Impedance Characterization of an LCO-NMC/Graphite Cell: Ohmic Conduction, SEI Transport and Charge-Transfer Phenomenon

Abstract

Currently, Li-ion cells are the preferred candidates as energy sources for existing portable applications and for those being developed. Thus, a proper characterization of Li-ion cells is required to optimize their use and their manufacturing process. In this study, the transport phenomena and electrochemical processes taking place in LiCoO2-Li(NiMnCo)O2/graphite (LCO-NMC/graphite) cells are identified from half-cell measurements by means of impedance spectroscopy. The results are calculated from current densities, instead of absolute values, for the future comparison of this data with other cells. In particular, impedance spectra are fitted to simple electrical models composed of an inductive part, serial resistance, and various RQ networks—the parallel combination of a resistor and a constant phase element—depending on the cell. Thus, the evolution of resistances, capacitances, and the characteristic frequencies of the various effects are tracked with the state-of-charge (SoC) at two aging levels. Concretely, two effects are identified at the impedance spectrum; one is clearly caused by the charge transfer at the positive electrode, whereas the other one is presumably caused by the transport of lithium ions across the solid electrolyte interphase (SEI) layer. Moreover, as the cells age, the characteristic frequency of the charge transfer is drastically reduced by a factor of around 70%.

Tailored Silicon/Carbon Compounds for Printed Li–Ion Anodes

“…This equivalent circuit comprises of a series connection of a contact resistance (R s ), two resistor (R)-constant phase elements (CPE) in parallel (R 1 , R 2 with Q 1 , Q 2 ), and a series constant phase element (Q 3 ). [22,23] The fits are added as solid lines to Figure 5a, showing very good agreement with the experiments. The high-frequency resistance values (R s ) of the Si/C and Si electrodes were found to be 22.6 Ω and 28.5 Ω, Figure S6.…”

EIS Study on the Electrode-Separator Interface Lamination

“…Using the described correlations of the EIS signal response, the equivalent circuit analysis of the signals shown in Figure 1 As shown in Figure 3, the capacitance fit parameters for the graphite anode and NMC cathode lie around 3-4 mF•s (a−1) •cm −2 with negligible dependence on SOC, which is in the range of typically reported values for EIS capacitance fit parameters, normalized to the geometric electrode area, of graphite anodes and composite cathodes in non-aqueous electrolytes [39][40][41]. The capacitance fit parameters of the surface resistance semicircle show no significant correlation to the SOC and lie around 3-5 µF•s (a−1) •cm −2 for all cells, which is in the range of double layer capacitances of nonaqueous electrolytes [39].…”

Three-Dimensional Fibrous Iron as Anode Current Collector for Rechargeable Zinc–Air Batteries

“…The equivalent circuit also used for fitting of the EIS plots and the fitted data for all circuit elements is shown in Table 3. The presence of an inductive part (L) at high frequencies is commonly related to the cables and connections [46]. The values of Rs were found to be relatively low for both cells after 200 cycles (<3 Ω), indicating a high electrochemical cycling stability presented by both electrodes.…”