Abstract
The present invention relates to an anode active material, containing fullerene, for a metal secondary battery and a metal secondary battery using the same. When the anode active material for a metal secondary battery of the present invention is nano-grained and used for an anode of a metal secondary battery, it has inherent electrochemical properties of C.sub.60 fullerene so that excellent specific capacity was exhibited and enables high coulombic efficiency to be exhibited even after not less than 1,000 redox cycles so that it is suitable for use in the anode for a metal secondary battery.
Claims
1. A method for preparing an anode active material for a metal secondary battery, comprising: 1) pulverizing a fullerene compound; 2) heating the pulverized fullerene compound under a nitrogen or argon atmosphere; and 3) obtaining a fullerene deposit evaporated by heating.
2. The method of claim 1, wherein the fullerene compound is C.sub.60.
3. The method of claim 1, wherein the heating is performed at a temperature of 700 to 900 C.
4. The method of claim 1, wherein the heating is performed for 90 to 150 minutes.
5. The method of claim 1, wherein the metal is any one selected from the group consisting of lithium, sodium, and potassium.
6. An anode active material for a metal secondary battery, which is prepared using the preparation method of claim 1.
7. An anode material for a metal secondary battery, comprising the anode active material for a metal secondary battery according to claim 6, an anode electrically conductive material, and a binder.
8. The anode material for a metal secondary battery of claim 7, wherein the anode electrically conductive material is carbon black.
9. The anode material for a metal secondary battery of claim 7, wherein the binder is methyl cellulose, styrene butadiene rubber, or a combination thereof.
10. The anode material for a metal secondary battery of claim 7, wherein the anode active material for a metal secondary battery is not chemically bonded with the anode electrically conductive material and the binder.
11-12. (canceled)
13. The anode material for a metal secondary battery of claim 7, wherein the metal secondary battery comprises an electrolyte which is liquid or solid.
Description
DESCRIPTION OF DRAWINGS
[0028] FIG. 1 shows schematic views, SEM images, a TEM image, XRD patterns, and Raman spectrum results of fullerene nanoparticles.
[0029] FIG. 2 is schematic views of the fullerene nanoparticle preparation process and evaluates the effects of the temperature gradients of the tube used in the preparation process.
[0030] FIG. 3 evaluates the degree of weight loss according to thermogravimetric analysis of HGC.sub.60 and fullerene nanoparticles.
[0031] FIG. 4 shows an SEM image, an optical image, XRD patterns, and Raman spectrum results of fullerene nanoparticles.
[0032] FIG. 5 shows XPS results and SIMS depth composition distribution results of fullerene nanoparticles.
[0033] FIG. 6 shows the electrochemical properties of fullerene nanoparticles.
[0034] FIG. 7 shows the results of GITT analysis of fullerene nanoparticles.
MODES OF THE INVENTION
Example 1. Preparation of Fullerene Nanopowder
[0035] FIG. 1A is a schematic view for the synthesis of fullerene nanoparticles (C.sub.60 NPs or C.sub.60 nanoparticles) of the present disclosure, FIG. 1B is a schematic diagram showing the structure of the fullerene nanoparticles of the present disclosure, and the specific synthesis method of the fullerene nanoparticles is as follows.
[0036] A fullerene mixture was obtained from the carbon product obtained using arc discharge, and then this was extracted using a Soxhlet extractor. Purification of low molecular weight fullerenes (C.sub.n<60) and high molecular weight fullerenes (C.sub.n>70) was performed using high performance liquid chromatography (HPLC).
[0037] Pure C.sub.60 fullerenes were collected using Buckyprep column HPLC with toluene as the mobile phase, where only pure C.sub.60 powder was collected. The pure C.sub.60 particles obtained were a bright black powder (0.15 g) (named raw C.sub.60 powder, FIG. 1C), and a relatively fine brown C.sub.60 powder (as a hand-milled C.sub.60 powder, this was named HGC.sub.60 powder, FIG. 1D) that was free from an agglomerated state by hand grinding it for up to 5 minutes was collected, and then transferred into a fused quartz tube electric furnace (FIG. 2A). Air was removed from the tube by filling it with N.sub.2 gas three times in vacuum, and then directly heated at a temperature of 800 C. for 2 hours in a nitrogen (99.999%) atmosphere (heating rate: 3 C. per minute, FIG. 2C). After the heating process was finished and the C.sub.60 powder was cooled to room temperature, the deposited resulting product was carefully collected, and this was named as C.sub.60 NPs (FIG. 1E), and the crystal phase was confirmed by XRD and Raman analysis (FIGS. 1G to 1H).
