NaFe₂PO₄(MoO₄)₂: A Promising NASICON-Type Electrode Material for Sodium-Ion Batteries

Searching for polyanionic electrode materials with high Na⁺ and electronic conductivity is pivotal to realize high-performance sodium-ion batteries. Here, we report a novel polyanionic-based NASICON-type compound, NaFe₂PO₄(MoO₄)₂ (NFPM), that does not crystallize in the common space group R-3c or C2/c but in the rare P2/c. The studies on bond valence sum maps show that NFPM has high Na⁺ conductivity because the large volumes of MoO₄ groups make the interstitial channels wider, thus making the energy barrier of Na⁺ diffusion decrease along these channels. Density functional theory calculations demonstrate that NFPM has high electronic conductivity because the contribution of Mo 4d orbitals on the formation of the bottom of the conduction band makes the connected MoO₄ groups take part in electron transport. Electrochemical tests exhibit that NFPM can deliver a capacity of ∼80 mAh g^-1 with good reversible cyclability utilizing the Fe³⁺/Fe²⁺ redox couple. In situ X-ray diffraction measurements indicate that NFPM undergoes one-phase reaction mechanism in the process of charge and discharge.