CoO2 Layers Research Articles

Microscopy Study of Structural Evolution in Epitaxial LiCoO2 Positive Electrode Films during Electrochemical Cycling.

The evolution of interface between the epitaxial thin film LiCoO2 (LCO) electrode and liquid electrolyte and inside the LCO film during electrochemical cycling has been analyzed by high resolution scanning transmission electron microscopy. Relaxation of sharp translational domain boundaries with mismatched layers of CoO2 octahedra occurs during cycling and results in formation of continuous CoO2 layers across the boundaries. The original trigonal layered structure of LiCoO2 tends to change into a spinel structure at the electrode/electrolyte interface after significant extraction of Li from LCO. This change is more pronounced at 4.2 V peak of CV, indicating lower stability of the layered LCO structure near its surface after Li is extracted above 60%. The transformed structure is identified to be close to Co3O4, with Co both on tetrahedral and octahedral sites, rather than to LiCo2O4 as it was suggested in earlier publications. Electron energy-loss spectroscopy measurements also show that Co ions oxidation state is reduced to mixed valence state Co(2+)/Co(3+) during the structure changes to spinel rather than oxidized.

Optical conductivity of layered calcium cobaltate Ca3Co4O9

We report the optical properties of layered calcium cobaltate, Ca3Co4O9, which is regarded as a promising candidate for use as a thermoelectric material. The optical conductivity shows three broad peaks related to the inter-band transition below 4 eV, which are quite similar to those in the spectra of NaxCoO2. This similarity implies that the CoO2 layer, which is an essential unit for both Ca3Co4O9 and NaxCoO2, is dominant in the energy band structure below 4 eV. In addition, we estimate the effective carrier number per Co site and find similarity between the CoO2 layers of Ca3Co4O9 and Na0.75CoO2, which is consistent with the similarity in their Seebeck coefficients. To discuss the contribution of the rocksalt-type Ca2CoO3 layer in Ca3Co4O9, we propose the concept of optical sheet conductivity in the layered materials and estimate its value in the Ca2CoO3 layer. A comparison with the spin-polarized band calculation of the LDA + Hubbard U formalism with U = 5 eV suggests that the Ca2CoO3 layer has the inter-band transition of 2.6 eV in the spin-down band structure. Evaluation of the valences of Co 3d orbitals indicates the existence of charge transfer from the Ca2CoO3 layer to the CoO2 layer and mixing of Co3+ and Co4+ in the CoO2 layer, which may be the origin of the large thermoelectric effect.

Electronic structure of iron-doped misfit-layered calcium cobaltite

The electronic structures of the undoped and Fe-doped misfit-layered calcium cobaltite are calculated using the first-principles calculations under DFT+U scheme. The calculated density of states (DOS) of the undoped sample is in a good agreement with the past theoretical calculation and experiments. The Fermi level falls at nearly the top of the valence band of the CoO2 sublattice. The obtained magnetization could also very well explain the previous experiments. With Fe doping, two obvious changes in the DOS can be observed. Firstly, the Fermi energy has shifted to the higher level value and intercepted at the edge of the valence band where the slope is maximum. This characteristic enhances the thermopower of the system. The second change is the additional states ∼0.1eV above the Fermi level coming from the states of Fe atoms. These states are interpreted as a source of charge reservoir providing holes for electrical conduction in the CoO2 layer. The electronic structures of the Fe-doped case show very good agreement with the previous thermoelectric measurements. The thermopower, calculated using Boltzmann transport theory, is significantly enhanced in the Fe-doped system compared to the undoped one.

Anisotropic electrical and thermal conductivity in Bi2AE2Co2O8+δ [AE = Ca, Sr1−xBax (x = 0.0, 0.25, 0.5, 0.75, 1.0)] single crystals

Bi2AE2Co2O8+δ (AE represents alkaline earth), constructed by stacking of rock-salt Bi2AE2O4 and triangle CoO2 layers alternatively along c-axis, is one of promising thermoelectric oxides. The most impressive feature of Bi2AE2Co2O8+δ, as reported previously, is their electrical conductivity mainly lying along CoO2 plane, adjusting Bi2AE2O4 layer simultaneously manipulates both thermal conductivity and electrical conductivity. It in turn optimizes thermoelectric performance of these materials. In this work, we characterize the anisotropic thermal and electrical conductivity along both ab-plane and c-direction of Bi2AE2Co2O8+δ (AE = Ca, Sr, Ba, Sr1−xBax) single crystals. The results substantiate that isovalence replacement in Bi2AE2Co2O8+δ remarkably modifies their electrical property along ab-plane; while their thermal conductivity along ab-plane only has a slightly difference. At the same time, both the electrical conductivity and thermal conductivity along c-axis of these materials also have dramatic changes. Certainly, the electrical resistance along c-axis is too high to be used as thermoelectric applications. These results suggest that adjusting nano-block Bi2AE2O4 layer in Bi2AE2Co2O8+δ cannot modify the thermal conductivity along high electrical conductivity plane (ab-plane here). The evolution of electrical property is discussed by Anderson localization and electron-electron interaction U. And the modification of thermal conductivity along c-axis is attributed to the microstructure difference. This work sheds more light on the manipulation of the thermal and electrical conductivity in the layered thermoelectric materials.

