Perspective on ultramicroporous carbon as sulphur host for Li–S batteries

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This article provides a comprehensive insight into the state of sulphur infused in ultramicroporous carbon (UMC), its electrochemical reaction mechanism with Li, and clarifies the role of SEI on the reactivity of UMC-S composites. Lithium–sulphur (Li–S) batteries are currently considered as next-generation battery technology. Sulphur is an attractive positive electrode for lithium metal batteries, mainly due to its high capacity (1675 mAh g −1 ) and high specific energy (2600 Wh kg −1 ). The electrochemical reaction of lithium with sulphur in non-aqueous electrolytes results in the formation of electrolyte soluble intermediate lithium–polysulphides. The dissolved polysulphides shuttle to the anode and get reduced at the anode resulting in Li metal corrosion. The solubility of polysulphide gradually reduces the amount of sulphur in the cathode, thereby limiting the cycle life of Li–S batteries. Several strategies have been proposed to improve the cycling stability of Li–S batteries. A unique approach to eliminate the polysulphide shuttle is to use ultramicroporous carbon (UMC) as a host for sulphur. The pore size of UMC which is below 7 Å, is the bottleneck for carbonate solvents to access sulphur/polysulphides confined in the pores, thereby preventing the polysulphide dissolution. This perspective article will emphasise the role of UMC host in directing the lithiation mechanism of sulphur and in inhibiting polysulphide dissolution, including the resulting parasitic reaction on the lithium anode. Further, the challenges that need to be addressed by UMC-S based Li–S batteries, and the strategies to realise high power density, high Coulombic efficiency, and resilient Li–S batteries will be discussed.