Positive electrode and secondary battery containing it

Abstract

This invention relates to a positive electrode and a secondary battery having the same. A positive electrode having a multilayered structure of positive electrode active material layer is provided, which imparts excellent cycle characteristics and internal short-circuit resistance to the secondary battery. The positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer has a first layer on the positive electrode current collector side and a second layer on its surface side. The ratio of the thickness of the second layer to the combined thickness of the first and second layers is 0.20 or more and 0.80 or less. For both the first and second layers, a charging voltage curve of 4.2V (vs Li / Li) is measured. + When considering the specific capacity of the potential flattening region near the first layer, the specific capacity of the potential flattening region of the second layer is greater than that of the first layer. The specific capacity of the potential flattening region of the second layer is greater than 17 mAh / g and less than 30 mAh / g. The specific capacity of the potential flattening region of the first layer is greater than 2 mAh / g and less than 17 mAh / g.

Description

Technical Field

[0001] This invention relates to a positive electrode. This invention also relates to a secondary battery having the positive electrode. Background Technology

[0002] In recent years, secondary batteries such as lithium-ion batteries have become suitable for use as portable power sources for personal computers, portable terminals, electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] The positive electrode of a secondary battery typically has a positive active material layer containing positive active material supported on a positive current collector. Previously, it was known to have a multi-layered structure for the positive active material layer, using different positive active materials in each layer. For example, Patent Document 1 discloses a technique in which a high-capacity positive active material with a high Ni content is used in the lower layer (i.e., the layer on the positive current collector side) and a high-output positive active material with a high Co content is used in the upper layer (i.e., the surface layer), thereby improving the output density and energy density of the lithium-ion secondary battery.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2018-198132 Summary of the Invention

[0007] However, the inventors conducted in-depth research and discovered that, in the aforementioned conventional cathode technology, repeated charging and discharging of a secondary battery equipped with such a cathode leads to capacity degradation. Specifically, insufficient cycle performance was found. On the other hand, it is desirable for the secondary battery to generate minimal heat even in the event of a very large internal short circuit, such as a metal object penetrating the electrode body. In other words, it is desirable for the secondary battery to have excellent internal short-circuit tolerance.

[0008] Therefore, the purpose of this invention is to provide a positive electrode having a multilayered structure of positive electrode active material layers, which can impart excellent cycle characteristics and resistance to internal short circuits to secondary batteries.

[0009] The positive electrode disclosed herein includes a positive current collector and a positive active material layer disposed on the positive current collector. The positive active material layer has a first layer located on the positive current collector side and a second layer located on the surface side of the positive active material layer. The ratio of the thickness of the second layer to the combined thickness of the first layer and the second layer is 0.20 or more and 0.80 or less. For both the first layer and the second layer, a charging voltage curve of 4.2V (vsLi / Li) is measured. + When considering the specific capacity of the potential flattening portion near the first layer, the specific capacity of the potential flattening portion of the second layer is greater than that of the first layer. The specific capacity of the potential flattening portion of the second layer is greater than 17 mAh / g and less than 30 mAh / g. The specific capacity of the potential flattening portion of the first layer is greater than 2 mAh / g and less than 17 mAh / g. Based on this configuration, a positive electrode having a multilayered structure of positive electrode active material layers can be provided, which can impart excellent cycle characteristics and internal short-circuit resistance to the secondary battery.

[0010] In a preferred embodiment of the positive electrode disclosed herein, the first layer and the second layer each contain a positive electrode active material. The positive electrode active material contained in the first layer and the second layer is a lithium composite oxide in which the content of Ni relative to metal atoms other than lithium is 75 mol% or more. With this configuration, the specific capacity of the potential flattening portion near 4.2V (e.g., the region from 4.18V to 4.22V) in the charging voltage curve can be easily adjusted; furthermore, it is advantageous to achieve a high capacity positive electrode.

[0011] In a preferred embodiment of the positive electrode disclosed herein, the first layer contains a Ti-doped positive electrode active material, and the second layer contains a Zr-doped positive electrode active material. With this configuration, it is advantageous to easily adjust the specific capacity of the potential flattening portion near 4.2V in the charging voltage curve.

[0012] In a preferred embodiment of the positive electrode disclosed herein, the first or second layer contains single-particle-shaped positive electrode active material. With this configuration, the specific capacity of the potential flattening portion near 4.2V in the charging voltage curve is easily adjusted, and capacity degradation can be further suppressed, which is advantageous.

[0013] In a preferred embodiment of the positive electrode disclosed herein, the thickness ratio is 0.23 or more and 0.50 or less. This configuration imparts excellent cycle characteristics and resistance to internal short circuits to the secondary battery.

[0014] On the other hand, the secondary battery disclosed herein includes the aforementioned positive electrode, negative electrode, and electrolyte. Based on this configuration, a secondary battery with excellent cycle characteristics and resistance to internal short circuits can be provided. Attached Figure Description

[0015] Figure 1 A cross-sectional view illustrating the structure of the positive electrode according to one embodiment of the present invention.

[0016] Figure 2 To demonstrate the use in LiNi 0.8 Co 0.1 Mn 0.1 A coordinate graph of the charge-discharge curves when O2 contains 0.5 atomic% ZrO2 as a secondary particulate positive electrode active material.

[0017] Figure 3 To demonstrate the use in LiNi 0.8 Co 0.1 Mn 0.1 A coordinate graph of the charge-discharge curves when O2 contains 0.5 atomic% WO3 as a secondary particulate positive electrode active material.

[0018] Figure 4 To demonstrate the use in LiNi 0.8 Co 0.1 Mn 0.1 A coordinate graph of the charge-discharge curves when O2 contains 3 atomic% TiO2 as a secondary particulate positive electrode active material.

[0019] Figure 5 To demonstrate the use in LiNi 0.8 Co 0.1 Mn 0.1 A coordinate graph of the charge-discharge curves when O2 contains 1 atom% Nb2O5 as a secondary particle-shaped positive electrode active material.

