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

Abstract

A positive electrode for a lithium secondary battery according to an exemplary embodiment includes a lithium metal oxide containing nickel, and has a strain (Q) satisfying a predetermined relational expression. The strain is the maximum value of the lattice strain measured within a prescribed voltage range, and the lattice strain can be calculated from the XRD pattern of the positive electrode. The present invention can provide a positive electrode having improved structural stability, high-temperature stability, and cycle characteristics, and having high capacity and high power characteristics.

Description

Technical Field

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. More specifically, the present invention relates to a positive electrode for a lithium secondary battery including a transition metal oxide and a lithium secondary battery including the same. 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 as power sources for portable electronic communication devices such as camcorders, mobile phones, and laptop computers. In addition, in recent years, battery packs including secondary batteries are being developed and used as power sources for eco-friendly vehicles such as hybrid vehicles.

[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, and are advantageous for charging speed and weight reduction, and thus are actively studied and developed.

[0004] A lithium secondary battery may include an electrode assembly and an electrolyte impregnating the electrode assembly. The electrode assembly includes a positive electrode, a negative electrode, and a separator (separation membrane). The lithium secondary battery may further include an exterior material that houses the electrode assembly and the electrolyte, such as an exterior material in the form of a pouch.

[0005] Lithium metal oxides can be used as positive electrode active materials for lithium secondary batteries. As the application range of lithium secondary batteries expands, longer life, higher capacity, and higher energy density are required. However, when the energy density, capacity, and power of the lithium metal oxide are increased, the stability and capacity retention rate of the lithium metal oxide may decrease.

[0006] For example, Korean Patent Publication No. 10-2017-0093085 discloses a positive electrode active material including a transition metal compound. Summary of the Invention

[0007] (I) Technical Problem to be Solved

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

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

[0010] (II) Technical Solution

[0011] The positive electrode for a lithium secondary battery according to an exemplary embodiment may include a lithium metal oxide. In the lithium metal oxide, x moles of nickel (Ni) are contained relative to 1 mole of the total metal excluding lithium, and the positive electrode for the lithium secondary battery may have a strain (Q) satisfying the following Equation 1.

[0012] [Equation 1]

[0013] 11.5x - 7.9 ≤ Q ≤ 11.5x - 7.5

[0014] In Equation 1, x may be the content of the nickel, and Q may be the maximum value of the lattice strain measured according to the voltage in the voltage range of 3.0 V to 4.3 V for a half-cell including the positive electrode for the lithium secondary battery and a lithium counter electrode.

[0015] The lattice strain may be 1 / 4 of the slope of a straight line obtained by plotting the diffraction angle θ (in radians (rad)) and the full width at half maximum β (in radians) of the XRD peak of the lithium metal oxide obtained by XRD analysis at each measurement voltage on a coordinate plane with sinθ on the horizontal axis and βcosθ on the vertical axis.

[0016] In some embodiments, the lattice strain may be calculated respectively using the following Equation 2.

[0017] [Equation 2]

[0018] βcosθ = 4ηsinθ + 0.9λ / D

[0019] In Equation 2, β may be the full width at half maximum of the XRD peak, θ may be the diffraction angle of the XRD peak, λ may be the wavelength (Å) of the X-ray used in the XRD analysis, D may be the crystallite size (Å) of the lithium metal oxide, and η may be the lattice strain (dimensionless number).

[0020] In some embodiments, the content (x) of the nickel may be 0.8 moles or more per 1 mole of the total metal excluding lithium.

[0021] In some embodiments, the lithium metal oxide may include a compound represented by the following Chemical Formula 1.

[0022] [Chemical Formula 1]

[0023] Li a Ni x M b O2

[0024] In the chemical formula 1, 0.8≤x≤0.94, 0.95≤a≤1.05, and 0.06≤b≤0.2, and M may be at least one selected from B, Al, Ti, V, Mn, Co, Zn, Y, Nb, Zr, Mo, Sn, Mg, Sr, Ba, and W.

[0025] In one embodiment, in the chemical formula 1, the molar ratio (x) of nickel may be from 0.83 to 0.94.

[0026] In some embodiments, the lattice strain may be a value measured at a time resolution of 12.5 minutes / scan in a voltage range of 3.0V to 4.3V.

[0027] In some embodiments, the maximum value (Q) in the lattice strain may be measured at a voltage less than 4.3V.

[0028] In one embodiment, the maximum value (Q) in the lattice strain may be measured at a voltage above 4.0V and less than 4.3V.

[0029] In some embodiments, the lattice strain may be a value measured while charging or discharging the half-cell at a C-rate of 0.1C in a voltage range of 3.0V to 4.3V.

[0030] In some embodiments, the lattice strain may be obtained by plotting the diffraction angle (θ) and full width at half maximum (β) of all XRD peaks appearing in the 2θ ranges of 15° to 20°, 35° to 40°, 45° to 50°, 55° to 60°, and 65° to 70° on the coordinate plane.

[0031] In some embodiments, the lattice strain may be obtained by plotting the diffraction angle (θ) and full width at half maximum (β) of the XRD peaks of the (003) plane, (101) plane, (105) plane, (107) plane, and (113) plane of the lithium metal oxide on the coordinate plane.

[0032] In some embodiments, the lithium metal oxide may have a layered crystal structure with the R-3m space group.

[0033] In some embodiments, the lithium metal oxide may further contain cobalt (Co) and manganese (Mn).

[0034] The lithium secondary battery according to an exemplary embodiment includes: the positive electrode for the lithium secondary battery described above; and a negative electrode disposed opposite to the positive electrode.

[0035] (III) Beneficial Effects

[0036] The positive electrode for a lithium secondary battery according to an exemplary embodiment includes a lithium metal oxide containing nickel, and the concentration of nickel has a specified relationship with the strain value of the positive electrode. Accordingly, a lithium secondary battery having high structural stability and high thermal stability while not reducing power and capacity can be provided.

[0037] The strain value of the positive electrode can be calculated as the maximum value of the lattice strain according to the voltage. Accordingly, the change in the lattice structure of the positive electrode according to the voltage range can be reduced, and the structural collapse of the positive electrode and the generation of gas due to charge and discharge can be prevented.

[0038] The lattice strain can be measured by XRD analysis while charging or discharging a half-cell. Accordingly, the strain of the lattice or crystal occurring during charging or discharging can be predicted, and by adjusting the strain value obtained thereby, the operation stability of the lithium secondary battery over the entire driving voltage range can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 and Figure 2 are a schematic plan view and a schematic cross-sectional view, respectively, showing a lithium secondary battery according to an exemplary embodiment.

[0040] Figure 3 is a view showing an XRD pattern of the positive electrode prepared according to Example 2.

[0041] Figure 4 is a view showing a plot according to Equation 2 of the positive electrode prepared according to Example 2.

[0042] Figure 5 is a view showing the crystallite size and lattice strain of the positive electrode prepared according to Example 2 according to the charging voltage. DETAILED DESCRIPTION

[0043] Embodiments of the present invention provide a positive electrode active material including a lithium metal oxide, a positive electrode including the positive electrode active material, and a lithium secondary battery.

[0044] Hereinafter, embodiments of the present invention will be described in detail. However, these are merely exemplary embodiments, and the present invention is not limited to the specific embodiments described exemplarily.

[0045] <Positive Electrode for Lithium Secondary Battery>

[0046] The positive electrode for a lithium secondary battery according to an exemplary embodiment (hereinafter, may be simply referred to as "positive electrode") may include a positive electrode active material for a lithium secondary battery (hereinafter, may be simply referred to as "positive electrode active material"). For example, the positive electrode may include a positive electrode active material layer formed by coating the positive electrode active material on a positive electrode current collector.

[0047] The positive electrode active material may include a compound that allows lithium ions to be reversibly intercalated and deintercalated.

[0048] In an exemplary embodiment, the positive electrode active material may include a lithium metal oxide. For example, the lithium metal oxide may include nickel (Ni), and may further include other transition metals.

[0049] According to an exemplary embodiment, the positive electrode may have a strain value represented by the following Formula 1.

[0050] [Formula 1]

[0051] 11.5x - 7.9 ≤ Q ≤ 11.5x - 7.5

[0052] In Formula 1, x is the content of the nickel. For example, x may be the content (x moles) of nickel relative to 1 mole of the total metals other than lithium in the lithium metal oxide. The terms "content" or "concentration" used in the present invention may represent the number of moles or molar ratio of any metal in the lithium metal oxide.

[0053] Q may be the maximum value of the lattice strain measured according to the voltage in the voltage range of 3.0 V to 4.3 V for a half-cell (coin-type half cell) including the positive electrode for the lithium secondary battery and a lithium counter electrode.

[0054] In one embodiment, the half-cell may be a 2032-type battery using the lithium metal oxide as the positive electrode and lithium metal as the negative electrode.

[0055] The lattice strain may be 1 / 4 of the slope of a straight line obtained by plotting the diffraction angle θ (radians) and the full width at half maximum β (radians) of the XRD peak of the lithium metal oxide obtained by XRD analysis at each measurement voltage on a coordinate plane with sin θ on the horizontal axis and βcos θ on the vertical axis.

[0056] Figure 3 and Figure 4 is an exemplary graph showing the application of the Williamson-Hall method to the values obtained by XRD analysis in an exemplary embodiment.

[0057] Figure 3 is a graph showing the XRD pattern of the positive electrode. Referring to Figure 3 , the XRD peak can be obtained by X-ray diffraction (XRD) analysis.

[0058] According to an exemplary embodiment, the lattice strain of the positive electrode can be obtained from the XRD profile of the positive electrode measured by XRD analysis.

[0059] For example, the lattice strain can be defined as 1 / 4 of the slope of a straight line plotted with sinθ on the horizontal axis and βcosθ on the vertical axis. The β can be the full width at half maximum (FWHM) (in radians) of the corresponding peak obtained by XRD analysis, and θ can be the diffraction angle (in radians) of the corresponding peak.

[0060] In some embodiments, the β can use the full width at half maximum that corrects the value from the device. In one embodiment, Si can be used as a standard substance to reflect the value from the device. In this case, by plotting the full width at half maximum curve of Si over the entire 2θ range, the full width at half maximum from the device can be expressed as a function of 2θ. Then, the value corrected by subtracting the full width at half maximum value from the device at the corresponding 2θ obtained from the function can be used as β.

[0061] For example, the XRD analysis can use Cu-Kα rays with a Ka1 wavelength of 1.540598 Å as a light source for the positive electrode in the 2θ range of 10 o to 75 o .

[0062] According to an exemplary embodiment, the lattice strain (η) can be calculated based on the XRD profile of the positive electrode measured by XRD analysis and using Equation 2 below. Equation 2 is an equation derived from the Williamson-Hall equation.

[0063] [Equation 2]

[0064] βcosθ = 4ηsinθ + 0.9λ / D

[0065] In Equation 1, β represents the full width at half maximum of the corresponding peak obtained by XRD analysis, θ represents the diffraction angle (in radians), η represents the lattice strain (dimensionless number), λ represents the X-ray wavelength (in Å), and D represents the crystallite size (in Å).

[0066] Figure 4 is a diagram showing the positive electrode according to Equation 2.

[0067] Referring to Figure 4 , substituting the measured values calculated from the XRD profile of the positive electrode into Equation 2, the slope (4η) is obtained through linear regression analysis, and thus the lattice strain (η) can be calculated.

[0068] The lattice strain can be measured in each voltage interval within the voltage range of 3.0 V to 4.3 V. For example, while charging or discharging the half-cell within the range of 3.0 V to 4.3 V, the XRD pattern of the positive electrode can be measured with a time resolution of 12.5 minutes per scan, and the lattice strain value according to the voltage can be obtained from the measured XRD pattern and the voltage at the time point of measuring the XRD pattern.

[0069] In one embodiment, the lattice strain can be measured at intervals of 0.1 V or at intervals of 0.3 V, 0.2 V, 0.05 V, 0.01 V, 0.005 V, 0.001 V within the voltage range of 3.0 V to 4.3 V.

