Electrochemical Energy Storage in Cs2Zn3[Fe(CN)6] and Their Use As Cathode at Na-Ion Batteries in Aqueous Media

The use of renewable energy resources represented by solar and wind must be the most ideal solution to reduce the serious energy and environmental problems. However, the available amounts of these resources are dependent of weather conditions, thus requiring energy storage systems (ESSs) for persistent power supply. While hydropower and pumped air systems are currently used as grid-scale ESSs, rechargeable batteries are expected to replace the existing technology utilizing their advantages related with higher efficiencies and simple installation. Among a variety of batteries, lithium-ion batteries (LIBs) are receiving the most significant attention due to their superior performance in various electrochemical aspects. In this context, batteries based on alternative carrier ion such as sodium (Na⁺) might be more suitable for grid-scale ESSs, linked with critical aspects such as abundance and cost of the raw materials. Indeed, raw materials for sodium-ion batteries (SIBs) are estimated to be about 30 times cheaper than those of LIBs. On the other hand, the use of radio-cesium (¹³⁷Cs) in nuclear-power plants and for military activities have caused a critical environmental hazard as main fission product in radioactive wastes.

From past few decades, transition metal hexacyanoferrates known as Prussian Blue Analogues (PBAs), have been widely used for Cs removal from waste waters. This open framework materials with the Prussian Blue crystal structure offer the high power capability and low cost synthesis. Herein, we report the synthesis of zinc hexacyanoferrate (ZHCF), which is endowed with enlarged ionic channels compared with typical PBAs. ZHCF was synthesized by precipitation method from a mixture of Na₄Fe(CN)₆ and Zn(NO₃)₂ solutions at 60°C. The solid formed was centrifuged and dried to room temperature. The solid material was mixed with a solution of CsNO₃ during 24 hr. Then  centrifuged again and dried. The obtained material was labeled as Cs-ZHCF. Using XRD, the crystal structure suggests that materials crystallize with a rhombohedral unit cell (R-3c space group). Their porous framework can be represented as the assembling of FeC₆ octahedral and ZnN₄ tetrahedral shapes_. Therefore, the synthesis process promotes the assembling of octahedral-anionic blocks ([Fe(CN)₆]^4-) through Zn²⁺ cations, which links neighboring blocks at the N ends. In the structure of these materials the N–Zn–N angle is near to 108°. The formation of elliptical-like windows and ellipsoidal cavities is related to such bonding angle. The calculated cavity dimension was ca. 15.5 x 11.1 x 7.9 Å. Six elliptical windows of ca. 6.8 x 8 Å communicate neighbor cavities. Within a given cavity two exchangeable Cs-ions are found, located close to the major ellipsoid axis in order to minimize the repulsive electrostatic interactions between them.

For the electrochemical characterization using cyclic voltammetry in NaNO₃ solutions, the Cs-ZHCF was mixed with amorphous carbon, polyvinylidenedifluoride, and graphite in a ratio of 80:9:9:2 were prepared in THF. The obtained suspension was deposited on a glassy carbon electrode, and dried in Argon atmosphere. It was found that the i-E profiles show a reversible capacity of 56.4 mAhg^-1. For the evaluation of the observed electrochemical data, it is important to note that in Cs-ZHCF, Zn is electrochemically inactive but rather only contributes to maintaining the framework structure, whereas, the iron in octahedral coordination can be oxidized and reduced during charge-discharge. These results indicate that only one Cs-ion was removed from the cavity of the material-structure, improving the diffusion of Na-ions.