JP2020161385A - Negative electrode material for lithium ion secondary battery and manufacturing method thereof, negative electrode for lithium ion secondary battery using the negative electrode material, and lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode material having a long life and excellent safety while using a graphite material having a modified surface of natural graphite, which is advantageous in cost.SOLUTION: In a negative electrode material for a lithium secondary battery in which a heat-treated product with a carbon-based isotropic pitch adheres to at least a part of the surface of natural graphite, the content of quinoline insoluble in the carbon-based isotropic pitch is 30 mass% or less when the total amount of the carbon-based isotropic pitch is 100% by mass.SELECTED DRAWING: None

Description

The present invention relates to a negative electrode material for a lithium ion secondary battery and a method for producing the same, and a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode material.

Lithium-ion secondary batteries are secondary batteries with high energy density and have been actively studied in recent years. In this lithium ion secondary battery, a carbon material or the like is generally used as a negative electrode material, a lithium transition metal composite oxide or the like is used as a positive electrode material, and an aprotic organic solvent in which various salts are dissolved is used as an electrolyte. Etc. are used.

Currently, the negative electrode materials of lithium ion secondary batteries are roughly classified into graphite materials and non-graphitizable carbon materials (hard carbon). Of these, the graphite material has the advantages that a high-capacity battery can be obtained because it has a large occlusion of lithium ions, and that the discharge curve is gentle and a stable high voltage is maintained until the end of the discharge. On the other hand, the graphitizable carbon material has many reaction sites related to occlusion and release of lithium ions, so that it can be charged and discharged with a large current, but it is difficult to obtain a high capacity depending on the heat treatment conditions. There are problems such as an increase in irreversible capacity.

Therefore, in consumer lithium-ion batteries used in mobile devices such as smartphones and notebook personal computers (notebook personal computers), a graphite material having a high capacity is generally used as a negative electrode material. Among these graphite materials, artificial graphite obtained by calcining a graphitizable carbon precursor at 2800 ° C. or higher has been the mainstream in the past, but in recent years, from the viewpoint of cost, natural graphite has been described in Patent Documents 1 to 3. Graphite materials with modified surfaces are becoming mainstream. The application of lithium-ion secondary batteries as a power source for automobiles and the like is also being studied. Longer life characteristics are required than in the case of small devices such as the above-mentioned mobile phones and notebook personal computers.

In response to the above requirements, Patent Document 4 proposes a graphite material that satisfies both capacity and life characteristics by modifying the surface of natural graphite using a coal-based isotropic pitch.

Japanese Unexamined Patent Publication No. 2016-081816 Japanese Unexamined Patent Publication No. 04-368778 Japanese Unexamined Patent Publication No. 04-370662 Japanese Unexamined Patent Publication No. 05-307959

Most recently, lithium metal precipitation on the negative electrode due to charging at low temperature has caused multiple events leading to battery ignition accidents, etc., and there is a growing demand for materials with better safety. .. Further, since such an event is likely to occur after repeating the charge / discharge cycle, it is required to further improve the life.

From the above, it is an object of the present invention to provide a negative electrode material having a long life and excellent safety while using a graphite material having a modified surface of natural graphite, which is advantageous in cost.

The present inventor has conducted extensive research in view of the above problems. As a result, in the graphite material whose surface is modified from natural graphite, the material covering the surface is a heat-treated product having a coal-based isotropic pitch in which the content of quinoline-insoluble matter is in a specific range, so that the life characteristics are longer than before. And found that safety can be improved. The present invention has been further studied and completed based on such findings. That is, the present invention includes the following configurations.
Item 1. A negative electrode material for a lithium secondary battery in which a heat-treated product having a carbon-based isotropic pitch is attached to at least a part of the surface of natural graphite.
A negative electrode material for a lithium ion secondary battery, wherein the total amount of the coal-based isotropic pitch is 100% by mass, and the content of the quinoline-insoluble content of the coal-based isotropic pitch is 30% by mass or less.
Item 2. Item 2. The negative electrode for a lithium ion secondary battery according to Item 1, wherein the total amount of the coal-based isotropic pitch is 100% by mass, and the content of the quinoline-insoluble content of the coal-based isotropic pitch is 15 to 28% by mass. material.
Item 3. Item 2. The negative electrode material for a lithium ion secondary battery according to Item 1 or 2, wherein the softening point of the coal-based isotropic pitch is 260 to 300 ° C.
Item 4. Item 1 of Item 1 to 3, wherein the total amount of the isotropic pitch of the coal system is 100% by mass, and the content of the acetone-soluble component of the isotropic pitch of the coal system is 7.5 to 14% by mass. Negative electrode material for lithium ion secondary batteries according to.
Item 5. Assuming that the total amount of the negative electrode material for the lithium ion secondary battery is 100% by mass, the content of the natural graphite is 95 to 99.9% by mass, and the content of the heat-treated product having a coal-based isotropic pitch is 0. Item 2. The negative electrode material for a lithium ion secondary battery according to any one of Items 1 to 4, which is 1 to 5% by mass.
Item 6. Item 2. The method for producing a negative electrode material for a lithium ion secondary battery according to any one of Items 1 to 5.
It is provided with a step of mixing (1) natural graphite and a coal-based isotropic pitch, and (2) a step of heat-treating the mixture at a temperature equal to or higher than the softening point of the coal-based isotropic pitch. A production method in which the total amount of isotropic pitch is 100% by mass and the content of quinoline insoluble content in the coal-based isotropic pitch is 30% by mass or less.
Item 7. Before the step (1),
Item 6. The production method according to Item 6, further comprising a step of removing the primary quinoline insoluble matter from coal tar and then distilling, wherein the distillation is performed in multiple stages and the final stage distillation is thin film distillation.
Item 8. Item 7. The production method according to Item 7, wherein the distillation is performed in the first stage and then in the second and subsequent stages while supplying air.
Item 9. Item 6 to 8, wherein the step (1) is a step of mixing the natural graphite and the coal-based isotropic pitch at a mixing ratio of 90 to 99.5: 0.5 to 10 (mass ratio). The production method according to any one of the above.
Item 10. Item 6. The production method according to any one of Items 6 to 9, wherein the step (2) is a step of performing a heat treatment in a non-oxidizing atmosphere.
Item 11. Item 2. The production method according to any one of Items 6 to 10, wherein the heating temperature in the step (2) is 800 to 1300 ° C.
Item 12. A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to any one of Items 1 to 5.
Item 13. Item 12. The lithium ion secondary battery provided with the negative electrode for the lithium ion secondary battery according to Item 12.

