JP5516929B2 - Carbon nanotube material for negative electrode and lithium ion secondary battery using the same as negative electrode - Google Patents

Description

  The present invention relates to a carbon material for a negative electrode used in a lithium ion secondary battery, and more particularly to a modified carbon nanotube and a lithium ion secondary battery using the carbon nanotube as a negative electrode.

A lithium ion secondary battery is a type of battery in which lithium ions in an electrolyte move between a positive electrode and a negative electrode during charging and discharging.
In such a lithium ion secondary battery, a material capable of reversibly storing lithium is used for both the positive electrode and the negative electrode, and the potential difference between insertion and desorption of lithium at both electrodes becomes the electromotive force of the battery. As the electrolytic solution, a liquid or solid electrolyte capable of moving lithium ions is used.

Among these, for the negative electrode material, the electrode material having a large capacity that operates at the lowest potential in theory is lithium metal. That is, the reaction in which lithium ions receive electrons and precipitate as lithium metal has the largest capacity, and the theoretical capacity reaches 3861 mAh / g. Moreover, it becomes a negative electrode which can operate at the lowest potential, and can give the highest electromotive force when combined with various positive electrodes.
However, since lithium metal precipitates as needle-like crystals (dendrites), there is a risk of short-circuiting with the positive electrode due to repeated use, and it is currently used only for very limited button batteries. . Therefore, a material capable of reversibly storing lithium with a large capacity is demanded instead.

Since the dissolution and precipitation potential of lithium is a very low potential, many compounds are corroded, but carbon materials are stable even at this potential and are recommended for batteries with high electromotive force.
Of these, graphite is a layered substance, and lithium ions are taken in between the layers, thereby producing a lithium graphite intercalation compound of composition formula LiC6. Since the potential at this time is only about +0.06 V away from the potential of lithium metal, a high electromotive force can be maintained when combined with the positive electrode reaction. In addition, the charge / discharge potential is almost constant, and the graphite itself has high conductivity, so the internal resistance loss is small, and the charge / discharge efficiency (ratio of charge to discharge). ) Exceeds 90%, and stable operation can be expected. For this reason, graphite materials are mostly used in current lithium ion batteries such as mobile phones.

  On the other hand, the theoretical capacity in the composition of LiC6 is only 372 mAh / g, and it cannot be said that the capacity is sufficient for designing a battery with a larger capacity, for example, for an electric vehicle.

Thus, attempts have been made for a long time to use the same carbon material, such as low-temperature calcined carbon and non-graphitizable carbon (hard carbon). These can be thought of as carbon materials with carbon hexagonal mesh surfaces that are not as large as graphite, and are laminated randomly. LiC6 intercalation compound formation reactions, along with voids and defects in carbon, etc. Can store 700-800 mAh / g more than twice that of graphite.
However, the charge / discharge efficiency is worse than graphite due to the fact that some of the inserted lithium cannot be removed, the amount of surface film (SEI) generated is large, the conductivity is poorer at low temperature firing, etc. Since the potential profile is not constant and the potential changes in proportion to the capacity, there is a disadvantage that it is not suitable for a battery such as a small device, and the product has not been commercialized to replace graphite.

From the above background, research on lithium ion secondary batteries using other carbon-based materials as negative electrode materials has been actively conducted.
Among these, since carbon nanotubes are one of the materials consisting of only carbon atoms, many studies have been conducted on lithium insertion / extraction from various nanotubes.

In the case of carbon nanotubes, as shown in FIG. 6, there are three possible lithium insertion spaces: (1) a wall layer, (2) a tube space, and (3) a bundle space.
Among these, since the technology for opening only the tip portion of the space inside the tube cleanly has not been established so far, reports that lithium is reversibly inserted and desorbed are still unreliable. In addition, single-walled carbon nanotubes do not exist between the wall layers, so they are limited to multi-walled carbon nanotubes. However, since the layers have a cylindrical structure, the expansion between the layers due to lithium insertion is hindered by stress and is unlikely to expand. There is a report that the wall portion cannot be expected to have a capacity as high as that of graphite. The bundle space is a unique space of nanotubes sandwiched between tubes where tubes are stacked, and is regarded as a van der Waals space as well as the graphite layer, and storage of materials here is expected from an early stage. Single-walled carbon nanotubes have the largest amount of bundle space. According to a paper by Non-Patent Document 1, the amount of lithium inserted into single-walled carbon nanotubes with a relatively bundle structure developed by laser ablation is 470 mAh / g. In-situ XRD method is used to insert lithium between tube bundles. Since then, lithium insertion into various tubes has been studied. At present, there are also reports of ~ 800 mAh / g regarding different tube manufacturing methods, structures and purity. (Non-patent documents 2-3)

