Water-in-Salt Electrolyte Promotes High-Capacity FeFe(CN)₆ Cathode for Aqueous Al-Ion Battery

Abstract

Prussian blue analogues (PBAs) are considered to be ideal multivalent cation host materials due to their unique open-framework structure. In aqueous solution, however, the PBAs’ cathodes have a low reversible capacity limited by the single electrochemical group Fe­(CN)6 3– and high crystal water content. They also suffer from fast cycle fading, resulting from significant oxygen/hydrogen evolution and cathode dissolution. In this work, a high-capacity PBA-type FeFe­(CN)6 cathode with double transition metal redox sites is successfully demonstrated in 5 m Al­(CF3SO3)3 Water-in-Salt electrolyte (Al-WISE). Due to Al-WISE having a wide electrochemical window (2.65 V) and low dissolution of the cathode, our PBA cathode exhibits a high discharge capacity of 116 mAh/g and the superior cycle stability >100 cycles with capacity fading of 0.39% per cycle.

Insights into the Structure Stability of Prussian Blue for Aqueous Zinc Ion Batteries

“…The favorable Zn electrodeposition in concentrated electrolyte might also benefit the long‐term cycling performance. [ 32,33 ] Note that, the coulombic efficiency of concentrated electrolyte is nearly 100% for over 1600 cycles (Figure 2i). However, the coulombic efficiency in 1 m Zn(TFSI) 2 electrolyte is only 85% for the first cycle, followed by capacity decreases for the subsequent cycles (Figure 2h).…”

Structure and Properties of Prussian Blue Analogues in Energy Storage and Conversion Applications

“…Furtherly, FeFe(CN) 6 also displayed capacity of 116 mAh g −1 in 5 m Al(CF 3 SO 3 ) 3 electrolyte. [ 79 ] The limited utilization of Fe LS result in insufficient lifespan, inferior rate capability, and low working voltage of FeHCF. Yang et al.…”

Cathodes for Aqueous Zn‐Ion Batteries: Materials, Mechanisms, and Kinetics

“…There are a large number of interstitial sites in Prussian blue analogs (PBAs) crystal structures, which can act as the storage sites for some ions, especially Mg 2+ , Zn 2+ , and Al 3+ [136–142] . The generic formula of a PBA system can be roughly represented as A x M A y [M B (CN) 6 ] z ⋅ n H 2 O, where M A and M B are usually transition metal atoms (Mn, Fe, Co, Ni, Cu, or Zn), and A is usually an alkali metal atom (Li, Na, or K).…”

“…There are al arge number of interstitial sites in Prussian blue analogs( PBAs)c rystal structures, which can act as the storage sites for some ions, especially Mg 2 + ,Z n 2 + ,a nd Al 3 + . [136][137][138][139][140][141][142] The generic formula of aP BA system can be roughlyr epresented as A x M Ay [M B (CN) 6 ]z•n H 2 O, where M A and M B are usually transition metal atoms (Mn, Fe, Co, Ni, Cu, or Zn), and Ai su sually an alkali metal atom (Li, Na,o rK ). M A and M B can be the same elemento rd ifferent elements, which are distinguishedb yt heir different oxidation states andt heir spin states.…”