Positive electrode material, positive electrode and lithium secondary battery comprising the same

Abstract

The present invention relates to a cathode active material for a lithium secondary battery that can suppress particle breakage within the cathode and improve the density of the cathode while exhibiting the electrical and chemical properties characteristic of a manganese-rich cathode active material, a cathode including the same, and a lithium secondary battery.

Description

Technology Field

[0001] The present invention relates to a cathode active material for a lithium secondary battery that can suppress particle breakage within the cathode and improve the density of the cathode while exhibiting the electrical and chemical properties characteristic of a manganese-rich cathode active material, a cathode including the same, and a lithium secondary battery. Background Technology

[0002] Recently, as the application areas of lithium-ion batteries have rapidly expanded to include not only power supply for electronic devices such as electrical, electronic, telecommunications, and computers, but also power storage for large-area devices such as automobiles and power storage systems, there is a growing demand for high-capacity, high-output, and high-stability secondary batteries.

[0003] The above lithium secondary battery generally consists of a positive electrode containing a positive active material, a negative electrode containing a negative active material, an electrolyte serving as a medium for transporting lithium ions, and a separator. In this case, carbon-based active materials, silicon-based active materials, etc., may be used as the negative active material. Additionally, lithium transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium nickel-cobalt-manganese composite oxide may be used as the positive active material.

[0004] Recently, lithium-rich manganese oxides have been attracting attention as next-generation cathode active materials. Lithium-rich manganese oxides have the advantage of high capacity due to the high content of manganese (Mn), which is relatively inexpensive and abundant. However, since these lithium-rich manganese oxides have compositional limitations with low rate characteristics, the structure is being controlled to increase the BET specific surface area of ​​secondary particles by making the primary particles smaller in order to improve this.

[0005] However, the above-mentioned lithium manganese-rich oxide has a low density and a relatively large BET specific surface area compared to conventional cathode active materials, such as lithium nickel-cobalt-manganese composite oxide, and has the disadvantage of being easily broken.

[0006] In particular, during the process of manufacturing the cathode, such as through rolling, the above-mentioned lithium manganese-rich oxide particles may easily break, which can degrade their electrical and chemical properties, lower the density of the cathode, and increase the porosity, thereby lowering the overall energy density of the lithium secondary battery. Furthermore, since the thickness of the cathode may inevitably increase due to such low cathode density, this can also act as a limit in increasing the energy density of the battery. The problem to be solved

[0007] Accordingly, the present invention provides a cathode active material for a lithium secondary battery that can suppress particle breakage within the cathode and improve the density of the cathode while exhibiting the electrical and chemical properties characteristic of a manganese-rich cathode active material.

[0008] In addition, the present invention provides a positive electrode and a lithium secondary battery comprising the above-mentioned positive electrode active material, which exhibit high density and excellent electrical, chemical, and mechanical properties. means of solving the problem

[0009] According to one embodiment of the invention, a positive active material comprising a first positive active material particle and a second positive active material particle having different average particle sizes (D50) and represented by the following chemical formula 1,

[0010] The above-described positive active material is provided as a positive active material for a lithium secondary battery having a bimodal particle size distribution when analyzed for volume cumulative particle size distribution:

[0011] [Chemical Formula 1]

[0012] Li a [Mn b Nic M d ] 2-a O2

[0013] In the above Chemical Formula 1, a is greater than 1, b is 0.5 or more and less than 1, and c and d are each 0 or more and 0.5 or less, provided that 0 <c+d≤0.5이고, M은 Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr 및 Zr로 이루어진 군에서 선택된 하나 이상이다.

[0014] In addition, 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 comprising a positive electrode active material of the first embodiment.

[0015] According to another embodiment of the invention, a lithium secondary battery comprising a positive electrode; a negative electrode; and an electrolyte of the other embodiment is provided. Effects of the invention

[0016] Experimental results by the inventors confirmed that by controlling the particle size distribution of manganese-rich positive active material particles and using a positive active material having different average particle sizes (D50) and a bimodal particle size distribution in the particle size distribution analysis results, the breakage of positive active material particles during rolling can be significantly reduced.

[0017] In particular, by applying a positive electrode active material using the above-mentioned bimodal particle size distribution, the contact area with the electrolyte within the positive electrode can be improved and rate characteristics can be improved. Accordingly, the BET specific surface area of ​​each positive electrode active material particle can be kept small, and the breakage of the positive electrode active material particles can be further reduced.

[0018] Accordingly, by using a manganese-rich cathode active material having a bimodal particle size distribution, it is possible to manufacture a good cathode with high density, low porosity, and excellent rate characteristics without particle breakage. Therefore, by using the cathode active material of this embodiment, it is possible to provide a next-generation lithium secondary battery having a thin thickness, high density, and excellent rate characteristics while exhibiting electrical and chemical properties, such as the high capacity characteristic unique to manganese-rich cathode active materials. Brief explanation of the drawing

[0019] Figure 1 shows the volume cumulative particle size distribution curves for the positive electrode active materials of Example 1, Example 2, and Comparative Example 1. Specific details for implementing the invention

[0020] The present invention will be described in more detail below.

