JP2009272041A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery enabling a heightened input and output with low-resistance and high-current charge/discharge and control of exothermic heat for safety. <P>SOLUTION: The lithium-ion secondary battery, in which a positive electrode material including a positive electrode active substance composed of a lithium-containing compound and a negative electrode material including a negative electrode active substance enabled to store and release lithium ions are laminated or wound around through a separator and are sealed up together with nonaqueous electrolyte solution in which lithium salt is dissolved, is characterized by fine carbon fiber adhered in a mesh state on particle surfaces of the positive electrode active substance and the negative electrode active substance, and it is desirable if all or a part of the surface of the fine carbon fiber are modified with a hydrophilic group. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium-ion secondary battery that has a low resistance and a high input / output in charge / discharge of a large current and that can suppress heat generation in safety.

2. Description of the Related Art Conventionally, along with the development of portable electronic devices such as mobile phones and notebook computers, secondary batteries with small size, light weight and high capacity are required. Currently, lithium ion secondary batteries using lithium-containing transition metal oxides such as LiCoO ₂ as the positive electrode material and carbon-based materials as the negative electrode active material are commercialized as high-capacity secondary batteries that meet this demand. . The lithium ion secondary battery has high energy density and can be reduced in size and weight, and thus is widely used as a power source for portable electronic devices.

  Furthermore, in recent years, lithium ion secondary batteries are being developed not only for consumer use but also for industrial use such as motorcycles and in-vehicle use, and there is a demand for higher capacity and higher input / output capabilities for lithium ion secondary batteries. Yes. For this purpose, the positive electrode composite metal lithium oxide used as a battery reactant and the negative electrode carbon material itself have a higher capacity and higher input / output, and an apparent charge by increasing the electrode specific surface area in terms of battery design. Improvements in capacity and durability during large current charge / discharge have been devised by reducing the current density of the discharge and further reducing the separator resistance by reducing the thickness of the separator.

  Electron conductivity of the positive electrode active material used in current lithium batteries is not so high, and many of them belong to semiconductors. Therefore, in order to ensure the conductivity of the electrode, a conductive material powder is added to the positive electrode active material powder, A positive electrode mixture formed by pasting with a binder (binder) is applied to a metal foil to form a positive electrode material. Conventionally, carbon, acetylene black, graphite or the like has been used for this conductive agent.

  For example, JP 2000-208147 A (Patent Document 1) discloses a positive electrode in which fine carbon black powder adheres to the surface of the positive electrode active material particles, and natural graphite and carbon fibers are filled in the gaps between the positive electrode active material particles. The structure is described. JP 2006-86116 A (Patent Document 2) describes a positive electrode structure including a carbon-based conductive material together with a positive electrode active material, and nano-sized carbon fibers are used as the carbon-based conductive material. Yes. Furthermore, JP 2004-220909 A (Patent Document 3) describes a positive electrode structure in which carbon nanofibers are filled between positive electrode active material particles.

  Japanese Patent Laid-Open No. 2008-66053 (Patent Document 4) discloses a composite carbon having extremely fine pores as a negative electrode active material and a fibrous carbon fiber having a special structure formed on the surface or inside of a core carbon particle. It describes what improved output characteristics and low temperature characteristics by using a material.

  The high rate discharge characteristics of the battery can be enhanced by increasing the content of the conductive material in the positive electrode. However, as the content of the conductive material increases, the content of the lithium-containing transition metal oxide in the positive electrode is relatively increased. This causes a problem that the discharge capacity is reduced. Also, a dispersing agent is used to disperse carbon, acetylene black, graphite, etc., which are conventionally used as conductive materials, and normal carbon nanofibers and carbon nanotubes in the positive electrode material, and the dispersing agent is used during the reaction. There were problems such as decomposition and generation of gas.

  Furthermore, the carbon-based material used as a conventional conductive material is filled in the gaps between the particles of the positive electrode active material, and thus a relatively large amount of conductive material is required. In addition, it is difficult to control the ultrafine pores using a composite carbon material having ultrafine pores made of fibrous carbon having a special structure formed on the surface and inside of the core carbon particles as the negative electrode active material. There is a problem.

