Li-ion batteries studied with Scanning Electron Microscopy and combined analytical methods

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The complex interaction in batteries is most clearly visible at the interfaces between anode, cathode, separator and electrolyte. A post-mortem microstructural examination of changes on the surfaces of the anode, cathode and separator offers great potential on the way to understand the complex interaction in batteries during ageing. The combination of scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), focused ion beam (FIB) and HIDEN secondary ion mass spectrometry (SIMS) technology is a powerful tool to examine ageing effects on the interfaces.
The following investigations were performed on commercial graphite/NMC 18650 cells (ICR18650-26F) with a nominal capacity of 2600mAh and a cycle window 2.75 V < V < 4.2 V. The cell was aged by storage at 60°C (73 days) while a reference cell was kept at 18°C. The electrochemical tests have been carried out at the Technische Hochschule Ingolstadt. Computer tomography reveals a triggered current disruptive device for the aged cell. This indicates gas evolution. The analysis of the cathode and anode surfaces delivers further insights: The morphology of the reference and the stored cell shows no difference, but in the material contrast image the surface of the NMC particle of the stored cell appears with a grey value which lies between the binder and the NMC particles. This suggests that elements with a lower atomic number than cobalt, manganese and nickel but higher than graphite are enriched on the surface. The anode surface is visualized in the same way. The comparison of reference and stored cell shows that the latter has a stronger incrustation, which is already visible by comparing the surface morphology. This can be seen in the material contrast by clearly lighter grey values. This means that the deposited elements have a higher atomic number than C, as the anode consists of graphite or the surface is less electronic conductiv (potential contrast). The EDX measurement at the same location reveals, that the surface of the stored cell is enriched with oxygen, phosphorus and fluorine. The elemental composition analysis of the negative ions with SIMS visualises the same event. The measurement of the storage cell shows newly added peaks for the element hydrogen, oxygen and phosphorus. Additionally, the proportion of fragments in the mass spectrum is increasing. The examination of the whole cell showed the following result: - Decomposition of electrolyte and deposition of products mainly on anode surface - Gas formation represented by the triggered current disruptive device - Occurance of water The mechanism of accelerated electrolyte decomposition in the presence of water has already been described in detail in the literature. The examination of the whole cell enables the recognition of different ageing fingerprints. SIMS complements the usual analysis methods and offers new insights into ageing mechanisms.

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