1388-2481/99/$ - see front matter q1999 Elsevier Science S.A. All rights reserved. PII S1388-2481 ( 99 ) 00107-1 Tuesday Oct 26 09:33 AM StyleTag -- Journal: ELECOM (Electrochemistry Communications) Article: 117 www.elsevier.nl/locate/elecom Electrochemistry Communications 1 (1999) 522–526 Insoluble Fe(VI) compounds: effects on the super-iron battery Stuart Licht *, Baohui Wang, Susanta Gosh, Jun Li, Vera Naschitz Department of Chemistry, Technion Israel Institute of Technology, 32000 Haifa, Israel Received 31 August 1999; accepted 3 September 1999 Abstract Cathodes composed of Fe(VI) salts are capable of three-electron reduction, and are useful for energetic super-iron batteries. This study investigates the solubility of BaFeO 4 and K 2 FeO 4 Fe(VI) salts. Electrolytes are determined in which Fe(VI) has a low aqueous or non- aqueous solubility, or is insoluble. Insoluble Fe(VI) salts have the duel benefits of preventing Fe(VI) solution-phase (i) decomposition and (ii) diffusion to the anode; thereby preventing super-iron battery self-discharge. BaFeO 4 is insoluble in water, and has a solubility of less than 2=10 y4 M in 5 M KOH containing Ba(OH) 2 . A BaFeO 4 super-iron battery has a high discharge efficiency when containing an electrolyte of either 12 M KOH, or 6 M KOH saturated in Ba(OH) 2 . Fe(VI) cathodes in non-aqueous media may be useful in providing a high-capacity Li or Li-ion super-iron battery. We illustrate that Fe(VI) salts are insoluble and chemically unreactive with a range of organic electrolytes, and can be discharged as cathodes in non-aqueous electrolytes. In acetonitrile containing 1 M LiClO 4 , the discharge of an Fe(VI) cathode is demonstrated to a capacity over 394 mAh g y1 K 2 FeO 4 . q1999 Elsevier Science S.A. All rights reserved. Keywords: Super-iron; Fe(VI); Ferrate; Battery; Primary battery; Secondary battery 1. Introduction We recently reported a class of batteries, referred to as super-iron batteries, containing a cathode utilizing a common material (iron) in an unusual (greater than 3) valence state [1]. The cathode is based on abundant starting materials, and is compatible with an alkaline electrolyte, and either a zinc or a metal hydride anode. The improved high specific charge capacity of metal hydride anodes [2] is greater than that of the nickel oxyhydroxide cathode in metal hydride batteries. Similarly, the high specific charge capacity of zinc anodes (820 mAh g y1 ) is greater than that of the manganese dioxide cathode in alkaline batteries. The storage capacity in both of these batteries is significantly cathode limited. Replacement of the manganese dioxide or nickel oxyhydroxide cathodes in these cells with a more energetic cathode such as Fe(VI) compounds can substantially increase the energy storage capacity of these cells. For example, using the same zinc anode and electrolyte, Fe(VI) cathode batteries were shown to provide 50% more energy capacity than in conventional alkaline batteries [1]. * Corresponding author. Tel.: q972-4-829-2963; fax: q972-4-823-3735; e-mail: chrlicht@techunix.technion.ac.il Iron typically occurs as a metal, or in the valence states Fe(II) or Fe(III). Fe(VI) species have been known for over a century, although its chemistry remains relatively unex- plored [3]. The term ‘ferrate’ has been variously applied to both Fe(II) and Fe(III) compounds. Instead, due to their highly oxidized iron basis, multiple electron transfer, and high intrinsic energy, we refer to the new cell containing these compounds as ‘super-iron’ batteries. The reduction of Fe(VI) represents an energetic and high capacity source of cathodic charge. The 406 mAh g y1 K 2 FeO 4 cathodic charge capacity is 32% greater than that of manganese dioxide, and we have demonstrated that this full capacity may be accessed in accord with: 2y y y FeO q5/2HOq3e ™1/2Fe O q5OH (1) 4 2 2 3 BaFeO 4 , although of lower intrinsic three-electron capacity (313 mAh g y1 ) than K 2 FeO 4 , is observed to support higher current densities than K 2 FeO 4 [1]. A BaFeO 4 cathode-based cell has substantially higher energy storage capacity than MnO 2 , particularly at high discharge rate [1]. In a zinc alka- line battery, the zinc anode generates a distribution of zinc oxide and zincate products, and similarly the final Fe(VI) product will depend on the depth of discharge. The general discharge of alkaline electrolyte cells utilizing a Zn anode and either a K 2 FeO 4 or a BaFeO 4 cathode is expressed as: