Method for preparing negative electrode

Abstract

The present invention relates to a method for preparing a negative electrode, the method comprising preparing a negative electrode structure comprising expanded natural graphite, impregnating the negative electrode structure with a pre-lithiation solution, and pre-lithiating the impregnated negative electrode structure by electrochemically charging the impregnated negative electrode structure to 10% to 20% of the charge capacity of the negative electrode structure.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from Korean Patent Application No. 10-2020-0139477, filed on October 26, 2020, the disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to a method for preparing a negative electrode. Background Art

[0005] Recently, with the rapid popularization of electronic devices using batteries (such as mobile phones, laptop computers and electric vehicles), the demand for secondary batteries with relatively high capacity, small size and light weight has been growing rapidly. In particular, lithium secondary batteries are the focus of public attention as driving power sources for portable devices because they are lightweight and have high energy density. Therefore, research and development work has been actively carried out to improve the performance of lithium secondary batteries.

[0006] Lithium secondary batteries generally include a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, an electrolyte, and an organic solvent. In addition, with respect to the positive electrode and the negative electrode, an active material layer comprising a positive electrode active material or a negative electrode active material may be formed on a current collector. Lithium-containing metal oxides (such as LiCoO2 and LiMn2O4) are generally used as the positive electrode active material in the positive electrode, and correspondingly, carbon-based active materials or silicon-based active materials that do not contain lithium are used as the negative electrode active material in the negative electrode.

[0007] Among the negative electrode active materials, natural graphite, expanded natural graphite or artificial graphite are known to be carbon-based active materials. Among them, expanded natural graphite is a carbon-based active material in which the spacing between the crystal lattice planes in natural graphite is increased by artificial treatment, such as treatment of natural graphite with acid or alkali. Since expanded natural graphite has a large spacing between the lattice planes to promote the insertion and extraction of lithium ions, it is advantageous that the output characteristics can be maximized. However, due to the large spacing between the lattice planes, the specific surface area increases and the irreversible capacity increases, so there is a limitation that the initial efficiency is very low and the life characteristics are poor.

[0008] Therefore, there is an urgent need to develop a method for preparing a negative electrode that can improve initial efficiency and life characteristics while exhibiting excellent levels of output characteristics of expanded natural graphite in a negative electrode using expanded natural graphite.

[0009] Korean Patent No. 10-0291067 discloses a method for pre-lithiating a carbon electrode and a method for preparing a lithium secondary battery using the method for pre-lithiating a carbon electrode.

[0010] [Prior Art Document]

[0011] [Patent Document]

[0012] Korean Patent No. 10-0291067 Summary of the Invention

[0013] Technical issues

[0014] One aspect of the present invention provides a method for preparing a negative electrode, which can improve initial efficiency and life characteristics while exhibiting output characteristics of expanded natural graphite by electrochemically charging the negative electrode structure including expanded natural graphite at a specific charge capacity to pre-lithiate the negative electrode structure.

[0015] Technical Solution

[0016] According to one aspect of the present invention, a method for preparing a negative electrode is provided, the method comprising: preparing a negative electrode structure comprising expanded natural graphite; impregnating the negative electrode structure with a pre-lithiation solution; and pre-lithiating the impregnated negative electrode structure by electrochemically charging the impregnated negative electrode structure to 10% to 20% of the charge capacity of the negative electrode structure.

[0017] Beneficial effects

[0018] According to the present invention, the method for preparing a negative electrode is characterized in that, after impregnating a negative electrode structure comprising expanded natural graphite with a pre-lithiation solution, the impregnated negative electrode structure is electrochemically charged to a specific charge level. The negative electrode prepared by pre-lithiation through electrochemical charging to the above charge level can exhibit excellent output characteristics, initial efficiency, and lifespan characteristics. In particular, expanded natural graphite is known to have lower initial efficiency and poorer lifespan characteristics than conventional natural graphite. However, since the irreversible capacity of the negative electrode comprising expanded natural graphite electrochemically charged to the above charge level is compensated to a desired level, the initial efficiency can be improved, and the low lithium ion diffusion resistance resulting from the larger spacing between the lattice planes of the expanded natural graphite can improve the lifespan characteristics to significantly better than those of conventional natural graphite. DETAILED DESCRIPTION

[0019] It will be understood that the words and terms used in the specification and claims should not be interpreted as having the meanings defined in commonly used dictionaries, and it will be further understood that, based on the principle that the inventor can appropriately define the meanings of words or terms in order to best explain the present invention, these words or terms should be interpreted as having meanings consistent with their meanings in the context of the art and the technical ideas of the present invention.

