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

Abstract

This positive electrode active material is a composite oxide having a crystal structure belonging to a space group R-3m and represented by a composition formula of Li1+aNibMncXdOe, in which: X is at least one type selected from the group consisting of typical elements and transition metal elements other than Li, Ni, and Mn; a≤1.15, 0.35≤b≤0.70, 0.30≤c≤0.65, and 0≤d≤0.07 are satisfied; and e is a value satisfying electrical neutrality. The positive electrode active material has a value (α×β) of 0.090 or lower, which is the product of a Ni mixing rate α determined through Rietveld analysis and a strain β determined using the Williamson-Hall method.

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 active material has a significant effect on battery performance, such as input / output characteristics, capacity, and durability, and therefore has been the subject of extensive research. Lithium transition metal composite oxides containing transition metal elements such as Ni and Mn are generally used as positive electrode active materials. The types and amounts of elements contained in the lithium transition metal composite oxide, as well as the crystalline structure of the composite oxide, significantly affect battery performance, and even slight changes in these physical properties can prevent the desired performance from being achieved.

[0003] For example, Patent Documents 1 to 3 disclose that, in order to improve battery performance such as charge / discharge cycle characteristics, attention is focused on lattice distortion in the crystal structure of the positive electrode active material, and the distortion is controlled within a specific range.

[0004] JP 2004-253169 A JP 2013-091581 A JP 2016-188168 A

[0005] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used as power sources for driving vehicles, and there is a demand for even higher capacity. The positive electrode active materials of Patent Documents 1 to 3 still have a large room for improvement in terms of increasing capacity.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure is a positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, and having the composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅIn the formula, X is at least one element selected from the group consisting of transition metal elements and typical elements other than Li, Ni, and Mn, a≦1.15, 0.35≦b≦0.70, 0.30≦c≦0.65, 0≦d≦0.07, and e are values ​​that satisfy electrical neutrality, and the value (α×β) obtained by multiplying the Ni mixing rate α determined by Rietveld analysis by the strain β determined by the Williamson-Hall method is 0.090 or less.

[0007] 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.

[0008] The positive electrode active material according to the present disclosure can achieve a high capacity non-aqueous electrolyte secondary battery.

[0009] 1 is a longitudinal cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention;

[0010] As a result of extensive research aimed at increasing the capacity of non-aqueous electrolyte secondary batteries, the present inventors have discovered that in a positive electrode active material having a crystal structure belonging to the space group R-3m, the charge / discharge capacity of the battery is significantly increased when the value (α×β) obtained by multiplying the mixing ratio α, which represents the proportion of Ni occupying the Li site of the crystal structure, by the strain β, which indicates the non-uniformity of the spacing of the crystal lattice planes, is 0.090 or less. The present inventors have found that in a layered rock salt structure belonging to the space group R-3m, the mixing ratio α and the strain β have a significant effect on the discharge capacity, and that controlling the value of α×β to 0.090 or less results in a specific improvement in the discharge capacity.

[0011] In particular, when the Ni content is 70 mol% or less of the total number of moles of metal elements excluding Li, the influence of the mixing ratio α and the strain β on the discharge capacity becomes significant, and the effect of controlling the value of α×β to 0.090 or less becomes significant. From the viewpoint of reducing battery material costs, etc., it is required to reduce the Ni content of the positive electrode active material, but if the Ni content is reduced, it becomes difficult to increase the capacity of the battery. The positive electrode active material according to the present disclosure is extremely useful in achieving both low cost and high capacity of the battery. Note that the value of α×β varies greatly depending on the synthesis conditions of the positive electrode active material, so adjusting the value of α×β to the desired value cannot be achieved unless the synthesis conditions are strictly controlled with attention to this value.

[0012] 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.

[0013] In the embodiment described below, 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. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] 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.

[0015] The non-aqueous electrolyte has lithium ion conductivity and may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0016] The liquid electrolyte (electrolytic solution) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0017] 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 a 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.

[0018] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length and width directions. 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, with the positive electrode 11 being particularly described in detail below.

[0023] [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.

[0024] 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, for example, 0.1% by mass or more and 5% by mass or less with respect to the mass of the positive electrode mixture layer.

[0025] 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 binder content 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.

[0026] The positive electrode active material has a crystal structure belonging to the space group R-3m and has the composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In this composition formula, X is at least one element selected from the group consisting of transition metal elements and typical elements other than Li, Ni, and Mn, and a≦1.15, 0.35≦b≦0.70, 0.30≦c≦0.65, 0≦d≦0.07, and e are values ​​that satisfy electrical neutrality. The composite oxide constituting the positive electrode active material contains Li, Ni, and Mn as essential elements. The composition of the positive electrode active material can be measured using an ICP optical emission spectrometer (e.g., iCAP6300 manufactured by Thermo Fisher Scientific).

[0027] The positive electrode active material has a layered rock salt structure belonging to the space group R-3m, has a composition that satisfies the above composition formula, and is characterized in that the value (α×β) obtained by multiplying the mixing ratio α, which represents the proportion of Ni occupying the Li site of the crystal structure as determined by Rietveld analysis, and the strain β, which represents the non-uniformity of the spacing of crystal lattice planes as determined by the Williamson-Hall method, is 0.090 or less. When the value of α×β is 0.090 or less, charge / discharge capacity is significantly improved.

[0028] Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In the formula, the molar ratio (b) of Ni is 0.35 or more and 0.70 or less (0.35≦b≦0.70). The molar ratio (c) of Mn is 0.30 or more and 0.65 or less (0.30≦c≦0.65). In this case, both low cost and high capacity of the battery can be achieved, and the effect of improving discharge capacity can be obtained by controlling the value of α×β to 0.090 or less. The Ni content is preferably 35 mol % or more and 70 mol % or less of the total number of moles of metal elements excluding Li, and is equal to or greater than the Mn content.

[0029] From the viewpoint of further increasing capacity, the Ni molar ratio (b) is preferably 0.40 or more, more preferably 0.45 or more, and particularly preferably 0.50 or more. Furthermore, from the viewpoint of reducing material costs, the Ni molar ratio (b) is preferably 0.65 or less, more preferably 0.60 or less. An example of a suitable range for the Ni molar ratio (b) is 0.45≦b≦0.65 or 0.50≦b≦0.60. In this case, both low cost and high capacity can be achieved to a higher degree.

[0030] The molar ratio (c) of Mn is preferably 0.35 or more, more preferably 0.40 or more. The molar ratio (c) of Mn is preferably 0.60 or less. An example of a suitable range for the molar ratio (c) of Mn is 0.35≦b≦0.60 or 0.40≦b≦0.60. In this case, both low cost and high capacity can be achieved to a higher degree.

[0031] Composition formula Li １＋ａ Ni ｂ Mn ｃ Xｄ O ｅ In the formula, X is, for example, at least one element selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al. When a small amount of X is added, the effect of improving charge / discharge capacity becomes more pronounced. Among them, at least one element selected from Al and Co is preferred. The molar ratio (d) of X is preferably 0.07 or less (0≦d≦0.07), more preferably 0.05 or less, and particularly preferably 0.03 or less.

[0032] Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In the formula (I), the molar ratio (e) of O is a value that satisfies electrical neutrality. In other words, it is a value that satisfies the valence of O in the positive electrode active material. The molar ratio (e) of O is, for example, 2.00 or more and 2.15 or less (2.00≦e≦2.15).

[0033] The positive electrode active material has a composition represented by the above composition formula and is composed mainly of a composite oxide (hereinafter referred to as "Li-Ni-Mn composite oxide") in which the value of α×β is 0.090 or less. Here, the term "main component" refers to the component that has the highest mass ratio among the components constituting the positive electrode active material. The mixture layer of the positive electrode 11 may contain a composite oxide other than the Li-Ni-Mn composite oxide as the positive electrode active material, but the content of the Li-Ni-Mn composite oxide is preferably 50 mass% or more, and may be substantially 100 mass%.

[0034] The Li-Ni-Mn composite oxide is, for example, a secondary particle formed by aggregation of a plurality of primary particles. The volume-based median diameter (D50) of the Li-Ni-Mn composite oxide is, for example, 1 μm or more and 30 μm or less, or 3 μm or more and 20 μm or less. The D50 of the composite oxide is the particle size at which the volume integrated value is 50% in the particle size distribution measured by the laser diffraction scattering method. The BET specific surface area of ​​the Li-Ni-Mn composite oxide is, for example, 0.1 m ２ / g or more 10m ２ / g or less, or 0.5m ２ / g or more 5m２ The BET specific surface area of ​​the composite oxide is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626. If the D50 and BET specific surface area are within the ranges, it is easy to increase the capacity.

[0035] As described above, the Li—Ni—Mn composite oxide has a layered rock salt structure belonging to the space group R-3m, and the product of the Ni mixing ratio α and the strain β (α×β) is 0.090 or less. The Ni mixing ratio α is the ratio of the amount of Ni occupying the Li layer of the layered rock salt structure to the total amount of Ni, and is determined by Rietveld analysis. The strain β indicates the non-uniformity of the lattice spacing of the crystal, and is determined by the Williamson-Hall method.

[0036] The powder X-ray diffraction pattern of the Li—Ni—Mn composite oxide was obtained using a desktop X-ray diffractometer (Rigaku Corporation, trade name "MiniFlex600"). Diffracted X-rays were detected with a high-speed one-dimensional detector (D / teX Ultra 2). The measurement conditions using the X-ray diffractometer were as follows: X-ray source: CuKα ray; Tube voltage: 40 kV; Tube current: 15 mA; Divergence slit (DS): 1 / 4°; Scattering slit (SS): 13 mm (open); Receiving slit (RS): 8 mm; Scan axis: 2θ / θ; Scan method: Continuous; 2θ scan range: 10-80°; Scan speed: 10° / min; Step width: 0.02°

[0037] The mixing ratio α in the crystal structure of the Li-Ni-Mn composite oxide is calculated by dividing the amount of Ni in the Li site refined by Rietveld analysis using Rigaku's analysis software "Smartlab Studio 2" by the total amount of Ni in the structure. In this specification, the Li site in the Rietveld analysis is the 3b site (0,0,0.5) in the layered rock salt structure (R-3m), and the transition metal site is the 3a site (0,0,0). Furthermore, the distortion β in the crystal structure of the Li-Ni-Mn composite oxide is calculated by the Williamson-Hall method using the 003, 101, 104, 015, and 113 diffraction lines in Smartlab Studio 2.

[0038] In the Li—Ni—Mn composite oxide, the value obtained by multiplying the mixing rate α and the strain β (α×β) may be 0.090 or less, preferably 0.072 or less, and more preferably 0.055 or less. The lower limit of the value of α×β is not particularly limited, but is preferably 0.002 or more, more preferably 0.005 or more. An example of a suitable range for the value of α×β is 0.002 or more and 0.072 or less, or 0.005 or more and 0.055 or less. In this case, both low cost and high capacity can be achieved to a higher degree.

[0039] When the Ni content in the Li-Ni-Mn composite oxide changes, the preferred value of α×β also changes slightly. １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In the formula (1), when the molar ratio (b) of Ni is 0.35 or more and 0.55 or less, the value of α×β is, for example, 0.035 or more and 0.090 or less, or 0.040 or more and 0.055 or less. When the molar ratio (b) of Ni is more than 0.55 and 0.70 or less, the value of α×β is, for example, 0.002 or more and 0.040 or less, or 0.005 or more and 0.035 or less.

[0040] The mixing ratio α is preferably 0.05 or more and 0.20 or less. The strain β is preferably 0.02 or more and 0.50 or less, and more preferably 0.05 or more and 0.30 or less. When the value of α×β is within the above range and the values ​​of the mixing ratio α and the strain β are each within the same range, the effect of improving the charge / discharge capacity becomes more significant.

[0041] The Li—Ni—Mn composite oxide can be synthesized, for example, by mixing a composite hydroxide or composite oxide containing Ni, Mn, etc. with a lithium raw material and calcining the mixture. The composite hydroxide containing Ni, Mn, etc. can be obtained by dropping an alkaline solution such as sodium hydroxide into a stirred solution of a metal salt containing Ni, Mn, etc., and adjusting the pH to the alkaline side (for example, 8.5 to 12.5), thereby causing precipitation (coprecipitation). The composite hydroxide can be calcined to obtain a composite oxide containing Ni, Mn, etc.

[0042] An example of a lithium raw material is Li２ CO ３ , LiOH, Li ２ O ２ , Li ２ O, LiNO ３ , LiNO ２ , Li ２ SO ４ , LiOH·H ２ Examples of suitable metal elements include O, LiH, LiF, etc. The composite hydroxide or composite oxide containing Ni, Mn, etc. and the lithium raw material are preferably mixed in such a manner that the molar ratio of the total amount of metal elements such as Ni, Mn, etc. to Li is 1:1.01 to 1:1.12.

[0043] In controlling the value of α×β, for example, the firing conditions are important. That is, in the synthesis process of the Li—Ni—Mn composite oxide, it is necessary to fire the mixture of raw materials so that the value of α×β is 0.090 or less. The mixture of raw materials is fired in the air or in an oxygen stream using a firing furnace. When the Ni content is about 50 mol% relative to the total number of moles of metal elements in the composite oxide, the firing temperature is preferably a high temperature of 800°C or higher. However, the required firing temperature varies depending on the composition of the raw materials, such as the Ni content. For example, if the Ni content increases, the firing temperature may be lowered. Therefore, adjusting the value of α×β to the desired value cannot be achieved unless the conditions are strictly controlled with attention to this value.

[0044] When the Ni content is about 50 mol%, the firing temperature is preferably 800°C or higher and 1100°C or lower. The temperature rise rate is, for example, 0.3°C / min or higher and 3.0°C / min or lower, or 0.5°C / min or higher and 2.0°C / min or lower. The firing time may be 3 hours or higher and 10 hours or lower. Here, the firing time refers to the time from when the temperature in the firing furnace reaches the maximum temperature in the firing step to when the firing is completed and cooling begins. The fired product may be rapidly cooled in the air upon removal from the firing furnace. The Li—Ni—Mn composite oxide can be obtained, for example, by rapidly cooling the fired product in the air, washing with water and drying as necessary, and pulverizing it by a known method.

[0045] When the Ni content in the composite hydroxide or composite oxide containing Ni, Mn, etc. is 35 mol % or more and 55 mol % or less, the firing temperature is preferably 900° C. or more and 1000° C. or less. In this case, if the firing temperature is 850° C., for example, the value of α×β cannot be made 0.090 or less, and a high capacity cannot be achieved.

[0046] [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 produced, 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. The negative electrode mixture layer may also contain a conductive agent such as CNT.

[0047] The negative electrode active material generally uses a carbon material that reversibly absorbs and releases lithium ions. Furthermore, the negative electrode active material may use an element that alloys with Li, such as Si or Sn, or a material containing such an element. Among these, a silicon-containing material containing Si is 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.

[0048] 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 Si-containing composite materials. A suitable silicon-containing material is a composite particle containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

[0049] As in the case of 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. One type of binder may be used alone, or multiple types may be used in combination. Furthermore, the negative electrode mixture layer preferably contains CMC or 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, for example, 0.1% by mass or more and 5% by mass or less with respect to the mass of the negative electrode mixture layer.

[0050] [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.

[0051] 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 and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. 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.

[0052] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0053] Example 1 Preparation of Positive Electrode Active Material A hydroxide containing Ni and Mn in a 1:1 molar ratio was mixed with lithium hydroxide in a molar ratio of Ni:Mn:Li = 0.5:0.5:1.05, and the resulting mixture was heated at a temperature increase rate of 1°C / min and calcined at 950°C for 10 hours in air. The calcined product was then quenched in air to obtain a Li-Ni-Mn composite oxide. The composition of the resulting Li-Ni-Mn composite oxide was analyzed using an ICP optical emission spectrometer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0054] [Fabrication of Positive Electrode] The Li—Ni—Mn composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 92:5:3, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. This positive electrode slurry was applied to a positive electrode core made of aluminum foil, and the coating film was dried. The coating film was then rolled with a rolling roller to obtain a positive electrode having a positive electrode mixture layer formed on the positive electrode core.

[0055] [Preparation of Non-Aqueous Electrolyte] A mixed solvent of fluoroethylene carbonate (FEC) and methyl propionate (FMP) in a volume ratio of 1:3 was dissolved in lithium hexafluorophosphate (LiPF ６ ) was dissolved in water to a concentration of 1 mol / liter to prepare a non-aqueous electrolyte solution.

[0056] [Preparation of Test Cell] A lithium metal foil was used as the negative electrode, and the positive and negative electrodes were arranged facing each other with a separator interposed therebetween to prepare an electrode assembly. This electrode assembly and the nonaqueous electrolyte solution were placed in a coin-shaped outer can, and the opening of the outer can was sealed with a gasket and a sealing member to prepare a test cell (nonaqueous electrolyte secondary battery).

[0057] Example 2 A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 1:1 and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Li=0.5:0.5:1.11, the resulting mixture was heated at a temperature increase rate of 1° C. / min, and then calcined at 900° C. for 3 hours in air.

[0058] Example 3 A positive electrode active material and a test cell were prepared in the same manner as in Example 2, except that in the synthesis step of the Li—Ni—Mn composite oxide, the firing temperature was changed to 1000° C. and the firing time was changed to 10 hours.

[0059] Example 4 A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 6:4 and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Li=0.6:0.4:1.00, the resulting mixture was heated at a temperature increase rate of 1° C. / min, and then calcined in air at 900° C. for 10 hours, and the calcined product was washed with a sufficient amount of water and then heat-treated in vacuum at 180° C. for 2 hours.

[0060] Example 5 A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 7:3 and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Li=0.7:0.3:1.05, the resulting mixture was heated at a temperature increase rate of 1° C. / min, and calcined at 800° C. for 3 hours in air, and the calcined product was washed with a sufficient amount of water and then heat-treated in vacuum at 180° C. for 2 hours.

[0061] Example 6 A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, a hydroxide containing Ni and Mn in a molar ratio of 7:3 was mixed with lithium hydroxide in a molar ratio of Ni:Mn:Li=0.7:0.3:1.09, the resulting mixture was heated at a temperature increase rate of 1° C. / min, and calcined at 900° C. for 3 hours in air, and the calcined product was washed with a sufficient amount of water and then heat-treated in vacuum at 180° C. for 2 hours.

[0062] Example 7 A positive electrode active material and a test cell were produced in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, a hydroxide containing Ni, Mn, and Co in a molar ratio of 60:35:5 and lithium hydroxide were mixed in a molar ratio of Ni:Mn:Co:Li=0.60:0.35:0.05:1.05, the resulting mixture was heated at a temperature increase rate of 1° C. / min, and calcined at 850° C. for 3 hours in air, and the calcined product was washed with a sufficient amount of water and then heat-treated in vacuum at 180° C. for 2 hours.

[0063] Comparative Example 1 A positive electrode active material and a test cell were prepared in the same manner as in Example 1, except that in the synthesis step of the Li—Ni—Mn composite oxide, the firing temperature was changed to 850° C. and the firing time was changed to 5 hours.

[0064] Comparative Example 2 A positive electrode active material and a test cell were prepared in the same manner as in Example 2, except that in the synthesis step of the Li—Ni—Mn composite oxide, the firing temperature was changed to 850° C. and the firing time was changed to 10 hours.

[0065] Comparative Example 3 A positive electrode active material and a test cell were prepared in the same manner as in Example 2, except that in the synthesis step of the Li—Ni—Mn composite oxide, the firing temperature was changed to 850° C. and the firing time was changed to 3 hours.

[0066] For each of the Li—Ni—Mn composite oxides of the Examples and Comparative Examples, the Ni mixing ratio α and strain β were determined by the above-mentioned method, and the values ​​and the value of α×β are shown in Table 1.

