Inverse vulcanization is a process that produces polysulfide polymers, which also contain some organic linkers. [1] In contrast, sulfur vulcanization produces material that is predominantly organic but has a small percentage of polysulfide crosslinks.
Like Thiokols and sulfur-vulcanization, inverse vulcanization uses the tendency of sulfur catenate. The polymers produced by inverse vulcanization consist of long sulfur linear chains interspersed with organic linkers. Traditional sulfur vulcanization produces a cross-linked material with short sulfur bridges, down to one or two sulfur atoms.
The polymerization process begins with the heating of elemental sulfur above its melting point (115.21 °C), to favor the ring-opening polymerization process (ROP) of the S8 monomer, occurring at 159 °C. As a result, the liquid sulfur is constituted by linear polysulfide chains with diradical ends, which can be easily bridged together with small dienes, such as 1,3-Diisopropylbenzene(DIB), [1] 1,4-diphenylbutadiyne, [2] limonene, [3] divinylbenzene (DVB), [4] dicyclopentadiene, [5] styrene, [6] 4-vinylpyridine, [7] cycloalkene [8] and ethylidene norbornene, [9] or longer organic molecules as polybenzoxazines, [10] squalene [11] and triglyceride. [12] Chemically, the diene carbon-carbon double bond (C=C) of the substitutional group disappears, forming the carbon-sulfur single bond (C-S) which binds together the sulfur linear chains. The advantage of such a polymerization is the absence of a solvent; Sulphur acts as comonomer and solvent. This makes the process highly scalable at the industrial level, and kilogram-scale synthesis of the poly(S-r-DIB) has already been accomplished. [13]
Vibrational spectroscopy was performed to investigate the chemical structure of the copolymers, and the presence of the C-S bonds was detected through Infrared or Raman spectroscopies. [14] The high amount of S-S bonds makes the copolymer highly IR-inactive in the near and mid-infrared spectrum. As a consequence, sulfur-rich materials made via inverse vulcanization are characterized by a high refractive index (n~1.8), whose value depends again upon the composition and crosslinking species. [15] As shown by thermogravimetric analysis (TGA), the copolymer thermal stability increases with the amount of added crosslinker; however, all the tested compositions degrade above 222 °C. [2] [4]
Copolymer behavior included that, the glass-transition temperature depends upon the composition and crosslinking species. For given comonomers, the behavior of the copolymers as a function of the temperature depends on the chemical composition; for example, the poly (sulfur-random-divinylbenzene) behaves as a plastomer for a diene content between 15-25%wt, and as a viscous resin with the 30–35%wt of DVB. On the other hand, the poly (sulfur-random-1,3-diisopropenylbenzene) acts as thermoplastic at 15–25%wt of DIB, while it becomes a thermoplastic-thermosetting polymer for a diene concentration of 30-35%wt. [16] The potential to break and reform the chemical bonds along the polysulfide chains (S-S) allows the repair of the copolymer by simply heating above 100 °C. This increases the ability to reform and recycle the high molecular weight copolymer. [17]
The sulfur-rich copolymers made via inverse vulcanization could in principle find diverse applications due to their simple synthesis process and thermoplasticity.
This new way of sulfur processing has been exploited for the cathode preparation of long-cycling lithium-sulfur batteries. Such electrochemical systems are characterized by a greater energy density than commercial Li-ion batteries, but they are not stable for long service life. Simmonds et al. first demonstrated improved capacity retention for over 500 cycles with an inverse vulcanization copolymer, suppressing the typical capacity fading of sulfur-polymer composites. [18] The poly (sulfur-random-1,3-diisopropenylbenzene), briefly defined as poly (S-r-DIB), showed a higher composition homogeneity compared with other cathodic materials, together with greater sulfur retention and an enhanced adjustment of the polysulfides' volume variations. These advantages made it possible to assemble a stable and durable Li-S cell. Subsequently, other copolymers were synthesized via inverse vulcanization and tested inside these electrochemical devices, again providing high stability over their cycles.
