Characterization of degeneration phenomena in lithium-ion batteries by combined microscopic techniques.

The application of detector strategies in scanning electron microscopy (SEM) correlated with computer tomography (CT) and light microscopy (LM) delivered unique new insights in degeneration effects of lithium-ion batteries. There we exemplary studied reference, cycled and storage cells. High-resolution SEM permit to visualize a coating on top of the cathode material of the treated cells for the first time, which also connects the conductive additives and battery active material. This confirms the assumption of a solid permeable interface on top of the cathode. The detection of low-loss reflected backscattered electrons for energies beyond 3 keV increases the available spatial resolution for material contrast. This offered the opportunity to address the atomic number of precipitates in the nm range inside the coating to be above carbon and below Li1-x(Ni1/3Mn1/3Co1/3)O2 (NMC). Applying voltage contrast enables to show the difference in electronic conductivity of plate-like features on top of the cycled cell anode, most likely lithium plating. Cross sectional images of the anode delivered a significant change of the surficial-area morphology for the treated cells with increasing porosity. Precipitates were detected on top of the separator foil. An increment in thickness of the entire treated cells by computer tomography was found, which can be explained by the alteration of the anode, separator and cathode.

Insights into the Impact of Impurities and Non-Stoichiometric Effects on the Electrochemical Performance of Li2 MnSiO4.

A series of Li2 MnSiO4 samples with various Li, Mn, and/or Si concentrations are reported to study for the first time the effect of impurities and deviation from ideal stoichiometry on electrochemical behavior. Carbon-coated and nanosized powders are obtained at 600 °C and compared with those synthetized at 900 °C. Samples are investigated using XRD, SEM, high-resolution TEM, attenuated total reflection infrared spectroscopy and Brunauer-Emmett-Teller surface area to characterize crystal structure, particle size, impurity amount, morphology, and surface area. Electrochemical performance depends on impurities such as MnO as well as crystallite size, surface area, and non-stoichiometric phases, which lead to the formation of additional polymorphs such as Pmnb and P21 /n of Li2 MnSiO4 at low calcination temperatures. A systematic analysis of the main parameters affecting the electrochemical behavior is performed and trends in synthesis are identified. The findings can be applied to optimize different synthesis routes for attaining stoichiometric and phase-pure Pmn21 Li2 MnSiO4 as cathode material for Li-ion batteries.

Determination of the oxidation state for iron oxide minerals by energy-filtering TEM.

The oxidation state of iron oxide nanoparticles was determined using the two principally different technical realisations of energy filtering TEM, in one case using the JEOL 3010 equipped with a LaB6 cathode and a post-column GIF and in the second, the newly designed LIBRA 200FE equipped with an corrected in-column 90 degrees energy filter and a field emission gun (Schottky emitter). The samples studied were oxide-coated iron nanoparticles, and iron oxide inclusions in feldspars in granites. Five possible candidates exist for the iron-oxide phases: FeO, alpha-Fe2O3 (hematite), gamma-Fe2O3 (maghemite), Fe3O4 (magnetite) or alpha-FeO(OH) (goethite). Fingerprinting the O K-edge ELNES allows to distinguish between oxide phases with the same stochiometry and enables to make a first selection of possible candidates. The additional determination of the chemical composition allows unique identification of the phase present. For the oxide coated iron nanoparticles the most probable iron oxide phase of the shell is maghemite, which was additionally confirmed by HRTEM studies. The second studied system were iron oxide needles in alkali feldspar, where we obtained hematite as the most probable phase. There we additionally demonstrated the drastic changes of the ELNES of the O K-edge for the alkali feldspar and iron oxide needle by spatially resolved EELS.

Laterally resolved EELS for ELNES mapping of the Fe L 2,3 - and O K-edge.

Nowadays fingerprinting techniques are well established for phase analysis. One of the common problems is the accurate calibration of the energy scale to compare the electron energy loss (ELNES) and to determine the energy shift precisely. One solution to this problem is laterally resolved electron energy loss spectroscopy (EELS), which involves orienting the specimen area or structure of interest, parallel to the energy dispersive direction and dispersing the intensity across the interface as a function of energy. This ELNES information can now be used to quantify and map changes in the electronic environment. The most critical instrumental performance for ELNES investigations is the available energy resolution, which for our instrument was estimated using the 0.5eV splitting of the Mn L(3)-edge of the mineral bixbyite. An ideal test sample for the ELNES investigations is a titanohematite, a solid solution between ilmenite (FeTiO(3)), with Fe in a divalent oxidation state, and hematite (Fe(2)O(3)) with Fe in a trivalent oxidation state. Using energy filtered imaging with a slit width of 4eV it is possible to map the Fe(2+)/Fe(3+) ratio as well as the near-edge structure of the O(K) signal and correlate these ELNES maps with a spatial resolution of a few nanometres. Quantitative compositional mapping on a nanometre scale was obtained by electron spectroscopic imaging. Quantitative point analyses also yield the chemical composition and the valence states. The precise knowledge of the energy shift and near edge structure enables us to select the characteristic ELNES structure and calculate jump ratio images. This yields quantitative valence state maps by using the Fe L(2,3)-edge, as well as phase maps by using the O K-edge.