Multivalent Cation Incorporated into Manganese-Iron Based NASICON Cathodes for High Voltage Sodium-Ion Batteries

Na 4 Mn 1.5 Fe 1.5 (PO 4 ) 2 P 2 O 7 (NMFPP), with its low cost and high energy density, is essential for accelerating the commercialization of sodium-ion batteries. However, its practical application is limited by serious voltage hysteresis and detrimental Jahn-Teller distortions. Herein, a high operating voltage and superior stable Nb-doped NMFPP with fewer intrinsic anti-site defects are elaborately designed by the reconstruction of the crystal lattice and electronic distribution. By introducing higher charge density Nb─O bonds, the lengths of Mn-O bonds are shortened, enhancing lattice stability. As a result, the lattice volume contracted during Na + extraction/insertion is decreased with niobium-modified Na 4 (Mn 0.5 Fe 0.5 ) 2.94 Nb 0.06 (PO 4 ) 2 P 2 O 7 , mitigating lattice distortion from the Jahn-Teller effect and increasing the capacity retention after 1000 cycles from 57.5% to 82.3%. More importantly, the delayed effect of Mn 2+ involvement in redox reactions is significantly reduced, raising the average operating voltage from 3.32 to 3.64 V and increasing the overall energy density by 13%. This study opens new avenues to develop advanced sodium-ion battery cathode materials with high energy density and long calendar life for energy storage.