Positive electrode for lithium secondary battery and lithium secondary battery including the same

Abstract

A positive electrode for a lithium secondary battery and a lithium secondary battery containing the same are provided. The positive electrode comprises: a current collector; and a positive electrode active material layer, provided with a plurality of positive electrode active materials located on at least one surface of the current collector, and an atomic layer deposition coating located on the surface of the positive electrode active material, within the pores, and in the gaps between the plurality of positive electrode active materials. The thickness of the atomic layer deposition coating is 0.2 nm to 1 nm. If the positive electrode active material layer is divided into five equal parts in the thickness direction, the portion of the positive electrode active material layer in surface contact with the current collector is referred to as the lowermost positive electrode active material layer, and the surface portion of the positive electrode active material layer farthest from the current collector is referred to as the uppermost positive electrode active material layer, then the ratio of the content of the atomic layer deposition coating of the lowermost positive electrode active material layer to the content of the atomic layer deposition coating of the uppermost positive electrode active material layer is at least 40% by weight. In addition, the porosity of the lithium secondary battery is 15% to 35%.

Description

Technical Field

[0001] The present disclosure relates to a positive electrode for a lithium secondary battery and a lithium secondary battery having the same. More particularly, the present disclosure relates to a positive electrode for a lithium secondary battery and a lithium secondary battery having the same, the positive electrode having improved cycle characteristics and storage stability.

[0002] This application claims the benefit of Korean Patent Application No. 10-2019-0064862, filed on May 31, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0003] Recently, there has been a surge in interest in energy storage technology. As its applications have expanded to mobile phones, camcorders, laptop computers, and even electric vehicles, the demand for high energy density in batteries used as power sources for electronic devices has increased. Lithium secondary batteries best meet this demand, and much research is currently underway on them.

[0004] Among currently available secondary batteries, lithium secondary batteries developed in the early 1990s comprise a negative electrode of a carbon material capable of intercalating and deintercalating lithium ions, a positive electrode containing a lithium oxide, and a nonaqueous electrolyte containing an optimal amount of a lithium salt dissolved in a mixed organic solvent.

[0005] In this case, the positive electrode suffers from severe electrolyte decomposition on the surface due to the high oxidation potential, leading to gas generation and consequent cell swelling. In addition, during lithium ion insertion and extraction, the structure becomes unstable and metal deposition on the electrode occurs, which degrades the battery. This phenomenon is particularly severe and rapid in high-energy active materials, reducing the battery's capacity retention and potentially posing a safety threat.

[0006] To improve electrode stability and safety of lithium secondary batteries, surface coating via vapor deposition or sputtering has been proposed, but this coating method has difficulty achieving deep penetration into the electrode to form a coating on the surface inside the pores.

[0007] Atomic layer deposition (ALD) is a technology that uses a gas-phase chemical vapor deposition reaction to inject precursors and reactants in a time-division manner to suppress gas-phase reactions and precisely control film thickness through self-controlled reactions on the substrate surface.

[0008] Coating using atomic layer deposition can be mainly divided into two categories: a method of coating an active material at a particle stage; and a method of coating an active material slurry on an electrode, drying it, and performing coating on the formed electrode.

[0009] When atomic layer deposition is performed on active material particles, the coating formed on the particle surface can act as an insulating layer that restricts electron movement. In contrast, electrode coating does not interrupt existing conductive paths, thereby not hindering electron movement, thereby minimizing the increase in cell resistance.

[0010] However, a general atomic layer deposition process is the most suitable atomic layer deposition process for flat plates, so when a general atomic layer deposition process is applied to the active material layer of an electrode, it is difficult to form an atomic layer deposition layer over the entire active material layer up to the deepest region of the active material layer of the electrode (i.e., the region in contact with the current collector). In most patents or papers, atomic layer deposition is applied to electrodes with relatively high porosity (void ratio), in which case atomic layer deposition is well performed deep inside the electrode active material layer, so the distribution of the atomic layer deposition layer in the depth direction of the active material layer has never been considered before.

[0011] In practice, electrodes used in batteries have very low porosity due to the high-pressure pressing after the active material layer is applied to the current collector. Consequently, it remains difficult to perform atomic layer deposition (ALD) in the thickness direction of the active material layer of low-porosity electrodes, extending to areas near the current collector. Furthermore, electrodes manufactured using typical ALD processes have an ALD layer formed only near the surface, resulting in limited effectiveness.

[0012] Therefore, there is still a great need for cathode technologies for improving the performance of secondary batteries. Summary of the Invention

[0013] Technical issues

[0014] The present disclosure aims to provide a positive electrode for a lithium secondary battery having high storage stability suitable for long-term storage and improved battery life, and a lithium secondary battery having the same.

[0015] Technical Solution

[0016] In order to solve the above problems, according to one aspect of the present disclosure, a positive electrode for a lithium secondary battery according to the following embodiments is provided.

[0017] The first embodiment relates to a positive electrode for a lithium secondary battery, comprising a current collector and a positive electrode active material layer, wherein the positive electrode active material layer comprises a plurality of positive electrode active materials arranged on at least one surface of the current collector, and an atomic layer deposition (ALD) coating arranged on the surface and pores of the positive electrode active materials and in the gaps between the plurality of positive electrode active materials, wherein the thickness of the atomic layer deposition coating is 0.2 nm to 1 nm, and when the positive electrode active material layer is divided into five equal parts in the thickness direction, the portion of the positive electrode active material layer in contact with the current collector is referred to as the lowermost positive electrode active material layer, and the surface portion of the positive electrode active material layer farthest from the current collector is referred to as the uppermost positive electrode active material layer, the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer is 40 weight % or more, and the positive electrode has a porosity of 15% to 35%.

[0018] According to the second embodiment, the porosity of the positive electrode may be 20% to 30%.

