JP2017142932A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which enables the achievement of both of high-rate endurance and high-temperature preservation characteristic.SOLUTION: A nonaqueous electrolyte secondary battery 100 provided according to the invention comprises a negative electrode active material layer including a graphite-based negative electrode active material and carbon particles. The content proportion (Graphite-based negative electrode active material:Carbon particles) of the graphite-based negative electrode active material to the carbon particles is 95:5 to 80:20 on a mass basis. The BET specific surface area A and the volume density B of the graphite-based negative electrode active material, the BET specific surface area C and the volume density D of the carbon particles, and the BET specific surface area weighted average efficiency E of the graphite-based negative electrode active material and the carbon particles satisfy the following conditions: 2 m/g≤A≤6 m/g; 0.44 g/cm≤B≤0.7 g/cm; 10 m/g≤C≤30 m/g; 0.05 g/cm≤D≤0.32 g/cm; and 4.8 m/g≤E≤7.9 m/g. In the nonaqueous electrolyte secondary battery, 3-6 pts.mass of SBR to a total mass of 100 pts.mass of the graphite-based negative electrode active material and the carbon particles is included.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  In recent years, lithium ion secondary batteries, nickel metal hydride batteries, and other nonaqueous electrolyte secondary batteries have become increasingly important as power sources for mounting on vehicles or as power sources for personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source mounted on a vehicle. In a lithium ion secondary battery, charging and discharging are performed by Li ions traveling between a positive electrode and a negative electrode.

  A typical negative electrode of this type of nonaqueous electrolyte secondary battery has a negative electrode active material layer containing a negative electrode active material capable of reversibly occluding and releasing charge carriers (lithium ions in the case of a lithium ion secondary battery). It has a structure held on the negative electrode current collector. Examples of the negative electrode active material mainly include various carbon materials, and for example, graphite particles are used. Patent document 1 is mentioned as a prior art regarding this kind of secondary battery.

International Publication No. 2013/002162

In Patent Document 1, scaly graphite particles and the surface of the graphite particles are coated with a coating layer containing amorphous carbon particles and an amorphous carbon layer, and the specific surface area is 4 to 6 m ² / g. It has been proposed to use a negative electrode active material mixed with coated graphite particles. According to the document, such a configuration can realize a non-aqueous electrolyte secondary battery excellent in capacity retention ratio and high IV characteristics after high rate cycle (that is, having high rate durability). However, according to the inventor's knowledge, even with such a technique, the capacity retention rate after high-temperature storage may decrease due to a side reaction between the coated graphite particle surface and the electrolytic solution, and there is room for further improvement. is there.

  This invention is made | formed in view of the said situation, The main objective is to provide the non-aqueous-electrolyte secondary battery which can make high-rate durability and high temperature storage characteristics compatible.

A nonaqueous electrolyte secondary battery provided by the present invention is a nonaqueous electrolyte secondary battery comprising a negative electrode in which a negative electrode active material layer containing a graphite-based negative electrode active material is formed on a negative electrode current collector, and a positive electrode. It is. The negative electrode active material layer further contains carbon particles. The ratio of the content of the graphite-based negative electrode active material and the carbon particles (graphite-based negative electrode active material: carbon particles) is 95: 5 to 80:20 on a mass basis. The BET specific surface area of the graphite-based negative electrode active material is A, the bulk density of the graphite-based negative electrode active material is B, the BET specific surface area of the carbon particles is C, the bulk density of the carbon particles is D, and the graphite When the weighted average value of the BET specific surface area A of the system negative electrode active material and the BET specific surface area C of the carbon particles is E, the following relationship:
2 m ² / g ≦ A ≦ 6 m ² / g
0.44 g / cm ³ ≦ B ≦ 0.7 g / cm ³
10 m ² / g ≦ C ≦ 30 m ² / g
0.05 g / cm ³ ≦ D ≦ 0.32 g / cm ³
4.8m ² /g≦E≦7.9m ² / g
Meet. And 3 mass parts-6 mass parts of styrene butadiene rubber are included with respect to 100 mass parts of total mass of the said graphite-type negative electrode active material and the said carbon particle.

