Active materials for rechargeable lithium-ion batteries are generally evaluated as porous composite electrodes including binder and conductive carbon. However, this method sometimes makes it difficult to understand the intrinsic electrochemical properties of active material because the electrochemical response of composite electrode is strongly influenced by the electrode structure and composition. In order to overcome this problem, we have focused on single particle measurement, in which a single particle of active material can be evaluated [1]. Lithium metal phosphates have been investigated as promising cathode active materials with high thermal and structural stability. Though their electronic and Li+ ion conductivities are very low, those drawbacks can be improved by carbon coating and formation of fine particles, respectively. It has been demonstrated that the electrochemical performance of LiFePO4 can be increased up to a practical use level by such particle design. However, its operating potential is low (around 3.5 V vs. Li /Li+) compared to conventional LiCoO2. Thus, LiCoPO4 have been particularly focused in recent years, due to its high operating potential (around 4.8 V vs. Li /Li+). In this study, the effect of carbon coating on the electrochemical properties of LiCoPO4 was investigated by single particle measurement. LiCoPO4 was synthesized by hydrothermal method using Li3PO4 as Li and P sources and CoSO4·7H2O as a Co source, then coated with carbon by using sucrose as a carbon source. The morphology of LiCoPO4 particles was characterized with a scanning electron microscope (SEM). The amount of carbon on LiCoPO4 was estimated by thermogravimetric analysis (TGA). The electrochemical properties of pristine and carbon-coated LiCoPO4 particles were evaluated by single particle measurement (Fig. 1), using a grass coated Au wire (Φ10 μm diameter) as a micro current collector. The single particle measurement was performed in a potential range of 2.5 ~ 5.1 V vs. Li /Li+ at room temperature under Ar atmosphere. A mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 in volume) containing 1 mol dm-3 LiPF6 was used as an electrolyte solution. Fig. 2 shows SEM images of pristine and carbon-coated LiCoPO4. The pristine LiCoPO4 particles were cubic and more than 5 μm in size, and those shape and size were maintained after the carbon coating. From TGA result, the amount of carbon on LiCoPO4 was estimated to be 0.5 wt%. Fig. 3 shows the charge – discharge curves of pristine LiCoPO4 particle at initial 3 cycles, in which the charge was carried out at 0.2 nA until the electrode potential reached to 5.1 V vs. Li /Li+ and then the potential was hold at 2 hours, followed by 0.2 nA discharge. It was hardly operated due to high ohmic resistance. Fig. 4 shows the charge – discharge curves of carbon-coated LiCoPO4 particle with 20 μm diameter at initial 3 cycles, in which the charge was carried out at 3 nA until the electrode potential reached to 5.1 V vs. Li /Li+ and then the potential was hold until the charge current dropped to 0.3 nA, followed by 3 nA discharge. The carbon-coated LiCoPO4 showed better electrochemical performance with the plateaus corresponding to Li+ extraction and insertion were clearly observed at 4.8 and 4.7 V vs. Li /Li+, respectively, although the irreversible capacity and capacity fading were observed due to the decomposition of electrolyte solution at high voltage [2]. Electrochemical properties of LiCoPO4 was greatly improved by the small amount of carbon even though the particle size was as large as 20 μm, suggesting that LiCoPO4 is a promising cathode material to realize high energy density lithium-ion batteries. References [1] H. Munakata et al. , Journal of Power Sources 217 (2012) 444 – 448. [2] E. Markevich et al. , Electrochemistry Communications 15 (2012) 22–25. Figure 1
The effect of carbon-coating on the electrochemical properties of LiCoPO 4 was investigated by single particle measurement. For this analysis, micrometer-scale LiCoPO 4 particles with and without carbon-coating were synthesized by hydrothermal method (LiCoPO 4 /C0 with 0.3 wt%, LiCoPO 4 /C1 with 0.8 wt%, LiCoPO 4 /C2 with 1.7 wt% carbon-coating and pristine LiCoPO 4). In the electrochemical tests using the conventional composite electrodes, all the samples showed the similar electrochemical properties with potential plateaus at~4.7 V vs. Li/Li +. In contrast, single particle measurement showed clear differences in the charge and discharge curves. The pristine LiCoPO 4 showed the potential plateau only in the discharge curve due to a large overpotential of charging. LiCoPO 4 /C0 also showed large overpotential. On the other hand, good electrochemical responses were obtained for the LiCoPO 4 /C1 and LiCoPO 4 /C2 particle electrodes even though their carbon-coating amounts were different. This result suggests that 0.8 wt% or higher carbon-coating enables to improve the electrochemical performance of one particle of LiCoPO 4 .
