JP6897253B2 - Negative electrode for lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a negative electrode for a lithium ion secondary battery.

International Publication No. 2016/121322 (Patent Document 1) describes a negative electrode active material containing a carbon material and silicon oxide, carboxymethyl cellulose (CMC), and partially neutralized polyacrylic acid (hereinafter, abbreviated as “PAA”). (In some cases), the negative electrode for a non-aqueous electrolyte secondary battery is disclosed.

International Publication No. 2016/121322

In Patent Document 1, the reason why silicon oxide is used as the negative electrode active material is that silicon oxide has a large charge / discharge capacity. The adoption of silicon oxide is expected to increase the capacity of non-aqueous electrolyte secondary batteries.

However, silicon oxide has a larger expansion rate and contraction rate during charging and discharging than carbon materials. Therefore, the carbon material may not sufficiently follow the expansion and contraction of silicon oxide during charging and discharging, and the silicon oxide in the negative electrode mixture layer may be isolated, resulting in a decrease in battery cycle durability. Further, silicon oxide has low electron conductivity. The low electron conductivity of silicon oxide is also considered to be a factor that lowers the cycle durability. In addition, in this specification, "cycle durability" means the resistance to the decrease of the discharge capacity of a battery when the number of cycles is increased, with charging and discharging (charging / discharging) as one cycle. That is, when the number of cycles increases, the cycle durability is high when the decrease in the discharge capacity of the battery is small as compared with the initial state of the battery (for example, the state immediately after the battery is manufactured).

In Patent Document 1, since the negative electrode mixture layer has a one-layer structure, silicon oxide and PAA are present in the entire negative electrode mixture layer. Therefore, there is a possibility that silicon oxide and PAA are separated from each other in the negative electrode mixture layer. Therefore, it is considered that the isolation of silicon oxide cannot be sufficiently suppressed. Furthermore, PAA increases battery resistance. That is, in the negative electrode for a non-aqueous electrolyte secondary battery disclosed in Patent Document 1, there is room for improvement in the decrease in cycle durability.

An object of the present disclosure is to suppress a decrease in cycle durability in a lithium ion secondary battery (hereinafter, may be abbreviated as "battery") containing silicon oxide in the negative electrode.

Hereinafter, the technical configuration and the action and effect of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The correctness of the mechanism of action should not limit the scope of this disclosure.

The negative electrode for a lithium ion secondary battery includes a current collector foil and a negative electrode mixture layer. The negative electrode mixture layer includes a first negative electrode mixture layer and a second negative electrode mixture layer. The first negative electrode mixture layer is arranged on the current collector foil, and the second negative electrode mixture layer is arranged on the first negative electrode mixture layer. The first negative electrode mixture layer contains graphite, silicon oxide and polyacrylic acid. The second negative electrode mixture layer contains graphite and does not contain silicon oxide and polyacrylic acid.

Silicon oxide and PAA are localized in the negative electrode mixture layer of the present disclosure. That is, the first negative electrode mixture layer contains silicon oxide and PAA, but the second negative electrode mixture layer does not contain silicon oxide and PAA. Here, PAA has a strong binding force. Therefore, it is considered that silicon oxide and PAA are bound in the first negative electrode mixture layer. Since PAA has a strong binding force, it is considered that the bonded state of silicon oxide and PAA is maintained even when the silicon oxide expands and contracts. It is considered that this prevents silicon oxide from being isolated in the negative electrode mixture layer.

The second negative electrode mixture layer containing graphite and not silicon oxide and PAA has good electron conductivity. Since the negative electrode mixture layer contains the second negative electrode mixture layer, the decrease in electrical conductivity due to the inclusion of silicon oxide and PAA in the first negative electrode mixture layer, in other words, the increase in battery resistance is suppressed. It is thought that. It is expected that the synergistic action of the above will suppress the decrease in the cycle durability of the battery.

FIG. 1 is a conceptual cross-sectional view showing the configuration of a negative electrode for a lithium ion secondary battery according to the embodiment of the present disclosure. FIG. 2 is a schematic view showing the configuration of the electrode group. FIG. 3 is a schematic view showing the configuration of a lithium ion secondary battery.

Hereinafter, embodiments of the present disclosure (hereinafter, also referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of the present disclosure.

<Negative electrode for lithium ion secondary battery>
FIG. 1 is a conceptual cross-sectional view showing the configuration of a negative electrode for a lithium ion secondary battery according to the embodiment of the present disclosure. The negative electrode 20 includes a current collecting foil 22 and a negative electrode mixture layer 21.

