Alleviating polarization by designing ultrasmall Li2S nanocrystals encapsulated in N-rich carbon as a cathode material for high-capacity, long-life Li-S batteries

Lithium sulfide (Li2S), which has a high theoretical specific capacity of 1166 mA h g(-1), has potential application in cathode materials because of its high safety and compatibility with lithium-free anodes for Li-S batteries. However, its low electron conductivity and lithium transfer cause significant polarization in Li2S electrodes. Here, we demonstrate the use of ultrasmall Li2S nanocrystals encapsulated in N-rich carbon (NRC) as a cathode material for Li-S batteries. By evaporating a mixture of polyacrylonitrile (PAN) and Li2S in dimethylformamide (DMF) solution and then subjecting the mixture to carbonization, a nano-Li2S@NRC composite with ultrasmall Li2S well dispersed in its carbon matrix was successfully synthesized. The obviously lower potential barriers and excellent cycling performance of nano-Li2S@NRC electrodes confirm their improved polarization because of the size effect of Li2S nanocrystals and the good electron transfer between Li2S and N-doped carbon. The nano-Li2S@NRC cathode delivers a high initial specific capacity of 1046 mA h g(-1) of Li2S (similar to 1503 mA h g(-1) of S) at 0.25C and 958 mA h g(-1) of Li2S (similar to 1376 mA h g(-1) of S) at 0.5C with a favorable cycling performance with an similar to 0.041% decay rate per cycle over 1000 cycles.