JP2015138644A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery superior in cycle characteristics.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode; and a negative electrode. The negative electrode includes a negative electrode current collector, a first layer provided on the negative electrode current collector and including a graphite-based negative electrode active material, and a second layer provided on the first layer and including a graphite-based negative electrode active material. When the ratio I/Iof a peak intensity Iof graphite crystal coming from (110)plane to a peak intensity Icoming from (002)plane according to powder X-ray diffraction measurement on a percentage basis is defined as an orientation degree, the orientation degree of the second layer is 0.01-0.4%, and the orientation degree of the first layer is 0.5% or less.

Description

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

Various attempts have been made to improve the lithium ion (Li ⁺ ) acceptability of the negative electrode in nonaqueous electrolyte secondary batteries. For example, in International Publication 2011/114433 (Patent Document 1), the negative electrode active material layer provided on the negative electrode current collector has a two-layer structure, and the average of the negative electrode active materials contained in the surface layer side active material layer A lithium secondary battery having a specific surface area larger than the average specific surface area of the negative electrode active material included in the current collector-side active material layer is disclosed.

International Publication No. 2011/114433

A negative electrode active material having a large specific surface area is advantageous for accepting Li ⁺ . Therefore adopting a two-layer structure as described above, by Li ⁺ acceptance to place a high negative electrode active material on the surface layer (upper layer), thereby improving the Li ⁺ acceptance of the negative electrode.

However, the two types of negative electrode active materials having different specific surface areas are different from each other in the amount of expansion / contraction caused by insertion / extraction of Li ⁺ . Therefore, due to the difference in expansion and contraction between the upper layer and the lower layer in the two-layer structure as described above, delamination may occur at the interface between the upper layer and the lower layer when the charge / discharge cycle is repeated. When delamination occurs, a part of the upper layer is electrically isolated, and the negative electrode active material cannot contribute to charging / discharging at that part, resulting in a significant capacity loss.

  The present invention has been made in view of the problems as described above, and an object of the present invention is to provide a nonaqueous electrolyte secondary battery excellent in cycle characteristics by preventing delamination of the negative electrode composite material layer. is there.

  The present inventor conducted earnest research to solve the above problems, and obtained knowledge that delamination can be suppressed by controlling the orientation of the negative electrode active material contained in the upper layer and the lower layer in the two-layer structure, The present invention has been completed by further research based on this finding. That is, the nonaqueous electrolyte secondary battery of the present invention has the following configuration.

The nonaqueous electrolyte secondary battery includes a positive electrode and a negative electrode, and the negative electrode includes a negative electrode current collector, a first layer provided on the negative electrode current collector and containing a graphite-based negative electrode active material, A peak intensity I ₍₁₁₀₎ derived from the (110) plane of the graphite crystal by powder X-ray diffraction measurement, and (002 _). ) When the percentage of the ratio I ₍₁₁₀₎ / I ₍₀₀₂₎ to the peak intensity I ₍₀₀₂₎ derived from the plane is the degree of orientation, the degree of orientation of the second layer is 0.01% or more and 0.00. 4% or less, and the degree of orientation of the first layer is 0.5% or less.

  By controlling the degree of orientation of the second layer (upper layer) and the first layer (lower layer) as described above, delamination can be prevented and cycle characteristics can be improved. The reason is as follows.

FIG. 12 is a schematic diagram showing a crystal structure of a graphite-based negative electrode active material. As shown in FIG. 12, the graphite crystal has a crystal structure in which a graphite layer in which six-membered rings of carbon are connected in the plane direction (ab-axis direction) is stacked in the vertical direction (c-axis direction). Graphite crystals can store Li ⁺ between graphite layers. At this time, the graphite crystals expand according to the amount of Li ⁺ inserted (that is, according to the amount of charge). FIG. 13 is a graph showing the relationship between the amount of charge (SOC) and the amount of expansion of the graphite crystal. As shown in FIG. 13, the average distance between the graphite layers increases stepwise as the SOC increases. This step-like change is considered to originate from the stage structure change of the intercalation compound composed of Li and graphite. On the other hand, the average distance between carbon and carbon constituting the six-membered ring hardly changes even when the SOC increases. As can be seen from FIG. 13, the volume change of the graphite crystal is mainly caused by the change of the average distance between the graphite layers.

  Here, the cause of delamination in the two-layer structure negative electrode is that the amount of expansion and contraction in the planar direction (parallel to the surface of the negative electrode) is different between the upper layer and the lower layer. It is considered that this is because distortion occurs due to deviation. Thus, in the present invention, the expansion and contraction directions of the graphite-based negative electrode active materials contained in the upper layer and the lower layer are regulated so as to be aligned with the thickness direction of the negative electrode (perpendicular to the negative electrode surface). As a result, the upper layer and the lower layer are both expanded and contracted in the thickness direction of the negative electrode, so that it is possible to prevent distortion at the interface between the upper layer and the lower layer.

The expansion / contraction direction can be regulated by controlling the orientation of the graphite-based negative electrode active material contained in each layer. That is, in each layer, the ratio I ₍₁₁₀₎ / I _{(002 )} between the peak intensity I ₍₁₁₀₎ derived from the (110) plane of the graphite crystal and the peak intensity I ₍₀₀₂₎ derived from the (002) plane by powder X-ray diffraction measurement. ₎ Is controlled. In this specification, the percentage of the peak intensity ratio I ₍₁₁₀₎ / I ₍₀₀₂₎ is referred to as “degree of orientation”.

Here, the peak intensity I ₍₁₁₀₎ is derived from the ab axis direction of the graphite crystal, and the peak intensity I ₍₀₀₂₎ is derived from the c axis direction. Therefore, it can be considered that the smaller the degree of orientation measured in the negative electrode mixture layer, the more negative electrode active material in which the c-axis is oriented in the thickness direction of the negative electrode.

  According to the study of the present inventors, the degree of orientation of the upper layer (second layer) is 0.01% or more and 0.4% or less, and the degree of orientation of the lower layer (first layer) is 0.5% or less. By regulating so as to be, delamination can be prevented, and thus cycle characteristics can be improved.

  The degree of orientation of each layer can be controlled in a desired range by applying a magnetic field to the negative electrode mixture paste before drying, for example.

  The nonaqueous electrolyte secondary battery of the present invention is excellent in cycle characteristics.

It is a typical fragmentary sectional view which shows an example of a structure of the negative electrode concerning one Embodiment of this invention. It is a typical fragmentary sectional view illustrating a part of manufacturing process of the negative electrode concerning one Embodiment of this invention. It is a typical fragmentary sectional view illustrating another part of manufacturing process of the negative electrode concerning one Embodiment of this invention. It is typical sectional drawing illustrating the preparation methods of the sample for orientation degree measurement. It is typical sectional drawing illustrating a part of measuring method of an orientation degree. It is typical sectional drawing illustrating other one part of the measuring method of orientation degree. It is a typical perspective view illustrating a part of manufacturing process of the negative electrode concerning one Embodiment of this invention. It is a typical perspective view illustrating other part of the manufacturing process of the negative electrode concerning one Embodiment of this invention. It is a typical perspective view which shows an example of a structure of the negative electrode concerning one Embodiment of this invention. It is a typical perspective view which shows an example of a structure of the positive electrode concerning one Embodiment of this invention. It is a typical perspective view which shows an example of a structure of the nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention. It is a schematic diagram which shows the crystal structure of a graphite type negative electrode active material. It is a graph which shows the relationship between a charge amount (SOC) and the expansion amount of a graphite crystal. It is a graph which shows an example of the relationship between the capacity retention after a cycle of the nonaqueous electrolyte secondary battery which concerns on one Embodiment of this invention, and the orientation degree of a negative electrode.

  Hereinafter, an embodiment of the present invention (hereinafter, also referred to as “this embodiment”) will be described in detail, but the present embodiment is not limited thereto.

<Nonaqueous electrolyte secondary battery>
FIG. 11 is a schematic perspective view showing an example of the configuration of the nonaqueous electrolyte secondary battery of the present embodiment. A battery 1000 shown in FIG. 11 is a prismatic lithium ion secondary battery, in which an electrode group 400 formed by winding a positive electrode 100 and a negative electrode 200 each having a long strip shape with a separator 300 interposed therebetween, together with a non-aqueous electrolyte, It is configured to be enclosed in an exterior body 500. As shown in FIGS. 9 and 10, the positive electrode 100 and the negative electrode 200 have a non-coated portion where the current collector is exposed on one side in the width direction, and the non-coated portion is a current collecting portion in the electrode group 400. The current collector is connected to the battery outer package 500 via a current collector. Hereinafter, each part which comprises the battery 1000 is demonstrated.

(Negative electrode)
FIG. 1 is a schematic cross-sectional view showing an example of the configuration of the negative electrode 200. Referring to FIG. 1, a negative electrode 200 includes a negative electrode current collector 201 and a negative electrode mixture layer 202 provided on the negative electrode current collector 201. The negative electrode mixture layer 202 is provided on the negative electrode current collector 201 and includes a first layer 202a containing the first graphite-based negative electrode active material 2a, and a second graphite-based layer provided on the first layer 202a. And a second layer 202b containing the negative electrode active material 2b.

  Here, the negative electrode 200 of the present embodiment only needs to include at least the first layer 202a and the second layer 202b, and is not necessarily limited to the two-layer structure. That is, the negative electrode of this embodiment may have a multilayer structure of three or more layers. However, also in the case of a multilayer structure of three or more layers, the first layer 202a and the second layer 202b are arranged in this order from the negative electrode current collector 201 side in the thickness direction of the negative electrode.

The second graphite-based negative electrode active material 2b included in the second layer 202b preferably has a specific surface area larger than that of the first graphite-based negative electrode active material 2a included in the first layer 202a. This is because the Li ⁺ acceptability of the negative electrode 200 is improved by disposing an active material having a large specific surface area in the upper layer (second layer 202b). In the present specification, the “specific surface area” indicates an average specific surface area obtained based on the adsorption isotherm of BET (Brunauer, Emmett, Teller).

  Furthermore, in this embodiment, the degree of orientation of the first layer 202a and the second layer 202b is controlled. As a result, distortion generated at the interface between the first layer 202a and the second layer 202b with the expansion and contraction of the graphite-based negative electrode active material can be suppressed, and thus excellent cycle characteristics can be obtained.

(Degree of orientation)
Here, the “degree of orientation” means the peak intensity I ₍₁₁₀₎ derived from the (110) plane of the graphite crystal and the peak intensity I ₍₀₀₂₎ derived from the (002) plane by powder X-ray diffraction (XRD) measurement. The ratio of the ratio I ₍₁₁₀₎ / I ₍₀₀₂₎ shall be indicated. Such degree of orientation can be measured as follows.

  First, referring to FIG. 4, the second layer 202b (upper layer) is peeled off by the adhesive tape 20 at the interface between the second layer 202b (upper layer) and the first layer 202a (lower layer). Next, referring to FIGS. 5 and 6, the second layer 202b and the first layer 202a are separately subjected to XRD analysis. That is, the XRD measurement is performed while maintaining the shape of the second layer 202b together with the adhesive tape (FIG. 6), and the XRD measurement is performed while maintaining the shape of the first layer 202a together with the negative electrode current collector 201. Perform (FIG. 5). For the measurement, a conventionally known XRD apparatus can be used.

In the XRD profile of each layer thus measured, the peak derived from the (110) plane of the graphite crystal appears in the vicinity of 2θ = 78 °, and the peak derived from the (002) plane appears in the vicinity of 2θ = 26 °. The percentage of the value obtained by dividing the peak intensity I ₍₁₁₀₎ near 2θ = 78 ° by the peak intensity I ₍₀₀₂₎ near 2θ = 26 ° can be used as the degree of orientation.

  In this embodiment, the degree of orientation of the second layer 202b needs to be 0.01% or more and 0.4% or less, and the degree of orientation of the first layer 202a needs to be 0.5% or less. . As a result, most of the negative electrode active material contained in the second layer 202b and the first layer 202a can be oriented so that the expansion / contraction direction is the thickness direction of the negative electrode 200.

  The smaller the degree of orientation of each layer, the more reliably the delamination is prevented and the cycle characteristics are improved. Therefore, the degree of orientation of the first layer 202a is preferably 0.01% or more and 0.4% or less similarly to the second layer 202b. More preferably, the degree of orientation of the first layer 202a and the second layer 202b is 0.01% or more and 0.2% or less, and particularly preferably 0.01% or more and 0.1% or less.

(First layer and second layer)
Each of the first layer 202a and the second layer 202b is formed by fixing a graphite-based negative electrode active material with a binder. Here, for example, styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene oxide (PEO), acrylic rubber (ACR), carboxymethyl cellulose (CMC), or the like can be used as the binder. CMC can also function as a thickener in the negative electrode mixture paste. The mass ratio between the graphite-based negative electrode active material and the binder in each layer is, for example, about graphite-based negative electrode active material: binder = 95: 5 to 99: 1.

The mass ratio of the first layer 202a and the second layer 202b is not particularly limited, but is preferably the first layer: second layer = 1: 9 to 9: 1, and more preferably the first layer: The second layer = 2: 8 to 8: 2, more preferably the first layer: the second layer = 3: 7 to 7: 3. The density of the negative electrode mixture layer (mass of the negative electrode mixture layer ÷ volume of the negative electrode mixture layer) is preferably about 0.5 to 2.5 g / cm ³ from the viewpoint of Li ⁺ acceptability and cycle characteristics. More preferably, it is about 0.8-2.0 g / cm < ³ >, More preferably, it is about 1.0-1.5 g / cm < ³ >.

(Graphite negative electrode active material)
As the graphite-based negative electrode active material of the present embodiment, either natural graphite or artificial graphite may be used, but natural graphite is preferably used from the viewpoint of theoretical capacity and raw material cost. The particle shape is not particularly limited, but scaly graphite is preferable from the viewpoint of easy orientation and uniform expansion and contraction.

Further, from the viewpoint of Li ⁺ acceptability, the average particle diameter of the second graphite-based negative electrode active material 2b is preferably about 5 to 15 μm, for example, about 10 μm. Further, from the viewpoint of high-temperature storage characteristics and the like, the average particle size of the first graphite-based negative electrode active material 2a is preferably about 15 to 30 μm, for example about 20 μm. In the present specification, the “average particle diameter” indicates a median diameter (d50) by a laser diffraction / scattering method.

As described above, in the present embodiment, the specific surface area of the second graphite-based negative electrode active material 2b is larger than the specific surface area of the first graphite-based negative electrode active material 2a. Here, from the viewpoint of further improving Li ⁺ acceptability, the specific surface area of the second graphite-based negative electrode active material 2b is preferably 3.50 m ² / g or more, more preferably 4.00 m ² / g or more. It is. The specific surface area of the first graphite-based negative electrode active material 2a from the viewpoint of suppressing the amount of gas generated during high temperature storage is preferably less than 3.50 m ² / g, more preferably 3.00 m ² / g It is as follows.

(Production method of negative electrode)
As described above, the first layer 202a and the second layer 202b having a specific degree of orientation can be manufactured by the following method.

  First, a negative electrode mixture paste is prepared by a conventionally known method. The negative electrode mixture paste can be produced, for example, by kneading a graphite-based negative electrode active material, a thickener, and a binder in water.

  Then, the negative electrode mixture paste is applied by a conventionally known method (for example, a die coating method). By applying a magnetic field before the negative electrode mixture paste is dried (solidified), the graphite-based negative electrode active material contained in the negative electrode mixture paste can be oriented.

