JP2013077391A - Negative electrode for lithium ion secondary battery, and lithium ion secondary battery including the negative electrode for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure appropriate resistance in a negative electrode by securing appropriate conductivity in a second layer, thereby preventing deterioration in initial charge/discharge efficiency and deterioration in battery characteristics such as large current input/output characteristics and cycle characteristics and also securing battery safety.SOLUTION: A negative electrode 1 for a lithium ion secondary battery includes: a negative electrode collector 1X; a first layer 1a formed on the negative electrode collector 1X and including a first negative electrode active material; and a second layer 1b formed on the first layer 1a and including a second negative electrode active material and a conductive agent. The first negative electrode active material includes carbon. The second negative electrode active material includes metal oxide capable of absorbing and desorbing lithium ions. A coverage factor for the conductive agent to cover a surface of a particle of the metal oxide is 10% or more and 20% or less of the surface area.

Description

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

  Lithium ion secondary batteries (hereinafter sometimes simply referred to as “batteries”) are actively researched and developed as batteries with high energy density. However, when a short circuit occurs, it is important to improve the safety of the battery because the battery may overheat.

For example, a non-aqueous electrolyte battery described in Patent Document 1 has been proposed for the purpose of improving safety at the time of an internal short circuit without reducing the large current performance of the battery. In the technique described in Patent Document 1, the negative electrode has a negative electrode current collector and a negative electrode layer formed on the negative electrode current collector. The negative electrode layer is formed on the surface of the negative electrode current collector and includes a main negative electrode layer including an active material, and a surface layer formed on the surface of the main negative electrode layer and including an active material different from the active material included in the main negative electrode layer; have. The active material contained in the surface layer is a lithium titanium composite oxide (Li _{4 + x} Ti ₅ O ₁₂ , −1 ≦ x ≦ 3) having a spinel structure. Thereby, the safety | security at the time of an internal short circuit is improved, without reducing the large current performance of a battery.

JP 2010-97720 A

  However, the technique described in Patent Document 1 has the following problems.

Lithium titanate _{(LTO: Li x Ti y O} z) has a high resistance. For this reason, if the resistance of the surface layer containing LTO is too high, the resistance of the entire negative electrode is increased, so that the current flowing through the battery is hindered. There is a risk that the overall battery characteristics such as current input / output characteristics and cycle characteristics may be deteriorated.

  As described above, the surface layer needs to have a high resistance in order to ensure the safety of the battery, but if the resistance of the surface layer is too high, there is a possibility that the overall battery characteristics may be deteriorated as described above. . For this reason, it is necessary to reduce the resistance of the surface layer to a certain extent (specifically, to the extent that the negative electrode can ensure appropriate conductivity). Thus, the surface layer needs to ensure appropriate conductivity.

  In view of the above, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery having a first layer formed on a negative electrode current collector and a second layer formed on the first layer. In the lithium ion secondary battery provided with the second layer, the second layer ensures appropriate conductivity, the negative electrode ensures appropriate resistance, the initial charge / discharge efficiency decreases, and the large current input / output characteristics and It is to prevent battery characteristics such as cycle characteristics from being deteriorated and to ensure battery safety.

  In order to achieve the above object, a negative electrode for a lithium ion secondary battery according to the present invention includes a negative electrode current collector, a first layer formed on the negative electrode current collector and including a first negative electrode active material, And a second layer including a second negative electrode active material and a conductive agent, wherein the first negative electrode active material includes carbon, and the second negative electrode active material includes: The coverage of the metal oxide containing metal oxide capable of inserting and extracting lithium ions, and the conductive agent covering the surface of the metal oxide particles is 10% or more and 20% or less of the surface area.

  According to the lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to the present invention, the second layer can ensure appropriate conductivity. As a result, the negative electrode secures an appropriate resistance to prevent the deterioration of the initial charge / discharge efficiency and the battery characteristics such as the large current input / output characteristics and the cycle characteristics, and to ensure the safety of the battery. it can.

FIG. 1 is a cross-sectional view showing the structure of a negative electrode for a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a diagram showing the structure of acetylene black. FIG. 3 is a diagram illustrating the structure of the metal oxide and the conductive agent included in the second layer. FIG. 4 is a cross-sectional view showing the structure of a lithium ion secondary battery according to an embodiment of the present invention.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the following embodiment is only a mere illustration form of this invention, and this invention is not limited to this. The present invention can be variously modified or changed without departing from the gist of the present invention, and such modified examples and modified examples are also included in the scope of the present invention. In the drawings, each component is illustrated in a dimensional ratio suitable for illustration, and the illustrated dimensional ratio may be different from the actual dimensional ratio.

