JP2002324577A - Lithium secondary battery and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery capable of suppressing reaction between a negative electrode and an electrolyte without degrading characteristics as a battery, and reducing the generated gas. SOLUTION: There are provided a positive electrode and a negative electrode capable of occlusion/release of lithium, and an electrolyte. Polyethylene glycol dimetacrylate (denoted as PEGDMA) or polyethylene glycol diacrylate (denoted as PEGDA) is added by 0.5-10 wt.% to the electrolyte. Acrylonitrile (denoted as AN) is added by 0.1-2 wt.%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, with the spread of portable electronic devices such as mobile phones, camcorders, and notebook personal computers, batteries with high energy density have been demanded, and demand for lithium secondary batteries has been increasing. In particular, in a lithium secondary battery including an electrolyte such as an organic electrolyte solution or a polymer electrolyte, it is important to suppress the reaction between the negative electrode and the electrolyte in order to exhibit high battery performance. In particular, a negative electrode that has a low potential when charged easily decomposes the electrolyte, and greatly affects battery performance, particularly battery capacity, battery storage characteristics, cycle characteristics, low-temperature characteristics, and the like.

[0003] Therefore, as the electrolyte of the lithium secondary battery, selection is made in consideration of the reactivity with the negative electrode in particular, and a number of solvents or combinations thereof that do not deteriorate the battery performance due to the reaction with the negative electrode have been studied. Furthermore, the selection of the solvent depends on the solubility of the supporting salt of the electrolyte, the reactivity with the positive electrode, the ion conductivity,
Cost and the like are taken into account. Specifically, examples of the non-aqueous solvent for the lithium secondary battery include organic solvents such as ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, methyl propionate, butyl propionate, and ethyl propionate. A solvent is used alone or in combination of two or more. Further, many attempts have been made to improve the battery performance by suppressing the reaction between the negative electrode and the electrolyte by adding a specific compound to the electrolyte as an additive.

[0004]

However, in some cases, the above-mentioned additives adversely affect the charge / discharge reaction and do not provide the original voltage or current of the battery. For example, JP-A-8-96852 discloses that
In a battery using a negative electrode having a material capable of doping and undoping lithium metal or lithium, a battery containing vinylene carbonate in a nonaqueous solvent is disclosed, but a carbonaceous material is used as the negative electrode. When this vinylene carbonate was used for a battery, the film-forming ability was not sufficient, and a sufficient improvement in battery characteristics could not be expected. Furthermore,
When vinylene carbonate is added, the amount of gas generated at the time of initial charging increases, and depending on the shape of the battery, the battery may be deformed due to an increase in the internal pressure of the battery. It is believed that this gas generation is caused by the simultaneous decomposition of the electrolyte during the formation of the film for the first charge, and this gas generation causes the deterioration of the electrolyte, which is one of the causes of the deterioration of the battery characteristics. Could have become.

The present invention has been made in view of the above circumstances, and it is possible to suppress the reaction between a negative electrode and an electrolyte without deteriorating the characteristics as a battery, and to further reduce gas generation. The purpose is to provide.

[0006]

In order to achieve the above object, the present invention employs the following constitution. The lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, wherein the electrolyte contains polyethylene glycol dimethacrylate (hereinafter referred to as PEGDMA) or polyethylene glycol diacrylate ( (Hereinafter referred to as PEGDA) in an amount of 0.5 to 10% by weight, and acrylonitrile (hereinafter referred to as AN) in an amount of 0.1 to 2% by weight.

According to the lithium secondary battery, PEGDMA or PEGDA is polymerized to form an electrolyte at the initial stage of the first charge, and PEGDMA or PEGDA and AN are polymerized to form an organic film on the surface of the negative electrode. Then, even when the charging voltage increases due to the progress of charging, the decomposition reaction of the electrolyte on the negative electrode surface is suppressed by the organic coating, so that gas generation due to the decomposition of the electrolyte and deterioration of the electrolyte itself are reduced, and lithium It is possible to prevent a decrease in the charge / discharge capacity of the secondary battery, improve the cycle characteristics, and prevent the battery from being deformed. Further, the high temperature storage characteristics of the lithium secondary battery are improved by the effect of suppressing the decomposition of the electrolyte by the organic coating. PEGDMA or PEGDA and A
When the amount of N added is in the above range, it is possible to form an organic film sufficient to suppress the decomposition of the electrolyte.

Further, the lithium secondary battery of the present invention is the lithium secondary battery described above, wherein acetonitrile (hereinafter referred to as ACN) is added to the electrolyte in a range of 0.1 to 5% by weight. It is characterized by having.

According to such a lithium secondary battery, by adding ACN to the electrolyte, ACN is formed during the formation of the organic film.
Is taken into the organic coating, thereby improving the lithium ion conductivity of the organic coating, so that the low-temperature characteristics can be further improved.

Next, the lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, wherein the electrolyte is made of polyethylene glycol dimethacrylate or polyethylene glycol diacrylate. And a polymer film made of polyethylene glycol dimethacrylate or polyethylene glycol diacrylate and acrylonitrile is formed on the surface of the negative electrode.