  According to such a configuration, it is possible to realize a high-performance nonaqueous electrolyte secondary battery that can suppress an increase in resistance after a high-rate cycle and that can maintain a high battery capacity even after high-temperature storage.

FIG. 1 is a cross-sectional view schematically showing a secondary battery according to an embodiment. FIG. 2 is a diagram schematically illustrating a wound electrode body according to an embodiment. FIG. 3 is a graph showing a charge / discharge cycle in a high-rate pulse cycle test.

  Hereinafter, a non-aqueous electrolyte secondary battery according to an embodiment of the present invention will be described. The embodiments described herein are, of course, not intended to limit the present invention in particular. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. Further, members / parts having the same action are denoted by the same reference numerals, and redundant description is omitted or simplified.

  In the following, an example of a lithium ion secondary battery in which a wound type electrode body (hereinafter referred to as “wound electrode body”) and a non-aqueous electrolyte are accommodated in a rectangular (here, rectangular box shape) case is shown as an example. To The battery structure is not limited to the illustrated example, and is not particularly limited to a prismatic battery.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 20 and a battery case 30. FIG. 2 is a view showing the wound electrode body 20. As shown in FIGS. 1 and 2, a lithium ion secondary battery 100 according to an embodiment of the present invention includes a flat wound electrode body 20 having a flat rectangular shape together with a liquid electrolyte (electrolytic solution) (not shown). The battery case (that is, the exterior container) 30 is accommodated.

  The battery case 30 has a box-shaped (that is, bottomed rectangular parallelepiped) case body 32 having an opening at one end (corresponding to the upper end in a normal use state of the battery), and the opening attached to the opening. It is comprised from the sealing board (lid body) 34 which consists of a rectangular-shaped plate member which plugs up a part. The material of the battery case 30 is exemplified by aluminum, for example. As shown in FIG. 1, a positive terminal 42 and a negative terminal 44 for external connection are formed on the sealing plate 34. A thin safety valve 36 is formed between the terminals 42 and 44 of the sealing plate 34 so as to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level.

  As shown in FIG. 2, the wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 50) and a long sheet-like negative electrode (negative electrode sheet 60) similar to the positive electrode sheet 50 in total. And a long sheet separator (separators 70 and 72).

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector 52 and a positive electrode active material layer 54. For example, a strip-shaped aluminum foil is used for the positive electrode current collector 52. An uncoated portion 52 a is set along the edge on one side in the width direction of the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is held on both surfaces of the positive electrode current collector 52 except for an uncoated portion 52 a set on the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material, a conductive material, and a binder.

As the positive electrode active material, a material used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of cathode active materials include lithium transition metal oxides such as LiNiCoMnO ₂ (lithium-nickel-cobalt-manganese composite oxide). For example, the positive electrode active material can be mixed with carbon black such as acetylene black (AB) or other (graphite or the like) powdery carbon material as a conductive material. In addition to the positive electrode active material and the conductive material, a binder such as polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), or polytetrafluoroethylene (PTFE) can be added. A positive electrode mixture (paste) can be prepared by dispersing these in a suitable dispersion medium and kneading. The positive electrode active material layer 54 is formed by applying this positive electrode mixture to the positive electrode current collector 52, drying it, and rolling (pressing) it to a predetermined thickness.

  As shown in FIG. 2, the negative electrode sheet 60 includes a strip-shaped negative electrode current collector 62 and a negative electrode active material layer 64. For the negative electrode current collector 62, for example, a strip-shaped copper foil is used. On one side in the width direction of the negative electrode current collector 62, an uncoated portion 62a is set along the edge. The negative electrode active material layer 64 is held on both surfaces of the negative electrode current collector 62 except for the uncoated portion 62 a set on the negative electrode current collector 62. The negative electrode active material layer 64 includes a graphite-based negative electrode active material, a thickener, a binder, and the like. As the binder, styrene butadiene rubber (SBR) is used. An example of the thickener is carboxymethyl cellulose (CMC).