[0038] In the process of heating up to 800 C. with N.sub.2 gas, structural collapse occurred due to the high temperatures, most of the C.sub.60 molecules sublimated and recrystallized from the bulk powder and deposited on the inner wall of the tube end, and this was due to a large temperature difference between the electric furnace and the portion of the tube exposed to the air (FIGS. 2A and 2B). The distribution of the highest temperature with respect to the tube length after reaching 800 C. is shown in FIG. 2D.
[0039] The TGA results for weight loss of HGC.sub.60 and C.sub.60 NPs in an N.sub.2 atmosphere are shown in FIG. 3. After cooling them to room temperature, it was confirmed that some black by-products remained without evaporation since the C.sub.60 molecules lost the ring and the FCC crystal structure and formed a porous carbon material (FIG. 4A). The amorphous phases of the by-products were further confirmed by XRD and Raman analysis (FIGS. 4C and 4D).
[0040] Regarding the formation mechanism, the fused quartz tube is an insulating material, and thus no free electrons can move along the tube. However, the relatively weak - interaction between the C.sub.60 molecules and the tubes induced the first thin film growth and the second layer, and then allowed C.sub.60 islands to be appeared and resulted in high molecular diffusivity. Therefore, C.sub.60 NPs with uniform morphology were formed layer by layer and could be easily collected from the surface of the fused quartz tube. The photographs of C.sub.60 NPs collected in the fused quartz tube in FIG. 2C showed a powder state as shown in FIGS. 1E and 4B.
Example 2. Analysis of Physicochemical Properties of Fullerene Nanopowder
[0041] (1) Analysis Method
[0042] The crystal structure of the C.sub.60 nanopowder prepared in Example 1 was irradiated by X-ray diffraction (XRD, XPERT-3, PANalytical) in the -2 scan mode in the 2 range of 1060 using Cu K1 X-rays. In situ XRD measurements were performed to observe the structural changes of the C.sub.60 NPs anode active material in the 2 range of 1040 during the charge/discharge process, and an in situ electrochemical analyzer and cell were used for this. A copper thin film (about 200 nm) sputtered onto a beryllium metal substrate (about 25 um thick) at 130 C. was prepared, and the slurry was applied thereon with a doctor blade and then dried. A Raman spectrometer (Raman, HORIBA Jobin Yvon, LabRam HR) using a 514 nm laser as an excitation source was used to obtain information on molecular vibrations and crystal structure. The surface morphologies of the C.sub.60 samples were characterized using a field emission scanning electron microscope (FESEM, S-4700, Hitachi). During the first discharge, lithiated anode samples prepared at different voltages were manufactured with a dual beam focused ion beam (FIB, Helios NanoLab 450, FEI) system. The thickness of the samples etched by the gallium ion beam was approximately less than 100 nm. The crystal structure of the samples and the elements carbon and lithium were analyzed using a transmission electron microscope (TEM, Titan3 G2 60-300 microscope, FEI) on which a dual Cs-aberration corrector and monochromator, and an UltraScan 1000 CCD and a Gatan Quantum 965 dual electron energy loss spectrometer (EELS) system were mounted. System conditions for TEM analysis such as accelerating voltage, exposure time and resolution were 80 kV, 0.2 s, and 20482048 pixels, respectively. High angle angular dark field (HAADF)-STEM imaging acquisition conditions included an acceleration voltage of 80 kV and a convergence angle of 26 mrad. The HAADF detector had an internal collection angle of 52 mrad and an external collection angle of 340 mrad. All images shown were 10241024 pixels across a 16 s dwell time. Selected area electron diffraction (SAED) patterns were acquired with Fast Fourier.
[0043] The corresponding domain axis and plane index were determined by performing analysis using HRTEM images' transformation (FFT) and CrysTBox software and quantifying the distance and angle between the diffraction spots. EELS was performed at an accelerating voltage of 80 kV. The energy spread of ZLP was 0.8 eV, the energy dispersion was 0.1 eV channel, and the exposure time ranged from 0.004 s (low loss) to 1 s (high loss).
[0044] The chemical bonding state of C60 NPs was analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo Fisher Scientific) using a multi-channel detector in the range of 0 to 1200 eV with monochromatic AlK radiation (1486.6 eV). The spectral binding energy was calibrated using the C1s peak (284.6 eV).
[0045] TOF-SIMS experiments on C.sub.60 NPs films on Pt/Si substrates were performed with a TOF-SIMS 5 (IONTOF GmbH, Mnster) using a pulsed 30 keV Bi.sup.3+ primary beam with a current of 0.60 pA. The analysis area used in this work was a square of 200 m200 m. Anion spectra were internally normalized to their respective secondary total ion yields using the H.sup., C.sup., C.sup.2, C.sup.3, and C.sup.4 peaks. Chemical images of the analyzed area were recorded at 128128 pixel resolution during data collection. The depth profile was a square of 500 m500 m using an Ar.sup.+ ion cluster of 20 keV and 13 nA.
[0046] Thermogravimetric analysis (TGA, STA 6000 thermal analyzer, PerkinElmer) of the samples was performed at 25 to 1,000 C. (heating rate of 10 C. min.sup.1) in an N.sub.2 atmosphere.
[0047] (2) Property Analysis Results
[0048] The SEM image of C.sub.60 NPs is shown in FIG. 1E, the morphology of C.sub.60 NPs is clearly different from that of the HGC.sub.60 powder, and the grain size of C.sub.60 NPs is greatly reduced so that they were more uniform and smooth by about hundreds of nanometers. C.sub.60 NPs were composed of small particle clusters as shown in the TEM image of FIG. 1F. The microstructure of the C.sub.60 sample was further analyzed using X-ray diffractometer (XRD) measurements. A typical XRD pattern is shown in FIG. 1G, and the diffraction peaks of (111), (220), (311), (222), (331), (420), (422), and (511) show a typical crystal structure of Fm-3m fullerene which is cubic. The results confirmed that C.sub.60 NPs perfectly maintained the original structure of the fullerene derived from the C.sub.60 powder. No impurity peak was detected, and thus it was suggested that FCC phase-pure C.sub.60 NPs could be obtained through evaporation and recrystallization process and that C.sub.60 NPs could be collected in large quantities.
[0049] The crystal qualities of raw C.sub.60 and HGC.sub.60 were similar to that of C.sub.60 NPs, as shown in FIG. 4C. The Raman spectrum showed typical characteristic peaks of C.sub.60 NPs mainly including 8 Hg bands and 2 Ag bands as shown in FIG. 1H. Eight Hg bands were present at 271, 431, 707, 773, 1100, 1249, 1424, and 1573 cm.sup.1, respectively. Another two Ag modes were located at 496 and 1468 cm.sup.1 corresponding to the breathing mode and the pentagonal pitch mode, respectively. The results indicated that C.sub.60 NPs derived from the HGC.sub.60 powder could well retain the FCC structure of the solid raw material C.sub.60 (FIG. 4D). This was a clear difference compared to previously published C.sub.60 related anodes in lithium ion batteries, and most of the previously published C.sub.60 related anodes have been changed to polymerized C.sub.60 or amorphous carbon.
[0050] The surface chemical bonding state and chemical composition of the C.sub.60 nanoparticles were analyzed using X-ray photoelectron spectroscopy (XPS), and are shown in FIGS. 5A to 5C. Referring to FIG. 5A, it can be confirmed that element C exists as the main peak and element oxygen exists as the only impurity peak with very weak intensity. FIG. 5B shows the high-resolution C is spectrum, and the peaks at 284.7 and 285.1 eV were assigned sp.sup.2 (CC bond) and sp.sup.3 (CC bond), respectively. The small peak located at 286.4 eV is due to physically adsorbed carbon oxides. To confirm this, the surface of C.sub.60 NPs was etched using Ar.sup.+ ions, and weak peaks could be removed after 30 seconds of etching. Therefore, it could be confirmed that the impurity peak detected at up to 532 eV (O1s, FIG. 5C) was oxygen due to the contribution of physically adsorbed CO molecules located on the C.sub.60 surface. Even after 60 seconds of etching, it could be confirmed that the main C1s peak was still present and the O1s peak could be no longer detected.
[0051] The purity of pure C.sub.60 NPs was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS), and the results are shown in FIG. 5D. C.sub.60 NPs were deposited for further use by putting Pt (100 nm)/Si into the cold part of a fused quartz tube as a substrate. Within about 700 seconds of sputter time, H.sup.+ and Pt.sup.+ were not completely present, and C.sub.60.sup.+ was uniformly distributed on the substrate surface. An image inserted in FIG. 5D shows a three-dimensional mapping image of positive charges of H.sup.+, Pt.sup.+, and C.sub.6.sup.+. Referring to FIG. 5D, it can be confirmed that C.sub.60H.sub.x is not present, and it can be confirmed that it is pure C.sub.60. As the sputter time increased to about 700 seconds or more, it could be confirmed that the intensity of C.sub.60.sup.+ gradually decreased and those of H.sup.+ and Pt.sup.+ gradually increased.
Example 3. Analysis of Electrochemical Properties of Fullerene Nanopowder
[0052] (1) Analysis Method
[0053] The electrochemical performance of the C.sub.60 active material was analyzed in a C.sub.60/Li half-cell. An anode electrode was prepared by mixing carbon black, an electrically conductive material, with carboxymethylcellulose/styrene butadiene rubber as a binder (CMC/SBR=1:1 wt %, using deionized water as a solvent). The weight ratio was 70:15:15 (In order to prepare raw C.sub.60 anode, carbon black and binder were mixed, dispersed, and then raw C.sub.60 powder was added thereto. In the case of HGC.sub.60 powder and C.sub.60 NPs, they were finely pulverized in a mortar and pestle). The slurry was spread on a copper foil with a doctor blade, dried in vacuum at 60 C. for 10 hours, and then punched into disks, each 1.4 cm in diameter. The electrolyte was a 1M LiPF.sub.6 solution of ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 vol %), containing 10% fluoroethylene carbonate. Lithium metal was used as the counter and reference electrodes. CR2032 coin cells were assembled into an argon-filled glove box. Electrochemical tests of the samples were investigated using a multi-channel potentiostat/galvanostat battery test station (Wonatech, WMPG1000) in the voltage range of 0.01 to 3.0 V (vs Li/Li.sup.+). Cyclic voltammetry (CV) tests were performed at scan rates of 0.1 to 5 mV s.sup.1. Galvanostatic Intermittent Titration Technique (GITT) was performed on a multi-channel electrochemical workstation (ZIVE MP1) after 4 hours of relaxation at 30 min intervals under a current density of 100 mA g.sup.1. Electrochemical impedance spectroscopy (EIS) analysis and temperature-dependent cycling performance were also performed using the ZIVE MP1 in the frequency range of 100 kHz to 0.01 Hz. For the whole cell test, the cell was assembled using a C.sub.60 NPs anode and an LiFePO.sub.4 cathode. The C.sub.60 NP anode was first pre-lithiated in a half-cell for 3 cycles at a current density of 170 mA g.sup.1, charged (delithiated) to 3.0 V, and then, as the anode of a full cell, paired with the LiFePO.sub.4 cathode. The specific capacity of the entire battery was evaluated based on the mass of the anode active material.
[0054] (2) Electrochemical Property Analysis
[0055] FIG. 6 showed the electrochemical performance of the C.sub.60 sample. FIG. 6A showed the CV curves of the C.sub.60 NPs anode during the initial 3 cycles at a scan rate of 0.1 mV s.sup.1. Compared to raw C.sub.60 and HGC.sub.60 samples, the reversible cathodic/anodic peaks of C.sub.60 NPs were much sharper and clearer in the second and third cycles. Oxidation peaks at 0.15, 0.19, 0.23, 0.72, 1.11, and 1.28 V overlapped during the cycle corresponding to delithiation on Li.sub.xC.sub.60. In the first anodic process, a small reduction peak of 2.09 V was assigned to the reductive decomposition of a carbonate solvent. The broad anodic peak located at 0.26 V was mainly due to the formation of a solid electrolyte interphase (SEI) film on the electrode surface, and it was disappeared during proceeding of subsequent cycles. Another small anodic peak corresponding to one in which Li.sup.+ ions were inserted into the C.sub.60 lattice appeared at about 0.05 V. FIG. 6B shows the rectifying charge-discharge curves of the initial three cycles at 0.1 A g.sup.1. The voltage flat section was well coincided with the peak positions of the CV curves to show high consistency. The discharge/charge specific capacity during the first cycle was 1130/674 mAh g.sup.1 with a coulombic efficiency of 59.6%. The large irreversible capacity may be since the Li.sup.+ ions are irreversibly intercalated due to the formation of the SEI layer and the reductive decomposition of the carbonate solvent. For the second and third cycles, the discharge/charge specific capacities and coulombic efficiencies were 754/706 and 752/722 mAh g.sup.1 and 93.6 and 96.0%, respectively. FIG. 6C provides Galvanostatic Intermittent Titration Technique (GITT) data related to Li.sup.+ ion diffusion coefficient (D.sub.Li+) values during the second discharge.
