JP5827193B2 - Method for producing negative electrode for non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a method for producing a negative electrode for a nonaqueous electrolyte secondary battery.

  A lithium ion secondary battery is generally composed of a positive electrode and a negative electrode capable of reversibly inserting and removing lithium ions, and a liquid, gel or solid electrolyte, and has advantages such as high output and high energy density. Yes.

In Patent Document 1, an active material paste in which active material particles, conductive material particles, and a polymer binder resin such as polyvinylidene fluoride (PVDF) are mixed is heated and applied on a current collector, A method for producing an electrode for a lithium ion secondary battery forming a negative electrode is disclosed (claim 1).
Patent Document 1 describes that the heating temperature of the active material paste when applying the active material paste on the current collector is preferably 20 to 55 ° C. (Claim 2).

  In lithium ion secondary batteries, carbon materials such as graphite are widely used as negative electrode active materials. For example, a negative electrode active material such as graphite, a binder such as styrene-butadiene rubber (SBR) or a modified product thereof (modified SBR), a thickener such as carboxymethylcellulose (CMC), and water are mixed. The paste is obtained, and the paste is applied onto the negative electrode current collector, dried, and pressed to produce a negative electrode comprising the current collector and the negative electrode active material layer.

Japanese Unexamined Patent Publication No. 2004-199916

  When a thickener such as CMC is used, the thickener does not dissolve well, and a microgel containing the thickener and water is easily generated in the paste. Furthermore, the microgel containing the aggregate which the fibers of the thickener entangled with each other may be generated.

  If there is a microgel containing an undissolved substance or an aggregate of the thickener in the paste, pinholes from which the active material has been removed will be formed in the resulting negative electrode active material layer. Since there is no active material in the pinhole portion, Li ions cannot be transferred during charging and discharging, and there is a possibility that the Li component may precipitate in the pinhole portion. For these reasons, the pinhole portion is not preferable because it causes a decrease in battery characteristics such as battery capacity.

  An undissolved thickener in which fibers are not agglomerated can be dissolved by heating, but agglomerates in which fibers of the thickener agglomerate are difficult to dissolve and dissolve only by heating. .

  This invention is made | formed in view of the said situation, and it aims at providing the manufacturing method of the negative electrode for nonaqueous electrolyte secondary batteries which can suppress formation of the pinhole from which the active material passed. Is.

The method for producing a negative electrode for a non-aqueous electrolyte secondary battery of the present invention is as follows.
A method for producing a negative electrode for a nonaqueous electrolyte secondary battery, comprising: a current collector; and a negative electrode active material layer formed on the current collector and including a negative electrode active material, a binder, and a thickener. ,
A step (A) of preparing a first paste having a solid content of 60% or more by mixing the negative electrode active material, the thickener, and water under a temperature of less than 40 ° C .;
Step (B) of adding a water and the binder to the first paste at a temperature of 40 to 90 ° C. to prepare a second paste having a solid content of 60% or less;
And (C) applying the second paste onto the current collector at a temperature lower than 40 ° C.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the negative electrode for nonaqueous electrolyte secondary batteries which can suppress formation of the pinhole from which the active material was missing can be provided.

In Example 1, it is a graph which shows the relationship between the preparation temperature (kneading temperature) of a 1st paste, and the number of microgels of 80 micrometers or more in the obtained 2nd paste. In Example 2, it is a graph which shows the relationship between the preparation temperature of a 2nd paste, and the number of 80 micrometer or more microgels in the obtained 2nd paste. In Example 3, it is a graph which shows the relationship between the temperature of a 2nd paste, and paste viscosity.

  Hereinafter, the present invention will be described in detail.

  The present invention relates to a method for producing a negative electrode for a nonaqueous electrolyte secondary battery.

Examples of the non-aqueous electrolyte secondary battery include a lithium ion secondary battery.
Hereinafter, main components will be described by taking a lithium ion secondary battery as an example.
A lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, a separator that insulates between them, a nonaqueous electrolyte, a battery container, and the like.

<Positive electrode>
The positive electrode can be manufactured by applying a positive electrode active material to a positive electrode current collector such as an aluminum foil by a known method.
Known no particular limitation on the positive electrode active material, for _{example, LiCoO 2, LiMnO 2, LiMn} 2 O 4, LiNiO 2, LiNi x Co (1-x) O 2, and _{LiNi x Co y Mn (1-} x-y _And lithium-containing composite oxides such as O ₂ (where 0 <x <1, 0 <y <1).

  For example, using a dispersing agent such as N-methyl-2-pyrrolidone, the above positive electrode active material, a conductive agent such as carbon powder, and a binder such as polyvinylidene fluoride (PVDF) are mixed to prepare a paste. The paste can be applied onto a current collector such as an aluminum foil, dried, and pressed to form a positive electrode active material layer.

  When metal lithium or lithium alloy is used as the positive electrode active material, it can be used as it is as the positive electrode.

<Negative electrode>
The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector and including a negative electrode active material, a binder, and a thickener.
The negative electrode current collector is not particularly limited, and examples thereof include a metal foil such as a copper foil.
The negative electrode active material is not particularly limited, and a material having a lithium storage capacity of 2.0 V or less on the basis of Li / Li + is preferably used. As the negative electrode active material, carbon such as graphite, metallic lithium, lithium alloy, transition metal oxide / transition metal nitride / transition metal sulfide capable of doping / dedoping lithium ions, and these A combination etc. are mentioned.
As the binder, a rubbery polymer, preferably styrene-butadiene rubber (SBR) or a modified product thereof (modified SBR) is used.
As the thickener, a water-soluble polymer, preferably carboxymethyl cellulose (CMC) is used.

  As described in the section “Problems to be Solved by the Invention”, in the conventional method, the thickener does not dissolve well, and a microgel containing the thickener and water tends to be generated in the paste. Furthermore, the microgel containing the aggregate which the fibers of the thickener entangled with each other may be generated.

The method for producing a negative electrode for a non-aqueous electrolyte secondary battery of the present invention is as follows.
Step (A) of preparing a first paste having a solid content of 60% or more by mixing a negative electrode active material, a thickener, and water under a temperature of less than 40 ° C .;
Step (B) of adding a water and a binder to the first paste at a temperature of 40 to 90 ° C., and preparing a second paste having a solid content of 60% or less;
And (C) applying the second paste onto the current collector at a temperature lower than 40 ° C.

Step (A) is a kneading step in which the negative electrode active material, the thickener, and a small amount of water are mixed.
In this step, a first paste having a solid content of 60% or more is prepared.
The mixing procedure of the negative electrode active material, the thickener and a small amount of water is not particularly limited, and may be mixed at once or separately.
For example, it is preferable to first mix a powdered negative electrode active material and a powder thickener, and then add and mix a small amount of water.
The temperature at which the powdered negative electrode active material and the powder thickener are mixed is not particularly limited, and may be room temperature.
The mixing temperature after adding water is less than 40 ° C, preferably less than 30 ° C.
As the paste temperature increases, the viscosity decreases, the shearing force during kneading decreases, and the thickener tends to aggregate.
By adjusting the mixing temperature of the negative electrode active material, the thickener and water (the preparation temperature of the first paste, the kneading temperature) to less than 40 ° C., preferably less than 30 ° C., the shear force during kneading is increased and increased It is possible to suppress the formation itself of the aggregate in which the fibers of the adhesive agent are aggregated, and even if the aggregate is generated, the aggregation of the fibers can be released. Due to these effects, it is possible to suppress the formation of a microgel containing aggregates in which the fibers of the thickener are aggregated.

Step (B) is a step of adding and mixing water and a binder to the first paste.
In this step, a second paste having a solid content of 60% or less is prepared.
The procedure for adding and mixing water and binder to the first paste is not particularly limited, and may be added and mixed all at once or separately.
For example, it is preferable to add and mix a binder after adding water to the first paste to obtain a desired solid content.
Regardless of the addition and mixing procedure, the addition and mixing temperature of water and binder to the first paste (preparation temperature of the second paste) is 40 to 90 ° C.
By adjusting the preparation temperature of the second paste to 40 ° C. or higher, the undissolved material of the thickener can be dissolved well, and the formation of the microgel containing the undissolved material of the thickener can be suppressed. .
If the preparation temperature of the second paste exceeds 90 ° C., the decomposition of the thickener such as CMC proceeds and the viscosity decreases rapidly, which is not preferable.

Step (C) is a step of applying the second paste prepared in step (B) onto the current collector.
Step (C) is carried out at a temperature below 40 ° C, preferably below 30 ° C.
When the temperature of the second paste increases, the viscosity decreases, and the second paste tends to flow down when applied onto the current collector. Therefore, it is difficult to form a high-weight negative electrode active material layer suitable for a lithium ion secondary battery mounted on a plug-in hybrid vehicle (PHV) or an electric vehicle (EV).
By carrying out the step (C) at a temperature of less than 40 ° C., preferably less than 30 ° C., a negative electrode active material layer having a basis weight of, for example, 7 mg / cm ² or more suitable for the above application can be stably formed. .

The negative electrode active material layer can be formed on one side or both sides of the current collector.
In this specification, the preferable basis weight of the active material layer is defined by the basis weight of the active material layer on one side.

  After the step (C), a step (D) of drying the coating film obtained by applying the second paste, and a step (E) of pressing the dried coating film as necessary are performed, A negative electrode including a current collector and a negative electrode active material layer can be obtained.

  As described above, in the method for producing a negative electrode for a non-aqueous electrolyte secondary battery of the present invention, the formation of thickener aggregates is suppressed in step (A), and the thickener undissolved material in step (B). Can be dissolved well. Therefore, it is possible to suppress the generation of pinholes in the negative electrode active material layer due to the formation of microgels containing undissolved or aggregated thickeners, and stably produce non-aqueous electrolyte secondary batteries having desired battery characteristics. Can be manufactured.

<Nonaqueous electrolyte>
As the non-aqueous electrolyte, known ones can be used, and liquid, gel-like or solid non-aqueous electrolytes can be used.
For example, a lithium-containing electrolyte is dissolved in a mixed solvent of a high dielectric constant carbonate solvent such as propylene carbonate or ethylene carbonate and a low viscosity carbonate solvent such as diethyl carbonate, methyl ethyl carbonate, or dimethyl carbonate. A water electrolysis solution is preferably used.

As the mixed solvent, for example, a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is preferably used.
Examples of the lithium-containing electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), LiPF _n {C _k F _{(2k + 1) )} (6-n) (} n = 1~5 integer, k = 1 to 8 integer) lithium salts such as, and combinations thereof.

<Separator>
The separator may be a film that electrically insulates the positive electrode and the negative electrode and is permeable to lithium ions, and a porous polymer film is preferably used.
As the separator, for example, a porous film made of polyolefin such as a porous film made of PP (polypropylene), a porous film made of PE (polyethylene), or a laminated porous film of PP (polypropylene) -PE (polyethylene) is preferably used. It is done.

<Battery container>
A well-known thing can be used as a battery container.
Secondary battery types include a cylindrical type, a coin type, a square type, a film type (laminate type), and the like, and a battery container can be selected in accordance with a desired type.

  Embodiments according to the present invention will be described.

Example 1
Only the preparation conditions of the negative electrode paste were changed, and the other conditions were the same, and a plurality of negative electrode paste samples were obtained.

  A graphite powder as a negative electrode active material and a carboxymethyl cellulose (CMC) powder as a thickener were mixed. Next, a small amount of water was added and mixed to obtain the first paste (step (A), kneading step). The solid content of the first paste was 65%. Mixing (solidifying) of graphite, CMC and a small amount of water was carried out for 30 minutes at a stirring speed of 50 rpm using a planetary mixer. The temperature (preparation temperature of the first paste, kneading temperature) at this time was shaken between 20 to 60 ° C., and the influence of the temperature was examined. The preparation temperature (kneading temperature) of the first paste was four conditions of 25 ° C., 30 ° C., 40 ° C., and 60 ° C.

  Next, water was added to and mixed with the first paste, and further styrene-butadiene rubber (SBR) as a binder was added and mixed to obtain a second paste (step (B)). Water and SBR were added and mixed using a planetary mixer for 10 minutes at a stirring speed of 15 rpm. The temperature at the time of addition and mixing (preparation temperature of the second paste) was 40 ° C.

  In any sample, the blending ratio of graphite, SBR, and CMC was 97/2/1 (mass ratio).

  The obtained second paste was pulled up with a particle gauge, and the number of microgels of 80 μm or more was visually measured.

FIG. 1 shows the relationship between the preparation temperature (kneading temperature) of the first paste and the number of microgels of 80 μm or more in the obtained second paste.
As shown in FIG. 1, the number of microgels in the second paste tended to decrease as the preparation temperature (kneading temperature) of the first paste was lower. By making the preparation temperature (kneading temperature) of the first paste less than 40 ° C., preferably less than 30 ° C., the number of microgels in the obtained second paste could be remarkably reduced.

(Example 2)
A plurality of negative electrode paste samples were obtained in the same manner as in Example 1 except that only the temperature conditions at the time of mixing the materials were changed and the other conditions were the same.

  In Example 2, the preparation temperature (kneading temperature) of the first paste was fixed at 30 ° C., the preparation temperature of the second paste was shaken between 20 ° C. and 60 ° C., and the influence of the temperature was examined. The preparation temperature of the second paste was four conditions of 25 ° C., 30 ° C., 40 ° C., and 60 ° C.

  The obtained second paste was pulled up with a particle gauge, and the number of microgels of 80 μm or more was visually measured.

FIG. 2 shows the relationship between the preparation temperature of the second paste and the number of microgels of 80 μm or more in the obtained second paste.
As shown in FIG. 2, the number of microgels tended to decrease as the preparation temperature of the second paste was higher. By setting the preparation temperature of the second paste to 40 ° C. or higher, the number of microgels in the obtained second paste can be significantly reduced.
If the preparation temperature of the second paste exceeds 90 ° C., the decomposition of CMC proceeds and the viscosity decreases rapidly. Therefore, the preparation temperature of the second paste is preferably 40 to 90 ° C.

(Example 3)
A negative electrode paste sample was obtained in the same manner as in Example 1 except that only the temperature conditions at the time of mixing the materials were changed and the other conditions were the same.

In Example 3, the first paste preparation temperature (kneading temperature) was 30 ° C., and the second paste preparation temperature was 40 ° C.
The temperature of the obtained second paste was changed, and the viscosity of each temperature sample was measured. The temperature conditions were 5 conditions of 10 ° C, 25 ° C, 40 ° C, 60 ° C, and 80 ° C.
The viscosity was measured using an E-type viscometer under conditions of a shear rate of 40 s ⁻¹ and 30 ° C.

The results are shown in FIG.
As shown in FIG. 3, the paste viscosity tended to increase as the temperature of the prepared second paste was lower.
In a lithium ion secondary battery mounted on a plug-in hybrid vehicle (PHV) or an electric vehicle (EV), the basis weight of the negative electrode active material layer is preferably 7 mg / cm ² or more, for example. In order to stably obtain a negative electrode active material layer having a basis weight of 7 mg / cm ² or more, the viscosity of the second paste is preferably 600 mPa · s or more.
By adjusting the temperature of the second paste after preparation to less than 40 ° C., preferably less than 30 ° C., the viscosity of the second paste can be stably set to 600 mPa · s or more, and the basis weight is 7 mg / cm ² or more. It was found that the negative electrode active material layer was stably obtained.

  The present invention is preferably applicable to a negative electrode such as a lithium ion secondary battery mounted on a plug-in hybrid vehicle (PHV) or an electric vehicle (EV).

Claims (7)

A method for producing a negative electrode for a nonaqueous electrolyte secondary battery, comprising: a current collector; and a negative electrode active material layer formed on the current collector and including a negative electrode active material, a binder, and a thickener. ,
A step (A) of preparing a first paste having a solid content of 60% or more by mixing the negative electrode active material, the thickener, and water under a temperature of less than 40 ° C .;
Step (B) of adding a water and the binder to the first paste at a temperature of 40 to 90 ° C. to prepare a second paste having a solid content of 60% or less;
And (C) applying the second paste onto the current collector at a temperature lower than 40 ° C. The method for producing a negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the weight of the negative electrode active material layer is 7 mg / cm ² or more.   The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the thickener is a water-soluble polymer.   The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to claim 3, wherein the thickener is carboxymethylcellulose.   The said binder is a rubber-like polymer, The manufacturing method of the negative electrode for nonaqueous electrolyte secondary batteries in any one of Claims 1-4.   The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the binder is styrene-butadiene rubber or a modified product thereof.   The method for producing a negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the nonaqueous electrolyte secondary battery is a lithium ion secondary battery.

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