Andreas Würsig

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.

Film Formation at Positive Electrodes in Lithium-Ion Batteries

Post mortem scanning electron microscopy was used to study the surface film formation on different cathode materials for lithium-ion batteries. The effect of the kind of oxide, the type of solvent and conducting salt, as well as the influence of additives on this process was investigated. In the case of an electrolyte containing ethylene carbonate (EC) and propylene carbonate (PC) (1:1, w/w) with 1 M LiPF 6 , film formation was observed on LiCoO 2 , LiMn 2 O 4 , and especially pronounced on LiNiO 2 oxide materials. In contrast, with EC, dimethyl carbonate (1:1, w/w) 1 M LiPF 6 containing electrolyte, no solid electrolyte interface formation was detected. The substitution of LiClO 4 for LiPF 6 with PC-based electrolytes leads to a less prominent layer. If 2% vinylene carbonate was added to the PC-based electrolyte a pronounced film, presumably consisting of polymerization products of this additive, was created.

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.