Activation of OxygenStabilized Sulfur for Li and Na Batteries

At present, the most promising alternative cathode material is sulfur due to its high theoretical capacity (1672 mAh g −1 ), low cost, high abundance in nature, and environmental benignity. [ 11–13 ] However, the rechargeable battery chemistry based on sulfur cathode still faces three intrinsic challenges: [ 14–19 ] (1) the formation of intermediate polysulfi de products and the parasitic shuttle reaction caused by them during lithiation/ delithiation process, resulting in low Coulombic effi ciency and rapid capacity fading; (2) the extremely low electronic and ionic conductivities of both starting material S and ending product Li 2 S, which are responsible for not only low capacity utilization but also poor power density; and (3) the stress/strain induced by the large volume difference (76%) between sulfur (2.03 g cm −3 ) and Li 2 S (1.66 g cm −3 ) during a complete lithiation/delithiation cycle, which destroys the physical integrity of sulfur cathode and results in fast capacity loss. Signifi cant efforts have been made to address these challenges, the most popular of which is to entrap sulfur into electronic conductive hosts of nanostructures, such as microporous carbon, carbon nanotube, graphene, graphene oxide, and carbon nanofi ber; [ 20–32 ] nevertheless, commercialization of sulfur cathode remains remote. In fact, since these three challenges are closely entangled, it is diffi cult to circumvent all of them with a single strategy. For example, adoption of electrolytes with high solubility for high-order polysulfi de effectively relieved the poor conductivity issue and reduced the stress/strain, [ 33 ] but it also accelerated the parasitic shuttle reaction, while the sulfur−TiO 2 yolk−shell nanoarchitecture with internal void space successfully accommodated the volume expansion of sulfur, [ 17 ] but the lower electronic conductivity of TiO 2 -host further worsened the utilization and reaction kinetics of S-TiO 2 . Carbon coating on Li 2 S mitigated the stress/strain and the loss of active species due to the physical disintegration of the electrode, but the large particle size (500 nm–2 μm) of Li 2 S reduced the utilization. [ 34 ] Activation of Oxygen-Stabilized Sulfur for Li and Na Batteries