Lithium secondary battery and its manufacturing method

Abstract

The present invention relates to a method for manufacturing a lithium secondary battery and a lithium secondary battery. The method for manufacturing a lithium secondary battery of the present invention includes a first step of arranging an electrode assembly in a battery case and injecting an electrolyte to manufacture a battery cell, and a second step of pre-charging and discharging the battery cell three or more times.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery and a method for manufacturing the same, and more particularly to a lithium secondary battery having excellent impact resistance and a method for manufacturing the same. [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] While 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, 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 lithium secondary battery in which the frictional force between the inner surface of the battery case and the electrode assembly is increased to 21 kgf or more by performing a pre-cycle process three or more times after the battery is manufactured, and a method for manufacturing the same. [Means for solving the problem]

[0006] In one embodiment, the present invention provides a method for manufacturing a lithium secondary battery, including: a first step of arranging an electrode assembly in a battery case and injecting an electrolyte to manufacture a battery cell; and a second step of pre-charging and pre-discharging the battery cell three or more times, preferably three to seven times, in a voltage range of 2.50 V to 4.35 V.

[0007] Here, 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.

[0008] In the first step, the electrolyte may be injected so that the amount of electrolyte per unit capacity of the secondary battery is 2.0 g / Ah to 2.5 g / Ah, preferably 2.1 g / Ah to 2.4 g / Ah.

[0009] Meanwhile, in the second step, 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, in a range of SOC 0 to 99, preferably 0 to 98.5. In addition, a step of activating the battery cell may be further performed between the first step and the second step.

[0010] In another aspect, the present invention provides a lithium secondary battery including a battery case, an electrode assembly and an electrolyte housed in the battery case, wherein the friction force between the inner surface of the battery case and the electrode assembly is 21 kgf or more, preferably 21 kgf to 31 kgf.

[0011] Here, 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, and the electrode assembly and the electrolyte may be accommodated in the cup portion.

[0012] The lithium secondary battery has excellent impact resistance, and when a crash shock test was carried out under the conditions of an acceleration of 133.7 G and a sustained time of 15.8 ms, the amount of electrolyte leakage was zero. [Effects of the Invention]

[0013] In the method for manufacturing a lithium secondary battery according to the present invention, when a pre-charge / discharge process is performed three or more times after manufacturing a battery cell, the frictional force between the electrode assembly and the inner surface of the battery case (e.g., the bottom surface of the cup portion) is significantly increased compared to the conventional method. As a result, separation of the electrode assembly and / or leakage of electrolyte upon external impact is suppressed, thereby significantly improving the impact resistance of the lithium secondary battery.

[0014] The lithium secondary battery of the present invention manufactured by the above method has a high friction force of 21 kgf or more between the electrode assembly and the inner surface of the battery case. Therefore, when a crash shock test is conducted under the conditions of an acceleration of 133.7 G and a retention time of 15.8 ms, electrolyte leakage due to damage to the pouch does not occur. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a flowchart showing a method for manufacturing a lithium secondary battery according to an embodiment of the present invention. [Figure 2] 1 is an exploded perspective view of a lithium secondary battery according to an embodiment of the present invention. [Figure 3] 1 is a cross-sectional view of a pouch according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0016] 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.

[0017] The present invention will now be described in more detail with reference to specific embodiments.

[0018] 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 battery case can lead to electrolyte leakage or deformation of the electrode assembly, resulting in serious problems with the performance and safety of the battery.

[0019] The inventors conducted extensive research to solve these problems and discovered that when pre-charging and discharging are performed three or more times during battery manufacturing, the frictional force between the electrode assembly and the battery case increases significantly compared to conventional methods. This reduces damage to the pouch caused by separation of the electrode assembly when subjected to external impact, thereby achieving excellent impact resistance, which led to the completion of the present invention.

[0020] Specifically, the method for manufacturing a lithium secondary battery according to the present invention includes a first step of disposing an electrode assembly in a battery case and injecting an electrolyte to manufacture a battery cell, and a second step of pre-charging and discharging the battery cell three or more times, preferably three to seven times, where the pre-charging and discharging may be performed in a voltage range of 2.50 V to 4.35 V.

[0021] When the frictional force between the electrode assembly and the inner surface of the battery case in contact with the electrode assembly increases, separation of the electrode assembly is suppressed when an external impact is applied. The frictional force between the electrode assembly and the inner surface of the battery case is significantly affected by the amount of electrolyte present at the interface between the battery case and the electrode assembly. When pre-charging and discharging are performed three or more times as in the present invention, the electrolyte present between the electrode assembly and the battery case is absorbed into the electrode assembly due to the battery's breathing process, reducing the amount of electrolyte at the interface between the electrode assembly and the battery case, thereby increasing the frictional force. Reducing the amount of electrolyte injected into the battery case can also increase the frictional force, but this method reduces the amount of available electrolyte available in the battery cell, resulting in a decrease in lifespan characteristics. In contrast, when pre-charging and discharging are performed three or more times as in the present invention, the amount of electrolyte between the electrode assembly and the battery case interface can be reduced without reducing the amount of available electrolyte, eliminating the problem of reduced lifespan.

[0022] Fig. 1 illustrates a method for manufacturing a lithium secondary battery according to the present invention. Fig. 2 illustrates a perspective view of a pouch-type lithium secondary battery according to an embodiment of the present invention, and Fig. 3 illustrates a cross section of a pouch according to the present invention. The present invention will be described in more detail below with reference to Figs. 1 to 3.

[0023] <Method of manufacturing lithium secondary batteries> First, a method for producing a lithium secondary battery according to the present invention will be described.

[0024] [Step 1: Battery cell manufacturing step] First, an electrode assembly is placed in a battery case and an electrolyte is injected to manufacture a battery cell (S1).

[0025] Here, the battery case may be, for example, a pouch.

[0026] Referring to Figures 2 and 3, the pouch 100 includes a barrier layer 20, a base 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 includes at least one cup portion 110 recessed in one direction.

[0027] 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.

[0028] 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.

[0029] 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 (registered trademark).

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] Meanwhile, the barrier layer 20 may be made of a metal material, specifically, an aluminum alloy thin film.

[0038] 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).

[0039] 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.

[0040] The sealant layer 30 is bonded by thermocompression to seal the pouch, and is located as the innermost layer of the pouch film laminate.

[0041] 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.

[0042] 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 (registered trademark). 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.

[0043] More specifically, the sealant layer 30 may include polypropylene, cast polypropylene (CPP), acid modified polypropylene, polypropylene-butylene-ethylene copolymer, or a combination thereof.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 occurs, resulting in wrinkles, while if the pressure is too high, the drawing process is not performed well, resulting in a reduced molding depth.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] The electrode assembly 200 is placed in the cup portion 110 of the pouch, an electrolyte is injected, the pouch is folded to bring the upper case 102 and the lower case 101 into contact with each other, and heat is applied to seal the sealant layer, thereby manufacturing a battery cell. Here, another gas pocket space may be formed in the terrace 120.

[0056] Here, the electrode assembly 200 may include a plurality of electrodes and a plurality of separators that are alternately stacked. The plurality of electrodes may include positive and negative electrodes having opposite polarities, which are alternately stacked with separators interposed therebetween.

[0057] 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 a plurality of electrodes, protrude from the electrode assembly 200, and act as a path through which electrons can move between the inside and outside of the electrode assembly 200. The plurality of electrode tabs 230 may be located inside the pouch 100.

[0058] 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.

[0059] A lead 240 for supplying electricity to the outside of the secondary battery may be connected to the plurality of electrode tabs 230 by spot welding, laser welding, ultrasonic welding, etc. One end of the lead 240 may be connected to the plurality of electrode tabs 230, and the other end may protrude outside the pouch 100.

[0060] 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.

[0061] Meanwhile, in the present invention, the ratio of the total length W1 to the total width W2 of the electrode assembly 200 may be 3 to 12, preferably 5 to 10. When the ratio of the total length W1 to the total width satisfies this range, it is possible to achieve a high energy density in a limited space. Here, the total length and total width may refer to the maximum lengths of the electrode assembly in the longitudinal and width directions, respectively, on a plane.

