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 (Ea) and lowest diffusion barrier (Eb), exhibits the best cycling and rate properties at low electrochemical polarization, which is related to the narrow Delta d-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. Equilibrium "Ea-Eb" relationship, i.e., the moderate adsorption energy and lowest diffusion barrier originated from narrow Delta p-d bandgap, as well as compatible electrolyte formula, is found to facilitate the most effective ionic transfer from the surface to the bulk of electrode with guaranteed capacity and redox kinetics. image