Pouch cells with 15% silicon calendar-aged for 4 years

TL;DR

This study investigates the four-year aging effects of silicon-containing lithium-ion pouch cells, revealing how storage conditions influence capacity loss and impedance growth, with implications for improving long-term battery performance.

Contribution

It provides the first detailed analysis of silicon's impact on calendar aging in commercial Li-ion pouch cells over four years, emphasizing the role of state of charge.

Findings

Severe silicon capacity loss occurs at high SOC due to particle isolation.

Impedance increase is mainly due to cathode phenomena, not anode electrolyte reduction.

Discharged cells prior to storage show less capacity fade.

Abstract

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.