JP2011216242A - Method of manufacturing nonaqueous electrolytic secondary battery and positive electrode therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a nonaqueous electrolytic secondary battery excellent in cycling characteristic.SOLUTION: The method of manufacturing the positive electrode for the nonaqueous electrolytic secondary battery includes: a step of forming a positive-electrode combining-agent layer 2 with a positive-electrode active material, an active-material conductive agent, and an active-material binding agent on a positive-electrode electric collector 1; and a step of removing a dispersion medium to form the covered layer after a covered-layer conductive agent and a covered-layer binding agent are deposited on the positive-electrode combining-agent layer 2, by soaking an counter electrode 3 and the positive-electrode electric collector 1 with the positive-electrode combining-agent layer 2 formed thereon, in dispersion liquid 5 in which the covered-layer conductive agent and the covered-layer binding agent are dispersed in the dispersion medium, and by applying a DC voltage in between the positive-electrode electric collector 1 and the counter electrode 3. Both the covered-layer conductive agent and the covered-layer binding agent are charged to the same polarity in the dispersion medium.

Description

  The present invention relates to an improvement in the positive electrode of a nonaqueous electrolyte secondary battery.

  Nonaqueous electrolyte secondary batteries have high energy density and high capacity, and are therefore widely used as drive power sources for portable devices. In addition, non-aqueous electrolyte secondary batteries are used for applications that require high output, such as electric tools, electric assist bicycles, and electric vehicles (EV, HEV).

  Lithium transition metal composite oxides such as lithium cobalt composite oxide are used for the positive electrode active material of the non-aqueous electrolyte secondary battery. However, since lithium cobalt composite oxide has low conductivity, it is made of a carbon material or the like. Conductivity is increased as a positive electrode by mixing a conductive agent or forming a conductive coating layer on the positive electrode surface.

  The following patent documents 1-5 are mentioned as a technique regarding such a coating layer.

JP 2007-176070 A JP 2006-179320 A JP 2002-121697 Japanese Unexamined Patent Publication No. 2000-331686 JP 2000-208135 A

  Patent Document 1 discloses a conductive material in which a composite particle having resin particles and conductive particles attached to the surface of the resin particles is formed by laminating and bonding the resins on the surface of the composite particles to each other. A composite membrane is disclosed. According to this technique, a conductive composite film having high conductivity suitable for a fuel cell can be obtained.

  Patent Document 2 discloses that an electrode active material layer is formed by adding a conductivity-imparting material in the voids of an active material layer intermediate containing at least an active material. According to this technique, the electrical conductivity in the electrode active material layer can be improved.

  Patent Document 3 discloses a fluororesin-containing porous body in which a gas diffusion electrode material mainly composed of fluororesin fine particles contained in a dispersion liquid is deposited on the surface of a conductive substrate by electrophoresis. According to this technique, a fluororesin-containing porous body suitable for a gas diffusion electrode used in a fuel cell or the like can be obtained.

  Patent Document 4 discloses that a conductive material layer made of a carbon material, metal powder, conductive ceramic, or the like is provided on the surface of a positive electrode active material layer or a negative electrode active material layer. According to this technique, the cycle life characteristics can be improved.

  Patent Document 5 discloses a nonaqueous electrolyte battery in which a layer made of a carbon material is provided in a porous structure including a lithium transition metal composite oxide that reversibly absorbs and releases lithium ions, a mixture containing a conductive agent and a binder. A positive electrode for use is disclosed. According to this technique, it is said that lithium metal can be prevented from being deposited on the surface of the negative electrode plate due to the uneven charging depth of the positive electrode active material.

  However, the techniques according to Patent Documents 1 and 3 are applied to an electrode used in a fuel cell that generates power while supplying a fuel and an oxidant, and an electrode for a non-aqueous electrolyte secondary battery that generates power using a potential difference. It cannot be applied as it is. Further, the techniques according to Patent Documents 2, 4, and 5 have a problem that it is difficult to form a uniform and thin conductive coating layer on the surface of the positive electrode. If the conductive coating layer is non-uniform, the electron conductivity near the positive electrode surface that is far from the highly conductive positive electrode current collector becomes non-uniform. Variations occur in the electrochemical reaction, and a portion in which the positive electrode active material is locally degraded is generated. And since the uniform electrochemical reaction of the positive electrode is further inhibited by the deteriorated positive electrode active material and the deterioration of the positive electrode active material is promoted, there is a problem that cycle deterioration is likely to occur. Further, if the conductive coating layer is non-uniform, the non-aqueous electrolyte holding ability of the conductive coating layer tends to be insufficient, and cycle deterioration due to the shortage of the non-aqueous electrolyte of the positive electrode tends to occur. Furthermore, if the thickness of the conductive coating layer is thick, the electrochemical reaction on the positive electrode surface may be hindered and the discharge characteristics may be deteriorated.

