JP2015138730A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery capable of effectively enhancing battery inner pressure during overcharging.SOLUTION: The secondary battery includes a positive electrode 50 and a negative electrode. The positive electrode 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 held on the positive electrode current collector 52. The positive electrode active material layer 54 includes a first positive electrode active material layer 54a formed on the positive electrode current collector 52 and a second positive electrode active material layer 54b formed on the first positive electrode active material layer 54a and the positive electrode current collector 52 around the layer 54a so as to cover the first positive electrode active material layer 54a. The first positive electrode active material layer 54a includes a first positive electrode active material capable of discharging oxygen in overcharging, and the second positive electrode active material layer 54b includes a second positive electrode active material whose humidity resistance is higher than the first positive electrode active material.

Description

  The present invention relates to a secondary battery including a positive electrode active material layer.

  Lithium ion secondary batteries and other secondary batteries are becoming increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is preferably used as a high-output power source mounted on a vehicle. When such a secondary battery is overcharged, excessive charge carriers are released from the positive electrode, and excessive charge carriers are inserted into the negative electrode. For this reason, both the positive electrode and the negative electrode are thermally unstable. When both the positive electrode and the negative electrode become thermally unstable, the organic solvent of the electrolytic solution is eventually decomposed, and an exothermic reaction occurs, thereby impairing the stability of the battery.

  To solve this problem, for example, the battery case is equipped with a current interruption mechanism that interrupts charging when the gas pressure inside the battery exceeds a predetermined pressure, and gas is generated when a predetermined overcharge state is reached in the electrolyte. A non-aqueous electrolyte secondary battery to which a gas generating agent is added is disclosed. As such a gas generating agent, for example, cyclohexylbenzene (CHB) or biphenyl (BP) is used (for example, Patent Document 1). CHB and BP activate the polymerization reaction during overcharge and generate hydrogen gas. Thereby, the pressure in a battery case becomes high, an electric current interruption mechanism act | operates, and an overcharge electric current is interrupted | blocked.

JP 2013-243020 A

  By the way, when the battery is overcharged, it is necessary to quickly increase the internal pressure of the battery in order to efficiently operate the current interruption mechanism. In order to quickly increase the internal pressure of the battery during overcharge, it is conceivable to increase the amount of the gas generating agent described above. However, since the gas generating agent functions as a resistance component of the battery, if the amount of the gas generating agent added is increased, battery performance (for example, input / output characteristics) may be a factor. The present invention solves the above problems.

  As a result of intensive studies, the present inventor has come up with the idea of effectively increasing the internal pressure of the battery during overcharge by using a positive electrode active material capable of releasing oxygen during overcharge. However, since the positive electrode active material capable of releasing oxygen during overcharge has poor moisture resistance, battery performance (for example, battery capacity) is reduced when exposed to the atmosphere. Therefore, further investigation is conducted, and the layer containing the positive electrode active material (first positive electrode active material layer) capable of releasing oxygen during overcharge is replaced with the layer containing the positive electrode active material having higher moisture resistance (second positive electrode active material layer). ), It was found that the internal pressure of the battery during overcharge can be effectively increased while maintaining good battery performance, and the present invention has been completed.

That is, the secondary battery provided by the present invention is a secondary battery including a positive electrode and a negative electrode.
The positive electrode includes a positive electrode current collector and a positive electrode active material layer held by the positive electrode current collector. The positive electrode active material layer includes a first positive electrode active material layer formed on the positive electrode current collector, and the first positive electrode active material layer and the surroundings thereof so as to cover the first positive electrode active material layer. And a second positive electrode active material layer formed on the positive electrode current collector. The first positive electrode active material layer includes a first positive electrode active material capable of releasing oxygen during overcharge, and the second positive electrode active material layer has a second moisture resistance higher than that of the first positive electrode active material. Contains a positive electrode active material. According to this configuration, while maintaining the battery performance satisfactorily, the internal pressure of the battery quickly increases when the battery is overcharged, and the current interrupt mechanism can be appropriately operated. Therefore, according to the present invention, a secondary battery having high performance and excellent stability can be provided.

