Secondary-battery positive electrode, method for producing same, and secondary battery

Abstract

A secondary-battery positive electrode comprising a positive-electrode current collector, and a positive-electrode active material layer that is supported by the positive-electrode current collector, wherein: the positive-electrode active material layer includes active material particles, and an oxide film that covers at least part of the surface of the active material particles; the active material particles include a lithium-containing transition metal oxide; the oxide film includes an oxide of a first element that is not a non-metallic element; and if the thickness of the positive-electrode active material layer is TA, the thickness Tb of the oxide film at a location that is 0.10TA from the surface of the positive-electrode current collector in the positive-electrode active material layer and the thickness Tt of the oxide film at a location that is 0.90TA from the surface of the positive-electrode current collector in the positive-electrode active material layer satisfy Tb<Tt.

Description

Positive electrode for secondary battery, method for producing the same, and secondary battery

[0001] The present disclosure relates primarily to a positive electrode for a secondary battery.

[0002] Patent Document 1 discloses a battery comprising a positive electrode, a non-aqueous electrolyte, and a negative electrode, the positive electrode comprising a conductive material made of a carbon material, aluminum oxide deposits scattered on the surface of the conductive material, and a positive electrode having an upper limit potential of 4.5 V (vs. Li / Li) relative to the oxidation-reduction potential of metallic lithium. ＋ and a lithium-containing oxide active material having a maximum diameter of 1 nm or more and 20 nm or less, wherein the aluminum oxide deposits have a maximum diameter of 1 nm or more and 20 nm or less.

[0003] Patent Document 2 discloses a battery comprising a positive electrode current collector and a positive electrode active material layer provided on the surface of the positive electrode current collector and containing a positive electrode material, the positive electrode material comprising positive electrode active material particles, a first coating film containing an oxide X of a metal element M1 and attached to the surface of the positive electrode active material particles, and a second coating film having lithium ion permeability and attached to the surface of the first coating film, the second coating film containing Li ｘ M ２ O ｙ (0.5≦x<4, 1≦y<6), and M2 is at least one element selected from the group consisting of B, Al, Si, P, S, Ti, V, Zr, Nb, Ta, and La.

[0004] JP 2017-174612 A International Publication No. 2017-174612

[0005] In Patent Document 1, aluminum oxide is deposited on the surface of a conductive material made of a carbon material, thereby suppressing decomposition of the non-aqueous electrolyte on the surface of the conductive material, thereby improving the capacity of the lithium-ion battery. However, the aluminum oxide deposits scattered on the surface of the conductive material have the disadvantage of increasing the resistance of the positive electrode. In Patent Document 2, the first coating and the second coating can also increase the resistance of the positive electrode.

[0006] On the other hand, the higher the energy density of a secondary battery, the more it is required to improve safety. In particular, it is important to reduce heat generation due to internal short circuits. When the temperature of active material particles (particles of positive electrode active material) near the short circuit point increases due to heat generation, the resistance of the active material particles tends to drop sharply.

[0007] One aspect of the present disclosure relates to a positive electrode for a secondary battery, including a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector, wherein the positive electrode active material layer includes active material particles and an oxide coating covering at least a portion of a surface of the active material particles, the active material particles including a lithium-containing transition metal oxide, and the oxide coating including an oxide of a first element other than a non-metallic element, wherein, when a thickness of the positive electrode active material layer is designated as TA, a thickness Tb of the oxide coating or an existence probability Pb ​​of the first element at a position 0.10 TA from the surface of the positive electrode current collector in the positive electrode active material layer, and a thickness Tt of the oxide coating or an existence probability Pt of the first element at a position 0.90 TA from the surface of the positive electrode current collector in the positive electrode active material layer satisfy Tb<Tt or Pb<Pt.

[0008] Another aspect of the present disclosure relates to a secondary battery including the above-described positive electrode for secondary batteries, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode.

[0009] Yet another aspect of the present disclosure provides a method for producing a cathode active material layer including a lithium-containing transition metal oxide, a cathode current collector, and a cathode active material layer including the active material particles on a surface of the cathode current collector, the cathode active material layer being formed by a supporting step of supporting the active material particles on a surface of the cathode current collector to form a precursor layer, a rolling step of rolling the precursor layer, and a step of exposing the active material particles to a gas phase including a first element other than a non-metallic element after the rolling step to coat at least a portion of the surface of the active material particles. and a coating formation step of forming an oxide coating so that the oxide coating contains an oxide of the first element, wherein, when a thickness of the positive electrode active material layer is TA, a thickness Tb of the oxide coating or an existence probability Pb ​​of the first element at a position 0.10 TA from the surface of the positive electrode current collector in the positive electrode active material layer and a thickness Tt of the oxide coating or an existence probability Pt of the first element at a position 0.90 TA from the surface of the positive electrode current collector in the positive electrode active material layer satisfy Tb<Tt or Pb<Pt.

[0010] According to the present disclosure, it is possible to suppress an increase in resistance while improving safety when an internal short circuit occurs in the battery.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a cross-sectional view schematically illustrating a main part of a positive electrode according to an embodiment of the present disclosure. 2 is an enlarged cross-sectional view illustrating a main part of the positive electrode shown in FIG. 3. 3 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0013] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0014] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] In the following description, the terms "contain" or "comprise" encompass "contain (or include)," "consist essentially of," and "consist of."

[0016] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries.

[0017] A. Positive Electrode for Secondary Battery A positive electrode for a secondary battery according to an embodiment of the present disclosure includes a positive electrode current collector and a positive electrode active material layer supported on the positive electrode current collector.