[0021] In the following specifications, "lithium manganese-rich oxide" or "manganese-rich cathode active material" may refer to a lithium metal oxide having a layered crystal structure, wherein the molar ratio of lithium to the molar amount of all metals excluding lithium exceeds 1, and the manganese content among all metals excluding lithium is 50 mol% or more.

[0022] Also, "primary particle" refers to a particle unit in which no grain boundaries appear when observed at a field of view of 5,000 to 20,000 times using a scanning electron microscope, and "secondary particle" refers to a particle formed by the aggregation of multiple primary particles.

[0023] In addition, "average particle size (D50)" refers to the particle size corresponding to 50% of the volume cumulative particle size distribution of the powder to be measured, and can be measured using the laser diffraction method. For example, after dispersing the positive active material powder in a dispersion medium, it can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S-3500), irradiated with ultrasound of about 28 kHz at an output of 60 W, and then a volume cumulative particle size distribution graph is obtained, and the particle size at the point where the volume cumulative amount is 50% is determined from the obtained graph.

[0024] In addition, the "BET specific surface area" is measured by the BET (Brunauer-Emmett-Teller) method, and specifically, can be calculated from the nitrogen adsorption isotherm obtained under a 77K liquid nitrogen atmosphere using BELSORP-MAX (MicrotracBEL corp.).

[0026] Meanwhile, a positive electrode active material according to one embodiment of the invention may include first and second positive electrode active material particles having different average particle sizes (D50), and these first and second positive electrode active material particles may include a lithium-rich manganese oxide represented by Chemical Formula 1:

[0027] [Chemical Formula 1]

[0028] Li a [Mn b Ni c M d ] 2-a O2

[0029] In the above Chemical Formula 1, a is greater than 1, b is 0.5 or more and less than 1, and c and d are each 0 or more and 0.5 or less, provided that 0 <c+d≤0.5이고, M은 Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr 및 Zr로 이루어진 군에서 선택된 하나 이상이다.

[0030] The positive active material of one embodiment, comprising these first and second positive active material particles, may each have the form of a secondary particle in which a plurality of primary particles are aggregated, and when the positive active material of one embodiment is analyzed for the overall volume cumulative particle size distribution, it may exhibit a bimodal particle size distribution including a plurality of separated peaks, for example, two peaks. At this time, the bimodal particle size distribution can be defined and confirmed by the appearance of two separated peaks corresponding to the first and second positive active material particles, respectively, in the volume cumulative particle size distribution curve of the positive active material of one embodiment.

[0031] Experimental results by the inventors confirmed that using the positive active material of one embodiment having the bimodal particle size distribution can suppress the breakage of the positive active material particles, particularly the breakage of particles during rolling. This is predicted to be because the stress applied to the particles during rolling, etc., can be effectively buffered by using the positive active material having the bimodal particle size distribution.

[0032] In addition, by applying a positive electrode active material using the above-mentioned bimodal particle size distribution, the contact area with the electrolyte within the positive electrode can be improved, and the rate characteristics can be improved in itself. Therefore, while solving the problem of low rate characteristics of existing lithium-rich manganese oxides, the BET specific surface area of ​​each positive electrode active material particle can be maintained small. For this reason, the breakage of the positive electrode active material particles can be further reduced.

[0033] Accordingly, when using the cathode active material of one embodiment, a good cathode having high density, low porosity, and thin thickness can be manufactured without particle breakage. Therefore, by using the cathode active material of this embodiment, it is possible to provide a next-generation lithium secondary battery having thin thickness, high density, and excellent rate characteristics while exhibiting electrical and chemical properties, such as the high capacity characteristic of manganese-rich cathode active materials.

[0035] Meanwhile, in the cathode material of the above embodiment, the molar ratio of lithium to the total number of moles of metals excluding lithium in the first and second cathode active material particles may be greater than 1, or 1.1 to 1.5, or 1.3 to 1.5. Such molar ratio of lithium can be calculated from the formula "a / (2-a)" in Chemical Formula 1. In addition, in Chemical Formula 1, a is the molar ratio of Li in the over-lithium manganese-rich oxide, and a in Chemical Formula 1 may satisfy 1.1≤a≤1.5, 1.1≤a≤1.4, 1.1≤a≤1.3, or 1.14≤a≤1.3.

[0036] As the range of a and the molar ratio of lithium to the remaining metal satisfy the range described above, the chemical and crystallographic stability of the first and second cathode active material particles can be maintained, while a higher capacity can be realized and excellent rate characteristics can be achieved. In addition, if the molar ratio of lithium to the remaining metal is excessively high, electrical conductivity may decrease and the rock salt phase (Li2MnO3) may increase, accelerating the degradation rate, and if it is too low, the effect of improving energy density may be negligible.

[0037] The above b is the molar ratio of Mn in the lithium-over-manganese-rich oxide, and in Chemical Formula 1, it can satisfy 0.5≤b<1, 0.5≤b≤0.9, or 0.55≤b≤0.8. As a result, excellent capacity characteristics unique to lithium-over-manganese-rich oxide can be exhibited.

[0038] The above c is the molar ratio of Ni in the lithium manganese-rich oxide, and can satisfy 0≤c≤0.5, 0.1≤c≤0.5, 0.1≤c≤0.45, or 0.3≤c≤0.4 in the above formula 1.

[0039] The above d is the molar ratio of element M added to the lithium manganese-rich oxide or added in the form of doping, and in the above chemical formula 1, it may be 0≤d≤0.5, 0≤d<0.5, 0≤d≤0.2, or 0≤d≤0.1. If the content of the added element M is too high, it may not only adversely affect the capacity of the active material, but also increase the oxygen-redox reaction, which may lead to gas generation and intensify the degradation of the cathode active material, thereby potentially degrading the lifespan characteristics.

[0040] More suitable examples of the above M may be one or more selected from the group consisting of Co, Cr, V, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and in a more specific embodiment, one or more selected from the group consisting of Co, Zn, Ti, Al, Mg, and B may be added to the first and second positive active material particles, respectively, in the form of doping.

[0041] In addition, in a specific embodiment, the first and second positive electrode active material particles may contain additional element M, etc., in the form of doping elements only on the surface, and may contain a molar ratio of nickel to manganese of 25:75 to 50:50, or 25:75 to 45:55, or 30:70 to 40:60. Within the above-described range, if the molar ratio of manganese becomes excessively small, the proportion of rock salt phase may be insufficient, resulting in insufficient capacity or reduced crystallographic and chemical stability. Conversely, if the molar ratio of manganese becomes excessively large, the stability of the first and second positive electrode active material particles may also be reduced.

[0042] Meanwhile, in one example of the above chemical formula 1, b+c+d may satisfy 1, but in another example, it may have a value of 0.9 or higher and less than 1. The fact that b+c+d is less than 1 may indicate that the above chemical formula 1 contains additional metal elements in addition to manganese, nickel, and additional element M. However, it goes without saying that such additional metal elements may be added to the extent that they maintain the crystal structure characteristic of lithium-rich manganese oxide.

[0043] In a specific embodiment of the positive electrode active material of one embodiment, the first and second positive electrode active material particles may be a compound represented by the following chemical formula 1a:

[0044] [Chemical Formula 1a]

[0045] Li a [Mn b Ni c M d ] 2-a O2

[0046] In the above chemical formula 1, 1.1 <a<1.3이고, 0.5≤b≤0.9, 0.1≤c≤0.5이고, 0≤d≤0.1이고, 이고, M은 Co, Cr, V, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr 및 Zr로 이루어진 군에서 선택된 하나 이상이다.

[0047] Meanwhile, in the case of a lithium-excess manganese-rich oxide containing an excess of lithium, a compound having a rock salt structure, e.g., Li2MnO3, and a compound having a layered structure, e.g., Li[NiwMnyMz]O2, may be included in a mixed state. Accordingly, the first and second cathode active material particles may each be represented by the following chemical formula 2:

[0048] [Chemical Formula 2]

[0049] X*Li2MnO3·(1-X)*Li[NiwMnyMz]O2

[0050] In the above chemical formula 2,

[0051] M is one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and 0.2≤X≤0.5, 0≤w≤0.5, 0.4≤y<1, 0≤z≤0.2, wherein 0 <w+z≤0.5임.

[0052] The above X represents the ratio of the rock salt phase (Li2MnO3 phase) in the lithium-rich manganese oxide, and the above w, y, and z represent the molar ratios of Ni, Mn, and additional element M in the layered structure compound, respectively.

[0053] Meanwhile, if necessary, a coating layer may be further included on the surface of the lithium-rich manganese oxide. In this case, contact between the lithium-rich manganese oxide and the electrolyte is suppressed by the coating layer, thereby reducing side reactions in the electrolyte and improving lifespan characteristics.

[0054] The above coating layer is, coating element M 1 It may include, and the coating element M 1 The coating element M may be, for example, at least one selected from the group consisting of Al, B, Co, W, Mg, V, Ti, Zn, Ga, In, Ru, Nb, Sn, Sr, and Zr. 1 It may include two or more types, for example, Al and Co.

[0055] The above coating element is in the form of an oxide within the coating layer, i.e., M 1 It can exist as Oz (1 ≤ z ≤ 4).

[0056] The above coating layer can be formed through methods such as dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). Among these, it is preferable to form it through atomic layer deposition in that it can form a large coating layer area.

[0057] The area of ​​the coating layer formed above may be 10% to 100%, 30% to 100%, or 50% to 100% based on the total surface area of ​​the lithium-over-manganese-rich oxide particles. When the area of ​​the coating layer formed satisfies the above range, the effect of improving lifespan characteristics is excellent.

[0058] Meanwhile, in the positive active material of the above embodiment, the first and second positive active material particles may be represented by the same or different chemical formulas within the range of the above-described chemical formula 1 or 2.

[0059] Additionally, the first positive active material particle may have an average particle size (D50) of, for example, 7 to 20 μm, or 8 to 15 μm, or 8.5 to 11 μm, and the second positive active material particle may have an average particle size (D50) smaller than that, for example, 1 to 6 μm, or 2 to 5 μm, or 2.5 to 4.5 μm.

[0060] As a result, stress applied during processes such as rolling can be more effectively relieved, further reducing breakage of the positive active material particles, and it becomes possible to provide a positive electrode with higher density and lower porosity.

[0061] In addition, in a specific embodiment of the invention, the first positive active material particle : the second positive active material particle may be included in the positive active material in a weight ratio of 50:50 to 95:5, or 55:45 to 95:5, or 60:40 to 90:10, or 65:35 to 70:30. By doing so, a positive electrode having a higher density and a lower porosity can be provided while maintaining the electrical and chemical properties characteristic of lithium-rich manganese oxide.

[0062] Meanwhile, if the average particle size (D50) of the first positive active material particle becomes excessively large or the average particle size (D50) of the second positive active material particle becomes excessively small, the electrical, chemical properties or mechanical properties of the positive active material of one embodiment may be degraded. Conversely, if the average particle size (D50) of the first positive active material particle becomes excessively small or the average particle size (D50) of the second positive active material particle becomes excessively large, the bimodal particle size distribution characteristics unique to the positive active material of one embodiment are not properly realized, and thus, particle breakage may occur during the rolling process, etc., or the density of the positive may not be sufficient.

[0063] Meanwhile, the positive active material of the above-described embodiment has an overall BET specific surface area of ​​0.1 m² while containing the first and second positive active material particles described above. 2 / g to 5.0m 2 / g, specifically 0.5m 2 / g to 3.0m 2 / g, more specifically 1.0m 2 / g to 2.0m 2 It can be / g.

[0064] In addition, the BET specific surface area of ​​the first cathode active material particle included in this cathode active material is 1.2 m 2 / g or more, preferably 1.3m 2 It may be greater than / g, which is desirable in that it enables the realization of high capacity and efficiency. However, considering electrode processability, large surface area, and issues such as increased surface side reactions and non-uniformity due to low sphericity, 4.0m 2 / g or less, or 3.0m 2 / g or less, or 2.0m 2 It may be / g or less. In addition, the BET specific surface area of ​​the second positive active material particle may be smaller than that of the first positive active material particle, specifically, 0.3m 2 / g to 1.7m 2 / g, or 1.0m2 / g to 1.5m 2 It can be / g.

[0065] The cathode active material of one embodiment has a bimodal particle size distribution including first and second cathode active materials, thereby securing a reaction area with the electrolyte and ensuring excellent capacity and rate characteristics. As a result, it can have the aforementioned low specific surface area, and particle breakage caused by rolling can be further reduced. In addition, as each specific surface area is optimized within the aforementioned range, the solid content of the slurry for forming the cathode active material layer can be optimized, and the amount of gas generated can be further reduced. However, if the aforementioned BET specific surface area is too low, it is difficult to achieve sufficient capacity due to insufficient reaction area with the electrolyte, and if the specific surface area is too high, moisture absorption is rapid and side reactions with the electrolyte are accelerated, making it difficult to secure lifespan characteristics.

[0066] The positive active material of the above-described embodiment, specifically the first and second positive active material particles, can be manufactured according to a general method for manufacturing lithium-rich manganese oxide. For example, each of the first and second positive active material particles can be manufactured by mixing a transition metal precursor containing manganese with a lithium raw material and then calcining it. Since the types of each precursor and raw material and the manufacturing conditions can follow the general manufacturing conditions for manganese-rich positive active materials, further explanation regarding this is omitted.

[0067] However, in this manufacturing process, the type or particle size of the transition metal precursor or the calcination temperature can be controlled to produce first and second positive active material particles that satisfy the average particle size (D50) described above, and the positive active material of one embodiment can be obtained by mixing them in a certain ratio. However, since the process conditions for manufacturing the positive active material to obtain a certain average particle size (D50) are obvious to those skilled in the art, further explanation regarding this is omitted.

[0068] Meanwhile, the positive active material of the above-described embodiment is mixed with a binder, a conductive material, a solvent, etc., to form a positive slurry, and this positive slurry is coated onto a positive current collector, dried, and rolled to produce a positive electrode. The positive slurry is described below.

