Positive electrode active material and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery comprising same, the positive electrode active material being capable of reducing the amount of gas generated in an activation process or the like of the lithium secondary battery, and further improving the energy density and discharge capacity characteristics of the lithium secondary battery. The positive electrode active material includes, in a mixed state, a first phase containing a lithium manganese oxide having a rock salt structure and a second phase containing a lithium metal oxide having a layered structure, wherein manganese constitutes at least about 50 mol% of the total content of metals other than lithium in the positive electrode active material, and the positive electrode active material may be further doped with about 2000 ppm to 9500 ppm of niobium (Nb) on the basis of the total content of the positive electrode active material.

Description

Cathode active material and lithium secondary battery containing the same

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0119293, filed September 7, 2023, the entire contents of which are incorporated herein by reference.

[0003] The present invention relates to a cathode active material and a lithium secondary battery including the same, which can reduce the amount of gas generated during an activation process of a lithium secondary battery and further improve the energy density and discharge capacity characteristics of the lithium secondary battery.

[0004] Recently, as the application areas of lithium secondary batteries have rapidly expanded to include not only power supply for electronic devices such as electric, electronic, communication, and computer devices, but also power storage for large-area devices such as automobiles and power storage devices, the demand for high-capacity, high-output, and high-stability secondary batteries is increasing.

[0005] The lithium secondary battery generally consists of a positive electrode containing positive active material, a negative electrode containing negative active material, an electrolyte that serves as a medium for transporting lithium ions, and a separator. Research is constantly being conducted to reduce the manufacturing costs of these positive electrodes, negative electrodes, electrolytes, and separators, and to increase the energy density and capacity of lithium secondary batteries.

[0006] The present invention provides a manganese-rich oxide-based cathode active material having a high energy density, while reducing the amount of gas generated during the activation process of a lithium secondary battery and exhibiting improved discharge capacity characteristics.

[0007] The present invention also provides a cathode and a lithium secondary battery including the cathode active material, which exhibit low gas generation and improved energy density and discharge capacity characteristics.

[0008] The present invention comprises a first phase comprising lithium manganese oxide having a rock salt structure; and

[0009] A positive electrode active material comprising a second phase including a lithium metal oxide of chemical formula 1 having a layered structure in a mixed state,

[0010] Manganese is included in an amount of about 50 mol% or more of the total metal content excluding lithium contained in the positive electrode active material.

[0011] Provided is a cathode active material further comprising niobium (Nb) additionally doped on the cathode active material in an amount of about 2000 ppm to 9500 ppm based on the total amount of the cathode active material.

[0012] [Chemical Formula 1]

[0013] Li[Ni 1-b-c Mn b M c ]O2

[0014] In the above chemical formula 1, b is 0.4 or more and less than 1, c is 0 or more and 0.2 or less, and M is at least one selected from the group consisting of Al, B, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr.

[0015] The present invention also comprises a positive electrode current collector; and

[0016] A positive electrode for a lithium secondary battery is provided, which includes a positive electrode active material layer formed on a positive electrode current collector and including the positive electrode active material, a binder, and a conductive material.

[0017] In addition, the present invention provides a lithium secondary battery including the positive electrode, negative electrode, and electrolyte.

[0018] These lithium secondary batteries can exhibit reduced gas generation of approximately 2.9 mL or less when stored for two weeks under conditions of 100% SOC (State of Charge) and 60°C.

[0019] The positive electrode active material of the present invention is a manganese-rich oxide, does not contain cobalt in its structure, and contains niobium (Nb) doped on the positive electrode active material in a predetermined amount.

[0020] The inventors' experimental results confirmed that this cathode active material exhibits the high energy density characteristic of manganese-rich oxides, while also exhibiting reduced gas generation under lithium secondary battery activation conditions. Furthermore, it was confirmed that coating with a predetermined niobium (Nb) content can improve the discharge capacity characteristics of lithium secondary batteries, particularly at high temperatures.

