Positive electrode and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode and a lithium secondary battery comprising same, the positive electrode comprising a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, a conductive material, and a binder, the positive electrode active material comprises a lithium iron phosphate-based compound, the conductive material comprises carbon black, and R represented by formula 1 below has a value of 500 to 1300. [Formula 1] R = {[oxygen content value of conductive material] × [BET specific surface area value of conductive material]} / [average particle diameter (D50) value of positive electrode active material] In formula 1, each of the oxygen content value (atomic %) of the conductive material, the BET specific surface area value (m2 / g) of the conductive material, and the average particle diameter (D50) value (㎛) of the positive electrode active material is a dimensionless number with units omitted.

Description

Anode and lithium secondary battery including the same

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0131919 filed on September 27, 2024, and all contents disclosed in said Korean Patent Application are incorporated herein as part of this specification.

[0002] The present invention relates to a positive electrode and a lithium secondary battery including the same.

[0003] A lithium secondary battery is generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode containing a positive active material containing lithium and a negative electrode containing a negative active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting a non-aqueous electrolyte that serves as a medium for transmitting lithium ions, and then sealing it.

[0004] These lithium secondary batteries are used in portable electronic devices such as mobile phones and laptops, as well as in electric vehicles, and demand is surging with the recent expansion of electric vehicle adoption. In particular, lithium secondary batteries used in electric vehicles require high energy density and excellent charging rate characteristics.

[0005] Meanwhile, lithium cobalt-based oxides, lithium nickel-based oxides, lithium manganese-based oxides, lithium nickel-cobalt-manganese-based oxides, lithium manganese phosphate compounds, and lithium iron phosphate compounds are used as positive electrode active materials for lithium secondary batteries.

[0006] Among these, lithium iron phosphate compounds are widely used as cathode active materials for lithium secondary batteries due to their excellent thermal stability, superior lifespan characteristics, safety, and low cost; however, compared to ternary cathode active materials, their relatively smaller particle size makes it difficult to form a conductive network, resulting in reduced conductivity.

[0007] Accordingly, there is a need to develop lithium secondary batteries containing lithium iron phosphate compounds that exhibit excellent thermal stability and superior lifespan characteristics despite the use of small-sized lithium iron phosphate compounds.

[0008] The present invention relates to an average particle size (D) of a positive electrode active material comprising a lithium iron phosphate-based compound. 50 The present invention aims to provide a cathode with improved lifespan characteristics and a lithium secondary battery containing the same by ensuring that the BET specific surface area and oxygen content of a conductive material including carbon black satisfy a specific equation.

[0009] [1] The present invention provides a positive electrode comprising a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, a conductive material and a binder, wherein the positive electrode active material comprises a lithium iron phosphate-based compound and the conductive material comprises carbon black, and the value of R represented by Formula 1 below is 500 to 1300.

[0010] [Equation 1]

[0011] R = {[Oxygen content of conductive material] × [BET specific surface area of ​​conductive material]} / [Average particle size of cathode active material (D 50 ) value]

[0012] In Equation 1 above, the oxygen content value (atomic%) of the conductive material, and the BET specific surface area value (m) of the conductive material 2 / g) and average particle size of the positive active material (D 50 Each of the ) values ​​(㎛) is a unitless number with the unit omitted.

[0013] [2] In the present invention, the BET specific surface area of ​​the conductive material is 200 m² in [1]. 2 / g to 450m 2 Provides a positive electrode of / g.

[0014] [3] The present invention provides an anode in which, in [1] or [2], the oxygen content of the conductive material is 2.0 atomic% to 4.5 atomic%.

[0015] [4] The present invention, in at least one of [1] to [3], wherein the average particle size (D) of the conductive material is 50 ) provides an anode having a diameter of 0.7㎛ to 1.3㎛.

[0016] [5] The present invention, in at least one of [1] to [4], wherein the average particle size (D) of the positive electrode active material is 50 ) provides an anode having a diameter of 0.6㎛ to 1.6㎛.

[0017] [6] The present invention provides an anode represented by the following chemical formula 1, wherein at least one of [1] to [5] above is a lithium iron phosphate compound.

[0018] [Chemical Formula 1]

[0019] Li 1+x [Fe 1-y M y ]PO4

[0020] In the above chemical formula 1, M comprises one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V, and -0.5≤x≤0.5, 0≤y<1.

[0021] [7] The present invention provides an anode in which, in at least one of [1] to [6], the conductive material is included in an amount of 0.5% to 5.0% by weight based on the total weight of the anode active material layer.

[0022] [8] The present invention provides a positive electrode in which, in at least one of [1] to [7], the positive electrode active material further comprises a conductive coating layer.

[0023] [9] The present invention provides a positive electrode in which, in at least one of [1] to [8], the weight ratio of the positive electrode active material and the conductive material is 90:10 to 99:1.

[0024]

[0010] The present invention provides an anode comprising at least one of [1] to [9], wherein the binder comprises one or more selected from the group consisting of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC).

[0025]

[0011] The present invention provides an anode having a porosity of 15% to 40% in at least one of [1] to

[0010] .

[0026]

[0012] The present invention provides a lithium secondary battery comprising a positive electrode according to at least one of [1] to

[0011] .

[0027] The anode according to the present invention has an average particle size (D 50 When mixing a positive electrode active material containing a small lithium iron phosphate-based compound with a conductive material containing carbon black, the BET specific surface area of ​​the conductive material is controlled to maintain uniform dispersibility, and the oxygen content with high electronegativity is controlled to increase the dispersibility of the conductive material in a polar solvent such as N-methylpyrrolidone (NMP). As a result, the contact area between the positive electrode active material containing the lithium iron phosphate-based compound and the conductive material containing carbon black is increased, which facilitates the formation of a conductive network, and consequently, excellent lifespan characteristics of the positive electrode and lithium secondary battery containing the same can be exhibited.

[0028] The present invention will be described in more detail below.

[0029] Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0030] In the present invention, terms such as “comprising,” “having,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should be understood as not excluding in advance the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0031] In the present invention, the “BET specific surface area” is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BEL Japan’s BELSORP-mino II.

[0032] In the present invention, "average particle size (D 50 )" refers to the particle size at the 50% reference of the volumetric cumulative particle size distribution. The above average particle size (D 50 ) can be measured using the laser diffraction method. The laser diffraction method generally enables the measurement of particle sizes ranging from the submicron range to several mm, and can obtain results with high reproducibility and high resolution.

[0033]

[0034] The present invention will be described in detail below.

[0035] The positive electrode and lithium secondary battery according to the present invention include at least one of the configurations disclosed below, and may include any combination of technically feasible configurations among the configurations below.

[0036]

[0037] <Bipolar>

[0038] Hereinafter, the anode according to the present invention will be described.

[0039] The positive electrode according to the present invention comprises a current collector and a positive electrode active material layer disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, a conductive material, and a binder, wherein the positive electrode active material comprises a lithium iron phosphate-based compound, the conductive material comprises carbon black, and the value of R represented by the following formula 1 is 500 to 1300.

[0040] [Equation 1]

[0041] R = {[Oxygen content of conductive material] × [BET specific surface area of ​​conductive material]} / [Average particle size of cathode active material (D 50 ) value]

[0042] In Equation 1 above, the oxygen content value (atomic%) of the conductive material, and the BET specific surface area value (m) of the conductive material 2 / g) and average particle size of the positive active material (D 50 Each of the ) values ​​(㎛) is a unitless number with the unit omitted.

[0043]

[0044] In the case of a positive electrode active material containing a lithium iron phosphate compound, the relative average particle size (D) compared to a nickel-cobalt-manganese ternary positive electrode active material 50 Because ) is small, in order to form a conductive network within the electrode, it must be uniformly dispersed, and the average particle size (D 50 A conductive material with a small ) is required. However, in the case of carbon nanotubes, which are widely used as conventional conductive materials, their length reaches several micrometers (㎛) but their diameter is only tens of nanometers (nm), so agglomeration occurs between particles; therefore, the average particle size (D 50 When this is mixed with a small positive electrode active material, it is difficult to achieve uniform dispersion, which has the disadvantage of potentially leading to a lack of electrical conduction pathways.

[0045] However, carbon black has an sp3 structure, which is a three-dimensional structure forming four σ-bonds, so it is relatively flexible and does not aggregate compared to carbon nanotubes with an sp2 structure, making it superior in terms of dispersibility. In addition, carbon black consists of very fine particles that are close to spherical, and since the particle size is small, it can be easily mixed with the cathode active material containing lithium iron phosphate compounds and dispersed uniformly to form a consistent conduction path within the electrode, thereby forming a conductive network.

