Modifying the morphology and structure of graphene oxide provides high-performance LiFePO4/C/rGO composite cathode materials

Abstract

In this study we synthesized LiFePO4/carbon/reduced graphene oxide (LFP/C/rGO) composite cathode materials using a method involving sol–gel processing, spray-drying, and calcination. To improve the electrochemical performance of LFP/C, we tested graphene oxides (GOs) of various morphologies as conductive additives, including pristine GO, three-dimensional GO, and hydrothermal porous GO (HTGO). Among our samples, the cathode material prepared through spray-drying with the addition of 1 wt% of HTGO (denoted SP-LFP/C/1%rHTGO) displayed the best electrochemical performance; its discharge capacities at 0.1C, 1C, 5C, and 10C were 160.5, 151.8, 138.8, and 130.3 mA h g−1, respectively. From measurements of its long-term cycling performance, the discharge capacity in the first cycle and the capacity retention after 30 cycles at 0.1C were 160.2 mA h g−1 and 99.6%, respectively; at 10C, these values were 132.2 mA h g−1 and 91.8%, respectively. The electronic conductivity of SP-LFP/C/1%rHTGO (6.58 × 10−5 S cm−1) was higher than that of the pristine LFP/C (9.24 × 10−6 S cm−1). The Li+ ion diffusivities (DLi+) of the SP-LFP/C/1%HTGO cathode, measured using AC impedance (3.91 × 10−13 cm2 s−1) and cyclic voltammetry (6.66 × 10−10 cm2 s−1 for discharge), were superior to those of the LFP/C cathode (9.31 × 10−15 cm2 s−1 and 1.79 × 10−10 cm2 s−1 for discharge, respectively). Galvanostatic intermittent titration revealed that the value of DLi+ was located in a reasonable range from 1 × 10−10 to 1 × 10−17 cm2 s−1; its value dropped to its lowest point when the state of charge was close to 50%. Thus, the use of spray-drying and the addition of conductive HTGO (having a 3D wrinkled morphology and interconnected pore structure) can enhance the electronic conductivity and Li+ ion diffusivity of LFP/C cathode materials, thereby improving the electrochemical performance significantly.