Thus, since carbon nanotubes are carbon-based materials, lithium can be inserted at a base potential close to that of lithium metal, the electromotive force can be increased, and the lithium insertion space is diverse. Greater capacity than can be expected. In addition, it itself has electrical conductivity, and has many merits such as no need for a conductive aid unlike an oxide electrode.
However, although the reversible capacity is larger than that of graphite, since the initial irreversible capacity is not so large as that of graphite, there is a fatal defect that a battery that can withstand actual use cannot be manufactured. Hereinafter, this will be described with reference to the drawings.

7 and 8 are charge and discharge curves of lithium using carbon nanotubes by graphite and a commercial HiPco method, respectively.
From FIG. 7, in the case of graphite, a very small plateau (flat portion) is generated around 0.8 V, and this corresponds to the amount of current used for forming the surface film (SEI). SEI is an organic compound containing lithium ions that are generated when some of the solvent molecules enter the interlayer near the surface of the graphite layer and undergo electrochemical decomposition there at the first lithium insertion. This SEI is not conductive, but lithium ions can pass through, so the solvent molecules are detached from the lithium solvated on the surface of the SEI, and only lithium passes through the SEI and is inserted into the graphite layer. Therefore, SEI must be generated at a minimum. After SEI is generated, a current flows around 0V, which corresponds to the insertion of lithium between the graphite layers. The amount of insertion near 0 V corresponds to about 300 mAh / g, which is very close to the value of graphite at the laboratory level. Since the SEI has already been completed in the second and subsequent charging / discharging, only a plateau near 0V is observed. The reason why the amount of SEI in graphite is small is that the exposed graphite surface is small.

  On the other hand, carbon nanotubes produced by the high-pressure carbon monoxide method (HiPco method) show an abnormally large and flat plateau near 1 V, and an initial irreversible capacity of 1500 mAh / g is generated from FIG. It can be seen that almost a single electrochemical decomposition reaction of the solvent corresponding to SEI occurs on the surface. In the case of the HiPco method, after SEI has been generated, a reversible capacity of about 400 mAh / g occurs near 0 V, which is the same as the literature value, which is the amount of lithium insertion / extraction into the tube bundle space. It is.

As described above, the carbon nanotube has a possibility of generating a reversible capacity larger than that of graphite, but the amount of SEI generated at the first insertion of lithium is abnormally large, and the initial irreversible capacity is 100 times or more that of graphite. Therefore, in the case of a product, there is a fatal defect that only about 10 to 20% can be discharged even if the manufactured battery is charged for the first time. Moreover, since SEI is insulative, if it is produced in a large amount, the internal resistance of the battery is lowered, and it cannot be used for an in-vehicle battery that requires high power.
Therefore, if the amount of SEI produced can be reduced to the same level as graphite and the initial irreversible capacity can be significantly reduced, the unique material storage space of the nanotube can be utilized well for application development using lithium storage, and useful lithium ion secondary The creation of the next battery can be expected.

A.S.Claye, et al., J. Electrochem.Soc. 147 (8) 2845-2852 (2000) Shingo Komiyama et al., `` Carbon '', No.216 (2005) p.25. Takeshi Abe, `` Journal of the Electrochemical Society '', Vol.76, No.5, p.354-357 (2008).

  The present invention relates to a carbon nanotube material for a negative electrode that has a variety of lithium insertion spaces, can have a larger electric capacity than graphite, and has many merits that a conductive auxiliary agent is unnecessary unlike an oxide electrode. An object of the present invention is to provide a novel lithium ion secondary battery using a carbon nanotube material as a negative electrode, in which the irreversible capacity in the vicinity of 1 V is significantly reduced by remarkably suppressing the generation of a huge SEI layer.

