  Small amounts of high-capacity silicon-based materials are already used in
the anode of commercial Li-ion batteries, helping increase their energy
density. Despite their remarkable storage capability, silicon continuously
reacts with the electrolyte, accelerating time-dependent cell performance fade.
Nevertheless, very limited information is available on the specific
consequences of this reactivity for the calendar aging of Li-ion cells. Here,
we analyze aging effects on 450 mAh pouch cells containing 15 wt% of Si (and 73
wt% graphite) after storage at 21 oC for four years. We show that severe losses
of Si capacity occurred due to particle isolation when cells were stored at
high states of charge (SOC), but not when cells were fully discharged prior to
storage. Impedance rise was also significantly higher when cells were kept at
high SOCs and was mostly due to phenomena taking place at the cathode; the
continuous electrolyte reduction at the anode did not lead to a major increase
in bulk electrode resistance. A series of post-test characterization provided
additional information on the effects of time and SOC on the calendar aging of
Si-containing cells. Our study highlights the many challenges posed by Si
during calendar aging and can inform future studies in the field.
