NaxCoO2 Research Articles

Thermoelectricity of p-NCO and n-ZAO Thin Films

Thin films of sodium cobalt oxide and zinc aluminium oxide were prepared onto ceramic substrates by a bipolar pulsed-dc magnetron sputtering system using a NaCoO2target and a ZnAlO target, respectively, under an argon atmosphere. Energy dispersive spectroscopy analysis revealed that the as-deposited films from the NaCoO2target comprised Na, Co, O elements, while those from the ZnAlO target contained Zn, Al, O elements. Cross-sectional view estimation by the scanning electron microscope indicated that the as-deposited Na-Co-O (NCO) and Zn-Al-O (ZAO) films had the thickness of 0.63 μm and 0.58 μm, respectively. X-ray diffraction analysis showed that the NCO thin films were grown in amorphous phase while the ZAO thin films exhibited hexagonal structure. From thermoelectric properties measurement, the p-NCO and n-ZAO films were found to exhibit the thermoelcectric power factor of 0.03 and 14.39 μW/mK2, respectively. A thermoelectric module made from three pairs of the p-n thin film stripes provided the open-circuit voltage up to 26.0 mV for a temperature difference of 79.3 K. However, the module was unable to produce useful electrical current due to its high internal series resistance contributed from the NCO films.

Getting back to NaxCoO2: Spectral and thermoelectric properties

Sodium cobaltate NaxCoO2 as dopable strongly correlated layered material with a triangular sublattice still poses a challenging problem in condensed matter. The intriguing interplay between lattice, charge, spin, and orbital degrees of freedom leads to a complex phase diagram bounded by a nominal Mott (x = 0) regime and a band‐insulating (x = 1) phase. By means of the charge self‐consistent density functional theory (DFT) plus dynamical mean‐field theory (DMFT) scheme, built on a pseudopotential framework combined with a continuous‐time quantum Monte‐Carlo solver, we here study the one‐particle spectral function A(k,ω) as well as the thermopower S(T). The computations may account for the suppression of the pockets in A(k,ω) at lower doping in line with photoemission experiments. Enhancement of the thermopower is verified within the present elaborate multi‐orbital method to treat correlated materials. In addition, the two‐particle dynamic spin susceptibility is investigated based on a simplified tight‐binding approach, yet by including vertex contributions in the DMFT linear response. Besides the identification of paramagnon branches at higher doping, a prominent high‐energy antiferromagnetic mode close to x = 0.67 is therewith identified in , which can be linked to extended hopping terms on the CoO2 sublattice.

Low-temperature hydrothermal synthesis of the three-layered sodium cobaltite P3-NaxCoO2 (x ∼ 0.60)

In order to obtain the layered sodium cobalt oxide materials by hydrothermal synthesis, the yield diagram for Na–Co–H2O system has been built and studied. In the same time, the well-known data of Co–H2O system have been extended at 250°C in basic solution.We had first synthesized directly the high temperature stable phase, P3-Na0.6CoO2 by a one-step low-temperature hydrothermal method. The rhombohedral structure of P3-Na0.6CoO2 has been determined by X-ray diffraction (XRD) and the purity of phases has been confirmed by XPS. The thermal stability of P3-Na0.6CoO2 has been investigated by means of high temperature X-ray diffraction in 298–873K range and when the temperature has reached 723K, the completely transformation of P3-Na0.6CoO2 in the rhombohedral stable phase α-NaCoO2 (space group R-3m) was observed. Also, a formation mechanism of P3-Na0.6CoO2 phase using the unit cell theory in the hydrothermal process was proposed.

Electrochemical performance of NaCo2O4 as electrode for supercapacitors

Abstract Sub-micron-scaled sodium cobalt oxide (NaCo2O4) powders are prepared by a solid-state reaction method. Characterization using X-ray diffraction indicates that the synthesized NaCo2O4 has a hexagonal layered structure. The electrochemical performance of the NaCo2O4 electrodes is investigated using cyclic voltammetry and galvanostatic charge/discharge in NaOH solution. The results show that the specific capacitance of the NaCo2O4 electrode reaches 337 F/g over the potential range of 0.15–0.65 V at a mass normalized current of 50 mA/g. Moreover, NaCo2O4 exhibits very good stability and cycling performance as a supercapacitor material.

Chemical Decomposition as a Likely Source of Ambient and Thermal Instabilities of Layered Sodium Cobaltate

With the application of an oxygen atmosphere, we synthesized a highly textured sodium cobaltate, Na0.75CoO2. At the same time, we identified its peculiarities that influence the measured parameters to a degree that poses serious questions about this material’s potential for use. We have systematically studied the influence of humidity on the ceramic pellets and identified the conditions under which the material completely deteriorates. By performing microstructural and thermal analyses, coupled with a determination of the evolved gases, we identified the chemical reactions that are involved in this process. In addition, we re-examined the performance of sodium cobaltate under the working conditions and found that the material behaves in a manner different from the expected manner. We have shown, in contrast to many other reports, that the oxygen vacancies do not play a very important role because the changes in the physical parameters can be attributed to the reduction of cobalt and consequently to the fo...

