Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode; and an electrolyte, in which the average resistance X1 of the positive electrode at room temperature and at a state of charge of 20% to 100% and the average resistance Y1 of the negative electrode at room temperature and at a state of charge of 20% to 100% satisfy the following expression (1), and the average resistance X2 of the positive electrode at room temperature and at a state of charge of 0% to 20% and the average resistance Y2 of the negative electrode at room temperature and at a state of charge of 0% to 20% satisfy the following expression (2): Expression (1): 0.6 Y1 < = X1 < = 0.9 Y1; expression (2): 2 * Y2 < = X2.

Description

BACKGROUND OF THE INVENTION

[0002] [CROSS-REFERENCE TO RELATED APPLICATIONS]

[0003] This application claims priority to Korean Patent Application No. 10-2023-0091706, filed on Jul. 14, 2023, the disclosure of which is incorporated herein by reference in its entirety. [TECHNICAL FIELD]

[0005] With the technological development of electric vehicles, mobile electronic devices, etc., the demand for lithium secondary batteries as an energy source is rapidly increasing. Among them, according to the recent technological development of electric vehicles, batteries with high energy density and high output are required.

[0006] Lithium secondary batteries with high energy density, such as those used in electric vehicles and power storage, may be easily exposed to an external high-temperature environment, and the temperature of the battery may increase due to instantaneous charging and discharging. In such a high-temperature environment, the battery life may be shortened, and the amount of stored energy may be reduced.

[0007] Therefore, there is a need to develop a high-capacity battery system that can improve the life characteristics at high temperatures as well as at room temperature.

[0008] Various factors may affect the life characteristics of lithium secondary batteries, but in addition to many other factors, it is necessary to design the positive electrode and the negative electrode in a balanced manner. Even when a secondary battery is prepared by selecting well-known positive electrode materials and negative electrode materials with good performance, problems of deteriorated life characteristics may occur in the absence of thermodynamic or electrochemical equilibrium between the positive electrode and the negative electrode. SUMMARY OF THE INVENTION

[0009] TECHNICAL PROBLEM

[0010] The inventors of the present invention sought to achieve thermodynamic or electrochemical equilibrium between the positive electrode and the negative electrode, thereby preventing rapid deterioration of the room-temperature life characteristics and high-temperature life characteristics of a lithium secondary battery including the positive electrode and the negative electrode.

[0011] TECHNICAL SOLUTION

[0012] [1] The present invention provides a lithium secondary battery including: a positive electrode; a negative electrode; and an electrolyte, wherein an average resistance X1 of the positive electrode at room temperature and a state of charge of 20% to 100% and an average resistance Y1 of the negative electrode at room temperature and a state of charge of 20% to 100% satisfy the following expression (1), and an average resistance X2 of the positive electrode at room temperature and a state of charge of 0% to 20% and an average resistance Y2 of the negative electrode at room temperature and a state of charge of 0% to 20% satisfy the following expression (2).

[0013] Expression (1): 0.6 × Y1 ≤ X1 ≤ 0.9 × Y1

[0014] Expression (2): 2 × Y2 ≤ X2

[0015] [2] The present invention can provide the lithium secondary battery of [1] above, wherein the average resistance X3 of the positive electrode at 40 °C and a state of charge of 20% to 100% and the average resistance Y3 of the negative electrode at 40 °C and a state of charge of 20% to 100% further satisfy the following Expression (3).

[0016] Expression (3): 0.75 × Y3 ≤ X3 ≤ Y3

[0017] [3] The present invention can provide the lithium secondary battery of [1] or [2] above, wherein the average resistance X4 of the positive electrode at 40 °C and a state of charge of 0% to 20% and the average resistance Y4 of the negative electrode at 40 °C and a state of charge of 0% to 20% further satisfy the following Expression (4).

[0018] Expression (4): 1.5 × Y4 ≤ X4

[0019] [4] The present invention can provide the lithium secondary battery of any one of [1] to [3] above, wherein the positive electrode includes a positive electrode active material layer and a positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder, and the positive electrode active material is a lithium nickel-based transition metal oxide, and the lithium nickel-based transition metal oxide contains a nickel content of 85 atm% or more in the transition metals other than lithium.

[0020] [5] The present invention can provide the lithium secondary battery of [4] above, wherein the positive electrode active material is a single particle or a particle-like single particle.

[0021] [6] The present invention can provide the lithium secondary battery of [4] or [5] above, wherein, based on the entire positive electrode active material layer, the conductive material is included in an amount of 0.1 wt% to 2.5 wt%.

[0022] [7] The present invention can provide the lithium secondary battery of any one of [4] to [6] above, wherein the positive electrode active material has a powder resistance of 100 Ω to 500 Ω.

[0023] [8] The present invention can provide the lithium secondary battery of any one of [1] to [7] above, wherein the negative electrode includes a negative electrode active material layer and a negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder, and the negative electrode active material includes a silicon-based negative electrode active material.

[0024] [9] The present invention can provide the lithium secondary battery of [8] above, wherein the silicon-based negative electrode active material is selected from SiO, SiC, and Si.

[0025]

[10] The present invention can provide the lithium secondary battery of [8] or [9] above, wherein, based on the entire negative electrode active material layer, the silicon-based negative electrode active material is included in an amount of 3 wt% or more.

[0026]

[11] The present invention can provide the lithium secondary battery of any one of [8] to

[10] above, wherein, based on the entire negative electrode active material layer, the conductive material is included in an amount of 0.05 wt% to 2.50 wt%.

[0027] Advantageous Effects

[0028] During the operation of the lithium secondary battery, the present inventors found that when both the positive electrode and the negative electrode are deteriorating, controlling the level of deterioration of the negative electrode relative to the level of deterioration of the positive electrode at a specific temperature and in a specific charging section can contribute to the stable operation of the lithium secondary battery.

[0029] When the lithium secondary battery operates, both the positive electrode and the negative electrode deteriorate, and by controlling the level of deterioration of the positive electrode and the negative electrode according to the charge / discharge region, the lithium secondary battery can have excellent long-life characteristics. In particular, by adjusting the resistance of the positive electrode / negative electrode in room temperature and high temperature environments to suppress the use of the negative electrode and inducing the positive electrode to deteriorate relatively further, the effect of excellent long-life characteristics can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Shows the results of measuring the resistance values of the positive electrode and the negative electrode of the lithium secondary battery according to Example 1 at 24 °C according to the state of charge.

[0031] Figure 2 Shows the results of measuring the resistance values of the positive electrode and the negative electrode of the lithium secondary battery according to Example 1 at 40 °C according to the state of charge.

[0032] Figure 3 Shows the results of measuring the resistance values of the positive electrode and the negative electrode of the lithium secondary battery according to Example 2 at 24 °C according to the state of charge.

[0033] Figure 4 Shows the results of measuring the resistance values of the positive electrode and the negative electrode of the lithium secondary battery according to Example 2 at 40 °C according to the state of charge.

[0034] Figure 5 Shows the results of measuring the resistance values of the positive electrode and the negative electrode of the lithium secondary battery according to Example 3 at 24 °C according to the state of charge.

[0035] Figure 6Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Example 3 at 40 °C according to the state of charge.

[0036] Figure 7 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 1 at 24 °C according to the state of charge.

[0037] Figure 8 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 1 at 40 °C according to the state of charge.

[0038] Figure 9 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 2 at 24 °C according to the state of charge.

[0039] Figure 10 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 2 at 40 °C according to the state of charge.

[0040] Figure 11 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 3 at 24 °C according to the state of charge.

[0041] Figure 12 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 3 at 40 °C according to the state of charge.

[0042] Figure 13 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 4 at 24 °C according to the state of charge.

[0043] Figure 14 Shows the results of measuring the resistance values of the positive and negative electrodes of the lithium secondary battery according to Comparative Example 4 at 40 °C according to the state of charge. Detailed Description

[0044] Hereinafter, the present invention will be described in more detail.

[0045] It will be understood that the terms or words used in the specification and claims should not be construed as having the meanings defined in a commonly used dictionary, and it will also be understood that based on the principle that the inventor can appropriately define the meanings of the terms or words to best explain the present invention, the terms or words should be construed as having meanings consistent with their meanings in the context of the relevant art and the technical concept of the present invention.

[0046] In the present invention, a "primary particle" refers to a particle unit that does not have grain boundaries in appearance when observed using a scanning electron microscope at a magnification of 5,000 to 20,000 times. The "average particle size of primary particles" refers to the arithmetic mean calculated from values obtained by measuring the particle sizes of primary particles observed from a scanning electron microscope image.

[0047] In the present invention, a "secondary particle" refers to a particle formed by aggregation of a plurality of primary particles. In the present invention, a secondary particle in which 10 or fewer primary particles are aggregated will be referred to as a pseudo single particle in order to distinguish it from a conventional secondary particle formed by aggregation of dozens to hundreds of primary particles.

[0048] In the present invention, "average particle size D 50 " refers to the particle size at 50% in the cumulative volume particle size distribution of the positive electrode active material powder. The average particle size D 50 can be measured by using the laser diffraction method. For example, after dispersing the positive electrode active material powder in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size measuring instrument (for example, Microtrac MT 3000), irradiated with ultrasonic waves of about 28 kHz with an output of 60 W, and then a cumulative-volume particle size distribution graph is obtained, and then the particle size corresponding to 50% of the cumulative volume can be calculated therefrom to measure D 50 .

[0049] The lithium secondary battery according to the present invention includes: a positive electrode; a negative electrode; and an electrolyte. The average resistance X1 of the positive electrode at room temperature and a charged state of 20% to 100% and the average resistance Y1 of the negative electrode at room temperature and a charged state of 20% to 100% satisfy the following expression (1), and the average resistance X2 of the positive electrode at room temperature and a charged state of 0% to 20% and the average resistance Y2 of the negative electrode at room temperature and a charged state of 20% to 100% satisfy the following expression (2).

[0050] Expression (1): 0.6×Y1 ≤ X1 ≤ 0.9×Y1

[0051] Expression (2): 2×Y2 ≤ X2

[0052] Room temperature refers to a constant temperature without heating or cooling, and the room temperature can be in the range of about 15°C to 25°C.

[0053] Charging and discharging of a lithium secondary battery occur as lithium ions move between a positive electrode and a negative electrode. When charging / discharging is performed, deterioration of the lithium secondary battery occurs due to side reactions with an electrolyte solution and structural collapse in the positive electrode and the negative electrode. At this time, most desirably, the positive electrode and the negative electrode deteriorate to a similar level. The positive electrode has a characteristic of rapidly deteriorating at a high voltage, and if the negative electrode contains a silicon-based active material, the negative electrode has a characteristic that the reaction of the silicon-based active material increases in a lower range of discharge (state of charge: 0% to 20%). In the lithium secondary battery of the present invention, deterioration of the positive electrode can be suppressed by making the resistance of the positive electrode slightly lower than that of the negative electrode when the state of charge is 20% to 100%, wherein a high voltage is applied in the case where the state of charge is 20% to 100%. In addition, in the lithium secondary battery of the present invention, the resistance of the positive electrode can be adjusted to be high when the state of charge is 0% to 20% which is a lower range of discharge, so as to prevent the potential of the negative electrode from rising during discharge, thereby suppressing deterioration of the negative electrode active material. Therefore, by minimizing the difference in the deterioration level between the positive electrode and the negative electrode, the life characteristics of the lithium secondary battery can be maximized.

[0054] The average resistance of the positive electrode and the average resistance of the negative electrode can be measured by using a first coin cell prepared from the positive electrode and a lithium metal counter electrode and a second coin cell prepared from the negative electrode and the lithium metal counter electrode, respectively. In particular, the first coin cell and the second coin cell can be prepared by the following process: detaching the positive electrode and the negative electrode from the prepared lithium secondary battery and connecting a lithium metal having a thickness of 200 μm to the counter electrode, and injecting an electrolyte containing a LiPF6 lithium salt and an organic solvent (a mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate) into the positive electrode and the negative electrode, and then aging for 24 hours.

[0055] In the above expression (1), X1 is the average resistance of the positive electrode at room temperature and when the state of charge is 20% to 100%, which means the average resistance value (Ω) in the region where the state of charge is 20% to 100% when the first coin cell is discharged at 1C at room temperature. In particular, X1 is the average resistance value for the region where the state of charge is 20% to 100%, and this average resistance value is calculated from the resistance values measured at room temperature by using a hybrid pulse power characteristic (HPPC) test according to the state of charge (SOC) when the first coin cell is fully charged and discharged at 1C.

[0056] In the above expression (1), Y1 is the average resistance of the negative electrode at room temperature and a state of charge of 20% to 100%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 20% to 100% when the second coin cell is discharged at 1C at room temperature. In particular, Y1 is the average resistance value for the region of a state of charge of 20% to 100%, and this average resistance value is calculated from the resistance values measured at room temperature using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the second coin cell is fully charged and discharged at 1C.

[0057] When the state of charge is 20% to 100%, due to the application of a high potential, the structural collapse in the positive electrode and the side reaction with the electrolyte solution are maximized. Therefore, the deterioration of the positive electrode proceeds rapidly, thereby reducing the capacity retention rate of the lithium secondary battery. In the lithium secondary battery of the present invention that satisfies the above expression (1) at room temperature, the positive electrode resistance and the negative electrode resistance can be adjusted to be at a similar level in the region of a state of charge of 20% to 100% to prevent the significant occurrence of overvoltage in the positive electrode during charging and to prevent the significant increase in the potential of the positive electrode during constant current (CC) charging, thereby reducing the structural collapse in the positive electrode material and the side reaction with the electrolyte solution.

[0058] In the above expression (2), X2 is the average resistance of the positive electrode at room temperature and a state of charge of 0% to 20%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 0% to 20% when the first coin cell is discharged at 1C at room temperature. In particular, X2 is the average resistance value for the region of a state of charge of 0% to 20%, and this average resistance value is calculated from the resistance values measured at room temperature using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the first coin cell is fully charged and discharged at 1C.

[0059] In the above expression (2), Y2 is the average resistance of the negative electrode at room temperature and a state of charge of 0% to 20%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 0% to 20% when the second coin cell is discharged at 1C at room temperature. In particular, Y2 is the average resistance value for the region of a state of charge of 0% to 20%, and this average resistance value is calculated from the resistance values measured at room temperature using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the second coin cell is fully charged and discharged at 1C.

[0060] The silicon-based negative electrode active material is highly activated when the state of charge is from 0% to 20%. If, during the discharge of a lithium secondary battery, the potential of the negative electrode significantly increases in the region where the state of charge is from 0% to 20%, the use of the silicon-based negative electrode active material increases, resulting in maximization of atomization and side reactions with the electrolyte solution, thereby deteriorating the performance of the lithium secondary battery. In the lithium secondary battery of the present invention that satisfies the above expression (2) at room temperature, by setting the resistance of the positive electrode to be higher than that of the negative electrode, during the discharge of the lithium secondary battery, the potential of the positive electrode can drop before the potential of the negative electrode rises, enabling the operation of the lithium secondary battery to be terminated quickly. Therefore, during discharge, the potential of the negative electrode in the region where the state of charge is from 0% to 20% is no longer high, thereby reducing the use of the silicon-based negative electrode active material, and thus suppressing the deterioration of the negative electrode, which can produce excellent life characteristics.

[0061] The lithium secondary battery according to the present invention can more preferably satisfy the following expression (2-1).

[0062] Expression (2-1): 2×Y2≤X2≤6.5×Y2

[0063] In the above expression (2-1), the definitions of X2 and Y2 are the same as those of X2 and Y2 in expression (2).

[0064] In addition, the lithium secondary battery according to the present invention can satisfy the following expression (A).

[0065] Expression (A): 0.9×Y1+Y2≤1.5×(X1+0.5×X2)

[0066] In the above expression (A), the definitions of X1, X2, Y1, and Y2 are the same as those of X1, X2, Y1, and Y2 in expressions (1) and (2).

[0067] In the lithium secondary battery according to the present invention, the average resistance X3 of the positive electrode at 40 °C and a state of charge of 20% to 100% and the average resistance Y3 of the negative electrode at 40 °C and a state of charge of 20% to 100% can satisfy the conditions of the following expression (3).

[0068] Expression (3): 0.75×Y3≤X3≤Y3

[0069] In the above expression (3), X3 is the average resistance of the positive electrode at 40 °C and a state of charge of 20% to 100%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 20% to 100% when the first coin cell is discharged at 1C at 40 °C. In particular, X3 is the average resistance value for the region of a state of charge of 20% to 100%, and this average resistance value is calculated from the resistance values measured at 40 °C using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the first coin cell is fully charged and discharged at 1C.

[0070] In the above expression (3), Y3 is the average resistance of the negative electrode at 40 °C and a state of charge of 20% to 100%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 20% to 100% when the second coin cell is discharged at 1C at 40 °C. Y3 is the average resistance value for the region of a state of charge of 20% to 100%, and this average resistance value is calculated from the resistance values measured at 40 °C using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the second coin cell is fully charged and discharged at 1C.

[0071] Since the lithium secondary battery of the present invention satisfies the above expression (3) at 40 °C, when the positive electrode resistance and the negative electrode resistance in the region of a state of charge of 20% to 100% are adjusted to be at a more similar level at 40 °C than at room temperature, the degradation of the positive electrode and the negative electrode can be adjusted to a similar level.

[0072] In the lithium secondary battery according to the present invention, the average resistance X4 of the positive electrode at 40 °C and a state of charge of 0% to 20% and the average resistance Y4 of the negative electrode at 40 °C and a state of charge of 0% to 20% may satisfy the conditions of the following expression (4).

[0073] Expression (4): 1.5 × Y4 ≤ X4

[0074] In the above expression (4), X4 is the average resistance of the positive electrode at 40 °C and a state of charge of 0% to 20%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 0% to 20% when the first coin cell is discharged at 1C at 40 °C. In particular, X4 is the average resistance value for the region of a state of charge of 0% to 20%, and this average resistance value is calculated from the resistance values measured at 40 °C using the Hybrid Pulse Power Characterization (HPPC) test according to the state of charge (SOC) when the first coin cell is fully charged and discharged at 1C.

[0075] In the above expression (4), Y4 is the average resistance of the negative electrode at 40 °C and a state of charge of 0% to 20%, which can be obtained by measuring the average resistance value (Ω) in the region of a state of charge of 0% to 20% when the second coin cell is discharged at 1C at 40 °C. In particular, Y4 is the average resistance value for the region of a state of charge of 0% to 20%, and this average resistance value is calculated from the resistance values measured at 40 °C using the hybrid pulse power characterization (HPPC) test according to the state of charge (SOC) when the second coin cell is fully charged and discharged at 1C.

[0076] The silicon-based negative electrode active material is highly activated at a state of charge of 0% to 20%. If, during the discharge of a lithium secondary battery, the potential of the negative electrode significantly increases in the region of a state of charge of 0% to 20%, the use of the silicon-based negative electrode active material increases, leading to maximization of atomization and side reactions with the electrolyte solution, thereby deteriorating the performance of the lithium secondary battery. By setting the resistance of the positive electrode to be higher than that of the negative electrode, during discharge, the potential of the positive electrode can drop before the potential of the negative electrode rises, enabling the operation of the lithium secondary battery to be terminated quickly. Therefore, since during discharge, the potential of the negative electrode in the region of a state of charge of 0% to 20% is no longer high, the use of the silicon-based negative electrode active material is reduced, and thus deterioration of the negative electrode is suppressed, which can result in excellent life characteristics. Under the 40 °C condition of expression (4), the resistance difference between the negative electrode and the positive electrode is smaller than that under the room temperature condition of expression (2), indicating that the resistance of the positive electrode may decrease slightly because the positive electrode deteriorates faster at high temperatures.

[0077] The lithium secondary battery according to the present invention can more preferably satisfy the following expression (4-1).

[0078] Expression (4-1): 1.5×Y4 ≤ X4 ≤ 6.5×Y4

[0079] In the above expression (4-1), X4 and Y4 are defined in the same way as X4 and Y4 in expression (4).

[0080] The lithium secondary battery according to the present invention can satisfy the following expression (B).

[0081] Expression (B): Y3 + 1.5×Y4 ≤ X3 + X4

[0082] In the above expression (B), X3, X4, Y3, and Y4 are defined in the same way as X3, X4, Y3, and Y4 in expressions (3) and (4).

[0083] The positive electrode of the present invention can have a structure in which a positive electrode active material layer is formed on one side or both sides of the positive electrode current collector, and the positive electrode active material layer can include a positive electrode active material, a conductive material, and a binder.

[0084] As the positive electrode current collector, various positive electrode current collectors used in the art can be used. For example, as the positive electrode current collector, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector generally can have a thickness of 3 μm to 500 μm, and microscopic irregularities can be formed on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material. The positive electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0085] Meanwhile, as the positive electrode active material, positive electrode active materials commonly used in the art can be used.