[0069] First, the positive active material of the above-described embodiment may be included in an amount of 90% to 99% by weight, or 95% to 99% by weight, or 97% to 98% by weight, based on the total weight of the solids (e.g., positive active material, conductive material, and binder) in the positive slurry. If the content of the positive active material in the solids is 90% by weight or less, the energy density may be lowered, and the capacity may be reduced.

[0070] Additionally, the binder may be one or more selected from the group consisting of 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, and various copolymers thereof, and preferably may be polyvinylidene fluoride (PVDF).

[0071] The binder may be included in an amount of 0.5% to 2.5% by weight, or 1% to 2% by weight, or 1.5% to 2% by weight, based on the total weight of the solid content in the anode slurry. When the content of the binder is within the above range, sufficient adhesion to the current collector and inter-particle bonding are secured, thereby improving the durability of the anode while maintaining a low initial resistance.

[0072] The conductive material may be one or more selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; carbon-based materials such as carbon fibers or carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives, and preferably carbon nanotubes or carbon black, most preferably carbon nanotubes.

[0073] The conductive material may be included in an amount of 0.1% to 2.5% by weight, or 0.3% to 2% by weight, or 0.5% to 1% by weight, based on the total weight of the solid content in the anode slurry. It is preferable that the content of the conductive material be within the above range in that it can reduce the dead volume while maintaining conductivity between active materials.

[0074] In addition, the anode slurry may optionally further include a dispersant, and the dispersant may be hydrogenated nitrile butadiene rubber (HNBR).

[0075] Meanwhile, the solvent of the anode slurry may be a solvent commonly used in the relevant technical field, for example, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, dimethyl formamide (DMF), acetone, water, or a mixture of two or more of these may be used. The solvent may be used in an amount adjusted to achieve the viscosity of the anode slurry described above.

[0076] Meanwhile, according to another embodiment of the invention, a positive electrode comprising the positive electrode active material described above is provided. Such a positive electrode may be manufactured from the positive electrode slurry and may include, for example, a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and comprising the positive electrode active material of one embodiment. In this case, the positive electrode active material layer may further include a binder and a conductive material by being formed by coating, drying, and rolling, etc., of the slurry described above.

[0077] The above positive current collector may include a highly conductive metal, and is not particularly limited as long as it facilitates the adhesion of the positive active material layer and is non-reactive within the voltage range of the battery. The above positive current collector may be, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. Additionally, the above positive current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. It may be used in various forms, such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0078] The above-described anode may be manufactured according to a conventional anode manufacturing method, except for using the anode active material of the above-described embodiment. Specifically, it may be manufactured by applying the anode slurry onto an anode current collector and then drying and rolling it, or by casting the anode slurry onto a separate support and then laminating the film obtained by peeling it off from the support onto an anode current collector.

[0079] At this time, the coating and drying process of the anode slurry may be carried out in a general manner, taking into account the content of the solvent and solids contained therein, and the rolling process may be carried out, for example, by a tandem rolling method, by rolling continuously two or more times, or two to four times, or two to three times. At this time, during the first rolling, the process may proceed to 60% to 90% or 70% to 90% of the target thickness reduction, and during the second rolling, the process may proceed to 70% to 100% or 80% to 100% of the target thickness reduction. In addition, if rolling is carried out three or more times, the process may be carried out so that 100% of the target thickness reduction is achieved during the final rolling.

[0080] In a more specific example, to prevent electrode breakage or the like from occurring during the above rolling, the applied pressure during rolling can be 1.0 ton / cm to 2.0 ton / cm, and the process can be carried out by applying a uniform pressure overall.

[0081] Meanwhile, according to another embodiment of the invention, a lithium secondary battery comprising a positive electrode of the other embodiment described above is provided. Such a lithium secondary battery may include, for example, the positive electrode described above, a negative electrode facing the positive electrode, a separator or an electrolyte layer interposed between the positive electrode and the negative electrode, and optionally an electrolyte. In this case, since the positive electrode is the same as previously described, a detailed description is omitted, and only the remaining components are described in detail below.

[0082] In the above lithium secondary battery, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0083] The above-mentioned negative 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., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector may typically have a thickness of 3 μm to 500 μm, and, similar to the positive current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0084] The above-mentioned cathode active material layer optionally includes a binder and a conductive material together with the cathode active material.

[0085] As the above-mentioned negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; and metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys.

[0087] Examples include metal oxides capable of doping and dedoping lithium, such as SiOβ (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the metal compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more of these may be used. Additionally, 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 80% to 99% by weight based on the total weight of the negative electrode active material layer.

[0088] The above binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1% to 10% by weight based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0089] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. Such conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives may be used.

[0090] The above-mentioned negative electrode active material layer may be manufactured by applying a negative electrode slurry, prepared by dissolving or dispersing a negative electrode active material and optionally a binder and a conductive material in a solvent, onto a negative electrode current collector and drying it, or by casting the negative electrode slurry onto a separate support and then laminating the film obtained by peeling it off from the support onto a negative electrode current collector.

