Method for producing positive electrode active material for non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery using a positive electrode active material containing a large amount of Ni, capable of suppressing an increase in a DC resistance during charge storage.SOLUTION: A positive electrode active material includes a lithium transition metal composite oxide containing 80 mol% or more of Ni and a Co content of less than 5 mol% with respect to the total number of moles of metal elements excluding Li. 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 with respect 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% or more and 2 mol% or less with respect to the total number of moles of Ni in a composite oxide.SELECTED DRAWING: Figure 2

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 greatly affects battery performance such as input / output characteristics, capacity, cycle characteristics, and storage characteristics. Generally, the positive electrode active material contains metal elements such as Ni, Co, Mn, and Al, and a lithium transition metal composite oxide composed of secondary particles formed by aggregation of primary particles is used. Since the properties of the positive electrode active material vary greatly depending on its composition, particle shape, etc., many studies have been conducted on various positive electrode active materials. In particular, lithium transition metal composite oxides with a high Ni content are expected as positive electrode active materials that contribute to 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, wherein the proportion of Ni is 30 mol% or more and 60 mol% or less, the proportion of Co is 15 mol% or more and 35 mol% or less, the proportion of Mn is 15 mol% or more and 35 mol% or less, and the proportion of W is more than 0 mol% and 5 mol% or less, and W is unevenly distributed on the surface layer of the positive electrode active material. Patent Document 1 also describes that by using this positive electrode active material, the output characteristics and cycle characteristics of the battery are improved.

[0004] Further, Patent Document 2 discloses a positive electrode active material having a layered crystal structure Li 1+a M 1-a O 2±b M’ k S m (-0.03 < a < 0.06, b ≒ 0), wherein M is a transition metal compound composed of at least one or more elements from the group of Ni, Mn, Co, and Ti in an amount of at least 95%, and M’ is a specific element present on the surface of the oxide. Patent Document 2 describes that by using this positive electrode active material, the performance of the lithium battery as a cathode is improved.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Since Co is rare and expensive, the manufacturing cost of the battery can be reduced by reducing the amount of Co used. However, when the amount of Co is reduced in the positive electrode active material with a large amount of Ni that contributes to the high capacity of the battery, there is a problem that the direct current resistance (DCR) increases during charging and storage of the battery. Note that the non-aqueous electrolyte secondary battery using the positive electrode active materials of Patent Documents 1 and 2 still has room for improvement in suppressing the increase in DCR.

Means for Solving the Problems

[0007] A method for manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery according to an aspect of the present disclosure includes manufacturing a composite oxide containing Ni and having a Co content of less than 5 mol%, mixing the composite oxide with a compound containing at least one element A selected from Ca and Sr, and a Li compound to produce a mixture, firing the mixture in an oxygen atmosphere, mixing a compound containing at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti with the fired mixture, and heat-treating the mixture to obtain a lithium transition metal composite oxide containing 80 mol% or more of Ni with respect to the total number of moles of metal elements excluding Li, 0.01 mol% or more and 1 mol% or less of the element A with respect to the total number of moles of metal elements excluding Li, and 0.05 mol% or more and 2 mol% or less of the element B with respect to the total number of moles of Ni.

[0008] A non-aqueous electrolyte secondary battery according to an aspect of the present disclosure includes a positive electrode containing the above positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

Advantages of the Invention

[0009] According to one aspect of the present disclosure, in a non-aqueous electrolyte secondary battery using a cathode active material with a high Ni content, an increase in DC resistance during charge storage can be suppressed. By using the cathode active material which is one aspect of the present disclosure, for example, a non-aqueous electrolyte secondary battery with high capacity and excellent storage characteristics can be provided.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0011] As described above, a lithium transition metal composite oxide with a large amount of Ni is a useful cathode active material that contributes to increasing the capacity and energy density of the battery. However, conversely, there is a problem that the increase in DC resistance (DCR) during charge storage of the battery is large.

[0012] As a result of intensive studies to solve this problem, the present inventors have found that in a lithium transition metal composite oxide with a large amount of Ni, by allowing at least one of Ca and Sr (element A) to be present in a predetermined amount on the surface of the primary particles of the composite oxide, and allowing at least one kind (element B) selected from B, Zr, W, Al, Nb, Mo, and Ti to be present in a predetermined amount on the surface of the secondary particles of the composite oxide, the increase in DCR during charge storage of the battery is specifically suppressed.

[0013] When the amount of Co is reduced in a cathode active material with a large amount of Ni, the stability of the surface of the active material decreases, making it easier for side reactions with the electrolyte to occur. Then, it is considered that the deterioration of the surface of the active material is promoted, and the DCR during charge storage increases significantly. In addition, the unstable surface of the active material is also easily eroded by HF that can be generated when a fluorine-containing compound in the electrolyte reacts with moisture. According to the cathode active material of the present disclosure, due to the interaction caused by the coexistence of element A and element B, a stable protective layer is formed on the surface of the secondary particles of the composite oxide, and the stability of the surface of the active material is greatly improved. As a result, it is considered that the reaction between the electrolyte and HF on the surface of the active material is suppressed, and the storage characteristics are improved.

[0014] In addition, when element A or element B is absent, a stable protective layer is not formed on the surface of the particles of the composite oxide, and the effects of the present disclosure cannot be obtained. As described above, the stability of the surface of the active material is specifically improved only when element A and element B coexist. In addition, since there are appropriate addition amounts for elements A and B, if the addition amounts are not strictly controlled, not only the effect of suppressing the increase in DCR cannot be obtained, but other battery performances will also deteriorate.

[0015] For example, when the addition amount of element A exceeds 1 mol% with respect to the total number of moles of metal elements excluding Li in the composite oxide, the layer of element A becomes a resistance layer and the battery capacity decreases. In addition, when the addition amount of element B exceeds 2 mol%, for example, Li is extracted from inside the particles of the composite oxide, and the layer of element B becomes a resistance layer, resulting in a decrease in battery capacity.

[0016] Hereinafter, with reference to the drawings, an example of an embodiment of a cathode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the cathode active material according to the present disclosure will be described in detail. It is initially assumed that a plurality of embodiments and modifications described below can be selectively combined.

[0017] Hereinafter, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical exterior can 16 will be exemplified. However, the exterior body of the battery is not limited to a cylindrical exterior can, and may be, for example, a rectangular exterior can (rectangular battery), a coin-shaped exterior can (coin-shaped battery), or an exterior body (laminated battery) composed of a laminated sheet including a metal layer and a resin layer. Further, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via a separator.

[0018] FIG. 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery 10 which is an example of an embodiment. As shown in FIG. 1, the non-aqueous electrolyte secondary battery 10 includes a wound electrode body 14, a non-aqueous electrolyte, and an exterior can 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 has 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 wound in a spiral shape via the separator 13. The exterior can 16 is a bottomed cylindrical metal container with one side in the axial direction open, and the opening of the exterior can 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the side of the battery where the sealing body 17 is located is taken as the upper side, and the bottom side of the exterior can 16 is taken as the lower side.

[0019] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode body 14 are all strip-shaped elongated bodies, and are alternately laminated in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed with dimensions slightly larger than those of the positive electrode 11 in order to prevent precipitation of lithium. That is, the negative electrode 12 is formed longer than the positive electrode 11 in the longitudinal direction and the width direction (short side direction). The two separators 13 are formed with dimensions at least slightly larger than those of the positive electrode 11 and are arranged, for example, so as to sandwich 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.

[0020] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. In the example shown in FIG. 1, the positive electrode lead 20 extends toward the sealing body 17 through the through-hole of the insulating plate 18, and the negative electrode lead 21 extends toward the bottom side of the outer can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0021] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness inside the battery. The outer can 16 is formed with a groove portion 22 for supporting the sealing body 17, with a part of the side surface portion projecting inward. The groove portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the outer can 16 by the groove portion 22 and the open end portion of the outer can 16 caulked to the sealing body 17.

[0022] 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 cap 27 are laminated in order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is 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 interposed between the peripheral edges of each. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks so as to push the upper valve body 26 toward the cap 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0023] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte will be described in detail, particularly the positive electrode active material constituting the positive electrode 11.

[0024] [Positive Electrode] The positive electrode 11 includes a positive electrode core 30 and a positive electrode mixture layer 31 provided on the surface of the positive electrode core 30. For the positive electrode core 30, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum or an aluminum alloy, a film having such a metal disposed on its surface layer, etc. can be used. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core 30. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive material, a binder, etc. onto the positive electrode core 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0025] Examples of the conductive material contained in the positive electrode mixture 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 mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, etc. These resins may be used in combination with a cellulose derivative such as carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc.

[0026] FIG. 2 is a diagram schematically showing a particle cross-section of a lithium transition metal composite oxide 35 constituting a positive electrode active material, which is an example of an embodiment. The positive electrode active material of this embodiment contains a lithium transition metal composite oxide 35 (hereinafter referred to as "composite oxide 35") containing 80 mol% or more of Ni with respect to the total molar number of metal elements excluding 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%. Also, as shown in FIG. 2, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36.

[0027] As described above, the composite oxide 35 with a high Ni content is a useful cathode active material that contributes to increasing the capacity and energy density of the battery. However, there is a problem that the DCR during charging and storage of the battery increases. 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. By applying this to the cathode active material, an increase in DCR can be highly suppressed.

[0028] The cathode active material of this embodiment contains the composite oxide 35 as the main component. Here, the main component means the component with the highest mass ratio among the materials constituting the cathode active material. The cathode composite material layer 31 may contain a composite oxide other than the composite oxide 35 as long as the object of the present disclosure is not impaired. However, the ratio of the composite oxide 35 is preferably 50% by mass or more, more preferably 80% by mass or more. In this embodiment, it will be described that the cathode active material is substantially composed of only the composite oxide 35. Further, the cathode active material may be composed of two or more types of composite oxides 35 having different compositions.

[0029] In addition to Li, Ni, and the above elements A and B, the composite oxide 35 preferably contains other metal elements. 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 at least Co, Al, and Mn among these. The total amount of metal elements other than Li, Ni, and 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, based on the total number of moles of metal elements excluding Li.

[0030] The Ni content of the composite oxide 35 is 80 mol% or more, preferably 85 mol% or more, more preferably 90 mol% or more, based on the total number of moles of metal elements excluding Li. The upper limit value of the Ni content is, for example, 95 mol%. If the Ni content is within this range, it is possible to achieve both high capacity and high energy density of the battery and good storage characteristics. A preferred composite oxide 35 contains Al and Mn in an amount of 5 mol% or more and 10 mol% or less based on the total number of moles of metal elements excluding Li. In this case, the structural stability of the composite oxide 35 is improved, contributing to the improvement of storage characteristics. The content of Al and Mn is, for example, 1 mol% or more and 5 mol% or less, respectively.

[0031] The Co content of the composite oxide 35 is less than 5 mol% based on the total number of moles of metal elements excluding Li, and it is preferable that the composite oxide 35 does not substantially contain Co. Since Co is rare and expensive, the manufacturing cost of the battery can be reduced by not using Co. When the amount of Co is reduced in a positive electrode active material with a large amount of Ni, generally the DCR tends to increase. However, when elements A and B coexist on the particle surface, even in a battery using a positive electrode active material with a low Co content or no Co, an increase in DCR can be suppressed. The molar fraction of the metal elements contained in the composite oxide 35 is measured by inductively coupled plasma (ICP) optical emission spectrometry.

[0032] 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 and a layered rock salt structure belonging to the space group C2 / m. Further, the half-value width n of the diffraction peak of the (208) 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 smaller than this range, the ion diffusion path in the composite oxide increases, resulting in a decrease in charge-discharge characteristics. If n is larger than this range, the ion conduction in the composite oxide is inhibited, resulting in a decrease in charge-discharge characteristics. If the half-value width n of the diffraction peak of the (208) plane is within this range, the insertion and extraction of Li become smooth, improving the charge-discharge capacity.

[0033] 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, radiation source Cu-Kα).

[0034] Measurement range: 15 - 120° Scan speed: 4° / min Analysis range: 30 - 120° Background: B-spline Profile function: Split pseudo-Voigt function Constraint condition: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the Ni content ratio of each) ICSD No.: 98-009-4814 As described above, the composite oxide 35 includes secondary particles 37 formed by aggregation of primary particles 36. The average particle diameter of the primary particles 36 is, for example, 200 nm or more and 500 nm or less. The average particle diameter of the primary particles 36 is determined by analyzing the SEM image of the 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 prepared by cross-section polisher (CP) processing, and this cross-section is photographed by SEM. From the SEM image, 30 primary particles 36 are randomly selected to observe the grain boundaries, the major axis (longest diameter) of each of the 30 primary particles 36 is obtained, and the average value thereof is taken as the average particle diameter.

[0035] The volume-based median diameter (hereinafter referred to as "D50") of the secondary particles 37 (composite oxide 35) is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 20 μm or less. D50 means the particle diameter at which the cumulative frequency in the volume-based particle size distribution becomes 50% from the smaller particle diameter side, and is also called the median diameter. The particle size distribution of the secondary particles 37 can be measured using a laser diffraction type particle size distribution measuring device (for example, manufactured by Microtrac Bell Corporation, MT3000II) with water as the dispersion medium.

[0036] On the surface of the primary particles 36 that make up the composite oxide 35, at least one element A selected from Ca and Sr is present in an amount of 0.01 mol% or more and 1 mol% or less with respect to the total molar amount 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 contact each other, and is present on the surface of the primary particles 36 that make up the secondary particles 37 of the composite oxide 35. It is considered that element A adheres to the surface of the primary particles 36 in a compound state, and a coating layer 36A containing element A is formed on the surface of the primary particles 36. The elemental distribution in the particle cross-section of the composite oxide 35 can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0037] Element A, for example, does not form a solid solution with Ni or the like and is substantially present only on the surface of the primary particles 36. Even a small addition of element A contributes to suppressing the increase in DCR due to the interaction with element B, but the effect becomes significant when added in an amount of 0.01 mol% or more with respect to the total molar amount of metal elements excluding Li. On the other hand, when the content of element A exceeds 1 mol%, the coating layer 36A containing element A becomes a resistance layer and the discharge capacity decreases.

[0038] The content of element A needs to be controlled to be 0.01 mol% or more and 1 mol% or less with respect to the total molar amount of metal elements excluding Li, more preferably 0.05 mol% or more, and particularly preferably 0.1 mol% or more. The upper limit value of the content of element A is more preferably 0.9 mol%, and particularly preferably 0.8 mol% from the viewpoint of achieving both high capacity and good storage characteristics of the battery. An example of a suitable content of element A is 0.05 mol% or more and 1 mol% or less, 0.1 mol% or more and 0.9 mol% or less, or 0.1 mol% or more and 0.8 mol% or less.

[0039] On the surface of the secondary particles 37 of the composite oxide 35, at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is present in an amount of 0.05 mol% or more and 2 mol% or less with respect to the total number of moles of Ni in the composite oxide 35. The element B may be present on the entire surface of the primary particles 36 including the inside of the secondary particles 37, similar to the element A, but it is preferably present only on the surface of the secondary particles 37 and substantially not present inside the secondary particles 37. In this case, the increase in DCR during charge storage can be efficiently suppressed. It is considered that the element B adheres uniformly to the surface of the secondary particles 37 in a compound state, and a coating layer 37B containing the element B is formed on the surface of the secondary particles 37.

[0040] On the surface of the secondary particles 37, there is an element B that is not solid-solved with Ni or the like. Even a small amount of the element B contributes to suppressing the increase in DCR due to the interaction with the element A, but the effect becomes remarkable when it is added in an amount of 0.05 mol% or more with respect to the total number of moles of Ni in the composite oxide 35. On the other hand, when the content of the element B exceeds 2 mol%, Li is extracted from the inside of the primary particles 36, and the coating layer 37B containing the element B becomes a resistance layer, resulting in a decrease in discharge capacity.

[0041] The content of the element B needs to be controlled to be 0.05 mol% or more and 2 mol% or less with respect to the total number of moles of Ni in the composite oxide 35, more preferably 0.06 mol% or more, and particularly preferably 0.08 mol% or more. The upper limit value of the content of the element B is more preferably 1.2 mol%, and particularly preferably 1 mol% from the viewpoint of achieving both high capacity and good storage characteristics of the battery. An example of a suitable content of the 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.

[0042] Element B preferably exists outside element A on the surface of the secondary particles 37. That is, in the particle cross-section of the composite oxide 35, element B and element A exist in layers in this order from the particle surface side. On the surface of the secondary particles 37, for example, a coating layer 37B containing element B is formed so as to cover a coating layer 36A containing element A. Note that a part of element B may be directly attached to the surface of the secondary particles 37. In the present embodiment, it is considered that the reaction with the electrolytic solution and HF is suppressed by the coexistence of element A and element B on the surface of the secondary particles 37, and the storage characteristics are greatly improved.

[0043] Although the preferred contents of element A and element B are as described above, the abundance ratio of element A and element B is also related to the improvement of the storage characteristics. The ratio (B / A) of the content of element B to the content of element A is, on a molar basis, 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. 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.

