For the practical applications of Li-S batteries, excellent energy/power densities and long cycling lifetimes along with high electrode mass loading must be achieved. This requires rational design of cathode’s architectures to facilitate fast electronic and ionic transport as well as accommodate the electrode’s volumetric change. Here we report a compact Li₂S/graphene cathode architectural design, where crystalline Li₂S nanoparticles wrapped by graphenes are generated simultaneously by burning lithium foil in CS₂ vapor, thus forming Li₂S@graphene nanocapsules. This structure represents the high possible volumetric efficiency for accommodating sulfur active species while providing superior electrical properties. As a result, the composite electrode exhibits remarkable electrochemical properties, where at a mass loading of 10 mg cm⁻², the electrode delivers high reversible capacity of 1160 mAh g⁻¹_S at 0.1 C rate and 800 mAh g⁻¹_S at 1.0 C rate as well as stable cycle performance.

Figs. 1a,b illustrate the nanocapsule structure of Li₂S@graphene. TEM images show that the particles have a compact core-shell shape, where the crystalline Li₂S cores display highly ordered lattice fringes, and they are surrounded by a contour coating of ~ 10 to 20 graphene layers, thus forming a capsule-like nanostructure. Figs. 1c-f show the electrochemical performance of Li₂S@graphene composite. Even at a high electrode loading of 10 mg_Li2S cm⁻², the cell delivers a high initial charge capacity of 1600 mAh g⁻¹ and a high discharge capacity of 1160 mAh g⁻¹. The capacity remains at 600 mAh g⁻¹ after 200 cycles. In addition, the cell exhibits a good rate capability with high discharge capacities of 1230 (0.1 C), 1105 (0.2 C), 984 (0.5 C), 802 (1.0 C), and 600 mAh g⁻¹ (2.0 C). This finding is mainly attributed to the improved electrical properties, electrochemical kinetics, and structural stability of Li₂S@graphene nanocapsules. In a word, this compact Li₂S@graphene capsule nanostructure exhibit good electrochemical performance.

Fig. 1 Structural and electrochemical characterization of Li₂S@graphene nanocomposite. (a) TEM images of Li₂S@graphene nanocapsules. (b) Schematic showing the compact nanocapsule structure. (c) Voltage profiles and (d) Cycling performance of Li₂S@graphene electrode based on electrode loading of 10 mg_Li2S cm⁻². (e) Voltage profiles at different charge/discharge current densities, and (f) Long rate cycling performance of Li₂S@graphene electrode based on electrode loading of 5 mg_Li2S cm⁻².