Promising Li-ion battery material derived from wood

In pursuit of batteries that deliver more power and operate more safely, researchers are working to replace the liquids commonly used in today’s lithium-ion (Li-ion) batteries with solid materials. Now, a research team led by Brown University and the University of Maryland has developed a new material for use in solid-state batteries that’s derived from an unlikely source: trees.

Today’s Li-ion batteries have electrolytes made from lithium salt dissolved in a liquid organic solvent. The electrolyte’s job is to conduct lithium ions between a battery’s cathode and anode. Liquid electrolytes work well but have some downsides; at high currents, tiny filaments of lithium metal, called 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 at conducting ions but are also thick, rigid and brittle. Stresses during manufacturing as well as charging and discharging can lead to cracks and breaks.

In a study published in the journal Nature, researchers demonstrate a solid ion conductor that combines copper with cellulose nanofibrils — polymer tubes derived from wood. The material is thin and flexible, almost like a sheet of paper, and its ion conductivity is on par with ceramics, at 10 to 100 times better than other polymer ion conductors.

The researchers 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 zip by relatively unimpeded.

“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 Maryland Professor Liangbing Hu. “In fact, we found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”

“The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity,” Brown Professor Yue Qi added. “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. In order to match the capacity of anodes, cathodes need to be substantially thicker. That thickness, however, can compromise ion conduction, reducing 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 thus hopeful that the new material could be a step toward bringing solid-state battery technology to the mass market.

Image credit: ©stock.adobe.com/au/malp

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