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

Abstract

This positive electrode active material for a non-aqueous electrolyte secondary battery contains a lithium transition metal composite oxide composed of secondary particles obtained by agglomerating primary particles, and is characterized in that the lithium transition metal composite oxide has a plurality of voids therein, and the median value of the envelope surface area of voids observed within the secondary particles as found through cross-sectional observation of the secondary particles is 0.80 or more.

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.

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, the positive electrode active material has a significant impact on battery performance, such as input / output characteristics, capacity, and durability, and therefore much research has been conducted on the positive electrode active material. 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. For example, Patent Document 1 discloses a positive electrode active material having a three-dimensional network structure consisting of a core-shell structure, with the aim of improving battery performance, such as output characteristics.

[0003] Special table 2018-531500 publication

[0004] 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 further increases in capacity and improvements in output characteristics. The positive electrode active material of Patent Document 1 still has a large room for improvement in terms of increasing capacity and improving output characteristics.

[0005] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a positive electrode active material for a non-aqueous electrolyte secondary battery, which contains a lithium transition metal composite oxide composed of secondary particles formed by aggregation of primary particles, wherein the lithium transition metal composite oxide has a plurality of voids therein, and the median area envelopment of the voids observed inside the secondary particles, as determined by cross-sectional observation of the secondary particles, is 0.80 or greater.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure can achieve a high capacity and improved output characteristics of a non-aqueous electrolyte secondary battery.

[0007] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment of the present invention; FIG. 2 is a diagram illustrating an imaginary envelope line surrounding a void and an envelope area;

[0008] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material according to the present disclosure and a nonaqueous 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.

[0009] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical, bottomed exterior body 16 is exemplified as a nonaqueous electrolyte secondary battery; however, the exterior body of the battery is not limited to a cylindrical exterior body. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery with a prismatic exterior body, a coin-type battery with a coin-type exterior body, or a pouch-type battery with an exterior body composed of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, but may also be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween. Furthermore, the design of the nonaqueous electrolyte secondary battery according to the present disclosure is not limited to the design of the exemplified nonaqueous electrolyte secondary battery, and known nonaqueous electrolyte secondary battery designs may also be applied.

[0010] FIG. 1 is an axial cross-sectional view of a cylindrical 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 exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. 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 exterior body 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior body 16 is closed by a sealing body 17. Hereinafter, for convenience of explanation, the sealing body 17 side of the battery will be referred to as the "top" and the bottom side of the exterior body 16 will be referred to as the "bottom."

[0011] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all rectangular, elongated bodies that are spirally wound in the longitudinal direction and stacked alternately in the radial direction of the electrode assembly 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. The electrode assembly 14 includes 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. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the lateral direction of the positive electrode 11 and the negative electrode 12 is the axial direction. That is, the lateral end faces of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode assembly 14.

[0012] 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 exterior body 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 exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0013] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 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 body 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0014] 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.

[0015] The positive electrode 11, the negative electrode 12, the separator 13, and the nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, with the positive electrode 11 being particularly described below.

[0016] The positive electrode 11 includes 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 such as aluminum, an aluminum alloy, stainless steel, or titanium that is stable within the potential range of the positive electrode 11, or a film having such a metal disposed on its surface.

[0017] The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent, and a binder, and is 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 the positive electrode active material, the conductive agent, and the binder onto the positive electrode core, drying the coating, and then compressing it to form positive electrode mixture layers on both sides of the positive electrode core.

[0018] 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 mass% or more and 5 mass% or less with respect to the mass of the positive electrode mixture layer.

[0019] 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.

[0020] The positive electrode active material includes a lithium transition metal composite oxide composed of secondary particles formed by aggregation of primary particles. A primary particle is a single particle with no grain boundary present inside the particle, and a secondary particle is formed by aggregation of, for example, two or more and 10,000 or less primary particles. The lithium transition metal composite oxide is a composite oxide containing a metal element in addition to Li. Examples of the metal element contained in the lithium transition metal composite oxide include at least one of Ni, Mn, Co, and Al, but are not limited to these elements. From the viewpoint of improving battery capacity, the metal element contained in the lithium transition metal composite oxide preferably includes at least Ni.

[0021] The lithium transition metal composite oxide has a plurality of voids therein. The median area envelopment ratio of the voids observed inside the secondary particles, as determined by cross-sectional observation of the secondary particles, is 0.80 or greater. As will be described in detail later, the area envelopment ratio is an index representing the smoothness of the contour shape of the voids, and the closer the ratio is to 1, the smoother the contour.

[0022] As a result of studies by the present inventors, it has been found that when the median area envelope ratio of voids observed inside the secondary particles is 0.80 or greater, the nonaqueous electrolyte secondary battery 10 can achieve high capacity and improved output characteristics. This is presumably because when the median area envelope ratio of voids is 0.80 or greater, the nonaqueous electrolyte easily penetrates into the secondary particles through the voids, improving the ionic conductivity of the positive electrode active material. In other words, when the median area envelope ratio of voids is less than 0.80, the nonaqueous electrolyte has difficulty sufficiently penetrating into the secondary particles, making it difficult to achieve high capacity and improved output characteristics of the nonaqueous electrolyte secondary battery 10. A method for calculating the area envelope ratio of voids observed inside the secondary particles is described below.

[0023] <Method for measuring area envelopment> (1) A cross section of a positive electrode active material (lithium transition metal composite oxide) is exposed. Examples of a method for exposing a cross section of a positive electrode active material include a method of processing the material with a focused ion beam (FIB) processing device (e.g., Thermo Fisher Scientific, Helios G3 UC) to expose the cross section, or a method of processing the positive electrode active material with an ion milling device (e.g., Hitachi High-Tech Corporation, ArBlade 5000) to expose the cross section of the positive electrode active material. (2) A backscattered electron image of the cross section of the exposed positive electrode active material is taken using a scanning electron microscope (e.g., Hitachi High-Tech Corporation, SU8600). The magnification when taking the backscattered electron image is, for example, 5,000 to 20,000 times. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health), to obtain a binarized image in which the cross sections of secondary particles in the cross-sectional image are colored white and the voids present in the cross sections of secondary particles are colored black. (4) From the binarized image, analysis software (e.g., Avizo-Materials Science, manufactured by Thermo Fisher Scientific) is used to measure the area A1 of each void and the value of the envelope area A2 enclosed by the virtual envelope line X surrounding each void, as shown in FIG. 2. Based on the measured values ​​of area A1 and envelope area A2, the area envelopment ratio (value A1 / A2) is calculated. Then, the area envelopment ratio is calculated for all voids present in an area of, for example, 7.6 μm × 10 μm on the cross section of the secondary particle, and the median value of the area envelopment ratio is calculated.

[0024] The median area envelopment ratio of the voids observed inside the secondary particles may be 0.80 or more, preferably 0.825 or more, more preferably 0.85 or more, and even more preferably 0.875 or more. In this case, the nonaqueous electrolyte can more easily penetrate into the secondary particles, thereby achieving a further increase in capacity and further improvement in output characteristics of the nonaqueous electrolyte secondary battery 10. The upper limit of the median area envelopment ratio of the voids observed inside the secondary particles is not particularly limited, and is, for example, 0.99.

[0025] In the lithium transition metal composite oxide, the median nearest neighbor gap distance of voids determined by cross-sectional observation of secondary particles is preferably 50 nm or more, more preferably 75 nm or more, and even more preferably 100 nm or more. If the median nearest neighbor gap distance of voids is less than 50 nm, the density of the positive electrode active material may be excessively reduced, resulting in a decrease in the capacity of the nonaqueous electrolyte secondary battery 10. Furthermore, in the lithium transition metal composite oxide, the median nearest neighbor gap distance of voids determined by cross-sectional observation of secondary particles is preferably 330 nm or less, more preferably 300 nm or less, and even more preferably 270 nm or less. By setting the median nearest neighbor gap distance of voids to 330 nm or less, an appropriate amount of voids can be formed within the secondary particles, making it easier for the nonaqueous electrolyte to penetrate into the secondary particles through the voids. As a result, the ionic conductivity of the positive electrode active material is improved, thereby achieving a high capacity and improved output characteristics of the nonaqueous electrolyte secondary battery 10. Therefore, in the lithium transition metal composite oxide, the median nearest neighbor gap distance of voids determined by cross-sectional observation of secondary particles is preferably 50 nm or more and 330 nm or less, more preferably 75 nm or more and 300 nm or less, and even more preferably 100 nm or more and 270 nm or less. Note that in the lithium transition metal composite oxide, the median nearest neighbor gap distance of voids determined by cross-sectional observation of secondary particles may be 50 nm or more and 300 nm or less, 50 nm or more and 270 nm or less, 100 nm or more and 330 nm or less, or 100 nm or more and 300 nm or less.

[0026] The nearest void distance of voids determined by cross-sectional observation of secondary particles can be calculated from the binarized image obtained by steps (1) to (3) of the above-mentioned area envelopment measurement method. Specifically, the centroid position of each void is derived from the obtained binarized image using analysis software (e.g., Avizo-Materials Science, manufactured by Thermo Fisher Scientific), and the distance between the centroid position of the void and the centroid position of the void closest to the centroid position of the void (nearest void distance) is measured. The centroid position of a void is defined as the center of the weight of the pixels contained in the area surrounded by the periphery of the void, assuming that the weights of the pixels constituting the image are uniform. Then, the nearest void distance is calculated for all voids present in an area of, for example, 7.6 μm × 10 μm on the cross section of the secondary particle, and the median of the nearest void distances is calculated.

[0027] The lithium transition metal composite oxide is, for example, a compound represented by the general formula Li ｘ Ni ａ Mn ｂ M1 ｃ M2 ｄ O ２ (wherein 0.80≦x≦1.20, 0.50≦a≦0.95, 0.05≦b≦0.50, 0≦c≦0.20, 0≦d≦0.10, M1 is at least one element selected from Co and Al, and M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Si, Fe, Cr, Mo, W, V, Nb, Ta, Sb, and Bi). The contents of the elements constituting the lithium transition metal composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0028] The Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more relative to the total number of moles of metal elements excluding Li. Increasing the Ni content can improve battery capacity. The Ni content may be 60 mol% or more, 70 mol% or more, or 80 mol% or more. The upper limit of the Ni content is, for example, 95 mol%. An example of a suitable range of the Ni content is 50 mol% or more and 95 mol% or less, or 70 mol% or more and 95 mol% or less, or 80 mol% or more and 95 mol% or less, or 50 mol% or more and 90 mol% or less, or 70 mol% or more and 90 mol% or less, or 80 mol% or more and 90 mol% or less.

[0029] The Mn content in the lithium transition metal composite oxide is 0 mol% or more and 50 mol% or less relative to the total number of moles of metal elements excluding Li, and Mn is an optional component. The lithium transition metal composite oxide can stabilize its crystal structure by containing Mn. The Mn content may be 5 mol% or more, 10 mol% or more, or 15 mol% or more. The upper limit of the Mn content is, for example, 50 mol%. An example of a suitable range for the Mn content is 5 mol% or more and 50 mol% or less, or 10 mol% or more and 50 mol% or less, or 15 mol% or more and 50 mol% or less, or 5 mol% or more and 40 mol% or less, or 10 mol% or more and 40 mol% or less, or 15 mol% or more and 40 mol% or less.

[0030] The content of M1 (M1 is at least one element selected from Co and Al) in the lithium transition metal composite oxide is 0 mol % or more and 20 mol % or less relative to the total number of moles of metal elements excluding Li, and M1 is an optional component. By containing Co, the lithium transition metal composite oxide can improve the heat resistance of the battery. Furthermore, by containing Al, the lithium transition metal composite oxide can stabilize the crystal structure.

[0031] The content of M2 (M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Si, Fe, Cr, Mo, W, V, Nb, Ta, Sb, and Bi) in the lithium transition metal composite oxide is 0 mol % or more and 10 mol % or less relative to the total number of moles of metal elements excluding Li, and M2 is an optional component. In other words, the lithium transition metal composite oxide does not need to contain M2. These elements may be contained inside the secondary particles of the lithium transition metal composite oxide or may be present on the surface of the secondary particles. When the lithium transition metal composite oxide contains these elements, it becomes easier to increase the area coverage of the voids inside the secondary particles.

