JP2012064820A - Manufacturing method for lithium ion capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion capacitor excellent in long-term stability and low-temperature characteristics by including propylene carbonate in organic electrolyte and inhibiting a reductive reaction of the propylene carbonate.SOLUTION: In a lithium ion capacitor that comprises a positive electrode including porous material; a negative electrode including carbon material; a separator; a lithium electrode; and an organic electrolyte, propylene carbonate is included in the organic electrolyte after predope processing is applied to the negative electrode in the organic electrolyte excluding propylene carbonate.

Description

  The present invention relates to a method for manufacturing a lithium ion capacitor, particularly a technique for impregnating an organic electrolyte, with respect to a lithium ion capacitor suitable for large current charge / discharge applications.

  An electric double layer capacitor capable of charging and discharging a large current includes an organic electrolyte solution impregnated in a polarizable electrode in addition to a polarizable electrode using activated carbon as an active material for both the positive electrode and the negative electrode. Such an electric double layer capacitor has a capacity density of 1/10 or less of a secondary battery such as a lead storage battery, a nickel hydride battery, or a lithium battery, and therefore further improvement in the capacity density is required.

  Under these circumstances, as one of the electrochemical capacitors that require an increase in capacity density, a so-called lithium ion capacitor has been proposed in which activated carbon is used for the positive electrode and graphite is used for the negative electrode, and the configuration of the positive electrode and the negative electrode is asymmetric. (For example, refer to Patent Document 1).

  In this lithium ion capacitor, an organic electrolytic solution in which a lithium salt is dissolved in an organic solvent is used. As the organic solvent, cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as diethyl carbonate and dimethyl carbonate are used. Of these, propylene carbonate is generally known as an excellent solvent in terms of physical properties such as electrical conductivity, potential window, freezing point, boiling point, and chemical stability such as hydrolysis resistance.

  On the other hand, since the lithium ion capacitor has a higher withstand voltage than the above-described general electric double layer capacitor, charging and discharging are performed at a high voltage. Then, in the first charge / discharge, the solvent contained in the organic electrolyte reacts with the graphite of the negative electrode, and a film of the reaction product is formed on the surface of the negative electrode. Such a film is called a SEI (Solid Electrolyte Interface) film and acts as a chemically stable passivation layer. Once formed, the SEI film inhibits the reduction reaction of the solvent on the graphite surface of the negative electrode. Therefore, the lithium ion capacitor can realize excellent long-term stability as well as high withstand voltage.

  Further, in the lithium ion capacitor, a pre-doping process is performed in which lithium ions are occluded in the graphite of the negative electrode to increase the capacitance of the negative electrode. And the SEI film | membrane is formed in the graphite surface of a negative electrode by the charging / discharging performed in the process of this pre dope process.

JP 2007-288017 A (published November 1, 2007)

  However, in the lithium ion capacitor, when propylene carbonate is used as the solvent contained in the organic electrolyte, the SEI film is not formed on the negative electrode. Therefore, in a lithium ion capacitor using propylene carbonate, the reduction reaction of propylene carbonate continuously occurs at the negative electrode during charging, and gas is generated, thereby impairing the long-term stability of the lithium ion capacitor.

  Under these circumstances, in the conventional lithium ion capacitor, propylene carbonate is not contained in the organic electrolyte, and it is the mainstream to use ethylene carbonate. Here, since ethylene carbonate has a higher freezing point than propylene carbonate, there is a problem that the low temperature characteristics of the lithium ion capacitor are impaired. Moreover, since the organic electrolyte does not contain propylene carbonate with excellent chemical stability, the reaction of the solvent accompanying charging and discharging of the lithium ion capacitor increases, and the reaction product, especially the long-term stability due to gas generation, is improved. There are concerns about adverse effects.

  Therefore, the present invention has been made to solve the above problems, and its purpose is to contain propylene carbonate in the organic electrolyte, suppress the reduction reaction of propylene carbonate on the surface of the negative electrode, The object is to provide a lithium ion capacitor having excellent low-temperature characteristics.