FIG. 1 is a diagram illustrating an example of the structure of a secondary battery. FIG. 2 is a cross-sectional view schematically showing the main part of the positive electrode sheet. FIG. 3 is a graph showing the relationship between SOC and internal pressure.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including a so-called storage battery such as a lithium ion secondary battery and a power storage element such as an electric double layer capacitor. The “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the movement of lithium ions between positive and negative electrodes. The electrode active material refers to a material capable of reversibly occluding and releasing chemical species (lithium ions in a lithium ion secondary battery) serving as a charge carrier.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified. Hereinafter, embodiments of the present invention will be described by taking as an example the case where the present invention is applied to a lithium ion secondary battery, but it is not intended to limit the application target of the present invention.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 20 and a battery case 30. As shown in FIG. 1, a lithium ion secondary battery 100 according to an embodiment of the present invention includes a flat rectangular battery case 20 having a flat wound electrode body 20 together with a liquid electrolyte (electrolytic solution) (not shown). That is, it is accommodated in the exterior container 30.

  The battery case 30 has a box-shaped (that is, bottomed rectangular parallelepiped) case body 32 having an opening at one end (corresponding to the upper end in a normal use state of the battery), and the opening attached to the opening. It is comprised from the cover body 34 which consists of a rectangular-shaped plate member which plugs up a part. The material of the battery case 30 is exemplified by aluminum, for example. As shown in FIG. 1, a positive electrode terminal 42 and a negative electrode terminal 44 for external connection are formed on the lid 34. A safety valve 36 is formed between both terminals 42 and 44 of the lid 34.

  The wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 50) and a long sheet-like negative electrode (negative electrode sheet 60) similar to the positive electrode sheet 50, in total, two long sheet-like separators (separators). 70).

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector 52 and a positive electrode active material layer 54. For the positive electrode current collector 52, for example, a strip-shaped aluminum foil having a thickness of about 15 μm is used. A positive electrode active material layer non-forming portion 52 a is set along an edge portion on one side in the width direction of the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is held on both surfaces of the positive electrode current collector 52 except for the positive electrode active material layer non-forming portion 52 a set in the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material, a conductive material, and a binder. Here, the positive electrode active material layer 54 is formed by applying a positive electrode mixture (paste) containing a positive electrode active material to the positive electrode current collector 52, drying it, and pressing it to a predetermined thickness.

  Here, FIG. 2 is a schematic view showing a cross section of the positive electrode sheet 50. In this embodiment, the positive electrode active material layer 54 has at least a two-layer structure of a first positive electrode active material layer 54a and a second positive electrode active material layer 54b. The positive electrode active material layer 54 will be described in detail later.

  The negative electrode sheet 60 includes a strip-shaped negative electrode current collector 62 and a negative electrode active material layer 64. For the negative electrode current collector 62, for example, a strip-shaped copper foil having a thickness of about 10 μm is used. On one side in the width direction of the negative electrode current collector 62, a negative electrode active material layer non-formation part 62a is set along the edge. The negative electrode active material layer 64 is held on both surfaces of the negative electrode current collector 62 except for the negative electrode active material layer non-forming portion 62 a set in the negative electrode current collector 62. The negative electrode active material layer 64 includes a negative electrode active material, a thickener, a binder, and the like.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon. And like a positive electrode, this negative electrode active material is disperse | distributed to a suitable dispersion medium with binders, such as PVDF, SBR, and CMC (it can function also as a thickener), and knead | mixing a negative electrode mixture (paste). Can be prepared. The negative electrode active material layer 64 is formed by applying this negative electrode mixture to the negative electrode current collector 62, drying it, and pressing it to a predetermined thickness.

  The separator 70 is a member that separates the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separator 70 is composed of a strip-shaped base material having a predetermined width and having a plurality of minute holes. Examples of the base material include a sheet material having a single layer structure (for example, a single layer structure of polyethylene) made of a porous polyolefin-based resin, or a sheet material having a laminated structure (for example, a three-layer structure of polypropylene, polyethylene, and polypropylene). Can be used.

  The wound electrode body 20 is attached to electrode terminals 42 and 44 attached to the battery case 30 (in this example, the lid body 34). The wound electrode body 20 is housed in the battery case 30 in a state where the wound electrode body 20 is flatly pushed and bent in one direction orthogonal to the winding axis. In the wound electrode body 20, the positive electrode active material layer non-formed portion 52 a of the positive electrode sheet 50 and the negative electrode active material layer non-formed portion 62 a of the negative electrode sheet 60 protrude on the opposite sides in the winding axis direction. Among these, one electrode terminal 42 is fixed to the positive electrode active material layer non-forming portion 52 a of the positive electrode current collector 52 via the positive electrode current collector tab 42 a, and the other electrode terminal 44 is the negative electrode current collector 62. The negative electrode active material layer non-forming portion 62a is fixed via a negative electrode current collecting tab 44a. The wound electrode body 20 is accommodated in the flat internal space of the case body 32. The case body 32 is closed by the lid 34 after the wound electrode body 20 is accommodated.