[0020] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. In the specification, terms in the singular form may include plural forms unless otherwise indicated.

[0021] It will be further understood that the terms “include,” “comprises,” or “has,” when used in this specification, indicate the presence of stated features, quantities, steps, elements, or a combination thereof, but do not preclude the presence or addition of one or more other features, quantities, steps, elements, or a combination thereof.

[0022] The expression "average particle size (D 50 The average particle size (D) can be defined as the particle size at 50% cumulative volume in the particle size distribution curve. For example, the average particle size (D) can be measured using a laser diffraction method. 50 Laser diffraction methods can typically measure particle sizes ranging from submicron levels to several mm and can obtain highly reproducible and high-resolution results.

[0023] Hereinafter, the present invention will be described in detail.

[0024] <Method for Preparing Negative Electrode>

[0025] The present invention relates to a method for preparing a negative electrode, and in particular, to a method for preparing a negative electrode for a lithium secondary battery.

[0026] The method for preparing a negative electrode of the present invention comprises the steps of: preparing a negative electrode structure comprising expanded natural graphite; impregnating the negative electrode structure with a pre-lithiation solution; and pre-lithiating the impregnated negative electrode structure by electrochemically charging the impregnated negative electrode structure to 10% to 20% of the charge capacity of the negative electrode structure.

[0027] According to the present invention, the method for preparing a negative electrode is characterized in that, after impregnating a negative electrode structure comprising expanded natural graphite with a pre-lithiation solution, the impregnated negative electrode structure is electrochemically charged to a specific charge level. The negative electrode prepared by pre-lithiation through electrochemical charging to the above charge level can exhibit excellent output characteristics, initial efficiency, and lifespan characteristics. In particular, expanded natural graphite is known to have lower initial efficiency and poorer lifespan characteristics than conventional natural graphite. However, since the irreversible capacity of the negative electrode comprising expanded natural graphite that is electrochemically charged to the above charge level is compensated to a desired level, the initial efficiency can be improved, and the low lithium ion diffusion resistance resulting from the larger spacing between the lattice planes of the expanded natural graphite can improve the lifespan characteristics to significantly better than those of conventional natural graphite.

[0028] The method for preparing a negative electrode of the present invention includes preparing a negative electrode structure including expanded natural graphite.

[0029] Expanded natural graphite can be included in the negative electrode structure as a negative electrode active material.

[0030] Expanded natural graphite is generally graphite in which an acid or base treated into the natural graphite chemically bonds with carbon atoms and then thermally decomposes to increase the spacing between the crystal lattice planes of the natural graphite. Generally, since the spacing between the crystal lattice planes of expanded natural graphite is larger than that of typical natural graphite, expanded natural graphite has better output characteristics than typical natural graphite. However, due to the increased side reactions of the electrolyte solution caused by the larger specific surface area, it has a larger irreversible capacity and lower initial efficiency and lifespan characteristics. However, expanded natural graphite that has undergone pre-lithiation by electrochemically charging a specific charge capacity, as will be described later, can exhibit excellent output characteristics while fully compensating for its irreversible capacity. Therefore, it has high initial efficiency and can exhibit better lifespan characteristics compared to typical natural graphite because lithium ion diffusion is smooth and the larger spacing between the crystal lattice planes can have low lithium ion diffusion resistance.

[0031] During X-ray diffraction (XRD) measurement, the interplanar spacing d002 of the (002) planes of expanded natural graphite can be in the range of 0.3370 nm to 0.3410 nm, for example, in the range of 0.3390 nm to 0.3405 nm. Since the expanded natural graphite has an interplanar spacing d002 within the above range, lithium ions can enter or diffuse into the expanded natural graphite more easily than in general natural graphite with a relatively small d002, and the expanded natural graphite can have excellent output characteristics. In addition, according to the method for preparing a negative electrode of the present invention, the irreversible capacity of the expanded natural graphite is compensated by the pre-lithiation process to be described later, wherein, since the expanded natural graphite with an interplanar spacing d002 in the above range has a low lithium ion diffusion resistance, it can also have excellent life characteristics even during repeated charge and discharge.

[0032] The crystal size Lc of the expanded natural graphite along the c-axis during XRD measurement may be in the range of 10 nm to 24 nm, for example, in the range of 16 nm to 21 nm. The expanded natural graphite may have excellent output characteristics by having the crystal size Lc in the above range.

[0033] The Brunauer-Emmett-Teller (BET) specific surface area of expanded natural graphite can be 4m 2 / g to 8m 2 / g, for example, within 4.5m 2 / g to 6.5m 2 When the BET specific surface area is within the above range, the ingress of lithium ions into the expanded natural graphite is facilitated, thereby improving output characteristics. Furthermore, according to the method for preparing a negative electrode of the present invention, the irreversible capacity of the expanded natural graphite is compensated by the pre-lithiation process described later. Since the expanded natural graphite having a specific surface area within the above range has low lithium ion diffusion resistance, it can have excellent lifespan characteristics even during repeated charge and discharge.

[0034] Considering the structural stability of the negative electrode active material during charge and discharge, the average particle size of expanded natural graphite (D 50 ) may be in the range of 8 μm to 20 μm, for example in the range of 12 μm to 18 μm.

[0035] The expanded natural graphite may contain oxygen (O) in an amount of 800 ppm to 3,000 ppm, particularly 1,000 ppm to 2,800 ppm, and more particularly 1,500 ppm to 2,500 ppm, based on the weight of the expanded natural graphite. Since the expanded natural graphite is subjected to acid treatment or alkali treatment during the preparation process, oxygen-containing functional groups such as hydroxyl groups (-OH) are formed on the surface and / or inside the expanded natural graphite. Therefore, the expanded natural graphite has a higher oxygen content than general natural graphite.

[0036] The oxygen content of the expanded natural graphite can be measured by an ONH analyzer or an X-ray photoelectron spectroscopy (XPS) analyzer.

[0037] The negative electrode structure may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer may include expanded natural graphite.

[0038] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause adverse chemical changes in the battery. Specifically, the negative electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, burned carbon, and aluminum-cadmium alloy, and may preferably include copper.

[0039] The negative electrode current collector may generally have a thickness of 3 μm to 500 μm.

[0040] Microscopic irregularities may be formed on the surface of the negative electrode current collector to improve adhesion of the negative electrode active material. For example, the negative electrode current collector may be used in various shapes, such as a film, sheet, foil, mesh, porous body, foam, nonwoven fabric, and the like.

[0041] The negative electrode active material layer may include the expanded natural graphite described above as a negative electrode active material.

[0042] From the perspective of fully exhibiting excellent output characteristics and capacity characteristics of the expanded natural graphite, the expanded natural graphite may be included in the negative electrode active material layer in an amount of 85 to 99 wt %, for example, 90 to 98 wt %.

[0043] The negative active material layer may further include, in addition to the expanded natural graphite, at least one additive selected from the group consisting of a binder, a conductive agent, and a thickener.

[0044] The binder is used to improve the performance of the battery by improving the adhesion between the negative electrode active material layer and the negative electrode collector, wherein, for example, the binder may include at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylate, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR) and fluororubber, and may preferably include styrene butadiene rubber.

[0045] The negative electrode active material layer may include a binder in an amount of 0.5 wt % to 10 wt %, for example, 1 wt % to 5 wt %, and when the amount of the binder is within the above range, the capacity of the negative electrode can be increased by increasing the solid content of the active material while exhibiting sufficient active material adhesion.

[0046] Any conductive agent can be used without particular limitation as long as it has conductivity and does not cause adverse chemical changes in the battery, and for example, conductive materials such as graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, conductive carbon black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; carbon fluoride; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives can be used.

[0047] The conductive agent may be included in the negative active material layer in an amount of 0.5 wt % to 10 wt %, for example, 1 wt % to 5 wt %.

[0048] A thickener is added to facilitate dispersion of expanded natural graphite during preparation of a negative electrode slurry for forming a negative electrode active material layer, wherein the thickener may include, for example, carboxymethyl cellulose (CMC). The thickener may be included in an amount of 0.1 wt % to 5 wt % in the negative electrode active material layer.