[0046] Accordingly, the inventors of the present invention have an average particle size (D) such as a positive electrode active material containing a lithium iron phosphate-based compound. 50 Carbon black was used as a conductive material that maintains dispersibility even when mixed with a small cathode active material. To optimize dispersibility, the BET specific surface area of ​​the carbon black conductive material was appropriately increased. Furthermore, to increase the dispersibility of the carbon black conductive material in the cathode slurry preparation step containing a polar solvent such as N-methylpyrrolidone (NMP), the content of highly electronegative atoms, specifically oxygen, was controlled, thereby developing a carbon black conductive material with optimized dispersibility with a cathode active material containing a lithium iron phosphate compound.

[0047] Accordingly, excellent electrical conductivity was achieved by connecting the positive electrode active materials with a conductive material to form a conductive network, and a lithium secondary battery exhibiting excellent lifespan characteristics was developed by including such a positive electrode.

[0048] Hereinafter, the anode according to the present invention will be described in more detail.

[0049]

[0050] The anode according to the present invention has a value of R represented by the following formula 1, which is 500 to 1300.

[0051] [Equation 1]

[0052] R = {[Oxygen content of conductive material] × [BET specific surface area of ​​conductive material]} / [Average particle size of cathode active material (D 50 ) value]

[0053] In Equation 1 above, the oxygen content value (atomic%) of the conductive material, and the BET specific surface area value (m) of the conductive material 2 / g) and average particle size of the positive active material (D 50 Each of the ) values ​​(㎛) is a unitless number with the unit omitted.

[0054] If the value of R above is less than 500, it corresponds to a case where the oxygen content or the BET specific surface area of ​​the conductive material is excessively low. If the oxygen content is low, the conductive material is not uniformly dispersed in polar solvents such as N-methylpyrrolidone (NMP), and the oxygen-based functional groups present on the surface of the conductive material decrease, resulting in a lower surface energy. Consequently, the chemical bonding force with the binder weakens, leading to reduced rigidity of the composite and, consequently, a problem of weakened adhesion of the electrode. Furthermore, if the BET specific surface area of ​​the conductive material is low, the average particle size (D) of the conductive material 50 The average particle size (D) of the positive electrode active material containing the lithium iron phosphate-based compound 50 Since it becomes excessively large compared to ), the contact area between the conductive material and the cathode active material containing a lithium iron phosphate compound is reduced, weakening the conductive network and causing a problem of reduced energy density. When the value of R exceeds 1300, it corresponds to a case where the oxygen content of the conductive material or the BET specific surface area of ​​the conductive material is excessively high. Due to the high oxygen content, the conductive material is excessively oxidized, and when applied to a cathode with a high cell potential, side reactions with the electrolyte increase, leading to a problem of gas generation. In addition, due to the high BET specific surface area, particle aggregation occurs, making uniform dispersion difficult, and consequently, there is a problem of non-uniform battery performance.

[0055] For example, the value of R may preferably be 600 or more, 650 or more, 700 or more, 750 or more, 800 or more, or 820 or more, and preferably 1,200 or less, 1,100 or less, 1,000 or less, 950 or less, 940 or less, 880 or less, or 870 or less. When the above range is satisfied, the conductive material is well dispersed in the polar solvent, enabling uniform coating of the anode slurry, balanced interaction between the conductive material and the anode active material is achieved, thereby increasing the electrical conductivity of the electrode, and an average particle size (D) similar to that of the anode active material containing a lithium iron phosphate-based compound 50 A conductive material having ) is formed, so that the contact area between the positive active material and the conductive material can be optimized. Accordingly, a stable conductive network is formed, and since the resistance is reduced, the lifespan characteristics are maximized.

[0056]

[0057] Meanwhile, the current collector according to the present invention 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.

[0058] In addition, the current collector may typically have a thickness of 3 μm to 500 μm, and preferably may have a thickness of 300 μm or less, 200 μm or less, 100 μm or less, or 80 μm or less. Fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0059] The positive active material layer is disposed on the current collector. The positive active material layer may be disposed on at least one surface of the current collector. Specifically, the positive active material layer may be disposed on one or both surfaces of the current collector.

[0060]

[0061] The above positive active material layer comprises a positive active material, a conductive material, and a binder.

[0062]

[0063] First, the conductive material of the present invention will be described.

[0064] The conductive material of the present invention comprises carbon black. Since carbon black is composed of fine particles, it provides a large specific surface area, which increases the contact area with the positive electrode active material. This results in a denser conductive path and an improved conductive network. Furthermore, because it can be evenly dispersed, it can maintain uniform performance of the entire electrode by uniformly distributing the conductive material even when applied to positive electrode active materials containing lithium iron phosphate-based compounds with small particle sizes. Additionally, since particle size control according to the size of the positive electrode active material is easy, it has the advantage of being effective for use with small particles, such as positive electrode active materials containing lithium iron phosphate-based compounds. Moreover, since it has a three-dimensional sp3 structure forming four σ-bonds, each bond is symmetrically distributed, giving it the characteristics of being relatively flexible and well dispersed.

[0065] The BET specific surface area of ​​the above conductive material is 200m² 2 / g to 450m 2 / g, preferably 250m 2 / g to 350m 2 / g, more preferably 280m 2 / g to 320m 2 / g. When included within the above range, the contact area between the conductive material and the cathode active material containing a lithium iron phosphate compound increases, thereby strengthening the conductive network. Consequently, electron transport pathways are effectively formed, improving the energy density and charge / discharge efficiency of the battery. Furthermore, since aggregation between the conductive materials does not occur, there is an effect that allows for uniform dispersion when manufacturing the cathode slurry later.

[0066] The oxygen content of the conductive material may be 2.0 atomic% to 4.5 atomic%, preferably 2.5 atomic% to 4.0 atomic%, 2.3 atomic% to 3.3 atomic%, 2.3 atomic% to 2.7 atomic%, or 3.1 atomic% to 3.8 atomic%. When the above range is satisfied, oxygen-based functional groups are appropriately present on the surface of the conductive material, thereby strengthening the interaction with polar solvents such as N-methylpyrrolidone (NMP). Consequently, the conductive material is better dispersed within the solvent, and the interaction between the conductive material and the binder is strengthened, resulting in an effect of increasing the stiffness of the composite. Additionally, since the carbon black conductive material is not excessively oxidized, the conductive network is maintained.

[0067] Average particle size (D) of the above conductive material 50 ) may be 0.7㎛ to 1.3㎛, preferably 0.8㎛ to 1.2㎛, and more preferably 0.85㎛ to 1.15㎛. When the above range is satisfied, the average particle size (D) with respect to the cathode active material containing a lithium iron phosphate-based compound is 50 Since the positive electrode active material and the conductive material are similar, the positive electrode active material and the conductive material can be evenly mixed to form an efficient conductive network within the electrode, and the conductive material can be uniformly mixed within the positive electrode slurry later on because it is easy to disperse.

[0068] The conductive material may be included in an amount of 0.5% to 5.0% by weight, preferably 0.8% to 4.0% by weight, and more preferably 1.0% to 2.0% by weight, based on the total weight of the positive electrode active material layer. When included within the above range, the electrode capacity is excellent, and the effect of forming a conductive path can be maximized by optimizing dispersibility.

[0069]

[0070] Next, the positive active material of the present invention will be described.

[0071] The positive electrode active material of the present invention comprises a lithium iron phosphate-based compound. The lithium iron phosphate-based compound has excellent thermal stability, which has the advantage of providing excellent lifespan characteristics and stability for lithium secondary batteries.

[0072] The above lithium iron phosphate-based compound may be represented by the following chemical formula 1.

[0073] [Chemical Formula 1]

[0074] Li 1+x [Fe 1-y M y ]PO4

[0075] In the above chemical formula 1, M may include one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V. The above lithium iron phosphate-based compound may be doped with M. In this case, the diffusivity of lithium ions is increased, and consequently, the electrochemical properties of a lithium secondary battery containing a positive electrode active material may be improved.

[0076] In addition, the above x may be -0.5 ≤ x ≤ 0.5, preferably -0.2 ≤ x ≤ 0.2, more preferably -0.1 ≤ x ≤ 0.1.

[0077] In addition, the above y may be 0≤y<1, preferably 0≤y<0.3, more preferably 0≤y<0.05.

[0078] The above lithium iron phosphate compound may be, for example, LiFePO4, Li(Fe, Mn)PO4, Li(Fe, Co)PO4, Li(Fe, Ni)PO4, or a mixture thereof, and preferably may be LiFePO4.

[0079] The above lithium iron phosphate-based compound may further include a conductive coating layer on its surface. The conductive coating layer is intended to improve the conductivity of the lithium iron phosphate-based compound and may include one or more mixtures selected from the group consisting of carbon-based materials, metals, and conductive polymers. Among these, when a conductive coating layer of a carbon-based material is included, the conductivity can be effectively improved without significantly increasing the weight of the lithium iron phosphate-based compound.

[0080] The conductive coating layer can be formed according to a conventional method for forming a coating layer and may be included in an amount of 1% to 7% by weight, preferably 1% to 5% by weight, based on the total weight of the lithium iron phosphate-based compound. When included within the above range, the problem of deterioration in battery characteristics due to a relative decrease in LFP content can be prevented, and the conductivity is improved due to the formation of the conductive layer.

[0081] Average particle size (D of the above positive active material) 50 The particle size may be 0.6 μm to 1.6 μm, preferably 0.7 μm to 1.4 μm, and more preferably 0.7 μm to 1.2 μm. When the above range is satisfied, the distance of lithium ions entering and exiting the positive electrode active material is shortened due to the small particle size, which is advantageous in terms of capacity and output.

[0082] The above positive active material may be included in an amount of 80% to 99% by weight, preferably 90% to 98% by weight, and more preferably 95% to 98% by weight, based on the total weight of the positive active material layer. When included within the above range, it is excellent in terms of the capacity and energy density of the electrode, and can exhibit excellent effects in terms of both the functional optimization of the conductive material and the binder.

[0083] The weight ratio of the positive active material and the conductive material may be 90:10 to 99:1, preferably 95:5 to 99:1, and more preferably 96:4 to 99:1. When the above range is satisfied, a balance is achieved between the positive active material and the conductive material, providing a sufficient electron transport path within the electrode, so that the electrode operates stably even during the charging and discharging process, thereby minimizing the decrease in capacity of the lithium secondary battery and suppressing the increase in resistance.

[0084]

[0085] Next, the binder of the present invention will be described.

[0086] The binder of the present invention serves to improve adhesion between positive active material particles and adhesion between the positive active material and the current collector. Preferably, the binder may include one or more selected from the group consisting of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

[0087] The binder may be included in an amount of 0.1% to 10.0% by weight, preferably 1.0% to 5.0% by weight, and more preferably 2.0% to 4.0% by weight, based on the total weight of the positive electrode active material layer. When the content of the binder is included in the above range, it has the effect of increasing the energy density per volume while lowering the resistance of the electrode and maintaining the adhesion of the electrode.

[0088]

[0089] The anode may have a porosity of 15% to 40%, preferably 20% to 35%, and more preferably 25% to 30%. When the above range is satisfied, the electrolyte impregnation is improved, and even better capacity characteristics can be achieved.

[0090]

[0091] The anode may be manufactured by methods known in the art. For example, the anode may be manufactured by mixing an anode active material, a binder, and a conductive material in a solvent to prepare an anode slurry, applying the anode slurry onto a current collector, and then drying and rolling, or by casting the anode slurry onto a separate support and then laminating the film obtained by peeling it off from the support onto a current collector. In this case, the solvent for the anode slurry may be any anode slurry solvents generally used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, or a mixture thereof, but is not limited thereto. The solvent may be used in an amount that dissolves or disperses the anode active material, the conductive material, and the binder, and has a viscosity such that the cathode slurry can be uniformly coated.

[0092]

[0093] Lithium secondary battery

[0094] Hereinafter, a lithium secondary battery according to the present invention will be described.

[0095] A lithium secondary battery according to the present invention comprises a positive electrode according to the present invention; a negative electrode disposed opposite to the positive electrode; and an electrolyte. Optionally, the lithium secondary battery according to the present invention may further comprise a separator interposed between the positive electrode and the negative electrode.

[0096] Since the anode above is the same as described above, the remaining components excluding the anode will be described below.

[0097]

[0098] (1) Cathode

[0099] In a lithium secondary battery according to the present invention, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, and the negative electrode active material layer may include a negative electrode active material, a binder, and a conductive material.

[0100] The above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, heat-treated carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used.

[0101] The above-mentioned negative electrode current collector can typically have a thickness of 3 μm to 500 μm, and preferably can have a thickness of 300 μm or less, 200 μm or less, 100 μm or less, or 80 μm or less. Fine irregularities may be formed on the surface of the current collector to strengthen the bonding force with the negative electrode active material. For example, it can be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0102] The above negative active material layer may be disposed on the negative current collector. The above negative active material layer may be disposed on at least one surface of the negative current collector. Preferably, the above negative active material layer may be disposed on one or both surfaces of the negative current collector.

