New Insights into Electrochemical Lithiation/Delithiation Mechanism of α-MoO3 Nanobelt by in Situ Transmission Electron Microscopy

Hexagonal MoO3 Anode with Extremely High Capacity and Cyclability for Lithium-Ion Battery: A Combined Theoretical and Experimental Study

It is essential and still remains a big challenge to obtain fast-charge anodes with large capacities and long lifespans for Li-ion batteries (LIBs). Among all of the alternative materials, molybdenum trioxide shows the advantages of large theoretical specific capacity, distinct tunnel framework, and low cost. However, there are also some key shortcomings, such as fast capacity decaying due to structural instability during Li insertion and poor rate performance due to low intrinsic electron conductivity and ion diffusion capability, dying to be overcome. A unique strategy is proposed to prepare Ti-h-MoO3-x@TiO2 nanosheets by a one-step hydrothermal approach with NiTi alloy as a control reagent. The density functional theory (DFT) calculations indicate that the doping of Ti element can make the hexagonal h-MoO3-x material show the best electronic structure and it is favor to be synthesized. Furthermore, the hexagonal Ti-h-MoO3-x material has better lithium storage capacity and lithium diffusion capacity than the orthogonal α-MoO3 material, and its theoretical capacity is more than 50% higher than that of the orthogonal α-MoO3 material. Additionally, it is found that Ti-h-MoO3-x@TiO2 as an anode displays extremely high reversible discharge/charge capacities of 1326.8/1321.3 mAh g-1 at 1 A g-1 for 800 cycles and 611.2/606.6 mAh g-1 at 5 A g-1 for 2000 cycles. Thus, Ti-h-MoO3-x@TiO2 can be considered a high-power-density and high-energy-density anode material with excellent stability for LIBs.

Unexpected Two-Dimensional Polarons Induced by Oxygen Vacancies in Layered Structure MoO3-x

Unveiling the effects of oxygen vacancies on the structural stability of layered α-MoO3 is critical for optimizing its physical and chemical properties. Herein, we present experimental evidence regarding the phase stability of α-MoO3 with ∼2% oxygen vacancy concentrations. Interestingly, we report a previously ignored oxygen-deficient orthorhombic MoO3-x phase in space group Cmcm. Further density functional theory calculations reveal a detailed phase transition mechanism from α-MoO3 to MoO3-x. More importantly, we demonstrate that two-dimensional (2D) large polarons must exist to stabilize the MoO3-x crystal structure. 2D large polarons are suspected to exist in numerous quasi-2D systems but have never been found in layered α-MoO3 or other molybdenum oxides. Our work contributes to a basic understanding of the polaronic behavior in MoO3-x and may broaden the application realm of molybdenum oxides.

Enhanced Lithium Storage Performance of α-MoO3/CNTs Composite Cathode

Orthorhombic molybdenum oxide (α-MoO3), as a one-layered pseudocapacitive material, has attracted widespread attention due to its high theoretical lithium storage specific capacity (279 mAh/g) for lithium-ion batteries' cathode. Nevertheless, low conductivity, slack reaction kinetics, and large volume change during Li+ ions intercalation and deintercalation seriously limit the practical application of α-MoO3. Herein, we added a small number of CNTs (1.76%) to solve these problems in a one-step hydrothermal process for preparing the α-MoO3/CNTs composite. Because of the influence of CNTs, the α-MoO3 nanobelt in the α-MoO3/CNTs composite had a larger interlayer spacing, which provided more active sites and faster reaction kinetics for lithium storage. In addition, CNTs formed a three-dimensional conductive network between α-MoO3 nanobelts, enhanced the electrical conductivity of the composite, accelerated the electron conduction, shortened the ion transport path, and alleviated the structural fragmentation caused by the volume expansion during the α-MoO3 intercalation and deintercalation of Li+ ions. Therefore, the α-MoO3/CNTs composite cathode had a significantly higher rate performance and cycle life. After 150 cycles, the pure α-MoO3 cathode had almost no energy storage, but α-MoO3/CNTs composite cathode still retained 93 mAh/g specific capacity.

