(Vulcanization is the chemical process that makes rubber more durable by adding a small amount of sulfur to rubber. The researchers dubbed their process “inverse vulcanization” because it requires mostly sulfur with a small amount of an additive.)

Lithium−sulfur (Li−S) batteries are considered one of the promising candidates for “beyond Li-ion” technology, given sulfur’s high theoretical specific capacity (1672 mAh/g) and high specific energy (~2600 Wh/kg). While Li-S batteries with capacities of 1200 mAh/g are fairly common, the authors noted, rapid fading of charge capacity is an issue.

The poor long-term performance has been associated with both the shuttling of polysulfides dissolved into the electrolyte medium, in addition to irreversible deposition of solid lithium sulfide (Li₂S) and other mixtures of insoluble discharge products on the cathode.

While electrolyte additives have suppressed polysulfide shuttling, repeated cycling ultimately leads to insoluble sulfide deposits encrusted on the carbon cathode framework resulting in both mechanical and electrical detachment from the electrode, leading to failure.

Other researchers have shown that sulfur-based nano-composite materials can improve the performance of Li−S batteries. However, the University of Arizona researchers note, challenges still persist in the creation of chemistry for sulfur-based cathode materials that are inexpensive and amenable to large scale production, while retaining high charge capacity and electrochemical stability.

Last year, the team reported the sulfur copolymers synthesized via inverse vulcanization exhibited high specific capacity (823 mAh/g at 100 cycles).

In this report, we explore for the first time with these sulfur copolymers a direct structure−property correlation of copolymer composition with electro-chemical properties to afford optimal polymeric materials for these battery systems. We further demonstrate improved Li−S battery lifetimes out to 500 charge−discharge cycles with excellent retention of charge capacity.

The enhanced battery performance observed with these polymeric active materials arises from in situ generation of organosulfur additives (from DIB units) and linear polysulfide segments (Li_xS_y) via electrochemical fragmentation of the initial poly(S-r-DIB) copolymer. We propose that these organosulfur species suppress irreversible deposition of insoluble discharge products (Li₂S₃, Li₂S₂, Li₂S) and are mechanistically distinct from recent Li−S battery systems that nanoencapsulate sulfur to suppress dissolution of linear polysulfides. This sulfur based copolymer is also a new addition to an emerging class of electroactive polymers that have been used as polymeric electrodes for Li batteries, examples of which include conjugated polymers and nitrosyl radical functional polymers.

To our knowledge, these novel sulfur copolymers exhibit one of the highest capacities of any wholly polymeric material serving as the active material in batteries cycled to extended lifetimes.

—Simmonds et al.

The Li−S batteries fabricated with poly(S-r-DIB) copolymers as the active cathode material are identical to traditional Li−S batteries using S₈, with the exception of soluble organosulfur species generated upon discharge of the copolymer. These organosulfur products co-deposit with other insoluble lower order polysulfides onto the carbon-binder cathode framework at the end of discharge; the researchers propose that this “plasticizes” these insoluble polysulfide discharge products, enabling more efficient battery cycling.

To investigate the effects of composition of the copolymer materials on battery performance, they fabricated a range of poly(S-r-DIB) copolymers into 2032-type battery coin cells and cycled them at a rate of C/10 (167.2 mA/g) with lithium foil employed as the anode.

They found that sulfur copolymers with 1% by mass DIB exhibited cycling perform- ance comparable to elemental sulfur, whereas copolymers with compositions of 20% or greater by mass DIB exhibited little to no improvement over elemental sulfur. However, poly(S-r- DIB) copolymers with compositions of 5, 10, and 15% by mass DIB all exhibited high initial capacities, low initial capacity loss, and consistently reduced capacity loss per cycle.

They concluded that copolymers with 10% by mass DIB performed the best. Cathodes made with this specific copolymer exhibited a specific capacity of 823 mAh/g at 100 cycles. Further optimization of cathode coating methods yielded significant improvement; an initial capacity of 1225 mAh/g was observed in the Li−S batteries fabricated in the present study and low capacity loss was exhibited, with capacities of 1005 mAh/g at 100 cycles and 817 mAh/g at 300 cycles with a Coulombic efficiency of 99% throughout. This system has been extended to 500 cycles while retaining a significant capacity of 635 mAh/g.

The results show that inexpensive, bulk copolymerization can sufficiently modify the properties of sulfur to improve the battery performance without the need for nanoscopic synthesis or processing. The team is exploring with other kinds of sulfur copolymers to further extend cycle life.

Resources

• Adam G. Simmonds, Jared J. Griebel, Jungjin Park, Kwi Ryong Kim, Woo Jin Chung, Vladimir P. Oleshko, Jenny Kim, Eui Tae Kim, Richard S. Glass, Christopher L. Soles, Yung-Eun Sung, Kookheon Char, and Jeffrey Pyun (2014) “Inverse Vulcanization of Elemental Sulfur to Prepare Polymeric Electrode Materials for Li–S Batteries,” ACS Macro Letters 3, pp 229–232 doi: 10.1021/mz400649w