Jan-Christoph Panitz

The complex electrochemistry of graphite electrodes in lithium-ion batteries

This paper discusses the interrelated phenomena of solid electrolyte interphase (SEI) formation and the irreversible charge consumption which occurs during the first cycle of a graphite electrode, as well as their relevance to the cycling stability of lithium-ion batteries. Thus, results from relevant characterization methods, namely, in situ mass spectrometry, in situ infrared spectroscopy, in situ Raman and video microscopy, in situ scanning probe microscopy, in situ quartz crystal microbalance, and differential scanning calorimetry were combined for a more thorough understanding of observations made in cycling experiments. From electrochemical cycling tests, we have learned that a high specific charge (∼360 Ah/kg of carbon), satisfactory cycle life of the graphite electrodes (1000 deep cycles), and an irreversible charge of <7% during SEI formation can only be obtained when water contamination of the cell is avoided. Under such conditions, a good-quality SEI film is formed on the carbon surface. We conclude that during SEI film formation, at first the carbonate solvent(s) are reduced, forming ethylene gas, organic radicals, oligomers, and polymers. Then a SEI film is precipitated on the surface via a nucleation and growth mechanism. The irreversible charge consumption due to SEI formation is proportional to the BET specific surface area of the graphite and rapidly increases with increasing water content in the cell.

In situ investigation of the interaction between graphite and electrolyte solutions

The formation of a solid electrolyte interphase (SEI) before and during lithium intercalation was studied on graphite electrodes in ethylene carbonate based electrolytes. We demonstrated by using in situ mass spectrometry that during the first charge of the graphite electrode ethylene gas is evolved in a potential window that corresponds to the formation of a SEI. Moreover, development of hydrogen gas was detected even in dry electrolytes containing <10 ppm H2O. No CO2 is developed however, as confirmed by two in situ methods, mass spectrometry and infrared spectroscopy. We conclude that the formation of the SEI is a complex process which depends among other things on the amount of trace water present in the cell. In addition, in situ Raman mapping experiments revealed that lithium intercalation into graphite does not proceed homogeneously.

Purely Hexagonal Graphite and the Influence of Surface Modifications on Its Electrochemical Lithium Insertion Properties

High-temperature treatment of the highly crystalline synthetic graphite TIMREX® SLX50 under inert gas atmosphere led to an increased crystallinity with no evidence of rhombohedral stacking defects in the hexagonal graphite crystal structure as well as a significantly lower specific BET (Brunauer-Emmett-Teller) surface area. The first electrochemical Li + insertion in this purely hexagonal graphite indicated coinsertion of solvated lithium ions which caused significant exfoliation of the graphite structure and an increased irreversible capacity compared to the untreated graphite. A progressive oxidation treatment of the heat-treated TIMREX® SLX50 in air preserved its purely hexagonal crystal structure. However, the exfoliation effects during the first electrochemical Li + insertion disappeared gradually with the oxidation temperature and finally vanished at oxidation temperatures above 800°C. Surface analysis investigations on TIMREX® SLX50 before and after heat-treatment indicated a surface curing effect. The amounts of prismatic surfaces (polar edges), low-energy defects located on the graphite basal planes, disordered carbon on the graphite particle surface, as well as the superficial oxygen concentration decreased as a result of the heat-treatment. A progressive oxidation of the heat-treated hexagonal graphite tends to increase the amount of disordered carbon atoms, the oxygen atom concentration, as well as the amount of prismatic surfaces, but keeps the number of low-energy defects unchanged. These results indicated that not the graphite crystal structure hut the surface properties are the responsible parameters for the exfoliation of the graphite structure and the irreversible capacity observed during the first electrochemical Li + insertion.

Raman microscopy as a quality control tool for electrodes of lithium-ion batteries

The Raman mapping technique is shown to be suitable for quality control of both positive and negative electrodes for lithium-ion batteries. An analysis of the spectral features observed at a multitude of locations on the electrode surface allows a distribution of band parameters to be obtained that is unique for a given kind of electrode. It is shown that band positions, linewidths, and intensity ratios can be employed as descriptors of the distribution. Such distributions can in turn be used to generate a fingerprint for a particular kind of electrode.

Raman Microscopy Applied to Rechargeable Lithium-Ion Cells-Steps towards in situ Raman Imaging with Increased Optical Efficiency

Important improvements have been made in an in situ Raman cell developed to monitor the processes of lithium intercalation into the carbon and metal oxide materials used in lithium-ion cells. By reducing the electrolyte gap in the cell and changing the optical arrangement, the signal-to-noise ratio could be improved by a factor of about 20. The optimized cell gives optical access to either electrode (anode or cathode) employed in commercial lithium-ion batteries. The optical efficiency is such that Raman maps of electrode surface segments can be recorded under in situ conditions with an acquisition time of about 30 s per spectrum.