[0020] Figure 6 To demonstrate the use of single-particle LiNi 0.8 Co 0.1 Mn 0.1 A coordinate graph of the charge-discharge curves when O2 is the positive electrode active material.

[0021] Figure 7 A cross-sectional view illustrating the internal structure of a lithium-ion secondary battery according to an embodiment of the present invention.

[0022] Figure 8 This is a schematic exploded view illustrating the configuration of the wound electrode body of a lithium-ion secondary battery according to an embodiment of the present invention.

[0023] Explanation of reference numerals in the attached figures

[0024] 20. Winded electrode body

[0025] 30 Battery casing

[0026] 36 Safety valve

[0027] 42 Positive extremes

[0028] 42a Positive Current Collector

[0029] 44 Negative extremes

[0030] 44a Negative Current Collector

[0031] 50 Positive Electrode

[0032] 52 Positive current collector

[0033] 52a Non-forming part of the positive electrode active material layer

[0034] 54 Positive electrode active material layer

[0035] 60 Negative electrode

[0036] 62 Negative current collector

[0037] 62a Non-forming part of negative electrode active material layer

[0038] 64 Negative Electrode Active Material Layer

[0039] 70 Separator

[0040] 80 Non-aqueous electrolyte

[0041] 100 Lithium-ion Secondary Battery Detailed Implementation

[0042] The following is a reference to the appendix. Figure 1 The embodiments of the present invention will be described below. It should be noted that matters not mentioned in this specification but necessary for the implementation of the present invention can be understood by those skilled in the art based on prior art. The present invention can be implemented based on the content disclosed in this specification and common technical knowledge in the field. Furthermore, in the following drawings, components and parts that perform the same function are labeled with the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships.

[0043] It should be noted that in this specification, "secondary battery" refers to an energy storage device capable of repeated charging and discharging, encompassing energy storage components such as batteries and double-layer capacitors. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that utilizes lithium ions as charge carriers, achieving charging and discharging through the movement of lithium ions between the positive and negative electrodes.

[0044] [positive electrode]

[0045] exist Figure 1The figure shows a positive electrode 50 of this embodiment, which is an example of the positive electrode disclosed herein. As shown, the positive electrode 50 of this embodiment includes a positive current collector 52 and a positive active material layer 54 supported on the positive current collector 52. In the example shown, the positive active material layer 54 is provided on both sides of the positive current collector 52, but it may also be provided on only one side. The positive active material layer 54 is preferably provided on both sides of the positive current collector 52.

[0046] As shown in the example, the positive electrode 50 may have a non-formed portion 52a of the positive active material layer 54 at at least one end, thus exposing the positive current collector 52. The non-formed portion 52a of the positive active material layer functions as a current collector (especially a current collector tab).

[0047] As the positive current collector 52, sheets or foils made of metals such as aluminum, nickel, titanium, and stainless steel can be used, with aluminum foil being preferred. When using aluminum foil as the positive current collector 52, there are no particular restrictions on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0048] As shown in the figure, the positive electrode active material layer 54 has a first layer (hereinafter also referred to as the "lower layer") 54A located on the side of the positive electrode current collector 52 and a second layer (hereinafter also referred to as the "upper layer") 54B located on its surface side. Therefore, the positive electrode active material layer 54 has a multilayer structure. To explain, in this embodiment, the two positive electrode active material layers 54 (i.e., on both sides of the positive electrode current collector 52) have a lower layer 54A and an upper layer 54B. However, it is also possible for only one positive electrode active material layer 54 (i.e., on only one side of the positive electrode current collector 52) to have a multilayer structure including a lower layer 54A and an upper layer 54B.

[0049] In this embodiment, when charging voltage curves are obtained for the lower layer 54A and the upper layer 54B, the 4.2V (vs Li / Li) in these charging voltage curves... + (Unless otherwise specified, "V" in this specification refers to the reference potential of metallic Li.) The specific capacity values ​​of the potential flattening portion near 4.2V are different. Specifically, the specific capacity of the potential flattening portion near 4.2V in the charging voltage curve of the upper layer 54B (i.e., the specific capacity of the aforementioned potential flattening portion of the upper layer 54B) is greater than that of the potential flattening portion near 4.2V in the charging voltage curve of the lower layer 54A (i.e., the specific capacity of the aforementioned potential flattening portion of the lower layer 54A). Furthermore, the specific capacity of the aforementioned potential flattening portion of the upper layer 54B is greater than 17 mAh / g and less than 30 mAh / g. On the other hand, the specific capacity of the aforementioned potential flattening portion of the lower layer 54A is greater than 2 mAh / g and less than 17 mAh / g.

[0050] The potential plateau near 4.2V in the charging voltage curve of the layer containing the positive electrode active material is caused by the structural phase transition of the positive electrode active material. Positive electrode active materials that are difficult to undergo this structural phase transition cannot mitigate changes in lattice constant, resulting in large volume changes in the positive electrode active material particles. Therefore, particle breakage accompanied by volume changes is prone to occur. In conventional technologies, because positive electrode active materials that are difficult to undergo structural phase transitions are used, particle breakage caused by volume changes in the positive electrode active material occurs during repeated charge-discharge cycles of the secondary battery, resulting in capacity degradation.

[0051] Therefore, in this embodiment, the specific capacity of the potential flat portion near 4.2V in the charging voltage curve of the upper layer 54B is increased to more than 17 mAh / g in a manner that easily induces the aforementioned structural phase transition, thereby suppressing particle breakage caused by volume changes in the positive electrode active material. As a result, when the secondary battery equipped with the positive electrode 50 of this embodiment is repeatedly charged and discharged, capacity degradation caused by particle breakage can be suppressed.

[0052] On the other hand, by making the specific capacity of the potential flat portion near 4.2V in the charging voltage curve of the lower layer 54A smaller than that of the upper layer 54B, for the electrode body of the secondary battery equipped with the positive electrode 50 of this embodiment, even in the event of a very large internal short circuit such as a metal object penetrating through it, the core material can easily melt due to heat dissipation near the core material, thereby suppressing the continuation of the internal short circuit. That is, the internal short circuit resistance of the secondary battery equipped with the positive electrode 50 of this embodiment can be improved.

