JP2018129212A - Negative electrode for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an increase in resistance due to charge and discharge in a negative electrode for a secondary battery containing silicon oxide in a negative electrode mixture layer.SOLUTION: A negative electrode for a secondary battery includes a negative electrode current collector foil and a negative electrode mixture layer formed on a surface of the negative electrode current collector foil. The negative electrode mixture layer contains a carbon-based negative electrode active material, silicon oxide, and expanded graphite. In the negative electrode mixture layer, an orientation degree of the expanded graphite in a long axis direction with respect to the surface of the negative electrode current collector foil is 30° or more and 90° or less.SELECTED DRAWING: Figure 1

Description

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

  Japanese Unexamined Patent Application Publication No. 2014-067583 (Patent Document 1) discloses a negative electrode active material that is a mixture of massive graphite, flaky graphite, and silicon oxide.

JP 2014-067583 A

  Silicon oxide has a higher capacity than carbon-based negative electrode active materials such as graphite, and has attracted attention as a material for negative electrode active materials from the viewpoint of increasing the capacity of secondary batteries. However, since silicon oxide has a larger expansion rate and shrinkage rate during charge / discharge than carbon-based negative electrode active material, carbon-based negative electrode active material does not sufficiently follow the expansion and contraction of silicon oxide due to charge / discharge cycles. Thus, the carbon-based negative electrode active material and silicon oxide in the negative electrode mixture layer are isolated, and the resistance of the negative electrode may increase.

  Patent Document 1 describes that scaly graphite has a function of improving the contact between particles in terms of shape and can follow expansion and contraction of silicon oxide. However, it is difficult to sufficiently follow the expansion and contraction of silicon oxide only by adding scaly graphite, and it is difficult to maintain a conductive path in the negative electrode mixture layer and suppress an increase in resistance. difficult.

  Therefore, the subject of this indication is suppressing the increase in resistance by charging / discharging in the negative electrode for secondary batteries containing a silicon oxide in a negative electrode compound material layer.

A negative electrode for a secondary battery of the present disclosure includes a negative electrode current collector foil and a negative electrode mixture layer formed on the surface of the negative electrode current collector foil.
The negative electrode mixture layer includes a carbon-based negative electrode active material, silicon oxide, and expanded graphite.
In the negative electrode mixture layer, the degree of orientation of the expanded graphite in the major axis direction with respect to the surface of the negative electrode current collector foil is 30 ° or more and 90 ° or less.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the action mechanism of the present disclosure may include estimation, and the scope of the present disclosure should not be limited by the correctness of the action mechanism.

  According to the present disclosure, an increase in resistance due to charge / discharge can be suppressed in a negative electrode for a secondary battery containing silicon oxide in the negative electrode mixture layer. The reason is considered as follows.

  Since silicon oxide has poor conductivity, the resistance of the negative electrode mixture layer is likely to increase by the addition of silicon oxide. Therefore, it is conceivable to suppress an increase in the resistance of the negative electrode mixture layer by adding a conductive material such as acetylene black or flaky graphite. However, silicon oxide has a very large volume change accompanying alloying and dealloying with Li. That is, silicon oxide expands greatly during charging and contracts greatly during discharging. For this reason, when charging / discharging is repeated, it is difficult for a normal conductive material to sufficiently follow the expansion and contraction of silicon oxide.

  Specifically, for example, referring to FIG. 5, when the silicon oxide 22 expands by charging, the state shown in FIG. 5 (a) is changed to the state shown in FIG. 5 (b). Thereafter, when the silicon oxide 22 contracts due to electric discharge and enters the state of FIG. 5C, the conductive material 24 may be separated from the silicon oxide 22. Thereby, it is considered that the conductive path in the negative electrode mixture layer cannot be maintained, and the resistance of the negative electrode is increased.

  On the other hand, in the negative electrode of the present disclosure, the negative electrode mixture layer includes swollen graphite as a conductive material. Expanded graphite has elastic stretchability (especially in the stacking direction of the constituent layers), so that expanded graphite expands and contracts, and reversible with respect to silicon oxide expansion and contraction. Therefore, it is considered easy to follow (see FIG. 1). Thereby, it is thought that the conductive path in the negative electrode mixture layer can be maintained even after the charge / discharge cycle, and the increase in resistance of the negative electrode is suppressed.

  However, expanded graphite often has an anisotropic shape (a shape that is longer in the plane direction than the stacking direction of the layers constituting the graphite). When a negative electrode mixture paste containing expanded graphite having such a shape is applied onto the negative electrode current collector foil to form a negative electrode mixture layer, as shown in FIG. The major axis direction of the expanded graphite 23 tends to be oriented in the direction parallel to the surface of the negative electrode current collector foil 1. When the expanded graphite 23 is oriented in this way, it is considered that the resistance is increased by inhibiting the movement of ions (for example, Li) in the negative electrode mixture layer.

  Therefore, in the negative electrode of the present disclosure, as shown in FIG. 1, the major axis direction 23 a (see FIG. 2) of the expanded graphite 23 is largely oriented in the direction perpendicular to the surface of the negative electrode current collector foil 1. That is, if the degree of orientation of the expanded graphite in the major axis direction with respect to the surface of the negative electrode current collector foil is 30 ° or more and 90 ° or less, the movement of ions in the negative electrode mixture layer is hardly inhibited, and the resistance of the negative electrode is increased. It is thought that it can suppress more reliably.

It is a conceptual diagram which shows the structure of the negative mix layer of embodiment. It is a schematic diagram for demonstrating the orientation angle of swollen graphite. It is a conceptual diagram for demonstrating the measuring method of the orientation angle of swollen graphite. It is a flowchart which shows the outline of the manufacturing method of the negative electrode for secondary batteries which concerns on embodiment. It is a conceptual diagram which shows the structure of the negative mix layer formed in the conventional manufacturing method. It is a conceptual diagram which shows another structure of the negative mix layer formed in the conventional manufacturing method.

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

  With reference to FIG. 1, the negative electrode for a secondary battery according to the present disclosure includes a negative electrode current collector foil 1 and a negative electrode mixture layer 2 formed on the surface of the negative electrode current collector foil. The negative electrode mixture layer 2 includes a carbon-based negative electrode active material 21, silicon oxide 22, and expanded graphite 23. The negative electrode mixture layer 2 may further contain other additives (binder, thickener, etc.).

[Negative electrode current collector foil]
The negative electrode current collector foil may be, for example, a copper (Cu) foil. The Cu foil may be a pure Cu foil or a Cu alloy foil. The negative electrode current collector foil may have a thickness of 5 to 30 μm, for example. The negative electrode mixture layer may be formed to have a thickness of 10 to 150 μm, for example.

[Negative electrode mixture layer]
(Carbon-based negative electrode active material)
The “carbon-based negative electrode active material” refers to a material that contains carbon and can absorb and release Li ions. The carbon-based negative electrode active material may be at least one selected from the group consisting of graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon), for example. When the negative electrode mixture layer contains the carbon-based negative electrode active material, there is a possibility that the balance of capacity, output characteristics, cycle characteristics, and the like is improved.

(Silicon oxide)
Silicon oxide is a compound represented by the general formula “SiO _x ” (where 0.01 ≦ x <2). In the general formula, preferably 0.1 ≦ x ≦ 1.5, more preferably 0.5 ≦ x ≦ 1.5, and further preferably 0.5 ≦ x ≦ 1.0. Hereinafter, silicon oxide may be abbreviated as “SiO”.

  The mass ratio of silicon oxide to carbon-based negative electrode active material is, for example, “silicon oxide: carbon-based negative electrode active material = 1: 99 to 30:70, preferably“ silicon oxide: carbon-based negative electrode active material = 5 ”. : 95-25: 75 ".

  Since SiO has a higher capacity than the carbon-based negative electrode active material, the capacity of the secondary battery can be improved when the negative electrode mixture layer contains SiO.

(Expanded graphite)
Expanded graphite is graphite having a lower bulk density than normal graphite such as natural graphite. The bulk density of expanded graphite is, for example, about 0.008 to 0.15. From the aspect of the effect of the present disclosure (suppression of increase in resistance due to charge / discharge), it is preferably 0.01 to 0.5.

  Further, in expanded graphite, the distance between constituent layers is wider than that of normal graphite such as natural graphite. In the expanded graphite, the distance between the constituent layers is, for example, about 300 nm to 30 μm. From the aspect of the effect of the present disclosure, the thickness is preferably 500 nm to 1000 nm.

Expanded graphite can be obtained, for example, by inserting sulfuric acid or the like between the constituent layers of graphite and heating to expand the layers by internal gasification. If such a process were produced, the expanded graphite, sulfur derived from sulfuric acid such as sulfuric acid group (-S0 ₄₎ is included about 15～200Ppm.

  Expanded graphite has elastic stretchability (particularly in the stacking direction of the layer structure forming expanded graphite). For this reason, it is considered that the expanded graphite easily follows expansion and contraction of silicon oxide by expanding and contracting or sliding.

  The expanded graphite is often derived from the shape of graphite that is widely used, and has an anisotropic shape (a shape that is longer in the surface direction than the stacking direction of the layers constituting the graphite).

  Referring to FIG. 2, in negative electrode mixture layer 2, the degree of orientation of expanded graphite 23 in the major axis direction 23 a with respect to the surface of negative electrode current collector foil 1 is 30 ° or more and 90 ° or less. Here, “the degree of orientation of the expanded graphite in the major axis direction relative to the surface of the negative electrode current collector foil” (hereinafter sometimes abbreviated as “the degree of orientation of the expanded graphite”) is the surface of the negative electrode current collector foil 1. Is an average value of an acute angle (or a right angle) formed by the long axis direction 23a of the expanded graphite 23 with respect to the average value of θ for the plurality of expanded graphites 23 in the negative electrode mixture layer 2.

  The degree of orientation of expanded graphite is preferably 40 ° or more and 90 ° or less. In this case, the effect of suppressing the increase in the resistance ratio after cycling is greater, the increase in the initial resistance is also suppressed, and the decrease in the capacity maintenance rate is also suppressed.

  The “major axis direction” means the direction of the longest line segment connecting two points on the surface of one expanded graphite (see the major axis direction 23a in FIG. 2).

  The addition rate of expanded graphite with respect to SiO is, for example, about 10 to 150% by mass. The addition ratio of expanded graphite to SiO is preferably 30 to 120% by mass. In this case, the effect of suppressing the increase in the resistance ratio after the charge / discharge cycle is greater, the increase in the initial resistance is also suppressed, and the decrease in the capacity retention rate after the charge / discharge cycle is also suppressed.

  The ratio of the average particle diameter of SiO to the average particle diameter of expanded graphite (average particle diameter of SiO / average particle diameter of expanded graphite) is, for example, about 0.25 to 4. The ratio of the average particle diameter is preferably more than 0.3 and less than 3.0. When the ratio of the average particle diameter is within this range, the effect of suppressing the increase in the resistance ratio after cycling is greater, the increase in the initial resistance is also suppressed, and the decrease in the capacity retention rate is also suppressed.

  In this specification, “average particle size” means a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method.

(Other ingredients)
The negative electrode mixture layer may contain a binder or the like as a component other than the above. Examples of the binder include styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), polytetrafluoroethylene (PTFE), and the like.

(Method for producing negative electrode)
Referring to FIG. 4, the negative electrode for the secondary battery of the present embodiment includes at least the following (A) preparation of negative electrode mixture paste, (B) application of negative electrode mixture paste, and (C) expanded graphite. It can be manufactured by a manufacturing method including orientation.

(A) Preparation of negative electrode mixture paste First, a negative electrode mixture paste containing a carbon-based negative electrode active material, silicon oxide, and expanded graphite is prepared. For example, a negative electrode mixture paste is prepared by mixing a carbon-based negative electrode active material, silicon oxide, expanded graphite, and a solvent. The solvent may be water, for example. For the preparation of the negative electrode paste, a general mixing apparatus can be used.

  Here, it is preferable to prepare a negative electrode mixture paste by first preparing a granulated body in which SiO particles are coated with expanded graphite, and mixing the granulated body with a carbon-based negative electrode active material, a solvent, and the like. . In this case, since the contact between the swollen graphite and SiO is good, the conductive path in the negative electrode mixture layer can be sufficiently formed, and the increase in initial resistance and the increase in resistance due to the charge / discharge cycle are more reliably ensured. Can be suppressed.

(B) Application of negative electrode composite paste The prepared negative electrode mixture paste is applied to the surface of the negative electrode current collector foil. For example, a die coater or the like can be used for applying the paste.

(C) Orientation of expanded graphite In the applied negative electrode composite paste, the expanded graphite is oriented by a magnetic field so that the degree of orientation of the expanded graphite is 30 ° or more and 90 ° or less. The magnetic field can be generated by, for example, a permanent magnet. The degree of orientation of expanded graphite can be adjusted by adjusting the strength of the magnetic field, the orientation treatment time, and the like. The strength of the magnetic field is, for example, about 0.6T to 1.2T. The alignment treatment time is, for example, about 3 to 60 seconds, and more preferably about 10 to 30 seconds. For example, when the orientation treatment is performed while moving the apparatus for performing the magnetic field orientation along the surface direction of the negative electrode current collector foil, the degree of orientation of the expanded graphite increases initially depending on the orientation treatment time. If the time exceeds a predetermined time, the degree of orientation of the expanded graphite may decrease with time, and therefore it is desirable to finish the orientation treatment at an appropriate time.

  After the expanded graphite is magnetically oriented, the coated negative electrode mixture paste is dried to form a negative electrode mixture layer on the surface of the negative electrode current collector foil. Thereby, a negative electrode is manufactured. The negative electrode can be compressed (rolled) and cut to a predetermined size in accordance with the specifications of the secondary battery. For rolling, for example, a roller mill or the like can be used.

  The negative electrode of this embodiment can be used as a negative electrode for a secondary battery such as a lithium ion secondary battery (nonaqueous electrolyte secondary battery). The secondary battery can be used, for example, as a power source for a hybrid vehicle (HV), an electric vehicle (EV), a plug-in hybrid vehicle (PHV), or the like. However, the negative electrode obtained by the manufacturing method of the present disclosure is not limited to such a use, and can be applied to any secondary battery.

  Examples will be described below. However, the following examples do not limit the scope of the present disclosure. Hereinafter, the secondary battery may be abbreviated as “battery”.

<Example 1>
<Manufacture of negative electrode>
The following materials were prepared:
Carbon-based negative electrode active material: natural graphite (average particle size: 10 μm)
Silicon oxide (average particle size: 10 μm)
Expanded graphite (average particle size: 10 μm)
Binder: SBR
Thickener: CMC-Na
Solvent: Water Negative electrode current collector foil: Copper foil (thickness: 10 μm)
[Note that the ratio of the average particle diameter of silicon oxide to the average particle diameter of expanded graphite (conductive material) (value in the column of “particle size ratio (SiO / conductive material)” in Table 1) is 1. ]

  10 parts by mass of silicon oxide, 5 parts by mass of expanded graphite, and 10 parts by mass of thickener were mixed. Here, the addition ratio of expanded graphite (conductive material) to silicon oxide (value in the column “Addition ratio to SiO” in Table 1) is 50 mass%. By adding 5 parts by mass of a solvent to the obtained mixture and further mixing, a granulated body in which the surface of silicon oxide was coated with expanded graphite was prepared.

  A carbon-based negative electrode active material in an amount of 90 parts by mass with respect to 10 parts by mass of silicon oxide was added to the obtained granulated body, and these were mixed. Here, the addition ratio of expanded graphite (conductive material) to the negative electrode active material (value in the column “Addition ratio to negative electrode active material” in Table 1) is 5 mass%. Furthermore, 1 part by mass of a thickener and a solvent were added to the mixture, and they were kneaded. A negative electrode mixture paste (slurry) was prepared by adding a solvent and 2 parts by mass of a binder to the obtained kneaded material for dilution and dispersion. In addition, the addition amount of the solvent was adjusted so that the non-volatile fraction of the obtained negative electrode mixture paste might be 54 mass%. “Nonvolatile fraction” means the mass ratio of components (nonvolatile components) other than the solvent to the total mass of all raw materials including the solvent.

The prepared negative electrode mixture slurry was applied to the surface (both front and back surfaces) of the negative electrode current collector foil using a die coater. The coating amount of the negative electrode mixture paste was adjusted so that the basis weight of the negative electrode mixture layer (excluding the solvent) (mass per unit area of the negative electrode current collector foil) was 18 mg / cm ² .

  With respect to the negative electrode mixture paste coated on the negative electrode current collector foil, before the negative electrode mixture paste is dried, orientation treatment is performed with a permanent magnet (magnetic field strength: 0.8 T, alignment treatment time: 12 seconds. The expanded graphite was oriented so that the degree of orientation of the expanded graphite was 30 ° or more and 90 ° or less. In addition, the orientation treatment was performed as a whole while moving the permanent magnet along the surface direction of the negative electrode current collector foil. Then, the negative electrode mixture layer was formed by drying the negative electrode mixture paste, and the negative electrode was produced. The negative electrode was compressed by a roll press so as to have a thickness of 87 μm and cut into a strip shape. In this way, the negative electrode of Example 1 was produced.

<< Manufacture of secondary batteries >>
Using the negative electrode of Example 1, a secondary battery (lithium ion secondary battery) was produced.
(Manufacture of positive electrode)
The following materials were prepared:
Cathode active material: Lithium nickel cobalt manganese composite oxide Conductive material: Acetylene black (AB)
Binder: Polyvinylidene fluoride (PVdF)
Solvent: N-methyl-2-pyrrolidone (NMP)
Positive electrode current collector foil: Aluminum foil (thickness: 15 μm)

100 parts by mass of a positive electrode active material, 4 parts by mass of a conductive material, and 2 parts by mass of a binder were mixed in a solvent. Thus, a positive electrode mixture paste was prepared. The positive electrode mixture paste was applied to the surface (both front and back surfaces) of the positive electrode current collector foil by a die coater and dried to form a positive electrode mixture layer. The coating amount of the positive electrode mixture paste was adjusted so that the basis weight of the positive electrode mixture layer (excluding the solvent) (mass per unit area of the positive electrode current collector foil) was 32 mg / cm ² . This produced a positive electrode. The positive electrode was further compressed by a roll press and cut into a strip shape.

(Manufacture of electrode groups)
A strip-shaped separator (porous membrane) was prepared. This separator has a thickness of 20 μm. This separator has a three-layer structure. The three-layer structure is configured by laminating a polypropylene porous layer, a polyethylene porous layer, and a polypropylene porous layer in this order.

  A positive electrode and a negative electrode were laminated with a separator in between, and wound in the longitudinal direction of each member. Thus, a wound electrode group was formed. A nickel-plated iron cylindrical outer case (diameter 18 mm, height 650 mm) was prepared. An external terminal was connected to the electrode group. The electrode group was housed in the outer case.

An electrolyte solution solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC). By dispersing and dissolving LiPF _{6 in the} electrolyte solvent, an electrolyte solution having the following composition was prepared.

Electrolyte solvent: [EC: EMC: DMC = 3: 4: 3 (volume ratio)]
LiPF ₆ : 1.0 mol / L

  A predetermined amount of electrolyte was injected into the outer case. The outer case was sealed. From the above, the secondary battery according to Example 1 was manufactured. The theoretical capacity of this battery was 550 mAh.

<Examples 2 to 5>
As shown in Table 1, the orientation treatment time of the expanded graphite (conductive material) was changed, and the orientation degree of the expanded graphite was changed to a value smaller than that of Example 1. Except for this point, the negative electrode of Examples 2 to 5 and a battery including the negative electrode were manufactured by basically the same manufacturing method as in Example 1. In addition, since the entire orientation treatment was performed while moving the permanent magnet along the surface direction of the negative electrode current collector foil, the degree of orientation of the expanded graphite increases according to the orientation treatment time at the beginning. Is too long, the degree of orientation of the expanded graphite decreases with time. For this reason, in Examples 2 to 5, the degree of orientation is smaller than that in Example 1.

<Examples 6 to 9>
As shown in Table 1, the amount of expanded graphite (conductive material) added was changed. Except for this point, the negative electrode of Examples 6 to 9 and a battery including the negative electrode were manufactured by basically the same manufacturing method as in Example 1.

<Examples 10 to 13>
As shown in Table 1, the particle diameter of expanded graphite (conductive material) was changed. Along with this, the bulk density and sulfuric acid content of expanded graphite have also changed. Except for this point, the negative electrode of Examples 10 to 13 and a battery including the negative electrode were manufactured by the same manufacturing method as in Example 1.

<Comparative Example 1>
As shown in Table 1, scaly graphite was used as the conductive material instead of expanded graphite. Along with this, the bulk density and sulfuric acid content of expanded graphite have also changed. Except for this point, the negative electrode of Comparative Example 1 and a battery including the negative electrode were manufactured by basically the same manufacturing method as in Example 1. The magnetic field orientation treatment for the negative electrode mixture paste applied to the negative electrode current collector foil was also performed in the same manner as in Example 1.

<Evaluation>
<< Measurement of bulk density of conductive material >>
A sufficient amount of the sample (conductive material) powder to conduct the test was passed through a sieve having an aperture of 1.0 mm or more. This operation was performed gently so as not to change the properties of the sample. Next, 100 g of the sample weighed with an accuracy of 0.1 mass% was gently put into a dry 250 mL graduated cylinder (minimum scale unit: 2 mL) without being compressed. The upper surface of the powder layer was then carefully leveled (so that the powder was not compressed) and the loose bulk volume was read to the smallest scale unit. From the volume and the mass (100 g) of the sample, the bulk density (g / cm ³ ) was obtained by calculation.

<Measurement of sulfuric acid content of conductive material>
The amount of sulfur element contained in the sample (conductive material) was measured with an ICP emission spectroscopic analyzer (manufactured by Shimadzu Corporation, ICPS-8100). By converting the measured value of sulfur element content in mass of S0 ₄ groups, sulfate content was determined.

<Measurement of orientation angle>
The negative electrode was cut in a cross section parallel to the thickness direction, and a SEM (Scanning Electron Microscope) image of the cut surface was taken. An SEM device (manufactured by Hitachi High-Technology, model number S-4300) was used as a measuring device. In the SEM image of the cut surface, 20 expanded graphites were selected in each of the regions (see the dotted lines in FIG. 3) obtained by dividing the negative electrode composite material layer formed on one side of the negative electrode current collector foil into 10 in the thickness direction, The angle of inclination (acute angle or right angle) in the major axis direction of each expanded graphite with respect to the surface of the negative electrode current collector foil was measured visually (see FIG. 3). The average value of the measured angle gradients of the total 200 was calculated as the degree of orientation of the expanded graphite (value in the column “Orientation degree” in Table 1).

<Measurement of initial resistance>
Under a 25 ° C. environment, the charging rate (SOC) of the battery was adjusted to 60% by charging. The battery was discharged at a current of 10 C for 10 seconds. “C” is a unit of current rate. “1C” indicates a current rate at which the charging rate (SOC) reaches 0% to 100% by charging for one hour.

  The voltage drop during discharge was measured, and the battery resistance (initial resistance) was calculated from the relationship between the voltage drop and the current during discharge. For each of the examples and comparative examples, the battery resistance was measured for 10 batteries, and the average value (value in the “initial resistance” column of Table 1) was calculated.

<Measurement of resistance ratio after cycle>
Under the environment of 25 ° C., the cycle characteristics of the battery were evaluated. The following “CC charge → pause → CC discharge → pause” was defined as one cycle for a battery whose SOC was adjusted to 60% by charging, and the cycle was repeated 3000 times. Here, “CC (Constant Current)” indicates a constant current method.
(Cycle conditions)
CC charging: Current = 10C, end voltage = 4.3V
Pause: 80 seconds CC discharge: Current = 2C, end voltage = 2.5V
Rest: 400 seconds After 3000 cycles, battery resistance was measured in the same procedure as the initial resistance. The ratio of the battery resistance after cycling to the initial resistance (value in the “Ratio after cycling ratio” column of Table 1) was calculated.

《High temperature storage durability test》
The battery was charged to 4.1V at a 1C current rate and then discharged to 3.0V at a 3C current rate. Thereby, the initial capacity (initial discharge capacity) of the battery was measured. In addition, about each of the Example and the comparative example, capacity | capacitance battery resistance was measured with respect to 50 batteries, and the average value was computed.

  In a 25 ° C. environment, the SOC of the battery was adjusted to 100% by charging. The battery was stored at 60 ° C. for 100 days, and then the battery capacity (discharge capacity) was measured by the same procedure as the initial capacity. The ratio of the battery capacity after high-temperature storage to the initial capacity was calculated as a capacity maintenance ratio (value in the “capacity maintenance ratio” column of Table 1).

<Result>
As shown in Table 1, in Examples 1 to 13, the ratio of the resistance after cycling to the initial resistance (the resistance ratio after cycling) is smaller than in Comparative Example 1. Therefore, as in Examples 1 to 13, when the negative electrode mixture layer contains expanded graphite and the orientation degree of the expanded graphite is 30 ° or more and 90 ° or less, silicon oxide is contained in the negative electrode mixture layer. It can be seen that even when the negative electrode is used, an increase in battery resistance due to charge / discharge can be suppressed.

  From the results of Examples 2 to 5, when the degree of orientation of expanded graphite is in the range of 40 ° or more and 90 ° or less, the effect of suppressing an increase in the resistance ratio after cycling is greater, and the initial resistance It can be seen that the increase is also suppressed and the decrease in the capacity maintenance rate is also suppressed. When the degree of orientation of the expanded graphite is 30 ° or less, the expanded graphite becomes a barrier to the movement of Li ions in the negative electrode mixture layer, and it is considered that the resistance is increased. Moreover, when the orientation degree of expanded graphite is 30 degrees or less, it is thought that the electrolyte solution in a negative electrode becomes easy to be extruded in the surface direction of negative electrode current collector foil by the expansion and contraction by a charging / discharging cycle. As a result, the electrolyte solution in the negative electrode is deficient, an unreacted portion that does not contribute to the capacity is generated, and the reaction at the time of charging and discharging becomes non-uniform. It is thought that it became large.

  From the results of Examples 6 to 9, when the addition ratio of expanded graphite to silicon oxide (SiO) is 30 to 120% by mass, the effect of suppressing the increase in the resistance ratio after cycling is greater, and the initial resistance is increased. It can be seen that the increase in the capacity is also suppressed, and the decrease in the capacity maintenance rate is also suppressed. In addition, when the addition rate of expanded graphite is too low, it is thought that a conductive path in the negative electrode mixture layer is not sufficiently formed, and the negative electrode active material is isolated by the charge / discharge cycle, resulting in an increase in resistance. On the other hand, if the addition rate of expanded graphite is too high, it is considered that the capacity is reduced due to side reactions during storage.

  From the results of Examples 10 to 13, the ratio of the average particle diameter of silicon oxide (SiO) to the average particle diameter of expanded graphite (conductive material) [particle diameter ratio (SiO / conductive material)] is more than 0.3. When it is in the range of less than 0, it can be seen that the effect of suppressing the increase in the resistance ratio after the cycle is greater, the increase in the initial resistance is also suppressed, and the decrease in the capacity retention rate is also suppressed. In addition, when the particle size ratio (SiO / conductive material) is smaller than 0.3 (when the average particle size of expanded graphite is larger than the average particle size of SiO particles), the SiO particles are not sufficiently covered with expanded graphite. Therefore, the conductive path in the negative electrode mixture layer is not sufficiently formed, the initial resistance increases, and it becomes difficult to suppress the increase in resistance due to the expansion and contraction of SiO due to the charge / discharge cycle, and the resistance ratio after cycle It is thought that the number also increased. On the other hand, when the particle size ratio (SiO / conductive material) is larger than 3.0 (when the average particle size of expanded graphite is smaller than the average particle size of SiO), the SiO particles are covered with expanded graphite. It is considered that the expanded graphite does not have sufficient stretchability, and as a result, the expansion and contraction of the SiO particles due to the charge / discharge cycle cannot be followed, and the post-cycle resistance ratio increases. In this case, it is considered that the battery capacity (capacity maintenance ratio) is also lowered because the reaction at the time of charging and discharging becomes uneven.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Negative electrode collector foil, 2 Negative electrode compound material layer, 21 Carbon type negative electrode active material, 22 Silicon oxide (SiO), 23 Expanded graphite, 24 Conductive material.

Claims (1)

A negative electrode for a secondary battery, comprising: a negative electrode current collector foil; and a negative electrode mixture layer formed on a surface of the negative electrode current collector foil,
The negative electrode mixture layer includes a carbon-based negative electrode active material, silicon oxide and expanded graphite,
The negative electrode for a secondary battery, wherein the degree of orientation of the expanded graphite in the major axis direction with respect to the surface of the negative electrode current collector foil is 30 ° or more and 90 ° or less.

Images