JP2009170348A - Durability-improving method for lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a durability-improving method for lithium-ion secondary battery, in which occurrence of peeling-off and slipping-down of a negative electrode is suppressed and durability is high, and to provide a manufacturing method of the lithium-ion secondary battery that uses the method. <P>SOLUTION: The method is for durability-improving of lithium-ion secondary battery which has a negative electrode body, having a negative electrode current collector and a negative electrode layer constituted of an element which can be alloyed with lithium; a positive electrode body having a positive electrode current collector and a positive electrode layer; a separator, arranged between the negative electrode layer and the positive electrode layer, and non-aqueous electrolytic solution containing lithium salt. By providing a means which at the initial discharge the initial period current density is made low, the current density is gradually increased, electricity is discharged, and aim is attained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for improving the durability of a lithium ion secondary battery, which improves the durability of the lithium ion secondary battery by preventing cracks and the like of a negative electrode layer.

  With the miniaturization of personal computers, video cameras, mobile phones, etc., in the fields of information-related equipment and communication equipment, lithium secondary batteries have been put into practical use because of their high energy density as the power source used for these equipment. It has become widespread. On the other hand, in the field of automobiles, the development of electric vehicles is urgently caused by environmental problems and resource problems, and lithium secondary batteries are also being studied as a power source for electric vehicles.

  Conventionally, carbon materials such as graphite have been widely used as negative electrode active materials used in lithium secondary batteries. However, since carbon materials generally have a small amount of Li storage, they have a large amount of Li storage compared to carbon materials. Sn, Sn alloy, etc. are attracting attention (for example, Patent Document 1).

  However, when charging and discharging are performed in such a lithium secondary battery, for example, in a conventional negative electrode body 7 (FIG. 3A) including a negative electrode current collector 5 and a negative electrode layer 6 as shown in FIG. The alloying active material alloyed with Li (lithium) in the layer 6 expands and contracts when occluding and releasing Li (lithium), and cracks are generated in the negative electrode layer 6 (FIG. 3B). ). If charging / discharging is further repeated in this state, the alloying active material cannot withstand rapid expansion and contraction, cracks in the negative electrode layer 6 propagate, and the negative electrode layer 6 peels off and slides down ( FIG. 3 (c)). For this reason, electrical conductivity is lost, charging and discharging cannot be performed, and cycle characteristics are lowered. Therefore, it is necessary to improve such a problem and improve the cycle characteristics of the lithium secondary battery.

Therefore, in order to improve the cycle characteristics, for example, in Patent Document 2, when the lithium secondary battery having an alloy electrode is initially charged, the negative electrode layer is charged by charging at a low current density while pressing in the thickness direction of the negative electrode. A method for controlling the micronization of particles is disclosed. This is a method of suppressing the occurrence of cracks by suppressing the expansion of the negative electrode due to the insertion of lithium ions during the first charge and by reducing the sudden volume change by charging at a low current density. .
However, even if the above-described method is used, cracking of the negative electrode layer occurs, so that deterioration of cycle characteristics cannot be sufficiently suppressed.

Japanese Patent Laid-Open No. 10-255768 JP 2005-32632 A JP 2006-24392 A

  The present invention has been made in view of the above problems, and suppresses the occurrence of peeling, slipping, etc. of the negative electrode layer in a lithium ion secondary battery, and a lithium ion secondary battery having a durable lithium ion secondary battery. It is a main object of the present invention to provide a method for improving durability and a method for producing a lithium ion secondary battery using the method.

The present inventor has conducted earnest research on the above problems, and as a result of investigating the occurrence frequency of cracks in the negative electrode layer during charging and discharging of the lithium ion secondary battery, It has been found that it occurs at the time of separation, and at the time of charging, that is, at the time of occlusion of lithium ions, it has been found that cracking hardly occurs regardless of the number of times.
Therefore, it is clear that if the discharge is performed by gradually increasing the current value from a low current density during the first discharge, the rapid volume change due to the desorption of lithium ions does not occur, so that the occurrence of cracks can be suppressed. became. The present inventor has completed the present invention based on these findings.

  That is, the present invention provides a negative electrode current collector and a negative electrode body having a negative electrode layer made of an alloyable element with lithium, a positive electrode body having a positive electrode current collector and a positive electrode layer, and the negative electrode layer and the positive electrode layer. Is a method for improving the durability of a lithium ion secondary battery comprising a separator and a non-aqueous electrolyte containing a lithium salt, wherein the initial current density is reduced and the current density is gradually increased during the first discharge. There is provided a method for improving the durability of a lithium ion secondary battery.

  According to the present invention, by performing the durability improvement method described above, even when the negative electrode layer is made of an element that can be alloyed with lithium, cracks due to expansion and contraction of the negative electrode that occur during charge and discharge are prevented, and cycle characteristics are deteriorated. Therefore, the conventional lithium ion secondary battery can be improved to a highly durable one.

  Moreover, in the said invention, how to raise the current density from the said initial current density detects the crack of the said negative electrode layer which generate | occur | produces at the time of the first discharge previously using a detection means, and is determined based on the detected result. It is preferable. This is because the negative electrode layer with fewer cracks can be obtained by accurately determining the value of the initial current density.

  In the said invention, it is preferable that the said detection means is based on detecting the acoustic emission which arises by a crack. This is because the occurrence of cracks can be accurately detected and can be easily detected.

  The present invention is arranged between a negative electrode current collector and a negative electrode body having a negative electrode layer made of an alloyable alloy with lithium, a positive electrode body having a positive electrode current collector and a positive electrode layer, and the negative electrode layer and the positive electrode layer. Method of improving the durability of the above-described lithium ion secondary battery after assembling the lithium ion secondary battery, comprising: The manufacturing method of the lithium ion secondary battery characterized by having the improvement process which improves the durability of a lithium ion secondary battery using is provided.

  According to the present invention, it is possible to manufacture a lithium ion secondary battery with high cycle characteristics by having an improvement process for improving the durability of the lithium ion secondary battery by the method described above.

  In the present invention, there is an effect that the durability of the lithium ion secondary battery is improved by controlling the current value at the first discharge and suppressing the occurrence of cracks in the negative electrode layer.

  The present invention relates to a method for improving the durability of a lithium secondary battery and a method for producing a lithium ion secondary battery using the method for improving the durability of a lithium secondary battery. Each will be described in detail below.

A. First, a method for improving the durability of the lithium ion secondary battery of the present invention will be described.
A method for improving the durability of a lithium ion secondary battery according to the present invention includes a negative electrode current collector and a negative electrode body having a negative electrode layer made of an alloyable alloy with lithium, a positive electrode current collector and a positive electrode body having a positive electrode layer, A method for improving the durability of a lithium ion secondary battery having a negative electrode layer and a separator disposed between the positive electrode layer and a non-aqueous electrolyte containing a lithium salt, wherein the initial current density is reduced during initial discharge. In this method, the current density is gradually increased and discharged.

FIG. 1 is a graph showing the change in potential of the negative electrode body with time and the measurement result of the acoustic emission signal when charging and discharging the lithium ion secondary battery at a constant current. The dotted line A indicates the change in potential with time, the solid line B indicates the peak of the acoustic emission signal, and the solid line C indicates the change in current value during charging / discharging. As illustrated in FIG. 1A, the acoustic emission signal showed a large peak at the beginning of the first charge (a in the figure). Therefore, it was found from this that the crack of the negative electrode layer mainly occurred at the beginning of the first discharge.
Therefore, as shown in FIG. 1 (b), the current value at the beginning of the first charge (a in the figure) is lowered to such an extent that the negative electrode layer is hardly cracked, and gradually increased to suppress the occurrence of cracks. The negative electrode layer can be prevented from peeling or sliding off due to the cause.

  As described above, in the first discharge, when an element that can be alloyed with lithium is used for the negative electrode layer by suppressing the sudden volume change of the negative electrode layer by gradually increasing the current density and preventing cracking However, high cycle characteristics can be achieved.

The method for increasing the current density from the initial current density in the present invention is not particularly limited as long as cracking does not occur in the negative electrode layer.
The method for increasing the current density from the initial current density is determined by the type and shape of the element of the negative electrode layer, the thickness of the negative electrode layer, and the like.
In the present invention, the method of increasing the current density from the initial current density is to change the slope of the current value.
The initial current density here means a current density of rate C / 100 based on the theoretical capacity of the negative electrode body, and is within a range of 0.001 mA / cm ^{2 to} 0.05 mA / cm ^2. 0.008mA ^/ cm ² ~0.03mA ^/ cm 2 in the range, preferably in the range particularly ^{0.005mA / cm 2 ~0.01mA / cm 2} .

  About how to increase the current density from the initial current density described above, a crack in the negative electrode layer that occurs at the time of the first discharge is detected in advance using a detection means, and is determined based on the detected result. More preferred. This is because it is easy to prevent cracking of the negative electrode layer by appropriately adjusting depending on the type and shape of the elements used in the negative electrode layer, the thickness of the negative electrode layer, and the like.

As described above, the detection means for detecting the occurrence frequency of cracks is not particularly limited as long as it is a means capable of detecting when a crack occurs. Specifically, a means for detecting acoustic emission generated at the time of cracking may be used, or a means for measuring a change in the thickness of the negative electrode layer due to cracking may be used. In the present invention, it is preferable to use a means for detecting acoustic emission. This is because it becomes possible to accurately and easily grasp when the occurrence of cracking occurs.
Here, the acoustic emission is an elastic wave generated with a crack or destruction of a material or the like.
In addition, the measurement method is a method in which an acoustic emission sensor is installed on the battery surface via grease, and an elastic wave generated from the object to be measured (negative electrode body) is detected.

  The method for improving the durability of a lithium ion secondary battery of the present invention is a method used for a lithium ion secondary battery having a negative electrode layer made of an element that can be alloyed with lithium. Below, the lithium ion secondary battery which can use this invention is demonstrated.

(Lithium ion secondary battery)
A lithium ion secondary battery that can utilize the present invention includes a negative electrode current collector and a negative electrode body having a negative electrode layer composed of an element that can be alloyed with lithium, a positive electrode current collector and a positive electrode body having a positive electrode layer, the negative electrode layer, It has a separator arrange | positioned between the said positive electrode layers, and the non-aqueous electrolyte containing a lithium salt.

A lithium secondary battery to which the present invention can be applied will be described with reference to the drawings. FIG. 2 is a schematic cross-sectional view showing an example of the lithium secondary battery of the present invention. A lithium ion secondary battery 1 shown in FIG. 2 includes a positive electrode current collector 2, a positive electrode body 4 having a positive electrode layer 3 containing a positive electrode active material formed on the positive electrode current collector 2, and a negative electrode current collector. 5, a negative electrode body 7 having a negative electrode layer 6 containing a negative electrode active material formed on the negative electrode current collector 5, a separator 8 disposed between the positive electrode layer 3 and the negative electrode layer 6, a positive electrode active material, and And a non-aqueous electrolyte (not shown) that conducts lithium ions between the negative electrode active materials.
Hereinafter, a lithium secondary battery to which the present invention can be applied will be described for each configuration.

(I) Negative Electrode Body A negative electrode body of a lithium ion secondary battery to which the present invention can be applied has a negative electrode current collector and a negative electrode layer. In the following, description will be given separately.

a. Negative electrode layer The negative electrode layer of the lithium ion secondary battery to which the present invention can be applied is composed of an element that can be alloyed with lithium.

  The element is not particularly limited as long as it can be alloyed with lithium ions, and examples thereof include lithium metal, silicon, tin, aluminum, and alloys thereof. In the present invention, tin is particularly preferable.

  The film thickness of the negative electrode layer is not particularly limited, and is appropriately adjusted depending on the use of the lithium ion secondary battery, but it is within the range of 0.1 μm to 5.0 μm, particularly 0.5 μm to It is preferably within the range of 3.0 μm. It is because it can ensure a capacity | capacitance and can make a negative electrode layer hard to generate | occur | produce a crack by being in the said range.

b. Negative electrode current collector A negative electrode current collector of a lithium ion secondary battery to which the present invention can be applied has a function of collecting current in the negative electrode layer.

  Examples of the material for the negative electrode current collector include copper, SUS, and nickel. Of these, copper is preferable. In addition, examples of the shape of the negative electrode current collector include a foil shape, a plate shape, and a mesh shape. Of these, a foil shape is preferable.

c. Method for Forming Negative Electrode Body As a method for forming the negative electrode layer of the present invention, a method similar to a general method for forming a negative electrode body can be used. Specifically, means such as electroplating, vacuum deposition, and sputtering are employed.

(Ii) Positive Electrode Body A positive electrode body to which the present invention can be applied has a positive electrode current collector and a positive electrode layer.
Each will be described below.

a. Positive electrode layer The positive electrode layer used for the positive electrode body contains a positive electrode active material capable of occluding and releasing lithium.

Examples of such a positive electrode active material include metal Li, LiCoO ₂ , LiCoO ₄ , LiMn ₂ O ₄ , LiNiO ₂ , LiFePO _{4, and the} like.

The positive electrode layer may further contain a conductive agent and a binder (binder).
Examples of the binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).
Examples of the conductive agent include carbon black such as acetylene black and ketjen black.

b. Positive Current Collector The positive current collector is a current collector for the positive electrode layer. The material of the positive electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include aluminum, SUS, nickel, iron, and titanium. Among them, aluminum and SUS are preferable. . Furthermore, the positive electrode current collector may be a dense metal current collector or a porous metal current collector.

c. Method for forming positive electrode body The method for forming the positive electrode body is not particularly limited, and a method similar to a method for forming a general positive electrode body can be used. Specifically, first, a positive electrode layer forming paste containing a positive electrode active material, a binder, a solvent, and the like is prepared, and then the positive electrode layer forming paste is applied onto the positive electrode current collector and dried. Can be mentioned. At this time, the positive electrode layer may be pressed in order to improve the electrode density of the positive electrode layer.

(iii) Separator Next, the separator used in the present invention will be described. As described above, the separator used in the present invention is installed between electrodes having different polarities, and has a function of holding an electrolyte described later.
Although it does not specifically limit as a material of the said separator, For example, resin, such as polyethylene (PE), polypropylene (PP), polyester, a cellulose, and polyamide, can be mentioned, Especially, a polypropylene is preferable. The separator may have a single layer structure or a multilayer structure. Examples of the separator having a multilayer structure include a separator having a two-layer structure of PE / PP and a separator having a three-layer structure of PP / PE / PP. Furthermore, in the present invention, the separator may be a nonwoven fabric such as a porous membrane, a resin nonwoven fabric, or a glass fiber nonwoven fabric. Among these, a porous film is preferable.

(iv) Nonaqueous Electrolyte In the present invention, a nonaqueous electrolyte containing a lithium salt is usually contained in the electrode and current collector in the electrode body described above, and further in the separator.
The non-aqueous electrolyte usually has a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited as long as it is a lithium salt used in a general lithium secondary battery. For example, LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ and LiClO ₄ . On the other hand, the non-aqueous solvent is not particularly limited as long as it can dissolve the lithium salt. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxy. Ethane, 1,2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. Can be mentioned. In the present invention, these non-aqueous solvents may be used alone or in combination of two or more. Moreover, room temperature molten salt can also be used as said non-aqueous electrolyte.

(v) Others When a lithium ion secondary battery to which the present invention can be applied is, for example, laminated, a lithium secondary battery as illustrated in FIG. It is made by sealing. As the battery case, generally, a metal case is used, for example, a stainless steel case. Further, the shape of the battery case used in the present invention is not particularly limited as long as it can accommodate the separator, the positive electrode layer, the negative electrode layer, and the like described above. , Coin type, laminate type and the like.

  Moreover, as a use of such a lithium ion battery, it uses for a motor vehicle etc., for example.

B. Next, a method for producing a lithium ion secondary battery of the present invention will be described.
The method for producing a lithium ion secondary battery of the present invention includes a negative electrode current collector and a negative electrode body having a negative electrode layer made of an alloyable alloy with lithium, a positive electrode body having a positive electrode current collector and a positive electrode layer, and the negative electrode layer And a method of manufacturing a lithium ion secondary battery having a separator disposed between the positive electrode layers and a non-aqueous electrolyte containing a lithium salt. It is a manufacturing method of a lithium ion secondary battery characterized by having the improvement process which improves the durability of a lithium ion secondary battery using the durability improvement method of an ion secondary battery.

According to the present invention, a lithium ion secondary battery having high cycle characteristics and a long life can be obtained by performing the above-described improvement process.
Hereinafter, each step will be described.

1. Improvement process This process consists in the lithium ion secondary battery described in the section “A. Method for improving durability of lithium ion secondary battery” after assembling the lithium ion secondary battery by “2. This is a step of improving the durability of the lithium ion secondary battery using the durability improving method.
The method for improving the durability of the lithium ion secondary battery has been described in detail in the section “A. Method for improving the durability of the lithium ion secondary battery”, and thus description thereof is omitted here.

  Moreover, the lithium ion secondary battery manufactured by the manufacturing method of the lithium ion secondary battery of this invention becomes a final product after improving by this process, and is shipped to the market.

2. Other Steps The method for producing a lithium ion secondary battery of the present invention includes a step of assembling a lithium ion secondary battery before the improvement step described above. About this process, it is the same as the process performed in the manufacturing method of a general lithium ion secondary battery. Specifically, under an inert atmosphere, first, the step of housing the positive electrode body, the negative electrode body and the separator in the battery case, the step of adding an organic electrolyte to the battery case, the step of sealing the battery case, etc. Can be mentioned.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  Hereinafter, the present invention will be described more specifically with reference to examples.

[Example 1]
(Formation of electrode body)
A tin plating film having a thickness of 2 μm was formed on a copper foil having a thickness of 10 μm to obtain a negative electrode body. Further, lithium metal was used as the positive electrode body. As the size of the electrode, a negative electrode body with a diameter of 16 mm and a positive electrode with a diameter of 19 mm were used.

(Battery formation)
A battery was formed using the negative electrode body, the positive electrode body, and a CR2032-type coin cell. In addition, the separator used the porous film made from a polypropylene (PP). The electrolyte used was a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 3: 4, and lithium hexafluorophosphate (LiPF) as a supporting salt. ₆ ) dissolved at a concentration of 1 mol / L was used.

(Battery evaluation method)
The battery was charged and discharged (upper and lower limit voltage: 1.0 V to 0.01 V) at a current density of 0.1 mA / cm ² . This charge cycle was repeated 20 times, and the capacity retention rate was evaluated from the ratio of the initial discharge capacity and the discharge capacity at the 20th cycle.
The time was increased when initial discharge by increasing the (lithium de Hanaretoki) gradually current value from the current density 0.01 mA / cm ² only to 0.1 mA / cm ^2.
The results are shown in Table 1.

[Comparative Example 1]
Using the same battery as in Example 1, the battery was charged and discharged (upper and lower limit voltage: 1.0 V to 0.01 V) at a current density of 0.1 mA / cm ² . This charge cycle was repeated 20 times, and the capacity retention rate was evaluated from the ratio of the initial discharge capacity and the discharge capacity at the 20th cycle.
Note that the current value during the first discharge was not controlled.
The results are shown in Table 1.

(Evaluation)
As in Example 1, by gradually increasing the current density only at the time of the first discharge, the capacity retention rate was improved as compared with the case of charging and discharging at a constant current from the time of the first discharge as in Comparative Example 1.

It is a graph which shows the relationship between the occurrence frequency of the crack of a negative electrode layer, and an electric current value. It is a schematic sectional drawing which shows an example of the lithium ion secondary battery which can use this invention. It is a figure explaining the crack of the conventional negative electrode layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery 2 ... Positive electrode collector 3 ... Positive electrode layer 4 ... Positive electrode body 5 ... Negative electrode collector 6 ... Negative electrode layer 7 ... Negative electrode body 8 ... Separator

Claims (4)

A negative electrode current collector and a negative electrode body having a negative electrode layer composed of an element that can be alloyed with lithium; a positive electrode current collector and a positive electrode body having a positive electrode layer; and a separator disposed between the negative electrode layer and the positive electrode layer; A method for improving the durability of a lithium ion secondary battery having a non-aqueous electrolyte containing a lithium salt,
A method for improving the durability of a lithium ion secondary battery, characterized by lowering an initial current density at the first discharge and gradually increasing the current density for discharge.   The method of increasing the current density from the initial current density is to detect cracks in the negative electrode layer that occur in advance at the time of initial discharge using a detection means, and is determined based on the detected result. The method for improving the durability of a lithium ion secondary battery according to claim 1.   3. The method for improving the durability of a lithium ion secondary battery according to claim 2, wherein the detecting means detects acoustic emission caused by cracking. A negative electrode current collector and a negative electrode body having a negative electrode layer composed of an element that can be alloyed with lithium; a positive electrode current collector and a positive electrode body having a positive electrode layer; and a separator disposed between the negative electrode layer and the positive electrode layer; A method for producing a lithium ion secondary battery having a non-aqueous electrolyte containing a lithium salt,
After assembling the lithium ion secondary battery, using the method for improving the durability of the lithium ion secondary battery according to claim 1, further comprising an improvement process for improving the durability of the lithium ion secondary battery, To manufacture a lithium ion secondary battery.

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