[0053] From the viewpoint of higher tolerance to capacity degradation, the specific capacity of the potential flat portion of the upper layer 54B is preferably 20 mAh / g or more, more preferably 22 mAh / g or more, even more preferably 24 mAh / g or more, and most preferably 26 mAh / g or more. On the other hand, the specific capacity of the potential flat portion of the upper layer 54B is preferably 30 mAh / g or less, more preferably 28 mAh / g or less.

[0054] From the viewpoint of higher resistance to internal short circuits, the specific capacity of the potential flat portion of the lower layer 54A is preferably 16 mAh / g or less, more preferably 15 mAh / g or less. On the other hand, the specific capacity of the potential flat portion of the lower layer 54A is preferably 5 mAh / g or more, more preferably 8 mAh / g or more, even more preferably 10 mAh / g or more, and most preferably 12 mAh / g or more.

[0055] There is no particular limitation on the specific capacity of the potential flat portion near 4.2V of the positive electrode active material layer 54 as a whole, but it is preferably 18 mAh / g or more, more preferably 20 mAh / g or more, and even more preferably 22 mAh / g or more.

[0056] To explain, the specific capacity of the potential flat portion near 4.2V in the overall charging voltage curve of the upper layer 54B, the lower layer 54A, and the positive electrode active material layer 54 can be determined by fabricating a positive electrode using only the same positive electrode active material as that contained in each layer, and a half-cell using Li metal as the counter electrode, according to known methods, and measuring the charging voltage curve of the half-cell.

[0057] Specifically, for example, a positive electrode is fabricated using only the same positive electrode active material as that contained in each layer, and a half-cell with Li metal as the counter electrode is fabricated using this positive electrode as a test cell. The electrolyte is, for example, a solution in which LiPF6, as the supporting salt, is dissolved at a concentration of 1.1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 3:3:4, and subsequently, ethylene carbonate (VC) is dissolved at a concentration of 2% by mass.

[0058] For the test battery, a known charge-discharge test apparatus was used, charging at a current of 0.05C. The battery voltage (V) was plotted relative to its specific capacity (mAh / g) on ​​the vertical axis and voltage (V) on the horizontal axis, thus obtaining the charging voltage curve. In this charging voltage curve, when viewed from the direction of increasing specific capacity, the slope of the charging voltage curve decreases near 4.2V when a structural phase transition of the positive electrode active material occurs, and then the slope of the charging voltage curve increases. The portion of the charging voltage curve with a decreasing slope is the potential flattening portion (see reference). Figures 2-6 The voltage at the center of the potential plateau can be set as V0, and the specific capacity in the range of V0 ± 0.02V (i.e., the difference between the specific capacity value (mAh / g) at V0 + 0.02V and the specific capacity value (mAh / g) at V0 - 0.02V) can be used as the specific capacity (mAh / g) of the potential plateau.

[0059] Here, the specific capacity of the potential flattening portion near 4.2V in the charging voltage curve depends on the type of positive electrode active material, and also on the mixing ratio when two or more positive electrode active materials are mixed. Therefore, when it is difficult to divide the positive electrode active material layer 54 into an upper layer 54B and a lower layer 54A for measurement, the specific capacity of the potential flattening portion of the upper layer 54B and the lower layer 54A can be determined as follows.

[0060] The composition of the positive electrode active material layer 54 is analyzed at certain intervals (e.g., every 5% of the thickness of the positive electrode active material layer 54) along its thickness direction. Based on the changes in composition, the existence of the upper layer 54B and the lower layer 54A is determined. Then, the average value of the composition analyzed along the thickness direction for the layer identified as the upper layer 54B is calculated as the composition of the upper layer 54B. Similarly, the average value of the composition analyzed along the thickness direction for the layer identified as the lower layer 54A is calculated as the composition of the lower layer 54A. A monolayer positive electrode active material layer having the same composition as the upper layer 54B is fabricated, and a charging voltage curve is generated for a test battery using it. The specific capacity of the potential flattening portion is then determined. Similarly, a monolayer positive electrode active material layer having the same composition as the lower layer 54A is fabricated, and a charging voltage curve is generated for a test battery using it. The specific capacity of the potential flattening portion is then determined.

[0061] Here, in order to fully obtain the capacity degradation suppression effect of using the upper layer 54B, the ratio of the thickness of the upper layer 54B to the total thickness of the upper layer 54B and the lower layer 54A is 0.20 or more. From the viewpoint of further improving the capacity degradation suppression effect, this thickness ratio is preferably 0.23 or more, and more preferably 0.25 or more.

[0062] On the other hand, in order to fully obtain the improved internal short-circuit resistance effect of the lower layer 54A, the ratio of the thickness of the upper layer 54B to the total thickness of the upper layer 54B and the lower layer 54A is 0.80 or less. From the viewpoint of a higher improvement in internal short-circuit resistance (especially a higher heat release suppression effect and gas generation suppression effect during internal short circuits), this thickness ratio is preferably 0.70 or less, more preferably 0.60 or less, further preferably 0.50 or less, and most preferably 0.35 or less.

[0063] There is no particular limitation on the total thickness of the positive electrode active material layer 54, for example, it is less than 10 μm and more than 300 μm, preferably more than 20 μm and less than 200 μm.

[0064] The positive electrode active material layer 54, namely the lower layer 54A and the upper layer 54B, each contains positive electrode active material. As described above, the potential flatness around 4.2V in the charging voltage curve is caused by the structural phase transition of the positive electrode active material. Therefore, for the positive electrode active material contained in the lower layer 54A, a positive electrode active material with a specific capacity of 2 mAh / g or more and 17 mAh / g or less during the aforementioned potential flatness is used; and for the positive electrode active material contained in the upper layer 54B, a positive electrode active material with a specific capacity of more than 17 mAh / g and 30 mAh / g or less during the aforementioned potential flatness is used.

[0065] As the positive electrode active material for the lower layer 54A and the upper layer 54B, a lithium composite oxide (hereinafter also referred to as "high-Ni content lithium composite oxide") with a Ni content of 75 mol% or more relative to metal atoms other than lithium is preferred. In this case, the specific capacity of the potential flat portion near 4.2V in the charging voltage curve can be easily adjusted, and the positive electrode 50 can be made to have a high capacity. The high-Ni content lithium composite oxide preferably has a layered rock salt type crystal structure. In the high-Ni content lithium composite oxide, the Ni content relative to metal atoms other than lithium is preferably 75 mol% or more and 95 mol% or less.

[0066] Examples of high-Ni-content lithium composite oxides include lithium nickel-cobalt-manganese composite oxides and lithium nickel-cobalt-aluminum composite oxides. It should be noted that in this specification, "lithium nickel-cobalt-manganese composite oxide" refers to oxides containing one or more additive elements other than Li, Ni, Co, Mn, and O. Similarly, "lithium nickel-cobalt-aluminum composite oxide" refers to oxides containing one or more additive elements other than Li, Ni, Co, Al, and O.

[0067] As a high-Ni-content lithium composite oxide, a lithium nickel cobalt manganese composite oxide is preferred. In this lithium nickel cobalt manganese composite oxide, the Ni content relative to metal atoms other than Li is 75 mol% or more as described above. There are no particular limitations on the Co content, but it is preferably 2 mol% or more, more preferably 5 mol% or more. There are no particular limitations on the Mn content, but it is preferably 2 mol% or more, more preferably 5 mol% or more.

[0068] Examples of methods for controlling the specific capacity of the aforementioned potential plateau include using a positive electrode active material doped with appropriate additive elements, or using a positive electrode active material with a compound (especially an oxide) containing appropriate additive elements attached to its surface (especially by coating the surface). By appropriately selecting the type and amount of additive elements, the specific capacity can be changed.

[0069] Specifically, for example, by using a positive electrode active material doped with elements such as W and Zr (especially a lithium composite oxide with high Ni content) as the positive electrode active material of the upper layer 54B, the specific capacity of the potential flat portion of the upper layer 54B can be easily adjusted to a range exceeding 17 mAh / g and below 30 mAh / g. Alternatively, by using a positive electrode active material with a surface coated with compounds containing W, Zr, etc. (especially oxides) (especially a lithium composite oxide with high Ni content) as the positive electrode active material of the upper layer 54B, the specific capacity of the potential flat portion of the upper layer 54B can be easily adjusted to a range exceeding 17 mAh / g and below 30 mAh / g. The amount of added elements can be appropriately set; for example, relative to a lithium composite oxide with high Ni content, the amount of added elements is 0.1 mol% or more and 1.0 mol% or less, preferably 0.2 mol% or more and 0.7 mol% or less.

[0070] Specifically, for example, by using a positive electrode active material doped with elements such as Ti and Nb (especially a lithium composite oxide with high Ni content) as the positive electrode active material of the lower layer 54A, the specific capacity of the potential plateau portion of the lower layer 54A can be easily adjusted to a range of 2 mAh / g or more and 17 mAh / g or less. Alternatively, by using a positive electrode active material with a compound (especially an oxide) containing Ti, Nb, etc., attached to its surface as the positive electrode active material of the lower layer 54A (especially a lithium composite oxide with high Ni content), the specific capacity of the potential plateau portion of the lower layer 54A can be easily adjusted to a range of 2 mAh / g or more and 17 mAh / g or less. The amount of added elements can be appropriately set; for example, relative to a lithium composite oxide with high Ni content, the amount of added elements is 1.0 mol% or more and 10 mol% or less, preferably 2.0 mol% or more and 5.0 mol% or less.

[0071] It should be noted that when excessive doping with added elements results in a mixed phase state in the positive electrode active material, the specific capacity value of the aforementioned potential flat portion can also change.

[0072] In addition, by mixing two or more positive electrode active materials with different compositions, the specific capacity can be adjusted.

[0073] As another example of a method for controlling the specific capacity described above, the method of using single-particle positive electrode active material can be cited. Specifically, when using single-particle positive electrode active material, the specific capacity of the positive electrode active material layer increases. Therefore, for example, by using single-particle positive electrode active material (especially single-particle high-Ni content lithium composite oxide) as the positive electrode active material of the upper layer 54B, the specific capacity of the potential flat portion of the upper layer 54B can be easily adjusted to a range of more than 17 mAh / g and less than 30 mAh / g. In addition, single-particle positive electrode active material is less prone to particle breakage, which can further suppress capacity degradation caused by particle breakage. The proportion of single-particle positive electrode active material in the positive electrode active material of the upper layer 54B is preferably 20% by mass or more, more preferably 40% by mass or more. On the other hand, when using (in combination) single-particle positive electrode active material (especially single-particle high-Ni content lithium composite oxide) as the positive electrode active material of the lower layer 54A, the occurrence of particle breakage of the positive electrode active material in the lower layer 54A can be reduced.

[0074] To clarify, generally speaking, positive electrode active materials are in the form of secondary particles formed by the aggregation of primary particles. In contrast, a "single particle" is a particle generated by the growth of a single crystal nucleus, and therefore a single crystal particle without grain boundaries. The fact that a particle is a single crystal can be confirmed, for example, by analyzing electron diffraction images obtained using a transmission electron microscope (TEM).

[0075] Single particles can individually constitute positive electrode active material particles, or they can aggregate to form positive electrode active material particles. However, when single particles aggregate to form positive electrode active material particles, the number of aggregated single particles is more than 2 and less than 10. Therefore, a positive electrode active material particle is composed of more than 1 and less than 10 single particles, a positive electrode active material particle can be composed of more than 1 and less than 5 single particles, more than 1 and less than 3 single particles, or even a single single particle. It should be noted that the number of single particles in a positive electrode active material particle can be confirmed by observation using a scanning electron microscope (SEM) at a magnification of 10,000 to 30,000 times.