The negative electrode material for a lithium ion secondary battery of the present invention has a quinoline-insoluble content on at least a part of the surface of natural graphite in order to improve the life characteristics and safety of surface-modified natural graphite which is cost-effective and has a high capacity. It is a carbon material to which a heat-treated product of a coal-based isotropic pitch having a specific content of graphite is attached. Since this negative electrode material for a lithium ion secondary battery is low in cost and excellent in capacity, life characteristics and safety, it is particularly useful as a negative electrode carbon material for a lithium ion secondary battery as a power source for automobiles and the like.

As used herein, "contains" is a concept that includes any of "comprise," "consist essentially of," and "consist of." Further, in the present specification, when the numerical range is expressed by A to B, it indicates A or more and B or less.

In the present specification, the "lithium ion secondary battery" means both a "non-aqueous lithium ion secondary battery" using a non-aqueous electrolyte solution and an "all-solid-state lithium ion secondary battery" using a solid electrolyte. To do.

1. 1. Negative electrode material for lithium ion secondary battery The negative electrode material for lithium ion secondary battery of the present invention has a coal-based isotropic pitch heat-treated material adhered to at least a part of the surface of natural graphite.

(1-1) Natural Graphite The shape and morphology of natural graphite are not particularly limited, and various types such as flaky, lumpy, fibrous, whiskers, spherical, and crushed ones can be adopted. However, from the viewpoint of further suppressing the orientation of the particles, making it easier for the electrolytic solution to penetrate, and further improving the battery characteristics such as the rate characteristics, spherical natural graphite which has been subjected to the spheroidizing treatment by a conventional method is preferable. In the spherical natural graphite, for example, when observed with a transmission electron microscope (TEM), a structure in which flat natural graphite is formed into a cabbage shape is observed, and a cleavage portion is present on the surface layer. Natural graphite can be used alone or in combination of two or more.

Natural graphite is usually flat and has a tap density of 0.6 g / cc or less. However, by performing a spheroidizing treatment, the tap density is preferably 0.62 to 1.3 g / cc, for example. Can be 0.65 to 1.2 g / cc, more preferably 0.7 to 1.1 g / cc. As a result, it is easy to prevent the particles from being oriented and the input / output characteristics from being lowered. The tap density of natural graphite is measured by "TAPDENSER KYT-4000" manufactured by Seishin Enterprise Co., Ltd.

The particle size of natural graphite is not particularly limited, but the central particle size (D50) is 5 to 40 μm from the viewpoints of electrode uniformity, bulk density of active material, handleability in the process of manufacturing the electrode, and the like. Is preferable, 5 to 30 μm is more preferable, and 7 to 25 μm is even more preferable. The centriole particle size (D50) of natural graphite is measured by "MT3000EXII" manufactured by Nikkiso Co., Ltd.

Further, in natural graphite, the average interplanar spacing d (002) of the (002) plane by the X-ray wide-angle diffraction method is a general index of the crystallinity, and the crystallinity is made higher sufficiently and lithium is dissolved. From the viewpoint of sufficiently increasing the capacity of the low potential portion (0 to 0.3 V based on the potential of graphite) close to precipitation, 0.335 to 0.337 nm is preferable, and 0.3354 to 0.3360 nm is more preferable.

Further, in natural graphite, the crystallite thickness Lc (004) in the c-axis direction by the X-ray wide-angle diffraction method has a sufficiently high crystallinity and a low potential portion close to the dissolution and precipitation of lithium (based on the potential of lithium). From the viewpoint of sufficiently increasing the capacitance of 0 to 0.3 V), 10 to 100 nm is preferable, and 20 to 95 nm is more preferable.

Natural graphite as described above is usually electrolyzed by reacting with an electrolyte used in a lithium ion secondary battery, for example, an electrolytic solution containing an aprotic organic solvent and a salt or an active point for lithium ions, that is, an electrolytic solution. It partially has active points that decompose the liquid and react with lithium ions that move during charging and discharging. Although the details of this active site are not clear, it is generally understood to be the end face of the crystallite, which is oriented outward of natural graphite.

(1-2) Coal-based isotropic pitch heat-treated product In the present invention, the coal-based isotropic pitch heat-treated product is a material having a lower crystallinity than natural graphite, and is subjected to X-ray wide-angle diffraction (002). ) The average surface spacing d (002) of the surfaces is preferably larger than 0.337 nm and smaller than 0.338 nm, more preferably 0.3371 to 0.3379 nm. Therefore, the reactivity of the electrolytic solution with the organic solvent and lithium ions is low, and decomposition of the electrolytic solution and destruction of particles are unlikely to occur. In the present invention, a heat-treated product having a coal-based isotropic pitch adheres to at least a part of natural graphite (particularly, the active points on the surface of natural graphite, and further all), and the heat-treated product having a coal-based isotropic pitch is the active points. By closing the above, there is an advantage that the charge / discharge efficiency and life characteristics of the battery are improved, and the safety thereof is improved.

The coal-based isotropic pitch used in the present invention has a quinoline insoluble content (QI) content of 30 mass by mass, where the total amount of the coal-based isotropic pitch is 100% by mass from the viewpoint of life characteristics, safety, etc. %, preferably 15 to 28% by mass, more preferably 17 to 25% by mass. The smaller the quinoline insoluble matter (QI), the smaller the number of components having a large molecular weight, and the molecular weight distribution tends to be narrow. When the content of quinoline insoluble matter (QI) in a coal-based isotropic pitch exceeds 30% by mass, the molecular weight distribution becomes wide and the uniformity of the components contained in the coal-based isotropic pitch is inferior. The uniformity of the components adhering to the graphite is also inferior, and the life characteristics and safety are insufficient. In the present invention, even when the content of quinoline insoluble matter (QI) is extremely low, the life characteristics and safety are excellent, but since the high molecular weight component is extremely small, the coal-based isotropic pitch Since the amount of the component adhering to the natural graphite contained in the graphite is small, an excessive coal-based pitch is required during the production of the negative electrode material for the lithium ion secondary battery of the present invention, and from the viewpoint of production efficiency, the quinoline insoluble matter (QI) It is preferable that the content of is not too small. The content of quinoline insoluble matter (QI) in a coal-based isotropic pitch is measured in accordance with the JIS K2415 standard.

The softening point of the coal-based isotropic pitch used in the present invention is preferably 260 to 300 ° C. from the viewpoint of more sufficiently closing the active site of natural graphite and improving the life characteristics while maintaining the capacity, preferably 270 ° C. ~ 295 ° C. is more preferable, and 275 to 290 ° C. is even more preferable. The softening point of the coal-based isotropic pitch is measured according to the standards of ASTM D3461.

The content of the acetone-soluble component (AcS) of the coal-based isotropic pitch of the present invention is 7.5, where the total amount of the coal-based isotropic pitch is 100% by mass from the viewpoint of life characteristics, safety, etc. ~ 14% by mass is preferable, and 8 to 13% by mass is more preferable. The smaller the acetone-soluble component (AcS), the smaller the number of components having a small molecular weight, and the molecular weight distribution tends to be narrowed. The content of acetone-soluble matter (AcS) is measured according to the JIS K2415 standard.

The coal-based isotropic pitch having the above-mentioned conditions can be used alone or in combination of two or more.

As described above, the negative electrode material for a lithium ion secondary battery of the present invention selectively reacts with a heat-treated product in which the active sites are mainly of natural graphite and have a coal-based isotropic pitch, and is incompatible with an electrolytic solution and lithium ions. It has been activated. That is, in this negative electrode material, the surface of the natural graphite is not completely coated with the coating layer containing the heat-treated product having the above-mentioned coal-based isotropic pitch, and the active point of the surface of the natural graphite is the above-mentioned coal-based isotropic. It is considered that the mixture is inactivated by being coated with an anisotropic heat-treated product.

The negative electrode material for a lithium ion secondary battery of the present invention is a material in which mainly active sites of natural graphite are selectively coated, that is, a material in which the surface of natural graphite is at least partially coated. .. The content of natural graphite is more likely to cause a decrease in mass, a decrease in life characteristics, and a decrease in safety due to cracking of the negative electrode material, peeling from the electrode, and side reactions with the electrolytic solution due to exposure of the active surface. In order to suppress this, the total amount of the negative electrode material for the lithium ion secondary battery of the present invention is 100% by mass, preferably 95 to 99.9% by mass, more preferably 96 to 99.5% by mass, and 97.5 to 99. Mass% is more preferred. For the same reason, the content of the heat-treated product having a coal-based isotropic pitch is preferably 0.1 to 5% by mass, more preferably 0.5 to 4% by mass, and 1 to 2.5% by mass. More preferred.

The negative electrode material for a lithium ion secondary battery of the present invention has a higher tap density than natural graphite because the active sites of natural graphite are coated with a heat-treated product having a low crystallinity and a coal-based isotropic pitch. From the viewpoint of further suppressing the orientation of particles and deterioration of input / output characteristics, 0.8 to 1.4 g / cc is preferable, 0.85 to 1.3 g / cc is more preferable, and 0.9 to 1 .2 g / cc is more preferable. The tap density of the negative electrode material for a lithium ion secondary battery of the present invention is measured by "TAPDENSER KYT-4000" manufactured by Seishin Co., Ltd.

The negative electrode material for a lithium ion secondary battery of the present invention has a central grain type (D50) more than that of natural graphite because the active sites of natural graphite are coated with a heat-treated product having an isotropic pitch with low crystallinity. Although it is large, 5 to 40 μm is preferable, 5 to 30 μm is more preferable, and 7 to 25 μm is further preferable, from the viewpoints of electrode uniformity, bulk density of active material, handleability in the process of producing the electrode, and the like. The central particle size (D50) of the negative electrode material for a lithium ion secondary battery of the present invention is measured by "MT3000EXII" manufactured by Nikkiso Co., Ltd.

In the negative electrode material for a lithium ion secondary battery of the present invention, d (002) is preferably 0.335 to 0.337 nm, more preferably 0.3354 to 0.3360 nm.

The negative electrode material for a lithium ion secondary battery of the present invention preferably has an Lc (004) of 10 to 100 nm, more preferably 20 to 90 nm.

The negative electrode material for a lithium ion secondary battery of the present invention as described above has high crystallinity and can realize a high capacity. In addition, since inexpensive natural graphite is used as a raw material, it is advantageous in terms of cost. Moreover, the long-term life characteristics (charge / discharge cycle characteristics) and safety, which have been problems, are also excellent, and the required performance as a power source for automobiles and the like can be satisfied.

2. 2. Method for Manufacturing Negative Electrode Material for Lithium Ion Secondary Battery The negative electrode material for lithium ion secondary battery of the present invention described above is not particularly limited, but the above-mentioned natural graphite is used with the above-mentioned coal-based isotropic pitch. It can be obtained by heat treatment in an atmosphere containing a heat-treated product.

More specifically, the negative electrode material for a lithium ion secondary battery of the present invention is
It is provided with a step of mixing (1) natural graphite and a coal-based isotropic pitch, and (2) a step of heat-treating the mixture at a temperature equal to or higher than the softening point of the coal-based isotropic pitch. It can be obtained by a method in which the total amount of the anisotropic pitch is 100% by mass and the content of the quinoline insoluble content (QI) of the coal-based isotropic pitch is 30% by mass or less.

As described above, in the step (1), natural graphite and a coal-based isotropic pitch having a specific quinoline insoluble content (QI) content are mixed, and in the subsequent step (2), heat treatment is performed. Therefore, the natural graphite can be heat-treated in an atmosphere containing the heat-treated product having the specific coal-based isotropic pitch.

(2-1) Natural graphite and coal-based isotropic pitch In the step (1), the natural graphite and coal-based isotropic pitch are as described above. The coal-based isotropic pitch having a quinoline insoluble content (QI) content of 30% by mass or less used in the present invention has a narrower molecular weight distribution and is composed of more uniform components, so that it can be used on the surface of natural graphite. It is considered that the components of the adhered coal-based isotropic pitch heat-treated product are also more uniform.

The content of quinoline insoluble matter (QI) in commercially available carbon-based isotropic pitch is usually larger than 30% by mass, where the total amount of carbon-based isotropic pitch is 100% by mass. A coal-based isotropic pitch having a quinoline insoluble (QI) content of 30% by mass or less can be obtained, for example, by removing the primary quinoline insoluble from coal tar and then distilling. However, in the distillation, a coal-based isotropic pitch having a quinoline insoluble content (QI) content of 30% by mass or less is obtained by performing multi-step distillation and thin-film distillation as the final-step distillation. be able to.

Examples of the coal tar used as a raw material include coal tar obtained by operating a coke oven; coal tar produced as a by-product by solvent extraction of coal tar pitch, thermal modification, etc., and can be used alone. 2 It is also possible to use a combination of seeds or more.

It is preferable that the coal tar first removes the primary quinoline insoluble component (primary QI) contained as a solid content sometimes referred to as free carbon. As a method for removing the primary quinoline insoluble matter (primary QI), for example, a static method, a centrifugation method, or the like can be adopted. These methods can be adopted alone or in combination. By removing the primary quinoline insoluble matter (primary QI) in this way, the isotropic structure can be more easily developed in a later step, and the carbon-based isotropic pitch used in the present invention can be easily obtained. It is also possible to remove impurities contained in coal tar as a raw material.

After that, coal tar from which the primary quinoline insoluble matter (primary QI) has been removed is distilled, and at this time, it is preferable to carry out multi-step distillation. At this time, first, a pitch having a softening point of 60 to 150 ° C. is easily produced by the first-stage distillation. After that, the second and subsequent stages of distillation are carried out. At this time, by carrying out the distillation a predetermined number of times while supplying air, an air bron reaction that increases the cross-linking reaction by oxygen is caused, and the softening point is higher (260- (300 ° C) A coal-based isotropic pitch can be obtained. From the viewpoint of efficiency, this air bron reaction step is preferably divided into 2 to 4 steps to gradually raise the softening point of the pitch.

However, since it is difficult to sufficiently reduce the content of quinoline insoluble matter (QI) by simply performing multi-step distillation, the final-step distillation is performed by thin-film distillation, for example, using a thin-film distillation apparatus. By performing the above, a coal-based isotropic pitch having a quinoline insoluble content (QI) content of 30% by mass or less can be obtained.

A batch type reaction tank is usually used for distillation, but even if stirring is performed, a temperature distribution often occurs in the reaction tank, and a carbon-based isotropic pitch produced in a low temperature part and a high temperature part is produced. Often have different physical properties. The low boiling point component remains in the low temperature part, while the low boiling point component remains in the high temperature part, and the distribution of the molecular weight of the resulting coal-based isotropic pitch tends to spread. On the other hand, when the final step is thin film distillation, it is possible to suppress variations in reaction temperature, and to produce a coal-based isotropic pitch having a narrower molecular weight distribution and a lower content of quinoline insoluble matter (QI). ..

(2-2) Step (1)
In the step (1), the mixing ratio of natural graphite and the above-mentioned coal-based isotropic pitch is not particularly limited, but from the viewpoint of obtaining the above-mentioned negative electrode material for a lithium ion secondary battery, 90 to 99. It is preferably 5: 0.5 to 10 (mass ratio), more preferably 90 to 99: 1 to 10 (mass ratio), and even more preferably 94 to 98: 2 to 6 (mass ratio).

The method for mixing natural graphite and coal-based isotropic pitch is not particularly limited, and can be carried out by a conventional method. For example, mixing can be performed by using a Nauta mixer, a ribbon mixer, a V-type mixer, a locking mixer, or the like.

(2-3) Step (2)
In the step (2), the atmosphere during the heat treatment is an inert gas atmosphere such as a nitrogen atmosphere or an argon atmosphere, a hydrogen atmosphere, or a mixture of the above-mentioned inert gas and hydrogen from the viewpoint of avoiding carbon combustion. It is preferably performed in a non-oxidizing atmosphere such as a reducing atmosphere such as a gas atmosphere.

Further, the heat treatment is preferably carried out under reduced pressure or normal pressure (about 0.1 Pa to 0.15 MPa), and the set temperature at that time is particularly high as long as it is equal to or higher than the pyrolysis temperature of the coal-based isotropic pitch. Although there is no limitation, it is usually preferably set to 800 to 1300 ° C, more preferably 850 to 1100 ° C. By keeping the heating temperature within this range, while maintaining the capacity, the natural graphite and the heat-treated product with an isotropic pitch of coal are more sufficiently reacted, and it is more difficult to react with lithium ions and the electrolytic solution, and the life characteristic. And a negative electrode material with further improved safety can be obtained.

The heating time (holding time at the maximum temperature reached) includes the concentration and heating temperature of the above-mentioned coal-based isotropic pitch heat-treated product, the content of the coal-based isotropic pitch heat-treated product in the negative electrode material to be obtained, and the like. It may be appropriately set according to the above, but 10 minutes to 5 hours is preferable, and 30 minutes to 2 hours is more preferable.

3. 3. Negative electrode for lithium ion secondary battery The negative electrode material for a lithium ion secondary battery of the present invention can be suitably used as a constituent material for a negative electrode for a lithium ion secondary battery (furthermore, a lithium ion secondary battery). For example, a method for molding a mixture containing a negative electrode material for a lithium ion secondary battery, a binder, etc. of the present invention; a paste for forming a negative electrode active material layer containing a negative electrode material for a lithium ion secondary battery, an organic solvent, a binder, etc. of the present invention. A negative electrode active material layer is formed on the negative electrode current collector by a method of applying the composition to the negative electrode current collector using a coating means (doctor blade or the like), and a negative electrode for a lithium ion secondary battery having an arbitrary shape is formed. can do. In forming the negative electrode, it may be combined with a terminal if necessary. In particular, a method of applying the paste composition for forming a negative electrode active material layer to the negative electrode current collector is preferable.

The negative electrode current collector is preferably a member made of a metal such as copper, silver, or gold, for example, in the form of a foil or a mesh, and a known negative electrode current collector can be used.

As the organic solvent, a solvent capable of dissolving or dispersing the binder is usually used, and examples thereof include organic solvents such as N-methylpyrrolidone and N, N-dimethylformamide. The organic solvent can be used alone or in combination of two or more. The amount of the organic solvent used is not particularly limited as long as it is in the form of a paste. For example, with respect to 100 parts by mass of the negative electrode material for a lithium ion secondary battery of the present invention, it is usually 60 to 150 parts by mass, particularly 60 to 100 parts by mass. Can be a part.

The binder is not particularly limited as long as it is a binder used in a lithium ion secondary battery, but specifically, a fluoropolymer (polyvinylidene fluoride, polytetrafluoroethylene, etc.) and a polyolefin polymer (polyethylene, polypropylene, etc.) Etc.), known binders such as synthetic rubber can be used. The amount of the binder in this case is not particularly limited, and is, for example, 0.1 to 20 parts by mass, particularly 1 to 10 parts by mass with respect to 100 parts by mass of the negative electrode material for the lithium ion secondary battery of the present invention. be able to.

The negative electrode active material layer (paste composition for forming the negative electrode active material layer) may further contain a conductive material (conductive carbon material or the like). Examples of the conductive material include carbon black such as acetylene black, thermal black, and furnace black. These conductive materials can be used alone or in combination of two or more. The proportion of the conductive material used is not particularly limited, but the total amount of the negative electrode material for the lithium ion secondary battery and the conductive material of the present invention is 100% by mass, and is usually 1 to 10% by mass, particularly 1 to 5% by mass. Can be done. Thereby, it is possible to further improve the conductivity as an electrode.

When forming such a negative electrode for a lithium ion secondary battery, the negative electrode material for a lithium ion secondary battery of the present invention is mixed with a binder and a conductive material as necessary to form a paste (for forming a negative electrode active material layer). It is preferable to prepare a paste composition) and apply the paste composition on the negative electrode current collector to form a negative electrode active material layer.

The coating amount of the negative electrode current collector of the paste composition is not particularly limited, usually, preferably ^{5~15mg / cm 2, 7~13mg / cm} 2 is more preferable. The thickness of the film applied to the negative electrode current collector (the film thickness of the paste composition) is, for example, preferably 30 to 300 μm, more preferably 50 to 200 μm. After coating, the negative electrode current collector may be subjected to a drying treatment (for example, vacuum drying).

4. Lithium Ion Secondary Battery The negative electrode material for a lithium ion secondary battery of the present invention can constitute a lithium ion secondary battery as a negative electrode constituent material as described above. In particular, the negative electrode material for a lithium ion secondary battery of the present invention can form a lithium ion secondary battery for enabling repeated charging and discharging with a large current as described above.

The lithium ion secondary battery of the present invention includes the negative electrode for the lithium ion secondary battery of the present invention described above. Further, the lithium ion secondary battery of the present invention includes, in addition to the negative electrode for the lithium ion secondary battery of the present invention, a positive electrode applied to a known lithium ion secondary battery, an electrolytic solution, and a container for storing these. be able to.

The positive electrode is not particularly limited, and a known positive electrode can be used, and the positive electrode can be composed of, for example, a positive electrode current collector, a positive electrode active material, a conductive agent, or the like. As the positive electrode current collector, for example, aluminum or the like can be exemplified. Examples of the positive electrode active material include metal chalcogens having a layered structure such as TiS ₂ , MoS ₃ , NbSe ₃ , FeS, VS ₂ , and VSe ₂ ; CoO ₂ , Cr ₃ O ₅ , TiO ₂ , CuO, and V ₃ O. Metals such as ₆ , Mo ₃ O, V ₂ O ₅ (・ P ₂ O ₅ ), Mn ₂ O (・ Li ₂ O), LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , lithium nickel manganese cobalt-based composite oxide Oxides: Conducting conjugated polymer substances such as polyacetylene, polyaniline, polyparaphenylene, polythiophene, and polypyrrole can be used. Preferably, a metal oxide (particularly V ₂ O ₅ , Mn ₂ O, LiCo O ₂ , lithium nickel manganese cobalt-based composite oxide) can be used.

Further, the electrolytic solution is an electrolytic solution in which a salt is dissolved in an aprotic organic solvent as described above, and is arranged between the positive electrode and the negative electrode. For example, in order to prevent a short circuit between the positive electrode and the negative electrode. It is impregnated and held in a separator made of a non-woven fabric or the like.

Examples of the aprotic organic solvent constituting the above-mentioned electrolytic solution include esters of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl formate, methyl acetate and the like. Classes; furans such as tetrahydrofuran and 2-methyltetraxyl; ethers such as dioxolane, 4-methyldioxolane, diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane and diethylene glycol; dimethyl sulfoxide; sulfolanes such as sulfolane and methylsulfolane. Kind; acetonitrile; dimethyl formamide and the like. These aprotic organic solvents can be used alone or in combination of two or more.

Meanwhile, salt dissolved in such aprotic organic solvents are, for _{example, LiPF 6, LiClO 4, LiBF} 4, LiAsF 6, LiSbF 6, LiAlO 4, LiAlCl 4, LiCl, hardly solvated such LiI anion Examples include salts that produce. These salts can be used alone or in combination of two or more. Preferred electrolytic solutions include ether solvents that are stable even in a strong reducing atmosphere, such as tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, and 4-methyldioxolane, and the aprotic solvents (preferably two types). The above mixed solvent) includes a solution or the like in which the above-exemplified salt is dissolved.

The lithium ion secondary battery may have any shape or form such as a cylindrical type, a square type, and a button type.

Since the lithium ion secondary battery of the present invention uses the negative electrode material for the lithium ion secondary battery of the present invention for the negative electrode, it has a large charge / discharge capacity and the negative electrode does not easily react with the electrolytic solution, so that it has a long life. High characteristics and safety.

The lithium ion secondary battery of the present invention can be similarly carried out when another electrolyte such as a known inorganic solid electrolyte or polymer solid electrolyte is used instead of the above-mentioned electrolytic solution.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

[Measurement of softening point]
The softening point was measured according to the standards of ASTM D3461.

[Measurement of quinoline insoluble matter (QI) content]
The content of quinoline insoluble matter (QI) was measured according to the JIS K2415 standard.

[Measurement of Acetone Soluble Content (AcS) Content]
The content of acetone-soluble matter (AcS) was measured according to the JIS K2415 standard.

[Measurement of particle size]
The particle size distribution and D50 of the particles were measured using "MT3000EXII" manufactured by Nikkiso Co., Ltd.

[Measurement method of specific surface area]
The specific surface area was measured using a "NOVA2000 / nitrogen adsorption BET specific surface area measuring device" manufactured by Canterchrome.

[Measurement method of tap density]
The tap density was measured using TAPDENSER "KYT-4000" manufactured by Seishin Enterprise Co., Ltd. The volume of the cylinder was 200 cc, the tapping distance was 50 mm, and the number of tapping times was 1200.

[Initial charge / discharge evaluation]
Preparation of Batteries 98 parts by mass of the carbon material obtained in Example 1 and Comparative Examples 1 and 2, 1 part by mass of carboxymethyl cellulose as a thickener, 1 part by mass of a styrene-butadiene copolymer as a binder, and an appropriate amount of water. Was added and kneaded to obtain a slurry. This slurry was applied onto a copper foil by a doctor blade method at a coating rate of 9.0 mg / cm ² . After drying at 60 ° C., it was compacted by a roll press to a density of 1.6 g / cc, and dried under reduced pressure at 150 ° C. to obtain a negative electrode.

In addition to the obtained negative electrode, a Li metal foil was used as the counter electrode, and a solution prepared by dissolving LiPF ₆ in a mixed solvent (volume ratio 1: 2) of ethylene carbonate and ethyl methyl carbonate at a ratio of 1 mol / L was used as the electrolytic solution. , A glass cell was prepared using a polypropylene non-woven fabric as a separator.

Evaluation method In the above glass cell, charge to 10 mV at 0.3 C (constant current constant voltage) for 8 hours at 25 ° C, discharge to 2 V at 0.3 C, and first discharge capacity (1.2 V cutoff voltage). Was calculated and shown in Table 2.

[Life characteristics and safety evaluation]
Preparation of Batteries 97.5 parts by mass of the carbon material obtained in Example 1 and Comparative Examples 1 and 2, 1 part by mass of carboxymethyl cellulose as a thickener, and 1.5 parts by mass of a styrene-butadiene copolymer as a binder. , An appropriate amount of water was added and kneaded to prepare a slurry. This slurry was applied to a copper foil at a size of 10.6 mg / cm ² by the doctor blade method. After drying at 60 ° C., it was compacted by a roll press to a density of 1.5 g / cc, cut into 30 mm × 50 mm squares, and dried under reduced pressure at 150 ° C. to obtain a negative electrode.

To 93 parts by mass of lithium nickel manganese cobalt-based composite oxide powder, 4 parts by mass of carbon black, 3 parts by mass of polyvinylidene fluoride (PVDF) and an appropriate amount of N-methylpyrrolidone were added and kneaded to prepare a slurry. This slurry was applied to an aluminum foil by a doctor blade method at a filling rate of 21.2 mg / cm ² . It was dried under reduced pressure at 130 ° C., and further consolidated by a roll press so that the density of the positive electrode layer was 2.7 mg / cm ³ . This was cut into 32 mm × 52 mm squares and dried at 150 ° C. to obtain a positive electrode.

As the electrolytic solution, one in which LiPF ₆ was dissolved in ethylene carbonate: ethyl methyl carbonate = 3: 7 (mass ratio) was used.

This battery was charged to 4.05 V at 0.2 C and discharged to 2.7 V at 0.2 C three times for initial adjustment.

Evaluation method 1: Under life characteristics of 50 ° C., charge to 4.05V at 0.5C and discharge to 2.7V at 0.5C as one cycle, repeat 1000 cycles, check the capacity retention rate, and check the result. It is shown in Table 2.

Evaluation method 2: Safety Charge / discharge is performed under the following conditions (1) to (6), and in order to judge the degree of deterioration (safety) of the battery due to the charge / discharge, before the start of charge / discharge and after the end of charge / discharge. The AC resistance of was measured. The AC resistance is measured under the conditions of a frequency of 500 kHz to 0.1 Hz and an amplitude of 10 mV at 25 ° C., and the degree of deterioration of the battery is increased by increasing the resistance value of 100 Hz to 0.1 Hz, which is the resistance on the negative electrode side. (Safety) was evaluated and the results are shown in Table 2.
(1) At 25 ° C., charging was performed at 0.2 C to 4.05 V, and discharging was performed at 0.2 C to 2.7 V as one cycle, and 10 cycles were repeated.
(2) At 25 ° C., charging to 4.15 V at 0.5 C and discharging to 2.7 V at 0.5 C were set as one cycle, and 10 cycles were repeated.
(3) At 25 ° C., 1C was charged to 4.15 V, and 1C was discharged to 2.7 V as one cycle, and 10 cycles were repeated.
(4) At 0 ° C., charging was performed at 0.5 C to 4.1 V, and discharging was performed at 0.5 C to 2.7 V as one cycle, and 10 cycles were repeated.
(5) Under 0 ° C., 1C was charged to 4.1 V, and 1C was discharged to 2.7 V as one cycle, and 10 cycles were repeated.
(6) Under 0 ° C., 1C was charged to 4.15 V, and 1C was discharged to 2.7 V as one cycle, and 10 cycles were repeated.

[Example 1]
Coal tar (quinoline insoluble (QI) content of 0.00% by mass or less; made in China) from which the primary quinoline insoluble (QI) has been removed by centrifugation is distilled to prepare a coal-based pitch with a softening point of about 80 ° C. did. Next, the obtained carbon-based pitch was distilled using a two-stage reaction tank and a thin-film distillation apparatus while supplying air at a flow rate of 3 L / min to obtain a coal-based isotropic pitch with a softening point of 286.5 ° C. Made. The obtained coal-based isotropic pitch had a quinoline insoluble content (QI) content of 20.4% by mass and an acetone-soluble content (AcS) content of 12.4% by mass.

Spherical natural graphite made in China [D50 = 11.5 μm, specific surface area = 8.8 m ² / g, tap density = 0.70 g / cc, d (002) = 0.335 nm, Lc (004) = 58 nm] 95 parts by mass And 5 parts by mass of the coal-based isotropic pitch obtained above were mixed with a Nauta mixer. A mixture of spherical natural graphite and a carbon-based isotropic pitch was heat-treated at 900 ° C. for 1 hour in a nitrogen atmosphere. Table 1 shows the physical property values of the obtained surface-coated graphite.

[Comparative Example 1]
Coal tar (quinoline insoluble (QI) content of 0.00% by mass or less; made in China) from which the primary quinoline insoluble (QI) has been removed by centrifugation is distilled to prepare a coal-based pitch with a softening point of about 80 ° C. did. Next, the obtained carbon-based pitch was distilled using a three-stage reaction tank (without using a thin film distillation apparatus) while supplying air at a flow rate of 3 L / min to coal having a softening point of 285.4 ° C. A system isotropic pitch was made. The obtained coal-based isotropic pitch had a quinoline-insoluble (QI) content of 35.7% by mass and an acetone-soluble content (AcS) content of 15.5% by mass. Surface-coated graphite was prepared in the same manner as in Example 1 except that the coal-based isotropic pitch obtained above was used, and the physical property values of the obtained surface-coated graphite are shown in Table 1.

[Comparative Example 2]
Coal tar (quinoline insoluble (QI) content 3.1% by mass; made in China) from which the primary quinoline insoluble (QI) was not removed was distilled to prepare a coal-based pitch having a softening point of about 80 ° C. Next, the obtained coal-based pitch was distilled using a three-stage reaction tank (without using a thin film distillation apparatus) while supplying air at a flow rate of 3 L / min to obtain coal having a softening point of 248.1 ° C. A system isotropic pitch was made. The obtained coal-based isotropic pitch had a quinoline insoluble content (QI) content of 40.7% by mass and an acetone-soluble content (AcS) content of 18.5% by mass. Surface-coated graphite was prepared in the same manner as in Example 1 except that the coal-based isotropic pitch obtained above was used, and the physical property values of the obtained surface-coated graphite are shown in Table 1.

Table 2 shows the battery characteristics of the lithium ion secondary batteries obtained in Example 1 and Comparative Examples 1 and 2.

From the results of the initial discharge capacity in Table 2, it can be seen that the difference in the quinoline insoluble content (QI) content does not affect the discharge capacity.

From the results of the life characteristics in Table 2, the negative electrode having excellent repetitive charge / discharge characteristics by coating natural graphite with a coal-based isotropic pitch heat-treated product having a quinoline insoluble content (QI) content of 30% by mass or less. It can be seen that the material is obtained and the life characteristics are excellent.

Further, in recent years, there have been cases where lithium metal is deposited on the negative electrode by charging at a particularly low temperature, which leads to a battery ignition accident or the like. The safety evaluation in Table 2 is based on the deterioration of the battery, that is, the negative electrode, by gradually setting the environmental temperature, charge / discharge rate, and charging voltage (that is, the utilization rate of the negative electrode material) to stricter conditions every 10 cycles. The ease with which lithium metal is deposited on the battery is evaluated by increasing the resistance of the battery. Specifically, the higher the resistance, the easier it is for lithium metal to precipitate, which means that safety is poor. According to this result, by coating natural graphite with a coal-based isotropic pitch heat-treated product having a quinoline insoluble content (QI) content of 30% by mass or less, a negative electrode material having excellent safety and less likely to precipitate lithium metal. Can be obtained.

According to the negative electrode material for a lithium ion secondary battery of the present invention, it is possible to provide an inexpensive lithium ion secondary battery negative electrode carbon material that satisfies the capacity, life characteristics and safety. In particular, it is useful as a power source for automobiles and the like, which require long-term life characteristics (charge / discharge cycle characteristics) and safety under severe conditions.

Claims (13)

A negative electrode material for a lithium secondary battery in which a heat-treated product having a carbon-based isotropic pitch is attached to at least a part of the surface of natural graphite.
A negative electrode material for a lithium ion secondary battery, wherein the total amount of the coal-based isotropic pitch is 100% by mass, and the content of the quinoline-insoluble content of the coal-based isotropic pitch is 30% by mass or less. The lithium ion secondary battery according to claim 1, wherein the total amount of the coal-based isotropic pitch is 100% by mass, and the content of the quinoline-insoluble component of the coal-based isotropic pitch is 15 to 28% by mass. Negative electrode material. The negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the softening point of the coal-based isotropic pitch is 260 to 300 ° C. Any 1 of claims 1 to 3, wherein the total amount of the coal-based isotropic pitch is 100% by mass, and the content of the acetone-soluble component of the coal-based isotropic pitch is 7.5 to 14% by mass. Negative electrode material for lithium ion secondary batteries according to the section. Assuming that the total amount of the negative electrode material for the lithium ion secondary battery is 100% by mass, the content of the natural graphite is 95 to 99.9% by mass, and the content of the heat-treated product having a coal-based isotropic pitch is 0. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, which is 1 to 5% by mass. The method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5.
It is provided with a step of mixing (1) natural graphite and a coal-based isotropic pitch, and (2) a step of heat-treating the mixture at a temperature equal to or higher than the softening point of the coal-based isotropic pitch. A production method in which the total amount of isotropic pitch is 100% by mass and the content of quinoline insoluble content in the coal-based isotropic pitch is 30% by mass or less. Before the step (1),
The production method according to claim 6, further comprising a step of removing the primary quinoline insoluble matter from coal tar and then distilling, wherein the distillation is carried out in multiple stages and the final stage distillation is thin film distillation. .. The production method according to claim 7, wherein the distillation is performed in the second and subsequent stages while supplying air after performing the first stage distillation. The step (1) is a step of mixing the natural graphite and the coal-based isotropic pitch at a mixing ratio of 90 to 99.5: 0.5 to 10 (mass ratio), claim 6 to 6. The production method according to any one of 8. The production method according to any one of claims 6 to 9, wherein the step (2) is a step of performing a heat treatment in a non-oxidizing atmosphere. The production method according to any one of claims 6 to 10, wherein the heating temperature in the step (2) is 800 to 1300 ° C. A negative electrode for a lithium ion secondary battery using the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5. A lithium ion secondary battery comprising the negative electrode for the lithium ion secondary battery according to claim 12.