JP7468067B2 - Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

In recent years, the demand for high-capacity secondary batteries has been increasing with the miniaturization of electronic devices. In particular, non-aqueous secondary batteries, especially lithium ion secondary batteries, have attracted attention because they have higher energy density and superior rapid charge/discharge characteristics than nickel-cadmium batteries and nickel-metal hydride batteries. In particular, non-aqueous lithium secondary batteries consisting of positive and negative electrodes capable of absorbing and releasing lithium ions, and a non-aqueous electrolyte solution in which lithium salts such as _LiPF6 and _LiBF4 are dissolved, have been developed and put to practical use.

Various materials have been proposed as anode materials for non-aqueous lithium secondary batteries, but graphitic carbon materials such as artificial graphite obtained by graphitizing natural graphite or coke, graphitized mesophase pitch, and graphitized carbon fiber are used because of their high capacity and excellent flatness of discharge potential. In this way, non-aqueous lithium ion secondary batteries that use graphite have small electrode expansion and excellent cycle characteristics. However, achieving both input/output characteristics and high-temperature storage characteristics is an issue, and to solve this problem, research is being conducted on anode materials that combine graphite with carbon materials with low crystallinity (degree of graphitization).

For example, it is described that by using "a negative electrode material for lithium ion secondary batteries containing a carbon material having an average interplanar spacing d002 of 0.335 nm to 0.340 nm as determined by X-ray diffraction, a volume average particle size (50% D) of 1 μm to 40 μm, a maximum particle size Dmax of 74 μm or less, and at least two exothermic peaks in a temperature range of 300° C. to 1000° C. in differential thermal analysis in an air stream," the distribution or arrangement of multiple carbon materials with different structures becomes uniform and consistent, improving the input/output characteristics, life characteristics, and thermal stability of the negative electrode (see Patent Document 1).

In addition, it is described that by using a carbon material for a non-aqueous electrolyte secondary battery, which is a composite particle of graphite particles and carbon fine particles having a primary particle diameter of 3 nm or more and 500 nm or less, and which is characterized in that the micro-Raman R value of 30 randomly selected composite particles is measured with a micro-Raman spectrometer and the Raman R (90/10) value represented by the following (Equation 2) is 1 or more and 4.3 or less, uniform and continuous fine flow paths are formed on the surface of the negative electrode active material, and input/output characteristics at low temperatures can be significantly improved (see Patent Document 2).
(Formula 2): Raman R(90/10) value=(the microscopic Raman R value of the (total number of measured particles×0.9)th smallest particle in terms of the microscopic Raman R value when the microscopic Raman R values are arranged in ascending order)/(the microscopic Raman R value of the (total number of measured particles×0.1)th smallest particle in terms of the microscopic Raman R value when the microscopic Raman R values are arranged in ascending order)

JP 2016-131159 A JP 2014-060148 A

For example, when a non-aqueous lithium ion secondary battery is used for a hybrid vehicle or an electric vehicle, a large amount of energy is required when the vehicle starts or accelerates, and the energy must be efficiently regenerated when the vehicle decelerates or stops, so that the battery must have extremely high input/output characteristics in addition to the high charge/discharge capacity, safety, and durability that have been required for mobile phones and notebook computers up to now. In particular, since the input/output characteristics of non-aqueous lithium ion secondary batteries tend to decrease at low temperatures, there is a demand for a technology that can maintain high input/output characteristics even at low temperatures while also achieving high-temperature storage characteristics.

As a result of the studies conducted by the present inventors, it has been found that the inventions disclosed in Patent Documents 1 and 2 have limitations in terms of improving the balance between input/output characteristics and high-temperature storage characteristics.
That is, an object of the present invention is to provide a nonaqueous electrolyte secondary battery with an improved balance between input/output characteristics and high-temperature storage characteristics.

As a result of extensive research aimed at solving the above problems, the inventors have discovered that by setting the distribution of interparticle voids on the negative electrode surface within a specific range, the balance between the input/output characteristics at low temperatures and the high-temperature storage characteristics of a non-aqueous electrolyte secondary battery can be significantly improved, and have completed the present invention.
The reason why the present invention exhibits such an effect is not yet clear, but is presumed to be as follows.
In other words, by setting the distribution of interparticle voids on the negative electrode surface to a specific range, the voids are distributed uniformly, the flow paths of lithium ions in the depth direction of the electrode plate are made uniform, and electronic conductivity between particles is ensured, making it possible to utilize the active material particle group uniformly, and it is presumed that the effects of the present invention are achieved.

That is, the present invention is as follows.
<1> A negative electrode for a non-aqueous electrolyte secondary battery comprising a current collector and a negative electrode active material formed on the current collector,
a negative electrode for a nonaqueous electrolyte secondary battery, wherein 15 surface SEM images obtained from different positions on the negative electrode surface are arbitrarily selected, each of the images is divided into a particle region and an interparticle void region, and each image is binarized to obtain a processed image, from which the standard deviation of the fractal dimension of the interparticle void region calculated by a box counting method is 0.1 or less.
<2> The negative electrode for a nonaqueous electrolyte secondary battery according to <1>, wherein an average value of a fractal dimension of the interparticle void regions is 0.1 or more and 3 or less.
<3> The negative electrode for a nonaqueous electrolyte secondary battery according to <1> or <2>, wherein an average area ratio of the interparticle void regions is 0.01 or more and 0.2 or less.
<4> The negative electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <3>, wherein the negative electrode active material contains composite particles having graphite and an amorphous carbon layer covering a surface of the graphite.
<5> The negative electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <4>, wherein the negative electrode active material particles constituting the negative electrode active material have a volume-based average particle size (D50) of 0.1 μm or more and 50 μm or less.
<6> The negative electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <5>, wherein the negative electrode active material has a BET specific surface area (SA) of 1 m ² /g or more and 20 m ² /g or less.
<7> The negative electrode for a nonaqueous electrolyte secondary battery according to any one of <1> to <6>, wherein the tap density of the negative electrode active material is 0.1 g/ ^cm3 or more and 2 g/ ^cm3 or less.
<8> A nonaqueous electrolyte secondary battery including a positive electrode and a negative electrode capable of absorbing and releasing metal ions, and an electrolyte, wherein the negative electrode is the negative electrode according to any one of <1> to <7>.

The present invention provides a nonaqueous electrolyte secondary battery that has excellent input/output characteristics even at low temperatures and also has excellent high-temperature storage characteristics.

1A shows an example of an acquired SEM image, FIG. 1B shows an example of a binarized image, and FIG. 1C shows an example of a graph for determining a fractal dimension. 1 is a graph showing the correlation between high-temperature storage characteristics and low-temperature output characteristics for an example, a comparative example, and a reference example.

The negative electrode for a non-aqueous electrolyte secondary battery and the non-aqueous electrolyte secondary battery according to the embodiment of the present invention will be described in detail below, but the present invention is not limited to these details as long as they are not contrary to the spirit of the present invention.

<1. Negative electrode for non-aqueous electrolyte secondary battery>
The negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present invention (hereinafter, sometimes abbreviated as "the negative electrode of this embodiment") comprises a current collector and a negative electrode active material formed on the current collector. The negative electrode of this embodiment is a negative electrode for a non-aqueous electrolyte secondary battery comprising a material. For the negative electrode of this embodiment, 15 surface SEM images obtained from different positions on the surface of the negative electrode are arbitrarily selected, and each image is classified into a particle region and a particle region. The standard deviation of the fractal dimension of the interparticle void region calculated from the processed images obtained by binarizing each image is 0.1 or less, as determined by a box counting method. By setting the standard deviation of the fractal dimension to 0.1 or less, that is, by setting the distribution of the interparticle voids present on the electrode surface within a desired range, the input/output characteristics at low temperatures and the high-temperature storage characteristics of the non-aqueous electrolyte secondary battery can be improved. The inventors have found that this balance can be significantly improved.

