Stabilizing Low-Spin Position by Iron–Manganese Coexistence in Hexacyanoferrate for Efficient Aqueous Nickel-Ion Storage

In this work, we designed a Mn-based hexacyanoferrate with partial substitution of MnIII by FeII. The presence of FeII at the low-spin position in Na2Mn1/6Fe5/6Fe(CN)6 plays an important role in inhibiting the Jahn–Teller effect and reducing the dissolution of Mn2+, contributing to the robustness of the crystal structure. As a result, Na2Mn1/6Fe5/6Fe(CN)6 can be qualified as an electrode material for aqueous nickel-ion batteries. A series of electrochemical evaluations show that it can achieve a charge capacity of 76.1 mAh g–1 at 100 mA g–1, as well as an excellent cycling performance with 69.1% capacity retention upon 500 cycles. Besides, the reversible insertion/extraction mechanism of Ni2+ in Na2Mn1/6Fe5/6Fe(CN)6 is investigated by in situ/ex situ methods, demonstrating its stability and reversibility for electrochemical energy storage. In summary, Fe2+ substitution enhances the structural integrity and electrochemical stability of Na2Mn1/6Fe5/6Fe(CN)6 as a host material for nickel-ion storage.