JP3503438B2 - Lithium ion secondary battery and method of manufacturing secondary battery - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention comprises an electrode containing active material particles and an organic binder , wherein the organic binder is polyvinyl fluoride.
Nilidene lithium-ion secondary battery and active material particles
Secondary battery provided with an electrode including a child and an organic binder (eg,
If, Ru <br/> Oh in a method of manufacturing a lithium-ion battery, etc.).

[0002]

2. Description of the Related Art As a secondary battery provided with electrodes (a positive electrode and a negative electrode) containing active material particles and an organic binder, for example, there is a lithium (Li) ion secondary battery having a high energy density. In the case of a Li-ion secondary battery, guests for the positive electrode and the negative electrode are Li and Li ions. Further, as the positive electrode active material, LiCoO ₂ , LiNi
Particles such as O ₂ , LiMnO ₂ and LiMn ₂ O ₄ are used, and carbon-based particles such as graphite are used as the negative electrode active material.

The positive and negative electrode active material particles are made of a current collector (Al,
It is held by Cu foil or the like to form an electrode, and the organic binder is polyvinylidene fluoride (PVDF).
Is often used. Examples of such a device include, for example, Japanese Patent Laid-Open Nos. 2-68855 and 8-250.
The one described in Japanese Patent Publication No. 127 has been proposed.

[0004]

The above organic binder is
As a so-called binder, which bonds the active material particles to each other so that the active material particles do not disperse, it plays a role of maintaining the binding property of the electrodes. However, on the other hand, since the organic binder covers the surface of the active material particles and inhibits the guest (Li ions or the like) from entering and leaving the active material particles (that is, the permeability of the guest is deteriorated). There is a problem that the discharge load characteristic of the battery is deteriorated (capacity is reduced during large current discharge).

By the way, in the conventional electrode structure containing the active material particles and the organic binder, there has been no proposal to achieve both the above-mentioned electrode binding property and guest permeability. In addition, maintaining the binding property of the electrode in the electrode configuration of only active material particles containing no organic binder is
It is difficult in reality. The present invention is made in view of the above disadvantages, a secondary battery comprising an electrode containing the active material particles and an organic binder, and an object that you realize the electrode configuration to achieve both binding property and guests permeability .

[0006]

The present inventors investigated and examined the electrode structure of a conventional secondary battery provided with an electrode containing active material particles and an organic binder by SEM observation or the like. As a result, PVDF which is a binder in the electrode
However, the surface of the active material particles is uniformly covered without any gaps,
Therefore, it was found that the guest was prevented from entering and leaving the active material, which is important for the battery reaction.

[0007] Therefore, based on the above-mentioned examination results, an earnest research is conducted by paying attention to the structure of the binder that coats the surface of the active material particle of the electrode. It was decided to adopt the technical means shown in. That is, the invention according to claim 1 is provided with an electrode containing active material particles and an organic binder, and the organic binder is a polyfluoride.
In a lithium ion secondary battery that is vinylidene chloride ,
It is characterized in that the organic binder is unevenly distributed so that it is thick at a binding contact portion between the active material particles of the electrode and thin at a portion other than the binding contact portion.

Here, the above-mentioned binding contact portion means a portion where the active material particles are in direct contact with each other or via an organic binding material, and thinning at a portion other than the binding contact portion means partially. It means that the organic binder does not exist (see FIG. 2B). Thereby, the amount of the binder is large in the part necessary for binding between the active material particles, while the amount of the binder is unevenly distributed in the part involved in the battery reaction so as to reduce the binding property and It is possible to realize an electrode configuration that is compatible with guest permeability, and it is possible to suppress a decrease in battery capacity during large current discharge.

Further, the degree of uneven distribution at the site of the others and the binder contacts, as a result of studying by using Auger electron spectroscopy, as claimed in claim 1, the binder contacts the binding It was found that better load characteristics (80% or more of discharge capacity at a discharge current of 4.5 A) can be realized when the elemental analysis peak intensity ratio to the portion other than the contact / contact portion is 2.0 or more.

Further, in a secondary battery including an electrode containing active material particles and an organic binder, an experimental study is advanced from the viewpoint of the manufacturing method, and the organic binder is the active material particles among the electrodes.
Thicker at the contact points between contacts, and thinner at areas other than the contact points
It has been found that the manufacturing method of the invention described in claims 2 to 4 can be used to manufacture the secondary battery unevenly distributed . That is, the invention according to claim 2 is a method of manufacturing a secondary battery, comprising: an electrode containing active material particles and an organic binder; and a current collector that holds the electrode and serves as an electron conductive path. While the organic binder is uniformly dissolved in the poor solvent of the organic binder by heating, active material particles are mixed to prepare a paste-like mixture, and the paste-like mixture is applied onto a current collector. The organic binder is then unevenly distributed by thickening the organic solvent by thickening the binding solvent between the active material particles and thinning it at a site other than the binding contact by evaporating the poor solvent. To do.

Here, the poor solvent has a poor solubility of the binder and is a term used in the opposite sense of a good solvent having a good solubility, and is also called a poor solvent. As a result, as shown in FIG. 4 described later, the solubility of the binder decreases in the process of vaporizing the poor solvent, so that uneven distribution (segregation) of the organic binder begins to occur in the initial stage of evaporation. Here, the binding joint between the active material particles is constricted with respect to the surface of the other active material particles, and this segregated organic binder easily gathers. An electrode structure in which the binder moves and finally the binder is unevenly distributed among the active material particles can be obtained.

[0012] According to a third aspect of the invention, among the manufacturing method according to claim 2, wherein the steps of applying a paste-like mixture on the current collector, a step of evaporating the poor solvent,
It is characterized in that the pasty mixture is cooled.

In the present invention, since the cooling step is performed between the both steps, the solubility of the poor solvent can be reduced more quickly, and the uneven distribution of the binder can be promoted. In the invention according to claim 4 , the organic binder is dissolved in a mixed solvent of a poor solvent for the organic binder and a good solvent having a lower boiling point than the poor solvent, and the active material particles are mixed. A paste-like mixture is prepared, the paste-like mixture is applied onto a current collector, and then the mixed solvent is evaporated, whereby the organic binder is formed at the binding contact portion between the active material particles of the electrode. It is characterized in that it is thick and unevenly distributed so as to be thin at a portion other than the binding contact portion.

In the present invention, since the mixed solvent of the poor solvent and the good solvent is used, the binder is uniformly dissolved by the action of the good solvent. Therefore, the above claims 4 and 5
As described above, the heating step for uniformly dissolving the binder can be omitted. Further, when the mixed solvent is evaporated, a good solvent having a low boiling point is evaporated first, so that the solubility becomes low with the evaporation and the segregation action described above finally causes uneven distribution of the binder between the active material particles. It can have an electrode structure. Therefore, also in the present invention, it is possible to provide the manufacturing method for manufacturing the secondary battery having the same effect as that of the invention described in claim 1.

Here, as the organic binder and the poor solvent in the above-mentioned claims 2 to 4 , those having a solubility parameter difference of 0.5 or more as described in claim 5 are used. Is preferred.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, but the present embodiment is a Li-ion secondary battery. This embodiment can be used, for example, in a mobile device such as a mobile phone or a portable personal computer. FIG. 1 shows the electrode structure of the secondary battery of this embodiment. Reference numeral 1 denotes a positive electrode of a battery, which is a main component of positive electrode active material particles (for example, lithium cobalt oxide), a conductive material (for example, graphite), and an organic binder as a binder (for example, polyvinylidene fluoride). Consists of. Reference numeral 2 denotes a positive electrode current collector (for example, aluminum foil) that structurally holds the positive electrode and serves as an electron conductive path, and the positive electrode 1 is fixed to the positive electrode current collector 2 by an organic binder.

Reference numeral 3 is a separator (for example, a polyethylene porous film) for electrically insulating the positive electrode and the negative electrode. Reference numeral 4 denotes a negative electrode of a battery, which is a main component of negative electrode active material particles (for example, spherical graphite) and an organic binder (for example, polyvinylidene fluoride, hereinafter P).
VDF). Reference numeral 5 denotes a negative electrode current collector (for example, copper foil) that structurally holds the negative electrode and serves as an electron conductive path, and the negative electrode 4 is fixed to the negative electrode current collector 5 with an organic binder.

Next, the binding structure of the active material particles and the organic binding material, which is a feature of the present invention, will be described with reference to FIG. As a comparison, FIG. 3 shows a binding structure of a conventional general secondary battery. The overall structure of the conventional secondary battery electrode structure of FIG. 3 is similar to that of FIG. The organic binder will be referred to as a binder hereinafter. In addition, the configuration, manufacturing method, and the like of the negative electrode 4 will be mainly described below.
The same can be said for.

