JP2017022076A - Negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a temperature rise during high rate overcharge while suppressing a decrease in battery capacity. A negative electrode for a lithium-ion secondary battery includes a negative electrode current collector foil 11 and a negative electrode mixture layer 12 formed on the negative electrode current collector foil 11. The negative electrode mixture layer 12 includes a first layer 1 formed on the negative electrode current collector foil 11 and a second layer 2 formed on the first layer 1. The ratio of the thickness (T2) of the second layer to the total of the thickness (T1) of the first layer and the thickness (T2) of the second layer is 0.01 or more and 0.1 or less. The first layer 1 contains a first binder component. The second layer 2 contains 0.5% by mass or more and 5.0% by mass or less of a second binder component having a composition different from that of the first binder component. The first binder component is composed of sodium polyacrylate. The second binder component is composed of at least one of polyacrylic acid, polymethacrylic acid, and a copolymer of maleic acid and acrylic acid. [Selection diagram] Figure 2

Description

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

  JP 2010-182479 A (Patent Document 1) discloses a negative electrode mixture layer having a two-layer structure. In the negative electrode mixture layer, the same binder component is used for each layer.

JP 2010-182479 A

  Conventionally, as a binder component for a negative electrode, a water-soluble polymer in which a carboxy group (—COOH) is neutralized with sodium hydroxide (NaOH) or the like has been used. Examples thereof include sodium carboxymethylcellulose (hereinafter sometimes abbreviated as “CMC-Na”), sodium polyacrylate (hereinafter sometimes abbreviated as “PAA-Na”), and the like. However, there are still many unclear points regarding the behavior of these binder components in the battery.

  The objective of this invention is suppressing the temperature rise at the time of high-rate overcharge, suppressing the fall of battery capacity.

  The lithium ion secondary battery includes a negative electrode current collector foil and a negative electrode mixture layer formed on the negative electrode current collector foil. The negative electrode mixture layer includes a first layer formed on the negative electrode current collector foil and a second layer formed on the first layer. The ratio of the thickness of the second layer to the total thickness of the first layer and the second layer is 0.01 or more and 0.1 or less. The first layer contains a first binder component. The second layer contains 0.5% by mass or more and 5.0% by mass or less of a second binder component having a composition different from that of the first binder component. The first binder component is composed of sodium polyacrylate. The second binder component is composed of at least one of polyacrylic acid, polymethacrylic acid, and a copolymer of maleic acid and acrylic acid.

  As a result of intensive studies by the inventor to suppress temperature rise during high-rate overcharge, a binder component in which the carboxy group is not neutralized, such as polyacrylic acid (hereinafter sometimes abbreviated as “PAA”). It was found that the temperature rise during high-rate overcharge can be suppressed by using. That is, the reaction shown in the following formula (I) proceeds between the metal lithium (Li) deposited during the high-rate overcharge and the carboxy group contained in the binder component, and the metal Li is consumed.

(—COOH) _n + nLi → (—COOLi) _n + nH (I)
Thereby, the heat generation accompanying accumulation of metal Li is suppressed. Here, when the carboxy group is neutralized, such as PAA-Na, the reaction of the above formula (I) does not occur, the metal Li accumulates, and the calorific value increases.

However, at the same time, a problem has been found that the battery capacity decreases when a binder component whose carboxy group is not neutralized, such as PAA, is used. That is, a reaction similar to the above formula (I) occurs between the carboxy group and the lithium ion (Li ⁺ ) in the electrolytic solution, and Li ⁺ that should contribute to the battery capacity is consumed.

  Therefore, the negative electrode for a lithium ion secondary battery of the present invention (hereinafter sometimes abbreviated as “negative electrode”) adopts a specific two-layer structure to suppress a decrease in battery capacity and at the time of high-rate overcharge. Reduces temperature rise.

