JP6848749B2 - Secondary battery - Google Patents

Description

The present disclosure relates to a secondary battery.

A secondary battery such as a lithium ion secondary battery is lighter in weight and has a higher energy density than an existing battery, and is therefore preferably used as a so-called portable power source or a high output power source for mounting on a vehicle. For such secondary batteries, further increase in energy density is being studied as part of improving the performance.

For example, Patent Document 1 discloses a secondary battery in which a particle coating film containing a metal salt is formed on the surface of negative electrode active material particles.

Japanese Unexamined Patent Publication No. 2009-021229

In a secondary battery, there is known a technique of impregnating a non-aqueous electrolyte solution with a fluorine-based solvent to increase the oxidation potential of the non-aqueous electrolyte solution and suppress oxidative decomposition in a high potential state. However, when a non-aqueous electrolytic solution containing a fluorine-based solvent is used, there is a problem that the electrolytic solution is reduced and decomposed and the amount of self-discharge increases.

The present disclosure has been made in view of the above problems, and when a non-aqueous electrolytic solution containing a fluorine-based solvent is used, it is possible to suppress the reductive decomposition of the electrolytic solution and suppress the increase in the amount of self-discharge. Its main purpose is to provide secondary batteries.

In order to achieve the above object, in the present disclosure, it is formed between the negative electrode active material layer having the negative electrode active material, the positive electrode active material layer having the positive electrode active material, and the negative electrode active material layer and the positive electrode active material layer. A secondary battery having an electrolyte layer having a non-aqueous electrolyte solution, wherein the non-aqueous electrolyte solution contains a fluorine-based solvent, the negative electrode active material is a carbon material, and the surface of the negative electrode active material is , A secondary battery characterized by being coated with a metal organic structure.

According to the present disclosure, since the surface of the negative electrode active material is coated with the metal-organic framework, it is possible to suppress the reductive decomposition of the electrolytic solution and suppress the increase in the amount of self-discharge.

The secondary battery of the present disclosure has an effect that even when an electrolytic solution having low reduction resistance is used, it is possible to suppress the reductive decomposition of the electrolytic solution and suppress an increase in the amount of self-discharge.

It is the schematic sectional drawing which shows an example of the secondary battery of this disclosure. It is a graph which shows the voltage difference before and after the high temperature storage of the secondary battery obtained in Example, Comparative Example and Reference Example 1-2. It is a graph which shows the charge resistance of the secondary battery obtained in Example, Comparative Example and Reference Example 1-2.

Hereinafter, the secondary battery according to the embodiment of the present disclosure will be described in detail.

FIG. 1 is a schematic cross-sectional view showing an example of the secondary battery of the present disclosure. The secondary battery 10 shown in FIG. 1 is formed between a negative electrode active material layer 1 having a negative electrode active material, a positive electrode active material layer 2 having a positive electrode active material, and a negative electrode active material layer 1 and a positive electrode active material layer 2. The non-aqueous electrolyte solution contains a fluorine-based solvent, the negative electrode active material is a carbon material, and the surface of the negative electrode active material is a metal organic structure. It is covered with. Further, the secondary battery 10 shown in FIG. 1 includes a negative electrode current collector 4 that collects electricity from the negative electrode active material layer 1, a positive electrode current collector 5 that collects electricity from the positive electrode active material layer 2, and members thereof. It has a battery case 6 for storing the above.

According to the present disclosure, since the surface of the negative electrode active material is coated with the metal-organic framework, it is possible to suppress the reductive decomposition of the electrolytic solution and suppress the increase in the amount of self-discharge. By exhibiting such an effect, the secondary battery of the present disclosure can solve a problem peculiar to a secondary battery in which a fluorine-based solvent is used as a non-aqueous electrolytic solution. Here, the problems peculiar to the secondary battery using the fluorine-based solvent as the non-aqueous electrolytic solution are as follows.

Conventionally, in a secondary battery, there is known a technique of impregnating a non-aqueous electrolyte solution with a fluorine-based solvent to increase the oxidation potential of the non-aqueous electrolyte solution and suppress oxidative decomposition in a high potential state. Therefore, when we examined a secondary battery in which a non-aqueous electrolyte solution contained a fluorine-based solvent, a secondary battery in which the non-aqueous electrolyte solution contained a non-fluorine-based solvent (the non-aqueous electrolyte solution did not contain a fluorine-based solvent). It was found that the electrolytic solution is more easily reduced and decomposed than the secondary battery), and the self-discharge amount is remarkably increased. It should be noted that such a problem is clear from the results of the high temperature storage test (FIG. 2) of Examples and Reference Examples 2 described later. As described above, the present disclosure has a problem peculiar to a secondary battery using a fluorine-based solvent as a non-aqueous electrolytic solution.

The present disclosure is an electrolyte formed between a negative electrode active material layer having a negative electrode active material, a positive electrode active material layer having a positive electrode active material, the negative electrode active material layer, and the positive electrode active material layer, and having a non-aqueous electrolyte solution. A secondary battery having a layer, the non-aqueous electrolyte solution contains a fluorine-based solvent, the negative electrode active material is a carbon material, and the surface of the negative electrode active material is coated with a metal organic structure. The above-mentioned problems can be solved by providing the secondary battery characterized by the above. The reason for this is presumed to be as follows.

Metal-organic frameworks (MOFs) have a porous coordination network structure with a high surface area due to the interaction between metals and organic ligands. By coating the surface of the negative electrode active material with such a metal-organic structure, electron insulation and ionic conductivity can be imparted, whereby the decomposition reaction on the negative electrode active material layer can be suppressed. It is presumed. Further, in the present disclosure, the metal-organic framework can impart electronic insulation and ionic conductivity, and can suppress deterioration of battery performance. This is because the metal-organic structure has a predetermined structure as described above, so that the metal-organic structure coated on the surface of the negative electrode active material is preferable when lithium ions move between the positive electrode and the negative electrode of the secondary battery. It is thought that this is due to the fact that it exerts a large Li storage capacity. Specifically, it is considered that when the metal-organic framework exhibits a good Li storage capacity, for example, when Li ⁺ is inserted into the negative electrode active material during charging, the movement of ^{Li + is accelerated.} It should be noted that it is clear from the result of the charging resistance described later that the deterioration of the battery performance can be suppressed as shown in FIG.

In Patent Document 1, since the negative electrode active material layer contains a conductive material, it is expected that the electron conductivity of the negative electrode is improved, while the self-discharge amount is significantly increased. In addition, the organic ligands need to be symmetrical in order to form a metal-organic framework. Here, although dilithium phthalate and dilithium naphthalenedicarboxylate are disclosed in Patent Document 1, since the organic ligands of the above compounds disclosed in Patent Document 1 are not symmetric , the metal-organic framework is disclosed in Patent Document 1. It seems difficult to form a structure.

Hereinafter, the secondary battery of the present disclosure will be described separately for each configuration.

1. 1. Negative electrode active material layer The negative electrode active material layer in the present disclosure includes a negative electrode active material.

The negative electrode active material in the present disclosure is a carbon material, and the surface of the negative electrode active material is coated with a metal-organic framework. Examples of the metal-organic framework include dilithium terephthalate, lithium 2,6-naphthalenedicarboxylate, lithium 4,4'-biphenyldicarboxylic acid and the like.

The thickness of the metal-organic framework coated on the surface of the negative electrode active material can be, for example, in the range of 5 nm or more and 200 nm or less, and can be in the range of 10 nm or more and 100 nm or less. The thickness of the metal-organic framework can be measured by, for example, observation with a transmission electron microscope (TEM) (for example, n ≧ 100) and calculated as an average value.

The coverage of the metal-organic framework is preferably higher, for example, 50% or more, and preferably 80% or more. Further, the coverage of the metal-organic framework may be 100%. The coverage of the metal-organic framework can be measured using, for example, a transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), or the like.

The method of coating the surface of the negative electrode active material with the metal-organic framework has been described in the section of Examples described later, and thus the description thereof is omitted here.

Examples of the negative electrode active material in the present disclosure include a graphite material, that is, graphite. Specific examples thereof include natural graphite, artificial graphite, a mixture of natural graphite and artificial graphite, and natural graphite coated with artificial graphite.