Controllable hot electron transfer in the Ag/MoO3 layer by layer system: Thickness-dependent MoO3 layer

The Ag and MoO₃ layer-by-layer sputtering method was employed to fabricate Ag/MoO₃ coated on a polystyrene (PS) array (Ag/MoO₃@PS), which exhibited excellent surface-enhanced Raman scattering (SERS) activity. The thickness of the MoO₃ layer was controlled by changing the sputtering power. The SERS intensity of 4-aminothiophenol (PATP) on Ag/MoO₃@PS with a 2 nm thickness of the MoO₃ layer was comparable to that on pure Ag coated on the PS array (Ag@PS). This is possible because hot electrons were injected from Ag into the MoO₃ layer to enhance the photocatalyst reaction; thus, the SERS spectra of coupled PATP were obtained. The transport of hot electrons rapidly decayed and was blocked with increasing thickness of the MoO₃ layer from 2 nm to 9 nm. Thus, the SERS intensity decreased, and interestingly, the b₂ mode of PATP decreased and nearly disappeared. This study provides new insight into the control of hot electron reduction for catalytic reduction process monitoring.

Nanostructured Molybdenum-Oxide Anodes for Lithium-Ion Batteries: An Outstanding Increase in Capacity

This work aimed at synthesizing MoO₃ and MoO₂ by a facile and cost-effective method using extract of orange peel as a biological chelating and reducing agent for ammonium molybdate. Calcination of the precursor in air at 450 °C yielded the stochiometric MoO₃ phase, while calcination in vacuum produced the reduced form MoO₂ as evidenced by X-ray powder diffraction, Raman scattering spectroscopy, and X-ray photoelectron spectroscopy results. Scanning and transmission electron microscopy images showed different morphologies and sizes of MoO_x particles. MoO₃ formed platelet particles that were larger than those observed for MoO₂. MoO₃ showed stable thermal behavior until approximately 800 °C, whereas MoO₂ showed weight gain at approximately 400 °C due to the fact of re-oxidation and oxygen uptake and, hence, conversion to stoichiometric MoO₃. Electrochemically, traditional performance was observed for MoO₃, which exhibited a high initial capacity with steady and continuous capacity fading upon cycling. On the contrary, MoO₂ showed completely different electrochemical behavior with less initial capacity but an outstanding increase in capacity upon cycling, which reached 1600 mAh g^-1 after 800 cycles. This outstanding electrochemical performance of MoO₂ may be attributed to its higher surface area and better electrical conductivity as observed in surface area and impedance investigations.

In Situ TEM Study on Conversion-Type Electrodes for Rechargeable Ion Batteries

Conversion-type materials have been considered as potentially high-energy-density alternatives to commercially dominant intercalation-based electrodes for rechargeable ion batteries and have attracted tremendous research effort to meet the performance for viable energy-storage technologies. In situ transmission electron microscopy (TEM) has been extensively employed to provide mechanistic insights into understanding the behavior of battery materials. Noticeably, a great portion of previous in situ TEM studies has been focused on conversion-type materials, but a dedicated review for this group of materials is missing in the literature. Herein, recent developments of in situ TEM techniques for investigation of dynamic phase transformation and associated structural, morphological, and chemical evolutions during conversion reactions with alkali ions in secondary batteries are comprehensively summarized. The materials of interest broadly cover metal oxides, chalcogenides, fluorides, phosphides, nitrides, and silicates with specific emphasis on spinel metal oxides and recently emerged 2D metal chalcogenides. Special focus is placed on the scientific findings that are uniquely obtained by in situ TEM to address fundamental questions and practical issues regarding phase transformation, structural evolution, electrochemical redox, reaction mechanism, kinetics, and degradation. Critical challenges and perspectives are discussed for advancing new knowledge that can bridge the gap between prototype materials and real-world applications.

