CHARACTERISATION OF Li-ION BATTERIES BY COMBINED LM, CT, EDS, WDS AND SIMS USING HELIUM-ION MICROSCOPY (CARL ZEISS ORION)

摘要

Today's battery technology is the bottleneck for advancements in a variety of applications ranging from consumer electronics to electronic vehicles. For Li-ion batteries the lifetime and safety are crucial concerns for their application. Depending on the cell chemistry, design and operation conditions several degradation processes can occur, which affect their performance and durability. The characterisation of structural and chemical properties of batteries from the macro- to the nanoscale with combined light microscopy (LM), computer tomography (CT), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) enable new insights for battery fabrication and enlightens battery aging mechanisms. The main studies on degradation effects were performed on commercial graphite/NMC (Li [Co_(1/3)Ni_(1/3)Mn_(1/3)] O_2) pouch cells treated in different scenarios and conditions. Increasing thickness of the whole cells due to storage and cycling processes are correlated to Li plating and the growth of the secondary electron image (SEI) layer can be detected. Morphology changes, formation of particle coatings on the cathode side (passivation of the active surface), a reaction rim on the anode side and the reduction of porosity of the separator are visualised [1]. The chemical analyses were performed by SEM with energy- (WDS) and wavelength-dispersive (WDS) X-ray spectrometry and additionally with the ORION NanoFab, a 3-in-l multibeam ion microscope for sub-10 nm nanostructuring, with an attached SIMS. This enables increasing resolution of the analytic studies and the spatial detection of Li. Figure 1 presents the investigation of the separator foils anode side. Figure 1a shows a top view image recorded with the SIGMA 300 VP SEM. Different precipitations formed by degeneration are marked as 1, 1b, 2 and 3. The EDS was performed with 5 kV accelerating voltage to improve the spatial resolution and decrease information depth. By phase mapping (Fig. lb) differences in the chemical composition are obtained, where the round precipitations (green and yellow) yield an enrichment in the Si and F content (Fig. 1c). The differences between 1 and lb are due to matrix effects, since the lateral information depth of the X-rays in the centre of particle 1b is beyond the particle size. The elongated precipitations (blue) show an enrichment in Cu and P and the original separator foil appears as the red phase (Fig. 1c). The ORION NanoFab equipped with a SIMS enables mapping of the lithium content with a high spatial resolution. All the newly created features on top of the separator foil showed an enrichment in the lithium content (Figs. 1d and 1e).