JP2014107029A - Negative electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a high output durable nonaqueous electrolyte secondary battery.SOLUTION: In the negative electrode of a nonaqueous electrolyte secondary battery using graphite having an amorphous carbon coat in a negative electrode active material, the negative electrode active material has the DDTA sum of 5.6-13.2 μV in the range of 620-640°C of the differential curve (DDTA curve) of a DTA (Differential Thermal Analysis) curve, obtained when TG-DTA measurement is performed. The amorphous carbon coating amount is 4.0-7.5 pts.mass for 100 pts.mass of the negative electrode active material. A binder for binding the negative electrode active material includes styrene-butadiene rubber having a glass transition point of -40°C through -10°C.

Description

  The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  In the negative electrode of a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, particulate graphite has been widely used as the negative electrode active material. In order to suppress the reactivity with the electrolyte, an amorphous carbon film is provided on the surface of the particulate graphite.

For example, Patent Document 1 has a specific surface area of 200 to 200 when sintered on the surface of particulate graphite for the purpose of providing a nonaqueous electrolyte secondary battery excellent in adhesion and load characteristics of the negative electrode. A negative electrode active for a non-aqueous electrolyte secondary battery that is coated with a substance that becomes 500 m ² / g, amorphous carbon having a molecular weight of 300 to 500, and whose coating amount is 0.1 to 10% by mass with respect to the graphite. Materials and negative electrodes are disclosed.
When an amorphous carbon film is formed on the surface of particulate graphite, the reactivity with the electrolyte is suppressed, and the performance is improved as compared with particulate graphite alone.

JP 2009-21118A

  In the nonaqueous electrolyte secondary battery, it is desirable that the reaction resistance is low and the capacity retention rate is high. When the surface of the particulate graphite has an amorphous carbon film, the state of the carbon film is considered to affect the reaction resistance and capacity retention rate. Which factor of the carbon film is involved in the reaction resistance and capacity retention rate? Since it is unknown and it is difficult to evaluate the state itself, there is a possibility that the reaction resistance increases and the capacity retention rate decreases depending on the state of the carbon film.

  The present invention has been made in view of the above circumstances, and by reducing the reaction resistance of a non-aqueous electrolyte secondary battery and improving the capacity retention rate, the non-aqueous electrolyte secondary has high output and durability. The object is to provide a negative electrode for a battery.

  The negative electrode of the non-aqueous electrolyte secondary battery of the present invention is a negative electrode of a non-aqueous electrolyte secondary battery using graphite having an amorphous carbon film as the negative electrode active material. The negative electrode active material was subjected to TG-DTA measurement. When the obtained DTA (Differential Thermal Analysis) curve differential curve (DDTA curve) DDTA total value in the range of 620-640 ° C. is 5.6 to 13.2 μV, the amount of the amorphous carbon coating is the negative electrode The binder that binds the negative electrode active material with respect to 100 parts by mass of the active material contains styrene butadiene rubber having a glass transition point of −40 to −10 ° C.

  More preferably, the total DDTA value is 8.7 μV. The amount of the amorphous carbon film is more preferably 6.0 to 7.0 parts by mass with respect to 100 parts by mass of the negative electrode active material. The glass transition point is preferably -20 ° C.

The negative electrode active material has a tap density of 0.85 to 1.17 g / cm ³ , a Kr adsorption specific surface area of 3.43 to 4.45 m ² / g, and the graphite is natural graphite. preferable. More preferably, the tap density is 0.89 to 1.01 g / cm ³ .
The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, the negative electrode, and a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery is preferably a lithium ion secondary battery.

  ADVANTAGE OF THE INVENTION According to this invention, the high output and durable negative electrode for nonaqueous electrolyte secondary batteries is provided.

It is a graph showing the example of a measurement of a DTA curve and a DDTA curve. It is a graph which shows the relationship between a DDTA total value and -30 degreeC reaction resistance (mohm). It is a graph which shows the relationship between the coating amount of a negative electrode active material, and a capacity | capacitance maintenance factor. It is a graph which shows the relationship between the glass transition point of a binder material, and a capacity | capacitance maintenance factor. It is a graph which shows the relationship between the Kr adsorption specific surface area of a negative electrode active material, and a capacity | capacitance maintenance factor.

