Structure and ion-transport performance of the Li–Cu–CNF solid-state ion conductor. (Image: Brown University)

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. Researchers have now developed a new material for use in solid-state batteries that’s derived from an unlikely source: trees.

The team demonstrated a solid ion conductor that combines copper with cellulose nanofibrils — 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. It could be used as either a solid battery electrolyte or as an ion-conducting binder for the cathode of an all-solid-state battery.

Today’s Li-ion batteries, which are widely used in everything from cellphones to cars, 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 they 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 that can catch fire.

Solid electrolytes have the potential to prevent dendrite penetration and can be made from nonflammable materials. Most of the solid electrolytes investigated so far are ceramic materials, which are good at conducting ions but they’re also thick, rigid, and brittle. Stresses during manufacturing as well as charging and discharging can lead to cracks and breaks. The new material, however, is thin and flexible — almost like a sheet of paper — and its ion conductivity is on par with ceramics.

Computer simulations of the microscopic structure of the copper-cellulose material were performed to understand why it is able to conduct ions so well.

The modeling study revealed that the copper increases the space between cellulose polymer chains, which normally exist in tightly packed bundles. The expanded spacing creates ion superhighways through which lithium ions can zip by relatively unimpeded.

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. That thickness, however, can compromise ion conduction, reducing efficiency. For thicker cathodes to work, they need to be encased in an ion-conducting binder. Using the new material as a binder, the team demonstrated what they believe to be one of the thickest functional cathodes ever reported.

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