[0044] An example of the preferred composite oxide 35 has the composition 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, or a > b or a < b may be satisfied, but it is preferable that they are less than the contents of Ni, Al, and Mn.

[0045] The composite oxide 35 can be produced, for example, through 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 firing the mixture, and a fourth step of adding a compound containing element B and performing heat treatment.

[0046] In the first step, for example, while stirring a solution of a metal salt containing Ni, Al, etc., an alkaline solution such as sodium hydroxide is dropped, and the pH is adjusted to the alkaline side (for example, 8.5 to 12.5) to precipitate (co-precipitate) a composite hydroxide containing metal elements such as Ni and Al. Then, by firing this composite hydroxide, a composite oxide containing metal elements such as Ni and Al is synthesized. The firing temperature is not particularly limited, but is, for example, 300°C or higher and 600°C or lower.

[0047] In the second step, the composite oxide obtained in the first step, a compound containing element A, and a 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, Sr(NO3)2, etc. The particle size of the compound containing element A is preferably 0.1 μm or more and 20 μm or less. When the compound containing element A contains moisture, in order to suppress the generation of moisture during firing, it may be used after dehydration treatment such as drying. Examples of the Li compound include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, LiF, etc.

[0048] The mixing ratio of the composite oxide obtained in the first step and the Li compound is, for example, preferably such that the molar ratio of the metal elements excluding Li to Li is in the range of 1:0.98 to 1:1.1, which makes it easy to adjust the above parameters within the specified ranges. Further, 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, for example, preferably such that the molar ratio of the metal elements excluding Li to (Sr + Ca) is in the range of 1:0.0005 to 1:0.006, which makes it easy to adjust the above parameters within the specified ranges. In the second step, when mixing the composite oxide obtained in the first step, the Li compound, and the Sr compound or the Ca compound, other metal raw materials may be added as necessary. The other metal raw materials are oxides or the like containing metal elements other than the metal elements constituting the composite oxide obtained in the first step.

[0049] In the third step, the mixture obtained in the second step is fired in an oxygen atmosphere. By this step, a coating layer 36A containing element A is formed on the surface of the primary particles 36. As an example of the firing conditions, the heating rate at 450°C or higher and 680°C or lower is 1.0°C / min or higher and 5.5°C / min or lower, and the maximum reaching temperature is 700°C or higher and 850°C or lower. The heating rate at 450°C or higher and 680°C or lower may be 0.1°C / min or higher and 5.5°C / min or lower, or may be 0.2°C / min or higher and 5.5°C / min or lower. The heating rate from 680°C to the maximum reaching temperature is, for example, 0.1°C / min or higher and 3.5°C / min or lower. The holding time at the maximum reaching temperature may be 1 hour or longer and 10 hours or shorter.

[0050] In the fourth step, a compound containing element B is mixed with the fired composite oxide, and the mixture is heat-treated. By this step, a coating layer 37B containing element B is formed on the surface of the secondary particles 37, and at this time, the coating layer 37B is formed on the coating layer 36A. The fired composite oxide obtained in the third step may be washed with water by a conventionally known method. After washing with water, in a state where the powder of the composite oxide is wet, a compound containing element B may be added, and then heat treatment (drying) may be performed. The compound containing element B may be added in a powder state, or may be added in a state dissolved or dispersed in water.

[0051] Examples of the compound containing element B include WO3, ZrO2, B2O3, (NH4)2[Zr(CO3)2(OH)2], Al(NO3)3, Nb2O5, MoO3, TiO2, etc. The particle size of the compound containing element B is preferably 0.1 μm or more and 20 μm or less. The heat treatment temperature is, for example, 150°C or more and 300°C or less in a vacuum atmosphere.

[0052] [Negative electrode] The negative electrode 12 has a negative electrode core 40 and a negative electrode composite layer 41 provided on the surface of the negative electrode core 40. For the negative electrode core 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper, a film having the metal disposed on the surface layer, etc. can be used. The negative electrode composite layer 41 contains a negative electrode active material and a binder, and is preferably 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 slurry containing a negative electrode active material, a conductive material, a binder, etc. on the surface of the negative electrode core 40, drying the coating film, and then compressing it to form the negative electrode composite layer 41 on both sides of the negative electrode core 40.

