Positive electrode and method for producing the same, and non-aqueous electrolyte secondary battery including the positive electrode and method for producing the same

Abstract

To provide is a positive electrode capable of suppressing an increase in reaction force that occurs with repeated charge-discharge cycles of a nonaqueous electrolyte secondary battery.SOLUTION: A positive electrode includes an active material layer. The active material layer includes an active material containing at least a first active material, and a binder. The first active material is a first secondary particle formed by agglomeration of the first primary particles, the surfaces of which are coated with a boron compound. The first secondary particles have a titanium compound at the grain boundaries of the first primary particles, and are a lithium transition metal composite oxide containing 75 mol% or more of Ni relative to the total number of moles of metal elements excluding Li. The binder coats the first active material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a positive electrode and a method for producing the same, and to a non-aqueous electrolyte secondary battery including a positive electrode and a method for producing the same. [Background technology]

[0002] Patent Document 1 discloses that a positive electrode plate is heat-treated within a specific temperature range in order to improve the storage characteristics and cycle characteristics of a nonaqueous electrolyte secondary battery (hereinafter also referred to as a "secondary battery") such as a lithium ion battery and to suppress expansion of the secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2007-273259 A Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, the capacity of secondary batteries for vehicles has been increased and they have a high energy density. In high-capacity secondary batteries, the batteries tend to expand more when they are repeatedly charged and discharged, and the restraining pressure to provide a reaction force against the expansion also increases. Therefore, a method for further suppressing the increase in the restraining pressure in high-capacity secondary batteries is required.

[0005] An object of the present disclosure is to provide a positive electrode capable of suppressing an increase in reaction force that occurs due to repeated charge-discharge cycles of a nonaqueous electrolyte secondary battery, a method for manufacturing the same, and a nonaqueous electrolyte secondary battery including the positive electrode and a method for manufacturing the same. [Means for solving the problem]

[0006] [1] A positive electrode having an active material layer, the active material layer includes an active material including at least a first active material and a binder, the first active material is a first secondary particle formed by agglomeration of first primary particles, the surface of which is coated with a boron compound; the first secondary particles have a titanium compound at the grain boundaries of the first primary particles, and are a lithium transition metal composite oxide containing 75 mol % or more of Ni with respect to the total number of moles of metal elements excluding Li, The binder covers the first active material. [2] The positive electrode according to [1], wherein the binder covers 60% or more of a surface of the first active material. [3] The positive electrode according to [1] or [2], wherein the boron compound contains Li. [4] The positive electrode according to any one of [1] to [3], wherein the content of B in the first active material is 0.5 to 3 mol % based on the total number of moles of metal elements excluding Li. [5] The positive electrode according to any one of [1] to [4], wherein Ti is further dissolved in the first secondary particles. [6] The positive electrode according to any one of [1] to [5], wherein the first secondary particles contain 1 to 5 mol % of Ti with respect to the total number of moles of metal elements excluding Li. [7] The binder is polyvinylidene fluoride, The positive electrode according to any one of [1] to [6], wherein the content of the polyvinylidene fluoride in the active material layer is 0.3 to 2 wt % with respect to the weight of the active material layer. [8] The positive electrode according to any one of [1] to [7], wherein the first secondary particles contain a lithium transition metal composite oxide represented by the following formula (I): Li x (Ni (1-y-z) Co y Me z )O 2 (I) [In formula (I), 1.0≦x≦1.2, 0.02≦y≦0.15, and 0.02≦z≦0.18; Me includes Ti and may include one or more elements selected from the group consisting of Mn, Al, Mg, Mo, Nb, and Zr.] [9] The positive electrode according to any one of [1] to [8], wherein the amount of residual alkali in the first active material is 0.03 to 0.3 wt %.

[10] The active material further includes a second active material, The first secondary particles are aggregates of more than 100 first primary particles, the second active material is a single particle or a second secondary particle formed by agglomeration of 100 or less second primary particles, The positive electrode according to any one of [1] to [9], wherein the average particle diameter (D50) of the second active material is ⅓ or less of the average particle diameter (D50) of the first active material.

[11] The positive electrode according to

[10] , wherein the second active material includes single crystal particles.

[12] The positive electrode according to any one of [1] to

[11] , wherein the positive electrode has an elastic modulus of 3 to 12 GPa.

[13] A non-aqueous electrolyte secondary battery comprising the positive electrode according to any one of [1] to

[12] .

[14] A method for producing a positive electrode having an active material layer, comprising the steps of: A step of applying a mixture onto a positive electrode current collector, drying and compressing the mixture to form a mixture layer; and heat-treating the mixture layer at a temperature within a range from the melting point to the thermal decomposition temperature of the binder. The mixture includes an active material including at least a first active material and the binder, the first active material is a first secondary particle formed by agglomeration of first primary particles, the surface of which is coated with a boron compound; the first secondary particles have a titanium compound at grain boundaries of the first primary particles, and are a lithium transition metal composite oxide containing 75 mol % or more of Ni with respect to the total number of moles of metal elements excluding Li.

[15] The content of B in the first active material is 0.5 to 3 mol %, The method for producing a positive electrode according to

[14] , wherein the first secondary particles contain 1 to 5 mol % of Ti with respect to the total number of moles of metal elements excluding Li.

[16] The binder is polyvinylidene fluoride, The method for producing a positive electrode according to

[14] or

[15] , wherein the content of the polyvinylidene fluoride in the active material layer is 0.3 to 2 wt % with respect to the weight of the active material layer.

[17] A method for producing a non-aqueous electrolyte secondary battery, comprising a step of producing a positive electrode by the method for producing a positive electrode according to any one of

[14] to

[16] . Effect of the Invention

[0007] The positive electrode of the present disclosure can suppress an increase in reaction force that occurs with repeated charge-discharge cycles of a nonaqueous electrolyte secondary battery. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is an explanatory view illustrating a schematic diagram of a first active material contained in an active material layer of a positive electrode according to an embodiment. [Diagram 2] 3 is a flowchart showing a method for manufacturing a positive electrode according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] In this specification, a numerical range such as "x to y" includes an upper limit and a lower limit unless otherwise specified. That is, "x to y" represents a numerical range of "not less than x and not more than y". A numerical value arbitrarily selected from within the numerical range may be set as a new upper limit or lower limit. For example, a new numerical range may be set by arbitrarily combining a numerical value within the numerical range with a numerical value described in another part of this specification, in a table, or in a figure, etc.

[0010] (positive electrode) FIG. 1 is an explanatory view that illustrates a schematic diagram of a first active material contained in an active material layer of a positive electrode according to an embodiment.

[0011] The positive electrode of this embodiment (hereinafter also referred to as the "positive electrode") is used in, for example, a nonaqueous electrolyte secondary battery (hereinafter also referred to as the "secondary battery") such as a lithium ion battery. The positive electrode has an active material layer, and the active material layer includes an active material including at least a first active material 10 and a binder 5. The first active material 10 is a first secondary particle 2 formed by agglomeration of the first primary particles 1, the surfaces of which are coated with a boron compound (hereinafter also referred to as a "B compound") 4. The first secondary particle 2 is a lithium transition metal composite oxide having a titanium compound (hereinafter also referred to as a "Ti compound") 3 at the grain boundary of the first primary particles 1 and containing 75 mol% or more of Ni with respect to the total molar number of metal elements excluding Li. The binder 5 coats the first active material 10. Thus, the active material layer includes a coated particle 11 in which the first active material 10, the surfaces of which are coated with the B compound 4, are further coated with the binder 5 (FIG. 1).

[0012] The elastic modulus of the positive electrode is preferably 3 to 12 GPa, and may be 5 to 10 GPa, 5 to 9 GPa, or 6 to 8 GPa. When the elastic modulus is within the above range, the positive electrode can have a suitable flexibility, and therefore it is easy to suppress the increase in the reaction force accompanying the repetition of the charge-discharge cycle of the secondary battery. The positive electrode having the elastic modulus within the above range can be adjusted by the type of the first active material contained in the active material layer, the contents of the Ti compound 3 and the B compound 4 in the first active material, and the like. The elastic modulus can be measured by the method described in the examples below, and is calculated based on the change in thickness of the laminate when a compressive force is applied in the lamination direction to the laminate in which the positive electrodes are laminated.

[0013] The positive electrode may have an active material layer on a positive electrode current collector. The active material layer may be formed on only one side of the positive electrode current collector, or on both sides. The positive electrode current collector is, for example, a metal foil made of an aluminum material such as aluminum or an aluminum alloy, and may be a metal foil that is stable in the potential range of the positive electrode. The active material layer is preferably formed on one or both sides of the surface of the positive electrode current collector, except for the part to which the positive electrode lead is connected.

[0014] The active material includes at least a first active material 10. The first active material 10 is in the form of particles. The first active material 10 is obtained by coating the surfaces of first secondary particles 2, which are aggregates of first primary particles 1, with a B compound 4. The first secondary particles 2 are preferably aggregates of more than 100 first primary particles 1, and the number of aggregates of the first primary particles may be 500 or more, 1,000 or more, or 10,000 or more, and is usually 5,000,000 or less.

[0015] The first secondary particles 2 contain a Ti compound 3 in the grain boundaries of the first primary particles 1. The presence of the Ti compound 3 in the grain boundaries of the first primary particles 1 makes it easier to obtain coated particles 11 that are well coated with the binder 5. This makes it easier to obtain a positive electrode that can suppress an increase in reaction force due to repeated charge-discharge cycles of the secondary battery, since the flexibility of the positive electrode can be improved.

[0016] The Ti compound 3 contained in the grain boundary of the first primary particle 1 may be Ti that is not dissolved in a metal element other than Ti during synthesis of the first secondary particle 2 (described later) and precipitates as the Ti compound 3. The Ti compound 3 may be present on the surface of the first primary particle 1 or on the surface of the first secondary particle 2. The Ti compound 3 present on the surface of the first primary particle 1 is preferably not unevenly distributed but is present all over the first secondary particle 2 when the first secondary particle 2 is viewed as a whole. When the Ti compound 3 is present on the surface of the first secondary particle 2, it may be present so as to cover the entire surface of the first secondary particle 2 or may be scattered on the surface of the first secondary particle 2. When the Ti compound 3 is particulate, the particle size of the particulate Ti compound 3 is preferably smaller than the particle size of the first primary particle 1.

[0017] In the first secondary particles 2, the Ti compound 3 is contained in the grain boundaries of the first primary particles 1, but a part of Ti may be present inside the first primary particles 1 or may form a solid solution. For example, Ti may form a solid solution with other metal elements, such as other transition metal elements other than Ti, contained in the first primary particles 1.

[0018] The Ti compound 3 may contain Ti, for example, Lip Ti q O r [wherein 1≦p≦4, 1≦q≦5, 1≦r≦12]. The lithium-containing titanium compound may be one produced by reacting a titanium source such as titanium oxide used in the synthesis of the first secondary particles 2 with Li during firing performed during the synthesis of the first secondary particles 2.

[0019] The first secondary particles 2 preferably contain 1 to 5 mol % of Ti relative to the total number of moles of metal elements excluding Li. The content of Ti in the first active material 10 may be 1 to 4 mol %, preferably 1 to 3 mol %, or may be 1.5 to 3 mol %, or may be 2 to 3 mol %, relative to the total number of moles of metal elements excluding Li. When the content of Ti in the first secondary particles 2 is within the above range, it becomes easier to obtain the coated particles 11, and it becomes easier to obtain a positive electrode that can suppress an increase in reaction force due to repeated charge-discharge cycles of the secondary battery.

[0020] The Ti compound 3 present on the surface of the first primary particle 1 or the first secondary particle 2 and the Ti dissolved therein can be confirmed, for example, by SEM (scanning transmission electron microscope) and quantified by ICP (inductively coupled plasma) emission spectrometry and XPS (X-ray photoelectron spectroscopy). The Ti dissolved in the first primary particle 1 or the first secondary particle 2 can be quantified, for example, by EDS (energy dispersive X-ray spectroscopy). The fact that the Ti compound present on the surface of the first primary particle 1 or the first secondary particle 2 exists in the form of a lithium-containing titanium compound can also be confirmed by Li mapping analysis using EPMA (electron beam microanalyzer), EELS (electron energy loss spectroscopy), TOF-SIMS (time-of-flight secondary ion mass spectrometry), XPS (X-ray photoelectron spectroscopy), XRD (X-ray diffraction method), and XAFS (X-ray absorption fine structure). In this specification, the Ti compounds present at the grain boundaries of the first primary particles 1 and the dissolved Ti were confirmed by SEM and EPMA, the Ti content was determined by ICP optical emission spectrometry, and the presence of the Ti compounds as lithium-containing titanium compounds was confirmed by EPMA and TOF-SIMS analysis.

[0021] The first secondary particles 2 are a lithium transition metal composite oxide, and the Ni content relative to the total mole number of metal elements excluding Li (hereinafter also referred to as "Ni content") is 75 mol% or more. The Ni content may be 78 mol% or more, 80 mol% or more, 82 mol% or more, 75 to 98 mol%, 80 to 95 mol%, or 82 to 90 mol%. When the Ni content of the first secondary particles 2 is within the above range, a secondary battery with high energy density can be obtained.

[0022] The first secondary particles 2 contain Li, Ni, and Ti, but may contain other metal elements such as one or more metal elements selected from the group consisting of Co, Mn, Al, Zr, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, and Si.

[0023] The first secondary particles 2 preferably contain a lithium transition metal composite oxide represented by the following formula (I). Li x (Ni (1-y-z) Co y Me z )O 2 (I) [In formula (I), 1.0≦x≦1.2, 0.02≦y≦0.15, and 0.02≦z≦0.18; Me includes Ti and may include one or more elements selected from the group consisting of Mn, Al, Mg, Mo, Nb, and Zr.]

[0024] In formula (I), x may be 1.01≦x≦1.09, 1.03≦x≦1.08, or 1.05≦x≦1.07. In formula (I), y may be 0.03≦y≦0.12, 0.05≦y≦0.10, or 0.06≦y≦0.09. In formula (I), Me preferably contains Ti and at least one selected from the group consisting of Mn and Al, and more preferably contains Ti and Mn.

[0025] The first secondary particles 2 may be, for example, a lithium transition metal composite oxide represented by the following formula (II). Li a Ni b Co c Mn d Ti e O f (II) [In formula (II), 0.8≦a≦1.2, b≧0.70, c≦0.10, 0.03≦d≦0.12, 0.01≦e≦0.05, 1≦f≦2, and b+c+d+e=1.]

[0026] The composition of the first secondary particles 2 can be determined by ICP (inductively coupled plasma) emission spectrometry.

[0027] The first secondary particles 2 can be synthesized, for example, by the following procedure. First, a Li source such as lithium hydroxide (LiOH) is added to a nickel compound containing at least Ni, and the mixture is fired to obtain a lithium nickel composite oxide. The nickel compound is, for example, a composite oxide or composite hydroxide containing Ni, Co, and Mn. Next, titanium dioxide (TiO 2 The first secondary particles 2 are obtained by adding a Ti source such as lithium hydroxide and firing the mixture to obtain a lithium transition metal composite oxide. A Li source such as lithium hydroxide may be added together with the Ti source. The firing temperature after adding the Ti source is, for example, 550 to 850° C. Examples of a method for causing a Ti compound to exist at the grain boundaries of the first primary particles 1 of the first secondary particles 2 include a method of adjusting the amount of Ti source added in the synthesis of the first secondary particles 2.

[0028] In the first active material 10, the surfaces of the first secondary particles 2 are coated with the B compound 4. The presence of the B compound 4 on the surfaces of the first secondary particles 2 makes it easier to obtain coated particles 11 that are well coated with the binder 5. This makes it easier to obtain a positive electrode that can suppress an increase in reaction force due to repeated charge / discharge cycles of the secondary battery, since the flexibility of the positive electrode can be improved.

[0029] The B compound 4 may preferably cover the entire surface of the first secondary particle 2, or may be scattered on the surface of the first secondary particle. The B compound 4 present on the surface of the first secondary particle 2 is preferably not unevenly distributed but is present all over the first secondary particle 2 when the first secondary particle 2 is viewed as a whole. However, it is preferable that the B compound 4 does not completely cover the entire surface of the first secondary particle 2, and it is preferable that there is an area on the surface of the first secondary particle 2 to which the B compound 4 is not attached. The proportion of the B compound 4 covering the first secondary particle is preferably 99% or less, may be 90% or less, may be 70% or less, and is usually 50% or more.

[0030] The B compound 4 may contain B, and may contain, for example, Li. The B compound 4 containing Li may be produced by reacting a boron source, such as boric acid, boron oxide, or lithium borate, used in the synthesis of the first active material 10 with Li during firing performed during the synthesis of the first active material 10.

[0031] The content of B (boron) in the first active material 10 may be 0.2 to 3 mol%, preferably 0.5 to 3 mol%, may be 0.7 to 2.5 mol%, or may be 0.8 to 2.3 mol%. When the content of B is within the above range, it becomes easier to obtain coated particles 11 that are well coated with the binder 5. This makes it easier to obtain a positive electrode that can suppress an increase in reaction force due to repeated charge-discharge cycles of a secondary battery.

[0032] The B compound in the first active material 10 can be confirmed, for example, by SEM (scanning electron microscope), and can be quantified by ICP emission spectrometry, EPMA, XPS, etc. The ratio of the area covered with the B compound 4 to the total surface area of ​​the above-mentioned first secondary particles can be measured by EPMA or XPS. In EPMA, the ratio of the peak area of ​​B (boron) to the total peak area of ​​metal elements other than Li may be calculated. In XPS, the ratio may be calculated from the molar fraction of B to the total mole number of metal elements other than Li. The inclusion of Li in the B compound can be confirmed by Li mapping analysis in EELS and TOF-SIMS analysis. In this specification, the B compound covering the first secondary particles was confirmed by SEM, the content of B was determined by ICP emission spectrometry, and the inclusion of Li in the B compound was confirmed by TOF-SIMS.

[0033] The first active material 10 can be synthesized, for example, by washing the first secondary particles 2 synthesized above with water, adding a B source (boron source) and firing the mixture. 3 BO 3 ), boron oxide (B 2 O 3 ), lithium borate (LiBO 2 , Li 2 B 4 O 7 ) and other boron compounds. Along with the addition of the B source, a Li source such as lithium hydroxide may be added. The firing temperature for firing after adding the B source is, for example, 200 to 500°C.

[0034] The amount of residual alkali in the first active material 10 is preferably 0.03 to 0.3 wt%, may be 0.05 to 0.25 wt%, or may be 0.1 to 0.2 wt%. When the amount of residual alkali in the first active material 10 is within the above range, it is possible to suppress expansion and deterioration of the secondary battery caused by gas generated due to the residual alkali. The amount of residual alkali in the first active material 10 is determined by measuring the pH of a suspension in which the first active material 10 is suspended in water, as described in the examples described later.

[0035] The active material may contain a second active material in addition to the first active material 10. The second active material is a single particle or a second secondary particle formed by agglomeration of 100 or less second primary particles. When the second active material is a second secondary particle, the number of second primary particles contained in the second active material is preferably less than the number of first primary particles 1 contained in the first active material 10, and may be 2 to 50, 2 to 30, 2 to 10, or 2 to 5. The average particle size (D50) of the second active material is preferably 1 / 3 or less, 1 / 4 or less, or 1 / 5 or less of the average particle size (D50) of the first active material. The average particle size (D50) in this specification is a particle size at which the cumulative frequency from the smaller particle size in the volume-based particle size distribution is 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device.

[0036] As described later, the active material layer is formed by applying a mixture containing an active material and a binder onto a positive electrode current collector, drying and compressing the mixture. The active material may crack during this compression. The active material may crack during the charge / discharge cycle of the secondary battery. When the active material cracks, the specific surface area of ​​the active material increases, which may cause the active material to react more easily with the electrolyte and generate gas or to expand during the charge / discharge cycle of the secondary battery. The second active material is less likely to crack than the first active material 10, so that the active material layer contains the second active material, which can suppress the above-mentioned gas generation and expansion of the secondary battery. From this viewpoint, the single particles and second primary particles constituting the second active material are preferably single crystal particles.

[0037] The second active material is preferably a lithium transition metal composite oxide. The surface of the second active material may or may not be coated with a B compound. The second secondary particles constituting the second active material include the first secondary particles 2 described in the first active material 10. However, the second secondary particles may or may not have a Ti compound at the grain boundaries of the second primary particles. The lithium transition metal composite oxide constituting the second active material and the lithium transition metal composite oxide constituting the first active material 10 may be the same or different.

[0038] The active material may contain other active materials other than the first active material 10 and the second active material, as long as the object of the present disclosure is not impaired. Examples of the other active materials include lithium transition metal composite oxides having a Ni content outside the range of the first secondary particles 2, or compounds other than lithium transition metal composite oxides. These other active materials may be primary particles or secondary particles.

[0039] The active material layer includes a binder 5. The binder 5 coats the first active material 10. When the charge / discharge cycle of the secondary battery is repeated, the active material expands and contracts, which may cause the active material to break and crack. When the binder enters and accumulates in the cracks, the active material layer hardens, the flexibility of the positive electrode is lost, and the elastic modulus of the positive electrode increases. A positive electrode with a large elastic modulus is less likely to absorb the expansion and contraction of the active material and is less likely to suppress the increase in the reaction force. In contrast, since the first active material 10 is coated with the binder 5, the elastic modulus of the positive electrode is less likely to increase. In particular, since the first active material 10 has a Ti compound 3 at the grain boundary of the first primary particle 1 and the surface of the first secondary particle 2 is coated with the B compound 4, the wettability between the first active material 10 and the binder 5 is improved, and the coated particles 11 in which the binder 5 is well coated can be easily obtained. Therefore, it is easier to suppress the increase in the reaction force accompanying the repeated charge / discharge cycle of the secondary battery.

[0040] The binder 5 preferably covers 60% or more of the surface of the first active material 10. The ratio of the binder 5 covering the surface of the first active material 10 may be 70% or more, 80% or more, or 85% or more, or may be, for example, 98% or less, or 95% or less. By being within the above ratio range, the flexibility of the positive electrode is improved, and it is easy to suppress the increase in the reaction force caused by repeated charge and discharge cycles of the secondary battery. The ratio of the binder 5 covering the first active material 10 is calculated by image analysis of the distribution of the binder by superimposing an image obtained by binarizing an SEM image of the active material layer and a mapping image of the binder corresponding to this SEM image, as described in the examples described later.

[0041] Examples of the binder 5 include fluororesins such as polyvinylidene fluoride (hereinafter also referred to as "PVdF") and polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, cellulose-based resins such as carboxymethyl cellulose (CMC), polyethylene oxide (POE), etc. The binder 5 is preferably PVdF.

[0042] When the binder 5 is PVdF, the PVdF content in the active material layer may be 0.3 to 2 wt%, 0.5 to 1.8 wt%, or 0.7 to 1.5 wt%, based on the weight of the active material. Even if the PVdF content in the active material layer is within the above range, coated particles 11 with a good coating state with PVdF can be obtained, so that an increase in reaction force due to repeated charge-discharge cycles of the secondary battery can be efficiently suppressed.

[0043] The active material layer may contain a conductive material in addition to the active material and the binder 5. The conductive material may be, for example, a carbon material. The carbon material may be, for example, one or more selected from the group consisting of fibrous carbon, carbon black (acetylene black, ketjen black, etc.), coke, and activated carbon. The fibrous carbon may be carbon nanotubes (CNT). The CNT may be a single-walled carbon nanotube (SWCNT) or a multi-walled carbon nanotube such as a double-walled carbon tube (DWCNT).

[0044] (Manufacturing method of positive electrode) 2 is a flow chart showing a method for manufacturing a positive electrode according to the embodiment. The method for manufacturing a positive electrode according to the embodiment can manufacture a positive electrode for a secondary battery, and can also manufacture the present positive electrode. The method for manufacturing a positive electrode includes a step of forming a mixture layer by applying a mixture onto a positive electrode current collector, drying and compressing the mixture, and a step of heat-treating the mixture layer at a temperature within the range from the melting point to the thermal decomposition temperature of the binder. The mixture includes an active material including at least a first active material, and a binder.

[0045] The first active material is a first secondary particle formed by agglomeration of the first primary particles, the surface of which is coated with a B compound. The first secondary particles have a Ti compound at the grain boundaries of the first primary particles, and are a lithium transition metal composite oxide containing 75 mol % or more of Ni relative to the total molar number of metal elements excluding Li. The binder coats the first active material.

[0046] The positive electrode current collector may be the metal foil described above. The first active material may be the first active material 10 described above, and the first primary particles and the first secondary particles may be the first primary particles 1 and the first secondary particles 2 described above. The B compound and the Ti compound may be the B compound 4 and the Ti compound 3 described above. The Ni content and the Ti content of the first secondary particles, and the B compound content in the first active material may also be within the ranges described above. The binder may be the binder 5 described above, and is preferably PVdF. The PVdF content in the active material layer may be within the ranges described above.

[0047] The mixture can be prepared by adding a solvent such as N-methyl-2-pyrrolidone (NMP) to materials that form an active material layer, such as an active material including a first active material, a binder, and, if necessary, a conductive material, and kneading them.

[0048] In the step of forming the mixture layer, the mixture may be applied to only one side or both sides of the positive electrode current collector. The mixture applied to the positive electrode current collector is dried and compressed to obtain the mixture layer.

[0049] In the heat treatment step, the mixture layer formed on the positive electrode current collector is heat treated at a temperature within the range of the melting point to the thermal decomposition temperature of the binder. The heat treatment temperature may be the melting point to the melting point + 100°C, or may be the melting point + 30°C to the melting point + 60°C. The melting point and thermal decomposition temperature of the binder are the melting point and thermal decomposition temperature of the binder with the largest content (weight) among the binders contained in the mixture. By heat treating the mixture layer in the above temperature range, the first active material can be well covered with the binder. In the first active material, the B compound is present on the surface of the first secondary particles having the Ti compound at the grain boundaries of the first primary particles, so that the wettability between the binder and the first active material is improved. As a result, the binder can well cover the first active material by the heat treatment step, and therefore it is easy to suppress the increase in the reaction force accompanying the repeated charge-discharge cycles of the secondary battery. The melting point of the binder can be measured by, for example, differential scanning calorimetry (DSC). The thermal decomposition temperature of the binder can be measured, for example, by a thermogravimetric differential thermal analyzer (TG-DTA).

[0050] (Nonaqueous electrolyte secondary battery) The nonaqueous electrolyte secondary battery of this embodiment (hereinafter also referred to as "the battery") includes the present positive electrode, and typically includes an electrode assembly including the present positive electrode, and a nonaqueous electrolyte. The battery may have a battery case that houses the electrode assembly and the nonaqueous electrolyte. The battery case may include an exterior body having an opening, and a sealing plate that seals the opening. The exterior body and the sealing plate are preferably made of metal, and may be formed using aluminum, an aluminum alloy, iron, an iron alloy, or the like, and may be formed using, for example, an aluminum laminate film. A resin sheet serving as an electrode holder may be disposed between the electrode body and the exterior body.

[0051] The electrode body may include the present positive electrode, the negative electrode, and a separator. In the electrode body, the active material layer of the present positive electrode and the negative electrode active material layer of the negative electrode face each other via the separator. The electrode body may be a laminated type in which the present positive electrode, the negative electrode, and the separator are laminated, or a wound type in which a laminate in which the present positive electrode, the negative electrode, and the separator are laminated is wound.

[0052] The negative electrode usually has a negative electrode current collector and a negative electrode active material layer, and the negative electrode current collector is, for example, a metal foil made of a copper material such as copper or a copper alloy. The negative electrode active material layer contains a negative electrode active material and may further contain a conductive material, a binder, and the like.

[0053] Examples of the negative electrode active material include carbon-based active material particles and metal-based active material particles. Examples of the carbon-based active material particles include one or more particles selected from the group consisting of graphite (graphite) such as natural graphite and artificial graphite, hard carbon, soft carbon, and carbon (C) such as amorphous coated graphite. Examples of the metal-based active material particles include particles of metal elements such as simple metals or metal oxides containing elements selected from the group consisting of silicon (Si), tin (Sn), antimony (Sb), bismuth (Bi), titanium (Ti), and germanium (Ge). Examples of the metal-based active material particles include one or more particles selected from the group consisting of Si, SiOx (x=0.5 to 1.5), a complex of Si and C (hereinafter also referred to as "SiC complex"), and Sn.

[0054] Examples of conductive materials include carbon materials such as fibrous carbon (CNT (SWCNT, DWCNT)), carbon black (e.g., acetylene black, ketjen black), coke, activated carbon, etc. Examples of binders include cellulose-based resins such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose, etc.; polyacrylic acid; SBR, etc.

[0055] The separator may have a substrate and a functional layer on at least one side of the substrate. The substrate may be a porous sheet such as a film made of a resin such as a polyolefin such as polyethylene or polypropylene, polyester, cellulose, or polyamide, or a nonwoven fabric. The substrate may have a single-layer structure or a multi-layer structure. Examples of the functional layer include an adhesive layer and a heat-resistant layer, and the separator may have one or both of these. The adhesive layer may be formed, for example, by an adhesive. The heat-resistant layer may include, for example, a filler and a binder.

[0056] The non-aqueous electrolyte is preferably a non-aqueous solvent such as an organic solvent containing a supporting salt. 6 , LiBF 4 , LiClO 4 , LiFSO 3 , LiBOB (lithium bis(oxalato)borate), etc. The non-aqueous electrolyte may contain one or more of these supporting salts. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate (PC), butylene carbonate (BC), and diethyl carbonate (DEC). The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. The non-aqueous electrolyte may further contain an additive such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), or fluoroethylene carbonate.

[0057] (Method of manufacturing non-aqueous electrolyte secondary battery) The method for producing the present battery includes a step of producing a positive electrode by the method for producing the present positive electrode, and may further include a step of obtaining an electrode assembly using the present positive electrode, the negative electrode, and a separator, and a step of housing the electrode assembly and the nonaqueous electrolyte in a battery case. EXAMPLES

[0058] The present disclosure will be described more specifically below with reference to examples and comparative examples. Example 1 (Synthesis of First Active Material) Nickel-cobalt-manganese hydroxide, lithium hydroxide, and titanium dioxide were mixed and baked in an oxygen atmosphere at 770°C for 10 hours to obtain a lithium transition metal composite oxide. The lithium transition metal composite oxide was crushed, washed with water, mixed with boric acid, and baked at 300°C for 1 hour to obtain a first active material. The amounts of titanium dioxide and boric acid added were adjusted so that the amounts of Ti and B contained in the first active material were as shown in Table 1.

[0059] (Preparation of positive electrode) The first active material, acetylene black, and polyvinylidene fluoride (PVdF, melting point 175°C, thermal decomposition temperature 420°C) as a binder were mixed at a ratio of 97.5:1.5:1.0 (weight ratio of solid content), and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and kneaded to obtain a mixture. The mixture was applied to both sides of an aluminum foil as a positive electrode current collector and dried to form a coating film. The coating film was rolled using a roller to form an active material layer, and after heat treatment at 230°C for 3 minutes, it was cut to an electrode size to obtain a positive electrode in which an active material layer was formed on both sides of the positive electrode current collector. The melting point of PVdF was measured by differential scanning calorimetry (DSC), and the thermal decomposition temperature of PVdF was measured by a thermogravimetric differential thermal analyzer (TG-DTA).

[0060] (Preparation of negative electrode) Natural graphite as the negative electrode active material, sodium carboxymethylcellulose (CMC-Na) as the binder, and styrene butadiene rubber (SBR) were mixed in water at a ratio of 100:1:1 (weight ratio of solids) to obtain a negative electrode mixture. The negative electrode mixture was applied to both sides of copper foil as the negative electrode current collector and dried to obtain a coating film. The coating film was rolled using a roller to form a negative electrode active material layer, and cut to an electrode size to obtain a negative electrode in which a negative electrode active material layer was formed on both sides of the negative electrode current collector.

[0061] (Preparation of non-aqueous electrolyte secondary battery) An aluminum lead was welded as a positive electrode lead to a portion of the positive electrode current collector where the positive electrode current collector on which the active material layer was not formed was exposed. A nickel lead was welded as a negative electrode lead to a portion of the negative electrode current collector where the negative electrode current collector on which the negative electrode active material layer was not formed was exposed. A positive electrode with a positive electrode lead attached and a negative electrode with a negative electrode lead attached were laminated via a polyolefin separator, and wound into a spiral shape, and then pressed in the radial direction to obtain a flat wound electrode body. The wound electrode body was housed in an exterior body made of an aluminum laminate sheet, and a nonaqueous electrolyte was injected, and the opening of the exterior body was sealed to obtain a secondary battery with a design capacity of 650 mAh.

[0062] The non-aqueous electrolyte used contained the following mixed solvent, supporting salt, and additives. The mixed solvent used was a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC=3:3:4. The supporting salt used was lithium hexafluorophosphate (LiPF 6 ) was used and dissolved in the above mixed solvent at a concentration of 1.0 mol / L. Vinylene carbonate (VC) was used as an additive and dissolved in the above mixed solvent at a concentration of 2.0 wt %.

[0063] [Examples 2 to 5, Comparative Examples 1 to 4] A first active material and a positive electrode were obtained and a nonaqueous electrolyte secondary battery was fabricated in accordance with the procedure of Example 1, except that the amounts of titanium dioxide and boric acid added were adjusted so that the Ti and B contained in the first active material were the amounts shown in Tables 1 and 2.

[0064] [Observation of the first active material and binder] (Check Ti) The first active material was observed using a scanning transmission electron microscope (SEM, JEOL Ltd. "JSM-7800F"), and an EPMA (electron probe microanalyzer) element mapping image corresponding to the 50,000x SEM image was obtained to confirm whether or not Ti compounds were present at the grain boundaries of the first primary particles, and to confirm the Ti that was dissolved in the first secondary particles. The results are shown in Tables 1 and 2. When the Ti compounds present at the grain boundaries of the first primary particles were confirmed by EPMA and TOF-SIMS, they were all lithium-containing titanium compounds.

[0065] The first active material was observed using the SEM, and a 10,000x SEM image was obtained at a low acceleration of 1 kV. The presence of B was estimated from the presence of black regions, and the coating state of the first secondary particles with the B compound was confirmed. The results are shown in Tables 1 and 2. When the B compound coating the first secondary particles was confirmed by TOF-SIMS, all of them contained Li.

[0066] The first active material was observed using the SEM for the positive electrodes obtained in Example 2 and Comparative Example 4 to confirm the state of coverage of the first active material with the binder (PVdF). The coverage ratio of the first active material with elemental fluorine was calculated by obtaining a mapping image of elemental fluorine (F) of the first active material covered with the binder, superimposing this on a binarized SEM image, and performing image analysis of the fluorine distribution on the surface of the first active material.

[0067] [Composition of first active material] The composition in the first active material was analyzed by ICP emission spectrometry, and the Ni content, B content, and Ti content were measured relative to the total mole number of metal elements excluding Li. The results are shown in Tables 1 and 2. The first secondary particles in Examples 1 to 5 and Comparative Examples 1 to 4 were all lithium transition metal composite oxides represented by the above formula (I). The composition of the first secondary particles obtained in Examples 1 to 5 was Li 1.04 Ni 0.82 Co 0.05 Mn 0.13 Ti 0.03 O 2 It was.

[0068] [Measurement of Residual Alkali Amount in First Active Material] A preset amount of the first active material was dispersed in pure water at 25°C to obtain a dispersion. Hydrochloric acid was added dropwise to the dispersion to perform neutralization titration, and the amount of alkali required for titration was calculated from the obtained titration curve, which was defined as the amount of residual alkali. The results are shown in Tables 1 and 2.

[0069] [Measurement of elastic modulus of positive electrode] A laminate of eight sheets of positive electrodes cut to a size of 30 × 40 mm was inserted into a laminate bag, 1 g of electrolyte was injected, and the laminate bag was vacuum-sealed. The electrolyte was a mixture of EC:EMC:DMC = 3:4:3 (volume ratio) and 1 mol / L LiPF 6The elastic modulus [GPa] was calculated from the change in thickness when a compressive force was applied to the laminate in the laminated direction in a vacuum-sealed laminated bag using an autograph AGX-10kNV2D (Shimadzu Corporation). Specifically, a compressive force was applied to the laminate, increasing by 0.5 kN every 1 h, and the elastic modulus [GPa] was calculated based on the change in strain when a compressive force was applied after 4 h (when a compressive force of 2 kN was applied). The results are shown in Tables 1 and 2.

[0070] [Measurement of the rate of increase in reaction force of secondary batteries] The secondary battery was set in the jig of an Autograph AGX-10kNV2D (manufactured by Shimadzu Corporation), and repeatedly charged and discharged at a current value of 1 / 3C rate while applying a restraining pressure of 1 MPa to obtain an SS curve (stress-strain curve). The load at this time was divided by the cross-sectional area of ​​the secondary battery (cell) to calculate the stress value. The rate of increase in the stress value when the secondary battery was charged after 200 cycles of charging and discharging was calculated relative to the stress value when the secondary battery was initially charged, and this was taken as the increase rate [%] of the reaction force of the secondary battery. The results are shown in Tables 1 and 2.

[0071] [Table 1]

[0072] [Table 2]

[0073] Example 6 A positive electrode was obtained by the procedure of Example 2, except that the ratio of polyvinylidene fluoride to the first active material was 100:1.5 (weight ratio of solid content). When the state of coating of the first active material with PVdF was confirmed by the above-mentioned method, 89% of the first active material was covered with PVdF.

[0074] Comparative Example 5 A positive electrode was obtained by the procedure of Comparative Example 4, except that the ratio of polyvinylidene fluoride to the first active material was 100:1.5 (weight ratio of solids). When the state of coating of the first active material with PVdF was confirmed by the above-mentioned method, 51% of the first active material was covered with PVdF. [Explanation of symbols]

[0075] 1 first primary particles, 2 first secondary particles, 3 titanium compound (Ti compound), 4 boron compound (B compound), 5 binder, 10 first active material, 11 coated particles.

Claims

1. A positive electrode having an active material layer, The active material layer includes an active material including at least a first active material and a binder, the first active material is a first secondary particle formed by agglomeration of first primary particles, the surface of which is coated with a boron compound; the first secondary particles have a titanium compound at grain boundaries of the first primary particles, and are lithium transition metal composite oxides containing 75 mol % or more of Ni with respect to the total number of moles of metal elements excluding Li, The binder covers the first active material.

2. The positive electrode according to claim 1 , wherein the binder covers 60% or more of a surface of the first active material.

3. 2. The positive electrode of claim 1, wherein the boron compound comprises Li.

4. 2. The positive electrode according to claim 1, wherein the content of B in the first active material is 0.5 to 3 mol % based on the total number of moles of metal elements excluding Li.

5. The positive electrode according to claim 1 , wherein the first secondary particles further contain Ti as a solid solution.

6. The positive electrode according to claim 1 , wherein the first secondary particles contain 1 to 5 mol % of Ti with respect to the total number of moles of metal elements excluding Li.

7. The binder is polyvinylidene fluoride, 2. The positive electrode according to claim 1, wherein the content of said polyvinylidene fluoride in said active material layer is 0.3 to 2 wt % with respect to the weight of said active material layer.

8. The positive electrode according to claim 1 , wherein the first secondary particles contain a lithium transition metal composite oxide represented by the following formula (I): Li ｘ (N （１－ｙ－ｚ） Co ｙ Me ｚ )O ２ (I) [In formula (I), 1.0≦x≦1.2, 0.02≦y≦0.15, and 0.02≦z≦0.18; Me contains Ti and may contain one or more elements selected from the group consisting of Mn, Al, Mg, Mo, Nb, and Zr.

9. 2. The positive electrode according to claim 1, wherein the amount of residual alkali in the first active material is 0.03 to 0.3 wt %.

10. The active material further includes a second active material, The first secondary particles are aggregates of more than 100 first primary particles, the second active material is a single particle or a second secondary particle formed by agglomeration of 100 or less second primary particles, 2. The positive electrode according to claim 1, wherein the second active material has an average particle diameter (D50) that is 1 / 3 or less of the average particle diameter (D50) of the first active material.

11. 11. The positive electrode of claim 10, wherein the second active material comprises single crystal particles.

12. The positive electrode according to claim 1, wherein the positive electrode has an elastic modulus of 3 to 12 GPa.

13. A non-aqueous electrolyte secondary battery comprising the positive electrode according to any one of claims 1 to 12.

14. A method for producing a positive electrode having an active material layer, comprising the steps of: A step of applying a mixture onto a positive electrode current collector, drying and compressing the mixture to form a mixture layer; and heat-treating the mixture layer at a temperature within a range from the melting point to the thermal decomposition temperature of the binder. The mixture includes an active material including at least a first active material and the binder, the first active material is a first secondary particle formed by agglomeration of first primary particles, the surface of which is coated with a boron compound; the first secondary particles have a titanium compound at grain boundaries of the first primary particles and are a lithium transition metal composite oxide containing 75 mol % or more of Ni with respect to the total number of moles of metal elements excluding Li.

15. The content of B in the first active material is 0.5 to 3 mol %, The method for producing a positive electrode according to claim 14, wherein the first secondary particles contain 1 to 5 mol % of Ti with respect to the total number of moles of metal elements excluding Li.

16. The binder is polyvinylidene fluoride, 15. The method for producing a positive electrode according to claim 14, wherein the content of said polyvinylidene fluoride in said active material layer is 0.3 to 2 wt % with respect to the weight of said active material layer.

17. A method for producing a non-aqueous electrolyte secondary battery, comprising the step of producing a positive electrode by the method for producing a positive electrode according to any one of claims 14 to 16.

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