[0062] For example, the electrode assembly may have a total length of 400 mm to 600 mm and a total width of 50 to 140 mm, preferably a total length of 450 mm to 550 mm and a total width of 60 to 130 mm. When the total width and length of the electrode assembly satisfy the above ranges, it can be more effective in increasing frictional force without increasing the amount of electrolyte. Here, the total length and total width refer to the maximum lengths of the electrode assembly in the longitudinal and width directions, respectively, on a plane.

[0063] Meanwhile, the weight of the electrode assembly may be 500 g to 1500 g, preferably 550 g to 1450 g, and more preferably 600 g to 1400 g. When the weight of the electrode assembly satisfies this range, a high capacity can be achieved, and the frictional force between the electrode assembly and the inner surface of the battery case can be increased, resulting in excellent impact resistance.

[0064] Meanwhile, at least one fixing member may be further included on the outer surface of the electrode assembly, which is wound and fixed in the entire width direction. In the case of a rectangular electrode assembly whose entire length is longer than its entire width (for convenience, referred to as a "long-cell"), a fixing member may be used to wrap and fix the electrode assembly in the entire width direction to prevent misalignment of the components of the electrode assembly, i.e., the positive electrode, the negative electrode, and the separator.

[0065] The fixing member may have a porous structure. When the fixing member has a porous structure, the electrolyte can pass through the fixing member and be impregnated into the electrode assembly, preventing the fixing member from reducing the electrolyte impregnation of the electrode assembly. Specifically, the fixing member may be, but is not limited to, a finishing tape having an adhesive layer formed on one side of a base layer made of a porous polymer material. The polymer material may be, but is not limited to, polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyethylene (PE), etc.

[0066] The fixing member preferably has a width of about 10 to 50 mm or 20 to 40 mm in the overall width direction of the electrode assembly. If the fixing member is too wide, the area of ​​the outer surface of the electrode assembly covered by the fixing member increases, reducing the contact area with the electrolyte, which may result in reduced electrolyte impregnation and reduced friction between the electrode assembly and the battery case, resulting in reduced impact resistance. On the other hand, if the fixing member is too thin, the fixing effect of the electrode assembly may be reduced.

[0067] Meanwhile, the number of fixing members arranged on the outer surface of the electrode assembly may be 2 to 10, preferably 2 to 8, and more preferably 3 to 7. The fixing members may be arranged symmetrically along the entire length, and preferably, the fixing members may be arranged at equal intervals. When a plurality of fixing members are provided and arranged as described above, an electrode assembly having a long-cell structure with a long overall length can be firmly fixed.

[0068] Meanwhile, the contact area between the fixing member and the electrode assembly may be 30% or less, 25% or less, or 20% or less of the total surface area of ​​the electrode assembly. Specifically, the contact area between the fixing member and the electrode assembly may be 0 to 30%, 1 to 30%, 5 to 30%, 5 to 25%, or 5 to 20% of the total surface area of ​​the electrode assembly.

[0069] The contact area between the fixing member and the electrode assembly can be adjusted by adjusting the width or number of fixing members used. Since commonly used fixing members are made of a material with a lower coefficient of friction than the separator disposed on the outermost surface of the electrode assembly, increasing the area of ​​the fixing member surrounding the electrode assembly can reduce the frictional force between the electrode assembly and the inner surface of the battery case. Therefore, when using a fixing member, it is preferable to limit the contact area between the electrode assemblies to 30% or less to prevent a decrease in frictional force.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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 the decrease in capacity of the battery, improving the discharge capacity of the battery, and the like.

[0074] Meanwhile, in the present invention, the electrolyte may be injected so that the amount of electrolyte per unit capacity of the secondary battery is 2.0 g / Ah to 2.5 g / Ah, preferably 2.1 g / Ah to 2.4 g / Ah, and more preferably 2.1 g / Ah to 2.3 g / Ah. 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. The "rated capacity of the secondary battery" refers to the electrical capacity exhibited when a fully charged battery is continuously discharged at 0.33 C until it reaches the end-of-discharge voltage. Here, the full charge voltage (end-of-charge voltage) and the end-of-discharge voltage can be appropriately selected depending on the type of secondary battery. For example, when the secondary battery is an NCM cell, the rated capacity may be the discharge capacity when the secondary battery is charged to 4.25 V and then discharged to 2.5 V at 0.33 C. The "unit capacity" refers to 1 Ah.

[0075] When the amount of electrolyte per unit capacity satisfies the above range, excellent impact resistance can be achieved without deteriorating the electrochemical properties of the secondary battery. If the amount of electrolyte is too small, the electrolyte may be insufficient during battery operation, resulting in a decrease in battery performance. If the amount of electrolyte is too large, the impact resistance is not significantly improved. Meanwhile, in the present invention, the lithium secondary battery may have a rated capacity of 50 Ah to 200 Ah, preferably 50 Ah to 150 Ah, and more preferably 60 Ah to 140 Ah.

[0076] Meanwhile, after manufacturing the battery cell, an activation process can be performed (S2).

[0077] 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.

[0078] 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.

[0079] 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 sufficiently 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.

[0080] 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.

[0081] 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.

[0082] [Second step: Pre-charge / discharge step] Next, the battery cell manufactured by the above method is subjected to pre-charge and discharge three or more times (S3).

[0083] According to the inventors' research, when pre-charge / discharge cycles are performed three or more times, the frictional force between the battery case and the electrode assembly increases significantly, thereby preventing the electrode assembly from separating when an external impact occurs, significantly improving impact resistance. When the pre-charge / discharge cycle is performed two or less times, the increase in frictional force is small, and the impact resistance is not significantly improved. However, if the pre-charge / discharge cycle is performed too many times, the consumption of the active material and electrolyte increases, which can lead to a deterioration in electrochemical properties. Therefore, it is preferable that the pre-charge / discharge cycle be 10 or 7 cycles or less.

[0084] The pre-charge / discharge may be performed in a voltage range of 2.50 V to 4.35 V. If the pre-charge / discharge voltage range is outside this range, deterioration of battery performance may occur due to overcharge or overdischarge.

[0085] Meanwhile, 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, in a range of SOC 0 to 100, preferably SOC 0 to 99, 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 small increase in frictional force.

[0086] The lithium secondary battery of the present invention manufactured through the above-described three or more pre-charge / discharge cycles has a higher friction force between the bottom surface of the cup portion and the electrode assembly than the conventional battery.

[0087] Specifically, the lithium secondary battery according to the present invention includes a battery case, an electrode assembly and an electrolyte housed in the battery case, and the frictional force between the battery case and the electrode assembly can be 21 kgf or more, preferably 21 kgf to 31 kgf. Here, the details of the battery case, electrode assembly and electrolyte are as described above, and therefore detailed description thereof will be omitted.

[0088] The frictional force between the electrode assembly and the battery case was measured in the following manner.

[0089] 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.

[0090] As described above, the lithium secondary battery according to the present invention has excellent impact resistance because the electrode assembly is less likely to separate due to external impact and damage to the pouch is minimized due to the high friction between the electrode assembly and the inner surface of the battery case. Specifically, the lithium secondary battery according to the present invention may have zero electrolyte leakage when subjected to a crash shock test under conditions of an acceleration of 133.7 G and a retention time of 15.8 ms.

[0091] The crash shock test can be performed by mounting a battery to be tested on a drop shock test fixture, allowing the battery to free-fall from a specific height toward the electrode tabs, and then determining whether the battery is damaged. The free-fall height is determined based on the crash conditions to be measured (acceleration x duration). 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 achieved is calculated based on the weight of the battery to be tested, and the free-fall height is set accordingly. Meanwhile, battery damage can be evaluated based on the presence or absence of electrolyte leakage.

[0092] [Example 1] A pouch was prepared by laminating nylon, polyethylene terephthalate, an aluminum alloy thin film, and polypropylene in this order to form a cup. A stacked electrode assembly measuring 548 mm in length and 99 mm in width was placed in the cup, and electrolyte was injected so that the amount of electrolyte per unit capacity was 2.3 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.

[0093] Next, the activated battery cell was charged to 4.25 V at 0.33 C in the range of SOC 0 to 100, and then discharged to 2.5 V at 0.33 C. This pre-charge-discharge process was carried out three times to manufacture a pouch-type lithium secondary battery.

[0094] [Example 2] A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the electrolyte was injected so that the amount of electrolyte per unit capacity was 2.2 g / Ah.

[0095] [Comparative Example 1] A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the pre-charge / discharge process was not performed.

[0096] Comparative Example 2 A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the pre-charge / discharge process was performed once.

[0097] Comparative Example 3 A pouch-type lithium secondary battery was manufactured in the same manner as in Example 1, except that the pre-charge / discharge process was performed twice.

[0098] [Experimental Example 1: Friction Test] A portion of the pouch of each of the lithium secondary batteries manufactured in Examples 1-2 and Comparative Examples 1-3 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 rate of 100 mm / min. The force applied was measured and evaluated as the frictional force between the electrode assembly and the inner surface of the battery case (bottom of the cup). The frictional force was measured for three secondary batteries manufactured in each Example and Comparative Example, and the average value of the measured frictional forces was used. The measurement results are shown in Table 1 below.

[0099] [Experimental example 2: Collision shock test] The pouch-type lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 to 3 were subjected to an impact shock test under conditions of an acceleration of 133.7 G and a retention time of 15.8 ms. The measurement results are shown in Table 1. After the test, if no electrolyte leakage or electrode assembly separation occurred, the battery was marked as Pass. If electrolyte leakage and / or electrode assembly separation occurred, the battery was marked as Fail.

[0100] [Table 1]

[0101] As shown in Table 1, the secondary batteries of Examples 1 and 2, which had undergone pre-charge / discharge three or more times, had a high frictional force between the electrode assembly and the battery case, which prevented the electrode assembly from separating due to external impact, demonstrating excellent impact resistance. In contrast, the secondary batteries of Comparative Examples 1 to 3, which had not undergone pre-charge / discharge or had undergone pre-charge / discharge two or less times, had a lower frictional force between the electrode assembly and the battery case than the batteries of the Examples, resulting in electrolyte leakage during the impact shock test.

Claims

1. a first step of manufacturing a battery cell by placing an electrode assembly in a battery case and injecting an electrolyte; a second step of pre-charging and pre-discharging the battery cell three or more times in a voltage range of 2.50V to 4.35V.

2. 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.

3. 2. The method for manufacturing a lithium secondary battery according to claim 1, wherein in the first step, the electrolyte is injected so that the amount of electrolyte per unit capacity of the lithium secondary battery 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 first step, the electrolyte is injected so that the amount of electrolyte per unit capacity of the lithium secondary battery is 2.1 g / Ah to 2.4 g / Ah.

5. 2. The method for manufacturing a lithium secondary battery according to claim 1, wherein in the second 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 100.

6. 2. The method for producing a lithium secondary battery according to claim 1, wherein in the second step, the pre-charge / discharge cycle is carried out 3 to 7 times.

7. 7. The method for producing a lithium secondary battery according to claim 1, further comprising an activation step between the first step and the second step.

8. a battery case; and an electrode assembly and an electrolyte housed in the battery case, The friction force between the inner surface of the battery case and the electrode assembly is 21 kgf or more.

9. 10. The lithium secondary battery of claim 8, wherein the battery case is 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.

10. 10. The lithium secondary battery according to claim 9, wherein the frictional force between the bottom surface of the cup portion and the electrode assembly is 21 kgf to 31 kgf.

11. 9. The lithium secondary battery of claim 8, wherein the electrode assembly has a ratio of overall length to overall width of 3 to 12.

12. 9. The lithium secondary battery of claim 8, wherein the electrode assembly has a total length of 400 mm to 600 mm and a total width of 50 mm to 140 mm.

13. 9. The lithium secondary battery according to claim 8, wherein the rated capacity of the lithium secondary battery is 50 Ah to 200 Ah.

14. 14. The lithium secondary battery according to claim 8, wherein the amount of electrolyte leakage is 0 when a crash shock test is performed under conditions of an acceleration of 133.7 G and a maintenance time of 15.8 ms.

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