JP2006338977A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of securing high capacity even in low-temperature environment and suppressing deterioration of cycle characteristics. <P>SOLUTION: The cylindrical lithium ion secondary battery has a wound group in which a positive and a negative electrode plates are wound around through a separator immersed into an non-aqueous electrolytic solution in a battery container. The positive electrode plate has a positive electrode mixture containing lithium-manganese cobalt-nickel compound oxide powder painted on an aluminum foil. The negative electrode plate has a negative electrode mixture containing two kinds or more of carbon materials capable of insertion and release of lithium ions painted on a rolled copper foil. One kind out of the carbon materials uses a graphite system carbon material of which the surface is amorphous carbon, and at least the other one kind uses amorphous carbon powder. In a low-temperature environment, when lithium ions in the graphite system carbon are released, electron conduction is performed through amorphous carbon on the surface of the graphite system carbon material and the amorphous carbon material. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery, and in particular, a lithium secondary battery in which a positive electrode plate containing a lithium transition metal composite oxide and a negative electrode plate containing a negative electrode material capable of inserting and releasing lithium ions are infiltrated into a non-aqueous electrolyte. Next battery.

  Lithium ion secondary batteries are mainly used as power sources for portable devices such as VTR cameras, notebook computers, and mobile phones, taking advantage of the high energy density. A typical lithium ion secondary battery has a diameter of 18 mm and a height of 65 mm, is called 18650 type, and is widely used as a small-sized consumer lithium ion secondary battery. In the 18650 type lithium ion secondary battery, the battery capacity is about 1.3 Ah to 1.8 Ah, and the output is about 10 W.

  On the other hand, in the automobile industry, in order to cope with environmental problems, an electric vehicle (EV) without exhaust gas in which the power source is completely a battery only, and a hybrid (electricity) with both an internal combustion engine and a battery as power sources The development of automobiles (HEV) has been accelerated and is in the stage of commercialization. A battery serving as a power source for the EV or HEV is naturally required to have a high capacity and a high output, and a lithium ion secondary battery is attracting attention as a battery that matches this requirement.

  Further, since EV and HEV may be used for a long time even in a cold region, a battery serving as a power source is required to have a high capacity and a long life under a low temperature environment. Under a low temperature environment, the lithium ion diffusibility in the non-aqueous electrolyte is very small and the electron conductivity is lowered, so that the output and capacity are lowered. In order to solve this, for example, a technique for suppressing a decrease in lithium ion diffusibility in a low temperature environment by using a mixed organic solvent in a nonaqueous electrolytic solution is disclosed (see Patent Document 1). However, with this technique, although the lithium ion diffusibility in the non-aqueous electrolyte is improved, the electron conductivity of the positive and negative electrode plates is not improved, so that it is not sufficient to increase the capacity in a low temperature environment. For this reason, it is important to improve the electron conductivity such as adding a conductive agent to the positive and negative electrode plates to ensure an electron conduction network.

  Generally, a lithium transition metal composite oxide is used as a positive electrode material of a lithium ion secondary battery, and lithium cobalt oxide is used among them in terms of balance of capacity and cycle characteristics. However, since cobalt has a small amount of resources and high costs, lithium transition metal composite oxides containing manganese typified by lithium manganate are considered promising as a positive electrode material for EV and HEV batteries.

  On the other hand, a carbon material capable of inserting and releasing lithium ions is used for the negative electrode material. Generally, natural graphite, artificial graphite such as flakes and blocks, graphite-based carbon materials such as mesophase pitch-based graphite, and furfuryl An amorphous carbon material obtained by firing a furan resin such as alcohol is used. An amorphous carbon material has a higher theoretical capacity value than a graphite carbon material, and thus has excellent capacity and cycle characteristics. However, since the irreversible capacity is large, it is difficult to increase the capacity of the battery. On the other hand, a graphite-based carbon material has a smaller irreversible capacity than an amorphous carbon material and excellent electronic conductivity, so that a high capacity can be obtained. However, since the volume change of the crystal accompanying charge / discharge is large, Decay or peeling of the negative electrode material occurs, and the electronic conductivity cannot be maintained for a long period of time, resulting in poor cycle characteristics. In order to solve these problems, a technique is disclosed in which natural graphite having excellent electronic conductivity and artificial graphite having a small degree of crystal volume change are used in combination (for example, see Patent Document 2).

JP 2001-155766 A JP 2004-127913 A

  However, in the technique of Patent Document 2, since only a graphite-based carbon material is used, it is impossible to avoid a decrease in electronic conductivity associated with a volume change, resulting in a decrease in cycle characteristics. In addition, since the nonaqueous electrolytic solution is decomposed on the surface of the graphite-based carbon material, a film that inhibits electron conduction is formed. For this reason, in a low temperature environment, the lithium ion diffusibility is lowered, and the film formed on the surface of the graphite-based carbon material impedes the electron conductivity, thereby causing a reduction in capacity and cycle characteristics. In the above-described EV and HEV batteries, it is important to maintain a high capacity and a high output for a long time even in a low temperature environment, but a battery that exhibits satisfactory performance has not been developed. is there.

  An object of the present invention is to provide a lithium secondary battery capable of securing a high capacity even in a low temperature environment and suppressing deterioration of cycle characteristics even in the low temperature environment.

  In order to solve the above problems, the present invention provides a lithium secondary battery in which a positive electrode plate containing a lithium transition metal composite oxide and a negative electrode plate containing a negative electrode material capable of inserting and releasing lithium ions are infiltrated into a non-aqueous electrolyte. In the secondary battery, two or more types of carbon materials are used for the negative electrode material, at least one of the carbon materials is a graphite-based carbon material whose surface is amorphous carbon, and at least one of the other carbon materials. It is a crystalline carbon material.

  In the present invention, two or more types of carbon materials are used for the negative electrode material, and at least one of the carbon materials is a graphite-based carbon material whose surface is amorphous carbon. Therefore, the graphite-based carbon material is a non-aqueous electrolyte. Since the non-aqueous electrolyte is not decomposed by the graphite-based carbon material because it is not in direct contact with the glass, the formation of a coating that inhibits electron conduction caused by the decomposition of the non-aqueous electrolyte is prevented, and the decrease in capacity is suppressed. Even if the volume of the graphite-based carbon material changes with charge / discharge, the amorphous carbon on the surface prevents the graphite-based carbon material from peeling off. It is possible to suppress the decrease, and at least one of the carbon materials is an amorphous carbon material. Therefore, even if the lithium ion diffusibility in the non-aqueous electrolyte decreases in a low temperature environment, the graphite carbon material The surface of graphite-based carbon material when lithium ions are released Since electronic conduction through the amorphous carbon or amorphous carbon material, it is possible to ensure the capacity in a low temperature environment.

  In this case, the ratio of the amorphous carbon material in the negative electrode material is preferably 5% to 90%, and the ratio may be 5% to 50%. Further, when the average particle size of the graphite-based carbon material is 1.0, and the average particle size of the amorphous-based carbon material is less than 1.0, the amorphous-based carbon material becomes the surface of the graphite-based carbon material. Since it becomes easy to contact with the amorphous carbon, the electron conductivity can be improved.

  According to the present invention, two or more carbon materials are used for the negative electrode material, and at least one of the carbon materials is a graphite-based carbon material whose surface is amorphous carbon. This prevents the formation of a coating that inhibits electron conduction and suppresses the decrease in capacity. Even if the volume of the graphite-based carbon material changes due to charge / discharge, the amorphous carbon on the surface does not peel off the graphite-based carbon material. Therefore, the deterioration of the cycle characteristics can be suppressed, and at least one of the carbon materials is an amorphous carbon material. Therefore, when lithium ions in the graphite carbon material are released, the graphite carbon Electron conduction is performed through the amorphous carbon or amorphous carbon material on the surface of the material, so that it is possible to obtain an effect that the capacity can be secured even in a low temperature environment.

  Embodiments of a cylindrical lithium ion secondary battery to which the present invention can be applied will be described below with reference to the drawings.

(Constitution)
As shown in FIG. 1, a cylindrical lithium ion secondary battery 20 of the present embodiment includes a nickel-plated steel bottomed cylindrical battery container 7 and a polypropylene hollow cylindrical shaft core 1. A strip-like positive and negative electrode plate has a wound group 6 wound in a spiral shape through a separator W5.

