Inactive Overhang in Silicon Anodes

TL;DR

This paper investigates the inactive overhang in silicon anodes of Li-ion batteries, showing that silicon's properties prevent lithiation at the overhang, which impacts capacity measurement and cell life estimation.

Contribution

It demonstrates that silicon anodes suppress Li+ exchange at the overhang due to large voltage hysteresis and thermodynamic barriers, differing from graphite anodes.

Findings

Crystalline Si overhangs are never lithiated after 4 months at 45°C.

Large voltage hysteresis reduces Li+ exchange efficiency in silicon anodes.

Silicon anodes require different life estimation methods than graphite anodes.

Abstract

Li-ion batteries contain excess anode area to improve manufacturability and prevent Li plating. These overhang areas in graphite electrodes are active but experience decreased Li+ flux during cycling. Over time, the overhang and the anode portions directly opposite to the cathode can exchange Li+, driven by differences in local electrical potential across the electrode, which artificially inflates or decreases the measured cell capacity. Here, we show that lithiation of the overhang is less likely to happen in silicon anodes paired with layered oxide cathodes. The large voltage hysteresis of silicon creates a lower driving force for Li+ exchange as lithium ions transit into the overhang, rendering this exchange highly inefficient. For crystalline Si particles, Li+ storage at the overhang is prohibitive, because the low potential required for the initial lithiation can act as thermodynamic barrier for this exchange. We use micro-Raman spectroscopy to demonstrate that crystalline Si particles at the overhang are never lithiated even after cell storage at 45 oC for four months. Since the anode overhang can affect the forecasting of cell life, cells using silicon anodes may require different methodologies for life estimation compared to those used for traditional graphite-based Li-ion batteries.