Triclinic Off-Stoichiometric Na_3.12Mn_2.44(P₂O₇)₂/C Cathode Materials for High-Energy/Power Sodium-Ion Batteries

The application of sodium-ion batteries (SIBs) requires a suitable cathode material with low cost, nontoxic, high safety, and high energy density, which is still a big challenge; thus, a basic research on exploring new types of materials is imperative. In this work, a manganic pyrophosphate and carbon compound Na_3.12Mn_2.44(P₂O₇)₂/C has been synthesized through a feasible sol-gel method. Rietveld refinement reveals that Na_3.12Mn_2.44(P₂O₇)₂ adopts a triclinic structure (P1 space group), which possesses spacious ion diffusion channels for facile sodium migration. The off-stoichiometric phase is able to offer more reversible Na⁺, delivering an enhanced reversible capacity of 114 mA h g^-1 at 0.1 C, and because of the strong "inductive effect" that (P₂O₇)^4- groups imposing on the Mn³⁺/Mn²⁺ redox couple, Na_3.12Mn_2.44(P₂O₇)₂/C presents high platforms above 3.6 V, contributing a remarkable energy density of 376 W h kg^-1, which is among the highest Fe-/Mn-based polyanion-type cathode materials. Furthermore, the off-stoichiometric compound also presents satisfactory rate capability and long-cycle stability, with a capacity retention of 75% after 500 cycles at 5 C. Ex situ X-ray diffraction demonstrates a single-phase reaction mechanism, and the density functional theory calculations display two one-dimensional sodium migration paths with low energy barriers in Na_3.12Mn_2.44(P₂O₇)₂, which is vital for the facile sodium storage. We believe that this compound will be a competitive cathode material for large-scale SIBs.