Effect of Sulfide Additives on the Discharge Characteristics of Iron Electrodes in Alkaline Batteries
Authors: Manohar Aswin K , Yang Chenguang , Narayanan S. R. 
Published: February 2020

Iron-based rechargeable alkaline batteries such as nickel-iron and iron-air are attractive candidates for large-scale energy storage applications because of the low cost and robustness of the iron electrode. [1] We have recently reported significant advancements in the charging efficiency and discharge rate capability of iron-based alkaline battery electrodes. [2, 3] By using carbonyl iron as the electrode active material and by including additives such as bismuth sulfide and bismuth oxide, charging efficiencies greater than 90% have been demonstrated.

To improve the utilization of the iron electrode and achieve high-discharge-rate capability, sulfide additives are necessary. The importance of sulfide additives in de-passivating the iron electrode during discharge has been reported in the literature. [4-6] Various sulfide-based additives such as sodium sulfide, bismuth sulfide, lead sulfide, and iron sulfide have been successfully employed as de-passivating agents. However, a mechanistic understanding of the effect of different sulfide additives on the polarization characteristics of the iron electrode during discharge is not available. The goal of the present study is to understand the effect of different sulfide additives on the anodic polarization, impedance response, and discharge characteristics of the iron electrode.

Iron electrodes were prepared by mixing carbonyl iron powder with polyethylene binder and hot-pressing the mixture onto a nickel grid. In some cases, additives such as bismuth sulfide, bismuth oxide or iron sulfide in the concentration range of 1-10 wt.% were added to the powder mixture prior to pressing. Electrochemical testing was performed in a three-electrode configuration with a mercury/mercuric oxide (MMO) reference electrode and nickel oxide counter electrodes.

The capacity of carbonyl iron electrodes with various sulfide additives at different discharge rates is shown in Figure 1. In the presence of bismuth sulfide additive, the discharge capacity of the iron electrode at the 1C rate was 0.2 Ah/g – significantly higher compared to that of the ‘sulfide-free’ iron electrode. With iron sulfide additive, the iron electrode was able to sustain discharge rates as high as 3C for similar levels of electrode utilization. (Figure 1)   

The results from the discharge rate capability experiments are consistent with the polarization behavior of the iron electrodes with different additives (Figure 2). When a ‘sulfide-free’ iron electrode is polarized positive to -0.85 V, the discharge current decreased signifying the onset of passivation (Figure 2). However, when sulfide additives are present, the polarization curves do not show any current limitation due to electrode passivation.

The amount of charge required for the iron electrode to undergo passivation has been measured at various states of charge. Analysis of this data is anticipated to provide insight into the process of passivation. An understanding of the effectiveness of various sulfide additives in de-passivating the iron electrode and improving the discharge rate capability will also be presented.

Acknowledgement:

The research reported here was supported by the U.S. Department of Energy ARPA-E (GRIDS program, DE-AR0000136), the Loker Hydrocarbon Research Institute, and the University of Southern California.

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Figure 1