Patent Application
20130337335
The present invention relates to a negative electrode material for a secondary battery and a method for manufacturing same, and more particularly, to a carbon based negative electrode material including a plurality of tin oxide nanorods formed on the graphite matrix to have improved Coulombic efficiency and high-rate capability, and a method for preparing the same capable of simply and easily controlling diameters and lengths of the tin oxide nanorods grown on the graphite matrix by a catalyst-assisted hydrothermal process.
Various kinds of metal oxides such as Co3O4, CuO, NiO, Fe3O4, SnO2, and the like, have been widely developed as an alternative electrode material of a lithium ion battery (LIB) due to high energy density and relatively low cost. Particularly, SnO2 based materials have a theoretical capacity (about 781 mAhg−1) about two times higher than a theoretical lithium storage capacity of graphite (about 372 mAhg−1) and advantages such as low cost, stability, and the like, such that the SnO2 based materials have been significantly spotlighted as a prominent alternative material replacing a currently commercialized graphite anode.
However, it is still difficult to actually implement an electrode using the SnO2 based material. The reason is that during a charging and discharging process, significant volume expansion of about 250% may occur, which may cause a problem in cyclability.
One way to solve this problem is to prepare the SnO2 based electrode material so as to have a nanostructure. More specifically, the SnO2 nanostructure may have a 1-dimensional structure such as a nanowire, a nanotube, and a nanorod. A technology of obtaining a higher lithium storage capacity of about 1134 mAhg−1 in an initial cycle and a lower capacity fading of 1.45% per cycle by synthesizing 1-dimensional SnO2 nanowires as described above has been disclosed [Park M. S. et al., Angew. Chem. Int. Ed., 2007, 119, 764-767].
In addition, a technology for 1-dimensional SnO2 nanorods having stable capacity retention in a relatively low electric potential window and a high initial capacity of 1100 mAhg−1 has been disclosed [Wang Y., Lee J. Y., J. Phys. Chem., B2004, 108, 17832-17837].
However, in the case of using the nanomaterials in these technologies as an electrode, there was a problem such as coagulation between the nanomaterials. In addition, there was a problem in that Coulombic efficiency and energy density were decreased by irreversible side reactions generated due to a large surface area of the nanomaterials.
In order to solve these problems, a technology of synthesizing a complex so that SnO2 nanoparticles are uniformly dispersed in a buffering matrix has been developed. Since carbon based materials used as the buffering matrix have high electric conductivity, excellent mechanical properties, and a reversible capacity retention property, SnO2 complexes containing the carbon based material have various advantages. In this sense, a SnO2-graphite complex containing SnO2 nanoparticles fixed onto a graphite surface to have more excellent capacity retention as compared to pure SnO2 nanomaterials has been announced [Wang Y., Lee J. Y., J. Power Sources 2005, 144, 220-225].
Even though complex may prevent a coagulation phenomenon, there were problems in that an amount of loaded SnO2 was significantly affected by an area of the graphite surface or dispersity of the nanoparticles.
An object of the present invention is to provide a negative electrode material for a secondary battery including a graphite matrix and a plurality of tin oxide nanorods formed on the graphite matrix to have a large storage capacity, high Coulombic efficiency and cycle stability, and high-rate capability.
Another object of the present invention is to provide a method for preparing a negative electrode material for a secondary battery capable of simply and easily controlling diameters and lengths of the tin oxide nanorods grown on the graphite matrix according to the use by a hydrothermal process.
In one general aspect, there is provided a negative electrode material for a secondary battery including: a graphite matrix; and a plurality of tin-oxide nanorods formed on the graphite matrix.
In another general aspect, there is provided a method for preparing a negative electrode material for a secondary battery, the method including: activating a graphite surface; coating tin oxide nanoparticles onto the activated graphite surface to prepare a tin oxide seed-type graphite; and hydrothermally heating the tin oxide seed-type graphite to grow a plurality of tin oxide nanorods.
The negative electrode material for a secondary battery according to the present invention may have improved Coulombic efficiency, excellent high-rate capability, and cycle stability.
In addition, the method for preparing a negative electrode material for a secondary battery according to the present invention may simply and easily control diameters or lengths of tin oxide nanorods grown on a graphite matrix by a catalyst assisted hydrothermal process.
The present invention may be variously modified and have various types, and exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawing. However, the present invention is not limited to the exemplary embodiments described herein, but all of the modifications, equivalents, and substitutions within the spirit and scope of the present invention are also included in the present invention. In describing each of the drawing, similar components will be denoted by similar reference numerals.
Unless otherwise defined herein, technical and scientific terms used in the present specification have the same meanings as those understood by specialists in the skilled art to which the present invention pertains. Generally used terms as defined in a dictionary should be construed as meanings equal to contextual meanings in the related art and not construed as ideal or excessively formal meanings as long as the meanings are not clearly defined in the present specification.
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Referring to
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Growth of the SnO2 nanorods is generated according to the following reactions.
Sn4++4OH−→Sn(OH)4 (1)
Sn(OH)4+2OH−→[Sn(OH)6]2− (2)
[Sn(OH)6]2−→SnO2+2H2O+2OH− (3)
Sn(OH)4 prepared by the reaction according to Formula (1) is decomposed by excessive OH− anions to form a compound ([Sn(OH)6]2−) of Formula (2). The compound ([Sn(OH)6]2−) is converted into SnO2 by a hydrothermal process according to Formula (3).
In this case, it is preferable that square pillar shaped SnO2 nanorods having a rectangular cross-section are obtained. To this end, a concentration of [Sn(OH)6]2− and a molar ratio of SnCl4.5H2O to NaOH may be controlled. The concentration of [Sn(OH)6]2− for growing the SnO2 nanorods is preferably higher than 0.05M. In the case in which the concentration of [Sn(OH)6]2− is lower than 0.1M, for example, in the case in which the concentration is 0.05M, the SnO2 nanorods are not formed until 24 hours elapse, but the SnO2 nanorods are formed after a longer time (48 hours) elapses.
As described above, the plurality of SnO2 nanorods may be vertically grown from the graphite surface by the hydrothermal process in the [Sn(OH)6]2− aqueous solution and the shapes, the diameters, and the lengths of the SnO2 nanorods may be easily controlled, respectively, while changing the concentration of the [Sn(OH)6]2− aqueous solution and the reaction time.
Meanwhile, the negative electrode material for a secondary battery according to the present invention contains a graphite matrix and a plurality of tin oxide nanorods formed on the graphite matrix. The graphite matrix may be a plurality of graphite cores, and each of the plurality of tin oxide nanorods may have a square pillar shape and a rectangular cross-section. In this case, the plurality of tin oxide nanorods formed on the plurality of graphite cores may be disposed so as to enclose the graphite core. A content of tin oxide may be preferably 50 to 90% by weight based on the total weight of the graphite matrix.
In addition, an average diameter of the plurality of tin oxide nanorods may be in a range of 28 to 84 nm, and an average length thereof may be in a range of 123 to 646 nm.
For example, in the case of applying the negative electrode material for a secondary battery to a lithium ion battery, during a charging process, that is, while Li+ ions are inserted, the Li+ ions may easily move into a spaced space between the grown nanorods to penetrate through each of the SnO2 nanorods, and the SnO2 nanorods may provide a relatively short diffusion length of the Li+ ions due to its structural characteristics.
In addition, since the graphite matrix has high electric conductivity, electrons may be effectively transferred by allowing the electrons to easily move through the 1-dimensional SnO2 nanorods. The plurality of SnO2 nanorods are spaced apart from each other by a predetermined distance to thereby be individually grown on the graphite matrix, such that electrolyte permeability for more rapidly moving the Li+ ions may be increased. In addition, since the plurality of SnO2 nanorods are firmly transplanted on the graphite surface to thereby be bonded thereto, breakdown of the SnO2 nanorods from the graphite may be prevented.
Movement of the electrons may be improved due to high electric conductivity of the graphite, and the SnO2 nanorods may serve as a buffer section moving mechanical stress during the charging/discharging process due to its high flexibility. Further, the negative electrode material for a secondary battery according to the present invention may increase Coulombic efficiency due to a decrease in unnecessary side reactions related to electrolysis on the graphite surface, as compared to a pure SnO2 electrode. Therefore, the negative electrode material for a secondary battery according to the present invention may have a significant influence on improving performance of the lithium ion battery.
Hereinafter, in order to assist in understanding of the present invention, preferable Example is described. However, the following Example is only for assisting in the understanding of the present invention, but the present invention is not limited thereto.
Graphite powder having an average diameter of 20 μm was stirred in an acidic solution in which HNO3 (70%, Aldrich) and HCl (30%, Aldrich) were mixed at a ratio of 1:3 (v/v) for 12 hours to activate a graphite surface. Then, the graphite powder was washed with distilled water (18.2MΩ cm) and dried by a vacuum freeze drying method.
SnO2 was seeded on the activated graphite surface by simple hydrolysis of SnCl4 using NaOH. To this end, first, 0.5 g of the activated graphite powder was dispersed in 4.1 mL of 0.054M SnCl4.5H2O (98%, Aldrich) aqueous solution. Then, 4.1 mL of 0.106M NaOH (99.99%, Aldrich) aqueous solution was added thereto while strongly stirring the solution. Precipitated colloidal SnO2 nanoparticles were formed by the above-mentioned processes. Subsequently, after magnetic stirring for 12 hours, SnO2 seed-type graphite powder was washed with distilled water and ethanol several times and then dried in a convection oven at 70° C. Then, the dried powder was heat treated at 400° C. under Ar atmosphere for 2 hours.
0.075 mol NaOH (99.99%, Aldrich) was mixed in 50 mL of 0.1M SnCl4.5H2O (98%, Aldrich) aqueous solution, and a tin precursor solution was injected into an Teflon inlet of an autoclave. The tin precursor solution was magnetically stirred for 20 minutes under air atmosphere to prepare a transparent and uniform precursor solution.
Meanwhile, after 0.1 g of the SnO2 seed-type graphite prepared above was added to the precursor solution, the mixture was hydrothermally heated at 200° C., and the temperature was maintained for 24 to 72 hours. A material obtained by the above-mentioned processes was washed with distilled water and ethanol and dried in a convection oven in which a temperature was maintained at 70° C.
In addition, SnO2 nanorods having various sizes were synthesized by changing the concentration of the solutions using 50 mL of 0.2M SnCl4.5H2O aqueous solution and 0.105 mol NaOH.
The average diameter and length of the SnO2 nanorods (S1 to S3) formed on the graphite by mixing 0.075 mol NaOH in 50 mL of 0.1M SnCl4.5H2O (98%, Aldrich) aqueous solution were 28 to 37 nm and 123 to 352 nm according to a growth time, respectively. Meanwhile, in the case of mixing 50 mL of 0.105 mol NaOH in 50 mL of 0.2M SnCl4.5H2O (98%, Aldrich) aqueous solution, the average diameter and length of the SnO2 nanorods (S4 to S6) were 62 to 84 nm and 409 to 646 nm, respectively.
In addition, SnO2 nanowires and nanoparticles were separately synthesized for comparison with other nanostructures. The SnO2 nanowires were synthesized using an Au catalyst in a vapor-liquid-solid growth mechanism using chemical vapor deposition (CVD). In this case, the synthesized SnO2 nanowire had a diameter of about 80 nm and a length of micrometer (μm). Further, the SnO2 nanoparticles having a diameter of about 100 nm were synthesized by a hydrothermal process of 50 mL of 0.01M SnCl4.5H2O aqueous solution containing 6.7 mol NaOH at 200° C. for 24 hours.
Measurement was performed using JEOL JSM-7500F as a scanning electron microscope (SEM). XRD patterns of powder samples were recorded with a diffractometer (Rigaku Rotalflex RU-200B) using a Cu Kα (λ=1.5418 ∪) source having a Ni filter at 40 kV, 40 mA, and a scanning rate of 0.02° s−1. Observation was performed using a transmission electron microscope (TEM) and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100) operated at 200 kV. A content (wt. %) of SnO2 was investigated by thermogravimetric analysis at a heating rate of 10° C. mim−1 under air atmosphere.
In addition, slurry obtained by mixing the graphite containing the SnO2 nanorods grown thereon, carbon black, carboxymethyl cellulose, and styrene butadiene rubber with one another at a weight ratio of 80:10:5:5 was pasted on a pure copper foil using a doctor blade method to prepare an electrode, and then dried in a vacuum oven at 145° C. for 3 hours. The resultant was used as a working electrode. Meanwhile, 1M LiPF6 in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1:1 was used as the electrolyte, and a pure lithium foil was used as a counter electrode. Further, Celgard 2400 was used as a separator. These materials were assembled in a glove box in which humidity and a concentration of oxygen were maintained lower than 1 ppm and Ar was filled, to thereby completing a cell of a 2-electrode system.