[0091] Meanwhile, in the above-mentioned lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. Any separator typically used in lithium secondary batteries can be used without special limitations, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and it may optionally be used in a single-layer or multi-layer structure.

[0092] Meanwhile, the lithium secondary battery may include the separator, but may also include an electrolyte layer separately from the separator, or include a laminate in which an electrolyte layer is laminated on the separator. In addition, in a specific example, the electrolyte layer may be a gel electrolyte layer containing a gel electrolyte or a solid electrolyte layer.

[0093] In a more specific example, the electrolyte layer including the gel electrolyte comprises, for example, a matrix including a polyurethane-based or polyacrylic-based crosslinked polymer, a lithium salt, and a non-aqueous organic solvent, and may have a form in which the lithium salt and the non-aqueous organic solvent are dispersed or encapsulated within the matrix. However, since the types of crosslinked polymers, lithium salts, and organic solvents that may be included in the gel electrolyte are obvious to those skilled in the art, further explanation regarding this is omitted.

[0094] In addition, in another specific example, the solid electrolyte layer may include one or more selected from the group consisting of any solid electrolyte, for example, polymer-based solid electrolytes, oxide-based solid electrolytes, sulfide-based solid electrolytes, and halogenated solid electrolytes. However, since the composition of such solid electrolyte layer may follow that of a general solid electrolyte layer known previously, further explanation is omitted.

[0095] Meanwhile, the above-described lithium secondary battery may further include an electrolyte comprising a lithium salt and a non-aqueous organic solvent.

[0096] The above-mentioned non-aqueous organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above-mentioned organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, 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 low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0097] The above lithium salt may be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, as the anion of the above lithium salt, 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 - It may be at least one selected from the group consisting of, and the lithium salt is, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2, etc., may be used. It is preferable to use the lithium salt within the range of 0.1 to 4.0 M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0098] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the above additives may be included in an amount of 0.1 to 5 weight% based on the total weight of the electrolyte.

[0099] Since the aforementioned lithium secondary battery stably exhibits excellent discharge capacity, output characteristics, and lifespan characteristics, it is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0100] In addition, there are no special restrictions on the external shape of the above lithium secondary battery, but it may be a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

[0101] In addition, the above-described lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but can also preferably be used as a unit cell in a medium-to-large battery module containing a plurality of battery cells.

[0102] Examples of the above-mentioned medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, but are not limited to these.

[0104] The present invention will be explained in more detail below through specific embodiments.

[0106] <Example - Preparation of Cathode Material>

[0107] The positive active materials A to D used in the following examples and comparative examples each have the characteristics shown in Table 1 below.

[0108] Cho Seong-sik D50 (㎛) Particle form BET specific surface area (m 2 / g) Positive active material A Li 1.36 [Mr 0.65 Ni 0.35 ]O2 9.46 secondary particles 1.56 Positive active material B Li 1.36 [Mr 0.65 Ni 0.35 ]O2 9.50 secondary particles 1.53 Positive active material C Li 1.36 [Mr 0.65 Ni 0.35 ]O2 3.77 secondary particles 1.28 Positive active material D Li 1.36 [Mr 0.65 Ni 0.35 ]O2 3.76 secondary particles 1.39

[0109] Example 1.

[0110] A cathode material was prepared by mixing the above-mentioned cathode active material A and cathode active material C in a weight ratio of 65:35. The total BET specific surface area of ​​this cathode material is approximately 1.46 m². 2 It was confirmed as / g.

[0112] Example 2.

[0113] A cathode material was prepared by mixing the above-mentioned cathode active material B and cathode active material D in a weight ratio of 50:50. The total BET specific surface area of ​​this cathode material is approximately 1.46 m². 2 It was confirmed as / g.

[0115] Comparative Example 1.

[0116] As the cathode material, only the above-mentioned cathode active material A was used alone.

[0118] Comparative Example 2.

[0119] A cathode material was prepared by mixing the above-mentioned cathode active material A and cathode active material B in a weight ratio of 65:35. The total BET specific surface area of ​​this cathode material is approximately 1.55 m². 2 It was confirmed as / g.

[0121] Manufacturing Example: Manufacturing of a positive electrode and a lithium secondary battery

[0122] An anode slurry was prepared by mixing the anode material of the example or comparative example, carbon nanotube, PVDF binder, and HNBR-containing dispersant in N-methylpyrrolidone in a weight ratio of 97.46:0.62:1.7:0.22. The anode slurry was applied onto an aluminum current collector sheet, dried, and then rolled at a pressure of about 1.5 ton / cm to produce an anode.

[0123] A cathode slurry was prepared by mixing graphite cathode active material, single-walled carbon nanotube, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) in water in a weight ratio of 96.2:0.8:2:1. The cathode slurry was applied onto a copper current collector sheet, dried, and then rolled to produce a cathode.

