Electrochemical Properties of Transition Metal-Doped LiCoPO4 Synthesized By Hydrothermal Method (2015)

Lithium metal phosphates (LiMPO 4 ) have attracted much attention as promising cathode materials to realize high safety in rechargeable lithium-ion batteries. Their thermal and structural stabilities are very high due to strong covalent bonds among oxygen and phosphorus atoms compared to those of currently used LiCoO 2 . Actually, the good cycle performance of LiFePO 4 has been demonstrated in many practical applications [1]. However, its operating potential (around 3.5 V vs. Li /Li + ) is lower than that of LiCoO 2 . Thus, it is difficult to improve the energy density of lithium-ion batteries by replacing LiCoO 2 with LiFePO 4 . In addition to LiFePO 4 , LiCoPO 4 has become of major interest lately due to a high operating potential at around 4.8 V vs. Li /Li + . The ionic and electronic conductivities of lithium metal phosphates are basically very low, and transition metal-doping has been studied as one of effective approaches for their improvement [2, 3]. In this study, we synthesized transition metal-doped LiCoPO 4 (LiCo 0.9 M 0.1 PO 4 ( M = Co, Fe, Mg)) by hydrothermal method and investigated those electrochemical properties thoroughly. Li 3 PO 4 , CoSO 4 ·7H 2 O and FeSO 4 ·7H 2 O or MgSO 4 ·7H 2 O 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 N 2 atmosphere to obtain the precursor solution with Co 2+ 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% H 2 atmosphere to promote the graphitization of carbon species on particle surface to obtain LiCo 0.9 M 0.1 PO 4 /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 LiCo 0.9 M 0.1 PO 4 /C: acetylene black: polyvinylidene difluoride = 75: 15: 10 in 2032 coin type cells. 1 mol dm -3 LiPF 6 / 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 LiCoPO 4 /C, LiCo 0.9 Fe 0.1 PO 4 /C and LiCo 0.9 Mg 0.1 PO 4 /C. Each sample was well-crystallized in an orthorhombic olivine structure with a Pnma space group. The XRD peaks of LiCoPO 4 /C shifted to larger and smaller 2θ in LiCo 0.9 Fe 0.1 PO 4 /C and LiCo 0.9 Mg 0.1 PO 4 /C, respectively. This result agrees with different ion sizes of Fe 2+ (larger) and Mg 2+ (smaller) compared to Co 2+ , and suggests that Co 2+ ions in the LiCoPO 4 are successfully substituted in the doped LiCo 0.9 M 0.1 PO 4 . Fig. 2 shows SEM images of LiCoPO 4 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 LiCoPO 4 /C. In addition, the irreversible capacity between charge and discharge capacities decreased by doping of Fe 2+ and Mg 2+ ions. References [1] A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc ., 144 (1997) 1188-1194. [2] L. Dimesso, C. Spanheimer, W. jaegermann, J Power sources, 254 (2014) 204-208. [3] J. L. Allen, T. R. Jow, J. Wolfenstine, J Power sources, 196 (2011) 8656-8661. Figure 1