In situ electrochemical Raman investigation of charge storage in rGO and N-doped rGO

In this study, in situ electrochemical Raman spectroscopy was applied to clarify the charge storage mechanism in three types of anodes, synthetic graphite, reduced graphene oxide (rGO), and nitrogen-doped reduced graphene oxide (N-rGO). The Li+ intercalation phenomenon was measured in LiPF6 electrolyte solution using a modified coin cell setup. The synthetic graphite anode showed the splitting of the G peak at the potential E < 0.2 V vs. Li/Li+, corresponding to the formation of a graphite intercalation compound (GIC) and its second-order 2D peak was found to be red-shifted due to charge transfer and induced strain in the potential region of 0.5 to 0.15 V vs. Li/Li+. In the case of rGO, the lattice defects assisted in large and early intercalation of electrolyte ions, which is confirmed by the red-shift in the G peak (∼36 cm-1) and its early disappearance below 0.3 V vs. Li/Li+, respectively. Unlike rGO, nitrogen vacancies in N-rGO provide active sites for Li+ intercalation, resulting in enhanced charge transfer, displayed by the large red-shift in the G peak (∼55 cm-1) and blue-shift in the D peak. In addition, a new Raman peak at 1850 cm-1 was observed in N-rGO for the first time, corresponding to the formation of a reversible intermediate species from the interaction between Li+ and nitrogen vacancies. This work demonstrates the use of a simple in situ technique to get insight into the nano-carbon electrodes during device operation and to reveal the role of doped nitrogen atoms for Li+ intercalation.

Morphology Evolution and Control of Mo-polydopamine Coordination Complex from 2D Single Nanopetal to Hierarchical Microflowers

Controllable synthesis of functional materials is of widespread interest for particle engineering. Such a method has not been widely promoted due to the lack of recognition of the fundamental principle, especially for organic-inorganic hybrid materials. Here, as an entrance, the controllable synthesis of Mo-polydopamine coordination flowers is realized through a facile foaming method, and a 2D nanopetal as the building monomer of the flower is synthesized. Depending on the morphology evolution of Mo-dopamine complex under different conditions, and the surface iterative topology growth of the Mo-polydopamine petal, the reasons of why the Mo-polydopamine complex self-assembles into a flower structure can be attributed to the synergistic effect of multicore symbiosis and structural self-protective growth behaviors. Benefiting from the strong structure stability of the Mo-polydopamine nanopetal, a hybrid structure of MoO₂ quantum dot in situ anchoring in the N-doped 2D carbon framework is prepared by direct pyrolysis, which shows a highly reversible performance in application for lithium-ion secondary batteries (LIBs). This work enhances the possibility for the controllable synthesis of organic-inorganic hybrid materials by adjusting the multicore intergrowth and inhibiting the interfacial assembly.

Hierarchical MoO2/Mo2C/C Hybrid Nanowires as High-Rate and Long-Life Anodes for Lithium-Ion Batteries

Hierarchical MoO2/Mo2C/C hybrid nanowires (MoO2/Mo2C/C HNWs) have been fabricated through facile calcination of Mo3O10(C6H5NH3)2·2H2O nanowires which serve as both precursors and self-templates. In the MoO2/Mo2C/C HNWs, nanoparticles dispersed in the nanowires are beneficial for Li(+) transportation due to the decreased diffusion paths. Moreover, hybridization with Mo2C and carbon facilitates the electron transfer and increases the structural stability without sacrifice of capacity. As anode materials for lithium-ion batteries, the MoO2/Mo2C/C HNWs exhibit a reversible capacity of 950 mA h g(-1) after 320 cycles at a current density of 200 mA g(-1). Even when cycled at 2000 mA g(-1), they maintained a reversible capacity of 602 mA h g(-1) after 500 cycles. By incorporation of Mo2C and C with MoO2, the MoO2/Mo2C/C HNWs show high-rate capability and long cycle life and can be a promising candidate for lithium-ion battery anodes.

Exfoliated Graphene Oxide/MoO2 Composites as Anode Materials in Lithium-Ion Batteries: An Insight into Intercalation of Li and Conversion Mechanism of MoO2

Exfoliated graphene oxide (EG)/MoO2 composites are synthesized by a simple solid-state graphenothermal reduction method. Graphene oxide (GO) is used as a reducing agent to reduce MoO3 and as a source for EG. The formation of different submicron sized morphologies such as spheres, rods, flowers, etc., of monoclinic MoO2 on EG surfaces is confirmed by complementary characterization techniques. As-synthesized EG/MoO2 composite with a higher weight percentage of EG performed excellently as an anode material in lithium-ion batteries. The galvanostatic cycling studies aided with postcycling cyclic voltammetry and galvanostatic intermittent titrations followed by ex situ structural studies clearly indicate that Li intercalation into MoO2 is transformed into conversion upon aging at low current densities while intercalation mechanism is preferably taking place at higher current rates. The intercalation mechanism is found to be promising for steady-state capacity throughout the cycling because of excess graphene and higher current density even in the operating voltage window of 0.005-3.0 V in which MoO2 undergoes conversion below 0.8 V.

