Carbonization-Temperature-Dependent Electrical Properties of Carbon Nanofibers-From Nanoscale to Macroscale.

An exact understanding of the conductivity of individual fibers and their networks is crucial to tailor the overall macroscopic properties of polyacrylonitrile (PAN)-based carbon nanofibers (CNFs). Therefore, microelectrical properties of CNF networks and nanoelectrical properties of individual CNFs, carbonized at temperatures from 600 to 1000 °C, are studied by means of conductive atomic force microscopy (C-AFM). At the microscale, the CNF networks show good electrical interconnections enabling a homogeneously distributed current flow. The network's homogeneity is underlined by the strong correlation of macroscopic conductivities, determined by the four-point-method, and microscopic results. Both, microscopic and macroscopic electrical properties, solely depend on the carbonization temperature and the exact resulting fiber structure. Strikingly, nanoscale high-resolution current maps of individual CNFs reveal a large highly resistive surface fraction, representing a clear limitation. Highly resistive surface domains are either attributed to disordered highly resistive carbon structures at the surface or the absence of electron percolation paths in the bulk volume. With increased carbonization temperature, the conductive surface domains grow in size resulting in a higher conductivity. This work contributes to existing microstructural models of CNFs by extending them by electrical properties, especially electron percolation paths.

Effect of Low Environmental Pressure on Sintering Behavior of NASICON-Type Li1.3Al0.3Ti1.7(PO4)3 Solid Electrolytes: An In Situ ESEM Study.

Solid-state sintering at high temperatures is commonly used to densify solid electrolytes. Yet, optimizing phase purity, structure, and grain sizes of solid electrolytes is challenging due to the lack of understanding of relevant processes during sintering. Here, we use an in situ environmental scanning electron microscopy (ESEM) to monitor the sintering behavior of NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) at low environmental pressures. Our results show that while no major morphological changes are observed at 10-2 Pa and only coarsening is induced at 10 Pa, environmental pressures of 300 and 750 Pa lead to the formation of typically sintered LATP electrolytes. Furthermore, the use of pressure as an additional parameter in sintering allows the grain size and shape of electrolyte particles to be controlled.

Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li10 GeP2 S12.

The structural and morphological changes of the Lithium superionic conductor Li10 GeP2 S12 , prepared via a widely used ball milling-heating method over a comprehensive heat treatment range (50 - 700 °C), are investigated. Based on the phase composition, the formation process can be distinctly separated into four zones: Educt, Intermediary, Formation, and Decomposition zone. It is found that instead of Li4 GeS4 -Li3 PS4 binary crystallization process, diversified intermediate phases, including GeS2 in different space groups, multiphasic lithium phosphosulfides (Lix Py Sz ), and cubic Li7 Ge3 PS12 phase, are involved additionally during the formation and decomposition of Li10 GeP2 S12 . Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li10 GeP2 S12 is fully crystalline, while the sample decomposed to complex phases at 650 °C with 30 wt.% impurities, including 20 wt.% amorphous phases. These findings over such a wide temperature range are first reported and may help provide previously lacking insights into the formation and crystallinity control of Li10 GeP2 S12 .

All-Solid-State Garnet-Based Lithium Batteries at Work-In Operando TEM Investigations of Delithiation/Lithiation Process and Capacity Degradation Mechanism.

Li7 La3 Zr2 O12 (LLZO)-based all-solid-state Li batteries (SSLBs) are very attractive next-generation energy storage devices owing to their potential for achieving enhanced safety and improved energy density. However, the rigid nature of the ceramics challenges the SSLB fabrication and the afterward interfacial stability during electrochemical cycling. Here, a promising LLZO-based SSLB with a high areal capacity and stable cycle performance over 100 cycles is demonstrated. In operando transmission electron microscopy (TEM) is used for successfully demonstrating and investigating the delithiation/lithiation process and understanding the capacity degradation mechanism of the SSLB on an atomic scale. Other than the interfacial delamination between LLZO and LiCoO2 (LCO) owing to the stress evolvement during electrochemical cycling, oxygen deficiency of LCO not only causes microcrack formation in LCO but also partially decomposes LCO into metallic Co and is suggested to contribute to the capacity degradation based on the atomic-scale insights. When discharging the SSLB to a voltage of ≈1.2 versus Li/Li+ , severe capacity fading from the irreversible decomposition of LCO into metallic Co and Li2 O is observed under in operando TEM. These observations reveal the capacity degradation mechanisms of the LLZO-based SSLB, which provides important information for future LLZO-based SSLB developments.

