JP6957333B2 - Negative electrode for lithium ion secondary battery - Google Patents

Description

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

Patent Document 1 discloses a negative electrode 90 for a lithium ion secondary battery as shown in FIG. A first layer 94 having graphite particles 93 as an active material is formed on the current collector 21. The first layer 94 is flattened by rolling. Further, a second layer 97 having LTO (lithium titanate) particles 96 as an active material is formed on the second layer 97. The binder is omitted in the figure.

Japanese Unexamined Patent Publication No. 2014-194842 Japanese Unexamined Patent Publication No. 2013-03928 JP-A-2017-022018

Consider a case where the foreign matter 29 has entered the negative electrode 90 from the outside of the second layer 97 as shown in FIG. The interface 95 between the first layer 94 and the second layer 97 is flat when viewed macroscopically. Therefore, the bite between the first layer 94 and the second layer 97 is poor. Therefore, the foreign matter 29 shears the second layer 97, and then the foreign matter 29 shears the first layer 94.

In the case shown in FIG. 1, an internal short circuit may occur between the foreign matter 29 and the first layer 94. Such an internal short circuit is considered to be one of the causes of heat generation of the lithium ion secondary battery. An object of the present invention is to provide a negative electrode suitable for suppressing heat generation of a lithium ion secondary battery.

[1] A current collector and an active material layer having a first layer and a second layer are provided.
The first layer has carbon material particles as an active material and is formed on the current collector.
The second layer has LTO (lithium titanate) particles as an active material and is formed on the first layer.
As the LTO particles permeate from the second layer toward the first layer, the second layer bites into the first layer.
The penetration depth of the LTO particles is 5 μm or more.
Negative electrode for lithium-ion secondary batteries.

[2] When considering the region including the second layer and occupying 1/5 of the thickness of the active material layer,
Binders are present between the active materials in the region.
The area ratio of the binder to the region in the cross section of the active material layer is 10% or less.
The negative electrode according to [1].

[3] The primary particle size of the LTO particles is 0.5 μm or less.
The average particle size of the primary particles of the carbon material particles is 1 μm to 20 μm.
The aspect ratio of the primary particles of the carbon material particles is 1.5 or more.
The negative electrode according to [2].

[4] The layer thickness of the second layer is 1 μm or more and 5 μm or less.
The penetration depth of the LTO particles is 14 μm or less.
The negative electrode according to [2] or [3].

[5] A lithium ion secondary battery having the negative electrode according to any one of [1] to [4].

[6] With the current collector
Granulated body Ga, which is a mixture of carbon material particles, which are active materials, binders, and volatile components,
Granulator Gb formed by mixing LTO (lithium titanate) particles, which are active materials, binders, and volatile components with each other.
It is a method of roll-molding a negative electrode using
Between the A roll and the B roll facing each other, the granulated body Ga is sprayed on the A roll side, and the granulated body Gb is further sprayed on the B roll side.
The first layer in which the carbon material particles are dispersed and the second layer in which the LTO particles are dispersed are formed by simultaneously sandwiching and pressing the A roll and the B roll and collapsing each granulated body. Formed at the same time
By allowing the LTO particles to permeate from the second layer toward the first layer with the pressing, the first layer is made to bite the second layer.
The active material layer composed of the first layer and the second layer is conveyed on the B roll, and the active material layer is conveyed.
The active material layer is supplied to the B roll side between the B rolls and the C rolls facing each other, and the current collector is supplied to the C roll side.
By pressing these with the B roll and the C roll, the side of the first layer of the active material layer is attached to the current collector.
Further, when the negative electrode is dried by removing the volatile component from the active material layer attached to the current collector.
In the dried negative electrode, the penetration depth of the LTO particles is 5 μm or more.
A method for manufacturing a negative electrode for a lithium ion secondary battery.

[7] When considering the region of the dried negative electrode that includes the second layer and occupies 1/5 of the thickness of the active material layer,
Binders are present between the active materials in the region.
In the cross section of the active material layer, the area ratio of the binder to the region is 10% or less, and here, the binder in the region has a region closer to the current collector than the region due to the removal of the volatile component. May contain binders that have been migrated from,
The method for manufacturing a negative electrode according to [6].

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a negative electrode suitable for suppressing heat generation of a lithium ion secondary battery.

Schematic diagram of the active material layer and foreign matter. Sectional view of the negative electrode. Schematic diagram of the active material layer. Schematic diagram of the active material layer and foreign matter. Side view of the manufacturing equipment. Schematic diagram of binder distribution. Schematic diagram of binder distribution. Binder occupancy graph. Graph of calorific value. Graph of capacity retention rate. Graph of capacity retention rate. Graph of interparticle distance. Graph of IV resistance.

In the following description, the same components will be designated by the same reference numerals, and duplicate description will be omitted.

FIG. 2 shows a cross section of the negative electrode 20 for a lithium ion secondary battery. The negative electrode 20 includes a current collector 21 and an active material layer 22. The active material layer 22 is formed on the surface 28a, which is one surface of the current collector 21 in the drawing.

The material of the current collector 21 shown in FIG. 2 is not particularly limited as long as it is suitable for roll molding of the negative electrode. The current collector 21 may be any of a metal foil, a film, a film and a sheet. The metal may be stainless steel, nickel, or copper.

As shown in FIG. 2, the active material layer 22 has a first layer 24 and a second layer 27. The first layer 24 is formed on the surface 28a of the current collector 21. The second layer 27 is formed on the first layer 24. The first layer 24 and the second layer 27 are preferably in direct contact with each other.

As shown in FIG. 2, the active material layer 22 may be formed on both sides of the current collector 21. For example, an equivalent active material layer 22 may be formed on the surface 28b, which is the other surface of the current collector 21 in the drawing. Further, another film may be formed on the outside of the second layer 27. Further, another layer may be formed between the first layer 24 and the current collector 21.

FIG. 3 shows an enlarged cross-sectional view of the negative electrode 20. The first layer 24 has carbon material particles 23 as an active material. The carbon material may be any of graphite, carbon black, acetylene black, and carbon fiber. The carbon material constituting the carbon material particles 23 may be a mixture thereof. The carbon material particles 23 may be particles in which particles made of different carbon materials are mixed with each other. When the carbon material particles 23 are made of graphite, the first layer 24 may be referred to as a graphite layer.

As shown in FIG. 3, the second layer 27 has LTO particles 26 as an active material. LTO represents lithium titanate. The LTO may be, for example, any of those represented by the _{chemical formulas Li 2} TiO ₃ , Li ₂ Ti ₃ O ₇ , Li ₂ Ti ₆ O ₁₃ , and Li ₄ Ti ₅ O _12. LTO may be a mixture of these. Other elements may be added to LTO. The LTO particles 26 may be a mixture of particles each consisting of different LTOs. The LTO is preferably _{Li 4} Ti ₅ O ₁₂ having a spinel structure. The second layer 27 may be referred to as an LTO layer.

A cell can be manufactured using the negative electrode 20 shown in FIG. At this time, the primary particle size of the LTO particles 26 is preferably 0.5 μm or less. This makes it possible to improve the input characteristics of the cell. The primary particle size of the LTO particle 26 may be larger than 0 μm. The primary particle size of the LTO particle 26 may be any of 0.4, 0.3, 0.2 and 0.1 μm. The description of the examples may be referred to to define the primary particle size of the LTO particle 26. The LTO particles 26 are smaller than the carbon material particles 23.

In FIG. 3, the binder and the conductive agent as an optional component are omitted. It is preferable that the first layer 24 and the second layer 27 contain a binder. The material of the binder may be any one of carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), fluororubber, styrene-butadiene rubber (SBR), nitrile butadiene rubber (NBR) and acrylic rubber (ACM). The CMC may be salt. The salt may be a sodium salt. The binder may be a combination of these. The binder may be a mixture of these. The type of binder may be different between the first layer 24 and the second layer 27. The binder may be mixed near the boundary between the first layer 24 and the second layer 27. The first layer 24 and the second layer 27 may further contain a conductive agent.

As shown in FIG. 3, the LTO particles 26 permeate from the second layer 27 toward the first layer 24. Such a portion in the first layer 24 is represented as a penetrating portion 25. As the LTO particles 26 permeate in this way, the second layer 27 bites the first layer 24.

FIG. 3 shows predetermined depths L1 and L2 from the outer surface of the active material layer 22. The depth L1 is a halfway depth. The second layer 27 and the penetrating portion 25 may be included in the range of the depth L1. The range of the depth L1 may include the boundary between the first layer 24 and the second layer 27. The depth L2 is the total depth of the active material layer 22. These parameters will be described in the examples.

In FIG. 3, the depth L2 corresponds to the layer thickness of the active material layer 22. The layer thickness of the active material layer 22 may be 10 μm to 200 μm as an example. The layer thickness of the active material layer 22 may be any of 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180 and 190 μm. For example, when a layer or film other than the first layer 24 and the second layer 27 is formed in the negative electrode 20, the layer thickness of the active material layer 22 is the layer thickness of the first layer 24 and the layer thickness of the second layer 27. You may think of it as the total of.

The layer thickness of the second layer 27 shown in FIG. 3 is preferably 5 μm or less. As a result, the electrical resistance, that is, the IV resistance of the cell can be reduced in the cell using the negative electrode 20. The layer thickness of the second layer 27 is preferably 1 μm or more. As a result, in the cell using the negative electrode 20, the place where the second layer 27 is not formed on the active material layer 22, the so-called defect, can be reduced. The layer thickness of the second layer 27 may be any of 2, 3 and 4 μm. The description of Examples may be referred to in order to define the layer thickness of the active material layer 22.

The layer thickness of the first layer 24 shown in FIG. 3 may be designed as the layer thickness of the active material layer 22 minus the layer thickness of the second layer 27.

The penetration depth of the LTO particles 26 in the penetration portion 25 shown in FIG. 3 is 5 μm or more. The penetration depth may be less than 20 μm. The penetration depth may be any of 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 and 19 μm. The description of the examples may be referred to to define the penetration depth of the LTO particles 26. In the figure, the permeation portion 25 is defined with reference to the LTO particles 26 that have permeated the deepest.

