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

Lithium metal phosphates (LiMPO₄) 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 LiCoO₂. Actually, the good cycle performance of LiFePO₄ has been demonstrated in many practical applications [1]. However, its operating potential (around 3.5 V vs. Li /Li⁺) is lower than that of LiCoO₂. Thus, it is difficult to improve the energy density of lithium-ion batteries by replacing LiCoO₂ with LiFePO₄. In addition to LiFePO₄, LiCoPO₄ 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 LiCoPO₄ (LiCo_0.9M_0.1PO₄ (M= Co, Fe, Mg)) by hydrothermal method and investigated those electrochemical properties thoroughly.

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 LiCo_0.9M_0.1PO₄/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 LiCo_0.9M_0.1PO₄/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, LiCo_0.9Fe_0.1PO₄/C and LiCo_0.9Mg_0.1PO₄/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 LiCo_0.9Fe_0.1PO₄/C and LiCo_0.9Mg_0.1PO₄/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 LiCo_0.9M_0.1PO₄. 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.

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.