  Continuous solid electrolyte interface (SEI) formation remains the limiting
factor of the lifetime of silicon nanoparticles (SiNPs) based negative
electrodes. Methods that could provide clear diagnosis of the electrode
degradation are of utmost necessity to streamline further developments. We
demonstrate that electron energy-loss spectroscopy (EELS) in a scanning
transmission electron microscope (STEM) can be used to quickly map SEI
components and quantify LixSi alloys from single experiments, with 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 aging mechanisms 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. Thanks to the retrieval of several results from
a single dataset, 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. This study emphasizes how initial heterogeneities in the
percolating electronic network and the porosity affect SiNPs aggregates along
cycling. These findings pinpoint the crucial role of an optimized formulation
in silicon-based thick electrodes.
