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 including a positive electrode active material including an overlithiated manganese-based oxide; a negative electrode including a negative electrode current collector and a negative electrode active material layer which is formed on the negative electrode current collector and includes a negative electrode active material; and a separator or an electrolyte layer between the positive electrode and the negative electrode, wherein the negative electrode active material layer has a porosity ratio of 1 to 1.4 between a 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 an upper layer that is not the lower layer, and the average porosity of the entire negative electrode active material layer may be 6% to 15%.

Description

lithium secondary battery

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

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0154056 filed November 4, 2024 and Korean Patent Application No. 10-2025-0142886 filed September 30, 2025, and all contents disclosed in the documents of said Korean patent applications are incorporated herein as part of this specification.

[0003] The present invention relates to a lithium secondary battery exhibiting excellent capacity characteristics and improved rapid charging characteristics.

[0004] A lithium secondary battery generally consists of a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive and negative electrodes include an active material capable of lithium ion intercalation and deintercalation.

[0005] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMnO4, etc.), and lithium iron phosphate compounds (LiFePO4) have been used as cathode active materials for lithium secondary batteries. Among these, lithium cobalt oxide has the advantage of high operating voltage and excellent capacity characteristics, but it is difficult to apply it commercially to high-capacity batteries due to the high cost and unstable supply of cobalt, which is the raw material. Lithium nickel oxide has poor structural stability, making it difficult to achieve sufficient lifespan characteristics. Meanwhile, lithium manganese oxide with a spinel structure has excellent stability but suffers from poor capacity characteristics. Accordingly, to compensate for the problems of lithium transition metal oxides containing Ni, Co, or Mn alone, lithium composite transition metal oxides containing two or more transition metals have been developed and are being used.

[0006] Among these, as it has become known that oxides containing an excess of lithium while having a higher Mn content than other metals excluding lithium (hereinafter referred to as “high-lithium manganese oxides”) can secure high energy density as high-capacity active materials, research and interest in this area are increasing significantly.

[0007] However, while lithium-rich manganese oxides have relatively high capacity and energy density, lithium secondary batteries containing them as cathode active materials have the disadvantage of insufficient rapid charging and output characteristics. This appears to be because, in addition to the above-mentioned lithium-rich manganese oxide cathode active material, a high-loading negative electrode must be applied, which may result in insufficient lithium ion mobility between the electrodes.

[0008] Accordingly, there is a need to develop a lithium secondary battery that exhibits excellent capacity characteristics and energy density unique to the above-mentioned lithium manganese-based oxide cathode active material, while also exhibiting improved rapid charging characteristics.

[0009] The present invention provides a lithium secondary battery that exhibits excellent capacity characteristics while also exhibiting improved rapid charging characteristics by increasing the movement speed of lithium ions.

[0010] According to one embodiment of the invention, an anode comprising an anode active material comprising a lithium manganese-based oxide having a layered crystal structure, wherein the molar ratio of lithium to the total number of moles of metals excluding lithium exceeds 1, and manganese among the total metals excluding lithium is contained in an amount of 50 mol% or more;

[0011] A cathode comprising a cathode current collector and a cathode active material layer formed on the cathode current collector and including a cathode active material; and

[0012] It includes a separator or electrolyte layer between the anode and the cathode, and

[0013] The above-mentioned negative electrode active material layer is in contact with the negative electrode current collector and, between the lower layer corresponding to 50% of the total thickness of the negative electrode active material layer and the remaining upper layer excluding the lower layer, the pore ratio of Formula 1 below is 1 to 1.4.

[0014] A lithium secondary battery is provided in which the average pore ratio of the entire negative electrode active material layer is 6% to 15%:

[0015] [Equation 1]

[0016] Pore ​​ratio (%) = (Pore ratio of the upper layer of the cathode active material layer) / (Pore ratio of the lower layer of the cathode active material layer)

[0017] In the above Equation 1, the pore ratio of each upper and lower layer is calculated from the area of ​​the pores relative to the total area of ​​the upper or lower layer when the cross-section in the thickness direction of the cathode active material layer is analyzed using an electron microscope.

[0018]

[0019] In a lithium secondary battery of such an embodiment, it can be represented by the following chemical formula 1:

[0020] [Chemical Formula 1]

[0021] Li 1+a [Mn 1-(b+c) Ni b M c ]O 2+d

[0022] In the above chemical formula 1, M comprises one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and 0.05≤a≤0.45, 0 <b≤0.5, 0≤c≤0.5, 0<b+c≤0.5, 0≤d≤1이고,

[0023] In addition, the above-mentioned lithium manganese-based oxide may have a structure in which rock salt-type lithium manganese oxide and layered lithium transition metal oxide are mixed, and in a more specific example, it may be represented by the following chemical formula 2:

[0024] [Chemical Formula 2]

[0025] X Li2MnO3· (1-X)Li[Ni 1-y-z-w Mn y Co z M' w ]O2

[0026] In the above chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' includes one or more selected from the group consisting of Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0027] In addition, in the lithium secondary battery of the above embodiment, among the negative electrode active material layers, the pore ratio of the lower layer may be 5% to 15%, and the pore ratio of the upper layer may be 6% to 18%.