[0103] The above-mentioned negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium, for example, carbon materials such as artificial graphite, natural graphite, Kish graphite, pyrolytic carbon, meso-carbon microbeads, mesophase pitches, petroleum or coal tar pitch-derived cokes, mesophase pitch-based carbon fiber, graphitized carbon fiber, amorphous carbon, softened carbon, or hardened carbon; (semi)metallic materials capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; SiO b (0 <b≤2), SnO2, 바나듐 산화물, 리튬 바나듐 산화물과 같이 리튬을 도프 및 탈도프할 수 있는 (준)금속 산화물 재료; Si-C 복합체 또는 Sn-C 복합체과 같은 이종 복합 재료; 또는 금속 리튬 박막 등을 들 수 있으며, 이들 중 어느 하나 또는 둘 이상의 혼합물이 사용될 수 있다.

[0104] Preferably, the negative electrode active material may comprise one or more selected from the group consisting of silicon-based active materials, carbon-based active materials, and silicon-carbon composite active materials; more preferably, the carbon-based active material may comprise one or more selected from the group consisting of artificial graphite, natural graphite, softened carbon, and hardened carbon, and the silicon-based active material may comprise pure Si particles and / or SiO₂ b (0 <b≤2)를 포함할 수 있으며, 상기 실리콘-탄소 복합계 활물질은 Si-C 복합체를 포함할 수 있다. 또한, 상기 음극 활물질은 상기한 물질들 중에서 2 이상이 혼합된 혼합 활물질이 적용될 수도 있다.

[0105] The above-mentioned negative electrode active material may be included in an amount of 60% to 99% by weight based on the total weight of the negative electrode active material layer, preferably in an amount of 70% or more, 80% or more, 85% or more, or 90% or more by weight, and may also be included in an amount of 98% or less, 97% or less, or 95% or less by weight.

[0106] The above binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and can typically be added in an amount of 0.1% to 10% by weight based on the total weight of the negative active material layer, and can be included in an amount of 0.2% or more, 0.3% or more, or 0.5% or more by weight, and can also be included in an amount of 8% or less, or 5% or less by weight. Examples of such binders may include one or more selected from the group consisting of styrene-butadiene copolymer, acrylate styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, acrylic rubber, butyl rubber, fluororubber, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene copolymer, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene propylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, and polyvinyl alcohol. Among these, it may include one or more selected from the group consisting of styrene-butadiene copolymer, acrylate styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, acrylonitrile-butadiene-styrene copolymer, carboxymethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylfluran, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, and cyanoethyl sucrose. Preferably, carboxymethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropyl cellulose, or a mixture thereof may be applied.

[0107] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10% by weight or less, preferably 5% by weight or less, 3% by weight or less, 2% by weight or less, or 1% by weight or less based on the total weight of the negative electrode active material layer, and may also be included in an amount of 0.01% by weight or more, 0.05% by weight or more, 0.08% by weight or more, 0.1% by weight or more, or 0.3% by weight or more. Such conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fiber or metal fiber; fluorinated carbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. may be used.

[0108] The above-mentioned cathode active material layer may be manufactured by applying a cathode slurry composition, prepared by dissolving or dispersing a cathode active material and optionally a binder and a conductive material in a solvent, onto a cathode current collector and drying it, or by casting the cathode slurry composition onto a separate support and then laminating the film obtained by peeling it off from the support onto a cathode current collector.

[0109]

[0110] (2) Electrolyte

[0111] Examples of the above electrolytes that can be used in the manufacture of lithium secondary batteries include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., but are not limited to these.

[0112] The above electrolyte may include a lithium salt and an organic solvent.

[0113] The above lithium salt may be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Preferably, the anion of the above lithium salt is F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The lithium salt may be at least one selected from the group consisting of LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. It is preferable to use the lithium salt within the range of 0.1 to 2.0 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 lithium ions can move effectively.

[0114] The above organic solvent may be used without special limitations as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Preferably, the above organic solvent is an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; or an aromatic hydrocarbon-based solvent such as benzene or fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0115] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the above additives may be included in an amount of 0.1 to 5 weight% based on the total weight of the electrolyte.