[0032] The lithium transition metal composite oxide preferably contains at least Sb as M2. As a result of the inventors' investigations, it has become clear that when the lithium transition metal composite oxide contains Sb, the capacity and output characteristics of the non-aqueous electrolyte secondary battery 10 can be further improved. When the lithium transition metal composite oxide contains Sb, a compound containing Sb is formed near the voids inside the secondary particles. Although the detailed mechanism is unclear, when the void contours are smooth, i.e., when the median void area envelopment ratio is 0.80 or greater, the presence of a compound containing Sb near the voids specifically improves the ionic conductivity of the positive electrode active material. As a result, the capacity and output characteristics of the non-aqueous electrolyte secondary battery 10 can be further improved. In other words, when the median void area envelopment ratio is less than 0.8, the capacity and output characteristics of the non-aqueous electrolyte secondary battery 10 cannot be further improved.

[0033] The positive electrode active material may include a positive electrode active material other than the lithium transition metal composite oxide of this embodiment, in which the median area envelopment ratio of voids observed inside the secondary particles is 0.80 or more, 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. Even when a lithium transition metal composite oxide having a median area envelopment ratio of voids observed inside the secondary particles of less than 0.80 is used as the positive electrode active material, the effects of increasing capacity and improving output characteristics described above can be achieved depending on the content of the positive electrode active material of this embodiment. The mass ratio of the lithium transition metal composite oxide of this embodiment, in which the median area envelopment ratio of voids observed inside the secondary particles is 0.80 or more, to the total mass of the positive electrode active material is, for example, 80 mass% or more, and may even be 90 mass% or more.

[0034] The cathode active material of the present embodiment can be manufactured by the following method. Note that the manufacturing method described here is merely an example, and the manufacturing method of the cathode active material is not limited to this method. The manufacturing method of the cathode active material includes, for example, a synthesis step of a lithium transition metal composite oxide, a washing step, and a drying step.

[0035] In the synthesis process of the lithium transition metal composite oxide, a metal hydroxide containing Ni and any metal such as Mn or Co relative to the total molar amount of metal elements excluding Li is mixed with a Li compound, and the mixture is fired 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.

[0036] The metal hydroxide can be obtained, for example, by adding an alkaline solution such as sodium hydroxide dropwise to a stirred solution of a metal salt such as Ni or Mn, adjusting the pH to the alkaline side (e.g., 8.5 or more and 12.5 or less), and then causing precipitation (coprecipitation). Note that, instead of the metal hydroxide, a metal oxide obtained by heat-treating the metal hydroxide may be used.

[0037] 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, Mg compounds, Ca compounds, Sr compounds, Ba compounds, Si compounds, Fe compounds, Cr compounds, Mo compounds, W compounds, V compounds, Nb compounds, Ta compounds, Sb compounds, Bi compounds, etc. may be added. These compounds are, for example, oxides, hydroxides, chlorides, carbonates, sulfates, or phosphates, and may also be composite compounds containing other metal elements such as Li. By adjusting the presence or absence and content of these additives, the area coverage of the voids and the nearest neighbor gap distance of the voids can be adjusted. For example, by adding W or Sb, the nearest neighbor gap distance of the voids can be reduced. When Sb is contained in the lithium transition metal composite oxide, it is preferable to mix an Sb compound when mixing the metal hydroxide with the Li compound, from the viewpoint of having Sb be present in the vicinity of the voids inside the secondary particles. Examples of the Sb compound include antimony trioxide, antimony tetroxide, antimony pentoxide, antimony trichloride, antimony ethoxide, and lithium antimonate. Among these, antimony trioxide is preferred.

[0038] The mixture of the metal hydroxide and the 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 a multi-stage firing. Alternatively, the metal hydroxide may be pre-fired at a temperature lower than that of the firing process. The pre-fire temperature is preferably 600°C or lower, and may be 300 to 500°C. Pre-fire or multi-stage firing facilitates increasing the area coverage of the voids inside the secondary particles. An example of multi-stage firing is a heating rate of 1.0°C / min or higher and 5.5°C / min or lower in the temperature range of 450°C to 680°C, with the maximum temperature being 850°C to 1100°C. The heating rate from 680°C to the maximum temperature may be 0.1°C / min or higher and 3.5°C / min or lower. The holding time at the maximum temperature may be 1 hour to 30 hours. The void area coverage and the nearest void distance can be adjusted by adjusting the firing conditions. For example, by lowering the maximum temperature, the void area coverage can be increased and the nearest void distance can be decreased.

