Edge-sharing CoO6 Research Articles

Water in the Alluaudite Type-Compounds: Synthesis, Crystal Structure and Magnetic Properties of Co3(AsO4)0.5+x(HAsO4)2−x(H2AsO4)0.5+x[(H,□)0.5(H2O,H3O)0.5]2x+

In this study, a new cobalt arsenate belonging to the alluaudite supergroup compounds with the general formula of Co3(AsO4)0.5+x(HAsO4)2−x(H2AsO4)0.5+x[(H,□)0.5(H2O,H3O)0.5]2x+ (denoted as CoAsAllu) was synthesized under hydrothermal conditions. Its crystal structure was determined by a room-temperature single-crystal X-ray diffraction analysis: space group C2/c, a = 11.6978(8), b = 12.5713(7), c = 6.7705(5) Å, β = 113.255(5)°, V = 914.76(11) Å3, Z = 2 for As6H8Co6O25. It represents a new member of alluaudite-like protonated arsenates and the first alluaudite-like phase showing both protonation of the tetrahedral site and presence of the H2O molecules in the channels. In the asymmetric unit of CoAsAllu, one of the two Co, one of the two As and one of the seven O atoms lie at 4e special positions (site symmetry 2). The crystal structure consists of the infinite edge-shared CoO6 octahedra chains, running parallel to the [101¯] direction. The curved chains are interconnected by [(As1O4)0.5(H2As1O4)0.5]2− and [HAs2O4]2− tetrahedra forming a heteropolyhedral 3D open framework with two types of parallel channels. Both channels run along the c-axis and are located at the positions (1/2, 0, z) and (0, 0, z), respectively. The H2 and H4 hydrogen atoms of O2H2 and O4H4 hydroxyl groups are situated in channel 1, while the uncoordinated water molecule H2O7 at half-occupied 4e special positions and hydrogen atoms of O6H6 hydroxyl group were found in channel 2. The results of the magnetic investigations confirm the quasi one-dimensional structure of divalent cobalt ions. They are antiferromagnetically coupled with the intrachain interaction parameter of J ≈ −8 cm−1 and interchain parameter of J’ ≈ −2 cm−1 that become effective below the Néel temperature of 3.4 K.

The Restructuring-Induced CoO x Catalyst for Electrochemical Water Splitting.

Restructuring is an important yet less understood phenomenon in the catalysis community. Recent studies have shown that a group of transition metal sulfide catalysts can completely or partially restructure during electrochemical reactions which then exhibit high activity even better than the best commercial standards. However, such restructuring processes and the final structures of the new catalysts are elusive, mainly due to the difficulty from the reaction-induced changes that cannot be captured by ex situ characterizations. To establish the true structure–property relationship in these in situ generated catalysts, we use multimodel operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity to investigate the restructuring of a representative catalyst, Co9S8, that shows better activity compared to the commercial standard RuO2 during the oxygen evolution reaction (OER), a key half reaction in water-splitting for hydrogen generation. We find that Co9S8 ultimately converts to oxide cluster (CoOx) containing six oxygen coordinated Co octahedra as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory calculations verify the in situ generated CoOx consisting of edge-sharing CoO6 octahedral clusters as the actual active sites. Our results also provide insights to design other transition-metal-based materials as efficient electrocatalysts that experience a similar restructuring in OER.

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.

Dicobalt (II) hydroxo-selenite: Hydrothermal synthesis, crystal structure and magnetic properties of Co2SeO3(OH)2

A previously unreported dicobalt hydroxo-selenite Co2SeO3(OH)2 was synthesized under mild hydrothermal conditions. Its crystal structure, determined by single-crystal X-ray diffraction at ambient temperature, is monoclinic, space group P21/n, with unit cell parameters a ​= ​7.716 ​Å, b ​= ​5.823 ​Å, c ​= ​9.556 ​Å, β ​= ​97.38°, and Z ​= ​4. Bond valence sums indicate that the cations have the oxidation states Co(II) and Se(IV), and allow us to locate the hydrogens in the structure. Chains of edge-sharing CoO6 octahedra extending along [010] are connected by pairs of edge sharing CoO6 octahedra, forming a two-dimensional Co-based magnetic plane. These magnetic planes are connected by two SeO32− pyramids. The magnetic properties of the Co2+ ions are dominated by antiferromagnetic interactions, evident by the negative Curie-Weiss temperature of −97.6 ​K. Long-range ordered antiferromagnetic ordering is observed below a transition temperature at 65 ​K.

