Effect of Sulfide Additives on the Cycling Behavior of Iron Electrodes in Alkaline Batteries

Abstract

Iron based alkaline batteries such as nickel-iron and iron-air are promising candidates for large scale energy storage applications [1]. The major impediments in the widespread deployment of iron-based battery systems are the low charging-efficiency and poor discharge- rate-capability of the iron electrode [1, 2]. We have recently reported several advances that address the foregoing issues [3, 4]. By using a high-purity active material such as carbonyl iron together with bismuth sulfide additive, charging efficiencies as high as 96% has been obtained. We have also observed that iron sulfide, when used as the electrode additive, results in an iron electrode that can sustain up to 3C rates during discharge. In addition to the high charging efficiency and good discharge rate capability, another major criterion for grid scale energy storage applications is the need for long cycle life. In present study, we aim at understanding the behavior of the iron electrode to repeated cycling and the effect of various sulfide additives on the cycling characteristics. The iron electrodes used in this study were prepared by mixing carbonyl iron powder with polyethylene binder and other additives and hot-pressing the mixture onto a nickel grid. After preparation, the iron electrodes were subjected to the formation process until a stable discharge capacity was observed. Cycling experiments were performed on such fully formed iron electrodes in a three-electrode set-up with nickel oxide counter electrodes and a mercury/mercuric oxide (MMO) reference electrode. An iron electrode prepared with iron sulfide additive does not show any loss in capacity even after 800 cycles of charge and discharge (Figure 1). An iron electrode with bismuth sulfide additive, however, lost nearly 50% of its initial capacity after about 150 cycles (Figure 1). The potential-time curves (Figure 2) at various stages of cycling of the iron electrode with bismuth sulfide additive indicate that the overpotential during charging increases continuously with cycling. For example, the charging overpotential of the iron electrode (measured at 50% state-of-charge) increased by more than 100 mV after 98 cycles. Upon addition of soluble sodium sulfide to the electrolyte, the capacity of the iron electrode can be recovered in a few cycles (Figure 1). This recovery of the discharge capacity after sulfide addition is also accompanied by a decrease in electrode overpotentials during charging. A mechanistic understanding of various aspects of the cycling behavior of iron electrodes with different sulfide additives will be presented. Various strategies to mitigate the capacity fade in iron electrodes will also be discussed. 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.