Manipulating Thermal Conductivity by Interfacial Modification of Misfit-Layered Cobaltites Ca3Co4O9

The phonon thermal conductivities of misfit-layered Ca3Co4O9, Sr3Co4O9, and Ba3Co4O9 were calculated using the perturbed molecular dynamics method to clarify the impact of lattice misfit on the phonon thermal conduction in misfit-layered cobaltites. Substitution of Sr and Ba for Ca substantially modified the magnitude of the lattice misfit between the CoO2 and rock salt (RS) layers, because of the different ionic radii, increasing overall phonon thermal conductivity. Further analyses with intentionally changed atomic masses of Ca, Sr, or Ba revealed that smaller ionic radius at the Ca site in the RS layer, instead of heavier atomic mass, is a critical factor suppressing the overall thermal conductivity of Ca3Co4O9, since it determines not only the magnitude of lattice misfit but also the dynamic interference between the two layers, which governs the phonon thermal conduction in the CoO2 and RS layers. This concept was demonstrated for Sr-doped Ca3Co4O9 as an example of atomistic manipulation for better thermoelectric properties. Phonon thermal conductivities not only in the RS layer but also in the CoO2 layer were reduced by the substitution of Sr for Ca. These results provide another strategy to improve the thermal conductivity of this class of misfit cobaltites, that is, to control the thermal conductivity of the CoO2 layer responsible for electronic and thermal conductivity by atomistic manipulation in the RS layer adjacent to the CoO2 layer.

Misfit-layered Bi1.85 Sr2 Co1.85 O7.7-δ for the hydrogen evolution reaction: beyond van der Waals heterostructures.

Recent research on stable 2D nanomaterials has led to the discovery of new materials for energy-conversion and energy-storage applications. A class of layered heterostructures known as misfit-layered chalcogenides consists of well-defined atomic layers and has previously been applied as thermoelectric materials for use as high-temperature thermoelectric batteries. The performance of such misfit-layered chalcogenides in electrochemical applications, specifically the hydrogen evolution reaction, is currently unexplored. Herein, a misfit-layered chalcogenide consisting of CoO2 layers interleaved with an SrO-BiO-BiO-SrO rock-salt block and having the formula Bi1.85 Sr2 Co1.85 O7.7-δ is synthesized and examined for its structural and electrochemical properties. The hydrogen-evolution performance of misfit-layered Bi1.85 Sr2 Co1.85 O7.7-δ , which has an overpotential of 589 mV and a Tafel slope of 51 mV per decade, demonstrates the promising potential of misfit-layered chalcogenides as electrocatalysts instead of classical carbon.

A molecular dynamics study of oxygen ion diffusion in A-site ordered perovskite PrBaCo(2)O(5.5): data mining the oxygen trajectories.

Molecular dynamics (MD) simulations have been widely used to study oxygen ion diffusion in crystals. In the data analysis, one typically calculates the mean squared displacements to obtain the self-diffusion coefficients. Further information extraction for each individual atom poses significant challenges due to the lack of general methods. In this work, oxygen ion diffusion in A-site ordered perovskite PrBaCo2O5.5 is studied using MD simulations and the oxygen migration is analyzed by k-means clustering, a machine learning algorithm. The clustering analysis allows the tracking of each individual oxygen jump along with its corresponding location, i.e., oxygen site in BaO, PrO0.5 and CoO2 layers. Therefore it increases the understanding of the factors influencing oxygen diffusion. For example, it is found that the oxygen occupation fraction in the PrO0.5 layers increases with temperature, while in the CoO2 layers it decreases with temperature. Additionally, the activation enthalpies of oxygen jumps from CoO2 to CoO2, CoO2 to PrO0.5 and PrO0.5 to CoO2 are 0.22 eV, 0.54 eV and 0.34 eV, respectively, exhibiting anisotropic characteristics. Furthermore, the dwell times of oxygen atoms suggest that they are highly mobile in PrO0.5 layers. Combining the analysis of activation enthalpies and dwell times, it is suggested that the oxygen transport is fast within the CoO2 layers while the PrO0.5 layers work as oxygen vacancy reservoirs.

Revealing the Origin of “Phonon Glass-Electron Crystal” Behavior in Thermoelectric Layered Cobaltate by Accurate Displacement Measurement

Measurement of local disorder and lattice vibrations is of great importance for understanding the mechanisms whereby thermoelectric materials efficiently convert heat to electricity. Calcium cobalt oxides (Ca2CoO3)0.62CoO2 is a model system in this regard with a figure of merit ZT above one. The compound has a complex misfit layered structure with significant lattice displacement (both static and dynamic) that is attributed to the reduced thermal conductivity. Its averaged structure consists of two interpenetrating subsystems of a CdI2-type CoO2 layer and a distorted tri-layered rock-salt-type Ca2CoO3 block (Fig.1,2), being incommensurately modulated along the b-axis. It is well known that both static displacement and thermal atomic vibration can effectively scatter phonons to reduce thermal conductivity, however, the exact scattering mechanisms are still unknown, largely because there is no reliable method available for such a measurement that can link the displacement to the phonon scattering.

Enhanced Thermoelectric Performance Induced by Cr Doping at Ca‐Sites in Ca3Co4O9 System

Thermoelectric performance of Ca3Co4O9 system has been enhanced through the misplaced substitution with Cr doping at Ca‐sites. The study of Cr doping at Co‐sites has also been performed for comparing. Base on the analysis of the structural data, we conclude that the doped Cr ions should be in the form of Cr+6, and enter into Ca‐sites of Ca2CoO3 layers in Ca3−xCrxCo4O9 lattice and Co‐sites of CoO2 layers in Ca3Co3.90Cr0.10O9 lattice, respectively. As Cr ions are doped into Ca‐sites in Ca3−xCrxCo4O9 lattice, both resistivity and thermopower increase obviously, while thermal conductivity decreases. Among all samples, Ca2.95Cr0.05Co4O9 shows the maximum ZT value, which is enhanced by 12% than that of Ca3Co4O9. The results show that the misplaced substitution with a proper Cr doping at Ca‐sites is beneficial to the enhancement of the thermoelectric performance in Ca3Co4O9 system compared to the traditional idea: Cr doping at Co‐sites, which is suggested to originate from the combined action of the carrier concentration, the electronic correlation, and the lattice disharmony.

Cr6+ ion doping effects in layered Ca3Co4O9 system

The effects of Cr doping on structural, magnetic, electrical and thermal transport properties of Ca3Co4−xCrxO9 (0⩽x⩽0.3) have been investigated systematically. Based on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data, the Cr ions are suggested to be doped at Co4+-sites in CoO2 layers in the form of Cr6+. The introduction of the nonmagnetic Cr6+ ions results in the decrease of the average effective magnetic moment, which confirming that the ferrimagnetic state of Ca3Co4O9 system can be well suppressed by Cr doping. The magnitude of resistivity and the metal–insulator-transition temperature increase monotonously with increasing x, indicating the enhanced carrier localization and the more stable spin-density-wave state in these Cr-doped samples. As Cr6+ ions are doped into system, the thermopower increases obviously. Correspondingly, the room-temperature thermopower reaches the maximum value of 140μV/K for x=0.3 sample. These results of magnetism and electrical/thermal transport are mainly originated from the variations of carrier concentration and electronic correlation resulted from the partial substitution of Cr6+ for Co ions.

Fabrication of all-solid-state battery using epitaxial LiCoO2 thin films

We demonstrate the orientation control of LiCoO2 epitaxial thin films deposited on (110)-2 × 1 reconstructed surfaces of Au and Pt by using pulsed laser deposition. The epitaxial LiCoO2 thin films have CoO2 layers tilted with respect to the surface normal, which is suitable for Li-ion insertion/extraction reactions in LiCoO2. We show the successful operation of all-solid-state thin-film Li-ion batteries by employing the epitaxial LiCoO2 thin films as cathodes. The electrochemical properties of epitaxial films in all-solid-state batteries are improved compared to the batteries using liquid electrolyte.