[0076] Such single particles differ from polycrystalline particles, which consist of multiple crystal grains, and secondary particles, which are aggregates of multiple (specifically more than 11) microparticles (primary particles). Single-particle positive electrode active materials can be produced using known methods for obtaining single-crystal particles (such as the molten salt method).

[0077] Furthermore, single particles are typically larger than the primary particles that constitute secondary particles when the crystal is a single crystal. Therefore, they are difficult to aggregate. The maximum diameter of a single particle can be greater than 0.5 μm, or exceed 1 μm, and further exceed 2 μm, or be greater than 3 μm but less than 7 μm. Additionally, the average maximum diameter of a single particle can be greater than 3 μm but less than 7 μm. It should be noted that the maximum diameter of a single particle can be determined in the SEM image of the single particle as the distance between the two furthest points on the particle's outline. This SEM image can be a two-dimensional projection image of the single particle or a cross-sectional image. The average maximum diameter of a single particle can be determined in the SEM image as the average of the maximum diameters of any arbitrarily selected 100 or more single particles.

[0078] There are no particular restrictions on the shape of a single particle; it can be spherical, columnar, plate-like, or irregular in shape.

[0079] In addition, by using secondary particle-shaped positive electrode active materials and single particle-shaped positive electrode active materials together, the specific capacity can be adjusted.

[0080] As an example, in Figures 2-6 The figure shows the charging voltage curve of a half-cell with a positive electrode active material layer containing the following positive electrode active material as the positive electrode and Li metal as the counter electrode.

[0081] Figure 2 In LiNi 0.8 Co 0.1 Mn 0.1 O2 contains 0.5 atomic% ZrO2 in the form of secondary particles as the positive electrode active material.

[0082] Figure 3 In LiNi 0.8 Co 0.1 Mn 0.1 O2 contains 0.5 atomic% WO3 as a secondary particle-shaped positive electrode active material.

[0083] Figure 4 In LiNi 0.8 Co 0.1 Mn 0.1 O2 contains 3 atomic% TiO2 secondary particle-shaped positive electrode active material.

[0084] Figure 5 In LiNi 0.8 Co 0.1 Mn 0.1 O2 contains 1 atom% of secondary particle-shaped positive electrode active material of Nb2O5.

[0085] Figure 6 Single-particle LiNi 0.8Co 0.1 Mn 0.1 O2 positive electrode active material

[0086] exist Figures 2-6 In the curve, the slope of the curve increases near 3.8V, then decreases near 4.2V, and then increases again. The portion of the curve with a decreasing slope near 4.2V is the potential plateau (see [reference needed]). Figures 2 to 6 (The arrow in the image).

[0087] There is no particular limitation on the content of the positive electrode active material in the positive electrode active material layer 54. The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., relative to the total mass of the positive electrode active material layer) is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more.

[0088] There are no particular limitations on the average particle size (median particle size D50) of the positive electrode active material. When the positive electrode active material is in the form of secondary particles, the average particle size (median particle size D50) is, for example, 0.05 μm or more and 25 μm or less, preferably 10 μm or more and 25 μm or less. When the positive electrode active material is in the form of single particles, it is preferably 2 μm or more and 5 μm or less. It should be noted that the average particle size (median particle size D50) of the positive electrode active material can be determined, for example, by laser diffraction scattering.

[0089] The positive electrode active material layer 54 may contain components other than the positive electrode active material. Examples include lithium phosphate (Li3PO4), conductive materials, and adhesives.

[0090] There is no particular limitation on the content of lithium phosphate in the positive electrode active material layer 54, but it is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 12% by mass or less.

[0091] As a conductive material, carbon black such as acetylene black (AB) or other carbon materials such as graphite are preferred. There are no particular limitations on the content of the conductive material in the positive electrode active material layer 54, for example, it is 0.1% by mass or more and 20% by mass or less, preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 10% by mass or less.

[0092] As a binder, polyvinylidene fluoride (PVdF) can be used, for example. There are no particular limitations on the content of the binder in the positive electrode active material layer 54, for example, it is 0.5% by mass or more and 15% by mass or less, preferably 1% by mass or more and 10% by mass or less, more preferably 1.5% by mass or more and 8% by mass or less.

[0093] The positive electrode 50 can be manufactured according to known methods.

[0094] The positive electrode constructed as described above can impart excellent cycle characteristics and resistance to internal short circuits to the secondary battery. Therefore, the positive electrode disclosed herein is preferably for use in secondary batteries, and more preferably for use in lithium-ion secondary batteries.

[0095] [Rechargeable Battery]

[0096] On the other hand, the secondary battery disclosed herein has the aforementioned positive electrode, negative electrode, and electrolyte.

[0097] The following describes in detail one embodiment of the secondary battery disclosed herein, using a flat square lithium-ion secondary battery having a flat-shaped wound electrode body and a flat-shaped battery casing as an example, but it is not intended to limit the secondary battery disclosed herein to the contents described in the embodiment.

[0098] Figure 7 The lithium-ion secondary battery 100 shown is a sealed battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte 80 within a flat, square battery casing (i.e., outer packaging container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 configured to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight and thermally conductive metal material, such as aluminum. Further explanation follows. Figure 7 The amount of non-aqueous electrolyte 80 is not accurately represented.

[0099] like Figure 7 and Figure 8As shown, the wound electrode body 20 has a configuration in which a positive electrode 50 and a negative electrode 60 are overlapped and wound along the length direction, separated by two elongated separators 70. The positive electrode 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive electrode current collector 52 along the length direction. The negative electrode 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative electrode current collector 62 along the length direction. The non-formed portions 52a of the positive electrode active material layer (i.e., the portions where the positive electrode active material layer 54 is not formed and thus the positive electrode current collector 52 is exposed) and the non-formed portions 62a of the negative electrode active material layer (i.e., the portions where the negative electrode active material layer 64 is not formed and thus the negative electrode current collector 62 is exposed) are formed to extend outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned length direction). The non-forming portion 52a of the positive electrode active material layer and the non-forming portion 62a of the negative electrode active material layer are respectively bonded with a positive electrode current collector 42a and a negative electrode current collector 44a.

