Positive-electrode active material for nonaqueous-electrolyte secondary battery, nonaqueous-electrolyte secondary battery, and method for producing positive-electrode active material for nonaqueous-electrolyte secondary battery

Abstract

A positive-electrode active material for a nonaqueous-electrolyte secondary battery as one embodiment comprises: a lithium / transition metal composite oxide in which the total content of Ni and Mn is 80 mol% or higher with respect to the total molar amount of the metallic elements excluding Li; and one or more sulfur compounds. The lithium / transition metal composite oxide has a single particle shape and has a volume-based median diameter of 0.5-5.0 μm and a crystallite size of 370-1,500 Å. The sulfur compounds are one or more members selected from the group consisting of compounds containing an S-S bond, compounds in which the valence of S is less than six, and products of decomposition of these compounds. The sulfur compounds are present in an amount of 0.1-2.0 mol% with respect to the total molar amount of the metallic elements, excluding Li, contained in the positive-electrode active material.

Description

Positive electrode active material for non-aqueous electrolyte secondary batteries, non-aqueous electrolyte secondary battery, and method for manufacturing positive electrode active material for non-aqueous electrolyte secondary batteries

[0001] This disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery using the positive electrode active material, and a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

[0002] In non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, the positive electrode significantly affects battery performance, including input / output characteristics, capacity, cycle characteristics, and thermal stability. Therefore, much research has been conducted on the positive electrode. For example, Patent Document 1 proposes a positive electrode active material having a structure in which a coating layer containing a sulfur compound is formed on the surface of lithium metal compound particles, with the aim of suppressing increases in resistance and leakage current.

[0003] Japanese Patent Publication No. 2021-86834

[0004] Incidentally, as the charging voltage of a battery increases, the resistance of the positive electrode increases, and this resistance increase is particularly pronounced in the high state of charge (SOC) region. Therefore, suppressing this resistance increase is an important challenge in order to achieve higher charging voltages. In particular, in order to use a battery for a long time, it is important not only to reduce the initial resistance, but also to suppress the resistance increase that occurs with the charge-discharge cycle, which in turn leads to an improvement in capacity retention.

[0005] The positive electrode active material for a non-aqueous electrolyte secondary battery according to this disclosure comprises a lithium transition metal composite oxide in which the combined content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li, and a sulfur compound, wherein the lithium transition metal composite oxide has a single-particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, and the sulfur compound is at least one selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds, and is present in an amount of 0.1 mol% or more and 2.0 mol% or less relative to the total molar amount of metal elements excluding Li contained in the positive electrode active material.

[0006] The non-aqueous electrolyte secondary battery according to this disclosure comprises a positive electrode containing the above-mentioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] The present disclosure relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, characterized by comprising the steps of: mixing a lithium transition metal composite oxide having a single-particle shape with a total content of Ni and Mn of 80 mol% or more relative to the total molar amount of metal elements excluding Li, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less; and a sulfur compound which is at least one selected from the group consisting of compounds containing S-S bonds and compounds with a S valence of less than 6 to obtain a mixture; and heat-treating the mixture.

[0008] The positive electrode active material described herein makes it possible to realize a non-aqueous electrolyte secondary battery in which the increase in resistance during high-potential charging is suppressed.

[0009] This is a cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. This is a cross-sectional view of a test cell prepared in the example. This is the XPS spectrum of the positive electrode active material synthesized in Comparative Example A1, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Comparative Example A5, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Example BB3, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B1, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B9, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B17, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B22, showing the binding energy range in which a peak originating from S appears. This is the XPS spectrum of the positive electrode active material synthesized in Comparative Example A1, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Reference Example A5, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Example BB3, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B1, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B9, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B17, showing the binding energy range in which a peak originating from B appears. This is the XPS spectrum of the positive electrode active material synthesized in Example B21, showing the binding energy range in which a peak originating from B appears.

[0010] In recent years, with the widespread use of non-aqueous electrolyte secondary batteries such as lithium-ion batteries in in-vehicle applications, power storage applications, etc., from the viewpoints of high capacity, rapid charging requirements, etc., an increase in the charging voltage to a higher voltage has been demanded. However, for example, when the charging voltage is increased to the 4.5 V (versus Li metal) level, the increase in the resistance of the positive electrode becomes particularly significant, especially in the region where the state of charge (SOC) is high. In addition, further improvement in the charge-discharge efficiency and durability of non-aqueous electrolyte secondary batteries has been demanded. Moreover, reduction of the manufacturing cost is also desired, and as the positive electrode active material, those mainly composed of Ni and Mn, which have high capacity and are relatively inexpensive, are preferable.

[0011] The inventors have succeeded in suppressing the increase in resistance during high-potential charging while ensuring excellent charge-discharge efficiency and durability by causing a specific sulfur compound to be present on the particle surface and in the vicinity of the surface of single-particle-shaped lithium transition metal composite oxides containing Ni and Mn. By using the positive electrode active material according to the present disclosure, the increase in resistance in the high SOC region is effectively suppressed. The effect of suppressing the increase in resistance by the addition of a specific sulfur compound is particularly remarkable in the single-particle-shaped positive electrode active material.

[0012] Hereinafter, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail while referring to the drawings. Note that a configuration formed by selectively combining each component of a plurality of embodiments and modification examples described below is included in the scope of the present disclosure.

[0013] Hereinafter, a non-aqueous electrolyte secondary battery 10, which is a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer can 16, will be exemplified, but the outer package of the battery is not limited to a cylindrical outer can. As other embodiments of the non-aqueous electrolyte secondary battery according to the present disclosure, for example, a rectangular battery provided with a rectangular outer can, a coin-shaped battery provided with a coin-shaped outer can, and a pouch-type battery provided with an outer package composed of a laminate sheet including a metal layer and a resin layer can be mentioned. Further, the electrode body is not limited to a wound type, and may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via separators.

[0014] 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 non-aqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. 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 with the separator 13 interposed therebetween. The exterior can 16 is a bottomed cylindrical metal container with one end 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 sealing body 17 of the battery is taken as the upper side, and the bottom side of the exterior can 16 is taken as the lower side.

[0015] 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 to be slightly larger in size than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed longer in the length direction (longitudinal direction) and the width direction (short transverse direction) than the positive electrode 11. The separator 13 is formed to be at least slightly larger in size than the positive electrode 11, and for example, two sheets are arranged 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.

[0016] 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 exterior can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected by welding or the like to the lower surface of the internal terminal plate 23 of the sealing body 17, 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 by welding or the like to the inner surface of the bottom of the exterior can 16, and the exterior can 16 serves as the negative electrode terminal.

[0017] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery and to prevent electrical contact between the outer casing 16 and the sealing body 17. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0018] 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 stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except 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, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular emphasis on the positive electrode active material that constitutes the positive electrode 11.

[0020] [Positive Electrode] The positive electrode 11 comprises a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, or titanium, or a film with the metal disposed on its surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core except for the portion to which the positive electrode lead 20 is connected. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder onto the positive electrode core, drying the coating, and then compressing it to form the positive electrode mixture layer on both sides of the positive electrode core.

[0021] Examples of conductive agents included in the positive electrode mixture layer include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, and conductive whiskers. One type of conductive agent may be used alone, or multiple types may be used in combination. The content of the conductive agent is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the positive electrode mixture layer.

[0022] Examples of binders included in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc. The binder may be used alone or in combination of multiple types. The binder content is not particularly limited, but is, for example, 0.1% to 5% by mass relative to the mass of the positive electrode mixture layer.

[0023] The positive electrode active material includes a lithium transition metal composite oxide in which the combined content of Ni and Mn relative to the total molar amount of metal elements excluding Li is 80 mol% or more. The positive electrode active material further includes a specific sulfur compound. The lithium transition metal composite oxide has a single-particle shape, a volume-based median diameter (D50) of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less. The sulfur compound is at least one selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds. With a positive electrode active material having this configuration, resistance increase during high-potential charging is effectively suppressed while ensuring excellent charge-discharge efficiency and durability. In particular, the effect of suppressing resistance increase in the high SOC region and the improvement of capacity retention rate are remarkable. Preferably, the positive electrode active material further includes a boron compound. By using a boron compound in combination, the effect of suppressing resistance increase becomes even more pronounced. However, the above effects can be sufficiently achieved with the specific sulfur compound alone. The following examples primarily illustrate forms containing sulfur compounds and boron compounds, but this disclosure is not limited to forms containing boron compounds.

[0024] As will be described in detail later, it is preferable that the above-mentioned specific sulfur compound exists substantially only on the particle surface of the lithium transition metal composite oxide and within a predetermined depth range from the particle surface. In the photoelectron spectrum (XPS spectrum) obtained by X-ray photoelectron spectroscopy (XPS) measurement of the positive electrode active material, a peak originating from S exists in the region of the lithium transition metal composite oxide particle surface and a depth of 150 nm or less from the surface, with a binding energy in the range of 172 eV to 176 eV. In addition, a peak originating from B exists in the region of the lithium transition metal composite oxide particle surface and a depth of 150 nm or less from the surface, with a binding energy in the range of 190 eV to 194 eV. Unless otherwise specified, the position of the peak in the XPS spectrum refers to the position of the peak top.

[0025] When a lithium transition metal composite oxide has a single-particle shape, a resistance reduction effect can be obtained by adding specific sulfur compounds and boron compounds. In this specification, a single particle refers to a particle formed from a single primary particle, not a secondary particle formed by the aggregation of many (e.g., 1000 or more) primary particles. In other words, there are substantially no particle interfaces of primary particles inside the particle. Note that a particle formed by the aggregation of 10 or fewer primary particles approximates a single-particle shape and can be considered substantially as a single particle. A single particle may be a single-crystal particle with substantially no grain boundaries inside, or a polycrystalline particle with several grain boundaries inside.

[0026] As described above, the D50 of the lithium transition metal composite oxide is 0.5 μm to 5.0 μm. In this case, a resistance reduction effect can be obtained by adding specific sulfur compounds and boron compounds. On the other hand, if specific sulfur compounds and boron compounds are added to single particles where D50 falls outside this range, or to secondary particles formed by the aggregation of many primary particles, the resistance reduction effect is small or substantially ineffective. From the viewpoint of improving the resistance reduction effect by adding specific sulfur compounds and boron compounds, the D50 of the lithium transition metal composite oxide is preferably 0.7 μm to 3.5 μm, more preferably 0.8 μm to 3.0 μm. Note that the specific sulfur compounds, boron compounds, and their decomposition products present on the particle surface do not substantially affect the D50 of the positive electrode active material, so the D50 of the positive electrode active material and the D50 of the lithium transition metal composite oxide are substantially the same.

[0027] In this specification, D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% of the volume-based particle size distribution. The particle size distribution of lithium transition metal composite oxides can be measured using a laser diffraction particle size distribution analyzer (e.g., the MT3000II manufactured by Microtrac-Bell Corporation) with water as the dispersion medium.

[0028] The crystallite size of the lithium transition metal composite oxide is 370 Å to 1500 Å, preferably 370 Å to 1000 Å, and more preferably 370 Å to 750 Å. If the crystallite size is within this range, a resistance reduction effect can be obtained by adding specific sulfur compounds and boron compounds. Note that the specific sulfur compounds, boron compounds, and their decomposition products present on the particle surface do not substantially affect the crystallite size of the positive electrode active material; therefore, the crystallite sizes of the positive electrode active material and the lithium transition metal composite oxide are substantially the same. The crystallite size is calculated from the full width at half maximum of the diffraction peak of the (10⁴) plane in the X-ray diffraction pattern obtained by X-ray diffraction, using Scherrer's equation, which is shown below. In the equation below, s is the crystallite size, λ is the wavelength of the X-ray, B is the full width at half maximum of the diffraction peak of the (10⁴) plane, θ is the diffraction angle (rad), and K is Scherrer's constant. In this specification, K is set to 0.9. s = Kλ / Bcosθ

[0029] The X-ray diffraction pattern is obtained by powder X-ray diffraction using a powder X-ray diffractometer (RINT-TTR, manufactured by Rigaku Corporation, Cu-Kα source) under the following conditions: Measurement range: 15–120° Scan rate: 4° / min Analysis range: 30–120° Background: B-spline profile Function: Split-type pseudo-Voigt function Constraints: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the respective Ni content) ICSD No.: 98-009-4814

[0030] The BET specific surface area of ​​a lithium transition metal composite oxide is, for example, 0.5 m². ２ / g or more 4m ２ It is less than or equal to / g, and more preferably 0.9m ２ / g or more 3m ２The value is less than or equal to / g. Secondary particles containing many primary particles have voids within the particles, so even if the particle size is large, the specific surface area is relatively large. On the other hand, single particles do not have voids within the particles, so the BET specific surface area decreases as the particle size increases. Furthermore, certain sulfur compounds, boron compounds, and their decomposition products present on the particle surface do not substantially affect the BET specific surface area of ​​the positive electrode active material, so the BET specific surface area of ​​the positive electrode active material and the lithium transition metal composite oxide are substantially the same.

[0031] The BET specific surface area can be measured using a Tristar II 3020 manufactured by Shimadzu Corporation under the following conditions: Number of measurement points: 11 points (P / P0: 0.05 to 0.3) Warm Free Space: Measured Equilibration Interval: 5s Analysis Adsorbent: N2 Analysis Bath Temp: 77.3K (liquid nitrogen temperature) Cold Free Space: Measured Low Pressure Done: None Analysis method: BET multi-point method

[0032] Lithium transition metal composite oxides preferably have a layered rock salt structure. When specific sulfur compounds and boron compounds are applied to composite oxides with a layered rock salt structure, the resistance reduction effect during high-potential charging becomes more pronounced. Examples of layered rock salt structures include those belonging to space group R-3m and those belonging to space group C2 / m. Among these, layered rock salt structures belonging to space group R-3m are preferred from the viewpoint of increasing capacity and crystalline structure stability.

[0033] Lithium transition metal composite oxides contain Ni and Mn as essential metal elements. The total content of Ni and Mn is 80 mol% or more, preferably 90 mol% or more, relative to the total molar amount of metal elements excluding Li. If the ratio of Ni and Mn to the metal elements excluding Li is 80 mol% or more, a resistance reduction effect can be obtained by adding specific sulfur compounds and boron compounds, and a high-capacity positive electrode active material can be obtained relatively inexpensively. When the lithium transition metal composite oxide contains Ni, Mn, and Co, their total content is preferably 90 mol% or more, or 95 mol% or more, relative to the total molar amount of metal elements excluding Li. Lithium transition metal composite oxides may contain substantially only Ni and Mn as transition metal elements.

[0034] Ni is preferably the most abundant of the metal elements other than Li that constitute the lithium transition metal composite oxide. From the viewpoint of increasing capacity, the Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more, more preferably 65 mol% or more, and particularly preferably 70 mol% or more or 75 mol% or more, relative to the total molar amount of the metal elements excluding Li. From the viewpoint of improving durability and suppressing resistance increase, the upper limit of the Ni content is preferably 90 mol% and more preferably 85 mol% or 80 mol%, relative to the total molar amount of the metal elements excluding Li. Examples of a suitable range for the Ni content are 60 mol% to 90 mol%, or 70 mol% to 90 mol%, or 70 mol% to 85 mol%, or 75 mol% to 85 mol%.

[0035] Mn is preferably the second most abundant element, after Ni, among the metal elements other than Li that constitute the lithium transition metal composite oxide. Mn stabilizes the crystal structure of the lithium transition metal composite oxide. The Mn content in the lithium transition metal composite oxide is, for example, 5 mol% to 50 mol%, or 10 mol% to 35 mol%, or 15 mol% to 30 mol%, relative to the total molar amount of the metal elements excluding Li. The lithium transition metal composite oxide may also contain Co in a smaller proportion than Ni. The Co content is preferably less than or equal to the Mn content, for example, 15 mol% or less, or 10 mol% or less, or 5 mol% or less, or 3 mol% or less.

[0036] The lithium transition metal composite oxide may further contain small amounts of elements other than Ni, Mn, and Co. Suitable other elements include at least one selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W. These elements may be contained within the particles of the lithium transition metal composite oxide or present on the particle surface. When these elements are included, for example, side reactions with the electrolyte are suppressed, improving the battery's durability. The lithium transition metal composite oxide may contain these elements in an amount of 0.01 mol% to 5 mol% relative to the total amount of Ni and Mn. The element content in the positive electrode active material can be measured using an ICP emission spectrometer (e.g., a CIROS-120 from SPECTRO).

[0037] As described above, a predetermined amount of at least one sulfur compound selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds is present on and near the particle surface of the lithium transition metal composite oxide. The sulfur compound may be a compound that contains S-S bonds and has a S valence of less than 6. The sulfur compound is expected to decompose in the heat treatment process described later, but it may also exist in the state of the added compound. In the synthesis process of the positive electrode active material of this embodiment, one type of compound may be used alone as the sulfur compound, or two or more types of compounds may be used.

[0038] Furthermore, to the extent that it does not impair the purpose of this disclosure, compounds containing S-S bonds, compounds with a S valence of less than 6, or other sulfur compounds other than decomposition products of these compounds may be present on and near the particle surface of lithium transition metal composite oxides. However, such other sulfur compounds do not contribute to suppressing the increase in resistance associated with charging and discharging, or their effect is small. For example, salts of sulfate esters with 5 or fewer carbon atoms (see the reference examples below) contribute to suppressing the increase in resistance, but their effect is smaller than that of the specific sulfur compounds described above. Hereinafter, unless otherwise specified, a sulfur compound means at least one selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds.

[0039] The sulfur compound content is preferably 0.1 mol% to 2.0 mol%, more preferably 0.2 mol% to 1.5 mol%, particularly preferably 0.3 mol% to 1.0 mol%, and most preferably 0.3 mol% to 0.8 mol%, or 0.3 mol% to 0.5 mol%, relative to the total molar amount of metal elements excluding Li that constitute the lithium transition metal composite oxide. When the sulfur compound content is within this range, resistance increase can be suppressed more effectively while maintaining good cycle characteristics at high capacity. Adding more than 2.0 mol% of sulfur compound is expected to plateau the resistance reduction effect and lead to problems such as a decrease in capacity; therefore, a sulfur compound content of 2.0 mol% or less is preferred.

[0040] It is preferable that the sulfur compound is distributed over a wide area of ​​the particle surface of the lithium transition metal composite oxide, rather than being concentrated in a specific area of ​​the particle surface. The sulfur compound is evenly distributed across the particle surface of the composite oxide. The sulfur compound may be present as a thin film or as granules on the particle surface of the composite oxide.

[0041] The sulfur compound is preferably present on the particle surface of the lithium transition metal composite oxide and in a depth range of 150 nm or less from the surface, and more preferably substantially present only on the particle surface and in a depth range of 150 nm or less from the particle surface. In other words, it is preferable that the sulfur compound is substantially absent from the inside of the particle beyond a depth of 150 nm from the particle surface. Since the presence of the sulfur compound on or near the particle surface of the composite oxide is thought to suppress the increase in resistance, in this case the increase in resistance can be suppressed efficiently. The content of the sulfur compound increases as you move from the inside of the particle towards the surface, and substantially the entire amount of the sulfur compound may be present only in a depth range of 150 nm or less from the particle surface. The sulfur compound may substantially be present only on the particle surface and in a depth range of 100 nm or less from the particle surface.

[0042] Elements present on or near the particle surface of lithium transition metal composite oxides can be identified by XPS measurement. The ESCA5600 from Ulvac Phi, Inc. can be used for XPS measurement. The depth from the particle surface obtained by XPS measurement of the positive electrode active material is SiO2. ２ These are converted values. The obtained data was smoothed (SG9) and then corrected so that C1s was 284.8 eV before use.

[0043] The XPS measurement conditions are as follows: X-ray source: Mg-conventional (fixed target type X-ray tube, 1253.6 eV) Neutralization: Electron gun only Etching conditions: Ar ion gun Acceleration voltage: 4 kV, 2.3 nm / min (SiO ２ (Conversion) Measurement conditions: Pass energy, 58.70 eV, Step 0.25 eV (outer surface, depth direction)

[0044] As shown in Figure 3 below, the XPS spectrum of the positive electrode active material contains a peak originating from sulfur (S) at the particle surface of the lithium transition metal composite oxide and in the depth range of 100 nm or less from the surface, in the bond energy range of 172 eV to 174 eV. The intensity of the S-derived peak increases as you approach the surface from the inside of the particle. The S-derived peak is SiO ２In terms of conversion, the peak may substantially exist only in a depth range of 150 nm or less from the particle surface, or only in a depth range of 100 nm or less from the particle surface. In addition to the peak originating from S, there may be another peak originating from S in a lower energy binding energy range (for example, between 168 eV and 171 eV). Furthermore, there may be yet another peak originating from S.

[0045] Since sulfates are used as coprecipitation raw materials when synthesizing lithium transition metal composite oxides, sulfur (S) may be detected at depths greater than 100 nm from the particle surface of the composite oxide, for example. However, even if the raw material contains about 2000 ppm of sulfate ions, it cannot be confirmed as a peak in the XPS spectrum.

[0046] Examples of suitable sulfur compounds include at least one selected from the group consisting of disulfites, dithionites, dithionates, and tetrathionates, and their hydrates. Among these, lithium salts, sodium salts, and potassium salts are preferred. When at least one selected from these is used, resistance increase can be more effectively suppressed while maintaining good cycle characteristics at high capacity.

[0047] The particle surface of the lithium transition metal composite oxide may also contain at least one of Na and K. At least one of Na and K is present only on the particle surface of the lithium transition metal composite oxide and in a depth range of less than 150 nm from the surface, or in a depth range of 100 nm or less from the particle surface. In the XPS spectrum of the positive electrode active material, for example, on the particle surface of the lithium transition metal composite oxide and in a depth range of less than 150 nm from the surface, there is a peak originating from Na in the bond energy range of 1074 eV to 1077 eV. The peak originating from Na is SiO ２ In terms of conversion, they may be present only in a depth range of less than 150 nm from the particle surface, or only in a depth range of 100 nm or less from the particle surface.

[0048] As described above, a predetermined amount of boron compound is present on and near the particle surface of the lithium transition metal composite oxide. The content of the boron compound is preferably 0.1 mol% to 2.0 mol%, more preferably 0.3 mol% to 1.5 mol%, and particularly preferably 0.5 mol% to 1.5 mol%, or 0.5 mol% to 1.0 mol%, relative to the total molar amount of metal elements excluding Li that constitute the lithium transition metal composite oxide. If the content of the boron compound is within this range, the increase in resistance can be suppressed more effectively while maintaining good cycle characteristics at high capacity. Adding more than 2.0 mol% of the boron compound is expected to result in a plateau in the resistance reduction effect and lead to problems such as a decrease in capacity; therefore, the content of the boron compound is preferably 2.0 mol% or less.

[0049] It is preferable that the boron compound is distributed over a wide area of ​​the particle surface of the lithium transition metal composite oxide, rather than being concentrated in a specific area. The boron compound is evenly distributed across the particle surface of the composite oxide. The boron compound may exist as a thin film or as granules on the particle surface of the composite oxide. Furthermore, the boron compound may be in solid solution with the composite oxide and bonded to the sulfur compound via oxygen.

[0050] The boron compound is preferably boron oxide, boric acid, or a borate. Preferred borates are lithium salts, sodium salts, and potassium salts. The boron compound is, for example, at least one selected from the group consisting of boron oxide, boric acid, and borates. Note that the boric acid may be metaboric acid. When at least one selected from these is used, the effect of suppressing resistance increase becomes more pronounced. Note that the boron compound is thought to decompose in the heat treatment process described later, but it may remain in the state of the added compound. In the synthesis process of the positive electrode active material of this embodiment, one type of compound may be used alone as the boron compound, or two or more types of compounds may be used.

[0051] The boron compound is preferably present on the particle surface of the lithium transition metal composite oxide and in the depth range of 150 nm or less from the surface, and more preferably substantially only on the particle surface and in the depth range of 150 nm or less from the particle surface. Since it is considered that the boron compound suppresses the increase in resistance by being present on the particle surface of the composite oxide or in its vicinity, in this case, the increase in resistance can be efficiently suppressed. The content of the boron compound increases as it approaches the surface from the inside of the particle, and all of the boron compound may be substantially present only in the depth range of 150 nm or less from the particle surface. The boron compound may be substantially present only on the particle surface and in the depth range of 100 nm or less from the particle surface.

[0052] In the XPS spectrum of the positive electrode active material, as shown in FIG. 4 described later, in the particle surface of the lithium transition metal composite oxide and in the depth range of 100 nm or less from the surface, a peak derived from B exists in the range of binding energy of 190 eV or more and 194 eV or less. And the intensity of the peak derived from B increases as it approaches the surface from the inside of the particle. The peak derived from B may be present only in the depth range of 150 nm or less from the particle surface or only in the depth range of 100 nm or less from the particle surface in terms of SiO ２ conversion. In addition to the peak derived from B, another peak derived from B may exist in a range on the lower energy side of the binding energy.

[0053] The positive electrode active material may contain a positive electrode active material other than the lithium transition metal composite oxide of the present embodiment in which a sulfur compound and a boron compound are present on the particle surface, as long as the object of the present disclosure is not impaired. In the non-aqueous electrolyte secondary battery 10, for example, depending on the required battery performance and the like, a plurality of types of positive electrode active materials can be used. Even when other composite oxides such as a composite oxide in which a sulfur compound or a boron compound is not present on the particle surface or a composite oxide in which the total amount of Ni and Mn is less than 80 mol% are used in combination as the positive electrode active material, the above resistance reduction effect can be obtained according to the content of the positive electrode active material of the present embodiment.

[0054] When the lithium transition metal composite oxide of this embodiment, in which sulfur compounds and boron compounds are present on the particle surface, is mixed with other composite oxides as the positive electrode active material, the lithium transition metal composite oxide of this embodiment may be present in an amount of 5% by mass or more of the total positive electrode active material, but it is preferably 10% by mass or more, and more preferably 15% by mass or more. As the other composite oxide, a composite oxide containing Ni is preferred. Furthermore, the particle size of the other composite oxide is preferably larger than the particle size of the lithium transition metal composite oxide of this embodiment, for example, 8 μm or more, or 10 μm or more. By mixing and using the lithium transition metal composite oxide of this embodiment with other composite oxides, effects such as improved electrode packing density, improved battery durability, and improved battery resistance suppression can be obtained.

[0055] The following describes an example of a method for producing a positive electrode active material. The method for producing a positive electrode active material includes, for example, a synthesis step of a lithium transition metal composite oxide, a washing step, a drying step, a crushing step, a sulfur compound addition step, and a heat treatment step after the addition of the sulfur compound. The positive electrode active material of this embodiment is produced by mixing a lithium transition metal composite oxide having a single-particle shape with a combined content of Ni and Mn of 80 mol% or more relative to the total molar amount of metal elements excluding Li, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, with a sulfur compound which is at least one selected from the group consisting of compounds containing S-S bonds and compounds with a S valence of less than 6 to obtain a mixture, and then heat treating the mixture. In the addition step, it is preferable to use a boron compound together with a specific sulfur compound.

[0056] In the synthesis process of lithium transition metal composite oxides, a metal hydroxide containing 80 mol% or more of Ni and Mn in total relative to the total molar amount of metal elements other than Li is mixed with a Li compound, and the mixture is calcined to obtain a lithium transition metal composite oxide. As the Li compound, for example, Li ２ CO ３ LiOH, Li ２ O ２ Li ２ O, LiNO３ LiNO ２ Li ２ SO ４ LiOH H ２ Examples include O, LiH, and LiF.

[0057] Metal hydroxides can be obtained, for example, by stirring a solution of a metal salt containing Ni, Mn, and any element, and adding an alkaline solution such as sodium hydroxide dropwise to adjust the pH to the alkaline side (for example, 8.5 to 12.5) to precipitate (coprecipitation). Alternatively, metal oxides obtained by heat-treating metal hydroxides may be used instead of metal hydroxides. Since smaller particle sizes of metal hydroxides facilitate the growth of primary particles and the acquisition of single particles, the D50 of the metal hydroxide is preferably 7 μm or less, and more preferably 5 μm or less.

[0058] The metal hydroxide and the Li compound are mixed in a ratio such that, for example, the molar ratio of the metal element excluding Li to Li is 1:0.98 to 1:1.05. When mixing the metal hydroxide and the Li compound, Mg compounds, Al compounds, Ca compounds, Nb compounds, Sr compounds, Zr compounds, W compounds, etc., may be added. These compounds are, for example, oxides, hydroxides, or carbonates, and may be composite compounds containing other metal elements such as Li.

[0059] A mixture of metal hydroxide and Li compounds, etc., is calcined, for example, under an oxygen atmosphere (under a gas flow with an oxygen concentration of 80% or more). The calcination process may be multi-stage calcination. An example of calcination conditions is to set the heating rate in the temperature range of 450°C to 670°C to 1.0°C / min to 5.5°C / min, and the maximum temperature reached to 850°C to 1100°C. The heating rate from 670°C to the maximum temperature may be 0.1°C / min to 3.5°C / min. The holding time at the maximum temperature may be 1 hour to 30 hours. By adjusting the calcination conditions, single particles can be produced and their particle size can be adjusted. For example, increasing the maximum temperature makes it easier to obtain single particles, and their particle size tends to be larger. The calcination process may be carried out in air or under an inert gas atmosphere such as nitrogen.

[0060] In the washing step, the lithium transition metal composite oxide obtained in the synthesis step is washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be carried out by known methods and conditions. Mg compounds, Al compounds, Ca compounds, Nb compounds, Sr compounds, Zr compounds, W compounds, etc., may be added to the cake-like composition. In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powder-like composition. The drying step may be carried out under a vacuum atmosphere. For example, the drying temperature is 150°C to 400°C, and the drying time is 0.5 hours to 15 hours. The washing step can be omitted.

[0061] The powdered composition obtained in the drying process is crushed using a pulverizer such as a jet mill. Crushing with a jet mill can be performed, for example, using a PJM-80 manufactured by Nippon Pneumatic Mfg. Co., Ltd., under the following conditions. Note that the crushing process can also be omitted. Compressed air consumption: 0.5 Nm³ ３ Gas supply pressure per minute: 0.53 MPa; Processing capacity: 2000 g / hour

[0062] By mixing a sulfur compound and a boron compound with a crushed lithium transition metal composite oxide and then performing a heat treatment, a positive electrode active material can be obtained in which the sulfur compound, boron compound, and their decomposition products are present on the particle surface of the composite oxide and in the vicinity of that surface. For example, after dry mixing the lithium transition metal composite oxide, sulfur compound, and boron compound, a heat treatment is performed at a temperature of 250°C to 600°C, or 250°C to 400°C, or 250°C to 350°C for 2 to 10 hours, or 2 to 5 hours. For dry mixing, mixers such as planetary mixers, rocking mills, and high-speed mixers can be used.

[0063] After adding a boron compound and performing heat treatment, a sulfur compound may be added, but it is preferable to mix the sulfur compound and the boron compound simultaneously and then perform heat treatment. In this case, the resistance reduction effect becomes more pronounced. It is also conceivable to add the sulfur compound and the boron compound before calcination (to the raw materials) of the lithium transition metal composite oxide, but in this case, the surface state of the composite oxide particles obtained will be different from the surface state of the composite oxide particles according to this disclosure, making it difficult to obtain a positive electrode active material with excellent capacity retention after charge-discharge cycles.

[0064] The heat treatment may be carried out in an oxygen atmosphere, in the air, or in an inert gas atmosphere such as nitrogen. Regardless of whether the heat treatment atmosphere is an oxygen atmosphere, an inert gas atmosphere, or air, the above effects can be obtained by adding sulfur compounds and boron compounds. However, when using a lithium transition metal composite oxide containing 80 mol% Ni as the transition metal, it is preferable to perform the heat treatment in an inert gas atmosphere or air. In other words, it is preferable to perform the heat treatment in an atmosphere with an oxygen concentration of 21% or less, or less than 20%. In this case, the effect of suppressing the increase in resistance becomes more pronounced.

[0065] [Negative Electrode] The negative electrode 12 comprises a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with the metal disposed on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the portion to which the negative electrode lead 21 is connected. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode core, drying the coating, and then compressing it to form the negative electrode mixture layer on both sides of the negative electrode core. A metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may consist only of a negative electrode core, and metallic lithium may be deposited on the core surface during battery charging.

[0066] The negative electrode active material is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally, carbon materials such as graphite are used. Alternatively, elements that alloy with Li, such as Si and Sn, or materials containing these elements may be used as the negative electrode active material. Silicon-containing materials, including Si, are preferred as negative electrode active materials. Furthermore, lithium titanate, which has a higher charge-discharge potential than carbon materials for metallic lithium, can also be used as the negative electrode active material. The negative electrode active material may be used alone or in combination of multiple types.

[0067] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. In particular, it is preferable to use artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and earthy graphite, or mixtures thereof. Examples of silicon-containing materials that function as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A preferred silicon-containing material is a composite particle containing an ionic conducting phase and a Si phase dispersed in the ionic conducting phase.

[0068] The binder included in the negative electrode mixture layer may be fluororesin, olefin resin, PAN, polyimide, polyamide, acrylic resin, etc., as in the case of the positive electrode 11, but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. may also be used. Among these, the use of SBR is preferred. One type of binder may be used alone, or multiple types may be used in combination. Furthermore, the negative electrode mixture layer preferably contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. These function as thickeners in the negative electrode mixture slurry. The binder content is not particularly limited, but is, for example, 0.1% by mass or more and 5% by mass or less relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer may also contain conductive agents such as CNTs.

[0069] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0070] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0071] [Non-aqueous electrolytes] Non-aqueous electrolytes are ionic conductive (for example, lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0072] The electrolyte solution includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0073] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters 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 linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0074] Examples of the above 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-dimethoxyethaneethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl 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.

[0075] The electrolyte salt is preferably a lithium salt. A suitable lithium salt is LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２ Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0076] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0077] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0078] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0079] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0080] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0081] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0082] <Example AA1> [Preparation of positive electrode active material] LiOH and Ni obtained by coprecipitation method ０．８０ Mn ０．２０ (OH) ２The powder was mixed with the total amount of Li, Ni, and Mn to obtain a mixture with a molar ratio of 1.05:1. This mixture was calcined in two stages under an oxygen stream with an oxygen concentration of 90% or higher (flow rate of 0.15 L / min to 0.2 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 880°C over 4 hours. After that, it was held at 880°C for 3 hours to obtain a lithium transition metal composite oxide.

[0083] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and after drying the washed composite oxide, it was crushed. Sodium disulfite was mixed with the crushed lithium transition metal composite oxide as a sulfur compound, and then heat-treated at 300°C for 4 hours under an oxygen atmosphere (flow rate: 4 L / min) to obtain a positive electrode active material in which at least one substance selected from the group consisting of sodium disulfite and its decomposition products was present on and near the particle surface of the lithium transition metal composite oxide. At this time, the amount of sodium disulfite and its decomposition products added was adjusted so that the content was 0.5 mol% relative to the total molar amount of metal elements excluding Li in the composite oxide.

[0084] SEM observation of the positive electrode active material confirmed that most of the lithium transition metal composite oxide particles were aggregated into several to several dozen single particles. The D50, crystallite size, and BET specific surface area of ​​the lithium transition metal composite oxide (positive electrode active material) were measured using the above method, and the results were: D50 4.9 μm, crystallite size 459 Å, and BET specific surface area 1.1 m². ２ The concentration was / g. The crystal structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to space group R-3m. Furthermore, in the XPS spectrum of the positive electrode active material, peaks originating from S were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from that surface (the same was true in Example AA2 described later).

[0085] [Preparation of Test Cell] The test cell shown in Figure 2 was prepared by following the procedure below. The working electrode 30 is a positive electrode using the positive electrode active material described above. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 100:5:2, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone as the dispersion medium. This slurry was applied to an aluminum foil which served as the positive electrode core, and the coating was dried at 80°C to obtain the working electrode 30.

[0086] Under dry air with a dew point of -50°C or lower, an electrode group was fabricated by interposing a separator 34 between each electrode: a working electrode 30, a counter electrode 31 (negative electrode), and a reference electrode 32, each fitted with electrode leads 38. This group was then housed in an outer casing 35. Subsequently, an electrolyte 36 was injected into the outer casing 35, and the outer casing 35 was sealed to obtain a test cell.

[0087] Details of each component of the test cell are as follows: Counter electrode 31: Lithium metal Reference electrode 32: Lithium metal Separator 34: Polyethylene separator Electrolyte 36: Non-aqueous solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 (25°C), with LiPF as the electrolyte salt. ６ A solution was used that was dissolved to a concentration of 1.0 mol / L.

[0088] The following performance evaluations were performed on the test cell of Example AA1, and the evaluation results, along with the composition and amount of additives, are shown in Table 1 (the same applies to Examples AA2, Reference Examples A1 to A5, and Comparative Examples A1 to A25).

[0089] [Evaluation of Charge / Discharge Efficiency and Capacity Retention Rate] Under a temperature environment of 25°C, the test cell was charged to 4.5V (against Li metal) with a constant current of 0.2C, and then charged to 0.02C with a constant voltage of 4.5V. After that, it was discharged to 2.5V with a constant current of 0.2C. The charge capacity and discharge capacity at this time were measured, and the initial charge / discharge efficiency was calculated by dividing the discharge capacity by the charge capacity. From the second cycle onward, the discharge current value was changed to 0.2C, and this charge / discharge cycle was performed 30 times. The capacity retention rate was calculated by dividing the discharge capacity at the 30th cycle by the discharge capacity at the 1st cycle.

[0090] [Evaluation of IV Resistance] For test cells after 10 and 30 charge-discharge cycles, the IV resistance (internal resistance) was measured for each state of charge (SOC) of 10%, 50%, and 100% (4.45V). The measurement was performed by setting the test cell to each SOC state, letting it rest for 15 minutes, and setting the voltage at that time as V0. The voltage drop during a 10-second discharge was defined as ΔV, and the IV resistance was calculated from the straight line obtained from each current value and each ΔV.

[0091] <Example AA2> The positive electrode active material and test cell were prepared in the same manner as in Example AA1, except that sodium dithionite was used instead of sodium disulfite, and the above performance evaluation was performed. The D50 of the positive electrode active material was 4.9 μm, the crystallite size was 459 Å, and the BET specific surface area was 1.1 m². ２ It was / g.

[0092] <Comparative Example A1> Except for the absence of sodium disulfite, an uncoated positive electrode active material and test cell were prepared in the same manner as in Example AA1, and the above performance evaluation was performed.

[0093] <Reference Examples A1-A5 and Comparative Examples A2-A25> The positive electrode active material and test cell were prepared in the same manner as in Example AA1, except that the compounds shown in Table 1 were used instead of sodium disulfite, and the above performance evaluation was performed.

[0094] Figures 3A and 3B show the XPS spectra of the positive electrode active materials synthesized in Comparative Example A1 and Reference Example A5, respectively, indicating the binding energy range in which peaks originating from S appear. Figures 4A and 4B show the XPS spectra of the positive electrode active materials synthesized in Comparative Example A1 and Reference Example A5, respectively, indicating the binding energy range in which peaks originating from B appear. As shown in Figure 3B, in the XPS spectrum of the positive electrode active material of Reference Example A5, a peak originating from S was confirmed in the depth range of less than 100 nm from the particle surface of the positive electrode active material. On the other hand, as shown in Figure 3A, no peak originating from S was confirmed in the XPS spectrum of the positive electrode active material of Comparative Example A1. Also, as shown in Figures 4A and 4B, no peak originating from B was confirmed in the XPS spectra of the positive electrode active materials of Comparative Example A1 and Reference Example A5.

[0095]

[0096] As shown in Table 1, in the test cells of Comparative Examples A1 to A26, the resistance of the positive electrode increases significantly as the number of charge-discharge cycles increases. However, in the test cell of Example AA1, which uses sodium disulfite as an additive to the positive electrode active material, and in the test cell of Example AA2, which uses sodium dithionite, this resistance increase is effectively suppressed. Many of the positive electrode active materials used in the test cells of the comparative examples contain sulfur, but the effect of suppressing the resistance increase after charge-discharge cycles was not obtained in the test cells of the comparative examples. Furthermore, in Reference Examples A1 to A4, alkyl sulfates are used as additives to the positive electrode active material, but even in this case, a sufficient effect of suppressing the resistance increase was not obtained compared to Examples AA1 and AA2.

[0097] These results indicate that when a specific sulfur compound, selected from compounds containing S-S bonds and compounds with a sulfur valency of less than 6, is used as an additive to the positive electrode active material, the increase in resistance after charge-discharge cycles can be specifically suppressed. Furthermore, the test cells of Examples AA1 and AA2 exhibited superior cycle characteristics, particularly a higher capacity retention rate after charge-discharge cycles compared to the test cells of Comparative Examples A1 to A26. Note that when only a boron compound was used as an additive to the positive electrode active material (Reference Example A5), the increase in resistance was suppressed compared to the case without additives (Comparative Example A1) when the number of cycles was small, but the increase in resistance could not be suppressed as the number of cycles increased.

[0098] <Examples BB1 to BB5> The positive electrode active material and test cell were prepared in the same manner as in Example AA1, except that the amount of sodium disulfite added was changed to the amount shown in Table 2, and the above performance evaluation was performed. The evaluation results are shown in Table 2 (the same applies to Examples B1 to B24 and Reference Examples B1 to B3 thereafter).

[0099] <Examples B1 to B23> The cathode active material and test cell were prepared in the same manner as in Example AA1, except that the amount of sodium disulfite added was as shown in Table 2, and boric acid was added in the amount shown in Table 2, and the above performance evaluation was performed.

[0100] <Reference Examples B1-B3> Except for the absence of sodium disulfite, the positive electrode active material and test cell were prepared in the same manner as in Examples B2-B4, and the above performance evaluation was performed.

[0101] Figures 3C to 3G show the XPS spectra of the positive electrode active materials synthesized in Examples BB3, B1, B9, B17, and B21, respectively, indicating the binding energy range in which peaks originating from S appear. Figures 4C to 4G show the XPS spectra of the positive electrode active materials synthesized in Examples BB3, B1, B9, B17, and B21, respectively, indicating the binding energy range in which peaks originating from B appear. As shown in Figures 3C to 3G, in the XPS spectra of the positive electrode active materials of Examples BB3, B1, B9, B17, and B21, peaks originating from S were confirmed in a depth range shallower than 100 nm from the particle surface of the positive electrode active material. Also, as shown in Figures 4D to 4G, in the positive electrode active materials of Examples B1, B9, B17, and B21, peaks originating from B were confirmed in a depth range shallower than 100 nm from the particle surface. On the other hand, as shown in Figure 4C, no peaks originating from B were confirmed in the XPS spectrum of the positive electrode active material of Example BB3.

[0102]

[0103] From the results shown in Table 2, it can be seen that the test cells of Examples B1 to B24, which use sodium disulfite and boric acid as additives to the positive electrode active material, can effectively suppress the increase in resistance after charge-discharge cycles compared to the test cell of Comparative Example A1 and the test cells of Reference Examples A1, B1 to B3, which do not use specific sulfur compounds as additives to the positive electrode active material. Furthermore, the test cells of Examples B1 to B24 have a high capacity retention rate after charge-discharge cycles and excellent cycle characteristics. In addition, the test cells of Examples B1 to B24 show a more significant effect in suppressing the increase in resistance compared to the test cells of Examples AA1, BB1 to BB5, which do not use boron compounds as additives to the positive electrode active material.

[0104] When a lithium transition metal composite oxide containing 80 mol% Ni and 20 mol% Mn as transition metals was used, the above effect was particularly pronounced when the amount of sodium disulfite added was 0.2 mol% to 0.5 mol% and the amount of boric acid added was 0.5 mol% to 1.5 mol%, relative to the total molar amount of metal elements excluding Li contained in the positive electrode active material.

[0105] <Example CC1> LiOH and Ni obtained by coprecipitation method ０．７５ Mn ０．２５ (OH) ２ The powder was mixed with the total amount of Li, Ni, and Mn to obtain a mixture with a molar ratio of 1.05:1. This mixture was calcined in two stages under an oxygen stream with an oxygen concentration of 90% or higher (flow rate of 0.15 L / min to 0.2 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 900°C over 4 hours. After that, it was held at 900°C for 3 hours to obtain a lithium transition metal composite oxide.

[0106] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and after drying the washed composite oxide, it was crushed using a jet mill. Sodium disulfite was mixed with the crushed lithium transition metal composite oxide as a sulfur compound, and then heat-treated at 300°C for 4 hours under an oxygen atmosphere (flow rate: 4 L / min) to obtain a positive electrode active material in which at least one substance selected from the group consisting of sodium disulfite and its decomposition products was present on and near the particle surface of the lithium transition metal composite oxide. At this time, the amount of sodium disulfite and its decomposition products added was adjusted so that the content was 0.2 mol% relative to the total molar amount of metal elements excluding Li in the composite oxide.

[0107] SEM observation of the positive electrode active material confirmed that most of the particles in the lithium transition metal composite oxide are single particles composed of a single primary particle. The D50 of the lithium transition metal composite oxide (positive electrode active material) was 1.4 μm, the crystallite size was 507 Å, and the BET specific surface area was 1.58 m². ２The concentration was / g. The crystal structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to space group R-3m. In addition, peaks originating from S were confirmed in the XPS spectrum of the positive electrode active material at the particle surface of the composite oxide and in a depth range shallower than 150 nm from that surface.

[0108] The performance evaluation described above was performed on the test cell of Example CC1, and the evaluation results, along with the composition and amount of additives, are shown in Table 3 (the same applies to Examples CC2, Examples C1 to C20, Reference Examples C1 to C5, and Comparative Example C1).

[0109] <Example CC2> The positive electrode active material and test cell were prepared in the same manner as in Example CC1, except that the amount of sodium disulfite added was changed to the amount shown in Table 3, and the above performance evaluation was performed.

[0110] <Examples C1 to C20> Except for the addition of sodium disulfite in the amount shown in Table 3 and the addition of boric acid in the amount shown in Table 3, the cathode active material and test cell were prepared in the same manner as in Example CC1, and the above performance evaluation was performed. In the XPS spectra of each cathode active material, peaks originating from S and B were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface, respectively.

[0111] <Comparative Example C1> The positive electrode active material and test cell were prepared in the same manner as in Example CC1, except that sodium disulfite was not added, and the above performance evaluation was performed.

[0112] <Reference Examples C1-C5> Except for the absence of sodium disulfite, the positive electrode active material and test cell were prepared in the same manner as in Examples C1, C5, C9, C13, and C17, and the above performance evaluation was performed.

[0113]

[0114] From the results shown in Table 3, it can be seen that even when using a lithium transition metal composite oxide containing 75 mol% Ni and 25 mol% Mn as the transition metal, the test cells of Examples C1 to C20 using sodium disulfite and boric acid can effectively suppress the increase in resistance after charge-discharge cycles compared to the test cell of Comparative Example C1 and the test cells of Reference Examples C1 to C5 that do not use sodium disulfite. Furthermore, the test cells of Examples C1 to C20 have a high capacity retention rate after charge-discharge cycles and excellent cycle characteristics. In addition, the test cells of Examples C1 to C20 suppress the increase in resistance after cycles more effectively than the test cells of Examples CC1 and CC2 that do not use boric acid.

[0115] <Example DD1> LiOH and Ni obtained by coprecipitation method ０．７０ Mn ０．３０ (OH) ２ The powder was mixed with the total amount of Li, Ni, and Mn to obtain a mixture with a molar ratio of 1.05:1. This mixture was calcined in two stages under an oxygen stream with an oxygen concentration of 90% or higher (flow rate of 0.15 L / min to 0.2 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 16 hours, and then raised from 670°C to 925°C over 4 hours. After that, it was held at 925°C for 3 hours to obtain a lithium transition metal composite oxide.

[0116] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and after drying the washed composite oxide, it was crushed using a jet mill. Sodium disulfite was mixed with the crushed lithium transition metal composite oxide as a sulfur compound, and then heat-treated at 300°C for 4 hours under an oxygen atmosphere (flow rate: 4 L / min) to obtain a positive electrode active material in which at least one substance selected from the group consisting of sodium disulfite and its decomposition products was present on and near the surface of the lithium transition metal composite oxide particles. At this time, the amount of sodium disulfite and its decomposition products added was adjusted so that the content was 0.2 mol% relative to the total molar amount of metal elements excluding Li in the composite oxide.

[0117] SEM observation of the positive electrode active material confirmed that most of the particles in the lithium transition metal composite oxide are single particles composed of a single primary particle. The D50 of the lithium transition metal composite oxide (positive electrode active material) was 1.42 μm, the crystallite size was 492 Å, and the BET specific surface area was 1.56 m². ２ The concentration was / g. The crystal structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to space group R-3m. In addition, peaks originating from S were confirmed in the XPS spectrum of the positive electrode active material at the particle surface of the composite oxide and in a depth range shallower than 150 nm from that surface.

[0118] The performance evaluation described above was performed on the test cell of Example DD1, and the evaluation results, along with the composition and amount of additives, are shown in Table 4 (the same applies to Examples DD2 to DD4, Examples D1 to D20, Reference Examples D1 to D5, and Comparative Example D1).

[0119] <Examples DD2 to DD4> The positive electrode active material and test cell were prepared in the same manner as in Example DD1, except that the amount of sodium disulfite added was changed to the amount shown in Table 4, and the above performance evaluation was performed.

[0120] <Examples D1 to D20> Except for the addition of sodium disulfite in the amount shown in Table 4 and the addition of boric acid in the amount shown in Table 4, the cathode active material and test cell were prepared in the same manner as in Example DD1, and the above performance evaluation was performed. In the XPS spectra of each cathode active material, peaks originating from S and B were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface, respectively.

[0121] <Comparative Example D1> The positive electrode active material and test cell were prepared in the same manner as in Example DD1, except that sodium disulfite was not added, and the above performance evaluation was performed.

[0122] <Reference Examples D1-D4> Except for the absence of sodium disulfite, the positive electrode active material and test cell were prepared in the same manner as in Examples D1, D5, D9, and D13, and the above performance evaluation was performed.

[0123]

[0124] From the results shown in Table 4, it can be seen that even when using a lithium transition metal composite oxide containing 70 mol% Ni and 30 mol% Mn as the transition metal, the test cells of Examples D1 to D20 using sodium disulfite and boric acid can effectively suppress the increase in resistance after charge-discharge cycles compared to the test cell of Comparative Example D1 and the test cells of Reference Examples D1 to D4 that do not use sodium disulfite. Furthermore, the test cells of Examples D1 to D20 have a high capacity retention rate after charge-discharge cycles and excellent cycle characteristics. In addition, the test cells of Examples D1 to D20 suppress the increase in resistance after cycles more effectively than the test cells of Examples DD1 to DD4 that do not use boric acid.

[0125] <Example GG1> LiOH and Ni obtained by coprecipitation method ０．６５ Co ０．０５ Mn ０．３０ (OH) ２ The powder was mixed with the total amount of Li, Ni, Co, and Mn in a molar ratio of 1.05:1 to obtain a mixture. This mixture was calcined in two stages under an oxygen stream with an oxygen concentration of 90% or more (flow rate of 0.15 L / min to 0.2 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 15 hours, and then raised from 670°C to 9500°C over 4 hours. After that, it was held at 950°C for 3 hours to obtain a lithium transition metal composite oxide.

[0126] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and after drying the washed composite oxide, it was crushed using a jet mill. Sodium disulfite was mixed with the crushed lithium transition metal composite oxide as a sulfur compound, and then heat-treated at 300°C for 3 hours under an oxygen atmosphere (flow rate: 4 L / min) to obtain a positive electrode active material in which at least one substance selected from the group consisting of sodium disulfite and its decomposition products was present on and near the surface of the lithium transition metal composite oxide particles. At this time, the amount of sodium disulfite and its decomposition products added was adjusted so that the content was 0.3 mol% of the total molar amount of metal elements excluding Li in the composite oxide.

[0127] SEM observation of the positive electrode active material confirmed that most of the particles in the lithium transition metal composite oxide are single particles composed of a single primary particle. The D50 of the lithium transition metal composite oxide (positive electrode active material) was 1.6 μm, the crystallite size was 467 Å, ​​and the BET specific surface area was 1.5 m². ２ The concentration was / g. The crystal structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to space group R-3m. In addition, peaks originating from S were confirmed in the XPS spectrum of the positive electrode active material at the particle surface of the composite oxide and in a depth range shallower than 150 nm from that surface.

[0128] The performance evaluation described above was performed on the test cell of Example GG1, and the evaluation results, along with the composition and amount of additives, are shown in Table 5 (the same applies to Examples GG2 to GG4, Examples G1 to G20, and Comparative Example G1).

[0129] <Examples GG2 to GG4> The cathode active material and test cell were prepared in the same manner as in Example GG1, except that the amount of sodium disulfite added was changed to the amount shown in Table 5, and the above performance evaluation was performed.

[0130] <Examples G1 to G20> Except for the addition of sodium disulfite in the amount shown in Table 5 and the addition of boric acid in the amount shown in Table 5, the cathode active material and test cell were prepared in the same manner as in Example GG1, and the above performance evaluation was performed. In the XPS spectra of each cathode active material, peaks originating from S and B were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface, respectively.

[0131] <Comparative Example G1> The positive electrode active material and test cell were prepared in the same manner as in Example GG1, except that sodium disulfite was not added, and the above performance evaluation was performed.

[0132]

[0133] From the results shown in Table 5, it can be seen that even when using a lithium transition metal composite oxide containing 65 mol% Ni, 30 mol% Mn, and 5 mol% Co as the transition metal, the test cells of Examples G1 to G20 using sodium disulfite and boric acid can effectively suppress the increase in resistance after charge-discharge cycles compared to the test cell of Comparative Example G1. Furthermore, the test cells of Examples G1 to G20 have a high capacity retention rate after charge-discharge cycles and excellent cycle characteristics. In addition, the test cells of Examples G1 to G20 suppress the increase in resistance after cycles more effectively than the test cells of Examples GG1 to GG4 which do not use boric acid.

[0134] <Examples BB3-2, BB3-3> The positive electrode active material and test cell were prepared in the same manner as in Example BB3, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed. The evaluation results are shown in Table 6 (the same applies to Examples B9-2, B9-3, etc.).

[0135] <Examples B9-2, B9-3> The cathode active material and test cell were prepared in the same manner as in Example B9, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0136] <Examples B10-2, B10-3> The positive electrode active material and test cell were prepared in the same manner as in Example B10, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0137] <Examples AA1-2, AA1-3> The positive electrode active material and test cell were prepared in the same manner as in Example AA1, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0138] <Examples B18-2, B18-3> The positive electrode active material and test cell were prepared in the same manner as in Example B18, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0139] <Examples B19-2, B19-3> The positive electrode active material and test cell were prepared in the same manner as in Example B19, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0140] <Comparative Examples A1-2 and A1-3> The positive electrode active material and test cell were prepared in the same manner as in Comparative Example A1, except that the lithium transition metal composite oxide was calcined in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0141]

[0142] As shown in Table 6, regardless of whether the heat treatment atmosphere was an oxygen atmosphere, a nitrogen atmosphere, or air, the addition of specific sulfur compounds and boron compounds was effective in suppressing the increase in resistance. Furthermore, when a lithium transition metal composite oxide containing 80 mol% Ni and 20 mol% Mn was used as the transition metal, it was found that increasing the amount of sulfur and boron compounds added made the effect of suppressing the increase in resistance more pronounced when the heat treatment atmosphere was changed to a nitrogen atmosphere (inert gas atmosphere) or when the heat treatment was performed in air, in other words, when the partial pressure of oxygen was reduced.

[0143] <Examples DD2-2, DD2-3> The positive electrode active material and test cell were prepared in the same manner as in Example DD2, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed. The evaluation results are shown in Table 7 (the same applies to Examples D10-2, D10-3, etc.).

[0144] <Examples D10-2, D10-3> The positive electrode active material and test cell were prepared in the same manner as in Example D10, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0145] <Examples D14-2, D14-3> The positive electrode active material and test cell were prepared in the same manner as in Example D14, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0146] <Examples DD4-2, DD4-3> The cathode active material and test cell were prepared in the same manner as in Example DD4, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0147] <Examples D12-2, D12-3> The positive electrode active material and test cell were prepared in the same manner as in Example D12, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0148] <Examples D16-2, D16-3> The positive electrode active material and test cell were prepared in the same manner as in Example D16, except that the calcination of the lithium transition metal composite oxide and the heat treatment after the addition of the sulfur compound were carried out in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0149] <Comparative Examples D1-2 and D1-3> The positive electrode active material and test cell were prepared in the same manner as in Comparative Example D1, except that the lithium transition metal composite oxide was calcined in air and under a nitrogen atmosphere, respectively, and the above performance evaluation was performed.

[0150]

[0151] As shown in Table 7, the effect of suppressing resistance increase by adding specific sulfur compounds and boron compounds was obtained regardless of whether the heat treatment atmosphere was an oxygen atmosphere, a nitrogen atmosphere, or air. Furthermore, when a lithium transition metal composite oxide containing 70 mol% Ni and 30 mol% Mn was used as the transition metal, no significant difference in the resistance increase suppression effect was observed depending on the heat treatment atmosphere.

[0152] <Examples EE1, EE2> The cathode active material and test cell were prepared in the same manner as in Example AA2, except that the amount of sodium dithionite added was changed to the amount shown in Table 8, and the above performance evaluation was performed. The evaluation results are shown in Table 8 (the same applies to Examples E1 to E4 thereafter). In the XPS spectra of each cathode active material, peaks originating from S were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface.

[0153] <Examples E1 to E4> Except for using sodium dithionite instead of sodium disulfite as the sulfur compound, the cathode active material and test cell were prepared in the same manner as in Examples B9, B10, B17, and B18, and the above performance evaluation was performed. In the XPS spectra of each cathode active material, peaks originating from S and B were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface, respectively.

[0154]

[0155] As shown in Table 8, the test cells of Examples E1 to E4, AA2, EE1, and EE2 (lithium transition metal composite oxide: containing 80 mol% Ni and 20 mol% Mn as transition metals), which used sodium dithionite as the sulfur compound, also showed resistance increase suppression and cycle characteristic improvement effects equivalent to or exceeding those of the case using sodium disulfite.

[0156] <Examples F1 to F12> Except for using sodium dithionite instead of sodium disulfite as the sulfur compound, and using the amounts of sulfur compound and boric acid added as shown in Table 9, the positive electrode active material and test cell were prepared in the same manner as in Example D1, and the above performance evaluation was performed. The evaluation results are shown in Table 9.

[0157]

[0158] As shown in Table 9, the test cells of Examples F1 to F12 (lithium transition metal composite oxide: containing 70 mol% Ni and 30 mol% Mn as transition metals) using sodium dithionite as the sulfur compound also showed resistance increase suppression and cycle characteristic improvement effects equivalent to or exceeding those of the case using sodium disulfite.

[0159] <Example HH1> The positive electrode active material and test cell were prepared in the same manner as in Example AA1, except that potassium disulfite was used instead of sodium disulfite as the sulfur compound, and the amount of sulfur compound added was as shown in Table 10. The performance evaluation described above was then performed. The evaluation results are shown in Table 10. In the XPS spectrum of the positive electrode active material, peaks originating from S were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface (the same applies to Examples HH2 and HH3).

[0160] <Example HH2> The positive electrode active material and test cell were prepared in the same manner as in Example HH1, except that sodium dithionate dihydrate was used instead of potassium disulfite as the sulfur compound, and the above performance evaluation was performed.

[0161] <Example HH3> The positive electrode active material and test cell were prepared in the same manner as in Example HH1, except that sodium tetrathionate dihydrate was used instead of potassium disulfite as the sulfur compound, and the above performance evaluation was performed.

[0162] <Examples H1-H3> Except for the addition of boric acid in the amounts shown in Table 10, the cathode active material and test cell were prepared in the same manner as in Examples HH1-HH3, and the above performance evaluation was performed. In the XPS spectra of each cathode active material, peaks originating from S and B were confirmed at the particle surface of the composite oxide and in a depth range shallower than 150 nm from the surface, respectively.

[0163]

[0164] As shown in Table 10, in Examples HH1 to HH3, where potassium disulfite, sodium dithionate dihydrate, and sodium tetrathionate dihydrate were used as sulfur compounds, respectively, and in the test cells of Examples H1 to H3 (lithium transition metal composite oxide: containing 80 mol% Ni and 20 mol% Mn as transition metals), the same or even greater resistance increase suppression effect and cycle characteristic improvement effect were obtained as when sodium disulfite was used.

[0165] <Example JJ1> The positive electrode active material and test cell were prepared in the same manner as in Example DD2, except that potassium disulfite was used instead of sodium disulfite as the sulfur compound, and the amount of sulfur compound added was as shown in Table 10. The performance evaluation described above was then performed. The evaluation results are shown in Table 11.

[0166] <Example JJ2> The cathode active material and test cell were prepared in the same manner as in Example JJ1, except that sodium dithionate dihydrate was used instead of potassium disulfite as the sulfur compound, and the above performance evaluation was performed.

[0167] <Example JJ3> The cathode active material and test cell were prepared in the same manner as in Example JJ1, except that sodium tetrathionate dihydrate was used instead of potassium disulfite as the sulfur compound, and the above performance evaluation was performed.

[0168] <Examples H1-H3> Except for the addition of boric acid in the amounts shown in Table 11, the positive electrode active material and test cell were prepared in the same manner as in Examples JJ1-JJ3, and the above performance evaluation was performed.

[0169]

[0170] As shown in Table 11, in Examples JJ1 to JJ3, where potassium disulfite, sodium dithionate dihydrate, and sodium tetrathionate dihydrate were used as sulfur compounds, respectively, and in the test cells of Examples J1 to J3 (lithium transition metal composite oxide: containing 70 mol% Ni and 30 mol% Mn as transition metals), the same or even greater resistance increase suppression effect and cycle characteristic improvement effect were obtained as when sodium disulfite was used.

[0171] This disclosure is further illustrated by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide in which the combined content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li, and a sulfur compound, wherein the lithium transition metal composite oxide has a single-particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, and the sulfur compound is at least one selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds, and is present in an amount of 0.1 mol% or more and 2.0 mol% or less relative to the total molar amount of metal elements excluding Li contained in the positive electrode active material. Configuration 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the sulfur compound is substantially present only on the particle surface of the lithium transition metal composite oxide and in a depth range of 150 nm or less from the particle surface. Configuration 3: A positive electrode active material for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the content of the sulfur compound is 0.3 mol% or more and 0.8 mol% or less relative to the total molar amount of metal elements excluding Li contained in the positive electrode active material. Configuration 4: A positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the sulfur compound is at least one selected from the group consisting of disulfite, dithionite, dithionate, tetrathionate, and hydrates thereof. Configuration 5: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of Configurations 1 to 4, a negative electrode, and a non-aqueous electrolyte. Configuration 6: A non-aqueous electrolyte secondary battery according to Configuration 5, wherein the proportion of the positive electrode active material for a non-aqueous electrolyte secondary battery among the positive electrode active material contained in the positive electrode is 5% by mass or more. Configuration 7: A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the steps of: mixing a lithium transition metal composite oxide having a single-particle shape with a combined content of Ni and Mn of 80 mol% or more relative to the total molar amount of metal elements excluding Li, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less; and a sulfur compound being at least one selected from the group consisting of compounds containing S-S bonds and compounds with a S valence of less than 6; and heat-treating the mixture.

[0172] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Working electrode, 31 Counter electrode, 32 Reference electrode, 34 Separator, 35 Outer casing, 36 Electrolyte, 38 Electrode lead

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide in which the combined content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li, and a sulfur compound, wherein the lithium transition metal composite oxide has a single-particle shape, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less, and the sulfur compound is at least one selected from the group consisting of compounds containing S-S bonds, compounds with a S valence of less than 6, and decomposition products of these compounds, and is present in an amount of 0.1 mol% or more and 2.0 mol% or less relative to the total molar amount of metal elements excluding Li contained in the positive electrode active material.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is substantially present only on the particle surface of the lithium transition metal composite oxide and in a depth range of 150 nm or less from the particle surface.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the sulfur compound is 0.3 mol% or more and 0.8 mol% or less with respect to the total molar amount of metal elements other than Li contained in the positive electrode active material.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is at least one selected from the group consisting of disulfites, dithionites, dithionates, tetrathionates, and hydrates thereof.

5. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material described in any one of claims 1 to 4; a negative electrode; and a non-aqueous electrolyte.

6. The non-aqueous electrolyte secondary battery according to claim 5, wherein the proportion of the positive electrode active material for non-aqueous electrolyte secondary batteries among the positive electrode active material contained in the positive electrode is 5% by mass or more.

7. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the steps of: mixing a lithium transition metal composite oxide having a single-particle shape with a combined content of Ni and Mn of 80 mol% or more relative to the total molar amount of metal elements excluding Li, a volume-based median diameter of 0.5 μm or more and 5.0 μm or less, and a crystallite size of 370 Å or more and 1500 Å or less; and a sulfur compound being at least one selected from the group consisting of compounds containing S-S bonds and compounds with a S valence of less than 6; and heat-treating the mixture.

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