Correlative inspection technologies from the macroscopic to nanoscopic scale for the understanding of performance loss in Li-Ion batteries

Summary:

The following investigations were performed on commercial available graphite/NMC (Li [Co1/3Ni1/3Mn1/3] O2) pouch cells[1]. Non-destructive Computer Tomography (CT) yields a thickness increment of the treated cells. Fig. 1 shows SEM images of cathode, separator foils in top-view and anode foil in top and cross-sectional view (SEM/FIB). The cathode foils consist of NMC active material particles and conductive additives with binder material (marked as (1) and (2), respectively). The comparison of the reference with the aged films shows a surface coating on the aged NMC grains (marked as (3, 4) semi permeable interface (SPI)) also visible on the magnified inset of a single NMC particle with a atomic number of the precipitates above carbon and below (Li [Co1/3Ni1/3Mn1/3] O2) [2]. On top of the separator foil of the treated cell (Fig. 1 d)) arise spherical particles in the order of 10 nm to 1 micron (labelled 5, 6) and net-like structures (labelled 7), which are not present on the reference once. The treated anode foils (Fig. 1 g) show with light microscopy bright and dark areas. Correlative SEM and EDS delivered changes in the surface consistence Fig.1 e), f), electronic conductivity (brighter apperearing SEM image Fig. 1 f)) and in the C, O, F, Cu, P content. These bright areas solely appear in treated cells. The bottleneck is the missing information on the Li content.
Phase mapping by EDS of the precipitates on top of the separator foil delivered a different chemistry for the round (1,1b) and flat (2) particles (Fig. 2 a), b)). In the mid position of particle (1b) the chemical information solely comes from the particle, because the particle size exceed the EDS information depth. There an enrichment of F, Si is detected, where again the information on the most important element Li is missing [3]. The ORION NanoFab (ZEISS) is an instrument based on helium and neon ion beams, newly equipped with a Secondary Ion Mass Spectrometer (SIMS) producing element maps with lateral resolution down to 15 nm [4]. This enables correlative SEM imaging and Li mapping (Fig. 3 a)-d)). To obtain semiquantitative results, image areas of the particles in the µm range (Precipitates, P), the small particles (Nanoparticles, NP) and pure separator foil (S) were summed up (Fig. 3 e)) and yield the diagrams shown in Fig. 3 f). There for (P) an increase of Li, F and O was found and for (NP) an increase of Li, F, Si and O in comparison to the surface of the separator (S). This agrees with the results from the EDS investigations except the Si and O content for (P), which indicates a core shell structure for the big particles with an oxidized rim and Si inside the particle.[5]

[1] M. Bauer J. Power Sources 317 (2016), p. 93-102
[2] U. Golla-Schindler et al, micron 113 (2018) p. 10-19, DOI: 10.1016/j.micron.2018.06.013
[3] U. Golla-Schindler et al, In: Microsc Microanal (2019), 25(S2), 1756-1757. doi:10.1017/S1431927619009516
[4] F. Khanom et al, In: Microsc Microanal (2019), 25 (S2), 866–867. doi: 10.1017/S1431927619005063.
[5] We thank the Zeiss GmbH for the SIMS investigations and the Federal Ministry for Economic Affairs and Energy (BMWi) for financial support.