  The nonaqueous electrolyte secondary battery of the present embodiment includes a positive electrode, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte. The nonaqueous electrolyte secondary battery is preferably a lithium ion secondary battery. Hereinafter, although the structure in the case of manufacturing as a lithium ion secondary battery is shown below, this invention is not limited to this.

  Further, the negative electrode of the nonaqueous electrolyte secondary battery of the present embodiment is obtained by binding a negative electrode active material with a binder and laminating the negative electrode current collector. A carbon material is preferably used as the negative electrode active material because it can occlude and release lithium ions and the like.

  As the core material of the carbon material, it is preferable to use graphite, and it is more preferable to use natural graphite because the discharge potential is flat, the true density is high, and the filling property is good. Moreover, it is preferable to use particulate graphite as the shape of the graphite.

  Graphite is preferably coated with a carbon film, and the carbon film is an amorphous carbon film made of amorphous carbon (hereinafter sometimes referred to as an amorphous coating) in order to suppress reactivity with the electrolyte. Preferably there is. In the present invention, amorphous carbon refers to carbon other than graphite, and refers to carbon having no crystallinity or low crystallinity.

  The negative electrode active material is preferably amorphous coated with a predetermined amount of amorphous carbon coating (hereinafter sometimes referred to as coating amount). In the present specification, the coating amount is represented by the amount (%) occupied by the mass part of the coating with respect to 100 parts by mass of the negative electrode active material.

  When the coating amount is too small, the effect of suppressing the reactivity with the electrolyte is not sufficiently exhibited. On the other hand, if the amount of coating is excessive, the coating becomes too thick and the initial resistance may increase. The predetermined coating amount is preferably 4.0 to 8.0 parts by mass, more preferably 4.0 to 7.5 parts by mass with respect to 100 parts by mass of the negative electrode active material, and 6.0 It is especially preferable that it is -7.0 mass parts.

By setting the coating amount within such a range, the nonaqueous electrolyte secondary battery of the present embodiment can suppress lithium deposition on the negative electrode even when it is repeatedly used at an extremely low temperature. Or the maintenance rate of the capacity of the battery (hereinafter sometimes referred to as the capacity maintenance rate) is increased.
The capacity maintenance rate is a value obtained from the capacity of the battery after the initial charge and the capacity of the battery after repeated charge / discharge. A high capacity retention rate indicates a small decrease. It also indicates that the battery characteristics after repeated use are maintained.

  When the negative electrode active material is amorphous coated at a coating amount of 4.0 to 8.0 parts by mass with respect to 100 parts by mass of the negative electrode active material, the amorphous coat has a predetermined crystallinity lower than that of graphite. It is preferable to have. As the predetermined degree of crystallinity, the DDTA total value is preferably in a predetermined range.

When TG-DTA measurement is performed on a negative electrode active material using graphite having an amorphous carbon coating, a DTA (Differential Thermal Analysis) curve is obtained. The DDTA total value is a total value in a range of 620 to 640 ° C. of a differential curve (DDTA curve) obtained from the DTA curve.
The total DDTA value is preferably 5.6 to 13.2 μV, and particularly preferably 8.7 μV. When the coating amount and crystallinity of the amorphous coating are within such ranges, the reaction resistance of the nonaqueous electrolyte secondary battery including the negative electrode is reduced. The crystallinity will be described later.

  The binder for binding the negative electrode active material (hereinafter sometimes referred to as a binder) is styrene butadiene rubber (hereinafter sometimes referred to as SBR or styrene-butadiene copolymer latex) or modified styrene-butadiene copolymer. It is preferable to contain united latex, and it is particularly preferable to contain SBR.

SBR preferably has a glass transition point of −40 to −10 ° C., more preferably −20 ° C. The higher the glass transition point, the softer the SBR and the better the deformation at the time of molding, so that the binding property to the negative electrode active material and the negative electrode current collector is increased. On the other hand, if it is deformed too much, it covers the surface of the negative electrode active material and impairs the reactivity of the negative electrode surface.
By binding the negative electrode active material coated with the amorphous coating having the predetermined crystallinity by such a binder, the capacity retention rate is increased. In addition, the non-aqueous electrolyte secondary battery retains battery characteristics even after repeated use at low temperatures.

