Decoupling the Kinetic Essence of Iron-Based Anodes through Anionic Modulation for Rational Potassium-Ion Battery Design

Potassium-ion batteries (PIBs) have favorable characteristics in terms of cell voltage and cost efficiency, making them a promising technology for grid-scale energy storage. The rational design of suitable electrode materials on a theoretical basis, aiming at high power and energy density, is of paramount importance to bring this battery technology to the practical market. In this paper, a series of iron-based compounds with different non-metal anions are selectively synthesized to investigate the nature of kinetic differences induced by anionic modulation. A combination of experimental characterization and theoretical calculation reveals that iron phosphide, with its moderate adsorption energy (E_a) and lowest diffusion barrier (E_b), exhibits the best cycling and rate properties at low electrochemical polarization, which is related to the narrow Δd-p band center gap that facilitates ion transfer. In addition, the optimization of the electrolyte formula results in the carbon-supported iron phosphide anode running stably over 2000 cycles at 0.5 A g⁻¹ and exhibiting a high rate capacity of 81.1 mAh g⁻¹ at 2 A g⁻¹. The superior electrochemical properties are attributed to the robust KF-rich solid electrolyte interphase formed by the highly compatible KFSI in ethylene carbonate (EC)/diethyl carbonate (DEC) configuration.