[0070] The lattice strain may refer to the internal stress in the structure generated when lithium enters and exits the lithium metal oxide particles during charge and discharge.

[0071] For example, when releasing a part of lithium, a nano-domain with a normal vector inclined by less than 1 degree ( o ) with respect to the normal vector of the main domain is formed within the crystal structure, thereby possibly generating stress.

[0072] In addition, as the charge and discharge are repeated and the lithium content in each domain within the crystal structure changes, each domain may have a different lattice constant. Therefore, domains with different lattice constants from each other may be adjacent, thereby possibly generating stress.

[0073] According to an exemplary embodiment, the maximum value (Q) of the lattice strain measured by the above method satisfies the formula 1, so the stability of the crystal structure within the positive electrode can be improved. Therefore, the particle crack phenomenon at the boundary region between grains or particles can be prevented. Therefore, the generation of gas in a high-temperature environment and / or the generation of gas during charge and discharge can be prevented, thereby improving the life characteristics of the secondary battery.

[0074] According to an exemplary embodiment, the content (x) of nickel in the lithium metal oxide may be 0.8 moles or more in 1 mole of the total metal excluding lithium.

[0075] For example, Ni can improve the power and capacity of the lithium secondary battery. Therefore, by adopting a high content of nickel (0.80 ≤ x) in the lithium metal oxide, a positive electrode and a lithium secondary battery with higher power and higher capacity can be provided.

[0076] However, when the content of Ni increases, the lattice size may change according to the voltage range during the charge and discharge of the lithium secondary battery. Therefore, the growth of microstrain within the crystal structure may increase, and the microstrain may cause the desorption of oxygen within the structure.

[0077] For example, in a high-Ni composition where the content of Ni in the transition metal is 80 mol% or more, a significant phase change from H2 (hexagonal) to H3 (hexagonal) may occur according to the change in the voltage range. Therefore, the lattice constant and lattice volume may decrease, which may lead to the collapse of the lattice structure within the particles, and particle cracks and the desorption of oxygen may occur. In addition, the desorbed oxygen may cause side reactions and the generation of gas.

[0078] The positive electrode active material according to an exemplary embodiment may satisfy a specified relationship between the maximum value of the lattice strain according to the change in the charging voltage and the Ni content. Therefore, even if it contains a high content of Ni, the lithium metal oxide particles can maintain crystallographic or structural stability within the driving voltage range of the secondary battery.

[0079] In one embodiment, when the strain (Q) value of the lithium metal oxide is less than 11.5x - 7.9, the capacity of the lithium secondary battery within the crystal structure may decrease, and the power may be reduced.

[0080] In one embodiment, when the strain (Q) value of the lithium metal oxide exceeds 11.5x - 7.5, the change in the crystal structure (e.g., lattice size or crystal phase) caused by charge and discharge may accelerate. Therefore, the desorption of oxygen within the crystal structure may accelerate, which may lead to an increase in the amount of gas generated. In addition, the cycle characteristics and high-temperature stability of the lithium secondary battery may decrease.

[0081] In some embodiments, the lattice strain of the positive electrode may be measured while charging or discharging the half-cell within a voltage range of 3.0V to 4.3V. For example, the lattice strain may be measured using in-situ XRD analysis, and the lattice strain may be measured while charging and discharging the half-cell. Therefore, the change in the lattice structure caused by charge and discharge within the above voltage range can be accurately measured.

[0082] Therefore, the lattice strain value of the positive electrode when the voltage of the secondary battery changes (increases or decreases) can be calculated. For example, the change in the lattice structure during charging or discharging can be measured at intervals of 0.01V or 0.001V.

[0083] Therefore, the changes in the lattice or crystal that may occur during charging or discharging of the secondary battery can be predicted based on the strain value (Q) of the lithium metal oxide. In addition, by adjusting the strain value obtained during the charging or discharging process, the operating stability of the lithium secondary battery can be further improved over the entire driving voltage range.

[0084] In one embodiment, charging and discharging can be performed at a rate of 0.1C.

[0085] In some embodiments, the lattice strain can be obtained by plotting the diffraction angle (θ) and the full width at half maximum (β) of all XRD peaks that appear in the 2θ ranges of 15° to 20°, 35° to 40°, 45° to 50°, 55° to 60°, and 65° to 70° on the coordinate plane, respectively.

[0086] For example, the lattice strain can be obtained by plotting the diffraction angle (θ) and the full width at half maximum (β) of the XRD peaks of the (003) plane, (101) plane, (105) plane, (107) plane, and (113) plane of the lithium metal oxide on the coordinate plane, respectively.

[0087] The (003) plane, (101) plane, (105) plane, (107) plane, and (113) plane of the lithium metal oxide can be planes related to the lattice structure and crystallinity. By selecting the XRD peaks of the above planes from the XRD peaks of the lithium metal oxide and calculating the strain (Q) of Equation 1, the structural stability of the lithium metal oxide can be further improved.

[0088] In some embodiments, the maximum value (Q) of the lattice strain can be measured at a voltage of less than 4.3V.

[0089] For example, the lattice constant can have a minimum value at the end of charging, so the change in the lattice structure at 4.3V can be the largest. However, the maximum value of the lattice strain can be observed in the region where the phase change is the largest, rather than at the end of charging. For example, the maximum value of the lattice strain can be measured at a voltage of less than 4.3V.

[0090] By measuring the lattice strain that has a direct impact on the charge-discharge performance of the lithium secondary battery instead of the lattice constant or lattice structure, the structural stability and the degree of oxygen desorption in the positive electrode active material can be quantified and controlled. Therefore, the amount of gas generated by the positive electrode can be compared or predicted without directly measuring the assembled battery.

[0091] In one embodiment, the maximum value (Q) of the lattice strain can be measured at a voltage above 4.0V and less than 4.3V.

