Negative electrode active material, method for preparing same, negative electrode and secondary battery including same

Abstract

The present invention relates to a negative electrode active material containing natural graphite, wherein D 90 and D 10 The ratio D 90 / D 10 is 2.20 or less, D 50 is from 6 μm to 11 μm, and the BET specific surface area is 2.2 m 2 / g or less.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of Korean Patent Application No. 10 - 2019 - 0038618, filed with the Korean Intellectual Property Office on April 2, 2019, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a negative electrode active material, a method for preparing the same, and a negative electrode and a lithium secondary battery including the same. Background Art

[0005] As energy prices rise due to the depletion of fossil fuels and concerns about environmental pollution intensify, environmentally friendly alternative energy has become an essential factor in future life.

[0006] In particular, with the technological development and increasing demand for mobile devices, the demand for secondary batteries as an environmentally friendly energy source has increased rapidly.

[0007] Typically, secondary batteries have used lithium metal as the negative electrode of the secondary battery. However, because battery short - circuit caused by the formation of dendrites and the resulting risk of battery explosion have become problems, carbon - based active materials that can reversibly intercalate and deintercalate lithium ions and maintain structural and electrical properties have emerged.

[0008] As carbon - based active materials, various types of carbon - based materials such as artificial graphite, natural graphite, and hard carbon have been applied. Among them, graphite - based active materials with excellent reversibility that can ensure the life characteristics of lithium secondary batteries have been most widely used. Since graphite - based active materials have a low discharge voltage of - 0.2V compared to lithium, batteries using graphite - based active materials can exhibit a high discharge voltage of 3.6V, thus providing many advantages in terms of the energy density of lithium batteries.

[0009] Among graphite - based active materials, in particular, natural graphite has higher output and capacity compared to other carbon - based active materials such as artificial graphite, and has excellent adhesion, thus having the advantages of reducing the amount of binder used and achieving a high - capacity, high - density negative electrode. However, when compared with artificial graphite, the problem with natural graphite is that as charging and discharging continue, cyclic swelling caused by electrolyte side reactions becomes severe, so its use may still be limited despite the above - mentioned advantages.

[0010] In order to prevent the problem of cyclic expansion of natural graphite, a mixture of natural graphite and artificial graphite mixed in a certain ratio can be used as the negative electrode active material. However, even in this case, compared with artificial graphite, natural graphite is still inferior in preventing cyclic expansion caused by charging and discharging. Therefore, when natural graphite is used in the negative electrode, thickness expansion cannot be sufficiently prevented.

[0011] Therefore, it is necessary to develop a negative electrode active material that can prevent the problem of cyclic expansion while contributing to the high output and high capacity of natural graphite.

[0012] Although Japanese Patent Unexamined Publication No. 4403327 discloses graphite powder for the negative electrode of a lithium ion secondary battery, no alternative solution to the above problems is proposed.

[0013] Prior Art Documents

[0014] [Patent Document]

[0015] Japanese Patent Unexamined Publication No. 4403327 Summary of the Invention

[0016] Technical Problem

[0017] One aspect of the present invention provides a negative electrode active material containing natural graphite, which can exhibit excellent output and capacity performance of natural graphite while effectively preventing the problem of cyclic expansion caused by electrolyte side reactions.

[0018] Another aspect of the present invention provides a method for preparing a negative electrode active material, wherein the method includes a step of pressurizing natural graphite by a cold isostatic pressing method, thereby effectively preventing the problem of cyclic expansion of natural graphite.

[0019] Still another aspect of the present invention provides a negative electrode and a secondary battery including the above negative electrode active material.

[0020] Technical Solution

[0021] According to one aspect of the present invention, there is provided a negative electrode active material containing natural graphite, wherein D 90 Compared with D 10 The ratio D 90 / D 10 Is 2.20 or less, D 50 Is from 6 μm to 11 μm, and the BET specific surface area is 2.2 m 2 / g or less.

[0022] According to another aspect of the present invention, there is provided a method for preparing a negative electrode active material, the method comprising the steps of: pressurizing natural graphite by cold isostatic pressing; and 90 With D 10 Ratio D 90 / D 10 Adjust to below 2.20 and reduce the D 50 Adjusted from 6μm to 11μm, with an adjusted D 90 / D 10 and D 50 The BET specific surface area of natural graphite is 2.2 m 2 / g or less.

[0023] According to another aspect of the present invention, there is provided a negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises the above-mentioned negative electrode active material.

[0024] According to another aspect of the present invention, there is provided a secondary battery including the above-mentioned negative electrode, a positive electrode opposite to the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0025] Beneficial Effects

[0026] The negative electrode active material of the present invention has a particle size distribution adjusted to a specific range of D 10 With D 90 The ratio of D 50 The high BET specific surface area allows for smooth accumulation of active material particles when applied to the negative electrode, effectively preventing cyclic expansion problems caused by electrolyte side reactions and thickness expansion of the negative electrode active material layer. This allows for improved lifespan characteristics while effectively demonstrating the output and capacity performance of natural graphite.

[0027] In addition, the method for preparing the negative electrode active material of the present invention achieves the above-mentioned negative electrode active material by applying cold isostatic pressing (CIP) to pressurize natural graphite and adjusting the particle size of the active material. Therefore, according to the preparation method of the present invention, the pores existing in the natural graphite or the negative electrode active material can be reduced or eliminated, thereby reducing the specific surface area of the active material and facilitating the accumulation of particles, thereby effectively preventing the expansion problem of the active material during charging and discharging. DETAILED DESCRIPTION

[0028] It will be understood that the words or terms used in the specification and claims of the present invention should not be construed as being limited to the meanings defined in a common dictionary. It will be further understood that, based on the principle that the inventor may appropriately define the meanings of the words or terms to best explain the invention, the words or terms should be construed as having meanings consistent with their meanings in the context of the relevant field and technical concept of the present invention.

[0029] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, singular terms may include plural forms.

[0030] In this specification, it should be understood that the terms "comprising," "including," or "having" are intended to specify the presence of the stated features, quantities, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, quantities, steps, elements, or combinations thereof.

[0031] In this specification, D 50 、D 10 and D 90 may be defined as the particle sizes corresponding to 50%, 10%, and 90% of the volume cumulative in the particle size distribution curve of the particles, respectively. D 50 、D 10 and D 90 For example, it can be measured by the laser diffraction method. The laser diffraction method can generally measure particle sizes from the sub-micron region to several millimeters, thereby obtaining highly reproducible and high-resolution results.

[0032] Hereinafter, the present invention will be described in more detail.

[0033] <Negative electrode active material>

[0034] The present invention relates to a negative electrode active material, specifically, to a negative electrode active material for a secondary battery, and more specifically, to a negative electrode active material for a lithium secondary battery.

[0035] Specifically, the negative electrode active material of the present invention contains natural graphite, wherein D 90 and D 10 The ratio D 90 / D 10 is 2.20 or less, D 50 is 6 μm to 11 μm, and the BET specific surface area is 2.2 m 2 / g or less.

[0036] The negative electrode active material of the present invention is a negative electrode active material containing natural graphite, and D 90 / D 10 is adjusted to 2.20 or less, D 50is adjusted to 6 μm to 11 μm, and at the same time, the BET specific surface area is adjusted to 2.2 m 2 / g or less. The negative electrode active material having D 90 / D 10 、D 50 and BET specific surface area adjusted to the above ranges is a negative electrode active material that sufficiently removes the micropores of natural graphite that cause side reactions of the electrolyte and cycle expansion problems and has a uniform particle size distribution. Therefore, when applied to the negative electrode, the packing degree between particles is excellent, and it is possible to effectively prevent the cycle expansion problem caused by charge / discharge of the negative electrode and the resulting increase in the thickness of the negative electrode.

[0037] The negative electrode active material contains natural graphite.

[0038] Since the negative electrode active material uses natural graphite, there are advantages of high output and high capacity compared with the case of using other carbonaceous active materials (such as artificial graphite). Specifically, the negative electrode active material may contain natural graphite having a theoretical capacity of 360 mAh / g or more.

[0039] The negative electrode active material may further contain a carbon coating formed on the natural graphite. The carbon coating may be an amorphous carbon coating. Specifically, the carbon coating may be formed by providing at least one precursor selected from pitch, rayon, and polyacrylonitrile resins to the natural graphite and performing heat treatment to thermally decompose the precursor.

[0040] The carbon coating can contribute to improving the structural stability of the active material. In addition, the carbon coating can reduce the pores present in the natural graphite and reduce the BET specific surface area to a desired level, so that side reactions with the electrolyte can be effectively prevented.

[0041] In terms of preventing side reactions with the electrolyte, sufficiently improving structural stability, and preventing the inhibition of lithium insertion / extraction due to overformation, the carbon coating may be contained in the negative electrode active material in an amount of 2 wt% to 7 wt%, preferably 3 wt% to 6 wt%, more preferably 4.5 wt% to 5.5 wt%.

[0042] The negative electrode active material may be spherical. In this specification, "spherical" is a concept that covers not only a completely spherical shape but also a slightly deformed but generally spherical form.

