Positive active material for lithium secondary battery and lithium secondary battery comprising the same

Abstract

Provided is a positive electrode active material for a lithium secondary battery, the positive electrode active material including lithium-transition metal composite oxide particles. The lithium-transition metal composite oxide particles have a grain size of 250 nm or more as measured by XRD analysis, and the lithium-transition metal composite oxide particles have an XRD peak intensity ratio of 9.8% or less. A lithium secondary battery is provided that includes the lithium-transition metal composite oxide particles and has improved cycle life and rate capability.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202110620746.2, filed on June 3, 2021, entitled "Cathode Active Material for Lithium Secondary Battery and Lithium Secondary Battery Comprising the Same," and claims priority to Korean Patent Application Nos. 10-2020-0067818, filed on June 4, 2020, and 10-2021-0017376, filed on February 8, 2021, in the Korean Intellectual Property Office (KIPO), the disclosures of which are incorporated herein in their entireties by reference. TECHNICAL FIELD

[0002] The present application relates to a cathode active material for a lithium secondary battery and a lithium secondary battery comprising the same. More particularly, the present application relates to a cathode active material comprising a lithium-transition metal composite oxide for a lithium secondary battery and a lithium secondary battery comprising the same. BACKGROUND

[0003] With the development of information technology and display technology, secondary batteries, which can be repeatedly charged and discharged, have been widely used as power sources for mobile electronic devices such as camcorders, mobile phones, notebook computers, etc. Recently, battery packs including secondary batteries have also been developed and applied to, for example, eco-friendly automobiles such as hybrid electric vehicles as their power sources.

[0004] Secondary batteries include, for example, lithium secondary batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Lithium secondary batteries are of great interest due to high operating voltage and energy density per unit weight, high charging rate, compact size, etc.

[0005] For example, a lithium secondary battery can include an electrode assembly including a cathode, an anode, and a separator layer (separator); and an electrolyte that immerses the electrode assembly. The lithium secondary battery can further include a case having, for example, a pouch shape.

[0006] Lithium metal oxides can be used as a cathode active material for a lithium secondary battery preferably having high capacity, power, and lifespan. However, if the lithium metal oxide is designed to have a high-power composition, the thermal stability and mechanical stability of the lithium secondary battery can be reduced, and thus the lifespan characteristics and operational reliability can also be deteriorated.

[0007] For example, Korean Patent Application No. 10-2017-0093085 discloses a cathode active material comprising a transition metal compound and an ion-absorbing binder, but it can not provide sufficient lifespan and stability. SUMMARY

[0008] According to an aspect of the present application, there is provided a positive electrode active material for a lithium secondary battery, which has improved operation stability and reliability.

[0009] According to an aspect of the present application, there is provided a lithium secondary battery including a positive electrode active material having improved operation stability and reliability.

[0010] According to an exemplary embodiment, a positive electrode active material for a lithium secondary battery includes lithium-transition metal composite oxide particles. A grain size of the lithium-transition metal composite oxide particles is 250 nm or more as measured by XRD analysis, and an XRD peak intensity ratio of the lithium-transition metal composite oxide particles defined by Equation 2 is 9.8% or less.

[0011] [Equation 2]

[0012] XRD peak intensity ratio (%) = 100 x I(110) / {I(110) + I(003)}

[0013] In Equation 2, I(110) is a maximum height of a peak corresponding to a (110) plane as obtained by XRD analysis of the lithium-transition metal composite oxide particles, and I(003) is a maximum height of a peak corresponding to a (003) plane as obtained by XRD analysis of the lithium-transition metal composite oxide particles.

[0014] In some embodiments, the grain size can be measured by the following Equation 1:

[0015]

[0016] In Equation 1 above, L is the grain size, λ is a wavelength of X-rays, β is a half-width of a peak corresponding to a (003) plane, and θ is a diffraction angle.

[0017] In some embodiments, the grain size of the lithium-transition metal composite oxide particles can be in a range of 250 nm to 1000 nm.

[0018] In some embodiments, the grain size of the lithium-transition metal composite oxide particles can be in a range of 300 nm to 1000 nm.

[0019] In some embodiments, the XRD peak intensity ratio of the lithium-transition metal composite oxide particles can be in a range of 4 to 9.8.

[0020] In some embodiments, the XRD peak area ratio of the lithium-transition metal composite oxide particles defined by Equation 3 can be 17% or less:

[0021] [Equation 3]

[0022] XRD peak area ratio (%) = 100 x A(110) / {A(110) + A(003)}

[0023] In Equation 3, A(110) is a peak area of a peak corresponding to the (110) plane obtained through XRD analysis of the lithium-transition metal complex oxide particles, and A(003) is a peak area of a peak corresponding to the (003) plane obtained through XRD analysis of the lithium-transition metal complex oxide particles.

[0024] In some embodiments, the XRD peak area ratio of the lithium-transition metal complex oxide particles can be in the range of 10% to 17%.

[0025] In some embodiments, the positive active material can include first lithium-transition metal complex oxide particles and second lithium-transition metal complex oxide particles having a secondary particle structure. The first lithium-transition metal complex oxide particles can have a grain size of 250 nm or more measured through XRD analysis, and an XRD peak intensity ratio of the first lithium-transition metal complex oxide particles can be 9.8% or less.