  2 and 3 schematically show the orientation of the graphite-based negative electrode active material 2b when a magnetic field is applied to the second layer 202b before drying. 2 and 3, when a magnetic field is applied to the negative electrode mixture paste before drying, the graphite-based negative electrode active material 2b contained in the negative electrode mixture paste is oriented so that the graphite layer is parallel to the magnetic force lines 51 and 61. To do. At this time, when a magnetic field is applied parallel to the surface of the negative electrode 200 as shown in FIG. 2, the degree of orientation of the second layer 202b decreases, and is perpendicular to the surface of the negative electrode 200 as shown in FIG. When a magnetic field is applied, the degree of orientation of the second layer 202b increases. And after orienting the graphite type negative electrode active material 2b in this way, the 2nd layer 202b by which the degree of orientation was controlled can be formed by drying the 2nd layer 202b. For the first layer 202a, the graphite-based negative electrode active material 2a can be oriented by the same method, and the degree of orientation can be controlled.

  Here, the strength of the magnetic field and the time for applying the magnetic field are not particularly limited, and may be set as appropriate so as to obtain a desired degree of orientation. For example, the strength of the magnetic field can be set so that the magnetic flux density is about 500 to 1000 mT with respect to the entire negative electrode 200, and the time for applying the magnetic field is 1 to 20 with respect to the entire negative electrode 200. It can be about a second.

(Positive electrode)
The positive electrode 100 is formed by fixing a positive electrode mixture layer 102 including a positive electrode active material, a conductive additive, and a binder on a positive electrode current collector 101. Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiNi _a Co _b O ₂ (a + b = 1, 0 <a <1, 0 <b <1), LiMnO ₂ , LiMn ₂ O ₄ , LiNi _a Co _b Mn _{cO 2} (a + b + c = 1, 0 <a <1, 0 <b <1, 0 <c <1), LiFePO ₄ or the like can be used. Moreover, acetylene black (AB) etc. can be used for a conductive support material, for example, and a polyvinylidene fluoride (PVdF) etc. can be used for a binder, for example.

(Separator)
The separator 300 transmits Li ⁺ and prevents electrical contact between the positive electrode 100 and the negative electrode 200. The separator 300 is preferably a microporous film made of a polyolefin-based material from the viewpoint of mechanical strength and chemical stability. Here, as the polyolefin-based material, for example, polyethylene (PE), polypropylene (PP) and the like can be used, and these can be used in combination.

(Nonaqueous electrolyte)
As the non-aqueous electrolyte, a lithium salt dissolved in an aprotic solvent can be used. Examples of the aprotic solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (γBL) and vinylene carbonate (VC), and dimethyl carbonate (DMC). , Chain carbonates such as ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) can be used. The solvent composition is, for example, EC: EMC: DEC = 3: 5: 2 (volume ratio). As the lithium salt, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ) or the like can be used. The salt concentration is, for example, about 0.5 to 2.0 mol / L. The nonaqueous electrolyte may be in the form of a gel or a solid.

  Although the present embodiment has been described with reference to a square battery as described above, the present embodiment is not limited to this, and for example, a cylindrical battery or a pouch-shaped battery may be used.

  Hereinafter, although this embodiment is described in detail using an example, this embodiment is not limited to these.

<Example 1>
A square lithium ion secondary battery was produced as follows, and a charge / discharge cycle test was performed to evaluate the cycle characteristics.

(Preparation of positive electrode)
A positive electrode mixture paste was obtained by kneading a positive electrode active material (LiCoO ₂ ), a conductive additive (AB), and a binder (PVdF) in a solvent (NMP).

Referring to FIG. 10, the positive electrode assembly was made so that the coating mass was 30 mg / cm ² (after drying) on one side in the width direction of the Al foil (width 77.0 mm, thickness 20 μm) as positive electrode current collector 101. The material paste was applied and dried to form the positive electrode mixture layer 102 (width 57.0 mm), and a hoop of the positive electrode 100 was obtained. As shown in FIG. 10, the positive electrode 100 has an uncoated portion (width 20.0 mm) where the positive electrode current collector 101 is exposed on one side in the width direction.

