Ultrathin Hexagonal 2D Co₂GeO₄ Nanosheets: Excellent Li-Storage Performance and ex Situ Investigation of Electrochemical Mechanism

Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. ThisB-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperatureTFN∼ 77 K due to uneven moments of divalent Co (↑ 5.33μB) and tetravalent Mn (↓ 3.87μB) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (JAB/kB∼ 4.3 K,JAA/kB∼ -6.2 K andJBB/kB∼ -3.3 K) between the cations, which is extracted following the Néel's expression for the two-sublattice model of FiM. Dynamical susceptibility (χac(f, T)) and relaxation of thermoremanent magnetization,MTRM(t) data have been analysed by means of the empirical scaling-laws such as Vogel-Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponentzυ= (6.7 ± 0.07), spin relaxation timeτ0= (2.33 ± 0.1) × 10-18s, activation energyEa/kB= (69.8 ± 0.95) K and interparticle interaction strength (T0= 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperatureTF= 72.85 K. The field dependence ofTFobtained fromχac(T) follows the irreversibility in terms of de Almeida-Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (HSF1) and 45.6 kOe (HSF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibilityχp-1χp=χ-χ0after subtracting the temperature independent diamagnetic termχ0(=-3 × 10-3emu mol-1Oe-1) results in the effective magnetic momentμeff= 7.654μB/f.u. This agrees well with the theoretically obtainedμeff= 7.58μB/f.u. resulting the cation distributionMn0.24+↓A[Co22+↑]BO4in support of the Hund's ground state spin configurationS=3/2andS= 1/2of Mn4+and Co2+, respectively. TheH-Tphase diagram has been established by analysing all the parameters (TF(H),TFN(H),HSF1(T) andHSF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4and also illustrates the demarcation between short-range and long-range ordered regions.

Structures and Properties of Planar Ge3O3 Cluster and Its Buckled Honeycomb Two-Dimensional Nanostructure

The discovery of graphene and its excellent properties inspired the search for more two-dimensional (2D) materials. Understanding the structures and properties of the smallest repeating units as well as crystal 2D materials is helpful for designing 2D materials. As germanium tends to form three-dimensional structures, the preparation of germanium-based 2D nanomaterials is still a challenge. Herein, we report a Ge3O3 cluster with the potential to construct a germanium oxide 2D nanostructure. We conduct a combined anion photoelectron spectroscopy and theoretical study on Ge3O3-/0. The structure of Ge3O3- is a Cs symmetric nonplanar six-membered ring, while that of Ge3O3 is a D3h symmetric planar six-membered ring. Chemical bonding analyses reveal that Ge3O3 exhibits π aromaticity. First-principle results suggest that a buckled honeycomb 2D nanostructure with a wide band gap of 3.14 eV may be produced based on Ge3O3, which is promising in optoelectronic applications especially in blue, violet, and ultraviolet regions.

Tuning the d-Band States of Ni-Based Serpentine Materials via Fe3+ Doping for Efficient Oxygen Evolution Reaction

The serpentine germanate materials are promising oxygen evolution reaction (OER) electrocatalysts due to their unique layered crystal structure and electronic structure. However, the catalytic activities still need to be improved to satisfy the practical applications. Adjusting the d-band center of metal active site to balance the adsorption and desorption of intermediates is considered an effective approach to improve the OER activity. In this work, an element dopant strategy was proposed to optimize the d-band state of Ni₃Ge₂O₅(OH)₄ serpentine to improve the OER activity. The density functional theory calculations revealed that Fe³⁺ doping increased the d-band center of the Ni₃Ge₂O₅(OH)₄ serpentine, which optimized the adsorption strength of intermediates on surface Ni and Fe atoms so that the Fe³⁺ doped Ni₃Ge₂O₅(OH)₄ (Ni_2.25Fe_0.75Ge₂O₅(OH)₄) exhibited much reduced Gibbs free energy changes in the rate-determining step compared with pristine serpentine. Inspired by the theoretical calculations, the Ni_xFe_3-xGe₂O₅(OH)₄ nanosheets with different amounts of doped Fe³⁺ were designed and synthesized. The structural characterizations indicated that Fe³⁺ was successfully doped into Ni₃Ge₂O₅(OH)₄ and replaced the Ni²⁺. The Fe³⁺ doped Ni_xFe_3-xGe₂O₅(OH)₄ nanosheets showed greatly improved OER activity than Ni₃Ge₂O₅(OH)₄ and Fe₃Ge₂O₅(OH)₄. Further electrochemical analysis illustrated that Fe³⁺ doping reduced the adsorptive/formative resistance of intermediates and the charge transfer resistance and facilitated the kinetic process of OER. The in situ Raman spectra indicated that the Fe³⁺ doped Ni₃Ge₂O₅(OH)₄ possesses a more active Ni-O bond than pristine Ni₃Ge₂O₅(OH)₄. This work provides an effective strategy to tune the d-band center of serpentines for efficient electrocatalytic OER.

Tailoring the electronic structure and magnetic properties of pyrochlore Co2Ti1-xGexO4: a GGA +Uab initiostudy

We report the electronic structure and magnetic properties of Co₂Ti_1-xGe_xO₄(0 ⩽x⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U_eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a≠ 1) due to the non-degeneratedz2anddx2-y2states (e_gorbitals) of the B-site Co. We observe large exchange splitting (Δ_EX∼ 9 eV) between the up and down spin bands oft_2gande_gstates, respectively, of tetrahedral and octahedral Co²⁺(⁴A_2(g)(F)) and moderate crystal-field splitting (Δ_CF∼ 4 eV) and the Jahn-Teller distortion (Δ_JT∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽E_g⩽ 3.3 eV). The exchange interaction (J_BB∼ -4.8 meV, for (x,y) = (0.25, 0)) between the Co²⁺dominates the overall antiferromagnetic behaviour of the system for all 'x' as compared toJ_AA(∼-2.2 meV, for (x,y) = (0.25, 0)) andJ_AB(∼-1.8 meV, for (x,y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasingxdue to the imbalance of Co spins between the A- and B-sites.

Co2GeO4 nanocomposites with reduced graphene oxide and carbon nanotubes as high-performance anodes for Na-ion batteries

The prepared nanocomposites show an enhanced electrochemical performance.

Achieving High Lithium Storage Capacity and Long-Term Cyclability of Novel Cobalt Germanate Hydroxide/Reduced Graphene Oxide Anodes with Regulated Electrochemical Catalytic Conversion Process of Hydroxyl Groups

To develop ternary transition-metal germanate anodes with superior lithium storage performances for lithium-ion batteries, a novel capacity counterbalance approach in one compound is designed by introducing an electrocatalytic conversion-type component with a positive cycling trend to compensate the negative cycling trend of the GeO₂ component. Novel cobalt germanate hydroxide (CGH) nanoplates chemically bonded on reduced graphene oxide (RGO) sheets are thus synthesized with a mild one-pot hydrothermal approach, constructing maximal face-to-face contact interfaces with interfacial bonds to boost the electrochemical conversion reactions. Furthermore, the hydroxyl groups (Co-OH) of CGH nanoplates are regulated by thermal annealing treatments, thus controlling the capacity contribution resulting from the electrocatalytic conversion reaction of LiOH to exactly offset the capacity fading of GeO₂. The results on the CGH electrodes at different cycling potentials confirm the stepwise electrochemical reactions of Co, GeO₂, and LiOH. The equilibrium of these electrochemical reactions ensures a stable cycling capacity without obvious fluctuations. Consequently, the optimal CGH/RGO hybrid anode delivers a reversible capacity as high as 1136 mA h g^-1 at 0.1 A g^-1 until 100 cycles. It also exhibits a long cyclability with a retained capacity of 560 mA h g^-1 at 1 A g^-1 until 1000 cycles. This work demonstrates a general and efficient capacity counterbalance method to highly boost lithium storage performances in terms of high capacity and long-term cyclability.

Co2TiO4/Reduced Graphene Oxide Nanohybrids for Electrochemical Sensing Applications

For the first time, the synthesis, characterization, and analytical application for hydrogen peroxide quantification of the hybrid materials of Co2TiO4 (CTO) and reduced graphene oxide (RGO) is reported, using in situ (CTO/RGO) and ex situ (CTO+RGO) preparations. This synthesis for obtaining nanostructured CTO is based on a one-step hydrothermal synthesis, with new precursors and low temperatures. The morphology, structure, and composition of the synthesized materials were examined using scanning electron microscopy, X-ray diffraction (XRD), neutron powder diffraction (NPD), and X-ray photoelectron spectroscopy (XPS). Rietveld refinements using neutron diffraction data were conducted to determine the cation distributions in CTO. Hybrid materials were also characterized by Brunauer-Emmett-Teller adsorption isotherms, Scanning Electron microscopy, and scanning electrochemical microscopy. From an analytical point of view, we evaluated the electrochemical reduction of hydrogen peroxide on glassy carbon electrodes modified with hybrid materials. The analytical detection of hydrogen peroxide using CTO/RGO showed 11 and 5 times greater sensitivity in the detection of hydrogen peroxide compared with that of pristine CTO and RGO, respectively, and a two-fold increase compared with that of the RGO+CTO modified electrode. These results demonstrate that there is a synergistic effect between CTO and RGO that is more significant when the hybrid is synthetized through in situ methodology.