[0019] According to a third embodiment, in the first embodiment, the atomic layer deposition coating may include at least one of oxides, nitrides, oxynitrides, sulfides, fluorides, and phosphates of metals or metalloids.

[0020] According to a fourth embodiment, in the third embodiment, the metal or metalloid may include at least one of Al, Zr, Si, Zn, Ti, Sn, Mn, Nb, W, and Li.

[0021] According to a fifth embodiment, in the third or fourth embodiment, the atomic layer deposition coating may include ZrO x 、AlO x 、SiO x 、ZnO x 、TiO x 、SnO x 、MnO x 、NbO x , WO x , at least one of lithium aluminum oxide, lithium zirconium oxide, lithium niobium oxide and lithium tungsten oxide, where x may be greater than 0 and 3 or less.

[0022] According to a sixth embodiment, in any one of the first to fifth embodiments, the thickness of the atomic layer deposition coating may be 0.2 nm to 1 nm.

[0023] According to a seventh embodiment, in any one of the first to sixth embodiments, the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer relative to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer may be 40 wt % to 120 wt %.

[0024] According to an eighth embodiment, in any one of the first to seventh embodiments, the amount of the atomic layer deposition coating layer may be 300 ppm to 6,000 ppm based on the weight of the positive electrode active material layer.

[0025] In order to solve the above-mentioned problems, according to one aspect of the present disclosure, a lithium secondary battery of the following embodiment is provided.

[0026] According to a ninth embodiment, the lithium secondary battery includes:

[0027] An electrode assembly comprising a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode;

[0028] a battery case accommodating the electrode assembly; and

[0029] The non-aqueous electrolyte is injected into the battery case,

[0030] The positive electrode may be the positive electrode for a lithium secondary battery according to any one of the first to eighth embodiments.

[0031] Beneficial effects

[0032] A positive electrode for a lithium secondary battery according to one embodiment of the present invention has an atomic layer deposition coating that is thinner than a conventional atomic layer deposition coating and is formed deep into the interior of the positive electrode active material layer in the direction of the current collector. The atomic layer deposition coating acts as a protective layer over the entire thickness of the positive electrode active material layer, thereby suppressing degradation of the positive electrode and improving stability. A lithium secondary battery having the positive electrode maintains capacity retention and exhibits long life characteristics over repeated cycles. When stored in a fully charged state at high temperatures, the atomic layer deposition coating helps stabilize the unstable structural state of the positive electrode active material during lithium ion insertion, thereby minimizing the increase in battery resistance after long-term storage, resulting in improved storage stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings illustrate a preferred embodiment of the present disclosure and, together with the detailed description, serve to provide a further understanding of the technical aspects of the present disclosure, and the present disclosure should not be construed as being limited to the accompanying drawings.

[0034] Figure 1is a scanning electron microscope (SEM) image of a cross section of a positive electrode having an atomic layer deposition coating prepared according to Example 3 of the present disclosure.

[0035] Figure 2 is an elemental mapping image of aluminum measured by energy dispersive X-ray spectroscopy of a cross section of a positive electrode having an atomic layer deposition coating prepared according to Example 3 of the present disclosure.

[0036] Figure 3 is a diagram showing five equal parts of a cross section of a positive electrode active material layer according to Experimental Example 2 of the present disclosure.

[0037] Figure 4 is a graph showing charge cycle and discharge cycle of Experimental Example 3 according to the present disclosure.

[0038] Figure 5 is a graph showing the resistance increase rate vs. storage time for storage stability evaluation according to Experimental Example 4 of the present disclosure. DETAILED DESCRIPTION

[0039] Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings. It should be understood that the terms or words used in the specification and the appended claims should not be interpreted as limited to the general and dictionary meanings, but should be interpreted based on the meanings and concepts corresponding to the technical aspects of the present disclosure based on the principle of allowing the inventor to appropriately define the terms for the best interpretation.

[0040] A positive electrode for a lithium secondary battery according to one aspect of the present disclosure includes:

[0041] current collector; and

[0042] a positive electrode active material layer having a plurality of positive electrode active materials disposed on at least one surface of the current collector, and an atomic layer deposition (ALD) coating disposed on the surface and in the pores of the positive electrode active materials and in the gaps between the plurality of positive electrode active materials,

[0043] wherein the atomic layer deposition coating has a thickness of 0.2 nm to 1 nm,

[0044] When the positive electrode active material layer is divided into five equal parts in the thickness direction, the portion of the positive electrode active material layer in contact with the current collector is referred to as the lowermost positive electrode active material layer, and the surface portion of the positive electrode active material layer farthest from the current collector is referred to as the uppermost positive electrode active material layer, the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer is 40% by weight or more, and

[0045] The positive electrode has a porosity of 15% to 35%.

[0046] The current collector is not limited to a specific type as long as it has high conductivity without causing chemical changes in the lithium secondary battery, and may include, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, and silver.

[0047] The current collector may increase the bonding strength of the positive electrode active material by forming fine textures on its surface, and may appear in various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0048] The current collector may have a thickness of 3 μm to 500 μm, for example 5 μm to 50 μm.

[0049] In one embodiment of the present disclosure, the positive electrode active material layer may include a positive electrode active material, a binder polymer, and a conductive material.

[0050] The positive electrode active material may be a lithium-containing oxide and a lithium-containing transition metal oxide, and may include but is not limited to lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium cobalt-nickel oxide, lithium cobalt-manganese oxide, lithium manganese-nickel oxide, lithium cobalt-nickel-manganese oxide, lithium cobalt-nickel-manganese-aluminum oxide, or at least one of oxides replaced or doped with other elements.

[0051] Here, the other elements may include but are not limited to at least one of Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti or Bi.

