KR102838736B1 - Positive electrode for secondary battery, method for preparing the same, and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a cathode for a secondary battery, comprising: a cathode current collector; a first cathode active material layer formed on the cathode current collector; and a second cathode active material layer formed on the first cathode active material layer, wherein the first cathode active material layer includes a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn), and the second cathode active material layer includes a second lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), and having a lithium excess of a molar ratio of lithium (Li) to total metals (M) excluding lithium (Li/M) of 1.1 or more.

Description

POSITIVE ELECTRODE FOR SECONDARY BATTERY, METHOD FOR PREPARING THE SAME, AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

The present invention relates to a positive electrode for a secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

Recently, with the rapid spread of electronic devices that use batteries, such as mobile phones, laptop computers, and electric vehicles, the demand for small, lightweight, and relatively high-capacity secondary batteries is rapidly increasing. In particular, lithium secondary batteries are attracting attention as a power source for portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts are actively being conducted to improve the performance of lithium secondary batteries.

A lithium secondary battery refers to a battery that includes an electrode assembly including a positive electrode active material capable of insertion/extraction of lithium ions, a negative electrode active material capable of insertion/extraction of lithium ions, and an electrolyte containing lithium ions, with a microporous separator interposed between the positive electrode and the negative electrode.

Lithium transition metal oxide is used as the positive electrode active material of a lithium secondary battery, and lithium metal, lithium alloy, crystalline or amorphous carbon, or carbon composite is used as the negative electrode active material. The above active material is applied to an electrode current collector in an appropriate thickness and length, or the active material itself is applied in a film shape and wound or laminated together with a separator, which is an insulator, to create an electrode group, which is then placed in a can or similar container, and an electrolyte is injected to manufacture a secondary battery.

The cathode active materials of lithium secondary batteries include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMnO ₂ or LiMn ₂ O _{4 ).} etc.), lithium iron phosphate compounds (LiFePO ₄ ), etc. were used. Among these, lithium cobalt oxide (LiCoO ₂ ) has the advantages of high operating voltage and excellent capacity characteristics, and is therefore widely used. However, due to the rising price of cobalt (Co) and unstable supply, there are limitations to its mass use as a power source in fields such as electric vehicles, and thus the need for the development of a cathode active material that can replace it has arisen. Accordingly, nickel-cobalt-manganese lithium composite transition metal oxides (hereinafter simply referred to as 'NCM-based lithium composite transition metal oxides') have been developed, in which some of the cobalt (Co) is replaced with nickel (Ni) and manganese (Mn).

However, the conventionally developed NCM-based lithium composite transition metal oxides had limitations in their application to high-voltage batteries because they did not have stability in the high-voltage range, and there were problems in that stability and thermal stability were not sufficiently secured when a metal body such as a nail penetrates the electrode from the outside.

Korean Patent Publication No. 2018-0118913

The present invention aims to provide a secondary battery cathode that implements high capacity while ensuring stability in a high-voltage range, and has improved safety and thermal stability when a metal body such as a nail penetrates the electrode from the outside.

The present invention provides a secondary battery cathode, comprising: a cathode current collector; a first cathode active material layer formed on the cathode current collector; and a second cathode active material layer formed on the first cathode active material layer, wherein the first cathode active material layer includes a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn), and the second cathode active material layer includes a second lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn), and having a lithium excess of a molar ratio of lithium (Li) to total metals (M) excluding lithium (Li/M) of 1.1 or more.

In addition, the present invention provides a method for manufacturing a positive electrode for a secondary battery, including the steps of forming a first positive electrode active material layer by coating a composition for forming a first positive electrode active material layer including a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn) on a positive electrode current collector; and the step of forming a second positive electrode active material layer by coating a composition for forming a second positive electrode active material layer including a second lithium composite transition metal oxide including nickel (Ni), cobalt (Co), and manganese (Mn) and having a lithium excess in which a molar ratio of lithium (Li) to total metals (M) excluding lithium (Li/M) is 1.1 or more on the first positive electrode active material layer.

In addition, the present invention provides a lithium secondary battery including the positive electrode.

The cathode for a secondary battery according to the present invention can realize high capacity while ensuring stability in a high voltage range, and can improve safety and thermal stability when a metal body such as a nail penetrates the electrode from the outside.

Figure 1 is a graph evaluating the thermal stability of the anodes manufactured in Example 1 and Comparative Examples 1 to 2.
Figures 2 to 4 are graphs showing penetration test evaluations of the anodes manufactured in Example 1 and Comparative Examples 1 and 2.

Hereinafter, the present invention will be described in more detail to help understand the present invention. At this time, the terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, and should be interpreted as meanings and concepts that conform to the technical idea of the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his own invention in the best way.

<Polar pole and its manufacturing method>

The cathode for a secondary battery of the present invention comprises: a cathode current collector; a first cathode active material layer formed on the cathode current collector; and a second cathode active material layer formed on the first cathode active material layer, wherein the first cathode active material layer comprises a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn), and the second cathode active material layer comprises a second lithium composite transition metal oxide having a lithium excess amount including nickel (Ni), cobalt (Co), and manganese (Mn), and having a molar ratio of lithium (Li) to total metals (M) excluding lithium (Li/M) of 1.1 or more.

The positive electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the positive electrode current collector may typically have a thickness of 3 to 500 ㎛, and fine unevenness may be formed on the surface of the positive electrode current collector to increase the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

The first cathode active material layer includes a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn). The first lithium composite transition metal oxide may have 70 mol% or less of nickel (Ni) among the total metal (M) excluding lithium, more preferably 60 mol% or less of nickel (Ni) among the total metal (M) excluding lithium, and even more preferably 50 mol% or less. The first lithium composite transition metal oxide may be represented by the following chemical formula 1, and as a specific example, may be LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

[Chemical Formula 1]

Li _p1 Ni _1-(x1+y1+z1) Co _x1 Mn _y1 M ^a _z1 O ₂

In the above formula, M ^a is at least one selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, Ti, Sr, Ba, F, P, S, and La, and 0.9≤p1<1.1, 0<x1≤0.5, 0<y1≤0.5, 0≤z1≤0.1, 0.3≤x1+y1+z1<1.

In the lithium composite transition metal oxide of the above chemical formula 1, Li may be included in a content corresponding to p1, that is, 0.9≤p1<1.1. If p1 is less than 0.9, there is a concern that the capacity may be reduced, and if p1 is 1.1 or more, the particles may be sintered during the sintering process, making it difficult to manufacture the positive electrode active material. Considering the balance between the remarkable effect of improving the capacity characteristics of the positive electrode active material according to the Li content control and the sinterability during the manufacture of the active material, the Li may be more preferably included in a content of 1.0≤p≤1.05.

In the lithium composite transition metal oxide of the above chemical formula 1, Ni may be included in an amount corresponding to 1-(x1+y1+z1), for example, 0<1-(x1+y1+z1)<0.7. More preferably, Ni may be included in an amount corresponding to 0<1-(x1+y1+z1)≤0.6.

In the lithium composite transition metal oxide of the above chemical formula 1, Co may be included in a content corresponding to x1, that is, 0<x1≤0.5. If the content of Co in the lithium composite transition metal oxide of the above chemical formula 1 exceeds 0.5, there is a concern of increased cost. Considering the remarkable effect of improving capacity characteristics due to the inclusion of Co, the Co may be included in a content of 0.05≤x1≤0.3, more specifically.

In the lithium composite transition metal oxide of the above chemical formula 1, Mn may be included in a content corresponding to y1, that is, 0<y1≤0.5. This metal element can improve the stability of the active material, and as a result, the stability of the battery. If y1 in the lithium composite transition metal oxide of the above chemical formula 1 exceeds 0.5, there is a concern that the output characteristics and capacity characteristics of the battery may be deteriorated, and more specifically, the Mn may be included in a content of 0.05≤y1≤0.3.

In the lithium composite transition metal oxide of the above chemical formula 1, M ^a may be a doping element included in the crystal structure of the lithium composite transition metal oxide, and M ^a may be included in a content corresponding to z1, that is, 0≤z1≤0.1.

The second cathode active material layer includes a second lithium composite transition metal oxide having a lithium excess amount, which includes nickel (Ni), cobalt (Co), and manganese (Mn), and a molar ratio of lithium (Li) to the total metal (M) excluding lithium (Li/M) of 1.1 or more. The second lithium composite transition metal oxide may more preferably have a molar ratio of lithium (Li) to the total metal (M) excluding lithium (Li/M) of 1.3 to 1.5, and even more preferably 1.3 to 1.4. For example, the second lithium composite transition metal oxide may be, but is not limited to, Li _1.13 Ni _0.2 Co _0.2 Mn _0.46 O _2, Li _1.17 Ni _0.17 Co _0.17 Mn _0.49 O _2, Li _1.13 Ni _0.21 Co _0.21 Mn _0.45 O ₂ , etc.

The present invention secures excellent thermal stability even in a high voltage range by forming a second positive active material layer including a second lithium composite transition metal oxide having a lithium excess amount where Li/M satisfies the above range on a first positive active material layer including a first lithium composite transition metal oxide of an NCM system having a layered structure, and the lithium excess NCM-based second lithium composite transition metal oxide on the upper side has slow conductivity and can suppress overcurrent even when a metal body such as a nail penetrates the electrode from the outside, which can be advantageous for ensuring stability.

The thickness of the first positive electrode active material layer after rolling may be 30 to 70 μm, more preferably 50 to 60 μm. The thickness of the second positive electrode active material layer after rolling may be 5 to 20 μm, more preferably 10 to 20 μm.

Additionally, a mixed layer comprising the first lithium composite transition metal oxide and the second lithium composite transition metal oxide may be further formed between the first cathode active material layer and the second cathode active material layer.

In addition, the first and second positive electrode active material layers may include a conductive material and a binder, respectively, together with the first and second positive electrode active materials described above.

At this time, the conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used. The conductive material may typically be included in an amount of 1 to 30 wt% with respect to the total weight of the positive electrode active material layer.

In addition, the binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the positive electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), 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. The binder may be included in an amount of 1 to 30 wt% with respect to the total weight of the positive electrode active material layer.

The positive electrode for a secondary battery of the present invention is manufactured by including the steps of: forming a first positive electrode active material layer by coating a composition for forming a first positive electrode active material layer, which includes a first lithium composite transition metal oxide having a layered structure, which includes nickel (Ni), cobalt (Co), and manganese (Mn), on a positive electrode current collector; and forming a second positive electrode active material layer by coating a composition for forming a second positive electrode active material layer, which includes a second lithium composite transition metal oxide, which includes nickel (Ni), cobalt (Co), and manganese (Mn), and has a lithium excess, wherein the molar ratio of lithium (Li) to total metals (M) excluding lithium (Li/M) is 1.1 or more, on the first positive electrode active material layer.

The first positive electrode active material layer can be manufactured by applying a composition for forming a first positive electrode active material layer, which includes the first positive electrode active material and optionally a binder and a conductive material, onto the positive electrode current collector, followed by drying and rolling. In addition, the second positive electrode active material layer forming composition, which includes the second positive electrode active material and optionally a binder and a conductive material, can be manufactured by applying the composition for forming a second positive electrode active material layer, which includes the second positive electrode active material and optionally a binder and a conductive material, onto the first positive electrode active material layer, followed by drying and rolling.

At this time, a second positive electrode active material layer forming composition may be coated before the first positive electrode active material layer forming composition is completely dried, thereby forming a mixed layer in which the first lithium composite transition metal oxide and the second lithium composite transition metal oxide are mixed between the first positive electrode active material layer and the second positive electrode active material layer.

The types and contents of the first and second positive electrode active materials, binder, and conductive agent are as described above.

<Lithium secondary battery>

According to another embodiment of the present invention, an electrochemical device including the positive electrode is provided. The electrochemical device may be specifically a battery or a capacitor, and more specifically, a lithium secondary battery.

The lithium secondary battery specifically includes an electrode assembly including a positive electrode, an anode positioned opposite to the positive electrode, and a separator interposed between the positive electrode and the negative electrode; a battery case housing the electrode assembly; and an electrolyte injected into the battery case; wherein the positive electrode is the same as the positive electrode pre-lithiated according to the present invention described above. In addition, the lithium secondary battery may further optionally include a battery case housing the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery case.

In the above 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.

The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change 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., an aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 to 500 ㎛, and, like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric.

The above negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material. The above negative electrode active material layer may be manufactured by, for example, applying a negative electrode forming composition including the negative electrode active material, and optionally a binder and a conductive material onto a negative electrode current collector and drying the same, or by casting the negative electrode forming composition onto a separate support and then laminating the resulting film onto a negative electrode current collector by peeling the resulting film from the support.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples thereof include: 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, and Al alloy; metallic oxides capable of doping and dedoping lithium, such as SiOα (0 < α < 2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used. In addition, a metallic lithium thin film can be used as the negative electrode active material. In addition, the carbon material can be both low-crystalline carbon and high-crystalline carbon. Representative examples of low-crystallization carbon include soft carbon and hard carbon, and representative examples of high-crystallization carbon include amorphous, plate-like, flaky, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase pitches, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

Additionally, the binder and the conductive material may be the same as those described above for the anode.

Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without special restrictions, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, 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, and an ethylene/methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. can also be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can optionally be used in a single-layer or multi-layer structure.

In addition, examples of the electrolyte used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, 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; Examples of solvents that can be used include 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 (wherein R represents a C2 to C20 linear, branched, or cyclic hydrocarbon group, which may include a double-bonded aromatic ring or an ether bond); amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane; and sulfolanes. Among these, a carbonate solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant capable of improving the charge/discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferable. In this case, the cyclic carbonate and the chain carbonate can be mixed and used in a volume ratio of about 1:1 to about 1:9 to exhibit excellent electrolyte performance.

The above lithium salt can be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ . LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ . It is preferable to use the concentration of the lithium salt within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance, and lithium ions can move effectively.

In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as, for example, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, 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. In this case, the additives may be contained in an amount of 0.1 to 5 wt% with respect to the total weight of the electrolyte.

As described above, a lithium secondary battery including a cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention, and is therefore useful in portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The above battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

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

Example 1

As a first cathode active material, LiNi _{0 . 5} Co _{0 . 2} Mn _{0 . 3} O _2, a carbon black conductive agent, and a PVdF binder were mixed in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent to prepare a composition for forming a first cathode active material layer, and as a second cathode active material, Li _{1 . 13} Ni _{0 . 2} Co _{0 . 2} Mn _{0 . 46} O ₂ (Li/M = 1.31), A composition for forming a second positive electrode active material layer was prepared by mixing carbon black conductive agent and PVdF binder in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent.

The first positive electrode active material layer forming composition manufactured as described above was applied to one side of an aluminum current collector, and the second positive electrode active material layer forming composition manufactured as described above was applied thereon, and then dried at 130° C. and rolled to manufacture a positive electrode. At this time, the second positive electrode active material layer forming composition was applied before the first positive electrode active material layer forming composition was completely dried so that the distinction between the two layers was not clear, and the positive electrode was manufactured so that the thickness of the first positive electrode active material layer was 60 μm and the thickness of the second positive electrode active material layer was 10 μm.

Comparative Example 1

LiNi _0.5 Co _0.2 Mn _0.3 O _{2 as the first cathode active material, and} Li _1.13 Ni _0.2 Co _0.2 Mn _0.46 O ₂ as the second cathode active material. With a weight ratio of 90:10 Mix, A composition for forming a positive electrode active material layer was prepared by mixing carbon black conductive material and PVdF binder in an N-methylpyrrolidone solvent at a weight ratio of 96:2:2 (total positive electrode active material: conductive material: binder).

After applying the composition for forming the positive electrode active material layer manufactured above to one side of an aluminum current collector, the composition was dried at 130°C and then rolled to manufacture a positive electrode.

Comparative Example 2

A composition for forming a first positive electrode active material layer was prepared by mixing LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as a first positive electrode active material, a carbon black conductive material, and a PVdF binder in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent, and a second positive electrode active material layer was prepared by mixing LiNi _0.5 Co _0.2 Mn _0.3 O ₂ as a second positive electrode active material, a carbon black conductive material, and a PVdF binder in a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent.

The first positive electrode active material layer forming composition manufactured as described above was applied to one side of an aluminum current collector, and the second positive electrode active material layer forming composition manufactured as described above was applied thereon, and then dried at 130° C. and rolled to manufacture a positive electrode. At this time, the second positive electrode active material layer forming composition was applied before the first positive electrode active material layer forming composition was completely dried so that the distinction between the two layers was not clear, and the positive electrode was manufactured so that the thickness of the first positive electrode active material layer was 60 μm and the thickness of the second positive electrode active material layer was 10 μm.

[Experimental Example 1: Electrode Resistance Evaluation]

The electrode resistance of the positive electrodes manufactured in the above Example 1 and Comparative Examples 1 and 2 was measured. Specifically, the total electrode resistance, electrode internal resistance, and electrode surface resistance were measured using an MP tester (KIKUSUI TOS5051) with a rolled cross-section electrode of 5*5 cm ² , and the results are shown in Table 1 below.

Resistance (mΩ cm ² ) Electrode surface resistance Electrode internal resistance Total electrode resistance Example 1 54.47 32.893 87.363 Comparative Example 1 46.068 37.692 83.760 Comparative Example 2 34.143 39.472 73.615

Referring to Table 1 above, it can be seen that the total electrode resistances of Example 1 and Comparative Examples 1 to 2 are at similar levels, but Example 1 is somewhat higher, and in particular, the electrode internal resistance is low while the electrode surface resistance is high. If the electrode surface resistance is low, it is easy to conduct electricity, which increases the possibility of ignition and lowers safety. However, in the case of Example 1, the electrode internal resistance is low while the electrode surface resistance is somewhat high, which can suppress conduction and improve safety.

