JP6787117B2 - Negative electrode for non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery capable of improving cycle characteristics.

In recent years, non-aqueous electrolyte secondary batteries (for example, Li-ion secondary batteries) have been used as secondary batteries that can be repeatedly charged and discharged with the aim of reducing oil consumption and greenhouse gases, and further diversifying and improving the efficiency of energy infrastructure. Attention is gathering. In particular, it is expected to be applied to electric vehicles, hybrid electric vehicles, and fuel cell vehicles. In electric vehicles, improvement in cruising range is required, and in the future, higher energy density of Li-ion secondary batteries will be further required.
Focusing on the negative electrode of the current Li-ion secondary battery, graphite electrodes are generally used. The theoretical capacity of graphite is 372 mAhg ^-1 (active material). On the other hand, Si and Sn have been attracting attention in recent years as active materials having a capacity higher than that of graphite. The theoretical capacity of Si is 4200 mAhg ^-1 (active material) and Sn is 990 mAhg ^-1 (active material). However, since Si has a capacity about 11 times that of graphite, the volume change due to occlusion and release of Li is also large. For example, Li storage increases the volume by about 3 times. Electrodes using active materials (Si, Sn), which have a larger capacity than graphite, undergo large volume changes due to charging and discharging, and desorption and current collection from the electrodes due to cutting of the conductive path of the electrodes and pulverization. There is a risk of peeling between the body and the mixture layer. This may cause a decrease in the life characteristics of the Li-ion secondary battery.

On the other hand, in recent years, it has been reported that the electrode structure is maintained and the life characteristics are improved by applying various polymer binders. For example, carboxymethyl cellulose, polyamide-imide, polyacrylic acid, sodium alginate can be mentioned.
In Patent Document 1, a high molecular weight crosslinked sodium polyacrylate is used as a main binder, and a low molecular weight non-crosslinked acrylic acid is used as an auxiliary binder. The main binder is responsible for maintaining the structure, and the auxiliary binder is responsible for adhesion to the active material. As described above, it has been reported that the improvement of the binder characteristics is effective for improving the life characteristics of the Li-ion secondary battery. On the other hand, in order to stably maintain the electrode structure, the electrode structure itself needs to be durable against volume changes.

As a result of diligent studies on the improvement of life characteristics, the present inventor has found that by keeping the composite elastic modulus and hardness of the active material layer of the negative electrode within an appropriate range, it is possible to suppress the change in the thickness of the mixture layer due to charge and discharge. It was found that it was possible to suppress the destruction of the mixture layer.

International Publication No. 2016/0518111

The present invention focuses on the above points, and an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery having excellent life characteristics.

In order to solve the problem, one aspect of the present invention is a negative electrode for a non-aqueous electrolyte secondary battery in which an active material layer is formed on a current collector, and the active material layer is composed of an active material and a conductive auxiliary agent. It is composed of a binder and the active material is SiOx (0 <x ≦ 1.5), and the composite elastic coefficient of the active material layer by nanoindentation measurement is 2.5 GPa or more and 3.5 GPa or less. It is characterized by being a range.

According to one aspect of the present invention, since the active material layer has a composite elastic modulus within the above-mentioned predetermined range, it is possible to provide a negative electrode for a non-aqueous electrolyte secondary battery having excellent life characteristics.

It is explanatory drawing which shows typically the cross section of the main part of the negative electrode for a non-aqueous electrolyte secondary battery which concerns on embodiment based on this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
It should be noted that the following detailed description describes many specific details to provide a complete understanding of embodiments of the present invention. However, it will be clear that one or more embodiments can be implemented without such specific details. Other well-known structures and devices are shown schematic to simplify the drawings.
FIG. 1 is an explanatory diagram schematically showing a cross section of a main part of a negative electrode 1 for a non-aqueous electrolyte secondary battery (hereinafter, also simply referred to as “negative electrode 1”) according to the present embodiment. As shown in FIG. 1, the negative electrode 1 has a structure in which the active material layer 3 is laminated on the current collector 2. Although FIG. 1 illustrates the case where the active material layer 3 is one layer, it may be two or more layers.

<Current collector>
The current collector 2 has a foil-like or plate-like shape. The material of the current collector 2 is not particularly limited as long as it is a conductive material, and for example, metal materials such as stainless steel, nickel, aluminum, iron, titanium, copper, palladium, gold and platinum may be used. it can.
<Active material layer>
Hereinafter, the structure of the active material layer 3 will be described.
The active material layer 3 of the present embodiment contains at least a binder, an active material, and a conductive auxiliary agent as main agents. The active material layer 3 according to the present embodiment is formed on the current collector 2 by coating and drying the electrode slurry formed by mixing these materials with a solvent on the current collector 2. ..

