JP2003007332A - Lithium secondary battery and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having a low internal resistance irrespective of the moisture content of the secondary battery, and a manufacturing method of the lithium secondary battery. SOLUTION: The lithium secondary battery comprises a positive electrode and a negative electrode, each of which is made of an activator capable of occluding or separating lithium, and a nonaqueous electrolytic solution composed by dissolving an electrolyte, which produces halogen acid in reacting with water, and an organic silicon compound, which has Si-N(silicon - nickel) bond, in an organic solvent. The organic silicon compound is added to the nonaqueous electrolytic solution after the lithium secondary battery is charged up at least once. In this manner, the formation of a high resistance film due to the organic silicon compound can be suppressed by charging the secondary battery more than once, so that a bad effect by water can be suppressed without adverse effect by the high resistance film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery which is less likely to be adversely affected by moisture inside the battery and a method for manufacturing the same.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, etc., in the field of information-related equipment and communication equipment, lithium secondary batteries are used as a power source for these equipment because of their high energy density. Has been put into practical use and has become widespread. On the other hand, also in the field of automobiles, the development of electric vehicles has been rushed due to environmental problems and resource problems, and lithium secondary batteries are also being considered as a power source for these electric vehicles.

Lithium secondary batteries have a layered rock salt structure of LiCoO ₂ , which can provide an operating voltage of 4V.
Positive electrode active materials such as LiNiO ₂ , LiMn ₂ O _{4 having} a spinel structure, and lithium transition metal composite oxides in which some of them are substituted with other elements are well known. Among these,
Batteries using LiCoO ₂ -based lithium-transition metal composite oxides as the positive electrode active material are now predominant because they are easy to synthesize and obtain the highest operating voltage.

By the way, in order to achieve an operating voltage of 4 V class, it is necessary to use a non-aqueous electrolyte as the electrolytic solution. LiPF ₆ , which is a halogen compound, is often used as the electrolyte in the non-aqueous electrolytic solution, but some water is inevitably mixed in the electrolytic solution which should be non-aqueous, or other batteries. When water is adsorbed on the material, a reaction as shown in the following formula (1) occurs, and a halogen acid such as hydrogen fluoride HF is generated.

LiPF ₆ + H ₂ O → 2HF + LiF + POF ₃ (1) The generated hydrogen fluoride deteriorates the materials constituting the battery,
Further, there is a problem that the battery performance is deteriorated. For example, internal resistance deteriorates as a result of deterioration of constituent materials.

In order to solve this problem, Japanese Patent Laid-Open No. 11-
Japanese Patent No. 16602 describes an invention of a non-aqueous electrolyte secondary battery in which an organosilicon compound having a Si—N bond is added to a non-aqueous electrolyte containing LiPF ₆ or the like as an electrolyte. The organosilicon compound having a Si—N bond reacts with water and hydrogen fluoride and is converted into a compound that does not adversely affect the battery material.

As another solution, there is a method of strictly drying each constituent element of the lithium secondary battery to limit the amount of mixed water.

[0008]

However, according to the research conducted by the present inventors, the addition of an organosilicon compound having a Si--N bond may rather increase the internal resistance of the battery. In addition, a large dry atmosphere is required to limit the water content of each component, which is one of the causes of cost increase.

Therefore, it is an object of the present invention to provide a lithium secondary battery having a low internal resistance regardless of the water content. Further, it is an object of the present invention to provide a method for producing a lithium secondary battery having a low internal resistance regardless of the water content.

[0010]

[Means for Solving the Problems] As a result of intensive research conducted by the present inventor for the purpose of solving the above problems, as a result
Non-aqueous electrolysis in which a positive electrode and a negative electrode each having a removable substance as an active material, an electrolyte capable of generating a halogen acid by reacting with water, and an organosilicon compound having a Si—N bond are dissolved in an organic solvent. A lithium secondary battery including a liquid, wherein the organosilicon compound is added to the non-aqueous electrolyte after charging the lithium secondary battery at least once. A characteristic lithium secondary battery has been invented.

