Method for predicting the cycle life of secondary batteries including carbon-based hybrid negative electrodes

Abstract

Disclosed is a method for predicting the cycle life of a secondary battery, the secondary battery including a carbon-based hybrid negative electrode, the method comprising: measuring the lattice spacing (d-spacing) of the carbon-based negative electrode active material of the target carbon-based hybrid negative electrode using an X-ray diffractometer during charge / discharge of a target secondary battery including a target carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, and plotting a curve showing the change in the lattice spacing value as a function of charge / discharge capacity (X-axis); calculating a target slope difference, the target slope difference corresponding to the difference in slope value that changes relative to an inflection point of the curve during discharge in the plotted curve; comparing the target slope difference with a reference slope difference, the reference slope difference corresponding to the difference in slope value that changes relative to an inflection point in a curve showing the change in the lattice spacing value as a function of charge / discharge capacity (X-axis) of a reference secondary battery; and predicting whether the cycle life of the target secondary battery is improved compared to the reference secondary battery based on the result of the comparison.

Description

Technical Field

[0001] The present disclosure relates to a method for predicting the cycle life of a secondary battery including a carbon-based hybrid anode.

[0002] This application claims the benefit of Korean Patent Application No. 10-2019-0139765, filed on November 4, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0003] Recently, with the rapid growth of the market for electric vehicles, robots, and energy storage systems, there is a demand for secondary batteries with high energy density, stability, compactness, light weight, and long service life. For large-scale applications, it is important to improve the performance of secondary batteries in order to achieve higher energy density per unit weight or volume than the current level.

[0004] Graphite is the negative electrode active material of lithium-ion batteries currently available on the market, and its limited theoretical capacity is 372 mAh / g (about 160 Wh / kg). As the next generation of negative electrode materials for non-aqueous electrolyte secondary batteries, the focus is on silicon (Si), which has a capacity at least 10 times higher than that of graphite (4200 mAh / g). In addition, various silicon-containing non-carbon-based materials that show high theoretical capacity when alloyed with lithium have been proposed as new materials for negative electrode active materials instead of carbon-based materials such as graphite.

[0005] However, silicon-containing non-carbon-based materials can cause cracks in and on the electrode surface and splitting failure of the active material during alloying with lithium due to the high volume expansion rate, resulting in reduced electrical contact and a sharp drop in the cycle capacity of the secondary battery. In order to solve the problem of silicon-containing non-carbon-based materials, many attempts have been made to apply hybrid negative electrodes including mixtures of non-carbon-based materials such as silicon-containing non-carbon-based materials and carbon-based materials.

[0006] Despite these attempts, simply mixing non-carbon-based materials with carbon-based materials has limitations in improving cycle life. Therefore, it is still necessary to design optimal anodes by predicting the cycle life of secondary batteries including hybrid anodes. Summary of the Invention

[0007] Technical issues

[0008] The present disclosure provides a method for predicting the cycle life of a secondary battery including a carbon-based hybrid anode.

[0009] These and other purposes and advantages of the present disclosure will be understood through the following description.At the same time, it is easy to understand that these purposes and advantages of the present disclosure can be achieved through the means and methods and their combinations set forth in the appended claims.

[0010] Technical Solution

[0011] In order to solve the above-mentioned problems of the present disclosure, according to one aspect of the present disclosure, a method for predicting the cycle life of a carbon-based hybrid negative electrode according to the following embodiment is provided.

[0012] According to the first embodiment,

[0013] A method for predicting the cycle life of a secondary battery is provided, wherein the secondary battery includes a carbon-based hybrid negative electrode, the method comprising:

[0014] During charge / discharge of a target secondary battery including a target carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, measuring the lattice spacing (d-spacing) of the carbon-based negative electrode active material of the target carbon-based hybrid negative electrode using an X-ray diffractometer, and plotting a curve showing changes in the lattice spacing value as a function of charge / discharge capacity (X-axis);

[0015] calculating a target slope difference value corresponding to a difference in slope value relative to an inflection point of a curve during discharge in the plotted curve;

[0016] comparing the target slope difference value with a reference slope difference value corresponding to a difference in slope value changed relative to an inflection point in a curve showing a change in lattice spacing value as a function of charge / discharge capacity (X-axis) of a reference secondary battery; and

[0017] Whether the cycle life of the target secondary battery is improved compared to the reference secondary battery is predicted based on the comparison result.

[0018] According to the second embodiment, in the first embodiment,

[0019] The reference slope difference can be obtained by the following method:

[0020] During charge / discharge of a reference secondary battery including a reference carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, measuring the d-spacing of the carbon-based negative electrode active material of the reference carbon-based hybrid negative electrode using an X-ray diffractometer, and plotting a curve showing changes in the d-spacing value as a function of charge / discharge capacity (X-axis); and

[0021] The reference slope difference value corresponding to the difference in slope values changed with respect to an inflection point of a curve during discharge in the plotted curve is calculated.

[0022] According to a third embodiment, in the first embodiment or the second embodiment,

[0023] In the plotted curve, the discharge capacity after the inflection point of the curve during discharge may correspond to the capacity contribution of the non-carbon-based negative electrode active material, and the discharge capacity before the inflection point may correspond to the capacity contribution of the carbon-based negative electrode active material.

[0024] According to a fourth embodiment, in any one of the first to third embodiments,

[0025] The target slope difference and the reference slope difference can be calculated by the following equation:

[0026] Target slope difference = absolute value of [(slope of the curve before the inflection point in the curve of the target secondary battery) - (slope of the curve after the inflection point in the curve of the target secondary battery)]

[0027] Reference slope difference = absolute value of [(slope of the curve before the inflection point in the curve of the reference secondary battery) - (slope of the curve after the inflection point in the curve of the reference secondary battery)]

[0028] According to a fifth embodiment, in any one of the first to fourth embodiments,

[0029] When the target slope difference is greater than the reference slope difference according to the comparison result, it may be determined that the cycle life of the target secondary battery is improved compared to the reference secondary battery.

[0030] According to a sixth embodiment, in any one of the first to fifth embodiments,

[0031] When the target slope difference is smaller than the reference slope difference according to the comparison result, it may be determined that the cycle life of the target secondary battery is reduced compared to the reference secondary battery.

[0032] According to a seventh embodiment, in any one of the first to sixth embodiments,

[0033] The carbon-based negative active material may include at least one of natural graphite, artificial graphite, soft carbon, hard carbon, pitch carbide, sintered coke, graphene, or carbon nanotubes.

[0034] According to an eighth embodiment, in any one of the first to seventh embodiments,

[0035] The non-carbon-based negative active material may include a metal or metalloid capable of forming an alloy with lithium.

[0036] According to a ninth embodiment, in any one of the first to eighth embodiments,

[0037] The non-carbon-based negative electrode active material may include a metal or metalloid selected from Si, Sn, In, Pb, Ga, Ge, Al, Bi, Sb, Ag, Mg, Zn, Pt, Ti, or a combination thereof; an oxide of the metal or the metalloid; a carbon composite of the metal or the metalloid; a carbon composite of the metal or the metalloid oxide; or a mixture thereof.

[0038] According to a tenth embodiment, in any one of the first to ninth embodiments,

[0039] The non-carbon-based negative electrode active material may include Si, SiO x (0<x<2), or a mixture thereof.

[0040] Beneficial effects

[0041] According to an embodiment of the present disclosure, as a result of separating the single behavior of non-carbon-based materials from the combined behavior of non-carbon-based materials by observing the change in lattice spacing of the carbon-based material as a function of charge / discharge capacity (X-axis) during charging and discharging of a full-battery secondary battery having a carbon-based hybrid negative electrode including carbon-based materials and non-carbon-based materials using a non-destructive analysis method, it is possible to predict whether a target secondary battery including a target hybrid negative electrode, whose cycle life is to be predicted, has an improved cycle life relative to a reference secondary battery including a reference hybrid negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the following detailed disclosure, are used to assist in understanding the technical aspects of the present disclosure, and therefore the present disclosure should not be interpreted as being limited thereto. At the same time, the shapes, sizes, scales, or proportions of the elements in the drawings may be exaggerated to enhance a clearer description.

[0043] Figure 1a and Figure 1b is a graph showing the results of observation of real-time changes in the lattice spacing of the artificial graphite included in each negative electrode with changes in capacity during charge / discharge of Sample A of Preparation Example 1 (reference secondary battery) and Sample B of Preparation Example 2 (target secondary battery).

[0044] Figure 2is a graph showing cycle life results of Sample A of Preparation Example 1 (reference secondary battery) and Sample B of Preparation Example 2 (target secondary battery). DETAILED DESCRIPTION

[0045] It should be understood that the terms or words used in the specification and the appended claims should not be interpreted as being limited to the common meaning and dictionary meaning, but should be interpreted based on the meaning and concept of the technical aspects of the present disclosure on the basis of the principle that allows the inventor to appropriately define the terms for the best interpretation. Therefore, the configuration shown in the embodiments described herein is only the most preferred embodiment of the present disclosure and is not intended to fully describe the technical aspects of the present disclosure. Therefore, it should be understood that various other equivalents or improvements may be made thereto at the time of filing this application.

[0046] In the present disclosure, the method of predicting the cycle life of a target secondary battery is a method of determining whether the cycle life of the target secondary battery is improved or reduced compared to a specific existing secondary battery, rather than a method of predicting the absolute value of the cycle life of the target secondary battery.

[0047] Here, a specific existing secondary battery used as a reference object for cycle life comparison is referred to as a “reference secondary battery”, and in this case, the carbon-based hybrid negative electrode of the reference secondary battery is referred to as a “reference carbon-based hybrid negative electrode”.

[0048] A secondary battery intended to predict a cycle life compared with a reference secondary battery is referred to as a “target secondary battery,” and in this case, the carbon-based hybrid negative electrode of the target secondary battery is referred to as a “target carbon-based hybrid negative electrode.”

[0049] The present disclosure is developed so that while observing the lithiation and delithiation behaviors of carbon-based materials and non-carbon-based materials during charge / discharge of a secondary battery having a carbon-based hybrid negative electrode in a charge / discharge environment, the cycle life of a target secondary battery including a target hybrid negative electrode is predicted compared with a reference secondary battery including a reference hybrid negative electrode based on the degree of the lattice spacing decrease slope in the late stage of discharge after an inflection point at which the lattice spacing of the carbon-based negative electrode active material in the negative electrode changes rapidly with changes in capacity.

[0050] Another cycle life prediction method according to an embodiment of the present disclosure can be performed by in situ (operando) analysis during charge / discharge of a secondary battery with a carbon-based hybrid negative electrode, i.e., by a non-destructive analysis method utilizing a full-cell secondary battery, by directly observing the behavior of a mixed electrode of carbon-based materials and non-carbon-based materials.

[0051] Secondary batteries using only carbon-based negative electrode active materials do not experience competitive lithiation and delithiation of lithium ions in the negative electrode active material. However, due to the competitive reactions of lithiation and delithiation of lithium ions in carbon-based negative electrode active materials and non-carbon-based negative electrode active materials depending on the reaction voltage, deceleration / acceleration of lithiation and delithiation of lithium ions is observed in hybrid negative electrodes including both carbon-based and non-carbon-based negative electrode active materials.

[0052] This change is reflected by an increase or decrease in the lattice spacing (d-spacing) of the carbon layer of the carbon-based negative electrode active material. Through this, the cycle life of a secondary battery including a target hybrid negative electrode (target secondary battery) can be predicted compared with a secondary battery including a reference hybrid negative electrode (reference secondary battery) as a comparison reference object by observing the behavior of the carbon-based negative electrode active material and the non-carbon-based negative electrode active material in the hybrid negative electrode and detecting the stress level (current impedance level, etc.) applied to the carbon-based negative electrode active material.

[0053] According to one aspect of the present disclosure, a method for predicting the cycle life of a secondary battery, wherein the secondary battery includes a carbon-based hybrid negative electrode, comprises:

[0054] During charge / discharge of a target secondary battery having a target carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, the lattice spacing (d-spacing) of the carbon-based negative electrode active material in the target carbon-based hybrid negative electrode is measured using an X-ray diffractometer to plot a change in the lattice spacing value as a function of charge / discharge capacity (X-axis);

[0055] calculating a target slope difference value corresponding to a difference in slope value relative to an inflection point of the curve during discharge in the plotted curve;

[0056] comparing the target slope difference value with a reference slope difference value corresponding to a difference in slope value changed relative to an inflection point in a curve showing a change in lattice spacing value as a function of charge / discharge capacity (X-axis) of a reference secondary battery; and

[0057] Whether the cycle life of the target secondary battery is improved compared to the reference secondary battery is predicted based on the comparison result.

[0058] The carbon-based hybrid negative electrode refers to a negative electrode that includes not only a carbon-based negative electrode active material but also a non-carbon-based negative electrode active material as a negative electrode active material.

[0059] Carbon-based negative electrode active materials include any type of carbon-based materials that allow lithiation and delithiation during the charge / discharge period of the battery, and are not limited to specific types. Carbon-based negative electrode active materials may include amorphous carbon, crystalline carbon, or a mixture thereof. Specifically, the carbon-based negative electrode active material may include at least one of natural graphite, artificial graphite, soft carbon, hard carbon, pitch carbide, sintered coke, graphene, or carbon nanotubes. In particular, graphite is a common negative electrode material in lithium secondary batteries, and when mixed with silicon to make an electrode, graphite stably ensures its capacity, has good initial efficiency, and can compensate for the low initial efficiency of silicon-based negative electrode materials. Because these advantages contribute to increasing the initial efficiency of the electrode, graphite-based materials such as artificial graphite or natural graphite are ideal.

[0060] The non-carbon-based negative electrode active material includes any material that can form an alloy with lithium and is not limited to a specific type. Specifically, the non-carbon-based negative electrode active material may include a metal or metalloid selected from Si, Sn, In, Pb, Ga, Ge, Al, Bi, Sb, Ag, Mg, Zn, Pt, Ti, or a combination thereof; such as SiO x (0<x<2), oxides of the metal or the metalloid such as SnO, SnO2, TiO2; carbon composites of the metal or the metalloid; carbon composites of the oxides of the metal or the metalloid; or mixtures thereof.

[0061] According to an embodiment of the present disclosure, the non-carbon-based negative electrode active material may include Si, SiO x (0<x<2), or a mixture thereof.

[0062] The carbon-based hybrid negative electrode according to an embodiment of the present disclosure can be prepared by coating a mixture of a negative electrode active material including a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, a conductive material, and a binder on a negative electrode collector and drying it, and if necessary, the mixture may further include a filler.

[0063] In the embodiments of the present disclosure, the thickness of the current collector is generally 3 μm to 500 μm. The current collector may include, without limitation, any type having high conductivity while not causing chemical changes to the corresponding battery, and for example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, and silver. They may be appropriately selected and used depending on the polarity of the positive electrode or negative electrode.

[0064] The binder contributes to the bonding between the active material and the conductive material and is bonded to the current collector, and is generally added in an amount of 1% to 50% by weight of the gross weight of the electrode material. The binder may include a high molecular weight polyacrylonitrile-acrylic acid copolymer, but is not limited thereto. In another example, the binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylate (salt), alkali cation or ammonium ion substituted polyacrylate (salt), alkali cation or ammonium ion substituted poly (alkylene-to-maleic anhydride), alkali cation or ammonium ion substituted poly (alkylene-to-maleic acid), polyethylene oxide, fluororubber, or a mixture thereof. More specifically, the alkali cation-substituted polyacrylate may include lithium-polyacrylate (Li-PAA), and the alkali cation-substituted poly(alkylene-co-maleic anhydride) may include lithium-substituted polyisobutylene-maleic anhydride.

[0065] The conductive material may include any type of material that does not cause chemical changes to the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black (trademark name), carbon nanotubes, carbon nanofibers, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum powder, nickel powder; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives.

[0066] According to an embodiment of the present disclosure, when a negative electrode is prepared by coating a mixture of a negative electrode active material including a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, a conductive material and a binder on a negative electrode current collector, the negative electrode can be prepared by a dry method in which a solid mixture including the negative electrode active material, the conductive material, and the binder is directly coated, and a wet method in which the negative electrode active material, the conductive material and the binder are added to a dispersion medium, stirred, coated in the form of a slurry, and dried to remove the dispersion medium. In this case, the dispersion medium used in the wet method may include an aqueous medium such as water (deionized water), or an organic medium such as N-methyl-2-pyrrolidone (NMP, N-methyl-2-pyrrolidone).

[0067] A secondary battery according to an embodiment of the present disclosure includes a carbon-based hybrid negative electrode, a positive electrode, and a separator interposed between the positive electrode and the negative electrode.

[0068] The positive electrode can be prepared by coating a mixture of a positive electrode active material, a conductive material, and a binder on a positive electrode current collector and drying the mixture, and if necessary, the mixture may further include a filler. The positive electrode active material may include a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; Li 1+x Mn 2-x Lithium manganese oxides such as LiMnO4 (x = 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7; 1-x Ni-site lithium nickel oxide represented by MxO2 (M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, x = 0.01 to 0.3); 2-x M x Lithium manganese composite oxide represented by Li2Mn3MO8 (M=Fe, Co, Ni, Cu or Zn); LiMn2O4 in which alkaline earth metal ions partially replace Li; disulfide compounds; Fe2(MoO4)3, but not limited to these.

[0069] For the conductive material, current collector, and binder of the positive electrode, reference may be made to the above description of the negative electrode.

[0070] The separator is interposed between the positive and negative electrodes and comprises an insulating film with a high dielectric constant and mechanical strength. The separator typically has a pore size of 0.01 to 10 μm and a thickness of 5 to 300 μm. For example, the separator may comprise a film, sheet, or nonwoven fabric made of a chemically resistant and hydrophobic olefin-based polymer such as polypropylene; fiberglass; or polyethylene. The separator may further comprise a porous layer on its outermost surface containing a mixture of inorganic particles and a binder resin.

[0071] In an embodiment of the present disclosure, the electrolyte solution includes an organic solvent and a predetermined amount of lithium salt, and the organic solvent includes, for example, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propionate (MP), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), vinylene carbonate (VC), gamma-martinolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, amyl acetate, methyl propionate, ethyl propionate, butyl propionate, or a mixture thereof. Halogenated derivatives of the organic solvent can be used, and linear esters can be used. The lithium salt may include a material that dissolves well in a non-aqueous electrolyte, and for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and imide.

[0072] The secondary battery according to an embodiment of the present disclosure can be manufactured by accommodating an electrode assembly including positive electrodes and negative electrodes stacked in an alternating manner together with a separator and an electrolyte solution in a package such as a battery case, followed by sealing. The method of manufacturing the secondary battery may include any general method without limitation.

[0073] According to an embodiment of the present disclosure, a battery module including the secondary battery as a unit cell and a battery pack including the battery module are provided. Since the battery module and the battery pack include secondary batteries that exhibit rapid charging characteristics under high loads, they can be used as power sources for electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0074] Meanwhile, for battery components not described herein, for example, conductive materials, reference may be made to descriptions of components generally used in the field of batteries, especially lithium secondary batteries.

[0075] In the life prediction method of the present disclosure, first, during the charge / discharge of a target secondary battery including a target carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, the lattice spacing (d-spacing) of the carbon-based negative electrode active material of the target carbon-based hybrid negative electrode is measured using an X-ray diffractometer, and a curve of the change in the lattice spacing value as a function of the charge / discharge capacity (X-axis) is plotted.

[0076] According to an embodiment of the present disclosure, during the charge / discharge period of the target secondary battery, a scan can be performed while tracking the (002) lattice interface peak (2θ=7.5~11 (Agλ=0.56)) of the carbon-based negative electrode active material (e.g., graphite) included in the target carbon-based hybrid negative electrode of the target secondary battery. The lattice d-spacing of the carbon-based negative electrode active material can be calculated by fitting the (002) lattice interface peak of the carbon-based negative electrode active material included in the negative electrode obtained by the scan (for details of the application example, see Figure 1a and Figure 1b ).

[0077] The difference in slope value relative to the inflection point of the slope of the curve during discharge in the plotted curve (inflection point where the absolute value of the slope changes from a large value to a small value) was calculated.

[0078] Subsequently, the target slope difference is compared with a reference slope difference, which corresponds to the difference in slope value relative to the inflection point in the curve showing the change in lattice spacing value as a function of the charge / discharge capacity (X-axis) of the reference secondary battery.

[0079] According to an embodiment of the present disclosure, the reference slope difference may be obtained by:

[0080] A reference secondary battery includes a reference carbon-based hybrid negative electrode, wherein the reference carbon-based hybrid negative electrode includes a carbon-based negative electrode active material and a non-carbon-based negative electrode active material, and an X-ray diffractometer is used to measure the lattice spacing (d-spacing) of the carbon-based negative electrode active material of the reference carbon-based hybrid negative electrode during charge / discharge of the reference secondary battery, and a curve of the change in the lattice spacing value as a function of charge / discharge capacity (X-axis); and

[0081] A reference slope difference value corresponding to a difference in slope values relative to an inflection point of a curve during discharge in the plotted curve is calculated.

[0082] In the plotted curve, during discharge, the discharge capacity after the inflection point of the curve may correspond to the capacity contribution of the non-carbon-based negative electrode active material, and the discharge capacity before the inflection point may correspond to the capacity contribution of the carbon-based negative electrode active material.

[0083] The target slope difference and the reference slope difference may be calculated by the following equations.

[0084] Target slope difference = absolute value of [(slope of the curve before the inflection point in the curve of the target secondary battery) - (slope of the curve after the inflection point in the curve of the target secondary battery)]

[0085] Reference slope difference = absolute value of [(slope of the curve before the inflection point in the curve of the reference secondary battery) - (slope of the curve after the inflection point in the curve of the reference secondary battery)]

[0086] For example, when the slope of the curve before the inflection point in the curve of the target secondary battery is -2 and the slope of the curve after the inflection point in the curve of the target secondary battery is 0, the target slope difference is the absolute value of [(-2)-(0)], which is equal to 2.

[0087] Meanwhile, when the slope of the curve before the inflection point in the curve of the reference secondary battery is -2 and the slope of the curve after the inflection point in the curve of the reference secondary battery is -1, the reference slope difference is the absolute value of [(-2)-(-1)], which is equal to 1.

[0088] Then, whether the cycle life of the target secondary battery is improved compared with the reference secondary battery is predicted based on the result of the comparison.

[0089] Specifically, according to an embodiment of the present disclosure, an inflection point where the slope changes from a large value to a small value in the late stage of discharge can be detected in a lattice spacing decrease curve as a function of the discharge capacity of a reference secondary battery and a target secondary battery including a carbon-based hybrid negative electrode, and the C rate applied to the carbon-based negative electrode active material in the hybrid negative electrode can be determined based on the slope change (slope difference) relative to the inflection point.

[0090] In this case, predicting whether the cycle life of the target secondary battery is improved or reduced compared to that of the reference secondary battery can be performed by comparing a reference slope difference value with a target slope difference value, wherein the reference target slope difference value corresponds to the difference in slope values that change relative to the inflection point in the curve showing the change in lattice spacing values as a function of the charge / discharge capacity (X-axis) of the reference secondary battery, and the target slope difference value corresponds to the difference in slope values that change relative to the inflection point in the curve showing the change in lattice spacing values as a function of the charge / discharge capacity (X-axis) of the target secondary battery.

[0091] When the target slope difference of the target secondary battery is much greater than the reference slope difference of the reference secondary battery, there may be a plateau range in which the slope of the lattice spacing decreases with capacity change in the late discharge stage after the inflection point in the curve of the target secondary battery is much smaller than before the inflection point. In this plateau range, the lattice spacing of the carbon-based material does not change or hardly changes, but the capacity changes, thereby minimizing the stress applied to the carbon-based material due to the very low delithiation rate of the carbon-based material.

[0092] When the stress applied to the carbon-based material of the target secondary battery is smaller than that of the reference secondary battery, it can be predicted that the cycle life of the target secondary battery is improved compared with the reference secondary battery.

[0093] Therefore, when the target slope difference value is greater than the reference slope difference value according to the comparison result, it can be determined that the cycle-life of the target secondary battery is improved compared with the reference secondary battery.

[0094] In contrast, when the target slope difference is smaller than the reference slope difference according to the comparison result, it can be determined that the cycle life of the target secondary battery is reduced compared with the reference secondary battery. In this case, the stress applied to the carbon-based material of the target secondary battery is greater than that of the reference secondary battery, and therefore it can be predicted that the cycle life of the target secondary battery is reduced compared with the reference secondary battery.

[0095] Hereinafter, in order to facilitate understanding of the present disclosure, the present disclosure will be described in detail by the following preparation examples and experimental examples. However, the preparation examples and experimental examples according to the embodiments of the present disclosure can be improved in a variety of different forms, and the scope of the present disclosure should not be interpreted as being limited to the following preparation examples and experimental examples. The preparation examples and experimental examples of the present disclosure are provided to help those skilled in the art to fully and completely understand the present disclosure.

[0096] Preparation Example 1—Sample A (reference secondary battery)

[0097] <Preparation of Negative Electrode>

[0098] A mixture of artificial graphite as a carbon-based active material and SiO (silicon oxide) as a non-carbon-based active material having a coulombic efficiency of 80% or more at 0.1C charge / discharge, a binder polymer (SBR (styrene-butadiene rubber) and CMC (carboxymethyl cellulose), and carbon black as a conductive material in a weight ratio of 95:3.5:1.5 was mixed with water as a dispersion medium in a weight ratio of 1:2 to prepare an active material layer slurry. In this case, the weight of SiO is 5% by weight based on the total weight of the mixed negative electrode active material composed of artificial graphite and SiO, and the weight ratio of SBR (styrene-butadiene rubber) and CMC (carboxymethyl cellulose) is 2.3:1.2.

[0099] The active material layer slurry was coated on one surface of a 10 μm thick copper (Cu) film as a negative electrode current collector using a slot die and dried under vacuum at 130° C. for 1 hour to form an active material layer on the copper film.

[0100] The active material layer was roll pressed to prepare a negative electrode having an 80 μm thick active material layer of a single layer structure. In this case, the loading amount was 17 mg / cm based on the dry weight of the negative electrode active material layer. 2 .

[0101] <Preparation of Positive Electrode>

[0102] As the positive electrode active material, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2 (NCM-811), carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to N-methylpyrrolidone (NMP) as a solvent in a weight ratio of 96:2:2 to prepare a positive electrode active material slurry. The slurry was applied to one surface of a 15 μm thick aluminum current collector and dried and pressed under the same conditions as the negative electrode to prepare a positive electrode. In this case, the loading amount was 20 mg / cm based on the dry weight of the positive electrode active material layer. 2 .

[0103] <Preparation of Lithium Secondary Battery>

[0104] LiPF 6 was dissolved in an organic solvent of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) mixed in a volume ratio of 3:1:6 so that the concentration was 1.0 M, thereby preparing a non-aqueous electrolyte solution.

[0105] A polyolefin separator was interposed between the positive electrode and the negative electrode prepared as above, and an electrolyte solution was injected to prepare a lithium secondary battery (Sample A, reference secondary battery).

[0106] Preparation Example 2—Sample B (Target Secondary Battery)

[0107] <Preparation of Negative Electrode>

[0108] A mixture of artificial graphite as a carbon-based active material, a binder polymer (SBR (styrene butadiene rubber) and CMC (carboxymethyl cellulose)) and carbon black as a conductive material in a weight ratio of 95:3.5:1.5 was mixed with water as a dispersion medium in a weight ratio of 1:2 to prepare a first active material layer slurry. In this case, the weight ratio of SBR (styrene butadiene rubber) to CMC (carboxymethyl cellulose) was 2.3:1.2.

[0109] The second active material layer slurry was prepared by the same method as the first active material layer slurry, except that a non-carbon-based active material, i.e., SiO (silicon oxide) having a coulombic efficiency of 80% or more at 0.1C charge / discharge was used instead of artificial graphite, and SiO was included in an amount of 5 wt % based on the total weight of the artificial graphite and SiO.

[0110] The first active material layer slurry was coated on one surface of a 10 μm thick copper (Cu) film as a negative electrode current collector using a slot die, and then the second active material layer slurry was coated on the first active material layer slurry and dried under vacuum at 130° C. for 1 hour to form the first active material layer and the second active material layer on the copper film.

[0111] The first active material layer and the second active material layer were simultaneously roll pressed to prepare a negative electrode having a double active material layer 80 μm thick. The loading amount was 17 mg / cm based on the dry weight of the negative electrode active material layer. 2 .

[0112] <Preparation of Positive Electrode>

[0113] The positive electrode was prepared by the same method as in Preparation Example 1.

[0114] <Preparation of Lithium Secondary Battery>

[0115] The non-aqueous electrolyte solution was prepared by the same method as in Preparation Example 1.

[0116] A polyolefin separator was interposed between the positive electrode and the negative electrode prepared as above, and an electrolyte solution was injected to prepare a lithium secondary battery (Sample B, target secondary battery).

[0117] Experimental example

[0118] Experimental Example 1

[0119] During the charge / discharge period of the secondary batteries of Preparation Example 1 and Preparation Example 2 under the same conditions as the cycle test (0.33 CC / CV charge, 0.33 CC discharge), a transmission X-ray diffractometer (manufacturer: Bruker, trade name: D8 Advance) was used to perform scanning while tracking the (002) lattice interface peak (2θ = 7.5 to 11 (Agλ = 0.56)) of the artificial graphite included in the negative electrode of each secondary battery. The lattice d-spacing of the artificial graphite was calculated by fitting the lattice interface peak of the artificial graphite included in the negative electrodes of Preparation Examples 1 and 2 obtained by scanning based on Bragg's law.

[0120] Figure 1a and Figure 1b is a graph showing the results of observation of real-time changes in the lattice spacing of the artificial graphite included in each negative electrode with changes in capacity during charge / discharge of Sample A of Preparation Example 1 (reference secondary battery) and Sample B of Preparation Example 2 (target secondary battery).

[0121] Reference Figure 1a and Figure 1b In the discharge curves of sample A (reference secondary battery) and sample B (target secondary battery), all discharge capacities have an inflection point where the slope of the curve changes rapidly at about 30 mAh. In a capacity range higher than the capacity at the inflection point (in the early stages of discharge), the graphite lattice changes significantly with the change in capacity, and the discharge capacity at that time is attributed to the delithiation of lithium ions in the graphite.

[0122] exist Figure 1a In the case of the reference secondary battery of Preparation Example 1, the lattice spacing decrease slope relative to the inflection point is smaller in the late discharge stage than in the early discharge stage, but not as Figure 1b That is, it can be seen that the target slope difference value is greater than the reference slope difference value.

[0123] exist Figure 1b In the case of a capacity range lower than the inflection point capacity, that is, a discharge capacity range of approximately 30 mAh to 0 mAh (at the late stage of discharge), the graphite lattice spacing does not change or barely changes, but the capacity changes. In the late stage of discharge, delithiation of the lithiated lithium ions in SiO occurs, rather than delithiation of the lithium ions in graphite.

[0124] Specifically, if Figure 1a As shown in, in the case of the secondary battery (sample A) of Preparation Example 1 of a mixed negative electrode including a single-layer structure, in which the carbon-based negative electrode active material and the non-carbon-based negative electrode active material are uniformly mixed, in the late stage of discharge, the lattice spacing change slope shows a rapid decline in range ② and a slope smaller than the slope of range ② when entering range ①, and delithiation of lithium ions occurs at a constant rate.

[0125] In contrast, in the target secondary battery (sample B) of Preparation Example 2 including the hybrid negative electrode of the double-layer structure, in which the carbon-based negative electrode active material layer and the non-carbon-based negative electrode active material layer are as follows: Figure 1b As shown in , the separation is complete, and a clear platform (step reaction) is observed in range ①. Figure 1b In range ②, all lithium ions remaining in the artificial graphite, which is a carbon-based negative electrode active material, are removed during delithiation. In range ①, they move to a reaction range where lithium ions are delithiated only in non-carbon-based negative electrode active materials (e.g., SiO). Because this reaction occurs stepwise from range ② to range ①, this platform can be called a "step reaction."

[0126] Specifically, it can be explained that when considering the Figure 1b In the data measurement interval of 4 minutes per point, the discharge capacity increased steadily in the range ① for approximately 20 to 25 minutes in the late stage of discharge, but the lattice spacing of the artificial graphite did not change. In the hybrid anode system of sample B, lithium ion delithiation occurred in the non-carbon-based anode active material earlier than the artificial graphite, which is the carbon-based anode active material. This is expected to alleviate the structural stress associated with the volume expansion of the artificial graphite.

[0127] As a result, as described above, it can be seen that the target slope difference of the target secondary battery (sample B) is greater than the reference slope difference of the reference secondary battery (sample A), and it can be predicted that the target secondary battery (sample B) with lower stress applied to the artificial graphite has a longer cycle life than the reference secondary battery (sample A).

[0128] Experimental Example 2

[0129] In order to see whether the cycle life predicted in Experimental Example 1 is consistent with the actual results, a cycle life test was performed by charging and discharging the reference secondary battery (sample A) of Preparation Example 1 and the target secondary battery (sample B) of Preparation Example 2 under the same conditions as the cycle test (0.33CC / CV charging and 0.33CC discharging conditions for 200 cycles).

[0130] Figure 2 is a graph showing cycle life results of Sample A of Preparation Example 1 (reference secondary battery) and Sample B of Preparation Example 2 (target secondary battery).

[0131] Reference Figure 2 It can be seen that the target secondary battery (sample B) including a negative electrode showing a platform from about 20 minutes to 25 minutes has excellent discharge capacity retention in terms of cycle life of the initial 200 cycles, compared with the reference secondary battery (sample A) in which no platform was observed in Experimental Example 1.

Claims

1. A method for predicting the cycle life of a secondary battery, wherein the secondary battery includes a carbon-based hybrid negative electrode, the method comprising: During charge / discharge of a target secondary battery of a target carbon-based hybrid negative electrode comprising a first active material layer and a second active material layer, measuring the d-spacing of the carbon-based negative electrode active material of the target carbon-based hybrid negative electrode using an X-ray diffractometer, and plotting a curve regarding changes in the d-spacing value with the charge / discharge capacity as the X-axis and the d-spacing value as the Y-axis; wherein the first active material layer contains a carbon-based negative electrode active material, and the second active material layer contains a non-carbon-based negative electrode active material; calculating a target slope difference, wherein the target slope difference=the absolute value of [(the slope of the curve before the inflection point in the curve of the target secondary battery)−(the slope of the curve after the inflection point in the curve of the target secondary battery)]; comparing the target slope difference with a reference slope difference, wherein the reference slope difference=the absolute value of [(the slope of the curve before the inflection point in the curve of the reference secondary battery)−(the slope of the curve after the inflection point in the curve of the reference secondary battery)]; as well as Whether the cycle life of the target secondary battery is improved compared to the reference secondary battery is predicted based on the comparison result.

2. The method for predicting the cycle life of a secondary battery according to claim 1, wherein the reference slope difference is obtained by: During charge / discharge of a reference secondary battery including a reference carbon-based hybrid negative electrode containing a carbon-based negative electrode active material and a non-carbon-based negative electrode active material uniformly mixed in a single-layer structure, measuring the lattice spacing (d-spacing) of the carbon-based negative electrode active material of the reference carbon-based hybrid negative electrode using an X-ray diffractometer, and plotting a curve regarding changes in lattice spacing values with charge / discharge capacity as an X-axis and lattice spacing values as a Y-axis; and The reference slope difference is calculated.

3. The method for predicting the cycle life of a secondary battery according to claim 1 , wherein the discharge capacity after the inflection point of the curve during discharge in the plotted curve corresponds to the capacity contribution of the non-carbon-based negative electrode active material, and the discharge capacity before the inflection point corresponds to the capacity contribution of the carbon-based negative electrode active material. 4 . The method for predicting a cycle-life of a secondary battery according to claim 1 , wherein when the target slope difference is greater than the reference slope difference according to the comparison result, it is determined that the cycle-life of the target secondary battery is improved compared with the reference secondary battery. 5 . The method for predicting a cycle life of a secondary battery according to claim 1 , wherein when the target slope difference is smaller than the reference slope difference according to the comparison result, it is determined that the cycle life of the target secondary battery is reduced compared to the reference secondary battery.

6. The method for predicting the cycle life of a secondary battery according to claim 1, wherein the carbon-based negative electrode active material comprises at least one of natural graphite, artificial graphite, soft carbon, hard carbon, pitch carbide, sintered coke, graphene, or carbon nanotubes. 7 . The method for predicting the cycle life of a secondary battery according to claim 1 , wherein the non-carbon-based negative electrode active material comprises a metal or metalloid capable of forming an alloy with lithium.

8. The method for predicting the cycle life of a secondary battery according to claim 1, wherein the non-carbon-based negative electrode active material comprises a metal or metalloid selected from Si, Sn, In, Pb, Ga, Ge, Al, Bi, Sb, Ag, Mg, Zn, Pt, Ti, or a combination thereof; an oxide of the metal or the metalloid; a carbon composite of the metal or the metalloid; a carbon composite of the oxide of the metal or the metalloid; or a mixture thereof.

9. The method for predicting the cycle life of a secondary battery according to claim 1, wherein the non-carbon-based negative electrode active material comprises Si, SiO x (0<x<2), or a mixture thereof.

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