Site-dependent electrochemical performance of Na and K co-doped LiFePO4/C cathode material for lithium-ion batteries

Abstract

Olivine-type lithium iron phosphate (LiFePO₄) has attracted much attention as a promising cathode material for lithium-ion batteries (LIBs). Na⁺ and K⁺ co-doped LiFePO₄/C composite materials in two discrete sites, namely the Li_1−x−yNa_xK_yFePO₄/C or Li-site and LiFe_1−x−yNa_xK_yPO₄/C or Fe-site (x = 0.02, y = 0.01) were synthesized via solid-state reaction route, which is a simple and convenient method, particularly for mass production. The effects of Na and K occupied sites are studied by X-ray diffraction (XRD), Rietveld analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) mapping, and transmission electron microscopy (TEM). Furthermore, the electrochemical properties are evaluated by cyclic voltammograms (CVs), constant current charge/discharge cycling tests and electrochemical impedance spectroscopy (EIS). It was observed that the co-doped samples exhibited improved electrochemical performances compared with the pristine LiFePO₄/C material. The experimental results revealed that the Li_0.97Na_0.02K_0.01FePO₄/C composite had a higher capacity delivery and kinetics than LiFe_0.97Na_0.02K_0.01PO₄/C sample. It delivered a discharge capacity of 159 (0.1C), 150 (0.5C), 146 (1C), 138 (3C) and 100 mA h g⁻¹ (5C).