JP2009272120A - Negative electrode material, negative electrode for lithium ion secondary battery, and lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode material and a negative electrode, capable of composing a lithium-ion secondary battery having a high charge/discharge efficiency and an excellent safety at a high temperature, and to provide a lithium-ion secondary battery equipped with the negative electrode. <P>SOLUTION: The negative electrode material contains a carbon material and a fluororesin and when a total of the carbon material and the fluororesin is 100 mass%, the fluororesin content is 0.5-10 mass% and at least a part of the fluororesin is deposited on a surface of the carbon material. The negative electrode for the lithium-ion secondary battery is provided on a surface of a collector with a negative electrode mixture layer formed with the negative electrode mixture containing styrene butadiene rubber or carboxymethyl cellulose as a binder, and the lithium-ion secondary battery is equipped with the above negative electrode for the lithium-ion secondary battery. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a negative electrode material and a negative electrode capable of constituting a lithium ion secondary battery having high charge / discharge efficiency and good safety at high temperatures, and a lithium ion secondary battery having the negative electrode.

  A lithium ion secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density.

In such lithium ion secondary batteries, various improvements have been made along with the improvement in performance of portable devices used. For example, Patent Document 1 discloses that a high current in a lithium ion secondary battery is obtained by treating the surface of a graphite material used as a negative electrode active material of a lithium ion secondary battery with a fluorinating agent such as fluorine gas or NF _3. Techniques have been proposed for reducing and suppressing the capacity drop due to density.

JP 2000-306582 A

  By the way, lithium ion secondary batteries are also required to have safety when exposed to high temperatures, and in particular, with the recent demand for improvement of various battery characteristics in lithium ion secondary batteries, the required safety is also required. It is becoming more advanced. For this reason, for example, development of a technology capable of improving safety while making good use of the capacity originally possessed by the lithium ion secondary battery is required.

  The present invention has been made in view of the above circumstances, and an object thereof is a negative electrode material and a negative electrode capable of constituting a lithium ion secondary battery having high charge / discharge efficiency and good safety at high temperature, and the negative electrode. It is providing the lithium ion secondary battery which has this.

  The negative electrode material of the present invention that has achieved the above object is a negative electrode material used for a negative electrode of a lithium ion secondary battery, and contains a carbon material and a fluororesin, and the carbon material and the fluororesin When the total amount is 100 mass%, the content of the fluororesin is 0.5 to 10 mass%, and at least a part of the fluororesin is attached to the surface of the carbon material. Is.

  Further, in the negative electrode for a lithium ion secondary battery of the present invention, a negative electrode material layer containing at least a negative electrode material containing an active material and styrene butadiene rubber or carboxymethyl cellulose as a binder is formed on one side or both sides of a current collector. A negative electrode for a lithium ion secondary battery, wherein the negative electrode material of the present invention is used as the negative electrode material.

  Furthermore, the lithium ion secondary battery of the present invention has the negative electrode for a lithium ion secondary battery of the present invention.

  According to the present invention, it is possible to provide a negative electrode material and a negative electrode that can form a lithium ion secondary battery having high charge / discharge efficiency and good safety at high temperatures, and a lithium ion secondary battery having the negative electrode. it can. That is, the lithium ion secondary battery of the present invention has high charge / discharge efficiency and good safety at high temperatures.

  The negative electrode material of the present invention is used for a negative electrode of a lithium ion secondary battery, contains a carbon material and a fluororesin, and at least a part of the fluororesin adheres to the surface of the carbon material. Is.

  The carbon material according to the negative electrode material functions as a negative electrode active material in the lithium ion secondary battery. Then, a negative electrode material is formed by attaching a fluororesin to the surface of the carbon material, and by using this for a lithium ion secondary battery, safety at the high temperature can be improved, and charging / discharging of the battery is also possible. Efficiency can also be increased.

  Examples of carbon materials that can be used for the negative electrode material include graphite such as natural graphite (flaky graphite), artificial graphite, and expanded graphite; graphitizable carbon materials such as coke obtained by calcining pitch; and furfuryl alcohol. Non-graphitizable carbon materials such as amorphous carbon obtained by low-temperature firing of resin (PFA), polyparaphenylene (PPP) and phenol resin, and the like. These may be used alone. Two or more kinds may be used in combination. Among these, graphite is particularly preferable because of its large capacity per unit mass and large capacity per unit volume.