2 and 3 are schematic views in which the negative electrode 4 is observed by an SEM (scanning electron microscope) or the like and its appearance is enlarged. In the conventional binding structure shown in FIG. 3, the surface of the negative electrode active material particles 4a is almost uniformly covered with the binder 4b, and no matter which part is compared, the coating film thickness of the binder 4b is It is almost the same. However, in the binding structure of the present embodiment shown in FIGS. 2A and 2B, the negative electrode active material particles 4a
The binding material 4b is thick on the binding contact portion (interface) 4c which is a narrowed portion, and the binding material 4b is formed on the other surface of the negative electrode active material particles 4a, that is, the portion other than the binding contact portion 4c.
The coating film thickness of is reduced, or as shown in FIG.
The unevenly distributed structure has a thin coating film thickness and partially does not have the binder 4b.

Therefore, a large amount of the binder 4b is unevenly distributed in the portion required for binding between the negative electrode active material particles 4a, and the binding property of the negative electrode 4 can be secured. Note that such an uneven distribution structure can be confirmed by SEM and Auger electron spectroscopy as described later. By the way, in the manufacturing method for binding the active material particles and the binder, conventionally, a good solvent (for example, N-methyl-2-pyrrolidone, hereinafter NM) for the binder (for example, PVDF) is used.
(Referred to as P) was used (see FIG. 5 described later). this is,
This is because the use of a good solvent that is excellent in the solubility of the binder has the advantage that a uniform paste is easily obtained in mixing with the active material particles to form a paste. Then, this paste was applied to a current collector, and a good solvent was dried and evaporated to form an electrode.

However, in the electrode binding structure according to the above conventional manufacturing method, as shown in FIG. 3, the surface of the negative electrode active material particles 4a is uniformly covered with the binding material 4b in the electrode state, and the battery is L to active material particles important for reaction
There is a problem that the i-ion is prevented from entering and exiting, and the capacity is lowered at the time of discharging a large current. On the other hand, in the present embodiment, a poor solvent (for example, ethyl acetoacetate) for the binder (for example, PVDF) is used as the manufacturing method for binding the active material particles and the binder. Here, the poor solvent has poor solubility in the binder, and is a term used in the opposite sense of the above good solvent, and is also called a non-good solvent.

Although various methods (described later) can be considered as the manufacturing method using this poor solvent, for example, the following method can be used. The binder is uniformly dissolved by heating the poor solvent to increase the solubility, and the active material particles are mixed while maintaining this uniform dissolved state to prepare a paste-like mixture. Then, this mixture is applied onto a current collector, and then the poor solvent is dried and evaporated. During this evaporation, the temperature of the poor solvent decreases and the solubility also decreases, so that the binder segregates in the poor solvent.

Incidentally, it has been conventionally considered that such segregation of the binding material is not preferable in order to realize a uniform binding property in the electrode. However, the present inventors have found that, as shown in FIG. 2, the binding joint portion 4c between the negative electrode active material particles 4a is constricted with respect to the surface of the other active material particles 4a, and the segregated binding material is We thought that it would be easier to get together, so we decided to change the way of thinking and rather to actively utilize this segregation.

The binding mechanism will be described with reference to the explanatory view shown in FIG. Since the binder has a low solubility in the drying process, uneven distribution (segregation) of the binder begins to occur in the initial stage of drying. Then, due to the difference in wettability between the active material particles and the binder, the segregated binder migrates to the stable binder contact portion, and finally the binder is unevenly distributed among the active material particles.
The electrode structure as shown in FIG.

In this production method, in order to reduce the solubility of the poor solvent and promote the uneven distribution of the binder, a cooling step may be provided between the mixture application step and the poor solvent evaporation step. The electrode structure can be realized. In this way, the positive electrode 1 and the negative electrode 4 can be formed on each of the current collectors 2 and 5 by utilizing the segregation of the binder. Then, as shown in FIG. 1, the electrodes 1, 4, the current collectors 2, 5, and the separator 3 are laminated to form a plurality of layers (laminates).
A lead (not shown) is connected to 4. Then, the laminated body is housed in a battery case (not shown), each lead is connected to a terminal (a positive electrode terminal, a negative electrode terminal) provided on the case, and an electrolytic solution is injected and sealed in the case. Li of the present embodiment
The ion secondary battery is completed.

