JP2022165614A - Graphite-based negative electrode active material and manufacturing method thereof - Google Patents

Abstract

To provide a graphite-based negative electrode active material capable of suitably reducing the low-temperature resistance of a secondary battery.SOLUTION: A graphite-based negative electrode active material 1 includes graphite particles 10 and an amorphous carbon layer 12 covering at least a part of the graphite particles. Here, in the cross-sectional TEM image of the graphite-based negative electrode active material 1, when a distance from the surface of the graphite-based negative electrode active material in the amorphous carbon layer 12 to the graphite particles 10 is divided into three equal layers, an A layer, a B layer, and a C layer from the surface, the progress of crystallization in the A layer, the B layer, and the C layer increases in this order.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a graphite-based negative electrode active material. Specifically, it relates to a graphite-based negative electrode active material in which graphite is coated with amorphous carbon. The present invention also relates to a method for producing the graphite-based negative electrode active material.

In recent years, secondary batteries such as lithium-ion secondary batteries have been used in portable power sources such as personal computers and mobile terminals, and power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). is preferably used for

Graphite-based negative electrode active materials are generally used for the negative electrodes of secondary batteries, particularly lithium ion secondary batteries. With the spread of secondary batteries, further improvement in performance is desired. One of the measures for improving the performance is improvement of the graphite-based negative electrode active material. As an example of improved graphite-based negative electrode active material, a multilayer negative electrode active material in which the surface of graphite is coated with amorphous carbon is known (see Patent Document 1 below).

JP 2012-074297 A

However, as a result of intensive studies by the present inventors, in the prior art, secondary batteries using a graphite-based negative electrode active material having a coating at low temperatures (for example, 0 ° C. or lower, particularly about -10 ° C. or lower) It has been found that there is a problem that the resistance of is insufficient.

The present invention has been made in view of such circumstances, and a main object thereof is to provide a graphite-based negative electrode active material that can suitably reduce the low-temperature resistance of secondary batteries.

In order to achieve this object, the present invention provides a graphite-based negative electrode active material comprising graphite particles and an amorphous carbon layer covering at least part of the surfaces of the graphite particles. Here, in the TEM image of the cross section of the graphite-based negative electrode active material, the distance from the surface of the graphite-based negative electrode active material in the amorphous carbon layer to the graphite particles is the nearer to the surface. When the layer is divided into three layers A, B, and C, the degree of progress of crystallization in the A layer, B layer, and C layer increases in this order.

Although not intended to be particularly limiting, such gradients in the crystallinity within the amorphous carbon layer (i.e., the crystallinity between the amorphous carbon layer and the graphitic particles) It can be thought that by suppressing the gap), the movement of ions (eg, lithium ions) within the amorphous carbon layer can be promoted. In addition, it can be considered that the number of ion insertion sites can be increased by reducing the crystallinity of the surface of the graphite-based negative electrode active material. It can be considered that these can reduce the low-temperature resistance of the secondary battery.

In a preferred embodiment of the graphite-based negative electrode active material disclosed herein, when electron diffraction images are obtained for the A layer, the B layer, and the C layer, at least the value of the lattice spacing d in the C layer can be determined. and the d value in the A layer is undeterminable.

In a preferred embodiment of the graphite-based negative electrode active material disclosed herein, the specific surface area based on the BET method (hereinafter also simply referred to as “specific surface area”) is 2.2 m ² /g to 8.3 m ² /g. Within range. When the specific surface area of the graphite-based negative electrode active material is within the above range, the low-temperature resistance of the secondary battery can be more suitably reduced.

In addition, the graphite-based negative electrode active material disclosed herein includes the following steps: a material preparation step of preparing an amorphous carbon material constituting an amorphous carbon layer and graphite particles; An adhesion step of mixing the material with the graphite particles and adhering the amorphous carbon material to at least a part of the surface of the graphite particles; and performing a microwave heat treatment after the adhesion step. a microwave heat treatment step for forming the amorphous carbon layer covering at least a portion of the graphite particles, wherein the graphite particles are heated from the surface of the graphite-based negative electrode active material in the amorphous carbon layer When the interval up to is divided into three equal layers, A, B, and C, from the side closest to the surface, the progress of crystallization in the A, B, and C layers increases in this order; It can be suitably manufactured by the included manufacturing method.

