Electron Energy Loss Spectroscopy Techniques Research Articles

Momentum-resolved EELS and CL study on 1D-plasmonic crystal prepared by FIB method.

We investigate a one-dimensional plasmonic crystal (1D PlC) using momentum-resolved electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) techniques, which are complementary in terms of available optical information. The PlC sample is fabricated from large aluminum grains through the focused ion beam (FIB) method. This approach allows curving nanostructures with high crystallinity, providing platforms for detailed analysis of plasmonic nanostructures using both EELS and CL. The momentum-resolved EELS visualizes dispersion curves outside the light cone, confirming the existence of the surface plasmon polaritons (SPP) and local modes, while the momentum-resolved CL mapping analysis identified these SPP and local modes. Such synergetic approach of two electron-beam techniques offers full insights into both radiative and non-radiative optical properties in plasmonic or photonic structures.

Nanoscale mapping of residual stresses in Al 2024 alloys using correlative and multimodal scanning transmission electron microscopy

A methodology for the mapping of residual stresses in metal alloys has been developed by analyzing an isotropic and homogeneous Al2024 alloy with scanning transmission electron microscopy (STEM), combined with diffraction (4DSTEM) and electron energy loss spectroscopy (STEM-EELS) techniques of TEM. The investigations on the alloy's microstructure and elemental distributions were also carried out with conventional dark-field STEM (DFSTEM) and X-ray energy dispersive (EDS) techniques, respectively. Using the STEM-EELS technique, the Young's modulus (YM) is mapped in the (001) plane of the Al alloy in the same regions where the residual strain maps are generated in [1‾ 00] and [010] directions by using 4DSTEM technique. The YM vs. residual strain plot for the Al 2024 alloy revealed that the value of YM decreased by about ∼ 7 % after the tensile residual strain reached 0.02 %. Whereas such a decrease in YM happens after the compressively residual strain reaches −0.015 %. The residual stress maps were also obtained in accordance with the Hooke's law i.e., by multiplying YM map with the corresponding residual strain maps.

Structural properties, bandgap, and complex dielectric function in Bi2Te3 thermoelectric by Valence Electron Energy Loss Spectroscopy (VEELS) analysis

This article describes the results and discussion performed to analyze the bandgap, optoelectronic properties, and complex dielectric function of Bi2Te3 thermoelectric material, using Valence Electron Energy Loss Spectroscopy. A key aspect of the report presented in this article is that the properties described above have not been analyzed and reported using the Valence Electron Energy Loss Spectroscopy technique. The material’s structure was analyzed using Rietveld and High-Resolution Transmission Electron Microscopy techniques. The band gap obtained value is 0.91 eV by Valence Electron Energy Loss Spectroscopy. Whereas, values obtained by other techniques are between 0.26 and 0.92 eV for Bi2Te3. The real part (ε1) and imaginary part (ε2) of the complex dielectric function (ε * ) were measured. For ε1, the principal peak at 1.03 eV and a second peak at 1.4 eV were observed. Furthermore, the optical dielectric constant is 8.6 for (ε0 = 0 eV), which is related to electronic contributions and is given by the refractive index squared (n2).

Structure of polymeric nanoparticles encapsulating a drug – pamoic acid ion pair by scanning transmission electron microscopy

Drug-delivery systems based on polymeric nanoparticles are useful for improving drug bioavailability and/or delivery of the active ingredient for example directly to the cancerous tumour. The physical and chemical characterization of a functionalized nanoparticle system is required to measure drug loading and dispersion but also to understand and model the rate and extent of drug release to help predict performance. Many techniques can be used, however, difficulties related to structure determination and identifying the precise location of the drug fraction make mathematical prediction complex and in many published examples the final conclusions are based on assumptions regarding an expected structure. Cryogenic scanning transmission electron microscopy imaging in combination with electron energy loss spectroscopy techniques are used here to address this issue and provide a multi-modal approach to the characterisation of a self-assembled polymeric nanoparticle system based upon a polylactic acid - polyethylene glycol (PLA-PEG) block copolymer containing a hydrophobic ion-pair between pamoic acid and an active pharmaceutical ingredient (API). Results indicate a regular dispersion of spherical nanoparticles of 88 ± 9 nm diameter. The particles are shown to have a multi-layer structure consisting of a 25 nm radius hydrophobic core of PLA and pamoic acid-API material with additional enrichment of the pamoic acid-API material within the inner core (that can be off-centre), surrounded by a 9 nm dense PLA-PEG layer all with a low-density PEG surface coating of around 10 nm thickness. This structure suggests that release of the API can only occur by diffusion through or degradation of the dense, 9 nm thick PLA-PEG layer either of which is a process consistent with the previously reported steady release kinetics of the API and counter ion from these nanoparticle formulations. Establishing accurate measures of product structure enables a link to performance by providing appropriate physical parameters for future mathematical modelling of barriers controlling API release in these nanoparticle formulations.

Observation of Ultrathin Sc-Containing Surface Layer on Life-Tested Scandate Cathodes

To explain enhanced levels of electron emission from scandate cathodes, models invoking surface layers ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 100 nm thick) of Ba-Sc-O, or alternatively a Ba-Sc-O monolayer, have been frequently employed to describe the lowered work function that is thought to lead to these improved emission capabilities. However, limited direct experimental proof has been provided, primarily involving Auger electron spectroscopy (AES) for depth profile analyses of elemental composition. In this article, we report direct observations of an Sc-containing surface layer on scandate cathodes, as indicated by independent, complementary measurements using scanning nanobeam AES and electron energy loss spectroscopy (EELS) techniques. The ultrathin Sc-containing layer on these cathodes is no more than 3–4 nm thick and is likely even thinner than this. The surface layer may consist of amorphous or nanoscale polycrystalline materials, as suggested by scanning nanobeam electron diffraction and analytical mapping.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Strain-induced Mn valence state variation in CaMnO3-δ/substrate interfaces: electronic reconstruction versus oxygen vacancies.

This study investigates the nanoscale crystalline and electronic structures of the interfaces between CaMnO3-δ and substrates such as SrTiO3 (001) and LaAlO3 (001) by employing advanced transmission electron microscopy and electron energy loss spectroscopy techniques. The objective is to comprehend the influence of different strains on the Mn valence state. Our findings reveal that the Mn valence state remains relatively stable in the region of a weakly tensile-strained interface, whereas it experiences a significant decrease from Mn4+ to Mn2.3+ in the region of a strongly tensile-strained interface. Although this reduction in valence appears to be consistent with the electron reconstruction scenario, the observed increase in the out-of-plane lattice constant at the interface implies the accumulation of oxygen vacancies at the interface. Consequently, the present study offers a comprehensive understanding of the intricate relationships among the Mn valence state, local structure, and formation of oxygen vacancies in the context of two distinct strain cases. This knowledge is essential for tailoring the interface properties and guiding future developments in the field of oxide heterostructures.

Correlating Structural Properties with Electrochemical Behavior of Non-graphitizable Carbons in Na-Ion Batteries.

We report on a detailed structural versus electrochemical property investigation of the corncob-derived non-graphitizable carbons prepared at different carbonization temperatures using a combination of structural characterization methodology unique to this field. Non-graphitizable carbons are currently the most viable option for the negative electrode in sodium-ion batteries. However, many challenges arise from the strong dependence of the precursor’s choice and carbonization parameters on the evolution of the carbon matrix and its resulting electrochemistry. We followed structure development upon the increase in carbonization temperature with thorough structural characterization and electrochemical testing. With the increase of carbonization temperature from 900 to 1600 °C, our prepared materials exhibited a trend toward increasing structural order, an increase in the specific surface area of micropores, the development of ultramicroporosity, and an increase in conductivity. This was clearly demonstrated by a synergy of small- and wide-angle X-ray scattering, scanning transmission electron microscopy, and electron-energy loss spectroscopy techniques. Three-electrode full cell measurements confirmed incomplete desodiation of Na+ ions from the non-graphitizable carbons in the first cycle due to the formation of a solid–electrolyte interface and Na trapping in the pores, followed by a stable second cycle. The study of cycling stability over 100 cycles in a half-cell configuration confirmed the observed high irreversible capacity in the first cycle, which stabilized to a slow decrease afterward, with the Coulombic efficiency reaching 99% after 30 cycles and then stabilizing between 99.3 and 99.5%. Subsequently, a strong correlation between the determined structural properties and the electrochemical behavior was established.

Several factors influencing energy-loss near-edge structure calculations using Wien2k.

Electron energy-loss spectroscopy (EELS) is widely applied combining with transmission electron microscopes with high spatial resolution, but its interpretation is a challenging task. One of the reasons is that the factors affecting EELS are very complicated. In this paper, we focus on the several factors involved in density functional theory (DFT) calculations. The sensitivity of calculated energy-loss near-edge structure (ELNES) to spin order, pressure and on-site Coulomb energy U has been discussed. Since EELS technique detects the local environment of atoms, the influence of spin order cannot be ignored. The chemical shifts and peak intensity of ELNES are also closely related to corresponding pressure. The correlation effects are very important for transition metal compounds and play a key role in EELS simulations. An overview of the effects of these factors on the ELNES is presented with the help of Wien2k code. The antiferromagnetic order results in the decreasing of intensities of related peaks and the moving of the peaks to high energy loss. The decreasing of lattice parameters causes the ELNES peaks to shift to high energy loss, and the peak shifts at the higher energy loss are more significant. The increase of correlation effect leads to the ELNES peaks to shift to high energy loss accompanied by the increase of the relative intensity of the peaks which locate at higher energy loss. Our work helps to understand how these factors affect EELS and to explain and predict the experimental EELS spectra. Through the discussion of these factors, we propose that some factors could not be ignored in EELS simulations.

Inorganic phosphate in growing calcium carbonate abalone shell suggests a shared mineral ancestral precursor

The presence of phosphate from different origins (inorganic, bioorganic) is found more and more in calcium carbonate-based biominerals. Phosphate is often described as being responsible for the stabilization of the transient amorphous calcium carbonate phase. In order to specify the composition of the mineral phase deposited at the onset of carbonated shell formation, the present study investigates, down to the nanoscale, the growing shell from the European abalone Haliotis tuberculata, using a combination of solid state nuclear magnetic resonance, scanning transmission electron microscope and spatially-resolved electron energy loss spectroscopy techniques. We show the co-occurrence of inorganic phosphate with calcium and carbonate throughout the early stages of abalone shell formation. One possible hypothesis is that this first-formed mixed mineral phase represents the vestige of a shared ancestral mineral precursor that appeared early during Evolution. In addition, our findings strengthen the idea that the final crystalline phase (calcium carbonate or phosphate) depends strongly on the nature of the mineral-associated proteins in vivo.

Measuring phonon dispersion at an interface.

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

Advanced Electron Energy Loss Spectroscopy for Battery Studies

AbstractAs a powerful tool for chemical compositional analyses, electron energy loss spectroscopy (EELS) can reveal an abundance of information regarding the atomic‐level electron state in a variety of materials, including the elemental types as well as their valence and concentration distributions, and the structure‐related atom radial distribution. Benefiting from its unique capabilities and the newly developed advanced transmission electron microscope (TEM) configurations (i.e., in situ bias, in situ heating, cryo‐TEM, etc.), EELS has facilitated the battery studies in various aspects. Here, a brief introduction of EELS is provided. Thereafter, a description of the recent progress in studying battery materials using EELS is provided, and finally a look ahead on the future development of EELS techniques and their applications in battery studies is provided.

