CoO2 Layers Research Articles

Influence of Hydration and Temperature on the NaxCO2 Based Transducer Voltage

This paper presents an experimental approach to maximizing the voltage generated by NaxCoO2 and improving the overall efficiency of the p-type thermoelectric leg by doping with Na up to x = 0.88. Two samples with different geometries were tested, each measured with and without an additional magnetic field applied in the direction of the temperature gradient. The properties of sodium cobaltite in response to hydration were explored, at temperatures between 300 and 380 K. Water injection boosted the current and power up to 75–100 µW at a temperature of 350–360 K. This power boost can be attributed to an electron-ion fluid flow pattern maintained by the longitudinal thermomagnetic effect and by water molecules forming hydrogen bonds with oxygen atoms in the CoO2 layers, inside the material. An electronic circuit was designed to boost the voltage to the desired level, for three or more sodium cobaltite samples mounted in parallel, and to store the energy in a supercapacitor. The output voltage and resistivity change of sodium cobaltite samples can be readily used as a humidity and temperature-sensing element in a transducer when paired with an appropriate electronic conditioning scheme.

Enhancing electrical and optical properties in layered NaxCoO2 through sodium non-stoichiometry: Insights into surface band bending and oxygen vacancies

The concentration of sodium ions (Na+) in layered sodium cobaltite (NaxCoO2) is a pivotal determinant influencing its physical and chemical properties. This study focused on NaxCoO2 with varying sodium content (x = 1, 0.9, 0.7, 0.5, 0.3, 0.1) to establish a correlation between the sodium concentration and the substantial modulation of the structural and electrical properties. X-ray diffraction and Raman studies confirmed the formation of hexagonal structured NaxCoO2 prepared by the sol-gel method, while a reduction in Na content increased the percentage of the Co3O4 phase. Notably, as the Na content was lowered, the Raman peak shifted towards a lower wavenumber due to the lattice alignment along the c-axis of the crystal plane. Morphological analysis displayed the growth of distinct plate-like structures, with a thickness ranging from 10 to 15 μm, which gradually decreased on lowering Na content. X-ray photoelectron spectroscopy studies revealed an increase in the relative concentration of Co4+/Co3+ with a decrease in sodium content implying a lattice distortion in CoO6 octahedra which led to the creation of additional oxygen vacancies in Na0.7CoO2 (NCO7). Optical studies highlighted a band bending phenomenon in the NCO7, while photoluminescence studies indicated an increase in emission intensity, suggesting the presence of oxygen vacancies in NCO7. Electrical measurements demonstrated the highest conductivity for NCO7 emphasizing the significance of the optimal lattice distortion, generation of oxygen vacancies, and surface band bending effect for efficient charge carrier transportation. This study highlights the crucial role of the sodium ion content between the CoO2 layers in precise tuning of the structural, electronic structure/distortion, optical, and electrical properties of NaxCoO2. The insights gained particularly in optimizing the optical and electrical properties through sodium non-stoichiometry, may hold significant implications for diverse energy conversion and storage devices, catalysis, and even optoelectronics devices.

Lithium-Ion Diffusion in Near-Stoichiometric Polycrystalline and Monocrystalline LiCoO2

Lithium-metal-oxide-based cathode materials like LiCoO2 are an essential part of lithium-ion batteries, which are intensively researched and continuously improved. For a basic understanding of kinetic processes that control lithium incorporation and removal into/from electrodes, the lithium-ion transport in the cathode material is of high relevance. This concerns lithium diffusivities, as well as defect structures, transport mechanisms, and confined diffusion paths, such as grain boundaries. In the present study, lithium tracer self-diffusion is investigated by means of isotope exchange and secondary ion mass spectrometry in polycrystalline sintered bulk samples of stoichiometric LiCoO2 with an average grain size of about 70 nm and in single crystalline LiCoO2 in the ab-plane and c-axis in the temperature range between 200 and 700 °C. For the polycrystals, we found an activation enthalpy of ΔH = 0.75 eV. In the single crystal, the lithium-ion diffusivities along the ab-plane are identical to the diffusivities in polycrystalline LiCoO2. This indicates that diffusion along grain boundaries is similar to bulk diffusion and does not play a dominating role for the overall lithium-ion migration. Along the c-axis, diffusivities are some orders of magnitude lower, but only a slightly higher activation energy of 0.94 eV is found. This provides the experimental evidence of the often-claimed sluggish lithium diffusion along the c-axis. We suggest that lithium diffusion along the c-axis is most likely determined by fast diffusion in the ab-plane and a slow transfer of lithium ions across the CoO2 layers.

Enhanced Thermoelectric Properties of Misfit Bi2Sr2-xCaxCo2Oy: Isovalent Substitutions and Selective Phonon Scattering.

Layered Bi-misfit cobaltates, such as Bi2Sr2Co2Oy, are the natural superlattice of an electrically insulating rocksalt (RS) type Bi2Sr2O4 layer and electrically conducting CoO2 layer, stacked along the crystallographic c-axis. RS and CoO2 layers are related through charge compensation reactions (or charge transfer). Therefore, thermoelectric transport properties are affected when doping or substitution is carried out in the RS layer. In this work, we have shown improved thermoelectric properties of spark plasma sintered Bi2Sr2-xCaxCo2Oy alloys (x = 0, 0.3 and 0.5). The substitution of Ca atoms affects the thermal properties by introducing point-defect phonon scattering, while the electronic conductivity and thermopower remain unaltered.

Charge order and structure transformation of hydrated cesium cobaltates revealed by variable‐temperature 133Cs NMR spectroscopy

AbstractVariable temperature 133Cs‐NMR experiments were performed to probe the local environment of the cesium ions in hydrated cesium cobaltates (Cs0.2CoO2·0.63H2O) from 170 to 390 K. Temperature dependence of the chemical shifts and linewidths reveals two‐phase transitions at a temperature near 200 and 350 K. Three inequivalent cesium sites are observed as the temperature is reduced below 200 K, indicating restricted motion of the cesium ions and a tendency to electron localization on cobalt ions. The results provide evidence for a definite order of the cesium ions and the charge order of the cobalt ions in the CoO2 layers. Two inequivalent cesium sites with larger chemical shift differences are observed as the temperature is above 350 K, indicating a spectacular change in the space between the CoO2 planes, which provides evidence for the transition between the bilayer‐hydrated and monolayer‐hydrated structures.

Converting Brownmillerite to Alternate Layers of Oxygen‐Deficient and Conductive Nano‐Sheets with Enhanced Thermoelectric Properties

AbstractIntroducing large oxygen deficiencies while retaining low resistivity is important for enhancing the overall thermoelectric properties in 3d transition‐metal oxides. In this study, a new synthesis route to reconstruct the insulating brownmillerite SrCoO2.5 is adapted. Through a step‐by‐step nano‐blocks modification, a series of highly‐conductive layered structures is evolved, which are [Sr2O2H2]0.5CoO2, [Sr2O2]0.4CoO2, and [Sr2CoO3]0.57CoO2, while still retaining considerable Seebeck coefficient (≈100 µV K−1). Coexistence of low resistivity and high oxygen deficiency is realized in the latter two polymorphs by forming a majority of sintered oxygen vacancies in the rock‐salt layer and a minority of normal oxygen vacancies in the CoO2 layer. A room‐temperature in‐plane power factor of 3.6 mW K−2 m−1, power output density of 4.5 W m−2 at a temperature difference of 28 K, and an out‐of‐plane thermal conductivity of 0.33 W K−1 m−1 are obtained in the [Sr2O2]0.4CoO2 thin film that exhibits the highest oxygen deficiency (δ = 2.95), which is on par with Bi2Te3, the benchmark. It is pointed out that proper distribution of oxygen vacancy is essential in tailoring the physical and chemical properties of transition‐metal oxides. The sintered/normal oxygen vacancy layer model provides guidance to the exploration of materials with both low electric resistivity and thermal conductivity.

Interplay between strong correlations and electronic topology in the underlying kagome lattice of Na2/3CoO2

Electronic topology in metallic kagome compounds is under intense scrutiny. We present transport experiments in Na2/3CoO2 in which the Na order differentiates a Co kagome sub-lattice in the triangular CoO2 layers. Hall and magnetoresistance (MR) data under high fields give evidence for the coexistence of light and heavy carriers. At low temperatures, the dominant light carrier conductivity at zero field is suppressed by a B-linear MR suggesting Dirac like quasiparticles. Lifshitz transitions induced at large B and T unveil the lower mobility carriers. They display a negative B^2 MR due to scattering from magnetic moments likely pertaining to a flat band. We underline an analogy with heavy Fermion physics.

Lattice strain and atomic replacement of CoO6 octahedra in layered sodium cobalt oxide for boosted water oxidation electrocatalysis

Layered alkali metal oxides have been emerged as an alternative group of low-cost and promising electrocatalysts in water oxidation. The distinct layered configuration may offer some interesting possibilities to tune the intrinsic activity by regulating the intralayer edge-shared CoO6 octahedra and the CoO2 interlayer spacing/strain. In this work, electrochemical desodiation tuning method is explored on intralayer Ag, Cu, Ce-doped Na0.7CoO2 for highly active OER catalysts. It is demonstrated that the ΔGOH* value in the volcano plot is optimized by proper desodiation. Meanwhile, the lattice strain introduced along with the desodiated process modulates the ΔGOH*, according to first principle calculations. It shows that ∼0.157 % compressive strain in the CoO2 layers and ∼1% tensile strain between CoO2 layers are introduced in the desodiated Ag doped Na0.7CoO2. Among these catalysts, the desodiated Ag-Na0.7CoO2 sample exhibits an optimal RuO2-beyond water oxidation (OER) activity with the lowest overpotential of 236 mV@10 mA/cm2, the smallest Tafel slope of 48 mV/dec and the highest mass current density of 227.8 A/g. This work provides an interesting avenues to optimize existing layered materials with inter/intralayer modifications for highly efficient water oxidation electrolysis.

Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.

Fe-doped calcium cobaltites as electrocatalysts for oxygen evolution reaction

The electrolysis of alkaline solutions is one of the most used strategies for producing hydrogen (H2). This process distinguishes itself by breaking the water molecule (water splitting) through two semi-reactions: Hydrogen Evolution Reaction (HER, cathodic reaction) and Oxygen Evolution Reaction (OER, anodic reaction). The OER is fundamental for several electrochemical technologies related to generate and store energy. New research for developing low-cost electrocatalysts with a good electrochemical activity using more earth-abundant elements has intensified in recent years. The current work aims to study the effect of Fe doping on the OER of calcium cobaltites, Ca3Co4-xFexO9 (x = 0, 0.1, 0.4, and 0.8). Powders were obtained by a proteic sol-gel method using gelatin with calcination at 900 °C for 2 h. The resultant samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). X-ray photoelectron spectroscopy (XPS) provided information on the surface chemical states, while Mössbauer spectroscopy indicated that Fe has a strong preference for octahedral sites in the CoO2 layer rather than in the Ca2CoO3 layer in the calcium cobaltite misfit-type structure. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were used to analyze the electrochemical performance. The results indicate that Ca3Co3·2Fe0·8O9 needs the lowest overpotential of 320 mV to generate a current density of 10 mA cm−2.

Stress-Driven Evolution on Mismatched Ca2Co2O5 Oxide Material: From Geometry to the Electronic States

The geometrical structures, phase stabilities, electron energy band structures, electron density of states, and atom recombination together with the electron conduction behaviors of the sandwiched Ca2Co2O5 with external stress of 1 GPa are intensively studied by the density functional theory method. The studying results show that the symmetry remains undisturbed; the strain to the stress response is anisotropic. The strain of microarchitecture induced by external stress is also anisotropic. There is stronger covalent binding between Co and O. The binding between Co and O within CdI2 like CoO2 is very much even covalent, and it is weakened under external stress. But the covalent Co-O binding within the rock salt like CaCoO layer is enhanced. The Ca-O binding strength is insensitive to external stress. An energy gap of 0.1 eV below Fermi level for the spin-up electron band disappears, and the two energy gaps are narrowed for the spin-down electron bands. The p orbital electrons form primarily the bands below Fermi level and the d orbital electrons form primarily the bands above Fermi level. The transitions from p orbital electrons to d orbital electrons produce the conduction. The CdI2 like CoO2 layer has been enhanced in terms of participating in the conduction properties with external stress of 1 GPa, and the capability of Co is enhanced while the capability of O is decreased.

Layer spacing gradient (NaLi)1−xCoO2 for electrochemical Li extraction

Layer spacing gradient (NaLi)1−xCoO2 for electrochemical Li extraction

Tunable Magnetic and Electronic Properties of the 2D CoO2 Layer

By performing first-principles calculations, we find that the predominant spin exchange of a hexagonal CoO2 layer is in proximity to the ferromagnetic-to-antiferromagnetic transition point. Its magnetic ground state can be easily altered by, e.g., substrate dielectricity and strain. In addition, the dopability of a stack of CoO2 layers is found to sensitively depend on the interlayer distance, which not only renders effective manipulation of the electronic property but also reveals an important intercalation effect in related bulk materials.

Suppression of Anisotropic Heat Conduction in Ion-Substituted Layered Cobalt Oxides

Layered compounds often show the anisotropic transport properties of both electron and phonon; the electrical/thermal conductivity along with the layer is high and the electrical/thermal conductivity perpendicular to the layer is low. For example, layered Na x CoO2, which is known as a promising candidate for oxide thermoelectric material [1], exhibits a rather large thermoelectric power factor along with the CoO2 layer, whereas it shows low electrical conductivity in perpendicular to the CoO2 layer. In addition, the thermal conductivity is large along with the CoO2 layer, whereas it is small in perpendicular to the CoO2 layer. Therefore, the anisotropic electron/phonon transport properties need to be suppressed to improve the thermoelectric figure of merit of Na x CoO2. Here we demonstrate that the anisotropy in thermal conductivity κ of Ax CoO2 (A = Li, Na, Ca, and Sr) can be suppressed by increasing the weight of Ax . We fabricated Ax CoO2 epitaxial films with two different crystallographic orientations with respect to the surface normal to the substrates by the Reactive Solid-Phase Epitaxy method [2,3]. We measured the electrical conductivity σ and thermopower S along with the substrate surface whereas we measured the κ in normal to the substrate surface. We extracted the κ parallel to the layers (κ p) from the comparison of two different crystallographic orientations [4]. The κ p was 3-6 times larger than that perpendicular to the layers (κ v). The κ p decreased with increasing the atomic mass of Ax though κ v was insensitive to it; anisotropy of κ was controlled by substituting heavier Ax -ions [5]. The results of this study will be of great value in understanding fundamental heat transport properties of layered structures as well as developing Ax CoO2 based thermoelectric materials. In addition, the analysis of the anisotropic thermal conductivity used in this study can be applied to other layered materials. [1] I. Terasaki et al., Phys. Rev. B 56, R12685(R) (1997).[2] H. Ohta et al., Cryst. Growth Des. 5, 25 (2005).[3] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 88, 082109 (2006).[4] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 94, 152105 (2009).[5] H.J. Cho, H. Ohta et al., Adv. Mater. Interfaces 7, 1901816 (2019). Figure 1

Interface Structure and Transport Properties of Thin-Film Solid-State Batteries with an Epitaxial Cathode

A problem of solid-state Li ion batteries is a relatively low ionic conductivity at the interface between electrodes and electrolytes that limits the charging/discharging rate. For designing the solid-state batteries, understanding the mechanism behind the low interface conductivity is crucial. Investigation of the atomic structure of the interfaces and its relation to the ionic conductivity would be helpful for this purpose. However, atomic-scale observation of the buried interfaces is often difficult in practical batteries which consist of grain aggregates. Recent progress in thin-film deposition techniques of electrodes and electrolyteshas opened a path to study the interface quantitatively. By using an all-in-vacuum battery fabrication system [1], we succeeded in fabricating a very low solid-electrolyte/electrode interface resistance in thin film batteries with an epitaxial cathode. The well-defined crystalline interfaces allow the analysis of atomic-scale structure and its transport properties. For the structure analysis, we used X-ray crystal truncation rod (CTR) scattering which enables the atomic-scale analysis in a non-destructive manner.The analysis of two interfaces will be presented: a solid-electrolyte/electrode interface and an electrode/current collector interface. In the former case, we studied Li(negative electrode)/Li3PO4(solid electrode)/LiCoO2(positive electrode) thin-film Li batteries with low and high (more than 10 times) interface resistances. It turned out that the low-resistance interface has an atomically well-ordered LiCoO2 surface and a disordered surface CoO2 layer can results in the high-resistance interface, demonstrating the crucial importance of the structural ordering in the interfacial layer for realizing a low-resistance interface [2]. In the second topic, we focused on electrode/current collector interface that can also affect the battery performance. We found a drastic improvement of the charging/discharging property by inserting 1.2 nm-thick band insulator LaAlO3 into the LiCoO2 (electrode)/Nb-doped SrTiO3 (current collector) interface. The CTR analysis revealed that a formation of electric dipole at the LaAlO3/Nb-doped SrTiO3 interface, which causes a energy band alignment preferable for the electronic conductance between the electrode and the current collector.

Protonation of Lix(Co,Ni,Mn)O2 compounds in acidic conditions

The easy intercalation/deintercalation of Li+ ions is the key factor for the functionality of the layered LixCoO2 compound as a Li-ion battery cathode material. However, the hexagonal CoO2-layer piling in the LixCoO2 structure is prone to incorporate not only monopositively-charged lithium ions but also monopositive protons. Here we employ a hydrothermal acid treatment for a gradual (controlled by the treatment time) de-intercalation of Li+ ions and a subsequent intercalation of protons between the CoO2 layers in LixCoO2. We demonstrate that the lithium-for-proton exchange reaction can be monitored by AAS for the Li-content, by XRD for the proton-containing phase formation, and by FTIR and TG for the appearance of specific features indicative of the incorporated protons. With XPS we verify that upon the lithium-for-proton exchange the valence of cobalt remains the same, i.e. the proton incorporation compensates the positive charge loss due to the Li+-ion extraction. Finally, we show that essentially parallel lithium-for-proton exchange reactions occur also for Ni- and Mn-substituted Li(Co,Ni,Mn)O2 samples, that is, for compounds that are highly relevant in current Li-ion battery technology.

High-pressure Synthesis of Ba2CoO2Ag2Te2 with Extended CoO2 Planes

Using a high-pressure synthesis method, we prepared the layered oxychalcogenide Ba2CoO2Ag2Te2 (space group: I4/mmm) with alternating stacks of CoO2 and Ag2Te2 layers, separated by Ba atoms. The CoO2 plane is greatly extended (Co-O = 2.19 Å on average) due to tensile strain from adjacent Ag2Te2 layers, causing displacement of oxide anions. Layered cobaltates with trans-CoO4X2 (X = chalcogen, halogen) coordination feature large spin-orbit coupling, which is linearly scaled by the tetrahedral factor of dCo-X/dCo-O. However, applying this relation to Ba2CoO2Ag2Te2 yields a magnetic moment of ∼4 μB, which is nearly twice the experimentally observed value of 1.87(17) μB. This result, along with a reduced Néel temperature (TN = 60 K), originates from the off-centered position of otherwise under-bonded oxide anions, which changes the crystal field splitting of Co d orbitals.

Atomic layer deposition of thermoelectric layered cobalt oxides

Layered cobalt oxides based on the hexagonal CoO2 layer, e.g., NaxCoO2 and [CoCa3O3]0.62CoO2 (or “Ca3Co4O9”), are promising thermoelectric materials. Here, the authors investigate the atomic layer deposition (ALD) of these materials in a thin-film form; this is not trivial, in particular, for the former compound, as both Na and Co are little challenged as components of ALD thin films. The authors employ diketonate precursors for all the metal constituents and ozone as the source of oxygen. In both cases, a postdeposition heat-treatment in O2 is applied to get crystalline coatings; the processes are found amazingly robust in terms of metal precursor pulsing ratios. A striking difference between the two processes is the resultant morphology: while the Ca3Co4O9 films grow highly homogeneous and smooth, the NaxCoO2 coatings exhibit a rather unique reproducible 10–20 μm scale channel-like island structure for all x values investigated. Finally, the authors characterized their ALD Ca3Co4O9 films for their chemical, structural, and physical property details not previously reported.

Electrical properties of Ca3–xSmxCo4O9+δ ceramics prepared under magnetic field**Project supported by the National Natural Science Foundation of China (Grant No. 51302055).

We fabricate Sm-doped Ca3Co4O9+δ (CCO) bulk materials in magnetic field during both processes of chemical synthesis and cold pressing. The structure and electrical performance of the samples are investigated. With the increasing Sm concentration, the electrical conductivity 1/ρ decreases and the Seebeck coefficient α increases. As a result, the power factor (PF = α2/ρ) is raised slightly. After applying magnetic field, the extent of texture, grain size and density of all the bulk materials are improved obviously, thereby an enhanced electrical conductivity can be gained. Additionally, the degeneracy of Co4+ state in the CoO2 layer of CCO is also increased as the magnetic field is used in the preparing process, which results in an enhanced α. The Ca2.85Sm0.15Co4O9+δ prepared in magnetic field shows the largest power factor (0.20 mW⋅m−1⋅K−2 at 1073 K).

Structural stability, defects and competitive oxygen migration in Pr1−xYxBaCo2O6−δ

The ab initio projector augmented wave method of the density functional theory is applied for studying energy characteristics of oxygen vacancies and migration pathways in Pr1−xYxBaCo2O6−δ. It is found that the negative formation energy leads to initial appearance of oxygen vacancies in PrO planes thus providing the pathway for oxygen migration at small δ's. The increase of oxygen vacancy population in PrO planes and replacement of Pr by Y both favor a weakening of CoO bonds, which facilitates the increase in the amount of oxygen vacancies in CoO2 layers. Moreover, the yttrium dopants enlarge the bottleneck for oxygen jumps along ⟨101⟩ directions over oxygen vacancies in PrO and CoO2 layers. As a result, the respective ‘zigzag’ oxygen migration pathway becomes predominant with increasing oxygen non-stoichiometry and yttrium doping. Thus, the layered perovskite-like cobaltites give an example of a peculiar diffusion process where minority defects provide the transport channel while majority defects maintain favorable geometry of the migration pathway.