Cell impedance

Cell voltage / V Internal cell Impedance / m 0 Cell voltage / V... 

In the overcharge tests we carried out, there was no fire or explosion. The cell impedance increased suddenly in every test. This was due to the oxidation of the electrolyte with a low charging current, or to the separator melting with a high charging current. In practical applications, an electronic device should be used to provide overcharge protection and ensure complete safety. 

In the measurements, one commonly determines the impedance of the entire ceU, not that of an individual (working) electrode. The cell impedance (Fig. 12.13) is the series combination of impedances of the working electrode (Z g), auxiliary electrode (Z g), and electrolyte (Z ), practically equal to the electrolyte s resistance (R). Moreover, between parallel electrodes a capacitive coupling develops that represents an impedance Z parallel to the other impedance elements. The experimental conditions are selected so that Z Z g Z g. To this end the surface area of the auxiliary electrode should be much larger than that of the working electrode, and these electrodes should be sufficiently far apart. Then the measured cell impedance... 

A reduction in conductance of portions of the cathode can easily explain the observed increase in cell impedance as well as loss of cathode capacity via isolation of oxide active material. Particle isolation is also in concert with our observations of carbon retreat or rearrangement in tested cathodes. These results represent the most clear and obvious difference (compared to other diagnostic studies) in cathode characteristics after prolonged cell tests at elevated temperatures. 

Electrode noise may not be the limiting noise anymore, but electronically generated noise. This noise may be amplified by the cell impedance, which means that the electronics have to be developed very carefully. 

Even though the effect of moisture on the anode kinetics is well known, interpretation of experimental results on the effect of moisture can be tricky. As Nakagawa et al. [52] pointed out, the measurement of the total cell impedance under the OCV condition is not convincing since the reduction of polarization could as well be due to the availability of H20 for the cathodic reaction. In addition, the measurement of cell performance under the constant voltage or constant current conditions may also lead to wrong conclusions about the effect of water, because the addition of H20 will... 

Dodge IL, Carr MW, Cernadas M, Brenner MB IL-6 production by pulmonary dendritic cells impedes Thl immune responses. J Immunol 2003 170 4457-4464. 

Partial pressure of gas compositions at anode (a) or cathode (c), atm. Polarization, V Current density, A/cm Cell impedance, Q-cm... 

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. 

On the other hand, the presence of these esters in the electrolyte solutions raised concern over the longterm performance at room temperatures, because EIS studies indicated that the resistance associated with the SEI film increased at a much higher rate for ester-based electrolytes as compared with the compositions that were merely based on carbonates. The authors attributed this rising cell impedance to the reactivity of these esters toward the electrode active material, which resulted in the continued growth of the SEI film in the long term and suggested that alkyl esters, especially those of acetic acid, might not be appropriate cosolvents for low-temperature application electrolytes. ... 

By EIS analysis of the corresponding lithium ion cells, Zhang et al. showed that the impact of SEI resistance on total cell impedance was rather negligible, and hence, they attributed the superior low-temperature behavior of LiBF4-based electrolytes to the lower resistance associated with the so-called charge-transfer processes , which are usually represented in impedance spectra by the semicircle at the lower frequency region. This suggestion could be viewed as a further extension of the conclusion... 

Another salt that is less sensitive to moisture than LiPFe, LiBF4, was also tested as an electrolyte solute intended for high-temperature applications. Zhang et al. reported that electrolytes based on this salt could allow the lithium ion cells to cycle at temperatures up to 70 Irreversible reactions occurred at temperatures above 80 °C, and the cells lost capacity rapidly, which was accompanied by the rise of cell impedance simultaneously. 

