Effect of Concentrated Electrolyte on High Voltage Aqueous Sodium-Ion Battery
Authors: Nakamoto Kosuke , Kitajou Ayuko , Ito Masato , Okada Shigeto 
Published: February 2020

Introduction

Na-ion batteries with aqueous electrolyte have attracted much attention, since it has 3 big advantages about the conductivity, non-flammability and cost. In addition, water is ideal solvent that can dissolve various salts in large amount. However, there is a severe restriction in the selection of the cathode and anode active materials, because of the narrow electrochemical window of water.
Most recently, the expansion of the working voltage in aqueous cell have been tried either by judicious choice of active materials [1] and by increasing electrolyte concentration [2], which effectively prevents the electrochemical decomposition of water.
Herein, new aqueous sodium-ion battery with inexpensive Na2MnFe(CN)6 (NMHCF) Prussian blue analogues as cathode and NASICON-type NaTi2(PO4)3 (NTP) as anode is introduced. Minor-metal free NMHCF and NTP are very attractive electrode active materials, because they have voltage plateaus near the upper/lower limit of electrochemical window of water. It has been reported that NTP anode can work in aqueous electrolyte reversibly at 2.1 V vs. Na/Na+ [3]. On the other hand, NMHCF has 2 high voltage plateaus at 3.5 V and 3.8 V vs. Na/Na+ and the higher voltage plateau is located above the electrochemical window of water. This is a reason that there are no reports about NMHCF cathode in aqueous electrolyte up to now.
Here, to suppress the electrochemical decomposition of water, highly concentrated NaClO4 aqueous electrolyte was tried to realize high voltage aqueous sodium-ion battery with NMHCF cathode and NTP anode.

Experimental

NaxMn[Fe(CN)6]y·zH2O (NMHCF, 0<x<2, 0<y<1, 0<z) cathode material was obtained by co-precipitating method written in the previous reports [4]. NaTi2(PO4)3 anode material was prepared by conventional solid-state reaction of stoichiometric starting materials. NaxMn[Fe(CN)6]y·zH2O molecular formula was determined by inductive coupled plasma atomic emission spectroscopy (ICP-AES) and thermogravimetric analysis (TGA). Both NMHCF and NTP were mixed with acetylene black (AB) in a weight ratio of active material/AB = 70/25, respectively. In addition, NTP/AB was annealed in Ar atmosphere at 800 °C for 12 h. The cathode and anode pellets were fabricated with 5 wt % of polytetrafluoroethylene binder and punched into disks. And then these pellets were sandwiched by titanium mesh. A three-electrode electrochemical cell (half-cell) and two electrode cell (full-cell) with aqueous electrolyte were used for the galvanostatic charge/discharge test. An Ag-AgCl electrode with saturated KCl and NTP were used as the reference and counter electrodes, respectively. The cathode/anode weight balance for this ion-type cell is 2:3, and the cathode/anode capacity balance is approximately 2:3 (anode capacity excess condition). Here, 1 mol/kg and 17 mol/kg NaClO4 diluted/concentrated aqueous solution were used as aqueous electrolytes.

Results and Discussion

By the ICP-AES and TGA, the NMHCF cathode molecular formula and theoretical capacity were determined as Na1.27Mn[Fe(CN)6]0.76·1.39H2O and 126 mAh/g, respectively.
Figure 1 compares the charge/discharge profiles of NMHCF//NTP full-cell with (a) 1 mol/kg and (b) 17 mol/kg NaClO4 aq. at the rate of 2.0 mA/cm2. In the case of 1 mol/kg electrolyte, the large irreversible capacity corresponding to the O2 gas evolution on cathode was observed on the 1st cycle between 0.5 and 2 V. On the other hand, the full-cell with 17 mol/kg electrolyte showed the reversible behavior in the wider voltage range than the electrochemical window of water. The drastic effect must be due to the suppression of the oxidation by the concentrated aqueous electrolyte.
To confirm the Fe2+/Fe3+ and Mn2+/Mn3+ redox reaction of NMHCF cathode in concentrated aqueous electrolyte, X-ray photoelectron spectroscopy and X-ray diffraction were measured during 1st cycle. These results will be discussed on the day.

References

[1] X. Wu, et al., ChemNanoMat., 1 (2015) 188.
[2] L. Suo, et al., Science, 350 (2015) 938.
[3] S. Park, et al., J. Electrochem. Soc., 158(10) (2011) A1067.
[4] L. Wang, et al., Angew. Chem. Int. Ed., 52 (2013) 1964.

Figure 1