JP2019029088A - Electrolyte for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

To provide an electrolyte for lithium ion secondary batteries excellent in charge/discharge characteristics, in which propylene carbonate is used, the propylene carbonate being high in flash point and high in safety and also low in melting point, so that it is excellent as a lithium ion secondary battery even under a low temperature environment.SOLUTION: An electrolyte (D) for lithium ion secondary batteries contains a nonaqueous solvent (A) containing propylene carbonate (a), a cyclic ketone compound (B) represented by the following general formula (1), and an electrolyte (C), the content of (a) being 25-100 wt.% based on the weight of (A).SELECTED DRAWING: None

Description

  The present invention relates to an electrolyte for a lithium ion secondary battery excellent in charge / discharge characteristics, and a lithium ion secondary battery using the same. In detail, it is related with said electrolyte solution for lithium ion secondary batteries containing a cyclic ketone compound.

Conventionally, diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate have been generally used as electrolytes for lithium ion secondary batteries, but these have a low flash point and a high risk of ignition in the event of an abnormality such as a short circuit.
Therefore, a technique for improving the safety of a lithium ion secondary battery by using propylene carbonate having a high flash point and a low melting point has been proposed.
However, it is known that repeated charge and discharge using an electrolytic solution containing propylene carbonate causes a side reaction of decomposition of propylene carbonate on the graphite surface of the negative electrode, which deteriorates the charge and discharge characteristics. Yes.
Therefore, it has been proposed to suppress deterioration of charge / discharge characteristics by adding an additive to the electrolytic solution (for example, Patent Documents 1 and 2), but the effect is not sufficient.

JP 2000-058122 A JP 2001-126761 A

  The present invention provides an electrolyte for a lithium ion secondary battery that uses propylene carbonate, which is excellent as a lithium ion secondary battery even in a low temperature environment because of its high flash point, high safety, and low melting point, and further excellent charge / discharge characteristics. The purpose is to do.

The inventors of the present invention have reached the present invention as a result of studies to achieve the above object.
That is, this invention contains the nonaqueous solvent (A) containing propylene carbonate (a), the cyclic ketone compound (B) represented by the following general formula (1), and the electrolyte (C), Electrolyte for lithium ion secondary battery (D) having a content of (a) of 25 to 100% by weight based on the weight of A); and electrolyte for lithium ion secondary battery (D), positive electrode and negative electrode A lithium ion secondary battery comprising:

[R ¹ and R ² are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. ]

  The electrolyte solution for a lithium ion secondary battery of the present invention has an effect of being able to suppress deterioration of charge / discharge characteristics when the battery is repeatedly charged and discharged.

The electrolyte solution for a lithium ion secondary battery of the present invention includes a nonaqueous solvent (A) containing propylene carbonate (a), a cyclic ketone compound (B) represented by the following general formula (1), and an electrolyte (C ) And. And based on the weight of (A), content of (a) is 25 to 100 weight%, It is characterized by the above-mentioned.

[R ¹ and R ² are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. ]

Graphite is a negative electrode active material commonly used in lithium ion secondary batteries. However, if propylene carbonate is contained in the electrolyte, the decomposition reaction of propylene carbonate tends to occur on the negative electrode surface, which deteriorates the charge / discharge characteristics of the battery. I will let you.
Since the cyclic ketone compound (B) contained as an essential component in the electrolyte solution for a lithium ion secondary battery of the present invention has an action of suppressing the decomposition reaction of propylene carbonate on the negative electrode active surface, propylene is used as a nonaqueous solvent. Even an electrolytic solution containing carbonate as an essential component can be used without deteriorating charge / discharge characteristics.

In the general formula (1) representing the cyclic ketone compound (B) of the present invention, R ¹ and R ² are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group.
Examples of the alkyl group having 1 to 4 carbon atoms include methyl group, ethyl group, n-propyl group, iso-propyl group, n-butyl group, iso-butyl group, sec-butyl group, t-butyl group and the like. .

From the viewpoint of steric hindrance, R ¹ and R ² are preferably a hydrogen atom, a methyl group, an ethyl group, and an n-propyl group, and both are particularly preferably a hydrogen atom.

The content of the ketone compound (B) is preferably 0.5 to 10% by weight based on the total weight of (A) to (C), from the viewpoint of charge / discharge characteristics. 5% by weight is more preferred.

As the non-aqueous solvent (A) of the present invention, propylene carbonate is contained as an essential component, and other non-aqueous solvents used in the electrolyte for lithium ion secondary batteries as required are based on the weight of (A). Up to 75% by weight.
Examples of the non-aqueous solvent used in combination include ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, γ-butyrolactone, and acetonitrile.
From the viewpoint of electrochemical stability, the above (A) is preferably propylene carbonate alone or a combination of propylene carbonate and ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate or the like.

