Electrochemical Properties of Transition Metal-Doped LiCoPO4 Synthesized By Hydrothermal Method
Li3PO4, CoSO4·7H2O and FeSO4·7H2O or MgSO4·7H2O as starting materials were mixed in molar ratio of 1: 0.9: 0.1. This mixture and carboxymethyl cellulose sodium salt (carbon coating) were added to degassed water under N2 atmosphere to obtain the precursor solution with Co2+ and M ions concentration = 3 mol dm-3. After hydrothermal treatment at 200 ºC for 24 h, the resulting precipitation was separated centrifugally and then freeze-dried. The obtained powder was then heated at 700 ºC for 1 h under 97% Ar + 3% H2 atmosphere to promote the graphitization of carbon species on particle surface to obtain LiCo0.9M0.1PO4/C. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TGA) and Raman spectroscopy, respectively. The electrochemical properties of sample were investigated using a composite electrode with the weight composition of LiCo0.9M0.1PO4/C: acetylene black: polyvinylidene difluoride = 75: 15: 10 in 2032 coin type cells. 1 mol dm-3 LiPF6/ ethylene carbonate: diethyl carbonate = 1: 2 (in volume) was used as an electrolyte solution. The charge - discharge measurement was performed in a potential range of 2.5 ~ 5.1 V at 30 ºC.
Fig. 1 shows XRD patterns of LiCoPO4/C, LiCo0.9Fe0.1PO4/C and LiCo0.9Mg0.1PO4/C. Each sample was well-crystallized in an orthorhombic olivine structure with a Pnma space group. The XRD peaks of LiCoPO4/C shifted to larger and smaller 2θ in LiCo0.9Fe0.1PO4/C and LiCo0.9Mg0.1PO4/C, respectively. This result agrees with different ion sizes of Fe2+ (larger) and Mg2+ (smaller) compared to Co2+, and suggests that Co2+ ions in the LiCoPO4 are successfully substituted in the doped LiCo0.9M0.1PO4. Fig. 2 shows SEM images of LiCoPO4 with and without M-doping. The size of particles became a little bit larger by Fe-doping. In contrast, very small particles were obtained by Mg-doping. Fig. 3 shows the charge - discharge curves of the samples at initial 3 cycles. The polarization became lower in both doped samples than that of pristine LiCoPO4/C. In addition, the irreversible capacity between charge and discharge capacities decreased by doping of Fe2+ and Mg2+ions.
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