[0052] According to one embodiment of the present disclosure, the lithium-containing transition metal oxide may include Li x CoO2(0.5 <x<1.3)、Li x NiO2(0.5 <x<1.3)、Li x MnO2(0.5 <x<1.3)、Li x Mn2O4(0.5 <x<1.3)、Li x (Ni a Co b Mn c )O2(0.5 <x<1.3,0<a<1,0<b<1,0<c<1,a+b+c=1)、Li x (Ni a Co b Mn c Al d)O2 (0.5 < x < 1.3, 0 < a < 1, 0 < b < 1, 0 < c < 1, 0 < d < 1, a + b + c + d = 1), Li x Ni 1-y Co y O2 (0.5 < x < 1.3, 0 < y < 1), Li x Co 1- y Mn y O2 (0.5 < x < 1.3, 0 ≤ y < 1), Li x Ni 1-y Mn y O2 (0.5 < x < 1.3, 0 ≤ y < 1), Li x (Ni a Co b Mn c )O4 (0.5 < x < 1.3, 0 < a < 2, 0 < b < 2, 0 < c < 2, a + b + c = 2), Li x Mn 2-z Ni z O4 (0.5 < x < 1.3, 0 < z < 2), Li x Mn 2-z Co z O4 (0.5 < x < 1.3, 0 < z < 2), Li x CoPO4 (0.5 < x < 1.3) or Li x FePO4 (0.5 < x < 1.3), and the lithium-containing transition metal oxide can be coated or doped with metals such as boron (B), aluminum (Al), and zirconium (Zr) or metal oxides. In addition to the lithium-containing transition metal oxide, sulfides, selenides, and halides can also be used.

[0053] The binder polymer can include at least one selected from the group consisting of polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene - propylene - diene monomer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), and fluororubber.

[0054] The conductive material generally includes conductive carbon and can include, for example, various types of conductive carbon materials such as graphite, carbon black, acetylene black, Ketjen black, super - P, and carbon nanotubes.

[0055] In the positive electrode active material layer, the binder polymer may be present in an amount of 0.1 to 10 parts by weight or 1 to 5 parts by weight, and the conductive material may be present in an amount of 0.5 to 10 parts by weight or 1 to 5 parts by weight, based on 100 parts by weight of the positive electrode active material. When the amounts of the binder polymer and the conductive material meet the above ranges, the content of the positive electrode active material is sufficient, and as a result, the capacity is not reduced, which is beneficial for electrode processes and can improve life performance.

[0056] The positive active material layer may be formed on one or both surfaces of the current collector, and the thickness of the positive active material layer formed on one surface of the current collector may be 20 to 200 μm, specifically 30 to 150 μm, more specifically 50 to 150 μm.

[0057] The positive electrode includes an atomic layer deposition coating disposed on a surface and in pores of a positive electrode active material layer.

[0058] The thickness of the atomic layer deposition coating may be 0.2 nm to 1 nm, or 0.2 nm to 0.8 nm, or 0.24 nm to 0.8 nm, or 0.24 nm to 0.55 nm. When the thickness of the atomic layer deposition coating satisfies the above range, the resistance of the battery hardly increases, and the atomic layer deposition coating can fully serve as a protective layer for the positive electrode active material layer. Specifically, in the case where the thickness of the atomic layer deposition coating does not meet the above range, when the thickness is less than 0.2 nm, the atomic layer deposition coating cannot fully serve as a protective layer, and when the thickness is greater than 1 nm, the resistance at the interface between the positive electrode active material layer and the electrolyte increases rapidly, the resistance of the battery increases, and the output characteristics decrease. In addition, when the atomic layer deposition coating is not formed to the positive electrode active material layer adjacent to the current collector, the function as a protective layer becomes weak, and in particular, the performance of delaying battery degradation may decrease as the charge / discharge cycle increases or decreases during long-term storage.

[0059] The atomic layer deposition is performed by injecting a precursor of the deposition material onto the surface of the positive electrode active material layer, injecting water vapor, and injecting an inert gas between each step, and the thickness of the atomic layer deposition coating can be adjusted by controlling the number of cycles of the series of steps. Here, the thickness of the atomic layer deposition coating refers to the thickness formed by performing atomic layer deposition on a silicon wafer under the same conditions as the atomic layer deposition performed on the positive electrode active material layer as defined above.

[0060] The thickness of the atomic layer deposition coating provided on the surface and in the pores of the positive electrode active material layer is defined as the thickness formed by performing atomic layer deposition on a silicon wafer under the same conditions as the atomic layer deposition performed on the positive electrode active material layer. The thickness of the atomic layer deposition coating can be measured by fitting the measured values ​​from 380 nm to 780 nm measured by spectroscopic ellipsometer with a modified Cauchy model.

[0061] The ALD coating is formed on the surface and in the pores of the positive electrode active material layer. The pores in the positive electrode active material layer have a small size ranging from a few micrometers to hundreds of nanometers, or even tens of nanometers, and the pores are not smoothly connected to each other. Therefore, it is difficult to form a uniform ALD coating across the entire positive electrode active material layer, from the surface of the positive electrode active material layer to the current collector. As a result, the ALD coating is formed with the greatest thickness in the uppermost positive electrode active material layer and with the least thickness in the lowermost positive electrode active material layer facing the current collector.

[0062] Therefore, the ratio of the amount of the atomic layer deposition coating of the lowermost positive active material layer to the amount of the atomic layer deposition coating of the uppermost positive active material layer indicates the extent to which the atomic layer deposition coating is uniformly formed in the thickness direction of the positive active material layer.

[0063] The present inventors found that the distribution of the ALD coating greatly affects battery performance, and therefore, they paid attention to the ratio of the amount of the ALD coating of the lowermost positive active material layer to that of the uppermost positive active material layer.

[0064] In the present disclosure, when the positive electrode active material layer is divided into five equal parts in the thickness direction, the portion of the positive electrode active material layer in contact with the current collector is referred to as the lowermost positive electrode active material layer, and the surface portion of the positive electrode active material layer farthest from the current collector is referred to as the uppermost positive electrode active material, the distribution uniformity of the atomic layer deposition coating is defined as the ratio (weight %) of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer.

[0065] For example, when the positive electrode active material layer is divided into five equal parts in the thickness direction, the five equal parts can be referred to as the first positive electrode active material layer, the second positive electrode active material layer, the third positive electrode active material layer, the fourth positive electrode active material layer, and the fifth positive electrode active material layer in order from the surface portion of the positive electrode active material layer farthest from the current collector to the portion of the positive electrode active material layer in contact with the current collector. In this case, the first positive electrode active material layer can correspond to the uppermost positive electrode active material layer, and the fifth positive electrode active material layer can correspond to the lowermost positive electrode active material layer. A cross-sectional electron microscope image of the positive electrode active material layer divided into the first positive electrode active material layer to the fifth positive electrode active material layer is shown in FIG. Figure 2 middle.

[0066] The inventors have repeatedly verified the gradual change in the distribution of the atomic layer deposition coating from the surface to the current collector. As a result, in the present disclosure, when evaluating the distribution uniformity of the atomic layer deposition coating, the positive electrode active material layer is divided into five equal parts in the thickness direction, and the distribution uniformity is defined as the ratio (weight %) of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer (first active material layer) to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer (fifth active material layer). The reason why the distribution uniformity of the atomic layer deposition coating is usually evaluated by dividing the positive electrode active material layer into five equal parts in the thickness direction is that: when the atomic layer deposition coating is divided into less than five equal parts, there is no discrimination for determining the distribution uniformity, and when the positive electrode active material layer is divided into more than five equal parts, the average particle size of the positive electrode active material particles contained in the positive electrode active material layer is greater than the height of one partition of the positive electrode active material layer, thereby there is a very large deviation in the distribution of the atomic layer deposition coating, and the reproducibility of the distribution evaluation is significantly reduced.

[0067] A general atomic layer deposition process cannot form atomic layer deposition on the active material layer of the electrode with low porosity in the thickness direction up to the area near the current collector. Therefore, the inventors improved the depth distribution of the atomic layer deposition coating by controlling various parameters of the atomic layer deposition process. To this end, the controllable atomic layer deposition process parameters mainly include the heating temperature of the substrate, the injection amount and concentration of the precursor and reaction gas, the time the precursor and reaction gas are exposed to the substrate, the flow rate of the precursor, reaction gas, carrier gas and purge gas, the injection time of the purge gas and the process pressure. The higher the heating temperature of the substrate, the smoother the atomic layer deposition can be carried out into the pores of the substrate, but the heating temperature of the substrate should be adjusted while considering the heat resistance temperature of the substrate. For example, in the positive electrode according to the embodiment of the present disclosure, the heating temperature of the substrate is preferably 120°C to 150°C. Relative to the specific surface area of ​​the substrate, it is necessary to supply a sufficient injection amount of precursor and reaction gas. For example, the number of trimethylaluminum molecules required per unit area during each atomic layer deposition can be calculated from the density of aluminum oxide formed as a film in the atomic layer deposition of aluminum oxide and the thickness formed during each atomic layer deposition. It is necessary to supply the injection amount at a sufficient ratio relative to the specific surface area of ​​the substrate. Here, the sufficient ratio is at least twice, preferably more than 10 times. In addition, in order to obtain such a sufficient injection amount, the vapor pressure can be adjusted by adjusting the temperature of the container for supplying the precursor and the reaction gas, and the injection time can be adjusted. It is not desirable for the concentration of the precursor and the reaction gas to be too low, and the concentration of the precursor and the reaction gas is determined by the ratio of the injection amount to the carrier gas adjusted by the temperature and the injection time. The exposure time of the precursor and the reaction gas is related to the reaction rate, and it is important to obtain sufficient time for the precursor and the reaction gas to penetrate deep into the pores of a substrate with a complex pore shape, such as a positive electrode. The time it takes for the gas to penetrate the pores is usually longer than the reaction rate. The exposure time can be shortened by increasing the temperature of the substrate. The flow rate of the precursor, reaction gas, carrier gas and purge gas greatly affects the depth-direction deposition distribution of the positive electrode substrate. As the process pressure increases, deposition inside the pores of the substrate is more favorable, but when the process pressure is too high, atomic layer deposition cannot be properly performed and powder may be formed by a gas phase reaction.

[0068] In the present disclosure, the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer is 40 wt % or more. According to one embodiment of the present disclosure, the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer may be 40 wt % to 120 wt %, or 49 wt % to 107 wt %, or 50 wt % to 100 wt %.

[0069] When the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer satisfies the above-mentioned range, the atomic layer deposition coating is uniformly formed from the surface of the positive electrode active material layer to the bottom in contact with the current collector, and the atomic layer deposition coating acts as a protective layer over the entire thickness of the positive electrode active material layer, thereby maintaining the protective function of the atomic layer deposition coating for the positive electrode active material layer without deterioration during hundreds of repeated charge and discharge cycles, stably maintaining the capacity during high-temperature storage, and suppressing the increase in resistance.

[0070] In this case, the positive electrode active material layer is divided into five equal parts in the thickness direction, and the amount of the atomic layer deposition coating in each of the first positive electrode active material layer, the second positive electrode active material layer, the third positive electrode active material layer, the fourth positive electrode active material layer, and the fifth positive electrode active material layer can be measured by quantifying the elements contained in the atomic layer deposition coating by energy dispersive X-ray spectroscopy using an electron microscope after cutting the positive electrode active material layer having the atomic layer deposition coating in a direction perpendicular to the current collector. Alternatively, the ratio of the amount of a specific element (e.g., aluminum (Al)) contained in the atomic layer deposition coating to the amount of a specific element (e.g., nickel (Ni)) in the positive electrode active material layer can be calculated from the value measured by the above method to observe whether the atomic layer deposition coating is uniformly formed in the thickness direction of the active material.

[0071] The amount of the metal element in the atomic layer deposition coating may be 300 ppm to 6,000 ppm, or 300 ppm to 4,000 ppm, or 400 ppm to 2,000 ppm, based on the total weight of the positive electrode active material layer having the atomic layer deposition coating. In this case, the positive electrode active material layer refers to all layer components formed on the current collector, including the positive electrode active material, conductive material, binder polymer, and atomic layer deposition coating of the positive electrode.

[0072] When the amount of the atomic layer deposition coating satisfies the above range, the resistance increase of the battery can be minimized without losing the function as a protective layer of the positive electrode active material layer, and problems that occur when the amount of the atomic layer deposition coating is too large, such as low energy density of the battery, increased resistance and reduced output characteristics, can be prevented.

[0073] The atomic layer deposition coating may include at least one of an oxide, nitride, oxynitride, sulfide, fluoride, and phosphate of a metal or metalloid, and in this case, the metal or metalloid may include at least one of Al, Zr, Si, Zn, Ti, Sn, Mn, Nb, W, and Li.

[0074] In addition, according to one embodiment of the present disclosure, the atomic layer deposition coating may be ZrO x 、AlO x 、SiO x 、ZnO x 、TiO x 、SnO x 、MnO x 、NbO x , WO x , at least one of lithium aluminum oxide, lithium zirconium oxide, lithium niobium oxide, and lithium tungsten oxide, and in this case, x may be greater than 0 and 3 or less.

[0075] The porosity of the positive electrode (including the porosity of the positive electrode active material layer and the atomic layer deposition coating provided on the surface and in the pores of the positive electrode active material layer) is 15% to 35%, or according to one embodiment of the present disclosure, the porosity can be 20% to 30%.

[0076] In addition, the porosity of the positive electrode active material layer before forming the atomic layer deposition coating is 15% to 35%, or according to one embodiment of the present disclosure, the porosity may be 20% to 30%. The porosity of the positive electrode can be calculated by the following method.

[0077] (1) The thickness and weight per unit area of ​​the positive electrode were measured.

[0078] (2) The density of the positive electrode (positive electrode active material layer) containing pores was calculated by dividing the measured weight per unit area by the volume obtained by multiplying the measured thickness by the unit area.

[0079] (3) Theoretical density is calculated by multiplying the distribution amount per unit area of ​​the constituent components of the positive electrode (positive electrode active material, conductive material, binder polymer, and atomic layer deposition coating in the case of atomic layer deposition) by the respective known true density values.

[0080] (4) The porosity is calculated by the following formula.

[0081] Porosity (%) = [(theoretical density calculated by (3)) / (measured density calculated by (2)) - 1] × 100 In addition, the porosity of the positive electrode can be measured based on the amount of non-reactive liquid that is not absorbed into the constituent components of the positive electrode and is injected into the positive electrode.

[0082] A method of manufacturing a positive electrode according to one embodiment of the present disclosure is as follows.

[0083] First, a positive electrode active material layer composition containing a positive electrode active material, a binder polymer, a conductive material, and a solvent is prepared, applied on at least one surface of a current collector, dried, and pressed by a roll press to form a positive electrode active material layer.

[0084] The solvent used in the present disclosure may include, but is not limited to, solvents commonly used in the art, such as N-methylpyrrolidone (NMP).

[0085] Subsequently, an atomic layer deposition coating is formed on the surface and in the pores of the positive electrode active material layer to obtain the positive electrode active material layer.

[0086] According to one embodiment of the present disclosure, the step of forming an atomic layer deposition coating may include installing a current collector (positive electrode substrate) having a pre-prepared positive electrode active material layer on an atomic layer deposition system, injecting a metal precursor into the system; using an inert gas (e.g., Ar) for purging, supplying water vapor to replace the binding groups of the metal precursor with OH groups; and using an inert gas for purging to remove impurities and unreacted components from the system to form an atomic layer deposition coating.

[0087] According to one embodiment of the present disclosure, in the step of injecting water vapor (H2O) into the metal precursor, the methyl groups of the metal precursor may be substituted with hydroxyl groups. A purge step using an inert gas may be performed to remove the remaining water vapor after all methyl groups of the oxide precursor are substituted with hydroxyl groups.

[0088] The metal precursor may include but is not limited to at least one of tetramethyl zirconium [Zr(CH3)4], tetrakis(ethylmethylamino) zirconium [Zr[N(CH3)(CH2CH3)]4(TEMAZ)], zirconium tert-butoxide [Zr(t-OC4H9)4(ZTB)], trimethylaluminum [Al(CH3)3(TMA)], tetramethyl silicon [Si(CH3)4], dimethyl zinc [Zn(CH3)2], diethyl zinc [Zn(C2H5)2(DEZn)], tetramethyl titanium [Ti(CH3)4], tetramethyl tin [Sn(CH3)4] or bis(ethylcyclopentadienyl) manganese [Mn(CpEt)2].

[0089] The inert gas may include, but is not limited to, He, N2, Ne, Ar, and Kr.

[0090] When an atomic layer deposition coating is formed on the surface and in the pores of the positive electrode active material layer, the method for manufacturing the positive electrode can be carried out as follows: applying and drying the positive electrode active material composition, performing an atomic layer deposition coating process and performing a pressing process, thereby fully forming an atomic layer deposition coating from the surface portion of the positive electrode active material layer farthest from the current collector to the portion of the positive electrode active material layer in contact with the current collector. By this method, the atomic layer deposition coating can be formed to the portion of the positive electrode active material layer in surface contact with the current collector in a state where the pore size and porosity of the positive electrode active material layer are the largest. Alternatively, when performing atomic layer deposition coating on a pressed positive electrode, the process pressure can be increased or a process pressure higher than normal pressure can be applied, and the incubation time can be increased to allow the metal precursor and water vapor or inert gas to reach the depth of the pores. The incubation time can be adjusted according to the porosity and thickness of the electrode.

