JP6937602B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

In recent years, electric vehicles (EVs) have been developed by various automobile manufacturers due to the problems of global warming and depleted fuels, and lithium ion secondary batteries having a high energy density are required as a power source for the electric vehicles.

An active material containing silicon (Si) is expected as a negative electrode active material that can be expected to increase the energy density of a lithium ion secondary battery. However, Si has a large volume change with charging and discharging. Therefore, there is a problem that the cycle deterioration is large due to the destruction of the conductive network between the active material particles and the peeling of the negative electrode mixture layer from the current collector due to charging and discharging.

Patent Document 1 includes a negative electrode, a positive electrode, and a separator, and the negative electrode includes a Si-based negative electrode active material containing silicon, graphite, and a negative electrode binder, and has a discharge capacity Q of the negative electrode. A lithium ion secondary battery is disclosed in which Ah / kg), the breaking strength A (MPa) of the negative electrode binder alone, and the breaking elongation B (%) satisfy a predetermined relational expression. According to Patent Document 1, it is possible to obtain a lithium ion secondary battery having excellent initial capacity and cycle characteristics.

JP-A-2017-50203

In recent years, the demand for high performance of lithium-ion secondary batteries has been increasing more and more. Therefore, it is desired to develop a lithium ion secondary battery that has both high capacity and high cycle characteristics at a level higher than that of Patent Document 1.

In view of the above circumstances, the present invention achieves both high capacity and high cycle characteristics at a high level in a lithium ion secondary battery having a negative electrode active material containing Si and a negative electrode containing polyimide, polyamide or polyamide-imide as a binder. It is an object of the present invention to provide a lithium ion secondary battery capable of producing a lithium ion secondary battery.

In order to achieve the above object, the present invention is a lithium ion secondary battery including a positive electrode, a negative electrode, and a separator that separates the positive electrode and the negative electrode. The negative electrode is a collection of Cu foils having a thickness of 5 μm or more and 15 μm or less. It has an electric body and a negative electrode mixture layer provided on the surface of the current collector, and the negative electrode mixture layer is a mixture of a negative electrode active material containing Si and natural graphite in a mass ratio of 3: 97 to 40:60. The negative electrode active material containing Si , including a binder containing polyimide, polyamide, polyamideimide or a mixture thereof, _{is Si X} M _1-X , where M is Al, Ni, Cu, Fe, Ti, It is at least one selected from Mn and O, and is a through hole in the current collector so that the discharge capacity Q (Ah / kg) of the negative electrode and the vacancy ratio Y (%) of the current collector satisfy the following formula 1. The breaking strength A (MPa) of the binder is 80 MPa or more and 400 MPa or less, the diameter of the through hole is the average particle size (5 μm) or more and 120 μm or less of the negative electrode active material, and the discharge capacity Q (Ah) of the negative electrode is provided. Provided is a lithium ion secondary battery characterized in that (/ kg) is 400 (Ah / kg) or more and 700 (Ah / kg) or less.
0.1 Q-40 ≤ Y ≤ 50 ... Equation 1

More specific configurations of the present invention are described in the claims.

According to the present invention, in a lithium ion secondary battery having a negative electrode active material containing Si and a negative electrode containing polyimide, polyamide or polyamide-imide as a binder, lithium ions capable of achieving both high capacity and high cycle characteristics at a high level can be achieved. A secondary battery can be provided.

Issues, configurations and effects other than those described above will be clarified by the description of the following embodiments.

It is sectional drawing (a) which shows typically a part of the negative electrode of the conventional lithium ion secondary battery, and top view (b) of the current collector after a cycle. It is sectional drawing (a) which shows typically a part of the negative electrode of the lithium ion secondary battery of this invention, and top view (b) of the current collector after a cycle. It is a top view which shows typically an example of the current collector which constitutes the lithium ion secondary battery of this invention. It is a top view schematically showing another example of the current collector which constitutes the lithium ion secondary battery of this invention. It is an exploded view which shows an example of the laminated electrode group inside the lithium ion secondary battery (laminate cell) of this invention. It is an exploded view which shows an example of the lithium ion secondary battery (laminate cell) of this invention. It is a graph which shows the relationship between the capacity retention rate of a lithium ion secondary battery, and the porosity Y of a current collector. It is a graph which shows the relationship between the porosity Y of the current collector of a lithium ion secondary battery, and the discharge capacity Q of a negative electrode.

[Basic Thought of the Present Invention]
The binder made of polyimide, polyamide or polyamideimide or a mixture thereof used in Patent Document 1 (hereinafter referred to as "polyimide-based binder") is a resin having excellent strength and binding force, and has a large volume change. Even when a negative electrode active material containing Si (hereinafter, referred to as “Si-based negative negative active material”) is used, it is possible to prevent the negative electrode mixture layer from peeling off from the current collector. However, as a result of the study by the present inventor, it has been found that a problem peculiar to the use of the polyimide-based binder arises.

FIG. 1 is a cross-sectional view (a) schematically showing a part of a negative electrode of a conventional lithium ion secondary battery, and a top view (b) of a current collector after a cycle. As shown in FIG. 1A, the negative electrode 10'has a current collector 1'and a negative electrode mixture layer 5'provided on the surface of the current collector 1'. The negative electrode mixture layer 5'contains Si-based negative electrode active material 2', graphite 3'and binder 4'. When a Si-based negative electrode active material 2'that has a large expansion and contraction during charging and discharging is contained, the negative electrode mixture layer 5'is expanded and contracted due to the expansion and contraction of the negative electrode active material 2', and the current collector 1'to the negative electrode mixture layer. 5'may peel off. However, as described above, when a polyimide-based binder is used as the binder 4', the adhesion between the current collector 1'and the negative electrode mixture layer 5'is high due to its high strength and binding force, so that the negative electrode is negative. Even if the mixture layer 5'expands and contracts, it is possible to prevent the negative electrode mixture layer 5'from peeling from the current collector 1'.

However, due to the high adhesion between the current collector 1'and the negative electrode mixture layer 5', the current collector 1'follows the expansion and contraction of the negative electrode mixture layer 5'and stresses the current collector 1'. It takes. As a result, as shown in FIG. 1B, it was found that wrinkles 7 may occur in the current collector 1'. If many such wrinkles 7 occur, it may cause a decrease in cycle characteristics.

As a result of diligent studies, the present inventor has found a configuration of a lithium ion secondary battery in which the current collector does not wrinkle after charging and discharging even when a Si-based negative electrode active material is used as the negative electrode active material. The invention was completed. FIG. 2 is a cross-sectional view (a) schematically showing a part of the negative electrode of the present invention, and a top view (b) of the current collector after the cycle. As shown in FIG. 2A, the negative electrode 10 has a current collector 1 and a negative electrode mixture layer 5 provided on the surface of the current collector 1. The negative electrode mixture layer 5 contains a Si-based negative electrode active material 2, graphite 3, and a binder 4. In the present invention, the current collector 1 is provided with a through hole 20 so that the discharge capacity (Q) of the negative electrode and the porosity (%) of the current collector 1 satisfy the above-mentioned equation 1. With such a configuration, as shown in FIG. 2B, it is possible to suppress the occurrence of wrinkles in the current collector 1 due to expansion and contraction of the Si-based negative electrode active material 2.

As shown in FIG. 2B, the lithium ion secondary battery of the present invention also has wrinkles 7 on the surface of the current collector 1, but compared to the conventional lithium ion secondary battery shown in FIG. 1B. It can be seen that wrinkles are significantly reduced.

In the present invention, Y = 0 may occur depending on the value of the discharge capacity Q of the negative electrode. That is, in this case, it is not necessary to have the through hole 20. It is of great significance that the present invention has found the above formula 1 for satisfying high capacity and high cycle characteristics in a lithium ion secondary battery having a negative electrode containing a Si-based negative electrode active material and a polyimide-based binder. ..

Hereinafter, the configuration of the negative electrode constituting the lithium ion secondary battery of the present invention will be described in detail.

(A) Current collector As described above, in the present invention, the current collector 1 may be provided with a through hole 20. FIG. 3A is a top view schematically showing an example of a current collector constituting the lithium ion secondary battery of the present invention, and FIG. 3B is another example of the current collector constituting the lithium ion secondary battery of the present invention. Is a top view schematically showing. As shown in FIGS. 3A and 3B, the shape of the through hole provided in the current collector may be circular or quadrangular. Further, it may be a polygon other than a quadrangle. The shape of the through hole 20 may be any shape as long as the above formula 1 is satisfied. From the viewpoint of ease of manufacture and stress relaxation, a circular shape is most preferable.

The diameter of the through hole 20 (r in FIG. 3A, the diameter r'of the inscribed circle in the case of a polygonal through hole as shown in FIG. 3B) is 120 μm or more, which is equal to or more than the average particle size of the Si-based negative electrode active material 2 described later. Is preferable. When the diameter of the through hole 20 is equal to or larger than the average particle size of the Si-based negative electrode active material 2, the particles of the negative electrode active material 2 on the front and back surfaces of the current collector 1 can come into contact with each other through the through hole 20, and the conductive network. Can be retained, which is preferable. On the other hand, if it is larger than 120 μm, the coatability of the negative electrode slurry described later is lowered, and the manufacturability of the battery is lowered. The thickness of the current collector is preferably 5 μm or more and 15 μm or less. If it is less than 5 μm, the strength of the current collector cannot be sufficiently maintained, and the cycle characteristics are deteriorated. Further, when it is larger than 15 μm, the wrinkle suppressing effect of the present invention is reduced.

As described above, in the present invention, the discharge capacity Q of the negative electrode and the porosity Y of the current collector 1 satisfy the relationship of the formula 1. If the porosity Y is smaller than 0.1Q-40, wrinkles are generated in the current collector 1 and the cycle characteristics are deteriorated. Further, when the porosity Y is larger than 0.1Q-40, the strength of the current collector 1 is lowered and the cycle characteristics are lowered.

(B) Negative electrode active material The lithium ion secondary battery of the present invention uses the Si-based negative electrode active material 2. Specifically, materials such as Si alloy and Si oxide can be used. Si alloys are usually in a state where fine particles of metallic silicon (Si) are dispersed in each particle of other metal elements, or other metal elements are dispersed in each particle of Si. It is in a state. The other metal element may contain any one or more of aluminum (Al), nickel (Ni), copper (Cu), iron (Fe), titanium (Ti), and manganese (Mn). ..

The method for producing a Si alloy can be carried out by mechanically synthesizing it by a mechanical alloy method or by heating and cooling a mixture of Si particles and other metal elements. The composition of the Si alloy preferably has an atomic ratio of Si: other metal elements of 50:50 to 90:10, more preferably 60:40 to 80:20. As Si alloys, Si ₇₀ Ti ₁₀ Fe ₁₀ Al ₁₀ , Si ₇₀ Al ₃₀ , Si ₇₀ Ni ₃₀ , Si ₇₀ Cu ₃₀ , Si ₇₀ Fe ₃₀ , Si ₇₀ Ti ₃₀ , Si ₇₀ Mn ₃₀ , Si ₇₀ Ti ₁₅ Fe ₁₅ , Si ₇₀ Al ₁₀ Ni _{20 and the} like can also be used.

(C) Graphite As the graphite 3 used as the conductive auxiliary agent, a graphite material such as natural graphite or artificial graphite can be used. Natural graphite is preferable from the viewpoint of cost, but the surface may be coated with non-graphitized carbon.

(D) Binder As the binder 4, polyimide, polyamide, polyamideimide, or a mixture thereof is used. Further, in addition to the polyimide-based binder, the binder may be a mixture of other binders such as polyvinylidene fluoride (PVDF) and styrene-butadiene copolymer (SBR).

The strict definition of polyamide-imide has not been particularly determined, and a mixture of polyimide and polyamide-imide is also called polyamide-imide. A structural example of polyamide-imide is shown in Structural Formula 1 below.

^{R 1} of the above structural formula 1 is an alkylene group having 1 to 18 carbon atoms, an arylene group, benzene or the like, and may contain nitrogen oxygen, sulfur, halogen and the like. ^{Further, R 2 to} R ¹⁰ of the structural formula 1 are hydrogen, alkyl group or aryl group. By increasing the number of carbon atoms of R ^{1 to} R ³ and increasing the n of structural formula 1 to change the amount of polymer, that is, by increasing the number of imide groups, the physical property values of the binder (break strength, break elongation, toughness and Elastic modulus) can be changed. In the structural formula (1), the ring structure at the center may be a benzene ring or another unsaturated ring.

The breaking strength A of the binder is preferably 80 to 400 MPa. If the breaking strength of the binder is less than 80 MPa, the negative electrode mixture layer is peeled off due to the volume change of the Si negative electrode active material, and the cycle characteristics are greatly deteriorated. Further, when the breaking strength A of the binder exceeds 400 MPa, the amount of imide groups in the binder is increased, so that lithium (Li) is trapped in the imide groups in the negative electrode binder, resulting in an irreversible capacity of the negative electrode, and the negative electrode becomes irreversible. The discharge capacity Q becomes low.

Further, it is preferable that the discharge capacity Q of the negative electrode, the breaking strength A (MPa) and the breaking elongation B (%) of the binder 4 constituting the negative electrode mixture layer 5 satisfy the relationship of the following formula 2.
Q ≦ A × B ÷ 10 ≦ 3 × Q ... Equation 2
In Equation 2, when the value of “A × B ÷ 10” indicating the toughness of the binder is smaller than Q, the effect of the binder on suppressing the peeling of the negative electrode mixture layer 5 is low. Further, if the value of "A × B ÷ 10" is larger than 3 × Q, the amount of imide groups in the binder in the binder is increased, so Li is trapped in the imide groups in the negative electrode binder and the negative electrode is negative. The irreversible capacity of the negative electrode becomes low, and the discharge capacity of the negative electrode becomes low.

(E) Negative electrode mixture layer The expansion coefficient of the negative electrode mixture layer 5 is preferably more than 110% and less than 150%. When the expansion coefficient is 110% or less, the wrinkle suppressing effect of the current collector 1 is reduced. Further, when the expansion coefficient is 150% or more, the porosity Y of the current collector 1 needs to be 50% or more, the strength of the current collector 1 is lowered, and the cycle characteristics are lowered.

Next, while explaining the procedure for actually manufacturing the lithium ion secondary battery, the configuration of the lithium ion secondary battery other than the negative electrode will be described in detail.

1. 1. Preparation of Lithium Ion Secondary Batteries of Examples 1 to 20, Reference Examples 1 to 3 and Comparative Examples 1 to 17 (1.1) Negative electrode configuration Pore ratio Y (%) and through hole diameter r (μm) of the current collector ) Was changed, and a current collector 1 made of Cu foil (thickness 10 μm) was prepared. The configurations of the current collectors constituting the lithium ion secondary batteries of Examples 1 to 20, Reference Examples 1 to 3 and Comparative Examples 1 to 17 are shown in Table 1 described later. The porosity Y of the current collector is calculated by subtracting the actual weight of the current collector from the product of the specific gravity of Cu and the volume of the current collector, and quoting it by the product of the specific gravity and the volume of the current collector. bottom.

_{Si alloy (Si 70} Ti ₃₀ ) synthesized by the mechanical alloy method and natural graphite as graphite are used as the Si-based negative electrode active material constituting the negative electrode mixture layer, and these are used in a mass ratio of 3: 97 to 40. It was mixed at 60. Specifically, Examples 1 to 6 are 3:97, Examples 7 to 11 and Comparative Examples 1 and 2 are 20:80, Examples 12 to 15, Comparative Examples 3 to 5 and Comparative Examples 17 to 19 are 30. : 70, Examples 16 to 18 and Comparative Examples 6 to 9 were prepared at 40:60, and Comparative Examples 10 to 16 were prepared at 50:50. The crystallinity of this graphite is 3.356 Å or less for d002, 1000 Å or more for Lc (002), and 1000 Å or more for La (110). Here, "d002" means the interplanar spacing distance (measured by an X-ray diffractometer) of the (002) plane of the graphite crystal, and "Lc (002)" is the 002 diffraction line (X-ray) of graphite. It means the crystallite diameter (thickness of the crystallite in the c-axis direction) obtained from (measured by a diffractometer), and "La (110)" is a crystallite obtained from 110 diffraction lines of graphite (measured by an X-ray diffractometer). It shall mean the diameter (width of the crystallite in the a-axis direction). _{The average particle size D 50} measured by the laser diffraction method of Si alloy and natural graphite is 5 μm, respectively.

Polyamideimide was used as the binder. A binder having different physical property values (break strength A and break elongation B) was prepared by adjusting the number of carbon atoms or the number of n ^{of R 1 to 3} of the above-mentioned structural formula 1. The breaking strength of the binder is calculated from the following formula 3 as the strength when the binder 4 is broken by pulling at a speed of 0.2 m / min using a tensile tester (manufactured by Shimadzu Corporation, Autograph AG-Xplus). Calculated.
A = (tensile load) ÷ (cross-sectional area of negative electrode binder piece) ... Equation 3
The breaking elongation (%) of the binder was calculated from the following formula 4 as the elongation at break when the negative electrode binder was pulled by pulling at a speed of 0.2 m / min using the above-mentioned tensile tester.
B = 100 × {(length of negative electrode binder piece after tension-length of negative electrode binder piece before tension)} ÷ (length of negative electrode binder piece before tension) ... Equation 4
The dimensions of the test piece are 3 cm × 3 cm, and the measurement temperature is 25 ° C.

Table 1 also shows the physical property values (breaking strength A, breaking elongation B and toughness “A × B ÷ 10”) of the negative electrode binders of Examples 1 to 20, Reference Examples 1 to 3 and Comparative Examples 1 to 17.

The negative electrode was prepared by preparing a slurry (negative electrode slurry) of the negative electrode mixture layer containing the above-mentioned negative electrode active material and binder, coating the slurry on the current collector, and pressing the slurry. In the negative electrode slurry, acetylene black was added as a conductive material in addition to the negative electrode active material and the binder described above. The slurry was prepared so that the mass ratio was 92: 5: 3 in order, and the NMP (N-methylpyrrolidone) solvent was mixed so that the viscosity was 6000 to 8000 mPa. Although NMP was used as the solvent this time, water, 2-butoxyethanol, butyl cellosolve, N, N-dimethylacetamide, diethylene glycol diethyl ether and the like may be used, or a mixture thereof may be used. A planetary mixer was used to prepare the slurry.

Using the obtained negative electrode slurry, the current collector 1 was coated with a desktop comma coater. The coating amount was 11 g / m ² . It was first dried at 90 ° C. through a drying oven. Then, the density of the coated negative electrode was adjusted by a roll press. The density was pressed so that the pores of the electrodes were about 20 to 40%, and the negative electrode was prepared at a ^{density of 1.9 g / cm 3.} Then, the polyamide-imide was heat-cured in vacuum at 300 ° C. for 1 hour. It does not matter whether it is in nitrogen or the curing time of the resin.

(1.2) Composition of Separator and Electrolyte The separator may be a material that does not allow lithium ions to pass through due to heat shrinkage. For example, polyolefin and the like are used. The polyolefin is characterized by containing at least one kind mainly of polyethylene, polypropylene and the like, but may also contain a heat-resistant resin such as polyamide, polyamideimide, polyimide, polysulfone, polyethersulfone, polyphenylsulfone and polyacrylonitrile. No.

Further, the inorganic filler layer may be applied to one side or both sides. The inorganic filler layer _{preferably contains at least one of SiO 2} , Al ₂ O ₃ , montmorillonite, mica, ZnO, TiO ₂ , BaTiO ₃ and ZrO ₂ , but from the viewpoint of cost and performance, SiO ₂ or Al ₂ O ₃ is most preferable. In this example, a three-layer film (thickness 25 μm) having polyethylene between polypropylene was used.

An electrolyte of 1MLiPF ₆ was used as the electrolytic solution, and one dissolved in a solvent of EC: EMC = 1: 3 vol% was used. Other electrolytes include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, γ-butyrolactone, γ-valerolactone, methyl acetate, ethyl acetate, methyl propionate, tetrahydrofuran, 2-Methyltetrahydrofuran, 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 3-methyltetrahydrofuran, 1,2-dioxane, 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane, _{LiPF 6} , LiBF ₄ , LiClO ₄ , LiN (C ₂ F ₅ SO _{2) are} added to a non-aqueous solvent selected from at least one of 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane and the like. ) It _{is possible to use an organic electrolyte solution in which at least one or more lithium salts selected from the second} grade are dissolved, a solid electrolyte having lithium ion conductivity, a gel electrolyte, a known electrolyte used in a molten salt battery, or the like. can.

(1.3) Structure of Positive Electrode The positive electrode has a positive electrode mixture layer formed on the surface of a positive electrode current collector foil made of aluminum foil. For the positive electrode mixture layer, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used as the positive electrode active material, carbon material was used as the conductive auxiliary agent, and PVDF was used as the binder. The mass ratio was sequentially prepared at 90: 5: 5, and the mixture coating amount was 240 g / m ² . When the positive electrode active material mixture was applied to the aluminum foil, the viscosity was adjusted with a dispersion solvent of NMP. The positive electrode after coating was dried at 120 ° C., and then the density was adjusted by a roll press to prepare ^{the positive electrode at a density of 3.0 g / cm 3 in this example.}

(1.4) Configuration of Lithium Ion Secondary Battery Using the negative electrode, separator, electrolytic solution and positive electrode described above, a laminated electrode group constituting a laminated laminated cell was produced. FIG. 4 is an exploded view showing an example of a laminated electrode group inside the lithium ion secondary battery (laminated cell) of the present invention. As shown in FIG. 4, in the laminated electrode group 40, a plate-shaped positive electrode 45 and a band-shaped negative electrode 46 are laminated with the separator 47 sandwiched between them. In the prepared positive electrode and negative electrode, an uncoated portion in which the active material mixture was not applied was formed on a part of the foil during processing. The positive electrode uncoated portion 43 and the negative electrode uncoated portion 44 are bundled and ultrasonically welded to the positive electrode terminal 41 and the negative electrode terminal 42 that electrically connect the inside and outside of the battery, respectively. The welding method may be another welding method such as resistance welding. The positive electrode terminal 41 and the negative electrode terminal 42 may be previously coated with a heat-welded resin or attached to the sealing portion of the terminal in order to further improve the sealing performance inside and outside the battery.

FIG. 5 is an exploded view showing an example of the lithium ion secondary battery (laminated cell) of the present invention. The laminate cell 50 was produced by heat-welding and sealing the peripheral portions of the laminate films 51 and 53 at 175 ° C. for 10 seconds to electrically insulate the electrode group 52, and penetrating the positive electrode terminal 41 and the negative electrode terminal 42. .. For sealing, in order to provide a liquid injection port, heat welding was performed first on all sides except one side, and after the electrolytic solution was injected, the remaining side was heat welded and sealed while vacuum pressurizing the remaining side.

2. Evaluation of Negative Electrodes and Lithium Ion Secondary Batteries of Examples 1 to 20, Reference Examples 1 to 3 and Comparative Examples 1 to 17 (2.1) Discharge Capacity Evaluation The produced negative electrode is processed to a size of φ16 mm, and a separator is sandwiched between them. , A single-pole small cell (single-pole battery) having a counter electrode of Li was prepared, and the discharge capacity Q of the negative electrode was measured. The charge / discharge conditions are the discharge capacity of the negative electrode when constant current charging and 2h constant voltage charging are performed at 0.2 CA up to the lower limit voltage of 5 mV and constant current discharge is performed at 0.2 CA up to the upper limit voltage of 2.0 V. And said.

Here, 1CA is a current value at which charging or discharging of the battery capacity is completed in 1 hour, and 0.2CA is a current value at which charging or discharging of the battery capacity is completed in 5 hours. In the case of 0.2CA, the influence of the thickness of the negative electrode can be ignored.

The design capacity (Ah / kg) of the negative electrodes of Examples 1 to 20, Reference Examples 1 to 3 and Comparative Examples 1 to 17 and the measured discharge capacity Q (Ah / kg) of the negative electrode are shown in Table 2 described later. do.

Further, regarding Examples 1 to 20, Reference Examples 1 to 3, and Comparative Examples 1 to 17, those satisfying Equations 1 and 2 are evaluated as "○", and those satisfying Equations 1 and 2 are evaluated as "○", respectively. × ”was evaluated. The evaluation results are also shown in Table 2.

(2.2) Evaluation of Cycle Characteristics Using the produced laminated cell, constant current charging with a voltage of 4.2 V and a current of 0.5 CA was performed, and then constant voltage charging was performed for 2 hours. For discharge, constant current discharge was performed at a voltage of 2 V and a current of 0.5 CA, and these were repeated 100 times, and the ratio of the 100th discharge capacity to the 1st discharge capacity was defined as the capacity retention rate after 100 cycles of the laminated cell. Table 2 also shows the capacity retention rates of Examples 1 to 20, Reference Examples 1 to 3, and Comparative Examples 1 to 17.

FIG. 6 is a graph showing the relationship between the capacity retention rate of the lithium ion secondary battery and the porosity Y of the current collector. 6 shows Examples 1 to 6 (negative electrode discharge capacity Q: 400 Ah / kg), Examples 7 to 11 and Comparative Examples 1 to 2 (negative electrode discharge capacity Q: 500 Ah / kg), Examples 12 to 15 and. Comparative Examples 3 to 5 (negative electrode discharge capacity Q: 600 Ah / kg), Examples 16 to 18 and Comparative Examples 6 to 9 (negative electrode discharge capacity Q: 700 Ah / kg) and Comparative Examples 10 to 16 (negative electrode discharge capacity). It is the graph which plotted the discharge capacity Q and the vacancy ratio Y of Q: 800Ah / kg).

As shown in FIG. 6, it can be seen that the relationship between the capacity retention rate and the porosity Y changes depending on the discharge capacity Q of the negative electrode. When the discharge capacity Q of the negative electrode is 400 Ah / kg, the porosity Y shows a high capacity retention rate of about 90% between 0 and 50%. When the discharge capacity Q of the negative electrode is 500 Ah / kg, when the porosity Y is 10 to 50% (region (I) in FIG. 6), a high capacity retention rate of about 90% is obtained. When the discharge capacity Q of the negative electrode is 600 Ah / kg, when the porosity Y is 20 to 50% (region (II) in FIG. 6), a high capacity retention rate of 80% or more is obtained. When the discharge capacity Q of the negative electrode is 700 Ah / kg, when the porosity Y is 30 to 50% (region (III) in FIG. 6), a high capacity retention rate of 76% or more is obtained.

FIG. 7 is a graph showing the relationship between the porosity Y of the current collector of the lithium ion secondary battery and the capacity Q of the negative electrode. FIG. 7 plots the data of Examples 1-18 with high cycle characteristics. From this graph, Equation 1 (0.1Q-40 ≦ Y ≦ 50) showing the relationship between Y and Q for realizing high cycle characteristics can be obtained. That is, it can be seen that Examples 1 to 18 satisfying the formula 1 can achieve both high capacity and high cycle characteristics.

From the comparison of Example 14, Example 19 and Example 20, if the through-hole diameter r is equal to or more than the average particle size (5 μm) of the Si-based negative electrode active material and 120 μm or less, both high capacity and high cycle characteristics can be achieved. I understood.

On the other hand, in Comparative Examples 1 to 17, the discharge capacity Q is the same as the design capacity, but since the equation 1 is not satisfied, the cycle characteristics are lower than those of the examples having the same discharge capacity Q. It is considered that the current collector with Y <0.1Q-40 cannot suppress wrinkles, while when Y is larger than 50%, the strength of the current collector is lowered and the cycle characteristics are lowered.

In Comparative Example 17, the diameter of the through hole of the current collector 1 was 130 μm, and the slurry fell off during coating, so that coating could not be performed.

Reference Example 1 satisfies the formula 1, but the breaking strength A of the binder is outside the preferable range of 80 to 400 MPa (less than 80 MPa) and does not satisfy the formula 2. Therefore, the cycle characteristics are deteriorated.

Reference Example 2 satisfies Equation 1 and shows high cycle characteristics, but the discharge capacity Q is lower than the design capacity. This is because the breaking strength A of the binder is outside the preferable range of 80 to 400 MPa (greater than 400 MPa) and the formula 2 is not satisfied, that is, because the amount of imide groups in the binder is large. It is considered that Li is trapped in the imide group in the negative electrode binder, the negative electrode has an irreversible capacity, and the discharge capacity of the negative electrode is low.

Reference Example 3 satisfies both Equation 1 and Equation 2, but has low cycle characteristics. It is considered that in Reference Example 3, the diameter of the through hole was smaller than the average particle size of the negative electrode active material of 5 μm, the stress on the current collector could not be relaxed, and the cycle characteristics were deteriorated.

As described above, according to the present invention, in a lithium ion secondary battery including a negative electrode active material containing Si and a polyimide, polyamide or polyamide-imide as a binder, both high capacity and high cycle characteristics are compatible at a high level. It has been demonstrated that a lithium ion secondary battery capable of being capable of being provided can be provided.

The present invention is not limited to the above-described examples, and includes various modifications. For example, the above-described embodiment has been described in detail in order to explain the present invention in an easy-to-understand manner, and is not necessarily limited to those having all the described configurations. Further, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Further, it is possible to add / delete / replace a part of the configuration of each embodiment with another configuration.

For example, in this embodiment. Although a laminated type laminated cell is manufactured as a lithium ion secondary battery, the same effect can be obtained regardless of whether the battery has a wound structure or is enclosed in a metal can.

1,1a, 1b, 1'... current collector, 2,2' ... negative electrode active material. 3,3'... Graphite, 4,4' ... Binder, 5,5' ... Negative electrode mixture layer, 7,7' ... Current collector wrinkles, 10,10' ... Negative electrode, 20, 20a, 20b ... Through holes , 40 ... Laminated electrode group, 41 ... Positive electrode terminal, 42 ... Negative electrode terminal, 43 ... Positive electrode uncoated part, 44 ... Negative electrode uncoated part, 45 ... Positive electrode, 46 ... Negative electrode, 47 ... Separator, 50 ... Laminate cell , 51 ... Laminated film (case side), 52 ... Electrode group, 53 ... Laminated film (lid side).

Claims (5)

In a lithium ion secondary battery including a positive electrode, a negative electrode, and a separator that separates the positive electrode and the negative electrode.
The negative electrode has a current collector made of Cu foil having a thickness of 5 μm or more and 15 μm or less, and a negative electrode mixture layer provided on the surface of the current collector.
The negative electrode mixture layer is a mixture of a negative electrode active material containing Si and natural graphite in a mass ratio of 3: 97 to 40: 6.
Mixing at 0 and containing a binder containing polyimide, polyamide, polyamideimide or a mixture thereof,
The negative electrode active material containing Si is Si _X M _1-X , where M is Al, Ni, Cu, Fe, T.
It is at least one selected from i, Mn, and O, and 0.5 ≦ x ≦ 0.9.
A through hole is provided in the current collector so that the discharge capacity Q (Ah / kg) of the negative electrode and the porosity Y (%) of the current collector satisfy the following equation 1.
The breaking strength A (MPa) of the binder is 80 MPa or more and 400 MPa or less.
Diameter of the through hole is not more than 120μm average particle size than the negative electrode active material,
The discharge capacity Q (Ah / kg) of the negative electrode is 400 (Ah / kg) or more and 700 (Ah / k).
g) A lithium ion secondary battery characterized by being less than or equal to.
0.1 Q-40 ≤ Y ≤ 50 ... Equation 1 In the lithium ion secondary battery according to claim 1, the discharge capacity Q of the negative electrode is 500 Ah.
A lithium ion secondary battery characterized by being / kg or more and 700 Ah / kg or less. In the lithium ion secondary battery according to claim 1 wherein, the expansion rate of the negative electrode mixture layer 11
A lithium ion secondary battery characterized by being more than 0% and less than 150%. In the lithium ion secondary battery according to claim 1,
A lithium ion secondary battery characterized in that the discharge capacity Q (Ah / kg) of the negative electrode, the breaking strength A (MPa) of the binder, and the breaking elongation B (%) of the binder satisfy the following formula 2. ..
Q ≦ A × B ÷ 10 ≦ 3 × Q ... Equation 2 In the lithium ion secondary battery according to claim 1,
M is at least one selected from Al, Ni, Cu, Fe, Ti, and Mn.
A lithium ion secondary battery characterized in that 0.6 ≦ x ≦ 0.8.

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