JPH10270076A - Secondary lithium battery electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery which has good cycle characteristics and has good battery characteristics such as the electric capacity or storage stability, by specifying the HF value in electrolyte. SOLUTION: The secondary lithium battery electrolyte contains nonaqueous solvent and fluorine-containing electrolyte which can release lithium ions, and concentration of an alcohol group in the nonaqueous solvent is less than 50 ppm, especially, concentration of a diol group is less than 20 ppm, and that of a moon-alcohol group is less than 30 ppm. HF value in the electrolyte which consists of nonaqueous solvent and fluorine containing electrolyte is less than 30 ppm, preferably less than 20 ppm, especially preferably less than 15 ppm. Battery members other than electrolyte are specified especially, various members used in the past can be used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery, which can form a lithium secondary battery having excellent cycle characteristics and also excellent battery characteristics such as electric capacity and storage stability.

[0002]

2. Description of the Related Art As an electrolyte for a lithium secondary battery, a non-aqueous electrolyte obtained by dissolving a fluorine-containing electrolyte such as LiPF _{6 in} a solvent such as a cyclic carbonate, a chain carbonate, or an ether has a high voltage and a high capacity. It is often used because it is suitable for obtaining batteries. However, lithium secondary batteries using such a non-aqueous electrolyte are not always satisfactory in battery characteristics such as cycle characteristics, electric capacity, storage stability, etc., and are particularly excellent in cycle characteristics which do not cause a decrease in battery performance. Further, an electrolyte for a lithium secondary battery has been desired. Various methods have been proposed to improve battery characteristics.
No. 49 discloses a method in which a non-aqueous electrolyte solution in which a fluorine-containing electrolyte is dissolved is treated with a metal oxide such as MgO as a fluorine adsorbent to reduce free acid such as HF to 20 to 25 ppm. However, it is conventionally known that a reaction between a metal oxide and HF generates a metal fluoride and water. In addition, water is LiPF ₆ , LiBF ₄ ,
It is known that HF is produced by reacting with a fluorinated electrolyte such as LiAsF ₆ .
Water is generated by this reaction, and the generated water and L in the electrolyte are removed.
HF is generated again by the hydrolysis reaction with iPF _6, and there is a concern that the amount of HF in the electrolytic solution will increase again with the passage of time. On the other hand, as a method for removing diols from cyclic carbonate, an adsorption method using an adsorbent such as silica gel, activated carbon, activated alumina, and molecular sieves is known. For example, in Japanese Patent Application Laid-Open No. 5-74485,
A solution containing a cyclic carbonate as a solvent of the non-aqueous electrolyte is disclosed, and in order to make the diol in the non-aqueous electrolyte a low concentration of 1500 ppm or less, a method of distilling the cyclic carbonate and a method of treating with a sorbent are described. Have been.
Further, a method has been reported in which a cyclic carbonate containing a diol as an impurity is brought into contact with a synthetic zeolite in the presence of a chain carbonate to remove the diol in the cyclic carbonate. That is, JP-A-8-325208
Japanese Patent Application Laid-Open No. H10-163,086 describes a method in which ethylene glycol and a chain carbonate are reacted with zeolite in the presence of a cyclic carbonate and a chain carbonate to form a monoalcohol, and the alcohol is removed with a molecular sieve.

[0003]

In view of the above-mentioned problems of the electrolyte solution for a lithium secondary battery, by removing alcohols in a non-aqueous solvent, an electrolyte comprising the non-aqueous solvent and a fluorine-containing electrolyte is removed. It is an object of the present invention to provide an electrolyte for a lithium secondary battery capable of reducing the amount of HF in a liquid and forming a lithium secondary battery having excellent battery characteristics, particularly excellent cycle characteristics.

