JP4281030B2 - Non-aqueous electrolyte battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nonaqueous electrolyte battery such as a lithium battery or a lithium ion battery.
[0002]
[Prior art]
With the recent miniaturization of portable electronic devices, the demand for lithium secondary batteries having high energy density is increasing. Application to large batteries for electric vehicles is also expected, and in these research and development, it is essential to establish technologies for increasing energy density, increasing capacity, and improving safety. At present, the development of a new supporting electrolyte is required from the viewpoint of safety and reliability of lithium secondary batteries. This is because LiPF _6, which is most frequently used as a supporting electrolyte for lithium secondary batteries, is not thermally and chemically stable. By using a new supporting electrolyte, safety in lithium secondary batteries and This is because improvement in reliability and further improvement in charge / discharge characteristics are expected.
[0003]
[Problems to be solved by the invention]
As a supporting electrolyte replacing LiPF ₆ , a supporting electrolyte made of a fluorine-containing organic anion typified by LiCF ₃ SO ₃ or LiN (SO ₂ CF ₃ ) ₂ (LiTFSI) is expected. LiCF ₃ SO ₃ and LiTFSI are thermally and chemically stable compared to LiPF ₆ and are considered to exhibit excellent characteristics in lithium secondary batteries. However, as a problem when using a supporting electrolyte containing these fluorine-containing organic anions, elution of an aluminum (Al) current collector can be mentioned. For example, in an electrolytic solution in which LiPF ₆ is dissolved in a mixed solvent of a low viscosity solvent such as propylene carbonate or ethylene carbonate and dimethyl carbonate, Al metal is applied even when a potential of 4 V is applied to the standard electrode potential of lithium. It exists stably. However, in the electrolytic solution in which LiCF ₃ SO ₃ or LiTFSI is dissolved in the mixed solvent, an oxidation current is observed when Al metal is applied with a potential of 4 V with respect to the standard electrode potential of lithium, and Al is considered to elute. It is done. Since Al metal is very lightweight and inexpensive, it has been widely used as a current collector for lithium secondary batteries. Practical use of supporting electrolytes composed of these fluorine-containing organic anions can suppress the elution of Al. Is the biggest issue.
[0004]
Although the influence of the supporting electrolyte on the elution of Al in the electrolytic solution is not clear, the stability of the supporting electrolyte has been pointed out by Kanamura et al. The presence of oxides and fluorides was confirmed on the surface of the Al electrode after anodic polarization at 5 V (vs. Li ^/ Li ⁺ ) in an electrolyte solution using LiPF _6, and depth analysis by XPS (photoelectron spectroscopy) was performed. From the profile, the main component of the film is changed to AlF ₃ , whereas when LiCF ₃ SO ₃ is used, there are many O elements and relatively few F elements. In addition, since the same spectrum as before the anodic oxidation treatment was observed after the treatment, the presence or absence or properties of the oxide and fluoride passivated films formed on the Al metal surface are related to the stability of the Al electrode. It has been reported. Since LiCF ₃ SO ₃ is stable, it does not generate a passive film on the Al surface, and LiPF ₆ decomposes to generate a passive film on the Al surface and stabilize the Al metal.
[0005]
Considering that the elution of Al in the case of using a supporting electrolyte such as LiCF ₃ SO ₃ or LiTFSI is because there is no supply of F and no passive film is formed on the surface of the Al metal, an additive was studied. It has been broken. An example is the addition of an HF aqueous solution, and it has been confirmed that although not sufficient, the elution of Al is somewhat suppressed. However, there is a concern that such an additive that forms a fluorine coating may affect the charge / discharge characteristics of the battery.
[0006]
Thus, supporting electrolytes such as LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), which have improved fluorine-containing organic anions, have been proposed. These supporting electrolytes are considered to suppress elution of Al by a mechanism different from stabilization by film formation. The mechanism is not clear, but the anion size may have an effect. Compared to LiCF ₃ SO ₃ and LiTFSI, the anion size in these supporting electrolytes is large and the charge density is expected to be low. This is thought to reduce the ability to elute Al. Alternatively, there is a possibility that a steric hindrance effect appears due to the longer fluoroalkyl chain. However, the larger the anion size, the greater the effect of suppressing the elution of Al. However, as the anion size increases, the mobility is lowered and the ionic conductivity is lowered.
[0007]
[Means for Solving the Problems]
In the present invention, by adding a solvent having an acceptor number greater than 18.3, specifically sultone, to the electrolyte solution or polymer gel electrolyte containing the above-mentioned supporting electrolyte containing a fluorine-containing organic anion, elution of Al is achieved. Can be suppressed. Therefore, a lithium secondary battery with high safety and reliability can be obtained by using a thermally and chemically stable supporting electrolyte.
[0008]
The number of acceptors is reported by V. Gultmann in The Donor-Acceptor Approch to Molecular Interactions, Plenum Press (1978). Gultmann shows the acceptor number using the 31P-NMR chemical shift of the adduct formed by its solvent and triethylphosphine oxide in 1,2-dichloroethane, and the chemistry of (C ₂ H ₅ ) PO: SbCl ₅ . The chemical shift of other (C ₂ H ₅ ) PO: acceptor solvents was defined as the acceptor number (AN) with a shift of 100. Therefore, it is expected that the solvent having a larger AN value is more strongly coordinated with the anion. In other words, it is considered that the solvent is strongly coordinated to the fluorine-containing organic anion in the electrolytic solution or the polymer gel electrolyte, thereby suppressing the elution of Al.
[0009]
Examples of the solvent having an acceptor number greater than 18.3 include amides such as dimethylformamide, and nitriles such as acetonitrile. However, there are not many solvents that can be used for lithium secondary batteries, and among these, sultone is included. Good battery characteristics were obtained in the solvent.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the nonaqueous electrolyte battery in the present invention will be described. FIG. 1 shows an evaluation cell for cyclic voltammetry used for evaluating corrosion of Al. a is a working electrode, and a 1 cm ² Al foil is used. b and c are a counter electrode and a reference electrode, respectively, and both are formed by pressure bonding lithium to a nickel plate. d is a non-aqueous electrolyte, and a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent or a polymer gel electrolyte that is a composite material of a non-aqueous solvent, a lithium salt and a polymer is used. The evaluation conditions are 30 ° C. atmosphere and potential sweep rate of 0.1 mV / s.
Example 1 As a non-aqueous electrolyte, sulfolane in which LiTFSI was dissolved at a concentration of 1 mol / l was used. The cyclic voltammetry measurement of this electrolyte solution was performed using the cell of FIG.
[0011]
(Comparative Example 1) As a non-aqueous electrolyte, γ-butyrolactone in which LiTFSI was dissolved at a concentration of 1 mol / l was used, and cyclic voltammetry measurement of this electrolyte was performed using the cell of FIG.
[0012]
FIG. 2 shows the results of cyclic voltammetry measurement in Example 1 and Comparative Example 1. The oxidation current at 3.7 V or higher seen in Comparative Example 1 was not observed in Example 1, and it was confirmed that Al elution was suppressed in Example 1. However, since the electrolytic solution in Example 1 has low ionic conductivity, it is necessary to use a mixed solvent of sulfolane and another solvent. Solvents used for mixing include cyclic carbonates such as ethylene carbonate and propylene carbonate, cyclic carboxylic acid esters such as γ-butyrolactone, cyclic ethers such as tetrahydrofuran and 1,3-dioxolane, and chain ethers such as 1,2-dimethoxyethane. , Chain carbonic acid esters such as dimethyl carbonate and ethyl methyl carbonate, and chain carboxylic acid esters such as methyl propionate, but are not limited thereto, and two or more types can be mixed. It is.
[0013]
Example 2 After mixing γ-butyrolactone and sulfolane at a weight ratio of 1: 4, LiTFSI was dissolved at a concentration of 1 mol / l to prepare an electrolytic solution. The cyclic voltammetry measurement of this electrolyte solution was performed using the cell of FIG.
[0014]
Example 3 After mixing γ-butyrolactone and sulfolane at a weight ratio of 2: 3, LiTFSI was dissolved at a concentration of 1 mol / l to prepare an electrolytic solution. The cyclic voltammetry measurement of this electrolyte solution was performed using the cell of FIG.
[0015]
(Comparative Example 2) As the non-aqueous electrolyte, γ-butyrolactone in which LiBF ₄ was dissolved at a concentration of 1 mol / l was used, and cyclic voltammetry measurement of this electrolyte was performed using the cell of FIG.
[0016]
FIG. 3 shows the results of cyclic voltammetry measurement in Examples 1, 2, 3 and Comparative Example 2. As the content of γ-butyrolactone increased, an increase in oxidation current value at 3.8 V or higher was confirmed. However, the oxidation peak current value was as small as about 1/100 that of Comparative Example 1 in FIG. 2, and it was confirmed that elution of Al was suppressed.
