Reduced Bond Covalency and Anisotropic Lattice Distortion Enable High Fe–Mn Redox Activation in a Mixed-Polyanionic Cathode

Fe- and Mn-based polyanionic electrode materials are important cathode materials for sodium-ion batteries (SIBs) due to their low cost. However, due to the low redox potential of Fe2+/3+ and the notorious Jahn–Teller (JT) effect, severe lattice distortion that is associated with Mn2+/3+ redox hinders Fe and Mn redox activation, leading to inadequate cycling stability and rate performance. Here, we discover both high Mn2+/3+ and Fe2+/3+ redox activation in a Na4Mn1.5Fe1.5(PO4)2(P2O7) (NMFPP) material. It is revealed that the substitution of Mn reduces the Fe–O bond covalency, elevating the Fe2+/3+ redox potential to improve the energy density. Furthermore, the JT effect is accompanied by Mn eg orbital splitting, with the dx2–y2 and dz2 orbitals being positioned at the top of the valence band and the bottom of the conduction band, respectively, which primary contributes to reduce the material band gap and facilitate electron transfer. Experimental and theoretical studies discover an anisotropic lattice distortion behavior, which enlarges Na+ diffusion pathways, lowering diffusion energy barriers and enabling rapid Na+ migration. As a consequence, high Fe–Mn redox activity is achieved and the NMFPP demonstrates enhanced energy density, rate performance, and exceptional cycling stability for sodium storage. These findings prove that the JT effect and lattice distortion could synergistically make positive impacts on transition-metal redox activation, which is informative for the design of high-performance electrode materials.