[0049] The thickness of the negative active material layer may be in the range of 10 μm to 100 μm, for example, in the range of 20 μm to 80 μm.

[0050] The negative electrode can be prepared by coating a negative electrode slurry on a negative electrode current collector, and then rolling and drying the coated negative electrode current collector. The negative electrode slurry is prepared by adding expanded natural graphite and an optional binder and / or conductive agent to a solvent (such as water) for forming the negative electrode slurry.

[0051] The method of preparing the negative electrode of the present invention comprises impregnating the negative electrode structure with a pre-lithiation solution.

[0052] The impregnation of the negative electrode structure is performed so that smooth pre-lithiation is performed during electrochemical charging to be described later by sufficiently wetting the negative electrode structure in the pre-lithiation solution.

[0053] The pre-lithiation solution may include a lithium salt and an organic solvent.

[0054] The organic solvent is not particularly limited as long as it can be used as a medium for electrochemical reaction and movement of ions, and in particular, an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene and fluorobenzene; or an aromatic hydrocarbon-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate can be used. Carbonate-based solvents include carbonate-based solvents such as isopropyl carbonate (PC); alcohol-based solvents such as ethanol and isopropyl alcohol; nitriles such as R-CN (wherein R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate-based solutions are ideal in terms of improving electrochemical stability, and in particular, ethylmethylcarbonate (EMC) or ethylene carbonate (EC) are more ideal.

[0055] The lithium salt may include at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI and LiB(C2O4)2, and may preferably include LiPF6.

[0056] Based on the pre-lithiation solution, the concentration of the lithium salt may be in the range of 0.1 M to 3 M, for example, 0.5 M to 1.5 M. When the concentration of the lithium salt is within the above range, it is ideal because the lithium salt can be sufficiently dissolved so that lithium ions can be smoothly intercalated into the active material.

[0057] The pre-lithiation solution may further include an additive comprising at least one selected from the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), polystyrene (PS), succinonitrile, ethylene glycol bis(propionitrile) ether, and lithium bis(fluorosulfonyl)imide (LiFSI). Fluorinated ethylene carbonate is preferred in terms of promoting pre-lithiation by stabilizing the surface of the negative electrode active material during pre-lithiation.

[0058] In terms of promoting pre-lithiation by stabilizing the surface of the negative active material, the additive is included in the pre-lithiation solution in an amount of 0.1 to 15 wt %, for example 0.5 to 5 wt %, based on the total weight of the pre-lithiation solution.

[0059] In terms of ensuring stable and uniform pre-lithiation, the impregnation of the negative electrode structure may be performed for 0.5 hours to 15 hours, for example, 1 hour to 5 hours.

[0060] The method of preparing the negative electrode of the present invention includes pre-lithiating the impregnated negative electrode structure by electrochemically charging the impregnated negative electrode structure to 10% to 20% of the charge capacity of the negative electrode structure.

[0061] When the impregnated negative electrode structure is pre-lithiated, the lithium ions in the pre-lithiated solution are embedded in the surface and / or interior of the expanded natural graphite, which can compensate for the irreversible capacity of the expanded natural graphite and form a solid electrolyte interface layer (SEI layer) on the expanded natural graphite in advance.

[0062] Pre-lithiation is performed by electrochemically charging the impregnated negative electrode structure. As a general pre-lithiation method, a method of inserting lithium metal into the negative electrode by bringing lithium metal into direct contact with the negative electrode or a method of electrochemically charging the negative electrode structure using lithium metal as a counter electrode is known. However, since the charge amount during electrochemical charging is important in order to simultaneously improve the initial efficiency, output characteristics, and life characteristics of the impregnated negative electrode structure as described later, it is desirable to use an electrochemical charging method in which the lithium charge amount can be easily controlled.

