Interfacial impedance

Li-ion secondary batteries Li(Ni,Co,Mn)02 surface coating interfacial impedance. 

At high frequencies, a semicircle is expected as a result of a parallel combination of R and Cg. At low frequencies a Warburg impedance may be found as part of the interfacial impedance. In some cases it may dominate the interfacial impedance as in Fig. 10.13(a), in which case only the diffusion coefficient of the salt will be determinable. It should be noted that, in the absence of a supporting electrolyte, the electroactive species, in this case Li, cannot diffuse independently of the anions. 

In other cases as in Fig. 10.13(b) the interfacial impedance will show a semicircle due to / <., and Cj, in parallel, with the Warburg impedance becoming apparent at significantly lower frequencies. In such cases R can be evaluated without difficulty. 

Fig. 10.13 Impedance plane diagrams for metal non-blocking electrodes with two mobile species in the electrolyte, (a) Interfacial impedance is only a Warburg impedance. (b) Interfacial impedance shows a charge transfer resistance semicircle.

In the ideal situation of 100% utilization x = 1.0), the capacity corresponding to the above anode half reaction is 372 mA h g However, due to the low ion conductivity of the polymer electrolyte and the high interfacial impedance between it and the graphite electrode, this elegant example of electrochemical preparation of lithiated graphite is of limited practical significance. 

Thus, at temperatures lower than the liquid us temperature (usually above —20 °C for most electrolyte compositions).EC precipitates and drastically reduces the conductivity of lithium ions both in the bulk electrolyte and through the interfacial films in the system. During discharge, this increase of cell impedance at low temperature leads to lower capacity utilization, which is normally recoverable when the temperature rises. However, permanent damage occurs if the cell is being charged at low temperatures because lithium deposition occurs, caused by the high interfacial impedance, and results in irreversible loss of lithium ions. An even worse possibility is the safety hazard if the lithium deposition continues to accumulate on the carbonaceous surface. 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

Workers have shown theoretically that this effect can be caused both at the microstructural level (due to tunneling of the current near the TPB) as well as on a macroscopic level when the electrode is not perfectly electronically conductive and the current collector makes only intermittent contact. ° Fleig and Maier further showed that current constriction can have a distortional effect on the frequency response (impedance), which is sensitive to the relative importance of the surface vs bulk path. In particular, they showed that unlike the bulk electrolyte resistance, the constriction resistance can appear at frequencies overlapping the interfacial impedance. Thus, the effect can be hard to separate experimentally from interfacial electrochemical-kinetic resistances, particularly when one considers that many of the same microstructural parameters influencing the electrochemical kinetics (TPB area, contact area) also influence the current constriction. 

Measurement of interfacial impedance and phase angle over a frequency range... 

Electrochemical noise consists of low-frequency, low-amplitude fluctuations of current and potential due to electrochemical activity associated with corrosion processes. ECN occurs primarily at frequencies less than 10 Hz. Current noise is associated with discrete dissolution events that occur on a metal surface, while potential noise is produced by the action of current noise on an interfacial impedance (140). To evaluate corrosion processes, potential noise, current noise, or both may be monitored. No external electrical signal need be applied to the electrode under study. As a result, ECN measurements are essentially passive, and the experimenter need only listen to the noise to gather information. 

Area normalization of ECN data is not as straightforward as with other type of electrochemical data (140). Current and potential noise may scale differently with electrode area. For example, if it is considered that the mean current is the sum of contributions from discrete events across the electrode surface, then the variance associated with the mean value will be proportional to the electrode area. The standard deviation of the current noise, o7, a measure of current amplitude, will then scale as the square root of the area. If is assumed that potential noise originates from current noise acting on the interfacial impedance, then aE will scale with the inverse root of the area. Therefore it is inappropriate to normalize current and potential noise by electrode area linearly. On the contrary, area normalization of noise resistance does appear to be appropriate. This is so because the potential and current noise have a constant relationship with one another. As a result, it is appropriate to report noise resistance in units of T> cm2, remembering that the total area for normalization is given by the sum of the areas on both working electrodes. 

Gerischer impedance — The Gerischer impedance is a transport-related interfacial impedance element which differs from the Warburg impedance in that the electroactive species taking part in the electrode process is chemically generated in a spatially homogeneous way prior to diffusing to the interface. It has the form ... 

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