KR101150252B1 - Process for preparation of lithium secondary battery for blocking spread of stress - Google Patents

Abstract

According to the present invention, after preparing an electrode by applying an electrode mixture including an active material and a binder to a current collector, preparing an electrode assembly through a separator (i), inserting the electrode assembly into a battery case to manufacture a battery cell And injecting an electrolyte solution (ii), and performing initial charging for activation of the battery cell (iii), wherein the electrode mixture layer is repeatedly charged and discharged in the course of using the secondary battery. In order to minimize the diffusion of stress due to the expansion and contraction of the, before the electrolyte is impregnated to the central portion of the electrode assembly in the step (ii), the active charge of the step (iii) is carried out It provides a method of manufacturing a lithium secondary battery.

Description

Process for Preparation of Lithium Secondary Battery for Blocking Spread of Stress

The present invention relates to a method for manufacturing a lithium secondary battery in which the diffusion of stress is blocked. More specifically, the step (i) of preparing an electrode assembly includes inserting an electrode assembly into a battery case to manufacture a battery cell and injecting an electrolyte solution. A method of manufacturing a lithium secondary battery comprising the step (ii) and performing initial charging for activation of a battery cell, wherein the expansion and contraction of the electrode mixture layer is repeated during charge and discharge in the course of using the secondary battery. In order to minimize the diffusion of the stress due to the lithium secondary battery, the active charge of the step (iii) is performed before the electrolyte is impregnated to the center of the electrode assembly in the step (ii). It relates to a manufacturing method of.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is increasing rapidly. Among them, many researches have been conducted and commercialized and widely used for lithium secondary batteries with high energy density and discharge voltage. It is used.

According to the shape of the battery case, secondary batteries are classified into cylindrical batteries and rectangular batteries in which the electrode assembly is embedded in a cylindrical or rectangular metal can, and pouch-type batteries in which the electrode assembly is embedded in a pouch type case of an aluminum laminate sheet. .

In addition, the electrode assembly embedded in the battery case is a generator capable of charging consisting of a laminated structure of the anode / separator / cathode, the jelly-roll-type wound through a separator between the long sheet-type anode and cathode coated with the active material and In addition, a plurality of positive and negative electrodes of a predetermined size are classified into a stack type which is sequentially stacked in a state interposed in a separator.

The lithium secondary battery uses a metal oxide such as LiCoO ₂ as a positive electrode active material and a carbon material as a negative electrode active material, a polyolefin-based porous separator is inserted between the negative electrode and the positive electrode, and a non-aqueous electrolyte solution containing a lithium salt such as LiPF ₆ is added thereto. To manufacture. During charging, lithium ions of the positive electrode active material are released and inserted into the carbon layer of the negative electrode, and during discharge, lithium ions of the negative electrode carbon layer are released and inserted into the positive electrode active material, wherein the non-aqueous electrolyte solution is lithium ions between the negative electrode and the positive electrode. It acts as a medium for moving the.

Such a lithium secondary battery is manufactured through, for example, a battery cell assembly process in which an electrode assembly is mounted in a battery case and an electrolyte is injected and sealed. At this time, a predetermined amount of electrolyte is injected and primary charging (pre-formation) is performed to activate the battery without sealing the electrolyte inlet, and the gas generated in the first activation process is filled with the electrolyte inlet and unsealing. After discharge through the part, etc., electrolyte solution is injected and sealed again. Then, secondary charging (secondary activation) by full charge / full discharge and high temperature aging is performed.

In general, a lithium secondary battery repeatedly contracts and expands as charging and discharging are performed during use, which causes problems such as peeling of the electrode active material from the current collector or wrinkles in the current collector due to diffusion of stress. . As a result, charge and discharge efficiency and cycle characteristics are lowered.

As described above, wrinkles may be formed in the negative electrode current collector due to various causes. As a related factor, charge and discharge depth, active material thickness, and negative electrode current collector strength are considered, but these and other factors appear in combination. It is understood that.

On the other hand, a technique for minimizing the phenomenon that the negative electrode active material layer is peeled off from the current collector due to the diffusion of stress during repeated charging and discharging has not been developed yet. Therefore, there is a high need for a technology that can fundamentally solve these problems.

The present invention aims to solve the problems of the prior art as described above and the technical problems that have been requested from the past.

After various experiments and in-depth studies, the inventors of the present application inject the electrolyte during manufacturing of the battery, and before the electrolyte is impregnated to the center portion of the electrode assembly, the initial charging for activation of the battery cell (Pre-Formation) ), It was confirmed that it is possible to minimize the phenomenon that the stress spread due to the expansion and contraction of the electrode mixture layer during repeated charging and discharging in the process of using the secondary battery, to complete the present invention.

Therefore, the method of manufacturing a lithium secondary battery according to the present invention,

After preparing an electrode by applying an electrode mixture including an active material and a binder to a current collector, and preparing an electrode assembly through a separator (i), inserting the electrode assembly into a battery case to produce a battery cell and the electrolyte solution A method of manufacturing a lithium secondary battery comprising the step of injecting (ii) and performing initial charging (iii) for activation of a battery cell (iii),

In the step (ii), before the electrolyte is impregnated to the center of the electrode assembly in order to minimize the spread of stress due to expansion and contraction of the electrode mixture layer during repeated charging and discharging during the use of the secondary battery. In this case, the activation charging of step (iii) is performed.

As described above, the conventional secondary battery has a problem in that partially aggregated portions are formed while the negative electrode active materials expand in the width direction of the negative electrode sheet due to the diffusion of stress during repeated charging and discharging, thereby degrading battery performance. The aggregation site of the negative electrode active material is concentrated between the areas where the electrode tabs are attached to the electrode sheet, because the stress is concentrated while the balance of the stress is broken at the site where the electrode tabs are formed. Therefore, the aggregation site of the negative electrode active material caused between the electrode tabs gradually spreads in the width direction, that is, the lower portion of the electrode sheet.

Thus, the inventors of the present application after various studies to minimize this phenomenon, when the active charge is performed before the electrolyte is impregnated to the center of the electrode assembly in the manufacturing process of the battery, the electrode mixture layer during repeated charge and discharge It was confirmed for the first time that the phenomenon of stress spreading due to expansion and contraction of the can be minimized. That is, in the secondary battery manufactured by the method according to the present invention, as can be specifically confirmed in the following examples, the aggregation region of the negative electrode active material is formed only to the center portion of the electrode assembly, and no longer diffused.

Conventionally, only techniques for sufficiently impregnating the electrolyte at the stage before the active charge of the battery have been studied, and there is no technique for performing the activation charge before fully impregnating the electrolyte as in the present invention.

The step (i) is a process of preparing an electrode by applying an electrode mixture including an active material and a binder to a current collector, and then manufacturing an electrode assembly through a separator, which is not particularly limited and is known in the art. It is possible to follow. The electrode assembly is not particularly limited as long as it is a structure consisting of a cathode and an anode, and a separator interposed therebetween, for example, a jelly-roll type, a stack type, or a stack / fold type structure.

Among them, in the case of a jelly-roll type electrode assembly, for example, an electrode mixture including an active material and a binder is applied to a sheet-shaped current collector having a long length and pressed to prepare a positive electrode and a negative electrode, and therebetween. It is manufactured by winding in a spiral after the separation membrane in the.

Such a jelly-roll type electrode assembly causes excessive stresses in the curved surface of the electrode due to a small radius of rotation of the central portion of the spiral, which causes a decrease in battery life and threatens battery safety. That is, the jelly-roll type electrode assembly has a problem that the stress due to the expansion and contraction of the electrode mixture layer is very large when repeated charging and discharging of the secondary battery, it can be preferably applied to the present invention that solves such problems of stress concentration and diffusion have.

As described above, the present invention is characterized in that the step (iii) is performed before the electrolyte solution injected in step (ii) is impregnated to the central portion of the electrode assembly.

As such, it may be advantageous to reduce the leaving time for impregnation of the electrolyte so as not to be impregnated to the central portion of the electrode assembly. In more detail, in general, a lithium secondary battery is manufactured by injecting an electrolyte into a battery cell, and then leaving the electrode assembly for a predetermined time (about 0.5 to 3 hours) so that the electrolyte is centered from the top and bottom of the electrode assembly. Impregnation into the area is followed by an activation process. That is, the manufacture of the secondary battery strictly includes a step (ii-1) for impregnation of the electrolyte between the electrolyte injection step (ii) and the activation step (iii). Therefore, if the leaving time for impregnation of the electrolyte in step (ii-1) is reduced, the electrolyte may not be impregnated to the central portion of the electrode assembly.

As such, when the electrolyte solution is not sufficiently impregnated to the center portion of the electrode assembly, the electrolyte impregnation rate is less than 100%, preferably 50 to 90%.

Step (iii) is a process of performing initial charging (Pre-Formation) for the activation of the battery cell. In this regard, the broad 'activation' process is charging performed in the process of manufacturing a battery, and means a process of supplying electrical energy to a lithium secondary battery and converting it into electrochemical energy. This activation process may be divided into a primary activation process and a secondary activation process, and the 'primary activation' process is a charging step performed for the first time in the battery manufacturing process and corresponds to the consultation process. The 'secondary activation' process is a charging and discharging process performed in the last step in the battery manufacturing process, and proceeds by the method of full charge, full discharge and high temperature aging (aging: Aging).

Therefore, step (iii) in the present invention corresponds to the first activation process. Meanwhile, in order to distinguish between the primary activation process and the secondary activation process, the secondary activation process is hereinafter referred to as an 'aging step'.

The inventors of the present application, as a result of performing the activation charge of the step (iii) in the state that the electrolyte is not sufficiently impregnated to the center portion of the electrode assembly, the negative electrode active material is deposited in the longitudinal direction (length direction) on the center portion of the negative electrode current collector It was confirmed that a deposited band (hereinafter, abbreviated as 'deposited band') was formed. Since the aggregation site of the negative electrode active material is hardly formed below the site where the deposition band is formed, it is presumed that blocking of the diffusion of the aggregation site of the negative electrode active material by the diffusion of the stress is blocked.

The process by which the deposition band is formed is not clear, but it is presumed to be based on the following principle.

That is, in the secondary battery, when electrons are charged and moved to the negative electrode through the wire during charging, lithium ions are intercalated in the negative electrode to achieve charge neutrality. At this time, the lithium ions can be occluded in the region in which the electrolyte is impregnated, that is, in the region in which the movement path of the ions is maintained, but in the electrolyte non-wetting region, occlusion becomes relatively difficult. Therefore, since the excess electrons are distributed in the electrolyte non-impregnation portion, the polarity of the cathode is instantaneously, and the neutral electrolyte impregnation portion is relatively polar in the anode.

In other words, when charging is performed in a state where the electrolyte solution is not sufficiently impregnated to the center portion of the electrode assembly, excess electrons are distributed on the surface of the negative electrode active material at the center portion of the electrode assembly, which is the electrolyte non-impregnation portion, and thus the polarity of the negative electrode is obtained. The upper and lower portions of the electrode assembly, which is an electrolyte impregnation site, have a relatively positive polarity. As the polarity is induced, an agglomeration phenomenon occurs as a face to edge network (see JALewis, JACS., 83, [10] (2000)) is formed for neutralization. It is assumed that a deposition band is formed by this.

The deposition band is formed by depositing the negative electrode active material, and thus the same as or similar to the material of the negative electrode active material. Therefore, as the negative electrode active material of the lithium secondary battery, a carbon-based material, a silicon-based material, a tin-based material, a silicon-carbon-based material, etc. may be used, and preferably a carbon-based material may be used, and thus, the deposition band is preferably May consist of carbon.

The size of the deposition band may vary depending on the distribution, content, or activation charge conditions of the negative electrode active material, but effectively prevents stress from spreading, while reducing the width of the negative electrode current collector in terms of preventing unnecessary material waste and battery capacity reduction. It is preferably formed in a size of 0.1 to 10% on the basis.

The initial charging condition for activation in step (iii) may be performed according to conditions known in the art, and may be performed by supplying a low current to a relatively low charging condition. Specifically, it can be performed by charging at low charging conditions of a low speed charging method and / or a multistage charging method so that the battery can be stably charged without difficulty and the deposition band can be formed.

In one preferred embodiment, the activated charging is preferably carried out at a charging condition of 1/30 to 1 / 2C at a 0.05 to 1.5C rate, more preferably of 1/25 to 1 / 5C at a 0.1 to 1C rate Can be performed under charging conditions.

Since a large amount of gas is generated in the activation process of step (iii), after discharging the gas, the electrolyte is injected again and then sealed. In this process, the electrolyte must be sufficiently impregnated to the center of the electrode assembly.

On the other hand, the method of manufacturing a secondary battery according to the present invention optionally, the step (iv) to activate the battery by repeating charging and discharging after the activation in step (iii), and full charge of the battery, It may further include an aging step (v) for finally charging and discharging for full discharge and high temperature aging.

The chemical conversion step (iv) is to activate the battery by repeating charging and discharging after assembling the battery, wherein lithium ions derived from the lithium transition metal oxide used as the anode during charging are moved and inserted into the carbon electrode used as the cathode. Lithium is highly reactive and reacts with the carbon anode to form compounds such as Li ₂ CO ₃ , LiO, LiOH, and these form a solid electrolyte interface (SEI) film on the surface of the cathode.

In addition, the aging step (v) is a process of stabilizing by leaving the activated battery as described above for a certain period. The charging and discharging of the aging step is carried out for the purpose of checking the capacity of the battery and sorting out defective products together with activation of the battery cell, and the high temperature aging is performed for the purpose of substantially sorting out defective products. For example, the high temperature aging process maintains a battery cell at a temperature of 50 to 60 ° C. for 24 hours, and then checks the voltage of the battery cell. Check for defects. That is, when a minute short is present in the manufacturing process of the battery, the battery cell is brought to a high temperature state and the defect can be confirmed within a short time by promoting the progress of the minute short.

The present invention also relates to a jelly-roll type electrode assembly manufactured by winding a laminated sheet of a cathode / separator / cathode structure, wherein, by the above production method, a negative electrode active material deposition band for preventing stress diffusion in the longitudinal direction at the center of the current collector of the negative electrode sheet It provides an electrode assembly and a secondary battery comprising the same, characterized in that is formed.

The electrode assembly may be blocked from spreading the stress in the longitudinal direction of the electrode assembly by the deposition band formed on the negative electrode sheet. Therefore, it is possible to prevent performance degradation of the battery due to the diffusion of stress during repeated charging and discharging during the use of the battery.

In one preferred example, the deposition band may be made of carbon. The deposition band may be formed by performing an activation process before the electrolyte is impregnated to the central portion of the electrode assembly as described above, but is not limited thereto. Details of the configuration and assembly of the electrode assembly is known to those skilled in the art, so a detailed description thereof will be omitted herein.

The secondary battery according to the present invention may be preferably a lithium secondary battery.

The lithium secondary battery may be variously applied regardless of the type and appearance of the battery cell. Since other components, such as an electrode assembly constituting the secondary battery, are known in the art, a detailed description thereof will be omitted herein.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

1-1. Manufacture of anode

95% by weight of LiCoO ₂ , 2.5% by weight of Denka Black (conductor), and 2.5% by weight of PVdF (binder) were added to NMP (N-methyl-2-pyrrolidone) as a positive electrode active material to prepare a positive electrode mixture slurry. A positive electrode was prepared by coating, drying and pressing on a long sheet aluminum foil.

1-2. Preparation of Cathode

As the negative electrode active material, 98% by weight of artificial graphite and 2% by weight of SBR (binder) were added to NMP as a solvent to prepare a negative electrode mixture slurry, and then coated, dried, and pressed on a long sheet copper foil to prepare a negative electrode.

1-3. Manufacture of batteries

The positive electrode and the negative electrode of 1-1 and 1-2 were interposed between the porous polyethylene separator (Celgard ^TM ) and then wound to prepare a jelly-roll type electrode assembly. It was inserted into a square can and then injected with 1M LiPF ₆ EC / EMC electrolyte. At about 1 hour after the injection of the electrolyte, a primary charge for activating the battery cell was performed under the condition of 1 / 12C to manufacture a lithium secondary battery.

Comparative Example 1

5 hours after the injection of the electrolyte, a lithium secondary battery was manufactured in the same manner as in Example 1 except that primary charging was performed for activation of the battery cell.