[0004]

The inventors of the present invention have found that when the content of HF in the electrolytic solution increases, the cycle characteristics of the lithium secondary battery deteriorate, and the battery characteristics such as electric capacity and storage stability also increase. Since it is decreasing, intensive studies were carried out to reduce HF in the electrolytic solution. As a result, diols or monoalcohols contained in a high-dielectric solvent such as ethylene carbonate or a low-viscosity solvent such as dimethyl carbonate gradually react with the fluorine-containing electrolyte at room temperature to generate HF. From this, it was found that the HF in the electrolytic solution increased with the passage of time, thereby deteriorating the cycle characteristics of the battery, leading to the present invention. The present invention relates to a lithium secondary battery electrolyte comprising a non-aqueous solvent in which alcohols are less than 50 ppm and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the HF content is 30%.
The present invention relates to an electrolyte solution for a lithium secondary battery, which has a concentration of less than 1 ppm.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous solvent used in the present invention is preferably a solvent comprising a high dielectric constant solvent and a low viscosity solvent. Preferred examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric constant solvents may be used alone or in combination of two or more.

As the low-viscosity solvent, for example, dimethyl carbonate (DMC), methyl ethyl carbonate (M
EC), chain carbonates such as diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane And lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl propionate, and amides such as dimethylformamide. 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 each arbitrarily selected and used in combination. In addition,
The high dielectric constant solvent and the low viscosity solvent are used in a volume ratio (high dielectric constant solvent: low viscosity solvent) of usually 1: 9 to 4: 1, preferably 1: 4 to 7: 3.

The fluorine-containing electrolyte capable of releasing lithium ions used in the present invention includes, for example, fluorophosphates such as LiPF ₆ , fluoroborates such as LiBF ₄ , and fluoroarsenic acids such as LiAsF ₆ salt,
Or a triflate salt such as LiOSO ₂ CF ₃ . At least one of these electrolytes is selected and used. These fluorinated electrolytes are usually added to the non-aqueous solvent in an amount of 0.1 M to 3 M, preferably 0.5 to 1.5 M.
It is used after being dissolved at a concentration of M.

The electrolyte for a lithium secondary battery of the present invention comprises a non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the content of alcohols in the non-aqueous solvent is less than 50 ppm. In particular, diols are less than 20 ppm and monoalcohols are less than 30 ppm, and the amount of HF in the lithium secondary battery electrolytic solution comprising the non-aqueous solvent and the fluorine-containing electrolyte is less than 30 ppm.
An electrolyte for a lithium secondary battery having a content of less than 20 ppm, preferably less than 20 ppm, and particularly preferably less than 15 ppm.

Therefore, each raw material used was purified by the following method to remove alcohols. Commercially available high-dielectric solvent and low-viscosity solvent constituting the non-aqueous solvent are preferably first subjected to crystallization treatment at room temperature such as ethylene carbonate, and also at room temperature as raw material such as diethyl carbonate. Is a reflux ratio of 0.01 to 300,
It is preferable to use one that has been subjected to precision distillation with 5 to 90 theoretical plates. The crystallization is desirably performed using a solvent such as acetonitrile, acetone, and toluene. The conditions for precision distillation vary depending on the type and amount of alcohol contained as impurities in the commercially available raw materials to be used, but it is usually preferable to purify under the above conditions. In the purification of commercial raw materials, precision distillation can be used instead of crystallization,
After crystallization, precision distillation can also be performed. Next, the high dielectric constant solvent and the low viscosity solvent constituting the non-aqueous solvent are purified by an adsorbent such as Molecular Sieves (trade name; the same applies hereinafter) 4A and / or Molecular Sieves 5A to remove alcohols. Is preferred. After preparing such a high dielectric constant solvent and a low viscosity solvent at a predetermined ratio, the alcohol is purified by an adsorbent such as Molecular Sieves 4A and / or Molecular Sieves 5A similar to the above for further purification. Classes may be removed. In the non-aqueous solvent treated as described above, a fluorinated electrolyte such as LiPF ₆ was dissolved to a predetermined concentration.

Hereinafter, a specific method for purifying a non-aqueous solvent will be described in detail. The respective purification treatments of the high dielectric constant solvent and the low viscosity solvent are performed in the same manner. Examples of the adsorbent used for the purification treatment include silica gel, alumina, activated carbon, molecular sieves 4A, and molecular sieves 5A. The contact method is a method in which a non-aqueous solvent is passed continuously (hereinafter, referred to as a continuous method), or a method in which an adsorbent is added to a non-aqueous solvent and allowed to stand or stirred (hereinafter, referred to as a batch method). Is mentioned. In the case of the continuous method,
The contact time is 0.1 to 4 / liquid hourly space velocity (LHSV).
It is preferably time. The contact temperature is 10 ° C ~
60 ° C. is preferred. In the case of a batch method, it is preferable to add 0.1 to 30% by weight based on the non-aqueous solvent, and to process for 0.5 to 24 hours. When the amount of alcohol contained in the non-aqueous solvent is large, distillation and crystallization are repeated, or the residence time or contact time of the adsorption method is lengthened to sufficiently purify the alcohol and the amount of alcohol in the non-aqueous solvent is reduced to less than 50 ppm. can do. Molecular sieves 4A among the adsorbents
The use of is preferred because the ability to selectively adsorb alcohols is high and the adsorption breakthrough time is long.

