Phase Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage

Achieving homogeneous phase transition and uniform charge distribution is essential for good cycle stability and high capacity when phase conversion materials are used as electrodes. Herein, we show that chemical lithiation of bulk 2H-MoS₂ distorts its crystalline domains in three primary directions to produce mosaic-like 1T′ nanocrystalline domains, which improve phase and charge uniformity during subsequent electrochemical phase conversion. 1T′-Li_<i>x</i>MoS₂, a macroscopic dense material with interconnected nanoscale grains, shows excellent cycle stability and rate capability in a lithium rechargeable battery compared to bulk or exfoliated-restacked MoS₂. Transmission electron microscopy studies reveal that the interconnected MoS₂ nanocrystals created during the phase change process are reformable even after multiple cycles of galvanostatic charging/discharging, which allows them to play important roles in the long term cycling performance of the chemically intercalated TMD materials. These studies shed light on how bulk TMDs can be processed into quasi-2D nanophase material for stable energy storage

Phase Transformations in TiS₂ during K Intercalation

The electrochemical performances of TiS₂ in potassium ion batteries (KIBs) are poor due to the large size of K ions, which induces irreversible structural changes and poor kinetics. To obtain detailed insights into the kinetics of phase changes, we investigated the electrochemical properties, phase transformations, and stability of potassium-intercalated TiS₂ (K_<i>x</i>TiS₂, 0 ≤ <i>x</i> ≤ 0.88). In situ XRD reveals staged transitions corresponding to distinct crystalline phases during K ion intercalation, which are distinct from those of Li and Na ions. Electrochemical (cyclic voltammetry and galvanostatic charge/discharge) studies show that the phase transitions among various intercalated stages slow down the kinetics of the discharge/charge in bulk TiS₂ hosts. By chemically prepotassiating the bulk TiS₂ (K_0.25TiS₂) to reduce the domain size of the crystal, these phase transitions are bypassed and more facile ion insertion kinetics can be obtained, which leads to improved Coulombic efficiency, rate capability, and cycling stability

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film

The catalytic and magnetic properties of molybdenum disulfide (MoS₂) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS₂ film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS₂ films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS₂ films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction

<i>In Situ</i> Observation and Electrochemical Study of Encapsulated Sulfur Nanoparticles by MoS₂ Flakes

Sulfur is an attractive cathode material for next-generation lithium batteries due to its high theoretical capacity and low cost. However, dissolution of its lithiated product (lithium polysulfides) into the electrolyte limits the practical application of lithium sulfur batteries. Here we demonstrate that sulfur particles can be hermetically encapsulated by leveraging on the unique properties of two-dimensional materials such as molybdenum disulfide (MoS₂). The high flexibility and strong van der Waals force in MoS₂ nanoflakes allows effective encapsulation of the sulfur particles and prevent its sublimation during <i>in situ</i> TEM studies. We observe that the lithium diffusivities in the encapsulated sulfur particles are in the order of 10^–17 m² s^–1. Composite electrodes made from the MoS₂-encapsulated sulfur spheres show outstanding electrochemical performance, with an initial capacity of 1660 mAh g^–1 and long cycle life of more than 1000 cycles