[0021] Figure 1 is a graph showing the results of a comparative evaluation of the gas generation amount of lithium secondary batteries manufactured using the positive electrode active materials of Comparative Example 1 and Example 2.

[0022] Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best possible manner.

[0023] It should be understood that the terms “include,” “comprising,” or “having” used in this specification are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0024] As used herein, “about,” “approximately,” and “substantially” are used to mean a range of or approximation to a numerical value or degree, taking into account inherent manufacturing and material tolerances.

[0025] Lithium transition metal oxides, such as lithium cobalt oxide, lithium nickel oxide, or lithium nickel-cobalt-manganese composite oxide, are mainly used as cathode active materials for lithium secondary batteries. However, lithium manganese-rich oxides are attracting attention to lower the manufacturing cost of cathode active materials and increase energy density and capacity. Lithium manganese-rich oxides can have relatively low manufacturing costs by increasing the content of manganese (Mn), which is relatively inexpensive and abundant, and decreasing the content of cobalt (Co).

[0026] The above lithium manganese-rich oxide may exist in a state in which a lithium manganese oxide phase (Li2MnO3) with a rock salt structure and a lithium metal oxide phase (LiMO2; M: one or more metal elements including manganese) with a layered structure are mixed within the positive electrode of a lithium secondary battery. At this time, due to the presence of the rock salt structure, a higher molar ratio of lithium can be included per unit mol of a metal such as manganese, and thus a higher energy density can be achieved compared to existing positive electrode active materials.

[0027] However, when an initial activation process is performed on a lithium secondary battery including the above lithium manganese-rich oxide, a large amount of gas may be generated, which may cause problems such as a decrease in the capacity of the secondary battery and a decrease in cell stability.

[0028] In one embodiment of the invention, a lithium manganese-rich oxide-based cathode active material is provided that can reduce the amount of gas generated during the activation process of the secondary battery, while improving energy density and capacity characteristics.

[0029]

[0030] Hereinafter, a positive electrode active material according to an embodiment of the invention, a positive electrode including the same, and a lithium secondary battery will be described.

[0031] According to one embodiment of the invention, a cathode active material comprising a first phase including lithium manganese oxide having a rock salt structure; and a second phase including lithium metal oxide of chemical formula 1 having a layered structure, in a mixed state,

[0032] Manganese is included in an amount of about 50 mol% or more of the total metal content excluding lithium contained in the positive electrode active material.

[0033] A cathode active material is provided which further comprises niobium (Nb) additionally doped on the cathode active material in an amount of about 2000 ppm to 9500 ppm based on the total amount of the cathode active material.

[0034] [Chemical Formula 1]

[0035] Li[Ni 1-b-c Mn b M c ]O2

[0036] In the above chemical formula 1, b is about 0.4 or more and less than 1, c is about 0 or more and 0.2 or less, and M is at least one selected from the group consisting of Al, B, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr.

[0037] According to one embodiment of the invention, a cathode active material is a manganese-rich oxide or a composite thereof, which comprises manganese in an amount of about 50 mol% or more, or from about 50 mol% to 95 mol%, or from about 50 mol% to 90 mol% of the total metal content excluding lithium. The cathode active material of one embodiment comprises a first phase comprising a lithium manganese oxide having a rock salt structure, for example, Li2MnO3, and a lithium metal oxide of the above chemical formula 1 having a layered structure, for example, Li[Ni 1-b-c Mn b M c]O2, and the first and second phases may exist in a mixed or mutually combined state within the positive electrode active material. At this time, due to the presence of the first phase containing lithium in a molar ratio greater than that of a metal such as manganese, the positive electrode active material of the above embodiment may contain a relatively large content of lithium and thus have a greater energy density than existing positive electrode active materials.

[0038] In addition, the cathode active material of the above embodiment can be a so-called “cobalt-free” cathode active material that does not include cobalt in the entire structure including the first and second phases. Depending on this “cobalt-free” form, when the cathode active material of the above embodiment is used, the energy density can be increased while reducing the manufacturing cost of the cathode active material, so that the manufacturing cost per unit energy can be relatively reduced.

[0039] However, while the positive electrode active material of the above embodiment can increase the energy density due to the above-described lithium and manganese-rich characteristics, for example, lithium ions may be desorbed from the transition metal oxide layer during an activation process, and the structure of the positive electrode may change. In addition, not only is oxygen radical generation / desorption caused by an oxygen redox reaction, but gases such as CO, CO2, or O2 are generated due to side reactions and decomposition of the electrolyte. Such gas generation not only causes a decrease in capacity due to gas traps, but also has a negative impact on cell stability.

[0040] In this regard, since the cathode active material of the above embodiment contains the doping element niobium (Nb) in a predetermined amount, for example, in an amount of about 2000 ppm to 9500 ppm, or about 3500 ppm to 8000 ppm, or about 4500 ppm to 5500 ppm based on the total amount of the cathode active material, the energy density and discharge capacity characteristics of the lithium secondary battery, particularly the discharge capacity characteristics at high temperatures, can be improved, and the amount of gas generated can be reduced. This is expected because, when niobium (Nb) is doped into the cathode active material, the crystal structure of the cathode active material becomes more stable, and the movement of lithium ions can occur more smoothly.

[0041] However, if the doping content is too small, for example, less than about 2000 ppm, the improvement in energy density and discharge capacity or the reduction in gas generation may not be substantially realized. Alternatively, if the doping content is too large, for example, more than about 9500 ppm, it was confirmed that the effect of niobium (Nb) doping is not realized, such as the discharge capacity actually decreasing.

[0042] Meanwhile, the positive electrode active material of the above embodiment basically includes nickel and manganese, and may include the total molar ratio of nickel: manganese included in the positive electrode active material to be 1:1 to 1:5, or 1:1 to 1:4. In addition, b corresponding to the molar ratio of manganese in the above chemical formula 1 may satisfy a range of about 0.4 or more and less than 1, or about 0.45 or more and 0.95 or less, or about 0.5 or more and 0.9 or less.

[0043] By meeting these ranges and including manganese in a larger molar ratio, the cathode active material of one embodiment can exhibit high energy density and capacity characteristics while maintaining the structural stability of the entire cathode active material including the first and second phases.

[0044] In the above chemical formula 1, M can define an additional doping element other than niobium (Nb), and c can define a molar ratio of this additional doping element M. The type of this additional doping element is not particularly limited, and c can satisfy a range of about 0 or more and 0.2 or less, or about 0 or more and 0.1 or less. If the content of the additional doping element is excessively large, it may not only have a negative effect on the capacity characteristics of the active material, but also, for example, the amount of gas generated may increase due to an increase in the oxygen redox reaction.

[0045] Meanwhile, the cathode active material of the above embodiment can exhibit high energy density and capacity characteristics by including an excess of lithium based on the total mole number of metals other than lithium, including nickel (Ni), manganese (Mn), and metal M. For example, the cathode active material may have a ratio of the total mole number of lithium of greater than about 1 and less than or equal to 1.5, or about 1.1 to 1.4, or about 1.2 to 1.4, based on the total mole number of metals other than lithium. In this way, by including an excess of lithium, the energy density and capacity characteristics can be excellently expressed. However, if the mole ratio of lithium becomes excessively large, the proportion of the first phase having a rock salt structure may increase excessively, thereby lowering the structural stability of the cathode active material and accelerating the degradation rate.

[0046] In addition, the positive electrode active material of the above embodiment may include the first phase having the rock salt structure and the second phase having the layered structure in a molar ratio of about 1:9 to 5:5, or about 2:8 to 4:6. Due to the content ratio of the first and second phases, the positive electrode active material may have excellent energy density and capacity characteristics by including an excess of lithium, while exhibiting overall structural stability.

