Dietrich Goers

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.

Raman spectroscopic and structural studies of heat-treated graphites for lithium-ion batteries

Standard graphite TIMREX® SLX 50 was oxidised at 500–800 °C under air atmosphere in a muffle and a rotary furnace. Scanning Electron Microscopy (SEM), Raman spectroscopy, and X-Ray Powder Diffraction (XRD) were used to study the changes in surface morphology and crystallinity. The results show a slight increase of the La value and a decrease of the rhombohedral fraction with increased heat-treatment temperature (HTT). XRD measurements show no significant change in La values within the bulk of graphite samples. Above 700 °C SEM images of graphite reveals holes and cavities, whereas heat-treatment temperatures below 700 °C do not significantly affect graphite materials parameters.