  This invention is made in view of the above, Comprising: It aims at providing the positive electrode for nonaqueous electrolyte secondary batteries provided with a uniform electroconductive coating layer, and a nonaqueous electrolyte secondary battery using the same. .

The first aspect of the present invention for solving the above problems is configured as follows.
A positive electrode mixture layer forming step of forming a positive electrode mixture layer having a positive electrode active material, an active material conductive agent, and an active material binder on the positive electrode current collector; and a coating layer conductive agent and a coating layer binder; Is immersed in a dispersion liquid in which a positive electrode mixture layer is formed and a counter electrode, and a DC voltage is applied between the positive electrode current collector and the counter electrode. A coating layer forming step of forming the coating layer by removing the dispersion medium after depositing the coating layer conductive agent and the coating layer binder on the positive electrode mixture layer, The method for producing a positive electrode for a non-aqueous electrolyte secondary battery, wherein the coating layer conductive agent and the coating layer binder are charged to the same polarity in the dispersion medium.

  In this configuration, the coating layer conductive agent having a charge of the same polarity in the dispersion medium and the coating layer binder are deposited by moving (electrophoresis) to the surface of the positive electrode mixture layer by applying a DC voltage, and thereafter By removing the dispersion medium, a conductive coating layer is formed. In this method, the thickness of the coating layer to be formed can be controlled by controlling the applied voltage and the application time, and the coating layer can be formed uniformly. A positive electrode with a uniform and thin conductive coating layer has uniform electron conductivity in the horizontal direction with the positive electrode current collector near the surface of the positive electrode mixture layer, that is, at a distance from the positive electrode current collector. Therefore, there is no variation in the charge / discharge depth of the positive electrode active material near the surface of the positive electrode mixture layer. Thereby, local deterioration of the positive electrode active material does not occur.

  In addition, a positive electrode having a uniform and thin conductive coating layer is excellent in liquid retention and can sufficiently hold a nonaqueous electrolyte therein. In the above configuration, a sufficient amount of nonaqueous electrolyte is present on the surface of the positive electrode active material, and a smooth electrochemical reaction proceeds at the positive electrode, so that the positive electrode reaction becomes more uniform. In the configuration of the first aspect of the present invention, the uniformity of electron conductivity near the surface of the positive electrode and the excellent liquid retention function act synergistically, and the cycle characteristics can be drastically improved. Moreover, since the thickness of the coating layer can be reduced, there is no possibility that the electrochemical reaction on the positive electrode surface is hindered.

  Here, when the dispersion medium is decomposed by a voltage or current accompanying electrophoresis, it becomes difficult to form a uniform and thin coating layer. In order to prevent this, it is preferable to use an aprotic organic solvent as the dispersion medium. Among these, it is more preferable to use acetone and / or N-methylpyrrolidone.

  In addition, as the coating layer conductive agent, non-graphitizable carbon, artificial graphite, natural graphite, coke, glassy carbon, carbon fiber, carbon black, acetylene black, ketjen black, or one or more of these fired bodies Mixed carbon materials can be used. Among these, acetylene black is more preferably used. Moreover, what was illustrated by the coating layer electrically conductive agent can be used as an active material electrically conductive agent.

  In addition, the coating layer itself can be formed with the coating layer conductive agent alone, but the adhesion between the coating layer and the positive electrode mixture layer is not sufficient, and the coating layer collapses. For this reason, it is necessary to use a coating layer binder in addition to the coating layer conductive agent. The coating layer binder is a single or plural fluorine resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), or a copolymer thereof. Can be used by mixing seeds. Among these, it is more preferable to use polyvinylidene fluoride. Moreover, what was illustrated by the coating layer binder can be used as an active material binder.

  When the molecular weight of the polyvinylidene fluoride is too small, the adhesion to the electrode plate is lowered, and when the molecular weight is too large, the swelling property with the solvent is lowered. For this reason, it is preferable to use those having a weight average molecular weight of 120,000 to 1,000,000.

  Further, in order to deposit the coating layer conductive agent and the coating layer binder uniformly on the surface of the positive electrode mixture layer by electrophoresis, a highly dispersible dispersion of the coating layer conductive agent and the coating layer binder Is preferably used. For example, it is difficult to uniformly disperse each other by a method of adding a coating layer conductive agent to a dispersion medium in which a coating layer binder is swollen and stirring at high speed or a method of performing ultrasonic irradiation. On the other hand, using a mixing method that mechanically collides each other and applies shear stress, such as a bead mill or ball mill, a slurry-like dispersion in which the coating layer conductive agent and the coating layer binder are more uniformly dispersed. It becomes easy to prepare the liquid. When this method is used, it is preferable to mix (wet mixing) the coating layer binder swollen in the dispersion medium and the coating layer conductive agent, and it is preferable that the solid content ratio in the wet mixing is high. Thereafter, at the time of electrophoresis, a dispersion medium is appropriately added to lower the solid content ratio so that the optimal solid content ratio for forming the coating layer on the positive electrode mixture layer is obtained. Here, when mixing and uniformly dispersing the coating layer binder and the coating layer conductive agent, the solid content (coating layer binding) with respect to the entire dispersion (dispersion medium + coating layer binder + coating layer conductive agent) The mass ratio of (agent + coating layer conductive agent) is preferably 8 to 30% by mass. Thereafter, at the time of electrophoretic deposition, it is preferable to add a dispersion medium so that the solid content ratio is 0.4 to 2% by mass.

  The mass mixing ratio of the coating layer binder and the coating layer conductive agent is preferably 1: 0.5 to 1: 2.

  In addition, it is preferable to use a metal, an alloy, or the like that is not easily ionized as the counter electrode used for electrophoresis, for example, platinum or graphite.

  In addition, the applied voltage is preferably 10 to 200 V as a direct current voltage from the viewpoint of suppressing the decomposition of the dispersion medium. Further, if the distance between the electrode plates is too close, there is a fear of contact, and if it is too far, the efficiency of electrophoresis is lowered. Therefore, the distance between the electrode plates is preferably 5 to 50 mm. The temperature during electrophoresis is preferably kept uniform at 0 to 30 ° C. in order to keep the electrophoresis constant.

  The voltage application time for electrophoresis is preferably 10 seconds to 100 minutes. Moreover, when the thickness of the coating layer formed is too thin, the positive electrode mixture layer surface cannot be uniformly covered, and the effect of the coating layer cannot be sufficiently obtained. On the other hand, if the coating layer is too thick, the electrochemical reaction on the positive electrode surface may be hindered. Therefore, the thickness of the coating layer is preferably 0.1 to 10 μm.

  Here, the positive electrode current collector (positive electrode for a non-aqueous electrolyte secondary battery) on which the positive electrode mixture layer is formed has a polarity opposite to that of the coating layer binder and the coating layer conductive agent in the dispersion medium. By applying a voltage between the counter electrode and the counter electrode, a coating layer by electrophoretic deposition can be formed on the positive electrode mixture layer. However, if electrophoresis is performed so that the positive electrode side for a nonaqueous electrolyte secondary battery is negative, the positive electrode material may be damaged. Therefore, a positive voltage is applied to the positive electrode for the non-aqueous electrolyte secondary battery, and the coating layer binder, the coating layer conductive agent, and the dispersion medium are the same in the dispersion medium. It is preferable to use a negatively charged one. Specifically, when a dispersion solution is used in which a fluorine resin as a coating layer binder and a coating layer conductive agent are uniformly dispersed in a dispersion medium composed of an aprotic organic solvent while applying a shear stress. The coating layer binder and the coating layer conductive agent are negatively charged in the dispersion medium. For this reason, it is particularly preferable to use a fluororesin as the coating layer binder.

  The second aspect of the present invention for solving the above problems is a method of manufacturing a non-aqueous electrolyte secondary battery comprising the method of manufacturing a positive electrode for a non-aqueous electrolyte secondary battery according to the first aspect of the present invention.

