Precursor for negative electrode material of lithium secondary battery, negative electrode material prepared from same, and method for preparing same

Abstract

One aspect of the present invention provides a precursor for a negative electrode material of a lithium secondary battery, the negative electrode material prepared from the precursor, and a method for preparing the same, the precursor for the negative electrode material having high capacity characteristics, excellent processability, and excellent lifespan characteristics by using artificial graphite as an inner core and natural graphite as a shell.

Description

TECHNICAL FIELD

[0001] The present application relates to a precursor for a negative electrode material of a lithium secondary battery and a negative electrode material prepared from the same. More particularly, the present application relates to a precursor for a negative electrode material of a lithium secondary battery formed by processing a mixture of artificial graphite and natural graphite and a negative electrode material prepared from the same. BACKGROUND

[0002] A lithium secondary battery is generally composed of a positive electrode including a positive electrode active material, a negative electrode including a negative electrode material, a separator, and an electrolyte, and is a secondary battery that enables charge and discharge through intercalation-decalation of lithium ions. The lithium secondary battery has advantages of high energy density, high electromotive force, and high capacity, and thus is applied to various fields such as mobile devices, electric vehicles, hybrid electric vehicles, etc.

[0003] In the lithium secondary battery, as the positive electrode active material constituting the positive electrode, metal oxides such as LiCoO2, LiMnO2, LiMn2O4, or LiCrO2 are used, and as the negative electrode material constituting the negative electrode, carbon-based materials such as metal lithium, graphite, or activated carbon, and metal-based materials such as silicon oxide (SiOx) are used.

[0004] As the above-described negative electrode material, metal lithium was primarily used, but as the charge and discharge cycle proceeds, lithium atoms grow on the surface of the metal lithium, damage the separator, and cause a phenomenon of battery breakage, and thus in recent years, carbon-based materials are primarily used.

[0005] The carbon-based negative electrode material used in the negative electrode of the lithium secondary battery has an electrode potential close to that of lithium metal, and thus has a small change in crystal structure during intercalation and decalation of ionic lithium. Therefore, the carbon-based negative electrode material can enable a continuous and repeated redox reaction on the electrode, thereby contributing to high capacity characteristics and excellent life characteristics of the lithium secondary battery.

[0006] As the carbon-based negative electrode material, various forms of materials are used, such as natural graphite and artificial graphite as crystalline carbon-based materials, or hard carbon and soft carbon as amorphous carbon-based materials, etc. Among them, the most widely used is a graphite-based active material, which has excellent reversibility, thus being advantageous to the life characteristics of a lithium secondary battery. In particular, the discharge voltage of the graphite-based active material with respect to lithium is as low as -0.2 V, so that a battery using the graphite-based active material can exhibit a high discharge voltage of 3.6 V, and thus a lithium secondary battery including the graphite-based active material has many advantages in terms of energy density.

[0007] However, the theoretical capacity value of the graphite-based active material (for example, about 372 mAh / g in the case of a LiC6 negative electrode) is relatively low, and thus it is still insufficient to meet the electrochemical characteristics required by the relevant market.

[0008] Therefore, many researchers are focusing on the Group IV elements (Si, Ge, Sn) in the periodic table, among which Si is particularly attractive as a material due to very high theoretical capacity (Li 15 Si4: 3600 mAh / g) and low operating voltage (~0.1 V vs. Li / Li+) characteristics. However, upon charging and discharging, Si experiences a large volume expansion and contraction due to a reaction with lithium, and thus there can be a problem of micro-pulverization of the silicon active material powder and poor electrical contact of the silicon active material powder with the current collector. Due to this phenomenon, a lithium secondary battery including Si has a problem in that the capacity of the battery can be sharply reduced as the charging and discharging cycles progress. SUMMARY

[0009] (1) Technical problem to be solved The present application aims to provide a precursor for a negative electrode material of a lithium secondary battery having high capacity characteristics, excellent processability, and excellent life characteristics, a negative electrode material prepared from the same, and a method of preparing the same.

[0010] The technical problem of the present application is not limited to the above.

[0011] (2) Technical solution One aspect of the present application provides a precursor for a negative electrode material of a lithium secondary battery, the precursor for a negative electrode material of a lithium secondary battery including: a core including artificial graphite; and a shell wrapping the core, and the shell including natural graphite.

[0012] The artificial graphite can incorporate a binder, and the artificial graphite can be randomly oriented within the core.

[0013] The artificial graphite can have a coefficient of thermal expansion of 3.0 x 10 -7 / ℃ or above.

[0014] The artificial graphite may be graphitized needle coke or pitch coke, waste coke that has undergone high-temperature heat treatment, or a combination thereof, and the artificial graphite may be flake artificial graphite.

[0015] Furthermore, the D90 value of the natural graphite can be above 20 μm and below 30 μm.

[0016] The R value of the precursor for the negative electrode material of the lithium secondary battery of the present invention can be 0.5 or more and 1.5 or less according to the following [relationship 1].

[0017] [Relation 1] R=B90 / A90 (In [Equation 1], A90 represents the diameter of the particle corresponding to D90 of artificial graphite, and B90 represents the diameter of the particle corresponding to D90 of natural graphite.) The adhesive can be a soft carbon-based adhesive, and the softening point of the adhesive can be above 100°C.

[0018] The content of the binder may be less than 25% by weight relative to the total weight of the artificial graphite and the natural graphite.

[0019] In the precursor for the negative electrode material of the lithium secondary battery of the present invention, SiOx (0≤x≤2) may be further contained at less than 10% by weight relative to the total weight of the artificial graphite and the natural graphite.

[0020] Another aspect of the present invention provides a negative electrode material for a lithium secondary battery, the negative electrode material comprising: a core comprising artificial graphite; and a natural graphite shell encapsulating the core, wherein the artificial graphite is randomly oriented within the core.

[0021] The artificial graphite is characterized by being bonded together by amorphous carbon atoms.

[0022] The specific surface area of ​​the negative electrode material of the lithium secondary battery of the present invention can be 4m². 2 For g and below, the span value can be 1.25 or below.

[0023] The coefficient of thermal expansion of the artificial graphite can be 3.0 × 10⁻⁶. -7 / ℃ or above.

[0024] Furthermore, the D90 value of the natural graphite can be above 20 μm and below 30 μm.

[0025] The R value of the negative electrode material of the lithium secondary battery of the present invention can be 0.5 or more and 1.5 or less according to the following [relationship 1].

[0026] [Relation 1] R=B90 / A90 (In [Equation 1], A90 represents the diameter of the particle corresponding to D90 of artificial graphite, and B90 represents the diameter of the particle corresponding to D90 of natural graphite.) The content of amorphous carbon may be less than 25% by weight relative to the total weight of the artificial graphite and the natural graphite.

[0027] In the negative electrode material of the lithium secondary battery of the present invention, relative to the total weight of the artificial graphite and the natural graphite, it may further contain less than 10% by weight of SiOx (0≤x≤2).

[0028] (III) Beneficial Effects In the precursor for the negative electrode material of the lithium secondary battery according to the present invention and the negative electrode material prepared from the precursor, high capacity characteristics and excellent processability can be ensured by using artificial graphite with high hardness as the inner core and natural graphite with excellent electrode processability as the shell.

[0029] Furthermore, in the precursor for the negative electrode material of the lithium secondary battery according to the present invention and the negative electrode material prepared from the precursor, the core-shell structure allows natural graphite to protect artificial graphite which is beneficial to lifespan characteristics, thereby ensuring excellent lifespan characteristics. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a precursor for the negative electrode material of a lithium secondary battery according to the present invention.

[0031] Figure 2 This is a schematic diagram of a precursor for the negative electrode material of a lithium secondary battery according to the present invention, which is embedded with SiOx.

[0032] Figure 3 This is a SEM image of the precursor for the negative electrode material of the lithium secondary battery according to the present invention. Best practice

[0033] The preferred embodiments of the present invention will be described below. However, the embodiments of the present invention can be modified into many other embodiments, and the scope of the present invention is not limited to the embodiments described below.

[0034] In this specification, unless otherwise stated to the contrary, the terms “comprising” and “including” mean that other constituent elements may be included, rather than excluded.

[0035] Furthermore, in the specification of this invention, unless otherwise specified, the % unit represents weight.

[0036] Artificial graphite, as a crystalline carbon-based material, is formed by applying high thermal energy above 2700°C to create a graphite crystal structure, thus possessing a more stable crystallographic structure than natural graphite. Therefore, even under repeated charging and discharging of lithium ions, the crystal structure of artificial graphite undergoes minimal change, resulting in a relatively longer lifespan compared to natural graphite. Typically, the lifespan of artificial graphite-based anode materials is 2 to 3 times that of natural graphite-based anode materials.

[0037] However, the electrode workability of the artificial graphite is worse than that of natural graphite, so it is preferable to use it mixed with a certain proportion of natural graphite.

[0038] Therefore, considering the uses and purposes of lithium secondary batteries, it is necessary to mix artificial graphite and natural graphite in an appropriate ratio to ensure the desired level of electrochemical properties.

[0039] The inventors of this invention have discovered that by placing artificial graphite with relatively high hardness in the inner core and natural graphite with relatively excellent electrode processability in the shell, a negative electrode material for lithium secondary batteries with high capacity characteristics and excellent processability can be provided.

[0040] Furthermore, the inventors of this invention have discovered that by using the core-shell structure to protect artificial graphite which is beneficial to lifespan characteristics with natural graphite, a negative electrode material for lithium secondary batteries with excellent lifespan characteristics can be provided.

[0041] Based on this viewpoint, a precursor for the negative electrode material of a lithium secondary battery according to a specific embodiment of the present invention may include: a core comprising artificial graphite; and a shell enclosing the core, wherein the shell comprises natural graphite.

[0042] Figure 1 This is a schematic diagram illustrating a precursor for a negative electrode material of a lithium secondary battery according to a specific embodiment of the present invention, featuring such a core-shell structure. As shown, the precursor for the negative electrode material consists of a shell structure of natural graphite 1 and a core structure of artificial graphite 2.

[0043] Preferably, the artificial graphite is randomly oriented within the core. This randomly oriented artificial graphite not only helps improve lifetime characteristics by suppressing silicon expansion, but also facilitates lithium ion insertion and extraction, thereby helping to ensure high power characteristics of the electrode.

[0044] The coefficient of thermal expansion can be one of the indicators for measuring the random orientation of the artificial graphite. This is because, generally, the more random the orientation of the artificial graphite, the greater its overall expansion at high temperatures. Therefore, the coefficient of thermal expansion of the artificial graphite may also increase proportionally. Specifically, in one embodiment of the present invention, the coefficient of thermal expansion of the artificial graphite may be 3 × 10⁻⁶. -7 / ℃ or above. There is no particular upper limit to the coefficient of thermal expansion of the artificial graphite, but as an example, the coefficient of thermal expansion of the artificial graphite can be 5 × 10⁻⁶. -7 / ℃ below.

[0045] The artificial graphite can be graphitized needle coke or pitch coke, high-temperature heat-treated waste coke, or a combination thereof. The waste coke can be coke used as packing material in an Acheson graphitization furnace. In this case, the present invention can use the waste coke used as packing material as a negative electrode material, thus having the environmental advantage of being recyclable.

[0046] Furthermore, the artificial graphite can be flake-shaped artificial graphite. This flake-shaped artificial graphite can be produced through… Figure 3 The negative electrode material of the lithium secondary battery was identified using SEM images of the precursor.

[0047] Furthermore, preferably, the D90 value of the natural graphite is 20 μm or more and 30 μm or less. D90 refers to the particle size corresponding to 90% of the volumetric cumulative distribution of the particle size distribution. When the D90 value of the natural graphite is less than 20 μm, the natural graphite shell may not be able to fully encapsulate the artificial graphite core. More preferably, the lower limit of the D90 value of the natural graphite is 21 μm, and even more preferably, the lower limit of the D90 value of the natural graphite is 23 μm. On the other hand, when the D90 value of the natural graphite exceeds 30 μm, the natural graphite becomes too long, which may cause some of the natural graphite to be exposed in the electrolyte. In this case, unnecessary side reactions may occur due to the contact between the natural graphite and the electrolyte. Therefore, in this invention, the upper limit of the D90 value of the natural graphite can be set to 30 μm. More preferably, the upper limit of the D90 value of the natural graphite is 29.5 μm, and even more preferably, the upper limit of the D90 value of the natural graphite is 28 μm.

[0048] As an example of the present invention, the precursor for the negative electrode material of a lithium secondary battery may comprise artificial graphite and natural graphite with an R value of 0.5 or more and 1.5 or less according to the following [Relationship 1]. When the R value according to the following [Relationship 1] is 0.5 or more and 1.5 or less, the shell containing the natural graphite can completely enclose the core surface, so that the artificial graphite used as the core cannot come into contact with the outside.

[0049] [Relation 1] R=B90 / A90 (In [Equation 1], A90 represents the diameter of the particle corresponding to D90 of artificial graphite, and B90 represents the diameter of the particle corresponding to D90 of natural graphite.) When the R value according to [Equation 1] is less than 0.5, the shell containing natural graphite cannot completely enclose the core, causing the artificial graphite inside the core to be exposed in the electrolyte, thus exhibiting a low initial efficiency. More preferably, the R value according to [Equation 1] can be 0.55 or higher, and even more preferably, the R value according to [Equation 1] can be 0.6 or higher. On the other hand, when the R value exceeds 1.5, the natural graphite shell will not enclose the artificial graphite core, but the natural graphite will aggregate together, which may cause the core to be exposed in the electrolyte, thus potentially leading to a decrease in initial efficiency. More preferably, the R value according to [Equation 1] can be 1.3 or lower, and even more preferably, the R value according to [Equation 1] can be 1.2 or lower.

[0050] Furthermore, the artificial graphite can be bonded using an adhesive. The adhesive allows the artificial graphite to form well on the carbon support layer after carbonization and also ensures the density within the negative electrode material. To achieve the above objective, in this invention, the content of the adhesive can be 10% by weight relative to the total weight of the artificial graphite and the natural graphite. On the other hand, the content of the adhesive can be 25% by weight or less relative to the total weight of the artificial graphite and the natural graphite. When the adhesive is added in excess at a content exceeding 25% by weight relative to the total weight of the artificial graphite and the natural graphite, it may lead to a problem where sufficient charge-discharge capacity of the negative electrode material cannot be ensured. More preferably, the upper limit of the adhesive content is 20% by weight; more preferably, the upper limit of the adhesive content is 15% by weight. (See schematic diagram of the invention.) Figure 1 Artificial graphite 2 bonded by adhesive 3 as described above is shown.

[0051] Furthermore, the adhesive can be a soft carbon-based adhesive or a hard carbon-based adhesive. However, since hard carbon-based adhesives do not possess the layered structure of graphite even at graphitization temperatures, they may be relatively less conducive to achieving high capacity in the anode material compared to soft carbon-based adhesives. Therefore, the adhesive is more preferably a soft carbon-based adhesive. However, this is not a limitation.

[0052] Specifically, soft carbon-based adhesives may include coal tar and petroleum-based pitch, while hard carbon-based adhesives may include polyvinyl chloride (PVC), phenolic resin and polyethylene terephthalate (PET).

[0053] The softening point of the adhesive can be above 100°C. This is because when the softening point of the adhesive is below 100°C, the granulation and spheroidization of artificial graphite and natural graphite may not proceed smoothly during the preparation of the precursor for the negative electrode material.

[0054] In a precursor for the negative electrode material of a lithium secondary battery according to another specific embodiment of the present invention, SiOx (0≤x≤2) may be further contained at less than 10% by weight relative to the total weight of the artificial graphite and the natural graphite.

[0055] That is, the present invention can improve the electrode capacity by adding Si in oxide form (Si has a higher theoretical capacity than graphite-based active materials) during the precursor stage. However, when the amount of SiOx added exceeds 10% by weight, particle cracking or loss of electrical contact may occur due to the expansion of SiOx caused by continuous charging and discharging, which may lead to a decrease in discharge capacity. Therefore, in a non-limiting embodiment of the present invention, the weight range of SiOx relative to the total weight of the artificial graphite and the natural graphite can be set to less than 10% by weight.

[0056] Furthermore, preferably, the SiOx is located within the core. This is to suppress structural expansion caused by the expansion of SiOx.

[0057] Figure 2 This is a schematic diagram of a precursor for a negative electrode material of a lithium secondary battery according to another specific embodiment of the present invention, incorporating SiOx, as shown below. Figure 2 As shown, it can be seen that in the existing core-shell structure, SiOx (0≤x≤2)4 is further dispersed in the core.

[0058] As described above, in the precursor for the negative electrode material of the lithium secondary battery according to the present invention, by using artificial graphite as the inner core and natural graphite as the shell, high capacity characteristics, excellent processability, and excellent lifespan characteristics can be ensured.

[0059] The negative electrode material of the lithium secondary battery according to the present invention will be described in detail below.

[0060] The negative electrode material for a lithium-ion secondary battery is prepared by carbonizing the aforementioned negative electrode material using a precursor. The negative electrode material may include: a core comprising artificial graphite; and a shell enclosing the core, wherein the shell comprises natural graphite. Furthermore, as an example of the negative electrode material of the present invention, the artificial graphite is characterized by being interposed of amorphous carbon atoms.

[0061] The artificial graphite can be randomly oriented within the core, and it can be in the form of sheet-like artificial graphite. Furthermore, the coefficient of thermal expansion of the artificial graphite can be 3.0 × 10⁻⁶. -7 / ℃ or above and 5.0×10 -7 / ℃ below.

[0062] The D90 value of the natural graphite can be above 20 μm and below 30 μm.

[0063] According to a specific embodiment of the present invention, the R value of the negative electrode material of the lithium secondary battery can be 0.5 or more and 1.5 or less according to the following [relationship 1].

[0064] [Relation 1] R=B90 / A90 (In [Equation 1], A90 represents the diameter of the particle corresponding to D90 of artificial graphite, and B90 represents the diameter of the particle corresponding to D90 of natural graphite.) The amorphous carbon is in the form of carbonized soft carbon-based binder, the soft carbon-based binder having a softening point of 100°C or higher, and the content of the amorphous carbon relative to the total weight of the artificial graphite and the natural graphite being 10% by weight or higher and 25% by weight or lower.

[0065] Furthermore, in the negative electrode material of the lithium secondary battery according to another specific embodiment of the present invention, relative to the total weight of the artificial graphite and the natural graphite, it may further contain less than 10% by weight of SiOx (0≤x≤2), wherein the SiOx may be located within the core.

[0066] The above content is the same as that for precursors for negative electrode materials in lithium secondary batteries, so it is omitted.

[0067] The specific surface area of ​​the negative electrode material of the lithium secondary battery according to the present invention, as measured by the BET method, can be 4 m². 2 / g or less. When the specific surface area of ​​the negative electrode material of the lithium secondary battery exceeds 4m². 2 When the concentration is / g, voids are generated inside the negative electrode material, causing some artificial graphite to be exposed to the outside, which may lead to the problem of artificial graphite coming into contact with the electrolyte.

[0068] Furthermore, as an example of the present invention, the span value of the negative electrode material of the lithium secondary battery can be 1.25 or less. The span value can be calculated by the following formula. In the following formula, D90, D50, and D10 represent the particle size corresponding to 90%, 50%, and 10% of the volumetric accumulation of the particle size distribution, respectively. When the span value exceeds 1.25, the particle size distribution of the negative electrode material is uneven, and therefore there is a risk of particle collapse when applied to the negative electrode, which may lead to a reduction in electrode life characteristics.

[0069] Span = (D90 - D10) / D50 Finally, the D10 value of the negative electrode material of the present invention can be 5.3 μm or higher. When the D10 value is less than 5.3 μm, the amount of micronized powder is excessive, which may lead to a sharp increase in the specific surface area of ​​the negative electrode material.

[0070] In the negative electrode material of the lithium secondary battery according to the present invention, similar to the precursor for negative electrode materials, a core-shell structure is formed to ensure high capacity characteristics, excellent processability, and excellent lifespan characteristics.

[0071] The following describes the preparation method of the precursor for the negative electrode material of the lithium secondary battery of the present invention.

[0072] The method for preparing a precursor for a negative electrode material of a lithium secondary battery according to the present invention may include the following steps: preparing natural graphite and artificial graphite; pulverizing the natural graphite and the artificial graphite; mixing the pulverized natural graphite and artificial graphite to prepare a mixture; adding a binder to the mixture; and granulating and spheroidizing the mixture with the binder added at a linear velocity of 40 m / s or more and 65 m / s or less using a device utilizing high-speed centrifugal force for a time of 1 minute or more and 15 minutes or less.

[0073] The pulverization step of the natural and artificial graphite can be performed using an air classifier mill (ACM) or a jet mill. Preferably, the D90 value of the pulverized natural graphite is 20 μm or more and 30 μm or less. This content is the same as the above content regarding precursors for anode materials and is therefore omitted. According to a specific embodiment of the invention, the natural and artificial graphite can be pulverized and screened separately to obtain natural and artificial graphite with desired particle sizes, but this is not necessarily the case.

[0074] As described above, in the step of adding a binder to a mixture of natural and artificial graphite and performing granulation and spheroidization, a device utilizing high-speed centrifugal force can be used to granulate and spheroidize the mixture with the binder at a linear velocity of 40 m / s or more and 65 m / s or less for a time of 1 minute or more and 15 minutes or less. When the linear velocity is less than 40 m / s or the duration is less than 1 minute, it may be difficult to ensure sufficient centrifugal force for granulation and spheroidization. More preferably, during granulation and spheroidization, the linear velocity can be 45 m / s or more, and the duration can be 3 minutes or more. Furthermore, more preferably, the linear velocity can be 50 m / s or more, and the duration can be 5 minutes or more. On the other hand, when the linear velocity exceeds 65 m / s or the duration exceeds 15 minutes, the granulated and spheroidized particles may break apart, making it difficult to ensure the electrochemical performance desired by the present invention. More preferably, during granulation and spheroidization, the linear velocity can be 60 m / s or less, and the duration can be 10 minutes or less. Furthermore, more preferably, the linear velocity can be less than 55 m / s and the duration can be less than 8 minutes.

[0075] Specifically, in the mixture, the content of natural graphite may be more than 42% by weight and less than 58% by weight relative to the total weight of the artificial graphite and the natural graphite.

[0076] When the content of natural graphite is less than 42% by weight or more than 58% by weight, the R value of the precursor for the negative electrode material of the prepared lithium secondary battery according to the following [Relationship 1] may not meet the range of 0.5 or more and 1.5 or less. As mentioned above, when the R value is less than 0.5, the natural graphite shell cannot adequately encapsulate the artificial graphite core. When the R value exceeds 1.5, the natural graphite aggregates due to the excessively large R value, which may lead to the problem of the core being exposed to the electrolyte. Therefore, in the preparation method of the present invention, preferably, the content of natural graphite is 42% by weight or more and 58% by weight or less relative to the total weight of artificial graphite and natural graphite. More preferably, the content of natural graphite can be 45% by weight or more and 55% by weight or less relative to the total weight of artificial graphite and natural graphite. More preferably, the content of natural graphite can be 48% by weight or more and 52% by weight or less.

[0077] [Relation 1] R=B90 / A90 (In [Equation 1], A90 represents the diameter of the particle corresponding to D90 of artificial graphite, and B90 represents the diameter of the particle corresponding to D90 of natural graphite.) In other words, in the method for preparing a precursor for a negative electrode material according to a specific embodiment of the present invention, the R value according to the above [relationship 1] can be controlled by adjusting the relative weight ratio of natural graphite to total graphite.

[0078] Finally, the granulation and spheroidization steps can be performed using equipment that utilizes horizontal or vertical centrifugal force. Examples of such equipment may include the NHS series from Nara Machinery Works, the Mechanofusion from Hosokawa Micron Corporation, or Faculty.

[0079] In addition, in another specific embodiment of the method for preparing a precursor for a negative electrode material of a lithium secondary battery according to the present invention, in the step of mixing natural graphite and artificial graphite, SiOx (0≤x≤2) of less than 10% by weight may be further mixed relative to the total weight of the artificial graphite and the natural graphite.

[0080] This is because, as mentioned above, the present invention can improve the electrode capacity by adding Si in oxide form during the precursor stage (Si has a higher theoretical capacity than graphite-based active materials). However, when the amount of SiOx added exceeds 10% by weight, the problem of Si particle expansion due to continuous charging and discharging may occur. Therefore, the upper limit of the amount of SiOx added can be set to 10% by weight.

[0081] In an example of the preparation method of the negative electrode material for a lithium secondary battery according to the present invention, the negative electrode material can be prepared by carbonizing the aforementioned negative electrode material with a precursor at a temperature above 800°C. In the present invention, by carbonizing the negative electrode material with a precursor at a temperature above 800°C, not only can the volatile components present inside the existing precursor evaporate, but the binder can also be cured, thereby ensuring the internal structure of the negative electrode material.

[0082] The manufacturing method of the negative electrode of the lithium secondary battery of the present invention will be described in detail below.

[0083] The negative electrode can be manufactured by mixing a negative electrode material prepared according to a specific embodiment of the present invention, an adhesive, and an optional conductive material to prepare a composition for forming a negative electrode material layer, and then coating the composition onto a negative electrode current collector.

[0084] The negative electrode current collector can be, for example, copper foil, nickel foil, stainless steel, titanium foil, foam nickel, foam copper, a polymer substrate coated with a conductive metal, or a combination thereof.

[0085] The adhesive may be polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropyl cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene, but is not limited thereto. The amount of the adhesive mixed relative to the total amount of the composition for forming the negative electrode material layer may be from 1% by weight to 30% by weight.

[0086] The conductive material is not particularly limited, as long as it does not cause chemical changes in the battery and is conductive. Specifically, the conductive material can be natural graphite, artificial graphite, etc.; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The mixing amount of the conductive material can be from 0.1% by weight to 30% by weight relative to the total amount of the composition for forming the negative electrode material layer. Detailed Implementation

[0087] (Example 1) (1) Preparation of negative electrode materials First, flake-shaped natural graphite and artificial graphite (waste coke used as filler material in the Atchison graphitization furnace) were prepared. Then, the natural and artificial graphite were pulverized, and 50% by weight of each were mixed to prepare a mixture. 15% by weight of coal-based pitch binder was added to the prepared mixture relative to the total weight of the artificial and natural graphite. The softening point of this binder was 110°C. Using a high-speed centrifugal apparatus, the mixture with the binder was granulated and spheroidized at a linear velocity of 50 m / s for 7 minutes to prepare a precursor for the negative electrode material of a lithium-ion battery. The granulation and spheroidization were performed using the NHS-5 equipment from Nara Machinery Works. The precursor was then carbonized at 850°C to prepare the negative electrode material for the lithium-ion battery. The particle size and specific surface area of ​​the prepared negative electrode material were measured and are shown in Table 1. The particle size was measured using a CILAS Model 1060 particle size analyzer.

[0088] (2) Evaluation of physical properties Subsequently, 97% by weight of the prepared negative electrode material, 2% by weight of the binder containing carboxymethyl cellulose and styrene-butadiene rubber, and 1% by weight of the Super P conductive material as carbon black conductive material were mixed in distilled water solvent to prepare the negative electrode material slurry.

[0089] The negative electrode material slurry was coated onto a copper (Cu) current collector, dried at 100°C for 10 minutes, and then pressed in a roller press. Afterward, it was vacuum dried in a vacuum oven at 100°C for 12 hours. The time point at which the negative electrode material separated from the copper (Cu) current collector, i.e., the detachment phenomenon, was measured was then recorded. This time point is shown as the electrode adhesion force (in hours) in Table 1. Furthermore, the adhesion force (gf) between the prepared negative electrode material and the copper (Cu) current collector was measured and is shown in Table 1 below.

[0090] [Table 1] The above experimental results show that, in Invention Examples 1 to 3, a core-shell structure with artificial graphite as the core and natural graphite as the shell was formed. Figure 3 The image is a photograph of Example 1 of the Invention, as observed by SEM, which confirms the core-shell structure. The above-described example of the invention exhibits a 4m... 2 While exhibiting a specific surface area of ​​less than / g, it also displays excellent electrode adhesion.

[0091] Furthermore, in Comparative Example 1, the span value exceeded 1.25, indicating an uneven particle size distribution in the negative electrode material, thus posing a risk of particle collapse when applied to the negative electrode. Additionally, in Comparative Example 1, the particle size corresponding to D10 in the negative electrode material was less than 5.3 μm, resulting in an excessive amount of microparticles and a specific surface area exceeding 4 m². 2 / g. Therefore, in Comparative Example 1, voids are generated within the negative electrode, causing a portion of the artificial graphite within the core to be exposed outside the shell, resulting in poor electrode adhesion.

[0092] (Example 2) (1) Preparation of negative electrode materials First, flake-like natural graphite and artificial graphite (waste coke used as filler material in the Atchison graphitization furnace) were pulverized and graded to prepare natural and artificial graphite with particle sizes shown in Table 2 below. Particle size was measured using a CILAS Model 1060 particle size analyzer. Next, the natural and artificial graphite were mixed according to the weight ratios shown in Table 3 relative to the total weight of the total graphite, and further mixed with 5% by weight of SiOx (0≤x≤2) relative to the total weight of the total graphite to prepare a mixture. A petroleum-based bitumen binder with the weight ratios shown in Table 3 was added to the prepared mixture according to the total weight of the artificial and natural graphite. The softening point of this binder is 170°C. Using a device utilizing high-speed centrifugal force, the mixture with the added binder was granulated and spheroidized at the linear velocity shown in Table 3 and for the time shown in Table 3 to prepare a precursor for the negative electrode material of a lithium secondary battery. The equipment used for granulation and spheroidization was the NHS-5 at Nara Machine Works. The precursor was then carbonized at 850°C to prepare the negative electrode material for lithium-ion batteries. After preparing the negative electrode material as described above, the particle size and specific surface area of ​​the negative electrode material were measured and are shown in Tables 3 and 4.

[0093] (2) Manufacturing of the negative electrode A negative electrode slurry was prepared by mixing 97% by weight of the prepared negative electrode material, 2% by weight of the binder containing carboxymethyl cellulose and styrene-butadiene rubber, and 1% by weight of Super P conductive material in distilled water solvent.

[0094] The negative electrode material slurry was coated onto a copper (Cu) current collector, dried at 100°C for 10 minutes, and then pressed in a roller press. Afterward, it was vacuum dried in a vacuum oven at 100°C for 12 hours to produce the negative electrode. The electrode density of the vacuum-dried negative electrode was set to be between 1.5 g / cm³ and 1.7 g / cm³.

[0095] (3) Electrochemical evaluation To evaluate the electrochemical characteristics, a lithium secondary battery was manufactured. Specifically, using a negative electrode manufactured by the above method, lithium metal (Li-metal) as the counter electrode, and an electrolyte (the electrolyte being an electrolyte in which 1 mole of LiPF6 solution is dissolved in a mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC) in a volume ratio of 1:1), a 2032 coin cell was manufactured according to conventional manufacturing methods.

[0096] The capacity, initial efficiency, and expansion rate of the above-mentioned batteries were measured and are shown in Table 4 below. The expansion rate was calculated using the following formula.

[0097] [Table 2] [Table 3] [Table 4] In Comparative Example 2, the D90 particle size of the natural graphite did not reach the range proposed in this invention. Therefore, the natural graphite shell could not adequately encapsulate the artificial graphite core, thus failing to ensure the desired level of electrode processability and reducing electrochemical properties.

[0098] On the other hand, in Comparative Example 3, the D90 particle size of the natural graphite exceeded the range proposed in this invention, thus exposing a portion of the natural graphite to the electrolyte, thereby generating unwanted side reaction sites. Therefore, it exhibited poor performance in terms of capacity and expansion rate.