Experimental Example 1

Each battery prepared in Example 1 and Comparative Example 1 was charged to 4.2V at 1C 50mA (cut off), 1C discharge was performed to 3V (cut off), and 500 cycles were repeated at room temperature.

The battery was disassembled and the negative electrode sheet was separated to observe the aggregation phenomenon of the negative electrode active material, and the results are shown in FIGS. 1 to 3.

First, FIG. 1 shows a negative electrode sheet obtained by decomposing a battery according to Comparative Example 1, and agglomeration sites of the negative electrode active material are shown in a wrinkled form in the width direction (vertical) of the negative electrode sheet. On the other hand, in the battery manufactured by the method of Example 1 according to the present invention, it can be seen that the aggregation site of the negative electrode active material is formed only up to approximately the center site, as shown in FIG.

In addition, as a result of peeling the negative electrode active material from the negative electrode sheet before performing a plurality of charge and discharge in the battery of Example 1, as shown in Figure 3, the deposition band of the carbon material (see red mark) in the center portion of the negative electrode sheet It can be confirmed that it is formed. This is performed by the initial activation process in the manufacture of the battery before the electrolyte is impregnated to the center portion of the electrode assembly, so that a deposition band is formed at the center portion of the negative electrode sheet. It is assumed that the stress diffusion can be blocked as in FIG.

Although described above with reference to embodiments of the present invention, those skilled in the art will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

As described above, in the method of manufacturing a secondary battery according to the present invention, before the electrolyte is impregnated to the center portion of the electrode assembly, by performing initial charging for activation of the battery cell, the negative electrode active material is placed on the center portion of the negative electrode sheet. Since the deposition band is formed and the stress is not diffused under the deposition band, the phenomenon in which the stress is diffused due to the expansion and contraction of the electrode mixture layer during charge and discharge may be prevented. Therefore, there is an advantage that it is possible to minimize the degradation of the battery performance, such as the increase in internal resistance and cycle characteristics thereby.

1 is an exploded photograph of a negative electrode sheet in a secondary battery according to Comparative Example 1;

2 is an exploded photograph of a negative electrode sheet in a secondary battery according to Example 1;

3 is a photograph of a negative electrode active material layer removed from a negative electrode sheet before performing a plurality of charge and discharge in the secondary battery according to Example 1.

Claims (10)

After preparing an electrode by applying an electrode mixture including an active material and a binder to a current collector, and preparing an electrode assembly through a separator (i), inserting the electrode assembly into a battery case to produce a battery cell and the electrolyte solution A method of manufacturing a lithium secondary battery comprising the step of injecting (ii) and performing initial charging for pre-formation of a battery cell (iii), Before the impregnation of the electrolyte in the step (ii) to the central portion of the electrode assembly in order to minimize the spread of the stress due to expansion and contraction of the electrode mixture layer during repeated charging and discharging during the use of the secondary battery, Perform the activation charge of step (iii), By the activation charge of the step (iii), the lithium secondary battery, characterized in that the band ('deposited band') in which the negative electrode active material is deposited in the longitudinal direction (lengthwise direction) is formed in the central portion of the negative electrode current collector Manufacturing method. The method of claim 1, wherein the electrode assembly is a jelly-roll type electrode assembly. The method of claim 1, wherein the activation charging of step (iii) is performed under a condition that an electrolyte impregnation rate of the electrode assembly is 50 to 90%. delete The method of claim 1, wherein the deposition band is made of carbon. The method of claim 1, wherein the deposition band is formed to a size of 0.1 to 10% based on the width of the negative electrode current collector. The method of claim 1, wherein the activation charging is performed at a condition of 1/20 to 1 / 5C. A secondary battery manufactured by the manufacturing method according to claim 1, comprising a jelly-roll structure manufactured by winding a laminated sheet of a cathode / separation membrane / cathode structure, and preventing stress diffusion in the longitudinal direction of the current collector central portion of the anode sheet. A secondary battery comprising an electrode assembly having a negative electrode active material deposition band formed thereon. The secondary battery of claim 8, wherein the negative electrode active material deposition band is made of carbon. The secondary battery of claim 8, wherein the battery is a lithium secondary battery.

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