JP2001068158A - Lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep sufficient battery performance even in high rate discharging, and obtain stable battery performance having a long life by making the gel electrolyte composition in a separator different from the gel electrolyte composition in at least one of positive electrode and a negative electrode, and specifying concentration of the polymers contained in the separator, the positive electrode, and the negative electrode. SOLUTION: The concentration of the polymers contained in the separator, the positive electrode, and the negative electrode is adjusted in the relation represented in the formula. In the formula, Cs is the weight percent of the polymer contained in the gel electrolyte in the separator, Cp is the weight percent of the polymer contained in the gel electrolyte in the positive electrode, and Cn is the weight percent of the polymer contained in the gel electrolyte in the negative electrode. Crosslinking density of the polymer contained in the separator is preferable to be lower than that of the polymer contained in at least one of the positive electrode and the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery, and more particularly, to an improvement in a gel electrolyte used for a lithium battery.

[0002]

2. Description of the Related Art In recent years, portable devices such as cellular phones, PHSs, and small personal computers have been significantly reduced in size and weight with the development of electronics technology. Miniaturization and weight reduction are required.

A lithium battery is one of the batteries that can be expected for such uses. In addition to the lithium primary battery that has already been put into practical use, a lithium secondary battery has been put to practical use, has a higher capacity, and has a longer life. Research is being pursued.

[0004] The above-mentioned various lithium batteries are mainly cylindrical or prismatic. On the other hand, in a lithium primary battery, a thin-shaped lithium-ion battery has been put into practical use by a manufacturing method using a printing technique and applying a printing technique. With the application of such technologies, various research and developments have been made on lithium secondary batteries and lithium ion secondary batteries in order to commercialize thin batteries using solid or gel electrolytes. I have.

[0005] In the case of a cylindrical or prismatic lithium secondary battery, a group of electrodes consisting of a positive electrode, a negative electrode, and a separator is inserted into a cylindrical or square battery case, and then a liquid electrolyte is injected. It is made. On the other hand, the solid electrolyte lithium secondary battery is manufactured by a method in which the positive electrode and the negative electrode are opposed to each other via a solid or gel electrolyte and then packed. However, such a solid electrolyte battery has a drawback that high-rate charge / discharge performance and cycle life are short as compared with a cylindrical or rectangular battery.

The following factors can be cited as the causes. That is, in the case of a cylindrical or prismatic battery, since a liquid electrolyte is injected, the lithium ion conductivity in the electrode and the separator is 1 × 10 ⁻³ S / cm, which is generally said to be a level necessary for battery operation. It is easy to secure orders and the diffusion speed of lithium ions is high. On the other hand, in the case of solid electrolyte batteries, since the electrolyte is a solid, the lithium ion conductivity must be lower than that of a liquid system, and a large amount of organic solvent is added to make it into a gel, improving the ion conductivity. 1 × 10 ⁻³ S / c
Although ion conductivity on the order of m can be ensured, there is a disadvantage that the diffusion rate of lithium ions is low.

As a practical example of a gel electrolyte having improved lithium ion conductivity, there is a gel electrolyte in which polyethylene oxide is used for a polymer skeleton, and an electrolyte comprising a lithium salt and an organic solvent is added thereto. By using polyethylene oxide having lithium ion conductivity as a polymer skeleton while being solid, and defining the mixing ratio with a lithium salt or an organic solvent, 1 × 10 ⁻³ S / cm, which is comparable to a liquid electrolyte to date, Lithium ion conductivity on the order of magnitude has been realized, and lithium batteries using this gel electrolyte have almost reached practical use.

[0008]

However, a lithium battery using a gel electrolyte typified by polyethylene oxide as described above exhibits sufficient battery performance during low-rate discharge, but still has high lithium ion discharge during high-rate discharge. Due to the low diffusion rate, there is a problem that lithium ions cannot move smoothly, and it is difficult to maintain battery performance at a sufficient level.

[0009] The present invention has been made in view of the above problems, and can reduce the ionic conductivity of the gel electrolyte in the separator to 1 × 10 ^-3 S / c without requiring a special manufacturing process or the like.
Lithium battery that can maintain the battery performance at a sufficient level even during high-rate discharge by maintaining the m-order and smooth movement of lithium ions in the gel electrolyte to obtain long-life and stable battery performance The purpose is to provide.