Ethanol thermal reduction synthesis of hierarchical MoO2–C hollow spheres with high rate performance for lithium ion batteries

Hierarchical MoO₂–C hollow spheres were initially synthesized, which exhibited large reversible capacity and excellent high rate performance for lithium-ion batteries.

A simple, one-pot synthesis of molybdenum oxide-reduced graphene oxide composites in supercritical methanol and their electrochemical performance

A simple and green supercritical methanol (scMeOH) route is developed to tightly anchor molybdenum oxide (MoO₂) nanoparticles on reduced graphene oxide (RGO).

MoO2 nanocrystals interconnected on mesocellular carbon foam as a powerful catalyst for vanadium redox flow battery

MoO₂ nanocrystals on mesocellular carbon foam are used for a high performance vanadium redox flow battery. This improves the slow reaction of the VO²⁺/VO²⁺ redox couple, inducing high efficiencies with high specific capacity.

Ultrahigh Capacity Due to Multi-Electron Conversion Reaction in Reduced Graphene Oxide-Wrapped MoO2 Porous Nanobelts

Multivalent transition metal oxides (MOx ) containing redox centers which can theoretically accept more than one electron have been suggested as promising anode materials for high-performance lithium ion batteries (LIBs). The Li-storage mechanism of these oxides is suggested to involve an unusual conversion reaction leading to the formation of metallic nanograins and Li2 O; however, a full-scale conversion reaction is seldom observed in molybdenum dioxide (MoO2 ) at room temperature due to slow kinetics. Herein, a full-scale multi-electron conversion reaction, leading to a high reversible capacity (974 mA h g(-1) charging capacity at 60 mA g(-1) ) in LIBs, is realized in a hybrid consisting of reduced graphene oxide (rGO) sheet-wrapped MoO2 porous nanobelts (rGO/MoO2 NBs). The rGO wrapping layers stabilize the nanophase transition in MoO2 and alleviate volume swing effects during lithiation/delithiation processes. This enables the hybrid to exhibit great cycle stability (tested to around 1900 cycles) and ultrafast rate capability (tested up to 50 A g(-1) ).

Nanostructured Mo-based electrode materials for electrochemical energy storage

This review focuses on the recent progress in nanostructured Mo-based electrode materials for rechargeable lithium/sodium-ion batteries, Mg batteries, and supercapacitors.

Enhanced electrochemical performances of MoO2 nanoparticles composited with carbon nanotubes for lithium-ion battery anodes

Lab-made CNT nanocomposites decorated with MoO₂ nanoparticles (MoO₂/CNTs) demonstrated superior cycling and rate performances as LIB anode materials.

Graphene-based nanocomposite anodes for lithium-ion batteries

Graphene-based nanocomposites have been demonstrated to be promising high-capacity anodes for lithium ion batteries to satisfy the ever-growing demands for higher capacity, longer cycle life and better high-rate performance. Synergetic effects between graphene and the introduced second-phase component are generally observed. In this feature review article, we will focus on the recent work on four different categories of graphene-based nanocomposite anodes by us and others: graphene-transitional metal oxide, graphene-Sn/Si/Ge, graphene-metal sulfide, and graphene-carbon nanotubes. For the supported materials on graphene, we will emphasize the non-zero dimensional (non-particle) morphologies such as two dimensional nanosheet/nanoplate and one dimensional nanorod/nanofibre/nanotube morphologies. The synthesis strategies and lithium-ion storage properties of these highlighted electrode morphologies are distinct from those of the commonly obtained zero dimensional nanoparticles. We aim to stress the importance of structure matching in the composites and their morphology-dependent lithium-storage properties and mechanisms.

Graphene: a two-dimensional platform for lithium storage

Lithium ion batteries (LIBs) have attracted great attention due to their high energy density, low maintenance requirements, and relatively low self-discharge. Since the electrode materials hold the key for the electrochemical performance of LIBs, the design and synthesis of unconventional electrode materials with high lithium-storage capacities are the current focus in LIB research. In the last few years, a great deal of effort has been directed toward graphene as the electrode material for LIBs owing to its high intrinsic surface area, high electrical conductivity, and good compatibility with other electrochemically active components. This review paper outlines the componential and structural design for graphene-based hybrids in LIBs with enhanced electrochemical performance. The typical fabrication methods and structure-property relationships of these hybrids are discussed.