Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery, which is one example of an embodiment, comprises: a positive electrode that has a positive electrode mixture layer; a negative electrode; and a non-aqueous electrolyte. The positive electrode mixture layer contains a first lithium-and-transition-metal composite oxide represented by general formula LixNi1-y-zCoyMzO2 (in the formula, 0.8≤x≤1.2, 0≤y≤0.2, 0<z≤0.5, and M is at least one metal element excluding Li, Ni, and Co), and a second lithium-and-transition-metal composite oxide represented by general formula LiaNi2-a-bMebO2 (in the formula, 0<a≤0.5, 0≤b≤0.5, and Me is at least one metal element excluding Li and Ni). The non-aqueous electrolyte contains a sulfonyl imide salt.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] A non-aqueous electrolyte secondary battery has a structure including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, all of which are housed in an exterior case. The configuration of the positive electrode, which is a main component of a non-aqueous electrolyte secondary battery, significantly affects battery characteristics, including charge-discharge cycle characteristics, and therefore, much research has been done on the positive electrode. For example, Patent Documents 1 and 2 disclose positive electrode active materials aimed at improving performance, such as charge-discharge cycle characteristics.

[0003] The positive electrode active material disclosed in Patent Document 1 is represented by the general formula Li x Ni 1-y-z-v-w Co y Al z M1 V M2 w O2, in which element M1 is at least one selected from Mn, Ti, Y, Nb, Mo, and W, element M2 is at least two selected from Mg, Ca, Sr, Ba, and Ra, and element M2 contains at least Mg and Ca.

[0004] In addition, the positive electrode active material disclosed in Patent Document 2 is [L] 3a [M] 3b [O2] 6c (L=Li, M=Ni, Mn, and Co, or Li, Ni, Mn, and Co), which contains one or more elements selected from Mo, W, Nb, Ta, and Re in an amount of 0.1 to 5 mol% relative to the total molar amount of Mn, Ni, and Co. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4781004

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, with the spread of non-aqueous electrolyte secondary batteries to in-vehicle applications, power storage applications, etc., there is a demand for further improvement in performance, particularly improvement in charge-discharge cycle characteristics, for non-aqueous electrolyte secondary batteries. Conventional technologies including the technologies of Patent Documents 1 and 2 still have room for improvement in terms of improving cycle characteristics.

Means for Solving the Problems

[0007] In the non-aqueous electrolyte secondary battery according to the present disclosure, which includes a positive electrode having a positive electrode mixture layer, a negative electrode, and a non-aqueous electrolyte, the positive electrode mixture layer contains a first lithium transition metal composite oxide represented by the general formula Li x Ni 1-y-z Co y M z O2 (where 0.8 ≤ x ≤ 1.2, 0 ≤ y ≤ 0.2, 0 < z ≤ 0.5, and M is at least one metal element other than Li, Ni, and Co), and a second lithium transition metal composite oxide represented by the general formula Li a Ni 2-a-b Me b O2 (where 0 < a ≤ 0.5, 0 ≤ b ≤ 0.5, and Me is at least one metal element other than Li and Ni), and the non-aqueous electrolyte contains a sulfonyl imide salt.

Advantages of the Invention

[0008] The non-aqueous electrolyte secondary battery according to the present disclosure is excellent in charge-discharge cycle characteristics.

Brief Description of the Drawings

[0009] [Figure 1] It is a cross-sectional view of a non-aqueous electrolyte secondary battery which is an example of an embodiment. [Figure 2] It is a view showing a partially enlarged cross-section of an electrode body which is an example of an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] As described above, improving the charge-discharge cycle characteristics is an important issue for non-aqueous electrolyte secondary batteries. In particular, it is required to achieve both high battery capacity and good cycle characteristics. As a result of extensive research into improving cycle characteristics, the present inventors have found that x Ni 1-y-z Co y M z The first lithium transition metal composite oxide represented by the general formula Li a Ni 2-a-b Me b It has been found that the cycle characteristics of a battery are significantly improved by using a positive electrode containing a second lithium transition metal composite oxide represented by O2 and a non-aqueous electrolyte containing a sulfonylimide salt.

[0011] Although the mechanism by which the combined use of the first lithium transition metal composite oxide, the second lithium transition metal composite oxide, and the sulfonylimide salt improves cycle performance is unclear, it is believed that the second lithium transition metal composite oxide protects the surface of the first lithium transition metal composite oxide, thereby suppressing degradation. Furthermore, the addition of the sulfonylimide salt to the non-aqueous electrolyte forms a high-quality protective coating on the particle surfaces of the positive electrode active material, suppressing side reactions of the electrolyte on the particle surfaces, resulting in a significant improvement in cycle performance.

[0012] Furthermore, the first lithium transition metal composite oxide contains Ni as an essential component, and increasing the Ni content makes it easier to achieve both high capacity and good cycle characteristics. The effect of adding the second lithium transition metal composite oxide is more pronounced when a first lithium transition metal composite oxide with a high Ni content is used.

[0013] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure also includes selective combinations of multiple embodiments and modified examples described below.

[0014] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can of the battery is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can made of a laminate sheet including a metal layer and a resin layer (laminated battery).The electrode assembly may also be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0015] FIG. 1 is a schematic diagram showing a cross section of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 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 spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 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.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1, the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and 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, and the outer can 16 serves as the negative electrode terminal.

[0018] As described above, the outer can 16 is a cylindrical metal container with a bottom and an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure sealing of the battery interior and insulation between the outer can 16 and the sealing body 17. The outer can 16 has a grooved portion 22 that supports the sealing body 17, with part of the side surface protruding inward. The grooved 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 top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If an abnormality occurs in the battery and the internal pressure increases, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below with appropriate reference to Figure 2. Figure 2 is a schematic diagram showing an enlarged portion of the cross section of an electrode assembly 14.