(One embodiment)
Below, the negative electrode for lithium ion secondary batteries which concerns on one Embodiment of this invention is demonstrated, referring FIG.1, FIG.2 and FIG.3.

  As shown in FIG. 1, a negative electrode 1 for a lithium ion secondary battery according to the present embodiment includes a negative electrode current collector 1X, a first layer 1a formed on the negative electrode current collector 1X, And a second layer 1b formed on the layer 1a. The first layer 1a includes at least a first negative electrode active material, and may further include a conductive agent, a binder, and the like. The second layer 1b includes at least a second negative electrode active material and a conductive agent, and may include a binder or the like in addition to these.

  The second layer 1b is thinner than the first layer 1a. The second layer 1b is preferably 3 μm or more and 30 μm or less, for example.

The first negative electrode active material has a higher theoretical capacity than the second negative electrode active material. The second negative electrode active material has a higher resistance than the first negative electrode active material. The first negative electrode active material contains carbon, for example, graphite. The second negative electrode active material includes a metal oxide capable of inserting and extracting lithium ions. Metal oxides, for example, lithium titanate: a _{(LTO Li x Ti y O z} ) , or molybdenum oxide.

  The conductive agent contained in the second layer is preferably, for example, acetylene black or carbon nanotube. FIG. 2 shows the structure of acetylene black. As shown in FIG. 2, acetylene black has a structure in which spherical primary particles are connected. The length of the acetylene black structure (linkage of primary particles) is preferably 0.4 μm or more and 0.6 μm or less. Similarly, the length of the carbon nanotube is preferably 0.4 μm or more and 0.6 μm or less. The “structure length” of acetylene black and the “length” of the carbon nanotube can be obtained from image analysis of a photographed scanning electron micrograph (SEM photograph).

  FIG. 3 shows the structure of the metal oxide and conductive agent contained in the second layer. As shown in FIG. 3, the metal oxide particles are spherical, and the conductive agent particles are chain-shaped. Conductive agent particles enter the gaps between adjacent metal oxide particles, and the adjacent metal oxide particles are electrically connected to each other.

  The covering ratio (hereinafter simply referred to as “covering ratio”) at which the conductive agent covers the surfaces of the metal oxide particles is 10% or more and 20% or less of the surface area of the metal oxide particles.

  In the present specification, the “covering ratio at which the conductive agent covers the surface of the metal oxide particles” refers to the ratio of the surface area of one particle of the metal oxide covered with the conductive agent. The coverage is a value obtained by multiplying the projected area s per particle of the conductive agent by the number m of the conductive agent particles divided by the projected area S per particle of the metal oxide. The projected area s, the number of particles m, and the projected area S are obtained from image analysis of the photographed SEM photograph. Individual particles were calculated as spheres with an average particle size.

  As a result of extensive studies by the inventors of the present application, it has been found that the second layer can ensure appropriate conductivity when the coverage is 10% or more and 20% or less.

  Specifically, as shown in [Table 1] described later, when the coverage is 10% or more and 20% or less, the resistance of the negative electrode is lower than the resistance value that cannot be used as a battery, and the internal short circuit test There is no battery that is short-circuited, and the initial charge / discharge efficiency (hereinafter sometimes simply referred to as “initial efficiency”) is high. As can be seen from the high initial efficiency, lithium (Li) ions are smoothly inserted and removed, and good battery characteristics such as cycle characteristics can be expected. In the case of Examples and Comparative Examples described later, a resistance value that cannot be used as a battery is set to, for example, 500 mΩ or more. However, since this resistance value is determined according to the battery design, it may be different from 500 mΩ. In the case of Examples and Comparative Examples described later, an initial efficiency of 95% or more was regarded as a high initial efficiency.

  On the other hand, when the coverage is less than 10%, there is no battery that is short-circuited, but the resistance of the negative electrode is not less than the unusable resistance value, and the initial efficiency is low. On the other hand, when the coverage is larger than 20%, the resistance of the negative electrode is lower than the unusable resistance value and the initial efficiency is high, but there is a battery that is short-circuited.

  As can be seen from [Table 1], when the coverage is 10% or more and 20% or less, the second layer can ensure appropriate resistance (in other words, conductivity). As a result, the negative electrode can ensure an appropriate resistance, prevent a decrease in initial efficiency, and ensure the safety of the battery. Further, by preventing a decrease in initial efficiency, it is possible to prevent a decrease in battery characteristics such as large current input / output characteristics and cycle characteristics.

  When the coverage is less than 10%, the resistance of the second layer is too high, and the negative electrode cannot secure an appropriate resistance, leading to a decrease in initial efficiency. When the coverage is larger than 20%, the resistance of the second layer is too low, and the short-circuit current cannot be interrupted by the second layer at the time of short circuit, and the safety of the battery cannot be ensured.

  Hereinafter, as a layer formed on the negative electrode current collector, (1) a case where a laminate of a first layer and a second layer is used as in this embodiment, and (2) this embodiment, Is different from the case of using a single layer of only the second layer.

  In the case of (2), as shown in [Table 3] described later, the resistance of the negative electrode when the coverage is 15% and 20% is higher than the unusable resistance value. The resistance of the negative electrode when the coverage was 10% was much higher than 200000 mΩ, and was not measurable.

  On the other hand, the resistance of the negative electrode when the coverage is 40% is lower than the unusable resistance value.

  As can be seen from [Table 3], in the case of (2), in order to make the resistance of the negative electrode lower than the unusable resistance value, the coverage needs to be 40% or more. On the other hand, in the case of (1), even when the coverage is 20% or less, the resistance of the negative electrode can be made lower than the unusable resistance value. The coverage in the case of (1) is smaller than the coverage in the case of (2). Since the second layer in the case of (1) is mainly a layer provided on the first layer in order to ensure the safety of the battery, the coverage of the second layer included in the laminate is The coverage of the second layer of the single layer body is smaller.

  For example, by using LTO as the second negative electrode active material, a high capacity can be obtained as follows, and lithium (Li) can be prevented from being deposited on the surface of the negative electrode during high input charging.

  In the case of the second layer containing LTO, for example, a higher capacity can be obtained than in the case of the second layer containing aluminum oxide instead of LTO. This is due to the following reason. Aluminum oxide cannot absorb and release Li ions and is not an active material. Therefore, the active material cannot be filled by the thickness of the second layer containing aluminum oxide, and the capacity is low. On the other hand, since LTO is an active material, a high capacity can be obtained.

  LTO can insert and desorb Li ions more efficiently than carbon. For this reason, by covering the surface of the first layer containing carbon with the second layer, it is possible to prevent Li from being deposited on the surface of the negative electrode during high input charging. Note that when the surface of the first layer is not covered with the second layer, Li is deposited on the surface of the first layer during high-input charging. The precipitated Li may break through the negative electrode and cause an internal short circuit. Further, once deposited Li does not contribute to charging / discharging of the battery, a high capacity cannot be obtained.

  The thickness of the second layer is preferably 3 μm or more and 30 μm or less.

  When the thickness of the second layer is smaller than 3 μm, a part of the surface of the first layer may be exposed. In this case, Li is deposited on the exposed portion during high input charging. Furthermore, at the time of a short circuit, a short circuit current flows into the exposed part, and the safety of the battery cannot be ensured.

  On the other hand, when the thickness of the second layer is larger than 30 μm, the thickness of the first layer is reduced by the thickness of the second layer, and the first negative electrode active material can be filled. Can not. Graphite containing carbon has a higher theoretical capacity than LTO. For this reason, there exists a possibility that a capacity | capacitance may fall. Since the second layer is mainly a layer provided on the first layer in order to ensure the safety of the battery, the thickness of the second layer is excessively increased (for example, more than 30 μm). There is no need to make it larger.

  In order to accurately cover the entire surface of the first layer with the second layer having a thickness of 3 μm or more and 30 μm or less, the average particle diameter of the second negative electrode active material is 1 μm or more and 2 μm or less. It is preferable. By making the average particle diameter of the second negative electrode active material sufficiently smaller than the thickness of the second layer (for example, 1 μm or more and 2 μm or less), a part of the surface of the first layer is exposed. It is possible to cover the entire surface of the first layer with high accuracy without making it.

  For example, when acetylene black is used as the conductive agent contained in the second layer, the length of the structure of acetylene black is preferably 0.4 μm or more and 0.6 μm or less. This is presumably due to the following reasons.

  First, (a) the second layer includes acetylene black having the length of the first structure of Y parts by weight with respect to X parts by weight of the metal oxide, and (b) the second layer. Is compared with the case of containing acetylene black having the length of the second structure of Y parts by weight with respect to X parts by weight of the metal oxide. The length of the second structure is longer than the length of the first structure. The length of the first structure is 0.4 μm or more and 0.6 μm or less. The length of the second structure is greater than 0.6 μm.

