Preparation of sulfur-based polymers for batteries

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

Researchers developed a high sulfur-containing copolymer that potentially can be used in batteries.

KEY CONCEPTS
• Sulfur is very difficult to work with because of its limited solubility in most organic solvents and incompatibility with other chemical raw materials.
• A new stable copolymer with sulfur contents up to 90 percent has been prepared through an inverse vulcanization process.
• This sulfur-based copolymer shows potential for use in batteries.

SULFUR HAS ESTABLISHED A POSITION in the lubricant industry as an element that is used in some critical additives such as sulfurized extreme pressure additives and overbased calcium sulfonates. But the presence of sulfur in fuels has been reduced significantly because of concern about emissions. This process is moving to marine fuel as regulations are in place to reduce the sulfur content from 3.5 to 0.1 percent in Emission Control Areas established within 200 miles of the North American continent by 2015.

In a previous TLT article, a new technique was discussed that is highly effective at adsorbing hydrogen sulfide and may be useful in removing sulfur compounds from fuel (1). The adsorbent contains nanofibers of zinc and titanium dioxide and shows good durability without loss of performance.

The emphasis to reduce sulfur content has led to the presence of a substantial excess of this element in the marketplace. Jeffrey Pyun, associate professor of chemistry and biochemistry at the University of Arizona in Tucson, Ariz., says, “More than 60 million tons of elemental sulfur is produced per year with much of it coming during petroleum refining.”

One of the problems with using sulfur is that it cannot readily be used in most common chemical processes. Pyun explains, “Sulfur is very difficult to handle because of its limited solubility in most organic solvents and incompatibility with many other chemical raw materials.”

But Pyun believes that sulfur has some very useful properties that can be exploited if it’s used in the proper manner. He says, “There is great interest in sulfur because of its high electrochemical capacities and good optical properties. For example, sulfur has the potential to be used as a cathode material in a battery because of its relatively low molecular weight and the fact that it has two unpaired electrons in its outermost orbital. This leads to a battery with a capacity that is four to five times higher than conventional lithium systems.”

If a process could be developed to incorporate sulfur in a form that can be readily incorporated into a battery, then a more efficient, less costly alternative could be developed. Such a process has now been developed.

INVERSE VULCANIZATION
Pyun and his fellow researchers have developed a new chemically stable material prepared through the copolymerization of elemental sulfur with vinyl monomers. He says, “At first, we tried to find a suitable solvent to react sulfur but gave up because there were no options. Instead we decided to just melt sulfur, dissolve the vinyl monomers in molten sulfur and do the reaction in the molten state.”

At ambient temperature, sulfur exists as an eight-membered ring that melts at a temperature between 120 and 124 C. Pyun says, “It has been known for 100 years that sulfur in this form can be polymerized to produce chemically stable polymers.”

Above 159 C sulfur undergoes a ring-opening polymerization to form a low molecular weight polymer that contains a radical at each end of the chain. This intermediate then is transformed into a high molecular weight polymer.

The researchers exploited the diradical nature of the intermediate by reacting molten sulfur with divinyl styrenic comonomers at 185 C. In producing the polymer, no additional initiators were needed in this free radical polymerization process.

The result is the formation of stable copolymers with sulfur contents up to 90 percent. Both step growth and condensation polymerization can be used to produce these high sulfur-containing materials.

One of the most common processes for using sulfur is in vulcanization where sulfur is used to cross-link rubber elastomers. Pyun says, “We are calling the formation of the sulfur-based polymers inverse vulcanization because we are, in essence, flipping the stoichiometry to maximize the number of sulfur-sulfur bonds.”

Figure 2 shows an image of the sulfur-based copolymer in a petri dish on the upper left and powdered sulfur on the lower right.


Figure 2. A new stable sulfur-based copolymer (shown in a petri dish on the upper left) prepared from sulfur (shown on the lower right) has the potential to be used as a more cost-effective material in batteries. (Courtesy of Jared Griebel/Pyun lab, University of Arizona - department of chemistry and biochemistry)

The polymers prepared have molecular weights below 10,000 and are amenable to be processed from solution or melt into patterned films. Pyun says, “The sulfur polymers are glassy and exhibit low glass transition temperatures.”

The researchers prepared a cathode with a sulfur polymer containing 90 percent sulfur and 10 percent 1, 3-diisopropenylbenzene. This material has similar electrochemical properties to elemental sulfur, according to Pyun.

In testing, the lithium-sulfur batter generated a charge of 800 milliampere-hour per gram after 100 charge, discharge cycles. Pyun says, “Lithium-sulfur batteries prepared with elemental sulfur do not provide this type of performance as they are quickly used up after 40 to 100 cycles.”

Pyun believes that this high sulfur-containing copolymer has strong potential for use in batteries. He adds, “We are trying to develop a new paradigm that is both sustainable and relevant to the development of a cost-effective alternative energy.”

Further information on this work can be found in a recent article (2) or by contacting Pyun at jpyun@email.arizona.edu.

REFERENCES
1. Canter, N. (2013), “Improved Sulfur Removal from Petroleum-Based Fuels,” TLT, 69 (3), pp. 8-9.
2. Chung, W., Griebel, J., Kim, E., Yoon, H., Simmonds, A., Ji, H., Dirlam, P., Glass, R., Wie, J., Nguyen, N., Guralnick, B., Park, J., Somogyi, A., Theato, P., Mackay, M., Sung, Y., Char, K. and Pyun, J. (2013), “The Use of Elemental Sulfur as an Alternative Feedstock for Polymeric Materials,” Nature Chemistry, 5, pp. 518-524.


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.