Resolution of Li deposition vs. intercalation of graphite anodes in lithium ion batteries - an in situ electron paramagnetic resonance study

The plating of lithium on a graphite anode is the main barrier to fast charging of lithium ion batteries (LIBs), in addition to causing serious capacity fade and safety issues. Here, in situ electron paramagnetic resonance (EPR) electrochemical spectroscopy is used to understand the mixed lithiation/deposition behavior occurring on the graphite anode during the charging process. The conductivity, degree of lithiation and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density and the EPR spectral change, respectively. Our results indicate that classical ‘over-charging’ (normally associated with potentials ≤ 0 V vs. Li+/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li+/Li) when the scan rate lowered to 0.04 mV s-1. The inhibition of Li deposition by the vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, which is attributed to a more mechanically flexible and polymeric SEI layer, with higher ionic conductivity. Finally a safe cut-off potential limit of +0.05 V for the graphite anode is suggested for high rate cycling, which is confirmed by the EPR response over prolonged cycling.