Boosting the electrochemical performance of MoO3 anode for long-life lithium ion batteries: Dominated by an ultrathin TiO2 passivation layer

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.

A Brief Review of MoO3 and MoO3-Based Materials and Recent Technological Applications in Gas Sensors, Lithium-Ion Batteries, Adsorption, and Photocatalysis

Molybdenum trioxide is an abundant natural, low-cost, and environmentally friendly material that has gained considerable attention from many researchers in a variety of high-impact applications. It is an attractive inorganic oxide that has been widely studied because of its layered structure, which results in intercalation ability through tetrahedral/octahedral holes and extension channels and leads to superior charge transfer. Shape-related properties such as high specific capacities, the presence of exposed active sites on the oxygen-rich structure, and its natural tendency to oxygen vacancy that leads to a high ionic conductivity are also attractive to technological applications. Due to its chemistry with multiple valence states, high thermal and chemical stability, high reduction potential, and electrochemical activity, many studies have focused on the development of molybdenum oxide-based systems in the last few years. Thus, this article aims to briefly review the latest advances in technological applications of MoO3 and MoO3-based materials in gas sensors, lithium-ion batteries, and water pollution treatment using adsorption and photocatalysis techniques, presenting the most relevant and new information on heterostructures, metal doping, and non-stoichiometric MoO3-x.

Flowers Like α-MoO3/CNTs/PANI Nanocomposites as Anode Materials for High-Performance Lithium Storage

Lithium-ion batteries (LIBs) have been explored to meet the current energy demands; however, the development of satisfactory anode materials is a bottleneck for the enhancement of the electrochemical performance of LIBs. Molybdenum trioxide (MoO₃) is a promising anode material for lithium-ion batteries due to its high theoretical capacity of 1117 mAhg^-1 along with low toxicity and cost; however, it suffers from low conductivity and volume expansion, which limits its implementation as the anode. These problems can be overcome by adopting several strategies such as carbon nanomaterial incorporation and polyaniline (PANI) coating. Co-precipitation method was used to synthesize α-MoO₃, and multi-walled CNTs (MWCNTs) were introduced into the active material. Moreover, these materials were uniformly coated with PANI using in situ chemical polymerization. The electrochemical performance was evaluated by galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). XRD analysis revealed the presence of orthorhombic crystal phase in all the synthesized samples. MWCNTs enhanced the conductivity of the active material, reduced volume changes and increased contact area. MoO₃-(CNT)_12% exhibited high discharge capacities of 1382 mAhg^-1 and 961 mAhg^-1 at current densities of 50 mAg^-1 and 100 mAg^-1, respectively. Moreover, PANI coating enhanced cyclic stability, prevented side reactions and increased electronic/ionic transport. The good capacities due to MWCNT_S and the good cyclic stability due to PANI make these materials appropriate for application as the anode in LIBs.

Thermionic Emission of Atomic Layer Deposited MoO3/Si UV Photodetectors

Ultrathin MoO₃ semiconductor nanostructures have garnered significant interest as a promising nanomaterial for transparent nano- and optoelectronics, owing to their exceptional reactivity. Due to the shortage of knowledge about the electronic and optoelectronic properties of MoO₃/n-Si via an ALD system of few nanometers, we utilized the preparation of an ultrathin MoO₃ film at temperatures of 100, 150, 200, and 250 °C. The effect of the depositing temperatures on using bis(tbutylimido)bis(dimethylamino)molybdenum (VI) as a molybdenum source for highly stable UV photodetectors were reported. The ON-OFF and the photodetector dynamic behaviors of these samples under different applied voltages of 0, 0.5, 1, 2, 3, 4, and 5 V were collected. This study shows that the ultrasmooth and homogenous films of less than a 0.30 nm roughness deposited at 200 °C were used efficiently for high-performance UV photodetector behaviors with a high sheet carrier concentration of 7.6 × 10¹⁰ cm^-2 and external quantum efficiency of 1.72 × 10¹¹. The electronic parameters were analyzed based on thermionic emission theory, where Cheung and Nord's methods were utilized to determine the photodetector electronic parameters, such as the ideality factor (n), barrier height (Φ₀), and series resistance (R_s). The n-factor values were higher in the low voltage region of the I-V diagram, potentially due to series resistance causing a voltage drop across the interfacial thin film and charge accumulation at the interface states between the MoO₃ and Si surfaces.

