Positive electrode active material for lithium secondary battery, and lithium secondary battery

Abstract

The present invention provides a positive electrode active material for a lithium secondary battery and a lithium secondary battery. The lithium secondary battery comprises: a positive electrode; a negative electrode; a separator that is interposed between the positive electrode and the negative electrode; and an electrolyte, in which the positive electrode contains a lithium composite transition metal oxide powder having a layered structure and having a nickel content of 85 atm% or more in the entire transition metal, and in which the lithium composite transition metal oxide powder undergoes a lithium-oxygen (Li-O) interlayer distance change of 3% or less in a state of charge (SOC) range of 58% to 72%. The lithium secondary battery according to the present invention can exhibit excellent capacity characteristics and excellent high-temperature life characteristics.

Description

[0001] This patent application for invention is a divisional application of a Chinese patent application with an application date of June 19, 2019, an application number of 201980034513.5, and an invention title of "Positive electrode active material for lithium secondary battery and lithium secondary battery".

[0002] Cross-reference to related applications

[0003] This application claims the priority and benefit of Korean Patent Application No. 10-2018-0071055, filed on June 20, 2018, the disclosure of which is incorporated herein by reference in its entirety. Technical field

[0004] The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery that exhibits excellent electrochemical performance even at high temperatures. Background art

[0005] Recently, as environmental problems have become important issues, the interest in renewable energy that can replace nuclear power generation or fossil fuels has increased. Among such renewable energies, the demand for secondary batteries that allow charging and discharging, thus having semi-permanent characteristics and allowing repeated use, is rapidly growing.

[0006] Lithium secondary batteries have become the most prominent secondary batteries due to their excellent life (cycle) characteristics and high energy density. As positive electrode active materials for such lithium secondary batteries, various lithium transition metal oxides have been developed, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4, Li(Ni a Co b Mn c )O2 (where a, b, and c are atomic fractions of transition metals, where 0 < a < 1, 0 < b < 1, 0 < c < 1, and a + b + c = 1; this compound is hereinafter referred to as Ni-containing NCM lithium oxide) and the like. At the same time, in recent years, for application to high-capacity batteries such as batteries for electric vehicles, the development of Ni-containing NCM lithium oxides with an increased nickel content for high energy density has been actively carried out.

[0007] However, in the application of Ni-rich NCM-type lithium secondary batteries, although excellent performance is achieved in terms of capacity, problems may arise due to the decreased structural and chemical stability of the active material with increasing nickel content. These problems include: repeated charging and discharging leading to a decline in the structural integrity of the active material surface; reduced battery stability due to exothermic reactions, resulting in a rapid decline in structural integrity; or a rapid decrease in lifetime characteristics due to structural degradation. These phenomena are exacerbated under high temperature and / or high voltage conditions, and are particularly pronounced when using cathode active materials with a nickel content of 80 atm% or higher in all transition metals. Furthermore, compared to cathode active materials with low nickel content, cathode active materials with a nickel content of 80 atm% or higher in all transition metals exhibit the following problems: decreased lifetime characteristics at high temperatures due to accelerated cation mixing, irreversible phase transitions, etc.

[0008] To address these issues, techniques have been explored to improve the structural stability of cathode active materials through doping or coating with metallic elements. However, the techniques proposed to date have not been able to fully realize the required capacity and high-temperature performance.

[0009] Therefore, there is a need to develop a lithium secondary battery that meets the high capacity requirements while also possessing excellent high-temperature characteristics. Summary of the Invention

[0010] Technical issues

[0011] This invention relates to a lithium secondary battery, which, because it contains a positive electrode active material with a nickel content of 85 atm% or more, exhibits high capacity characteristics and excellent lifespan characteristics even at high temperatures.

[0012] Technical solution

[0013] One aspect of the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode comprises lithium composite transition metal oxide powder having a layered structure and having a nickel content of 85 atm% or more in all transition metals as a positive electrode active material, and wherein the lithium composite transition metal oxide powder undergoes a lithium-oxygen (Li-O) interlayer distance (i.e., LiO6 plate thickness) variation of less than 3%, preferably less than 1%, in the range of 58% to 72% state of charge (SOC).

[0014] In lithium composite transition metal oxide powders, the Li-O interlayer distance at 100% SOC can be greater than or equal to the Li-O interlayer distance at 0% SOC.

[0015] In addition, the lithium composite transition metal oxide can be represented by the following Chemical Formula 1.