[0067] [Evaluation of Discharge Capacity] Each test cell of the Examples and Comparative Examples was charged at a constant current of 0.2 C at 25° C. until the battery voltage reached 4.5 V, and then charged at a constant voltage of 4.65 V until the current value reached 0.02 C. After a 20-minute rest, the test cell was discharged at a constant current of 0.2 C until the battery voltage reached 2.5 V, and the discharge capacity was determined. The measurement results of the discharge capacity are shown in Table 1, along with the composition of the positive electrode active material, the mixing ratio α, and the strain β.

[0068]

[0069] As shown in Table 1, all of the test cells of the examples have higher capacities than the test cells of the comparative examples. That is, in the layered rock salt structure of the Li—Ni—Mn composite oxide, when the value obtained by multiplying the mixing rate α and the strain β (α×β) is 0.090 or less, the battery capacity can be significantly improved. When the value of α×β exceeds 0.090, as in the positive electrode active material of the comparative example, it is not possible to achieve the high capacity achieved when the positive electrode active material of the examples is used.

[0070] Furthermore, the value of α×β of a Li—Ni—Mn composite oxide varies significantly depending on the synthesis conditions of the composite oxide. For example, as can be seen from Example 2 and Comparative Example 3, for Li—Ni—Mn composite oxides of the same composition, a 50°C change in the firing temperature results in a significant change in the value of α×β (Example 2: 0.0872, Comparative Example 3: 0.1100). As a result, the discharge capacity also differs significantly (Example 2: 174.0 mAh / g, Comparative Example 3: 153.9 mAh / g). Furthermore, when the firing temperature is increased to 1000°C and the firing time is extended, the value of α×β becomes 0.0423, and the discharge capacity becomes 189.9 mAh / g (see Example 3).

[0071] The present disclosure is further described by the following embodiments. Configuration 1: A positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, the positive electrode active material having the composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅwherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Ni, and Mn, a≦1.15, 0.35≦b≦0.70, 0.30≦c≦0.65, 0≦d≦0.07, and e are values ​​satisfying electrical neutrality, and the product (α×β) of the Ni mixing ratio α determined by Rietveld analysis and the strain β determined by the Williamson-Hall method is 0.090 or less. A positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein the product (α×β) of the Ni mixing ratio α and the strain β is 0.002 or more and 0.072 or less. A positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1 or Aspect 2, wherein the mixing ratio α is 0.05 or more and 0.20 or less. Configuration 4: Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In the formula (1), the molar ratio (b) of Ni is 0.50≦b≦0.60. １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ In the formula (I), X is at least one selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al. １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ wherein X is at least one selected from Al and Co. Aspect 7: A nonaqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material of any one of Aspects 1 to 6, a negative electrode, and a nonaqueous electrolyte.

[0072] 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

Claims

1. A positive electrode active material for a non-aqueous electrolyte secondary battery having a crystal structure belonging to the space group R-3m, the positive electrode active material having the composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ wherein X is at least one selected from the group consisting of transition metal elements and typical elements other than Li, Ni, and Mn, a≦1.15, 0.35≦b≦0.70, 0.30≦c≦0.65, 0≦d≦0.07, and e are values ​​that satisfy electrical neutrality, and a value (α×β) obtained by multiplying a Ni mixing rate α determined by Rietveld analysis by a strain β determined by the Williamson-Hall method is 0.090 or less.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the value obtained by multiplying the mixing rate α and the strain β (α×β) is 0.002 or more and 0.072 or less.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the mixing ratio α is 0.05 or more and 0.20 or less.

4. Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ 4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 3, wherein the molar ratio (b) of Ni is 0.50≦b≦0.

60.

5. Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ 4. The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 3, wherein X is at least one selected from Mg, Ca, Sr, Ba, Sn, Ti, Si, V, Cr, Fe, Cu, Zn, Bi, Sb, B, Ga, In, P, Zr, Hf, Nb, Ta, Mo, W, Co, and Al.

6. Composition formula Li １＋ａ Ni ｂ Mn ｃ X ｄ O ｅ 6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 5, wherein X is at least one selected from the group consisting of Al and Co.

7. A non-aqueous electrolyte secondary battery comprising: a positive electrode containing the positive electrode active material according to any one of claims 1 to 6; a negative electrode; and a non-aqueous electrolyte.

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