Influence of Carbon Content on LiFePO4 / C Samples Synthesized by Freeze-Drying Process

Abstract

The influence of the carbon content in composites synthesized by the freeze-drying method was studied by varying the citric acid (chelating agent): Fe ratio. Diminishing this ratio from 1:1 to 0.33:1 led to a gradual reduction of the carbon content from 16.1 to 7.2% wt and different morphologies. Transmission electron microscopy micrographs of the composite with the greatest carbon percentage (16%) show mainly 30 nm particles homogeneously embedded in a carbon network. Samples containing less carbon exhibit only one type of morphology, 200–700 nm aggregates made up of an intimate mixture of particles and carbon. Galvanostatic cycling from 2 to 4 V vs evidences the typical redox behavior at 3.4 V, and a second contribution at 2.65 V probably related to the carbon content. At a high rate, a good specific capacity value is observed for the nanoparticulate sample (16% wt C), whereas poorer performance is observed for low carbon content samples (11 and 7.2 wt % C). Heterogeneous and insufficient carbon covering together with phosphate particle aggregation in these latter samples can account for this behavior. Two carbon distribution models are proposed to explain different electrochemical responses. In all cases, a good capacity retention is observed after prolonged cycling.