Reversible, stable Li-ion storage in 2 D single crystal orthorhombic α-MoO3 anodes

Engineering two dimensional (2D) materials at atomic level is a key factor to achieve enhanced electrochemical Li-ion storage properties. This work demonstrates that single crystals of orthorhombic α-MoO₃ phase can preferentially grow with a 2D nanoarchitecture via a ball-milling process, followed by heat treatment at elevated temperature. Detailed FE-SEM and TEM micrographs proved the 2D architecture of α-MoO₃ nanoparticles and Raman spectroscopy evidenced the active vibration modes that correspond to the orthorhombic α-MoO₃ phase. Single crystalline MoO₃ belts depicted high intensity of (0 2 0) and (0 4 0) indexed planes indicating a preferential arrangement. As Li-ion host anode, the 2D α-MoO₃ nanostructure delivered high reversible specific discharge capacity of ~540 mA h g^-1 at 0.2 C-rate with 99.9% coulombic efficiency as well as 63% capacity retention after 200 charge-discharge cycles. An excellent reversible Li-ion storage performance (high capacity, longer cycle life and good rate capability) was attributed to the 2D α-MoO₃ arrangement consists of MoO₆ octahedron by corner sharing chains.

Flowerlike Ti-Doped MoO3 Conductive Anode Fabricated by a Novel NiTi Dealloying Method: Greatly Enhanced Reversibility of the Conversion and Intercalation Reaction

Anodes made of molybdenum trioxide (MoO₃) suffer from insufficient conductivity and low catalytic reactivity. Here, we demonstrate that by using a dealloying method, we were able to fabricate anode of Ti-doped MoO₃ (Ti-MoO₃), which exhibits high catalytic reactivity, along with enhanced rate performance and cycling stability. We found that after doping, interestingly, the Ti-MoO₃ forms nanosheets and assembles into a micrometer-sized flowerlike morphology with enhanced interlayer distance. The density functional theory result has further concluded that the band gap of the Ti-doped anode has been reduced significantly, thus greatly enhancing the electronic conductivity. As a result, the structure maintains stability during the Li⁺ intercalation/deintercalation processes, which enhances the cycling stability and rate capability. This engineering strategy and one-step synthesis route opens up a new pathway in the design of anode materials.

Self-templated construction of 1D NiMo nanowires via a Li electrochemical tuning method for the hydrogen evolution reaction

NiMo based materials have been widely recognized as the most promising alternatives to noble Pt electrocatalysts used in alkaline electrolytes for the hydrogen evolution reaction. However, it is difficult to construct a nanostructure, especially 1D morphology, for NiMo materials via an electrochemical method. Herein, a novel Li electrochemical tuning method, for the first time, is introduced to synthesize 1D NiMo nanowires by insertion of lithium ions into parent NiMoO₄ nanorods. The as-prepared NiMo catalyst exhibits high HER activity in 1 M KOH, in terms of low overpotential (73 mV) at a current density of 10 mA cm^-2 and a small Tafel slope (37.2 mV dec^-1) and charge transfer resistance (11.3 Ω). Furthermore, no decay in catalytic performance is observed for this material when it is operated at -0.125 V (vs. RHE) for 1250 min and a high Faraday efficiency (96%) is achieved. The high activity of NiMo is ascribed to the synergistic interplay between Ni and Mo and its unique nanostructure, which can expose more active sites and facilitate the mass transfer and hydrogen bubble release.

In Situ Transmission Electron Microscopy for Energy Materials and Devices

Energy devices such as rechargeable batteries, fuel cells, and solar cells are central to powering a renewable, mobile, and electrified future. To advance these devices requires a fundamental understanding of the complex chemical reactions, material transformations, and charge flow that are associated with energy conversion processes. Analytical in situ transmission electron microscopy (TEM) offers a powerful tool for directly visualizing these complex processes at the atomic scale in real time and in operando. Recent advancements in energy materials and devices that have been enabled by in situ TEM are reviewed. First, the evolutionary development of TEM nanocells from the open-cell configuration to the closed-cell, and finally the full-cell, is reviewed. Next, in situ TEM studies of rechargeable ion batteries in a practical operation environment are explored, followed by applications of in situ TEM for direct observation of electrocatalyst formation, evolution, and degradation in proton-exchange membrane fuel cells, and fundamental investigations of new energy materials such as perovskites for solar cells. Finally, recent advances in the use of environmental TEM and cryogenic electron microscopy in probing clean-energy materials are presented and emerging opportunities and challenges in in situ TEM research of energy materials and devices are discussed.

Two-Dimensional Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode Material for Lithium-Ion Batteries

α-MoO₃ has gained growing attention as an anode material of lithium-ion batteries (LIBs) because it has a high theoretical specific capacity of 1111 mA h g^-1 and unique layer structure. However, the electrochemical reactions of MoO₃ exhibit sluggish kinetics and structural instability caused by pulverization during charge and discharge. Herein, we report new two-dimensional Cr-doped MoO_2.5(OH)_0.5 (doped MoO_2.5(OH)_0.5) ultrathin nanosheets prepared by a facile hydrothermal process. The formation of the ultrathin nanosheets was clarified by a "doping-adsorption" model. Compared with doped MoO₃, doped MoO_2.5(OH)_0.5 has larger expanded spacing of the {0 l0} crystal planes for fast Li⁺ storage. The electrodes after cycling were investigated by ex situ transmission electron microscopy in combination with X-ray photoelectron spectroscopy analysis to reveal the reversible conversion reaction mechanism of doped MoO_2.5(OH)_0.5 nanosheets. Interestingly, for doped MoO_2.5(OH)_0.5 nanosheet electrodes, it was found that the as-formed intermediate Li _xMoO₃ nanodots were well-dispersed in the mesoporous amorphous matrix and had an expanded (040) crystal plane after 10 cycles. These unique structural features increased the effective surface of intermediate products Li _xMoO₃ to react with Li⁺ and shortened the diffusion length and thus promoted the electrochemical reactions of doped MoO_2.5(OH)_0.5. Additionally, the presence of Cr also played a critical role in the reversible decomposition of Li₂O and enhanced specific capacity. When employed as an anode in LIBs, doped MoO_2.5(OH)_0.5 nanosheets show superior reversible capacity (294 mA h g^-1 at 10 A g^-1 after 2000 cycles). Moreover, the reversible capacity after electrochemical activation is quite stable throughout the cycling, thereby presenting a potential candidate anode material for LIBs.

Recent Progress on Molybdenum Oxides for Rechargeable Batteries

Diminishing fossil-fuel resources and a rise in energy demands has required the pursuit of sustainable and rechargeable energy-storage materials, including batteries and supercapacitors, the electrochemical properties of which depend largely on the electrode materials. In recent decades, numerous electrode materials with excellent electrochemical energy-storage capabilities, long life spans, and environmentally acceptable qualities have been developed. Among existing materials, molybdenum oxides containing MoO₃ and MoO₂ , as well as their composites, are very fascinating contenders for competent energy-storage devices because of their exceptional physicochemical properties, such as thermal stability, high theoretical capability, and mechanical strength. This Minireview mainly focuses on the latest progress for the use of molybdenum oxides as electrode materials for lithium-ion batteries; sodium-ion batteries; and other novel batteries, such as lithium-sulfur batteries, lithium-oxygen batteries, and newly developed hydrogen-ion batteries, with a focus on studies of the reaction mechanism, design of the electrode structures, and improvement of the electrochemical properties.