Microstructure and Thermoelectric Properties of Na<sub>0.5</sub>CoO<sub>2</sub>

The polycrystalline of sodium cobalt oxide (Na0.5CoO2) was synthesized by solid state reaction method and sintering method. The microstructure was composed of powder size and crystal structure. The Seebeck coefficient and electrical resistivity are measured. We found that the concentration of sodium ions sandwiched between two neighboring CoO2layers played a crucial role in transport properties. The results showed small particle size, single phase and orthorhombic structure. The Seebeck coefficient of Na0.5CoO2increased as the temperature increased. The electrical resistivity was decreased as temperature decreased from the range 300-500 K.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Selecting the suitable dopants: electronic structures of transition metal and rare earth doped thermoelectric sodium cobaltate

Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co with formation energy $-2.35$ eV, $2.08$ eV when Fermi level equals to the valence band maximum. While Eu is more stable when it substitutes Na having formation energy of $-2.64$ eV. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high-performance sodium cobaltate based multifunctional materials.

Ab-initio investigation of spin states of sodium cobaltate Na2/3CoO2

Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U > 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.

Theoretical analysis of oxygen vacancies in layered sodium cobaltate, NaxCoO2−δ

Sodium cobaltate with high Na content is a promising thermoelectric material. It has recently been reported that oxygen vacancies can alter the material properties, reducing its figure of merit. However, experimental data concerning the oxygen stoichiometry are contradictory. We therefore studied the formation of oxygen vacancies in NaxCoO2 with first principles calculations, focusing on x = 0.75. We show that a very low oxygen vacancy concentration is expected at the temperatures and partial pressures relevant for applications.

ChemInform Abstract: Standard Enthalpy of Formation of NaxCoO2 System

Abstract[solution calorimetric determination for x = 0.1, 0.2, ...

Synthesis and Battery Studies of Sodium Cobalt Oxides, NaCoO<sub>2</sub> Cathodes

Sodium based batteries presented a new and promising battery system. This work attempted to develop NaCoO2 material using a new sol-gel method. The characterization of this sodium based material was done by using Thermogravimetric Analysis (TGA), X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). Cyclic voltammetry showed that the material exhibit reversible oxidation and reduction peaks which meant that a sodium based battery was possible for this material. Electrochemical charge-discharge profile was performed to study the voltage plateaus exhibited by the material. It showed discharge plateaus of between 3.2 to 2.3 V and another plateau at 0.6 V.

Standard enthalpy of formation of NaxCoO2 system

Layered sodium cobalt oxide (NaxCoO2) materials have attracted lot of attention in recent times because of their large thermoelectric power. Information on thermodynamic properties is necessary to predict the stability of these compounds. The present study deals with determination of enthalpy of formation of sodium-cobalt oxides [NaxCoO2 (x=0.1, 0.2…0.9)] using a solution calorimeter. The standard molar heat of dissolutions of NaxCoO2 (x=0.1, 0.2…0.9), NaCl(s), CoCl3(s) and H2O (l) in 0.150dm3 (1:1 ratio) aqueous HCl solution of concentration 5.96moldm−3 were measured by an isoperibol solution calorimeter. Using the values of the measured enthalpy of dissolution and other auxiliary data, the standard molar enthalpies of formation (ΔfHm°298) of the compounds were determined. The values of ΔfHm°298 for the Na0.1CoO2(s), Na0.2CoO2(s), Na0.3CoO2(s), Na0.4CoO2(s), Na0.5CoO2(s), Na0.6CoO2(s), Na0.7CoO2(s), Na0.8CoO2(s) and Na0.9CoO2(s) compounds are found to be −(598.61±6.03), −(637.40±6.04), −(676.26±6.04), −(716.53±6.05), −(758.03±6.06), −(800.04±6.07), −(841.58±6.08), −(883.38±6.09), −(924.93±6.10)kJmol−1, respectively.

Sol–gel synthesis of sodium and lithium based materials

Sodium and lithium cobaltates are important materials for thermoelectric and battery applications due to their large thermoelectric power and ability to (de-) intercalate the alkali metal. For these applications, phase pure materials with controlled microstructure are required. We report on the sol–gel synthesis of sodium- and lithium-based materials by using acetate precursors. The produced Na2/3CoO2, Li(Ni1/3Mn1/3Co1/3)O2, and Li(Ni1/2Co1/2)O2 powders are phase pure with grain sizes below 1 μm. X-ray diffraction and energy-dispersive spectral analyses show that the cation stoichiometry is preserved in the lithium-based compounds. Despite the low temperatures, the sodium content is reduced by 1/3 as compared to the initial value. Chemical phases of the investigated powders are formed in the sol–gel route at temperatures typically 100–200 K lower than those used in the conventional solid-state synthesis of these materials. The suggested sol–gel synthesis is a low temperature process suited for production of phase pure and homogeneous materials with volatile cations.