[0039] 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. The water used for washing may contain Mg compounds, Ca compounds, Sr compounds, Ba compounds, Si compounds, Fe compounds, Cr compounds, Mo compounds, W compounds, V compounds, Nb compounds, Ta compounds, Sb compounds, Bi compounds, etc. By adjusting the presence or absence and content of these compounds, the area coverage of the voids and the nearest void distance of the voids can be adjusted. For example, the area coverage of the voids can be reduced by adding Sr. The above-mentioned compounds may also be added to the cake-like composition. That is, the above-mentioned compounds may be added in the washing step in addition to or instead of the synthesis step.

[0040] In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powdery composition. The drying step may be performed under 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. The washing step and the drying step may be omitted.

[0041] [Negative Electrode] The negative electrode 12 may have, for example, a negative electrode core and a negative electrode mixture layer formed on the surface of the negative electrode core, or a metal Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode core, and lithium metal may be deposited on the surface of the negative electrode core upon charging. When the negative electrode 12 has a negative electrode mixture layer, the negative electrode mixture layer is preferably formed on both sides of the negative electrode core. The negative electrode core may be a foil of a metal stable within the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with such a metal disposed on the surface. The thickness of the negative electrode core is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side 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, a binder, etc. to the surface of a negative electrode core, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0042] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly absorb and release lithium ions, and generally, carbon materials such as graphite are used. Graphite may be any of natural graphite such as flake graphite, lump graphite, and amorphous graphite, or artificial graphite such as lump artificial graphite and graphitized mesophase carbon microbeads. Furthermore, metals that alloy with Li, such as Si and Sn, metal compounds containing Si, Sn, and lithium-titanium composite oxides may also be used as the negative electrode active material. Furthermore, those provided with a carbon coating may also be used. For example, SiO ｘ (0.5≦x≦1.6) or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0<y<2) may be used in combination with graphite.

[0043] Examples of binders contained in the negative electrode mixture layer include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), etc. These may be used alone or in combination of two or more.

[0044] [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.

[0045] 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.

[0046] [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.

[0047] The liquid electrolyte (electrolytic solution) contains, 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).

[0048] 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).

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] Examples of phenolic compounds include phenol, hydroxytoluene, etc. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, cyclohexylbenzene (CHB), etc.

[0054] 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.

[0055] 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.

[0056] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0057] Example 1 Preparation of Positive Electrode Active Material Ni Obtained by Coprecipitation Method ０．８２ Mn ０．１８ (OH) ２The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ２ O ３ And WO ３ The mixture was mixed so that the molar ratio of Li, Ni, and Mn to Sb and W was 1.05:1:0.003:0.005. 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 860°C at a rate of 1°C / min. The mixture was then held at 860°C for 5 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0058] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.808. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 148 nm.

[0059] [Preparation of Positive Electrode] The positive electrode active material, acetylene black (AB), and polyvinylidene fluoride were mixed in a mass ratio of 92:5:3, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was disposed on both sides of the positive electrode core. In addition, an exposed portion in which the surface of the positive electrode core was exposed was provided in a part of the positive electrode.

[0060] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75. Lithium hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1.35 mol / L to prepare a non-aqueous electrolyte.

[0061] [Preparation of Test Cell] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the lithium metal foil serving as the negative electrode. The positive electrode and the negative electrode were spirally wound with a polyolefin separator interposed therebetween to prepare a wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell.

[0062] [Evaluation of Charge Capacity and Discharge Capacity] The prepared test cell was charged to 4.5 V (vs. Li metal) at a constant current of 0.2 C in a temperature environment of 25°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 0.2 C. This charge / discharge cycle was repeated three times, and the discharge capacity at the third cycle was measured. The charge capacity was calculated as the integrated capacity at the third cycle using the following formula: Charge capacity = (charge capacity at the first cycle - discharge capacity at the first cycle) + (charge capacity at the second cycle - discharge capacity at the second cycle) + (charge capacity at the third cycle).

