JP2017182946A - Electrolyte solution for lithium secondary battery and lithium secondary battery including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electrolyte solution arranged by use of an ammonium salt which suppresses the worsening of a discharge rate characteristic; and a lithium secondary battery using the electrolyte solution.SOLUTION: An electrolyte solution for a lithium secondary battery comprises: an ammonium salt; and a solvent. The solvent has a highest occupied molecular orbital (HOMO) lower than that of the ammonium salt.SELECTED DRAWING: None

Description

  The present invention relates to an electrolytic solution for a lithium secondary battery and a lithium secondary battery including the same.

  In recent years, increasing the energy density of lithium-ion batteries for mobile devices has become a problem, but with higher energy densities, batteries using organic solvent electrolytes have had problems with safety. Therefore, a means for solving this problem has been studied by using, as an electrolytic solution, a salt that is in a liquid state at room temperature, called an ionic liquid, instead of the organic solvent electrolytic solution. The ionic liquid has excellent safety such as nonflammability, non-volatility, and high thermal stability. Taking advantage of this feature, extensive research has been conducted to use ionic liquids as electrolytes for various electrochemical devices such as lithium secondary batteries, dye-sensitized solar cells, actuators, and electric double layer capacitors.

  Among the ionic liquids, those having a structure with a nitrogen atom as a cation as typified by an ammonium salt are excellent in thermal stability, and are particularly being researched. Among them, the cyclic ammonium salt is excellent in thermal stability and is expected to be a promising material as an electrolyte for a lithium secondary battery. (See Patent Document 1)

JP 2013-020835 A

  However, the battery using the ammonium salt of the prior art has a problem of poor discharge rate characteristics. As a result of intensive studies, the present inventors have found that lithium ions are trapped in the SEI film by forming a SEI film having a high electron donor property in a battery using an ammonium salt.

  The present invention has been made in view of the above-described problems of the prior art, and provides an electrolytic solution that suppresses a decrease in discharge rate characteristics while maintaining the thermal stability of the electrolytic solution, and a lithium secondary battery using the electrolytic solution. The purpose is to do.

In order to solve the above problems, an electrolyte for a lithium secondary battery according to the present invention includes an ammonium salt and a solvent, and the solvent has a highest occupied orbit (HOMO) lower than that of the ammonium salt. .

According to such a configuration, since a solvent having a lower HOMO than an ammonium salt has a low electron donor property, an SEI film having a low trapping property for lithium ions can be formed. For this reason, even in an electrolytic solution using an ammonium salt, it is possible to suppress a decrease in discharge rate characteristics while maintaining the thermal stability of the electrolytic solution.

  Solvents according to the present invention are 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), vinyl ethylene carbonate (VEC), propylene carbonate (PC), diethyl carbonate (DEC), tetrahydrofuran (THF), It is preferably at least one selected from the group consisting of 2-methyltetrahydrofuran (2-MeTHF) and γ-butyrolactone (BL).

  According to this, the electrolyte solution for lithium secondary batteries which is more suitable as a solvent of this invention and suppresses the fall of a discharge rate characteristic more is obtained.

  It is preferable that content of the said solvent is 0.010-9.0 volume% with respect to the whole electrolyte solution for lithium secondary batteries.

  According to this, an electrolytic solution for a lithium secondary battery that is optimal as an addition amount and further suppresses a decrease in discharge rate characteristics can be obtained.

  The ammonium salt is preferably cyclic ammonium.

  The cyclic ammonium salt is preferably a pyrrolidinium salt or a piperidinium salt.

  According to this, it is suitable as an ammonium salt and the electrolyte solution for lithium secondary batteries which suppresses the fall of a discharge rate characteristic more is obtained.

  It is preferable that the lithium secondary battery which concerns on this invention has the said electrolyte solution for lithium secondary batteries.

  According to this, the lithium secondary battery which suppresses the fall of a discharge rate characteristic more is provided.

ADVANTAGE OF THE INVENTION According to this invention, the electrolyte solution for lithium ion secondary batteries which suppresses the fall of a discharge rate characteristic, maintaining a thermal stability of electrolyte solution, and a lithium secondary battery using the same are provided.

