JP2013073830A - Electrode for secondary battery and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an electrode for secondary battery having a multi-layer structure in which the drying time is shortened.SOLUTION: A conductive intermediate layer containing conductive particles is interposed between an electrode active material layer containing an electrode active material and a collector of a secondary battery. In the manufacturing method of an electrode for the secondary battery, a composition for forming a conductive intermediate layer produced by dispersing an intermediate layer material containing the conductive particles and a thermoplastic polymer into a predetermined solvent contains a thermoplastic polymer having a number-average molecular weight of 630,000 to 1,000,000. A composition for forming an electrode active material layer produced by dispersing an electrode active material and a binder into a predetermined solvent contains a thermoplastic polymer having a number-average molecular weight of 1,000,000 or more as the binder.

Description

  The present invention relates to an electrode for a secondary battery and a manufacturing method thereof. Specifically, the present invention relates to a secondary battery electrode having a conductive intermediate layer containing conductive particles and a method for manufacturing the same.

In recent years, lithium secondary batteries (typically lithium ion batteries) have become increasingly important as vehicle-mounted power supplies or personal computers and portable terminals. In particular, a lithium secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source mounted on a vehicle.
In one typical configuration of this type of secondary battery, an electrode material (electrode active material layer) including an electrode active material capable of reversibly occluding and releasing lithium ions serving as charge carriers is formed by a conductive member (current collector). Body). This electrode typically includes a paste-like electrode active material by first dispersing an electrode active material, a highly conductive material, and a solid material such as a binder (binder) in an appropriate solvent. A layer-forming composition is prepared, and this composition is applied in a layered manner on the surface of the current collector. Next, the applied composition is dried to remove the solvent, and an electrode active material layer containing the electrode active material is formed on the current collector (application method).

  Such a secondary battery electrode has a laminated structure in which a conductive intermediate layer is provided between an electrode active material layer and a current collector. Such a conductive intermediate layer ensures electrical conductivity between the electrode active material layer and the current collector in a normal charging state and discharging state. However, when the temperature of the secondary battery rises during overcharge, the resin (typically, the thermoplastic polymer) contained in the conductive intermediate layer crystallizes to become a high-resistance material, reducing current flow or (This function is hereinafter referred to as a “shutdown function”). Therefore, the conductive intermediate layer prevents further temperature rise of the secondary battery due to overcharging.

  As a resin in such a conductive intermediate layer, Patent Document 1 and Patent Document 2 disclose that a thermoplastic elastomer resin such as a fluorine resin, or a rubber resin such as a fluorine rubber can be used. Yes. Patent Document 3 discloses that a fluorine-containing polymer having a number average molecular weight Mw of 100,000 to 500,000 is used as a binder in an adhesion layer composed of a binder and a conductive auxiliary agent.

JP 2000-164206 A JP 2000-277146 A JP 2003-257433 A

  The electrode having a laminated structure having the conductive intermediate layer includes, for example, a composition prepared in the form of a paste for forming the conductive intermediate layer on the surface of the current collector (including slurry and ink. Hereinafter, simply. In some cases, it is referred to as “paste”.) Is applied and dried to form a conductive intermediate layer, and then a paste-like composition for forming an electrode active material layer is applied onto the intermediate layer and dried to form an electrode active material. It is common to form layers. However, in the step of drying the electrode active material layer, a situation may occur in which the thermoplastic polymer contained in the conductive intermediate layer diffuses into the electrode active material layer. Thereby, for example, it is conceivable that the shutdown function by the conductive intermediate layer is lowered.

  The present invention has been created in view of such a conventional situation, and an object thereof is a secondary battery capable of producing an electrode having a conductive intermediate layer without deteriorating the above-described shutdown function. It is providing the manufacturing method of the electrode for a vehicle.

In order to achieve the above object, the present invention provides a method for manufacturing an electrode for a secondary battery in which a conductive intermediate layer including conductive particles is interposed between an electrode active material layer including an electrode active material and a current collector. The method includes the following steps.
1: Step of preparing a composition for forming a conductive intermediate layer in which an intermediate layer material containing the conductive particles and the thermoplastic polymer is dispersed in a predetermined solvent 2: The composition for forming the conductive intermediate layer is collected as described above. A step of applying a layer on the surface of the electric body,
3: Step of drying the conductive intermediate layer forming composition applied to the current collector to form the conductive intermediate layer made of the intermediate layer material 4: Electrode active comprising the electrode active material and a binder Step of preparing an electrode active material layer forming composition in which a material is dispersed in a predetermined solvent 5: Step of applying the electrode active material layer forming composition on the upper surface of the conductive intermediate layer 6: Above A step of drying the composition for forming an electrode active material layer applied to a current collector to form the electrode active material layer made of the electrode active material.

In such a production method, the conductive intermediate layer forming composition contains a thermoplastic polymer having a number average molecular weight of 630,000 to less than 1 million as the thermoplastic polymer.
Thereby, in the drying process of the electrode active material layer, the thermoplastic polymer in the conductive intermediate layer is prevented from diffusing into the electrode active material layer, and the electrode for the secondary battery is preferably used without impairing the shutdown characteristics of the electrode. Can be manufactured.

  In a preferred embodiment of the method for producing an electrode for a secondary battery disclosed herein, the conductive intermediate layer forming composition is dried at a high temperature of 100 ° C. or higher and 150 ° C. or lower. By drying at such a high temperature, the time required for forming (drying) the conductive intermediate layer can be shortened.

  In a preferred aspect of the method for producing an electrode for a secondary battery disclosed herein, the composition for forming an electrode active material layer is dried at a high temperature of 120 ° C. or higher and 150 ° C. or lower. By enabling drying at such a high temperature, the time required for forming (drying) the electrode active material layer can be shortened.

  In a preferred embodiment of the method for producing an electrode for a secondary battery disclosed herein, the composition for forming an electrode active material layer is prepared using a thermoplastic polymer having a number average molecular weight of 1,000,000 or more as the binder. It is preferable. As a result, it is possible to manufacture a secondary battery electrode in which a decrease in capacity retention rate and a deterioration in load characteristics are suppressed.

  In another preferred embodiment of the method for producing an electrode for a secondary battery disclosed herein, the conductive intermediate layer forming composition is preferably prepared using polyvinylidene fluoride as the thermoplastic polymer. Polyvinylidene fluoride improves the stability and cycle characteristics of the conductive intermediate layer in the secondary battery electrode. Moreover, the shutdown effect which an electroconductive intermediate layer bears is exhibited favorably. Accordingly, an electrode for a secondary battery excellent in stability, cycle characteristics, and reliability can be manufactured.

  In another preferred embodiment of the method for producing an electrode for a secondary battery disclosed herein, the composition for forming a conductive intermediate layer is prepared so that a ratio of a solid material is 8% by mass or more. According to such a configuration, since the amount of the solvent for the conductive intermediate layer forming composition is reduced, it is possible to further shorten the time required for drying when forming the conductive intermediate layer.

  In the secondary battery electrode disclosed herein, a conductive intermediate layer including conductive particles and a thermoplastic polymer is interposed between an electrode active material layer including an electrode active material and a binder and a current collector. Has been. In such a configuration, the thermoplastic polymer contained in the conductive intermediate layer has a number average molecular weight of 630,000 or more and less than 1 million.

It is a typical sectional view showing the structure of the electrode concerning one embodiment. It is a flowchart of the manufacturing method of the electrode which concerns on one Embodiment. It is a typical perspective view which fractures | ruptures and shows a part of lithium secondary battery which concerns on one Embodiment. It is a figure which shows an example of the relationship between the number average molecular weight of PVdF, and the viscosity of the composition for electroconductive intermediate layer formation using the same. It is a figure which shows F element distribution in the cross section of electrode (A) (B) (D) which concerns on one Embodiment. It is a figure which shows an example of the relationship between the number average molecular weight of PVdF, and F element density | concentration in the electroconductive intermediate | middle layer of the electrode produced using this. It is a figure which shows an example of the relationship between environmental temperature and the absolute value of the internal resistance of an electrode.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

In the present specification, the “secondary battery” refers to a battery that can be repeatedly charged, such as a lithium secondary battery and a nickel metal hydride battery. In the present specification, the term “lithium secondary battery” refers to a general battery that can be repeatedly charged using lithium ions as a charge carrier. Typically, a secondary battery called a lithium ion battery, a lithium polymer battery, or the like is used. Include.
Further, in this specification, the “active material” can reversibly occlude and release (typically insertion and removal) chemical species (for example, lithium ions in a lithium secondary battery) that become charge carriers in the secondary battery. Refers to a substance.

  Here, as an embodiment of the electrode for a secondary battery, the present invention will be described by taking the production of a positive electrode for a lithium secondary battery as an example. FIG. 1 is a schematic cross-sectional view showing a configuration of a positive electrode for a lithium secondary battery. FIG. 2 is a flowchart for explaining the electrode manufacturing method disclosed herein. Hereinafter, the configuration and manufacturing method of the positive electrode 12 will be described in detail with reference to FIGS. 1 and 2.

≪Positive electrode manufacturing method≫
As shown in FIG. 1, in the positive electrode 12, a conductive intermediate layer 123 including conductive particles 50 is interposed between a positive electrode active material layer 124 including a positive electrode active material (not shown) and a current collector 122. . In manufacturing the positive electrode 12 having such a laminated structure (specifically, a two-layer structure), as shown in FIG. 2, first, in step S10, the conductive intermediate layer 123 is layered on the surface of the current collector 122. Form. Next, in step S <b> 20, the positive electrode active material layer 124 is formed on the conductive intermediate layer 123. The conductive intermediate layer 123 and the positive electrode active material layer 124 can be formed by applying a composition obtained by dispersing the constituent materials of each layer in a solvent to a predetermined range and drying it.

<1. Step of preparing conductive intermediate layer forming composition: S12>
The conductive intermediate layer 123 typically contains the conductive particles 50 and the thermoplastic polymer 60. The conductive intermediate layer 123 functions as a good conductor in a normal charge state and a discharge state, but increases resistance in an overcharge state, and exhibits a function of reducing or shutting down (shutdown) a current flow. Further, the thermoplastic polymer 60 functions as a binder that bonds the conductive particles 50 to each other and as a fixing agent that fixes the conductive intermediate layer 123 to the current collector 122. Although the scope of the present invention is not particularly limited, the reason why the resistance of the conductive intermediate layer increases during overcharge is that the conductive particles 50 are scattered on the surface of the conductive particles 50 during normal use. This is considered to be because the thermoplastic polymer 60 that has been bound to crystallizes due to a temperature rise accompanying overcharge to form a resistor, thereby preventing a high current from flowing between the conductive particles 50. .
In order to form such a conductive intermediate layer 123, first, as shown in step S12 of FIG. 1, a conductive layer in which an intermediate layer material containing conductive particles 50 and a thermoplastic polymer 60 is dispersed in a predetermined solvent is used. A composition for forming a conductive intermediate layer is prepared.

  As the conductive particles 50, for example, carbon powder can be preferably used. Further, as the conductive particles 50, conductive metal powder such as nickel powder may be used. Any of these may be used alone or in combination of two or more. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, and the like can be used. Of these, acetylene black can be particularly preferably employed. For example, a powdery conductive material (for example, a powdery carbon material such as acetylene black) having an average particle size of constituent particles (typically primary particles) in the range of about 10 nm to 200 nm, preferably about 20 nm to 100 nm. Can be used.

  Although the invention disclosed herein is not specified, the conductive intermediate layer 123 may include conductive particles 50 having different average particle diameters. In FIG. 1, as the conductive particles 50, first conductive particles 51 whose average particle size is a first particle size value, and second conductive particles 52 whose average particle size is a second particle size value, An example is included. Here, it is assumed that the second particle size value is larger than the first particle size value. For example, the mass ratio of the second conductive particles 52 in the intermediate layer 123 may be 10% to 60% (typically more than 10% and less than 60%, preferably 14% or more and 59% or less). . Note that the first conductive particles 51 and the second conductive particles 52 may be made of the same material or different materials. In addition, the conductive intermediate layer 123 may include three or more types of conductive particles 50 having different average particle diameters. In this case, for example, the mass ratio of the conductive particles having the largest average particle diameter may be 10% to 60% (typically more than 10% and less than 60%, preferably 14% or more and 59% or less). The conductive particles having the smallest and largest average particle diameter may be regarded as the first and second conductive particles, respectively.

  As the thermoplastic polymer 60, various conventionally known materials can be used. For example, halogen resins such as polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polytetrafluoroethylene (PTFE); polyolefin resins such as polyethylene and polypropylene; rubber systems such as styrene butadiene rubber (SBR) and fluoro rubber Various thermoplastic resins such as elastomers; acrylic resins; ethylene-vinyl acetate copolymers; and the like can be used. It is preferable that the thermoplastic polymer 60 is PVDF because it is easy to form a low-resistance conductive intermediate layer during normal use and the resistance is likely to increase greatly during overcharging.

  In the invention disclosed herein, the thermoplastic polymer 60 has a number average molecular weight of 630,000 or more and less than 1 million. In the present invention, the definition of the number average molecular weight of the thermoplastic polymer 60 is important. When the number average molecular weight is 1,000,000 or more, the viscosity at the time of preparing the composition for forming a conductive intermediate layer is too high, so that it is difficult to obtain applicability and it becomes impossible to produce an electrode suitably. Further, if the number average molecular weight is 1,000,000 or more, it is not preferable because the sensitivity of the swellability of the thermoplastic polymer as the temperature rises increases and the internal resistance of the battery is increased. The lower limit of the number average molecular weight of the thermoplastic polymer 60 is 630,000. When the thermoplastic polymer 60 having a number average molecular weight of less than 630,000 is used, when an electrode is produced by the production method disclosed herein, a post-process (specifically, a step of drying the composition for forming a positive electrode active material layer: This is not preferable because the phenomenon that the thermoplastic polymer 60 diffuses to the electrode active material layer 124 is noticeable in S26). When the thermoplastic polymer 60 scattered on the surface of the conductive particles 50 moves to the electrode active material layer 124, the binding force between the conductive particles 50 and the adhesiveness to the current collector 122 in the normal state. In addition to being weakened, the shutdown function is not sufficiently exhibited during overcharge. From the above, the number average molecular weight of the thermoplastic polymer 60 constituting the conductive intermediate layer 123 is set to be 630,000 or more and less than 1 million. More preferably, it is 650,000 to 950,000, and more specifically 700,000 to less than 850,000. By controlling the number average molecular weight of the thermoplastic polymer 60 within the above range, the fluidity of the composition for forming a conductive intermediate layer is increased, so that the amount of solvent in this composition can be reduced. .

In the intermediate layer material, the mass ratio of the conductive particles 50 is typically equal to or greater than the mass ratio of the thermoplastic polymer 60. That is, the mass ratio (conductive particles: thermoplastic polymer) between the conductive particles 50 and the thermoplastic polymer 60 in the intermediate layer material constituting the conductive intermediate layer 123 is 98: 2 to 50:50. Preferably, it is typically 96: 4 to 70:30.
As a solvent in which the intermediate layer material is dispersed, a solvent capable of dissolving or dispersing the thermoplastic polymer 60 (which may be a mixed solvent) can be appropriately selected in consideration of a combination with the thermoplastic polymer 60 to be used. . For example, when PVDF is used as the thermoplastic polymer 60, an organic solvent (non-aqueous solvent) used for preparing a conventional composition for forming a solvent-based active material layer can be suitably used. Examples of the organic solvent include N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone, toluene and the like. Among these, for example, NMP can be preferably employed.

  In the invention disclosed herein, the solid content concentration of the intermediate layer material in the conductive intermediate layer forming composition (non-volatile content, that is, the ratio of the intermediate layer material to the entire conductive intermediate layer forming composition. "NV") can be, for example, 8 mass% or more. By setting NV to 8% by mass or more, the amount of solvent in the composition for forming a conductive intermediate layer is reduced, and the time required for removing (ie, drying) the solvent from the composition for forming a conductive intermediate layer is effective. Can be shortened. When NV is less than 8% by mass, the effect of shortening the drying time cannot be obtained sufficiently, which is not preferable. NV is preferably 8.5% by mass or more, more preferably 9% by mass or more, and further preferably 9.3% by mass or more. On the other hand, if NV is too high, the handleability of the composition for forming a conductive intermediate layer (for example, coating properties when the composition is applied to a current collector (particularly a foil-shaped current collector)) decreases. May be easier. Although the upper limit of NV is not particularly defined, it is exemplified that it is 30% by mass or less, preferably about 20% by mass or less as an approximate guide.

The dispersion of the above intermediate layer material in the solvent is, for example, the intermediate layer material such as the conductive particles 50 and the thermoplastic polymer 60, and various additives such as a dispersant and a thickener as necessary. It can be carried out by putting a solvent into a mixer and mixing them.
Although not particularly limited, in preparing a composition for forming a conductive intermediate layer in the case where two or more kinds of particles having different average particle sizes are used as the conductive particles 50, first, a second particle having a larger average particle size is used. The conductive particles 52 and the thermoplastic polymer 60 are mixed with a solvent. Thereafter, it is possible to preferably adopt a mode in which the first conductive particles 51 having a small average particle diameter are charged and mixed and dispersed by this mixture. According to such an embodiment, it is possible to suitably prepare a conductive intermediate layer composition in which the first conductive particles 51 having a relatively small average particle size (and thus uniform dispersion tends to be difficult) are uniformly dispersed. . The introduction of the first conductive particles 51 may be performed at a time, may be divided into several times, or may be performed gradually (continuously). For example, the total amount of the first conductive particles 51 to be used is divided into 2 to 50 times (more preferably 3 to 20 times, for example, 5 to 10 times) (typically equally divided), and they are separated at a predetermined interval ( For example, it is preferable to supply at intervals of about 3 to 10 minutes. Note that either the thermoplastic polymer 60 or the second conductive particles 52 may be mixed with the solvent first, or both may be mixed with the solvent substantially simultaneously.

  About said mixing, it can carry out using a well-known various apparatus. For example, a general mixer or kneader used for the preparation of the active material layer forming paste can be used. For example, an apparatus capable of preparing a paste called a kneader, a stirrer, a disperser, a mixer, or the like can be used.

<2. Step of applying conductive intermediate layer forming composition: S14>
Next, the conductive intermediate layer forming composition prepared as described above is applied in layers to the surface of the current collector 122 as shown in step S14.
As the current collector 122, similarly to the current collector used for the positive electrode of a conventional secondary battery (typically a lithium secondary battery), a metal having good conductivity, an alloy thereof, or the like can be used. For example, a metal mainly composed of copper, aluminum, nickel, titanium, iron or the like or an alloy thereof, and more preferably aluminum or an aluminum alloy. Various shapes can be taken into consideration for the shape of the current collector 122 depending on the shape of the desired secondary battery and the like. For example, it may be in various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. In the present embodiment, a sheet-like aluminum current collector 122 is used. For example, an aluminum sheet having a thickness of about 10 μm to 30 μm can be suitably used.

About application | coating of the composition for electroconductive intermediate | middle layer formation to the collector 122 surface, it can carry out using well-known various coating apparatuses. For example, the paste can be applied to one side or both sides of the current collector using a coater. The coater only needs to be able to apply the paste to the current collector. For example, a slit coater, die coater, gravure coater, roll coater, doctor blade coater, comma coater, etc. Can do. Application of the composition for forming a conductive intermediate layer may be performed on both surfaces of the current collector 122, or may be performed on either one surface. The coating amount of the intermediate layer forming composition is not particularly limited. However, if the coating amount is too small, it is difficult to obtain an effect of reducing or blocking current during overcharge. If the coating amount is too large, the sheet resistance of the positive electrode 10 is large. It tends to be. Usually, the coating amount (weight per unit area) can be about 0.1 to 10 g / m ² (solid content basis) per one side of the current collector, for example, about 1 to 5 g / m ² (solid content basis). It is preferable that

<3. Step of drying conductive intermediate layer forming composition: S16>
Next, in step S <b> 16, the conductive intermediate layer forming composition applied to the current collector 122 is dried to form the conductive intermediate layer 123 made of the above intermediate layer material on the surface of the current collector 122. The drying is not particularly limited as long as it can remove excess volatile components, and an appropriate means can be adopted as necessary. For example, as the means for performing the drying, an appropriate drying device such as a hot air device, various infrared devices, an electromagnetic induction device, a microwave device, or a drying accelerating device such as a blower can be used. After drying, the shape of the conductive intermediate layer 123 may be adjusted to a desired shape by pressing the whole as necessary.
In the invention disclosed herein, since the amount of the solvent in the composition for forming a conductive intermediate layer is reduced, drying can be performed in a shorter time. For example, compared with the case where the composition in which the amount of solvents is not reduced is used, for example, the drying time can be shortened by about 20 to 50%. Furthermore, in the manufacturing method disclosed herein, high-speed drying at a high temperature can be employed. The high-speed drying of the composition for forming a conductive intermediate layer can be appropriately set, for example, in a high temperature range of 100 ° C. or higher and 150 ° C. or lower. Specifically, when the basis weight of the composition for forming a conductive intermediate layer is about 0.1 to 0.5 mg / cm ² , drying can be performed at 100 to 150 ° C. for 20 to 40 seconds. Therefore, efficient drying can be realized in a short time.

<4. Step of preparing a composition for forming a positive electrode active material layer: S22>
In step S22, typically, a positive electrode active material material including a positive electrode active material, a conductive material, and a binder is dispersed in a predetermined solvent to prepare a positive electrode active material layer forming composition.
As the positive electrode active material, a material capable of inserting and extracting lithium is used, and one or more of various materials conventionally used in lithium secondary batteries can be used without particular limitation. As such a positive electrode active material, a lithium transition metal oxide (typically in particulate form) is preferably used. Typically, a layered structure oxide or a spinel structure oxide is appropriately selected and used. can do. For example, selected from a lithium nickel oxide (typically LiNiO ₂ ), a lithium cobalt oxide (typically LiCoO ₂ ), and a lithium manganese oxide (typically LiMn ₂ O ₄ ). The use of one or more lithium transition metal oxides is preferred.

  Here, the “lithium nickel oxide” refers to an oxide having Li and Ni as constituent metal elements, and one or more metal elements other than Li and Ni (that is, other than Li and Ni). The ratio of transition metal element and / or typical metal element) to the same level as Ni or less than Ni (in terms of the number of atoms. When two or more kinds of metal elements other than Li and Ni are included, both of them are less than Ni. It is also meant to include composite oxides contained in a ratio). The metal element is selected from the group consisting of, for example, Co, Al, Mn, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, copper, Zn, Ga, In, Sn, La, and Ce. Or one or more elements.

Other general formulas:
_{Li (Li a Mn x Co y} Ni z) O 2
(A, x, y, z in the previous equation are real numbers satisfying a + x + y + z = 1)
A so-called ternary lithium transition metal oxide containing three kinds of transition metal elements, and a general formula:
xLi [Li _1/3 Mn _2/3 ] O _2. (1-x) LiMeO ₂
(In the above formula, Me is one or more transition metals, and x satisfies 0 <x ≦ 1)
It may be a so-called solid solution type lithium-excess transition metal oxide or the like.

Further, the positive electrode active material has a general formula of LiMAO ₄ (where M is at least one metal element selected from the group consisting of Fe, Co, Ni and Mn, and A is P, Si, S and And an anion selected from the group consisting of V.).

  The compound which comprises such a positive electrode active material can be prepared and prepared by a well-known method, for example. For example, some raw material compounds appropriately selected according to the composition of the target positive electrode active material are mixed at a predetermined ratio, and the mixture is fired by an appropriate means. Thereby, the oxide as a compound which comprises a positive electrode active material can be prepared. In addition, the preparation method of a positive electrode active material (typically lithium transition metal oxide) itself does not characterize this invention at all.

  Moreover, although there is no strict restriction | limiting about the shape of a positive electrode active material, the positive electrode active material prepared as mentioned above can be grind | pulverized, granulated, and classified by an appropriate means. For example, a lithium transition metal oxide powder substantially composed of secondary particles having an average particle size in the range of about 1 μm to 25 μm (typically about 2 μm to 15 μm) is used as the positive electrode in the technology disclosed herein. It can preferably be employed as an active material. Thereby, the granular positive electrode active material powder substantially comprised by the secondary particle which has a desired average particle diameter and / or particle size distribution can be obtained.

  In the case where a conductive material is included in the positive electrode active material layer formed in the positive electrode disclosed herein, the conductive material is not limited to a specific material and is conventionally used in this type of secondary battery. Various kinds of conductive materials can be used. For example, a carbon material such as carbon powder or carbon fiber is used as the conductive material. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, and the like can be used. Among these, you may use together 1 type, or 2 or more types. The conductive material is, for example, a granular conductive material (for example, a granular carbon material such as acetylene black) in which the average particle size of constituent particles (typically primary particles) is in the range of about 10 nm to 200 nm (for example, about 20 nm to 100 nm). Is preferred.

  When a carbon material is used as the conductive material used for manufacturing the electrode, it is preferable to select a carbon material (for example, acetylene black) having a small volatile content. The low volatile content of the carbon material can be related to the low functional groups on the surface of the carbon material. A carbon material having a small number of surface functional groups, for example, generates a gas by contacting the carbon material with an electrolyte (typically liquid) when a battery is constructed using the carbon material and conditioning is performed by a conventional method. This is preferable because the action tends to be small (as a result, the amount of gas generated by conditioning is small). For example, it is preferable to use a carbon material whose volatile content measured in accordance with JIS K6221 is approximately 1% or less (typically approximately 0.1 to 1%).

The binder has a function of binding each particle of the positive electrode active material and the conductive material contained in the positive electrode active material layer, or binding these particles and the positive electrode current collector. As such a binder, a polymer that can be dissolved or dispersed in a solvent used for forming the positive electrode active material layer can be used, and the material used for the composition for forming a conductive intermediate layer is similarly adopted. can do. Specifically, when an aqueous solvent is used as a solvent, carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose are used as water-soluble (water-soluble) polymer materials. Examples thereof include cellulose polymers such as (HPMC); polyvinyl alcohol (PVA); Examples of polymer materials that are dispersed in water (water-dispersible) include vinyl polymers such as polyethylene (PE) and polypropylene (PP); polyethylene oxide (PEO), polytetrafluoroethylene (PTFE), and tetrafluoroethylene. -Fluororesin such as hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA); vinyl acetate copolymer; styrene butadiene rubber (SBR), acrylic acid modified SBR resin Examples thereof include rubbers such as (SBR latex).
When a non-aqueous solvent is used as a solvent, a polymer (polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), etc.) can be preferably used.
In addition, the polymer material exemplified as the binder exhibits a function as a thickener and other additives prepared in the electrode active material layer forming composition for forming the electrode active material layer in addition to the function as the binder. May also be used.

  As the solvent, any of an aqueous solvent and a non-aqueous solvent can be used. Examples of the aqueous solvent include water and a composition using a mixed solvent mainly composed of water (aqueous solvent). As a solvent other than water constituting the mixed solvent, one or two or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP).

The above-described dispersion of the positive electrode active material in the solvent can be carried out in the same manner as the preparation of the conductive intermediate layer forming composition. Specifically, for example, a positive electrode active material such as the above positive electrode active material, conductive material, and binder, and various additives such as a dispersant and a thickener as necessary, and a solvent are charged into a mixer. Knead.
The ratio of the positive electrode active material to the entire positive electrode active material layer 124 (typically substantially the same as the ratio of the positive electrode active material to the solid content of the positive electrode active material composition) is about 50% by mass or more (typical Is preferably 50 to 95% by mass), more preferably about 75 to 90% by mass. In the positive electrode active material layer 124 having a composition including a conductive material, the proportion of the conductive material in the active material layer 124 can be, for example, approximately 3 to 25% by mass, and preferably approximately 3 to 15% by mass. In this case, the proportion of the positive electrode active material in the active material layer 124 is suitably about 80 to 95% by mass (for example, 85 to 95% by mass).
Moreover, there is no restriction | limiting in particular about the ratio (solid content concentration) of the positive electrode active material material in the composition for positive electrode active material layer formation. For example, it is preferable to adjust the solid content concentration with about 50 to 60% as a guide.
As a mixer for kneading, a general kneader used for preparing an active material layer forming paste can be used. For example, an apparatus capable of preparing a paste called a kneader, a stirrer, a disperser, or a mixer can be used.

<5. Step of applying composition for forming positive electrode active material layer: S24>
Next, the positive electrode active material layer forming composition prepared as described above is applied in layers on the upper surface of the conductive intermediate layer 123 formed as described above in step S24.
The application of the composition for forming a positive electrode active material layer can also be performed using various known coating apparatuses, and can be performed in the same manner as the application of the composition for forming a conductive intermediate layer. The application amount of the composition for forming an active material layer is not particularly limited, and can be arbitrarily set depending on the use of a secondary battery including a target electrode, for example. For example, it can be appropriately set within a range of about 3 to 50 mg / cm ² .

  The applied composition for forming a positive electrode active material layer is pressed as necessary or cut into a desired size to obtain an electrode having a desired thickness and size. As a pressing (compression) method, conventionally known compression methods such as a roll pressing method and a flat plate pressing method can be employed. Thereby, the thickness of the positive electrode active material layer 124 and the positive electrode 12 can be adjusted. In adjusting the thickness of the positive electrode active material layer 124, the thickness may be measured with a film thickness measuring instrument, and may be compressed a plurality of times until a desired thickness is obtained by adjusting the press pressure.

<6. Step of drying positive electrode active material layer forming composition: S26>
Finally, in step S26, the applied composition for forming a positive electrode active material layer is dried to form a positive electrode active material layer made of a positive electrode active material on the conductive intermediate layer. This drying is similar to the drying of the conductive intermediate layer, and is not particularly limited as long as it can remove excess volatile components, and an appropriate means can be adopted as necessary. And in the manufacturing method disclosed here, since the number average molecular weight of the thermoplastic polymer of an electroconductive intermediate | middle layer is prepared as mentioned above, the high-speed drying which performs drying for a short time at high temperature is employable. About the conditions of the high-speed drying of the composition for positive electrode active material layer formation, a drying temperature can be suitably set to the temperature range of about 120 degreeC-150 degreeC, for example, More preferably, about 135 degreeC-150 degreeC Temperature range. Thereby, drying time can be shortened at the rate of about 20 to 50% compared with the past, for example. Specifically, for example, when the basis weight of the composition for forming a positive electrode active material is about 5 to 10 mg / cm ² , drying can be performed for about 15 to 25 seconds in an atmosphere of 100 to 150 ° C. Thereby, the positive electrode is completed.

  The flow diagram of FIG. 2 shows an embodiment of the electrode manufacturing method disclosed herein, and the invention disclosed herein is not limited to this. For example, the process invention for preparing the electrode active material forming composition in step S22 need not be performed after step S16, and can be performed at any timing prior to step S24. Further, the method for producing a positive electrode for a lithium secondary battery having a two-layer structure has been described as an example. In addition, the electrode structure described above has a two-layer structure including a conductive intermediate layer (typically including a conductive intermediate layer on the surface of the current collector) between the active material layer and the current collector. In addition to these layers, a functional layer having some function may be further provided.

  FIG. 3 shows a lithium secondary battery 10 as an example of a secondary battery provided with the electrode provided by the present invention as a positive electrode. The lithium secondary battery 10 has a configuration in which an electrode body 11 is accommodated in a battery case 15 together with a nonaqueous electrolyte 20. At least a part of the nonaqueous electrolyte 20 is impregnated in the electrode body 11. The electrode body 11 includes a positive electrode 12, a negative electrode 14, and a separator 13.

  The positive electrode 12 is constituted by an electrode provided by the present invention. The positive electrode 12 includes a long sheet-shaped positive electrode current collector 122 and a positive electrode active material layer 124 that includes the positive electrode active material and is provided on the positive electrode current collector 122. A conductive intermediate layer 123 is provided between the positive electrode current collector 122 and the positive electrode active material layer 124. The negative electrode 14 includes a long sheet-like negative electrode current collector 142 and a negative electrode active material layer 144 that includes the negative electrode active material and is provided on the negative electrode current collector 142. Like the positive electrode 12 and the negative electrode 14, the separator 13 is formed in a long sheet shape. The positive electrode 12 and the negative electrode 14 are wound in a cylindrical shape together with the two separators 13 so that the separator 13 is interposed therebetween. Thereby, the electrode body 11 is formed.

  The battery case 15 includes a bottomed cylindrical case main body 152 and a lid 154 that closes the opening. The lid 154 and the case main body 152 are both made of metal and insulated from each other. The lid 154 is electrically connected to the positive electrode current collector 122, and the case body 152 is electrically connected to the negative electrode current collector 142. In the lithium secondary battery 10, the lid body 154 also serves as a positive electrode terminal, and the case main body 152 serves as a negative electrode terminal.

  On one edge (upper edge in FIG. 3) along the longitudinal direction of the positive electrode current collector 122, a portion where the positive electrode active material layer 124 is not provided and the current collector 122 is exposed is provided. A lid 154 is electrically connected to the exposed portion. On one edge (the lower edge in FIG. 3) along the longitudinal direction of the negative electrode current collector 142, a portion where the current collector 142 is exposed without the negative electrode active material layer 144 being provided. The case main body 152 is electrically connected to the exposed portion.

