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

Abstract

A positive electrode active material for a lithium secondary battery includes lithium metal oxide particles containing nickel (Ni) and cobalt (Co). The lithium metal oxide particles include a concentration gradient region formed in at least one region between a center of the lithium metal oxide particles and a surface of the lithium metal oxide particles. A ratio of a concentration at the surface expressed in atomic percentage of Co to a concentration at the center expressed in atomic percentage of Co is 6.7 or more.

Description

[0001] Cross-reference and priority claims of related applications

[0002] This application claims priority to Korean Patent Application No. 10-2020-0118216, filed with the Korean Intellectual Property Office (KIPO) on September 15, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention relates to a positive electrode active material for lithium secondary batteries and a lithium secondary battery including the positive electrode active material. More specifically, this invention relates to a lithium metal oxide-based positive electrode active material for lithium secondary batteries and a lithium secondary battery including the positive electrode active material. Background Technology

[0004] With the development of information and display technologies, rechargeable and dischargeable secondary batteries have been widely used as power sources for mobile electronic devices such as portable cameras, mobile phones, and laptops. Recently, secondary batteries or battery packs incorporating them have also been developed and applied to environmentally friendly vehicles, such as hybrid vehicles, as their power source.

[0005] Secondary batteries include, for example, lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Lithium-ion batteries have attracted much attention due to their high operating voltage and energy density per unit weight, high charge rate, and compact size.

[0006] For example, a lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator layer; and an electrolyte immersing the electrode assembly. The lithium secondary battery may further include a housing having, for example, a pouch shape.

[0007] As the application of lithium-ion rechargeable batteries continues to expand, there is a need for rechargeable batteries with greater capacity, power, and lifespan within a limited unit volume. For example, there is a need to construct rechargeable batteries to increase the capacity of the positive and negative electrode active materials, while also improving storage and mechanical stability.

[0008] For example, Korean Patent Application No. 10-2017-0093085 discloses a positive electrode active material comprising a transition metal compound and an ion adsorption binder, which may not adequately provide high energy density, lifetime, and high-temperature stability. Summary of the Invention

[0009] According to one aspect of the present invention, a positive electrode active material for a lithium secondary battery having improved electrical reliability, mechanical reliability, and operational stability is provided.

[0010] According to one aspect of the present invention, a lithium secondary battery is provided, which has improved electrical reliability, mechanical reliability, and operational stability.

[0011] A positive electrode active material for a lithium secondary battery includes lithium metal oxide particles containing nickel (Ni) and cobalt (Co). The lithium metal oxide particles include a concentration gradient region formed in at least one region between a center of the lithium metal oxide particles and a surface of the lithium metal oxide particles. A ratio of a concentration at the surface expressed in atomic percentage of Co with respect to an average concentration expressed in atomic percentage of Co in an entire region of the lithium metal oxide particles is 1.8 or more.

[0012] In some embodiments, a ratio of a concentration at the surface expressed in atomic percentage of Co with respect to a concentration at the center expressed in atomic percentage of Co can be 5.3 or more.

[0013] In some embodiments, a ratio of a concentration at the surface expressed in atomic percentage of Co with respect to an average concentration expressed in atomic percentage of Co in an entire region of the lithium metal oxide particles can be 2.0 or more.

[0014] In some embodiments, a ratio of a concentration at the surface expressed in atomic percentage of Co with respect to a concentration at the center expressed in atomic percentage of Co is 6.0 or more.

[0015] In some embodiments, in the lithium metal oxide particles, a concentration of Ni in the concentration gradient region can decrease in a direction from the center to the surface, and a concentration of Co in the concentration gradient region can increase in the direction from the center to the surface.

[0016] In some embodiments, the lithium metal oxide particles can further include manganese (Mn), and a concentration of manganese (Mn) from the center to the surface can be constant.

[0017] In some embodiments, Ni and Co can each have a slope of a concentration gradient in the concentration gradient region.

[0018] In some embodiments, the lithium metal oxide particles can have a core region occupying 50% or more of a radius of the lithium metal oxide particles from the center, and a concentration of the metal elements in the core region can be constant.

[0019] In some embodiments, the concentration gradient region can extend from a surface of the core region.

[0020] In some embodiments, the lithium metal oxide particles can have a shell region extending in a direction from the surface to the center, and a concentration of the metal elements in the shell region can be constant.

[0021] In some embodiments, the concentration gradient region can extend from the surface of the core region to the inner surface of the shell region.

[0022] In some embodiments, the distance of the shell region from the surface of the lithium metal oxide particle can be 10 nm to 200 nm.

[0023] In some embodiments, the overall average composition of the lithium metal oxide particle can be represented by Chemical Formula 1:

[0024] [Chemical Formula 1]

[0025] Li x Ni a Co b M3 c O y

[0026] In Chemical Formula 1, M3 includes at least one element selected from Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, and B, and 0 < x ≤ 1.2, 2 ≤ y ≤ 2.02, 0 ≤ a ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, and 0 < a + b + c ≤ 1.

