Secondary battery

Abstract

The present invention relates to a secondary battery comprising: a positive electrode comprising a positive electrode active material; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, in which the positive electrode contains a positive electrode active material containing 15 wt% or more of a first positive electrode active material in the form of a single particle, and the negative electrode contains a carbon-based active material and a silicon-based active material, and the ratio EN / EP of the weight EN of the electrolyte immersed in the negative electrode to the weight EP of the electrolyte immersed in the positive electrode is 0.9 or more.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the priority and benefit of Korean Patent Application No. 10-2022-0183720, filed on December 23, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery. Background Art

[0004] With the rapid spread of electronic devices using batteries (such as mobile phones, laptop computers, and electric vehicles), the demand for small and lightweight lithium secondary batteries with relatively high capacity has increased rapidly. In particular, with the rapid development of portable electronic devices, it is necessary to develop lithium secondary batteries with high energy density that can be used at high voltages.

[0005] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. In addition, the positive electrode and the negative electrode may have an active material layer including a positive electrode active material or a negative electrode active material on a current collector. In the positive electrode, lithium-containing metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium nickel cobalt manganese oxide are generally used as the positive electrode active material, and in the negative electrode, lithium-free carbon-based active materials and silicon-based active materials are used as the negative electrode active material.

[0006] Recently, in order to increase the capacity of the positive electrode, the use of lithium transition metal oxides with a high nickel content has been considered. In particular, in order to improve the diffusion rate of lithium ions, the material is used in the form of secondary particles. However, lithium transition metal oxides in the form of secondary particles rapidly deteriorate due to charge and discharge. To solve this problem, the use of lithium transition metal oxides in the form of single particles as the positive electrode active material has been considered.

[0007] On the other hand, in the case of the negative electrode, carbon-based active materials (such as graphite) with excellent service life performance and stability are mainly used, but silicon-based active materials are also considered to achieve high energy density and increase capacity. However, silicon-based active materials expand greatly in volume due to charge and discharge, which shortens the service life performance and increases the resistance, so they cannot be widely used. Summary of the Invention

[0008] [Technical Problem]

[0009] An object of the present invention is to provide a secondary battery with high energy density and improved service life performance.

[0010] [Technical Solution]

[0011] The present invention provides a secondary battery, comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode includes the positive electrode active material, the positive electrode active material includes 15% by weight or more of a first positive electrode active material in the form of single particles, wherein the negative electrode includes a carbon-based active material and a silicon-based active material, and wherein the weight E of the electrolyte impregnated into the negative electrode N with respect to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P is 0.9 or more.

[0012] [Advantageous Effects]

[0013] The secondary battery of the present invention includes: a positive electrode including a positive electrode active material, which includes a specific amount of a first positive electrode active material in the form of single particles; and a negative electrode including a carbon-based active material and a silicon-based active material, wherein the weight E of the electrolyte impregnated into the negative electrode N with respect to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P is within a specific range. The secondary battery according to the present invention can achieve a high energy density and exhibit excellent life performance through the above characteristics. Detailed Embodiments

[0014] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings or concepts consistent with the technical gist of the invention based on the principle that the inventor can appropriately define the concepts of the terms in order to best describe their invention.

[0015] The terms used herein are only for describing exemplary embodiments and are not intended to limit the present invention. Unless otherwise clearly specified in the context, singular expressions include plural expressions.

[0016] In this specification, the term "comprising", "provided with" or "having" is intended to specify the presence of the implemented features, numbers, steps, components or combinations thereof, and it should be understood that this does not preclude the presence or addition of other features, numbers, steps, components or combinations thereof in advance.

[0017] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size (D 50) It can be measured using, for example, the laser diffraction method. The laser diffraction method is generally used to measure particle sizes in the submicron region to several millimeters and can obtain results with high reproducibility and high resolution.

[0018] Hereinafter, the present invention will be described in detail.

[0019] <Secondary battery>

[0020] The present invention relates to a secondary battery, and more particularly to a lithium secondary battery.

[0021] Specifically, the secondary battery according to the present invention includes: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode contains a positive electrode active material, the positive electrode active material contains 15% by weight or more of a first positive electrode active material in the form of single particles, wherein the negative electrode contains a carbon-based active material and a silicon-based active material, and wherein the weight E of the electrolyte impregnated into the negative electrode N to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P is 0.9 or more.

[0022] The secondary battery according to the present invention includes: a positive electrode containing a positive electrode active material, which contains a specific amount of a first positive electrode active material in the form of single particles; and a negative electrode containing a carbon-based active material and a silicon-based active material, wherein the weight E of the electrolyte impregnated into the negative electrode N to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P is within a specific range. Since the first positive electrode active material is included in the positive electrode, the life performance of the positive electrode can be improved and the energy density can be increased. However, due to the use of the first positive electrode active material in the form of single particles, the amount of electrolyte impregnated into the positive electrode increases, and the amount of electrolyte impregnated into the negative electrode relatively decreases. The decrease in the amount of electrolyte impregnated into the negative electrode causes the life performance of the negative electrode containing the carbon-based active material and the silicon-based active material to rapidly deteriorate. To prevent this problem, the present invention adjusts the E N / E P ratio to a specific range, which ensures that the positive electrode and the negative electrode are sufficiently impregnated with the electrolyte, thereby enabling a secondary battery with high energy density and excellent life performance to be realized.

[0023] (1) Positive electrode

[0024] The positive electrode contains a positive electrode active material.

[0025] The positive electrode active material includes a first positive electrode active material in the form of single particles. The positive electrode active material contains 15% by weight or more of the first positive electrode active material.

[0026] The first positive electrode active material is in the form of single particles and has excellent durability because it is less likely to crack compared to other active materials in the form of secondary particles. However, the problem with the first positive electrode active material is that due to the osmotic pressure, a large amount of electrolyte is impregnated, which may cause an imbalance in the amount of electrolyte impregnated in the positive electrode and the negative electrode. However, as described later, the present invention can adjust the ratio of the amount of electrolyte impregnated in the negative electrode and the positive electrode so that both the negative electrode and the positive electrode are sufficiently impregnated with the electrolyte.

[0027] The term "single particle" as used herein refers to a primary structure of a single particle, which is composed of primary particles rather than secondary particles. On the other hand, the term "secondary particle" as used herein refers to an aggregate (i.e., a secondary structure) in which primary particles are held together by physical or chemical bonding without any intentional agglomeration or granulation process of the primary particles constituting the secondary particles.

[0028] The average particle diameter (D 50 ) of the first positive electrode active material may be from 0.1 μm to 5 μm, specifically from 0.5 μm to 4.5 μm, and more specifically from 1.5 μm to 4.5 μm.

[0029] The first positive electrode active material may include a lithium transition metal composite oxide represented by the following Chemical Formula 1.

[0030] [Chemical Formula 1]

[0031] Li a1 Ni b1 Co c1 M 1 d1 M 2 e1 O2

[0032] In Chemical Formula 1, M 1 may be Mn, Al, or a combination thereof, with Mn or a combination of Mn and Al being preferred.

[0033] M 2 may be one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, preferably one or more selected from the group consisting of Zr, Y, Mg, and Ti, and more preferably Zr, Y, or a combination thereof. The M 2 element is not necessarily included, but when the M 2 element is included in an appropriate amount, it can play a role in promoting the growth of crystal grains during the sintering process or improving the stability of the crystal structure.

[0034] a1 represents the molar ratio of lithium in the lithium transition metal oxide and can satisfy 0.8 ≤ a1 ≤ 1.3, 0.9 ≤ a1 ≤ 1.3, or 1.0 ≤ a1 ≤ 1.2. When the lithium molar ratio satisfies the above range, the crystal structure of the lithium transition metal oxide can be stably formed.

[0035] b1 represents the molar ratio of nickel in all metals other than lithium in the lithium transition metal oxide and can satisfy 0.8 ≤ b1 < 1, 0.82 ≤ b1 < 1, 0.83 ≤ b1 < 1, or 0.85 ≤ b1 < 1. When the nickel molar ratio satisfies the above range, a high energy density is exhibited, enabling high capacity to be achieved.

[0036] c1 represents the molar ratio of cobalt in all metals other than lithium in the lithium transition metal oxide and can satisfy 0 < c1 < 0.2, 0 < c1 < 0.18, or 0.01 ≤ c1 ≤ 0.17. When the cobalt molar ratio satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0037] d1 represents M in all metals other than lithium in the lithium transition metal oxide 1 the molar ratio of the element and can satisfy 0 < d1 < 0.2, 0 < d1 < 0.18, or 0.01 ≤ d1 ≤ 0.17. When the molar ratio of the M 1 element satisfies the above range, the structural stability of the positive electrode active material is excellent.

[0038] e1 represents M in all metals other than lithium in the lithium transition metal oxide 2 the molar ratio of the element and can satisfy 0 ≤ e1 ≤ 0.1 or 0 ≤ e1 ≤ 0.05.

[0039] The positive electrode active material contains 15% by weight or more of the first positive electrode active material. When the content of the first positive electrode active material is less than 15% by weight, the lithium mobility is significantly reduced, making it impossible to achieve the desired improvement in life performance. The positive electrode active material can contain the first positive electrode active material in an amount of 100% by weight or less. More specifically, the positive electrode active material can be composed of the first positive electrode active material.

[0040] On the basis of containing the first positive electrode active material, the positive electrode active material can further contain a second positive electrode active material in the form of secondary particles. Specifically, the positive electrode active material can be composed of the first positive electrode active material, or composed of a mixture of the first positive electrode active material and the second positive electrode active material.

[0041] When the second positive electrode active material in the form of secondary particles is used together with the first positive electrode active material in the form of single particles, it is preferred because the lithium mobility of the positive electrode can be further improved.

[0042] The second positive electrode active material may include a lithium transition metal composite oxide represented by Chemical Formula 2 below.

[0043] [Chemical Formula 2]

[0044] Li a2 Ni b2 Co c2 M 3 d2 M 4 e2 O2

[0045] In Chemical Formula 2, M 3 may be Mn, Al, or a combination thereof, with Mn or a combination of Mn and Al being preferred.

[0046] M 4 may be one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, preferably one or more selected from the group consisting of Zr, Y, Mg, and Ti, and more preferably Zr, Y, or a combination thereof. It is not necessarily included in the M 2 element, but when included in an appropriate amount, it can play a role in promoting the growth of crystal grains during the sintering process or improving the crystal structure stability.

[0047] a2 represents the molar ratio of lithium in the lithium transition metal oxide and may satisfy 0.8 ≤ a2 ≤ 1.3, 0.9 ≤ a2 ≤ 1.3, or 1.0 ≤ a2 ≤ 1.2. When the lithium molar ratio satisfies the above range, the crystal structure of the lithium transition metal oxide can be stably formed.

[0048] b2 represents the molar ratio of nickel in all metals other than lithium in the lithium transition metal oxide and may satisfy 0.8 ≤ b2 < 1, 0.82 ≤ b2 < 1, 0.83 ≤ b2 < 1, or 0.85 ≤ b2 < 1. When the nickel molar ratio satisfies the above range, a high energy density is exhibited, enabling a high capacity to be achieved.

[0049] c2 represents the molar ratio of cobalt in all metals other than lithium in the lithium transition metal oxide and may satisfy 0 < c2 < 0.2, 0 < c2 < 0.18, or 0.01 ≤ c2 ≤ 0.17. When the cobalt molar ratio satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0050] d2 represents M in all metals other than lithium in the lithium transition metal oxide 3The molar ratio of the elements and can satisfy 0 < d2 < 0.2, 0 < d2 < 0.18 or 0.01 ≤ d2 ≤ 0.17. When M 3 When the molar ratio of the elements satisfies the above range, the structural stability of the positive electrode active material can be excellent.

[0051] e2 represents the molar ratio of the M element among all the metals other than lithium in the lithium transition metal oxide and can satisfy 0 ≤ e2 ≤ 0.1 or 0 ≤ e2 ≤ 0.05. 4 The molar ratio of the elements and can satisfy 0 ≤ e2 ≤ 0.1 or 0 ≤ e2 ≤ 0.05.

[0052] The average particle size (D 50 ) of the second positive electrode active material can be greater than the average particle size (D 50 ) of the first positive electrode active material. In this case, the positive electrode active material according to the present invention has a bimodal structure, including a second positive electrode active material in the form of secondary particles with a relatively large particle size and a first positive electrode active material in the form of single particles with a relatively small particle size, thereby improving the energy density of the positive electrode, minimizing the cracking of the particles of the second positive electrode active material, improving the life performance by the introduction of the first positive electrode active material (single particles), and minimizing the reduction of lithium mobility by using relatively small particles as the first positive electrode active material.

[0053] The average particle size (D 50 ) of the second positive electrode active material can be 6 μm to 30 μm, specifically 8 μm to 20 μm.

