The National Institute for Computational Sciences

A Plant Polymer with Potential

Lignin Could Improve Lithium-ion Batteries

Lignin—it’s the second-most-abundant renewable carbon source on the planet, and it might be the key to making lithium-ion batteries more earth friendly and cost-effective.

An organic substance, lignin binds the cells and fibers that constitute wood and rigid elements of plants, such as straw. More than 40 million tons are produced worldwide each year, mostly as a low-cost waste product from the pulp, paper, and biofuels industries.

Recently, scientists have begun researching ways to use lignin in structural applications, such as automobiles. Orlando Rios, a materials scientist at Oak Ridge National Laboratory (ORNL), says he was examining the substance to see if it could be used in place of steel. During his study, he noticed something entirely unexpected: part of lignin’s microstructure resembled that of graphite, a key element in battery technology.

Long used to make markings on paper, graphite has experienced increasing demand during the last 30 years due to growing markets for lithium-ion batteries. These batteries, found in most mp3 players and cell phones, move electricity from a graphite-based negative terminal (anode) to a lithium-based positive terminal (cathode) via lithium ions. Graphite has become the most common anode because it naturally holds the ideal shape: a crystalline microstructure made up of two-dimensional sheets of carbon stacked on top of each other. This layered structure allows lithium ions to be inserted and removed from between the individual sheets in a process called intercalation. During battery usage, the intercalating lithium ions carry electric charge from the graphite anode to the lithium cathode. And when it’s time to charge the battery, the ions flow in the reverse direction.

Rios realized that lignin’s microstructure contained tiny regions of parallel planes of atoms within a larger, non-crystalline matrix. These nanoscale regions were similar to graphite’s overall structure, so Rios decided to investigate if lignin could be implemented in batteries. By melting and burning the lignin fibers together, he and his colleagues created a fused structure that could serve the same role as highly engineered graphite at a fraction of the industrial—and environmental—cost. Initial testing of the lignin-derived battery provided encouraging results, but Rios and his team still had questions as to how and why the lignin material was behaving differently from other graphites.

For help in pursuing the answers, Rios reached out to the University of Tennessee’s Computational Materials Group, led by David Keffer, a professor in the Department of Materials Science & Engineering. The two had met in 2011 through a mutual acquaintance at ORNL, and Keffer was happy to provide the analysis Rios needed.

“The collaboration between the Rios and Keffer research groups is exemplary,” Keffer said in an email interview, “because it highlights the ability to match strengths at ORNL—in the synthesis and characterization of complex materials—with multiscale modeling techniques at UT.”

Keffer and his group analyzed the microstructure of the burned lignin fibers, modeling its two distinct components. To model the non-crystalline component, they used randomly arranged fragments of graphite sheets (individually called graphene); to model the nanocrystalline component, they used nanoscale crystallites of graphite. Keffer said the model looks like marshmallows in jello, with the nanocrystalline components as marshmallows and the non-crystalline component as jello.

By running supercomputer simulations on the NICS-managed Kraken (now decommissioned) and ORNL’s Titan, their work contributed unprecedented clarity to the position of each atom in the structure and its calculated energy.

“We really started to understand how the lignin structure works,” Rios says. “You build a structure based on the data you measure.”

They also used ORNL’s Spallation Neutron Source to run neutron scattering experiments on three lignin-derived carbon samples, hoping to obtain experimental models for comparison with their simulation results. Though Keffer noted that his team needed experimental input, the level of precision they achieved was not possible with experimental data alone.

“The take-home message is that, taken jointly, simulation and experiment provide a comprehensive ability to understand structure/property relationships that lead to new materials discovery,” Keffer said.

The real-world and computer models matched, confirming the accuracy of their research and allowing its use in materials development. Keffer said they are now interested in generating structure/property relationships.

“How do changes in the material’s structure—at the nanoscale and mesoscale—impact the properties that govern its performance in operation?” Keffer explained. “We want to understand how the unique amorphous/crystalline structure of Orlando Rios’ materials impacts both the charge capacity as well as the charging rates.”

First, they needed a reliable model of the material. Now that they have one, Rios says they’ve made prototype batteries and are testing them.

“We’re trying to work with various companies to use some of these materials,” he says. “But more so, we’d like to understand the dynamics of these types of structures.”

The investigation is already underway on Titan.

R. J. Vogt, science writer, NICS, JICS

Article posting date: 25 October 2014

About JICS and NICS: The Joint Institute for Computational Sciences (JICS) was established by the University of Tennessee and Oak Ridge National Laboratory (ORNL) to advance scientific discovery and state-of-the-art engineering, and to further knowledge of computational modeling and simulation. JICS realizes its vision by taking full advantage of petascale-and-beyond computers housed at ORNL and by educating a new generation of scientists and engineers well versed in the application of computational modeling and simulation for solving the most challenging scientific and engineering problems. JICS runs the National Institute for Computational Sciences (NICS), which had the distinction of deploying and managing the Kraken supercomputer. NICS is a leading academic supercomputing center and a major partner in the National Science Foundation's eXtreme Science and Engineering Discovery Environment, known as XSEDE. In November 2012, JICS sited the Beacon system, which set a record for power efficiency and captured the number one position on the Green500 list of the most energy-efficient computers.