  The graphite-based negative electrode active material contained in the negative electrode active material layer 64 is preferably a material mainly composed of natural graphite or artificial graphite, and more preferably natural graphite. Further, various graphites such as natural graphite and artificial graphite processed into particles (spherical) (pulverization, spherical molding, etc.) can be used. For example, flaky graphite can be used. The graphite-based active material may be composite particles formed by coating the surface of graphite particles (core core) with a carbon material (for example, an amorphous carbon material or a non-graphitizable carbon material). The negative electrode active material layer 64 will be further described later.

  As illustrated in FIG. 2, the separators 70 and 72 are members that separate the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separators 70 and 72 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separators 70 and 72, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIG. 2, the width b1 of the negative electrode active material layer 64 is wider than the width a1 of the positive electrode active material layer 54 (b1> a1). Further, the widths c1 and c2 of the separators 70 and 72 are wider than the width b1 of the negative electrode active material layer 64 (c1, c2> b1> a1).

As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. Moreover, as said support salt, lithium salts, such as LiPF ₆ , can be used, for example.

  Hereinafter, the negative electrode sheet 60 will be described in more detail.

  As described above, the negative electrode sheet 60 has a structure in which the negative electrode active material layer 64 containing the graphite-based negative electrode active material is held on the negative electrode current collector 62. The negative electrode active material layer 64 disclosed herein contains carbon particles in addition to the graphite-based negative electrode active material. The carbon particles may be amorphous carbon particles that do not participate in charge and discharge, but are preferably carbon particles that can occlude and release Li ions (that is, carbon particles that can function as a negative electrode active material). In a preferred embodiment, the carbon particles can function as a negative electrode active material and a conductive material.

The BET specific surface area A of the graphite-based negative electrode active material is smaller than the BET specific surface area C of the carbon particles (A <C) and is approximately 6 m ² / g or less. The BET specific surface area A of the graphite-based negative electrode active material is preferably 5.5 m ² / g or less, more preferably 5 m ² / g or less, and still more preferably 4.5 m ² / g or less, from the viewpoint of improving high-temperature storage characteristics. It is. Further, from the viewpoint of achieving both high-temperature storage characteristics and high-rate durability, the BET specific surface area A of the graphite-based negative electrode active material is approximately 2 m ² / g or more, preferably 2.5 m ² / g or more, more preferably 3 m ^2. / G or more. In this specification, “BET specific surface area” is a value obtained by analyzing a gas adsorption amount measured by a gas adsorption method (constant capacity adsorption method) using nitrogen (N ₂ ) gas as an adsorbate by a BET method. Say.

The bulk density B of the graphite-based negative electrode active material is larger than the bulk density D of the carbon particles (B> D) and is generally about 0.44 g / cm ³ or more. Bulk density B of the graphite-based negative active material, from the viewpoint of high-temperature storage characteristics improved, preferably 0.48 g / cm ³ or more, more preferably 0.52 g / cm ³ or more, more preferably 0.55 g / cm ³ That's it. From the viewpoint of achieving both high-temperature storage characteristics and high-rate durability, the bulk density B of the graphite-based negative electrode active material is approximately 0.7 g / cm ³ or less, preferably 0.65 g / cm ³ or less, more preferably 0. 0.6 g / cm ³ or less. In this specification, “bulk density” refers to a value measured by the method defined in JIS K1469 (2003).

