Graphite Anodes for Li-Ion Batteries: An Electron Paramagnetic Resonance Investigation

Graphite is the most commercially successful anode materialforlithium (Li)-ion batteries: its low cost, low toxicity, and high abundancemake it ideally suited for use in batteries for electronic devices,electrified transportation, and grid-based storage. The physical andelectrochemical properties of graphite anodes have been thoroughlycharacterized. However, questions remain regarding their electronicstructures and whether the electrons occupy localized states on Li,delocalized states on C, or an admixture of both. In this regard,electron paramagnetic resonance (EPR) spectroscopy is an invaluabletool for characterizing the electronic states generated during electrochemicalcycling as it measures the properties of the unpaired electrons inlithiated graphites. In this work, ex situ variable-temperature(10-300 K), variable-frequency (9-441 GHz) EPR was carriedout to extract the g tensors and line widths and understandthe effect of metallicity on the observed EPR spectra of electrochemicallylithiated graphites at four different states of lithiation. We showthat the increased resolution offered by EPR at high frequencies (>300GHz) enables up to three different electron environments of axialsymmetry to be observed, revealing heterogeneity within the graphiteparticles and the presence of hyperfine coupling to Li nuclei. Importantly,our work demonstrates the power of EPR spectroscopy to investigatethe local electronic structure of graphite at different lithiationstages, paving the way for this technique as a tool for screeningand investigating novel materials for use in Li-ion batteries.