Cycle Performance of Metal-Substituted LiCoPO4 of Coarse Particles

Abstract

Introduction LiCoPO4 has been focused as a promising cathode material to realize lithium-ion batteries with high energy density due to the high specific capacity of 167 mA h g-1 and a high operating potential of Co2+/Co3+ redox at 4.8 V vs. Li/Li+. However, the high operating potential sometimes results in a large irreversible capacity from the oxidative decomposition of electrolyte solutions during charge process. The decomposition occurs on the surface of electrode, so that coarse particles of active materials with low surface areas reduces the irreversible capacity. Based on this discussion, we synthesized the size-controlled LiCoPO4 and LiCoPO4 partially substituted with Fe and Ni by hydrothermal method1) . The particle sizes can be controlled by hydrothermal duration. The effect of particle size on the irreversible capacity derived from the oxidative decomposition of electrolyte solutions was investigated. Experimental LiCoPO4 was prepared from Li3PO4, CoSO4･7H2O. LiCo0.9M0.1PO4 (M=Fe or Fe) were prepared by replacing 10% of CoSO4･7H2O with FeSO4･7H2O or NiSO4･5H2O. Li3PO4, CoSO4･7H2O, FeSO4･7H2O (NiSO4･5H2O) were mixed in a molar rate of 1 : 0.9 : 0.1. The prepared solution was heated for 48 hours at 200 ºC with stirring in an autoclave. The product was collected by centrifugation and freeze-drying. Then, the resulting sample was mixed with sucrose and heated at 700 ºC for 1 h under 97 % Ar + 3% H2 atmosphere to form a carbon layer on the particle surface. The crystal structure, morphology, composition, and surface area of each sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma emission spectrometer (ICP) and the BET method, respectively. The state of carbon on the LiCo0.9M0.1PO4 particle was investigated by Raman spectroscopy. The carbon-coated samples were mixed with acetylene black and polyvinylidene fluoride in a weight ratio of 15% and 10% to make composite electrodes, The electrochemical properties were characterized with 2032 coin type cell in a potential range of 2.5 ~ 5.1 V at 30 ºC. Results and discussions Figure 1 shows XRD patterns of the products. All peaks were assigned to those of LiCoPO4 and no impurity was observed. In addition, the substitutions of Co in LiCoPO4 with Ni and Fe were confirmed from the peak shifts to higher and lower angles, respectively. Those substitutions were also confirmed by ICP analysis. The carbon-coating on the particle surface was confirmed by Raman spectroscopy for all samples after the heat treatment at 700 ºC with sucrose. The specific surface areas of samples were characterized by BET measurements to be 8.75 m2 g-1(LiCoPO4), 5.40 m2 g-1 (LiCo0.9Ni0.1PO4), 1.00 m2 g-1 (LiCo0.9Fe0.1PO4), respectively. The particle size was increased by the metal replacement. Figure 2 shows the cycleability of the cells for LiCoPO4, LiCo0.9Ni0.1PO4 and LiCo0.9Fe0.1PO4 cathodes. The initial discharge capacity of LiCo0.9Fe0.1PO4 / C represented the largest value of 148 mAh g-1. The irreversible capacity of LiCoPO4 in the initial cycle was about 196 mAh g-1. In contrast, LiCo0.9Ni0.1PO4 / C and LiCo0.9Fe0.1PO4 / C showed the smaller irreversible capacities of 112 mAh g-1 and 76 mAh g-1, respectively. This result suggests that the irreversible capacity of the metal substitution products in the initial cycle was less than that of no-metal substitution product. Figure 3 shows the SEM image of the LiCo0.9Fe0.1PO4 / C particle after the cycle test. No crack in the particle was observed. Thus, the deterioration of the discharge capacity may not be due to the deterioration of the particles. LiCo0.9M0.1PO4 particle size was increased by both longer hydrothermal treatment duration and metal-substitution. The BET surface area of particle was decreased to 1.00 m2 g-1 from 8.75 m2 g-1. The decomposition of the electrolytic solution during charge was reduced with decreasing of BET surface area corresponding to electrochemical surface area. The irreversible capacity in the initial cycle was decreased to 76 mAh g-1 from 196 mAh g-1. Furthermore, it has been reported that Fe-doping improves the Li conductivity and electronic conductivity of LiCoPO4 2). These improvements are also possible reasons. Reference 1) J. L. Allen, T. R. Jow, J. Wolfenstine, J Power sources, 196 (2011) 8656-8661. 2) Y. M. Kang, Energy Environ. Sci, 4 (2011) 4978-4983 Figure 1