Built‐In Electric Field Modulates Phase Transition and Suppresses Voltage Hysteresis in Mn‐Based Phosphates Cathode for Sodium–Ion Batteries

ABSTRACT The voltage hysteresis and sluggish kinetics of Mn‐based mixed phosphate cathode significantly hinder their development and application. Herein, this work fabricates a heterogeneous composite cathode (NFMVP (9‐1)‐rGO) with a built‐in electric field and accelerated electronic pathway. As the key kinetic driving force, the built‐in electric field, can selectively promote the diffusion of Na + in the Na 3 MnFe(PO 4 )P 2 O 7 phase while simultaneously suppress the structural degradation of the Na 4 MnV(PO 4 ) 3 phase caused by the “avalanche extraction” of Na + under high‐voltage. The introduction of the dual‐carbon layer further establishes a robust conductive framework throughout the electrode. Crucially, in situ electrochemical impedance spectra based on distribution relaxation time reveal that the built‐in electric field modulates the phase transition mechanism from a two‐phase reaction to a solid‐solution behavior in the high‐voltage region, significantly accelerating the reaction kinetics and greatly suppressing voltage hysteresis. As a result, NFMVP (9‐1)‐rGO exhibits excellent capacity delivery (120.2 mAh g −1 ), high practical energy density (378.3 Wh kg −1 ), and outstanding long‐term cycling stability. Our findings enlighten a new paradigm in the kinetic driving force from built‐in electric field and the phase transition regulation mechanism in heterogeneous structures toward high‐energy Mn‐based sodium–ion batteries.