The negative electrode active material having the predetermined coating amount preferably has a predetermined spherical property. The degree of a predetermined sphericity, it is preferable that the tap density of 0.85~1.17g / cm ^3, particularly preferably 0.89~1.01g / cm ^3.
This is because when the coating amount and the tap density are within such ranges, the capacity retention rate is increased, and the battery characteristics of the nonaqueous electrolyte secondary battery of the present embodiment are maintained even after repeated use at low temperatures.

  In addition, when the coating amount and the tap density are in such ranges, the paste containing the negative electrode active material has preferable filter permeability. Therefore, the negative electrode manufactured by applying the paste to the negative electrode current collector, The production efficiency of the battery including the negative electrode is improved.

The negative electrode active material having the predetermined coating amount and having the predetermined sphericity preferably has a predetermined specific surface area. As the specific surface area of the negative electrode active material increases, the number of active points increases and the reaction area tends to increase. As the predetermined specific surface area, the krypton (Kr) adsorption specific surface area is preferably 3.43 to 4.45 m ² / g.
When the coating amount, tap density, and specific surface area are in such ranges, the non-aqueous electrolyte secondary battery of this embodiment has a high capacity retention rate, and battery characteristics are maintained even after repeated use at extremely low temperatures. It is.

<Crystallinity of negative electrode active material>
The crystallinity of the negative electrode active material according to the negative electrode of this embodiment will be described in more detail with a focus on TG-DTA measurement.
The negative electrode active material relating to the negative electrode of the present embodiment, which has desirable crystallinity, is evaluated and selected by differential thermal thermogravimetric simultaneous analysis (hereinafter sometimes abbreviated as TG-DTA) measurement. Can do. In the TG-DTA measurement, as the temperature rises, each particle of the negative electrode active material starts to burn from the surface side.

  Therefore, in the DTA curve, the first peak derived from the amorphous carbon film appears on the relatively low temperature side, and the second peak derived from the particulate graphite appears on the relatively high temperature side. The characteristic of the negative electrode active material concerning the negative electrode of this Embodiment is acquired by analyzing the 1st peak by the side of the low temperature which appears relatively small derived from an amorphous carbon film in a DTA curve.

  Here, since the amount of the amorphous carbon coating is much smaller than the particulate graphite, the first peak derived from the amorphous carbon coating is different from the second peak derived from the particulate graphite. It will be much smaller. In the DTA curve, a broad peak may appear in a relatively low temperature region of room temperature to about 500 ° C. due to impurities or the like, but this broad peak is ignored.

  The first peak on the relatively low temperature side derived from the amorphous carbon coating and the second peak on the relatively high temperature side derived from the particulate graphite usually appear at about 500 ° C. or more. As shown in FIG. 1, in the negative electrode active material of the present embodiment, the first peak on the relatively low temperature side derived from the amorphous carbon coating appears in the range of 620 to 640 ° C., for example, and the particulate graphite The second peak on the relatively high temperature side derived from is, for example, appears at 700 ° C. or higher.

  In order to obtain the DDTA total value, first, a temperature range of 20 ° C. is specified from the low temperature side to the high temperature side so as to include the maximum region of the DDTA curve corresponding to the first peak of the DTA curve. Next, the DDTA total value is obtained in this temperature range.

By performing TG-DTA on the negative electrode active material, a temperature change curve (DTA curve) of the DTA value (μV) is obtained. Furthermore, a time change curve (DDTA curve) of a DDTA value (μV / sec) that is a time differential value of the DTA value can be obtained from the DTA curve.
When the DDTA total value (μV) in the range of 620 to 640 ° C. at which the first peak on the relatively low temperature side derived from the amorphous carbon coating appears, this DDTA total value is the value of the nonaqueous electrolyte secondary battery. Correlates with reaction resistance. A preferable range of the DDTA total value is as described above. The DDTA total value is obtained by adding the DDTA values (μV / second) every second in the range of 620 to 640 ° C.

  There appears to be a correlation between the thermal stability and crystallinity of the amorphous carbon coating. The higher the crystallinity of the amorphous carbon film, the higher its thermal stability. In the DTA curve, the first peak on the relatively low temperature side derived from the amorphous carbon film appears on the higher temperature side, And the peak becomes sharper and the peak area tends to be larger.

