Lithium secondary battery manufacturing method

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery, which includes a first step of preparing a battery case, a second step of assembling a battery cell by placing an electrode assembly in the battery case and injecting an electrolyte so that the mass of the electrolyte per unit capacity is a (g / Ah), a third step of activating the battery cell, and a fourth step of pre-charging and pre-discharging the activated battery cell b times, and satisfies the following formula (1): Formula (1): 15≦486.77-373.09×e (-0.006b) ×a 0.29 ≦30 In the formula (1), a is an integer of 2.0 to 3.0, and b is an integer of 0 to 3.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a lithium secondary battery, and more particularly to a method for manufacturing a lithium secondary battery having excellent impact resistance. [Background technology]

[0002] Lithium secondary batteries are generally manufactured by coating a positive electrode active material slurry on a positive electrode current collector and a negative electrode current collector to form a positive electrode and a negative electrode, laminating these on both sides of a separator to form an electrode assembly of a predetermined shape, and then placing the electrode assembly in a pouch and injecting an electrolyte solution.

[0003] Secondary batteries are divided into pouch types and can types depending on the material of the case that houses the electrode assembly. Pouch-type secondary batteries are manufactured by pressing a flexible pouch film laminate to form a cup, placing an electrode assembly in the cup, injecting electrolyte, and then sealing the seal. Can-type secondary batteries are manufactured by placing an electrode assembly in a metal can, injecting electrolyte, and then assembling a top cap on the top of the can to seal it.

[0004] Pouch-type secondary batteries have the advantages of being lightweight, having excellent space utilization, and being able to achieve high energy density using a stacked electrode assembly, but they have the disadvantage of being more vulnerable to external impacts than can-type secondary batteries.Recently, as the environments in which secondary batteries are used have become more diverse, there has been a demand for them to have excellent safety even in harsh environments, and therefore there has been a demand for improved impact resistance for lithium secondary batteries. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention is intended to solve the above problems, and relates to a method for manufacturing a lithium secondary battery in which the frictional force between a battery case and an electrode assembly is increased by ensuring that the amount of electrolyte injected and the number of pre-charge / discharge cycles satisfy a specific relationship during battery manufacturing. [Means for solving the problem]

[0006] In one aspect, the present invention provides a method for manufacturing a lithium secondary battery, including a first step of preparing a battery case; a second step of assembling a battery cell by placing an electrode assembly in the battery case and injecting an electrolyte so that the mass of the electrolyte per unit capacity is a (g / Ah); a third step of activating the battery cell; and a fourth step of pre-charging and pre-discharging the activated battery cell b times, wherein the method satisfies the following formula (1): Formula (1): 15≦486.77-373.09×e (-0.006b) ×a 0.29 ≦30 In the formula (1), a is an integer of 2.0 to 3.0, preferably 2.0 to 2.5, and b is an integer of 0 to 3.

[0007] Preferably, the method for producing a lithium secondary battery satisfies the following formula (1-1): Formula (1-1): 15≦486.77-373.09×e (-0.006b) ×a 0.29 ≦27 In the formula (1-1), a and b are the same as in the formula (1).

[0008] Meanwhile, in the fourth step, the pre-charge / discharge may be performed at a C-rate of 0.1C to 1C in a range of SOC 0 to 99.

[0009] Preferably, the pre-charge / discharge is performed in a voltage range of 2.50V to 4.35V.

[0010] Meanwhile, the third step of activating the battery cells may include charging and discharging the battery cells.

[0011] Meanwhile, in the present invention, the battery case may be a pouch including a barrier layer, a substrate layer formed on one side of the barrier layer, and a sealant layer formed on the other side of the barrier layer, and including at least one cup portion recessed in one direction.

[0012] The electrode assembly may have a ratio of total length to total width of 5 to 10, for example, a total length of 400 mm to 600 mm and a total width of 50 to 150 mm.

[0013] In the lithium secondary battery of the present invention produced by the above-described method, the friction force between the inner surface of the battery case and the electrode assembly can be 15 kgf or more, preferably 15 kgf to 30 kgf.

[0014] Furthermore, the lithium secondary battery of the present invention manufactured by the above method does not leak electrolyte when subjected to a crash shock test under the crash condition of 133.7G x 15.8ms, i.e., the amount of electrolyte leakage is 0.

[0015] The lithium secondary battery may have a rated capacity of 50 Ah to 200 Ah. [Effects of the Invention]

[0016] When a battery is manufactured so that the amount of electrolyte injected and the number of pre-charge / discharge cycles satisfy a specific relationship as in the present invention, the frictional force between the electrode assembly and the inner surface of the battery case is significantly increased compared to conventional methods. As a result, separation of the electrode assembly and / or leakage of the electrolyte upon external impact is suppressed, thereby significantly improving the impact resistance of the lithium secondary battery.

[0017] The lithium secondary battery of the present invention manufactured by the above method has a high friction force of 15 kgf or more between the electrode assembly and the battery case. Therefore, when a crash shock test is conducted under the impact condition of 133.7 G x 15.8 ms, no electrolyte leakage due to damage to the battery case occurs. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a flowchart showing a method for manufacturing a lithium secondary battery according to the present invention. [Figure 2] 1 is an exploded perspective view of a secondary battery according to an embodiment of the present invention; [Figure 3] 1 is a cross-sectional view of a pouch according to one embodiment of the present invention. [Figure 4] FIG. 1 is a graph showing the results of measuring the frictional forces of the secondary batteries manufactured by the methods of Examples 1 to 4. [Figure 5] FIG. 1 is a diagram showing the results of measuring the frictional forces of the secondary batteries manufactured by the methods of Comparative Examples 1 and 2 and Examples 5 and 6. DETAILED DESCRIPTION OF THE INVENTION

[0019] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0020] The present invention will now be described in more detail with reference to specific embodiments.

[0021] Recently, with the increasing demand for high-capacity batteries, such as those for electric vehicles, the rated capacity of secondary battery cells has increased, and the weight of electrode assemblies has also tended to increase. However, as the weight of the electrode assembly increases, the electrode assembly is more likely to move and damage or penetrate the battery case when subjected to external impact. Such damage to the pouch can lead to electrolyte leakage or deformation of the electrode assembly, causing serious problems in the performance and safety of the battery.

[0022] As a result of extensive research to solve these problems, the inventors have found that the amount of electrolyte (a) injected and the number of pre-charge / discharge cycles (b) are closely related to the frictional force between the battery case and the electrode assembly. Specifically, the inventors discovered that when a and b satisfy a specific relationship, the frictional force between the electrode assembly and the battery case is significantly increased compared to conventional methods, thereby suppressing damage to the battery case due to separation of the electrode assembly when an external impact is applied, and achieving excellent impact resistance, which led to the completion of the present invention.

[0023] Specifically, the method for manufacturing a lithium secondary battery according to the present invention includes a first step of preparing a battery case, a second step of assembling a battery cell by placing an electrode assembly in the battery case and injecting an electrolyte so that the mass of the electrolyte per unit capacity is a (g / Ah), a third step of activating the battery cell, and a fourth step of pre-charging and pre-discharging the activated battery cell b times, wherein a and b satisfy the following formula (1), preferably the following formula (1-1):

[0024] Formula (1): 15≦486.77-373.09×e (-0.006b) ×a 0.29 ≦30

[0025] Formula (1-1): 15≦486.77-373.09×e (-0.006b) ×a 0.29 ≦27

[0026] In the formula (1) and formula (1-1), a is an integer of 2.0 to 3.0, and b is an integer of 0 to 3.

[0027] When the amount of electrolyte injected per unit capacity (a) and the number of pre-charge / discharge cycles (b) satisfy the relationship of the above formula (1) or formula (1-1) during battery manufacturing, the frictional force between the battery case and the electrode assembly increases significantly to 15 kgf or more, resulting in a significant improvement in impact resistance.

[0028] Fig. 1 illustrates a method for manufacturing a lithium secondary battery according to the present invention, Fig. 2 illustrates a perspective view of a secondary battery according to an embodiment of the present invention, and Fig. 3 illustrates a cross section of a battery case (pouch). Hereinafter, the present invention will be described in more detail with reference to Figs. 1 to 3.

[0029] <Method of manufacturing lithium secondary batteries> First, a method for producing a lithium secondary battery according to the present invention will be described.

[0030] (Step 1: Battery case preparation step) First, a battery case for accommodating an electrode assembly and an electrolyte is prepared (S1). Here, the battery case is preferably a pouch, but is not limited thereto, and may be a prismatic or cylindrical battery case.

[0031] Referring to Figures 2 and 3, the battery case may be a pouch 100 including a barrier layer 20, a substrate layer 10 formed on one side of the barrier layer, and a sealant layer 30 formed on the other side of the barrier layer, and including at least one cup portion 110 recessed in one direction.

[0032] Specifically, the pouch 100 can be manufactured by inserting a flexible pouch film laminate, in which a base layer 10, a barrier layer 20, and a sealant layer 30 are laminated in that order, into a press molding device, and applying pressure to a portion of the pouch film laminate with a punch to stretch it, thereby forming a cup portion having a recessed shape in one direction.

[0033] The base layer 10 is disposed on the outermost layer of the pouch to protect the electrode assembly from external impact and to provide electrical insulation.

[0034] The substrate layer 10 may be made of a polymer material, for example, one or more polymer materials selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon.

[0035] The substrate layer 10 may have a single layer structure, or may have a multi-layer structure in which different polymer films 12 and 14 are laminated, as shown in Fig. 3. When the substrate layer 10 has a multi-layer structure, an adhesive layer 16a may be interposed between the polymer films.

[0036] Meanwhile, the substrate layer 10 may have a total thickness of 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the substrate layer has a multilayer structure, the thickness includes the thickness of the adhesive layer. When the substrate layer 10 satisfies the above range, it exhibits excellent durability, insulating properties, and moldability. If the thickness of the substrate layer is too thin, durability may decrease and the substrate layer may be damaged during the molding process. If the thickness of the substrate layer is too thick, moldability may decrease, the total thickness of the pouch may increase, the battery storage space may decrease, and the energy density may decrease.

[0037] According to one embodiment, the base layer 10 may have a laminated structure of a polyethylene terephthalate (PET) film and a nylon film. Preferably, the nylon film is disposed on the barrier layer 20 side, i.e., the inner side, and the polyethylene terephthalate film is disposed on the outer surface side of the pouch.

[0038] Polyethylene terephthalate (PET) has excellent durability and electrical insulation properties, and when a PET film is placed on the surface side, it also has excellent durability and insulation properties. However, PET film has weak adhesion to the aluminum alloy thin film that constitutes the barrier layer 20 and its stretching behavior is different. Therefore, when a PET film is placed on the barrier layer side, peeling between the substrate layer and the barrier layer can occur during the molding process, resulting in an uneven stretching of the barrier layer and reduced formability. In contrast, nylon film has a similar stretching behavior to the aluminum alloy thin film that constitutes the barrier layer 20, so when a nylon film is placed between the polyethylene terephthalate and the barrier layer, it can improve formability.

[0039] The polyethylene terephthalate film may have a thickness of 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 7 μm to 15 μm, and the nylon film may have a thickness of 10 μm to 40 μm, preferably 10 μm to 35 μm, and more preferably 15 μm to 25 μm. When the thicknesses of the polyethylene terephthalate film and the nylon film satisfy the above ranges, excellent formability and rigidity after forming are achieved.

[0040] The barrier layer 20 is intended to ensure the mechanical strength of the pouch 100, to block the entry and exit of gas or moisture from the exterior of the secondary battery, and to prevent leakage of the electrolyte.

[0041] The barrier layer 20 may have a thickness of 40 μm to 100 μm, more preferably 50 μm to 80 μm, and even more preferably 60 μm to 80 μm. When the thickness of the barrier layer satisfies the above range, moldability is improved, the molding depth of the cup portion is increased, and even during two-cup molding, cracks and / or pinholes are reduced, and resistance to external stress after molding is improved.

[0042] Meanwhile, the barrier layer 20 may be made of a metal material, specifically, an aluminum alloy thin film.

[0043] The aluminum alloy thin film may contain aluminum and one or more metal elements other than aluminum, such as iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0044] Preferably, the aluminum alloy thin film has an iron (Fe) content of 1.2 wt% to 1.7 wt%, preferably 1.3 wt% to 1.7 wt%, more preferably 1.3 wt% to 1.45 wt%. When the iron (Fe) content in the aluminum alloy thin film satisfies this range, it is possible to minimize the occurrence of cracks and pinholes even when the cup portion is formed deep.

[0045] The sealant layer 30 is bonded by thermocompression to seal the pouch, and is located as the innermost layer of the pouch film laminate 1.

[0046] Since the sealant layer 30 is the surface that comes into contact with the electrolyte and the electrode assembly after the pouch is formed, it must have insulating and corrosion-resistant properties, and must completely seal the interior to prevent the movement of substances between the inside and the outside, so it must have high sealing properties.

[0047] The sealant layer 30 may be made of a polymer material, for example, one or more selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon. Among these, it is particularly preferable to use polypropylene (PP), which has excellent mechanical properties such as tensile strength, rigidity, surface hardness, abrasion resistance, and heat resistance, and chemical properties such as corrosion resistance.

[0048] More specifically, the sealant layer 30 may include polypropylene, cast polypropylene (CPP), acid modified polypropylene, polypropylene-butylene-ethylene copolymer, or a combination thereof.

[0049] The sealant layer 30 may have a single layer structure or a multi-layer structure including two or more layers made of different polymer materials.

[0050] The sealant layer may have a total thickness of 60 μm to 100 μm, preferably 60 μm to 90 μm, and more preferably 70 μm to 90 μm. If the sealant layer is too thin, the seal durability and insulation properties may decrease, while if it is too thick, the flexibility may decrease, the total thickness of the pouch film laminate may increase, and the energy density per volume may decrease.

[0051] Meanwhile, the pouch film laminate may be manufactured by a method known in the art for manufacturing a pouch film laminate, for example, by attaching the substrate layer 10 to the upper surface of the barrier layer 20 with an adhesive, and forming the sealant layer 30 on the lower surface of the barrier layer 20 by coextrusion or adhesive, but is not limited thereto.

[0052] The pouch film laminate described above is inserted into a molding device, and pressure is applied to a portion of the pouch film laminate with a punch to form a cup portion, thereby producing a pouch 100. Here, the pressure may be about 0.3 MPa to 1 MPa, preferably 0.3 MPa to 0.8 MPa, and more preferably 0.4 MPa to 0.6 MPa. If the pressure is too low during the formation of the cup portion, excessive drawing may occur, resulting in wrinkles, while if the pressure is too high, the drawing process may not be performed well, resulting in a reduced molding depth.

[0053] Meanwhile, the moving speed of the punch may be 20 mm / min to 80 mm / min, preferably 30 mm / min to 70 mm / min, and more preferably 40 mm / min to 60 mm / min. If the pressure is too low or the moving speed of the punch is too fast during molding, wrinkles may occur due to buckling. If the pressure is too high or the moving speed of the punch is too slow during molding, stress may be concentrated at the corners of the cup portion during molding, which may increase the occurrence of pinholes and cracks.

[0054] The pouch 100 manufactured by the above method includes a lower case 101, an upper case 102, and a folding part 130 connecting the lower cases, and the upper case and / or the lower case includes a cup part 110 having a recessed shape in one direction.

[0055] Specifically, the pouch 100 may be a one-cup type in which a cup portion 110 is formed only in the lower case 101 as shown in Fig. 2, but is not limited thereto and may be a two-cup type in which cup portions are formed in both the upper and lower cases. In the case of a two-cup type pouch, after the electrode assembly and electrolyte are accommodated, the upper case is folded so that the cup portion of the upper case and the cup portion of the lower case face each other. This allows for the accommodation of a thicker electrode assembly than a one-cup type pouch, which is advantageous in achieving a high energy density.

[0056] The cup part 110 has an accommodating space for accommodating the electrode assembly 200. Meanwhile, the pouch 100 may include a terrace 120 around the periphery of the cup part 110. The terrace 120 refers to a portion not formed by the pouch film laminate, i.e., the remaining area other than the cup part 110. The terrace 129 is a portion that is sealed by thermal bonding in a sealing process after accommodating the electrode assembly 200 in the cup part 110 and injecting an electrolyte.

[0057] The cup portion 110 may include a bottom surface and a peripheral surface. The peripheral surface may connect the bottom surface to the terrace 120. There may be a plurality of peripheral surfaces, more specifically, four peripheral surfaces. The bottom surface may cover one side of the electrode assembly 200, and the peripheral surface may surround the periphery of the electrode assembly 200.

[0058] Meanwhile, the folding part 130 connects the lower case 101 and the upper case 102, and after the electrode assembly 200 is accommodated in the cup part 110 and the electrolyte is injected, the folding part 130 is folded so that the upper case 102 can seal the cup part 110 of the lower case 101. When the folding part 130 is included, the lower case 101 and the upper case 102 are connected integrally, which reduces the number of sides to be sealed during a subsequent sealing process, thereby improving processability.

[0059] The folding portion 130 is formed to be spaced apart from the cup portion 110, and the distance between the folding portion 130 and the cup portion 110 may be about 0.5 mm to 3 mm, and preferably about 0.5 mm to 2 mm. If the folding portion 130 is formed too close to the cup portion 110, folding may not be performed smoothly, and if the folding portion 130 is formed too far from the cup portion 110, the total volume of the secondary battery may increase, and the energy density per volume may decrease. In the case of a two-cup case, the folding portion may be formed to satisfy the above-mentioned distance from each cup portion.

[0060] (2nd step: battery cell assembly step) Next, after the battery case is prepared, an electrode assembly is placed in the battery case, and an electrolyte is injected to assemble a battery cell (S2).