<1-1. Fractal dimension>
If a certain pattern is given, and even if you enlarge a part of the pattern, the property that it cannot be distinguished from the original pattern is considered a kind of symmetry, and is called self-similarity. A pattern with self-similarity is called a self-similar fractal, and the amount that quantifies and quantifies the difference in the density and degree of inclusion of the pattern is called fractal dimension. For example, even if you enlarge the coastline of the west coast of the Oshika Peninsula in eastern Miyagi Prefecture, it looks the same as the original pattern, and can be quantified as fractal dimension (for example, see pages 9, 18, 27 of "Physics of Fractals (I) Basics" published by Shokabo Physics Library).

<1-1-1. Standard deviation of fractal dimension>
The standard deviation of the fractal dimension represents the degree of distribution of the shape of the voids, and a small standard deviation of the fractal dimension means that the shape of the voids is less varied and the voids are more uniformly distributed. The standard deviation of the fractal dimension is 0.1 or less, preferably 0.09 or less, more preferably 0.08 or less, even more preferably 0.07 or less, particularly preferably 0.06 or less, and most preferably 0.04 or less, and is usually 0 or more, preferably 0.001 or more, more preferably 0.0015 or more, even more preferably 0.01 or more, particularly preferably 0.015 or more, and most preferably 0.018 or more. If the standard deviation of the fractal dimension is within the above range, the shape and distribution of the voids are less varied, and the balance between low-temperature input/output characteristics and high-temperature storage characteristics is good.

<1-1-2. Average value of fractal dimension>
The average value of the fractal dimension represents the average shape distribution of the voids, and a small average value of the fractal dimension means that the shape of the voids is surrounded by a curve with low regularity of unevenness. The average value of the fractal dimension is not particularly limited, but is preferably 0.1 or more and 3 or less. More preferably, it is 0.5 or more, even more preferably 1.0 or more, particularly preferably 1.1 or more, and most preferably 1.18 or more. More preferably, it is 2.5 or less, even more preferably 2 or less, particularly preferably 1.5 or less, and most preferably 1.26 or less. If the average value of the fractal dimension is in the above range, it is preferable because it is possible to achieve both electronic conductivity between particles and diffusion of lithium ions in the electrode plate.

<1-1-3. How to measure fractal dimension and standard deviation of fractal dimension>
The fractal dimension can be quantified by the following procedure.
(1) Acquiring an image of the negative electrode surface
An SEM image (secondary electron image) of the negative electrode surface is obtained under the following measurement conditions.
Measurement conditions: accelerating voltage 5 kV, magnification 500x, 1280 x 960 pixels, 8-bit image,
One image contains 200 or more particles, and surface images of different locations on the electrode are acquired at any desired 15 or more locations.
(2) Binarization process For each image, binarization process is performed on the particle region and the interparticle void region using image analysis software. ImageJ is used as the image analysis software, and the brightness threshold is adjusted so that the particles and the interparticle voids can be clearly distinguished. The Othu method and Yen method can be suitably used to set the threshold. Binarization process can also be performed manually.
(3) Obtaining fractal dimension and standard deviation The fractal dimension of the binarized image is obtained by the box counting method using image analysis software. The box size (ε) is set to 2 to 64 pixels, and the slope of the linear region of the graph with the horizontal axis log(ε) and the vertical axis log(n) is set to -D (D is the fractal dimension). The fractal dimension D is obtained for each of the 15 images, and its standard deviation σ is set to the standard deviation of the fractal dimension.
FIG. 1 shows an example of an acquired SEM image, a binarized image, and a graph for determining the fractal dimension.

<1-1-4. Standard deviation of area ratio of interparticle void regions>
The standard deviation of the area ratio of the interparticle void region indicates that the interparticle void area is uniformly distributed, and the standard deviation of the area ratio of the interparticle void region is preferably 0.8 or less, more preferably 0.7 or less, even more preferably 0.6 or less, particularly preferably 0.5 or less, and most preferably 0.4 or less, and is usually 0 or more, preferably 0.001 or more, more preferably 0.0015 or more, even more preferably 0.01 or more, particularly preferably 0.015 or more, and most preferably 0.018 or more. If the standard deviation of the area ratio of the interparticle void region is within the above range, local current concentration is unlikely to occur, and the active material particle group can be used uniformly, resulting in good output.

<1-1-5. Average area ratio of interparticle void regions>
The average value of the area ratio of the interparticle void region represents the projected area ratio of the entrance for ion transport on the electrode surface, and a small average value of the area ratio of the interparticle void region means that the total amount of the opening of the electrode surface in ion transport into the electrode plate is small. The average value of the area ratio of the interparticle void region is not particularly limited, but is preferably 0.01 or more and 0.2 or less. More preferably, it is 0.04 or more, even more preferably 0.08 or more, particularly preferably 0.1 or more, and most preferably 0.11 or more. More preferably, it is 0.19 or less, even more preferably 0.18 or less, particularly preferably 0.17 or less, and most preferably 0.16 or less. If the average value of the area ratio of the interparticle void region is in the above range, it is preferable because it is possible to achieve both the electronic conductivity between the particles and the transport of lithium ions into the electrode plate.

<1-1-6. Method for measuring the average area ratio of interparticle void regions>
Using the above images in which the particle regions and inter-particle void regions have been binarized, the number of pixels in the inter-particle void regions/total number of pixels is regarded as the area ratio of the inter-particle void regions, and the average of the 15 images is regarded as the average area ratio of the inter-particle void regions.

<1-1-7. Means for setting the fractal dimension value and the area ratio of interparticle void regions to desired values>
In this embodiment, the fractal dimension of the negative electrode, specifically the standard deviation of the fractal dimension and the average value of the fractal dimension, can be set to a desired value by appropriately adjusting the particle size and shape of the active material particles, the amount of amorphous carbon coated thereon, the slurry preparation conditions, the coated electrode drying conditions, and the pressing conditions.
In addition, the average area ratio of the interparticle void regions can be adjusted to a desired value by appropriately adjusting the particle size and shape of the active material particles, the amount of amorphous carbon coated thereon, the slurry preparation conditions, the coated electrode drying conditions, and the pressing conditions.

<1-2. Negative electrode active material>
The negative electrode active material forming the negative electrode of this embodiment is not limited, but is preferably a graphitic carbon material particle. The graphite may be either natural graphite or artificial graphite. The natural graphite is, for example, Any of flake graphite, lump graphite, and amorphous graphite may be used, but graphite with few impurities is preferred, and it is preferable to use it after subjecting it to a known purification treatment as necessary. Pitch, coal-based heavy oil, atmospheric residual oil, petroleum-based heavy oil, aromatic hydrocarbons, nitrogen-containing cyclic compounds, sulfur-containing cyclic compounds, polyphenylene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyvinyl butyral, natural Examples of such graphitized materials include organic materials such as polymers, polyphenylene silicide, polyphenylene oxide, furfuryl alcohol resin, phenol-formaldehyde resin, and imide resin that are baked at a temperature usually in the range of 2500° C. or higher and usually 3200° C. or lower. In this case, a silicon-containing compound, a boron-containing compound, or the like can be used as a graphitization catalyst.

The graphite carbon particles are preferably a multi-layered carbon material partially or entirely coated with amorphous carbon. In the case of a multi-layered carbon material, the thickness of the amorphous carbon is usually 0.1 nm or more, preferably 1 nm or more, more preferably 3 nm or more, and usually 3 μm or less, preferably 1 μm or less, even more preferably 100 nm or less, and particularly preferably 50 nm or less. The particle size of the graphite particles and the thickness of the amorphous carbon can be measured by observation with an electron microscope such as SEM or TEM. The presence or absence of amorphous carbon can be confirmed by Raman spectroscopy, true density measurement, etc., to confirm its presence and amount.

The shape of the particles constituting the negative electrode active material is not particularly limited, but spherical, spindle-shaped, rod-shaped, and columnar ones can be used, and spherical is preferable. As a method for spheroidizing the graphite particles, it is possible to carry out a spheroidizing process using a well-known technique. For example, it can be carried out using a device that repeatedly applies mechanical actions such as compression, friction, shear force, etc., including particle interactions, mainly impact force, to the particles. In addition, it can be granulated by using a granulating agent to form the graphite raw material into a spherical shape.
Concave or convex portions may be formed on the surface of the negative electrode active material. In the case of an active material having complex concave and convex portions, the absolute value of the fractal dimension of the interparticle voids in the negative electrode of this embodiment tends to be large.

The average circularity of the particles constituting the negative electrode active material is usually 0.7 to 1.0, preferably 0.8 to 0.97, and more preferably 0.85 to 0.95. When the average circularity is within this range, the gaps between the particles in the negative electrode plate tend to be uniform, and the area ratio of the interparticle gap region is easily within an appropriate range.

The volume-based average particle size (D50) of the particles constituting the negative electrode active material is usually 0.1 μm or more, preferably 1 μm or more, more preferably 2 μm or more, even more preferably 3 μm or more, particularly preferably 4 μm or more, most preferably 5 μm or more, and usually 50 μm or less, preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, particularly preferably 15 μm or less, and most preferably 10 μm or less. If it is below the above range, the irreversible capacity may increase and the capacity may decrease. On the other hand, if it exceeds the above range, the input/output characteristics tend to decrease, and at the same time, when it is made into an electrode plate, there are often process problems such as streaking, and further, when it is applied to a current collector, unevenness in the film thickness is easily generated.

The particle size (D10) based on volume of the particles constituting the negative electrode active material is usually 0.1 μm or more, preferably 1 μm or more, more preferably 2 μm or more, even more preferably 3 μm or more, particularly preferably 4 μm or more, and most preferably 5 μm or more, and usually 30 μm or less, preferably 20 μm or less, more preferably 15 μm or less, even more preferably 10 μm or less, particularly preferably 9 μm or less, and most preferably 8 μm or less. If it is below the above range, the irreversible capacity increases and there is a risk of capacity reduction. On the other hand, if it exceeds the above range, the input/output characteristics tend to decrease, and at the same time, when it is made into an electrode plate, there are often process problems such as streaking, and furthermore, when it is applied to the current collector, unevenness in the film thickness is easily generated.

The particle size (D90) based on volume of the particles constituting the negative electrode active material is usually 0.1 μm or more, preferably 1 μm or more, more preferably 5 μm or more, even more preferably 8 μm or more, particularly preferably 10 μm or more, and most preferably 12 μm or more, and usually 50 μm or less, preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, particularly preferably 18 μm or less, and most preferably 15 μm or less. If it is below the above range, the irreversible capacity increases and there is a risk of capacity reduction. On the other hand, if it exceeds the above range, the input/output characteristics tend to decrease, and at the same time, when it is made into an electrode plate, there are often process problems such as streaking, and furthermore, when it is applied to the current collector, unevenness in the film thickness is easily generated.

The particle size (Dmax) of the particles constituting the negative electrode active material is usually 0.1 μm or more, preferably 1 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, particularly preferably 15 μm or more, and most preferably 20 μm or more, and usually 60 μm or less, preferably 55 μm or less, more preferably 50 μm or less, even more preferably 45 μm or less, particularly preferably 40 μm or less, and most preferably 35 μm or less. If it is below the above range, the irreversible capacity increases and there is a risk of capacity reduction. On the other hand, if it exceeds the above range, the input/output characteristics tend to decrease, and at the same time, when it is made into an electrode plate, there are often process problems such as streaking, and furthermore, when it is applied to the current collector, unevenness in the film thickness is easily generated.

The ratio (D90/D10) of the volumetric particle diameter (D10) and (D90) of the particles constituting the negative electrode active material is usually 1 or more, preferably 1.5 or more, more preferably 1.8 or more, even more preferably 2 or more, particularly preferably 2.2 or more, most preferably 2.4 or more, and usually 10 or less, preferably 5 or less, more preferably 4 or less, even more preferably 3.5 or less, particularly preferably 3 or less, most preferably 2.8 or less. If it is below the above range, the irreversible capacity increases and the capacity may decrease. On the other hand, if it exceeds the above range, the input/output characteristics tend to decrease, and at the same time, when it is made into an electrode plate, there are often process problems such as streaking, and further, when it is applied to the current collector, unevenness in the film thickness is easily generated.
The volume-based average particle size is determined by mixing the measurement target with an aqueous surfactant solution (about 1 mL), measuring the volume-based particle size (D10, median diameter D50, D90, Dmax) using a laser diffraction particle size distribution analyzer (for example, HORIBA, Ltd.'s "LA-920") with ion-exchanged water as a dispersion medium.

The negative electrode active material may be a mixture of active material particles with different average circularity and volume-based average particle size. This can increase the ratio of active material in the negative electrode and may result in a more uniform distribution of voids between particles, which is preferable for forming the negative electrode.

The BET specific surface area (SA) of the particles constituting the negative electrode active material is usually 1 m ² /g or more, preferably 2 m ² /g or more, more preferably 3 m ² /g or more, and further preferably 4 m ² /g or more.
Particularly preferably, the surface area is 4.5 m ² /g or more, and although there is no particular upper limit, the surface area is usually 20 m ² /g or less, preferably 15 m ² /g or less, more preferably 10 m ² /g or less, even more preferably 6 m ² /g or less, and particularly preferably 5 m ² /g or less. If the surface area is in the above range, the charge/discharge efficiency and discharge capacity of the battery are high, lithium ions are quickly absorbed and released during high-speed charge/discharge, and the rate characteristics are excellent, which is preferable.
The BET specific surface area is measured by a surface area meter (automatic surface area measurement device manufactured by Ohkura Riken) after pre-drying a sample at 350° C. for 15 minutes under a nitrogen flow, and then measuring the value by a nitrogen adsorption BET single-point method using a gas flow method, using a nitrogen-helium mixed gas accurately adjusted so that the relative pressure of nitrogen to atmospheric pressure is 0.3.

The particles constituting the negative electrode active material preferably have a Raman R value calculated as follows of 0.35 or more and 1 or less.
Raman R value=(Intensity IB of peak PB at about 1360 cm ⁻¹ in Raman spectrum analysis)/(Intensity IA of peak PA at about 1580 cm ⁻¹ )
The Raman R value is more preferably 0.40 or more, even more preferably 0.45 or more, particularly preferably 0.48 or more, more preferably 0.8 or less, and even more preferably 0.6 or less. If it is below the above range, the crystallinity of the particle surface becomes too high, and when densified, the crystals tend to be oriented in a direction parallel to the electrode plate, which may lead to a decrease in input/output characteristics. On the other hand, if it exceeds the above range, the crystals on the particle surface are disturbed, and the reactivity with the electrolyte increases, which may lead to a decrease in charge/discharge efficiency and an increase in gas generation.

The tap density of the particles constituting the negative electrode active material is usually 0.1 g/cm ³ or more, preferably 0.5 g/cm ³ or more, more preferably 0.7 g/cm ³ or more, and even more preferably 0.9 g/cm ³ or more, and usually 2 g/cm ³ or less, preferably 1.8 g/cm ³ or less, more preferably 1.6 g/cm ³ or less, even more preferably 1.4 g/cm ³ or less, particularly preferably 1.2 g/cm ³ or less, and most preferably 1.1 g/cm ³ or less. If it is below the above range, it is difficult to increase the packing density when used as a negative electrode, and a high-capacity battery may not be obtained. Also, if it exceeds the above range, the gap between the particles in the electrode becomes too small, making it difficult to ensure the conductivity between the particles, and it may be difficult to obtain preferable battery characteristics.
The tap density is measured by passing a sample through a sieve with 300 μm openings, dropping the sample into a ²⁰ cm3 tapping cell to fill the cell up to the upper end surface of the cell with the sample, and then tapping 1000 times with a stroke length of 10 mm using a powder density meter (e.g., Tap Denser manufactured by Seishin Enterprise Co., Ltd.), and measuring the volume and mass of the sample at that time.

When the negative electrode active material is graphitic, the type and physical properties are not particularly limited as long as the crystallinity (degree of graphitization) is relatively high, but specifically, it means carbon particles in which the interplanar spacing (d002) of the (002) plane measured by wide-angle X-ray diffraction method is 0.335 nm or more and less than 0.340 nm. In addition, the d002 value is preferably 0.338 nm or less, more preferably 0.337 nm or less, and even more preferably 0.336 nm or less.

<1-3. Method for manufacturing negative electrode>
The negative electrode of this embodiment can be manufactured by any known method as long as it does not significantly impair the effects of the present invention. The negative electrode means a state in which a current collector and a negative electrode active material formed on the current collector are provided. For example, the negative electrode can be formed by adding a binder, a solvent, and if necessary, a thickener, a conductive material, a filler, etc. to the negative electrode active material to form a slurry, which is applied to a current collector, dried, and then pressed.

The binder used in the production of the negative electrode is not particularly limited as long as it is a material that is stable to the nonaqueous electrolyte solution and the solvent used in the production of the electrode.
Specific examples include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, polyimide, polyacrylic acid, cellulose, and nitrocellulose; rubber-like polymers such as SBR (styrene-butadiene rubber), isoprene rubber, butadiene rubber, fluororubber, NBR (acrylonitrile-butadiene rubber), and ethylene-propylene rubber; styrene-butadiene-styrene block copolymers or hydrogenated products thereof; thermoplastic elastomeric polymers such as EPDM (ethylene-propylene-diene terpolymer), styrene-ethylene-butadiene-styrene copolymer, and styrene-isoprene-styrene block copolymer or hydrogenated products thereof; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, and propylene-α-olefin copolymer; fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer; and polymer compositions having ionic conductivity for alkali metal ions (particularly lithium ions). These may be used alone or in any combination of two or more in any ratio.

The ratio of the binder to the negative electrode active material is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and particularly preferably 0.6% by mass or more, and is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 8% by mass or less. If the ratio of the binder to the negative electrode active material exceeds the above range, the ratio of the binder that does not contribute to the battery capacity increases, which may lead to a decrease in the battery capacity. Also, if it is below the above range, it may lead to a decrease in the strength of the negative electrode.

In particular, when the main component is a rubber-like polymer such as SBR, the ratio of the binder to the negative electrode active material is usually 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and usually 6% by mass or less, preferably 5% by mass or less, and more preferably 4% by mass or less. When the main component is a fluorine-based polymer such as polyvinylidene fluoride, the ratio to the negative electrode active material is usually 1% by mass or more, preferably 2% by mass or more, and more preferably 3% by mass or more, and usually 15% by mass or less, preferably 10% by mass or less, and more preferably 8% by mass or less.

The solvent for forming the slurry is not particularly limited as long as it is capable of dissolving or dispersing the negative electrode active material, the binder, and the thickener and conductive material used as needed, and either an aqueous solvent or an organic solvent may be used.
Examples of the aqueous solvent include water and alcohol, and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, tetrahydrofuran (THF), toluene, acetone, diethyl ether, dimethylacetamide, hexamethylphosphatamide, dimethylsulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, and hexane.
In particular, when an aqueous solvent is used, it is preferable to add a dispersant together with the thickener and form a slurry using a latex such as SBR. These solvents may be used alone or in any combination and ratio of two or more.

Thickeners are usually used to adjust the viscosity of the slurry. There are no particular limitations on the thickeners, but specific examples include carboxymethylcellulose, methylcellulose, hydroxymethylcellulose, ethylcellulose, polyvinyl alcohol, oxidized starch, phosphated starch, casein, and salts thereof. These may be used alone or in any combination and ratio of two or more.

Furthermore, when a thickener is used, the ratio of the thickener to the negative electrode active material is usually 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and usually 5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or less. If the ratio of the thickener to the negative electrode active material is below the above range, the coating property may be significantly reduced. If the ratio exceeds the above range, the ratio of the negative electrode active material in the negative electrode active material layer may be reduced, causing a problem of a decrease in the capacity of the battery and an increase in the resistance between the negative electrode active materials.

The conductive agent may be any electronically conductive material that does not undergo chemical changes at the charge/discharge potential. For example, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, conductive fibers such as carbon fiber, vapor-grown carbon fiber (VGCF), and metal fiber, metal powders such as carbon fluoride, and copper, may be included alone or as a mixture of these. Among these conductive agents, acetylene black and VGCF are particularly preferred. These may be used alone or in combination of two or more. The amount of conductive agent added is not particularly limited, but is preferably 1 to 30% by mass, and more preferably 1 to 15% by mass, based on the negative electrode active material.

Any known current collector can be used to hold the negative electrode active material. Examples of current collectors include metal materials such as aluminum, copper, nickel, stainless steel, and nickel-plated steel, with copper being particularly preferred in terms of ease of processing and cost.

When the current collector is made of a metal material, the shape of the current collector may be, for example, a metal foil, a metal cylinder, a metal coil, a metal plate, a metal thin film, an expanded metal, a punched metal, a foamed metal, etc. Among these, a metal thin film is preferable, a copper foil is more preferable, and a rolled copper foil produced by a rolling method and an electrolytic copper foil produced by an electrolytic method are further preferable, and either of them can be used as the current collector.
The thickness of the current collector is usually 1 μm or more, preferably 5 μm or more, and usually 500 μm or less, preferably 30 μm or less. If the thickness of the negative electrode current collector is too thick, the capacity of the entire battery may be too low, and conversely, if it is too thin, handling may be difficult.

The porosity of the negative electrode is usually 10% or more, preferably 20% or more, and usually 50% or less, preferably 40% or less. If the porosity of the negative electrode is below this range, the voids in the negative electrode are small, making it difficult for the electrolyte to penetrate, and it may be difficult to obtain desirable battery characteristics. On the other hand, if the porosity exceeds this range, the voids in the negative electrode are large, making the negative electrode strength too weak, and it may be difficult to obtain desirable battery characteristics. The porosity of the negative electrode is calculated as a percentage by dividing the total pore volume obtained by measuring the pore distribution of the negative electrode using a mercury porosimeter by the apparent volume of the negative electrode active material layer excluding the current collector.

The weight of the negative electrode active material formed on the current collector is preferably 1 mg/ ^cm2 or more per side. More preferably, it is 3 mg/ ^cm2 or more, and even more preferably, it is 5 mg/ ^cm2 or more. Also, it is usually 30 mg/ ^cm2 or less, more preferably, it is 20 mg/ ^cm2 or less, even more preferably, it is 15 mg/ ^cm2 or less, and most preferably, it is 12 mg/ ^cm2 or less. If the weight of the negative electrode active material is below this range, the filling amount of the active material is low, and the capacity of the battery is small. If the weight of the negative electrode active material exceeds this range, the ion diffusion distance in the negative electrode is large, and the capacity at high rates may be small.

<2. Non-aqueous secondary battery>
The negative electrode of this embodiment is useful as a negative electrode for a nonaqueous electrolyte secondary battery as described above. A nonaqueous electrolyte secondary battery using the negative electrode of this embodiment is also one aspect of the present invention. (Hereinafter, this may be abbreviated as "the nonaqueous electrolyte secondary battery of this embodiment.") The basic structure of the nonaqueous electrolyte secondary battery of this embodiment, particularly the lithium ion secondary battery, is as follows: The lithium ion secondary battery is similar to a conventionally known lithium ion secondary battery, and generally includes a positive electrode and a negative electrode capable of absorbing and releasing metal ions, and an electrolyte. The negative electrode of the present embodiment described above is used as the negative electrode.

<2-1. Positive electrode>
The positive electrode can be manufactured by any known method as long as it does not significantly impair the effects of the present invention. For example, a binder, a solvent, a conductive material, a thickener, etc. are added to a positive electrode active material to form a slurry. This may be applied to the current collector, dried, and then pressed to form the current collector.

The positive electrode active material (lithium transition metal compound) used in the positive electrode will be described below. The lithium transition metal compound is a compound having a structure capable of absorbing and releasing lithium ions, and examples thereof include sulfides, phosphate compounds, and lithium transition metal composite oxides. Examples of sulfides include compounds having a two-dimensional layered structure such as TiS ₂ and MoS ₂ , and Chevrel compounds having a strong three-dimensional skeletal structure represented by the general formula Me _x Mo ₆ S ₈ (Me is various transition metals including Pb, Ag, and Cu). Examples of phosphate compounds include those belonging to an olivine structure, generally represented by LiMePO ₄ (Me is at least one or more transition metals), and specifically LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , LiMnPO ₄ , and the like. Examples of lithium transition metal composite oxides include those belonging to a spinel structure capable of three-dimensional diffusion and a layered structure that enables two-dimensional diffusion of lithium ions. Those having _a spinel structure _are generally represented as _LiMe2O4 (Me is at least one or more transition metals), and specific examples include _LiMn2O4 , _LiCoMnO4 , _{LiNi0.5Mn1.5O4} , and _LiCoVO4 . Those having a layered structure are generally represented _{as LiMeO2} (Me _is at least one or more transition metals). _Specific examples include _LiCoO2 , _{LiNiO2 , LiNi1 - xCoxO2 , LiNi1 - xyCoxMnyO2 , LiNi0.5Mn0.5O2 , Li1.2Cr0.4Mn0.4O2 , Li1.2Cr0.4Ti0.4O2 , and LiMnO2 .}

The lithium-containing transition metal compound may be, for example, a lithium transition metal compound represented by the following composition formula (A) or (B).
1) In the case of a lithium transition metal compound represented by the following composition formula (A): Li1 _{+xMO2 ...} (A)
However, x is usually 0 or more and 0.5 or less. M is an element composed of Ni and Mn, or Ni, Mn and Co, and the Mn/Ni molar ratio is usually 0.1 or more and 5 or less. The Ni/M molar ratio is usually 0 or more and 0.5 or less. The Co/M molar ratio is usually 0 or more and 0.5 or less. The Li-rich portion represented by x may be substituted with the transition metal site M.

In the above composition formula (A), the atomic ratio of the oxygen amount is described as 2 for convenience, but some non-stoichiometry is acceptable. In addition, x in the above composition formula is the composition charged at the manufacturing stage of the lithium transition metal compound. Usually, batteries on the market are aged after assembly. Therefore, the amount of Li in the positive electrode may be lost due to charging and discharging. In that case, in the composition analysis, x may be measured to be -0.65 or more and 1 or less when discharged to 3V.
Furthermore, the lithium transition metal compound has excellent battery characteristics when it is sintered at a high temperature in an oxygen-containing gas atmosphere in order to increase the crystallinity of the positive electrode active material.

Furthermore, the lithium transition metal compound represented by the composition formula (A) may be a solid solution with Li ₂ MO ₃ , which is called the 213 layer, as shown in the following general formula (A′).
_{αLi2MO3 .} (1-α) _LiMO2 ...(A')
In the general formula, α is a number satisfying 0<α<1. M is at least one metal element having an average oxidation number of ⁴⁺ , specifically at least one metal element selected from the group consisting of Mn, Zr, Ti, Ru, Re and Pt. M′ is at least one metal element having an average oxidation number of ³⁺ , preferably at least one metal element selected from the group consisting of V, Mn, Fe, Co and Ni, more preferably at least one metal element selected from the group consisting of Mn, Co and Ni.

2) In the case of a lithium transition metal compound represented by the following general formula (B): Li[Li _a M _b Mn _2-b-a ]O _4+δ (B)
Here, M is an element composed of at least one transition metal selected from Ni, Cr, Fe, Co, Cu, Zr, Al and Mg.
The value of b is usually 0.4 or more and 0.6 or less. If the value of b is in this range, the energy density per unit weight in the lithium transition metal compound is high. The value of a is usually 0 or more and 0.3 or less. In addition, a in the above composition formula is the composition at the manufacturing stage of the lithium transition metal compound. Usually, batteries on the market are aged after assembling the battery. Therefore, the amount of Li in the positive electrode may be missing due to charging and discharging. In that case, in the composition analysis, a may be measured to be -0.65 or more and 1 or less when discharged to 3V. If the value of a is in this range, the energy density per unit weight in the lithium transition metal compound is not significantly impaired and good load characteristics are obtained. Furthermore, the value of δ is usually in the range of ±0.5. If the value of δ is in this range, the stability as a crystal structure is high, and the cycle characteristics and high-temperature storage of a battery having an electrode made using this lithium transition metal compound are good.

Here, the chemical meaning of the lithium composition in the lithium nickel manganese composite oxide, which is the composition of the lithium transition metal compound, will be described in more detail below. To obtain the a and b in the composition formula of the lithium transition metal compound, each transition metal and lithium are analyzed by an inductively coupled plasma atomic emission spectrometry (ICP-AES) and the Li/Ni/Mn ratio is calculated. From a structural viewpoint, it is considered that the lithium related to a is substituted and inserted into the same transition metal site. Here, the lithium related to a causes the average valence of M and manganese to be greater than 3.5 due to the principle of charge neutrality. In addition, the lithium transition metal compound may be fluorine-substituted and is represented as LiMn ₂ O _4-x F _2x .

Specific examples of the lithium transition metal compound having _the above composition include Li1 _{+ xNi0.5Mn0.5O2 , Li1 + xNi0.85Co0.10Al0.05O2 , Li1 + xNi0.33Mn0.33Co0.33O2 , Li1 + xNi0.45Mn0.45Co0.1O2 , Li1 +xMn1.8Al0.2O4, Li1+ xMn1.5Ni0.5O4 , etc. These lithium transition} metal compounds may be used alone _or in combination of _{two or} more.

In addition, a foreign element may be introduced into the lithium transition metal compound. The foreign element is selected from one or more of B, Na, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Sr, Y, Zr, Nb, Ru, Rh, Pd, Ag, In, Sb, Te, Ba, Ta, Mo, W, Re, Os, Ir, Pt, Au, Pb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, N, F, S, Cl, Br, I, As, Ge, P, Pb, Sb, Si, and Sn. These foreign elements may be incorporated into the crystal structure of the lithium transition metal compound, or may be unevenly distributed as a single element or compound on the particle surface or grain boundary without being incorporated into the crystal structure of the lithium transition metal compound.

The content of the lithium transition metal compound powder in the positive electrode active material layer is usually 10% by weight or more and 99.9% by weight or less. If the content of the lithium transition metal compound powder in the positive electrode active material layer is too high, the strength of the positive electrode tends to be insufficient, and if the content is too low, the capacity may be insufficient.

There are no particular limitations on the binder used in the manufacture of the positive electrode active material layer. In the case of the coating method, any material that is stable in the liquid medium used in the manufacture of the electrode may be used. Specific examples include resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), fluororubber, isoprene rubber, butadiene rubber, and ethylene-propylene rubber; styrene-butadiene-styrene block copolymers and their hydrogenated products; and EPDM (ethylene polypropylene dipropylene glycol). Examples of such materials include thermoplastic elastomer-like polymers such as styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers and hydrogenated products thereof, soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers, fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers, and polymer compositions having ionic conductivity for alkali metal ions (especially lithium ions). These substances may be used alone or in any combination and ratio of two or more.

The proportion of the binder in the positive electrode active material layer is usually 0.1% by weight or more and 80% by weight or less. If the proportion of the binder is too low, the positive electrode may not be able to sufficiently retain the positive electrode active material, resulting in insufficient mechanical strength of the positive electrode and possibly deteriorating battery performance such as cycle characteristics, while if the proportion is too high, it may lead to a decrease in battery capacity and conductivity.

The liquid medium for forming the slurry is not particularly limited as long as it is a solvent capable of dissolving or dispersing the lithium transition metal compound powder, the binder, and the conductive material and thickener used as necessary, and either an aqueous solvent or an organic solvent may be used. Examples of aqueous solvents include water and alcohol, and examples of organic solvents include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF), toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphatamide, dimethylsulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, and hexane. In particular, when an aqueous solvent is used, a dispersant is added together with the thickener, and a slurry is formed using a latex such as SBR. Note that these solvents may be used alone or in any combination and ratio of two or more.