[0092] In some embodiments, the lithium metal oxide may have a single-particle, secondary-particle, or a mixed morphology thereof. The single particles and the secondary particles can be distinguished by the morphology of the particles. For example, the secondary particles and the single particles can be distinguished based on the cross-sectional images of the particles measured using a scanning electron microscope (SEM).

[0093] The secondary 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 secondary particles, the boundaries of the primary particles can be observed in the SEM cross-sectional image.

[0094] In one embodiment, more than 10, 30 or more, 50 or more, or 100 or more primary particles may be aggregated in the secondary particles.

[0095] The single particles may refer to non-aggregated particles (monolith). For example, in the case of the single particles, different from the secondary particles, the boundaries of the primary particles may not be observable in the SEM cross-sectional image.

[0096] In some embodiments, the average particle diameter (D 50 ) of the lithium metal oxide may be from 0.5 μm to 35 μm. In one embodiment, the average particle diameter (D 50 ) of the lithium metal oxide may be from 1 μm to 35 μm, from 3 μm to 30 μm, or from 5 μm to 10 μm. The "average particle diameter (D 50 )" may be the particle diameter at which the volume fraction in the volume particle size distribution obtained by cumulating the particles in ascending order of particle diameter is 50%.

[0097] For example, when the average particle diameter of the lithium metal oxide is less than 0.5 μm, the mechanical and physical properties and the structural stability of the particles may be relatively reduced, which may lead to a reduction in the cycling characteristics of the lithium secondary battery. For example, when the average particle diameter of the lithium metal oxide exceeds 35 μm, as the particle size increases, the migration distance of lithium ions may become longer, and the capacity characteristics and power characteristics of the lithium secondary battery may be relatively reduced.

[0098] In some embodiments, each of the lithium metal oxide particles may include a compound represented by the following Chemical Formula 1.

[0099] [Chemical Formula 1]

[0100] Li a Ni x Mb O2

[0101] In the formula (1), 0.8 ≤ x ≤ 0.94, 0.95 ≤ a ≤ 1.05, and 0.06 ≤ b ≤ 0.2, and M may be at least one selected from B, Al, Ti, V, Mn, Co, Zn, Y, Nb, Zr, Mo, Sn, Mg, Sr, Ba, and W.

[0102] The chemical structure represented by formula (1) represents the bonding relationship included in the layered structure or crystal structure of the positive electrode active material, and does not exclude other additional elements. For example, M may be provided as the main active element of the positive electrode active material. Formula (1) is provided to represent the bonding relationship of the main active element, and it should be understood that formula (1) is a formula including the introduction and substitution of additional elements.

[0103] In one embodiment, in addition to including the main active element, an auxiliary element for enhancing the chemical stability of the positive electrode active material or the layered structure / crystal structure may be further included. The auxiliary element may be mixed into the layered structure / crystal structure together and form a bond, and it should be understood that this situation is also included within the scope of the chemical structure represented by formula (1).

[0104] The auxiliary element may include at least one selected from, for example, Na, Ca, Hf, Ta, Cr, Fe, Cu, Ag, Ga, C, Si, Ra, P, and S. The auxiliary element may act as an auxiliary active element that contributes to the capacity / power activity of the positive electrode active material together with, for example, Co, Ni, or Mn.

[0105] In some embodiments, in formula (1), x may be from 0.83 to 0.94. For example, x may be 0.88 ≤ x ≤ 0.94. Within the above range, the content of nickel increases, so that the energy density and power of the lithium secondary battery can be further improved. In addition, since the positive electrode has a strain value (Q) that satisfies formula (1), even if the content of nickel increases, the lithium secondary battery can have improved cycle characteristics, as well as thermal stability and chemical stability.

[0106] In some embodiments, the lithium metal oxide may have a layered crystal structure with the R-3m space group.

[0107] For example, the lithium metal oxide may have a hexagonal crystal system - NaFeO2 structure, which has a space group R-3m in which Li layers and oxide layers containing at least one metal element other than Li continuously cross.

[0108] The transition metal precursor (e.g., Ni-Co-Mn precursor) for preparing the lithium metal oxide can be prepared by a co-precipitation reaction.

[0109] The transition metal precursor can be prepared by a co-precipitation reaction of metal salts. The metal salts can include nickel salts, manganese salts, and cobalt salts.

[0110] Examples of the nickel salt can include nickel sulfate, nickel hydroxide, nickel nitrate, nickel acetate, and their hydrates, etc. Examples of the manganese salt can include manganese sulfate, manganese acetate, and their hydrates, etc. Examples of the cobalt salt can include cobalt sulfate, cobalt nitrate, cobalt carbonate, and their hydrates, etc.

[0111] The metal salts can be mixed with a precipitating agent and / or a chelating agent in a proportion that satisfies the content or concentration ratio of each metal described with reference to Chemical Formula 1 to prepare an aqueous solution. The aqueous solution can be subjected to co-precipitation in a reactor to prepare a transition metal precursor.

[0112] The precipitating agent can include basic compounds such as sodium hydroxide (NaOH), sodium carbonate (Na2CO3), etc. The chelating agent can include, for example, ammonia water (e.g., NH3·H2O), ammonium carbonate (e.g., NH3HCO3), etc.

[0113] The temperature of the co-precipitation reaction can be adjusted within a range of, for example, about 30°C to 70°C. The reaction time can be adjusted within a range of about 24 hours to 60 hours.

[0114] For example, lithium metal composite particles can be prepared by reacting a transition metal precursor with a lithium precursor. The lithium precursor compound can include, for example, lithium carbonate, lithium nitrate, lithium acetate, lithium oxide, lithium hydroxide, etc. These can be used alone or in combination of two or more.

[0115] After that, the crystallinity of the metal particles can be improved by a heat treatment (calcination) process of the lithium metal composite particles. In one embodiment, the heat treatment temperature can be in the range of about 600°C to 1000°C.

[0116] In one embodiment, a water washing process of the lithium metal composite particles can be carried out to remove lithium impurities or unreacted precursors. In one embodiment, the water washing process can be omitted. In one embodiment, a post-treatment or surface treatment process can also be carried out on the lithium metal composite particles.

[0117] In one example, when the positive electrode active material contains a high content of Ni, a low calcination temperature is required, and the content of residual lithium present on the surface may increase. The residual lithium may react with the electrolyte to generate gas and may reduce the energy density of the positive electrode. When a water washing process is performed to remove the residual lithium, the structure of the positive electrode active material may become unstable, and oxygen within the lattice structure may desorb or gas may be generated.

[0118] According to an exemplary embodiment, the lithium metal oxide may have a stable structure throughout the driving voltage range, thereby preventing the collapse and deterioration of the lattice structure during battery driving. Therefore, by using the lithium metal oxide as the positive electrode active material, it is possible to improve the capacity retention rate and gas generation amount while increasing the capacity and power characteristics of the lithium secondary battery.

[0119] For example, the strain (Q) of the above lithium metal oxide may vary depending on the co-precipitation reaction time, reaction temperature, heat treatment temperature, water washing process, surface treatment process, etc.

[0120] <Lithium secondary battery>

[0121] Hereinafter, with reference to Figure 1 and Figure 2 , a lithium secondary battery is provided, which includes a positive electrode containing the positive electrode active material for the above lithium secondary battery.

[0122] With reference to Figure 1 and Figure 2 , the lithium secondary battery may include a positive electrode 100 and a negative electrode 130 disposed opposite to the positive electrode 100. The positive electrode 100 includes a positive electrode active material, and the positive electrode active material includes the above lithium metal oxide particles.

[0123] The positive electrode 100 may include a positive electrode active material layer 110, which is formed by coating the positive electrode active material containing the above lithium metal oxide particles on the positive electrode current collector 105.

[0124] For example, the positive electrode active material may be mixed and stirred with a binder, a conductive material, and / or a dispersion material, etc. in a solvent to prepare a positive electrode paste. After coating the positive electrode paste on at least one surface of the positive electrode current collector 105, it may be dried and calendered to manufacture the positive electrode 100.

[0125] The positive electrode current collector 105 may include stainless steel, nickel, aluminum, titanium, or an alloy thereof. The positive electrode current collector 105 may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver.

[0126] The binder may include, for example, organic binders such as vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, etc., or water - based binders such as styrene - butadiene rubber (SBR), and may be used together with thickeners such as carboxymethyl cellulose (CMC).

[0127] For example, as a binder for forming the positive electrode, a PVDF - based binder can be used. In this case, the amount of the binder used for forming the positive electrode active material layer 110 can be reduced, and the amount of the positive electrode active material can be relatively increased, so that the power and capacity of the secondary battery can be improved.

[0128] The conductive material can be included to facilitate electron migration between the active material particles. For example, the conductive material may include carbon - based conductive materials such as graphite, carbon black, graphene, carbon nanotubes, etc., and / or metal - based conductive materials including perovskite substances such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.

[0129] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120, and the negative electrode active material layer 120 is formed by coating a negative electrode active material on at least one surface of the negative electrode current collector 125.

[0130] The negative electrode active material can be a material known in the art for allowing lithium ions to be intercalated and de - intercalated without particular limitation. For example, the negative electrode active material can use carbon - based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium alloys; silicon or tin, etc. As examples of the amorphous carbon, hard carbon, coke, mesocarbon microbead (MCMB) calcined at 1500 °C or lower, mesophase pitch - based carbon fiber (MPCF), etc. can be cited. As examples of the crystalline carbon, natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc., which are graphite - based carbons, can be cited. As elements included in the lithium alloy, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium, etc. can be cited.

[0131] The negative electrode current collector 125 may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or their alloys. For example, the negative electrode current collector 125 may include copper or a copper alloy.

[0132] In some embodiments, the negative electrode active material can be mixed and stirred with a binder, a conductive material, and / or a dispersing material, etc. in a solvent to prepare a slurry. The slurry can be coated on the negative electrode current collector and then dried and calendered to fabricate the negative electrode 130.

[0133] The binder and the conductive material can use substances that are substantially the same as or similar to the above substances. In some embodiments, for compatibility with the carbon-based active material, the binder used to form the negative electrode can include, for example, an aqueous binder such as styrene-butadiene rubber (SBR), and can be used together with a thickener such as carboxymethyl cellulose (CMC).

[0134] In some embodiments, a 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 140 can also include a non-woven fabric formed of glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0135] According to an exemplary embodiment, the 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 the battery cells. For example, the electrode assembly 150 can be formed by winding, laminating, folding, etc. of the separator 140.

[0136] The electrode assembly 150 can be accommodated in a housing 160 together with an electrolyte, thereby defining a lithium secondary battery. According to an exemplary embodiment, a non-aqueous electrolyte can be used for the electrolyte.

[0137] The non-aqueous electrolyte can contain a lithium salt as an electrolyte and an organic solvent. The lithium salt can be represented, for example, by Li + X - and, 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.

[0138] The organic solvent can be, 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, and tetrahydrofuran, etc. These can be used alone or in combination of two or more.

[0139] As Figure 1 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 one side of the housing 160 to form electrode leads (the positive lead 107 and the negative lead 127) extending to the outside of the housing 160 or exposed to the outside of the housing 160.

[0140] 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 type, etc.

[0141] Hereinafter, in order to help understand the present invention, preferred embodiments are presented, but these embodiments are only used to illustrate the present invention and are not used to limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope and technical concept of the present invention, which is obvious to those skilled in the art, and such variations and modifications are also naturally within the scope of the claims.

[0142] Examples and Comparative Examples

[0143] Example 1

[0144] Preparation of Lithium Metal Oxide Particles

[0145] NiSO4, CoSO4, and MnSO4 were mixed using distilled water that had been bubbled with N2 for 24 hours to remove internal dissolved oxygen, such that the ratios of Ni, Co, and Mn satisfied Table 2 below. The above solution was added to a reactor at 55 °C, and a coprecipitation reaction was carried out for 36 hours using NaOH and NH3·H2O as precipitants and chelating agents to obtain Ni 0.88 Co 0.09 Mn 0.03 (OH)2. The obtained precursor was dried at 80 °C for 12 hours and then dried again at 110 °C for 12 hours.

