The Li-ion battery has played an important role in portable electronic devices and is being developed actively for plug-in hybrid vehicles. However, limits of the natural abundance and uneven global distribution of lithium-containing precursors has raised a motivation to find a rechargeable Na-ion battery as an alternative due to the more abundant reserves and lower price of Na than Li^[1]. Advance in cathode materials is vital for the development of Na-ion battery due to their relatively low capacity value or poor cycling stability compared to the anode counterparts^[2]. Prussian blue (PB, A_xFe[Fe(CN)₆]_y· nH₂O, A = alkaline metal) and its analogues o(PBAs) are considered as promising cathode materials for Na-ion batteries owing to the advantages of the rigid open framework for Na-ion diffusion, the tunable composition, the non-toxicity and the low cost^[3]. Therein, PB is under most investigation due to its two-electron-redox processes and the corresponding high theoretical capacity of ~170 mAh g^-1 (based on Na₂FeFe(CN)₆). Despite the fact that the two Fe atoms in Na₂FeFe(CN)₆ are in the same oxidation state, they exhibit different redox activity. The low-spin Fe²⁺/Fe³⁺ couple bonding to the C atoms generally shows higher reaction potential but contributes very less capacity in comparison with the high-spin Fe²⁺/Fe³⁺ couple coordinated with N atoms. Such relatively low contribution of low spin Fe²⁺/Fe³⁺ couple will not only result in lower capacity compared to the theoretical capacity, but also drag down the average discharge plateau, leading to a serious reduction in working voltage. In addition, due to the structural imperfection and involved water of the PB, it is also suffering from relatively poor cycling stability. While by the addition of different portion of nickel ions in PB lattice, it can improve the integrity of structural and reduce the water content, which will assure better electrochemical performance.

Herein, we report a series of promising cathode material of iron-nickel hexacyanoferrates (FeNiHCFs) in NIB for the first time. When doping a portion of ~20% nickel ions to replace iron ions coordinated with N atoms in PB lattice, the FeNiHCF cathode exhibits a high discharge capacity of 106 mAh g^-1 and excellent cycling stability. The Fe²⁺/Fe³⁺ redox couple at higher voltage (3.43/3.35 V) provides a specific capacity of 48 mAh g^-1, which is much higher than the corresponding capacity of 24 mAh g^-1 for FeHCF. Then with the gradually increasing of nickel content in PB lattice, the Fe²⁺/Fe³⁺ redox couple at higher voltage provides an increasing capacity, but the total capacities of various FeNiHCFs have a decreasing trend since Ni²⁺ is electrochemical inactive for sodium reaction between 2.0 and 4.0 V (vs. Na/Na⁺). When the nickel content reaches to 100%, namely generated nickel hexacyanoferrate (NiHCF), there is only one-electron-redox process of Fe²⁺/Fe³⁺ active for Na-storage, which results in a low specific capacity of ~70 mAh g^-1.