(Binder)
The binder preferably contains an ethylenically unsaturated carboxylic acid compound as a repeating unit and is a crosslinked polymer.
Specifically, as the binder of the present embodiment, as a polymer composed of an ethylenically unsaturated carboxylic acid compound, for example, polyacrylic acid, maleic acid maleic acid copolymer, styrene acrylate copolymer, acetic acid acrylate. Sodium salts such as vinyl polymers, lithium salts, potassium salts, magnesium salts, calcium salts, ammonium salts and the like are desirable. In particular, sodium polyacrylate is desirable from the viewpoint of improving the life characteristics as described later. Further, the polymer composed of these ethylenically unsaturated carboxylic acid compounds may be subjected to a cross-linking treatment with a cross-linking agent. The cross-linking agent is not particularly limited as long as it is an aqueous cross-linking agent that reacts with a carboxylic acid, but a carbodiimide-based compound or an aziridine-based compound that can react in a few minutes at room temperature is preferably used. In particular, an aziridine compound is desirable.

Further, as an auxiliary binder, a polycarboxylic acid having a molecular weight of 10,000 or less may be used as an auxiliary binder for adhering to the active material and adhering to the current collector. The polycarboxylic acid is not particularly limited as long as it is a polymer containing a repeating unit having a carboxylic acid group, but for example, alginic acid, polyacrylic acid, maleic acid maleic acid copolymer, carboxymethyl cellulose and the like are desirable. In particular, a maleic anhydride copolymer is desirable.

(Active material)
The active material of the present embodiment is not particularly limited as long as it can reversibly occlude and release Li, and known materials can be used, but it is desirable to use a material that alloys with Li. In particular, if the material has a larger capacity than graphite, the effect of the present embodiment can be remarkably obtained.
As the material to be alloyed with Li, one or more alloys selected from the group consisting of Si, Ge, Sn, Pb, Al, Ag, Zn, Hg, and Au can be used. Preferably, it is SiOx. At this time, x is preferably larger than 0 and 1.5 or less. When x is larger than 1.5, a sufficient amount of Li storage and release cannot be secured. Moreover, not only such an active material but also graphite may be added as an active material. x is more preferably in the range of 0 <x ≦ 0.5.

(Conductive aid)
As the conductive auxiliary agent, carbon black, natural graphite, artificial graphite, metal oxides such as titanium oxide and ruthenium oxide, metal fibers and the like can be used. Of these, carbon black, which exhibits a structural structure, is preferable, and one of them, furnace black, ketjen black, and acetylene black (AB) are particularly desirable. A mixed system of carbon black and other conductive agent, for example, vapor-grown carbon fiber (VGCF) is also preferable.

As the solvent of the electrolytic solution used for the non-aqueous electrolyte secondary battery, a low-viscosity chain carbonate such as dimethyl carbonate and diethyl carbonate, a cyclic carbonate having a high dielectric constant such as ethylene carbonate, propylene carbonate and butylene carbonate, and γ- Butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, methyl acetate, methyl propionate, vinylene carbonate, dimethylformamide, sulfolane, and a mixed solvent thereof and the like can be mentioned.
The electrolyte contained in the electrolytic solution is not particularly limited, and known ones can be used, but LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiI, LiAlCl. ₄ mag and a mixture thereof can be used. Preferably, a lithium salt obtained by mixing one or more of LiBF ₄ and LiPF ₆ is preferable.

The active material layer 3 made of the above material preferably has a composite elastic modulus in the range of 2.5 GPa or more and 3.5 GPa or less as measured by nanoindentation.
By adopting the active material layer 3 having a composite elastic modulus in such a range, the active material layer 3 is appropriately expanded, the collapse of the active material layer 3 due to internal stress can be suppressed, and the contraction after expansion is also performed. It is done moderately and the change in film thickness can be minimized.
Further, the hardness of the active material layer 3 is preferably in the range of 0.030 GPa or more and 0.050 GPa or less as measured by nanoindentation.
By defining the hardness of the active material layer 3 in the above range, the internal stress due to the expansion of the active material layer 3 is dispersed throughout the mixture layer, and local stress concentration can be avoided. ..

(Effect of this embodiment)
The invention according to the present embodiment has the following effects.
(1) The negative electrode 1 for a non-aqueous electrolyte secondary battery according to the present embodiment is a negative electrode for a non-aqueous electrolyte secondary battery in which an active material layer 3 is formed on a current collector 2, and the active material layer 3 is The active material is SiOx (0 <x ≦ 1.5), and is composed of a conductive additive and a binder, and the composite elastic coefficient of the active material layer 3 by nanoindentation measurement is 2.5 to 2. It is in the range of 3.5 GPa.
According to such a configuration, the active material layer 3 is appropriately expanded, the collapse of the active material layer 3 due to internal stress can be suppressed, and the contraction after expansion is also appropriately performed to minimize the change in film thickness. it can.

