Lithium ion battery has been a popular energy storage technology for its high energy density and long cycle life. Up to now, hundreds of materials have been proposed as the cathode materials [1]. Among them, Prussian Blue (PB) and its analogues have recently been studied and gained considerable attention for rechargeable batteries applications because of their ability to accommodate a wide variety of ions, low cost, enviromental friendliness and ease for processing [2,3,4]. In addition, the redox reactions of PB are believed to be very fast and reversible. Previous investigations on PB and its analogues as electrodes in lithium ion batteries have mostly been aimed at electrochemical lithiation and delithiation [5].The intercalation reaction can be written as

Fe₄[Fe(CN)₆]₃+4Li⁺+4e^-→Li₄Fe₄[Fe(CN)₆]₃                                                  (1)

It is reported that commercial insoluble PB Fe₄[Fe(CN)₆]₃ can deliver a specific capacity of about 100mAhg^-1 owing to one-electron transfer per formula unit. Lithium ion batteries utilizing Fe₄[Fe(CN)₆]₃ as cathode material, however, suffer from a low ratio of available capacity to theoretical capacity and a limited cycle life which may result from its insulating nature. In addition, PB cathodes have a traditional problem of low mass loading, which also limits the application in the field of large scale energy storage [5]. Very recently, redox flow lithium battery (RFLB) based on the concept of redox targeting reaction has emerged [6,7,8,9,10] as an implementable solution for large-scale energy storage, in which the insulating or poorly conducting Li⁺ electrode material can be reversibly delithiated/lithiated via chemical reactions without being attached to the current collector (Figure 1a).

Inspired by that, we report here for the first time of the reversible chemical lithiation and delithiation of PB, and the application in the redox flow lithium batteries for cheaper, large-scale energy storage. Fe₄[Fe(CN)₆]₃ shows reversible lithium insertion potential at ~2.90 V vs. Li/Li⁺. The redox potentials of the Ethyl viologen (EV) diiodide are ~2.65V for EV⁺/EV²⁺ and ~3.1Vfor I^-/I₃^-respectively, perfectly straddling that of PB. Therefore, Fe₄[Fe(CN)₆]₃ could be reduced by EV⁺, and Li₄Fe₄[Fe(CN)₆]₃ could be oxidized by I₃^- thermodynamically. The chemical lithiation and delithiation of PB can be described as the following chemical equations:

   Fe₄[Fe(CN)₆]₃ (PB)+4Li⁺+4EV⁺→Li₄Fe₄[Fe(CN)₆]₃(PW)+4EV²⁺                                     (2)

   Li₄Fe₄[Fe(CN)₆]₃(PW)+ 2I₃^-→Fe4[Fe(CN)6]3(PB)+4Li⁺+6I^-                                          (3)

The reversible phase transition between PB and Prussian White (PW) was clearly observed by UV-Vis, because the redox targeting reactions accompany striking color changing. It was also confirmed by XPS test through monitoring the appearance and disappearance of the peak attributed to Li 1s.Moreover, cyclic voltammetry(CV) measurement was conducted to further investigate the kinetic process of the redox targeting reactions. When the CV was conducted on an FTO-Pt-Al₂O₃-PB electrode in Ethyl viologen diiodide electrolyte, we observed current plateaus with significantly enhanced current density instead of the reduction/oxidation peaks for EV²⁺/ EV⁺ and I^-/I₃^-. A redox-flow lithium battery was assembled using Ethyl viologen diiodide electrolyte as redox mediator and Fe₄[Fe(CN)₆]₃ as solid energy storage material in tank. The battery showed considerably extended capacity beyond that of the redox mediators (Figure 1b). It was estimated that almost 60% Fe₄[Fe(CN)₆]₃ was reversibly charged/discharged. We anticipate the PB-based RFLB would provide a low-cost means for high-density large-scale energy storage.

This research was supported by the National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP8–2011–04).

Reference:

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[7]Pan, F., Yang, J., Huang, Q., Wang, X., Huang, H., Wang, Q., Advanced Energy Materials, 2014, 15, 1400567-1400574.

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