From another aspect, the present invention provides a secondary battery comprising a positive electrode, a negative electrode containing any of the graphite-based negative electrode active materials disclosed herein, and an electrolyte. According to such a secondary battery, the low-temperature resistance can be suitably reduced.

1 is a diagram schematically showing the configuration of a graphite-based negative electrode active material according to one embodiment; FIG. FIG. 2 is a diagram schematically showing a TEM image obtained for a cross section of the graphite-based negative electrode active material of FIG. 1; 1 is a flow chart showing rough steps of a method for producing a graphite-based negative electrode active material according to one embodiment. 1 is a cross-sectional view schematically showing the internal structure of a lithium ion secondary battery according to one embodiment; FIG. FIG. 2 is a diagram schematically showing the configuration of a wound electrode assembly included in a lithium ion secondary battery according to one embodiment; 4 is a TEM image (4 million times) of a cross section of a thin piece obtained after FIB treatment of the graphite-based negative electrode active material according to Sample 5. FIG. 7 is an electron diffraction image of A ₁ layer and C ₁ layer in FIG. 6. FIG. 4 is a TEM image (4 million times) of a cross-section of a thin piece obtained after FIB treatment of the graphite-based negative electrode active material according to Sample 1. FIG. 9 is an electron diffraction image of _two layers A and _two layers B in FIG. 8. FIG.

Hereinafter, a preferred embodiment of the graphite-based negative electrode active material disclosed herein, a method for producing the graphite-based negative electrode active material, and a secondary battery using the graphite-based negative electrode active material will be described with reference to the drawings as appropriate. will be described in detail. Matters other than those specifically referred to in this specification that are necessary for implementation can be grasped as design matters for those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. It should be noted that the following embodiments are not intended to limit the technology disclosed herein. In addition, in the drawings shown in this specification, members and parts having the same function are denoted by the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.
In the present specification and claims, when a predetermined numerical range is described as A to B (A and B are arbitrary numerical values), it means A or more and B or less. Therefore, the case above A and below B is included.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors. In this specification, the term “lithium ion secondary battery” refers to a secondary battery that utilizes lithium ions as a charge carrier and is charged/discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

<Structure of graphite-based negative electrode active material 1>
FIG. 1 is a diagram schematically showing the configuration of a graphite-based negative electrode active material according to one embodiment. As shown in the figure, the graphite-based negative electrode active material 1 according to this embodiment is roughly composed of graphite particles 10 and an amorphous carbon layer 12 covering at least part of the surface of the graphite particles. It has Further, the graphite negative electrode active material 1 can be said to be a material capable of reversibly releasing charge carriers. Each component will be described below.

(Graphite particles 10)
The graphite particles 10 mean particles composed mainly of graphite. Here, "mainly composed of graphite" means that among the components constituting the graphite particles 10, graphite is the most contained component on a mass % basis. Although not particularly limited, the proportion of graphite constituting the graphite particles 10 can be approximately 70% by mass or more, for example, 80% by mass or more, 90% by mass, when the entire graphite particles are 100% by mass. As mentioned above, it can be 95% by mass or more (for example, it may be 100% by mass).
As the graphite particles 10, for example, graphite particles including natural graphite, artificial graphite, and mixtures thereof can be used. Alternatively, graphite particles containing a fired material obtained by combining one or more of coal-based coke, petroleum-based coke, acetylene black, pitch-based carbon fiber, etc., which have lower crystallinity than these, can also be used.

In addition, the shape of the graphite particles 10 is not particularly limited as long as the effect of the technology disclosed herein is exhibited. Graphite particles 10 are preferably substantially spherical. From this point of view, the average aspect ratio of the graphite particles 10 is approximately 1.3 or less, preferably 1.2 or less, and more preferably 1.1 or less. The average aspect ratio can be the arithmetic average value of the ratio (a/b) of the major axis a to the minor axis b of 10 or more graphite particles 10 based on electron microscope observation.

The average particle size of the graphite particles 10 is not particularly limited, but may be, for example, 0.1 μm or more and 50 μm or less, preferably 1 μm or more and 25 μm or less, more preferably 5 μm or more and 20 μm or less. In the present specification and claims, the "average particle size" refers to particles corresponding to cumulative 50% from the fine particle side with a small particle size in the volume-based particle size distribution measured based on the laser diffraction/light scattering method Diameter D50 value (also called median diameter) is meant.