Thereafter, cyclic voltammetry (CVs) was performed in a range of 2.5 to 0.01V at a scanning rate of 0.05 mVs−1 using AMETEK Solartron analytical 1400. In addition, the manufactured cells were cycled at 0.01 to 1.5V and 72 mAg−1 in a constant current system using WBCS 3000 battery tester. After the initial cycle, measurement results of electrochemical impedance spectroscopy (EIS) using a multi-impedance test system were recorded. The frequency range was 100 kHz to 10 mHz with AC amplitude of 5 mV.
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CVs of S1 at an electric potential of 2.5 to 0.01V and a scanning rate of 0.05 mVs−1 were shown. CVs behavior indicates electrochemical reactions induced by graphite and SnO2 during a charge/discharge cycle. The following Reaction Formulas indicate electrochemical reactions of Li+ ions with SnO2 and graphite in the lithium ion battery.
4Li++4e−+SnO2→2Li2O+Sn (4)
xLi++xe−+SnLixSn (0≦x≦4.4) (5)
Li++e−+C6LiC6 (6)
The CV curves indicated formation of a solid electrolyte interface during an initial discharge cycle and a cathodic peak generated at about 0.75V while SnO2 was reduced into Sn and Li2O (Reaction Formula (4)). In addition, relatively weak peaks were observed at about 0.7 to 0.2V, which relates to formation of LixSn (Reaction Formula (5)). Peaks at about 0V were generated by formation of LiC6 due to insertion of Li into graphite (Reaction Formula (6)).
In an anodic graph, peaks at 0.2 and 0.5V may be generated due to separation of Li from LiC6 and dealloying of Li from LixSn, respectively. As a result, it may be appreciated that, the charge/discharge process of the synthetic compound was a stepwise process. That is, it may be confirmed that first, after Li was alloyed with Sn, Li was inserted into the graphite for a cathodic process, and after Li was firstly separated from LiC6, the dealloying of the LixSn for an anodic reaction was performed.
In addition, it may be confirmed that as the number of cycles was increased, the current density in a CV loop was increased, which means that an activation process may be present during the initial charge/discharge cycle. Since the lithiation/delithiation process generates a structural change of electrically active materials, the activation process may be associated with a reconstruction of internal crystalline structure of the graphite containing the SnO2 nanorods grown thereon. As a result, activation characteristics were determined by a movement rate of Li+ or a formation rate of LiC6 and Li4.4Sn. Therefore, a movement barrier was gradually activated during each cycle, and the current density was continuously increased until a degradation process of the electrode was superior to the activation process.
Meanwhile, in the case of the SnO2 nanowire, since a degradation process of the electrode was superior to an activation process, severe performance deterioration may be generated. On the other hand, in the case of the graphite containing the plurality of SnO2 nanorods grown thereon prepared in Example of the present invention, since the activation process was superior to the degradation process during initial fifth cycles, it may be appreciated that performance degradation was not severe as compared to the SnO2 nanowire.
Referring to
Further, stable capacity retention after the initial cycle indicates that electrically active synthetic compounds were uniformly dispersed in an electrode membrane without coagulation. In the graphite electrode containing the plurality of SnO2 nanorods grown thereon, Li was completely alloyed/dealloyed, and initial Coulombic efficiency (59.2%) higher than theoretical value (52%) for the SnO2 electrode was obtained.
Therefore, it may be appreciated that the Coulombic efficiency of the graphite electrode containing the plurality of SnO2 nanorods grown thereon was high as compared to SnO2 based material of which the Coulombic efficiency was generally 40 to 50%. An initial irreversible capacity loss was mainly generated due to electrolysis of the electrically active materials. Since the graphite suppresses irreversible side reactions as compared to other materials such as Si, Fe2O3, Co3O4, and the like, the graphite containing the plurality of SnO2 nanorods grown thereon may have higher Coulombic efficiency.
After the first discharging process, the SEI layers covering the graphite surface on which the plurality of SnO2 nanorods were grown may prevent the electrolyte from being further decomposed. As a result, the Coulombic efficiency was significantly increased up to 94.2% at the second cycle.
