Copper Sulfide (CuxS) Nanowire‐in‐Carbon Composites Formed from Direct Sulfurization of the Metal‐Organic Framework HKUST‐1 and Their Use as Li‐Ion Battery Cathodes

Li‐ion batteries containing cost‐effective, environmentally benign cathode materials with high specific capacities are in critical demand to deliver the energy density requirements of electric vehicles and next‐generation electronic devices. Here, the phase‐controlled synthesis of copper sulfide (Cu_xS) composites by the temperature‐controlled sulfurization of a prototypal Cu metal‐organic framework (MOF), HKUST‐1 is reported. The tunable formation of different Cu_xS phases within a carbon network represents a simple method for the production of effective composite cathode materials for Li‐ion batteries. A direct link between the sulfurization temperature of the MOF and the resultant Cu_xS phase formed with more Cu‐rich phases favored at higher temperatures is further shown. The Cu_xS/C samples are characterized through X‐ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy, and energy dispersive X‐ray spectroscopy (EDX) in addition to testing as Li‐ion cathodes. It is shown that the performance is dependent on both the Cu_xS phase and the crystal morphology with the Cu_1.8S/C‐500 material as a nanowire composite exhibiting the best performance, showing a specific capacity of 220 mAh g^?1 after 200 charge/discharge cycles.     

Displacive Order–Disorder Behavior and Intrinsic Clustering of Lattice Distortions in Bi‐Substituted NaNbO3

Perovskite‐like NaNbO₃‐Bi_1/3NbO₃ solid solutions are studied to understand the interactions between octahedral rotations, which dominate the structural behavior of NaNbO₃ and displacive disorder of Bi present in Bi_1/3NbO₃. Models of instantaneous structures for representative compositions are obtained by refining atomic coordinates against X‐ray total scattering and extended X‐ray‐absorption fine structure data, with additional input obtained from transmission electron microscopy. A mixture of distinct cations and vacancies on the cuboctahedral A‐sites in Na_1?3xBi_xNbO₃ (x ≤ 0.2) results in 3D nanoscale modulations of structural distortions. This phenomenon is determined by the inevitable correlations in the chemical composition of adjacent unit cells according to the structure type—an intrinsic property of any nonmolecular crystals. Octahedral rotations become suppressed as x increases. Out‐of‐phase rotations vanish for x > 0.1, whereas in‐phase tilts persist up to x = 0.2, although for this composition their correlation length becomes limited to the nanoscale. The loss of out‐of‐phase tilting is accompanied by qualitative changes in the probability density distributions for Bi and Nb, with both species becoming disordered over loci offset from the centers of their respective oxygen cages. Symmetry arguments are used to attribute this effect to different strengths of the coupling between the cation displacements and out‐of‐phase versus in‐phase rotations. The displacive disorder of Bi and Nb combined with nanoscale clustering of lattice distortions are primarily responsible for the anomalous broadening of the temperature dependence of the dielectric constant.     

The Systematic Refinement for the Phase Change and Conversion Reactions Arising from the Lithiation of Magnetite Nanocrystals

Nanostructured materials can exhibit phase change behavior that deviates from the macroscopic phase behavior. This is exemplified by the ambiguity for the equilibrium phases driving the first open‐circuit voltage (OCV) plateau for the lithiation of Fe₃O₄ nanocrystals. Adding complexity, the relaxed state for Li_xFe₃O₄ is observed to be a function of electrochemical discharge rate. The phases occurring on the first OCV plateau for the lithiation of Fe₃O₄ nanocrystals have been investigated with density functional theory (DFT) through the evaluation of stable, or hypothesized metastable, reaction pathways. Hypotheses are evaluated through the systematic combined refinement with X‐ray absorption spectroscopy (XAS), X‐ray diffraction (XRD) measurements, neutron‐diffraction measurements, and the measured OCV on samples lithiated to x = 2.0, 3.0, and 4.0 in Li_xFe₃O₄. In contrast to the Li–Fe–O bulk phase thermodynamic pathway, Fe⁰ is not observed as a product on the first OCV plateau for 10–45 nm nanocrystals. The phase most consistent with the systematic refinement is LiFe₃O₄, showing Li+Fe cation disorder. The observed equilibrium concentration for conversion to Fe⁰ occurs at x = 4.0. These definitive phase identifications rely heavily on the systematic combined refinement approach, which is broadly applicable to other nano‐ and mesoscaled systems that have suffered from difficult or crystallite‐size‐dependent phase identification.     

