MoS2, WS2, and MoWS2 Flakes as Reversible Host Materials for Sodium-Ion and Potassium-Ion Batteries

Abstract

Transition-metal dichalcogenides (TMDs) and their alloys are vital for the development of sustainable and economical energy storage alternatives due to their large interlayer spacing and hosting ability for alkali-metal ions. Although the Li-ion chemically correlates with the Na-ion and K-ion, research on batteries with TMD anodes for K⁺ is still in its infancy. This research explores TMDs such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) and TMD alloys such as molybdenum tungsten disulfide (MoWS₂) for both sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs). The cyclic stability test analysis indicates that in the initial cycle, the MoS₂ NIB demonstrates exceptional performance, with a peak charge capacity of 1056 mAh g^-1, while retaining high Coulombic efficiency. However, the WS₂ KIB underperforms, with the least charge capacity of 130 mAh g^-1 in the first cycle and exceptionally low retention at a current density of 100 mA g^-1. The MoWS₂ TMD alloy exhibits a moderate charge capacity and cyclic efficiency for both NIBs and KIBs. This comparison study shows that decreasing sizes of alkali-metal ions and constituent elements in TMDs or TMD alloys leads to decreased resistance and slower degradation processes as indicated by cyclic voltammetry and electrochemical impedance spectroscopy after 10 cycles. Furthermore, the study of probable electrochemical intercalation and removal processes of Na-ions and K-ions demonstrates that large geometrically shaped TMD flakes are more responsive to intercalation for Na-ions than K-ions. These performance comparisons of different TMD materials for NIBs and KIBs may promote the future development of these batteries.

Figure 1

Low-resolution TEM images of (a, b) MoS₂ nanosheets and (c) corresponding SAED pattern; low-resolution TEM images of (d, e) WS₂ powders and (f) corresponding SAED pattern; high-resolution TEM images of (g, h) MoWS₂ powders depicting fringe distances; low-resolution TEM images of (i, j) MoWS₂ powders and (k) corresponding SAED pattern.

Figure 2

(a) Raman spectra of MoS₂, WS₂, and MoWS₂ powders and (b) XRD reflection of MoS₂, WS₂, and MoWS₂ electrodes.

Figure 3

(a) XPS survey scan representing the presence of different elements within MoWS₂; high-resolution (b) Mo 3d, (c) W 4f, and (d) S 2p XPS spectra of the MoWS₂ material demonstrating the chemical bonding states.

Figure 4

(a1) GCD and (a2) differential capacity curves of MoS₂ NIB electrodes; (a3) GCD and (a4) differential capacity curves of MoS₂ KIB electrodes; (b1) GCD and (b2) differential capacity curves of WS₂ NIB electrodes; (b3) GCD and (b4) differential capacity curves of WS₂ KIB electrodes; (c1) GCD and (c2) differential capacity curves of MoWS₂ NIB electrodes, and (c3) GCD and (c4) differential capacity curves of MoWS₂ KIB electrodes.

Figure 5

Cycling stability test and corresponding Coulombic efficiency of MoS₂, WS₂, and MoWS₂ electrodes in (a) Na-ion half-cells and (b) K-ion half-cells, along with rate constant (k) and half-life values (t_1/2) determined by exponential decay fitting.

Figure 6

First-cycle charge capacity and Coulombic efficiency comparison between all of the half-cells of this study and the MoWSe₂ NIB and KIB from another study reported in ref (48)

Figure 7

(a1) CV, (a2) capacitive vs diffusive contribution, (a3) fitting of anodic and cathodic voltammetric sweep data for MoS₂ NIB; (b1) CV, (b2) capacitive vs diffusive contribution, (b3) fitting of anodic and cathodic voltammetric sweep data for WS₂ NIB; (c1) CV, (c2) capacitive vs diffusive contribution, and (c3) fitting of anodic and cathodic voltammetric sweep data for MoWS₂ NIB.

Figure 8

(a1) CV, (a2) capacitive vs diffusive contribution, and (a3) fitting of anodic and cathodic voltammetric sweep data for MoS₂ KIB; (b1) CV, (b2) capacitive vs diffusive contribution, and (b3) fitting of anodic and cathodic voltammetric sweep data for WS₂ KIB; and (c1) CV, (c2) capacitive vs diffusive contribution, and (c3) fitting of anodic and cathodic voltammetric sweep data for MoWS₂ KIB.

Figure 9

Nyquist plot at the 11th cycle for MoS₂, WS₂, and MoWS₂ electrodes in (a) Na-ion half-cells and (b) K-ion half-cells (electrode area = 1.6 cm²).

Figure 10

Charge and discharge cycles with 15 min of current pulse at a 100 mA g^–1 electrode followed by 12 h of relaxation for (a) MoS₂ NIB, (b) WS₂ NIB, (c) MoWS₂ NIB, (d) MoS₂ KIB, (e) WS₂ KIB, and (f) MoWS₂ KIB.

Figure 11

Diffusion coefficients as a function of discharge and charge for MoS₂: (a) NIB and (b) KIB half-cells.

Figure 12

(a1, a2–f1, f2) SEM micrographs of pristine electrodes indicating structural integrity before cycling, (a3–f3) digital camera image of the cycled electrodes, (a4, a5–f4, f5) SEM micrographs of cycled electrodes, and (a6–f6) XRF spectra of the electrodes to identify the elemental composition.