Examples of the electrolyte (C) of the present invention include lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), lithium bis (fluorosulfonyl) imide (LiFSI), and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI). , Lithium bis (oxalate) borate (LiBOB), lithium trifluoromethanesulfonate, and the like. From the viewpoint of charge / discharge characteristics, LiPF ₆ , LiBF ₄ , LiFSI, LiTFSI, and the like are preferable.

  The lithium ion secondary battery of the present invention comprises the above-described electrolyte (D) for a lithium ion secondary battery, a positive electrode, a negative electrode, a separator, and a battery can or an aluminum laminate film as an outer package.

The positive electrode in the lithium ion secondary battery of the present invention comprises a current collector, a positive electrode active material, a conductive aid, and a binder.
The current collector is not particularly limited as long as it is a metal foil that is electrochemically stable in the range of 2.5 V to 5.0 V on the basis of metallic lithium, and examples thereof include aluminum and nickel. The aluminum foil used is mentioned.

As the positive electrode active material, a composite oxide of lithium and a transition metal (a composite oxide having one kind of transition metal (such as LiCoO ₂ , LiNiO ₂ , LiAlMnO ₄ , LiMnO _2, and LiMn ₂ O ₄ ), a transition metal element is used. Two kinds of complex oxides (for example, LiFeMnO ₄ , LiNi _1-x Co _x O ₂ , LiMn _1-y Co _y O ₂ , LiNi _1/3 Co _1/3 Al _1/3 O ₂ and LiNi _0.8 Co _0.15 Al _0.05 O ₂ ) and a composite oxide having three or more metal elements [for example, LiM _a M ′ _b M ″ _c O ₂ (M, M ′, and M ″ are different transition metal elements, respectively) And satisfies a + b + c = 1, such as LiMn _1/3 Ni _1/3 Co _1/3 O ₂ ) etc.] and lithium-containing transition metal phosphate (LiFePO ₄ , LiCoPO _{4 4,} LiMnPO _4, and LiNiPO _4, etc.) and the like, be used These positive electrode active material alone, or in combination of two or more. Among these, LiCoO ₂ and LiNi _1/3 Co _1/3 Al _1/3 O ₂ are preferable from the viewpoint of storage capacity.

  As the conductive assistant, conductive powder material or fibrous material can be used, and preferable ones are carbon blacks (carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black and Thermal black), metal powder (aluminum powder, nickel powder, etc.) and conductive metal oxide (zinc oxide, titanium oxide, etc.). A conductive support agent may be used individually by 1 type, or may use 2 or more types together.

  As the binder, a known binder used for forming an electrode of a lithium ion secondary battery can be used, and preferable ones include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene. Styrene butadiene rubber, polyethylene, polypropylene and the like. A binder may be used individually by 1 type, or may use 2 or more types together.

The negative electrode in the lithium ion secondary battery of the present invention comprises a current collector, a negative electrode active material, a conductive additive, and a binder.
The current collector is not particularly limited as long as it is a metal foil that is electrochemically stable in the range of 0 V to 2.0 V on the basis of metallic lithium, and generally used copper foil is preferable.

Examples of the negative electrode active material include a polymer compound fired body (phenol resin and furan resin fired and carbonized), coke (pitch coke, needle coke, petroleum coke, etc.), carbon fiber, conductive polymer (polyacetylene). And natural graphite, artificial graphite, tin, silicon and metal alloys (lithium-aluminum alloy, lithium-aluminum-manganese alloy, lithium-tin alloy, lithium-silicon alloy, etc.) and the like. A substance may be used individually by 1 type, or may use 2 or more types together.

It is not always necessary to use a conductive additive for the negative electrode. However, if it is used, a conductive powder material or fibrous material can be used as the conductive additive, and carbon blacks (carbon Black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc.), metal powder (aluminum powder, nickel powder, etc.) and conductive metal oxide (zinc oxide, titanium oxide, etc.) It is done. A conductive support agent may be used individually by 1 type, or may use 2 or more types together.
As the binder, a known binder used for forming an electrode of a lithium ion secondary battery can be used, and preferable ones include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene. Styrene butadiene rubber, polyethylene, polypropylene and the like. A binder may be used individually by 1 type, or may use 2 or more types together.

The separator in the lithium ion secondary battery of the present invention includes a microporous film made of polyethylene film, a microporous film made of polypropylene film, a multilayer film of porous polyethylene film and polypropylene, polyester fiber, aramid fiber and glass fiber And non-woven fabrics composed of these materials, and those having ceramic fine particles such as silica, alumina and titania attached to their surfaces.