When the carbon material is graphite, for example, the plane spacing (d ₀₀₂ ) of the 002 plane is preferably 0.338 nm or less. This is because the higher the crystallinity, the higher the density of the negative electrode mixture layer described later, and the higher the capacity of the lithium ion secondary battery.

  Further, the crystallite size (Lc) in the c-axis direction of graphite is preferably 70 nm or more, more preferably 80 nm or more, and further preferably 90 nm or more. This is because the larger the Lc, the flatter the charging curve, the easier to control the positive electrode potential, and the larger the capacity. On the other hand, if Lc is too large, the battery capacity tends to decrease particularly when the negative electrode mixture layer has a high density. Therefore, Lc is preferably less than 200 nm.

  Furthermore, since the irreversible capacity | capacitance will become large if the average particle diameter of a carbon material is too small, it is preferable that it is 5 micrometers or more, It is more preferable that it is 12 micrometers or more, It is still more preferable that it is 18 micrometers or more. Moreover, from the viewpoint of increasing the density of the negative electrode mixture layer, the average particle size of the carbon material is preferably 30 μm or less, more preferably 25 μm or less, and further preferably 20 μm or less.

  The average particle diameter of the graphite is determined by adding a predetermined amount of a carbon material to a liquid containing a surfactant in water and dispersing it by ultrasonic treatment, and using this dispersion liquid, a laser diffraction scattering particle size distribution This is a 50% diameter value (median diameter) in a volume-based integrated fraction when an integrated volume is determined from particles having a small particle size distribution measured by a measuring device (“MICROTRAC” manufactured by Honeywell).

  Examples of the fluororesin that can be used for the negative electrode material include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, and tetrafluoroethylene. -Hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer (E / TFE), polychlorotrifluoroethylene, fluororubber, etc., may be used alone or in combination of two or more May be used in combination. Among these, PVDF is particularly preferable.

  In the negative electrode material, the content of the fluororesin in the total of 100% by mass of the carbon material and the fluororesin is the effect of the use of the fluororesin, that is, the safety improving effect and the charge / discharge efficiency at a high temperature of the lithium ion secondary battery. From the viewpoint of reliably exhibiting the improving effect, it is 0.5% by mass or more, and preferably 1% by mass or more. In addition, if the amount of the fluororesin in the negative electrode material is too large, the charge / discharge efficiency of the lithium ion secondary battery is reduced, and the actual discharge capacity is smaller than the design capacity of the battery. The content of the fluororesin in the total of 100% by mass is 10% by mass or less, and preferably 8% by mass or less.

  In the negative electrode material, it is sufficient that at least a part of the fluororesin is attached to the surface of the carbon material. The negative electrode material may include a fluororesin that is not attached to the surface of the carbon material. It is preferable that all of the fluororesin contained in the material adheres to the surface of the carbon material. In the negative electrode material, the fact that the fluororesin adheres to the surface of the carbon material is obtained by cutting the negative electrode material or an electrode (negative electrode) formed using the negative electrode material by a cross section polisher (CP) method. The cross section can be confirmed by observing the cross section with a scanning electron microscope and an energy dispersive X-ray analyzer.

  The negative electrode material of the present invention can be prepared, for example, by the following method. First, a carbon material and a solution containing a fluororesin [for example, N-methyl-2-pyrrolidone (NMP) solution] are kneaded.

  The kneading is preferably carried out in a state where the solid content concentration of the kneaded product is high, and is preferably carried out in an apparatus having a small dead space. Specifically, a planetary mixer, a kneader, a twin-screw kneader, etc. It is desirable to implement with an apparatus. The solid content concentration of the kneaded product may vary depending on the type of fluororesin and solvent used, the particle size distribution of the carbon material particles, etc., but for example, the state when mixed until the whole is evenly moistened is clay-like. It is desirable that

  Next, the kneaded product is dried under reduced pressure at 60 to 120 ° C. for 8 to 15 hours to remove the solvent component related to the fluororesin solution. For drying, for example, a vacuum dryer can be used when performing batch processing, and a continuous drying facility equipped with a decompression zone can be used when performing continuous processing.

