Post-mortem identification of lithium plating in high energy automotive batteries

Abstract

The performance of LIBs deteriorates over time due to various aging mechanisms, among which lithium plating (LP) is critical. This study investigates LP in commercial high-energy graphite-SiO_x/NMC pouch LIBs cycled until end-of-life (EOL) under LP-inducing conditions. Employing post-mortem analysis techniques such as ⁷Li nuclear magnetic resonance (NMR) spectroscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), and scanning electron microscopy (SEM), we aim to provide a comprehensive understanding of LP. Electrochemical methods such as incremental capacity analysis (ICA) and differential voltage analysis (DVA) were first used to identify LP occurrence in a cell during artificial ageing (AA). Subsequently, the cells were dissected to prepare post-mortem analysis samples. ICP-OES revealed an increased soluble lithium (Li) content on the anode surface compared to a fresh cell, which is attributed to LP. Metallic Li was identified on the anode surface of the cycled cell by ⁷Li NMR at Knight shifts in the range from 245 to 270 ppm, whereas no metallic Li was detected in fresh cell. Post-mortem SEM analysis revealed a mossy layer growth on anode sample of the artificially aged cells that is absent on the anode surface of a fresh cell. This mossy growth is attributed to LP. Elemental mapping also revealed fluorine hotspots on the mossy metallic Li layer, indicating the formation of lithium fluoride (LiF) as a reaction product between metallic Li and the cell electrolyte. Additionally, as the SEM sample was exposed to air during transfer, oxygen hotspot on mossy Li layer in elemental mapping indicates the reaction of oxygen and moisture with metallic Li.