Characterization of the sol-gel process using raman spectroscopy organically modified silica gels prepared via the formic acid-alkoxide route

For precursor mixtures containing tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhTREOS), time of gelation can be reduced by up to two orders of magnitude depending on reaction conditions employed when reacting the silicon alkoxide mixture with formic acid instead of water. Results indicate that time of gelation depends on the amount of PhTREOS in the precursor mixture. Within the range of concentrations investigated, an exponential law describes best the dependence of reduced time of gelation on the molar fraction of PhTREOS. Therefore, we conclude that the phenyl ring acts as a steric hindrance to network formation. Raman spectroscopy is used to characterize the reaction between the alkoxide mixture and formic acid. During the acidolysis reaction, ethanol is formed as an intermediate. A preliminary reaction scheme is proposed to account for the time dependence of species involved. Furthermore, Raman spectroscopy is successfully employed to monitor the effects of post-gelation thermal treatment of the gel samples. The effects observed are interpreted with a model of a phenyl ring trapped in a siloxane cage.

Leaching of the Anthraquinone Dye Solvent Blue 59 Incorporated into Organically Modified Silica Xerogels

The effects of preparation method and precursor composition on the leaching behavior of the anthraquinone dye Solvent Blue 59 incorporated into silica based xerogels have been studied. Xerogels were prepared under acidic conditions from mixtures of 20 mol% of organically modified silicon alkoxides, R′–Si(OR)3, in Si(OR)4 (R = methyl or ethyl, R′ = methyl, vinyl, phenyl). The dye was added at the beginning of the sol-gel reaction. The reaction was carried out by either hydrolysis under acidic conditions or acidolysis by formic acid. The dye incorporated was leached with refluxing ethanol using a Soxhlet extraction procedure to simulate the long-term stability of the samples prepared. With increasing size of organic substituent (methyl < vinyl < phenyl), the amount of dye leached decreases. Results from nitrogen adsorption experiments show that all samples characterized have about the same average pore diameter, but they differ in total pore volume and BET surface area. With increasing size of the organic residue, the pore volume decreases by an order of magnitude. Therefore, it is concluded that the microstructure of the xerogels prepared determines the retention behavior of dyes incorporated during the sol-gel reaction.

Calcination of a Molybdenum/Silicon Mixed Oxide Xerogel Followed by in situ Raman Spectroscopy:

The calcination of materials derived by sol-gel reactions is important for the evolution of the ® nal structure.1 Especially for applications in catalysis, knowledge of how to control the calcination process is of interest for an optimization of the materials properties. The in situ investigation of the calcination process by vibrational spectroscopy yields information about the species present in the sample and the conditions needed to prepare them. Two advantages of the in situ method in comparison to off-line studies should be mentioned. First, in ̄ uences of atmospheric humidity may be effectively ruled out or controlled under in situ conditions;2,3 second, as compared to results from off-line experiments, results obtained with in situ spectroscopy can be more reliably correlated with results obtained by thermal analysis or temperature-programmed methods used for the characterization of catalysts and catalytic reactions. In this work, the calcination of a mixed molybdenum oxide/silica xerogel has been investigated by in situ Raman microscopy. It is shown that this method can be successfully used to characterize the dispersion of the molybdenum species present at high temperatures. The preparation of a mixed molybdenum oxide/silica xerogel by a nonaqueous sol-gel route is reported here for the ® rst time. Results of the characterization of this material are compared to results obtained from studies of molybdenum oxides supported on silica.4± 11

Characterization of ytterbium–yttrium mixed oxides using Raman spectroscopy and x-ray powder diffraction

Ytterbium–yttrium mixed oxides, which are potential emitter materials for thermophotovoltaic energy converters, were prepared using two different synthesis methods. The results of a characterization using Raman spectroscopy and x-ray powder diffraction (XRD) are presented. The purpose of this study was to confirm whether the materials prepared are indeed substitutional solid solutions of ytterbium oxide and yttrium oxide. Both XRD and Raman spectroscopy are established techniques for the analysis of solid solutions. Linear relationships between the Raman band position and the composition of mixed crystals are known for solid solutions with one active Raman mode. An aspect of this study was the investigation of the relationship between composition and band position and linewidth of vibrational modes when applying Raman techniques to a system with several vibrational modes. A linear relationship between the band position of a vibrational mode at about 600 cm−1, which is assigned to a stretching vibration, and the ytterbium content of the material was observed. Using an empirical fit to the most intense band around 370 cm−1, even a substitution of 2 mol% Yb may be detected using non-linear least-squares data analysis. In addition, a discussion of additional features observed in the Raman spectra at Raman shifts > 650 cm−1 is presented. Copyright