Then, in the Li ion secondary battery of the present embodiment and the Li ion secondary battery having the conventional binding structure, the chemical diffusion coefficient of Li ion (guest) was measured by the current pulse relaxation method described later, It was confirmed that this embodiment has a larger chemical diffusion coefficient than the conventional one. Therefore, the active material particles 4a other than the binder contact portion 4c
It is possible to reduce the thickness of the binder 4b on the surface, and to perform lithium intercalation on the active material particles 4a,
Deintercalation can be facilitated, and good guest transparency can be realized in each of the electrodes 1 and 4.

As described above, according to the present embodiment, in the secondary battery provided with the electrodes 1 and 4 containing the active material particles and the organic binder, the portion necessary for the binding between the active material particles 4a is formed. ,
Since the amount of binder is large and the amount of binder is unevenly distributed in the part that is involved in the battery reaction so that the amount of binder is small, it is possible to realize an electrode configuration that achieves both binder properties and guest permeability, resulting in high current discharge. It is possible to suppress a decrease in battery capacity at that time.

Next, each Example 1 showing the present embodiment will be described below.
5 and a comparative example, a more detailed description will be given, but the present embodiment is not limited to these examples. (Example 1) In this example, in the electrode structure of the Li-ion secondary battery of FIG. 1, the negative electrode 4 is the binding structure of this embodiment shown in FIG. 2, and the positive electrode 1 is the conventional binding structure shown in FIG. It is what The positive electrode 1 is composed of positive electrode active material particles of 94 wt% lithium cobalt oxide and conductive material of graphite 4.
wt% and PVDF 2 wt% as a binder. The positive electrode current collector 2 is made of Al foil, and the separator 3 is made of polyethylene porous film. The negative electrode 4 was composed of 92.5 wt% of spherical graphite as the negative electrode active material particles and P as the binder.
It is composed of 7.5 wt% of VDF. The negative electrode current collector 5 is made of copper (C
u) foil.

Next, a method for manufacturing the negative electrode 1 of this embodiment will be described. In this example, ethyl acetoacetate, which is a poor solvent for PVDF, was selected as the solvent for the binder PVDF. The solubility parameter of ethyl acetoacetate is 10, which is about 1 distant from 11 of PVDF.
DF cannot be dissolved. First, ethyl acetoacetate and PVDF
PVDF by mixing with and heating to 120 ℃
Was dissolved uniformly.

Next, in order to prevent this solution from gelling,
The mixture was mixed with spherical graphite that is the negative electrode active material particles 4a in a state of being maintained at 110 ° C., and uniformly dispersed by a kneader, a stirrer, etc. to obtain a paste-like mixture (hereinafter referred to as paste). Subsequently, the paste was applied onto the negative electrode current collector 5 and dried, and then the electrode was compressed by a roll press or the like to increase the electrode density, and punched into an electrode shape by a blank die. It should be noted that the paste temperature is set to 1 in order to prevent the aggregation due to gelation before the application, in order to apply the paste to the negative electrode current collector 5.
While maintaining at 10 ° C., the negative electrode current collector 5 serving as a base material is also 90
It was carried out with the temperature kept at ℃.

The negative electrode 1 of this example manufactured as described above
Has the binding structure shown in FIG. (Comparative Example) In the electrode structure shown in FIG. 1, a specific conventional manufacturing method of the negative electrode 4 will be described below. In this conventional negative electrode manufacturing method, the binding structure is as shown in FIG. In addition, the negative electrode 4 is the same as in the above-described first embodiment, 92.5 wt% of spherical graphite as the negative electrode active material particles, and P as the binder.
The negative electrode current collector 5 is made of copper foil and is made of 7.5 wt% VDF.

The binder PVDF is uniformly dissolved in NMP, which is a good solvent for PVDF, at room temperature, and the spherical graphite, which is the negative electrode active material particles 4a, is mixed and uniformly dispersed by a kneader, a stirrer, etc. The paste was applied on the current collector 5 and dried, and then the electrode was compressed by a roll press or the like to increase the electrode density and punched into an electrode shape by a blank mold. According to this method, the binder has a high solubility in a solvent, a uniform binder distribution is obtained when the electrode is dried (see FIG. 5), and finally an electrode having the conventional binder structure shown in FIG. 3 is obtained.

