Reports: ND1051026-ND10: Novel Utilization of Elemental Sulfur for Nanocomposite Materials and Energy Storage

Jeffrey Pyun, PhD, University of Arizona

Executive Summary: In the funding period of 2012-2013 for this ACS-PRF ND award, we have continued developing our new synthetic advanced, termed inverse vulcanization to utilize elemental sulfur as a novel sustainable feedstock for polymeric materials.  We have further demonstrated the useful electrochemical and optical properties of these sulfur plastics for next generation Li-S batteries and IR photonics. 

Utilization of Waste Sulfur for Polymeric Materials.  We have developed a new area of chemistry at the interface of petrochemicals, sustainability, materials and energy to utilize elemental sulfur as an alternative feedstock to prepare a novel class of sulfur plastics and materials. Our efforts in this area capitalize on the incredible abundance (nearly 7 million tons annually) of excess and unused elemental sulfur generated from petroleum refining.  Due to the limited number of applications/used of elemental sulfur, these excess amounts at simply stored as bricks, or powders in megaton, exposed storage deposits.    We have developed a single step, scalable process to convert sulfur into polymeric materials with a wide range of useful properties.  The key advance has been the direct use of molten liquid sulfur as an unconventional solvent for these polymerizations and chemical reactions.

Elemental sulfur exhibits useful electrochemical and optical properties.  However, the preparation of sulfur-rich copolymers, with a very high content of sulfur (50-95 wt%) have not been explored, due to the limited synthetic methods that have been developed for the polymerization of elemental sulfur.    To obviate these synthetic challenges, we have developed a new bulk polymerization method, termed, inverse vulcanization, that for the first time uses molten liquid sulfur as the reaction medium and comonomer for the synthesis of sulfur-rich copolymers.  While it has been known that elemental sulfur can undergo homopolymerize via ring-opening processes to form polymeric sulfur, this form of polymeric sulfur is chemically unstable and depolymerize back to monomeric S8.  Our new inverse vulcanization methodology allowed for the first time the preparation of a chemically stable copolymer with a very high content of sulfur and S-S bonds in the copolymer backbone.  Additionally, this modification of elemental sulfur enabled the production of melt and solution processable sulfur polymers.  With the developed of this new synthetic approach to prepare sulfur materials, we demonstrated that these materials are useful for two emerging applications in batteries and optical materials as described below:

A. High Capacity Sulfur Polymeric Electrodes for Next Generation Batteries. We have demonstrated the viability of inverse vulcanization to prepare cathodes for Li-S batteries using sulfur copolymers as the electroactive material, which excellent electrochemical performance using a very inexpensive polymeric material.  Li-S batteries have generated considerable interest due to their high theoretical specific capacity (1672 mAh/g) and energy density (2800 Wh/L) for the creation of light weight energy storage systems. These systems have charge capacities 4-5 times greater than current Li-ion technology, which is one of many next generation systems being investigated to address future battery needs in electric vehicles and electrical grid storage, which require storage systems with higher energy densities.   These sulfur copolymers exhibit the highest charge capacity and cycle performance of any polymeric electrode material for Li-batteries.  Future efforts in this area will focus on developing new chemistry and polymeric materials based on sulfur the following targets:

1)      Synthesis of conductive polymers with a high content of sulfur exhibiting enhanced Li-S battery performance with enhanced capacity retention (above 800 mAh/g beyond 500 charge-discharge cycles)

2)      New, very inexpensive condensation polymers with a high content of sulfur (50-99 wt% sulfur) with exhibiting enhanced Li-S battery performance with enhanced capacity retention (above 800 mAh/g beyond 500 charge-discharge cycles)

B. High refractive index polymers for IR optical materials. We seek to create a new class and application of high refractive index polymers for Infrared Optics.  To date, only inorganic semiconductors (e.g., Germanium-Ge; chalcogenide glasses) are used for IR optical applications, which are typically expensive and difficult to process (polishing-grinding).  For IR imaging, we propose to create a new class of high refractive index polymers that are completely melt, or solution processable into free standing films, or lenses.  We have demonstrated that sulfur copolymers prepared by inverse vulcanization have tunable refractive indices from n = 2.1 to 1.8, due to the very high fraction of polymeric S-units in the copolymer.  While the incorporation of sulfur into polymers has long been known to enhance the refractive index of polymeric materials, the proposed research enables a significantly higher fraction of S-units into copolymers, which accordingly affords very high n-values.  The applications of these materials as processable lenses and components in IR-optical devices are currently being explored.  (work in this area has just been submitted to Adv. Mater. 

Broader Impact of ACS-PRF ND Support:  The broader impacts of this work resulting from ACS-PRF support have been significant to develop a potentially transformative technology for sulfur utilization resulting in new intellectual property by Pyun at the University of Arizona, a high profile publication (Nature Chemistry, 2013, 5, 518) and numerous press releases (highlighted in Nature Materials 2013, 12, 472; ACS Macro Letters 2013, 2, 839; NPG Asia Materials 2013, 5, e64); search “Better Batteries using Waste Sulfur” for on-line press).  Furthermore, preliminary data from this award enabled further support thru a recent grant from the National Science Foundation (CHEM-MSN, “Electroactive Polymers via Inverse Vulcanization of Elemental Sulfur; Sept. 1, 2013-Aug. 31, 2016; $673,000).