  Next, the dried product is passed through a sieve to remove coarse particles, and the negative electrode material of the present invention can be obtained.

  The negative electrode for a lithium ion secondary battery of the present invention has a negative electrode material layer containing at least a negative electrode material containing an active material and styrene butadiene rubber (SBR) or carboxymethyl cellulose (CMC) as a binder. Alternatively, it is formed on both surfaces, and the negative electrode material of the present invention is used as the negative electrode material.

  As the binder, only one of SBR and CMC may be used, or both may be used in combination.

  The negative electrode of the present invention is, for example, a negative electrode mixture-containing composition (paste, slurry, etc.) by dispersing a negative electrode mixture containing the negative electrode material of the present invention, a binder, and, if necessary, a conductive additive in a solvent. Is prepared and applied to one or both sides of the current collector and dried, followed by a step of forming a negative electrode mixture layer while adjusting the thickness and density by pressing. Examples of the solvent used in the negative electrode mixture-containing composition include water; organic solvents such as NMP, toluene, and xylene; In preparing the negative electrode mixture-containing composition, it is preferable to use a solution previously dissolved or dispersed in an organic solvent or water as a binder, and mix these with a negative electrode material or the like. Furthermore, a conductive additive may or may not be used for the negative electrode. As a conductive support agent, the same thing as the conductive support agent for positive electrodes mentioned later can be used, for example. Note that the method for manufacturing the negative electrode is not limited to this, and other methods may be adopted.

  The composition of the negative electrode mixture constituting the negative electrode mixture layer is 97 to 99% by mass of the negative electrode material of the present invention, and SBR or CMC as a binder (the total when both are used in combination) of 1 to 3% by mass. When the conductive auxiliary is used, the amount is preferably 0.5 to 1% by mass. For example, when preparing the above-mentioned negative electrode mixture-containing composition, It is recommended to adjust the content ratio.

  Examples of a method for applying the negative electrode mixture-containing composition to the current collector include various known application methods such as an extrusion coater, a reverse roller, a doctor blade, and an applicator.

  As the negative electrode current collector, for example, a metal, a punched metal, a foam metal, a foil obtained by processing a metal conductive material such as aluminum, stainless steel, nickel, titanium, or copper into a plate shape is used. The thickness of the negative electrode current collector is preferably, for example, 5 to 30 μm.

  The thickness of the negative electrode mixture layer (the thickness per one side of the current collector) is the thickness after drying, and is preferably 90 to 120 μm, for example.

Further, the negative electrode mixture layer preferably has a density of 1.4 g / cm ³ or more, and more preferably 1.5 g / cm ³ or more. By increasing the density of the negative electrode mixture layer in this manner, the filling amount of the active material carbon material in the negative electrode mixture layer is increased, and the capacity of the negative electrode is increased, that is, the capacity of the lithium ion secondary battery is increased. be able to. However, if the density of the negative electrode mixture layer is too high, metallic lithium may precipitate on the negative electrode surface and cause a decrease in charge / discharge capacity and charge / discharge cycle characteristics, and therefore may be 1.8 g / cm ³ or less. Preferably, it is 1.7 g / cm ³ or less.

  In addition, the density of the negative mix layer as used in this specification is a value measured by the following method. The negative electrode is cut into a predetermined area, the mass is measured using an electronic balance with a minimum scale of 1 mg, and the mass of the negative electrode mixture layer is calculated by subtracting the mass of the current collector. On the other hand, the total thickness of the negative electrode was measured at 10 points with a micrometer having a minimum scale of 1 μm, and the volume of the negative electrode mixture layer was determined from the average value obtained by subtracting the thickness of the current collector from these measured values and the area. calculate. Then, the density of the negative electrode mixture layer is calculated by dividing the mass of the negative electrode mixture layer by the volume.

  The lithium ion secondary battery of the present invention only needs to have the above-mentioned negative electrode, and other configurations and structures are not particularly limited, and the configurations and structures employed in conventionally known lithium ion secondary batteries are the same. Can be applied.