[0061] When the battery case is a pouch, the electrode assembly 200 may be placed in the cup portion 110 of the pouch and an electrolyte (not shown) may be injected to manufacture a battery cell. Specifically, the electrode assembly may be placed in the cup portion of the pouch, the electrolyte may be injected, the pouch may be folded to bring the upper case and the lower case into contact, and heat may be applied to seal the sealant layer to manufacture a battery cell. Here, a separate gas pocket space may be formed in the terrace 120.

[0062] The electrode assembly 200 may include a plurality of electrodes and a plurality of separators stacked alternately, and the plurality of electrodes may include positive and negative electrodes having opposite polarities, stacked alternately with separators interposed therebetween.

[0063] 2, the electrode assembly 200 may include a plurality of electrode tabs 230 welded to one another. The plurality of electrode tabs 230 may be connected to the plurality of electrodes 210 and may protrude from the electrode assembly 200 to serve as a path for electrons to move between the inside and outside of the electrode assembly 200. The plurality of electrode tabs 230 may be located inside the pouch 100.

[0064] The electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode may protrude in different directions from each other with respect to the electrode assembly 200. However, without being limited thereto, the electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode may protrude in the same direction in parallel to each other.

[0065] Leads 240 for supplying electricity from outside the secondary battery may be connected to the electrode tabs 230 by spot welding, etc. One end of the lead 240 may be connected to the electrode tabs 230, and the other end may protrude outside the pouch 100.

[0066] A portion of the lead 240 may be surrounded by an insulating portion 250. For example, the insulating portion 250 may include insulating tape. The insulating portion 250 may be located between the terrace 120 of the first case 101 and the second case 102, and in this state, the terrace 120 and the second case 102 may be heat-sealed to each other. In this case, portions of the terrace 120 and the second case 102 may be heat-sealed to the insulating portion 250. Therefore, the insulating portion 250 prevents electricity generated from the electrode assembly 200 from flowing to the pouch 100 via the lead 240, thereby maintaining the seal of the pouch 100.

[0067] Meanwhile, in the present invention, the ratio of the total length W1 to the total width W2 of the electrode assembly 200 may be 5 to 10, preferably 5 to 8. When the ratio of the total length to the total width satisfies this range, it is possible to obtain an effect of realizing high energy density in a limited space.

[0068] For example, the electrode assembly may have a total length of 400 mm to 600 mm and a total width of 50 to 150 mm, preferably a total length of 500 mm to 600 mm and a total width of 50 to 100 mm.

[0069] Next, the electrolyte is used to transfer lithium ions generated by electrochemical reactions at the electrodes during charging and discharging of the secondary battery, and may include an organic solvent and a lithium salt.

[0070] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0071] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(CF, SO), LiN(CF, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt concentration is preferably within a range of 0.1 to 5.0 M, and more preferably 0.1 to 3.0 M. When the lithium salt concentration is within this range, the electrolyte exhibits appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0072] In addition to the constituent components of the electrolyte, the electrolyte may further contain additives for the purposes of improving the life characteristics of the battery, suppressing a decrease in the capacity of the battery, and improving the discharge capacity of the battery.

[0073] Meanwhile, in the present invention, the electrolyte is injected so that the amount of electrolyte per unit capacity of the secondary battery is a (g / Ah), where a can be 2.0 to 3.0, preferably 2.0 to 2.5, and more preferably 2.1 to 2.4. The amount of electrolyte per unit capacity refers to the ratio of the amount of secondary electrolyte injected (unit: g) to the rated capacity (unit: Ah) of the secondary battery. When the amount of electrolyte per unit capacity satisfies the above range, the electrochemical properties of the secondary battery are not deteriorated and excellent impact resistance can be achieved. If the amount of electrolyte is too small, there is a possibility that the electrolyte solution will be insufficient during battery operation, resulting in a deterioration in battery performance, and if the amount of electrolyte is too large, the impact resistance is not significantly improved.

[0074] (Third step: Activation step) After the battery cells are assembled, an activation process is performed (S3).

[0075] The activation process is intended to activate the assembled battery cell to impart electrical properties and stabilize the electrodes and electrolyte, and may be performed by a common activation method known in the art. For example, the activation process may be performed by charging and discharging the battery cell, and may include an aging process and / or a degassing process, as necessary.

[0076] The charge / discharge process is for charging and discharging the battery cell to impart electrical characteristics and form a solid electrolyte interphase (SEI) film on the surface of the electrode, and the charge / discharge conditions can be appropriately adjusted depending on the composition of the positive electrode, negative electrode, and electrolyte of the battery cell. For example, in the case of a battery using lithium nickel cobalt manganese oxide as the positive electrode active material and graphite as the negative electrode active material, charge / discharge can be performed at 20 to 60°C and in a voltage range of 2.0 V to 4.4 V.

[0077] The aging process is performed by storing the battery cell at a predetermined temperature and humidity for a predetermined period of time, so that the injected electrolyte is uniformly dispersed inside the cell and thoroughly permeates the positive and negative electrodes. For example, the aging process may be performed at 10°C to 70°C for 0 to 72 hours, but is not limited thereto.

[0078] Meanwhile, the order and number of times of the charge / discharge process and the aging process are not particularly limited and may be appropriately adjusted as needed. For example, the charge / discharge process and the aging process may each be independently performed one or more times, and the aging process may be performed before and / or after the charge / discharge process.

[0079] The degassing process is performed to remove gas from the battery cell. During the aging and charge / discharge processes, the electrolyte and electrodes chemically react to generate gas. If gas remains inside the battery cell, it may cause a decrease in electrochemical performance, so it is preferable to remove the gas from the battery cell. The degassing process may be performed using a common method used in the industry without any particular limitations. For example, the degassing process may be performed by forming a gas outlet in a portion of the pouch, discharging the gas in a decompression chamber, and then sealing the gas outlet by heat sealing, or by forming a gas pocket in a portion of the pouch, applying pressure to move the gas to the gas pocket, and then cutting and removing the gas pocket.

[0080] (4th step: Pre-charge / discharge step) Next, the activated battery cell is subjected to pre-charge and discharge b times (S4), where b is an integer between 0 and 3, preferably between 1 and 3. When b=0, this means that pre-charge and discharge are not performed.

[0081] According to the inventors' research, when pre-charging and discharging are performed, additional electrolyte is impregnated during the pre-charging and discharging process, reducing the amount of electrolyte at the interface between the electrode assembly and the battery case. This increases the friction between the battery case and the electrode assembly, thereby suppressing the electrode assembly from moving due to external impact. However, if the amount of electrolyte injected is insufficient or if the number of pre-charging and discharging cycles is too many, the remaining electrolyte may be too small, resulting in a deterioration in the electrochemical properties of the battery. Therefore, the pre-charging and discharging cycles must be performed an appropriate number of times, taking into account the amount of electrolyte injected. Specifically, the number of pre-charging and discharging cycles b must be performed to satisfy the above formula (1) or (1-1), and is preferably performed three or fewer times. If the number of pre-charging and discharging cycles exceeds three, the battery manufacturing time increases, reducing productivity and increasing manufacturing costs due to the need for multiple charging and discharging equipment. Furthermore, increasing the number of pre-charging and discharging cycles can lead to swelling and an increase in battery thickness.

[0082] The pre-charge / discharge may be performed at a C-rate of 0.1 C to 1 C, preferably 0.1 C to 0.5 C, and in a range of SOC 0 to 100, preferably SOC 0 to 99, and more preferably SOC 0 to 98.5. If the current rate during pre-charge / discharge is too fast, the battery degradation reaction may be accelerated, resulting in a decrease in battery performance, while if the SOC range is too narrow, the battery may not breathe sufficiently, resulting in a minimal increase in frictional force.

[0083] Meanwhile, the pre-charge / discharge may vary depending on the active material and electrolyte used, and may be performed within a voltage range of, for example, 2.50 V to 4.35 V. If the pre-charge / discharge voltage range deviates from the above range, deterioration of battery performance may occur due to overcharge or overdischarge.

[0084] When a lithium secondary battery is manufactured so that the amount of electrolyte injected per unit capacity, a, and the number of pre-charge / discharge cycles, b, satisfy the relationship of formula (1) or formula (1-1) as described above, the frictional force between the electrode assembly and the inner surface of the battery case (e.g., the bottom surface of the cup portion of the pouch) with which the electrode assembly is in contact increases significantly.

[0085] Specifically, the lithium secondary battery produced by the method of the present invention can have a frictional force between the inner surface of the battery case and the electrode assembly of 15 kgf or more, preferably 15 kgf to 30 kgf, and more preferably 17 kgf to 30 kgf. Here, the frictional force between the electrode assembly and the battery case was measured by the following method.

[0086] A portion of the battery case (pouch) of the secondary battery was cut open, and the welded portion between the negative electrode lead and the electrode assembly was cut. The positive electrode tab was then connected to a universal testing machine (UTM) and pulled at a speed of 100 mm / min, while measuring the force applied, which was evaluated as the friction force between the electrode assembly and the inner surface of the battery case.