[0016] [Chemical Formula 1]

[0017] Li x [Ni a Co b Mn c M d O2

[0018] In Chemical Formula 1, M is one or more elements selected from the following: W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and 0.9 ≤ x ≤ 1.2, 0.85 ≤ a ≤ 0.99, 0 < b < 0.15, 0 < c < 0.15, and 0 < d < 0.15.

[0019] In addition, the lithium composite transition metal oxide may include a coating on its surface, and the coating includes one or more elements selected from the following: Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S.

[0020] Another aspect of the present invention provides a positive electrode active material for a lithium secondary battery, the positive electrode active material including lithium composite transition metal oxide powder having a layered structure and having a nickel content of 85 atm% or more among all transition metals, wherein the lithium composite transition metal oxide powder undergoes a change in the Li - O interlayer distance (i.e., the LiO6 plate thickness) of less than or equal to 3% within the SOC range of 58% to 72%.

[0021] Advantageous Effects

[0022] Due to the positive electrode including the positive electrode active material having a nickel content of 85 atm% or more and undergoing a small change in the Li - O interlayer distance due to the insertion and extraction of lithium ions during charging and discharging, the lithium secondary battery of the present invention can exhibit excellent capacity characteristics and excellent high - temperature life characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a graph showing the change in the Li - O interlayer distance of the positive electrode active materials of Preparation Examples 1 to 4 according to the state of charge. DETAILED DESCRIPTION

[0024] Hereinafter, the present invention will be described in more detail.

[0025] The terms and words used in this specification and claims should not be construed as limited to their common or dictionary meanings, and, based on the principle that inventors are able to appropriately define the concepts of terms in order to best describe their invention, these terms and words should be interpreted in a meaning and concept consistent with the technical spirit of the invention.

[0026] In this specification, particle size Dn refers to the particle size corresponding to the n% point in the cumulative number distribution of particles according to particle size. That is, D50 is the particle size corresponding to the 50% point in the cumulative number distribution of particles according to particle size, D90 is the particle size corresponding to the 90% point in the cumulative number distribution of particles according to particle size, and D10 is the particle size corresponding to the 10% point in the cumulative number distribution of particles according to particle size. Dn can be determined using laser diffraction. Specifically, the powder to be analyzed is dispersed in a dispersion medium, and the dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), in which the particle size distribution is obtained by measuring the change in the diffraction pattern as the particles pass through a laser beam. D10, D50, and D90 can be determined by calculating the particle sizes corresponding to the 10%, 50%, and 90% points in the cumulative number distribution of particles according to particle size obtained by the analyzer.

[0027] As a result of long-term research into developing lithium secondary batteries with excellent capacity and high-temperature life characteristics, the inventors of this invention have discovered that the goal can be achieved by using a Ni-rich cathode active material that undergoes small Li-O interlayer distance variations within a specific SOC range, thus completing this invention.

[0028] Conventionally, X-ray diffraction (XRD) is used to analyze the crystal structure of cathode active materials. However, this conventional method is not yet able to measure changes in the crystal structure of cathode active materials according to the degree of charging due to the difficulty of accurate measurement.

[0029] Therefore, the inventors of this invention used high-resolution powder diffraction (HRPD) data obtained by synchrotron radiation to measure the crystal structure changes of the cathode active material according to the degree of charge, and discovered that within a specific range corresponding to the state of charge, the degree of change in the Li-O interlayer distance of the cathode active material closely affects the high-temperature lifetime characteristics. Specifically, it was found that when a cathode active material with a layered structure, a nickel content of 85 atm% or more in all transition metals, and a Li-O interlayer distance change of less than 3% within a SOC range of 58% to 72% is used, excellent high-temperature lifetime characteristics are exhibited.

[0030] According to research conducted by the inventors of this invention, even in positive electrode active materials with similar compositions and the same Li-O interlayer distance before charging and discharging, the high-temperature lifetime characteristics and capacity characteristics differ depending on the degree of variation in the Li-O interlayer distance within the SOC range of 58% to 72%.

[0031] In Ni-rich cathode active materials with layered structures, the mechanism by which the degree of variation in Li-O interlayer distance in the 58% to 72% SOC range affects high-temperature lifetime and capacity characteristics, although unclear, is hypothesized as follows: Lithium composite transition metal oxides with layered crystal structures have alternating stacked Li-O layers and transition metal layers, and when charged, the interlayer distance changes as a result of charge compensation to compensate for lithium-ion loss after lithium deintercalation from the Li-O layers. Typically, the Li-O interlayer distance increases in the 50% to 60% SOC range and decreases with further charging. In the case of cathode active materials with a nickel content of 85 atm% or higher, a decrease in the Li-O interlayer distance in the SOC range above 58% has been observed. However, when the Li-O interlayer distance decreases sharply during such a change, capacity and lifetime characteristics deteriorate, possibly due to irreversible structural changes occurring in the lattice structure due to additional strain.

[0032] Therefore, in this invention, by using a Ni-rich cathode active material that undergoes a small change in Li-O interlayer distance within a SOC range of 58% to 72%, the capacity characteristics and high-temperature lifetime characteristics of lithium secondary batteries are improved.

[0033] More specifically, the lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a separator inserted between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode comprises lithium composite transition metal oxide powder having a layered structure and having a nickel content of more than 85 atm% in all transition metals and undergoing a Li-O interlayer distance change of less than 3% in the range of 58% to 72% SOC as the positive electrode active material.

[0034] Each component of the invention will be described in detail below.

[0035] (1) Positive electrode

[0036] The positive electrode of the present invention comprises a positive electrode active material layer, and the positive electrode active material layer comprises lithium composite transition metal oxide powder having a layered structure and having a nickel content of more than 85 atm% in all transition metals as the positive electrode active material.

[0037] For example, lithium complex transition metal oxides can be represented by the following chemical formula 1.

[0038] [Chemical Formula 1]

[0039] Li x [Ni a Co b Mn c M d O2

[0040] In Chemical Formula 1, M is a doping element substituted at the transition metal site and is used to improve the structural stability of the positive electrode active material. M can be, for example, one or more elements selected from the following: W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. Preferably, M contains two or more elements selected from the following: W, Zr, Al, Ti, and Mg. More preferably, M contains W and one or more elements selected from the following: Zr, Al, Ti, and Mg.

[0041] x represents the atomic fraction of lithium in the lithium composite transition metal oxide and can be 0.9 ≤ x ≤ 1.2, preferably 1.0 ≤ x ≤ 1.2, and more preferably 1.0 ≤ x ≤ 1.1.

[0042] a represents the atomic fraction of nickel in the lithium composite transition metal oxide and can be 0.85 ≤ a ≤ 0.99, preferably 0.85 ≤ a ≤ 0.95. When nickel is contained in such a large amount, excellent capacity characteristics can be obtained.

[0043] b represents the atomic fraction of cobalt in the lithium composite transition metal oxide and can be 0 < b < 0.15, preferably 0 < b ≤ 0.12.

[0044] c represents the atomic fraction of manganese in the lithium composite transition metal oxide and can be 0 < c < 0.15, preferably 0 < c ≤ 0.1.

[0045] d represents the atomic fraction of the doping element M in the lithium composite transition metal oxide and can be 0 < d < 0.15, preferably 0 < d ≤ 0.1.

[0046] Meanwhile, the lithium composite transition metal oxide may contain a coating on its surface, and the coating contains one or more elements selected from the following: Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S. When such a coating is contained, since the contact between the transition metal in the lithium composite transition metal oxide and the electrolyte is suppressed, a decrease in the structural stability of the lithium composite transition metal oxide due to the reaction with the electrolyte can be prevented.

[0047] Meanwhile, the content of transition metal elements in lithium complex transition metal oxides can be constant regardless of their position, or the content of one or more transition metal elements can vary depending on their position within the particle. For example, lithium complex transition metal oxides can have a concentration gradient in which the concentrations of one or more components of Ni, Mn, and Co gradually change. Here, "a concentration gradient in which the concentrations gradually change" refers to a concentration distribution in which the concentration of a component continuously and gradually changes throughout the particle or in a specific region of the particle.

[0048] Meanwhile, in this invention, lithium composite transition metal oxide powder that undergoes a Li-O interlayer distance change of less than 3%, preferably less than 1%, within a SOC range of 58% to 72% is used as the positive electrode active material.

[0049] In this case, the extent of the change in Li-O interlayer distance in the SOC range of 58% to 72% can be calculated according to the following equation (1).

[0050] Equation (1): The degree of change in Li-O interlayer distance (%) = {(Li-O interlayer distance at 58% SOC – Li-O interlayer distance at 72% SOC) / Li-O interlayer distance at 58% SOC} × 100

[0051] When the Li-O interlayer distance of lithium composite transition metal oxide powder changes by more than 3% within the SOC range of 58% to 72%, strain occurs in the lattice structure during high-temperature charging and discharging, causing the Li-O layer to move. As a result, the channels through which lithium migrate (i.e., lithium pathways) are blocked, and lithium ions cannot migrate smoothly, thus reducing lifetime and capacity characteristics.

