Negative electrode and lithium secondary battery comprising same

Abstract

The present invention provides a negative electrode and a lithium secondary battery comprising same. The negative electrode includes a negative electrode mixture layer containing a Si-C composite and a graphite-based negative electrode active material, wherein the graphite-based negative electrode active material includes artificial graphite, and Y defined by Equation (1) is 3.5 or less. (1): Y = (T × S2) / P In Equation (1), T is the curvature of the negative electrode mixture layer, S is the swelling degree of the negative electrode, and P is the porosity of the negative electrode mixture layer.

Description

Negative electrode and lithium secondary battery including the same

[0001] The present application claims the benefit of priority based on Korean Patent Application No. 10-2024-0171336 filed on November 26, 2024 and Korean Patent Application No. 10-2025-0181126 filed on November 25, 2025, and all contents disclosed in said Korean patent application documents are incorporated herein as part of the specification.

[0002] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, and more specifically, to a negative electrode having excellent capacity and lifespan characteristics and a lithium secondary battery including the same.

[0003] 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.

[0004] The electrode of a lithium secondary battery is manufactured by applying an electrode slurry containing an electrode active material, a conductive material, and a binder onto an electrode current collector, drying it, rolling the electrode until it reaches a desired thickness, and vacuum drying it.

[0005] With the recent increase in demand for electric vehicles and the like, there is a growing need for batteries with high energy density and excellent rapid charging performance. Accordingly, there are active attempts to apply Si-based anode active materials as cathode materials, which have a large theoretical capacity and a fast reaction rate with lithium ions, resulting in superior rapid charging performance.

[0006] However, compared to carbon-based anode active materials, silicon-based anode active materials exhibit greater volume change during charging and discharging, leading to swelling and a deterioration in long-term lifespan characteristics. Consequently, silicon oxide (SiO), which shows relatively less volume change among silicon-based anode active materials, has been primarily used to date. However, silicon oxide-based anode active materials have lower capacity characteristics compared to other silicon-based anode active materials, such as Si or Si-C composites, which limits the increase in energy density.

[0007] The present invention aims to solve the above-mentioned problems by designing the cathode to satisfy specific conditions, thereby providing a cathode capable of realizing excellent lifespan characteristics while including a Si-C composite with excellent capacitance characteristics.

[0008] In addition, the present invention aims to provide a lithium secondary battery having excellent energy density and lifespan characteristics by including a cathode as described above.

[0009] [1] The present invention provides a cathode comprising a cathode composite layer including a Si-C composite and a graphite-based cathode active material, wherein the graphite-based cathode active material includes artificial graphite and Y defined by the following formula (1) is 3.5 or less.

[0010] Equation (1): Y = (T × S 2 ) / P

[0011] In the above equation (1), T is the curvature of the cathode composite layer, S is the swelling degree of the cathode, and P is the porosity of the cathode composite layer.

[0012] [2] The present invention provides a cathode in which Y', defined by the following formula (2), is 0.8 or less, in the above [1].

[0013] Equation (2): Y' = 10Y / W1

[0014] In Equation (2), Y is the same as defined by Equation (1), and W1 is the weight% of artificial graphite among the graphite-based cathode active materials.

[0015] [3] The present invention provides a cathode in which no peak appears between 0.4V and 0.5V in the dQ / dV graph of a coin half cell manufactured using the cathode in [1] or [2].

[0016] [3] The present invention provides a cathode in which, in at least one of [1] to [3], the Si-C composite has a structure in which Si is deposited or embedded within a carbon matrix.

[0017] [5] The present invention provides a cathode in which, in at least one of [1] to [4], the graphite-based cathode active material is a mixture of artificial graphite and natural graphite.

[0018] [6] The present invention provides a cathode in which, in at least one of [1] to [5], the Si-C composite and the graphite-based cathode active material are included in a weight ratio of 4:96 to 10:90.

[0019] [7] The present invention provides a cathode in which T is 4 or less in at least one of [1] to [6].

[0020]

[0078] The present invention provides a cathode in which S is 6.5 or less in at least one of [1] to [7].

[0021] [9] The present invention provides a cathode in which P is 20 to 35 in at least one of [1] to [8].

[0022]

[0010] The present invention provides a cathode in which W1 is 40 to 60, in accordance with [2].

[0023]

[0011] The present invention provides a lithium secondary battery comprising a cathode according to at least one of [1] to

[0010] ; a positive electrode; and an electrolyte.

[0024] The cathode according to the present invention can achieve high energy density by including a Si-C composite with excellent capacity characteristics as a cathode active material.

[0025] The cathode according to the present invention is designed so that the cathode porosity, cathode curvature, and cathode swelling degree satisfy specific conditions, and thus has excellent lifespan characteristics despite containing a Si-C composite.

[0026] Therefore, when the cathode according to the present invention is applied to a lithium secondary battery, high energy density and excellent lifespan characteristics can be realized.

[0027] Figure 1 is a dQ / dV graph of a coin-half cell with the cathodes of Examples 1 to 4 applied.

[0028] Figure 2 is a dQ / dV graph of a coin-half cell with the cathodes of Comparative Examples 1 to 4 applied.

[0029] Figure 3 is a graph showing the capacity retention rate of a coin-half cell with the cathodes of Examples 1 to 4 applied.

[0030] Figure 4 is a graph showing the capacity retention rate of coin-half cells with the cathodes of Comparative Examples 1 to 4 applied.

[0031] The present invention will be described in more detail below.

[0032] In the present invention, "swelling degree of the cathode (S)" refers to a value defined by the following formula (A).

[0033] Equation (A): S = {(t - t0) / t} × 100

[0034] In the above equation (A), t is the thickness of the test cell measured after manufacturing a test cell containing a cathode for which swelling degree is to be measured, and then performing one cycle of charge-discharge at 0.05C in a voltage range of 4.25V to 2.50V, and t0 is the thickness of the test cell before charge-discharge (t0). The test cell can be manufactured by placing a 2-stack bicell, which is stacked in the order of separator / single-sided cathode / separator / double-sided anode / separator / double-sided cathode / separator / double-sided anode / separator / single-sided cathode / separator, into a battery case and then injecting an electrolyte. Here, the single-sided cathode refers to a cathode in which a cathode composite layer is formed on one side of the cathode current collector, and the double-sided cathode refers to a cathode in which a cathode composite layer is formed on both sides of the cathode current collector. The double-sided anode refers to an anode in which an anode composite layer is formed on both sides of the anode current collector. Meanwhile, single-sided cathodes, double-sided cathodes, and double-sided anodes may be manufactured by applying an electrode slurry to one or both sides of a current collector, or they may be electrodes separated from a previously manufactured secondary battery. If the cathode separated from the secondary battery is a double-sided cathode, a single-sided cathode can be obtained by removing the cathode composite layer formed on one side of the current collector.

[0035] In the present invention, "bendability (T)" is an indicator representing the degree to which the internal flow path of the cathode composite layer is bent, and is a value defined by the following formula (B).

[0036] Equation (B): Curvature (T) = L e / L0

[0037] In the above equation (B), L0 is the theoretical resistance to lithium migration within the cathode composite layer, which can be calculated through the loading amount of the cathode composite layer, the thickness of the cathode composite layer, and the conductivity of the electrolyte. e is the actual distance that lithium ions move within the cathode composite layer, and is a value calculated by measuring the electrode pore resistance through AC impedance measurement after manufacturing a cell with a cathode / separator / cathode structure (symmetric cell).

[0038] In the present invention, "porosity (P)" is a percentage of the volume of pores relative to the total volume of the electrode composite layer, and is a value calculated by measuring the actual density of the electrode, dividing it by the theoretical electrode density, and then multiplying by 100.

[0039]

[0040] As a result of repeated research to improve the lifespan characteristics of a cathode using a Si-C composite with excellent capacitance characteristics, the inventors discovered that when the porosity, curvature, and swelling degree of the cathode satisfy specific conditions, the capacitance of the Si-C composite can be reduced in the voltage range where rapid volume expansion of the Si-C composite occurs. Consequently, the inventors found that the lifespan characteristics can be improved by suppressing cathode degradation caused by the volume expansion of the Si-C composite, and thus completed the present invention.

[0041]

[0042] cathode

[0043] First, the cathode according to the present invention will be described.

[0044] The cathode according to the present invention comprises a cathode composite layer comprising a Si-C composite and a graphite-based cathode active material. The cathode composite layer may further comprise a cathode conductive material and a cathode binder in addition to the cathode active material.

[0045] Si-C composites are materials having a structure in which silicon and carbon are combined by depositing or embedding Si within a carbon matrix. Compared to silicon oxide (SiO), which was conventionally used as a Si-based anode active material, they have lower irreversible capacity and superior conductivity. Therefore, using Si-C composites allows for the realization of higher energy density than using silicon oxide.

[0046] Preferably, the Si-C composite may have a grain size of 20 nm or less, preferably 1 nm to 20 nm, and more preferably 1 nm to 18 nm. When the grain size of the Si-C composite satisfies the above range, excellent improvement effects on cell resistance characteristics and lifespan characteristics are observed.

[0047] In addition, the above Si-C composite is D 50 This can be 1㎛ to 15㎛, preferably 2㎛ to 10㎛, more preferably 3㎛ to 10㎛. In addition, the Si-C composite is D 10 This can be 5㎛ or less, preferably 1 to 5㎛, and D 90 This can be 6㎛ to 20㎛, preferably 6㎛ to 15㎛. When the particle size distribution of the Si-C composite satisfies the above range, the cathode electrode density is increased, and high energy density can be achieved.

[0048] However, compared to silicon oxide, Si-C composites exhibit a large volume change during charging and discharging, which leads to rapid cathode degradation. Consequently, the lifespan characteristics of secondary batteries are inferior, making commercialization difficult. The volume change of Si-C composites occurs rapidly in the region where the cathode potential is 0.4 to 0.5 V. When the volume change of Si-C composites occurs, the cathode expands, destroying the conductive network, which in turn causes the cathode to rapidly degrade. Through continuous research, the inventors discovered that the capacity development of Si-C composites in the 0.4 V to 0.5 V potential range is related to cathode properties such as porosity, curvature, and swelling. They also found that if the cathode is designed so that these factors satisfy specific conditions, the volume expansion of the Si-C composite is suppressed, thereby reducing cathode degradation caused by volume expansion.

[0049] Specifically, the cathode according to the present invention may have Y defined by the following formula (1) as 3.5 or less, 1 to 3.5, or 2 to 3.5. When Y satisfies the above range, the capacity expression of the Si-C composite in the potential range of 0.4V to 0.5V is reduced, thereby minimizing the volume change of the Si-C composite.

[0050] Equation (1): Y = (T × S 2 ) / P

[0051] In the above equation (1), T is the curvature of the cathode composite layer, S is the swelling degree of the cathode, and P is the porosity of the cathode composite layer.

[0052] The curvature T of the above-mentioned cathode composite layer is an indicator representing the degree of bending of the internal flow path of the cathode composite layer, and is a value defined by the above-mentioned formula (B). The above-mentioned T may be 4 or less, preferably 1 to 4, and more preferably 1 to 3. When the T value satisfies the above range, excellent capacitance and resistance characteristics are exhibited. If the T value is too large, the lithium ion diffusion and electrolyte impregnation characteristics within the cathode composite layer deteriorate, which may lead to an increase in resistance. The curvature of the above-mentioned cathode composite layer is determined by the combined influence of factors such as the type and mixing ratio of the cathode active material used, the distribution of the cathode active material, the particle size of the cathode active material, the particle size of the cathode conductive material, and / or the porosity of the cathode composite layer, and a cathode composite layer having a desired curvature can be manufactured by appropriately adjusting the above factors.

[0053] The swelling degree S of the above-mentioned cathode is an indicator representing the change in thickness of the cathode after one cycle of charge and discharge, and is a value defined by the above-mentioned formula (A). The above-mentioned S may be 6.5 or less, preferably 6 or less, and more preferably 4 to 6. When S satisfies the above range, excellent capacity and lifespan characteristics are exhibited. If S is too large, side reactions with the electrolyte and cathode volume expansion increase, which may cause cathode degradation and degrade lifespan characteristics. The swelling degree S of the above-mentioned cathode is determined by the combined influence of factors such as the type and mixing ratio of the cathode active material used, physical properties of the cathode active material such as orientation or specific surface area, distribution of the cathode active material, and / or porosity of the cathode composite layer, and by appropriately adjusting the above factors, a cathode composite layer having a desired swelling degree can be manufactured. Here, the orientation degree (OI) of the cathode active material refers to the ratio I(004) / (110) of the peak area I(004) of the (004) plane to the peak area I(110) of the (110) plane appearing in the XRD spectrum measured by X-ray diffraction analysis of the cathode active material.

[0054] The porosity P of the above-mentioned cathode composite layer is an indicator representing the ratio of pores within the cathode composite layer. The above-mentioned P may be 20% to 35%, preferably 22% to 35%, and more preferably 23% to 30%. When P satisfies the above range, excellent capacity and lifespan characteristics are exhibited. If P is too small, lithium ion mobility is reduced, and reaction uniformity within the cathode composite layer is reduced, which may lead to a decrease in resistance and lifespan characteristics; if P is too large, capacity may decrease. The porosity of the above-mentioned cathode composite layer is determined by a complex influence of factors such as the type and mixing ratio of the cathode active material, the distribution of the cathode active material, physical properties of the cathode active material such as orientation or specific surface area, and / or the rolling rate during cathode manufacturing. By appropriately adjusting these factors, a cathode composite layer having a desired porosity can be manufactured.

[0055] In addition, the cathode may have a Y' defined by Equation (2) of 0.8 or less, preferably 0.4 to 0.75, and more preferably 0.5 to 0.7. When Y' satisfies the above range, the rate characteristics of the cathode are improved, and the capacity development of the Si-C composite at a potential of 0.4V to 0.5V can be suppressed more effectively.

[0056] Equation (2): Y' = 10Y / W1

[0057] The above W1 is a weight percentage of artificial graphite relative to the total weight of the graphite-based negative electrode active material, and may be 40 to 60, preferably 45 to 55, and more preferably 47 to 53. When W1 satisfies the above range, the effect of improving the rate characteristics and processability of the negative electrode can be obtained. Meanwhile, if the negative electrode includes a plurality of negative electrode composite layers, the above W1 refers to the weight percentage of artificial graphite among the graphite-based negative electrode active material contained in the entire plurality of negative electrode composite layers.

[0058]

[0059] Meanwhile, the cathode according to the present invention uses a graphite-based cathode active material together with a Si-C composite as the cathode active material. When a graphite-based cathode active material is used together, the volume change of the Si-C composite during charging and discharging is suppressed due to the graphite-based cathode active material, thereby improving lifespan characteristics.

