Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries

Advanced electrodes with a high energy density at high power are urgently needed for high-performance energy storage devices, including lithium-ion batteries (LIBs) and supercapacitors (SCs), to fulfil the requirements of future electrochemical power sources for applications such as in hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles. Metal sulfides with unique physical and chemical properties, as well as high specific capacity/capacitance, which are typically multiple times higher than that of the carbon/graphite-based materials, are currently studied as promising electrode materials. However, the implementation of these sulfide electrodes in practical applications is hindered by their inferior rate performance and cycling stability. Nanostructures offering the advantages of high surface-to-volume ratios, favourable transport properties, and high freedom for the volume change upon ion insertion/extraction and other reactions, present an opportunity to build next-generation LIBs and SCs. Thus, the development of novel concepts in material research to achieve new nanostructures paves the way for improved electrochemical performance. Herein, we summarize recent advances in nanostructured metal sulfides, such as iron sulfides, copper sulfides, cobalt sulfides, nickel sulfides, manganese sulfides, molybdenum sulfides, tin sulfides, with zero-, one-, two-, and three-dimensional morphologies for LIB and SC applications. In addition, the recently emerged concept of incorporating conductive matrices, especially graphene, with metal sulfide nanomaterials will also be highlighted. Finally, some remarks are made on the challenges and perspectives for the future development of metal sulfide-based LIB and SC devices.

Nanostructured metal sulfides for energy storage

SnO2: A comprehensive review on structures and gas sensors

The development of new electrode materials for lithium-ion batteries (LIBs) has always been a focal area of materials science, as the current technology may not be able to meet the high energy demands for electronic devices with better performance. Among all the metal oxides, tin dioxide (SnO₂) is regarded as a promising candidate to serve as the anode material for LIBs due to its high theoretical capacity. Here, a thorough survey is provided of the synthesis of SnO₂-based nanomaterials with various structures and chemical compositions, and their application as negative electrodes for LIBs. It covers SnO₂ with different morphologies ranging from 1D nanorods/nanowires/nanotubes, to 2D nanosheets, to 3D hollow nanostructures. Nanocomposites consisting of SnO₂ and different carbonaceous supports, e.g., amorphous carbon, carbon nanotubes, graphene, are also investigated. The use of Sn-based nanomaterials as the anode material for LIBs will be briefly discussed as well. The aim of this review is to provide an in-depth and rational understanding such that the electrochemical properties of SnO₂-based anodes can be effectively enhanced by making proper nanostructures with optimized chemical composition. By focusing on SnO₂, the hope is that such concepts and strategies can be extended to other potential metal oxides, such as titanium dioxide or iron oxides, thus shedding some light on the future development of high-performance metal-oxide based negative electrodes for LIBs.

SnO2‐Based Nanomaterials: Synthesis and Application in Lithium‐Ion Batteries

We successfully synthesized large-scale and highly pure ultrathin SnO2 nanosheets (NSs), with a minimum thickness in the regime of ca. 2.1 nm as determined by HRTEM and in good agreement with XRD refinements and AFM height profiles. Through TEM and HRTEM observations on time-dependent samples, we found that the as-prepared SnO2 NSs were assembled by “oriented attachment” of preformed SnO2 nanoparticles (NPs). Systematic trials showed that well-defined ultrathin SnO2 NSs could only be obtained under appropriate reaction time, solvent, additive, precursor concentration, and cooling rate. A certain degree of nonstoichiometry appears inevitable in the well-defined SnO2 NSs sample. However, deviations from the optimal synthetic parameters give rise to severe nonstoichiometry in the products, resulting in the formation of Sn3O4 or SnO. This finding may open new accesses to the fundamental investigations of tin oxides as well as their intertransition processes. Finally, we investigated the lithium-ion storage of...

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Ultrathin SnO2 Nanosheets: Oriented Attachment Mechanism, Nonstoichiometric Defects, and Enhanced Lithium-Ion Battery Performances

Metal sulfides are a type of potential anode materials for lithium-ion batteries (LIBs). However, their electrochemical behaviors and mechanism during the charge and discharge process remain unclear. In the present paper, we use CoS2 as a model material to investigate their electrochemical process using in situ transmission electron microscopy (TEM). Two kinds of reaction behaviors are revealed. The pure CoS2 particles show a side-to-side conversion process, in which large and anisotropic size expansion (47.1%) occurs that results in the formation of cracks and fractures in CoS2 particles. In contrast, the CoS2 particles anchored on reduced graphene oxide (rGO) sheets exhibit a core–shell conversion process involving small and homogeneous size expansion (28.6%) and few fractures, which attributes to the excellent Li+ conductivity of rGO sheets and accounts for the improved cyclability. Single-crystalline CoS2 particle converts to Co nanocrystals of 1–2 nm embedded within Li2S matrix after the first lithia...

In Situ Transmission Electron Microscopy Observation of Electrochemical Behavior of CoS2 in Lithium-Ion Battery

The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, the electrochemical sodiation process and microstructure evolution of individual Co9S8-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co9S8-filled CNT with an open end, the sodiated Co9S8 filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between th...

In Situ Transmission Electron Microscopy Observation of Electrochemical Sodiation of Individual Co9S8-Filled Carbon Nanotubes

Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co...

Yijun Zhong

Papers

Ultrathin SnO2 Nanosheets: Oriented Attachment Mechanism, Nonstoichiometric Defects, and Enhanced Lithium-Ion Battery Performances

In Situ Transmission Electron Microscopy Observation of Electrochemical Behavior of CoS2 in Lithium-Ion Battery

In Situ Transmission Electron Microscopy Observation of Electrochemical Sodiation of Individual Co9S8-Filled Carbon Nanotubes

In Situ Transmission Electron Microscopy Investigation of the Electrochemical Lithiation–Delithiation of Individual Co9S8/Co-Filled Carbon Nanotubes