Polyethylene Glycol for LiFePO4/C Composites Preparation: Large or Small Molecular Weight

Abstract

Olivine LiFePO4 is challenged by its poor electronic and ionic conductivities for lithium-ion batteries. Polyethylene glycol (PEG) has been applied for LiFePO4 preparation by different research groups, but there is no consensus on the influence of the mean molecular weight of PEG on the structure and electrochemical performances of LiFePO4/C composites. In this work, LiFePO4/C composites were prepared by using micronsized FePO4·2H2O powder as starting material, PEG (mean molecular weight of 200, 400, 4000 or 10000) and citric acid as complex carbon source. The structure and electrochemical performances of LiFePO4/C composites would be decided considerably by the mean molecular weight of PEG, and the sample using PEG200 exhibited the least inter-particle agglomeration, the smallest charge transfer resistance and the highest discharge capacity. A probable growth mechanism is also proposed based on SEM images and electrochemical results: with the assistance of citric acid, PEG molecule with small molecular weight tends to cover one or only a few micron-sized FePO4·2H2O particles, significantly suppress the agglomeration of primary LiFePO4 particles and thus result in uniform particle-size distribution and carbon coating.