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 (Li₂S_n, 4 ≤ n ≤ 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 Li₂S_n (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 g_Li2S⁻¹ (427 mA h g_cathode⁻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 Li₂S_n species dissolution.