[0116]

[0117] (3) Separator

[0118] The above separator physically 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. In this case, the separator may be interposed between the positive electrode and the negative electrode.

[0119] Preferably, a porous polymer film made of a polyolefin-based polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, a coated separator containing a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0120]

[0121] The lithium secondary battery according to the present invention as described above can be usefully applied to portable devices such as mobile phones, laptop computers, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs). Since the lithium secondary battery according to the present invention can achieve excellent output characteristics even under low temperature conditions, it can be particularly usefully applied in the field of electric vehicles.

[0122] According to another embodiment of the present invention, a battery module comprising a lithium secondary battery according to the present invention as a unit cell and a battery pack comprising the same are provided.

[0123] The above battery module or battery pack can be used as a power source for one or more medium-to-large devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0124]

[0125] The present invention will be explained in more detail below through specific embodiments. However, the following embodiments are intended only to enable a person skilled in the art to fully understand and easily implement the present invention, and the scope of the present invention is not limited to the following embodiments.

[0126]

[0127] Examples 1 to 5 and Comparative Examples 1 to 4

[0128] Examples 1 to 5 and Comparative Examples 1 to 4 were prepared by applying a lithium iron phosphate-based compound having the same composition as LiFePO4 as the positive electrode active material having particle size characteristics as described in Table 1 below, and applying carbon black having the same particle size characteristics and specific surface area as described in Table 1 below as the conductive material.

[0129] Specifically, the above-mentioned positive active material, conductive material, and PVDF binder were mixed in a weight ratio of 96:1.2:2.8, N-methylpyrrolidone solvent was added to obtain a solid content of 60%, and the mixture was mixed using a homogenizer at 2,500 rpm for 1 hour to prepare a positive slurry.

[0130] Afterwards, the anode slurry was applied to one side of an aluminum current collector, dried at 130°C, and then rolled twice using a roll-to-roll rolling machine at a linear pressure of 1 ton / cm to manufacture the anode.

[0131] In addition, the value of R, represented by the following Equation 1 of the above anode, was calculated and shown in Table 1.

[0132] [Equation 1]

[0133] R = [Average particle size of positive active material (D 50 ) value] / {[Oxygen content of conductive material] × [BET specific surface area of ​​conductive material]}

[0134] In Equation 1 above, the oxygen content value (atomic%) of the conductive material, and the BET specific surface area value (m) of the conductive material 2 / g) and average particle size of the positive active material (D 50 Each of the ) values ​​(㎛) is a unitless number with the unit omitted.

[0135] Oxygen content of conductive material [Atomic %] BET specific surface area of ​​conductive material [m² 2 / g] Average particle size of positive active material (D 50) [㎛]R Value Example 1 2.73 10 1.176 0.9 Example 22.627 5 1.165 0.0 Example 32.33 0 40.799 8.9 Example 43.118 5 1.152 1.4 Example 53.34 25 1.1127 5.0 Comparative Example 10.626 5 1.114 4.5 Comparative Example 27.527 1.118 40.9 Comparative Example 33.0267 1.844 5.0 Comparative Example 43.127 20.45187 3.8

[0136] 1) The specific surface area was measured by the BET method and calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BEL Japan's BELSORP-mino II.2) Average particle size (D 50 The positive active material powder and the conductive material powder were each dispersed in a dispersion medium, then introduced into a laser diffraction particle size measuring device (Microtrac MT 3000), irradiated with ultrasound of approximately 28 kHz at an output of 60 W, obtained a volume cumulative particle size distribution graph, and then measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0137]

[0138] Experimental Example 1: Evaluation of Dose Retention Rate

[0139] Manufacture of Lithium Secondary Batteries

[0140] Artificial graphite was used as the negative electrode active material. A negative electrode was prepared comprising a negative electrode active material layer containing the above negative electrode active material, negative electrode binders CMC and SBR, and negative electrode conductive material carbon black in a weight ratio of 96 : 1.0 : 2.5 : 0.5. Copper foil was used as the negative electrode current collector.

[0141] An electrode assembly was prepared by interposing a separator between the anode prepared in Examples 1 to 5 and Comparative Examples 1 to 4 and the cathode, and then the assembly was placed inside a battery case, and an electrolyte was injected into the case to manufacture a lithium secondary battery. The electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.2 M in a solvent mixed with ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3.