On the upper side of the winding group 6, an aluminum positive electrode current collecting ring 4 for collecting the electric potential from the positive electrode plate is disposed on an almost extension line of the shaft core 1. The positive electrode current collecting ring 4 is fixed to the upper end portion of the shaft core 1. The edge part of the positive electrode lead piece 2 led out from the positive electrode plate is joined by ultrasonic welding to the peripheral edge of the flange part integrally protruding from the periphery of the positive electrode current collecting ring 4. A disc-shaped battery lid serving as a positive electrode external terminal is disposed above the positive electrode current collecting ring 4. The battery lid includes a lid case 12, a lid cap 13, a valve retainer 14 that keeps airtightness, and a cleavage valve (internal gas discharge valve) 11 of an internal pressure release mechanism that cleaves when the internal pressure rises. Then, the lid case 12 is assembled by caulking the periphery. The cleavage pressure of the cleavage valve 11 is set to about 9 × 10 ⁵ Pa. One end of two positive electrode lead plates 9 formed by overlapping a plurality of aluminum ribbons is fixed to the upper portion of the positive electrode current collecting ring 4, and another one is fixed to the lower surface of the lid case 12. One end of the book is welded. The other ends of the two positive electrode lead plates 9 are joined by welding.

  On the other hand, a copper negative electrode current collecting ring 5 for collecting a potential from the negative electrode plate is disposed below the winding group 6. The outer peripheral surface of the lower end portion of the shaft core 1 is fixed to the inner peripheral surface of the negative electrode current collecting ring 5. The end of the negative electrode lead piece 3 led out from the negative electrode plate is joined to the outer peripheral edge of the negative electrode current collecting ring 5 by welding. A negative electrode lead plate 8 made of copper for electrical conduction is welded to the lower part of the negative electrode current collecting ring 5, and the negative electrode lead plate 8 is joined to the inner bottom portion of the battery container 7 by welding. In this example, the battery container 7 has an outer diameter of 40 mm and an inner diameter of 39 mm.

The battery lid is fixed by caulking to the upper part of the battery container 7 via an insulating and heat resistant EPDM resin gasket 10. For this reason, the positive electrode lead 9 is accommodated in the battery container 7 so as to be folded, and the lithium ion secondary battery 20 is sealed. Further, a non-aqueous electrolyte (not shown) is injected into the battery container 7. In the non-aqueous electrolyte, 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) as a lithium salt is dissolved in a 1: 1: 1 volume ratio of ethylene carbonate, dimethyl carbonate, and diethyl carbonate. Is used. The lithium ion secondary battery 20 has a PTC (Positive Temperature Coefficient) element that operates electrically in response to an increase in battery temperature, or a positive or negative electrical lead in response to an increase in battery internal pressure. A current interrupting mechanism to be disconnected can be arranged as necessary.

  In the winding group 6, the positive electrode plate and the negative electrode plate are wound around the shaft core 1 via a separator W5 made of porous polyethylene having a width of 90 mm and a thickness of 40 μm so that the two electrode plates do not directly contact each other. Yes. The positive electrode lead piece 2 and the negative electrode lead piece 3 are respectively disposed on opposite end surfaces of the wound group 6. Insulation coating is applied to the entire circumference of the collar surface of the winding group 6 and the positive electrode current collector ring 4. For the insulation coating, an adhesive tape in which a hexamethacrylate adhesive is applied to one side of a polyimide base material is used. The pressure-sensitive adhesive tape is wound one or more times from the collar surface to the outer periphery of the wound group 6. By adjusting the length of the positive electrode plate, the negative electrode plate, and the separator W5, the diameter of the wound group 6 is set to 38 ± 0.1 mm.