Constructing Sub 10 nm Scale Interfused TiO2/SiOx Bicontinuous Hybrid with Mutual-Stabilizing Effect for Lithium Storage

TiO₂ has been considered as a promising intercalation lithium-ion-battery (LIB) anode material owing to its robust cyclability. However, it suffers from low capacity. Herein, we construct a sub 10 nm scale interfused TiO₂/SiO_x hybrid with a bicontinuous structure, in which bridged TiO₂ nanoparticles (over 80 wt %) are densely packed within a wormlike SiO_x network, through the simple oxidation of MAX Ti₃SiC₂ ceramic. State-of-the-art in situ microscopy characterization unravels a "mutual-stabilizing" effect from the interfused TiO₂/SiO_x hybrid upon lithiation. That is to say, the two interpenetrated active components restrain the volume expansion of each other with the stress being relieved through abundant interfaces. Meanwhile, the stress generated from one phase functioned as the compressive force on the other phase and vice versa, offsetting the overall volume effect and synergistically reinforcing the structure integrity. Benefiting from the "mutual-stabilizing" effect, the TiO₂/SiO_x composite manifests a high and stable specific capacity (∼671 mAh g^-1 after 580 cycles at 0.1 A g^-1) with a low volume expansion of ∼14% even in an extended potential window of 0.01-3.0 V (vs Li⁺/Li). The concept of mutual-stabilizing effect, in principle, applies to a wide class of interfused bicontinuous hybrids, providing insight into the design of LIB anode materials with high capacity and longevity.

Atomic and Molecular Layer Deposition as Surface Engineering Techniques for Emerging Alkali Metal Rechargeable Batteries

Alkali metals (lithium, sodium, and potassium) are promising as anodes in emerging rechargeable batteries, ascribed to their high capacity or abundance. Two commonly experienced issues, however, have hindered them from commercialization: the dendritic growth of alkali metals during plating and the formation of solid electrolyte interphase due to contact with liquid electrolytes. Many technical strategies have been developed for addressing these two issues in the past decades. Among them, atomic and molecular layer deposition (ALD and MLD) have been drawing more and more efforts, owing to a series of their unique capabilities. ALD and MLD enable a variety of inorganic, organic, and even inorganic-organic hybrid materials, featuring accurate nanoscale controllability, low process temperature, and extremely uniform and conformal coverage. Consequently, ALD and MLD have paved a novel route for tackling the issues of alkali metal anodes. In this review, we have made a thorough survey on surface coatings via ALD and MLD, and comparatively analyzed their effects on improving the safety and stability of alkali metal anodes. We expect that this article will help boost more efforts in exploring advanced surface coatings via ALD and MLD to successfully mitigate the issues of alkali metal anodes.

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.

Advances in Electrochemical Energy Devices Constructed with Tungsten Oxide-Based Nanomaterials

Tungsten oxide-based materials have drawn huge attention for their versatile uses to construct various energy storage devices. Particularly, their electrochromic devices and optically-changing devices are intensively studied in terms of energy-saving. Furthermore, based on close connections in the forms of device structure and working mechanisms between these two main applications, bifunctional devices of tungsten oxide-based materials with energy storage and optical change came into our view, and when solar cells are integrated, multifunctional devices are accessible. In this article, we have reviewed the latest developments of tungsten oxide-based nanostructured materials in various kinds of applications, and our focus falls on their energy-related uses, especially supercapacitors, lithium ion batteries, electrochromic devices, and their bifunctional and multifunctional devices. Additionally, other applications such as photochromic devices, sensors, and photocatalysts of tungsten oxide-based materials have also been mentioned. We hope this article can shed light on the related applications of tungsten oxide-based materials and inspire new possibilities for further uses.

Flake (NH4)6Mo7O24/ Polydopamine as a High Performance Anode for Lithium Ion Batteries

Ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄) (AMT) is commonly used as the precursor to synthesize Mo-based oxides or sulfides for lithium ion batteries (LIBs). However, the electrochemical lithium storage ability of AMT itself is unclear so far. In the present work, AMT is directly examined as a promising anode material for Li-ion batteries with good capacity and cycling stability. To further improve the electrochemical performance of AMT, AMT/polydopamine (PDA) composite was simply synthesized via recrystallization and freeze drying methods. Unlike with block shape for AMT, the as-prepared AMT/PDA composite shows flake morphology. The initial discharge capacity of AMT/PDA is reached up to 1471 mAh g⁻¹. It delivers a reversible discharge capacity of 702 mAh g⁻¹ at a current density of 300 mA g⁻¹, and a stable reversible capacity of 383.6 mA h g⁻¹ is retained at a current density of 0.5 A g⁻¹ after 400 cycles. Moreover, the lithium storage mechanism is fully investigated. The results of this work could potentially expand the application of AMT and Mo-based anode for LIBs.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

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.