Enhanced lithium storage performances of novel layered nickel germanate anodes inspired by the spatial arrangement of lotus leaves

The rapid capacity degradation of Ge-based materials hinders their practical application for next generation lithium ion batteries, which could be solved by synthesizing Ge-containing ternary oxides, with new structures and hybridizing with carbon nanomaterials. Herein, novel Ni3Ge2O5(OH)4 nanosheets were synthesized and distributed in situ on reduced graphene oxide (RGO) sheets, with both flat-lying and vertically-grown spatial distributions to imitate the growth of lotus leaves. These two types of Ni3Ge2O5(OH)4 nanosheets enhance their efficient contact with RGO, and increase the mass loading of active materials. Furthermore, the interfacial bonds between RGO sheets and Ni3Ge2O5(OH)4 nanosheets are introduced to improve the diffusion rate of lithium ions. The RGO sheets act as a buffer matrix to sustain the volume change and prevent the nanosheets from aggregation. Consequently, the chemically bonded Ni3Ge2O5(OH)4/RGO hybrid delivers a high specific capacity of 863 mA h g-1 over 75 cycles, which is much higher than those for neat Ni3Ge2O5(OH)4 nanosheets or the hybrid without the interfacial bonding. This study provides a novel perspective for designing high-performance Ge-based anode materials for advanced lithium ion batteries.

Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries

The exponential increase in research focused on two-dimensional (2D) metal oxides has offered an unprecedented opportunity for their use in energy conversion and storage devices, especially for promising next-generation rechargeable batteries, such as lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs), as well as some post-lithium batteries, including lithium-sulfur batteries, lithium-air batteries, etc. The introduction of well-designed 2D metal oxide nanomaterials into next-generation rechargeable batteries has significantly enhanced the performance of these energy-storage devices by providing higher chemically active interfaces, shortened ion-diffusion lengths, and improved in-plane carrier-/charge-transport kinetics, which have greatly promoted the development of nanotechnology and the practical application of rechargeable batteries. Here, the recent progress in the application of 2D metal oxide nanomaterials in a series of rechargeable LIBs, NIBs, and other post lithium-ion batteries is reviewed relatively comprehensively. Current opportunities and future challenges for the application of 2D nanomaterials in energy-storage devices to achieve high energy density, high power density, stable cyclability, etc. are summarized and outlined. It is believed that the integration of 2D metal oxide nanomaterials in these clean energy devices offers great opportunities to address challenges driven by increasing global energy demands.

Self-Assembled LiNi1/3Co1/3Mn1/3O2 Nanosheet Cathode with High Electrochemical Performance

We have fabricated self-assembled LiNi_1/3Co_1/3Mn_1/3O₂ nanosheets via a facile synthesis method combining coprecipitation with the hydrothermal method. Scanning electron microscopic images show that the self-assembly processes for the LiNi_1/3Co_1/3Mn_1/3O₂ nanosheets depend on the reaction time and temperature. The nanosheet structure is uniform, and the width and thickness of the nanosheets are in the ranges of 0.7-1.5 μm and 10-100 nm, respectively. As a cathode material, the as-synthesized LiNi_1/3Co_1/3Mn_1/3O₂ nanosheets have demonstrated outstanding electrochemical performance. The initial specific capacity was 193 mAh g^-1, and the capacity was maintained at 189 mAh g^-1 after 100 cycles at 0.2 C, and 155 mAh g^-1 at 1 C (after 1000 cycles). The LiNi_1/3Co_1/3Mn_1/3O₂ nanosheets have efficient contact with the electrolyte and short Li⁺ diffusion paths, as well as sufficient void spaces to accommodate large volume variation. The nanosheets are thus beneficial to the diffusion of Li⁺ in the electrode. The enhanced electrical conductance and excellent capacity demonstrate the great potential of LiNi_1/3Co_1/3Mn_1/3O₂ nanosheets for energy storage applications.

