JP4432397B2 - Nonaqueous electrolyte and lithium secondary battery using the same - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte that can provide a lithium secondary battery excellent in battery characteristics such as battery cycle characteristics, electric capacity, and storage characteristics, and a lithium secondary battery using the same.

In recent years, lithium secondary batteries have been widely used as driving power sources for small electronic devices and the like. Lithium secondary batteries are mainly composed of a positive electrode, a non-aqueous electrolyte, and a negative electrode. In particular, lithium secondary batteries using a lithium composite oxide such as LiCoO ₂ as a positive electrode and a carbon material or lithium metal as a negative electrode are used. It is preferably used. As the non-aqueous electrolyte for the lithium secondary battery, carbonates such as ethylene carbonate (EC) and propylene carbonate (PC) are preferably used.

However, there is a demand for a secondary battery having more excellent battery characteristics such as battery cycle characteristics and electric capacity.
A lithium secondary battery using, for example, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ or the like as a positive electrode is partially decomposed by oxidation when a solvent in a non-aqueous electrolyte is locally charged. In order to inhibit the desired electrochemical reaction, the battery performance is degraded. This seems to be due to the electrochemical oxidation of the solvent at the interface between the positive electrode material and the non-aqueous electrolyte.
In addition, a lithium secondary battery using a highly crystallized carbon material such as natural graphite or artificial graphite as the negative electrode is reduced and decomposed on the negative electrode surface when the solvent in the non-aqueous electrolyte is charged. Even in EC that is generally widely used, reductive decomposition occurs partly during repeated charging and discharging, resulting in a decrease in battery performance.
For this reason, at present, battery characteristics such as battery cycle characteristics and electric capacity are not always satisfactory.

  The present invention solves the above-mentioned problems related to the non-aqueous electrolyte for a lithium secondary battery, has excellent battery cycle characteristics, and has excellent battery characteristics such as electric capacity and storage characteristics in a charged state. It aims at providing the nonaqueous electrolyte for lithium secondary batteries which can comprise a secondary battery, and a lithium secondary battery using the same.

  The present invention relates to a non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, and the non-aqueous electrolyte contains the following formula (I):

(Wherein R ₁ , R ₂ , R ₃ , R ₄ and R ₅ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms, provided that R ₁ , R ₂ , R ₃ , At least one of R ₄ and R ₅ is a hydrocarbon group having 1 to 12 carbon atoms). provide.
Further, in the lithium secondary battery comprising a non-aqueous electrolyte in which an electrolyte is dissolved in a positive electrode, a negative electrode, and a non-aqueous solvent, the non-aqueous electrolyte includes the following formula (I)

(Wherein R ₁ , R ₂ , R ₃ , R ₄ and R ₅ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms, provided that R ₁ , R ₂ , R ₃ , At least one of R ₄ and R ₅ is a hydrocarbon group having 1 to 12 carbon atoms.) A lithium secondary battery comprising a tert-butylbenzene derivative represented by provide.

  The nonaqueous electrolytic solution of the present invention is used as a constituent member of a lithium secondary battery. The constituent members other than the non-aqueous electrolyte constituting the secondary battery are not particularly limited, and various conventionally used constituent members can be used.

  According to the present invention, it is possible to provide a lithium secondary battery excellent in battery characteristics such as battery cycle characteristics, electric capacity, and storage characteristics.

In the tert-butylbenzene derivative represented by the above general formula (I) contained in a nonaqueous electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ are: Independently, a linear alkyl group such as a hydrogen atom, a methyl group, an ethyl group, a propyl group or a butyl group, an iso-propyl group, an iso-butyl group, a sec-butyl group or a tert-butyl group Branched alkyl groups are preferred. Further, it may be a cycloalkyl group having 3 to 6 carbon atoms such as a cyclopropyl group or a cyclohexyl group. Further, it may be a phenyl group, a benzyl group, an alkyl-substituted phenyl group such as a tosyl group, a tert-butylbenzene group, or a tert-butylbenzyl group, or a benzyl group. It is preferable to have such a hydrocarbon group having 1 to 12 carbon atoms.

Specific examples represented by tert- butyl benzene derivative by the general formula (I), For example, 2-tert-butyl toluene [R ₁ = _{methyl, R 2 = R 3 = R} 4 = R 5 = hydrogen Atoms], 3-tert-butyltoluene [R ₂ = methyl group, R ₁ = R ₃ = R ₄ = R ₅ = hydrogen atom], 4-tert-butyltoluene [R ₃ = methyl group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen atom], 1- (tert-butyl) -2-ethylbenzene [R ₁ = ethyl group, R ₂ = R ₃ = R ₄ = R ₅ = hydrogen atom], 1- (tert- Butyl) -3-ethylbenzene [R ₂ = ethyl group, R ₁ = R ₃ = R ₄ = R ₅ = hydrogen atom], 1- (tert-butyl) -4-ethylbenzene [R ₃ = ethyl group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen atom], 3-tert-butyl-o-xylene [R ₁ = R ₂ = methyl group, R ₃ = R ₄ = R ₅ = hydrogen atom], 4-tert-butyl-o-xylene [R ₂ = R ₃ = methyl group, R ₁ = R ₄ = R ₅ = hydrogen atom], 4-tert-butyl-m-xylene [R ₁ = R ₃ = methyl group, R ₂ = R ₄ = R ₅ = hydrogen atom], 5-tert-butyl-m-xylene [R ₂ = R ₄ = methyl group R ₁ = R ₃ = R ₅ = hydrogen atom], 2-tert-butyl-p-xylene [R ₁ = R ₄ = methyl group, R ₂ = R ₃ = R ₅ = hydrogen atom], 3-iso- Propyl-1-tert-butylbenzene [R ₂ = iso-propyl group, R ₁ = R ₃ = R ₄ = R ₅ = hydrogen atom], 4-iso-propyl-1-tert-butylbenzene [R ₃ = iso - _{propyl, R 1 = R 2 = R} 4 = R 5 = hydrogen], 4-n-butyl -1-tert Buchirubenze [R ₃ = n-butyl _{group, R 1 = R 2 = R} 4 = R 5 = hydrogen], 4-an iso-butyl -1-tert-butylbenzene [R ₃ = an iso-butyl group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen], 4-sec-butyl -1-tert-butylbenzene [R ₃ = sec-butyl _{group, R 1 = R 2 = R} 4 = R 5 = hydrogen], 3 -Cyclohexyl-1-tert-butylbenzene [R ₂ = cyclohexyl group, R ₁ = R ₃ = R ₄ = R ₅ = hydrogen atom], 4-cyclohexyl-1-tert-butylbenzene [R ₃ = cyclohexyl group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen atom], 4,4′-di-tert-butyldiphenylmethane [R ₃ = 4-tert-butylphenyl group, R ₁ = R ₂ = R ₄ = R ₅ = Hydrogen atom], 4,4′-di-tert-butylbiphenyl [R ₃ = 4-tert-butylbenzene group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen atom], 1,3-di-tert-butylbenzene [R ₂ = tert-butyl group, R ₁ = R ₃ = R ₄ = R ₅ = hydrogen atom], 1,4-di-tert-butylbenzene [R ₃ = tert-butyl group, R ₁ = R ₂ = R ₄ = R ₅ = hydrogen atom], 1,2,4- Tri-tert-butylbenzene [R ₁ = R ₃ = tert-butyl group, R ₂ = R ₄ = R ₅ = hydrogen atom], 1,2,3-tri-tert-butylbenzene [R ₁ = R ₂ = tert-butyl group, R ₃ = R ₄ = R ₅ = hydrogen atom], 1,3,5-tri-tert-butylbenzene [R ₂ = R ₄ = tert-butyl group, R ₁ = R ₃ = R ₅ = hydrogen], 1,2,3,5-tetra -tert- butyl benzene _{[R 1 = R 2 = R 4} = tert- Bed Le group, R ₃ = R ₅ = hydrogen], 1,2,3,4 -tert- butyl benzene _{[R 1 = R 2 = R 3} = tert- butyl, R ₄ = R ₅ = hydrogen ] 1,2,4,5-tetra-tert-butylbenzene [R ₁ = R ₃ = R ₄ = tert-butyl group, R ₂ = R ₅ = hydrogen atom], 3,5-di-tert-butyl And toluene [R ₂ = methyl, R ₄ = tert-butyl group, R ₁ = R ₃ = R ₅ hydrogen atom] and the like.

