Negative electrode for lithium secondary battery and lithium secondary battery comprising same

Abstract

A negative electrode for a lithium secondary battery according to an exemplary embodiment may include: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material. The preferred orientation (P / O) factor of the negative electrode active material layer obtained by the March-Dollase method may be less than 0.67. Accordingly, the lithium secondary battery may have high stability and cycle characteristics, and may provide high energy density and high power characteristics.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery. More specifically, it relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including active material particles. Background Art

[0002] A secondary battery is a battery that can be repeatedly charged and discharged. With the development of the information communication and display industries, secondary batteries are widely used in portable electronic communication devices such as camcorders, mobile phones, and laptop computers (PCs). In addition, as a power source for eco-friendly vehicles such as hybrid vehicles, battery packs including secondary batteries are being developed and applied.

[0003] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc. Among them, lithium secondary batteries have a high operating voltage and energy density per unit weight, so they are beneficial for charging speed and weight reduction. Therefore, lithium secondary batteries are being developed and applied as, for example, a power source for hybrid vehicles.

[0004] For example, a lithium secondary battery may include: an electrode assembly including a positive electrode, a negative electrode, and a separator (separating membrane); and an electrolyte impregnating the electrode assembly. In addition, the lithium secondary battery may further include an outer packaging material such as a pouch type for accommodating the electrode assembly and the electrolyte.

[0005] For example, the negative electrode may include active material particles such as carbon-based particles or silicon-based particles. However, when the lithium secondary battery is repeatedly charged and discharged, the active material particles shrink / expand, which may cause problems such as poor contact between the particles and short circuits. When changing the composition and structure of the negative electrode active material to improve the stability of the negative electrode, the power and life of the secondary battery may be reduced.

[0006] Therefore, there is a need to develop a negative electrode that can ensure life stability and power / capacity characteristics. For example, Korean Patent Publication No. 10-2017-0099748 discloses an electrode assembly for a lithium secondary battery and a lithium secondary battery including the electrode assembly. Summary of the Invention

[0007] (1) Technical Problem to be Solved

[0008] One technical problem of the present invention is to provide a negative electrode for a lithium secondary battery having improved stability and electrical characteristics.

[0009] One technical problem of the present invention is to provide a lithium secondary battery having improved stability and electrical characteristics.

[0010] (2) Technical Solution

[0011] The negative electrode for a lithium secondary battery according to an exemplary embodiment may include: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material.

[0012] According to an exemplary embodiment, the preferred orientation factor (P / O) of the negative electrode active material layer obtained by the March-Dollase method may be less than 0.67.

[0013] In some embodiments, the preferred orientation factor (P / O) of the negative electrode active material layer may be from 0.30 to 0.64.

[0014] In some embodiments, the preferred orientation factor (P / O) of the negative electrode active material layer may be calculated as a 1-orientation factor (r). The orientation factor (r) may be obtained by the following Equation 1.

[0015] [Equation 1]

[0016]

[0017] In Equation 1, r may be the orientation factor, α may be the diffraction angle of the X-ray of the negative electrode active material layer measured by X-ray diffraction analysis, and P k may be a value obtained by the following Equation 2.

[0018] [Equation 2]

[0019]

[0020] In Equation 2, θ may be the incident angle of the X-ray at which the diffraction angle α appears, and θ k may be the reflection angle of the X-ray incident at an angle of θ.

[0021] Y ci may be the diffraction intensity at 2θ calculated by the Rietveld refinement method using the X-ray diffraction curve of the negative electrode active material layer.

[0022] Y bi may be the background intensity.

[0023] S may be the scale factor, and L k may be the Lorenz factor, and F k may be the structure factor.

[0024] It may be the reflection intensity of X-rays measured for the negative electrode active material layer by XRD, and A may be the absorption factor.

[0025] In some embodiments, the preferred orientation plane of the negative electrode active material layer may be the crystal plane in the c-axis direction of the negative electrode active material.

[0026] According to an exemplary embodiment, the negative electrode active material may include artificial graphite, natural graphite, or a mixture thereof.

[0027] In some embodiments, the negative electrode active material may include single particles.

[0028] In some embodiments, the negative electrode active material may include assembled particles formed by aggregation of a plurality of primary particles.

[0029] In some embodiments, the negative electrode active material may include single particles and assembled particles formed by aggregation of a plurality of primary particles.

[0030] In one embodiment, the negative electrode active material may have a bi-modal distribution.

[0031] In one embodiment, the average particle size of the single particles may be 7 μm to 15 μm, and the average particle size of the assembled particles may be 10 μm to 20 μm.

[0032] In one embodiment, the ratio of the content of the assembled particles to the content of the single particles may be 0.5 to 4.

[0033] In some embodiments, the negative electrode active material may include a core and a carbon coating formed on the core.

[0034] In one embodiment, the content of the carbon coating may be 0.5 wt% to 10 wt% of the total weight of the negative electrode active material.

[0035] According to an exemplary embodiment, the negative electrode active material layer may include pores formed between the negative electrode active materials. In one embodiment, the pore volume of the negative electrode active material layer may be 0.16 ml / g to 0.30 ml / g.

[0036] The lithium secondary battery according to an exemplary embodiment may include: the negative electrode for a lithium secondary battery as described above; and a positive electrode disposed opposite to the negative electrode for a lithium secondary battery.

[0037] (III) Beneficial Effects

[0038] The negative electrode for a lithium secondary battery according to an exemplary embodiment may include a negative electrode active material layer formed on a negative electrode current collector. The preferred orientation factor (P / O) of the negative electrode active material layer obtained by the March-Dollase method may have a specified range. Accordingly, the degree and direction of orientation of the active material particles in the negative electrode active material layer can be adjusted within a specified range.

[0039] For example, crystals having an isotropic structure and crystals having an anisotropic structure may coexist appropriately in the negative electrode active material layer. Accordingly, volume expansion of the negative electrode during charge and discharge can be suppressed, and life characteristics, high-temperature stability, and power characteristics can be improved simultaneously.

[0040] The negative electrode active material may include artificial graphite or natural graphite. Accordingly, the life characteristics and high-temperature operating stability of the secondary battery can be improved. In addition, the negative electrode active material may further include a carbon coating. Accordingly, the strength of the negative electrode active material can be further increased, and volume expansion can be suppressed, thereby further improving cycle characteristics and structural stability.

[0041] Pores may be formed within the negative electrode active material layer. Accordingly, volume expansion of the negative electrode active material caused by charge and discharge can be further alleviated, and occurrence of cracks and short circuits in the negative electrode active material layer can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic plan view showing a secondary battery according to an exemplary embodiment.

[0043] Figure 2 is a schematic cross-sectional view showing an electrode assembly according to an exemplary embodiment. DETAILED DESCRIPTION

[0044] The negative electrode for a lithium secondary battery according to an embodiment of the present invention (hereinafter, may be simply referred to as a negative electrode) may include: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector.

[0045] In addition, a lithium secondary battery according to an embodiment of the present invention may include the negative electrode for a lithium secondary battery described above.

[0046] Hereinafter, with reference to the drawings, the negative electrode for a lithium secondary battery and the lithium secondary battery according to an embodiment of the present invention will be described in detail. However, this is merely exemplary, and the present invention is not limited to the specific embodiments described exemplarily.

[0047] <Negative Electrode for Lithium Secondary Battery>

[0048] Figure 1 and Figure 2They are a schematic plan view and a schematic cross-sectional view showing a secondary battery according to an exemplary embodiment. For example, Figure 2 is a cross-sectional view taken along the Figure 1 I-I' line shown in the thickness direction of the lithium secondary battery.

[0049] In addition, for ease of explanation, Figure 1 the illustrations of the positive electrode and the negative electrode are omitted in

[0050] Referring to Figure 1 and Figure 2 , the secondary battery can be provided as a lithium secondary battery. The secondary battery may include an electrode assembly 150 and a housing 160 that houses the electrode assembly 150. The electrode assembly 150 may include a positive electrode 100, a negative electrode 130, and a separator 140.

[0051] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed on at least one surface of the negative electrode current collector 125.

[0052] According to an exemplary embodiment, the negative electrode active material layer 120 may be formed on both sides (e.g., the upper and lower sides) of the negative electrode current collector 125. The negative electrode active material layer 120 may be coated on the upper surface and the bottom surface of the negative electrode current collector 125, respectively. For example, the negative electrode active material layer 120 may be in direct contact with the surface of the negative electrode current collector 125.

[0053] The negative electrode current collector 125 may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably may include copper or a copper alloy.

[0054] The negative electrode active material layer 120 may contain a negative electrode active material. A crystal structure in which the negative electrode active material is oriented in a specified direction may be formed in the negative electrode active material layer 120.

[0055] According to an exemplary embodiment, the preferred orientation (P / O) factor of the negative electrode active material layer 120 obtained by the March-Dollase method may be less than 0.67.

[0056] The preferred orientation factor may be a value representing the degree of orientation of the crystal structure of the negative electrode active material layer 120 along a specific crystal plane. For example, when the preferred orientation factor is high, the orientation of the negative electrode active material layer 120 with respect to a specific crystal plane may be high.

[0057] The preferred orientation factor can be obtained by using the March-Dollase method of X-ray diffraction analysis. For example, the preferred orientation factor can be calculated from the scattering vector and peak obtained by X-ray diffraction analysis of the surface or cross-section of the negative electrode active material layer 120.

[0058] As the preferred orientation factor increases, the crystals in the negative electrode active material layer 120 can be oriented toward specific crystal planes. When the crystal structure has a high degree of orientation, the negative electrode can expand toward the corresponding crystal planes during charge and discharge. Therefore, the stress caused by the volume change may act excessively on the negative electrode 130, which may reduce the life characteristics and structural stability.

[0059] In addition, due to the volume expansion of the negative electrode 130, the energy density of the lithium secondary battery may be reduced, and the charge-discharge capacity and power characteristics may be reduced.

[0060] According to an exemplary embodiment, the negative electrode active material layer 120 has a preferred orientation factor of less than 0.67, so that the orientation of the crystals in the negative electrode active material layer 120 can be reduced. Therefore, the volume expansion of the negative electrode active material layer 120 can be suppressed, and the cycle characteristics and stability can be improved.

[0061] In some embodiments, the preferred orientation factor of the negative electrode active material layer 120 can be from 0.30 to 0.67. For example, it can be from 0.35 to 0.64 or from 0.39 to 0.61.

[0062] For example, the higher the preferred orientation factor, the more the crystals in the negative electrode active material layer 120 can have a preferred orientation, for example, can have an anisotropic structure. In addition, the lower the preferred orientation factor, the more the crystals in the negative electrode active material layer 120 can have an isotropic structure.

[0063] When the preferred orientation factor of the negative electrode active material layer 120 is within the above range, isotropic crystal regions and anisotropic crystal regions can coexist appropriately within the negative electrode active material layer 120. Therefore, through the anisotropic structure within the negative electrode active material layer 120, the cycle performance and high-temperature storage performance can be improved, and through the isotropic structure, the initial energy density and power characteristics can be improved.

[0064] For example, when the preferred orientation factor of the negative electrode active material layer 120 is less than 0.30, the initial porosity of the negative electrode active material layer 120 increases, the energy density may decrease relatively, and the structural stability of the negative electrode active material layer 120 may decrease relatively.

[0065] In some embodiments, the preferred orientation factor (P / O) of the negative electrode active material layer 120 can be calculated as 1 - orientation factor (r).

[0066] For example, the orientation factor (r) can be obtained by Equation 1 below.

[0067] [Equation 1]

[0068]

[0069] In Equation 1, r is the orientation factor.

[0070] α can be the diffraction angle of the X - ray of the negative electrode active material layer 120 measured by X - ray diffraction analysis (XRD). For example, the X - ray scattering vector can be the angle formed with the direction perpendicular to the measurement plane of the negative electrode active material layer 120. In one embodiment, the X - ray can be irradiated in the direction perpendicular to the measurement plane of the negative electrode active material layer 120. For example, the incident angle of the X - ray can be 90°.

[0071] P k can be calculated by Equation 2 below.

[0072] [Equation 2]

[0073]

[0074] In Equation 2, θ can be the incident angle of the X - ray at which the diffraction angle α appears. For example, θ can be the angle formed between the measurement plane of the negative electrode active material layer 120 and the X - ray. θ k can be the reflection angle of the X - ray incident at an angle of θ.

[0075] Y ci is the intensity of the diffracted X - ray calculated by the Rietveld refinement method. For example, the diffraction curve of the negative electrode active material layer 120 obtained by XRD is calculated by the Rietveld refinement method, so that the X - ray diffraction intensity at the diffraction peak i can be obtained. The diffraction peak i can be the peak that appears at 2θ in the XRD pattern of the negative electrode active material layer 120.

[0076] For example, the diffraction pattern of the negative electrode active material layer is measured with a 2θ step size of 0.01° to 0.05°, and the pattern calculated from the measured diffraction pattern and the internal crystal structure of the negative electrode active material layer 120 is used, so that Y ci .

[0077] In one embodiment, the difference between the measured diffraction pattern and the calculated pattern can be repeatedly calculated by the least - square method. For example, Y ci can be calculated by Equation 3 below.

[0078] [Formula 3]

[0079]

[0080] In the said Formula 3, Y i is the diffraction intensity measured at diffraction peak i, and Y ci is the diffraction intensity at diffraction peak i calculated according to the internal crystal structure. The said Formula 3 can be repeatedly calculated to minimize the value of Sy, so as to finally calculate Y ci .

[0081] In the said Formula 2, can be the reflection intensity of the X-ray at 2θ measured for the negative electrode active material layer 120 by XRD. For example, can be the X-ray reflection intensity at 2θ obtained from the reflection profile of the negative electrode active material layer 120 obtained by XRD.

[0082] Y bi is the background intensity of the said negative electrode active material layer. For example, Y bi is the X-ray intensity at 2θ from the background, rather than from the negative electrode active material layer 120.

[0083] S is the scale factor, L k is the Lorentz factor, F k is the structure factor, and A is the absorption factor. S, L k , F k and A, as correction factors for the diffraction pattern, can be physical properties measured from the negative electrode active material layer 120.

[0084] In one embodiment, S, L k , F k and A can be values deduced or calculated by using the Rietveld refinement method of X-ray diffraction software. For example, by performing the Rietveld refinement method of "Highscore Plus" of Malvern Panalytical on the measured values obtained from the negative electrode active material layer 120, S, L k , F k and A can be calculated respectively.

[0085] In one embodiment, L k and F k can be calculated values when the reflection angle of the X-ray incident at an angle of θ is θ k , from which a function or value of P k can be obtained.

[0086] The orientation plane can refer to the plane of crystal orientation (e.g., crystal plane). The preferred orientation plane (H) can refer to the crystal plane when the negative electrode active material in the negative electrode active material layer 120 is preferentially oriented or grows along a specific crystal plane. For example, the preferred orientation plane (H) can refer to the crystal plane with the largest relative presence among the crystal planes in the negative electrode active material layer 120.

[0087] In one embodiment, the preferred orientation plane of the negative electrode active material layer 120 can be the crystal plane corresponding to the c-axis direction of the negative electrode active material. For example, when the negative electrode active material layer 120 contains natural graphite or artificial graphite as the negative electrode active material, the preferred orientation plane (H) of the negative electrode active material layer 120 can be the 002 plane, 004 plane, 006 plane, etc.

[0088] For example, the preferred orientation of the negative electrode active material layer 120 can occur on the (004) plane near 2θ = 53.5° to 56.0°.

[0089] In some embodiments, the preferred orientation (P / O) factor of the negative electrode active material layer 120 can be measured on the surface (out-of-plane) of the negative electrode active material layer 120. For example, the preferred orientation factor can be measured by X-ray diffraction analysis of the upper surface of the negative electrode active material layer 120.

[0090] In some embodiments, the preferred orientation (P / O) factor of the negative electrode active material layer 120 can be measured at the cross-section (in-plane) of the negative electrode active material layer 120. For example, the preferred orientation factor can be measured by X-ray diffraction analysis of the cross-section of the negative electrode active material layer 120.

[0091] In some embodiments, both the surface (out-of-plane) and cross-section (in-plane) of the negative electrode active material layer 120 can have the preferred orientation (P / O) factor within the above range. For example, the preferred orientation (P / O) factor measured by X-ray diffraction analysis of the upper surface of the negative electrode active material layer 120 and the preferred orientation (P / O) factor measured by X-ray diffraction analysis of the cross-section of the negative electrode active material layer 120 can both be 0.30 to 0.64.

[0092] Both the surface and the interior of the negative electrode active material layer 120 have the preferred orientation (P / O) factor within the above range, so the overall structural stability and volume expansion inhibition force of the negative electrode active material layer 120 can be further increased. Therefore, the cycle characteristics and high-temperature stability of the lithium secondary battery can be further improved.

[0093] In some embodiments, the surface (out-of-plane) and cross-section (in-plane) of the negative electrode active material layer 120 may have different preferred orientation planes (H) from each other. For example, when the preferred orientation plane at the surface of the negative electrode active material layer 120 is the (004) plane, the preferred orientation plane at the cross-section may be the (110) plane.

[0094] According to an exemplary embodiment, the negative electrode active material may include artificial graphite, natural graphite, or a mixture thereof. Artificial graphite and natural graphite have high heat resistance and high stability, and thus can further improve the life characteristics and high-temperature operating stability of the secondary battery.

[0095] In some embodiments, the negative electrode active material may include artificial graphite. Artificial graphite may have relatively higher chemical stability and thermal stability compared to natural graphite. Therefore, by using artificial graphite, the high-temperature storage characteristics or high-temperature life characteristics of the secondary battery can be further improved.

[0096] In some embodiments, the negative electrode active material may further include a carbon coating. For example, the negative electrode active material may include active material particles, and the active material particles include a core and a carbon coating formed on the core. The core is the part that provides negative electrode activity. For example, it may include artificial graphite, natural graphite, or the like.

[0097] Due to the carbon coating, the strength of the negative electrode active material particles can be further improved, and volume expansion can be inhibited. Therefore, the generation of cracks in the negative electrode active material particles caused by stress can be further inhibited, and the volume change of the negative electrode active material layer can be reduced, thereby further improving the energy density.

[0098] In one embodiment, the content of the carbon coating may be 0.5 wt% to 10 wt% of the total weight of the negative electrode active material. Within the above range, the cycle characteristics and high-temperature stability of the lithium secondary battery can be further improved without hindering the negative electrode activity.

[0099] For example, the content of the carbon coating may be 0.5 wt% to 6 wt% of the total weight of the negative electrode active material, or may be 1 wt% to 6 wt%.

[0100] In one embodiment, when the core is natural graphite, the content of the carbon coating may be 3 wt% to 6 wt%. In one embodiment, when the core is artificial graphite, the content of the carbon coating may be 0.5 wt% to 4 wt%. Within the above range, the power characteristics and life characteristics of the negative electrode active material can be improved simultaneously.

[0101] In one embodiment, the carbon coating may have an amorphous structure. The amorphous structure may also include cases where the carbon coating has partial crystallinity but can be regarded as amorphous as a whole. For example, compared with the core, the carbon coating may be relatively amorphous.

[0102] For example, the carbon coating may include amorphous base carbons such as hard carbon, soft carbon, calcined coke, mesophase pitch carbide, etc. In this case, the durability of the negative electrode active material is increased, so that the generation of cracks caused by charge and discharge or external impact can be further suppressed.

[0103] According to an exemplary embodiment, the negative electrode active material may include single particles, assembled particles, or a mixture thereof. The single particles and the assembled particles can be distinguished by the morphology of the particles. For example, the assembled particles and the single particles can be distinguished based on the cross-sectional image of the particles measured by a scanning electron microscope (SEM; Scanning Electron Microscope).

[0104] The assembled particles may refer to particles in which a plurality of primary particles are aggregated and are substantially regarded as or observed as one particle. For example, in the case of the assembled particles, the boundary of the primary particles can be observed from the SEM cross-sectional image.

[0105] In one embodiment, the assembled particles may aggregate more than 2, more than 10, more than 30, or more than 50 primary particles. In one embodiment, the assembled particles may aggregate 500 or fewer, 300 or fewer, or 100 or fewer primary particles.

[0106] The single particles may refer to particles (monolith) rather than aggregates. For example, different from the assembled particles, in the case of the single particles, the boundary of the primary particles may not be observable from the SEM cross-sectional image.

[0107] In one embodiment, the negative electrode active material may include only single particles. The single particles have relatively high structural stability, so that the long-term performance and life characteristics of the lithium secondary battery can be improved.

[0108] In one embodiment, the negative electrode active material may include only assembled particles. Since the assembled particles aggregate a plurality of primary particles, they may have relatively high conductivity. Therefore, the power characteristics of the lithium secondary battery can be improved.

[0109] In one embodiment, the negative electrode active material may include a mixture of single particles and assembled particles. Accordingly, the cycle characteristics of the lithium secondary battery can be improved while increasing the energy density and power.

[0110] In some embodiments, when the negative electrode active material includes both single particles and assembled particles, the particle size distribution of the negative electrode active material may have a bimodal distribution.

[0111] For example, the average particle diameter (D 50 ) of the single particles and the average particle diameter (D 50 ) of the assembled particles may be different from each other. The average particle diameter (D 50 ) may refer to the particle diameter at which the volume fraction is 50% in the cumulative particle size distribution of the negative electrode active material.

[0112] In one embodiment, the average particle diameter of the single particles may be smaller than the average particle diameter of the assembled particles. Since the average particle diameter of the single particles is relatively small, the high-rate characteristics can be further improved. In addition, since the average particle diameter of the assembled particles is relatively large, the stability and life characteristics can be further improved.

[0113] In one embodiment, the average particle diameter of the single particles may be 7 μm to 15 μm. In one embodiment, the average particle diameter of the assembled particles may be 10 μm to 20 μm.

[0114] In some embodiments, when the negative electrode active material includes both single particles and assembled particles, the particle size distribution of the negative electrode active material may have a monomodal distribution. In this case, the average particle diameter of the negative electrode active material may be 10 μm to 16 μm.

[0115] In one embodiment, the ratio of the content of the assembled particles to the content of the single particles may be 0.1 to 20. For example, it may be 1 to 10 or 0.5 to 4. Within the above range, the density of the negative electrode can be further increased, and the cycle characteristics and power characteristics can be improved simultaneously.

[0116] In one embodiment, the negative electrode active material may further include a silicon-based active material.

[0117] For example, the silicon-based active material may further include silicon (Si), silicon oxide (SiO X , 0 < x < 2), a silicon-metal alloy, or a silicon-carbon composite (Si-C). These may be used alone or in combination of two or more.

[0118] The silicon oxide (SiO X , 0 < x < 2) may include a lithium compound. For example, SiO containing a lithium compoundX It can be SiO pretreated with lithium X . For example, SiO containing a lithium compound X can contain lithium silicate.

[0119] In some embodiments, the silicon-carbon composite may include silicon carbide (SiC) formed by mechanical alloying of silicon and carbon or silicon-carbon particles having a core-shell structure.

[0120] According to an exemplary embodiment, the negative electrode active material layer 120 may include pores. For example, the pores may be empty spaces formed between the negative electrode active material particles.

[0121] For example, the pores can accommodate the volume expansion of the negative electrode active material. Therefore, the volume expansion of the negative electrode 130 can be further reduced, and the effect of internal stress in the negative electrode 130 can be alleviated, thereby further suppressing the occurrence of cracks and short circuits.

[0122] In one embodiment, the initial pore volume of the negative electrode active material layer 120 may be from 0.15 ml / g to 0.30 ml / g. For example, the pore volume may be the value obtained by dividing the total pore volume (ml) of the negative electrode active material layer 120 measured at the SOC 0% state by the total weight (g) of the negative electrode active material layer 120. The total pore volume of the negative electrode active material layer 120 can be measured using a specific surface area measuring device.

[0123] When the pore volume of the negative electrode active material layer 120 is less than 0.15 ml / g, cracks may occur in the negative electrode active material layer 120 due to the stress generated during charge and discharge. In addition, the life characteristics of the lithium secondary battery may be relatively reduced.

[0124] When the pore volume of the negative electrode active material layer 120 exceeds 0.30 ml / g, the energy density of the negative electrode active material layer 120 may be reduced. Therefore, the power characteristics and charge-discharge capacity of the lithium secondary battery may be relatively reduced. In addition, the tap density of the negative electrode active material layer 120 is reduced, so the structural stability may be reduced.

[0125] In some embodiments, the increase rate of the pore volume of the negative electrode active material layer 120 measured at the SOC 100% state relative to the initial pore volume may be 20% or less, preferably 10% or less. Therefore, the lithium secondary battery can suppress expansion even when fully charged (fully lithiated), and can further improve the life characteristics and operating stability.

[0126] <Lithium Secondary Battery>

[0127] According to an exemplary embodiment, the secondary battery may include an electrode assembly 150 and a case 160 accommodating the electrode assembly 150. The electrode assembly 150 may include a positive electrode 100, a negative electrode 130, and a separator 140.

[0128] The positive electrode 100 may include a positive electrode current collector 105 and a positive electrode active material layer 110 formed on at least one surface of the positive electrode current collector 105. According to an exemplary embodiment, the positive electrode active material layer 110 may be formed on both surfaces (e.g., the upper and lower surfaces) of the positive electrode current collector 105. For example, the positive electrode active material layer 110 may be coated on the upper and bottom surfaces of the positive electrode current collector 105, respectively, and may be directly coated on the surface of the positive electrode current collector 105.

[0129] The positive electrode current collector 105 may include, for example, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably may include aluminum or an aluminum alloy.

[0130] The positive electrode active material layer 110 may contain a lithium metal oxide as a positive electrode active material, and according to an exemplary embodiment, may contain a lithium (Li)-nickel (Ni)-based oxide.

[0131] In some embodiments, the lithium metal oxide contained in the positive electrode active material layer 110 may be represented by the following Chemical Formula 1.

[0132] [Chemical Formula 1]

[0133] Li 1+a Ni 1-(x+y) Co x M y O2

[0134] In Chemical Formula 1, -0.05 ≤ a ≤ 0.15, 0.01 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, and M may be one or more elements selected from Mn, Mg, Sr, Ba, B, Al, Si, Ti, Zr, and W. In one embodiment, 0.01 ≤ x ≤ 0.20 and 0.01 ≤ y ≤ 0.15.

[0135] Preferably, in Chemical Formula 1, M may be manganese (Mn). In this case, a nickel-cobalt-manganese (NCM)-based lithium oxide may be used as the positive electrode active material.

[0136] For example, nickel (Ni) may be provided as a metal related to the capacity of the lithium secondary battery. The higher the content of nickel, the more the capacity and power of the lithium secondary battery can be increased, but when the content of nickel is excessively increased, the life is reduced, and it may be disadvantageous in terms of mechanical stability and electrical stability. For example, cobalt (Co) may be a metal related to the conductivity or resistance of the lithium secondary battery.

[0137] In one embodiment, M may include manganese (Mn), and Mn may be provided as a metal related to the mechanical and electrical stability of the lithium secondary battery.

[0138] Through the interaction of the above nickel, cobalt, and manganese, the capacity, power, low resistance, and life stability of the positive electrode active material layer 110 can be improved simultaneously.

[0139] For example, the positive electrode paste can be prepared by mixing and stirring the positive electrode active material with a binder, a conductive material, and / or a dispersion material, etc. in a solvent. The positive electrode paste is coated on the positive electrode current collector 105 and then pressed and dried, thereby forming the positive electrode active material layer 110.

[0140] For example, the binder may include a vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene, polypropylene, polyacrylic acid, or styrene-butadiene rubber (SBR), etc., and can be used together with a thickener such as carboxymethyl cellulose (CMC).

[0141] For example, a PVDF-based binder can be used as the binder for forming the positive electrode. In this case, the amount of the binder used to form the positive electrode active material layer 110 can be reduced, and the amount of the positive electrode active material or lithium metal oxide particles can be relatively increased. Therefore, the power and capacity characteristics of the secondary battery can be improved.

[0142] For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, carbon nanofibers, carbon nanotubes, and / or metal-based conductive materials such as tin, tin oxide, zinc oxide, titanium oxide, and metal fibers.

[0143] In some embodiments, the electrode density of the positive electrode 100 may be 3.0 - 3.9 g / cubic centimeter (cc), for example, it may be 3.2 - 3.8 g / cubic centimeter.

[0144] In some embodiments, the negative electrode active material layer 120 can be formed by coating (spreading) the negative electrode composition on the negative electrode current collector 125 and then pressing (rolling) and drying.

[0145] The negative electrode composition may be provided in the form of a slurry, and the negative electrode composition may include a negative electrode active material, a binder, a conductive material, and a thickener. The binder and the conductive material may use substances that are substantially the same as or similar to those used for forming the positive electrode 100.

[0146] In some embodiments, the negative electrode composition may be coated on the negative electrode current collector 125, and then a magnetic field may be applied to the negative electrode current collector 125 coated with the negative electrode composition.

[0147] For example, a pair of magnetic field generating devices may be provided, and the negative electrode current collector 125 may be disposed therebetween. Magnetic field lines may be applied through the magnetic field generating devices, so that the arrangement direction of the crystals in the negative electrode active material layer 120 may be changed. Therefore, the preferred orientation factor of the negative electrode active material layer 120 may be adjusted to the above range by the intensity, application direction, and changing direction of the magnetic field lines, etc.

[0148] The magnetic field generating device is not limited as long as it can generate a magnetic field. For example, an electromagnet and a superconducting magnet, etc. may be used.

[0149] In some embodiments, a magnetic field may be applied to the negative electrode current collector 125, and then the negative electrode composition coated on the negative electrode current collector 125 may be dried and calendered. The orientation direction of the crystal structure may be fixed through the drying process.

[0150] The drying method may use natural drying, heat drying, vacuum drying, air drying, etc., and the drying process may be carried out step by step.

[0151] In some embodiments, for the compatibility with the graphite-based active material, the binder used for forming the negative electrode 130 may include, for example, styrene-butadiene rubber (SBR) or an acrylic-based binder, and may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0152] In an exemplary embodiment, the electrode density of the negative electrode 130 may be 1.0 - 1.9 g / cm³.

[0153] In some embodiments, the area (e.g., the contact area with the separator 140) and / or volume of the negative electrode 130 may be larger than that of the positive electrode 100. In this case, the lithium ions generated from the positive electrode 100 can migrate smoothly to the negative electrode 130 without precipitation in the middle, and the power and capacity characteristics of the secondary battery can be further improved.

[0154] The separator 140 can be disposed between the positive electrode 100 and the negative electrode 130. The separator 140 can include a porous polymer membrane made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc. The separator may further include a non-woven fabric formed of glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0155] The separator 140 can extend between the positive electrode 100 and the negative electrode 130, and be folded and wound along the thickness direction of the lithium secondary battery. Thus, a plurality of positive electrodes 100 and negative electrodes 130 can be laminated along the thickness direction through the separator 140.

[0156] According to an exemplary embodiment, a battery cell can be defined by the positive electrode 100, the negative electrode 130, and the separator 140, and an electrode assembly 150 in the form of, for example, a jelly roll can be formed by laminating a plurality of battery cells. For example, the electrode assembly 150 can be formed by winding, laminating, folding, etc. of the separator 140.

[0157] The electrode assembly 150 is accommodated in the housing 160, and an electrolyte can be injected into the housing 160 together. The housing 160 can include, for example, a pouch, a can, etc.

[0158] According to an exemplary embodiment, the electrolyte can use a non-aqueous electrolyte.

[0159] The non-aqueous electrolyte can contain a lithium salt as an electrolyte and an organic solvent, and the lithium salt can be represented, for example, by Li + X - As the anion (X - ) of the lithium salt, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N -, (FSO2)2N - 、 CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - etc.

[0160] As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, ethyl difluoroacetate (DFEA), propyl difluoroacetate (DFPA), and ethyl fluoroacetate (FEA) series, etc. can be used. These can be used alone or in combination of two or more.

[0161] As Figure 1 As shown, the tabs (the positive tab and the negative tab) can respectively protrude from the positive current collector 105 and the negative current collector 125 belonging to each battery cell and extend to one side of the housing 160. The tabs can be fused to the said one side of the housing 160 to connect to the electrode leads (the positive lead 107 and the negative lead 127) extending to the outside of the housing 160 or exposed outside the housing 160.

[0162] Figure 1 shows that the positive electrode lead 107 and the negative lead 127 are formed on the same side of the lithium secondary battery or the housing 160, but they can also be formed on the opposite sides.

[0163] For example, the positive lead 107 can be formed at the said one end of the housing 160, and the negative lead 127 can be formed at the said other end of the housing 160.

[0164] The lithium secondary battery can be made into, for example, a cylindrical shape using a can, a prismatic shape, a pouch type, or a coin shape, etc.

[0165] Hereinafter, for the purpose of helping to understand the present invention, experimental examples including specific examples and comparative examples are presented, but they are only used to illustrate the present invention and are not used to limit the claims. Various changes and modifications can be made to the examples within the scope and technical concept of the present invention, which are obvious to those skilled in the art. Such variations and modifications are naturally within the scope of the claims.

[0166] Example 1

[0167] (1) Fabrication of the negative electrode for a lithium secondary battery

[0168] Prepare artificial graphite single particles with an average particle diameter (D 50 ) of 14.8 μm as the negative electrode active material. The negative electrode active material, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) as a thickening agent are mixed at a mass ratio of 97.3:1.5:1.2 and then dispersed in deionized distilled water to prepare a negative electrode slurry, and the slurry is coated on one side of a Cu-foil current collector.

[0169] A magnetic field is set for the current collector coated with the negative electrode slurry so that the magnetic force penetrates the negative electrode active material layer in the vertical direction, thereby performing an orientation process through the magnetic field.

[0170] After the orientation process, drying and rolling are performed to fabricate a negative electrode having a mixture density of 13.0 mg / cm 2 (based on the cross-section) and 1.7 g / cm³.

[0171] (2) Measurement of the preferred orientation (P / O) factor

[0172] The preferred orientation (P / O) factor is measured for the surface of the fabricated negative electrode active material layer by the March-Dollase method using XRD analysis. XRD analysis is performed under the following specific measurement conditions, and the orientation factor (r) is measured according to Equation 1. The preferred orientation factor is calculated as 1 - r.

[0173] X-Ray Diffractometer (XRD) Empyrean

[0174] Maker: PANalytical

[0175] Anode material: Cu

[0176] K-Alpha1 wavelength:

[0177] Generator voltage: 45 kV

[0178] Tube current: 40 mA

[0179] Scan Range: 10 - 80°

[0180] Scan Step Size: 0.026°

[0181] Divergence slit: 1 / 4°

[0182] Antiscatter slit: 1°

[0183] Time per step: 100 seconds

[0184] The measurement results are shown in Table 1 below.

[0185] (3) Measurement of the pore volume (ml / g)

[0186] Using porosimetry (AutoPore V 9600), the pore volume of the negative electrode active material layer was measured by mercury intrusion porosimetry. The pore volume (ml / g) was measured by dividing the measured volume (ml) by the total weight (g) of the negative electrode active material layer.

[0187] (3) Fabrication of the secondary battery

[0188] LiNi 0.88 Co 0.10 Mn 0.02 O2, a conductive material, CNT as a dispersion material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a weight ratio of 98.18:0.6:0.12:1.1 to prepare a positive electrode paste, and then the positive electrode paste was coated on an aluminum substrate and dried and pressed to fabricate a positive electrode.

[0189] The fabricated positive electrode and the negative electrode were set, and a polyethylene (PE) separator (13 μm) was placed in the middle to form an electrode core, and the electrode cores were stacked to form an electrode assembly. The electrode assembly was housed in a soft package and the tab portions were fused.

[0190] Thereafter, an electrolyte solution containing a mixed solvent of 1.0 M LiPF6 and ethylene carbonate / ethyl methyl carbonate (EC / EMC, 25 / 75; volume ratio) was prepared. A life additive (fluoroethylene carbonate (FEC)) of 1% by weight of the total weight of the electrolyte solution was added to the prepared electrolyte solution. Thereafter, the electrolyte solution was injected into the electrode assembly and then sealed to fabricate a secondary battery.

[0191] Examples 2 to 12 and Comparative Examples 1 to 6

[0192] The negative electrode was fabricated by the same method as in Example 1, except that artificial graphite having the type and average particle diameter described in Table 1 below was used as the negative electrode active material.

[0193] In the types in Table 1, "single particle" means single particles of artificial graphite, "assembled" means assembled particles of artificial graphite, and "mixed" means an active material in which single particles of artificial graphite and assembled particles of artificial graphite are mixed at a weight ratio of 60:40 to 70:30.

[0194] The preferred orientation (P / O) factor was adjusted as shown in Table 1 below by changing the magnetic field line intensity, direction, process time, etc. of the orientation process.

[0195] In the cases of Examples 8 to 10 and Comparative Example 6, in order to form a carbon coating on the surface of the artificial graphite, soft carbon or hard carbon was heat-treated at 1100°C. The content of the remaining carbon coating after the heat treatment was as shown in Table 1 below.

[0196] [Table 1]

[0197]

[0198] Experimental Example 1

[0199] (1) Evaluation of room temperature life characteristics

[0200] For the secondary batteries according to the examples and comparative examples, charging (CC / CV, 1C, 4.2V, cut-off 0.05C) and discharging (CC, 1C, 2.5V cut-off) were performed at 25°C as one cycle, and the charging / discharging was repeated at 10-minute intervals. Thereafter, the capacity retention rate was measured by calculating the ratio (%) of the discharge capacity measured in 50 cycles to the discharge capacity in the first cycle.

[0201] The evaluation results are shown in Table 2 below.

[0202] (2) Evaluation of high temperature life characteristics

[0203] For the secondary batteries according to the examples and comparative examples, charging (CC / CV, 1C, 4.2V, cutoff at 0.05C) and discharging (CC, 1C, cutoff at 2.5V) were carried out at 25 °C to measure the initial discharge capacity.

[0204] After that, the secondary batteries were charged to SOC 100%, and then placed at 60 °C for 10 weeks. After the placement, the secondary batteries were discharged (CC, 1C, cutoff at 2.5V), and the discharge capacity after the placement was measured. The high-temperature stability was evaluated as the percentage of the value obtained by dividing the discharge capacity after the placement by the initial discharge capacity.

[0205] The evaluation results are shown in Table 2 below.

[0206] (3) Measurement of the increase rate of the pore volume

[0207] For the secondary batteries according to the examples and comparative examples, the pore volume (ml / g) was measured after charging to SOC 100% at 25 °C. The increase rate of the pore volume was evaluated as the percentage of the value obtained by dividing the increase amount of the pore volume by the initial pore volume.

[0208] The evaluation results are shown in Table 2 below.

[0209] [Table 2]

[0210]

[0211] Referring to Table 1 and Table 2, the examples include a negative electrode active material layer having a P / O factor of less than 0.67, and the room-temperature life characteristics and high-temperature life characteristics are improved. In addition, the increase rate of the pore volume is 20% or less when fully charged.

[0212] The comparative examples include a negative electrode active material layer having a high P / O factor, and the room-temperature life characteristics and high-temperature stability are reduced. In addition, the comparative examples show a high increase rate of the pore volume when fully charged.

Claims

1. A negative electrode for a lithium secondary battery, comprising: Negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material, wherein a preferred orientation (P / O) factor of the negative electrode active material layer obtained by the March-Dollase method is less than 0.

67.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein, The preferred orientation factor of the negative electrode active material layer is 0.30 to 0.

64.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein, The preferred orientation factor of the negative electrode active material layer is calculated by a 1-orientation factor r, and the orientation factor r is obtained by the following formula 1, [Formula 1] In the formula 1, r is the orientation factor, α is the diffraction angle of the X-ray of the negative electrode active material layer measured by X-ray diffraction analysis (XRD), and P k is the value obtained by the following formula 2, [Formula 2] In the formula 2, θ is the incident angle of the X-ray at which the diffraction angle α appears, θ k is the reflection angle of the X-ray incident at an angle of θ, Y ci The diffraction intensity at 2θ calculated by the Rietveld refinement method using the X-ray diffraction curve of the negative electrode active material layer Y bi is the background intensity, S is the scale factor, L k is the Lorentz factor, F k is the structure factor, The reflected intensity of the X-ray at 2θ measured for the negative electrode active material layer by XRD, and A is the absorption factor.

4. The negative electrode for a lithium secondary battery according to claim 1, wherein, The preferred orientation plane of the negative electrode active material layer is a crystal plane in the c-axis direction of the negative electrode active material.

5. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material includes artificial graphite, natural graphite, or a mixture thereof.

6. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material includes single particles.

7. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material includes aggregated particles formed by aggregation of a plurality of primary particles.

8. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material includes single particles and aggregated particles formed by aggregation of a plurality of primary particles.

9. The negative electrode for a lithium secondary battery according to claim 8, wherein, The negative electrode active material has a bimodal distribution.

10. The negative electrode for a lithium secondary battery according to claim 8, wherein, The average particle size of the single particles is 7 μm to 15 μm, and the average particle size of the aggregated particles is 10 μm to 20 μm.

11. The negative electrode for a lithium secondary battery according to claim 8, wherein, The ratio of the content of the aggregated particles to the content of the single particles is 0.5 to 4.

12. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material includes a core and a carbon coating formed on the core.

13. The negative electrode for a lithium secondary battery according to claim 12, wherein, The content of the carbon coating is 0.5 wt% to 10 wt% of the total weight of the negative electrode active material.

14. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode active material layer includes pores formed between negative electrode active material particles, and the pore volume of the negative electrode active material layer is 0.15 ml / g to 0.30 ml / g.

15. A lithium secondary battery, comprising: The negative electrode for a lithium secondary battery according to claim 1; and a positive electrode disposed opposite to the negative electrode for a lithium secondary battery.

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