As a result of intensive studies over many years on the improvement of the characteristics of carbon nanotubes as a negative electrode material, the present inventors have mixed a thermoplastic resin into a carbon nanotube bulk material and heated it in an inert gas, thereby enabling thermal decomposition. The carbon precursor material, which is obtained by coating the surface of the nanotube material with the carbon precursor that has become a liquid phase in the process and carbonizing in that state, exhibits extremely useful characteristics as a negative electrode material. I found.
The present invention has been made based on such novel findings.
That is, this application provides the following inventions.
< 1 > A method for producing a carbon material for a negative electrode comprising a carbon-coated carbon nanotube material in which an exposed surface of the carbon nanotube is coated with amorphous carbon, wherein the mixture of the carbon nanotube and the thermoplastic resin is mixed with an inert gas. subjected to heat treatment, and, a thermoplastic resin, characterized in that it is a resin carbonized via liquid phase, method of manufacturing the negative electrode carbon material consisting of carbon-containing coated carbon nanotube material (where "lithium The alloy particles that can be alloyed and the fibrous graphite material are mixed and stirred to form composite particles, and then the composite particles and the carbonaceous material precursor are mixed and heat-treated. .)
< 2 > The method for producing a carbon material for a negative electrode comprising the carbon-coated carbon nanotube material according to < 1 >, wherein the resin to be carbonized via the liquid phase is polyvinyl chloride or polyvinyl alcohol. .

The carbon material for negative electrodes of the present invention can suppress direct contact of the electrolyte solution with the nanotubes by covering the exposed surface of the carbon nanotube material with pyrolytic carbon. As a result, SEI is generated only on pyrolytic carbon and can suppress the generation of SEI layers generated at 1 V on the nanotube surface, and the initial irreversible capacity is significantly reduced to, for example, 1/4 to 1/5. Can be reduced. On the other hand, since pyrolytic carbon as a covering material allows lithium to pass through, the carbon nanotubes contained therein can exhibit a conventional lithium insertion / extraction behavior.
Carbon generated by thermal decomposition of the polymer covering the surface itself is an active material capable of lithium insertion / extraction, but it is generally low in conductivity because of low-temperature firing, and can be used alone or as an oxide active material. An electrode coated with a carbon material has difficulty in high power characteristics. However, when the nanotube is coated in this method, since the highly conductive nanotube is contained inside, the thin pyrolytic carbon layer whose surface is coated can be used as an active material.
Therefore, in this invention, it becomes possible to use as a C / C composite negative electrode material which can make up for each other's fault from both the nanotube side and the pyrolytic carbon side.
In addition, since the modified carbon nanotube of the present invention is a composite material electrode made of only carbon, unlike a composite material electrode in which an organic polymer or an oxide and a nanotube are combined, a further secondary treatment can be performed by heat treatment at a high temperature. Structure control is possible.

The carbon material for a negative electrode of the present invention is characterized in that the exposed surface of the carbon nanotube is coated with amorphous carbon.
Here, the exposed surface of the carbon nanotube is a portion that is usually in contact with the outside air, and in the case of a battery, it means a portion that gets wet with the electrolyte. In other words, the tube present in the bundle is not exposed to the outside, and the tube is normally closed so that the inner wall of the tube is not exposed, so the exposed surface is only the outermost surface of the bundle. Means.
A typical example of such a carbon-coated carbon nanotube material is obtained by heat-treating a mixture of carbon nanotubes and a thermoplastic resin in an inert gas and coating the exposed surface of the carbon nanotubes with amorphous carbon. .
A schematic view of a typical carbon-coated carbon nanotube material according to the present invention is shown in FIG.
In general, nanotubes often form bundles rather than exist independently and individually. This bundle further aggregates and is industrially provided as a structure such as powder or paper. The nanotube material is mixed with polyvinyl chloride, polyvinyl alcohol, or the like, which is a typical thermoplastic polymer, and heated to carbonize the thermoplastic polymer via the liquid phase. In this process, the carbon precursor in a liquid phase has a large number of exposed nanotube surfaces, and carbonization can be performed by carbonizing the carbon precursor. The resulting carbon-coated carbon nanotubes are covered with amorphous carbon that can insert lithium ions but not solvent molecules, so the decomposition of the solvent, i.e. the SEI phase, is only produced on the amorphous carbon, Lithium ions reach the nanotubes and are stored. It can be provided as a C / C composite material composed only of such carbonaceous matter.

As the carbon nanotube used as a raw material, either a conventionally known single-layer or multi-layer can be used. Examples of single-walled carbon nanotubes include those by chemical vapor pyrolysis methods such as HiPco method, super growth method, vapor phase flow growth method, and those by laser ablation method. The same applies to multi-walled carbon nanotubes, which include double-walled nanotubes called by other names.
Among these, the present invention is most effective when single-walled carbon nanotubes having many SEI formation sites and a large specific surface area are used. In addition, nanotubes with a bundle space that can store lithium are preferable in terms of capacity over nanotubes that are separated from each other, but the space inside the tube can be expected as a lithium storage space in the future. In particular, the presence or absence of a bundle is not limited.

The carbon-coated carbon nanotube material of the present invention can suppress direct contact of the electrolyte solution with the nanotube by covering the exposed surface of the carbon nanotube aggregated powder with pyrolytic carbon. As a result, SEI is generated only on pyrolytic carbon, suppresses the generation of a huge SEI layer generated at 1 V, which is characteristic of the carbon nanotube electrode shown in FIG. 8, and greatly reduces its initial irreversible capacity. be able to. On the other hand, since lithium can pass through both SEI and pyrolytic carbon, the encapsulated carbon nanotube can exhibit the conventional lithium insertion / extraction behavior.
Carbon generated from the thermal decomposition of the polymer that coats the surface itself is an active material capable of lithium insertion / extraction, but it has low conductivity because of low-temperature treatment, and by itself is not suitable for high-power batteries. Although unsuitable, in this method, a thin pyrolytic carbon layer containing conductive nanotubes can be used as an active material with low internal resistance.

In addition, in the lithium ion secondary battery as the graphite negative electrode mentioned in the background art, various methods for coating the graphite surface with carbon have been reported. This is to increase the degree of freedom in selecting the solvent used. It is about technology.
That is, in these technologies, propylene carbonate (PC) solvent is an excellent solvent that has excellent low-temperature characteristics and does not solidify even in cold regions, but when used as an electrolytic solvent for a lithium ion secondary battery using a graphite negative electrode In this case, a phenomenon occurs in which PC becomes a complex ion with lithium ions and is inserted between graphite layers, resulting in peeling and destruction of the graphite layer. Therefore, by covering the graphite surface with carbon that does not decompose PC, The main purpose is to suppress such a phenomenon and enable the use of a PC solvent (for example, Japanese Patent Application Laid-Open No. 2002-141062 and M. Yoshio, et al., J. Electrochem. Soc., 147 ( 4) See 1245-1250 (2000)).
Therefore, this technique remarkably suppresses the generation of a huge SEI layer, and the technique of the carbon-coated carbon nanotube material of the present invention, which aims to significantly reduce the irreversible capacity near 1 V, its object and problem, etc. Are clearly different, and the two technologies are clearly distinguished.

  The provided shape of the carbon nanotube is not particularly limited, and may be in the form of powder, paper, compression-molded body, etc., but it is easy to mix with the resin powder, the liquid phase carbon precursor penetrates well, and the nanotube Since wetting the surface facilitates efficient carbon coating, it is preferable to use a raw material in a granular or powder form. Since the powder-like aggregate is the form most easily obtained in the synthesis of nanotubes, the raw material form cannot be limited in the present invention.

Any method such as vapor phase impregnation method or liquid phase pyrolysis method may be used to coat carbon nanotubes with carbon, but the simplest and most effective way to coat carbon nanotubes is during the carbonization process. This is a liquid phase pyrolysis method via a liquid phase.
The carbon source polymer used for the liquid phase pyrolysis may be any thermoplastic resin that melts during the carbonization process and can wet the nanotube, and examples thereof include polyvinyl chloride and polyvinyl alcohol. On the other hand, even if the same thermoplastic resin is used, polyethylene, polystyrene, or the like that evaporates without being carbonized is not preferable.
In the examples described below, as a representative example, oxygen is used for the purpose of eliminating oxygen-containing functional group-generating elements that affect a lithium insertion property via a low-viscosity liquid phase suitable for wettability. Although polyvinyl chloride not containing was used, it is needless to say that the present invention is not limited to this.
The amount of carbon to be coated is not limited. That is, since the coated carbon material can also be used as an active material, the battery characteristics are not extremely lowered. In other words, the ratio of the nanotube to the coated carbon is determined by the required battery performance, such as capacity density and power density.

  A carbon material for a negative electrode made of a carbon-coated carbon nanotube material in which the exposed surface of the carbon nanotube is coated with amorphous carbon can be manufactured by various methods. For example, carbon nanotube powder and a thermoplastic resin are mixed with nitrogen. It can be obtained by heat treatment in an inert gas such as a gas.

  The use ratio of the carbon nanotube and the thermoplastic resin can be appropriately determined by considering the required battery performance, for example, the capacity density and the power density, as long as the reduction in the initial irreversible capacity is not impaired. In the examples of the present invention described later, the use ratio of the carbon nanotubes and the thermoplastic resin is 1: 0.5 to 1: 4, but the present invention is not limited to these ranges.

  The heat treatment temperature is possible as long as it is higher than the temperature at which the resin is thermally decomposed and carbonized, and generally a carbonaceous coating is observed at 500 ° C. or higher. However, in general, high-temperature treatment improves the conductivity of the coated carbon, so the final temperature should be higher. On the other hand, when the angle is 1500 degrees or more, the nanotube is denatured, and therefore, practically, 1000 to 1500 degrees is preferable.

  The specific surface area (BET method) of the carbon-coated carbon nanotube material according to the present invention is reduced depending on the coating amount by being coated with carbon, and if the nanotube is completely included in a large amount of coating, Since the surface area is close to zero, in the present invention, from the viewpoint of taking advantage of the characteristics of the nanotubes, it is preferable that the carbon is uniformly coated in a state close to the specific surface area of the nanotubes, but the coated carbon is also used as the battery active material. Therefore, even if the specific surface area is reduced, it cannot be used as an electrode.

  In the carbon-coated carbon nanotube material according to the present invention, the coating state can be confirmed by, for example, an electron microscope or a nitrogen adsorption method, and the effect can be remarkably confirmed by actually measuring the charge / discharge characteristics of the battery.

The carbon-coated carbon nanotube material of the present invention can suppress the direct contact of the electrolyte solution with the nanotube by covering the exposed surface of the carbon nanotube material with pyrolytic carbon. As a result, SEI is generated only on pyrolytic carbon, suppresses the generation of a huge SEI layer generated at 1 V, which is characteristic of the carbon nanotube electrode shown in FIG. 8, and greatly reduces its initial irreversible capacity. be able to. On the other hand, since lithium can pass through both SEI and pyrolytic carbon, the encapsulated carbon nanotube can exhibit the conventional lithium insertion / extraction behavior.
Carbon generated from the thermal decomposition of the polymer that coats the surface itself is an active material capable of lithium insertion / extraction, but it has low conductivity because of low-temperature treatment, and by itself is not suitable for high-power batteries. Although unsuitable, in this method, a thin pyrolytic carbon layer containing conductive nanotubes can be used as an active material with low internal resistance.
Therefore, in this invention, it becomes possible to use as a C / C composite negative electrode material which can make up for each other's fault from both the nanotube side and the pyrolytic carbon side.
In addition, the carbon-coated carbon nanotube material obtained by the present invention has a wide variety of lithium insertion spaces, and can have a larger electric capacity than graphite, and remarkably suppress the formation of a huge SEI layer, greatly increasing the irreversible capacity around 1V. Therefore, it is extremely useful as a negative electrode material for a lithium ion secondary battery because it has many advantages that it has excellent electrical conductivity, and unlike a oxide electrode, does not require a conductive aid.

  In the lithium ion secondary battery using the carbon-coated carbon nanotube material of the present invention as a negative electrode material, the positive electrode material, the electrolytic solution (electrolyte, solvent), and the like are used as constituent members of this type of lithium ion secondary battery having a carbon-based material as a negative electrode. Those used in the secondary battery can be applied as they are.

  As the positive electrode material, all conventionally known materials are applied, and examples thereof include lithium cobalt oxide, lithium manganese oxide, and lithium iron oxide.

The electrolyte to be contained in the electrolytic solution is not particularly limited as long as it forms lithium ions in the electrolytic solution. For _{example, LiPF 6, LiClO 4, LiBF} 4, LiAsF 6, LiAlCl 4, LiCF 3 SO 3, LiSbF 6 , and the like. These electrolytes may be used alone or in combination.
In addition, as the solvent for the electrolytic solution, all known organic solvents of this type can be used. For example, propylene carbonate, dimethyl carbonate, ethylene carbonate, tetrahydrofuran, dimethyl sulfoxide, γ-butyrolaclone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, sulfolane, Examples include diethyl carbonate, dimethylformamide, and acetonitrile. These organic solvents may be used alone or in combination.
What should be noted here is that the operating characteristics at low temperatures are extremely useful solvents, but intercalated with lithium ions between the graphite layers to cause delamination of the graphite, and the lithium ion secondary with graphite as the negative electrode. The propylene carbonate solvent which cannot be used in a battery can be used for both carbon nanotubes and coated amorphous carbon, and an electrode based on a combination thereof can inevitably use propylene carbonate.
In addition, peripheral members such as separators and binders in the lithium ion secondary battery of the present invention, current collector metal materials, and the like are the same as those used in this type of lithium ion secondary battery having a carbon-based material as a negative electrode. It can be used and is not subject to any restrictions.

Hereinafter, the present invention will be described in more detail with reference to examples.
(Production of carbon-coated carbon nanotube material and electrode using the same)
A commercially available HiPco single-walled carbon nanotube (Unidym, Purified Grade, hereinafter abbreviated as SWNTs) and polyvinyl chloride mixed in the mixing ratio shown in Table 1 in a nitrogen stream (200 mL / min.) And the heating rate Heating was performed at 900 ° C. for 30 minutes at 5 ° C./min. The BET specific surface area of the obtained sample was measured by a nitrogen adsorption method at 77K. This sample was made into a slurry in N-methylpyrrolidone together with 10 wt% polyvinylidene fluoride (PVdF) serving as a binder, applied onto a nickel mesh, and dried at 150 ° C. to obtain an electrode. Using this electrode as a working electrode, lithium metal as a counter electrode and a reference electrode, the negative electrode charge / discharge characteristics were evaluated in a mixed electrolyte of ethylene carbonate / diethyl carbonate (50:50) in which 1M LiBF4 was dissolved. The current density was 50 to 2000 mA / g, and 3 V to 0 V was the charge / discharge measurement range (cut-off potential).

  FIG. 2 shows a nitrogen adsorption isotherm before and after the coating treatment of a representative sample, and Table 1 shows a BET specific surface area. From FIG. 1, it can be seen that after carbon coating, the nitrogen adsorption amount is greatly reduced even with a small amount of PVC mixing ratio. In addition, from Table 1, the BET specific surface area shows a considerable decrease even when the amount of coated carbon is small, so that the carbon surface covers the nanotube surface, fills the pores in the tube aggregate powder, and accesses the nitrogen molecules. You can easily imagine that it is blocking. At the stage where the amount of PVC is small, the SWNTs isotherm is simply lowered. However, if the amount of PVC exceeds a certain level, the isotherm itself will change, and it will decrease further above the middle relative pressure range. The low relative pressure range will rather increase. This indicates an increase in the micropores of the PVC-derived carbon and a decrease in the mesopores derived from the SWNTs, which means that the surface properties of the PVC carbon gradually become main as PVC carbon increases on the surface. Yes. On the other hand, since the surface area of PVC charcoal is almost zero, it can be considered that PVC carbon is attached to the base material of the nanotube having a large surface area. Thus, it is possible to make a composite with another kind of carbonaceous material without damaging the large surface area of SWNTs. Thus, it can be seen that the surface properties can be controlled by embedding the nanotube powder with another carbon by carbon coating.

  3 and 4 show the charge / discharge curves (50 mA / g) of 1000 degree-treated SWNTs and some carbon-coated SWNTs. In the first charge curve, SWNTs have a plateau with a large irreversible capacity near 1500 mAh / g near 1V, and then a plateau with reversible lithium insertion as it approaches 0V. In contrast, with PVC-coated SWNTs, the 1V plateau decreases dramatically as the amount of PVC mixed increases, but all curves near 0V are similar. This indicates that the amount of electrochemical decomposition of the electrolytic solution generated at 1 V is reduced without changing the lithium insertion characteristics of SWNTs. Considering that the carbon yield from PVC at 1000 degrees is about 5 wt%, the amount of coated carbon in PVC-4 is only about 20% of the total, so a very small amount of carbon derived from PVA is SWNTs powder. It means that the body surface is covered and the contact between the electrolyte and the SWNTs surface is suppressed. Table 2 lists the initial charge capacity, initial irreversible capacity, reversible capacity after the second time, discharge capacity calculated only from 0-1.0 V after the second time, and initial charge / discharge efficiency of each sample. Since there is no definition for the initial irreversible capacity, the charge capacity between 0.5 and 1.0 V was used. Clearly, as the PVC addition rate increases, the initial irreversible capacity decreases dramatically from 1323 mAh / g before coating to about 1/5 of 251 mAh / g. Regarding the discharge capacity, when the amount of PVC is small, the capacity is almost the same as that of SWNTs. Although the amount of PVC decreases as the amount of PVC increases, the amount of discharge limited to around 0V to 1V, which corresponds to lithium desorption from SWNTs, does not change much across all samples. Is decreasing. As the amount of PVC increases, the discharge curve changes linearly as seen in Fig. 3, so the proportion of PVC charcoal has increased, and the properties of PVC charcoal that causes lithium desorption even at high potentials have emerged. I can guess that. As described above, by covering the SWNTs with the carbonaceous material by thermal decomposition of PVC, the amount of solvent decomposition at 1V can be dramatically reduced to one third or less without impairing the original lithium insertion / release characteristics of SWNTs. It became possible to reduce it. On the other hand, the initial charge / discharge efficiency increased from 0.26 to 0.4 for SWNTs alone.

  FIG. 5 shows capacity retention rates of 1000 degree SWNTs and carbon-coated SWNTs (PVC-4) at various current densities. SWNTs alone shows a large capacity at a low current density, but only a very small capacity can be obtained at a high current density, and a discharge capacity can hardly be obtained at a large current density such as 1000 mA / g. This seems to be due to the large amount of solvent-decomposable organisms at 1 V that are generated during the first charge, reducing the conductivity of the SWNTs electrode. In contrast, SWNTs coated with PVC charcoal can maintain capacity even at relatively large current densities. With PVC charcoal alone, the capacity cannot be calculated even at 50 mA / g. This is thought to be because the current was lost due to heat consumption because PVC charcoal alone has low conductivity. Therefore, SWNTs and PVC charcoal alone cannot express capacity at a large current density, but by covering SWNTs with a small amount of PVC carbon, excessive production of surface film products is suppressed, and It is clear that charging / discharging at large current density is possible because the conductivity of SWNTs supports lithium insertion into coated PVC charcoal. In this way, it is possible to make up for each other's drawbacks and show the potential as a high-capacity high-power battery application.

Schematic diagram of carbon-coated carbon nanotube material for negative electrode of the present invention Nitrogen adsorption tower hot spring (77K) of raw material SWNT, PVC and 1000 ° C treated carbon-coated carbon nanotube material for negative electrode of the present invention Charging curve at a current density of 50 mA / g of the carbon-coated carbon nanotube material of the present invention Discharge curve of carbon-coated carbon nanotube material of the present invention at a current density of 50 mA / g Rate characteristics of uncoated SWNTs (PVC-0) and carbon-coated SWNTs (PVC-4) Schematic diagram of the carbon nanotube lithium insertion site Charge / discharge curves of conventional lithium-ion batteries using graphite Charging / discharging curves of lithium-ion batteries using carbon nanotubes by conventional HiPco method

Claims (2)

A method for producing a carbon material for a negative electrode made of a carbon-coated carbon nanotube material, wherein an exposed surface of the carbon nanotube is coated with amorphous carbon, and heat-treating a mixture of the carbon nanotube and the thermoplastic resin in an inert gas and, a thermoplastic resin, characterized in that it is a resin carbonized via liquid phase, method of manufacturing the negative electrode carbon material consisting of carbon-containing coated carbon nanotube material (however, "capable of being alloyed with lithium Excluding those that are mixed and agitated with a mixture of metallic particles and fibrous graphite material to form composite particles, and then mixed and heat-treated with the composite particles and a precursor of the carbonaceous material . Resin carbonized via liquid phase, method of manufacturing the negative electrode carbon material consisting of carbon coated carbon nanotube material according to claim 1, wherein the polyvinyl chloride or polyvinyl alcohol.