Aqueous Na-Ion/ K-Ion Battery with Cyano-Bridged MOF Cathode and Bis(pyrazolate)-Bridged MOF Anode
Authors: Zhao Minyan , Nakamoto Kosuke , Sakamoto Ryo , Ito Masato , Okada Shigeto 
Published: December 2020

Aqueous Na-ion/ K-ion batteries with organic active material have gained intensive attention in the field of electric energy storage because of their abundant resources. However, the rate capability and cyclability of organic active material are relatively poor because of their low diffusivity and undesired miscibility with aqueous electrolyte. To address these issues, we are committed to using organic active material embedded in metal organic frameworks (MOFs), whose structures are open and robust. In this work, we introduce aqueous Na-ion/ K-ion batteries with a cyano-bridged MOF cathode and a bis(pyrazolate)-bridged MOF anode, which proved to be both adequately alkaline ion-diffusive and immiscible with aqueous electrolyte.
Prussian blue-type alkaline metal nickel hexacyanoferrate A2Ni[Fe(CN)6] (ANHCF, A = Na, K), in which redox-active Ni and Fe are bridged by κ2-cyano ligand, was synthesized according to previous reports [1,2], while Zn bis(pyrazolate) MOF (ZnNDI), in which redox-silent Zn are bridged by κ2-bis(pyrazolate) ligand containing redox-active naphthalenetetracarboxylic-diimide (NDI) linker, was obtained by the literature method [3]. Electrochemical measurements were carried out by using a three-electrode half cell and a two-electrode full cell with alkaline metal trifluoromethanesulfonates (ASO3CF3) aqueous electrolyte in the molality of 8 mol/kg, which was the most suitable concentration in terms of ionic conductivities and stability window. Electrode pellet were prepared by mixing active materials, conductive additives (acetylene black) and binder (polytetrafluoroethylene) in a weight ratio of 70:25:5, and then these pellets were sandwiched by titanium mesh as current collector. An Ag-AgCl electrode with saturated KCl and activated carbon were used as reference and counter electrodes, respectively.
Figure 1a compares 1st and 2nd charge/discharge curves of Na-ion (blue lines) and K-ion (red lines) cells. In contrast to Na-ion cell, K-ion cell displayed higher voltage and larger reversible capacity, possibly due to the fewer [Fe(CN)6]α
- (3 < α < 4) defects in the structure of KNHCF than those in NNHCF. After 400 charge/discharge cycles, the Na-ion cell maintained the capacity more than 96% of that at the initial cycle, while the K-ion cell was only 69%, as shown in Figure 1b. These differences may arise from the electrochemical stability of water molecule in the electrolyte, provided hydration number of K-ion is lower than that of Na-ion and hence the number of free water molecules in K-ion electrolyte is higher.
Acknowledgement
This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003.
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Figure 1