  As the positive electrode, for example, one formed by forming a positive electrode mixture layer containing a positive electrode active material, a conductive additive, a binder, and the like on one surface or both surfaces of a positive electrode current collector can be used. Examples of the positive electrode active material include lithium cobaltate, nickel cobaltate, lithium nickelate, lithium manganate, nickel cobalt lithium manganate, and the like. A positive electrode active material used in a conventionally known lithium secondary battery, such as a substituted compound or an olivine compound, can be used without particular limitation. These positive electrode active materials may be used alone or in combination of two or more.

  As the conductive auxiliary agent used for the positive electrode, for example, carbon materials such as carbon black, acetylene black, ketjen black, graphite, and carbon fiber are preferable. Among the carbon materials described above, acetylene black, ketjen black, and graphite are particularly preferable from the viewpoint of the amount of addition and conductivity, and the productivity of the positive electrode mixture layer-containing composition (described later).

  As the binder used for the positive electrode, for example, a polyvinylidene fluoride polymer (a polymer of a fluorine-containing monomer group containing 80% by mass or more of the main component monomer) is a rubber polymer. . These binders may use only 1 type and may use 2 or more types together. The binder may be provided in the form of a dispersion in a dispersion medium or a solution dissolved in a solvent, in addition to a powdery one.

  Examples of the fluorine-containing monomer group for synthesizing the polyvinylidene fluoride-based polymer include vinylidene fluoride; a mixture of vinylidene fluoride and another monomer, and a monomer mixture containing 80% by mass or more of vinylidene fluoride; Is mentioned. Examples of the other monomer include vinyl fluoride, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoroalkyl vinyl ether.

  Examples of the rubber-based polymer include SBR, ethylene propylene diene rubber, and fluorine rubber.

  The content of the positive electrode active material in the positive electrode mixture layer is preferably 96% by mass or more, more preferably 97.0% by mass or more, preferably 99.4% by mass or less, more preferably 98.0% by mass. It is as follows.

  Further, the content of the conductive additive in the positive electrode mixture layer is, for example, preferably 0.5% by mass or more, more preferably 0.8% by mass or more, and preferably 18% by mass or less, more preferably It is 15 mass% or less. If the amount of the conductive auxiliary agent in the positive electrode mixture layer is too small, the electron conductivity of the positive electrode may be insufficient and the load characteristics of the battery may be deteriorated, and the amount of the conductive auxiliary agent in the positive electrode mixture layer is too large. In addition, since the amount of the active material in the positive electrode mixture layer is reduced, the battery capacity may be reduced.

  Further, the binder content in the positive electrode mixture layer is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and preferably 5% by mass or less, more preferably 2% by mass or less. It is. If the content of the binder in the positive electrode mixture layer is too small, the mechanical strength of the positive electrode mixture layer is insufficient, and the positive electrode mixture layer may be peeled off from the current collector. If the content is too large, the amount of the active material in the positive electrode mixture layer is decreased, and the capacity of the battery may be reduced.

  The positive electrode having the positive electrode mixture layer is, for example, a positive electrode mixture-containing composition prepared by dispersing the positive electrode active material, a conductive additive, a binder, and the like in a solvent (some components may be dissolved). After applying (paste, slurry, etc.) on one or both sides of the current collector and drying, it can be produced by pressing as necessary to adjust the thickness and density of the positive electrode mixture layer. Note that the method for manufacturing the positive electrode is not limited to this, and other methods may be adopted. Examples of the solvent that can be used in the positive electrode mixture-containing composition include water; organic solvents such as NMP, toluene, and xylene.

  As the method for applying the positive electrode mixture-containing composition to the surface of the positive electrode current collector, various methods exemplified as the method for applying the negative electrode mixture-containing composition to the surface of the negative electrode current collector can be employed.

  As the positive electrode current collector, for example, a metal, a punched metal, a foam metal, a foil obtained by processing a metal conductive material such as aluminum, stainless steel, or titanium into a plate shape is used. The thickness of the positive electrode current collector is preferably, for example, 8 to 16 μm.

