I. Rom

Electron microscopical characterization of Sn/SnSb composite electrodes for lithium-ion batteries

Abstract Lithium storage alloys such as Sn/SnSb are promising new anode materials for Li-ion batteries. Due to a proper design of the active Sn/SnSb material as well as the composite electrode, capacities exceeding 500 mAh g −1 have been achieved with this system for more than 30 cycles. The observation of micro- and nano-structural changes in the composite electrode during charge/discharge cycling is of immense importance for a further improvement of the cycling performance. Electron microscopy (SEM and TEM) in combination with analytical techniques (EFTEM, EDXS and EELS) has been used for the characterization of Sn/SnSb raw powder as well as the Sn/SnSb composite electrodes. The pristine morphology and the changes of morphology during cycling of the electrode material have been studied. Furthermore, the chemical composition and particularly compositional fluctuations within the composite material have been investigated using EFTEM and EDXS. The electron microscopy results indicate that parts of the active material get finer during the initial cycles. Moreover, amorphous regions are detected in the cycled material. The experimental results are discussed with regard to the reaction mechanism of SnSb with Li.

Microstructure and ionic conductivity of strontium-substituted lanthanum cobaltites

Abstract La 0.4 Sr 0.6 CoO 3− δ powders were synthesized by the glycine nitrate process. X-ray powder diffraction and selected area electron diffraction (SAED) in the transmission electron microscope (TEM) were used to determine the basic crystal structure of the perovskite samples. Additionally, energy-filtering transmission electron microscopy (EFTEM) was used to evaluate the homogeneity of the samples at the nanometer level. The ionic conductivity σ i of La 0.4 Sr 0.6 CoO 3− δ was obtained from galvanostatic polarization experiments as a function of oxygen non-stoichiometry for 3− δ values between 2.70 and 2.81 (corresponding to p O 2 values between 10 −4 and 10 −1 bar) at 775 and 825 °C. It could be observed that σ i shows a maximum which shifts towards larger values of the oxygen non-stoichiometry with increasing temperature. The maximum value was observed at 3− δ =2.79 and 2.77 for 775 and 825 °C, respectively. This behavior has been reported to be indicative of the formation of vacancy-ordered structures which are expected to lower the mobility of oxygen vacancies. The TEM investigations revealed a superstructure within microdomains which were crystallographically oriented perpendicular to each other and were found to be around 100 nm in size.

Transport properties of La0.4Sr0.6CoO3−δ

La 0.4 Sr 0.6 CoO 3-δ was prepared by the glycine nitrate process and characterized in respect of purity and phase composition by powder X-ray diffraction and analytical transmission electron microscopy. The oxygen-nonstoichiometry, as a function of oxygen partial pressure (10 -4 bar <P O2 < 10 2 bar) and temperature (300°C < T < 900°C) as well as the chemical diffusion coefficient (725°C and 825°C), were determined by means of oxygen exchange measurements. Electronic conductivity as measured with the van der Pauw-technique is given as a function of temperature (between 300°C and 900°C) and oxygen partial pressure. With the obtained values for the oxygen-nonstoichiometry of La 0.4 Sr 0.6 CoO 3-δ , it is possible to take into account the change of the oxygen content of the sample with temperature and to calculate isostoichiometric (δ = constant) electronic conductivities. Additionally, chemical diffusion and surface exchange coefficients could be obtained from oxygen exchange measurements using simplified and exact fitting procedures.

Conductivity relaxation experiments on Ni1−δO

The chemical diffusion coefficient D of polycrystalline nickel oxide was studied as a function of oxygen activity (10 5 atm < p O2 < 1 atm) and temperature (700°C < T < 1200°C) employing electronic conductivity relaxation experiments in the van der Pauw configuration. With this technique the sample is equilibrated under a given oxygen pressure. The oxygen content of the atmosphere surrounding the sample tablet is then changed stepwise and re-equilibration of the sample is followed by monitoring the conductivity relaxation. The chemical diffusion coefficient is obtained from the time-dependent variation of the electronic conductivity. The temperature dependence of D between 700 and 1200°C can be described by D(cm 2 s -1 ) = 2.40 × 10 -3 exp(-0.70±0.05 eV/kT). The chemical diffusion coefficient is found to be almost independent of the oxygen partial pressure and thus of oxide composition. The electronic conductivity at 1000°C shows a 1/6 dependence on the oxygen partial pressure, indicating the presence of doubly ionized nickel vacancies and electron holes as predominant defects. The D values are compared with literature results on single crystals.

Composition dependence of chemical diffusion coefficient and ionic conductivity of α′- and α-Ag2Te

Abstract The chemical diffusion coefficient and the ionic conductivity of Ag2Te have been measured simultaneously with high stoichiometric resolution between 160 and 300°C as a function of composition by a recently developed polarization technique. The composition of the system was varied by coulometric titration employing AgI as solid ionic conductor. The results indicate the existence of two structurally cationic-disordered, mixed conducting phases of α-Ag2Te with narrow homogeneity ranges and high ionic conductivities. Between 160 and 220°C the chemical diffusion coefficient of α′-Ag2Te shows a distinct maximum at the inflexion point of the coulometric titration curve with values between 0.02 and 0.35 cm2s−1. At temperatures between 260 and 350°C a monotonically decreasing function of the chemical diffusion coefficient could be observed for α-Ag2Te. For both phases, the ionic conductivity was found to be almost independent of the silver activity showing rather high values between 1.0 and 1.6 S cm−1.

Composition dependent ionic and electronic conductivities and chemical diffusion coefficient of silver selenide at 160°C

Abstract The transport properties of α-Ag 2 Se at 160°C as a function of nonstoichiometry have been measured in the whole range of homogeneity using a symmetric solid state electrochemical cell with ionic and electronic probes as well as ionic and electronic electrodes. Chemical diffusion coefficients as well as ionic and electronic conductivities were obtained from polarization experiments with high stoichiometric resolution. The composition of the sample was varied in situ by means of the coulometric titration technique. The chemical diffusion coefficient of α-Ag 2 Se shows a distinct maximum in the vicinity of the stoichiometric point with values between 0.04 and 0.12 cm 2 s −1 . The ionic conductivity of α-Ag 2 Se is constant within the whole range of homogeneity with values around 2.2 S cm −1 , whereas the electronic conductivity increases almost linearly from about 600 to 2600 S cm −1 with increasing silver activity.

Formation of interfacial nano-layers in the system Pt-Ni-O

Abstract Solid state electrochemical cells have been applied to oxidize Pt–Ni alloys and NiO in contact with Pt. The resulting interfaces have been analyzed by analytical transmission electron microscopy. Highly oxidized phases and complex morphologies have been identified on a nanometer scale in both cases. These phenomena are discussed in the context of solid state pattern formation and characteristic reactions of the system Pt–Ni–O, such as alloy oxidation and reactions between NiO and Pt. Reaction and diffusion mechanisms, which can explain the formation of the complex microstructures, are introduced for each case.