Method for manufacturing lithium secondary battery, and lithium secondary battery

Abstract

The present specification relates to a method for manufacturing a lithium secondary battery and a lithium secondary battery. According to one exemplary embodiment of the present invention, there is provided a method for manufacturing a lithium secondary battery, comprising the steps of: injecting a first electrolyte into a battery case accommodating an electrode assembly including a positive electrode and a negative electrode; after the first electrolyte is injected, performing a first activation process; injecting a second electrolyte different from the first electrolyte into the battery shell subjected to the first activation process; and after the second electrolyte is injected, a second activation process is carried out, the lithium salt concentration of the first electrolyte is larger than that of the second electrolyte, and the lithium salt concentration difference of the first electrolyte and the second electrolyte meets the specific range.

Description

Manufacturing method of lithium secondary battery and lithium secondary battery Technical Field

[0001] This application claims priority and benefit to Korean Patent Application No. 10-2024-0098612, filed with the Korean Intellectual Property Office on July 25, 2024, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to a method for manufacturing a lithium secondary battery and the lithium secondary battery itself. Background Technology

[0003] Recently, with the rapid proliferation of battery-powered electronic devices such as mobile phones, laptops, electric vehicles, power tools, and cleaners, the demand for small, lightweight rechargeable batteries with relatively high capacity and / or high output is increasing rapidly. In particular, lithium-ion batteries are attracting significant attention as power sources for electronic devices due to their light weight and high energy density. Therefore, research and development are actively underway to improve the performance of lithium-ion batteries.

[0004] In a lithium secondary battery, an organic electrolyte or polymer electrolyte is filled between the positive and negative electrodes. Both the positive and negative electrodes are made of active materials that can insert and extract lithium ions. When lithium ions are inserted / extracted in the positive and negative electrodes, electrical energy is generated through a redox reaction.

[0005] During the initial charging of a lithium-ion secondary battery, lithium ions released from the positive electrode active material (e.g., lithium metal oxide) migrate to the negative electrode active material and embed themselves in the interlayer of the negative electrode active material. In this case, due to the strong reactivity of lithium ions, an electrolyte-derived film is formed on the surface of the electrode active material, such as a positive electrode electrolyte interface (CEI) film formed between the positive electrode and the electrolyte, or a solid electrolyte interface (SEI) film formed between the negative electrode and the electrolyte.

[0006] The electrolyte-derived film acts as a channel for lithium ions, preventing direct contact between the electrolyte and the electrode active material, thereby reducing side reactions during charging and discharging. However, the problem is that, depending on the electrolyte composition, the electrolyte-derived film cannot form uniformly on the surface of the electrode active material, or the formation of the electrolyte-derived film on the electrode surface is unstable, leading to deterioration of battery characteristics. Summary of the Invention

[0007] [Technical Issues]

[0008] This invention aims to provide a method for manufacturing a lithium secondary battery and a lithium secondary battery. The method improves the initial efficiency, fast charging performance and lifespan characteristics of the lithium secondary battery by forming a uniform electrolyte-derived film on the surface of the positive electrode active material or the negative electrode active material.

[0009] The technical problem to be solved by the present invention is not limited to the above-mentioned content. Other problems not explicitly mentioned will become obvious to those skilled in the art through the following description.

[0010] [Technical Solution]

[0011] An exemplary embodiment of the present invention provides a method for manufacturing a lithium secondary battery, comprising: injecting a first electrolyte into a battery casing containing an electrode assembly including a positive electrode and a negative electrode; performing a first activation process after injecting the first electrolyte; injecting a second electrolyte, different from the first electrolyte, into the battery casing after the first activation process; and performing a second activation process after injecting the second electrolyte, wherein the first electrolyte and the second electrolyte each independently contain lithium salts, the lithium salt concentration of the first electrolyte is greater than the lithium salt concentration of the second electrolyte, and the lithium salt concentration difference between the first electrolyte and the second electrolyte is 0.5 M to 2 M.

[0012] An exemplary embodiment of the present invention provides a lithium secondary battery manufactured by the above-described manufacturing method.

[0013] [Beneficial Effects]

[0014] The lithium secondary battery manufacturing method and lithium secondary battery according to exemplary embodiments of the present invention can stably form a high-density, thin-film and high-strength CEI film or SEI film derived from a first electrolyte and a second electrolyte on the surface of the electrode active material.

[0015] The lithium secondary battery manufacturing method and lithium secondary battery of the exemplary embodiments of the present invention can improve initial efficiency, fast charging characteristics and lifespan characteristics. Attached Figure Description

[0016] Figure 1 is a simplified schematic diagram of the secondary battery electrodes.

[0017] Figure 2 is a flowchart illustrating the theoretical formation process of the electrolyte-derived membrane.

[0018] Figure 3 is a flowchart illustrating the experimental formation process of the electrolyte-derived membrane.

[0019] Figure 4 is a schematic diagram illustrating an electrolyte-derived membrane according to an exemplary embodiment of the present invention.

[0020] <Explanation of Figure Markers>

[0021] 1: Current collector

[0022] 2: Electrode active materials

[0023] 3: Electrolyte-derived membrane

[0024] 31: First electrolyte-derived membrane

[0025] 32: Second electrolyte-derived membrane Detailed Implementation

[0026] The invention will be described in more detail below.

[0027] In this specification, when a part is referred to as "comprising" a component, unless otherwise stated, it means that the part may also contain other parts, rather than excluding other parts.

[0028] In this specification, when an element is referred to as being "on" another element, the element may be in direct contact with the other element, or there may be an intermediate element present.

[0029] It should be understood that the terms or words used throughout this specification should not be construed as limited to their conventional or dictionary meanings, but rather should be interpreted as having meanings and concepts consistent with the technical ideas of this invention, based on the inventor's appropriate definition of the concepts of the words or terms to best explain the invention.

[0030] In this specification, the phrase "different composition" between the first electrolyte and the second electrolyte may refer to the different types, concentrations, or weight ratios of lithium salts, solvents, and / or additives contained in the first electrolyte and the second electrolyte.

[0031] In this specification, "p to q" refers to the range of "p and below q".

[0032] In this specification, "electrolyte-derived film" refers to a film (layer) derived from electrolyte formed on the surface of the electrode active material during battery charging and discharging. For example, the electrolyte-derived film may include a solid electrolyte interface (SEI) film formed on the surface of the negative electrode active material, or a positive electrode electrolyte interface (CEI) film formed on the surface of the positive electrode active material.

[0033] In an exemplary embodiment of the present invention, the difference between the first electrolyte and the second electrolyte refers to the different types of lithium salts contained in the first electrolyte and the second electrolyte. For example, the first electrolyte may contain lithium salt A, and the second electrolyte may contain lithium salt B; or the first electrolyte may contain lithium salts A and C, and the second electrolyte may contain lithium salts B and C; or the first electrolyte may contain lithium salt A, and the second electrolyte may contain lithium salts B and C; but it is not limited to these.

[0034] In an exemplary embodiment of the present invention, the difference between the first electrolyte and the second electrolyte refers to the difference in the concentration of lithium salt contained in the first electrolyte and the second electrolyte. For example, when the first electrolyte and the second electrolyte each contain lithium salt A and the lithium salt concentration in the first electrolyte is "a M", the lithium salt concentration in the second electrolyte can be "b M"; or when the first electrolyte contains lithium salt A and the second electrolyte contains lithium salt B, and the lithium salt concentration in the first electrolyte is "a M", the lithium salt concentration in the second electrolyte can be "b M"; but it is not limited to these.

[0035] As used herein, singular terms such as “a,” “an,” and “the” are intended to include plural forms as well, unless the context otherwise clearly indicates otherwise.

[0036] Preferred exemplary embodiments of the present invention will be described in detail below. However, it should be understood that embodiments of the present invention can be modified in various ways, and the scope of the present invention is not limited to the exemplary embodiments described below.

[0037] Figure 1 is a simplified schematic diagram of the secondary battery electrodes.

[0038] During the charging and discharging process of a lithium secondary battery, the electrolyte-derived film 3 formed on the surface of the electrode active material 2, such as a solid electrolyte interphase (SEI) film or a positive electrode electrolyte interphase (CEI) film, acts as a lithium-ion transport channel, thereby preventing direct contact between the electrolyte and the electrode active material 2. This plays an important role in reducing possible side reactions during charging and discharging.

[0039] However, if the electrolyte-derived film 3 does not form stably due to the composition of the electrolyte, for example, if it is not formed uniformly on the surface of the electrode active material 2, or if it is formed too thickly, or if its strength is low, problems may arise, and the required characteristics of each interface of the battery may not be met.

[0040] Referring to Figure 1, at the interface (point a) near the electrode active material 2, lithium ions should transfer smoothly to facilitate their insertion and extraction during charging and discharging. Therefore, the characteristic of lithium ion transport from the electrolyte through the electrolyte-derived film 3 to the active material is required. On the other hand, at the interface (point b) relatively far from the electrode active material 2, the transport characteristics of lithium ions through the electrolyte itself become more important. However, if the electrolyte-derived film 3 is not formed uniformly, the required characteristics at each interface may not be met. For example, the electrolyte-derived film 3 may not form properly at point a, or the electrolyte-derived film 3 may form an excessively thick layer in a local area, extending to point b. Consequently, the reaction rate between the interfaces in the battery may be delayed, or the reaction may be restricted, and in the worst case, the reaction may stop, leading to battery performance degradation.

[0041] Figure 2 is a process diagram illustrating the theoretical formation process of the electrolyte-derived membrane 3, and Figure 3 is a process diagram illustrating the experimental formation process of the electrolyte-derived membrane 3. Figure 2(a) and Figure 3(a) show the electrodes in the electrode assembly before electrolyte injection. The electrodes include a current collector 1 and an electrode active material 2 (positive or negative electrode active material) disposed on the current collector 1. Figure 2(b) and Figure 3(b) show the electrodes immediately after electrolyte injection. Figure 2(c) and Figure 3(c) show that the electrolyte-derived membrane 3 is formed by performing an activation process under the electrolyte injection state shown in Figure 2(b) or Figure 3(b).

[0042] To address this issue, a method was proposed: injecting the electrolyte into the battery casing containing the electrode components in a single step, followed by an activation process to pre-form an electrolyte-derived film 3. Theoretically, as shown in Figure 2(c), it was expected that the electrolyte-derived film 3 would form uniformly and stably on the surface of the electrode active material 2. However, in practice, as shown in Figure 3(c), it was confirmed that the film was not formed uniformly, and the uniform electrolyte-derived film 3 shown in Figure 2(c) could not be formed.

[0043] Therefore, the inventors conceived of forming a robust and uniform electrolyte-derived film 3 on the electrode active material 2 by injecting two types of electrolytes that meet specific conditions in two separate injections and performing an activation process after each injection.

[0044] Manufacturing method of secondary batteries

[0045] An exemplary embodiment of the present invention provides a method for manufacturing a lithium secondary battery, comprising: injecting a first electrolyte into a battery casing containing an electrode assembly including a positive electrode and a negative electrode; performing a first activation process after injecting the first electrolyte; injecting a second electrolyte, different from the first electrolyte, into the battery casing that has undergone the first activation process; and performing a second activation process after injecting the second electrolyte, wherein the first electrolyte and the second electrolyte each independently contain lithium salts, the lithium salt concentration of the first electrolyte is greater than the lithium salt concentration of the second electrolyte, and the lithium salt concentration difference between the first electrolyte and the second electrolyte is 0.5 M to 2 M.

[0046] According to an exemplary embodiment of the present invention, the lithium salt concentration of the first electrolyte is greater than the lithium salt concentration of the second electrolyte.

[0047] When the activation process is carried out after the electrolyte is injected, an electrolyte-derived film 3 (SEI film or CEI film) is formed on the surface near the electrode active material 2. In this case, since the electrolyte-derived film with a sufficiently high lithium salt concentration has high viscosity, it has the advantage of ensuring a large number of ion transport channels inside the electrode active material 2. However, in this type of high-concentration electrolyte-derived film 3, due to the high viscosity of the first electrolyte, the impregnation rate of the electrolyte into the porous electrode layer becomes low, making it difficult to uniformly form the electrolyte-derived film 3 on the surface of the electrode active material 2. Therefore, in some areas on the surface of the electrode active material 2, the electrolyte-derived film 3 may not be formed, or it may be formed too thickly.

[0048] Referring to Figure 4, the lithium secondary battery preparation method of an exemplary embodiment of the present invention is characterized in that, after injecting and activating a first electrolyte with a high lithium salt concentration, a second electrolyte with a lower lithium salt concentration is injected, and a second activation process is performed, thereby forming a second electrolyte-derived film 32. This second electrolyte-derived film complements the unevenly formed first electrolyte-derived film 31, thereby enabling the electrolyte-derived film 3 to be uniformly formed on the surface of the electrode active material 2. In other words, because the lithium salt concentration of the second electrolyte is lower than that of the first electrolyte, the second electrolyte-derived film 32 can be uniformly formed on the unevenly formed first electrolyte-derived film 31 on the surface of the electrode active material 2.

[0049] In an exemplary embodiment of the present invention, the lithium salt concentration difference between the first electrolyte and the second electrolyte is 0.5 M to 2 M. For example, the lithium salt concentration difference between the first electrolyte and the second electrolyte can be greater than 0.5 M or greater than 0.7 M, or less than 2 M, less than 1.5 M, less than 1.2 M, or less than 1 M. That is, in an exemplary embodiment of the present invention, the lithium salt concentration difference between the first electrolyte and the second electrolyte can be 0.5 M to 1 M.

[0050] When the lithium salt concentration difference between the first electrolyte and the second electrolyte is within the aforementioned range, the concentration difference between the first electrolyte and the second electrolyte is sufficiently large, allowing the low-concentration second electrolyte to easily form a second electrolyte-derived film 32 that supplements the first electrolyte-derived film 31. This ultimately enables the electrolyte-derived film 3 to be uniformly formed across the entire electrode active material 2. On the other hand, if the lithium salt concentration difference between the first electrolyte and the second electrolyte exceeds the aforementioned range, the lithium salt concentration of the first electrolyte may become too high. In this case, the ionic conductivity of the first electrolyte may decrease, and the first electrolyte-derived film may not be formed. Alternatively, the lithium salt concentration of the second electrolyte may become too low, in which case the second electrolyte-derived film may not be formed normally after the second activation process.

[0051] In an exemplary embodiment of the present invention, the lithium salt concentration of the first electrolyte can be from 0.8 M to 2 M. For example, the lithium salt concentration of the first electrolyte can be 1 M or more, 1.1 M or more, or 1.2 M or more, or less than 2 M, less than 1.8 M, less than 1.6 M, or less than 1.5 M.

[0052] When the lithium salt concentration of the first electrolyte is within the above range, a first electrolyte-derived film can be formed after the first activation process. This film is derived from the high-viscosity first electrolyte and ensures that there are a sufficient number of ion channels at the position closest to the active material in the electrode and improves the ion transport characteristics, so that lithium ions can be smoothly transported at the interface of the electrode active material.

[0053] In an exemplary embodiment of the present invention, the lithium salt concentration of the second electrolyte can be from 0.3 M to 1 M. For example, the lithium salt concentration of the second electrolyte can be 0.3 M or more, 0.4 M or more, 0.5 M or more, or 0.7 M or more, or less than 1 M, less than 0.8 M, less than 0.75 M, or less than 0.7 M.

[0054] When the second electrolyte contains lithium salts within the aforementioned concentration range, because the viscosity of the second electrolyte is lower than that of the first electrolyte, a thinner second electrolyte-derived film can be uniformly formed on the unevenly formed first electrolyte-derived film to supplement the thickness of the first electrolyte-derived film. On the other hand, if the lithium salt concentration of the second electrolyte exceeds the upper limit, the concentration difference between it and the first electrolyte is insufficient, making it difficult for the second electrolyte-derived film to form between the regions of the unevenly formed first electrolyte-derived film, which can further exacerbate the non-uniformity of the electrolyte-derived film. If the lithium salt concentration of the second electrolyte is below the lower limit, the lithium salt concentration is too low, making it difficult to fully form the second electrolyte-derived film even after the activation process.

[0055] In exemplary embodiments of the present invention, the lithium salt in the first electrolyte and the lithium salt in the second electrolyte may be independently the same or different, and each lithium salt can be used without any particular limitation, as long as it is a compound capable of providing lithium ions for use in lithium secondary batteries. For example, the lithium salt in the first electrolyte and the lithium salt in the second electrolyte may be independently the same or different, and LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO2, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, LiFSI, LiTFSI, LiBF4, LiDFOB, or combinations thereof can be used. Specifically, the lithium salt in the first electrolyte and the lithium salt in the second electrolyte may be independently the same or different, and LiPF6, LiFSI, or combinations thereof can be used.

[0056] In an exemplary embodiment of the present invention, the lithium salt type in the first electrolyte may be the same as or different from the lithium salt type in the second electrolyte.

[0057] In an exemplary embodiment of the present invention, the lithium salt type in the first electrolyte can be the same as the lithium salt type in the second electrolyte.

[0058] In an exemplary embodiment of the present invention, the lithium salt type in the first electrolyte may be different from the lithium salt type in the second electrolyte.

[0059] In an exemplary embodiment of the present invention, the thermal decomposition temperature of the lithium salt in the first electrolyte can be lower than that of the lithium salt in the second electrolyte. That is, the thermal decomposition temperature of the lithium salt in the second electrolyte can be higher than that of the lithium salt in the first electrolyte. When a lithium salt with a higher thermal decomposition temperature is used as the lithium salt in the second electrolyte, a second electrolyte-derived film with higher thermal stability can be formed on the outer side of the electrolyte-derived film (the side opposite to the active material), thereby preventing the thermal decomposition of the electrolyte-derived film in the battery operating environment.

[0060] In an exemplary embodiment of the present invention, the lithium salt anion in the first electrolyte can be smaller than the lithium salt anion in the second electrolyte. That is, the lithium salt anion in the second electrolyte can be larger than the lithium salt anion in the first electrolyte. In other words, when the lithium salt anion size in the second electrolyte is larger than the lithium salt anion size in the first electrolyte, the ion mobility of the second electrolyte is further improved, thereby making it easier to form a second electrolyte-derived film.

[0061] Specifically, when a lithium salt with a larger anion size is used in the second electrolyte, compared to using a lithium salt with a smaller anion size, the migration characteristics of the lithium salt in the second electrolyte can be improved due to size resistance. This allows the second electrolyte to exhibit relatively high ionic conductivity and low viscosity even at low concentrations. For example, when LiPF6 is used as the lithium salt in the first electrolyte, and LiFSI is used instead of LiPF6 in the second electrolyte, the second electrolyte exhibits relatively higher ionic conductivity, thereby improving battery characteristics.

[0062] However, in the high-concentration first electrolyte, as the lithium salt concentration in the electrolyte becomes excessively high, the Debye length between ions in the electrolyte shortens, and due to the resulting ion relaxation effect and the electrophoretic effect caused by increased viscosity, the size resistance caused by changing the lithium salt type has little effect on improving ionic conductivity. Therefore, the above-mentioned effect can be achieved by changing the lithium salt type in the relatively low-concentration second electrolyte.

[0063] In an exemplary embodiment of the present invention, the lithium salt in the first electrolyte can be LiPF6, and the lithium salt in the second electrolyte can be LiFSI. When LiPF6 is used as the lithium salt in the first electrolyte, the first electrolyte-derived film can be stably formed on the current collector or active material during the formation of the first electrolyte-derived film. When the lithium salt in the second electrolyte is LiFSI, side reactions between the current collector and the lithium salt, such as corrosion, can be prevented, and even if the lithium salt concentration of the second electrolyte is low, its ionic conductivity is still high, and an electrolyte-derived film can be uniformly formed on the surface of the active material.

[0064] In an exemplary embodiment of the present invention, the viscosity of the first electrolyte may be higher than that of the second electrolyte.

[0065] In an exemplary embodiment of the present invention, the viscosity of the first electrolyte can be from 1 cP to 5 cP. For example, the viscosity of the first electrolyte can be 1 cP or more, 1.5 cP or more, 2 cP or more, 2.1 cP or more, 2.5 cP or more, 2.7 cP or more, or 2.8 cP or more, or it can be 5 cP or less, 4 cP or less, 3 cP or less, or 2 cP or less.

[0066] In an exemplary embodiment of the present invention, the viscosity of the second electrolyte can be from 0.5 cP to 4 cP. For example, the viscosity of the second electrolyte can be 0.5 cP or more, 0.7 cP or more, 1 cP or more, or 1.2 cP or more, or 4 cP or less, 3 cP or less, 2 cP or less, 1.5 cP or less, or 1.4 cP or less.

[0067] When the viscosity of the first electrolyte is higher than that of the second electrolyte, a large number of ion channels can be ensured in the first electrolyte-derived film, thereby forming a robust electrolyte-derived film. When the viscosity of the second electrolyte is lower than that of the first electrolyte, the ion mobility is improved, allowing the second electrolyte-derived film to form on the surface of the active material in areas where the first electrolyte-derived film has not formed, or on the first electrolyte-derived film with uneven thickness, thereby forming a thin and uniform electrolyte-derived film.

[0068] In an exemplary embodiment of the present invention, the viscosity of the electrolyte refers to the viscosity of the electrolyte measured at room temperature (25°C). For example, the viscosity of an electrolyte containing an organic solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75 can be measured at 25°C using an RS150 viscometer (manufacturer: McIntosh).

[0069] In an exemplary embodiment of the present invention, the ionic conductivity of the first electrolyte may be higher than that of the second electrolyte. That is, the ionic conductivity of the second electrolyte may be lower than that of the first electrolyte.

[0070] In an exemplary embodiment of the present invention, the ionic conductivity of the first electrolyte can be from 10 mS / cm to 15 mS / cm. For example, the ionic conductivity of the first electrolyte can be 10 mS / cm or more, 11 mS / cm or more, or 12 mS / cm or more, or 15 mS / cm or less, 14 mS / cm or less, or 13 mS / cm or less.

[0071] In an exemplary embodiment of the present invention, the ionic conductivity of the second electrolyte can be from 7 mS / cm to 13 mS / cm. For example, the ionic conductivity of the second electrolyte can be 7 mS / cm or more, 7.5 mS / cm or more, 8 mS / cm or more, 8.5 mS / cm or more, 9 mS / cm or more, or 9.5 mS / cm or more, and can be 13 mS / cm or less, 12 mS / cm or less, 11 mS / cm or less, or 10 mS / cm or less. When the ionic conductivity of the second electrolyte falls within the above range, the ion mobility in the second electrolyte is improved, causing the second electrolyte-derived film to form on the surface of the active material in areas where the first electrolyte-derived film has not formed, or on the first electrolyte-derived film with uneven thickness, thereby forming a thin and uniform electrolyte-derived film.

[0072] In an exemplary embodiment of the present invention, the ionic conductivity of the electrolyte refers to the ionic conductivity of the electrolyte measured at room temperature (25°C). For example, the ionic conductivity of an electrolyte containing an organic solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75 can be measured using a probe-type ionic conductivity measuring device (InoLab 731, model: S470, manufacturer: Mettler Torodo).

[0073] In an exemplary embodiment of the present invention, the first electrolyte may also contain additives.

[0074] In an exemplary embodiment of the present invention, the first electrolyte may contain an SEI film-forming agent as an additive. The SEI film-forming agent refers to a compound that promotes SEI film formation through electrochemical oxidation or reductive decomposition reactions, and may be at least one selected from the group consisting of cyclic carbonate compounds, halogenated carbonate compounds, sulfonyl lactone compounds, sulfate ester / salt compounds, borate ester / salt compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds different from the lithium salt contained in the first electrolyte.

[0075] The SEI film-forming agent can be selected based on the type of lithium salt or solvent in the first electrolyte. Specifically, the SEI film-forming agent can be one or more selected from the group consisting of: vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), succinic anhydride (SA), propylphosphonic anhydride (T3P), 1,3-propane sulpholone (PS), 1,4-butane sulpholone, ethane sulpholone, 1,3-propene sulpholone (PRS), 1,4-butene sulpholone, 1-methyl-1,3-propene sulpholone, ethylene sulfate, trimethylene sulfate (TMS), and methyltrimethylene sulfate. Ester (MTMS), tetraphenylborate, lithium difluorooxalate borate, succinic acid, adiponitrile, acetonitrile, propionitrile, butyric acid, valerate, octanoic acid, heptanoic acid, cyclopentaneformitrile, cyclohexaneformitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, fluorobenzene, triethanolamine, ethylenediamine, tetravinylsilane, LiN(SO2F)2 (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI), LiPO2F2, LiODFB, LiBOB and LiBF4.

[0076] Alternatively, the SEI film-forming agent may be vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propane sulpholactone (PS), succinic anhydride (SA), propylphosphonic anhydride (T3P), 1,3-propane sulpholactone (PS), or a combination thereof.

[0077] The content of the SEI film-forming agent in the first electrolyte can be less than 3.0% by weight, less than 2.0% by weight, less than 1.5% by weight, less than 1.2% by weight, or less than 1.0% by weight. When the content of this additive is within the above range, it can exhibit effects such as improving the battery's low-temperature output, high-temperature storage characteristics, and high-temperature lifespan characteristics. In addition, since the amount of SEI film-forming agent is not excessive, it can fully decompose at high temperatures, so that the SEI film-forming agent will not exist as unreacted substances or precipitates at room temperature, thereby preventing side reactions in the electrolyte during battery charging and discharging.

[0078] In addition to the components described above in the first electrolyte, the first electrolyte may also contain one or more additives, such as alkyl halogenated carbonate compounds (e.g., ethylene difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol diether, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, aluminum trichloride, lithium salts for corrosion protection, or HF scavengers, to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity.

[0079] In an exemplary embodiment of the present invention, the second electrolyte may also contain additives.

[0080] In an exemplary embodiment of the present invention, the second electrolyte may contain an HF scavenger as an additive.

[0081] The purpose of adding an HF remover is to remove HF generated by the reaction of incompletely dissociated lithium salts with residual H2O inside the battery. There are no particular restrictions on the type of HF remover, as long as it is a substance that does not participate in the battery's electrochemical reaction and can remove HF. Specifically, it can be a substance containing functional groups that can remove HF, or a substance containing Si-O bonds, a silane compound, or a silazane compound.

[0082] According to an exemplary embodiment of the present invention, the HF scavenger may be a silane compound.

[0083] According to an exemplary embodiment of the present invention, the HF scavenger may be at least one of trimethoxysilane (TMS), triethoxysilane (TES), hexamethyldisilazane (HMDS), or bis(trimethylsilyl)amine (HMDSA).

[0084] In this specification, the term "removal" refers to the function of using HF scavengers to eliminate, adsorb, or capture byproducts.

[0085] The content of HF scavenger in the second electrolyte can be less than 2% by weight, preferably less than 1.9% by weight, or less than 1.8% by weight. If the content of HF scavenger is less than 0.1% by weight, the effect of removing by-products by scavenger may be insufficient; if the content of HF scavenger exceeds 10% by weight, the HF scavenger material itself may act as a by-product in the battery, thereby leading to a decrease in the battery's electrochemical performance.

[0086] When the content of HF scavenger is within the above range, it can effectively remove the byproducts generated after the first activation process, thereby improving the battery characteristics of the final product.

[0087] In addition to the components mentioned above in the second electrolyte, the second electrolyte may also contain one or more additives, such as alkylene carbonates (e.g., ethylene difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol diether, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, aluminum trichloride, lithium salts for corrosion protection, or SEI forming agents, to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity.

[0088] In an exemplary embodiment of the present invention, the first electrolyte and the second electrolyte each independently contain the same or different solvents. There are no restrictions on the solvent used, as long as it is commonly used in the art.

[0089] For example, the solvent can be a non-aqueous organic solvent. Specifically, the solvent can include cyclic carbonate solvents, linear carbonate solvents, ester solvents, or combinations thereof.

[0090] The solvent may include: a mixture comprising cyclic carbonate solvents and linear carbonate solvents, a mixture comprising cyclic carbonate solvents and ester solvents, or cyclic carbonate solvents, linear carbonate solvents and ester solvents.

[0091] The content of cyclic carbonate solvent relative to 100 parts by weight of total solvent can be from 10 parts by weight to 45 parts by weight. For example, the content of cyclic carbonate solvent relative to 100 parts by weight can be more than 10 parts by weight, more than 15 parts by weight, more than 20 parts by weight, or more than 30 parts by weight, or less than 45 parts by weight, less than 40 parts by weight, less than 35 parts by weight, or less than 30 parts by weight. When the content of cyclic carbonate solvent is within the above range, the electrolyte can readily dissociate the lithium salt, thereby ensuring sufficient ionic conductivity and cation transport capacity.

