JP2018073464A - Method for manufacturing lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery in which the entry of 1,2-dimethoxy ethane together with lithium ions is suppressed.SOLUTION: A method for manufacturing a lithium ion secondary battery comprises the steps of: (A) manufacturing a negative electrode including graphite; (B) forming an electrode group including the negative electrode and a positive electrode; (C) preparing an electrolyte solution including ethylene carbonate and no 1,2-dimethoxy ethane; (D) impregnating the electrode group with the electrolyte solution; (E) charging the electrode group impregnated with the electrolyte solution, thereby causing the reductive decomposition of at least part of the ethylene carbonate; and (F) adding 1,2-dimethoxy ethane to the electrolyte solution after the charging step, thereby manufacturing the lithium ion secondary battery.SELECTED DRAWING: Figure 1

Description

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

  JP-A-4-284373 (Patent Document 1) discloses an electrolytic solution containing both ethylene carbonate (EC) and 1,2-dimethoxyethane (DME).

JP-A-4-284373

  DME is known as an electrolyte solvent. Since DME has a large number of donors, it promotes dissociation of lithium (Li) salts. Furthermore, DME has a low viscosity. Therefore, it is expected that the ionic conductivity of the electrolytic solution is improved by using DME as the electrolytic solution solvent.

  However, DME tends to cause co-insertion. “Co-insertion” refers to a phenomenon in which solvent molecules are inserted between graphite layers together with Li ions. Since DME has a large number of donors, it is considered that the desolvation of Li ions hardly proceeds. As the solvent molecules are inserted between the graphite layers, the graphite layers are peeled off. As a result, the charge / discharge capacity decreases.

  Therefore, an object of the present disclosure is to provide a lithium ion secondary battery in which co-insertion of DME is suppressed.

The manufacturing method of the lithium ion secondary battery of this indication contains the following (A)-(F).
(A) A negative electrode containing graphite is produced.
(B) An electrode group including a negative electrode and a positive electrode is formed.
(C) An electrolytic solution containing ethylene carbonate and not containing 1,2-dimethoxyethane is prepared.
(D) The electrode group is impregnated with an electrolytic solution.
(E) At least a part of ethylene carbonate is reductively decomposed by charging the electrode group impregnated with the electrolytic solution.
(F) A lithium ion secondary battery is manufactured by adding 1,2-dimethoxyethane to the electrolytic solution after charging.

  In the production method of the present disclosure, as shown in the above (C) and (D), the electrode group is first impregnated with an electrolytic solution containing EC. At this time, the electrolytic solution does not contain DME. Thereafter, as shown in (E) above, the first charge is performed on the electrode group. By charging, Li ions are inserted between the graphite layers from the edge surface of the graphite.

  By charging, the potential of the negative electrode decreases. When the potential of the negative electrode falls to a predetermined potential, a part of EC starts to be reduced and decomposed. The decomposition product of EC forms a film on the edge surface of graphite. This coating is called SEI (Solid Electrolyte Interface). SEI promotes desolvation of solvated Li ions as solvated Li ions pass through SEI.

  In the manufacturing method of the present disclosure, as shown in the above (F), after the SEI is formed, DME is added to the electrolytic solution. SEI promotes the desolvation of Li ions solvated in DME. Therefore, it is considered that co-insertion of DME is suppressed in the second and subsequent charging of the lithium ion secondary battery.

From the beginning, when an electrolyte containing both EC and DME is used, it is considered that DME co-insertion cannot be suppressed. The formation of SEI by the reductive decomposition of EC is 0.6 V [vs. Li / Li ⁺ ] is considered to start. The co-insertion of DME is 0.9V [vs. Li / Li ⁺ ] is considered to start. That is, the DME co-insertion potential is considered to be higher than the SEI formation potential. Therefore, in an electrolyte solution containing both EC and DME, it is considered that co-insertion of DME proceeds before the formation of SEI.

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 diagram illustrating an example of the configuration of the electrode group. FIG. 3 is a schematic cross-sectional view showing an example of the configuration of the lithium ion secondary battery. FIG. 4 is a graph showing the relationship between the ratio of DME in the final solvent composition and the discharge capacity when the second capacity is confirmed. FIG. 5 is a graph showing the relationship between the ratio of DME in the final solvent composition and the IV resistance.

  Hereinafter, an embodiment of the present disclosure (hereinafter referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of the present disclosure. Hereinafter, a lithium ion secondary battery may be abbreviated as “battery”.

<Method for producing lithium ion secondary battery>
FIG. 1 is a flowchart showing an outline of a method for manufacturing a lithium ion secondary battery of the present embodiment. The manufacturing method of this embodiment includes (A) manufacture of negative electrode, (B) configuration of electrode group, (C) preparation of electrolytic solution, (D) impregnation of electrolytic solution, (E) initial charge, (F) DME Includes additions. Hereinafter, the manufacturing method of the present disclosure will be described in order.

<< (A) Production of negative electrode >>
The manufacturing method of this embodiment includes manufacturing the negative electrode 20 containing (A) graphite.
FIG. 2 is a schematic diagram illustrating an example of the configuration of the electrode group. The negative electrode 20 is a strip-shaped sheet. The negative electrode 20 can be manufactured by a conventionally known method. The negative electrode 20 can be manufactured by the following procedure, for example. A negative electrode mixture paste is prepared. The negative electrode mixture paste is applied to the surface of the current collector foil 21 and dried, whereby the negative electrode mixture layer 22 is formed. Thereby, the negative electrode 20 is manufactured. For example, a die coater or the like can be used for coating. The current collector foil 21 may be a copper (Cu) foil, for example. The Cu foil may be a pure Cu foil or a Cu alloy foil. The current collector foil 21 may have a thickness of 5 to 30 μm, for example.

  The negative electrode 20 is processed into a predetermined dimension according to the specifications of the electrode group 50. Processing here includes rolling and cutting. For example, the negative electrode 20 may be rolled so that the negative electrode mixture layer 22 has a thickness of 10 to 150 μm.

  The negative electrode mixture paste is prepared by mixing graphite, a conductive material, a binder, a solvent, and the like. For mixing, for example, a planetary mixer or the like is used. The solid content of the negative electrode mixture paste includes, for example, 90 to 99.5% by mass of graphite, 0 to 5% by mass of a conductive material, and 0.5 to 5% by mass of a binder. Solid content shows components other than a solvent. The graphite may have an average particle diameter of 1 to 20 μm, 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.

  Graphite functions as a negative electrode active material. Graphite may be coated with amorphous carbon. The negative electrode 20 of this embodiment may further include other negative electrode active materials as long as it includes graphite. Examples of other negative electrode active materials include graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, tin, and tin oxide. The conductive material may be, for example, acetylene black or furnace black. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), or the like. The solvent of the paste is selected according to the binder. For example, when CMC and SBR are used, water can be used as the solvent.

<< (B) Structure of electrode group >>
The manufacturing method of the present embodiment includes (B) configuring an electrode group 50 including the negative electrode 20 and the positive electrode 10.

(Manufacture of positive electrode)
The positive electrode 10 is a strip-shaped sheet. The positive electrode 10 can be manufactured by a conventionally known method. The positive electrode 10 can be manufactured, for example, by the following procedure. A positive electrode mixture paste is prepared. The positive electrode mixture paste is applied to the surface of the current collector foil 11 and dried, whereby the positive electrode mixture layer 12 is formed. Thereby, the positive electrode 10 is manufactured. For example, a die coater or the like can be used for coating. The current collector foil 11 may be, for example, an aluminum (Al) foil. The Al foil may be a pure Al foil or an Al alloy foil. The current collector foil 11 may have a thickness of 5 to 30 μm, for example.

  The positive electrode 10 is processed into a predetermined dimension according to the specifications of the electrode group 50. Processing here includes rolling and cutting. For example, the positive electrode 10 may be rolled so that the positive electrode mixture layer 12 has a thickness of 10 to 150 μm.

  The positive electrode mixture paste is prepared by mixing a positive electrode active material, a conductive material, a binder, a solvent, and the like. For mixing, for example, a planetary mixer or the like can be used. The solid content of the positive electrode mixture paste includes, for example, 80 to 98% by mass of a positive electrode active material, 1 to 15% by mass of a conductive material, and 1 to 5% by mass of a binder.

The positive electrode active material may have an average particle diameter of 1 to 20 μm, for example. The positive electrode active material should not be particularly limited. The positive electrode active material may be, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiFePO ₄ or the like. A positive electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type. The conductive material may be, for example, carbon black such as acetylene black (AB) or furnace black, vapor grown carbon fiber (VGCF), or the like. For example, the binder may be polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like. The solvent for the paste is selected according to the type of binder. For example, when PVdF is used, N-methyl-2-pyrrolidone (NMP) can be used as the solvent.

(Configuration of electrode group)
The electrode group 50 includes the negative electrode 20 and the positive electrode 10. The electrode group 50 typically further includes a separator 30. For example, the positive electrode 10 and the negative electrode 20 are laminated so as to face each other with the separator 30 interposed therebetween, and are further wound in the longitudinal direction of each member, whereby the electrode group 50 can be configured. The electrode group 50 may be formed in a flat shape. Here, the wound electrode group 50 is described as an example. The electrode group may be a stacked electrode group. A stacked electrode group can be configured by alternately stacking positive and negative electrodes with a separator in between.

  The separator 30 is a belt-like porous film. Separator 30 may have a thickness of, for example, 5 to 30 μm. The separator 30 is electrically insulating. For example, a resin such as polyethylene (PE) or polypropylene (PP) can constitute the separator 30. The separator may have a multilayer structure. For example, the separator 30 may be configured by laminating a PP porous film, a PE porous film, and a PP porous film in this order.

<< (C) Preparation of electrolyte >>
The production method of the present embodiment includes (C) preparing an electrolytic solution containing ethylene carbonate (EC) and not containing 1,2-dimethoxyethane (DME).

  The electrolytic solution is prepared by dispersing and dissolving Li salt in a solvent. The solvent at this point is prepared to contain EC and no DME. That is, the electrolyte solution at this time is prepared so as to contain EC and not DME. The electrolyte solution finally contains DME by “addition of (F) DME” described later. EC is reductively decomposed in “(E) initial charge” described later. The decomposition product of EC forms SEI on the edge surface of graphite. SEI is thought to suppress DME co-insertion.

  The solvent is aprotic. The solvent is preferably prepared so as to contain 33.3 vol% or more and 60 vol% or less of EC, and the remaining low viscosity solvent. When EC is 33.3 vol% or more, formation of SEI tends to be promoted. However, EC has a high viscosity. For this reason, when the EC ratio is high, there may be inconveniences such as a long time required for impregnation. When EC is 60 vol% or less, it can be suppressed to an appropriate viscosity. The solvent more preferably contains 37.5 vol% or more and 60 vol% or less EC, and even more preferably 42.9 vol% or more and 60 vol% or less EC. The volume ratio (vol%) of the solvent can be measured by a gas chromatograph mass spectrometer (GCMS).

  The low-viscosity solvent of the present embodiment is a solvent other than DME and has a viscosity at 25 ° C. of 1.0 mPa · s or less. Examples of the low viscosity solvent include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). A low viscosity solvent may be used individually by 1 type, and 2 or more types may be used in combination.

In the present embodiment, for example, LiPF _6, LiBF _4, Li salts such as _{Li [N (FSO 2) 2} ] can be used. Li salt may be used individually by 1 type, and 2 or more types may be used in combination. The electrolytic solution is prepared so as to contain, for example, 0.5 to 2.5 mol / l Li salt.

<< (D) Impregnation of electrolyte >>
The manufacturing method of the present embodiment includes (D) impregnating the electrode group 50 with an electrolytic solution. Typically, the electrode group 50 is impregnated with the electrolytic solution while the electrode group 50 is housed in the housing 90.

  FIG. 3 is a schematic cross-sectional view showing an example of the configuration of the lithium ion secondary battery. The housing 90 is a container that stores the electrode group 50 and the electrolytic solution. The casing 90 may be rectangular (flat rectangular parallelepiped) or cylindrical. For example, a metal such as Al alloy or stainless steel (SUS) can constitute the housing 90. However, a metal and resin composite material or a resin alone may constitute the casing as long as it has a predetermined sealing property. Examples of the composite material of metal and resin include an aluminum laminate film.

  The housing 90 includes a case 91 and a lid 92. The case 91 includes a bottom, a side wall, and an opening. The side wall continues to the bottom. The opening is located on the opposite side of the bottom. The lid 92 is configured to close the opening. The lid 92 may include an external terminal 80, a liquid injection hole, and the like. The liquid injection hole is a hole for pouring the electrolytic solution into the housing 90. The liquid injection hole can be configured to be sealed by a stopper, for example. Case 91 and lid 92 are joined by, for example, laser welding, crimping, or the like. The housing 90 may include a safety valve, a current interrupt mechanism (CID), and the like.

  The electrode group 50 is housed in the housing 90. The electrode group 50 is connected to the external terminal 80. For example, a predetermined amount of electrolytic solution is poured into the housing 90 from the liquid injection hole. Before the case 91 and the lid 92 are joined, the electrolytic solution may be poured from the opening of the case 91. The electrolyte solution poured into the housing 90 is impregnated in the electrode group 50. The impregnation may be performed in a temperature environment of about 20 to 40 ° C. Impregnation may be performed in an atmospheric pressure environment, a reduced pressure environment, or a pressurized environment.

<< (E) Initial charge >>
The manufacturing method of this embodiment includes (E) reductively decomposing at least a part of EC by charging the electrode group 50 impregnated with the electrolytic solution.

In the electrode group 50, for example, the potential of the negative electrode 20 is 0.6 V [vs. Li / Li ⁺ ] is charged to Thereby, reductive decomposition of EC is promoted. However, as long as at least a part of EC is reductively decomposed, charging conditions should not be particularly limited. During charging, the housing 90 may be in an open state or in a sealed state.

The potential of the negative electrode 20 is theoretically 0 V [vs. Li / Li ⁺ ] or less. That is, in the electrode group 50, the potential of the negative electrode 20 is 0 V [vs. Li / Li ⁺ ] higher than 0.6 V [vs. Li / Li ⁺ ] or less. In the electrode group 50, the potential of the negative electrode 20 is 0.4 V [vs. Li / Li ⁺ ] or less, and the potential of the negative electrode 20 is 0.2 V [vs. Li / Li ⁺ ] or less, or the potential of the negative electrode 20 is 0.1 V [vs. Li / Li ⁺ ] or less may be charged. The potential of the negative electrode 20 is a potential difference between the negative electrode 20 and lithium metal in a state where the negative electrode 20 and lithium metal (reference electrode) are in liquid junction. The potential difference can be measured with a common voltmeter.

  Charging is performed by a general charging / discharging device. The charging may be constant current charging, constant voltage charging, or charging in which constant current charging and constant voltage charging are combined. However, uniform SEI tends to be easily formed by constant current charging.

  Here, the current rate at which the state of charge (SOC) of the lithium ion secondary battery reaches 0% to 100% after one hour of charging is defined as “1C”. The current rate during charging may be, for example, 0.1 C or more and 2.0 C or less. The current rate during charging is preferably 0.1 C or more and 0.5 C or less. The current rate may be less than 0.1C. In this case, however, charging takes a long time. If the current rate exceeds 0.5 C, depending on the battery specifications, there may be spots in the SEI. The current during charging can be measured by an ammeter attached to the charging / discharging device.

  Charging may be performed, for example, in a temperature environment of 0 ° C. or higher and 45 ° C. or lower. Charging is preferably performed in a temperature environment of 20 ° C. or higher and 45 ° C. or lower. Charging may be performed in a temperature environment of less than 20 ° C. However, depending on the battery specifications, the SEI may be uneven. Charging may be performed in a temperature environment higher than 45 ° C. However, depending on the composition of the solvent, the oxidative decomposition of the solvent may become active. The temperature environment during charging can be controlled by charging the battery in a temperature-controlled room. For example, if the temperature in the temperature-controlled room is 20 ° C., it is considered that charging is performed in a temperature environment of 20 ° C. The temperature in the temperature-controlled room can be measured with a general thermometer.

<< (F) Addition of DME >>
The manufacturing method of this embodiment includes (F) manufacturing a lithium ion secondary battery by adding DME to the electrolytic solution after charging. In the present embodiment, the electrode group 50 may be discharged between “(E) initial charge” and “(F) addition of DME”. Further, after “(E) initial charge”, a part of the electrolytic solution may be discharged from the housing 90. After DME is added to the electrolytic solution, the housing 90 is sealed. Thereby, a lithium ion secondary battery is completed.

  The DME added to the electrolyte is mixed with the electrolyte and becomes a part of the electrolyte. As a result, the electrolytic solution has a final solvent composition. Preferably, DME is added so that DME is 10 vol% or more and 50 vol% or less in the final solvent composition. When DME is 10 vol% or more, an improvement in ionic conductivity is expected. DME is more preferably 20% by volume or more, still more preferably 30% by volume or more, and most preferably 40% by volume or more.

  In the final solvent composition, EC may be, for example, 10 vol% or more and 40 vol% or less. In the final solvent composition, the low-viscosity solvent (excluding DME) may be, for example, 10% by volume or more and 80% by volume or less. The electrolytic solution may finally contain 0.5 to 2.0 mol / l Li salt.

  In the present embodiment, as long as DME is added to the electrolytic solution, a mixture of DME and another solvent may be added to the electrolytic solution. Examples of other solvents include EC, DMC, EMC, DEC, and propylene carbonate (PC). Or the mixture (namely, electrolyte solution containing DME) of DME and Li salt may be added.

<Applications of lithium ion secondary batteries>
The lithium ion secondary battery of this embodiment contains DME as an electrolyte solution solvent. Therefore, the ionic conductivity of the electrolytic solution is high. That is, the lithium ion secondary battery of this embodiment is expected to show a high output. In addition, in the lithium ion secondary battery of the present embodiment, the capacity reduction due to co-insertion of DME is small. That is, the lithium ion secondary battery of this embodiment can have a high capacity. Therefore, the lithium ion secondary battery of the present embodiment is suitable as a power source for hybrid vehicles (HV), plug-in hybrid vehicles (PHV), and electric vehicles (EV). However, the use of the lithium ion secondary battery of this embodiment should not be limited to such in-vehicle use. The lithium ion secondary battery of this embodiment is applicable to all uses.

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

<Example 1>
<< (A) Production of negative electrode >>
The following materials were prepared:
Negative electrode active material: Graphite (average particle size: 10 μm)
Binder: CMC and SBR
Solvent: Water Current collector foil: Cu foil (thickness: 10 μm)

  A negative electrode mixture paste was prepared by mixing graphite, CMC, SBR and water. The mass ratio of graphite, CMC and SBR was “graphite: CMC: SBR = 99: 0.5: 0.5”. The negative electrode paste was applied to the surface of the Cu foil and dried to form a negative electrode mixture layer. This produced a negative electrode. The negative electrode was rolled and cut into strips. The width of the negative electrode mixture layer was 105 mm.

<< (B) Structure of electrode group >>
The following materials were prepared:
Positive electrode active material: LiNiCoMnO ₂ (average particle size: 5 μm)
Conductive material: AB
Binder: PVdF
Solvent: NMP
Current collector foil: Al foil (thickness: 15 μm)

A positive electrode mixture paste was prepared by mixing LiNiCoMnO ₂ (hereinafter abbreviated as “NCM”), AB, PVdF, and NMP. The mass ratio of NCM, AB and PVdF was “NCM: AB: PVdF = 92: 5: 3”. The positive electrode mixture paste was applied to the surface of the Al foil and dried to form a positive electrode mixture layer. This produced a positive electrode. The positive electrode was rolled and cut into strips. The width of the positive electrode mixture layer was 100 mm.

  A strip separator was prepared. The separator has a three-layer structure. The separator is formed by laminating a porous film of PP, a porous film of PE, and a porous film of PP in this order. The separator and the negative electrode were laminated so that the positive electrode and the negative electrode face each other, and further wound. Thus, a wound electrode group was formed.

<< (C) Preparation of electrolyte >>
An electrolyte solution containing the following components was prepared.
Li salt: LiPF ₆ (1.11 mol / l)
Solvent: EC (33.3 vol%), EMC (66.7 vol%)

<< (D) Impregnation of electrolyte >>
A square housing was prepared. The housing includes a case and a lid. The lid includes a liquid injection hole and an external terminal. An electrode group was connected to the external terminal. An electrode group was inserted into the case. The case and lid were joined by laser welding. A predetermined amount of electrolytic solution was poured from the injection hole. The electrolyte solution was impregnated in the electrode group.

<< (E) Initial charge >>
The injection hole was sealed with a stopper. A charge / discharge device was prepared. An external terminal was connected to the charge / discharge device. In a temperature environment of 25 ° C., the electrode group was charged to 4.1 V at a current rate of 0.2 C. Charging is constant current charging. Thereby, a part of EC was reductively decomposed. Next, the electrode group was discharged to 3.0 V at a current rate of 0.2 C. The discharge is a constant current discharge. The discharge capacity is shown in the column “First capacity check” in Table 1 below.

<< (F) Addition of DME >>
The injection hole was opened by removing the stopper. A predetermined amount of DME was poured into the casing from the liquid injection hole. That is, DME was added to the electrolyte. DME was mixed in the electrolyte. Thus, a battery was manufactured. This battery is a prismatic lithium ion secondary battery having a design capacity of 5 Ah. This battery is configured to operate in a voltage range of 3.0 to 4.1V.

The electrolytic solution finally includes the following components.
Li salt: LiPF ₆ (1.0 mol / l)
Solvent: EC (30 vol%), EMC (60 vol%), DME (10 vol%)

<Examples 2 to 5>
The electrolyte solution having the solvent composition shown in the column of “(D) Electrolyte solution impregnation” in Table 1 below is prepared, and the addition amount of DME is changed in “(F) Addition of DME”. A battery was produced by the same production method as in Example 1 except that the typical solvent composition was changed.

<Examples 6 to 9>
As shown in Table 1 below, a battery was manufactured by the same manufacturing method as in Example 4 except that the current rate was changed in “(E) Initial charge”.

<Examples 10 to 13>
As shown in Table 1 below, a battery was manufactured by the same manufacturing method as in Example 4 except that the temperature environment was changed in “(E) Initial charging”.

<Comparative Examples 1-6>
An electrolyte solution containing the solvent shown in the column of “(D) Electrolyte solution impregnation” in Table 1 below and 1.0 mol / l Li salt was prepared. After the electrolyte group was impregnated with the electrode group, the casing was sealed. After the housing was sealed, the electrode group was charged. A battery was manufactured by the same manufacturing method as in Example 1 except for these. Comparative Example 1 corresponds to an example in which an electrolytic solution containing no DME is used. Comparative Examples 2 to 6 correspond to examples in which an electrolytic solution containing both EC and DME was used from the beginning.

<Evaluation>
《Second capacity check》
The capacity of the finished battery was confirmed again. In a temperature environment of 25 ° C., the battery was charged to 4.1 V at a current rate of 0.2 C. Charging is constant current charging. The battery was then discharged to 3.0V at a current rate of 0.2C. The discharge is a constant current discharge. Thereby, the discharge capacity was measured. The results are shown in the column “Second capacity check” in Table 1 below. It shows that the larger the capacity, the more the DME co-insertion is suppressed.

<< Measurement of IV resistance >>
The SOC of the battery was adjusted to 60%. The battery was discharged for 10 seconds at a current rate of 20 C in a temperature environment of 25 ° C. The discharge is a constant current discharge. The amount of voltage drop after 10 seconds of discharge was measured. The IV resistance was calculated by dividing the voltage drop by the current. The results are shown in the column of “IV resistance” in Table 1 below. The lower the IV resistance, the higher the ionic conductivity of the electrolyte.

<Result>
FIG. 4 is a graph showing the relationship between the ratio of DME in the final solvent composition and the discharge capacity when the second capacity is confirmed. FIG. 4 shows the results of Examples 1 to 5 and the results of Comparative Examples 1 to 6. As shown in FIG. 4, in the comparative example, the discharge capacity is greatly reduced as the amount of DME increases. In the comparative example, it is considered that the irreversible capacity is increased by co-insertion of DME. On the other hand, in the embodiment, the discharge capacity is substantially constant with respect to the ratio of DME. In the example, since DME is added after SEI is formed, it is considered that co-insertion of DME is suppressed.

  FIG. 5 is a graph showing the relationship between the ratio of DME in the final solvent composition and the IV resistance. FIG. 5 shows the results of Examples 1 to 5 and the results of Comparative Examples 1 to 6. As shown in FIG. 5, in the comparative example, when the ratio of DME exceeds 20 vol%, the IV resistance is significantly increased. The increase in IV resistance is thought to be due to co-insertion of DME. On the other hand, in the embodiment, the IV resistance decreases as the amount of DME increases. Since co-insertion of DME is suppressed, it is considered that the effect of improving the ionic conductivity by DME is easily exhibited.

  In Table 1, from the results of Examples 4 and 6 to 9, when the current rate of “(E) initial charge” is 0.1 C or more and 0.5 C or less, a tendency that the IV resistance is lowered is recognized. This is probably because the SEI is more easily formed as the current rate is lower.

  In Table 1, from the results of Examples 4 and 10 to 13, when the temperature environment of “(E) initial charge” is less than 20 ° C., the IV resistance tends to be slightly increased. This is probably because the reductive decomposition reaction of EC becomes inactive at low temperatures. On the other hand, in a temperature environment exceeding 45 ° C., there is a possibility that the oxidative decomposition reaction of the electrolytic solution becomes active. Therefore, the temperature environment of “(E) initial charge” is preferably 20 ° C. or higher and 45 ° C. or lower.

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

  DESCRIPTION OF SYMBOLS 10 Positive electrode, 11, 21 Current collecting foil, 12 Positive electrode compound material layer, 20 Negative electrode, 22 Negative electrode compound material layer, 30 Separator, 50 Electrode group, 80 External terminal, 90 Case, 91 Case, 92 Lid.

Claims (1)

Producing a negative electrode comprising graphite,
Constituting an electrode group including the negative electrode and the positive electrode;
Preparing an electrolyte solution containing ethylene carbonate and free of 1,2-dimethoxyethane;
Impregnating the electrode group with the electrolyte solution;
By charging the electrode group impregnated with the electrolytic solution, at least a part of the ethylene carbonate is reduced and decomposed, and after charging, the 1,2-dimethoxyethane is added to the electrolytic solution. Including producing an ion secondary battery,
A method for producing a lithium ion secondary battery.