High-temperature solid-phase synthesis of lithium iron phosphate using polyethylene glycol grafted carbon nanotubes as the carbon source for rate-type lithium-ion batteries

Abstract

• The presence of PEG effectively solves the problem of agglomeration of CNTs. • LiFePO 4 cathode material synthesized by high-temperature solid-phase method using PEG/CNTs as carbon source, forming uniform tube- net -like 3D conductive network on the surface. • In the synthesis of LiFePO 4 cathode material, PEG plays the role of dispersant, binder and conductive agent. Lithium iron phosphate (LiFePO 4 )/polyethylene glycol (PEG)/carbon nanotubes (CNTs) are successfully synthesized by the high-temperature solid-phase. PEG grafted onto CNTs surface by covalent functionalization. During the high-temperature sintering process, PEG/CNTs form the uniform tube- net -like 3D conductive network, significantly improving electron mobility. Beneficial effects of PEG include: (1). The addition of PEG improves the dispersion of CNTs by increasing the number of surface defects and groups. (2). The cracking PEG forms porous carbon layer uniformly coated around LiFePO 4 to improve its electrical conductivity. (3). As a thermoplastic polymer, PEG becomes a binder at sintering temperatures up to around 70 °C, making the CNTs and LiFePO 4 contact more closely. Compared to LiFePO 4 /CNTs, LiFePO 4 /PEG/CNTs has superior electrochemical performance, as evidenced by improving rate performance (LiFePO 4 /PEG/CNTs discharges 113.5 mAh g −1 at 15C, LiFePO 4 /CNTs discharges 71.6 mAh g −1 at 15C) and high rate cycling retention (capacity retention of 96.6% at 5 C after 300 cycles for LiFePO 4 /PEG/CNTs, capacity retention of 90.6% at 5 C after 300 cycles for LiFePO 4 /CNTs). The architecture of the composite conducting network effectively solves the issue of the poor electron migration of LiFePO 4 .