JP2018113220A - Method for manufacturing lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten a time required for pre-doping by use of an electrolyte solution.SOLUTION: A lithium ion secondary battery manufacturing method comprises at least the steps of: (A) manufacturing a negative electrode by disposing, on the surface of a negative electrode current collector, a negative electrode mixture layer including a first negative electrode active material and a second negative electrode active material; (C) disposing a lithium metal at a position where the lithium metal is electrically connected to the negative electrode current collector and is out of contact with the negative electrode mixture layer; (D) charging the negative electrode by a predetermined amount by immersing the negative electrode and the lithium metal in an electrolyte solution; and (E) manufacturing a lithium ion secondary battery including a negative electrode charged by the predetermined amount. The second negative electrode active material is lithium titanate. The first negative electrode active material has a larger charge capacity per unit mass in comparison to that of the second negative electrode active material. In the negative electrode charged by the predetermined amount, the difference in potential between the second negative electrode active material and the lithium metal is larger than that between the first negative electrode active material and the lithium metal.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a method for manufacturing a lithium ion secondary battery.

  Japanese Patent Laying-Open No. 2006-076333 (Patent Document 1) discloses that a lithium metal surface is coated with lithium titanate.

JP 2006-076333 A

  Improvement in the energy density of a lithium ion secondary battery (hereinafter sometimes abbreviated as “battery”) is required. One of the factors that hinder the improvement of energy density is irreversible capacity. The irreversible capacity is derived from Li ions that cannot be used thereafter among lithium (Li) ions supplied from the positive electrode to the negative electrode during the first charge. For example, the higher the capacity of the negative electrode active material such as silicon (Si), the larger the irreversible capacity. Due to the presence of irreversible capacity, the effective charge / discharge capacity of the battery is limited.

  Before the battery is completed, it has been studied to supply an amount of Li ion corresponding to the irreversible capacity to the negative electrode active material in advance (referred to as “pre-doping”). For example, Li ions can be supplied to the negative electrode active material by allowing Li metal (Li foil or the like) to adhere to the surface of the negative electrode and allowing it to stand. However, with this method, it is difficult to control the amount of charge. Furthermore, unevenness in the charge amount tends to occur in the thickness direction of the negative electrode.

  A pre-dope using an electrolytic solution is also conceivable. That is, the negative electrode and the Li metal are not directly contacted, and these are immersed in the electrolytic solution in a state where the negative electrode and the Li metal are short-circuited. According to this method, it is considered that Li ions can be supplied uniformly to the entire negative electrode. However, in this method, the diffusion rate of Li ions is slow, and it takes a long time to charge the irreversible capacity.

  An object of the present disclosure is to shorten the time required for pre-doping using an electrolytic solution.

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

[1] The method for producing a lithium ion secondary battery of the present disclosure includes at least the following (A), (C), (D), and (E).
(A) A negative electrode is produced by disposing a negative electrode mixture layer containing a first negative electrode active material and a second negative electrode active material on the surface of the negative electrode current collector.
(C) Lithium metal is disposed at a position that is electrically connected to the negative electrode current collector and does not contact the negative electrode mixture layer.
(D) A predetermined amount of the negative electrode is charged by immersing the negative electrode and lithium metal in the electrolytic solution.
(E) A lithium ion secondary battery including a negative electrode charged by a predetermined amount is manufactured.
The second negative electrode active material is lithium titanate. The first negative electrode active material has a larger charge capacity per unit mass than the second negative electrode active material. In the negative electrode charged by a predetermined amount, the potential difference between the second negative electrode active material and lithium metal is larger than the potential difference between the first negative electrode active material and lithium metal.

  Li ion diffusion so that the potential difference between the negative electrode and the Li metal is canceled by immersing the negative electrode and the Li metal in the electrolyte while the negative electrode and the Li metal are electrically connected (short circuit state). Happens. That is, Li ions are released from the Li metal into the electrolytic solution, and Li ions are supplied from the electrolytic solution to the negative electrode. Eventually, substantially all of the Li metal elutes. This completes pre-doping. It is considered that the amount of pre-doping (charge amount) can be controlled by the amount of Li metal. Moreover, since electrolyte solution can osmose | permeate the inside of a negative electrode composite material layer, it is thought that Li ion is supplied uniformly to the whole negative electrode composite material layer.