  In the case of (a), since the length of the structure is shorter than in the case of (b), the number of acetylene black particles entering the gap between adjacent metal oxide particles is (b) in the case of (b). ) More than in the case of For this reason, the number of conduction paths that electrically connect adjacent metal oxide particles is larger in the case of (a) than in the case of (b). Therefore, in the case of (a), the conductivity is higher than in the case of (b) (see [Table 2] described later).

  As described above, when the second layer includes the same weight part of acetylene black with respect to the same weight part of metal oxide, the structure has a length of 0.4 μm or more and 0.6 μm or less. Compared with the case where the length of the second layer is larger than 0.6 μm, the number of conduction paths is large and the conductivity of the second layer is high.

  When the length of the structure is not less than 0.4 μm and not more than 0.6 μm, the second layer can ensure proper conductivity with a small amount of acetylene black. On the other hand, when the length of the structure is larger than 0.6 μm, a large amount of acetylene black is required for the second layer to ensure appropriate conductivity.

  Secondly, (a) the second layer includes acetylene black having a length of the first structure of Y parts by weight with respect to X parts by weight of the metal oxide, and (c) the second layer. Is compared with the case of containing acetylene black having the length of the third structure of Y parts by weight with respect to X parts by weight of the metal oxide. The length of the third structure is shorter than the length of the first structure. The length of the first structure is 0.4 μm or more and 0.6 μm or less. The length of the third structure is smaller than 0.4 μm.

  In the case of (c), since the length of the structure is shorter than in the case of (a), the number of acetylene black particles entering the gap between adjacent metal oxide particles is (a), ) More than in the case of However, since the length of the structure is short, it is difficult to electrically connect adjacent metal oxide particles, and the number of conduction paths is as in (c), (a) and Compared to less. Therefore, in the case of (c), the conductivity is lower than in the case of (a) (see [Table 2] described later).

  Thus, when the second layer contains the same part by weight of acetylene black with respect to the same part by weight of metal oxide, the length of the structure is 0.4 μm or more and 0.6 μm or less. When the length of the structure is smaller than 0.4 μm, the number of conduction paths is small and the conductivity of the second layer is low.

  When the length of the structure is smaller than 0.4 μm, it is difficult to electrically connect between adjacent metal oxide particles, and the second layer can ensure proper conductivity. Have difficulty.

  For example, when carbon nanotubes are used as the conductive agent contained in the second layer, the length of the carbon nanotubes is preferably 0.4 μm or more and 0.6 μm or less. This is due to the same reason as the above-mentioned acetylene black.

  Below, the formation method of the negative electrode for lithium ion secondary batteries which concerns on one Embodiment of this invention is demonstrated.

  First, for example, a first slurry containing a first negative electrode active material is prepared. The first negative electrode active material contains carbon, for example, graphite. The average particle diameter of graphite is preferably 10 μm or more and 30 μm or less, for example. The first slurry may contain a conductive agent, a binder and the like in addition to the first negative electrode active material. Next, the first slurry is applied on the negative electrode current collector and then dried.

  Next, for example, when acetylene black is used as a conductive agent contained in the second slurry described later, the length of the structure of acetylene black is set to 0.4 μm or more and 0.6 μm or less. Specifically, for example, a dispersant and acetylene black are added to water, and these are mixed using a pulverizer such as a ball mill. As a result, the acetylene black is pulverized and dispersed, and the length of the pulverized and dispersed acetylene black structure is 0.4 μm or more and 0.6 μm or less.

  Secondly, for example, when carbon nanotubes are used as the conductive agent, the carbon nanotubes are pulverized and dispersed in the same manner as acetylene black so that the length is 0.4 μm or more and 0.6 μm or less. Alternatively, carbon nanotubes having a length of 0.4 μm or more and 0.6 μm or less are selected from carbon nanotubes having various lengths manufactured by each company.

  Next, for example, a second slurry containing the second negative electrode active material, the above-described conductive agent, and the like is prepared. The second negative electrode active material includes a metal oxide, and the metal oxide is, for example, LTO. The average particle diameter of LTO is preferably 1 μm or more and 2 μm or less, for example. The second slurry may contain a binder or the like in addition to the second negative electrode active material and the conductive agent. Next, the second slurry is applied on the dried first slurry and then dried.

  Next, the negative electrode current collector on which the first slurry and the second slurry are sequentially applied and dried is rolled. Thus, as shown in FIG. 1, the negative electrode current collector 1X, the first layer 1a formed on the negative electrode current collector 1X, and the second layer 1b formed on the first layer 1a The negative electrode 1 for lithium ion secondary batteries which has is formed. The thickness of the second layer 1b is preferably 3 μm or more and 30 μm or less, for example. In addition, after apply | coating and drying a 1st slurry, and before apply | coating and drying a 2nd slurry, you may roll the negative electrode collector which the 1st slurry was apply | coated and dried. .