[0056] FIGS. 7A and 7B show schematic diagrams for GITT measurement of voltage changes with respect to input current pulses during 2 cycles. The DLi.sup.+ values increased due to the chemical reaction between the active material and Li.sup.+ ions compared to other regions. In the second discharge process, the DLi.sup.+ values were 3.9410.sup.10 cm.sup.2s.sup.1 (maximum value) for Li.sub.9.4C.sub.60 and 1.0810.sup.11 cm.sup.2s.sup.1 (minimum value) for Li.sub.20.1C.sub.60, respectively. The GITT data during the first charge/discharge and second charge processes were shown in FIGS. 7C to 7Ee, and the DLi.sup.+ values showed almost similar behaviors except for the high specific capacities of the first discharge. The CV curves at different scan rates of 0.5-5 mV s.sup.1 were shown in FIG. 6D, and it could be confirmed that the cathodic/anodic peaks representing Li.sup.+ intercalation/deintercalation behaviors shifted to lower and higher potentials, respectively. As the scan rate increased, all redox peaks were still well maintained with a small potential difference at a high scan rate of 5 mV s.sup.1. The diffusion-controlled and pseudocapacitive contributions of the C.sub.60 NPs anode at various current densities from 0.1 to 5 A g.sup.1 were shown in FIG. 6E. Although the specific capacity decreases gradually with increasing current density, the lithiation/delithiation stability cycles were still clearly identified and were almost the same even at a high current density of 5 A g.sup.1. A cycling test was performed to evaluate the cycling stability. In the case of C.sub.60 NP, it exhibited a low current density of 0.1 A g.sup.1 and discharge specific capacity of 786 mAh g.sup.1 after 50 cycles due to excellent stability and low retention rate (FIG. 6F). FIG. 6G showed the rate performance of three C.sub.60 anodes as predicted by other analyses. The C.sub.60 NPs electrode exhibited relatively high reversible discharge specific capacities of 778, 734, 620, 499, 440, and 359 mAh g.sup.1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g.sup.1. As the current density returned to 2.0, 1.0, 0.5, 0.2, and 0.1 A g.sup.1, reversible specific capacities of 464, 595, 666, 731, and 782 mAh g.sup.1, respectively, could be obtained. For comparison, the HGC.sub.60 powder exhibited reversible discharge specific capacities of 615, 550, 408, 343, 257, and 107 mAh g.sup.1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g.sup.1, respectively. Reversible specific capacities of 258, 350, 434, 581, and 643 mAh g.sup.1 could be obtained, respectively, with current densities gradually returning to low current densities of 2, 1, 0.5, 0.2, and 0.1 A g.sup.1. The values of raw C.sub.60 powder showed only 422, 351, 260, 198, 126, and 43 mAh g.sup.1 for reversible discharge capacities at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A g.sup.1. The current densities recovered to 2, 1, 0.5, 0.2, and 0.1 A g.sup.1, but the values of raw C.sub.60 powder remained at 141, 219, 274, 368, and 437 mAh g.sup.1, respectively. For long cycling performance at high current densities, the anode was evaluated at 5 A g.sup.1 (after 5 cycles at 0.1 A g.sup.1). As shown in FIG. 6H, C.sub.60 NP maintained a reversible capacity of 408 mAh g.sup.1 after 500 cycles. HGC.sub.60 powder and raw C.sub.60 powder showed 154 and 81 mAh g.sup.1, respectively. Up to 1000 cycles, it was confirmed that C.sub.60 NP still maintained 373 mAh g.sup.1 and the HGC.sub.60 powder and raw C.sub.60 powder remained at only 105 and 64 mAh g.sup.1 with coulombic efficiencies of 98.7, 98.6, and 94.4%, respectively. It could be seen that the significant improvement of C.sub.60 NPs as an anode material (e.g., excellent cycling stability and rate capability) is related to the shorter path (transportation of both electrons and Li.sup.+ ions) due to the uniform particle size compared to other C.sub.60 powders.