[0026] In some embodiments, the first lithium-transition metal complex oxide particles can have a single particle shape having a crystallographic single crystal or polycrystal structure.

[0027] In some embodiments, the second lithium-transition metal complex oxide particles can have a grain size of less than 250 nm measured through XRD analysis.

[0028] In some embodiments, the XRD peak intensity ratio of the second lithium-transition metal complex oxide particles can exceed 9.8%.

[0029] In some embodiments, a weight ratio of the first lithium-transition metal complex oxide particles to the second lithium-transition metal complex oxide particles can be 3:7 to 7:3.

[0030] In some embodiments, a particle size (D50) of the second lithium-transition metal complex oxide particles can be greater than a particle size of the first lithium-transition metal complex oxide particles.

[0031] In some embodiments, the lithium-transition metal complex oxide particles can have a composition represented by Chemical Formula 1:

[0032] [Chemical Formula 1]

[0033] Li x Ni a M 1-a O 2+z

[0034] In Chemical Formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, -0.1≤z≤0.1, and M can be at least one element selected from Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, and Zr.

[0035] In some embodiments, 0.8≤a≤0.95 in Chemical Formula 1.

[0036] According to an exemplary embodiment, a lithium secondary battery includes a cathode including a cathode active material layer including a cathode active material as described above; and an anode opposite to the cathode.

[0037] In some embodiments, the cathode active material can include first lithium-transition metal composite oxide particles and second lithium-transition metal composite oxide particles having a secondary particle shape. The first lithium-transition metal composite oxide particles can have a grain size of 250 nm or more, and an XRD peak intensity ratio of the first lithium-transition metal composite oxide particles can be 9.8% or less, as measured by XRD analysis.

[0038] In some embodiments, in a scanning electron microscope (SEM) cross-section of the cathode active material layer, a ratio of a cross-sectional area of the first lithium-transition metal composite oxide particles to a cross-sectional area of the second lithium-transition metal composite oxide particles can be 1:4.5 to 4.5:1.

[0039] In some embodiments, in a scanning electron microscope (SEM) cross-section of the cathode active material layer, a ratio of a cross-sectional area of the first lithium-transition metal composite oxide particles to a cross-sectional area of the second lithium-transition metal composite oxide particles can be 2:3.7 to 3.7:2.

[0040] The lithium secondary battery according to the above-described exemplary embodiment can include lithium-transition metal composite oxide particles as a cathode active material, the particles having a grain size of 250 nm or more and satisfying an XRD peak intensity ratio and / or an XRD peak area ratio of less than or equal to a predetermined value. Accordingly, a contact area between the particles can be reduced, and a particle strength can be increased.

[0041] The grain size can be increased to 250 nm or more, such that a space / contact area between the particles or between the crystals can be reduced. Accordingly, particle breakage during an electrode pressing process can be prevented, and gas generation during repeated charging and discharging can also be prevented, thereby improving a lifespan characteristic of the secondary battery.

[0042] Further, the XRD peak ratio can be controlled to be below a predetermined value, so that a lithium diffusion distance can be reduced. Accordingly, while suppressing a power decrease due to an increase in grain size, a lithium secondary battery having improved life and power characteristics can be implemented.

[0043] For example, the lithium-transition metal composite oxide particles can include a high Ni composition to provide increased power and capacity, and can have the above-described grain size to provide improved operation stability and life characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 and Figure 2 are a schematic plan view and a schematic cross-sectional view, respectively, illustrating a lithium secondary battery according to an exemplary embodiment. DETAILED DESCRIPTION

[0045] According to an example embodiment of the present application, a lithium secondary battery is provided, which includes lithium-transition metal composite oxide particles having a predetermined range of grain size and XRD peak ratio as a positive electrode active material.

[0046] Hereinafter, the present application will be described in detail with reference to the accompanying drawings. However, those skilled in the art will appreciate that these embodiments described with reference to the drawings are provided to further understand the spirit of the present application, and do not limit the subject matter to be protected disclosed in the specification and the appended claims.

[0047] Figure 1 and Figure 2 are a schematic plan view and a schematic cross-sectional view, respectively, illustrating a lithium secondary battery according to an exemplary embodiment. Hereinafter, a positive electrode active material for a lithium secondary battery and a lithium secondary battery including the same will be described with reference to Figure 1 and Figure 2

[0048] Referring to Figure 1 and Figure 2 , the lithium secondary battery can include an electrode assembly, which can include a positive electrode 100, a negative electrode 130, and a separator layer 140 interposed between the positive electrode 100 and the negative electrode 130. The electrode assembly can be placed in a case 160 together with an electrolyte solution, so that the electrode assembly is immersed in the electrolyte solution.

[0049] The positive electrode 100 can include a positive electrode active material layer 110 formed by coating a positive electrode active material on a positive electrode current collector 105. The positive electrode active material can include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0050] ​In an exemplary embodiment, the positive active material can include lithium-transition metal composite oxide particles. For example, the lithium-transition metal composite oxide particles can include nickel (Ni), and can further include at least one of cobalt (Co) and manganese (Mn).

