Positive electrode for secondary battery and method for manufacturing the 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

[Technical Field]

[0001] This disclosure primarily relates to positive electrodes for secondary batteries. [Background technology]

[0002] Patent Document 1 describes a device comprising a positive electrode, a non-aqueous electrolyte, and a negative electrode, wherein the positive electrode comprises a conductive material made of carbon material, deposits of aluminum oxide scattered on the surface of the conductive material, and an upper limit potential of 4.5V (vs.Li / Li) for the oxidation-reduction potential of metallic lithium. + A lithium-ion battery is proposed that includes a lithium-containing oxide active material having a particle size of 1 nm or more, wherein the aluminum oxide deposit has a maximum diameter of 1 nm or more and 20 nm or less.

[0003] Patent Document 2 comprises a positive electrode current collector and a positive electrode active material layer containing a positive electrode material provided on the surface of the positive electrode current collector, wherein the positive electrode material comprises positive electrode active material particles, a first coating containing an oxide X of metal element M1 adhering to the surface of the positive electrode active material particles, and a second coating having lithium ion permeability adhering to the surface of the first coating, wherein the second coating is Li x M2O y We propose a positive electrode for secondary batteries that contains an oxide Y represented by (0.5 ≤ x < 4, 1 ≤ y < 6), and M2 is at least one selected from the group consisting of B, Al, Si, P, S, Ti, V, Zr, Nb, Ta, and La. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-174612 [Patent Document 2] International Publication No. 2017-174612 [Overview of the project] [Problems that the invention aims to solve]

[0005] Patent Document 1 improves the capacity of a lithium-ion battery by suppressing the decomposition of a non-aqueous electrolyte on the surface of a conductive material made of a carbon material by adhering aluminum oxide to the surface of the conductive material. However, the deposits of aluminum oxide scattered on the surface of the conductive material cause the disadvantage of increasing the resistance of the positive electrode. In Patent Document 2 as well, the first coating and the second coating can increase the resistance of the positive electrode.

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

Means for Solving the Problem

[0007] One aspect of the present disclosure relates to a positive electrode for a secondary battery, which includes 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-metal element. When the thickness of the positive electrode active material layer is TA, the thickness Tb or the presence probability Pb of the first element of the oxide film 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 or the presence probability Pt of the first element of the oxide film at a position 0.90TA from the surface of the positive electrode current collector of 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 positive electrode for a secondary battery, 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 includes a step of preparing active material particles containing a lithium-containing transition metal oxide, a step of preparing a positive electrode current collector, and a step of forming a positive electrode active material layer containing the active material particles on the surface of the positive electrode current collector. The step of forming the positive electrode active material layer includes a loading step of loading 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, and a film forming step of exposing the active material particles to a gas phase containing a first element other than a non-metal element after the rolling step to form an oxide film so as to cover at least a part of the surface of the active material particles. 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 satisfy Tb < Tt or Pb < Pt. The present disclosure relates to a method for manufacturing a positive electrode for a secondary battery.

Advantages of the Invention

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

[0011] The novel features of the present invention are described in the appended claims. However, the present invention will be better understood from the following detailed description in conjunction with the drawings, in terms of both its structure and content, along with other objects and features of the present invention.

Brief Description of the Drawings

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

Embodiments for Carrying Out the Invention

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

[0014] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0015] In the following explanation, the terms "contains" or "includes" encompass expressions such as "contains (or includes)," "substantially consists of," and "consists of."

[0016] Secondary batteries include non-aqueous electrolyte secondary batteries such as lithium-ion batteries and lithium metal secondary batteries.

[0017] A. Positive electrode for secondary batteries The positive electrode for a secondary battery according to the embodiment of this disclosure comprises 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 composed of a sheet-like conductive material. As the positive electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. [Cathode 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 composed of a positive electrode mixture, and is in the form of a film or membrane. The positive electrode mixture contains active material particles (particles of positive electrode active material) as an essential component.

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

[0020] [Oxide film] The oxide film covers at least a portion of the surface of the active material particles, which are secondary particles. The oxide film contains an oxide of a primary element other than a nonmetallic element. The oxide of the primary element other than a nonmetallic element has a different crystalline structure from the active material particles or is amorphous. The oxide of the primary element other than a nonmetallic element is electrochemically inert and does not need to exhibit substantial capacity. Such an oxide film has the effect of suppressing the increase in resistance of the positive electrode while increasing safety in the event of an internal short circuit in the battery. When an internal short circuit occurs, current concentrates and flows 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 rises due to the heat generation, the resistance of the active material particles tends to drop sharply. On the other hand, the oxide film covering at least a portion of the surface of the active material particles does not experience a sharp drop in resistance even when the temperature rises due to heat generation, and acts as a resistive component that suppresses the 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 film at the 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 at the position 0.90TA from the surface of the positive electrode current collector of the positive electrode active material layer satisfy Tb < Tt. For example, the oxide film may be formed thicker on the surface of the active material particles as it is farther from the surface of the positive electrode current collector. At this time, the existence probability Pb of the first element at the position 0.10TA and the existence probability Pt of the first element at the position 0.90TA satisfy Pb < Pt.

[0022] By making the oxide film have the above-mentioned distribution in thickness, an increase in the resistance of the positive electrode can be suppressed. When relatively reducing the resistance component covering the active material particles close to the surface of the positive electrode current collector, the current collection property from the active material particles to the positive electrode current collector is not significantly impaired. That is, the oxide film hardly acts as a resistance component of the positive electrode during normal use of the battery. On the other hand, when an internal short circuit occurs, the resistance of the oxide film compensates for the resistance of the active material particles with a sharp decrease, 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. The position 0.10TA from the surface of the positive electrode current collector of the positive electrode active material layer is synonymous with the position 0.10TA from the interface between the positive electrode active material layer and the positive electrode current collector. The position 0.90TA from the surface of the positive electrode current collector of the positive electrode active material layer is synonymous with the position 0.90TA from the interface between the positive electrode active material layer and the positive electrode current collector.

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

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

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

[0027] The thickness of the oxide film may vary such that it increases from the surface of the positive electrode current collector outwards. This change may be continuous, stepwise, or to the extent that it can be observed as an overall trend.

[0028] For example, active material particles are selected from multiple locations (e.g., 5 locations) at different distances from the surface of the positive electrode current collector, which lie on a straight line in the thickness direction of the positive electrode active material layer. The thickness of the oxide film is then measured at one or two intersections of this line and the outer edge of the active material particles. Multiple straight lines (e.g., 5 lines) are drawn in the thickness direction of the positive electrode active material layer, and the thickness of the oxide film is measured in the same manner. The film thickness calculated in this way is plotted on a graph where the horizontal axis is the distance from the surface of the positive electrode current collector and the vertical axis is the film thickness. From this graph, if the approximate straight line or approximate curve obtained by the least squares method is upward sloping, it can be determined that, as an overall trend, the thickness of the oxide film increases as you move outward from the surface of the positive electrode current collector.

[0029] The thickness Tb of the oxide film located 0.10 TA from the surface of the positive electrode current collector in the positive electrode active material layer is not particularly limited. From the viewpoint of improving safety during internal short circuits, 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 the increase in positive electrode resistance and lithium ion diffusion, the thickness Tb of the oxide film may be 50 nm or less, 10 nm or less, or 2 nm or less. For example, the thickness Tb of the oxide film may be 0.1 nm or more and 50 nm or 0.1 nm or more and 10 nm or less.

[0030] The thickness Tt of the oxide film located at a position 0.90 TA from the surface of the positive electrode current collector in the positive electrode active material layer is not particularly limited. From the viewpoint of suppressing the increase in positive electrode resistance and lithium ion diffusion, 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 during internal short circuits, the thickness Tt may be 0.1 nm or more, 0.5 nm or more, or 1 nm or more. For example, the thickness Tt of the oxide film may be 0.1 nm or more and 50 nm or 0.1 nm or more and 30 nm or less.

[0031] The average thickness Ta of the oxide film is not particularly limited. From the viewpoint of improving safety during internal short circuits, the average thickness Ta of the oxide film may be 0.1 nm or more, 0.5 nm or more, or 1 nm or more. From the viewpoint of suppressing the increase in positive electrode resistance and lithium ion diffusion, the average thickness Ta of the oxide film may be 50 nm or less, 10 nm or less, or 2 nm or less. For example, the average thickness Ta of the oxide film is 0.1 nm or more and 50 nm or less.

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

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

[0034] The positive electrode whose thickness Tb and Tt, or existence probability Pb ​​and Pt, is to be measured may be taken from a secondary battery with a depth of discharge (DOD) of 90% or more. Depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity held in a fully charged battery. Note that the amount of electricity charged when charging a fully discharged battery (DOD=100%) to a fully charged state (SOC=100%, DOD=0%) corresponds to the rated capacity. The voltage of a fully charged battery corresponds to the charge termination voltage. The voltage of a fully discharged battery corresponds to the discharge termination voltage.

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

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

[0037] Figure 1 is a schematic cross-sectional view showing the main part of a positive electrode according to one embodiment of the present disclosure. Figure 2 is a schematic cross-sectional view showing a further enlarged view of the main part of the positive electrode shown in Figure 1. The positive electrode 10 comprises 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 film. The active material particles 20 having an oxide film comprises active material particles 23 and an oxide film 27 covering at least a portion of their surface.

[0038] The method for calculating the thickness Tb of the oxide film 27 covering the active material particles 23 located 0.10TA from the surface of the positive electrode current collector 11 in the positive electrode active material layer 12 will be explained with reference to Figure 2. For convenience, only two active material particles 20 with oxide films are shown in Figure 2. The thickness Tt can be calculated in the same manner.

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

[0040] B. Secondary battery The secondary battery according to the embodiment of this disclosure comprises the 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-type secondary battery containing an electrolyte as the non-aqueous electrolyte, or an all-solid-state secondary battery containing a solid electrolyte as the non-aqueous electrolyte.

[0041] The following describes these configurations in detail, using a lithium-ion secondary battery according to the embodiments of this disclosure as an example.

[0042] [Positive electrode] As the positive electrode, a positive electrode having the characteristics described above is used. 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 binders, thickeners, etc. as optional components.

[0043] The positive electrode active material layer may be a multilayer structure having a first positive electrode mixture layer with a different morphology from the other, and one or more other positive electrode mixture layers. When the first positive electrode mixture layer is in close contact with 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. Different morphologies include, for example, cases where the particle size of the active material particles is different, or cases where the type or composition of the positive electrode active material is different. It can be easily determined that the positive electrode active material layer is a multilayer structure having a first positive electrode mixture layer with a different morphology from the other, and one or more other positive electrode mixture layers, by observing the cross-section in the thickness direction when the positive electrode active material layer and the positive electrode current collector are cut simultaneously.

[0044] The positive electrode active material layer may contain 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% or more by mass or 80% or more by mass of the active material particles contained in the first positive electrode mixture layer may be first active material particles. For example, 60% or more by mass or 80% or more by mass of the active material particles contained in one or more positive electrode mixture layers other than the first positive electrode mixture layer may be second active material particles. For example, if the positive electrode active material layer has a two-layer structure comprising a first positive electrode mixture layer on the positive electrode current collector side and a second positive electrode mixture layer above it, then 80% by mass or more of the active material particles contained in the first positive electrode mixture layer may be 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 second active material particles.

[0045] The first average particle size D1 and the second average particle size 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). In this case, a thermosetting resin may be filled into the positive electrode mixture layer and cured. Next, a scanning electron microscope image (hereinafter referred to as SEM image) of the cross-section is taken. The SEM image is taken so that 10 or more first positive electrode active material particles and 10 or more second positive electrode active material particles are observed. By image processing, the first equivalent circle diameter of the cross-section of 10 or more first positive electrode active material particles is determined, and their average value is taken as D1. By image processing, the second equivalent circle diameter of the cross-section of 10 or more second positive electrode active material particles is determined, and their average value is taken as D2. Here, the equivalent circle diameter refers to the diameter of a circle having the same area as the area of ​​the particle's cross-section (the area of ​​the particles observed in the cross-section of the positive electrode mixture layer).

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

[0047] The positive electrode active material constituting the active material particles contains a lithium-containing transition metal oxide. From the viewpoint of increasing capacity, it may also contain a lithium-nickel composite oxide (composite oxide N) containing nickel as at least as a transition metal. The proportion of composite oxide N in the positive electrode active material is, for example, 70% by mass or more, may be 90% by mass or more, or 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-type crystalline structure. The proportion of Ni among the metal elements other than Li in the lithium transition metal oxide may be 50 atomic% or more. The lithium transition metal oxide may also contain Co, but from the viewpoint of cost reduction and high capacity, the proportion of Co among the metal elements other than Li 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] Generally, the composite oxide N in which the proportion of Ni in the metal elements other than Li is 50 atomic% or more is considered to be likely to change its crystal structure and increase its resistance with repeated charge and discharge. In this embodiment, since at least a part of the surface of the active material particles is coated and protected with an oxide film, it is considered that such an increase in resistance is also suppressed.

[0050] The composite oxide N may contain Ni and at least one selected from the group consisting of Co, Mn, and Al. Co, Mn, and Al contribute to the stabilization of 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. The proportion of Mn in the metal elements other than Li may be 1 atomic% or more, 3 atomic% or more, or 5 atomic% or more. [[ID=​​​​​​​​​​​​​​​​​​​​​​ x1, which represents the atomic ratio of Co, is, for example, less than or equal to 0.1 (0 ≤ x1 ≤ 0.1), and may also be less than or equal to 0.08, less than or equal to 0.05, or less than or equal to 0.01. When x1 is 0, it includes the case where Co is below the detection limit.

[0055] x², which represents the atomic ratio of Mn, is, for example, less than or equal to 0.1 (0 ≤ x² ≤ 0.1), and may also be less than or equal to 0.08, less than or equal to 0.05, or less than or equal to 0.03. x² may also be greater than or equal to 0.01, or greater than or equal to 0.03.

[0056] The atomic ratio of Al, represented by y, is, for example, less than or equal to 0.1 (0 ≤ y ≤ 0.1), and may also be less than or equal to 0.08, less than or equal to 0.05, or less than or equal to 0.03. y may also be greater than or equal to 0.01, or greater than or equal to 0.03.

[0057] The value of z that represents the atomic ratio of element M is, for example, 0 ≤ z ≤ 0.10, and 0 <z≦0.05でもよく、0.001≦z≦0.01でもよい。

[0058] 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 the aggregation of multiple 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 also be 5 μm or more and 25 μm or less.

[0060] The average particle size of secondary particles refers to the particle size at which the integrated volume value in the particle size distribution measured by laser diffraction scattering (volume-average particle size) becomes 50%. Such particle sizes are sometimes referred to as D50. For measuring devices, for example, the "LA-750" manufactured by HORIBA, Ltd. can be used.

[0061] When the positive electrode active material layer includes a first active material particle having a first average particle size D1 and a second active material particle having a second average particle size D2 (D1 > D2), for example, D1 may be 10 μm or more, 11 μm or more, 12 μm or more, or 15 μm or more. Alternatively, D1 may be 30 μm or less, or 25 μm or less. On the other hand, 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 3 μm or more. The D1 / D2 ratio may be, for example, 2 to 6, or 3 to 5.

[0062] For example, a resin material can be used as the binder for the positive electrode. Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, and vinyl resins. The binder may be used alone or in combination of two or more types.

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

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

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

[0066] [Negative electrode] The negative electrode may include, for example, a negative electrode current collector and 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 film or membrane. The negative electrode mixture contains particles of the negative electrode active material as an essential component and may contain binders, conductive agents, thickeners, etc. as optional components. Alternatively, lithium metal foil or 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 dispersing a negative electrode slurry containing particles of negative electrode active material, a binder, etc., in a dispersion medium onto the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary.

[0069] The negative electrode active material includes materials that electrochemically intercalate and release lithium ions, lithium metals, lithium alloys, etc. Examples of electrochemically intercalating and releasing lithium ions include carbon materials and alloying materials. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Among these, graphite is preferred due to its excellent charge-discharge stability and low irreversible capacity. Examples of alloying materials include those containing at least one metal capable of alloying with lithium, specifically silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxide and tin oxide may also be used.

[0070] As alloy materials containing silicon, for example, a composite material can be used in which a lithium-ion conductive phase is present and silicon particles are dispersed in the lithium-ion conductive phase. As the lithium-ion conductive phase, for example, a silicon oxide phase, a silicate phase, or a carbon phase can be used. The main component of the silicon oxide phase (for example, 95-100% by mass) may be silicon dioxide. Among these, composite materials composed of a silicate phase and silicon particles dispersed in the silicate phase are preferred because they have high capacity and low irreversible capacity. Furthermore, as the silicate phase, a lithium silicate phase (lithium-containing silicate phase) is preferred because it has low irreversible capacity and 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). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. Examples of elements other than Li, Si, and O that may be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), and the like.

[0072] The carbon phase may be composed of, for example, low-crystalline amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or the like.

[0073] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, punching sheet, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, and the like.

[0074] Examples of the binder include at least one selected from the group consisting of polyacrylic acid, polyacrylate, and their derivatives. As the polyacrylate, a Li salt or a Na salt is preferably used. Among them, it is preferable to use crosslinked lithium polyacrylate.

[0075] Examples of the conductive material include carbon nanotubes (CNT), carbon fibers other than CNT, and conductive particles (such as carbon black, graphite).

[0076] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as sodium salts), cellulose derivatives such as methylcellulose (such as cellulose ethers); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (such as polyalkylene oxides like polyethylene oxide).

[0077] [Separator] A separator is interposed between the positive and negative electrodes. The separator has high ion permeability and appropriate mechanical strength and insulating properties. Examples of separators include microporous thin films, woven fabrics, and nonwoven fabrics. Polyolefins such as polypropylene and polyethylene can be used as materials for the separator. The separator may have a heat-resistant insulating layer on at least one surface layer. The heat-resistant insulating layer may contain inorganic oxide fillers as the main component (e.g., 80% by mass or more) or a heat-resistant resin as the main component (e.g., 40% by mass or more). The heat-resistant resin may be polyamide resin such as aromatic polyamide (aramid), polyimide resin, or polyamide-imide resin.

[0078] [Non-aqueous electrolytes] The non-aqueous electrolyte includes, 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 include cyclic carbonate esters, linear carbonate esters, and cyclic carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters 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). Non-aqueous solvents may be used individually or in combination of two or more.

[0080] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Examples include lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(SO2F)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), and lithium halides (LiCl, LiBr, LiI, etc.). Lithium salts may be used individually or in combination of two or more types.

[0081] One example of a lithium-ion secondary battery structure is one in which an electrode group, consisting of a positive electrode and a negative electrode wound around a separator, is housed in an outer casing along with a non-aqueous electrolyte. However, the structure is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The shape of the battery is also not limited; for example, it may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0082] Below, the structure of a rectangular non-aqueous secondary battery, as an example of a lithium-ion secondary battery according to the present invention, will be described with reference to Figure 3.

[0083] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and an electrolyte (not shown). The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them. The negative electrode current collector of the negative electrode 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 of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, 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 portion is laser-welded. The sealing plate 5 has an injection hole for a non-aqueous electrolyte, which is sealed by a seal 8 after injection.

[0084] <Manufacturing method for positive electrode> A method for manufacturing a positive electrode according to the embodiments of this disclosure comprises 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 for forming the positive electrode active material layer comprises a supporting step of supporting 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, and a film forming step after the rolling step of exposing the active material particles to a gas phase containing a first element other than a nonmetallic element to form an oxide film that covers at least a portion of the surface of the active material particles.

[0086] (I) Supporting process (S1) The precursor layer can be formed by coating the surface of the positive electrode current collector with a positive electrode slurry, which is obtained by dispersing the components of the positive electrode mixture in a dispersion medium, and then drying it. The positive electrode mixture contains active material particles (particles of positive electrode active material) as an essential component, and may contain binders, thickeners, etc. as optional components.

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

[0088] (II) Rolling process (S2) The coating film (i.e., precursor layer) of the dried positive electrode slurry is rolled. The rolling conditions are not particularly limited. Rolling is performed when the density of the positive electrode active material in the precursor layer is 2.5 g / cm³. 3 More than 4.0g / cm 3 Preferably, 2.9 g / cm³ 3 More than 3.7g / cm 3 It is preferable to continue until the following conditions are met. In rolling, it is desirable to set conditions that primarily densify the surface of the precursor layer. This allows the gas phase containing the first element to come into contact with more active material particles located near the outer surface of the positive electrode active material layer in the coating formation process described later. Therefore, a thicker oxide film is more likely to form on the active material particles that are relatively spaced further away from the surface of the positive electrode current collector.

[0089] (III) Oxide film formation process (S3) Next, the active material particles supported on the positive electrode current collector are exposed to a gas phase containing the first element. This causes an oxide film containing the oxide of the first element to form 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). The ALD method is particularly preferred because it allows for the formation of oxide films at relatively low temperatures. The ALD method enables the formation of oxide films in an atmosphere below 200°C.

[0091] In the ALD method, an organometallic compound (precursor) containing the first element is used as the raw material for the oxide film. In the ALD method, vaporized precursor (raw material gas) and an oxidizing agent are alternately supplied to the reaction chamber where the object to be processed is placed. As a result, an oxide film of the first element is formed on the surface of the object.

[0092] In the ALD method, a self-limiting mechanism is in operation, causing the first element to deposit on the surface of the object at the atomic layer level. In the ALD method, the overall thickness of the oxide film is controlled by the number of cycles, which consist of supplying the raw material gas (pulse) → exhausting the raw material gas (purge) → supplying the oxidizer (pulse) → exhausting the oxidizer (purge).

[0093] The precursor is an organometallic compound containing the first element. Various organometallic compounds conventionally used in the ALD method can be used as precursors.

[0094] Examples of Ti-containing precursors include bis(t-butylcyclopentadienyl)titanium(IV) dichloride (C 18 H 26 C l2 Examples include titanium (Ti), tetrakis(dimethylamino)titanium(IV) ([(CH3)2N]4Ti, TDMAT), tetrakis(diethylamino)titanium(IV) ([(C2H5)2N]4Ti), tetrakis(ethylmethylamino)titanium(IV) (Ti[N(C2H5)(CH3)]4), titanium(IV) (diisopropoxide-bis(2,2,6,6-tetramethyl-3,5-heptanedionate(Ti[OCC(CH3)3CHCOC(CH3)3]2(OC3H7)2), titanium tetrachloride (TiCl4), titanium(IV) isopropoxide (Ti[OCH(CH3)2]4), and titanium(IV) ethoxide (Ti[O(C2H5)]4). An example of an Al-containing precursor is trimethylaluminum ((CH3)3Al, TMA).

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

[0096] As the oxidizing agent, conventional oxidizing agents used in the ALD method can be used. Examples of oxidizing agents include water, oxygen, and ozone. The oxidizing agent may also 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 to 200°C, 25°C to 200°C, 100°C to 200°C, or 120°C to 200°C. From a similar viewpoint, the pressure in the reaction chamber during processing should be 1 × 10⁻⁶ -5 Pa or more 1×10 5 It may be less than or equal to Pa, and 1 × 10 -4 Pa or more 1×10 4 It may be less than or equal to Pa.

[0098] The temperature of the atmosphere containing the precursor or oxidizing agent in the reaction chamber is between 10°C and 200°C (for example, between 120°C and 200°C), and the pressure in the reaction chamber during processing is 1 × 10⁻⁶ -5 Pa or more 1×10 5 When the Pa is less than or equal to Pa, the pulse time of the raw material gas may be 0.01 seconds or longer, or 0.05 seconds or longer. The pulse time of the raw material 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 rolling conditions are not particularly limited and should be set appropriately so that the positive electrode active material layer reaches a predetermined thickness or density.

[0100] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0101] Example 1 [Fabrication of the positive electrode] The positive electrode active material constituting the active material particles has a layered rock salt-type crystalline structure and is a composite oxide N(LiNi) containing lithium and Ni. 0.85 Co0.10 Al 0.05 O2 was used. In addition, a 15 μm thick aluminum foil was prepared as the positive electrode current collector.

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

[0103] A cathode slurry was prepared by adding NMP to a cathode mixture containing active material particles (D1=13μm), acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5, and stirring.

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

[0105] (II) Rolling process (S2) Next, the dried precursor layer is rolled, and the density of the positive electrode active material in the precursor of the rolled positive electrode active material layer is 3.65 g / cm³. 3 The adjustments were made to achieve this. The overall thickness of the positive electrode after rolling was 160 μm.

[0106] (III) Oxide film formation process (S3) A laminate of a positive electrode current collector and a precursor layer (positive electrode precursor) was placed in a designated reaction chamber, and a portion of the active material particles in the precursor layer were coated with an oxide film using the ALD method according to the following procedure.

[0107] In the reaction chamber containing the cathode precursor, tetrakis(dimethylamino)titanium(IV) (TDMAT), a precursor that serves as a source of the primary element (Ti), was supplied in vapor form. The pulse time was set to 0.1 seconds. The temperature of the atmosphere containing the precursor in the reaction chamber was controlled to 200°C and the pressure to 260 Pa. After 30 seconds, the surface of the cathode precursor was assumed to be covered with a monolayer of the precursor, and the excess precursor was purged with nitrogen gas.

[0108] Next, an oxidizing agent (H2O) was vaporized and supplied to the reaction chamber containing the cathode precursor. The pulse time was set to 0.015 seconds. The temperature of the atmosphere containing the oxidizing agent was controlled to 200°C and the pressure to 260 Pa. After 30 seconds, the excess oxidizing agent was purged with nitrogen gas.

[0109] A titanium-containing oxide film was formed by repeating a series of operations (ALD cycle) consisting of precursor supply, purging, oxidizing agent supply, and purging 200 times.

[0110] The oxide film was analyzed using SEM, EDS, ICP, etc. The oxide film contained Ti. The thickness Tb of the oxide film covering the active material particles at a position 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 film covering the active material particles at a position 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 film was 18 nm. The Pb / Pt ≈ Tb / Tt = 0.2 obtained by EPMA analysis.

[0111] [Fabrication of the negative electrode] A negative electrode slurry was prepared by adding water to a negative electrode mixture containing graphite (the negative electrode active material), sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) in a mass ratio of 96:2:2, and stirring. Next, the negative electrode slurry was applied to the surface of a copper foil, which was the negative electrode current collector. After the coating was dried, the foil was rolled to form negative electrode active material layers 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 / cm³. 3 The adjustments were made to achieve this. The total thickness of the negative electrode was 170 μm.

[0112] [Preparation of non-aqueous electrolytes] A non-aqueous 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] [Manufacturing of secondary batteries] An electrode group was fabricated by attaching tabs to each electrode and spirally winding the positive and negative electrodes via a separator so that the tabs were located on the outermost periphery. The electrode group was inserted into an aluminum laminate film casing, vacuum-dried at 105°C for 2 hours, then electrolyte was injected, and the opening of the casing was sealed to obtain secondary battery A1.

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

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

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

[0117] Example 5 The positive electrode and 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 film formation step S3. The thicknesses Tb, Tt, and Ta of the oxide film were 3 nm, 15 nm, and 9 nm, respectively.

[0118] Example 6 The positive electrode and 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 film formation step S3. The thicknesses of the oxide film Tb, Tt, and Ta were 2 nm, 10 nm, and 6 nm, respectively.

[0119] Example 7 The positive electrode and 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 film formation step S3. The thicknesses of the oxide film Tb, Tt, and Ta were 1 nm, 5 nm, and 3 nm, respectively.

[0120] Comparative Example 1 The positive electrode and secondary battery B1 were fabricated in the same manner as in Example 1, except that the order of the rolling process S2 and the oxide film formation process S3 was reversed. The thicknesses Tb, Tt, and Ta of the oxide film were 30 nm, 30 nm, and 30 nm, respectively.

[0121] Comparative Example 2 The positive electrode and 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 film formation step S3. The oxide film thicknesses Tb, Tt, and Ta were 15 nm, 15 nm, and 15 nm, respectively.

[0122] Comparative Example 3 The positive electrode and 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 film formation step S3. The thicknesses of the oxide film Tb, Tt, and Ta were 10 nm, 10 nm, and 10 nm, respectively.

[0123] Comparative Example 4 The positive electrode and 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 film formation step S3. The thicknesses of the oxide film Tb, Tt, and Ta were 5 nm, 5 nm, and 5 nm, respectively.

[0124] Comparative Example 5 The positive electrode and secondary battery B5 were fabricated in the same manner as in Comparative Example 1, except that the oxide film formation step S3 was omitted.

[0125] [evaluation] The secondary batteries obtained in the examples and comparative examples were evaluated as follows. (1) DC resistance value (DCIR) In a 25°C environment, the battery was charged with a constant current of 0.3 It until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.05 It. Subsequently, it was discharged at a constant current of 0.3 It for 100 minutes to bring the State of Charge (SOC) to 50%.

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

[0127] (2) Nail penetration test (a) The battery was charged at a constant current of 0.3It in an environment of 25℃ until the battery voltage reached 4.2V, and then continued with constant voltage charging until the current value reached 0.05It. (b) In a 25°C environment, the tip of a round nail (2.7 mm in diameter) was brought into contact with the center of the battery charged in (a), and pierced at a speed of 1 mm / second. The piercing of the round nail was stopped immediately after detecting the battery voltage drop (Δ50 mV) due to an internal short circuit. The surface temperature of the battery was measured 1 minute after the battery short-circuited.

[0128] [Table 1]

[0129] Table 1 shows that batteries A1 to A7 were able to improve safety in the event of an internal short circuit while suppressing the increase in resistance. Batteries B1 to B3 were able to improve safety in the event of an internal short circuit, but it was difficult to suppress the increase in resistance. Batteries B4 and B5 were able to suppress the increase in resistance, but it was difficult to improve safety in the event of an internal short circuit. [Industrial applicability]

[0130] The positive electrode for secondary batteries and secondary batteries containing the same relating to this disclosure are useful as primary power sources for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0131] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention. [Explanation of Symbols]

[0132] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing 10 positive electrode 11 Positive electrode current collector 12 Cathode active material layer 20 Active material particles having an oxide coating 23 Active material particles 27 Oxide film

Claims

1. It comprises 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 portion of the surface of the active material particles. The active material particles include a lithium-containing transition metal oxide. The oxide film contains an oxide of a first element other than a nonmetallic element. When the thickness of the positive electrode active material layer is TA, The thickness Tb of the oxide film located at a position 0.10TA from the surface of the positive electrode current collector in the positive electrode active material layer, or the probability of the presence of the first element Pb, The thickness Tt of the oxide film located at a position 0.90TA from the surface of the positive electrode current collector in the positive electrode active material layer, or the probability of existence Pt of the first element, is defined as follows: Satisfying 0.2 ≤ Tb / Tt or Pb / Pt ≤ 0.8, Positive electrode for secondary batteries.

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

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

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

5. The secondary battery comprises a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode, according to claim 1 or 2. Secondary battery.

6. A process for preparing active material particles containing lithium-containing transition metal oxides, The process of preparing the 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, It is equipped with, The process of forming the positive electrode active material layer is, 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 for rolling the precursor layer, After the rolling process, a coating formation step is performed in which the active material particles are exposed to a gas phase containing a first element other than a nonmetallic element to form an oxide film that covers at least a portion of the surface of the active material particles. It is equipped with, The aforementioned coating formation step is carried out by atomic layer deposition. The atomic layer deposition method is performed in a temperature range of 120°C to 200°C, where one cycle consists of supplying and exhausting a gas containing the first element, followed by supplying and exhausting an oxidizing agent. The process is carried out for a range of 20 to 200 cycles. The pulse time for supplying the oxidizing agent is 0.01 seconds or more and 3 seconds or less. The oxide film comprises 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 located at a position 0.10TA from the surface of the positive electrode current collector in the positive electrode active material layer, or the probability of the presence of the first element Pb, The thickness Tt of the oxide film located at a position 0.90TA from the surface of the positive electrode current collector in the positive electrode active material layer, or the probability of existence Pt of the first element, is defined as follows: Satisfying 0.2 ≤ Tb / Tt or Pb / Pt ≤ 0.8, A method for manufacturing a positive electrode for a secondary battery.

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