[0054] In the present invention, the ratio of the average particle size (D 50 ) of the second positive electrode active material to the average particle size (D 50 ) of the first positive electrode active material can be 1.1 or more, specifically 2 or more, more specifically 3 or more. When the average particle sizes (D 50 ) of the first positive electrode active material and the second positive electrode active material satisfy the above range, the tap density of the positive electrode active material increases, and thus, the electrode density during electrode manufacturing is improved, and a high energy density can be achieved. In addition, since the reduction of lithium mobility caused by the use of single particles can be minimized by adjusting the average particle size (D 50 ) of the first positive electrode active material to the above range, the above range is preferred. The upper limit of the ratio of the average particle size (D 50 ) of the second positive electrode active material to the average particle size (D 50 ) of the first positive electrode active material is not particularly limited and can be, for example, 10 or less, 8 or less, 6 or less, 4 or less.

[0055] When the positive electrode active material further includes a second positive electrode active material, the weight ratio of the first positive electrode active material to the second positive electrode active material may be from 15:85 to 99:1, specifically from 20:80 to 80:20, more specifically from 25:75 to 60:40, and even more specifically from 28:72 to 45:55. Within the above ranges, the electrode density of the positive electrode can be further increased, and the life performance and lithium mobility can be improved in a balanced manner, which is desirable.

[0056] The positive electrode may include a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector.

[0057] The positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. Specifically, the positive electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, and specifically may include aluminum.

[0058] The thickness of the positive electrode current collector is generally from 3 to 500 μm.

[0059] The positive electrode current collector may have fine irregularities formed on its surface to enhance the bonding force of the positive electrode active material. For example, the positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, and non-woven fabric material.

[0060] The positive electrode active material layer may be provided on the positive electrode current collector, specifically on one or both surfaces of the positive electrode current collector.

[0061] The positive electrode active material layer may include the above positive electrode active material.

[0062] Considering the sufficient capacity of the positive electrode active material, the positive electrode active material may be included in the positive electrode active material layer in an amount of 80% by weight to 99% by weight, preferably 92% by weight to 98% by weight.

[0063] On the basis of including the positive electrode active material, the positive electrode active material layer may further include a positive electrode binder and a positive electrode conductive material.

[0064] The positive electrode binder is a component that helps the binding of the active material and the conductive material and the binding of the current collector, and specifically may include at least one selected from the group consisting of the following substances: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, and polyvinylidene fluoride is preferred.

[0065] To ensure sufficient binding force between the components including the positive electrode active material, the positive electrode binder may be included in the positive electrode active material layer in an amount of 0.1% to 10% by weight, preferably 0.1% to 3% by weight, more specifically 0.5% to 2.5% by weight.

[0066] The positive electrode conductive material can be used to assist and improve the conductivity in the secondary battery and is not particularly limited as long as it has conductivity and does not cause chemical changes. Specifically, the positive electrode conductive material may include at least one selected from the group consisting of: graphite, including natural graphite or artificial graphite; carbon black, including acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, including carbon fibers and metal fibers; carbon nanotubes, such as single-walled carbon nanotubes and multi-walled carbon nanotubes; fluorocarbons; metal powders, including aluminum powder and nickel powder; conductive whiskers, including zinc oxide and potassium titanate; conductive metal oxides, including titanium oxides; and polyphenylene derivatives. To improve the conductivity, specifically, carbon black and carbon nanotubes may be included, and more specifically, carbon black and multi-walled carbon nanotubes may be included.

[0067] To ensure sufficient conductivity, the positive electrode conductive material may be included in the positive electrode active material layer in an amount of 0.1% to 10% by weight, specifically 0.1% to 3.0% by weight, more specifically 0.5% to 2.5% by weight.

[0068] The thickness of the positive electrode active material layer may be 30 μm to 400 μm, preferably 60 μm to 200 μm.

[0069] The loading amount of the positive electrode may be 2 mAh / cm 2 to 6 mAh / cm 2 specifically 2.5 mAh / cm 2 to 4.5 mAh / cm 2 .

[0070] The porosity of the positive electrode may be 18% to 32%, specifically 20% to 30%, more specifically 20% to 25%, and further more specifically 21% to 23%.

[0071] The porosity of the positive electrode can be calculated by the following formula A.

[0072] [Formula A]

[0073] Porosity of the positive electrode (%) = {1 - (electrode density of the positive electrode / true density of the positive electrode)} × 100

[0074] In Formula A, the true density of the positive electrode is the density of the positive electrode active material layer measured by collecting a positive electrode of a certain size and pressing it with a press until the thickness of the positive electrode no longer changes, and the electrode density of the positive electrode is the density of the positive electrode active material layer measured by collecting a positive electrode of a certain size.

[0075] The positive electrode can be manufactured by the following steps: coating the positive electrode current collector with a positive electrode paste formed by including a positive electrode active material and optionally a positive electrode binder, a positive electrode conductive material, and a solvent for the positive electrode paste, followed by drying and calendering.

[0076] The solvent for forming the positive electrode paste may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that a desired viscosity is obtained when the positive electrode active material and optionally the positive electrode binder, the positive electrode conductive material, etc. are included. For example, the solvent for forming the positive electrode paste is included in the positive electrode paste such that the solid content including the positive electrode active material and optionally the positive electrode binder and the positive electrode conductive material is 50% by weight to 95% by weight, preferably 70% by weight to 90% by weight.

[0077] (2) Negative electrode

[0078] The negative electrode may be disposed facing the positive electrode.

[0079] The negative electrode includes a negative electrode active material.

[0080] The negative electrode active material includes a carbon-based active material and a silicon-based active material.

[0081] The negative electrode according to the present invention can be thin and has a high energy density by using a silicon-based active material with high capacity characteristics in addition to the carbon-based active material. In particular, when the negative electrode according to the present invention is used together with the above positive electrode, it is preferred because the secondary battery can achieve a high level of energy density. When only a carbon-based active material is used as the negative electrode active material, it is difficult to achieve a high unit volume capacity compared to using a combination of a carbon-based active material and a silicon-based active material. In order to achieve a high capacity, the thickness of the negative electrode inevitably increases, but as the thickness of the negative electrode increases, the speed of lithium insertion from the positive electrode inevitably slows down, and thus the fast charging performance cannot be improved.

[0082] The carbon-based active material may include at least one selected from the group consisting of graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and specifically may include graphite. For example, the graphite may include at least one selected from the group consisting of artificial graphite and natural graphite.

[0083] To ensure the structural stability during charge and discharge and reduce side reactions with the electrolyte, the average particle size (D 50 ) of the carbon-based active material may be 5 μm to 35 μm, preferably 10 μm to 20 μm.