Investigating the Interface between Ceramic Particles and Polymer Matrix in Hybrid Electrolytes by Electrochemical Strain Microscopy.

The interface between ceramic particles and a polymer matrix in a hybrid electrolyte is studied with high spatial resolution by means of Electrochemical Strain Microscopy (ESM), an Atomic Force Microscope (AFM)-based technique. The electrolyte consists of polyethylene oxide with lithium bis(trifluoromethanesulfonyl)imide (PEO6-LiTFSI) and Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta). The individual components are differentiated by their respective contact resonance, ESM amplitude and friction signals. The ESM signal shows increased amplitudes and higher contact resonance frequencies on the ceramic particles, while lower amplitudes and lower contact resonance frequencies are present on the bulk polymer phase. The amplitude distribution of the hybrid electrolyte shows a broader distribution in comparison to pure PEO6-LiTFSI. In the direct vicinity of the particles, an interfacial area with enhanced amplitude signals is found. These results are an important contribution to elucidate the influence of the ceramic-polymer interaction on the conductivity of hybrid electrolytes.

Signal Origin of Electrochemical Strain Microscopy and Link to Local Chemical Distribution in Solid State Electrolytes.

Electrochemical strain microscopy (ESM) is a distinguished method to characterize Li-ion mobility in energy materials with extremely high spatial resolution. The exact origin of the cantilever deflection when the technique is applied on solid state electrolytes (SSEs) is currently discussed in the literature. Understanding local properties and influences on ion mobility in SSEs is of utmost importance to improve such materials for next generation batteries. Here, the exact signal formation process of ESM when applied on sodium super ionic conductor (NASICON)-type SSE containing Na- and Li-ions is investigated. Changes in the dielectric properties, which are linked to the local chemical composition, are found to be responsible for the observed contrast in the deflection of the cantilever instead of a physical volume change as a result of Vegard´s Law. The cantilever response is strongly reduced in areas of high sodium content which is attributed to a reduction of the tip-sample capacitance in comparison to areas with high lithium content. This is the first time a direct link between electrostatic forces in contact mode and local chemical information is demonstrated on SSEs. The results open up new possibilities in information gain since dielectric properties are sensitive to subtle changes in local chemical composition.

Oxygen Nonstoichiometry and Valence State of Manganese in La1-x Ca x MnO3+δ.

Perovskites of the ABO3 type, such as LaMnO3, can be used as air electrodes in solid oxide fuel cells and electrolyzers. Their properties can be tuned by A- and B-site substitutions. The influence of La substitution by Ca on the oxygen nonstoichiometry has been investigated frequently, but the results depend highly on the synthesis and atmospheric conditions. In this work, a series of La1-x Ca x MnO3+δ (x = 0-0.5) was synthesized using conventional solid-state synthesis under an air atmosphere. The structures of the materials were studied in detail with powder X-ray diffraction. The initial oxygen nonstoichiometries were determined using thermogravimetric reduction. The samples were subsequently analyzed in terms of defect chemistry in dependence of temperature, atmosphere, and Ca content via thermogravimetric analysis. The changes in the manganese charge states were investigated by X-ray absorption near-edge spectroscopy experiments. The influence of intrinsic and extrinsic effects on the Mn-valence state of the differently Ca-substituted samples as calculated from thermogravimetric analysis and as determined directly from X-ray absorption near-edge spectroscopy is presented.

Microstructural details of spindle-like lithium titanium phosphate revealed in three dimensions.

