JP2008108649A - Method for manufacturing positive electrode for lithium secondary battery for vehicle - Google Patents

Abstract

The present invention provides a method for efficiently producing a positive electrode suitable for use in constructing a high-performance lithium secondary battery for a vehicle using an aqueous positive electrode active material composition.
In the positive electrode manufacturing method provided by the present invention, a lithium transition metal oxide powder as a positive electrode active material and a carbon powder as a conductive material are formed on the surface of an alkali-reactive positive electrode current collector (for example, an aluminum foil). And an aqueous positive electrode active material composition containing an aqueous medium. The applied positive electrode active material composition is dried by induction heating to form an uncompressed positive electrode active material layer, which is compressed to a density of the positive electrode active material layer of 2.3 to 2.6 g / cm. Adjust to ³ . This method can be particularly preferably employed as a method for producing a lithium battery positive electrode for vehicles in which the lithium transition metal oxide is a lithium nickel oxide.
[Selection] Figure 1

Description

  The present invention relates to a method of manufacturing a positive electrode for constructing a lithium secondary battery mounted on a vehicle. Moreover, this invention relates to the lithium secondary battery for vehicles provided with the positive electrode manufactured by this method.

Lithium secondary batteries that are charged and discharged by lithium ions traveling between a positive electrode and a negative electrode are known. In one typical configuration of this type of secondary battery, the positive electrode includes a positive electrode having a configuration in which a material capable of reversibly occluding and releasing lithium ions (positive electrode active material) is held in a positive electrode current collector. . As a typical example of the positive electrode active material, an oxide containing lithium and one or more transition metal elements as constituent metal elements (hereinafter also referred to as “lithium transition metal oxide”) can be given.
Patent Document 1 discloses a technique for manufacturing a positive electrode for a lithium secondary battery by fixing a thick film-shaped molded body obtained by extruding a kneaded material obtained by adding an organic solvent to LiCoO ₂ particles to a positive electrode current collector. Are listed. Further, Patent Document 2 is cited as a prior art document relating to a method for manufacturing an electrode used for an alkaline secondary battery or the like.

JP-A-9-171823 JP 2005-133255 A

  By the way, in a lithium secondary battery for electric appliances (consumer use) used in a general household such as a mobile phone, improvement of energy density (increase in capacity) is particularly emphasized. For this reason, the amount of the positive electrode active material present per unit volume of the positive electrode active material layer is increased by increasing the density of the active material layer or increasing the content ratio of the positive electrode active material in the active material layer. Efforts are made. It goes without saying that high energy density is also required for lithium secondary batteries for vehicles (for example, lithium secondary batteries used as power sources for hybrid vehicles, electric vehicles, etc., typically lithium ion batteries). However, in lithium secondary batteries for vehicles, the importance of the ability to extract a large amount of power in a short time (that is, high output) tends to be more important than lithium secondary batteries for consumer use. It is also important that such high output can be stably exhibited in a wide temperature range (for example, even at a low temperature of about −30 ° C.).

  On the other hand, as one of the typical methods for holding the positive electrode active material on the positive electrode current collector in manufacturing a positive electrode for a lithium secondary battery, a paste-like material in which a positive electrode active material powder or the like is dispersed in an appropriate medium is used. A method of forming a positive electrode active material layer by applying a composition (positive electrode active material composition) to a positive electrode current collector such as an aluminum foil, passing it through a hot air dryer or the like, and drying it. The positive electrode active material composition used in such a method includes the above-mentioned medium (positive electrode active material powder, etc.) from the viewpoints of reducing environmental burden, reducing material costs, simplifying equipment, reducing waste, improving handling properties An aqueous positive electrode active material composition in which the dispersion medium is an aqueous medium (that is, a composition in which the solvent constituting the medium is an aqueous solvent) is preferable.

  Therefore, the present invention provides a positive electrode suitable for use in constructing a lithium secondary battery for a vehicle having higher performance (for example, capable of stably exhibiting higher output) by using an aqueous positive electrode active material composition. It aims at providing the method of manufacturing efficiently. Another object of the present invention is to provide a lithium secondary battery for a vehicle constructed using the positive electrode produced by the above method and a vehicle including the battery.

  When the present inventor applies an aqueous positive electrode active material composition containing a lithium transition metal oxide to a general positive electrode current collector (typically made of aluminum) for a lithium secondary battery to produce a positive electrode We focused on the unique events that occur in And, by suppressing the occurrence of such an event and forming the positive electrode active material layer so as to have a predetermined property, a positive electrode capable of constructing a higher performance lithium secondary battery for vehicles can be efficiently manufactured. As a result, the present invention has been completed.

According to the present invention, a method for producing a positive electrode used in a lithium secondary battery (typically a lithium ion battery) for vehicles is provided. The positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the current collector. In this manufacturing method, a current collector made of a metal material that reacts with alkali to generate gas (hereinafter sometimes referred to as “alkali-reactive current collector”) is used as the positive electrode current collector. In this method, a positive electrode active material composition containing a lithium transition metal oxide powder as a positive electrode active material, a carbon powder as a conductive material, and an aqueous medium is applied to the surface of the positive electrode current collector (typical). Application). Further, the method includes drying the positive electrode active material composition applied to the current collector by induction heating to form an uncompressed positive electrode active material layer on the current collector. Furthermore, the uncompressed positive electrode active material layer is compressed to adjust the density of the active material layer to about 2.3 to 2.6 g / cm ³ .
Thus, by drying the positive electrode active material composition using induction heating and adjusting the density of the positive electrode active material layer to the above range, an electrolyte (typically a liquid electrolyte, that is, a liquid electrolyte, that is, The positive electrode active material layer in which the permeability of the electrolytic solution) and the conductivity of the active material layer are balanced at a higher level can be formed more accurately (more reliably, that is, efficiently). According to the positive electrode provided with such a positive electrode active material layer, for example, it shows a good initial output even at a low temperature (for example, −30 ° C.) and outputs after repeated charge and discharge (output after endurance, especially output after endurance at low temperature). In addition, an excellent lithium secondary battery for a vehicle can be constructed.

  In a preferred embodiment, as the positive electrode active material composition, a composition containing the carbon powder (conductive material) at a ratio of approximately 10 to 20% by mass in terms of solid content of the composition is used. Increasing the content of the conductive material in the solid content (positive electrode active material layer forming component) contained in the positive electrode active material composition can advantageously contribute from the viewpoint of constructing a higher-power lithium secondary battery. . The positive electrode active material layer, which has been dried by induction heating and adjusted to an appropriate property (density) as described above, has a composition with a relatively high content of the conductive material as described above, but has an electrolyte solution permeability. It can be good. Therefore, the output improvement effect by employ | adopting this composition can be exhibited more efficiently.

  The method disclosed herein is, for example, a method for producing a positive electrode in which the lithium transition metal oxide is one or more selected from lithium nickel oxide, lithium cobalt oxide and lithium manganese oxide. Can be preferably employed. In particular, it is suitable as a method for producing a positive electrode in which the lithium transition metal oxide is a lithium nickel oxide. Lithium nickel-based oxides are more likely to elute lithium when dispersed in aqueous solvents than lithium cobalt-based oxides and lithium-manganese oxides (thus reacting with an alkali-reactive current collector to release gas. In particular, the effect of applying the manufacturing method disclosed herein is particularly well exhibited.

  In a preferred embodiment, the current collector is an aluminum current collector (eg, aluminum foil). In the method disclosed herein, the positive electrode active material composition applied to the current collector is dried by induction heating. Therefore, even when an aluminum current collector having high alkali reactivity is used, a high performance is obtained. A positive electrode suitable for constructing a lithium secondary battery for vehicles can be accurately manufactured.

  In the compression, the volume ratio of pores having a diameter of 0.3 to 2.0 μm is approximately 70% or more of the total pore volume in the pore distribution of the positive electrode active material layer after compression (typically approximately 75 to 85). %) Is preferable. The positive electrode active material layer adjusted to have such properties is excellent in lithium ion conductivity. Therefore, the positive electrode having the positive electrode active material layer can be used to construct a higher performance lithium secondary battery for vehicles.

  According to the present invention, a positive electrode for a lithium secondary battery for vehicles manufactured by any of the methods disclosed herein is provided. The positive electrode can be used to construct a lithium secondary battery for a vehicle (typically a lithium ion battery) that exhibits a good initial output even at a low temperature and has an excellent output after repeated charge and discharge.

The invention disclosed herein includes a positive electrode used in a lithium secondary battery for a vehicle, and a lithium nickel-based oxide powder as a positive electrode active material and a carbon powder as a conductive material (typically average particles A carbon powder having a diameter of about 1/50 or less, for example, about 1/50 to 1/1000, preferably about 1/50 to 1/500 of the average particle diameter of the lithium nickel oxide powder. Is applied to an aluminum current collector, and the applied composition is applied to the aluminum positive electrode active material composition containing a water-based solvent and an alkaline positive electrode active material composition containing about 10 to 200 nm, preferably about 20 to 200 nm. A positive electrode for a lithium secondary battery for vehicles formed by forming a positive electrode active material layer on the surface of the current collector by drying by induction heating is included. The content ratio of the carbon powder in the positive electrode active material composition is about 10 to 20% by mass in terms of solid content. The density of the positive electrode active material layer is about 2.3 to 2.6 g / cm ³ (preferably about 2.3 to 2.5 g / cm ³ ).

According to the present invention, any positive electrode disclosed herein (which can be a positive electrode manufactured by any method disclosed herein), a negative electrode capable of inserting and extracting lithium ions, There is provided a lithium secondary battery for vehicles comprising an aqueous electrolyte. Since the battery having such a configuration includes the positive electrode, it can exhibit a good initial output even at a low temperature and can be excellent in output after repeated charge and discharge.
In another aspect, the invention disclosed herein provides any positive electrode disclosed herein (which can be a positive electrode manufactured by any method disclosed herein); and A method for producing a lithium secondary battery for a vehicle, comprising: constructing a lithium secondary battery using a positive electrode.

In a preferable aspect of the lithium secondary battery for vehicles disclosed herein, the positive electrode constituting the battery is fully wetted by heat of wetting generated for 2 seconds after the positive electrode is immersed in the electrolyte at a measurement temperature of 25 ° C. It is in the range of about 43-50% of heat. The positive electrode constituting the battery is a positive electrode obtained by compressing the uncompressed positive electrode active material layer so as to exhibit such characteristics (that is, the ratio of wet heat generated in 2 seconds after immersion to the total wet heat). It can be.
A lithium secondary battery for a vehicle constructed using such a positive electrode can achieve better output (especially initial low temperature output) because the positive electrode active material layer provided in the positive electrode has excellent electrolyte permeability. .

  According to the present invention, there is provided a vehicle including any of the vehicle lithium secondary batteries disclosed herein (which may be a lithium secondary battery including a positive electrode manufactured by any of the methods disclosed herein). Is done. The above-mentioned lithium secondary battery for vehicles has performance suitable as a lithium secondary battery mounted on a vehicle (for example, it exhibits a good initial output even at low temperatures and is excellent in output after repeated charge and discharge). It can be well balanced at a high level. Therefore, such a lithium secondary battery can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile.

  Hereinafter, preferred embodiments of the present invention will be described. In addition, matters other than matters specifically mentioned in the present specification (for example, characteristics such as density of positive electrode active material layer, pore distribution, electrolyte permeability, and adjustment method thereof) are necessary for the implementation of the present invention. (For example, specific configurations of induction heating devices, configurations and manufacturing methods of negative electrodes, separators, and electrolytes, and other general technologies related to the construction of lithium secondary batteries, etc.) It can be grasped as a design matter. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The method disclosed herein is a method for producing a positive electrode for a lithium secondary battery for a vehicle including a process of forming a positive electrode active material layer by applying a positive electrode active material composition to a positive electrode current collector (further, a lithium secondary battery for a vehicle). Applicable to manufacturing). As the positive electrode current collector, a positive electrode current collector made of a metal material that reacts with alkali to generate gas is used. As the positive electrode active material composition, an aqueous positive electrode active material composition containing a lithium transition metal oxide powder as a positive electrode active material, a carbon powder as a conductive material, and an aqueous medium is used. In particular, the alkaline positive electrode active material composition (for example, a positive electrode active material composition having a pH of about 10 to 14) is applied to the surface of a positive electrode current collector made of a metal material that reacts with alkali to generate gas. It is suitable as a method for producing a positive electrode for a lithium secondary battery for vehicles including the process.

  As the above-mentioned “metal material that reacts with alkali to generate gas” as the constituent material of the positive electrode current collector, aluminum (Al), an alloy containing aluminum as a main component (from the viewpoint of workability, conductivity, specific gravity, etc.) An aluminum material such as an aluminum alloy can be preferably used. Examples of other metal materials that can be used as the constituent material of the positive electrode current collector include amphoteric metals such as zinc (Zn) and tin (Sn), and alloys containing any one of these metals as main components.

  The shape of the positive electrode current collector to be used is not particularly limited because it can vary depending on the shape of the lithium secondary battery for vehicles constructed using the obtained positive electrode, and is in the form of a rod, plate, sheet, foil It can be in various forms such as mesh. The production method disclosed herein can be preferably applied to the production of a positive electrode using, for example, a sheet-like or foil-like current collector. As a preferred embodiment of the lithium secondary battery for vehicles constructed using the positive electrode manufactured by such a method, a lithium secondary battery including a wound electrode body can be mentioned. The outer shape of the battery is not particularly limited, and may be, for example, a rectangular parallelepiped shape, a flat shape, a cylindrical shape, or the like.

  As a lithium transition metal oxide (typically particulate) as a constituent component of the positive electrode active material composition used in the production of the positive electrode, a positive electrode of this type of lithium secondary battery (typically a lithium ion battery) is used. An oxide having a layered structure or an oxide having a spinel structure that can function as an active material can be appropriately selected and used. For example, a lithium nickel oxide (referred to as a lithium transition metal oxide in which the main transition metal element constituting the oxide is nickel; the same applies hereinafter), a lithium cobalt oxide, and a lithium manganese oxide are selected. The use of one or more lithium transition metal oxides is preferred. A particularly preferred application target of the production method according to the present invention is a positive electrode using a lithium nickel-based oxide as a positive electrode active material (typically, the positive electrode active material substantially consists of a lithium nickel-based oxide).

  Here, the “lithium nickel oxide” means an oxide having lithium and nickel as constituent metal elements, and at least one other metal element in addition to lithium and nickel (that is, a transition metal element other than lithium and nickel) And / or a composite oxide containing a typical metal element) in a proportion smaller than nickel (in terms of the number of atoms. If two or more metal elements other than lithium and nickel are contained, the total amount thereof is a proportion less than nickel). Is also meant to encompass. Such metal elements include, for example, cobalt (Co), aluminum (Al), manganese (Mn), chromium (Cr), iron (Fe), vanadium (V), magnesium (Mg), titanium (Ti), zirconium (Zr). ), Niobium (Nb), molybdenum (Mo), tungsten (W), copper (Cu), zinc (Zn), gallium (Ga), indium (In), tin (Sn), lanthanum (La) and cerium (Ce) ) Or one or more metal elements selected from the group consisting of: Similarly, the lithium-cobalt-based oxide is meant to include a composite oxide containing at least one other metal element in addition to lithium and cobalt in a proportion smaller than cobalt. In addition, it is meant to include composite oxides containing at least one other metal element in addition to manganese in a smaller proportion than manganese.

  As such a lithium transition metal oxide (typically in a particulate form), for example, a lithium transition metal oxide powder (hereinafter sometimes referred to as an active material powder) prepared and provided by a conventionally known method is used. It can be used as it is. For example, the oxide can be prepared by mixing several raw material compounds appropriately selected according to the atomic composition at a predetermined molar ratio and firing by an appropriate means. Further, the fired product is pulverized, granulated and classified by an appropriate means to obtain a lithium transition metal oxide powder substantially composed of secondary particles having a desired average particle size and / or particle size distribution. be able to. The method disclosed herein uses, for example, a lithium transition metal oxide powder substantially composed of secondary particles having an average particle size in the range of about 1 to 25 μm (typically about 2 to 15 μm). Can be preferably implemented. Since the preparation method of such a lithium transition metal oxide powder itself does not characterize the present invention at all, further detailed description is omitted.

  The positive electrode active material composition used in the method disclosed herein includes carbon powder as a conductive material that increases the conductivity of the positive electrode active material layer formed from the composition. 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 preferably employed. The average particle size of particles (carbon particles) constituting the carbon powder is approximately 1/50 of the average particle size of particles (lithium transition metal oxide particles, typically secondary particles) constituting the active material powder. It is preferable that the average particle size corresponds to ˜1 / 1000 (more preferably about 1/100 to 1/500). Moreover, carbon powder (acetylene black etc.) whose particle diameter (typically primary particle) which comprises this carbon powder is about 10-200 nm (for example, about 20-100 nm) can be used preferably.

  The positive electrode active material composition is an aqueous composition containing the above-described lithium transition metal oxide powder, carbon powder, and an aqueous medium. Typically, an aqueous positive electrode active material composition in which lithium transition metal oxide powder and carbon powder are dispersed in the aqueous medium is used. Here, the “aqueous solvent” is a concept indicating water or a mixed solvent mainly composed of water. As a solvent other than water constituting such a mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, more preferably 95% by mass or more) of the aqueous solvent is water. A particularly preferred example is an aqueous solvent substantially consisting of water.

  In addition to lithium transition metal oxide powder (positive electrode active material), carbon powder (conductive material), and aqueous solvent, the positive electrode active material composition forms a positive electrode active material layer in the production of a general lithium secondary battery positive electrode. As needed, it can contain the 1 type (s) or 2 or more types of material which can be mix | blended with the liquid composition (typically aqueous composition) used for (1). Examples of such materials include binders and flowability (typically viscosity) modifiers. As the binder, a water dispersible polymer such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) can be preferably selected. As the fluidity modifier, a polymer (typically water-soluble) that can be dissolved in an aqueous solvent to be used, such as carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA), can be preferably used.

Although not particularly limited, the solid content concentration of the positive electrode active material composition (nonvolatile content, that is, the ratio of the positive electrode active material layer forming component) can be, for example, about 40 to 60% by mass. The content ratio of the positive electrode active material in the solid content (positive electrode active material layer forming component) is preferably about 50% by mass or more (typically 50 to 95%), preferably about 75 to 90% by mass. It is more preferable. Moreover, the content rate of the electrically-conductive material which occupies for the said solid content can be about 5-25 mass%, for example. From the viewpoint of producing a positive electrode suitable for constructing a battery having performances particularly suitable as a lithium secondary battery for vehicles (for example, excellent output characteristics such as initial output at low temperature and durability of the output). A positive electrode active material composition having a conductive material content of about 10 to 20% by mass (for example, about 10 to 15% by mass) can be preferably used. In this case, it is appropriate that the content ratio of the positive electrode active material in the composition is about 80 to 90% by mass (for example, 85 to 90% by mass).
Moreover, in the composition containing the positive electrode active material layer forming component other than the positive electrode active material and the conductive material, the total content ratio of these optional components (the ratio of the total positive electrode active material layer forming component) is about 7% by mass or less. It is preferably about 5% by mass or less (for example, about 1 to 5% by mass). The total content of the above optional components may be about 3% by mass or less (for example, about 1 to 3% by mass).

In the method disclosed herein, an aqueous positive electrode active material composition as described above is prepared by using an alkali-reactive current collector (for example, at least the surface where the positive electrode active material composition is applied is made of aluminum or an aluminum alloy). Current collector, typically an aluminum foil). The application of the positive electrode active material composition is performed, for example, by using a suitable coating apparatus (slit coater, die coater, comma coater, etc.) and laminating a predetermined amount of the positive electrode active material composition on the surface of the current collector. It can carry out in the aspect to apply | coat. The coating amount of the composition (the coating amount per unit area of the current collector) is not particularly limited, and may be appropriately varied depending on the shape of the positive electrode and the battery. For example, when producing a positive electrode used in the construction of a lithium secondary battery for a vehicle having a wound electrode body as described above, a foil-like current collector (for example, aluminum having a thickness of about 10 to 30 μm) A metal foil such as a foil can be preferably used.) When the above composition is applied to the surface so that the coating amount in terms of solid content (that is, the mass after drying) is about 5 to 20 mg / cm ^2. Good. Such a coating amount of the positive electrode active material composition may be applied to one side of the current collector, or the positive electrode active material composition may be applied to both sides of the current collector so that the total coating amount falls within the above range. Good. The aspect which apply | coats a positive electrode active material composition to both surfaces of a collector can be employ | adopted preferably.
Thus, an uncompressed positive electrode active material layer is formed by drying the positive electrode active material composition provided to the positive electrode current collector.

Here, in an aqueous positive electrode active material composition using a lithium transition metal oxide as a positive electrode active material, a part of lithium constituting the lithium transition metal oxide (particularly lithium nickel oxide) is eluted in an aqueous medium. Thus, the liquidity of the composition tends to be alkaline (for example, alkaline having a pH of about 10 to 14). When such an alkaline positive electrode active material composition is applied to an alkali-reactive current collector (for example, an aluminum foil) and dried by means of a general hot air drier or the like, it contains macroscopic voids inside. The positive electrode active material layer is formed. Such vacancies are considered to be caused by bubbles of a reaction gas (typically H ₂ gas) between the alkaline positive electrode active material composition and the positive electrode current collector. In such a positive electrode active material layer containing many macroscopic voids, it is difficult to balance the permeability of the electrolytic solution into the active material layer and the conductivity of the active material layer at a high level.

In the manufacturing method according to the present invention, induction heating (IH heating) is employed as the drying means. According to such IH heating, the metal current collector can be efficiently heated. Further, the positive electrode active material composition applied to the current collector can be quickly dried by heat transfer from the heated current collector. Since heat transfer from the current collector is used in this way, the contact portion (interface) between the current collector and the positive electrode active material composition can be preferentially and efficiently dried. As a result, the time during which the positive electrode active material composition containing water (typically alkaline) is in contact with the current collector can be reduced as much as possible, and the gas from the current collector (typically Can highly suppress the generation of H ₂ gas). As a result, it is possible to form a positive electrode active material layer in which the generation of the macro voids is effectively reduced (good uniformity).

  Such IH heating itself can be performed using a conventionally known IH heating technique. For example, as shown in FIG. 2, one or a plurality of coils 65 (three are shown in FIG. 2) arranged along the transport path of the positive electrode current collector 32 to which the positive electrode active material composition is applied. An induction heating device 64 having a configuration including the above can be preferably used. Both ends of the coil 65 are connected to a high-frequency power source (not shown), whereby a high-frequency current can be applied to the coil 65 at an arbitrary timing. As shown in FIG. 2, the induction heating device 64 has a configuration in which a coil 65 wound in a planar shape (for example, in a spiral shape substantially parallel to the surface of the current collector 32) has one surface of the current collector 32. It is not limited to the structure arrange | positioned at the side. For example, the coil 65 wound similarly may be provided on both sides of the current collector 32, or the current collector 32 may be passed through the coil 65 wound in a cylindrical shape. May be.

  In the technique disclosed herein, typically, the density of the uncompressed positive electrode active material layer (that is, the positive electrode active material layer as dried) obtained by drying the positive electrode active material composition by the IH heating described above. Is adjusted to form a positive electrode active material layer having a predetermined density. As a method for adjusting the density, a method in which a current collector in which an uncompressed positive electrode active material layer is formed on one surface or both surfaces (preferably both surfaces) is compressed in the thickness direction can be preferably employed. The compression can be performed by applying a compression method such as a conventionally known roll press method or flat plate press method.

With reference to FIG. 1, a preferred embodiment example in which a positive electrode is produced by applying a water-based positive electrode active material composition to the surface of a positive electrode current collector by the method of the present invention will be described.
A positive electrode preparation device 60 shown in FIG. 1 includes two coaters (coating devices, for example, slit coaters) 62 and 66 for applying a positive electrode active material composition to one surface and the other surface of the positive electrode current collector 32, respectively. The current collector 32 is provided with two induction heating devices 64 and 68 provided to be capable of electromagnetic induction heating, and the positive electrode current collector 32 having a dried positive electrode active material composition (uncompressed positive electrode active material layer) in the thickness direction. And a roll press machine 75 configured to be able to apply a stress to the head.

  The positive electrode current collector (typically aluminum foil) 32 unwound from the unwinding roll 72 is conveyed to the first coater 62, and an aqueous positive electrode active material is provided on one surface of the current collector 32 by the coater 62. A composition (for example, an alkaline composition having a composition in which lithium nickel oxide and acetylene black are dispersed in an aqueous medium) is applied. The current collector 32 coated with the composition on one side is then sent to the first induction heating device 64. The apparatus 64 may be an induction heating apparatus having a configuration in which, for example, a coil 65 having a structure shown in FIG. 2 is arranged along the conveyance path of the current collector 32. The current collector 32 is heated by electromagnetic induction caused by the induction heating device 64, whereby the positive electrode active material composition applied to the one surface is dried, and the uncompressed positive electrode active material is applied to the one surface. A layer is formed. A suitable distance from the coater 62 to the induction heating device 64 may vary depending on the conveying speed of the positive electrode current collector 32, the coating amount of the positive electrode active material composition, and the like. After the material is applied, until the induction heating of the current collector 32 is started (typically, the timing at which at least the interface portion of the applied positive electrode active material composition with the current collector 32 is dried is approximately It is preferable to set the arrangement of the coater 62 and the induction heating device 64 so that the time of the coincidence) is approximately 30 seconds or less (for example, approximately 1 to 30 seconds, preferably approximately 3 to 15 seconds). Moreover, it is preferable to set the conveyance speed of the positive electrode current collector 32 so that the contact time between the undried positive electrode active material composition and the current collector 32 can be the above time.

  In this way, the positive electrode current collector 32 in which the uncompressed positive electrode active material layer is formed on one surface (while being dried by induction heating) passes through the transport rolls 74 and 74 to the second coater 66. Sent. The positive electrode active material composition is applied to the other surface of the current collector 32 by the coater 66. The current collector 32 is then sent to a second induction heating device 68 (which may be a device having a configuration similar to that of the first induction heating device 64), where the positive electrode active material applied to the other surface. The composition is dried. In this way, an uncompressed positive electrode active material layer is formed on both the one surface and the other surface of the current collector 32. This is supplied to a roll press machine 75 to adjust the properties of the positive electrode active material layer. More specifically, the current collector 32 having uncompressed positive electrode active material layers on both sides is passed between the compression rolls 76 and 76 provided in the roll press machine 75, and stress in the thickness direction is applied thereto. To do. As a result, as shown in FIG. 3, the positive electrode active material layer 35 having the preferable properties disclosed herein (for example, at least one, preferably two or more of density, pore distribution, and electrolyte permeability). , 35 are formed on both surfaces of the positive electrode current collector 32. As shown in FIG. 1, the positive electrode 30 having such a configuration is wound around a winding roll 78 downstream of the roll press machine 75. In this way, the positive electrode 30 can be manufactured.

The degree of compression (compression condition) is such that the density of the positive electrode active material layer can be adjusted to about 2.3 to 2.6 g / cm ³ (more preferably about 2.3 to 2.5 g / cm ³ ). It is appropriate to do. A positive electrode active material layer obtained by drying by induction heating and having the above density (typically, a positive (uncompressed) positive electrode active material layer that has been dried by the above induction heating) The positive electrode provided with a positive electrode active material layer obtained by appropriately compressing the material shows a good initial output not only at room temperature but also at a low temperature (for example, a low temperature of about −30 ° C.), and after repeated charging and discharging. It is possible to construct a high-performance lithium secondary battery for a vehicle that is excellent in output. Therefore, a lithium secondary battery suitable for constructing such a high-performance lithium secondary battery for vehicles by managing the manufacturing conditions of the positive electrode active material layer (and thus the positive electrode) so as to have a density in the above range. The positive electrode can be produced efficiently (stable).

  The density of the positive electrode active material layer can be measured by a conventional method. In practice, the amount of the positive electrode active material composition applied per unit area of the current collector (in terms of solid content, the total amount when the positive electrode active material composition is applied to both sides of the current collector) , The thickness of the positive electrode (the total thickness of the positive electrode including the positive electrode current collector and the positive electrode active material layer) and the thickness of the positive electrode current collector constituting the positive electrode (usually the positive electrode current collector used for the production of the positive electrode) The method of calculating the density of the positive electrode active material layer from the general thickness of the body is simple and preferable. For example, the technique disclosed herein is a value calculated by the following formula: {(thickness of positive electrode) − (thickness of positive electrode current collector)} / (coating amount in terms of solid content per unit area); The density value of the positive electrode active material layer can be preferably adopted. In addition, the method of managing the density of the positive electrode active material layer using the coating amount and the thickness of the positive electrode as described above has an advantage that the density can be easily managed when performing production management of the positive electrode. For example, in the manufacturing apparatus 60 shown in FIG. 1, a discharge meter for detecting the discharge amount of the positive electrode active material composition (amount applied to the positive electrode current collector) is installed in the coaters 62 and 66, and downstream of the roll press machine 75. By providing a film thickness measuring device for detecting the thickness of the positive electrode 30 on the winding roll 78 side, it is monitored whether or not a positive electrode active material layer having a density within the target range is appropriately formed. When it is likely to fall out of the above range, the information can be fed back and control can be performed to correct the discharge amount, press conditions, and the like. By this, the positive electrode which has the density of the preferable range mentioned above can be manufactured stably and efficiently (reliably).

The degree of compression is such that the volume ratio of pores having a diameter of 0.3 to 2.0 μm is approximately 70 to 85% of the total pore volume (more preferably approximately) in the pore distribution of the obtained positive electrode active material layer. 70 to 80%). In other words, in one preferred embodiment of the positive electrode produced by the method disclosed herein, the pore volume of a diameter of 0.3 to 2.0 μm in the pore distribution of the positive electrode active material layer constituting the positive electrode. The proportion is about 70 to 85% (more preferably about 70 to 80%) of the total pore volume.
The pores of the above size can particularly effectively contribute to the movement (diffusion) of lithium ions in a lithium secondary battery in which the positive electrode active material layer is impregnated with an electrolyte (typically liquid). Accordingly, a positive electrode including a positive electrode active material layer having a relatively large proportion of pores having such a size (for example, approximately 70% or more as described above) exhibits good lithium ion transportability, and thus is rapidly charged / discharged. It is suitable as a component of a lithium secondary battery having excellent performance. In addition, it is difficult to manufacture a positive electrode active material layer in which the volume ratio of pores having a diameter of 0.3 to 2.0 μm greatly exceeds the above range, and in a positive electrode active material layer having such a pore distribution, The balance between electrolyte permeability and conductivity tends to be impaired. Therefore, in practical use, it is appropriate that the volume ratio of pores having a diameter of 0.3 to 2.0 μm is about 85% or less (more preferably about 80% or less). The pore distribution can be measured using a commercially available mercury porosimeter or the like. Such a positive electrode active material layer having a preferable pore distribution can be efficiently produced by, for example, obtaining appropriate compression conditions by preliminary experiments and applying the above conditions during actual positive electrode production to perform the above compression. it can.