  Below, the lithium ion secondary battery using the negative electrode for lithium ion secondary batteries which concerns on this embodiment is demonstrated, referring FIG.

  The lithium ion secondary battery according to the present embodiment includes a battery case 7 and an electrode group 4 accommodated in the battery case 7. A sealing body 8 is caulked to the upper open end of the battery case 7 via a gasket 9.

  The electrode group 4 includes a positive electrode 2, a negative electrode 1, and a separator 3. The electrode group 4 is configured by winding the positive electrode 2 and the negative electrode 1 through the separator 3 between them. An upper insulating plate 10 is disposed at the upper end of the electrode group 4, and a lower insulating plate 11 is disposed at the lower end of the electrode group 4.

  One end of the positive electrode lead 5 is attached to the positive electrode 2, and the other end of the positive electrode lead 5 is connected to a sealing body 8 that also serves as a positive electrode terminal. One end of the negative electrode lead 6 is attached to the negative electrode 1, and the other end of the negative electrode lead 6 is connected to a battery case 7 that also serves as a negative electrode terminal.

  Although not shown in detail, the positive electrode 2 has a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a conductive agent, a binder, and the like.

  As described above, the negative electrode 1 includes the negative electrode current collector 1X, the first layer 1a, and the second layer 1b.

  Below, the manufacturing method of the lithium ion secondary battery using the negative electrode for lithium ion secondary batteries which concerns on this embodiment is demonstrated.

-Positive electrode-
A positive electrode is formed as follows. First, a positive electrode active material, a binder, a conductive agent, and the like are mixed in a solvent to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry is applied on the positive electrode current collector and then dried. Next, the positive electrode current collector coated with the positive electrode mixture slurry and dried is rolled.

-Negative electrode-
The negative electrode for a lithium ion secondary battery according to this embodiment is formed by the formation method described above.

-Battery-The lithium ion secondary battery according to the present embodiment is manufactured as follows. First, the positive electrode lead is attached to the positive electrode current collector, while the negative electrode lead is attached to the negative electrode current collector. Next, the positive electrode and the negative electrode are wound through a separator between them to constitute an electrode group. Next, an upper insulating plate is disposed at the upper end of the electrode group, and a lower insulating plate is disposed at the lower end of the electrode group. Next, the negative electrode lead is welded to the battery case, and the positive electrode lead is welded to the sealing body, and the electrode group is accommodated in the battery case. Next, a non-aqueous electrolyte is injected into the battery case. Next, the upper open end of the battery case is caulked to the sealing plate through a gasket.

  According to this embodiment, the coverage is 10% or more and 20% or less of the surface area of the metal oxide particles. Thereby, the second layer can ensure appropriate conductivity. For this reason, a negative electrode can ensure appropriate resistance, and can prevent the fall of the initial charging / discharging efficiency. Further, by preventing the initial charge / discharge efficiency from decreasing, it is possible to prevent deterioration of battery characteristics such as large current input / output characteristics and cycle characteristics. Furthermore, even if a short circuit may occur, the short circuit current can be cut off by the second layer, and the safety of the battery can be ensured.

  Further, LTO is used as the second negative electrode active material contained in the second layer. As a result, the capacity can be increased. Furthermore, it is possible to prevent Li from being deposited on the surface of the negative electrode during high input charging.

  Hereinafter, Examples 1 to 5 and Comparative Examples 1 to 8 will be described.

<Example 1>
(Preparation of positive electrode)
First, 100 parts by weight of lithium nickelate as a positive electrode active material, 5 parts by weight of acetylene black as a conductive agent, and 3 parts by weight of polyvinylidene fluoride (PVDF) as a binder, N-methylpyrrolidone (NMP) To obtain a positive electrode mixture slurry. The positive electrode mixture slurry was applied to both surfaces of a positive electrode current collector made of aluminum and then dried. Next, the positive electrode current collector on which both surfaces of the positive electrode mixture slurry were applied and dried was rolled.

(Preparation of negative electrode)
First, graphite was pulverized and classified so that the average particle diameter was 17 μm. Next, 100 parts by weight of graphite as a negative electrode active material and 1 part by weight of styrene butadiene rubber as a binder were mixed in water to obtain a first slurry. The first slurry was applied to both surfaces of a negative electrode current collector made of copper and then dried.

  Next, acetylene black was pulverized and dispersed so that the length of the structure was 0.4 μm. Specifically, a dispersant and acetylene black are added to water, and these are mixed using a ball mill. As a result, acetylene black was pulverized and dispersed, and the length of the structure of the acetylene black thus pulverized and dispersed was set to 0.4 μm. Next, 100 parts by weight of lithium titanate as a negative electrode active material, 5 parts by weight of acetylene black as a conductive agent, and 2 parts by weight of styrene butadiene rubber as a binder are mixed in water, and the second slurry Got. This second slurry was applied onto the dried first slurry and then dried. Next, the negative electrode current collector in which the first slurry and the second slurry were sequentially applied to both surfaces and dried was rolled. The thickness of the first layer was 75 μm, and the thickness of the second layer was 5 μm.

(Preparation of non-aqueous electrolyte)
LiPF ₆ is dissolved in a mixed solvent in which ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate are mixed so that the volume ratio is 2: 1: 7 as a non-aqueous solvent so that the concentration is 1.0 mol / m ^3. A nonaqueous electrolytic solution was obtained.

(Manufacture of lithium ion secondary batteries)
First, a positive electrode lead made of aluminum was attached to the positive electrode current collector, and a negative electrode lead made of nickel was attached to the negative electrode current collector. Next, the positive electrode and the negative electrode were wound through a polyethylene separator between them to form an electrode group. Next, an upper insulating plate was disposed at the upper end of the electrode group, while a lower insulating plate was disposed at the lower end of the electrode group. Next, the negative electrode lead was welded to the battery case, and the positive electrode lead was welded to the sealing body to accommodate the electrode group in the battery case. Next, a non-aqueous electrolyte was injected into the battery case. Next, a battery was manufactured by caulking the upper open end of the battery case to a sealing plate via a gasket.

<Example 2>
A battery was manufactured in the same manner as in Example 1 except that the thickness of the applied first slurry was reduced and the thickness of the applied second slurry was increased in (Preparation of negative electrode). The thickness of the first layer was 65 μm, and the thickness of the second layer was 10 μm.

<Example 3>
A battery was produced in the same manner as in Example 1 except that in the production of the negative electrode, 10 parts by weight of carbon nanotubes having a length of 0.6 μm were used instead of acetylene black. The coverage was 15%.

<Example 4>
In (Preparation of negative electrode), a battery was produced in the same manner as in Example 1, except that 10 parts by weight of acetylene black having a structure length of 0.5 μm was used. The coverage was 17%.

<Example 5>
A battery was manufactured in the same manner as in Example 1 except that in the preparation of the negative electrode, 8 parts by weight of carbon nanotubes having a length of 0.4 μm were used instead of acetylene black. The coverage was 20%.

<Comparative Example 1>
In (Preparation of negative electrode), a battery was produced in the same manner as in Example 1, except that 5 parts by weight of carbon nanotubes having a length of 0.6 μm were used instead of acetylene black. The coverage was 5%.

<Comparative example 2>
A battery was manufactured in the same manner as in Example 1 except that 20 parts by weight of acetylene black having a structure length of 0.4 μm was used in (Preparation of negative electrode). The coverage was 40%.

  The resistance of each negative electrode of Examples 1 to 5 and Comparative Examples 1 and 2 was measured. An internal short circuit test was performed on each of the batteries of Examples 1 to 5 and Comparative Examples 1 and 2. The initial charge / discharge efficiencies of the batteries of Examples 1 to 5 and Comparative Examples 1 and 2 were measured. These results are shown in the following [Table 1].

  The resistance of the negative electrode was measured as follows.

<Resistance>
Each of the negative electrodes of Examples 1 to 5 and Comparative Examples 1 and 2 was cut out to prepare, for example, a 20 mm square negative electrode for measurement. A predetermined current was applied to the negative electrode for measurement with a low resistance measuring instrument HIOKI-HiTESTER, and the resistance value was read.

  The internal short circuit test was conducted as follows.

<Internal short circuit test>
A plurality (for example, five) of batteries of Examples 1 to 5 and Comparative Examples 1 and 2 were prepared. For each battery, according to JIS C 8712 “Safety of sealed small-sized secondary batteries” and JIS C 8714 “Safety tests of single-cell batteries and assembled batteries for portable electronic devices” A forced internal short circuit test was performed. Among a plurality of (for example, five) batteries, the number of batteries ignited by the internal short circuit test was examined.

  The initial charge / discharge efficiency was measured as follows.

<Initial charge / discharge efficiency>
Charge capacity (mAh) when each battery of Examples 1 to 5 and Comparative Examples 1 and 2 was charged at a constant current of 0.2 C until the voltage value reached 4.2 V, and Examples 1 to 5 and Comparison The discharge capacity (mAh) when each battery of Examples 1 and 2 was discharged at a constant current of 0.2 C until the voltage value reached 2.5 V was introduced into [Formula 1] below, The charge / discharge efficiency was determined.

