JPH1055820A - Nonaqueous electrolyte for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the high electrical conductivity without lowering flame resistance by including the specified cyclic ester phosphate and ester carbonate at the specified ratio. SOLUTION: Nonaqueous electrolyte for secondary battery is obtained by including the cyclic ester phosphate, which is expressed with a formula, and carbonate at 90/10-10/90 of weight ratio. The electrolyte solvent of chained ester phosphate group, ether group and lactone group, which are used in usual, can be added. Desirable weight ratio of the chained ester phosphate and ester carbonate is 75/25-25/75, and 50/50 is more desirable. As the solute, any one of LiPF6 , LiClO4 , LiBF4 , LiAsF6 , LiCF3 SO3 , LiAlCl3 can be used. The electrolyte is dissolved in the solute at 0.05-3 mol/l of concentration in usual, and 0.1-2mol/l is desirable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel non-aqueous electrolyte for a secondary battery.

[0002]

2. Description of the Related Art A lithium secondary battery, which is a typical example of a secondary battery using a non-aqueous electrolyte, has a high voltage and a high energy density, and is widely used as a power source for consumer electronic devices. There is a lot of research going on.
Examples of the non-aqueous electrolyte include a solvent obtained by mixing a low-viscosity solvent such as dimethyl carbonate and diethyl carbonate with a high dielectric constant solvent such as ethylene carbonate and propylene carbonate, and an electrolyte such as LiPF ₆ , LiBF ₄ , and LiClO _4. Is used.

[0003]

However, the above-mentioned electrolytic solution does not always have a satisfactory high flash point in terms of safety, and cannot be said to be a flame-retardant solvent. Therefore, trimethyl phosphate, which is a chain phosphate ester known as a flame retardant, is used as a solvent and as a cosolvent,
%, Preferably 30% or more (Japanese Patent Laid-Open No. 4-184870). However, even if the flame retardancy is improved, the electric conductivity is lowered, which is not preferable as an electrolyte solvent. Therefore, a mixture of the chain carbonate and the cyclic carbonate is contained in a chain phosphate ester of 10% or less of trimethyl phosphate, so that a relatively high electric conductivity can be obtained without lowering the flame retardancy. A liquid mixture has been proposed (JP-A-8-22839). However, there is a limit in improving the electric conductivity. Therefore, an object of the present invention is to provide an electrolyte mixture that can obtain high electric conductivity without lowering flame retardancy.

[0004]

Means for Solving the Problems Accordingly, the present inventors have:
As a result of diligent studies, cyclic phosphate esters alone have high flame retardancy alone and do not have sufficient electrical conductivity.However, when carbonate esters are mixed in an appropriate ratio, the flame retardancy is maintained while maintaining the flame retardancy. It has been found that the conductivity is dramatically improved. That is, in the present invention, the cyclic phosphate ester and carbonate ester represented by the general formula [I] are used in a weight ratio of 90/10 to 1
The non-aqueous electrolyte for a secondary battery is characterized by containing 0/90.

(Equation 2) [R ₁ in the formula is an alkyl group having 1 to 4 carbon atoms, R _2,
R ₃ is the same or different and each represents hydrogen or a methyl group. ]

[0005]

According to the present invention, a cyclic phosphate having a high dielectric constant due to a cyclic structure alone cannot be expected to have a high electric conductivity due to a high viscosity. Thereby, it is possible to have both high electrical conductivity and flame retardancy. In particular, the weight ratio is 75/2.
Higher electrical conductivity can be exhibited in the range of 5 to 25/75.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The cyclic phosphate represented by the general formula [I] is

(Equation 3) [R ₁ in the formula is an alkyl group having 1 to 4 carbon atoms, R _2,
R ₃ is the same or different and each represents hydrogen or a methyl group. ]. R ₁ in the formula is an alkyl group having 1 to 4 carbon atoms, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl group and the like.
And R ₂ and R ₃ in the formula are preferably a hydrogen atom or a methyl group. Specific examples include ethylene methyl phosphate, ethylene ethyl phosphate, methyl trimethylene phosphate, and the like.

[0007] These cyclic phosphates can be prepared by a conventionally known method [for example, R.S. Kluger J. Am. Chem. Soc. 1
07,6009 (1985)].

Although these are flame-retardant phosphoric ester compounds, they have a high dielectric constant due to their cyclic structure, but high electrical conductivity cannot be expected due to their high viscosity alone. However, by mixing a carbonate with this, an electrolyte solvent that can have both high electric conductivity and flame retardancy is obtained.

The carbonate to be mixed is exemplified by dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate and methyl isopropyl carbonate as chain carbonates, and ethylene carbonate, propylene carbonate, butylene carbonate and vinylene as cyclic carbonates. Carbonate is exemplified.

The cyclic phosphate of the present invention can be used alone or in combination of two or more. Further, the carbonates to be mixed can be used alone or in combination of two or more. In addition, a linear phosphate ester-based, ether-based, lactone-based, or other electrolyte solvent that is normally used within a range that does not impair the object of the present invention (the purpose of improving electrical conductivity while maintaining high flame retardancy) is used. It can also be added.

The mixing ratio of the cyclic phosphate and the carbonate in the electrolytic solution is 90/10 to 10/9.
0, preferably 75/25 to 25/75, more preferably 50/50%.

As the solute, a conventionally known solute such as L
iPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ ,
Any electrolyte such as LiAlCl ₃ can be used.
Species or two or more species may be used in combination. The concentration at which the electrolyte is dissolved in the solvent can be generally used at 0.05 to 3 mol / l, and preferably 0.1 to 2 mol / l.

The secondary battery using the non-aqueous electrolyte of the present invention is not particularly limited as a negative electrode material, but is a conventionally known material such as a carbon material or lithium metal capable of doping and undoping lithium ions. A metal lithium alloy or the like can be used, and a conventionally known composite oxide or polymer compound of lithium and a transition metal can be used regardless of the material of the positive electrode. Further, the non-aqueous electrolyte battery of the present invention includes the non-aqueous electrolyte described above as the electrolyte, and its shape, form, and the like are applicable to any of a cylindrical shape, a square shape, a coin shape, a large size, and the like. You.

[0014]

EXAMPLES The present invention will be described in more detail below, but the present invention is not limited to these examples. Also,
In the examples, "parts" means "parts by weight" unless otherwise specified.
Means Examples 1 to 5 Ethylene methyl phosphate / dimethyl carbonate
(Weight ratio shown in Table 1 below) 1 mol / l of LiPF ₆ in the mixed solvent
It melt | dissolved and adjusted electrolyte solution. Manila paper having a thickness of 0.04 mm was formed into a strip having a width of 15 mm and a length of 320 mm, which was immersed in the above-mentioned electrolyte for 1 minute. Thereafter, it was suspended vertically for 3 minutes to remove excess electrolyte. The manila paper was horizontally fixed to a sample holder having supporting needles at intervals of 25 mm, and one end was lit with a lighter, and the burning length was measured. The electrical conductivity was measured at 100 kHz using an impedance meter. Example 6 In Example 3, ethylene ethyl phosphate was used instead of ethylene methyl phosphate (weight ratio = 1/1).
Except for 1), an electrolytic solution was prepared in the same manner as in Example 3, and the combustion length and electric conductivity were measured. Comparative Example 1 Except for ethylene methyl phosphate in Example 1,
An electrolytic solution was prepared in the same manner as in Example 1 except that only dimethyl carbonate was used, and the burning length and the electric conductivity were measured. Comparative Example 2 An electrolytic solution was prepared in the same manner as in Example 1 except that dimethyl carbonate was used and ethylene methyl phosphate was used, and the burning length and the electric conductivity were measured. Comparative Example 3 An electrolyte was prepared in the same manner as in Example 1 except that an ethylene carbonate / dimethyl carbonate = 1/1 (volume ratio) solvent was used instead of ethylene methyl phosphate / trimethyl phosphate, and combustion was performed. Length and electrical conductivity were measured. Table 1 shows the results.

[Table 1] As is clear from Table 1, by containing the cyclic phosphate [I] and the chain phosphate [II], both high electrical conductivity and flame retardancy are achieved, and the mixing ratio is 75 /.
The electrical conductivity increases from 25 to 50/50, and then decreases with a decrease in the mixing ratio of the cyclic phosphate [I], but up to 25/75 shows an excellent value which cannot be obtained by the conventional method. Production Example 1 The following battery was constructed using this electrolytic solution. The negative electrode was prepared by adding 90 parts of graphite NG-12 manufactured by Kansai Thermochemical Co., Ltd. to 10 parts of polyvinylidene fluoride as a binder, using dimethylformamide to form a paste, applying the paste to a stainless steel net, and then applying 1 t / cm ^2. Pressure.
After drying, the negative electrode was punched into an appropriate shape. On the other hand, the positive electrode is Li
A mixture consisting of 88 parts of Co ₂ , 6 parts of acetylene black, and 6 parts of polyvinylidene fluoride was put into a shaper, and 1 t / c
shaping at a pressure of m ^2, and create a disc-shaped electrode. A coin battery was prepared using the positive electrode and the negative electrode thus obtained and the electrolytic solution shown in Examples 1 to 6 above, and a battery performance test was performed. Even better batteries could be created.

Claims (2)

[Claims] A weight ratio of the cyclic phosphate ester and the carbonate ester represented by the general formula [I] to 90/10 to 10/9.
A non-aqueous electrolyte for a secondary battery, characterized by being contained at 0. (Equation 1) [R ₁ in the formula is an alkyl group having 1 to 4 carbon atoms, R _2,
R ₃ is the same or different and each represents hydrogen or a methyl group. ] 2. A cyclic phosphate ester and a carbonate ester represented by the general formula [I] are used in a weight ratio of 75/25 to 25/7.
5. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein