Growth of FePO<Subscript>4</Subscript> nanoparticles on graphene oxide sheets for synthesis of LiFePO<Subscript>4</Subscript>/graphene

FePO₄·xH₂O/graphene oxide (FePO₄·xH₂O/GO) composites were prepared by a facile chemical precipitation method. Using the as-prepared FePO₄·xH₂O/GO and LiOH·H₂O as precursors and followed by carbothermal reduction, LiFePO₄/graphene composites were obtained. Scanning electron microscope (SEM) images indicated that the graphene had very good dispersity and uniformly attached to the LiFePO₄ particles. The conductive framework of graphene improved the electrochemical properties of the composites. The composites deliver high initial discharge capacity of 163.4 mAh g^?1 as well as outstanding rate performance.     

Ultrasonic-assisted solution combustion synthesis of porous Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C: formation mechanism and sodium storage performance

Solution combustion synthesis (SCS) is an effective and rapid method for synthesizing nanocrystalline materials. However, the control over size, morphology, and microstructure are rather limited in SCS. Here, we develop a novel ultrasonic-assisted solution combustion route to synthesize the porous and nano-sized Na₃V₂(PO₄)₃/C composites, and reveal the effects of ultrasound on the structural evolution of NVP/C. Due to the cavitation effects generated from ultrasonic irradiation, the ultrasonic-assisted SCS can produce honeycomb precursor, which can be further transformed into porous Na₃V₂(PO₄)₃/C with reticular and hollow structures after thermal treatment. When used as cathode material for Na-ion batteries, the porous Na₃V₂(PO₄)₃/C delivers an initial discharge capacity of 118 mAh g^?1 at 0.1 C and an initial coulombic efficiency of 85%. It can retain 93.8% of the initial capacity after 120 cycles at 0.2 C. The results demonstrate that ultrasonic-assisted SCS can be a new strategy to design crystalline nanomaterials with tunable microstructures.

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Graphical abstract Porous and nano-sized Na₃V₂(PO₄)₃/C composites with reticular and hollow structures are synthesized by an ultrasonic-assisted solution combustion route due to the cavitation effects, and exhibit excellent electrochemical performance as cathode in sodium ion battery.

     

Electrical properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-based composite systems

Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at certain second-phase concentration. An attempt has been made to investigate a number of sodium sulfate (Na₂SO₄)-based composite systems. The dispersoids that have been used are MgO, Al₂O₃, and SiO₂. The samples have been characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum conductivity has been observed for MgO dispersed system, and the percolation threshold has been observed at 30-mol% dispersoid, MgO concentration. Interestingly, two maxima have been observed in case of the Na₂SO₄–SiO₂ and Na₂SO₄–Al₂O₃ composite systems. In the Na₂SO₄–SiO₂ system, the first maximum occurs at lower concentration, i.e., in the range between 10 and 20 mol%, whereas the second occurs at the 40-mol% dispersoid concentration. For the Na₂SO₄–Al₂O₃ system, although slightly indistinguishable, two peaks in the conductivity vs composition plot have been observed around 12- and 30-mol% Al₂O₃ concentrations.     

Polyol technique synthesis of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> coated on LiFePO<Subscript>4</Subscript> cathode materials for Li-ion storage

A novel approach has been made to tailor Niobium pentoxide (Nb₂O₅) as a coating material on the surface of lithium iron phosphate (LiFePO₄) via a facile polyol technique. The coating content was optimized at 1 wt%. The superficial coating demonstrated superior discharge capacity than the pristine LiFePO₄. However, increasing the coating content further would result in a capacity loss. This may be due to the electrochemical inactiveness that increases with the content of the coating material, and 1 wt% of Nb₂O₅-coated LiFePO₄ sample exhibits initial discharge capacity of 163 mAh g^?1 at a current of 0.1 C and retains a stable discharge capacity of 143 mAh g^?1 up to 400 cycles at 1 C rate with a coulombic efficiency of 98%.

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Graphical abstract ?

     

Polyaspartic-acid-pyrolysis route for the synthesis of nanocrystalline LiCo<Subscript>0.15</Subscript>Mn<Subscript>1.85</Subscript>O<Subscript>4</Subscript> powder for Li-ion batteries

Nanocrystalline cubic spinel LiCo_0.15Mn_1.85O₄ powder was prepared by a novel method based on in situ polymerization of aspartic acid along with metal salts. Thermal study shows that the complete crystallization and/or formation of the compound is at 358 °C. The structural property of the synthesized material was characterized by X-ray diffraction studies. The X-ray diffractogram reveals the single-phase formation of the product. Scanning electron microscope study shows that the average grain size of the powder is less than 1 μm. To assess the electrochemical performance of the synthesized cathode material, the C/LiCo_0.15Mn_1.85O₄ cell with 1 M LiPF₆ in 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate as the electrolyte was assembled, and the charge and discharge studies were made in between 3.0 and 4.8 V at a constant current density of 0.1 mAcm⁻². It shows that capacity loss is only 2% even after the 50th cycle. As this preparation method is simple and particularly suitable for preparation of highly homogeneous mixed metal oxides for Li-ion batteries.     

Thermal conductivity of Na<Subscript>2</Subscript>W<Subscript>2</Subscript>O<Subscript>7</Subscript> crystal

The thermal conductivity of Na₂W₂O₇ single crystal has been studied along the main crystallographic directions at temperatures of 50–573 K. A low thermal conductivity is found to correlate with a significant difference in the cation weight.     

Antisite defects and valence state of vanadium in Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Electron paramagnetic resonance (EPR) studies have been performed with the aim of determining the valence state and local crystal structure of the nearest environment of vanadium ions in the initial, charged, and discharged samples of the cathode material Na_xV₂(PO₄)₃ (1 ≤ x ≤ 3). It has been found that the charged sample (x = 1) is characterized by an intense signal corresponding to V⁴⁺ ions located in a highly distorted octahedral crystal field. An EPR signal with the g-factor close to the g-factor of the V⁴⁺ ion has also been observed in the initial sample (x = 3), where the intensity of the resonance signal is one order of magnitude lower than that in the charged sample. It has been revealed that the resonance signal under consideration is associated with the formation of antisite defects when a part of vanadium ions are located in sites of sodium ions. It has also been found that the intensity of this signal increases after a complete charge–discharge cycle (x = 3).     

Dipole ordering and ionic conductivity in NASICON-Type Na<Subscript>3</Subscript>Cr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structures

The aspects of structure, dipole ordering, and ionic conductivity of the Na₃Cr₂(PO₄)₃ crystal with the four polymorphic phases (α, α', β, and γ) have been investigated. The features of the α-Na₃Cr₂(PO₄)₃ crystal structure and its dipole ordering and relaxation polarization in the low-temperature α and α' phases have been refined. The occurrence of Na₃Cr₂(PO₄)₃ dipole ordering in the α and α' phases and high ionic conductivity in the β and γ phases is attributed to the structural changes in the rhombohedral [Me₂(PO₄)₃]^–3_3∞ crystal frame upon phase transformations α → α', α' → β, and β → γ. A model for explaining the dipole ordering and ionic conductivity phenomena in Na₃Cr₂(PO₄)₃ is proposed.     

Computer calculations for thermal behavior of Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>CO<Subscript>3</Subscript> melt

The thermal behavior of Na₂CO₃+Li₂CO₃ melt is studied by the method of thermodynamic simulation. The equilibrium compositions of the gas and salt phases are calculated at different temperatures in the initial argon atmosphere. Basic trends of the variation in the compositions of the melts and the gas phase above the melts in the presence of carbon are determined. The obtained results characterizing the stability of carbonate components in the melt are analyzed.     

Electrosprayed NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles for long cycle life and high-power Li-ion battery anode

Electrospraying-based synthesis of NiCo₂O₄ (NCO-ES) nanoparticles that exhibit long cycle life and high rate capability is reported. The results are compared with a conventionally prepared NiCo₂O₄ sample by direct annealing (NCO-DA). The structure and morphology of NCO-ES and NCO-DA nanoparticles have been characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy to confirm the size, morphology, structure, and surface chemistry of the as-prepared samples. Electrochemical testing established that the NCO-ES sample displayed enhanced Li-ion storage performance. The NCO-ES delivered a discharge capacity of almost 370 mAh/g at the end of 50 cycles at 1C rate (890 mA/g) while only 180 mAh/g was retained for the NCO-DA sample at the same condition. At a high rate of 5C (4450 mA/g), NCO-ES electrodes delivered a stabilized specific capacity of 225 mAh/g with almost 100% Coulombic efficiency over 1000 cycles. Its rate capability and cycle life were found to be superior to NCO-DA electrodes. The nanoscale grain boundaries in the NCO-ES sample enhanced the lithium-ion diffusion and enabled high rate capability. The impedance analysis at different stages of lithiation/delithiation indicates a lower impedance and better kinetics as one of the reasons for better performance of the NCO-ES sample.     