  The negative electrode mixture layer in the negative electrode of the present invention has a two-layer structure including a first layer and a second layer. Here, the ratio of the thickness of the second layer to the total thickness of the first layer and the second layer (hereinafter also referred to as “second layer ratio”) is 0.01 or more and 0.1 or less. . That is, the negative electrode mixture layer includes a first layer (base layer portion) that occupies most of the negative electrode mixture layer, and a second layer (outermost portion) that is disposed on the first layer and is thinner than the first layer. Consists of

  The first layer contains a first binder component, and the second layer contains a second binder component. The first binder component and the second binder component have different compositions.

The first binder component is composed of PAA-Na. The neutralized carboxy group (—COONa) contained in PAA-Na is very stable and has low reactivity with Li ⁺ in the electrolyte. For this reason, in the first layer containing the first binder component, the capacity reduction derived from the binder component is suppressed. As described above, the first layer that is the base layer occupies most of the negative electrode mixture layer. Therefore, the capacity | capacitance fall resulting from a binder component is suppressed also as the whole negative electrode compound-material layer.

  The second binder component may be abbreviated as PAA, polymethacrylic acid (hereinafter sometimes abbreviated as “PMA”), and a copolymer of maleic acid and acrylic acid (hereinafter referred to as “P (MA / AA)”). ). PAA, PMA and P (MA / AA) contain abundant carboxy groups in the molecular chain and have a high consumption capacity of metallic Li.

The outermost surface portion of the negative electrode mixture layer is a portion where Li precipitation is most likely to occur during high-rate overcharge. In the negative electrode of the present invention, the second layer containing the second binder component is disposed on the outermost portion. Thereby, the metal Li produced | generated at the time of high-rate overcharge can be consumed efficiently. However, as described above, when the amount of the carboxy group in the negative electrode mixture layer is excessively increased, the battery capacity is reduced due to the reaction between Li ⁺ and the carboxy group in the electrolytic solution. Therefore, in the negative electrode of the present invention, the content of the second binder component is limited to a range of 0.5% by mass or more and 5.0% by mass or less, and the second layer ratio is limited as described above.

  According to the above, it is possible to suppress an increase in temperature during high-rate overcharge while suppressing a decrease in battery capacity.

It is the schematic which shows an example of a structure of the negative electrode for lithium ion secondary batteries which concerns on embodiment of this invention. It is a schematic sectional drawing which shows an example of a structure of the negative electrode for lithium ion secondary batteries which concerns on embodiment of this invention. It is the schematic which shows an example of a structure of a positive electrode. It is the schematic which shows an example of a structure of an electrode group. It is the schematic which shows an example of a structure of a lithium ion secondary battery.

  Hereinafter, an example of an embodiment of the present invention (hereinafter also referred to as “this embodiment”) will be described. However, the present embodiment is not limited to the following description. In the following description, the lithium ion secondary battery may be abbreviated as “battery”.

  In the present embodiment, for example, “PAA” and “PAA-Na” are used to distinguish high molecular compounds depending on the presence or absence of neutralization treatment. COOH group-containing polymer compounds such as PAA are those having a neutralization degree of substantially 0%. Here, the degree of neutralization indicates the ratio (percentage) of COOH groups converted to COONa groups by neutralization treatment among all COOH groups contained in the polymer compound. A polymer compound in which a carboxyl group such as PAA-Na is neutralized, that is, a COONa group-containing polymer compound has a neutralization degree of 50 to 100%. In this embodiment, the higher the degree of neutralization of the COONa group-containing polymer compound, the more desirable, and ideally 100% (completely neutralized product).

[Anode for lithium ion secondary battery]
FIG. 1 is a schematic diagram illustrating an example of the configuration of the negative electrode according to the present embodiment. As shown in FIG. 1, the negative electrode 10 includes a negative electrode current collector foil 11 and a negative electrode mixture layer 12 formed on the negative electrode current collector foil 11. The negative electrode current collector foil is, for example, a copper (Cu) foil. The thickness of the negative electrode current collector foil may be about 5 to 20 μm, for example. The exposed portion 13 where the negative electrode current collector foil 11 is exposed from the negative electrode mixture layer 12 is provided for connection to an external terminal 70 (see FIG. 5).