[0084] The silicon-based active material may include a silicon-based compound represented by SiO x (0 ≤ x < 2). The silicon-based compound may be represented by the chemical formula SiO x (0 ≤ x < 2), and specifically may be represented by the chemical formula SiO x (0 < x < 2). Considering that SiO2 (when x = 2) does not react with lithium ions and thus cannot store lithium, x is preferably within the above range. Specifically, the silicon-based compound may be represented by the chemical formula SiO x (0.5 ≤ x ≤ 1.5).

[0085] More specifically, the silicon-based active material may include a silicon-based compound represented by SiO x (0 ≤ x < 2) and a metal doped into the silicon-based compound. Generally, a problem with silicon-based active materials is that irreversible reactions occur, where some lithium ions that have moved to the negative electrode during the initial charge cannot return to the positive electrode during discharge due to the presence of irreversible sites in the silicon-based active material. To prevent this problem, a metal can be doped into the silicon-based compound to reduce the irreversible phase of the silicon-based compound and improve efficiency.

[0086] The metal can be doped into the silicon-based compound. Specifically, the metal can be doped into the silicon-based compound and be located inside, on the surface, or both inside and on the surface of the silicon-based compound. The metal can be doped into the silicon-based compound to form a metal silicate with the silicon oxide contained in the first silicon-based compound.

[0087] The metal may include at least one metal selected from the group consisting of Li, Mg, Ca, and Al. Specifically, to control the volume expansion of silicon-based oxide particles, prevent damage, and improve the initial efficiency to a good level, it may include at least one metal selected from the group consisting of Li and Mg, and more specifically may include Mg.

[0088] Based on the total weight of the silicon-based compound and the metal, the weight of the metal may be 1 wt% to 30 wt%, specifically 5 wt% to 20 wt%. Within the above range, the irreversible capacity of the first silicon-based active material can be sufficiently removed, and a capacity reduction caused by excessive metal doping can be prevented. The metal content can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0089] The silicon-based active material may further include a carbon coating provided on the surface. The carbon coating may serve as a protective layer that suppresses the volume expansion of the silicon-based active material and prevents side reactions with the electrolyte.

[0090] The carbon coating may be included in the silicon-based active material in an amount of 0.1% to 10% by weight, preferably 3% to 7% by weight. Since the carbon coating can control the volume expansion of the silicon-based active material to a good degree and prevent side reactions with the electrolyte, this range is preferred.

[0091] The carbon coating may be an amorphous carbon coating. Specifically, the carbon coating may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0092] To ensure the structural stability of the active material during charge and discharge, and to prevent large volume expansion / contraction caused by an overly large particle size and a decrease in initial efficiency caused by an overly small particle size, the average particle size (D 50 ) of the silicon-based active material may be 1 μm to 15 μm, more preferably 2 μm to 10 μm.

[0093] The weight ratio of the carbon-based active material to the silicon-based active material may be 60:40 to 99:1, specifically 83:17 to 99:1, and more specifically 88:12 to 98:2. Within the above range, sufficient capacity of the negative electrode can be ensured while reducing the influence of the silicon-based active material on volume expansion, and a high-load negative electrode can be achieved.

[0094] The negative electrode may include a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector.

[0095] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. Specifically, the negative electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, and specifically may include copper.

[0096] The thickness of the negative electrode current collector is generally 3 to 500 μm.

[0097] The negative electrode current collector may have fine irregularities formed on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, and non-woven fabric material.

[0098] The negative electrode active material layer can be formed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer can be formed on one or both surfaces of the negative electrode current collector.

[0099] The negative electrode active material layer contains the above-mentioned negative electrode active material.

[0100] The negative electrode active material can be contained in the negative electrode active material layer in an amount of 65% to 98% by weight, specifically 80% to 95% by weight.

[0101] On the basis of containing the negative electrode active material, the negative electrode active material layer can further contain a negative electrode binder and a negative electrode conductive material.

[0102] In order to further improve electrode adhesion and provide sufficient resistance to the volume expansion / contraction of the active material, the negative electrode binder can contain at least one selected from the group consisting of: polyvinylidene fluoride (PVdF), styrene-butadiene rubber (SBR), nitrile rubber, acrylic rubber, butyl rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacrylamide (PAM), and specifically contains styrene-butadiene rubber (SBR).

[0103] The negative electrode binder can be contained in the negative electrode active material layer in an amount of 0.1% to 10% by weight, specifically 2% to 8% by weight. Within this range, it is preferred because a high-capacity negative electrode can be achieved, adhesion can be improved, and the thickness expansion of the negative electrode can be controlled.

[0104] The negative electrode conductive material can be used to improve the conductivity of the negative electrode active material layer, and preferably has conductivity without causing chemical changes. Specifically, the negative electrode conductive material can be at least one selected from the group consisting of: natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, conductive fiber, single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT), fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivative.

[0105] The conductive material can be contained in the negative electrode active material layer in an amount of 0.1% to 10% by weight. Within this range, it is preferred because while preventing the disconnection of the conductive network caused by the volume expansion of the negative electrode active material, the volume expansion caused by charge and discharge can be controlled.

[0106] The negative electrode active material layer may further include a thickener. The thickener may include carboxymethyl cellulose (CMC).

[0107] The thickener may be included in the negative electrode active material layer in an amount of 0.5 wt% to 5 wt%, but is not limited thereto.

[0108] The loading amount of the negative electrode active material layer may be 2 mAh / cm 2 to 6 mAh / cm 2 Specifically, it is 3 mAh / cm 2 to 5 mAh / cm 2 .

[0109] The thickness of the negative electrode active material layer may be 20 μm to 200 μm, specifically 50 μm to 130 μm. By using a carbon-based active material and a silicon-based active material, the negative electrode according to the present invention can be a thin negative electrode with a high energy density, and lithium insertion from the above-mentioned positive electrode is smooth, thereby improving the fast charging performance.

[0110] The negative electrode active material layer can be formed by the following steps: preparing a negative electrode slurry by adding the carbon-based active material, the silicon-based active material, and optionally a negative electrode binder, a negative electrode conductive material, and / or a thickener to a solvent (such as water), then coating the negative electrode current collector with the negative electrode slurry, and then calendering and drying.

[0111] The porosity of the negative electrode may be 20% to 40%, specifically 23% to 32%.

[0112] The porosity of the negative electrode can be calculated by the following formula B.

[0113] [Formula B]

[0114] Porosity of negative electrode (%) = {1 - (electrode density of negative electrode / true density of negative electrode)} × 100

[0115] In Formula B, the true density of the negative electrode is the density of the negative electrode active material layer measured by collecting a negative electrode of a certain size and pressing it with a press until the thickness of the negative electrode no longer changes, and the electrode density of the negative electrode is the density of the negative electrode active material layer measured by collecting a negative electrode of a certain size.

