A High-Capacity Nonstoichiometric Na_2.67Fe_2.67(P₂O₇)₂ Cathode Material of a Sodium Ion Battery

Iron-based pyrophosphates, known for their open 3D channels and low cost, have garnered significant research attention. However, the poor intrinsic conductivity and low theoretical capacity hinder their further development in sodium-ion battery applications. In this study, we designed a sodium ferric pyrophosphate, Na_2.67Fe_2.67(P₂O₇)₂ (termed NFPO-2.67), with a theoretical specific capacity of 128 mA h g^-1 by optimizing the Na/Fe stoichiometric ratio of the substrate material, which expanded the conventional solid solution range in Na_4-αFe_2+α/2(P₂O₇)₂ from (0 ≤ α ≤ 1) to (0 ≤ α ≤ 4/3). The pure-phase NFPO-2.67 exhibits an operating voltage of ∼3.0 V (vs Na⁺/Na) and delivers a specific capacity of 124.4 mA h g^-1 at 0.1 C (close to its theoretical specific capacity) and an excellent rate performance (93.6 and 70.6 mA h g^-1 at 1 and 10 C, respectively), as well as an outstanding cycling stability (70% capacity retention after 5000 cycles at 20 C). The possible mechanisms of their excellent electrochemical performance, including the material structure, energy storage mechanism, and Na⁺ diffusion kinetics, were further elucidated by using density functional theory (DFT). This work enriches the knowledge on solid solution systems and provides insights for exploring the nonstoichiometric ratios of iron-based pyrophosphate cathode materials in sodium-ion batteries.