  In the manufacturing method of the present disclosure, the first negative electrode active material is the object of pre-doping. The first negative electrode active material has a larger charge capacity per unit mass than the second negative electrode active material. The driving force of pre-doping is considered to be a potential difference between the object of pre-doping and Li metal. However, when a general negative electrode active material is charged even a little, the electric potential rapidly decreases. Therefore, it is considered that the driving force for pre-doping is reduced and the required time for pre-doping is increased.

  Therefore, in the manufacturing method of the present disclosure, the second negative electrode active material (lithium titanate) is used. The potential difference between lithium titanate and Li metal is about 1.5 V, which is relatively large. Furthermore, the potential drop of lithium titanate is very gradual until it is fully charged. By coexisting with the second negative electrode active material, it is considered that even if the potential of the first negative electrode active material is reduced, the driving force of the pre-dope is suppressed to be reduced. As a result, it is considered that the time required for pre-doping is shortened.

[2] The first negative electrode active material is preferably at least one selected from the group consisting of graphite, amorphous carbon, silicon, silicon oxide, and silicon alloy.
When these negative electrode active materials are charged even a little, the potential drops rapidly. During the pre-doping, the potential difference between the negative electrode active material and the Li metal is about 0.05 to 0.3V. Therefore, it is considered that the time required for pre-doping is usually increased. However, as described above, it is considered that the time required for pre-doping is shortened by using the second negative electrode active material (lithium titanate).

[3] The surface of the second negative electrode active material is preferably covered with a lithium ion permeable film.
During the pre-doping, the second negative electrode active material (lithium titanate) is also charged. Due to the charging, the potential of lithium titanate gradually decreases, albeit slowly. Furthermore, when fully charged, the potential of lithium titanate decreases rapidly. It is considered that the driving force of the pre-dope decreases as the potential of lithium titanate decreases. It is considered that the lithium titanate is suppressed from being charged when the lithium titanate is covered with the Li ion permeable coating.

[4] The second negative electrode active material may have a mass ratio of 1% by mass to 5% by mass with respect to the total of the first negative electrode active material and the second negative electrode active material.
The higher the mass ratio of the first negative electrode active material to be pre-doped (that is, the lower the mass ratio of the second negative electrode active material), the higher the energy density is expected. On the other hand, the lower the mass ratio of the second negative electrode active material, the longer the required pre-doping time. When the second negative electrode active material has a mass ratio of 1% by mass or more and 5% by mass or less, a balance between the energy density and the time required for pre-doping is good.

FIG. 1 is a flowchart illustrating an outline of a method for manufacturing a lithium ion secondary battery according to an embodiment of the present disclosure. FIG. 2 is a schematic plan view showing an example of the configuration of the negative electrode. FIG. 3 is a conceptual cross-sectional view showing an example of the configuration of the negative electrode. FIG. 4 is a schematic plan view showing an example of the configuration of the positive electrode. FIG. 5 is a schematic cross-sectional view showing an example of the configuration of the electrode group.

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

<Method for producing lithium ion secondary battery>
FIG. 1 is a flowchart illustrating an outline of a method for manufacturing a lithium ion secondary battery according to an embodiment of the present disclosure. The manufacturing method of the present embodiment includes “(A) production of negative electrode”, “(B) configuration of electrode group”, “(C) arrangement of Li metal”, “(D) pre-dope” and “(E) battery”. Including "manufacturing". That is, the manufacturing method of the present embodiment includes at least “(A) production of negative electrode”, “(C) arrangement of Li metal”, “(D) pre-dope” and “(E) production of battery”. “(C) Li metal arrangement” may be performed between “(A) Production of negative electrode” and “(B) Configuration of electrode group”.

  Below, the manufacturing method of a battery provided with a laminated electrode group is demonstrated as an example. However, the electrode group of this embodiment may be a wound type. Hereinafter, the manufacturing method of this embodiment will be described in order.

<< (A) Production of negative electrode >>
The manufacturing method of this embodiment includes manufacturing a negative electrode by disposing a negative electrode mixture layer containing a first negative electrode active material and a second negative electrode active material on the surface of a negative electrode current collector.

  FIG. 2 is a schematic plan view showing an example of the configuration of the negative electrode. The negative electrode 20 is a rectangular sheet. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22 disposed on the surface (front and back surfaces) of the negative electrode current collector 21.

  The negative electrode current collector 21 may be a copper (Cu) foil, for example. The Cu foil may be a pure Cu foil or a Cu alloy foil. The negative electrode current collector 21 may have a thickness of 5 to 30 μm, for example. A lead tab 24 is joined to the end of the negative electrode current collector 21. The lead tab 24 is made of nickel, for example.

  The negative electrode mixture layer 22 is formed by applying a negative electrode mixture paste to the surface of the negative electrode current collector 21 and drying it. The negative electrode mixture paste is prepared by mixing a first negative electrode active material, a second negative electrode active material, a binder, a solvent, and the like. An appropriate solvent should be selected depending on the binder type. The solvent may be water, for example. For coating and drying, for example, a die coater and a hot air drying furnace are used.

  FIG. 3 is a conceptual cross-sectional view showing an example of the configuration of the negative electrode. The negative electrode mixture layer 22 may be formed to have a thickness of 10 to 100 μm, for example. The negative electrode mixture layer 22 includes a first negative electrode active material 1 and a second negative electrode active material 2. The negative electrode mixture layer 22 may include, for example, a total of 80% by mass or more and 98% by mass or less of the first negative electrode active material and the second negative electrode active material.

(First negative electrode active material)
The first negative electrode active material 1 is a target of pre-doping. The first negative electrode active material 1 mainly contributes to the charge / discharge reaction of the battery. The first negative electrode active material 1 has a larger charge capacity than the second negative electrode active material 2 per unit mass. From the viewpoint of energy density, the first negative electrode active material 1 is preferably at least one selected from the group consisting of graphite, amorphous carbon, silicon, silicon oxide, and silicon alloy. More preferably, the first negative electrode active material 1 is at least one selected from the group consisting of silicon, silicon oxide, and silicon alloys.

  Examples of the amorphous carbon include easily graphitizable carbon and non-graphitizable carbon. Examples of the silicon alloy include a Si—Cu alloy and a Si—Cu—Sn alloy. The 1st negative electrode active material 1 may have an average particle diameter of 3-30 micrometers, for example. The average particle size indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method.

(Second negative electrode active material)
The second negative electrode active material 2 is lithium titanate (Li ₄ Ti ₅ O ₁₂ ). In the present embodiment, the potential difference between the second negative electrode active material 2 and the Li metal 23 is the potential difference between the first negative electrode active material 1 and the Li metal 23 in the negative electrode 20 (predetermined amount of charged negative electrode 20) that has been pre-doped. Bigger than. That is, the potential difference between the second negative electrode active material 2 and the Li metal 23 is kept large until pre-doping is completed. This is thought to reduce the time required for pre-doping.

  Lithium titanate can maintain a potential of about 1.5 V with respect to Li metal during pre-doping. Thereby, it is considered that the driving force of the pre-doping is suppressed. The 2nd negative electrode active material 2 may have an average particle diameter of 0.1-3 micrometers, for example.

  From the viewpoint of energy density, the smaller the second negative electrode active material 2, the better. From the viewpoint of the time required for pre-doping, the more the second negative electrode active material 2 is, the better. The second negative electrode active material 2 may have a mass ratio of 1% by mass to 5% by mass with respect to the total of the first negative electrode active material 1 and the second negative electrode active material 2. When the second negative electrode active material 2 has a mass ratio of 1% by mass or more and 5% by mass or less, a balance between the energy density and the time required for pre-doping is good. In the present specification, when the mass ratio calculation result has a value after the decimal point, the first decimal place is rounded off.

  The surface of the second negative electrode active material 2 is preferably covered with a Li ion permeable coating 3. Accordingly, it is considered that the potential of the second negative electrode active material 2 is suppressed from being reduced during pre-doping. The coating 3 preferably covers the entire surface of the second negative electrode active material 2. However, a part of the surface of the second negative electrode active material 2 that is not covered with the coating 3 may be present.

  The coating 3 is desirably insoluble in the solvent used for the negative electrode mixture paste. When the film 3 is dissolved in the solvent, the surface of the second negative electrode active material 2 is exposed, and the potential reduction suppressing effect may be reduced. The coating 3 may be, for example, polyvinylidene fluoride (PVdF), polyimide, or the like.

  The coating 3 may have a mass ratio of 3% by mass to 20% by mass with respect to the second negative electrode active material 2. When the coating 3 is 3% by mass or more, it is expected that the effect of suppressing the potential decrease is increased. When it exceeds 20 mass% of the film 3, since the 2nd negative electrode active material 2 is insulated from the periphery, the diffusion promotion effect of Li ion may become small.

(Other ingredients)
The negative electrode mixture layer 22 may include a conductive material, a binder, and the like that are not illustrated. The conductive material should not be particularly limited. The conductive material may be, for example, acetylene black, thermal black, furnace black, and the like. The negative electrode mixture layer 22 may include, for example, 1 to 10% by mass of a conductive material. The binder should not be particularly limited. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), or the like. The negative electrode mixture layer 22 may include, for example, 1 to 10% by mass of a binder.

<< (B) Structure of electrode group >>
The manufacturing method of this embodiment includes constituting an electrode group including a positive electrode and a negative electrode.

  First, a positive electrode is manufactured. FIG. 4 is a schematic plan view showing an example of the configuration of the positive electrode. The positive electrode 10 is a rectangular sheet. The positive electrode 10 includes a positive electrode current collector 11 and a positive electrode mixture layer 12 disposed on the surface (front and back surfaces) of the positive electrode current collector 11. The positive electrode current collector 11 may be, for example, an aluminum (Al) foil. The Al foil may be a pure Al foil or an Al alloy foil. The positive electrode current collector 11 may have a thickness of 10 to 30 μm, for example. A lead tab 14 is joined to the end of the positive electrode current collector 11. The lead tab 14 is made of, for example, Al.

  The positive electrode mixture layer 12 is formed by applying a positive electrode mixture paste to the surface of the positive electrode current collector 11 and drying it. The positive electrode mixture paste is prepared by mixing a positive electrode active material, a conductive material, a binder, a solvent, and the like. An appropriate solvent should be selected depending on the binder type. The solvent may be, for example, N-methyl-2-pyrrolidone (NMP). For coating and drying, for example, a die coater and a hot air drying furnace are used.

The positive electrode mixture layer 12 may have a thickness of 10 to 100 μm, for example. The positive electrode mixture layer 12 includes, for example, 85 to 98% by mass of a positive electrode active material, 1 to 10% by mass of a conductive material, and 1 to 5% by mass of a binder. The positive electrode active material should not be particularly limited. Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.80 Co _0.15 Al _0.05 O _2, LiFePO _{4, and the} like. May be. One type of positive electrode active material may be used alone, or two or more types of positive electrode active materials may be used in combination. The conductive material should not be particularly limited. The conductive material may be, for example, acetylene black. The binder should not be particularly limited. The binder may be, for example, PVdF, polytetrafluoroethylene (PTFE), or the like.

  FIG. 5 is a schematic cross-sectional view showing an example of the configuration of the electrode group. The positive electrode 10 and the negative electrode 20 are stacked with the separator 30 interposed therebetween. Thus, the electrode group 40 is configured. In FIG. 5, two positive electrodes 10 and one negative electrode 20 are laminated. However, this is merely an example, and a larger number of positive electrodes 10 and negative electrodes 20 may be stacked. The separator 30 is an electrically insulating porous film. The separator 30 can be constituted by, for example, a polyethylene (PE) porous film, a polypropylene (PP) porous film, or the like. For example, the positive electrode 10 and the negative electrode 20 may be bonded to the separator 30 with an adhesive.

<< (C) Arrangement of lithium metal >>
The manufacturing method of this embodiment includes disposing lithium metal at a position that is electrically connected to the negative electrode current collector and does not contact the negative electrode mixture layer. The Li metal 23 may be disposed before the electrode group 40 is configured, or may be disposed after the electrode group 40 is configured.

  Li metal 23 may be, for example, a Li foil. For example, the Li metal 23 is disposed on the surface of the negative electrode current collector 21. However, as shown in FIG. 2, the Li metal 23 needs to be disposed at a position where it does not come into contact with the negative electrode mixture layer 22. The Li foil can be bonded to the surface of the negative electrode current collector 21 by being pressed against the surface of the negative electrode current collector 21. The Li metal 23 may be an amount capable of generating Li ions exceeding the irreversible capacity of the first negative electrode active material 1.

<< (D) Pre-Dope >>
The manufacturing method of this embodiment includes charging a predetermined amount of the negative electrode by immersing the negative electrode and lithium metal in an electrolytic solution.

  The electrode group 40 is accommodated in a predetermined exterior body. The exterior body may be, for example, a metal container, an Al laminate film bag, or the like. An electrolytic solution is injected into the exterior body. The exterior body is sealed. Thereby, the negative electrode 20 and the Li metal 23 are immersed in the electrolytic solution.

  When the anode 20 and the Li metal 23 are immersed in the electrolytic solution, they are allowed to stand, so that Li ions are eluted from the Li metal 23 into the electrolytic solution, and Li ions are supplied from the electrolytic solution to the negative electrode 20. Is done. In the present embodiment, it is considered that the supply of Li ions is promoted because the second negative electrode active material 2 (lithium titanate) has a high potential. The pre-doping amount (charge amount) may be, for example, 1 to 50%, 1 to 30%, or 1 to 20% with respect to the charge capacity of the negative electrode 20. And it may be 1 to 10% or 1 to 5%.

  Preferably, the negative electrode 20, the Li metal 23, and the electrolytic solution are heated. This is thought to further promote the supply (diffusion) of Li ions. The negative electrode 20, the Li metal 23 and the electrolytic solution are preferably heated to about 45 ° C. to 70 ° C., more preferably 50 ° C. to 65 ° C., and even more preferably 55 ° C. to 65 ° C. The In the present embodiment, the time required for pre-doping may be, for example, about 27.5 to 37 days.

The electrolytic solution is a liquid electrolyte in which a Li salt is dissolved in a solvent. The solvent may be a mixed solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The Li salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ] (LiFSI) or the like. The electrolytic solution may contain additives such as vinylene carbonate (VC) and bis (oxalato) lithium borate (LiBOB).

<< (E) Battery Production >>
The manufacturing method of this embodiment includes manufacturing a lithium ion secondary battery including a negative electrode (pre-doped negative electrode) charged with a predetermined amount.

  After completion of pre-doping, the first charge / discharge is performed. Thereby, the positive electrode active material and the negative electrode active material are activated, and a lithium ion secondary battery is completed. In this embodiment, since pre-doping is performed, the irreversible capacity is small, and it is expected that the initial charge / discharge efficiency (= initial discharge capacity / initial charge capacity) is improved. That is, it is expected that the energy density of the battery is improved.

  Examples will be described below. However, the following examples do not limit the scope of the present disclosure.

<Example 1>
<< (A) Production of negative electrode >>
The following materials were prepared:
Second negative electrode active material: lithium titanate (average particle size: 0.5 μm)
Coating material: NMP solution of polyamic acid (solid content ratio: 10% by mass)

  With a planetary mixer, 60 parts by mass of lithium titanate (hereinafter sometimes abbreviated as “LTO”) and 40 parts by mass of the coating material were stirred and mixed for 1 hour. This prepared a paste. A stainless steel plate was prepared. The paste obtained above was applied to the surface of the stainless steel plate. As a result, a coating film having a thickness of about 0.5 mm was formed. The coating film was dried for 20 minutes in a thermostat set at 80 ° C. Next, the coating film and the stainless steel plate were placed in a heating furnace. The coating was heated at 250 ° C. for 30 minutes in an argon atmosphere. As a result, the polyamic acid was imidized to produce polyimide.

  Thereafter, the coating film was scraped off from the stainless steel plate. The stripped paint film pieces were crushed in a mortar. A mesh having an opening of 30 μm was prepared. Coarse particles were removed by passing the crushed material through a mesh. From the above, LTO coated with a polyimide coating was obtained. Hereinafter, the LTO covered with the coating is referred to as “coated LTO”.

The following materials were prepared:
First negative electrode active material: silicon oxide (average particle size: 5 μm)
Conductive material: Acetylene black Binder: CMC and SBR
Negative electrode current collector: Cu foil (thickness: 10 μm)
Solvent: ion exchange water

  With a planetary mixer, 83 parts by mass of silicon oxide (SiO), 3 parts by mass of coated LTO, 4 parts by mass of CMC, 5 parts by mass of SBR, 5 parts by mass of acetylene black and ion-exchanged water were mixed. Thereby, a negative electrode mixture paste was prepared. The solid content ratio of the negative electrode mixture paste was 43% by mass.

The negative electrode mixture paste was applied to the surface (both front and back surfaces) of the negative electrode current collector 21 by a comma coater (registered trademark) and dried at 120 ° C. Thereby, the negative electrode mixture layer 22 was formed. The mass per unit area of the negative electrode mixture layer 22 was 3.0 mg / cm ^{2 on} one side. The negative electrode mixture layer 22 was compressed by a roll press. The density of the negative electrode mixture layer 22 after compression was 1.4 g / cm ³ . The planar size of the negative electrode mixture layer 22 was 15.4 cm in the vertical dimension (corresponding to the dimension in the Y-axis direction in FIG. 2) and 10.4 cm in the lateral dimension (corresponding to the dimension in the X-axis direction in FIG. 2). Thus, negative electrode 20 was manufactured.

<< (B) Structure of electrode group >>
The following materials were prepared:
Positive electrode active material: LiCoO ₂ (average particle size: 10 μm)
Conductive material: Acetylene black Binder: PVdF
Solvent: NMP
Positive electrode current collector: Al foil (thickness: 15 μm)

With a planetary mixer, 90 parts by mass of LiCoO ₂ , 6 parts by mass of acetylene black, 4 parts by mass of PVdF, and NMP were mixed. Thus, a positive electrode mixture paste was prepared. The solid content ratio of the positive electrode mixture paste was 65% by mass.

The positive electrode mixture paste was applied to the surface (one surface) of the positive electrode current collector 11 by a comma coater (registered trademark) and dried at 120 ° C. As a result, the positive electrode mixture layer 12 was formed. The mass per unit area of the positive electrode mixture layer 12 was 3.9 mg / cm ^{2 on} one side. The positive electrode mixture layer 12 was compressed by a roll press. The density of the positive electrode mixture layer 12 after compression was 2.7 g / cm ³ . The planar size of the positive electrode mixture layer 12 was 15 cm in vertical dimension (corresponding to the dimension in the Y-axis direction in FIG. 4) and 10 cm in horizontal dimension (corresponding to the dimension in the X-axis direction in FIG. 4). Thus, the positive electrode 10 was manufactured.

  A separator 30 having a thickness of 25 μm was prepared. The planar size of the separator 30 was set to a vertical dimension of 16 cm and a horizontal dimension of 10.7 cm. The negative electrode 20 was sandwiched between the two positive electrodes 10. A separator 30 was disposed between the positive electrode 10 and the negative electrode 20. Thereby, the stacked electrode group 40 was configured.

<< (C) Li metal arrangement >>
In an argon atmosphere, 20 mg of Li metal 23 (Li foil) was attached to the surface of the negative electrode current collector 21. The Li metal 23 was disposed at a position not in contact with the negative electrode mixture layer 22. A bag made of an Al laminate film was prepared as an exterior body. The electrode group 40 was accommodated in the exterior body.

<< (D) Pre-Dope >>
An electrolyte solution containing the following components was prepared.
Solvent: [EC: EMC: DMC = 1: 1: 1 (volume ratio)]
Li salt: LiPF ₆ (1.1 mol / l)

  4 g of electrolyte was injected into the outer package. That is, the negative electrode 20 and the Li metal 23 were immersed in the electrolytic solution. The outer package was sealed with a vacuum sealing device. This produced a pre-activation battery. Similarly, a plurality of pre-activation batteries were manufactured. A plurality of pre-activation batteries were placed in a thermostat set at 60 ° C.

<< (E) Battery Production >>
Each time a certain time elapses, one pre-activation battery was taken out from the thermostatic chamber (60 ° C.), and the first charge / discharge was performed under the following conditions.
Temperature: 25 ° C
Charging: CCCV charging (CC current = 0.1A, CV voltage = 4.1V, end current = 5mA)
Discharge: CC discharge (CC current = 0.1A, end voltage = 2.5V)

The initial charge capacity was calculated by dividing the initial discharge capacity by the initial charge capacity. The first elapsed time when the initial charge / discharge efficiency was not improved by 0.5% or more was determined as the time required for pre-doping. The required time is shown in Table 1 below.
As described above, a lithium ion secondary battery was manufactured.

<Example 2>
A battery was produced by the same production method as in Example 1 except that the amount of the coated LTO was changed to 1 part by mass.

<Example 3>
A battery was produced by the same production method as in Example 1 except that the amount of the coated LTO was changed to 5 parts by mass.

<Example 4>
A battery is manufactured by the same manufacturing method as in Example 1 except that polycrystalline silicon (Si) having an average particle diameter of 6 μm is used as the first negative electrode active material instead of silicon oxide (SiO). It was done.

<Example 5>
A battery was produced by the same production method as in Example 1, except that an NMP solution of PVdF was used as a coating material instead of the polyamic acid NMP solution and the coating film was heated at 370 ° C. for 30 minutes. It was done.

<Comparative Example 1>
A battery was manufactured by the same manufacturing method as in Example 1 except that the negative electrode was manufactured so as not to contain the coated LTO (second negative electrode active material and film) and pre-doping was not performed.

<Comparative example 2>
A battery was manufactured by the same manufacturing method as in Example 4 except that the negative electrode was manufactured so as not to contain the coated LTO and pre-doping was not performed.

<Comparative Example 3>
A battery was produced by the same production method as in Example 1 except that the negative electrode was produced so as not to contain the coated LTO.

<Comparative example 4>
A battery was produced by the same production method as in Example 4 except that the negative electrode was produced so as not to contain the coated LTO.

<Example 6>
A battery was produced by the same production method as in Example 1 except that the coating treatment was not performed on the LTO.

<Result>
As shown in Table 1 above, in the example in which the second negative electrode active material (lithium titanate) was used, the pre-doping required as compared with Comparative Examples 3 and 4 in which the second negative electrode active material was not used. There is a tendency for time to be short. From the results of Example 1 and Example 6, it is recognized that the pre-doping time tends to be further shortened when the second negative electrode active material is covered with a Li ion permeable film (polyimide or the like).

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

  DESCRIPTION OF SYMBOLS 1 1st negative electrode active material, 2nd 2nd negative electrode active material, 3 film | membrane, 10 positive electrode, 11 positive electrode collector, 12 positive electrode composite material layer, 20 negative electrode, 21 negative electrode current collector, 22 negative electrode composite material layer, 23 Li metal , 14, 24 Lead tab, 30 separator, 40 electrode group.

Claims (1)

Producing a negative electrode by disposing a negative electrode mixture layer containing a first negative electrode active material and a second negative electrode active material on the surface of the negative electrode current collector;
Disposing lithium metal at a position electrically connected to the negative electrode current collector and not in contact with the negative electrode mixture layer;
Charging at least a predetermined amount of the negative electrode by immersing the negative electrode and the lithium metal in an electrolyte, and producing a lithium ion secondary battery comprising the negative electrode charged at a predetermined amount;
The second negative electrode active material is lithium titanate;
The first negative electrode active material has a larger charge capacity per unit mass than the second negative electrode active material,
In the negative electrode charged by a predetermined amount, a potential difference between the second negative electrode active material and the lithium metal is larger than a potential difference between the first negative electrode active material and the lithium metal.
A method for producing a lithium ion secondary battery.

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