It is a schematic cross section of the lithium ion secondary battery of this embodiment.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments according to the invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Lithium ion secondary battery>
As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A laminated body 30 including a plate-like separator 18, an electrolytic solution containing lithium ions, a case 50 containing these in a sealed state, and one end of the negative electrode 20 being electrically connected. A lead 62 whose other end protrudes outside the case and a lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case are provided.

The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.
<Positive electrode>

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a necessary amount of positive electrode conductive additive.

(Positive electrode active material)
As the positive electrode active material, lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a counter anion (for example, PF ₆ ⁻ ) of the lithium ion are reversibly performed. If it can be made to advance, it will not specifically limit, A well-known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the chemical formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1) , 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Product, lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO) shown), and composite metal oxides such as lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1)

(Binder for positive electrode)
The positive electrode binder bonds the positive electrode active materials to each other and bonds the positive electrode active material layer 14 to the positive electrode current collector 12. The binder is not particularly limited as long as it can be bonded as described above. For example, fluorine resin such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide A resin, a polyamideimide resin, or the like may be used. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene, polythiophene, and polyaniline. Examples of the ion conductive conductive polymer include those obtained by combining a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as LiClO ₄ , LiBF ₄ , and LiPF _6. It is done.

  Although content of the binder in the positive electrode active material layer 14 is not specifically limited, When adding, it is preferable that it is 0.5-5 mass parts with respect to the mass of a positive electrode active material.

(Conductive aid for positive electrode)
The conductive aid for positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive aid can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, and conductive oxides such as ITO.

  <Negative electrode>

(Negative electrode current collector)
The negative electrode current collector 22 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as copper can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material and, if necessary, a negative electrode binder and a negative electrode conductive additive.

(Negative electrode active material)
The negative electrode active material is not particularly limited as long as it can reversibly advance metal lithium, lithium ion occlusion and release, and lithium ion desorption and insertion (intercalation). Can be used. For example, carbon materials such as graphite and hard carbon, silicon materials such as silicon oxide (SiO _x ) metal silicon (Si), metal oxides such as lithium titanate (LTO), metal materials such as lithium, tin, and zinc Is mentioned.

(Binder for negative electrode)
There is no limitation in particular as a binder for negative electrodes, The thing similar to the binder for positive electrodes described above can be used.

(Conductive aid for negative electrode)
There is no limitation in particular as a conductive support agent for negative electrodes, The thing similar to the conductive support agent for positive electrodes described above can be used.

  When metallic lithium is used as the negative electrode active material, formation of lithium dendrite is suppressed, and a safer and higher energy density negative electrode is obtained. In this case, the negative electrode binder and the negative electrode conductive additive are not used.

The metal lithium is more preferably a metal lithium foil having a thickness of 100 μm or less.

<Electrolyte>
The electrolytic solution according to the present embodiment contains an ammonium salt and a solvent having a lower maximum occupied orbital (HOMO) than the ammonium salt.

According to this, even if it is the electrolyte solution which uses ammonium salt, the fall of a discharge rate characteristic can be suppressed. A solvent having a HOMO lower than that of an ammonium salt has a low electron donor property, and thus has a low trapping property for lithium ions, and is preferentially decomposed to form an SEI film over an ammonium salt during charging.

  It is preferable that the highest occupied orbital (HOMO) of the solvent is −12 eV or more.

  According to this, it is suitable as a value of the HOMO of the solvent, and an electrolytic solution for a lithium secondary battery that further suppresses a decrease in discharge rate characteristics can be obtained.

The solvent is 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), vinyl ethylene carbonate (VEC), propylene carbonate (PC), diethyl carbonate (DEC), tetrahydrofuran (THF), 2-methyl It is preferably at least one selected from the group consisting of tetrahydrofuran (2-MeTHF) and γ-butyrolactone (BL).

In the battery using the ammonium salt, an SEI film having a high electron donor property is formed, and lithium ions are trapped in the SEI film at a high discharge rate, resulting in poor discharge rate characteristics. By mixing the solvent according to the present embodiment, an SEI film having a low electron donor property and a low trapping property for lithium ions can be formed, and a decrease in discharge rate characteristics can be suppressed.

  It is preferable that content of the said solvent is 0.010-9.0 volume% with respect to the said electrolyte solution for lithium secondary batteries.

  According to this, the amount of addition is optimal, and an electrolytic solution for a lithium secondary battery that further suppresses a decrease in discharge rate characteristics can be obtained.

  The ammonium salt is preferably a cyclic ammonium salt, more preferably a pyrrolidinium salt or a piperidinium salt.

  According to this, it is suitable as ammonium salt which comprises an ionic liquid, and the electrolyte solution for lithium secondary batteries which suppresses the fall of a discharge rate characteristic more is obtained.

Further, an electrolyte that becomes a carrier of lithium ions may be added to the electrolyte solution. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , and lithium bisoxalate borate, LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, organic acid anion salts such as (FSO ₂ ) ₂ NLi, and the like can be mentioned, and these can be used alone or in admixture of plural components.

  As mentioned above, although preferred embodiment which concerns on this invention was described, this invention is not limited to the said embodiment.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Preparation of positive electrode)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM): carbon black: PVDF = 80: 10: 10 (mass%) The mixture was mixed to obtain N-methyl-2-pyrrolidone (NMP ) Was applied to an aluminum metal foil having a thickness of 20 μm, and NMP was evaporated to obtain a positive electrode sheet.

(Preparation of negative electrode)
A negative electrode sheet was obtained by sticking a metal lithium foil having a thickness of 100 μm onto a copper foil having a thickness of 16 μm.

(Electrolyte adjustment)
Using N-methyl-N-propylpyrrolidinium bis (fluorosulfonyl) imide (P ₁₃ -FSI) as an ammonium salt, LiPF ₆ was dissolved to a concentration of 1 mol / L, and then the concentration was 0.010. 1,3-Dioxolane (DOL) was mixed as a solvent so that it might become volume%.

(Production of evaluation lithium-ion secondary battery)
The positive electrode and negative electrode produced above, and a separator made of a polyethylene microporous film sandwiched between them and put in an aluminum laminate pack, into this aluminum laminate pack, after injecting the electrolyte prepared above, vacuum sealed, A lithium ion secondary battery for evaluation was produced.

(Thermal stability test)
About the lithium ion secondary battery for evaluation produced above, the temperature was set to 80 ° C. using a thermostatic bath, and left in the thermostatic bath for 6 hours to observe gas generation, and the thermal stability of the electrolytic solution was confirmed. In the lithium ion secondary battery produced in Example 1, no gas generation was observed, and the thermal stability of the electrolyte was confirmed.

(Measurement of discharge capacity maintenance rate)
About the lithium ion secondary battery for evaluation produced above, using a secondary battery charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.), the voltage range was 2.5 V to 4.2 V, and 1 C = 3.06 mA. The battery was charged and discharged at a current value of 0.1 C, and the initial discharge capacity was determined. Subsequently, charging / discharging was performed at a current value of 1 C, and a discharge capacity retention ratio (discharge capacity at 1 C / initial discharge capacity × 100) was obtained. A higher value means that the deterioration of the discharge capacity accompanying the increase in the discharge rate is suppressed. The obtained results are shown in Table 1.

  From Table 1, it was revealed that the battery of Example 1 had a higher discharge capacity retention rate than the battery of Comparative Example 1, and the deterioration of the discharge rate characteristics was suppressed.

[Examples 2 to 5, 17 to 18]
Lithium ion secondary batteries for evaluation of Examples 2 to 5 and 17 to 18 were the same as Example 1 except that the amount of 1,3-dioxolane (DOL) was changed as shown in Table 1 below. Was made.

[Examples 6 to 12, 14]
Lithium ion secondary batteries for evaluation of Examples 6 to 12 and 14 were prepared in the same manner as in Example 1 except that the solvent was changed as shown in Table 1 below.

[Examples 13, 15-16]
Lithium ion secondary batteries for evaluation of Examples 13 to 15 were produced in the same manner as in Example 1 except that the ammonium salt was changed as shown in Table 1 below.

  As a result of performing the thermal stability measurement described in Example 1 for the lithium ion secondary batteries for evaluation produced in Examples 2 to 18, no gas generation was observed, and the thermal stability of the electrolyte was confirmed. It was done.

  Table 1 shows the results of measurement of the discharge capacity retention rate described in Example 1 for the evaluation lithium ion secondary batteries produced in Examples 2 to 18.

[Comparative Example 1]
As the electrolytic solution, one not mixed with a solvent was used. That is, an electrolytic solution in which LiPF ₆ was dissolved to 1 mol / L using N-methyl-N-propylpyrrolidinium bis (fluorosulfonyl) imide (P ₁₃ -FSI) as an ammonium salt was used. A laminate film type battery was produced in the same manner as in Example 1 except for the above.

  As a result of measuring the thermal stability described in Example 1 for the lithium ion secondary battery for evaluation produced in Comparative Example 1, no gas was generated and the thermal stability of the electrolyte was confirmed. .

  Table 1 shows the results of measuring the discharge capacity retention rate described in Example 1 for the lithium ion secondary battery for evaluation prepared in Comparative Example 1.

The lithium ion secondary battery for evaluation produced in Comparative Example 1 has a lower discharge capacity maintenance rate than the batteries of Examples 1 to 18, and therefore does not mix a solvent having a lower maximum occupied orbit (HOMO) than the ammonium salt. It has been clarified that the battery using the ammonium salt does not suppress the deterioration of the discharge rate characteristics.

  From the results of Examples 2 and 6 to 12, it was found that the use of DOL, DME, and VEC as solvents can further suppress the deterioration of the discharge rate characteristics.

  Since the lithium ion secondary batteries for evaluation created in Examples 1 to 5 and 17 to 18 have a higher discharge capacity retention rate than Comparative Example 1, the mixing amount of 1,3-dioxolane (DOL) is 0.010. It became clear that the deterioration of the discharge rate characteristics can be suppressed if it is ˜9.0 vol%.

  Since the evaluation lithium ion secondary batteries produced in Examples 2-3 have a higher discharge capacity retention rate than Examples 1 and 4-5, the amount of 1,3-dioxolane (DOL) mixed is 1. It has been clarified that the deterioration of the discharge rate characteristics can be further suppressed when the content is 0 to 2.0% by volume.

From the results of Examples 2, 15, and 16, it has been clarified that the deterioration of the discharge rate characteristics is further suppressed when the cyclic ammonium salt is used rather than the chain ammonium salt.

  According to the present invention, an electrolytic solution that suppresses a decrease in discharge rate characteristics and a lithium secondary battery using the electrolytic solution are provided.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 60 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (6)

  An electrolyte for a lithium secondary battery, comprising an ammonium salt and a solvent, wherein the solvent has a highest occupied orbit (HOMO) lower than that of the ammonium salt.   The solvent is 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), vinyl ethylene carbonate (VEC), propylene carbonate (PC), diethyl carbonate (DEC), tetrahydrofuran (THF), 2-methyl. 3. The electrolytic solution for a lithium secondary battery according to claim 1, which is at least one selected from the group consisting of tetrahydrofuran (2-MeTHF) and γ-butyrolactone (BL).   The electrolyte solution for a lithium secondary battery according to any one of claims 1 to 3, wherein a content of the solvent is 0.010 to 9.0% by volume with respect to the entire electrolyte solution for the lithium secondary battery.   The electrolyte solution for a lithium secondary battery according to claim 1, wherein the ammonium salt is a cyclic ammonium salt.   The electrolyte solution for a lithium secondary battery according to claim 4, wherein the cyclic ammonium salt is a pyrrolidinium salt or a piperidinium salt. A lithium secondary battery provided with the electrolyte solution for lithium secondary batteries as described in any one of Claims 1-5.

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