  Further, the higher the crystallinity of the amorphous carbon film, the larger the DDTA total value. It is considered that the higher the DDTA combined value and the higher the crystallinity of the amorphous carbon film, the more improved the properties such as the conductivity of the film and the lower the reaction resistance.

<Sphericality of negative electrode active material>
The sphericity of the negative electrode active material according to the negative electrode of the present embodiment will be described in more detail. The negative electrode active material applied to the negative electrode of the present embodiment and having a desirable sphericity can be evaluated and selected by measuring tap density.

  The tap density can be obtained, for example, as a bulk density when a predetermined container is filled with a negative electrode active material and then tapped to reduce gaps between the negative electrode active materials to be densely filled with the negative electrode active material. The number of tappings may be any number as long as the tap density with a desired accuracy can be measured. For example, it can be selected between 250 and 1000 times.

<Specific surface area of negative electrode active material>
The specific surface area of the negative electrode active material according to the negative electrode of the present embodiment will be described in more detail. The negative electrode active material according to the present embodiment, which has a desirable specific surface area, can be evaluated and selected by a gas adsorption method. In the present specification, unless otherwise specified, the specific surface area represents the Kr adsorption specific surface area. The specific surface area can be determined as a BET specific surface area by Kr adsorption.

<Manufacture of negative electrode active material>
The negative electrode active material of the present embodiment is a carbon material such as pitch produced using oil such as heavy oil, polymer such as polyvinyl alcohol (PVA), coal or petroleum as a raw material on the surface of particulate graphite. And a coating material containing an additive such as a solvent, if necessary, can be produced by coating by a vapor phase method or a liquid phase method and firing in an inert atmosphere.

Examples of the vapor phase method include a CVD (Chemical Vapor Deposition) method. Examples of the inert atmosphere at the time of firing include an N ₂ atmosphere, a rare gas atmosphere such as Ar, and combinations thereof. By adjusting the manufacturing conditions including the firing profile, a negative electrode active material having a predetermined DDTA total value can be generated.

  As a baking profile, baking temperature can be 800-1000 degreeC and it is preferable to set it as 840-920 degreeC. For example, as shown in Examples and Table 1 described later, the baking temperature can be 800, 840, 880, 920, 960, or 1000 ° C.

<Preparation of coating amount of negative electrode active material>
In the present embodiment, the coating amount represents a part by mass of the coating film per 100 parts by mass of the negative electrode active material. A desired coating amount can be obtained by appropriately adjusting the amount of the coating material used when forming the coating.

  When a large amount of amorphous carbon is added to form a thick film, the amorphous carbon does not cover the graphite, and a lump is formed only with non-graphitic carbon. The battery characteristics may not be obtained. The gas phase method is preferred as the coating method because there is little risk of forming a lump consisting only of non-graphitic carbon and an amorphous coat can be formed uniformly.

<Manufacture of negative electrode>
The negative electrode of the present embodiment can be manufactured by applying the negative electrode active material for the nonaqueous electrolyte secondary battery to the negative electrode current collector. In the present specification, the negative electrode may refer to a negative electrode.
When applying the negative electrode active material to the negative electrode current collector, it is preferable to apply a paste or slurry that can be obtained by adding the negative electrode active material and the binder to a dispersant (solvent) and mixing them. It is preferable to further mix a thickener with the paste or slurry. After coating, it is preferable to dry and press.

  As the negative electrode current collector, a metal foil is preferable, and a copper foil is more preferable. The dispersant is preferably water. As the binder, SBR or polyvinylidene fluoride (PVDF) is preferable, and SBR is particularly preferable. The glass transition point of SBR that can be preferably used as a binder is as described above. As the thickener, carboxymethyl cellulose Na salt (CMC) is preferable.

  A negative electrode active material other than the above negative electrode active material can be used in combination with the negative electrode of the present embodiment. There is no restriction | limiting in particular as a negative electrode active material which can be used together, What has a lithium occlusion ability to 2.0V or less on the basis of Li / Li + is used preferably. As the negative electrode active material to be used in combination, lithium metal, lithium alloy, transition metal oxide / transition metal nitride / transition metal sulfide capable of doping / dedoping lithium ions, and combinations thereof are preferable. .

<Manufacture of positive electrode>
The positive electrode is preferably manufactured by applying a positive electrode active material to a positive electrode current collector. When applying the positive electrode active material to the positive electrode current collector, the paste or slurry that can be obtained by adding the positive electrode active material, the conductive material and the binder to the dispersant (solvent) and mixing them is applied. preferable. After coating, it is preferable to dry and press.

The positive electrode current collector is preferably a metal foil, and more preferably an aluminum foil. Examples of the positive electrode active material include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Co _(1-x) O ₂ , and LiNi _x Co _y Mn _(1-xy) O _2. A lithium-containing composite oxide is preferred. As the dispersant, N-methyl-2-pyrrolidone is preferable. Carbon powder is preferable as the conductive material. As the binder, polyvinylidene fluoride (PVDF) is preferable.

<Composition of non-aqueous electrolyte>
The non-aqueous electrolyte is preferably a liquid, gel or solid, and more preferably a non-aqueous electrolyte in which a lithium-containing electrolyte is dissolved in a mixed solvent of two or more carbonate solvents. The mixed solvent is preferably a mixture of a high dielectric constant carbonate solvent and a low viscosity carbonate solvent.

  As the high dielectric constant carbonate solvent, propylene carbonate, ethylene carbonate or a combination thereof is preferable. As the low viscosity carbonate solvent, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate or a combination thereof is preferable. As a combination of two or more carbonate solvents, ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is particularly preferable.

The lithium-containing electrolyte is preferably a lithium salt. LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), or LiPF _n {C _k F _{(2k + 1)} } _(6-n) (n is an integer of 1 to 5, k is an integer of 1 to 8), or a combination thereof is preferable.

<Composition of separator>
As the separator, a porous polymer film is preferable because it electrically insulates the positive electrode and the negative electrode and allows lithium ions to pass therethrough. The film is preferably a polyolefin porous film, particularly preferably a PP (polypropylene) porous film, a PE (polyethylene) porous film, or a laminated porous film of PP (polypropylene) -PE (polyethylene). .
<Case>
As the case, it is preferable to use a case suitable for the type of the secondary battery. As the secondary battery type, a cylindrical type, a coin type, a square type, or a film type is preferable.

  Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention.

1. Production of negative electrode active material <graphite>
In Examples and Comparative Examples, natural graphite having an average particle diameter (nominal diameter) of 10 μm was used as particulate graphite.

<Adjustment of coat amount>
The surface of the particulate graphite was coated with a pitch as a coating material by a CVD method, and fired in an N ₂ atmosphere to produce a negative electrode active material. In Examples 1 to 3 and 9 to 12 and Comparative Examples 1 to 3 and 26 to 45, the coating amount of the non-graphite carbon coating on the particulate graphite was 5% by mass. It is about 4.8% in terms of coat amount.

  The coating amounts of Examples 4 to 8 and Comparative Examples 4 to 25 were 2 to 9%, and are shown in Table 4 described later. The coating amounts of Examples 13 to 42 and Comparative Examples 46 to 116 were 2 to 10%, and are shown in Tables 6 to 9 described later.

<Baking profile>
The firing profile affects the total DDTA value. Coating was performed in the gas phase. The firing profiles according to Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1. Comparative Examples 4 to 12 and 26 to 33 are the same as Comparative Example 1. Examples 4 to 12 and Comparative Examples 13 to 16 and 34 to 37 are the same as Example 2. Comparative Examples 17 to 25 and 38 to 45 are the same as Comparative Example 2.

  In Examples 13 to 42 and Comparative Examples 46 to 116, the profile was fired at 880 ° C. as in Example 2.

2. Characterization of negative electrode active material <TG-DTA>
About the negative electrode active material of Examples 1-12 and Comparative Examples 1-45, TG-DTA measurement was performed on the following measurement conditions using the smart loader TG-DTA8120 by Rigaku Corporation.

Firing atmosphere: air (supply speed 650 ml / min)
Temperature increase rate: 20 ° C / min
Measurement temperature range: 30-1000 ° C

  From the differential curve (DDTA curve) of the obtained DTA curve, the DDTA total value in the range of 620 to 640 ° C. was determined as described above.

<Tap density>
About the negative electrode active material of Examples 13-42 and Comparative Examples 46-116, tap density was measured on the following measurement conditions, and the coated graphite was selected.
The tap density was the tap density when the number of taps n was 200 under the condition of a tap speed of 60 times / minute. Although it does not specifically limit as a tap density measuring apparatus, For example, the model "TPM-3" by a Tsutsui scientific chemical company, or its equivalent can be used.

<Filter permeability>
About Examples 13-25 and Comparative Examples 46-68, the following 3. According to the manufacture of the battery, a paste containing a negative electrode active material was prepared, and then a filter transmission test was performed. In the test, the filter permeability of each paste was measured under the following measurement conditions.

  300 ml of the paste was filtered with a filter having a pore size of 50 μm, and it was determined whether or not the entire amount could pass through the filter. As shown in the filter permeation test of Table 6 described later, the result was expressed as “◯” when the light was transmitted and “X” when it was not.

<Specific surface area>
About the negative electrode active material of Examples 26-42 and Comparative Examples 69-116, the specific surface area was measured on the following measurement conditions, and the coated graphite was selected.
Using the Kr adsorption method, the BET specific surface area of the negative electrode active material was measured as follows. As a sample, the negative electrode active material concerning the said Example and each comparative example was used. The sample was precisely weighed and then sealed in a test tube, and the BET specific surface area was measured by adsorption of krypton gas. The measurement conditions were as follows.

・ Adsorption gas: Kr
・ Dead volume: He
・ Adsorption temperature: Liquid nitrogen temperature (77K)
・ Measurement pretreatment: 200 ℃
Measurement mode: Isothermal adsorption Measurement range: Relative pressure (P / P0) = 0.01-0.4
P: Measurement pressure P0: Saturated vapor pressure and equilibrium time of adsorbed gas: 180 seconds for each equilibrium relative pressure

The specific surface area was calculated by applying the BET theory. The specific surface area was calculated by analyzing the relative pressure range of about 0.05 to 0.3 in the BET plot according to the same theoretical formula. The BET specific surface areas of Examples 1 to 25 and Comparative Examples 1 to 68 were 3.5 m ² / g. The BET specific surface areas of Examples 26 to 42 and Comparative Examples 69 to 116 were as shown in Tables 7 to 9 described later.

3. Battery manufacturing <Negative electrode>
In Examples and Comparative Examples, water was used as a dispersant, and the negative electrode active material, the binder, and the thickener were mixed at 98/1/1 (mass ratio) to obtain a paste. As the binder, a modified styrene-butadiene copolymer latex (SBR) having a glass transition point of −50 to −25 ° C. was used.

  The glass transition points of SBR used in Examples 9 to 12 and Comparative Examples 26 to 45 are as shown in Table 5 described later. The glass transition point of SBR used in other examples and comparative examples is −30 ° C. Carboxymethylcellulose Na salt (CMC) was used as the thickener. About Examples 13-25 and Comparative Examples 46-68, the filter permeation | transmission test of the paste was done. The determination results are as shown in Table 6.

The paste was applied to both sides of a copper foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer. A negative electrode was obtained as described above. The negative electrode layer had a basis weight of 18 mg / cm ² and a density of 1.4 g / cm ³ per side.

<Positive electrode>
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used as the positive electrode active material. Using N-methyl-2-pyrrolidone as a dispersant, mixing the above positive electrode active material, acetylene black as a conductive agent, and PVDF as a binder at 93/4/3 (mass ratio), An electrode layer forming paste was obtained.

The electrode layer forming paste was applied to both surfaces of an aluminum foil as a current collector by a doctor blade method, dried at 150 ° C. for 30 minutes, and pressed using a press machine to form an electrode layer. As described above, a positive electrode was obtained. The positive electrode layer had a basis weight of 30 mg / cm ² and a density of 2.8 g / cm ³ per side.

<Nonaqueous electrolyte>
A mixed solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) = 3/4/3 (volume ratio) was used as a solvent, and LiPF ₆ which is a lithium salt as an electrolyte was 1.1 mol / L. A non-aqueous electrolysis solution was prepared by dissolving at a concentration.

<Package>
The separator made of the positive electrode, the negative electrode, the non-aqueous electrolyte, and the PE (polyethylene) porous film was packaged in a cylindrical case to obtain a lithium ion secondary battery.

4). Evaluation of battery characteristics <Measurement of low-temperature reaction resistance>
The low temperature reaction resistance was measured as follows. First, the lithium secondary battery after the conditioning treatment was adjusted to a SOC (State of Charge) 60% charge state. Thereafter, the electrical resistance was measured by an AC impedance method at a frequency of 10 mHz to 1 MHz under a temperature condition of −30 ° C.

The relationship between the DDTA total value of the negative electrode active material and the −30 ° C. reaction resistance of the lithium ion secondary battery in Examples 1 to 3 and Comparative Examples 1 to 3 is shown in FIG. In FIG. 2, Examples 1 to 3 are surrounded by a broken line. As shown in FIG. 2 and Table 2, the lithium ion secondary batteries of Examples 1 to 3 in which the total DDTA value of the negative electrode active material is in the range of 5.6 to 13.2 μV are compared with Comparative Examples 1 to 3. The reaction resistance decreased and was below 460 mΩ.
From this, it was found that when the crystallinity of the amorphous coat is in such a range, the reaction resistance of the nonaqueous electrolyte secondary battery including the negative electrode is reduced, and the battery performance is improved.

<Measurement of capacity retention>
After the battery was manufactured, the battery was charged at a constant current up to 4.1 V at a charge rate of 1 C, and then subjected to a conditioning process by charging at a constant voltage for 2 hours. Thereafter, the battery was discharged to 3 V at a discharge rate of 0.3 C, and the discharge capacity at this time was defined as the initial battery capacity.

Then, after performing a charge / discharge test on each sample, the battery capacity after the test was obtained. Table 3 shows the number of each charge / discharge test, the temperature, the pulse, and the number of cycles in which the pulse was executed. The battery capacity after the test was determined from the discharge capacity at this time by setting each battery to SOC = 100% and then discharging to 3 V at a discharge rate of 0.3 C. And the capacity | capacitance maintenance factor (%) was calculated | required using the following formula.

Capacity maintenance rate (%) = (battery capacity after test / initial battery capacity) × 100
In general, if the capacity retention rate exceeds 98%, there is no visible Li deposition, and therefore it is estimated that the battery characteristics are maintained even after repeated use.

(1) Evaluation of influence of DDTA total value and coating amount of negative electrode active material For lithium ion secondary batteries of Examples 4 to 8 and Comparative Examples 4 to 25, the capacity was maintained after repeating charge / discharge in the cycle of test number 1. The rate was measured. FIG. 3 and Table 4 show the relationship between the total DDTA value and the coating amount of the negative electrode active material and the capacity retention rate of the lithium ion secondary battery. In FIG. 3, Examples 4 to 8 are surrounded by a broken line.

  As shown in FIG. 3 and Table 4, the lithium ion secondary batteries of Examples 4 to 8 in which the total DDTA value of the negative electrode active material is 8.7 μV and the coating amount is included in the range of 4.0 to 7.5%. The capacity retention rate was higher than that of the comparative example, which was 98% or more.

  Therefore, a battery in which the total value of DDTA and the coating amount of the negative electrode active material are in the above range has a low resistance on the negative electrode reaction surface, and the battery characteristics are maintained even after repeated use. It was found that the required level of durability could be secured.

(2) Evaluation of Influence of DDTA Total Value of Negative Electrode Active Material and Glass Transition Temperature of SBR After Lithium Ion Secondary Batteries of Examples 9-12 and Comparative Examples 26-45 were repeatedly charged and discharged in the cycle of test number 2 The capacity retention rate was measured. FIG. 4 and Table 5 show the relationship between the DDTA total value of the negative electrode active material and the glass transition point of SBR, and the capacity retention rate of the lithium ion secondary battery.

  In FIG. 4, Examples 9 to 12 are surrounded by a broken line. As shown in FIG. 4 and Table 5, the lithium ion secondary of Examples 9 to 12 in which the total DDTA value of the negative electrode active material is 8.7 μV and the glass transition point of SBR is in the range of −40 to −10 ° C. The battery had a capacity retention rate higher than that of the comparative example, which was 98% or more.

  From this, it was found that the battery having the DDTA combined value of the negative electrode active material and the glass transition point of SBR in the above range has a low resistance on the outermost surface of the negative electrode and retains the battery characteristics even after repeated use.