As the battery can in the lithium ion secondary battery of the present invention, metal materials such as stainless steel, iron, aluminum and nickel-plated steel can be used, but plastic materials can also be used depending on the battery application. Further, the battery can can be formed into a cylindrical shape, a coin shape, a square shape, or any other shape depending on the application. Although it will not specifically limit if it is a thing for lithium ion secondary batteries as an aluminum laminate film, The aluminum laminate film for lithium ion secondary batteries which can be obtained from Showa Denko etc. and can be heat-sealed can be used.

  Hereinafter, although an example and a comparative example explain the present invention further, the present invention is not limited to these. Hereinafter, unless otherwise specified, “%” represents “% by weight” and “parts” represents “parts by weight”.

Example 1
In a reaction vessel equipped with a thermometer, a heating / cooling device, a dropping device, and a stirrer, 100 parts of propylene carbonate (manufactured by Kishida Chemical), cyclic ketone compound (B-1) represented by the following general formula (2) (3- 2 parts of methylene-2-norbornanone (manufactured by Tokyo Chemical Industry Co., Ltd.) and 13 parts of LiPF ₆ (manufactured by Kishida Chemical) were added and stirred for 3 hours while maintaining the temperature at 25 ° C. 1) 115 parts were obtained. It was confirmed that there was no insoluble matter in appearance.

Examples 2-5 and Comparative Examples 1-5
The nonaqueous solvent (A), compound, and electrolyte (C) listed in Table 1 were blended in the number of parts and stirred to obtain electrolytic solutions of Examples 2 to 5 and Comparative Examples 1 to 5.
The external appearance of the electrolyte solution of Comparative Example 3 was solid without dissolving ethylene carbonate in propylene carbonate, but all other electrolyte solutions were confirmed to be completely liquid with the electrolyte salt completely dissolved. .

In addition, the nonaqueous solvent (A), compound, and electrolyte (C) used in Table 1 are as follows.
(A): Propylene carbonate [relative dielectric constant 64.4, manufactured by Kishida Chemical]
(A′-1): Diethyl carbonate [relative permittivity 2.8, manufactured by Kishida Chemical]
(A'-2): Ethyl methyl carbonate [relative permittivity 2.9, manufactured by Kishida Chemical]
(A'-3): Dimethyl carbonate [relative dielectric constant 2.8, manufactured by Kishida Chemical]
(A′-4): Ethylene carbonate [relative permittivity 95.3, manufactured by Kishida Chemical]
(B-1): Cyclic ketone compound of the above chemical formula (2) [3-methylene-2-norbornanone, manufactured by Tokyo Chemical Industry]
(B′-1): Cyclic ketone compound of the following chemical formula (3) [manufactured by Tokyo Chemical Industry]
(B′-2): carbonate compound of the following chemical formula (4) (B′-3): carbonate compound of the following chemical formula (5) [manufactured by Tokyo Chemical Industry]

(C-1): LiPF ₆ [lithium hexafluorophosphate, manufactured by Kishida Chemical]
(C-2): LiFSI [Lithium bis (fluorosulfonyl) imide, manufactured by Nippon Shokubai]
(C-3): LiTFSI [Lithium bis (trifluoromethanesulfonyl) imide, manufactured by Kishida Chemical]
(C-4): LiBF ₄ [lithium borofluoride, manufactured by Kishida Chemical]

<Production of negative electrode>
The negative electrode of the battery for evaluation of the electrolyte for the lithium ion secondary battery was obtained by punching a negative electrode sheet of spherical graphite (manufactured by Hosen, HS-LIB-N-Gr-001) into a disk shape having a diameter of 16 mm and an electrode for the negative electrode. did.

<Preparation of positive electrode>
The positive electrode of the battery for evaluation of the electrolyte for a lithium ion secondary battery is a positive electrode made by punching a lithium cobaltate positive electrode sheet (made by Hosen, HS-LIB-P-Co-001) into a disk shape having a diameter of 15 mm. It was.

<Production of evaluation lithium-ion secondary battery>
Separators (made by Asahi Kasei, Cellguard 2500 (made of polypropylene)), with a negative electrode and a positive electrode punched in a disc shape placed at both ends in a 2032 type coin cell so that the coated surfaces face each other, and punched to a diameter of 17 mm between the electrodes ) Was inserted to prepare a cell for evaluating an electrolyte for a lithium ion secondary battery.
After injecting the electrolytic solutions of Examples 1 to 5 and Comparative Examples 1 to 5 into this cell, a battery for electrolytic solution evaluation was produced.

Using the electrolyte evaluation battery, the performance of the initial discharge capacity and the discharge capacity retention rate after 100 cycles was evaluated.
In addition, since it was confirmed that the electrolyte solution (D′-3) of Comparative Example 3 was insoluble in appearance, the battery evaluation was not performed.
<Measurement of initial discharge capacity>
Keep the battery at 25 ° C., charge / discharge measuring device (Battery Analyzer Model 1470, manufactured by Toyo Technica Co., Ltd.) with a current of 0.1 C to 4.2 V, and after 10 minutes of rest, with a current of 0.1 C The battery was discharged to 2.5V. The discharge capacity at this time was defined as the initial discharge capacity and is shown in Table 1.
The larger this value, the larger the initial battery capacity. Under this measurement condition, 6 mAh or more is preferable.

<Method of charge / discharge at 2nd cycle to 99th cycle>
The battery was kept at 25 ° C., charged to 4.2 V with a current of 0.2 C using a charge / discharge measuring device (manufactured by Toyo Technica, battery analyzer 1470 type), rested for 10 minutes, and then with a current of 0.2 C The battery was discharged to 2.5V. After 10 minutes of rest, charging, 10 minutes of rest and discharge were repeated with the same current value, charge upper limit voltage and discharge lower limit voltage.
<Measurement of discharge capacity at 100th cycle>
Charging and discharging were performed in the same manner as when measuring the initial discharge capacity, and the discharge capacity at this time was taken as the 100th cycle discharge capacity. The larger this value, the greater the battery capacity after 100 cycles.

<Calculation of discharge capacity maintenance rate after 100 cycles>
The value of “100th cycle discharge capacity / initial discharge capacity” was calculated using the discharge capacity data measured by the above method, and the results obtained as the discharge capacity retention rate (%) after 100 cycles are shown in Table 1. Under these measurement conditions, 90% or more is preferable.

In the electrolyte solutions for lithium ion secondary batteries of Examples 1 to 5 of the present invention, since the side reaction of propylene carbonate in the negative electrode by the cyclic ketone compound (B-1) is sufficiently suppressed, the discharge capacity is maintained after 100 cycles. All the rates are 90% or more, and the deterioration of the charge / discharge characteristics is small.
On the other hand, since the electrolytic solution (D′-1) of Comparative Example 1 containing no cyclic ketone compound does not sufficiently suppress the side reaction of propylene carbonate in the negative electrode, the discharge capacity retention rate after 100 cycles is 10%, and the charge / discharge characteristics Degradation is large.
In addition, the electrolytic solution (D′-2) of Comparative Example 2 in which the cyclic ketone compound (B′-1) having a carbon-carbon double bond inside the ring was added instead of the cyclic ketone compound of the present invention was propylene carbonate in the negative electrode. Since the side reaction is not sufficiently suppressed, the discharge urine volume retention rate after 100 cycles is 25%, and the charge / discharge characteristics are greatly deteriorated.
Since the electrolytic solution (D′-3) of Comparative Example 3 in which the propylene carbonate / nonaqueous solvent (A) was less than 30% by weight was a non-uniform solid at room temperature, it could not be used as the electrolytic solution.
The cyclic carbonate compound (B′-2) described in Patent Document 1 was used as the electrolytic solution (D′-4) of Comparative Example 4, but the side reaction suppression performance of propylene carbonate in the negative electrode was not sufficient, and after 100 cycles. The discharge capacity maintenance rate was 55%, which was improved as compared with Comparative Examples 1 to 5, but the maintenance rate was insufficient.
As the electrolytic solution (D′-5) of Comparative Example 5, the cyclic carbonate compound (B′-3) described in Patent Document 2 was used, but the discharge capacity retention rate after 100 cycles was insufficient at 60%.

Since the electrolyte solution for lithium ion secondary batteries of the present invention has an excellent ability to suppress side reaction of propylene carbonate in the negative electrode, the lithium ion secondary battery having improved safety by using propylene carbonate having a high flash point and a low melting point. It can be suitably used as an electrolyte for a secondary battery.
The lithium ion secondary battery using the electrolytic solution for a lithium ion secondary battery of the present invention is useful as an in-vehicle secondary battery or a secondary battery for mobile devices that require high safety.

Claims (4)

A non-aqueous solvent (A) containing propylene carbonate (a), a cyclic ketone compound (B) represented by the following general formula (1), and an electrolyte (C), and based on the weight of (A) The electrolyte solution for lithium ion secondary batteries (D) whose content of (a) is 25-100 weight%.

[R ¹ and R ² are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenyl group. ] 2. The electrolyte solution for a lithium ion secondary battery according to claim 1, wherein the content of (B) is 0.5 to 10 wt% based on the total weight of (A) to (C). The electrolytic solution for a lithium ion secondary battery according to claim 1 or 2, wherein R ¹ and R ² in the general formula (1) are hydrogen atoms.   The lithium ion secondary battery which consists of electrolyte solution (D) for lithium ion secondary batteries in any one of Claims 1-3, a positive electrode, and a negative electrode.