[Negative electrode mixture layer]
FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the negative electrode according to the present embodiment. As shown in FIG. 2, the negative electrode mixture layer 12 includes a first layer 1 and a second layer 2 having a binder component different from that of the first layer 1. The thickness of the negative electrode mixture layer is, for example, about 10 to 150 μm. In FIG. 2, “T1” indicates the thickness of the first layer, and “T2” indicates the thickness of the second layer.

[First layer]
The first layer 1 is formed on the negative electrode current collector foil 11. The first layer contains a first binder component. For example, the first layer contains about 0.5 to 5.0% by mass of the first binder component.

[First binder component]
The first binder component is composed of sodium polyacrylate (PAA-Na). PAA-Na has low reactivity with Li ⁺ and is unlikely to cause a decrease in capacity. The weight average molecular weight of PAA-Na can be changed as appropriate in consideration of the aqueous solution viscosity and binding properties. The weight average molecular weight of PAA-Na may be, for example, about 10,000 to 1,000,000.

  The remainder other than the first binder component in the first layer is composed of a negative electrode active material, a conductive material and the like. For example, the first layer contains about 90 to 99% by mass of the negative electrode active material. The negative electrode active material may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, or the like. The conductive material may be carbon blacks such as acetylene black and thermal black.

  The first binder component may contain other binder components in addition to PAA-Na. Examples of other binder components include styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), polytetrafluoroethylene (PTFE), and the like.

  However, it is desirable that the first binder component does not contain a binder component including a carboxy group that has not been neutralized, such as polyacrylic acid (PAA). This is to suppress a decrease in battery capacity. Moreover, it is desirable that the first binder component does not contain sodium carboxymethyl cellulose (CMC-Na). CMC-Na is a polymer compound containing a carboxy group that has been neutralized in the same manner as PAA-Na, but when the first binder component contains CMC-Na, the first layer and the second layer described later are included. Peeling may occur at the interface.

[Second layer]
The second layer 2 is formed on the first layer 1. The second layer contains 0.5% by mass or more and 5.0% by mass or less of a second binder component having a composition different from that of the first binder component. Similar to the first layer, in the second layer, the remainder other than the second binder component is composed of the above-described negative electrode active material, conductive material, and the like.

  The second binder component is composed of at least one of polyacrylic acid (PAA), polymethacrylic acid (PMA), and a copolymer of maleic acid and acrylic acid (P (MA / AA)). These polymer compounds contain abundant carboxy groups in the molecular chain and have a high consumption capacity of metal Li.

  If the content of the second binder component is less than 0.5% by mass, the effect of suppressing the temperature rise during high-rate overcharge may be insufficient. On the other hand, if the content of the second binder component exceeds 5.0% by mass, the battery capacity may decrease. The content of the second binder component may be 0.5% by mass or more and 2.0% by mass or less, or 2.0% by mass or more and 5.0% by mass or less. The weight average molecular weights of PAA, PMA and P (MA / AA) can be appropriately changed in consideration of the aqueous solution viscosity, binding property and the like. The weight average molecular weight of PAA, PMA and P (MA / AA) may be, for example, about 10,000 to 1,000,000.

  P (MA / AA) may be a block copolymer, a graft copolymer, an alternating copolymer, or a random copolymer. The constituent ratio (molar ratio) of maleic acid (MA) and acrylic acid (AA) in the copolymer may be, for example, about MA: AA = 1: 9 to 9: 1.

  The second binder component may also contain other binder components such as SBR described above in addition to PAA, PMA and P (MA / AA). However, it is desirable that the second binder component does not contain carboxymethyl cellulose (CMC). In CMC, there are few carboxy groups contained in a molecular chain. Therefore, when the second binder component contains CMC, the effect expected in the present embodiment may be reduced. The second binder component preferably contains no binder component including a neutralized carboxy group such as PAA-Na. This is because when the second binder component contains PAA-Na or the like, the effect of suppressing the temperature rise is reduced.

