Rational Design of a Ni₃N_0.85 Electrocatalyst to Accelerate Polysulfide Conversion in Lithium-Sulfur Batteries

Slow kinetics of polysulfide conversion reactions lead to severe issues for lithium-sulfur (Li-S) batteries, for example, low rate capability, polysulfide migration, and low Coulombic efficiencies. These challenges hinder the practical applications of Li-S batteries. In this study, we proposed a rational strategy of tuning the d-band of catalysts to accelerate the conversion of polysulfides. Nitrogen vacancies were engineered in hexagonal Ni₃N (space group P6₃22) to tune its d-band center, leading to the strong interaction between polysulfides and Ni₃N. Because of the greater electron population in the lowest occupied molecular orbital of Li₂S₄, the terminal S-S bonds were weakened for breaking. Temperature-dependent experiments confirm that Ni₃N_0.85 demonstrates a much low activation energy, thereby accelerating the conversion of polysulfides. A Li-S cell using Ni₃N_0.85 can deliver a high initial discharge capacity of 1445.9 mAh g^-1 (at 0.02 C) and low decay per cycle (0.039%). The Ni₃N_0.85 cell can also demonstrate an initial capacity of 1200.4 mAh g^-1 for up to 100 cycles at a high loading of 5.2 mg cm^-2. The high efficiency of rationally designed Ni₃N_0.85 demonstrates the effectiveness of the d-band tuning strategy to develop low-activation-energy catalysts and to promote the atomic understanding of polysulfide conversion in Li-S batteries.