[0018] [Positive electrode current collector] The positive electrode current collector is made of a sheet-like conductive material. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. [Positive electrode active material layer] The positive electrode active material layer is supported on one or both surfaces of the positive electrode current collector. The positive electrode active material layer is usually a positive electrode mixture layer made of a positive electrode mixture, and is in the form of a membrane or film. The positive electrode mixture contains active material particles (particles of the positive electrode active material) as an essential component.

[0019] The positive electrode active material layer includes active material particles and an oxide coating that covers at least a portion of the surface of the active material particles. The active material particles include a lithium-containing transition metal oxide. The positive electrode active material layer is formed on the surface of a positive electrode current collector. The positive electrode active material layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0020] [Oxide Coating] The oxide coating covers at least a portion of the surface of the active material particles, which are secondary particles. The oxide coating includes an oxide of a first element other than a non-metallic element. The oxide of the first element other than a non-metallic element has a different crystal structure from that of the active material particles or is amorphous. The oxide of the first element other than a non-metallic element may be electrochemically inactive and not exhibit substantial capacity. Such an oxide coating acts to suppress an increase in the resistance of the positive electrode while improving safety in the event of an internal short circuit. When an internal short circuit occurs, current concentrates near the short circuit point, generating heat. When the temperature of the active material particles (positive electrode active material particles) near the short circuit point increases due to heat generation, the resistance of the active material particles tends to suddenly decrease. On the other hand, the oxide coating covering at least a portion of the surface of the active material particles does not suddenly decrease in resistance even when the temperature increases due to heat generation, and acts as a resistance component that suppresses an increase in short circuit current.

[0021] Here, when the thickness of the positive electrode active material layer is TA, the thickness Tb of the oxide coating at a position 0.10 TA from the surface of the positive electrode current collector of the positive electrode active material layer and the thickness Tt of the oxide coating at a position 0.90 TA from the surface of the positive electrode current collector of the positive electrode active material layer satisfy the relationship Tb<Tt. For example, the oxide coating may be formed on the surface of the active material particles so that the thickness increases with increasing distance from the surface of the positive electrode current collector. In this case, the existence probability Pb ​​of the first element at the position 0.10 TA and the existence probability Pt of the first element at the position 0.90 TA satisfy the relationship Pb<Pt.

[0022] By providing the above-described thickness distribution for the oxide coating, an increase in the resistance of the positive electrode can be suppressed. When the resistive component covering the active material particles near the surface of the positive electrode current collector is relatively reduced, the current collection performance from the active material particles to the positive electrode current collector is not significantly impaired. In other words, the oxide coating is unlikely to act as a resistive component for the positive electrode during normal battery use. On the other hand, when an internal short circuit occurs, the resistance of the oxide coating compensates for the sudden drop in the resistance of the active material particles, suppressing an increase in the short-circuit current.

[0023] The surface of the positive electrode current collector is synonymous with the interface between the positive electrode active material layer and the positive electrode current collector. A position of the positive electrode active material layer 0.10 TA from the surface of the positive electrode current collector is synonymous with a position 0.10 TA from the interface between the positive electrode active material layer and the positive electrode current collector. A position of the positive electrode active material layer 0.90 TA from the surface of the positive electrode current collector is synonymous with a position 0.90 TA from the interface between the positive electrode active material layer and the positive electrode current collector.

[0024] The thicknesses Tb and Tt may satisfy 0≦Tb / Tt<1, 0.02≦Tb / Tt≦0.7, or 0.1≦Tb / Tt≦0.5. Similarly, the existence probabilities Pb and Pt may satisfy 0≦Pb / Pt<1, 0.02≦Pb / Pt≦0.8, or 0.1≦Pb / Pt≦0.5.

[0025] The oxide film thicknesses Tb and Tt can be measured by observing the cross-section of the active material particles using an SEM or TEM. First, the secondary battery is disassembled to remove the positive electrode, and a cross-section of the positive electrode active material layer is obtained using a cross-section polisher (CP). From the cross-sectional image obtained using the SEM or TEM, 10 active material particles with a maximum diameter of 5 μm or more are selected that partially overlap a line drawn from the surface of the positive electrode current collector of the positive electrode active material layer at a position 0.10 TA. For each particle, the oxide film thickness is measured at one or two intersections between the line and the outer edge of the active material particle. The average thickness at up to 20 of these points is calculated. After calculating this average, data that differ from the obtained average by 20% or more are excluded, and the average is calculated again. This corrected average is the oxide film thickness Tb at the 0.10 TA point. Similarly, the oxide film thickness Tt at the 0.90 TA point is calculated using a line drawn from the surface of the positive electrode current collector of the positive electrode active material layer at a position 0.90 TA.

[0026] The starting point of the oxide coating is the interface between the active material particle and the oxide coating. For example, the starting point of the oxide coating can be determined by SEM-EDS analysis, where the peak intensity attributable to the constituent elements of the active material particle is 1 / 10 or less of the peak intensity attributable to the first element. The ending point of the oxide coating can be determined by SEM-EDS analysis, where the peak intensity attributable to the first element is 5% or less of its maximum value.

[0027] The thickness of the oxide coating may change so that it increases from the surface of the positive electrode current collector toward the outside. This change may be continuous or stepwise, as long as it can be grasped as an overall trend.

[0028] For example, active material particles are selected at multiple locations (e.g., five locations) on a line in the thickness direction of the positive electrode active material layer, at different distances from the surface of the positive electrode current collector, and the oxide film thickness is measured at one or two intersections between the line and the outer edge of the active material particle. Multiple lines (e.g., five lines) are drawn in the thickness direction of the positive electrode active material layer, and the oxide film thickness is measured in the same manner. The film thickness calculated in this manner is plotted on a graph with the horizontal axis representing the distance from the surface of the positive electrode current collector and the vertical axis representing the film thickness. If the approximated line or curve obtained from the least squares method slopes upward to the right, it can be determined that the oxide film thickness generally increases from the surface of the positive electrode current collector toward the outside.

[0029] The thickness Tb of the oxide coating at a position 0.10 TA from the surface of the positive electrode current collector of the positive electrode active material layer is not particularly limited. From the viewpoint of improving safety in the event of an internal short circuit, the thickness Tb may be 0.1 nm or more, 0.5 nm or more, or 1 nm or more. From the viewpoint of suppressing an increase in the resistance of the positive electrode and improving lithium ion diffusibility, the thickness Tb of the oxide coating may be 50 nm or less, 10 nm or less, or 2 nm or less. The thickness Tb of the oxide coating may be, for example, 0.1 nm or more and 50 nm or less, or 0.1 nm or more and 10 nm or less.

[0030] The thickness Tt of the oxide coating at a position 0.90 TA from the surface of the positive electrode current collector of the positive electrode active material layer is not particularly limited. From the viewpoints of suppressing an increase in the resistance of the positive electrode and improving lithium ion diffusibility, the thickness Tt may be 50 nm or less, 30 nm or less, 10 nm or less, or 2 nm or less. From the viewpoint of improving safety in the event of an internal short circuit, the thickness Tt may be 0.1 nm or more, 0.5 nm or more, or 1 nm or more. The thickness Tt of the oxide coating may be, for example, 0.1 nm or more to 50 nm or less, or 0.1 nm or more to 30 nm or less.

[0031] The average thickness Ta of the oxide coating is also not particularly limited. From the viewpoint of improving safety in the event of an internal short circuit, the average thickness Ta of the oxide coating may be 0.1 nm or more, 0.5 nm or more, or 1 nm or more. From the viewpoint of suppressing an increase in the resistance of the positive electrode and improving lithium ion diffusibility, the average thickness Ta of the oxide coating may be 50 nm or less, 10 nm or less, or 2 nm or less. The average thickness Ta of the oxide coating is, for example, 0.1 nm or more and 50 nm or less.

[0032] The average thickness Ta of the oxide coating can be calculated by averaging the thickness Tb and the thickness Tt.

[0033] The presence probabilities Pb and Pt can be measured by SEM-EDS analysis or EPMA analysis of the cross section of the active material particle. The presence probability may be calculated using the spectral intensity ratio of each element at each depth, or, for example, by the following method. The presence probability Pb ​​may be calculated as the ratio (Rb = Lb / L) of the total length Lb of the portions of a line of length L drawn from the surface of the positive electrode current collector of the positive electrode active material layer to the length L that intersects with the first element, to the length L. The length L is 100 μm or more. Similarly, the presence probability Pt may be calculated as the ratio (Rb = Lt / L) of the total length Lt of the portions of a line of length L drawn from the surface of the positive electrode current collector of the positive electrode active material layer to the length L that intersects with the first element, to the length L. Furthermore, the ratio of Pb to the existence probability Pt: Pb / Pt may be calculated as the ratio of the frequencies at which the first element is detected (the ratio of the detection frequency at the position of 0.90TA to the detection frequency at the position of 0.10TA).

[0034] The positive electrode for measuring the thicknesses Tb and Tt or the existence probabilities Pb and Pt may be taken from a secondary battery with a depth of discharge (DOD) of 90% or more. The depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity in a fully charged battery. Note that the amount of electricity charged (i.e., the fully charged amount) when a battery in a fully discharged state (DOD = 100%) is charged to a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-discharge voltage.

[0035] The first element is an element other than a non-metallic element, and includes a metallic element and a so-called metalloid element. In particular, the first element preferably includes at least one element selected from the group consisting of Groups 3, 4, 5, and 6 of the periodic table, because this element has a significant effect of improving safety. In particular, the first element preferably includes at least one element selected from the group consisting of Al, Ti, Si, Zr, Mg, Nb, Ta, Sn, Ni, and Cr.

[0036] When the oxide coating contains two or more types of oxides, the oxides may be mixed together or may be arranged in layers.

[0037] Fig. 1 is a cross-sectional view schematically showing a main portion of a positive electrode according to an embodiment of the present disclosure. Fig. 2 is a schematic cross-sectional view further enlarging the main portion of the positive electrode shown in Fig. 1. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode active material layer 12 includes active material particles 20 having an oxide coating. The active material particles 20 having an oxide coating include active material particles 23 and an oxide coating 27 covering at least a portion of the surface of the active material particles 23.

[0038] A method for calculating the thickness Tb of the oxide coating 27 that covers the active material particles 23 located 0.10 TA from the surface of the positive electrode current collector 11 in the positive electrode active material layer 12 will be described with reference to Fig. 2. For convenience, Fig. 2 shows only two active material particles 20 with oxide coatings. The thickness Tt can also be calculated in a similar manner.

[0039] When the thickness of the positive electrode active material layer is designated as TA, ten active material particles 23 with a maximum diameter of 5 μm or greater are selected, each of which partially overlaps a line drawn from the surface of the positive electrode current collector at a position 0.10 TA. For each particle, the thickness of the oxide coating is measured at one or two intersections of the line and the outer edge of the active material particle 23 (T11, T12, T13, T14, ...). The average thickness at up to 20 of these points is calculated. After calculating this average, data that differ from the obtained average by 20% or more are excluded, and the average is calculated again. This corrected average is designated as the thickness Tb of the first coating at the 0.10 TA point.

[0040] B. Secondary Battery A secondary battery according to an embodiment of the present disclosure includes the above-described positive electrode, negative electrode, non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode. The secondary battery may be a liquid secondary battery containing an electrolytic solution as the non-aqueous electrolyte, or may be an all-solid-state secondary battery containing a solid electrolyte as the non-aqueous electrolyte.

[0041] These configurations will be specifically described below using a lithium ion secondary battery according to an embodiment of the present disclosure as an example.

[0042] [Positive Electrode] The positive electrode has the above-described characteristics. The positive electrode active material layer may be composed of a positive electrode mixture. The positive electrode mixture contains active material particles (particles of positive electrode active material) as an essential component, and may contain a binder, a thickener, etc. as optional components.

[0043] The positive electrode active material layer may have a multilayer structure having a first positive electrode mixture layer and one or more other positive electrode mixture layers that are different in form from each other. When the first positive electrode mixture layer is closest to the positive electrode current collector, the thickness of the first positive electrode mixture layer may be 10 μm or more and 40 μm or less, or 10 μm or more and 30 μm or less. Examples of different forms include cases where the particle diameters of the active material particles are different, or cases where the type or composition of the positive electrode active material is different. Whether the positive electrode active material layer has a multilayer structure having a first positive electrode mixture layer and one or more other positive electrode mixture layers that are different in form from each other can be easily determined by observing a cross section in the thickness direction obtained by simultaneously cutting the positive electrode active material layer and the positive electrode current collector.

[0044] The positive electrode active material layer may include first active material particles having a first average particle size D1 and second active material particles having a second average particle size D2 (D1 > D2). In this case, the first positive electrode mixture layer and one or more other positive electrode mixture layers may contain the first active material particles and the second active material particles in different proportions. For example, 60% by mass or more or 80% by mass or more of the active material particles contained in the first positive electrode mixture layer may be the first active material particles. For example, 60% by mass or more or 80% by mass or more of the active material particles contained in one or more positive electrode mixture layers other than the first positive electrode mixture layer may be the second active material particles. For example, when the positive electrode active material layer has a two-layer structure including a first positive electrode mixture layer on the positive electrode current collector side and a second positive electrode mixture layer on top of the first positive electrode mixture layer, 80% by mass or more of the active material particles contained in the first positive electrode mixture layer may be the first active material particles, and 80% by mass or more of the active material particles contained in the second positive electrode mixture layer may be the second active material particles.

[0045] The first average particle diameter D1 and the second average particle diameter D2 can be measured from a cross-section in the thickness direction obtained by simultaneously cutting the positive electrode mixture layer and the positive electrode current collector. The cross-section may be formed using a cross-section polisher (CP). At this time, a thermosetting resin may be filled into the positive electrode mixture layer and cured. Next, a scanning electron microscope (SEM) photograph (hereinafter, SEM image) of the cross-section is taken. The SEM image is taken so that at least 10 first positive electrode active material particles and at least 10 second positive electrode active material particles are observed. First circle-equivalent diameters of the cross-sections of at least 10 first positive electrode active material particles are determined by image processing, and their average value is calculated as D1. Second circle-equivalent diameters of the cross-sections of at least 10 second positive electrode active material particles are determined by image processing, and their average value is calculated as D2. Here, the circle-equivalent diameter refers to the diameter of a circle having the same area as the cross-section of the particle (the area of ​​the particle observed in the cross-section of the positive electrode mixture layer).

[0046] When the first positive electrode active material particles and the second positive electrode active material particles can be separated and recovered from the positive electrode mixture layer, the median diameters (particle diameters at 50% cumulative volume) in the volume-based particle size distributions of the first positive electrode active material particles and the second positive electrode active material particles may be determined as D1 and D2, respectively. The volume-based particle size distributions can be measured by a laser diffraction scattering method.

[0047] The positive electrode active material constituting the active material particles contains a lithium-containing transition metal oxide. From the viewpoint of increasing capacity, the positive electrode active material may contain a lithium-nickel composite oxide (composite oxide N) containing at least nickel as a transition metal. The proportion of the composite oxide N in the positive electrode active material is, for example, 70% by mass or more, or 90% by mass or more, or even 95% by mass or more.

[0048] The composite oxide N may be a lithium transition metal oxide containing lithium and Ni and having a layered rock salt crystal structure. The proportion of Ni in the metal elements other than Li contained in the lithium transition metal oxide may be 50 atomic % or more. The lithium transition metal oxide may contain Co, but from the viewpoint of cost reduction and high capacity, the proportion of Co in the metal elements other than Li contained in the lithium transition metal oxide is preferably 0 atomic % or more and 20 atomic % or less, and more preferably 0 atomic % or more and 15 atomic % or less.

[0049] In general, it is believed that a composite oxide N in which the proportion of Ni among metal elements other than Li is 50 atomic % or more is likely to undergo a change in crystal structure with repeated charge and discharge, and thus to become highly resistive. In this embodiment, since at least a portion of the surface of the active material particles is covered and protected by an oxide film, it is believed that such a high resistance is also suppressed.

[0050] The composite oxide N contains Ni and may contain at least one element selected from the group consisting of Co, Mn, and Al. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide N.

[0051] The proportion of Mn in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less, or 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0052] The proportion of Al in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less. The proportion of Al in the metal elements other than Li may be 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0053] The composite oxide N is, for example, a compound represented by the formula: Li αNi(1-x1-x2-yz)Co ｘ１ Mn ｘ２ Al ｙ M ｚ O ２＋β The element M is an element other than Li, Ni, Co, Mn, Al, and oxygen. α, which indicates the atomic ratio of lithium, is, for example, 0.95≦α≦1.05, and increases or decreases with charge and discharge. In (2+β), which indicates the atomic ratio of oxygen, β satisfies −0.05≦β≦0.05. 1-x1-x2-y-z (=v), which indicates the atomic ratio of Ni, is, for example, 0.8 or more, or may be 0.85 or more, 0.90 or more, or 0.95 or more. Furthermore, v, which indicates the atomic ratio of Ni, may be 0.98 or less, or may be 0.95 or less.

[0054] The x1, which indicates the atomic ratio of Co, is, for example, 0.1 or less (0≦x1≦0.1), and may be 0.08 or less, 0.05 or less, or 0.01 or less. When x1 is 0, this includes cases where Co is below the detection limit.

[0055] The atomic ratio x2 of Mn is, for example, 0.1 or less (0≦x2≦0.1), and may be 0.08 or less, 0.05 or less, or 0.03 or less. x2 may be 0.01 or more, or 0.03 or more.

[0056] The atomic ratio y of Al is, for example, 0.1 or less (0≦y≦0.1), may be 0.08 or less, 0.05 or less, or 0.03 or less. y may be 0.01 or more, or 0.03 or more.

[0057] The atomic ratio z of the element M is, for example, 0≦z≦0.10, may be 0<z≦0.05, or may be 0.001≦z≦0.01.

[0058] The element M may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y.

[0059] The composite oxide N is, for example, a secondary particle formed by aggregation of a plurality of primary particles. The average particle size of the secondary particles of the composite oxide N contained in the entire positive electrode active material is, for example, 3 μm or more and 30 μm or less, and may be 5 μm or more and 25 μm or less.

[0060] The average particle size of secondary particles refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. Such a particle size is sometimes referred to as D50. For example, an "LA-750" manufactured by Horiba Ltd. can be used as a measuring device.

[0061] When the positive electrode active material layer includes first active material particles having a first average particle size D1 and second active material particles having a second average particle size D2 (D1>D2), for example, D1 is 10 μm or more, or may be 11 μm or more, 12 μm or more, or 15 μm or more. Furthermore, D1 may be 30 μm or less, or may be 25 μm or less. Meanwhile, D2 may be less than 10 μm, 8 μm or less, 6 μm or less, or 5 μm or less. From the viewpoint of improving charge / discharge cycle characteristics, D2 may be 1 μm or more, or may be 3 μm or more. The D1 / D2 ratio may be, for example, 2 or more and 6 or less, or 3 or more and 5 or less.

[0062] As the binder for the positive electrode, for example, a resin material is used. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, etc. One type of binder may be used alone, or two or more types may be used in combination.

[0063] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0064] The dispersion medium used for the positive electrode slurry is not particularly limited, but examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0065] Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0066] [Negative Electrode] The negative electrode may have, for example, a negative electrode current collector and may include a negative electrode active material layer. The negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0067] The negative electrode active material layer may be a negative electrode mixture layer composed of a negative electrode mixture. The negative electrode mixture layer is in the form of a membrane or film. The negative electrode mixture contains particles of the negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector as the negative electrode active material layer.

[0068] The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry, which is a negative electrode mixture containing particles of a negative electrode active material, a binder, etc., dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0069] Negative electrode active materials include materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. Materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide, tin oxide, etc. may also be used.

[0070] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of lithium ion conductive phases that can be used include a silicon oxide phase, a silicate phase, and a carbon phase. The silicon oxide phase may contain silicon dioxide as the main component (e.g., 95 to 100% by mass). Among these, a composite material composed of a silicate phase and silicon particles dispersed in the silicate phase is preferred because it has a high capacity and a small irreversible capacity. Furthermore, a lithium silicate phase (a silicate phase containing lithium) is preferred because it has a small irreversible capacity and a high initial charge / discharge efficiency.

[0071] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0072] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0073] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0074] The binder may be at least one selected from the group consisting of polyacrylic acid, polyacrylic acid salts, and derivatives thereof. As the polyacrylic acid salts, Li salts or Na salts are preferably used. Among them, cross-linked lithium polyacrylate is preferably used.

[0075] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0076] Examples of thickeners include carboxymethyl cellulose (CMC) and modified products thereof (including salts such as the Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (polyalkylene oxides such as polyethylene oxide, etc.).

[0077] [Separator] The separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulation. Examples of the separator include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is made of a polyolefin such as polypropylene or polyethylene. The separator may have a heat-resistant insulating layer on at least one surface layer. The heat-resistant insulating layer may contain an inorganic oxide filler as a main component (e.g., 80% by mass or more) or a heat-resistant resin as a main component (e.g., 40% by mass or more). The heat-resistant resin may be a polyamide resin such as aromatic polyamide (aramid), a polyimide resin, or a polyamideimide resin.

[0078] [Non-aqueous electrolyte] The non-aqueous electrolyte contains, for example, a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less. The non-aqueous electrolyte may contain known additives.

[0079] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0080] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2 etc.), lithium salts of fluorine-containing acid imides (LiN(SO ２ F) ２ , LiN(CFSO)2, LiN(CFSO)(CFSO), LiN(CFSO)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salt may be used alone or in combination of two or more.

[0081] An example of the structure of a lithium-ion secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0082] Hereinafter, the structure of a prismatic non-aqueous secondary battery will be described as an example of the lithium ion secondary battery according to the present invention with reference to FIG.

[0083] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and an electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0084] <Method for manufacturing positive electrode> A method for manufacturing a positive electrode according to an embodiment of the present disclosure includes the steps of preparing active material particles containing a lithium-containing transition metal oxide, preparing a positive electrode current collector, and forming a positive electrode active material layer containing the active material particles on the surface of the positive electrode current collector.

[0085] The process of forming the positive electrode active material layer includes a supporting process of supporting active material particles on the surface of a positive electrode current collector to form a precursor layer, a rolling process of rolling the precursor layer, and a coating process of exposing the active material particles to a gas phase containing a first element other than non-metallic elements after the rolling process to form an oxide coating so as to cover at least a portion of the surface of the active material particles.

[0086] (I) Supporting Step (S1) The precursor layer can be formed by applying a positive electrode slurry, in which the components of the positive electrode mixture are dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the slurry. The positive electrode mixture contains active material particles (particles of the positive electrode active material) as an essential component and may contain a binder, a thickener, etc. as optional components.

[0087] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0088] (II) Rolling step (S2) The dried coating film of the positive electrode slurry (i.e., precursor layer) is rolled. The conditions for rolling are not particularly limited. The rolling is performed when the density of the positive electrode active material in the precursor layer is 2.5 g / cm ３ 4.0g / cm or more ３ Preferably 2.9 g / cm or less ３ 3.7g / cm or more ３ It is preferable to perform the rolling until the oxide film thickness reaches or exceeds 1000 nm. In the rolling, it is desirable to set conditions that mainly densify the surface layer portion of the precursor layer. This allows the gas phase containing the first element to come into contact with more of the active material particles present near the outer surface of the positive electrode active material layer in the coating formation step described below. Therefore, a thicker oxide film is more likely to be formed on the active material particles that are relatively far from the surface of the positive electrode current collector.

[0089] (III) Oxide Coating Formation Step (S3) Next, the active material particles supported on the positive electrode current collector are exposed to a gas phase containing the first element, thereby forming an oxide coating containing an oxide of the first element on at least a portion of the surface of the active material particles.

[0090] Examples of vapor-phase methods include CVD, ALD, and physical vapor deposition (PVD). ALD is particularly preferred because it can form an oxide film at a relatively low temperature. ALD can form an oxide film in an atmosphere of 200° C. or less.

[0091] In the ALD method, an organometallic compound (precursor) containing a first element is used as a raw material for the oxide film. In the ALD method, a vaporized precursor (source gas) and an oxidizing agent are alternately supplied to a reaction chamber in which a target object is placed. This results in the formation of an oxide film of the first element on the surface of the target object.

[0092] In the ALD method, the first element is deposited on the surface of the target object in atomic layers due to a self-limiting mechanism. In the ALD method, the overall thickness of the oxide film is controlled by the number of cycles, which is defined as one cycle of supplying (pulsing) the source gas, exhausting (purging) the source gas, supplying (pulsing) the oxidant, and exhausting (purging) the oxidant.

[0093] The precursor is an organometallic compound containing the first element, and various organometallic compounds that have been conventionally used in the ALD method can be used as the precursor.

[0094] Examples of precursors containing Ti include bis(t-butylcyclopentadienyl)titanium(IV) dichloride (C １８ H ２６ C ｌ２ Ti), tetrakis(dimethylamino)titanium(IV) ([(CH ３ ) ２ N] ４ Ti, TDMAT), tetrakis(diethylamino)titanium(IV) ([(C ２ H ５ ) ２ N] ４ Ti), tetrakis(ethylmethylamino)titanium(IV) (Ti[N(C ２ H ５ ) (CH ３ )] ４ ), titanium(IV) (diisopropoxide-bis(2,2,6,6-tetramethyl-3,5-heptanedionate) (Ti[OCC(CH ３ ) ３ CHCOC (CH ３ ) ３ ] ２ (OC ３ H ７ ) ２ ), titanium tetrachloride (TiCl ４ ), titanium(IV) isopropoxide (Ti[OCH(CH ３ ) ２ ] ４ ), titanium(IV) ethoxide (Ti[O(C ２ H ５ )] ４ ) is an example of a precursor containing Al. ３ ) ３ Al, TMA).

[0095] The source gas may contain multiple types of precursors. Different types of precursors may be supplied to the reaction chamber simultaneously or sequentially. Alternatively, the type of precursor contained in the source gas may be changed for each cycle.

[0096] The oxidizing agent may be any oxidizing agent that has been conventionally used in the ALD method. Examples of the oxidizing agent include water, oxygen, ozone, etc. The oxidizing agent may be supplied to the reaction chamber as plasma using the oxidizing agent as a raw material.

[0097] The conditions for the ALD method are not particularly limited. In order to facilitate the formation of a thicker oxide film on the active material particles near the positive electrode current collector, the temperature of the atmosphere containing the precursor or oxidizing agent may be 10° C. or higher and 200° C. or lower, 25° C. or higher and 200° C. or lower, 100° C. or higher and 200° C. or lower, or 120° C. or higher and 200° C. or lower. From the same viewpoint, the pressure in the reaction chamber during the treatment may be 1×10 －５ Pa or more 1×10 ５ Pa or less, and －４ Pa or more 1×10 ４ Pa or less.

[0098] The temperature of the atmosphere containing the precursor or oxidizing agent in the reaction chamber is 10° C. or higher and 200° C. or lower (for example, 120° C. or higher and 200° C. or lower), and the pressure in the reaction chamber during the treatment is 1×10 －５ Pa or more 1×10 ５ When the pressure is 0.01 Pa or less, the pulse time of the source gas may be 0.01 seconds or more, or 0.05 seconds or more. The pulse time of the source gas may be 5 seconds or less, or 3 seconds or less.

[0099] After forming the oxide film, the positive electrode active material layer may be further rolled. The conditions for rolling are not particularly limited, and may be appropriately set so that the positive electrode active material layer has a predetermined thickness or density.

[0100] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0101] Example 1 [Preparation of Positive Electrode] The positive electrode active material constituting the active material particles is a composite oxide N (LiNi) having a layered rock salt type crystal structure and containing lithium and Ni. 0.85 Co 0.10 Al 0.05O2) was used. Furthermore, an aluminum foil having a thickness of 15 μm was prepared as a positive electrode current collector.

[0102] The median diameter D1 in the volume-based particle size distribution of the active material particles measured by a laser diffraction scattering method was 13 μm.

[0103] NMP was added to a positive electrode mixture containing active material particles (D1=13 μm), acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5, and the mixture was stirred to prepare a positive electrode slurry.

[0104] (I) Supporting Step (S1) The positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried to form a precursor layer of the positive electrode active material layer on both sides of the aluminum foil.

[0105] (II) Rolling step (S2) Next, the dried precursor layer is rolled to reduce the density of the positive electrode active material of the precursor of the positive electrode active material layer after rolling to 3.65 g / cm ３ The thickness of the entire positive electrode after rolling was 160 μm.

[0106] (III) Oxide Coating Formation Step (S3) The laminate (positive electrode precursor) of the positive electrode current collector and the precursor layer was placed in a predetermined reaction chamber, and a portion of the active material particles in the precursor layer was coated with an oxide coating by the ALD method according to the following procedure.

[0107] Tetrakis(dimethylamino)titanium(IV) (TDMAT), a precursor serving as a source of the first element (Ti), was vaporized and supplied to the reaction chamber containing the positive electrode precursor. The pulse time was 0.1 seconds. The temperature of the atmosphere containing the precursor in the reaction chamber was controlled to 200°C, and the pressure was controlled to 260 Pa. After 30 seconds, the surface of the positive electrode precursor was assumed to be covered with a monolayer of the precursor, and excess precursor was purged with nitrogen gas.

[0108] Next, an oxidizing agent (H ２ O) was vaporized and supplied. The pulse time was 0.015 seconds. The temperature of the atmosphere containing the oxidizing agent was controlled to 200°C, and the pressure was controlled to 260 Pa. After 30 seconds, excess oxidizing agent was purged with nitrogen gas.

[0109] A series of operations (ALD cycle) consisting of supplying a precursor, purging, supplying an oxidizing agent, and purging was repeated 200 times to form an oxide film containing titanium.

[0110] The oxide coating was analyzed by SEM, EDS, ICP, etc. The oxide coating contained Ti. The thickness Tb of the oxide coating covering the active material particles located 0.10 TA from the surface of the positive electrode current collector in the positive electrode active material layer was 6 nm. The thickness Tt of the oxide coating covering the active material particles located 0.90 TA from the surface of the positive electrode current collector in the positive electrode active material layer was 30 nm. The average thickness Ta of the oxide coating was 18 nm. The Pb / Pt ≒ Tb / Tt = 0.2 as determined by EPMA analysis.

[0111] [Fabrication of Negative Electrode] A negative electrode mixture containing graphite, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) as negative electrode active materials in a mass ratio of 96:2:2 was added with water and stirred to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of copper foil as a negative electrode current collector, and the coating was dried and then rolled to form a negative electrode active material layer on both sides of the copper foil. The density of the negative electrode active material in the negative electrode active material layer was 1.6 g / cm ３ The total thickness of the negative electrode was 170 μm.

[0112] [Preparation of Nonaqueous Electrolyte] A nonaqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0113] [Preparation of Secondary Battery] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween so that the tabs were positioned at the outermost periphery to prepare an electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film, and vacuum dried at 105°C for 2 hours. After that, an electrolyte solution was poured into the exterior case, and the opening of the exterior case was sealed to obtain a secondary battery A1.

[0114] Example 2 A positive electrode and a secondary battery A2 were fabricated in the same manner as in Example 1, except that in the oxide coating formation step S3, the temperature of the atmosphere containing the precursor and the oxidizing agent in the reaction chamber was changed to 180° C. The thicknesses Tb, Tt, and Ta of the oxide coating were 12 nm, 30 nm, and 21 nm, respectively.

[0115] Example 3 A positive electrode and a secondary battery A3 were fabricated in the same manner as in Example 1, except that in the oxide coating formation step S3, the temperature of the atmosphere containing the precursor and the oxidizing agent in the reaction chamber was changed to 150° C. The thicknesses Tb, Tt, and Ta of the oxide coating were 18 nm, 30 nm, and 24 nm, respectively.

[0116] Example 4 A positive electrode and a secondary battery A4 were fabricated in the same manner as in Example 1, except that in the oxide coating formation step S3, the temperature of the atmosphere containing the precursor and the oxidizing agent in the reaction chamber was changed to 120° C. The thicknesses Tb, Tt, and Ta of the oxide coating were 24 nm, 30 nm, and 27 nm, respectively.

[0117] Example 5 A positive electrode and a secondary battery A5 were fabricated in the same manner as in Example 1, except that the ALD cycle was repeated 100 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 3 nm, 15 nm, and 9 nm, respectively.

[0118] Example 6 A positive electrode and a secondary battery A6 were fabricated in the same manner as in Example 1, except that the ALD cycle was repeated 60 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 2 nm, 10 nm, and 6 nm, respectively.

[0119] Example 7 A positive electrode and a secondary battery A7 were fabricated in the same manner as in Example 1, except that the ALD cycle was repeated 20 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 1 nm, 5 nm, and 3 nm, respectively.

[0120] Comparative Example 1 Except for switching the order of the rolling step S2 and the oxide coating film forming step S3, a positive electrode and a secondary battery B1 were fabricated in the same manner as in Example 1. The thicknesses Tb, Tt, and Ta of the oxide coating film were 30 nm, 30 nm, and 30 nm, respectively.

[0121] Comparative Example 2 A positive electrode and a secondary battery B2 were fabricated in the same manner as in Comparative Example 1, except that the ALD cycle was repeated 100 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 15 nm, 15 nm, and 15 nm, respectively.

[0122] Comparative Example 3 A positive electrode and a secondary battery B3 were fabricated in the same manner as in Comparative Example 1, except that the ALD cycle was repeated 60 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 10 nm, 10 nm, and 10 nm, respectively.

[0123] Comparative Example 4 A positive electrode and a secondary battery B4 were fabricated in the same manner as in Comparative Example 1, except that the ALD cycle was repeated 20 times in the oxide coating formation step S3. The thicknesses Tb, Tt, and Ta of the oxide coating were 5 nm, 5 nm, and 5 nm, respectively.

[0124] Comparative Example 5 A positive electrode and a secondary battery B5 were fabricated in the same manner as in Comparative Example 1, except that the oxide film forming step S3 was not performed.

[0125] [Evaluation] The secondary batteries obtained in the examples and comparative examples were evaluated as follows. (1) Direct current resistance (DCIR) In a temperature environment of 25°C, the battery was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It. Next, the battery was discharged at a constant current of 0.3 It for 100 minutes to bring the state of charge (SOC) to 50%.

[0126] The voltage value was measured when a battery with an SOC of 50% was discharged for 10 seconds at current values ​​of 0 A, 0.1 A, 0.5 A, and 1.0 A. The DCIR (initial DCIR) was calculated from the absolute value of the slope when the relationship between the discharge current value and the voltage value after 10 seconds was approximated to a straight line using the least squares method. Table 1 shows the relative values ​​when the DCIR of battery B5 of Comparative Example 5 is set to 100%. The larger the relative value, the greater the resistance.

[0127] (2) Nail Penetration Test (a) In an environment of 25°C, the battery was charged at a constant current of 0.3 It until the battery voltage reached 4.2 V, and then continued to be charged at a constant voltage until the current value reached 0.05 It. (b) In an environment of 25°C, the tip of a round nail (diameter 2.7 mm) was contacted with the center of the battery charged in (a) and pierced at a speed of 1 mm / sec. Immediately after detecting a battery voltage drop (Δ50 mV) due to an internal short circuit, the nail penetration was stopped. The surface temperature of the battery was measured 1 minute after the battery shorted.

[0128]

[0129] Table 1 shows that for Batteries A1 to A7, it was possible to suppress an increase in resistance while improving safety in the event of an internal short circuit. For Batteries B1 to B3, it was possible to improve safety in the event of an internal short circuit, but it was difficult to suppress an increase in resistance. For Batteries B4 and B5, it was possible to suppress an increase in resistance, but it was difficult to improve safety in the event of an internal short circuit.

[0130] The positive electrode for a secondary battery according to the present disclosure and a secondary battery including the same are useful as main power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0131] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0132] REFERENCE SIGNS LIST 1 electrode group 2 positive electrode lead 3 negative electrode lead 4 battery case 5 sealing plate 6 negative electrode terminal 7 gasket 8 sealing plug 10 positive electrode 11 positive electrode current collector 12 positive electrode active material layer 20 active material particle having oxide coating 23 active material particle 27 oxide coating

Claims

1. A positive electrode current collector, and a positive electrode active material layer supported on the positive electrode current collector, The positive electrode active material layer includes active material particles and an oxide film covering at least a part of the surface of the active material particles, The active material particles include a lithium-containing transition metal oxide, The oxide film includes an oxide of a first element other than a non-metallic element, When the thickness of the positive electrode active material layer is TA, The thickness Tb of the oxide film or the existence probability Pb of the first element at a position 0.10TA from the surface of the positive electrode current collector of the positive electrode active material layer, The thickness Tt of the oxide film or the existence probability Pt of the first element at a position 0.90TA from the surface of the positive electrode current collector of the positive electrode active material layer, A positive electrode for a secondary battery that satisfies Tb < Tt or Pb < Pt.

2. The thickness Tb and the thickness Tt, or the existence probability Pb and the existence probability Pt, The positive electrode for a secondary battery according to claim 1, which satisfies 0 ≦ Tb / Tt < 1 or 0 ≦ Pb / Pt < 1.

3. The positive electrode for a secondary battery according to claim 1 or 2, wherein the first element includes at least one selected from the group consisting of Group 3 elements, Group 4 elements, Group 5 elements, and Group 6 elements of the periodic table.

4. The positive electrode for a secondary battery according to claim 1 or 2, wherein the first element includes at least one selected from the group consisting of Al, Ti, Si, Zr, Mg, Nb, Ta, Sn, Ni, and Cr.

5. The positive electrode for a secondary battery according to claim 1 or 2, wherein the average thickness Ta of the oxide film is 0.1 nm or more and 50 nm or less.

6. A secondary battery including the positive electrode for a secondary battery according to claim 1 or 2, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode.

7. A step of preparing active material particles containing a lithium-containing transition metal oxide; A step of preparing a positive electrode current collector; A step of forming a positive electrode active material layer containing the active material particles on the surface of the positive electrode current collector; comprising: The step of forming the positive electrode active material layer includes: A supporting step of supporting the active material particles on the surface of the positive electrode current collector to form a precursor layer; A rolling step of rolling the precursor layer; After the rolling step, a film forming step of exposing the active material particles to a gas phase containing a first element other than a non-metal element to form an oxide film so as to cover at least a part of the surface of the active material particles; comprising: The oxide film contains an oxide of the first element; When the thickness of the positive electrode active material layer is TA, The thickness Tb of the oxide film or the presence probability Pb of the first element at a position 0.10TA from the surface of the positive electrode current collector of the positive electrode active material layer, and The thickness Tt of the oxide film or the presence probability Pt of the first element at a position 0.90TA from the surface of the positive electrode current collector of the positive electrode active material layer are A method for manufacturing a positive electrode for a secondary battery, satisfying Tb < Tt or Pb < Pt.

8. The method for manufacturing a positive electrode for a secondary battery according to claim 7, wherein the film forming step is performed by an atomic layer deposition method.