The non-aqueous electrolyte includes a lithium salt as a supporting salt in an organic solvent (non-aqueous solvent). A nonaqueous electrolyte that is liquid at room temperature (that is, an electrolytic solution) can be preferably used. As the lithium salt, for example, a known lithium salt conventionally used as a supporting salt for a non-aqueous electrolyte of a lithium secondary battery can be appropriately selected and used. Examples of such lithium salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO _{3 and the} like. These supporting salts can be used alone or in combination of two or more. A particularly preferred example is LiPF ₆ . The non-aqueous electrolyte is preferably prepared, for example, so that the concentration of the supporting salt is within a range of 0.7 to 1.6 mol / L.

  As the non-aqueous solvent, an organic solvent used in a general lithium secondary battery can be appropriately selected and used. Particularly preferred non-aqueous solvents include carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and propylene carbonate (PC). These organic solvents can be used alone or in combination of two or more.

  As the negative electrode current collector 142, a conductive member made of a highly conductive metal is preferably used. For example, copper or an alloy containing copper as a main component can be used. The shape of the negative electrode current collector 142 may vary depending on the shape of the lithium secondary battery and the like, and thus is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. In the present embodiment, a sheet-like copper negative electrode current collector 142 is used. For example, a copper sheet having a thickness of about 5 μm to 30 μm can be suitably used.

  The negative electrode active material layer 144 can contain a conductive material, a binder, and the like similar to those of the positive electrode active material layer 124 as necessary, in addition to the negative electrode active material. Although it does not specifically limit, the usage-amount of the binder with respect to 100 mass parts of negative electrode active materials can be 0.5-10 mass parts, for example. For the negative electrode active material layer 144, similarly to the positive electrode active material layer 124, a composition in which the negative electrode active material is dispersed in a liquid medium containing an appropriate solvent and a binder is prepared, and the composition is used as the negative electrode current collector 142. It can be preferably prepared by applying to the surface of the material, drying, and pressing as desired.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium secondary batteries can be used without any particular limitation. For example, a carbon particle is mentioned as a suitable negative electrode active material. A particulate carbon material (carbon particles) containing a graphite structure (layered structure) at least partially is preferably used. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. Can be done.

The separator 13 is formed in a long sheet shape. However, since the shape of the separator 13 may vary depending on the shape of the lithium secondary battery, it is not particularly limited to a sheet shape. As the separator 13, for example, a porous film made of a polyolefin resin such as polyethylene or polypropylene can be suitably used.
The electrode provided by the present invention is preferably used as an electrode (for example, positive electrode) for constructing various types of batteries. For example, the positive electrode using the above electrode, the negative electrode in which the negative electrode active material layer is held on the negative electrode current collector, the electrolyte disposed between the positive and negative electrodes, and typically the positive and negative electrode current collectors are separated. It is suitable as a component of a lithium secondary battery including a separator (which may be unnecessary when the electrolyte is solid). Structure (for example, metal casing or laminate film structure) and size of an outer container constituting such a battery, or structure of an electrode body (for example, a wound structure or a laminated structure) having a positive / negative electrode current collector as a main component There is no particular restriction on the etc.

  Such a lithium secondary battery 10 can be used as a secondary battery for various applications. For example, it can be suitably used as a power source for a vehicle driving motor (electric motor) mounted on a vehicle such as an automobile. The type of vehicle is not particularly limited, but is typically a hybrid vehicle, an electric vehicle, a fuel cell vehicle, or the like. Such lithium secondary battery 10 may be used alone, or may be used in the form of an assembled battery that is connected in series and / or in parallel.

Hereinafter, the present invention will be further described with reference to the accompanying drawings. However, it goes without saying that the present invention is not limited to these examples.
≪Viscosity of composition for forming conductive intermediate layer≫
A conductive intermediate layer forming composition for forming a conductive intermediate layer was prepared as follows. That is, carbon black having an average particle size of 35 nm was used as the conductive particles. As the thermoplastic polymer, four types of polyvinylidene fluoride (PVdF) having different number average molecular weights were used. That is, four types of PVdF having a number average molecular weight of PVdF (A): 280,000, PVdF (B): 500,000, PVdF (C): 630,000, and PVdF (D): 1 million were prepared. Moreover, N-methylpyrrolidone (NMP) was used as a solvent.
These intermediate layer materials were mixed so that the mass ratio of the conductive particles, the thermoplastic polymer, and the solvent was 10: 2: 88 (that is, the solid content concentration NV was about 13.6% by mass). A composition for forming a conductive intermediate layer was prepared. Composition (A) is PVdF (A) as the thermoplastic polymer, Composition (B) is PVdF (B) as the thermoplastic polymer, Composition (C) is PVdF (C) as the thermoplastic polymer, Composition ( D) uses PVdF (D) as the thermoplastic polymer. The viscosities of these compositions (A) to (D) were examined. Viscosity was measured using a B-type viscometer under the conditions of 25 ° C. and 0.1 s ⁻¹ . The results of measuring the viscosity of the compositions (A) to (D) are shown in FIG. 4 as the relationship between the number average molecular weight of the thermoplastic polymer and the viscosity of the conductive intermediate layer forming composition.

As shown in FIG. 4, it was confirmed that the viscosity of the composition for forming a conductive intermediate layer increases almost linearly as the number average molecular weight of PVdF increases. The viscosity of the composition for forming a conductive intermediate layer using carbon black as the conductive particles is significantly higher than the viscosity of a carbon black simple substance dispersed in an NMP solvent (so-called carbon paste). The viscosity of the composition further increases as the number average molecular weight of PVdF increases. The present inventor believes that these phenomena have a higher adsorbability with carbon as PVdF having a higher molecular weight, and a greater effect of causing steric hindrance when adsorbed, resulting in an increase in the viscosity of the composition. ing.
≪Preparation of secondary battery electrode≫
An aluminum foil is used as a current collector, and a lithium nickel cobalt manganese composite oxide having a composition represented by the general formula LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is used as a positive electrode active material layer. A sheet-like electrode provided with a conductive intermediate layer was produced.

<Formation of conductive intermediate layer>
A conductive intermediate layer forming composition for forming a conductive intermediate layer was prepared as follows. That is, carbon black having an average particle size of 35 nm was used as the conductive particles. As the thermoplastic polymer, four types of polyvinylidene fluoride (PVdF) having different number average molecular weights were used. That is, PVdF prepared four types of number average molecular weights: PVdF (A): 280,000, PVdF (B): 500,000, PVdF (C): 630,000, PVdF (D): 1 million. Moreover, N-methylpyrrolidone (NMP) was used as a solvent.
These intermediate layer materials are blended so that the mass ratio of the conductive particles to the thermoplastic polymer is 28:72, and this is mixed in a solvent so that the solid content concentration NV is 9.5% by mass. Four types of compositions for forming a conductive intermediate layer were prepared. Composition (a) is PVdF (A) as a thermoplastic polymer, Composition (b) is PVdF (B) as a thermoplastic polymer, Composition (c) is PVdF (C) as a thermoplastic polymer, Composition ( d) uses PVdF (D) as the thermoplastic polymer.