[0063] [Evaluation of Output Characteristics] The fabricated nonaqueous electrolyte secondary battery was charged to 4.5 V (vs. Li metal) at a constant current of 0.2 C in a temperature environment of 25° C., and then charged to 0.02 C at a constant voltage of 4.5 V. Subsequently, it was discharged to 2.5 V at a constant current of 0.2 C, and the discharge capacity at 0.2 C was measured. Next, it was charged to 4.5 V at a constant current of 0.2 C, and then charged to 0.02 C at a constant voltage of 4.5 V. Subsequently, it was discharged to 2.5 V at a constant current of 1 C, and the discharge capacity at 1 C was measured. The output characteristics were evaluated using the following formula: Output characteristic [%] = (discharge capacity at 1 C) / (discharge capacity at 0.2 C) × 100

[0064] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0065] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ２ O ３ And WO ３ The above was mixed so that the molar ratio of Li, Ni, and Mn to Sb and W was 1.05:1:0.005:0.005 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 860°C at a rate of 1°C / min. The mixture was then held at 860°C for 5 hours to obtain a lithium transition metal composite oxide.

[0066] The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. During the washing, Al nitrate, Ca nitrate, and Sr nitrate were mixed with the water used for washing so that the molar ratio of the total amount of Ni and Mn in the lithium transition metal composite oxide to Al, Ca, and Sr was 1:0.0075:0.0075:0.003.

[0067] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.912. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 112 nm.

[0068] Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0069] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ２ O ３ And WO ３ and Nb ２ O ５ And, Bi ２ O ３ The mixture was mixed so that the molar ratio of Li, Ni, and Mn to Sb, W, Nb, and Bi was 1.05:1:0.0025:0.003:0.003:0.000125. This mixture was fired in two stages under an oxygen stream with an oxygen concentration of 90% or more (a flow rate of 0.15 L / min to 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 840°C at a rate of 1°C / min. The mixture was then held at 840°C for 5 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0070] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.905. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 153 nm.

[0071] Example 4 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0072] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ２ O ３ And WO ３ The mixture was mixed so that the molar ratio of Li, Ni, and Mn to Sb and W was 1.05:1:0.003:0.003. 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 820°C at a rate of 1°C / min. The mixture was then held at 820°C for 5 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0073] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.875. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 261 nm.

[0074] Example 5 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0075] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ２ O ３ And WO ３ A test cell was produced and evaluated in the same manner as in Example 1, except that the above was mixed to obtain a mixture such that the molar ratio of Li, the total amount of Ni and Mn, Sb, and W was 1.05:1:0.003:0.003.

[0076] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.850. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 138 nm.

[0077] Example 6 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0078] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ３ The above was mixed so that the molar ratio of Li, Ni, and Mn to W was 1.05:1:0.005 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 840°C at a rate of 1°C / min. The mixture was then held at 840°C for 5 hours to obtain a lithium transition metal composite oxide.

[0079] The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. During the washing, Al nitrate, Ca nitrate, and Sr nitrate were mixed with the water used for washing so that the molar ratio of the total amount of Ni and Mn in the lithium transition metal composite oxide to Al, Ca, and Sr was 1:0.0025:0.0075:0.003.

[0080] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.913. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 157 nm.

[0081] Example 7 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0082] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ３ and Ca(OH) ２ and Sr(OH) ２ The above mixtures were mixed so that the molar ratio of Li, Ni, and Mn to W, Ca, and Sr was 1.05:1:0.005:0.0075:0.003. 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 860°C at a rate of 1°C / min. The mixture was then held at 860°C for 5 hours to obtain a lithium transition metal composite oxide.

[0083] The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. During the washing, Al nitrate was mixed with the water used for washing so that the molar ratio of Al to the total amount of Ni and Mn in the lithium transition metal composite oxide was 1:0.0025.

[0084] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.889. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 210 nm.

