Quantitative Operando EPR Method on Graphite Anodes: Electronic Properties, Lithiation Kinetics, and Lithium Deposition

Graphite is currently an irreplaceable anode material for lithium-ion batteries due to its many advantages. Despite decades of extensive study, real-time investigation of its electrochemical processes, especially during fast charging, has been lacking. In this work, we develop a quantitative operando electron paramagnetic resonance (EPR) method and standardize data analysis for researching graphite anodes. For the first time, the density of states at the Fermi level is determined under different charge rates, revealing a consistently homogeneous electronic property across the graphite lattice. However, the lithiation shows inhomogeneity with increasing charge current, as evidenced by the EPR line width which correlates with Li-ion mobility. During fast charge, it is found that the lithiation kinetics is limited by bulk diffusion and Li deposition may commence once the surface layer reaches full lithiation at stage 1. Further analysis methods effectuate the identification of the plating onset and dead Li. Additionally, we preliminarily explore lithiation homogeneity across the electrode plane by spectral-spatial EPR imaging. At last, the competition between Li intercalation and Li deposition is elucidated by quantifying the plating current. The versatile EPR paradigm is anticipated to benefit the further development of graphite anodes and other carbon-based anodes.