[0060] The graphite-based cathode active material described above includes artificial graphite, and preferably may be a mixture of artificial graphite and natural graphite.

[0061] The artificial graphite may be included in an amount of 40% to 60% by weight, preferably 45% to 55% by weight, and more preferably 47% to 53% by weight, based on the total weight of the graphite-based negative electrode active material. When the content of artificial graphite in the graphite-based negative electrode active material satisfies the above range, the rate characteristics and processability of the negative electrode are excellent, and the capacity development of the Si-C composite at a potential of 0.4V to 0.5V can be effectively suppressed. If the content of artificial graphite is too low, the effect of suppressing the capacity development of the Si-C composite at a potential of 0.4V to 0.5V is negligible, and if it is too high, processability may be reduced during the manufacture of the negative electrode.

[0062] When the graphite-based cathode active material is a mixture of artificial graphite and natural graphite, the weight ratio of artificial graphite to natural graphite may be 40:60 to 60:40, preferably 45:55 to 55:45, more preferably 47:53 to 53:47. When the mixing ratio of artificial graphite and natural graphite satisfies the above range, the rate characteristics and processability of the cathode are excellent, and the capacity development of the Si-C composite at a potential of 0.4V to 0.5V can be effectively suppressed.

[0063]

[0064] Meanwhile, in the present invention, the Si-C composite and the graphite-based negative electrode active material may be included in a weight ratio of 4:96 to 10:90, preferably 5:95 to 10:90, and more preferably 5:95 to 8:92. When the mixing ratio of the Si-C composite and the graphite-based negative electrode active material satisfies the above range, both capacity characteristics and lifespan characteristics are excellent.

[0065] The total content of the total negative electrode active material, which is the sum of the graphite-based negative electrode active material and the Si-C composite, may be 80 to 99 weight%, preferably 85 to 99 weight%, and more preferably 90 to 99 weight% with respect to the total weight of the negative electrode composite layer.

[0066]

[0067] Meanwhile, the above-mentioned cathode conductive material is used to impart conductivity to the cathode, and can be used without special restrictions as long as it is used as a conductive material for a lithium secondary battery. Specific examples of the above-mentioned cathode conductive material include carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or 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. Preferably, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube, or a combination thereof may be used as the above-mentioned cathode conductive material, and it is more preferable to use point-type conductive materials and linear-type conductive materials together in terms of improving conductivity. At this time, the point-shaped conductive material is a material having a particle shape in which the contact form with the negative electrode active material is in the form of a point, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc., and the linear conductive material is a material in which the contact form with the negative electrode active material is in the form of a line, such as carbon fiber, carbon nanotube, etc.

[0068] The above cathode conductive material may typically be included in an amount of 1 to 30 weight%, preferably 1 to 20 weight%, and more preferably 1 to 10 weight% based on the total weight of the cathode composite layer.

[0069]

[0070] Next, the above-mentioned negative electrode binder serves to improve adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector, and any material used as a negative electrode binder for a lithium secondary battery can be used without special restrictions. Specific examples of negative electrode binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0071] The above cathode binder may be included in an amount of 1 to 30 weight%, preferably 1 to 20 weight%, more preferably 1 to 10 weight% based on the total weight of the cathode composite layer.

[0072] Meanwhile, the above-mentioned cathode composite layer may be formed as a single-layer structure or a multi-layer structure. When the cathode composite layer is a multi-layer structure composed of two or more layers, the types and / or contents of the cathode active material, cathode binder, and / or cathode conductive material in each layer may differ from one another. By forming the cathode active material layer as a multi-layer structure and varying the composition of each layer, the performance characteristics of the battery, such as rapid charging performance and output characteristics, can be appropriately controlled.

[0073]

[0074] Meanwhile, a coin half cell manufactured using the cathode according to the present invention does not show a peak between 0.4V and 0.5V in the dQ / dV graph. This means that the cathode according to the present invention exhibits almost no capacity at a potential of 0.4V to 0.5V. As described above, the 0.4V to 0.5V potential range is the range where the volume expansion of the Si-C composite occurs most rapidly. If there is no capacity development in this range, the volume expansion of the Si-C composite during charging and discharging is significantly reduced, and accordingly, the effect of improving the cathode lifespan characteristics can be obtained.

[0075]

[0076] The above-mentioned cathode can be manufactured according to a conventional cathode manufacturing method. Specifically, the cathode according to the present invention can be manufactured by preparing a Si-C composite and a graphite-based cathode active material as cathode active materials, dissolving or dispersing the cathode active material, a cathode binder, a cathode conductive material, and / or a dispersant in a solvent to prepare a cathode slurry composition, applying the cathode slurry onto a cathode current collector and then drying and rolling, or by casting the cathode slurry composition onto a separate support and then laminating the film obtained by peeling off from the support onto a cathode current collector.

[0077] At this time, the above-mentioned negative current collector may be any negative current collectors commonly used in the relevant technical field, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy may be used. 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 force 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.

[0078] As the solvent for the above-mentioned cathode slurry, common solvents used in the art for manufacturing cathode slurries, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), dimethyl formamide (DMF), acetone, water, or mixtures thereof may be used. The amount of the solvent used is sufficient to dissolve or disperse the cathode active material, conductive material, binder, and dispersant, taking into account the coating thickness and manufacturing yield of the slurry, and to have a viscosity that provides excellent thickness uniformity when coated for cathode manufacturing thereafter.

[0079]

[0080] lithium secondary battery

[0081] Next, a lithium secondary battery according to the present invention will be described.

[0082] The lithium secondary battery according to the present invention comprises a negative electrode, a positive electrode, and an electrolyte, and may further comprise a separator as needed. In this case, since the negative electrode is identical to the negative electrode according to the present invention described above, a detailed description is omitted, and the remaining components excluding the negative electrode will be described below.

[0083]

[0084] anode

[0085] The anode according to the present invention comprises an anode composite layer comprising an anode active material, an anode conductive material, and an anode binder. The anode can be manufactured by a method of forming an anode composite layer by coating an anode slurry comprising an anode active material, an anode binder, an anode conductive material, and a solvent, etc., onto an anode current collector and then rolling it.

[0086]

[0087] The above positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used.

[0088]

[0089] The above-mentioned cathode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium metal oxide may be a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), or a lithium-nickel-manganese-based oxide (e.g., LiNi1-Y Mn Y O2(here, 0 <Y<1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2(here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2(here, 0 <Y2<1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r )O2(where, 0<p<1, 0<q<1, 0<r<1, p+q+r=1) or Li(Ni p1 Co q1 Mn r1)O4 (where 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 Examples include )O2(wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r2 and s2 are each atomic fractions of independent elements, such that 0<p2<1, 0<q2<1, 0<r2<1, 0<s2<1, p2+q2+r2+s2=1), etc., and any one or more of these compounds may be included.

[0090] Specifically, the above positive active material may include a lithium transition metal oxide represented by the following [Chemical Formula 1].

[0091] [Chemical Formula 1]

[0092] Li x Ni a Co b M 1 c M 2 d O2

[0093] In the above chemical formula 1, the M 1 It is one or more selected from Mn and Al, and preferably, for durability, it may be Mn or a combination of Mn and Al.

[0094] M 2 It may be one or more selected from the group consisting of Zr, Y, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S.

[0095] The above x represents the atomic fraction of lithium in the lithium transition metal oxide, and may be 0.90≤x≤1.1, preferably 0.95≤x≤1.08, and more preferably 1.0≤x≤1.08.

[0096] The above a represents the atomic fraction of nickel among metal elements excluding lithium in the lithium transition metal oxide, and may be 0.50≤a<1.0, 0.60≤a≤0.95, 0.65≤a≤0.95, or 0.80≤a≤0.95. When the nickel content satisfies the above range, high capacity characteristics can be achieved.

[0097] The above b represents the atomic fraction of cobalt among the metal elements excluding lithium in the lithium transition metal oxide, where 0 <b<0.5, 0<b<0.4, 또는 0.01≤b≤0.3일 수 있다.

[0098] The above c is M among the metal elements excluding lithium in the lithium transition metal oxide. 1 Representing the atomic fraction of, 0 <c<0.5, 0<c<0.4, 또는 0.01≤c≤0.3일 수 있다.

[0099] The above d is M among the metal elements excluding lithium in the lithium transition metal oxide. 2 It represents the atomic fraction of , which can be 0≤d≤0.1 or 0≤d≤0.05.

[0100] The above positive active material may be included in an amount of 60 to 99 weight%, preferably 70 to 99 weight%, and more preferably 80 to 98 weight% based on the total weight of the positive composite layer.

[0101]

[0102] The above-mentioned anode binder is a component that assists in the bonding of the anode active material and the anode conductive material, as well as in the bonding to the current collector.

[0103] Examples of such anode binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers.

[0104] Typically, the anode binder may be included in an amount of 1 to 20 weight%, preferably 1 to 15 weight%, and more preferably 1 to 10 weight% based on the total weight of the anode composite layer.

[0105] The above-mentioned positive electrode conductive material is a component intended to further enhance the conductivity of the positive electrode active material, and is not particularly limited as long as it is conductive without causing chemical changes in the battery. For example, carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; fluorocarbon powder; conductive powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0106] Typically, the anode conductive material may be included in an amount of 1 to 20 weight%, preferably 1 to 15 weight%, and more preferably 1 to 10 weight% based on the total weight of the anode composite layer.

[0107]

[0108] The solvent for the anode slurry may include organic solvents such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), and acetone, and may be used in an amount that results in a desirable viscosity when including the anode active material, anode binder, and anode conductive material. For example, the concentration of the solid component, which includes the anode active material and optionally the anode binder and anode conductive material, may be 50 to 95 weight%, preferably 70 to 95 weight%, and more preferably 70 to 90 weight%.

[0109]

[0110] electrolytes

[0111] The electrolyte used in the present invention may be any of the various electrolytes usable in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., and the types thereof are not particularly limited.

[0112] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0113] 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 C2 to C20 structures 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.

[0114] The above lithium salt can be used without special limitations as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or combinations thereof. The concentration of the lithium salt is preferably used within the range of 0.1 to 5.0 M, more preferably 0.1 to 3.0 M. 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 allow lithium ions to move effectively.

[0115] Meanwhile, in addition to the above components, the electrolyte may additionally include 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. For example, the electrolyte may include at least one additive selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sulfone compounds, sulfate compounds, phosphate compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0116] Examples of the above-mentioned cyclic carbonate compounds include vinylene carbonate (VC) or vinylethylene carbonate.

[0117] Examples of the above-mentioned halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC).

[0118] Examples of the above sulfone-based compounds include at least one compound selected from the group consisting of 1,3-propane sulfone (PS), 1,4-butane sulfone, ethen sulfone, 1,3-propene sulfone (PRS), 1,4-butene sulfone, and 1-methyl-1,3-propene sulfone.

[0119] Examples of the above sulfate compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0120] Examples of the above-mentioned phosphate compounds include one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyl trimethylsilyl phosphate, trimethylsilyl phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0121] Examples of the above borate compounds include tetraphenylborate, lithium oxalyl difluoroborate (LiODFB), and lithium bisoxalate toborate (LiB(C2O4)2, LiBOB).

[0122] Examples of the above nitrile compounds include at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0123] Examples of the above benzene-based compounds include fluorobenzene, examples of the above amine-based compounds include triethanolamine or ethylenediamine, and examples of the above silane-based compounds include tetravinylsilane.

[0124] The above lithium salt-based compound is a compound different from the lithium salt included in the above-mentioned non-aqueous electrolyte, and examples include lithium difluorophosphate (LiDFP), LiPO2F2, or LiBF4.

[0125] The above additive may be included in an amount of 0.1 to 10 weight%, preferably 0.1 to 5 weight%, based on the total weight of the electrolyte.

[0126]

[0127] Separator

[0128] The above 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 any special restrictions. Specifically, the separator may be 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. Alternatively, 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.

[0129]

[0130] A lithium secondary battery according to the present invention as described above can be used to manufacture a battery pack. The battery pack comprises an assembly of lithium secondary batteries electrically connected according to the present invention and a pack housing that accommodates the same, wherein the pack housing may include a busbar for electrically connecting the lithium secondary batteries, a cooling unit, an external terminal, etc. The battery pack may be mounted in a vehicle. The vehicle may be, for example, an electric vehicle, a hybrid vehicle, or a plug-in hybrid vehicle. The vehicle includes a four-wheeled vehicle or a two-wheeled vehicle. In particular, the lithium secondary battery according to the present invention has high energy density and excellent rapid charging performance, so it can be usefully used as a battery for an electric vehicle.

[0131]

[0132] The present invention will be explained in more detail below through specific embodiments. However, the following embodiments are intended only to aid in understanding the present invention, and the scope of the present invention is not limited to these embodiments.

[0133]

[0134] Example 1

[0135] A first cathode slurry was prepared by adding cathode active material : cathode conductive material : cathode binder to distilled water in a weight ratio of 97.9 : 0.1 : 2.0.

[0136] As the cathode active material of the first cathode slurry, a Si-C composite and a graphite-based cathode active material were mixed in a weight ratio of 6:94, and as the graphite-based cathode active material, artificial graphite and natural graphite were mixed in a weight ratio of 10:90. Single-walled CNTs were used as the cathode conductive material. In addition, as the cathode binder, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed in a weight ratio of 3:2.

[0137] A second cathode slurry was prepared by adding cathode active material : cathode conductive material : cathode binder to distilled water in a weight ratio of 98.1 : 0.1 : 1.8.

[0138] As the cathode active material of the second cathode slurry, a Si-C composite and a graphite-based cathode active material were mixed in a weight ratio of 6:94 and as the graphite-based cathode active material, artificial graphite and natural graphite were mixed in a weight ratio of 90:10.

[0139] Meanwhile, natural graphite with an orientation degree I (004) / I (110) of 26 was used as the natural graphite, and single-walled CNTs were used as the cathode conductive material. In addition, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed in a weight ratio of 3:2 and used as the cathode binder.

[0140] Meanwhile, when the first cathode slurry and the second cathode slurry were combined, the weight ratio of artificial graphite to natural graphite in the graphite-based cathode active material was 50:50.

[0141] After sequentially applying the first cathode slurry and the second cathode slurry onto a copper current collector, drying and rolling were performed, and vacuum drying was carried out to produce a cathode having a porosity of 25% in the cathode composite layer. At this time, rolling was performed so that the rolling rate was 33%. At this time, the rolling rate is a value calculated by the following formula (C).

[0142] Formula (C): Rolling rate = {(Application thickness of first cathode slurry and second cathode slurry - Thickness of cathode composite layer after rolling) / Application thickness of first cathode slurry and second cathode slurry} × 100

[0143]

[0144] Example 2

[0145] A cathode with a porosity of 24.8% in the cathode composite layer was manufactured using the same method as in Example 1, except that the rolling was performed to a rolling rate of 34% during the manufacturing of the cathode.

[0146]

[0147] Example 3

[0148] A cathode with a porosity of 25.5% in the cathode composite layer was prepared in the same manner as in Example 1, except that artificial graphite and natural graphite were mixed in a weight ratio of 40:60 as the graphite-based cathode active material when preparing the first cathode slurry and the second cathode slurry.

[0149]

[0150] Example 4

[0151] A cathode with a porosity of 24.0% in the cathode composite layer was prepared in the same manner as in Example 3, except that Si-C composite and graphite-based cathode active material were mixed and used in a weight ratio of 5:95 when preparing the first cathode slurry and the second cathode slurry.

[0152]

[0153] Comparative Example 1

[0154] A cathode with a porosity of 25.3% in the cathode composite layer was prepared in the same manner as in Example 1, except that artificial graphite and natural graphite were mixed in a weight ratio of 20:80 as the graphite-based cathode active material when preparing the first cathode slurry and the second cathode slurry.

[0155]

[0156] Comparative Example 2

[0157] A cathode with a porosity of 24.8% in the cathode composite layer was prepared in the same manner as in Example 1, except that when preparing the first cathode slurry and the second cathode slurry, a mixture of artificial graphite and natural graphite was used as the graphite-based cathode active material in a weight ratio of 30:70.

[0158]

[0159]

[0160] Comparative Example 3

[0161] A cathode with a porosity of 26.8% in the cathode composite layer was manufactured in the same manner as in Example 1, except that when preparing the first cathode slurry and the second cathode slurry, a mixture of artificial graphite and natural graphite was used as the graphite-based cathode active material in a weight ratio of 40:60, natural graphite with an orientation degree I(004) / I(110) of 36 was used, and rolling was performed at a rolling rate of 38%.

[0162]

[0163] Comparative Example 4

[0164] A cathode with a porosity of 26% in the cathode composite layer was prepared using the same method as in Example 1, except that artificial graphite and natural graphite were mixed in a weight ratio of 30:70 as the graphite-based cathode active material when preparing the first cathode slurry, and artificial graphite and natural graphite were mixed in a weight ratio of 70:30 when preparing the second cathode slurry.

[0165]

[0166] Experimental Example 1: Measurement of Cathode Curvature and Swelling

[0167] The curvature and swelling of the cathodes prepared according to the above examples and comparative examples were measured in the following manner, and the Y value of Equation (1) and the Y' value of Equation (2) were calculated using the measured values. The measurement results are shown in [Table 1] below.

[0168] (1) Curvature (T): An electrode assembly was manufactured by interposing a separator between two negative electrodes, the electrode assembly was placed in a battery case, an electrolyte that does not contain lithium salt (ethylene carbonate : ethylmethyl carbonate = 3 : 7 v / v%) was injected, and the symmetric coin cell was manufactured by aging for 12 to 24 hours. Then, a current of 0.1 to 1000000 Hz and 10 Mv was applied to the manufactured symmetric coin cell, and the electrode pore resistance was measured using a graph measured by EIS (Electrochemical Impedance Spectroscopy) to calculate the average movement distance Ls of lithium ions inside the negative electrode composite layer, and the curvature was calculated by dividing this by the thickness L0 of the negative electrode composite layer.

[0169] (2) Swelling degree (S): After manufacturing a single-sided cathode and a double-sided cathode in the same manner as in the example and comparative example, a 2-stack bicell stacked in the order of separator / single-sided cathode / separator / double-sided anode / separator / double-sided cathode / separator / double-sided anode / separator / single-sided cathode / separator was placed in a battery case and an electrolyte was injected to manufacture a test cell.

[0170] The above-mentioned double-sided anode is an anode active material (LiNi 0.8 Co 0.1 Mn 0.1 O2): Anode conductive material (Super P): Anode binder (PVdF) was added to N-methylpyrrolidone in a weight ratio of 98:0.6:1.4 to prepare an anode slurry, and then the anode slurry was applied to both sides of an aluminum current collector, followed by drying and rolling. The porosity of the double-sided anode was 24.0%.

[0171] After charging and discharging the above test cell in a voltage range of 4.25V to 2.50V at 0.05C for one cycle, the thickness t0 of the test cell before charging and discharging and the thickness t of the test cell after one cycle of charging and discharging were measured, and the swelling degree S of the cathode was measured by substituting them into the following equation (A).

[0172] Equation (A): Swelling (S) = {(t - t0) / t} × 100

[0173]

[0174] Rolling rate (%) Si-C: Graphite W1PTSYY'=10Y / W1 Example 1336 : 9450252.85.53.390.68 Example 2346 : 945024.82.55.42.940.59 Example 3336 : 944025.52.75.12.750.69 Example 4335 : 954024.03.04.42.420.61 Comparative Example 1336 : 942025.35.75.77.323.66 Comparative Example 2336 : 943024.84.766.822.27 Comparative Example 3386 : 944026.82.26.83.800.95Comparative Example 4336 : 945026.02.56.54.0600.81

[0175] Experimental Example 2: dQ / dV Measurement

[0176] An electrode assembly was prepared by interposing a separator between the negative electrode and the lithium counter electrode prepared according to Examples 1 to 4 and Comparative Examples 1 to 4, and a coin-half cell was prepared by placing the electrode assembly in a battery case and injecting an electrolyte.

[0177] The above coin half cell was charged in a voltage range of 1.5 to 0.005V in 0.1C, CC mode to obtain a voltage-capacity profile, and then differentiated to obtain a dQ / dV graph.

[0178] FIG. 1 shows a dQ / dV graph of a coin-half cell with the cathodes of Examples 1 to 4, and FIG. 2 shows a dQ / dV graph of a coin-half cell with the cathodes of Comparative Examples 1 to 4.

[0179] As shown in FIG. 1, for coin-half cells with cathodes of Examples 1 to 4 designed so that Y is 3.5 or less, no peak appeared in the range of 0.4 to 0.5 V. In contrast, as shown in FIG. 2, for coin-half cells with cathodes of Comparative Examples 1 to 4 designed so that Y exceeds 3.5, it can be confirmed that a peak appeared in the range of 0.4 to 0.5 V.

[0180]

[0181] Experimental Example 3: Life Performance Evaluation

[0182] <Anode Manufacturing>

[0183] A cathode slurry was prepared by adding cathode active material, cathode conductive material, and cathode binder to N-methylpyrrolidone in a weight ratio of 98:0.6:1.4. LiNi was used as the cathode active material. 0.8 Co 0.1 Mn 0.1 O2 was used, and Super P was used as the anode conductive material. In addition, PVdF was used as the anode binder.

[0184] The above anode slurry was applied onto an aluminum current collector, and then dried and rolled to manufacture an anode.

[0185] An electrode assembly was manufactured by stacking the anode prepared as described above and the respective cathodes prepared according to Examples 1 to 4 and Comparative Examples 1 to 4 with a separator interposed therebetween and then winding them, and the electrode assembly was placed in a cylindrical battery case and then an electrolyte was injected to manufacture a cylindrical cell.

[0186] The above cylindrical cell was charged in 0.33C, CCCV mode with a maximum charging voltage of 4.25V and a cut-off condition of 1 / 20C, and then discharged to 2.5V in 1C, CC mode. The charge and discharge cycles were repeated up to 400 cycles, with 1 cycle constituting the discharge, and the capacity retention rate (%) was measured. The measurement results are shown in Figures 3 and 4.

[0187] FIG. 3 is a graph showing the capacity retention rate of a cylindrical cell with the cathodes of Examples 1 to 4, and FIG. 5 is a graph showing the capacity retention rate of a cylindrical cell with the cathodes of Comparative Examples 1 to 5.

[0188] Through FIGS. 3 and 4, it can be seen that cylindrical cells with the cathodes of Examples 1 to 4 have a higher capacity retention rate than cylindrical cells with the cathodes of Comparative Examples 1 to 4 at the same number of cycles, and that the difference in capacity retention rate becomes larger as the number of cycles increases.

Claims

1. A cathode composite layer comprising a Si-C composite and a graphite-based cathode active material, and The above graphite-based negative electrode active material includes artificial graphite, and A cathode in which Y, defined by the following equation (1), is 3.5 or less. Equation (1): Y = (T × S 2 ) / P In the above equation (1), T is the curvature of the cathode composite layer, S is the swelling degree of the cathode, and P is the porosity of the cathode composite layer.

2. In Paragraph 1, A cathode in which Y' defined by the following equation (2) is 0.8 or less. Equation (2): Y' = 10Y / W1. In Equation (2), Y is the same as defined by Equation (1), and W1 is the weight% of artificial graphite among the graphite-based cathode active materials.

3. In Paragraph 1, A cathode in which no peak appears between 0.4V and 0.5V in the dQ / dV graph of a coin half cell manufactured using the above cathode.

4. In Paragraph 1, The above Si-C composite is a cathode having a structure in which Si is deposited or embedded within a carbon matrix.

5. In Paragraph 1, The above graphite-based cathode active material is a cathode comprising artificial graphite and natural graphite.

6. In Paragraph 1, A cathode comprising the above Si-C composite and the above graphite-based cathode active material in a weight ratio of 4:96 to 10:

90.

7. In Paragraph 1, The above T is a cathode of 4 or less.

8. In Paragraph 1, The above S is a cathode with a value of 6.5 or less.

9. In Paragraph 1, The above P is a cathode with a value of 20 to 35.

10. In Paragraph 2, The above W1 is a cathode with a value of 4 to 6.

11. A lithium secondary battery comprising a negative electrode according to any one of claims 1 to 10; a positive electrode; and an electrolyte.

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