  In order to solve the above problems, the method for producing a lithium ion capacitor of the present invention can occlude and desorb lithium ions, and a positive electrode including a porous material capable of reversibly adsorbing and desorbing lithium ions and / or anions. A method for producing an electricity storage device comprising: a negative electrode including a carbon material; a lithium electrode that supplies lithium ions; a separator interposed between the positive electrode and the negative electrode that are stacked; and an organic electrolyte that includes a lithium salt. The organic electrolyte containing no propylene carbonate includes a step of performing a pre-doping process for occluding lithium ions in the negative electrode, and then a step of including propylene carbonate in the organic electrolyte.

  According to the present invention, since the propylene carbonate is included in the organic electrolyte after the SEI film is formed on the surface of the negative electrode by the pre-doping treatment, the reduction reaction of propylene carbonate on the surface of the negative electrode is suppressed by the SEI film. Therefore, generation of gas or the like due to the reaction is also suppressed. For this reason, a lithium ion capacitor excellent in long-term stability and low temperature characteristics can be realized.

  Moreover, in the said invention, it is preferable that the porous material contained in the said positive electrode is activated carbon, and the carbon material contained in the said negative electrode is graphite.

  Moreover, in the said invention, in the step which performs the said pre dope process, it is preferable that the said organic electrolyte solution which does not contain propylene carbonate contains either or multiple in cyclic carbonates or chain carbonates except propylene carbonate.

  By producing a lithium ion capacitor using the method for producing a lithium ion capacitor of the present invention, the solvent contains propylene carbonate, suppresses the reduction reaction of propylene carbonate on the surface of the negative electrode, and has excellent long-term stability and low temperature characteristics. A lithium ion capacitor can be obtained.

It is a schematic diagram which shows the structure of the lithium ion capacitor in one Embodiment of this invention. It is a figure which shows the evaluation result of the pre dope process in one Embodiment of this invention. It is a figure which shows the evaluation result of the pre dope process in one Embodiment of this invention. It is a figure which shows the evaluation result of long-term stability in one Embodiment of this invention.

  An example of an embodiment in the method for producing a lithium ion capacitor of the present invention will be described with reference to FIG. Referring to FIG. 1, the lithium ion capacitor according to the present embodiment includes an electrode laminate including a polarizable electrode 1, a positive electrode current collector 2, a carbon electrode 3, a negative electrode current collector 4, and a separator 5. 6, a lithium metal 7, and a lithium electrode 9 composed of a lithium electrode current collector 8. Further, the electrode laminate 6 is impregnated with an organic electrolytic solution not containing propylene carbonate before the pre-doping treatment, and propylene carbonate is contained after the pre-doping treatment.

  The polarizable electrode 1 used for the positive electrode includes a porous material capable of adsorbing and desorbing lithium ions and / or anions as an active material.

The porous material is preferably activated carbon having a specific surface area of 800 to 3000 m ² / g, and other carbon nanomaterials, mesoporous carbon, etc. having a large specific surface area and conductivity can be used. As a raw material of activated carbon, palm resin, phenol resin, coke, or the like can be used, and it is preferably activated by a steam activation method, an alkali activation method, or the like.

  In order to increase the electric conductivity, the polarizable electrode 1 preferably contains a conductive additive. The conductive assistant is carbon black, graphite, or vapor grown carbon fiber (VGCF), and is preferably contained in the polarizable electrode in an amount of 0.1 to 20% by weight.

  The polarizable electrode 1 can be manufactured using a technique that has been widely known in the art. For example, the polarizable electrode 1 can be produced by rolling a composite material obtained by mixing and kneading a conductive additive such as activated carbon, carbon black, and polytetrafluoroethylene (PTFE) and forming it into a sheet shape. . In this case, the polarizable electrode 1 is bonded to the positive electrode current collector 2 made of a metal foil such as aluminum via a conductive adhesive. The positive electrode current collector 2 may be a mesh or an expanded metal.

  Also, a binder such as styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyamide imi or polyimide, a dispersion medium such as water or N-methyl-2-pyrrolidone (NMP), activated carbon and a conductive auxiliary agent. The polarizable electrode 1 can also be produced by a method of coating the positive electrode current collector 2 by using a coater such as a die coater, a comma coater, or a roll coater in addition to a mixture obtained by dispersing. it can.

  The carbon electrode 3 used for the negative electrode is made of a carbon material capable of inserting and extracting lithium ions as an active material, and the others are manufactured in the same manner as the positive electrode. The carbon material is preferably natural graphite, artificial graphite, non-graphitizable carbon, graphitizable carbon, low-temperature calcined carbon, or the like. The negative electrode current collector 4 is preferably a copper foil, and may be a mesh or expanded metal.

  The lithium electrode 9 serving as a lithium ion supply source during the pre-doping process on the negative electrode is configured by supporting the lithium metal 6 on the lithium electrode current collector 7. The lithium electrode current collector 7 is preferably a porous body such as a mesh made of nickel or stainless steel. Nickel is particularly preferred because it has low reactivity with lithium and is chemically stable. The lithium electrode 9 may be covered with a separator 5 in order to ensure insulation from the positive electrode and the negative electrode.

  The electrode laminate 6 is formed by laminating or winding a separator 5 between a positive electrode and a negative electrode. The lithium electrode 9 is provided in the vicinity of the electrode laminate 6. The position at which the lithium electrode 9 is disposed is not particularly limited, but may be provided close to the side surface of the electrode stack 6 so as to face substantially parallel.

  The electrode laminate 6 and the lithium electrode 9 are accommodated in an exterior body made of an aluminum laminate film or a metal can. At this time, lead terminals are attached to the positive electrode current collector 2, the negative electrode current collector 4, and the lithium electrode current collector 7, and the lead terminals are drawn out of the exterior body.

  The organic electrolyte solution is impregnated into the electrode laminate 6 at least twice before and after the pre-doping treatment. It is preferable to use cyclic carbonates or chain carbonates excluding propylene carbonate as a solvent contained in the organic electrolyte before the pre-doping treatment. Examples of the cyclic carbonates include ethylene carbonate, butylene carbonate, vinylene carbonate, and derivatives thereof. Examples of the chain carbonates include diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, di-n-propyl carbonate, and derivatives thereof. These solvents are used alone or in combination of two or more.

Examples of the electrolyte contained in the organic electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , CF ₃ SO ₃ Li, and LiC (SO ₂ CF ₃ ) ₃ , LiAsF ₆ , LiSbF ₆ , LiI, LiCF ₃ CO ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₄ (C ₂ F ₅ ) ₂ , LiPF ₄ ( Examples include lithium salts such as CF ₃ ) ₂ , LiPF ₅ (C ₂ F ₅ ), LiPF ₅ (CF ₃ ), and the like. These lithium salts are used alone or in combination of two or more. The concentration of the lithium salt in the organic electrolyte is preferably 0.5 to 2.5 mol / L, and more preferably 0.75 to 2.0 mol / L.

  Using the lithium ion capacitor having the configuration of the present embodiment described above, a pre-doping process is performed on the carbon material contained in the carbon electrode 3 of the negative electrode. The method of the pre-doping treatment is not particularly limited. For example, by short-circuiting the negative electrode and the lithium electrode 9 through an external circuit, lithium ions corresponding to a predetermined amount of charge generated from the lithium electrode 9 are made into the carbon material of the negative electrode. This is done by occlusion.

  Then, after completion of the pre-doping treatment, propylene carbonate is included in the organic electrolytic solution impregnated in the electrode laminate 6. At this time, the aspect of propylene carbonate to be added may be either propylene carbonate alone, a mixed solvent of propylene carbonate and another solvent, or a solution of these and an electrolyte, and the composition and concentration of the target organic electrolyte solution The combination and amount are appropriately determined according to the above.

  Although the method of including propylene carbonate in the electrode laminate 6 is not particularly limited, for example, when an aluminum laminate film is used for the exterior body of a lithium ion capacitor, an opening is provided by opening a part of the heat seal portion, Propylene carbonate may be injected from the opening. And if the said opening part is heat-sealed again and the exterior body is sealed after the impregnation process of propylene carbonate, the lithium ion capacitor in embodiment of this invention can be obtained.

  In the pre-doping process described above, the solvent contained in the organic electrolyte solution undergoes a reduction reaction on the surface of the carbon material of the negative electrode, whereby an SEI film is formed on the surface. At this time, propylene carbonate that does not form the SEI film is not included in the organic electrolyte. Therefore, an SEI film sufficient to suppress the reduction reaction of the solvent can be formed on the surface of the carbon material of the negative electrode. Therefore, even if the lithium ion capacitor is charged and discharged after the pre-doping treatment, the reduction reaction of the solvent on the surface of the carbon material of the negative electrode is suppressed, so that the reaction product amount of the solvent that causes deterioration can be reduced.

  In addition, since propylene carbonate is included in the organic electrolyte solution after the pre-dope treatment for forming the SEI film, the reduction reaction of propylene carbonate on the surface of the carbon material of the negative electrode is performed in advance even if the lithium ion capacitor is charged / discharged. Is inhibited by the modified SEI membrane. Therefore, the lithium ion capacitor according to the present embodiment can reduce the amount of reaction product of the solvent that causes deterioration as described above even if propylene carbonate that does not form an SEI film is included in the organic electrolyte, and can be used for a long time. Stable operation against charging / discharging is possible. In addition, propylene carbonate has the characteristics that the potential window is wider and the freezing point is lower than that of ethylene carbonate. Therefore, the lithium ion capacitor can be improved in long-term stability and low-temperature characteristics.

  Next, the lithium ion capacitor obtained by the method for producing a lithium ion capacitor of the present invention will be specifically described with reference to examples.

[Example 1]
(Preparation of positive electrode)
80% by weight of activated carbon having a specific surface area of 1500 m ² / g obtained by steam activation of petroleum coke as an active material, 10% by weight of acetylene black as a conductive material, 10% by weight of polytetrafluoroethylene (PTFE) as a binder and ethanol The kneaded mixture was roll-rolled and formed into a sheet to obtain a polarizable electrode having a thickness of 400 μm. And the positive electrode was produced by bonding the said polarizable electrode on both surfaces of the aluminum foil of a collector using a conductive adhesive, and drying and pressing this.

(Preparation of negative electrode)
A slurry obtained by dispersing and mixing 90% by weight of artificial graphite as an active material and 10% by weight of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP) as a binder is used as a collector copper foil. Coating was performed on one side using a doctor blade, and this was dried and pressed to produce a negative electrode. The thickness of the carbon electrode excluding the negative electrode current collector was 140 μm.

(Production of lithium electrode)
A lithium electrode having a thickness of 5 mm × 40 mm was sandwiched and bonded to a nickel mesh welded with a nickel plate serving as a lead terminal, thereby producing a lithium electrode.

(Production of cell)
An electrode laminate having a configuration in which a pair of positive and negative electrodes is laminated on both sides of a positive electrode formed in a size of 80 mm × 100 mm with a negative electrode formed in the same size interposed through a cellulose separator is obtained. It was. And the lead terminal which consists of aluminum and copper, respectively was attached to the positive electrode of the electrode laminated body, and the negative electrode, the lithium electrode was arrange | positioned adjacent to the electrode laminated body, and these were accommodated in the exterior body which consists of an aluminum laminated film. In addition, the electrode laminate was impregnated with an organic electrolyte solution in which LiPF ₆ was dissolved in a 1: 1 mixed solvent of ethylene carbonate and diethyl carbonate for 1 M for a sufficient time to produce a cell.

(Pre-doping treatment)
The negative electrode and the lithium electrode were short-circuited through an external circuit, whereby lithium ions were occluded in the artificial graphite of the negative electrode. The pre-doping process was terminated when the amount of lithium ions stored in the negative electrode measured using an external circuit reached 60 mAh.

(Addition of propylene carbonate)
After the pre-doping treatment, an organic electrolytic solution in which LiPF ₆ was dissolved to 1 M was injected into propylene carbonate through an opening provided by cutting off a part of the outer package of the cell, and impregnation treatment was performed under reduced pressure. At this time, the injection amount of the organic electrolyte was adjusted so that the weight ratio of ethylene carbonate, diethyl carbonate, and propylene carbonate contained in the cell was 3: 3: 1. Then, the cell of Example 1 was obtained by sealing the said opening part again by heat sealing.

[Comparative Example 1]
Using an electrolytic solution in which LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate, diethyl carbonate, and propylene carbonate (weight ratio 3: 3: 1) to 1 M, and adding propylene carbonate after pre-doping treatment. A cell of Comparative Example 1 was obtained in the same manner as Example 1 except that there was no cell.

[Comparative Example 2]
Example 1 except that an organic electrolytic solution in which LiPF ₆ was dissolved to 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate (weight ratio is 1: 1) was used, and propylene carbonate was not added after the pre-doping treatment. In the same manner, a cell of Comparative Example 2 was obtained.

(Evaluation of pre-doping treatment)
FIG. 2 and FIG. 3 show the results of evaluating the amount of gas generated in the cell and the negative electrode potential during the pre-doping process for the cells of Example 1 and Comparative Example 1.

  As apparent from FIG. 2, the total amount of gas generated during the pre-doping treatment in the cell of Example 1 was about 1/7 as compared with the cell of Comparative Example 1.

Further, from FIG. 3, in the cell of Example 1, the negative electrode potential steadily decreased to about 0.1 V (vs. Li ^/ Li ⁺ ), whereas in the cell of Comparative Example 1, the negative electrode potential was 0.8 V ( vs. Li ^/ Li ⁺ ) and then increased, and the pre-doping treatment was not completed normally. Thereafter, the cell of Example 1 could be charged, whereas the cell of Comparative Example 1 could not be charged.

  Therefore, in the pre-doping treatment of the cell of Example 1, the reduction reaction of the solvent in the negative electrode is suppressed, but in the pre-doping treatment of the cell of Comparative Example 1, the reduction reaction of the solvent in the negative electrode is caused by the presence of propylene carbonate. You can see that it is going. This suggests that a stable SEI film was formed on the surface of the artificial graphite of the negative electrode by using an organic electrolyte containing no propylene carbonate as a solvent.

(Evaluation of long-term stability)
For the cells of Example 1 and Comparative Example 2, the result of float voltage application by applying a voltage of 3.8 V in an atmosphere of 50 ° C. is shown in FIG. The cell was evaluated by measuring the amount of gas generated in the cell.

  As is clear from FIG. 4, the cell of Example 1 in which propylene carbonate was added after the pre-doping treatment was compared with the cell of Comparative Example 2 in which propylene carbonate was not included in the organic electrolyte, and the total amount of gas generated during float charging. Became about a quarter.

  Therefore, in the cell of Example 1, in addition to the formation of the SEI film on the negative electrode by the pre-doping treatment, the gas generation reaction was suppressed by including the chemically stable propylene carbonate in the organic electrolyte after the pre-doping treatment, and the lithium It can be seen that the long-term stability of the ion capacitor has been improved.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiment but by the scope of claims for patent, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims for patent.

DESCRIPTION OF SYMBOLS 1 Polarization electrode 2 Positive electrode collector 3 Carbon electrode 4 Negative electrode collector 5 Separator 6 Electrode laminated body 7 Lithium metal 8 Lithium electrode current collector 9 Lithium electrode

Claims (3)

A positive electrode including a porous material capable of reversibly adsorbing and desorbing lithium ions and / or anions, a negative electrode including a carbon material capable of absorbing and desorbing lithium ions, and a lithium electrode supplying lithium ions are laminated. A method for producing a lithium ion capacitor comprising a separator interposed between the positive electrode and the negative electrode, and an organic electrolyte containing a lithium salt,
Performing a pre-dope treatment for occluding lithium ions in the negative electrode in the organic electrolyte without propylene carbonate;
And a step of including propylene carbonate in the organic electrolyte solution.   The method for producing a lithium ion capacitor according to claim 1, wherein the porous material contained in the positive electrode is activated carbon, and the carbon material contained in the negative electrode is graphite.   3. The lithium ion capacitor according to claim 1, wherein in the step of performing the pre-doping treatment, the organic electrolyte solution that does not include propylene carbonate includes one or more of cyclic carbonates and chain carbonates excluding propylene carbonate. Manufacturing method.

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