Understanding the Structural Evolution and Redox Mechanism of a NaFeO2–NaCoO2 Solid Solution for Sodium‐Ion Batteries

Na‐ion batteries have become promising candidates for large‐scale energy‐storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe³⁺/Fe⁴⁺ redox couple. The high‐rate performance derived from the low Lewis‐acidity of the Na⁺ ions is another advantage of Na‐ion batteries and has been demonstrated in NaFe_1/2Co_1/2O₂ solutions. Here, a solid solution of NaFeO₂‐NaCoO₂ is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end‐members. The combined analysis of operando X‐ray diffraction, ex situ X‐ray absorption spectroscopy, and density functional theory (DFT) calculations for Na_{1– x} Fe_1/2Co_1/2O₂ reveals that the O3‐type phase transforms into a P3‐type phase coupled with Na⁺/vacancy ordering, which has not been observed in O3‐type NaFeO₂. The substitution of Co for Fe stabilizes the P3‐type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3‐type NaFeO₂, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte‐Carlo simulations, the formation of a superlattice originating from the Na⁺/vacancy ordering is found by synchrotron X‐ray diffraction for Na_0.5Fe_1/2Co_1/2O₂, which may involve a potential step in the charge/discharge profiles.     

Analysis of Phase Separation in High Performance PbTe–PbS Thermoelectric Materials

Phase immiscibility in PbTe–based thermoelectric materials is an effective means of top‐down synthesis of nanostructured composites exhibiting low lattice thermal conductivities. PbTe_1‐x S_x thermoelectric materials can be synthesized as metastable solid solution alloys through rapid quenching. Subsequent post‐annealing induces phase separation at the nanometer scale, producing nanostructures that increase phonon scattering and reduce lattice thermal conductivity. However, there has yet to be any study investigating in detail the local chemical structure of both the solid solution and nanostructured variants of this material system. Herein, quenched and annealed (i.e., solid solution and phase‐separated) samples of PbTe–PbS are analyzed by in situ high‐resolution synchrotron powder X‐ray diffraction, solid‐state ¹²⁵Te nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy analysis. For high concentrations of PbS in PbTe, e.g., x >16%, NMR and IR analyses reveal that rapidly quenched samples exhibit incipient phase separation that is not detected by state‐of‐the‐art synchrotron X‐ray diffraction, providing an example of a PbTe thermoelectric “alloy” that is in fact phase inhomogeneous. Thermally‐induced PbS phase separation in PbTe–PbS occurs close to 200 °C for all compositions studied, and the solubility of the PbS phase in PbTe at elevated temperatures >500 °C is reported. The findings of this study suggest that there may be a large number of thermoelectric alloy systems that are phase inhomogeneous or nanostructured despite adherence to Vegard's Law of alloys, highlighting the importance of careful chemical characterization to differentiate between thermoelectric alloys and composites.     

Microstructure and Superconductivity of La1.85Sr0.15CuO4 Nanowire Arrays

Superconducting La_1.85Sr_0.15CuO₄ nanowire arrays are successfully synthesized through a sol–gel method combined with porous alumina as a morphology‐directing hard template for the first time. The morphology, structure, and composition of the as‐prepared nanowire arrays are characterized by field‐emission scanning electron microscopy, transmission electron microscopy, high‐resolution transmission electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy. These experimental results indicate that the nanowires are well crystallized with an approximately uniform diameter of about 30 nm. The superconducting transition temperature T_c (ca. 30 K) of the annealed nanowires is lower than that in bulk La_1.85Sr_0.15CuO₄. It is suggested that this superconductivity suppression is derived from the weakening of in‐plane hybridization in the nanowire system.     

Br‐Doped Li4Ti5O12 and Composite TiO2 Anodes for Li‐ion Batteries: Synchrotron X‐Ray and in situ Neutron Diffraction Studies

Synchrotron X‐ray diffraction data were used to determine the phase purity and re‐evaluate the crystal‐structure of Li₄Ti₅O_12‐xBr_x electrode materials (where the synthetic chemical inputs are x = 0.05, 0.10 0.20, 0.30). A maximum of x′ = 0.12 Br, where x′ is the Rietveld‐refined value, can be substituted into the crystal structure with at least 2% rutile TiO₂ forming as a second phase. Higher Br concentrations induced the formation of a third, presumably Br‐rich, phase. These materials function as composite anodes that contain mixtures of TiO₂, Li₄Ti₅O_12‐xBr_x, and a Br‐rich third, unknown, phase. The minor quantities of the secondary phases in combination with Li₄Ti₅O_12‐xBr_x where x′ ～ 0.1 were found to correspond to the optimum in electrochemical properties, while larger quantities of the secondary phases contributed to the degradation of the performance. In situ neutron diffraction of a composite anatase TiO₂/Li₄Ti₅O₁₂ anode within a custom‐built battery was used to determine the electrochemical function of the TiO₂ component. The Li₄Ti₅O₁₂ component was found to be electrochemically active at lower voltages (1.5 V) relative to TiO₂ (1.7 V). This enabled Li insertion/extraction to be tuned through the choice of voltage range in both components of this composite or in the anatase TiO₂ phase only. The use of composite materials may facilitate the development of multi‐component electrodes where different active materials can be cycled in order to tune power output.     

On the role of Bi2Sr2Ca1Cu2O8 and Bi2Sr2Cu1O6 on the weak links and critical currents in (Bi,Pb)2Sr2Ca2Cu2O10/Ag superconducting tapes

Silver-sheated (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconducting tapes with different Bi₂Sr₂CaCu₂O₈ (Bi2212) and Bi₂Sr₂Cu₁O₆ (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.     

Bioinspired Design of SrAl2O4:Eu2+ Phosphor

A phosphor based on Sr_0.97Al₂O₄:Eu_0.03 with a biomorphous morphology is manufactured via vacuum assisted infiltration of wood tissue (Pinus sylvestris) with a precursor nitrate solution. The nitrate solution penetrates homogeneously into the uniform arrangement of rectangular shaped tracheidal cells of the wood tissue. According to scanning electron microscopy, the original wood cell walls are completely transformed retaining the original wood structure. The major crystalline phase is monoclinic SrAl₂O₄, detected by X‐ray diffraction and confirmed by Rietveld refinement. Energy‐dispersive X‐ray analysis proves the homogeneous conversion of the original wood cell wall into Sr_0.97Al₂O₄:Eu_0.03 struts. The optical properties of the resulting phosphor material are determined by photoluminescence and cathode‐luminescence spectroscopy in scanning electron microscopy. The biotemplated Sr_0.97Al₂O₄:Eu_0.03 shows a characteristic green emission at 530 nm (2.34 eV). Shaping biomorphous SrAl₂O₄:Eu²⁺ phosphor with a microstructure pseudomorphous to the bioorganic template anatomy offers a novel approach for designing micropatterned phosphor materials.     

High‐Performance Manganese Hexacyanoferrate with Cubic Structure as Superior Cathode Material for Sodium‐Ion Batteries

Sodium manganese hexacyanoferrate (Na_xMnFe(CN)₆) is one of the most promising cathode materials for sodium‐ion batteries (SIBs) due to the high voltage and low cost. However, its cycling performance is limited by the multiple phase transitions during Na⁺ insertion/extraction. In this work, a facile strategy is developed to synthesize cubic and monoclinic structured Na_xMnFe(CN)₆, and their structure evolutions are investigated through in situ X‐ray diffraction (XRD), ex situ Raman, and X‐ray photoelectron spectroscopy (XPS) characterizations. It is revealed that the monoclinic phase undergoes undesirable multiple two‐phase reactions (monoclinic ? cubic ? tetragonal) due to the large lattice distortions caused by the Jahn–Teller effects of Mn³⁺, resulting in poor cycling performances with 38% capacity retention. The cubic Na_xMnFe(CN)₆ with high structural symmetry maintains the structural stability during the repeated Na⁺ insertion/extraction process, demonstrating impressive electrochemical performances with specific capacity of ≈120 mAh g^?1 at 3.5 V (vs Na/Na⁺), capacity retention of ≈70% over 500 cycles at 200 mA g^?1. In addition, the TiO₂//C‐MnHCF full battery is fabricated with an energy density of 111 Wh kg^?1, suggesting the great potential of cubic Na_xMnFe(CN)₆ for practical energy storage applications.     

Strong Band Bowing Effects and Distinctive Optoelectronic Properties of 2H and 1T′ Phase‐Tunable MoxRe1–xS2 Alloys

Structure and energy band engineering of 2D materials via selective doping or phase modulation provide a significant opportunity to design them for optoelectronic devices. Here, the synthesis of high‐quality Mo_xRe_1–xS₂ alloys with tunable composition and phase structure via chemical vapor deposition growth is reported, and their novel energy band structures and optoelectronic properties are explored. The phase separation and structure reconstruction, which are found to be two serious problems in the synthesis of these alloys, are successfully suppressed through tuning their growth thermodynamics. As a result, the obtained Mo_xRe_1–xS₂ alloys have uniform composition, phase structure, and crystal orientation. Together with X‐ray photoelectron spectroscopy analysis and first‐principle calculation, the Re/Mo doping‐induced Fermi level up‐shift/down‐shift, new electronic states, and “sub‐gap” formation in Mo_xRe_1–xS₂ alloys are revealed. Especially, a strong band bowing effect is discovered in the Mo_xRe_1–xS₂ alloys with structure transition between 1T′ and 2H phases. Furthermore, these alloys reveal tunable conduction behavior from n‐type to bipolar and p‐type in 1T′ phase, as well as novel “bipolar‐like” electron conduction behavior in 2H alloys. The results highlight the unique alloying effects, which do not exist in the single‐phase 2D alloys, and provide the feasibility for potential applications in building novel electronic and optoelectronic devices.     

Facile Synthesis of Manganese‐Loaded Mesoporous Silica Materials by Direct Reaction Between KMnO4 and an In‐Situ Surfactant Template

A series of manganese oxide‐loaded SBA‐15 (MnSBA‐xh, x = 1, 2, 3, 4, 5, 6; h: hour(s)) mesoporous materials are synthesized via a facile, in‐situ reduction method with a surfactant template. The composite materials are characterized using Fourier‐transform infrared spectroscopy, X‐ray diffraction, N₂ sorption isotherms, X‐ray photoelectron spectroscopy (XPS), transmission electron microscopy, energy‐dispersive spectroscopy, and CO oxidation catalysis. The results show that a high content of manganese (an atomic ratio of Mn/Si from 0.12 up to approximately 1) could be loaded into the channels of SBA‐15 when treated with an aqueous solution of potassium permanganate, while retaining the ordered mesostructure and large surface area of SBA‐15. Increasing the manganese oxide content results in a gradual decrease in the specific surface area, pore size, and pore volume. XPS spectra are employed to confirm the redox reaction between KMnO₄ and the surfactant. CO‐conversion tests on the calcined MnSBA‐2h sample (MnSBA‐2h‐cal) shows that it has a repeatable, and relatively high, catalytic activity.     

Revealing the Pseudo‐Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte

Since the discovery of Ti₃C₂T_x in 2011, the family of two‐dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non‐rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X‐ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X‐ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti₃C₂T_x in 1 m H₂SO₄. Results reveal that a 0.5 Å expansion and shrinkage of the c‐lattice parameter of Ti₃C₂T_x occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.     

Role of Excess FAI in Formation of High‐Efficiency FAPbI3‐Based Light‐Emitting Diodes

Introducing extensively excess ammonium halides when forming perovskites has recently been demonstrated as an effective approach to improve the performance of perovskite light‐emitting diodes (PeLEDs). Here, Cs_0.17FA_0.83PbI_2.5Br_0.5 is used as a model system to elucidate the impact of introducing excess formamidinium iodide (FAI) on the crystallization process of the perovskite film and operation of the corresponding PeLED. The excess FAI ratio is varied from 0 to 120 mol% and the crystallization process of the perovskite through in situ absorbance, in situ photoluminescence, and ex situ X‐ray diffraction measurements is systematically monitored. The results suggest that excess FAI triggers formation of a compact wide‐bandgap intermediate phase in the as‐deposited film, which then transforms to isolated and highly crystalline perovskite grains upon annealing. Using excitation correlation photoluminescence spectroscopy it is found that excess FAI results in a lower density of deep trap states and therefore a reduction of nonradiative losses in the material. This leads to a greatly enhanced maximum external quantum efficiency (EQE) from 0.25% (stoichiometric) to 12.7% (90 mol% excess). Furthermore, the FAI‐excess perovskite film is optimized with Pb(SCN)₂ and 5‐ammonium valeric acid iodide additives and achieve a record radiance of 965 W Sr^?1 m^?2 for near‐infrared PeLEDs and a high EQE of 17.4%.     

Microstructures and superconducting properties of V doped Bi2223 tapes

We prepared Ag-sheathed tapes by the powder-in-tube method using V doped Bi2223 powder. We found that V doping into Bi oxide superconductors greatly enhances the Bi2223 phase formation of the tapes without any degradation of the superconducting critical temperature T_c. The doped V is not included in the Bi2223 (or Bi2212) grains, but instead exists as Sr₆V₂O₁₁ particles with diameters of 0.1 to several micrometers, in which the larger plate-like particles play an important role to promote the formation of the Bi2223 phase. With V doping, the optimum critical current density J_c value is obtained for a short time heat treatment.     

Origin of a Tetragonal BiFeO3 Phase with a Giant c/a Ratio on SrTiO3 Substrates

A tetragonal BiFeO₃ phase with giant c/a of approximately 1.25 has been of great interest recently as it potentially possesses a giant polarization and much enhanced electromechanical response. This super‐tetragonal phase is known to be a stable phase only under high compressive strains of above approximately 4.5%, according to first principle calculations. However, in previous work, this super‐tetragonal BiFeO₃ phase was obtained in films deposited at high growth rate on SrTiO₃ substrates with compressive strain of only around 1.5%. By detailed structure analysis using high resolution synchrotron X‐ray diffraction, atomic force microscopy, and transmission electron microscopy, the parasitic β‐Bi₂O₃ phase is identified as the origin inducing the formation of super‐tetragonal BiFeO₃ phase on SrTiO₃ substrates. In addition, ab initio calculations also confirm that this super‐tetragonal phase is more stable than monoclinic phase when Bi₂O₃ is present. Using Bi₂O₃ as a buffer layer, an alternative route, not involving strain engineering, is proposed to stabilize this promising super‐tetragonal BiFeO₃ phase at low growth rates.     

Martensitic Phase Transformation of Isolated HfO2, ZrO2, and HfxZr1 – xO2 (0 < x < 1) Nanocrystals

We previously reported that, during the reactions to make nanocrystals of HfO₂ and Hf‐rich Hf_xZr_{1 – x}O₂, a tetragonal‐to‐monoclinic phase transformation occurs that is accompanied by a shape change of the particles (faceted spherical to nanorods) when the temperature at which the reaction is conducted is changed from 340 to 400 °C. We now conclude that this concomitant phase and shape change is a result of the martensitic transformation of isolated nanocrystals in a hot liquid, where twinning plays a crucial role in accommodating the shape‐change‐induced strain. That such change was not observed during the reactions forming ZrO₂ and Zr‐rich Hf_xZr_{1 – x}O₂ nanocrystals is attributed to the higher driving force needed in those instances compared to that needed for producing HfO₂ and Hf‐rich Hf_xZr_{1 – x}O₂ nanocrystals. We also report here the post‐synthesis, heat‐induced phase transformation of Hf_xZr_{1 – x}O₂ (0 < x < 1) nanocrystals. As temperature increases, all the tetragonal nanocrystals transform to the monoclinic phase accompanied by an increase in particle size (as evidenced by X‐ray diffraction and transmission electron microscopy), which confirms that there is a critical size for the phase transformation to occur. When the monoclinic nanorods are heated above a certain temperature the grains grow considerably; under certain conditions a small amount of tetragonal phase appears.     

Synthesis of CeOx‐Decorated Pd/C Catalysts by Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

Due to the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline electrolytes, the development of more efficient HOR catalysts is essential for the next generation of anion‐exchange membrane fuel cells (AEMFCs). In this work, CeO_x is selectively deposited onto carbon‐supported Pd nanoparticles by controlled surface reactions, aiming to enhance the homogenous distribution of CeO_x and its preferential attachment to Pd nanoparticles, to achieve highly active CeO_x‐Pd/C catalysts. The catalysts are characterized by inductively coupled plasma–atomic emission spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy, scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, and X‐ray photoelectron spectroscopy to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation, and surface chemical states, respectively. The intimate contact between Pd and CeO_x is shown through high‐resolution STEM maps. The oxophilic nature of CeO_x and its effect on Pd are probed by CO stripping. The interfacial contact area between CeO_x and Pd nanoparticles is calculated for the first time and correlated to the electrochemical performance of the CeO_x‐Pd/C catalysts. Highest recorded HOR specific exchange current (51.5 mA mg^?1_Pd) and H₂–O₂ AEMFC performance (peak power density of 1,169 mW cm^?2 mg_Pd^?1) are obtained with a CeO_x‐Pd/C catalyst with Ce_0.38/Pd bulk atomic ratio.     

Bismuth–Ceramic Nanocomposites with Unusual Thermal Stability via High‐Energy Ball Milling

Electrically conducting nanocomposites of bismuth metal and insulating ceramic phases of SiO₂ and MgO were generated via high‐energy ball milling for 24 h using zirconia milling media. The resulting nanocomposites contain Bi nanoparticles with sizes down to 5 nm in diameter. The morphology is a strong function of the oxide phase: specifically, the Bi appears to wet MgO while it forms spherical nanoparticles on the SiO₂. X‐ray diffraction measurements indicate a nominal bismuth grain size of 50 nm, and peak fitting to a simple bidisperse model yields a mixture of approximately 57 % bulk bismuth and 43 % 27 nm diameter crystallites. Nanoparticles as small as 5 nm are observed in transmission electron microscopy (TEM), but may not constitute a significant volume fraction of the sample. Differential scanning calorimetry reveals dramatic broadening in the temperatures over which melting and freezing occur and a surprising persistence of nanostructure after thermal cycling above the melting point of the Bi phase.     

In Situ X‐Ray Diffraction Studies on Structural Changes of a P2 Layered Material during Electrochemical Desodiation/Sodiation

Sodium layered oxides with mixed transition metals have received significant attention as positive electrode candidates for sodium‐ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X‐ray absorption spectroscopy reveals the phase transition mechanism for the ternary transition metal system (Fe–Mn–Co) with P2 stacking. In situ synchrotron X‐ray diffraction using a capillary‐based microbattery cell shows a structural change from P2 to O2 in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ at the voltage plateau above 4.1 V on desodiation. The P2 structure is restored upon subsequent sodiation. The lattice parameter c in the O2 structure decreases significantly, resulting in a volumetric contraction of the lattice toward a fully charged state. Observations on the redox behavior of each transition metal in P2–Na_0.7Fe_0.4Mn_0.4Co_0.2O₂ using X‐ray absorption spectroscopy indicate that all transition metals are involved in the reduction/oxidation process.