[0092] Cyclic carbonate solvents are high-viscosity organic solvents. When contained in a solvent, cyclic carbonate solvents have a high dielectric constant, which allows lithium salts in the electrolyte to dissociate smoothly, thereby improving ionic conductivity and cation transport capacity.

[0093] In an exemplary embodiment of the present invention, the cyclic carbonate solvent may be at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene carbonate, 2,3-pentene carbonate, and vinylene carbonate.

[0094] The content of linear carbonate solvent relative to 100 parts by weight of total solvent can be from 55 parts by weight to 90 parts by weight. For example, the content of linear carbonate solvent relative to 100 parts by weight can be more than 55 parts by weight, more than 60 parts by weight, more than 65 parts by weight, or more than 70 parts by weight, or less than 90 parts by weight, less than 80 parts by weight, less than 70 parts by weight, or less than 60 parts by weight. If the content of linear carbonate solvent relative to 100 parts by weight of total solvent is not within the above range, the viscosity of the electrolyte will be too high, which may not adequately guarantee the impregnation performance of the electrolyte.

[0095] Therefore, by mixing and adding cyclic carbonate solvents with high dielectric constant and high viscosity, as well as linear carbonate solvents with relatively low dielectric constant and relatively low viscosity, an electrolyte with fully dissociated lithium salt can be provided, thereby giving the electrolyte both excellent ionic conductivity and excellent impregnation performance.

[0096] The linear carbonate solvent can be at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0097] The content of ester solvent relative to 100 parts by weight of total solvent can be from 1 part by weight to 70 parts by weight. For example, the content of ester solvent relative to 100 parts by weight of total solvent can be more than 1 part by weight, more than 5 parts by weight, more than 10 parts by weight, or more than 20 parts by weight, or less than 70 parts by weight, less than 60 parts by weight, less than 50 parts by weight, or less than 40 parts by weight. When the content of ester solvent relative to 100 parts by weight of total solvent is within the above range, the impregnation properties of the electrolyte can be improved and its low-temperature characteristics can be enhanced.

[0098] Ester solvents are low-density, low-viscosity, and low-melting-point organic solvents. When included in a solvent, they can prevent the electrolyte from becoming too viscous and improve low-temperature properties.

[0099] The ester solvent can be at least one selected from the group consisting of methyl acetate, ethyl acetate (EA), dimethyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), decanolide, valprolide, methavalprolide, and caprolactone.

[0100] The ester solvent can be a linear ester solvent. Specifically, the ester solvent can be at least one selected from the group consisting of methyl acetate, ethyl acetate (EA), dimethyl acetate, methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0101] An exemplary embodiment of the present invention provides a method for manufacturing a lithium secondary battery, comprising the steps of manufacturing an electrode assembly including a positive electrode and a negative electrode, and of housing the electrode assembly in a battery casing. The battery casing housing the electrode assembly can be prismatic, coin-shaped, cylindrical, pouch-shaped, or other types. The method of housing the electrode assembly in the casing may vary slightly depending on the type.

[0102] An exemplary embodiment of the present invention provides a method for manufacturing a lithium secondary battery, including the step of injecting a first electrolyte into a battery casing. The step of injecting the first electrolyte is the process of injecting the first electrolyte into a battery casing housing an electrode assembly containing a positive electrode and a negative electrode, and the electrolyte can be injected using various methods known in the art.

[0103] In an exemplary embodiment of the present invention, the first activation process includes the step of charging the battery casing that has been injected with the first electrolyte.

[0104] By performing a first activation process, a first electrolyte-derived film can be formed on various interfaces inside the electrode (e.g., the surface of the active material).

[0105] In an exemplary embodiment of the present invention, the first activation process includes the step of charging the battery casing that has been injected with the first electrolyte.

[0106] In an exemplary embodiment of the present invention, the first activation process includes the steps of aging, degassing, charging, and discharging the battery casing that has been injected with the first electrolyte. The aging and degassing methods are not limited, as long as they are known in the art.

[0107] In an exemplary embodiment of the present invention, the first activation process includes the steps of sealing the battery casing into which the first electrolyte has been injected and charging the sealed battery casing.

[0108] The first activation process may include: sealing the electrode assembly that has been injected with the first electrolyte, and then charging it under a constant current condition of 0.01C to 5C within a charging voltage range of 1.0 V to 4.5 V. Specifically, the charging voltage may be above 1.0 V or above 1.2 V, or below 4.5 V, below 4.2 V, or below 4.0 V. The constant current condition may be above 0.5 C, above 0.2 C, or above 0.1 C, or below 1 C, below 2 C, or below 3 C.

[0109] In an exemplary embodiment of the present invention, the step of injecting the second electrolyte is to inject the second electrolyte into the battery casing that has already undergone the first electrolyte injection and the first activation process. That is, since this is a step of injecting the second electrolyte when the first electrolyte is already present inside the battery casing, the first electrolyte and the second electrolyte can mix inside the battery casing where the second electrolyte has been injected.

[0110] In an exemplary embodiment of the present invention, the second activation process includes the step of charging the battery casing that has been injected with the second electrolyte.

[0111] By performing a second activation process, a second electrolyte-derived film can be formed at various interfaces inside the electrode (such as the surface of the first electrolyte-derived film or the active material) to supplement the first electrolyte-derived film.

[0112] In an exemplary embodiment of the present invention, the second activation process includes the step of charging the battery casing that has been injected with the second electrolyte.

[0113] The second activation process may be the same as or different from the first activation process.

[0114] In an exemplary embodiment of the present invention, the second activation process includes the steps of charging and discharging the battery casing that has been injected with the second electrolyte.

[0115] In an exemplary embodiment of the present invention, the second activation process includes the steps of aging, degassing, charging, and discharging the battery casing that has been injected with the second electrolyte. The aging and degassing methods are not limited, as long as they are known in the art.

[0116] In an exemplary embodiment of the present invention, the second activation process includes the steps of sealing the battery casing into which the second electrolyte has been injected and charging the sealed battery casing.

[0117] The second activation process may include: sealing the electrode assembly into which the second electrolyte has been injected, and then charging it under a constant current condition of 0.01C to 5C within a charging voltage range of 1.0 V to 4.5 V. Specifically, the charging voltage may be above 1.0 V or above 1.2 V, or below 4.5 V, below 4.2 V, or below 4.0 V. The constant current condition may be above 0.5 C, above 0.2 C, or above 0.1 C, or below 1 C, below 2 C, or below 3 C.

[0118] By injecting a first electrolyte into the electrode assembly and then performing a first activation process, the lithium secondary battery manufacturing method of the exemplary embodiment of the present invention can stably form an anti-corrosion film (electrolyte-derived film) on the surface of the electrode active material. By injecting a second electrolyte while the anti-corrosion film is stably formed in this manner and then performing a second activation step, the output characteristics and lifespan characteristics of the lithium secondary battery can be improved simultaneously.

[0119] In other words, the lithium secondary battery manufacturing method of the exemplary embodiment of the present invention is characterized by forming a robust and uniform electrolyte-derived film by sequentially injecting two different types of electrolytes and performing corresponding activation processes, thereby supplementing the areas where the electrolyte-derived film is unevenly formed due to local electrolyte concentration gradients.

[0120] Therefore, it can improve the stagnation phenomenon that often occurs when using a single type of electrolyte due to reduced reactivity or reaction overload, and can smoothly provide film-forming resources (the film breaks or forms as charge-discharge cycles proceed), and can also improve the ion diffusion capacity and conductivity of the electrolyte.

[0121] Secondary batteries

[0122] According to an exemplary embodiment of the present invention, a lithium secondary battery manufactured by the above method is provided.

[0123] The lithium secondary battery of an exemplary embodiment of the present invention includes a positive electrode and a negative electrode.

[0124] In an exemplary embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material stacked on the positive electrode current collector.

[0125] In an exemplary embodiment of the present invention, the positive electrode includes a current collector layer and a positive electrode active material layer disposed on the current collector layer. The positive electrode active material layer includes a positive electrode active material, a first electrolyte derivative film disposed on the positive electrode active material, and a second electrolyte derivative film disposed on the positive electrode active material or the first electrolyte derivative film.

[0126] The first electrolyte-derived membrane and the second electrolyte-derived membrane are membranes derived from the first electrolyte and the second electrolyte, respectively, and their shape, composition, and properties differ according to the composition of the first electrolyte and the second electrolyte. The first electrolyte-derived membrane and the second electrolyte-derived membrane can be distinguished by analyzing the remaining electrolyte or by depth profiling the electrolyte-derived membrane using NMR or XPS.

[0127] In the lithium secondary battery manufacturing method of an exemplary embodiment of the present invention, the first electrolyte-derived film is formed during the first activation process. That is, according to an exemplary embodiment of the present invention, the first electrolyte-derived film is an electrolyte-derived film derived from the first electrolyte.

[0128] In the lithium secondary battery manufacturing method of an exemplary embodiment of the present invention, the second electrolyte-derived film is formed during the second activation process. That is, according to an exemplary embodiment of the present invention, the second electrolyte-derived film is an electrolyte-derived film derived from the second electrolyte.

[0129] In an exemplary embodiment of the present invention, both the first electrolyte-derived membrane and the second electrolyte-derived membrane are CEI membranes.

[0130] The positive electrode current collector layer contains the positive electrode current collector, which is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the thickness of the positive electrode current collector can typically range from 1 μm to 500 μm, and microscopic irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector can take various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0131] The positive electrode active material may include a lithium complex transition metal compound containing nickel (Ni), cobalt (Co), and manganese (Mn). Furthermore, the positive electrode active material includes nickel, cobalt, and manganese, and may also include aluminum, but is not limited thereto.

[0132] In addition, the positive electrode active material may contain more than 80 mol% and less than 100 mol% nickel in metals other than lithium, and the lithium complex transition metal compound containing more than 80 mol% and less than 100 mol% nickel in metals other than lithium may contain one compound represented by the following chemical formula 1 or a mixture of two or more thereof.

[0133] [Chemical Formula 1]

[0134] Li a Ni 1-b-c-d Cob Mn c Q d O 2+δ

[0135] In this chemical formula, Q is any one or more elements selected from the group consisting of Na, K, Mg, Ca, Sr, Ni, Co, Ti, Al, Si, Sn, Mn, Cr, Fe, V, and Zr, where 1 ≤ a ≤ 1.5, 0 < b ≤ 0.5, 0 < c ≤ 0.5, 0 ≤ d ≤ 0.1, 0 < b + c + d ≤ 20, and -0.1 ≤ δ ≤ 1.0.

[0136] According to an exemplary embodiment of the present invention, the positive electrode of the above exemplary embodiment further includes a positive electrode binder and a conductive material.

[0137] The positive electrode binder can be used to improve the adhesion between the particles of the positive electrode active material and the adhesion force between the particles of the positive electrode active material and the positive electrode current collector. For the positive electrode binder, those known in the art can be used. Non-limiting examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene monomer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, various copolymers thereof, etc., and any one of them can be used alone or a mixture of two or more of them.

[0138] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode and can be used without particular limitation as long as the conductive material has electronic conductivity and does not cause chemical changes in the battery. Specific examples may include: graphite, such as natural graphite and artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, etc., and any one of them or a mixture of two or more of them can be used.

[0139] In an exemplary embodiment of the present invention, the negative electrode may include a negative electrode current collector and a negative electrode active material stacked on the negative electrode current collector.

[0140] In an exemplary embodiment of the present invention, the negative electrode includes a current collector layer and a negative electrode active material layer provided on the current collector layer. The negative electrode active material layer includes a negative electrode active material, a first electrolyte-derived film provided on the negative electrode active material, and a second electrolyte-derived film provided on the negative electrode active material or the first electrolyte-derived film.

[0141] The first electrolyte-derived membrane and the second electrolyte-derived membrane are membranes derived from the first electrolyte and the second electrolyte, respectively, and their shape, composition, and properties differ according to the composition of the first electrolyte and the second electrolyte. The first electrolyte-derived membrane and the second electrolyte-derived membrane can be distinguished by analyzing the remaining electrolyte or by depth profiling the electrolyte-derived membrane using NMR or XPS.

[0142] In the lithium secondary battery manufacturing method according to an exemplary embodiment of the present invention, the first electrolyte-derived film is formed during the first activation process. That is, according to an exemplary embodiment of the present invention, the first electrolyte-derived film is an electrolyte-derived film derived from the first electrolyte.

[0143] In the lithium secondary battery manufacturing method according to an exemplary embodiment of the present invention, the second electrolyte-derived film is formed during the second activation process. That is, according to an exemplary embodiment of the present invention, the second electrolyte-derived film is an electrolyte-derived film derived from the second electrolyte.

[0144] In an exemplary embodiment of the present invention, the first electrolyte-derived membrane and the second electrolyte-derived membrane are SEI membranes.

[0145] The negative electrode current collector layer contains a negative electrode current collector, and there are no particular limitations on the negative electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, charred carbon, or aluminum or stainless steel that has been surface-treated with carbon, nickel, titanium, silver, etc., can be used for the current collector. Specifically, the negative electrode current collector can be a transition metal with excellent carbon adsorption performance, such as copper and nickel. The thickness of the current collector can range from 1 μm to 500 μm. However, the thickness of the current collector is not limited to this.

[0146] In an exemplary embodiment of the present invention, the negative electrode active material is a carbon-based active material or a silicon-based active material.

[0147] The carbon-based active material can be graphite. Graphite can be natural graphite, artificial graphite, or a mixture of natural and artificial graphite. Furthermore, when the graphite is a mixture of natural and artificial graphite, the weight ratio of natural to artificial graphite can be from 50:50 to 90:10, specifically from 60:40 to 80:20, or from 65:35 to 75:25.

[0148] In an exemplary embodiment of the present invention, the average particle size of graphite (D) 50) It can be from 10 μm to 20 μm. Specifically, it can be from 15 μm to 20 μm. When the average particle size of the graphite falls within the above range, the influence of particle aggregation can be reduced, and the slurry dispersibility can be improved.

[0149] In an exemplary embodiment of the present invention, the silicon-based active material can be SiO x (x = 0), SiO x (0 < x < 2) or a silicon-carbon composite.

[0150] The silicon-carbon composite can be an Si / C-based active material. The silicon-carbon composite can be composed of Si and C that are not bonded to each other, but can also contain additional components as needed. For example, the silicon-carbon composite may or may not include silicon carbide represented as SiC. When the silicon-carbon composite contains silicon carbide, its content is 3 wt% or less. The silicon-carbon composite can exist in a crystalline state, an amorphous state, or a mixed state thereof. According to one example, C in the silicon-carbon composite can exist in an amorphous state.

[0151] The silicon-carbon composite can be a composite of silicon, carbon, etc., and can form a structure in which a core composite of silicon, carbon, etc. is surrounded by graphite, graphene, or amorphous carbon, etc. In the silicon-carbon composite, the silicon can be nano-silicon.

[0152] The silicon-carbon composite can be a physical composite or a chemical composite of the above carbon material and silicon material, and there is no limitation as long as it has a structure in which the carbon material and the silicon material form a composite.

[0153] In an exemplary embodiment of the present invention, in the lithium secondary battery of the above exemplary embodiment, in addition to the negative electrode active material, the negative electrode active material layer can further contain a negative electrode binder.

[0154] The negative electrode binder can be used to improve the adhesion between the negative electrode active material particles and the adhesion force between the negative electrode active material particles and the negative electrode current collector. The negative electrode binder can be those known in the art, and non-limiting examples thereof can include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogen in them is replaced by Li, Na, Ca, etc., and can also include various copolymers thereof.

[0155] The negative electrode active material layer may not contain conductive materials, but it can include them if necessary. There are no particular restrictions on the conductive materials contained in the negative electrode active material layer, as long as the conductive material is conductive and does not cause chemical changes in the battery. Examples include: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; metal powders, such as fluorocarbon powders, aluminum powder, and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials such as polyphenylene derivatives; and so on.

[0156] In an exemplary embodiment of the present invention, the positive electrode further includes a positive electrode active material layer containing a positive electrode active material, and the negative electrode further includes a negative electrode active material layer containing a negative electrode active material. The thicknesses of the positive and negative electrode active material layers are each 10 μm or more and 500 μm or less. The thickness of the positive electrode active material layer can be 90% to 110% of the thickness of the negative electrode active material layer, for example, 95% to 105%, and their thicknesses can be the same. Specifically, the thicknesses of the positive and negative electrode active materials can each be 15 μm or more and 400 μm or less, 20 μm or more and 300 μm or less, 25 μm or more and 200 μm or less, or 30 μm or more and 100 μm or less.

[0157] In an exemplary embodiment of the present invention, the electrode assembly includes a positive electrode and a negative electrode, and may also include a diaphragm.

[0158] The separator is used to separate the negative and positive electrodes and provide a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is typically used as a separator in a secondary battery. In particular, separators with high electrolyte retention capacity and low resistance to ion migration in the electrolyte are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes made from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers), or membranes having two or more stacked layers. Furthermore, commonly used porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, coated separators including ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0159] In an exemplary embodiment of the present invention, the electrode assembly is a power generation element capable of being charged and discharged, and includes a positive electrode and a negative electrode. The electrode assembly is not particularly limited, as long as it has a structure including a positive electrode and a negative electrode, and the electrode assembly can be manufactured by various methods known in the art.

[0160] Preferred embodiments will be provided below to better understand the invention. It will be apparent to those skilled in the art that the embodiments are provided merely to illustrate the invention, and various modifications and changes can be made within the scope and spirit of the invention. Such modifications and changes naturally fall within the scope of the claims included herein.

[0161] <Preparation Example>

[0162] Example 1

[0163] 1) Fabrication of electrode components

[0164] Using LiNi 0.92 Co 0.06 Mn 0.02 O2 (average particle size (D50): 15 μm) was used as the positive electrode active material, carbon black (product name: Super C65, manufactured by Timcal) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder, in a weight ratio of 97:1.5:1.5 to prepare a positive electrode active material layer composition. The positive electrode active material layer composition was added to N-methyl-2-pyrrolidone (NMP) as a solvent to form a positive electrode slurry, thereby preparing a positive electrode slurry with a solids content of 78% by weight. The positive electrode slurry was then subjected to a reaction at 537 mg / 25 cm⁻¹. 2 The loading amount was coated on both sides of the aluminum current collector (thickness: 12 μm) used as the positive electrode current collector, and then rolled and dried in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer with a thickness of 65 μm, thereby obtaining the positive electrode (porosity 26%).

[0165] Using SiO x A negative electrode active material layer composition was prepared using (x=0) (average particle size (D50): 8 μm) as the negative electrode active material, single-walled carbon nanotubes (SWCNTs) as the conductive material, and polyacrylamide (PAM) as the binder, at a weight ratio of 89:1:19. The negative electrode active material layer composition was added to distilled water as a solvent for forming the negative electrode slurry to prepare a negative electrode slurry with a solids content of 25% by weight. The negative electrode slurry was then mixed with distilled water at a concentration of 85 mg / 25 cm⁻¹. 2The loading amount was coated on both sides of the copper current collector (thickness: 8 μm) used as the negative electrode current collector, and then rolled and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer with a thickness of 33 μm, thereby obtaining the negative electrode (porosity 55%).

[0166] An electrode assembly is prepared by placing a polyethylene (PE) diaphragm between the positive and negative electrodes.

[0167] 2) Preparation of non-aqueous electrolyte

[0168] Preparation of the first electrolyte

[0169] The first electrolyte was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75 to obtain an organic solvent, and then adding ethylene carbonate (VC) as an SEI film-forming agent at a weight of 1.5% based on the total weight of the electrolyte, and adding 1.2 M of lithium salt LiPF6.

[0170] Preparation of the second electrolyte

[0171] The second electrolyte was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75 to obtain an organic solvent, and then adding trimethoxysilane (TMS) as an HF scavenger at a weight of 1.8% based on the total weight of the electrolyte, and adding 0.5 M of lithium salt LiFSI.

[0172] 3) Preparation of secondary batteries

[0173] The electrode assembly prepared as described above was placed in the battery casing. Then, 70% by volume of the first electrolyte was injected, relative to the total electrolyte injection volume of 100% by volume. The battery casing with the first electrolyte injected was left to stand at 25°C for 40 hours, and then charged to 4.2 V at a constant current of 1 C to perform the first activation process.

[0174] A second electrolyte of 30% volume was injected into the battery casing after the first activation process, relative to a total electrolyte injection of 100% volume. The battery was then charged at a constant current of 1.0 C to 4.2 V for a second activation process, thereby preparing a secondary battery.

[0175] Examples 2 to 4

[0176] The secondary battery was prepared in the same manner as in Example 1, except that in the preparation of the electrolyte in 2) of Example 1, the first electrolyte and the second electrolyte had the compositions shown in Table 1 below.

[0177] Comparative Example 1

[0178] The secondary battery was prepared in the same manner as in Example 1, except that in the preparation of the electrolyte in 2) of Example 1, the electrolyte was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 20:5:75 to obtain an organic solvent, and then adding ethylene carbonate (VC) as an SEI film-forming agent at a weight of 1.5% based on the total weight of the electrolyte, and adding 1.2 M of lithium salt LiPF6.

[0179] Comparative Examples 2 to 6

[0180] The secondary battery was prepared in the same manner as in Example 1, except that in the preparation of the electrolyte in 2) of Example 1, the first electrolyte and the second electrolyte had the compositions shown in Table 1 below.

[0181] [Table 1]

[0182] <Experimental Example>

[0183] The following items were evaluated for the above-prepared Examples 1 to 4 and Comparative Examples 1 to 6.

[0184] Experimental Example 1: Evaluation of discharge capacity, initial efficiency, and lifetime (capacity retention) characteristics

[0185] The secondary batteries prepared in the examples and comparative examples were charged and discharged to evaluate the discharge capacity, initial efficiency and capacity retention, and the results are listed in Table 2 or Table 3 below.

[0186] The first and second cycles were charged and discharged at 0.1 C, and the third to 49th cycles were charged and discharged at 0.5 C. The charging and discharging process was terminated at the 50th cycle in a charged state (lithium-containing negative electrode).

[0187] Charging conditions: CC (constant current) / CV (constant voltage) (5 mV / 0.005 C current cutoff)

[0188] Discharge conditions: CC (constant current) condition 1.5 V

[0189] The discharge capacity (mAh / g) and initial efficiency (%) are calculated based on the results of the first charge and discharge. Specifically, the initial efficiency (%) is obtained using the following formula.

[0190] Initial efficiency (%) = (First discharge capacity / First charge capacity) × 100

[0191] The capacity retention rate is obtained using the following formula.

[0192] Capacity retention rate (%) = (Capacity at 49th discharge / Capacity at first discharge) × 100

[0193] Experimental Example 2: Evaluation of the Lithium Plating Starting Point

[0194] The secondary batteries prepared in the examples and comparative examples were charged and discharged to evaluate the lithium plating initiation point, and the results are listed in Table 2 or Table 3 below.

[0195] The first and second cycles were performed at 0.1 C, and from the third cycle onwards, the cycles were performed at 3 C to measure the battery resistance.

[0196] Charging conditions: CC (constant current) / CV (constant voltage) (5 mV / 0.005 C current cutoff)

[0197] Discharge conditions: CC (constant current) condition 1.5 V

[0198] The point at which lithium plating occurs is defined as the state of charge (SOC) at which the battery resistance drops sharply and the current value changes abruptly.

[0199] [Table 2]

[0200] According to Table 2 above, it is confirmed that Examples 1 to 4 of the exemplary embodiments of the present invention are superior to Comparative Examples 1 to 5 in terms of discharge capacity, initial efficiency, capacity retention, and lithium plating initiation point. On the other hand, in Comparative Examples 1 to 5, the lithium salt concentration difference between the first electrolyte and the second electrolyte did not fall within the range of the present invention, confirming that an electrolyte-derived film was not stably formed on the electrode surface, resulting in poor performance. In particular, in Comparative Example 1, the electrolyte was injected only once and activation was performed only once, resulting in insufficient degassing during battery preparation, leading to the worst battery performance compared to Examples 1 to 4 and Comparative Examples 2 to 5.

[0201] In Examples 1, 3 and Comparative Example 6, the lithium salt concentration difference between the first electrolyte and the second electrolyte was 0.7 M. The ionic conductivity and viscosity of the first electrolyte and the second electrolyte were also measured, and the results are listed in Table 3 along with the experimental results above.

[0202] For each electrolyte, ionic conductivity was measured at 25°C using a probe-type ionic conductivity meter (InoLab 731, model: S470, manufacturer: Mettler Torodo), and viscosity was measured at 25°C using an RS150 viscometer (manufacturer: McIntosh).

[0203] [Table 3]

[0204] According to Table 3 above, when comparing Examples 1 and 3 with Comparative Example 6, it was confirmed that Examples 1 and 3 were superior to Comparative Example 6 in terms of discharge capacity, initial efficiency, capacity retention, and lithium plating initiation point. This can be understood as the lithium salt concentration of the first electrolyte in Comparative Example 6 being lower than that of the second electrolyte, resulting in the inability to stably form the first electrolyte-derived film initially on the electrode surface. Furthermore, it was confirmed that Example 1 exhibited superior battery characteristics compared to Example 3. This can be understood as follows: even though the lithium salt concentrations of the second electrolytes in Examples 1 and 3 were the same, the anion size of the lithium salt contained in the second electrolyte of Example 1 was larger than that of the lithium salt contained in the second electrolyte of Example 3. Therefore, even with the same lithium salt concentration in the second electrolyte, the second electrolyte of Example 1 exhibited higher ionic conductivity and lower viscosity.

[0205] Although the present invention has been described with reference to its exemplary embodiments, those skilled in the art can make various applications and modifications within the scope of the present invention based on the above description.

Claims

1. A method for manufacturing a lithium secondary battery, comprising: A first electrolyte is injected into a battery casing containing an electrode assembly comprising a positive electrode and a negative electrode. After the first electrolyte is injected, the first activation process is carried out; A second electrolyte, different from the first electrolyte, is injected into the battery casing that has undergone the first activation process; After the second electrolyte is injected, a second activation process is carried out, wherein the first electrolyte and the second electrolyte each independently contain lithium salts, the lithium salt concentration of the first electrolyte is greater than the lithium salt concentration of the second electrolyte, and the lithium salt concentration difference between the first electrolyte and the second electrolyte is 0.5 M to 2 M.

2. The method as described in claim 1, wherein, The lithium salt concentration of the first electrolyte is 0.8 M to 2 M.

3. The method as described in claim 1, wherein, The lithium salt concentration of the second electrolyte is 0.3 M to 1 M.

4. The method of claim 1, wherein, The lithium salt type in the first electrolyte is the same as the lithium salt type in the second electrolyte.

5. The method of claim 1, wherein, The lithium salt type in the first electrolyte is different from the lithium salt type in the second electrolyte.

6. The method of claim 5, wherein, The thermal decomposition temperature of lithium salt in the first electrolyte is lower than that of lithium salt in the second electrolyte.

7. The method of claim 1, wherein, The viscosity of the first electrolyte is higher than that of the second electrolyte.

8. The method of claim 1, wherein, The ionic conductivity of the first electrolyte is higher than that of the second electrolyte.

9. The method of claim 1, wherein, The first electrolyte also contains a solid electrolyte interphase (SEI) film-forming agent.

10. The method of claim 1, wherein, The second electrolyte also contains an HF scavenger.

11. The method of claim 10, wherein, The HF scavenger is a silane compound.

12. A lithium secondary battery manufactured by the method of manufacturing a lithium secondary battery according to any one of claims 1 to 11.

13. The lithium secondary battery as described in claim 12, wherein, The lithium secondary battery includes a positive electrode and a negative electrode. The positive electrode includes a current collector layer and a positive electrode active material layer disposed on the current collector layer. The positive electrode active material layer includes: a positive electrode active material, a first electrolyte-derived film disposed on the positive electrode active material, and a second electrolyte-derived film disposed on the positive electrode active material or the first electrolyte-derived film.

14. The lithium secondary battery as described in claim 12, wherein, The lithium secondary battery includes a positive electrode and a negative electrode. The negative electrode includes a current collector layer and a negative electrode active material layer disposed on the current collector layer. The negative electrode active material layer includes: a negative electrode active material, a first electrolyte-derived film disposed on the negative electrode active material, and a second electrolyte-derived film disposed on the negative electrode active material or the first electrolyte-derived film.

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