A negative electrode for a secondary battery with improved fast charging performance, and a secondary battery including the same.

Abstract

This invention relates to a negative electrode for a secondary battery and a secondary battery including the same. The negative electrode for the secondary battery comprises: a current collector; and a negative electrode active material layer formed on the current collector, containing a negative electrode active material, satisfying the following relation 1: [Relation 1] D < 1.9·10 ‑3 In Equation 1, D is the diffusion coefficient of lithium ions within the negative electrode, derived mathematically as follows: D = σ 2 ·τ d ‑γ σ is the thickness (cm) of the negative electrode active material layer, and τ d Here, γ is the diffusion time (s), and γ is the dispersion parameter, satisfying 0.8 ≤ γ ≤ 0.95. The electrode of this invention forms fine and dense pores, thereby reducing diffusion resistance within the electrode and thus enabling a battery with high output performance.

Description

Technical Field

[0001] This invention relates to a negative electrode for a secondary battery with improved fast charging performance and a secondary battery including the same. Background Technology

[0002] Recently, with the increasing demand for electronic devices such as mobile devices, the development of lightweight and miniaturized electrochemical batteries (secondary batteries) to improve the portability of electronic devices is expanding. Along with this trend, there is a need to develop secondary batteries with high capacity and high output.

[0003] Research on lithium-ion rechargeable batteries is progressing towards increasing the density of the anode mixture to achieve higher capacity. Increasing the density of the anode mixture allows for a greater amount of active material within the same anode volume, thus improving energy density. However, as the mixture density increases, the porosity of the anode decreases, leading to a slower lithium-ion diffusion rate and less efficient lithium-ion adsorption / release reactions, resulting in reduced output performance. This problem is particularly severe during high-speed charge-discharge processes; even with increased active material content in the anode, discharge capacity and cycle characteristics significantly decrease during high-speed charging and discharging. Therefore, there is a need to develop an anode that can simultaneously exhibit high mixture density and high output.

[0004] The importance of high-output electrodes and the development of pores within the electrodes is recognized, and efforts are underway to develop electrode pore structures that are conducive to high performance. In the past, factors such as pore quantity and average pore diameter have been considered as indicators of high-performance pore structures. However, these factors are analyzed by filling the electrode with nitrogen or mercury, which differs from actual electrode-electrolyte systems and is therefore difficult to consider as representative of the actual system in operation.

[0005] Prior art 1 (KR 10-2019-0042335) clarifies the diffusion of lithium ions within active material particles, showing that the diffusion coefficient is caused by the active material, rather than by the development of pores in the electrode active material layer. Summary of the Invention

[0006] Technical issues

[0007] The pores within the electrodes of lithium-ion batteries play a crucial role in cell performance. Since these pores serve as channels for supplying lithium ions within the electrode, their formation within the electrode requires careful consideration. Factors contributing to pore formation include active materials, conductive materials, and binders—all components of the electrode—as well as the loading and density—structural elements of the electrode. It can be seen that, aside from structural factors and factors present in relatively small amounts within the electrode, such as conductive materials and binders, the active material itself has the greatest impact on pore formation within the electrode (active material layer).

[0008] On the other hand, the supply of lithium ions to the electrode pores is accomplished through a diffusion mechanism. In particular, lithium ion diffusion within the electrode is spatially confined diffusion. Therefore, the diffusion coefficient (= diffusion rate) and diffusion time reflect the diffusion space (the longer the diffusion space, the longer the required diffusion time). Thus, if we can know the time required for lithium ions to diffuse within the pores of the electrode, we can explain the formation of the pores.

[0009] This invention provides an active material layer with a suitable porous structure within an electrode to achieve high electrode performance. In particular, the electrode containing the active material layer is characterized by a relatively long diffusion time. Generally, a shorter diffusion time is considered to indicate a more easily diffused structure, but due to the spatially confined diffusion mechanism of the electrode, this can be interpreted as the opposite. That is, it can be analyzed that a longer diffusion time within the confined diffusion space results in a narrower and longer diffusion space (pore length). A long diffusion space in the electrode means that the pores formed within the electrode are fine and uniform. Therefore, it has the advantage of being able to uniformly supply (diffuse) lithium ions throughout the entire space of the electrode active material layer.

[0010] The purpose of this invention is to provide an active material and electrode that achieve the above-mentioned characteristics through manufacturing, having a porous structure that is beneficial to high performance, and in particular, having excellent high-speed charge and discharge performance.

[0011] Technical solution

[0012] In one implementation example, a negative electrode for a secondary battery is provided, comprising: a current collector; and a negative electrode active material layer formed on the current collector, containing a negative electrode active material, which satisfies the following relation 1:

[0013] [Relation 1]

[0014] D<1.9·10 -3

[0015] In Equation 1, D is the diffusion coefficient of lithium ions within the negative electrode, derived through the following mathematical formula.

[0016] D = σ 2 ·τ d -γ

[0017] σ is the thickness (cm) of the negative electrode active material layer, τ d is the diffusion time (s), and γ is the dispersion parameter, which satisfies 0.8≤γ≤0.95.

[0018] The negative electrode can also satisfy the following relationship 2.

[0019] [Relation 2]

[0020] 1.10 -3 ≤D<1.9·10 -3

[0021] In Equation 2, D is the diffusion coefficient of lithium ions within the negative electrode, derived through the following mathematical formula.

[0022] D = σ 2 ·τ d -γ

[0023] σ is the thickness (cm) of the negative electrode active material layer, τ d is the diffusion time (s), and γ is the dispersion parameter, which satisfies 0.8≤γ≤0.95.

[0024] The negative electrode may have a lithium-ion diffusion time τ of 0.020 seconds or more. d The negative electrode.

[0025] The negative electrode may have a lithium-ion diffusion time τ of 0.025 to 0.041 seconds. d The negative electrode.

[0026] The negative electrode active material may include at least one carbon-based active material.

[0027] The negative electrode active material layer may have a thickness of 0.20 to 0.23 cm. 3 / g of total pore volume of the negative electrode active material layer.

[0028] The negative electrode active material layer can be a negative electrode active material layer with a porosity of 20 to 30%.

[0029] The negative electrode active material layer may be a negative electrode active material layer with an average pore size (D50) of 550 to 800 nm.

[0030] The negative electrode active material layer can have a concentration of 6 to 15 g / cm³. 2 The load.

[0031] The negative electrode active material layer may have a concentration of 1.4 to 1.8 g / cm³. 3 The negative electrode active material layer with negative electrode density.

[0032] In another embodiment, a secondary battery is provided, comprising: the negative electrode; the positive electrode; a separator and an electrolyte.

[0033] Technical effect

[0034] When the lithium-ion diffusion coefficient of the electrode meets the numerical range proposed in this invention, fine and dense pores are formed inside the electrode, thereby reducing the diffusion resistance inside the electrode and thus providing a battery with high output performance. Detailed Implementation

[0035] The advantages, features, and implementation methods of the present invention will become clear from the following detailed description of the embodiments. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in many different forms. The embodiments provided here are intended to make the disclosure of the present invention complete and to fully inform those skilled in the art of the scope of the invention, which is defined only by the scope of the claims. The following detailed description is for implementing the specific content of the present invention.

[0036] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) are to be used in the sense that would be readily understood by one of ordinary skill in the art. Throughout this specification, when a part is referred to as "comprising" a constituent element, it means, unless specifically stated otherwise, that other constituent elements may also be included, rather than excluded. Furthermore, unless specifically stated otherwise, the singular form also includes the plural form.

[0037] In this specification, when describing layers, films, regions, plates, etc., as being located "above" or "on top of" other parts, this includes not only the case where they are "directly" located "on top of" other parts, but also the case where there are other parts between them.

[0038] One embodiment of the present invention provides a negative electrode for a secondary battery. The negative electrode includes: a current collector; and a negative electrode active material layer formed on the current collector, containing a negative electrode active material.

[0039] The current collector may be selected from, but is not limited to, the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0040] The negative electrode active material layer includes a negative electrode active material and a binder, and may also include a conductive material.

[0041] The negative electrode active material can be a substance comprising at least one carbon-based active material, preferably a substance comprising at least two carbon-based active materials. For example, the negative electrode active material can be at least one carbon-based active material, more preferably at least two carbon-based active materials. Thus, the lithium-ion diffusion coefficient of the negative electrode of the present invention can be adjusted.

[0042] The carbon-based active material may include at least one selected from the group consisting of natural graphite, artificial graphite, graphitized carbon fibers, graphitized mesophase carbon microspheres, and amorphous carbon, preferably artificial graphite. Additionally, the carbon-based active material may also contain primary particles, preferably secondary particles granulated from at least two primary particles. Therefore, the lithium-ion diffusion coefficient of the negative electrode of the present invention can be adjusted to a desired range.

[0043] The carbon-based active material may include a coating on the particle surface containing amorphous carbon composed of soft carbon and / or hard carbon. Therefore, the lithium-ion diffusion coefficient of the negative electrode of the present invention can be adjusted to a preferred range.

[0044] As an example, the method for preparing the artificial graphite is described below, but the present invention is not limited thereto. Generally, artificial graphite particles can be obtained by mixing graphitizable raw materials (aggregates), graphitization catalysts, and binders for binding these, followed by pre-calcination and graphitization. Various types of coke, such as gold coke and needle coke, can be used as graphitizable aggregates. Furthermore, pre-graphitized natural graphite or artificial graphite can also be used. Petroleum, coal, and artificial pitch and tar can be used as binders, preferably graphitizable materials such as the aggregates described above. Carbides, oxides, and nitrides of silicon, iron, nickel, titanium, boron, etc., can be used as graphitization catalysts. Regarding pre-calcination and graphitization, it is preferable to carry out the process in an atmosphere where aggregates and binders are difficult to oxidize, such as a nitrogen atmosphere, argon atmosphere, or vacuum. Pre-calcination is preferably performed at a temperature range of 400–1,000°C, and graphitization at a temperature above 2,000°C. In terms of graphitization, the graphitization catalyst is removed at temperatures above 2,000°C, thus allowing for the formation of fine pores. Furthermore, a graphitization temperature above 2,500°C is more preferred, and at temperatures above 2,800°C, highly crystalline graphite can be obtained, which is likely the most preferred. When the graphitization temperature is below 2,000°C, graphite crystallization development deteriorates, and graphitization catalyst residues remain in the prepared graphitic particles, thus tending to reduce charge / discharge capacity. Preferably, the amount of graphitization catalyst added is 1–50% by weight of the total weight of the graphitizable aggregate or graphite and binder. Below 1% by weight, there is a tendency for poor crystallization development of the artificial graphite particles and a reduction in charge / discharge capacity. On the other hand, when the content exceeds 50% by weight, there is a tendency for uniform mixing to be difficult and for workability to decrease. Since the artificial graphite obtained above is in block form, it is preferable to pulverize it. The pulverization method is not particularly limited, but devices such as jet mills, vibratory mills, and hammer mills can be used. The average particle size (D50) after pulverization is preferably less than 100 μm, and the coatability is good in the range of 10 to 50 μm, which may be more preferred. In addition, the pulverized powder can be subjected to cold homogenization molding as needed, but the present invention is not limited thereto.

[0045] The negative electrode active material may also include a metal-based active material, which is an active material that includes a metal. Generally, it refers to an active material that includes an element capable of intercalating lithium, and whose theoretical capacitance per unit mass is 500 mAh / g or more when lithium is intercalated. Examples of such metal-based active materials include lithium metal, monomeric metals capable of forming lithium alloys (e.g., Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, Ti, etc.) and their alloys, oxides, sulfides, nitrides, silicides, carbides, phosphides, etc. of the metal or alloy.

[0046] When a silicon-based active material containing silicon is used as the active material of the metal-based negative electrode, it is preferable in terms of increasing the capacity of the battery. Examples of the silicon-based active material include silicon (Si), an alloy of silicon and cobalt, nickel, iron, aluminum, etc., SiOx (0 < x < 2), a mixture of a Si-containing material and a carbon material, a composite of a Si-containing material and conductive carbon obtained by coating or compounding the Si-containing material with conductive carbon, etc.

[0047] The binder can be a water-soluble binder. Specifically, it can be styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, a copolymer of propylene and an olefin having 2 to 8 carbon atoms, polyacrylamide, a copolymer of (meth)acrylic acid and an alkyl (meth)acrylate, or a combination thereof. When the water-soluble binder is used as the negative electrode binder, the water-soluble binder can well bond the electrode active material and the current collector without affecting the viscosity of the slurry, so it has advantages. However, as the electrode active material and the conductive material in the form of fine particles are likely to cause gelation of the electrode slurry, a thickener can also be included to form a stable slurry by imparting viscosity to the electrode slurry. As an example, a cellulose-based compound can be used as the thickener. Specifically, one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts can be mixed and used. Here, the alkali metal can be Na, K, or Li.

[0048] The conductive material is used to impart conductivity to the negative electrode. As long as it is an existing electronic conductive material that does not cause chemical changes in the battery, there is no particular limitation. As an example, it can be natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, and their combination, but is not limited thereto.

[0049] The negative electrode satisfies the following relational expression 1, and more preferably may also satisfy the following relational expression 2.

[0050] [Relational expression 1]

[0051] D < 1.9·10 -3

[0052] [Relational expression 2]

[0053] 1·10 -3 ≤ D < 1.9·10 -3

[0054] In the above relational expressions 1 and 2, D is the diffusion coefficient of lithium ions in the negative electrode, and can be derived from the following mathematical formula.

[0055] ]>D = σ 2 ·τ d -γ

[0056] σ is the thickness (cm) of the negative electrode active material layer, τ d γ is the diffusion time constant (s), and γ is the dispersion parameter, which satisfies 0.8≤γ≤0.95.

[0057] In one embodiment of the present invention, the diffusion coefficient (D, cm⁻¹) of the relation 1 in the negative electrode is... 2 ·s -γ The diffusion coefficient can represent the rate at which lithium ions are supplied through the pores of the negative electrode via diffusion. In a non-chargeable / dischargeable system (negative electrode / separator / negative electrode), since no charging or discharging occurs, lithium ions do not react with the active material. Therefore, it can be said that the coefficient fully reflects the situation where lithium ions diffuse only through the electrolyte (liquid) via the porous structure of the negative electrode active material layer.

[0058] That is, the diffusion coefficient of the present invention reflects the diffusion rate of lithium ions through the electrolyte (liquid) to the interior of the pores before reaching the active material. Compared with the prior art for evaluating the diffusion of lithium ions in the structure of a rechargeable system (including a battery with a positive electrode / separator / negative electrode and electrolyte), it can more accurately evaluate the pore structure inside the negative electrode.

[0059] Specifically, the diffusion coefficient D can be 1.1 * 10⁻⁶. -3 ≤D<1.9*10 -3 1.15*10 -3 ≤D<1.9*10 -3 1.2*10 -3 ≤D<1.9*10 -3 Or 1.5*10 -3 ≤D<1.9*10 -3 Specifically, it can be 1*10 -3 ≤D≤1.85*10 -3 1*10 -3 ≤D≤1.8*10 -3 1*10 -3 ≤D≤1.5*10 -3 1*10 -3 ≤ D≤1.4*10 -3 1*10 -3 ≤D≤1.3*10 -3 Or 1*10 -3 ≤D≤1.2*10 -3Analysis shows that the lower the diffusion coefficient within the specified range, the narrower and longer the diffusion space (pore length) becomes in a negative electrode (with the same negative electrode active material layer loading and density, and also the same porosity) as the lithium ion diffusion time increases. Therefore, with the formation of fine and uniform pores within the electrode, the entire electrode can uniformly supply lithium ions, thereby improving the high output characteristics of the battery. It is evident that the diffusion coefficient, calculated considering the diffusion time and diffusing space, represents a preferred range for negative electrode formation, where an appropriate diffusion coefficient range as described above results in the appropriate formation of pores. However, an excessively low diffusion coefficient indicates overly fine pore development. In this case, the ion diffusion path may be too long, potentially increasing diffusion resistance and reducing output characteristics.

[0060] Various methods can be employed to prepare a negative electrode active material layer with a diffusion coefficient within the aforementioned range. For example, when artificial graphite is used as the active material, differences in the hardness, strength, and other physical properties of the prepared artificial graphite can be achieved by changing the type of raw material, such as coke, thereby altering the porosity of the prepared negative electrode active material layer. For instance, isotropic coke can be considered to have relatively higher strength characteristics compared to needle coke. Furthermore, the porosity of the prepared negative electrode active material layer can be altered by mixing two or more active material particles with different average particle sizes (D50) or by adjusting the mixing weight ratio of these active material particles. Moreover, when the surface of the active material includes a coating containing amorphous carbon composed of soft carbon and / or hard carbon, the strength of the active material particles increases, thereby creating conditions favorable for pore formation. However, the present invention is not limited to the aforementioned characteristics of the negative electrode active material and its preparation method.

[0061] The thickness (σ) of the negative electrode active material layer can represent the thickness of the negative electrode active material layer measured after immersing the prepared negative electrode in the electrolyte and storing it at room temperature for 10 hours. The thickness can be measured by subtracting the foil thickness from the negative electrode thickness using a thickness gauge, or by measuring the thickness of the negative electrode active material layer after peeling off the foil, but the present invention is not limited to these methods. For example, an electrolyte solution can be used by mixing 1.0 M lithium salt LiPF6 with an organic solvent (EC (ethylene carbonate): EMC (ethyl methyl carbonate) = 1:4 vol%), followed by 1 vol% of electrolyte additive FEC (fluoroethylene carbonate), but the present invention is not limited to this method. Furthermore, analysis shows that the migration velocity of ions changes with different electrolyte compositions, and the time for ion diffusion varies depending on the electrolyte, but the tendency of the lithium ion diffusion coefficient (D) shown in the present invention is not reversed or shows different results. Here, the degree of variation in the numerical range of the diffusion coefficient (D) can be ±0.1% to 5% as an error range level.

[0062] The diffusion time constant (τ) can be determined by performing Nyquist plotting and fitting on the data calculated using electrochemical impedance spectroscopy (EIS). d ).

[0063] The EIS can be performed using a biologic VMP3 device, with a frequency range of 500 kHz to 50 MHz, but the invention is not limited thereto. The Nyquist plot can be performed using a common method of graphically observing the EIS results, and fitting can be done using the Z-fit tool in EC-Lab software, but the invention is not limited thereto.

[0064] The dispersion parameter (γ) represents the parameter used to correct the analysis model (τ). d The difference between the ideal and experimental environments is obtained through data fitting. The closer the value is to 1, the more ideal the conditions; conversely, the closer it is to 0, the more non-ideal the conditions. The secondary battery system only restricts the dispersion parameter to values ​​between 0.8 and 0.95. The dispersion parameter described in this invention uses a value in the range of 0.88 to 0.92, but this invention is not limited to this.

[0065] In one embodiment of the present invention, the negative electrode can have a lithium-ion diffusion time (τ) of 0.020 seconds (s) or more. dMore preferably, it can have a lithium-ion diffusion time (τ) of 0.025 to 0.050 seconds (s). d When lithium ions diffuse inside the electrode, the diffusion time is proportional to the volume (diffusible space or area) formed by each pore structure path. When the diffusion time falls within the specified range, it can be inferred that fine and dense pores have formed. This effect can be achieved by preparing a negative electrode with a lithium ion diffusion time within this range. However, exceeding this diffusion time range indicates that the pores are too fine. This may lead to long-term performance degradation due to excessive reaction surface area, and the diffusion resistance may increase excessively.

[0066] The negative electrode active material layer can have a thickness of 0.18 to 0.25 cm. 3 The total pore volume per gram is preferably 0.20 to 0.25 cm³. 3 / g, 0.20 to 0.23cm 3 / g of total pore volume. This allows for further improvement of the aforementioned effects.

[0067] The negative electrode active material layer can have a porosity of 20% to 30%, preferably 20% to 25% or 21% to 25%. This further improves the aforementioned effects.

[0068] The negative electrode active material layer can have an average pore size (D50) of 500 to 1000 nm, preferably 500 to 850 nm, 550 to 800 nm, or 550 to 750 nm, and more preferably 550 to 700 nm. This further improves the aforementioned effects.

[0069] The negative electrode active material layer can have a thickness of 2.0 to 6.0 μm. 2 The specific surface area per g is preferably between 2.1 and 6.0 m². 2 / g or 3.0 to 6.0m 2 The specific surface area is / g. Therefore, the above effects can be further improved.

[0070] The negative electrode active material layer can have a density of 5.0 to 25.0 g / cm³. 2 The loading amount (based on the solid composition of the negative electrode active material layer) is preferably 5.0 to 20.0 g / cm³. 2 5.0 to 15.0 g / cm³ 2 Or 6.0 to 15.0 g / cm³ 2The loading capacity can have a thickness of 50 to 150 μm, preferably 50 to 120 μm or 70 to 120 μm. This further improves the aforementioned effects.

[0071] The negative electrode active material layer can have a content of 1.4 to 1.8 g / cm³. 3 The negative electrode density is preferably 1.6 to 1.8 g / cm³. 3 Or 1.4 to 1.7 g / cm³ 3 As an example, it can have 1.7 g / cm³. 3 The negative electrode density. Therefore, the above effects can be further improved.

[0072] In this invention, the lithium-ion diffusion coefficient (D) may be difficult to deduce from individual factors such as the total pore volume, porosity, average pore size, specific surface area, loading, and negative electrode density of the aforementioned negative electrode active material layer. Similarly, as mentioned above, even if the specifications of the active materials are the same, the diffusion coefficient may differ because the characteristics of the pores are derived from the complex effects of the particle size, shape (spherical, plate-like, deformed, etc.), density, strength, and raw material properties of the active materials constituting the specifications of the active materials. Therefore, rather than defining the properties of the negative electrode active material layer, such as pore size, by several active material factors, it is preferable to define the pore performance by its characteristics. Here, when evaluating the pore characteristics, it is more preferable to evaluate them using the aforementioned diffusion coefficient than by evaluating them using the numerical ranges of individual factors such as the total pore volume, porosity, average pore size, specific surface area, loading, and negative electrode density of the aforementioned negative electrode active material layer.

[0073] In addition, the total pore volume, porosity, average pore size and specific surface area of ​​the negative electrode active material layer can be determined by the mercury porosimeter method, but the present invention is not limited thereto.

[0074] Another embodiment of the present invention provides a secondary battery, including the negative electrode, positive electrode, separator and electrolyte.

[0075] The negative electrode is as described above.

[0076] The positive electrode includes a current collector and a layer of positive electrode active material formed by coating the current collector with a positive electrode slurry containing positive electrode active material.

[0077] The current collector can be the aforementioned negative current collector, or it can be a material known in the art, but the present invention is not limited thereto.

[0078] The positive electrode active material layer includes a positive electrode active material, and optionally may also include a binder and a conductive material. It is permissible to use positive electrode active materials known in the art, for example, preferably a composite oxide of metals selected from cobalt, manganese, nickel, and combinations thereof, and lithium, but the invention is not limited thereto.

[0079] The adhesive and conductive material may be the aforementioned negative electrode adhesive and negative electrode conductive material. It is permissible to use substances known in the art, but the present invention is not limited thereto.

[0080] The separator can be selected from materials such as glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof, and can be in the form of nonwoven or woven fabric. For example, polyolefin polymer separators such as polyethylene and polypropylene are primarily used in lithium secondary batteries, and to ensure heat resistance or mechanical strength, separators coated with compositions containing ceramic components or polymeric substances can be used. Selectively, they can be used in single-layer or multi-layer structures. It is permissible to use separators known in the art, but the invention is not limited thereto.

[0081] The electrolyte comprises an organic solvent and a lithium salt.

[0082] The organic solvent serves as a medium that allows ions participating in the electrochemical reaction of the battery to move. For example, carbonates, esters, ethers, ketones, alcohols, or aprotic solvents can be used. These organic solvents can be used alone or in mixtures of two or more. When using mixtures of two or more, the mixing ratio can be adjusted appropriately according to the desired battery performance. Furthermore, the use of organic solvents known in the art is permissible, but the invention is not limited thereto.

[0083] The lithium salt is dissolved in an organic solvent and serves as a lithium-ion supply source within the battery, enabling the basic operation of the lithium secondary battery and promoting the movement of lithium ions between the positive and negative electrodes. Examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO3C2F5)2, LiN(CF3SO2)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C x F 2x+ 1SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, LiB(C2O4)2, or combinations thereof, but the present invention is not limited thereto.

[0084] The concentration of the lithium salt can be used in the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte exhibits excellent electrolyte performance due to its appropriate conductivity and viscosity, and can effectively move lithium ions.

[0085] Furthermore, as needed, to improve charge / discharge characteristics, flame retardant properties, etc., the electrolyte may further contain pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether (glyme), triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. Depending on the circumstances, it may also contain halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride to impart non-flammability, and may also contain fluoroethylene carbonate (FEC), propene sulfone (PRS), fluoropropylene carbonate (FPC), etc., to improve high-temperature storage properties.

[0086] The secondary battery can be a lithium secondary battery.

[0087] The method for preparing the secondary battery of the present invention to achieve the above-described objectives may involve sequentially stacking a negative electrode, a separator, and a positive electrode to form an electrode assembly, placing the prepared electrode assembly into a cylindrical battery casing or a angular battery casing, and then injecting an electrolyte to prepare the battery. Alternatively, the stacked electrode assembly may be immersed in an electrolyte, and the resulting material may be placed into a battery casing for sealing.

[0088] The battery casing used in this invention can be any casing commonly used in the art, and there are no shape restrictions for the purpose of the battery. For example, it can be cylindrical, angular, pouch-shaped, or coin-shaped, etc., using a metal casing.

