High-energy-density aqueous sodium-ion batteries enabled by chromium hexacycnochromate anodes

Aqueous sodium-ion batteries (A-SIBs) are a cost-effective and safe battery candidate for stationary energy storage systems. Prussian blue and its analogues (PBAs) have been studied as cathode materials in A-SIBs. However, few studies have used PBAs as anode materials because of their small specific capacity and low cell voltages. Herein, we report that chromium hexacycnochromate (CrCr PBA) can serve as an efficient anode material to enable high-performance A-SIBs. CrCr PBA contains Cr ions at two types of sites coordinated by C and N, offering two redox pairs at -0.62/-0.65 V and -0.73/-0.77 V (vs. Ag/AgCl electrode), respectively. X-ray photoelectron and electron energy loss spectroscopic characterizations of CrCr PBA at different charge/discharge stages show that Cr at both types of coordinated sites can efficiently participate in Na+ storage, delivering a high specific capacity of 108.2 mAh g-1 at a discharge rate of 0.5 A g-1. Besides, low redox potentials of CrCr PBA also contribute to a higher battery voltage. Coupled with a manganese hexacyanoferrate cathode in a water-in-salt electrolyte (17 M NaClO4), full coin cells exhibit a high energy density of 81.6 Wh kg- 1 and a high capacity of 52.8 mAh g-1 at an average voltage of 1.55 V. They also display a long-cycle-life with 93.01% capacity retention over 500 cycles at 30C with around 100% Coulombic efficiency. Their performance is one of the best among recently reported A-SIBs. Because of their similar energy storage mechanisms, CrCr PBA may also serve as promising anode materials in other types of aqueous alkaline metal-ion batteries (e.g., lithium-ion battery and potassium-ion battery).