In addition, inserted SEM images shown in
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An average capacity fading ratio of the graphite (S1) containing the plurality of SnO2 nanorods grown thereon was 0.85% at each cycle after a second cycle, and it may be confirmed that capacity retention was excellent. Since elastic force of carbon is larger than that of SnO2, elastic graphite on which the nanorods are spaced apart from each other may effectively receive strain energy when the SnO2 nanorods and the graphite are reacted with Li+. Therefore, the graphite containing the plurality of SnO2 nanorods grown thereon has excellent cyclability.
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In the case in which high-rate capabilities of the SnO2 nanowire and the SnO2 nanoparticle were compared under the same test conditions in order to find out a cause of improved performance of the graphite containing the SnO2 nanorods grown thereon, the synthesized SnO2 nanowire had a diameter of about 80 nm and a length of a micrometer, and the SnO2 nanoparticles had a diameter of about 100 nm. The SnO2 nanowire and the SnO2 nanoparticle had discharge capacities of 242.5 mAhg−1 and 192.9 mAhg−1 at the current density of 288 mAg−1, respectively, and 58.8% and 34.4% of their capacities were recovered, respectively.
In addition, referring to
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It may be confirmed in the Nyquist plots that as the size of the SnO2 nanorods was increased (that is, in a sequence of S1 to S5), a diameter of the semi-circle was increased. This was associated with a surface region because an amount of the decomposed electrolyte was in proportion to a contact region of the electrolyte. In addition, it may be confirmed that a first semi-circle (RSEI) and a second semi-circle (Rct) of the graphite containing the SnO2 nanorods grown thereon were significantly smaller as those of the SnO2 nanowires and nanoparticles. In the case of SnO2 nanowires and SnO2 nanoparticles, sums of RSEI and Rct were 2.38 Ωm2g−1 and 1.19 Ωm2g−1, respectively, and in the case of the graphite containing the plurality of SnO2 nanorods grown thereon, a sum of RSEI and Rct was decreased to 0.26 Ωm2g−1. This indicates that electric conductivity was improved by combination of 1-dimensional SnO2 nanorods and the graphite. Further, in the case of comparing the Li+ movement rate of the graphite containing the plurality of SnO2 nanorods grown thereon with those of the SnO2 nanowires and nanoparticles, it may be appreciated that Li+ was more rapidly moved due to thinner SEI layers in the graphite. Therefore, the graphite containing the plurality of SnO2 nanorods grown thereon has improved high-rate capability and cycle stability at a slightly higher current rate as compared to the SnO2 nanowires and nanoparticles.
The negative electrode material for a secondary battery according to the present invention contains the graphite matrix and the plurality of tin oxide nanorods formed on the graphite matrix, such that the negative electrode material has a larger capacity than that of the graphite and higher Coulombic efficiency and rate capability than those of the SnO2 based material. The excellent performance as described above may be attributed to the peculiar structure of the negative electrode material. The poor cyclability of the SnO2 based material is attributed to large change in volume during the charge/discharge process, and accordingly pulverization of electrodes was generated. However, in the negative electrode material for a secondary battery according to the present invention, mechanical stress caused by a rapid change in volume may be decreased due to the vertically grown 1-dimensional SnO2 nanorods and the elastic graphite, thereby making it possible to decrease degradation of the electrode.
In addition, homogeneous interconnection between electrode membranes are generated due to high affinity between the SnO2 nanorods and the graphite, such that coagulation or separation of the SnO2 nanorods during the charge/discharge process may be prevented, thereby making it possible to obtain excellent capacity retention.
Further, the graphite matrix improves conductivity of the electrode, which may improve movement of electrons and decrease a resistance loss. Theoretical Coulombic efficiency of the SnO2 based materials was 52% due to irreversible formation of Li2O during the complete Li alloying/dealloying process. However, in the case of the graphite containing the SnO2 nanorods grown thereon, since the stable SEI is formed on the graphite, the Coulombic efficiency thereof is higher than that of the SnO2 based materials, which corresponds to an increase in energy density. Therefore, excellent high-rate capability and stable cyclability may be secured.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1020110017452 | Feb 2011 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR2011/009655 | 12/15/2011 | WO | 00 | 8/23/2013 |