That is, even if the organosilicon compound having the Si--N bond is not added, the internal resistance does not decrease as expected because the organosilicon compound having the Si--N bond is electrochemically unstable. It is considered that the internal resistance of the lithium secondary battery increases on the contrary, as a result of being decomposed to form a high resistance film on the surface of the active negative electrode active material at the initial stage of charging the lithium secondary battery. Based on this finding, the present inventor has conducted an investigation for the purpose of suppressing the formation of a high resistance film formed by a decomposition product of an organosilicon compound having a Si—N bond, and as a result, decomposed the organosilicon compound having a Si—N bond. It has been found that the film formation by the method is concentrated during the first charge after the lithium secondary battery is manufactured. Therefore, the addition of the organosilicon compound having a Si—N bond into the electrolytic solution is performed after the first charging after the lithium secondary battery is created, so that the organosilicon compound having a Si—N bond is derived. Since the formation of a high resistance film is suppressed and the function of the organosilicon compound is not affected, it has succeeded in suppressing the adverse effect of water.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the lithium secondary battery of the present invention,
Known battery structures such as a coin battery, a button battery, a cylindrical battery, and a prismatic battery can be adopted. Whichever shape is adopted, the positive electrode and the negative electrode are laminated or wound with a separator to form an electrode body, and the positive electrode current collector and the negative electrode current collector are connected to the positive electrode terminal and the negative electrode terminal which communicate with the outside. After connecting between the two using a current collecting lead, etc., insert this electrode body together with the non-aqueous electrolyte into the battery case,
By sealing this, a lithium battery can be completed.

The feature of the lithium secondary battery of the present invention is that
This is to add the organosilicon compound having a -N bond to the non-aqueous electrolyte after charging the present lithium secondary battery at least once. Further, the charging of the lithium secondary battery before adding the organosilicon compound can be carried out for several cycles with discharging, and preferably the charging and discharging is carried out 5 times. Here, "to charge a lithium secondary battery" means to insert an electrode body consisting of a positive electrode, a negative electrode and a separator into a battery case, and to charge after sealing, as well as after forming the electrode body, the battery. The charging may be performed without sealing the case or in the non-aqueous electrolyte solution stored in a container other than the battery case. That is, it is not necessary to perform charging in a state in which all of the constituent elements used in the final lithium secondary battery are included or in a form completed as a lithium secondary battery. Furthermore, it suffices to charge only at least the negative electrode. A film is formed on the surface of the negative electrode by the first charge, and this film suppresses the formation of a high resistance film derived from an organosilicon compound. Therefore, to put it in a polar theory, even if charging is not performed in the form of the electrode body used in the final lithium secondary battery, charging is performed after the electrode body is created by another combination, and only the negative electrode is used (further To put it to the extreme, the lithium secondary battery of the present invention can be obtained by taking out only the negative electrode active material used for the negative electrode.

As a method of adding the organosilicon compound to the non-aqueous electrolyte, a hole is provided in the battery case (for example, a lid of the battery case) when charging is performed in the battery case, and the organosilicon compound is introduced from the hole. You can add or charge without sealing the lid and open the lid to add, and when charging in a container other than the battery case, when inserting the electrode body etc. in the battery case The organosilicon compound can be added to the non-aqueous electrolyte solution at the same time. Here, the non-aqueous electrolyte in the case of “charging the lithium secondary battery” does not have to be the one used in the final lithium secondary battery. Therefore, the non-aqueous electrolyte can be exchanged before and after charging.
The constituent elements of the lithium secondary battery of the present invention will be described in detail below.

The positive electrode is prepared by mixing a conductive material and a binder with a positive electrode active material capable of occluding and desorbing lithium ions, and adding a suitable solvent if necessary to obtain a paste-like positive electrode mixture, which is made of aluminum. Coated on the surface of a metal foil collector such as
It is formed by drying and then increasing the density of the active material by pressing.

As the positive electrode active material, a known positive electrode active material such as a lithium transition metal composite oxide can be used. The lithium-transition metal composite oxide is a preferable material for the positive electrode active material because it has low electric resistance, excellent lithium ion diffusion performance, high charge-discharge efficiency, and good charge-discharge cycle characteristics. For example, lithium cobalt oxide,
Lithium nickel oxide, lithium manganese oxide,
A material in which a transition metal such as Li, Al, and Cr is added or replaced is used. When these lithium-metal composite oxides are used as the positive electrode active material, they can be used not only individually but also as a mixture of two or more kinds.

The conductive material is for ensuring the electrical conductivity of the positive electrode, and it is preferable to use one kind or a mixture of two or more kinds of carbon material powders such as carbon black, acetylene black and graphite. it can. The binder plays a role of binding the active material particles and the conductive material particles together, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene can be used. . As a solvent for dispersing the active material, the conductive material, and the binder, N-
An organic solvent such as methyl-2-pyrrolidone can be used.

The negative electrode is not particularly limited in material constitution as long as it can use a negative electrode active material which occludes lithium ions during charging and releases lithium ions during discharging, and a known material constitution is used. You can For example, it is a carbon material such as lithium metal, graphite or amorphous carbon. Among them, it is particularly preferable to use a carbon material. Since the specific surface area can be made relatively large, and the absorption and desorption rates of lithium are fast, the charge and discharge characteristics at a large current and the output and regeneration density are good. In particular,
Considering the balance between the output and the regenerative density, it is preferable to use a carbon material having a relatively large voltage change with charge and discharge. Above all, it is preferable to use one made of natural graphite or artificial graphite having high crystallinity. By using such a carbon material having high crystallinity, the lithium ion delivery efficiency of the negative electrode can be improved.

When a carbon material is used as the negative electrode active material as described above, the negative electrode mixture obtained by mixing the conductive material and the binder as described in the positive electrode, if necessary, with the current collector. It is preferable to use the one applied to the body.

The non-aqueous electrolyte is composed of an organic solvent, an electrolyte and Si-
It is a solution of an organosilicon compound having an N bond.

The organic solvent is not particularly limited as long as it is an organic solvent usually used in a non-aqueous electrolyte of a lithium secondary battery, and examples thereof include carbonates, halogenated hydrocarbons, ethers, ketones and nitriles. , Lactones, oxolane compounds and the like can be used. In particular,
Propylene carbonate, ethylene carbonate, 1,
2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran and the like and mixed solvents thereof are suitable. For example, a mixed organic solvent of a main solvent having a high dielectric constant such as ethylene carbonate and propylene carbonate and a sub-solvent having a low viscosity such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate is preferable. Moreover, you may use dimethoxyethane, tetrahydrofuran, butyl lactone, etc. as a subsolvent.

The electrolyte contains LiPF ₆ . Furthermore, L
Inorganic salt selected from iBF ₄ , LiClO ₄ and LiAsF ₆ , derivatives of the inorganic salt, LiSO ₃ CF ₃ , LiC
(SO ₃ CF ₃ ) ₂ , LiN (SO ₃ CF ₃ ) ₂ , LiN (S
O ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F
_At least one kind of organic salt selected from ₉ ) and a derivative of the organic salt can be contained if necessary.

By using these electrolytes, the battery performance can be further improved, and the battery performance can be maintained even higher in a temperature range other than room temperature. The concentration of the electrolyte is also not particularly limited, and it is preferable to appropriately select the concentration of the electrolyte and the organic solvent in consideration of the use. These electrolytes generate halogen acid by reacting with water.

The organosilicon compound is a compound having a Si-N bond. In this organosilicon compound, the Si—N bond is cleaved and reacts with water or a halogen acid, whereby water or the like is converted into a lithium secondary battery by the reaction of the following formula (2): water and (3): halogen acid. Change to a compound that does not adversely affect. _{R 3 -Si-N-R 2} + H 2 O → R 3 -Si-OH + HN-R 2 ... (2) R 3 -Si-N-R 2 + HX → R 3 -Si-X + HN- R ₂ (3) Here, R is a group in which both formulas (2) and (3) are independently modifiable, and is selected from an alkyl group capable of substituting a hydrogen atom with any substituent. It Therefore, the organosilicon compound usable in the lithium secondary battery of the present invention is not particularly limited in terms of other chemical structures as long as it has a Si—N bond, and may be a mixture of a plurality of compounds. There may be a plurality of Si-N bonds in the molecule, and further includes a structure such as Si-N-Si. Some examples of organosilicon compounds are 1,1,3,3,5,5
-Hexamethylcyclotrisilazane, 1,1,1,3
3,3-hexamethyldisilazane, (N, O-bistrimethylsilyl) acetamide, (N, N-diethylamino) trimethylsilazane, bis (trimethylsilyl)-
1,4-butanediamine, N-trimethylsilylimidazole and the like. Even if it is assumed that all of the water content reacts with the electrolyte to form a halogen acid, it produces only twice the halogen acid content in terms of the molar ratio of the water content, so the concentration of the organosilicon compound added is: A sufficient effect can be exhibited by adding at most twice the molar ratio of the water content expected in the final lithium secondary battery.

The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a porous film of polyolefin polymer (polyethylene, polypropylene) may be used. The separator is preferably larger than the positive electrode and the negative electrode in order to ensure the insulation between the positive electrode and the negative electrode.

The battery case is not particularly limited and can be made of known materials and forms.

The gasket ensures electrical insulation between the case and both the positive and negative terminal portions and the hermeticity of the inside of the case. For example, the electrolyte may be composed of a polymer such as polypropylene that is chemically and electrically stable.

[0028]

EXAMPLES The lithium secondary battery of the present invention will be described below based on examples.

<Lithium Secondary Battery of Test Example 1> The lithium secondary battery of this test example uses a lithium nickel composite oxide represented by the composition formula LiNiO ₂ as a positive electrode active material,
It is a lithium secondary battery using carbon black as a negative electrode active material.

The positive electrode of the lithium secondary battery of this test example was prepared by mixing 85 parts by mass of the above LiNiO ₂ , 10 parts by mass of acetylene black as a conductive material, and 5 parts by mass of polyvinylidene fluoride as a binder to prepare an appropriate amount. A paste-like positive electrode mixture is obtained by adding and kneading N-methyl-2-pyrrolidone, and the positive electrode mixture is applied to both surfaces of a 15 μm thick Al foil positive electrode current collector, dried, and pressed. After that, a sheet-shaped positive electrode was produced.

The negative electrode was prepared by mixing 92.5 parts by mass of carbon black and 7.5 parts by mass of polyvinylidene fluoride as a binder, adding an appropriate amount of N-methyl-2-pyrrolidone, and kneading the paste. A negative electrode composite material was obtained, and the negative electrode composite material was applied onto both surfaces of a Cu foil negative electrode current collector having a thickness of 10 μm, dried, and subjected to a pressing step to produce a sheet-shaped negative electrode.

The positive electrode and the negative electrode are each cut into a predetermined size, and the cut positive electrode and negative electrode have a thickness of 2 mm therebetween.
Wound by sandwiching a polyethylene separator of 5 μm,
A roll-shaped electrode body was formed. A current-collecting lead was attached to this electrode body and inserted into a 18650 type battery case, and then a non-aqueous electrolyte was injected into the battery case. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 1 was used. Finally, the battery case was sealed to complete the lithium secondary battery of this test example. The positive electrode and the negative electrode contained water in a weight ratio of 2000 ppm.

Then, 4.1 is used for conditioning.
Charge / discharge was performed for 5 cycles at 1 C between V and 3V.

<Lithium Secondary Battery of Test Example 2> A lithium secondary battery of Test Example 1 was manufactured except that the positive electrode and the negative electrode contained water of 6000 ppm in mass ratio.

<Lithium Secondary Battery of Test Examples 3, 4, and 5>
A lithium secondary battery was manufactured in the same manner as in Test Example 1 except that the water contents of the positive electrode and the negative electrode were not specified. One of them was subjected to conditioning, and then 1,1,3,3,5,5-hexamethylcyclotrisilazane as an organic silicon compound was added to the non-aqueous electrolyte solution in a mass ratio of 0.5%. The battery of Test Example 3 was used. The organosilicon compound was added after conditioning the battery in a dry atmosphere without sealing the lid of the battery case.

Another condition is that before conditioning, the organosilicon compound is added to the non-aqueous electrolyte in a mass ratio of 0.
5% was added to obtain a battery of Test Example 4. Then, the battery to which the above-mentioned organosilicon compound was not added was used as a battery of Test Example 5.
The batteries of Test Examples 3 to 5 are considered to have the same water content in the electrodes.

<Measurement of Internal Resistance> The internal resistance at 25 ° C. of the batteries of each test example was measured. The internal resistance was measured by measuring the value of the terminal voltage of the battery when the value of the current applied to the battery was changed and deriving from the slope of the line connecting each current value and the voltage value.

Regarding the batteries of Test Examples 3 to 5-3
The output at 0 ° C was measured. The output value was obtained by cooling the battery of each test example fully charged to −30 ° C. and then obtaining the current value (I) at which the terminal voltage was 3 V, and was obtained from 3 × I.

These measurements were carried out 24 hours after the battery conditioning was completed.

<Results> The results are shown in Table 1.

[0041]

[Table 1]

From the table, the internal resistance of the battery of Test Example 1 (61 m
Ω) was compared with the internal resistance (74 mΩ) of the battery of Test Example 2, it was revealed that the internal resistance of the battery of Test Example 2 was higher. It is considered that this is because the battery of Test Example 2 had a large amount of water and thus a large amount of halogen acid (hydrogen fluoride) produced by the reaction of the formula (1), and the positive electrode active material was damaged by the produced hydrogen fluoride. To be

Comparing the batteries of Test 4 and Test Example 5, the internal resistance of the battery of Test Example 4 to which the organosilicon compound was added was rather increased. It was speculated that this was because the potential stability of the organosilicon compound was low, and the products decomposed during conditioning formed a high resistance film. On the other hand, the internal resistance (50 mΩ) of the battery of Test Example 3 in which the organosilicon compound was added after conditioning was lower than the internal resistance (60 mΩ) of the battery of Test Example 4, as well as the internal resistance of the battery of Test Example 5. Resistance (53mΩ)
It became clear that it is lower than that. It was speculated that this is because the negative electrode surface was covered with the film formed in the initial stage of conditioning, and thus the progress of decomposition of the organosilicon compound or the formation of the film was suppressed.

The same thing can be seen from the measurement results of the low temperature output characteristics at -30 ° C. That is, since the battery of Test Example 3 has a higher output than the batteries of Test Examples 4 and 5, the internal resistance of the battery of Test Example 3 was similar to that found from the result of the internal resistance at 25 ° C. Was low even at -30 ° C.

[0045]

INDUSTRIAL APPLICABILITY In the lithium secondary battery of the present invention, the organosilicon compound is added to the non-aqueous electrolyte after charging the lithium secondary battery at least once. By having such a configuration, the lithium secondary battery of the present invention,
It becomes unnecessary to strictly control the water content of the constituent materials of the battery, and it becomes possible to provide a lithium secondary battery having a low internal resistance at a lower cost.

Similarly, in the method for producing a lithium secondary battery of the present invention, the organosilicon compound is added to the non-aqueous electrolyte after charging the lithium secondary battery at least once. The lithium secondary battery manufactured by the manufacturing method of the present invention having such a structure does not require strict water content management for battery constituent materials, and provides a lithium secondary battery with low internal resistance at lower cost. It becomes possible to do.

Claims (3)

[Claims] 1. A positive electrode and a negative electrode each having a substance capable of occluding / desorbing lithium as an active material, an electrolyte capable of generating a halogen acid by reacting with water, and an organosilicon compound having a Si—N bond. A non-aqueous electrolyte solution dissolved in an organic solvent, wherein the lithium secondary battery comprises the non-aqueous electrolyte after charging the lithium secondary battery at least once. A lithium secondary battery, which is added in an electrolytic solution. 2. The lithium secondary battery according to claim 1, wherein the organosilicon compound is added to the lithium secondary battery after repeating charging and discharging at least 5 times. 3. A positive electrode and a negative electrode each having a substance capable of occluding / desorbing lithium as an active material, an electrolyte capable of generating a halogen acid by reacting with water, and an organosilicon compound having a Si—N bond. A method for producing a lithium secondary battery comprising a non-aqueous electrolyte solution dissolved in an organic solvent, wherein the organosilicon compound is obtained by charging the lithium secondary battery at least once. A method for manufacturing a lithium secondary battery, which is added in the non-aqueous electrolyte.