According to the lithium secondary battery, an organic film made of PEGDMA or PEGDA and AN is formed on the surface of the negative electrode, and the decomposition reaction of the electrolyte on the surface of the negative electrode is suppressed by the organic film. Gas generation due to the decomposition of the electrolyte and deterioration of the electrolyte itself are reduced, the reduction of the charge / discharge capacity of the lithium secondary battery can be prevented, the cycle characteristics can be improved, and the battery can be prevented from being deformed. Further, the high temperature storage characteristics of the lithium secondary battery are improved by the effect of suppressing the decomposition of the electrolyte by the organic coating.

Further, a lithium secondary battery according to the present invention is the above-described lithium secondary battery, characterized in that at least acetonitrile is contained in the organic coating.

According to such a lithium secondary battery, since ACN is contained in the organic coating, the lithium ion conductivity of the organic coating is improved, the charge / discharge efficiency of the lithium secondary battery is increased, and the low-temperature characteristics are further improved. Can be improved.

Next, a method for manufacturing a lithium secondary battery according to the present invention is a method for manufacturing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte. In a state where polyethylene glycol dimethacrylate is added in a range of 0.5 to 10% by weight and acrylonitrile is added in a range of 0.1 to 2% by weight, the electrolyte is disposed at least between the positive electrode and the negative electrode. Performing a heat treatment at a temperature in the range of 40 to 120 ° C., and performing constant current charging until the potential of the negative electrode when metal lithium is used as the reference electrode reaches a range of 0.8 V or more and 1.3 V or less. A first charging step of performing constant voltage charging for 0.1 to 8 hours while maintaining the potential of the negative electrode. After the first charging step, constant current charging is performed until the potential of the negative electrode reaches the range of 0 V or more and 0.1 V or less. It is preferable to perform a second charging step of performing voltage charging.

According to the method for manufacturing a lithium secondary battery, PEGDMA or PEGDA is thermally polymerized by heat treatment to form an electrolyte.
And AN are adsorbed on the surface of the negative electrode, and then the PEGDMA or PEGDA and AN adsorbed in the first charging step are polymerized to form an organic film, so that the organic material formed on the surface of the negative electrode before the previously formed electrolyte decomposes. A coating can be formed.
In addition, since the constant voltage charging in the first charging step is performed for a relatively long time, the polymerization reaction of PEGDMA or PEGDA and AN is sufficiently performed, the reaction yield of the organic coating is increased, and the sufficient organic coating is obtained. Is formed. Further, the formation of the organic coating makes it possible to suppress the decomposition of the electrolyte in the second charging step, thereby preventing gas generation and deterioration of the electrolyte. Further, by performing the first charging step, a part of the electrolyte is absorbed by the organic coating, so that the affinity between the organic coating and the electrolyte is improved, and the charge / discharge efficiency can be improved. If the amount of PEGDMA or PEGDA and AN is within the above range, it is possible to form an organic film sufficient to suppress the decomposition of the electrolyte.

The method for manufacturing a lithium secondary battery according to the present invention is a method for manufacturing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte. The substance is at least one of a composite oxide of lithium and at least one selected from cobalt, manganese, and nickel, and the electrolyte contains 0.5 to 10% by weight of polyethylene glycol dimethacrylate or polyethylene glycol diacrylate. A step of performing a heat treatment at a temperature of 40 to 120 ° C. by disposing the electrolyte at least between the positive electrode and the negative electrode while adding acrylonitrile in a range of 0.1 to 2% by weight while adding acrylonitrile in a range of 0.1 to 2% by weight; After performing constant current charging until the battery voltage reaches a range of 2.5 V or more and 3.1 V or less, the battery voltage is maintained. In which it characterized in that it consists of a first charging step of performing constant-voltage charging at 0.1 to 8 hours. Further, after the first charging step, the battery voltage is 4.0 V or more and 4.3 or more.
After performing constant current charging until the voltage reaches the range of V or less,
It is preferable to perform the second charging step of performing constant voltage charging for 1 to 8 hours while maintaining the battery voltage.

Further, a method of manufacturing a lithium secondary battery according to the present invention is the method of manufacturing a lithium secondary battery described above,
Acetonitrile is added to the electrolyte in a range of 0.1 to 5% by weight.

According to the method of manufacturing a lithium secondary battery, ACN can be incorporated into the organic film at the time of forming the organic film by adding ACN to the electrolyte, and thereby the lithium ion conductivity of the organic film can be improved. Therefore, the low temperature characteristics of the lithium secondary battery can be improved.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. The lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, and a polyethylene glycol dilute in the electrolyte in a state where an organic film is not formed on the surface of the negative electrode. It contains methacrylate (hereinafter referred to as PEGDMA) or polyethylene glycol diacrylate (hereinafter referred to as PEGDA) and acrylonitrile (hereinafter referred to as AN). Acetonitrile (hereinafter ACN) is contained in the electrolyte.
May be included).

The lithium secondary battery of the present invention comprises a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, wherein the electrolyte is made of polyethylene glycol dimethacrylate or polyethylene glycol diacrylate. The organic electrolyte is impregnated in the coalescence,
An organic film composed of PEGDMA or PEGDA and AN is formed on the surface of the negative electrode. Further, at least ACN may be contained in the organic coating.

PEGDMA is a so-called bifunctional acrylate derivative having a structure represented by the following formula (1) and having two carbon-carbon double bonds in the molecule. this
PEGDMA is an anion addition polymerizable monomer that performs anionic polymerization, and undergoes radical polymerization to form a polymer when heated. In addition, an organic film is formed on the surface of the negative electrode that exhibits a low potential during charging. When this PEGDMA undergoes anionic polymerization, two double bonds in the molecule are cleaved, and a reaction in which each of the double bonds is bonded to another PEGDMA occurs in a chain, and a film formed by polymerization of PEGDMA is formed on the negative electrode surface. PEGDA has a structure represented by the following formula (2), is a bifunctional acrylate derivative similarly to PEGDMA, and is an anion-addition polymerizable monomer that performs anionic polymerization. To form Further, an organic film is formed on the surface of the negative electrode which exhibits a low potential during charging.

[0022]

Embedded image

[0023]

Embedded image

Further, PEGDMA or PEGDA forms an organic film according to the present invention together with AN in a state where AN coexists. The mechanism of this film formation is the same as in the case where PEGDMA or PEGDA is used alone, and anion polymerization is carried out on the negative electrode surface showing a negative potential during charging to form the organic film according to the present invention. Although the detailed structure of this organic coating is unknown, it is likely to be a copolymer of PEGDMA or PEGDA with AN. The organic coating composed of PEGDMA or PEGDA and AN has a high lithium ion conductivity and is a strong coating that does not undergo electrolysis even when a voltage of 4.2 V or more is applied.

Further, the organic coating according to the present invention may comprise PEGD
ACN may be included in addition to MA or PEGDA and AN. The organic coating containing ACN improves the ionic conductivity of lithium as compared with the case where ACN is not included, reduces the internal impedance of the battery, and improves the charging / discharging efficiency. ACN is P
Either reacting with EGDMA or PEGDA and AN and present in the organic coating, or present in the organic coating dissolved in a copolymer consisting of PEGDMA or PEGDA and AN alone, or both It is considered to be in a state. The unreacted PEGDMA or PEGDA and AN contained in the electrolyte with the formation of the coating in the first charging step.
Is significantly reduced. Therefore, the residual monomer does not deteriorate the battery characteristics.

The thickness of the organic film is about several to several tens of nm, and is an extremely thin film. When the film thickness is on the order of several μm, it is difficult to allow lithium ions to pass therethrough, and the charge / discharge reaction cannot be performed smoothly. Further, when the thickness is, for example, about 1 nm or less, it is difficult to maintain the shape of the film, which is not preferable.

Since the organic coating is formed on the surface of the negative electrode, it functions to prevent direct contact between the negative electrode and the electrolyte. As a result, the reductive decomposition reaction of the electrolyte on the negative electrode surface is suppressed, the generation of gas due to the decomposition of the electrolyte is reduced, and the deterioration of the electrolyte itself is prevented. Due to this reduction in gas generation, the internal pressure of the battery does not increase, and the battery does not deform. Further, by preventing the deterioration of the electrolyte, the charge / discharge reaction proceeds smoothly without reducing the electrolytic mass, the charge / discharge efficiency is increased, and the cycle characteristics are improved. Furthermore, since the reaction between the electrolyte and the negative electrode is suppressed, even when the battery is stored at a high temperature for a long period of time, the electrolyte does not deteriorate, and the battery characteristics such as charge / discharge efficiency and cycle characteristics do not deteriorate. .

Further, since the above-mentioned organic coating has excellent lithium ion conductivity, it also has a function of transporting lithium ions between the electrolyte and the negative electrode. Therefore, even if the negative electrode surface is covered with the organic coating, there is no hindrance to the transport of lithium ions, the charge / discharge reaction proceeds smoothly, the charge / discharge efficiency is increased, and the cycle characteristics are improved. Also, the internal impedance of the battery does not increase, and the charge / discharge capacity does not significantly decrease.

The above-mentioned electrolyte is a polymer electrolyte obtained by impregnating a polymer made of PEGDMA or PEGDA with an organic electrolyte. Examples of the organic electrolyte include an organic electrolyte obtained by dissolving a lithium salt in an aprotic solvent.
As aprotic solvents, propylene carbonate,
Ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-
Methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethylcarbonate, methylethyl carbonate Aprotic solvents such as diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or a mixture of two or more of these solvents Examples of solvents and those already known as solvents for lithium secondary batteries Can, in particular propylene carbonate, ethylene carbonate, dimethyl carbonate with including any one of butylene carbonate, methyl ethyl carbonate, those comprising any one of the diethyl carbonate preferred.