Lithium titanium phosphate LiTi2(PO4)3 is an electrode material for lithium-ion batteries with a specific capacity of 138 mA h g-1. Owing to its potential of 2.5 V vs. Li/Li+ it provides an electrochemically stable interface when used as an anode in all-solid state batteries with NASICON type lithium aluminium titanium phosphate electrolyte. High performance has been identified for in situ carbon coated LiTi2(PO4)3 synthesized via a hydrothermal route, resulting in micro-scaled spindle shaped particles consisting of nano-scaled sub-particles. To elucidate the internal microstructure of these spindle-like particles in three dimensions we applied tomographic Focused Ion Beam - Scanning Electron Microscopy. For more detailed chemical analysis we performed electron-energy loss spectroscopy and energy dispersive X-ray spectroscopy in the scanning electron microscope as well as high resolution (scanning) transmission electron microscopy for structural insight. It could be clearly shown that the spindle-like particles mainly are made up of LiTi2(PO4)3 sub-particles in the 100 to 400 nm range. Additionally, two types of secondary phase materials were identified. LiTiOPO4, which shows different surface morphology, as a volume component of the spindles and TiO2 nanoparticles (anatase), which are not only present at the particle surface but also inside the spindle, were detected. Reconstruction from tomography reveals the nanoparticles form a three-dimensionally interconnected network even though their phase fraction is low.

Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15†nm): Synthesis and Thermal Conversion by In†Situ Transmission Electron Microscopy.

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1-2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

The carbonization of polyacrylonitrile-derived electrospun carbon nanofibers studied by in situ transmission electron microscopy.

Cathode structures derived from carbonized electrospun polyacrylonitrile (PAN) nanofibers are a current line of development for improvement of gas diffusion electrodes for metal-air batteries and fuel cells. Diameter, surface morphology, carbon structure and chemical composition of the carbon based fibers play a crucial role for the functionality of the resulting cathodes, especially with respect to oxygen adsorption properties, electrolyte wetting and electronic conductivity. These functionalities of the carbon fibers are strongly influenced by the carbonization process. Hitherto, fibers were mostly characterized by ex situ methods, which require great effort for statistical analysis in the case of microscopy. Here, we show the morphological and structural evolution of nanofibers during their carbonization at up to 1000 °C by in situ transmission electron microscopy (TEM). Changes in fiber diameter and surface morphology of individual nanofibers were observed at 250 °C, 600 °C, 800 °C and 1000 °C in imaging mode. The structural evolution was studied by concomitant high resolution TEM and electron diffraction. The results show with comparatively little effort shrinkage of the nanofiber diameter, roughening of the surface morphology and formation of turbostratic carbon with increasing carbonization temperature at identical locations.

Carbonisation temperature dependence of electrochemical activity of nitrogen-doped carbon fibres from electrospinning as air-cathodes for aqueous-alkaline metal-air batteries.

Poly-acrylonitrile (PAN)-derived carbon fibres were characterised as air electrode frameworks for aqueous-alkaline metal-air batteries, focussing on the influence of the carbonisation temperature on the structure and electrochemical properties. Elemental composition, (atomic) structure, electrical conductivity, and electrochemical performance related to the oxygen reduction were investigated for electrodes carbonised in the range from 300 °C to 1400 °C. Chemical and structural properties were analysed using elemental analysis, XPS, SEM, and Raman spectroscopy; electrical conductivities of the fibre networks were examined by four-point probe measurements. Electrochemical properties were evaluated using linear sweep voltammetry in 6 M KOH by the open circuit potentials, the cathodic current densities at given overpotentials, and required overpotentials at given current densities. The highest current density was obtained from fibres carbonised at 850 °C. The connection between the fibre characteristics and electrochemical properties are discussed, highlighting the importance of the nitrogen bonding state. The results provide a base for thedevelopment of high performance air electrodes.

Transformation of carbon-supported Pt-Ni octahedral electrocatalysts into cubes: toward stable electrocatalysis.

