JP2008181739A - Positive electrode for non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode for manufacturing a non-aqueous electrolyte secondary battery having a high output and a high capacity as well as a long life to be realized. <P>SOLUTION: The electrode for the non-aqueous electrolyte secondary battery is obtained by filling a positive electrode active material in a current collector which is obtained by laminating a nickel plating layer and a chrome plating layer in order on a porous non-woven fabric. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a novel positive electrode for a nonaqueous electrolyte secondary battery.

In recent years, non-aqueous electrolyte secondary batteries typified by lithium ion batteries have become widespread for reasons such as high energy density. Such a lithium ion battery utilizes the principle of charging and discharging by moving lithium ions between a positive electrode and a negative electrode. Lithium ion batteries include LiCoO ₂ and LiMn ₂ O ₄ as positive electrode materials, carbon materials capable of occluding and releasing lithium ions as negative electrode materials, microporous thin films as separators, and LiBF ₄ and LiPF ₆ as electrolytes. An organic solvent in which a lithium salt is dissolved is used.

In particular, for positive electrode materials, LiCoO ₂ is the mainstream and widely used. LiMn ₂ O ₄ has also been put to practical use because it can greatly solve the resource depletion problem and the price problem.

However, even in these materials, further improvement in discharge capacity is required as a future problem. In addition, LiMn ₂ O ₄ has a problem that Mn dissolves in the electrolyte due to an increase in battery temperature. In addition to these materials, LiNiO _{2 and the} like have also been developed, but both the discharge capacity and voltage are low, and further improvement is necessary.

On the other hand, in a lithium ion battery, a metal foil is generally used as a current collector (support) to which a positive electrode material or a negative electrode material is attached. Such current collectors have many porous bodies such as punching metal, screen, and expanded metal, as well as three-dimensional porous bodies such as foam and nonwoven fabric, for the purpose of high output, high capacity, and long life. It has been proposed (Patent Documents 1 to 4). For example, Patent Document 1 discloses a three-dimensional network porous body whose surface is made of aluminum, an alloy, or stainless steel as a positive electrode current collector. Patent Document 2 discloses a current collector such as a sheet made of a metal such as aluminum, copper, zinc, or iron, or a conductive polymer such as polypyrrole or polyaniline, or a mixture thereof, a sheet having holes, or a three-dimensional porous body. It is disclosed. Patent Document 3 discloses a current collector such as aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony alone or an alloy, or a stainless alloy. Patent Document 4 discloses foamed aluminum, foamed nickel, and the like as the positive electrode current collector.
JP-A-11-233151 JP2000-195522 JP2005-078991 JP2006-032144

  In order to increase the output and capacity of the secondary battery as a whole, it is desired that the current collector adopts a three-dimensional structure having a larger porosity than the two-dimensional structure. In particular, a positive electrode current collector is required to have oxidation resistance and electrolytic solution resistance because it is oxidized by an electrolyte under a high charging voltage.

  However, for the following reasons, the lithium-based non-aqueous electrolyte secondary battery has an oxidation resistance and an electrolyte resistance, a large porosity, and a positive electrode current collector suitable for industrial production. Is not provided.

  In order to increase the porosity of the current collector, the surface of the porous organic resin is generally plated, as represented by a nickel porous body, and the organic resin is removed by incineration if necessary. Is called.

  However, the nickel porous body is easily oxidized in the lithium non-aqueous electrolyte secondary battery, and is dissolved in the electrolyte solution, so that sufficient charging cannot be performed by long-term charge / discharge.

  On the other hand, in aluminum, which is the main material of the current positive electrode current collector, it is necessary to process in a very high temperature molten salt state for the plating process, so organic resin cannot be used as the object to be plated. It is difficult to plate the surface of the organic resin. Therefore, a highly porous current collector made of aluminum is not currently provided.

  Stainless steel is also widely used as a material for the positive electrode current collector. For the same reason as stainless steel, this stainless steel should be made a highly porous current collector by plating the surface of the organic resin. It is difficult. Further, there has been provided a method for obtaining a porous body of stainless steel by applying it to powder and applying it to an organic resin porous body and sintering it.

  However, stainless steel powder is very expensive. In addition, since the organic resin is incinerated and removed, there is a problem that the strength deteriorates and it cannot be used.

  Therefore, it is desired to provide a current collector that has oxidation resistance and electrolytic solution resistance, has a high porosity, and is suitable for industrial production, and further provides a positive electrode obtained by using this current collector.

  In view of the above problems, the present inventors have intensively researched and, as a result, have adopted the current collector having a specific structure and have solved the above problems. That is, the present invention relates to the following positive electrode for a nonaqueous electrolyte secondary battery.

Item 1. A positive electrode for a non-aqueous electrolyte secondary battery, which is obtained by filling a current collector obtained by sequentially laminating a nickel plating layer and a chromium plating layer on a porous nonwoven fabric with a positive electrode active material.
Item 2. Item 2. The positive electrode according to Item 1, wherein the positive electrode active material contains olivine-type lithium phosphate.
Item 3. Item 2. The positive electrode according to Item 1, wherein the positive electrode active material contains a lithium metal oxide.
Item 4. Item 4. The positive electrode according to Item 3, wherein the lithium metal oxide includes an oxide of at least one metal selected from the group consisting of cobalt, manganese, and nickel.
Item 5. Item 5. The positive electrode according to any one of Items 1 to 4, wherein the nonwoven fabric is made of polyolefin resin fibers.
Item 6. Item 6. The positive electrode according to any one of Items 1 to 5, wherein a weight per unit area of the nickel plating layer is 50 to 100 g / m ² .
Item 7. Item 7. The positive electrode according to any one of Items 1 to 6, wherein the basis weight of the chromium plating layer is 50 to 150 g / m ² .
Item 8. Item 8. The positive electrode according to any one of Items 1 to 7, wherein the conductive additive is contained in an amount of 0.5 to 5 parts by weight with respect to 100 parts by weight of the positive electrode active material.

  The positive electrode for a nonaqueous electrolyte secondary battery according to the present invention is obtained by filling a positive electrode active material into a current collector obtained by sequentially laminating a nickel plating layer and a chromium plating layer on a porous nonwoven fabric. To do. This feature makes it possible to produce a positive electrode with a highly porous current collector that excels in oxidation resistance, electrolyte resistance, strength, industrial production, etc., and increases the output and capacity of nonaqueous electrolyte secondary batteries. And longer life. The present invention is described in detail below.

Nonwoven fabric The porous nonwoven fabric used in the present invention is not limited, and known or commercially available ones can be used. In particular, from the viewpoint of excellent oxidation resistance and electrolyte resistance, polyolefin resin fibers are the main component. It is preferable. Such polyolefin resin fibers are preferably thermoplastic, for example, fibers made of olefin homopolymers such as polyethylene, polypropylene, polybutene, ethylene-propylene copolymers, ethylene-butene copolymers, propylene- Examples thereof include fibers made of an olefin copolymer such as a butene copolymer, and a mixture of these fibers. Moreover, the core-sheath-type fiber which the olefin resin (core component) coat | covered with the kind of olefin resin (sheath component) different from the said core component is also mentioned.

  Among these, a core-sheath fiber having polypropylene as a core component and polyethylene as a sheath component is particularly preferable. In this case, the blending ratio (weight ratio) of polypropylene resin: polyethylene resin is usually about 20:80 to 80:20, preferably about 40:60 to 70:30.

  The average fiber diameter of the resin fibers is usually about 9 μm to 50 μm, preferably about 10 μm to 40 μm. The average fiber length is usually about 5 mm to 100 mm, preferably about 30 mm to 70 mm. The molecular weight and density of the polyolefin resin constituting the polyolefin resin fiber are not particularly limited.

  The porosity of a nonwoven fabric is about 85-98 vol% normally, Preferably it is about 86-96 vol%. By setting it within this range, the current collector can be filled with a large amount of the positive electrode active material while maintaining the strength as an electrode, and the battery can have high output and high capacity.

  The thickness of the non-woven fabric is not limited, and may be appropriately determined according to the use, purpose, etc. of the nonaqueous electrolyte secondary battery to be manufactured, but is usually about 100 μm to 600 μm, preferably about 150 μm to 500 μm.