[0142]

[0143] <Position Retention Rate Evaluation>

[0144] Each of the lithium secondary batteries manufactured above was charged to 3.8V at 45℃ under CC / CV and 0.5C conditions using an electrochemical charge / discharger, and then discharged to 2.5V under CC and 1.0C conditions as one cycle, and the capacity retention rate was measured.

[0145] The capacity retention rate was calculated using the formula below, and the capacity retention rate at 100 cycles is shown in Table 2 below.

[0146] Capacity Retention Rate (%) = (Discharge Capacity after 100 Cycles / Discharge Capacity after 1 Cycle) × 100

[0147]

[0148] Experimental Example 2: Evaluation of Resistance Increase Rate

[0149] Each of the lithium secondary batteries manufactured above was charged to 3.8V at 25℃ under CC / CV, 0.5C conditions using an electrochemical charge / discharger, and then discharged to 2.5V under CC, 1.0C conditions, with this being one cycle of charge / discharge.

[0150] After one cycle of charging and discharging, the discharge capacity after one cycle was measured using an electrochemical charge / discharger, and the SOC was adjusted to 50%. Then, a pulse of 2.5C was applied for 10 seconds, and the initial resistance was calculated through the difference between the voltage before and after the pulse application.

[0151] In addition, the resistance was calculated using the same method as above after cycle charging and discharging, and the resistance growth rate was calculated using the formula below. The resistance growth rate at 100 cycles is shown in Table 2 below.

[0152] Resistance increase rate (%) = {(Resistance after 100 charge / discharge cycles - Initial resistance) / Initial resistance} × 100

[0153] Capacitance retention rate (%, @100th cycle) Resistance increase rate (%, @100th cycle) Example 192.52.67 Example 292.03.21 Example 391.02.70 Example 490.33.85 Example 591.53.90 Comparative Example 185.47.40 Comparative Example 284.87.58 Comparative Example 382.16.91 Comparative Example 483.48.23

[0154] Referring to Table 2 above, it can be seen that the lithium secondary batteries of Examples 1 to 5, in which the value of R is included in the range of 500 to 1300, exhibit a superior cycle capacity retention rate and a low resistance increase rate compared to the lithium secondary batteries of Comparative Examples 1 to 4, in which they are not.

Claims

1. The entire house; and A positive active material layer disposed on the above current collector; comprising, The above positive active material layer comprises a positive active material, a conductive material, and a binder, and The above positive active material includes a lithium iron phosphate-based compound, and The above conductive material includes carbon black, and An anode having a value of R expressed by the following Equation 1, ranging from 500 to 1300. [Equation 1] R = {[Oxygen content of conductive material] × [BET specific surface area of ​​conductive material]} / [Average particle size of cathode active material (D 50 ) value] In Equation 1 above, the oxygen content value (atomic%) of the conductive material, and the BET specific surface area value (m) of the conductive material 2 / g) and average particle size of the positive active material (D 50 Each of the ) values ​​(㎛) is a unitless number with the unit omitted.

2. In Claim 1, The BET specific surface area of ​​the above conductive material is 200m² 2 / g to 450m 2 Anode in / g.

3. In Claim 1, An anode having an oxygen content of 2.0 atomic% to 4.5 atomic% of the above conductive material.

4. In Claim 1, Average particle size (D) of the above conductive material 50 ) is an anode with a diameter of 0.7㎛ to 1.3㎛.

5. In Claim 1, Average particle size (D of the above positive active material) 50 ) is an anode with a diameter of 0.6㎛ to 1.6㎛.

6. In Claim 1, The above lithium iron phosphate-based compound is an anode represented by the following chemical formula 1: [Chemical Formula 1] Li 1+x [Fe 1-y M y ]PO4 In the above chemical formula 1, M comprises one or more selected from the group consisting of Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V, and -0.5≤x≤0.5, 0≤y<1.

7. In Claim 1, The above conductive material is included in an amount of 0.5% to 5.0% by weight based on the total weight of the above positive active material layer.

8. In Claim 1, The above positive active material is a positive electrode further comprising a conductive coating layer.

9. In Claim 1, A positive electrode in which the weight ratio of the positive electrode active material and the conductive material is 90:10 to 99:

1.

10. In Claim 1, The above binder comprises one or more selected from the group consisting of styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

11. In Claim 1, The above anode is an anode having a porosity of 15% to 40%.

12. A lithium secondary battery comprising the positive electrode of Claim 1.