(Preparation of negative electrode)
(I) Production of First Negative Electrode Mixture Paste Natural graphite powder (average particle size 20 μm, specific surface area 2.8 m ² / g) was prepared as the first graphite-based negative electrode active material 2a. The first graphite-based negative electrode active material 2a, the binder (SBR), and the thickener (CMC) were kneaded in water to obtain a first negative electrode mixture paste.

(Ii) Production of Second Negative Electrode Mixture Paste Natural graphite powder (average particle diameter 10 μm, specific surface area 3.6 m ² / g) was prepared as the second graphite-based negative electrode active material 2b. The second graphite-based negative electrode active material 2b, the binder (SBR), and the thickener (CMC) were kneaded in water to obtain a second negative electrode mixture paste.

(Iii) Formation of first layer First, the first negative electrode mixture paste is coated on one surface of a Cu foil (thickness 10 μm) as the negative electrode current collector 201 with a single-side coating mass of 5 mg / cm ² (dried). The first layer 202a was formed by solidifying the paste by coating with hot air drying.

(Iv) Formation of Second Layer Next, with reference to FIG. 7, the second negative electrode mixture paste was applied onto the first layer 202a so that the single-sided coating mass was 5 mg / cm ² . Before the second negative electrode mixture paste solidified, a magnetic field was applied in the width direction of the Cu foil with reference to FIGS. 2 and 8 to orient the second graphite-based negative electrode active material 2b. Thereafter, the second negative electrode mixture paste was solidified by hot air drying to form the second layer 202b. Furthermore, according to the same procedure, the 1st layer 202a and the 2nd layer 202b were formed also on the other surface (back surface) of Cu foil. At this time, the strength of the magnetic field was set so that a magnetic field having a magnetic flux density of 750 mT was applied to the entire negative electrode for 5 seconds.

(V) Slit and Rolling Thereafter, the negative electrode was cut into a width of 80.9 mm with a slitter with reference to FIG. 9, and further rolled with a roll mill to obtain a hoop of negative electrode 200. As shown in FIG. 9, the negative electrode 200 has a non-coated portion where the negative electrode current collector 201 is exposed on one side in the width direction. The width of the non-coated portion is 20.0 mm. The width was 60.9 mm.

(Nonaqueous electrolyte adjustment)
EC, EMC, and DEC were mixed so that EC: EMC: DEC = 3: 5: 2 (volume ratio) to obtain an aprotic solvent. By then dissolving the LiPF ₆ (1.0mol / L) as a solute in the aprotic solvent, to obtain a non-aqueous electrolyte.

(assembly)
A PE microporous membrane separator (width 63.0 mm) was prepared as the separator 300. Referring to FIG. 11, the electrode group 400 was obtained by winding the positive electrode 100 and the negative electrode 200 so as to face each other with the separator 300 interposed therebetween. At this time, it wound so that the non-coating part of the positive electrode 100 and the negative electrode 200 could be taken out from a mutually different direction in a winding axis | shaft. Then, the non-coated portion and the electrode terminal were connected, and the electrode group 400 was inserted into the battery outer package 500 (height 75 mm, width 120 mm, thickness 15 mm, case thickness 1 mm). Further, a nonaqueous electrolyte was injected, and the battery outer package 500 was sealed to obtain a prismatic lithium ion secondary battery (rated capacity 4.5 Ah) according to Example 1.

<Examples 2 to 4>
In the above “formation of the second layer”, the example in which the magnetic field is applied is 3 seconds (Example 2), 2 seconds (Example 3), and 1 second (Example 4). In the same manner as in Example 1, the prismatic lithium ion secondary batteries according to Examples 2 to 4 were obtained.

<Example 5>
In Example 5, a prismatic lithium ion secondary battery was obtained in the same manner as in Example 1 except that a magnetic field was applied to the first layer 202a to orient the graphite-based negative electrode active material 2b.