As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. As the non-aqueous solvent, for example, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like can be used. Moreover, as said support salt, lithium salts, such as LiPF ₆ , can be used, for example.

  The non-aqueous electrolyte contains a gas generating agent that reacts and generates gas when the battery voltage becomes equal to or higher than a predetermined voltage. As such a gas generating agent, for example, cyclohexylbenzene (CHB) or biphenyl (BP) can be used. For example, when cyclohexylbenzene (CHB) and biphenyl (BP) are overcharged at about 4.35V to 4.6V, for example, the polymerization reaction is activated, and gas (here, hydrogen gas) is generated. The amount of the gas generating agent added to the non-aqueous electrolyte is preferably about 0.05% by mass or more and 10% by mass or less, for example. The amount of CHB added may be, for example, about 1% by mass to 5% by mass (eg, 4 ± 1% by mass). For example, the amount of BP added is preferably about 0.1% by mass to 2% by mass (eg, 1 ± 0.5% by mass). In addition, the addition amount of a gas generating agent is not limited to this, It is good to adjust so that a predetermined amount of gas may be generated on predetermined conditions. Further, the gas generating agent is not limited to cyclohexylbenzene (CHB) and biphenyl (BP).

  Further, the lithium ion secondary battery 100 includes a current interruption mechanism 90. The current interruption mechanism 90 is a mechanism that interrupts the current path when the pressure in the battery case becomes abnormally high. In this embodiment, as shown in FIG. 1, the current interruption mechanism 90 is constructed inside the positive electrode terminal 42 so that the conduction path of the battery current in the positive electrode is interrupted.

  Hereinafter, the positive electrode active material layer 54 will be described in more detail.

  FIG. 2 is a schematic view showing a cross section of the positive electrode sheet 50, showing the positive electrode current collector 52 and the positive electrode active material layer 54 formed on one side thereof. Since the positive electrode active material layer 54 formed on the other side of the positive electrode current collector 52 has the same configuration, illustration and description thereof are omitted. As shown in FIG. 2, the positive electrode active material layer 54 includes a first positive electrode active material layer 54a and a second positive electrode active material layer 54b. Hereinafter, the first positive electrode active material layer 54a and the second positive electrode active material layer 54b will be described in this order.

  The first positive electrode active material layer 54 a is formed on the positive electrode current collector 52. The first positive electrode active material layer 54a includes a first positive electrode active material, a conductive material, and a binder (not shown). As the first positive electrode active material, a material capable of releasing oxygen during overcharge is used.

The first positive electrode active material may be any material that can occlude and release lithium and can release oxygen during overcharge. It is also preferable to use a positive electrode active material that can release oxygen only during overcharge. Furthermore, it is a positive electrode active material that has a higher charge capacity density (charge energy density) per unit mass than the second positive electrode active material used for the second positive electrode active material layer 54b described later, and can contribute to an increase in battery capacity. Preferably there is. A positive electrode active material that satisfies such conditions can be used without particular limitation. Examples of the positive electrode active material include lithium transition metal composite oxides such as Li ₂ NiO ₂ , Li ₅ FeO ₄ , Li ₂ CuO ₂ , and Li ₆ CoO ₄ . Since these composite oxides have a high charge energy density and do not catch up with the change in the valence of the cation associated with Li release during overcharge, they release oxygen, which is suitable as the first positive electrode active material suitable for the purpose of the present invention. Can be used. One kind of these complex oxides may be used alone, or two or more kinds may be used in combination.

  The second positive electrode active material layer 54b is formed on the first positive electrode active material layer 54a and the positive electrode current collector 52 around it so as to cover the first positive electrode active material layer 54a. The second positive electrode active material layer 54b covers the entire first positive electrode active material layer 54a so that the first positive electrode active material layer 54a is not exposed. The second positive electrode active material layer 54b includes a second positive electrode active material, a conductive material, and a binder (not shown). A material having higher moisture resistance than the first positive electrode active material is used for the second positive electrode active material.

The second positive electrode active material may be any material that can occlude and release lithium and has higher moisture resistance than the first positive electrode active material. Further, it is preferable to use a positive electrode active material having higher electrical conductivity than the first positive electrode active material (typically contributing to the construction of a battery having excellent load characteristics). Furthermore, a positive electrode active material having higher durability than the first positive electrode active material (typically contributing to construction of a battery having excellent cycle characteristics) is preferable. A positive electrode active material that satisfies such conditions can be used without particular limitation. Examples of the positive electrode active material include lithium transition metal composite oxides such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , and LiCoO ₂ . Since these composite oxides have higher moisture resistance than the first positive electrode active material described above and are excellent in cycle characteristics and load characteristics, they can be suitably used as a second positive electrode active material suitable for the purpose of the present invention. One kind of these complex oxides may be used alone, or two or more kinds may be used in combination.

  In addition, the magnitude relationship of the moisture resistance of a 1st positive electrode active material and a 2nd positive electrode active material can be grasped | ascertained by a moisture resistance test. For example, a positive electrode using each of the first positive electrode active material and the second positive electrode active material may be prepared, and a moisture resistance test may be performed in which the positive electrode active material is stored in a constant temperature bath at a temperature of 60 ° C. and a humidity of 90% for 48 hours. Then, a coin cell is constructed using the positive electrode before and after the moisture resistance test, the battery capacity before and after the moisture resistance test is measured, and the capacity deterioration rate = [(battery capacity before the moisture resistance test−battery capacity after the moisture resistance test) / before the moisture resistance test. Battery capacity] × 100. In this case, the magnitude of the moisture resistance can be grasped as a relationship that reverses the magnitude of the capacity deterioration rate. That is, it can be understood that the smaller the capacity deterioration rate, the higher the moisture resistance.

In the lithium ion secondary battery having such a configuration, as described above, the first positive electrode active material layer 54a formed on the positive electrode current collector 52 and the first positive electrode active material layer 54a are covered so as to cover the first positive electrode active material layer 54a. An active material layer 54a and a second positive electrode active material layer 54b formed on the positive electrode current collector 52 around the active material layer 54a. The first positive electrode active material layer 54a includes a first positive electrode active material capable of releasing oxygen during overcharge, and the second positive electrode active material layer 54b has a second moisture resistance higher than that of the first positive electrode active material. Contains a positive electrode active material. Thus, by using the first positive electrode active material that can release oxygen during overcharge in the first positive electrode active material layer 54a, the internal pressure of the battery case during overcharge can be effectively increased. Therefore, when the battery is overcharged, the internal pressure of the battery case quickly increases, and the current interrupt mechanism 90 (FIG. 1) can be appropriately operated. Further, since the first positive electrode active material tends to be inferior in moisture resistance, the second positive electrode active material layer 54b containing the second positive electrode active material having higher moisture resistance covers the first positive electrode active material layer 54a. Battery performance deterioration (for example, capacity deterioration) can be prevented. Therefore, according to this configuration, it is possible to provide a secondary battery having high performance and excellent stability.

  In a preferred embodiment of the technology disclosed herein, as shown in FIG. 2, the thickness T2 of the second positive electrode active material layer 54b is thicker than the thickness T1 of the first positive electrode active material layer 54b. For example, the thickness T2 of the second positive electrode active material layer 54b may be thicker than the thickness T1 of the first positive electrode active material layer 54b, and is, for example, about 30 μm to 100 μm, and preferably 50 μm to 80 μm. The thickness T1 of the first positive electrode active material layer 54b only needs to be smaller than the thickness T2 of the second positive electrode active material layer 54b. For example, approximately 1/10 to 2 of the thickness T2 of the second positive electrode active material layer 54b. / 3, preferably 1/5 to 1/2, more preferably 1/4 to 1/3. When the thickness is within the thickness range of the second positive electrode active material layer 54b and the first positive electrode active material layer 54b, the first positive electrode active material layer 54b can appropriately protect the first positive electrode active material layer 54b. .

  In a preferred embodiment of the technology disclosed herein, the coating width W2 of the second positive electrode active material layer 54b is wider than the coating width W1 of the first positive electrode active material layer 54b. For example, the coating width W2 of the second positive electrode active material layer 54b may be wider than the coating width W1 of the first positive electrode active material layer 54b, for example, about 10 cm to 30 cm, and preferably 15 cm to 20 cm. Moreover, the coating width W1 of the 1st positive electrode active material layer 54b should just be narrower than the coating width W2 of the 2nd positive electrode active material layer 54b, for example, is about 3 cm-10 cm, Preferably it is 5 cm-8 cm. . When the coating width of the second positive electrode active material layer 54b and the first positive electrode active material layer 54b is within such a range, the second positive electrode active material layer 54b appropriately protects the first positive electrode active material layer 54b. Can do.

  In the positive electrode active material layer 54 as a whole, the first positive electrode active material layer 54b exists in the central portion C in the width direction of the positive electrode active material layer 54, but at both ends E in the width direction of the positive electrode active material layer 54 The first positive electrode active material layer 54b does not exist. Therefore, the positive electrode active material layer 54 has different charge capacities per unit area at the central portion C and both end portions E in the width direction. If the charge capacity distribution is uneven in the positive electrode as described above, Li may partially precipitate on the negative electrode, which may be a factor in reducing battery performance. In order to eliminate such an inconvenience, the charging capacity per unit length of the first positive electrode active material layer 54b (per unit length in the longitudinal direction of the positive electrode sheet) is set to be per unit length of the first positive electrode active material layer 54a. The positive electrode active material layers 54a and 54b have a charge capacity of 5% or less (for example, 1% to 5%, preferably 3% or less) of the charge capacity per unit length of the second positive electrode active material layer 54b. It is desirable to adjust the weight per unit area.

  Examples of the conductive material used for the first positive electrode active material layer 54a and the second positive electrode active material layer 54b include carbon black such as acetylene black (AB) and ketjen black and other powdery carbon materials (graphite and the like). The Examples of the binder used for the first positive electrode active material layer 54a and the second positive electrode active material layer 54b include polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC).

  An evaluation cell was constructed using the positive electrode provided with the first positive electrode active material layer 54a and the second positive electrode active material layer 54b, and the moisture resistance of the positive electrode was evaluated. A specific method will be described below.

<Example 1>
In this example, the first positive electrode active material was Li ₂ NiO ₂ and the second positive electrode active material was LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (charge capacity: 158 mAh / g).

Li ₂ NiO ₂ as the first positive electrode active material was produced as follows. First, the reagent LiO ₂ powder and the reagent NiO ₂ powder were mixed at a molar ratio of 1: 1, pulverized and mixed in a mortar for 20 minutes, and then a cylindrical pellet having a diameter of 20 mm and a thickness of 5 mm was formed by press molding. The pellet was fired at 800 ° C. for 12 hours in a nitrogen atmosphere. The fired pellets were pulverized in a mortar to obtain a green powder. From the X-ray diffraction pattern of this powder, it was confirmed to be Li ₂ NiO ₂ . 92 parts by mass of Li ₂ NiO ₂ obtained, 3 parts by mass of PVDF, and 4 parts by mass of AB were mixed to prepare a paste, which was applied onto an aluminum foil, and then pressed with 300 kg weight to produce a positive electrode. A coin cell was constructed using the positive electrode and the negative electrode (Li metal), and the charge capacity of Li ₂ NiO ₂ was determined by charging and discharging in the range of 4.1 V to 2.5 V. The results are shown in Table 1.

The same paste as the above-mentioned charge capacity measurement was applied on an aluminum foil (positive electrode current collector: thickness 15 μm, width 20 cm) with a coating width of 7 cm, and dried at 120 ° C. for 10 minutes, so that it was applied on the positive electrode current collector. A first positive electrode active material layer was formed. In addition, a paste containing LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as the second positive electrode active material (LiNi _1/3 Co _1/3 Mn _1/3 so as to cover the first positive electrode active material layer. By coating O ₂ 92 parts by mass, PVDF 3 parts by mass, AB 4 parts by mass on the first positive electrode active material layer and the surrounding positive electrode current collector with a coating width of 17 cm and drying at 120 ° C. for 10 minutes. Then, a second positive electrode active material layer was formed on the first positive electrode active material layer. Then, it pressed so that the thickness (one side) of the whole positive electrode active material layer might be set to 60 micrometers by roll press. The basis weight of the first positive electrode active material layer and the second positive electrode active material layer, and the ratio of the charge capacity per unit length of the second positive electrode active material layer to the charge capacity per unit length of the first positive electrode active material layer are shown. It is shown in 1. The first positive electrode active material layer and the second positive electrode active material layer were formed on the back surface of the positive electrode current collector in the same manner. In this way, a positive electrode according to Example 1 was obtained.

<Example 2>
A positive electrode was produced in the same procedure as in Example 1 except that the first positive electrode active material was Li ₅ FeO ₄ . Li ₅ FeO ₄ as the first positive electrode active material was produced as follows. First, the reagent LiO ₂ powder and the reagent Fe ₂ O ₃ powder were mixed at a molar ratio of 5: 1, pulverized and mixed in a mortar for 20 minutes, and then a cylindrical pellet having a diameter of 20 mm and a thickness of 5 mm was formed by press molding. . The pellet was fired at 900 ° C. for 12 hours in a nitrogen atmosphere. The fired pellets were pulverized in a mortar to obtain a yellowish white powder. From the X-ray diffraction pattern of this powder, it was confirmed to be Li ₅ FeO ₄ .

<Example 3>
A positive electrode was produced in the same procedure as in Example 1 except that the first positive electrode active material was Li ₆ CoO ₄ . Li ₆ CoO ₄ as the first positive electrode active material was produced as follows. First, a reagent LiO ₂ powder and a reagent Co (OH) ₂ powder are mixed at a molar ratio of 3: 1, pulverized and mixed in a mortar for 20 minutes, and then a cylindrical pellet having a diameter of 20 mm and a thickness of 5 mm is formed by press molding. did. The pellet was fired at 850 ° C. for 12 hours in a nitrogen atmosphere. The fired pellets were pulverized in a mortar to obtain a blue powder. From the X-ray diffraction pattern of this powder, it was confirmed to be Li ₆ CoO ₄ .

<Example 4>
A positive electrode was produced in the same procedure as in Example 1 except that the first positive electrode active material was Li ₂ CuO ₂ . Li ₂ CuO ₂ as the first positive electrode active material was produced as follows. First, the reagent LiO ₂ powder and the reagent CuO powder were mixed at a molar ratio of 2: 1, pulverized and mixed for 20 minutes in a mortar, and then a cylindrical pellet having a diameter of 20 mm and a thickness of 5 mm was formed by press molding. The pellet was fired at 750 ° C. for 12 hours in a nitrogen atmosphere. The fired pellets were pulverized in a mortar to obtain a gray powder. It was confirmed that the X-ray diffraction pattern of this powder is _Li 2 CuO _2.

<Comparative Example 1>
In this example, a positive electrode was produced in the same procedure as in Example 1 except that the first positive electrode active material layer was not formed and only the second positive electrode active material layer was used.

<Comparative Examples 2-5>
In Comparative Examples 2 to 5, positive electrodes having the first positive electrode active material layer as an upper layer and the second positive electrode active material layer as a lower layer were produced. Specifically, in Comparative Examples 2 to 5, after the second positive electrode active material layer was formed on the positive electrode current collector with a coating width of 17 cm, the first positive electrode active material layer was applied thereon with a coating width of 17 cm. Formed. The other conditions were the same as in Examples 1 to 4 (see Table 1).

<Comparative Example 6>
A positive electrode was produced in the same procedure as in Example 1 except that the basis weight of the first positive electrode active material layer was changed as shown in Table 1.

<Moisture resistance evaluation>
A moisture resistance test was carried out in which the positive electrode of each example was stored in a thermostatic chamber at a temperature of 60 ° C. and a humidity of 90% for 48 hours. Moreover, the coin cell was constructed | assembled using the positive electrode before and behind the said moisture resistance test, and the battery capacity before and behind a moisture resistance test was measured. The capacity deterioration rate = [(battery capacity before the moisture resistance test−battery capacity after the moisture resistance test) / battery capacity before the moisture resistance test] × 100 was calculated. Here, for the negative electrode, an amorphous coated graphite, CMC, and SBR as a negative electrode active material were kneaded with water at a mass ratio of 98: 1: 1 to prepare a paste, and this paste was made into a copper foil (negative electrode current collector). : The thickness of 10 μm) was applied on both sides with a basis weight of 14.7 mg / cm ² and dried to form a negative electrode active material layer on both sides of the negative electrode current collector. The density of the negative electrode active material layer was adjusted to 1.3 g / cm ³ . The results are shown in Table 2.

  As shown in Table 1 and Table 2, in the batteries according to Examples 1 to 4 in which the first positive electrode active material layer is covered with the second positive electrode active material layer, the second positive electrode active material layer is used as the lower layer. Compared to the batteries according to Comparative Examples 1 to 5 having the positive electrode active material layer as an upper layer, the capacity deterioration after the moisture resistance test was small and the moisture resistance was excellent. From this result, it was confirmed that the moisture resistance can be improved by covering the first cathode active material layer with the second cathode active material layer containing the cathode active material having more excellent moisture resistance.

Next, the positive electrode sheet in which the positive electrode of each example was formed into a 5 m sheet and the negative electrode sheet in which the negative electrode used in the moisture resistance evaluation was formed into a 5.3 m sheet were wound through two separators, By flattening the wound body from the side, a flat wound electrode body was produced. The wound electrode body thus obtained was housed in a box-type battery case together with the non-aqueous electrolyte, and the opening of the battery case was hermetically sealed. As the separator, one having a heat-resistant porous layer containing alumina particles formed on the surface of a PE substrate was used. Further, as the non-aqueous electrolyte, a non-aqueous electrolyte in which LiPF ₆ as a supporting salt is contained at a concentration of about 1 mol / liter in a mixed solvent containing EC, DMC, and EMC in a volume ratio of 3: 4: 3. used. 4% by mass of CHB and 1% by mass of BP were added to the non-aqueous electrolyte. In this way, a lithium ion secondary battery (evaluation cell) was assembled. In this battery, 4.1V was set to 100% SOC (State Of Charge), and 3V was set to 0% SOC.

<Overcharge test>
The evaluation cells constructed using the positive electrodes of Examples 1 to 4 and Comparative Examples 1 and 6 were charged at a current value of 1 C, and charged to a capacity corresponding to SOC 160%. At this time, an internal pressure change in the battery was measured by attaching an internal pressure sensor to the battery. FIG. 3 shows the results of Example 1 and Comparative Example 1. Further, the SOC at which the internal pressure reached 0.8 MPa was determined. The results are shown in Table 2.

  As shown in Table 1 and Table 2, the batteries according to Examples 1 to 4 provided with the first positive electrode active material layer were more in comparison with the battery according to Comparative Example 1 that was not provided with the first positive electrode active material layer. It was confirmed that the internal pressure increased in the low SOC region. Further, the battery according to Comparative Example 6 in which the charge capacity ratio (first positive electrode active material layer / second positive electrode active material layer) exceeded 0.005 was short-circuited during the overcharge test. This is because the battery according to Comparative Example 6 is presumed that the deposition of Li partially occurred on the negative electrode and the micro short circuit occurred because the charge capacity distribution in the positive electrode was too uneven (width). The From this result, the charge capacity ratio (first positive electrode active material layer / second positive electrode active material layer) is preferably 0.005 or less (particularly 0.045 or less).

  Although the lithium ion secondary battery according to one embodiment of the present invention has been described above, the secondary battery according to the present invention is not limited to any of the above-described embodiments, and various modifications can be made. For example, in the above-described embodiment, a lithium ion secondary battery has been described as a typical example of a non-aqueous electrolyte secondary battery. However, the embodiment is not limited to the non-aqueous electrolyte secondary battery. For example, a secondary battery using a metal ion (for example, sodium ion) other than lithium ion as a charge carrier may be used.

  Since the secondary battery according to the present invention exhibits good battery performance as described above, it can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Therefore, the present invention provides a vehicle (typically an automobile such as an automobile, particularly a hybrid automobile, an electric automobile, or a fuel cell automobile) provided with such a secondary battery (typically, a plurality of batteries connected in series) as a power source. Vehicle).

20 wound electrode body 50 positive electrode sheet 52 positive electrode current collector 54 positive electrode active material layer 54a first positive electrode active material layer 54b second positive electrode active material layer 60 negative electrode sheet 62 negative electrode current collector 64 negative electrode active material layer 70 separator 90 current interruption Mechanism 100 Lithium ion secondary battery

Claims (1)

A secondary battery comprising a positive electrode and a negative electrode,
The positive electrode is
A positive electrode current collector;
A positive electrode active material layer held on the positive electrode current collector,
The positive electrode active material layer is
A first positive electrode active material layer formed on the positive electrode current collector;
A first positive electrode active material layer and a second positive electrode active material layer formed on the positive electrode current collector around the first positive electrode active material layer so as to cover the first positive electrode active material layer;
The first positive electrode active material layer includes a first positive electrode active material capable of releasing oxygen during overcharge,
The second positive electrode active material layer is a secondary battery including a second positive electrode active material having higher moisture resistance than the first positive electrode active material.

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