Monitoring SEI Formation on Graphite Electrodes in Lithium-Ion Cells by Impedance Spectroscopy

In this work, we target to isolate the SEI resistance of graphite electrodes in lithium-ion cells by impedance spectroscopy measured in blocking conditions (here = 0% SOC), where the charge transfer resistance RCT is significantly enlarged (∼104 Ω cm2geom) and thus the corresponding semicircle shifted to very low frequencies. Therefore, we measure impedance spectra of graphite/LFP full cells with a gold-wire reference electrode (GWRE) in blocking conditions (graphite potential 2 V vs Li+/Li, 0% SOC) before and after formation. As electrolytes, we use LP57 (EC:EMC 3:7 + 1 M LiPF6) either without additive or with 1 wt% VC, 1 wt% FEC, or 1 wt% DiFEC as additive. By fitting the impedance data to a transmission line-based model, we show that the SEI resistance RSEI can be extracted from blocking condition impedance spectra, whereas SEI and charge transfer resistance are inseparable in non-blocking conditions. We validate our approach by determining the activation energies for the obtained ionic and SEI resistances. Finally, we introduce a potential-controlled cycling procedure which allows to assess RSEI during formation. Here, we show that SEI resistance evolution follows the electrolyte reduction potentials, which makes this method a useful tool to study film formation on Li-ion battery anodes.