(2) In the negative electrode 1 for a non-aqueous electrolyte secondary battery according to the present embodiment, the hardness of the active material layer 3 is in the range of 0.030 to 0.050 GPa as measured by nanoindentation.
According to such a configuration, the internal stress accompanying the expansion of the active material layer 3 is dispersed in the entire mixture layer, and local stress concentration can be avoided.
(3) In the negative electrode 1 for a non-aqueous electrolyte secondary battery according to the present embodiment, the binder is a polymer containing an ethylenically unsaturated carboxylic acid compound as a repeating unit and crosslinked.
According to such a configuration, in the active material layer 3 having the composite elastic modulus and hardness shown in claims 1 and 2, the carboxylic acid group of the binder is firmly bonded to the current collector 2 and the active material and crosslinked. Depending on the structure, it can withstand an internal stress load, so that an active material layer having the smallest change in film thickness is provided.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to any examples.
(Example 1)
24.63 g of sodium polyacrylate (manufactured by Nippon Shokubai Co., Ltd.) was added to 471.66 g of water and stirred with a dispa to prepare a polymer solution. 3.71 g of a 10% aqueous solution of an aziridine compound (PZ-33) was added to the polymer solution, and the mixture was stirred at room temperature for 20 minutes to prepare an aqueous sodium polyacrylate solution.

Subsequently, in 33.53 g of the prepared sodium polyacrylate aqueous solution, 5.88 g of Si (average particle size 200 nm), 1.18 g of AB, 1.18 g of VGCF, and 0.18 g of a 50% aqueous solution of maleic acid maleic acid copolymer. , 8.06 g of water was added and stirred. Subsequently, this dispersion was carried out with a fill mix to obtain an electrode slurry.
The obtained slurry was applied to a current collector. A copper foil having a thickness of 12 μm was used as the current collector. The slurry was applied with a doctor blade so as to have a basis weight of 0.71 mg / cm ² . Subsequently, it was pre-dried at 80 ° C. for 30 minutes. This was pressed so that the density was 0.4 g / cm ³ . Finally, it was dried under reduced pressure at 105 ° C. for 5 hours to obtain an electrode.

(Comparative Example 1)
24.63 g of sodium polyacrylate (manufactured by Nippon Shokubai Co., Ltd.) was added to 471.66 g of water and stirred with a dispa to prepare a polymer solution. 3.71 g of a 10% aqueous solution of an aziridine compound (PZ-33) was added to the polymer solution, and the mixture was stirred at room temperature for 20 minutes to prepare an aqueous sodium polyacrylate solution.
Subsequently, in 33.53 g of the prepared sodium polyacrylate aqueous solution, 5.88 g of Si (average particle size 200 nm), 1.18 g of AB, 1.18 g of VGCF, and 0.18 g of a 50% aqueous solution of maleic acid maleic acid copolymer. , 8.06 g of water was added and stirred. Subsequently, this dispersion was carried out with a fill mix to obtain an electrode slurry.
The obtained slurry was applied to a current collector. A copper foil having a thickness of 12 μm was used as the current collector. The slurry was applied with a doctor blade so as to have a basis weight of 0.71 mg / cm ² . Subsequently, it was pre-dried at 80 ° C. for 30 minutes. This was pressed so that the density was 1.5 g / cm ³ . Finally, it was dried under reduced pressure at 105 ° C. for 5 hours to obtain an electrode.

(Comparative Example 2)
25.00 g of sodium polyacrylate (manufactured by Nippon Shokubai Co., Ltd.) was added to 475.00 g of water and stirred with a dispa to prepare an aqueous sodium polyacrylate solution.
Subsequently, in 33.53 g of the prepared sodium polyacrylate aqueous solution, 5.88 g of Si (average particle size 200 nm), 1.18 g of AB, 1.18 g of VGCF, and 0.18 g of a 50% aqueous solution of maleic acid maleic acid copolymer. , 8.06 g of water was added and stirred. Subsequently, this dispersion was carried out with a fill mix to obtain an electrode slurry.