[0052] Meanwhile, the change in Li-O interlayer distance of lithium composite transition metal oxide powder during charging and discharging is determined by the composition of lithium composite transition metal oxide, the type of doping and coating elements, particle size, etc., rather than by a single factor.

[0053] The change in Li-O interlayer distance of lithium composite transition metal oxide powder during charging can be measured by obtaining high-resolution powder diffraction (HRPD) data using synchrotron radiation and analyzing the obtained data according to the Rietveld method. Specifically, the change in Li-O interlayer distance can be measured in the charged state by the following method: First, a coin-shaped half-cell is fabricated by inserting a separator between a positive electrode and a lithium metal negative electrode, wherein the positive electrode contains the lithium composite transition metal oxide to be analyzed as the positive electrode active material. Multiple coin-shaped half-cells fabricated as described above are charged to different capacities. Subsequently, each half-cell is disassembled to separate the positive electrode from it. The positive electrode active material layer is scraped off from the separated positive electrode to obtain the lithium composite transition metal oxide powder in the charged state, which is then exposed to synchrotron radiation to obtain HRPD data. By analyzing the obtained data using the space group R-3m in the crystal structure model according to the method of analyzing crystal structure by the Reidberg method, the Li-O interlayer distance of lithium composite transition metal oxide powder at each charging degree can be determined, and based on this result, the degree of variation of Li-O interlayer distance with charging degree can be determined.

[0054] Furthermore, in lithium composite transition metal oxide powders, the Li-O interlayer distance at 100% SOC (i.e., fully charged state) can be greater than or equal to the Li-O interlayer distance at 0% SOC.

[0055] In lithium-complex transition metal oxides with high nickel content, the Li-O interlayer distance typically decreases during charging and discharging, becoming smaller than the distance before charging. When the Li-O interlayer distance decreases so significantly, lithium ions cannot migrate smoothly, potentially leading to problems such as a rapid decline in lifetime characteristics. This reduction in Li-O interlayer distance is due to the abrupt changes in the crystal structure during charging and discharging. However, when using the aforementioned lithium-complex transition metal oxides with a Li-O interlayer distance at 100% SOC greater than or equal to that at 0% SOC, the rapid decline in lifetime characteristics can be prevented because smooth lithium ion migration is allowed.

[0056] Based on the total weight of the positive electrode active material layer, the positive electrode active material can be included in an amount of 80% to 99% by weight, more specifically 85% to 98.5% by weight. When the content of the positive electrode active material meets the above range, excellent capacity characteristics can be exhibited.

[0057] In addition to including a positive electrode active material in the positive electrode active material layer, the positive electrode of the present invention may further include a conductive material and / or a binder.

[0058] Conductive materials are used to impart conductivity to electrodes, and their use is not particularly restricted, provided they do not cause chemical changes in the manufactured battery and possess electronic conductivity. Specific examples of conductive materials include: graphite, such as natural graphite and artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, and carbon fiber; metal powders or fibers of copper, nickel, aluminum, silver, etc.; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination of two or more of them. Based on the total weight of the positive electrode active material layer, conductive materials can be included in an amount from 0.1% to 15% by weight.

[0059] Adhesives are used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of adhesives include: polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers of the above materials. These materials can be used alone or in combination of two or more of them. Based on the total weight of the positive electrode active material layer, the adhesive can be included in an amount from 0.1% to 15% by weight.

[0060] The above-mentioned positive electrode can be manufactured by common methods for manufacturing positive electrodes, for example, by coating a positive electrode mixture prepared by dissolving or dispersing positive electrode active material, binder and / or conductive material in a solvent onto a positive electrode current collector, and then drying and rolling it.

[0061] In this case, the solvent can be one commonly used in the art. Examples of solvents include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, water, etc., which can be used alone or in combination of two or more of them. The amount of solvent used is not particularly limited, and is sufficient as long as it allows for appropriate adjustment of the viscosity of the cathode mixture, taking into account factors such as coating thickness, manufacturing yield, and processability.

[0062] Meanwhile, the positive electrode current collector is not particularly restricted, as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel; aluminum; nickel; titanium; calcined carbon; or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the positive electrode current collector can typically have a thickness from 3 μm to 500 μm and can have micro-protrusions on its surface to increase adhesion to the positive electrode material. The positive electrode current collector can be used in any of the following forms: film, sheet, foil, mesh, porous materials, foam, nonwoven fabric, etc.

[0063] Alternatively, the positive electrode can be manufactured by preparing a membrane by casting the positive electrode mixture onto a separate carrier and then removing it from the carrier, and then laminating the membrane onto the positive electrode current collector.

[0064] (2) Negative electrode

[0065] In this invention, the negative electrode can be used without particular limitation, as long as it is a negative electrode commonly used in lithium secondary batteries, and the negative electrode can include, for example, a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0066] The negative electrode current collector is not particularly limited, as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper; stainless steel; aluminum; nickel; titanium; calcined carbon; or copper or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, etc., can be used. Furthermore, the negative electrode current collector can typically have a thickness from 3 μm to 500 μm, and similar to the positive electrode current collector, it can have micro-protrusions on its surface to increase adhesion to the negative electrode active material. The negative electrode current collector can be used in any of the following forms: film, sheet, foil, mesh, porous material, foam, non-woven fabric, etc.

[0067] In addition to the negative electrode active material, the negative electrode active material layer may also include optional binders and optional conductive materials.

[0068] As a negative electrode active material, any of the various negative electrode active materials used in this art can be used without particular restriction. Specific examples of negative electrode active materials include: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, amorphous carbon, etc.; (semi)metallic materials capable of forming alloys with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, Al alloys, etc.; and metal oxides capable of being doped or undoped with lithium, such as SiO2. y(0 < y < 2), SnO2, vanadium oxides or lithium vanadium oxides; composites containing (semi) metallic materials and carbonaceous materials such as Si-C composites or Sn-C composites, etc., and these materials can be used alone or in combinations of two or more of them. Additionally, a thin film of metallic lithium can be used as the negative electrode active material.

[0069] Meanwhile, based on the total weight of the negative electrode active material layer, the negative electrode active material can be included in an amount of 80% to 99% by weight.

[0070] The binder is a component that helps the adhesion between the conductive material, the active material and the current collector, and based on the total weight of the negative electrode active material layer, the binder is usually added in an amount of 0.1% to 10% by weight. Examples of such binders include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, nitrile rubber, fluororubber, various copolymers of the above materials, etc.

[0071] The conductive material is a component for further improving the conductivity of the negative electrode active material, and based on the total weight of the negative electrode active material layer, the conductive material can be added in an amount of 10% by weight or less, preferably 5% by weight or less. Such conductive materials are not particularly limited as long as they do not cause chemical changes in the battery and have conductivity, and examples of materials that can be used as conductive materials include: graphite such as natural graphite, artificial graphite, etc.; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, etc.; conductive fibers such as carbon fibers, metal fibers, etc.; carbon fluoride; metal powders such as aluminum powder, nickel powder, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide, etc.; and conductive materials such as polyphenylene derivatives, etc.

[0072] The negative electrode active material layer can be prepared by coating a composition for forming the negative electrode active material layer, which is prepared by dissolving or dispersing the negative electrode active material, an optional binder and an optional conductive material in a solvent, onto the negative electrode current collector and then drying it, or the negative electrode active material layer can be prepared by casting the composition for forming the negative electrode active material layer on a separate carrier, then removing it from the carrier to prepare a film, and laminating the film on the negative electrode current collector.

[0073] (3) Separator

[0074] A separator is inserted between the negative and positive electrodes, thereby separating the positive and negative electrodes and providing a channel for lithium-ion migration. As the separator, separators commonly used in lithium secondary batteries can be used without particular restriction, and in particular, separators exhibiting low resistance to electrolyte ion migration and excellent electrolyte permeation capability are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc.); or stacked structures having two or more layers of the above porous polymer membranes. Alternatively, common porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymer materials and optionally having a single-layer or multi-layer structure can be used.

[0075] (4) Electrolyte

[0076] As an electrolyte, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, inorganic solid electrolytes, molten inorganic electrolytes, etc., available in lithium secondary batteries can be used without particular restrictions.

[0077] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0078] As organic solvents, solvents that can act as a medium through which ions participating in the electrochemical reactions of the battery can migrate can be used without particular restriction. Specifically, organic solvents can be: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether solvents, such as dibutyl ether, tetrahydrofuran, etc.; ketone solvents, such as cyclohexanone, etc.; aromatic solvents, such as benzene, fluorobenzene, etc.; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.; alcohol solvents, such as ethanol, isopropanol, etc.; nitriles, such as Ra-CN (Ra is a C2-C20 hydrocarbon group with a straight chain, branched chain or cyclic structure and may contain double bonds, aromatic rings or ether bonds), etc.; amides, such as dimethylformamide, etc.; dioxolane, such as 1,3-dioxolane, etc.; or sulfolane.