[0087] As described above, the secondary battery according to the present invention has excellent impact resistance because the electrode assembly is less likely to separate from the battery case even when subjected to an external impact due to the high friction between the electrode assembly and the battery case. Specifically, the lithium secondary battery according to the present invention did not leak electrolyte when subjected to a crash shock test under impact conditions of 133.7 G x 15.8 ms (acceleration x sustained time).

[0088] The crash shock test can be performed by mounting a battery to be tested on a jig for drop shock testing, allowing the battery to be dropped from a specific height, and then determining whether the battery is damaged. Here, the height of the free fall is set in consideration of the crash conditions to be measured (acceleration x duration time). Specifically, the impact energy under the crash conditions to be measured is converted into potential energy, and the height at which the converted potential energy can be held is calculated in consideration of the weight of the battery to be tested, and the height of the free fall can be set. Meanwhile, whether the battery is damaged can be evaluated by checking whether the electrolyte is leaking.

[0089] The present invention will be described in more detail below with reference to specific examples.

[0090] Example 1 A pouch was prepared in which nylon, polyethylene terephthalate, an Al alloy thin film, and polypropylene were laminated in this order to form a cup portion. A stacked electrode assembly measuring 548 mm in length and 99 mm in width was placed in the cup portion, and electrolyte was injected so that the amount of electrolyte per unit capacity was 2.19 g / Ah. The pouch was then sealed to produce a battery cell. The battery cell was then charged and discharged at a temperature range of 10 to 70°C and a voltage range of 2.0 to 4.25 V, and then activated by a degassing process. No pre-charge or pre-discharge was performed.

[0091] Example 2 An activated battery cell was manufactured in the same manner as in Example 1, and then a pre-charge / discharge process was performed once, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0092] Example 3 An activated battery cell was manufactured in the same manner as in Example 1, and then the battery cell was subjected to two pre-charge / discharge processes, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0093] Example 4 An activated battery cell was manufactured in the same manner as in Example 1, and then the battery cell was subjected to a pre-charge / discharge process three times, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0094] (Comparative Example 1) A pouch was prepared in which nylon, polyethylene terephthalate, an Al alloy thin film, and polypropylene were laminated in this order to form a cup portion. A stacked electrode assembly measuring 548 mm in length and 99 mm in width was placed in the cup portion, and electrolyte was injected so that the amount of electrolyte per unit capacity was 2.30 g / Ah. The pouch was then sealed to produce a battery cell. The battery cell was then charged and discharged at a temperature range of 10 to 70°C and a voltage range of 2.0 to 4.25 V, and then activated by a degassing process. No pre-charge or pre-discharge was performed.

[0095] (Comparative Example 2) An activated battery cell was manufactured in the same manner as in Comparative Example 1, and then a pre-charge / discharge process was performed once, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0096] Example 5 An activated battery cell was manufactured in the same manner as in Comparative Example 1, and then the battery cell was subjected to two pre-charge / discharge processes, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0097] Example 6 An activated battery cell was manufactured in the same manner as in Comparative Example 1, and then the battery cell was subjected to a pre-charge / discharge process three times, in which the battery cell was charged to 4.25 V at 0.33 C in the SOC range of 0 to 98.3, and then discharged to 2.5 V at 0.33 C.

[0098] Table 1 below shows the amount of electrolyte injected (a), the number of pre-charge / discharge cycles (b) and the values ​​calculated by Equation 1 for Examples 1 to 6 and Comparative Examples 1 and 2.

[0099] [Table 1]

[0100] (Experimental Example 1: Friction Force Test) A portion of the pouch of each lithium secondary battery manufactured in Examples 1 to 6 and Comparative Examples 1 and 2 was cut open, and the welded portion between the negative electrode lead and the electrode assembly was cut. The positive electrode tab was then connected to a universal testing machine (UTM) and pulled at a speed of 100 mm / min to measure the force applied, which was used to evaluate the frictional force between the electrode assembly and the inner surface of the battery case (bottom of the cup). For accurate measurements, two or three secondary batteries were measured for each Example and Comparative Example. The measurement results are shown in Figures 4 and 5.

[0101] As shown in FIGS. 4 and 5, the lithium secondary batteries of Examples 1 to 6, in which the amount of electrolyte injected (a) and the number of pre-charge / discharge cycles (b) satisfy the condition of formula (1), are shown to have a frictional force between the electrode assembly and the bottom surface of the cup portion exceeding 15 kgf, whereas the lithium secondary batteries of Comparative Examples 1 and 2, in which the amount of electrolyte injected (a) and the number of pre-charge / discharge cycles (b) do not satisfy the condition of formula (1), are shown to have a frictional force between the electrode assembly and the bottom surface of the cup portion of 15 kgf or less.

[0102] (Experimental example 2: Collision shock test) A crash shock test was conducted on the pouch-type secondary batteries manufactured in Examples 1 to 6 and Comparative Examples 1 and 2 under the impact condition of 133.7 G x 15.8 ms. The measurement results are shown in Table 2 below. If no electrolyte leakage or electrode assembly separation occurred after the test, it was marked as Pass, and if electrolyte leakage and / or electrode assembly separation occurred, it was marked as Fail.

[0103] [Table 2]

[0104] As shown in Table 1, the lithium secondary batteries of Examples 1 to 6, in which the amount of electrolyte injected (a) and the number of pre-charge / discharge cycles (b) satisfied the condition of formula (1), exhibited a high frictional force between the electrode assembly and the bottom surface of the cup, thereby suppressing detachment of the electrode assembly due to external impact and exhibiting excellent impact resistance.In contrast, the lithium secondary batteries of Comparative Examples 1 and 2, in which the amount of electrolyte injected (a) and the number of pre-charge / discharge cycles (b) did not satisfy the condition of formula (1), exhibited a lower frictional force between the electrode assembly and the bottom surface of the cup compared to the batteries of the Examples, resulting in electrolyte leakage during the impact shock test. [Explanation of symbols]

[0105] 10 Base material layer 12 Polymer Film 14 Polymer Film 16a Adhesive layer 20 Barrier Layer 30 Sealant Layer 100 pouches 101 Lower case 102 Upper case 110 Cup section 120 Terrace 129 Terrace 130 Folding section 200 electrode assembly 230 Electrode Tab 240 leads 250 Insulation

Claims

1. A first step of preparing a battery case; a second step of assembling a battery cell by placing an electrode assembly in the battery case and injecting an electrolyte so that the mass of the electrolyte per unit capacity becomes a (g / Ah); a third step of activating the battery cell; a fourth step of pre-charging and pre-discharging the activated battery cell b times; A method for producing a lithium secondary battery, which satisfies the following formula (1): Equation (1): 15≦486.77-373.09×e （－０．００６ｂ） ×a ０．２９ ≤30 In the formula (1), a is an integer of 2.0 to 3.0, and b is an integer of 0 to 3.

2. The method for producing a lithium secondary battery according to claim 1, wherein the following formula (1-1) is satisfied: Equation (1-1): 15 ≤ 486.77 - 373.09 × e （－０．００６ｂ） ×a ０．２９ ≤27 In the formula (1-1), a is an integer of 2.0 to 3.0, and b is an integer of 0 to 3.

3. 2. The method for producing a lithium secondary battery according to claim 1, wherein the a is 2.0 g / Ah to 2.5 g / Ah.

4. 2. The method for manufacturing a lithium secondary battery according to claim 1, wherein in the fourth step, the pre-charge / discharge is performed at a C-rate of 0.1 C to 1 C in a range of SOC 0 to 99.

5. 2. The method for producing a lithium secondary battery according to claim 1, wherein in the fourth step, the pre-charge and pre-discharge are performed in a voltage range of 2.50V to 4.35V.

6. The method for manufacturing a lithium secondary battery according to claim 1 , wherein the third step includes a step of charging and discharging the battery cell.

7. 2. The method of claim 1, wherein the friction force between the inner surface of the battery case and the electrode assembly is 15 kgf or more.

8. 2. The method of claim 1, wherein the friction force between the inner surface of the battery case and the electrode assembly is 15 kgf to 30 kgf.

9. 2. The method of claim 1, wherein the lithium secondary battery has zero electrolyte leakage when subjected to a crash shock test under a crash condition of 133.7 G x 15.8 ms.

10. 2. The method of claim 1, wherein the battery case is a pouch including a barrier layer, a substrate layer formed on one surface of the barrier layer, and a sealant layer formed on the other surface of the barrier layer, and including at least one cup portion recessed in one direction.

11. 2. The method of claim 1, wherein the electrode assembly has a ratio of overall length to overall width of 5 to 10.

12. 12. The method of claim 11, wherein the electrode assembly has a total length of 400 mm to 600 mm and a total width of 50 mm to 150 mm.

13. 2. The method for producing a lithium secondary battery according to claim 1, wherein the lithium secondary battery has a rated capacity of 50 Ah to 200 Ah.

Images