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

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 Na3MnFe(PO4)P2O7 phase while simultaneously suppress the structural degradation of the Na4MnV(PO4)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.