[0051] For example, the lithium-transition metal composite oxide particles can be represented by the following Chemical Formula 1.

[0052] [Chemical Formula 1]

[0053] Li x Ni a M 1-a O 2+z

[0054] In Chemical Formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, and -0.1≤z≤0.1, M can represent at least one element selected from Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, or Zr.

[0055] Preferably, the molar ratio (a) of Ni in Chemical Formula 1 can be 0.8 or more, for example, 0.8≤a≤0.95. In one embodiment, the molar ratio (a) of Ni can exceed 0.8.

[0056] Ni can serve as a transition metal related to the power and capacity of a lithium secondary battery. Accordingly, as described above, a high Ni composition can be applied to the lithium-transition metal composite oxide particles, and thus a positive electrode and a lithium secondary battery having high power can be provided.

[0057] However, as the content of Ni increases, the long-term storage stability and life stability of the positive electrode or the secondary battery can be relatively deteriorated. However, according to an exemplary embodiment, while maintaining the electrical conductivity by including Co, the life stability and capacity retention characteristics can be improved by using Mn.

[0058] In some embodiments, the positive active material or the lithium-transition metal composite oxide particles can further include a coating element or a doping element. For example, the coating element or the doping element can include Al, Ti, Ba, Zr, Si, B, Mg, P, an alloy thereof, or an oxide thereof. These substances can be used alone or in combination. The positive active material particles can be passivated by the coating element or the doping element, and the stability and life characteristics against penetration of external objects can be further improved.

[0059] According to an exemplary embodiment, the lithium-transition metal composite oxide particles can have a grain size of 250 nm or more.

[0060] For example, as shown in Chemical Formula 1, when the Ni content in the lithium-transition metal composite oxide particles is relatively increased (e.g., a Ni-rich composition), high capacity and high power characteristics can be easily achieved. However, in this case, the mechanical strength and chemical stability of the positive electrode active material can be deteriorated, and thus, particle breakage can occur during, for example, a pressing process for forming the positive electrode active material layer 110. Accordingly, the electrolyte solution can penetrate into the particles, and gas can be generated due to a side reaction.

[0061] In addition, as repeated charging and discharging operations are repeated, volume shrinkage / expansion of the positive electrode active material can be repeatedly generated. Accordingly, in the case of a high-Ni positive electrode active material, particle breakage can be further generated.

[0062] However, according to an exemplary embodiment, by reducing the inter-grain or inter-particle boundary region, a lithium-transition metal composite oxide having a grain size of 250 nm or more can be used to increase the particle strength and reduce particle breakage.

[0063] Accordingly, gas generation due to repeated charging / discharging can be inhibited, and breakage generation due to volume shrinkage / expansion can also be prevented. Accordingly, even in a high-temperature environment, a stable capacity characteristic can be provided while improving the life characteristic of the lithium secondary battery.

[0064] In an exemplary embodiment, the "grain size" is a value measured through X-ray diffraction (XRD) analysis. The grain size can be calculated and obtained using a full width at half maximum (FWHM) obtained through XRD analysis according to a Scherrer equation (Formula 1 below).

[0065] [Formula 1]

[0066]

[0067] In Formula 1 above, L is the grain size, λ is the wavelength of X-rays, β is the half-width of the corresponding peak, and θ is the diffraction angle. In an exemplary embodiment, the half-width in the XRD analysis for measuring the grain size can be measured from the peak of the (003) plane.

[0068] In some embodiments, in Formula 1 above, β can be a half-width obtained by correcting a value derived from a device. In one embodiment, Si can be used as a standard material reflecting a device-derived value. In this case, a half-width profile of Si over the entire 2θ range can be fitted, and the device-derived half-width can be expressed as a function of 2θ. Thereafter, a value obtained by subtracting and correcting the device-derived half-width value from the corresponding 2θ of the above function can be used as β.

[0069] If the grain size is excessively increased, the power characteristics of the lithium-transition metal composite oxide particles can be deteriorated. In an exemplary embodiment, the grain size can be in the range of 250 nm to 1000 nm. Within the above range, it can be possible to effectively maintain the thermal stability and life characteristics while fully achieving high power and high capacity from a high Ni composition. In a preferred embodiment, the grain size can be in the range of 300 nm to 1000 nm. For example, the grain size can be in the range of 350 nm to 600 nm.

[0070] For example, a nickel-manganese-cobalt precursor (e.g., nickel-cobalt-manganese hydroxide) and a lithium precursor (e.g., lithium hydroxide or lithium carbonate) can be reacted by wet mixing or dry mixing, and then the reaction product can be fired to prepare the lithium-transition metal composite oxide particles.

[0071] In one embodiment, the grain size of the lithium-transition metal composite oxide particles can be controlled by adjusting the firing temperature.

[0072] In terms of crystallography, the lithium-transition metal composite oxide particles can have a single crystal and / or a polycrystal structure. In one embodiment, the positive active material can include a mixture or a blend of single crystal particles and polycrystal particles having the aforementioned grain size.

[0073] In terms of morphology, the lithium-transition metal composite oxide particles can have a single particle shape or a primary particle shape.

[0074] The term "single particle shape" herein can be used to exclude a secondary particle structure in which a plurality of primary particles can be agglomerated or combined with each other. In one embodiment, the single particle shape can include a monolithic shape in which several (e.g., 2 to 10) independent particles are adjacent to or attached to each other.

[0075] According to an exemplary embodiment, the lithium-transition metal composite oxide particles can have an XRD peak intensity ratio of 9.8% or less, defined by Equation 2 below.

[0076] [Equation 2]

[0077] XRD peak intensity ratio (%) = 100 x I(110) / {I(110) + I(003)}

[0078] In Equation 2, I(110) denotes a peak intensity or maximum height of a (110) plane obtained by X-ray diffraction (XRD) analysis of the lithium-transition metal composite oxide particles, and I(003) denotes a peak intensity or maximum height of a (003) plane obtained by XRD analysis of the lithium-transition metal composite oxide particles.

[0079] For example, XRD analysis of the dried powder of the lithium-transition metal composite oxide particles can be performed using copper Kα rays as a light source, in a diffraction angle (2θ) range of 10 to 120, at a scan rate of 0.0065° / step.

[0080] Within the above range of the XRD peak intensity ratio, the ion propagation length and the ion diffusion length on the (110) plane through which lithium ions diffuse can be reduced. In addition, the peak intensity ratio with respect to the (003) plane intersecting the (110) plane can be adjusted to reflect the aspect ratio of the particles.

[0081] Accordingly, power reduction due to an increase in lithium diffusion length or an excessive increase in the aspect ratio of the particles can be prevented. Furthermore, as described above, the relative reduction in power / capacity due to an increase in the grain size to 250 nm or more, preferably 300 nm or more, can be mitigated or compensated for by adjusting the XRD peak intensity ratio.

[0082] Accordingly, by controlling the grain size, gas generation due to particle breakage can be prevented, thereby improving life stability, and by adjusting the XRD peak intensity ratio, the mobility of lithium ions can be improved, thereby also improving power / capacity.

[0083] In one embodiment, the XRD peak intensity ratio can be 4 to 9.8, preferably 5 to 9.8. Within the above range, power characteristics can be improved while maintaining surface stability and life characteristics of the lithium-transition metal composite oxide particles.

[0084] In some embodiments, the lithium-transition metal composite oxide particles can have an XRD peak area ratio of 17% or less, defined by Equation 3 below.

[0085] [Equation 3]

[0086] XRD peak area ratio (%) = 100 x A(110) / {A(110) + A(003)}

[0087] In Equation 3, A(110) denotes the peak area of the (110) plane obtained through XRD analysis of the lithium-transition metal composite oxide particles, and A(003) denotes the peak area of the (003) plane obtained through XRD analysis of the lithium-transition metal composite oxide particles.

[0088] The peak area ratio can be controlled together with the above-described peak intensity ratio, so that the effects of controlling the diffusion length of lithium ions and the aspect ratio through XRD values can be more fully achieved.

[0089] In preferred embodiments, the XRD peak area ratio can be 10% to 17%, more preferably 12% to 17%.

[0090] For example, a transition metal precursor (e.g., a Ni-Co-Mn precursor) for preparing lithium-transition metal complex oxide particles can be prepared through a co-precipitation reaction.

[0091] The above-described transition metal precursor can be prepared through a co-precipitation reaction of metal salts. The metal salts can include nickel salts, manganese salts, and cobalt salts.

[0092] Examples of the nickel salts can include nickel sulfate, nickel hydroxide, nickel nitrate, nickel acetate, and hydrates thereof, etc. Examples of the manganese salts can include manganese sulfate, manganese acetate, and hydrates thereof, etc. Examples of the cobalt salts can include cobalt sulfate, cobalt nitrate, cobalt carbonate, and hydrates thereof, etc.

[0093] The metal salts can be mixed in a ratio to satisfy the content or concentration ratio of each metal described with reference to Chemical Formula 1 with a precipitant and / or a chelating agent to prepare an aqueous solution. The aqueous solution can be co-precipitated in a reactor to prepare a transition metal precursor.

[0094] The precipitant can include basic compounds such as sodium hydroxide (NaOH), sodium carbonate (Na2CO3), etc. The chelating agent can include, for example, ammonia (e.g., NH3H2O), ammonium carbonate (e.g., NH3HCO3), etc.

[0095] For example, the temperature of the co-precipitation reaction can be adjusted in the range of about 40 to 60°C. The reaction time can be adjusted in the range of about 24 to 72 hours.

[0096] For example, lithium-transition metal complex oxide particles can be prepared by the mutual reaction of a transition metal precursor and a lithium precursor. The lithium precursor compound can include, for example, lithium carbonate, lithium nitrate, lithium acetate, lithium oxide, lithium hydroxide, etc. These substances can be used alone or in combination.

[0097] Thereafter, for example, lithium impurities or unreacted precursors can be removed through a washing process, and through a heat treatment (firing) process, metal particles can be fixed or crystallinity can be increased. In one embodiment, the temperature of the heat treatment can be in the range of about 600 to 1000°C.

[0098] For example, the above-described XRD peak ratio can vary with the above-described co-precipitation reaction time, reaction temperature, heat treatment temperature, etc.

[0099] In some embodiments, in addition to the lithium-transition metal complex oxide particles having the above-described grain size and XRD peak ratio, the positive active material can further include an active material having a morphologically secondary particle shape.

[0100] For example, the positive active material can include first lithium-transition metal composite oxide particles and second lithium-transition metal composite oxide particles. The first lithium-transition metal composite oxide particles can have a single particle shape having a crystallographic single crystal or polycrystal structure, and can have the above-described grain size and XRD peak ratio. The second lithium-transition metal composite oxide particles can have a composition of Formula 1, and can have a secondary particle structure.

[0101] In one embodiment, the grain size of the second lithium-transition metal composite oxide particles can be less than 250 nm. In one embodiment, the XRD peak intensity ratio of the second lithium-transition metal composite oxide particles can exceed 9.8.

[0102] In one embodiment, the second lithium-transition metal composite oxide particles can include a concentration gradient region between the particle center and the particle surface. In this case, for example, the concentration or molar ratio of Ni can decrease in the concentration gradient region in the direction from the particle center to the particle surface.

[0103] In this case, the concentration of Mn or Co can increase in the concentration gradient region in the direction from the particle center to the particle surface.

[0104] When the first lithium-transition metal composite oxide particles and the second lithium-transition metal composite oxide particles are used together, the mixed weight ratio (first lithium-transition metal composite oxide particles: second lithium-transition metal composite oxide particles) can be 1:9 to 9:1.

[0105] In one embodiment, in a scanning electron microscope (SEM) cross-section of the positive active material layer 110, the ratio of the cross-sectional area of the first lithium-transition metal composite oxide particles (sum of cross-sectional areas of particles) to the cross-sectional area of the second lithium-transition metal composite oxide particles (sum of cross-sectional areas of particles) can be 1:4.5 to 4.5:1.

[0106] In a preferred embodiment, the weight ratio can be adjusted in the range of 3:7 to 7:3 to achieve a balance between sufficient power and life performance through the above-described grain size and XRD peak ratio.

[0107] In one embodiment, in a scanning electron microscope (SEM) cross-section of the positive active material layer 110, the ratio of the cross-sectional area of the first lithium-transition metal composite oxide particles to the cross-sectional area of the second lithium-transition metal composite oxide particles can be 2:3.7 to 3.7:2.

[0108] As described above, even when lithium-transition metal composite oxide particles in the form of high-Ni secondary particles having a relatively small grain size are used, by using the first lithium-transition metal composite oxide particles, the mechanical stability and thermal stability of the entire positive electrode active material can be improved to provide stable life characteristics.

[0109] In one embodiment, the particle diameter (D50) (e.g., the average particle diameter in the cumulative volume distribution) of the lithium-transition metal composite oxide particles can be about 1 μm to 20 μm. If the first lithium-transition metal composite oxide particles and the second lithium-transition metal composite oxide particles are used together, the particle diameter of the second lithium-transition metal composite oxide particles can be greater than the particle diameter of the first lithium-transition metal composite oxide particles.

[0110] For example, the particle diameter of the first lithium-transition metal composite oxide particles can be about 1 μm to 10 μm, preferably about 3 μm to 7 μm. The particle diameter of the second lithium-transition metal composite oxide particles can be about 11 μm to 20 μm, preferably about 11 μm to 16 μm.

[0111] In one embodiment, the specific surface area (BET) of the lithium-transition metal composite oxide particles can be about 0.1 m 2 / g to 1 m 2 / g.

[0112] The positive electrode active material particles including the above-described lithium-transition metal composite oxide particles can be mixed and stirred together with a binder, a conductive agent, and / or a dispersant in a solvent to form a slurry. The slurry can be coated on the positive electrode current collector 105, dried, and pressed to obtain the positive electrode 100.

[0113] The positive electrode current collector 105 can include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or an alloy thereof can be used.

[0114] The binder can include an organic-based binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or an aqueous binder such as styrene butadiene rubber (SBR), which can be used together with a thickening agent such as carboxymethyl cellulose (CMC).

[0115] For example, a PVDF-based binder can be used as the positive electrode binder. In this case, the amount of the binder used to form the positive electrode active material layer can be reduced, and the amount of the positive electrode active material can be relatively increased. Accordingly, the capacity and power of the secondary battery can be further improved.

[0116] A conductive agent can be added to facilitate electron migration between the active material particles. For example, the conductive agent can include carbon-based materials, such as graphite, carbon black, graphene, carbon nanotubes, etc., and / or metal-based materials, such as tin, tin oxide, titanium oxide, perovskite materials (e.g., LaSrCoO3 or LaSrMnO3).

[0117] In an exemplary embodiment, the negative electrode 130 can include a negative electrode current collector 125 and a negative electrode active material layer 120 formed by coating a negative electrode active material on the negative electrode current collector 125.

[0118] The negative electrode active material can include a substance capable of adsorbing and ejecting lithium ions. For example, a carbon-based substance such as crystalline carbon, amorphous carbon, carbon composite, or carbon fiber, a lithium alloy, a silicon-based compound, tin, etc. can be used. The amorphous carbon can include hard carbon, coke, meso-carbon microbeads (MCMB), meso-phase pitch-based carbon fiber (MPCF), etc.