[0089] The secondary battery of the present invention can be used not only as a battery cell for powering small devices, but also preferably as a unit battery in medium to large-sized battery modules comprising multiple battery cells. Preferred examples of such medium to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems, but are not limited thereto.

[0090] The present invention will now be described in detail with reference to the embodiments, but these are for the purpose of further elaborating the invention, and the scope of the invention is not limited to the following embodiments.

[0091] Example

[0092] (Example 1)

[0093] A slurry was prepared by adding water to 97.3 wt% of negative electrode active material, 1.5 wt% of SBR binder, and 1.2 wt% of CMC, and mixing at room temperature for 120 minutes.

[0094] Here, the negative electrode active material is artificial graphite using petroleum-based needle coke, with a central particle size D50 of 14 μm. Artificial graphite is produced by granulating small-diameter particles and coating each particle surface with amorphous carbon (asphalt), with the coating amount being approximately 3% by weight.

[0095] The prepared slurry was prepared at 13 mg / cm³. 2 The loading amount was coated onto the Cu foil current collector and dried, and then rolled until the density of the negative electrode mixture (negative electrode density) was 1.7 g / cc, thereby preparing the negative electrode.

[0096] (Example 2)

[0097] Except for using artificial graphite anode active material made from petroleum-based isotropic coke (with a change in raw material coke), the anode was prepared in the same manner as in Example 1. Here, the center particle size D50 of the artificial graphite and the amorphous carbon coating on the surface are the same as in Example 1.

[0098] (Example 3)

[0099] The negative electrode was prepared in the same manner as in Example 1, except that a negative electrode active material consisting of a 3:7 weight ratio mixture of artificial graphite (D50 14 μm) and artificial graphite (D50 11 μm) used in Example 1 was used. Here, the artificial graphite (D50 11 μm) is composed of granulated small-diameter particles coated with amorphous carbon (asphalt) on the surface of each particle, with a coating amount of approximately 3% by weight.

[0100] (Comparative Example 1)

[0101] The negative electrode was prepared in the same manner as in Example 3, except that the artificial graphite with D50 14um and artificial graphite with D50 11um used as artificial graphite active materials in Example 3 were mixed in a weight ratio of 7:3 to be used as negative electrode active materials.

[0102] (Comparative Example 2)

[0103] Except for the artificial graphite used in Example 3, which has no amorphous carbon layer on its surface, the negative electrode was made in the same manner as in Example 3.

[0104] (Comparative Example 3)

[0105] The negative electrode was fabricated in the same manner as in Example 1, except that the artificial graphite used in Example 1 had no amorphous carbon layer on its surface and was used as the negative electrode active material.

[0106] (Comparative Example 4)

[0107] The negative electrode was fabricated in the same manner as in Example 3, except that artificial graphite with a central particle size of D50 11 μm was used as the single negative electrode active material.

[0108] Evaluation example

[0109] (Evaluation Method)

[0110] 1. Determination of negative polarity properties

[0111] The dimensions of the pores within the negative electrodes prepared in Examples 1-3 and Comparative Examples 1-4, as well as the porosity (mL / g) and specific surface area (m²) were obtained using a mercury porosimeter (product name: Auto pore V9620). 2 The average porosity ( / g) and average pore diameter (nm) are shown in Table 1 below.

[0112] 2. Determination of negative electrode diffusion coefficient (D)

[0113] After the negative electrodes prepared in Examples 1-3 and Comparative Examples 1-4 were configured into symmetrical units of the coin cell type, they were filled with electrolyte. An electrolyte was used in which 1.0 M lithium salt LiPF6 was mixed with an organic solvent (EC:EMC = 1:4 Vol%) and electrolyte additive FEC 1 Vol%.

[0114] i) The data of the prepared symmetric units were determined using an EIS (electrochemical impedance spectroscopy) analyzer (product name: Biologic VMP3), with a frequency range of 500 kHz to 50 MHz.

[0115] The diffusion time constant (τ) is obtained by performing Nyquist plotting and fitting on the calculated data. d ).

[0116] ii) Nyquist plot is a way to visualize EIS results using a general approach.

[0117] iii) Fitting was performed using the Z-fit tool in EC-Lab software (generally, results from the Nyquist plot within the range of 500kHz to 100mHz were used as the target). The results were simulated using an equivalent circuit with a resistance-constrained diffusion series connection for the corresponding data region. In the simulation results, χ... 2 The / |Z| value is 1.10 -2 The following levels indicate that the results are highly reliable.

[0118] After filling with electrolyte, the applicable negative electrode active material layer thickness (σ) is 80 μm, the dispersion parameter (γ) is 0.91, and the obtained diffusion time constant (τ) is... d The formula for the calculation is shown in Table 1 below.

[0119] 3. Evaluation of the high-speed charging characteristics of the half-cell battery

[0120] CR2016 coin cells were prepared by injecting electrolyte after placing a PE separator between the negative lithium metal positive and negative electrodes prepared in Examples 1-3 and Comparative Examples 1-4. Half-cells were prepared by placing the assembled coin cells at room temperature for 24 hours. Here, an electrolyte was used, consisting of 1.0 M lithium salt LiPF6 mixed with an organic solvent (EC:EMC = 1:4 vol%) and 1 vol% electrolyte additive FEC.

[0121] For each half-cell prepared using low-speed (0.1C) charge-discharge for the first 3-5 cycles, high-speed charging performance was evaluated using high-speed (2C) charging after electrode stabilization. The results are shown in Table 1 below. Here, the high-speed charging rate (%) is calculated by measuring the high-speed (2C) charging amount relative to the low-speed (0.1C) charging amount.

[0122] (Evaluation Results)

[0123] For the negative electrodes and half-cells prepared in Examples 1-3 and Comparative Examples 1-4, the negative electrode properties, diffusion coefficients, and fast charging rates of the half-cells are shown in Table 1 below.

[0124] Table 1

[0125]

[0126]

[0127] Examples 1-3 and Comparative Examples 1-4 are electrodes prepared with identical loading and density of each negative electrode active material layer, thus confirming that the negative electrode active material layer has the same porosity. Analysis shows that the negative electrodes prepared in the examples satisfy Equation 1 of the present invention, thus improving fast charging performance (high-speed charging rate) compared to Comparative Examples 1-4. Conversely, analysis shows that in Comparative Examples 1-4, which have the same average pore size and porosity, the pores are unevenly formed, failing to satisfy Equation 1 of the present invention, thus deteriorating the high-speed charging rate.

[0128] Furthermore, the analysis revealed that when the average particle size of the pores exceeded the preferred range (Comparative Example 4), the pores became coarser and developed into a branch-like shape, the ion diffusion length within the pores shortened, and the high-speed charging rate deteriorated further as the diffusion coefficient increased excessively.

[0129] Furthermore, it can be confirmed that Example 1 exhibits further improved performance compared to Examples 2 to 3 due to having a diffusion coefficient within the preferred range of the present invention.

[0130] The embodiments of the present invention have been described above, but the present invention is not limited to the described embodiments and can be prepared in various different forms. It is understood that those skilled in the art can implement the present invention in other specific forms without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not limiting.

Claims

1. A negative electrode for a secondary battery, comprising: Current collector; as well as A layer of negative electrode active material containing negative electrode active material is formed on the current collector. The negative electrode active material includes artificial graphite. The negative electrode of the secondary battery satisfies the following relationship: [Relational Formula] 1·10 -3 ≤ D < 1.9·10 -3 In the aforementioned relationship, D is the diffusion coefficient of lithium ions within the negative electrode, derived through the following mathematical formula: D = σ 2 ·t d -γ σ is the thickness of the negative electrode active material layer, and its unit is cm; τ d γ is the diffusion time, in seconds; γ is the dispersion parameter and is equal to 0.

91. Wherein, the dispersion parameter γ represents the parameter used to correct the diffusion time τ. d The difference between the ideal environment and the experimental environment, The thickness σ of the negative electrode active material layer represents the thickness of the negative electrode active material layer measured after the negative electrode has been immersed in the electrolyte and stored at room temperature for 10 hours. The negative electrode has a lithium-ion diffusion time τ of more than 0.020 seconds. d .

2. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode has a lithium-ion diffusion time τ of 0.025 to 0.041 seconds. d .

3. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode active material layer has a thickness of 0.20 to 0.23 cm. 3 / g of total pore volume.

4. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode active material layer has a porosity of 20 to 30%.

5. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode active material layer has an average pore size of 550 to 800 nm.

6. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode active material layer has a content of 6 to 15 g / cm³. 2 The load.

7. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode active material layer has a content of 1.4 to 1.8 g / cm³. 3 The negative electrode density.

8. A secondary battery, comprising a negative electrode as described in any one of claims 1-7; a positive electrode; a separator; and an electrolyte.

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