Understanding of Li‐plating on graphite electrode: detection, quantification and mechanism revelation

Abstract

The detrimental lithium (Li) plating is considered as the main cause inducing capacity degradation and safety issue of lithium‐ion battery. This study presents an underlying understanding in detecting, quantifying and revealing mechanism of Li plating on graphite electrode driven by over‐lithiation focused on Li/graphite coin cell by adequate experimental methods assisted with a finite element model. Firstly, Li plating signal is extracted from negative graphite voltage with the distinctive Li stripping plateau converted into increment capacity peak in the subsequent delithiation procedure; Afterwards, in‐situ thermal spectrum with an extra heat peaks corresponding to Li stripping further provides a convincing evidence for Li plating. An intriguing phenomenon is discovered that the presence of Li stripping heat peak will weaken the first delithiation heat peak, which can act as characteristic traits for Li plating diagnostics from the thermal perspective. Subsequently, Coulomb efficiency of Li plating/stripping is calculated to keep below 98.3% over cycling and the decline after 20 cycles implies that capacity loss is ascribed to the aggravated irreversible Li plating. Next, with the complement of post‐mortem approaches represented by 7Li NMR (Nuclear magnetic resonance), Li plating is semi-quantified to unveil Li plating/stripping mechanism over cycling. Finally, a 2D electrochemical model is presented for this Li/graphite cell to further unravel Li plating. Results shows that the lowest graphite voltage, as well as the highest Li plating current density and thickness of plated Li always occurs at the graphite/separator interface. The results can also provide guidance for full/commercial cell.