Negative electrode active material, method for producing a negative electrode active material, negative electrode composition, negative electrode for lithium secondary battery containing the same, and lithium secondary battery containing the negative electrode

Abstract

This application relates to a negative electrode active material, a method for producing a negative electrode active material, a negative electrode composition, a negative electrode for a lithium secondary battery containing the same, and a lithium secondary battery containing the negative electrode.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2023-0140879, filed with the Korean Intellectual Property Office on October 20, 2023, and all of its content is incorporated herein.

[0002] This application relates to a negative electrode active material, a method for manufacturing the negative electrode active material, a negative electrode composition, a negative electrode for a lithium secondary battery including the same, and a lithium secondary battery including the negative electrode.

Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for alternative and clean energy has been increasing, and among them, the field most actively studied is the field of power generation and power storage using electrochemical reactions.

[0004] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its usage area is increasingly expanding.

[0005] As the technology development and demand related to mobile devices increase, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density, voltage, long cycle life, and low self-discharge rate have been commercialized and widely used. Also, research on methods for manufacturing high-density electrodes with an even higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries has been actively conducted.

[0006] Generally, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and desorbs lithium ions emitted from the positive electrode, and as the negative electrode active material, silicon-based particles with a large discharge capacity can be used.

[0007] In particular, with the recent demand for high-density energy batteries, Si / C and SiO2, which have more than 10 times the capacity of graphite-based materials, are being used as negative electrode active materials. x Active research is being conducted on methods to increase capacity by using silicon-based compounds such as those mentioned above. However, while silicon-based compounds are high-capacity materials, they have the problem that, compared to conventionally used graphite, they rapidly expand in volume during the charging process, disrupting the conductive path and degrading battery performance.

[0008] Therefore, in order to resolve the problems that arise when silicon-based compounds are used as negative electrode active materials, various methods have been discussed, such as methods to adjust the driving potential, methods to further coat a thin film on the active material layer, methods to suppress volume expansion itself such as adjusting the particle size of the silicon-based compound, or methods to prevent the conduction path from being interrupted. However, in the case of the above methods, there is a limit to their application because they may actually degrade the performance of the battery, and there are still limitations to the commercialization of negative electrode battery manufacturing with a high content of silicon-based compounds.

[0009] Furthermore, in the case of silicon-based active materials, the reactivity is very high, resulting in non-uniformity of the reaction at the upper and lower ends of the electrode. In other words, in the case of silicon-based negative electrodes, the rapid reaction on the surface of lithium ions concentrates the reaction only at the upper end of the electrode, causing non-uniformity of the reaction and resulting in problems with the life characteristics, such as desorption at the upper end of the electrode.

[0010] In particular, in silicon-based negative electrodes, the more metal impurities there are, the more electrochemically inert FeSi2 is present as an interfering substance within the electrode. This further hinders the diffusion of lithium ions, causing the reaction to concentrate only in the silicon-based active material in the upper layer of the electrode, resulting in a decrease in lifespan performance.

[0011] Furthermore, the HF generated by the LiPF6 + H2O reaction within the cell can attack metal foreign matter contained in the NCMA active material or silicon-based active material of the positive electrode, leading to the problem of metal ion elution. In other words, if metal foreign matter is present in the positive electrode area, it becomes metal ions through oxidation, chelates with the electrolyte, and moves to the negative electrode side. There, it precipitates on the negative electrode surface through reduction, and if this process is repeated, the grown precipitate can penetrate the separator, causing a short circuit in the cell or generating a low voltage.

[0012] Consequently, in the case of silicon-based active materials, methods such as using magnetism to remove metallic foreign matter during the manufacturing process are under research. However, if metallic foreign matter is introduced during the manufacturing process of silicon-based active materials, or if it is a weakly magnetic metal present in the material itself, it cannot be removed, resulting in the aforementioned problems.

[0013] Therefore, research is being conducted to effectively remove metal impurities and solve the aforementioned problems when applying silicon-based negative electrodes. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2009-080971 [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] When using silicon-based negative electrodes, problems arose when metallic foreign matter in the negative electrode active material was exposed to HF generated inside the cell, causing it to dissolve in the form of metal ions and subsequently precipitate. Research to solve this problem revealed that removing metallic foreign matter by simple magnetic force was difficult. It was found that this was due to the contamination of metal components by friction between the machine and the silicon material during the pulverization process of the silicon-based raw material, or by weakly magnetic metal components contained in the silicon-based active material.

[0016] Through this research, we investigated the efficient removal of the aforementioned metal components and found that when the silicon-based active material undergoes a cleaning process with two or more acids during the manufacturing process of the negative electrode active material, the aforementioned metal components are effectively removed, and the Si-Fe phase or Si-Fe-Ti phase, which can cause problems such as short circuits or dendrites, is efficiently removed.

[0017] Therefore, this application relates to a negative electrode active material that can solve the aforementioned problems, a method for producing a negative electrode active material, a negative electrode composition, a negative electrode for a lithium secondary battery containing the same, and a lithium secondary battery containing the negative electrode. [Means for solving the problem]

[0018] One embodiment of this specification provides a negative electrode active material comprising a silicon-based active material and a metal impurity, wherein the proportion of the Si-Fe phase or Si-Fe-Ti phase in the negative electrode active material is 2% or less, based on 100% of the total phase of the silicon-based active material and the metal impurity contained in the negative electrode active material.

[0019] Another embodiment provides a method for producing a negative electrode active material, comprising the steps of preparing a silicon raw material, grinding the silicon raw material, and pickling the ground silicon raw material, wherein the pickling step includes adding an acid to the ground silicon raw material, and the acid is a mixture of two or more different acids.

[0020] Another embodiment provides a negative electrode composition comprising a negative electrode active material, a negative electrode conductive material, and a negative electrode binder according to the present application.

[0021] Another embodiment provides a negative electrode for a lithium secondary battery, comprising a negative electrode current collector layer and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer, wherein the negative electrode active material layer comprises the negative electrode composition or a cured product thereof according to this application.

[0022] Finally, the present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode for a lithium secondary battery according to the present application, a separator provided between the positive electrode and the negative electrode for a lithium secondary battery, and an electrolyte. [Effects of the Invention]

[0023] In the case of a negative electrode active material according to one embodiment of the present invention, the content of metal impurities is low, below a certain range, and the proportion of the Si-Fe phase or Si-Fe-Ti phase is adjusted to 2% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material. In other words, by having the proportion of metal impurities having a specific phase within the aforementioned range, it is possible to prevent cell short circuits due to the deposition of metal foreign matter.

[0024] Furthermore, in the case of silicon-based active materials, it is important that the tortuosity of the lithium ion pathway is simplified. In the case of Si-Fe or Si-Fe-Ti phases, these are electrochemically inert phases and exist as interference within the electrode. However, the negative electrode active material according to this application satisfies the aforementioned ratio through a cleaning process with two or more acids, so that lithium ions are not concentrated only in the silicon of the upper layer of the electrode, but can diffuse well to the vicinity of the current collector of the electrode. This has the characteristic of improving the lifespan performance of the negative electrode.

[0025] Furthermore, while Si-Fe phases or Si-Fe-Ti phases cause a decrease in the capacity per gram of the silicon-based active material when they are included in a silicon-based active material, the negative electrode active material according to this application does not experience a decrease in the capacity per gram of the silicon-based active material itself. Therefore, it is possible to achieve high energy density, increased lifetime, and suppression of dendrites. [Brief explanation of the drawing]

[0026] [Figure 1]This figure shows a stacked structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. [Figure 2] This figure shows a stacked structure of a lithium secondary battery according to one embodiment of the present application. [Figure 3] This figure shows an image of the EDS analysis according to Example 1. [Figure 4] This figure shows an image of EDS analysis using Comparative Example 1. [Modes for carrying out the invention]

[0027] Before describing the present invention, let us first define some terms.

[0028] In this specification, when a part "includes" a component, it means that it may include other components rather than excluding them, unless otherwise stated.

[0029] In this specification, "p~q" means the range "p or greater and q or less".

[0030] In this specification, "specific surface area" is measured by the BET method, specifically calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-mino II manufactured by BEL Japan. In other words, in this application, BET specific surface area may mean the specific surface area measured by the above measurement method.

[0031] In this specification, "Dn" refers to the particle size distribution, specifically the particle size at the n% point of the cumulative particle number distribution corresponding to the particle size. That is, D50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution corresponding to the particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution corresponding to the particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution corresponding to the particle size. On the other hand, the average particle size can be measured using the laser diffraction method. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the difference in diffraction patterns corresponding to the particle size is measured as the particles pass through the laser beam to calculate the particle size distribution.

[0032] In one embodiment of this application, particle size may refer to the average diameter or representative diameter of individual particles making up the metal powder.

[0033] In this specification, when a polymer is said to contain a monomer as a monomer unit, it means that the monomer participates in the polymerization reaction and is included in the polymer as a repeating unit. In this specification, when a polymer is said to contain a monomer, it is interpreted the same way as when a polymer contains a monomer as a monomer unit.

[0034] In this specification, unless otherwise specified, the term "polymer" is understood to be used in a broad sense, including copolymers.

[0035] In this specification, weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers of various degrees of polymerization (standard samples) commercially available for molecular weight measurement as standard substances. In this specification, molecular weight refers to weight-average molecular weight unless otherwise specified.

[0036] Hereinafter, a detailed description will be given with reference to the drawings so that those having ordinary knowledge in the technical field to which the present invention pertains can easily implement the present invention. However, the present invention may be realized in various different forms and is not limited to the following description.

[0037] One embodiment of the present specification provides a negative electrode active material containing a silicon-based active material and a metal impurity, wherein the ratio of the Si-Fe phase or the Si-Fe-Ti phase is 2% or less based on 100% of the total phase of the silicon-based active material and the metal impurity contained in the negative electrode active material.

[0038] In the case of the negative electrode active material according to one embodiment of the present invention, by having the ratio of the metal impurity having the specific phase as described above within the above range, it is possible to prevent a short circuit of the cell due to the precipitation of metal foreign matter, ensure a lithium ion path, improve the life performance, and provide a negative electrode with a high energy density due to an increase in the capacity of the negative electrode active material itself.

[0039] This application provides a negative electrode active material containing a silicon-based active material and a metal impurity.

[0040] In one embodiment of this application, the silicon-based active material includes SiO x (x = 0), SiO x (0 < x < 2), a SiC composite, or a Si alloy, to provide a negative electrode active material.

[0041] That is, any type of silicon-based active material can be included without limitation.

[0042] In the present specification, the SiC composite, as a silicon-carbon composite, is a composite of Si and C and is distinguished from silicon carbide represented by SiC. The silicon carbide does not react electrochemically with lithium, and all performances such as life can be measured as 0.

[0043] In the present application, the silicon-based active material may be a SiC composite including porous carbon and silicon deposited on the porous carbon.

[0044] The SiC composite may be one in which silicon and graphite or the like are composite, and may form a structure surrounded by graphene or amorphous carbon or the like centering on a core in which silicon and graphite or the like are composite. The silicon in the SiC composite may be nanosilicon. For example, the nanosilicon may be silicon in the range of 1 nm to 999 nm.

[0045] According to one embodiment of the present specification, the SiO x (0 < x < 2) can be represented as silicon oxide. At this time, the SiO x (0 < x < 2) corresponds to an amorphous-phase matrix in the silicon oxide particles. The SiO x (0 < x < 2) may be in a form including a part of Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2). When the silicon oxide particles contain the SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0046] In one embodiment of the present application, the silicon-based active material includes one or more selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2), and may contain 70 parts by weight or more of the SiO x (x = 0) based on 100 parts by weight of the silicon-based active material.

[0047] In one embodiment of the present application, the silicon-based active material contains SiO x (x = 0), and may contain 70 parts by weight or more of the SiO x (x = 0) based on 100 parts by weight of the silicon-based active material.

[0048] In another embodiment, the SiO is used based on 100 parts by weight of the silicon-based active material. x The solution may contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, of (x=0), and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0049] In one embodiment of this application, pure silicon (Si) particles may be used as the silicon-based active material. Using pure silicon (Si) particles as the silicon-based active material means that, as described above, when 100 parts by weight of the silicon-based active material is used, pure Si particles (SiO) that are not bonded to other particles or elements are used. x This could mean that (x=0) is included in the aforementioned range.

[0050] In one embodiment of this application, the silicon-based active material is SiO2 based on 100 parts by weight of silicon-based active material. x It may consist of silicon-based particles having 100 parts by weight of (x=0).

[0051] On the other hand, the average particle size (D50) of the silicon-based active material in the present invention may be 1 μm to 10 μm, more specifically 2 μm to 7 μm, and more specifically 4 μm to 5 μm. If the average particle size is less than the above range, the specific surface area of ​​the particles will increase excessively, and the viscosity of the negative electrode slurry will increase excessively. As a result, the dispersion of the particles constituting the negative electrode slurry will not be smooth. Also, if the size of the silicon-based active material is excessively small, it will have the same porosity, the straightness of lithium ion diffusion (tortuosity) will be poor, charging and discharging to the lower end of the electrode will not be easy, and the battery capacity retention rate will decrease. On the other hand, if the average particle size exceeds the above range, there will be excessively large silicon particles, the surface of the negative electrode will not be smooth, and as a result, non-uniformity of current density will occur during charging and discharging. Also, if the silicon particles are excessively large, the phase stability of the negative electrode slurry will be unstable, and processability will decrease. As a result, the battery capacity retention rate will decrease.

[0052] In one embodiment of this application, the silicon-based active material generally has a characteristic BET specific surface area. The BET specific surface area of ​​the silicon-based active material is preferably 10 m². 2 / g or more 40m 2 / g or less, more preferably 15m 2 / g~35m 2 / g, particularly preferably 20m 2 / g~30m 2 / g, most preferably 25m 2 / g~30m 2 The value is / g. The BET specific surface area is measured according to DIN 66131 (using nitrogen).

[0053] While silicon-based active materials with a large specific surface area can improve the reaction uniformity of the material and prevent particle cracking due to volume changes during charging and discharging, a large specific surface area leads to the formation of many solid electrolyte interface (SEI) layers on the surface of the silicon-based active material. This consumes a large amount of usable lithium in the cell, resulting in a reduced cell lifespan. By satisfying the specific surface area within the aforementioned range, the aforementioned problems are avoided.

[0054] In one embodiment of this application, the silicon-based active material may exist, for example, in a crystalline or amorphous form, and is preferably non-porous. The silicon particles are preferably spherical or fragmentary particles. Alternatively, but less conveniently, the silicon particles may have a fibrous structure or exist in the form of a silicon-containing film or coating.