| Cathode | Date | Source | Specific Capacity after cycling |
|---|---|---|---|
| Poly (sulfur-random-1,3-diisopropylbenzene) | 2014 | University of Arizona [18] | 800 mA⋅h/g after 300 cycles (at 0.1 C) |
| Poly (sulfur-random-1,4-Diphenyl-1,3-butadiene) | 2015 | University of Arizona [2] | 800 mA⋅h/g after 300 cycles (at 0.2 C) |
| Poly (sulfur-random-divinylbenzene) | 2016 | University of the Basque Country [19] | 700 mA⋅h/g after 500 cycles (at 0.25 C) |
| Poly (sulfur-random-diallyl disulfide) | 2016 | University of the Basque Country [20] | 616 mA⋅h/g after 200cycles (at 0.2 C) |
| Poly (sulfur-random-bismaleimide-divinylbenzene) | 2016 | Istanbul Technical University [21] | 400 mA⋅h/g after 50 cycles (at 0.1 C) |
| Poly (sulfur-random-styrene) | 2017 | University of Arizona [6] | 485 mA⋅h/g after 1000 cycles (at 0.2 C) |
In order to overcome the disadvantages related to the materials' low electrical conductivity (1015–1016 Ω·cm), [16] researchers have started to add special carbon-based particles to increase electron transport inside the copolymer. Furthermore, such carbonaceous additives improve the polysulfides' retention at the cathode through the polysulfides-capturing effect, increasing the battery performances. Examples of employed nanostructures are long carbon nanotubes, [22] graphene, [11] and carbon onions. [23]
The new materials could be used to remove toxic metals from soil or water. Pure sulfur cannot be employed to manufacture a functional filter because of its low mechanical properties; therefore, inverse vulcanization was investigated to produce porous materials, in particular for the mercury capturing process. The liquid metal binds together with the sulfur-rich copolymer, remaining mostly inside the filter. [3] [24] [25]
Sulfur-rich copolymers, made via inverse vulcanization, have advantages over traditional IR optical materials due to the simple manufacturing process, low cost reagents, and high refractive index. As mentioned before, the latter depends upon the S-S bonds concentration, leading to the ability to tune the optical properties of the material by modifying the chemical formulation. The ability to change the material's refractive index to fulfill the specific application requirements makes these copolymers applicable in military, civil or medical fields. [15] [26] [27] [28]
The inverse vulcanization process can also be employed for the synthesis of activated carbon with narrow pore-size distributions. The sulfur-rich copolymer acts as a template where the carbons are produced. The final material is doped with sulfur and exhibits a micro-porous network and high gas selectivity. Therefore, inverse vulcanization could also be used for gas separation applications. [29]
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. In comparison with other rechargeable batteries, Li-ion batteries are characterized by a higher specific energy, higher energy density, higher energy efficiency, longer cycle life and longer calendar life. Also noteworthy is a dramatic improvement in lithium-ion battery properties after their market introduction in 1991: within the next 30 years their volumetric energy density increased threefold, while their cost dropped tenfold.
A sodium–sulfur (NaS) battery is a type of molten-salt battery that uses liquid sodium and liquid sulfur electrodes. This type of battery has a similar energy density to lithium-ion batteries, and is fabricated from inexpensive and non-toxic materials. However, due to the high operating temperature required, as well as the highly corrosive and reactive nature of sodium and sodium polysulfides, these batteries are primarily suited for stationary energy storage applications, rather than for use in vehicles. Molten Na-S batteries are scalable in size: there is a 1 MW microgrid support system on Catalina Island CA (USA) and a 50 MW/300 MWh system in Fukuoka, Kyusyu, (Japan).
An elastomer is a polymer with viscoelasticity and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of elastic polymer, is often used interchangeably with rubber, although the latter is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature, so that considerable molecular reconformation is feasible without breaking of covalent bonds. At ambient temperatures, such rubbers are thus relatively compliant and deformable. Their primary uses are for seals, adhesives and molded flexible parts.
Polysulfides are a class of chemical compounds derived from anionic chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. The inorganic polysulfides have the general formula S2−
n. These anions are the conjugate bases of polysulfanes H2Sn. Organic polysulfides generally have the formulae R1SnR2, where R = alkyl or aryl.
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The element sulfur exists as many allotropes. In number of allotropes, sulfur is second only to carbon. In addition to the allotropes, each allotrope often exists in polymorphs delineated by Greek prefixes.
The lithium–sulfur battery is a type of rechargeable battery. It is notable for its high specific energy. The low atomic weight of lithium and moderate atomic weight of sulfur means that Li–S batteries are relatively light. They were used on the longest and highest-altitude unmanned solar-powered aeroplane flight by Zephyr 6 in August 2008.