[0047] An embodiment of a cathode active material exhibiting the above-described characteristics may, in one embodiment, include a lithium metal oxide complex of the following chemical formula 2, and may include niobium (Nb) additionally doped on the lithium metal oxide complex in an amount of about 3500 ppm to 8000 ppm based on the total amount of the cathode active material:

[0048] [Chemical Formula 2]

[0049] X{Li2MnO3}·(1-X){Li[Ni 1-b-c Mn b M c ]O2}

[0050] In the above chemical formula 1, X is about 0.1 or more and 0.5 or less, b is about 0.45 or more and 0.95 or less, c is about 0 or more and 0.2 or less, and M is at least one selected from the group consisting of Al, B, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr.

[0051] Additionally, the positive electrode active material may further include a coating layer formed on the lithium metal oxide composite, if necessary. When the positive electrode active material includes a coating layer, contact with the electrolyte is suppressed, thereby reducing side reactions and further reducing the amount of gas generated.

[0052] The coating layer may include a coating element M1, and the coating element M1 may be at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr, and may be, for example, Al, Co, W, and combinations thereof. Such a coating element may exist in the coating layer in the form of an oxide, for example, M1Oz (1≤z≤4).

[0053] In addition, the coating layer can be formed to cover about 10% to 100%, or about 30% to 100%, or about 50% to 100% of the total surface area of ​​the positive electrode active material, for example, the lithium metal oxide composite particle, and thereby the life characteristics of the positive electrode active material can be further improved.

[0054] Meanwhile, the above-described positive electrode active material can be manufactured by mixing and calcining metal precursors such as nickel, manganese, and M to manufacture a precursor, and then additionally mixing and calcining the niobium precursor and lithium raw material.

[0055] In this manufacturing method, as the lithium raw material, for example, lithium-containing carbonates (e.g., lithium carbonate, etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH·H2O), etc.), hydroxides (e.g., lithium hydroxide, etc.), nitrates (e.g., lithium nitrate (LiNO3), etc.), chlorides (e.g., lithium chloride (LiCl), etc.), etc., etc., can be used, and one type alone or a mixture of two or more types of these can be used.

[0056] Additionally, the metal precursor such as nickel, manganese, M or niobium (Nb) may be in the form of a hydroxide, oxide or carbonate. When a precursor in the form of a carbonate is used, a cathode active material having a relatively high specific surface area can be manufactured.

[0057] Each of these metal precursors and lithium raw materials can be mixed according to the molar ratio of each element defined in the above chemical formula 2, etc., and each firing step can be performed. This firing step can be performed at a temperature of about 600°C to 1000°C or about 700°C to 950°C, and the firing time can be about 5 hours to 30 hours or about 5 hours to 20 hours. In addition, the firing atmosphere can be an air atmosphere or an oxygen atmosphere, and for example, it can be an atmosphere containing about 20% to 100% by volume of oxygen.

[0058] However, since the positive electrode active material of one embodiment can be manufactured according to a general positive electrode active material manufacturing process and conditions in the form of lithium metal oxide, further description thereof will be omitted.

[0059] Meanwhile, according to another embodiment of the invention, a positive electrode for a lithium secondary battery is provided, comprising: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material, binder, and conductive material of the above-described embodiment.

[0060] In the positive electrode of the above other embodiments, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the positive electrode current collector can typically have a thickness of about 3 ㎛ to 500 ㎛, and can have various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

[0061] In addition, in the positive electrode active material layer, the conductive material may include, for example, spherical or flaky graphite; carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, etc.; metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide, etc.; or conductive polymers such as polyphenylene derivatives, etc., and one of these may be used alone or a mixture of two or more thereof may be used. The conductive material may be included in an amount of about 0.1 wt% to 20 wt%, or about 1 wt% to 20 wt%, or about 1 wt% to 10 wt%, based on the total weight of the positive electrode active material layer.

[0062] In addition, examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used. The above binder may be included in an amount of about 0.1 wt% to 20 wt%, or about 1 wt% to 20 wt%, or about 1 wt% to 10 wt% based on the total weight of the positive electrode active material layer.

[0063] Additionally, the positive electrode active material layer may include the positive electrode active material of one embodiment in an amount of about 80 wt% to 99 wt% based on the total weight.

[0064] The positive electrode of the above other embodiment can be manufactured according to a general positive electrode manufacturing process, except that it includes the positive electrode active material of one embodiment, for example, by mixing each of the above-described components in a solvent to prepare a slurry composition, and then applying, drying, and rolling the slurry composition onto a positive electrode current collector.

[0065] Meanwhile, according to a further embodiment of the invention, a lithium secondary battery comprising the positive electrode, negative electrode, and electrolyte is provided. Such a lithium secondary battery, by including the positive electrode active material of the above-described embodiment, may exhibit reduced gas generation during an activation process and the like, and may have improved discharge capacity characteristics.

[0066] In such a lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector.

[0067] At this time, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. The thickness and shape of other negative electrode current collectors can be the same as or corresponding to those of the positive electrode current collector.

[0068] Additionally, the negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material.

[0069] As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. For example, carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metallic oxides capable of doping and dedoping lithium, such as SiOβ (0 <β< 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, may be used, and any one or a mixture of two or more of these may be used. In addition, a metallic lithium thin film may be used as the negative electrode active material. The negative electrode active material may be included in an amount of about 80 wt% to 99 wt% based on the total weight of the negative electrode active material layer.

[0070] Meanwhile, in the negative electrode active material layer, the type and content range of the binder and conductive material may be the same or similar to those described for the positive electrode active material layer, and therefore, further description thereof will be omitted.

[0071] In addition, the negative electrode active material layer and the negative electrode can be manufactured by mixing the above-described negative electrode active material, binder, and conductive material components in a solvent to prepare a slurry composition, and then applying, drying, and rolling the slurry composition onto a negative electrode current collector. In another example, the negative electrode composite including the negative electrode active material, binder, and conductive material can be manufactured by casting the negative electrode composite including the above-described negative electrode active material, binder, and conductive material onto a separate support, and then laminating the resulting film onto a negative electrode current collector by peeling it off from the support.

[0072] Meanwhile, in the lithium secondary battery, the electrolyte may be an organic liquid electrolyte, an inorganic liquid electrolyte, a polymer solid electrolyte, a gel-type polymer electrolyte, an inorganic solid electrolyte, or a molten inorganic electrolyte that can be used in the manufacture of a secondary battery.

[0073] In one example, the electrolyte may be a liquid electrolyte comprising an organic solvent and a lithium salt.

[0074] As the organic solvent, any solvent that can serve as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. For example, the organic solvent may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used.

[0075] Among these, a carbonate solvent can be appropriately used, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and a linear carbonate compound having low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) can be used.

[0076] The above lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. For example, the anion of the lithium salt may be at least one selected from the group consisting of F-, Cl-, Br-, I-, NO3-, N(CN)2-, BF4-, CF3CF2SO3-, (CF3SO2)2N-, (FSO2)2N-, CF3CF2(CF3)2CO-, (CF3SO2)2CH-, (SF5)3C-, (CF3SO2)3C-, CF3(CF2)7SO3-, CF3CO2-, CH3CO2-, SCN-, and (CF3CF2SO2)2N-. For example, the lithium salt may include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2. LiCl, LiI, or LiB(C2O4)2.

[0077] In addition, it is recommended that the concentration of the lithium salt in the liquid electrolyte be within the range of about 0.1 M to 4.0 M. When the concentration of the lithium salt is within the above range, the electrolyte can exhibit excellent electrolyte performance because it has appropriate conductivity and viscosity, and lithium ions can move effectively.

[0078] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. At this time, the additives may be included in an amount of about 0.1 wt% to 5 wt% based on the total weight of the electrolyte.

[0079] Meanwhile, the lithium secondary battery described above may further include a separator that separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. As such a separator, a porous polymer film having low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may also be used.

[0080] Additionally, a coated separator containing a ceramic component or polymer material may be used to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0081] There is no particular limitation on the external shape of the above lithium secondary battery, but it may be a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

[0082] The lithium secondary battery described above can exhibit excellent energy density and discharge capacity characteristics and reduced gas generation by including the cathode active material of one embodiment. For example, when the lithium secondary battery is stored for two weeks under the conditions of 100% SOC (State of Charge) and 60°C, it can exhibit a small gas generation amount of about 0.29 mL or less, or 0.25 mL or less.

[0083] Accordingly, the lithium secondary battery is useful in portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0084] Accordingly, the lithium secondary battery can be used not only as a battery cell used as a power source for a small device, but can also be preferably used as a unit battery in a medium- to large-sized battery module including a plurality of battery cells.

[0085] Examples of the above medium and large devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0086]

[0087] Hereinafter, embodiments of the invention will be described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0088]

[0089] Comparative Example 1: 0.3{Li2MnO3}·0.7{Li[Ni 0.5 Mn 0.5 ]O2} production

[0090] Ni 0.35 Mn 0.65(OH)2 and lithium hydroxide (LiOH) were added to a Henschel mixer (700 L) so that the molar ratio of Li:transition metal (Ni+Mn) was 1.3:1, and mixed at 300 rpm for 20 minutes. The mixed powder was placed in an alumina crucible measuring 330 mm × 330 mm, and calcined at 850°C for 10 hours in an oxygen atmosphere to manufacture a cathode active material of Comparative Example 1.

[0091]

[0092] Example 1: Nb (2500 ppm) doped 0.3{Li2MnO3}·0.7{Li[Ni 0.5 Mn 0.5 ]O2} production

[0093] Ni 0.35 Mn 0.65 (OH)2 and lithium hydroxide (LiOH) were added to a Henschel mixer (700 L) so that the molar ratio of Li:transition metal (Ni+Mn) was 1.3:1, and mixed at 300 rpm for 20 minutes. During this mixing process, niobium oxide (NbO) was additionally mixed so that the content of niobium (Nb) in the final cathode active material was 2500 ppm.

[0094] The above mixed powder was placed in an alumina crucible measuring 330 mm × 330 mm and fired at 850°C for 10 hours in an oxygen atmosphere to manufacture the positive electrode active material of Example 1.

[0095]

[0096] Example 2: Nb (5000 ppm) doped 0.3{Li2MnO3}·0.7{Li[Ni 0.5 Mn 0.5 ]O2} production

[0097] Ni 0.35 Mn 0.65(OH)2 and lithium hydroxide (LiOH) were added to a Henschel mixer (700 L) so that the molar ratio of Li:transition metal (Ni+Mn) was 1.3:1, and mixed at 300 rpm for 20 minutes. During this mixing process, niobium oxide (NbO) was additionally mixed so that the content of niobium (Nb) in the final cathode active material was 5000 ppm.

[0098] The above mixed powder was placed in an alumina crucible measuring 330 mm × 330 mm and fired at 850°C for 10 hours in an oxygen atmosphere to manufacture the positive electrode active material of Example 2.

[0099]

[0100] Example 3: Nb (9000 ppm) doped 0.3{Li2MnO3}·0.7{Li[Ni 0.5 Mn 0.5 ]O2} production

[0101] Ni 0.35 Mn 0.65 (OH)2 and lithium hydroxide (LiOH) were added to a Henschel mixer (700 L) so that the molar ratio of Li:transition metal (Ni+Mn) was 1.3:1, and mixed at 300 rpm for 20 minutes. During this mixing process, niobium oxide (NbO) was additionally mixed so that the content of niobium (Nb) in the final cathode active material was 9000 ppm.

[0102] The above mixed powder was placed in an alumina crucible measuring 330 mm × 330 mm and fired at 850°C for 10 hours in an oxygen atmosphere to manufacture the positive electrode active material of Example 3.

