A new anode material for inorganic-based rechargeable batteries

Abstract

Iron hexacyanochromate (FeHCC) has been introduced as a new anode material for the fabrication of high voltage all-solid-state batteries. As the redox potential of FeHCC is sufficiently negative, it can be used as the anode in inorganic complex-based rechargeable batteries to gain high-voltage performances. Material chemistry has an important role in introducing new materials with acceptable properties for battery performance. The most important factor in candidate materials is the potential of their electrochemical redox systems to offer high-voltage performances. Among different types of rechargeable batteries, all-solid-state batteries are very interesting due to their advantages for commercial applications. An interesting type of solidstate battery is based on electrochemical redox systems of two solid materials utilized as anode and cathode. Therefore, two materials with a significant difference between their redox potentials are required for this purpose. Among inorganic materials, transition metal hexacyanoferrates are one of the most famous materials in solid-state electrochemistry due to their excellent properties [1, 2] (but less attention has been paid to other hexacyanometallates). There is considerable potential in this class of inorganic compounds for use as electrode materials in rechargeable batteries, due to their excellent cycleability. It has been reported that Prussian blue (PB) can intercalate/deintercalate potassium ions in an aqueous medium for 105 reversible cycles [3]. This ability can be increased up to 107 cycles by using a suitable substrate such as ITO and SnO2 for the deposition of the electroactive film [4]. The importance of substrate surfaces to deposit highly stable surfaces has been described for the deposition of a variety of materials onto aluminum substrate [5–7] based on the direct modification method [8]. Several hexacyanoferrate-based rechargeable batteries have been reported in the literature [3, 4, 9–12]. However, the problem is that hexacyanometallates have redox potentials located in the same range. Therefore, finding two hexacyanometallates with significantly different redox potentials to produce a secondary cell with acceptable voltage is difficult. To overcome this problem, the use of other hexacyanometallates has been suggested to offer greater variation of cathode materials for the fabrication of high-voltage secondary cells [13]. In the present communication, we introduce a transition metal hexacyanometallate with excellent properties for fabrication of a high-voltage battery. PB is a known cathode material for such batteries, due to its high redox potential ca. 0.9 V vs. SCE (saturated calomel electrode). It also should be noted that PB oxidation/reduction occurs in two steps, however, the upper redox can be applied when used as a cathode material. Thus, we need to find an anode material with sufficient negative redox potential to produce a high-voltage cell. Investigations of elecrochemical properties of different hexacyanometallate ions in aqueous solution showed that Cr(CN)3− 6 has a noticeable negative potential ca. −1.140 V vs. SHE (saturated hydrogen electrode), due to the Cr(CN)3− 6 /Cr(CN) 4− 6 redox system, which is in agreement with the literature [14]. Studies of electrochemical behavior of different transition metal hexacyanochromates were indicative of the fact that iron hexacyanochromate (FeHCC) has the most negative potential. Such behavior has been reported in a comprehensive study of redox potentials of hexacyanometallates [15]. Based on this fact, in the present communication, we examine the possibility of iron hexacyanochromate as a potential anode material for all-solid-state redox batteries in accordance with its negative redox potential. The iron hexacyanochromate (FeHCC) was synthesized according to the methods previously described in the literature [16, 17]. Cyclic voltammetric measurements were performed using an electrode by mechanical immobilization of FeHCC as a thin solid film, according to the method described in the literature [1]. The electrodes (anode and cathode) were prepared by mixing 0.25 g powder of FeHCC or PB with 2.0 g graphite powder. Then, a paste form was obtained by adding a few drops of diluted HCl (0.1 M). In addition, as proton transfer is strongly involved in the cell processes, the acidic medium is very helpful in avoiding any local pH changes. The cell design was similar to those reported in the literature [11, 12]. The all-solid-state cell was prepared by using two solid films and conditioning a Nafion layer between two electrodes to separate two half-cell reactions to avoid any material exchange. Thus, this ion-exchange membrane just allowed proton-exchange between two electrodes to maintain the charge balance. This means that potassium ions are the only ions involved in the intercalating and deintercalating processes. The battery investigations were performed at different charge/discharge rates (Note: 1C = 1 mA cm−2) at room temperature. The electrochemical measurements were carried out using a homemade potentiostat connected to a computer running CorrView software. All chemical reagents were of analytical grade. Fig. 1 shows a typical cyclic voltammetric characteristic of the FeHCC mechanically immobilized onto an