JP2014044921A - Lithium ion secondary battery, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion battery improved in battery characteristics and reliability by appropriately dispersing an active material in an electrode material, and to provide a method for manufacturing such a lithium ion battery.SOLUTION: A lithium ion secondary battery comprises: an electrode for positive polarity; an electrode for negative polarity; and a separator for insulation between the electrodes for positive and negative polarities. The electrode for negative polarity has first active material particles, and second active material particles smaller than the first ones in the volume expansion at the time of charging. If the degree of particle dispersion of the first active material particles is expressed by a dispersion exponent (D) defined below, the dispersion exponent is smaller than 0.6: (Defined formula) Dispersion exponent(D)=Standard deviation of distances between the active material particles/Average of distances between the active material particles.

Description

  The present invention relates to a lithium ion secondary battery and a method for manufacturing the same.

  Although progress in lithium ion battery manufacturing technology is remarkable, important factors that determine the performance of lithium ion batteries include higher capacity and smaller size. In order to satisfy these performances, it is essential to increase the energy that can be extracted per unit weight or unit deposition, that is, the energy density. Since the energy density can be secured as the theoretical capacity value determined by the active material constituting the battery increases, an active material having a large theoretical capacity is desired for both the positive electrode active material and the negative electrode active material. .

  A lithium ion battery incorporates a positive electrode, a negative electrode, and a separator provided between the two electrodes in a metal can structure or a laminated film battery case having a metal film such as aluminum as a base. In general, it is manufactured by sealing after pouring. The manufacturing method and appearance vary greatly depending on the can structure or laminated film structure, but the basic parts for operating the battery are common. For example, a positive electrode active material is applied to the positive electrode and a negative electrode active material is applied to the negative electrode on the current collector foil, and a separator is inserted to prevent the positive electrode active material and the negative electrode active material from being short-circuited.

  Among the active material materials described above, carbon materials are widely used as active materials for negative electrodes because of the stability of the reaction in the charge / discharge process and the stability of the cycle characteristics, and graphite (theoretical capacity value: 372 mAh / g) has been used. Recently, silicon (theoretical capacity value 4200mAh / g) -based material has been developed as a material that greatly exceeds the theoretical capacity value of carbon materials. As described above, there is Patent Document 1 as a background art in this technical field.

JP 2006-92969 A

  It is known that a silicon-based negative electrode material, which is one of the negative electrode constituent materials of a lithium ion battery, has a very large volume expansion of about 400% during battery charging. This repeated expansion and contraction due to charging and discharging is critical to battery characteristics and reliability, such as collapse of the active material particle structure and lack of adhesion between the active material and the current collector foil coated with the active material. It has become a problem to have a negative effect. As long as a silicon-based material is used, in principle, volume expansion and contraction during charging and discharging are inevitable. Therefore, even if expansion and contraction occur, it is necessary to avoid and reduce the influence from the viewpoint of improving battery characteristics and battery reliability.

  Patent Document 1 shows an example of composite component particles of silicon and silicon oxide, and the composite component particles have a structure to be included in the carbonaceous material. Thus, in order to achieve both increase in battery capacity and relaxation of volume expansion, a silicon-based material having a large theoretical capacity but a large volume expansion during charging and a carbon material having a small theoretical capacity but a small volume expansion during charging. Both have been optimized.

  On the other hand, it has been found that the dispersion state of the target active material particles is also closely related to battery characteristics and battery reliability. When the electrode is composed of a plurality of types of particles, it is not sufficient to relax the inherent expansion and contraction characteristics, especially when active material particles that have a large volume expansion effect are locally aggregated and segregated. Structural breakdown occurs starting from the local volume expansion of the part, and as a result, battery characteristics and battery reliability are affected.

  In view of the above problems, the present invention is a lithium ion battery having improved battery characteristics and battery reliability by appropriately dispersing an active material having a large theoretical capacity but a large volume expansion during charging in the electrode material, And an object of the present invention.

In order to solve the above problems, the present invention comprises a positive electrode, a negative electrode, a separator that insulates the positive electrode and the negative electrode, and the negative electrode comprises first active material particles, The second active material particles having a smaller volume expansion during charging than the first active material, and the dispersion index (D) defined below as the particle dispersity of the first active material particles, Provided is a lithium ion secondary battery wherein the dispersion index is less than 0.6.
(Definition formula)
Dispersion index (D) = standard deviation of distance between active material particles / average distance of distance between active material particles In the present invention, the step of kneading the first active material particles and the viscosity modifier, A step of mixing and kneading substance particles, a step of mixing and kneading a binder composition dispersed in an aqueous solution, and applying the kneaded product to the electrode current collector foil, followed by drying treatment A method for producing a negative electrode for a lithium ion secondary battery is provided.

  Further, in the present invention, the step of mixing the first active material particles and the viscosity modifier, and the mixed liquid in which the first active material particles and the viscosity modifier are mixed are more charged than the first active material. A method for producing a lithium ion secondary battery, comprising: mixing a second active material particle having a small volume expansion; and applying a mixed solution mixed with the second active material particle to an electrode current collector foil. provide.

  In the present invention, the step of mixing the first active material particles, the second active material particles having a smaller volume expansion during charging than the first active material, and a viscosity modifier at a shear rate of 8 m / s or more, And providing a method of manufacturing a lithium ion secondary battery, the method comprising: applying a mixed liquid obtained by mixing the first active material particles, the second active material particles, and the viscosity modifier to an electrode current collector foil.

  In the present invention, the step of mixing the first active material particles, the second active material particles having a smaller volume expansion during charging than the first active material, and the viscosity modifier, and the first active material Lithium ions having a step of dispersing particles by applying ultrasonic waves to a mixed liquid in which particles, the second active material particles, and the viscosity modifier are mixed, and a step of applying the mixed liquid to an electrode current collector foil A method for manufacturing a secondary battery is provided.

  In the present invention, the step of mixing the first active material particles, the second active material particles having a smaller volume expansion during charging than the first active material, and the viscosity modifier, and the first active material A step of dispersing a particle by mixing a surfactant in a mixed solution in which particles, second active material particles and the viscosity modifier are mixed, and applying the mixed solution containing the surfactant to the electrode current collector foil The manufacturing method of the lithium ion secondary battery which has a process is provided.

  According to the present invention, a lithium ion battery having improved battery characteristics and battery reliability by appropriately dispersing an active material having a large theoretical capacity but a large volume expansion during charging in an electrode material, and its manufacture A method can be provided.

It is a figure which shows the lithium ion battery in the Example of this invention. It is a figure which shows the negative electrode of the lithium ion battery in the Example of this invention. It is a figure which shows the negative electrode of the lithium ion battery in the Example of this invention. It is a figure which shows the manufacturing process of the electrode for negative electrodes in the Example of this invention. It is a figure which shows the manufacturing process modification 1 of the electrode for negative electrodes in the Example of this invention. It is a figure which shows the manufacturing process modification 2 of the electrode for negative electrodes in the Example of this invention. It is a figure which shows the manufacturing process modification 3 of the electrode for negative electrodes in the Example of this invention. It is a conceptual diagram which shows the evaluation criteria of the battery characteristic in the Example of this invention. It is a figure which shows the correlation of the parameter regarding the 1st active material in the Example of this invention, and a battery characteristic.

  Hereinafter, details of the present invention will be described in Examples.

  The negative electrode in this example is used as a negative electrode for a lithium ion battery. A structure of a lithium ion battery will be described as an example with reference to FIG.

  A laminate of a positive electrode 11, a negative electrode 12, and a separator 13 that insulates the positive electrode 11 and the negative electrode 12 is disposed inside the first housing 15. For simplification of description, in the drawing, one set of a positive electrode 11, a negative electrode 12, and a separator 13 is laminated inside the first casing 15, but the positive electrode 11 Even if a plurality of sets of negative electrode 12 and separator 13 are laminated inside the housing 15, there is no problem as long as it functions as a lithium ion battery.

  The positive electrode 11 is a positive current collector foil (not shown) and a positive electrode active material (not shown), and the negative electrode 12 is a negative electrode current collector foil (not shown) and a negative electrode active material (not shown). ). The positive electrode active material and the negative electrode active material are electromotive materials that generate electric power by directly participating in the lithium ion battery reaction. In general, when the battery is discharged, the positive electrode active material reduces itself and the negative electrode active material plays a role of being oxidized. The positive electrode current collector foil and the negative electrode current collector foil supply electrons received from the outside during charging and discharging of the battery to the positive electrode active material and the negative electrode active material.

  As the positive electrode active material of the lithium ion battery in this example, a spinel-structure lithium-containing composite oxide containing Mn is used. In addition to the spinel-structured lithium-containing composite oxide containing Mn, other materials may be used in combination for the positive electrode active material. As such another substance, for example, an olivine type compound or a lithium-containing transition metal oxide having a layered structure can be used.

  As the negative electrode active material, for example, a material such as silicon or silicon oxide is used as the first active material 22 or 32 having a large theoretical capacity and a large volume expansion coefficient during charging. Further, as the second active materials 23 and 33 having a small theoretical capacity and a volume expansion coefficient at the time of charging, for example, a graphite material such as natural graphite or artificial graphite is used. In this case, the first active materials 22 and 32 have a theoretical capacity approximately 10 times larger than that of the second active materials 23 and 33, but expand by about 400% during charging. On the other hand, the second active materials 23 and 33 have a smaller theoretical capacity than the first active materials 22 and 32, but the volume expansion during charging is about 10 to 20%. Is also small.

  In this embodiment, an aluminum foil is used for the positive electrode current collector foil, and a copper foil is used for the negative electrode current collector foil.

  In this embodiment, a laminate film in which the surface of aluminum is coated with an insulating film such as nylon, polypropylene (commonly called PP) or polyethylene (commonly called PE) is used as the casing 15. An electrolytic solution 14 is injected into the housing 15. The electrolyte solution 14 in a lithium ion battery is generally an ionic conductor for causing ion migration between a positive electrode and a negative electrode, in which a solute lithium salt is dissolved in an organic solvent. Generally, organic solvent species and solute species are selected in consideration of electrical conductivity, chemical stability, temperature stability, price, and the like.

  Next, an example of the structure of the negative electrode of this example will be described with reference to FIGS. FIGS. 2 and 3 compare the mixed states of the first active materials 22 and 32 and the second active materials 23 and 33.

  The negative electrode is a negative electrode current collector foil 21, 31, a first active material 22, 32 having a large theoretical capacity and a volume expansion coefficient during charging, and a second active material 23 having a small theoretical capacity and a volume expansion coefficient during charging. , 33.

  In this example, silicon particles are used as the first active materials 22 and 32, and graphite is used as the second active materials 23 and 33. The application of the negative electrode active material onto the negative electrode current collector foils 21 and 31 is usually performed by applying a liquid paste or slurry (not shown) to the current collector foil and then solidifying it by arbitrary drying. When mixing a plurality of types of active material particles, the mixed state immediately after application is reflected even after drying. Therefore, before applying the liquid slurry, the first active material 22, 32 and the second active material particle are mixed. It is desirable that the active materials 23 and 33 are sufficiently mixed.

  In the present embodiment, the silicon-based negative electrode material, which is the first active material 22, 32, has a very large volume expansion at the time of battery charging of about 400% during discharging. Due to repeated expansion and contraction due to charging and discharging, the active material particle structure collapses and the adhesion between the active material and the current collector foil coated with the active material is lost, which is fatal to battery characteristics and reliability. It will have an effect. In principle, volume expansion and contraction during charging and discharging is inevitable as long as silicon-based materials are used. Therefore, even if expansion or contraction occurs, it is necessary to avoid the effect of improving battery characteristics and battery reliability. It is necessary from. Due to the aggregation and segregation of the target particles, excessive volume expansion occurs locally, and structural destruction occurs starting from that portion, and as a result, battery characteristics and battery reliability are affected.

  Comparing the states of FIG. 2 and FIG. 3, it can be said that the silicon that is the first active material 22 of FIG. 2 is in a better mixed state than the silicon that is the first active material 32 of FIG. In other words, the first active material 22 in FIG. 2 is better dispersed than the first active material 32 in FIG.

In the present invention, the dispersion index (D) is defined by the following equation in order to clearly determine the mixed state, quantitatively determine the quality of the dispersion, and compare with the battery characteristics described later.
(Definition formula) Variance index (D)
= Standard deviation of the distance between the first active material particles / Average distance of the distance between the first active material particles Although the example in which the first active material particles in the present invention are silicon is shown, silicon and silicon oxide The second active material particles may be natural graphite, but carbon-based particles can be used as an alternative material. For example, artificial graphite, There is no problem even if hard carbon, soft carbon, or a mixture of the aforementioned carbon-based particles is used.

  Further, in this example, the weight ratio of the first active material particles to the weight sum of the first active material and the second active material particles is limited to a dispersion state of 0.04 or more or 0.4 or less. This is because when the weight ratio is less than 0.04 or greater than 0.4, the correlation between the superiority of the battery characteristics and the dispersion index does not appear experimentally.

  Next, FIG. 4 shows a production flow for an example of a method for producing a negative electrode sheet of a lithium ion battery in this example.

  First, the first active material particles and the viscosity modifier are sufficiently mixed by a step of kneading the first active material particles and the viscosity modifier. In this embodiment, silicon particles whose surfaces are covered with a carbon film are used as the first active material particles, and a 1.0 wt% carboxymethylcellulose aqueous solution is used as the viscosity modifier. A planetary mixer is used as the mixer. Here, the second active material particles are not mixed at the same time when the wettability of the first active material and the second active material to the aqueous solution is different, preventing either of the mixing from proceeding preferentially. It is to do.

  The second active material is mixed in a mixed solution in which the first active material and the aqueous carboxymethyl cellulose solution are mixed, and again mixed by a planetary mixer. In this embodiment, a mixture particle of artificial graphite and natural graphite is used as the second active material particle.

  After sufficient mixing has progressed, the first active material particles and the binder material for bonding the second active material particles and the current collector foil are added to the mixed solution, and further mixed by a planetary mixer. In this embodiment, a 50% by weight styrene butadiene rubber aqueous solution is used as the binder material.

  Since the mixed liquid produced in this manner is in a paste or slurry at this point, the negative electrode is completed by applying the paste or slurry to the current collector foil and drying the current collector foil.

  According to the manufacturing flow shown in FIG. 4, since the second active material is mixed after the first active material is sufficiently dispersed by the planetary mixer, the first active material having a large volume expansion due to charging is more reliably obtained. Can be dispersed.

  Next, FIG. 5 shows a manufacturing flow of Modification 1 of the method for manufacturing the negative electrode sheet of the lithium ion battery according to the present invention.

  First, the first active material particles, the second active material particles, and the viscosity modifier are mixed. A mixer that can apply a high shearing force to the particles is desirable so that the first active material particles and the second active material particles are sufficiently mixed. Specifically, particle mixing is performed at a shear rate of 8 m / s or more. In the present invention, the viscosity of the first active material particles is adjusted with silicon particles whose surfaces are covered with a carbon film, the mixed particles of artificial graphite and natural graphite are used as the second active material particles, and the 1.0 wt% carboxymethyl cellulose aqueous solution is adjusted in viscosity. It is used as an agent.

  A binder material that bonds the first active material particles and the second active material particles to the current collector foil is added to the mixed solution of the first active material, the second active material, and the carboxymethyl cellulose aqueous solution. And further mixing with a planetary mixer. In Modification 1, a 50% by weight styrene butadiene rubber aqueous solution is used as the binder material.

  Since the mixed liquid produced in this manner is in a paste or slurry at this point, the negative electrode is completed by applying the paste or slurry to the current collector foil and drying the current collector foil.

  According to the manufacturing flow of FIG. 5, the dispersion of the first active material having a particularly large volume expansion by charging can be promoted by applying a particularly high shearing force.

  FIG. 6 shows a manufacturing flow for Modification 2 of the method for manufacturing the negative electrode sheet of the lithium ion battery according to the present invention.

  First, the first active material particles, the second active material particles, and the viscosity modifier are mixed. As an example, mixing is performed using a planetary mixer. In Modification 2, the viscosity of silicon particles whose surfaces are covered with a carbon film as the first active material particles, mixed particles of artificial graphite and natural graphite as the second active material particles, and 1.0 wt% carboxymethylcellulose aqueous solution are adjusted. It is used as an agent.

  The mixture of the first active material, the second active material, and the carboxymethyl cellulose aqueous solution is subjected to ultrasonic pulling treatment, and then the first active material particles and the second active material particles are collected in the mixture. A binder material for bonding the electric foil is added and further mixed by a planetary mixer. In the second modification, the processing is performed under the condition of ultrasonic waves of 20 kHz. A 50 wt% styrene butadiene rubber aqueous solution is used as the binder material.

  Since the mixed liquid produced in this manner is in a paste or slurry at this point, the negative electrode is completed by applying the paste or slurry to the current collector foil and drying the current collector foil.

  According to the manufacturing flow of FIG. 6, the aggregated particles are unraveled by applying ultrasonic waves, and in particular, the dispersion of the first active material having a large volume expansion due to charging can be promoted.

  Furthermore, a manufacturing flow is shown in FIG. 7 for Modification 3 of the method for manufacturing the negative electrode sheet of the lithium ion battery in the present invention.

  First, the first active material particles, the second active material particles, and the viscosity modifier are mixed. As an example, mixing is performed using a planetary mixer. In Modification 3, silicon particles whose surfaces are covered with a carbon film are used as the first active material particles, mixed particles of artificial graphite and natural graphite are used as the second active material particles, and 1.0% by weight carboxymethylcellulose is used as the thickener. An aqueous solution is used as a viscosity modifier.

  A surfactant is mixed as a dispersion accelerator in a mixed liquid in which the first active material, the second active material, and the carboxymethyl cellulose aqueous solution are mixed, and the mixture is stirred with a planetary mixer. In Modification 3, a 0.1% by weight aqueous solution of Triton-X100, which is one of nonionic surfactants as a surfactant and has a hydrophilic polyethylene oxide group, is added.

  Thereafter, a binder material is added and further mixed by a planetary mixer. In Modification 3, a 50% by weight styrene butadiene rubber aqueous solution is used as a binder material for adhering the first active material particles and the second active material particles and the current collector foil to the mixed solution.

  Since the mixed material manufactured in this way is in a paste form or a slurry at this point, the negative electrode is completed by applying the paste or slurry to the current collector foil and drying the current collector foil.

  According to the manufacturing flow of FIG. 7, by mixing a surfactant as a dispersion accelerator, the active material particles can be made to conform to the mixed solution, and the active material particles are less likely to aggregate, and in particular, volume expansion due to charging is prevented. Dispersion of the large first active material can be promoted.

  Next, with respect to the battery manufactured using the negative electrode according to this example, the results of the battery characteristics and reliability tests of the active material of the negative electrode are shown. A test battery having the structure shown in FIG. 1 is used.

  In order to examine battery characteristics under various conditions, the ratio of the weight of the first active material to the sum of the weight of the first active material and the weight of the second active material of the negative electrode is set to 0.02 to 0.4. The electrode material composition was controlled. Further, the correlation between the degree of dispersion and the battery characteristics was analyzed by changing the degree of dispersion of the first active material particles by the manufacturing method described above.

  As the evaluation criteria for battery characteristics, the capacity change was observed by repeating the charge and discharge cycles, and those that could not maintain 90% capacity with respect to the initial capacity after the test of at least 200 cycles were determined as defective samples. did. FIG. 8 is a conceptual diagram showing determination criteria, and FIG. 9 shows the correlation between the weight ratio of the first active material particles, the degree of dispersion of the first active material, and the battery characteristics.

  For the first active material weight ratio of 0.02, no defect occurred even if the dispersity was changed, and the dispersibility changed because the original abundance was small for the small active material weight ratio. However, it is thought that it did not appear as a battery failure. From this examination result, it was found that the first active material weight ratio can be considered to be 0.04 or more. Moreover, the theoretical capacity of the whole negative electrode can be increased by setting the weight ratio of the first active material having a large theoretical capacity to at least 0.04 or more. It was also found that good battery characteristics can be obtained by setting the dispersity of at least the first active material to 0.6 or less within the range of the weight ratio of the active material.

  In addition, this invention is not limited to above-described embodiment, Various modifications are included. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described.

11: Electrode for positive electrode 12: Electrode for negative electrode 13: Separator 14: Electrolytic solution 15: Housing 21: Current collector foil for negative electrode 22: First active material for negative electrode 23: Second active material for negative electrode 31: Current collector for negative electrode Electric foil 32: first active material for negative electrode 33: second active material for negative electrode

Claims (9)

A positive electrode, a negative electrode, a separator that insulates the positive electrode and the negative electrode,
The negative electrode has first active material particles and second active material particles whose volume expansion during charging is smaller than that of the first active material, and the degree of particle dispersion of the first active material particles Is a dispersion index (D) defined below, the dispersion index is less than 0.6.
(Definition formula)
Dispersion index (D) = standard deviation of distance between active material particles / average distance of distance between active material particles The first active material particles include Si particles or particles having a mixed composition of Si and SiO2,
2. The lithium ion secondary battery according to claim 1, wherein the second active material particles include particles of at least one of natural graphite, artificial graphite, hard carbon, and soft carbon.   3. The lithium ion secondary material according to claim 2, wherein the weight ratio of the first active material particles to the sum of the weight of the first active material and the weight of the second active material particles is 0.04 or more and 0.4 or less. Next battery. Mixing the first active material particles and the viscosity modifier;
Mixing the second active material particles having a smaller volume expansion at the time of charging than the first active material into a mixed liquid obtained by mixing the first active material particles and the viscosity modifier;
A method of manufacturing a lithium ion secondary battery, comprising: applying a mixed liquid mixed with the second active material particles to an electrode current collector foil. Mixing the first active material particles and the second active material particles having a smaller volume expansion during charging than the first active material and the viscosity modifier at a shear rate of 8 m / s or more;
The manufacturing method of a lithium ion secondary battery which has the process of apply | coating the liquid mixture which mixed said 1st active material particle, said 2nd active material particle, and the said viscosity modifier to electrode current collector foil. Mixing the first active material particles, the second active material particles having a smaller volume expansion during charging than the first active material, and a viscosity modifier;
Applying ultrasonic waves to a mixed liquid in which the first active material particles, the second active material particles, and the viscosity modifier are mixed, and dispersing the particles;
The manufacturing method of a lithium ion secondary battery which has the process of apply | coating the said liquid mixture to electrode current collection foil. Mixing the first active material particles, the second active material particles having a smaller volume expansion during charging than the first active material, and a viscosity modifier;
A step of dispersing the particles by mixing a surfactant in a mixed liquid obtained by mixing the first active material particles, the second active material particles, and the viscosity modifier;
A method of manufacturing a lithium ion secondary battery, comprising: applying a mixed solution mixed with the surfactant to an electrode current collector foil. The first active material particles include Si particles or particles having a mixed composition of Si and SiO2,
The lithium ion secondary battery according to any one of claims 4 to 7, wherein the second active material particles include particles of at least one of natural graphite, artificial graphite, hard carbon, and soft carbon. Production method.   9. The lithium ion secondary material according to claim 8, wherein the weight ratio of the first active material particles to the sum of the weight of the first active material and the weight of the second active material particles is 0.04 or more and 0.4 or less. A method for manufacturing a secondary battery.

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