[0146] Lithium hydroxide and the transition metal precursor were added to a dry high-speed mixer at a ratio of 1.05:1 and mixed evenly for 5 minutes. The above mixture was placed in a calcination furnace, and the temperature was raised to 950 °C at a heating rate of 2 °C / minute. After maintaining at 950 °C for 5 hours, it was naturally cooled to 900 °C and maintained for 5 hours. During the heating and maintaining processes, oxygen was continuously introduced at a flow rate of 10 mL / minute. After calcination, it was naturally cooled to room temperature and then pulverized and classified to obtain lithium metal oxide particles (LiNi 0.88 Co 0.09 Mn 0.03 O2) having an R-3m crystal structure.

[0147] Addition and Precoating of Lithium Metal Oxide

[0148] The lithium metal oxide particles prepared according to the preparation example and 0.3 mol% of Al2O3 powder relative to the total molar amount of the lithium metal oxide particles were mixed to form a precoat on the surface of the lithium metal oxide particles.

[0149] Mixing and Calcination of Aqueous Sulfur Compound Solution

[0150] An aqueous sulfur compound solution was added to the precoated lithium metal oxide particles and then mixed.

[0151] At this time, the aqueous sulfur compound solution was prepared by adding ammonium sulfate ((NH4)2SO4) powder at 1.5 wt% relative to the total weight of the lithium metal oxide particles to pure water at 8 wt% relative to the total weight of the lithium metal oxide particles.

[0152] Put the obtained mixture into a calcination furnace, supply oxygen at a flow rate of 10 mL / min, while heating it at a rate of 2 °C / min to 400 °C, and maintain it at this temperature for 8 hours. After calcination, classify it with 325 mesh (mesh) to obtain the positive electrode active material.

[0153] Example 2

[0154] The positive electrode active material was obtained by the same method as in Example 1, except that ammonium sulfate ((NH4)2SO4) powder at 1 wt% relative to the total weight of the lithium metal oxide particles was added to pure water at 8 wt% relative to the total weight of the lithium metal oxide particles.

[0155] Example 3

[0156] The positive electrode active material was obtained by the same method as in Example 1, except that ammonium sulfate ((NH4)2SO4) powder at 0.5 wt% relative to the total weight of the lithium metal oxide particles was added to pure water at 8 wt% relative to the total weight of the lithium metal oxide particles.

[0157] Example 4

[0158] The positive electrode active material was obtained by the same method as in Example 1, except that LiNi 0.80 Co 0.10 Mn 0.10 O2 was used as the lithium metal oxide particles.

[0159] Example 5

[0160] The positive electrode active material was obtained by the same method as in Example 4, except that ammonium sulfate ((NH4)2SO4) powder at 1 wt% relative to the total weight of the lithium metal oxide particles was added to pure water at 8 wt% relative to the total weight of the lithium metal oxide particles.

[0161] Example 6

[0162] The positive electrode active material was obtained by the same method as in Example 4, except that ammonium sulfate ((NH4)2SO4) powder at 0.5 wt% relative to the total weight of the lithium metal oxide particles was added to pure water at 8 wt% relative to the total weight of the lithium metal oxide particles.

[0163] Example 7

[0164] The positive electrode active material was obtained by the same method as in Example 1, except that LiNi 0.94 Co 0.05 Mn 0.01 O2 was used as the lithium metal oxide particles.

[0165] Example 8

[0166] The positive electrode active material was obtained by the same method as in Example 7, except that 1 wt% of ammonium sulfate ((NH4)2SO4) powder was added to 8 wt% of pure water relative to the total weight of the lithium metal oxide particles.

[0167] Example 9

[0168] The positive electrode active material was obtained by the same method as in Example 7, except that 0.5 wt% of ammonium sulfate ((NH4)2SO4) powder was added to 8 wt% of pure water relative to the total weight of the lithium metal oxide particles.

[0169] Comparative Example 1

[0170] The lithium metal oxide particles prepared according to the preparation example were added to 100 wt% of pure water relative to the total weight of the lithium metal oxide particles and stirred for 10 minutes, then the filtered particles were dried at 170 °C for 12 hours and mixed with 0.25 wt% of boric acid (H3BO3) relative to the total weight of the lithium metal oxide particles, and calcined at 200 - 300 °C to obtain the positive electrode active material.

[0171] Comparative Example 2

[0172] The lithium metal oxide particles prepared according to the preparation example were placed without performing the post-treatment step to obtain the positive electrode active material.

[0173] Comparative Example 3

[0174] The positive electrode active material was obtained by the same method as in Comparative Example 1, except that LiNi 0.80 Co 0.10 Mn 0.10 O2 was used as the lithium metal oxide particles.

[0175] Comparative Example 4

[0176] The positive electrode active material was obtained by the same method as in Comparative Example 2, except that LiNi 0.80 Co 0.10 Mn 0.10 O2 was used as the lithium metal oxide particles.

[0177] Comparative Example 5

[0178] The positive electrode active material was obtained by the same method as in Comparative Example 1, except that LiNi 0.94 Co 0.05 Mn0.01 O2 as lithium metal oxide particles.

[0179] Comparative Example 6

[0180] The positive electrode active material was obtained by the same method as in Comparative Example 2, except that LiNi 0.94 Co 0.05 Mn 0.01 O2 was used as lithium metal oxide particles.

[0181] Measurement of residual lithium

[0182] 2.5 g of the positive electrode active material according to each example and comparative example was added to a 250 mL beaker, 100 g of deionized water was added, and then a magnetic stir bar was placed and stirred at a speed of 300 rpm for 10 minutes. After that, filtration was carried out using a vacuum flask, and then 100 g was aliquoted. The aliquoted solution was added to the container of an automatic measuring instrument (Auto titrator), and automatic titration was carried out with 0.1 N HCl with reference to the Wader Method to measure the contents of Li2CO3 and LiOH in the solution.

[0183] Measurement of strain (Q)

[0184] 1) Fabrication of coin-type half cell

[0185] A coin-type battery for in-situ XRD was fabricated using the positive electrode active material according to each example and comparative example, and the strain (Q) was measured by XRD analysis. The coin-type battery for in-situ XRD was made into a coin-type half cell (2032 type) using the lithium metal oxide particles as the positive electrode and lithium metal as the counter electrode.

[0186] Specifically, the positive electrode active material, acetylene black (Denka Black) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of 93:5:2, respectively, to prepare a positive electrode paste. The positive electrode paste was coated on an aluminum current collector and then dried and calendered to prepare a positive electrode. After the calendering, the density of the positive electrode was adjusted to 3.3 g / cubic centimeter (cc).

[0187] A separator (polyethylene, thickness 13 μm) was disposed between the positive electrode and the negative electrode to form an electrode core. The electrode core was assembled by placing it in an outer package material and injecting an electrolyte, and aged for more than 12 hours so that the electrolyte could be impregnated into the electrode interior. The electrolyte used was an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of EC / EMC (30 / 70; volume ratio).

[0188] 2) Measurement of lattice strain value

[0189] Drill a hole with a diameter of 10π on the outer surface of the exterior material in contact with the positive electrode, and seal the hole with a Kapton film to prevent the leakage of the electrolyte present inside the battery. Thereafter, perform XRD analysis on the positive electrode active material to measure the lattice strain value. Pass X-rays through the hole through the aluminum substrate and generate diffraction in the positive electrode active material particles, and the detector reads this signal, so that the structural change of the positive electrode active material particles can be confirmed in real time.

[0190] Charge (CC-CV 0.1C, 4.3V, 0.005C cut-off) and discharge (CC 0.1C, 3.0V cut-off) the coin-type half-cell, and measure the initial discharge capacity. Thereafter, while charging the coin-type half-cell at 3.0V (CC-CV 0.1C, 4.3V, 0.005C cut-off), measure the lattice strain of the positive electrode active material according to the voltage at a time resolution of 12.5 minutes / scan. Use the formula 2 when calculating the lattice strain value, and select the diffraction angles and full widths at half maximum of the XRD peaks of the (003) plane, (101) plane, (105) plane, (107) plane, and (113) plane of the positive electrode active material particles when calculating according to formula 2.

[0191] Figure 3 It is a figure showing the XRD pattern of the positive electrode prepared according to Example 2 at a specific voltage. Figure 4 It is a figure showing the figure according to formula 2 of the positive electrode prepared according to Example 2 at the above voltage.

[0192] Thereafter, calculate the maximum value (Q) of the lattice strain measured according to the voltage and show it in Table 2 below.

[0193] Figure 5 It is a figure showing the crystallite size (c) and lattice strain of the positive electrode prepared according to Example 2 according to the charging voltage. Refer to Figure 5 , and the strain (Q) of the lithium metal oxide particles of Example 2 measured is 2.42.

[0194] In Example 2, the content of nickel is 0.88 moles relative to 1 mole of the total metal other than lithium, the minimum value of formula 1 of Example 2 is 2.22, and the maximum value is 2.62.

[0195] The specific XRD analysis equipment / conditions are shown in Table 1 below, and the highscore program is used when calculating the lattice strain.

[0196] [Table 1]

[0197]

[0198] [Table 2]

[0199]

[0200] Experimental Example

[0201] (1) Initial Capacity of Coin-Type Half-Cell (2 mAh)

[0202] After fabricating the coin-type half-cell, the first charge (CC-CV 0.1C, 4.3V, cutoff at 0.005C) and discharge (CC 0.1C, cutoff at 3.0V) were performed, and the initial discharge capacity was measured.

[0203] (2) Fabrication of Lithium Secondary Battery (20 Ah)

[0204] A secondary battery was fabricated using the lithium metal oxide particles prepared above as the positive electrode active material. Specifically, the positive electrode active material, acetylene black (Denka Black) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder were mixed at a mass ratio of 97:2:1, respectively, to obtain a positive electrode mixture. Then, the positive electrode mixture was coated on an aluminum current collector and dried and calendered to obtain a positive electrode. After calendering, the target electrode density of the positive electrode was adjusted to 3.69 g / cm³ (cc).

[0205] A negative electrode slurry was prepared, which contained 93 wt% natural graphite as the negative electrode active material, 5 wt% flake type conductive material KS6 as the conductive material, 1 wt% styrene-butadiene rubber (SBR) as the binder, and 1 wt% carboxymethyl cellulose (CMC) as the thickener. The negative electrode slurry was coated on a copper substrate and dried and calendered to obtain a negative electrode.

[0206] Fourteen positive electrodes and fifteen negative electrodes prepared as described above were each subjected to specified notching and laminated. A separator (polyethylene, thickness 25 μm) was disposed between the positive and negative electrodes to form an electrode assembly, and then the tabs of the positive and negative electrodes were welded respectively. The welded positive electrode / separator / negative electrode assembly was placed in a soft package, and three sides (sealing part) other than the electrolyte injection part surface were sealed. At this time, the part having the tab was included in the sealing part. The electrolyte was injected through the remaining surfaces other than the sealing part, and the remaining surfaces were sealed, and then impregnated for 12 hours or more.

[0207] The electrolyte used was an electrolyte in which 1 M LiPF6 was dissolved in a mixed solvent of EC / EMC (25 / 75; volume ratio), and then 1 wt% vinylene carbonate (VC) and 0.5 wt% 1,3-propane sultone (PRS) were added.

[0208] (3) Measurement of 60°C life (capacity retention rate)

[0209] Charge (CC-CV 0.1C, 4.2V, cut-off at 0.05C) and discharge (CC 0.1C, 2.5V) the lithium secondary batteries of the above-mentioned examples and comparative examples, and then measure the initial discharge capacity. After that, charge the battery (CC-CV 0.1C, 4.2V, cut-off at 0.05C) and perform a discharge with a DOD of 3% to prepare a battery with an SOC of 97%.

[0210] Store the prepared battery in a constant temperature chamber at 60°C. At the 16th week, perform charging (CC-CV 0.1C 4.2V 0.05C cut-off) and discharging (CC 0.1C cut-off at 2.5V) respectively, and measure the discharge capacity. Calculate the capacity retention rate by calculating the percentage (%) of this discharge capacity relative to the initial discharge capacity.

[0211] (4) Measurement of gas generation amount and carbon dioxide content in the gas at high temperature

[0212] Store the battery with an SOC of 97% prepared as described above in a constant temperature chamber at 60°C. Measure the gas generation amount inside the battery at the 4th week, 8th week, and 16th week. Place each battery in a fixture designed according to the battery size and pierce it with a needle. Measure the pressure inside the changing fixture and convert it into the gas generation amount.

[0213] At this time, collect the gas taken at the 16th week into a cylinder. Fix the cylinder to a gas chromatography-residual gas analyzer (GC-RGA) to analyze the CO2 content (volume ratio) in the total gas amount.

[0214] The evaluation results are shown in Table 3 below.

[0215] [Table 3]

[0216]

[0217] Referring to Table 1 and Table 2, the strain (Q) value of the positive electrode active material prepared according to the examples satisfies the range of Formula 1, has a high initial capacity, and the capacity retention rate and gas generation amount are improved.

[0218] For example, the positive electrode active material containing 0.88 moles of Ni relative to 1 mole of the total metal has a Q value of 2.22 to 2.62, the lithium metal oxide particles containing 0.80 moles of Ni have a Q value of 1.3 to 1.7, and the positive electrode active material containing 0.94 moles of Ni has a Q value of 2.91 to 3.31.

[0219] On the other hand, in the comparative example, the Q value is not within the range of Formula 1, and the initial discharge capacity decreases or the gas generation amount and the capacity retention rate deteriorate. For example, in the comparative example, the gas generation amount caused by high-temperature storage and charge-discharge is relatively high, and the gas generation accelerates as the storage time increases.

[0220] Although the positive electrode active materials used in the examples and the comparative example have substantially similar residual lithium contents, the gas generation amount in the examples is further reduced, and the capacity and power characteristics are improved.

[0221] In addition, the gas generated by the secondary battery according to the example has a low CO2 content. During charge-discharge, when oxygen desorbed from the positive electrode reacts with the solvent in the electrolyte and is oxidized, CO2 may be generated. Therefore, in the secondary battery according to the example, it can be confirmed that the side reaction between the positive electrode and the electrolyte is reduced.

Claims

1. A positive electrode for a lithium secondary battery, which comprises a lithium metal oxide, Among them, In the lithium metal oxide, x moles of nickel (Ni) are contained relative to 1 mole of the total metal other than lithium, The positive electrode for the lithium secondary battery has a strain Q of the following formula 1, [Formula 1] In the formula 1, x is the content of the nickel, and Q is the maximum value among the lattice strains measured according to the voltage in the voltage range of 3.0 V to 4.3 V for a half-cell including the positive electrode for the lithium secondary battery and a lithium counter electrode, The lattice strains are each 1 / 4 of the slope of a straight line obtained by plotting the diffraction angle θ and the full width at half maximum β of the XRD peak of the lithium metal oxide obtained by XRD analysis at each measurement voltage on a coordinate plane with the horizontal axis being sinθ and the vertical axis being βcosθ, where the unit of the diffraction angle θ is radian and the unit of the full width at half maximum β is radian.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lattice strains are each calculated using the following formula 2, [Formula 2] βcosθ = 4ηsinθ + 0.9λ / D In the formula 2, β is the full width at half maximum of the XRD peak, θ is the diffraction angle of the XRD peak, η is the lattice strain and is a dimensionless number, λ is the wavelength of the X-ray used in the XRD analysis and the unit is Å, and D is the crystallite size of the lithium metal oxide and the unit is Å.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein, The content x of nickel in the lithium metal oxide is 0.8 moles or more in 1 mole of the total metal other than lithium.

4. The positive electrode for a lithium secondary battery according to claim 3, wherein, The lithium metal oxide is represented by the following chemical formula 1, [Chemical formula 1] Li a Ni x M b O2 In the chemical formula 1, 0.8 ≤ x ≤ 0.94, 0.95 ≤ a ≤ 1.05, and 0.06 ≤ b ≤ 0.2, and M is at least one selected from B, Al, Ti, V, Mn, Co, Zn, Y, Nb, Zr, Mo, Sn, Mg, Sr, Ba, and W.

5. The positive electrode for a lithium secondary battery according to claim 4, wherein, In the chemical formula 1, x is 0.83 to 0.

94.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lattice strain is measured at a time resolution of 12.5 minutes / scan in the voltage range of 3.0 V to 4.3 V.

7. The positive electrode for a lithium secondary battery according to claim 1, wherein The maximum value Q of the lattice strain is measured at a voltage less than 4.3 V.

8. The positive electrode for a lithium secondary battery according to claim 7, wherein, The maximum value Q of the lattice strain is measured at a voltage of more than 4.0 V and less than 4.3 V.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lattice strain is measured while charging or discharging the half-cell at a rate of 0.1C in the voltage range of 3.0 V to 4.3 V.

10. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lattice strains are each obtained by plotting the diffraction angle θ and the full width at half maximum β of all XRD peaks appearing in the 2θ range of 15° to 20°, 35° to 40°, 45° to 50°, 55° to 60°, and 65° to 70° on the coordinate plane.

11. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lattice strains are each obtained by plotting the diffraction angle θ and the full width at half maximum β of the XRD peaks of the (003) plane, (101) plane, (105) plane, (107) plane, and (113) plane of the lithium metal oxide on the coordinate plane.

12. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide has a layered crystal structure with an R-3m space group.

13. The positive electrode for a lithium secondary battery according to claim 1, wherein, The lithium metal oxide further contains cobalt (Co) and manganese (Mn).

14. A lithium secondary battery, which includes: The positive electrode for a lithium secondary battery according to claim 1; and a negative electrode, wherein the negative electrode is disposed opposite to the positive electrode.

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