The BET specific surface area B of the carbon particles is larger than the BET specific surface area A of the graphite-based negative electrode active material (A <C), and is generally about 10 m ² / g or more. The BET specific surface area B of the carbon particles is preferably 15 m ² / g or more, more preferably 18 m ² / g or more, and still more preferably 20 m ² / g or more from the viewpoint of improving high-rate durability. Further, from the viewpoint of improving high-temperature storage characteristics, etc., the BET specific surface area B of the carbon particles is approximately 30 m ² / g or less. For example, carbon particles having a BET specific surface area B of 20 m ² / g or more and 25 m ² / g or less are suitable from the viewpoint of achieving both high temperature storage characteristics and high rate durability at a high level. As the carbon particles having a BET specific surface area B of 10 m ² / g or more and 20 m ² / g or less, those obtained by pulverizing natural graphite or highly crystalline artificial graphite can be preferably used. Further, as carbon particles having a BET specific surface area B of 20 m ² / g or more and 30 m ² / g or less, carbon black (CB) or CB graphitized can be preferably used.

The bulk density D of the carbon particles is smaller than the bulk density B of the graphite-based negative electrode active material (B> D), and is generally 0.32 g / cm ³ or less. The bulk density D of the carbon particles is preferably 0.3 g / cm ³ or less, more preferably 0.25 g / cm ³ or less, and still more preferably 0.2 g / cm ³ or less from the viewpoint of high rate durability and the like. From the viewpoint of high-temperature storage characteristics and the like, the bulk density D of the carbon particles is approximately 0.05 g / cm ³ or more. For example, carbon particles bulk density D is not more than 0,1g / cm ³ or more 0.2 g / cm ³ are preferred from the viewpoint of achieving both high-temperature storage characteristics and high-rate durability at a high level.

Weighted average value E of BET specific surface area A of graphite-based negative electrode active material and BET specific surface area C of carbon particles
(That is, E = α × A + β × C, where α is the mass ratio of the graphite-based negative electrode active material when the total mass of the graphite-based negative electrode active material and the carbon particles is 1, and β is the graphite-based negative electrode active material and carbon. The mass ratio of carbon particles, α + β = 1) when the total mass with the particles is 1, is approximately 4.8 m ² / g or more. The weighted average value E of the BET specific surface area is preferably 5 m ² / g or more, more preferably 5.5 m ² / g or more, and further preferably 6 m ² / g or more from the viewpoint of improving high-rate durability. Further, from the standpoint of a high-temperature storage characteristics improve, a weighted average value E of the BET specific surface area is approximately 7.9 m ² / g or less, preferably 7.5 m ² / g, more preferably 6.5m ² / g or less. For example, the graphite-based negative electrode active material and carbon particles having a weighted average value E of the BET specific surface area of 5.5 g / cm ³ or more and 6.5 g / cm ³ or less have high temperature storage characteristics and high rate durability. It is preferable from the viewpoint of achieving both levels.

  The ratio (mass basis) of the content of the graphite-based negative electrode active material and the carbon particles is 95: 5 to 80:20. The mass ratio of the graphite-based negative electrode active material and the carbon particles is preferably 90:10 to 80:20, for example, 85 from the viewpoint of better exhibiting the effect of using the graphite-based negative electrode active material and the carbon particles in combination. : 15-80: 20.

  The negative electrode active material layer 64 disclosed here contains SBR as described above. The content of SBR contained in the negative electrode active material layer is generally 3 parts by mass to 6 parts by mass with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles. The content of the SBR is preferably 3.5 parts by mass or more, more preferably 4 parts by mass or more from the viewpoint of improving the adhesion of the negative electrode active material layer. Further, from the viewpoint of suppressing Li precipitation at a low temperature, the SBR content is preferably 5.5 parts by mass or less with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles. Preferably it is 5 mass parts or less. The technique disclosed here can be preferably implemented in an embodiment in which the content of SBR is 4 parts by mass or more and 6 parts by mass or less with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles.

  The graphite-based negative electrode active material and carbon particles can be separated from the completed negative electrode active material layer of the lithium ion secondary battery as follows. That is, the lithium ion secondary battery is disassembled, and the negative electrode active material layer is peeled off from the negative electrode current collector. Next, the negative electrode active material layer is dissolved in water to wash away SBR and a thickener (for example, CMC). Thereafter, the residue is dried, pulverized, and classified by airflow to separate the graphite-based negative electrode active material from the carbon particles.