[0124] Meanwhile, the above anode and cathode were manufactured such that the ratio of the cathode discharge capacity to the anode discharge capacity (N / P ratio) was 115% by adjusting the loading amount.

[0125] An electrode assembly was manufactured by interposing a polyethylene separator between the anode and cathode manufactured as described above, and after inserting the electrode assembly into a battery case, an electrolyte was injected, and an activation process was performed by charging at 45°C with a constant current of 0.1C until the voltage reached 4.6V, and then discharging at a constant current of 0.1C until the voltage reached 2.0V to manufacture a lithium secondary battery.

[0127] Experimental Example 1: Analysis of Volume Cumulative Particle Size Distribution and Average Particle Size (D50) of Anode Active Material

[0128] First, regarding the powder of the cathode material included in the example or comparative example, a Particle Size Distribution (PSD) was obtained using Microtrac's S-3500. This PDS curve is shown in FIG. 1. From this PDS curve, it was confirmed that the cathode materials of Examples 1 and 2 have a bimodal particle size distribution. In addition, the average particle size (50) of each cathode active material included in the cathode material was analyzed from the PDS curve, and the analysis results were as described in Table 1 above.

[0129] In addition, the PSD of the entire cathode material contained in each cathode slurry prepared in the manufacturing example was analyzed in the same way and derived as pre-rolling data. From this, the volume ratio of particles having a particle size of 1 μm or less and the volume ratio of coarse particles (e.g., cathode active material A or B) among the cathode materials contained in the cathode slurry were evaluated, respectively.

[0131] Experimental Example 2: Evaluation of Volume Cumulative Particle Size Distribution and Particle Breakage Rate of Anode

[0132] The anode prepared in the preparation example using the anode material of the example or comparative example was heat-treated in a furnace at a temperature of 700°C for 10 hours to collect the anode material, and then finely ground using a mortar and pestle, and classified using a 250 mesh sieve to obtain the anode material powder contained in each anode. For the obtained powder, the Particle Size Distribution (PSD) was obtained using a Microtrac S-3500.

[0133] From this, the volume ratio of particles having a particle size of 1 μm or less and the volume ratio of coarse particles (e.g., positive active material A or B) among the positive materials contained in the above positive electrode were evaluated, respectively. The volume ratio of particles having a particle size of 1 μm or less and the volume ratio of coarse particles contained in the positive electrode after rolling are shown in Table 2 below.

[0134] In addition, compared with the data obtained in Experimental Example 1, the extent to which the volume ratio of particles with a particle size of 1 μm or less and the volume ratio of coarse particles changed in the anode manufactured through rolling was evaluated and is shown together in Table 2 below.

[0136] division Example 1 Example 2 Comparative Example 2 Evaluation of particle breakage rate Volume ratio of particles smaller than 1㎛ (fine powder) (Anode after rolling; %) 8.4 13.0 7.2 Change rate (%) in differential volume ratio before / after rolling 0 4.6% increase 1.2% decrease Opto-alphabetic volume ratio (anode after rolling; %) 18.6 23.9 28.5 Change rate (%) in the volume ratio of opposing particles before / after rolling 0 5.3% decrease 9.9% decrease

[0137] Referring to Table 2 above, it was confirmed that when using the cathode material of Example 1, there is no substantial change in the volume ratio of particles with a particle size of 1 μm or less and the volume ratio of large particles even when the cathode is manufactured through rolling. From this, it can be confirmed that particle breakage can be suppressed during the cathode manufacturing process through rolling.

[0138] It was confirmed that when using the cathode material of Example 2, particle breakage occurs more than in Example 1, but compared to Comparative Example 2 containing only the co-positive cathode active material, the change in the volume ratio of co-positive materials before and after rolling is small, so particle breakage can be relatively suppressed.

[0140] Experimental Example 3: Evaluation of Gas Generation Amount

[0141] For the lithium secondary battery prepared in the preparation example using the cathode material of the above example or comparative example, gas generated during an additional 100 charge-discharge cycles (charge-discharge cycles performed under conditions of 2.0 to 4.6 V and 45°C) was collected, and the amount of gas generated was quantitatively analyzed using GC-FID / TCD. The results of the analysis of the amount of active gas generated are shown in Table 3 below.

[0142] Gas generation amount (μl) total H2 CO CO2 CH4 C2H4 C2H6 Example 1 474 67 104 69 15 212 7 Example 2 476 64 102 103 15 185 7 Comparative Example 1 499 64 117 130 14 168 6

[0143] Referring to Table 3 above, it was confirmed that the lithium secondary battery manufactured using the cathode materials of Examples 1 and 2 showed a reduced amount of gas generation compared to Comparative Example 1.