[0079] As a lithium salt, lithium salts conventionally used in electrolytes for lithium secondary batteries can be used without particular restriction. For example, the lithium salt can be one containing Li + Lithium salts that are cations and have at least one of the following as anions: F - Cl - ,Br -I - NO3 - N(CN)2 - BF4 - ClO4 - AlO4 - AlCl4 - PF6 - SbF6 - AsF6 - BF2C2O4 - BC4O8 - PF4C2O4 - PF2C4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - C4F9SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - , CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - Specifically, the lithium salt may contain one or more mixtures selected from the following: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10The electrolyte salts used in lithium secondary batteries can be unrestricted, including LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiAlO4, and LiCH3SO3. Additionally, lithium imide salts commonly used in lithium secondary battery electrolytes, such as those represented as LiN(SO2C2F5)2 (lithium bis(perfluoroethylsulfonyl)imide (LiBETI)), LiN(SO2F)2 (lithium fluorosulfonylimide (LiFSI)), or LiN(SO2CF3)2 (lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)). Specifically, the electrolyte salt may contain one or more mixtures selected from: LiPF6, LiBF4, LiCH3CO2, LiCF3CO2, LiCH3SO3, LiFSI, LiTFSI, and LiN(C2F5SO2)2.

[0080] The amount of lithium salt can be appropriately varied within the generally available range; specifically, lithium salt can be included in the electrolyte in amounts from 0.8 M to 3 M, and more specifically from 0.1 M to 2.5 M.

[0081] In addition to the electrolyte components mentioned above, various additives can be used in the electrolyte to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity.

[0082] As additives, the following materials can be used: imide salts, such as lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide, etc.; borate salts, such as lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiOdFB), lithium tri(trimethylsilyl)borate (TMSB), etc.; phosphate salts, such as difluorophosphate or tri(trimethylsilyl)phosphate; haloalkyl carbonate compounds, such as ethylene difluorocarbonate, etc.; or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imide dyes, N-substituted β-azolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. These materials can be used alone or in combination. In this case, each additive may be included in amounts ranging from 0.1% to 10% by weight, based on the total weight of the electrolyte.

[0083] The lithium secondary battery described above is useful in the following fields: portable devices, such as mobile phones, laptop computers, digital cameras, etc.; electric vehicles, such as hybrid electric vehicles (HEVs), etc.

[0084] The lithium secondary battery of the present invention can be of the type, but is not particularly limited to cylindrical, prismatic, pouch, coin-shaped, etc., using a can.

[0085] The lithium secondary battery of the present invention can be used not only in battery cells used as power sources for small devices, but also preferably as a unit battery in medium to large battery modules containing multiple battery cells.

[0086] [Mode of the Invention]

[0087] The invention will be described in more detail below through specific embodiments.

[0088] Preparation Example 1

[0089] By Ni 0.88 Co 0.09 Mn 0.03 Lithium-doped Al and Zr composite transition metal oxide powder was prepared by dry mixing (OH)2, LiOH, ZrO2 and Al(OH)3 and calcining the mixture at 760°C for 12 hours.

[0090] After mixing the lithium composite transition metal oxide prepared as described above with H3BO3, the mixture was heat-treated at 350°C for three hours to form a B-containing coating on the surface of the lithium composite transition metal oxide.

[0091] The obtained lithium composite transition metal oxide powder has a bimodal particle size distribution that mixes 16 μm and 5 μm particles.

[0092] Preparation Example 2

[0093] By Ni 0.88 Co 0.09 Mn 0.03 Lithium composite transition metal oxides doped with Al, Zr, and W were prepared by dry mixing (OH)2, LiOH, ZrO2, WO3, and Al(OH)3 and calcining the mixture at 740°C for 12 hours.

[0094] After mixing the lithium composite transition metal oxide prepared as described above with H3BO3, the mixture was heat-treated at 350°C for three hours to form a B-containing coating on the surface of the lithium composite transition metal oxide.

[0095] The obtained lithium composite transition metal oxide powder has a single-peak particle size distribution with a D50 of 5 μm.

[0096] Preparation Example 3

[0097] By Ni 0.90 Co 0.08 Mn 0.02Lithium-doped composite transition metal oxides were prepared by dry mixing (OH)2, LiOH, TiO2 and WO3 and calcining the mixture at 760°C for 12 hours.

[0098] After mixing the lithium composite transition metal oxide prepared as described above with H3BO3, the mixture was heat-treated at 350°C for three hours to form a B-containing coating on the surface of the lithium composite transition metal oxide.

[0099] The obtained lithium composite transition metal oxide powder has a single-peak particle size distribution with a D50 of 16 μm.