Unfortunately, TMP was found to be cathodically unstable on a graphitic anode surface, where, in a manner very similar to PC, it cointercalated into the graphene structure at 1.20 V and then decomposed to exfoliate the latter, although its anodic stability did not seem to be a problem. Eor this reason, TMP has to be used in amounts less than 10% with EC and other carbonates in high concentration in order to achieve decent performance in lithium ion cells. However, capacity fading caused by the increase of cell impedance cast doubt on the application of this flame retardant in a lithium ion cell. To avoid the poor cathodic stability of TMP on graphitic anodes, the possibility of using it with other amorphous carbon electrodes was also explored by the authors. ... 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

The lithium-ion cells have demonstrated power loss when aged and/or cycled at high temperatures. Norin et al." demonstrated that the separator is at least partly responsible for the power loss due to the intrinsic increase in its ionic resistance. They showed that impedance increased significantly upon cycling and/or aging of lithium-ion cells at elevated temperatures and that separators accounts for 15% of the total cell impedance rise. They later reported that the loss in ionic conductivity of the separator was due to blocking of the separator pores with the products formed due to electrolyte decomposition, which was significantly accelerated at elevated temperatures. 

Figure 53. Idealized half-cell response of a thin solid electrolyte cell, (a) Cell geometry including working electrodes A and B and reference electrode (s). (b) Equivalent circuit model for the cell in a, where the electrolyte and two electrodes have area-specific resistances and capacitances as indicated, (c) Total cell and half-cell impedance responses, calculated assuming the reference electrode remains equipotential with a planar surface located somewhere in the middle of the active region, halfway between the two working electrodes, as shown in a.

Figure 54. Measured (a) and simulated (b) effect of electrode misalignment, (a) Total-cell and balf-cell impedances of a symmetric LSC/rare-earth-doped ceria/LSC cell with nominally identical porous LSC x= 0.4) electrodes, measured at 750 °C in air based on tbe cell geometry shown. (b) Finite-element calculation of tbe total-cell and half-cell impedances of a symmetric cell with identical R—C electrodes, assuming a misalignment of the two working electrodes (d) equal to the thickness of the electrolyte (L). ...

Furthermore, Boukamp and Adler showed that when the electrodes on opposite sides of a cell are different from each other (as they are in a fuel cell), errors may not only involve a numerical scaling factor but also cross-contamination of anode and cathode frequency response in the measured half-cell impedances. For example. Figure 55a shows the calculated half-cell impedance of the cell idealized in Figure 53a, assuming alignment errors of 1 electrolyte thickness. Significant distortion of the halfcell impedances (Za and Zb) from the actual impedance of the electrodes are apparent, including cross-talk of anode and cathode frequency response (1 and 10 Hz, respectively), as well as a... 

Figure 55. Simulated half-cell impedances of the cell shown in Figure 53, calculated using finite-element analysis. (a) Half-cell responses assuming an electrode misalignment dlL equal to 1, as defined in Figure 54c. (b) Half-cell responses assuming perfect electrode alignment [dlL = 0).

ACEA A549 cell impedance measures over 7... 

Further problems can exist in the electrofluorination of aromatics arising from polymer formation on the anode surface, causing increased cell impedance and low current efficiencies. 

It is easily argued that, if the voltage measured by the detector D equals zero, the impedance of the cell must be equal to the impedance of the series combination, i.e. Z = Rs and Z" = (coCs) 1. Usually, the values of Rs and Cs needed to balance the bridge vary with the applied frequency as a consequence of the fact that the cell impedance has a frequency dependence different from that of a simple RC series combination. 

Another technique that is useful at frequencies of less than —10 kHz is phase-selective demodulation. A distinct advantage of this technique is that it enables the separation of the resistive (in-phase or real ) and capacitive (90°-out-of-phase or quadrature ) components of the cell impedance. This is accomplished through the process of cross-correlation [13] (selecting that component of es that correlates with the phase of i). The excitation voltage waveform is multiplied by a square wave that is in phase with the cell current waveform. 

We can understand how this is carried out by considering the waveforms of Figure 8.13a. At frequencies for which parallel capacitive components of the conductance cell impedance are negligible, sinusoidal excitation of the cell produces the waveforms of A, where es, eR, ec, and i have the same significance as previously discussed. In order to measure the real component of the impedance, the magnitude of the correlation integral cc must be determined. 

Therefore, the magnitude of the correlation integral is proportional to the resistive component of the cell impedance at constant frequency. This is shown qualitatively in waveforms A to C of Figure 8.13a. Graphical multiplication of waveform B and es in A gives the correlation waveform C. The shaded areas represent areas that cancel upon integration and the unshaded area represents res dt. 

---