[0017]
Examples 4 and 5 Dimethyl carbonate and sulfolane were mixed at a weight ratio of 1: 4 and 2: 3, respectively, and then LiTFSI was dissolved at a concentration of 1 mol / l to prepare an electrolytic solution. Table 1 shows the results of measuring the ionic conductivity at 20 ° C. using the electrolytic solutions of Examples 1, 2, 3, 4 and 5. Improvement of ionic conductivity was confirmed by adding γ-butyrolactone or dimethyl carbonate.
[0018]
[Table 1]
[0019]
Moreover, it was confirmed that the same elution of Al was suppressed also in the polymer gel electrolyte which is a composite of the polymer and the electrolyte solution as in Examples 1 to 5. Hereinafter, a battery using the polymer gel electrolyte will be described. FIG. 4 shows a battery used for evaluation, in which 1 is an Al current collector, 2 is a positive electrode active material, 3 is a polymer gel electrolyte, 4 is a negative electrode active material, 5 is a negative electrode current collector, and 6 is an exterior.
[0020]
(Example 6) As a polymer gel electrolyte, a composite material composed of polyether, LiTFSI, γ-butyrolactone and sulfolane was used, and a charge / discharge cycle test was conducted using the battery shown in FIG. The mixing ratio of γ-butyrolactone and sulfolane was 2: 3 by weight. LiCoO ₂ was used as the positive electrode active material, and carbon was used as the negative electrode active material. Charging / discharging was performed in the range of 2.7 to 4.2 V at a current value of 1 mA.
[0021]
(Comparative Example 3) A charge / discharge cycle test was performed using a composite material composed of polyether, LiTFSI, and γ-butyrolactone as the polymer gel electrolyte, using the battery shown in FIG. LiCoO ₂ was used as the positive electrode active material, and carbon was used as the negative electrode active material. Charging / discharging was performed in the range of 2.7 to 4.2 V at a current value of 1 mA.
[0022]
The results of the charge / discharge cycle test in Example 6 and Comparative Example 3 are shown in FIG. In Comparative Example 3, the discharge capacity was low from the first cycle, and corrosion of Al was confirmed after the battery was disassembled, whereas in Example 6, a higher discharge capacity was obtained than in the initial cycle, and the capacity was deteriorated by repeating the cycle. The result was small. Moreover, corrosion of Al was not confirmed after the battery was disassembled.
[0023]
【The invention's effect】
As described above, LiN (CF ₃ SO ₂ ) ₂ or LiOSO ₂ Rf1 (wherein Rf1 is —F, —C _k F _{2k + 1} , or —OC _m H _2m C _n F _{2n + 1} , and k = 1 to 5 In a non-aqueous electrolyte or polymer gel electrolyte using a supporting electrolyte composed of a fluorine organic anion represented by m = 1 or 2, and n = 1 to 5, a lithium battery can be obtained by containing sultone in a solvent. Corrosion of Al used for the current collector is suppressed. Therefore, by using a lithium salt composed of a fluorine-containing organic anion that is thermally and chemically stable compared to LiPF ₆ and LiBF ₄ , a secondary battery excellent in safety and reliability can be provided.
[Brief description of the drawings]
FIG. 1 is a perspective view of a cyclic voltammetry measurement cell.
FIG. 2 is a comparison diagram of cyclic voltammograms of Al electrodes when various electrolytes are used.
FIG. 3 is a comparative diagram of cyclic voltammograms of Al electrodes when various electrolytes are used.
FIG. 4 is a cross-sectional view of a battery used for evaluation.
FIG. 5 is a diagram showing the relationship between the number of cycles and the discharge capacity in a charge / discharge cycle test.

Claims (3)

In a battery in which aluminum or an aluminum alloy is used as a positive electrode current collector, an electrolytic solution or a polymer gel electrolyte is LiN (CF ₃ SO ₂ ) ₂ or LiOSO ₂ Rf1 (where Rf1 is −F, −C _k F _{2k + 1} , Or —OC _m H _2m C _n F _{2n + 1} , k = 1 to 5, m = 1 or 2, and n = 1 to 5) , and the solvent contains sultone. nonaqueous electrolyte battery characterized by containing. The nonaqueous electrolyte battery according to claim 1, wherein the content of the sultone in all the solvents is 60% or more by weight. The non-aqueous electrolyte battery according to claim 1 , wherein the sultone is a compound represented by Chemical Formula 1 .
(Wherein R 9 to R 14 are —H or —CH ₃ ).