Observation of a Large Magnetic Anisotropy and a Field-Induced Magnetic State in SrCo(VO4)(OH): A Structure with a Quasi One-Dimensional Magnetic Chain.

A new member of the descloizite family, a cobalt vanadate, SrCo(VO4)(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. SrCo(VO4)(OH) crystallizes in the orthorhombic crystal system in space group P212121 with the following unit cell parameters: a = 6.0157(2) Å, b = 7.645(2) Å, c = 9.291(3) Å, V = 427.29(2) Å3, and Z = 4. It contains one-dimensional Co-O-Co chains of edge-sharing CoO6 octahedra along the a-axis connected to each other via VO4 tetrahedra along the b-axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO4)(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co2+ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO6 octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a-axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO4)(OH).

Crystal Structure of Baryum Dicobalt Iron (III) Three (Orthophosphate) Belonging to <i>α</i>-CrPO<SUB>4</SUB> Family

The new compound, BaCo2Fe(PO4)3, has been synthesized by solid state reaction. The analysis by X-ray diffraction technique showed that it crystallizes in the orthorhombic system with the space group Imma. The structure is closely linked to that of α-CrPO4 type structure. In this structure, two oxygen atoms (O1, O2) are in general position and the others in special positions. The three-dimensional network of the crystal structure is made up of two types of chains running to [001] direction. The first chain is originated from two edge-sharing CoO6 octahedra leading to the formation of Co2O10 dimers that are connected to two PO4 tetrahedra by a common edge. The junction of FeO6 octahedra and PO4 tetrahedra sharing vertices allowed to built the second chain. These chains are linked together by common vertices of tetrahedra PO4 to form an open three-dimensional framework that delimits two types of tunnels parallel to [010] and [100] where the BaII cations are located. Indeed, each Barium cation is surrounded by eight oxygen atoms.

Activation of Catalytically Active Edge-Sharing Domains in Ca2FeCoO5 for Oxygen Evolution Reaction in Highly Alkaline Media

Rechargeable zinc-air batteries are considered as one of the possible candidates to replace conventional lithium-ion batteries. One of the requirements for effective battery operation is an oxygen evolution reaction (OER) that needs to be generated in a highly alkaline electrolyte. The A2BB'O5 brownmillerite-type Ca2FeCoO5 electrocatalyst having a 57 Pbcm symmetry exhibits very high electrocatalytic activity toward OER in 4 mol dm-3 KOH. Our studies show that the electrocatalyst undergoes bulk amorphization upon OER and adequately activates catalytically active domains. The synchrotron radiation studies using the extended X-ray absorption fine structure (EXAFS) technique show that the central structural unit found in the polarized Ca2FeCoO5 is a cluster of edge-sharing CoO6 octahedra. The electrochemical data indicate that OER preferentially takes place on the edge-sharing CoO6 octahedra catalytic centers reconstructed in the brownmillerite-type electrocatalyst. The EXAFS second shell peaks at an interatomic distance of 2.8 Å are the fingerprints of the catalytically active domains.

Operando Observation of Cobalt Catalysts for Water Splitting By X-Ray and Infrared Absorption Spectroscopy

Recently, electrochemical hydrogen production by water splitting has been receiving attention with regard to achieving a sustainable society. For commercial applications, improving overall water splitting efficiency has been required for the development of highly active oxygen evolution electrocatalysts. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. In this situation, we have developed many operando observation techniques for XAFS and infrared absorption spectroscopy using attenuated total reflection mode (ATR-IR) to observe the oxygen evolution electrocatalysts. For example, hard X-ray XAFS can observe the electronic states and local structures of metal species in the catalysts. On the other hand, tender and soft X-ray XAFS and ATR-IR can reveal the chemical states of light elements in the catalysts. Therefore, we investigated the reaction mechanism of Co-KPi and similar catalysts for water splitting using various kinds of operando XAS and ATR-IR spectroscopy (Figure 1). At first, we prepared for Co-KPi and some similar cobalt catalysts in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution by electrodeposition method. It was confirmed that these cobalt catalysts are composed of the edge-sharing CoO6 octahedral structure (CoOOH) by Co K-edge XAFS using hard X-ray. Next, operando O K-edge XAFS spectra for cobalt catalysts were taken using soft X-ray under electrochemical control. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). The CoO2 peak was obviously observed for Co-KPi of highly active OER electrocatalyst, compared with Co-KOAc and Co-KCl. Finally, operando P K-edge XAFS and ATR-IR revealed the phosphate ion is bounded on the cobalt clusters to stabilize the CoO2 structure. Therefore, we revealed that Co-KPi can function as highly active OER electrocatalyst due to the presence of CoO2 strucutre stabilized by phosphate ion. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). Figure 1

Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured CayCoO2 thermoelectric ceramics

A brucite-type hydroxide of CazCo1−z(OH)2 with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite CayCoO2. Plate-like CazCo1−z(OH)2 particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like CazCo1−z(OH)2 particles (z = 0.22), which was calcined at 373 K beforehand, at a pressure as high as 500 MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923 K into an oxide ceramic of preferentially (010)-oriented CayCoO2 via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO6 octahedra. Although the ceramic also contained a minor insulating Co3O4 phase (16.7 ± 0.7 mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the CayCoO2 phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.

Layered P2-NaxCoO2 and NaxFeO2 as Cathode Materials for Potassium-Ion Batteries

Electrochemical intercalation of sodium and potassium-ion in NaxCoO2 and NaxFeO2 was successfully studied in this work. The Na0.84CoO2 and NaxFeO2 were synthesized by solution combustion synthesis technique using urea as fuel. The as-prepared Na0.84CoO2 assumed a hexagonal structure with P 63/mmc space group built from slabs of edge-sharing CoO6 octahedra accommodating Na in prismatic sites. Hexagonal platelet-like morphology was observed by SEM and TEM micrography. Firstly, the Na-ion intercalation in P2-type Na0.84CoO2 layered material was examined. The P2-type Na0.84CoO2 exhibited a reversible capacity of 148 mA h g-1 in Na-ion cell. Following, the K-ion (de)intercalation behavior of the material was investigated for the first time. The reversible K-ion (de)intercalation was observed with the high capacity of 71 mA h g-1 at C/10 current rate. Na0.84CoO2 cathode act as a suitable host for K+ intercalation delivering robust cycling, rate capability and Coulombic efficiency. In contrast, NaxFeO2 was found to be electrochemically inactive for K-ion insertion.

Cobalt Catalysts for Oxygen Evolution Studied By Operando Soft X-Ray Absorption Spectroscopy

Recently, hydrogen production system by electrochemical water splitting has received attractive attention to realize a sustainable society that does not depend on fossil fuels. For commercial applications, the improvement of overall water splitting efficiency has been required by the development of highly active oxygen evolution electrocatalyst. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. Thus, we have investigated the reaction mechanism by measuring operando O K-edge XAFS spectra for Co-KPiand similar catalysts under electrochemical potential control. Electrochemical O K-edge XAFS measurements using soft X-rays were performed with transmission mode at BL3U in the UVSOR Synchrotron, according to the previous works [3]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt counter electrode, and a Ag/AgCl reference electrode. Co-KPi electrocatalyst was electrodeposited on the Au/Cr/SiC working electrode in potassium phosphate buffered electrolyte containing Co(NO3)2. The O K-edge XAFS spectra for Co-KPi catalyst were taken under electrode potential control in potassium phosphate buffered electrolyte (Figure 1). At 0.0 V, an absorption peak attributed to oxygen species of CoOOH was detected around 531 eV. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). Next, to discuss the relationship between spectroscopic result and catalytic activity, the O K-edge XAFS spectra were taken for the low active cobalt oxide catalysts with CoOOH local structures electrodeposited in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution. Then, the intensities of CoO2 species for Co-KOAc and Co-KCl were much lower than that for Co-KPi. Therefore, we found that Co-KPi can function as highly active OER electrocatalysts due to the presence of CoO2species. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). [3] (a) M. Nagasaka et al, J. Phys. Chem. C 117 16343 (2013). (b) M. Yoshida et al, J. Phys. Chem. C 119, 19279 (2015). Figure 1

Calcium-induced cation ordering and large resistivity decrease in Pr0.3CoO2

Structure of layered cobaltates Pr0.3CoO2 and (PrCa)0.3CoO2 were investigated by electron diffraction tomography and powder X-ray diffraction. The effect of the calcium substitution on thermoelectric properties was evaluated. The structure consists of hexagonal sheets of edge-sharing CoO6 octahedra interleaved by cationic monolayers. The cations form a 2-dimensional a√3×a√3 superstructure in the a–b plane. While Pr0.3CoO2 showed no layer order in the [001] direction, introduction of calcium resulted in the formation of a superstructure spanning over six cationic layers along the [001]. This superstructure model appears to be valid also for the description of the superstructures of CaxCoO2 and SrxCoO2 with x about 1/3. Thanks to the increased number of charge carriers, the substitution of Ca2+ for Pr3+ significantly lowers the electric resistivity, while keeping quite high thermopower around 100μVK−1, though the character of resistivity remained semiconducting.

PH-Controlled assembly of two novel Dawson-sandwiched clusters involving the in situ reorganization of trivacant α-[P2W15O56](12-) into divacant α-[P2W16O57](8.).

Modified classical trivacant Wells-Dawson α-[P2W15O56](12-) and the assembly of related sandwiched transition metal clusters are of interest, but surprisingly few reports of these materials exist because of the sensitivity of α-[P2W15O56](12-) to the assembly environment. Herein, we describe the pH-controlled assembly of two novel Dawson-sandwiched clusters, (H2bpz)6[Co2(P2W16O57)2]·22H2O (1, bpz = 3,3',5,5'-tetramethyl-4,4'-bipyrazole) and (H2bpz)6[Co3H2(P2W16O57)(P2W15O56)(H2O)]·12H2O (2), involving the in situ transformation of α-[P2W15O56](12-). Both clusters were characterized by X-ray single-crystal diffraction, FT-IR spectroscopy, UV-Visible spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and elemental analyses. X-ray crystallography showed that both heteropolytungstates become divacant α-[P2W16O57](8-) and symmetrically encapsulate two edge-shared CoO6 octahedra in the interior in 1, while only one divacant α-[P2W16O57](8-) is observed in 2, which combined with another trivacant α-[P2W15O56](12-) to asymmetrically clamp three edge-shared CoO6 octahedra. The α-[P2W16O57](8-) heteropolytungstate should be generated in situ from α-[P2W15O56](12-)via self-decomposition equilibria in solution. Their electrochemical behaviors reveal characteristic multi-electron redox processes related to W(VI) centers. The electrocatalytic reduction performances toward nitrite, hydrogen peroxide, chlorate, bromate and iodate were fully measured and discussed; among these species, both clusters exhibit the best electrocatalytic activity towards the reduction of bromate. Magnetic measurements indicate weak ferromagnetic exchange interactions between Co atoms sandwiched by vacant polyoxometalates.

Single crystal magnetic structure and susceptibility of CoSe2O5

The structure of CoSe2O5 consists of one-dimensional ribbons of edge-sharing CoO6 octahedra bound together by polyanionic subunits of Se2O5. Previous work on polycrystalline samples reported a canted antiferromagnetic arrangement of the magnetic moments below the ordering temperature of 8.5K. Here, we report a single crystal investigation using variable temperature and field magnetic susceptibility and low-temperature neutron diffraction to more precisely characterize the nature of the magnetic ground state of CoSe2O5. Contrary to previous reports, we find that the single crystal magnetic structure shows no canting of the antiferromagnetic ground state, and in the process have identified several field-induced changes to the magnetization. We discuss these results in the context of the revised magnetic structure and highlight the importance of crystal growth for the accurate characterization of these properties.

Mapping of reciprocal space of La0.30CoO2 in 3D: Analysis of superstructure diffractions and intergrowths with Co3O4

We have used electron diffraction tomography and powder X-ray diffraction to elucidate the structural properties of layered cobaltate γ-La0.30CoO2. The structure consists of hexagonal sheets of edge-sharing CoO6 octahedra interleaved by lanthanum monolayers. The La3+ cations occupy only one third of available P2 sites, forming a 2-dimensional a√3×a√3 superstructure in a–b plane. The results show that there exists no order in the mutual relative shift between the neighbouring La interlayers within the a–b plane. This is manifested in the observed monotonous decrease of the diffracted intensity of the superstructure diffractions along c⁎ in both X-ray and electron diffraction data. The observed lack of stacking order differentiates the LaxCoO2 from its Ca and Sr analogues where at least a partial stacking order of the cationic interlayers is manifested in experimental data published in literature.

Enhanced Performance of IrOx-Modified Copi Electrodes and the Characterization

The CoPi electrode films supported on ITO glass plates were modified with iridium oxide nano nanoparticles. This IrOx-modified CoPi electrodes showed Tafel slopes of ~30 mV per decade during OER at the pH between 4.5 and 13.9. The overpotentials were less than 0.2 V and the turnover frequency (TOF) was 68,631 h-1 (based on the Ir atom) at 1.3 V vs. Ag/AgCl at pH 7. The TOF is the highest value ever observed from IrO2 catalysts tested for electrochemical water oxidation. The mechanism during water oxidation was studied by an in-situ X-ray absorption spectroscopy to explain the unusually high catalytic activity. In the fully oxidized state the amounts of edge-sharing CoO6 octahedra bond and extended planes of CoO6 octahedra increased whereas those of IrO6 octahedra bond and extended planes of IrO6 octahedra decreased. These results demonstrate that the water oxidation occurred by transferring oxygen from Ir-O to Co-O generating O2 molecules. The details of the mechanism and the related research results obtained from our lab will be presented.

Magnetic Structure of Ground and Field Induced Ordered States of Low-Dimensional γ-CoV2O6

The structural and magnetic properties of γ-CoV2O6 low-dimensional magnetic oxide have been investigated with an emphasis on its magnetic structure. Our main results have been obtained by powder neutron diffraction measurements carried out under magnetic fields of 0, 0.52, and 1 T, corresponding to the antiferromagnetic, ferrimagnetic, and ferromagnetic configurations of the magnetic moments. The magnetic moments are not collinear, but lie mostly along the b direction (i.e., inside structural edge-sharing CoO6 chains), which corresponds to the experimental easy magnetization axis. In the ground state, the moments are ferromagnetically ordered along the b and c directions, and antiferromagnetically ordered along a. The ferromagnetic sheets (bc plane) containing the chains are separated by nonmagnetic sheets containing VO6 octahedra and VO4 tetrahedra. The ground state magnetic structure can be described in a (4a, 2b, c) supercell using two antiferromagnetic propagation vectors kAF1 = (1/2, 0, 0) and kAF2 = (1/4, 1/2, 0). The ferri- and ferro-magnetic structures are characterized by kFi = (1/3, 0, −1/3) and k = (0, 0, 0) propagation vectors, respectively. These results are in agreement with macroscopic magnetic measurements performed on single crystals.

Layered Sodium Cobalt Oxides Anchored on Graphene for Improvement of Capacity Retention in Sodium Ion Batteries

NaxCoO2 is a well-known material for a promising cathode in sodium ion batteries. The crystal structures of NaxCoO2 have been reported to have four different phases. Each phase is built up by sheets of edge-sharing CoO6 octahedra, allowing Na+ ions to be intercalated with octahedral and/or trigonal prismatic surroundings. According to the stacking sequence of the oxygen layers, two or three repeating CoO2 sheets per unit cell can be present. Two-layer structure has γ phase in 0.6≤x≤0.75 and three-layer structure has β phase in 0.55≤x≤0.6, α phase in 1≤x≤0.9, and α’ phase at x=0.75. In this study, stoichiometric NaCoO2 (NCO) was anchored on graphene oxide (GO) and used as a cathode for sodium ion batteries. Co3O4 was first prepared on GO and thermally converted to NCO in an inert atmosphere (to minimize the loss of GO). During this process, GO was simultaneously reduced to graphene to provide NCO with a conducting network and possibly structural stability. Since the stoichiometric NCO, which has an O3-type layered structure, is in an electronically semiconducting state in contrast to non-stoichiometric NCO (metallic), the improvement of electrochemical property of NCO could be expected by incorporating graphene. Preliminary study showed that the capacity retention of NCO anchored on graphene was substantially better than the pristine NCO. Multi-step phase transition was not altered, but the capacity fading was negligible during 50 cycles. We are currently focusing on the synthesis of phase-pure NCO in an inert atmosphere.

Crystal electric field ofP2-NaxCoO2for the intermediate spin state of Co3+

The magnetic moment per Co${}^{4+}$ of $P2$($\ensuremath{\gamma}$)-type Na${}_{x}$CoO${}_{2}$ in the Curie-Weiss metal regime is revisited and examined under a proposed modification of the octahedral crystal electric field (CEF). The proposed model explains the origin of the existence of the intermediate state $S=1$ for Co${}^{3+}$ through an exciton-like elementary excitation of the narrow gap between the split ${e}_{g}$ and ${t}_{2g}$ groups. The CoO${}_{2}$ layer is proposed to be constructed from the tilted edge-sharing square-planar CoO${}_{2}$ chains with interchain coupling. The square-planar CEF of CoO${}_{2}$ requires covalent bond formation between Co and the four in-plane neighboring oxygens, while the oxygens sitting in the neighboring chains can be viewed as apical oxygens of low effective charge for a CoO${}_{6}$ pseudo-octahedron. The reason why angle-resolved photoemission spectroscopy failed to observe the local-density approximation (LDA) predicted ${e}_{g}^{\ensuremath{'}}$ hole pockets at ${k}_{z}=0$ and the reversed order of the ${t}_{2g}$ splittings between LDA and CEF calculations can also be resolved using the proposed model.

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.