Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery exhibiting excellent capacity characteristics and improved rapid charging characteristics. The lithium secondary battery comprises: a positive electrode containing a positive electrode active material comprising a lithium-rich manganese-based oxide; a negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and containing a negative electrode active material; and a separator or an electrolyte layer interposed between the positive electrode and the negative electrode, wherein the ratio of the porosity between the upper layer of the negative electrode active material layer and the remaining lower layers other than the upper layer, which are in contact with the negative electrode current collector and correspond to 50% of the total thickness of the negative electrode active material layer, is 1 to 1.4, and wherein the average porosity of the entire negative electrode active material layer can be 6% to 15%.

Description

Technical Field

[0001] Cross-references to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2024-0154056, filed on November 4, 2024, and Korean Patent Application No. 10-2025-0142886, filed on September 30, 2025, the entire contents of which are incorporated herein by reference.

[0003] This invention relates to a lithium secondary battery that exhibits excellent capacity characteristics and improved fast charging characteristics. Background Technology

[0004] Lithium-ion batteries typically consist of a positive electrode, a negative electrode, a separator, and an electrolyte, with the positive and negative electrodes containing active materials capable of inserting and deintercalating lithium ions.

[0005] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4, etc.), or lithium iron phosphate compounds (LiFePO4) have been used as positive electrode active materials for lithium-ion batteries. Among these, lithium cobalt oxide has the advantages of high operating voltage and excellent capacity characteristics; however, due to the high price and unstable supply of cobalt as a raw material, it is difficult to use commercially in high-capacity batteries. Lithium nickel oxide suffers from poor structural stability, making it difficult to achieve sufficient lifetime characteristics. On the other hand, lithium manganese oxide, with its spinel structure, exhibits excellent stability but suffers from poor capacity characteristics. Therefore, lithium composite transition metal oxides containing two or more transition metals have been developed and used to compensate for the problems of lithium transition metal oxides containing only Ni, Co, or Mn.

[0006] Among them, oxides containing excess lithium and with a higher Mn content than other metals excluding lithium (hereinafter referred to as "lithium-rich manganese oxides") are known to ensure high energy density as high-capacity active materials, and research and interest in this area are increasing significantly.

[0007] However, while lithium-rich manganese oxides possess relatively high capacity and energy density, lithium-ion secondary batteries incorporating them as positive electrode active materials suffer from insufficient fast-charging and output characteristics. This is believed to be because the application of lithium-rich manganese oxide positive electrode active materials necessitates the use of a highly loaded negative electrode, and under these conditions, the lithium-ion migration rate between the electrodes is insufficient.

[0008] Therefore, there is a need to develop a lithium secondary battery that exhibits improved fast-charging characteristics while also demonstrating the excellent capacity characteristics and energy density characteristic of lithium-rich manganese oxide cathode active materials. Summary of the Invention

[0009] Technical issues

[0010] The present invention provides a lithium secondary battery that exhibits excellent capacity characteristics and also improves the migration speed of lithium ions, thereby exhibiting improved fast charging characteristics.

[0011] Technical solution

[0012] According to one embodiment of the present invention, a lithium secondary battery is provided, the lithium secondary battery comprising: The positive electrode contains a positive electrode active material comprising a lithium-rich manganese oxide, the lithium-rich manganese oxide comprising a layered crystal structure, wherein the ratio of the molar number of lithium to the molar number of all metals excluding lithium is greater than 1, and the content of manganese in the all metals excluding lithium is more than 50 mol%. The negative electrode includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being formed on the negative electrode current collector and comprising negative electrode active material; and The membrane or electrolyte layer between the positive and negative electrodes The porosity ratio between the upper layer of the negative electrode active material layer and the remaining lower layers (excluding the upper layer) according to the following mathematical formula 1 is 1 to 1.4, wherein the lower layers are in contact with the negative electrode current collector and correspond to 50% of the total thickness of the negative electrode active material layer, and The average porosity of the entire negative electrode active material layer is 6% to 15%.

[0013] [Mathematical Expression 1]

[0014] Porosity ratio (%) = (Porosity of the upper layer in the negative electrode active material layer) / (Porosity of the lower layer in the negative electrode active material layer)

[0015] In Equation 1, the porosity of the upper and lower layers is calculated by the area of ​​the pores in the upper or lower layer relative to the total area when the cross-section in the thickness direction of the negative electrode active material layer is analyzed using an electron microscope.

[0016] In one embodiment of the lithium secondary battery, lithium-rich manganese oxides can be represented by the following chemical formula 1: [Chemical Formula 1] Li 1+a [Mn 1-(b+c) Ni b M c ]O 2+d In chemical formula 1, M contains at least one element selected from the following: Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr. 0.05≤a≤0.45,0 <b≤0.5,0≤c≤0.5,0<b+c≤0.5,0≤d≤1。

[0017] Furthermore, lithium-rich manganese oxides can have a structure in which rock-salt type lithium manganese oxides are mixed with layered lithium transition metal oxides. In a more specific embodiment, lithium-rich manganese oxides can be represented by the following chemical formula 2: [Chemical Formula 2] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M' w O2 In chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' contains at least one element selected from the following: Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0018] In one embodiment of the lithium secondary battery, the porosity of the lower layer in the negative electrode active material layer can be 5% to 15%, and the porosity of the upper layer in the negative electrode active material layer can be 6% to 18%.

[0019] In one embodiment, the negative electrode active material layer may comprise a negative electrode active material, a conductive material, and a binder, and in order to satisfy the porosity ratio of Formula 1, for example, the upper and lower layers of the negative electrode active material layer may each comprise negative electrode active materials having different compositions, particle morphologies, sphericity, or particle hardness.

[0020] In a more specific embodiment, the upper and lower layers may contain carbon-based anode active materials having different particle morphologies or sphericity in a calendered state, and the carbon-based anode active materials may contain natural graphite, artificial graphite, or mixtures thereof.

[0021] In a more specific embodiment, the lower layer may contain a greater amount of natural graphite than the upper layer. In another embodiment, the carbon-based negative electrode active material of the lower layer may be made of natural graphite, and the carbon-based negative electrode active material of the upper layer may be made of a mixture of natural and artificial graphite, or made entirely of artificial graphite.

[0022] Beneficial effects

[0023] In one embodiment of the lithium-ion secondary battery, a negative electrode satisfying a certain porosity ratio can be combined with a lithium-rich manganese oxide positive electrode active material. This negative electrode has a larger porosity in the upper layer than in the lower layer, thus enabling it to satisfy a specific range of interlayer porosity ratios according to Formula 1. When this negative electrode is combined, it has been confirmed that even when excess lithium ions migrate to the negative electrode from the lithium-rich manganese oxide, these lithium ions diffuse very rapidly on the negative electrode, resulting in a fast lithium-ion migration rate and improved fast-charging characteristics.

[0024] Furthermore, since this improved fast charging speed can be achieved even when a high-load negative electrode is applied, the lithium secondary battery according to one embodiment can exhibit enhanced fast charging characteristics as well as excellent capacity characteristics and energy density. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating the technical principle that enables rapid lithium-ion diffusion and fast charging within the negative electrode of a lithium secondary battery according to an embodiment of the present invention.

[0026] Figure 2a and 2b The images are electron microscope images of the cross-sections of the negative electrode active material layer in the thickness direction of the lithium secondary batteries contained in Comparative Examples 1 and 2, respectively.

[0027] Figure 2c and 2d These are electron microscope images of the cross-sections in the thickness direction of the negative electrode active material layer contained in the lithium secondary batteries of Examples 1 and 2, respectively. Detailed Implementation

[0028] The terms or words used in this specification and claims should not be construed as limited to their common or dictionary meanings, but rather should be interpreted as meanings and concepts consistent with the technical concept of the invention, based on the principle that inventors can appropriately define the concepts of the terms in order to best describe their own invention.

[0029] As used herein, the term "lithium-rich manganese oxides" can refer to lithium metal oxides that contain a layered crystal structure and wherein the ratio of the molar number of lithium to the molar number of all metals excluding lithium is greater than 1, and the content of manganese in the all metals excluding lithium is greater than 50 mol%.

[0030] As used in this article, the term "D" 50 "" refers to the particle size corresponding to 50% of the volumetric cumulative particle size distribution. D 50The measurement can be performed using laser diffraction. For example, the measurement can be performed by a method including the following steps: dispersing the positive electrode active material powder in a dispersion medium, introducing it into a commercially available laser diffraction particle size measurement device (e.g., Microtrac MT 3000), irradiating it with ultrasound at approximately 28 kHz at an output power of 60 W to obtain a volumetric cumulative particle size distribution map, and determining the particle size corresponding to 50% of the cumulative volume.

[0031] The specific embodiments of the present invention will be described in detail below.

[0032] A lithium secondary battery according to one embodiment of the present invention comprises: The positive electrode contains a positive electrode active material comprising lithium-rich manganese oxides; The negative electrode includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being formed on the negative electrode current collector and comprising negative electrode active material; and The membrane or electrolyte layer between the positive and negative electrodes The porosity ratio between the upper layer of the negative electrode active material layer and the remaining lower layers (excluding the upper layer) according to the following mathematical formula 1 is 1 to 1.4, wherein the lower layers are in contact with the negative electrode current collector and correspond to 50% of the total thickness of the negative electrode active material layer, and The average porosity of the entire negative electrode active material layer is 6% to 15%.

[0033] [Mathematical Expression 1]

[0034] Porosity ratio (%) = (Porosity of the upper layer in the negative electrode active material layer) / (Porosity of the lower layer in the negative electrode active material layer)

[0035] In Equation 1, the porosity of the upper and lower layers is calculated by the area of ​​the pores in the upper or lower layer relative to the total area when the cross-section in the thickness direction of the negative electrode active material layer is analyzed using an electron microscope.

[0036] The lithium-ion secondary battery according to this embodiment essentially comprises lithium-rich manganese oxide as the positive electrode active material. The lithium-rich manganese oxide contains excess lithium and also has a structure in which a rock salt phase and a layered structure are mixed, primarily activated within mutually different voltage ranges, thereby exhibiting improved capacity characteristics and energy density.

[0037] However, due to the characteristics of this lithium-rich manganese oxide, it needs to be combined with a highly loaded anode (with a high loading of anode active material and relatively low porosity) to increase the overall energy density of the lithium secondary battery. However, when combined with such a highly loaded anode, the migration rate of lithium ions between electrodes may decrease, and lithium ion deposition may occur on the anode surface due to lithium ion supersaturation. As a result, lithium secondary batteries containing lithium-rich manganese oxides may exhibit relatively poor output and fast-charging characteristics.

[0038] In one embodiment of the lithium secondary battery, in order to overcome these disadvantages, the porosity ratio between the upper and lower layers in the negative electrode active material layer, as defined by mathematical formula 1, is optimized to be 1 to 1.4, or greater than 1 and less than 1.39, or 1.2 to 1.4, or 1.2 to 1.38, while the total average porosity is optimized to be 6% to 15%, or 7% to 13%, or 8% to 12%.

[0039] According to mathematical formula 1, the range of interlayer porosity ratios can be defined, for example, by adjusting the particle morphology of the calendered negative electrode active material to make the upper layer have higher porosity and the lower layer have lower porosity, such as... Figure 1 As shown in the figure, it was confirmed that by controlling the porosity of the upper layer to a high level in this manner, even if excess lithium ions migrate to the negative electrode from the lithium-rich manganese oxide, these lithium ions diffuse very rapidly on the surface of the negative electrode, thus exhibiting high lithium ion migration rate and improved fast charging characteristics.

[0040] In contrast, by controlling the porosity of the lower layer to be relatively low, the entire negative electrode active material layer exhibits low porosity, thereby improving the energy density and capacity characteristics of the lithium-ion secondary battery. Specifically, this total average porosity takes into account the battery characteristics combined with the positive electrode active material; if the average porosity is too high, even with the use of lithium-rich manganese oxides, the thickness of the negative electrode may become too large, and the energy density, defined as energy per unit electrode volume, may also decrease. Conversely, if the total average porosity is too low, it may be difficult to sufficiently increase the porosity of the upper layer, which could lead to a reduction in the fast-charging characteristics of the lithium-ion secondary battery.

[0041] The following will describe in detail one embodiment of a lithium secondary battery for each of its constituent elements.

[0042] Lithium-ion secondary batteries may contain lithium-rich manganese oxides represented by the following chemical formula 1 as positive electrode active materials: [Chemical Formula 1] Li 1+a [Mn 1-(b+c) Ni b M c ]O 2+d wherein, in Chemical Formula 1, M comprises at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr and Zr, and 0.05≤a≤0.45, 0<b≤0.5, 0≤c≤0.5, 0<b+c≤0.5, 0≤d≤1.

[0043] In Chemical Formula 1, a is an excess molar ratio of Li in the lithium-rich manganese-based oxide, and may be 0.05 to 0.45, or 0.1 to 0.4, or 0.12 to 0.35. When a satisfies the above range, high capacity characteristics and high volumetric energy density can be achieved.

[0044] b is the molar ratio of Ni in the lithium-rich manganese-based oxide, and may satisfy 0<b≤0.5, 0.10≤b≤0.40, or 0.2≤b≤0.39.

[0045] c is the molar ratio of the additional metal M in the lithium-rich manganese-based oxide, and may satisfy 0≤c≤0.5, 0≤c≤0.2, or 0.05≤c≤0.15. If the proportion of M represented by cobalt or other additional doping elements becomes too large, it is difficult to ensure high capacity, gas generation and degradation of the positive electrode active material become intensified, and the lifespan characteristics may be degraded.

[0046] In this case, M may be at least one selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr and Zr. In specific embodiments, M may optionally comprise additional doping elements and Co.

[0047] 1-(b+c) is the molar ratio of Mn in the lithium-rich manganese-based oxide, and may be 0.5 or more and less than 1.0, 0.53 to 0.99, or 0.55 to 0.90. If the molar ratio of Mn becomes too small, the proportion of the rock salt phase becomes too small, so the effect of improving capacity is weak. In addition, d is a value indicating the proportion of excess oxygen, and can be appropriately adjusted in some cases.

[0048] The lithium-rich manganese-based oxide has a structure in which layered lithium transition metal oxide is mixed with rock salt-type lithium manganese oxide (e.g., Li2MnO3). During the initial activation process, the rock salt phase is activated and excess lithium ions are generated. In addition, an oxygen-redox reaction occurs during the activation of the rock salt phase, and this generation of excess lithium ions and the oxygen-redox reaction can contribute to improving the capacity of the positive electrode active material.

[0049] In a more specific instance, lithium-rich manganese oxides can be represented by the following chemical formula 2.

[0050] [Chemical Formula 2]

[0051] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mn y Co z M' w O2

[0052] In chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' contains at least one element selected from the following: Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0053] X refers to the proportion of rock salt phase (Li₂MnO₃) in lithium-rich manganese oxides, and can be 0.2≤X≤0.5, 0.25≤X≤0.5, or 0.25≤X≤0.4. When the proportion of rock salt phase (Li₂MnO₃) in lithium-rich manganese oxides meets the above range, high capacity characteristics can be achieved.

[0054] y is the molar ratio of Mn in the layered phase, and can be 0.4≤y<1, 0.4≤y≤0.8, or 0.4≤y≤0.7.

[0055] z is the molar ratio of Co in the layered phase, and can be 0 ≤ z ≤ 0.1, 0 ≤ z ≤ 0.08, or 0 ≤ z ≤ 0.05. When z exceeds 0.1, gas generation and degradation of the positive electrode active material become more severe, and lifetime characteristics may deteriorate.

[0056] w is the molar ratio of the additional element M' in the layered phase, and can be 0≤w≤0.2, 0≤w≤0.1, or 0≤w≤0.05.

[0057] Furthermore, according to one aspect, the positive electrode active material can take the form of secondary particles in which multiple primary particles are aggregated. In this case, the particle size of the primary particles can be 50 nm to 200 nm, or 60 nm to 180 nm, or 70 nm to 150 nm, and the average particle size D of the secondary particles... 50 It can be 2 µm to 10 µm, or 3 µm to 9 µm, or 4 µm to 7 µm.

[0058] Because the positive electrode active material with secondary particle morphology has the above-mentioned D 50The range allows for the achievement of excellent post-calendering cathode density and improved capacity characteristics. If the D of the cathode active material... 50 If D is too low, it will be difficult to achieve high calendering density, which may lead to a decrease in calendering characteristics and energy density. 50 If the value is too high, the lithium mobility in the positive electrode active material will decrease, which may increase the resistance of the lithium secondary battery containing it.

[0059] The aforementioned positive electrode active material can be manufactured according to previously known methods for producing lithium-rich manganese oxides, therefore its detailed description will be omitted.

[0060] Next, the positive electrode containing the above-mentioned positive electrode active material will be described.

[0061] This positive electrode comprises the aforementioned lithium-rich manganese oxide positive electrode active material. Specifically, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer comprises the aforementioned positive electrode active material. Since the positive electrode active material has already been described above, its detailed description will be omitted, and only the remaining components will be described in detail below.

[0062] The positive electrode current collector can contain a highly conductive metal, and there are no particular limitations, as long as it is non-reactive within the battery's voltage range and the positive electrode active material layer can easily adhere to it. Examples of materials that can be used as the positive electrode current collector include: stainless steel, aluminum, nickel, titanium, heat-treated carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. Furthermore, the positive electrode current collector can typically have a thickness from 3 µm to 500 µm and can have fine irregularities formed on its surface to enhance the adhesion of the positive electrode active material. For example, the current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven structures.

[0063] In addition to the positive electrode active material, the positive electrode active material layer may optionally contain conductive materials and adhesives as needed.

[0064] In this case, based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be from 80% to 99% by weight, more specifically from 90% to 98% by weight.

[0065] Conductive materials are used to provide conductivity to the electrodes. Any conductive material can be used without particular restriction, provided it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of conductive materials include: graphite, such as natural or artificial graphite; carbonaceous materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive tubes, such as carbon nanotubes; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; or conductive polymers, such as polyphenylene derivatives, and any one or a mixture of two or more thereof can be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material can be from 0.01% to 10% by weight, or from 0.1% to 9% by weight, or from 0.1% to 5% by weight.

[0066] Adhesives are used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of adhesives may be: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers in which hydrogen is replaced by lithium (Li), sodium (Na), or calcium (Ca), or various copolymers thereof, and any one or a mixture of two or more thereof may be used. Based on the total weight of the positive electrode active material layer, the adhesive content may be from 1% to 30% by weight, or from 1% to 20% by weight, or from 1% to 10% by weight.

[0067] On the other hand, a lithium secondary battery according to one embodiment includes a positive electrode, a negative electrode positioned facing the positive electrode, a separator or electrolyte layer between the positive and negative electrodes, and optionally, may further include an electrolyte.

[0068] Since the positive electrode is the same as described above, the remaining components will only be described in detail below.

[0069] The negative electrode includes a negative electrode current collector and a layer of negative electrode active material located on the negative electrode current collector.

[0070] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and will not cause adverse chemical changes in the battery. For example, materials such as copper, stainless steel, aluminum, nickel, titanium, heat-treated carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the negative electrode current collector can typically have a thickness from 3 µm to 500 µm, and similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. The negative electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven structures.

[0071] In addition to the negative electrode active material, the negative electrode active material layer may optionally include a binder and a conductive material.

[0072] In one embodiment of the lithium secondary battery, as described above, the negative electrode active material layer can have a porosity such that the porosity of the lower layer, which is in contact with the negative electrode current collector and corresponds to 50% of the total thickness of the negative electrode active material layer, is less than the porosity of the upper layer corresponding to the remaining negative electrode active material layers. This satisfies the structural characteristics defined by Mathematical Formula 1, where the porosity ratio between the upper and lower layers is 1 to 1.4, or greater than 1 and less than 1.39, or 1.2 to 1.4, or 1.2 to 1.38. Furthermore, the total average porosity of the negative electrode active material layer can be 6% to 15%, or 7% to 13%, or 8% to 12%. Therefore, the lithium secondary battery of one embodiment can exhibit excellent capacity and energy density, as well as improved fast charging and output characteristics.