  The positive electrode plate constituting the wound group 6 has an aluminum foil W1 as a positive electrode current collector, and the negative electrode plate has a rolled copper foil W3 as a negative electrode current collector. In this example, the thicknesses of the aluminum foil W1 and the rolled copper foil W3 are set to 20 μm and 10 μm, respectively. On the side edges on one side in the longitudinal direction of the aluminum foil W1 and the rolled copper foil W3, uncoated portions of the positive electrode mixture W2 and the negative electrode mixture W4 are formed with a width of 30 mm, respectively. The uncoated part is cut out in a comb shape, and the positive electrode lead piece 2 and the negative electrode lead piece 3 are formed in the notch remaining part, respectively. The interval between the adjacent positive electrode lead pieces 2 and the interval between the negative electrode lead pieces 3 is set to 50 mm, and the width of each of the positive electrode lead piece 2 and the negative electrode lead piece 3 is set to 5 mm.

A positive electrode mixture W2 including lithium transition metal composite oxide lithium manganese cobalt nickel composite oxide (LiMnCoNiO) powder as a positive electrode active material (positive electrode material) is applied to both surfaces of the aluminum foil W1 substantially evenly. In the positive electrode mixture W2, for example, 8 parts by mass of graphite powder as a conductive material, 2 parts by mass of acetylene black (hereinafter abbreviated as AB), and a binder (binding) with respect to 85 parts by mass of the positive electrode active material. As a material, 5 parts by mass of polyvinylidene fluoride (hereinafter abbreviated as PVDF) is blended. When the positive electrode mixture W2 is applied to the aluminum foil W1, the viscosity is adjusted with a dispersion solvent N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP). The coating amount of the positive electrode mixture W2 is set to be 180 g / m ^{2 by} weight after drying. After drying, the positive electrode plate is pressed so that the bulk density of the positive electrode mixture W2 layer is 2.65 g / cm ³ and cut into a width of 82 mm.

  On the other hand, a negative electrode mixture W4 containing two or more kinds of carbon materials capable of inserting and releasing lithium ions as a negative electrode active material (negative electrode material) is coated on both surfaces of the rolled copper foil W3 substantially evenly. One of the carbon materials is a graphite-based carbon material whose surface is amorphous carbon, and at least one of the other carbon materials is an amorphous carbon powder (Kureha Chemical). Kogyo Co., Ltd., trade name Carbotron) is used. The graphite-based carbon material whose surface is amorphous carbon is prepared by mixing natural graphite and coal tar pitch and firing at 1000 to 1500 ° C. It was confirmed by a transmission electron microscope (TEM) photograph, an X-ray diffraction method, a Raman spectroscopic analysis method and the like that the surface of the obtained carbon material was amorphized.

  In addition to the above-mentioned carbon material, PVDF as a binder is blended in the negative electrode mixture W4, and vapor-grown carbon fiber (trade name VGCF, manufactured by Showa Denko KK) or AB as a conductive material is blended as necessary. The When the negative electrode mixture W4 is applied to the rolled copper foil W3, the viscosity is adjusted with NMP as a dispersion solvent. The coating amount of the negative electrode mixture W4 is adjusted to be 1.1 times the capacity per unit area of the positive electrode plate. After drying, the negative electrode plate is pressed in the same manner as the positive electrode plate with a constant linear pressure, and cut to a width of 86 mm.

  Next, an example of the lithium ion secondary battery 20 produced by changing the average particle diameter and the blending ratio of two or more kinds of carbon materials blended in the negative electrode mixture W4 according to the present embodiment will be described. In addition, it describes together about the battery of the comparative example produced for the comparison.

Example 1
As shown in Table 1 below, in Example 1, the average particle diameter was 15 μm and the surface of the graphite-based carbon material whose surface was amorphous carbon (hereinafter referred to as amorphous surface-graphite carbon) was 70 parts by weight. 30 parts by weight of amorphous carbon having a particle diameter of 10 μm was mixed, and the PVDF solution was blended to 7 parts by weight in terms of solid content.

(Example 2 to Example 4)
As shown in Table 1, Examples 2 to 4 were the same as Example 1 except that the blending ratio of amorphous surface-graphite carbon and amorphous carbon was changed. In Example 2, 5 parts by weight of amorphous carbon is used for 95 parts by weight of amorphous surface graphite-based carbon. In Example 3, 50 parts by weight of amorphous carbon is used for 50 parts by weight of amorphous surface-graphite carbon. In Example 4, 90 parts by weight of amorphous carbon was used per 10 parts by weight of amorphous surface graphite-based carbon.

