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Electrochem 2020, 1 242 One rather novel approach to growing a cathode material was to grow carbon fibers from the vapor phase to penetrate sulfur crystals [92]. A reasonable areal mass loading and capacity (3.4–4.4 mAh cm−2) were achieved with this cathode. Between 93–97% of capacity is retained at low current densities (0.2–0.5 C) Concerning the stability of the capacity, 90% of the initial 920 mAh g−1 capacity is retained after 200 cycles (0.1 C), with a reported coulombic efficiency of 100%. Electrochem 2020, 2, FOR PEER REVIEW 18 4.3. Less Common Allotropic Forms of Carbon and Sulfur 4.3. Less Common Allotropic Forms of Carbon and Sulfur Although carbon nanotubes and graphene elements play a central role in Li-S and many other Although carbon nanotubes and graphene elements play a central role in Li-S and many other emerging electrochemical technologies, more recently engineered allotropes of carbon, such as emerging electrochemical technologies, more recently engineered allotropes of carbon, such as graphynes, may well prove to be important elements in such technologies as well. An interesting graphynes, may well prove to be important elements in such technologies as well. An interesting example is the recent exploration of graphdiyne nanosheets imbued on a polypropylene membrane example is the recent exploration of graphdiyne nanosheets imbued on a polypropylene membrane (GDY-PP, Figure 14) as a separator [62]. This design has merit in improving the adsorption of polysulfide (GDY-PP, Figure 14) as a separator [62]. This design has merit in improving the adsorption of species on the basis of the increased electronegativity of sp-hybridized carbon atoms in the diyne polysulfide species on the basis of the increased electronegativity of sp-hybridized carbon atoms in units. These sp-hybridized carbon atoms are not present in typical graphene or carbon nanotubes, the diyne units. These sp-hybridized carbon atoms are not present in typical graphene or carbon which comprise exclusively sp2-hybridized carbon atoms. This chemical intuition was born out in nanotubes, which comprise exclusively sp2-hybridized carbon atoms. This chemical intuition was the performance of the cell, which displayed a high initial capacity (1262 mAh g−1, 0.1 C) and after born out in the performance of the cell, which displayed a high initial capacity (1262 mA h g−1, 0.1 C) 500 cycles still exhibited a capacity of 412 mAh g−1 at 1 C. As illustrated by the performance of this and after 500 cycles still exhibited a capacity of 412 mA h g−1 at 1 C. As illustrated by the performance initial cell, utilizing additional graphyne and related emerging carbon allotropes should certainly be of this initial cell, utilizing additional graphyne and related emerging carbon allotropes should pursued as an avenue to find the optimal separator layers for Li-S batteries. certainly be pursued as an avenue to find the optimal separator layers for Li-S batteries. FigFuigreur1e4.1S4c.hSecmheamticatiilcluisltlurastiroantionfthoferthoelerooflethoefsetphearsaetoprarcaotmorprciosminpgrgisrianpghgdriaypnhedniaynoeshnaeentossihmeebtused oniampbouleydproonpaylpeonleypmreompyblreanneem(GemDbYr-aPnPe).(RGeDpYri-nPtPe)d. Rfreopmrinrteefderfernocme [r6e2fe],re©nc2e0[2602u],s©ed2w02i0thupsedrmwiistshion permission from the American Chemical Society. from the American Chemical Society. JusJut satsansenwewalalolltortorpoepsesoof fcacarbrboonnaarreebeiing developed forusseeininLLi-iS-Sbabtattetreireise,sn, enwewfofromrsmosf osuf lsfulrfur arearaelsaolsporipmriemteartagregtestsfofroreveavlauluataitoionn..Atstandardtemperraattuurreeaannddppreressusurere(S(TSPT)P,)s,uslufulfru’sr’msomstost prevalent allotrope is as an S8 ring in a solid, orthorhombic crystalline state. In 2019, it was reported prevalent allotrope is as an S8 ring in a solid, orthorhombic crystalline state. In 2019, it was reported that a liquid form of sulfur can be generated in an electrochemical cell in a supercooled state well that a liquid form of sulfur can be generated in an electrochemical cell in a supercooled state well below the melting point of orthorhombic sulfur [93]. The researchers who made this remarkable below the melting point of orthorhombic sulfur [93]. The researchers who made this remarkable discovery quickly recognized that such a new form of sulfur should be evaluated in Li-S battery discovery quickly recognized that such a new form of sulfur should be evaluated in Li-S battery contexts [94]. In the initial study in this vein, it was revealed that very different areal capacities are contexts [94]. In the initial study in this vein, it was revealed that very different areal capacities are achieved with the supercooled liquid sulfur versus the typical solid sulfur, but most of the study achieved with the supercooled liquid sulfur versus the typical solid sulfur, but most of the study focused on understanding the morphology and phase changes in space during operation of the cell, focused on understanding the morphology and phase changes in space during operation of the cell, so a so a full picture of how supercooled liquid sulfur might perform in Li-S batteries of various full picture of how supercooled liquid sulfur might perform in Li-S batteries of various configurations configurations remains to be elucidated. Much more work will need to be done to evaluate the remains to be elucidated. Much more work will need to be done to evaluate the performance of this performance of this new form of sulfur and other forms of sulfur that may be attainable. new form of sulfur and other forms of sulfur that may be attainable. An emerging technique for the synthesis of high sulfur-content materials is by inverse An emerging technique for the synthesis of high sulfur-content materials is by inverse vulcanization [63]. Since these materials can often stabilize oligomeric or polymeric forms of sulfur vulcanization [63]. Since these materials can often stabilize oligomeric or polymeric forms of sulfur that that may have different electrochemical performance than the usual S8 allotrope, materials made by mayhavedifferentelectrochemicalperformancethantheusualS allotrope,materialsmadebyinverse 8 inverse vulcanization should be carefully and thoroughly pursued as an element of lithium sulfur vulcanization should be carefully and thoroughly pursued as an element of lithium sulfur batteries. batteries. The utility of high sulfur-content materials prepared via inverse vulcanization for Li-S batteries has been so hotly pursued recently that the area was recently reviewed [95]. Since that review, however, other reports have emerged. One example [96] employs poly(sulfur-co-1-vinyl-3- allylimidazolium bromide) made by inverse vulcanization of S8 with an ionic polymer. The dual role of this material as cathode and polysulfide suppression material was demonstrated and supported by DFT calculations as well. The cell employing this cathode retained over 90% of its initial capacityPDF Image | Lithium-Sulfur Batteries: Advances and Trends
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