JP6769451B2 - Lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a lithium ion secondary battery.

Japanese Unexamined Patent Publication No. 2017-0377776 (Patent Document 1) discloses a positive electrode active material represented by the general formula Li _x Ni _1-ab Co _a Mn _b M _c O ₂ (1-ab ≧ 0.6). are doing.

Japanese Unexamined Patent Publication No. 2017-0377776

A lithium transition metal composite oxide is known as a positive electrode active material for a lithium ion secondary battery (hereinafter, may be abbreviated as "battery"). The transition metal contained in the lithium transition metal composite oxide can be, for example, nickel (Ni), cobalt (Co), manganese (Mn), or the like. Lithium transition metal composite oxides with a high Ni content are also referred to as "high nickel positive electrode active materials". Generally, in a high nickel positive electrode active material, the Ni content is, for example, 50 mL% or more with respect to the total of transition metals.

The high nickel positive electrode active material can have a high capacity. However, the high nickel positive electrode active material tends to adsorb water easily. When the positive electrode active material on which water is adsorbed is brought into the battery, it is considered that an acid (for example, hydrofluoric acid) is generated by the water reacting with the supporting salt in the electrolytic solution. It is considered that the transition metal is eluted from the positive electrode active material into the electrolytic solution when the positive electrode active material is attacked by the generated acid. It is considered that the transition metal eluted in the electrolytic solution will be deposited on the surface of the counter electrode (negative electrode). It is considered that lithium (Li) is likely to be locally deposited in the region of the negative electrode where the transition metal is deposited.

When the battery contains a winding type electrode group and the positive electrode plate contains a high nickel positive electrode active material, Li tends to be locally deposited at the end portion in the width direction of the negative electrode plate. The "width direction" indicates a direction orthogonal to the longitudinal direction. The positive electrode plate used for the winding type electrode group is strip-shaped. Normally, the strip-shaped positive electrode plate is stored in a state of being wound around a winding core (hereinafter, also referred to as a “wrapped state”). In the wound state, it is considered that water penetrates from the end portion in the width direction of the positive electrode plate toward the center portion. Therefore, it is considered that water is likely to be locally adsorbed on the edge of the positive electrode plate in the width direction. As a result, it is considered that the transition metal is likely to elute locally at the end of the positive electrode plate in the width direction in the battery.

An object of the present disclosure is to suppress the precipitation of lithium in a lithium ion secondary battery in which the electrode group is a winding type and the positive electrode plate contains a high nickel positive electrode active material.

Hereinafter, the technical configuration and the action and effect of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of claims should not be limited by the correctness of the mechanism of action.

The lithium ion secondary battery of the present disclosure includes at least an electrode group and an electrolytic solution. The electrolytic solution contains at least a solvent and a supporting salt. The electrode group is a winding type. The electrode group includes at least a positive electrode plate and a negative electrode plate. The positive electrode plate contains at least a positive electrode active material layer. The positive electrode active material layer has a strip-shaped planar shape. The positive electrode active material layer includes a central portion and an end portion. In the width direction of the positive electrode active material layer, the ends are arranged at both ends of the positive electrode active material layer. The central portion is the portion of the positive electrode active material layer excluding the end portion. Moreover, the ratio of the width of the edge to the sum of the width of the edge and the width of the center is 4.3% or more.
The positive electrode active material layer contains at least a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is contained in the central portion. The second positive electrode active material is contained in the end portion.
The first positive electrode active material is the following formula (I):
LiNi _x M ¹ _(1-x) O ₂ … (I)
[However, x in the formula satisfies 0.5 ≦ x ≦ 0.8, and M ¹ is at least one selected from the group consisting of Co, Mn and Al]
Represented by.
The second positive electrode active material is the following formula (II):
LiNi _y M ² _(1-y) O ₂ … (II)
[However, y in the formula satisfies 0.2 ≦ y ≦ 0.4, and M ² is at least one selected from the group consisting of Co and Mn]
Represented by.
The ratio of the resistance value of the second positive electrode active material to the resistance value of the first positive electrode active material is 0.88 or more and 1.13 or less.

In the positive electrode active material layer of the present disclosure, the high nickel positive electrode active material (first positive electrode active material) is arranged in the central portion in the width direction. The use of high nickel positive electrode active material is expected to increase battery capacity.

It is considered that the end portion of the positive electrode active material layer in the width direction is likely to come into contact with moisture. A second positive electrode active material is arranged at the end of the positive electrode active material layer. Since the second positive electrode active material has a low Ni content, it is considered that water is not easily adsorbed. Therefore, it is considered that the elution of the transition metal is unlikely to occur at the end of the positive electrode active material layer.

In the width direction of the positive electrode active material layer, the central portion exists inside the end portion. Since the edges have a predetermined width, it is considered that moisture does not easily reach the center. That is, it is considered that the adsorption of water to the first positive electrode active material contained in the central portion is suppressed. Therefore, it is considered that the elution of the transition metal is unlikely to occur even in the central portion of the positive electrode active material layer.

As described above, in the battery of the present disclosure, it is considered that the elution of the transition metal is unlikely to occur over the entire area of the positive electrode active material layer. Therefore, it is considered that the precipitation of Li is suppressed.

However, the ratio of the resistance value of the second positive electrode active material to the resistance value of the first positive electrode active material (hereinafter, also referred to as “resistance ratio”) is 0.88 or more and 1.13 or less. If the resistivity ratio is less than 0.88, Li may easily precipitate. Even if the resistivity ratio exceeds 1.13, Li may easily precipitate. It is considered that the farther the resistivity ratio is from 1, the larger the difference in electrode reaction speed and the like between the central portion and the end portion, and the more likely the current concentration is to occur.

Furthermore, the ratio of the width of the edge to the sum of the width of the edge and the width of the center is 4.3% or more. If the ratio of the width of the edges (hereinafter also referred to as the "ratio of the edges") is less than 4.3%, moisture may reach the center. This can lead to elution of transition metals.

FIG. 1 is a schematic view showing an example of the configuration of the lithium ion secondary battery of the present embodiment. FIG. 2 is a schematic view showing an example of the configuration of the electrode group of the present embodiment. FIG. 3 is a schematic plan view showing an example of the configuration of the positive electrode plate of the present embodiment.

Hereinafter, embodiments of the present disclosure (also referred to as “the present embodiment” in the present specification) will be described. However, the following description does not limit the scope of claims.

<Lithium-ion secondary battery>
FIG. 1 is a schematic view showing an example of the configuration of the lithium ion secondary battery of the present embodiment.
The battery 100 includes a case 90. The case 90 is square (flat rectangular parallelepiped). The case 90 may be cylindrical, for example. The case 90 is made of metal, for example. The case 90 may be a pouch made of an aluminum laminated film or the like. The case 90 is hermetically sealed. The case 90 houses the electrode group 50 and the electrolytic solution (not shown). That is, the battery 100 includes at least the electrode group 50 and the electrolytic solution.

FIG. 2 is a schematic view showing an example of the configuration of the electrode group of the present embodiment.
The electrode group 50 is a winding type. The electrode group 50 is formed by laminating a positive electrode plate 10, a separator 30, a negative electrode plate 20, and a separator 30 in this order, and further winding them in a spiral shape. That is, the electrode group 50 includes at least a positive electrode plate 10 and a negative electrode plate 20. It is considered that even in the electrode group 50, water can enter from the end portion of the positive electrode plate 10 in the width direction (x-axis direction in FIG. 2).

《Positive plate》
FIG. 3 is a schematic plan view showing an example of the configuration of the positive electrode plate of the present embodiment.
The positive electrode plate 10 is a strip-shaped sheet. The positive electrode plate 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. That is, the positive electrode plate 10 includes at least the positive electrode active material layer 12. The positive electrode active material layer 12 is formed on the surface of the positive electrode current collector 11. The positive electrode active material layer 12 may be formed on both the front and back surfaces of the positive electrode current collector 11. The positive electrode current collector 11 may be, for example, an aluminum (Al) foil or the like. The positive electrode current collector 11 may have a thickness of, for example, 5 μm or more and 50 μm or less.

