Towards advanced Fe-based phosphates cathode with ultra-fast redox reaction kinetics and suppressed voltage hysteresis

Abstract

Voltage hysteresis has been regarded as a crucial challenge to achieve the application of ultra-fast charge/discharge sodium-ion batteries (SIBs), which drastically minimizes capacity output and degrades energy efficiency. Unfortunately, relevant research for suppressing the issue is mainly conducted in layered oxide cathodes, but the internal mechanism and suppression strategy of voltage hysteresis in polyanionic systems are often overlooked. Herein, this work proposes a dual strategy of Fe-defect and V-substitution to improve the redox reaction kinetics and suppress voltage hysteresis in Na₃Fe_1.85V_0.1(PO₄)P₂O₇ (NF1.85V0.1PP). The dual strategy can facilitate the sluggish diffusion kinetics of sodium ions and improve redox reversibility while enhancing the stability of crystal structure by constructing rigid VO₆. Consequently, NF1.85V0.1PP cathode exhibits excellent high-rate capability (101.3 mAh g⁻¹ at 1C and 68.8 mAh g⁻¹ at 50C) and remarkable cycling stability (decay-free for 3000 cycles under an ultra-high current density of 50C). Moreover, systematic in-situ/ex-situ characterizations reveal that the suppressed voltage hysteresis originates primarily from improved diffusion kinetics of the two-phase reaction process rather than the solid solution reaction process. This work sheds new light on the targeted improvement of redox reaction kinetics and suppression of voltage hysteresis to achieve high-performance SIBs.