  Prior to the plating described below, the nonwoven fabric may be subjected to a pretreatment such as a confounding treatment such as a needle punch method or a hydroentanglement method; a heat treatment near the softening temperature of the resin fiber. By such pretreatment, the bonding between the fibers becomes strong and the strength of the nonwoven fabric can be improved. As a result, when filling the positive electrode active material into the nonwoven fabric, the three-dimensional structure of the nonwoven fabric can be sufficiently retained.

  The nonwoven fabric is usually produced by any one of the known dry method and wet method, but may be produced by any method in the present invention. Examples of the dry method include a cart method, an air lay method, a melt blow method, and a spun bond method. Examples of the wet method include a method in which a single fiber is dispersed in water and rolled up on a net-like net. In the present invention, it is preferable to use a nonwoven fabric obtained by a wet method from the viewpoint of producing a current collector with a small basis weight and a small variation in thickness and a uniform thickness.

Current Collector In the current collector of the present invention, a nickel plating layer and a chromium plating layer are sequentially laminated on the surface of the porous nonwoven fabric. Since it has a structure in which a chromium plating layer is formed on such a nickel plating layer, the current collector of the present invention has oxidation resistance, electrolyte resistance, and the like.

  The nickel plating layer is a layer provided by a known nickel plating process. Such nickel plating treatment is not limited, and examples thereof include electrolytic plating, electroless plating, and sputtering. A plurality of nickel plating layers may be formed by forming the nickel plating layer by a plurality of plating methods.

The weight per unit area (adhesion amount) of the nickel plating layer is not limited, but is usually about ²⁰ to 300 g / m ^{2 with} respect to the nonwoven fabric from the viewpoints of conductivity, porosity, strength, economy, corrosion resistance, etc., preferably It may be about 50 to 150 g / m ² (when a plurality of nickel plating layers are formed, the total amount is indicated).

  In the electroless plating method, for example, the nonwoven fabric may be immersed in a known electroless nickel plating bath such as a nickel sulfate aqueous solution containing sodium hypophosphite as a reducing agent. Moreover, you may immerse a nonwoven fabric in the activation liquid (Kanizen Co., Ltd.) etc. which contain a trace amount palladium ion before immersion in a plating bath as needed.

  In the sputtering method, for example, after attaching a non-woven fabric to a substrate holder, a DC voltage is applied between the holder and the target (nickel) while introducing an inert gas so that the ionized inert gas collides with the nickel. Then, the blown nickel may be deposited on the nonwoven fabric surface.

  The electroplating method may be performed in a known nickel electroplating bath, and examples of the nickel plating bath to be used include a watt bath, a chloride bath, a sulfamic acid bath, and the like.

  When nickel plating is applied to the nonwoven fabric, an electroless plating method or a sputtering method may be used in combination with the electrolytic plating method. That is, after a conductive layer (nickel plating layer) is provided by an electroless plating method or a sputtering method, a desired amount of a nickel plating layer may be formed by performing an electrolytic plating method. Thereby, a desired amount of nickel plating layer can be provided.

  In the current collector of the present invention, a chromium plating layer is formed on the nickel plating layer. The chromium plating layer is a layer provided by a known chromium plating process. Such a chromium plating treatment is not limited, and examples thereof include an electrolytic plating method, an electroless plating method, and a sputtering method. In the present invention, a chromium plating layer formed by an electrolytic plating method is preferable.

As the electroplating bath used in the electroplating method, a known or commercially available bath can be used. For example, a sergeant bath (typical composition is CrO ₃ : 250 g / l and sulfuric acid H ₂ SO ₄ : 2.5 g / l). And a fluorination bath (typical composition is CrO ₃ : 250 g / l and fluorine component F: 0.6 g / l (or SiF ₆ : 2.5 g / l)).

The basis weight (attachment amount) of the chromium plating layer is not limited, but from the viewpoint of conductivity, porosity, strength, economy, corrosion resistance, and the like, for example, generally about ²⁰ to 200 g / m ^{2 with} respect to the nonwoven fabric, Preferably, it may be about 50 to 150 g / m ² . In addition, it is good also as a some chromium plating layer by forming a chromium plating layer by a some plating method.

