We show that a combination of selected techniques, such as 7Li, 19F MAS NMR, XPS, TOF-SIMS and STEM-EELS, provides an in-depth characterization at different scales of the SEI layer forming on the surface of silicon particles as well as its evolution upon cycling.
We demonstrate that STEM-EELS can be used to quickly map SEI components and quantify LixSi alloys from single experiments with nanoscale resolutions down to 5 nm. Exploiting the low-loss part of the EEL spectrum allowed us to circumvent the degradation phenomena that have so far crippled the application of this technique on such beam-sensitive compounds. Our results provide unprecedented insight into silicon failure mechanisms explored at the nanometer scale in full cell configuration. We observe the morphology of the SEI to be extremely heterogeneous at the particle scale but with clear chemical evolutions with extended cycling coming from both SEI accumulation and a transition from lithium-rich carbonate-like compounds to lithium-poor ones. In addition, we were able to correlate local discrepancies in lithiation to the initial crystallinity of silicon as well as to the local SEI chemistry and morphology. Electrode failure mechanisms at the particle scale were directly observed to be related to SiNP disconnection from both electronic and ionic conduction pathways, creating strong heterogeneous lithiation processes leading to some overlithiated SiNPs. It leads to differential swelling, amplified pulverization and enhanced SEI formation. These findings suggest that capacity retention could be improved through a better control of the nanoscale homogeneity of the electrical wiring and accessibility of porosity in these highly electrolyte-reactive electrodes.
Such results emphasize the importance of achieving multiprobe, multiscale and nanoscale analysis by using various advanced characterization tools.