JP7092525B2 - Lithium Ion Secondary Battery Electrolyte Composition and Lithium Ion Secondary Battery - Google Patents

Description

The present disclosure relates to an electrolytic solution composition for a lithium ion secondary battery.

Japanese Patent Application Laid-Open No. 2001-229966 (Patent Document 1) discloses a gel-like electrolyte composed of an aprotic organic solvent, a lithium salt, and inorganic oxide fine particles.

2001-229966 Publication No.

Generally, the electrolytic solution of a lithium ion secondary battery is combustible. It is considered that the solvent of the electrolytic solution is an organic solvent having a low boiling point and a low flash point. It is considered that the gel-like electrolyte disclosed in Patent Document 1 also exhibits combustibility. Therefore, there is a demand for an electrolytic solution composition for a lithium ion secondary battery having self-extinguishing properties. In the present specification, "combustibility" means that if there is an ignition source, it ignites, and combustion does not stop even if the ignition source is moved away, and "self-extinguishing" means that there is an ignition source. If it ignites, it will ignite, but if the ignition source is far away, combustion will stop.

An object of the present disclosure is to provide an electrolytic solution composition for a lithium ion secondary battery having self-extinguishing properties.

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

[1] The electrolytic solution composition of the present disclosure contains an aprotic organic solvent, a lithium salt, silica nanoparticles, and diphosphorus pentoxide ( _{P4O 10} ).

Since the electrolytic solution composition has the above structure, it is expected to have self-extinguishing property. The lithium ion secondary battery containing the electrolytic solution composition is expected to suppress thermal runaway at the time of abnormality.

In the coexistence of an aprotonic organic solvent (hereinafter, also simply referred to as "solvent") and oxygen, and in the presence of an ignition source, a rapid oxidation reaction (that is, a combustion reaction) occurs, and hydrogen radicals (H. ) And hydroxy radicals (OH ·) and the like are considered to be generated. It is considered that the combustion reaction continues due to the chain reaction of these radicals.

It is considered that a large number of hydroxyl groups (-OH) are present on the surface of the silica nanoparticles. It is considered that the electrolytic solution composition contains diphosphorus pentoxide in addition to the silica nanoparticles, so that an interaction occurs between the surface of the silica nanoparticles and diphosphorus pentoxide. As a result of such an interaction, a P—OH bond and a P—O—Si bond are formed from the P—O—P bond possessed by diphosphorus pentoxide, as represented by the following general formula (1). it is conceivable that.

The surface of the silica nanoparticles dispersed in the solvent can be the reaction field of the above-mentioned combustion reaction. In the above general formula (1), the P—OH bond formed from the P—O—P bond possessed by diphosphorus pentoxide is considered to function as a radical trap group for trapping radicals. It is expected that the radical chain reaction is suppressed by the selective and high concentration of P-OH bonds in the reaction field of the combustion reaction (that is, the surface of the silica nanoparticles). That is, it is considered that the electrolytic solution composition of the present disclosure may have self-extinguishing property.

FIG. 1 is a schematic view showing an example of the configuration of the lithium ion secondary battery of the present embodiment.

Hereinafter, embodiments of the present disclosure (also referred to as “the present embodiment” in the present specification) will be described. However, the following explanation does not limit the scope of claims.

<Electrolytic solution composition>
The electrolyte composition of this embodiment contains at least an aprotic organic solvent, a lithium salt, silica nanoparticles, and diphosphorus pentoxide. The electrolytic solution composition may further contain other components (described later).

The electrolyte composition of the present embodiment may have self-extinguishing properties. Further, it is considered that the electrolytic solution composition may also have sufficient conductivity.

<< Aprotic organic solvent >>
The aprotic organic solvent should not be particularly limited. The aprotic organic solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio may be, for example, cyclic carbonate / chain carbonate = 10/90 to 50/50 in terms of volume ratio.

The cyclic carbonate may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), or the like. One type of cyclic carbonate may be used alone. Two or more cyclic carbonates may be used in combination.

The chain carbonate may be, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or the like. One type of chain carbonate may be used alone. Two or more chain carbonates may be used in combination.

The solvent may contain, for example, a lactone, a cyclic ether, a chain ether, a carboxylic acid ester and the like. The lactone may be, for example, γ-butyrolactone (GBL), δ-valerolactone and the like. The cyclic ether may be, for example, tetrahydrofuran (THF), 1,3-dioxolane, 1,4-dioxane or the like. The chain ether may be, for example, 1,2-dimethoxyethane (DME) or the like. The carboxylic acid ester may be, for example, methylformate (MF), methylacetate (MA), methylpropionate (MP) or the like.

《Lithium salt》
Lithium salts are supporting salts. The lithium salt is dissolved in the solvent. The electrolytic solution composition may contain, for example, a lithium salt of 0.5 mL / l or more and 2 mL / l or less. The lithium salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ], Li [N (CF ₃ SO ₂ ) ₂ ], or the like. One type of lithium salt may be used alone. Two or more kinds of lithium salts may be used in combination.

Lithium salts dissociate into lithium ions and counter anions in the solvent. It is considered that the more lithium ions generated by the dissociation of the lithium salt, the higher the conductivity of the electrolytic solution composition. In the electrolytic solution composition of the present embodiment, for example, LiPF ₆ is expected to promote the dissociation of the lithium salt because the counter anion is a molecular ion and the terminal of the molecular ion is a fluorine atom. Since a hydrogen bond is easily formed between the fluorine atom (F) of the counter anion (PF ₆ ⁻ ) and the hydroxyl group (OH) of the silica nanoparticles, it is considered that lithium ion (Li ⁺ ) is easily desorbed.

《Silica nanoparticles》
Silica nanoparticles impart self-extinguishing properties to the electrolyte composition. The silica nanoparticles are dispersed in the solvent. The silica nanoparticles may have a ratio of, for example, 3% by mass or more and 5% by mass or less with respect to the electrolytic solution composition. Good charge / discharge characteristics and conductivity are also expected in this range. If the mass ratio of the silica nanoparticles is excessively high, for example, the viscosity of the electrolytic solution composition may increase and the conductivity may decrease.

The silica nanoparticles may have an average particle size of, for example, 1 nm or more and 100 nm or less. The average particle size of silica nanoparticles is measured, for example, by dynamic light scattering or electron microscopy. The electron microscope may be a transmission electron microscope (TEM) or a scanning electron microscope (SEM). The measurement can be performed according to, for example, "JIS Z8826". The average particle size can be measured at least 3 times. An arithmetic average of at least 3 times can be adopted as the measurement result.

Silica nanoparticles can have a large number of hydroxyl groups on their surface. In silica nanoparticles, the hydroxyl groups may have, for example, a number density of 3 / nm ² or more and 5 / nm ² or less. It is expected that trapping of hydrogen radicals and hydroxyl radicals will be promoted in this range. The concentration of hydroxyl groups can be measured, for example, by potentiometric titration, infrared spectroscopy or solid-state nuclear magnetic resonance. The concentration of hydroxyl groups can be measured at least 3 times. At least three arithmetic means can be adopted as the measurement result.

《Phosphorus pentoxide》
Diphosphorus pentoxide imparts self-extinguishing properties to the electrolyte composition. Diphosphorus pentoxide is dispersed in the solvent. Diphosphorus pentoxide may have a ratio of, for example, 6% by mass or more and 10% by mass or less with respect to the electrolytic solution composition. If the mass ratio of diphosphorus pentoxide is excessively high, for example, the conductivity may decrease.

<< Other ingredients >>
The electrolytic solution composition may further contain other components in addition to the above components. Examples of other components include various functional additives and the like. The functional additive may have a ratio of, for example, 0.1% by mass or more and 10% by mass or less with respect to the electrolytic solution composition. Examples of the functional additive include a gas generating agent (so-called overcharge additive), a SEI (solid electrolyte engine) film forming agent, and the like.

Examples of the gas generating agent include cyclohexylbenzene (CHB), biphenyl (BP) and the like. Examples of the SEI film-forming agent include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), Li [B (C ₂ O ₄ ) ₂ ] (LiBOB), LiPO ₂ F ₂ , propane sulton (PS), and ethylene. Sulfite (ES) and the like can be mentioned.

<Lithium-ion secondary battery>
FIG. 1 is a schematic view showing an example of the configuration of the lithium ion secondary battery of the present embodiment.
Hereinafter, the lithium ion secondary battery may be abbreviated as "battery". The battery 100 includes a case 50. The case 50 is hermetically sealed. The case 50 is a rectangular parallelepiped (flat rectangular parallelepiped). The case 50 may be formed of, for example, an aluminum (Al) alloy or the like. Of course, the case should not be limited to squares. The case may be cylindrical. The case may be, for example, a pouch made of an Al laminated film. That is, the battery may be a laminated battery.

The case 50 houses the electrode group 40 and the electrolytic solution composition. The alternate long and short dash line in FIG. 1 indicates the liquid level of the electrolytic solution composition. The electrolytic solution composition is the electrolytic solution composition of the present embodiment described above. That is, the battery 100 contains at least the electrolytic solution composition of the present embodiment. The details of the electrolytic solution composition are as described above. The battery 100 is expected to be resistant to abnormal use accompanied by heating, short circuit, overcharging and the like. This is because the electrolytic solution composition may have self-extinguishing properties.

The electrode group 40 is electrically connected to an external terminal. The electrode group 40 includes a positive electrode, a negative electrode, and a separator. The voids in the electrode group 40 are impregnated with the electrolytic solution composition. The electrode group 40 is a stacked type. That is, the electrode group 40 is formed by alternately laminating the positive electrode and the negative electrode while sandwiching the separator between the positive electrode and the negative electrode. Of course, the electrode group may be a winding type. The winding type electrode group can be formed, for example, by laminating a positive electrode, a separator and a negative electrode in this order, and further winding them in a spiral shape.

《Positive electrode》
The positive electrode can be in the form of a sheet. The positive electrode may include, for example, a positive electrode current collector and a positive electrode mixture layer. The positive electrode current collector may be, for example, an Al foil or the like. The positive electrode mixture layer can be formed on the surface of the positive electrode current collector. The positive electrode mixture layer may be formed on both the front and back surfaces of the positive electrode current collector. The positive electrode mixture layer contains at least the positive electrode active material. The positive electrode mixture layer may contain, for example, 80% by mass or more and 98% by mass or less of the positive electrode active material, 1% by mass or more and 10% by mass or less of the conductive material, and the balance binder.

The positive electrode active material should not be particularly limited. The positive electrode active material is, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _1/3 Co _1/3 Mn _1/3 ) O ₂ , LiNi _0.8 Co _0.15 Al _0.05 . It may be O ₂ , LiFePO ₄ , or the like. One kind of positive electrode active material may be used alone. Two or more kinds of positive electrode active materials may be used in combination. The conductive material should not be particularly limited. The conductive material may be, for example, acetylene black (AB) or the like. Binders should not be particularly limited either. The binder may be, for example, polyvinylidene fluoride (PVdF) or the like.

《Negative electrode》
The negative electrode can be in the form of a sheet. The negative electrode may include, for example, a negative electrode current collector and a negative electrode mixture layer. The negative electrode current collector may be, for example, a copper (Cu) foil or the like. The negative electrode mixture layer can be formed on the surface of the negative electrode current collector. The negative electrode mixture layer may be formed on both the front and back surfaces of the negative electrode mixture layer. The negative electrode mixture layer contains at least the negative electrode active material. The negative electrode mixture layer may contain, for example, 80% by mass or more and 99.5% by mass or less of the negative electrode active material, and the balance binder.

The negative electrode active material should not be particularly limited. The negative electrode active material may be, for example, graphite, soft carbon, hard carbon, silicon, silicon oxide, a silicon-based alloy, tin, tin oxide, a tin-based alloy, or the like. One kind of negative electrode active material may be used alone. Two or more kinds of negative electrode active materials may be used in combination. Binders should not be particularly limited either. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), or the like.

《Separator》
The separator can be a porous film. The separator is electrically insulating. The separator may be made of, for example, polyethylene (PE), polypropylene (PP), or the like. The separator may have, for example, a single layer structure. The separator may be formed only of, for example, a porous film made of PE. The separator may have, for example, a multi-layer structure. The separator may be formed, for example, by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order.

Hereinafter, examples of the present disclosure will be described. However, the following explanation does not limit the scope of claims.

<Preparation of electrolyte composition and manufacture of lithium-ion secondary battery>
<< Example 1 >>
1. 1. Preparation of electrolyte composition The following materials were prepared.
Aprotic organic solvent: EC, DMC, EMC
Lithium salt: LiPF ₆
Silica nanoparticles (SiO ₂ , average particle diameter 7 nm)
Flame Retardant: Diphosphorus pentoxide

The solvent was prepared by mixing EC, DMC and EMC. The mixing ratio is EC: DMC: EMC = 30:40:30 in terms of volume ratio. LiPF ₆ was dissolved in the solvent. The concentration of LiPF ₆ is 1 mol / l. As a result, a standard electrolyte was prepared.

Silica nanoparticles and diphosphorus pentoxide were added to the standard electrolytic solution to prepare an electrolytic solution composition. The silica nanoparticles have a ratio of 3% by mass with respect to the electrolytic solution composition. Diphosphorus pentoxide has a ratio of 6% by mass with respect to the electrolytic solution composition.

2. 2. Manufacture of positive electrode The following materials were prepared.
Positive electrode active material: Li (Ni _1/3 Co _1/3 Mn _1/3 ) O ₂ (NCM)
Conductive material: AB
Binder: PVdF
Solvent: N-methyl-2-pyrrolidone (NMP)
Positive electrode current collector: Al foil (thickness = 12 μm)

A positive electrode mixture slurry was prepared by mixing NCM, AB, PVdF, and NMP. The mixing ratio of the solid content is "NCM: AB: PVdF = 90: 4: 3 (mass ratio)". The positive electrode mixture slurry was applied to the surface (both front and back surfaces) of the positive electrode current collector (weight: 39.5 mg / cm ² ) and dried. As a result, a positive electrode mixture layer was formed. The positive electrode mixture layer was rolled. The positive electrode mixture layer after rolling had a density of 2.9 g / cm ³ . This produced a positive electrode. The positive electrode was cut into a sheet.

3. 3. Manufacture of negative electrode The following materials were prepared.
Negative electrode active material: Amorphous coated graphite binder: CMC, SBR
Solvent: Water Negative electrode current collector: Cu foil (thickness 10 μm)

Amorphous coated graphite, CMC, SBR, and water were mixed to prepare a negative electrode mixture slurry. The mixing ratio of the solid content is "amorphous coated graphite: CMC: SBR = 99: 0.5: 0.5 (mass ratio)". The negative electrode mixture slurry was applied to the surface (both front and back surfaces) of the negative electrode current collector (weight: 16.9 mg / cm ² ) and dried. As a result, a negative electrode mixture layer was formed. The negative electrode mixture layer was rolled. The negative electrode mixture layer after rolling had a density of 1.4 g / cm ³ . As a result, the negative electrode was manufactured. The negative electrode was cut into a sheet.

4. Battery manufacturing Separator (polyethylene porous film) was prepared. The separator was impregnated with the electrolyte composition under an argon atmosphere. The positive electrode, the separator and the negative electrode were laminated so that the positive electrode and the negative electrode faced each other across the separator. As a result, a group of electrodes was formed. A case (pouch made of Al laminated film) was prepared. The electrode group was housed in a case. The case was sealed. From the above, a bipolar electrochemical cell (lithium ion secondary battery) was manufactured.

<< Example 2 to Example 5 >>
As shown in Table 1 below, the electrolytic solution composition was prepared in the same manner as in Example 1 except that the ratios of silica nanoparticles and diphosphorus pentoxide to the electrolytic solution composition were changed, and the electrolytic solution was prepared. A battery containing the composition was manufactured.

<< Comparative Example 1 >>
As shown in Table 1 below, the electrolytic solution composition was prepared in the same manner as in Example 1 except that the standard electrolytic solution was used as the electrolytic solution composition, and a battery containing the electrolytic solution composition was manufactured. Was done.

<< Comparative Example 2 >>
As shown in Table 1 below, the same as in Example 1 except that the ratio of silica nanoparticles to the electrolytic solution composition was 5% by mass and the ratio of diphosphorus pentoxide was 0% by mass. An electrolytic solution composition was prepared, and a battery containing the electrolytic solution composition was manufactured.

<< Comparative Example 3 >>
As shown in Table 1 below, the same as in Example 1 except that the ratio of silica nanoparticles to the electrolytic solution composition was 0% by mass and the ratio of diphosphorus pentoxide was 10% by mass. An electrolytic solution composition was prepared, and a battery containing the electrolytic solution composition was manufactured.

<Evaluation>
《Ignition test》
Quartz filter paper was prepared. The quartz filter paper has external dimensions of width 10 mm × length 50 mm × thickness 0.42 mm. Quartz filter paper was hung vertically. 100 μl of the electrolyte composition was impregnated from the upper end side of the quartz filter paper. The lighter's fire was brought close to the bottom edge of the quartz filter paper. In that state, the presence or absence of ignition was confirmed for 1 second. The confirmation results are shown in Table 1 below. "A" and "B" shown in the column of "Ignition test" in Table 1 below indicate the following contents. If the confirmation result is "A", it is considered that the electrolytic solution composition has "self-extinguishing property".

A: It ignites, but the fire goes out soon afterwards.
B: Ignite and continue to burn.

<< Charge / discharge efficiency test >>
The charge / discharge efficiency of the electrochemical cell was measured. Charging was carried out by a constant current constant voltage (CCCV) method. The current at the time of constant current charging is 0.3 mA. The voltage at the time of constant voltage charging is 4.2V. Charging was terminated when the constant voltage charging time reached 2 hours. The discharge was carried out by the CCCV method. The current at the time of constant current discharge is 0.3 mA. The voltage at the time of constant voltage discharge is 3.0V. The discharge was terminated when the constant voltage discharge time reached 2 hours.

This cycle of charging and discharging was regarded as one cycle, and three cycles were carried out. The charge / discharge efficiency of the third cycle was calculated by dividing the discharge capacity of the third cycle by the charge capacity of the third cycle. The results are shown in Table 1 below. It is considered that the higher the charge / discharge efficiency, the better. Although the charge / discharge efficiency test was not performed on Comparative Example 2 and Comparative Example 3, it is estimated that the charge / discharge efficiency is about 92% to 100%.

<Result>
As shown in Table 1 above, Examples 1 to 5 in which the electrolytic solution composition contained an aprotic organic solvent, a lithium salt, silica nanoparticles, and diphosphorus pentoxide had self-extinguishing properties. .. That is, it was shown that an electrolytic solution composition for a lithium ion secondary battery having self-extinguishing property is provided.

Examples 1 to 5 had good charge / discharge efficiency in addition to self-extinguishing property. A large number of hydroxyl groups (-OH) are present on the surface of the silica nanoparticles. It is considered that the hydrogen atom at the end of the hydroxyl group is positively charged (δ ⁺ ) due to polarization. On the other hand, it is considered that the fluorine atom of LiPF ₆ is negatively charged (δ- ⁾ . It is considered that a hydrogen bond acts between the hydroxyl group (-OH) on the surface of the silica nanoparticles and the fluorine atom in LiPF ₆ to attract PF ₆ ^- to the surface of the silica nanoparticles and promote the dissociation from Li ⁺ . Be done. As a result, the movement of Li ⁺ in the electrolytic solution did not decrease, and it is considered that good charge / discharge efficiency was obtained.

When the electrolyte composition did not contain either silica nanoparticles or diphosphorus pentoxide, no self-extinguishing property was observed (Comparative Example 1). No self-extinguishing property was observed when the silica nanoparticles were used alone (Comparative Example 2). In addition, no self-extinguishing property was observed even when diphosphorus pentoxide was used alone (Comparative Example 3). In the examples, the inclusion of silica nanoparticles and diphosphorus pentoxide in the electrolyte composition allows the combustion reaction field on the surface of the silica nanoparticles to selectively and highly concentrate P-OH bonds having a radical trapping effect. However, it is considered that the self-extinguishing effect was exhibited.

The embodiments and examples disclosed this time are exemplary in all respects and are not restrictive. The technical scope defined by the description of the scope of claims includes all changes within the meaning and scope equivalent to the scope of claims.

40 electrodes, 50 cases, 100 batteries.

Claims (3)

Aprotic solvent,
Lithium salt,
Silica nanoparticles and
Contains diphosphorus pentoxide,
An electrolytic solution composition for a lithium ion secondary battery .
The silica nanoparticles have a ratio of 3% by mass or more and 5% by mass or less with respect to the electrolytic solution composition.
The silica nanoparticles have a hydroxyl group on the surface thereof, and the silica nanoparticles have a hydroxyl group on the surface thereof.
The hydroxyl group has a number density of 3 / nm ² or more, and has a number density of 3 or more.
The diphosphorus pentoxide has a ratio of 6% by mass or more and 10% by mass or less with respect to the electrolytic solution composition.
Electrolyte composition. The silica nanoparticles have an average particle size of 1 nm or more and 100 nm or less.
The electrolytic solution composition according to claim 1. The electrolyte composition according to claim 1 or 2.
Lithium-ion secondary battery.

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