[0053] The negative electrode composite layer 41 contains, as the negative electrode active material, for example, a carbon-based active material that can reversibly occlude and release lithium ions. Suitable carbon-based active materials are graphite such as flaky graphite, massive graphite, and earthy graphite, artificial graphite such as massive artificial graphite (MAG), and graphitized mesophase carbon microbeads (MCMB). Further, as the negative electrode active material, a Si-based active material composed of at least one of Si and Si-containing compounds may be used, or a carbon-based active material and a Si-based active material may be used in combination.

[0054] As the conductive material contained in the negative electrode composite layer 41, carbon materials such as carbon black, acetylene black, ketjen black, and graphite can be used in the same manner as in the case of the positive electrode 11. As the binder contained in the negative electrode composite layer 41, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc. can be used in the same manner as in the case of the positive electrode 11, but it is preferable to use styrene-butadiene rubber (SBR). Further, the negative electrode composite layer preferably further contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among them, it is preferable to use SBR in combination with CMC or its salt, PAA or its salt.

[0055] [Separator] For the separator 13, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefin, cellulose, etc. are suitable. The separator 13 may have either a single-layer structure or a laminated structure. On the surface of the separator 13, a heat-resistant layer containing inorganic particles, a heat-resistant layer composed of a highly heat-resistant resin such as aramid resin, polyimide, polyamideimide, etc. may be formed.

[0056] [Non-aqueous electrolyte] The non-aqueous electrolyte contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these can be used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. Examples of the halogen-substituted product include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0057] 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.

[0058] 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-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, 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.

[0059] 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), LiPF6-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, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, and imide salts such as these can be mentioned. The lithium salt may be used alone or in combination of multiple types. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is, for example, 0.8 mol or more and 1.8 mol or less per 1 L of the non-aqueous solvent. Furthermore, vinylene carbonate, propane sultone-based additives, etc. may be added.

Example

[0060] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0061] <Example 1> [Synthesis of Cathode Active Material] The composite hydroxide containing Ni, Co, and Al obtained by the coprecipitation method was calcined at 500 °C for 8 hours to obtain a composite oxide (molar ratio of Ni, Co, and Al is 91.75:4:4) (the first step). Next, LiOH, the above composite oxide, and Ca(OH)2 were mixed so that the molar ratio of Li, the total amount of Ni, Co, and Al, and Ca became 1.02:0.9975:0.0025 to obtain a mixture (the second step). The mixture was heated from room temperature to 650 °C at a heating rate of 2.0 °C / min in an oxygen stream, and then calcined from 650 °C to 730 °C at a heating rate of 0.5 °C / min to obtain a calcined product (the third step). After washing the calcined product with water, a predetermined amount of tungsten oxide was added and dried at 180 °C for 1 hour to obtain a lithium transition metal composite oxide (cathode active material) containing the elements shown in Table 1 (the fourth step).

[0062] The obtained positive electrode active material contains secondary particles with a D50 of 12 μm formed by aggregation of primary particles with an average particle diameter of 350 nm. From the measurement results of TEM-EDX of the particle cross-section, it was confirmed that Ca was present substantially uniformly on the surface of the primary particles, and W was present substantially uniformly on the surface of the secondary particles. Incidentally, W was present outside the particles rather than Ca so as to cover Ca.

[0063] [Fabrication of Positive Electrode] The above lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed at a predetermined solid content mass ratio, and a positive electrode composite material slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode composite material slurry was applied onto a positive electrode core made of aluminum foil, and after drying and compressing the coating film, it was cut into a predetermined electrode size to obtain a positive electrode.

[0064] [Fabrication of Negative Electrode] Graphite, a dispersion of styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) were mixed at a predetermined solid content mass ratio, and a negative electrode composite material slurry was prepared using water as a dispersion medium. Next, this negative electrode composite material slurry was applied to both sides of a negative electrode core made of copper foil, and after drying and compressing the coating film, it was cut into a predetermined electrode size to fabricate a negative electrode having negative electrode composite material layers formed on both sides of the negative electrode core.

[0065] [Preparation of Non-aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a predetermined volume ratio. LiPF6 was added to the mixed solvent to obtain a non-aqueous electrolyte.

[0066] [Fabrication of Test Cell (Non-aqueous Electrolyte Secondary Battery)] The positive electrode with the aluminum positive electrode lead attached and the negative electrode with the nickel negative electrode lead attached were wound in a spiral shape with a polyethylene separator in between, and formed into a flat shape to produce a wound electrode body. This electrode body was housed in an exterior body composed of an aluminum laminate. After injecting the non-aqueous electrolyte, the opening of the exterior body was sealed to produce a test cell for evaluation.