Since the purification method and purification conditions vary depending on the type of each raw material used and the type and amount of alcohol contained therein, it is necessary to appropriately select appropriate purification methods and purification conditions. The alcohol content in the non-aqueous solvent produced using these raw materials is less than 50 ppm, and the HF content in the electrolytic solution comprising the non-aqueous solvent and the fluorinated electrolyte is less than 30 ppm. Particularly, the amount of HF after the preparation of the electrolytic solution hardly increases with time, and the cycle characteristics are improved when a lithium secondary battery is formed.

A lithium secondary battery using the electrolyte solution for a lithium secondary battery of the present invention has good cycle characteristics, and also has excellent battery characteristics such as electric capacity and storage stability. The constituent members of the lithium secondary battery other than the electrolyte are not particularly limited, and various conventionally used constituent members can be used. For example, as the positive electrode material (positive electrode active material), a composite metal oxide of lithium and at least one metal selected from the group consisting of chromium, vanadium, manganese, iron, cobalt, and nickel is used. Examples of such a composite metal oxide include LiCoO ₂ , L
iMn ₂ O ₄ , LiNiO _{2 and the} like can be mentioned.

The positive electrode is made of a conductive material such as acetylene black or carbon black, polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (P).
VDF) and the like to form a positive electrode mixture, and this positive electrode mixture is rolled as a current collector into an aluminum or stainless steel foil or lath plate at a temperature of about 50 to 250 ° C. It is produced by heating under vacuum for about an hour.

Examples of the negative electrode material (negative electrode active material) include lithium metals, lithium alloys, carbon materials (pyrolytic carbons, cokes, graphites, glassy carbons, organic polymer compound burners, carbon fibers, activated carbon, etc. ) Or a compound capable of occluding and releasing lithium, such as a tin composite oxide. A powder material such as a carbon material is kneaded with a binder such as ethylene propylene diene monomer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF) to be used as a negative electrode mixture.

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, a cylindrical battery having a positive electrode, a negative electrode and a roll-shaped separator, and the like. A prismatic battery is an example. In addition, as a separator,
Known microporous polyolefin membranes, woven fabrics, nonwoven fabrics and the like are used.

[0016]

EXAMPLES Next, the present invention will be described specifically with reference to examples and comparative examples, but these do not limit the present invention in any way. Example 1 [Preparation of electrolytic solution] Commercially available EC was crystallized twice with acetonitrile, and then adsorbed with molecular sieves 4A (50).
C., LHSV; 1 / hour). On the other hand, DMC was subjected to sufficiently precise distillation at a reflux ratio of 1 and a theoretical plate number of 30.
An adsorption treatment (25 ° C.) was performed with molecular sieves 4A at an SV of 1 / hour. After that, EC: DMC (volume ratio)
A 1: 2 non-aqueous solvent was prepared. At that time, diols and monoalcohols in the non-aqueous solvent were not detected. LiPF ₆ was dissolved therein to a concentration of 0.8M. As a result, the amount of HF in the electrolyte one day later was 10 ppm
Met. Two weeks later, the amount of HF in the electrolytic solution was measured again, but was unchanged at 10 ppm.

[Preparation of Lithium Secondary Battery and Measurement of Battery Characteristics] LiCoO ₂ (cathode active material) was 70% by weight, acetylene black (conductive agent) was 20% by weight, and polytetrafluoroethylene (binder) was 10% by weight. The mixture was mixed at a ratio of weight%, and this was compression molded to prepare a positive electrode. 95% by weight of natural graphite (negative electrode active material) and 5% by weight of ethylene propylene diene monomer (binder) were mixed and compression molded to prepare a negative electrode. Then, using a separator made of polypropylene microporous film, impregnated with the above-mentioned electrolytic solution, a coin-type battery (diameter: 20 mm, thickness: 3.2 m)
m) was prepared. Room temperature (2
0 ° C.) at a constant current of 0.8 mA and a charge end voltage of 4.2
When charge and discharge were repeated as a potential regulation of 2.7 V and a discharge end voltage of 2.7 V, the discharge capacity retention rate after 100 cycles was 90%.

Example 2 Commercially available EC and DMC were each separately subjected to precision distillation at a reflux ratio of 1 and a theoretical plate number of 30 and then each was subjected to LHSV.
2 / hour, the molecular sieve 4A adsorption treatment was performed at 50 ° C. for EC and 25 ° C. for DMC. Thereafter, a non-aqueous solvent of EC: DMC (volume ratio) = 1: 2 was prepared, and treated with Molecular Sieves 4A (25 ° C.) at an LHSV of 2 / hour. At that time, the diol content in the non-aqueous solvent was 2 ppm, the monoalcohol content was 1 ppm, and the total content of alcohols was 3 ppm.
LiPF ₆ was dissolved therein to a concentration of 0.8M. As a result, the HF content of the electrolytic solution after 1 day was 11 ppm and after 2 weeks was 11 ppm. Using this electrolytic solution, a coin-type battery was produced in the same manner as in Example 1, and charge and discharge were repeated. As a result, the discharge capacity retention rate after 100 cycles was 88%.

Example 3 Commercially available EC was crystallized once with acetonitrile, and then treated with molecular sieves 5A (50 ° C., LHSV; 2 / hour). On the other hand, DEC was subjected to sufficiently precise distillation at a reflux ratio of 0.5 and a theoretical plate number of 30 and then subjected to an adsorption treatment (25 ° C.) with molecular sieves 4A at a LHSV of 2 / hour. Thereafter, a non-aqueous solvent of EC: DEC (volume ratio) = 1: 2 was prepared, and treated with molecular sieves 4A (2
5 ° C., LHSV; 2 / hour). At that time, the diol content in the non-aqueous solvent was 3 ppm, and the monoalcohol content was 2 p.
pm, and the total content of alcohols was 5 ppm. LiPF ₆ was dissolved therein to a concentration of 0.8M.
As a result, the amount of HF in the electrolytic solution after 1 day was 13 ppm, and after 2 weeks, it was 13 ppm. Using this electrolytic solution, a coin-type battery was manufactured in the same manner as in Example 1, and charge and discharge were repeated. As a result, the discharge capacity retention rate after 100 cycles was 8%.
7%.

Example 4 Commercially available EC and DMC were each separately subjected to a reflux ratio of 0.1.
5. Precision distillation was performed with 30 theoretical plates. After that, EC: D
A non-aqueous solvent having a MC (volume ratio) of 1: 2 was prepared and treated with Molecular Sieves 5A (25 ° C., LHSV; 4 / hour) and 4A (25 ° C., LHSV; 4 / hour). At that time, the diol content in the non-aqueous solvent was 3 ppm, the monoalcohol content was 3 ppm, and the total content of alcohols was 6 ppm.
Met. LiPF ₆ was dissolved therein to a concentration of 0.8M. As a result, the amount of HF in the electrolyte one day later was 1
It was 3 ppm and 14 ppm after 2 weeks. Using this electrolytic solution, a coin-type battery was produced in the same manner as in Example 1, and charge and discharge were repeated. As a result, the discharge capacity retention rate after 100 cycles was 87%.

Example 5 Commercially available EC was treated with Molecular Sieves 4A (50
C., LHSV; 1 / hour). On the other hand, DMC was subjected to precision distillation at a reflux ratio of 0.5 and a theoretical plate number of 30 and then subjected to adsorption treatment with molecular sieves 4A (25 ° C., LHSV; 1).
/ Hour). Then, EC: DMC (volume ratio) =
A 1: 2 non-aqueous solvent was prepared, and molecular sieves 4A was prepared.
(25 ° C., LHSV; 2 / hour). At that time,
Diol content in non-aqueous solvent 10 ppm, monoalcohol content 11 ppm, total content of alcohols 21 pp
m. LiPF ₆ was dissolved therein to a concentration of 0.8M. As a result, the amount of HF in the electrolytic solution after one day was 16 ppm, and the amount after two weeks was 18 ppm. Using this electrolytic solution, a coin-type battery was fabricated in the same manner as in Example 1, and charge and discharge were repeated. As a result, the discharge capacity retention rate after 100 cycles was 83%.

Example 6 A commercially available EC was subjected to precision distillation with a reflux ratio of 0.5 and a theoretical plate number of 30 to prepare DMC, which was prepared as a non-aqueous solvent of EC: DMC (volume ratio) = 1: 2. Treatment with Sieves 5A (25 ° C., LHSV; 4 / hour) was followed by treatment with molecular sieves 4A (25 ° C., LHSV; 4 / hour). At that time, the diol content in the non-aqueous solvent was 16p
pm, the content of monoalcohol was 19 ppm, and the total content of alcohols was 35 ppm. LiPF ₆ was dissolved therein to a concentration of 0.8M. As a result, the HF content of the electrolyte solution after 1 day was 18 ppm, and 2 hours after 2 weeks.
It was 5 ppm. Using this electrolytic solution, a coin-type battery was produced in the same manner as in Example 1, and charging and discharging were repeated.
The discharge capacity retention rate after the 00 cycles was 79%.

Example 7 Commercially available EC and 1,2-dimethoxyethane (DME) were precision distilled separately at a reflux ratio of 0.7 and a theoretical plate number of 30. Thereafter, EC: DME (capacity ratio) = 1: 2
Of a non-aqueous solvent of molecular sieves 5A (25
° C, LHSV; 4 / hour) and 4A (25 ° C, LHS
V; 4 / hour). At that time, the diol content in the non-aqueous solvent was 2 ppm, the monoalcohol content was 0 ppm,
The total content of alcohols was 2 ppm. This is Li
The PF ₆ was dissolved to a concentration of 0.8M. As a result, the HF content of the electrolytic solution after 1 day was 11 ppm, and after 2 weeks was 12 ppm. Using this electrolytic solution, a coin-type battery was produced in the same manner as in Example 1, and charge and discharge were repeated. As a result, the discharge capacity retention rate after 100 cycles was 88%.

Comparative Example 1 Commercially available EC and DMC were mixed to prepare a non-aqueous solvent of EC: DMC (volume ratio) = 1: 2, and treated with Molecular Sieves 5A (25 ° C.). LHSV was 5 / hr. At that time, the diol content in the electrolytic solvent was 40 pp.
m, the monoalcohol content was 45 ppm, and the total content of alcohols was 85 ppm. To this, LiPF ₆ was added at 0.
It was dissolved to a concentration of 8M. As a result, the HF content of the electrolytic solution after one day was 51 ppm, and the
pm. Using this electrolytic solution, a coin-type battery was produced in the same manner as in Example 1, and charging and discharging were repeated.
The discharge capacity retention after the cycle was 58%.

[0025]

According to the present invention, it is possible to provide an electrolyte for a lithium secondary battery capable of forming a lithium secondary battery having excellent cycle characteristics and also excellent battery characteristics such as electric capacity and storage stability. .

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yosuke Ueno 1078, Ogushi, Ogushi, Ube City, Yamaguchi Prefecture Ube Industries, Ltd. No. 10 Ube Kosan Co., Ltd. Ube Chemical Factory Ube Integration Plant (72) Inventor Masahiko Watanabe 1978 Kogushi Oaza, Ube City, Yamaguchi Prefecture 10 Ube Kosan Ube Chemical Plant Ube Integration Plant

Claims (3)

[Claims] 1. An electrolyte for a lithium secondary battery comprising a non-aqueous solvent containing less than 50 ppm of an alcohol and a fluorine-containing electrolyte capable of releasing lithium ions, has an HF content of 30.
An electrolyte solution for a lithium secondary battery, wherein the electrolyte solution is less than 1 ppm. 2. An electrolyte for a lithium secondary battery comprising a non-aqueous solvent containing less than 20 ppm of a diol and a fluorine-containing electrolyte capable of releasing lithium ions, has an HF content of 30 p.
pm, which is less than pm. 3. A lithium secondary battery electrolyte comprising a non-aqueous solvent containing less than 30 ppm of a monoalcohol and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the HF content is less than 30 ppm. Electrolyte for batteries.