REDOX POTENTIALS AND DIFFUSION OF LITHIUM IN LAMELLAR COMPOUNDS

Abstract

Themodynamic and dynamic properties of intercalation products of lithium into MoS₂ are strongly determined by the coordination of lithium in the interlaminar spaces. Lithium redox potentials as well as lithium diffusion coefficients in MoS₂ pure, exfoliated, as well as in compounds where lithium is co-intercalated with the polymeric electron pair donors, poly(ethylene oxide) and poly-acrylonitrile, and discrete species, OH^? ions and secondary amines, were analyzed comparatively. Reduction potentials in pure or exfoliated MoS₂ are always much lower than those observed in lithium-donor co-intercalates. Thus, donors appear to effectively stabilize higher lithium oxidation states. The donors also influence lithium migration properties, with lithium diffusion coefficients in general higher than in pure MoS₂. Lithium diffusion activation energy in pure MoS₂ is constant in a relatively large lithium concentration range, while for co-intercalates it often depends on lithium intercalation degree. These more complex diffusion mechanisms probably arise from changes in the donor conformation in the interlaminar spaces, which affect the lithium first coordination sphere.     

Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods

MoS₂ has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS₂, obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge‐transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron‐transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS₂‐based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS₂. The electrochemical tuning of electrocatalytic properties of MoS₂ opens the doors to scalable and facile tailoring of MoS₂‐based electrochemical devices.     

花瓣状微球MoS2/石墨烯复合材料的制备及其电化学性能

采用有利于二维层状结构形成的L-半胱氨酸作为硫源,钼酸钠作为钼源,制备聚乙烯基吡咯烷酮（PVP）辅助水热合成花瓣状微球形貌的MoS₂/还原氧化石墨烯复合电极材料（PVP-MoS₂/RGO）. X射线衍射（XRD）及透射电子显微镜（TEM）证实,经过PVP的适量添加,MoS₂有序堆垛结构的片层数目明显减少. 扫描电子显微镜（SEM）显示,添加适量PVP的MoS₂/石墨烯材料具有分散性更好的花瓣状微球形貌. 上述的少层有序堆垛结构及复合材料的良好分散性缩短了MoS₂中锂离子的嵌入/脱出路径,使其具有更高的容量、循环稳定性和倍率性能.     

类石墨烯二硫化钼及其在光电子器件上的应用

由单层或几层二硫化钼构成的类石墨烯二硫化钼(graphene-like MoS₂)是一种具有类似石墨烯结构和性能的新型二维(2D)层状化合物, 近年来以其独特的物理、化学性质而成为新兴的研究热点. 本文综述了近年来类石墨烯二硫化钼常见的几种制备方法, 包括以微机械力剥离、锂离子插层和液相超声法等为主的“自上而下”的剥离法, 以及以高温热分解、水热法等为主的“自下而上”的合成法; 介绍了其常用的结构表征方法, 包括原子力显微镜(AFM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和拉曼光谱等; 概述了类石墨烯二硫化钼的紫外-可见(UV-Vis)吸收、荧光发射等基本光物理性质及其相关机理; 总结了类石墨烯二硫化钼在二次电池、场效应晶体管、传感器、有机电致发光二极管和电存储等光电子器件领域的应用原理及其研究进展, 展望了这类新型二维层状化合物的研究前景.     

Hierarchical MoS2@Carbon Microspheres as Advanced Anodes for Li‐Ion Batteries

Hierarchical hybridized nanocomposites with rationally constructed compositions and structures have been considered key for achieving superior Li‐ion battery performance owing to their enhanced properties, such as fast lithium ion diffusion, good collection and transport of electrons, and a buffer zone for relieving the large volume variations during cycling processes. Hierarchical MoS₂@carbon microspheres (HMCM) have been synthesized in a facile hydrothermal treatment. The structure analyses reveal that ultrathin MoS₂ nanoflakes (ca. 2–5 nm) are vertically supported on the surface of carbon nanospheres. The reversible capacity of the HMCM nanocomposite is maintained at 650 mA h g^?1 after 300 cycles at 1 A g^?1. Furthermore, the capacity can reach 477 mA h g^?1 even at a high current density of 4 A g^?1. The outstanding electrochemical performance of HMCM is attributed to the synergetic effect between the carbon spheres and the ultrathin MoS₂ nanoflakes. Additionally, the carbon matrix can supply conductive networks and prevent the aggregation of layered MoS₂ during the charge/discharge process; and ultrathin MoS₂ nanoflakes with enlarged surface areas, which can guarantee the flow of the electrolyte, provide more active sites and reduce the diffusion energy barrier of Li⁺ ions.     

Revisiting lithium-storage mechanisms of molybdenum disulfide

Molybdenum disulfide（MoS₂）,a typical two-dimensional transition metallic layered material,attracts tremendous attentions in the electrochemical energy storage due to its excellent physicochemical properties.However,with the deepening of the research and exploration of the lithium storage mechanism of these advanced MoS₂-based anode materials,the complex reaction process influenced by internal and external factors hinders the exhaustive understanding of the lithium storage p...     

Covalent Assembly of MoS2 Nanosheets with SnS Nanodots as Linkages for Lithium/Sodium‐Ion Batteries

Weak van der Waals interactions between interlayers of two‐dimensional layered materials result in disabled across‐interlayer electron transfer and poor layered structural stability, seriously deteriorating their performance in energy applications. Herein, we propose a novel covalent assembly strategy for MoS₂ nanosheets to realize unique MoS₂/SnS hollow superassemblies (HSs) by using SnS nanodots as covalent linkages. The covalent assembly based on all‐inorganic and carbon‐free concept enables effective across‐interlayer electron transfer, facilitated ion diffusion kinetics, and outstanding mechanical stability, which are evidenced by experimental characterization, DFT calculations, and mechanical simulations. Consequently, the MoS₂/SnS HSs exhibit superb rate performance and long cycling stability in lithium‐ion batteries, representing the best comprehensive performance in carbon‐free MoS₂‐based anodes to date. Moreover, the MoS₂/SnS HSs also show excellent sodium storage performance in sodium‐ion batteries.     

Exfoliated transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2): An electrochemical impedance spectroscopic investigation

Owing to the enthralling properties which transition metal dichalcogenides present, they are facing immense scientific interest from researchers. Till date, these two-dimensional materials have been assessed for a wide array of different applications and there are various synthetic methods of attaining them in their respective bulk and exfoliated forms. Herein, we explore the effects of lithium ion intercalation exfoliation process on the charge transfer resistance of transition metal dichalcogenide materials (MoS₂, MoSe₂, WS₂ and WSe₂). We also show that electrochemical activation of the transition metal dichalcogenides results in decreased resistance towards charge transfer, as demonstrated by electrochemical impedance spectroscopy.     

<Emphasis Type="Italic">In-situ</Emphasis> TEM investigation of MoS<Subscript>2</Subscript> upon alkali metal intercalation

Phase transition in two dimensional molybdenum disulfide (MoS₂) can be induced by several methods and has been investigated for decades. Alkali metal insertion of MoS₂ had been proved an effective method to cause phase transition early in 1970s, and has been gaining renewed interest recently, due to the possible application of MoS₂ in energy storage. The alkali metal intercalation of MoS₂ has been studied by various techniques, among which in-situ transmission electron microscopy (TEM) provides unique capability of real time resolving the structural evolution of the materials at high spatial resolutions. Here by in-situ TEM technique we investigated the structural evolution of MoS₂ upon lithium and sodium intercalation, along with transformation of the nanosheet and variation of the electron diffraction patterns. The intercalation process is accompanied by emergence of superstructures, which exist in several forms. The ion intercalation results in phase transition of MoS₂ from 2H to 1T, and the driving mechanism of the phase transition are discussed. The work provides a more comprehensive understanding of ion intercalation induced phase transition of MoS₂.     

Nanohybridization of CoS2/MoS2 Heterostructure with Polyoxometalate on Functionalized Reduced Graphene Oxide for High-Performance LIBs

To address the poor cycling stability and low rate capability of MoS₂ as electrode materials for lithium-ion batteries (LIBs), herein, the CoS₂/MoS₂/PDDA-rGO/PMo₁₂ nanocomposites are constructed via a simple hydrothermal process, combining the advantages of all three, namely, CoS₂/MoS₂ heterojunction and polyoxometalates (POMs) provide abundant catalytically active sites and increase the multi-electron transfer ability, and the positively charged poly(diallyldimethylammonium chloride) modified reduced graphene oxide (PDDA-rGO) improve electronic conductivity and effectively prevent the aggregation of MoS₂, meanwhile stabilize the negatively charged [PMo₁₂O₄₀]³⁻. After the electrochemical testing, the resulting CoS₂/MoS₂/PDDA-rGO/PMo₁₂ nanocomposite achieved 1055 mA h g⁻¹ initial specific capacities and stabilized at 740 mA h g⁻¹ after 150 cycles at 100 mA g⁻¹ current density. And the specific capacities of MoS₂, MoS₂/PDDA-rGO, CoS₂/MoS₂, and CoS₂/MoS₂/PDDA-rGO were 201, 421, 518, and 589 at 100 mA g⁻¹ after 150 cycles, respectively. The fact of the greatly improving capacity of MoS₂-based nanocomposites suggests its potential for high performance electrode materials of LIBs. Moreover, the lithium storage mechanism of CoS₂/MoS₂/PDDA-rGO/PMo₁₂ has been discussed on the basis of cyclic voltammetry with different scan rates.     

