Carboxylic acid-assisted solid-state synthesis of LiFePO₄/C composites and their electrochemical properties as cathode materials for lithium-ion batteries

To enhance the capability of LiFePO₄ materials, we attempted to coat carbon by incorporating various organic carboxylic acids as carbon sources. The purity of LiFePO₄ was confirmed by XRD analysis. Galvanostatic cycling, cyclic voltammetry, electric impedance spectroscopy, and conductivity measurements were used to evaluate the material's electrochemical performance. The best cell performance was delivered by the sample coated with 60 wt.% malonic acid. Its first-cycle discharge capacity was 149 mA h g^-1 at a 0.2 C rate or 155 mA h g^-1 at a 0.1 C rate. The presence of carbon in the composite was verified by total organic carbon and Raman spectral analysis. The actual carbon content of LiFePO₄ was 1.90 wt.% with the addition of 60 wt.% malonic acid. The LiFePO₄/C samples sintered with 60 wt.% various carboxylic acids were measured by Raman spectral analysis. The intense broad bands at 1,350 and 1,580 cm^-1 are assigned to the D and G bands of residual carbon in LiFePO₄/C composites, respectively. The peak intensity (I _D/I _G) ratio of the synthesized powders is from 0.907 to 0.935. Carbon coatings of LiFePO ₄ with low I _D/I _G ratios can be produced by incorporating carboxylic acid additives before the final calcining process. The use of carboxylic acid as a carbon source increases the overall conductivity (10^-4 S cm^-1) of the material.