Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

This positive electrode active material contains a lithium transition metal composite oxide in which the total content of Ni and Mn is at least 80 mol% with respect to the total molar amount of metal elements excluding Li. 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. Predetermined amounts of a boron compound and a sulfur compound are present on the particle surface of the lithium transition metal composite oxide.

Description

Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

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

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, the positive electrode has a significant effect on battery performance, such as input / output characteristics, capacity, cycle characteristics, and thermal stability, and therefore much research has been conducted on the positive electrode. For example, Patent Document 1 proposes a non-aqueous electrolyte secondary battery containing a sodium salt and at least one of a sulfate and a sulfite, with the aim of improving high-temperature durability. Patent Document 1 describes fabricating a positive electrode by applying sodium sulfate, sodium sulfite, or the like to the surface of a positive electrode mixture layer.

[0003] JP 2018-60693 A

[0004] However, as the charging voltage of a battery increases, the resistance of the positive electrode increases, and the increase in resistance becomes particularly noticeable in the region of high state of charge (SOC). Therefore, in order to achieve a high charging voltage, it is important to suppress such an increase in resistance. In particular, in order to use a battery for a long period of time, it is important not only to reduce the initial resistance but also to suppress the increase in resistance due to charge / discharge cycles, which results in an improvement in the capacity retention rate.

[0005] The positive electrode active material according to the present disclosure includes a lithium transition metal composite oxide in which the total content of Ni and Mn is 80 mol % or more relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide having a single particle shape and 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, a boron compound and a sulfur compound are present on the particle surfaces of the lithium transition metal composite oxide, and the content of the boron compound is 0.1 mol % or more and 2.0 mol % or less, and the content of the sulfur compound is 0.1 mol % or more and 2.0 mol % or less, relative to the total molar amount of metal elements excluding Li constituting the lithium transition metal composite oxide.

[0006] A non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] The positive electrode active material according to the present disclosure can realize a nonaqueous electrolyte secondary battery in which the increase in resistance during high-potential charging is suppressed.

[0008] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; FIG. 2 is a cross-sectional view of a test cell produced in an Experimental Example; FIG. 3 is an XPS spectrum of a positive electrode active material produced in Experimental Example F9, showing the binding energy range in which a peak derived from B appears; FIG. 4 is an XPS spectrum of a positive electrode active material produced in Experimental Example G3, showing the binding energy range in which a peak derived from B appears; FIG. 5 is an XPS spectrum of a positive electrode active material produced in Experimental Example G8, showing the binding energy range in which a peak derived from B appears; FIG. 6 is an XPS spectrum of a positive electrode active material produced in Experimental Example F9, showing the binding energy range in which a peak derived from S appears; FIG. 7 is an XPS spectrum of a positive electrode active material produced in Experimental Example F4, showing the binding energy range in which a peak derived from S appears; FIG. 8 is an XPS spectrum of a positive electrode active material produced in Experimental Example G8, showing the binding energy range in which a peak derived from S appears; FIG. 9 is an XPS spectrum of a positive electrode active material produced in Experimental Example F9, showing the binding energy range in which a peak derived from Ni appears. 1 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G3, showing the binding energy range in which a peak derived from Ni appears. 2 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Ni appears. 3 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from Ni appears. 4 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from Mn appears. 5 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G3, showing the binding energy range in which a peak derived from Mn appears. 6 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Mn appears. 7 is an XPS spectrum of the positive electrode active material prepared in Experimental Example G8, showing the binding energy range in which a peak derived from Mn appears. 8 is an XPS spectrum of the positive electrode active material prepared in Experimental Example F9, showing the binding energy range in which a peak derived from Na appears. 1A and 1B are XPS spectra of the positive electrode active material prepared in Experimental Example F4, showing the binding energy range in which a peak derived from Na appears; 1C and 1D are backscattered electron images of the positive electrode active material prepared in Experimental Example F9; and 1D and 1E are backscattered electron images of the positive electrode active material prepared in Experimental Example F2.1 is a backscattered electron image of the positive electrode active material prepared in Experimental Example G8.

[0009] In recent years, with the widespread use of non-aqueous electrolyte secondary batteries such as lithium ion batteries for automotive applications and power storage applications, there has been a demand for higher charging voltages in terms of requirements for higher capacity and rapid charging. However, for example, when the charging voltage is increased to a level of 4.5 V (vs. Li metal), the increase in the resistance of the positive electrode becomes significant, especially in the high SOC range. Furthermore, non-aqueous electrolyte secondary batteries are required to further improve charge / discharge efficiency and durability. In addition, there is a demand for reduced manufacturing costs, and a positive electrode active material mainly composed of high-capacity, relatively inexpensive Ni and Mn is preferred.

[0010] The present inventors have succeeded in suppressing resistance increase during high-potential charging while ensuring excellent charge / discharge efficiency and durability by adding a predetermined amount of a boron compound and a sulfur compound to the particle surface of a single-particle composite oxide containing Ni and Mn. By using the positive electrode active material according to the present disclosure, resistance increase in the high SOC region is effectively suppressed. The effect of suppressing resistance increase by adding a boron compound and a sulfur compound is specifically achieved in single-particle composite oxides. In other words, the presence of a boron compound and a sulfur compound on the particle surface of a secondary-particle composite oxide formed by the aggregation of numerous primary particles does not effectively suppress resistance increase.

[0011] Hereinafter, with reference to the drawings, 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. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and modifications described below are included within the scope of the present disclosure.

[0012] In the following, a nonaqueous electrolyte secondary battery 10 is exemplified, which is a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom, but the outer can of the battery is not limited to a cylindrical outer can. Other embodiments of the nonaqueous electrolyte secondary battery according to the present disclosure include, for example, a prismatic battery having a prismatic outer can, a coin-shaped battery having a coin-shaped outer can, and a pouch-shaped battery having an outer can made of a laminate sheet including a metal layer and a resin layer. In addition, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The nonaqueous electrolyte secondary battery 10 is, for example, a lithium-ion secondary battery. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container with a bottom and an open end in the axial direction. The opening of the outer can 16 is closed by a sealing member 17. Hereinafter, for convenience of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0014] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction (longitudinal direction) and width direction (transverse direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0015] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0016] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0017] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10, particularly the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0019] [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface layer. 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. The positive electrode 11 can be produced, 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 a positive electrode mixture layer on both sides of the positive electrode core.

[0020] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, 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.

[0021] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins 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 carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination. The content of the binder 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] The positive electrode active material includes a lithium transition metal composite oxide in which the total content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide has a single particle shape, a volume-based median diameter (D50) of 0.5 μm to 5.0 μm, and a crystallite size of 370 Å to 1500 Å. Furthermore, a predetermined amount of a boron compound and a sulfur compound are present on the particle surface of the lithium transition metal composite oxide. This effectively suppresses resistance increase during high-potential charging 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.

[0023] As will be described in more detail later, the photoelectron spectrum obtained by X-ray photoelectron spectroscopy (XPS) measurement of the positive electrode active material shows a peak attributable to S in the binding energy range of 172 eV to 176 eV at the surface of the lithium transition metal composite oxide particles and in a region less than 100 nm deep from the surface, and a peak attributable to B in the binding energy range of 194 eV to 198 eV. Furthermore, in the photoelectron spectrum, a peak attributable to Ni in the binding energy range of 858 eV to 862 eV at the surface of the lithium transition metal composite oxide particles and in a region less than 50 nm deep from the surface, and a peak attributable to Mn in the binding energy range of 645 eV to 649 eV. Unless otherwise specified, the position of the peak in the XPS spectrum refers to the position of the peak top.

[0024] When the lithium transition metal composite oxide has a single particle shape, the effect of reducing resistance due to the addition of a boron compound and a sulfur compound can be obtained. In this specification, a single particle means a particle formed from a single primary particle, not a secondary particle formed by the aggregation of a large number (e.g., 1,000 or more) of primary particles. In other words, the particle interior does not substantially have a particle interface between primary particles. Note that particles formed by the aggregation of 10 or fewer primary particles approximate a single particle shape and can be considered to be substantially single particles. The single particle may be a single crystal particle with substantially no crystal grain boundaries within the particle, or a polycrystalline particle with several crystal grain boundaries within the particle.

[0025] In the lithium transition metal composite oxide having a boron compound and a sulfur compound attached to the particle surface, for example, 80% or more of all particles preferably have a single particle shape. In other words, the composite oxide in the form of secondary particles formed by aggregation of primary particles may be contained in an amount of 20% or less of all particles, and at this level, the same effect as when all particles have a single particle shape can be obtained.

[0026] As described above, the D50 of the lithium transition metal composite oxide is 0.5 μm or more and 5.0 μm or less. In this case, the resistance reduction effect of adding a boron compound and a sulfur compound is obtained. On the other hand, even if a boron compound and a sulfur compound are added to single particles having a D50 outside this range or to secondary particles formed by agglomeration of many primary particles, the resistance reduction effect is not substantially obtained. Based on the resistance reduction effect of adding a boron compound and a sulfur compound, the D50 of the lithium transition metal composite oxide is preferably 0.7 μm or more and 3.5 μm or less, more preferably 0.8 μm or more and 3.0 μm or less.

[0027] In this specification, D50 means the particle size at which the cumulative frequency of particles in a volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., 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 or more and 1500 Å or less, preferably 370 Å or more and 1000 Å or less, and more preferably 370 Å or more and 750 Å or less. If the crystallite size is within this range, the effect of reducing resistance due to the addition of a boron compound and a sulfur compound can be obtained. The crystallite size is calculated using the Scherrer equation expressed as the following formula from the half-width of the diffraction peak of the (104) plane in the X-ray diffraction pattern obtained by X-ray diffraction. In the formula below, s is the crystallite size, λ is the wavelength of the X-ray, B is the half-width of the diffraction peak of the (104) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this specification, K is set to 0.9. s = Kλ / B cos θ

[0029] The X-ray diffraction pattern was 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 speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: divided pseudo-Voigt function Constraint conditions: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = α (α is the Ni content of each element) ICSD No.: 98-009-4814

[0030] The BET specific surface area of ​​the lithium transition metal composite oxide is, for example, 0.5 m ２ / g or more 4m ２ / g or less, more preferably 0.9m ２ / g or more 3m ２ / g or less. Secondary particles containing many primary particles have voids within them, so even if the particle size is large, the specific surface area is relatively large. On the other hand, single particles have no voids within them, so the larger the particle size, the smaller the BET specific surface area.

[0031] The BET specific surface area can be measured using a Tristar II3020 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: 5 seconds Analysis Adsorbent: N2 Analysis Bath Temp.: 77.3 K (liquid nitrogen temperature) Cold free space: Measured Low pressure done: None Analysis method: BET multipoint method

[0032] The lithium transition metal composite oxide preferably has a layered rock salt structure. When a boron compound and a sulfur compound are applied to a composite oxide having a layered rock salt structure, the effect of reducing resistance during high-potential charging becomes more pronounced. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of increasing capacity and stability of the crystal structure.

[0033] The lithium transition metal composite oxide contains Ni and Mn as essential metal elements. The total content of Ni and Mn is 80 mol% or more relative to the total molar amount of metal elements excluding Li. The proportion of Ni and Mn in the metal elements excluding Li needs only to be 80 mol% or more. In this case, the resistance reduction effect due to the addition of a boron compound and a sulfur compound can be obtained, and a high-capacity composite oxide can be obtained relatively inexpensively. When the lithium transition metal composite oxide contains Ni, Mn, and Co, the total content thereof is preferably 90 mol% or more relative to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide may contain only Ni and Mn as metal elements excluding Li.

[0034] Ni is preferably contained in the largest amount among the metal elements other than Li constituting the lithium transition metal composite oxide. From the viewpoint of increasing capacity, etc., the Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more, more preferably 70 mol% or more, and particularly preferably 75 mol% or more, relative to the total molar amount of metal elements excluding Li. The upper limit of the Ni content is, for example, 95 mol%. Examples of suitable ranges of the Ni content are 70 mol% or more and 95 mol% or less, or 75 mol% or more and 95 mol% or less, or 75 mol% or more and 90 mol% or less, or 80 mol% or more and 90 mol% or less.

[0035] Among the metal elements other than Li that constitute the lithium transition metal composite oxide, Mn is preferably the second most abundant element after Ni. 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, 3 mol% to 50 mol% or 5 mol% to 30 mol% relative to the total molar amount of metal elements excluding Li. The lithium transition metal composite oxide may also contain Co at a rate less than that of Ni. The Co content is preferably equal to or less than the Mn content, for example, 15 mol% or less, 10 mol% or less, 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 inside the lithium transition metal composite oxide particles or may be present on the particle surface. When these elements are contained, for example, side reactions with the electrolyte are suppressed, improving the durability of the battery. 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 content of the elements contained in the positive electrode active material can be measured using an ICP optical emission spectrometer (e.g., CIROS-120 manufactured by SPECTRO).

[0037] As described above, a predetermined amount of boron compound is present on the particle surface of the lithium transition metal composite oxide. The content of the boron compound is preferably 0.005 mol% or more and 2.0 mol% or less, more preferably 0.1 mol% or more and 1.5 mol% or less, and particularly preferably 0.3 mol% or more and 1.0 mol% or less, relative to the total molar amount of metal elements excluding Li constituting the lithium transition metal composite oxide. If the content of the boron compound is within this range, the resistance reduction effect becomes significant. Note that, even if the boron compound is added in an amount exceeding 2.0 mol%, the resistance reduction effect will plateau, and it is expected that problems such as a decrease in capacity will occur. Therefore, the content of the boron compound is preferably 2.0 mol% or less.

[0038] The boron compound is preferably present over a wide area of ​​the particle surface of the lithium transition metal composite oxide, without being concentrated in a specific area. The boron compound is present evenly over the particle surface of the composite oxide. The boron compound may be present in the form of a thin film or particles on the particle surface of the composite oxide. The boron compound may also be in solid solution with the composite oxide and bonded to the sulfur compound via oxygen. The average particle size of the boron compound on the particle surface of the lithium transition metal composite oxide is, for example, 1 nm to 500 nm, or 2 nm to 100 nm, which is smaller than the average particle size of the sulfur compound. The average particle size of the boron compound can be measured in the same manner as the average particle size of the sulfur compound described below.

[0039] The boron compound is preferably boron oxide, boric acid, or a borate salt. Suitable 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 a borate salt. The boric acid may be metaboric acid. When at least one selected from these is used, the effect of suppressing the increase in resistance becomes more pronounced.

[0040] The boron compound is preferably present on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, and may be present substantially only in a region less than 100 nm deep from the particle surface. The presence of the boron compound on or near the particle surface of the composite oxide is thought to suppress an increase in resistance, so in this case, the increase in resistance can be efficiently suppressed. The content of the boron compound increases from the interior of the particle toward the surface, and substantially the entire amount of the boron compound may be present only in a region 50 nm deep or less from the particle surface.

[0041] The elements present on or near the particle surface of the lithium transition metal composite oxide can be confirmed by X-ray photoelectron spectroscopy (XPS). For the XPS measurement, an ESCA 5600 manufactured by Ulvac Phi, Inc. can be used. The depth from the particle surface obtained by XPS measurement of the positive electrode active material is determined by the depth of the SiO ２ The obtained data was smoothed (SG9) and then corrected so that C1s was 284.8 eV.

[0042] The XPS measurement conditions were 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 Measurement conditions: pass energy, 58.70 eV, step 0.25 eV (top surface, depth direction)

[0043] In the photoelectron spectrum (XPS spectrum) obtained by XPS measurement of the positive electrode active material, as shown in FIG. 3A described later, peaks derived from B exist in the binding energy range of 194 V or more and 198 eV or less on the particle surface of the lithium transition metal composite oxide and in the region at a depth of less than 100 nm from the surface. The intensity of the peak derived from B increases from the interior of the particle toward the surface. The peak derived from B is due to SiO ２ In terms of SiO, it may be present only in a region having a depth of less than 100 nm from the particle surface, or only in a region having a depth of 50 nm or less from the particle surface. ２ In terms of B, a peak derived from B may be present only in a region at a depth of 30 nm or less from the particle surface, or only in a region at a depth of 20 nm or less from the particle surface. At the particle surface of the lithium transition metal composite oxide and in a region at a depth of less than 100 nm from the surface, it is sufficient that the peak derived from B is present in a binding energy range of 194 V or more and 198 eV or less. However, in addition to the peak derived from B, another peak derived from B may be present in a lower binding energy range (for example, 190 eV or more and 193 eV or less). Furthermore, another peak derived from B may also be present.

