JP3244017B2 - Method for producing electrolyte solution for lithium secondary battery - Google Patents

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, such an electrolyte contains a large amount of HF, and a lithium secondary battery using this electrolyte is not always satisfactory in battery characteristics such as cycle characteristics, electric capacity, and storage stability. There is a demand for an electrolyte for a lithium secondary battery having excellent cycle characteristics that does not cause a decrease in battery performance. As a method for removing diols from the cyclic carbonate, an adsorption method using an adsorbent such as silica gel, activated carbon, activated alumina, and molecular sieves is known. For example, JP-A-5-74485 discloses a solution containing a cyclic carbonate as a solvent of a non-aqueous electrolyte,
In order to reduce the diol concentration in the nonaqueous electrolyte to a low level of 1500 ppm or less, a method of distilling a cyclic carbonate and a method of treating with a sorbent are described. On the other hand, there has been reported a method of removing a diol in a cyclic carbonate by bringing a cyclic carbonate containing a diol as an impurity into contact with a synthetic zeolite in the presence of a chain carbonate. That is, JP-A-8-325208 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 produce a monoalcohol, and the alcohol is removed with a molecular sieve. ing.

[0003]

In view of the above-mentioned problems of the electrolyte for a lithium secondary battery, H in the electrolyte has been considered.
It is an object of the present invention to provide an electrolyte for a lithium secondary battery capable of forming a lithium secondary battery having a low F and excellent battery characteristics, particularly excellent cycle characteristics.

[0004]

Means for Solving the Problems The present inventors have conducted intensive studies in view of the above-mentioned problems of the electrolyte solution for a lithium secondary battery, and as a result, have found that the alcohols in the non-aqueous solvent are different from the fluorine-containing electrolyte at room temperature. It was found that HF was formed by reacting slowly. When the content of the alcohol increases, the lithium secondary battery using the electrolytic solution composed of the nonaqueous solvent and the fluorinated electrolyte has reduced cycle characteristics, and further has reduced battery characteristics such as electric capacity and storage stability. Therefore, the present inventors have studied to reduce the content in the electrolytic solvent, and have reached the present invention. The present invention provides a method for producing an electrolyte solution for a lithium secondary battery comprising a non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the non-aqueous solvent contains a high dielectric constant solvent and a low-viscosity solvent; It is produced by separately purifying a dielectric solvent and a low-viscosity solvent, and then mixing the non-aqueous solvent obtained by mixing with the fluorinated electrolyte, wherein the diols in the non-aqueous solvent are less than 20 ppm.
The present invention relates to a method for producing an electrolyte for a lithium secondary battery, wherein the electrolyte is full . Further, the present invention relates to a lithium secondary battery using the electrolytic solution obtained by the production method.

[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 according to the present invention contains the above-mentioned non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the alcohol in the non-aqueous solvent is less than 50 ppm, 20 ppm
And the content of monoalcohols is preferably less than 30 ppm. Examples of the diols include ethylene glycol and / or diethylene glycol, propylene glycol, and dipropylene glycol, and examples of the monoalcohol include methanol and / or ethanol, propanol, and butanol.

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 these high-dielectric solvent and low-viscosity solvent so as to have a predetermined ratio, for further purification, purification treatment with an adsorbent such as molecular sieves 4A and / or molecular sieves 5A similar to the above, Alcohols 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 left standing 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 amount of alcohol in the lithium secondary battery electrolyte prepared using these materials is less than 50 ppm, and the cycle characteristics are improved when a lithium secondary battery is configured.

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 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 30 theoretical plates, and then subjected to an adsorption treatment with molecular sieves 4A (25 ° C., LHSV; 1 /
Time). Then, 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.

[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. 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 with molecular sieves 4A (25 ° C., LHSV; 2 / hour). Thereafter, a non-aqueous solvent of EC: DEC (volume ratio) = 1: 2 was prepared and treated with molecular sieves 4A (25
° C, LHSV; 2 / hour). At that time, the diol content in the non-aqueous solvent was 3 ppm and the monoalcohol content was 2 pp.
m and the total content of alcohols was 5 ppm. LiPF ₆ was dissolved therein to a concentration of 0.8M. 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 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. 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 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. 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 0 cycles was 83%.

[0022]

Example 6 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
PF6 was dissolved to a concentration of 0.8M. Using this electrolyte solution, a coin-type battery was produced in the same manner as in Example 1,
When the charge and discharge were repeated, 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. LiPF ₆ was added to this in 0.1.
It was dissolved to a concentration of 8M. 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 58%.
%Met.

[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 Continuing on the front page (72) Inventor Noriyuki Ohira 10-78, Ogushi, Oji, Ube City, Yamaguchi Prefecture Ube Industries, Ltd. 1978 Ube Industries, Ltd. Ube Chemical Plant Ube Chemical Plant Ube Integrated Plant Examiner Mitsuji Uemae (56) References JP-A-5-74485 (JP, A) JP-A-59-81869 (JP, A) JP-A-61-284062 (JP, A) JP-A-8-325208 (JP, A) JP-A-5-226004 (JP, A) JP-A-10-116632 (JP, A) JP-A-10-270076 ( JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 10/40

Claims (7)

(57) [Claims] 1. A method for producing an electrolyte for a lithium secondary battery comprising a non-aqueous solvent and a fluorine-containing electrolyte capable of releasing lithium ions, wherein the non-aqueous solvent contains a high dielectric constant solvent and a low viscosity solvent, It is produced by separately purifying a high dielectric constant solvent and a low viscosity solvent, and then mixing the non-aqueous solvent obtained by mixing with the fluorinated electrolyte, wherein the diols in the non-aqueous solvent are 20 ppm. A process for producing an electrolyte for a lithium secondary battery. 2. The method according to claim 1, wherein the purification of the high dielectric constant solvent is carried out by crystallization.
The method for producing an electrolyte for a lithium secondary battery according to claim 1, wherein the method is at least one kind of purification treatment selected from precision distillation and purification treatment with an adsorbent. 3. The method for producing an electrolyte for a lithium secondary battery according to claim 1, wherein said high dielectric constant solvent contains at least one kind of cyclic carbonates. 4. The method for producing an electrolyte solution for a lithium secondary battery according to claim 1, wherein the purification treatment of the low-viscosity solvent is at least one purification treatment selected from crystallization, precision distillation, and purification treatment with an adsorbent. 5. The lithium secondary battery according to claim 1, wherein the low-viscosity solvent contains at least one selected from the group consisting of chain carbonates, ethers, lactones, nitriles, esters, and amides. Method for producing electrolyte solution for secondary battery. 6. An electrolytic solution for a lithium secondary battery obtained by the method according to claim 1. 7. A lithium secondary battery using the electrolyte for a lithium secondary battery obtained by the method according to claim 1.