In addition, the proposed improvement measures for higher capacity and higher input / output can increase the capacity, but from the viewpoint of cycle life performance and safety in charge / discharge of large current, There is a problem when considering the application of.
JP 2000-208147 A JP 2006-086116 A JP 2004-220909 A JP 2008-066053 A

  An object of the present invention is to provide a lithium ion secondary battery that has a low resistance and can achieve high input / output in charge / discharge of a large current and suppression of heat generation for safety.

According to this invention, the lithium ion secondary battery which solved the said subject with the following structures is provided.
[1] A positive electrode material including a positive electrode active material composed of a lithium-containing compound and a negative electrode material including a negative electrode active material capable of occluding and releasing lithium ions are laminated or wound through a separator, and a non-aqueous electrolyte solution In the lithium ion secondary battery which is sealed together and the lithium salt is dissolved in the non-aqueous electrolyte, fine carbon fibers are attached to the surface of the particles of the positive electrode active material and the negative electrode active material. Lithium ion secondary battery.
[2] The lithium ion secondary battery according to the above [1], wherein all or part of the surface of the fine carbon fiber is modified with a hydrophilic group.
[3] Fine carbon fibers having an average fiber diameter of 1 nm to 100 nm and an aspect ratio of 5 or more adhere to the positive electrode active material having an average particle diameter of 60 nm to 10 μm and the negative electrode active material having an average particle diameter of 3 μm to 7 μm. The lithium ion secondary battery according to the above [1] or [2].
[4] The content of the fine carbon fiber is described in any one of [1] to [3] above, which is 0.5 to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material or the negative electrode active material. Lithium ion secondary battery.
[5] The lithium ion secondary battery according to any one of [1] to [4], wherein the positive electrode material and the negative electrode material further contain carbon powder finer than the active material.
[6] The lithium ion secondary battery according to any one of [1] to [5] above, wherein the positive electrode active material is lithium-containing transition metal oxide particles.
[7] The lithium-containing transition metal oxide positive electrode active material is _{LiCoO 2, Li (Ni x /} Mn y / Co z) O 2 (x + y + z = 1), LiMn 2 O 4, LiCoPO 4, LiFePO 4 consists _{LiNiVO 4, LiCoVO 4, LiMnCoO 4} , LiMnCrO 4, LiMn 1.5 Ni 0.5 O 4 at least one selected from the group consisting of, or non-stoichiometric compounds a portion of the composition was replaced with a metal element The lithium ion secondary battery according to the above [6], which is a compound containing at least one selected from the group or both.
[8] The negative electrode active material is natural graphite, artificial graphite, synthetic graphite, mesocarbon microbeads, organic graphitized material, coal, coke, PAN-based carbon fiber, pitch-based carbon fiber, or Li ₄ Ti ₅ O ₁₂ , or The lithium ion secondary battery according to any one of [1] to [7], which is an alloy system such as Sn or Si.

  The lithium ion secondary battery of the present invention uses a positive carbon active material and a negative electrode active material in which fine carbon fibers are dispersed and attached to the surface of the particles in a mesh form, so that high conductivity with a relatively small amount of carbon fibers. Therefore, since the content of the lithium-containing compound in the positive electrode can be sufficiently ensured, the charge / discharge capacity is large and the output of the battery can be increased.

  In addition, the lithium ion secondary battery of the present invention is made uniform on the surface of the active material without using a dispersant by making the fine carbon fibers on the surface of the active material hydrophilic by acid treatment and modifying the surface with hydrophilic groups. A carbon fiber film having a simple network structure can be formed, and there is no problem of gas generation caused by the dispersant, and a battery having a high cycle life and high safety can be obtained.

  In the lithium ion secondary battery of the present invention, preferably, an average fiber diameter of 1 nm to 100 nm as fine carbon fibers and a positive electrode active material particle having an average particle diameter of 60 nm to 10 μm and a negative electrode active material having an average particle diameter of 3 μm to 7 μm By using carbon nanofibers having an aspect ratio of 5 or more, a uniform network layer of fine carbon fibers can be formed on the particle surface of the active material, and a small amount of carbon fibers, for example, 100 parts by mass of the active material A secondary battery having excellent charge / discharge capacity, output characteristics, and cycle characteristics can be obtained with a fine carbon fiber content of 0.5 to 15 parts by mass, preferably 1 to 10 parts by mass.

  In the lithium ion secondary battery of the present invention, preferably, the positive electrode material and the negative electrode material further contain carbon powder finer than the active material, so that the fine carbon powder enters the gaps between the particles of the active material. By adding 0.5 to 5 parts by mass, preferably 1 to 3 parts by mass of carbon powder with respect to 100 parts by mass of the positive electrode active material, the conductivity of the positive electrode material and the negative electrode material can be further improved. it can.

In the positive electrode material of the present invention, a lithium-containing compound is used as a positive electrode active material, and lithium-containing transition metal oxide particles are mainly used. Specifically, for _{example, LiCoO 2, Li (Ni x} / Mn y / Co z) O 2 (x + y + z = 1), LiMn 2 O 4, LiCoPO 4, LiFePO 4, LiNiVO 4, LiCoVO 4, A compound in which at least one selected from the group consisting of LiMnCoO ₄ , LiMnCrO ₄ and LiMn _1.5 Ni _0.5 O ₄ , or a part of the above composition is substituted with a metal element is used. In addition to Li-containing fiber metal oxide particles, sulfides such as LiTiS ₂ can also be used. By using these lithium-containing compounds, it is possible to obtain a lithium ion battery having an excellent charge / discharge cycle.

  In the negative electrode material of the present invention, a material capable of occluding and releasing lithium ions is used as the negative electrode active material. Specifically, for example, natural graphite, artificial graphite, synthetic graphite, mesocarbon microbeads, organic graphitized materials, mainly carbon-based materials, coal, coke, PAN-based carbon fibers, pitch-based carbon fibers As the non-carbon-based material, a Li-occlusion material such as Si-based, Sn-based, or Ti-based material can be preferably used.

Hereinafter, the present invention will be specifically described based on embodiments.
The lithium ion secondary battery of the present invention comprises a positive electrode material containing a positive electrode active material made of a lithium-containing compound and a negative electrode material containing a negative electrode active material capable of occluding and releasing lithium ions, stacked or sandwiched through a separator. In a lithium ion secondary battery that is rotated and sealed together with a non-aqueous electrolyte and a lithium salt is dissolved in the non-aqueous electrolyte, fine carbon fibers adhere to the surface of the particles of the positive electrode active material and the negative electrode active material. It is the lithium ion secondary battery characterized by the above-mentioned.

  An example of the structure of a lithium ion secondary battery is shown in FIG. As shown in the figure, a laminate 14 in which positive electrode materials 11 and negative electrode materials 12 are alternately laminated inside a container 10 is sealed together with a non-aqueous electrolyte. The positive electrode material 11 includes a positive electrode active material and a conductive material contained in a binder paste, and is basically formed by applying the paste to both surfaces of an aluminum alloy foil. As the positive electrode active material, a lithium transition metal double oxide powder or the like that mainly serves as a lithium ion source is used. The negative electrode material 12 includes a negative electrode active material contained in a binder paste, and is basically formed by applying the paste to both surfaces of a copper alloy foil. As the negative electrode active material, graphite powder capable of inserting and extracting lithium ions is used. Further, if necessary, carbon powder is included in the paste together with the graphite powder in order to increase conductivity.

  The positive electrode material 11 and the negative electrode material 12 have a band shape, and are laminated in a roll shape with a separator 13 interposed therebetween. The separator 13 is a porous film such as polyolefin resin. A pair of positive electrode material 11 and negative electrode material 12 stacked via a separator 13 are wound and stacked via an insulating material (not shown), and this roll-shaped stacked body 14 is placed in the container 10 together with a nonaqueous electrolyte. It is stored. A positive electrode 20 is formed on the top side of the laminate 14 by connecting a lead to the winding center axis, and a negative electrode 21 is formed on the bottom side of the laminate 14 via the lead.

As the nonaqueous electrolytic solution, a solution obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) or the like as a lithium salt in an organic solvent such as ethylene carbonate (EC) is used. In addition, the structure shown in figure is an example and the structure of the lithium ion secondary battery of this invention is not restricted to what is shown in figure.

  The lithium ion secondary battery of the present invention is characterized in that fine carbon fibers are attached in a network form on the particle surface of the positive electrode active material and the particle surface of the negative electrode active material. The fine carbon fibers are preferably carbon nanofibers having an average fiber diameter of 1 nm to 100 nm and an aspect ratio of 5 or more. The carbon nanofibers have a volume resistance value of 1.0 Ωcm or less of the compact of the fiber powder so that good conductivity is obtained, and the stacking interval of the [002] plane of the graphite layer by X-ray diffraction measurement is 0.35 nm. The following are preferred. By forming a uniform network coating of fine carbon fibers on the surface of the positive electrode active material and the negative electrode active material, the conductivity is remarkably improved.

  The fine carbon fiber is preferably made by hydrophilizing the surface by oxidation treatment and modifying the surface with a hydrophilic group. Generally, carbon powder and carbon fiber have a strong tendency to agglomerate in water, and it is difficult to uniformly disperse them. Therefore, conventional carbon powder and carbon fiber require a dispersant. On the other hand, since fine carbon fibers having hydrophilic groups on the surface are uniformly dispersed in an aqueous solution without using a dispersant, a carbon fiber film having a uniform network structure is formed on the active material surface without the need for a dispersant. Can do.

  The oxidation treatment method may be either a dry method or a wet method. For example, a sulfur-containing strong acid such as sulfuric acid is added to fine carbon fibers, an oxidizing agent such as nitric acid is added, the slurry is stirred under heating, and then filtered, and the remaining acid is washed and removed. By this oxidation treatment, polar functional groups such as a carbonyl group, a carboxyl group or a nitro group are formed on the surface of the fine carbon fiber, so that it can be hydrophilized.

  By dispersing fine carbon fibers whose surface is oxidized and hydrophilized in a dispersion medium such as an organic solvent or water, and adding an electrode active material (a positive electrode active material and a negative electrode active material) together with a binder to the dispersion, Fine carbon fibers maintained in a dispersed state adhere to the surface of the electrode active material particles, and a uniform network-like film is formed.

  As the binder, polyvinylidene fluoride (PVdF), carboxymethyl cellulose (CMC), styrene butadiene copolymer emulsion (SBR), polyvinyl alcohol (PVA), silicon emulsion, and the like are used. The amount of the binder is suitably 0.5 to 10 parts by mass, preferably 1 to 5 parts by mass with respect to 100 parts by mass of the active material. If this amount is less than 0.5 parts by mass, poor adhesion tends to occur, and if it exceeds 10 parts by mass, the effect of a decrease in conductivity will appear due to the binder, which is not appropriate.

In the lithium ion secondary battery of the present invention, a lithium-containing oxide serving as a lithium ion source is mainly used as the positive electrode active material. For example, a lithium-containing transition metal oxide is used. Specifically, for _{example, LiCoO 2, Li (Ni x} / Mn y / Co z) O 2 (x + y + z = 1), LiMn 2 O 4, LiCoPO 4, LiFePO 4, LiNiVO 4, LiCoVO 4, At least one selected from the group consisting of LiMnCoO ₄ , LiMnCrO ₄ , LiMn _1.5 Ni _0.5 O ₄ , or at least one selected from the group consisting of non-stoichiometric compounds in which a part of the above composition is substituted with a metal element. Compounds containing either or both species can be used.

Among the lithium-containing transition metal oxides exemplified above, LiFePO ₄ or a compound in which a part of Fe of the compound is substituted with a metal element has a primary particle size as compared with other lithium-containing transition metal oxide particles. It is 50 to 100 nm, and the aggregated secondary particle diameter is very small as 1 to 3 μm, and it is more preferable because it is easy to form a fine network as the conductive material of the fine carbon fiber of the present invention.

  As the negative electrode active material, carbon materials such as natural graphite, artificial graphite, synthetic graphite, mesocarbon microbeads, organic graphitized material, coal, coke, PAN-based carbon fiber, and pitch-based carbon fiber can be mainly used.

  The positive electrode active material particles preferably have an average particle size of 60 nm to 10 μm, and the negative electrode active material particles preferably have an average particle size of 3 μm to 7 μm. Fine carbon fibers having an average fiber diameter of 1 nm to 100 nm and an aspect ratio of 5 or more are used for the active material particles having this particle diameter, and fine carbon fibers having an average fiber diameter of several tens of nanometers are preferably used for the active material of 1 μm or less. .

  The content of the fine carbon fiber is suitably 0.5 to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material or the negative electrode active material. If the content of the fine carbon fiber is less than 0.5 parts by mass, the conductivity cannot be sufficiently increased, and if it is more than 15 parts by mass, for example, the content of the positive electrode active material is relatively small, which is not preferable. .

  A carbon powder finer than the active material, for example, carbon black having an average primary particle size of 10 nm can be used in combination with the fine carbon fibers. When the fine carbon powder is used in combination, the carbon powder enters the gaps between the particles of the active material, and the conductivity can be further increased. The carbon powder may be added to the dispersion together with the fine carbon fibers or before and after the addition of the fine carbon fibers.

  0.5-5 mass parts is suitable with respect to 100 mass parts of active materials, and, as for content of carbon powder, 1-3 mass parts is preferable. If the amount is less than 0.5 parts by mass, the effect of using the carbon powder together is poor. If the amount is more than 5 parts by mass, the total amount of fine carbon fibers increases, and the amount of the positive electrode active material becomes relatively small. . In order to increase the advantage of using fine carbon fibers, the amount of the carbon powder is preferably smaller than the amount of fine carbon fibers.

  Examples of the carbon powder include conductive carbon black, ketjen black, acetylene black, coal, coke, polyacrylonitrile carbon fiber, pitch carbon fiber, organic carbonized product, natural graphite, artificial graphite, synthetic graphite, and mesocarbon. Powders composed of microbeads, graphitized organic materials, graphite fibers, and the like can be used.

An example of a method for producing the positive electrode material and the negative electrode material is shown below.
[Production of fine carbon fiber dispersion]
Carbon nanofibers (CNF: average fiber diameter 20 nm) in a mixed solution of nitric acid (concentration 60%) and sulfuric acid (concentration 95% or more) at a ratio of CNF: nitric acid: sulfuric acid = 1 part by weight: 5 parts by weight: 15 parts by weight It mixed and heated and the surface oxidation process was performed. The resulting solution was filtered and washed several times with water to wash away the remaining acid. Thereafter, the powder is dried and powdered, and the powder is dissolved in N-methylpyrrolidone (NMP) to prepare a CNF dispersion.

[Production of positive electrode material]
The positive electrode active material powder (lithium-containing transition metal oxide powder: LiFePO ₄ or the like) is added to the CNF dispersion together with the binder (polyvinylidene fluoride: PVdF or the like) and stirred to form CNF on the surface of the positive electrode active material powder. A positive electrode active material slurry in which is uniformly attached in a network form is prepared. This positive electrode active material slurry is applied and dried on both sides of an aluminum foil (positive electrode current collector), rolled to produce a positive electrode film, and this positive electrode film is cut to produce a positive electrode.

(Production of negative electrode material)
A negative electrode active material (graphite powder or the like) and a binder (PVdF or the like) are added to the CNF dispersion and stirred to prepare a negative electrode active material slurry in which CNF is uniformly attached to the surface of the graphite powder. . The negative electrode active material slurry is applied and dried on both sides of a copper foil (negative electrode current collector), rolled to produce a negative electrode film, and the negative electrode film is cut to produce a negative electrode.

[Production of battery]
The positive electrode and the negative electrode are wound through a polyethylene separator to form an electrode group. The electrode group is inserted into a cylindrical battery container, and a predetermined amount of electrolyte is injected and sealed to form a cylindrical lithium ion secondary battery. Can be obtained. 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a solution in which EC, DMC, and DEC are mixed at a volume ratio of 25:60:15, and vinylene carbonate is added to the solution. Can be used.

[Production of positive electrode]
Lithium iron phosphate powder (D50: 100 nm) was used as a positive electrode active material, and 90 parts by mass of this active material was mixed with 2 parts by mass of a conductive material and 8 parts by mass of a polyvinylidene fluoride binder. A carbon nanotube having a diameter of 10 to 20 nm, a length of 0.1 to 10 μm, a specific surface area of 150 to 200 m ² / g, and a part of the surface substituted with a hydrophilic group was used as a conductive material. To this was added N-methylpyrrolidone as a dispersion solvent and kneaded to prepare a positive electrode mixture (slurry). This slurry was applied to both sides of an aluminum foil having a thickness of 20 μm, dried, rolled and cut to produce a positive electrode having a thickness of about 150 μm.

(Production of negative electrode)
94 parts by mass of graphite powder (D50: 5 μm) was used as an active material, and 1 part by mass of a conductive material and 5 parts by mass of polyvinylidene fluoride as a binder were mixed therewith. The same conductive material as that of the positive electrode was used. N-methylpyrrolidone was added thereto as a dispersion solvent and kneaded to prepare a negative electrode mixture (slurry). This slurry was applied to both sides of a 10 μm thick copper foil, dried, rolled and cut to prepare a negative electrode having a thickness of about 110 μm.

[Production of battery]
The positive electrode and the negative electrode are wound through a polyethylene separator having a thickness of 20 μm to form an electrode group. This electrode group is inserted into a cylindrical battery container, and after a predetermined amount of electrolyte is injected, it is sealed and cylindrical lithium ion is sealed. A secondary battery was produced. 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) is dissolved in a solution in which EC, DMC, and DEC are mixed at a volume ratio of 25:60:15, and vinylene carbonate is added to the solution. Used. The design capacity of this battery is 1000 mAh.

[Test Example 1]
Cylindrical lithium ion battery in the same manner as in Example 1 except that a carbon nanotube material having a diameter of 100 to 200 nm, a length of 5 μm, a specific surface area of 20 m ² / g, and having no hydrophilic group on the surface was used as the conductive material. Got. The design capacity of this battery is 1000 mAh as in Example 1.

[Test Example 2]
A cylindrical lithium ion battery was obtained in the same manner as in Example 1 except that graphite powder having a D50 of 15 μm was used as the negative electrode material and the conductive material used in Comparative Example 1 was used. The design capacity of this battery is 1000 mAh as in Example 1.

[Test Example 3]
A cylindrical lithium ion battery was obtained in the same manner as in Test Example 1 except that 0.3% by mass of the conductive material used in Example 1 was added to the positive electrode material and the negative electrode material, respectively. The design capacity of this battery is 1000 mAh.

[Test Example 4]
An active material slurry was prepared in the same manner as in Test Example 1 except that 15% by mass of the conductive material used in Example 1 was added to the positive electrode material and the negative electrode material, respectively. The battery could not be manufactured. Therefore, the content of the conductive material (fine carbon fiber) is suitably less than 15% by mass.

  The batteries of Example 1 and Test Examples 1 to 3 were subjected to charge / discharge tests, and the DC resistance values calculated from the discharge rate test capacity and current-voltage characteristics were compared.

[Capacity test]
The batteries in the 4.0V state are charged with 5 hour rate (0.2C, 0.2A), 1 hour rate (1C, 1A), 1/2 hour rate (2C, 2A), 1/3 hour rate (3C 3A), and 1/5 hour rate (5C, 5A), the battery was discharged to a final voltage of 2.0 V, and the capacity was determined from the product of the current value and time. The results of the discharge rate capacity test are shown in FIG. Regarding the capacity, the 5-hour rate discharge capacity in each battery was set as a reference value, and the capacity at each discharge rate was plotted as a ratio to the 5-hour rate capacity.

  As shown in FIG. 2, in Test Example 3, the amount of the conductive material is small, the electron conduction resistance in the electrode during discharge increases, and the capacity decreases during large current discharge. More than 3% by weight is suitable. Since Test Example 1 has a lower capacity retention rate when the discharge current is higher than that of Example 1, it is preferable that the fine carbon fiber used as the conductive material has a hydrophilic surface.

  In Example 1 of the present invention, in addition to the conductivity of the fine carbon fiber used as the conductive material, the positive electrode active material particles and the negative electrode active material particles are in good contact with each other and have high conductivity. Large capacity during discharge. In addition, the battery of Example 1 has a heat generation during 5C discharge of about 5 ° C. lower than that of the batteries of Test Examples 1 to 3, and the electronic conductivity has good heat dissipation and also has the effect of suppressing the heat generation of the battery. It was confirmed.

[DC resistance comparison test]
At an ambient temperature of 25 ° C. ± 2 ° C., 50% of the capacity of the battery is discharged at a constant current (1A) for 1 hour, and then at a current of 0.2A, 1A, 2A, 3A, 5A. After discharging for 10 seconds, the battery voltage after 10 seconds was measured. At that time, after each discharge for 10 seconds, a test was performed by charging each predetermined amount of electricity corresponding to the electric capacity discharged at each current with a current of 1 A and providing a 10-minute rest period. . Using the obtained voltage value, a horizontal axis is plotted with the current plotted on the horizontal axis and the voltage is plotted on the vertical axis, and the slope of the first-order approximate linear relationship was determined and the value was taken as the DC resistance. The results are shown in Table 1.

  As shown in Table 1, it can be seen that the direct current resistance value of the battery is the smallest in Example 1, and the conductivity at the positive electrode and the negative electrode is the best. The small DC resistance value can generate a large current, and as a result, a large battery output can be obtained. Therefore, it can be seen that the battery of the present invention is not only a high capacity, but also has an effect of suppressing the heat generation of the battery and is a high input / output battery.

  The above-mentioned effects are not limited to the positive electrode lithium lithium phosphate of this example, but also to lithium cobaltate, lithium nickelate, lithium manganate, and further to these composite metal lithium oxides and other lithium-containing compounds. Similar effects were obtained when the negative electrode was not only graphite but also amorphous carbon material, alloy material, and oxide material.

Carbon nanofibers (CNF: average fiber diameter 20 nm) in a mixed solution of nitric acid (concentration 60%) and sulfuric acid (concentration 95% or more) at a ratio of CNF: nitric acid: sulfuric acid = 1 part by weight: 5 parts by weight: 15 parts by weight It mixed and heated and the surface oxidation process was performed. The resulting solution was filtered and washed several times with water to wash away the remaining acid. Then, it dried and pulverized, the powder was dissolved in N-methylpyrrolidone (NMP), and the CNF dispersion liquid was obtained. On the other hand, LiCoO ₂ (capacity 140 mAh / g) having an average particle diameter of 15 μm and polyvinylidene fluoride (PVdF) were mixed at a solid content weight ratio of 1: 1. This mixture was added to the CNF dispersion, and LiCoO ₂ , PVdF, and CNF were adjusted to a ratio of 100 parts by weight: 5 parts by weight: 5 parts by weight and stirred to prepare a positive electrode active material-containing slurry. This slurry was applied to both sides of an aluminum foil, dried, and then rolled to prepare a positive electrode material film having a thickness of 0.09 cm. An electron micrograph of the surface of the positive electrode active material is shown in FIG. As shown in the figure, it is observed that a mesh-like coating is formed of fine carbon fibers (CNF) on the surface of the LiCoO ₂ particles of the positive electrode active material.

Cross-sectional schematic diagram of a lithium ion secondary battery Graph showing the results of the discharge capacity test Electron micrograph of the surface of the positive electrode active material of Example 2

Explanation of symbols

10-container, 11-positive electrode material, 12-negative electrode material, 13-separator, 14-laminate

Claims (8)

A positive electrode material including a positive electrode active material composed of a lithium-containing compound and a negative electrode material including a negative electrode active material capable of occluding and releasing lithium ions are laminated or wound through a separator and sealed together with a non-aqueous electrolyte. In the lithium ion secondary battery in which a lithium salt is dissolved in the non-aqueous electrolyte, fine carbon fibers are attached in a network shape on the surface of the positive electrode active material and the negative electrode active material. Next battery.
The lithium ion secondary battery according to claim 1, wherein all or part of the surface of the fine carbon fiber is modified with a hydrophilic group.
2. Fine carbon fibers having an average fiber diameter of 1 nm to 100 nm and an aspect ratio of 5 or more adhere to the positive electrode active material having an average particle diameter of 60 nm to 10 μm and the negative electrode active material having an average particle diameter of 3 μm to 7 μm. Alternatively, a lithium ion secondary battery according to claim 2.

The lithium ion secondary battery according to any one of claims 1 to 3, wherein a content of the fine carbon fiber is 0.5 to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material or the negative electrode active material. .
The lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode material and the negative electrode material further contain carbon powder finer than the active material.
The lithium ion secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material is a lithium-containing transition metal oxide particle.
LiCoO ₂ lithium-containing transition metal oxide positive electrode active _{material, Li (Ni x / Mn y} / Co z) O 2 (x + y + z = 1), LiMn 2 O 4, LiCoPO 4, LiFePO 4, LiNiVO 4 _{, LiCoVO 4, LiMnCoO 4, LiMnCrO} 4, LiMn 1.5 Ni 0.5 O 4 at least one selected from the group consisting of or selected from the group consisting of a portion of the composition from the non-stoichiometric compounds substituted with a metal element The lithium ion secondary battery according to claim 6, wherein the lithium ion secondary battery is a compound containing either or both of at least one kind.
The negative electrode active material is natural graphite, artificial graphite, synthetic graphite, mesocarbon microbead, organic graphitized material, coal, coke, PAN-based carbon fiber, pitch-based carbon fiber, or Li ₄ Ti ₅ O ₁₂ , or Sn, Si The lithium ion secondary battery according to any one of claims 1 to 7, wherein the lithium ion secondary battery is an alloy system.