[0027] In some embodiments, the average concentration of Ni in the entire region of the lithium metal oxide particle can be in the range of 0.6 atomic % to 0.99 atomic %.

[0028] A lithium secondary battery includes a cathode including a cathode active material according to the above-described embodiments and an anode opposite the cathode.

[0029] According to an exemplary embodiment of the present application, a cathode active material of a lithium secondary battery can include a concentration gradient region in at least one region between a center portion and a surface portion. For example, a high-nickel (high-Ni) composition can be formed in the center portion, and a relatively high-cobalt (high-Co) composition can be formed in the surface portion. Accordingly, a cathode or cathode active material that provides high capacity and high energy density by the center portion and improved chemical and mechanical stability by the surface portion can be obtained.

[0030] In some embodiments, the concentration gradient of the active metal in the concentration gradient region can remain substantially constant, such that the composition and performance change between the center portion and the surface portion can be stably and gradually achieved.

[0031] In some embodiments, a mixture of natural graphite and artificial graphite can be used as an anode active material of a lithium secondary battery. Accordingly, one structure that capacity and life stability are improved and balanced can be achieved by each of the cathode and the anode. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 and Figure 2 are schematic plan and sectional views illustrating a lithium secondary battery according to an exemplary embodiment.

[0033] Figure 3 and Figure 4 is a graph illustrating a concentration distribution of a positive active material included in an embodiment. DETAILED DESCRIPTION

[0034] According to embodiments of the present application, a positive active material including lithium metal oxide particles having a concentration gradient region and a relatively high cobalt composition on a surface thereof is provided. Accordingly, a lithium secondary battery having improved and balanced energy density, lifespan, and high-temperature stability is also provided.

[0035] Hereinafter, embodiments of the present application will be described in detail. However, the embodiments to be disclosed herein are exemplary, and the present application is not limited to specific embodiments. The terms "first" and "second" used herein are not intended to limit the number or order of elements or objects, but are used only for relatively distinguishing different elements or objects.

[0036] Figure 1 and Figure 2 are schematic plan and sectional views illustrating a lithium secondary battery according to an exemplary embodiment.

[0037] Referring to Figure 1 and Figure 2 , a lithium secondary battery can include a positive electrode 100, a negative electrode 130, and a separator 140 interposed between the positive electrode and the negative electrode.

[0038] The positive electrode 100 can include a positive electrode current collector 105 and a positive active material layer 110 formed by coating a positive active material on the positive electrode current collector 105. In an exemplary embodiment, the positive active material can include lithium metal oxide particles including a concentration gradient region between a center and a surface (e.g., an outermost surface) of the particles.

[0039] In the lithium metal oxide particles, the concentrations of lithium and oxygen are substantially fixed throughout the region of the particles, and at least one of the metal elements other than lithium and oxygen can have a continuous concentration gradient in the concentration gradient region from the center to the surface in a direction.

[0040] The term "continuous concentration gradient" used herein can mean a concentration distribution that continuously changes in a constant tendency between the surface and the center. The constant tendency includes a decreasing tendency or an increasing tendency of the concentration change.

[0041] In some embodiments, a substantially linear concentration gradient can be formed in the concentration gradient region. Accordingly, a substantially constant slope of the concentration gradient can be defined in the concentration gradient region. In one embodiment, the concentration gradient region can have a curved concentration change profile.

[0042] In one embodiment, the concentration of at least one metal element among the metal elements other than lithium included in the lithium metal oxide particle can continuously increase in the concentration gradient region, and the concentration of the at least one metal element can continuously decrease.

[0043] In one embodiment, at least one metal among the metals other than lithium included in the lithium metal oxide particle can have a substantially constant concentration from the center to the surface.

[0044] In one embodiment, the metal elements other than lithium included in the lithium metal oxide particle can include a first metal M1 and a second metal M2. The concentration of the first metal M1 can continuously decrease in the concentration gradient region, and the concentration of the second metal M2 can continuously increase in the concentration gradient region.

[0045] In one embodiment, the metal elements other than lithium included in the lithium metal oxide particle can further include a third metal M3. The third metal M3 can have a substantially constant or uniform concentration from the center to the surface.

[0046] The term "constant concentration" used herein means a substantially uniform concentration without forming a constant increasing or decreasing trend from the center to the surface. Accordingly, it should be understood that the constant concentration can include a concentration increase, a concentration decrease, a concentration outlier, etc. situation locally generated by a deviation of a formation process of the lithium metal oxide particle.

[0047] Further, the term "continuous increase" or "continuous decrease" of the concentration used herein is also understood to include a generally increasing trend or a decreasing trend on the whole, even if there are points partially deviating from the trend due to process deviation.

[0048] For example, the term "concentration" can mean a molar ratio or an atomic ratio of, for example, the first metal to the third metal.

[0049] In an exemplary embodiment, the total average composition of the lithium metal oxide particle can be represented by the following Chemical Formula 1.

[0050] [Chemical Formula 1]

[0051] Li x Ni a Cob M3 c O y

[0052] In the above Chemical Formula 1, M1, M2, and M3 can each include at least one element selected from the group consisting of Ni, Co, Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, and B.