The degree of compression is about 2 seconds after the positive electrode is immersed in an electrolyte (typically a liquid electrolyte, ie, an electrolytic solution) provided in a lithium secondary battery for vehicles constructed using the positive electrode obtained. It is preferable that the heat of wetting generated between them be in the range of about 43 to 50% of the total heat of wetting (the measurement temperature is 25 ° C.). In other words, in one preferred embodiment of the positive electrode produced by the method disclosed herein, the heat of wetting generated for 2 seconds after immersing the positive electrode in the electrolyte is approximately 43 to 50% of the total wet heat. Is in range.
The wetting heat can be grasped as the amount of heat generated when the electrolyte wets the surface (including the pore surface) of the positive electrode active material layer. This amount of heat can be detected (measured) using a commercially available microcalorimeter or the like. For example, from the measurement chart in which the elapsed time from the start of immersion is the horizontal axis and the amount of heat generated at each elapsed time or the integrated value thereof is the vertical axis, the ratio of the heat of wetting for 2 seconds after the immersion to the total wet heat (below) , This ratio may be referred to as “initial impregnation rate”).
Such a positive electrode active material layer having a high initial impregnation rate has a high electrolyte permeability (wetability with respect to the electrolyte solution), and thus has a performance of appropriately spreading the electrolyte solution to the details (in the pores) of the active material layer. Excellent. For this reason, since the positive electrode active material which comprises a positive electrode active material layer can be utilized effectively to every corner, the lithium secondary battery which exhibits a favorable output can be constructed | assembled. Moreover, since the positive electrode active material can be used more uniformly, it is advantageous in terms of durability of battery performance (output, etc.). The positive electrode active material layer exhibiting such a preferable initial impregnation rate is efficiently manufactured by, for example, obtaining appropriate compression conditions by a preliminary experiment and applying the above conditions during the actual positive electrode manufacturing to perform the above compression. Can do.

  The positive electrode provided by the present invention is preferably used as a positive electrode for constructing various types of vehicle lithium secondary batteries. For example, the positive electrode, a negative electrode having a negative electrode active material layer on the surface of the negative electrode current collector, an electrolyte disposed between the positive and negative electrodes, and a separator that typically separates the positive and negative electrode current collectors (the electrolyte is a solid In other words, it may be unnecessary as a component of a lithium secondary battery for vehicles. 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. For example, suitable for a positive electrode sheet formed by forming a positive electrode active material layer on the surface of a foil-like current collector for positive electrode made of aluminum or the like, and a foil-like current collector for negative electrode made of metal (for example, copper) or carbon Constituent elements of a wound electrode body obtained by stacking and winding a negative electrode sheet formed by attaching a negative electrode active material with a separator such as a porous polyolefin sheet (positive electrode sheet constituting the electrode body) As described above, the positive electrode disclosed herein can be preferably used. A lithium secondary battery for a vehicle is constructed by housing the wound electrode body having such a configuration in an appropriate container together with an electrolytic solution (typically a nonaqueous electrolytic solution as described later).

Hereinafter, an embodiment of a positive electrode provided by the present invention and a lithium secondary battery for a vehicle including the positive electrode will be described with reference to schematic diagrams shown in FIGS.
As shown in the figure, the lithium secondary battery 10 for a vehicle according to the present embodiment includes a housing (outer container) 12 made of metal (resin or laminate film is also suitable). The casing 12 accommodates a wound electrode body 20 configured by laminating a long sheet-like positive electrode sheet 30, a separator 50A, a negative electrode sheet 40, and a separator 50B in this order and then winding them into a flat shape. Is done.

  The positive electrode 30 is manufactured by applying any one of the methods disclosed herein, and is a long sheet-like positive electrode current collector 32 and a positive electrode active material layer 35 formed on one or both sides thereof. With.

On the other hand, the negative electrode 40 includes a long sheet-like negative electrode current collector 42 and a negative electrode active material layer 45 formed on the surface thereof. As the negative electrode current collector 42, a sheet material (typically a metal foil such as a copper foil) made of a metal such as copper can be preferably used. As a negative electrode active material capable of inserting and extracting lithium ions, a carbon material containing a graphite structure (layered structure) at least partially can be preferably used. Any carbon material such as so-called graphitic (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), or a combination of these can be used. It is. For example, natural graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), etc. can be used. Such a negative electrode active material is mixed with a binder (the same material as the positive electrode side can be used) and a conductive material used as necessary (the same material as the positive electrode side can be used). The composition for forming the negative electrode active material layer (negative electrode active material composition) was applied to one or both surfaces of the negative electrode current collector 42, and then the coated material was dried, whereby the current collector 42 The negative electrode active material layer 45 can be formed at a desired site (FIG. 5). 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 made into the range of 0.5-10 mass parts, for example. As a method of drying the negative electrode active material composition, induction heating similar to that of the positive electrode active material composition can be preferably employed. Alternatively, the negative electrode active material composition may be dried by hot air drying or other methods, and these methods may be appropriately combined and dried.
Moreover, as separator 50A, 50B used by overlapping with a positive / negative electrode sheet, the porous film which consists of polyolefin resin, such as polyethylene and a polypropylene, can be used conveniently, for example.

  As shown in FIG. 5, the active material composition is not applied to one end portion along the longitudinal direction of the positive electrode sheet 30 and the negative electrode sheet 40, so that a portion where the active material layers 35 and 45 are not formed is formed. When the positive and negative electrode sheets 30 and 40 are overlapped together with the two separators 50A and 50B, both the active material layers 35 and 45 are overlapped, and at the same time, the active material layer unformed portion of the positive electrode sheet and the active material layer of the negative electrode sheet are not yet formed. The positive and negative electrode sheets 30 and 40 are slightly shifted and overlapped so that the formation part is separately disposed at one end and the other end along the longitudinal direction. In this state, a total of four sheets 30, 40, 50A, 50B are wound, and then the obtained wound body is crushed from the side surface direction and crushed to obtain the flat wound electrode body 20.

Next, the obtained wound electrode body 20 is accommodated in the housing 12 (FIG. 6), and the positive electrode active material layer unformed portion 32 and the negative electrode active material layer unformed portion 42 are partly formed in the housing 12. The external connection positive terminal 14 and the external connection negative terminal 16 are electrically connected to each other.
Then, a suitable liquid electrolyte or solid (or gel) electrolyte, here a suitable non-aqueous electrolyte (for example, a non-aqueous electrolyte such as a mixed solvent of diethyl carbonate and ethylene carbonate containing a suitable amount of lithium salt such as LiPF ₆₎ Liquid) is disposed (injected) in the housing 12, and the opening of the housing 12 is sealed by welding or the like between the housing and the corresponding lid member 13, and the lithium secondary according to the present embodiment. Construction (assembly) of the battery 10 is completed. In addition, the sealing process of the housing | casing 12 and an electrolyte arrangement | positioning (liquid injection) process may be the same as the method currently performed by manufacture of the conventional lithium secondary battery, and do not characterize this invention.

  Since the lithium secondary battery for vehicles according to the present invention has the excellent performance (output performance, durability, etc.) as described above, it is suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Can be done. Such lithium secondary batteries may be used in the form of an assembled battery formed by connecting a plurality of them in series and / or in parallel. Accordingly, as schematically shown in FIG. 7, the present invention provides a vehicle (typically an automobile, particularly a hybrid automobile, an electric automobile, etc.) having such a lithium secondary battery (which may be in the form of an assembled battery) 10 as a power source. An automobile equipped with an electric motor such as a fuel cell automobile) 1 is provided.

Although it is not necessary to clarify the reason why the above-described effects of the present invention are exerted in implementing the present invention, examples of factors that may be related to the effects include the following.
As described above, when an alkaline positive electrode active material composition is applied to an alkali-reactive current collector such as an aluminum foil and dried by a general hot air dryer or the like, a reaction gas (typically H ₂ gas) is obtained. ), A positive electrode active material layer having a property including macroscopic pores is formed. Such a positive electrode active material layer containing a large number of macropores adjusts the properties of the active material layer (for example, the density is adjusted by a technique such as compression) to balance the electrolyte solution permeability and conductivity. Even so, it is difficult to achieve such a balance at a high level. This is because even if the positive electrode active material layer is compressed, the macro vacancies are not easily lost. The density of the positive electrode active material layer is compressed to a higher density (excessively) or higher, and as a result, the permeability of the electrolytic solution decreases before reaching a density that sufficiently improves the conductivity. It is done. Further, such a positive electrode active material layer in which the density non-uniformity is conspicuous is disadvantageous from the viewpoint of durability of battery performance (for example, output performance).

  Such an event is caused by an aqueous positive electrode active material composition having a composition containing a conductive material at a higher rate than a lithium secondary battery for consumer use (such as a lithium secondary battery for a mobile phone) (for example, a positive electrode active material powder). The positive electrode active material composition in which carbon powder having an average particle size of about 1/50 to 1/500 of the average particle size is blended at a ratio of about 10 to 20% by mass in terms of solid content is particularly easily manifested. This is because the density of the positive electrode active material layer mainly depends on the particle size of the positive electrode active material particles in a composition having a relatively low content of the conductive material, such as a positive electrode active material layer provided in a lithium secondary battery for consumer use. On the other hand, in a composition having a relatively high content of the conductive material such as a positive electrode active material layer preferred in a lithium secondary battery for vehicles, the density of the positive electrode active material layer tends to be influenced by the particle size of the conductive material. It is to become. For example, when the conductive material particles are filled in the gaps between the positive electrode active material particles, the density of the positive electrode active material layer in portions other than the macro vacancies tends to be excessively increased.

  In the production method according to the present invention, the water-based positive electrode active material composition is dried by induction heating to form an uncompressed positive electrode active material layer, thereby preventing the generation of macroscopic voids ( An uncompressed positive electrode active material layer) is formed, and then the uncompressed positive electrode active material layer is appropriately compressed (for example, so as to achieve the appropriate density described above). This makes it possible to efficiently produce a positive electrode active material layer having properties suitable for exhibiting high performance (for example, having few macro vacancies due to the reaction gas and having an appropriate density). . According to such a positive electrode active material layer, electrolyte permeability and conductivity can be balanced at a higher level. Therefore, according to the technique disclosed herein, it is obtained by applying a water-based positive electrode active material composition to an alkali-reactive positive electrode current collector, and also has a composition containing a relatively large amount of conductive material. Even if it exists, the positive electrode suitable for the use which constructs a lithium secondary battery for vehicles with higher performance can be manufactured exactly.

  Hereinafter, experimental examples relating to the present invention will be described. However, the present invention is not intended to be limited to the specific examples.

<Experimental example 1>
[Production of positive electrode]
A lithium ion battery positive electrode having a lithium nickel-based oxide (lithium nickelate) having a composition represented by LiNiO ₂ as a positive electrode active material was produced as follows.
That is, lithium nickelate powder (average particle size of about 10 μm), acetylene black (average particle size of primary particles of about 40 nm, average particle size of secondary particles of about 500 nm), carboxymethylcellulose (CMC) and polytetrafluoroethylene, A paste-like aqueous positive electrode active material composition was prepared by mixing with ion-exchanged water so that the mass ratio of these materials was 87: 11: 1: 1 and the solid content concentration was 45 mass%.

Using the positive electrode manufacturing apparatus 60 having the schematic configuration shown in FIG. 1, the positive electrode active material composition is applied to both surfaces of a positive electrode current collector (15 μm thick aluminum foil is used here) 32, and dried and compressed. Thus, the positive electrode 30 was produced.
That is, the positive electrode current collector 32 is conveyed from the unwinding roll 72 to the winding roll 78 at a traveling speed of 150 cm / min, and the positive electrode active material composition is formed on one surface of the positive electrode current collector 32 by the first coater 62. Was applied. Next, the current collector 32 to which the composition is applied is arranged such that three coils 65 having the size and structure shown in FIG. 2 are spaced from each other along the transport path of the current collector 32 by about 10 cm. It introduced into the 1st induction heating apparatus 64 of the structure comprised. The distance from the coil 65 to the current collector 32 was 4 cm, and the application condition of the high frequency current to the coil 65 was 100 kHz, 3A. The positive electrode active material composition applied to the one surface was dried by passing through the induction heating device 64. At this time, the time from when the positive electrode active material composition is applied to the current collector 32 until it is dried by the first induction heating device 64 (more specifically, the predetermined location of the current collector 32 is the positive electrode active material in the coater 62. The time from the application position of the substance composition to the upstream end of the induction heating device 65 (that is, the time to reach the coil 65 arranged closest to the coater 62) was about 10 seconds.

Next, the positive electrode active material composition is applied to the other surface of the current collector 32 by the second coater 66, and applied to the other surface by the second induction heating device 68 configured similarly to the first induction heating device 64. The prepared positive electrode active material composition was dried. In this way, an uncompressed positive electrode active material layer was formed on both the one surface and the other surface of the current collector 32. Here, the coating amount of the positive electrode active material composition is adjusted so that the total coating amount (in terms of solid content) of the positive electrode active material composition coated on both surfaces of the current collector 32 is 12.8 g / cm ^2. did.

Then, the current collector 32 having an uncompressed positive electrode active material layer on both sides obtained by drying by induction heating was compressed through a roll press machine 75. At this time, by adjusting the compression conditions of the roll press machine 75 (the thickness of the positive electrode 30 obtained after compression), five types of positive electrode samples 1 to 5 having different positive electrode active material layer densities were produced. Among these, the positive electrode sample 1 is a sample obtained by performing compression (roll press) so that the thickness of the positive electrode (the total thickness including the current collector and the positive electrode active material layers on both sides) becomes 73 μm. The density of the positive electrode active material layer calculated from the coating amount and the thickness of the positive electrode is 2.21 g / cm ³ . The positive electrode sample 2 is a sample obtained by compressing so that the thickness of the positive electrode is 71 μm, and the density of the positive electrode active material layer calculated in the same manner as described above is 2.29 g / cm ³ . The positive electrode sample 3 is a sample obtained by compressing so that the thickness of the positive electrode becomes 69 μm, and the density of the positive electrode active material layer is 2.37 g / cm ³ . The positive electrode sample 4 is a sample obtained by compression so that the thickness of the positive electrode is 66 μm, and the density of the positive electrode active material layer is 2.51 g / cm ³ . The positive electrode sample 5 is a sample obtained by compressing the positive electrode to have a thickness of 64 μm, and the density of the positive electrode active material layer is 2.61 g / cm ³ . In addition, when the cross section of the obtained sample was observed, the thickness of the positive electrode current collector 32 constituting each sample was maintained at substantially the same thickness (15 μm) as that before forming the positive electrode active material layer. The thickness of the positive electrode (uncompressed) before passing through the roll press 75 was about 110 μm (the density of the positive electrode active material layer was 1.35 g / cm ³ ).

[Production of lithium-ion batteries]
In order to evaluate the performance of the positive electrode samples 1 to 5 obtained as described above, each sample was used as a positive electrode, and a general cylindrical shape having a diameter of 18 mm and a height of 65 mm (that is, 18650 type) was performed according to the following procedure. A lithium ion battery 100 (see FIG. 8) was produced.
The following were used as the negative electrode. That is, natural graphite (powder), styrene butadiene rubber and carboxymethyl cellulose are mixed with ion-exchanged water so that the mass ratio of these materials is 98: 1: 1 and the solid content concentration is 45% by mass, A paste-like negative electrode active material composition was prepared. This composition was applied to both sides of a long copper foil having a thickness of about 15 μm as a negative electrode current collector and dried to form a negative electrode active material layer having a thickness of 120 μm on both sides of the copper foil current collector. Subsequently, it pressed so that the whole thickness might be set to 85 micrometers.

As a non-aqueous electrolyte (electrolytic solution), a supporting salt (here, LiPF ₆ ) was dissolved at a concentration of 1 mol / L in a 3: 7 (volume ratio) mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). A non-aqueous electrolyte (electrolytic solution) having a composition was used.
And the positive electrode sample and negative electrode which were produced above were laminated | stacked with two separators (here the porous polyethylene sheet was used), this laminated sheet was wound, and the wound electrode body was produced. This electrode body was housed in a container together with a nonaqueous electrolyte, and the container opening was sealed to construct a lithium ion battery. Thereafter, an appropriate conditioning process (for example, a constant current charging for 3 hours at a charging rate of 1/10 C, and then charging at a constant current constant voltage up to 4.1 V at a charging rate of 1/3 C; The initial charge / discharge treatment was repeated 2 to 3 times with a constant current discharge operation up to 3.0 V at a discharge rate of 5 to obtain 18650 type cylindrical lithium ion batteries corresponding to each positive electrode sample. . Hereinafter, the obtained lithium ion batteries are also referred to as samples 1 to 5 in association with the types of positive electrode samples used for the construction of these lithium ion batteries.

[Measurement of initial low temperature output]
This evaluation test was performed at −30 ° C.
The lithium ion batteries (samples 1 to 5) obtained above were adjusted to a 40% SOC (State of Charge) charge state by constant current constant voltage (CC-CV) charging. Each battery was discharged at a constant power (W) of 40 W, 60 W, 80 W and 100 W, and the time (discharge seconds) from the start of discharge until the battery voltage dropped to 2.5 V (discharge cut voltage) was measured. . By plotting the power value (W) in the constant power discharge against the discharge seconds, the power value at which the discharge seconds become 2 seconds (that is, 2.5 V in 2 seconds from the state of SOC 40% at −30 ° C. Output) until the initial CP (Constant Power, constant power) discharge output at −30 ° C. of the battery was obtained.

[Measurement of low temperature output after endurance]
This evaluation test was performed at −30 ° C.
The lithium ion batteries (samples 1 to 5) obtained above were charged with a constant current (10 A) from 3.0 V to 4.1 V, and then to a constant current (10 A) up to 3.0 V. The charge / discharge cycle for discharging was repeated 150 times. For these batteries after 150 cycles, the low-temperature output (CP output after endurance at −30 ° C.) was measured in the same manner as the measurement of the initial low-temperature output.
Further, for the lithium ion batteries according to Samples 1 to 5, the low-temperature output of the battery after 500 cycles (CP after endurance at −30 ° C.) was the same as above except that the number of charge / discharge cycles was 500. Output).

The results are shown in FIG. 9 in terms of relative output (output ratio) with the initial low-temperature output value of the lithium ion battery according to Sample 4 as 1. In the figure, the plots indicated by black circles indicate the initial low temperature output, the plots indicated by black squares indicate the low temperature output after 150 cycles endurance, and the plots indicated by crosses indicate the low temperature output after 500 cycles endurance.
As can be seen from this figure, the initial low temperature output is particularly good when the positive electrode active material layer has a density of 2.25 to 2.6 g / cm ³ (more preferably 2.3 to 2.55 g / cm ³ ). Results were obtained. On the other hand, as for the low temperature output after endurance, better results were obtained as the density increased in the range of the active material layer density of the positive electrode produced in this example after both endurance for 150 cycles and endurance for 500 cycles. Considering these performances comprehensively, the positive electrode active material layer density is 2.3 to 2.6 g / cm ³ (more preferably 2.3 to 2.5 g / cm ³ , still more preferably 2.4 to 2. 5 g / cm ³ ), it can be seen that a positive electrode suitable for the construction of a high-performance vehicle lithium ion battery in which the initial low temperature output and the low temperature output after endurance are balanced at a high level can be obtained. .

As a reference example, a total of 12.8 g / cm ² of the positive electrode active material composition was applied to both surfaces of the positive electrode current collector as in Samples 1 to 5, except that the composition was applied instead of induction heating. After the current collector is passed through a hot-air drying furnace to be dried to form an uncompressed positive electrode active material layer, compression is performed in a range where the density of the positive electrode active material layer becomes 2.0 to 2.7 g / cm ³ . Several types of positive electrodes were produced at different degrees. Using these positive electrodes, lithium ion batteries (hereinafter sometimes referred to as “lithium ion batteries according to reference examples”) were produced in the same manner as described above, and the initial low-temperature output was measured in the same manner. As a result, unlike the lithium ion batteries according to Samples 1 to 5 provided with the positive electrode active material layer obtained by drying after induction heating (see FIG. 9), the material is compressed after drying by hot air drying. In the lithium ion battery of the reference example provided with the obtained positive electrode active material layer, the density of the positive electrode active material layer having a peak output ratio in the graph of density versus output ratio (that is, the density at which the highest initial low-temperature output is obtained) is The output was about 2.2 g / cm ³ (the thickness of the positive electrode was about 73 μm), and the output tended to decrease even when the density was higher or lower than that. Moreover, the lithium ion battery constructed | assembled using the positive electrode provided with the positive electrode active material layer (The positive electrode active material layer of density 2.2g / cm < ³ > formed by compressing after hot-air drying) corresponding to the peak of the said output ratio. The initial low temperature output of was less than 0.9 in terms of relative output (output ratio) where the initial low temperature output value of the lithium ion battery according to Sample 4 was 1.

[Measurement of initial normal temperature output]
This evaluation test was performed at 25 ° C.
The lithium ion batteries (samples 1 to 5) obtained above were adjusted to a SOC of 60% by CC-CV charging. Each battery was discharged at a constant power (W) of 40 W, 60 W, 80 W and 100 W, and the time (discharge seconds) from the start of discharge until the battery voltage dropped to 2.5 V (discharge cut voltage) was measured. . By plotting the power value (W) in the constant power discharge against the discharge seconds, a power value at which the discharge seconds becomes 2 seconds was obtained, and this was used as the initial CP discharge output at room temperature of the battery. .

These results are shown in FIG. 10 in terms of relative output (output ratio) where the initial normal temperature output value of the lithium ion battery according to Sample 2 is 1. As can be seen from this figure, good results were obtained when the density of the positive electrode active material layer was in the range of 2.2 to 2.5 g / cm ³ . Considering this result and the result of the low-temperature output (initial and after durability), the positive electrode active material layer density is 2.3 to 2.5 g / cm ³ (more preferably 2.4 to 2.5 g / cm ³ ). By adjusting to this range, it can be seen that a positive electrode suitable for construction of a high-performance lithium-ion battery for vehicles in which the initial output from normal temperature to low temperature and the low-temperature output after durability are balanced at a high level can be obtained.

In the lithium ion battery according to the above reference example, the density of the positive electrode active material layer having a peak output ratio (that is, the density at which the highest initial normal temperature output is obtained) is about 2.1 g / cm ³ (the thickness of the positive electrode). The output tends to decrease even when the density is higher or lower than that. Moreover, the lithium ion battery constructed | assembled using the positive electrode provided with the positive electrode active material layer (The positive electrode active material layer of density 2.1g / cm < ³ > formed by compressing after hot-air drying) corresponding to the peak of the said output ratio. The initial normal temperature output was less than 0.9 in terms of relative output (output ratio) where the initial low temperature output value of the lithium ion battery according to Sample 2 was 1.

<Experimental example 2>
[Production of positive electrode]
The positive electrode sample was prepared in the same manner as the positive electrode samples 1 to 5 prepared in Experimental Example 1, except that the positive electrode was compressed to have a thickness of 77 μm (the density of the positive electrode active material layer was 2.06 g / cm ³ ). 6 was produced. Moreover, the positive electrode sample 7 was produced like the positive electrode samples 1-5 except having compressed so that the thickness of a positive electrode might be set to 65 micrometers (the density of a positive electrode active material layer is 2.56 g / cm < ³ >).

[Pore distribution measurement]
Each positive electrode is composed of positive electrode sample 1 (density 2.21 g / cm ³ ), positive electrode sample 3 (density 2.37 g / cm ³ ) prepared in Experimental Example 1, and positive electrode sample 6 and positive electrode sample 7 prepared above. The pore distribution of the positive electrode active material layer was measured. For the pore distribution measurement, a fully automatic pore distribution measuring device manufactured by Yuasa Ionics Co., Ltd., model number “Pore Master 33P” is used. Pore with a diameter exceeding 2 μm, diameter 0.3 μm to 2 μm The total pore volume by size was determined for each of the pores and pores having a diameter of less than 0.3 μm.

The results are shown in FIG. In this figure, the pore distribution of the positive electrode active material layer provided in each positive electrode sample is represented by the total volume of pores having a diameter of less than 0.3 μm, the total volume of pores having a diameter of 0.3 μm to 2 μm, and a diameter of 2 μm. As a bar graph stacked from the bottom in the order of the total volume of the pores exceeding, it is displayed in the order of sample 6, sample 1, sample 3, and sample 7 from the left. In the positive electrodes according to Sample 3 and Sample 7, pores having a diameter of less than 0.3 μm were not detected.
Further, assuming that the total pore volume of each positive electrode sample was 100%, the ratio of the total volume of pores having a diameter of 0.3 μm to 2 μm to the total pore volume was calculated. The results are shown in Table 1 together with the sample name and the density of the positive electrode active material layer.

As can be seen from Table 1, FIG. 11, and FIG. 9, in the positive electrode (samples 3 and 7) in which the density of the positive electrode active material layer was adjusted to ^2.3 to 2.6 g / cm ³ , all of the active material layer The volume ratio of pores having a diameter of 0.3 μm to 2 μm (pores having a size that effectively contributes to the movement of lithium ions) is 70% or more (more specifically 70 to 80%, more preferably 70 to 75%). A high percentage. These results show that by compressing the positive electrode active material layer so as to achieve the above density and / or volume ratio, higher performance (for example, initial low temperature output and post-endurance low temperature output at a higher level). It supports that a balanced lithium-ion battery can be manufactured stably.

<Experimental example 3>
With respect to the positive electrodes according to Samples 1, 3, 6, and 7 prepared in Experimental Example 1 or 2, the electrolyte solution impregnation property (penetration) into the positive electrode active material layer provided in each positive electrode was evaluated. The evaluation of the impregnation property is carried out by immersing the positive electrode in an electrolytic solution having a composition corresponding to its use at the measurement temperature of 25 ° C. (here, the electrolytic solution used in the construction of the lithium ion battery in Experimental Example 1). The heat was measured, and the initial impregnation rate (the ratio of the wet heat generated in 2 seconds after immersion to the total wet heat) was determined from the result. A microcalorimeter manufactured by Tokyo Riko Co., Ltd., model number “Multi-micro calorimeter MMC-5112I” was used for measurement of heat of wetting. The obtained results are shown in FIG. 12 with the density of the positive electrode active material layer of each sample as the horizontal axis and the initial impregnation rate as the vertical axis.
As can be seen from this figure, in all of the positive electrodes (samples 3 and 7) in which the density of the positive electrode active material layer was adjusted to ^2.3 to 2.6 g / cm ³ , the initial impregnation rate of the active material layer was 43. % Or more (more specifically, 43 to 45%, more preferably 43 to 44%). This result shows that by compressing the cathode active material layer so as to achieve the above-mentioned density and / or impregnation rate, higher performance (for example, initial low temperature output and post-endurance low temperature output balance at a higher level) It supports that lithium ion batteries can be manufactured stably.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here. For example, in FIG. 1, a positive electrode active material composition is first applied to one surface of the positive electrode current collector 32 and dried by induction heating, and then the positive electrode active material composition is applied to the other surface of the positive electrode current collector 32. In the embodiment, the positive electrode manufacturing apparatus 60 is coated and dried by induction heating. For example, the positive electrode active material composition is applied to both surfaces of the positive electrode current collector 32 almost simultaneously or successively, and the positive electrode current collector is applied. The positive electrode active material composition applied to both surfaces of the current collector 32 may be simultaneously dried by induction heating 32.

It is typical explanatory drawing which shows the outline of the manufacturing method of the lithium secondary battery positive electrode which concerns on one Embodiment. It is typical explanatory drawing which illustrates the principal part of an induction heating device. It is typical sectional drawing which shows the lithium secondary battery positive electrode which concerns on one Embodiment. 1 is a perspective view schematically showing a lithium secondary battery according to an embodiment. It is a partially broken top view which shows the positive / negative electrode and separator which comprise the wound electrode body which concerns on one Embodiment. It is the VI-VI sectional view taken on the line in FIG. It is a side view which shows typically the vehicle (automobile) provided with the lithium secondary battery which concerns on this invention. It is a perspective view which shows typically the lithium secondary battery which concerns on other embodiment. It is a graph which shows the output characteristic in -30 degreeC of the battery which concerns on an experiment example. It is a graph which shows the output characteristic in 25 degreeC of the battery which concerns on an experiment example. It is a graph which shows the pore distribution of the positive electrode which concerns on an experiment example. It is a graph which shows the electrolyte solution impregnation rate of the positive electrode which concerns on an experiment example.

Explanation of symbols

1 Vehicle (Automobile)
DESCRIPTION OF SYMBOLS 10,100 Lithium secondary battery 12 Case 20 Winding electrode body 30 Positive electrode 32 Positive electrode collector 35 Positive electrode active material layer 40 Negative electrode 50A, 50B Separator 60 Positive electrode preparation apparatus 62, 66 Coater (coating apparatus)
64, 68 Induction heating device 65 Coil 75 Roll press machine (compression device)
76 Compression roll

Claims (9)

A method for producing a positive electrode used in a lithium secondary battery for vehicles, comprising a positive electrode current collector and a positive electrode active material layer formed on the current collector:
Positive electrode active material composition comprising lithium transition metal oxide powder as a positive electrode active material, carbon powder as a conductive material, and an aqueous medium on the surface of the positive electrode current collector made of a metal material that reacts with alkali to generate gas Giving things;
Drying the positive electrode active material composition applied to the current collector by induction heating to form an uncompressed positive electrode active material layer on the current collector; and
Compressing the uncompressed positive electrode active material layer to adjust the density of the active material layer to 2.3 to 2.6 g / cm ³ ;
The manufacturing method of the lithium secondary battery positive electrode for vehicles including this.   The said positive electrode active material composition is a method of Claim 1 containing the said carbon powder in the ratio of 10-20 mass% in conversion of solid content of this composition.   The method according to claim 1, wherein the lithium transition metal oxide is a lithium nickel oxide.   The method according to claim 1, wherein the current collector is made of aluminum.   The compression is performed such that the volume ratio of pores having a diameter of 0.3 to 2.0 μm is 70 to 85% of the total pore volume in the pore distribution of the positive electrode active material layer after compression. 5. The method according to any one of items 1 to 4.   The positive electrode for lithium secondary batteries for vehicles manufactured by the method according to any one of claims 1 to 5.   A lithium secondary battery for vehicles, comprising: the positive electrode according to claim 6; a negative electrode capable of inserting and extracting lithium ions; and a non-aqueous electrolyte.   The positive electrode was obtained by performing the compression so that the heat of wetting generated in 2 seconds after immersion of the positive electrode in the electrolyte at a measurement temperature of 25 ° C. was 43 to 50% of the total heat of wetting. The battery according to claim 7.   A vehicle comprising the lithium secondary battery according to claim 7 or 8.