[0028] In a battery of such an embodiment, the negative electrode active material layer may include the negative electrode active material, a conductive material, and a binder, and to satisfy the pore ratio of Formula 1, for example, in the negative electrode active material layer, the upper layer and the lower layer may each include negative electrode active materials having different compositions, particle shapes, sphericity, or particle hardness.

[0029] In a more specific example, the upper layer and the lower layer may comprise carbon-based negative electrode active materials having different particle shapes or sphericities in a rolled state, and such carbon-based negative electrode active materials may comprise natural graphite, artificial graphite, or a mixture thereof.

[0030] In a more specific embodiment, the lower layer may contain a greater amount of natural graphite than the upper layer, and in an additional embodiment, the carbon-based negative electrode active material of the lower layer may be composed of natural graphite, and the carbon-based negative electrode active material of the upper layer may be composed of a mixture of natural graphite and artificial graphite or artificial graphite.

[0031] In a lithium secondary battery of one embodiment, a negative electrode satisfying the pore ratio of calcination is combined with a positive electrode active material of a lithium-over-manganese-based oxide. In this negative electrode, the pore ratio of the upper layer is higher than that of the lower layer, so the interlayer pore ratio of Formula 1 can satisfy a certain range. It has been confirmed that when such a negative electrode is combined, even if excess lithium ions move from the lithium-over-manganese-based oxide to the negative electrode, these lithium ions diffuse very rapidly on the negative electrode, thereby exhibiting a fast lithium ion migration speed and improved rapid charging characteristics.

[0032] In addition, since such an improved rapid charging speed can be achieved even when a high-loading cathode is applied, the lithium secondary battery of one embodiment can exhibit excellent capacity characteristics and energy density, as well as improved rapid charging characteristics.

[0033] FIG. 1 is a schematic diagram illustrating the technical principle that enables rapid diffusion of lithium ions and rapid charging within the negative electrode in a lithium secondary battery of one embodiment of the invention.

[0034] Figures 2a and 2b are electron microscope images of cross-sections in the thickness direction of the negative electrode active material layer included in the lithium secondary batteries of Comparative Examples 1 and 2, respectively.

[0035] Figures 2c and 2d are electron microscope images of cross-sections in the thickness direction of the negative electrode active material layer included in the lithium secondary batteries of Examples 1 and 2, respectively.

[0036] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0037] In the following specification, “lithium manganese oxide” may refer to a lithium metal oxide that includes a layered crystal structure, has a molar ratio of lithium to the total number of moles of metals excluding lithium exceeding 1, and contains manganese in an amount of 50 mol% or more among the total number of metals excluding lithium.

[0038] Also, "D 50 " refers to the particle size at the 50% standard of the volume cumulative particle size distribution of the positive active material. The above D 50 It can be measured using the laser diffraction method. For example, after dispersing the positive active material powder in a dispersion medium, it can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), irradiated with ultrasound of about 28 kHz at an output of 60 W, obtained a volume cumulative particle size distribution graph, and then measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0039]

[0040] Specific embodiments of the invention will be described in detail below.

[0041] A lithium secondary battery according to one embodiment of the invention comprises: a positive electrode comprising a positive active material including a lithium-rich manganese oxide; a negative electrode comprising a negative current collector and a negative active material layer formed on the negative current collector and including a negative active material; and a separator or electrolyte layer between the positive electrode and the negative electrode.

[0042] The above-mentioned negative electrode active material layer is in contact with the negative electrode current collector and, between the lower layer corresponding to 50% of the total thickness of the negative electrode active material layer and the remaining upper layer excluding the lower layer, the pore ratio of Formula 1 below is 1 to 1.4, and the average pore ratio of the entire negative electrode active material layer is 6% to 15%:

[0043] [Equation 1]

[0044] Pore ​​ratio (%) = (Pore ratio of the upper layer of the cathode active material layer) / (Pore ratio of the lower layer of the cathode active material layer)

[0045] In the above Equation 1, the pore ratio of each upper and lower layer is calculated from the area of ​​the pores relative to the total area of ​​the upper or lower layer when the cross-section in the thickness direction of the cathode active material layer is analyzed using an electron microscope.

[0046] The lithium secondary battery of this embodiment basically includes the above-mentioned over-lithium manganese-based oxide as a positive electrode active material. While containing excess lithium, it has a crystal structure in which a rock salt phase and a layered structure that are mainly activated in different voltage ranges are mixed, thereby exhibiting improved capacity characteristics and energy density.

[0047] However, due to the characteristics of such over-lithium manganese-based oxides, in order to increase the overall energy density of the lithium secondary battery, it is necessary to combine them with a high-loading anode that has a large loading amount of negative electrode active material and a relatively low porosity ratio. However, when combined with such a high-loading anode, the movement speed of lithium ions between electrodes may decrease, and lithium ion precipitation may occur due to lithium ion supersaturation on the surface of the anode. As a result, the lithium secondary battery containing the above-mentioned over-lithium manganese-based oxide may exhibit relatively poor output characteristics and rapid charging characteristics.

[0048] In order to solve these disadvantages, the lithium secondary battery of one embodiment optimizes the pore ratio between the upper and lower layers defined by Equation 1 within the negative electrode active material layer to 1 to 1.4, or greater than 1 and less than or equal to 1.39, or 1.2 to 1.4, or 1.2 to 1.38, while optimizing the overall average pore ratio to 6% to 15%, or 7% to 13%, or 8% to 12%.

[0049] The range of the interlayer pore ratio in Equation 1 above can be defined such that, for example, as shown in Fig. 1, the particle shape after rolling of the cathode active material is controlled, resulting in a high pore ratio in the upper layer and a low pore ratio in the lower layer. As such, it has been confirmed that when the pore ratio of the upper layer is controlled to be high, even if excess lithium ions move from the over-lithium manganese-based oxide to the cathode, these lithium ions diffuse very rapidly on the cathode surface, thereby exhibiting a fast lithium ion migration speed and improved rapid charging characteristics.

[0050] In contrast, the pore ratio of the lower layer is controlled to be relatively low, so that the entire negative electrode active material layer exhibits a low pore ratio, thereby improving the energy density and capacity characteristics of the lithium secondary battery. In particular, this overall average pore ratio is considered in relation to the battery characteristics combined with the positive electrode active material of lithium-rich manganese oxide. If the above average pore ratio becomes excessively high, the thickness of the negative electrode becomes excessively large despite the use of the lithium-rich manganese oxide, and the energy density, defined as unit energy per electrode volume, may decrease. Conversely, if the above overall average pore ratio becomes excessively low, it becomes difficult to sufficiently increase the pore ratio of the upper layer, which may degrade the rapid charging characteristics of the lithium secondary battery.

[0051] Hereinafter, a lithium secondary battery of one embodiment will be described by each component.

[0052] The above lithium secondary battery may include a lithium-over-manganese oxide represented by the following chemical formula 1 as a positive electrode active material:

[0053] [Chemical Formula 1]

[0054] Li 1+a [Mn 1-(b+c) Ni b M c ]O 2+d

[0055] In the above chemical formula 1, M comprises one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and

[0056] 0.05≤a≤0.45, 0 <b≤0.5, 0≤c≤0.5, 0<b+c≤0.5, 0≤d≤1이고,

[0057] In this chemical formula 1, a may be 0.05 to 0.45, or 0.1 to 0.4, or 0.12 to 0.35, as the molar ratio of excess Li in the lithium-over-manganese oxide. When a satisfies the above range, high capacity characteristics and high energy density per unit volume can be achieved.

[0058] The above b is the molar ratio of Ni in the lithium-over-manganese oxide, 0 <b≤0.5, 0.10≤b≤0.40, 또는 0.2≤b≤0.39일 수 있다.

[0059] The above c is the molar ratio of additional metal M in the lithium-over-manganese oxide, and may be 0≤c≤0.5, 0≤c≤0.2, or 0.05≤c≤0.15. If the ratio of M, represented by cobalt or additional doping elements, becomes excessively large, it is difficult to secure high capacity, and gas generation and degradation of the cathode active material may worsen, leading to a decrease in lifespan characteristics.

[0060] At this time, M may be one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and in a specific example, may optionally include additional doping elements along with Co.

[0061] The above 1-(b+c) is the molar ratio of Mn in the lithium-over-manganese oxide, which may be 0.5 or more and less than 1.0, 0.53 to 0.99, or 0.55 to 0.90. If this molar ratio of Mn becomes excessively small, the proportion of rock salt phase becomes too small, resulting in a negligible effect on capacity improvement. Additionally, the above d is a value representing the ratio of oxygen excess, which can be appropriately adjusted depending on the case.

[0062] The above-mentioned lithium manganese oxide has a structure in which layered lithium transition metal oxides and rock salt-type lithium manganese oxides (e.g., Li2MnO3) are mixed, and during the initial activation process, an excess amount of lithium ions is generated as the rock salt phase is activated. In addition, an oxygen-redox reaction occurs during the activation process of the rock salt phase, and this generation of excess lithium ions and the oxygen-redox reaction can contribute to the capacity enhancement of the cathode active material.

[0063] In a more specific example, the above-mentioned lithium manganese oxide may be represented by the following chemical formula 2.

[0064] [Chemical Formula 2]

[0065] X Li2MnO3· (1-X)Li[Ni 1-y-z-w Mn y Co z M' w ]O2

[0066] In the above chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' includes one or more selected from the group consisting of Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

[0067] The above X represents the ratio of the rock salt phase (Li2MnO3) in the lithium-over-manganese oxide, and may be 0.2≤X≤0.5, 0.25≤X≤0.5, or 0.25≤X≤0.4. When the ratio of the rock salt phase (Li2MnO3) in the lithium-over-manganese oxide satisfies the above range, high capacity characteristics can be achieved.

[0068] The above y is the molar ratio of Mn in the layer, and may be 0.4≤y<1, 0.4≤y≤0.8, or 0.4≤y≤0.7.

[0069] The above z is the molar ratio of Co in the layer, and may be 0≤z≤0.1, 0≤z≤0.08, or 0≤z≤0.05. If z exceeds 0.1, gas generation and degradation of the cathode active material may be intensified, and the lifespan characteristics may be reduced.

[0070] The above w is the molar ratio of additional element M' in the layer, and may be 0≤w≤0.2, 0≤w≤0.1, or 0≤w≤0.05.

[0071] In addition, the positive electrode active material of the above-mentioned aspect may be in the form of secondary particles in which a plurality of primary particles are aggregated. In this case, the primary particles may have a particle size of 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㎛ to 10㎛, or 3㎛ to 9㎛, or 4㎛ to 7㎛.

[0072] The positive active material having the above secondary particle form is the D described above 50 By having a range, excellent anode density after rolling can be achieved, and capacitance characteristics can be improved. D of the above-mentioned anode active material 50 If this is excessively small, it is difficult to achieve high rolling density, which may lead to lower rolling characteristics and energy density, and D 50 If this is excessively large, the lithium mobility in the positive electrode active material decreases, which may increase the resistance of the lithium secondary battery containing it.

[0073] The aforementioned cathode active material can be manufactured according to a previously known method for manufacturing lithium-over-manganese oxides, so a detailed explanation regarding this will be omitted.

[0074] Next, a positive electrode containing the aforementioned positive electrode active material is described.

[0075] Such anodes include the anode active material of the lithium-over-manganese oxide described above. Specifically, the anode comprises an anode current collector and an anode active material layer formed on the anode current collector, and the anode active material layer comprises the anode active material described above. Since the anode active material has been described above, a detailed explanation is omitted, and only the remaining components will be described in detail below.

[0076] The above positive current collector may include a highly conductive metal, and is not particularly limited as long as it facilitates the adhesion of the positive active material layer and is non-reactive within the voltage range of the battery. The above positive current collector may be, for example, stainless steel, aluminum, nickel, titanium, heat-treated carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. Additionally, the above positive current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. It may be used in various forms, such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0077] The above positive active material layer may, together with the positive active material, optionally include a conductive material and a binder as needed.

[0078] At this time, the positive active material may be included in an amount of 80% to 99% by weight, more specifically 90% to 98% by weight, based on the total weight of the positive active material layer.

[0079] The above conductive material is used to impart conductivity to the electrode, and in the battery being constructed, it may be used without special limitations as long as it possesses electronic conductivity without causing chemical changes. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powder or metal fibers such as copper, nickel, aluminum, and silver; conductive tubes such as carbon nanotubes; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The above conductive material may be included in an amount of 0.01% to 10% by weight, or 0.1% to 9% by weight, or 0.1% to 5% by weight based on the total weight of the positive electrode active material layer.

[0080] The above binder serves to improve the adhesion between positive active material particles and the adhesion between the positive active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polymethyl methacrylate, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and polymers in which hydrogens thereof are substituted with Li, Na, or Ca, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 1% to 30% by weight, or 1% to 20% by weight, or 1% to 10% by weight, based on the total weight of the positive active material layer.

[0081] Meanwhile, a lithium secondary battery of one embodiment comprises a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator or electrolyte layer interposed between the positive electrode and the negative electrode, and may optionally further comprise an electrolyte.

[0082] Since the above anode is the same as previously described, only the remaining components will be described in detail below.

[0083] The above cathode includes a cathode current collector and a cathode active material layer located on the cathode current collector.

[0084] The above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, heat-treated carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector may typically have a thickness of 3 μm to 500 μm, and, similar to the positive current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0085] The above-mentioned cathode active material layer optionally includes a binder and a conductive material together with the cathode active material.

[0086] In a lithium secondary battery of one embodiment, as described above, the negative electrode active material layer may have a lower layer corresponding to 50% of the total thickness of the negative electrode active material layer in contact with the negative electrode current collector, having a smaller pore ratio than the upper layer corresponding to the remaining negative electrode active material layer. As a result, the structural characteristics defined by Equation 1, such that the pore ratio between the upper and lower layers is 1 to 1.4, greater than 1 and less than or equal to 1.39, or 1.2 to 1.4, or 1.2 to 1.38, can be satisfied. Additionally, the overall average pore ratio of the negative electrode active material layer may be 6% to 15%, or 7% to 13%, or 8% to 12%. Thus, the lithium secondary battery of one embodiment can exhibit improved rapid charging characteristics and output characteristics, along with excellent capacity and energy density.

[0087] At this time, the pore ratio of the entire cathode active material layer or the upper and lower layers can be calculated from the results of analyzing the cross-section in the thickness direction of the cathode active material layer using an electron microscope, for example, as shown in FIG. 2a to 2b. From the results of the electron microscope analysis of the cross-section in the thickness direction, cross-sectional image data can be extracted using digital transformation technology, and the pore ratio of the cathode active material layer, the upper layer, or the lower layer can be calculated from the ratio of the pore area to the total cross-sectional area. This pore ratio can be derived by averaging the values ​​calculated from at least 10, or 10 to 100, or 20 to 50 cross-sectional image data, and then deriving this as the respective pore ratio.

[0088] In a more specific example, among the negative electrode active material layers, the pore ratio of the lower layer may be 5% to 15%, or 5.5% to 13%, or 5.7% to 12%, and the pore ratio of the upper layer may be 6% to 18%, or 7% to 17%, or 8% to 16.5%. If the pore ratio of the lower layer and the upper layer becomes excessively large, the overall energy density of the lithium secondary battery may decrease, and conversely, if the pore ratio of the lower layer and the upper layer becomes excessively small, the rapid charging characteristics and output characteristics of the lithium secondary battery may decrease.

[0089] Each of the above-described pore ratios can be defined as the pore ratio of the final cathode active material layer produced by applying a slurry for forming a cathode active material layer on a cathode current collector, and then drying and rolling it.

[0090] Accordingly, to satisfy the pore ratio of the upper and / or lower layers of the above-mentioned cathode active material layer, the upper layer and the lower layer of the cathode active material layer may each include cathode active materials having different compositions, particle shapes, sphericity, or particle hardness. For example, as shown in FIG. 1, the pore ratio of the lower layer can be controlled to be small by using a cathode active material having a particle shape in which the long axis diameter is relatively larger than the short axis diameter in the lower layer, while the pore ratio of the upper layer can be controlled to be high by using a cathode active material having a particle shape close to spherical in the upper layer. Alternatively, the above-mentioned pore ratios can be achieved by using a cathode active material in the lower layer that exhibits relatively low particle hardness and exhibits large particle deformation after rolling, and a cathode active material in the upper layer that exhibits relatively high particle hardness and exhibits large particle deformation after rolling.

[0091] In a more specific example, the upper layer and the lower layer may comprise carbon-based negative electrode active materials having different particle shapes or sphericities in a rolled state, and such carbon-based negative electrode active materials may be selected from natural graphite, artificial graphite, or a mixture thereof.

[0092] It has been known for some time that artificial graphite has a higher particle hardness than natural graphite and does not undergo significant particle deformation after rolling. Accordingly, the lower layer may contain a larger amount of natural graphite than the upper layer, and as a result, the negative electrode active material of the lower layer after rolling has a particle shape in which the long axis diameter is relatively larger than the short axis diameter, thereby allowing the porosity ratio to be controlled to be low. Conversely, the upper layer containing a relatively large amount of artificial graphite maintains a particle shape close to spherical even after rolling, so the porosity ratio can be maintained at a high level.

[0093] In a more specific embodiment, the carbon-based negative electrode active material of the lower layer is composed of natural graphite, and the carbon-based negative electrode active material of the upper layer may be composed of a mixture of natural graphite and artificial graphite, or may be composed of artificial graphite.

[0094] Although the above description primarily describes a method for satisfying the pore ratios of the cathode active material layer, upper layer, and lower layer using carbon-based cathode active materials, even when using other cathode active materials such as silicon-based cathode active materials, the pore ratios described above can be achieved by a similar method by considering the particle characteristics and particle shape of each cathode active material.

[0095] Meanwhile, examples of such various cathode active materials are not particularly limited and may include carbon-based cathode active materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon, as well as metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium such as SiOβ (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the said metallic compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one or more of these may be used. In addition, as the said carbon-based cathode active material, low-crystallinity carbon and high-crystallinity carbon may both be used. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature heat-treated carbons such as petroleum and coal tar pitch-derived cokes.

[0096] The above-mentioned negative electrode active material may be included in an amount of 80% to 99% by weight, 82% to 99% by weight, or 84% to 99% by weight based on the total weight of the negative electrode active material layer.

[0097] The above binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1% to 10% by weight based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0098] The above conductive material is a component for further improving the conductivity of the negative electrode active material and may be included in an amount of 1% to 30% by weight, 1% to 20% by weight, or 1% to 10% by weight based on the total weight of the negative electrode active material layer. Such conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powders such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives may be used.

[0099] The above-mentioned cathode active material layer may be manufactured by applying a cathode slurry composition, prepared by dissolving or dispersing a cathode active material and optionally a binder and a conductive material in a solvent, onto a cathode current collector, and then drying and rolling, or by casting the cathode slurry composition onto a separate support and then laminating and rolling the film obtained by peeling it off from the support onto a cathode current collector.

[0100] Meanwhile, in the above-mentioned lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. Any separator typically used in lithium secondary batteries can be used without special restrictions, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and it may optionally be used in a single-layer or multi-layer structure.

[0101] In addition, the electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which are usable when manufacturing a lithium secondary battery, but is not limited to these. In this case, when the electrolyte is a solid electrolyte or a gel-type electrolyte, an electrolyte layer may be included to replace the separator described above, or a combination of the separator and the electrolyte layer described above may be included.

[0102] Meanwhile, if the above lithium secondary battery includes a separator, the lithium secondary battery may further include an electrolyte (liquid electrolyte) comprising a non-aqueous organic solvent and a lithium salt.

[0103] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0104] The above lithium salt may be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, as the anion of the above lithium salt, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - It may be at least one selected from the group consisting of, and the lithium salt is, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2, etc., may be used. It is preferable to use the lithium salt in a concentration range of 0.1M to 4.0M, preferably 0.5M to 3.0M, and more preferably 1.0M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0105] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, haloalkylene carbonate-based compounds like difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be included in an amount of 0.1 to 10.0 weight% based on the total weight of the electrolyte.

[0106] As the lithium secondary battery of the above-described embodiment exhibits excellent energy density, capacity characteristics, and rapid charging characteristics, it is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0107] Accordingly, according to an additional embodiment, a battery module comprising the lithium secondary battery as a unit cell and a battery pack comprising the same are provided.

[0108] The above battery module or battery pack can be used as a power source for one or more medium-to-large devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0109]

[0110] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily practice the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0111]

[0112] Examples and Comparative Examples

[0113] Example 1

[0114] First, as the cathode active material, Li 1.16 Ni 0.305 Co 0.004 Mn 0.531 A lithium-ion manganese oxide having an O2 composition was used. An anode slurry (solid content 75.5 wt%) was prepared by adding carbon black as a conductive material and polyvinylidene fluoride as a binder to N-methyl-2-pyrrolidone (NMP), a solvent, in a weight ratio of 96.6:2.1:1.3, along with the anode active material. The anode slurry was applied to an anode current collector (Al thin film) with a thickness of 12 μm, and an anode was manufactured by drying and rolling.

[0115] 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 negative electrode slurry for the lower layer (solid content: 60 wt%).

[0116] Meanwhile, the upper layer cathode slurry was prepared in the same manner as the lower layer cathode slurry, except that a mixture of natural graphite and artificial graphite in a weight ratio of 60 to 40 was used as the cathode active material. When the lower layer cathode slurry and the upper layer cathode slurry were considered together, the weight ratio of natural graphite to artificial graphite of the cathode active material was 80 to 20.

[0117] The above lower layer cathode slurry and upper layer cathode slurry were coated and dried to the same thickness on a copper (Cu) thin film, which is a cathode current collector with a thickness of 8 μm. Subsequently, a roll press was performed with a rolling rate of 37.3% to manufacture a cathode containing a lower layer and an upper layer in a cathode active material layer with a total thickness of 180 μm.

[0118] The electrolyte was prepared by mixing ethylene carbonate (EC):ethyl methyl carbonate (EMC):diethyl carbonate (DEC) in a volume ratio of 20:70:10 and then dissolving LiPF6 to a concentration of 1.2 M.

[0119] An electrode assembly was manufactured by sequentially stacking the above-mentioned anode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a cathode. The assembled electrode assembly was placed inside a pouch-type battery case, and the above-mentioned non-aqueous electrolyte was injected to manufacture a lithium secondary battery.

[0120]

[0121] Example 2

[0122] The anode, electrolyte, and separator were prepared in the same manner as in Example 1, except for the cathode.

[0123] 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 negative electrode slurry for the lower layer (solid content: 60 wt%).

[0124] Meanwhile, the upper layer cathode slurry was prepared in the same manner as the lower layer cathode slurry, except that only artificial graphite was used as the cathode active material. When the lower layer cathode slurry and the upper layer cathode slurry were considered together, the weight ratio of natural graphite to artificial graphite of the cathode active material was 50:50.

[0125] The above lower layer cathode slurry and upper layer cathode slurry were coated and dried to the same thickness on a copper (Cu) thin film, which is a cathode current collector with a thickness of 8 μm. Subsequently, a roll press was performed at a rolling rate of 35% to manufacture a cathode containing a lower layer and an upper layer in a cathode active material layer with a total thickness of 180 μm.

[0126] An electrode assembly was manufactured by sequentially stacking the above-mentioned anode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a cathode. The assembled electrode assembly was placed inside a pouch-type battery case, and the above-mentioned non-aqueous electrolyte was injected to manufacture a lithium secondary battery.

[0127]

[0128] Comparative Example 1

[0129] The anode, electrolyte, and separator were prepared in the same manner as in Example 1, except for the cathode.

[0130] 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 negative electrode slurries for the lower layer and the upper layer (solid content: 60 wt%) with the same composition.

[0131] The above lower layer cathode slurry and upper layer cathode slurry were coated and dried to the same thickness on a copper (Cu) thin film, which is a cathode current collector with a thickness of 8 μm. Subsequently, a roll press was performed with a rolling rate of 31.3% to manufacture a cathode containing a lower layer and an upper layer in a cathode active material layer with a total thickness of 180 μm.

[0132] An electrode assembly was manufactured by sequentially stacking the above-mentioned anode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a cathode. The assembled electrode assembly was placed inside a pouch-type battery case, and the above-mentioned non-aqueous electrolyte was injected to manufacture a lithium secondary battery.

[0133]

[0134] Comparative Example 2

[0135] The anode, electrolyte, and separator were prepared in the same manner as in Example 1, except for the cathode.

[0136] 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 negative electrode slurries for the lower layer and the upper layer (solid content: 60 wt%) with the same composition.

[0137] The above lower layer cathode slurry and upper layer cathode slurry were coated and dried to the same thickness on a copper (Cu) thin film, which is a cathode current collector with a thickness of 8 μm. Subsequently, a roll press was performed with a rolling rate of 29.2% to manufacture a cathode containing a lower layer and an upper layer in a cathode active material layer with a total thickness of 180 μm.

[0138] An electrode assembly was manufactured by sequentially stacking the above-mentioned anode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a cathode. The assembled electrode assembly was placed inside a pouch-type battery case, and the above-mentioned non-aqueous electrolyte was injected to manufacture a lithium secondary battery.

[0139]

[0140] Experimental Example 1: Evaluation of pore ratios in the cathode active material layer, upper layer, and lower layer

[0141] In the cathodes obtained in the above examples and comparative examples, first, the binder and conductive material present in the cathode active material layer were stained, and the cross-section of the cathode active material layer was photographed using an electron microscope. For reference, Figures 2a and 2b are electron microscope images of the cross-section in the thickness direction of the cathode active material layer included in the lithium secondary batteries of Comparative Examples 1 and 2, respectively, and Figures 2c and 2d are electron microscope images of the cross-section in the thickness direction of the cathode active material layer included in the lithium secondary batteries of Examples 1 and 2, respectively.

[0142] From the cross-sectional analysis results using the electron microscope above, the area ratios of the cathode active material, binder (and conductive material), and pores were obtained through image processing using digital transformation technology. Each of these area ratios was calculated not only for the entire cathode active material layer, but also for the lower layer which is in contact with the cathode current collector and corresponds to 50% of the total thickness of the cathode active material layer, and for the upper layer excluding the lower layer.

[0143] The area ratio of the pores was calculated in the same way for about 30 different cross-sections in the same cathode active material layer. From the average values ​​of these, the (average) pore ratios of the cathode active material layer, the upper layer, and the lower layer were each calculated, and the interlayer pore ratios were calculated together by substituting them into Equation 1.

[0144] The average pore ratio of the entire cathode active material layer derived in this way, the pore ratios of the upper and lower layers respectively, and the interlayer pore ratio derived from Equation 1 are summarized and shown in Table 1 below.

[0145] Comparative Example 1 Comparative Example 2 Example 1 Example 2 Total average porosity ratio (%) of the cathode active material layer 15.7 14.3 8.4 8.4 Porosity ratio of the upper layer (%) 14.3 14.2 9.7 9.1 Porosity ratio of the lower layer (%) 17.1 14.5 7.1 5.9 Interlayer porosity ratio of Formula 1 0.8 36 0.9 86 1.3 66 1.3 72

[0146] It was confirmed that in Examples 1 and 2 above, the upper layer has a larger pore ratio than the lower layer, whereas in Comparative Example 1, the lower layer has a larger pore ratio, and in Comparative Example 2, the pore ratios of the upper and lower layers are similar.

[0147] Experimental Example 2: Evaluation of Rapid Charging Characteristics

[0148] For the lithium secondary batteries of the above examples or comparative examples, the voltage profile of the negative electrode was compared and evaluated according to C-rate from 0.3C to 3C at 25℃ through a three-electrode experiment. During this comparative evaluation process, the time was calculated to locate the lithium plating point.

[0149] More specifically, the lithium plating point was detected by differentiating the cathode potential from a point below 0V in the cathode voltage profile. The SOC at the point where lithium plating occurs for each C-rate was identified, and charging was performed by the delta SOC at which lithium plating does not occur as the C-rate increased by 0.1 C-rate increments from the low C-rate to the high C-rate. In this process, the delta SOC at which lithium plating does not occur at the corresponding C-rate was identified, and the QC (fast charging) time was calculated using the formula “[(60 / C-rate)*delta SOC] / 100”. The results of the calculation are shown in Table 2 below.

[0150] Comparative Example 1 Comparative Example 2 Example 1 Example 2 Total average pore ratio (%) of cathode active material layer 15.7 14.3 8.4 8.4 Interlayer pore ratio of Formula 1 0.8 36 0.9 86 1.3 66 1.3 72 QC Time (min) 42 36 30 22

[0151] Referring to Table 2 above, it was confirmed that the lithium secondary battery of the example exhibited a shorter rapid charging time than the comparative example, and thus showed superior rapid charging characteristics compared to the comparative example.

Claims

1. A positive electrode comprising a positive electrode active material comprising a lithium manganese-based oxide having a layered crystal structure, wherein the molar ratio of lithium to the molar amount of all metals excluding lithium exceeds 1, and the manganese content among all metals excluding lithium is 50 mol% or more; A cathode comprising a cathode current collector and a cathode active material layer formed on the cathode current collector and including a cathode active material; and It includes a separator or electrolyte layer between the anode and the cathode, and The above negative electrode active material layer is in contact with the negative electrode current collector and between the lower layer corresponding to 50% of the total thickness of the negative electrode active material layer and the remaining upper layer excluding the lower layer, the pore ratio of Formula 1 below is 1 to 1.

4. A lithium secondary battery in which the average pore ratio of the entire negative electrode active material layer is 6% to 15%: [Equation 1] Pore ​​ratio (%) = (Pore ratio of the upper layer of the cathode active material layer) / (Pore ratio of the lower layer of the cathode active material layer) In the above Equation 1, the pore ratio of each upper and lower layer is calculated from the area of ​​the pores relative to the total area of ​​the upper or lower layer when the cross-section in the thickness direction of the cathode active material layer is analyzed using an electron microscope.

2. In claim 1, the over-lithium manganese-based oxide is a lithium secondary battery 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 the above chemical formula 1, M comprises one or more selected from the group consisting of Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr, and 0.05≤a≤0.45, 0 <b≤0.5, 0≤c≤0.5, 0<b+c≤0.5, 0≤d≤1이고, 3. In claim 1, the lithium secondary battery having a structure in which the over-lithium manganese oxide comprises a rock salt-type lithium manganese oxide and a layered lithium transition metal oxide.

4. In claim 3, the above-mentioned lithium manganese-based oxide is a positive active material 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 the above chemical formula 2, 0.2≤X≤0.5, 0.4≤y<1, 0≤z≤0.1, 0≤w≤0.2, and M' includes one or more selected from the group consisting of Fe, Cr, V, Cu, Zn, Ti, Al, Mg, B, W, Ga, In, Ru, Nb, Sn, Sr, and Zr.

5. A lithium secondary battery according to claim 1, wherein the pore ratio of the lower layer among the negative electrode active material layers is 5% to 15%.

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

7. In claim 1, the negative electrode active material layer comprises the negative electrode active material, the conductive material, and the binder, in a lithium secondary battery.

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

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

10. A 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. A lithium secondary battery according to claim 10, wherein the lower layer comprises a greater content of natural graphite than the upper layer.

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

13. A lithium secondary battery according to claim 1, further comprising an electrolyte including a lithium salt and a non-aqueous organic solvent.

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