Fast and Universal Approach to Encapsulating Transition Bimetal Oxide Nanoparticles in Amorphous Carbon Nanotubes under an Atmospheric Environment Based on the Marangoni Effect

Transition metal oxide nanoparticles capsuled in amorphous carbon nanotubes (ACNTs) are attractive anode materials of lithium-ion batteries (LIBs). Here, we first designed a fast and universal method with a hydromechanics conception which is called Marangoni flow to fabricate transition bimetal oxides (TBOs) in the ACNT composite with a better electrochemistry performance. Marangoni flows can produce a liquid column with several centimeters of height in a tube with one side immersed in the liquid. The key point to induce a Marangoni flow is to make a gradient of the surface tension between the surface and the inside of the solution. With our research, we control the gradient of the surface tension by controlling the viscosity of a solution. To show how our method could be generally used, we synthesize two anode materials such as (a) CoFe₂O₄@ACNTs, and (b) NiFe₂O₄@ACNTs. All of these have a similar morphology which is ∼20 μm length with a diameter of 80-100 nm for the ACNTs, and the particles (inside the ACNTs) are smaller than 5 nm. In particular, there are amorphous carbons between the nanoparticles. All of the composite materials showed an outstanding electrochemistry performance which includes a high capacity and cycling stability so that after 600 cycles the capacity changed by less than 3%.

Carbon-Free Porous Zn2GeO4 Nanofibers as Advanced Anode Materials for High-Performance Lithium Ion Batteries

In this work, carbon-free, porous, and micro/nanostructural Zn₂GeO₄ nanofibers (p-ZGONFs) have been prepared via a dissolution-recrystallization-assisted electrospinning technology. The successful electrospinning to fabricate the uniform p-ZGONFs mainly benefits from the preparation of completely dissolved solution, which avoids the sedimentation of common Ge-containing solid-state precursors. Electrochemical tests demonstrate that the as-prepared p-ZGONFs exhibit superior Li-storage properties in terms of high initial reversible capacity of 1075.6 mA h g^-1, outstanding cycling stability (no capacity decay after 130 cycles at 0.2 A g^-1), and excellent high-rate capabilities (e.g., still delivering a capacity of 384.7 mA h g^-1 at a very high current density of 10 A g^-1) when used as anode materials for lithium ion batteries (LIBs). All these Li-storage properties are much better than those of Zn₂GeO₄ nanorods prepared by a hydrothermal process. The much enhanced Li-storage properties should be attributed to its distinctive structural characteristics including the carbon-free composition, plentiful pores, and macro/nanostructures. Carbon-free composition promises its high theoretical Li-storage capacity, and plentiful pores cannot only accommodate the volumetric variations during the successive lithiation/delithiation but can also serve as the electrolyte reservoirs to facilitate Li interaction with electrode materials.

Influence of annealing temperature on microstructure and lithium storage performance of self-templated CuxCo3−xO4 hollow microspheres

Spinel Cu_xCo_3−xO₄ (x ≤ 0.30) hollow microspheres have been readily prepared via a self-templated solvothermal reaction followed by a thermal annealing step.

Synthesis and electrochemical performance of Co2TiO4and its core–shell structure of Co2TiO4@C as negative electrodes for Li-ion batteries

Spinel Co₂TiO₄is synthesised using a polymeric precursor method and used as an efficient negative electrode for Li-ion batteries.

Green synthesis of GeO2/graphene composites as anode material for lithium-ion batteries with high capacity

A facile green solution route using only GeO₂ powder, graphene oxide and purified water has been developed to prepare a GeO₂/graphene composite, in which the GeO₂ particles are wrapped in graphene nanosheets.

2D materials for renewable energy storage devices: Outlook and challenges

We review cost-effective, clean and durable alternative energy devices based on 2D materials.

Hybrid two-dimensional materials in rechargeable battery applications and their microscopic mechanisms

Advances in two-dimensional (2D) hybrid nanomaterials in electrochemical energy storage and their microscopic mechanisms are summarized and reviewed.

Design and synthesis of one-dimensional Co3O4/Co3V2O8 hybrid nanowires with improved Li-storage properties

A new nanostructure of one-dimensional Co₃O₄/Co₃V₂O₈ hybrid nanowires directly grown on Ti substrates with improved electrochemical Li-storage properties are successfully prepared by a simple hydrothermal strategy.

Facile synthesis of a nickel vanadate/Ni composite and its electrochemical performance as an anode for lithium ion batteries

Ni₃V₂O₈/Ni composites are synthesized by a simple hydrothermal route, and show high-rate capability and outstanding long-life cycling stability as a new anode material for Li-ion batteries.

An overview of AB2O4- and A2BO4-structured negative electrodes for advanced Li-ion batteries

Energy-storage devices are state-of-the-art devices with many potential technical and domestic applications.