[0073] In this case, the porosity of the entire negative electrode active material layer, or the upper and lower layers, can be calculated, for example, from the results of electron microscopy analysis of the thickness-direction cross-section of the negative electrode active material layer. Figure 2a and 2b As shown in the figure. Based on electron microscopy analysis of the thickness-direction cross-section, cross-sectional image data can be extracted using digital conversion technology, and the porosity of the negative electrode active material layer, upper layer, or lower layer can be calculated from the ratio of pore area to total cross-sectional area. This porosity can be derived by averaging the values ​​obtained from at least 10, 10 to 100, or 20 to 50 cross-sectional image data, and then deriving the corresponding porosity.

[0074] In a more specific example, the porosity of the lower layer in the negative electrode active material layer can be 5% to 15%, or 5.5% to 13%, or 5.7% to 12%, and the porosity of the upper layer can be 6% to 18%, or 7% to 17%, or 8% to 16.5%. If the porosity of the lower and upper layers is too large, the overall energy density of the lithium-ion battery may decrease. Conversely, if the porosity of the lower and upper layers is too small, the fast-charging and output characteristics of the lithium-ion battery may deteriorate.

[0075] The porosity mentioned above can be defined as the porosity of the final negative electrode active material layer formed by coating a negative electrode active material layer slurry onto a negative electrode current collector, followed by drying and calendering.

[0076] Therefore, in order to meet the porosity requirements of the negative electrode active material layer, the upper layer, and / or the lower layer, the upper and lower layers of the negative electrode active material layer can each contain negative electrode active materials with different compositions, particle morphologies, sphericity, or particle hardness. For example, such as Figure 1 As shown, the lower layer can be made of an anode active material with a particle morphology where the major axis diameter of the particles is relatively larger than the minor axis diameter, thereby controlling the porosity of the lower layer to be low. The upper layer can be made of an anode active material with a particle morphology closer to spherical, thereby controlling the porosity of the upper layer to be high. Alternatively, the lower layer can use an anode active material with relatively low particle hardness and large particle deformation after calendering, and the upper layer can use an anode active material with relatively high particle hardness and small particle deformation after calendering, thereby achieving the aforementioned porosity.

[0077] In a more specific example, the upper and lower layers may comprise carbon-based anode active materials with distinct particle morphologies or sphericity in a calendered state. These carbon-based anode active materials may be selected from natural graphite, artificial graphite, or mixtures thereof.

[0078] It has long been known that synthetic graphite particles are harder than natural graphite particles, and that particle deformation is not significant after calendering. Therefore, the lower layer can contain a larger amount of natural graphite compared to the upper layer. As a result, after calendering, the lower layer's anode active material has a particle morphology where the major axis diameter is relatively larger than the minor axis diameter, thus enabling low porosity control. Conversely, the upper layer containing a relatively large amount of synthetic graphite allows the anode active material to maintain a near-spherical particle morphology even after calendering, thus keeping porosity at a high level.

[0079] In a more specific embodiment, the lower carbon-based negative electrode active material can be composed of natural graphite, and the upper carbon-based negative electrode active material can be composed of a mixture of natural graphite and artificial graphite, or can be composed of artificial graphite.

[0080] The above mainly describes the method of satisfying the porosity of the negative electrode active material layer, the upper layer, and the lower layer. However, even when using other negative electrode active materials such as silicon-based negative electrode active materials, the above porosity can be achieved in a similar way, taking into account the particle characteristics and particle morphology of each negative electrode active material.

[0081] On the other hand, there are no particular limitations on examples of such various negative electrode active materials, and they may include: carbon-based negative electrode active materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; (semi-)metallic materials capable of forming alloys with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and (semi-)metal oxides that can be doped with lithium and de-doped with lithium, such as SiO2. β (0<β<2), SnO2, vanadium oxides, and lithium vanadium oxides; or composite materials containing (semi-)metallic materials and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more thereof can be used. Furthermore, both low-crystallinity carbon and high-crystallinity carbon can be used as carbon-based anode active materials. Typical examples of low-crystallinity carbon include soft carbon and hard carbon. Typical examples of high-crystallinity carbon include irregular, plate-like, sheet-like, spherical, or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, high-temperature sintered carbon such as mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0082] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 80% to 99% by weight, 82% to 99% by weight, or 84% to 99% by weight.

[0083] Adhesives are components that facilitate bonding between conductive materials, active materials, and current collectors. The amount of adhesive added is typically from 0.1% to 10% by weight, based on the total weight of the negative electrode active material layer. Examples of adhesives include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0084] Conductive materials are components used to further improve the conductivity of the negative electrode active material. Based on the total weight of the negative electrode active material layer, the content of the conductive material can be from 1% to 30% by weight, 1% to 20% by weight, or 1% to 10% by weight. There are no particular limitations on the conductive material, as long as it is conductive and will not cause adverse chemical changes in the battery. For example, conductive materials such as: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0085] The negative electrode active material layer can be prepared by coating a negative electrode slurry composition onto a negative electrode current collector and drying and calendering the coated negative electrode current collector, wherein the negative electrode slurry composition is prepared by dissolving or dispersing the negative electrode active material, along with optional binders and conductive materials, in a solvent, or by casting the negative electrode slurry composition onto a separate carrier and then pressing the film layer separated from the carrier onto the negative electrode current collector and calendering it.

[0086] On the other hand, in lithium-ion secondary batteries, the separator separates the negative electrode from the positive electrode and provides a migration path for lithium ions. The separator can be used without particular restriction, as long as it is commonly used in lithium-ion secondary batteries. In particular, separators with high electrolyte retention capacity and low resistance to electrolyte ion migration can be used. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures having two or more layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Additionally, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can optionally be used.

[0087] Furthermore, the electrolyte may include: organic liquid electrolyte, inorganic liquid electrolyte, solid polymer electrolyte, gel polymer electrolyte, solid inorganic electrolyte, or molten inorganic electrolyte, which can be used to prepare lithium secondary batteries, but the present invention is not limited thereto. In this case, if the electrolyte is a solid electrolyte or gel electrolyte, it may include an electrolyte layer instead of the above-described separator, or it may combine and include the above-described separator and electrolyte layer.

[0088] On the other hand, if the lithium secondary battery includes a separator, it may further contain an electrolyte (liquid electrolyte) comprising a non-aqueous organic solvent and a lithium salt.

[0089] Organic solvents can be used without particular restrictions, as long as they can serve as a medium through which ions participating in the electrochemical reactions of the battery can migrate. Specifically, organic solvents that can be used 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 solvents, such as benzene and fluorobenzene; or carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethanol and isopropanol; and nitriles, such as R-CN (where R is a linear, branched, or cyclic C2-C). 20 The solvents can contain hydrocarbon groups (and may include double bonds, aromatic rings, or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant and low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred, as they can improve the charge / discharge performance of the battery.

[0090] Lithium salts can be used without particular restrictions, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, the anion of the lithium salt can be at least one selected from the following anions: F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N -As lithium salts, the following can be used: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. The concentration range of the lithium salt can be from 0.1 M to 4.0 M, preferably from 0.5 M to 3.0 M, and more preferably from 1.0 M to 2.0 M. When the concentration of the lithium salt is within the above range, excellent electrolyte performance can be obtained because the electrolyte can have appropriate conductivity and viscosity, and lithium ions can migrate efficiently.

[0091] To improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, in addition to the electrolyte component, the electrolyte may also contain at least one additive, such as: alkylene carbonate halide compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphonotriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted... The additives may be 0.1% by weight to 10% by weight, based on the total weight of the electrolyte. Other additives include oxaliplatinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride.

[0092] As described above, since the lithium secondary battery according to one embodiment exhibits excellent energy density, capacity characteristics, fast charging characteristics, etc., it can be used in portable devices such as mobile phones, laptops and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0093] Therefore, according to another embodiment of the present invention, a battery module comprising a lithium secondary battery as a unit battery and a battery pack comprising the same are provided.

[0094] Battery modules or battery packs can be used as a power source for at least one medium to large-sized device among the following: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0095] Embodiments of the present invention will be described in detail below, enabling those skilled in the art to readily implement them. However, the present invention can be modified in many different ways and is not limited to the embodiments set forth herein.

[0096] Examples and Comparative Examples

[0097] Example 1

[0098] First, the composition will be Li 1.16 Ni 0.305 Co 0.004 Mn 0.531 Lithium-rich manganese oxides containing O2 were used as the positive electrode active material. This positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 96.6:2.1:1.3 to prepare a positive electrode slurry (75.5% by weight solids). The positive electrode slurry was coated onto a 12 µm thick positive electrode current collector (Al film), dried, and calendered to manufacture the positive electrode.

[0099] Natural graphite was used as the negative electrode active material. This negative electrode active material, SBR-CMC as a binder, and carbon black as a conductive material were added to water as a solvent in a weight ratio of 95.7:2.3:2.0 to prepare a lower negative electrode slurry (solids content: 60% by weight).

[0100] On the other hand, except for using a mixture of natural graphite and artificial graphite in a weight ratio of 60:40 as the negative electrode active material, the upper negative electrode slurry is prepared in the same manner as the lower negative electrode slurry. Considering both the lower and upper negative electrode slurries, the weight ratio of natural graphite to artificial graphite in the negative electrode active material is 80:20.

[0101] The lower and upper negative electrode slurries were coated onto an 8 µm thick copper (Cu) film serving as the negative electrode current collector and dried. Then, the film was calendered at a rolling rate of 37.3% to prepare a negative electrode comprising a lower and upper layer of negative electrode active material layers with a total thickness of 180 µm.

[0102] An electrolyte was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 20:70:10, and then dissolving LiPF6 to a concentration of 1.2 M.

[0103] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane coated with inorganic particles (Al2O3), and a negative electrode. The assembled electrode assembly is then housed in a pouch-type battery case, and a prepared non-aqueous electrolyte is injected into it to prepare a lithium secondary battery.

[0104] Example 2

[0105] Except for the negative electrode, the positive electrode, electrolyte, and separator were prepared in the same manner as in Example 1.

[0106] Natural graphite was used as the negative electrode active material. This negative electrode active material, SBR-CMC as a binder, and carbon black as a conductive material were added to water as a solvent in a weight ratio of 95.7:2.3:2.0 to prepare a lower negative electrode slurry (solids content: 60% by weight).

[0107] On the other hand, except that only artificial graphite is used as the negative electrode active material, the upper negative electrode slurry is prepared in the same manner as the lower negative electrode slurry. Considering both the lower and upper negative electrode slurries together, the weight ratio of natural graphite to artificial graphite in the negative electrode active material is 50:50.

[0108] The lower and upper negative electrode slurries were coated onto an 8 µm thick copper (Cu) film serving as the negative electrode current collector and dried. Then, the film was calendered at a 35% calendering rate to prepare a negative electrode comprising the lower and upper layers of a negative electrode active material layer with a total thickness of 180 µm.

[0109] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane coated with inorganic particles (Al2O3), and a negative electrode. The assembled electrode assembly is then housed in a pouch-type battery case, and a prepared non-aqueous electrolyte is injected into it to prepare a lithium secondary battery.

[0110] Comparative Example 1

[0111] Except for the negative electrode, the positive electrode, electrolyte, and separator were prepared in the same manner as in Example 1.

[0112] Natural graphite was used as the negative electrode active material. This negative electrode active material, SBR-CMC as a binder, and carbon black as a conductive material were added to water as a solvent in a weight ratio of 95.7:2.3:2.0 to prepare a lower negative electrode slurry and an upper negative electrode slurry (solids content: 60 wt%) with the same composition.

[0113] The lower and upper negative electrode slurries were coated onto an 8 µm thick copper (Cu) film serving as the negative electrode current collector and dried. Then, the film was calendered at a calendering rate of 31.3% to prepare a negative electrode comprising a lower and upper layer of negative electrode active material layers with a total thickness of 180 µm.

[0114] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane coated with inorganic particles (Al2O3), and a negative electrode. The assembled electrode assembly is then housed in a pouch-type battery case, and a prepared non-aqueous electrolyte is injected into it to prepare a lithium secondary battery.

[0115] Comparative Example 2

[0116] Except for the negative electrode, the positive electrode, electrolyte, and separator were prepared in the same manner as in Example 1.

[0117] Natural graphite was used as the negative electrode active material. This negative electrode active material, SBR-CMC as a binder, and carbon black as a conductive material were added to water as a solvent in a weight ratio of 95.7:2.3:2.0 to prepare a lower negative electrode slurry and an upper negative electrode slurry (solids content: 60 wt%) with the same composition.

[0118] The lower and upper negative electrode slurries were coated onto an 8 µm thick copper (Cu) film serving as the negative electrode current collector and dried. Then, the film was calendered at a calendering rate of 29.2% to prepare a negative electrode comprising a lower and upper layer of negative electrode active material layers with a total thickness of 180 µm.

[0119] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane coated with inorganic particles (Al2O3), and a negative electrode. The assembled electrode assembly is then housed in a pouch-type battery case, and a prepared non-aqueous electrolyte is injected into it to manufacture a lithium secondary battery.

[0120] Experimental Example 1: Evaluation of the porosity of the negative electrode active material layer, upper layer, and lower layer

[0121] In the negative electrodes obtained in the examples and comparative examples, firstly, the binder and conductive material present in the negative electrode active material layer were stained, and the cross-section of the negative electrode active material layer was photographed using an electron microscope. For reference, Figure 2a and 2b These are electron microscope images of the cross-sections in the thickness direction of the negative electrode active material layer contained in the lithium secondary batteries of Comparative Examples 1 and 2, respectively. Figure 2c and 2d These are electron microscope images of the cross-sections of the negative electrode active material layers contained in the lithium secondary batteries of Examples 1 and 2 in the thickness direction.

[0122] Based on cross-sectional analysis obtained using electron microscopy, the area ratios of the negative electrode active material, binder (and conductive material), and pores were determined using image processing employing digital conversion techniques. These area ratios were calculated not only for the entire negative electrode active material layer but also for the lower layer, which is in contact with the negative electrode current collector and corresponds to 50% of the total thickness of the negative electrode active material layer, and for the remaining upper layers excluding the lower layer.

[0123] Within the same negative electrode active material layer, the pore area ratio was calculated using the same method for approximately 30 cross-sections in mutually different directions. The (average) porosity of the negative electrode active material layer, the upper layer, and the lower layer was calculated from these average values, and then the values ​​were substituted together into Equation 1 to calculate the interlayer porosity ratio.

[0124] The average porosity of the entire negative electrode active material layer, the porosity of the upper and lower layers, and the ratio of interlayer porosity derived from Equation 1 are summarized in Table 1 below.

[0125] [Table 1]

[0126] It was confirmed that Examples 1 and 2 have greater porosity in the upper layer than in the lower layer, while Comparative Example 1 has greater porosity in the lower layer, and Comparative Example 2 has similar porosity in both the upper and lower layers.

[0127] Experiment Example 2: Evaluation of Fast Charging Characteristics

[0128] For the lithium secondary batteries of the examples and comparative examples, the voltage curves of the negative electrode were compared and evaluated by a three-electrode experiment at 25°C and various C rates from 0.3C to 3C. During this comparative evaluation, lithium deposition points were identified and the time was calculated.

[0129] More specifically, the lithium (Li) deposition point is detected by differentiating the negative electrode potential from the point below 0 V on the negative electrode voltage curve. For each C rate, the State of Charge (SOC) at the point where lithium deposition occurs is determined, and charging continues only up to the ΔSOC where lithium deposition does not occur when the C rate increases from low to high in 0.1C increments. Through this process, the ΔSOC at which lithium deposition does not occur at the corresponding C rate is found, and the mathematical formula [(60 / C rate)] is used to determine this. The QC (fast charging) time is calculated using ΔSOC / 100. The calculation results are shown in Table 2 below.

[0130] [Table 2]

[0131] Referring to Table 2, it was confirmed that the lithium secondary battery of the embodiment exhibited a shorter fast charging time than the comparative example, thus demonstrating superior fast charging characteristics compared to the comparative example.

Claims

1. A lithium secondary battery, the lithium secondary battery comprising: The positive electrode contains a positive electrode active material comprising a lithium-rich manganese oxide, the lithium-rich manganese oxide comprising a layered crystal structure, wherein the ratio of the molar number of lithium to the molar number of all metals excluding lithium is greater than 1, and the content of manganese in the all metals excluding lithium is more than 50 mol%. The negative electrode includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being formed on the negative electrode current collector and comprising negative electrode active material; and The membrane or electrolyte layer between the positive electrode and the negative electrode The porosity ratio between the upper layer of the negative electrode active material layer and the remaining lower layers (excluding the upper layer) according to the following mathematical formula 1 is 1 to 1.4, and the lower layers are in contact with the negative electrode current collector and correspond to 50% of the total thickness of the negative electrode active material layer. The average porosity of the entire negative electrode active material layer is 6% to 15%. [Mathematical Expression 1] Porosity ratio (%) = (Porosity of the upper layer in the negative electrode active material layer) / (Porosity of the lower layer in the negative electrode active material layer) In Formula 1, the porosity of the upper and lower layers is calculated by the area of ​​the pores in the upper or lower layer relative to the total area when the cross-section in the thickness direction of the negative electrode active material layer is analyzed using an electron microscope.

2. The lithium secondary battery according to claim 1, wherein the lithium-rich manganese oxide is represented by the following chemical formula 1: [Chemical Formula 1] Li 1+a [Mr 1-(b+c) Ni b M c ]O 2+d in, In chemical formula 1, M contains at least one element selected from the following: Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr. 0.05≤a≤0.45,0 <b≤0.5,0≤c≤0.5,0<b+c≤0.5,0≤d≤1。 3. The lithium secondary battery according to claim 1, wherein the lithium-rich manganese oxide has a structure in which rock-salt type lithium manganese oxide is mixed with layered lithium transition metal oxide.

4. The lithium secondary battery according to claim 3, wherein the lithium-rich manganese oxide is represented by the following chemical formula 2: [Chemical Formula 2] X Li2MnO3·(1-X)Li[Ni 1-y-z-w Mr y Co z m' w ]O2 In chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' contains at least one element selected from the following: Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

5. The lithium secondary battery according to claim 1, wherein the porosity of the lower layer in the negative electrode active material layer is 5% to 15%.

6. The lithium secondary battery according to claim 1, wherein the porosity of the upper layer in the negative electrode active material layer is 6% to 18%.

7. The lithium secondary battery according to claim 1, wherein the negative electrode active material layer comprises the negative electrode active material, a conductive material, and a binder.

8. The lithium secondary battery according to claim 1, wherein in the negative electrode active material layer, the upper layer and the lower layer comprise negative electrode active materials having different compositions, particle morphologies, sphericity, or particle hardness.

9. The lithium secondary battery according to claim 1, wherein in the negative electrode active material layer, the upper layer and the lower layer comprise carbon-based negative electrode active materials having different particle morphologies or sphericity in a rolled state.

10. The lithium secondary battery according to claim 9, wherein the carbon-based negative electrode active material comprises natural graphite, artificial graphite, or a mixture thereof.

11. The lithium secondary battery of claim 10, wherein the lower layer contains a greater amount of natural graphite than the upper layer.

12. The lithium secondary battery according to claim 10, wherein the lower carbon-based negative electrode active material is made of natural graphite, and The upper carbon-based negative electrode active material is made of a mixture of natural graphite and artificial graphite, or is made of artificial graphite.

13. The lithium secondary battery according to claim 1, wherein the lithium secondary battery further comprises an electrolyte containing lithium salt and a non-aqueous organic solvent.

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