As the lithium salt, LiPF ₆ ,
_{LiBF 4, LiSbF 6, LiAsF 6} , LiClO 4,
LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉
SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , Li
One or more lithium salts of N (C _x F _{2x + 1} SO ₂ ) (C _y F _{2 y10 1} SO ₂ ) (where x and y are natural numbers), LiCl, LiI, etc. are mixed. And those conventionally known as lithium salts for lithium secondary batteries can be exemplified. Particularly, those containing any one of LiPF ₆ and LiBF ₄ are preferable.

Further, as another example of the polymer electrolyte, the above-mentioned organic electrolyte and PEO, PPO, PAN, PVDF, PMA, PMM having a high swelling property with respect to the above-mentioned organic electrolyte.
A polymer such as A or a polymer electrolyte obtained by mixing the polymer can be exemplified.

It is preferable that PEGDMA or PEGDA is added to the above electrolyte in a range of 0.5 to 10% by weight before the formation of the organic film. PEGDMA or PEGD
If the addition amount of A is less than 0.5% by weight, the organic film is not sufficiently formed, which is not preferable. If the addition amount exceeds 10% by weight, the thickness of the organic film increases and the internal impedance increases. Is not preferred. Also AN
Is preferably added to the above electrolyte in a range of 0.1 to 2% by weight before the formation of the organic film. If the amount of AN is less than 0.1% by weight, an organic film is not sufficiently formed, which is not preferable.
Exceeding the weight percent is not preferred because the thickness of the organic coating increases and the internal impedance increases. Further
ACN is preferably added to the above electrolyte in a range of 0.1 to 5% by weight before the formation of the organic film. If the addition amount of ACN is less than 0.1% by weight, it is not preferable because the ionic conductivity of lithium in the organic coating cannot be sufficiently increased, and if the addition amount exceeds 5% by weight, the electrolyte solution at high temperature Is unfavorable because the vapor pressure becomes high.

Next, the negative electrode is formed into a sheet-shaped, flat circular shape by mixing a binder such as polyvinylidene fluoride and a conductive auxiliary material such as carbon black with a negative electrode active material powder capable of absorbing and releasing lithium. One molded into a plate or the like can be exemplified. Examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, graphitized mesocarbon microbeads, and amorphous carbon. Further, a simple substance of a metal which can be alloyed with lithium or a composite containing this metal and a carbonaceous material can also be exemplified as the negative electrode active material. Metals that can be alloyed with lithium include Al, Si, Sn, P
Examples thereof include b, Zn, Bi, In, Mg, Ga, and Cd. Further, a metal lithium foil can be used as the negative electrode.

Examples of the form in which the organic film is formed on the surface of the negative electrode include, for example, a state in which the organic film is formed on the surface of the negative electrode active material and a state in which the organic film is formed on the surface of the metallic lithium foil. Conceivable.

Next, the positive electrode was prepared by adding polyfluoride to the positive electrode active material powder.
Binder such as vinylidene and conductive assistant such as carbon black
Example of mixing into a sheet, flat disk, etc. by mixing
Can be shown. As the positive electrode active material, cobalt, man
At least one selected from gun and nickel and lithium
And any one or more of the complex oxides with
Specifically, LiMn_TwoO_Four, LiCoO_Two, LiNi
O_Two, LiFeO_Two, V_TwoO _FiveIs preferred. Also, Ti
S, MoS, organic disulfide compound or organic police
Those capable of absorbing and releasing lithium such as sulfide compounds
May be used.

Next, a method for manufacturing the lithium secondary battery of the present invention will be described. The method for manufacturing a lithium secondary battery according to the present invention includes a step of placing an electrolyte to which PEGDMA or PEGDA and AN is added between a positive electrode and a negative electrode and performing a heat treatment, a first charging step, and a second charging step.

First, an electrolyte is prepared by adding PEGDMA or PEGDA and AN. This electrolyte is prepared by adding PEGDMA or PEGDA and AN to the above-mentioned organic electrolyte as described above. ACN may be added together with PEGDMA or PEGDA and AN. The addition amount of PEGDMA or PEGDA is preferably in the range of 0.5 to 10% by weight, and more preferably in the range of 2 to 5% by weight. The amount of AN added is 0.1 to
A range of 2% by weight is preferable, and a range of 0.2 to 0.5% by weight is more preferable. Further, the addition amount of ACN is preferably in the range of 0.1 to 5% by weight, and more preferably in the range of 0.2 to 1% by weight.

Next, this electrolyte is disposed between the positive electrode and the negative electrode. When the electrolyte is an organic electrolyte, these may be impregnated with an organic electrolyte while a separator is interposed between the positive electrode and the negative electrode. When the electrolyte is a polymer electrolyte, the polymer electrolyte may be sandwiched between the positive electrode and the negative electrode,
Further, the positive and negative electrodes may be impregnated with the organic electrolyte separately from the polymer electrolyte.

Next, PEGDMA or PEGDA and AN and ACN
In the state where the electrolyte containing
Heat treatment is performed in a temperature range of 20 ° C. By this heat treatment,
PEGDMA in the electrolyte radically polymerizes to form a polymer,
The polymer is impregnated with an organic electrolyte to form an electrolyte. If the heating temperature is lower than 40 ° C., PEGDMA or
It is not preferable because radical polymerization of PEGDA does not sufficiently proceed. On the other hand, if the heating temperature exceeds 120 ° C., it is not preferable because the electrolyte is altered and the battery characteristics are deteriorated.

Next, in the first charging step, the potential of the negative electrode when metal lithium is used as the reference electrode is 0.8 V or more and 1.3 V or more.
After performing constant current charging until the voltage reaches the range of V or less,
The constant voltage charging is performed for 0.1 to 8 hours while the voltage of the negative electrode is maintained. The current at the time of constant current charging is 0.01-0.3C
The degree is preferred. In the first charging step, PEGDMA or PEGDA and AN are anionically polymerized to form an organic film on the surface of the negative electrode before reductive decomposition of the electrolyte occurs. That is, PEGDMA or PEGDA and AN perform anion addition polymerization when the potential of the negative electrode in the case where metal lithium is used as the reference electrode is in the range of 0.8 to 1.3 V, and the potential is 0.8
Above V, the electrolyte does not undergo reductive decomposition, so the lower limit of the charging voltage must be limited to 0.8V. In addition, since the progress of the anion polymerization on the surface of the negative electrode is relatively slow, constant voltage charging is required for 1 to 8 hours while maintaining the above charging voltage in order to sufficiently advance the polymerization reaction. If the potential of the negative electrode is less than 0.8 V, it is not preferable because a reductive decomposition reaction of the electrolyte occurs simultaneously.

On the other hand, if the potential of the negative electrode exceeds 1.3 V during constant current charging, the polymerization reaction of PEGDMA or PEGDA and AN does not start, which is not preferable. Next, if the charging time in constant voltage charging is less than 0.1 hour, PEGDMA or PE
Since the polymerization reaction of GDA and AN does not proceed sufficiently, there is a possibility that a defect may occur in the organic coating, which is not preferable. There is no real benefit of charging in a time.

In the first charging step, the positive electrode is Li
When any one of CoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ was used, the battery voltage was maintained after constant-current charging was performed until the battery voltage reached a range of 2.5 V or more and 3.1 V or less. It is preferable to perform constant voltage charging for 0.1 to 8 hours as it is.

When ACN is added to the organic electrolyte together with PEGDMA or PEGDA and AN, an organic film containing ACN together with PEGDMA or PEGDA and AN is formed. When ACN is contained, the lithium ion conductivity of the organic coating is improved, the internal impedance of the battery is reduced, and the charge / discharge efficiency is improved. ACN reacts with PEGDMA or PEGDA and AN and is present in the organic coating, or is present in the organic coating dissolved in a copolymer consisting of PEGDMA or PEGDA and AN alone. Or it is considered to be in both states. Incidentally, the concentration of PEGDMA or PEGDA, AN and ACN contained in the electrolyte is significantly reduced with the formation of the film.

Next, in the second charging step, the potential of the negative electrode when metal lithium is used as the reference electrode is 0.0V or more and 0.1V or less.
After performing constant current charging until the voltage reaches the range of V or less,
Constant voltage charging is performed for 1 to 8 hours while maintaining the negative electrode potential at 0.0 V or more and 0.1 V or less. The current during constant current charging is preferably about 0.1 to 0.5C. In the second charging step, since the organic film has already been formed, the electrolyte does not come into direct contact with the negative electrode, and the reductive decomposition of the electrolyte is suppressed. If the potential of the negative electrode exceeds 0.1 V during constant current charging, the battery capacity becomes insufficient, which is not preferable. If it is less than 0.0 V, the crystal structure of the positive electrode may be broken, which is not preferable. Further, if the charging time in the constant voltage charging is less than 1 hour, the charging becomes insufficient, which is not preferable. If the charging time exceeds 8 hours, the positive electrode deteriorates due to overcharging, which is not preferable.

In the second charging step, the positive electrode is Li
When any one of CoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ was used, constant current charging was performed until the battery voltage reached a range of 4.0 V to 4.3 V, and then the battery voltage was maintained. It is preferable to perform constant voltage charging for 1 to 8 hours as it is. Also, between the first charging step and the second charging step,
Providing a rest time of about 1 to 8 hours is preferable in that the polymerization reaction sufficiently proceeds when the first charging time is not sufficiently long.

According to the method for producing a lithium secondary battery described above, PEGDMA or PEGDA is radically polymerized by heat treatment to form a polymer, and the polymer is impregnated with an organic electrolyte to form an electrolyte. In addition, PEGDMA or PEGDA and AN are adsorbed on the surface of the negative electrode, and then the PEGDMA, PEGDA and AN adsorbed in the first charging step are polymerized to form an organic film. Therefore, the surface of the negative electrode is formed before the generated electrolyte is decomposed. An organic coating can be formed thereon. Further, since the constant voltage charging in the first charging step is performed for a relatively long time, PEGDMA or PEGDA and A
The polymerization reaction of N is sufficiently performed, the reaction yield of the organic film is increased, and a sufficient organic film can be formed. Further, the formation of the organic coating makes it possible to suppress the decomposition of the electrolyte in the second charging step, thereby preventing gas generation and deterioration of the electrolyte.

Further, by adding ACN to the electrolyte, ACN can be taken into the organic film at the time of forming the organic film, thereby improving the lithium ion conductivity of the organic film and increasing the charge / discharge efficiency. Thus, the cycle characteristics can be further improved.

[0048]

EXAMPLES [Production of lithium secondary batteries of Examples 1 to 4]
First, 4.95% by weight of PEGDMA having an average molecular weight of 550, AN
Were mixed at a ratio of 0.5% by weight, 0.05% by weight of a polymerization initiator AIBN, and 94.5% by weight of an organic electrolyte,
The mixture was mixed for minutes to prepare an electrolyte precursor. The composition of the organic electrolytic solution is such that 1 mol / L of Li is added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DEC) in a volume ratio of 3: 7.
Using a mixture of the PF _6. Next, a positive electrode using LiCoO ₂ as a positive electrode active material and a negative electrode using carbon fiber as a negative electrode active material were inserted into a battery container, and after the electrolyte was injected, the battery container was sealed, and the width was 30 mm, the height was 60 mm, Thickness 4
mm square batteries were manufactured.

The obtained prismatic battery was subjected to a heat treatment at 70 ° C. for 5 hours to radically polymerize PEGDMA to form an electrolyte comprising a PEGDMA polymer and an organic electrolyte. Unreacted PEGDMA and AN are polymerized in the first charging step in which constant current charging is performed until the battery voltage reaches 3 V (the potential of the negative electrode with respect to lithium metal is 0.8 V) and then constant voltage charging is performed for 4 hours. Thus, an organic film was formed. Next, a second charging step of performing constant-current charging at a current of 0.2 C until the battery voltage reaches 4.2 V (the potential of the negative electrode with respect to metallic lithium is 0.1 V) and then performing constant-voltage charging for 9 hours is performed. Thereby, the lithium secondary batteries of Examples 1 and 2 were manufactured. Note that the battery of Example 2 was subjected to a process of releasing the internal gas of the battery after the completion of the second charging step.

Further, 0.2% by weight of AN and 9% of organic electrolyte were used.
A battery of Example 3 was manufactured in the same manner as described above except that the amount was changed to 4.8% by weight. Furthermore, 0.2% by weight of AN, 1% by weight of ACN, 0.05% by weight of polymerization initiator AIBN, and 93.8% by weight of an organic electrolyte were mixed, and mixed for 30 minutes to prepare an electrolyte precursor. Was prepared in the same manner as described above, except that was prepared. In addition, it was confirmed that there was almost no change in ionic conductivity when 5% by weight or less of ACN was added to the organic electrolytic solution.

[Production of lithium secondary batteries of Comparative Examples 1 to 3] 4.95% by weight of PEGDMA having an average molecular weight of 550, 0.05% by weight of a polymerization initiator AIBN, and 95% by weight of an organic electrolyte were used. An electrolyte precursor was prepared in the same manner as in Example 1 except for mixing and mixing for 30 minutes.
A prismatic battery was manufactured in the same manner as described above. The obtained prismatic battery is subjected to a heat treatment at 70 ° C. for 5 hours to radically polymerize PEGDMA to form an electrolyte composed of a PEGDMA polymer and an organic electrolyte. By performing constant-current charging until the voltage reaches 4.2 V (the potential of the negative electrode with respect to metallic lithium is 0.1 V) and then performing 4.2-hour constant-voltage charging for 9 hours, lithium secondary batteries of Comparative Examples 1 and 2 were obtained. A battery was manufactured. The battery of Comparative Example 1 was subjected to a process of releasing the internal gas of the battery after charging was completed.

A prismatic battery was manufactured in the same manner as in Comparative Examples 1 and 2, and a battery voltage of 3 V (current of the negative electrode with respect to metallic lithium of 0.8
V), and then heat-treated at 75 ° C. for 4 hours. Further, at a current of 0.2 C, the battery voltage becomes 4.2 V (when the potential of the negative electrode with respect to lithium metal is 0.1 V).
The lithium secondary battery of Comparative Example 3 was manufactured by performing a constant current charge until the voltage reached 1 V) and then performing a constant voltage charge for 9 hours.

[Production of Lithium Secondary Battery of Comparative Example 4] First, a prismatic battery was produced in the same manner as in Examples 1 to 3, except that PEGDMA, AN and AIBN were not added. This prismatic battery is charged at a constant current of 0.2 C until the battery voltage reaches 3 V (the potential of the negative electrode with respect to lithium metal is 0.8 V), and then heat-treated at 75 ° C. for 4 hours. Further, a constant current was performed at a current of 0.2 C until the battery voltage reached 4.2 V (the potential of the negative electrode with respect to lithium metal was 0.1 V), and then a constant voltage charge was performed for 9 hours. A lithium secondary battery was manufactured.

Regarding the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 4, the thickness and internal impedance of the batteries immediately after production, the discharge capacity before and after storage at 85 ° C. for 24 hours, the remaining capacity and the recovery capacity were as follows. investigated. Table 1 shows the results. 1 and 2 show Examples 1 and 2 and Comparative Example 1.
4 shows the Coulomb efficiency with respect to the charging voltage in the first and second charging steps after the heat treatment. Further, Example 1
And 2 and Comparative Examples 1 and 2, the charge / discharge current 1C,
A cycle characteristic test was performed under the conditions of a charge end voltage of 4.2 V and a discharge end voltage of 2.5 V. The results are shown in FIG. Further, the discharge capacity at −20 ° C. of Examples 3 and 4 and Comparative Example 1 was measured. Table 2 shows the results.

[0055]

[Table 1]

[0056]

[Table 2]

As shown in FIG. 1, in the first and second embodiments,
In the first charging step, a peak corresponding to the polymerization reaction of PEGDMA and AN is observed at a charging voltage of around 2.9 to 3.0 V. Then, in the second charging step, it is considered that the Coulomb efficiency is gradually increased with the improvement of the charging voltage, and the decomposition of the electrolyte is suppressed. On the other hand, as shown in FIG. 1 and FIG.
A large peak was observed in the range of 5 V, which indicates that the decomposition of the electrolyte had occurred. Therefore, Examples 1 and 2
Thus, it is considered that an organic film is formed on the negative electrode surface by the first charging step, and the decomposition of the electrolyte is suppressed by the presence of the organic film.

The above is supported by a comparison of battery thickness. That is, as shown in Table 1, when comparing the thickness of each battery, the thickness of Example 1 without internal gas removal was as follows:
The thickness is smaller than the thickness of Comparative Example 2 from which the internal gas has not been removed, and is substantially the same as the thickness of Comparative Example 1 from which the internal gas has been removed. From these comparisons, it is considered that in Example 1, the organic film was formed by the addition of PEGDMA and AN, and the decomposition of the electrolyte was suppressed, so that the generation of internal gas was significantly reduced. Further, in Comparative Example 3, although PEGDMA and AN were added, the thickness of the battery was found to be large. This is presumably because the heat treatment was performed during the charging, the organic film was not sufficiently formed, and the electrolyte was decomposed to generate a large amount of gas.

Next, comparing the internal impedance,
It can be seen that there is no significant difference in the internal impedance between Examples 1 and 2 and Comparative Examples 1 and 2, and no increase in the internal impedance due to the organic coating is observed.

Next, when the remaining capacity after storage at 85 ° C. for 24 hours was compared, the remaining capacity of Examples 1 and 2 was
Is higher. Also, the recovery capacities of Examples 1 and 2 are higher than Comparative Examples 1 to 3. Therefore, PE
It can be seen that by adding GDMA and AN to form an organic film, contact between the negative electrode and the electrolyte is prevented, reductive decomposition of the electrolyte is suppressed, and high-temperature storage characteristics are improved.

Next, as shown in FIG. 3, the cycle characteristics of Examples 1 and 2 and Comparative Examples 1 and 2
2, the difference between Examples 1 and 2 and Comparative Examples 1 and 2 gradually increased from around 50 times, and the difference between Examples 1 and 2 near 200 times. Are Comparative Examples 1 and 2.
The discharge capacity is larger than that. This is considered to be because in the case of Examples 1 and 2, the decomposition of the electrolyte was suppressed by the presence of the organic coating, and the charge and discharge efficiency was increased without causing the deterioration of the electrolyte. On the other hand, in Comparative Examples 1 and 2, it is considered that the reason is that the electrolyte was gradually deteriorated with an increase in the number of cycles because the anode was in direct contact with the electrolyte, and the charge / discharge efficiency was lowered.

Next, from Table 2, it can be seen that in Examples 3 and 4, the discharge capacity at −20 ° C. was improved as compared with Comparative Example 1, and the discharge capacity at −20 ° C. was particularly large in Example 4 to which ACN was added. It can be seen that the number has increased. This shows that the low-temperature characteristics can be improved by adding ACN.

[0063]

As described above in detail, according to the lithium secondary battery of the present invention, PEGDMA, PEGDA and AN are polymerized at the initial stage of the initial charge, and an organic film is formed on the negative electrode surface at an early stage. Therefore, even when the charging voltage increases due to the progress of charging, the decomposition reaction of the electrolyte on the negative electrode surface is suppressed by the organic coating, so that gas generation due to decomposition of the electrolyte and deterioration of the electrolyte itself are reduced. In addition, cycle characteristics can be improved, and deformation of the battery can be prevented. Further, the high temperature storage characteristics of the lithium secondary battery can be improved by the effect of suppressing the decomposition of the electrolyte by the organic coating.

Further, according to the lithium secondary battery of the present invention, by adding ACN to the electrolyte, ACN is taken into the organic film when the organic film is formed, thereby improving the lithium ion conductivity of the organic film. Therefore, the charge / discharge efficiency of the lithium secondary battery is increased, and the cycle characteristics can be further improved.

Further, according to the method for producing a lithium secondary battery of the present invention, a heat treatment is performed to form a more electrolyte of PEGDMA or PEGDA and an organic electrolyte, and PEGDMA or PEGDA and AN are adsorbed on the negative electrode surface. Then, the organic film is formed by polymerizing PEGDMA or PEGDA and AN adsorbed in the first charging step, so that the organic film can be formed on the surface of the negative electrode before the previously formed electrolyte is decomposed. In addition, since the constant voltage charging in the first charging step is performed for a relatively long time, the polymerization reaction of PEGDMA and AN is sufficiently performed, the reaction yield of the organic coating increases, and a sufficient organic coating is formed. Is done. Further, the formation of the organic coating makes it possible to suppress the decomposition of the electrolyte in the second charging step, thereby preventing gas generation and deterioration of the electrolyte. Further, by performing the second charging step, a part of the electrolyte is adsorbed to the organic coating, so that the affinity between the organic coating and the electrolyte is improved, and the charge / discharge efficiency can be improved.

Further, according to the method of manufacturing a lithium secondary battery of the present invention, by adding ACN to the electrolyte, ACN can be incorporated into the organic film at the time of forming the organic film. Since the ion conductivity is improved, the charge and discharge efficiency of the lithium secondary battery is increased, and the cycle characteristics can be further improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing Coulomb efficiencies with respect to charging voltage in Examples 1 and 2 and Comparative Examples 1 and 2.

FIG. 2 is a diagram illustrating Coulomb efficiency with respect to charging voltage in Comparative Examples 3 and 4.

FIG. 3 is a diagram showing the relationship between the number of cycles and discharge capacity in Examples 1 and 2 and Comparative Examples 1 and 2.

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) GA26 GA27 HA01 HA14 HA18 HA20

Claims (9)

[Claims] 1. An electrolyte comprising: a positive electrode and a negative electrode capable of inserting and extracting lithium; and an electrolyte, wherein the electrolyte contains polyethylene glycol dimethacrylate or polyethylene glycol diacrylate.
A lithium secondary battery characterized in that it is added in a range of 5 to 10% by weight and acrylonitrile is added in a range of 0.1 to 2% by weight. 2. An acetonitrile containing 0.1% of acetonitrile in the electrolyte.
2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery is added in an amount of about 5% by weight. 3. 3. An electrolyte comprising: a positive electrode and a negative electrode capable of inserting and extracting lithium; and an electrolyte, wherein the electrolyte is obtained by impregnating a polymer made of polyethylene glycol dimethacrylate or polyethylene glycol diacrylate with an organic electrolyte. A lithium secondary battery, wherein an organic coating comprising polyethylene glycol dimethacrylate or polyethylene glycol diacrylate and acrylonitrile is formed on the surface of the negative electrode. 4. The lithium secondary battery according to claim 3, wherein the organic coating contains at least acetonitrile. 5. A method for producing a lithium secondary battery comprising: a positive electrode and a negative electrode capable of inserting and extracting lithium; and an electrolyte, wherein the electrolyte comprises polyethylene glycol dimethacrylate or polyethylene glycol diacrylate. 5
The electrolyte is placed at least between the positive electrode and the negative electrode in a state where acrylonitrile is added in the range of 0.1 to 2% by weight and acrylonitrile is added in the range of 0.1 to 2% by weight.
Performing a heat treatment at a temperature in the range of 0 to 120 ° C .;
After performing constant current charging until the voltage reaches a range of 0.8 V or more and 1.3 V or less, 0.1 is charged while the potential of the negative electrode is maintained.
And a first charging step of performing constant-voltage charging for up to 8 hours. 6. After the first charging step, after performing a constant current charging until the potential of the negative electrode reaches a range of 0 V or more and 0.1 V or less, 1 to 1 while maintaining the potential of the negative electrode.
The method for manufacturing a lithium secondary battery according to claim 5, wherein a second charging step of performing constant-voltage charging for 8 hours is performed. 7. A method for manufacturing a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and an electrolyte, wherein the active material of the positive electrode is selected from cobalt, manganese, and nickel. At least one kind of complex oxide of lithium and at least one kind thereof, wherein the electrolyte contains polyethylene glycol dimethacrylate or polyethylene glycol diacrylate in an amount of 0.5
The electrolyte is placed at least between the positive electrode and the negative electrode in a state where acrylonitrile is added in the range of 0.1 to 2% by weight and acrylonitrile is added in the range of 0.1 to 2% by weight.
A step of performing a heat treatment in the range of 0 to 120 ° C., and performing a constant current charge until the battery voltage reaches a range of 2.5 V or more and 3.1 V or less, and then 0.1 to 8 while maintaining the battery voltage. A method for producing a lithium secondary battery, comprising: a first charging step of performing constant voltage charging for a long time. 8. After the first charging step, constant current charging is performed until the battery voltage reaches a range of 4.0 V or more and 4.3 V or less. 8. The method for manufacturing a lithium secondary battery according to claim 7, wherein a second charging step of performing constant voltage charging is performed. 9. An acetonitrile containing 0.1% of acetonitrile in the electrolyte.
6. The method according to claim 5, wherein the addition is in the range of from 5 to 5% by weight.
A method for manufacturing a lithium secondary battery according to claim 8.