Factors Influencing the Quality of Carbon Coatings on LiFePO[sub 4]

Several LiFePO4/C composites were prepared and characterized electrochemically in lithium half-cells. Pressed pellet conductivities correlated well with the electrochemical performance in lithium half-cells. It was found that carbon structural factors such as sp2/sp3, D/G, and H/C ratios, as determined by Raman spectroscopy and elemental analysis, influenced the conductivity and rate behavior strongly. The structure of the residual carbon could be manipulated through the use of additives during LiFePO4 synthesis. Increasing the pyromellitic acid (PA) content in the precursor mix prior to calcination resulted in a significant lowering of the D/G ratio and a concomitant rise in the sp2/sp3 ratio of the carbon. Addition of both iron nitrate and PA resulted in higher sp2/sp3 ratios without further lowering the D/G ratios, or increasing carbon contents. The best electrochemical results were obtained for LiFePO4 processed with both ferrocene and PA. The improvement is attributed to better decomposition of the carbon sources, as evidenced by lower H/C ratios, a slight increase of the carbon content (still below 2 wt. percent), and more homogeneous coverage. A discussion of the influence of carbon content vs. structural factors on the composite conductivities and, by inference, the electrochemical performance, is included.