The d002 value (that is, the lattice spacing) of the lattice plane (002 plane) in the graphite particles 10 determined by electron diffraction is not particularly limited as long as the technique disclosed herein exhibits the effect, but is generally 0.00. 335 nm or greater and may be generally less than 0.340 nm. If the d002 value is too large, the crystallinity tends to decrease and the initial irreversible capacity tends to increase, which is considered unfavorable. In addition, the crystallite size (Lc) of graphitic carbon can be approximately 30 nm or more. Acquisition of such a d002 value can be carried out, for example, using a commercially available electron diffraction device according to the procedure manual for the electron diffraction device.

(Amorphous carbon layer 12)
The amorphous carbon layer 12 means a layer mainly composed of an amorphous carbon material with low crystallinity. Here, “mainly composed of an amorphous carbon material” means that, among the components that constitute the amorphous carbon layer 12, the component that is contained in the largest amount on a mass % basis is the amorphous carbon material. It means that there is Although not particularly limited, the ratio of the amorphous carbon material constituting the amorphous carbon layer 12 can be approximately 70% by mass or more, for example, 80% by mass, when the entire amorphous carbon layer is taken as 100% by mass. % by mass or more, 90% by mass or more, or 95% by mass or more (for example, it may be 100% by mass).

The amorphous carbon layer 12 is a layer covering at least part of the surface of the graphite particles 10 . By covering the surface of the graphite particles with the amorphous carbon layer, even if only slightly, the low-temperature resistance of the secondary battery can be suitably reduced. Further, by covering the entire surface of the graphite particles with an amorphous carbon layer, the above effect can be more suitably achieved.
In addition, a transmission electron microscope (TEM) image is obtained for the cross section of the graphite-based negative electrode active material 1 (for example, the graphite-based negative electrode active material 1 is thinned to a thickness of several μm to several tens of μm. The cross section of one thin piece out of the thin pieces may be obtained, or the cross section of a plurality of thin pieces (about 2 to 5) may be obtained), the graphite-based negative electrode active material in the amorphous carbon layer 12 When the space from the surface to the graphite particles 10 is divided into three equal layers A, B, and C from the side closer to the surface, the progress of crystallization in the A, B, and C layers are characterized by increasing in this order. Here, FIG. 2 is a diagram schematically showing a TEM image obtained for a cross section of the graphite-based negative electrode active material 1 according to one embodiment. In FIG. 2, the degree of progress of crystallization is represented by the number of dots and lines, and the higher the degree of progress, the smaller the number of dots and the greater the number of lines. In addition, it is preferable to apply Os (osmium) dyeing treatment to the flake to be observed, because the degree of progress of crystallization can be visually confirmed easily.

Here, conventional graphite-based negative electrode active materials (typically manufactured by furnace firing) have a large difference in crystallinity between the graphite particles and the amorphous carbon layer, and the intra-particle diffusion of lithium ions There is a problem that resistance increases. In addition, if external heat firing is performed to eliminate such a difference in crystallinity, the crystallinity of the outermost surface of the graphite-based negative electrode active material increases, and the lithium ion insertion sites decrease, resulting in an increase in resistance. There was also such a problem. On the other hand, the graphite-based negative electrode active material disclosed herein is preferably produced by microwave heating and sintering, and the crystallinity in the amorphous carbon layer is graded (that is, the amorphous carbon layer and the By suppressing the crystallinity gap between graphite particles), the movement of lithium ions in the amorphous carbon layer can be promoted. In addition, the number of ion insertion sites can be increased by reducing the crystallinity of the surface of the graphite-based negative electrode active material. It is considered that these can reduce the low-temperature resistance of the secondary battery. Note that the above description is based on considerations made by the present inventor, and the technology disclosed herein should not be construed as being limited to the above description.

As described above, the difference in progress of crystallization in the A layer, B layer, and C layer in the amorphous carbon layer 12 can be confirmed by observing the cross section of the graphite-based negative electrode active material 1 with a TEM. can. Further, for example, when electron diffraction images are obtained for the A layer, the B layer, and the C layer, the d value (that is, the lattice spacing) can be determined at least in the C layer, and the d value in the A layer can be determined. It can be confirmed suitably also from the fact that it cannot be determined.

Regarding the layer whose d value can be determined, the d002 value (that is, the lattice spacing) of the lattice plane (002 plane) obtained by electron diffraction is not particularly limited as long as the effect of the technology disclosed herein is exhibited. is approximately 0.340 nm or more, preferably 0.345 nm or more. Also, it is approximately 0.380 nm or less, preferably 0.370 nm or less, and 0.360 nm or less. In addition, the crystallite size (Lc) of amorphous carbon can be approximately 1 nm or more. Acquisition of such a d002 value can be carried out, for example, using a commercially available electron diffraction device according to the procedure manual for the electron diffraction device.

The thickness of the amorphous carbon layer 12 is not particularly limited as long as the effect of the technology disclosed herein is exhibited, but is, for example, 10 nm or more and 500 nm or less, preferably 30 nm or more and 400 nm or less, more preferably 30 nm. It may be greater than or equal to 300 nm or less. The thickness of the amorphous carbon layer 12 can be obtained by observing the cross section of the graphite-based negative electrode active material 1 using, for example, a TEM.

The specific surface area of the graphite-based negative electrode active material 1 is not particularly limited as long as the effect of the technology disclosed herein is exhibited, but can be approximately 0.1 m ² /g or more and 100 m ² /g. In addition, from the viewpoint of suppressing the low-temperature resistance of the resulting battery, it is preferably 0.5 m ² /g or more, 1.0 m ² /g or more, 2.0 m ² /g or more (for example, 2.2 m ² /g g or more). From the viewpoint of suppressing excessive reactivity with the electrolytic solution, it is preferably 50 m ² /g or less, 20 m ² /g or less, 10 m ² /g or less (for example, 9.0 m ² /g or less). can do. The specific surface area of the graphite-based negative electrode active material 1 is preferably in the range of 2.2 m ² /g to 8.3 m ² /g, and in the range of 4.5 m ² /g to 7.5 m ² /g. It can also be inside.

In the present specification and claims, the term "specific surface area" refers to, for example, a gas adsorption amount measured by a gas adsorption method (constant volume adsorption method) using nitrogen (N ₂ ) gas as an adsorbate, It means the surface area calculated by analysis by the BET method (for example, BET single-point method). Such measurements can be made, for example, by using a commercially available surface area measurement device and following the procedure manual for the surface measurement device.

<Method for producing graphite-based negative electrode active material 1>
A preferred method for producing the graphite-based negative electrode active material 1 according to this embodiment will be described below, but the production method is not intended to be limited to the one shown below.

FIG. 3 is a flow chart showing rough steps of a method for producing a graphite-based negative electrode active material according to one embodiment. As shown in the figure, the graphite-based negative electrode active material 1 according to the present embodiment includes the following steps: a material preparation step (step S1); an adhesion step of mixing the amorphous carbon material and the graphite particles, and adhering the amorphous carbon material to at least part of the surface of the graphite particles (step S2); After the adhesion step, a microwave heat treatment step (step S3) of forming the amorphous carbon layer covering at least a part of the graphite particles by performing microwave heat treatment. It can be manufactured suitably. Each step will be described below.

(Step S1: material preparation step)
In step S1, an amorphous carbon material forming an amorphous carbon layer and graphite particles are prepared. As the graphite particles, the particles described in the description of the graphite particles can be used. In addition, as the amorphous carbon material, conventionally known materials used as this type of material can be used. Aromatic hydrocarbon compounds such as ethylene tar, acenaphthylene, decacyclene, anthracene, and phenanthrene; N-ring compounds such as phenazine and acridine; S-ring compounds such as thiophene and bithiophene; , thermoplastic resins such as polyphenylene oxide, and thermosetting resins such as phenol-formaldehyde resins and imide resins. As the graphite particles and the amorphous carbon material, for example, commercially available ones can be purchased and used. Alternatively, the amorphous carbon material described above can be dissolved in an organic solvent and used as a solution or the like. Examples of such organic solvents include benzene, toluene, xylene, quinoline, n-hexane and the like.

(Step S2: Adhesion step)
In step S2, the amorphous carbon material and the graphite particles are mixed, and the amorphous carbon material is adhered to at least part of the surface of the graphite particles.
Specifically, first, an amorphous carbon material, graphite particles, and, if necessary, other components (for example, a solvent, etc.) are mixed using a commercially available stirrer, mixer, or kneader. Here, the mixing ratio of the amorphous carbon layer material and the graphite particles can be appropriately adjusted depending on the type of material used, the coverage of the amorphous carbon layer in the graphite particles, the value of the specific surface area, and the like. preferable. Also, mixing conditions (specifically, mixing time, mixing speed, etc.) are preferably adjusted as appropriate according to the types and amounts of the materials used. The above mixing ratio and mixing conditions can be appropriately determined by conducting preliminary experiments, for example.

(Step S3: Microwave heat treatment step)
Next, in step S3, microwave heat treatment is performed after the adhesion step (step S2) to form the amorphous carbon layer covering at least part of the graphite particles. The output (W) of the microwave baking apparatus is not particularly limited as long as the effect of the technology disclosed herein is exhibited, but it can be generally within the range of 1000 W to 2000 W (for example, about 1500 W). . Also, the firing time is not particularly limited as long as the effect of the technique disclosed herein is exhibited, but it can be approximately 20 to 240 minutes. And it is preferable to perform such baking in a gas atmosphere such as nitrogen gas, carbon dioxide gas, argon gas, or the like. Thus, the graphite-based negative electrode active material 1 can be obtained.

Also, if necessary, a pulverization step may be provided after step S3. Such pulverization can be performed using, for example, commercially available crushers, ball mills, stirring mills, and the like. Further, a classification treatment step can be provided. Such classification can be performed using, for example, a commercially available classifier.

From another aspect, according to the technology disclosed herein, it is possible to provide a secondary battery including a positive electrode, a negative electrode containing any of the graphite-based negative electrode active materials disclosed herein, and an electrolyte. can. That is, using the graphite-based negative electrode active material 1 produced as described above, a secondary battery can be constructed by a conventionally known method.

Hereinafter, the secondary battery according to the present embodiment will be described in detail by taking as an example a flat prismatic lithium ion secondary battery having a flat wound electrode assembly and a flat battery case.

The lithium ion secondary battery 100 shown in FIG. 4 is constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (that is, an outer container) 30. It is a sealed battery that is The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 that is set to release the internal pressure of the battery case 30 when it rises above a predetermined level. It is The positive and negative terminals 42 and 44 are electrically connected to the positive and negative current collecting plates 42a and 44a, respectively. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used.

As shown in FIGS. 4 and 5, the wound electrode body 20 is formed by stacking a positive electrode sheet 50 and a negative electrode sheet 60 with two elongated separator sheets 70 interposed therebetween and winding them in the longitudinal direction. morphology. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long positive electrode current collector 52 . The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long negative electrode current collector 62 . The positive electrode active material layer non-formed portion 52a (that is, the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the negative electrode active material layer non-formed portion 62a (that is, the negative electrode active material layer 64 is formed). The portion where the negative electrode current collector 62 is exposed without being wound) is formed so as to protrude outward from both ends of the wound electrode body 20 in the winding axial direction (that is, the sheet width direction orthogonal to the longitudinal direction). there is A positive collector plate 42a and a negative collector plate 44a are joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a, respectively.

As the positive electrode current collector 52, a known positive electrode current collector used for lithium ion secondary batteries may be used, and examples thereof include metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). ) sheets or foils. Aluminum foil is preferable as the positive electrode current collector 52 .

The dimensions of the positive electrode current collector 52 are not particularly limited, and may be appropriately determined according to the battery design. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

The positive electrode active material layer 54 contains a positive electrode active material. Examples of positive electrode active materials include lithium-nickel-based composite oxides (eg, _LiNiO2 , etc.), lithium-cobalt-based composite oxides (eg, LiCoO2, etc.), lithium _- nickel-cobalt-manganese-based composite oxides (eg, _LiNiCoMnO2 , etc.). LiNi _1/3 Co _1/3 Mn _1/3 O ₂ etc.), lithium nickel cobalt aluminum composite oxides (e.g., LiNi _0.8 Co _0.15 Al _0.5 O ₂ etc.), lithium manganese composite oxides Lithium transition metal composite oxides such as substances (e.g., LiMn ₂ O ₄ etc.), lithium nickel manganese composite oxides (e.g. LiNi _0.5 Mn _1.5 O ₄ etc.); lithium transition metal phosphate compounds (e.g. , _LiFePO4 , etc.).

The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, a binder, and the like. Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.

The content of the positive electrode active material in the positive electrode active material layer 54 (that is, the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass. It is 97% by mass or more, more preferably 85% by mass or more and 96% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 12% by mass or less. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 13% by mass or less. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, more preferably 1.5% by mass or more and 10% by mass or less.

