Monitoring SEI Formation on Graphite Electrodes in Lithium-Ion Cells by Impedance Spectroscopy
Sophie Solchenbach, Xinyi Huang, Daniel Pritzl, Johannes Landesfeind, Hubert A. Gasteiger
Abstract
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.