[0055] In one embodiment of this application, the silicon-based active material may have a non-spherical shape, and its sphericity is, for example, 0.9 or less, for example 0.7 to 0.9, for example 0.8 to 0.9, for example 0.85 to 0.9.

[0056] In this application, the degree of sphericity is determined by the following formula 1, where A is the area and P is the boundary line.

[0057] [Formula 1] 4πA / P2

[0058] In one embodiment of this application, the size of the crystal grains of the silicon-based active material may be 1 nm or more and 200 nm or less.

[0059] In another embodiment, the size of the crystal grains of the silicon-based active material may be 150 nm or less, 100 nm or less, 90 nm or less, 80 nm or less, or 1 nm or more, 5 nm or more, 7 nm or more, or 10 nm or more.

[0060] In another embodiment, the silicon-based active material may have crystal grains of 10 nm to 80 nm in size.

[0061] In this context, the size of the crystal grains of the silicon active material can refer to the size of the crystal grains of the Si particles contained within the silicon active material.

[0062] The silicon-based active material has the aforementioned grain size, and by filling the aforementioned range, grain boundaries are distributed abundantly within the particles, allowing lithium ions to be inserted uniformly. This reduces the stress exerted on the silicon particles during lithium ion insertion, thereby mitigating particle cracking. As a result, it has the characteristic of improving the lifetime stability of the negative electrode. If the grain size exceeds the aforementioned range, the grain boundaries within the particles become narrowly distributed. In this case, lithium ions are inserted non-uniformly within the particles, the stress due to ion insertion is large, and particle cracking occurs.

[0063] In this application, "crystal grain" refers to a crystalline particle in a metal or material that is an aggregate of microscopically irregular shapes, and "size of the crystal grain" may refer to the diameter of the observed crystal grain. In other words, in this application, "size of the crystal grain" refers to the size of domains within a particle that share the same crystal orientation, and is a different concept from particle size or particle size, which represents the size of a substance.

[0064] In one embodiment of this application, the grain size can be calculated using the FWHM (Full Width at Half Maximum) value obtained by XRD analysis. The remaining value, excluding L, is measured by XRD analysis of the silicon-based active material, and the grain size can be determined from the Debey-Scherrer equation, which shows that FWHM and grain size are inversely proportional. In this case, the Debey-Scherrer equation is as shown in Equation 1-1 below.

[0065] [Formula 1-1] FWHM = Kλ / Lcosθ

[0066] In the above formula 1-1, L represents the size of the crystal grain, K is a constant, θ is the Bragg angle, and λ is the wavelength of the X-ray.

[0067] Furthermore, the shape of the crystal grains varies and can be measured three-dimensionally. Generally, the size of the crystal grains can be measured using commonly used circle methods or diameter measurement methods, but is not limited to these.

[0068] The aforementioned diameter measurement method involves drawing 5 to 10 parallel lines, each with a length of L mm, on a micrograph of the target particle, and counting the number of crystal grains z along each line and averaging the results. In this process, only grains that fit entirely within the line are counted, and those that do not are excluded. If the number of lines is P and the magnification is V, the average grain size can be calculated using the following equation 1-2.

[0069] [Formula 1-2] Dm = (L × P × 10) 3 ) / (zV)(μm)

[0070] Furthermore, the circle method involves drawing a circle of a predetermined diameter on a micrograph of the target particles, and then determining the average area of ​​the crystal grains by the number of crystal grains that fall within the circle and the number of crystal grains that fall on the boundary line. This can be calculated using the following equations 1-3.

[0071] [Formula 1-3] F m =(F k ×10 6 ) / ((0.67n+z)V 2 )(μm 2 )

[0072] In the above formula 1-3, F m F is the average particle area. k ∫ represents the measurement area on the photograph, z represents the number of particles within the circle, n represents the number of particles along the arc, and V represents the magnification of the microscope.

[0073] In one embodiment of this application, a negative electrode active material is provided, wherein the silicon-based active material is a single-crystal silicon-based active material; or a polycrystalline silicon-based active material.

[0074] In one embodiment of this application, the silicon-based active material may be a single-crystal silicon-based active material.

[0075] In one embodiment of this application, the silicon-based active material may be a polycrystalline silicon-based active material.

[0076] In this application, the present invention provides a negative electrode active material in which the metallic impurity is 0.2 parts by weight or less based on 100 parts by weight of the negative electrode active material.

[0077] In another embodiment, the metal impurity may be 0.2 parts by weight or less, 0.19 parts by weight or less, 0.18 parts by weight or less, or 0.01 parts by weight or more, or 0.03 parts by weight or more, based on 100 parts by weight of the negative electrode active material.

[0078] In this application, in the case of the metal impurity, electrochemically inert FeSi2 is present as an obstruction within the electrode, further hindering the diffusion of lithium ions, causing the reaction to concentrate only in the silicon-based active material in the upper layer of the electrode, resulting in a problem of reduced lifetime performance.

[0079] Furthermore, the HF generated by the LiPF6 + H2O reaction within the cell can attack metal foreign matter contained in the NCMA active material or silicon-based active material of the positive electrode, leading to the problem of metal ion elution. In other words, if metal foreign matter is present in the positive electrode area, it becomes metal ions through oxidation, chelates with the electrolyte, and moves to the negative electrode side. There, it precipitates on the negative electrode surface through reduction, and if this process is repeated, the grown precipitate can penetrate the separator, causing a short circuit in the cell or generating a low voltage.

[0080] The negative electrode active material of this application is characterized by effectively removing metallic impurities, particularly impurities of a specific phase, through a manufacturing process described later, thereby solving the aforementioned problems.

[0081] In one embodiment of this application, the metal impurity is a metal impurity that may be included in general, and specifically, it may include one or more selected from the group consisting of Fe, Al, Ca, Ti, Zn, Ni, Cu, Cr, Co, Mg, Sn, Ba, Zr, and Mn.

[0082] In this application, the metal impurity may include one or more selected from the group consisting of Fe, Al, and Ca.

[0083] In this application, the proportion of the Si-Fe phase or Si-Fe-Ti phase in the negative electrode active material may be 2% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material.

[0084] In this application, the proportion of the Si-Fe phase in the negative electrode active material may be 2% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material.

[0085] In another embodiment, the proportion of the Si-Fe phase in the negative electrode active material may be 2% or less, preferably 1.5% or less, more preferably 1.2% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material, and may also satisfy 0.005% or more, 0.007% or more, or 0.01% or more.

[0086] In this application, the proportion of the Si-Fe-Ti phase in the negative electrode active material may be 2% or less, based on 100% of the total phase of silicon-based active material and metal impurities.

[0087] In another embodiment, the proportion of the Si-Fe-Ti phase in the negative electrode active material may be 2% or less, preferably 1.5% or less, more preferably 1.3% or less, or 0.7% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material, and may also satisfy 0.005% or more, 0.007% or more, or 0.01% or more.

[0088] This application provides a negative electrode active material in which the proportion of Si-Fe phase and Si-Fe-Ti phase is 3% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material.

[0089] In another embodiment, the proportion of Si-Fe phase and Si-Fe-Ti phase in the negative electrode active material may be 3% or less, preferably 2% or less, more preferably 1.7% or less, or 0.005% or more, or 0.1% or more, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material.

[0090] In other words, the negative electrode active material of this application is characterized by having a metal impurity content within the aforementioned range through a manufacturing process described later, and by adjusting the ratio of the Si-Fe phase or Si-Fe-Ti phase within the aforementioned range based on a total phase (phase) of 100% of the silicon-based active material and metal impurities contained in the negative electrode active material.

[0091] When a mixture of two or more acids is not used in the manufacturing process described later, the content of metal impurities can be reduced to the aforementioned range, but the ratio of the Si-Fe phase or Si-Fe-Ti phase cannot be adjusted. In other words, this application applies a mixture of two or more acids to the pickling process to adjust the ratio of the Si-Fe phase or Si-Fe-Ti phase to below the aforementioned range. This prevents cell short circuits due to the deposition of metal foreign matter, ensures the lithium ion pathway, improves lifespan performance, and provides a high-energy-density negative electrode due to an increase in the capacity of the negative electrode active material itself.

[0092] In one embodiment of this application, a negative electrode active material is provided having a discharge capacity of 3600 mAh / g or more and 3700 mAh / g or less.

[0093] In one embodiment of this application, the charging capacity of the negative electrode active material may be 3850 mAh / g or more and 3950 mAh / g or less.

[0094] In one embodiment of this application, the initial efficiency of the negative electrode active material may be 93% or higher.

[0095] When SiO2 is present at the negative electrode, oxygen and lithium react, making the initially charged lithium irreversible. Therefore, in the manufacturing process described later, the acid containing HF, in particular, etches the SiO2 inside and outside the silicon, thereby increasing the initial charge-discharge efficiency of the silicon-based active material. Furthermore, the inert Si-Fe phase or Si-Fe-Ti phase, which does not participate in the reaction and accounts for only the weight of the silicon-based active material itself, can be efficiently removed by a specific acid. As a result, the capacity per unit weight of the negative electrode active material itself can satisfy the aforementioned range.

[0096] This application provides a method for producing a negative electrode active material, comprising the steps of preparing a silicon raw material, grinding the silicon raw material, and pickling the ground silicon raw material, wherein the pickling step includes adding an acid to the ground silicon raw material, and the acid is a mixture of two or more different acids.

[0097] It is recognized that metal impurities are a problem when using silicon-based negative electrodes, and conventionally, this has been addressed by removing metals using magnetism. However, in the case of silicon-based active materials, the silicon-based raw material is crushed during the process, and metal foreign matter is introduced due to friction between the milling machine and the silicon-based raw material. Furthermore, there are also weakly magnetic metal foreign matter contained in the silicon-based raw material itself, making it difficult to remove metals using magnetism.

[0098] Therefore, this application includes a step of pickling after crushing the silicon-based raw material, and in particular, by using a mixture of two or more different acids as the acid, it has the characteristic of being able to effectively remove metal impurities and impurities of a specific phase.

[0099] In one embodiment of this application, the present invention provides a method for producing a negative electrode active material, wherein the acid comprises at least hydrofluoric acid (HF).

[0100] In another embodiment, the acid is a mixture of two different acids, and the acid may include at least hydrofluoric acid (HF).

[0101] In particular, research has shown that the removal of the Si-Fe or Si-Fe-Ti phase becomes more efficient when hydrofluoric acid (HF) is included, as described above. That is, when the acid is a mixture of two types, and at least HF is included, the SiO2 inside and outside the silicon is etched, thereby increasing the initial charge-discharge efficiency of the silicon-based active material. Furthermore, the aforementioned inert Si-Fe or Si-Fe-Ti phase, which does not participate in the reaction and only accounts for the weight of the silicon-based active material itself, can be efficiently removed by a specific acid.

[0102] Simply using one type of acid (e.g., HCl, sulfuric acid) is insufficient to perform the aforementioned role, and it becomes impossible to efficiently remove the specific phase that causes problems at the negative electrode.

[0103] In this application, the acid may include two or more selected from the group consisting of HCl, HNO3, H2SO4, CH3COOH, and HF, and may include at least hydrofluoric acid (HF).

[0104] In one embodiment of this application, the hydrofluoric acid may have an aqueous solution concentration of 1% or more and 10% or less.

[0105] In another embodiment, the hydrofluoric acid may have an aqueous solution concentration of 1% to 10% or 2% to 7%.

[0106] As described above, by adjusting the concentration of hydrofluoric acid to an optimal range, the Si-Fe phase or Si-Fe-Ti phase can be efficiently removed.

[0107] One embodiment of this application provides a negative electrode composition comprising the negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

[0108] In one embodiment of this application, a negative electrode composition is provided in which the silicon-based active material is 60 parts by weight or more based on 100 parts by weight of the negative electrode composition.

[0109] In another embodiment, the silicon-based active material may be 60 parts by weight or more, preferably 63 parts by weight or more, more preferably 65 parts by weight or more, even more preferably 70 parts by weight or more, based on 100 parts by weight of the negative electrode composition, and may be 95 parts by weight or less, preferably 90 parts by weight or less, and more preferably 85 parts by weight or less.

[0110] The negative electrode composition according to this application has the characteristic of not degrading the performance of the negative electrode and having excellent output characteristics in charging and discharging, even when a silicon-based active material with a remarkably high capacity is used within the aforementioned range, by using a negative electrode active material that satisfies a specific surface area size that can suppress the volume expansion rate during the charging and discharging process.

[0111] Traditionally, graphite-based compounds were used exclusively as the negative electrode active material. However, in recent years, with the increasing demand for high-capacity batteries, there has been a growing trend to mix in silicon-based active materials to increase capacity. However, in the case of silicon-based active materials, even if the properties of the silicon-based active material itself are adjusted as described above, a problem can still occur where the volume expands rapidly during the charge / discharge process, damaging the conductive paths formed in the negative electrode active material layer.

[0112] Therefore, in one embodiment of this application, the negative electrode conductive material may include one or more selected from the group consisting of point conductive materials, sheet conductive materials, and linear conductive materials.

[0113] In one embodiment of this application, the point-shaped conductive material means a point-shaped or spherical conductive material that can be used to improve conductivity in a negative electrode, does not cause chemical changes, and is conductive. Specifically, the point-shaped conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and preferably contains carbon black in that it achieves high conductivity and has excellent dispersibility.

[0114] In one embodiment of this application, the point conductive material has a BET specific surface area of ​​40 m². 2 / g or more 70m 2 It may be less than / g, preferably 45m 2 / g or more 65m 2 / g or less, more preferably 50m 2 / g or more 60m 2 It may be less than / g.

[0115] In one embodiment of this application, the point-shaped conductive material may satisfy the requirement that the content of functional groups (volatile matter) be 0.01% or more and 1% or less, preferably 0.01% or more and 0.3% or less, and more preferably 0.01% or more and 0.1% or less.

[0116] In particular, when the functional group content of the point-like conductive material satisfies the aforementioned range, functional groups are present on the surface of the point-like conductive material, and when water is used as the solvent, the point-like conductive material can be smoothly dispersed in the solvent. In particular, the present invention makes it possible to lower the functional group content of the point-like conductive material by using a specific silicon-based active material, thereby providing an excellent improvement in dispersibility.

[0117] In one embodiment of this application, a silicon-based active material is provided along with a point-type conductive material having a functional group content within the range described above, wherein the content of the functional group can be adjusted according to the degree of heat treatment of the point-type conductive material.

[0118] In one embodiment of this application, the particle size of the dot-like conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 20 nm to 60 nm.

[0119] In one embodiment of this application, the conductive material may include a sheet-like conductive material.

[0120] The sheet-like conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, and can also suppress the disruption of the conductive path due to volume expansion. The sheet-like conductive material can also be described as a plate-like conductive material or a bulk conductive material.

[0121] In one embodiment of this application, the sheet-like conductive material may include at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and preferably plate graphite.

[0122] In one embodiment of this application, the average particle size (D50) of the sheet-like conductive material may be 2 μm to 7 μm, more specifically 3 μm to 6 μm, and more specifically 3.5 μm to 5 μm. When this range is met, the particle size is sufficient to prevent an excessive increase in the viscosity of the negative electrode slurry and facilitates dispersion. Therefore, the dispersion effect is excellent when dispersed using the same apparatus and time.

[0123] In one embodiment of this application, a negative electrode composition is provided in which the sheet-like conductive material has a D10 of 0.5 μm or more and 2.0 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 6.5 μm or more and 15.0 μm or less.

[0124] In one embodiment of this application, the sheet-like conductive material may be a high-specific-surface-area sheet-like conductive material with a high BET specific-surface-area; or a low-specific-surface-area sheet-like conductive material.

[0125] In one embodiment of this application, a sheet-like conductive material with a high specific surface area or a sheet-like conductive material with a low specific surface area can be used without limitation as the sheet-like conductive material. However, in particular, since the sheet-like conductive material according to this application may be affected to some extent by dispersion, it is especially preferable to use a sheet-like conductive material with a low specific surface area that does not cause dispersion problems.

[0126] In one embodiment of this application, the sheet-like conductive material has a BET specific surface area of ​​0.25 m². 2 It may be more than / g.

[0127] In another embodiment, the sheet-like conductive material has a BET specific surface area of ​​1 m². 2 / g or more 500m 2 It may be less than / g, preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 It may be less than / g.

[0128] The sheet-like conductive material relating to this application may be a sheet-like conductive material with a high specific surface area, or a sheet-like conductive material with a low specific surface area.

[0129] In another embodiment, the sheet-like conductive material is a sheet-like conductive material with a high specific surface area, and has a BET specific surface area of ​​50 m². 2 / g or more 500m 2 / g or less, preferably 80mg 2 / g or more 300m 2 Less than / g, more preferably 100m 2 / g or more 300m 2 The range of / g or less may also be satisfied.

[0130] In another embodiment, the sheet-like conductive material is a low specific surface area sheet-like conductive material, with a BET specific surface area of ​​1 m². 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m 2 The range of / g or less may also be satisfied.

[0131] Other conductive materials include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain multiple carbon nanotube units. Specifically, unless otherwise specified, "bundle type" here refers to a bundle-like or rope-like secondary shape in which multiple carbon nanotube units are arranged side by side or intertwined with substantially the same orientation along the length direction of the carbon nanotube units. The carbon nanotube units have a graphite sheet that is cylindrical with a nanoscale diameter and has an sp2 bond structure. In this case, the graphite sheet can exhibit conductive or semiconductor properties depending on the angle and structure at which it is curled. Compared to entangled-type carbon nanotubes, the bundle-type carbon nanotubes can be dispersed more uniformly during anode manufacturing, smoothly forming a conductive network within the anode and improving the conductivity of the anode.

[0132] In one embodiment of this application, the negative electrode conductive material is provided in an amount of 0.1 parts by weight or more and 40 parts by weight or less based on 100 parts by weight of the negative electrode composition.

[0133] In another embodiment, the negative electrode conductive material may be 0.1 parts by weight or more and 40 parts by weight or less, preferably 0.2 parts by weight or more and 30 parts by weight or less, more preferably 0.4 parts by weight or more and 25 parts by weight or less, and most preferably 0.4 parts by weight or more and 10 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0134] In one embodiment of this application, a negative electrode composition is provided in which the negative electrode conductive material includes a sheet-like conductive material; or a linear conductive material.

[0135] In one embodiment of this application, the negative electrode composition is provided, wherein the negative electrode conductive material includes a sheet-like conductive material and a linear conductive material.

[0136] In one embodiment of this application, the negative electrode conductive material is provided as a negative electrode composition comprising 80 to 99.9 parts by weight of the sheet-like conductive material and 0.1 to 20 parts by weight of the linear conductive material, based on 100 parts by weight of the negative electrode conductive material.

[0137] In another embodiment, the negative electrode conductive material may contain 80 to 99.9 parts by weight, preferably 85 to 99.9 parts by weight, and more preferably 95 to 98 parts by weight, of the sheet-like conductive material based on 100 parts by weight of the negative electrode conductive material.

[0138] In another embodiment, the negative electrode conductive material may contain 0.1 to 20 parts by weight, preferably 0.1 to 15 parts by weight, and more preferably 0.2 to 5 parts by weight, of the linear conductive material based on 100 parts by weight of the negative electrode conductive material.

[0139] In one embodiment of this application, the negative electrode conductive material includes a sheet-like conductive material and a linear conductive material, satisfying the aforementioned composition and proportions, thereby not significantly affecting the life characteristics of conventional lithium secondary batteries. In particular, when a sheet-like conductive material and a linear conductive material are included, the number of charge and discharge points increases, resulting in excellent output characteristics at a high C-rate and reduced high-temperature gas generation.

[0140] In one embodiment of this application, the negative electrode conductive material may be a linear conductive material.

[0141] In particular, when linear conductive materials are used alone, the tortuosity of the electrode, which is a problem with silicon-based negative electrodes, can be simplified, thereby improving the electrode structure and reducing the resistance to lithium ion movement within the electrode.

[0142] In one embodiment of this application, when the negative electrode conductive material includes a linear conductive material alone, the negative electrode conductive material may be included in an amount of 0.1 parts by weight or more and 5 parts by weight or less, preferably 0.2 parts by weight or more and 3 parts by weight or less, and more preferably 0.4 parts by weight or more and 1 part by weight or less, based on 100 parts by weight of the negative electrode composition.

[0143] The negative electrode conductive material described in this application has a completely different structure from the positive electrode conductive material applied to the positive electrode. Specifically, the negative electrode conductive material described in this application plays the role of providing contact between silicon-based active materials, which undergo very large volume expansion during charging and discharging, while the positive electrode conductive material plays the role of a buffer during rolling and imparts some conductivity, and its structure and role are completely different from the negative electrode conductive material of the present invention.

[0144] Furthermore, the negative electrode conductive material described in this application is applied to silicon-based active materials and has a completely different structure from conductive materials applied to graphite-based active materials. That is, conductive materials used in electrodes with graphite-based active materials simply have particles that are smaller than the active material, and thus have the properties of improving output characteristics and imparting some conductivity. This is completely different in structure and role from negative electrode conductive materials applied together with silicon-based active materials, as in the present invention.

[0145] In one embodiment of this application, the sheet-like conductive material used as the negative electrode conductive material described above has a different structure and role from the carbon-based active material generally used as a negative electrode active material. Specifically, the carbon-based active material used as the negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or dot shape to facilitate the storage and release of lithium ions.

[0146] In contrast, sheet-like conductive materials used as negative electrode conductive materials are substances that have a sheet-like or plate-like shape and can be described as plate-like graphite. That is, they are substances included in the negative electrode active material layer to maintain conductive pathways, and do not play a role in the storage and release of lithium, but rather are substances that secure conductive pathways in a sheet-like form within the negative electrode active material layer.

[0147] In other words, in this application, when plate-shaped graphite is used as a conductive material, it means that it is processed into a sheet or plate shape and used not to store or release lithium, but as a material to secure a conductive path. In this case, the negative electrode active material included together has high capacity characteristics for lithium storage and release, and plays a role in storing and releasing all lithium ions transmitted from the positive electrode.

[0148] In contrast, in this application, the use of a carbon-based active material as an active material means that it is processed into a point-like or spherical shape and used as a substance that stores or releases lithium.

[0149] In other words, in one embodiment of this application, the carbon-based active material, artificial graphite or natural graphite, is point-like and has a BET specific surface area of ​​0.1 m². 2 / g or more 4.5m 2 The range of less than or equal to / g may also be satisfied. In addition, plate graphite, which is a sheet-like conductive material, is in sheet form and has a BET specific surface area of ​​5m². 2 It may be more than / g.

[0150] In one embodiment of this application, the negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, and may also contain various copolymers thereof.

[0151] The negative electrode binder according to one embodiment of this application plays a role in holding the active material and conductive material to prevent distortion and structural deformation of the negative electrode structure during the volume expansion and relaxation of the silicon-based active material. Any general binder that fulfills the above role can be applied, and specifically, an aqueous binder may be used, or more specifically, a polyacrylamide (PAM) binder may be used.

[0152] In one embodiment of this application, the negative electrode binder may be 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode composition, and may also be 5 parts by weight or more, or 10 parts by weight or more.

[0153] One embodiment of this application provides a negative electrode for a lithium secondary battery, comprising a negative electrode current collector layer and a negative electrode active material layer formed on one or both sides of the negative electrode current collector layer, the negative electrode composition according to this application or a cured product thereof.

[0154] Figure 1 shows a laminated structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. Specifically, a negative electrode 100 for a lithium secondary battery can be seen, which includes a negative electrode active material layer 20 on one side of a negative electrode current collector layer 10. Although Figure 1 shows the negative electrode active material layer formed on one side, it may also be included on both sides of the negative electrode current collector layer.

[0155] In one embodiment of this application, the negative electrode for the lithium secondary battery may be formed by applying and drying a negative electrode slurry containing the negative electrode composition to one or both sides of a negative electrode current collector layer.

[0156] In this case, the negative electrode slurry may contain the negative electrode composition described above and a slurry solvent.

[0157] In one embodiment of this application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0158] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0159] The solid content of the negative electrode slurry refers to the content of the negative electrode composition contained in the negative electrode slurry, and may refer to the content of the negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0160] When the solid content of the negative electrode slurry satisfies the aforementioned range, the viscosity during the formation of the negative electrode active material layer is suitable, minimizing particle aggregation of the negative electrode composition and enabling efficient formation of the negative electrode active material layer.

[0161] In one embodiment of this application, the slurry solvent can be used without limitation as long as it is capable of dissolving the negative electrode composition, and specifically, water or N-methyl-2-pyrrolidinone (NMP) may be used.

[0162] In one embodiment of this application, the negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. Furthermore, fine irregularities may be formed on the surface to strengthen the bonding force of the negative electrode active material, and it may be used in various forms such as film, sheet, foil, mesh, porous body, foam, and nonwoven fabric.

[0163] In one embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer is 5 μm or more and 500 μm or less.

[0164] However, the thickness can vary considerably depending on the type and application of the negative electrode used, and is not limited thereto.

[0165] In one embodiment of this application, the porosity of the negative electrode active material layer may be in the range of 10% to 60%.

[0166] In another embodiment, the porosity of the negative electrode active material layer may be in the range of 10% to 60%, preferably 20% to 50%, and more preferably 30% to 45%.

[0167] The aforementioned porosity varies depending on the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer. In particular, the aforementioned range is satisfied by including the silicon-based active material and conductive material according to this application in specific compositions and content portions, thereby ensuring that the electrical conductivity and resistance of the electrode are within a suitable range.

[0168] One embodiment of this application provides a lithium secondary battery comprising a positive electrode, a negative electrode for a lithium secondary battery according to this application, a separator provided between the positive electrode and the negative electrode for a lithium secondary battery, and an electrolyte.

[0169] Figure 2 shows a stacked structure of a lithium secondary battery according to one embodiment of the present application. Specifically, a negative electrode 100 for a lithium secondary battery, including a negative electrode active material layer 20, can be seen on one side of the negative electrode current collector layer 10, and a positive electrode 200 for a lithium secondary battery, including a positive electrode active material layer 40, can be seen on one side of the positive electrode current collector layer 50. The negative electrode 100 and the positive electrode 200 for a lithium secondary battery are formed in a stacked structure with a separator 30 interposed between them.

[0170] A secondary battery according to one embodiment of this specification may include the negative electrode for lithium secondary batteries described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode for lithium secondary batteries, and an electrolyte, wherein the negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, a detailed explanation will be omitted.

[0171] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0172] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0173] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); or LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion, etc., are examples, but are not limited thereto. The positive electrode may also be lithium metal (Li-metal).

[0174] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0175] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitation as long as it has electronic conductivity without causing a chemical change in the battery that is constructed. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0176] Furthermore, the positive electrode binder plays a role in improving the bonding between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0177] The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in secondary batteries is acceptable without particular limitations, and it is especially preferable that it has low resistance to electrolyte ion movement and excellent electrolyte moisture retention capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymeric substances may be used, and they may be selectively used as single-layer or multi-layer structures.

[0178] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0179] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0180] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0181] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents because they have high dielectric constants and dissociate lithium salts well. Furthermore, when such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be produced, making them even more preferable.

[0182] As the metal salt, a lithium salt may be used, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte, for example, as the anion of the lithium salt, F - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3- 、(CF3SO2)2N - 、(FSO2)2N - 、CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - 、CF3(CF2)7SO3 - 、CF3CO2 - 、CH3CO2 - 、SCN - 、and (CF3CF2SO2)2N - One or more selected from the group consisting of may be used.

[0183] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, etc., for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc.

[0184] One embodiment of the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, they can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

Examples

[0185] The following are preferred embodiments to aid in understanding the present invention. However, these embodiments are merely illustrative examples, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of this description and the technical concept, and such variations and modifications will naturally fall within the scope of the appended claims.

[0186] <Manufacturing example> <Manufacturing of silicon-based active materials> <Example 1> MG-Si 553 (metallurgical silicon, 98.7 wt% purity, Fe 0.5 wt%, Al 0.5 wt%, Ca 0.3 wt%) raw material was crushed in three stages: jaw crusher (several millimeters size), disk mill (tens of micrometers to hundreds of micrometers), and spiral jet mill (0 nm to tens of nanometers). After crushing, Si material with a central particle size of 4 μm to 5 μm was produced using an air jet classifier.

[0187] Subsequently, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, and 200 g of pulverized Si material was added to the aqueous solution. Then, the mixture was stirred for 5 hours at 250 rpm while maintaining a temperature of 60°C using a magnetic stirrer. After that, the mixture was washed three times with distilled water using a centrifuge to remove any metallic foreign matter and acid components that may remain on the surface of the silicon particles, and the negative electrode active material was produced through a drying process.

[0188] <Example 2> MG-Si 553 (metallurgical silicon, 98.7 wt% purity, Fe 0.5 wt%, Al 0.5 wt%, Ca 0.3 wt%) raw material was crushed in three stages: jaw crusher (several millimeters size), disk mill (tens of micrometers to hundreds of micrometers), and spiral jet mill (0 nm to tens of nanometers). After crushing, Si material with a central particle size of 4 μm to 5 μm was produced using an air jet classifier.

[0189] Subsequently, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, and 200 g of pulverized Si material was added to the aqueous solution. Then, the mixture was stirred for 5 hours at 250 rpm using a magnetic stirrer while maintaining a temperature of 25°C. After that, the mixture was washed three times with distilled water using a centrifuge to remove any metallic foreign matter and acid components that may remain on the surface of the silicon particles, and the negative electrode active material was produced through a drying process.

[0190] <Example 3> MG-Si 553 (metallurgical silicon, 98.7 wt% purity, Fe 0.5 wt%, Al 0.5 wt%, Ca 0.3 wt%) raw material was crushed in three stages: jaw crusher (several millimeters size), disk mill (tens of micrometers to hundreds of micrometers), and spiral jet mill (0 nm to tens of nanometers). After crushing, Si material with a central particle size of 4 μm to 5 μm was produced using an air jet classifier.

[0191] Subsequently, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, and 200 g of pulverized Si material was added to the aqueous solution. Then, the mixture was stirred for 1 hour at 250 rpm while maintaining a temperature of 60°C using a magnetic stirrer. After that, the mixture was washed three times with distilled water using a centrifuge to remove any metallic foreign matter and acid components that may remain on the surface of the silicon particles, and the negative electrode active material was produced through a drying process.

[0192] <Example 4> SiC (silicon-carbon composite, 99.5 wt% purity, 0.2 wt% Fe, 0.2 wt% Al, 0.1 wt% Ca) raw material was crushed in three stages: Jaw Crusher (several millimeters in size), Disk Mill (tens of micrometers to hundreds of micrometers), and Spiral Jet Mill (0 nm to tens of nanometers). After crushing, Si material with a central particle size of 4 μm to 7 μm was produced using an air-flow classifier.

[0193] Subsequently, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, and 200 g of pulverized Si material was added to the aqueous solution. Then, the mixture was stirred for 1 hour at 250 rpm while maintaining a temperature of 60°C using a magnetic stirrer. After that, the mixture was washed three times with distilled water using a centrifuge to remove any metallic foreign matter and acid components that may remain on the surface of the silicon particles, and the negative electrode active material was produced through a drying process.

[0194] <Example 5> SiO (silicon oxide, 99.7 wt% purity, 0.1 wt% Fe, 0.15 wt% Al, 0.05 wt% Ca) raw material was crushed in three stages: jaw crusher (several millimeters in size), disk mill (tens of micrometers to hundreds of micrometers), and spiral jet mill (0 nm to tens of nanometers). After crushing, Si material with a central particle size of 1 μm to 3 μm was produced using an air jet classifier.

[0195] Subsequently, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, and 200 g of pulverized Si material was added to the aqueous solution. Then, the mixture was stirred for 1 hour at 250 rpm while maintaining a temperature of 60°C using a magnetic stirrer. After that, the mixture was washed three times with distilled water using a centrifuge to remove any metallic foreign matter and acid components that may remain on the surface of the silicon particles, and the negative electrode active material was produced through a drying process.

[0196] <Comparative Example 1> The preparation was carried out in the same manner as in Example 1, except that distilled water was used for washing instead of the aqueous solutions containing hydrochloric acid and hydrofluoric acid.

[0197] <Comparative Example 2> The preparation was carried out in the same manner as in Example 1, except that instead of using the aqueous solutions containing hydrochloric acid and hydrofluoric acid, 500 g of an aqueous solution containing 25 wt% hydrochloric acid by weight was prepared and used for washing.

[0198] <Comparative Example 3> In Example 1, 500 g of an aqueous solution containing 25 wt% hydrochloric acid and 5 wt% hydrofluoric acid by weight was prepared, 200 g of pulverized Si material was added to the aqueous solution, and the mixture was not stirred for 5 hours at 250 rpm while maintaining a temperature of 60°C using a magnetic stirrer, except that the process was carried out in the same manner as in Example 1. In other words, this corresponds to a case where the pickling step is not included.

[0199] The proportions of the Si-Fe phase and Si-Fe-Ti phase in the negative electrode active material of the above examples and comparative examples are shown in Table 1 below.

[0200] [Table 1]

[0201] In Table 1 above, the proportions of the Si-Fe phase and the Si-Fe-Ti phase refer to the weight ratio based on the total phase contained in the negative electrode active material.

[0202] This corresponds to a method where the area of ​​silicon active material within a certain size region is defined by EDS mapping, and the proportion of the relevant phase within that area is identified.

[0203] Specifically, Figure 3 shows an EDS analysis image from Example 1, confirming that no other metallic foreign substances other than Si or O were detected. For reference, in the case of C, it originates from the carbon tape. Figure 4 shows an EDS analysis image from Comparative Example 1. In Figure 4, it can be confirmed that Si, Ti, and Fe are observed at the same positions.

[0204] <Example of experiment> [Experimental Example 1: Lifetime Performance Results of Monocell] The secondary batteries containing the negative electrodes manufactured in the above examples and comparative examples were evaluated for their lifespan using an electrochemical charger / discharger, and their capacity retention rate was assessed. The secondary batteries underwent in-situ cycle testing at 4.2-3.0V 1C / 0.5C, and during the test, they were charged / discharged at 0.33C / 0.33C (4.2-3.0V) every 50 cycles to measure the capacity retention rate, which is shown in Table 2.

[0205] Life retention rate (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the first cycle)} × 100

[0206] [Table 2]

[0207] [Experimental Example 2: Evaluation of Discharge Capacity and Initial Efficiency] Using the negative electrode manufactured as described above, a circularly cut lithium metal thin film was used as the positive electrode. An electrolyte was injected using this positive-negative electrode to manufacture a lithium coin half-cell.

[0208] The evaluation method involved charging the manufactured lithium-ion secondary battery at a 0.1C rate (0.1C constant current charging, 5mV constant voltage charging, and 0.005C cutoff), discharging it at a 0.1C rate with a constant current (1.5V cutoff), and measuring the discharge capacity. The discharge capacity was calculated based on the weight of the negative electrode active material, and the capacity development rate was calculated according to the following formula.

[0209] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off) Discharge condition: CC (constant current) condition 1.5V The discharge capacity (mAh / g) and initial efficiency (%) were derived from the results of a single charge-discharge cycle, and the results are shown in Table 3.

[0210] [Table 3]

[0211] In Table 3 above, the discharge capacity (mAh / g) of the negative electrode active material was calculated as the measured discharge capacity divided by the loading amount of the negative electrode active material, the charge capacity (mAh / g) of the negative electrode active material was calculated as the measured charge capacity divided by the loading amount of the negative electrode active material, and the initial efficiency (%) was calculated as (discharge capacity (mAh / g) of the negative electrode active material / charge capacity (mAh / g) of the negative electrode active material) × 100.

[0212] [Experimental Example 3: Evaluation of Metal Contamination Content by ICP Analysis] The types and total content of doping elements were confirmed by ICP analysis using an inductively coupled plasma emission spectrometer (ICP-OES, AVIO 500, manufactured by Perkin-Elmer 7300).

[0213] Specifically, a certain amount (approximately 0.01 g) of the negative electrode active material was separated, transferred to a platinum crucible, and completely decomposed by adding nitric acid, hydrofluoric acid, and sulfuric acid to a hot plate at 130°C. After confirming that the sample was completely dissolved, it was allowed to cool completely at room temperature.

[0214] Subsequently, a reference calibration curve was created by measuring the intensity of standard solutions prepared using standard solutions (5 mg / kg) at wavelengths specific to the element to be analyzed, using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300).

[0215] Subsequently, the pre-treated sample solution and empty sample were introduced into the instrument, their respective intensities were measured to calculate the actual intensities, and the concentrations of each component were calculated against the calibration curve created above. These were then converted so that the total sum equaled the theoretical value, and the elemental content contained in the manufactured negative electrode active material was analyzed. The results are shown in Table 4.

[0216] [Table 4]

[0217] In Table 4, values ​​below 20 mg / kg were deemed unreliable as detection limits and are indicated as "<20". Furthermore, in the case of Sn, Ba, and Zr, some were present in trace amounts or were not detected, and are therefore not shown in Table 4.

[0218] In the ICP measurement described in Table 4, the weight of the first sample before the pickling process was measured, and then the amount of each element present in ppm levels was evaluated by ICP analysis. If the total content of metallic foreign matter is 10,000 mg / kg or less, it can be determined to be 1% or less based on 100 parts by weight of the negative electrode active material, and these parts by weight are shown in Table 4.

[0219] As can be seen from Tables 2 to 4 above, in the case of the negative electrode active material according to one embodiment of the present invention, it can be confirmed that the content of metal impurities is low and below a certain range, and that the proportion of the Si-Fe phase or Si-Fe-Ti phase is adjusted to 2% or less, based on 100% of the total phase of silicon-based active material and metal impurities contained in the negative electrode active material. In other words, by having the proportion of metal impurities having a specific phase as described above within the aforementioned range, it is possible to prevent cell short circuits due to the deposition of metal foreign matter, and it was confirmed that the embodiment according to this application is evaluated more highly than the comparative example in the lifetime evaluation.

[0220] Furthermore, Examples 4 and 5 use SiC composite and silicon oxide, respectively, and since these involve a silicon deposition process, it can be seen that there are fewer metal impurities compared to Examples 1-3. In particular, compared to Examples 1-3 of the present invention, it was confirmed that SiC and SiO have less volume expansion problems than Si, have a better lifespan than Examples 1-3, and have superior lifespan performance compared to Comparative Examples 1-3, which do not undergo a pickling process.

[0221] However, in Example 4, because the silicon-carbon composite contains carbon used as a support, the charging and discharging capacities were measured to be lower than in the other examples. In Example 5, due to the characteristics of the mixed Si and SiO2 material, it was confirmed that the initial efficiency decreased. Nevertheless, Examples 1 to 5, which underwent the pickling process, all demonstrated superior lifespan performance compared to the comparative examples.

[0222] Furthermore, in the case of silicon-based active materials, it is important that the tortuosity of the lithium ion path is simplified. In the case of the Si-Fe phase or Si-Fe-Ti phase, these are electrochemically inert phases and exist as interference within the electrode. However, the negative electrode active material according to this application, by undergoing a cleaning process with two or more acids to satisfy the aforementioned ratio, allows lithium ions to diffuse well to the vicinity of the electrode's current collector, rather than being concentrated only in the silicon of the upper layer of the electrode. This has been confirmed to have the characteristic of improving the lifespan performance of the negative electrode.

[0223] Furthermore, in the case of the Si-Fe phase or Si-Fe-Ti phase, when included in the silicon-based active material, it causes a problem of reduced capacity per gram of the silicon-based active material itself. As can be seen from Table 3 above, in the case of Examples 1 to 3 of this application, a reduction in capacity per gram of the silicon-based active material itself does not occur compared to Comparative Examples 1 to 3. Therefore, it was confirmed that high energy density, increased lifetime, and dendrite suppression are possible. [Explanation of Symbols]

[0224] 10 ···Negative electrode current collector layer 20...Negative electrode active material layer 30 ···Separator 40...Cathode active material layer 50 ···Positive electrode current collector layer 100 ···Negative electrode for lithium secondary batteries 200 ···Positive electrode for lithium secondary batteries

Claims

1. A negative electrode active material comprising a silicon-based active material and a metal impurity, A negative electrode active material in which the proportion of the Si-Fe phase or Si-Fe-Ti phase is 2% or less, based on 100% of the total phase of the silicon-based active material and the metal impurities contained in the negative electrode active material.

2. The aforementioned silicon-based active material is SiO ｘ (x=0), SiO ｘ The negative electrode active material according to claim 1, comprising (0 < x < 2), a SiC composite, or a Si alloy.

3. The aforementioned silicon-based active material is SiO ｘ (x=0) and SiO ｘ It includes one or more selected from the group consisting of (0 < x < 2), and the SiO is based on 100 parts by weight of the silicon-based active material. ｘ The negative electrode active material according to claim 1, comprising 90 parts by weight or more of (x=0).

4. The negative electrode active material according to claim 1, wherein the metallic impurity is 0.2 parts by weight or less based on 100 parts by weight of the negative electrode active material.

5. The negative electrode active material according to claim 1, wherein the metal impurity comprises one or more selected from the group consisting of Fe, Al, and Ca.

6. The negative electrode active material according to claim 1, wherein the proportion of the Si-Fe phase and the Si-Fe-Ti phase is 2% or less based on 100% of the total phase of the silicon-based active material and the metal impurities contained in the negative electrode active material.

7. The anode active material according to claim 1, wherein the silicon-based active material has a crystal grain size of 1 nm or more and 200 nm or less.

8. The negative electrode active material according to claim 1, wherein the discharge capacity of the negative electrode active material is 3600 mAh / g or more and 3700 mAh / g or less.

9. The stage of preparing silicon raw materials, The steps include: pulverizing the aforementioned silicon raw material, The step of pickling the crushed silicon raw material, A method for producing a negative electrode active material containing, A method for producing a negative electrode active material, wherein the pickling step includes adding an acid to the pulverized silicon raw material, and the acid is a mixture of two or more different acids.

10. The method for producing a negative electrode active material according to claim 9, wherein the acid comprises at least hydrofluoric acid (HF).

11. The method for producing a negative electrode active material according to claim 10, wherein the hydrofluoric acid has an aqueous solution standard concentration of 1% or more and 10% or less.

12. A negative electrode composition comprising a negative electrode active material according to any one of claims 1 to 8, a negative electrode conductive material, and a negative electrode binder.

13. The negative electrode composition according to claim 12, wherein the negative electrode active material is 60 parts by weight or more based on 100 parts by weight of the negative electrode composition.

14. The negative electrode composition according to claim 12, wherein the negative electrode conductive material comprises one or more selected from the group consisting of a sheet-like conductive material and a linear conductive material.

15. It includes a negative electrode current collector layer and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer, The negative electrode active material layer comprises the negative electrode composition or a cured product thereof according to claim 12, for use as a negative electrode for a lithium secondary battery.

16. The thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less. The negative electrode for a lithium secondary battery according to claim 15, wherein the thickness of the negative electrode active material layer is 5 μm or more and 500 μm or less.

17. Positive electrode and, The negative electrode for a lithium secondary battery according to claim 15, A separator is provided between the positive electrode and the negative electrode for the lithium secondary battery, Electrolytes, Lithium-ion batteries, including lithium-ion batteries.

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