[0010]

In order to solve the above problems, a first aspect of the present invention is to form a positive electrode and a negative electrode containing at least an electrode active material, a polymer, a lithium salt and a gel electrolyte comprising an organic solvent with a polymer and a lithium electrolyte. In a lithium battery opposed via a separator containing a gel electrolyte comprising a salt and an organic solvent, the gel electrolyte composition in the separator and the gel electrolyte composition of at least one of the positive electrode and the negative electrode are different. And a separator, a positive electrode, and a negative electrode, wherein the polymer concentration is within the following range.

7 ≦ (Cs + Cp + Cn) / 3 ≦ 20 where Cs = weight percentage of polymer contained in gel electrolyte in separator Cp = weight percentage of polymer contained in gel electrolyte in positive electrode Cn = gel percentage in gel electrolyte in negative electrode Weight percent of polymer included The second aspect of the present invention is a lithium battery, wherein the crosslink density of the polymer included in the separator is lower than the crosslink density of the polymer included in at least one of the positive electrode and the negative electrode. It is.

A third aspect of the present invention is that the polymer used for the gel electrolyte contained in the separator has a higher affinity for the electrolyte than the polymer used for the gel electrolyte contained in at least one of the positive electrode and the negative electrode. A lithium battery mainly having a structure.

[0013]

According to the present invention, the following effects can be expected. In general, the lower the polymer concentration in the gel electrolyte, the higher the performance of the gel electrolyte is governed by the performance of the liquid electrolyte than the performance of the polymer, so the ionic conductivity in the gel electrolyte increases, The diffusion rate of lithium ions also increases. Therefore, when the battery is used for a battery, it is easily expected that the use of a gel electrolyte having a lower polymer concentration facilitates the movement of lithium ions during charge and discharge, and that a battery having excellent charge and discharge performance is obtained. On the other hand, the polymer in the electrode is also expected to function as a binder that binds active material particles by a polymer network and suppresses expansion and contraction due to a charge / discharge reaction. Therefore, the polymer concentration in the electrode needs to be a certain level or more. Therefore, as one means for maintaining the ion conductivity and diffusion rate of lithium ions necessary for a battery, when the polymer concentration in the electrode is higher than a certain level, the polymer concentration in the separator is lowered, so that the battery Can be maintained. Also, as a second method, by using a polymer skeleton in the electrode having a low affinity for the electrolytic solution, the ion conductivity of the gel electrolyte in the electrode and the diffusion rate of lithium ions are increased, and the battery is charged. Discharge performance can be maintained.

Therefore, first, the gel electrolyte composition in the separator is different from the gel electrolyte composition of at least one of the positive electrode and the negative electrode. By defining the range within the range shown in the invention, lithium ions in the electrolyte in the battery system can be smoothly moved, and sufficient supply of lithium ions to the electrode during charging and discharging can be realized. The gel electrolyte composition at this time means any of the type of polymer skeleton, the crosslink density of the polymer, and the mixing ratio of the polymer to the other electrolyte components.

In the present invention, when defining the polymer concentration contained in the separator, the positive electrode and the negative electrode within the range shown in the present invention, the polymer concentration contained in the separator, the positive electrode and the negative electrode is not individually specified. However, to maintain the charge / discharge performance as a battery, 5
By setting the content within the range of ２５25% by weight, the battery performance can be maintained at a sufficient level even at the time of high-rate discharge, and a long life and stable battery performance can be obtained. In particular, a lower polymer concentration is desirable for maintaining charge / discharge performance as a battery, but when the polymer concentration is 5% by weight or less,
Problems such as liquid leakage occur, which is not desirable.

Second, the above effect can be more effectively obtained by making the crosslink density of the polymer contained in the separator lower than the crosslink density of the polymer contained in at least one of the positive electrode and the negative electrode. Can be.

Third, the gel electrolyte contained in the separator is mainly composed of a polymer having a structure having a high affinity for a lithium salt, an organic solvent, and an electrolyte obtained by dissolving the lithium salt in the organic solvent. In the case where the polymer concentration in the separator is reduced, or when the polymer skeleton in the electrode has low affinity with the electrolytic solution, the polymer skeleton in the separator is In addition, it is possible to easily gel, and to retain an electrolyte sufficient for the progress of the battery reaction, and to maintain the charge / discharge performance of the battery. As a result, not only can stable battery performance be obtained, but there is no danger of liquid leakage or the like.