[0116] The negative electrode active material layer may have a bilayer structure. Specifically, the negative electrode active material layer may include a first negative electrode active material layer provided on the negative electrode current collector and a second negative electrode active material layer provided on the first negative electrode active material layer.

[0117] Specifically, the negative electrode active material may include a first negative electrode active material and a second negative electrode active material. The first negative electrode active material may include a first carbon-based active material and a first silicon-based active material. The second negative electrode active material may include a second carbon-based active material and a second silicon-based active material. The first negative electrode active material layer may include the first carbon-based active material and the first silicon-based active material, and the second negative electrode active material layer may include the second carbon-based active material and the second silicon-based active material.

[0118] In this specification, "the first negative electrode active material" and "the second negative electrode active material"; "the first carbon-based active material" and "the second carbon-based active material"; and "the first silicon-based active material" and "the second silicon-based active material" are terms used to describe where the carbon-based active material and the silicon-based active material are included, and the above descriptions regarding the carbon-based active material and the silicon-based active material can be equally applied.

[0119] The first negative electrode active material layer may include the first carbon-based active material and the first silicon-based active material in a weight ratio of 60:40 to 99:1, specifically 83:17 to 99:1, and more specifically 88:12 to 98:2. The second negative electrode active material layer may include the second carbon-based active material and the second silicon-based active material in a weight ratio of 60:40 to 99:1, specifically 83:17 to 99:1, and more specifically 88:12 to 98:2. Within the above range, while reducing the influence of the silicon-based active material on volume expansion, sufficient capacity of the negative electrode can be ensured and a high-load negative electrode can be achieved.

[0120] When the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, and the negative electrode active material layer includes the above-mentioned negative electrode binder, negative electrode conductive material, and / or thickening agent, the negative electrode binder may include a first negative electrode binder and a second negative electrode binder, the negative electrode conductive material may include a first negative electrode conductive material and a second negative electrode conductive material, and the thickening agent may include a first thickening agent and a second thickening agent. On the basis of including the first carbon-based active material and the first silicon-based active material, the first negative electrode active material layer may further include the first negative electrode binder, the first negative electrode conductive material, and / or the first thickening agent. On the basis of including the second carbon-based active material and the second silicon-based active material, the second negative electrode active material layer may further include the second negative electrode binder, the second negative electrode conductive material, and / or the second thickening agent.

[0121] In this specification, the "first negative electrode binder" and the "second negative electrode binder"; the "first negative electrode conductive material" and the "second negative electrode conductive material"; and the "first thickener" and the "second thickener" are terms used to describe where the negative electrode binder, the negative electrode conductive material, and the thickener are included, and the above descriptions regarding the negative electrode binder, the negative electrode conductive material, and the thickener can be equally applied.

[0122] The first negative electrode binder may be included in the first negative electrode active material layer in an amount of 0.1% by weight to 10% by weight, specifically 2% by weight to 8% by weight. The second negative electrode binder may be included in the second negative electrode active material layer in an amount of 0.1% by weight to 10% by weight, specifically 2% by weight to 8% by weight.

[0123] The first conductive material may be included in the first negative electrode active material layer in an amount of 0.1% by weight to 10% by weight. The second conductive material may be included in the second negative electrode active material layer in an amount of 0.1% by weight to 10% by weight.

[0124] The first thickener may be included in the first negative electrode active material layer in an amount of 0.5% by weight to 5% by weight. The second thickener may be included in the second negative electrode active material layer in an amount of 0.5% by weight to 5% by weight.

[0125] When the negative electrode active material layer includes the first negative electrode active material layer and the second negative electrode active material layer, the manufacture of the negative electrode may not be particularly limited as long as the first negative electrode active material layer and the second negative electrode active material layer having the above characteristics can be achieved. For example, the negative electrode according to the present invention can be manufactured as follows: dispersing a first negative electrode active material (a first carbon-based active material and a first silicon-based active material), a first binder, a first conductive material, and / or a thickener in a solvent (such as water) to prepare a slurry for the first negative electrode active material layer, dispersing a second negative electrode active material (a second carbon-based active material and a second silicon-based active material), a second binder, and / or a second conductive material in a solvent (such as water) to prepare a slurry for the second negative electrode active material layer, and then coating the negative electrode current collector with the slurry. More specifically, the negative electrode according to the present invention can be manufactured as follows: after applying the above-prepared slurry for the first negative electrode active material layer to the negative electrode current collector, calendering and drying to form the first negative electrode active material layer, applying the above-prepared slurry for the second negative electrode active material layer to the first negative electrode active material layer, calendering and drying to form the second negative electrode active material layer. In addition, the negative electrode according to the present invention can be manufactured as follows: applying the slurry for the first negative electrode active material layer to the negative electrode current collector, while applying the slurry for the second negative electrode active material layer to the slurry for the first negative electrode active material layer, and then calendering and drying.

[0126] (3) Separator

[0127] The separator separates the negative electrode and the positive electrode and provides a movement path for lithium ions. It can be used without particular limitation as long as it is commonly used as a separator in a lithium secondary battery. In particular, the separator preferably has low ion movement resistance in the electrolyte and excellent electrolyte impregnation ability. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. In addition, commonly used porous non-woven fabric materials such as non-woven fabric materials made of high melting point glass fibers and polyethylene terephthalate fibers can be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer can be used, and a single-layer or multi-layer structure can be optionally used.

[0128] (4) Electrolyte

[0129] The electrolyte can be contained in the secondary battery, and the positive electrode and the negative electrode can be impregnated with the electrolyte.

[0130] The electrolyte used in the present invention can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, an inorganic solid electrolyte, and an inorganic molten electrolyte that can be used in the manufacture of a secondary battery, but is not limited thereto.

[0131] Specifically, the electrolyte can contain an organic solvent and a lithium salt.

[0132] The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably used in the range of 0.1 M to 2.0 M. When the lithium salt concentration is within the above range, since the electrolyte has appropriate conductivity and viscosity, it exhibits excellent performance and enables effective movement of lithium ions.

[0133] The organic solvent can contain at least one selected from linear carbonates, cyclic carbonates, linear esters, cyclic esters, ethers, diglyme, and nitriles.

[0134] The linear carbonate may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0135] The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate.

[0136] Specific examples of the linear ester may include, but are not limited to, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0137] Specific examples of the cyclic ester may include, but are not limited to, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0138] Specific examples of the ether may include, but are not limited to, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL).

[0139] Specific examples of the glycol dimethyl ether may include, but are not limited to, dimethoxyethane (ethylene glycol dimethyl ether, DME), diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether (TEGDME).

[0140] Specific examples of the nitrile may include, but are not limited to, acetonitrile, propionitrile, butyronitrile, valeronitrile, octanenitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0141] More specifically, the organic solvent may include a cyclic carbonate, a linear ester, and a halogenated cyclic carbonate. In this case, not only the high dielectric constant and the effect of improving ion conductivity of the cyclic carbonate are achieved, but also the appropriate viscosity effect of the linear ester is achieved, and it is possible to simultaneously improve electrolyte impregnation and reduce the degree of reductive decomposition at high voltage, which is more preferable in terms of improving stability and life performance at high voltage. In particular, the halogenated cyclic carbonate is preferred because it can form a stable solid electrolyte interface layer (SEI layer) on the negative electrode using a silicon-based active material in the present invention.

[0142] When the organic solvent contains a cyclic carbonate, a linear ester, and a halogenated cyclic carbonate, the cyclic carbonate may include at least one selected from ethylene carbonate and propylene carbonate as the halogen-free cyclic carbonate; the linear ester may include at least one selected from ethyl propionate and propyl propionate; and the halogenated cyclic carbonate may include at least one selected from fluoroethylene carbonate and difluoroethylene carbonate.

[0143] When the organic solvent contains a cyclic carbonate, a linear ester, and a halogenated cyclic carbonate, the cyclic carbonate may be contained in the organic solvent in an amount of 10% to 50% by weight, specifically 15% to 30% by weight; the linear ester may be contained in the organic solvent in an amount of 30% to 80% by weight, specifically 40% to 70% by weight; and the halogenated cyclic carbonate may be contained in the organic solvent in an amount of 5% to 30% by weight, specifically 10% to 25% by weight.

[0144] Based on the lithium salt and the organic solvent, the electrolyte may further contain an additive.

[0145] The additive may include at least one selected from the group consisting of vinylene ethylene carbonate, propanesultone, lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiODFB), 1,3,6-hexanetricarbonitrile (HTCN), and sodium superoxide (NaO2). Specifically, it may include fluoroethylene carbonate, difluoroethylene carbonate, vinylene ethylene carbonate, propanesultone, lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiODFB), 1,3,6-hexanetricarbonitrile (HTCN), succinonitrile, 1,4-dicyano-2-butyne, adiponitrile, lithium difluorophosphate (LiPO2F2), and sodium superoxide (NaO2).

[0146] The additive may be contained in the electrolyte in an amount of 0.1% to 20% by weight, specifically 1% to 10% by weight, but is not limited thereto.

[0147] (5) E N / E P

[0148] According to the present invention, the weight E of the electrolyte impregnated into the negative electrode N to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P is 0.9 or more.

[0149] As described above, when the first positive electrode active material in the form of single particles is included in the positive electrode active material in an amount of 15% by weight or more, the amount of the electrolyte impregnated into the positive electrode increases due to the osmotic pressure. This non-uniform distribution of the electrolyte in the positive electrode causes a decrease in the amount of the electrolyte impregnated into the negative electrode. When the negative electrode is not sufficiently impregnated with the electrolyte, problems such as a decrease in charge-discharge performance and an increase in resistance cause rapid deterioration of the performance of the secondary battery. These problems are more serious especially when a silicon-based active material with poor durability is used as the negative electrode active material.

[0150] To solve the above problems, the secondary battery of the present invention uses the above positive electrode and negative electrode, and adjusts the weight E of the electrolyte impregnated into the negative electrode N to the weight E of the electrolyte impregnated into the positive electrode P ratio E N / E P to 0.9 or more. When the E N / E P ratio is less than 0.9, the amount of the electrolyte impregnated into the positive electrode increases excessively, causing an imbalance in the electrolyte impregnation between the positive electrode and the negative electrode. As a result, the life performance may deteriorate rapidly.

[0151] The E N / E P ratio may be 0.9 or more, specifically 1 or more, and more specifically 1.3 or more. When within the above range, since the positive electrode and the negative electrode are impregnated with the electrolyte in a balanced manner, a secondary battery with high energy density and significantly improved life performance can be achieved. The upper limit of the E N / E P ratio is not particularly limited, and the E N / E P ratio may be, for example, 1.6 or less, specifically 1.5 or less.

[0152] The E N / E P ratio can be adjusted by changing the thickness, loading amount, and porosity of the positive electrode and the negative electrode, and the electrolyte impregnation properties of the positive electrode and the negative electrode, but are not limited thereto.

[0153] The E N / E P ratio can be measured more than one week after injecting the electrolyte into the secondary battery, or can be measured after performing an activation process on the secondary battery, or can be measured in a state where the SOC of the secondary battery is greater than 0% and less than 30% after performing the activation process on the secondary battery. Specifically, the E N / E PThe ratio can be measured after performing an activation process on the secondary battery. Measure the E after performing an activation process on the secondary battery N / E P ratio. There are no restrictions on the conditions of the activation process. For example, measure the E in the beginning of life (BOL) state or in the state when the secondary battery is shipped after manufacture N / E P ratio.

[0154] The E N / E P ratio can be calculated in the following manner:

[0155] (a1) Separate the negative electrode and the positive electrode from the secondary battery and measure the weight of the negative electrode E N1 and the weight of the positive electrode E P1 ;

[0156] (a2) Wash the separated negative electrode and positive electrode with a solvent to remove the electrolyte, dry them, and then measure the weight of the negative electrode E N2 and the weight of the positive electrode E P2 ; and

[0157] (a3) Calculate the E N / E P ratio using Equation 1 below.

[0158] [Equation 1]

[0159] E N / E P =(E N1 -E N2 ) / (E P1 -E P2 )

[0160] In the E N / E P calculation method, a solvent such as dimethyl carbonate can be used to wash the separated negative electrode and positive electrode.

[0161] After washing the negative electrode and the positive electrode, a drying process can be performed under vacuum and at a temperature of 25°C to 55°C. The drying process can be performed for 0.5 to 12 hours.

[0162] In the secondary battery of the present invention, the N / P ratio calculated by Equation C below can be 1.0 to 1.2, preferably 1.02 to 1.10.

[0163] [Equation C]

[0164] N / P ratio = {(discharge capacity per unit area of the negative electrode) / (discharge capacity per unit area of the positive electrode)}

[0165] Specifically, the discharge capacity per unit area of the negative electrode can be obtained as follows. First, a negative electrode sample identical to the negative electrode used is prepared. After manufacturing a coin-shaped half-cell including the negative electrode sample, a lithium metal counter electrode opposite to the negative electrode, a separator interposed between the negative electrode and the lithium metal counter electrode, and an electrolyte, the discharge capacity is calculated. The discharge capacity per unit area of the negative electrode can be obtained by dividing the discharge capacity by the area of the negative electrode sample.

[0166] In addition, the discharge capacity per unit area of the positive electrode can be obtained as follows. First, a positive electrode sample identical to the positive electrode used is prepared. After manufacturing a coin-shaped half-cell including the positive electrode sample, a lithium metal counter electrode opposite to the positive electrode, a separator interposed between the positive electrode and the lithium metal counter electrode, and an electrolyte, the discharge capacity is calculated. The discharge capacity per unit area of the positive electrode can be obtained by dividing the discharge capacity by the area of the positive electrode sample.

[0167] The secondary battery may further include a battery case that houses the negative electrode, the positive electrode, the separator, and the electrolyte.

[0168] The secondary battery can be manufactured as follows: manufacturing an electrode assembly including the positive electrode, the negative electrode opposite to the positive electrode, and a separator interposed between the positive electrode and the negative electrode, housing the electrode assembly in a battery case, injecting an electrolyte into the battery case, and sealing it.

[0169] The present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same.

[0170] The battery module or the battery pack can be used as a power source for one or more medium and large-sized devices among the following devices: power tools; electric vehicles (EVs), including hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0171] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but may be cylindrical, square, pouch-shaped, or coin-shaped using a can.

[0172] The lithium secondary battery according to the present invention can be used not only as a battery cell for a small device power source, but also preferably as a unit cell in a medium and large-sized battery module including a plurality of battery cells.

[0173] As an example, the medium and large-sized device may include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, but is not limited thereto.

[0174] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in various forms and is not limited to the embodiments described herein.

[0175] Embodiments

[0176] Embodiment 1: Manufacture of secondary battery

[0177] 1. Fabrication of the positive electrode

[0178] As the first positive electrode active material, Li[Ni 0.8 Co 0.1 Mn 0.1 O2 is prepared. The first positive electrode active material is in the form of single particles and has an average particle diameter (D 50 ) of 3 μm.

[0179] As the second positive electrode active material, Li[Ni 0.8 Co 0.1 Mn 0.1 O2 is prepared. The second positive electrode active material is in the form of secondary particles and has an average particle diameter (D 50 ) of 10 μm.

[0180] The first positive electrode active material and the second positive electrode active material are mixed at a weight ratio of 50:50 to prepare a positive electrode active material.

[0181] The positive electrode active material, carbon nanotubes as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are added to N-methyl-2-pyrrolidone (NMP) as a solvent for forming a positive electrode slurry at a weight ratio of 98:1:1 to prepare a positive electrode slurry.

[0182] The positive electrode slurry is used to coat an aluminum current collector (thickness: 15 μm) as a positive electrode current collector at a loading amount of 4 mAh / cm 2 , calendered, and dried in a vacuum oven at 130 °C for 10 hours to form a positive electrode active material layer (thickness: 110 μm) and manufacture a positive electrode. The porosity of the positive electrode is adjusted to 21%.

[0183] 2. Fabrication of the negative electrode

[0184] Graphite (average particle diameter (D 50 ): 18 μm) as a carbon-based active material and SiO (average particle diameter (D 50 ): 3 μm) as a silicon-based active material are prepared. The carbon-based active material and the silicon-based active material are mixed at a weight ratio of 95:5 to prepare a negative electrode active material.

[0185] Mix the negative electrode active material, styrene-butadiene rubber (SBR) as the negative electrode binder, carboxymethyl cellulose (CMC) as the thickener, and carbon black as the negative electrode conductive material in a weight ratio of 95.3:2.5:1.2:1.0 and add them to water as the solvent to form a negative electrode slurry.

[0186] Coat a copper foil (thickness: 8 μm) serving as the negative electrode current collector with the negative electrode slurry at a loading of 4.3 mAh / cm 2 , roll it, and dry it in a vacuum oven at 130 °C for 10 hours to form a negative electrode active material layer (thickness: μm) and fabricate a negative electrode. The porosity of the negative electrode is 25%.

[0187] 3. Fabrication of a secondary battery

[0188] Fabricate the secondary battery of Example 1 by inserting a polyethylene separator between the fabricated negative electrode and positive electrode and injecting an electrolyte. As the electrolyte, use an electrolyte obtained by adding LiPF6 as a lithium salt to an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 30:70 at a concentration of 1.0 mol / L.

[0189] The N / P ratio of the secondary battery described in Example 1 is 1.08.

[0190] 4. E N / E P Measurement

[0191] Activate the fabricated secondary battery to the state at the beginning of life (BOL). Separate the negative electrode and the positive electrode from the secondary battery in the BOL state, and measure the weight E N1 of the negative electrode and the weight E P1 of the positive electrode.

[0192] Wash the separated negative electrode and separated positive electrode with a solvent (dimethyl carbonate) to remove the electrolyte, dry them under vacuum at 25 °C for 6 hours, and then measure the weight E N2 of the negative electrode and the weight E P2 of the positive electrode.

[0193] Calculate E N / E P using Equation 1 below, and its value is 1.21.

[0194] [Equation 1]

[0195] E N / E P =(E N1 -E N2 ) / (E P1 -EP2 )

[0196] Embodiment 2: Manufacture of secondary battery

[0197] A secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode active material was prepared by mixing the first positive electrode active material and the second positive electrode active material in a weight ratio of 30:70. In Example 2, the loading amount of the positive electrode paste was 4 mAh / cm 2 , the thickness of the positive electrode active material layer was 112 μm, and the porosity of the positive electrode was adjusted to 22%.

[0198] E in Example 2 N / E P was 1.46. The N / P ratio of the secondary battery of Example 2 was 1.08.

[0199] Embodiment 3: Manufacture of secondary battery

[0200] The positive electrode and the secondary battery were manufactured in the same manner as in Example 1, except that only the first positive electrode active material was used as the positive electrode active material without using the second positive electrode active material. In Example 3, the loading amount of the positive electrode paste was 4 mAh / cm 2 , the thickness of the positive electrode active material layer was 118 μm, and the porosity of the positive electrode was adjusted to 28%.

[0201] E in Example 3 N / E P was 1.25. The N / P ratio of the secondary battery of Example 3 was 1.08.

[0202] Comparative Example 1: Manufacture of secondary battery

[0203] The positive electrode and the secondary battery were manufactured in the same manner as in Example 1, except that only the first positive electrode active material was used as the positive electrode active material without using the second positive electrode active material, and the porosity of the positive electrode was adjusted to 20% by adjusting the pressing degree of the roll press. In Comparative Example 1, the loading amount of the positive electrode paste was 4 mAh / cm 2 , the thickness of the positive electrode active material layer was 112 μm, and the porosity of the positive electrode was 20%.

