Electrochemical Properties of Transition Metal-Doped LiCoPO4 Synthesized By Hydrothermal Method
Authors: Noda Yusaku , Yamada Yuto , Miyamoto Shohei , Munakata Hirokazu , Ohira Koji , Yoshida Shuhei , Shibata Daisuke , Kanamura Kiyoshi 
Published: February 2020

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 (LiCo0.9
M
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 LiCo0.9
M
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 LiCo0.9
M
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, 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 LiCo0.9
M
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.

Figure 1