Preparation of lithium-sulfur batteries from lignosulfonate

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2018

Potentially a more cost-effective option.

KEY CONCEPTS
One factor in developing high-performance batteries is producing them in a cost-effective manner to enable them to become competitive with existing energy storage technologies. A readily available raw material that is low cost and renewable is lignin, which represents about 30%-40% of the dry mass of wood that comes from trees. Lignin is a polymer with molecular weights ranging from 200 to more than one million Da. It is an undefined structure that contains a variety of aromatic and phenolic groups.

In a previous TLT article, researchers converted lignin into lignin-based carbon fibers that were used as an anode material for a lithium-ion battery (1). Superior performance was achieved with lignin-based carbon fibers compared to the currently used anode material, graphite.

Lithium-sulfur batteries are an alternative technology that can replace conventional lithium-ion batteries for certain applications. Interest in this technology has grown as they theoretically can hold more than double the energy compared to conventional chemistries. The use of sulfur, an abundant, low-cost raw material, also is an advantage for lithium-sulfur batteries.

Dr. Trevor Simmons, research scientist at Rensselaer Polytechnic Institute in Troy, N.Y., says, “The cathode of a lithium-sulfur battery contains a sulfur-carbon composite, while the anode is prepared typically with a conventional lithium metal oxide. Sulfur has a number of advantages including high affinity for lithium and good compatibility with carbon. But the main concern with using sulfur is the element’s low conductivity.”

At high temperatures, sulfur can be combined with carbon and result in a highly conductive material. Simmons says, “In a lithium-sulfur battery, sulfur is converted from its original all-otrope (S₈) through a series of reduction steps as it forms a number of intermediate polysulfides. Eventually sulfur is reduced to a sulfide that exhibits a 2:1 ratio of lithium:sulfur.”

The challenge in preparing a lithium-sulfur battery is to control the process so the intermediate sulfides present during cycling of the cell do not solubilize in the battery’s electrolyte. One of the keys, according to Simmons, is incorporating sulfur in a cathodic material that is a porous carbon structure with mesoscale and microscale features. Simmons says, “The key is to trap sulfur in a carbon matrix so that the lithium ions cannot escape. Currently the best form of carbon to achieve this result has been a variety of activated carbons.”

Based on previous experience with lignin, Simmons realized that using lignosulfonate, a byproduct of paper manufacturing, could provide some benefits with the carbon and sulfur already in place in the same molecule. The ready availability of this material led Simmons and his colleagues to determine how it could be used in lithium-sulfur batteries.

Activated pyrolytic lignosulfonate
The researchers prepared a cathode for a rechargeable lithium-sulfur battery using lignosulfonate as both the carbon and the sulfur source. Simmons says, “We sourced the lignosulfonate from a local paper company that produced it as a byproduct of a sulfite process for manufacturing paper.”

After drying a water-based solution of the lignosulfonate known as brown liquor, the researchers mechanically pulverized it and then pyrolyzed it at 700 C for several hours in a quartz tube under an inert argon atmosphere. The pyrolytic lignosulfonate was placed in an alumina jar with zirconia balls and milled. Further treatment with potassium hydroxide was used to produce activated pyrolytic lignosulfonate.

Simmons says, “During the pyrolysis step, most but not all of the sulfur was lost as a gas. As a final step, we then placed the activated pyrolytic lignosulfonate down to the cold zone of the quartz tube during pyrolysis of a fresh batch of lignosulfonate. This enabled us to seed sulfur for a new batch of activated pyrolytic lignosulfonate.”

One concern the researchers had was whether the sulfur might aggregate into large clusters. Simmons says, “We found that the sulfur clusters were trapped deep inside the carbon pores and did not further aggregate. This made it more difficult for sulfur to leach into the electrolyte during the charge-discharge cycles.”

The researchers built 2032 coin cells (size of a watch battery) as a half-cell prototype to test the concept. A slurry containing activated pyrolytic lignosulfonate, additional sulfur, carbon black and polyvinylidene fluoride was cast onto an aluminum foil to produce the cathode. In preliminary testing, this battery displayed superior durability to a battery prepared with a pure sulfur, or bulk sulfur carbon cathode. A decay rate of 0.1% per cycle was obtained over 200 cycles during initial testing.

Figure 1 shows images of the water-based lignosulfonate solution, the activated pyrolytic lignosulfonate and battery prototype cells. Simmons says, “In the future, this lithium-sulfur battery will need to be scaled up in an effort to increase the discharge rate and the battery’s cycle life. Our hope is that the strong initial performance observed, combined with the availability of a low-cost raw material that is a byproduct in an existing industrial process, will lead to further interest in commercializing this concept.”

Figure 1. A cathode for a lithium-ion battery was prepared from a water-based lignosulfonate solution (shown in the beaker on the left) that produced an activated pyrolytic lignosulfonate black powder (to the right of the beaker). The latter was placed in 2032 coin cells for evaluation (shown below the beaker and the black powder). (Figure courtesy of Rensselaer Polytechnic Institute.)

Additional information can be found in a recently published article (2) or by contacting Trevor J. Simmons at simmot@rpi.edu.

REFERENCES
1. Canter, N. (2013), “Preparation of lithium-ion battery anodes using lignin,” TLT, 69 (12), pp. 16-17.
2. Li, L., Huang, L., Linhardt, R., Koratkar, N. and Simmons, T. (2017), “Repurposing paper by-product lignosulfonate at a sulfur donor/acceptor for high performance lithium-sulfur batteries,” Sustainable Energy Fuels, 2 (2), pp. 422-429.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.