The lithium ion secondary battery disclosed herein has a graphite-based negative electrode active material and carbon particle content ratio (graphite-based negative electrode active material: carbon particles) of 95: 5 to 80:20 on a mass basis, and graphite. The negative electrode active material BET specific surface area is A, the graphite negative electrode active material bulk density is B, the carbon particle BET specific surface area is C, the carbon particle bulk density is D, and the graphite negative electrode active material BET. a weighted average value of the BET specific surface area C of the specific surface area a and the carbon particles is taken as E, the following ^{relationship: 2m 2 / g ≦ a ≦} 6m 2 /g;0.44g/cm 3 ≦ B ≦ 0. met ^{4.8m 2 /g≦E≦7.9m 2 / g; 7g} / cm 3; 10m 2 / g ≦ C ≦ 30m 2 /g;0.05g/cm 3 ≦ D ≦ 0.32g / cm 3 , SBR with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles Comprising 3 parts by 6 parts by weight. By using such a graphite negative electrode active material and carbon particles having a specific BET specific surface area and bulk density in combination with a predetermined amount of SBR, an increase in resistance after a high rate cycle can be suppressed, and even after high-temperature storage. A high-performance nonaqueous electrolyte secondary battery capable of maintaining a high battery capacity can be realized.

The reason why the above effects can be obtained is not particularly limited, but is considered as follows, for example. That is, in a conventional non-aqueous electrolyte secondary battery using a graphite-based negative electrode active material, when charging and discharging at a high rate are repeated, the non-aqueous electrolyte is pushed out of the negative electrode due to expansion and contraction of the negative electrode, As a result of unevenness (unevenness) in the salt concentration depending on the location, resistance is likely to increase. On the other hand, since the carbon particles described above are bulky (low bulk density) compared to the graphite-based negative electrode active material, when the carbon particles are mixed with the graphite-based negative electrode active material, the filling properties of the graphite-based negative electrode active material Decreases, the negative electrode active material layer is thick, and the pore volume of the negative electrode active material layer is increased. As a result, the diffusion rate of Li ions in the negative electrode active material layer vacancies increases, and an increase in resistance due to uneven salt concentration of the electrolyte in the negative electrode can be suppressed.
Here, in order to suppress an increase in resistance due to unevenness (unevenness) in the salt concentration of the electrolytic solution, if the bulk density of the carbon particles is lowered, the BET specific surface area of the carbon particles increases as a contradiction. A side reaction (typically a film formation reaction on the surface of the carbon particles) between the carbon particles and the electrolytic solution is promoted, and the capacity retention rate after high-temperature storage is lowered.
Therefore, according to this aspect, 3 to 6 parts by mass of SBR is contained with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles. According to the inventor's knowledge, since SBR has high affinity with carbon particles having a large bulk and a large BET specific surface area, when 3 parts by mass to 6 parts by mass of SBR is contained in the negative electrode active material layer, SBR is It preferentially adsorbs to carbon particles over the graphite-based negative electrode active material, and coats the surface of the carbon particles. By coating the surface of the carbon particles with SBR, it is possible to suppress side reactions between the carbon particles and the electrolytic solution in a high temperature environment while suppressing an increase in resistance due to uneven salt concentration of the electrolytic solution during high-rate charge / discharge. . Therefore, according to this aspect, it is possible to provide a high-performance lithium ion secondary battery in which high-temperature storage characteristics and high-rate charge / discharge cycle durability are compatible at a high level.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Negative electrode sheet>
A plurality of types of graphite-based negative electrode active materials and carbon particles having different BET specific surface areas and bulk densities were prepared. As the graphite-based negative electrode active material, a graphite material coated with spheroidized natural graphite was used. A carbon material was used as the carbon particles. A negative electrode mixture is prepared by dispersing 100 parts by mass of a mixture of these graphite-based negative electrode active materials and carbon particles, dispersing SBR as a binder, and 0.7 parts by mass of CMC as a thickener in water. did. This negative electrode mixture was applied to both sides of a long sheet-like copper foil and dried to prepare each negative electrode sheet in which negative electrode active material layers were provided on both sides of the negative electrode current collector. For the negative electrode sheet according to each example, the BET specific surface area A and bulk density B of the graphite-based negative electrode active material used, the BET specific surface area C and bulk density D of the carbon particles, the mixing ratio of the graphite-based negative electrode active material and carbon particles, Table 1 to Table 1 show the addition amount of SBR with respect to 100 parts by mass of the total mass of the graphite-based negative electrode active material and the carbon particles, and the weighted average value E of the BET specific surface area A of the graphite-based negative electrode active material and the BET specific surface area C of the carbon particles. 10 is shown collectively.

