scientists demonstrated in the paper that the electroplated cathode could store 30% more energy than a cathode fabricated by the conventional process. A battery with the novel cathode was also found to charge and discharge faster because the current was not hindered by the inactive glue or the porous structure that the slurry would create.
Instead, the layer of cathode material was found to be nearly solid, which makes it more stable and allows for more energy storage.
The good contact between the cathode material and the aluminium foils also made the cathode very flexible, which allows for flexible battery designs. And there is yet another advantage: The electroplating process created a pure cathode material, even when the starting materials were not that pure. For manufacturers, this means that they can use materials of lower quality, which are cheaper, and still produce an end product that is high in performance, as Braun explains.
Electroplating: Dipping the substrate in a liquid bath of the metal ions
The researchers could achieve all this by a process called electroplating. Electroplating is widely used in industry for coating a metal with a thin film of another metal in order to add a property that the first metal lacks. One example is to prevent corrosion by plating car parts or bath taps with chromium. Electroplating is also used to make less expensive jewellery by adding a layer of gold to a cheaper metal.
In the current study, electroplating was achieved by dipping the substrate into a liquid bath containing the metal ions and applying an electron current. In this way, the substrate was coated by the metal. “This is an entirely new approach to manufacturing battery cathodes, which resulted in batteries with previously unobtainable forms and functionalities”, says Paul V. Braun, principal investigator of the study and professor of materials science and engineering at Illinois.
Apart from using conventional aluminium foil as the current collector, the researchers also demonstrated that electroplating works with other substrates, such as carbon foam, which is a lightweight and inexpensive material. They further found that the electroplated porous substrate could achieve cathodes with much greater flexibility.
“This method opens the door to flexible and three-dimensional battery cathodes”, says co-author Huigang Zhang, a professor at Nanjing University. Braun adds: “These designs are impossible to achieve by conventional processes. But what’s really important is that it’s a high-performance material and that it’s nearly solid. By using a solid electrode rather than a porous one, you can store more energy in a given volume. At the end of the day, people want batteries to store a lot of energy.”
- Zhang et al. (2017) Electroplating lithium transition metal oxides, Science Advances 3, e1602427, DOI: 10.1126/sciadv.1602427.
- Liz Ahlberg (2017) Electroplating delivers high-energy, high-power batteries, phys.org, available at: https://phys.org/news/2017-05-electroplating-high-energy-high-power-batteries.html (assessed: 15/05/2017).
- Image Credit: Shutterstock.com/axyse