Initial charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 100 [Equation 1]
As shown in Table 1, when the coverage is 10% or more and 20% or less, the resistance is lower than the unusable resistance value (for example, 500 mΩ), there is no short circuit battery, and the initial efficiency is high ( For example, 95% or more). On the other hand, when the coverage is less than 10%, although there is no battery that is short-circuited, the resistance is not less than the unusable resistance value, and the initial efficiency is low. On the other hand, when the coverage is larger than 20%, the resistance is lower than the unusable resistance value and the initial efficiency is high, but there is a battery that is short-circuited.

<Comparative Example 3>
A battery was manufactured in the same manner as in Example 1 except that 20 parts by weight of acetylene black having a structure length of 0.2 μm was used in (Preparation of negative electrode). The coverage was 87%.

<Comparative example 4>
In (Preparation of negative electrode), a battery was produced in the same manner as in Example 1 except that 20 parts by weight of carbon nanotubes having a length of 1.0 μm were used instead of acetylene black. The coverage was 17%.

  The resistance of each negative electrode of Comparative Examples 3 and 4 was measured. An internal short circuit test was performed on each of the batteries of Comparative Examples 3 and 4. The initial efficiency of each battery of Comparative Examples 3 and 4 was measured. These results are shown in the following [Table 2] together with the results of Examples 1, 3, 4, and 5. In Table 2, “length” means the length of the acetylene black structure or the length of the carbon nanotube. The method of measuring resistance, the method of performing an internal short circuit test, and the method of measuring initial efficiency are the same as those described above.

  As shown in Table 2, when the length is 0.4 μm or more and 0.6 μm or less, the resistance is lower than the unusable resistance value, no battery is short-circuited, and the initial efficiency is high.

  When the length is smaller than 0.4 μm (for example, 0.2 μm), the resistance cannot be used even though a large amount of conductive agent (for example, 20 parts by weight of acetylene black) is used. It is higher than the value, and the negative electrode cannot secure an appropriate resistance. This is presumably because it is difficult to electrically connect adjacent metal oxide particles because of the short length.

  When the length is larger than 0.6 μm (for example, 1.0 μm), in order to make the coverage ratio 10% or more and 20% or less (for example, 17%), a large amount of conductive agent (for example, 20 parts by weight of carbon black) was required. This is presumably because the number of conduction paths is small because the length is long.

  As can be seen from the above, when the length is smaller than 0.4 μm (for example, 0.2 μm), the negative electrode cannot secure an appropriate resistance even when a large amount of conductive agent is used. Therefore, the length is preferably at least larger than 0.2 μm, and more preferably 0.4 μm or more.

  On the other hand, if the length is larger than 0.6 μm (for example, 1.0 μm), a large amount of conductive agent is required to make the coverage rate 10% or more and 20% or less. . Therefore, the length is preferably at least smaller than 1.0 μm, and more preferably 0.6 μm or less.

  When the length is 0.4 μm or more and 0.6 μm or less, a small amount of conductive agent (for example, 5 or 10 parts by weight of acetylene black or 8 or 10% by weight is not required, without using a large amount of conductive agent. Even if some carbon nanotubes are used, the coverage can be made 10% or more and 20% or less.

  In the case of Comparative Example 4, the resistance of the negative electrode is 100 mΩ, and the initial efficiency is 89% despite being relatively low, which is lower than 95%. This is presumably due to the following reasons. In the case of the comparative example 4, since the length is larger than 0.6 μm, a large amount (for example, 20 parts by weight) of a conductive agent is required to set the resistance to 100 mΩ, for example. For this reason, since the ratio of the second negative electrode active material is reduced by the amount of the conductive agent, the initial efficiency is lower than 95%.

  The reason why acetylene black is used as an example of the conductive agent having a length of 0.2 μm is as follows. It is technically difficult to set the length of the carbon nanotube to 0.2 μm, for example. On the other hand, in the case of acetylene black, the length of the structure can be easily reduced to 0.2 μm.

  In addition, the reason for using carbon nanotubes as an example of the conductive agent having a length of 1.0 μm is as follows. When the length of the acetylene black structure is long (for example, 1.0 μm), there is a risk of causing poor dispersion. In contrast, in the case of carbon nanotubes, the length can be accurately adjusted to 1.0 μm.