[0063] During pre-lithiation, the impregnated negative electrode structure is electrochemically charged to 10% to 20% of the negative electrode structure's charge capacity. Since the irreversible capacity of the expanded natural graphite is fully compensated when the negative electrode structure is electrochemically charged at this charge ratio, initial efficiency can be improved, the high output characteristics of expanded natural graphite can be smoothly exhibited, and the low lithium ion diffusion resistance caused by the larger spacing between lattice planes can exhibit better lifespan characteristics than typical natural graphite. Accordingly, not only can the drawbacks of expanded natural graphite (which is generally known to have lower lifespan characteristics and initial efficiency than typical natural graphite) be overcome, but expanded natural graphite can also exhibit significantly improved output characteristics and lifespan characteristics compared to typical natural graphite.

[0064] If the impregnated negative electrode structure is electrochemically charged to less than 10% of the negative electrode structure's charge capacity, the extent to which the irreversible capacity of the expanded natural graphite is compensated is minimal, making it difficult to improve initial efficiency and, consequently, long-term cycle characteristics. If the impregnated negative electrode structure is electrochemically charged to more than 20% of the negative electrode structure's charge capacity, excessive lithium insertion causes an increase in resistance, resulting in a decrease in output. Consequently, there is a problem in which the decrease in output and the increase in resistance accumulate, deteriorating long-term cycle characteristics.

[0065] Preferably, the impregnated negative electrode structure can be electrochemically charged to 13% to 16% of the charge capacity of the negative electrode structure, and when the impregnated negative electrode structure is electrochemically charged within the above range, the effect of simultaneously improving the initial efficiency, output characteristics and life characteristics of the negative electrode can be maximized.

[0066] Electrochemical charging can be performed using lithium metal as a counter electrode, which is disposed in a pre-lithiation solution and spaced apart from the impregnated negative electrode structure. Since the lithium metal used as the counter electrode is disposed in the pre-lithiation solution so as to be spaced apart from the negative electrode structure, an electrode short circuit phenomenon that can occur when the lithium metal and the negative electrode structure come into direct contact with each other during electrochemical charging can be prevented.

[0067] Electrochemical charging can be performed using an electrochemical charger / discharger. Specifically, WOCS3000s (manufactured by WonATech Co., Ltd.) can be used as the electrochemical charger / discharger.

[0068] 0.1mA / cm 2 Up to 3mA / cm 2 The current density is 0.3 mA / cm 2 Up to 2mA / cm 2 The electrochemical charge is performed at a current density of 100 Å to 200 Å, and when the electrochemical charge is performed within the above range, stable and uniform pre-lithiation can be performed on the negative electrode active material.

[0069] Electrochemical charging may be performed at 10° C. to 70° C., for example, 20° C. to 40° C., wherein the negative electrode structure is stably charged and discharged at such a temperature to enable formation of a uniform SEI layer, and the above temperature is ideal in terms of preventing damage to the negative electrode structure caused by electrochemical charging.

[0070] The pre-lithiated negative electrode structure can be used as a negative electrode for a secondary battery, more specifically, a negative electrode for a lithium secondary battery, and can have high levels of initial efficiency and lifespan characteristics while smoothly exhibiting the excellent output characteristics of expanded natural graphite.

[0071] The negative electrode prepared by the preparation method of the present invention can be preferably used in a secondary battery, in particular in a lithium secondary battery.

[0072] The secondary battery may include a negative electrode prepared by the preparation method described above; a positive electrode facing the negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte. The positive electrode, separator, and electrolyte used in conventional lithium secondary batteries may be used without limitation.

[0073] The secondary battery is suitable for portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0074] In addition, the secondary battery may be used in a battery module including the secondary battery as a unit battery or a battery pack including the battery module.

[0075] The battery module or battery pack can be used as a power source for at least one medium-sized and large-sized device in a power tool; an electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0076] Below, the embodiment of the present invention will be described in detail in a manner that can be easily performed by those skilled in the art to which the present invention belongs. However, the present invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein.

[0077] Example

[0078] Example 1: Preparation of negative electrode

[0079] <Preparation of negative electrode structure>

[0080] Expanded natural graphite (average particle size (D 50 ):16μm) as a negative electrode active material. During XRD measurement, the interplanar spacing d002 of the (002) plane of the expanded natural graphite was 0.3402nm, the crystal size along the c-axis direction of the expanded natural graphite was 17.73nm during XRD measurement, and the BET specific surface area was 5.4m 2 / g, and the oxygen (O) content based on the weight of the expanded natural graphite measured by an ONH analyzer (manufacturer: LECO, name: ONH 836) was 2,100 ppm.

