Post-Lithium Technology: Polymer cathode for long-life sodium ion batteries

Post-Lithium Technology: Polymer cathode for long-life sodium ion batteries

Technology News |
By Christoph Hammerschmidt

If the breakthrough of electromobility is to succeed, electric vehicles will have to have a longer range in the future, charging must be faster and, in addition, the price of the battery must fall significantly. The next generation battery type therefore needs to have a higher energy density and better capacity, and be made of inexpensive, safer and more environmentally friendly materials. Rechargeable batteries, which basically function according to the same technology as lithium-ion batteries, but in which lithium-ion is replaced by cheaper metal ions such as sodium, magnesium and aluminium ions, are being intensively investigated.

Organic materials are ideal for the electrodes because they usually do not contain any harmful and expensive heavy metals and can easily be adapted for different purposes. Unfortunately, they dissolve in the liquid electrolyte, making such electrodes unstable.

Chunsheng Wang and his team from the University of Maryland (USA) along with an international group of scientists have developed an organic polymer cathode that does not dissolve, but at the same time has a high capacity for rapid discharge and charging. For the sodium ion, it showed a significantly better short- and long-term capacity than other cathodes made of polymeric or inorganic materials. According to the study, the approach also achieved excellent results for magnesium and aluminum ions.

The scientists identified the organic compound hexaazatrinaphthalene (HATN) as a cathode material with high energy density and good storage readiness for methallions. HATN has already been tested in lithium ion batteries and supercapacitors, but, like most organic materials, it dissolved in the electrolyte over time. In order to stabilize the material, the researchers linked the individual molecules with binding bridges. The result was an organic polymer called polymeric HATN or PHATN. The capacity for sodium, aluminium and magnesium ions was excellent.

The PHATN cathode was then tested in metal ion batteries with a highly concentrated electrolyte. The tests showed excellent battery data. The sodium ion battery could be operated at a voltage of 3.5 volts and, according to the data, even after 50,000 charge-discharge cycles, still had a proud capacity of over 100 milliamperes per gram. The magnesium ion battery and the aluminum ion battery were only slightly worse.

Such advanced pyrazine polymer cathodes (pyrazine is the basic substance of HATN, it is an aromatic benzene-like but nitrogen-rich organic substance with a fruity fragrance) could be used in next-generation environmentally friendly, long-lasting, rechargeable high-performance batteries, the scientists assume.

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