Michael E. Spahr

REDOX-ACTIVE NANOTUBES OF VANADIUM OXIDE

Unlike many small carbon nanotubes, VOx nanotubes (shown on the right) are obtained as the main product of a direct chemical synthesis at relatively low temperatures. The multiwalled material contains template molecules between the individual shells, which by a simple cation exchange can be removed without destruction of the tubes.

High Rate Capability of Graphite Negative Electrodes for Lithium-Ion Batteries

The rate capability of various lithium-ion half-cells was investigated. Our study focuses on the performance of the carbon negative electrode, which is composed of TIMREX SFG synthetic graphite material of varying particle size distribution. All cells showed high discharge and comparatively low charge rate capability. Up to the 20 C rate, discharge capacity retention of more than 96% was found for SFG6. The rate capability of the half-cells is a function of both the particle size distribution of the graphite material and the preparation method of the electrode. A transport limitation model is proposed to explain the restrictions of the high current performance of graphite electrodes. The key parameters found to influence the performance of a graphite negative electrode were the loading, the thickness, and the porosity of the electrode.

Exfoliation of Graphite during Electrochemical Lithium Insertion in Ethylene Carbonate-Containing Electrolytes

Post mortem scanning electron microscopy, X-ray diffraction analysis, and Raman spectroscopy were applied to study the exfoliation tendency of a high temperature-treated graphite negative electrode material during the first electrochemical lithium insertion in various carbonate electrolyte systems. Exfoliation of the heat-treated graphite electrode material was observed in propylene carbonate (PC)- and ethylene carbonate (EC)-containing electrolytes. Using acyclic carbonates and 1-fluoro ethylene carbonate, exfoliation of the graphite structure could be avoided. LiPF 6 used as a conducting salt in the EC-based electrolyte increased the exfoliation tendency of the graphite material. Differential electrochemical mass spectrometry was performed to study the passivation of the untreated and heat-treated graphite surface during the first electrochemical Li + insertion. The heat-treated graphite surface showed a reduced reactivity towards EC, which hindered the graphite surface passivation in EC-based electrolyte systems and led to the exfoliation of the graphite structure so far known only for PC-containing electrolytes.

Safety Aspects of Graphite Negative Electrode Materials for Lithium-Ion Batteries

Safety aspects of different graphite negative electrode materials for lithium-ion batteries have been investigated using differential scanning calorimetry. Heat evolution was measured for different types of graphitic carbon between 30 and 300°C. This heat evolution, which is irreversible, starts above 100°C. From the values of energy evolved, the temperature rise in complete lithium-ion cells was estimated. The heat evolved between 80 and 220°C is a linear function of the irreversible charge capacity of the carbon. The specific Brunauer, Emmett, and Teller method surface area measured by nitrogen gas adsorption, which is usually also a linear function of irreversible charge capacity, may be used with certain reservations to calculate approximately the heat evolution of graphitic carbon negative electrode materials in lithium-ion batteries. Graphite materials are usually safer if their irreversible charge capacity during the first cycle is low.

Redoxaktive Nanoröhren aus Vanadiumoxid

Anders als die viel kleineren Kohlenstoffnanorohren erhalt man VOx-Nanorohren (Bild rechts) als Hauptprodukt einer direkten chemischen Synthese bei recht niedrigen Temperaturen. Die Rohrenwande sind mehrschichtig, und zwischen den Schichten befinden sich Templatmolekule, die durch Kationenaustausch unter Erhaltung der Rohren entfernt werden konnen.

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.

Key factors for the cycling stability of graphite intercalation electrodes for lithium-ion batteries

Aspects of the charge loss during the first cycle and the cycling stability of lithium-ion batteries are discussed as functions of water and oxygen impurities in their electrolyte solutions. Differential electrochemical mass spectrometry revealed different decomposition products depending on the water content. Carbon black, copper particles, and nickel particles were added to the graphite electrodes in order to improve their cycling stability.