[0081] A negative electrode active material, carbon black as a conductive agent, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose as a thickener were added to water at a weight ratio of 96.0:1.0:1.3:1.1 to prepare a negative electrode slurry.

[0082] The negative electrode slurry was coated on one surface of a copper negative electrode collector (thickness: 15 μm), and the coated negative electrode collector was roll-pressed and dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer (thickness: 111 μm) on one surface of the copper negative electrode collector and to form a 1.4875 μm thick negative electrode active material layer. 2 In this case, the loading amount of the negative electrode active material is 3.61 mAh / cm 2 .

[0083] <Impregnation of Negative Electrode Structure>

[0084] The prepared anode structure was immersed in the pre-lithiation solution for 3 h.

[0085] LiPF6 as a lithium salt was added at a concentration of 1 M to an organic solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed in a volume ratio of 30:70 as a pre-lithiation solution, and fluoroethylene carbonate (FEC) was added as an additive in an amount of 2 wt % based on the total weight of the pre-lithiation solution.

[0086] <Pre-lithiation Treatment>

[0087] A lithium metal counter electrode is immersed in the pre-lithiation solution and spaced a predetermined distance from the negative electrode structure.

[0088] Afterwards, the negative electrode structure was subjected to a pre-lithiation treatment. The pre-lithiation treatment was performed at 25° C. Specifically, in the pre-lithiation treatment, the current was 0.5 mA / cm 2 The impregnated negative electrode structure was electrochemically charged at a current density of 15.7% of the charge capacity of the negative electrode structure.

[0089] The negative electrode structure was cleaned with ethyl methyl carbonate solvent and dried at room temperature to obtain the negative electrode of Example 1.

[0090] Example 2: Preparation of negative electrode

[0091] The negative electrode of Example 2 was prepared in the same manner as Example 1, except that the impregnated negative electrode structure was electrochemically charged to a charge of 17.5% of the charge capacity of the negative electrode structure.

[0092] Example 3: Preparation of negative electrode

[0093] The negative electrode of Example 3 was prepared in the same manner as Example 1, except that the impregnated negative electrode structure was electrochemically charged to a charge of 11.4% of the charge capacity of the negative electrode structure.

[0094] Comparative Example 1: Preparation of negative electrode

[0095] The negative electrode structure that was not subjected to the impregnation and pre-lithiation treatment in Example 1 was used as the negative electrode of Comparative Example 1.

[0096] Comparative Example 2: Preparation of negative electrode

[0097] In addition to using general natural graphite (D 50 The negative electrode of Comparative Example 2 was prepared in the same manner as in Example 1, except that expanded natural graphite (0.16 μm) was used as the negative electrode active material and the impregnation and pre-lithiation treatments were not performed on the negative electrode structure.

[0098] During the XRD measurement, the interplanar spacing d002 of the (002) plane of the natural graphite is 0.3355 nm, the crystal size Lc along the c-axis direction of the natural graphite is 27.57 nm, and the BET specific surface area is 2.3 m 2 / g, and the oxygen (O) content based on the weight of general natural graphite measured by an ONH analyzer (manufacturer: LECO, name: ONH 836) is 300 ppm.

[0099] Comparative Example 3: Preparation of negative electrode

[0100] The negative electrode of Comparative Example 3 was prepared in the same manner as Example 1, except that the general natural graphite used in Comparative Example 2 was used as the negative electrode active material and the impregnated negative electrode structure was electrochemically charged to 9.3% of the charge capacity of the negative electrode structure.

[0101] Comparative Example 4: Preparation of negative electrode

[0102] The negative electrode of Comparative Example 4 was prepared in the same manner as Example 1, except that the general natural graphite used in Comparative Example 2 was used as the negative electrode active material and the impregnated negative electrode structure was electrochemically charged to a charge amount of 15.7% of the charge capacity of the negative electrode structure.

[0103] Comparative Example 5: Preparation of negative electrode

[0104] The negative electrode of Comparative Example 5 was prepared in the same manner as Example 1, except that the impregnated negative electrode structure was electrochemically charged to a charge of 24.2% of the charge capacity of the negative electrode structure.

[0105] Comparative Example 6: Preparation of negative electrode

[0106] The negative electrode of Comparative Example 6 was prepared in the same manner as Example 1, except that the impregnated negative electrode structure was electrochemically charged to a charge of 8.9% of the charge capacity of the negative electrode structure.

[0107] Test example

[0108] Test Example 1: Initial Efficiency Evaluation

[0109] <Preparation of lithium secondary battery>

[0110] Lithium metal was prepared as the positive electrode.

[0111] A propylene polymer separator was placed between the negative electrode prepared in Example 1 and the positive electrode prepared above, and an electrolyte was injected to prepare a coin-type lithium secondary battery. LiPF6 as a lithium salt was added to an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70 at a concentration of 1 M as an electrolyte, and fluoroethylene carbonate (FEC) was added as an additive in an amount of 0.5% by weight based on the total weight of the electrolyte.

[0112] A coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the negative electrodes prepared in Examples 2 and 3 and Comparative Examples 1 to 6 were used.

[0113] <Initial Efficiency Evaluation>

[0114] The charge capacity and discharge capacity of the coin-type half-cells of these Examples and Comparative Examples prepared above were measured, the initial efficiency was calculated from the following formula, and the results are presented in Table 1. The charge and discharge conditions were as follows.

[0115] Charging conditions: CCCV (constant current constant voltage) mode, 0.1C charging, 5mV and 0.005C cut-off Discharge conditions: CC mode, 0.1C discharge, 1.5V cut-off

[0116] Initial efficiency = (discharge capacity in the first cycle / charge capacity) × 100

[0117] Test Example 2: Output Characteristics Evaluation

[0118] <Preparation of lithium secondary battery>

[0119] An aluminum current collector was coated with a positive electrode slurry in which a positive electrode active material LiCoO 2 , a conductive agent Super C, and a binder PVdF were mixed in a weight ratio of 96:1:3, subjected to roll pressing, and dried at 130° C. to prepare a positive electrode.

[0120] The propylene polymer separator was placed between the negative electrode prepared in Example 1 and the positive electrode prepared above, and an electrolyte was injected to prepare a coin-type lithium secondary battery. LiPF6 as a lithium salt was added to an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70 at a concentration of 1 M as an electrolyte, and fluoroethylene carbonate (FEC) was added as an additive in an amount of 0.5% by weight based on the total weight of the electrolyte.

[0121] A coin-type lithium secondary battery was prepared in the same manner as in Example 1, except that the negative electrodes prepared in Examples 2 and 3 and Comparative Examples 1 to 6 were used.

[0122] <Output Characteristics Evaluation>

[0123] The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 were cyclically charged and discharged at a large current to measure the capacity retention ratio in the 100th cycle and evaluate the output characteristics.

[0124] Specifically, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 were charged and discharged under charging (CC / CV mode, 5C charging, 4.2V, 0.005C cutoff) and discharging (CC mode, 5C discharging, 3.0V cutoff) conditions for the 100th cycle.

[0125] The capacity retention after 100 cycles was evaluated by the following equation, and the results are presented in Table 1.

[0126] Capacity retention (%) = {(discharge capacity at the 100th cycle) / (discharge capacity at the 1st cycle)} × 100

[0127] Test Example 3: Cycling Capacity Retention Evaluation

[0128] <Preparation of Lithium Secondary Battery>

[0129] The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 were prepared in the same manner as the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 prepared in Experimental Example 2.

[0130] <Cycle Capacity Retention Evaluation>

[0131] The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 were evaluated for cycle capacity retention.

[0132] Specifically, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 were charged and discharged at 45° C. under charging (CC / CV mode, 1C charging, 4.2 V, 0.005 C cutoff) and discharging (CC mode, 1C discharging, 3.0 V cutoff) conditions for the 400th cycle.

[0133] The capacity retention after 400 cycles was evaluated by the following equation, and the results are presented in Table 1.

[0134] Capacity retention (%) = {(discharge capacity at the 400th cycle) / (discharge capacity at the 1st cycle)} × 100

[0135] [Table 1]

[0136]

[0137] Referring to Table 1, it can be confirmed that the secondary batteries of Examples 1 to 3, which were pre-lithiated by electrochemically charging the expanded natural graphite to a desired charge amount, have excellent initial efficiency, output characteristics, and lifespan performance.

