Improving the lithium-ion diffusion and electrical conductivity of LiFePO4 cathode material by doping magnesium and multi-walled carbon nanotubes

Abstract

This paper reports a co-modification approach to improve both electronic conductivity and lithium-ion diffusion of lithium iron phosphate (LiFePO4) via doping magnesium (Mg2+) and multi-walled carbon nanotubes (MWCNTs). A series of LiFe1−xMgxPO4 composites consisting of various amounts of MWCNTs were synthesized by using a facile hydrothermal method which involves an in-situ MWCNTs embedding process. The structure and morphology of prepared composites were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical performance was tested via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge tests. All composites exhibit good crystallinity without any impurity phases. Besides, a slight shrinkage in the crystal lattice was observed after Mg2+ doping. Mg2+ is uniformly dispersed in the composites in which the formation of a three-dimensional conductive network enhances electronic conductivity and lithium-ion diffusion especially at high current densities. Among LiFe1−xMgxPO4/yMWNT composites, LiFe0.98Mg0.02PO4 with 1.5 wt% MWCNTs displays the highest electrochemical performance, offering a discharge capacity of 142 mA h g−1 at 0.1 C and exhibiting a good rate capability with a capacity of 120 mA h g−1 at a high rate of 2 C and a stable long cycle life (94.5 % capacity retention over 150 cycles). The co-modified composite cathode displays high discharge capacity, good rate capability, and excellent cycling stability compared to pure LiFePO4, rendering the co-modification approach a promising strategy for the preparation of high-performance electrode materials.