Competing orders in NaxCoO2from strong correlations on a two-particle level

Based on dynamical mean-field theory with a continuous-time quantum Monte-Carlo impurity solver, static as well as dynamic spin and charge susceptibilites for the phase diagram of the sodium cobaltate system Na$_x$CoO$_2$ are discussed. The approach includes important vertex contributions to the q-dependent two-particle response functions by means of a local approximation to the irreducible vertex function in the particle-hole channel. A single-band Hubbard model suffices to reveal several charge- and spin-instability tendencies in accordance with experiment, including the stabilization of an effective kagome sublattice close to x=0.67, without invoking the doping-dependent Na-potential landscape. The in-plane antiferromagnetic-to-ferromagnetic crossover is additionally verified by means of the computed Korringa ratio. Moreover an intricate high-energy mode in the transverse spin susceptiblity is revealed, pointing towards a strong energy dependence of the effective intersite exchange.

Synthesis and Properties of Boron Doped NaxCo2O4 Nanocrystalline Ceramics

Sodium cobalt oxide (NaCo2O4) nanofibers with diameters ranging between 20 and 200 nm were prepared by electrospinning a precursor mixture of PVA/(Na–Co) acetate. This was the first time any such attempt was made. Afterwards, the electrospun nanofibers were subjected to calcination treatment. The characteristics of the fibers were investigated using a Fourier transform infrared spectroscopy, a X-ray diffractometer, and a scanning electron microscopy. The boron doped and undoped NaCo2O4 nanofibers calcined at 850 °C were polycrystalline of the γ NaxCo2O4 phase having diameters ranging between 20 and 60 nm with grain sizes of 5–10 nm, and the nanofibers calcined at 800 °C were single crystals having linked particles or crystallites with particle sizes ranging between 60 and 200 nm. The results indicated a significant effect of calcination temperature on the crystalline phase and morphology of the nanofibers. It could be seen in the SEM micrograph of the fibers that when boron was added, this resulted in the formation of cross-linked bright-surfaced fibers. The average fiber diameter for boron doped and undoped fiber mats were 204 and 123 nm, respectively. The grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 140 and 118 nm, respectively.

Synthesis of submicron tubules of sodium cobaltate using a natural template of bamboo charcoal as supporting templates

Direction conversion of heat to electricity using thermoelectric materials has attracted considerable attention. The γ-phase of Na xCoO 2 is a potential candidate for thermoelectric applications at high temperatures. In this work, we report a simple approach of fabricating submicron tubules of γ-Na 0.7CoO 2 using the urea-based sol–gel method with the assistance of porous bamboo charcoal as supporting templates. The synthesized tubules are characterized by X-ray diffractometer, field-emission scanning electron microscope and transmission electron microscope. The resulting tubules exhibit a single phase of γ-Na 0.7CoO 2 and have various sizes ranging from about 0.43 to 1.44 μm with the length reaching up to 10 μm.

Role of electronic correlations in the Fermi surface formation of Na x CoO2

Band structure of metallic sodium cobaltate Na$_x$CoO$_2$ ($x$=0.33, 0.48, 0.61 0.72) has been investigated by local density approximation+Hubbard $U$ (LDA+$U$) method and within Gutzwiller approximation for the Co-$t_{2g}$ manifold. Correlation effects being taken into account results in suppression of the $e'_g$ hole pockets at the Fermi surface in agreement with recent angle-resolved photo-emission spectroscopy (ARPES) experiments. In the Gutzwiller approximation the bilayer splitting is significantly reduced due to the correlation effects. The formation of high spin (HS) state in Co $d$-shell was shown to be very improbable.

The P2-Na2/3Co2/3Mn1/3O2 phase: structure, physical properties and electrochemical behavior as positive electrode in sodium battery

Manganese substituted sodium cobaltate, Na(2/3)Co(2/3)Mn(1/3)O(2), with a layered hexagonal structure (P2-type) was obtained by a co-precipitation method followed by a heat treatment at 950 °C. Powder X-ray diffraction analysis revealed that the phase is pure in the absence of long-range ordering of Co and Mn ions in the slab or Na(+) and vacancy in the interslab space. The oxidation states of the transition metal ions were studied by magnetic susceptibility measurements, electron paramagnetic resonance (ESR) and (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The charge compensation is achieved by the stabilization of low-spin Co(3+) and Mn(4+) ions. The capability of Na(2/3)Co(2/3)Mn(1/3)O(2) to intercalate and deintercalate Na(+) reversibly was tested in electrochemical sodium cells. It appears that the P2 structure is maintained during cycling, the cell parameter evolution versus the sodium amount is given. From the features of the cycling curve the formation of an ordered phase for the Na(0.5)Co(2/3)Mn(1/3)O(2) composition is expected.

Synthesis and Characterization of CoO<sub>2</sub> Nanosheets

The layered sodium cobalt oxide Na0.6CoO2 phase has been synthesized from Na2O2 and Co3O4 by solid-state reaction at 550°C for 12h with the heating rate about 2°C /min. The colloidal suspension of exfoliated, layered cobalt oxide nanosheets was prepared through the intercalation of tetrabutyl ammonium hydroxide ions (TBA) into protonated sodium cobalt oxide. The as-obtained composites were analyzed by powder X-ray diffraction (XRD). They were also cross-confirmed by a range of methods including UV/Vis spectroscopy, and Transmission Electron Microscopy (TEM). The Zeta-potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets.