Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

The positive electrode active material includes a lithium transition metal composite oxide containing 80 mol% or more of Ni relative to the total molar number of metal elements other than Li, and a Co content of less than 5 mol%. The lithium transition metal composite oxide is a secondary particle formed by aggregation of primary particles, and at least one element A selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.01 mol% or more and 1 mol% or less relative to the total molar number of metal elements other than Li. In addition, at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present on the surface of the secondary particles in an amount of 0.05 mol% or more and 2 mol% or less relative to the total molar number of Ni in the composite oxide.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. Background Art

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion batteries, the positive electrode active material has a great influence on battery performance such as power characteristics, capacity, cycle characteristics, storage characteristics, etc. Generally, a lithium transition metal composite oxide containing metal elements such as Ni, Co, Mn, Al, etc. and composed of secondary particles aggregated from primary particles is used as the positive electrode active material. The positive electrode active material varies greatly in properties depending on its composition, particle shape, etc. Therefore, many studies have been conducted on various positive electrode active materials. In particular, a lithium transition metal composite oxide with a high Ni content is expected as a positive electrode active material that is advantageous for increasing the capacity of the battery.

[0003] For example, Patent Document 1 discloses a positive electrode active material containing Li, Ni, Co, Mn, and W, where the ratio of Ni is 30 mol% or more and 60 mol% or less, the ratio of Co is 15 mol% or more and 35 mol% or less, the ratio of Mn is 15 mol% or more and 35 mol% or less, the ratio of W exceeds 0 mol% and is 5 mol% or less, and W is unevenly present on the surface layer of the positive electrode active material. In addition, Patent Document 1 states that by using this positive electrode active material, the power characteristics and cycle characteristics of the battery are improved.

[0004] In addition, Patent Document 2 discloses a positive electrode active material having a layered crystal structure Li ,

[0009] , ,

[0008] , M 1-a O 2± b M’ k S m (-0.03 < a < 0.06, b ≈ 0), M is a transition metal compound formed from any one or more of the elements in a group consisting of at least 95% of Ni, Mn, Co, and Ti, and M’ is a specific element present on the surface of the oxide. Patent Document 2 states that by using this positive electrode active material, the performance of the cathode of a lithium battery can be improved.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-91626

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2010-535699 Summary of the Invention

[0009] Co is rare and expensive, so by reducing the amount of Co, the manufacturing cost of the battery can be reduced. However, if the amount of Co is reduced in the positive electrode active material with a high amount of Ni, which is conducive to the high capacity of the battery, there is a problem of increasing the direct current resistance (DCR) of the battery during charging and storage. It should be noted that in the non-aqueous electrolyte secondary battery using the positive electrode active material of Patent Documents 1 and 2, there is still room for improvement in suppressing the increase in DCR.

[0010] A positive electrode active material for a non-aqueous electrolyte secondary battery as one embodiment of the present disclosure includes a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide contains: Ni in an amount of 80 mol% or more relative to the total molar number of metal elements other than Li, and at least one selected from Co, Mn, Al, Ti, Nb, Fe, and Zn, with the Co content being less than 5 mol%. The lithium transition metal composite oxide includes secondary particles formed by aggregation of primary particles, at least one element A selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.01 mol% or more and 1 mol% or less relative to the total molar number of metal elements other than Li, and at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present on the surface of the secondary particles in an amount of 0.05 mol% or more and 2 mol% or less relative to the total molar number of Ni in the composite oxide.

[0011] A nonaqueous electrolyte secondary battery as one embodiment of the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a nonaqueous electrolyte.

[0012] According to one embodiment of the present disclosure, a non-aqueous electrolyte secondary battery using a positive electrode active material having a high Ni content can suppress an increase in DC resistance during storage under charge. By using the positive electrode active material as one embodiment of the present disclosure, a non-aqueous electrolyte secondary battery with high capacity and excellent storage characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a cross-sectional view of a nonaqueous electrolyte secondary battery as an example of an embodiment.

[0014] Figure 2 This is a diagram schematically showing a cross section of a particle of a lithium-transition metal composite oxide constituting a positive electrode active material as an example of an embodiment. DETAILED DESCRIPTION

[0015] As described above, lithium transition metal composite oxides containing a large amount of Ni are useful positive electrode active materials that contribute to higher capacity and higher energy density of batteries. However, they suffer from a problem of a significant increase in direct current resistance (DCR) during battery storage under charge.

[0016] The present inventors conducted intensive research to solve this problem and found that, in a lithium transition metal composite oxide containing a large amount of Ni, by having a specified amount of at least one of Ca and Sr (element A) present on the surface of the primary particles of the composite oxide, and by having a specified amount of at least one selected from B, Zr, W, Al, Nb, Mo and Ti (element B) present on the surface of the secondary particles of the composite oxide, the increase in the DCR of the battery during charged storage can be specifically suppressed.