(Example 5 to Example 6)
As shown in Table 1, Examples 5 to 6 were the same as Example 1 except that the average particle diameters of amorphous surface graphite-based carbon and amorphous carbon were changed. In Example 5, the average particle diameter of amorphous surface graphite-based carbon was 10 μm and the average particle diameter of amorphous carbon was 15 μm. In Example 6, the average particle diameter of amorphous surface graphite-based carbon was 15 μm, The average particle diameter of the amorphous carbon was 15 μm.

(Example 7)
As shown in Table 1, in Example 7, 35 parts by weight of graphite-based carbon not having an amorphous surface with an average particle diameter of 15 μm, an average of 35 parts by weight of amorphous-surface graphite-based carbon with an average particle diameter of 15 μm, The procedure was the same as Example 1 except that 30 parts by weight of amorphous carbon having a particle diameter of 10 μm was blended.

(Comparative Example 1)
As shown in Table 1, Comparative Example 1 was the same as Example 1 except that only 100 parts by weight of amorphous surface graphite-based carbon was used.

(Comparative Example 2)
As shown in Table 1, Comparative Example 2 was the same as Example 1 except that 30 parts by weight of amorphous carbon was mixed with 70 parts by weight of graphite-based carbon having no amorphous surface.

[Test / Evaluation]
About the battery of each Example and the comparative example, 25 degreeC continuous discharge capacity, 0 degreeC division | segmentation discharge capacity, and -10 degreeC division | segmentation discharge capacity were measured as follows, respectively, and the temperature characteristic was evaluated. The continuous discharge capacity at 25 ° C is that each battery is charged at an ambient temperature of 25 ± 2 ° C with a current of 6A, a constant voltage of 4.15V and a state of charge (SOC) of 100% in 4 hours, and then continuously with a current of 6A. The discharge capacity when discharged to a final voltage of 3.0 V was measured. The divided discharge capacity is determined by charging each battery to an SOC of 100% in an ambient temperature of 25 ± 2 ° C at a current of 6A and a constant voltage of 4.15V for 4 hours, and then standing at 0 ° C and −10 ° C for 6 hours. Later, a pattern of discharging at 30 A for 5 minutes and resting for 1 hour was repeatedly performed, and the discharge capacity when 3.0 V was reached was measured. From the measurement results of the discharge capacity, a relative value with the 25 ° C. continuous discharge capacity of Comparative Example 1 as 100% was obtained to evaluate the temperature characteristics. Moreover, about the battery of each Example and the comparative example, the charging / discharging cycle characteristic was evaluated as follows. Each battery was charged at an ambient temperature of 50 ± 2 ° C at a current of 6 A, a constant voltage of 4.15 V, and SOC was charged to 100% in 4 hours, and then repeatedly discharged to a current of 12 A and a final voltage of 3.0 V, followed by 200 cycles. The discharge capacity at the time was measured. From the measurement results of the discharge capacity, the charge / discharge cycle characteristics were evaluated by obtaining a relative value with the 25 ° C. continuous discharge capacity of Comparative Example 1 as 100%. The results of temperature characteristics and charge / discharge cycle characteristics are shown in Table 2 below.

  The results of temperature characteristics and charge / discharge cycle characteristics shown in Table 2 are graphed and shown in FIG. As shown in FIG. 2, the battery of Comparative Example 1 in which the negative electrode material is only amorphous surface graphite-based carbon, and the battery of Comparative Example 2 in which graphite-based carbon having no amorphous surface and amorphous carbon are blended. Then, with respect to the 25 ° C continuous discharge capacity, the 0 ° C divided discharge capacity and the -10 ° C divided discharge capacity are greatly reduced. In addition, the charge / discharge cycle characteristics are greatly reduced. This is because, in the battery of Comparative Example 2, since the graphite-based carbon does not have amorphous carbon on the surface, the graphite-based carbon partially peels off due to the volume change accompanying charging / discharging, so the charge / discharge cycle characteristics are large. It is thought that it fell. In addition, the formation of a coating that accompanies the decomposition of the non-aqueous electrolyte on the surface of the graphite-based carbon hinders electronic conductivity, resulting in a large decrease in capacity, particularly in a low-temperature environment. It is considered a thing. In the battery of Comparative Example 1, the surface of the graphite-based carbon is amorphous carbon, but the graphite-based carbon is prevented from peeling off. However, the amorphous surface graphite-based carbon alone has electron conductivity in a low-temperature environment. It is considered that the low-temperature characteristics were greatly deteriorated because of insufficient. On the other hand, in the batteries of Examples 1 to 7 in which amorphous surface graphite-based carbon and amorphous carbon were blended, the decrease in 0 ° C. divided discharge capacity and −10 ° C. divided discharge capacity was suppressed. It can also be seen that the decrease in the capacity retention rate is suppressed in the charge / discharge cycle characteristics.