[0043] When the negative electrode active material is spherical, it can contribute to the packing between the active material particles, so that the thickness expansion problem of the negative electrode active material caused by charge / discharge can be reduced to an even more excellent level.

[0044] The spherical negative electrode active material can be realized by spherical natural graphite or by a negative electrode active material including spherical natural graphite and a carbon coating formed on the spherical natural graphite.

[0045] The D of the negative electrode active material 50 is 6 μm to 11 μm. When D 50 is less than 6 μm, the specific surface area of the active material increases, so the side reaction with the electrolyte may increase, and the occurrence of cyclic swelling may be aggravated. When it is greater than 11 μm, due to the increase in the particle size of the negative electrode active material, the volume expansion caused by charging / discharging may be further aggravated, and the diffusion distance of lithium becomes longer, so that the fast charging performance may deteriorate.

[0046] The D of the negative electrode active material 50 can be preferably 7.5 μm to 10.5 μm, more preferably 8.5 μm to 9.5 μm. When within the above range, the packing performance of the particles is improved and the pores between the particles are reduced, so that the anti-swelling property is excellent, and it is also preferable in improving the fast charging performance, because due to the small particle size, lithium can be smoothly intercalated / deintercalated even at a high C rate.

[0047] The D of the negative electrode active material 90 and D 10 The ratio D 90 / D 10 is 2.20 or less.

[0048] By adjusting D 90 / D 10 to the above level, the negative electrode active material can have active material particles with uniform particle size. Therefore, when applying the negative electrode active material to the negative electrode, the packing degree of the active material particles can be improved, and particle deformation can be prevented when rolling the negative electrode. Therefore, mechanical swelling of the negative electrode and / or cyclic swelling caused by side reactions of the electrolyte can be prevented.

[0049] If the D of the negative electrode active material 90 / D 10 exceeds 2.20, it is not conducive to the packing between particles, and because the pores between particles increase, the side reaction of the electrolyte may be aggravated, and the thickness swelling of the negative electrode caused by charging / discharging may not be effectively prevented. In addition, the volume expansion caused by the charging / discharging of particles with relatively large particle size may partially cause the aggravation of the swelling phenomenon.

[0050] The D of the negative electrode active material 90 / D 10It can be 2.20 or less, preferably 1.98 or less, more preferably 1.80 to 1.98. When within the above range, the problem of cycle expansion caused by side reactions of the electrolyte can be prevented at an even better level.

[0051] In the negative electrode active material, D 90 and D 10 The difference (D 90 - D 10 ) can be 7.5 μm or less, preferably 7.1 μm or less, more preferably 6.5 μm to 7.1 μm. When within the above range, the influence of volume expansion caused by charge / discharge of relatively large-sized particles can be minimized, thereby minimizing the occurrence of expansion.

[0052] The D of the negative electrode active material 90 can be 11 μm to 17 μm, preferably 13 μm to 16 μm, more preferably 14 μm to 15 μm. The D of the negative electrode active material 10 can be 5 μm to 9 μm, preferably 6 μm to 8 μm, more preferably 6.5 μm to 7.5 μm.

[0053] The BET specific surface area of the negative electrode active material is 2.2 m 2 / g or less. Generally, natural graphite has a large specific surface area due to the presence of pores on its surface and inside, so there is a problem that the problem of cycle expansion caused by side reactions of the electrolyte is aggravated. However, although the negative electrode active material of the present invention uses natural graphite, the range of the BET specific surface area is adjusted to the above level, so side reactions of the electrolyte can be effectively prevented.

[0054] If the BET specific surface area of the negative electrode active material exceeds 2.20 m 2 / g, side reactions of the electrolyte are aggravated, and irreversible reactions of the negative electrode active material increase, resulting in reduced efficiency, and thus the output and capacity performance of natural graphite may not be fully realized.

[0055] The BET specific surface area of the negative electrode active material can be 2.2 m 2 / g or less, preferably 1.5 m 2 / g to 2.2 m 2 / g, more preferably 1.7 m 2 / g to 2.15 m 2 / g. When within the above range, it is advantageous in improving the life performance and high-temperature storage performance by preventing the expansion problem without deteriorating the output performance of natural graphite.

[0056] It can be achieved by making the D of the negative electrode active material 50The BET specific surface area is achieved by adjusting within the above range and treating natural graphite by the subsequent cold isostatic pressing (CIP) method. Generally, natural graphite has a relatively large BET specific surface area due to the pores existing on its surface and inside, and thus has the problem of intensified side reactions with the electrolyte. However, in the present invention, through the CIP process, D 50 of the negative electrode active material can be controlled within the above range, and the number and area of pores can be effectively controlled. Therefore, the BET specific surface area of the negative electrode active material can be reduced to a desired level to prevent side reactions with the electrolyte, thereby improving the life characteristics of the negative electrode active material and preferably achieving high output performance of natural graphite.

[0057] The BET specific surface area of the negative electrode active material can be measured by using an adsorption gas such as nitrogen and the BELSORP (BET device) of BEL JAPAN through, for example, the Brunauer-Emmett-Teller (BET) measurement method.

[0058] d2 / d1 (the ratio of the pellet density d2 of the negative electrode active material under 3000 kg to the pellet density d1 under 400 kg) can be 1.1 or more. When d2 / d1 is 1.1 or more, during the roll pressing for preparing the negative electrode, the active material particles can be smoothly stacked, which is preferable for achieving a negative electrode with high energy density. The pellet density can be measured by, for example, using a powder resistance measuring device (product name: HPRM-1000, manufacturer: HANTECH Corporation).

[0059] <Method for preparing negative electrode active material>

[0060] In addition, the present invention provides a method for preparing a negative electrode active material. Specifically, the method for preparing the negative electrode active material can be the above method for preparing the negative electrode active material.

[0061] Specifically, the method for preparing the negative electrode active material of the present invention includes the following steps: pressurizing natural graphite by the cold isostatic pressing (CIP) method; and adjusting the ratio of D 90 to D 10 to be 2.20 or less, and adjusting D 90 / D 10 of the pressurized natural graphite to be 6 μm to 11 μm, wherein the BET specific surface area of the natural graphite having the adjusted D 50 / D 90 / D 10 and D 50 is 2.2 m 2 / g or less.

[0062] Natural graphite can be spherical. Spherical natural graphite can be prepared by spheroidizing flaky natural graphite, and when the natural graphite is spherical, it can contribute to the packing between active material particles, thereby reducing the thickness expansion problem of the negative electrode active material caused by charging / discharging to an even more excellent level.

[0063] Flaky natural graphite is prepared from natural graphite raw materials (such as natural graphite raw materials collected from graphite ore). Specifically, it can be prepared through processes such as crushing the natural graphite raw materials, removing impurities by alkali treatment and / or acid treatment, washing, drying, and sieving.

[0064] Spherical natural graphite can be prepared by spheroidizing flaky natural graphite. Specifically, a vortex mill can be used for spheroidization.

[0065] The method for preparing the negative electrode active material of the present invention includes the step of pressurizing natural graphite by a cold isostatic pressing method. By the cold isostatic pressing method, pores present in the negative electrode active material containing natural graphite can be effectively removed or reduced, and the specific surface area of natural graphite can be reduced to an appropriate level, thereby effectively preventing the cycle expansion problem (which is caused by side reactions of the electrolyte) of the negative electrode active material prepared thereby.

[0066] The "cold isostatic pressing" method or CIP refers to a molding method in which powder is filled into a mold and compressed along infinitely many axial directions by hydrostatic pressure. That is, according to the CIP process, the powder can be compressed in an isostatic manner, so that pores present in the molded negative electrode active material can be uniformly removed or reduced. Due to the reduction of pores, the reaction area between the natural graphite pressurized by CIP and the electrolyte is reduced. Therefore, side reactions with the electrolyte can be significantly reduced, and the life characteristics of the battery can be improved.

[0067] The pressurization can be carried out at a pressure of 80 MPa to 150 MPa, preferably 85 MPa to 135 MPa. When within the above pressure range, this is preferred because the pores of the particles can be filled to the required level, thereby effectively preventing side reactions with the electrolyte and preventing damage to the negative electrode active material that may be caused by using too high a pressure.

[0068] The pressurization can be carried out for 0.5 minutes to 30 minutes, preferably 1 minute to 10 minutes. When within the above range, this is preferred because the BET specific surface area of the negative electrode active material can be reduced to the required level.

[0069] The method for preparing the negative electrode active material of the present invention may further include the following steps: after pressurizing natural graphite, a carbon coating is formed on the natural graphite. The carbon coating can compensate for the damage or breakage of natural graphite that may occur during the processes of pressurizing, pulverizing, sieving, acid / alkali treatment, etc. of natural graphite, reduce the BET specific surface area of natural graphite increased due to pulverization, etc., and can improve the mechanical stability of the negative electrode active material.

[0070] The carbon coating can be formed by providing at least one precursor selected from pitch, rayon, and polyacrylonitrile resins to spherical natural graphite and performing heat treatment to thermally decompose the precursor. In terms of forming a uniform carbon coating and preventing excessive formation of the carbon coating, the heat treatment process for forming the carbon coating can be carried out in the temperature range of 1100 °C to 1500 °C.

[0071] Based on the weight of the negative electrode active material, the weight of the carbon coating can be 2 wt% to 7 wt%, preferably 3 wt% to 6 wt%, more preferably 4.5 wt% to 5.5 wt%. When within the above range, it is possible to sufficiently improve the effect of preventing side reactions with the electrolyte and the structural stability while preventing the inhibition of lithium insertion / extraction due to excessive formation.

[0072] The method for preparing the negative electrode active material of the present invention includes the following steps: adjusting the ratio D 90 of the pressurized natural graphite to D 10 to be 2.20 or less, and adjusting the D 90 of the pressurized natural graphite to be 6 μm to 11 μm. 10 The negative electrode active material with D 50 / D

[0073] and D 90 / D 10 adjusted within the above range has a uniform particle size distribution. Therefore, when applied to the negative electrode, the packing degree between particles is excellent, and it is possible to effectively prevent the cyclic swelling problem caused by charging / discharging of the negative electrode and the resulting increase in the thickness of the negative electrode. 50 The step of adjusting the particle size distribution of the pressurized natural graphite can be carried out by, for example, sieving, which is a well-known method in the art for adjusting the particle size distribution of particles.

[0074] The step of adjusting the particle size distribution can be carried out before or after forming the carbon coating when forming the carbon coating on the pressurized natural graphite.

[0075] When forming the carbon coating on the pressurized natural graphite, the step of adjusting the particle size distribution can be carried out before or after forming the carbon coating.

[0076] The negative electrode active material having the adjusted D 90 / D 10 and D 50The BET specific surface area of the carbon coating can be 2.2 m 2 / g or less, preferably 1.5 m 2 / g to 2.2 m 2 / g, more preferably 1.7 m 2 / g to 2.15 m 2 / g. When within the above range, the pores present in natural graphite are controlled to the desired level, thereby effectively preventing side reactions with the electrolyte.

[0077] The above BET specific surface area range can be achieved, for example, by pressing natural graphite by the above cold isostatic pressing method or by adjusting the processing conditions of cold isostatic pressing.

[0078] <Negative electrode>

[0079] In addition, the present invention provides a negative electrode, which comprises the above negative electrode active material.

[0080] Specifically, the negative electrode of the present invention comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises the above negative electrode active material.

[0081] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has high electrical conductivity and does not cause chemical changes in the battery. For example, it is possible to use: copper; stainless steel; aluminum; nickel; titanium; fired carbon; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloy, etc. In addition, as in the case of the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to improve the binding force of the negative electrode active material, and the negative electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric bodies.

[0082] The negative electrode active material layer comprises the above negative electrode active material.

[0083] Within the range that does not deteriorate the effects of the present invention, the negative electrode active material layer may further comprise other active materials known in the art. Specifically, it may comprise one or more selected from the following: carbonaceous materials; lithium-containing titanium composite oxides (LTO); (semi) metals such as Si, Sn, Li, Zn, Mg, Cd, Ce, Ni, or Fe; alloys composed of (semi) metals; oxides of (semi) metals; and composites of (semi) metals and carbon.

[0084] The negative electrode conductive material may be contained in the negative electrode active material layer in an amount of 80% to 99% by weight, preferably 80% to 99% by weight.

[0085] In addition, while containing the negative electrode active material, the negative electrode active material layer may selectively contain at least one additive selected from the following: a binder, a thickener, and a conductive material.

[0086] The binder is a component that helps the binding between the conductive material, the active material, and the current collector, and is generally added in an amount of 1 wt% to 30 wt% in the negative electrode active material layer. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, nitrile butadiene rubber, fluororubber, various copolymers thereof, and the like.

[0087] As the thickener, any thickener used in a typical lithium secondary battery may be used, and one example is carboxymethyl cellulose (CMC).

[0088] The conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 1 wt% to 20 wt% in the negative electrode active material layer. The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, the following can be used: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, such as carbon fibers and metal fibers; carbon fluoride; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive materials, such as polyphenylene derivatives, etc. Specific examples of commercially available conductive materials may include the acetylene black series products of Chevron Chemical Company, Denka Black of Denka Singapore Private Limited, products of Gulf Oil Company, etc., Ketjen black, the EC series products of Armak Company, Vulcan XC-72 of Cabot Company, and Super-P of Timcal Company.

[0089] Secondary battery

[0090] In addition, the present invention provides a lithium secondary battery comprising the negative electrode for a secondary battery described above.

[0091] Specifically, the lithium secondary battery includes the negative electrode described above, a positive electrode opposite to the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

[0092] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.

[0093] The positive electrode active material layer may include a positive electrode active material, and selectively include a binder and a conductive material.

[0094] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel; aluminum; nickel; titanium; fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0095] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel; aluminum; nickel; titanium; fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0096] The positive electrode current collector generally may have a thickness of 3 μm to 500 μm.

[0097] The positive electrode active material layer is formed on the positive electrode current collector and includes a positive electrode active material.

[0098] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium. Specifically, it may include a lithium composite metal oxide containing one or more metals (such as cobalt, manganese, nickel, or aluminum) and lithium. More specifically, the lithium composite metal oxide may be: lithium-manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (such as LiCoO2, etc.), lithium-nickel-based oxides (such as LiNiO2, etc.), lithium-nickel-manganese-based oxides (such as LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt-based oxides (such as LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (such as LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt-based oxides (such as Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1 and p + q + r1 = 1) or Li(Ni p1 Co q1 Mnr2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2 and p1 + q1 + r2 = 2), etc. or lithium-nickel-cobalt-transition metal (M) oxides (such as Li(Ni p2 Co q2 Mn r3 M s2 )O2 (where M is selected from: Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 are each the atomic fraction of an independent element, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1), etc., and may include any one of the above materials or a composite of two or more thereof. Among them, due to the fact that the capacity performance and stability of the battery can be improved, the lithium composite metal oxide can be LiCoO2, LiMnO2, LiNiO2, lithium nickel-manganese-cobalt oxide (such as Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2) or lithium nickel cobalt aluminum oxide (such as Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.) etc.

[0099] Based on the total weight of the positive electrode active material layer, the positive electrode active material can be included in an amount of 80% to 99% by weight.

[0100] While including the above positive electrode active material, the positive electrode active material layer may also selectively include at least one additive selected from the following: a binder and a conductive material.

[0101] The binder is a component that helps the binding of the active material, conductive material, etc. and the binding to the current collector, and based on the total weight of the positive electrode active material layer, the binder is usually added in an amount of 1% to 30% by weight. Examples of the binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, various copolymers thereof, etc.

[0102] The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. Examples of the conductive material can include: graphite; carbonaceous materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives, etc. Specific examples of commercially available conductive materials can include the acetylene black series products of Chevron Chemical Company, Denka Black of Denka Singapore Private Limited, products of Gulf Oil Company, etc., Ketjen black, the EC series products of Armak Company, Vulcan XC-72 of Cabot Company, and Super-P of Timcal Company.

[0103] Based on the total weight of the positive electrode active material layer, the conductive material can be included in an amount of 1% by weight to 30% by weight.

[0104] The positive electrode active material layer can be prepared by adding additives including the positive electrode active material and optionally a binder and / or a conductive material to a solvent to prepare a positive electrode slurry, then coating the slurry onto a positive electrode current collector, followed by roll pressing and drying.

[0105] The solvent can include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and can be used in an amount such that a preferred viscosity is obtained when the positive electrode active material and optionally a binder and a conductive material are included. For example, the solvent can be included in an amount such that the concentration of the solids including the positive electrode active material and optionally a binder and a conductive material is 50% by weight to 95% by weight, preferably 70% by weight to 90% by weight.

[0106] In a lithium secondary battery, a separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used without particular limitation as long as it is a separator commonly used in secondary batteries. In particular, a separator having excellent electrolyte moisturizing ability and low resistance to ion movement in the electrolyte is preferred. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane made of an olefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer); or a laminated structure having two or more layers of the above porous polymer membranes. In addition, typical porous non-woven fabrics can be used, such as non-woven fabrics formed of high melting point glass fibers, polyethylene terephthalate fibers, etc. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0107] In addition, the electrolyte used in the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc., which can all be used for manufacturing lithium secondary batteries, but is not limited thereto.

[0108] Specifically, the electrolyte can contain an organic solvent and a lithium salt.

[0109] Any organic solvent can be used without particular limitation as long as it can be used as a medium through which ions participating in the battery electrochemical reaction can move. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane, etc. Among the above solvents, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having high ion conductivity and high dielectric constant, which can improve the charge / discharge performance of the battery, and a linear carbonate compound having low viscosity (such as ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate). In this case, when the cyclic carbonate and the chain carbonate are mixed at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0110] Any compound can be used as the lithium salt without particular limitation as long as it can provide lithium ions used in the lithium secondary battery. Specifically, as the lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. can be used. The lithium salt can be used in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent performance, and lithium ions can move efficiently.

[0111] As described above, the secondary battery according to the present invention can be used in: portable devices such as mobile phones, notebook computers, and digital cameras; and the field of electric vehicles, such as hybrid electric vehicles (HEVs). In particular, the secondary battery according to the present invention can preferably be used as a component battery of a medium to large-sized battery module. Therefore, the present invention also provides a medium to large-sized battery module including the secondary battery as described above as a unit cell.

[0112] The medium to large-sized battery module as described above can preferably be applied to power sources that require high output and high capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices.

[0113] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0114] Examples and Comparative Examples

[0115] Example 1: Preparation of Negative Electrode Active Material

[0116] <Preparation of Natural Graphite>

[0117] Use the flotation method to collect natural graphite raw materials from graphite ore, and treat the natural graphite raw materials with acid or alkali to remove impurities, and then wash and dry to prepare flaky natural graphite. Use a vortex mill to spheroidize the obtained flaky natural graphite, treat it with sulfuric acid to remove impurities therefrom, and then dry it at 500 °C to prepare spherical natural graphite.

[0118] Fill the spherical natural graphite into a mold and pressurize it by the cold isostatic pressing (CIP) method. During pressurization, the pressurization pressure is 90 MPa, and the pressurization is carried out for 100 seconds.

[0119] The pressurized spherical natural graphite is mixed with pitch having a softening point of 130 °C, and the mixture is heat-treated in an inert atmosphere at 1250 °C in a dry manner for 24 hours to form a carbon coating on the spherical natural graphite.

[0120] The spherical natural graphite having a carbon coating formed thereon is crushed, screened, and subjected to removal of metal impurities (iron removal), and 10% volume cumulative diameter D 10 、50% volume cumulative diameter D 50 and 90% volume cumulative diameter D 90 are respectively adjusted to 6.3 μm, 9 μm and 13.5 μm to prepare the negative electrode active material of Example 1.

[0121] At this time, based on the total weight of the negative electrode active material of Example 1, the carbon coating is formed to be 5% by weight.

[0122] Example 2: Preparation of negative electrode active material

[0123] Except that after forming the carbon coating in Example 1, when screening, D 10 、D 50 and D 90 of the spherical natural graphite having a carbon coating formed thereon are respectively adjusted to 7.3 μm, 9.0 μm and 14.3 μm, the negative electrode active material of Example 2 is prepared in the same manner as in Example 1.

[0124] Comparative Example 1: Preparation of negative electrode active material

[0125] Except that in Example 1, the spherical natural graphite is not pressurized by the cold isostatic pressing (CIP) method and after forming the carbon coating, when screening, D 10 、D 50 and D 90 of the spherical natural graphite having a carbon coating formed thereon are respectively adjusted to 7.0 μm, 12 μm and 19.5 μm, the negative electrode active material of Comparative Example 1 is prepared in the same manner as in Example 1.

[0126] Comparative Example 2: Preparation of negative electrode active material

[0127] Except that after forming the carbon coating in Example 1, when screening, D 10 、D 50 and D 90 of the spherical natural graphite having a carbon coating formed thereon are respectively adjusted to 6.7 μm, 12 μm and 18.5 μm and the carbon coating is formed to be 4% by weight based on the total weight of the negative electrode active material, the negative electrode active material of Comparative Example 2 is prepared in the same manner as in Example 1.

[0128] Comparative Example 3: Preparation of Anode Active Material

[0129] Except that in Example 1, the spherical natural graphite is not pressurized by the cold isostatic pressing (CIP) method, and after forming the carbon coating, during sieving, the D 10 , D 50 and D 90 of the spherical natural graphite with the carbon coating formed thereon are adjusted to 6.3 μm, 9 μm, and 15.5 μm respectively, the anode active material of Comparative Example 3 is prepared in the same manner as in Example 1.

[0130] Comparative Example 4: Preparation of Anode Active Material

[0131] Except that in Example 1, after forming the carbon coating, during sieving, the D 10 , D 50 and D 90 of the spherical natural graphite with the carbon coating formed thereon are adjusted to 6.2 μm, 9 μm, and 14.0 μm respectively, the anode active material of Comparative Example 4 is prepared in the same manner as in Example 1.

[0132] The D 50 , D 10 , D 90 and BET specific surface area of each of the anode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4 are measured respectively, and the measurement results are shown in Table 1 below.

[0133] [Table 1]

[0134]

[0135] Experimental Example 1: Swelling Evaluation

[0136] <Manufacture of Lithium Secondary Battery>

[0137] Each of the anode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4, Super C65 as a conductive material, styrene-butadiene rubber (SBR) as an adhesive, and carboxymethyl cellulose (CMC) as a thickener are mixed at a weight ratio of 95.9:1:1.9:1.2, and then water is added to prepare an anode slurry. Thereafter, the anode slurry is coated on a copper foil to a thickness of 65 μm, then vacuum dried at about 130 °C for 8 hours, and then roll pressed to manufacture an anode. At this time, the anode is manufactured to have a loading of 3.61 mAh / cm 2 .

[0138] LiCoO2 as the positive electrode active material, Li-435 of Denka Company as the conductive material, KF9700 of Kureha Company as the binder, and BH-730H of Zeon Company as the thickener were mixed at a weight ratio of 96.25:1.0:1.5:1.25, and then water was added to prepare a positive electrode paste. The positive electrode paste was coated on an aluminum foil and then vacuum dried at about 130 °C for 8 hours, followed by roll pressing to manufacture the positive electrode. At this time, the positive electrode was manufactured to have a loading of 3.61 mAh / cm 2 The load amount of.

[0139] A polyolefin separator was disposed between each negative electrode and positive electrode manufactured in Examples 1 and 2 and Comparative Examples 1 to 4, and then an electrolytic solution was injected therein to manufacture secondary batteries for each example and comparative example. The electrolytic solution used was prepared as follows: Based on a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:4, vinylene carbonate (VC) was added to the non-aqueous electrolyte solvent in an amount of 0.5 wt%, and then 1 M LiPF6 was dissolved therein.

[0140] <Swelling evaluation>

[0141] Each lithium secondary battery manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 was charged and discharged at 0.1 C in the first cycle, 0.2 C in the second cycle, and 0.5 C from the 3rd cycle to the 30th cycle through a charging range with SOC from 0 to SOC 95. Thereafter, the swelling rate was calculated by the following Equation 1.

[0142] [Equation 1]

[0143] Swelling rate (%) = {(t2 - t1) / t1} × 100

[0144] (t1 is the thickness of the negative electrode for the secondary battery before the first charge / discharge cycle,

[0145] t2 is the thickness of the negative electrode for the secondary battery after the 30th charge / discharge cycle)

[0146] [Table 2]

[0147] Expansion rate (%) Example 1 22 Example 2 21 Comparative Example 1 27 Comparative Example 2 25 Comparative Example 3 26 Comparative Example 4 24

[0148] Referring to Table 2, by preventing the natural graphite from being pressurized by the cold isostatic pressing method and making the D of the negative electrode active material 50 and D 90 / D 10Each of the negative electrode active materials of Examples 1 and 2 adjusted to the desired range is affected by side reactions of the electrolyte, and when compared with the Comparative Examples, the swelling phenomenon is reduced to an excellent level.

Claims

1. A negative electrode active material, which comprises natural graphite and a carbon coating formed on the natural graphite, wherein: D 90 compared with D 10 the ratio of D 90 / D 10 is 2.20 or less; D 50 is from 6 μm to 11 μm; and The BET specific surface area is 2.2 m 2 / g or less, where D 90 and D 10 the difference D 90 -D 10 is 7.5 μm or less, The negative electrode active material is spherical.

2. The negative electrode active material according to claim 1, wherein the D 90 and D 10 The ratio of D 90 / D 10 is 1.98 or less.

3. The negative electrode active material according to claim 1, wherein the carbon coating is contained in the negative electrode active material in an amount of 2% to 7% by weight.

4. A method for preparing the negative electrode active material according to claim 1, the method comprising the following steps: Pressurizing natural graphite by a cold isostatic pressing method; and Adjust the ratio of D of the pressurized natural graphite 90 to D 10 to be below 2.20, and adjust the D of the pressurized natural graphite 90 / D 10 to be 6 μm to 11 μm, 50 ​ Among them, there is adjusted D 90 / D 10 and D 50 The BET specific surface area of the natural graphite is 2.2 m 2 / g or less where D 90 and D 10 the difference D 90 -D 10 is 7.5 μm or less, The natural graphite is spherical, The method further includes: The step of forming a carbon coating on the natural graphite after pressurizing the natural graphite.

5. The method according to claim 4, wherein based on the weight of the negative electrode active material, the weight of the carbon coating is 2% to 7% by weight.

6. The method according to claim 4, wherein the pressurization is carried out at a pressure of 80 MPa to 150 MPa.

7. The method according to claim 4, wherein the pressurization is carried out for 0.5 minutes to 30 minutes.

8. A negative electrode, the negative electrode comprising: A negative electrode current collector; and A negative electrode active material layer formed on the negative electrode current collector, Wherein the negative electrode active material layer contains the negative electrode active material according to claim 1.

9. A secondary battery, the secondary battery comprising: The negative electrode according to claim 8; A positive electrode opposite to the negative electrode; A separator interposed between the positive electrode and the negative electrode; and An electrolyte.

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