Each of the compositions (a) to (d) is coated on one side of a 15 μm-thick long aluminum foil (current collector) and dried to thereby form a conductive intermediate layer (a) on one side of the current collector. ~ (D) was formed. The conductive intermediate layer (a) is formed using the composition (a). The conductive intermediate layer (b) is formed using the composition (b). The conductive intermediate layer (c) is formed using the composition (c). The conductive intermediate layer (d) is formed using the composition (d). For reference, only the composition (a) was applied on both sides of the current collector, and a conductive intermediate layer was formed on both sides of the current collector. A gravure coater is used for coating the compositions (a) to (d), and the coating amount (basis weight) is about 0.3 mg / cm ² (solid content basis) per one side of the current collector. It was adjusted. According to this coating amount, a conductive intermediate layer having a thickness of about 2 μm is formed on the current collector.
As drying conditions, drying was performed in a drying furnace at 115 ° C. for 30 seconds.

<Formation of positive electrode active material layer>
The positive electrode active material layer was formed as follows. That is, lithium nickel cobalt manganese composite oxide powder represented by the above formula (positive electrode active material, average particle size 8 μm), acetylene black (conductive material, average particle size 48 nm) and PVdF (binder) A composition for forming a positive electrode active material layer was prepared by mixing with ion-exchanged water so that the mass ratio was 88: 10: 2 and NV was 58 mass%. And the positive electrode active material layer (a)-(d) was formed by apply | coating and drying the composition for active material layer formation from on electroconductive intermediate | middle layer (a)-(d), respectively. The coating amount (weight per unit area) of the composition for forming a positive electrode active material layer was adjusted to be about 5.6 mg / cm ² (based on solid content). Moreover, the drying conditions for formation of a positive electrode active material layer employ | adopted the high-speed drying which performs drying for 20 second at 150 degreeC. This drying time is shortened by 100 seconds or more (that is, about 84%) as compared with normal drying (typically, 95 ° C. for 125 seconds).
The conductive intermediate layer (a) is formed on both sides of the current collector, but here, the positive electrode active material layer is formed only on the conductive intermediate layer (a) formed on one side. The total thickness including the current collector and the electrode films (conductive intermediate layer and positive electrode active material layer) formed on both sides of the current collector in a state where the composition for forming the positive electrode active material layer is dried is about 100 μm. there were.

<EPMA analysis>
Electrodes provided with the positive electrode active material layers (a) to (d) formed as described above were used as positive electrode samples (A) to (D), respectively. These electrode samples (A) to (D) were subjected to CP treatment (Cross Section Polisher treatment) to obtain a cross section. Then, these sections were analyzed by EPMA (Electron Probe Micro Analyzer), and quantitative analysis and element distribution on element F (fluorine) were examined. The results are shown in FIGS. 5A, 5B, and 5D as F element maps. Table 1 and FIG. 6 show the average values of the detected concentrations of element F in the conductive intermediate layers of the electrode samples (A) to (D).

In FIG. 5, (A) is a figure which shows distribution of F element regarding an electrode sample (A), (B) is a figure which shows distribution of F element regarding an electrode sample (B), (D) is an electrode sample. It is a figure which shows distribution of F element regarding (D). In these figures, the F density is shown in black and white contrast, and the correspondence between the F density and contrast can be confirmed by the legend shown on the right side of each figure.
In FIGS. 5A, 5B, and 5D, the element F is contained in PVdF used as a thermoplastic polymer in the conductive intermediate layer and fluorine (F) contained in PVdF used as a binder in the positive electrode active material layer. Derived from. That is, from FIG. 5 (A) (B) (D), the location and concentration of the thermoplastic polymer and binder can be confirmed. In addition, the density | concentration of PVdF in each layer is 72 mass% in a conductive intermediate | middle layer, and 2 mass% in a positive electrode active material layer as a simple arithmetic mean value.

  In FIGS. 5A, 5B, and 5D, a black belt-like portion extending in a substantially horizontal direction near the center corresponds to an aluminum foil (cross section) that is a current collector, and one side of the aluminum foil (in FIG. 5) A conductive intermediate layer and a positive electrode active material layer are provided on the upper side. 5A, a conductive intermediate layer is also provided on the other surface (downward in FIG. 5) of the aluminum foil for reference. The white belt-like portion extending horizontally below the aluminum foil portion in FIG. 5A corresponds to the F element contained in the conductive intermediate layer provided on the other surface. From this, it can be seen that in the F element distribution diagram, the appropriately formed conductive intermediate layer can be clearly confirmed as a white belt.

On the other hand, the F element contained in each of the conductive intermediate layer and the positive electrode active material layer is confirmed above the aluminum foil that looks like a black belt. For example, in FIG. 5D, the F element in the conductive intermediate layer can be seen as a white band above the aluminum foil that looks like a black band, and further above that, the positive electrode active material in a wide black and white spotted band. It can be seen that the F element in the layer is present. In FIG. 5D, the boundary between the conductive intermediate layer and the positive electrode active material layer can be confirmed relatively clearly by the difference in F element concentration (difference in distribution).
On the other hand, in FIG. 5B, the distribution of the F element in the conductive intermediate layer and the distribution of the F element in the positive electrode active material layer can not be distinguished by the roughness of the contrast pattern, It is difficult to clearly identify the boundary. Further, in FIG. 5A, the boundary between the distribution of F element in the conductive intermediate layer and the distribution of F element in the positive electrode active material layer can hardly be confirmed. That is, in the electrode samples (A) and (B), it can be seen that there is no difference in the concentration of the F element in the vicinity of the boundary between the conductive intermediate layer and the positive electrode active material layer as compared with the electrode sample (D). It can also be seen that the concentration of the F element in the positive electrode active material layer is clearly high. That is, it is expected that the F element (that is, PVdF) in the conductive intermediate layer has moved into the positive electrode active material layer.

Table 1 shows, in order from the top, the types and number average molecular weights of PVdF used for the conductive intermediate layers of the electrode samples (A) to (D), and the average value of the detected concentration of F element in the conductive intermediate layer. ing. FIG. 6 is a graph showing the relationship between the number average molecular weight of PVdF used in the conductive intermediate layers of the electrode samples (A) to (D) and the detected concentration of the F element concentration in the conductive intermediate layers. From Table 1 and FIG. 6, it can confirm that the density | concentration of F element detected in a conductive intermediate layer is so low that the number average molecular weight of PVdF used for the conductive intermediate layer is small. Further, it can be said that the decrease in the concentration of the F element tends to be remarkable in the region where the number average molecular weight of PVdF is less than 630,000.
From the above Table 1, FIGS. 5A, 5B, and D, and FIG. 6, the smaller the number average molecular weight of PVdF used for the conductive intermediate layer, the more PVdF in the conductive intermediate layer is in the positive electrode active material layer. It was confirmed that it was moving.

≪Production of lithium secondary battery≫
Lithium secondary batteries (A), (B), and (D) were constructed using the electrode samples (A), (B), and (D) as positive electrodes. The negative electrode is provided with a negative electrode mixture layer containing natural graphite, SBR, and CMC at a mass ratio of 98: 1: 1 on both sides of a negative electrode current collector made of a long copper foil having a thickness of about 15 μm. I used one. The negative electrode and the positive electrode were laminated together with two long separators (here, a porous polyethylene sheet was used), and the laminated sheet was wound in the long direction to produce a wound electrode body. This electrode body was housed in an outer case together with a non-aqueous electrolyte, and a 18650 type (18 mm diameter, 65 mm height) lithium secondary battery was constructed. As the non-aqueous electrolyte, a composition containing LiPF ₆ at a concentration of 1.0 M in a mixed solvent containing EC, DMC, and EMC at a volume ratio of 3: 3: 4 was used. The lithium secondary batteries thus obtained are referred to as batteries (A), (B), and (D) in association with the numbers of the electrode samples used for the batteries.

<Evaluation of shutdown characteristics>
For the batteries (A), (B), and (D), the DC resistance value was measured when the environmental temperature was raised from 50 ° C to 150 ° C. Specifically, each battery is adjusted to 30% SOC, and a direct current of 1 C is passed to each battery in a high temperature environment where the temperature is increased from 50 ° C. to 150 ° C. at a temperature rising rate of 2 ° C./sec. The change in absolute value of was measured. The measurement results are shown in FIG. In the graph of FIG. 7, the horizontal axis represents the environmental temperature (° C.), and the vertical axis represents the absolute value of internal resistance (| Z | (mΩ)).

As shown in FIG. 7, the battery (A) in which PVdF has moved in a large amount from the conductive intermediate layer to the positive electrode active material layer shows no change in the internal resistance even when the environmental temperature is raised. It was confirmed that the shutdown effect by the intermediate layer was not obtained at all. Regarding the battery (B) and the battery (D) whose degree of movement of PVdF is relatively low, the internal resistance gradually increases from the start of the temperature rise, gradually increases in resistance, and the shutdown function by the conductive intermediate layer is achieved. It was confirmed that it was expressed. In particular, the battery (B) is considered to have a good shutdown function due to a large rise in resistance near 100 ° C. and a large increase in resistance. On the other hand, the internal resistance of the battery (D) increases relatively linearly as the temperature rises, and it is considered that the shutdown function is inferior to that of the battery (B). The battery (D) has the highest absolute value of the internal resistance, but the original internal resistance is also high. Therefore, the size of the shutdown function is defined as (resistance value at the temperature at which the resistance curve reaches a peak) ÷ (resistance value at the measurement start temperature (50 ° C.)), and the batteries (A), (B), and (D) The shutdown function was evaluated. As a result, the shutdown function was larger in the order of battery (B)> battery (D)> battery (A). From these things, it can be considered that the shutdown effect of the battery (B) is most excellent. The reason why the battery (B) has a greater shutdown effect than the battery (D) considered to have the least amount of PVdF transfer from the conductive intermediate layer to the positive electrode active material layer is considered as follows. . That is, the battery (D) uses PVdF having a number average molecular weight as large as 1,000,000 as the thermoplastic polymer of the conductive intermediate layer. As PVdF has a higher molecular weight, the sensitivity of swelling with increasing temperature becomes higher. Therefore, the molecule is swollen with increasing temperature, and it is considered that a clear shutdown was not expressed. Therefore, in view of the internal resistance, it is understood that PVdF having a number average molecular weight of less than 1 million is preferably used as the thermoplastic polymer of the conductive intermediate layer.
From the above, it was confirmed that the number average molecular weight of PVdF used for the conductive intermediate layer is appropriately 630,000 to less than 1 million.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

  According to the technology disclosed herein, a method for manufacturing an electrode for a secondary battery including a conductive intermediate layer can be provided without degrading shutdown characteristics. Moreover, according to this manufacturing method, high-speed drying can be realized without significantly impairing battery performance. Therefore, it is possible to reduce manufacturing time and cost.

10 Lithium secondary battery 12 Positive electrode (battery electrode)
50 conductive particles 51 first conductive particles 52 second conductive particles 60 binder 122 current collector 123 intermediate layer 124 active material layer

Claims (7)

A method for producing an electrode for a secondary battery in which a conductive intermediate layer containing conductive particles is interposed between an electrode active material layer containing an electrode active material and a current collector,
A step of preparing a composition for forming a conductive intermediate layer in which an intermediate layer material containing the conductive particles and the thermoplastic polymer is dispersed in a predetermined solvent;
Applying the conductive intermediate layer forming composition in a layered manner to the surface of the current collector,
Drying the conductive intermediate layer forming composition applied to the current collector to form the conductive intermediate layer made of the intermediate layer material;
Preparing an electrode active material layer forming composition in which an electrode active material containing the electrode active material and a binder is dispersed in a predetermined solvent;
Applying the electrode active material layer forming composition to the upper surface of the conductive intermediate layer in layers,
Including drying the composition for forming an electrode active material layer applied to the current collector to form the electrode active material layer made of the electrode active material.
The said composition for conductive intermediate | middle layer formation is a manufacturing method of the electrode for secondary batteries using the thermoplastic polymer whose number average molecular weight is 630,000 or more and less than 1 million as said thermoplastic polymer.   The method for producing an electrode for a secondary battery according to claim 1, wherein the conductive intermediate layer forming composition is dried at a high temperature of 100 ° C. or higher and 150 ° C. or lower.   The method for producing an electrode for a secondary battery according to claim 1 or 2, wherein the electrode active material layer forming composition is dried at a high temperature of 120 ° C or higher and 150 ° C or lower.   The electrode for secondary battery according to claim 1, wherein the composition for forming an electrode active material layer is prepared using a thermoplastic polymer having a number average molecular weight of 1,000,000 or more as the binder. Manufacturing method.   The method for producing an electrode for a secondary battery according to any one of claims 1 to 4, wherein the composition for forming a conductive intermediate layer is prepared using polyvinylidene fluoride as the thermoplastic polymer.   The method for producing an electrode for a secondary battery according to any one of claims 1 to 5, wherein the composition for forming a conductive intermediate layer is prepared such that a ratio of a solid material is 8% by mass or more. . An electrode for a secondary battery in which a conductive intermediate layer containing conductive particles and a thermoplastic polymer is interposed between an electrode active material layer containing an electrode active material and a binder, and a current collector,
The thermoplastic polymer contained in the conductive intermediate layer is an electrode for a secondary battery having a number average molecular weight of 630,000 to less than 1 million.