[0100] Positive electrode 50 uses the positive electrode described above.

[0101] On the other hand, the negative current collector 62 constituting the negative electrode 60 can be made of metal sheets or foils such as copper, nickel, titanium, or stainless steel, with copper foil being preferred. When using copper foil as the negative current collector 62, there are no particular limitations on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0102] As the negative electrode active material, known negative electrode active materials used in lithium-ion secondary batteries can be used, such as carbon materials like graphite, hard carbon, and soft carbon. Graphite can be natural graphite or artificial graphite, or it can be graphite coated with an amorphous carbon material, in the form of amorphous carbon-coated graphite.

[0103] There is no particular limitation on the content of negative electrode active material in the negative electrode active material layer, but it is preferably 90% by mass or more, and more preferably 95% by mass or more.

[0104] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as adhesives, tackifiers, etc.

[0105] As an adhesive, styrene-butadiene rubber (SBR) and its modifiers, acrylonitrile-butadiene rubber and its modifiers, acrylic rubber and its modifiers, fluororubber, etc., can be used. Among them, SBR is preferred. There are no particular limitations on the content of the adhesive in the negative electrode active material layer 64, but it is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.2% by mass or more and 3% by mass or less.

[0106] As a thickener, cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropyl methyl cellulose (HPMC) can be used; polyvinyl alcohol (PVA) is also acceptable. CMC is preferred. There are no particular limitations on the content of the thickener in the negative electrode active material layer 64, but it is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.4% by mass or more and 2% by mass or less.

[0107] There are no particular limitations on the thickness of the negative electrode active material layer 64, for example, it is 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

[0108] Examples of separators 70 include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. These porous sheets can be single-layered or multi-layered (e.g., a three-layered structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0109] There are no particular limitations on the thickness of the separator 70, for example, it is 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less.

[0110] Non-aqueous electrolyte 80 typically contains a non-aqueous solvent and an electrolyte salt (in other words, a supporting salt). As the non-aqueous solvent, various organic solvents used in common lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular restriction. Among these, carbonates are preferred, and specific examples include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (MFEC), ethylene difluorocarbonate (DFEC), methyl monofluorodifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more.

[0111] As the electrolyte salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) can be used, with LiPF6 being preferred. There are no particular limitations on the concentration of the electrolyte salt, but it is preferably 0.7 mol / L or higher and 1.3 mol / L or lower.

[0112] It should be noted that, as long as the effect of the present invention is not significantly impaired, the above-mentioned non-aqueous electrolyte 80 may also contain components other than those mentioned above, such as film-forming agents such as oxalate complexes and vinylene carbonate (VC), gas-generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), thickeners, and various additives.

[0113] The lithium-ion secondary battery 100 configured as described above suppresses capacity degradation during repeated charge and discharge cycles. That is, the lithium-ion secondary battery 100 exhibits excellent cycle characteristics. Furthermore, the lithium-ion secondary battery 100 generates minimal heat even in the event of a very large internal short circuit, such as a metal object penetrating the electrode body. That is, the lithium-ion secondary battery 100 exhibits excellent internal short-circuit tolerance.

[0114] The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for driving electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can be used as a battery for small energy storage devices. Typically, the lithium-ion secondary battery 100 can be used in the form of a battery pack consisting of multiple batteries connected in series and / or in parallel.

[0115] As an example, a square lithium-ion secondary battery 100 having a flat, wound electrode body 20 has been described. However, the lithium-ion secondary battery disclosed herein can also be configured as a lithium-ion secondary battery having a stacked electrode body (i.e., an electrode body consisting of multiple positive electrodes and multiple negative electrodes stacked alternately). Furthermore, the non-aqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium-ion secondary battery, a laminated shell type lithium-ion secondary battery, a coin-shaped lithium-ion secondary battery, etc.

[0116] Alternatively, non-aqueous electrolyte secondary batteries other than lithium-ion secondary batteries can be constructed using the aforementioned positive electrode according to known methods. Furthermore, all-solid-state secondary batteries (especially all-solid-state lithium-ion secondary batteries) can be constructed using a solid electrolyte instead of the non-aqueous electrolyte 80 according to known methods.

[0117] The following describes embodiments of the present invention, but it is not intended to limit the present invention to the contents shown in these embodiments.

[0118] <Example 1>

[0119] As the lower layer positive electrode active material, it is prepared for use in LiNi 0.8 Co 0.1 Mn 0.1A lithium nickel manganese cobalt composite oxide (NCM811-Ti) doped with 3 mol% Ti in O2. This lithium nickel manganese cobalt composite oxide is in the form of secondary particles. A slurry for forming the lower layer is prepared by mixing the lower layer positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of AB:PVDF = 97.5:1.5:1.0. An appropriate amount of N-methyl-2-pyrrolidone (NMP) is added to the resulting mixture.

[0120] As the upper layer positive electrode active material, it is prepared for use in LiNi 0.8 Co 0.1 Mn 0.1 A lithium nickel manganese cobalt composite oxide (NCM811-Zr) doped with 0.3 mol% Zr in O2 is prepared. This lithium nickel manganese cobalt composite oxide is in the form of secondary particles. A slurry for upper layer formation is prepared by mixing the upper layer positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of AB:PVDF = 97.5:1.5:1.0. An appropriate amount of N-methyl-2-pyrrolidone (NMP) is added to the resulting mixture.

[0121] A lower layer forming slurry is coated on both sides of a 15μm thick aluminum foil positive electrode current collector and dried. Next, an upper layer forming slurry is coated on the dried lower layer forming slurry and dried. At this point, the coating thickness of the lower layer forming slurry and the upper layer forming slurry are set to be the same. Then, the coating is rolled using calendering rolls to produce a positive electrode sheet. In the rolled positive electrode active material layer, the thickness of the lower layer formed by the lower layer forming slurry and the thickness of the upper layer formed by the upper layer forming slurry are the same. The dimensions of the fabricated positive electrode are 6150mm in length × 117mm in width × 120μm in thickness.