[0067] [Evaluation of DCR increase rate] The produced test cell was subjected to constant current charging at a current of 0.5It until the battery voltage reached 4.2V in a temperature environment of 25°C, and then low voltage charging was performed at 4.2V until the current value reached 1 / 50It. Subsequently, discharging was performed at a current of 0.5It, and the voltage before discharging and the voltage 10 seconds after the start of discharging were measured. The DCR before the storage test was calculated by the following formula.

[0068] DCR (Ω) = (voltage before discharging - voltage 10 seconds after the start of discharging) / current value Thereafter, the test cell was subjected to constant current charging at a constant current of 0.5It until the battery voltage reached 4.2V, and then low voltage charging was performed at 4.2V until the current value reached 1 / 50It. After that, it was left standing in a high temperature environment of 60°C for 45 days. The DCR of the test cell after storage was calculated by the above method, and the DCR increase rate after storage was calculated. The evaluation results are shown in Table 1.

[0069] The DCR increase rate shown in Table 1 is a relative value based on the DCR increase rate of the test cell of Comparative Example 1 described later.

[0070] [Example 2] In the synthesis of the positive electrode active material, a test cell was produced in the same manner as in Example 1 except that ammonium zirconium carbonate was added instead of tungsten oxide, and the DCR increase rate was evaluated.

[0071] [Example 3] In the synthesis of the positive electrode active material, a test cell was produced in the same manner as in Example 1 except that boron oxide was added instead of tungsten oxide, and the DCR increase rate was evaluated.

[0072] <Example 4> A test cell was fabricated in the same manner as in Example 1, except that aluminum nitrate was added instead of tungsten oxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0073] <Example 5> A test cell was fabricated 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.

[0074] <Example 6> A test cell was fabricated 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.

[0075] <Example 7> A test cell was fabricated 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.

[0076] <Example 8> A test cell was fabricated 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.

[0077] <Example 9> A test cell was fabricated 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.

[0078] <Examples 10 to 18> A test cell was fabricated in the same manner as in Examples 1 to 9, except that strontium hydroxide was added instead of calcium hydroxide in the synthesis of the positive electrode active material, and the DCR increase rate was evaluated.

[0079] <Examples 19 to 36> In the synthesis of the positive electrode active material, a composite oxide containing Ni, Al, and Mn (molar ratio of Ni, Al, and Mn is 93.75:3:3) was used instead of the composite oxide containing Ni, Co, and Al. Except for this, test cells were fabricated in the same manner as in Examples 1 to 18, and the DCR increase rate was evaluated.

[0080] <Comparative Example 1> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Example 1 except that calcium hydroxide and tungsten oxide were not added, and the DCR increase rate was evaluated.

[0081] <Comparative Example 2> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Example 1 except that tungsten oxide was not added, and the DCR increase rate was evaluated.

[0082] <Comparative Example 3> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Example 10 except that tungsten oxide was not added, and the DCR increase rate was evaluated.

[0083] <Comparative Examples 4 to 8> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Examples 1 to 5 except that calcium hydroxide was not added, and the DCR increase rate was evaluated.

[0084] <Comparative Example 9> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Example 19 except that calcium hydroxide and tungsten oxide were not added, and the DCR increase rate was evaluated.

[0085] <Comparative Example 10> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Example 20 except that tungsten oxide was not added, and the DCR increase rate was evaluated.

[0086] <Comparative Example 11> In the synthesis of the positive electrode active material, a test cell was fabricated in the same manner as in Example 28 except that tungsten oxide was not added, and the DCR increase rate was evaluated.

[0087] <Comparative Examples 12 - 16> In the synthesis of the positive electrode active material, test cells were fabricated in the same manner as in Examples 19 - 23 respectively except that calcium hydroxide was not added, and the DCR increase rate was evaluated.

[0088]

Table 1

[0089] As shown in Table 1, all of the test cells of the examples have a lower DCR increase rate after charge storage and better storage characteristics compared to the test cells of the comparative examples. From the results shown in Table 1, it can be seen that when using a positive electrode active material in which Ca or Sr (Element A) does not exist 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) does not exist on the surface of the secondary particles (Comparative Examples 2, 3, 10, 11), and a positive electrode active material in which neither Element A nor Element B exists (Comparative Example 1, 9), the DCR after storage increases significantly. Moreover, when only one of Element A and Element B exists on the particle surface of the positive electrode active material, the DCR after storage increases more than when neither Element A nor Element B exists.