[0138] However, it was confirmed that the secondary battery of Comparative Example 1, in which pre-lithiation was not performed on the expanded natural graphite, exhibited remarkably low initial efficiency and lifespan performance.

[0139] In addition, the secondary battery of Comparative Example 2 in which pre-lithiation was not performed on general natural graphite not only exhibited relatively lower output characteristics than the examples but also exhibited a decrease in lifespan performance.

[0140] Furthermore, the secondary batteries of Comparative Examples 3 and 4, in which general natural graphite was pre-lithiated, may have slightly improved initial efficiency, but have significantly decreased output characteristics and high lithium ion diffusion resistance. This decrease in output characteristics leads to a decrease in lifespan characteristics due to repeated charge and discharge, and it was confirmed that Comparative Examples 3 and 4 have decreased lifespan characteristics compared to Examples 1 to 3 using expanded natural graphite.

[0141] Furthermore, in Comparative Example 5, in which expanded natural graphite was pre-lithiated by electrochemically charging it to a very high charge, it was confirmed that the output characteristics significantly decreased due to the excessive lithium charge acting as resistance, and that the lifespan performance decreased due to the decreased output characteristics. In Comparative Example 6, in which expanded natural graphite was pre-lithiated by electrochemically charging it to a very low charge, the irreversible capacity of the expanded natural graphite was not sufficiently removed, resulting in a significant decrease in initial efficiency and a significant decrease in lifespan performance due to insufficient removal of the irreversible capacity.

[0142] Therefore, it can be determined that the expanded natural graphite prepared by the preparation method of the present invention can exhibit excellent output characteristics while having high initial efficiency due to sufficient compensation of irreversible capacity, and because the larger spacing between lattice planes allows lithium ion diffusion to be smooth, it can exhibit better life characteristics than general natural graphite.

Claims

1. A method for preparing a negative electrode, the method comprising: preparing a negative electrode structure comprising expanded natural graphite; impregnating the negative electrode structure with a pre-lithiation solution; and pre-lithiating the impregnated negative electrode structure by electrochemically charging the negative electrode structure to 10% to 20% of the charge capacity of the negative electrode structure, wherein the crystal size Lc of the expanded natural graphite along the c-axis direction is in the range of 10 nm to 24 nm during XRD measurement, wherein the interplanar spacing d002 of the (002) planes of the expanded natural graphite is in the range of 0.3370 nm to 0.3410 nm during XRD measurement, and The BET specific surface area of the expanded natural graphite is 4m 2 / g to 8m 2 / g range.

2. The method according to claim 1, wherein In the pre-lithiation, the impregnated negative electrode structure is electrochemically charged to 13% to 16% of the charge capacity of the negative electrode structure.

3. The method of claim 1, wherein the electrochemical charging is performed using lithium metal as a counter electrode, the lithium metal being disposed in the pre-lithiation solution and spaced apart from the impregnated negative electrode structure. The method of claim 1 , wherein the soaking is performed for 0.5 to 15 hours.

5. The method of claim 1, wherein the pre-lithiation solution comprises a lithium salt and an organic solvent.

6. The method of claim 1, wherein the 2 Up to 3mA / cm 2 The electrochemical charging is performed at a current density of . 7 . The method of claim 1 , wherein an interplanar spacing d002 of (002) planes of the expanded natural graphite is in the range of 0.3390 nm to 0.3405 nm during XRD measurement.

8. The method of claim 1, wherein the BET specific surface area of the expanded natural graphite is 4.5 m 2 / g to 6.5m 2 / g range.

9. The method of claim 1, wherein a crystal size Lc of the expanded natural graphite along a c-axis direction is in a range of 16 nm to 21 nm during XRD measurement. 10 . The method of claim 1 , wherein the expanded natural graphite includes oxygen (O) in an amount of 800 ppm to 3,000 ppm based on the weight of the expanded natural graphite.

11. The method according to claim 1, wherein the average particle size D of the expanded natural graphite is 50 In the range of 8μm to 20μm.

12. The method of claim 1, wherein the negative electrode structure comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, The negative electrode active material layer includes the expanded natural graphite.

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