[2nd layer ratio]
The ratio of the second layer thickness (T2) to the second layer ratio, that is, the sum of the first layer thickness (T1) and the second layer thickness (T2) is obtained by the following formula (II). It is done.

[Second layer ratio] = T2 ÷ (T1 + T2) (II)
In the present embodiment, the second layer ratio is 0.01 or more and 0.1 or less. When the second layer ratio is less than 0.01, the effect of suppressing the temperature rise may be reduced. If the second layer ratio exceeds 0.1, the battery capacity may decrease. The second layer ratio may be 0.01 or more and 0.05 or less, or 0.05 or more and 0.1 or less.

[Method for producing negative electrode for lithium ion secondary battery]
The negative electrode of this embodiment can be manufactured by a conventionally known method. For example, a first negative electrode paint to be the first layer and a second negative electrode paint to be the second layer may be respectively prepared, and sequentially coated and dried on the negative electrode current collector foil. At this time, after the first layer was dried, when the second negative electrode paint was applied thereon, the first layer again became a paint state at the interface between them, and the first layer and the second layer were compatible with each other. A region may be formed. However, as long as the negative electrode mixture layer includes the first layer and the second layer described above, the effect expected in the present embodiment is shown.

  Further, in the present embodiment, there is also an aspect such as forming a negative electrode mixture layer by producing a granulated body that is an aggregate of granulated particles without going through a paint, and roll-forming the granulated body. Can be adopted.

  Hereinafter, although this embodiment is described using an example, this embodiment is not limited to these. In the following, an example of application to a prismatic battery is shown as an example, but this embodiment is not limited to a prismatic battery, and can also be applied to a cylindrical battery, a laminated battery, and the like.

[Production of lithium ion secondary battery]
Various negative electrodes and evaluation batteries (design capacity: 25 Ah) using the negative electrodes were prepared as follows, and the battery performance was evaluated.

[Comparative Example 1]
1. Preparation of negative electrode The following materials were prepared: Negative electrode active material: Natural graphite Binder component: CMC-Na, SBR
Solvent: Water Negative electrode current collector foil: Cu foil (thickness: 10 μm).

  Using a kneading apparatus, a negative electrode active material (97 parts by mass), CMC-Na (2 parts by mass) and SBR (1 part by mass) are kneaded in water to obtain a negative electrode paint having a solid content concentration of 50% by mass. Produced. Using a die coater, a negative electrode paint was applied to both surfaces of the negative electrode current collector foil and dried to form a negative electrode mixture layer as a single layer. The negative electrode mixture layer was rolled using a roll mill. The thickness (one side) of the negative electrode mixture layer after rolling was adjusted to 100 μm. Further, it was cut into predetermined dimensions. From the above, a negative electrode according to Comparative Example 1 was produced. The dimensions of each part shown in FIG. 1 were as follows.

Width W12 of negative electrode mixture layer 12: 80.9 mm
The width W13 of the exposed portion 13: 20 mm.

2. Preparation of positive electrode The following materials were prepared: Positive electrode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: Acetylene black Binder component: Polyvinylidene fluoride Solvent: N-methyl-2-pyrrolidone Positive electrode current collector foil: Aluminum (Al) foil (thickness: 20 μm).

  A positive electrode paint was prepared by kneading the positive electrode active material, the conductive material, and the binder component in a solvent using a kneader. Using a die coater, a positive electrode paint layer was applied to both surfaces of the positive electrode current collector foil and dried to form a positive electrode mixture layer. The positive electrode mixture layer was rolled using a roll rolling machine, and further cut into predetermined dimensions. From the above, the positive electrode 20 shown in FIG. 3 was produced. The dimensions of each part shown in FIG. 3 were as follows.

Width W22 of positive electrode mixture layer 22: 78 mm
The width W23 of the exposed part 23: 20 mm
Here, the exposed portion 23 indicates a portion where the positive electrode current collector foil 21 is exposed from the positive electrode mixture layer 22 as shown in FIG.

