Exploring the physicochemical role of Pd dopant in promoting Li-ion diffusion dynamics and storage performance of NbS2 at the atomic scale†
The two-dimensional layered niobium disulfide (NbS2), as a kind of anode material for Li-ion batteries, has received great attention because of its excellent electronic conductivity and structural stability. However, its ionic conductivity is far from desirable. Herein, we have proposed an effective way to acquire the rapid promotion of its Li-ion diffusion dynamics from the palladium doping effect. By first-principle calculations, we firstly investigated quantitative relations among lattice constants, mechanical properties, and Pd-doped concentration (x) for Pd doped NbS2 (PdxNbS2). It is found that the interlayer spacing of PdxNbS2 undergoes dramatic expansion, which contributes to affording its large space for Li-ion storage. And Pd0.25NbS2 has the best ductility, exhibiting its excellent destruction-resistant properties. Among PdxNbS2 (x = 0, 0.083, 0.167, 0.250, 0.333, and 0.417), it is also proved that Pd0.25NbS2 is the easiest to be prepared with the introduction of NbPd3 as the raw material for the Pd-dopant and it also exhibits excellent thermal stability at room temperature (300 K). Most importantly, by analysis with the climbing-image nudged elastic band method (CI-NEB), it is revealed that Pd0.25NbS2 shows the lowest Li-ion diffusion energy barrier of 0.26 eV, which is also much lower than that of NbS2 (0.43 eV). This is attributed to the inductive effect of the Pd-dopant in its layered structure, trying to maintain the Li–S six-coordinated structure at the initial state when Li-ions transfer to the saddle point. Accordingly, it induces a small structural difference in coordinate structures between initial states and transition states. Moreover, Pd0.25NbS2 undergoes a less obvious oxidation and reduction reaction, maintaining its excellent structural stability during Li intercalation/deintercalation. Additionally, the theoretical average voltage of Pd0.25NbS2 (1.75 V for Li0.75Pd0.25NbS2vs. Li/Li+) is also much lower than that of NbS2 (2.41 V vs. Li/Li+), implying that it can provide a higher power density. Therefore, our theoretical results pave a distinctive way to develop an ultrahigh-rate and long-life anode material for Li-ion batteries.