Cellulose nanofibrils take charge in solid-state batteries

Researchers have developed a new material for use in solid-state batteries that is derived cellulose nanofibrils found in trees.

cellulose nanofibrils
Image by PublicDomainPictures from Pixabay

In research published in Nature, the team from Brown University and the University of Maryland demonstrated a solid ion conductor that combines copper with cellulose nanofibrils, which are polymer tubes derived from wood.

The paper-thin material has an ion conductivity that is 10 to 100 times better than other polymer ion conductors, the researchers said. It could be used as a solid battery electrolyte or as an ion-conducting binder for the cathode of an all-solid-state battery.

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“By incorporating copper with one-dimensional cellulose nanofibrils, we demonstrated that the normally ion-insulating cellulose offers a speedier lithium-ion transport within the polymer chains,” said Liangbing Hu, a professor in the University of Maryland’s Department of Materials Science and Engineering. “In fact, we found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”

The work was a collaboration between Hu’s lab and the lab of Yue Qi, a professor at Brown’s School of Engineering.

Current lithium-ion batteries contain electrolytes made from lithium salt dissolved in a liquid organic solvent. At high currents, tiny filaments of lithium metal – dendrites – can form in the electrolyte leading to short circuits. In addition, liquid electrolytes are made with flammable and toxic chemicals, which can catch fire.

Solid electrolytes have the potential to prevent dendrite penetration and can be made from non-flammable materials.  Most of the solid electrolytes investigated so far are ceramic materials, which are good conductors but also thick, rigid and brittle. Stresses during manufacturing as well as charging and discharging can lead to cracks and breaks.

The material introduced in this study is thin, flexible and its ion conductivity is on par with ceramics.

Qi and Qisheng Wu, a senior research associate at Brown, performed computer simulations of the microscopic structure of the copper-cellulose material to understand why it is able to conduct ions so well. The modelling study revealed that the copper increases the space between cellulose polymer chains, which normally exist in tightly packed bundles. The expanded spacing creates what amount to ion superhighways through which lithium ions can move relatively unimpeded.

“The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity,” Qi said in a statement. “Using materials nature provides will reduce the overall impact of battery manufacture to our environment.”

In addition to working as a solid electrolyte, the new material can also act as a cathode binder for a solid-state battery. To match the capacity of anodes, cathodes need to be substantially thicker. According to Brown, that thickness can compromise ion conduction, which reduces efficiency. In order for thicker cathodes to work, they need to be encased in an ion-conducting binder. Using their new material as a binder, the team demonstrated what they believe to be one of the thickest functional cathodes ever reported.

The researchers are hopeful that the new material could be a step toward making bringing solid state battery technology to the mass market.