Hollow Ni 3 Se 4 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 (Ni 3 Se 4 ) with various architectures was designed and employed as a carbon-free sulfur immobilizer. Among them, hollow sea urchins like Ni 3 Se 4 with high porosity (0.39 cm 3 g -1 ) and large specific surface area (82.7 m 2 g -1 ) exhibit abundant adsorptive and electrocatalytic sites, which pledge excellent electrochemical performances of the Li-S battery. Correspondingly, the Ni 3 Se 4 -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 (Ni 3 Se 4 /S: 1.57 g cm -3 vs super P/S: 0.7 g cm -3 ), the volumetric specific capacity of Ni 3 Se 4 -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.