Three-Dimensional Insights into Interfacial Segregation at the Atomic Scale in a Nanocrystalline Glass-Ceramic.

The distribution of dopant atoms plays a key role in the effectiveness of doping, thereby requiring delicate characterizations. In this study, we found that energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) techniques in scanning transmission electron microscopy (STEM) were not adequate to reveal the distribution of yttrium and the chemical composition of the ZrO2/SiO2 heterophase interface in an yttrium-doped ZrO2-SiO2 nanocrystalline glass-ceramic. Atom probe tomography (APT) is rarely utilized to characterize ceramics due to some inherent difficulties. However, we successfully revealed the three-dimensional distribution of ZrO2 nanocrystallites and SiO2 matrix at the atomic scale with APT under optimized and well-controlled conditions. We also found that the ZrO2 nanocrystallites had a special core-shell structure, with a thin Zr/Si interfacial layer as a shell and a ZrO2 solid solution as a core. Yttrium dopants showed interfacial segregation at both ZrO2 grain boundaries and the ZrO2/SiO2 heterophase interfaces.

Electronic and optical properties of ZnSe by theoretical simulation TB-mBJ (Tran-Blaha modified Becke Johnson) associated to analysis techniques XPS (X-Ray Photoelectron Spectroscopy); REELS (Reflective Electron Energy Loss Spectroscopy) and PLS (Photoluminescence Spectroscopy)

Our work aims to establish a correspondence between the obtained theoretical and experimental results on the electronic and optical properties of ZnSe.The calculated DOS (Density of States) using the FP-LAPW method with the Tran-Blaha modified Becke-Johnson exchange-correlation potential reveal that the upper valence energy levels involve an important contribution from the Se-(4p and 3d) electron states with a small contribution from the Zn-2s states.The real (ε1(ω)) and imaginary (ε2(ω)) parts of the dielectric function show the anisotropy character of w-ZnSe.The main peaks appearing in the ε2(ω) spectrum, are calculated by summing over all contributions of electronic transitions between the pairs of bands Vn→Cn′, where Vn is an occupied valence band and Cn′ is an unoccupied conduction band. Experimentally, the XPS (X-Ray Photoelectron Spectroscopy) analysis technique is used to show the clean and homogeneous state of ZnSe. To confirm the calculated electron transitions, we use the REELS (Reflected Electron Energy Loss Spectroscopy) technique of great sensitivity for displaying the electron interband transitions. The electron states in the w-ZnSe gap are determined through the Gaussian deconvolution of the photoluminescence spectrum (PLS).

Current Density Induced Microstructure Evolution on Li Dendrite and Solid Electrolyte Interphase Revealed By Cryogenic Transmission Electron Microscopy

The ultralow negative electrochemical potential of -3.040 V versus standard hydrogen electrode, extremely high theoretical specific capacity of 3860 mAh g−1, and low gravimetric density of 0.534 g cm−3 make lithium (Li) metal has re-emerged as an ideal choice as anode material. However, the aggressive dendritic growth of Li metal during cycling makes it too dangerous for practical use. To address this critical issue, we need not only understanding on the macroscopic but also on the microscopic, nanoscopic and atomistic features to probe more fundamental aspects of dendritic growth process. [1-4] Herein, by applying high resolution transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) techniques under cryogenic (cryo) conditions, [5-8] we characterize the detailed structure of electrochemically deposited Li metal (EDLi) and its solid electrolyte interphase (SEI) layers with systematic control of current density, to establish the correlation between electrochemical performance (interfacial impedance) and current density induced structure and chemical evolution.We discover that increasing current density leads to increased overpotential for Li nucleation and growth, leading to the transition from growth-limited to nucleation-limited mode for Li dendrite. Independence of current density, the EDLi exhibits crystalline whisker-like morphology. The SEI formed at low current density (0.1 mA cm-2) is monolithic amorphous; while, a current density of above 2 mA cm-2 leads to a mosaic structured SEI, featuring an amorphous matrix with Li2O and LiF dispersoids, and the thickness of the SEI increases with the increase of current density. Specifically, the Li2O particles is spatially located at the top surface of the SEI, while LiF is spatially adjacent to the Li-SEI interface. [9] These results offer possible ways of regulating crucial microstructural and chemical features of EDLi and SEI through altering deposit conditions and consequently direct correlation with battery performance.