[Experimental Example 2: Thermal Stability Evaluation]

The thermal stability of the positive electrodes manufactured in the above Example 1 and Comparative Examples 1 to 2 was evaluated. Specifically, each positive electrode was prepared in a charged state with the same weight and measured by TPD-MS (Temperature Programmed Desorption-Mass Spectrometry, EQC-0295). The thermal stability was evaluated by measuring the amount of oxygen generated while increasing the temperature from 50 to 600°C in a vacuum or He atmosphere, and the results are shown in Fig. 1.

Referring to FIG. 1, it can be confirmed that in the case of Example 1, the peak appears at a higher temperature than in Comparative Examples 1 and 2, and in particular, it can be confirmed that the first peak appears at a higher temperature than in Comparative Examples 1 and 2, indicating excellent thermal stability.

[Experimental Example 3: Penetration Test]

The positive electrodes manufactured in Example 1 and Comparative Examples 1 to 2 were used, respectively, and lithium metal was used as the negative electrode. An electrode assembly was manufactured by interposing a porous polyethylene separator between the positive and negative electrodes manufactured as described above, and the electrode assembly was placed inside a case, and an electrolyte was injected into the case to manufacture a lithium secondary battery. At this time, the electrolyte was manufactured by dissolving 1.0 M lithium hexafluorophosphate (LiPF ₆ ) in an organic solvent composed of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate (mixing volume ratio of EC/DMC/EMC = 3/4/3).

As for the lithium secondary battery manufactured as described above, an explosion was evaluated when a metal body with a diameter of 5-8 mm was dropped at a speed of 25±5 mm/sec to penetrate the cell, which is the same as the Chinese GB/T certification conditions. The results are shown in Table 2 below and Fig. 2 (Example 1), Fig. 3 (Comparative Example 1), and Fig. 4 (Comparative Example 2).

Whether it exploded or not Example 1 No explosion, no abnormality Comparative Example 1 There is some delay after penetration, but it explodes. Comparative Example 2 Explosion immediately after penetration

Referring to Table 2 and FIGS. 2 to 4 above, in Example 1, in which a second positive electrode active material layer including a second lithium composite transition metal oxide with an excess of lithium was formed on a first positive electrode active material layer including a first lithium composite transition metal oxide having a layered structure of NCM according to the present invention, no explosion occurred during a penetration test. However, it was confirmed that an explosion occurred in the cases of Comparative Examples 1 and 2.

Claims (8)

Bipolar collector;
A first cathode active material layer formed on a cathode current collector; and
A second positive electrode active material layer formed on the first positive electrode active material layer;
The first cathode active material layer includes a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn),
The second cathode active material layer includes a second lithium composite transition metal oxide having a lithium excess, which includes nickel (Ni), cobalt (Co) and manganese (Mn), and a molar ratio of lithium (Li) to the total metal (M) excluding lithium (Li/M) of 1.1 or more,
A secondary battery cathode, wherein a mixed layer comprising the first lithium composite transition metal oxide and the second lithium composite transition metal oxide is formed between the first cathode active material layer and the second cathode active material layer.
In the first paragraph,
The above first lithium composite transition metal oxide is a positive electrode for a secondary battery, wherein nickel (Ni) is 70 mol% or less among the total metal (M) excluding lithium.
In the first paragraph,
The above second lithium composite transition metal oxide is a positive electrode for a secondary battery having a molar ratio (Li/M) of lithium (Li) to the total metal (M) excluding lithium of 1.3 to 1.5.
In the first paragraph,
A cathode for a secondary battery, wherein the second lithium composite transition metal oxide is Li _{1 . 13} Ni _{0 . 2} Co _{0 . 2} Mn _{0 . 46} O ₂ .
delete A step of forming a first cathode active material layer by coating a composition for forming a first cathode active material layer including a first lithium composite transition metal oxide having a layered structure including nickel (Ni), cobalt (Co), and manganese (Mn) on a cathode current collector; and
A step of forming a second positive electrode active material layer by coating a composition for forming a second positive electrode active material layer, which includes a second lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), and having a molar ratio of lithium (Li) to the total metal (M) excluding lithium (Li/M) of 1.1 or more, on the first positive electrode active material layer; comprising;
A method for manufacturing a positive electrode for a secondary battery, characterized in that the composition for forming the second positive electrode active material layer is coated before the composition for forming the first positive electrode active material layer is completely dried after coating, thereby forming a mixed layer in which the first lithium composite transition metal oxide and the second lithium composite transition metal oxide are mixed between the first positive electrode active material layer and the second positive electrode active material layer.
delete A lithium secondary battery comprising a positive electrode according to claim 1.