[0044] A predetermined amount of sulfur compound is present on the particle surface of the lithium transition metal composite oxide. The sulfur compound may be any compound containing sulfur, preferably a sulfur oxide, and more preferably a compound containing a functional group represented by formula (1). The functional group represented by formula (1) is generally called a sulfate ester group. In the formula, R is an alkyl group having 5 or less carbon atoms, preferably an alkyl group having 3 or less carbon atoms. By having a predetermined amount of the sulfur compound present on the surface of the composite oxide particles together with the boron compound, an increase in the resistance of the positive electrode 11 during high-potential charging, particularly in a high SOC region, is effectively suppressed.

[0045] The content of the sulfur compound is preferably 0.005 mol% or more and 2.0 mol% or less, more preferably 0.1 mol% or more and 1.5 mol% or less, and particularly preferably 0.3 mol% or more and 1.0 mol% or less, relative to the total molar amount of metal elements excluding Li constituting the lithium transition metal composite oxide. If the content of the sulfur compound is within this range, the resistance reduction effect becomes significant. Note that, even if the sulfur compound is added in an amount exceeding 2.0 mol%, the resistance reduction effect will plateau, and it is expected that problems such as a decrease in capacity will occur. Therefore, the content of the sulfur compound is preferably 2.0 mol% or less.

[0046] The sulfur compound is preferably present over a wide area of ​​the particle surface of the lithium transition metal composite oxide, rather than being concentrated in a specific area. As shown in FIG. 8 (described later), the sulfur compound is present in granular form and is evenly scattered across the particle surface of the composite oxide. The granular presence of the sulfur compound allows for adequate contact between the particle surface of the lithium transition metal composite oxide and the electrolyte, making it easier to achieve a resistance reduction effect. The average particle size of the sulfur compound is preferably 0.005 μm or more and 0.5 μm or less, more preferably 0.007 μm or more and 0.2 μm or less, and particularly preferably 0.01 μm or more and 0.1 μm or less. By using a sulfur compound with an average particle size within this range, the resistance reduction effect due to the addition of the sulfur compound becomes more pronounced. The average particle size of the sulfur compound can be determined by observing the particle surface of the composite oxide using a scanning electron microscope (SEM). The average particle size of the sulfur compounds can be calculated by selecting any 100 sulfur compounds from an SEM image of the particle surface, measuring the diameters of the circumscribed circles, and averaging the measured values.

[0047] The measurement conditions for the SEM were as follows: Apparatus: FE-SEM manufactured by JEOL Ltd. Detector: UED (upper electron detector) Acceleration voltage: 1 kV WD: 3 mm

[0048] It is preferable that the sulfur compound is substantially present only on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. In other words, it is preferable that the sulfur compound is substantially absent from the interior of the particle at a depth of 100 nm or more from the particle surface. Since the sulfur compound is believed to suppress the increase in resistance by being present on or near the particle surface of the composite oxide, in this case, the increase in resistance can be efficiently suppressed. The content of the sulfur compound increases from the interior of the particle toward the surface, and substantially the entire amount of the sulfur compound may be present only in a region 50 nm deep or less from the particle surface.

[0049] In the XPS spectrum of the positive electrode active material, as shown in FIG. 4A described later, peaks derived from S exist in the binding energy range of 172 eV to 174 eV in the region on the particle surface of the lithium transition metal composite oxide and at a depth of less than 100 nm from the surface. The intensity of the peak derived from S increases as it approaches the surface from the inside of the particle. The peak derived from S is ２ In terms of SiO2, it may be present substantially only in a region having a depth of less than 100 nm from the particle surface, or only in a region having a depth of 50 nm or less from the particle surface. ２ In terms of S, a peak derived from S may be present only in a region at a depth of 30 nm or less from the particle surface, or only in a region at a depth of 20 nm or less from the particle surface. On the other hand, as shown in FIG. 8, no peak is observed if a sulfur compound is not contained. Note that, at the particle surface of the lithium transition metal composite oxide and in a region at a depth of less than 100 nm from the surface, it is sufficient that the peak derived from S is present in a binding energy range of 172 eV or more and 174 eV or less. However, in addition to the peak derived from S, another peak derived from S may be present in a lower binding energy range (for example, 168 eV or more and 171 eV or less). Furthermore, another peak derived from S may also be present.

[0050] Since sulfate is used as the coprecipitated raw material for synthesizing lithium transition metal composite oxides, S may be detected at a depth of more than 100 nm from the particle surface of the composite oxide, but even if the raw material contains about 2000 ppm of sulfate ions, it cannot be confirmed as a peak in the XPS spectrum. Similarly, Na is contained in the raw material at about 80 ppm, but it is not detected as a peak in the XPS spectrum at a depth of more than 100 nm from the particle surface. XPS spectra can be used to qualitatively characterize elements present on the particle surface.

[0051] At least one of Na and K may further be present on the particle surface of the lithium transition metal composite oxide. At least one of Na and K is present only on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, or in a region 50 nm deep or less from the particle surface. In the XPS spectrum of the positive electrode active material, as shown in FIG. 7A described later, a peak derived from Na exists in the binding energy range of 1074 eV or more and 1077 eV or less on the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface. The peak derived from Na is present in the region of SiO ２ In terms of Na, the peak may be present only in a region less than 100 nm deep from the particle surface, or only in a region 50 nm or less, 30 nm or less, or 20 nm or less deep from the particle surface. At the particle surface of the lithium transition metal composite oxide and in a region less than 100 nm deep from the surface, it is sufficient that the peak derived from Na exists in a binding energy range of 1074 eV or more and 1077 eV or less. However, in addition to the peak derived from Na, another peak derived from Na may also exist in a range of lower binding energy. Furthermore, another peak derived from Na may also exist.

[0052] The sulfur compound is preferably a salt of a sulfate ester having 5 or less carbon atoms, particularly a lithium salt, a sodium salt, or a potassium salt. Suitable sulfur compounds include at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate. When at least one selected from these compounds is used, the effect of suppressing the increase in resistance becomes more pronounced.

[0053] In the XPS spectrum of the positive electrode active material, as shown in Figures 5A and 6A described below, at the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, a peak derived from Ni is present in the binding energy range of 858 eV to 862 eV, and a peak derived from Mn is present in the binding energy range of 645 eV to 649 eV. The peaks derived from Ni and Mn present in each range may be present only in a region 30 nm or less, 20 nm or less, or 10 nm or less deep from the particle surface of the composite oxide. The peak derived from Ni is present, for example, in the range of 853 eV to 856 eV in a region more than 50 nm deep from the particle surface of the composite oxide. Furthermore, the peak derived from Mn is present, for example, in the range of 640 eV to 643 eV in a region more than 50 nm deep from the particle surface of the composite oxide. At the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, a peak derived from Ni may be present in a binding energy range of 858 eV to 862 eV, but in addition to the peak derived from Ni, another peak derived from Ni may be present in a lower binding energy range (e.g., 853 eV to 856 eV). Furthermore, another peak derived from Ni may also be present. Similarly, at the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, a peak derived from Mn may be present in a binding energy range of 645 eV to 649 eV, but in addition to the peak derived from Mn, another peak derived from Mn may be present in a lower binding energy range (e.g., 640 eV to 643 eV). Furthermore, another peak derived from Mn may also be present.

[0054] That is, at the particle surface of the lithium transition metal composite oxide and in a region less than 50 nm deep from the surface, the peaks derived from Ni and Mn are significantly shifted to higher energy compared to the interior of the particles. This large peak shift indicates that the Ni and Mn on the particle surface have a higher electron binding energy in the 2p orbital than the Ni and Mn inside the particles, or that they are in a high acid value state. It is believed that the use of a positive electrode active material exhibiting this peak shift, i.e., a positive electrode active material with high activity of Ni and Mn on the particle surface, significantly suppresses the increase in resistance. Note that the peaks derived from Ni and Mn gradually shift to higher energy toward the particle surface, for example, in a region more than 10 nm but less than 50 nm deep from the particle surface of the composite oxide.

[0055] The large shifts in the peaks derived from Ni and Mn appear specifically when B and S coexist on the particle surface of the lithium transition metal composite oxide. Figures 5B to 5D and Figures 6B to 6D show XPS spectra when B or S, or both, are not present on the particle surface of the lithium transition metal composite oxide, and in this case, the large peak shifts shown in Figures 5A and 6A are not observed.

[0056] That is, by mixing a composite oxide, a boron compound, and a sulfur compound and heat-treating them at a predetermined temperature, the state of Ni and Mn on the particle surface of the composite oxide is thought to change, resulting in a highly active state. The effect of the boron compound and the sulfur compound is thought to be due to the bond between the boron compound and an oxo acid, and boron oxide is preferably used as the boron compound, with boric acid and borate salts being particularly preferred. Furthermore, preferred oxo acids include titanic acid, molybdic acid, phosphoric acid, and alkali salts thereof. It is thought that the presence of these on the particle surface of the composite oxide, or their bonding with Ni and Mn, results in a positive electrode active material that exhibits a large peak shift.

[0057] The positive electrode active material may contain a positive electrode active material other than the lithium transition metal composite oxide of this embodiment in which a boron compound and a sulfur compound are present on the particle surface, as long as the object of the present disclosure is not impaired. Multiple types of positive electrode active materials can be used in the nonaqueous electrolyte secondary battery 10, for example, depending on the required battery performance, etc. Even when other composite oxides are used in combination as the positive electrode active material, such as a composite oxide in which a boron compound or a sulfur 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%, the above-mentioned resistance reduction effect can be obtained depending on the content of the positive electrode active material of this embodiment.

[0058] An example of a method for producing a positive electrode active material will be described below. The method for producing a positive electrode active material includes, for example, a step of synthesizing a lithium transition metal composite oxide, a step of washing, a step of drying, a step of crushing, and a step of adding a boron compound and a sulfur compound.

[0059] In the synthesis step of the lithium transition metal composite oxide, a metal hydroxide containing Ni and Mn in a total amount of 80 mol % or more relative to the total molar amount of metal elements excluding Li is mixed with a Li compound, and the mixture is calcined to obtain a lithium transition metal composite oxide. ２ CO ３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ Examples of the lithium ion include O, LiH, and LiF.

[0060] The metal hydroxide can be obtained, for example, by dropping an alkaline solution such as sodium hydroxide into a stirred solution of a metal salt containing Ni, Mn, and an optional element, adjusting the pH to the alkaline side (e.g., 8.5 to 12.5), and allowing precipitation (coprecipitation). Note that, instead of the metal hydroxide, a metal oxide obtained by heat-treating the metal hydroxide may be used. Since the smaller the particle size of the metal hydroxide, the easier it is for primary particles to grow and single particles to be obtained, the D50 of the metal hydroxide is preferably 7 μm or less, more preferably 5 μm or less.

[0061] The metal hydroxide and the Li compound are mixed, for example, in a molar ratio of metal elements excluding Li to Li of 1:0.98 to 1:1.05. When mixing the metal hydroxide and the Li compound, an Mg compound, an Al compound, a Ca compound, an Nb compound, a Sr compound, a Zr compound, a W compound, or the like may be added. These compounds are, for example, oxides, hydroxides, or carbonates, and may also be composite compounds containing other metal elements such as Li.

[0062] The mixture of metal hydroxide and Li compound or the like is fired, for example, in an oxygen atmosphere (under a gas flow with an oxygen concentration of 80% or more). The firing process may be multi-stage firing. An example of firing conditions is a temperature rise rate of 1.0°C / min to 5.5°C / min in the temperature range of 450°C to 680°C, and a maximum temperature of 850°C to 1100°C. The temperature rise rate from 680°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. Adjusting the firing conditions allows the production of single particles, and the particle size can be adjusted. For example, increasing the maximum temperature makes it easier to obtain single particles, and the particle size tends to be larger.

[0063] 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 performed using known methods and conditions. Note that 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 performed in a vacuum atmosphere. For example, the drying temperature is 150°C or higher and 400°C or lower, and the drying time is 0.5 hours or higher and 15 hours or lower. Note that the washing step may be omitted.

[0064] The powder composition obtained in the drying step is pulverized using a pulverizer such as a jet mill. The pulverization using a jet mill can be carried out using, for example, a PJM-80 manufactured by Nippon Pneumatic Mfg. Co., Ltd. under the following conditions. The pulverization step can also be omitted. Compressed air consumption: 0.5 Nm ３ / min Supply gas pressure: 0.53 MPa Processing capacity: 2000 g / hour

[0065] A boron compound and a sulfur compound are added to a crushed lithium-transition metal composite oxide, mixed, and then heat-treated to obtain a positive electrode active material in which the boron compound and sulfur compound are attached to the particle surfaces of the composite oxide. For example, the lithium-transition metal composite oxide, the boron compound, and the sulfur compound are dry-mixed, and then heat-treated at a temperature of 250°C to 600°C for 2 hours to 5 hours. This heat treatment causes the boron compound and sulfur compound to adhere to the particle surfaces of the composite oxide. For dry mixing, a mixer such as a planetary mixer, a rocking mill, or a high-speed mixer can be used. The heat treatment is performed in an oxygen atmosphere, but may also be performed in air.

[0066] Although a sulfur compound may be added after heat treatment with a boron compound, it is preferable to simultaneously mix the boron compound and the sulfur compound and then perform the heat treatment. In this case, the resistance reduction effect becomes more pronounced. The boron compound and the sulfur compound may be added before calcination of the lithium transition metal composite oxide (raw material). However, the surface state of the composite oxide particles obtained in this case differs from that of the composite oxide particles according to the present disclosure, making it difficult to obtain a positive electrode active material with excellent capacity retention after charge-discharge cycling.

[0067] [Negative Electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer disposed on the negative electrode core. The negative electrode core can be a foil of a metal stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film with such a 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. The negative electrode 12 can be fabricated, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core. Metallic lithium foil can also be used as the negative electrode 12. Alternatively, the negative electrode 12 may be composed only of a negative electrode core, with metallic lithium being deposited on the core surface during battery charging.

[0068] The negative electrode active material is not particularly limited as long as it reversibly absorbs and releases lithium ions, and typically, carbon materials such as graphite are used. Furthermore, elements that alloy with Li, such as Si and Sn, or materials containing such elements, may also be used as the negative electrode active material. Among these, silicon-containing materials containing Si are preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination.

[0069] The carbon material functioning 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. Among them, 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 amorphous graphite, or a mixture thereof. Examples of the silicon-containing material functioning as the negative electrode active material include silicon alloys, silicon compounds, and composite materials containing Si. A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0070] As with the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, an olefin resin, PAN, a polyimide, a polyamide, an acrylic resin, or the like. However, polyvinyl acetate, styrene-butadiene rubber (SBR), or the like may also be used. Among these, SBR is preferably used. A single binder may be used, or multiple binders may be used in combination. The negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. These function as thickeners in the negative electrode mixture slurry. The content of the binder 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 a conductive agent such as CNT.

[0071] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0072] 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 phosphate compounds. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0073] [Non-aqueous electrolyte] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0074] The electrolyte solution includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0075] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0076] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane ethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0077] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ Among these, LiPF is preferred from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ The concentration of the lithium salt may be, for example, 4 mol or less, or 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less, per 1 L of the non-aqueous solvent.

[0078] The non-aqueous electrolyte may contain an additive such as an unsaturated carbonate ester, an acid anhydride, a phenol compound, a benzene compound, a nitrile compound, an isocyanate compound, a sultone compound, a sulfate compound, a borate ester compound, a phosphate ester compound, or a phosphite ester compound.

[0079] Examples of unsaturated cyclic carbonates 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. One type of unsaturated cyclic carbonate may be used alone, or two or more types may be used in combination. Some of the hydrogen atoms in the unsaturated cyclic carbonate may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by intermolecular condensation of multiple carboxylic acid molecules, but is preferably an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0080] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0081] 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 bisisocyanatomethylcyclohexane (BIMCH). Examples of sultone compounds include propane sultone and propene sultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethyl borate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethyl phosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethyl phosphite and tris(trimethylsilyl)phosphite.

[0082] 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, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, 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 the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0083] The present disclosure will be further explained below with reference to experimental examples, but the present disclosure is not limited to these experimental examples.

[0084] Experimental Example A1 [Preparation of Positive Electrode Active Material] LiOH and Ni obtained by coprecipitation method ０．８ Mn ０．２ (OH) ２The powders were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1.05:1 to obtain a mixture. This mixture was fired in two stages under an oxygen stream with an oxygen concentration of 90% or higher (a 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 from 650°C to 880°C over 2 hours. The mixture was then held at 880°C for 3 hours to obtain a lithium transition metal composite oxide.

[0085] The resulting lithium-transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Sodium ethyl sulfate was added to the crushed lithium-transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere (flow rate: 4 L / min) to obtain a positive electrode active material in which sodium ethyl sulfate was adhered to the particle surfaces of the lithium-transition metal composite oxide. The amount of sodium ethyl sulfate added was adjusted so that the content of sodium ethyl sulfate was 0.7 mol% relative to the total molar amount of metal elements in the composite oxide excluding Li.

[0086] Observation of the positive electrode active material using an SEM confirmed that most of the particles of the lithium transition metal composite oxide were single particles consisting of a single primary particle. When the D50 and crystallite size of the lithium transition metal composite oxide (positive electrode active material) were measured using the above method, the D50 was 1.1 μm, the crystallite size was 459 Å, and the BET specific surface area was 1.9 m. ２ / g. The crystalline structure of the lithium transition metal composite oxide is a layered rock salt structure belonging to the space group R-3m. It was also confirmed that sodium ethyl sulfate particles were scattered on the particle surface of the composite oxide. In the XPS spectrum of the positive electrode active material, peaks derived from S and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

[0087] [Preparation of Test Cell] The test cell shown in FIG. 2 was prepared by the following procedure. The working electrode 30 was a positive electrode using the above-described positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 80:10:10, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone as a dispersion medium. This slurry was applied to an aluminum foil positive electrode core, and the coating was vacuum-dried at 110°C to obtain the working electrode 30.

[0088] An electrode group was prepared in dry air with a dew point of −50° C. or lower, with separators 34 interposed between the working electrode 30, counter electrode 31 (negative electrode), and reference electrode 32, each of which had an electrode lead 38 attached thereto, and housed in an exterior case 35. Thereafter, an electrolyte solution 36 was poured into the exterior case 35, and the exterior case 35 was sealed to obtain a test cell.

[0089] 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 Non-aqueous electrolyte: a non-aqueous solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 (25°C), to which LiPF 6 was added as an electrolyte salt. ６ was dissolved to a concentration of 1.0 mol / L.

[0090] The test cell of Experimental Example A1 was subjected to the following performance evaluation, and the evaluation results, along with the composition and amount of additive, are shown in Table 1-1.

[0091] [Evaluation of charge-discharge efficiency and capacity retention] In a temperature environment of 25°C, the test cell was charged to 4.5 V (vs. Li metal) at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.5 V. It was then discharged to 2.5 V at a constant current of 0.1 C. The charge capacity and discharge capacity at this time were measured, and the discharge capacity was divided by the charge capacity to calculate the charge-discharge efficiency. From the second cycle onwards, the discharge current was changed to 0.2 C, and this charge-discharge cycle was repeated 30 times, and the capacity retention was calculated by dividing the discharge capacity at the 30th cycle by the discharge capacity at the first cycle.

[0092] [Evaluation of IV Resistance] For the test cell after the above-mentioned initial charge / discharge and after 30 charge / discharge cycles, the IV resistance (internal resistance) was measured at SOCs of 10%, 50%, and 100% (4.45 V). The test cell was placed in each SOC state, then rested for 15 minutes, and the voltage at that state was defined as V0. The amount of voltage drop upon 10-second discharge was defined as ΔV. The IV resistance was calculated from the straight line obtained from each current value and each ΔV.

[0093] Experimental Example A2: A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that sodium methyl sulfate was used instead of sodium ethyl sulfate, the amount added was adjusted to 0.5 mol %, and the crushing step of the composite oxide was omitted. The evaluation results, along with the composition and amount of additive added, are shown in Table 1-1 (the same applies to Experimental Examples B1 to B23). The D50 of the positive electrode active material was 4.9 μm, and the BET specific surface area was 1.1 m. ２ / g. It was confirmed that sodium methyl sulfate particles (average particle size: 0.09 μm) were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks due to S and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

[0094] Experimental Examples B1 to B23: Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A2, except that the compounds shown in Table 1-1 were used instead of sodium methyl sulfate. The presence or absence of a crushing step for the composite oxide is as shown in Table 1-1.

[0095] <Experimental Examples B24 to B41> Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that instead of sodium ethyl sulfate, compounds shown in Table 1-2 were used and the amounts added were adjusted to those shown in Table 1-2, and the performance evaluations were carried out as described above. The evaluation results are shown in Table 1-2. The presence or absence of a crushing step for the composite oxide is as shown in Table 1-2.

[0096] Experimental Example B42 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0097]

[0098]

[0099] The results shown in Tables 1-1 and 1-2 indicate that the test cells of Experimental Examples B1 to B42 show a large increase in the positive electrode resistance as the number of cycles increases, whereas the test cells of Experimental Examples A1 and A2 effectively suppress the increase in resistance. Furthermore, the test cells of Experimental Examples A1 and A2 have a higher capacity retention rate after charge / discharge cycling and superior cycle characteristics compared to the test cells of Experimental Examples B1 to B42. Even when sodium ethyl sulfate is used, the resistance-reducing effect cannot be obtained if the amount added is less than 0.1 mol% (see Experimental Example B30).

[0100] Experimental Example C1: A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that in the sodium ethyl sulfate addition step, sodium ethyl sulfate was mixed to a content of 0.5 mol % relative to the total molar amount of metal elements excluding Li in the composite oxide, and heat treatment was performed for 3 hours at 500°C under an oxygen atmosphere, and the composite oxide crushing step was omitted. The evaluation results, along with the composition and amount of additive added, are shown in Table 2 (similar for Experimental Examples D1 to D15). It was confirmed that sodium ethyl sulfate particles were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks attributable to S and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface.

[0101] Experimental Example D1 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example C1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0102] Experimental Examples D2 to D15 Positive electrode active materials and test cells were prepared and the performance evaluations were carried out in the same manner as in Experimental Example C1, except that the compounds shown in Table 2 were used instead of sodium ethyl sulfate. The amount of each compound added was 0.5 mol % relative to the total molar amount of metal elements in the composite oxide excluding Li.

[0103]

[0104] From the results shown in Table 2, it can be seen that the test cell of Experimental Example C1 had a lower positive electrode resistance than the test cells of Experimental Examples D1 to D15. Furthermore, from these results, it can be seen that the resistance reduction effect can be obtained regardless of whether the heat treatment temperature in the sulfur compound addition step is 300°C or 500°C. The difference is particularly noticeable in the high SOC region (Spectris).

[0105] Experimental Examples E1 to E3: In the sodium ethyl sulfate addition step, sodium ethyl sulfate was mixed in amounts of 0.3 mol %, 0.5 mol %, and 1.0 mol %, respectively, relative to the total molar amount of metal elements excluding Li in the composite oxide. Heat treatment was performed in air at 300°C for 3 hours, and the composite oxide crushing step was omitted. Cathode active materials and test cells were prepared in the same manner as in Experimental Example A1, and the performance evaluations were carried out. The evaluation results are shown in Table 3. It was confirmed that sodium ethyl sulfate particles were scattered on the surface of the composite oxide particles in all of the cathode active materials. Furthermore, peaks attributable to S and Na were confirmed in the XPS spectrum of the cathode active materials at the surface of the composite oxide particles and in a region shallower than 100 nm from the surface.

[0106] Experimental Examples E4 to E6: Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that boric acid was added together with sodium ethyl sulfate, and the performance evaluations were carried out as described above. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 3, along with the evaluation results.

[0107] In all of the positive electrode active materials of Experimental Examples E4 to E6, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks attributable to B, S, and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV, on the particle surface of the composite oxide and in a region shallower than 50 nm from the surface.

[0108] Experimental Example EE1 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example E1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0109]

[0110] The results shown in Table 3 demonstrate that an excellent effect of suppressing resistance increase can be achieved even when the mixture is heat-treated in the air during the sodium ethyl sulfate addition step. In particular, the effect of suppressing resistance increase was significant when the positive electrode active materials of Experimental Examples E4 to E6 were used. The oxygen concentration during the heat treatment step is preferably 5% or higher, more preferably 10% or higher, and particularly preferably 18% or higher.

[0111] Experimental Examples F1 to F7: Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that sodium ethyl sulfate was added in the amounts shown in Table 4, and the water-washed composite oxide was dried, and then the crushing step using a jet mill was omitted. The D50 of the lithium transition metal composite oxide was 4.9 μm.

[0112] 4B is an XPS spectrum of the positive electrode active material of Experimental Example F4, showing the binding energy range in which a peak derived from S appears. As shown in Fig. 4B, a peak derived from S was observed in the binding energy range of 168 eV to 172 eV on the particle surface of the lithium transition metal composite oxide and in a region 20 nm deep from the surface (particularly noticeable at 10 nm).

[0113] 7B is an XPS spectrum of the positive electrode active material of Experimental Example F4, showing the binding energy range in which a peak derived from Na appears. As shown in Fig. 7B, a peak derived from Na was observed in the binding energy range of 1070 eV to 1073 eV on the particle surface of the lithium transition metal composite oxide and in a region 100 nm deep from the surface.

[0114] Figure 5C (a) shows the XPS spectrum of the cathode active material of Experimental Example F4, showing the binding energy range in which a peak derived from Ni appears. Figure 5C (b) shows the XPS spectrum of the cathode active material of Experimental Example G8 (described later) containing no S. Figure 6C (a) shows the XPS spectrum of the cathode active material of Experimental Example F4, showing the binding energy range in which a peak derived from Mn appears. Figure 6C (b) shows the XPS spectrum of the cathode active material of Experimental Example G8 (described later) for comparison.

[0115] 5C and 6C, the XPS spectrum of the positive electrode active material of Experimental Example F4 shows a shift of the peaks derived from Ni and Mn to higher energy (0.4 eV for Ni, 0.2 eV for Mn) near the particle surface of the composite oxide when compared with the spectrum of the positive electrode active material of Experimental Example G8, which does not contain S. The depth at which this peak shift is observed roughly coincides with the depth (10 nm) at which the peak derived from S shown in FIG. 4B clearly appears, indicating that S is dissolved in the particle surface of the composite oxide and in a depth range of at least 10 nm therearound, forming a surface layer in which the bonding state of Ni and Mn is changed.

[0116] Furthermore, in the XPS spectrum of the positive electrode active material prepared in Experimental Example F2, clear peaks due to S were confirmed in the binding energy range of 168 eV to 172 eV at the surface of the composite oxide particles and in the region 70 nm deep from the surface. As in Experimental Example F4, S was dissolved in the composite oxide at a depth of 70 nm from the surface where S was present, forming a surface layer in which the bonding state of Ni and Mn was changed. Furthermore, an XPS peak due to Na was confirmed even at 100 nm.

[0117] Figure 9 is a backscattered electron image of the positive electrode active material prepared in Experimental Example F2. As shown in Figure 9, the small particles present on the particle surface of the lithium transition metal composite oxide are sodium ethyl sulfate. The sodium ethyl sulfate particles are not concentrated in a part of the particle surface of the composite oxide, but are scattered evenly over a wide area of ​​the particle surface. The average particle size of the sodium ethyl sulfate was approximately 0.02 μm. Figure 10 is a backscattered electron image of the positive electrode active material prepared in Experimental Example G8, and no granular deposits are observed on the particle surface.

[0118] Experimental Examples F8 to F21: Cathode active materials and test cells were prepared in the same manner as Experimental Example F1, except that boric acid was added along with sodium ethyl sulfate. The performance evaluations were then conducted. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 4, along with the evaluation results. In Experimental Examples 11 to 13, boron was added to an uncrushed composite oxide, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere. Then, sodium ethyl sulfate was added, and heat treatment at 300°C for 3 hours in an oxygen atmosphere was conducted to prepare cathode active materials. Furthermore, in the XPS spectrum of the cathode active material, peaks attributable to B, S, and Na were observed at the surface of the composite oxide particles and in a region 100 nm deep from the surface. Furthermore, at the surface of the composite oxide particles and in a region less than 50 nm deep from the surface, a peak attributable to Ni was observed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was observed in the binding energy range of 645 eV to 649 eV.

[0119] 3A, 4A, 5A, 6A, and 7A are XPS spectra of the positive electrode active material of Experimental Example F9, showing the binding energy ranges in which a peak derived from B appears, the binding energy range in which a peak derived from S appears, the binding energy range in which a peak derived from Ni appears, the binding energy range in which a peak derived from Mn appears, and the binding energy range in which a peak derived from Na appears. In the XPS spectrum, new peaks derived from B, S, and Na were observed at the surface of the composite oxide particles and in a region shallower than 100 nm from the surface, at 194.5 eV to 197 eV, 172 eV to 174 eV, and 1074 eV to 1077 eV, respectively. Furthermore, new peaks derived from Ni were observed at the surface of the composite oxide particles and in a region 50 nm deep from the surface, at binding energy ranges of 858 eV to 862 eV, and 645 eV to 649 eV, respectively, resulting in roughly two peaks being observed. Furthermore, by incorporating both B and S, the low-energy peak weakens closer to the particle surface, and the new high-energy peak becomes stronger. Therefore, new bonds consisting of B and S are formed on the particle surface, and their presence suppresses structural destruction and improves resistance and capacity retention. B and S are bonded via oxygen, shifting the energy toward the high-energy side.

[0120] In the XPS spectra (see FIGS. 5B to 5D and 6B to 6D) of Experimental Examples G3, F4, and G8 (positive electrode active materials that do not contain B, S, or both), a peak derived from Ni can be seen in the binding energy range of 853 eV to 855.5 eV, a peak derived from Mn can be seen in the binding energy range of 640 eV to 643 eV, a peak derived from S can be seen in the binding energy range of 167 eV to 170 eV, a peak derived from B can be seen in the binding energy range of 190 eV to 193 eV, and a peak derived from Na can be seen in the binding energy range of 1071 eV to 1072 eV.

[0121] When the spectra of Experimental Example F4 and Experimental Example G8, which did not contain S, were compared, a shift to a higher energy side of the peaks derived from Ni and Mn (0.4 eV for Ni, 0.2 eV for Mn) was observed near the particle surface of the composite oxide. The depth at which this peak shift was observed roughly coincided with the depth (10 nm) at which the peak derived from S clearly appeared. This indicates that S was dissolved at the particle surface of the composite oxide and in a depth range of at least 10 nm nearby, forming a surface layer in which the bonding state of Ni and Mn was changed.

[0122] On the other hand, in the XPS spectrum of the positive electrode active material of Experimental Example F9, in addition to the peaks attributable to Ni and Mn mentioned above (a Ni-derived peak in the binding energy range of 853 eV to 855.5 eV and a Mn-derived peak in the binding energy range of 640 eV to 643 eV), new Ni-derived peaks appear in the binding energy range of 858 eV to 862 eV and a Mn-derived peak in the binding energy range of 645 eV to 649 eV, each of which exhibits at least two Ni and Mn bonding states. On the surface of the composite oxide particles, the newly emerged peaks are observed in a range shallower than 50 nm from the surface (clear at 30 nm). The depth at which this peak shift is confirmed is approximately the same as the depth (50 nm) at which the new boron peak shown in Figure 3A appears, indicating that B, S, and Na are bonded via oxygen at the surface of the composite oxide particles and at least 50 nm deep in the vicinity. Therefore, in the area shallower than 50 nm from the particle surface of the composite oxide, a new layer consisting of at least three of S, B, Ni, Mn, and Na is formed or solid-solved. The thickness of this layer is at least 5 nm or more, preferably 10 nm or more, and more preferably 30 nm or more. This composite oxide having a particle surface where both B and S are present is highly effective in suppressing electrolyte decomposition during high-potential charging, suppressing resistance increase, and maintaining a high capacity retention rate.

[0123] 10 is a backscattered electron image of the positive electrode active material of Experimental Example F9. From the backscattered electron image of the positive electrode active material, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the surface of the composite oxide particles. The average particle size of the sodium ethyl sulfate was approximately 0.05 μm.

[0124] In addition, in all of the positive electrode active materials of Experimental Examples F8, F10 to F21, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the particle surface of the composite oxide. Furthermore, in the XPS spectrum of the positive electrode active material, peaks derived from B, S, and Na were confirmed on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface. Furthermore, in the particle surface of the composite oxide and in a region less than 50 nm from the surface, a peak derived from Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak derived from Mn was confirmed in the binding energy range of 645 eV to 649 eV.

[0125] Experimental Example G1: A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example F5, except that sodium ethyl sulfate was added before calcining the lithium transition metal composite oxide and the water washing step was omitted, and the performance evaluation was carried out. Because the surface condition of the composite oxide particles differed from that of the composite oxide particles according to the present disclosure, it was difficult to obtain a positive electrode active material with excellent capacity retention after charge / discharge cycling. Note that, if water washing was not performed, the resistance value would be small, so evaluation of IV resistance was not performed.

[0126] 3B, 5B, and 6B show the XPS spectra of the positive electrode active material of Experimental Example G3, respectively, showing the binding energy ranges in which the peaks attributable to B, Ni, and Mn appear.

[0127] Experimental Examples G4 to G7 Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example F1, except that instead of sodium ethyl sulfate, compounds shown in Table 4 were used and the amounts added were adjusted to the amounts shown in Table 4. The evaluation results are shown in Table 4.

[0128] 3C, 4C, 5D, and 6D show the XPS spectra of the positive electrode active material of Experimental Example G3, respectively, showing the binding energy ranges in which the peaks derived from B, Ni, and Mn appear.

[0129]

[0130] From the results shown in Table 4, it can be seen that the test cells of Experimental Examples F1 to F21, and particularly the test cells of Experimental Examples F8 to F21, have lower positive electrode resistance compared to the test cells of Experimental Examples G1 to G8. This difference is particularly pronounced in the high SOC range. Even when sodium ethyl sulfate is used, adding it during the calcination of the lithium transition metal composite oxide does not provide a resistance-reducing effect. Furthermore, adding boron alone does not provide a resistance-reducing effect. Furthermore, when sodium dodecyl sulfate is used with boron, the capacity is lower and the resistance-reducing effect is also smaller than when no additive is added (Experimental Example G8). Sodium dodecyl sulfate has a long carbon number of 12, which results in carbon coating the particle surface of the composite oxide, inhibiting the bond between boron and sulfur and preventing the synergistic effect. Therefore, it is desirable to use a sulfate ester with a carbon number of 5 or less.

[0131] <Experimental Examples H1 to H8> LiOH and Ni obtained by coprecipitation method ０．９１ Co ０．０４ Al ０．０５ (OH) ２ The powders were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1 to obtain a mixture. This mixture was then fired in two stages under an oxygen stream with an oxygen concentration of 90% or higher (a 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 from 670°C to 720°C over 50 minutes. The mixture was then held at 720°C for 3 hours to obtain a lithium transition metal composite oxide. The obtained composite oxide particles have a secondary particle shape formed by the aggregation of numerous primary particles.

[0132] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dried. The compounds shown in Table 5 were added to and mixed with the dried lithium transition metal composite oxide, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere to obtain a positive electrode active material in which the compounds shown in Table 5 were attached to the particle surfaces of the lithium transition metal composite oxide. The D50 of the positive electrode active material was 11 μm. Except for using this positive electrode active material, a positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, and the performance evaluation was carried out. The performance evaluation of the test cell was performed by charging to 4.4 V (vs. Li metal) at the temperature environment (25°C or 45°C) shown in Table 5. The IV resistance was evaluated with a cell voltage of 4.4 V defined as 100% SOC.

[0133]

[0134] From the results shown in Table 5, it can be seen that adding sodium ethyl sulfate to a lithium transition metal composite oxide in the form of secondary particles that does not contain Mn does not provide a resistance-reducing effect, and the discharge capacity decreases and the resistance increases.

[0135] Experimental Examples J1 to J25: Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that the jet mill crushing step was omitted after drying the washed composite oxide, and that boric acid was added and mixed with sodium ethyl sulfate, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere (flow rate: 3 L / min). The amounts of boric acid and sodium ethyl sulfate added are shown in Table 6, along with the evaluation results.

[0136] In all of the positive electrode active materials of Experimental Examples J1 to J25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the particle surface of the composite oxide. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV on the particle surface of the composite oxide and in a region shallower than 50 nm from the surface.

[0137] Experimental Examples K1 to K3: Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example A1, except that boric acid was added instead of sodium ethyl sulfate, the jet mill crushing step was omitted, and heat treatment was performed at 300°C for 3 hours in an oxygen atmosphere (flow rate: 3 L / min). The amount of boric acid added is shown in Table 6, along with the evaluation results.

[0138] Experimental Example K4 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example A1, except that the step of adding sodium ethyl sulfate, the step of crushing using a jet mill, and heat treatment was performed at 300°C for 3 hours in an oxygen atmosphere (flow rate: 3 L / min) were omitted, and the performance evaluation was carried out as described above.

[0139]

[0140] The results shown in Table 6 indicate that the test cells of Experimental Examples J1 to J25 have lower positive electrode resistance than the test cells of Experimental Examples K1 to K3. This difference is particularly noticeable after charge-discharge cycling. The test cells of Experimental Examples J1 to J25 also have excellent charge-discharge cycle characteristics.

[0141] <Experimental Example L1> [Preparation of Positive Electrode Active Material] LiOH and Ni obtained by coprecipitation method ０．９ Co ０．０５ Mn ０．０５ (OH) ２ powder and Zr(OH) ４The above was mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.05:1:0.003 to obtain a mixture. This mixture was then fired in two stages under an oxygen stream with an oxygen concentration of 90% or higher (a 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 from 650°C to 810°C over 2 hours. The mixture was then held at 810°C for 3 hours to obtain a lithium transition metal composite oxide.

[0142] Observation of the obtained lithium transition metal composite oxide using an SEM confirmed that most of the particles of the composite oxide were single particles consisting of a single primary particle. When the D50 and crystallite size of the lithium transition metal composite oxide were measured by the above-mentioned method, the D50 was 1.8 μm and the BET specific surface area was 1.1 m. ２ The crystalline structure of the lithium transition metal composite oxide was a layered rock salt structure belonging to the space group R-3m.

[0143] The resulting lithium-transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Boric acid and sodium ethyl sulfate were added to the crushed lithium-transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere to obtain a positive electrode active material in which boric acid and sodium ethyl sulfate were attached to the particle surfaces of the lithium-transition metal composite oxide. The amounts of boric acid and sodium ethyl sulfate added were adjusted to 0.50 mol% and 0.10 mol%, respectively, relative to the total molar amount of metal elements in the composite oxide excluding Li.

[0144] A test cell was prepared in the same manner as in Experimental Example A1 except that the above positive electrode active material was used, and the performance evaluation was carried out as described above.

[0145] Experimental Examples L2 to L25 Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example L1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to the amounts shown in Table 7. The performance of the test cells was evaluated by charging up to 4.4 V (vs. Li metal). Note that the state where the cell voltage was 4.4 V was defined as 100% SOC, and the IV resistance was evaluated.

[0146] In all of the positive electrode active materials of Experimental Examples L1 to L25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the particle surface of the composite oxide. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the particle surface of the composite oxide and in a region shallower than 100 nm from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV on the particle surface of the composite oxide and in a region shallower than 50 nm from the surface.

[0147] Experimental Example M1 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example L1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0148]

[0149] The results shown in Table 7 indicate that the test cells of Experimental Examples L1 to L25 have lower positive electrode resistance than the test cell of Experimental Example M1. Furthermore, the test cells of Experimental Examples L1 to L25 also have excellent charge-discharge cycle characteristics. It was also confirmed that the effect was observed even when the Zr content was 0.05 mol% or less.

[0150] <Experimental Example N1> [Preparation of Positive Electrode Active Material] LiOH and Ni obtained by coprecipitation method ０．７５ Co ０．０５ Mn ０．２０ (OH) ２ The powders were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Mn was 1.05:1 to obtain a mixture. This mixture was fired in two stages under an oxygen stream with an oxygen concentration of 90% or higher (a 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 from 650°C to 900°C over 2 hours. The mixture was then held at 900°C for 3 hours to obtain a lithium transition metal composite oxide.

[0151] Observation of the obtained lithium transition metal composite oxide using an SEM confirmed that most of the particles of the composite oxide were single particles consisting of a single primary particle. When the D50 and crystallite size of the lithium transition metal composite oxide were measured by the above method, the D50 was 1.6 μm and the BET specific surface area was 1.5 m. ２ The crystalline structure of the lithium transition metal composite oxide was a layered rock salt structure belonging to the space group R-3m.

[0152] The resulting lithium-transition metal composite oxide was washed with water to remove excess lithium, dried, and then crushed using a jet mill. Boric acid and sodium ethyl sulfate were added to the crushed lithium-transition metal composite oxide and mixed, followed by heat treatment at 300°C for 3 hours in an oxygen atmosphere to obtain a positive electrode active material in which boric acid and sodium ethyl sulfate were attached to the particle surfaces of the lithium-transition metal composite oxide. The amounts of boric acid and sodium ethyl sulfate added were adjusted to 0.50 mol% and 0.10 mol%, respectively, relative to the total molar amount of metal elements in the composite oxide excluding Li.

[0153] A test cell was prepared in the same manner as in Experimental Example A1 except that the above positive electrode active material was used, and the performance evaluation was carried out as described above.

[0154] <Experimental Examples N2 to N25> Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example N1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to the amounts shown in Table 8, and the performance evaluations were carried out as described above.

[0155] In all of the positive electrode active materials of Experimental Examples N1 to N25, it was confirmed that particles of boric acid and sodium ethyl sulfate were scattered on the particle surface of the composite oxide. Furthermore, XPS measurements confirmed peaks attributable to B, S, and Na on the particle surface of the composite oxide and in a region 100 nm deep from the surface. Furthermore, a peak attributable to Ni was confirmed in the binding energy range of 858 eV to 862 eV, and a peak attributable to Mn was confirmed in the binding energy range of 645 eV to 649 eV on the particle surface of the composite oxide and in a region less than 50 nm deep from the surface.

[0156] Experimental Example P1 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example N1, except that the step of adding sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0157]

[0158] From the results shown in Table 8, it can be seen that the test cells of Experimental Examples N1 to N25 have lower positive electrode resistance than the test cell of Experimental Example N25. Furthermore, the test cells of Experimental Examples N1 to N25 also have excellent charge-discharge cycle characteristics. Therefore, it is estimated that even a Ni content of 50% of the total transition metal content is effective.

[0159] Experimental Examples R1 to R8: Positive electrode active materials and test cells were prepared and the performance evaluations were carried out in the same manner as in Experimental Example A1, except that boric acid was added together with sodium ethyl sulfate, the performance evaluations were carried out in a 45°C environment, and the crushing step using a jet mill was omitted. The amounts of boric acid and sodium ethyl sulfate added are shown in Table 9, along with the evaluation results.

[0160] Experimental Examples S1 to S8 Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example R1, except that the additives shown in Table 9 were used instead of boric acid and sodium ethyl sulfate, and the amounts added were adjusted to the amounts shown in Table 9, and the performance evaluations were carried out as described above.

[0161] Experimental Example S9 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example R1, except that the step of adding boric acid and sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0162]

[0163] From the results shown in Table 9, it can be seen that the test cells of Experimental Examples R1 to R8 have lower positive electrode resistance than the test cell of Experimental Example S9. The test cells of Experimental Examples R1 to R8 also have lower positive electrode resistance than the test cells of Experimental Examples S1 to S8. Furthermore, the test cells of Experimental Examples R1 to R8 also have excellent charge-discharge cycle characteristics.

[0164] <Experimental Examples T1 to T8> Positive electrode active materials and test cells were prepared in the same manner as in Experimental Example L1, except that the amounts of boric acid and sodium ethyl sulfate added were adjusted to those shown in Table 7, and the performance evaluation was performed in a temperature environment of 45°C.

[0165] Experimental Example U1 A positive electrode active material and a test cell were prepared in the same manner as in Experimental Example T1, except that the step of adding boric acid and sodium ethyl sulfate was omitted, and the performance evaluation was carried out as described above.

[0166]

[0167] From the results shown in Table 10, it can be seen that the test cells of Experimental Examples T1 to T8 have lower positive electrode resistance than the test cell of Experimental Example U1. Furthermore, the test cells of Experimental Examples T1 to T8 also have excellent charge-discharge cycle characteristics.

[0168] The present disclosure is further described by the following embodiments. Aspect 1: A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide having a combined content of Ni and Mn of 80 mol % or more relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide having a single particle shape, a volume-based median diameter of 0.5 μm to 5.0 μm, and a crystallite size of 370 Å to 1500 Å, a boron compound and a sulfur compound present on the particle surfaces of the lithium transition metal composite oxide, and the content of the boron compound is 0.005 mol % to 2.0 mol % and the content of the sulfur compound is 0.005 mol % to 2.0 mol % relative to the total molar amount of metal elements excluding Li constituting the lithium transition metal composite oxide. Aspect 2: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein the lithium transition metal composite oxide further comprises at least one element selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W. Aspect 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1 or Aspect 2, wherein at least one of Na and K is present on the particle surfaces of the lithium transition metal composite oxide. Aspect 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 3, wherein the sulfur compound is a sulfur oxide. Aspect 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 4, wherein the sulfur compound contains a functional group represented by formula (1). wherein R is an alkyl group having 5 or less carbon atoms. Aspect 6: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 5, wherein the sulfur compound is at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate. Aspect 7: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 6, wherein the sulfur compound is present in the form of particles on the surface of the lithium transition metal composite oxide particles, and the average particle size of the sulfur compound present in the form of particles is 0.005 μm or more and 0.5 μm or less. Aspect 8: A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to any one of Aspects 1 to 7; a negative electrode; and a non-aqueous electrolyte.

[0169] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Working electrode, 31 Counter electrode, 32 Reference electrode, 34 Separator, 35 Outer can, 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 total content of Ni and Mn is 80 mol % or more relative to the total molar amount of metal elements excluding Li, the lithium transition metal composite oxide having 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, a boron compound and a sulfur compound being present on the particle surfaces of the lithium transition metal composite oxide, and the content of the boron compound is 0.005 mol % or more and 2.0 mol % or less, and the content of the sulfur compound is 0.005 mol % or more and 2.0 mol % or less, relative to the total molar amount of metal elements excluding Li constituting the lithium transition metal composite oxide.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains at least one element selected from the group consisting of Mg, Al, Ca, Nb, Sr, Zr, and W.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein at least one of Na and K is present on the particle surfaces of said lithium transition metal composite oxide.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is a sulfur oxide.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound contains a functional group represented by formula (1). In the formula, R is an alkyl group having 5 or less carbon atoms.

6. 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 lithium methyl sulfate, lithium ethyl sulfate, lithium propyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, sodium propyl sulfate, potassium methyl sulfate, potassium ethyl sulfate, and potassium propyl sulfate.

7. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the sulfur compound is present in the form of particles on the surfaces of the lithium transition metal composite oxide particles, and the average particle size of the sulfur compound present in the form of particles is 0.005 μm or more and 0.5 μm or less.

8. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to any one of claims 1 to 7; a negative electrode; and a non-aqueous electrolyte.

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