  As shown in Table 2 and FIG. 2, in the continuous discharge capacity at 25 ° C., 5 parts by weight of amorphous carbon was used for each capacity of the battery of Comparative Example 1 in which amorphous surface graphite carbon was 100 parts by weight. , 30 parts by weight, 50 parts by weight, and 90 parts by weight, the battery capacities of Example 2, Example 1, Example 3, and Example 4 were sequentially decreased. From this, it can be seen that the 25 ° C. continuous discharge capacity is reduced by increasing the proportion of amorphous carbon. This is because amorphous carbon has a lower true density than graphite-based carbon and is less likely to be crushed by the press, so that the electrode density does not increase. However, in the 0 ° C. divided discharge capacity and the −10 ° C. divided discharge capacity, the capacity of the battery of Comparative Example 1 is greatly reduced, whereas the capacity of the batteries of Examples 1 to 7 is the capacity. The decrease is suppressed. From this, it can be seen that the low temperature characteristics are improved by blending amorphous surface-graphite carbon and amorphous carbon. This is because the diffusion rate of lithium ions (diffusibility) in the non-aqueous electrolyte decreases in a low-temperature environment, but the electron conduction is amorphous on the surface of the graphite carbon when releasing lithium ions in the graphite carbon. It is considered that there is a function that tends to occur through carbon or amorphous carbon.

  In addition, in the battery of Example 2 in which the blending ratio of amorphous carbon was reduced to 5 parts by weight, the effect of suppressing the decrease in capacity under a low temperature environment was slightly reduced, and conversely, the blending ratio was increased to 90 parts by weight. In the battery of Example 4, the 25 ° C continuous discharge capacity is slightly reduced. For this reason, the blending ratio of amorphous carbon is preferably 5 to 90%, and more preferably 5 to 50%.

  Example 1 in which the amorphous carbon was made smaller than the amorphous surface graphite carbon, and the average particle size of the amorphous surface graphite carbon and the amorphous carbon was smaller than the amorphous surface graphite carbon. From the results of Example 5 which was increased and Example 6 which was the same, the average particle size of amorphous carbon was made smaller than that of amorphous surface graphite-based carbon, that is, the average particle size of amorphous surface graphite-based carbon. When 1.0 is set to 1.0, it can be seen that when the average particle diameter of amorphous carbon is less than 1.0, both temperature characteristics and charge / discharge cycle characteristics can be improved. This means that the amorphous carbon and the amorphous carbon on the surface of the graphite-based carbon can easily come into contact efficiently by making the average particle diameter of the amorphous carbon smaller than that of the amorphous-surface graphite-based carbon. It is thought that.

  Furthermore, even in the battery of Example 7 in which graphite-based carbon having no amorphous surface was blended, the results show that the temperature characteristics and charge / discharge cycle characteristics under a low temperature environment are improved as compared with the battery of Comparative Example 1. Therefore, even if other carbon materials are blended in addition to amorphous surface graphite-based carbon and amorphous carbon, two types of amorphous surface graphite-based carbon and amorphous carbon exist. It can be seen that there is an effect of improving the discharge performance in a low temperature environment.

  Moreover, in the charge / discharge cycle characteristics, the battery of each example in which the amorphous surface graphite-based carbon and the amorphous carbon were blended was the amorphous surface graphite-based carbon and the amorphous surface. Compared to the battery of Comparative Example 2 in which graphite-based carbon having no carbon and amorphous carbon were blended, the performance excellent in capacity retention was exhibited. This is also considered to be due to the fact that due to the excellent electron conductivity, the expansion and contraction of the electrode is suppressed, and the mixture (active material) peeling from the current collector hardly occurs.

