Enhanced Charge-Transfer Kinetics by Anion Surface Modification of LiFePO₄

Despite the great achievement in understanding the materials properties and powder engineering of LiFePO 4, the chemical bonding at the surface has been almost ignored. Herein, we demonstrate that the undercoordinated Fe 2+ /Fe 3+ redox couple at the surface gives a high barrier for charge transfer, but it can be stabilized by nitrogen or sulfur adsorption. The surface modification improves greatly the charge transfer kinetics and the charge/discharge performance of a LiFePO 4 cathode. Density functional theory (DFT) calculation estimates the origin of the improvement in terms of an electronic and ionic contribution based on a surface model probed by time of flight secondary ion mass spectrometry (TOF-SIMS); the calculation agrees well with an experimental rate-constant analysis.