3D ordered mesoporous TiO2@CMK-3 nanostructure for sodium-ion batteries with long-term and high-rate performance

Sodium ion battery is abundant in resources and costs low, making it very competitive in the large-scale energy storage devices. The anatase TiO₂ electrode material with insertion/extraction mechanism shows stable cycling performance, which is more in line with the technical requirements of large-scale energy storage batteries. To improve the electrical conductivity and stability of the TiO₂ electrode materials, we have synthesized anatase TiO₂ and CMK-3 composite. TiO₂ nanoparticles were deposited on the surface of CMK-3 by hydrothermal reaction, and the anode material of the SIBs with 3D network structure was prepared. With the CMK-3, the structure stability, conductivity and reaction kinetics of TiO₂@CMK-3 composite is improved. The electrochemical behavior is dominated by pseudocapacitance, which gives the material excellent high-rate performance. It delivers a reversible specific capacity of 186.3 mA h g^-1 after 100 cycles at the current density of 50 mA g^-1, 124.5 mA h g^-1 after 500 long-term cycles, meanwhile it shows an outstanding rate performance, a reversible specific capacity of 105.9 mA h g^-1 at 1600 mA g^-1, 177.3 mA h g^-1 when the current density drops to 50 mA g^-1.

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.

C@TiO2 /MoO3 Composite Nanofibers with 1T-Phase MoS2 Nanograin Dopant and Stabilized Interfaces as Anodes for Li- and Na-Ion Batteries

Integrating layered nanostructured MoS₂ with a structurally stable TiO₂ backbone to construct reciprocal MoS₂ /TiO₂ -based nanocomposites is an effective strategy. C@TiO₂ /MoO₃ composite nanofibers doped with 1T-phase MoS₂ nanograins were fabricated by partially sulfurizing MoO_x /TiO₂ precursors. By controlling a suitable preoxidation temperature before severe thermolysis of polyvinylpyrrolidone (PVP), the MoO_x /TiO₂ precursors formed a polymer-embedded array through coordination of the Mo source and pyrrolidyl groups of PVP. Sulfidation under water/solvent hydrothermal conditions led to partial formation of metallic 1T-phase MoS₂ from the MoO_x precursor with preoxidation at 200 °C. After carbonization, the TiO₂ /MoO₃ /MoS₂ nanograins were encapsulated in a carbon backbone in a vertical pattern, providing both chemical contact for confined electron transport and sufficient space to adapt to volume changes. The obtained carbon-based platform not only has the advantages of an integral structure, but also exhibited ultrastable specific capacities of 540 and 251 mAh g^-1 for Li-ion batteries and Na-ion batteries, respectively, after 100 cycles.

Heterostructure Manipulation via in Situ Localized Phase Transformation for High-Rate and Highly Durable Lithium Ion Storage

Recently, heterostructures have attracted much attention in widespread research fields. By tailoring the physicochemical properties of the two components, creating heterostructures endows composites with diverse functions due to the synergistic effects and interfacial interaction. Here, a simple in situ localized phase transformation method is proposed to transform the transition-metal oxide electrode materials into heterostructures. Taking molybdenum oxide as an example, quasi-core-shell MoO₃@MoO₂ heterostructures were successfully fabricated, which were uniformly anchored on reduced graphene oxide (rGO) for high-rate and highly durable lithium ion storage. The in situ introduction of the MoO₂ shell not only effectively enhances the electronic conductivity but also creates MoO₃@MoO₂ heterojunctions with abundant oxygen vacancies, which induces an inbuilt driving force at the interface, enhancing ion/electron transfer. In operando synchrotron X-ray powder diffraction has confirmed the excellent phase reversibility of the MoO₂ shell during charge/discharge cycling, which contributes to the excellent cycling stability of the MoO₃@MoO₂/rGO electrode (1208.9 mAh g^-1 remaining at 5 A g^-1 after 2000 cycles). This simple in situ heterostructure fabrication method provides a facile way to optimize electrode materials for high-performance lithium ion batteries and possibly other energy storage devices.