[0100] Preparation Example 4

[0101] By Ni 0.90 Co 0.08 Mn 0.02 Lithium composite transition metal oxides doped with Al, Zr, and W were prepared by dry mixing (OH)2, LiOH, ZrO2, WO3, and Al(OH)3 and calcining the mixture at 760°C for 12 hours.

[0102] After mixing the lithium composite transition metal oxide prepared as described above with H3BO3, the mixture was heat-treated at 350°C for three hours to form a B-containing coating on the surface of the lithium composite transition metal oxide.

[0103] The obtained lithium composite transition metal oxide powder has a single-peak particle size distribution with a D50 of 16 μm.

[0104] Experimental Example 1

[0105] Each of the lithium composite transition metal oxides prepared in Examples 1 to 4 was mixed with carbon black conductive material and PVdF binder in an N-methylpyrrolidone solvent at a weight ratio of 96.5:1.5:2.0 to prepare a positive electrode mixture. The positive electrode mixture was then coated onto one surface of an aluminum current collector, dried at 130°C, and then rolled to manufacture a positive electrode.

[0106] An electrode assembly is manufactured by inserting a porous polyethylene diaphragm between the positive and negative electrodes, which are manufactured as described above. After placing the electrode assembly in a housing, an electrolyte is injected into the housing to manufacture a lithium secondary battery (coin-type half-cell).

[0107] In this case, lithium metal is used as the negative electrode, and an electrolyte is used as the electrolyte prepared by dissolving lithium hexafluorophosphate (LiFP6) at a concentration of 1 M in an organic solvent consisting of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:4:3.

[0108] Eight lithium-ion secondary batteries were prepared for each lithium-ion composite transition metal oxide, and charged at 0 mA / g, 40 mA / g, 80 mA / g, 120 mA / g, 160 mA / g, 200 mA / g, 240 mA / g, and 278 mA / g, respectively. After separating the positive electrode from each lithium-ion secondary battery, the positive electrode active material layer was scraped off to obtain lithium-ion composite transition metal oxide powder. The collected lithium-ion composite transition metal oxide powder was placed in a radiation accelerator and subjected to X-ray analysis to determine the Li-O interlayer distance according to the degree of charging.

[0109] The measurement results are shown in Figure 1 In addition, by converting the charging capacity value into a SOC value, based on... Figure 1 The Li-O interlayer distance (in Å) based on SOC was calculated, and the results are shown in Table 1.

[0110] [Table 1]

[0111] As shown in Table 1 and Figure 1 As shown, the lithium composite transition metal oxides prepared in Preparation Examples 1 to 3 experienced a Li-O interlayer distance change of less than 1% within a SOC range of 58% to 72%. However, the lithium composite transition metal oxide prepared in Preparation Example 4 experienced a Li-O interlayer distance change of approximately 4.8% within a SOC range of 58% to 72%.

[0112] Furthermore, in the case of lithium composite transition metal oxides prepared in Examples 1 and 2, the Li-O interlayer distance increased or remained almost unchanged when fully charged (100% SOC) compared to before charging (0% SOC). However, in the case of lithium composite transition metal oxides prepared in Examples 3 and 4, the Li-O interlayer distance decreased when fully charged compared to before charging (0% SOC).

[0113] Example 1

[0114] A positive electrode mixture was prepared by mixing lithium composite transition metal oxide, carbon black conductive material and PVdF binder prepared in Preparation Example 1 as positive electrode active material in N-methylpyrrolidone solvent at a weight ratio of 96.5:1.5:2.0. The positive electrode mixture was then coated onto one surface of an aluminum current collector, dried at 130°C and then rolled to manufacture a positive electrode.

[0115] An electrode assembly is manufactured by inserting a porous polyethylene diaphragm between the positive and negative electrodes, which are manufactured as described above. After placing the electrode assembly in a housing, an electrolyte is injected into the housing to manufacture a lithium secondary battery (coin-type half-cell).

[0116] In this case, lithium metal is used as the negative electrode, and an electrolyte is used as the electrolyte prepared by dissolving lithium hexafluorophosphate (LiFP6) at a concentration of 1 M in an organic solvent consisting of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:4:3.

[0117] Example 2

[0118] Except that the lithium composite transition metal oxide prepared in Preparation Example 2 was used as the positive electrode active material, the lithium secondary battery was prepared in the same manner as in Example 1.

[0119] Example 3

[0120] Except that the lithium composite transition metal oxide prepared in Preparation Example 3 was used as the positive electrode active material, the lithium secondary battery was prepared in the same manner as in Example 1.

