3D graphene framework supported Li2S coated with ultra-thin Al2O3 films: binder-free cathodes for high-performance lithium sulfur batteries

Lithium sulfide (Li2S) has drawn special attention as a promising cathode material for emerging energy storage systems due to its high theoretical specific capacity and great compatibility with lithium metal-free anodes. However, Li2S cathodes urgently require a solution to increase their poor electrical conductivity and to suppress the dissolution of long-chain polysulfide (Li2Sn, 4 less than or equal to n less than or equal to 8) species into electrolyte. To this end, we report a free-standing Al2O3-Li2S-graphene oxide sponge (GS) composite cathode, in which ultrathin Al2O3 films are preferentially coated on Li2S by an atomic layer deposition (ALD) technique. As a result, a combination of high electron conductivity (from GS) and strong binding with Li2Sn (from ultrathin Al2O3 films) was designed for cathodes. The newly developed Al2O3-Li2S-GS cathodes are able to deliver a highly reversible capacity of 736 mA h gLi2S-1 (427 mA h gcathode-1) at 0.2C, which is much higher than that of corresponding cathodes without Al2O3 (59%). Also, the long-term cycling stability of Al2O3-Li2S-GS cathodes was demonstrated up to 300 cycles at 0.5C with an excellent capacity retention of 88%. In addition, combined with density functional theory calculations, the promotional mechanism of ultrathin Al2O3 films was elucidated using extensive characterization. The ultra-thin Al2O3 film with optimal thickness not only acts as a physical barrier to Li2S nanoparticles, but provides a strong binding interaction to suppress Li2Sn species dissolution.