The shape of the negative electrode active material is not particularly limited, and examples thereof include a spherical shape. When the shape of the negative electrode active material is spherical, the average particle size (D ₅₀ ) of the negative electrode active material is, for example, 1 nm or more, 10 nm or more, or 100 nm or more. On the other hand, the average particle size (D ₅₀ ) of the negative electrode active material is, for example, 50 μm or less, and may be 20 μm or less.

The content of the negative electrode active material in the negative electrode active material layer is preferably higher from the viewpoint of capacity, for example, 60% by weight or more and 99% by weight or less, 70% by weight or more and 95% by weight or less. May be good. The negative electrode active material layer may contain additives such as a binder and a thickener, if necessary, in addition to the negative electrode active material.

The thickness of the negative electrode active material layer varies greatly depending on the configuration of the battery, but can be, for example, 0.1 μm or more and 1000 μm or less.

The negative electrode active material layer may contain, for example, a binder or a thickener as other components.

2. Electrolyte layer The electrolyte layer in the present disclosure is a layer formed between a negative electrode active material layer and a positive electrode active material layer and having a non-aqueous electrolyte solution.

The non-aqueous electrolyte solution conducts metal ion conduction between the negative electrode active material layer and the positive electrode active material layer. The non-aqueous electrolyte solution is typically liquid at room temperature (for example, 25 ° C.), preferably always in the operating temperature range (for example, -30 to 60 ° C.). The non-aqueous electrolyte solution in the present disclosure contains a fluorine-based solvent, and usually also contains a supporting salt.

Examples of the fluorine-based solvent include fluorinated carbonate. Specific examples of the fluorinated carbonate include monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), and trifluoroethylmethyl. Examples thereof include carbonate (TFEMC). As such a fluorine-based solvent, one type can be used alone, or two or more types can be used in combination as appropriate.

The supporting salt is, for example, a supporting salt in the case of a lithium ion battery, which is appropriately selected according to the type of the target secondary battery. Lithium salts include, for example, inorganic lithium salts such as _{LiPF 6} , LiBF ₄ , LiClO ₄ and LiAsF _{6 ; and LiCF 3} SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiC. (CF ₃ SO ₂ ) Organic lithium salts such as _{3 and the like can be mentioned.} Further, for example, in the case of a sodium ion battery, examples of the sodium salt as a supporting salt include inorganic sodium salts such as _{NaPF 6} , NaBF ₄ , NaClO ₄ and NaAsF _{6 ; and NaCF 3} SO ₃ and NaN (CF ₃ SO ₂ ). _2. Organic sodium salts such as NaN (C ₂ F ₅ SO ₂ ) ₂ and NaC (CF ₃ SO ₂ ) _{3 can be mentioned.}

The concentration of the supporting salt in the non-aqueous electrolytic solution may be, for example, 0.3 mol / L or more and 5.0 mol / L or less, or 0.8 mol / L or more and 1.5 mol / L or less. If the concentration of the supporting salt is too low, the capacity at high rate may decrease, while if the concentration of the supporting salt is too high, the viscosity may increase and the capacity at low temperature may decrease. .. In the present disclosure, a low volatile liquid such as an ionic liquid may be used as the non-aqueous electrolytic solution.

The non-aqueous electrolytic solution may contain various additives such as a film-forming agent, a dispersant, and a thickener as other components. Since general materials can be used for these additives, the description here is omitted.

3. 3. Positive Electrode Active Material Layer The positive electrode active material layer in the present disclosure includes a positive electrode active material. The positive electrode active material layer in the present disclosure can be the same as that used in a general secondary battery, and is not particularly limited.

Examples of the positive electrode active material in the present disclosure include LiMn ₂ O ₄ , LiCoO ₄ , LiNiO ₂ , LiNi _0.5 Mn _1.5 O _4, LiFePO _{4, and the} like. Examples of the binder used for the positive electrode layer include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). The positive electrode active material layer may contain a conductive material, if necessary.

The thickness of the positive electrode active material layer varies greatly depending on the configuration of the battery, but can be, for example, 0.1 μm or more and 1000 μm or less.

4. Secondary battery In addition to the above-mentioned negative electrode active material layer, positive electrode active material layer and electrolyte layer, the secondary battery of the present disclosure includes, for example, a negative electrode current collector, a positive electrode current collector, a separator, a battery case, a spacer and a wave washer. Etc.

As the negative electrode current collector in the present disclosure, the same negative electrode current collector as that used in a general secondary battery can be used. Examples of the negative electrode current collector include a foil obtained by processing a metal such as copper and nickel into a plate shape. Further, as the positive electrode current collector in the present disclosure, for example, a foil obtained by processing a metal such as aluminum or stainless steel into a plate shape can be mentioned.

The separator in the present disclosure is a member having a function of separating the negative electrode active material layer and the positive electrode active material layer and holding a non-aqueous electrolytic solution. The separator in the present disclosure can be the same as that used in a general secondary battery, and is not particularly limited. Examples of the separator include a porous membrane such as polyethylene and polypropylene.

The battery case, spacer, wave washer, etc. in the present disclosure can be the same as the members used in a general secondary battery, and thus the description thereof is omitted here.

Examples of the secondary battery of the present disclosure include a lithium ion battery, a sodium ion battery, a magnesium ion battery, a calcium ion battery and the like.

The secondary battery of the present disclosure can be repeatedly charged and discharged, and is useful as, for example, an in-vehicle battery. The secondary battery referred to in the present disclosure also includes use as a primary battery (use for the purpose of discharging only once after charging).

The shape of the secondary battery of the present disclosure is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a cylindrical type, and a square type.

Since the method for manufacturing the secondary battery of the present disclosure can be the same as the method for manufacturing a general secondary battery, the description here is omitted.

The present disclosure is not limited to the above embodiment. The above embodiment is an example, and any object having substantially the same structure as the technical idea described in the claims of the present disclosure and exhibiting the same effect and effect is the present invention. Included in the technical scope of the disclosure.

The present disclosure will be described in more detail with reference to Examples below.

[Example]
<Manufacturing of negative electrode>
Natural graphite was prepared as the negative electrode active material. Next, terephthalic acid and lithium hydroxide were dissolved in water, and natural graphite was added thereto. After refluxing in an oil bath (temperature: 100 ° C.) for about 30 minutes, the solvent was removed using an evaporator. In this way, the surface of the negative electrode active material (natural graphite) was coated with a metal-organic framework (dilithium terephthalate). Next, the obtained negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with each other in a mass ratio of C: SBR: CMC = 98: 1: 1. Weighed so that it was mixed with water to prepare a negative electrode thriller. This negative electrode thriller was applied to the surface of a negative electrode current collector (copper foil), dried, and then pressed to prepare a negative electrode having a negative electrode active material layer on the negative electrode current collector.

<Cathode preparation>
_{NiMn spinel (LiNi 0.5} Mn _1.5 O ₄ , LNM) as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder, LNM: AB: PVdF. Weighed so as to have a mass ratio of = 87: 10: 3, and mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode thriller. This positive electrode thriller was applied to the surface of a positive electrode current collector (aluminum foil), dried, and then pressed to prepare a positive electrode having a positive electrode active material layer on the positive electrode current collector.

<Preparation of non-aqueous electrolyte solution>
1. _{LiPF 6} as a supporting salt is added to a mixed solvent containing monofluoroethylene carbonate (MFEC) and trifluoroethyl methyl carbonate (TFEMC) as a fluorosolvent in a volume ratio of MFEC: TFEMC = 50: 50. It was dissolved to a concentration of 0 mol / L.

<Making secondary batteries>
The obtained negative electrode and the positive electrode were opposed to each other with a separator sheet interposed therebetween, and were housed and sealed in a laminated battery case together with the non-aqueous electrolytic solution prepared as described above. In this way, the secondary battery of the present disclosure was produced.

[Comparison example]
A secondary battery was produced in the same manner as in Examples except that the surface of the negative electrode active material was not coated with a metal-organic framework.

[Reference example 1]
A non-aqueous electrolytic solution was prepared in the same manner as in Examples except that a non-fluorine-based solvent was used as the solvent contained in the non-aqueous electrolytic solution. As the non-fluorine-based solvent, a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and methyl carbonate (DMC) was used.

[Reference example 2]
A non-aqueous electrolytic solution was prepared in the same manner as in Comparative Example except that a non-fluorine-based solvent was used as the solvent contained in the non-aqueous electrolytic solution. As the non-fluorine solvent, a mixed solvent of EC, EMC, and DMC was used.

[Evaluation]
(High temperature storage test)
The secondary batteries obtained in Examples, Comparative Examples and Reference Examples 1 and 2 were subjected to a high temperature storage test, and the voltage difference before and after storage was measured. A large voltage difference before and after storage means a large amount of self-discharge, while a small voltage difference before and after storage means a small amount of self-discharge. In the high temperature storage test, the battery is stored in an environment of 60 ° C. with 100% SOC, and the difference (voltage drop) between the voltage before storage and the voltage one week after storage is measured and evaluated. did. The results are shown in FIG.

(Charging resistance)
The resistance of the secondary batteries obtained in Examples, Comparative Examples and Reference Examples 1 and 2 during charging was evaluated. Specifically, the resistance value after 10 seconds was calculated by energizing at 5C for 10 seconds from the state of SOC 60%. The secondary batteries obtained in Examples, Comparative Examples and Reference Examples 1 and 2 were discharged at a low current from a state of being charged to 60% of the capacity at 25 ° C., and the resistance was calculated from the voltage drop at that time. The results are shown in FIG.

When the self-discharge amounts of the examples and the comparative examples were compared, as shown in FIG. 2, in the example in which the surface of the negative electrode active material was coated with the metal-organic framework (MOF), the surface of the negative electrode active material was covered with the metal-organic structure. Compared with the comparative example in which the body did not cover the body, the increase in the amount of self-discharge could be suppressed. In addition, Examples and Comparative Examples in which a fluorine-based solvent was used as the non-aqueous electrolyte solution were compared with Reference Example 1 and Reference Example 2 in which the non-fluorine-based solvent was used as the non-aqueous electrolyte solution. As a result, it was found that the amount of self-discharge tends to increase when a fluorine-based solvent is used as the non-aqueous electrolytic solution. Further, as in the examples and the comparative examples, in Reference Example 1 and Reference Example 2 in which the non-fluorine-based solvent was used as the non-aqueous electrolytic solution, the surface of the negative electrode active material was coated with the metal-organic framework of Reference Example 1. Compared with Reference Example 2 in which the surface of the negative electrode active material was not coated with the metal-organic framework, the increase in the amount of self-discharge could be suppressed, but a fluorine-based solvent was used as the non-aqueous electrolyte solution. In some cases, the effect was found to be significant.

Using the secondary batteries obtained in Examples, Comparative Examples and Reference Examples 1 and 2, the effect (increased resistance) on the battery performance when the surface of the negative electrode active material was coated with the metal-organic framework was confirmed. As a result, as shown in FIG. 3, a slight increase in resistance was observed by coating the surface of the negative electrode active material with the metal-organic framework, and the influence on the battery performance could be suppressed.

1 ... Negative electrode active material layer 2 ... Electrolyte layer 3 ... Positive electrode active material layer 4 ... Negative electrode current collector 5 ... Positive electrode current collector 10 ... Secondary battery

Claims (1)

A negative electrode active material layer having a negative electrode active material, a positive electrode active material layer having a positive electrode active material, and an electrolyte layer formed between the negative electrode active material layer and the positive electrode active material layer and having a non-aqueous electrolyte solution. It is a secondary battery that has
The non-aqueous electrolytic solution contains a fluorine-based solvent and contains a fluorine-based solvent.
The negative electrode active material is a carbon material and
The surface of the negative electrode active material is coated with a metal-organic framework .
A secondary battery, wherein the metal-organic framework is dilithium terephthalate, lithium 2,6-naphthalenedicarboxylate, or lithium 4,4'-biphenyldicarboxylic acid.

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