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

Abstract

Several 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 and disordered/graphene (D/G), as determined by Raman spectroscopy, and ratios determined from elemental analysis influenced the conductivity and rate behavior strongly. The structure of the residual carbon could be manipulated through the use of additives during 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 ratio of the carbon. Addition of both iron nitrate and PA resulted in higher ratios without further lowering the D/G ratios or increasing carbon contents. The best electrochemical results were obtained for processed with both ferrocene and PA. The improvement is attributed to better decomposition of the carbon sources, as evidenced by lower ratios, a slight increase of the carbon content (still below ), 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.