Anomalous Shape Evolution of Ag2O2 Nanocrystals Modulated by Surface Adsorbates during Electron Beam Etching

An understanding of nanocrystal shape evolution is significant for the design, synthesis, and applications of nanocrystals with surface-enhanced properties such as catalysis or plasmonics. Surface adsorbates that are selectively attached to certain facets may strongly affect the atomic pathways of nanocrystal shape development. However, it is a great challenge to directly observe such dynamic processes in situ with a high spatial resolution. Here, we report the anomalous shape evolution of Ag₂O₂ nanocrystals modulated by the surface adsorbates of Ag clusters during electron beam etching, which is revealed through in situ transmission electron microscopy (TEM). In contrast to the Ag₂O₂ nanocrystals without adsorbates, which display the near-equilibrium shape throughout the etching process, Ag₂O₂ nanocrystals with Ag surface adsorbates show distinct facet development during etching by electron beam irradiation. Three stages of shape changes are observed: a sphere-to-a cube transformation, side etching of a cuboid, and bottom etching underneath the surface adsorbates. We find that the Ag adsorbates modify the Ag₂O₂ nanocrystal surface configuration by selectively capping the junction between two neighboring facets. They prevent the edge atoms from being etched away and block the diffusion path of surface atoms. Our findings provide critical insights into the modulatory function of surface adsorbates on the shape control of nanocrystals.

Single-Nanostructured Electrochemical Detection for Intrinsic Mechanism of Energy Storage: Progress and Prospect

Energy storage appliances are active by means of accompanying components for renewable energy resources that play a significant role in the advanced world. To further improve the electrochemical properties of the lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and lithium-sulfur (Li-S) batteries, the electrochemical detection of the intrinsic mechanisms and dynamics of electrodes in batteries is required to guide the rational design of electrodes. Thus, several researches have conducted in situ investigations and real-time observations of electrode evolution, ion diffusion pathways, and side reactions during battery operation at the nanoscale, which are proven to be extremely insightful. However, the in situ cells are required to be compatible for electrochemical tests and are therefore often challenging to operate. In the past few years, tremendous progresses have been made with novel and more advanced in situ electrochemical detection methods for mechanism studies, especially single-nanostructured electrodes. Herein, a comprehensive review of in situ techniques based on single-nanostructured electrodes for studying electrodes changes in LIBs, SIBs, and Li-S batteries, including structure evolution, phase transition, interface formation, and the ion diffusion pathway is provided, which is instructive and meaningful for the optimization of battery systems.

A 3D MoOx/carbon composite array as a binder-free anode in lithium-ion batteries

A 3D aligned MoO_x/carbon composite anode displays good cycle capacity in binder free lithium ion battery applications.

Atomic layer deposition of TiO2 shells on MoO3 nanobelts allowing enhanced lithium storage performance

Atomic layer deposition of TiO₂ shells on MoO₃ nanobelts greatly improved the lithium storage performance.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Direct Realization of Complete Conversion and Agglomeration Dynamics of SnO2 Nanoparticles in Liquid Electrolyte

The conversion reaction is important in lithium-ion batteries because it governs the overall battery performance, such as initial Coulombic efficiency, capacity retention, and rate capability. Here, we have demonstrated in situ observation of the complete conversion reaction and agglomeration of nanoparticles (NPs) upon lithiation by using graphene liquid cell transmission electron microscopy. The observation reveals that the Sn NPs are nucleated from the surface of SnO₂, followed by merging with each other. We demonstrate that the agglomeration has a stepwise process, including rotation of a NP, formation of necks, and subsequent merging of individual NPs.

New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C Yolk-Shell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries

In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk-shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk-shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk-shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mAh g^-1), high rate capability (specific capabilities of 622, 557, 496, 439, and 384 mAh g^-1 at 100, 200, 500, 1000, and 2000 mA g^-1, respectively) and stable cycling performance (a specific capacity of 405 mAh g^-1 after long 300 cycles at 1000 mA g^-1). As for Na-ion storage, these yolk-shell Sb@C particles also maintained a reversible capacity of approximate 280 mAh g^-1 at 1000 mA g^-1 after 200 cycles.