[0017] In a positive electrode active material with a large amount of Ni, if the amount of Co is reduced, the stability of the active material surface is reduced, and it becomes easy to cause side reactions with the electrolyte. It is also believed that: the degradation of the active material surface is promoted, and the DCR during charging and storage increases significantly. In addition, the fluorine-containing compounds in the electrolyte react with water to produce HF, and the unstable active material surface is also easily corroded by the generated HF. According to the positive electrode active material disclosed in the present invention, the interaction generated by the coexistence of element A and element B is utilized to form a stable protective layer on the surface of the secondary particles of the composite oxide, and the stability of the active material surface is greatly improved. It is therefore believed that the reaction with the electrolyte and HF on the surface of the active material is suppressed, and the storage characteristics are improved.

[0018] It should be noted that in the absence of element A or element B, a stable protective layer is not formed on the surface of the composite oxide particles, and the effects of the present disclosure cannot be achieved. As mentioned above, the stability of the active material surface is specifically improved only when element A and element B coexist. In addition, there are appropriate addition amounts of elements A and B. Therefore, if the addition amount is not strictly controlled, not only will the effect of suppressing the increase in DCR not be achieved, but other battery performance will also be reduced.

[0019] For example, if the amount of element A added exceeds 1 mol% relative to the total molar number of metal elements other than Li in the composite oxide, the layer of element A becomes a resistance layer, reducing the battery capacity. Furthermore, if the amount of element B added exceeds 2 mol%, for example, Li is removed from the interior of the composite oxide particles, and the layer of element B becomes a resistance layer, reducing the battery capacity.

[0020] Hereinafter, an embodiment of the positive electrode active material for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the positive electrode active material disclosed herein will be described in detail with reference to the accompanying drawings. It should be noted that the multiple embodiments and modifications described below are initially envisioned to be selectively combined.

[0021] The following example illustrates a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16. However, the outer can of the battery is not limited to a cylindrical outer can. For example, a rectangular outer can (a rectangular battery), a coin-shaped outer can (a coin-shaped battery), or an outer can composed of a laminate sheet including a metal layer and a resin layer (a laminated battery) may also be used. Furthermore, the electrode body may be a stacked type in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0022] Figure 1 FIG is a cross-sectional view of a non-aqueous electrolyte secondary battery 10 as an example of an embodiment. Figure 1 As shown, the nonaqueous electrolyte secondary battery 10 includes a wound electrode body 14, a nonaqueous electrolyte, and an outer can 16 for housing the electrode body 14 and the nonaqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a metal container with a bottom and a cylindrical shape, open on one side in the axial direction. The opening of the outer can 16 is closed by a sealing member 17. For ease of description, the battery's sealing member 17 side is considered the top, and the bottom side of the outer can 16 is considered the bottom.

[0023] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all long, strip-shaped bodies, and are alternately stacked along the radial direction of the electrode body 14 by being spirally wound. The negative electrode 12 is formed to be one circle larger than the positive electrode 11 to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer in the length direction and the width direction (width direction) than the positive electrode 11. The two separators 13 are formed to be at least one circle larger than the positive electrode 11, for example, and are arranged in a manner of clamping the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0024] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14 . Figure 1 In the example shown, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 toward the sealing body 17, while the negative electrode lead 21 extends through the outside of the insulating plate 19 toward the bottom of the outer can 16. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like. The top plate of the sealing body 17, or lid 27, which is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, with the outer can 16 serving as the negative electrode terminal.

[0025] A gasket 28 is provided between the outer can 16 and the sealing member 17 to ensure the airtightness of the battery interior. The outer can 16 has a groove 22 formed on its side, with a portion of the side protruding inward to support the sealing member 17. The groove 22 is preferably formed in an annular shape along the circumference of the outer can 16, and its upper surface supports the sealing member 17. The sealing member 17 is secured to the upper portion of the outer can 16 by the groove 22 and the open end of the outer can 16, which is pressed against the sealing member 17.

[0026] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26 and a cover 27 are stacked in sequence from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat release, the lower valve body 24 is deformed and broken in a manner that pushes the upper valve body 26 toward the cover 27 side, thereby blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and the gas is discharged from the opening of the cover 27.

[0027] Hereinafter, the positive electrode 11 , the negative electrode 12 , the separator 13 , and the non-aqueous electrolyte, in particular, the positive electrode active material constituting the positive electrode 11 will be described in detail.

[0028] [positive electrode]

[0029] The positive electrode 11 includes a positive electrode core 30 and a positive electrode composite material layer 31 provided on the surface of the positive electrode core 30. The positive electrode core 30 may be made of a foil of a metal such as aluminum or an aluminum alloy that is stable within the potential range of the positive electrode 11, or a thin film having the metal provided on the surface. The positive electrode composite material layer 31 preferably includes a positive electrode active material, a conductive material, and a binder, and is provided on both sides of the positive electrode core 30. The positive electrode 11 can be manufactured, for example, by coating a positive electrode composite material slurry including a positive electrode active material, a conductive material, and a binder on the positive electrode core 30, drying the coating, and then compressing the coating to form the positive electrode composite material layers 31 on both sides of the positive electrode core 30.

[0030] Examples of the conductive material contained in the positive electrode composite material layer 31 include carbon materials such as carbon black, acetylene black, Ketjen black, graphite, and carbon nanotubes. Examples of the binder contained in the positive electrode composite material layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins can be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0031] Figure 2 This figure schematically shows a cross section of a particle of a lithium transition metal composite oxide 35 constituting a positive electrode active material as an example of an embodiment. The positive electrode active material of this embodiment includes a lithium transition metal composite oxide 35 (hereinafter referred to as "composite oxide 35"), wherein the lithium transition metal composite oxide 35 contains 80 mol% or more of Ni relative to the total molar number of metal elements other than Li. The composite oxide 35 further contains at least one selected from Co, Mn, Al, Ti, Nb, Fe, and Zn, and the content of Co is less than 5 mol%. In addition, as Figure 2 As shown, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36 .

[0032] As mentioned above, the nickel-rich composite oxide 35 is a useful positive electrode active material that contributes to higher battery capacity and energy density. However, it also poses the problem of increasing the battery's DCR during charged storage. In the composite oxide 35, at least one element A selected from Ca and Sr is present on the surface of the primary particles 36, and at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present on the surface of the secondary particles 37. Using this composite oxide as a positive electrode active material significantly suppresses increases in DCR.

[0033] The positive electrode active material of this embodiment has the composite oxide 35 as the main component. Here, the main component refers to the component with the largest mass ratio in the materials constituting the positive electrode active material. The positive electrode composite material layer 31 may contain composite oxides other than the composite oxide 35 as the positive electrode active material within the scope that does not impair the purpose of the present disclosure, but the ratio of the composite oxide 35 is preferably 50% by mass or more, more preferably 80% by mass or more. In this embodiment, the positive electrode active material is described as being essentially composed only of the composite oxide 35. In addition, the positive electrode active material may also be composed of two or more composite oxides 35 with different compositions.

[0034] The composite oxide 35 preferably contains other metal elements in addition to Li, Ni, and the aforementioned elements A and B. Suitable other metal elements include at least one selected from Co, Mn, Al, Ti, Nb, Fe, and Zn. The composite oxide 35 preferably contains at least one selected from Co, Al, and Mn. The total amount of metal elements other than Li, Ni, and the elements A and B contained in the composite oxide 35 is preferably 15 mol% or less, more preferably 10 mol% or less, for example, 5 mol% or more and 10 mol% or less, relative to the total molar amount of the metal elements other than Li.

[0035] The Ni content of the composite oxide 35 is 80 mol% or more, preferably 85 mol% or more, more preferably 90 mol% or more, relative to the total number of moles of metal elements other than Li. The upper limit value of the Ni content is, for example, 95 mol%. If the Ni content is within this range, high capacity / high energy density of the battery and good storage characteristics can be achieved simultaneously. The suitable composite oxide 35 contains Al and Mn in an amount of 5 mol% or more and 10 mol% or less relative to the total number of moles of metal elements other than Li. In this case, the structural stability of the composite oxide 35 is improved, which is beneficial to improving the storage characteristics. The contents of Al and Mn are, for example, 1 mol% or more and 5 mol% or less, respectively.

[0036] The Co content of the composite oxide 35 is less than 5 mol% relative to the total number of moles of metal elements other than Li, and preferably the composite oxide 35 substantially does not contain Co. Since Co is rare and expensive, by not using Co, the manufacturing cost of the battery can be reduced. In a positive electrode active material with a large amount of Ni, if the amount of Co is reduced, the DCR usually tends to increase. However, when elements A and B coexist on the particle surface, in a battery using a positive electrode active material with a small Co content or no Co, an increase in DCR can also be suppressed. The molar fractions of the metal elements contained in the composite oxide 35 are determined by inductively coupled plasma (ICP) emission spectrophotometric analysis.

[0037] The composite oxide 35 preferably has a layered rock salt structure. Examples of the layered rock salt structure of the composite oxide 35 include a layered rock salt structure belonging to the space group R-3m, a layered rock salt structure belonging to the space group C2 / m, etc. In addition, the half-value width n of the diffraction peak of the (2\(\theta\)8) plane in the X-ray diffraction pattern of the composite oxide 35 is preferably 0.30° < n < 0.50°, more preferably 0.35° < n < 0.50°. If n is less than this range, the ion diffusion path in the composite oxide increases, so the charge-discharge characteristics deteriorate. In addition, if n is greater than this range, the ion conductivity in the composite oxide is hindered, so the charge-discharge characteristics will deteriorate. If the half-value width n of the diffraction peak of the (2\(\theta\)8) plane is within this range, the deintercalation / insertion of Li becomes smooth and the charge-discharge capacity is improved.

[0038] The X-ray diffraction pattern of the composite oxide 35 is obtained under the following conditions using a powder X-ray diffractometer (manufactured by Rigaku Corporation, RINT-TTR, X-ray source Cu-K\(\alpha\)).

[0039] Measurement range: 15 - 120°

[0040] Scanning speed: 4° / min

[0041] Analysis range: 30 - 120°

[0042] Background: B-Splines

[0043] Curve function: segmented pseudo-Voig function

[0044] Binding condition: Li(3a)+Ni(3a)=1

[0045] Ni(3a)+Ni(3b)=α (α is the Ni content ratio)

[0046] ICSD No.:98-009-4814

[0047] As described above, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36. The average particle size of the primary particles 36 is, for example, greater than 200 nm and less than 500 nm. The average particle size of the primary particles 36 is determined by analyzing an SEM image of a particle cross section observed by a scanning electron microscope (SEM). For example, the positive electrode 11 is embedded in a resin, a cross section is made using a cross-section polisher (CP), and the cross section is photographed using an SEM. From the SEM image, 30 primary particles 36 are randomly selected and the grain boundaries are observed to determine the major axis (longest diameter) of each of the 30 primary particles 36, and the average value thereof is taken as the average particle size.

[0048] The volume-based median diameter (hereinafter referred to as "D50") of the secondary particles 37 (composite oxide 35) is, for example, 1 μm to 30 μm, preferably 5 μm to 20 μm. D50 is the particle diameter at which the cumulative frequency of the smallest particle in the volume-based particle size distribution reaches 50%, also known as the median diameter. The particle size distribution of the secondary particles 37 can be measured using a laser diffraction particle size analyzer (e.g., MT3000II manufactured by MicrotracBEL Corp.) using water as the dispersion medium.

[0049] At least one element A selected from Ca and Sr is present on the surface of the primary particles 36 constituting the composite oxide 35 in an amount of 0.01 mol% to 1 mol% relative to the total molar number of metal elements excluding Li. Element A is present on the surface of the secondary particles 37 and at the particle interface where the primary particles 36 are in contact with each other, and on the surface of the primary particles 36 constituting the secondary particles 37 of the composite oxide 35. It is believed that element A adheres to the surface of the primary particles 36 in the form of a compound, forming a coating 36A containing element A on the surface of the primary particles 36. It should be noted that the element distribution in the particle cross-section of the composite oxide 35 can be confirmed by energy dispersive X-ray spectroscopy (TEM-EDX).

[0050] For example, element A is essentially present only on the surface of primary particles 36 and does not form a solid solution with Ni or the like. Even small amounts of element A contribute to suppressing increases in DCR through interaction with element B. This effect becomes significant when the element A content exceeds 0.01 mol% relative to the total molar amount of metal elements excluding Li. On the other hand, if the element A content exceeds 1 mol%, the coating layer 36A containing element A becomes a resistive layer, reducing discharge capacity.

[0051] The content of element A needs to be controlled to be 0.01 mol% to 1 mol% relative to the total molar number of metal elements excluding Li, but is more preferably 0.05 mol% or more, and particularly preferably 0.1 mol% or more. From the perspective of achieving both high battery capacity and good storage characteristics, the upper limit of the content of element A is more preferably 0.9 mol% and particularly preferably 0.8 mol%. Examples of suitable content of element A are 0.05 mol% to 1 mol%, 0.1 mol% to 0.9 mol%, or 0.1 mol% to 0.8 mol%.

[0052] At least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present on the surface of the secondary particles 37 of the composite oxide 35 in an amount of 0.05 mol% to 2 mol% relative to the total molar number of Ni in the composite oxide 35. Like element A, element B can be present on the entire surface of the primary particles 36, including the interior of the secondary particles 37. However, it is preferably present only on the surface of the secondary particles 37 and not substantially within the interior of the secondary particles 37. In this case, an increase in the DCR during charged storage can be effectively suppressed. It is believed that element B uniformly adheres to the surface of the secondary particles 37 in the form of a compound, forming a coating 37B containing element B on the surface of the secondary particles 37.

[0053] Element B, which does not form a solid solution with Ni or the like, is present on the surface of the secondary particles 37. Even a small amount of element B contributes to suppressing an increase in DCR through interaction with element A, but this effect becomes significant when the element B content exceeds 0.05 mol% relative to the total molar amount of Ni in the composite oxide 35. On the other hand, if the element B content exceeds 2 mol%, Li is removed from the interior of the primary particles 36, and the coating layer 37B containing element B becomes a resistance layer, reducing the discharge capacity.

[0054] The content of element B relative to the total mole number of Ni in the composite oxide 35 must be controlled to be 0.05 mol% or more and 2 mol% or less, more preferably 0.06 mol% or more, and particularly preferably 0.08 mol% or more. From the viewpoint of achieving both high capacity and good storage characteristics of the battery, the upper limit value of the content of element B is more preferably 1.2 mol%, and particularly preferably 1 mol%. An example of a suitable content of element B is 0.05 mol% or more and 1.2 mol% or less, 0.08 mol% or more and 1 mol% or less, 0.09 mol% or more and 1 mol% or less, or 0.1 mol% or more and 1 mol% or less.

[0055] Element B preferably exists outside element A on the surface of the secondary particle 37. That is, in the cross-section of the particle of the composite oxide 35, element B and element A exist in layers in sequence from the particle surface side. For example, a coating 37B containing element B is formed on the surface of the secondary particle 37 so as to cover the coating 36A containing element A. It should be noted that a part of element B may also be directly attached to the surface of the secondary particle 37. In the present embodiment, since element A and element B coexist on the surface of the secondary particle 37, it is considered that the reaction with the electrolyte and HF is suppressed, and the storage characteristics are greatly improved.

[0056] The suitable contents of element A and element B are as described above, but the molar ratio of the contents of element B to element A (B / A) also relates to the improvement of the storage characteristics. The ratio (B / A) of the content of element B to the content of element A is, for example, 0.05 or more and 200 or less, preferably 0.08 or more and 50 or less, more preferably 0.1 or more and 10 or less, and particularly preferably 0.16 or more and 2.2 or less, based on the mole number. The content of element A is, for example, more than the content of element B, and an example of the ratio (B / A) is 0.1 or more and less than 1.

[0057] An example of a suitable composite oxide 35 is represented by the compositional formula Li α Ni β Co x Al y Mn z A a B b O2 (where 0.9 ≤ α ≤ 1.2, 0.80 ≤ β ≤ 0.95, 0 ≤ x < 0.05, 0.01 < y ≤ 0.07, 0 ≤ z ≤ 0.05, 0.0001 ≤ a ≤ 0.01, 0.0005 ≤ b ≤ 0.02). The contents of elements A and B may be substantially the same, may be a > b, or a < b, but are preferably less than the contents of Ni, Al, and Mn.

[0058] The composite oxide 35 can be produced, for example, through the following steps: a first step of obtaining a composite oxide containing metal elements such as Ni and Al; a second step of mixing the composite oxide obtained in the first step, a compound containing element A, and a Li compound to obtain a mixture; a third step of calcining the mixture; and a fourth step of adding a compound containing element B and performing heat treatment.

[0059] In the first step, for example, a solution containing a metal salt such as Ni or Al is stirred while an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to alkaline (e.g., 8.5 to 12.5), thereby precipitating (coprecipitating) a composite hydroxide containing metal elements such as Ni and Al. Thereafter, the composite hydroxide is calcined to synthesize a composite oxide containing metal elements such as Ni and Al. The calcination temperature is not particularly limited, and is, for example, 300° C. or higher and 600° C. or lower.

[0060] In the second step, the composite oxide obtained in the first step, the compound containing element A, and the Li compound are mixed to obtain a mixture. Examples of the compound containing element A include Ca(OH)2, CaO, CaCO3, CaSO4, Ca(NO3)2, Sr(OH)2, Sr(OH)2·H2O, Sr(OH)2·8H2O, SrO, SrCO3, SrSO4, and Sr(NO3)2. The particle size of the compound containing element A is preferably 0.1 μm or more and 20 μm or less. If the compound containing element A contains water, it may be dried or dehydrated before use to suppress the generation of water during calcination. In addition, examples of the Li compound include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF.

[0061] In order to facilitate adjustment of the above parameters within the above-defined ranges, the mixing ratio of the composite oxide obtained in the first step and the Li compound is preferably set to a ratio such that the molar ratio of the metal element other than Li:Li is within the range of 1:0.98 to 1:1.1. In addition, in order to facilitate adjustment of the above parameters within the above-defined ranges, the mixing ratio of the composite oxide obtained in the first step and at least one of the Sr compound and the Ca compound is preferably set to a ratio such that the molar ratio of the metal element other than Li:(Sr+Ca) is within the range of 1:0.0005 to 1:0.006. In the second step, when mixing the composite oxide obtained in the first step with the Li compound and the Sr compound or the Ca compound, other metal raw materials may be added as needed. Other metal raw materials are oxides of metal elements other than the metal element constituting the composite oxide obtained in the first step, etc.

[0062] In the third step, the mixture obtained in the second step is calcined under an oxygen atmosphere. Through this step, a coating 36A containing element A is formed on the surface of the primary particle 36. As an example of the calcination conditions, the heating rate from 450°C to 680°C is set to 1.0°C / minute to 5.5°C / minute, and the maximum temperature reached is set to 700°C to 850°C. The heating rate from 450°C to 680°C can be 0.1°C / minute to 5.5°C / minute, or 0.2°C / minute to 5.5°C / minute. The heating rate from 680°C to the maximum temperature reached is, for example, 0.1°C / minute to 3.5°C / minute. The holding time of the maximum temperature reached can be 1 hour to 10 hours.

[0063] In the fourth step, a compound containing element B is mixed with the fired composite oxide, and the mixture is heat-treated. Through this step, a coating 37B containing element B is formed on the surface of the secondary particles 37, and at this time, a coating 37B is formed on the coating 36A. The fired composite oxide obtained in the third step can be washed with water by a conventionally known method. After washing, a compound containing element B can be added to the composite oxide powder in a wet state, and then heat-treated (dried). The compound containing element B can be added in a powdered state or in a dissolved or dispersed state in water.

[0064] Examples of compounds containing element B include WO3, ZrO2, B2O3, (NH4)2[Zr(CO3)2(OH)2], Al(NO3)3, Nb2O5, MoO3, and TiO2. The particle size of the compound containing element B is preferably 0.1 μm to 20 μm. The heat treatment temperature is, for example, 150°C to 300°C in a vacuum atmosphere.

[0065] [negative electrode]

[0066] The negative electrode 12 includes a negative electrode core 40 and a negative electrode composite material layer 41 provided on the surface of the negative electrode core 40. The negative electrode core 40 may be made of a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a thin film having the metal disposed on the surface. The negative electrode composite material layer 41 preferably contains a negative electrode active material and a binder, and is provided on both sides of the negative electrode core 40. The negative electrode 12 can be produced, for example, by applying a negative electrode composite material slurry containing a negative electrode active material, a conductive material, and a binder to the surface of the negative electrode core 40, drying the coating, and then compressing the coating to form the negative electrode composite material layers 41 on both sides of the negative electrode core 40.

[0067] The negative electrode composite material layer 41 includes, for example, a carbon-based active material that reversibly absorbs and releases lithium ions as the negative electrode active material. Suitable carbon-based active materials include natural graphites such as flaky graphite, bulk graphite, and earthy graphite, as well as artificial graphites such as bulk artificial graphite (MAG) and graphitized mesocarbon microbeads (MCMB). Furthermore, a Si-based active material composed of at least one of Si and a Si-containing compound can be used as the negative electrode active material, or a carbon-based active material and a Si-based active material can be used in combination.

[0068] As the conductive material contained in the negative electrode composite material layer 41, carbon materials such as carbon black, acetylene black, Ketjen black, and graphite can be used, as in the case of the positive electrode 11. Among the binding materials contained in the negative electrode composite material layer 41, fluororesins, PAN, polyimide, acrylic resins, polyolefins, etc. can also be used, as in the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. In addition, the negative electrode composite material layer preferably further includes CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. Among them, SBR is suitably used in combination with CMC or a salt thereof, and PAA or a salt thereof.

[0069] [Separator]

[0070] A porous sheet having ion permeability and insulation is used in the separator 13. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, and the like. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may be a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, a heat-resistant layer composed of a heat-resistant resin such as an aromatic polyamide resin, polyimide, or polyamide-imide, and the like may be formed on the surface of the separator 13.

[0071] [Non-aqueous electrolyte]

[0072] The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more thereof. The non-aqueous solvent may also contain a halogen substituted substance in which at least a portion of the hydrogen of these solvents is replaced by a halogen atom such as fluorine. Examples of the halogen substituted substance include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylates such as methyl fluoropropionate (FMP).

[0073] Examples of the above esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, etc., chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, etc., cyclic carboxylic acid esters such as γ-butyrolactone (GBL), γ-valerolactone (GVL), etc., and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate (EP), etc.

[0074] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ethers, etc., and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0075] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), etc., imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, etc. These lithium salts can be used alone or in combination of multiple kinds. Among them, from the viewpoints of ionic conductivity, electrochemical stability, etc., LiPF6 is preferably used. The concentration of the lithium salt is, for example, 0.8 mol / L or more and 1.8 mol / L or less with respect to each 1 L of the non-aqueous solvent. Further, vinylene carbonate, propanesultone-based additives, etc. can be added.

[0076] <Example>

[0077] Hereinafter, the present disclosure will be further described based on examples, but the present disclosure is not limited to these examples.

[0078] <Example 1>

[0079] [Synthesis of positive electrode active material]

[0080] The composite hydroxide containing Ni, Co, and Al obtained by the coprecipitation method is calcined at 500°C for 8 hours to obtain a composite oxide (the molar ratio of Ni, Co, and Al is 91.75:4:4) (first step). Next, LiOH, the above composite oxide, and Ca(OH)2 are mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al and Ca becomes 1.02:0.9975:0.0025 to obtain a mixture (second step). The mixture is heated from room temperature to 650°C at a heating rate of 2.0°C / min in an oxygen gas flow, and then calcined from 650°C to 730°C at a heating rate of 0.5°C / min to obtain a calcined product (third step). After washing the calcined product with water, a specified amount of tungsten oxide is added and dried at 180°C for 1 hour to obtain a lithium transition metal composite oxide (positive electrode active material) containing the elements shown in Table 1 (fourth step).

[0081] The obtained positive electrode active material comprises secondary particles with a D50 of 12 μm, formed by aggregation of primary particles with an average particle size of 350 nm. TEM-EDX measurements of particle cross sections confirmed that Ca is roughly uniformly present on the surface of the primary particles, and W is roughly uniformly present on the surface of the secondary particles. It should be noted that W is present on the outside of the particles, covering Ca.

[0082] [Production of positive electrode]

[0083] As the positive electrode active material, the above-mentioned lithium transition metal composite oxide is used. The positive electrode active material is mixed with acetylene black and polyvinylidene fluoride at a specified solid content mass ratio, and N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium to prepare a positive electrode composite material slurry. Next, the positive electrode composite material slurry is applied to the positive electrode core formed by aluminum foil, the coating film is dried, compressed, and cut into a specified electrode size to obtain a positive electrode.

[0084] [Production of negative electrode]

[0085] Graphite, a dispersant for styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) are mixed at a specified solid content ratio by mass, and water is used as the dispersion medium to prepare a negative electrode composite material slurry. This negative electrode composite material slurry is then applied to both sides of a negative electrode core formed of copper foil. The coating is dried, compressed, and then cut into the specified electrode size to produce a negative electrode having a negative electrode composite material layer formed on both sides of the negative electrode core.

[0086] [Preparation of non-aqueous electrolyte]

[0087] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) are mixed at a predetermined volume ratio. LiPF6 is added to the mixed solvent to obtain a non-aqueous electrolyte solution.

[0088] [Fabrication of Test Battery Cell (Non-aqueous Electrolyte Secondary Battery)]

[0089] The positive electrode, attached with an aluminum positive lead, and the negative electrode, attached with a nickel negative lead, were spirally wound with a polyethylene separator interposed therebetween to form a flat shape, thereby producing a wound electrode assembly. This electrode assembly was housed in an outer casing made of an aluminum laminate sheet, and after injecting the nonaqueous electrolyte, the opening of the outer casing was sealed to produce a test cell for evaluation.

[0090] [Evaluation of DCR Rising Rate]

[0091] The prepared test battery cells were charged at a constant current of 0.5 It at a temperature of 25°C until the battery voltage reached 4.2V. Low-voltage charging was then performed at 4.2V until the current value reached 1 / 50 It. Discharge was then performed at a current of 0.5 It, and the voltage before discharge and the voltage 10 seconds after the start of self-discharge were measured. The DCR before the storage test was calculated using the following formula.

[0092] DCR (Ω) = (voltage before discharge - voltage 10 seconds after self-discharge) / current value

[0093] The test cells were then charged at a constant current of 0.5 It until the battery voltage reached 4.2 V. They were then charged at a low voltage of 4.2 V until the current value reached 1 / 50 It. The cells were then left standing in a high-temperature environment at 60°C for 45 days. The DCR of the test cells after storage was calculated using the above method, and the DCR increase rate after storage was also calculated. The evaluation results are shown in Table 1.

[0094] The DCR increase rates shown in Table 1 are relative values ​​based on the DCR increase rate of a test cell of Comparative Example 1 described later.

[0095] <Example 2>

[0096] A test cell was prepared in the same manner as in Example 1 except that ammonium zirconium carbonate was added instead of tungsten oxide during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0097] <Example 3>

[0098] A test cell was prepared in the same manner as in Example 1 except that boron oxide was added instead of tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0099] <Example 4>

[0100] A test cell was prepared in the same manner as in Example 1 except that aluminum nitrate was added instead of tungsten oxide during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0101] <Example 5>

[0102] A test cell was prepared in the same manner as in Example 1 except that niobium hydroxide was added instead of tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0103] <Example 6>

[0104] A test cell was prepared in the same manner as in Example 1 except that ammonium zirconium carbonate was added together with tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0105] <Example 7>

[0106] A test cell was prepared in the same manner as in Example 1 except that boron oxide was added together with tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0107] <Example 8>

[0108] A test cell was prepared in the same manner as in Example 1 except that aluminum nitrate was added together with tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0109] <Example 9>

[0110] A test cell was prepared in the same manner as in Example 1 except that niobium hydroxide was added together with tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0111] <Examples 10 to 18>

[0112] Test cells were prepared in the same manner as in Examples 1 to 9 except that strontium hydroxide was added instead of calcium hydroxide during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0113] <Examples 19 to 36>

[0114] In the synthesis of the positive electrode active material, a composite oxide containing Ni, Al, and Mn (the molar ratio of Ni, Al, and Mn is 93.75:3:3) was used instead of the composite oxide containing Ni, Co, and Al. Otherwise, test battery cells were prepared in the same manner as in Examples 1 to 18, and the DCR increase rate was evaluated.

[0115] <Comparative Example 1>

[0116] A test cell was produced in the same manner as in Example 1 except that calcium hydroxide and tungsten oxide were not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0117] <Comparative Example 2>

[0118] A test cell was produced in the same manner as in Example 1 except that tungsten oxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0119] <Comparative Example 3>

[0120] A test cell was produced in the same manner as in Example 10 except that tungsten oxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0121] <Comparative Examples 4 to 8>

[0122] Test cells were prepared in the same manner as in Examples 1 to 5 except that calcium hydroxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0123] <Comparative Example 9>

[0124] A test cell was produced in the same manner as in Example 19 except that calcium hydroxide and tungsten oxide were not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0125] <Comparative Example 10>

[0126] A test cell was produced in the same manner as in Example 20 except that tungsten oxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0127] <Comparative Example 11>

[0128] A test cell was produced in the same manner as in Example 28 except that tungsten oxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0129] <Comparative Examples 12 to 16>

[0130] Test cells were prepared in the same manner as in Examples 19 to 23 except that calcium hydroxide was not added during the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0131] [Table 1]

[0132]

[0133] As shown in Table 1, the test cells of the examples all showed a lower rate of increase in DCR after charge storage than the test cells of the comparative examples, and all exhibited excellent storage characteristics. The results shown in Table 1 indicate that the DCR after storage significantly increased when using a positive electrode active material in which Ca or Sr (element A) was not present on the surface of the primary particles of the lithium transition metal composite oxide (Comparative Examples 4-8, 12-16), a positive electrode active material in which W, Zr, B, Al, or Nb (element B) was not present on the surface of the secondary particles (Comparative Examples 2, 3, 10, 11), and a positive electrode active material in which both elements A and B were absent (Comparative Examples 1 and 9). Furthermore, when only one of elements A or B was present on the particle surface of the positive electrode active material, the DCR after storage increased compared to when neither element A or B was present.

[0134] In other words, it is believed that the interaction between elements A and B improves the stability of the active material surface, specifically suppressing the increase in DCR after storage. While reducing the amount of Co in a positive electrode active material with a high Ni content generally tends to increase DCR, when elements A and B coexist on the particle surface, DCR increases can also be suppressed in batteries using positive electrode active materials with low or no Co content. This allows for a further increase in the Ni ratio, reducing battery manufacturing costs while achieving higher capacity.

[0135] <Examples 37 to 39, Comparative Example 17>

[0136] In the synthesis of the positive electrode active material, the amount of the Ca raw material charged was changed so that the amount of element A became the value shown in Table 2. A test cell was prepared in the same manner as in Example 19, and the DCR increase rate was evaluated.

[0137] <Examples 40-42, Comparative Example 18>

[0138] In the synthesis of the positive electrode active material, the amount of W raw material charged was changed so that the amount of element B became the value shown in Table 2. A test cell was prepared in the same manner as in Example 19, and the DCR increase rate was evaluated.

[0139] [Table 2]

[0140]

[0141] As shown in Table 2, when a specified amount of element A is included, both high battery capacity and good storage characteristics can be achieved. However, if the amount of element A is too low, the effect of suppressing the increase in DCR is small. If the amount of element A is too high, the DCR suppression effect is achieved, but the battery discharge capacity decreases. (Examples 19, 37-39, Comparative Example 17). Furthermore, when a specified amount of element B is included, both high battery capacity and good storage characteristics can be achieved. However, if the amount of element B is too high, the DCR suppression effect is achieved, but the battery discharge capacity decreases. (Examples 19, 40-42, Comparative Example 18). Therefore, controlling the amounts of element A and element B appropriately is crucial for both high battery capacity and good storage characteristics.

[0142] Description of Reference Numerals

[0143] 10 Non-aqueous electrolyte secondary batteries

[0144] 11. Positive electrode

[0145] 12 negative electrode

[0146] 13 Dividers

[0147] 14 Electrode body

[0148] 16 outer cans

[0149] 17 Sealing body

[0150] 18, 19 Insulation board

[0151] 20 Positive lead

[0152] 21 Negative lead

[0153] 22 groove part

[0154] 23 Internal terminal board

[0155] 24 Lower valve body

[0156] 25 Insulation components

[0157] 26 Upper valve body

[0158] 27 lid

[0159] 28 gasket

[0160] 30 positive electrode core

[0161] 31. Positive electrode composite material layer

[0162] 35 Lithium transition metal composite oxides (composite oxides)

[0163] 36 primary particles

[0164] 36A, 37B coating

[0165] 37 Secondary particles

[0166] 40 negative electrode core

[0167] 41 negative electrode composite material layer

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide, wherein the lithium transition metal composite oxide contains: Ni in an amount of 80 mol% or more relative to the total molar amount of metal elements other than Li, and at least one selected from Co, Mn, Al, Ti, Nb, Fe, and Zn, wherein the content of Co is less than 5 mol%. The lithium transition metal composite oxide comprises secondary particles formed by aggregation of primary particles. At least one element A selected from Ca and Sr is present on the surface of the primary particles in an amount of 0.01 mol% or more and 1 mol% or less relative to the total number of moles of metal elements excluding Li, At least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present on the surface of the secondary particles in an amount of 0.05 mol% to 2 mol% based on the total moles of Ni in the composite oxide.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein The element B exists on the surface of the secondary particles and outside the element A.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein The element B exists only on the surface of the secondary particles and does not substantially exist inside the secondary particles.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein The content of the element B is 1.2 mol% or less relative to the total molar number of Ni in the composite oxide.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein The ratio of the content of the element B to the content of the element A is 0.05 or more and 200 or less on a molar basis.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein The half-value width n of the diffraction peak of the (208) plane in the X-ray diffraction spectrum of the lithium transition metal composite oxide is 0.30° <n<0.55°。 7 . A non-aqueous electrolyte secondary battery comprising: a positive electrode comprising the positive electrode active material according to claim 1 , a negative electrode, and a non-aqueous electrolyte.

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