《Current collector foil》
The current collector foil 22 may be, for example, a copper (Cu) foil or the like. The Cu foil may be a pure Cu foil or a Cu alloy foil. The current collector foil 22 may have a thickness of, for example, 5 to 30 μm.

<< Negative electrode mixture layer >>
The negative electrode mixture layer 21 is arranged on the current collector foil 22. The negative electrode mixture layer 21 includes at least a two-layer structure. That is, the negative electrode mixture layer 21 includes the first negative electrode mixture layer 23 and the second negative electrode mixture layer 24. The first negative electrode mixture layer 23 and the second negative electrode mixture layer 24 are laminated on the current collector foil 22 in this order. That is, the first negative electrode mixture layer 23 is arranged on the current collector foil 22, and the second negative electrode mixture layer 24 is arranged on the first negative electrode mixture layer 23. The negative electrode mixture layer 21 may have a thickness of, for example, 10 to 150 μm.

<< 1st negative electrode mixture layer >>
The first negative electrode mixture layer 23 contains graphite 25 and silicon oxide 26, which are negative electrode active materials, and PAA 27, which is a binder. That is, the first negative electrode mixture layer 23 contains graphite 25, silicon oxide 26, and PAA 27. Silicon oxide 26 has a larger charge / discharge capacity than graphite 25, and PAA27 has a strong binding force.

(graphite)
Graphite 25 functions as a negative electrode active material. Graphite 25 may have an average particle size of, for example, 1 to 30 μm. The "average particle size" in the present specification indicates a cumulative 50% particle size from the fine particle side in the volume-based particle size distribution measured by the laser diffraction / scattering method. The graphite 25 may be natural graphite or artificial graphite. The shape of the graphite 25 is not particularly limited, and may be spherical, scaly, scaly, flaky, or any other shape. You may. The content of graphite 25 may be, for example, 90 parts by weight or more and 99 parts by weight or less with respect to a total of 100 parts by weight of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21.

(Silicon oxide)
The silicon oxide 26 functions as a negative electrode active material and has a charge / discharge capacity larger than that of the graphite 25. Therefore, when the first negative electrode mixture layer 23 contains silicon oxide 26 as the negative electrode active material, it is expected that the charge / discharge capacity of the battery will increase. Silicon oxide 26 contains silicon (Si) and oxygen (O). Silicon oxide 26 can have any conventionally known atomic ratio. The silicon oxide 26 may be represented by, for example, the composition formula SiO _{x (where x in the} formula satisfies 0.5 ≦ x ≦ 1.5). The silicon oxide 26 may have an average particle size of, for example, 0.1 to 30 μm.

The content of silicon oxide 26 is preferably 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight in total of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21. When the content of silicon oxide 26 is less than 1 part by weight, it may not be desired to increase the charge / discharge capacity of the battery. When the content of the silicon oxide 26 exceeds 10 parts by weight, it tends to be difficult to suppress the decrease in the cycle durability of the battery.

(PAA)
PAA27 has a strong binding force. Here, the above-mentioned silicon oxide 26 has a property that the expansion rate and the contraction rate at the time of charging / discharging are large. In the present embodiment, it is considered that the state in which the PAA 27 and the silicon oxide 26 are bonded is maintained even when the silicon oxide 26 expands. It is considered that the isolation of the silicon oxide 26 in the negative electrode mixture layer 21 can be suppressed by the inclusion of the PAA 27 in the first negative electrode mixture layer 23. The weight average molecular weight of PAA27 may be, for example, 5000 or more and 1 million or less.

The content of PAA27 is preferably 2 parts by weight or more and 12 parts by weight or less with respect to 100 parts by weight in total of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21. When the content of PAA27 is less than 2 parts by weight, the absolute amount of PAA27 contained in the first negative electrode mixture layer 23 is small, so that the binding force between PAA27 and silicon oxide 26 becomes insufficient, and the negative electrode mixture layer 21 Silicon oxide 26 may be isolated in. When the content of PAA27 exceeds 12 parts by weight, the battery resistance tends to increase.