When the content of the tert-butylbenzene derivative represented by the formula (I) contained in the non-aqueous electrolyte is excessively large, the battery performance may be deteriorated. Battery performance is not obtained. Accordingly, the content thereof is based on the weight of the non-aqueous electrolyte 0. 2-10 wt%, good Mashiku may in the range of 0.5 to 5% by weight to improve the cycle characteristics.

As the non-aqueous solvent used in the present invention, a solvent composed of a high dielectric constant solvent and a low viscosity solvent is preferable.
Preferred examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). These high dielectric constant solvents may be used alone or in combination of two or more.

Examples of the low viscosity solvent include chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2- Ethers such as dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl propionate, amides such as dimethylformamide Kind. These low viscosity solvents may be used alone or in combination of two or more.
The high dielectric constant solvent and the low viscosity solvent are arbitrarily selected and used in combination. The high dielectric constant solvent and the low viscosity solvent are usually used in a volume ratio (high dielectric constant solvent: low viscosity solvent) of 1: 9 to 4: 1, preferably 1: 4 to 7: 3. The

Examples of the electrolyte used in the present invention include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₄ (C ₂ F ₅ ) ₂ , LiPF ₃ (iso-C ₃ F ₇ ) ₃ , LiPF ₅ (iso-C ₃ F ₇ ) Etc. These electrolytes may be used alone or in combination of two or more. These electrolytes are used by being dissolved in the non-aqueous solvent usually at a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.

  The non-aqueous electrolyte of the present invention is prepared by, for example, mixing the above-mentioned high dielectric constant solvent or low-viscosity solvent, dissolving the above-mentioned electrolyte in this, and dissolving the tert-butylbenzene derivative represented by the above formula (I) Can be obtained.

For example, a composite metal oxide of at least one metal selected from the group consisting of cobalt, manganese, nickel, chromium, iron, and vanadium and lithium is used as the positive electrode active material. Examples of such composite metal oxides include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _0.8 CO _0.2 O _{2 and the} like. Only one type of these positive electrode active materials may be selected and used, or two or more types may be used in combination.

  The positive electrode is obtained by kneading the positive electrode active material with a conductive agent such as acetylene black or carbon black, a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF), and a solvent to form a positive electrode mixture. By applying this positive electrode material to an aluminum foil or stainless steel lath plate as a current collector, and after drying and pressure molding, heat treatment is performed under vacuum at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. Produced.

Examples of the negative electrode active material include lithium metal and lithium alloy, and carbon materials having a graphite-type crystal structure capable of occluding and releasing lithium (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), Materials such as molecular compound combustor, carbon fiber] and composite tin oxide are used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which a lattice spacing ( ₀₀₂ ) (d ₀₀₂ ) is 0.335 to 0.340 nm. Only one kind of these negative electrode active materials may be selected and used, or two or more kinds may be used in combination. A powder material such as a carbon material is kneaded with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) and used as a negative electrode mixture. The manufacturing method of a negative electrode is not specifically limited, It can manufacture with the method similar to the manufacturing method of said positive electrode.

  The structure of the lithium secondary battery is not particularly limited, and a coin-type battery having a positive electrode, a negative electrode, and a single-layer or multi-layer separator, and a cylindrical battery or a square type having a positive electrode, a negative electrode, and a roll separator. An example is a battery. A known polyolefin microporous film, woven fabric, non-woven fabric or the like is used as the separator.

Next, an Example and a comparative example are given and this invention is demonstrated concretely.
Example 1
(Preparation of non-aqueous electrolyte)
A nonaqueous solvent of EC: PC: DEC (volume ratio) = 30: 5: 65 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1 M to prepare a nonaqueous electrolytic solution. Tert-butyltoluene was added so that it might become 2.0 weight% with respect to a non-aqueous electrolyte.

[Production of lithium secondary battery and measurement of battery characteristics]
80% by weight of LiCoO ₂ (positive electrode active material), 10% by weight of acetylene black (conductive agent), and 10% by weight of polyvinylidene fluoride (binder) are mixed, and this is mixed with 1-methyl-2-pyrrolidone. What mixed and added the solvent was apply | coated on the aluminum foil, and it dried, press-molded, and heat-processed, and prepared the positive electrode. 90% by weight of artificial graphite (negative electrode active material) and 10% by weight of polyvinylidene fluoride (binder) were mixed, and 1-methyl-2-pyrrolidone solvent was added to this, and the resulting mixture was added to the copper foil. The negative electrode was prepared by drying, pressure molding, and heat treatment. And using the separator of a polypropylene microporous film, said nonaqueous electrolyte solution was inject | poured and the coin battery (diameter 20mm, thickness 3.2mm) was produced.
Using this coin battery, it was charged at a constant current and a constant voltage of 0.8 mA at room temperature (20 ° C.) for 5 hours to a final voltage of 4.2 V, and then at a constant current of 0.8 mA and a final voltage of 2. The battery was discharged to 7 V, and this charge / discharge was repeated. The initial charge / discharge capacity is compared with the case where 1M LiPF ₆ -EC / PC / DEC (capacity ratio 30/5/65) without addition of 4-tert-butyltoluene is used as the non-aqueous electrolyte (Comparative Example 1). The relative value was 1.03, and the battery characteristics after 50 cycles were measured, and the discharge capacity retention rate was 92.2% when the initial discharge capacity was 100%. Also, the low temperature characteristics were good. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 2
A coin battery was prepared by preparing a non-aqueous electrolyte in the same manner as in Example 1 except that 4-tert-butyltoluene was used as an additive at 5.0% by weight based on the non-aqueous electrolyte. When the battery characteristics were measured, the discharge capacity retention rate was 91.7%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 3
A coin battery was prepared by preparing a non-aqueous electrolyte in the same manner as in Example 1 except that 4-tert-butyltoluene was used as an additive in an amount of 0.5% by weight based on the non-aqueous electrolyte. When the battery characteristics of were measured, the discharge capacity retention rate was 90.1%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Comparative Example 1
A non-aqueous solvent of EC: PC: DEC (volume ratio) = 30: 5: 65 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1M. At this time, no tert-butylbenzene derivative was added. Using this non-aqueous electrolyte, a coin battery was produced in the same manner as in Example 1, and the battery characteristics were measured. The discharge capacity retention ratio after 50 cycles was 82.6% with respect to the initial discharge capacity. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 4 (not an example of the present invention but a reference example)
A non-aqueous solvent of EC: PC: DEC (volume ratio) = 30: 5: 65 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1 M to prepare a non-aqueous electrolyte solution. Butylbenzene was added to 2.0 wt% with respect to the non-aqueous electrolyte. The non-aqueous electrolyte solution using a similarly prepare a coin battery of Example 1 was measured for battery characteristics, the initial discharge capacity 4-tert-butylbenzene derivatives without additives 1M LiPF ₆ -EC / PC / When DEC (capacity ratio 30/5/65) was used as a non-aqueous electrolyte (Comparative Example 1), the relative value was 1.02, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate when the capacity was 100% was 91.8%. Also, the low temperature characteristics were good. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 5
A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that 4-tert-butyl-m-xylene was used in an amount of 2.0% by weight based on the non-aqueous electrolyte. When the battery characteristics after 50 cycles were measured, the discharge capacity retention rate was 91.6%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 6
As in Example 1, except that EC / PC / DEC / DMC (capacity ratio 30/5/30/35) was used as the nonaqueous solvent and natural graphite was used as the negative electrode active material instead of artificial graphite. A coin battery was prepared by preparing a nonaqueous electrolytic solution, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 92.6%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 7
1M LiPF ₆ -EC / PC / MEC / DMC (capacity ratio 30/5/50/15) is used as the non-aqueous electrolyte, and LiNi _0.8 Co _0.2 O ₂ is used instead of LiCoO ₂ as the positive electrode active material. In the same manner as in Example 1, a non-aqueous electrolyte was prepared to produce a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 90.8%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 8
1M LiBF ₄ -EC / PC / DEC / DMC (capacity ratio 30/5/30/35) was used as the non-aqueous electrolyte, and LiMn ₂ O ₄ was used instead of LiCoO ₂ as the positive electrode active material. Prepared a coin battery by preparing a non-aqueous electrolyte in the same manner as in Example 1 and measured the battery characteristics after 50 cycles. As a result, the discharge capacity retention rate was 92.3%. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 9 to Example 11
Instead of 4-tert-butyltoluene, in each example, 4,4-di-tert-butylbiphenyl, 1,3-di-tert-butylbenzene, 1,3,5-tri-tert-butylbenzene was used. A coin battery was prepared by preparing a nonaqueous electrolytic solution in the same manner as in Example 1 except that it was used, and the battery characteristics after 50 cycles were measured. The production conditions and battery characteristics of the coin battery are shown in Table 1.

Example 12
Non-aqueous electrolysis as in Example 1 except that 3,5-di-tert-butyltoluene was used instead of 4-tert-butyltoluene and natural graphite was used as the negative electrode active material instead of artificial graphite. The liquid was prepared to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The production conditions and battery characteristics of the coin battery are shown in Table 1.

In addition, this invention is not limited to the Example described, The various combination which can be easily guessed from the meaning of invention is possible. In particular, the combination of solvents in the above examples is not limited. Furthermore, although the said Example is related with a coin battery, this invention is applied also to a cylindrical and prismatic battery.

Claims (8)

In a non-aqueous electrolyte for a lithium secondary battery in which an electrolyte is dissolved in a non-aqueous solvent, the non-aqueous electrolyte contains the following formula (I)

(Wherein R ₁ , R ₂ , R ₃ , R ₄ and R ₅ each independently represents a hydrogen atom or a hydrocarbon group having 1 to 12 carbon atoms, provided that R ₁ , R ₂ , R ₃ , And at least one of R ₄ and R ₅ is a hydrocarbon group having 1 to 12 carbon atoms.) 0.2 to 10% by weight of tert-butylbenzene derivative represented by Non-aqueous electrolyte.   The nonaqueous electrolytic solution according to claim 1, wherein the nonaqueous solvent contains at least a cyclic carbonate.   The nonaqueous electrolytic solution according to claim 2, wherein the nonaqueous solvent contains at least two kinds of cyclic carbonates.   The nonaqueous electrolytic solution according to claim 2 or 3, wherein the cyclic carbonate is a compound selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate.   The nonaqueous electrolytic solution according to any one of claims 2 to 4, wherein the nonaqueous solvent further contains a chain carbonate. tert-Butylbenzene derivatives are 4-tert-butyltoluene, 4-tert-butyl-m-xylene, 4,4′-di-tert-butylbiphenyl, 1,3-di-tert-butylbenzene, 1,3 The nonaqueous electrolytic solution according to any one of claims 1 to 5, which is 1,5-tri-tert-butylbenzene or 3,5-di-tert-butyltoluene. The nonaqueous electrolytic solution according to any one of claims 1 to 6 , wherein the content of the tert-butylbenzene derivative is in the range of 0.5 to 5% by weight . A lithium secondary battery comprising a positive electrode, a negative electrode, and the nonaqueous electrolytic solution according to any one of claims 1 to 7.