The impedance of the pouch-type full cell can be measured by electrochemical impedance spectroscopy, e.g. using an AutoLab modular potentiostat/galvanostat (Metrohm AG) over a 1 MHz to 0.01 Hz frequency range with an amplitude of 10 mV.
Electrolyte Conductivity Measurement
Li-ion conductivity of a solid-state electrolyte (SSE) can be measured by the same method:
- Pellet of SSE preparation: a SSE pellet can be prepared from powder by using a uniaxial press at 300 MPa in a stainless-steel die (diameter of 13 mm).
- Thickness, Weight, and Density measurement
- Lamination of charge collectors: A foil (thickness of 50 μm) was attached on both sides of the sample pellet, with a pressure of 30 MPa to make contact.
- Load in testing jig: The prepared sample was set in an in-house test cell, which consisted of a polytetrafluoroethylene body, stainless-steel pins and a small spring.
- Encapsulation: The test cell was mounted and enclosed in an Al-laminate bag to protect against air.
- Preheat: The cell was then preheated at 80 °C for 12 h
- Measurement: the impedance was measured from 80 to −20 °C.
- Result extraction: From the obtained impedance data, the Li-ion conductivity and activation energy were calculated
EIS measurement set-up
To perform EIS on batteries, a frequency response analyser (FRA) is typically used in combination with an electrochemical interface. The electrochemical interface applies a constant voltage (CV) or constant current (CC) and the FRA superimposes an ac signal. A multiplexer connects the FRA to the battery test system. A typical setup is shown in Fig. 1. The battery is connected using four wires — two for current flow and two for cell potential. The FRA outputs a signal to the cell via the counter electrode (I-) and the signal is returned through the working electrode (I+). It also connects to a pair of reference electrode points (V+ and V-) to measure the voltage across the cell.
The FRA applies a small ac signal, usually in the range mHz–MHz, and measures the response. EIS can be used in either potentiostatic (constant voltage) or galvanostatic (constant current) mode. Potentiostatic mode, where a voltage is applied at each frequency and the resultant current measured, is most common. The current responds at the same frequency as the applied voltage but may be shifted in phase
The impedance, Z, is a frequency-dependent complex number characterized by the ratio of voltage to current and the phase angle shift between them, Φ.
The generated impedance spectrum consists of resistive and reactive components. The resistance is made up of a combination of electronic and ionic resistances. The most common form of plotting impedance data is as a Z∗ complex plane plot of Z′′ against Z′ on linear scales, also known as a Nyquist plot.
By fitting the data with the equilibrium circuit model, resistance and reactance can be calculated.[1]
To account for, and model, a non-ideal or distorted semi-circle, a constant phase element (CPE), which is a combined variable resistor and variable capacitor, is added in parallel with the parallel RC element. The impedance of a constant phase element is defined as:
Assignment of element vs frequency[2]
- Very high frequency
- an inductive behavior is measured caused by measurement setup such as connecting lines and cable wiring
- charge transport process in electrolyte, SEI, active materials
- Middle frequency range
- Charge transfer process from electrode to electrolyte or vice versa
- Low frequency range
- Diffusion process in anode and cathode
- Warburg, 45 degree rising
[1] Energy Reports 6 (2020) 232–241
[2] DOI: 10.1002/ente.201600154