[0119] The crystalline carbon can include graphite-based substances such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc. The lithium alloy can further include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0120] The silicon-based compound can include, for example, silicon oxide or a silicon-carbon composite such as silicon carbide (SiC).

[0121] For example, the negative electrode active material can be mixed and stirred together with a binder, a conductive agent, and / or a dispersant in a solvent to form a slurry. The slurry can be coated on at least one surface of the negative electrode current collector 125, dried, and pressed to obtain the negative electrode 130.

[0122] The binder and the conductive agent used can be substantially the same as or similar to those used in the positive electrode active material layer 110. In some embodiments, the binder used for the negative electrode can include an aqueous binder such as styrene butadiene rubber (SBR), which can be used together with a thickening agent such as carboxymethyl cellulose (CMC), whereby the compatibility with carbon-based active materials can be improved.

[0123] A separator layer 140 can be interposed between the positive electrode 100 and the negative electrode 130. The separator layer 140 can include a porous polymer film made of, for example, a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc. The separator layer 140 can also be formed of a non-woven fabric including high-melting point glass fibers, polyethylene terephthalate fibers, etc.

[0124] In some embodiments, the area and / or volume of the negative electrode 130 (e.g., the contact area with the separator layer 140) can be greater than that of the positive electrode 100. Thus, lithium ions generated from the positive electrode 100 can be easily transferred to the negative electrode 130 without being lost due to, for example, precipitation or sedimentation.

[0125] In an exemplary embodiment, an electrode unit can be defined by the positive electrode 100, the negative electrode 130, and the separator layer 140, and a plurality of electrode units can be stacked to form an electrode assembly having, for example, a jelly roll shape. For example, the electrode assembly 150 can be formed by winding, laminating, or folding the separator layer 140.

[0126] The electrode assembly 150 can be accommodated in the case 160 together with an electrolyte solution to form a lithium secondary battery. In an exemplary embodiment, the electrolyte solution can include a non-aqueous electrolyte solution.

[0127] The non-aqueous electrolyte solution can include a lithium salt and an organic solvent. The lithium salt can be represented by Li + X - The anion X - may include, for example, 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 - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , (CF3CF2SO2)2N- wait.

[0128] Organic solvents can include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. These solvents can be used alone or in combination.

[0129] like Figure 1 As shown, tabs (positive and negative tabs) can protrude from the positive current collector 105 and negative current collector path 125 of each electrode unit to extend to one side of the housing 160. The tabs can be fused to one side of the housing 160 to form electrode leads (positive lead 107 and negative lead 127) that extend or are exposed outside the housing 160.

[0130] Lithium-ion batteries can be manufactured in various shapes, including cylindrical (canned), triangular (prismatic), pouch, and coin-shaped.

[0131] Preferred embodiments are presented below to describe the invention in more detail. However, the following examples are merely illustrative, and those skilled in the art will clearly understand that various changes and modifications can be made within the scope and spirit of the invention. Such changes and modifications are suitably included within the appended claims.

[0132] Preparation of first lithium-transition metal complex oxide particles

[0133] NiSO4, CoSO4, and MnSO4 were mixed with distilled water in a ratio of 0.8:0.1:0.1, respectively. The dissolved oxygen in the distilled water was removed by bubbling with N2 for 24 hours. This solution was then added to a reactor at 50°C, and a co-precipitation reaction was carried out for 48 hours using NaOH and NH3H2O ​​as precipitating and chelating agents to obtain Ni as a transition metal precursor. 0.8 C 0.1 Mn 0.1 (OH)2. The obtained precursor was dried at 80°C for 12 hours, and then dried at 110°C for another 12 hours.

[0134] Lithium hydroxide and transition metal precursors were added to a dry high-speed mixer at a ratio of 1.05:1 and mixed uniformly for 5 minutes. The mixture was put into a kiln and heated to 950°C at a heating rate of 2°C / minute and maintained at 950°C for 10 hours. During the heating and maintaining, oxygen was continuously passed at a flow rate of 10 mL / minute. After the firing was completed, it was naturally cooled to room temperature, pulverized and distributed to obtain first lithium-transition metal composite oxide particles in a single particle shape (including single crystal structure and polycrystal structure) as LiNi 0.8 C 0.1 Mn 0.1 O2 (Particle 1-1) represents a positive electrode active material.

[0135] The reaction time or reaction temperature in the reactor, or the firing time or firing temperature in the firing process was changed to further prepare particles 1-2 to 1-4. For each of the first lithium-transition metal composite oxide particles, XRD analysis was performed to calculate the grain size using Formula 1 described above and to calculate the XRD peak ratio.

[0136] Preparation of second lithium-transition metal complex oxide particles

[0137] Nickel precursor (NiSO4), manganese precursor (MnSO4), and cobalt precursor (CoSO4) were mixed by continuously changing the mixing ratio to form a precipitate so that a concentration gradient region is formed between the particle center and the particle surface. The total composition was LiNi 0.80 Co 0.11 Mn 0.09 O2, the composition of the particle center was LiNi 0.802 Co 0.11 Mn 0.088 O2, and the composition of the particle surface was LiNi 0.77 Co 0.11 Mn 0.12 O2. By mixing the precipitate with LiOH as a lithium source and then performing firing, second lithium-transition metal composite oxide particles (Particle 2-1) having a secondary particle structure and a particle size of 206 nm and second lithium-transition metal composite oxide particles (Particle 2-2) having a secondary particle structure and a particle size of 248 nm were obtained. The XRD peak ratio of each particle was calculated.

[0138] The XRD analysis values and particle diameters of the lithium-transition metal composite oxide particles are shown in Table 2.

[0139] The detailed XRD analysis equipment / conditions are shown in Table 1 below.

[0140] [Table 1]

[0141]

[0142] Fifty particles were randomly selected from the scanning electron microscope cross-sectional image analysis, and the average of the lengths of the long axis and the short axis was defined as the particle diameter. The average particle diameter of the 50 particles was calculated.

[0143] [Table 2]

[0144]

[0145] Manufacture of secondary battery

[0146] Based on the combination shown in Table 3, a secondary battery was manufactured using the lithium-transition metal complex oxide particles of Table 2 as a positive active material. Specifically, the positive active material particles, ethylene carbon black (Denka Black) as a conductive additive, and PVDF as a binder were mixed in a weight ratio of 97:2:1 to form a positive electrode slurry. The positive electrode slurry was coated on an aluminum substrate, dried, and pressed to form a positive electrode. The density of the positive electrode after pressing was controlled to be 3.55 g / cc or more.

[0147] A negative electrode slurry was prepared by mixing 93 wt% of natural graphite as a negative active material, 5 wt% of flaky conductive additive KS6, 1 wt% of SBR as a binder, and 1 wt% of CMC as a thickening agent. The negative electrode slurry was coated on a copper substrate, dried, and pressed to form a negative electrode.

[0148] The positive electrode and the negative electrode obtained as described above were notched in an appropriate size and stacked, with a separator (polyethylene, thickness: 25 μm) interposed between the positive electrode and the negative electrode to form an electrode unit. Each tab portion of the positive electrode and the negative electrode was welded. The welded positive electrode / separator / negative electrode assembly was inserted into a pouch, and three sides (for example, except for the electrolyte injection side) of the pouch were sealed. The tab portion was also included in the sealed portion. Electrolyte was injected through the electrolyte injection side, and then the electrolyte injection side was also sealed. Subsequently, the above structure was immersed for more than 12 hours.

[0149] An electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio) and adding 1 wt% of vinylene carbonate, 0.5 wt% of 1,3-propene sultone (PRS), and 0.5 wt% of lithium bis(oxalato)borate (LiBOB).

[0150] The lithium secondary battery manufactured as above was pre-charged by applying a pre-charge current (5 A) corresponding to 0.25C for 36 minutes. After 1 hour, the battery was degassed, aged for more than 24 hours, and then subjected to formation charge-discharge (charge condition CC-CV 0.2C 4.2V 0.05C cutoff, discharge condition CC 0.2C 2.5V cutoff).

[0151] [Table 3]

[0152]

[0153]

[0154] Experimental Example

[0155] (1) Evaluation of high-temperature gas generation

[0156] After charging (1C 4.2V 0.1C cutoff) of the lithium secondary batteries of the examples and comparative examples having the positive electrode compositions in Table 2, the amount of gas generated after storage for 1 week and 4 weeks in a constant temperature chamber at 60°C was measured using gas chromatography (GC) analysis. In order to measure the total amount of generated gas, a hole was formed through a vacuum chamber having a predetermined volume (V), and the pressure change was measured to calculate the volume of the generated gas.

[0157] (2) Evaluation of high-temperature gas generation after repeated charge / discharge

[0158] At 45°C in a room, the lithium secondary batteries of the examples and comparative examples having the positive electrode compositions in Table 2 were repeatedly charged (CC-CV 1.0C 4.2V 0.05C cutoff) and discharged (CC 1.0C 2.7V cutoff) 100 times and 300 times, and then the amount of gas was measured by the same method as in (1) above.

[0159] (3) Evaluation of life at 45°C (capacity retention rate)

[0160] At 45°C in a room, the lithium secondary batteries of the examples and comparative examples having the positive electrode compositions in Table 2 were repeatedly charged (CC-CV 1.0C 4.2V 0.05C cutoff) and discharged (CC 1.0C 2.7V cutoff) 300 times, and then the capacity retention was measured by the percentage (%) of the discharge capacity of the 300th cycle with respect to the discharge capacity of the 1st cycle.

[0161] (4) Evaluation of DCIR at room temperature

[0162] The lithium secondary batteries of the examples and comparative examples having the positive electrode compositions in Table 2 were charged / discharged twice (SOC 100%) under the conditions of 25°C, 0.5C, and CC-CV, and then charged again under the conditions of 0.5C CC-CV. Subsequently, 0.5C discharging was performed until the SOC reached 50%. Thereafter, the voltage was measured after standing for 30 minutes (first voltage).

[0163] Thereafter, i) 1C, discharging for 10 seconds and standing for 40 seconds, ii) 0.75C, charging for 10 seconds, standing for 40 seconds, and then the voltage (second voltage) was measured. The difference between the first voltage and the second voltage was used to measure the DCIR.

[0164] The results are shown in Table 4 below.

[0165] [Table 4]

[0166]

[0167]

[0168] Referring to Table 4, in the examples in which the lithium-transition metal composite oxide having a grain size of 250 nm or more and satisfying the XRD peak ratio below a predetermined value was used, the gas evolution was suppressed compared to the comparative examples, and improved capacity retention and DCIR properties were obtained at room temperature.

[0169] In the case of Example 8, as the amount of the first lithium-transition metal composite oxide particles having a single particle shape increased, the gas evolution was suppressed, but the capacity retention and resistance properties were slightly reduced.

Claims

1. A positive electrode active material for a lithium secondary battery, comprising lithium-transition metal composite oxide particles, the lithium-transition metal composite oxide particles having a composition represented by Chemical Formula 1: [Chemical Formula 1] Li x Ni a M 1-a O 2+z wherein in Chemical Formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, -0.1≤z≤0.1, and M is at least one element selected from the group consisting of Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, and Zr, wherein a grain size of the lithium-transition metal composite oxide particles measured by Formula 1 is in the range of 300 nm to 1000 nm, and wherein an XRD peak area ratio of the lithium-transition metal composite oxide particles defined by Formula 3 is in the range of 10% to 17%: [Formula 1] wherein, in Formula 1, L is the grain size, λ is a wavelength of X-rays, β is a half-width of a peak corresponding to a (003) plane, and θ is a diffraction angle, [Formula 3] XRD peak area ratio (%) = 100 x A(110) / {A(110)+A(003)} wherein, in Formula 3, A(110) is a peak area of a peak corresponding to a (110) plane obtained by XRD analysis of the lithium-transition metal composite oxide particles, and A(003) is a peak area of a peak corresponding to a (003) plane obtained by XRD analysis of the lithium-transition metal composite oxide particles.

2. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the grain size of the lithium-transition metal composite oxide particles is in the range of 350 nm to 1000 nm.

3. The positive electrode active material for a lithium secondary battery according to claim 1, wherein an XRD peak intensity ratio of the lithium-transition metal composite oxide particles defined by Formula 2 is 9.8% or less: [Formula 2] XRD peak intensity ratio (%) = 100 x I(110) / {I(110)+I(003)} wherein in Formula 2, I(110) is a maximum height of a peak corresponding to a (110) plane obtained by XRD analysis of the lithium-transition metal composite oxide particles, and I(003) is a maximum height of a peak corresponding to a (003) plane obtained by XRD analysis of the lithium-transition metal composite oxide particles.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the positive electrode active material comprises first lithium-transition metal composite oxide particles and second lithium-transition metal composite oxide particles having a secondary particle structure, wherein a grain size of the first lithium-transition metal composite oxide particles measured by Formula 1 is in the range of 300 nm to 1000 nm, and an XRD peak area ratio of the lithium-transition metal composite oxide particles defined by Formula 3 is in the range of 10% to 17%. 5.The positive electrode active material for a lithium secondary battery according to claim 4, wherein the first lithium-transition metal composite oxide particles have a single particle shape having a crystallographic single crystal or polycrystal structure. 6.The positive electrode active material for a lithium secondary battery according to claim 4, wherein the second lithium-transition metal composite oxide particles have a grain size of less than 250 nm as measured by XRD analysis. 7.The positive electrode active material for a lithium secondary battery according to claim 4, wherein a weight ratio of the first lithium-transition metal composite oxide particles to the second lithium-transition metal composite oxide particles is 3:7 to 7:

3. 8.The positive electrode active material for a lithium secondary battery according to claim 4, wherein a particle size D50 of the second lithium-transition metal composite oxide particles is greater than a particle size of the first lithium-transition metal composite oxide particles. 9.A lithium secondary battery comprising: a positive electrode including a positive electrode active material layer including the positive electrode active material according to claim 1; and a negative electrode opposite to the positive electrode. 10.The lithium secondary battery according to claim 9, wherein the positive electrode active material includes first lithium-transition metal composite oxide particles and second lithium-transition metal composite oxide particles having a secondary particle shape, wherein a grain size of the first lithium-transition metal composite oxide particles as measured by Equation 1 is in a range of 300 nm to 1000 nm, and an XRD peak area ratio of the lithium-transition metal composite oxide particles defined by Equation 3 is in a range of 10% to 17%. 11.The lithium secondary battery according to claim 10, wherein a ratio of a cross-sectional area of the first lithium-transition metal composite oxide particles to a cross-sectional area of the second lithium-transition metal composite oxide particles is 1:4.5 to 4.5:1 in a scanning electron microscope (SEM) cross-sectional image of the positive electrode active material layer. 12.The lithium secondary battery according to claim 10, wherein a ratio of a cross-sectional area of the first lithium-transition metal composite oxide particles to a cross-sectional area of the second lithium-transition metal composite oxide particles is 2:3.7 to 3.7:2 in a scanning electron microscope (SEM) cross-sectional image of the positive electrode active material layer. ​

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