<Peel strength test>
Using a universal testing machine, the tensile strength when the negative electrode active material layer was peeled by 90 ° from the negative electrode current collector of the negative electrode sheet was measured and taken as the peel strength. The results are shown in Tables 1-10. Here, the case where the peel strength was 1.5 N / m or more was evaluated as “◯”, and the case where the peel strength was less than 1.5 N / m was evaluated as “X”.

<Lithium ion secondary battery>
A lithium ion secondary battery for evaluation was produced using the negative electrode sheet of each example, and the battery performance was evaluated. Here, the positive electrode sheet of the lithium ion secondary battery for evaluation used aluminum foil for the positive electrode current collector. The solid content of the positive electrode mixture prepared when forming the positive electrode active material layer was positive electrode active material: conductive material: binder = 90: 8: 2 in mass ratio. As the positive electrode active material, particles of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (lithium nickel cobalt manganese composite oxide) were used, and a common positive electrode active material was used in each evaluation cell. Acetylene black (AB) was used as the conductive material, and polyvinylidene fluoride (PVDF) was used as the binder.

A wound electrode body was produced by winding the positive electrode sheet and the negative electrode sheet in a flat shape via two separators. The wound electrode body was accommodated in a rectangular battery case together with a non-aqueous electrolyte, and the opening of the battery case was hermetically sealed. As the separator, a porous film having a three-layer structure made of polypropylene (PP) / polyethylene (PE) / polypropylene (PP) was used. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3, and about 1 mol of LiPF ₆ as a supporting salt. The one contained at a concentration of 1 / liter was used. In this way, a lithium ion secondary battery for evaluation was constructed.

<Initial capacity measurement>
For each of the lithium ion secondary batteries of each example thus produced, the total charging time was 2 hours at 25 ° C. using a constant current / constant voltage method with a current of 0.7 C (2.8 A) and a voltage of 4.1 V. Charged until After 10 minutes of rest, the charged battery was discharged to 3 V at a constant current and a constant voltage of 0.5 C (2 A) at 25 ° C., and the discharge capacity at this time was measured as the initial capacity.

<Low-temperature pulse cycle test>
A low temperature pulse cycle test was performed on each of the lithium ion secondary batteries of each example. Specifically, after charging so that the charging depth (SOC) of the battery in each example is 60% of the initial capacity, high-rate pulse charging at 20 C (80 A) for 20 seconds is performed in a low temperature environment of −10 ° C. The charging / discharging cycle which performs high-rate pulse discharge for 40 seconds at 10C (40) A was repeated 1000 times continuously. Then, the discharge capacity after the low temperature pulse cycle test is obtained by the same method as the initial capacity measurement, and the capacity retention rate after the low temperature pulse cycle test (= [low temperature pulse cycle] Discharge capacity after test / initial capacity] × 100) was calculated. The results are shown in Tables 1-10. Here, a capacity maintenance rate of 98% or more was evaluated as “◯”, and a capacity maintenance rate of less than 98% was evaluated as “X”.

<High temperature holding test>
Each of the lithium ion secondary batteries in each example was subjected to a high-temperature storage test in which the batteries were stored at 80 ° C. for 60 days. Specifically, charging is performed so that the charging depth of each example battery is 80% of the initial capacity (current 0.7C (2.8A), voltage 3.9V constant current constant voltage method, total charging time 3 The battery in a charged state was stored in an environment at 80 ° C. for 60 days. Then, the discharge capacity after the high-temperature storage test is obtained by the same method as the initial capacity measurement, and the capacity retention ratio (= [discharge capacity after the high-temperature storage test / initial capacity] is calculated from the discharge capacity and the initial capacity after the high-temperature storage test. × 100) was calculated. The results are shown in Tables 1-10. Here, a capacity maintenance rate of 80% or more was evaluated as “◯”, and a capacity maintenance rate of less than 80% was evaluated as “×”.

<High-rate pulse cycle test (excessive charge)>
A high rate pulse cycle test was performed on each of the lithium ion secondary batteries of each example. Specifically, after charging the charging depth (SOC) of the battery in each example from the discharged state to 60% of the initial capacity, the charge / discharge cycle consisting of the following (I) to (II) is repeated 3000 times. A high rate pulse cycle test was conducted. FIG. 3 shows a charge / discharge cycle in the evaluation test. Then, the rate of increase in resistance (= [IV resistance after high-rate pulse cycle test / initial IV resistance] × 100) was calculated from the IV resistance after the high-rate pulse cycle test and the initial IV resistance. In addition, IV resistance was calculated | required from the inclination of the linear approximation line of the current (I) -voltage (V) plot value when discharging for 10 second at 10C. The results are shown in Tables 1-10. Here, a resistance increase rate of 120% or less was evaluated as “◯”, and a resistance increase rate exceeding 120% was evaluated as “x”.
(I) Charge for 10 seconds at a constant current of 10 C (40 A).
(II) Discharge for 400 seconds at a constant current of 2C (8A).

As shown in Tables 1 to 10, the graphite-based negative electrode active material has a BET specific surface area A of 2 m ² / g ≦ A ≦ 6 m ² / g and a bulk density B of 0.44 g / cm ³ ≦ B ≦ 0. and 7 g / cm ^3, a BET specific surface area C of the carbon particles and ^{10m 2 / g ≦ C ≦ 30m} 2 / g, a bulk density D and ^{0.05g / cm 3 ≦ D ≦ 0.32g} / cm 3, graphite The weighted average value E between the BET specific surface area A of the negative electrode active material and the BET specific surface area C of the carbon particles is 4.8 m ² /g≦E≦7.9 m ² / g, and the graphite negative electrode active material and the carbon particles are contained. Samples in which the ratio of the amounts (graphite-based negative electrode active material: carbon particles) was 95: 5 to 80:20 on a mass basis and the amount of SBR added was 3 parts by mass to 6 parts by mass were the capacity retention ratio after high-temperature storage Is as high as 80% or more, and the rate of increase in resistance after a high-rate pulse cycle is also 120% It was suppressed to below. Further, the peel strength of the negative electrode active material layer was 1.5 N / m or more, and the adhesion of the negative electrode active material layer was good. Furthermore, the capacity retention after the low-temperature pulse cycle was 98% or more, and the Li precipitation resistance was also excellent. When SBR in the negative electrode active material layer of these samples was stained with Br and the SBR distribution was observed by a cross section with an electron beam microanalyzer (EPMA), more SBR was present on the surface of the carbon particles than the graphite-based negative electrode active material. It was. From this result, in the above sample, SBR preferentially adsorbs to the carbon particles over the graphite-based negative electrode active material, thereby suppressing side reactions between the carbon particles and the electrolytic solution, and maintaining the capacity retention rate after high-temperature storage. It is presumed that the decrease could be suppressed.

For example, when the weighted average value E of the BET specific surface area of the graphite-based negative electrode active material and the carbon particles is in the range of 4.8 m ² /g≦E≦7.9 m ² / g, the amount of SBR added is 1 to 6 parts by mass. When increased to parts by mass (see Table 3, Table 6, Table 7, and Table 9), it can be seen that the capacity retention rate after high-temperature storage is improved. At this time, since the capacity retention ratios after the low-temperature pulse cycle are both 98% or more, it can be said that there is almost no decrease in Li precipitation resistance due to the increase in the SBR.

On the other hand, in the sample in which the weighted average value E of the BET specific surface area of the graphite-based negative electrode active material and the carbon particles was 8.1 m ² / g or more, the capacity retention rate after high temperature storage was less than 80%. In such a sample, since a part of the surface of the carbon particles was not completely covered with SBR, a side reaction between the carbon particles and the electrolytic solution occurred, and it is considered that the capacity retention rate after high temperature storage was lowered.

Further, in the sample in which the weighted average value E of the BET specific surface area of the graphite-based negative electrode active material and the carbon particles was 4.7 m ² / g or less, the resistance increase rate after the high-rate pulse cycle significantly exceeded 120%. In such a sample, since the effect of increasing the void volume of the negative electrode active material layer due to the use of bulky carbon particles having a large BET specific surface area was insufficient, the electrolyte was pushed out of the negative electrode due to the expansion of the negative electrode, It is considered that the increase in resistance was caused as a result of the occurrence of unevenness (unevenness) in the salt concentration of the electrolytic solution depending on the location.

  Further, in the sample in which the amount of SBR added was 1 part by mass, the peel strength of the negative electrode active material layer was less than 1.5 N / m, and the adhesion of the negative electrode active material layer was lacking. Insufficient adhesion of the negative electrode active material layer is not preferable because problems such as slipping of the negative electrode active material layer occur when the wound electrode body is wound. Further, the sample in which the amount of SBR added was 8 parts by mass had a capacity retention rate of less than 98% after the low-temperature pulse cycle and lacked Li precipitation resistance. In such a sample, the adsorption of SBR on the surface of the carbon particles was saturated, and the surface of the graphite-based negative electrode active material was also coated with SBR, so that lithium ions were less likely to enter the graphite-based negative electrode active material and Li deposition occurred. It is considered a thing.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The invention disclosed herein may include various modifications and alterations of the specific examples described above.

20 Winding electrode body 50 Positive electrode sheet 52 Positive electrode current collector 54 Positive electrode active material layer 60 Negative electrode sheet 62 Negative electrode current collector 64 Negative electrode active material layer 70, 72 Separator 100 Lithium ion secondary battery

Claims (1)

A negative electrode active material layer containing a graphite-based negative electrode active material is a non-aqueous electrolyte secondary battery comprising a negative electrode formed on a negative electrode current collector and a positive electrode,
The negative electrode active material layer further contains carbon particles,
The ratio of the content of the graphite-based negative electrode active material and the carbon particles (graphite-based negative electrode active material: carbon particles) is 95: 5 to 80:20 on a mass basis,
The BET specific surface area of the graphite-based negative electrode active material is A, the bulk density of the graphite-based negative electrode active material is B, the BET specific surface area of the carbon particles is C, the bulk density of the carbon particles is D, and the graphite When the weighted average value of the BET specific surface area A of the system negative electrode active material and the BET specific surface area C of the carbon particles is E, the following relationship:
2 m ² / g ≦ A ≦ 6 m ² / g
0.44 g / cm ³ ≦ B ≦ 0.7 g / cm ³
10 m ² / g ≦ C ≦ 30 m ² / g
0.05 g / cm ³ ≦ D ≦ 0.32 g / cm ³
4.8m ² /g≦E≦7.9m ² / g
The filling,
A non-aqueous electrolyte secondary battery comprising 3 to 6 parts by mass of styrene butadiene rubber with respect to 100 parts by mass of a total mass of the graphite-based negative electrode active material and the carbon particles.

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