Any known current collector can be used to hold the positive electrode. Examples of the current collector for the positive electrode include metal materials such as aluminum, stainless steel, and nickel-plated steel, and aluminum is particularly preferred in terms of ease of processing and cost. A carbon material may be formed on the current collector to improve the conductivity of the positive electrode active material and the current collector.
The thickness of the positive electrode active material layer is usually about 10 to 200 μm. The electrode density after pressing of the positive electrode is usually 2.2 g/cm ³ or more and 4.2 g/cm ³ or less. The positive electrode active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the positive electrode active material.

<2-2. Nonaqueous electrolyte>
As the non-aqueous electrolyte, for example, a known non-aqueous electrolyte solution, a polymer solid electrolyte, a gel electrolyte, an inorganic solid electrolyte, etc. can be used, among which a non-aqueous electrolyte solution is preferable. It is composed of a solute (electrolyte) dissolved in a solvent.

(Electrolytes)
There is no limitation on the electrolyte used in the non-aqueous electrolyte, and any known electrolyte may be used and contained. When the non-aqueous electrolyte is used in a non-aqueous electrolyte secondary battery, the electrolyte is preferably a lithium salt. Specific examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithium difluorobis(oxalato)phosphate, and lithium fluorosulfonate. These electrolytes may be used alone or in any combination and ratio of two or more.

The concentration of the lithium salt in the electrolyte is arbitrary, but is usually 0.5 mol/L or more, preferably 0.6 mol/L or more, more preferably 0.8 mol/L or more, and is usually 3 mol/L or less, preferably 2 mol/L or less, more preferably 1.5 mol/L or less. By having the total molar concentration of lithium within the above range, the electrical conductivity of the electrolyte is sufficient, while at the same time, a decrease in electrical conductivity due to an increase in viscosity and a decrease in battery performance can be prevented.

(Non-aqueous solvent)
The non-aqueous solvent contained in the non-aqueous electrolyte solution is not particularly limited as long as it is a solvent that does not adversely affect the battery characteristics when used as a battery. Examples of commonly used non-aqueous solvents include chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, cyclic carbonates such as ethylene carbonate, fluoroethylene carbonate, ethynylethylene carbonate, propylene carbonate, and butylene carbonate, chain carboxylate esters such as methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate, and γ-butyrolactone. Examples of the solvent include cyclic carboxylates such as cyclic carboxylates, chain ethers such as dimethoxyethane and diethoxyethane, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, and tetrahydropyran, nitriles such as acetonitrile, propionitrile, benzonitrile, butyronitrile, and valeronitrile, phosphates such as trimethyl phosphate and triethyl phosphate, and sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, methyl methanesulfonate, sulfolane, and dimethylsulfone, and the hydrogen atoms of these compounds may be partially substituted with halogen atoms. These may be used alone or in combination of two or more kinds, but it is preferable to use two or more kinds of compounds in combination. For example, it is preferable to use a high-dielectric constant solvent such as a cyclic carbonate or a cyclic carboxylate in combination with a low-viscosity solvent such as a chain carbonate or a chain carboxylate.

Here, a high dielectric constant solvent means a compound with a relative dielectric constant of 20 or more at 25°C. Among high dielectric constant solvents, ethylene carbonate, propylene carbonate, and compounds in which the hydrogen atoms of these solvents are substituted with other elements such as halogens or alkyl groups are preferably contained in the electrolyte. The proportion of the high dielectric constant solvent in the electrolyte is preferably 15% by weight or more, more preferably 20% by weight or more, and most preferably 25% by weight or more. If the content of the high dielectric constant solvent is less than the above range, the desired battery characteristics may not be obtained.

(Auxiliary Agent)
In addition to the above-mentioned electrolyte and non-aqueous solvent, the non-aqueous electrolyte may contain an auxiliary agent according to the purpose. Examples of the auxiliary agent that has the effect of improving the life of the battery by forming a film on the surface of the negative electrode include unsaturated cyclic carbonates such as vinylene carbonate, vinylethylene carbonate, and ethynylethylene carbonate, cyclic carbonates having fluorine atoms such as fluoroethylene carbonate, and fluorinated unsaturated cyclic carbonates such as 4-fluorovinylene carbonate. Examples of the overcharge inhibitor that effectively suppresses the explosion and ignition of the battery when the battery is in an overcharged state include aromatic compounds such as biphenyl, cyclohexylbenzene, diphenyl ether, t-butylbenzene, t-pentylbenzene, diphenyl carbonate, and methylphenyl carbonate. Examples of the auxiliary agent for improving the cycle characteristics and low-temperature discharge characteristics include lithium salts such as lithium monofluorophosphate, lithium difluorophosphate, lithium fluorosulfonate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, and lithium difluorobis(oxalato)phosphate. Examples of the auxiliary agent for improving the capacity maintenance characteristics and cycle characteristics after high-temperature storage include sulfur-containing compounds such as ethylene sulfite, propane sultone, and propene sultone, carboxylic acid anhydrides such as succinic anhydride, maleic anhydride, and citraconic anhydride, and nitrile compounds such as succinonitrile, glutaronitrile, adiponitrile, and pimelonitrile. The blending amount of these auxiliary agents is not particularly limited and may be any amount as long as it does not significantly impair the effects of the present invention.

The non-aqueous electrolyte may contain an organic polymer compound to form a gel-like, rubber-like, or solid sheet-like solid electrolyte. In this case, specific examples of the organic polymer compound include polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide; crosslinked polymers of polyether-based polymer compounds; vinyl alcohol-based polymer compounds such as polyvinyl alcohol and polyvinyl butyral; insolubilized vinyl alcohol-based polymer compounds; polyepoxy chlorohydrin; polyphosphazene; polysiloxane; vinyl-based polymer compounds such as polyvinylpyrrolidone, polyvinylidene carbonate, and polyacrylonitrile; and polymer copolymers such as poly(ω-methoxyoligooxyethylene methacrylate) and poly(ω-methoxyoligooxyethylene methacrylate-co-methyl methacrylate).

<2-3. Separator>
A separator is usually interposed between the positive electrode and the negative electrode to prevent short circuiting. In this case, the non-aqueous electrolyte is usually impregnated into the separator before use.

There are no particular limitations on the material or shape of the separator, and any known material may be used as long as it does not significantly impair the effects of the present invention. Among these, it is preferable to use materials that are stable against non-aqueous electrolytes, such as resins, glass fibers, and inorganic substances, and to use porous sheets or nonwoven fabrics with excellent liquid retention.

Materials that can be used for the resin and glass fiber separator include, for example, polyolefins such as polyethylene and polypropylene, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, and glass filters. Among these, glass filters and polyolefins are preferred, and polyolefins are even more preferred. These materials may be used alone or in any combination and ratio of two or more.

The thickness of the separator is arbitrary, but is usually 1 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and is usually 50 μm or less, preferably 40 μm or less, more preferably 30 μm or less. If the separator is thinner than the above range, the insulation properties and mechanical strength may decrease. Furthermore, if the separator is thicker than the above range, not only may the battery performance such as rate characteristics decrease, but the energy density of the nonaqueous electrolyte secondary battery as a whole may decrease.

Furthermore, when a porous material such as a porous sheet or nonwoven fabric is used as the separator, the porosity of the separator is optional, but is usually 20% or more, preferably 35% or more, more preferably 45% or more, and is usually 90% or less, preferably 85% or less, more preferably 75% or less. If the porosity is too small, the membrane resistance tends to increase and the rate characteristics tend to deteriorate. If the porosity is too large, the mechanical strength of the separator tends to decrease and the insulating properties tend to decrease.

The average pore size of the separator can also be any size, but is usually 0.5 μm or less, preferably 0.2 μm or less, and usually 0.05 μm or more. If the average pore size exceeds the above range, short circuits are more likely to occur. If the average pore size is below the above range, the membrane resistance increases and the rate characteristics may deteriorate.

On the other hand, inorganic materials include, for example, oxides such as alumina and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates such as barium sulfate and calcium sulfate, and are used in particulate or fibrous form.

As for the form, a thin film such as a nonwoven fabric, woven fabric, or microporous film is used. In the thin film form, a film with a pore size of 0.01 to 1 μm and a thickness of 5 to 50 μm is preferably used. In addition to the above independent thin film form, a separator can be used in which a composite porous layer containing the above inorganic particles is formed on the surface layer of the positive electrode and/or negative electrode using a resin binder. For example, a porous layer can be formed on both sides of the positive electrode using alumina particles with a 90% particle size of less than 1 μm and a fluororesin as a binder.

The characteristics of the separator in a non-electrolyte secondary battery can be understood by the Gurley value. The Gurley value indicates the difficulty of air passing through in the film thickness direction, and is expressed as the number of seconds required for 100 ml of air to pass through the film, so a smaller value means that air passes through easily, and a larger value means that air passes through less easily. In other words, a smaller value means that the film has better interconnectivity in the thickness direction, and a larger value means that the film has worse interconnectivity in the thickness direction. Interconnectivity is the degree of connection of pores in the film thickness direction. If the separator of the present invention has a low Gurley value, it can be used for various purposes. For example, when used as a separator for a non-aqueous lithium secondary battery, a low Gurley value means that lithium ions are easily transferred, and is preferable because it provides excellent battery performance. The Gurley value of the separator is arbitrary, but is preferably 10 to 1000 seconds/100 ml, more preferably 15 to 800 seconds/100 ml, and even more preferably 20 to 500 seconds/100 ml. If the Gurley value is 1000 seconds/100 ml or less, the electrical resistance is substantially low and it is preferable as a separator.

<2-4. Battery Design＞
The electrode group may have a laminated structure in which the positive electrode plate and the negative electrode plate are sandwiched between the separator, or a structure in which the positive electrode plate and the negative electrode plate are spirally wound with the separator between them. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as the electrode group occupancy rate) is usually 40% or more, preferably 50% or more, and usually 90% or less, and preferably 80% or less. % or less is preferable.

If the electrode group occupancy rate falls below the above range, the battery capacity will be small. If it exceeds the above range, there will be little void space, and the battery will become hot, causing the components to expand and the vapor pressure of the electrolyte liquid components to increase, resulting in an increase in internal pressure. This will reduce the battery's various characteristics, such as repeated charge/discharge performance and high-temperature storage, and may even activate the gas release valve that releases internal pressure to the outside.

<2-5. Outer case>
The material of the exterior case is not particularly limited as long as it is a material that is stable against the non-aqueous electrolyte used.Specifically, metals such as nickel-plated steel sheet, stainless steel, aluminum or aluminum alloy, magnesium alloy, or a laminate film of resin and aluminum foil (laminate film) are used.From the viewpoint of weight reduction, metals such as aluminum or aluminum alloy, and laminate film are preferably used.

In the case of an exterior case using metals, the metals are welded together by laser welding, resistance welding, or ultrasonic welding to form a sealed structure, or the metals are used via a resin gasket to form a crimped structure. In the case of an exterior case using the above-mentioned laminate film, the resin layers are heat-sealed together to form a sealed structure. In order to improve the sealing properties, a resin different from the resin used in the laminate film may be interposed between the resin layers. In particular, when the resin layers are heat-sealed via a current collecting terminal to form a sealed structure, a resin having a polar group or a modified resin into which a polar group has been introduced is preferably used as the interposed resin, since the metal and resin are joined.

<2-6. Protection elements>
As the protective element, a PTC (Positive Temperature Coefficient) whose resistance increases when abnormal heat generation or excessive current flows, a temperature fuse, a thermistor, a valve (current cutoff valve) that cuts off the current flowing in the circuit due to a sudden increase in the internal pressure or temperature of the battery when abnormal heat generation occurs, etc. It is preferable to select the above protective element under conditions that do not operate under normal use at high current, and it is more preferable to design it so that abnormal heat generation or thermal runaway does not occur even without the protective element.

<2-7. Exterior body>
The nonaqueous electrolyte secondary battery of this embodiment is usually configured by housing the above-mentioned nonaqueous electrolyte, negative electrode, positive electrode, separator, etc. in an exterior body. This exterior body is not particularly limited, and any known exterior body can be used as long as it does not significantly impair the effects of the present invention. Specifically, the material of the exterior body is arbitrary, but usually, for example, nickel-plated iron, stainless steel, aluminum or its alloy, nickel, titanium, etc. are used.
The shape of the exterior body may also be arbitrary, and may be, for example, cylindrical, rectangular, laminated, coin-shaped, large, or the like.

The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples in any way as long as the gist of the invention is not exceeded.

[Image analysis]
Image analysis of the negative electrode surfaces obtained in the Examples, Comparative Examples, and Reference Examples was carried out as follows.
(1) Acquiring an image of the negative electrode surface
A scanning electron microscope was used to obtain an SEM image (secondary electron image) of the negative electrode surface. The measurement conditions were as follows: 15 surface SEM images were obtained at random from different locations on the electrode surface so that each image contained 200 or more particles.
Conditions: Acceleration voltage 5 kV, magnification 500x, 1280 x 960 pixels, 8-bit image,
(2) Binarization process Each image was binarized using image analysis software (ImageJ) by adjusting the brightness threshold value according to the Yen method so that particles and interparticle voids could be clearly distinguished.
(3) Obtaining Fractal Dimension and Standard Deviation The fractal dimension was obtained by the box counting method for the binarized image. Image J was used as the image analysis software. The box size (ε) was set to 2 to 64 pixels, and the slope of the linear region of the graph with the horizontal axis log(ε) and the vertical axis log(n) was set to -D (D is the fractal dimension). The fractal dimension D was obtained for each of the 15 images obtained, and the standard deviation σ was set to the standard deviation of the fractal dimension.

[Comparative Example 1]
<Preparation of negative electrode active material>
Spheroidized natural graphite was mixed with petroleum-based heavy oil obtained during naphtha pyrolysis as an amorphous carbon precursor, and the mixture was subjected to heat treatment at 1100°C in an inert gas. The fired product was then pulverized and classified to obtain composite particles (multilayered carbon material) having a volume-based average particle size (D50) of 11.6 μm, a BET specific surface area (SA) of 4.2 m ² /g, and a tap density of 1.08 g/cm ³ , which include graphite and an amorphous carbon layer covering the surface of the graphite. From the firing yield, it was confirmed that the obtained multilayered carbon material was covered with 3.5 parts by mass of amorphous carbon per 100 parts by mass of graphite. The results are shown in Table 1.

<Preparation of negative electrode sheet>
The multilayer carbon material prepared in the above Comparative Example 1 was used as the negative electrode active material, and an electrode plate having an active material layer with an active material layer density of 1.20±0.03 g/cm ³ was prepared. Specifically, 20.00±0.02 g of a 1% by mass carboxymethylcellulose sodium salt aqueous solution (0.200 g in terms of solid content) and 0.50±0.05 g of a styrene-butadiene rubber aqueous dispersion with a weight average molecular weight of 270,000 (0.2 g in terms of solid content) were added to 20.00±0.02 g of the multilayer carbon material, and the mixture was stirred for 5 minutes with a Keyence hybrid mixer and degassed for 30 seconds to obtain a slurry. This slurry was applied to a width of 5 cm with a die coater on a copper foil with a thickness of 10 μm, which is a current collector, so that the negative electrode active material adhered to 6.0±0.3 mg/cm ² , and then dried with hot air. After drying, the active material layer was roll pressed using a roller having a diameter of 20 cm at a press load (linear pressure) of 331 kgf/5 cm to adjust the density of the active material layer to 1.20±0.03 g/cm ^{3 ,} thereby obtaining an electrode sheet.

Fifteen different locations on the surface of the electrode sheet obtained after pressing were randomly selected, and surface SEM images were obtained under the measurement conditions described above. Each of the obtained SEM images was binarized using the image analysis software described above, and the fractal dimension and the standard deviation of the fractal dimension were determined using the box counting method. The results are shown in Table 2.

<Preparation of Positive Electrode Sheet>
The positive electrode was prepared by mixing 90% by mass of lithium nickel-manganese-cobalt oxide (LiNiMnCoO ₂ ) as a positive electrode active material, 7% by mass of acetylene black as a conductive material, and 3% by mass of polyvinylidene fluoride (PVdF) as a binder in an N-methylpyrrolidone solvent to obtain a slurry.
This slurry was applied onto a 15 μm thick aluminum foil current collector so that the positive electrode active material was attached at 13.7±0.5 mg/cm ² , and then dried. Further, roll pressing was performed to adjust the positive electrode density to 2.60±0.05 g/cm ³ , thereby obtaining an electrode sheet.

<Preparation of non-aqueous electrolyte secondary battery (laminated type battery)>
The positive electrode sheet and the negative electrode sheet prepared by the above method, and the polyethylene separator were laminated in the order of the negative electrode, the separator, and the positive electrode. The battery element thus obtained was wrapped in a cylindrical aluminum laminate film, and LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio = 3: 7) so as to be 1 mol / L, and an electrolyte solution containing vinylene carbonate (VC) and lithium bis (oxalato) borate (LiBOB) was further injected, followed by vacuum sealing to prepare a sheet-shaped non-aqueous electrolyte secondary battery. Furthermore, in order to increase the adhesion between the electrodes, the sheet-shaped battery was sandwiched between glass plates and pressure was applied.

<Low temperature output characteristics>
Using the laminate type nonaqueous electrolyte secondary battery prepared above, low-temperature output characteristics were measured by the following measurement method.
For a non-aqueous electrolyte secondary battery that has not undergone charge/discharge cycles, the voltage range is 4.1 V at 25°C.
Three cycles of initial charge and discharge were performed at up to 3.0 V and a current value of 0.2 C (the current value at which the rated capacity based on the discharge capacity at a one-hour rate is discharged in one hour is defined as 1 C, the same applies below). The discharge capacity at the third cycle was defined as the initial capacity.

Furthermore, after charging at a current value of 0.2C up to an SOC (charge rate) of 50%, constant current discharge was performed for 10 seconds at current values of 1/8C, 1/4C, 1/2C, 1.5C, 2.5C, 3.5C, and 5C in a low-temperature environment of -30°C, and the drop in battery voltage after 2 seconds of charging under each condition was measured. From these measurements, the current value I that could be passed for 2 seconds when the lower discharge voltage limit was set to 3.0V was calculated, and the value calculated using the formula 3.0 x I (W) was used as the low-temperature output characteristic of each battery. The low-temperature output value of the battery in Comparative Example 1 was set to 100, and the batteries in the Examples and Reference Examples described below were shown as low-temperature output ratios.

<High temperature storage characteristics>
The battery for which the low-temperature output measurement was performed was charged to 80% SOC at a current value of 0.2 C in an environment of 25° C., and then stored for 2 weeks in an environment of 60° C. After storage, the battery was discharged to 3.0 V at a current value of 0.2 C in an environment of 25° C., and the capacity (remaining capacity) was measured, and further charging and discharging was performed from 4.1 V to 3.0 V at a current value of 0.2 C to measure the discharge capacity (recovery capacity).
The ratio of the recovered capacity after storage at 60°C to the initial capacity was calculated and taken as the high-temperature storage capacity retention rate. The high-temperature storage capacity retention rate of the battery of Comparative Example 1 was set to 100, and the batteries of the Examples and Reference Examples described below were shown in terms of the high-temperature storage capacity retention rate ratio. The results are shown in Table 2. Also, Fig. 2 shows the correlation between high-temperature storage characteristics and low-temperature output characteristics.

Example 1
A nonaqueous secondary battery was fabricated in the same manner as in Comparative Example 1, except that composite particles having graphite with a volume-based average particle size (D50) of 10.6 μm, a BET specific surface area (SA) of 4.6 m ² /g, and a tap density of 1.07 g/cm ³ and an amorphous carbon layer covering the surface of the graphite were used and roll pressed with a press load (linear pressure) of 272 kgf/5 cm. The fabricated nonaqueous secondary battery was evaluated in the same manner as in Comparative Example 1. The results are shown in Table 2.

Example 2
A nonaqueous secondary battery was fabricated in the ^same manner as in Comparative Example ¹ , except that composite particles coated with 4.1 parts by mass of amorphous carbon per 100 parts by mass of graphite, with a volume-based average particle size (D50) of 8.2 μm, a BET specific surface area (SA) of 4.8 m 2 /g, and a tap density of 1.06 g/cm 3, were used and roll pressed with a press load (linear pressure) of 397 kgf/5 cm, and the fabricated nonaqueous secondary battery was evaluated in the same manner as in Comparative Example 1. The results are shown in Table 2.

<Reference Example 1>
A nonaqueous secondary battery was fabricated in the same manner as in Comparative Example 1, except that composite particles having a volume-based average particle diameter (D50) of 11.6 μm, a BET specific surface area (SA) of 5.3 m ² /g, and a tap density of 0.96 g/cm ³ , which were coated with 4.3 parts by mass of amorphous carbon and 3 parts by mass of carbon black per 100 parts by mass of graphite, were used and roll pressed with a press load (linear pressure) of 495 kgf/5 cm, and the fabricated nonaqueous secondary battery was evaluated in the same manner as in Comparative Example 1. The results are shown in Table 2.

<Reference Example 2>
A nonaqueous secondary battery was fabricated in the same manner as in Comparative Example 1, except that composite particles having a volume-based average particle diameter (D50) of 8.1 μm, a BET specific surface area (SA) of 5.2 ^m2 /g, and a tap density of 1.06 g/ ^cm3 , coated with 3.7 parts by mass of amorphous carbon per 100 parts by mass of graphite, were used and roll pressed with a press load (linear pressure) of 300 kgf/5 cm. The fabricated nonaqueous secondary battery was evaluated in the same manner as in Comparative Example 1. The results are shown in Table 2.

As is clear from Figure 2 and Table 2, the high-temperature storage characteristics and low-temperature output characteristics of a battery are contradictory characteristics, and the nonaqueous secondary battery produced in the examples has an improved balance between high-temperature storage characteristics and low-temperature output characteristics compared to the secondary batteries produced in the comparative examples and reference examples.

By using the negative electrode according to the embodiment of the present invention, a non-aqueous electrolyte secondary battery that is excellent in high-temperature storage and has excellent output characteristics even at low temperatures can be realized, making it suitable for use in the field of automobile secondary batteries, which require large-scale high-input/output characteristics and are used even at low temperatures.

Claims (8)

A negative electrode for a non-aqueous electrolyte secondary battery comprising a current collector and a negative electrode active material formed on the current collector,
The negative electrode for a non-aqueous electrolyte secondary battery has interparticle voids,
a negative electrode for a nonaqueous electrolyte secondary battery, wherein 15 surface SEM images obtained from different positions on the negative electrode surface are arbitrarily selected, each of the images is divided into a particle region and an interparticle void region, and each image is binarized to obtain a processed image, from which the standard deviation of the fractal dimension of the interparticle void region calculated by a box counting method is 0.1 or less.
(However, when the fractal dimension is calculated from one image, the magnification of the surface SEM image is 500 times, the box size (ε) of the box counting method is 2 pixels to 64 pixels, and n is the number of boxes including particle regions, and the fractal dimension is calculated by multiplying the slope of the straight line of the graph of horizontal axis log(ε) and vertical axis log(n) by -1.) The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the average value of the fractal dimension of the interparticle void regions is 0.1 or more and 3 or less. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the average area ratio of the interparticle void regions is 0.01 or more and 0.2 or less. The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material includes composite particles having graphite and an amorphous carbon layer covering the surface of the graphite. The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material particles constituting the negative electrode active material have a volume-based average particle size (D50) of 0.1 μm or more and 50 μm or less. 6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material particles constituting the negative electrode active material have a BET specific surface area (SA) of 1 ^m2 /g or more and 20 ^m2 /g or less. 7. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the tap density of the negative electrode active material particles constituting the negative electrode active material is 0.1 g/ ^cm3 or more and 2 g/ ^cm3 or less. A non-aqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of absorbing and releasing metal ions, and an electrolyte, the negative electrode being the negative electrode according to any one of claims 1 to 7.