The average particle size of the primary particles of the carbon material particles 23 shown in FIG. 3 may be 1 μm to 20 μm, preferably 10 μm to 15 μm. The particle size distribution of the primary particles of the carbon material particles 23 may be as follows. D10 may be 0.7 times or more of D50 (D10 ≧ 0.7 × D50). Further, D90 may be 1.6 times or less of D50 (D90 ≦ 1.6 × D50). By adjusting the particle size distribution, it may be possible to prevent the small particles of the carbon material particles 23 from inhibiting the penetration of the LTO particles.

Further, the aspect ratio of the primary particles of the carbon material particles 23 is preferably 1.5 or more. When the primary particle diameter of the LTO particles 26 is 0.5 μm, the permeation of the LTO particles 26 can be promoted by having the carbon material particles 23 having the above dimensions. The carbon material particles 23 having these dimensions are also suitable when the primary particle size of the LTO particles 26 is smaller than 0.5 μm. The description of the examples may be referred to in order to define the definition of the average particle size and the aspect ratio of the primary particles of the carbon material particles 23.

The aspect ratio of the carbon material particles 23 can also be 5 or less. The aspect ratio of the carbon material particles 23 is 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 3. It may be any of 0, 3.5, 4.0 and 4.5.

Consider a case where the foreign matter 29 has entered the negative electrode 20 from the outside of the second layer 27 as shown in FIG. A penetrating portion 25 is formed on the outermost side of the first layer 24. Therefore, the second layer 27 bites the first layer 24. Therefore, even if the second layer 27 and the first layer 24 are sheared by the foreign matter 29, the coating of the first layer 24 formed by the second layer 27 is not easily broken.

In the case shown in FIG. 4, the possibility of an internal short circuit occurring between the foreign matter 29 and the first layer 24 is relatively low. In the present embodiment, it is easy to suppress heat generation of the lithium ion secondary battery due to such an internal short circuit. The present embodiment is suitable for providing a negative electrode suitable for suppressing heat generation of a lithium ion secondary battery.

FIG. 5 shows a manufacturing apparatus 30 for roll-forming the negative electrode 20. In order to form the negative electrode 20 in the manufacturing apparatus 30, the current collector 21, the granulated body Ga, and the granulated body Gb are prepared as materials. Hereinafter, the granulated material may be referred to as granulated particles. As a method for producing an electrode sheet using granulated particles or granulated materials, a method as shown in Patent Document 3 can be mentioned. This embodiment is different from Patent Document 3 in that two types of granulated bodies, a granulated body Ga and a granulated body Gb, are charged. In Patent Document 3, the granulated body has a coating, but in the present embodiment, the granulated body may or may not have a coating. Hereinafter, a method of roll-molding an electrode sheet using a granulated body is referred to as a “granulated body method” in the present specification.

The granulated body Ga shown in FIG. 5 is formed by mixing carbon material particles, a binder, and a volatile component with each other. The volatile component may be water. The proportion of carbon material particles may be 95-99% of the total solid content, and may be any of 96, 97 and 98%. The proportion of the binder may be 1 to 5% with respect to the total solid content, or may be any of 2, 3 and 4%. The binder may be either or both of CMC and SBR. The proportion of CMC may be 0.5 to 5% with respect to the total solid content, and may be any of 1, 2, 3 and 4%. The proportion of SBR may be 0.5 to 5% with respect to the total solid content, and may be any of 1, 2, 3 and 4%.

The solid content of the granulated body Ga shown in FIG. 5 may be 70 to 90%, and may be 75, 76, 77, 77.5, 77.6, 77.7, 77.8, 77.9, 78.0, It may be any of 78.1, 78.2, 78.3, 78.4, 78.5, 79, 80, 81, 82, 83, 84 and 85%. The description of the examples may be referred to to define the solids content.

The granulated body Gb shown in FIG. 5 is formed by mixing LTO particles, a binder, and a volatile component with each other. The volatile component may be water. The proportion of LTO particles may be 95-99% of the total solid content and may be any of 96, 97 and 98%. The proportion of the binder may be 1 to 5% with respect to the total solid content, or may be any of 2, 3 and 4%. The binder may be acrylic as shown in Examples.

In FIG. 5, the solid content of the granulated product Gb may be 70 to 90%, 75, 76, 77, 77.5, 77.6, 77.7, 77.8, 77.9, 78.0, It may be any of 78.1, 78.2, 78.3, 78.4, 78.5, 79, 80, 81, 82, 83, 84 and 85%. The description of the examples may be referred to to define the solids content.

In FIG. 5, in the production method (granulation method) of the present embodiment, these granulation bodies are formed by giving moisture (Moisture) to the powder of each component by the volatile components. The volatile components are finally removed from the negative electrode.

As shown in FIG. 5, the A roll and the B roll face each other. Between these rolls, the granulated body Ga is sprayed on the A roll side. Further, the granulated body Gb is sprayed on the B roll side. The granulated bodies thus supplied by the A roll and the B roll are pressed together. Further, the pressure causes each granulation body to collapse.

As shown in FIG. 5, the first layer 24 and the second layer 27 are simultaneously formed by the A roll and the B roll. On the other hand, the method using a slurry mixture cannot form the first layer and the second layer at the same time. Therefore, the granulation method is more efficient than the method using a slurry mixture.

In FIG. 5, carbon material particles are dispersed in the first layer 24 while the granulated body is pressed by the A roll and the B roll. LTO particles are dispersed in the second layer 27. In each layer, it is preferable that the particles are evenly dispersed to the extent that the original shape of the granulated product is not retained.

In FIG. 5, LTO particles permeate from the second layer 27 toward the first layer 24 as the pressure is applied between the A roll and the B roll (FIG. 3). As a result, the second layer 27 is made to bite into the first layer 24. As a result, the active material layer 22 composed of the first layer 24 and the second layer 27 is formed.

In FIG. 5, a velocity difference may be provided in the flow on the roll surface between the A roll and the B roll. It is preferable that the B roll is faster than the A roll. The peripheral speed ratio (B / A) may be 1.1 to 1.3, for example, 1.15, 1.2 or 1.25. The active material layer 22 is supported on the B roll. The active material layer 22 is conveyed on the B roll. The second layer 27 is attached to the surface of the B roll. The first layer 24 is exposed.

In FIG. 5, the penetration depth of the LTO particles may be determined based on the spacing between the A and B rolls (FIG. 3). The distance between the A roll and the B roll may be larger than 80 μm and may be 105 μm or less. The interval between the A roll and the B roll may be any of 85, 90, 95, 96, 97, 98, 99, 100, 101, 102, 103 and 104.

As shown in FIG. 5, the B roll and the C roll face each other. The active material layer 22 is supplied to the B roll side, and the current collector 21 is supplied to the C roll side. Press them with the B roll and the C roll. As a result, the side of the first layer 24 of the active material layer 22 is attached onto the surface 28a of the current collector 21. The peripheral speed ratio (C / B) may be 1.2 to 1.6, for example, any of 1.3, 1.4 and 1.5. The active material layer 22 is supported on the current collector 21. The negative electrode composed of the active material layer 22 and the current collector 21 is conveyed on a roll. The volatile components may be removed from the active material layer 22 formed on the surface 28a. After removing the volatile components, the penetration depth of the LTO particles is preferably 5 μm or more. As described above, an electrode sheet-like negative electrode having an active material layer on at least one side is formed.

In FIG. 5, the composition ratio (wt%) of the solid content of the entire active material layer including the first layer 24 and the second layer 27 is referred to as a mixture composition. The mixture composition reflects the respective compositions of the granulated body Ga and the granulated body Gb. The layer thickness of the second layer 27 can be controlled by changing the balance between the supply rate of the granulated body Ga and the supply rate of the granulated body Gb.

In FIG. 5, the supply rate ratio (weight ratio) of the granulated body Ga / granulated body Gb may be 96.7 / 3.3 to 93.7 / 6.2. The weight ratio may be any of 96/4, 95/5 and 94/6. The supply rate ratio (weight ratio) of the carbon material particles / LTO particles in the mixture composition may be 94.9 / 3.2 to 91.9 / 6.1. The weight ratio may be any of 94/4, 93/5 and 92/6.

The supply rates of the granulated body Ga and the granulated body Gb can be set to 50 g / min to 500 g / min, respectively. At this time, the peripheral speeds of the A roll and the B roll can be set to 60 mm / sec to 90 mm / sec, respectively.

In FIG. 5, the layer thickness of the active material layer 22 may be determined based on the distance between the B rolls and the C rolls and the thickness of the current collector 21. The thickness of the current collector 21 may be 5 to 20 μm, or may be any of 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 and 19 μm.

In FIG. 5, the value obtained by subtracting the thickness of the current collector 21 from the size of the interval between the B rolls and the C rolls is smaller than the interval between the A rolls and the B rolls. The interval between the B roll and the C roll may be 10 μm or more and less than 150 μm. The interval between the B roll and the C roll may be any of 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130 and 140 μm.

In FIG. 5, an active material layer may be further formed on the surface 28b of the current collector 21 by using a roll group equivalent to the A roll, the B roll, and the C roll. Further, the target electrode sheet-shaped negative electrode may be obtained by removing the volatile component from the active material layer on the surface 28b. The volatile components may be simultaneously removed from the active material layer 22 on the surface 28b and each on the surface 28b. The layer thickness of each active material layer 22 on the surface 28b and on the surface 28b may be determined based on the distance between the B roll and the C roll through which the negative electrode passes last and the thickness of the current collector.

The present invention is not limited to the above embodiment, and can be appropriately modified without departing from the spirit.

Negative electrodes were prepared according to Table 1 in Examples 1 to 14.

In Table 1, the column of granulated body Ga represents the composition in the solid component of the granulated particles for forming the graphite layer. Water was used as a volatile component to give the granulated particles moisture. The solid content column shows the ratio of the solid component to the entire granulated particle component including water as a weight ratio. Such a solid content may be defined as the solid content of the granulated body Ga in the present specification. The solid component is superior to the volatile component in the granulated particles of this example as compared with the slurry mixture and the like. CMC represents the sodium salt of carboxymethyl cellulose. SBR represents styrene-butadiene rubber.

In Table 1, the graphite aspect ratio represents the major / minor axis ratio when the primary graphite particles are approximated by an elongated ellipsoid. Such an aspect ratio is defined by (maximum major axis) / (width orthogonal to maximum major axis). Particles with a smaller aspect ratio have a shape closer to a sphere. The graphite primary particles represent the graphite primary particles before the formation of the granulated body Ga.