Study on electrochemical performance of Mg-doped Li<Subscript>2</Subscript>FeSiO<Subscript>4</Subscript> cathode material for Li-ion batteries

In this paper, Li₂Fe_1?yMg_ySiO₄/C (y?=?0, 0.01, 0.02, 0.03, 0.05), a cathode material for lithium-ion battery was synthesized by solid-state method and modified by doping Mg²⁺ on the iron site. The effects of Mg²⁺ doping on the crystal structure and electrochemical performance Li₂FeSiO₄ was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical tests. Electrochemical methods of measurement were applied including constant current charge–discharge test, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), to determine the electrochemical performance of the material and the optimal doping ion and ratio. The results showed that Li₂Fe_0.98Mg_0.02SiO₄/C has the higher specific capacity and better cycle stability as well as lower impedance and better reversibility. The enhanced electrochemical performance can be attributed to the increased electronic conductivity, the decreased charge transfer impedance, and the improved Li-ion diffusion coefficient. Then, further study on the synthesis conditions was performed to find the optimal combustion temperature and time. According to the study, the material which has the best electrochemical performance, shows initial discharge specific capacity of 142.3 mAh g^?1 at 0.1 C (1 C?=?166 mA g^?1) and coulomb efficiency of 95.6%, under the condition that the temperature is 700 °C and the calcining time is 10 h.     

Improved structural stability and electrochemical performance of Na<Subscript>3</Subscript>V<Subscript>2</Subscript> (PO<Subscript>4</Subscript>)<Subscript>3</Subscript> cathode material by Cr doping

Cr-doped sodium vanadium phosphate (NVP) in the form of Na₃V_2-xCr_x(PO₄)₃ (x = 0, 0.02, 0.04, 0.08, 0.10) is synthesized via a facile sol-gel route as cathode materials for sodium ion batteries. The structure and morphology of these materials are systematically characterized by x-ray diffraction (XRD), Fourier-infrared spectra (FT-IR), and scanning electron microscope (SEM). XRD analysis reveals that with the increasing amount of Cr, the crystallographic parameters show a descending trend. Electrochemical tests show that the cycle stability and the specific capacity of the sodium ion batteries can be significantly improved by doping Cr into NVP. Among all the Cr-doped cathode materials, Na₃V_1.92Cr_0.08(PO₄)₃ achieves the highest capacity of 112.2 mAh g^?1 and the capacity retention is 97.2 % after 50 cycles. Electrochemical impedance spectroscopy measurements demonstrate that Cr doping is an effective method to reduce the contact resistance of interparticles by suppressing irreversible phase transformation at low sodium contents.     

Multi-hierarchical nanosheet-assembled chrysanthemum-structured Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as electrode materials for high-performance sodium-ion batteries

The structure and morphology of sodium vanadium phosphate (Na₃V₂(PO₄)₃) play a vital role in enhancing the electrochemical performance of sodium-ion batteries due to the inherent poor electronic conductivity of the phosphate framework. In order to improve this drawback, a new chrysanthemum-structured Na₃V₂(PO₄)₃/C material has been successfully assembled with multi-hierarchical nanosheets via a hydrothermal method. Continuous scattering nanosheets in chrysanthemum petals are beneficial in reducing energy consumption during the process of sodium ion diffusion, on which the carbon-coated surface can significantly increase overall conductivity. The as-prepared sample exhibits outstanding electrochemical performance due to its unique structure. It rendered a high initial specific capacity of 117.4?mAh?g^?1 at a current density of 0.05 C. Further increasing the current density to 10 C, the initial specific capacity still achieves 101.3?mAh?g^?1 and remains at 87.5?mAh?g^?1 after 1000 cycles. In addition, a symmetrical sodium-ion full battery using the chrysanthemum-structured Na₃V₂(PO₄)₃/C materials as both the cathode and anode has been successfully fabricated, delivering the capacity of 62?mAh?g^?1 at 1?C and achieving the coulombic efficiency at an average of 96.4% within 100 cycles. These results indicate that the new chrysanthemum-structured Na₃V₂(PO₄)₃/C can provide a new idea for the development of high-performance sodium-ion batteries.     

AC conductivity evolution in bulk and grain boundary response of sodium tungstate Na<Subscript>2</Subscript>WO<Subscript>4</Subscript>

The Na₂WO₄ compound has been obtained by the conventional solid-state reaction and characterized by X -ay powder diffraction. The title material crystallizes in the cubic system with Fd-3m space group. The electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 586–679 K. Two semicircles are observed in impedance plot indicating the presence of two relaxation processes in the compound associated with the grain and grain boundary. The relaxation behavior of the grain and grain boundary of the Na₂WO₄ are also obtained from the analyzed electrical modulus data. AC conductivity measured follows the power-law dependence σ_AC～ω^s typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process. The mechanism of conduction is probably due from the displacements of the Na+ ion in the tunnel-type cavities along [111] direction.     

AlF<Subscript>3</Subscript> coating of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> for high-performance Li-ion batteries

AlF₃-coating is attempted to improve the performance of LiNi_0.5Mn_1.5O₄ cathode materials for Li-ion batteries. The prepared powders are characterized by scanning electron microscope, powder X-ray diffraction, charge/discharge, and impedance. The coated LiNi_0.5Mn_1.5O₄ samples show higher discharge capacity, better rate capability, and higher capacity retention than the uncoated samples. Among the coated samples, 1.0 mol% AlF₃-coated sample shows highest capacity after charge–discharged at 30 mA/g for 3 cycles, but 4.0 mol% coated sample exhibits the highest capacity and cycling stability when cycled at high rate of 150 and 300 mA/g. The 40th cycle discharge capacity at 300 mA/g current still remains 114.8 mAh/g for 4.0 mol% AlF₃-coated LiNi_0.5Mn_1.5O₄, while only 84.3 mAh/g for the uncoated sample.     

Insertion properties of LiFe<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>PO<Subscript>4</Subscript> electrode materials for Li-ion batteries

This paper addresses the synthesis structural and electrochemical properties of LiFe_0.5Mn_0.5PO₄ electrode materials for Li-ion batteries. The charge–discharge reaction of Li/LiPF₆-EC–DEC/LiFe_0.5Mn_0.5PO₄ cell carried out at the 1-C rate shows a capacity retention of 128 mAh/g. The local structure of the delithiated Li _x Fe_0.5Mn_0.5PO₄ phases have been studied by Fourier transform infrared spectroscopy and magnetometry. Spectral features indicate that the structure of the delithiated phase remains in the orthorhombic system. The compositional dependence of the magnetic moment is found to be in quantitative agreement with the theoretical value predicted for oxidation of M ²⁺ ions in the high spin state. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007     

Impedance spectroscopy study of Na<Subscript>1/2</Subscript>Sm<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> ceramic

Complex impedance analysis of a valence-compensated perovskite ceramic oxide Na_1/2Sm_1/2TiO₃, prepared by a mixed oxide (solid-state reaction) method, has been carried out. The formation of single-phase material was confirmed by X-ray diffraction studies, and it was found to be an orthorhombic phase at room temperature. In a scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with that of impedance analysis. Alternating current impedance measurements were made over a wide temperature range (31–400 °C) in an air atmosphere. Complex impedance and modulus plots helped to separate out the contributions of grain and grain boundaries to the overall polarization or electrical behavior. The physical structure of the samples was visualized most prominently at higher temperatures (275 °C) from the Nyquist plots showing inter- and intragranular impedance present in the material. The frequency dependence of electrical data is also analyzed in the framework of the conductivity and modulus formalisms. The bulk resistance, evaluated from the impedance spectrum, was observed to decrease with rise in temperature, showing a typical negative temperature coefficient of resistance-type behavior like that of semiconductors. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by the impedance data. PACS 77.22.Ch; 77.22.Ej; 77.22.Gm; 77.22.Jp; 77.84.Bw     

Electrochemical behavior of carbonic precursor with Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>nanostructured material in hybrid battery system

The nanostructured Na₃V₂(PO₄)₃ (NVP) cathode material has been synthesized using the sol-gel route for different molar fractions of citric acid as a carbon source during the synthesis. The nanostructured NVP as cores with carbonic shell structures are fabricated with two different citric acid molar ratios. The thermal treatment has been optimized to convert the amorphous carbon shell into graphitic carbon to realize the better electrical conductivity and thus effective electron transfer during the electrochemical charge transfer process. The X-ray diffraction measurements confirmed the rhombohedral crystallographic phase (space group R-3c) with average crystallite size ~28 ± 5 nm. The coin cells are assembled in a hybrid rechargeable electrochemical cell configuration with lithium as a counter electrode and LiPF₆-EC:DEC:DMC (1:1:1 ratio) as the electrolyte. The electrochemical charge/discharge measurements are carried out at C/10 and C/20 rates and the measured specific capacities are 80 and 120 mAhg^?1 for samples with lower and higher citric acid molar ratios, respectively. The studies suggest that NVP can be used as an effective cathode material in hybrid electrochemical cells, and a higher concentration of citric acid may result in the effective carbonic shell for optimal electron transfer and thus enhanced electrochemical performance.     

First-principle calculations for electronic structure and bonding properties in layered Na<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>7</Subscript>

First-principles calculations of Na₂Ti₃O₇ have been carried out with density-functional theory (DFT) and ultrasoft pseudopotentials. The electronic structure and bonding properties in layered Na₂Ti₃O₇ have been studied through calculating band structure, density of states, electron density, electron density difference and Mulliken bond populations. The calculated results reveal that Na₂Ti₃O₇ is a semiconductor with an indirect gap and exhibits both ionic and covalent characters. The stability of the (Ti₃O₇)²⁻ layers is attributed to the covalent bonding of strong interactions between O 2p and Ti 3d orbitals. Furthermore, the O atoms located in the innerlayers interact more strongly with the neighboring Ti atoms than those in the interlayer regions. The ion-exchange property is due to the ionic bonding between the Na⁺ and (Ti₃O₇)²⁻ layers, which can stabilize the interlayers of layered Na₂Ti₃O₇ structure.