  As explained above, according to the present invention, a non-aqueous electrolyte secondary battery having excellent cycle characteristics can be obtained.

FIG. 1 is a diagram illustrating an electrophoretic deposition method. FIG. 2 is a schematic diagram of the nonaqueous electrolyte secondary battery according to Example 1. FIG. 2A shows the electrode plate configuration of the electrode body, and FIG. 2B shows the appearance of the nonaqueous electrolyte secondary battery. Indicates.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated in detail through a following example. In addition, this invention is not limited to the following form, In the range which does not change the summary, it can change suitably and can implement.

(Example)
[Example 1]
[Production of positive electrode]
90 parts by mass of lithium nickel cobalt manganese composite oxide (LiNi _0.5 Co _0.25 Mn _0.25 O ₂ ) as a positive electrode active material, 5 parts by mass of carbon powder as an active material conductive agent, and active material binding 5 parts by mass of polytetrafluoroethylene as an agent and N-methyl-2-pyrrolidone were mixed to obtain a positive electrode active material slurry. The positive electrode active material slurry was applied to both sides of an aluminum current collector having a thickness of 15 μm by the doctor blade method, dried, rolled by a roller press, and cut into a size of 10 × 50 mm. The positive electrode plate which has 1 and the positive mix layer 2 was obtained.

(Coating layer forming process)
A dispersion solution (polyvinylidene fluoride concentration of 7% by mass) in which polyvinylidene fluoride (PVDF) having a weight average molecular weight of 300,000 as a coating layer binder is dispersed in acetone as a dispersion medium is rotated at a rotational speed of 400 rpm. While stirring, acetylene black having a specific surface area of 80 m ² / g as a coating layer conductive agent was gradually added and mixed. At this time, the mass ratio of PVDF to acetylene black was set to 1: 1. Then, the premixed solution for electrophoretic deposition in which PVDF and acetylene black were uniformly dispersed was obtained by mixing the above mixed solution while applying shear stress using a bead mill. The solid content ratio of the pre-solution (the total ratio of PVDF and acetylene black to the entire pre-solution) was 11% by mass.

  Thereafter, acetone was added and diluted so that the solid content ratio was 0.8% by mass to prepare a dispersion 5 for electrophoretic deposition. And as shown in FIG. 1, the said dispersion liquid 5 was put in the bathtub 4, and the positive electrode and the counter electrode 3 were immersed in the dispersion liquid. At this time, only the portion of the positive electrode plate where the positive electrode mixture layer 2 was formed was immersed in the dispersion 5. Thereafter, the positive electrode current collector 1 and the counter electrode (platinum electrode) 3 were connected to a DC power source 6 and applied with a DC voltage to perform electrophoretic deposition. Electrophoretic deposition conditions are as follows: a voltage is applied so that the positive electrode current collector 1 is positive and the counter electrode 3 is negative; the applied voltage is 100 V; the application time is 120 seconds; and the distance between the positive electrode mixture layer 2 and the counter electrode 3 is It was 10 mm. Since both PVDF and acetylene black are negatively charged in acetone, PVDF and acetylene black are deposited on the positively charged positive electrode mixture layer 2 surface. Thereafter, the positive electrode plate was pulled up from the dispersion and treated at 110 ° C. for 2 minutes to remove acetone as a dispersion medium, whereby a positive electrode 11 having a coating layer formed on the surface was obtained. In addition, the thickness of the coating layer was 4 μm.

(Production of negative electrode)
95 parts by mass of natural graphite powder, 5 parts by mass of polyvinylidene fluoride, and N-methyl-2-pyrrolidone were mixed to obtain a negative electrode active material slurry. This negative electrode active material slurry was applied to both surfaces of a 15 μm thick copper current collector by the doctor blade method, dried, rolled with a roller press, and cut into a size of 20 × 60 mm to obtain the negative electrode 12. Two negative electrodes are used per battery.

  Note that the potential of graphite during charging is 0.1 V on the basis of Li. In addition, the active material filling amount of the positive electrode and the negative electrode was adjusted so that the charge capacity ratio (negative electrode charge capacity / positive electrode charge capacity) of the positive electrode and the negative electrode was 1.20 at the potential of the positive electrode active material as a design standard.

(Production of electrode body)
Current collecting tabs 11a and 12a were attached to the positive electrode 11 and the negative electrode 12, respectively. As shown in FIG. 2A, the positive electrode 11, the two negative electrodes 12, the separator 13 made of two polypropylene microporous films, and the two glass plates 14 are connected to the glass plate 14 / the negative electrode 12. The electrode body was manufactured by superimposing / separator 13 / positive electrode 11 / separator 13 / negative electrode 12 / glass plate 14 in this order.

[Nonaqueous electrolyte adjustment]
Ethylene carbonate and dimethyl carbonate were mixed at a mass ratio of 3: 7, and LiPF ₆ as an electrolyte salt was dissolved to 1.0 M (mol / liter) to obtain a nonaqueous electrolyte.

[Assembling the battery]
The electrode body 16 is inserted into an exterior body 15 made of a laminate resin film, the electrolyte is injected, and the opening portion of the exterior can is sealed by ultrasonic welding to form a sealed portion 15a. A battery according to Example 1 was manufactured (see FIG. 2B). The design capacity of the nonaqueous electrolyte secondary battery according to Example 1 is 40 mAh.

[Example 2]
A nonaqueous electrolyte secondary battery according to Example 2 was fabricated in the same manner as in Example 1 except that the coating layer forming step was performed as follows.

(Coating layer forming process)
A dispersion (polyvinylidene fluoride concentration of 10% by mass) in which polyvinylidene fluoride (PVDF) having a weight average molecular weight of 300,000 as a coating layer binder is dispersed in N-methylpyrrolidone (NMP) as a dispersion medium While stirring at a rotational speed of 400 rpm, acetylene black having a specific surface area of 80 m ² / g as a coating layer conductive agent was gradually added and mixed. At this time, the mass ratio of PVDF to acetylene black was set to 1: 1. Then, the premixed solution for electrophoretic deposition in which PVDF and acetylene black were uniformly dispersed was obtained by mixing the above mixed solution while applying shear stress using a bead mill. In that case, solid content ratio (total ratio of PVDF and acetylene black with respect to the whole pre-solution) was 17 mass%.

  Thereafter, NMP was added and diluted so that the solid content ratio was 0.8% by mass to prepare a dispersion 5 for electrophoretic deposition. And as shown in FIG. 1, the said dispersion liquid 5 was put in the bathtub 4, and the positive electrode and the counter electrode 3 were immersed in the dispersion liquid. At this time, only the portion of the positive electrode plate where the positive electrode mixture layer 2 was formed was immersed in the dispersion 5. Thereafter, the positive electrode current collector 1 and the counter electrode (platinum electrode) 3 were connected to a DC power source 6 and applied with a DC voltage to perform electrophoretic deposition. The electrophoretic deposition conditions are as follows: a voltage is applied so that the positive electrode current collector 1 is positive and the counter electrode 3 is negative; the applied voltage is 110 V; the application time is 100 seconds; and the distance between the positive electrode mixture layer 2 and the counter electrode 3 is It was 10 mm. In NMP, both PVDF and acetylene black are negatively charged. Therefore, PVDF and acetylene black are deposited on the positively charged positive electrode mixture layer 2 surface. Thereafter, the positive electrode plate was pulled up from the dispersion and treated at 110 ° C. for 2 minutes to remove NMP as a dispersion medium, whereby a positive electrode 11 having a coating layer formed on the surface was obtained. In addition, the thickness of the coating layer was 4 μm.

[Comparative Example 1]
Comparative Example 1 was carried out in the same manner as in Example 1 except that the pre-solution for electrophoretic deposition was applied to the surface of the positive electrode mixture layer using a doctor blade, and acetone was volatilized and removed to form a coating layer. A non-aqueous electrolyte secondary battery was produced. The thickness of the coating layer was 4 μm.

[Comparative Example 2]
Comparative Example 2 was carried out in the same manner as in Example 2 except that the pre-solution for electrophoretic deposition was applied to the surface of the positive electrode mixture layer using a doctor blade, and NMP was volatilized and removed to form a coating layer. A non-aqueous electrolyte secondary battery was produced. The thickness of the coating layer was 4 μm.

[Comparative Example 3]
A non-aqueous electrolyte secondary battery according to Comparative Example 3 was produced in the same manner as in Example 1 except that a positive electrode plate without a coating layer was used as the positive electrode.

[Surface roughness measurement]
Positive electrodes were produced under the same conditions as in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, and the surface roughness of the coating layer surface was measured using a Keyence laser microscope (VK-9700). The results are shown in Table 1 below. As the surface roughness parameter, arithmetic average roughness Ra (a value obtained by summing and averaging the absolute values of deviations from the reference surface to the measurement curved surface) was used.

[Measurement of liquid retention]
A positive electrode was produced under the same conditions as in Examples 1 and 2 and Comparative Examples 1 to 3. The positive electrode was washed with diethyl carbonate and then dried. Then, the mass of each positive electrode was made constant, it was immersed in the said nonaqueous electrolyte until the liquid was completely absorbed, and the mass at this time was measured. Then, the liquid was exuded by pressurizing at a constant pressure of 5 kN / cm ² for 30 seconds, and the mass at this time was measured. And the liquid retention property was computed with the following formula | equation. The results are shown in Table 1 below.

Liquid retention (%) = the amount of liquid remaining after pressurization ÷ the amount of liquid when the liquid is completely sucked × 100

[High duty cycle characteristics test]
Batteries were produced under the same conditions as in Examples 1 and 2 and Comparative Examples 1 to 3. After that, each battery was subjected to constant current charging (current 40 mA, final voltage 4.2 V) -constant voltage charging (voltage 4.2 V, final current 0.8 mA) in a temperature environment of 25 ° C., and then 2 at a current value of 400 mA. Discharged to 50V. This charge / discharge cycle was performed 500 times. And the high duty cycle characteristic was computed with the following formula | equation. The results are shown in Table 1 below.

High duty cycle characteristics (%) = 1st cycle discharge capacity / 500th cycle discharge capacity x 100

  As shown in Table 1, Examples 1 and 2 in which the coating layer was formed by electrophoretic deposition had a liquid retention of 71% and 68%, a high duty cycle characteristic of 75% and 70%, and a doctor blade. It can be seen that the liquid retaining properties 53% and 55% of Comparative Examples 1 and 2 in which the coating layer was applied and formed were higher than the high duty cycle characteristics 59% and 61%. Moreover, it turns out that the comparative example 3 using the positive electrode which does not provide a coating layer is inferior to the comparative examples 1 and 2 with a liquid retention property of 49% and a high duty cycle characteristic of 51%.

  This is considered as follows. In Comparative Example 3 in which the coating layer is not formed, the liquid retention property improvement effect by the coating layer cannot be obtained, and thus the liquid retention property is significantly lowered. Moreover, since the coating layer having the coating layer conductive agent is not formed, the conductivity in the vicinity of the surface of the positive electrode mixture layer (the portion far from the positive electrode current collector) is low, so the vicinity of the surface of the positive electrode mixture layer In this case, the electron conductivity in the horizontal direction is different from that of the positive electrode current collector, and the charge / discharge depth is likely to vary. For this reason, the part which the positive electrode active material deteriorated locally will arise. The deteriorated positive electrode active material inhibits the uniform charge / discharge reaction of the positive electrode, and further promotes the deterioration of the positive electrode. In the case of performing high-load discharge, such a deterioration of the positive electrode is likely to occur, so that the high-duty cycle characteristics are remarkably deteriorated.

  In Examples 1 and 2 in which the coating layer was formed on the positive electrode surface using the electrophoretic deposition method, the arithmetic average roughness (Ra) of the coating layer was 0.7 and 0.6, and the doctor blade method was used. It is smaller than 1.3 and 1.4 of the comparative examples 1 and 2 which apply | coated the coating layer. A small arithmetic average roughness (Ra) means that the surface is smooth and uniform. Since the coating layer having a smooth surface and a uniform surface has gaps between the coating layer conductive agent and the coating layer binder, the liquid retention of the positive electrode is improved. In addition, since the conductivity near the surface of the positive electrode mixture layer is improved by the coating layer having a smooth surface, the positive electrode current collector and the horizontal electron conductivity may vary near the surface of the positive electrode mixture layer. Absent. For this reason, the problem as in Comparative Example 3 does not occur, and a smooth charge / discharge reaction is performed by the nonaqueous electrolyte that is sufficiently held inside the positive electrode. These synergistic actions dramatically improve the high duty cycle characteristics.

  On the other hand, in Comparative Examples 1 and 2, since the surface of the coating layer is rougher than in Examples 1 and 2, the liquid retention property and the conductivity improving effect near the surface of the positive electrode mixture layer are higher than those in Examples 1 and 2. Becomes smaller. For this reason, the high duty cycle characteristics are inferior to those of Examples 1 and 2 although they are superior to those of Comparative Example 3.

(extra content)
As the positive electrode active material, it is preferable to use a lithium transition metal composite oxide, a lithium transition metal phosphate compound having an olivine structure, or the like. Examples of lithium transition metal composite oxides include lithium cobalt composite oxide, lithium nickel composite oxide, spinel-type lithium manganese composite oxide, and some transition metal elements contained in these compounds substituted with other metal elements. Compounds are preferred. The lithium transition metal phosphate compound having an olivine structure is preferably lithium iron phosphate. These can be used alone, or can be used in combination of two or more. Moreover, you may add well-known additives, such as lithium carbonate, to a positive electrode.

As the negative electrode active material, it is preferable to use a carbon material, a titanium oxide, a metalloid element, an alloy, or the like. As the carbon material, natural graphite, artificial graphite, non-graphitizable carbon and the like are preferable. As the titanium oxide, LiTiO ₂ , TiO ₂ or the like is preferable. As the metalloid element, silicon, tin and the like are preferable. As the alloy, a Sn—Co alloy or the like is preferable. These can be used alone, or can be used in combination of two or more.

Further, as nonaqueous electrolyte solvents, cyclic carbonates typified by propylene carbonate / ethylene carbonate / butylene carbonate / vinylene carbonate, lactones typified by γ-butyrolactone / γ-valerolactone, diethyl carbonate, dimethyl carbonate, methyl ethyl Chain carbonate typified by carbonate, tetrahydrofuran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,3-dioxolane, 2-methoxytetrahydrofuran, ether typified by diethyl ether, etc. alone or in combination of two or more. Can be used. As the electrolyte salt in the nonaqueous _electrolyte, it is possible to use _{LiPF 6, LiAsF 6, LiClO 4} , LiBF 4, LiCF 3 SO 3, LiN (CF 3 SO 2) 2 and the like.

  As described above, according to the present invention, a coating layer having good conductivity and a nonaqueous electrolyte retention function can be formed on the surface of the positive electrode, and thereby the cycle characteristics of the battery can be dramatically improved. Therefore, industrial applicability is great.

1: Positive electrode current collector 2: Positive electrode mixture layer 3: Counter electrode (platinum electrode)
4: Bath 5: Dispersion 6: DC power source 11: Positive electrode 11a: Positive electrode tab 12: Negative electrode 12a: Negative electrode tab 13: Separator 14: Glass plate 15: Laminate outer package 15a: Sealing portion 16: Electrode body

Claims (4)

A positive electrode mixture layer forming step of forming a positive electrode mixture layer having a positive electrode active material, an active material conductive agent, and an active material binder on the positive electrode current collector;
A positive electrode current collector on which a positive electrode mixture layer is formed and a counter electrode are immersed in a dispersion liquid in which a coating layer conductive agent and a coating layer binder are dispersed in a dispersion medium. By applying a direct current voltage between the counter electrode and the coating layer conductive agent and the coating layer binder are deposited on the positive electrode mixture layer, the dispersion medium is removed to form a coating layer. A coating layer forming step to be formed;
With
The coating layer conductive agent and the coating layer binder are charged to the same polarity in the dispersion medium,
The manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries characterized by the above-mentioned. In the manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries of Claim 1,
The dispersion medium is an aprotic organic solvent,
The coating layer conductive agent is a carbon material,
The said coating layer binder is a fluorine resin. The manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries characterized by the above-mentioned. In the manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries of Claim 2,
The aprotic organic solvent is acetone and / or N-methylpyrrolidone;
The carbon material is acetylene black;
The fluororesin is polyvinylidene fluoride,
The manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries characterized by the above-mentioned.   The manufacturing method of a nonaqueous electrolyte secondary battery provided with the manufacturing method of the positive electrode for nonaqueous electrolyte secondary batteries of Claim 1, 2, or 3.