Although the thickness of the positive electrode active material layer 54 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

As the negative electrode current collector 62, a known negative electrode current collector used for a lithium ion secondary battery may be used. ) sheets or foils. A copper foil is preferable as the negative electrode current collector 52 .

The dimensions of the negative electrode current collector 62 are not particularly limited, and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

The negative electrode active material layer 64 contains the above-described graphite-based negative electrode active material as a negative electrode active material. The negative electrode active material layer 64 may contain other negative electrode active materials in addition to the graphite-based negative electrode active material described above within a range that does not significantly impair the effects of the present invention.

The negative electrode active material layer 64 may contain components other than the active material, such as binders and thickeners. As the binder, for example, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), etc. can be used. As a thickening agent, for example, carboxymethyl cellulose (CMC) or the like can be used.

The content of the negative electrode active material in the negative electrode active material layer is preferably 90% by mass or more, and more preferably 95% by mass or more and 99% by mass or less. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less. The content of the thickener in the negative electrode active material layer is preferably 0.3% by mass or more and 3% by mass or less, more preferably 0.5% by mass or more and 2% by mass or less.

Although the thickness of the negative electrode active material layer 64 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

Examples of the separator 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70 .

Although the thickness of the separator 70 is not particularly limited, it is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less.

A non-aqueous electrolyte typically contains a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in electrolytes of general lithium ion secondary batteries, can be used without particular limitation. can be done. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC ), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC) and the like. Such non-aqueous solvents can be used singly or in combination of two or more.

Lithium salts (preferably LiPF ₆ ) such as LiPF ₆ , LiBF ₄ and lithium bis(fluorosulfonyl)imide (LiFSI) can be suitably used as supporting salts. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

In addition, the non-aqueous electrolyte includes components other than the components described above, such as film-forming agents such as oxalato complexes; agent; thickener; and other various additives may be included.

The lithium ion secondary battery 100 can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Moreover, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can also be used typically in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

The prismatic lithium ion secondary battery including the flattened wound electrode assembly has been described above as an example. However, the graphite-based negative electrode active material according to this embodiment can also be used for other types of lithium ion secondary batteries according to conventionally known methods. For example, using the graphite-based negative electrode active material according to the present embodiment, a lithium ion secondary battery having a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated) is manufactured. You can also build. In addition, a cylindrical lithium ion secondary battery, a laminate case type lithium ion secondary battery, or the like can be constructed using the graphite-based negative electrode active material according to the present embodiment. Furthermore, using the graphite-based negative electrode active material according to the present embodiment, a non-aqueous electrolyte secondary battery other than a lithium ion secondary battery can be constructed according to a conventionally known method.

Further, according to a conventionally known method, a solid electrolyte (e.g., sulfide solid electrolyte, oxide solid electrolyte, etc.) is used instead of the non-aqueous electrolyte, and the solid electrolyte is interposed between the positive electrode and the negative electrode instead of the separator. It is also possible to construct an all-solid secondary battery (in particular, an all-solid lithium ion secondary battery).

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Preparation of graphite-based negative electrode active material>
(Sample 1)
5.0 g of commercially available coal tar pitch as an amorphous carbon material and 50 g of graphite particles having an average particle size of 10 μm (CGB-10 manufactured by Nippon Graphite Industry Co., Ltd.) were mixed with a commercially available stirring device (LAB MILL, rotating speed 20000 rpm for 30 seconds) was fired in a tube furnace at 1000° C. for 60 minutes under a nitrogen atmosphere. Thus, a graphite-based negative electrode active material according to sample 1 was obtained.

(Samples 2-9)
Commercially available coal tar pitch as an amorphous carbon material and graphite particles with an average particle size of 10 μm (CGB-10 manufactured by Nippon Graphite Industry Co., Ltd.) were mixed with a commercially available stirring device (LAB MILL, rotation speed 20000 rpm, 30 seconds) in the amount shown in the corresponding column of Table 1, and baked in a microwave baking apparatus (MDS-2015-X02 manufactured by Hirotech Co., Ltd.) under the baking conditions shown in the corresponding column of Table 1 in a nitrogen atmosphere. As a result, graphite-based negative electrode active materials of Samples 2 to 9 were obtained.