  As described above, in the lithium ion secondary battery 20 of the present embodiment, two or more kinds of carbon materials are used for the negative electrode active material. Among these carbon materials, amorphous surface graphite carbon is used as one kind, and amorphous carbon is used as at least one kind. Since amorphous surface graphite-based carbon is used, graphite carbon does not come into direct contact with the non-aqueous electrolyte due to the amorphous carbon on the surface, so decomposition of the non-aqueous electrolyte by graphite-based carbon is suppressed. . For this reason, since the formation of the film which is generated by the decomposition of the non-aqueous electrolyte and inhibits the electron conduction is prevented, the decrease in the discharge capacity can be suppressed. Graphite carbon expands and contracts due to the insertion and release of lithium ions accompanying charge and discharge, resulting in a volume change. However, because the surface of the graphite carbon is amorphous carbon, the internal graphite carbon partially It is prevented from peeling off and falling off. Thereby, since the electronic conductivity of graphite-type carbon is maintained, the fall of cycling characteristics can be suppressed.

  In addition, diffusibility of lithium ions in the non-aqueous electrolyte is reduced in a low-temperature environment, but since amorphous surface graphite carbon and amorphous carbon are blended, lithium ions are released from graphite carbon. Occasionally, electron conduction occurs through amorphous carbon on the surface of the amorphous surface graphite-based carbon or amorphous carbon added separately. Thereby, the fall of a capacity | capacitance can be suppressed also in a low temperature environment. Therefore, the lithium ion secondary battery 20 of the present embodiment can secure a high capacity even in a low temperature environment, and can suppress a decrease in capacity maintenance rate due to a charge / discharge cycle.

  Furthermore, when the blending ratio of amorphous carbon in the negative electrode material is too small, the capacity under a low temperature environment decreases, and on the contrary, the continuous discharge capacity at 25 ° C. decreases. In the lithium ion secondary battery 20 of the present embodiment, since the blending ratio of amorphous carbon in the negative electrode material was adjusted to 5 to 90%, both the 25 ° C continuous discharge capacity and the capacity in a low temperature environment were used. The decrease can be suppressed. In addition, the amorphous surface graphite-based carbon prevents the graphite-based carbon inside from being peeled off, so that the electronic conductivity of the graphite-based carbon is maintained. be able to. By setting the blending ratio of amorphous carbon to 5 to 50%, it is possible to further suppress a decrease in capacity and charge / discharge cycle characteristics under a low temperature environment.

  Moreover, in the lithium ion secondary battery 20 of this embodiment, the average particle diameter of amorphous carbon is set smaller than the average particle diameter of amorphous surface graphite carbon. For this reason, amorphous carbon and amorphous surface graphite-based carbon can be efficiently contacted in the negative electrode mixture W4. As a result, when lithium ions are released from the graphite-based carbon inside the amorphous-surface graphite-based carbon, electron conduction occurs through the amorphous carbon on the surface and the amorphous carbon that is blended separately, The capacity can be ensured without lowering the electron conductivity.

  In the lithium ion secondary battery 20 of the present embodiment, an example in which natural graphite and coal tar pitch are mixed and fired for preparation of amorphous surface graphite-based carbon has been shown. It is not limited to the method for preparing the system carbon. As a method for obtaining a graphite-based carbon material having an amorphous surface, for example, a method in which a graphite-based carbon material is mixed with coal-based heavy oil, direct-current heavy oil, petroleum-based heavy oil, and the like and fired. And a method of mixing and baking with organic substances such as aromatic hydrocarbons, nitrogen-containing cyclic compounds, and sulfur-containing cyclic compounds. The graphite-based carbon material to be used is not limited to natural graphite, and may be any graphitized material such as artificial graphite and vapor-grown carbon fiber. Further, the shape of the carbon material is not particularly limited, such as a scale shape, a spherical shape, a fiber shape, or a lump shape.