The positive electrode active material layer 12 may have a thickness of, for example, 10 μm or more and 200 μm or less. The positive electrode active material layer 12 has a strip-shaped planar shape. The z-axis direction in FIG. 3 corresponds to the longitudinal direction of the positive electrode active material layer 12. The x-axis direction in FIG. 3 corresponds to the width direction of the positive electrode active material layer 12. The positive electrode active material layer 12 includes a central portion 12C and an end portion 12E. In the width direction of the positive electrode active material layer 12, the end portions 12E are arranged at both ends of the positive electrode active material layer 12, respectively. The central portion 12C is a portion of the positive electrode active material layer 12 excluding the end portion 12E.

In the wound state or the electrode group 50, it is considered that water penetrates from the widthwise end portion 12E of the positive electrode active material layer 12 toward the central portion 12C. In the present embodiment, a second positive electrode active material that is difficult to adsorb water is arranged at the end 12E. A first positive electrode active material (high nickel positive electrode active material) is arranged in the central portion 12C. It is considered that water is easily adsorbed on the first positive electrode active material. However, since the end portion 12E has a predetermined width, it is considered that the moisture does not easily reach the central portion 12C. Therefore, it is considered that it is difficult for water to be adsorbed on the entire positive electrode active material layer 12.

(End ratio)
The end ratio is 4.3% or more. End ratio, the width of the end portion 12E (W _E) and the central portion 12C of the width (W _C) of the width of the end portion 12E of the total (W _E) ratio of _{[W E / (W E + W} C) ] Shown. If the edge ratio is less than 4.3%, Li precipitation may occur. It is considered that the water can reach the central portion 12C. The edge ratio may be, for example, 8.5% or less. This is expected to increase the battery capacity, for example.

(1st positive electrode active material)
The positive electrode active material layer 12 contains at least a first positive electrode active material and a second positive electrode active material. The first positive electrode active material is contained in the central portion 12C. For example, the central portion 12C may be formed by applying a first coating material containing a first positive electrode active material, a conductive material, a binder, and a solvent to a predetermined region on the surface of the positive electrode current collector 11 and drying the first coating material. ..

The first positive electrode active material is typically a particle group (powder). The first positive electrode active material may have, for example, D50 of 1 μm or more and 30 μm or less. “D50” indicates a particle size in which the integrated particle volume from the fine particle side is 50% of the total particle volume in the volume-based particle size distribution. D50 can be measured by, for example, a laser diffraction type particle size distribution measuring device or the like.

The first positive electrode active material is the following formula (I):
LiNi _x M ¹ _(1-x) O ₂ … (I)
[However, x in the formula satisfies 0.5 ≦ x ≦ 0.8, and M ¹ is at least one selected from the group consisting of Co, Mn and Al]
Represented by.

In the above formula (I), x may satisfy 0.6 ≦ x ≦ 0.8. x may satisfy 0.7 ≦ x ≦ 0.8. M ¹ may be at least one selected from the group consisting of Co and Mn.

The first positive electrode active material is, for example, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _0.7 Co _0.1 Mn _0.2 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.82 Co _0.15 Al _0.03. It may be O ₂ or the like. The central portion 12C may contain one kind of first positive electrode active material alone. Two or more kinds of first positive electrode active materials may be contained in the central portion 12C. The first positive electrode active material is selected from the group consisting of, for example, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _0.7 Co _0.1 Mn _0.2 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ and LiNi _0.5 Co _0.2 Mn _0.3 O _2. It may be at least one kind.

(Second positive electrode active material)
The second positive electrode active material is contained in the end portion 12E. For example, a second coating material containing a second positive electrode active material, a conductive material, a binder and a solvent is applied to both sides of the central portion 12C on the surface of the positive electrode current collector 11 and dried to form the end portion 12E. You may.

The second positive electrode active material is typically a particle swarm. The second positive electrode active material may have, for example, a D50 of 1 μm or more and 30 μm or less.

