Mixed Transition Metal Oxides Research Articles

Azadirachta indica leaf-extract-assisted synthesis of CoO–NiO mixed metal oxide for application in a microbial fuel cell as a cathode catalyst

In this study, a highly efficient yet low-cost mixed transition metal oxide of Ni and Co as CoO–NiO was synthesized using Azadirachta indica leaf extract as a bioactive chelating agent.

A facile route to prepare mixed transition metal oxide yolk-shell microspheres for enhanced lithium storage.

A coordination co-precipitation route followed by a calcination process was developed to prepare mixed transition metal oxide microspheres with a yolk-shell structure. ZnCo2O4, NiCo2O4, MnCo2O4, ZnMn2O4 and CoMn2O4 have been successfully fabricated, which demonstrates the universality of this route. As a promising anode for lithium-ion batteries (LIBs), ZnCo2O4 was investigated as a typical example, which showed a remarkable reversible capacity of 1063 mA h g-1 after 50 cycles at a current density of 200 mA g-1 and a good rate capability of 839 mA h g-1 after 200 cycles at a large current density of 900 mA g-1. This work shows the extensive potential of a general route for the synthesis of yolk-shell microspheres for next-generation energy conversion and storage devices.

Transition Metal Dissolution in State-of-the-Art and Next Generation Li-Ion Batteries Studied By Spatially Resolved Operando X-Ray Absorption Spectroscopy

High energy Li-Ion batteries rely on the pairing of a transition metal oxide cathode with a graphite anode. Pushing the upper cut-off voltage and state of charge (SOC) to higher values, the cycle-life of the battery is drastically reduced. Besides electrochemical electrolyte oxidation leading to poor coulombic efficiencies and a large growth of cell impedance, leaching of transition metal ions from the cathode active materials is one of the major obstacles to increase the power density of the cell without change of active materials. While the capacity loss which can be ascribed to the overall loss of active material is minor, cell aging is largely aggravated by liberated transition metal ions present in the electrolyte, as they diffuse through the separator and deposit on the graphite anode due to its low potential. A severe impedance growth of the anode and loss of active lithium by chemical delithiation of the graphite has been observed by many studies. Especially for manganese, the amount of active lithium loss has been found to be a multifold higher than the total amount of manganese ions present in the electrolyte and deposited onto the graphite anode, indicating a catalytic role of transition metal ions on active lithium loss (1). Comparisons of the transition metal leaching behavior for different cathode active materials is missing so far, as studies vary widely in test procedures (potential range, SOC) and environment (electrolyte and temperature). We herein present operando analysis of the intrinsic stability of several cathode active materials upon charge to high SOC and potential with our unique spectroscopic cell enabling spatially resolved operando X-ray absorption spectroscopy, monitoring the concentration and oxidation state of transition metals in the electrolyte and in the graphite anode (2, 3). Aside layered mixed transition metal oxide materials (Li1+w[NixCoyMnz]1-xO2) also the spinel structure is tested to better understand the effect of crystal destabilization at high SOC. The formerly standard layered oxide NCM111 (x=y=z=0.33) was already studied earlier, focusing on manganese dissolution (3). In this talk we present time and potential resolved analysis of all transition metals released from NCM111 (see Fig. 1) in comparison to a nickel-rich NCM811 (x=0.8, y=z=0.1), a lithium- and manganese-rich layered oxide material (HE-NCM; w=0.17, x=0.22, y=0.12, z=0.66 (4)) and the high-voltage spinel LiNi0.5Mn1.5O4, (LNMO). This data allows conclusions on the extent of the two major causes for transition metal leaching from oxide based cathode materials: chemical decomposition induced by protic electrochemical electrolyte oxidation products (5) and crystal lattice destabilization of layered transition metal oxides caused by high degrees of delithiation (high SOC). References J. A. Gilbert, I. A. Shkrob and D. P. Abraham, Journal of The Electrochemical Society, 164, A389 (2017). Y. Gorlin, A. Siebel, M. Piana, T. Huthwelker, H. Jha, G. Monsch, F. Kraus, H. A. Gasteiger and M. Tromp, Journal of the Electrochemical Society, 162, A1146 (2015).J. Wandt, A. Freiberg, R. Thomas, Y. Gorlin, A. Siebel, R. Jung, H. A. Gasteiger and M. Tromp, J. Mater. Chem. A, 4, 18300 (2016). B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, Journal of The Electrochemical Society, 164, A400 (2017). M. Metzger, B. Strehle, S. Solchenbach and H. A. Gasteiger, Journal of The Electrochemical Society, 163, A798 (2016). Acknowledgements Financial support for S. S. and B. S. by the BASF SE through its Research Network on Electrochemistry and Batteries is gratefully acknowledged. XAS data were gathered at the European Synchrotron Radiation Facility (ESRF) and SOLEIL Synchrotron (France).We are grateful to Dr. Debora Motta Meira at the ESRF, Grenoble (France) for providing assistance in using beamline BM 23 as well as Dr. Emiliano Fonda for assistance on the SAMBA beamline at SOLEIL Synchrotron. Fig. 1: Transition metal dissolution of an NCM111 (5.5 mAh/cm2)//C6 cell upon cycling in the conventional potential range (0-6.5 h: 60% SOC until ≈4.2 VLi) and upon high degrees of delithiation (7-11 h: ≈100% SOC until 5.1 VLi) measured in our operando XAS cell using glass fiber separators and standard LP57 electrolyte (1 M LiPF6 in EC:EMC [3:7]) at 25 °C. The concentration of transition metal ions within the graphite electrode (filled symbols) and in the electrolyte (hollow stars, shown as electrolyte share in the graphite pores [concentration measured in the separator multiplied by the porosity of the graphite anode]) is monitored operando. Figure 1

Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems.

Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3 ̅m LiNi0.5M0.5O2 positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiM+3O2 with metals active for +3/+4 redox couples is extended to ternary oxides with LiA0.5B0.5O2 with A = Ni2+ and B = Ti4+, Zr4+, and V+4 to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3̅ m cation-disordered rock-salt positive electrode materials is presented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg-1 corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V4+/V5+. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration.

Characterization of Lithium Ion Battery Materials with Valence Electron Energy-Loss Spectroscopy.

Cutting-edge research on materials for lithium ion batteries regularly focuses on nanoscale and atomic-scale phenomena. Electron energy-loss spectroscopy (EELS) is one of the most powerful ways of characterizing composition and aspects of the electronic structure of battery materials, particularly lithium and the transition metal mixed oxides found in the electrodes. However, the characteristic EELS signal from battery materials is challenging to analyze when there is strong overlap of spectral features, poor signal-to-background ratios, or thicker and uneven sample areas. A potential alternative or complementary approach comes from utilizing the valence EELS features (<20 eV loss) of battery materials. For example, the valence EELS features in LiCoO2 maintain higher jump ratios than the Li-K edge, most notably when spectra are collected with minimal acquisition times or from thick sample regions. EELS maps of these valence features give comparable results to the Li-K edge EELS maps of LiCoO2. With some spectral processing, the valence EELS maps more accurately highlight the morphology and distribution of LiCoO2 than the Li-K edge maps, especially in thicker sample regions. This approach is beneficial for cases where sample thickness or beam sensitivity limit EELS analysis, and could be used to minimize electron dosage and sample damage or contamination.

Porous NiCoMn ternary metal oxide/graphene nanocomposites for high performance hybrid energy storage devices

Mixed transition metal oxides have attracted great attention due to their improved properties over simple oxides in energy storage applications. Herein, we report facile hydrothermal synthesis of novel porous NiCoMn ternary metal oxide flakes with reduced graphene oxide (NCMO_rGO) and their application in high performance aqueous-based energy storage devices. The effect of reaction time on composition and morphology was studied by X-ray diffraction, Raman spectroscopy, thermal gravimetric analysis, transmission electron microscopy, and N2 ad/de-sorption measurements. Accordingly, a high specific surface area of 200 m2 g−1 was achieved for the optimized sample. The sample was electrochemically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. Porous NCMO_rGO hybrid exhibited a high capacity of 115 mAh·g−1 at 1 A g−1. Subsequent integration of the hybrid material as a positive electrode with rGO negative electrodes resulted in asymmetric aqueous devices having a specific energy as high as 27 Wh·kg−1. Moreover, this NCMO_rGO//rGO hybrid device showed an excellent cycling stability of 96% over 2000 cycles. This work not only reports for the first time the rational design and fabrication of porous NCMO_rGO nanoflakes as high performance electrodes for energy storage but also paves the path towards facile synthesis of mixed metal oxide nanocomposites for other applications.

Engineered Electrolyte-Electrocatalyst Nanocomposites for Enhanced CO2 Electroreduction

The development of a viable approach to capture, store, and convert carbon dioxide into fuels and chemicals may offer a means to mitigate climate change while also creating value added commodity chemicals. Critical bottlenecks in existing technologies that have impeded commercialization of CO2 electrosynthesis include catalysts with high overpotentials and limited selectivities. Furthermore, current approaches often employ PEM-based electrolyzers that suffer from low CO2 solubility in acidic aqueous solutions, limiting mass transport and reaction rates, or AEM-based systems which have much higher CO2 solubilites in alkaline electrolytes, but have the added problem of membrane poisoning by bicarbonate and carbonate. We have recently developed an electrochemical reactor that overcomes many of the shortcomings that have limited the above technologies. Our preliminary results suggest that onset potentials for CO2 reduction are more than 200 mV lower than with current densities in excess of 150 mA/cm2 at -1 V vs RHE. We have examined several cathode electrocatalysts and some of the most promising include a series of nanostructured mixed and/or doped transition metal and main group metal oxides that also suppress the hydrogen evolution reaction. We will present detailed electrochemical and structural studies and will comment on possible reaction mechanisms as well as the key features of the reactor that enable the enhanced kinetics.

Comparative Study Nickel Rich Layered Oxides: NMC 622, NMC 811 and NCA Cathode Materials for Lithium Ion Battery

LiCoO2 has conquered the cathode market for lithium ion batteries; recently the mixed transition metal oxide lithium nickel manganese cobalt dioxide have gained attention due to their superior electrochemical properties, better thermal stability, and lower cost. Nickel-rich compositions of lithium nickel manganese cobalt oxide (NMC) have been developed as promising high-energy, high-voltage and improved rate cathode materials capable of significantly increasing the energy density of lithium-ion batteries. Here we report a systematic comparative study on rate capability, charge discharge behavior and nickel/lithium mixing of the nickel-rich layered oxides, NMC622, NMC811 and NCA. The results showed that both NMC622 and NMC811 have pure hexagonal layered phase with Ni/Li mixing below 4%. Both samples have similar specific capacity of about 220 mAh/g at current densities, C/10; however, NMC811 demonstrated superior electrochemical performance compared to NMC611 at higher rates. The 622 sample showed a strong dependence on the electrode loading, higher electrode loading above 15 mg/cm2 had poor performance compared to lower loading at the same current density in mA/cm2 while NMC811 sample does not show such loading dependence. This work was supported by the DOE-EERE-Battery500 consortium.

Three-dimensional porous self-assembled chestnut-like nickel-cobalt oxide structure as an electrochemical sensor for sensitive detection of hydrazine in water samples

Three-dimensional NiCo2O4 is a kind of superior sensing material owing to its high electron transfer capability, large available surface area and numbers of active sites. In this work, NiCo2O4 of the three-dimensional chestnut-like structure were easily achieved through a one step hydrothermal process. Afterwards, the morphology and structure were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Based on the three-dimensional porous chestnut-like NiCo2O4, an electrochemical sensor for hydrazine (N2H4) detection is fabricated. This electrochemical platform can realize good selectivity, excellent stability, high sensitivity (∼2154.4 μA mM-1 cm-2), and low detection limit (0.3 μM), as well as a wide linear range from 1 μM to 1096 μM. The synergistic effect of nickel-cobalt in such mixed transition metal oxides which Co in Co3O4 is partially replaced by Ni are beneficial for enhancing sensing properties. This study proves that three-dimensional porous chestnut-like NiCo2O4 is electrochemically active for catalytic performance which is particular and promising material for good application in the practical detection of N2H4.

Synthesis and application of NiMnO3-rGO nanocomposites as electrode materials for hybrid energy storage devices

Demand for more efficient and ecofriendly energy storage systems arouse research efforts in seeking to develop new energy materials with promising properties. In this regard, mixed transition metal oxides have recently attracted great attention due to their improved electrochemical and electrical properties in comparison with simple oxides. Herein, NiMnO3 and their composites with reduced graphene oxide (NiMnO3-rGO) were synthesized via a facile hydrothermal route, followed by a thermal treatment and their electrochemical properties have been evaluated as electrode materials for hybrid energy storage devices. The prepared samples were characterized by using X-ray diffraction (XRD), Raman spectroscopy, Thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and N2 adsorption measurements. The energy storage behavior of the samples was investigated using different electrochemical techniques including cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. Accordingly, a NiMnO3-rGO nanocomposite showed a high capacity of 91 mAh g−1 at a scan rate of 5 mV s−1, 48% higher than that of the pure NiMnO3 sample (47.7 mAh g−1). Furthermore, this nanocomposite was integrated as a positive electrode with reduced graphene oxide nanosheets as the negative electrode in an aqueous hybrid energy storage device. This system displayed a high specific energy of 23.5 Wh kg−1 and a maximum specific power of 7.64 kW kg−1.

Synthesis of highly crystalline octahedron 3D-Zn2SnO4 as an advanced high-performance anode material for lithium ion batteries

A promising class of mixed transition-metal oxides (MTMOs) with stoichiometric or non-stoichiometric ratios has recently received notable attention worldwide. Benefiting from their excellent electrochemical properties, these MTMOs will play a substantial role for low-cost and ecologically responsive energy storage/conversion technologies. In this work, we have developed highly crystalline octahedron 3D-Zn2SnO4 by hydrothermal synthesis and evaluated their application in Li-ion batteries (LIBs). Several characterization techniques have been used to analyze structural, compositional, and morphological features of the material. The highly crystalline octahedron 3D-Zn2SnO4 micro-particle attained a specific discharge capacity of 739 mAh g−1 with a coulombic efficiency of 99% after 100 cycles at a current density of 100 mA g−1. These attractive results suggest that 3D-Zn2SnO4 shows good performance as a negative electrode material for high-performance LIBs.

Graphene oxide-decorated Fe2(MoO4)3 microflowers as a promising anode for lithium and sodium storage

Mixed transition metal oxides (MTMOs) have received intensive attention as promising anode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). In this work, we demonstrate a facile one-step water-bath method for the preparation of graphene oxide (GO) decorated Fe2(MoO4)3 (FMO) microflower composite (FMO/GO), in which the FMO is constructed by numerous nanosheets. The resulting FMO/GO exhibits excellent electrochemical performances in both LIBs and SIBs. As the anode material for LIBs, the FMO/GO delivers a high capacity of 1,220 mAh·g–1 at 200 mA·g–1 after 50 cycles and a capacity of 685 mAh·g–1 at a high current density of 10 A·g–1. As the anode material for SIBs, the FMO/GO shows an initial discharge capacity of 571 mAh·g–1 at 100 mA·g–1, maintaining a discharge capacity of 307 mAh·g–1 after 100 cycles. The promising performance is attributed to the good electrical transport from the intimate contact between FMO and graphene oxide. This work indicates that the FMO/GO composite is a promising anode for high-performance lithium and sodium storage.

Template-assisted synthesis of NiCoO2 nanocages/reduced graphene oxide composites as high-performance electrodes for supercapacitors.

Here we reported a coordinating etching and precipitating method to synthesize a complex binary metal oxides hollow cubic structure. A novel NiCoO2/rGO composite with a structure of NiCoO2 nanocages anchored on layers of reduced graphene oxide (rGO) were synthesized via a simple template-assisted method and the electrochemical performance was investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy tests as a faradaic electrode for supercapacitors at a graphene weight ratio of 1 wt% (1%). When used as electrode materials for electrochemical capacitors, the NiCoO2/rGO composites achieved a specific capacity of 1375 F g−1 at the current density of 1 A g−1 and maintained 742 F g−1 at 10 A g−1. After 3000 cycles, the supercapacitor based on these nanocage structures shows long-term cycling performance with a high capacity of 778 F g−1 at a current density of 1 A g−1. These outstanding electrochemical performances are primarily attributed to the special morphological structure and the combination of mixed transition metal oxides and rGO, which not only maintains a high electrical conductivity for the overall electrode but also prevents the aggregation and volume expansion of electrochemical materials during the cycling processes.

Electrochemical performances of iron-cobalt oxides nanoparticles loaded crumpled graphene for supercapacitor

Mixed transition metal oxides (MTMOs) have been explored as attractive electrode materials for supercapacitors owing to their higher electronic conductivity and larger specific capacitance compared with simple transition metal oxides. Among MTMOs, cobalt-based binary metal oxides such as Fe-Co oxides have emerged as alternative electrode materials for supercapacitors because they have the advantages of low cost, natural abundance, and environmental friendliness. Recently, graphene-based Fe-Co oxides composites have become of particular interest for their improved electrochemical performance of supercapacitors due to the synergetic effect of both materials. In this study, we present three-dimensional (3D) crumpled graphene (CGR) decorated with Fe-Co oxides nanoparticles to determine which molar ratio of Fe/Co can exhibit higher electrochemical performance. The Fe-Co oxides/CGR composites were synthesized in different molar ratios of Fe/Co via aerosol spray pyrolysis and post heat treatment. Sizes of Fe-Co oxides nanoparticles ranged from 5 to 10 nm when loaded onto 500 nm CGR. The electrochemical properties of the Fe-Co oxides/CGR composites with different molar ratios of Fe/Co were examined. The Fe-Co oxides/CGR electrodes fabricated at the Fe/Co ratio of 0.1 showed the highest performances in the capacitance and electrical conductivity among those electrodes with different molar ratios of Fe/Co.

Two-step method to synthesize spinel Co3O4-MnCo2O4 with excellent performance for lithium ion batteries

Spinel Co3O4-MnCo2O4 was synthesized by a two-step method, with a post annealing step after loading MnO2 on the surface of Co-MOF (ZIF-67). For the first step, the methyl (-CH3) groups of ZIF-67 were oxidized to carboxyl (-COOH) groups, with MnO2 uniformly loaded on ZIF-67. Benefiting from this step, the uniform distribution of spinel Co3O4-MnCo2O4 was achieved after calcination. Based on the stable framework of Co-MOF, a nanoparticle spinel Co3O4-MnCo2O4 was obtained with an average diameter of 20–50 nm. This nano-sized characteristic not only shortens the diffusion lengths of lithium ion, but also alleviates the volume expansion of material during the Li+ insertion/extraction process. The as-synthesized spinel Co3O4-MnCo2O4 exhibited superior rate stability and cycle stability with high energy density and power density. Even at 5000 mA g−1, a specific capacity of 614 mA h g−1 can be maintained. In view of the remarkable performance of spinel Co3O4-MnCo2O4, it can be concluded that spinel Co3O4-MnCo2O4 has a potential to be an anode material in LIBs. Further, this two-step method can be used to prepare other mixed transition-metal oxides in various fields.

Porous TiO2 nanobelts coated with mixed transition-metal oxides Sn3O4 nanosheets core-shell composites as high-performance anode materials of lithium ion batteries

A novel type of porous TiO2 nanobelts anchored with Sn3O4 nanosheets (TiO2 nanobelts @Sn3O4 nanosheets) core-shell composites were developed via two-step hydrothermal methods. The structure, morphology, formation mechanism and electrochemical performance of the porous TiO2 nanobelts coated with mixed transition-metal oxides Sn3O4 nanosheets core-shell composites were researched. The results illustrated that the Sn3O4 nanosheets were uniformly anchored on the surface of the porous TiO2 nanobelts, which were almost perpendicular to the porous TiO2 nanobelts. The composites as anode materials exhibited more remarkable electrochemical performances than the Sn3O4 nanosheet and the porous TiO2 nanobelts, in terms of the cycling performance and rate capability. The initial discharge capacity was 1513.7 mAh/g and the reversible capacity was 659 mAh/g after 50 cycles. This study perhaps inspired the designing of other transition metal oxide-based anode materials with desirable electrochemical performance for applications of lithium ion batteries.

Frequency and temperature dependent conductivity spectra of mixed transition metal oxide doped semiconducting glassy system

Frequency and temperature dependent conductivity of mixed transition metal oxide (TMO) doped semiconducting glassy system, xV2O5–(1−x) (0.05MoO3–0.95ZnO) have been investigated in the wide range of frequency and temperature. The dc conductivity (σdc), crossover frequency (ωH), frequency exponent (n) and power law exponent (s) have been computed from the best fitted plots of experimental data. We have estimated the values of activation energy of ac conduction (Eac) and free energy of polaron migration (EH). Dc conductivity of the as-prepared samples shows thermally activated non–linear nature, which has been interpreted with Vogel–Tammann–Fulcher (VTF) model. From the fitting, pseudo activation energy (Ea) and thermodynamic ideal glass transition temperature (T0) have been estimated. Nature of variation of conductivity and the power law exponent (s) data exhibit that non–overlapping small polaron tunnelling (NSPT) model is suitable to interpret these data for compositions x=0.3, 0.5 and 0.7. On the other hand, correlated barrier hopping (CBH) model is most applicable mechanism for ac conduction for compositions x=0.9 and 0.93. Formation of some complex Mo-O-V structures may be possible reason for applicability of CBH model for composition x=0.9, 0.93. It is also observed that conductivity relaxation process of charge carriers (polarons) is independent of temperature, but depends upon composition.

Hierarchically three-level Ni3(VO4)2@NiCo2O4 nanostructure based on nickel foam towards highly efficient alkaline hydrogen evolution

Designing mixed transition metal oxides with advantageous nanostructure is efficient access to highly-efficient electrocatalysts for hydrogen evolution reaction (HER) in water electrolysis due to more exposed active sites and synergistic effects in mixed metals. A hierarchically three-level nanostructure combining two types of mixed transition metal oxides on nickel foam (Ni3(VO4)2@NiCo2O4/NF) has been synthesized through a two-step hydrothermal process. The hierarchically three level nanostructure comprises porous nickel foam as robust and conductive substrate in the bottom, vertical and uniform NiCo2O4 nanowires arrays in the intermediate, and top-coated Ni3(VO4)2 nanoparticles in 10nm. The triple hierarchical nanostructure of Ni3(VO4)2@NiCo2O4/NF is favorable for enlarging surface areas, exposing more active sites, promoting mass and charge transfer and accelerating HER process. Ni3(VO4)2@NiCo2O4/NF electrode can afford a current density of 10mAcm−2 at a moderate overpotential of 113mV with robust stability for at least 12h. It may provide a new strategy to design triple hierarchical nanostructures based on earth-abundant transition metals to achieve large-scale production of hydrogen through water electrolysis in alkaline.

Rapid One-Pot Solvothermal Batch Synthesis of Porous Nanocrystal Assemblies Composed of Multiple Transition-Metal Elements.

The ability of a rapid-heating solvothermal process to synthesize porous nanocrystal assemblies composed of the multiple transition metals was demonstrated. The rapid heating facilitated the quick formation of nascent nanocrystals to generate homogeneous mixed transition-metal oxides. Systematic studies of the synthesis of mixed-metal oxides under various experimental conditions indicated that the present simple method is suitable to develop a wide variety of binary and ternary transition-metal systems such as Co/Mn, Ni/Mn, and Co/Mn/Fe mixed-metal oxides. The products obtained from the rapid heating process were hierarchically assembled porous nanospheres composed of sub-10 nm nanocrystals, which had an extraordinarily high surface area and nano/mesopores. Electrochemical tests revealed the high catalytic ability of the porous nanocrystal assemblies in water oxidation.

Enthalpies of formation of layered LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) compounds as lithium ion battery cathode materials

AbstractLayer-structured mixed transition metal oxides with the formula LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) are considered as important cathode materials for lithium-ion batteries. In an effort to evaluate the relative thermodynamic stabilities of individual compositions in this series, the enthalpies of formation of selected stoichiometries are determined by high temperature oxide melt drop solution calorimetry and verified by ab-initio calculations. The measured and calculated data are in good agreement with each other, and the results show that LiCoO2–LiNi0.5Mn0.5O2 solid solution approaches ideal behavior. By increasing x, i. e. by equimolar substitution of Mn4+ and Ni2+ for Co3+, the enthalpy of formation of LiNixMnxCo1–2xO2 from the elements becomes more exothermic, implying increased energetic stability. This conclusion is in agreement with the literature results showing improved structural stability and cycling performance of Ni/Mn-rich LiNixMnxCo1–2xO2 compounds cycled to higher cut-off voltages.