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 Fe^2+/3+ and the notorious Jahn-Teller (JT) effect, severe lattice distortion that is associated with Mn^2+/3+ redox hinders Fe and Mn redox activation, leading to inadequate cycling stability and rate performance. Here, we discover both high Mn^2+/3+ and Fe^2+/3+ redox activation in a Na₄Mn_1.5Fe_1.5(PO₄)₂(P₂O₇) (NMFPP) material. It is revealed that the substitution of Mn reduces the Fe-O bond covalency, elevating the Fe^2+/3+ redox potential to improve the energy density. Furthermore, the JT effect is accompanied by Mn e_g orbital splitting, with the d_{<i>x</i>2-<i>y</i>2} and d_z2 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.