Sodium and potassium ion storage in cation substituted 2D MoWSe2: insights into the effects of upper voltage cut-off.

The superior properties, such as large interlayer spacing and the ability to host large alkali-metal ions, of two-dimensional (2D) materials based on transition metal di-chalcogenides (TMDs) enable next-generation battery development beyond lithium-ion rechargeable batteries. In addition, compelling but rarely inspected TMD alloys provide additional opportunities to tailor bandgap and enhance thermodynamic stability. This study explores the sodium-ion (Na-ion) and potassium-ion (K-ion) storage behavior of cation-substituted molybdenum tungsten diselenide (MoWSe2), a TMD alloy. This research also investigates upper potential suspension to overcome obstacles commonly associated with TMD materials, such as capacity fading at high current rates, prolonged cycling conditions, and voltage polarization during conversion reaction. The voltage cut-off was restricted to 1.5 V, 2.0 V, and 2.5 V to realize the material's Na+and K+ion storage behavior. Three-dimensional (3D) surface plots of differential capacity analysis up to prolonged cycles revealed the convenience of voltage suspension as a viable method for structural preservation. Moreover, the cells with higher potential cut-off values conveyed improved cycling stability, higher and stable coulombic efficiency for Na+and K+ion half-cells, and increased capacity retention for Na+ion half-cells, respectively, with half-cells cycled at higher voltage ranges.

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

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.

Addressing Irreversibility and Structural Distortion in WS2 Inorganic Fullerene-Like Nanoparticles: Effects of Voltage Cutoff Experiments in Beyond Li+-Ion Storage Applications.

Large interlayer spacing beneficially allows Na+- and K+-ion storage in transition-metal dichalcogenide (TMD)-based electrodes, but side reactions and volume change, which pulverize the TMD crystalline structure, are persistent challenges for the utilization of these materials in next-generation devices. This study first determines whether irreversibility due to structural distortion, which results in poor cycling stability, is also apparent in the case of inorganic fullerene-like (IF) tungsten disulfide (WS2) nanocages (WS2IF). To address these problems, this study proposes upper and lower voltage cutoff experiments to limit specific reactions in Na+/WS2IF and K+/WS2IF half-cells. Three-dimensional (3D) differential capacity curves and derived surface plots highlight the continuation of reversible reactions when a high upper cutoff technique is applied, thereby indirectly suggesting restricted structural dissolution. This resulted in improved capacity retention with stable performance and a higher Coulombic efficiency, laying the ground for the use of TMD-based materials beyond Li+-ion storage devices.

WS2 Nanotube-Embedded SiOC Fibermat Electrodes for Sodium-Ion Batteries.

Layered transition metal dichalcogenides (TMDs) such as tungsten disulfide (WS2) are promising materials for a wide range of applications, including charge storage in batteries and supercapacitors. Nevertheless, TMD-based electrodes suffer from bottlenecks such as capacity fading at high current densities, voltage hysteresis during the conversion reaction, and polysulfide dissolution. To tame such adverse phenomena, we fabricate composites with WS2 nanotubes. Herein, we report on the superior electrochemical performance of ceramic composite fibers comprising WS2 nanotubes (WS2NTs) embedded in a chemically robust molecular polymer-derived ceramic matrix of silicon-oxycarbide (SiOC). Such a heterogeneous fiber structure was obtained via electrospinning of WS2NT/preceramic polymer solution followed by pyrolysis at elevated temperatures. The electrode capacity fading in WS2NTs was curbed by the synergistic effect between WS2NT and SiOC. As a result, the composite electrode exhibits high initial capacity of 454 mAh g-1 and the capacity retention approximately 2-3 times higher than that of the neat WS2NT electrode.

WS2 Nanosheet Loaded Silicon-Oxycarbide Electrode for Sodium and Potassium Batteries.

Transition metal dichalcogenides (TMDs) such as the WS2 have been widely studied as potential electrode materials for lithium-ion batteries (LIB) owing to TMDs' layered morphology and reversible conversion reaction with the alkali metals between 0 to 2 V (v/s Li/Li+) potentials. However, works involving TMD materials as electrodes for sodium- (NIBs) and potassium-ion batteries (KIBs) are relatively few, mainly due to poor electrode performance arising from significant volume changes and pulverization by the larger size alkali-metal ions. Here, we show that Na+ and K+ cyclability in WS2 TMD is improved by introducing WS2 nanosheets in a chemically and mechanically robust matrix comprising precursor-derived ceramic (PDC) silicon oxycarbide (SiOC) material. The WS2/SiOC composite in fibermat morphology was achieved via electrospinning followed by thermolysis of a polymer solution consisting of a polysiloxane (precursor to SiOC) dispersed with exfoliated WS2 nanosheets. The composite electrode was successfully tested in Na-ion and K-ion half-cells as a working electrode, which rendered the first cycle charge capacity of 474.88 mAh g-1 and 218.91 mAh g-1, respectively. The synergistic effect of the composite electrode leads to higher capacity and improved coulombic efficiency compared to the neat WS2 and neat SiOC materials in these cells.

Enhanced Li-Ion Rate Capability and Stable Efficiency Enabled by MoSe2 Nanosheets in Polymer-Derived Silicon Oxycarbide Fiber Electrodes.

Transition metal dichalcogenides (TMDs) such as MoSe2 have continued to generate interest in the engineering community because of their unique layered morphology-the strong in-plane chemical bonding between transition metal atoms sandwiched between two chalcogen atoms and the weak physical attraction between adjacent TMD layers provides them with not only chemical versatility but also a range of electronic, optical, and chemical properties that can be unlocked upon exfoliation into individual TMD layers. Such a layered morphology is particularly suitable for ion intercalation as well as for conversion chemistry with alkali metal ions for electrochemical energy storage applications. Nonetheless, host of issues including fast capacity decay arising due to volume changes and from TMD's degradation reaction with electrolyte at low discharge potentials have restricted use in commercial batteries. One approach to overcome barriers associated with TMDs' chemical stability functionalization of TMD surfaces by chemically robust precursor-derived ceramics or PDC materials, such as silicon oxycarbide (SiOC). SiOC-functionalized TMDs have shown to curb capacity degradation in TMD and improve long term cycling as Li-ion battery (LIBs) electrodes. Herein, we report synthesis of such a composite in which MoSe2 nanosheets are in SiOC matrix in a self-standing fiber mat configuration. This was achieved via electrospinning of TMD nanosheets suspended in pre-ceramic polymer followed by high temperature pyrolysis. Morphology and chemical composition of synthesized material was established by use of electron microscopy and spectroscopic technique. When tested as LIB electrode, the SiOC/MoSe2 fiber mats showed improved cycling stability over neat MoSe2 and neat SiOC electrodes. The freestanding composite electrode delivered a high charge capacity of 586 mAh g-1electrode with an initial coulombic efficiency of 58%. The composite electrode also showed good cycling stability over SiOC fiber mat electrode for over 100 cycles.