Solution deposition of thin carbon coatings on LiFePO₄

We report the synthesis of ultrathin carbon coatings on polycrystalline LiFePO4 via solution deposition and subsequent annealing. The annealing temperature was systematically investigated with polymer systems on LiFePO4 nanostructures. The crystal structures, sizes, and morphologies were monitored and analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Micro-Raman and TEM were used to interrogate the carbon coatings after heat-treatments. Electrochemical performance of coated materials was investigated by cyclic voltammograms (CVs) and galvanostatic charge-discharge analysis. The olivine structured LiFePO4 remained stable up to 600 °C but underwent a rapid reduction reaction from LiFePO4 to Fe2P above 700 °C. The good compatibility between polyethylene glycol (PEG) and the surface of LiFePO4 enabled the formation of core-shell structure, which was transformed into a thin carbon coating on LiFePO4 after annealing. Both PEG and sucrose carbon-based sources yielded high-quality carbon coatings after annealing, as determined by the graphitic/disordered (G/D) ratios of 1.30 and 1.20, respectively. By producing more uniform and coherent coatings on LiFePO4 particles, batteries with significantly less carbon (i.e., 0.41 wt %) were fabricated and demonstrated comparable performance to traditionally synthesized carbon-coated LiFePO4 with higher carbon loadings (ca. 2.64 wt %). This will enable development of batteries with higher active material loading and therefore significantly larger energy densities.