The obtained slurry was applied to a current collector. A copper foil having a thickness of 12 μm was used as the current collector. The slurry was applied with a doctor blade so as to have a basis weight of 0.71 mg / cm ² . Subsequently, it was pre-dried at 80 ° C. for 30 minutes. This was pressed so that the density was 0.4 g / cm ³ . Finally, it was dried under reduced pressure at 105 ° C. for 5 hours to obtain an electrode.
(Evaluation by nanoindentation measurement)
The obtained electrodes were fixed to the stage, and continuous rigidity measurement (MTS Systems patented technology) was performed 5 times each. Then, the measured data was analyzed to obtain the composite elastic modulus and the distribution of hardness in the depth direction. An Indenter XP manufactured by MTS Systems Corporation was used as the apparatus, and Berkovich (trigonal pyramidal shape) was used as the indenter used.

(Cell preparation and charge / discharge evaluation)
A coin cell was produced using the obtained electrode and Li foil, and charge / discharge evaluation of Example 1 and Comparative Examples 1 and 2 was performed. Charging and discharging were repeated 10 times in a voltage range of 0.03-1.0 V at 2100 mA / g for charging and 2100 mA / g for discharging. Then, the coin cell was disassembled and the film thickness of the electrode was measured. A 2032 type coin cell was used. The electrode was punched into a disk having a diameter of 15 mm, and the Li foil was punched into a disk having a diameter of 16 mm. The basic configuration of the coin cell was an electrode, Li foil, and a separator (Asahi Kasei Corporation, Hypore ND525). The electrolytic solution is a mixed solution of ethylene carbonate (EC) containing 10 wt% of fluoroethylene carbonate (FEC) and diethyl carbonate (DMC) in a ratio of 3: 7 (v / v), to which LiPF6 is added so as to be 1 M. used.

The evaluation results are shown in Table 1.

In Table 1, nanoindentation measurements of the electrodes were performed. In the evaluation of the film thickness change, the film thickness after one charge / discharge is measured with respect to the film thickness of the active material layer before the battery evaluation, and those having a film thickness increase rate of less than 15% are ○ and 15%. The above items were evaluated as x.
The contact depth was approximately 1.57 nm. The electrode of Example 1 had a composite elastic modulus of 2.6 GPa and a hardness of 0.037 GPa. In Comparative Example 1, the composite elastic modulus was 4.0 GPa and the hardness was 0.060 GPa. In Comparative Example 2, the composite elastic modulus was 1.9 GPa and the hardness was 0.028 GPa.

In Example 1, the film thickness did not change.
On the other hand, the film thicknesses of Comparative Example 1 and Comparative Example 2 changed significantly. In particular, in Comparative Example 1, many cracks were observed on the electrode surface, and some of them were peeled off from the current collector. Compared with Example 1, Comparative Example 1 comprises a harder active material layer. From this, it was clarified that if the values of the composite elastic modulus and the hardness are too high, the active material layer collapses due to the internal stress due to the volume change of the active material layer.
Further, Comparative Example 2 is provided with a soft active material layer as compared with Example 1. Although the number of cracks and peeling points from the current collector was smaller than that in Comparative Example 1, the increase in film thickness was the largest. From this, if the values of the composite elastic modulus and the hardness are too low, the contraction of the active material layer after the volume expansion is insufficient, and the active material layer remains expanded as compared with Example 1 and Comparative Example 2. It became clear that

The negative electrode for a non-aqueous electrolyte secondary battery according to the present invention is a power source for various portable electronic devices, a storage battery for driving an electric vehicle or the like that requires high energy density, and various energies such as solar energy and wind power generation. It is used as an electrode for a power storage device or a storage power source for household electric appliances.

1 Negative electrode for non-aqueous electrolyte secondary battery 2 Current collector 3 Active material layer

Claims (6)

A negative electrode for a non-aqueous electrolyte secondary battery in which an active material layer is formed on a current collector.
The active material layer is composed of an active material, a conductive auxiliary agent, and a binder.
The active material is SiOx (0 <x ≦ 1.5).
The composite elastic modulus of the active material layer measured by nanoindentation is in the range of 2.5 GPa or more and 3.5 GPa or less .
The binder is a polymer containing an ethylenically unsaturated carboxylic acid compound as a repeating unit and crosslinked using a crosslinking agent.
The crosslinking agent, a negative electrode for a nonaqueous electrolyte secondary battery, wherein the water-based crosslinking agent der Rukoto of reacting with the carboxylic acid. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the hardness of the active material layer is in the range of 0.030 GPa or more and 0.050 GPa or less as measured by nanoindentation. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the cross-linking agent is a carbodiimide-based compound or an aziridine-based compound. The active material layer is further composed of an auxiliary binder.
The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the auxiliary binder is a polycarboxylic acid having a molecular weight of 10,000 or less. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 4, wherein the auxiliary binder is alginic acid, polyacrylic acid, a maleic acid maleic acid copolymer, or carboxymethyl cellulose. The negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the content of the conductive auxiliary agent is larger than the content of the binder.

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