Quantum chemical study of MoS2 hydro desulfurization catalysts

The electronic and bonding properties of MoS₂, thiophene, tetrahydrothiophene and their related adsorption systems are studied by DV-X_α calculations on model clusters. The calculated results indicate that for MoS₂ the Mo-S bonding is not very strong. The Mo atoms are held together through bonding with S and the S atoms are held together through bonding with Mo. Three types of S atoms may be distinguished by their location, coordination and bond type. The d band of MoS₂ is different from that of Group VIII transition metals and markedly influence its adsorptive properties. The adsorption of a thiophene molecule on MoS₂ is weak, the interaction is complicated, and little activation is indicated. It may be inferred that thiophene is first hydrogenated to tetrahydrothiophene and then desulfurized.     

Approaching the Lithiation Limit of MoS2 While Maintaining Its Layered Crystalline Structure to Improve Lithium Storage

MoS₂ holds great promise as high‐rate electrode for lithium‐ion batteries since its large interlayer can allow fast lithium diffusion in 3.0–1.0 V. However, the low theoretical capacity (167 mAh g^?1) limits its wide application. Here, by fine tuning the lithiation depth of MoS₂, we demonstrate that its parent layered structure can be preserved with expanded interlayers while cycling in 3.0–0.6 V. The deeper lithiation and maintained crystalline structure endows commercially micrometer‐sized MoS₂ with a capacity of 232 mAh g^?1 at 0.05 A g^?1 and circa 92 % capacity retention after 1000 cycles at 1.0 A g^?1. Moreover, the enlarged interlayers enable MoS₂ to release a capacity of 165 mAh g^?1 at 5.0 A g^?1, which is double the capacity obtained under 3.0–1.0 V at the same rate. Our strategy of controlling the lithiation depth of MoS₂ to avoid fracture ushers in new possibilities to enhance the lithium storage of layered transition‐metal dichalcogenides.     

Functionalization of Two‐Dimensional MoS2: On the Reaction Between MoS2 and Organic Thiols

Two‐dimensional layered transition metal dichalcogenides (TMDs) have attracted great interest owing to their unique properties and a wide array of potential applications. However, due to their inert nature, pristine TMDs are very challenging to functionalize. We demonstrate a general route to functionalize exfoliated 2H‐MoS₂ with cysteine. Critically, MoS₂ was found to be facilitating the oxidation of the thiol cysteine to the disulfide cystine during functionalization. The resulting cystine was physisorbed on MoS₂ rather than coordinated as a thiol (cysteine) filling S‐vacancies in the 2H‐MoS₂ surface, as originally conceived. These observations were found to be true for other organic thiols and indeed other TMDs. Our findings suggest that functionalization of two‐dimensional MoS₂ using organic thiols may not yield covalently or datively tethered functionalities, rather, in this instance, they yield physisorbed disulfides that are easily removed.     

Three‐Dimensional MoS2 Hierarchical Nanoarchitectures Anchored into a Carbon Layer as Graphene Analogues with Improved Lithium Ion Storage Performance

Much attention has recently been focused on the synthesis and application of graphene analogues of layered nanomaterials owing to their better electrochemical performance than the bulk counterparts. We synthesized graphene analogue of 3D MoS₂ hierarchical nanoarchitectures through a facile hydrothermal route. The graphene‐like MoS₂ nanosheets are uniformly dispersed in an amorphous carbon matrix produced in situ by hydrothermal carbonization. The interlaminar distance between the MoS₂ nanosheets is about 1.38 nm, which is far larger than that of bulk MoS₂ (0.62 nm). Such a layered architecture is especially beneficial for the intercalation and deintercalation of Li⁺. When tested as a lithium‐storage anode material, the graphene‐like MoS₂ hierarchical nanoarchitectures exhibit high specific capacity, superior rate capability, and enhanced cycling performance. This material shows a high reversible capacity of 813.5 mAh g^?1 at a current density of 1000 mA g^?1 after 100 cycles and a specific capacity as high as 600 mAh g^?1 could be retained even at a current density of 4000 mA g^?1. The results further demonstrate that constructing 3D graphene‐like hierarchical nanoarchitectures can effectively improve the electrochemical performance of electrode materials.     

Hybrid Structures of Sisal Fiber Derived Interconnected Carbon Nanosheets/MoS2/Polyaniline as Advanced Electrode Materials in Lithium-Ion Batteries

In this work, we designed and successfully synthesized an interconnected carbon nanosheet/MoS₂/polyaniline hybrid (ICN/MoS₂/PANI) by combining the hydrothermal method and in situ chemical oxidative polymerization. The as-synthesized ICNs/MoS₂/PANI hybrid showed a “caramel treat-like” architecture in which the sisal fiber derived ICNs were used as hosts to grow “follower-like” MoS₂ nanostructures, and the PANI film was controllably grown on the surface of ICNs and MoS₂. As a LIBs anode material, the ICN/MoS₂/PANI electrode possesses excellent cycling performance, superior rate capability, and high reversible capacity. The reversible capacity retains 583 mA h/g after 400 cycles at a high current density of 2 A/g. The standout electrochemical performance of the ICN/MoS₂/PANI electrode can be attributed to the synergistic effects of ICNs, MoS₂ nanostructures, and PANI. The ICN framework can buffer the volume change of MoS₂, facilitate electron transfer, and supply more lithium inset sites. The MoS₂ nanostructures provide superior rate capability and reversible capacity, and the PANI coating can further buffer the volume change and facilitate electron transfer.     

Fluoroethylene Carbonate Enabling a Robust LiF‐rich Solid Electrolyte Interphase to Enhance the Stability of the MoS2 Anode for Lithium‐Ion Storage

As a high‐capacity anode for lithium‐ion batteries (LIBs), MoS₂ suffers from short lifespan that is due in part to its unstable solid electrolyte interphase (SEI). The cycle life of MoS₂ can be greatly extended by manipulating the SEI with a fluoroethylene carbonate (FEC) additive. The capacity of MoS₂ in the electrolyte with 10 wt % FEC stabilizes at about 770 mAh g^?1 for 200 cycles at 1 A g^?1, which far surpasses the FEC‐free counterpart (ca. 40 mAh g^?1 after 150 cycles). The presence of FEC enables a robust LiF‐rich SEI that can effectively inhibit the continual electrolyte decomposition. A full cell with a LiNi_0.5Co_0.3Mn_0.2O₂ cathode also gains improved performance in the FEC‐containing electrolyte. These findings reveal the importance of controlling SEI formation on MoS₂ toward promoted lithium storage, opening a new avenue for developing metal sulfides as high‐capacity electrodes for LIBs.     

Solution‐Processed Two‐Dimensional MoS2 Nanosheets: Preparation,Hybridization, and Applications

As one member of the emerging class of ultrathin two‐dimensional (2D) transition‐metal dichalcogenide (TMD) nanomaterials, the ultra‐thin MoS₂ nanosheet has attracted increasing research interest as a result of its unique structure and fascinating properties. Solution‐phase methods are promising for the scalable production, functionalization, hybridization of MoS₂ nanosheets, thus enabling the widespread exploration of MoS₂‐based nanomaterials for various promising applications. In this Review, an overview of the recent progress of solution‐processed MoS₂ nanosheets is presented, with the emphasis on their synthetic strategies, functionalization, hybridization, properties, and applications. Finally, the challenges and opportunities in this research area will be proposed.     

Vertical crosslinking MoS2/three-dimensional graphene composite towards high performance supercapacitor

The vertical crosslinking MoS₂/three-dimensional graphene composite has been prepared by hydrothermal method, which delivered a superior and stable electrochemical capacitive performance.     

Carbon Nanofibers Decorated with Molybdenum Disulfide Nanosheets: Synergistic Lithium Storage and Enhanced Electrochemical Performance

Traditional lithium‐ion batteries that are based on layered Li intercalation electrode materials are limited by the intrinsically low theoretical capacities of both electrodes and cannot meet the increasing demand for energy. A facile route for the synthesis of a new type of composite nanofibers, namely carbon nanofibers decorated with molybdenum disulfide sheets (CNFs@MoS₂), is now reported. A synergistic effect was observed for the two‐component anode, triggering new electrochemical processes for lithium storage, with a persistent oxidation from Mo (or MoS₂) to MoS₃ in the repeated charge processes, leading to an ascending capacity upon cycling. The composite exhibits unprecedented electrochemical behavior with high specific capacity, good cycling stability, and superior high‐rate capability, suggesting its potential application in high‐energy lithium‐ion batteries.     

Ultrathin MoS2 Nanosheets Supported on N‐doped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties

Molybdenum disulfide (MoS₂) has received considerable interest for electrochemical energy storage and conversion. In this work, we have designed and synthesized a unique hybrid hollow structure by growing ultrathin MoS₂ nanosheets on N‐doped carbon shells (denoted as C@MoS₂ nanoboxes). The N‐doped carbon shells can greatly improve the conductivity of the hybrid structure and effectively prevent the aggregation of MoS₂ nanosheets. The ultrathin MoS₂ nanosheets could provide more active sites for electrochemical reactions. When evaluated as an anode material for lithium‐ion batteries, these C@MoS₂ nanoboxes show high specific capacity of around 1000 mAh g^?1, excellent cycling stability up to 200 cycles, and superior rate performance. Moreover, they also show enhanced electrocatalytic activity for the electrochemical hydrogen evolution.