This talk will discuss electrochemical impedance spectroscopy (EIS) tracking of aqueous alkaline Zn-MnO₂ cells cycled at 20% depth of discharge (DOD) based on cathode capacity. Shallow cycled alkaline batteries have previously been reported as cost effective and safe options for large-scale electrical storage.^1-3 Periodically collected EIS data was used to fit a full battery model based on Voigt elements, and fitted parameters were tracked over time. These were used as a real-time diagnostic to assess performance and predict future performance in advance of any degradation of the cell voltage.

The cell model was based on individual electrode models developed previously by Donne and co-workers for γ-MnO₂ and Hampson and McNeil for Zn.^4,5 Two prismatic cell builds were compared using electrodes fabricated by two different commercial sources with identical compositions. Both cell performance and EIS response were distinctly different between the electrode sources. The model provided an acceptable fit of the experimental data in both cases, as shown in Figure 1. The parameters of the model corresponded to physical phenomena, allowing an analysis of the performance difference despite the fact that all electrode fabrication variables could not be known unless provided by the commercial sources.

The combined anode and cathode interfacial models were incorporated into a transmission line porous electrode, shown in Figure 2. Each anode + cathode fit involved a combined 15 parameters, which was the minimum number of parameters that would fit data for all cells in all states of charge. Performance analysis was accomplished by comparing a) the individual parameters, b) lumped parameters such as the RC time constants and RLC Q factors, and c) features of the cycling potential such as the discharge end voltage (DEV).⁶ Use of a reference electrode with EIS has been shown to be highly dependent on electrode placement.^7,8 Battery EIS also faces a challenge in that electrodes may have similar capacity, while ideally the counter electrode should be non-limiting.⁹ We will address these factors and discuss steps taken to obtain repeatable data free of inductive loops caused by capacitive coupling with current collectors and electrode tabs.¹⁰

Experimental procedure

The cycling profile used constant-current/constant-voltage (CCCV) taper charging and constant current (CC) discharging. The cycled capacity was 20% of the one-electron MnO₂ capacity in the cathode. The CC discharge was completed in 4 hours. The CCCV charge had a 1% overcharge, with the same current as discharge and an upper voltage limit of 1.65 V. If this voltage limit was tripped, voltage was held at 1.65 V until the desired charge capacity was reached or current dropped to below 20% of the CC stage. Every tenth cycle the cell was paused after the OCV period following discharging, and an EIS measurement was performed. The EIS potential was the sampled OCV when EIS began, with an amplitude of 5 mV. Frequencies were scanned from 50 kHz to 20 mHz at a spacing of 10 points per decade. Upon completion the cell was CCCV charged as usual, there was a 10 minute OCV period, and another EIS measurement was performed in the charged state.

Acknowledgement

This work was supported by the National Science Foundation under Grant No.1332030 in the Small Business Technology Transfer Research (STTR) program.

References

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