  In the present embodiment, the lithium transition metal composite oxide of the positive electrode active material is exemplified by a composite oxide of lithium, manganese, cobalt, and nickel, but the present invention is not limited to this. The lithium transition metal composite oxide that can be used in other embodiments is a material capable of inserting and releasing lithium, and a sufficient amount of lithium may be inserted in advance. For example, a spinel crystal structure or a layered crystal Lithium manganese complex oxide having a structure, a material in which a part of manganese or lithium in a crystal is substituted or doped with other elements such as Fe, Co, Ni, Cr, Al, Mg, etc., oxygen in a crystal A material in which a part of is substituted or doped with an element such as S or P can be given. In addition to the above, the effect of the present invention is not changed even when a lithium manganese complex oxide capable of 5V class as the battery voltage is used.

Furthermore, in this embodiment, a solution obtained by dissolving 1 mol / liter of lithium hexafluorophosphate in a mixed solution of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 1: 1: 1 was used as the nonaqueous electrolytic solution. However, the battery of the present invention is not particularly limited, and a battery in which a general lithium salt is used as an electrolyte and this is dissolved in an organic solvent can be used. The lithium salt and organic solvent to be used are not particularly limited, and examples of the electrolyte include LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, and CF ₃ SO _3. Li or the like or a mixture thereof can be used. As the organic solvent, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether , Sulfolane, methylsulfolane, acetonitrile, propionitrile, etc., or a mixed solvent of two or more of these may be used, and the mixing ratio is not limited.

  Furthermore, as binders (binders) that can be used in other embodiments, polytetrafluoroethylene (PTFE), polyethylene, polystyrene, polybutadiene, butyl rubber, nitrile rubber, styrene / butadiene rubber, polysulfide rubber, nitrocellulose -Polymers such as sulfur, cyanoethyl cellulose, various latexes, acrylonitrile, vinyl fluoride, vinylidene fluoride, propylene fluoride, chloroprene fluoride, and mixtures thereof.

  Furthermore, in the present embodiment, the cylindrical lithium ion secondary battery 20 having the wound group 6 in which the positive and negative electrode plates are wound is illustrated, but the present invention is not limited to the shape and structure of the battery. . For example, it may be a rectangular shape or other polygonal shapes, and can be applied to a stacked type battery in which positive and negative electrode plates are stacked. Further, the battery size and the battery capacity are not particularly limited. Furthermore, as a battery structure to which the present invention can be applied, in addition to the structure of the bottomed cylindrical container (can) of the present embodiment in which the battery upper cover is sealed by caulking, for example, positive and negative external terminals penetrate the battery cover and the battery An example is a structure in which positive and negative external terminals are pressed against each other through a shaft core in the container.

  The present invention contributes to the manufacture and sale of lithium secondary batteries in order to provide a lithium secondary battery that can secure a high capacity and suppress the deterioration of cycle characteristics even in a low temperature environment. Have sex.

It is sectional drawing which shows the cylindrical lithium ion secondary battery of embodiment which can apply this invention. It is a graph which shows the evaluation result of the temperature characteristic and charging / discharging cycle characteristic of the lithium ion secondary battery of embodiment.

Explanation of symbols

W2 Positive electrode mixture W4 Negative electrode mixture 1 Axle core 6 Winding group 7 Battery container 20 Cylindrical lithium ion secondary battery (lithium secondary battery)

Claims (4)

  In the lithium secondary battery in which a positive electrode plate containing a lithium transition metal composite oxide and a negative electrode plate containing a negative electrode material capable of inserting and releasing lithium ions are infiltrated into a non-aqueous electrolyte, the negative electrode material includes two or more types The carbon material is used, and at least one of the carbon materials is a graphite-based carbon material whose surface is amorphous carbon, and at least one of the other carbon materials is an amorphous carbon material. Lithium secondary battery.   The lithium secondary battery according to claim 1, wherein a ratio of the amorphous carbon material in the negative electrode material is 5% to 90%.   The lithium secondary battery according to claim 2, wherein a ratio of the amorphous carbon material in the negative electrode material is 5% to 50%.   The average particle diameter of the amorphous carbon material is less than 1.0 when the average particle diameter of the graphite-based carbon material is 1.0. The lithium secondary battery according to claim 1.

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