  In the following, as a layer formed on the negative electrode current collector, (1) a case where a laminate of the first layer and the second layer is used, and (2) a single layer body including only the second layer. Compare the difference in coverage with that used. Therefore, batteries of Comparative Examples 5 to 8 (that is, batteries including a negative electrode in which only the second layer was formed without forming the first layer on the negative electrode current collector) were manufactured.

<Comparative Example 5>
In (Preparation of negative electrode), a battery was produced in the same manner as in Example 1, except that only the second layer was formed on the negative electrode current collector without forming the first layer. Specifically, the method for producing the negative electrode is as follows.

(Preparation of negative electrode)
First, acetylene black was pulverized and dispersed so that the length of the structure was 0.4 μm. Next, 100 parts by weight of lithium titanate as a negative electrode active material, 5 parts by weight of acetylene black as a conductive agent, and 2 parts by weight of styrene butadiene rubber as a binder are mixed in water, and the second slurry Got. This second slurry was applied to both sides of the negative electrode current collector and then dried. Next, the negative electrode current collector on which both surfaces of the second slurry were applied and dried was rolled. The thickness of the second layer was 80 μm.

<Comparative Example 6>
In (Preparation of negative electrode), a battery was produced in the same manner as in Comparative Example 5 except that 10 parts by weight of carbon nanotubes having a length of 0.6 μm were used instead of acetylene black. The coverage was 15%.

<Comparative Example 7>
In (Preparation of negative electrode), a battery was produced in the same manner as in Comparative Example 5 except that 8 parts by weight of carbon nanotubes having a length of 0.4 μm were used instead of acetylene black. The coverage was 20%.

<Comparative Example 8>
In (Preparation of negative electrode), a battery was produced in the same manner as in Comparative Example 5 except that 20 parts by weight of acetylene black having a structure length of 0.4 μm was used. The coverage was 40%.

  The resistance of each negative electrode of Comparative Examples 5 to 8 was measured. The results are shown in [Table 3] below. The method for measuring the resistance is the same as that described above.

  As shown in Table 3, in the case of a single layer body (comparative example), in order to make the resistance of the negative electrode lower than the unusable resistance value, the coverage needs to be 40% or more. On the other hand, as shown in Table 1, in the case of the laminate (Example), even when the coverage is 20% or less, the resistance of the negative electrode can be made lower than the unusable resistance value. Thus, the coverage in the case of a laminated body is small compared with the coverage in the case of a single layer body.

  The present invention ensures the appropriate conductivity of the second layer, thereby preventing a decrease in initial charge / discharge efficiency and a decrease in battery characteristics such as a large current input / output characteristic and a cycle characteristic. A negative electrode for a lithium ion secondary battery having a first layer formed on the negative electrode current collector and a second layer formed on the first layer It is useful for lithium ion secondary batteries.

DESCRIPTION OF SYMBOLS 1 Negative electrode 1X Negative electrode collector 1a 1st layer 1b 2nd layer 2 Positive electrode 3 Separator 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Battery case 8 Sealing body 9 Gasket 10 Positive electrode insulating plate 11 Negative electrode insulating plate

Claims (7)

A negative electrode current collector;
A first layer formed on the negative electrode current collector and including a first negative electrode active material;
A second layer formed on the first layer and including a second negative electrode active material and a conductive agent;
The first negative electrode active material includes carbon,
The second negative electrode active material includes a metal oxide capable of inserting and extracting lithium ions,
The negative electrode for a lithium ion secondary battery, wherein a covering ratio of the conductive agent covering the surface of the metal oxide particles is 10% or more and 20% or less of a surface area. The negative electrode for a lithium ion secondary battery according to claim 1,
The negative electrode for a lithium ion secondary battery, wherein the thickness of the second layer is 3 μm or more and 30 μm or less. The negative electrode for a lithium ion secondary battery according to claim 1,
The conductive agent is acetylene black,
The length of the acetylene black structure is 0.4 μm or more and 0.6 μm or less, and the negative electrode for a lithium ion secondary battery. The negative electrode for a lithium ion secondary battery according to claim 1,
The conductive agent is a carbon nanotube,
The length of the carbon nanotube is 0.4 μm or more and 0.6 μm or less, and the negative electrode for a lithium ion secondary battery. The negative electrode for a lithium ion secondary battery according to claim 1,
The negative electrode for a lithium ion secondary battery, wherein the metal oxide is lithium titanate (Li _x Ti _y O _z ). The negative electrode for a lithium ion secondary battery according to claim 1,
The negative electrode for a lithium ion secondary battery, wherein the metal oxide has an average particle size of 1 μm or more and 2 μm or less.   The lithium ion secondary battery provided with the negative electrode for lithium ion secondary batteries of Claim 1.

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