[0145] Experimental Example 4: Evaluation of Life Characteristics

[0146] For the lithium secondary battery prepared in the preparation example using the cathode material of the above example or comparative example, a charge and discharge test of 100 cycles was conducted at 25°C and a voltage of 2.5 to 4.35V. While performing these charge and discharge cycles for 100 cycles, the energy retention rate and capacity retention rate were evaluated and are shown in Table 4 below.

[0147] @100cycles Example 1 Example 2 Comparative Example 3 Average voltage (V) 3.482 3.489 3.488 Energy Retention Rate (%) 90.18 91.36 89.71 Capacity retention rate (%) 92.06 93.46 9.143

[0148] Referring to Table 4 above, it was confirmed that the lithium secondary battery containing the cathode material of the example exhibits a higher energy retention rate and capacity retention rate after 100 charge and discharge cycles compared to the comparative example, and shows improved lifespan characteristics.

Claims

Claim 1 A positive active material comprising a first positive active material particle and a second positive active material particle having different average particle sizes (D50), represented by the following chemical formula 1, wherein the positive active material has a bimodal particle size distribution when analyzed for cumulative volume particle size distribution, and the positive active material comprising the first and second positive active material particles has a total of 0.1 m 2 / g to 3.0m 2 Cathode active material for lithium secondary batteries having a BET specific surface area of ​​ / g: [Chemical Formula 1]Li a [Mn b Ni c M d ] 2-a O2 In the above chemical formula 1, a is greater than 1, b is 0.5 or more and less than 1, and c and d are each 0 or more and 0.5 or less, wherein 0 <c+d≤0.5이고, M은 Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr 및 Zr로 이루어진 군에서 선택된 하나 이상이다. Claim 2 In claim 1, the first and second positive active material particles are positive active materials for a lithium secondary battery, wherein the molar ratio of lithium to the total number of moles of metal excluding lithium is 1.3 to 1.

5. Claim 3 A positive electrode active material for a lithium secondary battery according to claim 1, wherein the molar ratio of nickel to manganese contained in the first and second positive electrode active material particles is 25:75 to 50:

50. Claim 4 In claim 1, the first and second positive active material particles are positive active materials for a lithium secondary battery comprising a compound represented by the following chemical formula 1a: [Chemical Formula 1a]Li a [Mn b Ni c M d ] 2-a O2 In the above chemical formula 1, 1.1 <a<1.3이고, 0.5≤b≤0.9, 0.1≤c≤0.5이고, 0≤d≤0.1이고, 이고, M은 Co, Cr, V, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr 및 Zr로 이루어진 군에서 선택된 하나 이상이다. Claim 5 In claim 1, the first and second positive active material particles comprise a positive active material for a lithium secondary battery comprising a rock salt structure compound and a layered structure compound in a mixed state. Claim 6 In claim 5, the first and second positive active material particles are positive active materials for a lithium secondary battery represented by the following chemical formula 2: [Chemical Formula 2]X*Li2MnO3·(1-X)*Li[NiwMnyMz]O2, wherein M is one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and 0.2≤X≤0.5, 0≤w≤0.5, 0.4≤y<1, 0≤z≤0.2, and 0 <w+z≤0.5임. Claim 7 A positive active material for a lithium secondary battery according to claim 1, wherein the first positive active material particle has an average particle size (D50) of 7 to 20 μm, and the second positive active material particle has an average particle size (D50) smaller than that of the first positive active material particle. Claim 8 In claim 7, the second positive active material particle is a positive active material for a lithium secondary battery having an average particle size (D50) of 1 to 6 μm. Claim 9 In claim 1, the first and second positive active material particles each have a secondary particle form in which a plurality of primary particles are aggregated, for a positive active material for a lithium secondary battery. Claim 10 A positive active material for a lithium secondary battery according to claim 1, wherein the first positive active material particle : the second positive active material particle is included in a weight ratio of 50 : 40 to 95 :

5. Claim 11 In claim 1, the first positive electrode active material particle is 1.2 m 2 / g to 3.0m 2 A positive electrode active material for a lithium secondary battery having a BET specific surface area of ​​ / g. Claim 12 In claim 11, the second positive active material particle is a positive active material for a lithium secondary battery having a smaller BET specific surface area than the first positive active material particle. Claim 13 In claim 1, the second positive active material particle is 0.3m 2 / g to 1.7m 2 A positive electrode active material for a lithium secondary battery having a BET specific surface area of ​​ / g. Claim 14 delete Claim 15 A positive electrode for a lithium secondary battery comprising: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and comprising a positive electrode active material according to any one of claims 1 to 13. Claim 16 In claim 15, the positive active material layer further comprises a binder and a conductive material, a positive electrode for a lithium secondary battery. Claim 17 A lithium secondary battery comprising: a positive electrode according to claim 15; a negative electrode facing the positive electrode; and a separator or electrolyte layer interposed between the positive electrode and the negative electrode. Claim 18 A lithium secondary battery according to claim 17, further comprising an electrolyte including a lithium salt and a non-aqueous organic solvent.

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