[0099] In Comparative Examples 4 and 5, the linear velocity during granulation and spheroidization exceeded 65 m / s, resulting in the problem that the granulated and spheroidized particles broke due to excessively fast circumferential motion, thus reducing the electrochemical properties.

[0100] In particular, in the case of Comparative Example 5, not only was D10 less than 5.3 μm, but the span value of the negative electrode material also exceeded 1.25, thus making it impossible to ensure the desired level of electrochemical characteristics.

[0101] In Comparative Example 6, the adhesive content was too high (more than 25% by weight), so high capacity characteristics could not be ensured.

[0102] In Comparative Examples 7 and 8, the R value according to [Relation 1] is less than 0.5 or more than 1.5, therefore the initial efficiency characteristics at the level desired by the present invention cannot be guaranteed.

[0103] On the other hand, Examples 4 and 5 of the invention simultaneously satisfy the structure and preparation method proposed in this invention, so the electrode containing the negative electrode material exhibits high capacity characteristics, excellent processability and excellent lifetime characteristics.

[0104] (Example 3) The coefficient of thermal expansion of the artificial graphite included in Example 4 of Embodiment 2 is 3.3 × 10⁻⁶. -7 / ℃.

[0105] The manufacturing method of the negative electrode in Comparative Example 9 is the same as that in Invention Example 4, except that the coefficient of thermal expansion of the artificial graphite contained in the negative electrode material is 1.7 × 10⁻⁶. -7 / ℃.

[0106] Subsequently, the electrochemical characteristics of the negative electrode of Invention Example 4 and the negative electrode of Comparative Example 9 were measured and are shown in Table 5 below.

[0107] [Table 5] In the case of the negative electrode in Comparative Example 9, the artificial graphite within the core is oriented in a specific direction, therefore the coefficient of thermal expansion does not reach 3.0 × 10⁻⁶. -7 / ℃, resulting in poor electrochemical characteristics.

[0108] On the other hand, in the case of the negative electrode in Invention Example 4, the artificial graphite within the core is randomly oriented, therefore the coefficient of thermal expansion is 3.0 × 10⁻⁶. -7 It has a temperature above ℃ and excellent electrochemical properties.

Claims

1. A precursor for a negative electrode material of a lithium secondary battery, comprising: a core including artificial graphite; and a shell that coats the core, and the shell includes natural graphite.

2. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein A binder is incorporated in the artificial graphite.

3. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The artificial graphite is randomly oriented within the core.

4. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The artificial graphite has a thermal expansion coefficient of 3.0 x 10 -7 / °C or more.

5. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The artificial graphite is graphitized needle coke or pitch coke, high-temperature heat-treated waste coke, or a combination thereof.

6. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The artificial graphite is flaky artificial graphite.

7. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The D90 value of the natural graphite is 20 μm or more and 30 μm or less.

8. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The R value of the precursor for a negative electrode material of a lithium secondary battery according to the following relation 1 is 0.5 or more and 1.5 or less, [Relation 1] R = B90 / A90 In the relation 1, A90 represents the diameter of a particle corresponding to the D90 of the artificial graphite, and B90 represents the diameter of a particle corresponding to the D90 of the natural graphite.

9. The precursor for a negative electrode material of a lithium secondary battery according to claim 2, wherein The binder is a soft carbon-based binder, and the softening point of the binder is 100°C or more.

10. The precursor for a negative electrode material of a lithium secondary battery according to claim 2, wherein The content of the binder is 10% by weight or more and 25% by weight or less with respect to the total weight of the artificial graphite and the natural graphite.

11. The precursor for a negative electrode material of a lithium secondary battery according to claim 1, wherein The precursor for a negative electrode material of a lithium secondary battery further includes 10% by weight or less of SiOx (0≤x≤2) with respect to the total weight of the artificial graphite and the natural graphite.

12. A negative electrode material of a lithium secondary battery, comprising: a core including artificial graphite; and a natural graphite shell that coats the core, wherein the artificial graphite is randomly oriented within the core.

13. The negative electrode material of a lithium secondary battery according to claim 12, characterized by, The artificial graphite is bound to each other by amorphous carbon.

14. The negative electrode material of the lithium secondary battery according to claim 12, wherein, The specific surface area of the negative electrode material of the lithium secondary battery is 4 m 2 / g or less, and the span value is 1.25 or less.

15. The negative electrode material of the lithium secondary battery according to claim 12, wherein, The negative electrode material of a lithium secondary battery further includes 10% by weight or less of SiOx (0≤x≤2) with respect to the total weight of the artificial graphite and the natural graphite.

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