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

Lithium iron phosphate (LiFePO4)/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 LiFePO4 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 LiFePO4 contact more closely. Compared to LiFePO4/CNTs, LiFePO4/PEG/CNTs has superior electrochemical performance, as evidenced by improving rate performance (LiFePO4/PEG/CNTs discharges 113.5 mAh g−1 at 15C, LiFePO4/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 LiFePO4/PEG/CNTs, capacity retention of 90.6% at 5 C after 300 cycles for LiFePO4/CNTs). The architecture of the composite conducting network effectively solves the issue of the poor electron migration of LiFePO4.