[0086] Preferably, the positive electrode active material can contain lithium nickel-based oxides, and in particular, can contain lithium nickel-based transition metal oxides, and the lithium nickel-based transition metal oxides contain a nickel content of 85 atm% or more or 90 atm% or more among the transition metals other than lithium. Preferably, the lithium nickel-based oxides can contain 90 mol% or more and less than 100 mol%, 93 mol% or more and less than 100 mol%, or 95 mol% or more and less than 100 mol% of Ni. When using the lithium nickel-based oxides with a high Ni content as described above, high capacity can be achieved.

[0087] More particularly, the positive electrode active material can contain lithium nickel-based oxides represented by the following [Chemical Formula 5].

[0088] [Chemical Formula 5]

[0089] Li a Ni b Co c M 1 d M 2 e O2

[0090] In the above Chemical Formula 5, M 1 can be Mn, Al or a combination thereof, and preferably is Mn or Mn and Al.

[0091] The above M 2 can be one or more selected from Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb. Preferably, it can be one or more selected from Zr, Y, Mg and Ti, and more preferably, can be Zr, Y or a combination thereof. The M 2 element is not necessarily included, but if included in an appropriate amount, it can be used to promote particle growth during calcination or improve crystal structure stability.

[0092] The a above refers to the molar ratio of lithium in the lithium nickel-based oxide, where 0.8 ≤ a ≤ 1.2, 0.85 ≤ a ≤ 1.15, or 0.9 ≤ a ≤ 1.2 can be satisfied. When the molar ratio of lithium falls within the above range, the crystal structure of the lithium nickel-based oxide can be stably formed.

[0093] The b above refers to the molar ratio of nickel in the total metals other than lithium in the lithium nickel-based oxide, where 0.85 ≤ b < 1, 0.9 ≤ b < 1, or 0.95 ≤ b < 0.98 can be satisfied. When the molar ratio of nickel falls within the above range, a high energy density can be achieved, and thus, a high capacity can be achieved.

[0094] The c above refers to the molar ratio of cobalt in the total metals other than lithium in the lithium nickel-based oxide, where 0 < c < 0.15, 0 < c < 0.1, or 0.02 ≤ c ≤ 0.07 can be satisfied. When the molar ratio of cobalt falls within the above range, good resistance characteristics and output characteristics can be achieved.

[0095] The d above refers to the molar ratio of the M 1 element in the total metals other than lithium in the lithium nickel-based oxide, where 0 < d < 0.15, 0 < d < 0.1, or 0.01 ≤ d ≤ 0.05 can be satisfied. When the molar ratio of the M 1 element falls within the above range, the structural stability of the positive electrode active material is excellent.

[0096] The e above refers to the molar ratio of the M 2 element in the total metals other than lithium in the lithium nickel-based oxide, where 0 ≤ e ≤ 0.1 or 0 ≤ e ≤ 0.01 can be satisfied.

[0097] Meanwhile, according to the present invention, the positive electrode active material may further include a coating on the surface of the particles of the lithium nickel-based oxide as needed, and the coating contains one or more coating elements selected from Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S. Preferably, the coating element may be Al, B, Co, or a combination thereof, and most preferably, the coating element may be B.

[0098] In the case where a coating exists on the surface of the particles of the lithium nickel-based oxide, the contact between the electrolyte and the lithium composite transition metal oxide can be suppressed by the coating, and thus, the effect of reducing the transition metal dissolution or gas generation caused by the side reaction with the electrolyte can be achieved.

[0099] Meanwhile, the positive electrode active material may have a form that is not particularly limited, including a secondary particle form in which a plurality of primary particles are aggregated, a single particle form composed of one primary particle, or a form that is a combination thereof.

[0100] Preferably, the positive electrode active material may include a positive electrode active material formed of single particles each composed of one primary particle and / or formed of pseudo single particles aggregated from 10 or fewer primary particles. By using a positive electrode active material formed of single particles each composed of one primary particle and / or formed of pseudo single particles aggregated from 10 or fewer primary particles as the positive electrode active material, a lithium secondary battery having excellent stability and providing a high capacity can be achieved.

[0101] Since the positive electrode active material in the form of single particles each composed of one primary particle or in the form of pseudo single particles in which 10 or fewer primary particles are aggregated has higher particle strength than the typical secondary particles in which dozens to hundreds of primary particles are aggregated, particle breakage hardly occurs during rolling. In addition, in the case of the positive electrode active material in the form of single particles or pseudo single particles, the number of primary particles constituting the particles is small, so that the change due to the volume expansion or contraction of the primary particles during charging and discharging is small, and thus the occurrence of internal cracks in the particles is significantly reduced.

[0102] Therefore, if the content of Ni in the positive electrode material is increased to achieve high energy, there is a problem of increasing the structural instability in the positive electrode. However, in the case of using a positive electrode active material in the form of single particles and / or pseudo single particles, the amount of gas generated due to particle breakage and the occurrence of internal cracks can be significantly reduced, and excellent stability can be achieved. However, in the case of using a positive electrode active material in the form of single particles having a high nickel content, there is a limitation in high resistance, so that it is necessary to adjust the resistance characteristics of the positive electrode and the negative electrode of the lithium secondary battery to satisfy the conditions of the above expressions (1) and (2).

[0103] In particular, the positive electrode active material may have a powder resistance of 100 Ω to 500 Ω, preferably 200 Ω to 400 Ω, and more preferably 300 Ω to 350 Ω. When the powder resistance falls within the above range, the level of deterioration can be adjusted to a level similar to that of the negative electrode.

[0104] Meanwhile, according to the present invention, the positive electrode active material in the form of single particles and / or pseudo single particles may have an average particle diameter D of 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less 50 , and the average particle diameter D 50 is, for example, 0.5 μm to 5 μm, preferably 1 μm to 5 μm, and more preferably 2 μm to 5 μm. When the average particle diameter D of the positive electrode active material 50 falls within the above range, the increase in resistance can be minimized.

[0105] Since the positive electrode active material in the form of single particles and / or single-particle-like has small grain boundaries between primary particles as paths for lithium ions to disperse inside the particles, there is a problem that the lithium mobility is lower than that of the positive electrode active material in the form of secondary particles, resulting in an increase in resistance. This increase in resistance is exacerbated as the particle size increases, and the increased resistance adversely affects the capacity and output characteristics. Therefore, in the present invention, by applying a positive electrode active material in the form of single particles and / or single-particle-like having a small average particle size D of 5 μm or less 50 , the distance for lithium ions to disperse inside the particles is minimized, thereby suppressing the increase in resistance.

[0106] The average particle size of the primary particles in the positive electrode active material in the form of single particles and / or single-particle-like can be 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less, and the average particle size is, for example, 0.5 μm to 5 μm, preferably 1 μm to 5 μm, more preferably 2 μm to 5 μm. When the average particle size of the primary particles falls within the above range, a positive electrode active material in the form of single particles and / or single-particle-like having excellent electrochemical characteristics can be formed. If the average particle size of the primary particles is too small, since the number of primary particles that aggregate to form the positive electrode active material becomes larger, the effect of suppressing particle breakage during rolling may be reduced, and if the average particle size of the primary particles is too large, since the path for lithium to disperse inside the primary particles becomes longer, the resistance may increase and the output characteristics may decrease.

[0107] In the present invention, the positive electrode active material in the form of single particles and / or single-particle-like preferably has a unimodal particle size distribution. Conventionally, in order to increase the electrode density of the positive electrode active material layer, a bimodal positive electrode active material is often used, in which a large-particle-size positive electrode active material having a large average particle size and a small-particle-size positive electrode active material having a small average particle size are mixed. However, in the case of a positive electrode active material in the form of single particles or single-particle-like, as the particle size increases, the movement path of lithium becomes longer, thereby significantly increasing the resistance, and therefore, if large-particle-size particles are used together, it may cause problems of reduced capacity and output characteristics. Therefore, in the present invention, by using a positive electrode active material having a unimodal distribution, the increase in resistance can be minimized.

[0108] Secondly, the conductive material is used to provide conductivity for the electrode, and any material can be used in the battery without particular limitation as long as the material does not cause chemical changes and conducts electrons. Specific examples can be: graphite, such as natural graphite or artificial graphite; carbon-based materials, which include carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, carbon nanotube, etc.; powders or fibers formed of metals including copper, nickel, aluminum, silver, etc.; conductive whiskers, which include zinc oxide, potassium titanate, etc.; conductive metal oxides, which include titanium oxide, etc.; or conductive polymers, which include polyphenylene derivatives, etc., and any one of them can be used alone or a mixture of two or more of them can be used.

[0109] Based on the total weight of the positive electrode active material layer, the conductive material can be included in an amount of 0.1 wt% to 2.5 wt%, preferably 0.5 wt% to 2.0 wt%, and more preferably 1.0 wt% to 1.8 wt%.

[0110] The binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector, and specific examples can include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one of them can be used alone or a mixture of two or more of them can be used.

[0111] Based on the total weight of the positive electrode active material layer, the binder can be included in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 10 wt%, and more preferably 1.5 wt% to 5.0 wt%.

[0112] The negative electrode of the present invention can have a structure in which a negative electrode active material layer is formed on one side or both sides of the negative electrode current collector, and the negative electrode active material layer can include a negative electrode active material, a conductive material, and a binder.

[0113] As the negative electrode current collector, those commonly used in the art can be used, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum cadmium alloy can be used. The negative electrode current collector generally can have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, microscopic irregularities can be formed on the surface of the current collector to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms including films, sheets, foils, meshes, porous bodies, foams, non-woven fibrous bodies, etc.

[0114] The negative electrode active material can include a silicon-based negative electrode active material, such as Si, Si-Me alloy (wherein, Me is one or more selected from Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (wherein, 0 < y < 2), and Si-C composite material, and most preferably, it can include a silicon-based negative electrode active material selected from SiO, SiC, and Si. The negative electrode containing the silicon-based negative electrode active material can have a high capacity. However, when a positive electrode with a high resistance is used together with a negative electrode containing a silicon-based negative electrode active material, there are the following limitations: due to the increased use of the negative electrode containing the silicon-based negative electrode active material at room temperature, the negative electrode deteriorates rapidly, resulting in a rapid deterioration of the performance of the lithium secondary battery. Therefore, it is necessary to adjust the resistance characteristics of the positive and negative electrodes of the lithium secondary battery to meet the conditions of the above expressions (1) and (2).

[0115] As the negative electrode active material other than the silicon-based negative electrode active material, compounds capable of reversibly inserting and extracting lithium can be used. Specific examples can include: carbon-based materials, including artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, etc.; lithium metal thin films; metal materials capable of forming alloys with lithium, including Sn, Al, etc.; and so on, and any one of them or a mixture of two or more of them can be used.

[0116] Based on the entire negative electrode active material layer, the silicon-based negative electrode active material can be included in an amount of 3 wt% or more, preferably 3 wt% to 10 wt%, and more preferably 3 wt% to 6 wt%.

[0117] The conductive material is used to provide conductivity to the negative electrode, and any material can be used in the battery without particular limitation as long as the material does not cause chemical changes and conducts electrons. Specific examples may include: graphite, such as natural graphite or artificial graphite; carbon-based materials, which include carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube, etc.; powders or fibers formed of metals including copper, nickel, aluminum, silver, etc.; conductive whiskers, which include zinc oxide, potassium titanate, etc.; conductive metal oxides, which include titanium oxide, etc.; or conductive polymers, which include polyphenylene derivatives, etc., and any one of them can be used alone or a mixture of two or more of them can be used.

[0118] Based on the total weight of the negative electrode active material layer, the conductive material can generally be included in an amount of 0.05 wt% to 2.50 wt%, preferably 0.5 wt% to 2.0 wt%, and more preferably 1.0 wt% to 1.5 wt%.

[0119] The binder is used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene monomer rubber (EPDM rubber), sulfonated - EPDM, styrene - butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one of them can be used alone or a mixture of two or more of them can be used.

[0120] Based on the total weight of the negative electrode active material layer, the binder can be included in an amount of 1 wt% to 10 wt%, preferably 1 wt% to 5 wt%, and more preferably 1.5 wt% to 3.0 wt%.

[0121] In addition to the positive electrode and the negative electrode, a separator is introduced between the positive electrode and the negative electrode to separate the negative electrode and the positive electrode and provide a path for the movement of lithium ions. Any separator can be used without particular limitation as long as it is commonly used as a separator in a lithium secondary battery. In particular, as the separator, a porous polymer membrane can be used, such as a porous polymer membrane prepared from a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers can be used. Additionally, ordinary porous non-woven fabrics can also be used, such as non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can also be used.

[0122] In addition, the electrolyte used in the present invention can include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten-type inorganic electrolyte, etc. The electrolyte can be used to prepare a lithium secondary battery, and its type is not particularly limited thereto.

[0123] In particular, the electrolyte can include an organic solvent and a lithium salt.

[0124] Any organic solvent can be used as the organic solvent without particular limitation as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. In particular, as the organic solvent, the following can be used: ester-based solvents, including methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether-based solvents, including dibutyl ether, tetrahydrofuran, etc.; ketone-based solvents, including cyclohexanone, etc.; aromatic hydrocarbon-based solvents, including benzene, fluorobenzene, etc.; carbonate-based solvents, including dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol-based solvents, including ethanol, isopropyl alcohol, etc.; nitriles, including R-CN (wherein R is a linear, branched or cyclic C2 to C20 hydrocarbon group and may include a double bond aromatic ring or an ether bond), etc.; amides, including dimethylformamide, etc.; dioxolanes, including 1,3-dioxolane, etc.; or sulfolane. Among them, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate having a high ionic conductivity and a high dielectric constant that can improve the charge and discharge performance of the battery (e.g., ethylene carbonate, propylene carbonate, etc.) and a low-viscosity linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) is more preferred.

[0125] Any compound capable of providing lithium ions used in a lithium secondary battery can be used as a lithium salt without particular limitation. In particular, the anion of the lithium salt can be selected from F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and at least one of (CF3CF2SO2)2N - , and LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. can be used for the lithium salt. The lithium salt can be used in a concentration range of 0.1 M to 2.0 M. When the concentration of the lithium salt falls within the above range, the electrolyte can have appropriate conductivity and viscosity, and thus, can exhibit excellent performance of the electrolyte, and lithium ions can move effectively.

[0126] To improve the life characteristics of the battery, suppress the reduction of the battery capacity, and improve the discharge capacity of the battery, in addition to the electrolyte components, at least one additive such as a halogenated alkylene carbonate compound, such as ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, N-glycol diamide, hexaphosphoric triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. can be further included in the electrolyte. At this time, based on the total weight of the electrolyte, the additive can be included in an amount of 0.1 wt% to 10 wt%.

[0127] The external shape of the lithium secondary battery of the present invention is not particularly limited, but can have a cylindrical shape, a prismatic shape, a pouch shape, a coin shape, etc. using a can.

[0128] The lithium secondary battery according to the present invention can be used not only in a battery cell serving as a power source for a small device, but also preferably as a unit cell in medium and large battery modules including a plurality of battery cells.

[0129] Examples of medium and large devices may include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc., but are not limited thereto.

[0130] Hereinafter, the present invention will be described in more detail by specific examples. However, the following examples are only for helping to understand the present invention, and the scope of the present invention is not limited thereto. It is obvious to those skilled in the art that various changes and modifications are possible within the scope and technical spirit of this specification, and such changes and modifications naturally fall within the scope of the appended claims.

[0131] Example

[0132] Example 1

[0133] Mix a positive electrode active material (LiNi 50 Co 0.958 Mn 0.015 O2) with a D of 10.8 μm, carbon nanotubes, and a PVDF binder in a weight ratio of 95.6:1.8:2.6 in N-methylpyrrolidone to prepare a positive electrode paste. Apply the positive electrode paste to both sides of an aluminum current collector sheet, then dry at 120 °C, and then press to prepare a positive electrode. 0.027

[0134] Mix a negative electrode active material (a mixture of natural graphite∶artificial graphite∶SiO with a weight ratio of 47.75:47.75:4.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a weight ratio of 95.95:1.05:1.70:1.30 in water to prepare a negative electrode paste. Apply the negative electrode paste to both sides of a copper current collector sheet, then dry at 150 °C, and then press to prepare a negative electrode.

[0135] Dissolve LiPF6 in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0136] ​A separator is introduced between the positive electrode and the negative electrode prepared as described above, and laminated in the order of separator / positive electrode / separator / negative electrode, and then the laminate is wound to prepare a wound electrode assembly. The electrode assembly prepared as described above is inserted into a cylindrical battery can. An electrolyte is injected into the battery can to prepare a 4680 battery cell, and the battery cell is activated to prepare a lithium secondary battery.

[0137] After the activation is terminated, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare a first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare a second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 1 The results are shown in.

[0138] In Figure 1 In the graph of, the average resistance value X1 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y1 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0139] In Figure 1 In the graph of, the average resistance value X2 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y2 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0140] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 2 The results are shown in.

[0141] In Figure 2 In the graph of, the average resistance value X3 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y3 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0142] In Figure 2 In the graph of, the average resistance value X4 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y4 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0143] Example 2

[0144] The positive electrode active material LiNi with D 50 being 8.5 μm 0.891 Co 0.066 Mn0.043 O2, carbon nanotubes, and a PVDF binder are mixed in N-methylpyrrolidone at a weight ratio of 95.95:1.50:2.55 to prepare a positive electrode paste. The positive electrode paste is applied to two sides of an aluminum current collector sheet, then dried at 120 °C, and then pressed to prepare a positive electrode.

[0145] The negative electrode active material (natural graphite), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are mixed in water at a weight ratio of 95.7:1.4:1.5:1.4 to prepare a negative electrode paste. The negative electrode paste is applied to two sides of a copper current collector sheet, then dried at 150 °C, and then pressed to prepare a negative electrode.

[0146] LiPF6 is dissolved in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0147] A separator is introduced between the positive electrode and the negative electrode prepared as described above, laminated in the order of separator / positive electrode / separator / negative electrode, and then the laminate is wound to prepare a wound-type electrode assembly. The electrode assembly prepared as described above is inserted into a cylindrical battery can. The electrolyte is injected into the battery can to prepare a 4680 battery cell, and the cell is activated to prepare a lithium secondary battery.

[0148] After the activation is terminated, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare a first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare a second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 3 The results are shown.

[0149] In Figure 3 In the graph of, the average resistance value X1 (Ω) of the first coin cell in the region of 20% to 100% state of charge and the average resistance value Y1 (Ω) of the second coin cell in the region of 20% to 100% state of charge are calculated and listed in Table 1 below.

[0150] In Figure 3 In the graph of, the average resistance value X2 (Ω) of the first coin cell in the region of 0% to 20% state of charge and the average resistance value Y2 (Ω) of the second coin cell in the region of 0% to 20% state of charge are calculated and listed in Table 1 below.

[0151] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 4 The results are shown.

[0152] In Figure 4 the curve graph of, the average resistance value X3 (Ω) of the first coin cell in the region of 20% to 100% state of charge and the average resistance value Y3 (Ω) of the second coin cell in the region of 20% to 100% state of charge are calculated and listed in Table 1 below.

[0153] In Figure 4 the curve graph of, the average resistance value X4 (Ω) of the first coin cell in the region of 0% to 20% state of charge and the average resistance value Y4 (Ω) of the second coin cell in the region of 0% to 20% state of charge are calculated and listed in Table 1 below.

[0154] Example 3

[0155] Mix the positive electrode active material (LiNi 50 Co 0.962 Mn 0.029 O2) with a D 0.009 of 11.2 μm, carbon nanotubes, and PVDF binder in a weight ratio of 95.69:1.30:3.01 in N-methylpyrrolidone to prepare a positive electrode paste. Apply the positive electrode paste to both sides of an aluminum current collector sheet, then dry at 120 °C, and then press to prepare a positive electrode.

[0156] Mix the negative electrode active material (a mixture of natural graphite∶artificial graphite∶SiO with a weight ratio of 46.75:46.75:6.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a weight ratio of 95.625:1.075:1.900:1.400 in water to prepare a negative electrode paste. Apply the negative electrode paste to both sides of a copper current collector sheet, then dry at 150 °C, and then press to prepare a negative electrode.

[0157] Dissolve LiPF6 in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0158] Introduce a separator between the positive electrode and the negative electrode prepared as described above, laminate in the order of separator / positive electrode / separator / negative electrode, and then wind the laminate to prepare a wound electrode assembly. Insert the electrode assembly prepared as described above into a cylindrical battery can. Inject the electrolyte into the battery can to prepare a 4680 cell, and activate the cell to prepare a lithium secondary battery.

[0159] After activation termination, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare the first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare the second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 5 The results are shown in

[0160] In Figure 5 In the graph of , the average resistance value X1 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y1 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0161] In Figure 5 In the graph of , the average resistance value X2 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y2 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0162] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 6 The results are shown in

[0163] In Figure 6 In the graph of , the average resistance value X3 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y3 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0164] In Figure 6 In the graph of , the average resistance value X4 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y4 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0165] Comparative Example 1

[0166] The positive electrode active material (LiNi 50 with D 0.962 Co 0.029 Mn 0.009 O2), carbon nanotubes, and PVDF binder are mixed in N-methylpyrrolidone at a weight ratio of 95.19:1.80:3.01 to prepare a positive electrode slurry. The positive electrode slurry is applied to both sides of an aluminum current collector sheet, then dried at 120 °C, and then pressed to prepare a positive electrode.

[0167] The negative electrode active material (a mixture of natural graphite∶artificial graphite∶SiO with a weight ratio of 46.75:46.75:6.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are mixed in water at a weight ratio of 95.625:1.0750:1.900:1.400 to prepare a negative electrode paste. The negative electrode paste is applied to both sides of a copper current collector sheet, then dried at 150 °C, and then pressed to prepare a negative electrode.

[0168] LiPF6 is dissolved in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0169] A separator is introduced between the positive electrode and the negative electrode prepared as described above, laminated in the order of separator / positive electrode / separator / negative electrode, and then the laminate is wound to prepare a wound-type electrode assembly. The electrode assembly prepared as described above is inserted into a cylindrical battery can. The electrolyte is injected into the battery can to prepare a 4680 cell, and the cell is activated to prepare a lithium secondary battery.

[0170] After the activation is terminated, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare a first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare a second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 7 The results are shown.

[0171] In Figure 7 the graph of, the average resistance value X1 (Ω) of the first coin cell in the region of 20% to 100% state of charge and the average resistance value Y1 (Ω) of the second coin cell in the region of 20% to 100% state of charge are calculated and listed in Table 1 below.

[0172] In Figure 7 the graph of, the average resistance value X2 (Ω) of the first coin cell in the region of 0% to 20% state of charge and the average resistance value Y2 (Ω) of the second coin cell in the region of 0% to 20% state of charge are calculated and listed in Table 1 below.

[0173] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 8 The results are shown.

[0174] In Figure 8In the curve graph, the average resistance value X3 (Ω) of the first coin cell in the region where the state of charge is from 20% to 100% and the average resistance value Y3 (Ω) of the second coin cell in the region where the state of charge is from 20% to 100% were calculated and listed in Table 1 below.

[0175] In Figure 8 the curve graph, the average resistance value X4 (Ω) of the first coin cell in the region where the state of charge is from 0% to 20% and the average resistance value Y4 (Ω) of the second coin cell in the region where the state of charge is from 0% to 20% were calculated and listed in Table 1 below.

[0176] Comparative Example 2

[0177] Mix the cathode active material (LiNi 50 Co 0.962 Mn 0.029 O2) with a particle size of 3.8 μm, carbon nanotubes, and PVDF binder in a weight ratio of 95.19:1.80:3.01 in N-methylpyrrolidone to prepare a cathode slurry. Apply the cathode slurry to both sides of an aluminum current collector sheet, then dry at 120 °C, and then press to prepare a cathode.

[0178] Mix the anode active material (a mixture of natural graphite∶artificial graphite∶SiO with a weight ratio of 44.75:44.75:10.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a weight ratio of 95.625:1.075:1.900:1.400 in water to prepare an anode slurry. Apply the anode slurry to both sides of a copper current collector sheet, then dry at 150 °C, and then press to prepare an anode.

[0179] Dissolve LiPF6 in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0180] Introduce a separator between the cathode and anode prepared as described above, laminate them in the order of separator / cathode / separator / anode, and then wind the laminate to prepare a wound electrode assembly. Insert the electrode assembly prepared as described above into a cylindrical battery can. Inject the electrolyte into the battery can to prepare a 4680 cell, and activate the cell to prepare a lithium secondary battery.

[0181] Figure 9 ​After activation termination, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare the first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare the second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 9 .

[0182] In the Figure 9 graph, the average resistance value X1 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y1 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0183] In the Figure 9 graph, the average resistance value X2 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y2 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0184] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 10 .

[0185] In the Figure 10 graph, the average resistance value X3 (Ω) of the first coin cell in the region where the state of charge is 20% to 100% and the average resistance value Y3 (Ω) of the second coin cell in the region where the state of charge is 20% to 100% are calculated and listed in Table 1 below.

[0186] In the Figure 10 graph, the average resistance value X4 (Ω) of the first coin cell in the region where the state of charge is 0% to 20% and the average resistance value Y4 (Ω) of the second coin cell in the region where the state of charge is 0% to 20% are calculated and listed in Table 1 below.

[0187] Comparative Example 3

[0188] A positive electrode active material (LiNi 50 Co 0.962 Mn 0.029 O2) with D 0.009 of 11.2 μm, carbon nanotubes, and a PVDF binder are mixed in a weight ratio of 95.6:1.8:2.6 in N-methylpyrrolidone to prepare a positive electrode slurry. The positive electrode slurry is applied to both sides of an aluminum current collector sheet, then dried at 120 °C, and then pressed to prepare a positive electrode.

[0189] The negative electrode active material (a mixture of natural graphite∶artificial graphite∶SiO with a weight ratio of 44.75:44.75:10.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) are mixed in water at a weight ratio of 95.625:1.075:1.900:1.400 to prepare a negative electrode slurry. The negative electrode slurry is applied to two sides of a copper current collector sheet, then dried at 150 °C, and then pressed to prepare a negative electrode.

[0190] LiPF6 is dissolved in an organic solvent (ethylene carbonate (EC)∶ethyl methyl carbonate (EMC)∶dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0191] A separator is introduced between the positive electrode and the negative electrode prepared as described above, laminated in the order of separator / positive electrode / separator / negative electrode, and then the laminate is wound to prepare a wound electrode assembly. The electrode assembly prepared as described above is inserted into a cylindrical battery can. The electrolyte is injected into the battery can to prepare a 4680 cell, and the cell is activated to prepare a lithium secondary battery.

[0192] After the activation is terminated, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare a first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare a second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the results are shown in Figure 11 The results are shown.

[0193] In Figure 11 In the graph of [], the average resistance value X1 (Ω) of the first coin cell in the region of 20% to 100% state of charge and the average resistance value Y1 (Ω) of the second coin cell in the region of 20% to 100% state of charge are calculated and listed in Table 1 below.

[0194] In Figure 11 In the graph of [], the average resistance value X2 (Ω) of the first coin cell in the region of 0% to 20% state of charge and the average resistance value Y2 (Ω) of the second coin cell in the region of 0% to 20% state of charge are calculated and listed in Table 1 below.

[0195] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the results are shown in Figure 12 The results are shown.

[0196] In Figure 12In the curve graph, the average resistance value X3 (Ω) of the first coin cell in the region of 20% to 100% state of charge and the average resistance value Y3 (Ω) of the second coin cell in the region of 20% to 100% state of charge are calculated and listed in Table 1 below.

[0197] In Figure 12 the curve graph, the average resistance value X4 (Ω) of the first coin cell in the region of 0% to 20% state of charge and the average resistance value Y4 (Ω) of the second coin cell in the region of 0% to 20% state of charge are calculated and listed in Table 1 below.

[0198] Comparative Example 4

[0199] Mix the positive electrode active material (LiNi 50 with a particle size of 4.5 μm, Co 0.962 Mn 0.029 O2), carbon nanotubes and PVDF binder in a weight ratio of 95.69:1.30:3.01 in N-methylpyrrolidone to prepare a positive electrode paste. Apply the positive electrode paste to both sides of an aluminum current collector sheet, then dry at 120 °C, and then press to prepare a positive electrode. 0.009 Mix the negative electrode active material (a mixture of natural graphite and SiO with a weight ratio of 93.5:6.5), super carbon, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in a weight ratio of 95.625:1.075:1.900:1.400 in water to prepare a negative electrode paste. Apply the negative electrode paste to both sides of a copper current collector sheet, then dry at 150 °C, and then press to prepare a negative electrode.

[0200] Dissolve LiPF6 in an organic solvent (ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) with a volume ratio of 20:5:75) to 1.3 M to prepare an electrolyte.

[0201] Introduce a separator between the positive electrode and the negative electrode prepared as described above, laminate in the order of separator / positive electrode / separator / negative electrode, and then wind the laminate to prepare a wound electrode assembly. Insert the electrode assembly prepared as described above into a cylindrical battery can. Inject the electrolyte into the battery can to prepare a 4680 cell, and activate the cell to prepare a lithium secondary battery.

[0202]

[0203] ​After the activation is terminated, the positive electrode and the negative electrode are separated from the lithium secondary battery. The separated positive electrode and the lithium metal counter electrode are used to prepare the first coin cell, and the separated negative electrode and the lithium metal counter electrode are used to prepare the second coin cell. When the prepared first coin cell and second coin cell are discharged at 1C at 24 °C, the resistance is measured, and the result is shown in Figure 13 The results are shown in

[0204] In Figure 13 In the graph of

[0205] In Figure 13 In the graph of

[0206] In addition, when the prepared first coin cell and second coin cell are discharged at 1C at 40 °C, the resistance is measured, and the result is shown in Figure 14 The results are shown in

[0207] In Figure 14 In the graph of

[0208] In Figure 14 In the graph of

[0209] [Table 1]

[0210]

[0211]

[0212] Experimental Example 1 - Evaluation of Life Characteristics at Room Temperature

[0213] The life characteristics of the 4680 cells according to Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated at room temperature.

[0214] Specifically, each of the 4680 cells according to Examples 1 to 3 and Comparative Examples 1 to 4 was charged and discharged 70 times. Here, 1 cycle means charging at a constant current of 0.5C to 4.2V at 24°C and discharging at a constant current of 0.5C to 2.5V, and then the capacity retention rate after 70 cycles compared to the initial capacity was measured. The results are listed in Table 2 below.

[0215] [Table 2]

[0216] Capacity retention rate (%) Example 1 97.4 Example 2 95.4 Example 3 92.4 Comparative Example 1 90.3 Comparative Example 2 86.4 Comparative Example 3 87.2 Comparative Example 4 77.4

[0217] As listed in Table 2 above, it can be observed that the lithium secondary batteries according to Examples 1 to 3 that satisfy Expression (1) and Expression (2) have more excellent life characteristics at room temperature than the lithium secondary batteries according to Comparative Examples 1 to 4 that do not satisfy any one or more of Expression (1) and Expression (2).

[0218] Experimental Example 2 - Evaluation of Life Characteristics at High Temperature

[0219] The life characteristics of the 4680 cells according to Examples 1 to 3 and Comparative Examples 1 to 4 were evaluated at high temperature.

[0220] Specifically, each of the 4680 cells according to Examples 1 to 3 and Comparative Examples 1 to 4 was charged and discharged 70 times. Here, 1 cycle means charging at a constant current of 0.5C to 4.2V at 40°C and discharging at a constant current of 0.5C to 2.5V, and then the capacity retention rate after 70 cycles compared to the initial capacity was measured. The results are listed in Table 3 below.

[0221] [Table 3]

[0222] Capacity retention rate (%) Example 1 96.5 Example 2 95.7 Example 3 94.1 Comparative Example 1 93.7 Comparative Example 2 92.9 Comparative Example 3 93.9 Comparative Example 4 86.2

[0223] As listed in Table 3 above, it can be observed that the lithium secondary batteries according to Examples 1 to 3 that satisfy Expression (1) and Expression (2) have more excellent life characteristics at high temperature than the lithium secondary batteries according to Comparative Examples 1 to 4 that do not satisfy any one or more of Expression (1) and Expression (2).

Claims

1. A lithium secondary battery, comprising: a positive electrode; a negative electrode; and an electrolyte, wherein an average resistance X1 of the positive electrode at room temperature and a state of charge of 20% to 100% and an average resistance Y1 of the negative electrode at room temperature and a state of charge of 20% to 100% satisfy the following expression (1), and an average resistance X2 of the positive electrode at room temperature and a state of charge of 0% to 20% and an average resistance Y2 of the negative electrode at room temperature and a state of charge of 0% to 20% satisfy the following expression (2): Expression (1): 0.6×Y1 ≤ X1 ≤ 0.9×Y1 Expression (2): 2×Y2 ≤ X2.

2. The lithium secondary battery according to claim 1, wherein, The average resistance X3 of the positive electrode at 40 °C and a state of charge of 20% to 100% and the average resistance Y3 of the negative electrode at 40 °C and a state of charge of 20% to 100% further satisfy the following expression (3): Expression (3): 0.75×Y3 ≤ X3 ≤ Y3.

3. The lithium secondary battery according to claim 1, wherein, The average resistance X4 of the positive electrode at 40 °C and a state of charge of 0% to 20% and the average resistance Y4 of the negative electrode at 40 °C and a state of charge of 0% to 20% further satisfy the following expression (4): Expression (4): 1.5×Y4 ≤ X4.

4. The lithium secondary battery according to claim 1, wherein, The positive electrode includes a positive electrode active material layer and a positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder, and the positive electrode active material is a lithium nickel-based transition metal oxide, and the lithium nickel-based transition metal oxide contains a nickel content of 85 atm% or more among transition metals other than lithium.

5. The lithium secondary battery according to claim 4, wherein, The positive electrode active material is a single particle or similar to a single particle.

6. The lithium secondary battery according to claim 4, wherein, Based on the entire positive electrode active material layer, the conductive material is included in an amount of 0.1 wt% to 2.5 wt%.

7. The lithium secondary battery according to claim 4, wherein, The positive electrode active material has a powder resistance of 100 Ω to 500 Ω.

8. The lithium secondary battery according to claim 1, wherein, The negative electrode includes a negative electrode active material layer and a negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, a conductive material, and a binder, and the negative electrode active material includes a silicon-based negative electrode active material.

9. The lithium secondary battery according to claim 8, wherein, The silicon-based negative electrode active material is selected from SiO, SiC, and Si.

10. The lithium secondary battery according to claim 8, wherein, Based on the entire negative electrode active material layer, the silicon-based negative electrode active material is included in an amount of 3 wt% or more.

11. The lithium secondary battery according to claim 8, wherein, Based on the entire negative electrode active material layer, the conductive material is included in an amount of 0.05 wt% to 2.50 wt%.

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