[0091] The positive electrode for the lithium secondary battery of the present invention as described above forms an electrode assembly together with the negative electrode and the separator sandwiched between the positive electrode and the negative electrode, and the electrode assembly is accommodated in a battery case, and a non-aqueous electrolyte is injected into the battery case to manufacture a lithium secondary battery. The negative electrode and separator and the non-aqueous electrolyte that form the electrode assembly together with the positive electrode according to the present disclosure may include those commonly used in the manufacture of lithium secondary batteries.

[0092] The negative electrode according to the present disclosure may be manufactured in the same manner as the positive electrode by mixing and stirring a negative electrode active material, a binder polymer, a solvent, and, if necessary, a conductive material and a dispersant to prepare a slurry, applying the slurry to a current collector, and drying.

[0093] The negative electrode active material may generally include a carbon material capable of intercalating and deintercalating lithium ions, lithium metal, silicon or tin, and may include a metal oxide having a potential of less than 2V relative to lithium, such as TiO2 and SnO2. The negative electrode active material preferably includes a carbon material, and the carbon material may include low-crystalline carbon and high-crystalline carbon. The low-crystalline carbon may generally include soft carbon and hard carbon, and the high-crystalline carbon may generally include high-temperature sintered carbon, such as natural graphite, artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microbeads, mesophase pitch, coke derived from petroleum, and coke derived from tar pitch.

[0094] The binder polymer, solvent, and conductive material of the negative electrode may be the same as those used in the fabrication of the positive electrode.

[0095] The separator may comprise a porous polymer film commonly used as a separator, for example, a porous polymer film made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer, used alone or in combination, or may include but is not limited to general porous non-woven fabrics, such as non-woven fabrics made of high-melting point glass fiber, polyethylene terephthalate fiber, etc.

[0096] According to one embodiment of the present disclosure, the separator may include only a porous substrate, such as a porous polymer film and a porous non-woven fabric, and in addition to the porous substrate, the separator may further include a porous coating layer including inorganic particles and a binder polymer on at least one surface of the porous substrate.

[0097] In the electrolyte used in the present disclosure, the electrolyte may include a lithium salt, and the lithium salt may include but is not limited to those commonly used in electrolytes for lithium secondary batteries. For example, the anion of the lithium salt may include an anion selected from the group consisting of F - 、Cl - Br - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - 、 CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - Any of the groups consisting of.

[0098] In the electrolyte used in the present disclosure, the organic solvent contained in the electrolyte may include but is not limited to those commonly used in lithium secondary battery electrolytes, and may generally include at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite and tetrahydrofuran. In particular, in carbonate organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are high viscosity organic solvents, and contribute to the dissociation of lithium salts in electrolytes due to high dielectric constant, and when low viscosity, low dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate are mixed with cyclic carbonates in an appropriate ratio, it is more preferred that the electrolyte has high conductivity.

[0099] Optionally, the electrolyte stored according to the present disclosure may further include additives commonly used in electrolytes, such as overcharge inhibitors.

[0100] The battery case used in the present disclosure may include those commonly used in the art and is not limited to a specific shape according to the purpose of use of the battery, and may include, for example, a cylindrical shape using a can, a prismatic shape, a bag shape, or a coin shape.

[0101] Hereinafter, the present disclosure will be described in detail by test examples and embodiments to help understand the present disclosure. However, the embodiments according to the present disclosure can be modified in many other forms, and the scope of the present disclosure should not be interpreted as being limited to the following embodiments. The embodiments of the present disclosure are provided to help those of ordinary skill in the art fully and comprehensively understand the present disclosure.

[0102] Comparative Example 1

[0103] LiNi as the positive electrode active material 0.8 Mn 0.1 Co 0.1 O2, carbon black (product name: DenkaBlack) as a conductive material, and polyvinylidene fluoride (PVdF) (product name: KF1300) as a binder polymer were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.5:1:1.5 to prepare a positive electrode slurry. The positive electrode slurry was coated on both surfaces of an aluminum current collector having a thickness of 12 μm and dried at 135 ° C for 3 hours, and then pressed to produce a positive electrode having a positive electrode active material layer having a thickness of 90 μm and a porosity of 25% on each of the two surfaces of the current collector. The prepared positive electrode was used without a separate atomic layer deposition process.

[0104] Comparative Example 2

[0105] The positive electrode substrate manufactured in the same manner as in Comparative Example 1 was mounted on an atomic layer deposition system (Lucida C200-PL, NCDTech), and trimethylaluminum (Al(CH3)3) as a metal precursor was injected into the system for 0.1 second. The precursor was filled in a steel container maintained at 14°C, and the vaporized precursor was injected into the system without using a separate carrier gas. In this case, the temperature inside the system was maintained at 120°C. Subsequently, after purging with Ar as an inert gas for 5 seconds, water vapor filled in a steel container maintained at 11°C was supplied for 0.2 seconds to replace the methyl group of aluminum with an OH group. Subsequently, impurities and unreacted components in the system were removed by purging with Ar as an inert gas for another 10 seconds. The atomic layer deposition process was repeated twice to produce a positive electrode having an aluminum oxide coating with a thickness of about 0.2 nm formed on the surface of the positive electrode active material layer.