[0090] That is, it is considered that the interaction between Element A and Element B improves the stability of the active material surface and specifically suppresses the increase in DCR after storage. When reducing the Co amount in a positive electrode active material with a large Ni amount, generally the DCR tends to increase. However, when Element A and Element B coexist on the particle surface, even in a battery using a positive electrode active material with a low Co content or no Co, the increase in DCR can be suppressed. Therefore, the Ni ratio can be further increased, and while suppressing the manufacturing cost of the battery, it is possible to increase the capacity.

[0091] <Examples 37 - 39, Comparative Example 17> In the synthesis of the positive electrode active material, test cells were prepared in the same manner as in Example 19 except that the input amounts of the Ca raw material were changed so that the amount of Element A became the values shown in Table 2, and the DCR increase rate was evaluated.

[0092] <Examples 40 to 42, Comparative Example 18> In the synthesis of the positive electrode active material, test cells were prepared in the same manner as in Example 19 except that the input amounts of the W raw material were changed so that the amount of Element B became the values shown in Table 2, and the DCR increase rate was evaluated.

[0093]

Table 2

[0094] As shown in Table 2, when a predetermined amount of Element A is contained, both high capacity and good storage characteristics of the battery can be achieved. However, when the amount of Element A is small, the effect of suppressing the DCR increase is small, and when the amount of Element A is large, although there is an effect of suppressing the DCR, the discharge capacity of the battery decreases. (Examples 19, 37 to 39, Comparative Example 17). Also, when a predetermined amount of Element B is contained, both high capacity and good storage characteristics of the battery can be achieved. However, when the amount of Element B is large, although there is an effect of suppressing the DCR, the discharge capacity of the battery decreases (Examples 19, 40 to 42, Comparative Example 18). Therefore, controlling the amounts of Element A and Element B appropriately is related to increasing the capacity and achieving good storage characteristics of the battery.

Explanation of Reference Numerals

[0095] 10 Non-aqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 Outer can 17 Sealing body 18, 19 Insulating plate 20 Positive electrode lead 21 Negative electrode lead 22 Grooved portion 23 Internal terminal board 24 Lower valve body 25 Insulating member Upper valve body Cap Gasket Positive electrode core Positive electrode composite material layer Lithium transition metal composite oxide (composite oxide) Primary particle Coating layers 36A, 37B Secondary particle Negative electrode core Negative electrode composite material layer

Claims

1. A composite oxide containing Ni and having a Co content of less than 5 mol% is produced, the composite oxide, a compound containing at least one element A selected from Ca and Sr, and a Li compound are mixed to produce a mixture, the mixture is fired in an oxygen atmosphere, a compound containing at least one element B selected from B, Zr, W, Al, Nb, Mo, and Ti is mixed with the fired mixture, and the mixture is heat-treated, to obtain a lithium transition metal composite oxide containing 80 mol% or more of Ni based on the total number of moles of metal elements excluding Li, 0.01 mol% or more and 1 mol% or less of the element A based on the total number of moles of metal elements excluding Li, and 0.05 mol% or more and 2 mol% or less of the element B based on the total number of moles of Ni, a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide contains at least one selected from Co, Mn, Al, Ti, Nb, Fe, and Zn.

3. The compound containing the element A is Ca(OH) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ), ２ , Sr(OH) ２ , Sr(OH) ２ ·H ２ O, Sr(OH) ２ ·8H ２ O, SrO, SrCO ３ , SrSO ４ , Sr(NO ３ ), ２ The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, which is at least one selected from

4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the firing has a heating rate of 0.1 °C / min or more and 5.5 °C / min or less at 450 °C or more and 680 °C or less, and a maximum reaching temperature of 700 °C or more and 850 °C or less.

5. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the fired mixture is washed with water and then the compound containing the element B is mixed.

6. The compound containing the element B is WO ３ , ZrO ２ , B ２ O ３ , (NH ４ ) ２ [Zr(CO ３ ) ２ (OH) ２ , Al(NO ３ ) ３ , Nb ２ O ５ , MoO ３ , TiO ２ The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, which is at least one selected from the group consisting of.

7. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the heat treatment is performed at a temperature of 150 °C or more and 300 °C or less in a vacuum atmosphere.

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