[0085] Example 8 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0086] LiOH and Ni obtained by coprecipitation method ０．８ Mn ０．２ (OH) ２ powder and SiO ２ and Ca(OH) ２ The mixture was mixed so that the molar ratio of the total amount of Li, Ni, and Mn to Si and Ca was 1.05:1:0.0075:0.01. This mixture was fired in two stages under an oxygen stream with an oxygen concentration of 90% or more (a flow rate of 0.15 L / min to 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 820°C at a rate of 1°C / min. The mixture was then held at 820°C for 10 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0087] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.875. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 199 nm.

[0088] Example 9 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0089] LiOH and Ni obtained by coprecipitation method ０．８ Mn ０．２ (OH) ２ Powder and WO ３ and Fe ２ O ３ And, MoO ３ The mixture was mixed so that the molar ratio of Li, Ni, and Mn to W, Fe, and Mo was 1.05:1:0.0025:0.005:0.005. 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 670°C to 800°C at a rate of 1°C / min. The mixture was then held at 800°C for 10 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0090] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.870. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 244 nm.

[0091] Example 10 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0092] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ３ The above was mixed so that the molar ratio of Li, Ni, and Mn to W was 1.05:1:0.005 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 860°C at a rate of 1°C / min. The mixture was then held at 860°C for 5 hours to obtain a lithium transition metal composite oxide.

[0093] The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. During the washing, Al nitrate, Ca nitrate, and Sr nitrate were mixed with the water used for washing so that the molar ratio of the total amount of Ni and Mn in the lithium transition metal composite oxide to Al, Ca, and Sr was 1:0.0075:0.0075:0.003.

[0094] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.904. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 124 nm.

[0095] Example 11 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0096] Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ The powder was calcined in air by increasing the temperature from room temperature to 150°C at a rate of 5.0°C / min, and then calcined again by increasing the temperature to 400°C at a rate of 1.0°C / min to obtain a Ni, Mn-containing composite oxide. ３ The above was mixed so that the molar ratio of the total amount of Li, Ni, and Mn to W was 1.05:1:0.003 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 820°C at a rate of 1°C / min. The mixture was then held at 820°C for 5 hours to obtain a lithium transition metal composite oxide.

[0097] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0098] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.875. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 171 nm.

[0099] Comparative Example 1 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0100] LiOH and Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ powder and Sb２ O ３ And WO ３ and Sr(OH) ２ The above mixtures were mixed so that the molar ratio of Li, Ni, and Mn to Sb, W, and Sr was 1.05:1:0.005:0.005:0.002. 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 820°C at a rate of 1°C / min. The mixture was then held at 820°C for 5 hours to obtain a lithium transition metal composite oxide.

[0101] The resulting lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. During the washing, Al nitrate and Ca nitrate were mixed with the water used for washing so that the molar ratio of the total amount of Ni and Mn in the lithium transition metal composite oxide to Al and Ca was 1:0.005:0.0025.

[0102] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.783. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 44 nm.

[0103] Comparative Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0104] LiOH and Ni obtained by coprecipitation method ０．８２ Mn ０．１８ (OH) ２ powder and Nb ２ O ５ And WO ３The above was mixed so that the molar ratio of Li, Ni, and Mn to Nb and W was 1.05:1:0.001:0.001 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 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then the temperature was raised from 670°C to 830°C at a rate of 1°C / min. The mixture was then held at 830°C for 5 hours to obtain a lithium transition metal composite oxide.

[0105] The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material. ３ was added so that the molar ratio of the total amount of Ni and Mn, Nb, and W in the lithium transition metal composite oxide was 1:0.001:0.002.

[0106] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.790. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 332 nm.

[0107] Comparative Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the positive electrode active material, a lithium transition metal composite oxide was prepared by the following method.

[0108] LiOH and Ni obtained by coprecipitation method ０．８ Mn ０．２ (OH) ２ powder and TiO ２ and Ca(OH) ２The mixture was mixed so that the molar ratio of the total amount of Li, Ni, and Mn to Ti and Ca was 1.05:1:0.02:0.01. This mixture was fired in two stages under an oxygen stream with an oxygen concentration of 90% or more (a flow rate of 0.15 L / min to 0.35 L / min per 1 L of furnace volume). Specifically, the temperature was raised from room temperature to 670°C over 5 hours, and then raised from 650°C to 820°C at a rate of 1°C / min. The mixture was then held at 820°C for 10 hours to obtain a lithium transition metal composite oxide. The obtained lithium transition metal composite oxide was washed with water to remove excess lithium, and the washed composite oxide was dehydrated, dried, and then crushed to obtain a positive electrode active material.

[0109] Observation of the positive electrode active material using a SEM confirmed that the produced lithium transition metal composite oxide was composed of secondary particles formed by aggregation of primary particles. Furthermore, the median area envelopment ratio of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 0.795. Furthermore, the median nearest neighbor gap distance of voids observed within the secondary particles was calculated using the above-mentioned method, and was found to be 517 nm.

[0110] The evaluation results of the charge capacity, discharge capacity, and output characteristics of the test cells of the examples and comparative examples are shown in Table 1. Table 1 also shows the molar ratio of Ni, the added elements, the firing temperature, the median of the area envelopment ratio, and the median of the nearest gap distance.

[0111]

[0112] As shown in Table 1, the test cells of the examples having a median area envelopment ratio of 0.80 or more have improved charge capacity, discharge capacity, and output characteristics compared to the test cells of the comparative examples having a median area envelopment ratio of less than 0.80. This is presumably because when the median area envelopment ratio of voids is 0.80 or more, the nonaqueous electrolyte easily penetrates into the secondary particles through the voids, improving the ionic conductivity of the positive electrode active material.

[0113] Furthermore, the test cells of Examples 2 and 4, which had a median area envelope ratio of 0.80 or more and contained Sb, had improved charge capacity, discharge capacity, and output characteristics compared to the test cells of Examples 10 and 11, which also had a median area envelope ratio of 0.80 or more and did not contain Sb. This is presumably because the presence of a compound containing Sb near the voids specifically improved the ionic conductivity of the positive electrode active material.

[0114] 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 composed of secondary particles formed by aggregation of primary particles, wherein the lithium transition metal composite oxide has a plurality of voids therein, and the median area envelopment ratio of the voids observed within the secondary particles, as determined by cross-sectional observation of the secondary particles, is 0.80 or greater. 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 contains at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Al, Si, Fe, Cr, Mo, W, V, Nb, Ta, Sb, and Bi. Aspect 3: The positive electrode active material for a non-aqueous electrolyte secondary battery according to Aspect 1 or 2, wherein the lithium transition metal composite oxide contains Sb. Configuration 4: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the lithium transition metal composite oxide has a median nearest neighbor gap distance of 50 nm or more and 330 nm or less between voids observed inside the secondary particles, as determined by cross-sectional observation of the secondary particles. Configuration 5: The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the lithium transition metal composite oxide contains at least one element selected from the group consisting of Ni, Co, Mn, and Al. Configuration 6: The lithium transition metal composite oxide has a general formula Li ｘ Ni ａ Mn ｂ M1 ｃ M2 ｄ O ２(wherein 0.80≦x≦1.20, 0.50≦a≦0.95, 0≦b≦0.50, 0≦c≦0.20, 0≦d≦0.10, M1 is at least one element of Co and Al, and M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Si, Fe, Cr, Mo, W, V, Nb, Ta, Sb, and Bi). Configuration 7: A nonaqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6.

[0115] REFERENCE SIGNS LIST 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 exterior body, 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, comprising a lithium transition metal composite oxide consisting of secondary particles formed by aggregation of primary particles, wherein the lithium transition metal composite oxide has a plurality of voids therein, and the median area envelopment ratio of the voids observed inside the secondary particles, as determined by cross-sectional observation of the secondary particles, is 0.80 or greater.

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

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide contains Sb.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide has a median nearest neighbor gap distance of voids observed inside the secondary particles, determined by cross-sectional observation of the secondary particles, of 50 nm or more and 330 nm or less.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide contains at least one element selected from the group consisting of Ni, Co, Mn and Al.

6. The lithium transition metal composite oxide has the general formula Li ｘ Ni ａ Mn ｂ M1 ｃ M2 ｄ O ２ (wherein 0.80≦x≦1.20, 0.50≦a≦0.95, 0≦b≦0.50, 0≦c≦0.20, 0≦d≦0.10, M1 is at least one element of Co and Al, and M2 is at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Si, Fe, Cr, Mo, W, V, Nb, Ta, Sb, and Bi).

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

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