In each example, the maximum major axis of the graphite particles is assumed to be obtained as the so-called Feret diameter measured by an image analysis method for each individual particle. It is assumed that the width orthogonal to the minor axis, that is, the maximum major axis is also obtained as the Feret diameter. In Example 1, the aspect ratio of the graphite particles is the average number. The same is true in other examples. The aspect ratio of the graphite particles obtained by measuring as described above may be defined as the aspect ratio of the graphite particles in the present specification. The definition of the aspect ratio can also be applied to carbon material particles made of other carbon materials.

In each example, the average particle diameter of the primary particles of the graphite particles is the arithmetic mean diameter of the area circle equivalent diameter measured by the laser diffraction scattering method for each individual particle. The average particle size is the volume average diameter. The average particle size of the primary particles of the graphite particles obtained by the measurement as described above may be defined as the primary particle size of the graphite particles in the present specification. The definition of the average particle size can also be applied to carbon material particles made of other carbon materials.

In each example, the average particle size of the primary particles of the graphite particles was as follows. In Example 1, the average particle size was 15 μm. In Example 9, the average particle size was 11 μm. In Example 10, the average particle size was 15 μm. In Example 11, the average particle size was 17 μm. In the other examples, the average particle size was the same as in Example 1.

In Table 1, the column of the granulated body Gb represents the composition in the solid component of the granulated particles for forming the LTO layer. Water was used as a volatile component to moisturize the granulated particles. The solid content column shows the ratio of the solid component to the entire granulated particle component including water as a weight ratio. Such solid content may be defined in the present specification as the solid content of the granulated product Gb. The solid component is superior to the volatile component in the granulated particles of this example as compared with the slurry mixture and the like. LTO is represented by the chemical formula Li ₄ Ti ₅ O _12. Acrylic represents (CH ₂ CHCOOX) _n . Here, X represents Na or Li. The LTO primary particle size represents the particle size of the primary particles of the LTO particles before the formation of the granulated body Gb.

In each example, the LTO primary particle size is the average of the individual particles measured by the laser diffraction / scattering method. The LTO primary particle diameter is the diameter equivalent to a volume sphere; the effective diameter is obtained by the so-called centrifugal sedimentation method or the laser diffraction / scattering method. The LTO primary particle size is an arithmetic mean diameter and a volume average diameter. The LTO primary particle size obtained by measuring as described above may be defined as the primary particle size of the LTO particles in the present specification.

SBR particles were measured in the same manner as the LTO primary particle size. The average particle size of SBR was 100 μm. These numbers are common in each example. Since CMC and acrylic are soluble binders, the particle size can be set arbitrarily.

In Table 1, the column of the mixture composition shows the composition ratio (wt%) of the solid content of the entire active material layer including the graphite layer and the LTO layer.

In Table 1, the column of roll GAP shows the distance between the A roll and the B roll (AB) and the distance between the B roll and the C roll (BC) shown in FIG. Since (BC) is narrower than (AB), the active material layer 22 can be attached to the current collector 21. The material of the metal foil used as the current collector 21 was copper, and its thickness was 10 μm. The peripheral speed ratio column represents the peripheral speed, that is, the ratio of the transport speeds on the outer circumference of each roll.

In Table 1, the column of construction method shows the following. With respect to Examples 1 and 3-14, "granulation" represents a granulation method. With respect to Example 2, coating represents slurry coating.

[Examples 1 and 2: Control of segregation of binder by granulation method]

In Example 1, the granulated body Ga and the granulated body Gb as shown in FIG. 5 were mixed according to the composition shown in Table 1. In this example, a commonly available granulated food processor was used. Next, using A roll and B roll as shown in FIG. 5, an active material layer 22 composed of a graphite layer, that is, a first layer 24 and an LTO layer, that is, a second layer 27 was formed. The supply speed of the granulated body Ga was set to 150 g / min with respect to the peripheral speed of the A roll of 65 mm / sec. The supply speed of the granulated body Gb was set to 100 g / min with respect to the peripheral speed of the B roll of 78 mm / sec.

Further, water was removed from the active material layer 22 by heating the active material layer 22 attached to the current collector 21 as shown in FIG. 5 under atmospheric pressure, a temperature of 90 ° C., and a time of 10 minutes. After removing the water, the roll-molded product was cut to a predetermined size to obtain a negative electrode.

In Example 2, a graphite layer was formed by applying a slurry containing graphite particles or the like on the current collector. Further, the LTO layer was formed by applying a slurry containing LTO particles or the like on the graphite layer.

FIG. 6 shows the distribution of the binder 35 after the active material layer 22 is prepared by the granulation method as in Example 1 and the volatile components are removed. The granulation method is characterized by the distribution of the binder 35 after the removal of the volatile components. The solid component is superior to the volatile component in the granulated particles of this example as compared with the slurry mixture and the like. That is, the removal of water, which is a volatile substance, is completed promptly. Therefore, segregation of the binder is unlikely to occur, and the bias of the binder 35 is not conspicuous in the active material layer 22.

FIG. 7 shows the distribution of the binder 35 after the active material layer 92 is prepared and the volatile components are removed by the method using a slurry mixture as in Example 2. In such a method, a volatile component is used as a dispersion medium for the slurry. A large amount of volatile components are added to apply the slurry smoothly. Therefore, the binder 35 segregates after removing the volatile components. As the volatile components move to the outer surface of the active material layer 92, the binder 35 also migrates toward the surface of the active material layer 92.

Here, as shown in FIGS. 6 and 7, the region from the surface of the active material layer to a certain depth L1 is considered. The range of depth L1 includes the entire LTO layer, which is the second layer. L2 is the thickness of the active material layer. Here, L1 / L2 is set to 1/5. Binders are present in all the regions up to the depth L1 in the active material layer. In addition, a binder exists between the active material particles in the region. This includes between LTO particles, between graphite particles, and between LTO particles and graphite particles. The binder occupancy in the region of L1 was different between Example 1 and Example 2.

FIG. 8 shows the binder occupancy in each example. In Example 1, the area ratio of the binder to the region up to the depth L1 in the cross section of the active material layer, that is, the binder occupancy was 10% or less. Note that Example 1, which relies on the granulation method, does not mean that segregation of the binder has not occurred at all. That is, it is considered that the binder in the region up to the depth L1 contains some binder that has migrated from the region deeper than the depth L1 due to the removal of the volatile component. However, the ratio is considered to be particularly small as compared with Example 2.

As shown in FIG. 8, in Example 2, the area ratio of the binder to the region up to the depth L1 in the cross section of the active material layer, that is, the binder occupancy rate was 10% or more. It is considered that this is because the binder in the region up to the depth L1 contains a large amount of the binder that has migrated from the region deeper than the depth L1 due to the removal of the volatile component.

In the granulation method, the density of graphite can be reduced by increasing the binder / graphite ratio in the granulation Ga. In this case, the penetration of LTO particles into the graphite layer can be promoted. In the granulation method, even when the amount of the binder is increased in this way, the migration of the binder can be suppressed by reducing the volatile components in the granulation Ga.

By suppressing the segregation of the binder, it is expected that good effects such as reduction of internal resistance and expression of excellent cycle characteristics by making the current distribution uniform can be obtained.

[Reference example: Patent Document 2]

Permeation of LTO particles into the graphite layer may also be obtained by the production method as described in Patent Document 2. However, in Patent Document 2, a negative electrode is manufactured using a slurry. Since it is necessary to remove the solvent or the dispersion medium from the slurry, it is considered that segregation of the binder as shown in FIG. 7 of the present application occurs even in the negative electrode of Patent Document 2.

[Examples 3 to 5: Effect of penetration depth on cell calorific value]

The negative electrodes of Examples 3 to 5 were prepared in the same manner as in Example 1. However, in Example 3, the roll GAP (AB) was set to a wide 115 μm. In Example 4, the roll GAP (AB) was set to 105 μm as in Example 1. Further, in Example 5, the roll GAP (AB) was narrowed to 95 μm.

FIG. 9 shows the penetration depth and calorific value of the LTO particles in each example. In Example 3, the penetration depth of the LTO particles was 2 μm. In Example 4, it was 5 μm. In Example 5, it was 7 μm. The smaller the roll GAP, the greater the penetration depth of the LTO particles.

It may be defined as the depth of penetration of LTO particles in the present specification. Here, the penetration depth of the LTO particles is an average value obtained by image analysis of the cross section of the active material layer. Explaining with reference to FIG. 3, the permeation depth of the LTO particles 26 in the permeation portion 25, that is, the average value of the distances from the second layer 27 is specified. In FIG. 3, the height of the penetrating portion 25 is schematically shown as being constant, but the actual penetrating depth varies depending on the position in the image. In this example, the maximum value of the distance from the upper end of the graphite particles in the graphite layer 24 to the penetrating LTO particles 26 was calculated like a length measurement to obtain the average penetration depth. The penetration depth of the LTO particles obtained by measuring as described above may be defined as the penetration depth of the LTO particles in the present specification.

Further, a cell material containing a positive electrode, a separator and an electrolytic solution was prepared to prepare a cell having the negative electrode. The material of the positive electrode active material was lithium nickel cobalt manganate. The material of the positive electrode current collector was aluminum. As the electrolytic solution, one having a general electrolytic solution composition can be used. In this example, 1.0 M LiPF ₆ EC / DMC / EMC was used. The separator was a porous polyolefin membrane. The physical characteristics of the film of the separator can be set arbitrarily. The cell was activated according to a known method. The same is true in the following example

In measuring the calorific value, the cell was used under the condition that the needle was passed through the center of the electrode body in a state of being charged to 25 ° C. and SOC 100%. The calorific value of the cell was measured by calculating ^{I 2} × R from the current value I flowing in the cell at the time of short circuit and the internal resistance R.

As shown in FIG. 9, when the penetration depth of the LTO particles was 5 μm or more, the calorific value of the cell could be suppressed to 0.25 J or less. One of the interpretations of the ability to suppress the calorific value of the cell is thought to be due to the conductive foreign matter.

Consider the case where the conductive foreign matter 29 has invaded the active material layer 22 as shown in FIG. In this case, the LTO particles 26 follow the foreign matter 29 including the LTO particles 26 in the penetrating portion 25. Therefore, heat generation due to an internal short circuit can be suppressed. If the penetration depth of the LTO particles is insufficient, such followability may decrease. Such an interpretation is not intended to limit the scope of the invention of the present application. Disclosure of such interpretations is to aid the technical understanding of those skilled in the art.

[Examples 6 to 8: Effect of LTO primary particle size on input characteristics]

The negative electrodes of Examples 6 to 8 were prepared in the same manner as in Example 1. However, in Example 6, the LTO primary particle size was set to 0.2 μm. In Example 7, it was set to 0.5 μm. Further, in Example 11, it was set to 1.0 μm. A cell was prepared and its capacity retention rate was measured.

The capacity retention rate of each example is shown in the graph of FIG. The capacity retention rate shown on the vertical axis is represented by (discharge capacity (mAh) of the cell after charging / discharging for 300 cycles) / (discharge capacity (mAh) of the cell immediately after activation). The horizontal axis represents the charging rate in C rate. The standard of C rate is the discharge capacity of the cell immediately after activation. The C rates in the graph are 1C, 2C, 3C, 4C and 5C, respectively, in ascending order. The discharge rate was 1C, respectively.

By setting the LTO primary particle size to 0.5 or less as shown in FIG. 10, the capacity retention rate after charging and discharging 300 times could be set to 85% or more regardless of the C rate. When the LTO primary particle size was 1.0 μm, the capacity retention rate after 300 charge / discharge in the high charge / discharge rate range (3C to 5C) was less than 85%. If the LTO primary particle size is 0.5 μm or less, the input characteristics of the cell can be enhanced.

[Supplementary example: Capacity maintenance rate after 300 cycles]

The capacity retention rate in each supplementary example is shown in FIG. The capacity retention rate of cells after 300 cycles was investigated. The ellipse represents Supplementary Example 15 and the triangle represents Supplementary Example 16. The negative electrode of Supplementary Example 15 had an LTO primary particle size of 0.5 μm. Other conditions were the same as in Example 1. Supplement No LTO particles were used in the negative electrode of Example 16. That is, the negative electrode has only a graphite layer. The negative electrode of Supplementary Example 16 was also prepared by the granulation method. That is, the active material layer was formed only by the granulated body Ga without using the granulated body Gb containing the LTO particles (FIG. 5). The composition of the granulated body Ga is the same as in Example 1.

A cell was prepared and its capacity retention rate was measured. The capacity retention rate was calculated in the same manner as in Examples 6 to 8. The charging rate was set to 3C. The discharge rate was 1C. The standard of C rate is the discharge capacity of the cell immediately after activation.

As shown in FIG. 11, in Supplementary Example 16, the capacity retention rate sharply decreased after 300 cycles. The alternate long and short dash line parallel to the horizontal axis shows the capacity retention rate in 300 cycles of Supplementary Example 16. In Supplementary Example 15, the capacity retention rate in 300 cycles of Supplementary Example 16 was not lower than that of Supplementary Example 16.

The cell of Supplementary Example 16 is different from Supplementary Example 15 in that the LTO layer is arranged on the surface layer of the active material layer. As one interpretation of the above result, it is considered that the LTO layer reduces the shear stress applied to the roll molded product by the C roll. It is considered that the reduction of the shear stress suppresses the deterioration of the input characteristics due to the surface collapse of the graphite layer. Therefore, it is considered that the negative electrode of Supplementary Example 15 is superior in precipitation resistance to the negative electrode of Supplementary Example 16. Here, the precipitation resistance represents the degree of capacity deterioration under a high current load.

[Examples 9 to 11: Effect of aspect ratio of graphite particles on distance between graphite particles]

The negative electrodes of Examples 9 to 11 were prepared in the same manner as in Example 1. However, in Example 9, the aspect ratio of the graphite particles was set to 1.1. In Example 10, it was set to 1.5. In Example 11, it was set to 2.5. The average particle size of the primary particles of the graphite particles of Examples 9 to 11 is 11 to 17 μm as described above.

The inter-particle distance shown on the vertical axis of the graph of FIG. 12 is the distance between graphite particles. Such a distance was obtained by measuring the minimum value of the distance between adjacent graphite particles from the cross-sectional SEM image of the electrode plate and averaging 100 points.

By setting the aspect ratio to 1.5 or more as shown in FIG. 12, the interparticle distance could be set to 0.5 μm or more. The average particle size of the graphite particles is in the range of 15 μm to 17 μm as described above. In such a case, if the distance between the particles is 0.5 μm or more, a graphite layer suitable for permeation of LTO particles having a primary particle size of 0.5 μm or less can be obtained.

[Examples 12 to 14: Effect of LTO layer thickness on cell resistance]

The negative electrodes of Examples 12 to 14 were prepared in the same manner as in Example 1. However, the supply rate of the granulated body Ga was relatively reduced, and the supply rate of the granulated body Gb was increased. Alternatively, the supply rate of the granulated body Ga was relatively increased, and the supply rate of the granulated body Gb was decreased. That is, as shown in Table 1, in Example 12, the ratio of LTO particles in the mixture composition was 3.2 wt%, and the ratio of graphite particles was 94.9 wt%. In Example 13, it was 6.1 wt%, and the ratio of graphite particles was 91.9 wt%. In Example 14, it was set to 11.5%, and the ratio of graphite particles was set to 86.5 wt%. The proportions of the binder are shown in Table 1.

The layer thickness of the LTO layer in the negative electrode was measured after the volatile components were dried. The measurement was performed as follows. The distance from the upper end particles of the graphite layer to the upper end particles of the LTO layer was measured from the cross-sectional SEM image of the electrode plate, and 100 points were averaged. The layer thickness of the LTO layer obtained by measuring as described above may be defined as the layer thickness of the first layer in the present specification. When the permeation portion and the LTO layer are separated with the upper end of the graphite layer as a boundary, the portion corresponding to the LTO layer thickness is the distance between the poles. This is based on the fact that the portion corresponding to the layer thickness affects the cell resistance.

FIG. 13 shows the IV resistance of the cell in each example. In Example 12, the layer thickness of the LTO layer was 1 μm. In Example 13, the layer thickness of the LTO layer was 5 μm. In Example 14, it was 10 μm. The layer thickness could be controlled by reducing the proportion of graphite particles and increasing the proportion of LTO particles in the mixture composition.

The vertical axis of FIG. 13 shows the relative value of the IV resistance of the cell. The IV resistance was measured by dividing the amount of voltage drop when discharging at 25 ° C., SOC 50%, and 10 seconds by the discharge current value. The alternate long and short dash line parallel to the horizontal axis indicates the IV resistance in the cell without the LTO layer (see Supplementary Example 16).

By setting the layer thickness of the LTO layer to 5 μm or less as shown in FIG. 13, the IV resistance could be reduced as compared with the cell without the LTO layer. However, if the layer thickness of the LTO layer becomes too small, a place where the LTO layer is not formed on the active material layer, that is, a so-called defect increases. Therefore, the layer thickness of the LTO layer is preferably 1 μm or more.

20 Negative electrode, 21 Collector, 22 Active material layer, 23 Carbon material particles, 24 First layer, 25 Penetration part, 26 LTO particles, 27 Second layer, 28a surface, 28b surface, 29 Foreign matter, 30 Manufacturing equipment, 35 Binder, 90 Negative electrode for battery, 92 Active material layer, 93 Graphite particles, 94 First layer, 95 Interface, 96 particles, 97 Second layer, A A roll, BB roll, CC roll, Ga granulation, Gb granulated body, L1 depth, L2 depth

Claims (7)

It is provided with a current collector and an active material layer having a first layer and a second layer.
The first layer has carbon material particles as an active material and is formed on the current collector.
The second layer has LTO (lithium titanate) particles as an active material and is formed on the first layer.
As the LTO particles permeate from the second layer toward the first layer, the second layer bites into the first layer.
The depth of penetration of the LTO particles Ri der than 5 [mu] m,
When considering the region including the second layer and occupying 1/5 of the thickness of the active material layer,
Binders are present between the active materials in the region.
Area ratio of the binder occupied in the area in the cross section of the active material layer is Ru der than 10%,
Negative electrode for lithium-ion secondary batteries. The region, which occupies 1/5 of the thickness of the active material layer, includes the entire second layer.
The negative electrode according to claim 1. The primary particle size of the LTO particles is 0.5 μm or less.
The average particle size of the primary particles of the carbon material particles is 1 μm to 20 μm.
The aspect ratio of the primary particles of the carbon material particles is 1.5 or more.
The negative electrode according to claim 1 or 2. The layer thickness of the second layer is 1 μm or more and 5 μm or less.
The penetration depth of the LTO particles is 14 μm or less.
The negative electrode according to any one of claims 1 to 3. A lithium ion secondary battery having the negative electrode according to any one of claims 1 to 4. With the current collector
Granulated body Ga, which is a mixture of carbon material particles, which are active materials, binders, and volatile components,
Granulator Gb formed by mixing LTO (lithium titanate) particles, which are active materials, binders, and volatile components with each other.
It is a method of roll-molding a negative electrode using
Between the A roll and the B roll facing each other, the granulated body Ga is sprayed on the A roll side, and the granulated body Gb is further sprayed on the B roll side.
The first layer in which the carbon material particles are dispersed and the second layer in which the LTO particles are dispersed are formed by simultaneously sandwiching and pressing the A roll and the B roll and collapsing each granulated body. Formed at the same time
By allowing the LTO particles to permeate from the second layer toward the first layer with the pressing, the first layer is made to bite the second layer.
The active material layer composed of the first layer and the second layer is conveyed on the B roll, and the active material layer is conveyed.
The active material layer is supplied to the B roll side between the B rolls and the C rolls facing each other, and the current collector is supplied to the C roll side.
By pressing these with the B roll and the C roll, the side of the first layer of the active material layer is attached to the current collector.
Further, when the negative electrode is dried by removing the volatile component from the active material layer attached to the current collector.
In the dried negative electrode, the penetration depth of the LTO particles is 5 μm or more.
A method for manufacturing a negative electrode for a lithium ion secondary battery. When considering the region of the dried negative electrode that includes the second layer and occupies 1/5 of the thickness of the active material layer,
Binders are present between the active materials in the region.
In the cross section of the active material layer, the area ratio of the binder to the region is 10% or less, and here, the binder in the region has a region closer to the current collector than the region due to the removal of the volatile component. May contain binders that have been migrated from,
The method for manufacturing a negative electrode according to claim 6.

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