[0103]

[0104] Manufacturing example: Manufacturing of lithium secondary batteries

[0105] A cathode slurry was prepared by mixing a cathode active material of an example or comparative example: a single-walled carbon nanotube: a PVDF binder in a weight ratio of 96.0:1.0:3.0 in N-methylpyrrolidone. The cathode slurry was applied onto an aluminum current collector sheet, dried, and then rolled to prepare a cathode.

[0106] Graphite negative electrode active material: single-walled carbon nanotube: styrene-butadiene rubber (SBR): carboxymethyl cellulose (CMC) were mixed in a weight ratio of 96.2:0.8:2:1 in water to prepare a negative electrode slurry. The negative electrode slurry was applied onto a copper current collector sheet, dried, and then rolled to prepare a negative electrode.

[0107] Meanwhile, the positive and negative electrodes were manufactured so that the ratio of the negative electrode discharge capacity to the positive electrode discharge capacity (N / P ratio) was 115% by controlling the loading amount.

[0108] An electrode assembly was manufactured by interposing a polyethylene separator between the positive and negative electrodes manufactured as described above, inserting the electrode assembly into a battery case, injecting an electrolyte, charging it to 4.6 V at a constant current of 0.1 C at 45°C, and then discharging it to 2.0 V at a constant current of 0.1 C to perform an activation process, thereby manufacturing a lithium secondary battery.

[0109]

[0110] Experimental Example 1: Gas Emission Evaluation

[0111] When lithium secondary batteries manufactured using the positive electrode active materials of Example 2 and Comparative Example 1 were stored for two weeks under conditions of 100% SOC (State of Charge) and 60°C, the amount of gas generated was evaluated. This amount of gas generated was measured and evaluated using gas chromatography, and the evaluation results are shown in Figure 1.

[0112] Referring to Fig. 1, it was confirmed that the lithium secondary battery manufactured using Example 2 showed a gas generation amount reduced by about 14% compared to the battery manufactured from Comparative Example 1.

[0113]

[0114] Experimental Example 2: Electrochemical Characteristics Evaluation

[0115] For lithium secondary batteries manufactured with the positive electrode active materials of the above examples or comparative examples, charge-discharge tests were performed in the voltage range of 4.4 V to 2.5 V under the conditions of 25°C, 0.1 C, and 0.33 C. From the results of these charge-discharge tests, the discharge capacity, average voltage, and energy density were evaluated, respectively, and the evaluation results are shown in Tables 1 and 2 below. Table 1 shows the results of comparative evaluation of average voltage and energy density for the batteries of Example 2 and Comparative Example 1, and Table 2 shows the results of comparative evaluation of discharge capacity for the batteries of Examples 1 to 3.

[0116] At this time, the energy density was calculated by multiplying the discharge capacity by the average voltage and dividing it by the unit volume of the secondary battery, and the average voltage was calculated by dividing the curve integral of the capacity-voltage profile by the capacity.

[0117]

[0118] Example 2 Comparative Example 1 Average voltage (V) 0.1 C 3.64 6 3.64 5 0.33 C 3.64 4 3.64 0 Energy density (mWh / g) (Average voltage x discharge capacity) 0.1 C 8 3 6.1 1 8 3 1.68 0.33 C 8 0 9.67 8 0 2.40

[0119] Example 1 Example 2 Example 3 Discharge capacity (mAh / g) 0.1C 2 2 8.8 2 2 9.2 2 7.4 0.33C 2 2 0.6 2 2 1.8 2 19.3 De-rating (0.33C / 0.1C;%) 96.4 96.8 96.4

[0120] Referring to Table 1 above, it was confirmed that the battery manufactured from Example 2 exhibited a relatively higher energy density than Comparative Example 1, and this difference was confirmed to be more significant under high discharge rates. For example, a higher energy density per weight means that a larger amount of active material can be used to produce a lighter battery than would be used in an actual battery, as a larger amount of energy can be produced.