By the way, as described above, in this embodiment, in order to evaluate the uneven distribution of the binder on the electrode surface, S
Elemental analysis is performed by EM observation and micro Auger electron spectroscopy. Next, this evaluation method will be described in the case of using the above-mentioned Example 1 and the comparative example. The sample was manufactured by using the negative electrodes of both examples and punching the electrode to a diameter of 15 mm. From the results of SEM observation, the analysis was performed by limiting the binding contact points between the negative electrode active material particles and the other places. The analyzer used was a model PHI670 manufactured by ULVAC-PHI, and the acceleration voltage was 10 KeV, the beam spot diameter was about 30 nm, and the electron beam current was 10 nA.

FIG. 6 is an SEM image of the binding structure,
(A) is the above example 1, and (b) is the comparative example.
Further, FIG. 7 is a schematic diagram of FIG. 6, in which (a) is the example 1 and (b) is the comparative example. The elemental analysis sites in each example are shown by crosses A1 and A in FIGS. 7 (a) and 7 (b).
2, B1 and B2. The results of the micro Auger analysis at these analysis sites are shown in FIG. Thus, at each analysis site, the carbon (C) peak in the negative electrode active material particles (spherical graphite) and the fluorine (F) peak in the binder appear. Therefore, the amount of the binder PVDF on the negative electrode active material particles is determined by the peak fluorine intensity in the binder. Here, since the carbon peak intensity is almost constant everywhere, the fluorine peak intensity was standardized as a ratio to the carbon peak intensity.

As shown in FIG. 8, in Example 1, the fluorine peak intensity of the binding contact portion (interface A1 in FIG. 7 (a)) was high, and the other portion (surface in FIG. 7 (a), surface). The fluorine peak intensity in A2) is so small that it is almost invisible, and the fluorine peak intensity ratio (fluorine peak intensity in A1 / fluorine peak intensity in A2) of both analysis sites is about 4
Met. Therefore, in Example 1, it was confirmed that a large amount of the binding material gathered at the binding contact portions of the active material particles, and the coating film thickness was thin in other places.

On the other hand, in the above-mentioned comparative example, that is, the negative electrode manufactured by the conventional method, the binding contact portion (in FIG.
1), the same fluorine peak intensity was obtained at other locations (surface B2 in FIG. 7B), and the intensity ratio of both fluorine peaks was about 1. Therefore, in the above comparative example, it is understood that the film thickness of the binder PVDF on the negative electrode active material particles is uniform regardless of the site.

Further, as described above, in this embodiment, in order to quantitatively grasp the permeability of the guest, the chemical diffusion coefficient (hereinafter, referred to as Li ion diffusion coefficient) D of Li ion by the current pulse relaxation method is used. Taking measurements. Next, this measurement will be described using the negative electrodes of Example 1 and Comparative Example described above. A pressure cell was used to measure the Li ion diffusion coefficient D. The same paste as in Example 1 was formed to a thickness of 18 μm.
Was coated on a copper foil (negative electrode current collector) and punched to a diameter of 15 mm to prepare a negative electrode having the binding structure shown in FIG. Lithium metal pressed onto a nickel mesh was used as the counter electrode, a separator having a thickness of 25 μm made of polyethylene was used, and an electrolytic solution was 1M LiPF ₆ / EC (ethylene carbonate): EMC (ethyl methyl carbonate) = 50: 50. . Also,
A pressure cell was similarly prepared for the comparative example.

To evaluate the effect of discharging a large current, the evaluation conditions were such that the potential of the graphite negative electrode was lowered to 0.005 V or less, a current of 20 mA was applied for 10 seconds in the direction of extracting Li ions from the graphite, and thereafter, It was determined by measuring the change with time of the graphite negative electrode potential.
The calculation of the Li ion diffusion coefficient D was performed using the following formula 1.

[0039]

[Equation 1]

Where V _M : molar volume of the electrode dE / dx: open circuit voltage-slope of Li composition x of composition (x) curve i: current (A) τ: duration of current pulse (s) n: reaction Number of electrons involved F: Faraday constant (C / m) a: Electrode area (cm ² ) ΔE: Potential change t: Time.

Then, the Li ion diffusion coefficient D (m ² / s) obtained for the negative electrodes of Example 1 and the comparative example, and the binding contact portion (interface) and the portion other than the binding contact portion of both of the above examples ( The relationship between the surface intensity) and the fluorine peak intensity ratio (hereinafter, simply referred to as peak intensity ratio) was plotted. The result is shown in FIG. Here, in FIG. 9, a ● mark indicates Example 1, and a □ mark indicates a comparative example.

