2 V-Class Aqueous Multi-Ion Batteries Realized By Superconcentrated Na/K Electrolytes
Authors: Hosaka Tomooki , Noda Ayumi , Kubota Kei , Matsuda Yuki , Ida Kazuhiko , Denzumi Shigenobu , Komaba Shinichi 
Published: February 2020

Aqueous superconcentrated electrolyte solutions have recently attracted attention for aqueous high-voltage Li-ion batteries due to their wider potential window than that of conventionally concentrated aqueous ones. According to literatures, 21 mol kg-1 LiN(SO2CF3)2 (LiTFSA)/H2O and 27.8 mol kg-1 Li(TFSA)0.7[N(SO2C2F5)2 (BETI)]0.3/H2O electrolytes show potential window wider than 2 V and 3.5 V, respectively (1, 2). As well as the Li electrolytes, superconcentrated aqueous Na and K electrolytes have been recently studied. Thanks to weak Lewis acidity of Na+ and K+ ions, aqueous Na and K solutions show higher ionic conductivity than Li counterparts in principle. Indeed, 17 mol kg-1 NaClO4/H2O solution exhibits a very high ionic conductivity of 108 mS cm-1; however, potential window wider than 2 V is still challenging in aqueous Na-ion batteries (3). In this study, we have developed superconcentrated mixed cation electrolytes demonstrating wide potential window and high ionic conductivity based on NaN(SO2F)2 (NaFSA), KFSA, and H2O system to realize 2 V-class aqueous Na/K multi-ion batteries.

Ionic conductivities of the electrolytes were measured at 25 °C. NaTi2(PO4)3 carbon composite (NTP/C) (4) and Na3V2(PO4)3 (NVP) (5) were used as negative electrode materials, and K2Mn[Fe(CN)6] (6) was as a positive electrode material. Al and Ti foils were used as a current collector for the negative and positive electrodes, respectively.

Figure 1a shows water content of saturated aqueous K
x
Na1-x
FSA solutions. The concentration of the saturated NaFSA and KFSA solutions as endmembers is 20 mol kg-1 and 31 mol kg-1, respectively. Higher concentration is realized at eutectic or close to eutectic composition of NaFSA - KFSA: 35 mol kg-1 for Na0.55K0.45FSA/H2O and 33 mol kg-1 for Na0.45K0.55FSA/H2O. Both Na/K mixed electrolytes show ionic conductivity of 20–25 mS cm-1 which is much higher than 3 mS cm-1 of 27.8 mol kg-1 Li(TFSA)0.7(BETI)0.3 (2). Figure 1b shows the LSV curves of Al and Ti foil for a cathodic and anodic scan, respectively, in the superconcentrated Na, K, and mixed electrolytes. 20 mol kg-1 NaFSA/H2O and 31 mol kg-1 KFSA/H2O as endmembers show potential window of 3.2 V and 3.4 V, respectively. As expected, the Na/K mixed electrolytes demonstrate a wider potential window of 3.5 V.

NTP//K2Mn[Fe(CN)6] and NVP//K2Mn[Fe(CN)6] cells were fabricated using 33 mol kg-1 K0.55Na0.45FSA/H2O electrolyte. The NTP//K2Mn[Fe(CN)6] cell shows reversible charge/discharge curves with mainly two discharge voltage plateaus located at 1.7 V and 1.3 V and delivers reversible capacity of ca. 130 mAh g (positive electrode)-1 and 70 mAh g (negative electrode)-1 with excellent capacity retention over 30 cycles as shown in Fig. 1c. On the other hand, the NVP//K2Mn[Fe(CN)6] cell demonstrate 2 V-class operation as shown in Fig. 1d. Electrochemical performance and charge/discharge mechanism of the electrode materials in terms of insertion/extraction of Na/K multi-ions will be discussed in the presentation.

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Figure 1