Positive electrode The positive electrode of the present invention is obtained by filling a positive electrode active material into a current collector in which a nickel plating layer and a chromium plating layer are sequentially laminated on a porous nonwoven fabric. Since the positive electrode of the present invention has a porous nonwoven fabric as a support for the current collector, it has high strength and porosity and can be filled with more positive electrode active material. Thereby, it is possible to increase the output and capacity of the battery.

The positive electrode active material is not particularly limited as long as it can be used for a non-aqueous electrolyte secondary battery. In the present invention, olivine type lithium phosphate (LiMPO ₄ ) is particularly preferable. Examples of the metal component M constituting the olivine-type lithium phosphate include iron (Fe), manganese (Mn), chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), and aluminum (Al). And at least one selected from the group consisting of and the like. Among these, iron and the like are particularly preferable, that is, olivine type lithium iron phosphate and the like are preferable. In this olivine type lithium iron phosphate, a part of iron may be substituted with other metals such as Mn, Cr, Cu, Ni, Zn, Al.

  In the present invention, a known or commercially available lithium metal oxide (LiM′Ox) (where 1 ≦ x ≦ 4) can also be used as the positive electrode active material. Examples of such a lithium metal oxide M ′ include at least one metal selected from the group consisting of Co, Mn, Ni, and the like.

The filling amount of the positive electrode active material is not limited, and may be appropriately determined according to the use, purpose, etc. of the nonaqueous electrolyte secondary battery to be produced, but is usually about 40 mg to 150 mg, preferably 50 mg per 1 cm ^{2 of} the nonwoven fabric. It should be about ~ 100mg.

  In addition to the positive electrode active material, for example, a known additive such as a conductive additive or a binder may be included as a material to be filled in the plated porous current collector.

  As the conductive assistant, known or commercially available ones can be used. For example, acetylene black, ketjen black, graphite and the like are preferable. The content of the conductive assistant is usually about 0.1 to 7 parts by weight, preferably about 0.5 to 5 parts by weight, and more preferably about 0.5 to 2 parts by weight with respect to 100 parts by weight of the positive electrode active material. Thereby, the discharge capacity etc. of a battery can be improved.

  A known or commercially available binder can be used. For example, polyvinylidene fluoride (PVdf), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC) and the like. Among these, PVdf is preferable. Thereby, the binding strength between the positive electrode active material and the current collector can be improved. The amount of the binder added is appropriately determined according to the type of the binder and the like, but is usually about 0.1 to 5 parts by weight with respect to 100 parts by weight of the positive electrode active material. By setting it as this range, it is possible to improve the binding strength while preventing an increase in electrical resistance and a decrease in discharge capacity.

  The positive electrode of the present invention is formed by filling the above-mentioned nickel-plated and chrome-plated porous nonwoven fabric with a positive electrode active material. Specifically, for example, a paste containing a positive electrode active material is known, such as a press-fitting method. What is necessary is just to fill the said porous nonwoven fabric by the method.

  Examples of the press-fitting method include a method of immersing a porous nonwoven fabric in a positive electrode active material paste and reducing the pressure as necessary, and a method of filling the positive electrode active material paste while pressing it with a pump from one side of a current collector. Can be mentioned.

  The positive electrode active material paste only needs to contain a positive electrode active material and a solvent, and the blending ratio thereof is not limited. The solvent is not limited, and examples thereof include N-methyl-2-pyrrolidone and water. In particular, when polyvinylidene fluoride is used as a binder, N-methyl-2-pyrrolidone may be used as a solvent. When polytetrafluoroethylene, polyvinyl alcohol, carboxymethyl cellulose, or the like is used as a binder, water may be used as a solvent.

  In the positive electrode of the present invention, the solvent in the paste may be removed by performing a drying treatment after filling the positive electrode active material paste, if necessary.

  If necessary, the positive electrode of the present invention may be compression molded by pressurizing the paste with a roller press after filling.

  According to the positive electrode of the present invention, the current collector has oxidation resistance, electrolyte resistance, porosity, and high strength. Therefore, the non-aqueous electrolyte secondary battery such as a lithium ion battery has high output and high power. Capacitance and life can be extended. Further, since the material is inexpensive and easy to manufacture, it is suitable for industrial production.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. The present invention is not limited to the following examples.

Examples 1-3
-Fabrication of non-woven fabric Non-woven fabric material used was a core-sheath fiber (polypropylene component was 15 μm, polypropylene component was 60% by weight, polyethylene component was 40% by weight) with polypropylene fiber as the core component and polyethylene as the sheath component. . Using this core-sheath fiber, a porous nonwoven fabric was prepared by a wet method so that the basis weight was 50 g / m ² and the average thickness was 1.3 mm. The produced porous nonwoven fabric had a porosity of about 94 vol% and a pore size of 15 μm to 200 μm.

-Preparation of current collector An electroless nickel plating method was applied to 1 m ^{2 of} this porous nonwoven fabric to form a conductive layer (nickel plating layer) on the surface of the nonwoven fabric. A nickel sulfate aqueous solution (commercially available) containing sodium hypophosphite as a reducing agent was used in the electroless nickel plating bath.

Next, an electrolytic nickel plating method was performed, and a nickel plating layer was further formed. The electrolytic nickel plating bath was a Watts bath (nickel sulfate 240 g / liter, nickel chloride 45 g / liter, boric acid 30 g / liter), pH was adjusted to 4-5, and temperature was adjusted to 45-55 ° C. The cathode current density was 2 A / dm ² . As the counter electrode, a nickel basket injected into a nickel basket was used. The total amount of the nickel plating layer formed by the electroless nickel plating method and the electrolytic plating method was 80 g / m ² .

Next, the obtained nickel plating layer-formed non-woven fabric was subjected to a chromium plating treatment to form a chromium plating layer of 150 g / m ² on the nickel plating layer. As an electrode, an electrode in which a titanium substrate was coated with a platinum-based metal and an electrode in which a copper substrate was coated with a lead alloy were used. As a plating bath, a Sargent bath (bath composition chromic acid 250 g / l, sulfuric acid 2.5 g / l) was used. As electrodeposition conditions, the bath temperature was 50 ° C. and the current density was 40 A / dm ² .

  The porous bodies thus obtained were adjusted to three thicknesses of 480 μm, 570 μm and 680 μm, respectively, and were designated as current collectors a, b and c of the present invention.

・ Production of positive electrode 100 parts by weight of LiFePO ₄ powder as a positive electrode active material, 2.5 parts by weight of ketjen black as a conductive additive, and 5 parts by weight of polyvinylidene fluoride as a binder are mixed and mixed as a solvent for the binder A positive electrode active material paste was prepared by adding 25 parts by weight of N-methyl-2-pyrrolidone.

This paste was filled into current collectors a, b, and c using a press-fitting method using a known pump. The filling amount of the positive electrode active material was 55 mg / cm ² for current collector a, 69 mg / cm ² for current collector b, and 90 mg / cm ² for current collector c.

  Next, these three types of current collectors were dried at 90 ° C. for 1 hour with a dryer, and then pressed under conditions of a slit of 100 μm with a roller press having a roller diameter of 500 mm. The thickness after pressing was 220, 310 μm, and 396 μm, respectively. Thereafter, it was further dried at 150 ° C. under reduced pressure for 3 hours to obtain positive electrodes a, b and c of Examples 1 to 3.

Comparative Examples 1-3
An aluminum foil (thickness: 25 μm) was used as a current collector. In this case, the positive electrode active material paste prepared in the example was applied by the doctor blade method so that the total on both sides was 40 mg / cm ² , but the positive electrode active material was sufficiently aluminum foil because the adhesive strength was insufficient. Could not be adhered to.

Therefore, a positive electrode active material paste similar to that produced in the example was produced except that the polyvinylidene fluoride was changed to 10 parts by weight. This paste was applied to both sides of the aluminum foil by the doctor blade method so that the coating amount (both sides) of the positive electrode active material was 21 mg / cm ² , 31 mg / cm ² and 43 mg / cm ² , respectively. By drying and pressurizing, positive electrodes d, e, and f of Comparative Examples 1 to 3 were produced. The thicknesses of these positive electrodes were 105 μm, 154 μm, and 215 μm, respectively.

Comparative Example 4
As the current collector, foamed nickel (commercial product, porosity 96 vol%, average pore diameter 150 μm, thickness 550 μm) was used. The positive electrode active material produced in Example 1 was filled in the same manner as in Example 1 so as to be 70 mg / cm ^2, and then further pressurized and dried to produce the electrode of Comparative Example 4.

Production of Battery and Tests Each of the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 3 was cut into 5 cm × 5 cm, and batteries A to C (Examples 1 to 3) and B to D (Comparative Examples 1 to 3) ) Was produced. As the negative electrode, lithium metal having a sufficiently large capacity compared to the positive electrode was used, and as the electrolyte, 1.0 mol / l of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 4: 6). An organic electrolyte was used, a microporous polyolefin film (thickness 20 μm, porosity 55%) was used as a separator, and a laminate film was used as a battery case.

  These batteries A to F were charged to 4.1 V with a current of 0.1 C, and charged and discharged to discharge to 3.0 V with a discharge current of 0.2 C for 10 times to form. In this case, the end-of-charge voltage was 4.1V and the end-of-discharge voltage was 3V. Next, each battery was charged to an ambient temperature of 40 ° C. to 4.1 V at 0.2 C, and discharged to a final voltage of 3 V at 0.5 C, 1 C, and 1.5 C. Table 1 shows the capacity per unit weight obtained from the measured values.

  Also, charging to 4.1 V at 0.2 C and discharging to 3 V at 0.5 C were performed at an ambient temperature of 25 ° C. Table 1 also shows the rate of decrease in capacity at 200 cycles with respect to the initial capacity.

  In addition, although the battery produced using the positive electrode of the comparative example 4 repeated formation and charging / discharging on the same conditions as the above, it became impossible to charge in only 10 cycles (not shown together in Table 1).

As is clear from the evaluation table 1, it was found that the batteries A to C of the present invention were superior in capacity density and capacity retention rate than the batteries D to F of the comparative example. Thus, it was found that if the positive electrode of the present invention is used, the non-aqueous electrolyte secondary battery can be increased in output, capacity, and life.

  In addition, in the batteries A to C of the present invention, the reason why the decrease in the capacity density is small even when the charge / discharge cycle is repeated is that the positive electrode active material of the present invention is a current collector having a two-dimensional structure like the batteries D to F of the comparative example. This is because it is not coated on the surface of the body but is wrapped in a three-dimensional porous skeleton (porous non-woven fabric), and as a result, electric resistance due to swelling of the positive electrode material (positive electrode active material, conductive additive and binder) It is thought that the increase in

  From the above, it has been clarified that the effects of the present invention are exhibited with respect to olivine-type lithium phosphate having a slightly lower charge / discharge potential than those of general-purpose lithium metal oxides in Examples 1-3.

  Further, when at least one lithium metal oxide selected from the group consisting of cobalt, manganese, nickel and the like is used as the other positive electrode active material, the effects of the present invention are also exhibited.

Claims (8)

A positive electrode for a non-aqueous electrolyte secondary battery, which is obtained by filling a current collector obtained by sequentially laminating a nickel plating layer and a chromium plating layer on a porous nonwoven fabric with a positive electrode active material. The positive electrode according to claim 1, wherein the positive electrode active material contains olivine type lithium phosphate. The positive electrode according to claim 1, wherein the positive electrode active material contains a lithium metal oxide. The positive electrode according to claim 3, wherein the lithium metal oxide includes an oxide of at least one metal selected from the group consisting of cobalt, manganese, and nickel. The positive electrode according to any one of claims 1 to 4, wherein the nonwoven fabric is made of a polyolefin resin fiber. The positive electrode according to claim 1, wherein a weight per unit area of the nickel plating layer is 50 to 100 g / m ² . The positive electrode according to claim 1, wherein a weight per unit area of the chromium plating layer is 50 to 150 g / m ² . The positive electrode according to claim 1, wherein the conductive additive is contained in an amount of 0.5 to 5 parts by weight with respect to 100 parts by weight of the positive electrode active material.