[0106] Example 1

[0107] The atomic layer deposition process was performed in the same manner as in Comparative Example 2, but the injection time of the precursor, reaction gas and purge gas, and the number of atomic layer deposition processes were different. The injection time of trimethylaluminum (Al(CH3)3) was set to 0.5 seconds, and after purging with Ar as an inert gas for 10 seconds, water vapor was supplied for 1 second to replace the methyl group of aluminum with an OH group. Subsequently, impurities and unreacted components in the system were removed by purging with Ar as an inert gas for another 20 seconds. The atomic layer deposition process was repeated 5 times to produce a positive electrode having an aluminum oxide coating with a thickness of about 0.55 nm formed on the surface of the positive electrode active material layer.

[0108] Example 2

[0109] The atomic layer deposition process was performed in the same manner as in Comparative Example 2, but the injection time of the precursor, reaction gas, and purge gas, as well as the number of atomic layer deposition processes, were different. The injection time of trimethylaluminum (Al(CH3)3) was set to 2 seconds, and after purging with Ar as an inert gas for 50 seconds, water vapor was supplied for 2 seconds to replace the methyl group of aluminum with an OH group. Subsequently, impurities and unreacted components in the system were removed by purging with Ar as an inert gas for another 50 seconds. The atomic layer deposition process was repeated twice to produce a positive electrode having an aluminum oxide coating with a thickness of about 0.24 nm formed on the surface of the positive electrode active material layer.

[0110] Example 3

[0111] The atomic layer deposition process was performed in the same manner as in Comparative Example 2, but the injection time of the precursor, reaction gas, and purge gas, as well as the number of atomic layer deposition processes, were different. The injection time of trimethylaluminum (Al(CH3)3) was set to 2 seconds, and after purging with Ar as an inert gas for 50 seconds, water vapor was supplied for 2 seconds to replace the methyl group of aluminum with an OH group. Subsequently, impurities and unreacted components in the system were removed by purging with Ar as an inert gas for another 50 seconds. The atomic layer deposition process was repeated 7 times to produce a positive electrode having an aluminum oxide coating with a thickness of about 0.84 nm formed on the surface of the positive electrode active material layer.

[0112] The cross-sectional electron microscope image of the positive electrode with the prepared atomic layer deposition coating and the elemental mapping image of aluminum measured by energy dispersive X-ray spectroscopy are shown in Figure 1 and Figure 2 middle.

[0113] The surface of the positive electrode active material layer (the uppermost portion of the positive electrode active material layer) of the positive electrode is located Figure 1 On the right vertical direction, and the collector is located Figure 1 On the left vertically. Figure 2 yes Figure 1 The lower part of the image is an elemental mapping image of aluminum measured by energy dispersive X-ray spectroscopy. Figure 2 , in the corresponding Figure 1 Al is observed in the vertical left region of the aluminum current collector, and regions of the same color are observed distributed in the active material layer, and they are atomic layer deposition coatings.

[0114] Comparative Example 3

[0115] The atomic layer deposition process was performed in the same manner as in Comparative Example 2, but the injection time of the precursor, reaction gas and purge gas, and the number of atomic layer deposition processes were different. The injection time of trimethylaluminum (Al(CH3)3) was set to 1 second, and after purging with Ar as an inert gas for 50 seconds, water vapor was supplied for 1 second to replace the methyl group of aluminum with an OH group. Subsequently, impurities and unreacted components in the system were removed by purging with Ar as an inert gas for another 50 seconds. The atomic layer deposition process was repeated 10 times to produce a positive electrode having an Al2O3 coating with a thickness of about 1.2 nm formed on the surface of the positive electrode active material layer.

[0116] Experimental Example 1: Measurement of Atomic Layer Deposition Coating Thickness

[0117] The thickness of the atomic layer deposition coating provided on the surface and in the pores of the positive electrode active material layer is defined as the thickness formed by atomic layer deposition on a silicon wafer under the same conditions as the atomic layer deposition performed on the positive electrode active material layer, and the thickness can be measured by fitting the ψ and δ measurements at 380 nm to 780 nm measured by a spectroscopic ellipsometer using an improved Cauchy model.

[0118] In order to determine the deposition rate (growth per cycle) under the atomic layer deposition coating conditions corresponding to the comparative examples and embodiments, the thickness measured using an ellipsometer by repeating the atomic layer deposition process 200 times under the atomic layer deposition conditions corresponding to each comparative example and embodiment and the coating thickness per atomic layer deposition cycle calculated therefrom are shown in Table 1 below.

[0119]

Table 1

[0120]

[0121] The thicknesses of the positive electrode active material layers in the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 3 were calculated by the above-described method, and the results are shown in Table 2.

[0122] Experimental Example 2: Distribution Uniformity of Atomic Layer Deposition Coating

[0123] In the positive electrodes manufactured in Examples 1 to 3 and Comparative Examples 1 to 3, when the positive electrode active material layer is divided into five equal parts in the thickness direction, and in these five equal parts, the part of the positive electrode active material layer in contact with the current collector is called the lowermost positive electrode active material layer, and the surface part of the positive electrode active material layer farthest from the current collector is called the uppermost positive electrode active material layer, the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer and the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer are measured using an electron microscope (FE-SEM, JEOL JSM-7200F) by energy dispersive X-ray spectrometry (Oxford EXTREME EDS system) under the conditions of an acceleration voltage of 15 kV and a working distance of 15 mm.

[0124] When the positive electrode active material layers are referred to as the first positive electrode active material layer, the second positive electrode active material layer, the third positive electrode active material layer, the fourth positive electrode active material layer, and the fifth positive electrode active material layer in order from the surface portion of the positive electrode active material layer farthest from the current collector to the portion of the positive electrode active material layer in contact with the current collector, the first positive electrode active material layer may correspond to the uppermost positive electrode active material layer, and the fifth positive electrode active material layer may correspond to the lowermost positive electrode active material layer. An electron microscope image of a cross section of the positive electrode active material layer divided into the first to fifth positive electrode active material layers is shown in FIG. Figure 3 middle.

