|Fig. 1: Sulfur, the cathode material used in lithium-sulfur batteries. (Source: Wikimedia Commons)|
Current lithium-ion batteries we use today, based on transition metal oxide cathodes and graphite anodes, have a theoretical specific energy of 387 Wh/kg.  Lithium-sulfur batteries, on the other hand, have a theoretical specific energy of 2,567 Wh/kg, which is about 6-7 times higher.  The successful development of lithium-sulfur batteries can help to meet our ever- increasing demand for high-energy storage in many modern-day applications, including portable electronics, electric vehicles and grid-scale storage. Moreover, sulfur is cheap, environmentally benign and readily abundant in the Earth's crust, which makes lithium-sulfur batteries particularly attractive.
However, there are several challenges that impede the successful commercialization of lithium- sulfur batteries. On the sulfur cathode side, both the charge product (sulfur) and the discharge product (lithium sulfide) are insulating in nature, resulting in poor material utilization.  Moreover, during the cycling process, they form a series of long chain lithium polysulfide species which dissolve into the electrolyte, leading to continual loss of active material and rapid capacity decay.  Finally, sulfur undergoes a large volumetric expansion of about 80% upon full lithiation to lithium sulfide, which causes pulverization and structural damage at the electrode level. 
On the lithium anode side, lithium metal is highly reactive and prone to formation of dendrites, which can potentially lead to short circuits and associated safety hazards.  Moreover, the long chain lithium polysulfides that dissolve into the electrolyte can diffuse to the lithium anode and become reduced to form short chain polysulfides on the surface. This causes the so-called shuttle effect and low Coulombic efficiency that plague lithium-sulfur batteries. 
The most common approach to alleviate these problems is to encapsulate sulfur with conducting materials such as mesoporous carbon or graphene. For example, Nazar and co-workers demonstrated lithium-sulfur batteries based on infiltration of sulfur into the pores of mesoporous carbon, CMK-3, for high capacity and stable cycling over 20 cycles.  The mesoporous carbon not only helps to improve the electrical conductivity of the composite, but also acts as a physical barrier to trap the lithium polysulfide species, reducing their dissolution into the electrolyte. Recently, the use of microporous carbon was also explored for even better physical confinement effect. Wan and co-workers found that by synthesizing small sulfur molecules in the confined space of a microporous carbon matrix, the unfavorable formation of long-chain polysulfides can be avoided, hence avoiding the dissolution problem.  Using this structure, they demonstrated a high specific capacity that is close to theoretical limit of sulfur cathodes as well as stable cycling over 200 cycles. 
However, further research and evidence suggests that physical entrapment alone is insufficient to prevent the dissolution of lithium polysulfides into the electrolyte. Strong chemical binding with the polysulfide species is also necessary, especially to achieve long cycle life in lithium-sulfur batteries. Unfortunately, carbon itself, being non-polar in nature, does not interact strongly with the strongly-polar lithium polysulfide species. As a result, extensive research has been focused on encapsulating sulfur with carbon-based materials that have been functionalized with polar groups (such as oxygen and nitrogen-rich groups) that can bind strongly with polysulfide species. For instance, Zhang and co-workers demonstrated the use of graphene oxide, with its oxygen-rich groups, to immobilize sulfur and polysulfide species in lithium-sulfur batteries, achieving high capacity and stable cycling over 50 cycles.  The same group further demonstrated that nitrogen-doped graphene is also an effective coating material for sulfur cathodes. In this case, sulfur nanoparticles were wrapped inside nitrogen-doped graphene sheets to form a nanocomposite cathode, achieving an ultralong cycle life of 2,000 cycles.  The results of theoretical ab initio simulations also support the strong binding capability of nitrogen functional groups with lithium polysulfides through lithium-nitrogen interaction, hence explaining the excellent cycling performance.  Overall, these works represent promising steps towards the development of high-performance and long- lasting lithium-sulfur batteries.
© Zhi Wei Seh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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