Hollow Ni₃Se₄ with High Tap Density as a Carbon-Free Sulfur Immobilizer to Realize High Volumetric and Gravimetric Capacity for Lithium–Sulfur Batteries

Despite that the practical gravimetric energy density of lithium sulfur batteries has exceeded that of the traditional lithium-ion battery, the volumetric energy density still pales due to the low density of carbonaceous materials. Herein, hollow polar nickel selenide (Ni3Se4) with various architectures was designed and employed as a carbon-free sulfur immobilizer. Among them, hollow sea urchins like Ni3Se4 with high porosity (0.39 cm3 g–1) and large specific surface area (82.7 m2 g–1) exhibit abundant adsorptive and electrocatalytic sites, which pledge excellent electrochemical performances of the Li–S battery. Correspondingly, the Ni3Se4-based sulfur electrode presents excellent rate endurability (581 mAh g–1–composite at 2.0 C) and superior cycle stability (ultralow fading rate of 0.042% per cycle during the 1000 cycles at 1.0 C). More importantly, thanks to the higher tap density (Ni3Se4/S: 1.57 g cm–3 vs super P/S: 0.7 g cm–3), the volumetric specific capacity of Ni3Se4-based cathodes is as high as 1699 mAh cm–3–composite at 0.1 C, which is almost 2.8 times that of the carbonaceous electrode. Hence, rational transition metal selenide architecture design with synergistic function of good conductivity, well-defined catalyst and adsorption, as well as high tap density provide a promising route toward high gravimetric and volumetric energy density of Li–S batteries.