[0121] Comparative Example 1

[0122] Except that the lithium composite transition metal oxide prepared in Preparation Example 4 was used as the positive electrode active material, the lithium secondary battery was prepared in the same manner as in Example 1.

[0123] Experimental Example 2: Evaluation of Capacity Characteristics

[0124] The capacity characteristics of the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1 were evaluated using the following methods: At room temperature (25°C), each lithium-ion battery was charged at a constant current of 0.2C until it reached 4.25 V under a 0.005C cutoff condition, and maintained for 20 minutes. Subsequently, the lithium-ion batteries were discharged at a constant current of 0.2C under a 2.5V cutoff condition, and maintained for 20 minutes. The charge and discharge capacity of the first cycle was measured by defining these charge and discharge behaviors as a cycle. The measurement results are shown in Table 2.

[0125] Experimental Example 3: Evaluation of High-Temperature Lifetime Characteristics

[0126] The high-temperature life characteristics of the lithium secondary batteries of Examples 1 to 3 and Comparative Example 1 were evaluated using the following methods: Each lithium-ion battery was charged at an elevated temperature (45°C) in CC / CV mode at 0.3C until it reached 4.25 V, and then discharged at a constant current of 0.3C until it reached 2.5 V. After 30 such charge-discharge cycles, the capacity retention was measured. The measurement results are shown in Table 2 below.

[0127] [Table 2]

[0128] As shown in Table 2, compared with the lithium secondary battery of Comparative Example 1 which used a positive electrode active material that experienced a Li-O interlayer distance change of more than 3% in the range of 58% to 72% SOC, the lithium secondary batteries of Examples 1 to 3 which used a positive electrode active material that experienced a Li-O interlayer distance change of less than 3% exhibited improved initial efficiency and improved high-temperature lifetime characteristics.

[0129] Furthermore, it was observed that, compared to the lithium secondary batteries of Examples 1 and 2, which used an increased Li-O interlayer distance under full charge compared to before charging, the lithium secondary battery of Example 3, which used a decreased Li-O interlayer distance under full charge compared to before charging, exhibited a slightly decreased high-temperature lifetime characteristic. This is presumably because, as in Example 3, when the Li-O interlayer distance is reduced under full charge, the lithium-ion mobility decreases.

Claims

1. A lithium secondary battery, the lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode contains a lithium composite transition metal oxide powder having a layered structure and having a nickel content of 85 atm% or more among all transition metals as a positive electrode active material, wherein the lithium composite transition metal oxide powder experiences a change in the lithium-oxygen (Li-O) interlayer distance of 3% or less within a state of charge (SOC) range of 58% to 72%.

2. The lithium secondary battery according to claim 1, wherein the lithium composite transition metal oxide powder experiences a change in the Li-O interlayer distance of 1% or less within a SOC range of 58% to 72%.

3. The lithium secondary battery according to claim 1, wherein in the lithium composite transition metal oxide powder, the Li-O interlayer distance at 100% SOC is greater than or equal to the Li-O interlayer distance at 0% SOC.

4. The lithium secondary battery according to claim 1, wherein the lithium composite transition metal oxide is represented by Chemical Formula 1: [Chemical Formula 1] Li x [Ni a Co b Mr c M d ]O2 in, In Chemical Formula 1, M is one or more elements selected from the following: W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and 0.9 ≤ x ≤ 1.2, 0.85 ≤ a ≤ 0.99, 0 < b < 0.15, 0 < c < 0.15, and 0 < d < 0.

15.

5. The lithium secondary battery according to claim 4, wherein the M contains two or more elements selected from the following: W, Zr, Al, Ti, and Mg.

6. The lithium secondary battery according to claim 4, wherein the M contains W and one or more elements selected from the following: Zr, Al, Ti, and Mg.

7. The lithium secondary battery according to claim 1, wherein the lithium composite transition metal oxide includes a coating on its surface, the coating containing one or more elements selected from the following: Al, Ti, W, B, F, P, Mg, Ni, Co, Fe, Cr, V, Cu, Ca, Zn, Zr, Nb, Mo, Sr, Sb, Bi, Si, and S.

8. A positive electrode active material for a lithium secondary battery, the positive electrode active material containing a lithium composite transition metal oxide powder having a layered structure and having a nickel content of 85 atm% or more among all transition metals, wherein the lithium composite transition metal oxide powder experiences a change in the Li-O interlayer distance (i.e., LiO6 plate thickness) of 3% or less within a SOC range of 58% to 72%.

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