[0121] In addition, referring to Table 2 above, it was confirmed that the battery manufactured with the positive active material of the example showed a relatively large discharge capacity, and in particular, it was confirmed that it was maximized the most in Example 2 where niobium (Nb) was doped at 5000 ppm. In addition, the De-rating value is a value expressed as a % of the 0.33C capacity / 0.1C capacity ratio. Here, C is the C rate value, which indicates the charge / discharge speed of the battery. In the case of the C-rate of 0.1C, the time used for charging / discharging the battery is 10 hours, and in the case of 0.33C, the time used for charging / discharging the battery is approximately 3 hours. This means that the higher the C rate value, the faster the charge / discharge speed can be used, and it means that it is advantageous to use it even in situations where a high current is required momentarily. Therefore, the closer the % of the discharge capacity expressed at 0.33C / the discharge capacity expressed at 0.1C is to 100%, the faster the charge / discharge is possible. This means that when 5000 ppm of niobium (Nb) is added, charging and discharging can be performed at a fast speed.

[0122] Table 1 confirms that energy density is improved, and Table 2 confirms that the derating value is improved when 5000 ppm is added. This suggests that adding an appropriate level of niobium (Nb) can improve energy density, create a lightweight battery, and enable fast charging and discharging. These conditions are expected to be even more advantageous when used in batteries for electric vehicles.

[0123] Although the present disclosure has been described above with reference to embodiments thereof, it will be understood by those skilled in the art or those with ordinary knowledge in the art that various modifications and changes to the present invention may be made without departing from the spirit and technical scope of the present invention as set forth in the claims below. Accordingly, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification, but should be defined by the claims.

Claims

1. A first phase comprising lithium manganese oxide having a rock salt structure; and A positive electrode active material comprising a second phase including a lithium metal oxide of chemical formula 1 having a layered structure in a mixed state, Manganese is included in an amount of 50 mol% or more of the total metal content excluding lithium contained in the positive electrode active material, A cathode active material further comprising niobium (Nb) additionally doped on the cathode active material in an amount of 2000 ppm to 9500 ppm based on the total amount of the cathode active material: [Chemical Formula 1] Li[Ni 1-b-c Mr b M c ]O2 In the above chemical formula 1, b is 0.4 or more and less than 1, c is 0 or more and 0.2 or less, and M is at least one selected from the group consisting of Al, B, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr.

2. In the first paragraph, the first phase is a positive electrode active material including lithium manganese oxide represented by Li2MnO3.

3. In the first paragraph, the positive electrode active material is comprised of the first phase and the second phase in a molar ratio of 1:9 to 5:

5.

4. In the first paragraph, a positive electrode active material in which the ratio of the total mole number of lithium contained in the positive electrode active material is greater than 1 and less than or equal to 1.5, based on the total mole number of nickel (Ni), manganese (Mn) and metal M contained in the positive electrode active material.

5. A positive electrode active material in the first paragraph, wherein the total molar ratio of nickel (Ni) : manganese (Mn) contained in the positive electrode active material is 1:1 to 1:

5.

6. In the first paragraph, a lithium metal oxide complex of the following chemical formula 2 is included, A cathode active material comprising niobium (Nb) in an amount of 3500 ppm to 8000 ppm based on the total amount of the cathode active material: [Chemical Formula 2] X{Li2MnO3}·(1-X){Li[Ni 1-b-c Mn b M c ]O2} In the above chemical formula 1, X is 0.1 or more and 0.5 or less, b is 0.45 or more and 0.95 or less, c is 0 or more and 0.2 or less, and M is at least one selected from the group consisting of Al, B, W, Mg, V, Ti, Zn, Ga, In, Ru, Sn, Sr, and Zr.

7. Anode current collector; and A positive electrode for a lithium secondary battery, comprising a positive electrode active material layer formed on a positive electrode current collector and including a positive electrode active material, a binder, and a conductive material according to any one of claims 1 to 6.

8. A lithium secondary battery comprising the positive electrode, negative electrode and electrolyte of Article 7.

9. A lithium secondary battery having a gas generation amount of 2.9 mL or less when stored for two weeks under conditions of SOC (State of Charge) 100% and 60°C in paragraph 8.

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