Sodium-rich iron hexacyanoferrate with nickel doping as a high performance cathode for aqueous sodium ion batteries

Na-rich NaxFeFe(CN)6 (NFF) in terms of low cost and high theoretical capacity has been considered to be a potential cathode material for aqueous sodium ion batteries (SIBs). However, low sodium content, poor Columbic efficiency (CE) and rapid capacity degradation impede their practical application. Herein, the effect of low-level Ni doping in NFF as cathode materials in 1 M NaNO3 electrolyte has been investigated, which were synthesized via a citrate-assisted co-precipitation route. NFF substituted with 23% Ni2+ exhibited the best electrochemical behaviors, which delivers a discharge capacity of 105.9 mAh g−1 at 200 mA g−1 and 73.1% capacity retention over 1000 cycles with an average CE of 99.8% at 1000 mA g−1. A reversible capacity of 55.5 mAh g−1 can be obtained even at a high current density of 2000 mA g−1. The Ni substitution strategy was evidenced to be a scalable method to prepare promising cathode materials with excellent cycling stability for high-performance aqueous SIBs.