Resolution of Lithium Deposition versus Intercalation of Graphite Anodes in Lithium Ion Batteries: An In Situ Electron Paramagnetic Resonance Study

In situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes 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. 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 is lowered to 0.04 mV s-1 . The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut-off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling.