JP2016091984A - Power storage element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a power storage element which uses metallic lithium for an anode active material is not put into practical use at present since the problem that dendrite is gradually grown by repeating charge/discharge, breaks through a separator and short-circuits positive and negative electrodes cannot be solved.SOLUTION: In the power storage element of the present invention, the negative electrode does not practically contain metallic lithium during complete discharge or metallic lithium is deposited from an electrolyte and forms the negative electrode. Therefore, even if dendrite is generated during charge, dendrite disappears during complete discharge. Namely, when charge/discharge is repeated, growth of dendrite can be suppressed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power storage device, and more particularly to a power storage device capable of suppressing lithium dendrite growth due to repeated charge and discharge.

  Currently, as power storage devices used in electronic devices such as mobile phones and personal computers, or electric vehicles, lithium metal compounds capable of occluding and releasing lithium ions are used as positive electrode active materials, carbon materials are used as negative electrode active materials, and non-aqueous Development and practical application (that is, widespread use as a product) of lithium ion secondary batteries (for example, Patent Document 1) including an electrolytic solution are progressing.

  In addition, as a storage element having an energy density equivalent to that of a lithium ion secondary battery and capable of high-speed charge / discharge, a storage element using a carbon material for both the positive electrode active material and the negative electrode active material has been developed. (For example, Non-Patent Document 1 and Patent Document 2).

  However, at present, these power storage elements have not achieved high energy density, which is a characteristic characteristic of metallic lithium.

  In other words, lithium is the least basic (low electrode potential) and has the smallest molar mass, and a battery using this metal can be expected to have a high energy density. Therefore, many studies have been conducted for a long time.

As a result, lithium batteries (metallic lithium primary batteries) using metallic lithium as the negative electrode active material have been put into practical use, but storage elements using metallic lithium (for example, Patent Document 3 and Patent Document 4) have been put into practical use to date. It has not been.

Patent No. 2668678 JP 2005-259378 A Japanese Patent Laid-Open No. 61-10882 JP 63-58763 A

Nissin Electric Technical Report Vol. 57, no. 2 (2012.11.1) P28-31

  As a cause of the fact that a storage element using metallic lithium as a negative electrode active material has not been put into practical use, metallic lithium grows in a dendritic shape due to repeated charge and discharge (this is called dendrite) and breaks the separator to short-circuit between the positive and negative electrodes. The problem cannot be solved.

  Therefore, an object of the present invention is to provide a power storage element that can solve the conventional problem of dendrite growth in repeated charge and discharge.

  In order to solve the above-described problems, a first aspect of the present invention is a positive electrode having a positive electrode active material, a negative electrode having a negative electrode current collector, a separator sandwiched between the positive electrode and the negative electrode, and an electrolyte containing lithium. The negative electrode is a power storage element that substantially does not have a negative electrode active material during complete discharge.

  According to a second aspect of the present invention, a positive electrode having a positive electrode active material, a negative electrode current collector, a separator sandwiched between the positive electrode and the negative electrode current collector, and an electrolyte containing lithium are provided. This is an electric storage element in which metallic lithium, which is a negative electrode active material, is deposited on the negative electrode current collector from the electrolyte by a voltage applied between the bodies to constitute the negative electrode.

  Here, the negative electrode active material is “substantially” not included in the electricity storage device of the present invention when the reserve battery configuration described later is adopted, even when the positive electrode is active. In some cases, anions may remain in the material, and in this case, an amount of lithium ions that are equal in charge to the remaining anions remains on the negative electrode current collector, so that the negative electrode is “completely” on the negative electrode current collector. It is because it does not become the situation which does not have a negative electrode active material.

  Therefore, it is assumed that the negative electrode active material is not “substantially”, and this state can be confirmed by disassembling the storage element after complete discharge and invisible lithium in the metal state on the negative electrode current collector. .

  In the electricity storage device of the present invention, the negative electrode is substantially free of metallic lithium, which is a negative electrode active material, at the time of complete discharge.

  Therefore, even if dendrite is generated during charging, it disappears during complete discharge. That is, there is an effect that dendrite growth can be suppressed when charging and discharging are repeated.

Schematic cross-sectional view of the electricity storage device of the present invention in Example 1 Charge / discharge curve (capacitance-voltage curve) of the electricity storage device of the present invention in Example 1 Charge / discharge curve (capacitance-voltage curve) of the electricity storage device of the present invention in Example 2 Charging / discharging curve (capacitance-voltage curve) of the storage element in the comparative example

  In the lithium ion secondary battery described in Patent Document 1 or the like, lithium ions move from the positive electrode to the negative electrode during charging, and from the negative electrode to the positive electrode during discharging. Therefore, the ion concentration in the non-aqueous electrolyte does not change.

  Such a power storage element is called a rocking chair type (although there are expressions such as a seesaw type and a shuttlecock type, they are unified as a rocking chair type in this specification).

In contrast, the electric storage device according to Non-Patent Document 1 and Patent Document 2, PF ₆ from the non-aqueous electrolyte solution during charging in the positive electrode ^- lithium anions or the like is inserted, and a nonaqueous electrolytic solution on the negative electrode Ions are inserted.

Further, at the time of discharging, anions such as PF _6- ^{and the} like from the negative electrode and lithium ions from the negative electrode are desorbed to the non-aqueous electrolyte (see the following reaction formulas 1 and 2, charging reaction from the left side to the right side and discharging reaction from the right side to the left side).

Reaction formula 1

Positive electrode: PF ₆ ⁻ + nC = Cn (PF ₆ ) + e ⁻

Reaction formula 2

Negative electrode: Li ⁺ + nC + e ⁻ = LiCn

  A power storage element having such a change in ion concentration in the non-aqueous electrolyte is called a reserve type.

  The power storage device of the present invention can adopt either a rocking chair type or a reserve type depending on selection other than the negative electrode.

  When the electricity storage device of the present invention is a rocking chair type, members other than the negative electrode can be preferably used for members of conventional lithium ion secondary batteries. The member used for the electrical storage element (a carbon material is used for both the positive electrode active material and the negative electrode active material) can be preferably used.

  Hereinafter, embodiments of the present invention will be described.

<Explanation of words>
First, the “negative electrode” in the electricity storage device of the present invention is defined. However, this definition is for convenience to explain the present invention, and is not necessarily electrochemically accurate.

1. The battery has a positive electrode through which electrons flow during discharge and a negative electrode through which electrons flow out.

2. A power storage element such as a lithium ion secondary battery has a positive electrode active material that accepts electrons at the time of discharge at the positive electrode (for example, a reaction field where lithium ions are inserted or anions are released), and discharge at the negative electrode Occasionally, it has a negative electrode active material that emits electrons (for example, a reaction field where lithium ions are desorbed or metallic lithium dissolves).

3. These active materials (positive electrode active material and negative electrode active material) are often used alone because of their low electrical conductivity or materials with insufficient physical strength. In order to improve the movement of electrons and the handling of batteries, the positive electrode and the negative electrode have a positive electrode current collector and a negative electrode current collector made of metal, respectively.

4. That is, the negative electrode in the electricity storage device of the present invention is composed of at least a negative electrode current collector and metallic lithium as a negative electrode active material.

5. Here, a state where the electricity storage device of the present invention is completely discharged is considered. In this case, the metal lithium on the negative electrode current collector is substantially dissolved, and the negative electrode current collector can be considered to exist alone.

6. However, since the negative electrode current collector in this state can sufficiently be expected to function as a negative electrode due to deposition and formation of metallic lithium by new charging, in the present invention, ( The negative electrode current collector (which can be regarded as existing alone) is collectively referred to as a negative electrode.

<Configuration of power storage element of the present invention>
(1) Negative electrode current collector As the material of the negative electrode current collector in the electricity storage device of the present invention, metals such as copper, nickel, stainless steel, titanium, etc. are used, and these are processed into a foil shape, a mesh shape, or the like. .

(2) Negative electrode active material The negative electrode active material in the electricity storage device of the present invention is metallic lithium formed on the negative electrode current collector in an amount that substantially dissolves during complete discharge. A specific example is metallic lithium (derived from an electrolyte) deposited on the negative electrode current collector by a voltage applied between the positive electrode and the negative electrode current collector.

(3) Negative electrode Usually, the negative electrode current collector of the above (1) and the negative electrode active material of the above (2) are included. The negative electrode current collector is also collectively referred to as the negative electrode.

(4) Positive electrode current collector As the material of the positive electrode current collector in the electricity storage device of the present invention, metals such as aluminum, nickel, stainless steel, and titanium are used, which are processed into a foil shape, a mesh shape, or the like. .

(5) Positive electrode active material The positive electrode active material in the electricity storage device of the present invention is a carbon material capable of inserting and removing lithium ions (in this case, a rocking chair type electricity storage device), or an anion can be inserted and removed. Carbon material (in this case, it becomes a reserve-type energy storage device).

  Examples of the carbon material capable of inserting and removing lithium ions include carbon fibers having a high graphitization rate, carbon fibers formed by fluidized vapor phase growth (for example, Patent Document 3 and Patent Document 4), and the like.

  Examples of the carbon material capable of inserting and removing anions include various graphite materials (for example, Patent Document 2) such as coke, artificial graphite, and natural graphite.

(6) Positive Electrode The positive electrode current collector of (4) above and the positive electrode active material of (5) above are generally included, but the positive electrode active material is generally included in part of the surface of the positive electrode current collector. A positive electrode active material layer is formed.

(7) Positive electrode active material layer A method for forming the positive electrode active material layer is described below.

  A conductive material for supplementing the positive electrode active material with the positive electrode active material as necessary, a binder for increasing the bondability between powders, and / or for increasing the adhesion to the positive electrode current collector, Then, a solvent or the like is added to form a slurry (positive electrode slurry).

  Examples of the conductive material include carbonaceous materials such as graphite and acetylene black.

  Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, butadiene rubber, cellulose compounds such as carboxymethyl cellulose and methyl cellulose, and polyvinyl alcohol.

  The solvent may be aqueous or organic. Examples of the aqueous solvent include water and alcohol, and examples of the organic solvent include N-methylpyrrolidone, N, N-dimethylformamide, and toluene.

  A positive electrode slurry is applied to the positive electrode current collector by a doctor blade method, a roll coating method, a bar coating method, a slit coating method, or the like, and the solvent is dried to form a positive electrode active material layer. Further, the thickness of the positive electrode active material layer may be adjusted by a roll press or a flat plate press.

(8) Separator In the electricity storage device of the present invention, a separator is usually sandwiched between the positive electrode and the negative electrode in order to prevent a short circuit between the positive electrode and the negative electrode.

  The separator may be of any material and / or shape that allows ion movement and hinders electron conduction, but the electrolyte used is a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt. In this case, it is preferable to select from materials that are stable with respect to the non-aqueous electrolyte and have excellent liquid retention properties, such as porous sheets or nonwoven fabrics made of polyolefin such as glass fiber, cellulose, or polyethylene. Preferably used.

(9) Electrolyte The electrolyte in the electricity storage device of the present invention contains lithium, and is most commonly a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt.

  Nonaqueous solvents include carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters, amides, phosphate ester compounds, etc. In particular, a mixed solvent in which ethylene carbonate and dimethyl carbonate are mixed at a volume ratio of 1: 1 to 1:10 is preferably used. Moreover, it is good also as a gel form by adding a polymer etc. to these.

As the lithium salt, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ), LiCl, LiBr, CH ₃ SO ₃ Li, or the like can be used.

  Furthermore, when the electricity storage device of the present invention is a rocking chair type, a solid electrolyte that selectively allows lithium ions to pass through may be used as the electrolyte.

Examples of the solid electrolyte include a solid electrolyte membrane obtained by adding a carboxylic acid group, a phosphoric acid group, or a sulfonic acid group to polyethylene oxide as a base material, an oxide solid electrolyte such as LaLiTiO and LiTiAl (PO ₄ ), Li A sulfide solid electrolyte such as ₂ S—P ₂ S ₅ can be mentioned.

  In addition, when a solid electrolyte is used as the electrolyte, this can be used as a separator. That is, the electrolyte and the separator can be shared by the same member.

(10) Exterior Member In the electricity storage device of the present invention, the exterior member that houses the positive electrode, the negative electrode, the separator, and the electrolyte can be of the same material and shape as a conventional lithium ion secondary battery.

  That is, a metal can obtained by processing a metal plate such as an iron plated with Ni or a stainless steel plate into a cylindrical shape, a rectangular tube shape, a coin shape or the like is preferably used.

  Alternatively, a laminate film obtained by laminating a metal foil such as aluminum or stainless steel and a plastic film such as polyethylene, polypropylene, or polyethylene terephthalate, and a part of which is laminated to form a bag is also preferably used.

<Charging method for the electricity storage device of the present invention>
As described above, in the electricity storage device of the present invention, even if dendrite is generated on the negative electrode current collector during charging, it disappears during complete discharge.

  Therefore, the dendrite growth can be further reliably suppressed by adding a process (dendrite disappearance process) that completely discharges the electric storage element of the present invention once during charging.

  However, it is not always necessary to completely discharge each time the battery is charged, and charging having a dendrite disappearance process may be performed when a predetermined number of times of charging and discharging are performed.

  In addition, since the dendrite disappearance process in the charging process of the present invention is complete discharge (discharge to a voltage of 0), refresh discharge (discharge) is performed when charging a storage element having a so-called memory effect such as a nickel cadmium battery or a nickel hydrogen battery. Discharge up to the end voltage).

  Embodiments of the present invention will be described below with reference to the drawings.

  Each of the power storage elements in this example is a reserve type, but may be a rocking chair type by changing the positive electrode active material.

<Production of test cell>
In order to confirm the present invention, a test cell shown in FIG. 1 was produced. The manufacturing process is shown below.

(1) Production of Negative Electrode Current Collector A 15 μm thick copper foil (commercially available product) was punched into a diameter of 16 mm to obtain a negative electrode current collector 1.

(2) Production of positive electrode Carbon powder (KS-6 manufactured by TIMCAL) as a positive electrode active material, conductive agent (acetylene black: AB) and binder (2% by weight aqueous solution of carboxymethyl cellulose: CMC) are mixed in a weight ratio of KS. -6: AB: CMC = 90: 7: 3 was mixed and kneaded with water to prepare a positive electrode slurry.

  This positive electrode slurry was applied to a positive electrode current collector 2 of 20 μm thick aluminum foil (commercially available product) at a thickness of 100 μm and dried at 140 ° C.

  Next, it was compressed to a thickness of 50 μm with a roller press to form a positive electrode active material layer 3, and further punched into a circle with a diameter of 14 mm together with an aluminum foil to form a positive electrode 4.

(3) Production of Separator A polyolefin-based porous film 5 (ND525 manufactured by Asahi Kasei E-Materials) having a thickness of 25 μm is sandwiched between two glass fiber filter papers 6 having a diameter of 21 mm and a thickness of 0.2 mm. It was.

(4) Cell assembly After the negative electrode current collector 1, the separator 7, and the positive electrode 4 are sequentially disposed in a stainless steel case 8, EC: DMC = 1 by volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). : 2M LiPF ₆ 1M solution (non-aqueous electrolyte) in a mixed solvent was injected.

  Further, a stainless sealing plate 10 was fixed through a polypropylene gasket 9 to complete a test cell 100 (test cell A). Needless to say, in the test cell having the above-described structure, both the case 8 and the sealing plate 10 constitute the exterior member 11.

<Measurement of charge / discharge capacity>
The initial charge / discharge capacity of the test cell produced as described above was measured.

Specifically, the charge end voltage was set to 5.2 V at 25 ° C., and constant current charging was performed at a current density of 0.04 mA / cm ² .

Next, the initial charge capacity (mAh / g) and the initial discharge capacity (mAh / g) per unit mass of the positive electrode active material when performing a constant current discharge at a current density of 0.04 mA / cm ² at a discharge end voltage of 3.0 V. mAh / g) was measured. The obtained charging / discharging curve (capacity-voltage curve) is shown in FIG.

  It can be seen that even a test cell that does not use metallic lithium, which is a negative electrode active material, in the manufacturing stage functions as a power storage element.

  In addition, the test cell A produced in the present Example 1 decomposes | disassembles after fully discharging at 25 degreeC, and it can confirm visually that there is no precipitation of metallic lithium on the negative electrode collector 1. FIG.

  A test foil (test cell B) according to Example 1 was prepared except for the copper foil as the negative electrode current collector, and the charge / discharge curve shown in FIG. 3 was obtained.

  In the test cell B produced in this example, the stainless steel case 8 functions as a negative electrode current collector, and metal lithium is deposited in the contact area with the nonaqueous electrolyte by the applied voltage. Therefore, it is considered that it also functions as a negative electrode.

  In addition, the test cell B produced in this Example 2 decomposed | disassembled after fully discharging at 25 degreeC, and it can confirm visually that there is no precipitation of metallic lithium inside a case.

  Further, the test cells produced in Examples 1 and 2 are reserve-type electricity storage elements as described above, and the metal lithium deposited on the negative electrode current collector is derived only from the electrolyte, and other than the electrolyte inside the electricity storage element. Does not have a lithium source.

  In such an element configuration, it is considered that the lithium ion and anion inside the electrolyte are always electrically matched regardless of the level of charge / discharge state of the electricity storage element. I can say that.

  Furthermore, in the test cell manufacturing method in Examples 1 and 2, metallic lithium is deposited from the electrolyte by the charging process inside the completed power storage element to constitute the negative electrode.

  In such a manufacturing method, the deposited metallic lithium is never exposed from the electrolyte (in the first and second embodiments, the nonaqueous electrolytic solution). Therefore, it can be said that the deterioration of the surface of metallic lithium in the production process can be suppressed, and it can be said that this is a preferred production method.

Comparative example

  A commercially available copper foil with a lithium metal thin film was used in place of the negative electrode current collector of Example 1, and a test cell (test cell C) according to Example 1 was prepared otherwise. The charge / discharge curve shown in FIG. Got.

  In the test cell C produced in this comparative example, metallic lithium can be visually confirmed on the copper foil even after complete discharge.

  As described above, the present invention has industrial applicability as a power storage element that can suppress dendrite growth, and an improvement in cycle life can be expected in a power storage element using metallic lithium as a negative electrode active material.

1: Negative electrode current collector 2: Positive electrode current collector 3: Positive electrode active material layer 4: Positive electrode 5: Porous film 6: Glass fiber filter paper 7: Separator 8: Case 9: Gasket 10: Sealing plate 11: Exterior 100: Test cell

Claims (9)

A positive electrode having a positive electrode active material;
A negative electrode having a negative electrode current collector;
A separator sandwiched between the positive electrode and the negative electrode;
An electrolyte containing lithium,
The power storage element, wherein the negative electrode has substantially no negative electrode active material during complete discharge. A positive electrode having a positive electrode active material;
A negative electrode current collector;
A separator sandwiched between the positive electrode and the negative electrode current collector;
An electrolyte containing lithium,
A power storage element, wherein a metal applied as a negative electrode active material is deposited on the negative electrode current collector from the electrolyte by a voltage applied between the positive electrode and the negative electrode current collector to form a negative electrode.   The electric storage element according to claim 1 or 2, wherein the positive electrode active material is a carbon material capable of inserting and removing lithium ions.   The power storage element according to claim 1 or 2, wherein the positive electrode active material is a carbon material capable of inserting and removing anions.   The electric storage element according to any one of claims 1 to 4, wherein the positive electrode further includes a positive electrode current collector.   The electrical storage element according to any one of claims 1 to 5, further comprising an exterior member capable of accommodating the positive electrode, the negative electrode, the separator, and the electrolyte.   The power storage element according to claim 6, wherein at least a part of the exterior member also serves as the negative electrode current collector. A positive electrode having a positive electrode active material;
A negative electrode current collector;
A separator;
A non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt;
A preparation step of preparing an exterior member;
A storage step of sandwiching the separator between the positive electrode and the negative electrode current collector and storing the separator together with the non-aqueous electrolyte;
An initial charging step of applying a voltage between the positive electrode and the negative electrode current collector to deposit lithium metal from the electrolyte on the negative electrode current collector to form a negative electrode;
A method for manufacturing a power storage element, comprising:   A method for charging a storage element, comprising a step of eliminating a dendrite that completely discharges the storage element when charging the storage element according to any one of claims 1 to 7.

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