Poly(ethylene glycol) (PEG) grafted multi-walled carbon nanotubes (MWCNTs-g-PEG or MP) were synthesized and used to modify LiFePO4 as cathodes for lithium-ion batteries (LIBs). The effects of different molecular weights of PEG grafted on MWCNTs and different mass fractions of MP on the properties of LiFePO4/MP composite cathodes were evaluated by their morphology, charge–discharge tests, electrochemical impedance spectroscopy, electrical and thermal conductivities. Their electrochemical behaviors at ambient temperature and low temperature, high rate capability and cycling performance were observed in the presence of the MP additives. The lithium ions diffusion in the LiFePO4/MP composite electrodes was almost 2 orders of magnitude higher than that in the LiFePO4/acetylene black (AB) electrode when the conductive additive content was 5 wt.%. Thermal studies of LiFePO4/MP were also examined by the heat-pole method, which showed higher thermal conductivity of the cathode in cases of MP being incorporated into LiFePO4 particles than LiFePO4 cathodes with AB or MWCNTs additives. These results suggest that MP is a promising conductive additive to increase the electrochemical performances, thermal transport and safety of LiFePO4 cathodes for LIBs.