The second positive electrode active material is the following formula (II):
LiNi _y M ² _(1-y) O ₂ … (II)
[However, y in the formula satisfies 0.2 ≦ y ≦ 0.4, and M ² is at least one selected from the group consisting of Co and Mn]
Represented by.

In the above formula (II), y may satisfy 0.2 ≦ y ≦ 1/3. The second positive electrode active material is, for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.2 Co _0.6 Mn _0.2 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O ₂ , LiNi _0.4 Co _0.3 Mn _0.3 O ₂ , LiNi. _It may be _0.4 Co _0.4 Mn _0.2 O ₂ or the like. The end portion 12E may contain one kind of second positive electrode active material alone. The end portion 12E may contain two or more kinds of second positive electrode active materials. The second positive electrode active material is, for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.2 Co _0.6 Mn _0.2 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O ₂ , LiNi _0.4 Co _0.3 Mn _0.3 O ₂ and It may be at least one selected from the group consisting of LiNi _0.4 Co _0.4 Mn _0.2 O ₂ .

(Resistance ratio)
The resistivity ratio is 0.88 or more and 1.13 or less. The resistance ratio indicates the ratio (R2 / R1) of the resistance value (R2) of the second positive electrode active material to the resistance value (R1) of the first positive electrode active material. The resistance value (R1) of the first positive electrode active material can be adjusted by the composition and crystallinity of the first positive electrode active material. The resistance value (R2) of the second positive electrode active material can be adjusted by the composition and crystallinity of the second positive electrode active material. The resistance value (R1) and the resistance value (R2) are measured as follows.

The measurement sample is prepared. The measurement sample is prepared by applying a coating material containing one kind of positive electrode active material, a conductive material, a binder and a solvent to the surface of the Al foil and drying it. That is, the measurement sample is a positive electrode plate containing either the first positive electrode active material or the second positive electrode active material alone as the positive electrode active material. A battery containing the measurement sample is prepared. The SOC (state of charge) of the battery is adjusted to 50%. The battery is discharged for 10 seconds by a predetermined discharge current in a temperature environment of 25 ° C. The amount of voltage drop 10 seconds after the start of discharge is measured. The resistance value (R1 or R2) is calculated by dividing the amount of voltage drop by the discharge current.

It is considered that the closer the resistivity ratio is to 1, the more desirable it is. It is considered that the closer the resistivity ratio is to 1, the more the precipitation of Li is suppressed. The resistivity ratio may be, for example, 0.94 or more. The resistivity ratio may be, for example, 1.06 or less. The resistivity ratio may be, for example, 1.03 or less. The resistivity ratio is ideally 1.

(Other ingredients)
The positive electrode active material layer 12 may further contain a conductive material, a binder, and the like. The conductive material may be, for example, carbon black, carbon short fibers, graphene flakes, graphite or the like. The positive electrode active material layer 12 may contain one kind of conductive material alone. The positive electrode active material layer 12 may contain two or more kinds of conductive materials. The content of the conductive material may be, for example, 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. The content of the conductive material may be the same in the central portion 12C and the end portion 12E. The content of the conductive material may be different between the central portion 12C and the end portion 12E.

The binder may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), polyacrylic acid (PAA) or the like. The positive electrode active material layer 12 may contain one kind of binder alone. The positive electrode active material layer 12 may contain two or more kinds of binders. The content of the binder may be, for example, 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. The content of the binder may be the same in the central portion 12C and the end portion 12E. The content of the binder at the central portion 12C and the end portion 12E may be different from each other.

《Negative electrode plate》
The negative electrode plate 20 is a strip-shaped sheet. The negative electrode plate 20 includes a negative electrode current collector 21 and a negative electrode active material layer 22. That is, the negative electrode plate 20 includes at least the negative electrode active material layer 22. The negative electrode active material layer 22 is formed on the surface of the negative electrode current collector 21. The negative electrode active material layer 22 may be formed on both the front and back surfaces of the negative electrode current collector 21. The negative electrode current collector 21 may be, for example, a copper (Cu) foil or the like. The negative electrode current collector 21 may have a thickness of, for example, 5 μm or more and 50 μm or less.

The negative electrode active material layer 22 may have a thickness of, for example, 10 μm or more and 200 μm or less. The negative electrode active material layer 22 has a strip-shaped planar shape. The negative electrode active material layer 22 contains at least the negative electrode active material. The negative electrode active material should not be particularly limited. The negative electrode active material may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, silicon-based alloy, tin, tin oxide, tin-based alloy or the like. The negative electrode active material layer 22 may contain one kind of negative electrode active material alone. The negative electrode active material layer 22 may contain two or more kinds of negative electrode active materials.

The negative electrode active material layer 22 may further contain a binder or the like. Binders should not be particularly limited. The binder may be, for example, CMC and styrene butadiene rubber (SBR). The content of the binder may be, for example, 0.1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the negative electrode active material.

<< Separator >>
The separator 30 has a strip-shaped planar shape. The separator 30 may have a thickness of, for example, 10 μm or more and 30 μm or less. The separator 30 is electrically insulating. The separator 30 is arranged between the positive electrode plate 10 and the negative electrode plate 20. The positive electrode plate 10 and the negative electrode plate 20 are separated from each other by a separator 30. The separator 30 is a porous membrane. The separator 30 may be, for example, a porous membrane made of polyolefin or the like.

The separator 30 may have a single-layer structure. The separator 30 may be formed only of, for example, a porous membrane made of polyethylene (PE). The separator 30 may have a multi-layer structure (for example, a three-layer structure or the like). The separator 30 may be formed, for example, by laminating a porous film made of polypropylene (PP), a porous film made of PE, and a porous film made of PP in this order. The separator 30 may include a heat-resistant film on its surface. The heat-resistant film contains a heat-resistant material. The heat-resistant material may be, for example, boehmite.

《Electrolytic solution》
The electrolytic solution contains at least a solvent and a supporting salt. The electrolytic solution may further contain various additives. The electrolytic solution may contain, for example, a supporting salt of 0.5 mL / L or more and 2 mL / L or less (0.5 M or more and 2 M or less). The supporting salt is dissolved in the solvent. The supporting salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ], Li [N (CF ₃ SO ₂ ) ₂ ], or the like. The electrolytic solution may contain one kind of supporting salt alone. The electrolytic solution may contain two or more kinds of supporting salts.

The solvent is aprotic. The solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio may be, for example, "cyclic carbonate: chain carbonate = 1: 9 to 5: 5 (volume ratio)".

The cyclic carbonate may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), or the like. The solvent may contain one type of cyclic carbonate alone. The solvent may contain two or more cyclic carbonates.

The chain carbonate may be, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or the like. The solvent may contain one kind of chain carbonate alone. The solvent may contain two or more chain carbonates.

The solvent may contain, for example, lactone, cyclic ether, chain ether, carboxylic acid ester and the like. The lactone may be, for example, γ-butyrolactone (GBL), δ-valerolactone and the like. The cyclic ether may be, for example, tetrahydrofuran (THF), 1,3-dioxolane, 1,4-dioxane or the like. The chain ether may be, for example, 1,2-dimethoxyethane (DME) or the like. The carboxylic acid ester may be, for example, methylformate (MF), methylacetate (MA), methylpropionate (MP) or the like.

Examples of the present disclosure will be described below. However, the following description does not limit the scope of claims.

<Manufacturing of lithium-ion secondary batteries>
Batteries 100 according to Examples 1 to 7, Comparative Examples 1 to 8, and Reference Examples 1 to 2, respectively, were manufactured. The configuration of the positive electrode active material layer 12 in each battery 100 is shown in Table 1 below.

<Evaluation>
The charge / discharge cycle was repeated 1000 cycles. One cycle shows the following cycle of discharge and charge. At a current of "20C", the rated capacity of the battery 100 is discharged in 3 minutes (180 seconds).

Discharge: Current = 20C, Discharge time = 100 seconds Charging: Current = 20C, Charging time = 100 seconds

After 1000 cycles, the battery 100 was disassembled and the negative electrode plate 20 was recovered. It was visually confirmed whether or not Li was deposited on the surface of the negative electrode plate 20. The results are shown in Table 1 below.

<Result>
In Comparative Examples 1 to 3, the precipitation of Li was confirmed. In Comparative Examples 1 to 3, the high nickel positive electrode active material is used alone. Therefore, it is considered that water is adsorbed on the high nickel positive electrode active material at the widthwise end of the positive electrode active material layer 12, and the transition metal is locally eluted.

No precipitation of Li was confirmed in Reference Examples 1 and 2. However, in Reference Examples 1 and 2, the high nickel positive electrode active material is not used. Therefore, it is considered that an increase in battery capacity cannot be expected in Reference Examples 1 and 2.

No precipitation of Li was confirmed in Examples 1 to 7. In Examples 1 to 7, in the width direction of the positive electrode active material layer 12, the first positive electrode active material (high nickel positive electrode active material) is arranged in the central portion, and the second positive electrode active material (Ni ratio is low) is arranged at the end portion. It is considered that this is because the positive electrode active material) is arranged.

In Comparative Examples 4 and 5, the precipitation of Li was confirmed. It is considered that the resistance ratio (R2 / R1) is less than 0.88. Precipitation of Li was also confirmed in Comparative Example 6. This is probably because the resistivity ratio exceeds 1.13. In Examples 1 to 7, the resistivity ratio (R2 / R1) is 0.88 or more and 1.13 or less.

In Comparative Example 7, precipitation of Li was confirmed. In Comparative Example 7, the positive electrode active materials are different from each other in the central portion 12C and the end portion 12E. However, in Comparative Example 7, the positive electrode active material contained in the central portion 12C and the positive electrode active material contained in the end portion 12E are both high nickel positive electrode active materials.

In Comparative Example 8, precipitation of Li was confirmed. It is considered that the edge ratio is less than 4.3% in Comparative Example 8. In Examples 1 to 7, the end ratio is 4.3% or more.

The embodiments and examples disclosed this time are exemplary in all respects and are not restrictive. The technical scope defined by the description of the scope of claims includes all changes within the meaning and scope equivalent to the scope of claims.

10 Positive electrode plate, 11 Positive electrode current collector, 12 Positive electrode active material layer, 12C central part, 12E end, 20 Negative electrode plate, 21 Negative electrode current collector, 22 Negative electrode active material layer, 30 Separator, 50 Electrode group, 90 cases, 100 batteries.

Claims (1)

Contains at least the electrode group and electrolyte,
The electrolyte contains at least a solvent and a supporting salt and contains at least.
The electrode group is a winding type and is
The electrode group includes at least a positive electrode plate and a negative electrode plate, and includes at least a positive electrode plate and a negative electrode plate.
The positive electrode plate contains at least a positive electrode active material layer and contains at least a positive electrode active material layer.
The positive electrode active material layer has a strip-shaped planar shape.
The positive electrode active material layer includes a central portion and an end portion.
In the width direction of the positive electrode active material layer, the end portions are arranged at both ends of the positive electrode active material layer, and the central portion is a portion of the positive electrode active material layer excluding the end portion and said. The ratio of the width of the end to the sum of the width of the end and the width of the center is 4.3% or more and 8.5% or less .
The positive electrode active material layer contains at least a first positive electrode active material and a second positive electrode active material.
The first positive electrode active material is contained in the central portion.
The second positive electrode active material is contained in the end portion, and the second positive electrode active material is contained in the end portion.
The first positive electrode active material has the following formula (I):
LiNi _x M ¹ _(1-x) O ₂ … (I)
[However, x in the formula satisfies 0.5 ≦ x ≦ 0.8, and M ¹ is at least one selected from the group consisting of Co, Mn and Al]
Represented by
The second positive electrode active material has the following formula (II):
LiNi _y M ² _(1-y) O ₂ … (II)
[However, y in the formula satisfies 0.2 ≦ y ≦ 0.4, and M ² is at least one selected from the group consisting of Co and Mn]
Represented by
The ratio of the resistance value of the second positive electrode active material to the resistance value of the first positive electrode active material is 0.88 or more and 1.13 or less.
Lithium-ion secondary battery.

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