Non-aqueous electrolyte secondary battery and method for manufacturing the same

Abstract

A non-aqueous electrolyte secondary battery includes: a positive electrode containing a positive electrode active material capable of inserting and extracting lithium ions; a negative electrode containing a negative electrode active material capable of inserting and extracting lithium ions; a non-aqueous electrolyte containing lithium ions; and an outer package, wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula: LiNi<subgt;x< / subgt;Co<subgt;y< / subgt;Mn<subgt;z< / subgt;O<subgt;2< / subgt>, provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and the battery is formed by using a non-aqueous electrolyte containing methylene methanedisulfonate, and the content of methylene methanedisulfonate is 2.0% by mass or more and 5.0% by mass or less relative to the solvent.

Description

[0001] This application is a divisional application of a Chinese patent application with an international filing date of February 23, 2017, an international application number of PCT / JP2017 / 006784, a national stage application number in China of 201780015613.4, and an invention title of "Non-aqueous electrolyte secondary battery and method for producing the same". Technical Field

[0002] The present invention relates to a non-aqueous electrolyte secondary battery and a method for producing the same. Background Art

[0003] As a non-aqueous electrolyte secondary battery, a lithium ion secondary battery has a high energy density and excellent charge / discharge cycle characteristics, and is therefore widely used as a power source for compact mobile devices such as mobile phones and laptop computers. In addition, recently increasing environmental considerations and growing energy conservation awareness have promoted the demand for large batteries with large capacity and long life in the fields of electric vehicles, hybrid electric vehicles, power storage, and the like.

[0004] Generally, a lithium ion secondary battery mainly includes the following components: a negative electrode containing a negative electrode active material of a carbon material capable of intercalating and deintercalating lithium ions; a positive electrode containing a positive electrode active material of a lithium composite oxide capable of intercalating and deintercalating lithium ions; a separator separating the negative electrode and the positive electrode; and a non-aqueous electrolyte prepared by dissolving a lithium salt in a non-aqueous solvent.

[0005] In order to improve the characteristics of such a lithium ion secondary battery, various studies have been conducted.

[0006] Patent Document 1 describes a non-aqueous electrolyte secondary battery in which the non-aqueous electrolyte contains a specific fluorinated cyclic carbonate, and the upper limit of the operating voltage of the positive electrode is 4.75 V or more and 4.90 V or less with respect to Li / Li + , and describes that a non-aqueous electrolyte secondary battery can be provided, which is designed for high voltage, has excellent cycle durability at high temperature, and has a high energy density. This document describes that the positive electrode active material in the battery is a specific lithium-transition metal compound (LiNi 0.5 Mn 1.5 O4 in Example 1) and graphite particles for the negative electrode active material (carbonaceous material based on natural graphite in the example). This document discloses that in addition to the fluorinated cyclic carbonate, various cyclic sulfonic esters such as 1,3-propane sultone and methanedisulfonic acid methylene ester can be blended in the electrolyte.

[0007] Amorphous carbon or graphite is used as the carbon material used as the negative electrode active material in a lithium ion secondary battery, and particularly, graphite is typically used in applications requiring high energy density.

[0008] For example, Patent Document 2 describes a carbon material for a negative electrode of a non-aqueous electrolyte secondary battery, which contains a mixture of artificial graphite particles and natural graphite particles in a ratio of 50:50 to 80:20 (by mass), wherein the artificial graphite particles have an interplanar spacing d of the (002) crystal plane in the X-ray diffraction pattern of 0.3354 to 0.3360 nm 002 , and an average aspect ratio of 1 to 5; the natural graphite particles have an interplanar spacing d of the (002) crystal plane in the X-ray diffraction pattern of 0.3354 to 0.3357 nm 002 , a median diameter (D 50 ) of 10 to 25 μm, and the relationship between D 50 , the diameter (D 10 ) at 10% cumulative and the diameter (D 90 ) at 90% cumulative. Specifically, D 90 / D 50 and D 50 / D 10 are each 1.6 or less. This document states that the object of the invention is to provide a non-aqueous electrolyte secondary battery having excellent charge / load characteristics in a low-temperature environment by using such a carbon material.

[0009] Patent Document 3 describes a negative electrode for a non-aqueous electrolyte secondary battery, which contains a first carbon capable of electrochemically inserting and extracting lithium ions; and a second carbon capable of electrochemically inserting and extracting lithium ions or substantially incapable of inserting lithium ions, wherein the aggregate of the second carbon particles is mainly located in the gaps between a plurality of first carbon particles, and the average particle size of the second carbon is 15% or less of the average particle size of the first carbon. This document states that the object of the invention is to use such a negative electrode to provide a non-aqueous electrolyte secondary battery in which peeling of the mixture layer caused by charge / discharge cycles can be prevented and which provides a high capacity.

[0010] Patent Document 4 describes a negative electrode material for a non-aqueous electrolyte secondary battery, which contains graphite particles (A) and a carbon material (B), wherein the graphite material (A) has an interplanar spacing (d002) of the 002 crystal plane measured by wide-angle X-ray diffraction method of or less, and an average roundness of 0.9 or more; the carbon material (B) has an interplanar spacing (d002) of the 002 crystal plane of or less, and a Raman R value of 0.18 to 0.7 in the Raman spectrum using an argon ion laser (peak intensity near 1360 cm -1 / peak intensity near 1580 cm -1the peak intensity in the vicinity thereof), an aspect ratio of 4 or more, and an average particle diameter (d50) of 2 to 12 μm; and the mass fraction of the carbon material (B) relative to the total amount of the graphite particles (A) and the carbon material (B) is 0.5 to 15% by mass. The document states that a non-aqueous electrolyte secondary battery using such a negative electrode material exhibits a low irreversible capacity and excellent performance in terms of charge / discharge efficiency.

[0011] Citation List

[0012] Patent Document

[0013] Patent Document 1: JP2014-86221A

[0014] Patent Document 2: JP2009-026514A

[0015] Patent Document 3: JP2012-014838A

[0016] Patent Document 4: JP2012-084519A Summary of the Invention

[0017] Technical Problem

[0018] An object of the present invention is to provide a non-aqueous electrolyte secondary battery having satisfactory cycle characteristics.

[0019] Solution to the Problem

[0020] According to an aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery including: a positive electrode including a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode including a negative electrode active material capable of intercalating and deintercalating lithium ions; a non-aqueous electrolyte containing lithium ions; and an outer package,

[0021] wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula:

[0022] LiNi x Co y Mn z O2

[0023] provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1 are satisfied, and

[0024] the battery is formed by using a non-aqueous electrolyte containing methylene methanedisulfonate and the content of the methylene methanedisulfonate is 2.0% by mass or more and 5.0% by mass or less relative to the solvent.

[0025] According to another aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery, which includes: a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium ions; a non-aqueous electrolyte containing lithium ions; and an outer package,

[0026] wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula:

[0027] LiNi x Co y Mn z O2

[0028] provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and

[0029] the content of methylene methanesulfonate in the non-aqueous electrolyte is not more than 5.0% by mass relative to the solvent.

[0030] According to another aspect of the present invention, there is provided a method for manufacturing a non-aqueous electrolyte secondary battery, which includes: a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium ions; a non-aqueous electrolyte containing lithium ions; and an outer package, and the method includes:

[0031] forming a positive electrode;

[0032] forming a negative electrode;

[0033] forming a non-aqueous electrolyte; and

[0034] placing the positive electrode, the negative electrode and the non-aqueous electrolyte into the outer package,

[0035] wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula:

[0036] LiNi x Co y Mn z O2

[0037] provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and

[0038] the non-aqueous electrolyte contains methylene methanesulfonate, and the content of methylene methanesulfonate is 2.0% by mass or more and 5.0% by mass or less relative to the solvent.

[0039] Advantages of the Invention

[0040] According to an exemplary embodiment, a non-aqueous electrolyte secondary battery having satisfactory cycle characteristics can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Figure 1 is a cross-sectional view illustrating an example of a non-aqueous electrolyte secondary battery according to one exemplary embodiment.

[0042] Figure 2A Figure 2A is a schematic view illustrating the distribution of particles in the negative electrode of a secondary battery according to the related art (the active material is in a contracted state due to discharge).