[0204] E in Comparative Example 1 N / E P was 0.84. The N / P ratio of the secondary battery of Comparative Example 1 was 1.08.

[0205] Comparative Example 2: Manufacture of secondary battery

[0206] A secondary battery was manufactured in the same manner as in Example 1, except that the first positive electrode active material and the second positive electrode active material were mixed at a weight ratio of 10:90 to prepare the positive electrode active material. In Comparative Example 2, the loading amount of the positive electrode paste was 4 mAh / cm 2 , and the thickness of the positive electrode active material layer was 122 μm, and the porosity of the positive electrode was adjusted to 28%.

[0207] E in Comparative Example 2 N / E P was 1.45. The N / P ratio of the secondary battery of Comparative Example 2 was 1.08.

[0208] Experimental Examples

[0209] Experimental Example 1: Measurement of capacity retention rate

[0210] Using an electrochemical charge-discharge device, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2 manufactured above were charged to 4.2 V and 1 / 40 C under the conditions of 25 °C, CC / CV, and 0.33 C, and then discharged to 2.5 V under the conditions of CC and 0.5 C as one cycle, and N charge-discharge cycles were performed.

[0211] (1) Capacity retention rate

[0212] The capacity retention rate was calculated using the following formula, and the results are shown in Table 1 below.

[0213] Capacity retention rate (%) = {(Discharge capacity after N cycles / Discharge capacity after the first cycle)} × 100

[0214] In the above formula, N is an integer of 1 or more.

[0215] (2) Resistance increase rate

[0216] After one charge-discharge cycle, the discharge capacity after the first cycle was measured using an electrochemical charge-discharge device. After adjusting the SOC to 50%, a pulse of 2.5 C was applied for 10 seconds, and the initial resistance was calculated from the voltage difference before and after the pulse application.

[0217] After N (N is an integer of 1 or more) charge-discharge cycles, the resistance after N cycles was calculated in the same manner as above, and the resistance increase rate was calculated using the following formula, and the results are shown in Table 1 below.

[0218] Resistance increase rate (%) = (Resistance after N cycles - Initial resistance) / Initial resistance × 100

[0219] (In the above formula, N is an integer of 1 or more.)

[0220]

[0221] Referring to Table 1, it can be seen that the secondary batteries according to Embodiments 1 to 3 of the present invention still have a high capacity retention rate and a low resistance increase rate even after multiple charge-discharge cycles. In particular, the secondary battery of Embodiment 2 still has significantly excellent life characteristics and resistance characteristics even after 900 charge-discharge cycles.

[0222] However, the secondary batteries of Comparative Examples 1 and 2 have a very low capacity retention rate and an excessively high resistance increase rate. Therefore, it can be confirmed that by combining the compositions of the positive and negative electrodes of the secondary battery according to the present invention with the E N / E P value, the life performance and resistance characteristics of the secondary battery can be significantly improved.

Claims

1. A secondary battery, comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode includes a positive electrode active material, wherein the positive electrode active material includes 15% by weight or more of a first positive electrode active material in the form of single particles, wherein the negative electrode includes a carbon-based active material and a silicon-based active material, and The weight E of the electrolyte impregnated into the negative electrode N to the weight E of the electrolyte impregnated into the positive electrode P The ratio E N / E P is 0.9 or more.

2. The secondary battery according to claim 1, wherein an average particle diameter (D 50 ) of the first positive electrode active material is from 0.1 μm to 5 μm.

3. The secondary battery according to claim 1, wherein the first positive electrode active material includes a lithium transition metal composite oxide represented by the following Chemical Formula 1: [Chemical Formula 1] Li a1 Ni b1 Co c1 M 1 d1 M 2 e1 O2 In Chemical Formula 1, M 1 is Mn, Al, or a combination thereof, M 2 is one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, a1, b1, c1, d1, and e1 are atomic fractions of respective independent elements and satisfy 0.8 ≤ a1 ≤ 1.3, 0.8 ≤ b1 < 1, 0 < c1 < 0.2, 0 < d1 < 0.2, 0 ≤ e1 ≤ 0.1, and b1 + c1 + d1 + e1 = 1.

4. The secondary battery according to claim 1, wherein the positive electrode active material further includes a second positive electrode active material in the form of secondary particles.

5. The secondary battery according to claim 4, wherein the second positive electrode active material includes a lithium transition metal composite oxide represented by the following Chemical Formula 2: [Chemical Formula 2] Li a2 Ni b2 Co c2 M 3 d2 M 4 e2 O2 In Chemical Formula 1, M 3 is Mn, Al or a combination thereof, M 4 is one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, a2, b2, c2, d2, and e2 are atomic fractions of respective independent elements and satisfy 0.8 ≤ a2 ≤ 1.3, 0.8 ≤ b2 < 1, 0 < c2 < 0.2, 0 < d2 < 0.2, 0 ≤ e2 ≤ 0.1, and b2 + c2 + d2 + e2 = 1.

6. The secondary battery according to claim 4, wherein the weight ratio of the first positive electrode active material to the second positive electrode active material is 15:85 to 99:

1.

7. The secondary battery according to claim 4, wherein an average particle diameter (D 50 ) of the second positive electrode active material is larger than an average particle diameter (D 50 ) of the first positive electrode active material.

8. The secondary battery according to claim 4, wherein a ratio of an average particle diameter (D 50 ) of the second positive electrode active material to an average particle diameter (D 50 ) of the first positive electrode active material is 1.1 or more.

9. The secondary battery according to claim 4, wherein an average particle diameter (D 50 ) of the second positive electrode active material is from 6 μm to 30 μm.

10. The secondary battery according to claim 1, wherein the porosity of the positive electrode is 18% to 32%.

11. The secondary battery according to claim 1, wherein the carbon-based active material includes at least one selected from the group consisting of graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon.

12. The secondary battery according to claim 1, wherein the silicon-based active material contains a silicon-based compound represented by SiO x (0 ≤ x < 2).

13. The secondary battery according to claim 1, wherein the weight ratio of the carbon-based active material to the silicon-based active material is 60:40 to 99:

1.

14. The secondary battery according to claim 1, wherein the porosity of the negative electrode is 20% to 40%.

15. The secondary battery according to claim 1, wherein the N / P ratio calculated by the following Equation C is 1.0 to 1.2, [Equation C] The N / P ratio = {(discharge capacity per unit area of the negative electrode) / (discharge capacity per unit area of the positive electrode)}.

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