Octahedral Pt-Ni catalyst nanoparticles (NPs) are predicted to exhibit high activity for the oxygen reduction reaction. However, until now this class of catalysts has been limited by its long-term performance, as a result of compositional and morphological instabilities of the NPs. In situ transmission electron microscopy (TEM) is a powerful technique for understanding morphological and compositional evolution under controlled conditions. It is of great importance to study the evolution of the morphology and elemental distribution in bimetallic NPs and their interaction with the support in reducing and oxidizing treatments at the atomic scale for the rational design of catalysts. Here, we use in situ TEM to follow dynamic changes in the NP morphology, faceting and elemental segregation under working conditions in previously unreported Pt-Ni core-shell octahedral structures. We follow changes in the Pt-Ni catalyst from a segregated structure to an alloyed shell configuration and then a more spherical structure as a function of temperature under reducing conditions. Exposure to an oxidizing environment then leads to oxidation of the C support, while the spherical NPs undergo a cycle of transformations into cubic NPs followed by the reaction to spherical NPs. The formation of the cubic NPs results from CO formation during C oxidation, before it is finally oxidized to CO2. Our observations may pave the way towards the design of optimized structure-stability electrocatalysts and highlight the importance of TEM visualization of degradation and transformation pathways in bimetallic Pt-Ni NPs under reducing and oxidizing conditions.

Correlative electrochemical strain and scanning electron microscopy for local characterization of the solid state electrolyte Li1.3Al0.3Ti1.7(PO4)3.

Correlative microscopy has been used to investigate the relationship between Li-ion conductivity and the microstructure of lithium aluminum titanium phosphate (Li1.3Al0.3Ti1.7(PO4)3, LATP) with high spatial resolution. A key to improvement of solid state electrolytes such as LATP is a better understanding of interfacial and ion transport properties on relevant length scales in the nanometer to micrometer range. Using common techniques, such as electrochemical impedance spectroscopy, only global information can be obtained. In this work, we employ multiple microscopy techniques to gain local chemical and structural information paired with local insights into the Li-ion conductivity based on electrochemical strain microscopy (ESM). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) have been applied at identical regions to identify microstructural components such as an AlPO4 secondary phase. We found significantly lower Li-ion mobility in the secondary phase areas as well as at grain boundaries. Additionally, various aspects of signal formation obtained from ESM for solid state electrolytes are discussed. We demonstrate that correlative microscopy is an adjuvant tool to gain local insights into interfacial properties of energy materials.

Monolithic All-Phosphate Solid-State Lithium-Ion Battery with Improved Interfacial Compatibility.

High interfacial resistance between solid electrolyte and electrode of ceramic all-solid-state batteries is a major reason for the reduced performance of these batteries. A solid-state battery using a monolithic all-phosphate concept based on screen printed thick LiTi2(PO4)3 anode and Li3V2(PO4)3 cathode composite layers on a densely sintered Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte has been realized with competitive cycling performance. The choice of materials was primarily based on the (electro-)chemical and mechanical matching of the components instead of solely focusing on high-performance of individual components. Thus, the battery utilized a phosphate backbone in combination with tailored morphology of the electrode materials to ensure good interfacial matching for a durable mechanical stability. Moreover, the operating voltage range of the active materials matches with the intrinsic electrochemical window of the electrolyte which resulted in high electrochemical stability. A highly competitive discharge capacity of 63.5 mAh g-1 at 0.39 C after 500 cycles, corresponding to 84% of the initial discharge capacity, was achieved. The analysis of interfacial charge transfer kinetics confirmed the structural and electrical properties of the electrodes and their interfaces with the electrolyte, as evidenced by the excellent cycling performance of the all-phosphate solid-state battery. These interfaces have been studied via impedance analysis with subsequent distribution of relaxation times analysis. Moreover, the prepared solid-state battery could be processed and operated in air atmosphere owing to the low oxygen sensitivity of the phosphate materials. The analysis of electrolyte/electrode interfaces after cycling demonstrates that the interfaces remained stable during cycling.

Photogrammetry of the three-dimensional shape and texture of a nanoscale particle using scanning electron microscopy and free software.

