JP3687712B2 - Method for producing carbon material for negative electrode of nonaqueous solvent secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a carbon material for a negative electrode of a non-aqueous solvent secondary battery having a large capacity and a small irreversible capacity.
[0002]
[Prior art]
Nonaqueous solvent secondary batteries using a carbon material for the negative electrode have already been put into practical use as lithium ion secondary batteries due to advantages such as high energy density, light weight, small size, and long-term storage. However, it is necessary to increase the capacity of the carbon material for the negative electrode in order to cope with the reduction in size and weight of electronic devices. Therefore, for example, as described in JP-A-6-187888, a high-capacity carbon material having a discharge capacity per weight exceeding 500 mAh / g is found by reacting pitch or tar with a nitro compound. Has been issued and reviewed. However, there is a great demand for the development of a lithium ion secondary battery that can operate for a longer time, and conventional materials have been insufficient to meet the demand in terms of capacity. So far, it has been found that coke baked at low temperature and polyacene baked phenol resin have a high capacity. However, while the capacity is improved, the irreversible capacity (first cycle), which is disadvantageous in making a secondary battery, has been found. The difference between the charge capacity and the discharge capacity at the time of increase was insufficient to meet the demand. Furthermore, since the potential of the negative electrode material with respect to lithium metal is high at the time of discharge, a major drawback is that the average voltage when the secondary battery is designed in combination with the positive electrode material is low.
[0003]
[Problems to be solved by the invention]
As described above, a nonaqueous solvent type lithium secondary battery using a conventional carbon material as a negative electrode material has not been sufficient for realizing the large capacity which is the feature thereof.
The present invention overcomes the conventional problems, and provides a high-performance non-aqueous solvent secondary battery having a large capacity, good charge / discharge cycle characteristics, and stable and safe. ) Having a high capacity of 500 mAh / g or more per weight, 2) reducing the irreversible capacity in the first cycle of the carbon material for negative electrode, and 3) the potential of the negative electrode material with respect to lithium metal during discharge being 0.2 V or less. An object is to provide a carbon material for a negative electrode having a large capacity in a certain region.
[0004]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have intensively studied a high-capacity negative electrode carbon material using pitch or tar as a raw material, and as a result, synthesized from a specific condensed polycyclic compound or a substance containing the same. The specific precursor pitch and tar obtained are modified into a specific modified pitch, which is infusibilized and then baked, and the resulting carbon material has excellent properties as a negative electrode for non-aqueous solvent secondary batteries As a result, the present invention has been completed.
[0005]
The carbon material for a negative electrode of a non-aqueous solvent secondary battery according to the present invention is a specific precursor pitch obtained by polymerizing a specific condensed polycyclic compound or a substance containing the same in the presence of hydrogen fluoride / boron trifluoride. Alternatively, a non-aqueous solvent secondary battery negative electrode prepared by preparing a specific modified pitch or tar by reforming tar, infusible treatment with an oxidizing gas, and then firing. Carbon material.
[0006]
A condensed polycyclic compound containing one or more methyl groups or a material containing the same as a precursor pitch or tar material to be synthesized includes methyl groups of condensed polycyclic hydrocarbons such as naphthalene, anthracene, pyrene and coronene. A derivative containing one or more methyl groups of a condensed heterocyclic compound such as benzofuran, quinoline, thiaphthalene, silanaphthalene, and the like, a mixture of these and a condensed polycyclic compound, and a coal tar fraction containing them. Fractions, petroleum fractions, oil processing residue, etc. are used. In particular, naphthalene derivatives such as methylnaphthalene and dimethylnaphthalene, and a coal tar methylnaphthalene fraction, ethylene bottom oil, and the like, which are mixtures thereof, are preferable for the performance as a carbon material for a negative electrode, which will be described later, and the infusibilization step.
[0007]
A method for synthesizing precursor pitch or tar from a condensed polycyclic compound under a hydrogen fluoride / boron trifluoride catalyst is not particularly limited. 1 mol to 0.1 mol of hydrogen fluoride, 0.01 to 1.0 mol of boron trifluoride, reaction temperature in the range of 0 to 300 ° C, preferably 40 to 200 ° C, more preferably It is carried out at 60 to 170 ° C.
[0008]
As the properties of the precursor pitch or tar obtained from the condensed polycyclic compound, the softening point is preferably 0 to 180 ° C, more preferably 20 to 150 ° C. Further, it is preferable that the atomic ratio of hydrogen to carbon is 0.7 to 1.10, the pyridine insoluble content is 1.0% or less, and the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 30 to 80%. . After polishing the precursor pitch or tar by a conventional method, the optical structure when observed under a polarizing microscope is 100% isotropic.
[0009]
Next, the precursor pitch or tar is treated with a modified pitch having a softening point of 150 ° C. or higher or with tar while maintaining optical isotropy. The modification can be performed by a known method such as distillation, air blowing, nitric acid addition, sulfur addition and the like. Among these methods, a method in which an oxidizing gas, generally air, is circulated through pitch or tar that is in a fluid state under heating is simple, inexpensive, and effective. The temperature at this time cannot be generally specified depending on the softening point of the precursor pitch or tar, but is 200 ° C. or higher, preferably 300 to 350 ° C. If the treatment temperature is too low, the reactivity is low, so that reforming with air is not sufficiently performed. On the other hand, if the temperature is too high, thermal polymerization of pitch or tar itself occurs, and reforming with air is not performed effectively. The air flow rate varies depending on the device shape and the like, but is about 0.5 to 50 ml / g with respect to pitch or tar. At this time, in order to increase the contact efficiency between pitch or tar and air, use of a mesh or a filter or stirring can be applied. Since the end point of reforming with air is accompanied by an increase in the softening point, it can be determined by measuring the softening point. Although the softening point of the end point of the reforming cannot be specified depending on the starting material or the like, it is 150 to 350 ° C, preferably 200 to 300 ° C. Further, the atomic ratio of hydrogen to carbon is 0.50 to 0.80, and the ratio of the absorption strength of aliphatic C—H stretching vibration measured by FT-IR to the absorption strength of aromatic C—H stretching vibration is 0.00. It is preferably 5 or more.
[0010]
For the infusibilization treatment, a compound containing nitrogen, sulfur, oxygen or the like can be used. For example, nitric acid, sulfuric acid, nitrating agent, nitro compound, ammonium sulfate, acidic ammonium sulfate, nitrogen dioxide gas, ozone, air, oxygen and the like and mixtures thereof can be mentioned. Of these, the use of an oxidizing gas such as air is convenient and inexpensive, and is preferable for the performance of the obtained carbon material with little residual impurities after firing. In particular, it is more preferable to use air gas because it is simple and inexpensive. The method of infusibilization with an oxidizing gas is not particularly limited, but after processing the modified pitch or tar into a powder, fiber, or thin film pulverized to a certain particle size or less, preferably in the temperature range of 100 to 400 ° C, preferably It is performed by circulating an oxidizing gas in a temperature range of 150 to 350 ° C. The pitch or tar infusibilization step using an oxidizing gas is a method generally used for carbon fiber production and the like. The higher the reactivity of pitch or tar to oxygen, the lower the temperature and the shorter the time. It is important for productivity improvement. Here, it is considered that the precursor pitch or tar described in the present invention and the methyl group contained in a large amount in the modified pitch or tar improve the infusibilities of the pitch or tar. Therefore, as can be seen from a comparison between Examples and Comparative Examples described later, a condensed polycyclic compound having no methyl group can be obtained by using a condensed polycyclic compound having a methyl group as a precursor pitch or tar raw material. Compared to the case of using, infusibilization is possible more quickly and under a condition with less heat history.
[0011]
The carbon material of the present invention is obtained by firing the raw material organic compound thus obtained in a non-oxidizing gas or under vacuum. The firing temperature is 800 to 1800 ° C., preferably 1000 to 1300 ° C., the firing time is 1 to 50 hours, and optimum conditions are appropriately selected according to the raw material organic compound. Moreover, you may pre-calculate at 800 degrees C or less. As the non-oxidizing gas, nitrogen and argon are preferable. A method in which a non-oxidizing gas is continuously supplied as an air flow and the gas generated by firing the raw organic compound is accompanied and discharged, or a method in which the generated gas is forced out of the system by vacuum evacuation can be applied as appropriate. .
[0012]
The carbon material for a non-aqueous solvent secondary battery negative electrode according to the present invention is characterized in that, in its production process, it is characterized by excellent infusibility of a modified pitch or tar by an oxidizing gas, and a potential of 0 to lithium metal potential. An emission capacity of 540 mAh / g or more is possible between 1.5 V, and at the same time, a capacity between 0 and 0.2 V at the lithium metal potential is 380 mAh / g or more, and the irreversible capacity in the first cycle is The maximum feature is 100 mAh / g or less. Hereinafter, the present invention will be described specifically and in detail with reference to examples, but the examples shown below are for specific description and limit the embodiments of the present invention and the scope of the invention. It is not intended to be. The pitch analysis method and analysis conditions in this example are described below.
[0013]
(Elemental analysis)
For simultaneous analysis of carbon, nitrogen, and hydrogen, a 2400CHN type elemental analyzer manufactured by PERKINELMER was used as an analyzer. The measurement was performed by measuring 1.5 ± 0.2 mg of the sample pitch in a tin container, setting it in the apparatus, burning it at a temperature of 975 ° C. for 5 minutes, and detecting and measuring by TCD with a He gas carrier. The sample was corrected in advance with the standard substance acetanilide (2.0 ± 0.1 mg).
[0014]
(NMR analysis)
The ¹ H-NMR method was used to determine the proportion of aliphatic hydrogen in the total hydrogen contained in the precursor pitch or tar. Since almost all pitch or tar is soluble in chloroform, the 1% deuterated chloroform solution was placed in an NMR sample tube and measured with JNM-EX270 manufactured by JEOL Ltd. Note that TMS (tetramethylsilane) was used as a reference substance, and this was set to 0 ppm.
[0015]
(FT-IR analysis)
1 part of modified pitch or tar powder was added to 100 parts of KBr powder and mixed in an agate mortar. Subsequently, this was set in FT / IR-5300 manufactured by JASCO Corporation and diffuse reflection method measuring apparatus DR-81, and measurement was performed. The resulting diffuse reflection spectrum, on spectra obtained by converting Kubelka-Munk, the peak intensity at around 2930 cm ^-1 (absorption intensity of aliphatic C over H stretching vibration), peak intensity in the vicinity of 3050 cm ^-1 ( The ratio to the absorption intensity of the aromatic CH stretching vibration) was determined.
[0016]
Example 1
An acid-resistant autoclave with an internal volume of 3 L was charged with 7 mol of methylnaphthalene, 3.68 mol of hydrogen fluoride (HF) and 1.16 mol of boron trifluoride (BF ₃ ), and heated to 100 ° C. under autogenous pressure. The reaction was further continued for 4 hours at 100 ° C. Then, in accordance with a conventional method, nitrogen was blown into the autoclave to recover HF and BF _3. Subsequently, low boiling point components were removed to obtain a precursor pitch having a softening point of 76 ° C. The ratio of hydrogen atoms to carbon atoms contained in the precursor pitch (H / C) is 0.87, the pyridine insoluble content is 0.0%, and the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 58%. Met. The obtained precursor pitch was charged into another autoclave, 2 L of air was blown per 100 g at 320 ° C. and reacted for 2 hours to obtain a modified pitch having a softening point of 200 ° C. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the modified pitch is 0.66, and the absorption strength of the aliphatic C—H stretching vibration measured by FT-IR is an aromatic C—H stretching vibration. The ratio of to the absorption intensity was 1.1. The modified pitch was pulverized into a powder of 200 μm or less, heated from 150 ° C. to 300 ° C. at 5 ° C./min, held for 10 minutes, and taken out. The obtained processed product was adjusted to an average particle size of 15 μm, and then baked at 1200 ° C. for 2 hours under a reduced pressure of 10 Torr while flowing a small amount of nitrogen to obtain a powdery carbon material.
[0017]
(Evaluation as negative electrode material)
To 90 parts by weight of the obtained carbon material, 10 parts by weight of polyvinylidene fluoride powder (binder) was added, and after blending and mixing dimethylformamide as a solvent, it was coated on copper foil, cut into 1 cm squares after drying, and evaluated. A test piece was obtained. Next, a solution (concentration of 1.0 mol / l) in which LiClO4 was dissolved in three kinds of mixtures having a blending ratio of ethylene carbonate / dimethyl carbonate / diethyl carbonate of 1 / 0.5 / 0.5 was used as the electrolyte, A half cell having a 50 μm polypropylene microporous membrane as a separator was produced. Note that lithium metal having a diameter of 16 mm and a thickness of 0.5 mm was used as a counter electrode. In addition, a small piece of lithium metal was used as a reference electrode in the same manner as the counter electrode.
[0018]
When the electrode potential of the test piece for evaluation with respect to the reference electrode was 1 mV at a current density of ² mA / cm ² and further constant potential charging was performed at an electrode potential of 1 mV for 40 hours, an occlusion capacity of 647 mAh / g was confirmed. It was. Subsequently, constant current discharge was performed until the electrode potential of the test piece for evaluation with respect to the reference electrode was 1.5 V at a current density of 1 mAh / cm ³ , and a discharge capacity: 562 mAh / g was confirmed. The capacity loss was 85 mAh / g, and the discharge capacity between 0 and 0.2 V with respect to the lithium metal potential was 417 mAh / g.
[0019]
Example 2
An acid-resistant autoclave having an internal volume of 3 L was charged with 7 mol of dimethylnaphthalene, 4.90 mol of hydrogen fluoride (HF) and 1.40 mol of boron trifluoride (BF ₃ ), and heated to 120 ° C. under autogenous pressure. The reaction was further continued for 4 hours at 120 ° C. Subsequently, in accordance with a conventional method, nitrogen was blown into the autoclave to recover HF and BF3, and subsequently, low-boiling components were removed to obtain a precursor pitch having a softening point of 40 ° C. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the precursor pitch is 0.91, the pyridine insoluble content is 0.0%, and the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 66%. Met. The obtained precursor pitch is charged into another autoclave, blown 2 L of air per 100 g at 320 ° C. and reacted for 2 hours to obtain a 100% optically isotropic modified pitch with a softening point of 249 ° C. It was. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the modified pitch is 0.65, and the absorption strength of the aliphatic C—H stretching vibration measured by FT-IR is an aromatic C—H stretching vibration. The ratio to the absorption intensity was 1.6. This modified pitch is pulverized into a powder of 200 μm or less, 10 g is put in a magnetic dish, and the temperature is raised from 150 ° C. to 300 ° C. at 5 ° C./min while flowing 1 L / min in a muffle furnace. Hold for minutes and remove. The obtained processed product was adjusted to an average particle size of 15 μm, and then baked at 1200 ° C. for 2 hours under a reduced pressure of 10 Torr while flowing a small amount of nitrogen to obtain a powdery carbon material. When the negative electrode material was evaluated in the same manner as in Example 1, the storage capacity: 635 mAh / g and the discharge capacity: 547 mAh / g were confirmed. The capacity loss was 88 mAh / g, and the discharge capacity between 0 and 0.2 V with respect to the lithium metal potential was 397 mAh / g.
[0020]
Example 3
An acid-resistant autoclave with an internal volume of 3 L is charged with 142 g of ethylene bottom oil (manufactured by Maruzen Petrochemical), 5.30 mol of hydrogen fluoride (HF), and 1.50 mol of boron trifluoride (BF ₃ ), and 120 ° C. under autogenous pressure. Then, the reaction was further continued for 4 hours while maintaining the temperature at 120 ° C. Subsequently, in accordance with a conventional method, nitrogen was blown into the autoclave to recover HF and BF _3. Subsequently, low boiling point components were removed to obtain a precursor pitch having a softening point of 107 ° C. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the precursor pitch is 0.95, the pyridine insoluble content is 0.0%, and the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 66%. Met. The obtained precursor pitch is charged into another autoclave, 2 L of air is blown per 100 g at 340 ° C. and reacted for 2 hours to obtain a 100% optically isotropic modified pitch with a softening point of 246 ° C. It was. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the modified pitch is 0.78, and the absorption strength of the aliphatic CH stretching vibration measured by FT-IR is the aromatic CH stretching vibration. The ratio to the absorption intensity was 2.9. This modified pitch is pulverized into a powder of 200 μm or less, 10 g is put into a magnetic dish, and the temperature is raised from 150 ° C. to 280 ° C. at 5 ° C./min while flowing 1 L / min in a muffle furnace. Hold for minutes and remove. The obtained processed product was adjusted to an average particle size of 15 μm, and then baked at 1200 ° C. for 2 hours under a reduced pressure of 10 Torr while flowing a small amount of nitrogen to obtain a powdery carbon material. When the negative electrode material was evaluated in the same manner as in Example 1, the storage capacity: 658 mAh / g and the discharge capacity: 600 mAh / g were confirmed. The capacity loss was 58 mAh / g, and the discharge capacity between 0 and 0.2 V with respect to the lithium metal potential was 429 mAh / g.
[0021]
Comparative Example 1
Softening point 76 ° C., ratio of hydrogen atoms contained in pitch to carbon atoms (H / C) is 0.55, pyridine insoluble content is 0.1%, aliphatic hydrogen in total hydrogen contained in pitch Coal tar pitch with a ratio of 3% is charged into an autoclave, 2 L of air is blown per 100 g at 340 ° C., reacted for 1 hour, and a 100% optically isotropic modified pitch with a softening point of 243 ° C. Got. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the modified pitch is 0.48, and the absorption strength of the aliphatic C—H stretching vibration measured by FT-IR is an aromatic C—H stretching vibration. The ratio to the absorption intensity was 0.0. This modified pitch is pulverized into a powder of 200 μm or less, 10 g is put in a magnetic dish, and the temperature is raised from 150 ° C. to 320 ° C. at 5 ° C./min while flowing 1 L / min in a muffle furnace. Hold for minutes and remove. The obtained processed product melted during the temperature rising process and became a lump. This was prepared into a powder having an average particle diameter of 15 μm, and then baked at 1200 ° C. for 2 hours under a reduced pressure of 10 Torr while flowing a small amount of nitrogen to obtain a powdery carbon material. When the negative electrode material was evaluated in the same manner as in Example 1, the storage capacity: 525 mAh / g and the discharge capacity: 398 mAh / g were confirmed. The capacity loss was as large as 127 mAh / g, and the charge / discharge capacity was also reduced. The discharge capacity between 0 and 0.2 V with respect to the lithium metal potential was as small as 230 mAh / g.
[0022]
Comparative Example 2
An acid-resistant autoclave with an internal volume of 3 L was charged with 7 moles of naphthalene, 2.45 moles of hydrogen fluoride (HF), and 0.77 moles of boron trifluoride (BF ₃ ), and the temperature was raised to 100 ° C. under autogenous pressure. The reaction was carried out while maintaining at 100 ° C. for 4 hours. Then, in accordance with a conventional method, nitrogen was blown into the autoclave to recover HF and BF _3. Subsequently, low boiling point components were removed to obtain a precursor pitch having a softening point of 82 ° C. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the precursor pitch is 0.76, pyridine insoluble matter is 0.0%, and the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 35%. Met. The obtained precursor pitch is charged into another autoclave, 2 L of air is blown per 100 g at 340 ° C., and reacted for 4 hours to obtain a 100% optically isotropic modified pitch with a softening point of 234 ° C. It was. The ratio of hydrogen atoms to carbon atoms (H / C) contained in the modified pitch is 0.49, and the absorption strength of the aliphatic C—H stretching vibration measured by FT-IR is an aromatic C—H stretching vibration. The ratio to the absorption intensity was 0.20. This modified pitch is pulverized into a powder of 200 μm or less, 10 g is put in a magnetic dish, heated at 150 ° C. to 320 ° C. at a rate of 1 ° C./min in a muffle furnace, and then 30 Hold for minutes and remove. The obtained processed product melted during the temperature rising process and became a lump. This was prepared into a powder having an average particle size of 15 μm, and then baked at 1200 ° C. for 2 hours under a reduced pressure of 10 Torr while flowing a small amount of nitrogen to obtain a powdery carbon material. When the negative electrode material was evaluated in the same manner as in Example 1, the storage capacity: 535 mAh / g and the release capacity: 403 mAh / g were confirmed. The capacity loss was as large as 132 mAh / g, and the charge / discharge capacity was also reduced. The discharge capacity between 0 and 0.2 V with respect to the lithium metal potential was as small as 240 mAh / g.
[0023]
【The invention's effect】
Compared with conventional carbon materials for negative electrodes of lithium secondary batteries, it is possible to realize a secondary battery having a large capacity and a low cost by being excellent in productivity, having a large discharge capacity, and reducing the irreversible capacity in the first cycle.

Claims (1)

In the presence of hydrogen fluoride / boron trifluoride, a condensed polycyclic compound containing one or more methyl groups is present in a molar ratio of the condensed polycyclic compound: hydrogen fluoride: boron trifluoride of 1: 0.1-10. : Obtained by polymerizing at 0 to 200 ° C. in the range of 0.01 to 1.0, the softening point is 0 to 180 ° C., the atomic ratio of hydrogen to carbon is 0.70 to 1.10, and the pyridine insoluble content is 1 The softening point is obtained by circulating an oxidizing gas in a flowing state at 200 ° C. or higher in a precursor pitch in which the proportion of aliphatic hydrogen in the total hydrogen contained in the pitch is 30% to 80%. Absorption of aromatic C—H stretching vibration of 150 to 350 ° C., atomic ratio of hydrogen to carbon of 0.50 to 0.80, and absorption intensity of aliphatic C—H stretching vibration measured by FT-IR A 100% optically isotropic modified pitch having a ratio to strength of 0.5 or more; After infusibilized at a temperature of 100 ° C. or higher 400 ° C. or less in the presence of an oxidizing gas reforming pitch, firing the preparation of non-aqueous solvent secondary battery negative electrode carbon material characterized in that.