Electrochemical Characterization of Carbon-Coated LiCoPO4 Synthesized By Hydrothermal Method Using Various Carbon Sources

Abstract

Introduction Lithium metal phosphates (LiMPO4, M = Mn, Fe, Co, Ni) are promising as excellent safety cathode materials for lithium-ion batteries owing to their high thermal and structural stability derived from the strong P-O covalent band. LiFePO4 has been deeply studied and some practical applications have been already started due to its long cycle life.1 However, the operating potential of LiFePO4 is restricted to around 3.5 V vs. Li / Li+. Consequently, LiCoPO4 has attracted great attention owing to high operating potential (4.8 V vs. Li / Li+) and good theoretical capacity (167 mA h g-1).2 However, the electrochemical performance of LiCoPO4 is still poor due to the intrinsically low electrical and lithium-ion conductivity. The cycling performance is also poor because of degradation of electrolyte at high voltage. Therefore, many efforts have been made in recent years on their improvement by decreasing the particle size and carbon-coating on the surface.3 In this study, carbon-coated LiCoPO4 (LiCoPO4/C) having fine particle size was synthesized by hydrothermal method using various carbon sources. The effect of carbon sources on particle size, carbon-coating, and electrochemical performance of the LiCoPO4/C was investigated. Experimental 0.09 mol of CoSO4·7H2O, 0.09 mol of Li3PO4 and a carbon source were dissolved into degassed water (30 ml) to prepare the precursor solution of LiCoPO4/C under N2 atmosphere. As the carbon source, 3.0 g of carboxymethylcellulose sodium salt (CMC), 3.0 g of glucose and 2.9 g of ascorbic acid were used. For comparison, a precursor solution without carbon source was also examined. After the hydrothermal treatment at 200°C for 24 h, the resulting precipitation was separated centrifugally followed by freeze-drying. The obtained powder was heat-treated at 700°C for 1 h under 97% Ar / 3% H2 atmosphere to promote the graphitization of carbon species on the surface of LiCoPO4. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy, respectively. The electrochemical properties of each sample were evaluated in 2032 coin cells, using a composite electrode with the composition of LiCoPO4/C : acetylene black : polyvinylidene difluoride = 75:15:10 in weight. 1 mol dm-3 LiPF6 / ethylene carbonate : diethyl carbonate = 1:2 (v/v) and Li foil were used as electrolyte solution and counter electrode. Charge and discharge tests of the cells were carried out in the potential range of 3.0 – 5.1 V at 30°C. Results and discussion XRD patterns of all the samples exhibited well-resolved diffraction peaks attributed to phospho-olivine LiCoPO4 without impurity phase, indicating that well-crystallized LiCoPO4 particles can be obtained by hydrothermal treatment. The average particle sizes of LiCoPO4 and LiCoPO4/C prepared with CMC, glucose, and ascorbic acid were determined to be 3.3, 0.65, 3.0, and 3.1 μm on the basis of SEM images, respectively. The decrease of the particle size by addition of CMC suggests that CMC suppresses the particle growth of LiCoPO4 by high viscosity in hydrothermal process.4 Raman spectra of the samples prepared with carbon sources showed broad peaks related to disorder and graphite carbon (D- and G-band), respectively, suggesting that all the carbon sources were decomposed to form carbon layer on the surface of LiCoPO4 particles during hydrothermal treatment. The highest ratio of the G-band to the PO4 3- peak was obtained for LiCoPO4/C prepared with CMC, implying that the carbon layer on the surface of particles is more uniform than those formed using the other carbon sources. Figure 1 shows the second charge and discharge curves of the samples in Li half-cells tested in the potential range of 3.0 – 5.1 V at 0.1 C. LiCoPO4/C prepared with CMC, which has the smaller particle size, showed much higher discharge capacity and lower electrochemical polarization than the other samples. This improvement in electrochemical property compared to pristine LiCoPO4 at the low current rate can be mainly attributed to the reduction of the particle size which decreases electron and lithium-ion diffusion lengths in the crystal. Moreover, LiCoPO4/C prepared with CMC, which has uniform carbon layer on the surface, showed the highest coulombic efficiency among all the samples despite its small particle size tending to cause the irreversible reaction due to the oxidative decomposition of electrolyte solution during charge process.