In Example 5, the first layer 202a was formed as follows. That is, after coating the first negative electrode mixture paste on one surface of the Cu foil as the negative electrode current collector 201 so that the single-sided coating mass is 5 mg / cm ² (after drying), Before the negative electrode mixture paste solidified, a magnetic field having a magnetic flux density of 750 mT was applied in the width direction of the Cu foil for 5 seconds. Thereafter, the first layer 202a was solidified by hot air drying. In the same manner as in Example 1, the second layer 202b was formed on the first layer 202a.

<Comparative Example 1>
A prismatic lithium ion secondary battery according to Comparative Example 1 was obtained in the same manner as in Example 1 except that in the “formation of the second layer”, the time for applying the magnetic field was 0.5 seconds. .

<Comparative Example 2>
A prismatic lithium ion secondary battery according to Comparative Example 2 was obtained in the same manner as in Example 1 except that in the “formation of the second layer”, a magnetic field was not applied.

<Comparative Example 3>
In the “formation of the second layer”, the direction in which the magnetic field is applied is the thickness direction of the negative electrode 200 with reference to FIG. 3, and the magnetic field having a magnetic flux density of 750 mT is applied for 1 second. In the same manner as in Example 1, a prismatic lithium ion secondary battery according to Comparative Example 3 was obtained.

<Comparative Example 4>
Before the first negative electrode mixture paste is solidified in the “formation of the first layer”, a magnetic field having a magnetic flux density of 750 mT is applied to the first negative electrode mixture paste in the thickness direction of the negative electrode 200 for 1 second. A prismatic lithium ion secondary battery according to Comparative Example 4 was obtained in the same manner as in Comparative Example 2 except that it was given.

<Comparative Example 5>
In the above “preparation of negative electrode”, a magnetic field is applied to the first negative electrode mixture paste in the thickness direction of the negative electrode 200 before the first negative electrode mixture paste is solidified, as in Comparative Example 4, and As in Comparative Example 3, Example 2 is the same as Example 1 except that a magnetic field is applied to the second negative electrode mixture paste in the thickness direction of the negative electrode 200 before the second negative electrode material paste is solidified. A square lithium ion secondary battery was obtained in the same manner.

<Evaluation>
Each negative electrode and each battery obtained as described above were evaluated as follows.

(Measurement of orientation)
4 to 6, the second layer 202 b is peeled off at the interface between the first layer 202 a and the second layer 202 b using an adhesive tape, and the degree of orientation of the second layer 202 b and The degree of orientation of the first layer 202a was measured separately. The results are shown in Table 1. For the XRD apparatus, a Rotaflex RU-200B manufactured by Rigaku Corporation was used.

(Measurement of initial capacity)
Under an environment of 25 ° C., each battery was charged to 4.2 V at a current value of 1 It (4.5 A), then rested for 5 minutes, then discharged to 2.5 V at a current value of 1 It, and then rested for 5 minutes. did.

  Next, each battery is charged by CC-CV charging (CC current value 1 It, CV voltage 4.2 V, end current 0.01 It), and then CC-CV discharge (CC current value 1 It, CV voltage 3.0 V, end current 0) .01 It) and the initial capacity of each battery was measured.

(Charge / discharge cycle test)
After measuring the initial capacity of each battery in a 25 ° C. environment, it was adjusted to a discharged state (SOC = 0%). Next, the following [1] to [4] were sequentially executed as one cycle, and the cycle was executed 1000 times.
[1] CC-CV charge (CC current value 1 It, CV voltage 4.2 V, end current 0.01 It)
[2] Pause (10 minutes)
[3] CC-CV discharge (CC current value 1 It, CV voltage 3.0 V, end current 0.01 It)
[4] Pause (10 minutes).

After the end of 1000 cycles, the post-cycle capacity was measured in the same manner as in the above “Measurement of initial capacity”. The capacity retention rate was calculated according to the following formula. The results are shown in Table 1.
Formula: (capacity maintenance rate [unit%]) = (capacity after cycle) / (initial capacity) × 100.

  Further, the battery after 1000 cycles was disassembled, the negative electrode was collected, and it was confirmed whether or not delamination occurred at the interface between the upper layer (second layer) and the lower layer (first layer). The results are shown in Table 1.

<Results and discussion>
As shown in Table 1, in Examples 1 to 5, no delamination occurred, and the capacity retention rate was high. On the other hand, in Comparative Examples 1 to 5, delamination occurred and the capacity retention rate was low. From these results, it is clear that capacity reduction is caused by delamination.

  That is, in Comparative Examples 1 to 5, since the distortion occurs between the upper layer and the lower layer due to the expansion and contraction of the graphite-based negative electrode active material accompanying the charge / discharge cycle, this causes delamination and a part of the upper layer is electrically It is considered isolated.

  On the other hand, in Examples 1-5, the orientation degree of the upper layer (second layer) was controlled to 0.01% or more and 0.4% or less, and the orientation degree of the lower layer (first layer) was controlled to 0.5% or less. Therefore, the negative electrode active materials contained in the upper layer and the lower layer are both regulated to expand and contract in the thickness direction of the negative electrode, so that no distortion occurs between the layers and no delamination occurs, so that a high capacity can be maintained. Conceivable.

  Here, FIG. 14 shows the degree of orientation of the second layer 202b and the capacity retention ratio after the cycle when no magnetic field was applied to the first layer 202a (Examples 1 to 4 and Comparative Examples 1 to 3). It is a graph which shows the relationship. From FIG. 14, the capacity retention ratio has changed greatly before and after the point where the degree of orientation of the second layer 202b is 0.4%, and the degree of orientation is 0.01% or more and 0.4%. It can be seen that the capacity retention ratio is high when it falls within the following range. Therefore, when the degree of orientation of the second layer 202b is 0.01% or more and 0.4% or less, it can be said that the cycle characteristics are excellent.

As described above, a positive electrode and a negative electrode are provided. The negative electrode includes a negative electrode current collector, a first layer provided on the negative electrode current collector and containing a graphite-based negative electrode active material, and the first layer. A peak intensity I ₍₁₁₀₎ derived from the (110) plane of the graphite crystal by powder X-ray diffraction measurement, and derived from the (002) plane. When the percentage of the ratio I ₍₁₁₀₎ / I _{(002) to} the peak intensity I ₍₀₀₂₎ is taken as the degree of orientation, the degree of orientation of the second layer is 0.01% or more and 0.4% or less. The non-aqueous electrolyte secondary battery according to the example in which the degree of orientation of the first layer is 0.5% or less is compared with the non-aqueous electrolyte secondary battery of the comparative example that does not satisfy the same condition. It was confirmed that it was excellent in characteristics.

  Although the present embodiment and examples have been described as described above, it should be considered that the embodiments and examples disclosed this time are illustrative and not restrictive in all respects. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  2a 1st graphite negative electrode active material, 2b 2nd graphite negative electrode active material, 100 positive electrode, 101 positive electrode current collector, 102 positive electrode composite material layer, 200 negative electrode, 201 negative electrode current collector, 202 negative electrode composite material layer, 202a 1st layer, 202b 2nd layer, 300 separator, 400 electrode group, 500 battery outer package, 1000 battery, 20 adhesive tape, 50a, 50b, 60a, 60b magnet, 51, 61 magnetic field line, 70 die head.

Claims (1)

A positive electrode and a negative electrode,
The negative electrode is
A negative electrode current collector;
A first layer provided on the negative electrode current collector and containing a graphite-based negative electrode active material;
A second layer containing a graphite-based negative electrode active material provided on the first layer,
The ratio I ₍₁₁₀₎ / I ₍₀₀₂₎ of the peak intensity I ₍₁₁₀₎ derived from the (110) plane of the graphite crystal and the peak intensity I ₍₀₀₂₎ derived from the (002) plane by powder X-ray diffraction measurement When the percentage is the orientation degree,
The degree of orientation of the second layer is 0.01% or more and 0.4% or less,
The non-aqueous electrolyte secondary battery, wherein the degree of orientation of the first layer is 0.5% or less.

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