Local structure determination of substitutional elements in Ca3Co4−xMxO9(M = Fe, Cr, Ga) using X-ray absorption spectroscopy

Ca3Co4O9 is one of the most promising thermoelectric oxide materials at high temperature. Its structure consists of two misfit layers: the CaOCoOCaO rocksalt-type (RS) layer and the CdI2-type CoO2 layer. In this paper, we reported the synthesis of single phase Ca3Co4−xMxO9 (where M = Fe, Cr, and Ga). Thermoelectric measurements showed the variation of thermoelectric properties depending on the charge state of the substitutional elements. We showed the direct evidence for the location of the substitutional elements in the system using an X-ray absorption spectroscopy technique. The extended X-ray absorption fine structure spectra were fitted with the models. Also, the X-ray absorption near edge structure spectra were compared with the simulated spectra from the first principle calculation. The data analysis show that for Fe and Cr substitution, the Fe and Cr atoms are more likely to be located in the RS layer rather than in the CoO2 as presumed previously. For the Ga case, Ga atoms accommodate both sites; still the majority stays in the RS layer. This is supported by the calculation of the total energy of the system, which showed that the total energy is lower when the substituted elements were located in the RS layer.

Impact of Dynamic Interlayer Interactions on Thermal Conductivity of Ca3Co4O9

The phonon thermal conductivity of misfit-layered Ca3Co4O9 has been calculated by perturbed molecular dynamics using a classical force field. Detailed numerical analyses reveal that, in spite of its smaller cross-sectional area, the CoO2 layer transports more heat than the thicker rock salt (RS) layer, although its local thermal conduction is more suppressed than in another layered cobaltite, NaxCoO2. The origins of these differences have been elucidated through careful examination of the atomic arrangements in each layer. Since thermal conduction in the RS layer can be reduced without deteriorating electronic properties for which the CoO2 layer is responsible, it is suggested that the RS layer should be modified to further suppress the overall in-plane thermal conductivity. Computational experiments with increasing number of Ca–O planes in the RS layer showed the opposite trend to what can be predicted based on the misfit between two dissimilar layers. Further analyses to reveal the origin of these unexpected results provide yet another strategy to further decrease the thermal conductivity, namely to control the dynamic interference between atoms across the interface between two layers.

A Comparative Study of LiCoO2 Polymorphs: Structural and Electrochemical Characterization of O2-, O3-, and O4-type Phases

O4-type LiCoO2 as a third polymorph of LiCoO2 is prepared by an ion-exchange method in aqueous media from OP4-[Li, Na]CoO2, which has an intergrowth structure of O3-LiCoO2 and P2-Na0.7CoO2. O4-type LiCoO2 is characterized by synchrotron X-ray diffraction, neutron diffraction, and X-ray absorption spectroscopy. Structural characterization reveals that O4-type LiCoO2 has an intergrowth structure of O3- and O2-LiCoO2 with stacking faulted domains. Three LiCoO2 polymorphs are formed from the close-packed CoO2 layers, which consist of edge-shared CoO6 octahedra, whereas the oxide-ion stacking is different: cubic in the O3-phase, cubic/hexagonal in the O2-phase, and alternate O3 and O2 in the O4-phase. Structural analysis using the DIFFaX program suggests that the O4-phase consists of approximately 30% of O12-domains, while stacking faults are not evidenced for O2-phase. The results suggest that a nucleation process for Na/Li ion-exchange kinetically dominates a growth process of ideal O4-domains because the presence of CoO2-Li-CoO2 blocks as O3-domains could be expected to prevent through-plane interaction of Na layers. Electrochemical behavior and structural transition processes for three LiCoO2 polymorphs are compared in Li cells. A new phase, OT(#)4-type Li0.5CoO2, is first isolated as an intergrowth phase of O3- and T(#)2-Li0.5CoO2. However, some deviations from ideal behavior as the O2/O3-intergrowth phase are also noted, presumably because of the existence of stacking faults.

Local structure of Fe in Fe-doped misfit-layered calcium cobaltite: An X-ray absorption spectroscopy study

Polycrystalline Ca3Co4−xFexO9+δ ceramics (x=0, 0.01, 0.03, 0.05) were fabricated using a simple thermal hydro-decomposition method and a spark plasma sintering technique. Thermoelectric property measurements showed that increasing Fe concentration resulted in a decrease in electrical resistivity, thermopower and thermal conductivity, leading to an improvement in the dimensionless figure-of-merit, >35% for x=0.05 at 1073 K. An X-ray absorption spectroscopy technique was used to investigate the local structure of Fe ions in the Ca3Co4−xFexO9+δ structure for the first time. By fitting data from the extended X-ray absorption fine structure (EXAFS) spectra and analyzing the X-ray absorption near-edge structure (XANES) spectra incorporated with first principle simulation, it was shown that Fe was substituted for Co in the the Ca2CoO3 (rocksalt, RS) layer rather than in the CoO2 layer. Variation in the thermoelectric properties as a function of Fe concentration was attributed to charge transfer between the CoO2 and the RS layers. The origin of the preferential Fe substitution site was investigated considering the ionic radii of Co and Fe and the total energy of the system.

Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co4+ Ions in the Ca3Co4–xRhxO9 System

The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4–xRhxO9 (0 ≤ x ≤ 0.4) samples have been investigated systematically. XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal–insulator transition (MIT) and an anomaly of the slope related to the transition from a Fermi liquid to an incoherent metal at low temperatures were observed in the resistivity curve for the undoped sample. As Rh ions were doped into the samples, an additional anomaly and MIT occurred in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition (SST) of Co ions. The low-temperature MIT temperature increased with increasing Rh-doping content, indicating that the spin-density-wave state became stable as a result of the enhanced random Coulomb potential in CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute at the Co3+ sites of CoO2 layers. The substitution induced a partial SST of Co4+ ions from the low-spin to the high-spin state, leading to the formation of a spin-state polaron. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4–xRhxO9. It should be noted that the thermopower of the system did not change obviously with Rh doping, but the thermal conductivity decreased significantly. As a result, the ZT value increased markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reached 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the thermoelectric performance of the Ca3Co4O9 system.

Synthesis and Electrochemical Properties of Novel Co2+/Co3+ Layered Ordered Rock-Salt Structure Type Li0.75Ca0.25CoO¬2

New layered transition metal oxide cathode material, Li0.75Ca0.25CoO2 (Space Group = P63/mmc; a = 2.817 Å, b = 2.817 Å, c = 10.79 Å ß = 120{degree sign}) was prepared by a multistep synthetic technique. Electrochemical cycling of 0.75 Li+ per formula unit utilizes both the Co3+/Co4+ and Co2+/Co3+ redox couples. The presence of Ca2+ between CoO2 layers results in low volume difference (~0.8%) between fully lithiated and delithiated materials.

First-Principle Calculation on Misfit Layered Cobaltite and La-Doped Series

The relations between electronic structure and thermoelectric property of misfit layered cobaltite of Ca3Co4O9 and La-doped series are studied from the calculation by density function and discrete variation method (DFT-DVM). The highest valence band (HVB) and the lowest conduction band (LCB) near Fermi level are only mainly from O 2p and Co 3d in Ca2CoO3 layer. Therefore, the semiconductor, or thermoelectric property of Ca3Co4O9 should be mainly from Ca2CoO3 layer, but have no direct relation to the CoO2 layer, which is consistent with that binary oxides hardly have thermoelectric property, but trinary oxide compounds have quite good thermoelectric property. With the amount of La-doped increase, the gap between HVB and LCB firstly decrease, then reaches the minimum, finally increase. The gap affects the thermoelectric property. Therefore, there is a best amount of Na-doped to improve thermoelectric property, which is consistent with the experiment.

Evolution of the thermoelectric performance in low Ca-doped layered cobaltite Bi2Sr2Co2Oy

We have synthesized a series of layered cobaltite Bi2Sr2−xCaxCo2Oy (0≤x≤0.3). Reduced resistivity was observed with x≤0.2 due to the possible chemical pressure induced charge transfer between the Bi2Sr2O4 layer and CoO2 layer. Enhanced thermoelectric power S was also observed in the Ca-doped samples with the largest S as for x=0.2. The variation of S(T) could reflect the change of electronic correlation or the modification of local density of states and band structure near the Fermi energy. In addition, magnetic measurements show that the samples with x=0.0 exhibit abnormal and abrupt changes, revealing a possible thermally excited spin-state transition. It is found that the thermoelectric performance is obviously enhanced by the low Ca-doping.

Water-induced two-dimensional Kondo-like scattering in Na0.23CoO2 single crystals

An in-plane Kondo-like resistivity minimum is found with a corresponding temperature Tmin, around 7 K for hydrated Na0.23CoO2 single crystals. Below 7 K, the resistivity exhibits logarithmic temperature dependence of α lnT, where α is independent of the external magnetic field up to 8 T. The feature is resulted in terms of the Kondo-like effect arising from the presence of spin disorder induced by water intercalation in the a-b plane. Our results indicate that the water intercalation weakens the coupling between the CoO2 layers and increases the two-dimensional characteristic of transport properties. The dependence of resistivity minimum on applied field proved also the suppression of localized Co(1) site on paramagnetic triangular superlattice. Further, the present experiments give the evidence of a more complex cooperative phenomena involving charge or spin ordering in the CoO2 layers as a dominating factor for the understanding of hydrated NaxCoO2 compounds.