[0053] In the above Chemical Formula 1, 0 < x ≤ 1.2, 2 ≤ y ≤ 2.02, 0 < a < 1, 0 < b < 1, 0 < c < 1, and 0 < a + b + c ≤ 1.

[0054] In the above Chemical Formula 1, M1, M2, and M3 can respectively represent the first metal, the second metal, and the third metal described above. In some embodiments, M1, M2, and M3 can respectively include nickel (Ni), cobalt (Co), and manganese (Mn).

[0055] In Chemical Formula 1, a, b, and c can respectively represent average concentrations of M1, M2, and M3 in the entire particle region. In some embodiments, the first metal M1 in Chemical Formula 1 can be nickel, and for example, 0.6 ≤ a ≤ 0.99 and 0.01 ≤ b + c ≤ 0.4.

[0056] When the concentration of nickel is less than about 0.6, the total capacity and power characteristics of the positive active material can be degraded. When the concentration of nickel exceeds about 0.99, the life and mechanical stability of the positive active material can be degraded.

[0057] In one embodiment, in consideration of both improving capacity and stability, 0.7 ≤ a ≤ 0.95 and 0.05 ≤ b + c ≤ 0.3, or 0.7 ≤ a ≤ 0.9 and 0.1 ≤ b + c ≤ 0.3. For example, 0.77 ≤ a ≤ 0.83, 0.07 ≤ b ≤ 0.13, and 0.07 ≤ c ≤ 0.13, or 0.79 ≤ a ≤ 0.81, 0.09 ≤ b ≤ 0.11, and 0.09 ≤ c ≤ 0.11.

[0058] In a non-limiting example, the average composition of nickel: cobalt: manganese of the lithium metal oxide particles can be substantially 8:1:1, and lithium metal oxide particles that provide improved capacity and power while maintaining long-term life stability and resistance uniformity can be achieved.

[0059] For example, nickel can be a metal related to the capacity of a lithium secondary battery. As the content of nickel increases, the capacity and power of the lithium secondary battery can be improved. However, if the content of nickel is excessively increased, the life, mechanical, and electrical stability can be degraded.

[0060] For example, if the content of nickel is excessively increased, defects such as ignition and short circuit can not be sufficiently suppressed when an external object penetration occurs, and sufficient capacity retention can not be provided during repeated charging and discharging at high temperature (e.g., 60°C or more).

[0061] However, according to some embodiments, nickel can be selected as the first metal M1, and the content of nickel at the particle center can be relatively increased to obtain sufficient capacity and power of the lithium secondary battery. The concentration of nickel at the surface can be relatively reduced to suppress the decrease in capacity and lifespan at high temperature.

[0062] For example, cobalt (Co) can be used as a metal related to the electrical conductivity or resistance of the lithium secondary battery. In some embodiments, the content of cobalt at the surface can be increased relative to the content of cobalt at the center to obtain stability of the lithium metal oxide while maintaining improved electrical conductivity and low resistance. Accordingly, the lifespan and capacity of the lithium secondary battery including the positive electrode active material can be improved.

[0063] For example, manganese (Mn) can be used as a metal related to the mechanical stability and electrical stability of the lithium secondary battery. In some embodiments, the concentration of manganese can be maintained substantially fixed or constant over substantially the entire region of the lithium metal oxide particle. Accordingly, the stability of the positive electrode active material at high temperature can be enhanced, and defects such as ignition and short circuit of the lithium secondary battery can be suppressed while increasing the lifespan of the lithium secondary battery.

[0064] In some embodiments, the lithium metal oxide particle can include a core region including a predetermined distance (first distance) from the center to the surface.

[0065] For example, the first distance can be 1 μm or more from the particle center. In one embodiment, the first distance can be 3 μm or more, preferably 5 μm or more, from the particle center.

[0066] In one embodiment, the core region can occupy 50% or more of the radius of the lithium metal oxide particle. In one embodiment, the core region can occupy 60% or more of the radius of the lithium metal oxide particle. In a preferred embodiment, the core region can occupy 70% or more of the radius of the lithium metal oxide particle, and more preferably, can occupy 80% or more of the radius of the lithium metal oxide particle.

[0067] In one embodiment, the first distance of the core region can be appropriately adjusted according to the lengths of the shell region and the concentration gradient region to be described hereinafter while occupying 50% or more of the radius of the lithium metal oxide particle as described above.

[0068] The core region can be provided as a constant concentration region (e.g., a first constant concentration region). For example, the molar ratio or atomic ratio of Ni, Co, and Mn in the core region can be maintained substantially constant, and a higher concentration of nickel can be maintained in the core region compared to the concentration of nickel in other regions.

[0069] As described above, the distance of the core region can occupy 50% or more of the radius of the lithium metal oxide particle, and thus a sufficiently high power and capacity can be achieved from the center of the particle.

[0070] In some embodiments, the lithium metal oxide particle can include a shell region including a predetermined distance (second distance) from the surface toward the center. The shell region can function as a region for extending or maintaining the metal concentration ratio of the surface to the predetermined distance.