  In addition, the thickness of the positive electrode mixture layer formed on the surface of the positive electrode current collector (thickness per side of the current collector) is the thickness after drying, and is preferably 40 to 150 μm, for example.

The density of the positive electrode mixture layer is preferably 3.75 g / cm ³ or more, and more preferably 3.80 g / cm ³ or more. By using a positive electrode having such a high-density positive electrode mixture layer, the capacity of the battery can be increased. However, if the density of the positive electrode mixture layer is too large, it is difficult to get wet with the electrolytic solution, and the productivity of the battery may be lowered. Therefore, the density is preferably 4.1 g / cm ³ or less. The density of the positive electrode mixture layer can be adjusted, for example, by adjusting the press conditions in the press process during positive electrode manufacture. The density of the positive electrode mixture layer here is a value measured by the same measurement method as the density of the negative electrode mixture layer.

  The lithium ion secondary battery of the present invention includes, for example, a laminated electrode body in which a separator is interposed between the negative electrode of the present invention and the positive electrode, and a winding in which the laminated electrode body is wound in a spiral shape. The electrode body is manufactured by inserting it into a battery case made of aluminum, aluminum alloy, nickel-plated iron or stainless steel, injecting an electrolytic solution, and sealing the battery body. The battery of the present invention usually has a conventionally known explosion-proof mechanism for discharging the gas generated in the battery to the outside of the battery at a stage where the pressure has risen to a certain pressure, and preventing the battery from bursting under high pressure. Is adopted.

  There is no restriction | limiting in particular about the separator interposed between a positive electrode and a negative electrode, A conventionally well-known thing is applicable. For example, a microporous polyethylene film having a thickness of 5 to 30 μm and a porosity of 30 to 70%, a microporous polypropylene film, a polyethylene polypropylene composite film, or the like is preferably used.

As the electrolytic solution, an organic solvent in which an electrolyte such as a lithium salt is dissolved is used. Examples of the electrolyte include a general formula LiXF _n (wherein X is P, As, Sb or B, n is 6 when X is P, As or Sb, and 4 when X is B). Inorganic lithium salts represented by (A) and fluorine-containing organic lithium imide salts. These electrolytes may be used alone or in combination of two or more.

  The organic solvent used for dissolving the electrolyte is not particularly limited, and examples thereof include 1,2-dimethoxyethane, 1.2-diethoxyethane, dimethoxypropane, 1,3-dioxolane, tetrahydrofuran, Ethers such as 2-methyltetrahydrofuran; esters such as propylene carbonate, ethylene carbonate, γ-butyrolactone, diethyl carbonate, dimethyl carbonate, and ethylmethyl carbonate; sulfur-containing compounds such as sulfolane; fluorinated chain carbonate (trifluoromethylethyl) Carbonates), fluorinated solvents such as fluorinated cyclic carbonates (such as perfluoroethylene carbonate), and fluorinated chain ethers (such as perfluorobutyl methyl ether). These organic solvents may be used alone or as a mixed solvent containing two or more kinds. Among the organic solvents, esters are preferable because they are less reactive with the positive electrode active material even under a high voltage and have a large effect of improving storage characteristics. From the viewpoint of improving the stability of the electrolyte during charging, the esters are preferably 20% by volume or more in the total electrolyte solvent.

  The concentration of the electrolyte in the electrolyte solution is preferably 0.4 to 1.8 mol / l as a whole, even if two or more different types of electrolytes are included, and is 0.6 to 1.6 mol / l. It is particularly preferred.

  The lithium ion secondary battery of the present invention can be applied to the same use as a conventionally known lithium ion secondary battery.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of negative electrode material>
An NMP solution of PVDF [“L # 1120 (trade name) manufactured by Kureha Chemical Co., Ltd., solid content concentration: 12% by mass]] is added to graphite (d ₀₀₂ = 0.3356 nm), and PVDF is 0 in the total of graphite and PVDF. The NMP was added so as to be 5% by mass, and an appropriate amount of NMP was added so that kneading was possible, and the mixture was kneaded for 1 hour under reduced pressure using a planetary mixer. The obtained kneaded material was taken out on a metal vat and dried under reduced pressure at 80 ° C. for 12 hours. The obtained dried product was crushed and passed through a 200 mesh (75 μm sieve opening) sieve to obtain a negative electrode material.

<Production of negative electrode>
Prepare the SBR suspension and the CMC aqueous solution so that the solid content is 1 part by mass (that is, 2 parts by mass as a whole of the binder solid content), and mix with 98 parts by mass of the negative electrode material to contain the negative electrode mixture A composition was prepared. This negative electrode mixture-containing composition was uniformly applied to both sides of a copper foil having a thickness of 8 μm using an applicator, and then rolled with a roll press to have a negative electrode mixture layer on both sides of the current collector. Then, a sheet-like negative electrode having a total thickness of 128 μm was produced. The negative electrode mixture layer density of the negative electrode thus produced was 1.6 g / cm ³ .

<Preparation of positive electrode>
A positive electrode mixture-containing composition was prepared using 98 parts by mass of lithium cobaltate as an active material, 1 part by mass of acetylene black as a conductive auxiliary agent, 1 part by mass of PVDF as a binder, and NMP as a solvent. The positive electrode mixture-containing composition is prepared by previously dissolving PVDF in NMP, adding the active material mixture and acetylene black to this solution, adding NMP while stirring, and adjusting the viscosity while sufficiently dispersing. Was done by. This positive electrode mixture-containing composition was uniformly applied to both sides of an aluminum foil having a thickness of 15 μm using an applicator, and then rolled with a roll press to have a positive electrode mixture layer on both sides of the current collector. Thus, a sheet-like positive electrode having a total thickness of 130 μm was obtained. The positive electrode mixture layer density of the positive electrode thus prepared was 3.85 g / cm ³ .

<Battery assembly>
A lead body is attached to the negative electrode and the positive electrode, and these are overlapped via a separator made of a microporous polyethylene-polypropylene composite film having a thickness of 14 μm, wound in a spiral shape, and pressurized to form a flat wound electrode Got the body. After the insulating tape is attached to this wound electrode body, the outer dimensions are inserted into a rectangular (square tube) battery case having a height of 50 mm, a width of 34 mm, and a thickness of 4 mm. Laser welding of the sealing cover plate to the opening end was performed. Thereafter, the electrolyte solution is injected into the battery case from the electrolyte solution injection port provided on the sealing lid plate, and after the electrolyte solution sufficiently penetrates into the separator and the like, the electrolyte solution injection port is sealed and sealed. . Note that the electrolytic solution 1 of ethylene carbonate and methyl ethyl carbonate: 2 (volume ratio) mixed solvent was used LiPF ₆ was dissolved at a concentration of 1.0 mol / l. Thereafter, precharging and aging were performed to obtain a prismatic lithium ion secondary battery having the structure shown in FIG. 1 and the appearance shown in FIG.

  Here, the battery shown in FIGS. 1 and 2 will be described. The positive electrode 1 and the negative electrode 2 are wound in a spiral shape through the separator 3 as described above, and then pressed so as to become a flat shape. The rotating electrode body 6 is accommodated in a rectangular battery case 4 together with an electrolytic solution. However, in FIG. 1, in order to avoid complication, a metal foil, an electrolytic solution, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated.

  The battery case 4 is made of an aluminum alloy and constitutes a battery exterior material. The battery case 4 also serves as a positive electrode terminal. And the insulator 5 which consists of a polyethylene sheet is arrange | positioned at the bottom part of the battery case 4, and from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3, it is to each one end of the positive electrode 1 and the negative electrode 2 The connected positive electrode lead body 7 and negative electrode lead body 8 are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the battery case 4 via a polypropylene insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

  And this cover plate 9 is inserted in the opening part of the battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, and the inside of the battery is sealed. Further, in the battery of FIG. 1, an electrolyte solution inlet 14 is provided in the lid plate 9, and the electrolyte solution inlet 14 is welded and sealed by, for example, laser welding or the like with a sealing member inserted. Thus, the sealing property of the battery is ensured (therefore, in the battery of FIGS. 1 and 2, the electrolyte inlet 14 is actually the electrolyte inlet and the sealing member, but the explanation is easy. In order to achieve this, it is shown as an electrolyte inlet 14). Further, the cover plate 9 is provided with an explosion-proof vent 15.

