Preparation of lithium-ion battery anodes using lignin

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2013

The new material is low cost, renewable and can be used without a binder or current collector.

KEY CONCEPTS
• Lignin has been converted into lignin-based carbon fibers that are suitable anode materials for use in a lithium-ion battery.
• The method used to prepare lignin-based carbon fibers involves preparation of a three-dimensional interconnected network by melt processing followed by oxidation and carbonization.
• Lignin-based carbon fibers used as an anode material displayed superior performance than the graphite currently used in commercial lithium-ion batteries.

LIGNIN IS A READILY AVAILABLE, RENEWABLE MATERIAL that is the main structural component in the wood that comes from trees. It is a polymeric material that has a range of molecular weights varying from 200 to more than a million. The chemical structure of lignin contains aromatic groups combined with such functionalities as phenols, alkoxy and hydroxyl groups.

Lignin is produced commercially as a byproduct in the manufacture of paper through the use of several pathways including the Kraft process. Due to lignin’s availability and low cost, this material has been evaluated for use in lubricant applications.

In a previous TLT article, the performance of lignin was assessed as an extreme pressure additive for use in greases (1). Four-ball performance testing showed that a lignin-based EP additive displayed comparable performance to molybdenum disulfide in an aluminum complex grease. In addition, the phenolic functionality in lignin contributes antioxidant characteristics that give the lignin derivatives the potential to provide multifunctional characteristics.

Development of improved performance for lithium-ion batteries is ongoing in a number of areas including improved battery anodes. Dr. Orlando Rios, Weinberg Fellow & Group Member in the Materials Science & Technology Division of Oak Ridge National Laboratory in Oak Ridge, Tenn., says, “There are three important areas where anode improvement can be realized. The biggest one is an improvement in rate performance. Two other characteristics that need to be enhanced are manufacturing materials and boosting the capacity of the anode.”

From the material standpoint, the active material in the lithium-ion anode is graphite. But there are several other components also required in an anode. Rios says, “Besides graphite, porosity agents, conductivity additives, a binder and a current collector are required. The problem with using these components is they add mass and cost to the overall battery without improving performance.”

One concern with using graphite is its incompatibility with electrolytes such as propylene carbonate. Rios says, “Propylene carbonate is a very attractive electrolyte because of its good performance at low temperatures and low melting point. The problem is that propylene carbonate will readily degrade a graphite electrode.”

If an alternative material can be developed for use in an anode that is low cost, renewable and can be used without a binder or a current collector, then there is potential for developing a lithium-ion battery with improved performance. Such a material has now been identified and evaluated.

NANOSCALE CRYSTALLINE DOMAINS 
Rios and his fellow researchers have determined that lignin is a suitable anode material for use in a lithium-ion battery. Through a low-cost, melt-processing technique combined with oxidation and carbonization, lignin is converted into a very effective anode material known as lignin-based carbon fibers (LCFs). Rios says, “LCFs are disordered materials that contain nanoscale crystalline domains in localized areas.”

Rios explains how the researchers decided to work with lignin. He says, “We were working on a project to evaluate the possibility of preparing carbon fiber from lignin. After evaluating the microstructure of lignin, we felt that this material would work well in lithium-ion batteries.”

After a three-dimensional interconnected LCF network is prepared by melt processing, the resulting lignin fiber mat is oxidized through heating in air. This step leads to breaking the weakest intermolecular bonds and crosslinks the lignin molecules.

As this occurs, the glass transition temperature of the LCF starts to increase. Rios says, “During the oxidation, we found that it is important to maintain the temperature of the process just below the glass transition temperature. This enables the structure formed to relax and stabilize in a low energy conformation that allows the aromatic rings derived from the lignin to easily stack one on top of another.”

Carbonization was then conducted at 1000 C, 1500 C and 2000 C. Rios indicates that as the temperature increases, the nanoscale crystalline domains are larger. He says, “This results in a more stable material but also hinders solid-state diffusion. We expect that in the rougher materials, reductions in fiber diameter will enable us to fully access the full capacity.”

LCFs produced at 1000 C display better performance as an anode material than graphite, which is prepared at 2000 C. Rios says, “LCFs generate approximately 350 milliampere-hour per gram of charge, while a commercial grade graphite anode produces between 200 and 310 milliampere-hour per gram.”

Figure 3 shows the progression of how the LCF is prepared, starting from a renewable resource and ending up with a disordered material containing local nanocrystalline domains. In the future, Rios has an objective of developing higher capacity materials for use in batteries.


Figure 3. Lignin-based carbon fibers, prepared from a readily available, renewable resource by the process shown, display promising performance as an anode material compared to graphite. (Courtesy of Oak Ridge National Laboratory)

Rios envisions evaluating LCFs as the sole material in a lithium-ion battery anode to see how it can perform versus a conventional graphite-based anode. He says, “We believe the fused mat electrode design can provide better performance than a slurry-coated graphite electrode because the LCF does not contain such materials as a binder, porosity agent and a copper current collector that do not contribute to the battery capacity.”

Additional information can be found in a recent article (2) or by contacting Rios at rioso@ornl.gov.

REFERENCES
1. Canter, N. (2010), “Environmentally Friendly Extreme Pressure Additive,” TLT, 67 (10), pp. 10-11.
1. Tenhaeff, W., Rios, O., More, K. and McGuire, M. (2013), “Highly Robust Lithium Ion Battery Anodes from Lignin: An Abundant, Renewable, and Low-Cost Material,” Advanced Functional Materials, DOI: 10.1002/adfm.201301420.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.