Surface, interface and electronic studies on anthracene derived polymeric thin films for OLED applications

We have investigated semiconducting aromatic aliphatic polyethers based on either distyrylanthracene (Polymer A) or distyrylanthracene, distyrylbenzothiadiazole and distyrylcarbazole (Terpolymer B) chromophores separated by aliphatic undecane spacers that were deposited as thin films onto PEDOT:PSS by X-ray Photoelectron spectroscopy (XPS), depth-profiling XPS, and Reflection Electron Energy Loss Spectroscopy (REELS) techniques. XPS measurements demonstrated the quality of the prepared films. They also showed that for the Terpolymer B sample there is an excess of PSS dopant in the film surface, which was further corroborated through argon ion sputtering process coupled to the XPS acquisition. N1s XPS results suggest that sulphonic groups present in the PSS molecule might interfere in the Terpolymer B backbone by protonating their nitrogen atoms. From REELS data, π − plasmon peaks were derived for Polymer A and Terpolymer B thin films. In addition, Spectroscopic Ellipsometry and Photoluminescence Spectroscopy were applied for the evaluation and the determination of the optical properties as well as the thickness of the photoactive polymeric films and the emission characteristics, respectively. Semiconducting aromatic-aliphatic polyethers based on anthracene were studied by spectroscopic techniques. Spectroscopic Ellipsometry and Photoluminescence Spectroscopy were applied for the evaluation and the determination of the optical properties. Information on film's electronic, interface and optical properties and electronic transitions were obtained.

Four-dimensional vibrational spectroscopy for nanoscale mapping of phonon dispersion in BN nanotubes

Directly mapping local phonon dispersion in individual nanostructures can advance our understanding of their thermal, optical, and mechanical properties. However, this requires high detection sensitivity and combined spatial, energy and momentum resolutions, thus has been elusive. Here, we demonstrate a four-dimensional electron energy loss spectroscopy technique, and present position-dependent phonon dispersion measurements in individual boron nitride nanotubes. By scanning the electron beam in real space while monitoring both the energy loss and the momentum transfer, we are able to reveal position- and momentum-dependent lattice vibrations at nanometer scale. Our measurements show that the phonon dispersion of multi-walled nanotubes is locally close to hexagonal-boron nitride crystals. Interestingly, acoustic phonons are sensitive to defect scattering, while optical modes are insensitive to small voids. This work not only provides insights into vibrational properties of boron nitride nanotubes, but also demonstrates potential of the developed technique in nanoscale phonon dispersion measurements.

Investigation on the interaction between a gallium nitride surface and H2O using a nanometer-scale GaN lamella structure

Gallium nitride (GaN) is exceedingly apposite for liquid-based sensor applications because of their high internal piezoelectric polarization, chemical and high temperature stability. In this work, the interaction between GaN and H2O has been investigated using a novel methodology. We report the fabrication of single crystal GaN lamella with thickness of few hundreds of nanometer using focused ion-beam milling technique, for sensing applications. Results signify that the device resistivity increases with time at room temperature during the GaN-H2O interaction. Such a change in electrical resistivity is explained based on the electron transfer and electrochemical reactions at the surface of GaN. Study of the surface chemistry transformation of the tested GaN lamella is conducted using high-angle annular dark-field scanning transmission electron microscopy coupled with electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDS) techniques. EDS and EELS results signify the presence of a region containing Ga and O at the interface of the H2O/GaN which is a result due to the adsorption of molecular H2O and its dissociation products implying the occurrence of GaN-water reaction.

A new technical approach for preparing frozen biological samples for electron microscopy

BackgroundMany methodological approaches have focused so far on physiological and molecular responses of plant tissues to freezing but only little knowledge is available on the consequences of extracellular ice-formation on cellular ultrastructure that underlies physiological reactions. In this context, the preservation of a defined frozen state during the entire fixation procedure is an essential prerequisite. However, current techniques are not able to fix frozen plant tissues for transmission electron microscopy (TEM) without interrupting the cold chain. Chemical fixation by glutaraldehyde and osmium tetroxide is not possible at sub-zero temperatures. Cryo-fixation methods, such as high pressure freeze fixation (HPF) representing the state-of-the-art technique for best structural preservation, are not equipped for freezing frozen samples. In order to overcome this obstacle, a novel technical approach for maintaining the cold chain of already frozen plant samples prior and during HPF is presented.ResultsDifferent algae (Micrasterias denticulata, Klebsormidium crenulatum) and higher plant tissues (Lemna sp., Ranunculus glacialis, Pinus mugo) were successfully frozen and prepared for HPF at freezing temperatures (− 2 °C, − 5 °C, − 6 °C) within a newly developed automatic freezing unit (AFU), that we manufactured from a standard laboratory freezer. Preceding tests on photosynthetic electron transport and ability to plasmolyse show that the temperatures applied did not impair electron transport in PSII nor cell vitality. The transfer of the frozen specimen from the AFU into the HPF-device and subsequently cryo-fixation were performed without intermediate thawing. After cryo-substitution and further processing, the resulting TEM-micrographs showed excellent ultrastructure preservation of the different organisms when compared to specimens fixed at ambient temperature.ConclusionsThe method presented allows preserving the ultrastructure of plant cells in the frozen state during cryo-fixation. The resulting high quality TEM-images represent an important step towards a better understanding of the consequences of extracellular ice formation on cellular ultrastructure. It has the potential to provide new insights into changes of organelle structure, identification of intracellular injuries during ice formation and may help to understand freezing and thawing processes in plant tissues. It may be combined with analytical TEM such as electron energy loss spectroscopy (EELS), X-ray analyses (EDX) and various other electron microscopic techniques.

Insights into the Enhanced Cycle and Rate Performances of the F‐Substituted P2‐Type Oxide Cathodes for Sodium‐Ion Batteries

AbstractA series of F‐substituted Na2/3Ni1/3Mn2/3O2−xFx (x = 0, 0.03, 0.05, 0.07) cathode materials have been synthesized and characterized by solid‐state 19F and 23Na NMR, X‐ray photoelectron spectroscopy, and neutron diffraction. The underlying charge compensation mechanism is systematically unraveled by X‐ray absorption spectroscopy and electron energy loss spectroscopy (EELS) techniques, revealing partial reduction from Mn4+ to Mn3+ upon F‐substitution. It is revealed that not only Ni but also Mn participates in the redox reaction process, which is confirmed for the first time by EELS techniques, contributing to an increase in discharge specific capacity. The detailed structural transformations are also revealed by operando X‐ray diffraction experiments during the intercalation and deintercalation process of Na+, demonstrating that the biphasic reaction is obviously suppressed in the low voltage region via F‐substitution. Hence, the optimized sample with 0.05 mol f.u.−1 fluorine substitution delivers an ultrahigh specific capacity of 61 mAh g−1 at 10 C after 2000 cycles at 30 °C, an extraordinary cycling stability with a capacity retention of 75.6% after 2000 cycles at 10 C and 55 °C, an outstanding full battery performance with 89.5% capacity retention after 300 cycles at 1 C. This research provides a crucial understanding of the influence of F‐substitution on the crystal structure of the P2‐type materials and opens a new avenue for sodium‐ion batteries.