Accordingly, the present invention can provide a lithium battery excellent in reliability and excellent in initial capacity, high-rate charge / discharge performance, cycle life, etc., because the above actions are synergistically obtained. You can do it.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below based on embodiments.

FIG. 1 is a sectional view of the lithium battery of the present invention.

In FIG. 1, reference numeral 1 denotes a positive electrode mixture mainly composed of lithium cobalt oxide as a positive electrode active material, which is applied on a positive electrode current collector 3 made of aluminum foil. Also, 2
Is a negative electrode mixture mainly composed of carbon, which is a negative electrode active material, and is applied on a negative electrode current collector 4 made of copper foil.
The positive electrode mixture 1 and the negative electrode mixture 2 are laminated via a separator 5 made of a gel electrolyte. Further, the electrode group thus laminated is covered with an aluminum laminated film 6, and the four sides are sealed by heat welding to obtain a lithium battery.

Next, a method of manufacturing the lithium battery having the above configuration will be described. First, the positive electrode mixture 1 was obtained as follows. First, lithium cobalt oxide which is a positive electrode active material,
A mixture of acetylene black, which is a conductive agent, and an N-methyl-2-pyrrolidone solution of polyvinylidene fluoride as a binder was applied on an aluminum foil as the positive electrode current collector 3, and then dried. By pressing the mixture so as to have a thickness of 0.1 mm, a positive electrode active material sheet was obtained. Next, 2 mol of L is added to 1 liter of γ-butyrolactone.
An electrolyte solution was prepared by mixing an acrylate monomer having the structure shown in Chemical Formula 1 with an electrolyte solution in which iBF ₄ was dissolved. At this time, the mixing ratio between the electrolytic solution and the acrylate monomer was 95% by weight of the electrolytic solution and 5% by weight of the acrylate monomer. The positive electrode active material sheet was immersed in this, and impregnated with an electrolyte solution. Subsequently, the positive electrode active material sheet was taken out of the electrolyte solution, and the monomer was polymerized by electron beam irradiation to form a polymer. Through the above steps, positive electrode mixture 1 was obtained. Therefore, the battery A1 of the present invention
The weight percentage of the polymer contained in the gel electrolyte in the positive electrode was Cp = 5. The negative electrode mixture 2 was obtained by the same method as the positive electrode mixture 1 except that carbon as the negative electrode active material was used and a copper foil was used for the negative electrode current collector 4. Therefore, the weight percentage of the polymer contained in the gel electrolyte in the negative electrode of the battery A1 of the present invention is Cn = 5.

[0023]

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On the other hand, the separator 5 was obtained as follows. First, an electrolyte solution was prepared by mixing a trifunctional acrylate monomer having the structure shown in Chemical Formula 2 with an electrolyte solution in which 2 mol of lithium salt LiBF ₄ was dissolved in 1 liter of γ-butyrolactone as an organic solvent. At this time, the mixing ratio of the electrolytic solution and the acrylate monomer was 75% by weight of the electrolytic solution and 25% by weight of the acrylate monomer. After this was applied on the positive electrode mixture 1, the monomer was polymerized by electron beam irradiation to form a polymer, thereby obtaining a gel electrolyte. Through the above steps, the separator 5 was obtained. Therefore, the weight percentage of the polymer contained in the gel electrolyte in the separator in the battery A1 of the present invention is Cs = 25.

[0025]

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The lithium battery having a capacity of 10 mAh produced by the above-described raw materials and the production method was designated as Battery A1 of the present invention. In the battery A1 of the present invention, the gel electrolyte (C
(s + Cp + Cn) / 3 value is 11.7.

The electrolyte used in the positive electrode mixture 1 and the negative electrode mixture 2 and the electrolyte used in the separator 5 are those shown in Table 1, and the other conditions are the same as those of the battery A1 of the present invention, and the capacity is 10 mAh. Of the present invention batteries A2 to A6 and comparative batteries B1 to B4
And

[0028]

[Table 1]

The lithium ion conductivity of the separator and the positive and negative electrode gel electrolytes used in the batteries A1 to A6 of the present invention and the comparative batteries B1 to B4 was at least 20 ° C.
In the vicinity, all of them maintain an order of 1 × 10 ⁻³ S / cm, and do not show much temperature dependency even at a low temperature. Therefore, the battery A1 of the present invention using these electrolytes
-A6 and Comparative Batteries B1 to B4 are expected to have performances near at least the initial capacity at the time of normal-temperature low-rate charge / discharge.

First, these batteries A1 to A of the present invention were used.
6 and comparative batteries B1 to B4 were discharged at various current values, and the relationship between the resulting discharge current and discharge capacity is shown in FIG. The test conditions were as follows: 1 at a temperature of 20 ° C.
The battery was charged to a final voltage of 4.2 V with a current of mA (corresponding to 0.1 CmA) and then discharged to a final voltage of 2.7 V with various currents. The discharge capacity was obtained when the battery was discharged at a current of 1 mA. Is expressed as a percentage with respect to 100. The batteries A1 to A6 of the present invention and the comparative battery B
In any one of 1 to B4, the discharge capacity at a discharge current of 1 mA is:
Almost 100% of the design capacity was obtained.

As shown in FIG. 2, when the discharge current is 10 mA, the comparative batteries B1 to B4 have a discharge capacity of 30 to 6 at a discharge current of 1 mA.
While only about 0% of the discharge capacity can be obtained, the batteries A1 to A6 of the present invention have a design capacity of 85% even at a discharge current of 10 mA.
It was found that a discharge capacity of ~ 95% was obtained.

As the cause, the following factors are considered. First, in the batteries A1 to A6 of the present invention, the polymer skeleton in the gel electrolyte in the separator is different from the polymer skeleton in the gel electrolyte in the positive electrode and the negative electrode, and the average polymer concentration of the gel electrolyte in the separator and the electrode is 10%. ~
It is in the range of 20 weight percent. Therefore, in the batteries A1 to A6 of the present invention, smooth movement of lithium ions in the gel electrolyte can be realized, and lithium ions on the positive electrode side are sufficiently supplied even at the time of high-rate discharge, so that sufficient discharge capacity can be obtained. It is thought that it is possible.

However, in the comparative batteries B1 to B4, similar to the batteries A1 to A4 of the present invention, although the polymer skeleton in the gel electrolyte in the separator is different from the polymer skeleton in the gel electrolyte in the positive electrode and the negative electrode, it is expected. No such performance improvement is seen. This is because the (Cs + Cp + Cn) / 3 value of the gel electrolyte in the separator and the electrode is 7 because the concentration of the polymer contained in either the separator or the gel electrolyte in the electrode is too high or too low.
It is thought to be due to being out of the range of ２０20.
If the polymer concentration is too high, the ionic conductivity will be low, so that the movement of lithium ions will be difficult, causing insufficient battery performance. Conversely, if the polymer concentration is too low, lithium ions can move easily, but the electrolyte retention of the gel electrolyte is insufficient, and the electrolyte is released in the battery, resulting in the electrolyte in the electrode group. Is insufficient to cause sufficient battery performance to be obtained.

From the above results, the polymer skeleton in the gel electrolyte in the separator was different from the polymer skeleton in the gel electrolyte in the positive electrode and the negative electrode, and (Cs + Cp + Cn) / 3 of the gel electrolyte in the separator and the electrode. It was found that by setting the value within the range of 7 to 20, smooth movement of lithium ions in the gel electrolyte can be realized. In particular, it was found that by setting the polymer concentration of the gel electrolyte contained in the separator and the electrode to 5 to 25% by weight, smooth movement of lithium ions can be optimally realized.

Further, among the batteries of the present invention, A5 and A
6 and the comparative batteries B1 and B2 were subjected to a charge / discharge cycle test, and the relationship between the number of cycles and the discharge capacity obtained as a result is shown in FIG. The test conditions were as follows: after charging at a temperature of 20 ° C. and a current of 2 mA to a final voltage of 4.2 V,
The battery was discharged to a cutoff voltage of 2.7 V at a current of 2 mA, and the discharge capacity is shown as a percentage when the designed capacity of the positive electrode was set to 100.

FIG. 3 shows that the batteries A5 and A6 of the present invention and the comparative batteries B1 and B2 each obtained at least 90% of the designed capacity at the initial stage of charge and discharge. It turns out that it works well in the early stage. However, the capacity of the comparative batteries B1 and B2 gradually decreases as the cycle passes, and the comparative battery B2 falls below 50% of the designed capacity at the 150th cycle and the comparative battery B1 falls below 50% of the designed capacity at the 300th cycle. On the other hand, the batteries A5 and A6 of the present invention have a design capacity of approximately 100 from the initial stage of charge and discharge.
%, And a slight decrease in capacity is observed even after 300 cycles, but it was found that 80% or more of the designed capacity was maintained.