3. Assembly A microporous membrane separator (width: 63 mm) made of polyethylene was prepared. As shown in FIG. 4, the negative electrode 10 and the positive electrode 20 were laminated | stacked on both sides of the separator 30, and the electrode group 80 was comprised by winding these.

  A battery case 50 made of an Al alloy shown in FIG. 5 was prepared. The plate thickness of the battery case was 1 mm, and the outer dimensions of the battery case were 75 mm long × 120 mm wide × 15 mm deep. As shown in FIG. 5, the exposed portions 13 and 23 and the external terminals 70 and 72 were connected, and the electrode group 80 was accommodated in the battery case 50.

4). Injection Solution A mixed solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of EC: EMC: DEC = 3: 5: 2. Furthermore, an electrolytic solution was prepared by dissolving LiPF ₆ in a mixed solvent so that its concentration was 1.0 mol / L.

  An electrolyte solution 81 was injected from a liquid injection port (not shown) provided in the battery case 50. Thereafter, the battery case 50 was sealed. As described above, the battery 100 according to Comparative Example 1 was produced.

[Comparative Examples 2 and 3]
As shown in Table 1, a negative electrode and a battery were produced in the same manner as in Comparative Example 1 except that PAA-Na or the like was used instead of CMC-Na. In Table 1, Comparative Examples 1 to 4 including a single negative electrode mixture layer have the binder component of the entire negative electrode mixture layer and the like written in the column of the first layer for convenience.

[Comparative Example 4]
As shown in Table 1, in place of CMC-Na (2 parts by mass) in Comparative Example 1, PAA (1 part by mass) and PAA-Na (1 part by mass) were used, except for Comparative Example 1. Similarly, a negative electrode and a battery were produced. In Table 1, “PAA: PAA-Na = 1: 1” indicates that the mass ratio of PAA to PAA-Na is 1: 1.

[Example 1]
Using a kneading apparatus, the negative electrode active material (97 parts by mass) and PAA-Na (2 parts by mass) and SBR (1 part by mass) as the first binder component are kneaded in water, so that the solid content concentration is 50. The 1st negative electrode coating material which is the mass% was produced. Using a die coater, the first negative electrode paint was applied to both surfaces of the negative electrode current collector foil and dried to form a first layer. The coating amount (mass per unit area) of the first layer was adjusted so that the thickness after rolling was 95 μm. Here, the density setting (after rolling) of the first layer was the same as the density setting (after rolling) of the negative electrode mixture layer of Comparative Example 1.

  Using a kneader, the negative electrode active material (97 parts by mass) and PAA (2 parts by mass) and SBR (1 part by mass) as the second binder component are kneaded in water, so that the solid content concentration is 50% by mass. A second negative electrode paint was prepared. The second negative electrode paint was applied on the first layer using a die coater and dried to form the second layer. The coating amount of the second layer was adjusted so that the thickness after rolling was 5 μm. Here, the density setting (after rolling) of the second layer was the same as the density setting (after rolling) of the negative electrode mixture layer of Comparative Example 1.

  After forming the second layer, the negative electrode mixture layer was rolled using a roll mill, and further cut into predetermined dimensions. From the above, the negative electrode according to Example 1 was produced. Further, a battery was produced in the same manner as in Comparative Example 1.

[Examples 2-7 and Comparative Examples 5-10]
As shown in Table 1, in the same manner as in Example 1, except that the types of the first binder component and the second binder component, the content of the second binder component, and the ratio of the second layer were changed, the negative electrode And batteries were prepared respectively.

[Evaluation]
Each battery produced above was evaluated as follows. In the following description, it is assumed that the unit of current “C” indicates a current that can completely discharge the design capacity of the battery in one hour. “CC” represents a constant current method, and “CCCV” represents a constant current-constant voltage method.

1. Measurement of battery capacity The battery was charged to 4.2 V with a current of 1C. The battery was discharged to 2.5 V with a current of 1 C with a 5-minute pause. Furthermore, charging / discharging was performed under the following conditions, and the initial discharge capacity (battery capacity) was measured. The results are shown in Table 1. As shown in Table 1, for example, in Comparative Example 3, the actual battery capacity is reduced to 20.5 Ah even though the design capacity is 25 Ah.

CCCV charge: CC current = 1.0C, CV voltage = 4.2V, end current = 0.01C
CCCV discharge: CC current = 1.0C, CV voltage = 3.0V, end current = 0.01C.

2. High-rate overcharge test A high-rate overcharge test was conducted under the following conditions. During the test, the temperature of the battery was measured with a thermocouple attached to the battery case. Table 1 shows the maximum temperature reached by the battery in this test. In Table 1, it can be said that the lower the maximum temperature reached, the smaller the temperature rise and the better.

CC current: 10C (250A)
Upper limit voltage: 4.5V.

〔Results and discussion〕
1. Examples 1-7
As can be seen from Table 1, in the lithium ion secondary battery provided with the negative electrode of the example that satisfies all the following conditions (a) to (e), the battery capacity decrease is small, and the temperature rise during high-rate overcharge Is small.

(A) The negative electrode mixture layer includes a first layer and a second layer.
(B) The second layer ratio is 0.01 or more and 0.1 or less.
(C) The first binder component is composed of PAA-Na.
(D) The second binder component is composed of at least one of PAA, PMA and P (MA / AA).
(E) Content of 2nd binder component is 0.5 mass% or more and 5.0 mass% or less.

2. Comparative Examples 1-4
When the negative electrode composite material layer is a single layer, it is difficult to achieve both suppression of capacity reduction and suppression of temperature increase during high-rate overcharge, regardless of which binder component is used.

3. Comparative Examples 5 and 6
Regarding (b) above, if the second layer ratio is less than 0.01, the temperature rise during overcharge is large (Comparative Example 5). On the other hand, when the ratio of the second layer exceeds 0.1, the capacity reduction is large (Comparative Example 6).

4). Comparative Example 7
In Comparative Example 7 using CMC-Na as the first binder component, the temperature rise during overcharge is large. It is considered that peeling occurred at the interface between the first layer and the second layer, thereby increasing the electrode resistance and increasing the amount of Li deposition.

5. Comparative Example 8
In Comparative Example 8 in which CMC is used as the second binder component, the temperature rise during overcharge is large. In CMC, since the number of carboxy groups contained in the molecular chain is small, it is considered that the consumption of metal Li is insufficient.

6). Comparative Examples 9 and 10
Regarding (e) above, when the content of PAA or the like is less than 0.5% by mass, the temperature rise during overcharge is large (Comparative Example 9). Moreover, when content, such as PAA, exceeds 5.0 mass%, a capacity | capacitance fall is large (comparative example 10).

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 1st layer, 2nd layer, 10 negative electrode, 11 negative electrode current collector foil, 12 negative electrode compound material layer, 13, 23 exposed part, 20 positive electrode, 21 positive electrode current collector foil, 22 positive electrode compound material layer, 30 separator, 50 Battery case, 70, 72 external terminal, 80 electrode group, 81 electrolyte, 100 battery.

Claims (1)

Negative electrode current collector foil;
A negative electrode mixture layer formed on the negative electrode current collector foil,
The negative electrode mixture layer includes a first layer formed on the negative electrode current collector foil, and a second layer formed on the first layer,
The ratio of the thickness of the second layer to the sum of the thickness of the first layer and the thickness of the second layer is 0.01 or more and 0.1 or less,
The first layer contains a first binder component;
The second layer contains a second binder component having a composition different from that of the first binder component, in a range of 0.5 mass% to 5.0 mass%,
The first binder component is composed of sodium polyacrylate,
The said 2nd binder component is a negative electrode for lithium ion secondary batteries comprised from at least 1 sort (s) among the copolymer of polyacrylic acid, polymethacrylic acid, and a maleic acid and acrylic acid.

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