[0071] The second distance of the shell region can be less than the first distance of the core region. In one embodiment, the second distance can be in the range of from about 10 nm to 200 nm. For example, the second distance can be in the range of from about 20 nm to 200 nm, or from about 30 nm to 100 nm. Preferably, the second distance can be in the range of from about 30 nm to 60 nm.

[0072] For example, the shell region can be provided as a constant concentration region (e.g., a second constant concentration region). For example, the molar ratio or atomic ratio of Ni, Co, and Mn in the shell region can be maintained substantially constant, and a relatively high Co concentration compared to the core region can be maintained.

[0073] The core region having a high-Ni composition can be extended from the center to occupy the largest area in the lithium metal oxide particle, and thus a high-capacity and high-output structure can be effectively achieved. In addition, the core region can be substantially covered by a shell region having a relatively small thickness to improve the lifespan and capacity retention performance at high temperatures.

[0074] A concentration gradient region can be formed between the core region and the surface of the lithium metal oxide particle. In an exemplary embodiment, the concentration gradient region of the lithium metal oxide particle can be formed in a specific region between the center of the particle and the surface of the particle.

[0075] For example, the concentration gradient region can be disposed between the outer surface of the core region and the outer surface of the lithium metal oxide particle.

[0076] In some embodiments, the concentration gradient region can be formed between the core region and the shell region. For example, the concentration gradient region can extend from the outer surface of the core region to the inner surface of the shell region.

[0077] The concentration gradient region can function as a transfer region or a buffer region for the concentration change between the core region and the shell region. Rapid change in concentration between the core region and the shell region can be buffered by the concentration gradient region, so that the capacity and power of the positive active material can become completely uniform or averaged. In addition, both the high-Ni region and the high-Mn region can exist simultaneously through the concentration gradient region, so that capacity and life stability can be improved.

[0078] In some embodiments, the length of the concentration gradient region in the direction from the center of the particle to the surface can be in the range of about 40 nm to 800 nm. Within the above range, the effect of increasing power / capacity by the high-Ni component in the core region can be effectively transferred to the entire particle.

[0079] In one embodiment, the length of the concentration gradient region can be in the range of from about 40 nm to 500 nm, preferably, from about 40 nm to 400 nm, more preferably, from about 40 nm to 300 nm.

[0080] However, the length of the concentration gradient region can be appropriately adjusted without being specifically limited by the above range, in consideration of the Ni and / or Co content in the core region, the distance or thickness of the core region, the Ni and / or Co content in the shell region, etc.

[0081] In an exemplary embodiment, in the concentration gradient region, the concentration (molar ratio or atomic ratio) of Ni can decrease in the direction from the center to the surface, and the content of Co can increase in the direction from the center to the surface.

[0082] As described above, the slope of the concentration gradient in the concentration gradient region can be defined. In some embodiments, the slopes of the concentration gradients of Ni and Co can be substantially the same.

[0083] For example, the concentration of Ni can decrease in the direction from the center to the surface according to the slope of the concentration gradient, and the concentration of Co can increase in the direction from the center to the surface according to the slope of the concentration gradient. The slope of the concentration gradient can represent the ratio of the change in atomic ratio (atom %) according to the increase in distance (nm).

[0084] In an exemplary embodiment, the slope of the concentration gradient can remain substantially constant to form a substantially linear concentration gradient. The term "constant slope of the concentration gradient" can mean a slope of the concentration gradient that can be defined as having a linear trend substantially overall. It should be understood that even if outliers are included in some parts due to process bias, a perfectly linear slope of the concentration gradient is still considered to be a "constant slope of the concentration gradient".

[0085] The surface (or shell region) of the lithium metal oxide particle can include a relatively high Co content compared to the center (or core region).

[0086] In an exemplary embodiment, the ratio of the Co concentration (at%) at the surface to the Co concentration (at%) at the center can be 5.3 or more. Preferably, the ratio of the Co concentration (at%) at the surface to the Co concentration (at%) at the center can be 6.0 or more. More preferably, the ratio of the Co concentration (at%) at the surface to the Co concentration (at%) at the center can be 7.5 or more, 15 or more, or 150 or more.

[0087] In an exemplary embodiment, the ratio of the Co concentration (at%) at the surface to the average Co concentration (at%) of the entire lithium metal oxide particle can be 1.8 or more. Preferably, the ratio of the Co concentration (at%) at the surface to the average Co concentration (at%) of the entire lithium metal oxide particle can be 2.0 or more. More preferably, the ratio of the Co concentration (at%) at the surface to the average Co concentration (at%) of the entire lithium metal oxide particle can be 2.2 or more, 5 or more, or 7.5 or more.

[0088] Within the above concentration ratio range, the relatively high Co composition of the surface or shell region can increase the lifespan and can more effectively enhance the capacity retention performance at high temperatures. In addition, the ignition stability at high temperatures can be maintained.

[0089] In some embodiments, the positive active material or lithium metal oxide particle 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, or alloys or oxides thereof. These coating elements or doping elements can be used alone or in combination. The positive active material particle can be passivated by the coating element or the doping element, and thus the stability against penetration of external objects and the lifespan can be further improved.

[0090] In some embodiments, the lithium metal oxide particle can have a secondary particle structure in which rod-type primary particles can be aggregated. The average particle diameter of the lithium metal oxide particle can be about 3 μm to about 17 μm.

[0091] In the formation of the positive active material or lithium metal oxide particles, a metal precursor solution having different concentrations can be prepared. The metal precursor solution can include a metal precursor to be included in the positive electrode active material. For example, the metal precursor can include a metal halide, a metal hydroxide, an acid salt of a metal, etc.

[0092] For example, the metal precursor can include a nickel precursor, a manganese precursor, and a cobalt precursor.

[0093] In an exemplary embodiment, a first precursor solution having a target composition at the center of the lithium metal oxide particles (e.g., a concentration of nickel, a concentration of manganese, and a concentration of cobalt at the center) and a second precursor solution having a target composition at the surface of the lithium metal oxide particles (e.g., a concentration of nickel, a concentration of manganese, and a concentration of cobalt at the surface) can be prepared.

[0094] Thereafter, the first precursor solution can be reacted and stirred to form a precipitate to form a core region, and the second precursor solution can be introduced from a certain time and mixed while continuously changing a mixing ratio, so that a concentration gradient can be continuously formed from the target composition of the core region to the surface.

[0095] Accordingly, the precipitate can be formed so that the concentration of the metal can form a concentration gradient region within the particle. Thereafter, the second precursor solution can be additionally added to fix or stabilize the target composition at the surface of the shell region.

[0096] In some embodiments, a chelating agent and a basic agent can be added while mixing the precursor solutions in the co-precipitation as described above. The precipitate can be heat-treated, then mixed with a lithium salt, and fired or heat-treated to obtain lithium metal oxide particles as a positive active material.

[0097] The positive electrode slurry can be prepared by mixing the positive active material as described above with a binder, a conductive agent, and / or a dispersing agent in a solvent and stirring. The positive electrode slurry can be coated on the positive current collector 105, then dried and pressed to form the positive electrode 100.

[0098] The positive current collector 105 can include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or an aluminum alloy can be used.

[0099] The binder can include an organic-based binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or the like, or a water-based binder such as styrene butadiene rubber (SBR) that can be used together with a thickening agent such as carboxymethyl cellulose (CMC).

[0100] For example, a PVDF-based binder can be used as a cathode binder. In this case, the amount of the binder used to form the cathode active material layer can be reduced, and the amount of the cathode active material or lithium metal oxide particles can be relatively increased. Accordingly, the capacity and power of the lithium secondary battery can be further improved.

[0101] A conductive agent can be added to facilitate electron migration between the active material particles. For example, the conductive agent can include a carbon-based material such as graphite, carbon black, graphene, carbon nanotube, or the like, and / or a metal-based material such as tin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO3or LaSrMnO3, or the like.

[0102] In some embodiments, the electrode density of the cathode 100 can be 3.0 g / cc (grams / cubic centimeter) to 3.9 g / cc, preferably 3.2 g / cc to 3.8 g / cc.

[0103] In an exemplary embodiment, the anode 130 can include an anode current collector 125 and an anode active material layer 120 formed by coating the anode active material on the anode current collector 125.

[0104] The anode active material can include a material capable of adsorbing and discharging lithium ions commonly used in the related art. For example, a carbon-based material such as crystalline carbon, amorphous carbon, carbon composite, or carbon fiber, a lithium alloy, a silicon-based compound, tin, or the like can be used.

[0105] The amorphous carbon can include hard carbon, coke, mesocarbon microbeads (MCMB) fired at a temperature of 1500℃ or less, mesophase pitch-based carbon fiber (MPCF), or the like. The crystalline carbon can include a graphite-based material such as natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, or the like. The lithium alloy can further include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, or the like.

[0106] The anode current collector 125 can include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and can preferably include copper or a copper alloy.

[0107] For example, a slurry can be prepared by mixing the anode active material with a binder, a conductive agent, a thickening agent, and stirring in a solvent. The slurry can be coated on at least one surface of the anode current collector 125, and then dried and pressed to form the anode 130.

[0108] The 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 prepared from, 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, or the like. The separator layer 140 can also include a non-woven fabric formed of high-melting point glass fibers, polyethylene terephthalate fibers, or the like.

[0109] 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 the area and / or volume 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. Accordingly, by the combination of the above-described positive electrode active material and negative electrode active material, improvement in power and stability can be effectively achieved.

[0110] 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 150 which can have, for example, a jelly roll shape. For example, the electrode assembly 150 can be formed by winding, laminating, or folding the separator layer 140.

[0111] The electrode assembly 150 can be accommodated in a case 160 together with an electrolyte solution to define a lithium secondary battery. In an exemplary embodiment, a non-aqueous electrolyte solution can be used as the electrolyte solution.

[0112] For example, the non-aqueous electrolyte solution can include a lithium salt and an organic solvent. The lithium salt and the organic solvent can be represented by Li + X - , respectively.

[0113] The anion of the lithium salt 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.

[0114] Organic solvents may include, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. These solvents may be used alone or in combination.

[0115] like Figure 1 As shown, tabs (positive and negative tabs) can protrude from the positive current collector 110 and negative current collector 120 included in each electrode unit to one side of the housing 170. The tabs can be welded to one side of the housing 170 to form electrode leads (positive lead 107 and negative lead 127) extending or exposed outside the housing 160.

[0116] Lithium-ion batteries can be manufactured in shapes such as cylindrical (using cans), square, pouch, or coin.

[0117] According to the above exemplary embodiments, lithium metal oxides with a predetermined composition ratio and concentration distribution can be used as the positive electrode active material. Accordingly, lithium secondary batteries with high capacity, long lifespan, and improved high-temperature storage performance can be realized.

[0118] Preferred embodiments are presented below to describe the invention in more detail. However, the following examples are given for illustrative purposes only, and those skilled in the art will clearly understand that various changes and modifications are possible within the scope and spirit of the invention. Such changes and modifications are suitably included in the appended claims.

[0119] Examples and Comparative Examples

[0120] (1) Manufacture of positive electrode active material and positive electrode

[0121] Example 1

[0122] By continuously changing the mixing ratio of the precursors to form the precipitate, a concentration gradient of nickel and cobalt is formed in the region between the central portion and the surface portion, and the overall composition is LiNi 0.80 Co 0.06 Mn 0.14 O2, the composition of the central portion is LiNi 0.84 Co 0.02 Mn 0.14 O2, the composition of the surface portion is LiNi 0.74 Co 0.12 Mn 0.14 O2. Thus, a lithium metal oxide particle (hereinafter, which can be abbreviated as Al) having a continuous concentration gradient between the central portion and the surface portion (average particle diameter (D50): 13 μm) was prepared as a positive electrode active material.

[0123] The positive electrode active material, ethylene carbon black (Denka Black) as a conductive material, and PVDF as a binder were mixed in a mass ratio of 92:5:3 to prepare a positive electrode mixture, and then the positive electrode mixture was coated on an aluminum base, dried, and pressed to prepare a positive electrode.

[0124] Example 2

[0125] In addition to the lithium metal oxide particle having a surface composition of LiNi 0.71 Co 0.15 Mn 0.14 O2, a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0126] Example 3

[0127] In addition to the lithium metal oxide particle having a surface composition of LiNi 0.56 Co 0.30 Mn 0.14 O2, a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0128] Figure 3 is a graph showing the concentration distribution of metal elements in the lithium metal oxide particle A3 of Example 3. For example, the concentration of metal elements was measured at intervals of 10 nm in the direction from the surface of the lithium metal oxide particle to the center. The measurement was stopped when the concentration became constant by passing through the concentration gradient region into the central portion. Refer to Figure 3A concentration gradient region of Ni and Co is formed in a region from about 90 nm to 350 nm.

[0129] Example 4

[0130] In addition to forming lithium metal oxide particles having a surface composition of LiNi 0.41 Co 0.45 Mn 0.14 O2, except that lithium metal oxide particles having a surface composition of LiNi

[0131] Example 5

[0132] In addition to forming lithium metal oxide particles having a surface composition of LiNi 0.31 Co 0.55 Mn 0.14 O2, except that lithium metal oxide particles having a surface composition of LiNi

[0133] Example 6

[0134] In addition to forming lithium metal oxide particles having a surface composition of LiNi 0.21 Co 0.65 Mn 0.14 O2, except that lithium metal oxide particles having a surface composition of LiNi

[0135] Example 7

[0136] In addition to forming lithium metal oxide particles (hereinafter, which can be referred to as A9) so that the overall composition is LiNi 0.88 Co 0.09 Mn 0.03 O2, except that the center composition is LiNi 0.94 Co 0.03 Mn 0.03 O2, and the surface composition is LiNi 0.81 Co 0.16 Mn 0.03 O2, an anode active material and an anode were prepared in the same manner as in Example 1.

[0137] Example 8

[0138] In addition to forming lithium metal oxide particles (hereinafter, which can be referred to as A10) so that the overall composition is LiNi 0.88 Co 0.09 Mn 0.03 O2, except that the center composition is LiNi0.94 Co 0.03 Mn 0.03 O2, the surface composition of which is LiNi 0.77 Co 0.20 Mn 0.03 O2, the surface composition of which is LiNi

[0139] Example 9

[0140] In addition to forming lithium metal oxide particles having a surface composition of Li 0.67 Co 0.30 Mn 0.03 O2, the surface composition of which is LiNi

[0141] Example 10

[0142] In addition to forming lithium metal oxide particles having a surface composition of Li 0.52 Co 0.45 Mn 0.03 O2, the surface composition of which is LiNi

[0143] Figure 4 is a graph showing the concentration profile of the metal elements in the lithium metal oxide particles A12 of Example 10. For example, the concentration of the metal elements was measured at intervals of 10 nm in the direction from the surface to the center of the lithium metal oxide particles. The measurement was stopped when the concentration became constant after passing through the concentration gradient region into the center portion. Referring to Figure 4 , concentration gradient regions of Ni and Co were formed in a region of about 90 nm to 250 nm.

[0144] Example 11

[0145] In addition to forming lithium metal oxide particles having a surface composition of Li 0.42 Co 0.55 Mn 0.03 O2, the surface composition of which is LiNi

[0146] Example 12

[0147] In addition to forming lithium metal oxide particles having a surface composition of Li 0.32 Co 0.65 Mn 0.03A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0148] Example 13

[0149] A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.96 Co 0.02 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.979 Co 0.001 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.83 Co 0.15 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0150] Example 14

[0151] A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.78 Co 0.20 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0152] Example 15

[0153] A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.68 Co 0.30 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0154] Example 16

[0155] A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.53 Co 0.45 Mn 0.02 O2, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1.

[0156] Example 17

[0157] A lithium metal oxide particle having a surface composition of LiNi0.8Co0.1Mn0.1O2 (hereinafter, which can be abbreviated as A14) was prepared in addition to the lithium metal oxide particle of Example 1, and a positive electrode active material and a positive electrode were prepared in the same manner as in Example 1. 0.43 Co 0.55 Mn 0.02In addition to the lithium metal oxide particles (hereinafter referred to as A22) on the surface of O2, the positive electrode active material and the positive electrode were prepared in the same manner as in Example 13.

[0158] Example 18

[0159] Besides forming LiNi 0.33 Co 0.65 Mn 0.02 In addition to the lithium metal oxide particles (hereinafter referred to as A23) on the surface of O2, the positive electrode active material and the positive electrode were prepared in the same manner as in Example 13.

[0160] Comparative Example 1

[0161] In addition to using LiNi 0.80 Co 0.06 Mn 0.14 The positive electrode was manufactured using the same method as in Example 1, except for O2 and lithium metal oxide particles (hereinafter referred to as A7) in which there is no concentration gradient.

[0162] Comparative Example 2

[0163] In addition to the lithium metal oxide particles (hereinafter referred to as A8) forming in the region between the central and surface portions with concentration gradients of nickel and manganese, the overall composition is LiNi. 0.80 Co 0.06 Mn 0.14 O2, with LiNi as its central component 0.82 Co 0.06 Mn 0.12 O2, surface composition is LiNi 0.64 Co 0.06 Mn 0.30 In addition to O2, the positive electrode is manufactured using the same method as in Example 1.

[0164] Comparative Example 3

[0165] In addition to using LiNi 0.88 Co 0.09 Mn 0.03 The positive electrode was prepared using the same method as in Example 7, except for O2 and lithium metal oxide particles (hereinafter referred to as A15) in which there is no concentration gradient.

[0166] Comparative Example 4

[0167] LiNi0.8Co0.1Mn0.1O2 0.89 Co 0.09 Mn 0.03 O2, the bulk composition is LiNi0.8Co0.1Mn0.1O2 0.90 Co 0.09 Mn 0.01 O2, the surface composition is LiNi0.8Co0.1Mn0.1O2 0.61 Co 0.09 Mn 0.30 O2. The positive electrode was produced in the same manner as in Example 7 except for the lithium metal oxide particles having a surface composition of LiNi0.8Co0.1Mn0.1O2

[0168] Comparative Example 5

[0169] The positive electrode was produced in the same manner as in Example 7 except for the lithium metal oxide particles having a surface composition of LiNi0.8Co0.1Mn0.1O2 0.82 Co 0.15 Mn 0.03 O2. The positive electrode was produced in the same manner as in Example 7 except for the lithium metal oxide particles having a surface composition of LiNi0.8Co0.1Mn0.1O2

[0170] Comparative Example 6

[0171] The positive electrode was produced in the same manner as in Example 13 except for the lithium metal oxide particles having a composition of LiNi0.8Co0.1Mn0.1O2 and having no concentration gradient therein (hereinafter, which can be abbreviated as A24). 0.96 Co 0.02 Mn 0.02 O2. The positive electrode was produced in the same manner as in Example 13 except for the lithium metal oxide particles having a composition of LiNi0.8Co0.1Mn0.1O2 and having no concentration gradient therein (hereinafter, which can be abbreviated as A24).

[0172] Comparative Example 7

[0173] The positive electrode was produced in the same manner as in Example 13 except for the lithium metal oxide particles having a composition of LiNi0.8Co0.1Mn0.1O2 and having no concentration gradient therein (hereinafter, which can be abbreviated as A24). 0.96 Co 0.02 Mn 0.02 O2, the bulk composition is LiNi0.8Co0.1Mn0.1O2 0.979 Co 0.02 Mn 0.001 O2, the surface composition is LiNi0.8Co0.1Mn0.1O2 0.78 Co 0.02 Mn 0.20 O2. The positive electrode was produced in the same manner as in Example 13 except for the lithium metal oxide particles having a composition of LiNi0.8Co0.1Mn0.1O2 and having no concentration gradient therein (hereinafter, which can be abbreviated as A24).

[0174] The compositions of the positive electrode active materials of Examples and Comparative Examples are shown in Table 1 below. In Table 1, the term "bulk composition" indicates the average composition of the entire region of the particles or the positive electrode active material.

[0175] [Table 1]

[0176]

[0177]

[0178] (2) Preparation of secondary battery (coin cell)

[0179] A secondary battery was manufactured using the positive electrode of the Examples and Comparative Examples. Lithium metal foil was used as a negative electrode active material, and a separator (polyethylene, thickness 12 μm) was interposed between the positive electrode and the lithium foil, and an electrolyte was injected and clamped to manufacture a coin cell. The electrolyte included a 1M LiPF6 solution using a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio). The coin cell product was immersed for more than 12 hours.

[0180] Experimental Examples

[0181] Evaluation of life characteristics and capacity characteristics

[0182] For each lithium secondary battery manufactured in the Examples and Comparative Examples, charge and discharge were repeated 300 times under the conditions shown in Table 2 below to evaluate the life and capacity characteristics. Characteristics

[0183] [Table 2]

[0184]

[0185] The results are shown in Table 3 below together with the content ratio of surface cobalt.

[0186] [Table 3]

[0187]

[0188] Referring to Table 3, the lithium secondary battery of the Examples provided improved life and capacity characteristics compared to the lithium secondary battery of the Comparative Examples.

[0189] Specifically, in the comparison of Example 1 to Example 6 and Comparative Example 1 to Comparative Example 2, when the total particle composition was Ni:Co:Mn = 80:6:14, the Co concentration in the surface portion was 6.0 times or more of the Co concentration in the center portion, and was 2.0 times or more of the overall Co concentration, in the Examples to provide improved life and capacity characteristics.

[0190] ​In comparison of Example 7 to Example 12 and Comparative Example 3 to Comparative Example 5, when the total particle composition is Ni:Co:Mn = 88:9:3, the cobalt concentration in the surface portion is 5.3 times or more than that of the center portion and 1.8 times or more than the overall Co concentration in the examples to provide improved life characteristics and capacity characteristics.

[0191] In comparison of Example 13 to Example 18 and Comparative Example 6 to Comparative Example 7, when the total particle composition is Ni:Co:Mn = 96:2:2, the cobalt concentration in the surface portion is 150 times or more than that of the center portion and 7.5 times or more than the overall Co concentration in the examples to provide improved life characteristics and capacity characteristics.

[0192] Furthermore, the lithium secondary batteries of the examples provide improved life characteristics and capacity characteristics compared to the lithium secondary batteries of Comparative Example 1, Comparative Example 3, and Comparative Example 6, which have a completely fixed concentration.

Claims

1. A positive electrode active material for a lithium secondary battery, comprising lithium metal oxide particles containing nickel (Ni), cobalt (Co), and manganese (Mn), wherein the lithium metal oxide particles comprise a core region occupying 50% or more of a radius of the lithium metal oxide particle from a center, and in which the concentrations of Ni and Co are constant; a shell region extending in a direction from a surface toward the center, in which the concentrations of Ni and Co are constant; and a concentration gradient region formed between the core region and the shell region, in which the concentration of Ni decreases in a direction from the center to the surface, and in which the concentration of Co increases in the direction from the center to the surface, wherein the concentration of Mn is constant from the center to the surface of the lithium metal oxide particle, and wherein a ratio of the concentration at the surface of the lithium metal oxide particle expressed in atomic percentage of Co to an average concentration expressed in atomic percentage of Co in an entire region of the lithium metal oxide particle is 5.0 or more. 2.The positive electrode active material for a lithium secondary battery according to claim 1, wherein a ratio of the concentration at the surface expressed in atomic percentage of Co to the concentration at the center expressed in atomic percentage of Co is 5.3 or more. 3.The positive electrode active material for a lithium secondary battery according to claim 1, wherein a ratio of the concentration at the surface expressed in atomic percentage of Co to the concentration at the center expressed in atomic percentage of Co is 6.0 or more.

4. The positive electrode active material for a lithium secondary battery according to claim 1, wherein, In the concentration gradient region, each of Ni and Co has a slope of a concentration gradient. 5.The positive electrode active material for a lithium secondary battery according to claim 1, wherein a distance of the shell region from the surface of the lithium metal oxide particle is 10 nm to 200 nm. 6.The positive electrode active material for a lithium secondary battery according to claim 1, wherein a total average composition of the lithium metal oxide particle is represented by Chemical Formula 1: [Chemical Formula 1] Li x Ni a Co b M3 c O y wherein In Chemical Formula 1, M 3 includes at least one element selected from the group consisting of Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, and B, and 0 < x ≤ 1.2, 2 ≤ y ≤ 2.02, 0 ≤ a ≤ 1, 0 ≤ b ≤ 1, 0 ≤ c ≤ 1, and 0 < a + b + c ≤ 1. 7.The positive electrode active material for a lithium secondary battery according to claim 1, wherein an average concentration of Ni in an entire region of the lithium metal oxide particle is in a range of 0.6 atomic% to 0.99 atomic%. 8.A lithium secondary battery, comprising: a positive electrode comprising the positive electrode active material according to claim 1; and a negative electrode opposite to the positive electrode.

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