The following factors are considered as the cause. First, the gel electrolyte in the electrode uses a polymer obtained by polymerizing a bifunctional acrylate monomer having the structure shown in Chemical Formula 1, and has a polymer skeleton having a relatively low affinity with the electrolyte, whereas the gel electrolyte in the separator is Of the gel electrolyte
This is a polymer obtained by polymerizing a trifunctional acrylate monomer having a structure represented by Chemical Formula 2. That is,
Since the polymer skeleton has an ethylene oxide structure and a propylene oxide structure with high affinity for the electrolyte, and has a three-dimensional network structure, it easily gels with the electrolyte and has sufficient electrolysis for the progress of the battery reaction. It is a gel electrolyte that can hold a liquid and has excellent mechanical strength.
Therefore, even if lithium ions and the electrolyte move repeatedly during charge and discharge, sufficient lithium ions and the electrolyte are retained in the separator, and not only stable battery performance is obtained, but also danger such as liquid leakage is caused. Absent. In addition, a trifunctional acrylate monomer having the structure shown in Chemical Formula 2 was polymerized compared to a polymer obtained by polymerizing a bifunctional acrylate monomer having the structure shown in Chemical Formula 1 used for the gel electrolyte in the electrode. In the polymer, the distance between the polymer groups is large, and the crosslinking density of the polymer is low. Therefore, despite having an ethylene oxide structure and a propylene oxide structure, which are generally considered to have a strong interaction between the polymer, the solvent, and the supporting salt, an increase in resistance of the separator portion is suppressed, and stable battery performance is obtained. Can be

On the other hand, in the electrode, the ease of movement of the ions is given priority over the ability to hold the electrolytic solution. Therefore, when a polymer having a high affinity for the electrolytic solution is used, the ions are restricted and the movement is restricted. .

In addition, in the batteries A5 and A6 of the present invention, the (Cs + Cp + Cn) / 3 value of the gel electrolyte in the separator and the electrode is in the range of 7 to 20, as in the above, so that the gel electrolyte It is considered that the movement of lithium ions in the interior was realized smoothly. Therefore, in the batteries A5 and A6 of the present invention, it is considered that sufficient lithium ions and the electrolytic solution are retained in the separator even after the progress of the charge / discharge cycle, and a decrease in capacity due to the progress of the cycle is suppressed.

[0040]

As described above, according to the present invention, a lithium battery having excellent initial capacity, high-rate charge / discharge performance, and cycle life can be provided without requiring a special manufacturing process. Things.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lithium battery of the present invention.

FIG. 2 is a diagram showing the relationship between discharge current and discharge capacity when discharging at various current values for the batteries A1 to A6 of the present invention and comparative batteries B1 to B4.

FIG. 3 is a diagram showing the relationship between the number of cycles and the discharge capacity when a charge / discharge cycle test is performed on batteries A5 and A6 of the present invention and comparative batteries B1 and B2.

[Explanation of symbols]

 1 Positive electrode mixture 2 Negative electrode mixture 5 Separator

Claims (3)

[Claims] 1. A lithium battery comprising: a positive electrode and a negative electrode each containing at least an electrode active material, a polymer, a lithium salt and a gel electrolyte made of an organic solvent; In a battery, a gel electrolyte composition in the separator,
A lithium battery, wherein a gel electrolyte composition of at least one of the positive electrode and the negative electrode is different, and a polymer concentration contained in the separator, the positive electrode, and the negative electrode is in the following range. 7 ≦ (Cs + Cp + Cn) / 3 ≦ 20 where Cs = weight percent of polymer contained in gel electrolyte in separator Cp = weight percent of polymer contained in gel electrolyte in positive electrode Cn = polymer contained in gel electrolyte in negative electrode Weight percent of 2. The lithium battery according to claim 1, wherein the crosslink density of the polymer contained in the separator is lower than the crosslink density of the polymer contained in at least one of the positive electrode and the negative electrode. 3. The polymer used for the gel electrolyte contained in the separator mainly has a structure having a higher affinity for the electrolyte than the polymer used for the gel electrolyte contained in at least one of the positive electrode and the negative electrode. The lithium battery according to claim 1, wherein: