Activating the Anomalous Jahn–Teller Effect in NASICON-Type Mixed Phosphate Cathode via Crystal Field Engineering

Abstract High-voltage Mn3+/Mn2+ redox reactions in polyanionic cathodes offer the potential to increase energy density while reducing the cost of batteries. However, its practical application is hindered by sluggish reaction kinetics and asymmetric Jahn–Teller distortion. Herein, we report a NASICON-type cathode, Na3.85□0.15Mn1.35Fe1.5V0.15(PO4)2(P2O7) (□ denotes Na vacancy, NFMPP-V), in which vanadium substitution reconfigures the Mn3+-induced Jahn–Teller distortion and couples it with multivalent redox processes, achieving enhanced energy density and reaction kinetics. Remarkably, the Na+ mobility increases with the Jahn–Teller distortion, exhibiting an anomalous behavior opposite to that of conventional Mn-based cathodes, which is associated with an ordered anisotropic distortion of the [MnO6] octahedra. As a result, NFMPP-V delivers a high reversible capacity of 120.2 mAh g–1 and achieves a capacity retention of 82.3% for 500 cycles at 5C. This work demonstrates that Jahn–Teller distortion can be rationally re-engineered into a kinetic advantage and provides new insights into the design of high-energy-density NASICON-type cathodes for batteries.