[0122] In addition, graphite (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the tackifier were mixed in ion-exchanged water at a mass ratio of C:SBR:CMC = 98:1:1 to prepare a slurry for forming the negative electrode active material layer. This slurry was then coated onto a copper foil with a thickness of 10 μm. After drying, it was rolled to a specified thickness to produce a negative electrode sheet. The dimensions of the fabricated negative electrode were 6300 mm in length × 122 mm in width × 130 μm in thickness.

[0123] As a separator, a porous polyolefin sheet with a thickness of 24 μm and a three-layer structure of PP / PE / PE is prepared. The positive electrode sheet and the negative electrode sheet are overlapped with the separator in between to obtain a laminate. Next, the laminate is wound to obtain a wound body, which is then pressed to form a flat shape, thus obtaining a flat wound electrode body.

[0124] The electrode terminals were installed on the electrode body, inserted into the battery casing, and fused together. Then, a non-aqueous electrolyte was injected. The non-aqueous electrolyte was prepared by dissolving LiPF6 (as a supporting salt) at a concentration of 1.1 mol / L and vinylene carbonate (VC) at a concentration of 2% by mass in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 3:3:4. The battery casing was then sealed, thus obtaining the lithium-ion secondary battery for evaluation in Example 1.

[0125] <Example 2>

[0126] In addition to using single-particle LiNi 0.8 Co 0.1 Mn 0.1 O2 and in LiNi 0.8 Co 0.1 Mn 0.1 Except for the mixed active material, which is a lithium nickel manganese cobalt composite oxide (secondary particle form) doped with 3 mol% Ti in O2 and mixed at a mass ratio of 50:50 as the lower layer positive electrode active material, the same operation as in Example 1 was performed to obtain the evaluation lithium-ion secondary battery of Example 2.

[0127] <Example 3>

[0128] Using single-particle LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) is used as the upper positive electrode active material, and single-particle LiNi is used. 0.8 Co 0.1 Mn 0.1 O2 and in LiNi 0.8 Co 0.1 Mn 0.1 A lithium nickel manganese cobalt composite oxide (secondary particle form) doped with 3 mol% Ti in O2 was mixed in a mass ratio of 50:50 to form a mixed active material as the lower positive electrode active material. The thickness ratio of the upper layer to the lower layer was changed to 25:75. Otherwise, the same operation as in Example 1 was performed to obtain the evaluation lithium-ion secondary battery of Example 3.

[0129] <Examples 4-7>

[0130] Using single-particle LiNi 0.8 Co 0.1 Mn 0.1 O2 and in LiNi 0.8 Co 0.1 Mn 0.1 A mixed active material consisting of lithium nickel manganese cobalt composite oxides (secondary particle form) doped with 0.3 mol% Zr in O2, mixed in a 50:50 mass ratio, is used as the upper positive electrode active material. Single-particle LiNi is also used. 0.8 Co 0.1 Mn 0.1 O2 and in LiNi 0.8 Co 0.1 Mn 0.1 A mixed active material consisting of a lithium nickel manganese cobalt composite oxide (secondary particle form) doped with 3 mol% Ti in O2, mixed at a mass ratio of 50:50, was used as the lower positive electrode active material. The thickness ratio of the upper and lower layers was changed to the values ​​shown in Table 1. Otherwise, the same procedure as in Example 1 was followed to obtain the evaluation lithium-ion secondary batteries of Examples 4 to 7.

[0131] <Comparative Example 1>

[0132] Using only the slurry for lower layer formation prepared in Example 1, a positive electrode with the same positive electrode thickness was prepared. Otherwise, the same procedure as in Example 1 was followed to obtain the lithium-ion secondary battery for evaluation of Comparative Example 1.

[0133] <Comparative Example 2>

[0134] Using only the slurry for upper layer formation prepared in Example 1, a positive electrode with the same positive electrode thickness was prepared. Otherwise, the same procedure as in Example 1 was followed to obtain the lithium-ion secondary battery for evaluation of Comparative Example 2.

[0135] <Comparative Example 3>

[0136] By reversing the coating order of the slurry for forming the lower layer and the slurry for forming the upper layer, the upper and lower layers of the positive electrode active material layer are interchanged. Otherwise, the same procedure as in Example 1 is followed to obtain the lithium-ion secondary battery for evaluation of Comparative Example 3.

[0137] <Comparative Example 4>

[0138] Using only the slurry for lower layer formation prepared in Example 2, a positive electrode with the same positive electrode thickness was prepared. Otherwise, the same procedure as in Example 1 was followed to obtain the lithium-ion secondary battery for evaluation of Comparative Example 4.

[0139] <Comparative Example 5>

[0140] In addition to single-particle LiNi 0.8 Co0.1 Mn 0.1 O2 and in LiNi 0.8 Co 0.1 Mn 0.1 Except for changing the mass ratio of the lithium nickel manganese cobalt composite oxide (secondary particle form) doped with 3 mol% Ti in O2 to 25:75, the same procedure as in Comparative Example 4 was followed to obtain the evaluation lithium-ion secondary battery of Comparative Example 5.

[0141] <Specific Capacitance Measurement of Potential Flatness>

[0142] Positive electrodes having the same positive electrode active material layer composition as Comparative Examples 1, 2, 4, and 5, and positive electrodes having the same upper and lower layers composition as Comparative Examples 3 and each embodiment were fabricated. Using these positive electrodes and Li negative electrodes, test lithium-ion secondary batteries were fabricated in the same manner as in Example 1 (i.e., using the same non-aqueous electrolyte). Each test lithium-ion secondary battery was charged at a current of 0.05C using a commercially available charge-discharge testing apparatus. The battery voltage (V) was plotted relative to the battery specific capacity (mAh / g) with specific capacity (V) on the vertical axis and voltage (V) on the horizontal axis to obtain a charging voltage curve. The voltage at the center of the potential flattening portion near 4.2V on the charging voltage curve was designated as V0. The specific capacity within the range of V0 ± 0.02V (i.e., the difference between the specific capacity value (mAh / g) at V0 + 0.02V and the specific capacity value (mAh / g) at V0 - 0.02V) was used as the specific capacity (mAh / g) of the potential flattening portion. The results are shown in Table 1.

[0143] <Fuse Test>

[0144] Each evaluation lithium-ion secondary battery was charged to 4.2V using a constant current. While measuring the voltage with a data logger, a nail (made by Daido Hand Co., Ltd., round nail, 3mm in diameter) was inserted into the center of the battery casing along the thickness direction of each evaluation lithium-ion secondary battery, penetrating both the positive and negative terminals, to create an internal short circuit. The voltage rise after the voltage drop was due to Joule heating caused by the internal short circuit; therefore, the heat release (J) was calculated using the voltage, current, and time during the voltage rise. The results are shown in Table 1.

[0145] <Cyclic Performance Evaluation>

[0146] Each evaluation lithium-ion secondary battery was charged to 4.2V at a constant current of 0.1C at room temperature, and then discharged to 2.5V at a constant current of 0.1C. The discharge capacity at this point was calculated and taken as the initial capacity.

[0147] Each evaluation lithium-ion secondary battery was placed at 25°C and charged at a constant current of 2C to 4.2V, paused for 10 minutes, and then discharged at a constant current of 2C to 3.0V, paused for 10 minutes, constituting one cycle. This charge-discharge cycle was repeated 500 times. The discharge capacity after 500 cycles was calculated using the same method as for the initial capacity. The capacity retention rate (%) was calculated by multiplying (discharge capacity after 500 charge-discharge cycles / initial capacity) by 100. The results are shown in Table 1.

[0148] [Table 1]

[0149]

[0150] The capacity retention of the lithium-ion secondary batteries in Comparative Examples 1-5 was as low as approximately 50% to approximately 70%. The lithium-ion secondary batteries in Comparative Examples 1-5 were disassembled after cycle performance evaluation, and the state of the positive electrode active material layer was observed. The results showed that the fragmentation of the positive electrode active material particles began from the surface of the positive electrode active material layer; the lower the capacity retention, the more fragmentation of the positive electrode active material particles. Furthermore, the lithium-ion secondary batteries in Comparative Examples 1-3 exhibited heat release exceeding 4 J.

[0151] On the other hand, the capacity retention rate of the lithium-ion secondary batteries in Examples 1-7 is as high as 75% or more. Furthermore, the lithium-ion secondary batteries in Examples 1-7 achieve a relatively low heat release of less than 3J. The lithium-ion secondary batteries in Examples 1-7, after cycle performance evaluation, were disassembled, and the state of the positive electrode active material layer was observed. The results showed that compared to Comparative Examples 1-5, there was less breakage of the positive electrode active material particles, resulting in a higher capacity retention rate. Additionally, in Example 2, single particles were mixed in the lower layer, and compared to Example 1, there was less breakage of the positive electrode active material particles in the lower layer. In Examples 3 and 4, single particles were used in the upper layer; the higher the mixing ratio of single particles, the less breakage occurred, and the higher the capacity retention rate. Furthermore, based on the comparison of Examples 4-7, from the viewpoint of internal short-circuit resistance (heat release suppression), the thickness ratio of the upper layer should preferably be 0.8 or less, and from the viewpoint of cycle performance, the thickness ratio of the upper layer should preferably be 0.2 or more.

[0152] In summary, it can be said that when the ratio of the thickness of the upper layer to the combined thickness of the upper and lower layers is 0.20 or more and 0.80 or less, the specific capacity of the potential flat portion of the upper layer is greater than that of the lower layer. Specifically, when the specific capacity of the potential flat portion of the upper layer is greater than 17 mAh / g and less than 30 mAh / g, and the specific capacity of the potential flat portion of the lower layer is 2 mAh / g or more and less than 17 mAh / g, heat dissipation is low and capacity retention is high. Therefore, it is evident that the cathode disclosed herein can impart excellent cycle characteristics and resistance to internal short circuits to the secondary battery.

[0153] The specific examples of the present invention have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technology described within the scope of the claims includes technologies obtained by various modifications and alterations to the specific examples described above.

Claims

1. A positive electrode is a positive electrode comprising a positive current collector and a layer of positive active material disposed on the positive current collector, wherein, The positive electrode active material layer has a first layer located on the positive electrode current collector side and a second layer located on the surface side of the positive electrode active material layer. The ratio of the thickness of the second layer to the combined thickness of the first and second layers is greater than 0.20 and less than 0.

80. For the first and second layers, the charging voltage curve is 4.2V (vs Li / Li). + When the voltage at the center of the potential flattening region near the potential flattening region is set as V0, and the specific capacitance at V0+0.02V and V0-0.02V is used as the difference to calculate the specific capacitance of the potential flattening region, the specific capacitance of the potential flattening region in the second layer is larger than that in the first layer. The specific capacity of the potential flat portion of the second layer is greater than 17 mAh / g and less than 30 mAh / g. The specific capacity of the potential flat portion of the first layer is 2 mAh / g or more and 17 mAh / g or less.

2. The positive electrode according to claim 1, wherein, The first layer and the second layer each contain a positive electrode active material, and the positive electrode active material contained in the first layer and the second layer is a lithium composite oxide in which the content of Ni relative to the metal atoms other than lithium is more than 75 mol%.

3. The positive electrode according to claim 1 or 2, wherein, The first layer contains Ti-doped positive electrode active material, and the second layer contains Zr-doped positive electrode active material.

4. The positive electrode according to claim 1 or 2, wherein, The first layer or the second layer contains single-particle positive electrode active material.

5. The positive electrode according to claim 1 or 2, wherein, The thickness ratio is 0.23 or higher and 0.50 or lower.

6. A secondary battery, which has the following features: The positive electrode according to any one of claims 1 to 5; Negative electrode; and Electrolytes.

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