2 V-Class Aqueous Multi-Ion Batteries Realized By Superconcentrated Na/K Electrolytes

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(SO 2 CF 3 ) 2 (LiTFSA)/H 2 O and 27.8 mol kg -1 Li(TFSA) 0.7 [N(SO 2 C 2 F 5 ) 2 (BETI)] 0.3 /H 2 O 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 NaClO 4 /H 2 O 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(SO 2 F) 2 (NaFSA), KFSA, and H 2 O system to realize 2 V-class aqueous Na/K multi-ion batteries. Ionic conductivities of the electrolytes were measured at 25 °C. NaTi 2 (PO 4 ) 3 carbon composite (NTP/C) (4) and Na 3 V 2 (PO 4 ) 3 (NVP) (5) were used as negative electrode materials, and K 2 Mn[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 Na 1- 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 Na 0.55 K 0.45 FSA/H 2 O and 33 mol kg -1 for Na 0.45 K 0.55 FSA/H 2 O. 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/H 2 O and 31 mol kg -1 KFSA/H 2 O 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//K 2 Mn[Fe(CN) 6 ] and NVP//K 2 Mn[Fe(CN) 6 ] cells were fabricated using 33 mol kg -1 K 0.55 Na