[0021] <Positive electrode> As shown in FIG. 2, the positive electrode 11 includes a positive electrode core 30 and a positive electrode mixture layer 31 formed on at least one 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 the metal disposed on the surface layer, etc. can be used. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both surfaces of the positive electrode core 30. A lithium transition metal composite oxide is used as the positive electrode active material. The positive electrode 11 can be manufactured by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, 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 surfaces of the positive electrode core 30.

[0022] 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, acrylic resin, and polyolefin. Further, these resins and cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc. may be used in combination. The content of the binder is, for example, 0.1 to 5% by mass, or 0.5 to 3% by mass based on the total mass of the positive electrode mixture layer 31.

[0023] The positive electrode mixture layer 31 contains at least two types of lithium transition metal composite oxides. The first lithium transition metal composite oxide (hereinafter referred to as "composite oxide (A)") has the general formula Li x Ni 1-y-z Co y M z O2 (where 0.8 ≦ x ≦ 1.2, 0 ≦ y ≦ 0.2, 0 < z ≦ 0.5, and M is at least one metal element other than Li, Ni, and Co) is a composite oxide represented by this formula. The second lithium transition metal composite oxide (hereinafter referred to as "composite oxide (B)") has the general formula Li a Ni 2-a-b Me bIt is a composite oxide represented by O2 (where 0 < a ≤ 0.5, 0 ≤ b ≤ 0.5, and Me is at least one metal element other than Li and Ni).

[0024] In the positive electrode mixture layer 31, the coexistence of the composite oxides (A, B) specifically improves the charge-discharge cycle characteristics. The composite oxide (B) is considered to protect the particle surface of the composite oxide (A) and effectively suppress the deterioration of the particle surface. As a result, the cycle characteristics are greatly improved. In particular, when the particles of the composite oxides (A, B) are in contact with each other or in contact via carbon nanotubes (CNT), this protective effect is considered to act more effectively. Further, the composite oxide (B) may be mixed with the composite oxide (A) by applying strong shear force and compressive force and immobilized on the surface of the composite oxide (A).

[0025] Although the composite oxide (B) exhibits the above effects with a small addition, there is a preferable range for the addition amount of the composite oxide (B) in order to improve the cycle characteristics while maintaining a high battery capacity. The content of the composite oxide (B) is preferably 0.05 to 10% by mass, more preferably 0.1 to 7% by mass, or 0.1 to 5% by mass with respect to the total mass of the positive electrode mixture layer 31. Similarly, it is preferably 0.05 to 10% by mass, more preferably 0.1 to 7% by mass, or 0.1 to 5% by mass with respect to the total mass of the positive electrode active material. If the amount of the composite oxide (B) is within this range, the cycle characteristics can be efficiently improved.

[0026] The positive electrode mixture layer 31 may contain a composite oxide other than the composite oxides (A, B) (for example, a lithium transition metal composite oxide that does not satisfy the above general formula) within a range that does not impair the object of the present disclosure. The composite oxides (A, B) are preferably contained in an amount of 50% by mass or more with respect to the total mass of the positive electrode mixture layer 31. The total content of the composite oxides (A, B) is, for example, 85% by mass or more, or 90% by mass or more, or 95% by mass or more with respect to the total mass of the positive electrode mixture layer 31. An example of a preferable content is 90 to 99% by mass, or 95 to 99% by mass.

[0027] [First lithium transition metal composite oxide (composite oxide (A))] The composite oxide (A) is a composite oxide represented by the above general formula and contains at least one metal element M excluding Li, Ni, and Co as an essential constituent element. The composite oxide (A) preferably contains Co. However, since Co is particularly rare and expensive, the composite oxide (A) may be substantially Co-free. When the composite oxide (A) contains Co, the Co content is 20 mol % or less, more preferably 0.1 to 10 mol %, or 0.5 to 5 mol %, based on the total molar amount of the metal elements excluding Li. The molar fraction of the metal elements in the composite oxide can be measured by inductively coupled plasma (ICP) atomic emission spectroscopy.

[0028] It is preferred that the composite oxide (A) has the highest Ni content among the metal elements excluding Li. The Ni content is preferably 50 mol% or more, more preferably 70 mol% or more, and particularly preferably 80 mol% or more, based on the total molar amount of the metal elements excluding Li. A suitable example of the Ni content is 80 to 97 mol%, or 85 to 95 mol%. That is, a suitable example of (1-yz), which represents the Ni content in the above general formula, is 0.80≦(1-yz)≦0.97, or 0.85≦(1-yz)≦0.95.

[0029] As described above, a suitable example of the composite oxide (A) is a composite oxide containing 80 mol% or more of Ni relative to the total molar amount of metal elements excluding Li. By increasing the proportion of Ni among the metal elements in the composite oxide (A), it is possible to increase the capacity of the battery. Furthermore, a Ni-rich composite oxide (A) has good compatibility with the composite oxide (B) and is also effective in improving cycle characteristics. In the above general formula, x, which indicates the Li content, is 0.8≦x≦1.2 or 0.97≦x≦1.2, and the composite oxide (A) may be a lithium-excess composite oxide in which the molar ratio of Li to transition metal exceeds 1.

[0030] The composite oxide (A) contains at least one metal element M other than Li, Ni, and Co. The metal element M is, for example, at least one selected from Mn, W, Mg, Mo, Nb, Ti, Si, Al, Zr, B, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, and Ca, and more preferably at least one selected from Mn, W, Mg, Mo, Nb, Ti, Si, Sr, Ca, and Al. Among these, it is preferable to contain at least one of Mn and Al. When there are multiple elements, the content of the metal element M is 50 mol % or less, more preferably 0.1 to 20 mol %, or 0.5 to 10 mol %, or 1 to 5 mol %, based on the total molar amount of the metal elements excluding Li.

[0031] The composite oxide (A) has, for example, a crystal structure belonging to the space group R3-m. The composite oxide (A) has a layered structure including a transition metal layer, a Li layer, and an oxygen layer. In this case, the protective effect of the composite oxide (B) is more effective, a stable crystal structure is maintained even after repeated charge and discharge, and cycle characteristics can be more effectively improved. The BET specific surface area of ​​the composite oxide (A) is, for example, 0.2 to 2.0 m 2 The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0032] The composite oxide (A) is, for example, a secondary particle formed by the aggregation of multiple primary particles. The volume-based median diameter (D50) of the composite oxide (A) is, for example, 3 to 20 μm or 5 to 15 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%, and is also called the median diameter. The particle size distribution of the secondary particles of the composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrack-Bell Corporation) with water as the dispersion medium. The particle size of the primary particles of the composite oxide (A) is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a cross-sectional image of the secondary particles observed with a scanning electron microscope (SEM).

[0033] Furthermore, a compound containing at least one selected from Sr, Ca, W, Mg, Nb, and Al (hereinafter referred to as "metal element M2") may be adhered to the particle surface of the composite oxide (A). The M2 compound containing the metal element M2 may be scattered on the particle surface of the composite oxide (A), or may be present in the form of a layer covering the entire particle surface. The thickness of the layer of the M2 compound is, for example, 0.1 to 5 nm. It is believed that the M2 compound protects the surface of the composite oxide (A) and also the surface of the composite oxide (B), thereby suppressing side reactions of the electrolyte on the particle surfaces of the composite oxides (A, B).

[0034] The M2 compound is an oxide, hydroxide, or carbonate. Specific examples of the M2 compound include SrO, CaO, Sr(OH)2, Ca(OH)2, SrCO3, and CaCO3. The amount of the M2 compound is, for example, 0.05 to 0.5 mol% in terms of the metal element M2 relative to the total number of moles of metal elements excluding Li constituting the composite oxide (A). The presence of the M2 compound can be confirmed by energy dispersive X-ray spectroscopy (TEM-EDX). The metal element M2 can also be measured by ICP emission spectroscopy of a solution obtained by dissolving the composite oxide (A) in nitric acid and hydrofluoric acid.

[0035] The composite oxide (A) is produced through a first step of obtaining a composite oxide containing, for example, Ni, a metal element M, etc., a second step of mixing the composite oxide with a Li raw material to obtain a mixture, and a third step of firing the mixture. When an M2 compound is to be fixed to the particle surface of the composite oxide (A), a raw material containing the metal element M2 (hereinafter referred to as the "M2 raw material") may be added in the second step. The composition, particle size, BET specific surface area, etc. of the composite oxide (A) and the M2 compound can be adjusted by controlling the mixing ratio of the raw materials, the firing conditions in the third step, etc.

[0036] In the first step, an alkaline solution such as sodium hydroxide is added dropwise to a stirred solution of a metal salt containing a metal element such as Ni or the metal element M, and the pH is adjusted to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a composite hydroxide containing the metal element. This composite hydroxide is then calcined to obtain a composite oxide containing Ni, the metal element M, etc. The calcination temperature is not particularly limited, but is, for example, 300 to 600°C.

[0037] In the second step, the composite oxide obtained in the first step, a Li raw material, and, if necessary, an M2 raw material are mixed to obtain a mixture. Examples of Li raw materials include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of M2 raw materials include oxides, hydroxides, carbonates, nitrates, and sulfates of M2. The mixing ratio of the composite oxide obtained in the first step and the Li raw material is adjusted, for example, so that the molar ratio of metal elements excluding Li to Li is 1:0.98 to 1:1.22. The mixing ratio of the composite oxide and the M2 raw material is adjusted, for example, so that the molar ratio of metal elements excluding Li to M2 is 1:0.0005 to 1:0.005.

[0038] In the third step, the mixture obtained in the second step is fired at a predetermined temperature for a predetermined time to obtain a fired product. The firing of the mixture is carried out by a multi-stage firing process, including, for example, a first firing step in which the mixture is fired in a firing furnace under an oxygen stream at a first temperature increase rate to a first set temperature of 450°C or higher and 680°C or lower, and a second firing step in which the mixture is fired in a firing furnace under an oxygen stream after the first firing step at a second temperature increase rate to a second set temperature of above 680°C and 800°C or lower. The first temperature increase rate is 1.5 to 5.5°C / min, and the second temperature increase rate may be slower than the first temperature increase rate and may be 0.1 to 3.5°C / min. Note that multiple temperature increase rates may be set in each firing step.

[0039] The holding time of the first set temperature in the first firing step is, for example, 0 to 5 hours, or 0 to 3 hours. Note that the holding time of the set temperature is the time for maintaining the set temperature after reaching the set temperature. The holding time of the second set temperature in the second firing step is, for example, 1 to 10 hours, or 1 to 5 hours. The firing of the mixture is carried out in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream may be 0.2 to 4 mL / min per 10 cm of the firing furnace 3 and may be 0.3 L / min or more per 1 kg of the mixture. The fired product may be washed with water, dehydrated, and dried to remove impurities.

[0040] Note that without adding the M2 raw material in the second step, the M2 raw material is added in the third step, during the washing with water of the fired product, or during drying, and, for example, a heat treatment is carried out at 150 to 400 °C for 0.5 to 15 hours in a vacuum atmosphere, whereby the M2 compound may be fixed to the particle surface of the composite oxide (A).

[0041] [Second lithium transition metal composite oxide (composite oxide (B))] As described above, the composite oxide (B) has the general formula Li a Ni 2-a-b Me b O2 (where 0 < a ≤ 0.5, 0 ≤ b ≤ 0.5, and Me is at least one metal element other than Li and Ni). The content of the metal element Me is preferably less than the contents of Li and Ni, and is, for example, less than 10 mol%, or less than 5 mol% with respect to the total molar amount of the metal elements. Examples of the metal element Me include at least one selected from Cu, Sr, Ca, Nb, Si, and Al.

[0042] The composite oxide (B) does not absorb or release Li during charging and discharging, and its composition does not change. When such composite oxide (B) is used in combination with composite oxide (A), the cycle characteristics of the battery are significantly improved. The composite oxide (B) may contain multiple composite oxides with similar compositions. In the above general formula, a is more preferably 0.1≦a≦0.5 or 0.2≦a≦0.4. When a is within this range, the cycle characteristics are more effectively improved. The composition of the composite oxide (B) can be identified from the X-ray diffraction pattern and analyzed using ICP atomic emission spectroscopy.

[0043] The composite oxide (B) is a composite oxide having at least one diffraction peak with a peak top at a diffraction angle (2θ) of 21.40° to 21.65° in synchrotron X-ray diffraction (light energy 16 keV). This diffraction peak may have a peak top in the range of 2θ = 21.40° to 21.65°, and may be a broad peak whose entire peak does not lie within this range. The X-ray diffraction pattern of the composite oxide (B) has, for example, one peak top in the range of 2θ = 21.40° to 21.65°.

[0044] The X-ray diffraction pattern of the composite oxide (B) is obtained by powder X-ray diffraction using a synchrotron radiation facility (beamline BL5S2 at the Aichi Synchrotron Radiation Center) under the following conditions. Light energy: 16 keV Scan range: 10~90° Analyzing optical system: Debye-Scherrer type The obtained data is subjected to a peak search using identification analysis software PDXL (manufactured by Rigaku Corporation) to identify the composite oxide (B). When measured with the above-mentioned device, NiO has a peak at 21.36°, and as the a in the above general formula increases, the peak shifts to a higher angle. If the a in the above general formula is within the above range, the main peak will be between 21.40° and 21.65°. Composite oxide (B) can be identified by comparing the other peaks with the JCPDS card.

[0045] The composite oxide (B) is, for example, a particle having a smaller particle diameter than the composite oxide (A), and is a secondary particle formed by aggregation of a plurality of primary particles. The D50 of the composite oxide (B) is, for example, 1 to 15 μm, or 1 to 10 μm, or 2 to 7 μm. The D50 of the composite oxide (B) may be ½ or less of the D50 of the composite oxide (A), and is preferably ⅕ to ½, or ⅓ to ½ of the D50 of the composite oxide (A). By making the particle diameter of the composite oxide (B) smaller than that of the composite oxide (A), good contact between the particles can be obtained, and the effect of improving cycle characteristics can be enhanced. Furthermore, the BET specific surface area of ​​the composite oxide (B) is, for example, 0.5 to 2.5 m. 2 / g.

[0046] In the positive electrode mixture layer 31, the composite oxide (B) is present, for example, surrounded by a plurality of composite oxides (A). The particle surfaces of the composite oxides (A, B) are in contact with each other. In this case, it is believed that the interaction between the composite oxides (A, B) is more effectively expressed, and the effect of improving cycle characteristics is enhanced. Note that the composite oxide (B) is not limited to being intentionally added, and may be a by-product of the composite oxide (A) or a composite oxide mixed as an impurity in other positive electrode materials.

[0047] The composite oxide (B) is produced, for example, through a first step in which predetermined amounts of a Li raw material and a Ni raw material are mixed to obtain a mixture, and a second step in which the mixture is fired at 500 to 800°C for 10 to 30 hours. In the first step, the raw materials may be pulverized as necessary, and a raw material containing the metal element Me may be added within a range in which the X-ray diffraction pattern of the composite oxide (B) satisfies the above-mentioned conditions. In the second step, the mixture may be formed into pellets and then fired, or a crushing treatment may be performed after firing. The firing in the second step is carried out, for example, in the air or in an oxygen atmosphere.

[0048] Examples of the Li raw material include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, LiF, etc. Examples of the Ni raw material include NiO, Ni(OH)2, NiCO3, NiSO4, Ni(NO3)2, etc. The mixing ratio of the Li raw material and the Ni raw material is adjusted so that, for example, the X-ray diffraction pattern of the composite oxide (B) satisfies the above conditions and a in the above general formula satisfies the condition of 0 < a ≤ 0.5.

[0049] [Conductive agent] As described above, the conductive agent is included in the positive electrode mixture layer 31. The conductive agent forms a good conductive path in the positive electrode mixture layer 31 and contributes to reducing the resistance of the positive electrode mixture layer 31. The positive electrode mixture layer 31 contains carbon nanotubes (CNT) as the conductive agent. Further, in the positive electrode mixture layer 31, together with CNT, particulate conductive agents such as carbon black, acetylene black, ketjen black, graphite, etc., fibrous conductive agents such as vapor-grown carbon fibers (VGCF), carbon fibers by electrospinning method, polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, graphene, etc. may also be included.

[0050] It is considered that CNT forms a good conductive path in the positive electrode mixture layer 31, connects the particles of the composite oxides (A, B), and improves the surface protection effect by the composite oxide (B). By coexisting the composite oxide (B) and CNT, the cycle characteristics of the battery are specifically improved. The content of CNT is, for example, 0.01 to 5% by mass, more preferably 0.04 to 2% by mass, or 0.06 to 0.8% by mass, based on the total mass of the positive electrode mixture layer 31. If the content of CNT is within this range, the cycle characteristics can be efficiently improved. The positive electrode mixture layer 31 may contain only CNT as the conductive agent.

[0051] The CNTs may be either single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs). Examples of MWCNTs that can be used include CNTs with a tubular structure in which graphene sheets made of six-membered carbon rings are wound parallel to the fiber axis, CNTs with a pullet structure in which graphene sheets made of six-membered carbon rings are aligned perpendicular to the fiber axis, and CNTs with a herringbone structure in which graphene sheets made of six-membered carbon rings are wound at an oblique angle to the fiber axis. Two or more types of CNTs may be added to the positive electrode mixture layer 31.

[0052] The average diameter of the CNTs is, for example, 50 nm or less, preferably 40 nm or less, more preferably 25 nm or less, or 20 nm or less. The lower limit of the average diameter of the CNTs is not particularly limited, but an example is 1 nm or 5 nm. An example of a suitable range for the average diameter of the CNTs is 1 to 20 nm, or 5 to 20 nm. When the average diameter of the CNTs is within this range, the effect of improving cycle characteristics is enhanced compared to when CNTs with an average diameter outside this range are used.

[0053] The average fiber length of the CNTs is, for example, 0.5 μm or more, preferably 0.7 μm or more, more preferably 0.8 μm or more, or 1 μm or more. The upper limit of the average fiber length of the CNTs is not particularly limited, but an example is 10 μm or 5 μm. An example of a suitable range for the average fiber length of the CNTs is 1 to 10 μm or 1 to 5 μm. When the average fiber length of the CNTs is within this range, the effect of improving cycle characteristics is enhanced compared to when CNTs with an average fiber length outside this range are used.

[0054] The average fiber length of the CNTs is, for example, shorter than the length corresponding to D50 of the composite oxide (B). The average diameter of the CNTs is determined by measuring the diameter and fiber length of 100 CNTs selected from a surface TEM image of the positive electrode mixture layer 31, and the average fiber length is determined by averaging the measured values.

[0055] <Negative electrode> As shown in FIG. 2 , the negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 formed on at least one surface of the negative electrode core 40. The negative electrode core 40 can be made of a foil of a metal that is stable within the potential range of the negative electrode, such as copper or a copper alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably formed on both surfaces of the negative electrode core 40. A conductive agent such as CNT may also be added to the negative electrode mixture layer 41. The negative electrode 12 can be manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both surfaces of the negative electrode core 40.

[0056] The negative electrode mixture layer 41 contains, as negative electrode active materials, a carbon-based active material and at least one of at least one element selected from Si, Sn, Sb, Mg, and Ge (hereinafter referred to as "metal element M3") and an M3 compound containing the metal element M3. The content of the metal element M3 and the M3 compound is, for example, 0.5 to 30 mass%, and preferably 1 to 15 mass%, relative to the total mass of the negative electrode active material.

[0057] A metal element M3 may be added to the negative electrode mixture layer 41, but an M3 compound is preferably added. Examples of the M3 compound include SiC, SnO2, a first silicon material (SiO) containing a silicon oxide phase and Si dispersed in the silicon oxide phase, a second silicon material (LSX) containing a lithium silicate phase and Si dispersed in the lithium silicate phase, and a third silicon material (Si-C) containing a carbon phase and Si dispersed in the carbon phase. Among these, SiO, LSX, and Si-C are preferred.

[0058] As the carbon-based active material, for example, natural graphite such as flaky graphite, massive artificial graphite, artificial graphite such as graphitized mesophase carbon microbeads, etc. are used. The content of the carbon-based active material (graphite) is, for example, 70 to 99.5% by mass, or 85 to 99% by mass with respect to the mass of the negative electrode active material. Also, an example of the D50 of the carbon-based active material is 1 to 20 μm, or 2 to 15 μm. By using the carbon-based active material and the M3 compound in combination, it is possible to achieve high capacity while maintaining good cycle characteristics.

[0059] SiO and LSX are, for example, particles with a D50 smaller than that of graphite. An example of the D50 of SiO and LSX is 1 μm to 15 μm, or 3 μm to 10 μm. A conductive layer composed of a material with high conductivity may be formed on the particle surface of SiO and LSX. An example of a suitable conductive layer is a carbon film composed of a carbon material. The thickness of the conductive layer is preferably 1 to 200 nm, or 5 to 100 nm in consideration of ensuring conductivity and the diffusibility of lithium ions into the particles.

[0060] SiO has a particle structure in which fine Si particles are dispersed in a silicon oxide phase. Suitable SiO has a sea-island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicon oxide matrix, and is represented by the general formula SiO x (0 < x ≦ 2). The silicon oxide phase is composed of an aggregate of particles finer than the Si particles. The content of the Si particles is preferably 35 to 75% by mass with respect to the total mass of SiO from the viewpoints of achieving both battery capacity and cycle characteristics.

[0061] The average particle diameter of the Si particles dispersed in the silicon oxide phase is, for example, 500 nm or less before charge and discharge, preferably 200 nm or less, or 50 nm or less. After charge and discharge, it is, for example, 400 nm or less, or 100 nm or less. The average particle diameter of the Si particles is obtained as the average value of the longest diameters of 100 Si particles by observing the cross-section of the SiO particles using SEM or a transmission electron microscope (TEM).

[0062] LSX has a particle structure in which fine Si particles are dispersed in a lithium silicate phase. Preferred LSX has a sea-island structure in which fine Si particles are substantially uniformly dispersed in a matrix of lithium silicate. The lithium silicate phase is composed of an aggregate of particles finer than the Si particles. The content of the Si particles is preferably 35 to 75% by mass based on the total mass of LSX, similar to the case of SiO. Also, the average particle size of the Si particles is, for example, 500 nm or less before charge and discharge, preferably 200 nm or less, or 50 nm or less.

[0063] The lithium silicate phase is preferably composed of a compound represented by the general formula Li 2z SiO (2+z) (0 < z < 2). That is, Li4SiO4 (Z = 2) is not included in the lithium silicate phase. Li4SiO4 is an unstable compound and reacts with water to show alkalinity, which may alter Si and cause a decrease in charge-discharge capacity. From the viewpoints of stability, ease of production, lithium ion conductivity, etc., it is preferable that the lithium silicate phase contains Li2SiO3 (Z = 1) or Li2Si2O5 (Z = 1 / 2) as the main component. When Li2SiO3 or Li2Si2O5 is the main component, the content of the main component is preferably more than 50% by mass based on the total mass of the lithium silicate phase, and more preferably 80% by mass or more.

[0064] Si-C has a carbon phase and silicon particles dispersed in the carbon phase. In terms of increasing the capacity, etc., the content of the silicon particles in preferred Si-C is preferably 30% by mass or more and 80% by mass or less, more preferably 35% by mass or more and 75% by mass or less, and even more preferably 55% by mass or more and 70% by mass or less. The average particle size of the preferred silicon particles is generally 500 nm or less before charge and discharge, preferably 200 nm or less, and more preferably 100 nm or less. After charge and discharge, it is preferably 400 nm or less, and more preferably 100 nm or less. The average particle size of the silicon particles is measured by observing the cross section of the Si-C particles using SEM or TEM, and specifically, it is obtained as the average value of the longest diameters of 100 silicon particles.