(Other ingredients)
The first negative electrode mixture layer 23 may contain a conductive material and a binder in addition to graphite 25, silicon oxide 26 and PAA27. The conductive material should not be particularly limited. The conductive material may be, for example, acetylene black (AB), scaly graphite, thermal black, furnace black, vapor-grown carbon fiber (VGCF), graphene flakes and the like. The content of the conductive material may be, for example, 3 parts by weight or more and 7 parts by weight or less with respect to 100 parts by weight of the total of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21. The binder should not be particularly limited either. The binder may be, for example, CMC, styrene-butadiene rubber (SBR), or the like. The content of the binder may be, for example, 0.3 parts by weight or more and 1 part by weight or less with respect to 100 parts by weight of the total of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21. The conductive material and the binder material may be used individually by 1 type, or may be used in combination of 2 or more types.

<< Second negative electrode mixture layer >>
The second negative electrode mixture layer 24 contains graphite 25, which is a negative electrode active material, but does not contain silicon oxide 26, which is a negative electrode active material, and PAA 27, which is a binder. That is, the second negative electrode mixture layer 24 contains graphite 25 and does not contain silicon oxide 26 and PAA27. Graphite 25 may have an average particle size of, for example, 1 to 30 μm. The graphite 25 may be natural graphite or artificial graphite. The shape of the graphite 25 is not particularly limited, and may be spherical, scaly, scaly, or any other shape. Further, the graphite 25 contained in the second negative electrode mixture layer 24 may be the same as or different from the graphite 25 contained in the first negative electrode mixture layer 23.

The second negative electrode mixture layer 24 may contain a binder in addition to the graphite 25. As the binder, the binder described in the first negative electrode mixture layer 23 can be used in the same manner. The content of the binder may be, for example, 0.5 parts by weight or more and 3 parts by weight or less with respect to 100 parts by weight of the total of graphite 25 and silicon oxide 26 contained in the entire negative electrode mixture layer 21.

The thickness of the second negative electrode mixture layer 24 may be the same as or different from the thickness of the first negative electrode mixture layer 23. The second negative electrode mixture layer 24 may be thicker than the first negative electrode mixture layer 23. For example, the ratio of the thickness of the second negative electrode mixture layer 24 to the thickness of the first negative electrode mixture layer 23 may be 1 or more and 4 or less, or 1.8 or more and 4 or less. It may be 2 or more and 4 or less, or 2.33 or more and 4 or less.

The second negative electrode mixture layer 24 does not contain silicon oxide 26 and PAA27, but as a result of migration, a part of silicon oxide 26 or PAA27 contained in the first negative electrode mixture layer 23 (for example, about 10%). However, it may be contained in the second negative electrode mixture layer 24. Even if a trace amount of silicon oxide 26 or PAA27 is contained in the second negative electrode mixture layer 24 due to migration, it is included in the scope of the present disclosure.

<Positive electrode>
The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the main surface of the positive electrode current collector. The positive electrode current collector may be, for example, an aluminum (Al) foil or the like. The Al foil may be a pure Al foil or an Al alloy foil. The positive electrode current collector may have a thickness of, for example, 10 to 30 μm.

<< Positive electrode mixture layer >>
The positive electrode mixture layer contains a positive electrode active material, a conductive material and a binder. The positive electrode mixture layer may contain, for example, 80 to 98% by weight of the positive electrode active material, 1 to 15% by weight of the conductive material, and 1 to 5% by weight of the binder. The positive electrode mixture layer may have a thickness of, for example, 100 to 200 μm.

(Positive electrode active material, conductive material and binder)
The positive electrode active material, the conductive material and the binder should not be particularly limited. The positive electrode active material may be, for example, LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM), LiMnO ₂ , LiMn ₂ O ₄ , LiFePO _4, or the like. The conductive material may be, for example, acetylene black (AB), furnace black, vapor-grown carbon fiber (VGCF), graphite or the like. The binder may be, for example, PVdF, PTFE or the like.

<Separator>
The separator is an electrically insulating porous membrane. The separator electrically separates the positive electrode and the negative electrode. The separator may have a thickness of, for example, 5 to 30 μm. The separator may be composed of, for example, a porous polyethylene (PE) film, a porous polypropylene (PP) film, or the like. The separator may include a multi-layer structure. For example, the separator may be composed of a porous PP film, a porous PE film, and a porous PP film laminated in this order. The separator may include a heat-resistant layer on its surface. The heat-resistant layer contains a heat-resistant material. Examples of the heat-resistant material include metal oxide particles such as alumina and a refractory resin such as polyimide.

<Electrolytic solution>
The electrolyte contains a lithium salt and a solvent. The solvent is aprotic. The solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio of the cyclic carbonate and the chain carbonate may be a volume ratio, for example, cyclic carbonate: chain carbonate = 1: 9 to 5: 5. The cyclic carbonate may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or the like. The chain carbonate may be, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or the like. The electrolytic solution contains, for example, 0.5 to 2.0 mL / L of lithium salt. The lithium salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ], Li [N (CF ₃ SO ₂ ) ₂ ], or the like.

