Transition metal nitrides embedded in N-doped porous graphitic Carbon: Applications as electrocatalytic sulfur host materials

Abstract

While transition metal nitrides (TMNs) are promising electrocatalysts, their widespread use is challenged by the complex synthetic methodology and a limited understanding of the underlying electrocatalytic mechanisms. Herein, we describe a novel synthesis of TMNs (including Mo2N, NbN, and ZrN) and explore their potential as electrocatalysts to affect sulfur cathode reactions. The TMNs were prepared in-situ using a process that simultaneously infuses nitrogen-doped porous graphitic carbon (designated as TMN@N-PGC). The methodology avoids the use of ammonia, which poses safety risks due to its flammability and toxicity. Analysis of the d-p hybridized orbitals formed between the transition metal ions and sulfur species revealed that the antibonding orbitals are empty. Thus, TMNs with more negative d-band centers exhibit stronger affinities towards polysulfides. NbN facilitated polysulfide conversion as well as Li2S detachment, and thus featured a high electrocatalytic capability for promoting cathode kinetics. Lithium−sulfur (Li–S) batteries containing NbN@N-PGC exhibited the highest performance metrics in terms of specific capacity (1488 mA h g−1 at 0.1 C), rate capacity (521 mA h g−1 at 6 C), and cycling stability (603 mA h g−1 at 0.5 C after 1300 cycles, corresponding a capacity decay of 0.030% per cycle). Li−S cells with high sulfur loadings also exhibit outstanding performance.