JP2013131432A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a 1.5 V class nonaqueous electrolyte secondary battery using, as a positive electrode, lithium titanate excellent in overcharge and overdischarge characteristics.SOLUTION: The nonaqueous electrolyte secondary battery is formed by encapsulating a power generation element disposed so that a positive electrode and a negative electrode face each other through a separator in an outer sheath together with nonaqueous electrolyte. The positive electrode is composed of lithium titanate as an active material, conductive assistant and binder. As the binder, fluorine not-contained rubber or olefin resin such as styrene-butadiene rubber, ethylene methacrylic acid copolymer is used. The negative electrode is formed from a negative electrode in which lithium is previously occluded. Positive electrode capacity is smaller than negative electrode capacity and battery capacity is determined by the positive electrode.

Description

  The present invention relates to a 1.5 V class non-aqueous electrolyte secondary battery excellent in overdischarge characteristics and overcharge characteristics, wherein the positive electrode active material is lithium titanate.

  In recent years, due to a decrease in the driving voltage of SRAM and RTC, a secondary battery having an operating voltage of 1.5 V and excellent charge / discharge characteristics at a low temperature has been demanded. In response to this demand, conventional aqueous secondary batteries such as nickel metal hydride batteries cannot cope with such reasons as the solidification of the electrolyte in a low temperature environment, and nonaqueous electrolyte secondary batteries are promising. As a battery that can cope with this, non-aqueous electrolyte secondary batteries in which lithium titanate is combined in the positive electrode and a carbon material in the negative electrode have been studied.

  This non-aqueous electrolyte secondary battery using lithium titanate as the positive electrode is excellent in charge / discharge cycle performance, but it can be used in various applications where long-term reliability is required due to large deterioration during overdischarge and overcharge. It has become a challenge.

  In Patent Document 1, in a secondary battery including a positive electrode using lithium titanate as an active material and a negative electrode including a carbon material in which lithium is occluded, the positive electrode capacity is 1.05-1.20 times the negative electrode capacity. Thus, it has been proposed to suppress the decrease in the positive electrode potential and suppress the capacity deterioration during overdischarge or overcharge.

JP-A-11-260412

  However, according to the study by the present inventors, it has been found that the effect of the non-aqueous electrolyte secondary battery of Patent Document 1 is insufficient, and the capacity deteriorates significantly during overdischarge and overcharge. .

  An object of the present invention is to solve the above-mentioned problems and to provide a 1.5 V class non-aqueous electrolyte secondary battery using lithium titanate excellent in overcharge and overdischarge characteristics as a positive electrode.

  In order to achieve the above object, the present invention provides a non-aqueous electrolyte secondary battery in which a power generation element in which a positive electrode and a negative electrode are opposed to each other via a separator is enclosed in a package together with a non-aqueous electrolyte. It consists of lithium titanate as an active material, a conductive additive, and a binder. As the binder, styrene-butadiene rubber, ethylene-methacrylic acid copolymer-free rubber or olefin resin is used, and the negative electrode Is a non-aqueous electrolyte secondary battery comprising a negative electrode in which lithium is previously occluded, a positive electrode capacity being smaller than a negative electrode capacity, and a battery capacity being determined by the positive electrode.

  According to the present invention, a 1.5 V class non-aqueous electrolyte secondary battery that can remarkably improve the overcharge / overdischarge characteristics of a battery using a lithium titanate positive electrode and can be used for various applications is provided. Can be obtained.

Sectional drawing of the nonaqueous electrolyte secondary battery in one embodiment of this invention

  According to a first aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery in which a power generation element in which a positive electrode and a negative electrode are opposed to each other via a separator is enclosed in a package together with a non-aqueous electrolyte, wherein the positive electrode is an active material As the binder, a fluorine-free rubber or olefin resin such as styrene butadiene rubber or ethylene / methacrylic acid copolymer is used as the binder. A nonaqueous electrolyte secondary battery comprising a negative electrode in which lithium is occluded, a positive electrode capacity being smaller than a negative electrode capacity, and a battery capacity being determined by the positive electrode.

  By adopting this configuration, the overcharge / overdischarge characteristics of the battery using the lithium titanate positive electrode can be remarkably improved, and it can be developed for various uses that require long-term reliability.

  The present inventors have confirmed that the type of binder mixed with lithium titanate greatly affects the characteristics of the positive electrode. As the binder material, as described in Patent Document 1, PTFE, which is a fluororesin, is mainly used. This fluororesin has a reaction potential with lithium that is equal to or lower than that of lithium titanate, which is a positive electrode active material. In addition, the fluorine-based resin reacts with lithium to become lithium fluoride and a carbon material, and expands greatly. When lithium is inserted into the positive electrode by discharge, it reacts with not only the active material lithium titanate but also the fluorine resin of the binder, so that the positive electrode is significantly expanded and lithium is consumed. Decrease and charge / discharge cycle performance will decrease. Although lithium titanate itself has little volume expansion and is an active material that consumes lithium and has a small irreversible reaction, it does not take advantage of its characteristics.

  Therefore, the positive electrode capacity is made larger than the negative electrode capacity (1.05 to 1.20 times), and the reaction between the binder and lithium can be suppressed without lowering the potential of the positive electrode. As a result, the irreversible capacity is reduced, the expansion of the electrode itself is suppressed, and the charge / discharge cycle characteristics are improved. As a problem, since 5 to 20% of the positive electrode capacity is not used, energy efficiency per volume is deteriorated. In addition, even if the capacity is restricted, the deterioration of the overdischarge characteristics and the continuous charge characteristics at high temperatures has progressed.

  In the present invention, the positive electrode capacity can be fully utilized effectively, and the negative electrode potential can be designed not to be increased by lowering the positive electrode potential, so that deterioration of overdischarge characteristics and continuous charge characteristics at high temperatures is suppressed. In addition, energy efficiency per volume is improved.

  A second invention in the present invention is a non-aqueous electrolyte secondary battery, wherein the negative electrode is made of a composite active material of Si and graphite. For stabilizing the potential of the negative electrode, it is preferable to use a composite active material rather than a single active material. Si and graphite have portions where the reaction potential of lithium is the same and different. For this reason, even in a battery having a large positive electrode capacity according to the present invention, the potential stability of the negative electrode is improved, and deterioration of overdischarge characteristics and continuous charge characteristics at high temperatures can be suppressed.

  According to a third aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery characterized in that Si is larger in the electric capacity ratio of Si and graphite of the negative electrode. The reaction potential of graphite is often flat, but in the case of Si, the reaction potential is inclined. Therefore, Si has higher potential stability than graphite, and it improves overdischarge characteristics and continuous charge characteristics at high temperatures. To do.

  In particular, when Si comprising an amorphous phase and a crystalline alloy phase having a large reaction area is used, the effect is great. Further, the ratio of Si to graphite is preferably in the range of 60:40 to 90:10 by mass ratio, and it is preferable to increase the ratio of Si.

  The mechanical alloying method for synthesizing Si consisting of an amorphous phase and a crystalline alloy phase involves mechanically stirring and mixing the raw material mixture using a ball mill, and applying energy to the raw material mixture to produce the alloy powder by solid-phase reaction. It is a manufacturing method. Examples of the ball mill used in the mechanical alloying method include a rolling ball mill, a vibration ball mill, and a planetary ball mill.

  The Si amorphous phase obtained by the mechanical alloying method has no diffraction peak on the Si (111) plane in the X-ray diffraction image obtained by the wide-angle X-ray diffraction method, and the maximum crystallite size is 200 nm. It is as follows.

Ti, Co, Ni, Cu, Mg, Zr, V, Mo, W, Mn, and Fe can be used as the metal that can be alloyed with Si. From the viewpoint of electrical conductivity of the negative electrode, a crystalline alloy phase of Si and Ti having high electron conductivity is preferable, and an intermetallic compound phase represented by the composition formula TiSi ₂ is particularly preferable.

  The mass ratio of Si to the metal element M to be alloyed is preferably 10:90 to 40:60. For the ternary alloy, the mass ratio of Si, the alloying element M1 and the alloying element M2 is 10:90 (the mass ratio of M1 and M2 is arbitrary) to 40:60 (the mass ratio of M1 and M2) Is optional).

  Hereinafter, preferred embodiments of the present invention will be described. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.

  FIG. 1 is a cross-sectional view of a coin-type lithium secondary battery which is an example of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention.

  The coin-type battery case container that houses the power generation element has a function of insulating the positive electrode can 1 made of stainless steel with excellent corrosion resistance, the stainless steel negative electrode can 2, and the positive electrode can 1 and the negative electrode can 2. In addition, it has a gasket 3 for physically sealing the power generation element in the battery container.

  For the gasket 3 interposed between the positive electrode can 1 and the negative electrode can 2, a material made of engineering plastics such as polypropylene resin, polyphenylene sulfide, polyether ether ketone, or fluororesin can be used. A sealant (not shown) is formed between the gasket 3 and the positive electrode can 1 and the negative electrode can 2.

  As the sealant, for example, a sealant made of a butyl rubber film can be formed by applying a solution obtained by diluting butyl rubber with toluene and evaporating the toluene.

  The positive electrode 4 is obtained by molding a mixture powder obtained by adding a conductive agent and a binder to lithium titanate as an active material, adding pure water, performing wet mixing, and drying.

  The conductive agent for the positive electrode 4 is preferably at least one selected from the group consisting of carbon black, acetylene black, and denka black. In addition, natural graphite, artificial graphite and the like can be mixed and used. As a compounding quantity of a electrically conductive agent, it is the range of 3-10 mass%.

As the binder of the positive electrode 4, a fluorine-free rubber or olefin resin is used. For example, nitrile butadiene rubber, methacrylate / butadiene rubber, styrene butadiene rubber is used as rubber, and ethylene / methacrylic acid copolymer is used as olefin resin. be able to.

  As the negative electrode 5, a carbon material such as natural graphite, artificial graphite, or non-graphitizable carbon is used as an active material, carbon black is used as a conductive agent, and a binder or polyvinylidene fluoride (PVDF) is used as a binder. Examples thereof include an alloy and a sheet-like Li-Si alloy. In particular, Si is composed of an amorphous phase of Si, a crystalline alloy phase of Si and Ti, graphite as an active material, carbon black as a conductive agent, and non-crosslinked polyacrylic acid as a binder, followed by drying. It is preferable to use a mixture powder obtained by molding.

  As this graphite, natural graphite, artificial graphite or the like can be used.

  A separator 6 disposed between the positive electrode 4 and the negative electrode 5 is filled with a non-aqueous electrolyte (not shown). The positive electrode current collector 7 is formed by applying a carbon paint on the inner surface of the positive electrode can 1.

  Examples of the material of the separator 6 such as a nonwoven fabric and a film that are porous insulators include olefin polymers such as polypropylene and polyethylene, engineering plastics such as polybutylene terephthalate, polyphenylene sulfide, and polyether ether ketone, and inorganic glass fibers. Etc. can be used.

Solutes constituting the non-aqueous electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN ( A single component such as CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) or a mixture of a plurality of components can be used.

  Further, as a solvent constituting the non-aqueous electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolane, dimethoxyethane, diethoxyethane, tetrahydrofuran, dioxolane, γ-butyrolactone However, the present invention is not limited to this.

  1 to 10% by mass of ethylene sulfide, 1,3 propane sultone, 1,4 butane sultone, sulfolene, vinylene carbonate and vinyl ethylene carbonate can be added to the organic electrolyte.

  Hereinafter, preferred embodiments of the present invention will be described.

Example 1
(Production of Inventive Battery 1)
FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery having a thickness of 1.4 mm and a diameter of 6.8 mm used in an example of the present invention.

The positive electrode 4 is composed of lithium titanate obtained by firing a mixture of lithium hydroxide and anatase-type titanium dioxide at 850 ° C. for 10 hours as an active material, carbon black having a specific surface area of 800 m ² / g as a conductive agent, A pure water was mixed with an aqueous binder in which an ethylene / methacrylic acid copolymer having a pH of 10 was dispersed, and then water was dried to obtain a positive electrode mixture. As the positive electrode mixture, an active material, a conductive agent, and a binder were mixed at a mass ratio of 85: 7: 8. This mixture was molded into a pellet having a diameter of 4 mm and a thickness of 0.7 mm, and then dried at 150 ° C. for 12 hours. The obtained pellet-like positive electrode material is in contact with the positive electrode current collector 7 formed by applying a carbon paint on the inner surface of the positive electrode can 1.

The negative electrode 5 is an aqueous binder composed of natural graphite having a (002) plane lattice constant of 3.35 as an active material, carbon black having a specific surface area of 800 m ² / g as a conductive material, and polyacrylic acid having a pH of 2. After adding pure water and wet mixing, the water was dried to obtain a negative electrode mixture. In the mixture composition, the mass ratio of carbon black: polyacrylic acid in the active material: conductive agent is 90: 5: 5. The negative electrode mixture was molded into a pellet shape having a diameter of 4 mm and a thickness of 0.3 mm, and then dried at 150 ° C. for 12 hours to obtain a negative electrode 5.

A lithium metal sheet was pressure-bonded to the surface of the negative electrode sealing plate. When the battery is assembled, by injecting a non-aqueous electrolyte, lithium and the negative electrode 5 are short-circuited, and lithium is occluded electrochemically in the natural graphite of the negative electrode 5 to form an intercalation compound represented by C ₆ Li. Obtained. The outer diameter and thickness of the lithium metal sheet were set so as to have a capacity corresponding to the theoretical capacity in which C ₆ Li was formed.

  The separator 6 disposed between the positive electrode 4 and the negative electrode 5 is made of a polypropylene film and a polypropylene non-woven fabric. The polypropylene non-woven fabric is placed on the negative electrode 5 side. Was arranged so as to face the positive electrode 4 side.

1 mol of LiN (CF ₃ SO ₂ ) ₂ was dissolved as a solute in a mixed solvent having a volume ratio of propylene carbonate (PC), ethylene carbonate (EC) and 1,2 · dimethoxyethane (DME) of 3: 2: 5. A non-aqueous electrolyte was used. This non-aqueous electrolyte is impregnated in a separator 6 in a battery container composed of a positive electrode can 1, a negative electrode can 2 and a gasket 3. The capacity ratio of positive electrode capacity / negative electrode capacity is 0.95.

  The non-aqueous electrolyte secondary battery thus obtained was designated as an inventive battery 1 according to Example 1.

(Production of Inventive Battery 2)
Inventive battery 2 having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of inventive battery 1 was 0.90.

(Production of Inventive Battery 3)
Inventive battery 3 having the same configuration was produced except that the capacity ratio of positive electrode capacity / negative electrode capacity of inventive battery 1 was changed to 0.85.

(Production of Inventive Battery 4)
Inventive battery 4 having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of inventive battery 1 was 0.80.

(Production of Inventive Battery 5)
Inventive battery 5 having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of inventive battery 1 was changed to 0.75.

(Production of Invention Battery 6)
Inventive battery 6 having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of inventive battery 1 was 0.70.

(Production of Inventive Battery 7)
Inventive battery 7 having the same configuration was prepared except that the negative electrode active material of inventive battery 1 was changed to Si.

Here, Si of the negative electrode active material is composed of an amorphous phase of Si and a crystalline alloy phase of Si and Ti. For the Ti—Si alloy containing amorphous Si, a Si wafer doped with 1 × 10 ¹⁸ P atoms per cm ³ of Si by a mother alloy doping method was crushed with a mortar to obtain a powder having an average particle diameter of 1 mm. Further, a Si wafer doped with 1 × 10 ¹⁸ B atoms per 1 cm ³ of Si by a mother alloy doping method was crushed in a mortar to obtain a powder having an average particle diameter of 1 mm.

  1.5 kg of this N-type semiconductor and P-type semiconductor Si powder mixed at a mass ratio of 10:90, 1 kg of Ti powder with an average particle size of 0.5 mm, and 300 kg of 1 inch stainless steel balls, The container was put in a container of a stainless steel vibrating ball mill (product code: FV-30, manufactured by Chuo Kaoki Co., Ltd.) having an internal volume of 95 liters and covered.

  The inside of the container was depressurized, and Ar gas was introduced until the inside of the container reached 1 atm. Next, the amplitude of the vibration ball mill was set to 8 mm and the rotational speed of the drive motor was set to 1200 rpm, respectively, and mechanical alloying was performed for 20 hours to produce a Ti 37 wt% -Si 63 wt% alloy powder used as a negative electrode active material.

  Using a wide angle X-ray diffractometer (product code: RINT-2500, manufactured by Rigaku Corporation) using a CuKα ray having a wavelength of 1.5405 mm as a radiation source, the diffraction intensity in a range of diffraction angle 2θ = 10 ° to 80 ° is measured. did. When the presence or absence of a peak near the diffraction angle attributed to the (111) plane of Si was examined, no peak was present.

  Moreover, when the obtained alloy powder was observed using TEM (transmission electron microscope), the maximum crystallite size was 40 nm and the average crystallite size was 10 nm. An active material comprising an amorphous phase of Si and an alloy phase of Ti and Si was obtained.

(Production of comparative battery A)
Comparative battery A having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of Invention Battery 1 was 1.05.

(Production of comparative battery B)
A comparative battery B having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of the inventive battery 1 was 1.10.

(Production of comparative battery C)
A comparative battery C having the same configuration was prepared except that the capacity ratio of positive electrode capacity / negative electrode capacity of the inventive battery 1 was 1.20.

(Production of comparative battery D)
A comparative battery D having the same structure was prepared except that polytetrafluoroethylene (PTFE) was used as the binder of the positive electrode of the inventive battery 1.

(Production of comparative battery E)
A comparative battery E having the same structure was prepared except that tetrafluoroethylene / hexafluoropropylene copolymer (FEP) was used as the binder of the positive electrode of the inventive battery 1.

The initial charge / discharge capacities of the inventive batteries 1 to 7 and the comparative batteries A to E were confirmed. First, a first cycle discharge was performed to 0.8 V with a constant current of 0.1 mA. Subsequently, as the second cycle, the battery was charged to 2.0 V with a constant current of 0.1 mA and then discharged to 0.5 V. 2 thus obtained
The discharge capacity at the cycle was set to 100 as the relative value of the initial charge / discharge capacity of the inventive battery 1. Moreover, about the capacity | capacitance of invention battery 2-7 and comparative battery AE, initial stage charge / discharge capacity was calculated | required as a relative value on the basis of invention battery 1. FIG.

  The charge / discharge cycle test was performed 100 times under the same conditions as the initial discharge capacity confirmation, and the discharge capacity was confirmed.

  Moreover, after discharging to 0V with a constant current of 0.1 mA to make it an overdischarged state, a 30 KΩ discharge resistor was connected and stored in an environmental tank at 60 ° C. in a completely discharged state. After 100 days, remove the discharge resistance from the environmental tank to room temperature, charge to 2.0 V with a constant current of 0.1 mA, discharge to 0.5 V with a constant current of 0.1 mA, and overdischarge. The subsequent discharge capacity was measured.

  In the overcharge evaluation, after the initial capacity was confirmed, the battery was discharged to 2.0 V at a constant current of 0.1 mA, and then stored in a state where a voltage of 2.0 V was continuously applied in a 60 ° C. environmental tank. After 100 days, the battery was taken out of the environmental tank, discharged to 0.5 V with a constant current of 0.1 mA, and the discharge capacity after overcharging was measured.

  Table 1 shows the results of the initial charge / discharge capacity, the discharge capacity after the charge / discharge cycle, the discharge capacity after overdischarge, and the discharge capacity after overcharge. The initial charge / discharge capacity was set to 100, and the relative value was shown.

  For the inventive batteries 1 to 7 using an ethylene / methacrylic acid copolymer containing no fluorine as the binder of the positive electrode and having a positive electrode capacity smaller than the negative electrode capacity, the charge / discharge cycle is suppressed by suppressing the deterioration of characteristics due to the positive electrode. Thereafter, a high discharge capacity could be maintained after 60 ° C. overdischarge and after overcharge.

  As a positive electrode binder, an ethylene / methacrylic acid copolymer that does not contain fluorine is used. For comparative batteries A to C having a positive electrode capacity larger than the negative electrode capacity, the deterioration of the characteristics due to the positive electrode is reduced. Thus, after the charge / discharge cycle, the discharge capacity decreased after overdischarge at 60 ° C. and after overcharge.

  As the positive electrode binder, polytetrafluoroethylene / tetrafluoroethylene / tetrafluoroethylene / hexafluoropropylene copolymer containing fluorine is used, and the comparative batteries D and E whose positive electrode capacity is smaller than the negative electrode capacity are caused by the positive electrode binder. As a result of the deterioration, the discharge capacity significantly decreased after the overdischarge at 60 ° C. and after the overcharge after the charge / discharge cycle.

(Example 2)
(Production of Invention Battery 8)
Inventive battery 8 having the same structure was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 7: 3.

  The inventive battery 8 was subjected to charge / discharge cycles, overdischarge and overcharge tests in the same manner as in Example 1. The results are shown in Table 2.

  In invention battery 8 using a composite active material for the negative electrode, a slight improvement in characteristics was observed.

(Example 3)
(Production of Invention Battery 9)
Inventive battery 9 having the same structure was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 6: 4.

(Production of Invention Battery 10)
Inventive battery 10 having the same configuration was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 5: 5.

(Production of Invention Battery 11)
Inventive battery 11 having the same structure was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 4: 6.

(Production of Invention Battery 12)
Inventive battery 12 having the same structure was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 3: 7.

(Production of Invention Battery 13)
Inventive battery 13 having the same configuration was prepared except for the composite active material blended in the negative electrode of inventive battery 1 so that the capacity ratio of graphite and Si was 1: 9.

  Inventive batteries 9 to 13 were subjected to charge / discharge cycles, overdischarge and overcharge tests in the same manner as in Example 1. The results are shown in Table 3.

  As the capacity ratio between Si and graphite of the negative electrode was increased, the characteristics were further improved.

  By providing a 1.5V class non-aqueous electrolyte secondary battery with excellent charge / discharge cycle performance, overcharge, and overdischarge characteristics, it can be used as a power source for various devices used over a long period of time. The industrial utility value is very high.

DESCRIPTION OF SYMBOLS 1 Positive electrode can 2 Negative electrode can 3 Gasket 4 Positive electrode 5 Negative electrode 6 Separator 7 Positive electrode collector

Claims (3)

In a non-aqueous electrolyte secondary battery in which a power generation element in which a positive electrode and a negative electrode are opposed to each other via a separator is enclosed in a package together with a non-aqueous electrolyte, the positive electrode includes lithium titanate as an active material, and a conductive assistant. It comprises an agent and a binder, and as the binder, a fluorine-free rubber such as styrene butadiene rubber or ethylene / methacrylic acid copolymer or an olefin-based resin is used, and the negative electrode includes a negative electrode in which lithium is previously occluded, A non-aqueous electrolyte secondary battery, wherein the positive electrode capacity is smaller than the negative electrode capacity, and the battery capacity is determined by the positive electrode. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is made of a composite active material of Si and graphite. The nonaqueous electrolyte secondary battery according to claim 2, wherein Si is larger in an electric capacity ratio of Si and graphite of the negative electrode.