We apply photogrammetry in a scanning electron microscope (SEM) to study the three-dimensional shape and surface texture of a nanoscale LiTi2(PO4)3 particle. We highlight the fact that the technique can be applied non-invasively in any SEM using free software (freeware) and does not require special sample preparation. Three-dimensional information is obtained in the form of a surface mesh, with the texture of the sample stored as a separate two-dimensional image (referred to as a UV Map). The mesh can be used to measure parameters such as surface area, volume, moment of inertia and center of mass, while the UV map can be used to study the surface texture using conventional image processing techniques. We also illustrate the use of 3D printing to visualize the reconstructed model.

Full solution processed mesostructured optical resonators integrating colloidal semiconductor quantum dots.

Herein we show a solution based synthetic pathway to obtain a resonant optical cavity with embedded colloidal semiconductor quantum dots (CSQDs). The optical cavity pore network, surrounded by two dense Bragg mirrors, was designed ad hoc to selectively host the quantum dots, while uncontrolled infiltration of those in the rest of the layered structure was prevented. Coupling between the optical resonant modes of the host and the natural emission of the embedded nanoparticles gives rise to the fine tuning of the luminescence spectrum extracted from the ensemble. Our approach overcomes, without the need for an encapsulating agent and exclusively by solution processing, the difficulties that arise from the low thermal and chemical stability of the CSQDs. It opens the route to achieving precise control over their location and hence over the spectral properties of light emitted by these widely employed nanomaterials. Furthermore, as the porosity of the cavity is preserved after infiltration, the system remains responsive to environmental changes, which provides an added value to the proposed structure.

On the origin of differential phase contrast at a locally charged and globally charge-compensated domain boundary in a polar-ordered material.

Differential phase contrast (DPC) imaging in the scanning transmission electron microscope is applied to the study of a charged antiphase domain boundary in doped bismuth ferrite. A clear differential signal is seen, which matches the expected direction of the electric field at the boundary. However, further study by scanned diffraction reveals that there is no measurable deflection of the primary diffraction disc and hence no significant free E-field in the material. Instead, the DPC signal arises from a modulation of the intensity profile within the primary diffraction disc in the vicinity of the boundary. Simulations are used to show that this modulation arises purely from the local change in crystallographic structure at the boundary and not from an electric field. This study highlights the care that is required when interpreting signals recorded from ferroelectric materials using both DPC imaging and other phase contrast techniques.

STEM-EELS analysis reveals stable high-density He in nanopores of amorphous silicon coatings deposited by magnetron sputtering.

A broad interest has been showed recently on the study of nanostructuring of thin films and surfaces obtained by low-energy He plasma treatments and He incorporation via magnetron sputtering. In this paper spatially resolved electron energy-loss spectroscopy in a scanning transmission electron microscope is used to locate and characterize the He state in nanoporous amorphous silicon coatings deposited by magnetron sputtering. A dedicated MATLAB program was developed to quantify the helium density inside individual pores based on the energy position shift or peak intensity of the He K-edge. A good agreement was observed between the high density (∼35-60 at nm(-3)) and pressure (0.3-1.0 GPa) values obtained in nanoscale analysis and the values derived from macroscopic measurements (the composition obtained by proton backscattering spectroscopy coupled to the macroscopic porosity estimated from ellipsometry). This work provides new insights into these novel porous coatings, providing evidence of high-density He located inside the pores and validating the methodology applied here to characterize the formation of pores filled with the helium process gas during deposition. A similar stabilization of condensed He bubbles has been previously demonstrated by high-energy He ion implantation in metals and is newly demonstrated here using a widely employed methodology, magnetron sputtering, for achieving coatings with a high density of homogeneously distributed pores and He storage capacities as high as 21 at%.

Analyzing the defect structure of CuO-doped PZT and KNN piezoelectrics from electron paramagnetic resonance.

The defect structure for copper-doped sodium potassium niobate (KNN) ferroelectrics has been analyzed with respect to its defect structure. In particular, the interplay between the mutually compensating dimeric (Cu(Nb)'''-V(O)··) and trimeric (V(O)··-Cu(Nb)'''-V(O)··)· defect complexes with 180° and non-180° domain walls has been analyzed and compared to the effects from (Cu'' - V(O)··)(x)× dipoles in CuO-doped lead zirconate titanate (PZT). Attempts are made to relate the rearrangement of defect complexes to macroscopic electromechanical properties.