In FIG. 9, it was found that the Li ion diffusion coefficient D of the negative electrode increases as the peak intensity ratio increases, and the two have a correlation. Here, FIG. 10 shows the relationship between the Li ion diffusion coefficient D of the negative electrode and the discharge capacity. Here, the negative electrode current density is 17 positive electrodes (positive electrode area 387).
cm ² = 3.70cm × 3.08cm × 17 sheets × 2) and the negative electrode (negative electrode area 422cm ² = 3.90cm × 3.18
cm × 17 sheets × 2), a negative electrode current density of 10.7 mA that realizes a high load discharge current of 4.5 A.
/ Cm ² (corresponding to a discharge current of 4.5 A). Then, two measured values (FIG.
It is obtained from the middle square mark) and the simulated value of Li ion transfer inside the battery (black mark in FIG. 10).

From FIG. 10, the discharge capacity at a discharge current of 4.5 A was calculated as 8% of the battery nominal capacity (0.2 C = 240 mA).
In order to achieve 0% (target value) or more, the negative electrode Li ion diffusion coefficient D is 3.10 × 10 ⁻¹⁴ m ² / s or more, and
In order to make it 90% or more of the battery's nominal capacity (0.2 C = 240 mA), the negative electrode Li ion diffusion coefficient D was set to 5.24.
It can be seen that it is necessary to set it to × 10 ⁻¹⁴ m ² / s or more.

In the relationship between the Li ion diffusion coefficient D and the peak intensity ratio shown in FIG. 9 described above, the peak intensity ratio must be set so that the negative electrode Li ion diffusion coefficient D becomes 3.10 × 10 ⁻¹⁴ m ² / s or more. Is preferably 2.0 or more. Therefore, by adopting the electrode structure of the present embodiment, the binder coating film thickness on the surface of the active material particles can be reduced, and the Li ion diffusion coefficient inside the battery can be increased.

Furthermore, the effect of the binding structure of this embodiment is
In order to confirm the actual discharge load characteristics of the battery,
In the electrode structure of No. 083, using the negative electrode of Example 1,
A 448 size prismatic battery (thickness 8 mm, width 34 mm, height 48 mm) was prototyped, and this battery was used as the battery of the present embodiment to evaluate the characteristics during large current discharge. The positive electrode 1 is composed of 94 wt% lithium cobalt oxide and 4 wt% conductive material (KS-6).
%, Binder PVDF 2 wt%, N-methyl 2 pyrrolidone 3
5 wt%, the negative electrode 4 is 92.5 wt% spherical graphite,
Binder PVDF 7.5wt%, ethyl acetoacetate 90w
An electrode was prepared by mixing t%. These electrodes are used as the positive electrode 17
A rectangular battery was manufactured by stacking 18 sheets of the negative electrode.

Here, in the battery of this embodiment,
Since the positive electrode 1 has a conventional binding structure and the negative electrode 4 uses the negative electrode of Example 1, it has the binding structure of this embodiment shown in FIG. The large-current discharge characteristics of this prismatic battery are shown in FIG. 11 (marked with ● in FIG. 11). The discharge capacity ratio on the vertical axis is a value normalized with the discharge capacity at a discharge load of 0.2 C as 1.

FIG. 11 shows the discharge load characteristics of a conventional battery in which the negative electrode of the prismatic battery is replaced with the negative electrode of the comparative example, that is, the prismatic battery having the conventional binding structure for both the positive electrode and the negative electrode. Values (□ mark in FIG. 11) are also shown. Further, the relationship between the Li ion diffusion coefficient D of the negative electrode and the large current characteristics in these batteries is shown in FIG.
2), the negative electrode of Example 1 is shown as a ● mark, and the negative electrode of Comparative Example is shown as a □ mark).

The battery of this embodiment is a battery using the negative electrode of Example 1, and the Li ion diffusion coefficient D on the negative electrode side.
As a result, the large current dischargeability is improved as compared with the conventional battery. Next, another example of this embodiment will be described below. (Example 2) In the negative electrode manufacturing method of Example 1 above, P
Even when propylene carbonate (solubility parameter 13.3) was used instead of ethyl acetoacetate as a poor solvent for VDF, the Li ion diffusion coefficient D was 2.50 × 1.
It shows almost the same value as 0 ^-13 (m ² / S). In addition, when various poor solvents were examined, it was found that the same effect as that of the above-mentioned Example 1 could be obtained by using those having a difference in solubility parameter between PVDF and solvent of 0.5 or more. . (Example 3) This example shows an example of using the good solvent and the poor solvent of the binder PVDF without heating during the preparation of the paste, as a method for unevenly distributing the binder in the negative electrode manufacturing method of the above-mentioned Example 1. . Dimethylimidazolidinone as a good solvent (boiling point 1
07.degree. C.), cyclohexanone (boiling point 155.degree. C.) as a poor solvent, and spherical graphite: PVDF: dimethylimidazolidinone: cyclohexanone 92.5:
Mix in a weight ratio of 7.5: 50: 50 and dissolve uniformly.

Next, this paste was applied to a copper foil (negative electrode current collector) and dried in an atmosphere kept at 80 ° C., so that dimethylimidazolidinone, which was a good solvent, was evaporated first, and as shown in FIG. The electrode structure in which the binder shown is unevenly distributed can be manufactured. The Li ion diffusion coefficient of the electrode according to this production method was 2.32 × 10 ⁻¹³ (m ² / S). (Example 4) An example in which a resin other than PVDF is used as the binder for the negative electrode in the above Example 1 will be described. Cellulose nitrate is used as a binder, and spherical graphite: cellulose nitrate: methyl acetate: ethanol: butanol: water:
Glycerin was added to 92.5: 7.5: 56: 26: 13.
Mix in a weight ratio of 6: 3.0: 1.4 and dissolve uniformly. Next, apply this paste to copper foil (negative electrode current collector),
By drying in an atmosphere kept at 80 ° C., the electrode structure in which the binder is unevenly distributed as shown in FIG. 2 can be realized.

The Li ion diffusion coefficient of the electrode prepared by this method was 2.40 × 10 ^-13 (m ² / S). (Example 5) An example in which the electrode manufacturing method of the present embodiment is applied to the positive electrode will be described. Lithium cobalt oxide: conductive material graphite: PVDF: ethyl acetoacetate 94: 4:
Mix in a weight ratio of 2: 100 and dissolve uniformly. next,
Apply this paste to aluminum foil (positive electrode current collector) and apply
By drying in an atmosphere kept at ° C, the electrode structure in which the binder is unevenly distributed as shown in Fig. 2 can be realized.

The Li ion diffusion coefficient of the electrode according to this manufacturing method is 2.40 × 10 ^-12 (m ² / S), and the Li ion diffusion coefficient of the positive electrode having the conventional structure is 2.80 × 10 ^-13 (m ² / S).
The value is one digit larger than that of S). In this embodiment, in addition to PVDF, as the binder, an olefin resin, a fluorine resin, an imide resin, an amide resin, a vinyl resin, and as a rubber binder, butyl rubber, butadiene rubber, SBR. , EPDM, etc. may be used. (Other Embodiments) In addition, in the electrode having the unevenly distributed binding structure shown in FIG. 2, an additive is added to the paste prepared by the above-described conventional electrode manufacturing method to activate the paste in which the binding material is melted. It can also be produced by reducing the wettability to the substance particles. Even with such a method, it was confirmed that the binder had an electrode structure as shown in FIG. 2 in which the binder was unevenly distributed among the active material particles. An example thereof is shown in Example 6 below. Example 6 A rectangular battery was manufactured according to the above method using an electrode prepared by adding 0.1% by weight of oxalic acid as an additive to the negative electrode paste prepared by the conventional method.
The large-current discharge characteristics of these batteries are shown in FIG. 13 (mark in FIG. 13: Example 6, mark: Comparative example). In this example, due to the uneven distribution of the binder that occurred in the drying step, the large current discharge property was improved as compared with the electrode having the conventional structure.

Other additives such as ethylene glycol, diiodomethane, tetrabromoethane, trinitrotoluene, nitroglycerin, nitrophenol, pyridazide, formamide, malonic acid nitrile, methanesulfonic acid and the like may be used. In each of the above embodiments and examples, a chalcogen compound such as lithium nickel oxide or lithium manganese oxide other than lithium cobalt oxide may be used as the positive electrode active material. As the negative electrode active material, carbon other than spherical graphite such as fibrous graphite, scaly graphite, lump graphite, amorphous carbon, and amorphous coated graphite may be used.

As the lithium salt of the electrolytic solution, Li
Other than PF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ S
O ₃ , the organic solvent is ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate,
It may be used by selecting from 1,2-dimethoxyethane, tetrahydrofuran, acetonitrile, ethylmethyl carbonate and the like.

In Example 1, since the binder was PVDF, the binder was constructed so that the analysis by Auger electron spectroscopy did not overlap with the carbon of the graphite of the negative electrode active material particles of the base. Although fluorine is used as the element to be applied, an element other than carbon may be selected in addition to fluorine, depending on the type of binder. Further, the above-described embodiments are not limited to those using Li ions as guests, and may be H ions, Na ions, or the like, for example.

[Brief description of drawings]

FIG. 1 is a sectional view showing an electrode structure of a secondary battery according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a binding structure of active material particles and an organic binding material in the above embodiment.

FIG. 3 is a schematic diagram showing a binding structure of conventional active material particles and an organic binding material.

FIG. 4 is an explanatory diagram showing a binding mechanism in the embodiment.

FIG. 5 is an explanatory diagram showing a conventional binding mechanism.

FIG. 6 is an SEM image of a binding structure, (a) shows Example 1 of the present invention, and (b) shows a comparative example.

7 is a schematic diagram of the SEM image of FIG. 6, in which (a) shows Example 1 of the present invention and (b) shows a comparative example.

8A and 8B are spectrum diagrams showing the results of micro-Auger analysis, in which FIG. 8A shows the example 1 and FIG. 8B shows the comparative example.

FIG. 9 is a graph showing a relationship between a negative electrode Li ion diffusion coefficient D and a fluorine peak intensity ratio (binding contact portion (interface) / portion other than binding contact portion (surface)).

FIG. 10 is a graph showing the relationship between the Li ion diffusion coefficient D of the negative electrode and the discharge capacity.

FIG. 11 is a graph showing large current discharge characteristics of the battery according to the above embodiment and the battery according to the comparative example.

FIG. 12 is a graph showing the relationship between negative electrode Li ion diffusion coefficient D and large current characteristics.

FIG. 13 is a graph showing large current discharge characteristics of a battery according to Example 6 of the present invention and a battery according to a comparative example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Positive electrode collector, 3 ... Separator, 4 ... Negative electrode, 4a ... Negative electrode active material particles, 4b ... Binder, 4c ... Binding contact part, 5 ... Negative electrode collector.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Manabu Yamada, 1-1, Showa-cho, Kariya city, Aichi Prefecture, DENSO CORPORATION (56) References JP-A-7-169465 (JP, A) JP-A-7-153453 (JP, A) JP 10-199569 (JP, A) JP 10-214629 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/00-4 / 62 H01M 10/40

Claims (5)

(57) [Claims] 1. An electrode comprising active material particles and an organic binder , wherein the organic binder is polyvinylidene fluoride.
In lithium ion secondary batteries, the organic binder is thicker in the binder contacts between the active material particles of the electrode, are unevenly distributed to be thinner at a site other than the binder contacts the As a degree of uneven distribution, the elements forming the organic binder are
Of the elements, fluorine was analyzed by Auger electron spectroscopy.
Of the binding contact part and a part other than the binding contact part.
A lithium ion secondary battery having an elemental analysis peak intensity ratio of 2.0 or more . 2. A method of manufacturing a secondary battery, comprising: an electrode containing active material particles and an organic binder; and a current collector that holds the electrode and serves as an electron conductive path. While uniformly dissolved in a poor solvent of the organic binder by heating, to prepare a paste mixture by mixing the active material particles, the paste mixture is applied on the current collector, then By evaporating the poor solvent, the organic binder is unevenly distributed so that it is thick at the binding contact portion between the active material particles of the electrode and thin at a portion other than the binding contact portion. And a method for manufacturing a secondary battery. 3. A step of applying the paste-like mixture on the current collector, according to claim 2, wherein the poor solvent with the step of evaporating, characterized by cooling the paste-like mixture A method for manufacturing a secondary battery according to 1. 4. A method for manufacturing a secondary battery, comprising: an electrode containing active material particles and an organic binder; and a current collector that holds the electrode and serves as an electron conductive path. In a mixed solvent of a poor solvent and a good solvent having a lower boiling point than this poor solvent, while dissolving the organic binder, to prepare a paste-like mixture by mixing the active material particles, the paste-like By applying a mixture on the current collector, and then evaporating the mixed solvent, the organic binder is thick at the binding contact portion between the active material particles of the electrode, the binding contact portion A method of manufacturing a secondary battery, characterized in that it is unevenly distributed so as to be thin in a region other than the above. 5. The organic binder and the poor solvent having a solubility parameter difference of 0.5 or more are used as the organic binder and the poor solvent according to any one of claims 2 to 4 . Manufacturing method of secondary battery.