  In the battery of Example 1, the battery case 4 and the cover plate 9 function as positive terminals by directly welding the positive electrode lead body 7 to the cover plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the battery case 4, the sign may be reversed. There is also.

  FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode body is not cross-sectional.

Examples 2 to 4 and Comparative Examples 1 to 4
A negative electrode material was prepared in the same manner as in Example 1 except that the content of PVDF in the total of 100% by mass of graphite and PVDF at the time of mixing graphite and PVDF with NMP solution was changed as shown in Table 1. A negative electrode and a lithium ion secondary battery were produced in the same manner as in Example 1 except that these negative electrode materials were used.

  The lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to the following charge / discharge efficiency measurement and safety evaluation. These results are also shown in Table 1.

<Measurement of charge / discharge efficiency>
About the lithium ion secondary battery of Examples 1-4 and Comparative Examples 1-4, charging / discharging was performed on the following conditions, charge capacity and discharge capacity were calculated | required, respectively, and the ratio of discharge capacity with respect to charge capacity is the initial charge / discharge efficiency ( Hereinafter, it was evaluated as “charge / discharge efficiency”. First, each battery was charged at a constant current of 180 mA (0.2 C) to 4.2 V, then charged at a constant voltage of 4.2 V until the total charging time was 8 hours, and then a constant current of 180 mA (0.2 C). Then, constant current discharge was performed until the battery voltage reached 3.0V.

<Safety evaluation>
About the lithium ion secondary battery (each 5 pieces) of Examples 1-4 and Comparative Examples 1-4, it charged on the same conditions as the time of the said charging / discharging efficiency measurement. Thereafter, these batteries were placed in a thermostatic bath, heated from 30 ° C. to 150 ° C. at a rate of 5 ° C. per minute, and then kept at 150 ° C. for 3 hours to measure the surface temperature of the batteries. In this test, a battery having a maximum surface temperature of 180 ° C. or higher was evaluated as inferior in safety.

  In Table 1, regarding the negative electrode material, the content of PVDF in 100% by mass of graphite and PVDF at the time of mixing graphite and PVDF with an NMP solution is described as “amount of fluororesin in negative electrode material”. Yes.

  As shown in Table 1, the lithium ion secondary batteries of Examples 1 to 4 have high charge / discharge efficiency, and even when held at 150 ° C. for 3 hours, the surface temperature does not exceed 180 ° C. and is safe. Is good.

  In contrast, the lithium ion secondary batteries of Comparative Examples 1 to 3 configured using a negative electrode material that does not contain a fluororesin or has a small amount of fluororesin have a surface temperature of 180 when held at 150 ° C. for 3 hours. Some of them exceed ℃, and safety is inferior, and charge / discharge efficiency is low. Furthermore, the lithium ion secondary battery of Comparative Example 4 configured using a negative electrode material having a large amount of fluororesin has low charge / discharge efficiency.

It is a figure which shows typically an example of the lithium ion secondary battery of this invention, (a) is the top view, (b) is the fragmentary longitudinal cross-sectional view. It is a perspective view of the lithium ion secondary battery shown in FIG.

Explanation of symbols

1 Positive electrode 2 Negative electrode 3 Separator

Claims (3)

A negative electrode material used for a negative electrode of a lithium ion secondary battery,
A carbon material and a fluororesin are contained, and when the total of the carbon material and the fluororesin is 100% by mass, the content of the fluororesin is 0.5 to 10% by mass, and the fluororesin At least a part of which adheres to the surface of the carbon material. A negative electrode material containing an active material and a negative electrode mixture layer containing at least styrene butadiene rubber or carboxymethyl cellulose as a binder is a negative electrode for a lithium ion secondary battery formed on one or both sides of a current collector,
A negative electrode for a lithium ion secondary battery, wherein the negative electrode material according to claim 1 is used as the negative electrode material.   A lithium ion secondary battery comprising the negative electrode for a lithium secondary battery according to claim 2.

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