JP2018133290A - Nonaqueous electrolyte and nonaqueous electrolyte battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte and a nonaqueous electrolyte battery using the same capable of improving low temperature cycle characteristics.SOLUTION: There is provided a nonaqueous electrolytic solution comprising an electrolytic solution containing a solvent composed of a cyclic carbonate and a chain carbonate, an additive (A) represented by the chemical formula (1), and an additive (B) selected from a monofluorophosphate or a difluorophosphate.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte battery using the same.

  In recent years, a lithium ion secondary battery used as a main power source for mobile communication devices and portable electronic devices has a feature of high electromotive force and high energy density.

  The electrolyte for a lithium ion secondary battery is composed of a lithium salt that is an electrolyte and a non-aqueous organic solvent. The non-aqueous organic solvent is required to have a high dielectric constant for dissociating the lithium salt, to exhibit high ionic conductivity in a wide temperature range, and to be stable in the battery. Since it is difficult to achieve these requirements with a single solvent, usually a combination of a high-boiling solvent typified by propylene carbonate and ethylene carbonate and a low-boiling solvent such as dimethyl carbonate and diethyl carbonate is used. ing.

  In addition, many additives are added to the electrolyte to improve various battery characteristics such as initial capacity, rate characteristics, cycle characteristics, high temperature storage characteristics, continuous charge characteristics, self-discharge characteristics, and overcharge prevention characteristics. Has been studied. For example, it has been reported that lithium fluorophosphates are added as a method for suppressing self-discharge at high temperatures. (Patent Document 1)

Japanese Patent Laid-Open No. 11-67270

  However, the conventional methods still do not satisfy various characteristics, and improvement of the low temperature cycle characteristics is demanded.

  As a result of intensive studies, the present inventors have found that lithium is deposited on the negative electrode during low-temperature charging and becomes dead lithium, thereby deteriorating the low-temperature cycle characteristics.

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide a nonaqueous electrolyte capable of improving low-temperature cycle characteristics and a nonaqueous electrolyte battery using the same. .

In order to solve the above problems, a nonaqueous electrolytic solution according to the present invention includes an electrolytic solution containing a solvent composed of a cyclic carbonate and a chain carbonate, an additive (A) represented by the following chemical formula (1), It contains an additive (B) selected from fluorophosphate or difluorophosphate.
(Here, R ^{1 to} R ⁴ are each independently hydrogen, an alkyl group having 1 to 3 carbon atoms, a halogen, an alkyl group that may contain a halogen element having 1 to 3 carbon atoms, a cycloalkyl group, or an alkenyl group. And n is an integer of 1 to 3.)

  According to this, low temperature cycle characteristics can be improved by including the additives (A) and (B) in the electrolytic solution.

  The details of the factors that cause these effects are not clear, but are considered as follows. That is, since the additive (A) contains a large amount of oxygen atoms in the molecule and generates a carboxylic acid residue upon decomposition, an SEI film having a high donor property and a high lithium trapping property is formed on the negative electrode. Furthermore, lithium ion conductivity of the SEI film is improved by the concerted decomposition of the additive (B). Thereby, even at the time of low temperature driving, desolvation of lithium ions on the negative electrode is performed quickly, Li dendride precipitation on the negative electrode is suppressed, and low temperature cycle characteristics are improved.

  In the non-aqueous electrolyte according to the present invention, the additive (A) is ethyl 3-methyl-2,5-dioxa-6-oxononanoate or methyl 3-methyl-2,5-dioxa-6-oxononanoate. Preferably there is.

According to this, it is suitable as the additive (A), and the low temperature cycle characteristics are further improved.

In the nonaqueous electrolytic solution according to the present invention, the additive (A) is preferably contained in the electrolytic solution in an amount of 1 × 10 ^{−4 to} 3 × 10 ⁻² mol / L.

  According to this, it is suitable as an addition amount, and the low-temperature cycle characteristics are further improved.

In the nonaqueous electrolytic solution according to the present invention, the additive (B) is preferably contained in the electrolytic solution in an amount of 1 × 10 ^{−3 to} 3 × 10 ⁻¹ mol / L.

  According to this, it is suitable as an addition amount, and the low-temperature cycle characteristics are further improved.

  In the nonaqueous electrolytic solution according to the present invention, it is further preferable that the additive (B) is lithium difluorophosphate.

  According to this, it is more suitable as an additive, and the low-temperature cycle characteristics are further improved.

  ADVANTAGE OF THE INVENTION According to this invention, the non-aqueous electrolyte which can improve a low-temperature cycle characteristic and a non-aqueous electrolyte 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. One end is electrically connected to the negative electrode 20, a laminate 30 including a plate-like separator 18, a non-aqueous electrolyte containing lithium ions, a case 50 containing these in a sealed state, and the negative electrode 20. And 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. .

  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 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 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, a positive electrode conductive additive, and a positive electrode 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) , Lithium vanadium compound Li _a (M) _b (PO ₄ ) _c (where M = VO or V, and 0.9 ≦ a ≦ 3.3, 0.9 ≦ b ≦ 2.2, 0.9 ≦ c ≦ 3.3), olivine-type LiMPO ₄ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, and Zr or VO), titanium Lithium acid (Li ₄ Ti ₅ O ₁₂ ), LiNi _x Co _y Al _z O ₂ (0.9 <x + y + z <1.1) and the like.

  Among the positive electrode active materials, particularly when a lithium vanadium compound is used, the low-temperature cycle characteristics are effectively improved. Although the exact mechanism of the above action is still unclear, it is considered that vanadium ions eluted from the positive electrode active material are carried on the negative electrode and have a significant effect on film formation.

(Binder for positive electrode)
As the positive electrode binder, the positive electrode active materials are bonded together, and the positive electrode active material layer 14 and the positive electrode current collector 12 are bonded. 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 auxiliary agent 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 auxiliary agent 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, 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 occlusion and release of lithium ions and desorption and insertion (intercalation) of lithium ions, and a known electrode active material 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.

  Of the above negative electrode active materials, the effect of the present invention can be further obtained when lithium is used. Although the details of the above factors are not clear, it is considered that the room temperature molten salt forms a SEI film having a higher donor property on lithium.

  When a metal material is not used as the negative electrode active material, the negative electrode active material layer 24 may further include a negative electrode binder and a negative electrode conductive additive.

(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.

<Non-aqueous electrolyte>
The nonaqueous electrolytic solution according to this embodiment includes an electrolytic solution containing a solvent composed of a cyclic carbonate and a chain carbonate, an additive (A) represented by the chemical formula (1), and a monofluorophosphate or difluorophosphorus. It contains an additive (B) selected from acid salts.

  According to this, low temperature cycle characteristics can be improved by including the additives (A) and (B) in the electrolytic solution.

  The details of the factors that cause these effects are not clear, but are considered as follows. That is, since the additive (A) contains a large amount of oxygen atoms in the molecule and generates a carboxylic acid residue upon decomposition, an SEI film having a high donor property and a high lithium trap property is formed on the negative electrode. Furthermore, lithium ion conductivity of the SEI film is improved by the concerted decomposition of the additive (B). Thereby, even at the time of low temperature driving, desolvation of lithium ions on the negative electrode is performed quickly, Li dendride precipitation on the negative electrode is suppressed, and low temperature cycle characteristics are improved.

  In the non-aqueous electrolyte according to this embodiment, the additive (A) is further methyl 3-methyl-2,5-dioxa-6-oxononanoate or methyl 3-methyl-2,5-dioxa-6-oxononanoate. It is preferable that

  According to this, it is suitable as the additive (A), and the low temperature cycle characteristics are further improved.

In the nonaqueous electrolytic solution according to this embodiment, the additive (A) is preferably contained in the electrolytic solution in an amount of 1 × 10 ^{−4 to} 3 × 10 ⁻² mol / L.

  According to this, it is suitable as an addition amount, and the low-temperature cycle characteristics are further improved.

In the nonaqueous electrolytic solution according to this embodiment, the additive (B) is preferably contained in the electrolytic solution in an amount of 1 × 10 ^{−3 to} 3 × 10 ⁻¹ mol / L.

  According to this, it is suitable as an addition amount, and the low-temperature cycle characteristics are further improved.

  In the nonaqueous electrolytic solution according to this embodiment, it is further preferable that the additive (B) is lithium difluorophosphate.

  According to this, it is more suitable as an additive, and the low-temperature cycle characteristics are further improved.

  In the non-aqueous electrolyte according to this embodiment, the additive (A) and the additive (B) are added in a molar ratio of additive (A): additive (B) = 1: 2-1. : 20 is preferable.

  According to this, it is suitable as a ratio of the addition amount, and the low temperature cycle characteristics are further improved.

(solvent)
Furthermore, the solvent generally used for the lithium ion secondary battery can be used as the solvent of the electrolytic solution. The solvent is not particularly limited. For example, cyclic carbonate compounds such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonate compounds such as diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), and γ-butyrolactone. A cyclic ester compound such as propyl propionate, a chain ester compound such as ethyl propionate and ethyl acetate, and the like can be mixed and used at an arbitrary ratio.

(Electrolytes)
The electrolyte is not particularly limited as long as it is a lithium salt used as an electrolyte of a lithium ion secondary battery. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , lithium bisoxalate borate, LiCF ₃ SO ₃ , An organic acid anion salt such as (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, or the like can be used.

  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)
As a positive electrode active material, 85 parts by mass of LiVOPO ₄ , 5 parts by mass of carbon black, and 10 parts by mass of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to one surface of an aluminum metal foil having a thickness of 20 μm so that the applied amount of the positive electrode active material was 9.0 mg / cm ² and dried at 100 ° C. to form a positive electrode active material layer. Then, it pressure-molded with the roller press and produced the positive electrode.

(Preparation of negative electrode)
90 parts by mass of natural graphite, 5 parts by mass of carbon black, and 5 parts by mass of PVDF were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a negative electrode active material layer. The slurry was applied to one surface of a copper foil having a thickness of 20 μm so that the amount of the negative electrode active material applied was 6.0 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. Then, it pressure-molded with the roller press and produced the negative electrode.

(Preparation of electrolyte)
Were mixed so that EC / DEC = 3/7 by volume and this dissolved LiPF ₆ at a concentration of 1 mol / L. After that, 1.0 × 10 ⁻³ mol / L of ethyl 2,5-dioxa-6-oxooctanoate and lithium difluorophosphate (LiPO ₂ F ₂ ) as an additive (A) were added to this solution. It added so that it might become a density | concentration of 0 * 10 ^<-2 > mol / L, and produced electrolyte solution.

(Production of evaluation lithium-ion secondary battery)
The positive electrode and negative electrode produced above and a separator made of a polyethylene microporous film were sandwiched between them to be put in an aluminum laminate pack. After injecting the electrolytic solution prepared above into this aluminum laminate pack, vacuum sealing was performed to prepare a lithium ion secondary battery for evaluation.

(Measurement of low temperature cycle characteristics)
The lithium ion secondary battery for evaluation produced above was installed in a thermostat (manufactured by Espec Corp.) set at 0 ° C. After the battery temperature was sufficiently stabilized, the secondary battery charge / discharge test apparatus (made by Hokuto Denko Co., Ltd.) was used to charge the battery until the battery voltage reached 4.2 V by constant current charging at a charge rate of 0.5 C. Thereafter, the battery was discharged with a constant current discharge at a discharge rate of 0.5 C until the battery voltage became 2.8 V, and the first cycle discharge capacity C ₁ was obtained. The charge and discharge is counted as one cycle, repeating the charge and discharge was determined 300th cycle discharge capacity C _2. In addition, nC (mA) of the said charging / discharging rate is an electric current value which can charge / discharge nominal capacity (mAh) by 1 / n (h).

From the first cycle discharge capacity C ₁ and 300 cycle discharge capacity C ₂ obtained above, in accordance with equation (1) to determine the low-temperature cycle property. The obtained results are shown in Table 1.
Low temperature cycle characteristics [%] = C ₂ / C ₁ × 100 (1)

[Examples 2 to 8]
Lithium secondary batteries for evaluation of Examples 2 to 8 were produced in the same manner as in Example 7 except that the type of additive (A) used in the production of the electrolytic solution was changed as shown in Table 1. .

[Examples 9 to 14]
The evaluation lithium ion secondary batteries of Examples 9 to 14 were produced in the same manner as in Example 7 except that the amount of additive (A) used in the production of the electrolytic solution was changed as shown in Table 1. did.

[Examples 15 to 18]
The evaluation lithium ion secondary batteries of Examples 15 to 18 were produced in the same manner as in Example 7 except that the amount of additive (B) used in the production of the electrolytic solution was changed as shown in Table 1. did. Here, Li ₂ PO ₃ F is lithium monofluorophosphate.

[Examples 19 to 23]
Lithium ion secondary batteries for evaluation of Examples 19 to 23 were the same as Example 7 except that the type and amount of additive (B) used in the preparation of the electrolytic solution were changed as shown in Table 1. Was made.

[Examples 24-27]
Lithium ion secondary batteries for evaluation of Examples 24-27 were produced in the same manner as in Example 7 except that the positive electrode active material used in the production of the positive electrode was changed as shown in Table 1.

[Comparative Example 1]
As shown in Table 1, a lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 7 except that the additive (B) was not added in the production of the electrolytic solution.

[Comparative Example 2]
As shown in Table 1, a lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 7 except that the additive (A) was not added in the production of the electrolytic solution.

[Comparative Example 3]
As shown in Table 1, a lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as in Example 7 except that no additive was added in the production of the electrolytic solution.

[Comparative Examples 4 to 7]
Lithium ion secondary batteries for evaluation of Comparative Examples 4 to 7 were produced in the same manner as Comparative Example 3 except that the positive electrode active material used in the production of the positive electrode was changed as shown in Table 1.

  About the lithium ion secondary battery for evaluation produced in Examples 2-27 and Comparative Examples 1-7, the low temperature cycling characteristic was measured similarly to Example 1. FIG. The results are shown in Table 1.

  In each of Examples 1 to 27, the low-temperature cycle characteristics were improved with respect to Comparative Examples 1 to 3 in which one or both of the additives were not added. From the results of Examples 1 to 8, by using ethyl 3-methyl-2,5-dioxa-6-oxononanoate or methyl 3-methyl-2,5-dioxa-6-oxononanoate as additive (A) It was confirmed that the low-temperature cycle characteristics were further improved. Furthermore, from the results of Examples 9 to 14, it was confirmed that the low temperature cycle characteristics were further improved by optimizing the addition amount of the additive (A).

Similarly, for the additive (B), it was confirmed from the results of Examples 15 to 18 that the low temperature cycle characteristics were further improved by optimizing the addition amount. Furthermore, from the results of Examples 19 to 23, it was confirmed that when LiPO ₂ F ₂ was used as an additive, the low-temperature cycle characteristics were further improved.

In addition, from the results of Examples 24-27, it was confirmed that the low temperature cycle characteristics were further improved when LiPOVO ₄ was used as the positive electrode active material.

  According to the present invention, a nonaqueous electrolytic solution capable of improving low temperature cycle characteristics is 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 electrolytic solution containing a solvent comprising a cyclic carbonate and a chain carbonate;
An additive (A) represented by the following chemical formula (1);
A nonaqueous electrolytic solution comprising an additive (B) selected from monofluorophosphate and difluorophosphate.

(Here, R ^{1 to} R ⁴ are each independently hydrogen, an alkyl group having 1 to 3 carbon atoms, a halogen, an alkyl group that may contain a halogen element having 1 to 3 carbon atoms, a cycloalkyl group, or an alkenyl group. And n is an integer of 1 to 3.)   2. The additive (A) is ethyl 3-methyl-2,5-dioxa-6-oxononanoate or methyl 3-methyl-2,5-dioxa-6-oxononanoate. Non-aqueous electrolyte. 3. The non-aqueous electrolyte according to claim 1, wherein the additive (A) is contained in the electrolyte at 1 × 10 ^{−4 to} 3 × 10 ⁻² mol / L. The non-aqueous electrolysis according to any one of claims 1 to 3, wherein the additive (B) is contained in the electrolytic solution in an amount of 1 x ^{10-3 to} 3 x ^10-1 mol / L. liquid.   The non-aqueous electrolyte according to any one of claims 1 to 4, wherein the additive (B) is lithium difluorophosphate.   A nonaqueous electrolyte battery comprising the nonaqueous electrolyte solution according to any one of claims 1 to 5.

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