Introduction Lithium ion batteries (LIB) have been widely used as power sources for portable electronic devices such as laptop computers and cellular phones due to their high energy density. A battery research has been become more important issue for large scale applications such as EVs and HEVs. LiMnPO4 has been focused as a promising cathode material to realize high performance of LIB. So far, many approaches have been done to improve the electrochemical properties of LiMnPO4 by carbon coating, cation doping and so on. An electric and Li+ conductivities of LiMnPO4 are intrinsically low. The Mn substitution with Fe has been done as one of effective approaches to improve the electrochemical properties of LiMnPO4. However, its substitution effect has not been fully understood. The conventional evaluation method using composite electrodes has some difficulties due to porous structure of composite electrode. Therefore, we have conducted a single particle measurement using one active material particle (mainly secondary particle) or a composite particle obtained by taking out a part of composite electrode. We have reported to reduce an influence of the electrode structure on the electrochemical response by using single particle measurement (1). A phosphate-based cathode materials have an excellent thermal stability. These materials are promising active materials for large batteries. However, a slow electrochemical reaction of LiMnPO4 should be improved to obtain high rate capability. A Fe substitution for LiMnPO4 has improved its rate capability (2). In this study, we synthesized carbon-coated LiFexMn1-xPO4. A single particle measurement was employed to understand the intrinsic effect of Mn substitution with Fe on the electrochemical properties of LiMnPO4. Experimental Li2SO4, MnSO4•5H2O, FeSO4•7H2O, (NH4)2HPO4, and carboxymethyl cellulose (CMC) were used as starting materials to synthesized carbon coated LiFexMn1-xPO4 by hydrothermal method. The hydrothermal treatment was performed at 200 ºC for 3 h. The reaction product was dried and then grounded with planetary ball mill (400 rpm, 1 hour × 10 times) to obtain fine powder. The obtained powder was then heated at 700 ºC for 1 h under 97% Ar + 3% H2 atmosphere for a graphitization of CMC on particle surface to obtain LiFex Mn1-xPO4/C. A small composite particle used for single particle measurement was obtained by stripping from the composite electrode with a weight ratio of LiFexMn1-xPO4/C: AB: PVdF = 85: 10: 5 on aluminum current collector. Au micro-electrode with 10 mm diameter was attached to a LiFexMn1-xPO4/C composite electrode particle in an electrolyte (1 mol dm-3 LiPF6 / EC : EMC = 3 : 7 in vol.) using a micromanipulator under optical microscope observation. All the experiments were carried out under Ar atmosphere to remove the effect of water and oxygen. Results and discussion Figure 1 shows the XRD patterns of LiFexMn1-xPO4 synthesized by hydrothermal method. All peaks are attributable to olivine structure of LiFexMn1-xPO4, suggesting that the synthesized LiFexMn1-xPO4 samples are single phase without impurity. The Fe substitution in LiMnPO4was also confirmed from Rietveld analysis. Figure 2 shows the rate capabilities of LiFexMn1-xPO4/C composite electrode particles by single particle measurement. Discharge capacities at 1.5 C and 15 C were plotted against x in LiFexMn1-xPO4/C. In single particle measurement, in order to normalize a variation of particle size, the capacities were indicated by depth of discharge (DOD). The capacity retention at 15 C was improved with increasing amount of Fe substitution. A comparison of discharge curves for each composite particle, a potential plateau corresponding to Mn2+/3+ appeared at more noble potential with increasing of Fe substitution amount. The improvement of rate capability may be due to the improved electronic conductivity and suppressed Jahn-Teller deformation of Mn by Fe substitution. Reference [1] H. Munakata, K. Annaka, K. Kanamura, The 81th Electrochemical Society of Japan, p.343(2Q01). [2] Seung-Min Oh et al , J. Power Sources 196 (2011) 6924-6928 Figure 1
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
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