pubs.acs.org/JPCL
Switching Redox-Active Sites by Valence Tautomerism in Prussian Blue Analogues AxMny[Fe(CN)6] 3 nH2O (A: K, Rb): Robust Frameworks for Reversible Li Storage )
M. Okubo,†, D. Asakura,† Y. Mizuno,† J.-D. Kim,‡ T. Mizokawa,§ T. Kudo,† and I. Honma*,# †
National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki, 305-8578 Japan, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan, §Department of Complexity Science and Engneering, University of Tokyo, Kashiwanoha 5-1-5, Chiba 277-8581, Japan, and #Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan ‡
ABSTRACT The discovery of a new electrode material that provides a reversible Li ion insertion/extraction reaction is of primary importance for Li ion batteries. In this report, electrochemical Li ion insertion/extraction in valence tautomeric Prussian blue analogues AxMny[Fe(CN)6] (A: K, Rb) was investigated. Ex situ X-ray diffraction experiments revealed that the K salt without the valence tautomerism exhibits the Li ion insertion/extraction with a redox process of an Fe ion, while the Rb salt with the valence tautomerism exhibits that with a redox process of a Mn ion. Regardless of the redox-active metal ions, highly reversible Li ion storage was achieved. The electronic structure changes during the Li ion insertion/extraction are confirmed by XPS experiments. SECTION Energy Conversion and Storage
Among the possible building blocks for the FeIII-baed MOFs, [FeIII(CN)6]3- has a redox potential of 3.4 V [versus Li/Liþ]. This suggests that MOFs consisting of [FeIII(CN)6]3- are more promising candidates for the cathode materials with the higher charge/discharge voltage than MIL-53. Therefore, we investigated the Li ion insertion/extraction in Prussian blue analogues (PBAs), AxMnIIy[FeIII(CN)6] 3 nH2O (A: K, Rb). PBAs are rather classical materials among MOF families, whose structural, physical, magnetic, and electrochemical properties have been exhaustively studied.11 Generally, the structural framework of PBAs is closely related to that of perovskites, in which the metal centers are connected by cyanide bridges instead of oxide ions (Scheme 1). The stoichiometry is strongly dependent on the oxidation states of the metal centers and the amount of alkali metal cations present. The generalized formula can be written as AxBII1.5-0.5x[B0 III(CN)6] 3 00.5-0.5x 3 nH2O (A: alkali metal; B, B0 : transition metal; 0 < x < 1), where [B0 III(CN)6]3- has fractional occupancy of the sites to form intrinsic vacancies (0) occupied with coordinating and zeolitic water molecules. The π orbital electron delocalization expected in cyanide has the possibility to stabilize the Li ion storage, and lightweight cyanide is also expected to exhibit high capacity. It should be mentioned that some studies on Li ion insertion/extraction in PBAs have been carried out already;12,13 however, according to these reports,
T
he study on a new class of electrode materials that provide reversible Li ion insertion/extraction reaction is of primary importance for rechargeable Li ion batteries because the automotive industry requires a reduction in conventionally used high-cost metal oxides (e.g., LiCoO2).1,2 The LiFePO4 family is now one of the most promising candidates for cathode materials3-7 and are beginning to be applied commercially. However, the Li ion insertion/extraction reaction in LiFePO4 proceeds mainly via a two-phase process, unless the crystallite size is reduced down to the extreme nanosize (∼