[0065] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 41 can be, for example, a fluororesin, PAN, polyimide, acrylic resin, or polyolefin. However, it is particularly preferable to use styrene-butadiene rubber (SBR). The negative electrode mixture layer 41 preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA). Among these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof. The content of the binder is, for example, 0.1 to 5 mass% relative to the mass of the negative electrode active material.

[0066] <Separator> As shown in FIG. 2, the separator 13 has a porous substrate 50 and a surface layer 51 formed on the surface of the substrate 50 facing the positive electrode 11. The surface layer 51 is a layer containing inorganic particles and a binder. The surface layer 51 may be formed on both sides of the substrate 50, but is preferably formed on only one side of the substrate 50 facing the positive electrode 11 from the viewpoint of increasing capacity, etc. The separator 13 is a porous sheet interposed between the positive electrode 11 and the negative electrode 12 to prevent electrical contact between the two electrodes, and has ion permeability and insulating properties. The porosity of the separator 13 is, for example, 30% to 70%. The porosity of the separator 13 is determined by the porosity of the substrate 50.

[0067] The substrate 50 is a porous resin sheet. The thickness of the substrate 50 is, for example, 5 to 50 μm, and more preferably 10 to 30 μm. The resin constituting the substrate 50 is not particularly limited, but specific examples include polyolefins such as polyethylene, polypropylene, copolymers of ethylene and α-olefin, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyether ether ketone, polyimide, fluororesin, and cellulose. The substrate 50 may have a single-layer structure or a laminated structure such as a three-layer structure of polyethylene / polypropylene / polyethylene.

[0068] The surface layer 51 is porous like the substrate 50, and has ion permeability and insulating properties. There are no particular limitations on the thickness of the surface layer 51, but it is preferably thinner than the thickness of the substrate 50, and is, for example, 0.5 to 10 μm, and preferably 1 to 6 μm. The surface layer 51 is in contact with the surface of the positive electrode mixture layer 31, and is preferably formed over substantially the entire area of ​​one side of the substrate 50. The surface layer 51 can be formed, for example, by applying a slurry containing inorganic particles and a binder to the entire surface of the substrate 50 and then drying the coating.

[0069] The surface layer 51 is a layer containing inorganic particles as a main component. The content of the inorganic particles is, for example, 70 mass % or more, and preferably 80 mass % or more, relative to the total mass of the surface layer 51. The content of the inorganic particles is preferably in the range of 70 to 99 mass %, 80 to 98 mass %, or 85 to 95 mass %. The surface layer 51 has the function of suppressing damage to the separator 13 due to conductive foreign matter, deformation of the separator 13 during abnormal heat generation, etc. Furthermore, the surface layer 51 in contact with the positive electrode 11 is thought to suppress side reactions of the electrolyte in the positive electrode 11 through interaction with the composite oxide (B), and the provision of the surface layer 51 significantly improves the cycle characteristics and storage characteristics of the battery.

[0070] Examples of inorganic particles contained in the surface layer 51 include particles of metal oxides, metal nitrides, metal fluorides, metal carbides, aluminum hydroxide (boehmite), metal hydroxides such as magnesium hydroxide, metal carbonates such as calcium carbonate, magnesium carbonate, and barium carbonate, and metal sulfates such as calcium sulfate, magnesium sulfate, and barium sulfate. One type of inorganic particle may be used alone, or two or more types may be used in combination. The D50 of the inorganic particles is, for example, 0.01 to 10 μm, and preferably 0.05 to 5 μm.

[0071] Examples of metal oxides include aluminum oxide (alumina), titanium oxide, magnesium oxide, zirconium oxide, nickel oxide, silicon oxide, and manganese oxide. Examples of metal nitrides include titanium nitride, boron nitride, aluminum nitride, magnesium nitride, and silicon nitride. Examples of metal fluorides include aluminum fluoride, lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, and barium fluoride. Examples of metal carbides include silicon carbide, boron carbide, titanium carbide, and tungsten carbide. From the viewpoint of improving cycle characteristics and storage characteristics, an example of a suitable inorganic particle is at least one selected from alumina, boehmite, and barium sulfate.

[0072] The binder contained in the surface layer 51 is not particularly limited as long as it can bond the inorganic particles to each other, to the substrate 50, and has electrolyte resistance. For example, the same type of binder as that used in the positive electrode mixture layer 31 and the negative electrode mixture layer 41 can be used. Specific examples include fluororesins such as PVdF and PTFE, PAN, and acrylic resin. Alternatively, a resin with high heat resistance such as aramid resin may be used. An example of a suitable binder is at least one selected from aramid resin and acrylic resin.

[0073] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP). The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0074] The non-aqueous electrolyte contains a sulfonylimide salt as an electrolyte salt. In the non-aqueous electrolyte secondary battery 10 provided with the positive electrode 11 containing the composite oxide (A, B), by adding a sulfonylimide salt to the non-aqueous electrolyte, a good protective film is formed on the particle surface of the positive electrode active material, and it is considered that the side reaction of the electrolyte on the particle surface is suppressed, and the cycle characteristics are specifically improved. The concentration of the sulfonylimide salt is preferably 0.05 to 2.5 mol / L, more preferably 0.1 to 2.0 mol / L, or 0.1 to 1.5 mol / L. If the content of the sulfonylimide salt is within this range, the cycle characteristics can be improved more effectively.

[0075] The sulfonylimide salt added to the non-aqueous electrolyte is preferably lithium sulfonylimide. Examples of lithium sulfonylimide include lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(nonafluorobutanesulfonyl)imide, lithium bis(pentafluoroethanesulfonyl)imide (LIBETI), and the like. Among them, at least one lithium sulfonylimide selected from LiFSI and lithium bis(trifluoromethanesulfonyl)imide is preferable. The sulfonylimide salt may be used alone or in combination of two or more.

[0076] The non-aqueous electrolyte may further contain other lithium salts. Examples of other lithium salts 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, and borates such as LiBO and Li(B(CO)F). Of these, LiPF is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc.

[0077] It is preferable that the non-aqueous electrolyte contains both lithium sulfonylimide and a second lithium salt. In this case, a combination of LiFSI and LiPF6 is particularly preferable. The concentration of the lithium sulfonylimide is adjusted, for example, within the above range, even when the second lithium salt is contained. Specific examples include a lithium sulfonylimide concentration of 0.1 to 1.5 mol / L and a total lithium salt concentration of 1.5 to 2.5 mol / L. The lithium sulfonylimide concentration is, for example, 30 to 70% of the lithium salt concentration contained in the non-aqueous electrolyte.

[0078] The non-aqueous electrolyte may also contain additives such as vinylene carbonate (VC), ethylene sulfite (ES), cyclohexylbenzene (CHB), ortho-terphenyl (OTP), and propane sultone compounds. Of these, adding VC is preferred from the viewpoint of achieving high capacity. The concentration of the additive is not particularly limited, but is, for example, 0.1 to 5% by mass.

[0079] As described above, esters and ethers are used for the non-aqueous solvent. Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate, chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone (GBL), and γ-valerolactone (GVL), and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0080] Examples of ethers include 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, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and 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, and tetraethylene glycol dimethyl ether.

[0081] (Variation) Instead of a compound containing the metal element M2, other compounds such as lanthanoid-containing compounds may be adhered to the particle surfaces of the composite oxide (A). Alternatively, the particle surfaces of the composite oxide (A) may be substantially free of adhered materials. Furthermore, the particle surfaces of the composite oxide (A) may have composite oxide (B) with a small particle size adhered thereto.

[0082] The positive electrode mixture layer may not contain CNTs, and may contain, as a conductive agent, carbon black, acetylene black, ketjen black, graphite, etc. The negative electrode mixture layer may not contain the metal element M3 and the M3 compound, and may contain, as the negative electrode active material, only a carbon-based active material such as graphite.

[0083] The separator may be a porous sheet that does not have a surface layer mainly composed of inorganic particles, or a surface layer mainly composed of inorganic particles may be provided on the surface of the positive electrode mixture layer. [Example]

[0084] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0085] Example 1 [Synthesis of composite oxide (A1)] [Ni 0.91 Co 0.04 Al 0.05 The composite hydroxide represented by ](OH)2 was calcined at 500°C for 8 hours to form the composite oxide (Ni 0.91 Co 0.04 Al 0.05 Next, lithium hydroxide (LiOH) and the above composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.02:1 to obtain a mixture. This mixture was heated under an oxygen flow (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired from room temperature to 650°C at a temperature increase rate of 2°C / min (flow rates of 2 mL / min per kg of mixture and 5 L / min per kg of mixture) and then fired again at a temperature increase rate of 0.5°C / min from 650°C to 720°C. The fired product was washed with water to remove impurities, yielding a composite oxide (A1).

[0086] The composition of the composite oxide (A1) was determined by ICP analysis to be LiNi 0.91 Co 0.04 Al 0.05 O2. Furthermore, X-ray diffraction identified the crystal structure of the first lithium transition metal composite oxide as belonging to the space group R-3m. The D50 of the composite oxide (A1), measured using a Microtrack Bell MT3000II with water as the dispersion medium, was 10 μm.

[0087] [Synthesis of composite oxide (B1)] Li2O and NiO were weighed out so that the molar ratio of Li to Ni was 0.3:1.7, and then mixed while being crushed to prepare a mixture. Next, this mixture was fired at 650°C for 20 hours in an oxygen atmosphere and further crushed to obtain composite oxide (B1). When powder X-ray diffraction measurement of composite oxide (B1) was performed using the synchrotron radiation facility, one peak with a peak top at 21.48° was confirmed in the range of 2θ = 21.40° to 21.65° in the obtained X-ray diffraction pattern. In addition, the composition of composite oxide (B1) was confirmed to be Li by comparing it with the JCPDS card including other peaks. 0.3 Ni 1.7 The D50 of the composite oxide (B1), measured using a Microtrac Bell MT3000II with water as the dispersion medium, was 5 μm.

[0088] [Preparation of positive electrode] The composite oxides (A1, B1) were mixed in a mass ratio of 99.9:0.1 to prepare a positive electrode active material. The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride (PVdF) were mixed in a solids mass ratio of 92:5:3, and an appropriate amount of N-methylpyrrolidone (NMP) was added. The mixture was then kneaded to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, and the coating was dried. The coating was then rolled using a rolling roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core. An exposed portion was provided on a portion of the positive electrode, exposing the surface of the positive electrode current collector.

[0089] [Preparation of negative electrode] The negative electrode active material is graphite and SiO xThe negative electrode active material was mixed with SiO (X = 1.0) in a mass ratio of 95:5. The negative electrode active material, a dispersion of styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC-Na) were mixed in a solids mass ratio of 98:1:1, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. Next, this negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried and compressed. The resulting film was then cut to the specified electrode size to produce a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode core.

[0090] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving LiFSI at a concentration of 0.5 mol / L and LiPF6 at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 (25°C, 1 atmosphere).

[0091] [Test cell construction] The positive electrode with an aluminum positive electrode lead attached and the negative electrode with a nickel negative electrode lead attached were spirally wound with a polyethylene separator interposed therebetween and flattened to prepare a wound electrode assembly. This electrode assembly was housed in an exterior body made of aluminum laminate, and after the nonaqueous electrolyte solution was poured into it, the opening of the exterior body was sealed to prepare a test cell for evaluation.

[0092] <Example 2> A test cell was produced in the same manner as in Example 1, except that in the production of the positive electrode, the composite oxides (A1, B1) were mixed in a mass ratio of 99.8:0.2.