(3) Evaluation of influence of tap density and coating amount of negative electrode active material For lithium ion secondary batteries of Examples 13 to 25 and Comparative Examples 46 to 68, after repeating charge and discharge in the cycle of test number 3, the capacity retention rate Was measured. Test number 3 is a pulse test with a relatively long charge / discharge time. The test at 0 ° C. is a test under conditions where lithium ions are relatively easily diffused. Table 6 shows the relationship between the tap density and the coating amount of the negative electrode active material and the capacity retention rate of the lithium ion secondary battery.

As shown in Table 6, the lithium ions of Examples 13 to 25 in which the tap density of the negative electrode active material is 0.86 to 1.17 g / cm ³ and the coating amount is included in the range of 4.0 to 8.0%. The secondary battery had a capacity retention rate higher than that of the comparative example, which was 98% or more.

From this, the battery in which the tap density and the coating amount of the negative electrode active material are in the above range has a high ability to receive lithium ions of the negative electrode active material even under time conditions in which lithium ions diffuse from the positive electrode toward the negative electrode, It was also found that the battery characteristics were maintained even after repeated use.
Moreover, it was found that the paste having the tap density and the coating amount of the negative electrode active material within the above ranges has preferable filter permeability and can be uniformly applied to the negative electrode current collector.

(4) Evaluation of influence of tap density, coating amount and specific surface area of negative electrode active material For lithium ion secondary batteries of Examples 26 to 42 and Comparative Examples 69 to 116, charge and discharge were repeated in cycles of test numbers 4 to 6. Thereafter, the capacity retention rate was measured. Test numbers 4-6 are pulse tests with a charge / discharge time of 1-2 seconds. FIG. 5 and Tables 7 to 9 show the relationship between the tap density, the coating amount and the specific surface area of the negative electrode active material and the capacity retention rate of the lithium ion secondary battery. In FIG. 5, examples and comparative examples selected from those having a coating amount of 6% in Table 8 are typically represented by graphs, and Examples 33 to 38 are surrounded by a broken line.

As shown in Tables 7 to 9, the tap density of the negative electrode active material is 0.85 to 1.17 g / cm ³ , the coating amount is 4.0 to 8.0%, and the specific surface area is 3.43 to 4.45 m ^2. The lithium ion secondary batteries of Examples 26 to 42 included in the range of / g have a higher capacity retention rate than the comparative example, which is 98% or more.

  From this, it was found that the battery having the negative electrode active material tap density, coating amount, and specific surface area within the above ranges has a reaction area optimized and retains battery characteristics even after repeated use.

  The present invention is not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit of the present invention.

Claims (8)

In the negative electrode of a non-aqueous electrolyte secondary battery using graphite having an amorphous carbon film as the negative electrode active material,
When the negative electrode active material is subjected to TG-DTA measurement, the total value of DDTA in the range of 620 to 640 ° C. of the differential curve (DDTA curve) of the obtained DTA (Differential Thermal Analysis) curve is 5.6 to 13.2 μV. Yes,
The amount of the amorphous carbon coating is 4.0 to 7.5 parts by mass with respect to 100 parts by mass of the negative electrode active material,
The binder for binding the negative electrode active material is a negative electrode for a non-aqueous electrolyte secondary battery, which contains styrene butadiene rubber having a glass transition point of -40 to -10 ° C.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1, wherein the total DDTA value is 8.7 μV.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the amount of the amorphous carbon film is 6.0 to 7.0 parts by mass with respect to 100 parts by mass of the negative electrode active material.   The negative electrode of the nonaqueous electrolyte secondary battery according to claim 1, wherein the glass transition point is −20 ° C. The negative electrode active material is
The tap density is 0.85 to 1.17 g / cm ³ ;
The Kr adsorption specific surface area is 3.43 to 4.45 m ² / g,
The negative electrode according to claim 1, wherein the graphite is natural graphite. The negative electrode of the nonaqueous electrolyte secondary battery according to claim 5, wherein the tap density is 0.89 to 1.01 g / cm ³ .   A nonaqueous electrolyte secondary battery comprising a positive electrode, the negative electrode according to any one of claims 1 to 6, and a nonaqueous electrolyte.   The nonaqueous electrolyte secondary battery according to claim 7, which is a lithium ion secondary battery.

Images