<TEM Observation of Amorphous Carbon Layer>
The graphite-based negative electrode active material of each sample obtained as described above was thinned by a focused ion beam (FIB) method (micro-sampling method). Then, one slice was randomly taken out of the plurality of slices obtained, and a TEM image (magnification: 4,000,000 times) was acquired after performing an appropriate Os-staining treatment on the slice. Such acquisition was performed using a JEM-ARM200F manufactured by JEOL Ltd. with an acceleration voltage of 200 kV.
Then, for the TEM image obtained by the above observation, the space from the surface of the graphite-based negative electrode active material to the graphite particles (hereinafter also simply referred to as "between the surface and the graphite particles") was observed. As a result, in the graphite-based negative electrode active material according to sample 1, the degree of crystallization between the surface and the graphite particles was generally large, and almost no crystallinity gradient was confirmed (barely divided into two layers). (It was a form that could be used). In addition, in the graphite-based negative electrode active material according to sample 2, since the firing time was insufficient, the crystallization between the surface and the graphite particles hardly progressed as a whole, and the crystallinity gradient was confirmed. I didn't. In the graphite-based negative electrode active materials according to samples 3 to 9, it was confirmed that the degree of crystallization between the surface and the graphite particles increased from the surface to the graphite particles. It was confirmed that it was divided into layers (that is, A layer, B layer, and C layer).

Here, as an example, TEM images of the flakes of the graphite-based negative electrode active materials according to Sample 5 and Sample 1 are shown in FIGS. 6 and 8, respectively. In FIG. 6, it was confirmed that the crystallinity increased from the _{A 1} layer to the _{C 1} layer in 12a. Almost the same crystallinity was confirmed.

<Electron Diffraction Measurement of Amorphous Carbon Layer>
In addition to the TEM observation, the A ₂ layers and B ₂ layers of Sample 1 and the A layers and C layers of Samples 3 to 9 were analyzed using an electron diffraction device attached to the TEM device. Images and d002 values were acquired. At this time, the beam diameter was set to Φ1 nm. As a result, the d002 value could be calculated for the two layers of sample 1. On the other hand, in the two layers of Samples 3 to 9, the d002 value could be calculated for the C layer, but the d002 value could not be calculated for the A layer.

Here, as an example, FIGS. 7 and 9 show electron diffraction images of two layers (A ₁ layer, C ₁ layer) according to sample 5 and two layers (A ₂ layers, B ₂ layers) according to sample 1, respectively. and d002 values are listed. Although not shown, the d002 value of the graphite particles 10a in FIG. 7 was 0.338 nm, and the d002 value of the graphite particles 10b in FIG. 9 was 0.338 nm.

<Measurement of specific surface area of graphite-based negative electrode active material>
The specific surface area of the graphite-based negative electrode active material for each sample obtained as described above was determined. As the specific surface area, a value analyzed by the BET method from the nitrogen gas adsorption isotherm was adopted. In addition, the specific surface area was measured using Macsorb manufactured by Mountech, Inc., and was measured according to the procedure manual of the apparatus. The results are shown in the "BET specific surface area" column of Table 1.

<Production of lithium-ion secondary battery for evaluation>
LiNiCoMnO ₂ (NCM) as a positive electrode active material powder, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a mass ratio of NCM: AB: PVdF = 92: 5: 3. was mixed with N-methylpyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to the surface of an aluminum foil having a thickness of 15 μm, dried, and then roll-pressed to prepare a positive electrode sheet.

The graphite-based negative electrode active material (C) of each example and each comparative example, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were combined into C:SBR:CMC=99. : 0.5:0.5 by mass ratio, and mixed in ion-exchanged water to prepare a slurry for forming a negative electrode active material layer. This slurry was applied to the surface of a copper foil having a thickness of 10 μm, dried, and then roll-pressed to prepare a negative electrode sheet.

Also, two separator sheets each having a 4 μm-thick ceramic particle layer (HRL) formed on a 24 μm-thick PP/PE/PP three-layer porous polyolefin layer were prepared.

The prepared positive electrode sheet, negative electrode sheet, and two prepared separator sheets were superimposed and wound to prepare a wound electrode assembly. At this time, the HRL of the separator sheet was opposed to the positive electrode sheet. Electrode terminals were respectively attached by welding to the positive electrode sheet and the negative electrode sheet of the wound electrode body thus produced, and this was housed in a battery case having a liquid inlet.