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Titanium disulfide is an inorganic compound with the formula TiS2. A golden yellow solid with high electrical conductivity, it belongs to a group of compounds called transition metal dichalcogenides, which consist of the stoichiometry ME2. TiS2 has been employed as a cathode material in rechargeable batteries.
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NASICON is an acronym for sodium (Na) Super Ionic CONductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. NASICON compounds have high ionic conductivities, on the order of 10−3 S/cm, which rival those of liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.
A magnesium sulfur battery is a rechargeable battery that uses magnesium ion as its charge carrier, magnesium metal as anode and sulfur as cathode. To increase the electronic conductivity of cathode, sulfur is usually mixed with carbon to form a cathode composite. Magnesium sulfur battery is an emerging energy storage technology and now is still in the stage of research. It is of great interest since in theory the Mg/S chemistry can provide 1722 Wh/kg energy density with a voltage at ~1.7 V.
Magnesium batteries are batteries that utilize magnesium cations as the active charge transporting agents in solution and often as the elemental anode of an electrochemical cell. Both non-rechargeable primary cell and rechargeable secondary cell chemistries have been investigated. Magnesium primary cell batteries have been commercialised and have found use as reserve and general use batteries.
Sulfur vulcanization is a chemical process for converting natural rubber or related polymers into materials of varying hardness, elasticity, and mechanical durability by heating them with sulfur or sulfur-containing compounds. Sulfur forms cross-linking bridges between sections of polymer chains which affects the mechanical and electronic properties. Many products are made with vulcanized rubber, including tires, shoe soles, hoses, and conveyor belts. The term vulcanization is derived from Vulcan, the Roman god of fire.
Calcium (ion) batteries are energy storage and delivery technologies (i.e., electro–chemical energy storage) that employ calcium ions (cations), Ca2+, as the active charge carrier in the electrolytes as well as in the electrodes (anode and cathode). Calcium (ion) batteries remain an active area of research, with studies and work persisting in the discovery and development of electrodes and electrolytes that enable stable, long-term battery operation.
Arumugam Manthiram is an American materials scientist and engineer, best known for his identification of the polyanion class of lithium ion battery cathodes, understanding of how chemical instability limits the capacity of layered oxide cathodes, and technological advances in lithium sulfur batteries. He is a Cockrell Family Regents Chair in engineering, Director of the Texas Materials Institute, the Director of the Materials Science and Engineering Program at the University of Texas at Austin, and a former lecturer of Madurai Kamaraj University. Manthiram delivered the 2019 Nobel Lecture in Chemistry on behalf of Chemistry Laureate John B. Goodenough.
A solid-state electrolyte (SSE) is a solid ionic conductor and electron-insulating material and it is the characteristic component of the solid-state battery. It is useful for applications in electrical energy storage (EES) in substitution of the liquid electrolytes found in particular in lithium-ion battery. The main advantages are the absolute safety, no issues of leakages of toxic organic solvents, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability. This makes possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte thanks to the property of lithium dendrite suppression in the presence of a solid-state electrolyte membrane. The use of a high capacity anode and low reduction potential, like lithium with a specific capacity of 3860 mAh g−1 and a reduction potential of -3.04 V vs SHE, in substitution of the traditional low capacity graphite, which exhibits a theoretical capacity of 372 mAh g−1 in its fully lithiated state of LiC6, is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. Moreover, this allows the reach of gravimetric and volumetric energy densities, high enough to achieve 500 miles per single charge in an electric vehicle. Despite the promising advantages, there are still many limitations that are hindering the transition of SSEs from academia research to large-scale production, depending mainly on the poor ionic conductivity compared to that of liquid counterparts. However, many car OEMs (Toyota, BMW, Honda, Hyundai) expect to integrate these systems into viable devices and to commercialize solid-state battery-based electric vehicles by 2025.
Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li
1+xAl
xGe
2-x(PO
4)
3. LAGP belongs to the NASICON family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10–5 - 10–4 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like Li
10GeP
2S
12 (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of Li
1.5Al
0.5Ge
1.5(PO
4)
3, which is also the typically used material in battery applications.
Zeta Energy is a Houston, Texas-based company that develops lithium-sulfur batteries based on two proprietary technologies: a sulfurized carbon cathode and a 3D-structured metallic lithium anode. This combination yields a battery with high energy density and lower cost than traditional lithium-ion batteries, while also offering comparable cyclability and better safety. The elimination of metals like cobalt, manganese and nickel also dramatically simplifies the supply chain for battery manufacturing.