<Battery case>
The battery case may be, for example, square (flat rectangular parallelepiped), cylindrical, or bag-shaped. For example, a metal such as aluminum (Al) or an Al alloy constitutes a battery case. However, as long as the battery case has a predetermined airtightness, for example, a composite material of metal and resin may form the battery case. Examples of the metal and resin composite material include an aluminum laminated film and the like. The battery case may include an external terminal, a liquid injection hole, a gas discharge valve, a current cutoff mechanism (CID), and the like.

<Lithium-ion secondary battery>
The lithium ion secondary battery includes an electrode body having a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode, and the electrode body is arranged in a battery case together with an electrolytic solution. The electrode body is configured by winding or laminating a positive electrode and a negative electrode with a separator interposed therebetween.

Hereinafter, examples will be described. However, the following examples do not limit the scope of this disclosure.

<Example 1>
1. 1. Manufacture of negative electrode The following materials were prepared.
Negative electrode active material: graphite, silicon oxide Binder: PAA, CMC, SBR
Conductive material: Scaly graphite Solvent: Water Current collector foil: Cu foil (thickness = 10 μm)

25 parts by weight of graphite, 5 parts by weight of silicon oxide, 0.45 parts by weight of CMC, 10.5 parts by weight of PAA, 5 parts by weight of scaly graphite by planetary mixer (Not included in graphite) and water were mixed. As a result, a paste-like first negative electrode mixture (hereinafter, referred to as “first negative electrode mixture paste”) was prepared.

70 parts by weight of graphite, 1.05 parts by weight of CMC, 0.7 parts by weight of SBR and water were mixed by a planetary mixer. As a result, a paste-like second negative electrode mixture (hereinafter, referred to as “second negative electrode mixture paste”) was prepared.

The first negative electrode mixture paste was applied to the surface (both front and back surfaces) of the current collector foil 22 by a die coater and dried. As a result, the first negative electrode mixture layer 23 was arranged on the current collector foil 22. The second negative electrode mixture paste was applied to the surface of the first negative electrode mixture layer 23 by a die coater, dried, and then pressed. As a result, the second negative electrode mixture layer 24 was arranged on the first negative electrode mixture layer 23, and the negative electrode mixture layer 21 was formed.

The negative electrode mixture layer 21 was compressed by the roll rolling mill. As a result, the negative electrode 20 was manufactured. The thickness ratio of the first negative electrode mixture layer to the second negative electrode mixture layer was 3: 7 (that is, the ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer). Is 2.33). The negative electrode 20 was cut to a predetermined size. The configuration of the negative electrode 20 is as follows.

Width of negative electrode mixture layer: 122 mm
Negative electrode mixture layer length: 5835 mm
Negative electrode thickness: 144 μm

2. Manufacture of positive electrode The following materials were prepared. Positive electrode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM)
Conductive material: AB
Binder: PVdF
Solvent: N-methyl-2-pyrrolidone Current collector foil: Aluminum (Al) foil (thickness = 15 μm)

NCM93 parts by weight, AB4 parts by weight, PVdF3 parts by weight and N-methyl-2-pyrrolidone were mixed by a planetary mixer. As a result, a paste-like positive electrode mixture (hereinafter, referred to as “positive electrode mixture paste”) was prepared. The positive electrode mixture paste was applied to the surface (both front and back sides) of the current collector foil by a die coater and dried. As a result, a positive electrode mixture layer was formed. The positive electrode mixture layer was compressed by the roll rolling mill. As a result, a positive electrode was manufactured. The positive electrode was cut to a predetermined size. The configuration of the positive electrode is as follows.

Width of positive electrode mixture layer: 117 mm
Length of positive electrode mixture layer: 5681 mm
Positive electrode thickness: 142 μm

3. 3. Assembly A separator with a thickness of 20 μm was prepared. The separator is composed of a polypropylene porous membrane, a polyethylene porous membrane, and a polypropylene porous membrane laminated in this order.

FIG. 2 is a schematic view showing the configuration of the electrode group. The positive electrode 10, the separator 30, the negative electrode 20, and the separator 30 were laminated so that the positive electrode 10 and the negative electrode 20 faced each other with the separator 30 interposed therebetween, and were further wound in a spiral shape. As a result, a cylindrical electrode group was formed. A group of cylindrical electrodes was formed into a flat shape by a flat plate press. The molding pressure was 4 kN / cm ² . The molding time was set to 2 minutes. As a result, the electrode group 50 was manufactured.

FIG. 3 is a schematic view showing the configuration of a lithium ion secondary battery. The battery case 80 was prepared. The battery case 80 is square. The battery case 80 is made of Al alloy. The electrode group 50 was housed in the battery case 80. An external terminal was connected to the electrode group 50. 120 ml of electrolyte was injected into the battery case 80. The electrolytic solution has the following composition.

Solute: LiPF ₆ (1.0 mol / l)
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)]
EC indicates ethylene carbonate, DMC indicates dimethyl carbonate, and EMC indicates ethyl methyl carbonate.

The battery case 80 was sealed. From the above, the battery 100 was manufactured. The battery 100 is designed to have a rated capacity of 35 Ah in the voltage range of 3.0 to 4.1 V. From the above, the battery according to Example 1 shown in Table 1 was obtained.

<Examples 2 to 4>
As shown in Table 1 below, the negative electrode was manufactured by the same manufacturing method as in Example 1 except that the content of PAA contained in the first negative electrode mixture layer was changed, and the battery was manufactured.

<Comparative example 1>
95 parts by weight of graphite, 5 parts by weight of silicon oxide, 1.50 parts by weight of CMC, 1 part by weight of SBR and water were mixed by a planetary mixer. This prepared the paste. The paste was applied to the surface of the current collector foil by a die coater and dried. As a result, a single negative electrode mixture layer was formed. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as in Example 1.

<Comparative example 2>
95 parts by weight of graphite, 5 parts by weight of silicon oxide, 1.50 parts by weight of CMC, 0.7 parts by weight of SBR, 10.5 parts by weight of PAA and water were mixed by a planetary mixer. This prepared the paste. The paste was applied to the surface of the current collector foil by a die coater and dried. As a result, a single negative electrode mixture layer was formed. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as in Example 1.

<Comparative example 3>
95 parts by weight of graphite, 5 parts by weight of silicon oxide, 1.50 parts by weight of CMC, 0.7 parts by weight of SBR, 10.5 parts by weight of PAA, 5 parts by weight of scaly graphite and water were mixed by a planetary mixer. This prepared the paste. The paste was applied to the surface of the current collector foil by a die coater and dried. As a result, a single negative electrode mixture layer was formed. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as in Example 1.

<Comparative example 4>
95 parts by weight of graphite, 5 parts by weight of silicon oxide, 1 part by weight of CMC, 5 parts by weight of PAA and water were mixed by a planetary mixer. This prepared the paste. The paste was applied to the surface of the current collector foil by a die coater and dried. As a result, a single negative electrode mixture layer was formed. Except for this, the negative electrode was manufactured and the battery was manufactured by the same manufacturing method as in Example 1.

<Evaluation of cycle durability>
One cycle of constant current charging and constant current discharging was defined as one cycle, and 500 cycles were carried out.

Constant current charging: Current = 2C, end voltage = 4.1V
Constant current discharge: Current = 2C, end voltage = 2.5V
The current of 1C is a constant current at which the battery whose charge state is adjusted to 100% is discharged and the discharge is completed in exactly one hour.

The capacity retention rate was calculated by dividing the discharge capacity after 500 cycles by the initial discharge capacity. The results are shown in Table 1 below. In Table 1 below, the values shown in the column of "cycle durability" are values obtained by dividing the capacity retention rate of each example by the capacity retention rate of Comparative Example 1. The larger the value, the more the decrease in cycle durability is suppressed.

<Initial IV resistance measurement test>
An IV resistance measurement test was performed on the batteries according to each of the examples and comparative examples in the initial state. The batteries according to the Examples and Comparative Examples in the initial state were charged in an environment of a temperature of 25 ° C., and adjusted to a charged state of SOC 50%. Then, pulse discharge was performed at 25 ° C. with a current of 5 C for 10 seconds, and the IV resistance value (mΩ) was calculated from the amount of voltage drop 10 seconds after the start of discharge. The results are shown in Table 1 below. In Table 1 below, the values shown in the “Initial IV resistance” column are the values obtained by dividing the IV resistance value of each example by the IV resistance value of Comparative Example 1. The smaller the value, the smaller the resistance.

<Result>
As shown in Table 1 above, in Examples 1 to 4, the decrease in cycle durability is suppressed as compared with Comparative Examples 1 to 4. In Examples 1 to 4, since silicon oxide and PAA are localized in the first negative electrode mixture layer, the negative electrode mixture is compared with Comparative Examples 1 to 4 in which silicon oxide and PAA are not localized. It is considered that a large amount of silicon oxide and PAA are bound in the layer. Therefore, it is expected that a battery containing silicon oxide in the negative electrode will be provided in which a decrease in cycle durability is suppressed.

Comparative Example 1 has low cycle durability. In Comparative Example 1, the negative electrode mixture layer is a single layer and does not contain PAA. Therefore, it is considered that the silicon oxide is isolated due to the expansion and contraction of the silicon oxide, and as a result, the cycle durability is deteriorated.

In Example 1 and Comparative Examples 2 and 3, the same amount of silicon oxide and PAA were contained in the negative electrode mixture layer, but the cycle durability of Example 1 was remarkably excellent. In Example 1, silicon oxide and PAA are contained only in the first negative electrode mixture layer. Here, the first negative electrode mixture layer has only 30% of the thickness of the entire negative electrode mixture layer. Therefore, it is considered that silicon oxide and PAA are present in close proximity to each other in the first negative electrode mixture layer, and a large amount of silicon oxide and PAA are bound to each other. On the other hand, in Examples 2 and 3, since silicon oxide and PAA were contained in the entire negative electrode mixture layer of the single layer, silicon oxide and PAA existed apart from each other as compared with Example 1. It is considered to be. Therefore, it is considered that the silicon oxide that did not bind to PAA was isolated and the cycle durability was inferior to that of Example 1.

Example 2 contains only 2.5 parts by weight of PAA with respect to 5 parts by weight of silicon oxide, but has a higher cycle durability than Comparative Examples 2 and 3 containing 10.5 parts by weight of PAA with respect to 5 parts by weight of silicon oxide. The decline in sex was suppressed. From this result, by localizing silicon oxide and PAA, even if the absolute amount of PAA with respect to silicon oxide is small, PAA follows the expansion and contraction of silicon oxide during charging and discharging, and the cycle durability is lowered. Is expected to be suppressed.

In Comparative Example 4, since PAA was contained in the negative electrode mixture layer, the cycle durability was improved as compared with Comparative Example 1, but the initial IV resistance value was increased as compared with Comparative Example 1.

Examples 1 to 3 were superior in initial IV resistance as compared with Comparative Example 1. It was shown that Example 4 also has an initial resistance comparable to that of Comparative Example 1 containing no PAA, even though the content of PAA in the first negative electrode mixture layer is 13.5 parts by weight. .. It is considered that the increase in battery resistance was suppressed by including the second negative electrode mixture layer containing graphite and not silicon oxide and PAA in the negative electrode mixture layer.

It should be considered that the above embodiments and examples are exemplary in all respects and are not restrictive. The scope of the present disclosure is indicated by the scope of claims, not the above description, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

10 positive electrode, 20 negative electrode, 21 negative electrode mixture layer, 22 current collector foil, 23 first negative electrode mixture layer, 24 second negative electrode mixture layer, 25 graphite, 26 silicon oxide, 27 PAA (polyacrylic acid), 30 separator , 50 electrode group, 80 battery case, 100 batteries.

Claims (1)

With the current collector foil
Equipped with a negative electrode mixture layer,
The negative electrode mixture layer includes a first negative electrode mixture layer and a second negative electrode mixture layer.
The first negative electrode mixture layer is arranged on the current collector foil, and the first negative electrode mixture layer is arranged on the current collector foil.
The second negative electrode mixture layer is arranged on the first negative electrode mixture layer.
The first negative electrode mixture layer contains graphite, silicon oxide and polyacrylic acid, and contains
The graphite content is 90 parts by weight or more and 99 parts by weight or less with respect to a total of 100 parts by weight of graphite and silicon oxide contained in the entire negative electrode mixture layer.
The content of the silicon oxide is 1 part by weight or more and 10 parts by weight or less with respect to a total of 100 parts by weight of graphite and silicon oxide contained in the entire negative electrode mixture layer.
The content of the polyacrylic acid is 2 parts by weight or more and 12 parts by weight or less with respect to a total of 100 parts by weight of graphite and silicon oxide contained in the entire negative electrode mixture layer.
The second negative electrode mixture layer contains graphite and does not contain silicon oxide and polyacrylic acid.
The ratio of the thickness of the second negative electrode mixture layer to the thickness of the first negative electrode mixture layer is 1.8 or more and 4 or less.
Negative electrode for lithium ion secondary battery.

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