Subsequently, a non-aqueous electrolyte was injected from the inlet of the battery case, and the inlet was airtightly sealed with a sealing screw. The non-aqueous electrolyte contains _LiPF6 as a supporting salt in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:3:4. A solution dissolved at a concentration of 1.0 mol/L was used. The wound electrode body was allowed to stand for a predetermined time to impregnate the non-aqueous electrolyte. Thereafter, this was subjected to initial charging and aging treatment at 60° C. to obtain a lithium ion secondary battery for evaluation.

<Low temperature resistance evaluation>
Each activated lithium ion secondary battery for evaluation was adjusted to have an SOC of 60%, and then placed in an environment of -10°C. Each lithium ion secondary battery for evaluation was charged at a current value of 10 C for 2 seconds. The amount of voltage change ΔV at this time was acquired, and the battery resistance was calculated using the current value and ΔV. Specifically, it was calculated as battery resistance=ΔV/current value. Then, when the resistance of the evaluation lithium ion secondary battery using the graphite-based negative electrode active material according to Sample 1 is set to 100, the evaluation lithium ion secondary battery using the graphite-based negative electrode active material according to the other samples A ratio of resistances of the following batteries was obtained. The results are shown in the column of "low temperature resistance ratio" in Table 1. In addition, when the value of the low temperature resistance ratio is 99.5 or less, it can be evaluated that the low temperature resistance is suitably reduced.

As shown in Table 1, the secondary batteries using the graphite-based negative electrode active material materials according to Samples 3 to 9, in which the crystallinity of the amorphous carbon layer was graded, had the crystallinity of the amorphous carbon layer of A secondary battery using the graphite-based negative electrode active material according to Sample 1, which is expensive, and the graphite according to Sample 2, which could not give a gradient to the crystallinity in the amorphous carbon layer due to insufficient firing time. It was confirmed that the low-temperature resistance was favorably reduced compared to a secondary battery using a negative electrode active material. Further, referring to samples 3 to 9 in which the crystallinity of the amorphous carbon layer is graded, when the specific surface area of the graphite-based negative electrode active material is 2.2 m ² /g to 8.3 m ² /g, , it was confirmed that the low-temperature resistance of the secondary battery is more preferably reduced.
From the above, it can be seen that the low-temperature resistance of the secondary battery is preferably reduced according to the graphite-based negative electrode active material disclosed herein.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

1 Graphite-based negative electrode active material 10, 10a, 10b Graphite particles 12, 12a, 12b Amorphous carbon layer 20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector Plate 50 positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formation portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium ion secondary battery

Claims (5)

graphite particles;
an amorphous carbon layer covering at least part of the surface of the graphite particles;
A graphite-based negative electrode active material comprising
In the TEM image of the cross section of the graphite-based negative electrode active material, the area from the surface of the graphite-based negative electrode active material to the graphite particles in the amorphous carbon layer is the A layer from the surface near the surface. , B layer, and C layer, the progress of crystallization in the A layer, B layer, and C layer increases in this order. When electron diffraction images are obtained for the A layer, the B layer, and the C layer, at least the value of the lattice spacing d in the C layer can be determined, and the d value in the A layer cannot be determined. The graphite-based negative electrode active material according to claim 1. 3. The graphite-based negative electrode active material according to claim 1, wherein the BET specific surface area is in the range of 2.2 m ² /g to 8.3 m ² /g. A method for producing a graphite-based negative electrode active material comprising graphite particles and an amorphous carbon layer covering at least a portion of the graphite particles, comprising the steps of:
a material preparation step of preparing an amorphous carbon material constituting the amorphous carbon layer and the graphite particles;
an adhesion step of mixing the amorphous carbon material and the graphite particles and adhering the amorphous carbon material to at least part of the surface of the graphite particles; and microwave heat treatment after the adhesion step A microwave heat treatment step for forming the amorphous carbon layer covering at least a portion of the graphite particles, wherein from the surface of the graphite-based negative electrode active material in the amorphous carbon layer When the space up to the graphite particles is divided into three equal layers from the side closer to the surface into A layer, B layer and C layer, the progress of crystallization in the A layer, B layer and C layer is in this order. growing;
A method for producing a graphite-based negative electrode active material, comprising: A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode comprises the graphite-based negative electrode active material according to any one of claims 1 to 3.

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