[0093] Example 3 A test cell was produced in the same manner as in Example 1, except that in the production of the positive electrode, the composite oxides (A1, B1) were mixed in a mass ratio of 99.7:0.3.

[0094] Example 4 A test cell was fabricated in the same manner as in Example 2, except that in preparing the non-aqueous electrolyte, LiFSI was dissolved at a concentration of 1.0 mol / L and LiPF6 was dissolved at a concentration of 0.5 mol / L.

[0095] <Example 5> A test cell was fabricated in the same manner as in Example 2, except that Li2O and NiO were mixed so that the molar ratio of Li to Ni was 0.2:1.8 to synthesize the composite oxide (B2).

[0096] Example 6 A test cell was prepared in the same manner as in Example 2, except that Li2O and NiO were mixed so that the molar ratio of Li to Ni was 0.5:1.5 to synthesize the composite oxide (B3).

[0097] <Comparative Example 1> A test cell was prepared in the same manner as in Example 4, except that the composite oxide (B1) was not used in preparing the positive electrode.

[0098] <Comparative Example 2> A test cell was fabricated in the same manner as in Comparative Example 1, except that in preparing the non-aqueous electrolyte, LiFSI was dissolved at a concentration of 0.5 mol / L and LiPF6 was dissolved at a concentration of 1.0 mol / L.

[0099] <Comparative Example 3> A test cell was fabricated in the same manner as in Example 2, except that in preparing the non-aqueous electrolyte, LiFSI was not used and LiPF6 was dissolved at a concentration of 1.5 mol / L.

[0100] <Comparative Example 4> A test cell was fabricated in the same manner as in Comparative Example 1, except that in preparing the non-aqueous electrolyte, LiFSI was not used and LiPF6 was dissolved at a concentration of 1.5 mol / L.

[0101] <Comparative Example 5> A test cell was produced in the same manner as in Example 2, except that the composite oxide (B10) below was used instead of the composite oxide (B1) in producing the positive electrode. [Synthesis of Composite Oxide (B10)] Li2O and NiO were weighed so that the molar ratio of Li to Ni was 0.7:1.3, and these were mixed while being pulverized to prepare a mixture. Next, this mixture was fired at 650 °C for 20 hours in an oxygen atmosphere and further crushed to obtain a composite oxide (B10).

[0102] For each test cell of the examples and comparative examples, the charge-discharge cycle characteristics were evaluated by the following method. The evaluation results are shown in Table 1 together with the compositions of the positive electrode and non-aqueous electrolyte.

[0103] [Evaluation of Cycle Characteristics (Capacity Retention Rate)] The test cell was subjected to constant current charging at 0.3C until the battery voltage reached 4.3V in a temperature environment of 40 °C, and then constant voltage charging was performed until the current value reached 0.02C at 4.3V. Thereafter, constant current discharge was performed at 0.05C until the battery voltage reached 2.5V. This charge-discharge was performed for 30 cycles, and the capacity retention rate after 30 cycles was determined based on the following formula. The capacity retention rate shown in Table 1 is the relative value when the capacity retention rate of the test cell of Comparative Example 4 was used as the reference (100). Capacity retention rate = (discharge capacity at the 30th cycle / discharge capacity at the 1st cycle) × 100

[0104]

Table 1

[0105] As understood from the results shown in Table 1, all of the test cells of the examples have a higher capacity retention rate and are superior in cycle characteristics compared to the test cells of the comparative examples. That is, when a positive electrode containing a composite oxide (A1) and a composite oxide (B1 to B3) represented by the general formula Li a Ni 2-a-b Me b O2 (where 0 < a ≤ 0.5, 0 ≤ b ≤ 0.5, and Me is at least one metal element other than Li and Ni) and a non-aqueous electrolyte to which LiFSI is added are used, a specific improvement in cycle characteristics is observed.

[0106] When the composite oxide (A1) was used alone as the positive electrode active material (Comparative Examples 1, 2, and 4), when LiFSI was not added to the non-aqueous electrolyte (Comparative Examples 3 and 4), and when the composite oxide (B10) that does not satisfy the conditions of the above general formula was used instead of the composite oxides (B1 to B3) (Comparative Example 5), the effect of improving the cycle characteristics as in the Examples was not obtained. [Explanation of symbols]

[0107] 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 plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core body, 31 positive electrode mixture layer, 40 negative electrode core body, 41 negative electrode mixture layer, 50 substrate, 51 surface layer

Claims

1. A non-aqueous electrolyte secondary battery including a positive electrode having a positive electrode mixture layer, a negative electrode, and a non-aqueous electrolyte, The positive electrode mixture layer includes General formula Li ｘ Ni １－ｙ－ｚ Co ｙ M ｚ O ２ (wherein 0.8≦x≦1.2, 0≦y≦0.2, 0<z≦0.5, and M is at least one metal element excluding Li, Ni, and Co), and General formula Li ａ Ni2-a O ２ (wherein 0<a≦0.5), and a second lithium transition metal composite oxide represented by the formula: Contains, The non-aqueous electrolyte secondary battery includes a sulfonylimide salt in the non-aqueous electrolyte.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the second lithium transition metal composite oxide is 0.1 to 10 mass % with respect to the total mass of the positive electrode mixture layer.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the second lithium transition metal composite oxide is a composite oxide having at least one diffraction peak having a peak top at a diffraction angle (2θ) of 21.40° to 21.65° in synchrotron X-ray diffraction (light energy 16 keV).

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the sulfonylimide salt is at least one lithium sulfonylimide selected from the group consisting of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

5. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfonylimide salt is contained in the non-aqueous electrolyte at a concentration of 0.1 to 1.5 mol / L.

6. The negative electrode includes, as negative electrode active materials, a carbon-based active material and a second active material which is at least one of at least one metal element selected from Si, Sn, Sb, Mg, and Ge and a compound containing the metal element; 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the second active material is 1 to 15 mass % with respect to the total mass of the negative electrode active material.

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