Complexing Mechanistic Impacts on the Microstrucutre and Performances of LiFePO4 Synthesized Via Sol-Gel Approaches
Authors: Huang Hong , Guan Chuang 
Published: February 2020

Olivine structured LiFePO4 is an attractive cathode material for lithium ion batteries for its low cost, environmental benign, good cycling performance, safety, etc. It intrinsic low electronic conductivity and lithium-ion diffusion demands exquisite synthesis to produce small particles with surface electrical coating. Wet chemical sol-gel route have indisputable advantages over solid-state reactions for fabricating LiFePO4 fine particles with narrow size distribution, in addition to its ease of mass production and capabilities of in-situ carbon coating as well as flexible doping. Understanding of the influences of the key processing parameters like species of chelating agent and PH value of solution on the microstructure and performances of materials is fundamentally important. Here we report our studies on correlating the stability of complexing compounds on the mircrostructure of LiFePO4 products, carbon contents, and hence the electrochemical properties.

LiOH.H2O and FeC2O4.2H2O were selected as the sources of lithium and iron. H3PO4 and/or NH4H2PO4 was the source of phosphate ions, which was also used to une the PH value of the sol. Basic complexing agents are citric acid (CA) and ethylene glycol (EG). In this work, individual complexing agent and the combination of the two with different ratios and their impacts on the product microstrucutre and electrochemical performances were investigated. The microstructure and composition of the LiFePO4/C products were investigated by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and thermal graviemtric analyses (TGA) etc. For galvanostatic charge-discharge and electrochemical impedance spectroscopic analyses, the synthesized LiFePO4/C powders were mixed with conductive agent and binder at a certain mass ratio in solvent and coated on Al foil.

Nanocrystalline LiFePO4/C materials with spherical particles of less than 500nm and appropriate amount of carbon coating were successfully obtained at the optimal synthesis parameters. It was found that carbon content remaining in the powder product after sintering varied from 10wt% and 1.2 wt%, depending on the chelating complex composition and the amount. A consistent trend was observed among the lithium-storage capacity, carbon content, and complex stability. The carbon content and the stability constants of the complex compounds are corroborative, while the Li-storage capacities of LiFePO4 products are inversely proportional to the stability constants of their corresponding complex compounds as well as the carbon content. It is well known that the types of central metallic ion and complex ligand have decisive influences on the stability of the formed complex compounds. The stability of complex compounds formed from Fe2+ are all small than that  formed from Fe3+ with the same complex agent. The complex agent CA forms five or six-membered rings during the complex process, so the corresponding stability constants of the formed complex compounds are all relatively larger. Consequently, the product contains large amounts of carbon remnants. Excessive carbon on the surface of LiFePO4 will block the diffusion of Li into the LiFePO4 crystal structure, leading to the reduced lithium storage capacity and rate capability. The PH value of the precursor solution affects two main reactions, i.e.  hydrolysis and polymerization, during the sol-gel process. Thereby, it affects the iorn valency and final particle size. Suitable PH environment is found to ensure uniform distribution of metal cations in the organic sol-gels leading to the uniformly fine and well dispersed LiFePO4 powders which are benefical to the high electrochemical performances.