[0043] Figure 2B Figure 2B is a schematic view illustrating the distribution of particles in the negative electrode of a secondary battery according to one exemplary embodiment (the active material is in a contracted state due to discharge). DETAILED DESCRIPTION

[0044] A non-aqueous electrolyte secondary battery (lithium ion secondary battery) according to an exemplary embodiment includes: a positive electrode containing a specific lithium-containing composite oxide as a positive electrode active material; a negative electrode containing a negative electrode active material; and a non-aqueous electrolyte, and the positive electrode, the negative electrode, and the non-aqueous electrolyte are housed in an outer package. A separator may be provided between the positive electrode and the negative electrode. A plurality of electrode pairs of the positive electrode and the negative electrode may be provided.

[0045] In consideration of achieving a high energy density, it is preferable to use a lithium-containing composite oxide containing nickel (lithium-nickel composite oxide) as the positive electrode active material. However, if a lithium-containing composite oxide having a high nickel content is used, the charge transfer resistance tends to increase in a region of a high state of charge (state of charge, SOC). Therefore, a non-aqueous electrolyte secondary battery using such a lithium-containing composite oxide cannot achieve sufficient cycle characteristics.

[0046] The inventors of the present invention have focused on this phenomenon and the composition of the electrolyte and have conducted intensive research. As a result, the inventors of the present invention have found that even when a lithium-containing composite oxide having a high nickel content is used as the positive electrode active material, a non-aqueous electrolyte secondary battery having improved cycle characteristics can be obtained, and thus the present invention has been completed.

[0047] Specifically, the main feature of the exemplary embodiment is to use a positive electrode active material containing a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula:

[0048] LiNixCo y Mn​​​​​​z O2

[0049] The conditions are that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1,

[0050] and a non-aqueous electrolyte solution containing 2.0 mass% or more and 5.0 mass% or less (0.13 to 0.31 mol / L) of methylene methane disulfonate relative to the solvent is used to form the battery.

[0051] When the content of methylene methane disulfonate is 2.0 mass% or more, sufficient improvement effects are provided, and the content is more preferably 2.3 mass% or more, and even more preferably 3.0 mass% or more. Considering preventing an increase in the viscosity or resistance of the electrolyte solution, the content is preferably 5.0 mass% or less, and more preferably 4.0 mass% or less.

[0052] The lithium-containing composite oxide with a high nickel content (represented by the above compositional formula) undergoes a phase change in the region of high SOC (such a phase change is not observed for other lithium-containing composite oxides below 4.3 V). It is considered that this phase change causes an increase in charge transfer resistance and deterioration of cycling characteristics (for example, see Comparative Examples 5 and 6 in Table 3). The swelling and shrinkage associated with the phase change cause frequent formation of new surfaces by generating cracks in the surface of the active material. Methylene methane disulfonate has high reactivity with the new surface and reacts with the new surface to form a satisfactory coating film. It is inferred that the formed coating film inhibits cracking of the active agent, decomposition of the solvent, and elution of alkaline components, thereby preventing an increase in charge transfer resistance and leading to an improvement in cycling characteristics. This improvement effect is particularly high when using methylene methane disulfonate (MMDS), and propiolactone as another sulfur-containing additive does not provide sufficient improvement effects (for example, see Example 1 and Comparative Example 8 in Table 3).

[0053] One exemplary embodiment of the present invention may be presented as follows: A non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode containing a negative electrode active material capable of intercalating and deintercalating lithium ions; a non-aqueous electrolyte solution containing lithium ions; and an outer package, wherein

[0054] the positive electrode active material contains a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula:

[0055] LiNi x Co y Mn z O2

[0056] The conditions are that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and

[0057] the content of methylene methanedisulfonate in the non-aqueous electrolyte is not more than 5.0% by mass relative to the solvent.

[0058] Although methylene methanedisulfonate in the non-aqueous electrolyte can react during the preparation of the battery (e.g., in the aging step) to form a coating film on the surface of the active material, methylene methanedisulfonate can be present in its non-aqueous electrolyte after the preparation of the battery and react during cycling to form a coating film. From this perspective, after the preparation of the battery (e.g., after the aging step), the content of methylene methanedisulfonate in the non-aqueous electrolyte can be 0.01% by mass or more, or even 0.05% by mass or more, and can be 0.1% by mass or more relative to the solvent; and considering preventing an increase in the viscosity or resistance of the electrolyte, the content is preferably 5.0% by mass or less, and more preferably 4.0% by mass or less. When the electrolyte contains methylene methanedisulfonate after the preparation of the battery (e.g., after the aging step), when cracks occur in the surface of the active material and new surfaces appear during the use of the battery (during charge / discharge cycling), the contained methylene methanedisulfonate can form a coating film on the new surfaces, resulting in satisfactory cycling characteristics.

[0059] Considering the achievement of high energy density, the negative electrode in the secondary battery using the positive electrode contains a graphite active material as the negative electrode active material. Considering the improvement of conductivity, the negative electrode preferably further contains a conductive additive.

[0060] Considering the further improvement of cycling characteristics, the negative electrode preferably contains a fine graphite material with an average particle size smaller than the average particle size of the negative electrode active material. The average particle size (median diameter D 50 ) of the fine graphite material is preferably in the range of 1 to 15 μm.

[0061] When an appropriate amount of fine graphite particles with appropriate sizes are added to the particles of the negative electrode, the fine graphite particles participate in the formation and maintenance of the conduction path of the negative electrode active material, thereby preventing the disconnection of the conduction path of the negative electrode active material during charge / discharge cycling, and thus tend to maintain the conduction path, which can contribute to the improvement of cycling characteristics.

[0062] The ratio Db / Da of the average particle size (D 50 ) Db of the fine graphite material to the average particle size (D 50 ) Da of the negative electrode active material is preferably in the range of 0.2 to 0.7.

[0063] The mass ratio of the fine graphite material to the conductive additive is preferably in the range of 1 to 10. Relative to the negative electrode active material, the content of the fine graphite material is preferably in the range of 0.1 to 6.0% by mass. Relative to the negative electrode active material, the content of the conductive additive is preferably in the range of 0.1 to 3.0% by mass.

[0064] The fine graphite material is preferably flaky particles, and the negative electrode active material is preferably spheroidized graphite particles.

[0065] A non-aqueous electrolyte secondary battery according to an exemplary embodiment may have the following suitable configuration.

[0066] (Positive electrode)

[0067] The positive electrode preferably has a structure including a current collector and a positive electrode active material layer formed on the current collector.

[0068] (Positive electrode active material)

[0069] A lithium-containing composite oxide having a layered rock salt structure is preferably used as the positive electrode active material. The positive electrode active material layer may contain additional active materials other than the lithium composite oxide, and considering the energy density, the content of the lithium-nickel composite oxide is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0070] The BET specific surface area of the positive electrode active material (obtained in the measurement at 77K according to the nitrogen adsorption method) is preferably in the range of 0.1 to 1 m 2 / g, and more preferably in the range of 0.3 to 0.5 m 2 / g. If the specific surface area of the positive electrode active material is too small, cracks may occur due to large particle diameters during pressing in electrode fabrication or during cycling, and significant deterioration of characteristics may occur, making it difficult to prepare an electrode with high density. On the contrary, if the specific surface area is too large, a large amount of conductive additive in contact with the active material is required, which complicates the achievement of high energy density. The positive electrode active material having a specific surface area within the above range provides a positive electrode excellent in terms of energy density and cycle characteristics.

[0071] The average particle diameter of the positive electrode active material is preferably 0.1 to 50 μm, more preferably 1 to 30 μm, and even more preferably 2 to 25 μm. Here, the average particle diameter refers to the particle diameter (median diameter D 50 ) at which 50% of the cumulative value is reached in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method. The positive electrode active material having a specific surface area within the above range and having an average particle diameter within the above range provides a positive electrode excellent in terms of energy density and cycle characteristics.

[0072] To form the positive electrode active material layer, first, a slurry containing a positive electrode active material, a binder, and a solvent (and a conductive additive as needed) is prepared, and the slurry is coated on the positive electrode current collector, dried, and pressed as needed. As the slurry solvent used in fabricating the positive electrode, N-methyl-2-pyrrolidone (NMP) can be used.

[0073] (Binder for positive electrode)

[0074] There is no limitation on the binder for the positive electrode, and binders commonly used for positive electrodes can be used. Among them, polyvinylidene fluoride (PVDF) is preferred in view of versatility and low cost. In addition, examples of binders other than polyvinylidene fluoride (PVdF) include vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer rubber, polytetrafluoroethylene (PTFE), polypropylene, polyethylene, polyimide, and polyamideimide.

[0075] In view of the adhesive strength and energy density in a balanced relationship, the content of the binder for the positive electrode is preferably in the range of 1 to 25% by mass, more preferably in the range of 2 to 20% by mass, and even more preferably in the range of 2 to 10% by mass, relative to the positive electrode active material.

[0076] (Conductive additive for positive electrode)

[0077] For the purpose of reducing impedance, a conductive additive can be added to the positive electrode active material layer. Examples of the conductive additive include carbon black such as acetylene black. The content of the conductive additive in the active material layer can be set in the range of 1 to 10% by mass relative to the positive electrode active material.

[0078] (Positive electrode active material layer)

[0079] The porosity of the positive electrode active material layer (excluding the current collector) constituting the positive electrode is preferably 30% or less, and more preferably 20% or less. Since there is a tendency for a high porosity (in other words, a low electrode density) to be accompanied by a higher contact resistance and a higher charge transfer resistance, it is preferred to set the porosity low as above, and thus the electrode density can be high. On the other hand, an excessively low porosity (an excessively high electrode density) provides a low contact resistance but causes an increase in charge transfer resistance and deterioration of rate characteristics, and thus it is necessary to ensure a certain degree of porosity. From this viewpoint, the porosity is preferably 10% or more, more preferably 12% or more, and can be set to 15% or more.

[0080] The porosity refers to the fraction of the volume excluding the volume occupied by the particles of the active material and the conductive additive in the overall apparent volume of the active material layer (see the following formula). Therefore, the porosity can be determined by calculating using the thickness of the active material layer, the mass of the active material layer per unit area, and the true density of the particles of the active material and the conductive additive.

[0081] Porosity = (Apparent volume of the active material layer - Volume of the particles) / (Apparent volume of the active material layer)

[0082] The "volume of the particles" in this formula (the volume occupied by the particles contained in the active material layer) can be calculated by using the following formula.

[0083] Volume of the particles = (Weight of the active material layer per unit area × Area of the active material layer × Content of the particles) / True density of the particles

[0084] Here, the "area of the active material layer" refers to the area of the plane opposite to the current collector side (separator side).

[0085] There is no limitation on the thickness of the positive electrode active material layer, and it can be appropriately set according to the required characteristics. For example, considering the energy density, the thickness can be set to be large, and considering the output characteristics, the thickness can be set to be small. For example, the thickness of the positive electrode active material layer can be appropriately set in the range of 10 to 250 μm, preferably 20 to 200 μm, and more preferably 40 to 180 μm.

[0086] (Positive current collector)

[0087] For the current collector used for the positive electrode, for example, aluminum, stainless steel, nickel, titanium, or their alloys can be used. Examples of the shape include foil, plate, and mesh. In particular, aluminum foil can be appropriately used.

[0088] (Negative electrode)

[0089] The negative electrode preferably has a structure including a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material layer contains a negative electrode active material and a binder, and preferably contains a conductive additive in consideration of improving the conductivity. Preferably, in consideration of improving the cycle characteristics, the negative electrode active material layer further contains a fine graphite material.

[0090] (Negative electrode active material)

[0091] There is no limitation on the negative electrode active material as long as it is an active material for a negative electrode capable of intercalating and deintercalating lithium ions. However, carbon-based active materials such as graphite materials and amorphous carbon (e.g., graphitizable carbon, non-graphitizable carbon) can be appropriately used. Substances commonly used as negative electrode active materials in lithium ion secondary batteries can be used for carbon-based active materials in preparation. Natural graphite and artificial graphite can be used for graphite materials, and inexpensive natural graphite is preferred in view of material cost. Examples of amorphous carbon include amorphous carbon obtained by heat treatment of coal tar pitch coke, petroleum pitch coke, or acetylene pitch coke.

[0092] In the case where a graphite material, especially natural graphite, is used as the negative electrode active material, the graphite material can be covered with amorphous carbon. The surface of the particles of the graphite material can be covered with amorphous carbon by using a conventional method. Examples of the methods that can be used include a method in which an organic substance such as tar pitch is attached to the surface of the particles and heat treatment is performed; and film formation methods such as chemical vapor deposition (CVD) using condensed hydrocarbons of organic substances such as benzene and xylene, sputtering (e.g., ion beam sputtering) using condensed hydrocarbons of organic substances such as benzene and xylene, vacuum deposition, plasma method, and ion plating method. The amorphous carbon covering the particles of the graphite material can suppress side reactions between the particles of the graphite material and the electrolyte, thereby enhancing the charge / discharge efficiency and increasing the reaction capacity, and additionally making the particles of the graphite material have a higher hardness.

[0093] In view of the charge / discharge efficiency, input / output characteristics, etc., the average particle diameter of the negative electrode active material is preferably in the range of 2 to 40 μm, more preferably in the range of 5 to 30 μm, and even more preferably in the range of 10 to 30 μm. Here, the average particle diameter refers to the particle diameter (median diameter D 50 ) when reaching the cumulative value of 50% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method.

[0094] In view of the charge / discharge efficiency and input / output characteristics, the specific surface area (BET specific surface area measured at 77K according to the nitrogen adsorption method) of the negative electrode active material is preferably in the range of 0.3 to 10 m 2 / g, more preferably in the range of 0.5 to 10 m 2 / g, and even more preferably in the range of 0.5 to 7.0 m 2 / g.

[0095] The ratio D 90 of the particle diameter (D 50 ) of the negative electrode active material at 90 cumulative% in the cumulative distribution to the median diameter (D 90 ) is D 50Preferably, it is 1.5 or less, and more preferably 1.3 or less. The negative electrode active material having a sharp particle size distribution enables the formation of a uniform negative electrode and provides improved charge / discharge characteristics to the resulting secondary battery.

[0096] Here, the particle size D 90 refers to the particle size at the cumulative value of 90% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method, and the median diameter D 50 refers to the particle size at the cumulative value of 50% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method.

[0097] The particles of the negative electrode active material are preferably spherical (non-scaly) particles, and the average particle roundness is preferably in the range of 0.6 to 1, more preferably in the range of 0.86 to 1, even more preferably in the range of 0.90 to 1, and particularly preferably in the range of 0.93 to 1. The spheroidization can be carried out by using a conventional method. Considering the achievement of high capacity in combination with cost reduction of raw materials, such negative electrode active material particles are preferably spherical natural graphite particles, and commercially available spherical natural graphite materials can be used.

[0098] The particle roundness is given as follows: project the particle image onto a plane; and when the perimeter of the corresponding circle having the same area as the projected particle image is represented as 1, and the perimeter of the projected particle image is represented as L, the ratio l / L is defined as the particle roundness.

[0099] The average particle roundness can be measured as follows by using a commercially available electron microscope (for example, a scanning electron microscope manufactured by Hitachi, Ltd., trade name: S-2500). First, observe the image of the particles (powder) at a magnification of 1000× by using the electron microscope, project it onto a plane, and determine the perimeter L of the projected figure; then determine the perimeter l of the corresponding circle having the same area as the projected image of the observed particles; calculate the ratio of the perimeter l to the perimeter L of the projected image of the particles, that is, l / L, for 50 randomly selected particles; and use the average value as the average particle roundness. Alternatively, this measurement can be carried out by using a flow particle image analyzer. For example, even when measuring the particle roundness by using a powder measuring device (trade name: FPIA-1000) available from Hosokawa Micron Corporation, almost the same value is obtained.

[0100] Among them, the configuration in which the negative electrode active material has a high roundness promotes the formation of voids between the particles of the negative electrode active material. As a result, the fine graphite material tends to be arranged in a uniformly dispersed manner, which contributes to the improvement of the cycle characteristics. In addition, the formation of voids between the particles is beneficial to the flow of the electrolyte, thereby contributing to the improvement of the output characteristics. In the case where natural graphite (which has a higher tendency to take a specific orientation by pressing in the preparation of the electrode compared to artificial graphite) is used as the negative electrode active material, the natural graphite takes a random orientation by spheroidization, and thus can contribute to the improvement of the output characteristics.

[0101] The negative electrode active material, the fine graphite material, and the conductive additive can be mixed together by using a known mixing method. If necessary, additional active materials can be mixed therein within a range that does not impair the desired effects.

[0102] In the case where a graphite material is used as the negative electrode active material, the content of the graphite material is preferably 90% by mass or more, and more preferably 95% by mass or more, based on the total amount of the negative electrode active material (excluding the fine graphite material). The negative electrode active material can consist only of the graphite material.

[0103] (Fine graphite material)

[0104] The negative electrode in the non-aqueous electrolyte secondary battery according to an exemplary embodiment preferably includes a negative electrode active material, a fine graphite material, a conductive additive, and a binder. The fine graphite material includes particles in contact with the particles of the negative electrode active material, or particles in contact with the particles of the conductive additive, and the particles of the conductive additive are in contact with the particles of the negative electrode active material, and a conduction path can be formed between the particles of the negative electrode active material via the particles of the fine graphite material (hereinafter also referred to as "fine graphite particles").

[0105] The average particle diameter (median diameter D 50 ) of the fine graphite material in the negative electrode is preferably smaller than the average particle diameter (median diameter D 50 ) of the negative electrode active material, and is also preferably in the range of 1 to 15 μm. The mass ratio of the fine graphite material to the conductive additive in the negative electrode is preferably in the range of 1 to 10.

[0106] The use of such a negative electrode provides a non-aqueous electrolyte secondary battery (lithium ion secondary battery) with improved cycle characteristics. This is presumably because adding an appropriate amount of fine graphite particles with an appropriate size to the particles of the negative electrode active material enables the fine graphite particles to participate in the formation and maintenance of the conduction path between the particles of the negative electrode active material, thereby preventing the disconnection of the conduction path between the particles of the negative electrode active material during the charge / discharge cycle, and thus tending to maintain the conduction path.

[0107] To ensure the conductive path during charge / discharge cycles, a large amount of conductive aids are accordingly required. The amount of conductive aids can be reduced by adding fine graphite materials. As a result, generation of gas derived from electrolyte decomposition caused by conductive aids (particularly, conductive aids having a large specific surface area or functional groups on the surface) can be suppressed, and reduction in peel strength and capacity caused by addition of a large amount of conductive aids can also be prevented. In addition, the fine graphite materials have capacity, whereby a decrease in capacity due to addition can be reduced. Furthermore, the fine graphite materials have excellent conductivity, whereby a conductive path with low resistance can be formed, contributing to improvement in cycle characteristics.

[0108] Conductive aids such as carbon black and Ketjen black (particularly, conductive aids having a primary particle diameter of about several tens of nanometers) have high aggregability. Thus, it is difficult to uniformly disperse the conductive aids in the interparticle space of the negative electrode active material, and non-uniformity may occur in the network of the conductive path. The conductive path formed via such fine conductive aid particles has effective conductivity in the initial stage of the cycle. However, as the charge / discharge cycles are repeated, disconnection of the conductive path related to, for example, volume change (swelling, shrinkage) of the negative electrode active material may occur, and a sharp increase in resistance or a decrease in capacity may be caused. In addition, the fine particles of the conductive aid can fill the interparticle gaps of the negative electrode active material, thereby disconnecting the flow path of the electrolyte. On the other hand, the fine graphite particles have a larger particle diameter. Therefore, their dispersibility is excellent, non-uniformity of the network of the conductive path can be reduced, and filling of the interparticle gaps of the negative electrode active material can also be prevented. As a result, disconnection of the conductive path or the flow path of the electrolyte is less likely to occur during charge / discharge cycles, whereby an increase in resistance or deterioration of capacity can be reduced.

[0109] In addition, an SEI film is formed on each of the fine graphite particles constituting the conductive path, and the SEI film formed on the uniformly dispersed fine graphite particles is also presumably available as a migration path for lithium ions, contributing to improvement in performance.

[0110] In the formation of the conductive path between the particles of the negative electrode active material, the fine graphite particles in contact with the particles of the negative electrode active material can directly contact another particle of the negative electrode active material, or a conductive path electrically connected to another particle of the negative electrode active material via conductive particles (e.g., conductive aid particles or another fine graphite particle) contained in the negative electrode can be formed. For example, the fine graphite particles in contact with the particles of the negative electrode active material can contact particles of a conductive aid (primary particles or secondary particles), and the particles of the conductive aid are in contact with another particle of the negative electrode active material. The fine graphite particles in contact with the particles of the negative electrode active material can contact another fine graphite particle, and the another fine graphite particle is in contact with another particle of the negative electrode active material.

[0111] In the formation of a conduction path between particles of the negative electrode active material, fine graphite particles in contact with particles of a conductive additive (primary particles or secondary particles, where the particles of the conductive additive are in contact with the particles of the negative electrode active material) can directly contact another particle of the negative electrode active material, or can form a conduction path that is electrically connected to another particle of the negative electrode active material via particles of a conductive material contained in the negative electrode (e.g., conductive additive particles or another fine graphite particle). For example, fine graphite particles in contact with particles of a conductive additive (primary particles or secondary particles, where the particles of the conductive additive are in contact with the particles of the negative electrode active material) can contact another particle of the conductive additive (primary particles or secondary particles), where the other particle of the conductive additive is in contact with another particle of the negative electrode active material. Fine graphite particles in contact with particles of a conductive additive (primary particles or secondary particles, where the particles of the conductive additive are in contact with the particles of the negative electrode active material) can contact another fine graphite particle, where the other fine graphite particle is in contact with another particle of the negative electrode active material.

[0112] Figure 2A and 2B are schematic diagrams for exemplifying the distribution state of particles in the negative electrode during discharge (when the active material is in a contracted state) after repeated charge / discharge cycles. Figure 2A shows the case without a fine graphite material, and Figure 2B shows the case of using a fine graphite material. In these figures, reference numeral 11 represents negative electrode active material particles, reference numeral 12 represents conductive additive particles, and reference numeral 13 represents fine graphite particles. In Figure 2A after the charge / discharge cycle, during discharge, the conduction path is disconnected due to the contraction of the active material. In contrast, Figure 2B shows a conduction path maintained along the arrow via fine graphite particles 13. Fine graphite particles in direct contact with negative electrode active material particles in the figure can contact conductive additive particles or their secondary particles that are in contact with the active material particles.

[0113] For the fine graphite material, graphite materials such as artificial graphite and natural graphite can be used. Substances commonly used as negative electrode active materials in lithium-ion secondary batteries can be used in the preparation of the graphite material.

[0114] Considering that artificial graphite contains fewer impurities, has an appropriate degree of graphitization, and has a low resistance, which is beneficial for improving battery performance such as cycle characteristics, the fine graphite material is preferably artificial graphite. Conventional artificial graphite that is commonly available can be applied.

[0115] The physical properties of artificial graphite depend on the type of raw material, the calcination temperature during preparation, the type of gas used for the atmosphere, and the pressure, and the required fine graphite material can be obtained by adjusting these preparation conditions. Examples thereof include: artificial graphite obtained by subjecting graphitizable carbon for graphitization such as coke (e.g., petroleum coke, coal coke) and pitch (e.g., coal pitch, petroleum pitch, coal tar pitch) to high-temperature heat treatment at a temperature of 2000 to 3000 °C, preferably at a high temperature of 2500 °C or higher; artificial graphite obtained by graphitizing two or more types of graphitizable carbon.

[0116] Alternatively, a material covered with amorphous carbon can be used, which is prepared by pyrolyzing hydrocarbons such as benzene and xylene and depositing them on the surface of a substrate containing natural graphite or artificial graphite by using the CVD method (chemical vapor deposition method).

[0117] The mass ratio of the fine graphite material to the conductive additive can be set within the range of 1 to 10. Considering obtaining a sufficient addition effect, the mass ratio of the fine graphite material to the conductive additive is preferably 1 or more, more preferably 1.5 or more, and even more preferably 2 or more. Considering preventing gas generation or preventing a decrease in peel strength, the mass ratio is preferably 10 or less, more preferably 8 or less, and even more preferably 7 or less.

[0118] The content of the fine graphite material relative to the negative electrode active material is preferably within the range of 0.1 to 6.0% by mass. Considering obtaining a sufficient addition effect, the content of the fine graphite material relative to the negative electrode active material is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.6% by mass or more. Considering preventing gas generation or preventing a decrease in peel strength, the content is preferably 6.0% by mass or less, more preferably 4.0% by mass or less, and even more preferably 3.0% by mass or less. The "content of the fine graphite material relative to the negative electrode active material" (% by mass) can be determined by 100×A / B, where A represents the mass of the fine graphite material and B represents the mass of the negative electrode active material.

[0119] The average particle size (median diameter D 50 ) of the fine graphite material is preferably smaller than the average particle size (median diameter D 50 ) of the negative electrode active material, and is more preferably within the range of 1 to 15 μm.

[0120] An arrangement in which the fine graphite material has a moderately small median particle size provides an increased number of particles per unit weight, and thus forms a large number of contact points even with a small addition, which provides an advantageous effect for the formation of a conductive path. Further, an arrangement in which the particles of the fine graphite material are smaller than the particles of the negative electrode active material is advantageous for the deposition of the particles of the fine graphite material in the inter-particle space or gap of the particles of the negative electrode active material, which provides an advantageous effect for the formation of a conductive path. Further, the influence on the peel strength can be reduced.

[0121] From such a viewpoint, the average particle size (D 50 ) of the fine graphite material is preferably 15 μm or less, and more preferably 10 μm or less. The average particle size (D 50 ) of the fine graphite material is preferably smaller than the average particle size (D 50 ) of the negative electrode active material, and more preferably, the ratio Db / Da of the average particle size (D 50 ) Db of the fine graphite material to the average particle size (D 50 ) Da of the negative electrode active material is 0.7 or less, and even more preferably 0.67 or less.

[0122] On the other hand, if the particle size of the fine graphite material is too small, the specific surface area is large, which easily leads to the generation of gas derived from the decomposition of the electrolyte, and the conductive path may be disconnected during the charge / discharge cycle. For these reasons, the average particle size (D 50 ) of the fine graphite material is preferably 1 μm or more, and more preferably 4 μm or more, and the BET specific surface area (obtained by measurement at 77K according to the nitrogen adsorption method) of the fine graphite material is preferably 45 m 2 / g or less, and more preferably 20 m 2 / g or less, and Db / Da is preferably 0.2 or more, and more preferably 0.3 or more. Considering the sufficient formation of contact points, the BET specific surface area of the fine graphite material is preferably greater than 1 m 2 / g, and more preferably 4 m 2 / g or more.

[0123] In the case where the conductive particles are granular, the particle size of the fine graphite material is preferably larger than the particle size of the conductive aid. In the case where the conductive particles are fibrous, the particle size of the fine graphite material is preferably larger than the average diameter of the conductive aid. The presence of the fine graphite material having a larger size enables the retention of the conductive path, even in a case where the conductive path formed by the fine conductive aid is disconnected due to the shrinkage of the negative electrode active material during discharge and the expansion of the inter-particle gap of the negative electrode active material as a result of the charge / discharge cycle.

[0124] The particle size (D 90 ) of the fine graphite material at 90% cumulative in the cumulative distribution and the average particle size (D50 ) ratio D 90 / D 50 Preferably greater than 1.5, and more preferably 1.65 or more. Adding fine graphite material with a smaller particle size and a wide particle size distribution to the negative electrode active material with a sharper particle size distribution can improve the packing factor and provide a mixture with a high density.

[0125] Here, D 90 refers to the particle diameter when reaching the cumulative value of 90% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method, and D 50 refers to the particle diameter (median diameter) when reaching the cumulative value of 50% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method.

[0126] The average particle roundness of the particles of the fine graphite material is preferably lower than the average particle roundness of the particles of the negative electrode active material, and this average particle roundness is preferably lower than 0.86, more preferably 0.85 or less, and even more preferably 0.80 or less. For example, graphite particles with an average particle roundness of 0.5 or more and less than 0.86, or graphite particles with an average particle roundness in the range of 0.6 to 0.85 can be used. For example, flaky particles can be appropriately used.

[0127] Using spheroidized particles (non-flaky particles) for the particles of the negative electrode active material and using particles with a roundness lower than that of the negative electrode active material particles (e.g., flaky particles) for the particles of the fine graphite material (preferably, controlling the mixing ratio, particle size distribution, etc. as described above) enables the fine graphite particles to fill the inter-particle space of the negative electrode active material in a uniformly dispersed manner, and the negative electrode active material particles and the fine graphite particles can be packed with a high density. Therefore, a sufficient number of contact points are formed between the particles, while the electrolyte penetrates sufficiently to prevent the disconnection of the conduction path, thereby suppressing the increase in resistance during cycling, and the capacity is unlikely to decrease.

[0128] (Conductive aid for negative electrode)

[0129] For the conductive aid, carbon materials commonly used as conductive aids for lithium-ion secondary batteries can be used, and examples thereof include conductive amorphous carbons such as Ketjen black, acetylene black, and carbon black; and conductive nanocarbon materials such as carbon nanofibers and carbon nanotubes. For the conductive aid, amorphous carbon with high conductivity and low graphitization degree can be used (e.g., amorphous carbon with an R value (I D / I G ) of 0.18 or more and 0.7 or less). I D is the peak intensity of the D band near 1300 to 1400 cm -1 in the Raman spectrum, and I Gis the peak intensity of the G band around 1500 to 1600 cm -1 in Raman spectroscopy.

[0130] The content of the conductive additive is preferably in the range of 0.1 to 3.0% by mass with respect to the negative electrode active material. Considering the sufficient formation of the conduction path, the content of the conductive additive is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and even more preferably 0.3% by mass or more with respect to the negative electrode active material. And considering the prevention of the generation of gas due to the decomposition of the electrolytic solution caused by the excessive addition of the conductive additive, or the prevention of the reduction of the peel strength or the decrease in the capacity, the content is preferably 3.0% by mass or less, more preferably 1.5% by mass or less, and even more preferably 1.0% by mass or less. The "content of the conductive additive (mass%) with respect to the negative electrode active material" can be determined by 100×A / B, where A represents the mass of the conductive additive and B represents the mass of the negative electrode active material.

[0131] The average particle diameter (primary particle diameter) of the conductive additive is preferably in the range of 10 to 100 nm. Considering the prevention of excessive aggregation of the conductive additive and the uniform dispersion of the conductive additive in the negative electrode, the average particle diameter (primary particle diameter) of the conductive additive is preferably 10 nm or more, and more preferably 30 nm or more. And considering the formation of a sufficient amount of contact points and the formation of a satisfactory conduction path, the average particle diameter is preferably 100 nm or less, and more preferably 80 nm or less. In the case where the conductive additive is fibrous, examples of such a conductive additive include fibrous conductive additives having an average diameter of 2 to 200 nm and an average fiber length of 0.1 to 20 μm.

[0132] Here, the average diameter of the conductive additive is the median diameter (D 50 ), that is, the particle diameter at which the cumulative value reaches 50% in the particle size distribution (based on volume) obtained by using the laser diffraction / scattering method.

[0133] (Method for fabricating the negative electrode)

[0134] For the negative electrode of a lithium-ion secondary battery according to an exemplary embodiment, for example, a negative electrode in which a negative electrode active material layer including the above-mentioned negative electrode active material, fine graphite material, conductive additive, and further including a binder is provided on a negative electrode current collector can be used.

[0135] The negative electrode can be formed by using a common slurry coating method. For example, a slurry containing a negative electrode active material, a fine graphite material, a binder, and a solvent is prepared, and the slurry is coated on a negative electrode current collector, dried, and pressed as needed to obtain a negative electrode in which a negative electrode active material layer is provided on the negative electrode current collector. Examples of the method for coating the negative electrode slurry include a doctor blade method, a die coater method, and a dip coating method. Alternatively, the negative electrode can be obtained by forming a thin film of aluminum, nickel, or an alloy thereof on a negative electrode active material layer that has been previously formed by a vapor deposition method, a sputtering method, etc.

[0136] (Binder for negative electrode)

[0137] There is no limitation on the binder for the negative electrode, and examples thereof include polyvinylidene fluoride (PVdF), vinylidene fluoride - hexafluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene copolymer, styrene - butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide, (meth)acrylic acid methyl ester, (meth)acrylic acid ethyl ester, (meth)acrylic acid butyl ester, (meth)acrylonitrile, isoprene rubber, butadiene rubber, and fluororubber. For the slurry solvent, N - methyl - 2 - pyrrolidone (NMP) or water can be used. In the case where water is used as the solvent, a thickener such as carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, and polyvinyl alcohol can be further used.

[0138] Considering the adhesive strength and energy density in a balanced relationship, the content of the binder for the negative electrode is preferably in the range of 0.5 to 30% by mass, more preferably in the range of 0.5 to 25% by mass, and even more preferably in the range of 1 to 20% by mass, relative to the negative electrode active material.

[0139] (Negative electrode current collector)

[0140] There is no limitation on the negative electrode current collector, but considering electrochemical stability, preferably, copper, nickel, stainless steel, titanium, molybdenum, tungsten, tantalum, or an alloy containing two or more of them can be used. Examples of the shape include foil, plate, and mesh.

[0141] (Non - aqueous electrolyte)

[0142] For the non - aqueous electrolyte, a non - aqueous electrolyte in which a lithium salt is dissolved in one or more non - aqueous solvents can be used.

[0143] There is no restriction on the non-aqueous solvent, and examples thereof include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC); chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC); aliphatic carboxylic acid esters such as methyl formate, methyl acetate, and ethyl propionate; γ-lactones such as γ-butyrolactone; chain ethers such as 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME); and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. One of these non-aqueous solvents can be used alone, or two or more thereof can be used as a mixture.

[0144] There is no restriction on the lithium salt dissolved in the non-aqueous solvent, and examples thereof include LiPF6, LiAsF6, LiAlCl4, LiClO4, LiBF4, LiSbF6, LiCF3SO3, LiCF3CO2, Li(CF3SO2)2, LiN(CF3SO2)2, and lithium bis(oxalato)borate. One of these lithium salts can be used alone, or two or more thereof can be used in combination. In addition, a polymer component can be contained as the non-aqueous electrolyte. The concentration of the lithium salt can be set in the range of 0.8 to 1.2 mol / L, and preferably 0.9 to 1.1 mol / L.

[0145] In order to form a high-quality SEI (solid electrolyte interface) film on the surface of the negative electrode and stably maintain it, an electrode protective film-forming agent as an additive can be added to the electrolyte. The SEI film has, for example, the effect of suppressing the reactivity (decomposition) of the electrolyte and the effect of suppressing the physical deterioration of the structure of the negative electrode active material by promoting desolvation related to the insertion and elimination of lithium ions. Examples of the electrode protective film-forming agent for forming and maintaining such a high-quality SEI film include: compounds having a sulfo group; fluorinated carbonates such as fluoroethylene carbonate; unsaturated cyclic carbonates such as vinylene carbonate; sultone compounds (cyclic monosulfonates) such as 1,3-propane sultone and 1,4-butane sultone; and cyclic disulfonates such as methylene methanedisulfonate, ethylene methanedisulfonate, and propylene methanedisulfonate. When the electrolyte contains an electrode protective film-forming agent as an additive, considering obtaining a sufficient addition effect, the content (mass ratio to the solvent) of the additive in the electrolyte is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, and even more preferably 0.1% by mass or more, and considering reducing the increase in, for example, the viscosity or resistance of the electrolyte, the content is preferably 10% by mass or less, and more preferably 5% by mass or less. Since methylene methanedisulfonate added to reduce the charge transfer resistance of the positive electrode and improve the cycle characteristics of the battery can form an SEI film on the surface of the negative electrode, the total amount of methylene methanedisulfonate and other electrode protective film-forming agents is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less.

[0146] (Separator)

[0147] A separator can be provided between the positive electrode and the negative electrode. For the separator, a porous film, a textile, or a non-woven fabric made of polyolefins such as polypropylene and polyethylene, fluororesins such as polyvinylidene fluoride, or polyimide can be used.

[0148] (Shape and Structure of the Battery)

[0149] Examples of the shape of the battery include cylindrical, rectangular, coin-type, button-type, and laminated-type. In the case of the laminated-type, a laminated film is preferably used for the outer package to accommodate the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte. This laminated film includes a resin substrate, a metal foil layer, and a heat-sealing layer (sealant). Examples of the resin substrate include polyester and nylon, and examples of the metal foil layer include aluminum foil, aluminum alloy foil, and titanium foil. Examples of the material for the heat-sealing layer include thermoplastic polymer materials such as polyethylene, polypropylene, and polyethylene terephthalate. Each of the resin substrate layer and the metal foil layer is not limited to a single-layer configuration and can be two or more layers. From the viewpoints of versatility and cost, an aluminum laminated film is preferred.

[0150] The positive electrode, negative electrode, and a separator disposed therebetween are accommodated in an outer packaging container made of a laminated film or the like, and an electrolytic solution is injected therein, and then the outer packaging container is sealed. A structure in which an electrode group having a plurality of laminated electrode pairs is accommodated therein may be employed.

[0151] For an apparatus for forming an active material layer on a current collector in the production of a positive electrode and a negative electrode, various apparatuses having a variety of coating methods including a doctor blade, a die coater, a gravure coater, a transfer method, and a vapor deposition method, and combinations of these coating apparatuses can be used. For the precise formation of the edge portion coated with the active material, the use of a die coater is particularly preferred. The method for coating the active material using a die coater is roughly divided into two types. Specifically, a continuous coating method for continuously forming the active material along the longitudinal direction of a long current collector, and an intermittent coating method for alternately and repeatedly forming portions coated with the active material and uncoated portions along the longitudinal direction of the current collector. These methods can be appropriately selected.

[0152] Figure 1 A cross-sectional view of an example (laminated type) of a non-aqueous electrolyte secondary battery (lithium ion secondary battery) according to an exemplary embodiment is illustrated. As Figure 1 illustrated, the lithium ion secondary battery of this embodiment includes: a positive electrode including a positive electrode current collector 3 made of a metal such as aluminum foil and a positive electrode active material layer 1 provided thereon and containing a positive electrode active material; and a negative electrode including a negative electrode current collector 4 made of a metal such as copper foil and a negative electrode active material layer 2 provided thereon and containing a negative electrode active material. The positive electrode and the negative electrode are laminated with a separator made of a non-woven fabric or a polypropylene microporous membrane interposed therebetween such that the positive electrode active material layer 1 and the negative electrode active material layer 2 are located on opposite surfaces of the separator 5. This electrode pair is accommodated in a container formed by outer packages 6, 7 made of an aluminum laminated film. The positive electrode current collector 3 is connected to a positive electrode tab 9, and the negative electrode current collector 4 is connected to a negative electrode tab 8, and these tabs are led out to the outside through the container. An electrolytic solution is injected into the container, and then the container is sealed. Alternatively, a structure in which an electrode group having a plurality of laminated electrode pairs is accommodated in the container can be used.

[0153] (Method for preparing a battery)

[0154] A battery having the above structure can be formed as follows.

[0155] First, the positive electrode and the negative electrode laminated together with the separator interposed therebetween are placed in a container, then an electrolytic solution is injected therein, and then vacuum impregnation is performed. In order to impregnate more fully with the electrolytic solution, the solvent may be allowed to stand for a certain period of time or pressurized before applying vacuum.

[0156] After vacuum impregnation, the unfused opening of the outer package is fused in vacuo for temporary sealing.

[0157] Pressurization is preferably carried out after temporary sealing. This pressurization can accelerate the infiltration of the electrolyte. Pressurization can be achieved by applying pressure from the outside of the container with the battery sandwiched between a pair of pressure plates. The battery is preferably left standing in a pressurized state (e.g., 2 to 6 N·m) for a specified time (e.g., more than 10 hours, preferably more than 18 hours, and preferably less than 30 hours in consideration of higher process efficiency).

[0158] Subsequently, pre-charging is carried out in a temporarily sealed state. Charging and discharging can be repeated a specified number of cycles. The pre-charged state is preferably maintained for a specified time (about 10 to 60 minutes). There is no limitation on the pressure during pre-charging. If pressurization has been carried out before pre-charging, the pressure can be set lower than the pressure during pressurization (e.g., a pressure of about 0.2 to 0.6 N·m).

[0159] Next, the temporarily sealed part is opened for degassing. After that, vacuum impregnation, temporary sealing, and pre-charging are carried out again as needed.

[0160] Subsequently, main sealing is carried out. After that, the surface of the container can be made uniform by rolling.

[0161] Subsequently, the battery is charged and then left standing in a charged state while being warmed (e.g., at 35 to 55 °C, preferably at 40 to 50 °C) for aging for a specified time (e.g., more than 7 days, preferably 7 to 30 days, more preferably 10 to 25 days). During this aging treatment, methylene methanesulfonate added to the electrolyte can form a coating film on the surface of the positive electrode. It is inferred that this formed coating film inhibits the cracking of the active material, the decomposition of the solvent, and the elution of the alkaline component caused by the phase change, resulting in an improvement in the cycle characteristics.

[0162] After that, the battery is discharged and, if necessary, a charge / discharge treatment (RtRc treatment) is carried out, whereby a desired battery can be obtained.

[0163] Examples

[0164] Hereinafter, exemplary embodiments will be further described with reference to the examples.

[0165] (Example 1)

[0166] Spheroidized natural graphite with high roundness (average particle diameter (D 50 ) : 15 μm) is provided as the negative electrode active material, and flaky artificial graphite (average particle diameter (D 50):10 μm) as fine graphite material (hereinafter referred to as "fine graphite"). Provide fine particles (carbon black) with an average particle diameter (D 50 ) of 100 nm or less as a conductive aid.

[0167] As a result of the above measurement method, it was confirmed that the average particle roundness of natural graphite was 0.86 or more, and higher than that of flaky fine graphite. In addition, by using a commercially available laser diffraction / scattering particle size analyzer, it was confirmed that the D 90 / D 50 of the negative electrode active material (natural graphite) was 1.3 or less, and the D 90 / D 50 of fine graphite (flaky artificial graphite) was 1.65 or more.

[0168] The amount of the added fine graphite material relative to the negative electrode active material was 2.0% by mass (mass ratio to the conductive aid: approximately 6.7). The amount of the added conductive aid relative to the negative electrode active material was 0.3% by mass.

[0169] Mix the negative electrode active material (natural graphite), fine graphite material (flaky artificial graphite) and conductive aid in the above mass ratio, and mix the mixture with an aqueous solution of 1.0 wt% carboxymethyl cellulose (thickener) to prepare a slurry. Mix styrene-butadiene copolymer (binder) into it. The amount of the added binder relative to the negative electrode active material was 2.0% by mass.

[0170] Coat this slurry on one surface of a copper foil with a thickness of 10 μm, and dry to form a coating film. Similarly, coat the slurry on the other surface and dry. After that, roll the coating film (negative electrode coating film) so that the density reaches 1.5 g / cm 3 , and process the obtained product into a predetermined shape to obtain a negative electrode sheet with a size of 130×69.0 mm.

[0171] Separately, disperse LiNi 0.8 C o0.1 Mn 0.1 O2 (positive electrode active material) and polyvinylidene fluoride (binder) in N-methyl-2-pyrrolidone to prepare a slurry. Coat this slurry on one surface of an aluminum foil, and dry to form a coating film. Similarly, coat the slurry on the other surface and dry. After that, roll the coating film (positive electrode coating film) so that the density reaches 3.0 g / cm 3 , and process the obtained product into a predetermined shape to obtain a positive electrode sheet with a size of 125×65.5 mm.

[0172] The five positive electrode plates and six negative electrode plates thus prepared are alternately laminated using a separator made of a porous polyethylene film with a thickness of 25 μm inserted between every two adjacent electrode plates. Lead-out electrodes for the positive electrode and lead-out electrodes for the negative electrode are provided, and then the laminate is covered with a laminate film, and the electrolyte is injected into it through an unfused opening.

[0173] The electrolyte used is a solution obtained by dissolving a lithium salt (LiPF6) as an electrolyte salt in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) with a volume ratio of 3:6:1 (EC:DEC:EMC) as a solvent so that the concentration of the lithium salt reaches 1.0 mol / L, and adding methylene methanesulfonate (MMDS) as an additive so that the content reaches 2.4 mass%.

[0174] After injecting the electrolyte, a vacuum impregnation treatment is performed, and then a temporary seal (temporary fusion) is carried out in a vacuum state. Subsequently, the obtained product is left standing under external pressure by a pair of pressing plates. After that, the pressing pressure is reduced, and a pre-charge is carried out in a fixed state.

[0175] After the pre-charge, the obtained product is left standing, and then the temporarily sealed part is opened to degas. After that, a second vacuum impregnation treatment is performed, and then a temporary seal is carried out in a vacuum state. Subsequently, a main seal (fusion) is performed on the temporarily sealed part.

[0176] Next, the surface of the laminate film forming the outer container is roll-pressed, and then a main charge for aging is carried out. Subsequently, the obtained product is left standing at 45 °C for 19 days for aging.

[0177] After aging, a charge / discharge treatment (RtRc treatment) is carried out in the order of discharge, charge, and discharge under predetermined conditions.

[0178] A charge / discharge cycle test is carried out on the lithium-ion secondary battery manufactured as described above (cycle rate: 1C, temperature: 25 °C, upper limit voltage: 4.15V, lower limit voltage: 2.5V), and the capacity retention rate after 300 cycles and the capacity retention rate after 500 cycles are determined. The results are shown in Tables 1 and 2.

[0179] In addition, an AC impedance measurement (SOC100%, 4.15V) is carried out, and the charge transfer resistance (after aging, after 500 cycles) is determined from the arc in the Cole-Cole plot. By using the fitting of an equivalent circuit, the resistance components from the positive electrode and the negative electrode are successfully separated from each other, and it is found that the main component of the charge transfer resistance comes from the positive electrode. The measurement results are shown in Tables 1 and 3.

[0180] (Example 2)

[0181] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the content of the additive (MMDS) was changed to 3.2% by mass. The results are shown in Tables 1 and 2.

[0182] (Comparative Example 1)

[0183] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the additive (MMDS) and the fine graphite were not used. The results are shown in Table 2.

[0184] (Comparative Example 2)

[0185] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the content of the additive (MMDS) was changed to 1.6% by mass and the fine graphite was not used. The results are shown in Table 2.

[0186] (Comparative Example 3)

[0187] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the content of the additive (MMDS) was changed to 1.6% by mass. The results are shown in Tables 1 and 2.

[0188] (Comparative Example 4)

[0189] Except that NCA (lithium-nickel-cobalt-aluminum composite oxide: LiNi 0.8 Co 0.15 Al 0.05 O2) was used as the positive electrode active material instead of NCM811, and VC (vinylene carbonate) was used as the additive instead of MMDS, a secondary battery was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0190] (Comparative Example 5)

[0191] Except that NCM523 (LiNi 0.5 Co 0.2 Mn 0.3 O2) was used as the positive electrode active material instead of NCM811, and VC was used as the additive instead of MMDS and the content was set to 1.5% by mass, a secondary battery was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0192] (Comparative Example 6)

[0193] Except that VC was used as the additive instead of MMDS and the content was set to 1.5% by mass, a secondary battery was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0194] (Comparative Example 7)

[0195] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that NCA was used for the positive electrode active material instead of NCM811, and PS (1,3 - propanesultone) was used for the additive instead of MMDS and the content was set to 2.0 mass%. The results are shown in Table 3.

[0196] (Comparative Example 8)

[0197] A secondary battery was fabricated and evaluated in the same manner as in Example 1, except that PS was used for the additive instead of MMDS and the content was set to 2.0 mass%. The results are shown in Table 3.

[0198] [Table 1]

[0199]

[0200] It is clear from the comparison between Comparative Example 3 and Examples 1 and 2 that the charge transfer resistance after 500 cycles in Examples 1 and 2 where the additive content is 2.0 mass% or more is lower than that in Comparative Example 3 where the additive content is less than 2.0 mass%, and correspondingly the capacity retention rate (cycle characteristics) is higher.

[0201] [Table 2]

[0202]

[0203] In Comparative Example 1 where neither the additive (MMDS) nor the fine graphite was used, the capacity retention rate dropped below 95% as early as after 25 cycles, and thus the cycle characteristics were significantly low.

[0204] It is clear from the comparison between Comparative Example 1 and Comparative Example 2 as described above that the use of the additive (MMDS) provides an improved capacity retention rate (cycle characteristics).

[0205] In addition, it is clear from the comparison between Comparative Example 2 and Comparative Example 3 that, in addition to the additive, the use of fine graphite for the negative electrode provides a further improved capacity retention rate (cycle characteristics).

[0206] Furthermore, it is clear from the comparison between Comparative Example 3 and Examples 1 and 2 that an additive (MMDS) content of 2.0 or more provides a further improved capacity retention rate (cycle characteristics).

[0207] [Table 3]

[0208]

[0209] It is clear from the measurement results of the charge transfer resistance in Comparative Examples 4 to 8 that the charge transfer resistance in Comparative Examples 6 and 8, each using NCM811 as the positive electrode active material, is significantly higher than that in Comparative Examples 4, 5, and 7, each using other positive electrode active materials (NCA, NCM523). Additionally, it is clear from the comparison between Comparative Examples 6 and 8 and Example 1 that additives different from MMDS (VC, PS) cannot provide a sufficient resistance reduction effect.

[0210] In the foregoing, the present invention has been described with reference to exemplary embodiments and examples; however, the present invention is not limited to the exemplary embodiments and examples. Various changes understandable to those skilled in the art can be made to its composition and details within the scope of the present invention.

[0211] This application claims priority based on Japanese Patent Application No. 2016-55699 filed on March 18, 2016, and incorporates its entire disclosure herein by reference.

[0212] List of Reference Numerals

[0213] 1 Positive electrode active material layer

[0214] 2 Negative electrode active material layer

[0215] 3 Positive electrode current collector

[0216] 4 Negative electrode current collector

[0217] 5 Separator

[0218] 6 Laminated outer packaging

[0219] 7 Laminated outer packaging

[0220] 8 Negative electrode tab

[0221] 9 Positive electrode tab

[0222] 11 Negative electrode active material particles

[0223] 12 Conductive additive particles

[0224] 13 Fine graphite particles

Claims

1. A non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode, the negative electrode comprising: a negative electrode active material capable of intercalating and deintercalating lithium ions, a fine graphite material, and a conductive additive; a non-aqueous electrolyte containing lithium ions; and an outer package, wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula: LiNi x Co y Mn z O2 provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, the battery is formed by using a non-aqueous electrolyte containing methylene methanedisulfonate and the content of methylene methanedisulfonate is 2.0% by mass or more and 5.0% by mass or less relative to the solvent, the negative electrode active material includes a graphite active material, The average particle size D of the graphite active material 50 is in the range of 10 to 30 μm, and the average particle size of the fine graphite material is smaller than the average particle size of the graphite active material. The ratio Db / Da of Db to Da is in the range of 0.2 to 0.7, where Db is the average particle size D of the fine graphite material 50 , and Da is the average particle size D of the graphite active material 50 , The average particle size D of the fine graphite material 50 is in the range of 1 to 15 μm, the content of the fine graphite material is in the range of 0.1 to 6.0% by mass relative to the negative electrode active material, The conductive additive contains amorphous carbon particles with an average particle size D 50 in the range of 10 to 100 nm, and the content of the conductive additive is in the range of 0.1 to 3.0% by mass relative to the negative electrode active material.

2. A non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery comprising: a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions; a negative electrode, the negative electrode comprising: a negative electrode active material capable of intercalating and deintercalating lithium ions, a fine graphite material, and a conductive additive; a non-aqueous electrolyte containing lithium ions; and an outer package, wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula: LiNi x Co y Mn z O2 provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, the battery is formed by using a non-aqueous electrolyte containing methylene methanedisulfonate and the content of methylene methanedisulfonate is 2.0% by mass or more and 5.0% by mass or less relative to the solvent, the negative electrode active material includes a graphite active material, The average particle diameter D of the graphite active material 50 is in the range of 10 to 30 μm, and the average particle diameter of the fine graphite material is smaller than the average particle diameter of the graphite active material. The ratio Db / Da of Db to Da is in the range of 0.2 to 0.7, where Db is the average particle size D of the fine graphite material 50 , and Da is the average particle size D of the graphite active material 50 , The average particle size D of the fine graphite material 50 is in the range of 1 to 15 μm, the content of the fine graphite material is in the range of 0.1 to 6.0% by mass relative to the negative electrode active material, the conductive additive contains a nano-carbon material, and the content of the conductive additive is in the range of 0.1 to 3.0% by mass relative to the negative electrode active material.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the mass ratio of the fine graphite material to the conductive additive is in the range of 1 to 10.

4. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode active material includes natural graphite.

5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the natural graphite is natural graphite covered with amorphous carbon.

6. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of methylene methanedisulfonate is 2.3% by mass or more and 4.0% by mass or less relative to the solvent.

7. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode active material includes spheroidized particles, The fine graphite material includes particles having an average particle roundness lower than that of the spherical particles of the negative electrode active material, and the average particle roundness of the spherical particles of the negative electrode active material is in the range of 0.86 to 1.

8. The non-aqueous electrolyte secondary battery according to claim 7, wherein the fine graphite material includes flaky particles.

9. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode active material includes spherical particles, and the fine graphite material includes flaky particles.

10. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein The particle size D of the negative electrode active material at 90% cumulative in the cumulative distribution 90 and the median diameter D 50 The ratio D 90 / D 50 is 1.5 or less, and The particle size D of the fine graphite material at 90% cumulative in the cumulative distribution 90 and the average particle size D 50 The ratio D 90 / D 50 is greater than 1.

5.

11. A method for manufacturing a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery including: a positive electrode including a positive electrode active material capable of inserting and extracting lithium ions; a negative electrode including a negative electrode active material capable of inserting and extracting lithium ions, a fine graphite material, and a conductive assistant; a non-aqueous electrolyte containing lithium ions; and an outer package, the method including: forming the positive electrode; forming the negative electrode; forming the non-aqueous electrolyte; and placing the positive electrode, the negative electrode, and the non-aqueous electrolyte into the outer package, wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula: LiNi x Co y Mn z O2 provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and the non-aqueous electrolyte contains methylene methanedisulfonate and the content of the methylene methanedisulfonate is 2.0 mass% or more and 5.0 mass% or less relative to the solvent, the negative electrode active material includes a graphite active material, The average particle size D of the graphite active material 50 is in the range of 10 to 30 μm, the average particle size of the fine graphite material is smaller than the average particle size of the graphite active material, The ratio Db / Da of Db to Da is in the range of 0.2 to 0.7, where Db is the average particle size D of the fine graphite material 50 , and Da is the average particle size D of the graphite active material 50 , The average particle size D of the fine graphite material 50 is in the range of 1 to 15 μm, the content of the fine graphite material is in the range of 0.1 to 6.0 mass% relative to the negative electrode active material, The conductive additive contains amorphous carbon particles with an average particle size D 50 in the range of 10 to 100 nm, and the content of the conductive assistant is in the range of 0.1 to 3.0 mass% relative to the negative electrode active material.

12. A method for manufacturing a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery including: a positive electrode including a positive electrode active material capable of inserting and extracting lithium ions; a negative electrode including a negative electrode active material capable of inserting and extracting lithium ions, a fine graphite material, and a conductive assistant; a non-aqueous electrolyte containing lithium ions; and an outer package, the method including: forming the positive electrode; forming the negative electrode; forming the non-aqueous electrolyte; and placing the positive electrode, the negative electrode, and the non-aqueous electrolyte into the outer package, wherein the positive electrode active material includes a lithium-containing composite oxide having a layered rock salt structure and represented by the following compositional formula: LiNi x Co y Mn z O2 provided that 0.7 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.2, 0.05 ≤ z ≤ 0.15 and x + y + z = 1, and the non-aqueous electrolyte contains methylene methanedisulfonate and the content of the methylene methanedisulfonate is 2.0 mass% or more and 5.0 mass% or less relative to the solvent, the negative electrode active material includes a graphite active material, The average particle size D of the graphite active material 50 is in the range of 10 to 30 μm, the average particle size of the fine graphite material is smaller than the average particle size of the graphite active material, The ratio Db / Da of Db to Da is in the range of 0.2 to 0.7, where Db is the average particle size D of the fine graphite material 50 and Da is the average particle size D of the graphite active material 50 , The average particle size D of the fine graphite material 50 is in the range of 1 to 15 μm, The content of the fine graphite material ranges from 0.1 to 6.0% by mass relative to the negative electrode active material. The conductive aid contains a nano-carbon material, and the content of the conductive aid ranges from 0.1 to 3.0% by mass relative to the negative electrode active material.

13. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, the method further comprising the step of maintaining a charged state under heating.

14. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, the method further comprising the step of maintaining a charged state under heating at 35 to 55 °C for more than 7 days after placing the positive electrode, the negative electrode, and the non-aqueous electrolyte into an outer package and after sealing the outer package.

15. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, wherein the mass ratio of the fine graphite material to the conductive aid ranges from 1 to 10.

16. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, wherein the content of methylene methanedisulfonate is 2.3% by mass or more and 4.0% by mass or less relative to the solvent.

17. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, wherein the negative electrode active material includes spheroidized particles, the fine graphite material includes particles having an average particle roundness lower than the average particle roundness of the spheroidized particles of the negative electrode active material, and the average particle roundness of the spheroidized particles of the negative electrode active material ranges from 0.86 to 1.

18. The method for preparing a non-aqueous electrolyte secondary battery according to claim 17, wherein the fine graphite material includes flaky particles.

19. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, wherein the negative electrode active material includes spheroidized particles, and the fine graphite material includes flaky particles.

20. The method for preparing a non-aqueous electrolyte secondary battery according to claim 11 or 12, wherein the negative electrode active material includes natural graphite.

21. The method for preparing a non-aqueous electrolyte secondary battery according to claim 20, wherein the natural graphite is natural graphite covered with amorphous carbon.

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