[0125] Subsequently, the distribution uniformity of the atomic layer deposition coating was calculated as the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer, and is shown in Table 2 below.

[0126] Experimental Example 3: Measurement of Porosity (%) of Positive Electrode

[0127] The porosity of the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 3 was measured by the following method, and the measurement results are shown in Table 2 below.

[0128] (1) The thickness and weight per unit area of ​​the positive electrode were measured.

[0129] (2) The density of the positive electrode (positive electrode active material layer) containing pores was calculated by dividing the measured weight per unit area by the volume obtained by multiplying the measured thickness by the unit area.

[0130] (3) Theoretical density is calculated by multiplying the distribution amount per unit area of ​​the constituent components of the positive electrode (positive electrode active material, conductive material, binder polymer, and atomic layer deposition coating in the case of atomic layer deposition) by the respective known true density values.

[0131] (4) The porosity is calculated by the following formula.

[0132] Porosity (%) = [(theoretical density calculated by (3)) / (measured density calculated by (2)) - 1] × 100

[0133] Experimental Example 4: Charge / Discharge Test (Capacity Retention)

[0134] For the charge / discharge test, a single cell was manufactured using the positive electrode manufactured in the examples and comparative examples, a negative electrode comprising graphite, a conductive material and a binder, a polypropylene separator sandwiched between the positive and negative electrodes, and an organic electrolyte of ethylene carbonate / dimethyl carbonate / LiPF6 (Merck battery grade, EC / DMC (volume ratio) = 1 / 1, 1M LiPF6). The single cell was activated and then charged / discharged at 45°C. Charging was performed at a current density of 0.3C rate until 4.25V, and discharging was performed at the same current density to 2.5V. 200 charge / discharge cycles were performed, and the capacity was measured every 100 cycles. The capacity cycle results during charge / discharge are shown in Figure 4 , and the capacity retention rates during 200 charge / discharge cycles are shown in Table 2 below.

[0135] Experimental Example 5: Storage Stability Evaluation

[0136] In order to evaluate the storage stability, the single cell manufactured in Experimental Example 1 was activated and the initial resistance (DCIR) was measured at 4.25 V in a 100% fully charged state. Subsequently, the DCIR was measured every week during storage at 60°C for 4 weeks. The storage stability of the single cell was evaluated as the ratio (%) of the resistance at the 4th week relative to the initial resistance and is shown in Table 2 below. The resistance increase rate (%) of the resistance from the 1st week to the 4th week relative to the initial resistance (i.e., the resistance increase rate (%) during the 60°C storage period (weeks) is shown in the figure Figure 5 middle.

[0137]

Table 2

[0138]

[0139] Referring to Table 2, compared with Comparative Examples 1 to 3, the positive electrodes of Examples 1 to 3, which simultaneously meet the thickness range and distribution uniformity range of the atomic layer deposition coating according to the present disclosure, exhibit high capacity retention and long life characteristics within 200 cycles, and exhibit significantly high storage stability because the battery resistance does not increase much even after long-term storage.

Claims

1. A positive electrode for a lithium secondary battery, comprising: current collector; and a positive electrode active material layer comprising a plurality of positive electrode active materials disposed on at least one surface of the current collector, and an atomic layer deposition coating disposed on the surface and in the pores of the positive electrode active materials and in the gaps between the plurality of positive electrode active materials, The thickness of the atomic layer deposition coating is 0.2 nm to less than 1 nm, When the positive electrode active material layer is divided into five equal parts in the thickness direction, the portion of the positive electrode active material layer in contact with the current collector is referred to as the lowermost positive electrode active material layer, and the surface portion of the positive electrode active material layer farthest from the current collector is referred to as the uppermost positive electrode active material layer, the ratio of the amount of the atomic layer deposition coating of the lowermost positive electrode active material layer to the amount of the atomic layer deposition coating of the uppermost positive electrode active material layer is 40 wt % to 120 wt %, and The positive electrode has a porosity of 15% to 35%, The atomic layer deposition is performed by injecting a precursor of a deposition material onto the surface of the positive electrode active material layer, injecting water vapor, and injecting an inert gas between each step. 2 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the positive electrode has a porosity of 20% to 30%. 3 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the atomic layer deposition coating comprises at least one of an oxide, a nitride, a nitrogen oxide, a sulfide, a fluoride, and a phosphate of a metal or a metalloid. 4 . The positive electrode for a lithium secondary battery according to claim 3 , wherein the metal or metalloid comprises at least one of Al, Zr, Si, Zn, Ti, Sn, Mn, Nb, W, and Li.

5. The positive electrode for lithium secondary battery according to claim 3, wherein the atomic layer deposition coating comprises ZrO x 、AlO x 、SiO x 、ZnO x 、TiO x 、SnO x 、MnO x 、NbO x , WO x , at least one of lithium aluminum oxide, lithium zirconium oxide, lithium niobium oxide and lithium tungsten oxide, wherein x is greater than 0 and is 3 or less. 6 . The positive electrode for a lithium secondary battery according to claim 1 , wherein the thickness of the atomic layer deposition coating is 0.2 nm to 0.8 nm. 7 . The positive electrode for a lithium secondary battery according to claim 1 , wherein an amount of the atomic layer deposition coating layer is 300 ppm to 6,000 ppm based on the total weight of the positive electrode active material layer.

8. A lithium secondary battery comprising: An electrode assembly comprising a positive electrode, a negative electrode, and a separator sandwiched between the positive electrode and the negative electrode; a battery case accommodating the electrode assembly; and The non-aqueous electrolyte is injected into the battery case, The positive electrode is a positive electrode for a lithium secondary battery according to any one of claims 1 to 7.

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