Positive electrode for lithium secondary battery and lithium secondary battery including the same

Abstract

To provide a positive electrode that ensures safety by suppressing internal heat generation and ignition in a lithium secondary battery in various situations such as penetration by a sharp object or exposure to high temperatures, and to provide a lithium secondary battery including the positive electrode.SOLUTION: There are provided a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, the positive electrode includes a positive electrode current collector, a safety functional layer located on the positive electrode current collector, and a positive electrode active material layer located on the safety functional layer, and the safety functional layer includes a lithium iron phosphate compound and an endothermic material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. [Background technology]

[0002] Recently, with the rapid spread of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles, the demand for high-energy-density, high-capacity secondary batteries has skyrocketed, leading to active research and development into improving the performance of lithium secondary batteries.

[0003] A lithium secondary battery is a battery that contains a cathode and an anode, which contain active materials that allow the intercalation and deintercalation of lithium ions, and an electrolyte. Electrical energy is produced through oxidation and reduction reactions that occur when lithium ions are inserted and deintercalated at the cathode and anode.

[0004] However, if a sharp object (e.g., a nail) penetrates a lithium secondary battery, the negative and positive electrodes come into contact, causing a short circuit. This short circuit can cause internal heat generation in the lithium secondary battery and even lead to a fire.

[0005] Meanwhile, when a lithium secondary battery is exposed to a high-temperature environment, the structure of the positive electrode active material is degraded, generating oxygen radicals, which cause oxidative decomposition of the electrolyte, which can lead to internal heat generation and fire in the lithium secondary battery.

[0006] Therefore, a method is needed to ensure safety by suppressing internal heat generation and ignition of lithium secondary batteries in various situations such as penetration by a sharp object or exposure to high temperatures. Summary of the Invention [Problem to be solved by the invention]

[0007] One embodiment provides a positive electrode that ensures safety by suppressing internal heat generation and ignition of a lithium secondary battery in various situations, such as penetration by a sharp object or exposure to high temperatures.

[0008] Another embodiment provides a lithium secondary battery including the positive electrode. [Means for solving the problem]

[0009] One embodiment provides a positive electrode for a lithium secondary battery, comprising: a positive electrode current collector; a safety functional layer disposed on the positive electrode current collector; and a positive electrode active material layer disposed on the safety functional layer, wherein the safety functional layer comprises a lithium iron phosphate-based compound and an endothermic material.

[0010] Another embodiment provides a lithium secondary battery comprising: the positive electrode; a negative electrode; and an electrolyte. [Effects of the Invention]

[0011] The positive electrode according to an embodiment can ensure safety by suppressing heat generation and ignition of the lithium secondary battery in various situations, such as when penetrated by a sharp object or exposed to high temperatures. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 2] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 3] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 4] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Specific examples of the present invention will be described in detail below, but these are presented as examples and are not intended to limit the present invention, which is defined only by the scope of the claims set forth below.

[0014] Unless otherwise specified in this specification, when a layer, film, region, plate, or other part is "on" another part, this includes not only when it is "directly on" the other part, but also when there is another part in between.

[0015] In this specification, unless otherwise specified, the singular can also include the plural. Also, unless otherwise specified, "A or B" can mean "including A, including B, or including A and B."

[0016] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0017] As used herein, the terms D50 particle size and D90 particle size refer to the diameter of the 50% (D50) and 90% (D90) cumulative volume particles in a particle size distribution, respectively. The D50 and D90 particle sizes can be measured by methods well known to those skilled in the art, such as using a particle size analyzer or a transmission electron microscope or scanning electron microscope. Alternatively, the D50 and D90 particle sizes can be calculated by counting the number of particles in each particle size range using a dynamic light scattering device and analyzing the data. Alternatively, the D50 and D90 particle sizes can be measured using a laser diffraction method. More specifically, in the case of measurement by laser diffraction, the particles to be measured are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. After that, the D50 particle size at 50% of the particle size distribution in the measuring device and the D90 particle size at 90% of the particle size distribution can be calculated.

[0018] (positive electrode) One embodiment provides a positive electrode for a lithium secondary battery, comprising: a positive electrode current collector; a safety functional layer disposed on the positive electrode current collector; and a positive electrode active material layer disposed on the safety functional layer, wherein the safety functional layer comprises a lithium iron phosphate-based compound and an endothermic material.

[0019] In one embodiment, the positive electrode has a safety function layer disposed between the positive electrode current collector and the positive electrode active material layer. The safety function layer has a current reducing function when a short circuit occurs and a heat absorbing function when a short circuit occurs and / or when exposed to high temperatures.

[0020] The lithium iron phosphate-based compound is a high-resistance positive electrode active material, and can reduce current when a short circuit occurs in a lithium secondary battery due to penetration by a sharp object.

[0021] In addition, the heat-absorbing material is a substance that has a heat-absorbing function, and can absorb heat generated inside the lithium secondary battery due to penetration by a sharp object, exposure to high temperatures, etc.

[0022] Therefore, in one embodiment, the positive electrode has a safety functional layer disposed between the positive electrode current collector and the positive electrode active material layer. The safety functional layer has a current reducing function when a short circuit occurs and a heat absorbing function when a short circuit occurs and / or exposure to high temperatures. As a result, the positive electrode can suppress heat generation and ignition of the lithium secondary battery even in various situations such as penetration by a sharp object or exposure to high temperatures, thereby ensuring safety.

[0023] Heat-absorbing material The heat-absorbing material may be composite particles containing a metal hydroxide and a phosphorus (P)-based flame retardant.

[0024] The metal hydroxide has a heat absorbing function, and the phosphorus-based flame retardant has an oxygen radical scavenging function and a combustion resistance function.

[0025] As a result, when a lithium secondary battery is exposed to a high-temperature environment, the heat-absorbing material captures oxygen radicals generated by the structural collapse of the positive electrode active material (oxygen radical scavenging function), and even if heat is generated inside the lithium secondary battery, it absorbs that heat (heat-absorbing function), and can delay combustion of the lithium secondary battery and suppress ignition (combustion-resistant function).

[0026] Here, "composite" means that the functional groups of the metal hydroxide (e.g., hydroxyl groups) and the functional groups of the phosphorus-based flame retardant (e.g., phosphate groups) are chemically bonded to each other, and multiple particles form a single mass.

[0027] Furthermore, the term "chemical bond" includes various types of bonds such as covalent bonds, ionic bonds, coordinate bonds, metallic bonds, etc. The bond type between particles can be confirmed by, for example, X-ray photoelectron spectroscopy.

[0028] Specifically, a plurality of the metal hydroxide particles and a plurality of the phosphorus-based flame retardant particles may be chemically bonded to each other to form secondary particles, and the phosphorus-based flame retardant particles within the secondary particles may be located on the surface and in the internal pores of the metal hydroxide particles.

[0029] When analyzed using a mass spectrometer by thermal desorption spectroscopy (TDS), the amount of P2 gas desorbed from 80°C to 1400°C (MS1) was 200 x 10 -6 ~2500×10 -6 mol / g, 300×10 -6 ~2000×10 -6 mol / g, or 400 × 10 -6 ~1800×10 -6 It can be in mol / g.

[0030] The endothermic material has a thermal desorption spectroscopy (TDS) analysis using a mass spectrometer, and the amount of H2O gas desorbed from 80°C to 200°C (MS2) is 50 x 10 -6 ~1000×10 -6 mol / g, 100 x 10 -6 ~9500×10 -6 mol / g, or 300 × 10 -6 ~900×10 -6 It can be in mol / g.

[0031] The heat-absorbing material may satisfy the following formula 1: [Formula 1] 0.5≦(MS1 / MS2)≦10.0 Preferably, the heat-absorbing material satisfies the following formula 1-1: [Formula 1-1] 0.5≦(MS1 / MS2)≦5.0 More preferably, the heat-absorbing material satisfies the following formula 1-2: [Formula 1-2] 0.8≦(MS1 / MS2)≦3.0 When the heat-absorbing material satisfies the formula 1, formula 1-1, or formula 1-2, the safety of the lithium secondary battery can be improved without deteriorating the performance.

[0032] The heat-absorbing material preferably has an Al (aluminum) element and P (phosphorus) element content within the following ranges as measured by analysis using an inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0033] The aluminum element may be contained in an amount of 5 to 30 wt %, or 5 to 25 wt %, relative to 100 wt % of the total amount of the heat absorbing material; and the phosphorus element may be contained in an amount of 5 to 30 wt %, or 5 to 25 wt %.

[0034] The content of aluminum and phosphorus in the heat-absorbing material can be controlled by the type and content of the metal hydroxide and flame retardant used in the production of the heat-absorbing material.

[0035] The metal hydroxide may be aluminum hydroxide, bohemite, pseudobohemite, alumina, kaolinite, or a combination thereof. For example, the metal hydroxide may be aluminum hydroxide.

[0036] The D50 particle size of the metal hydroxide may be 10 nm to 10 μm, 50 nm to 5 μm, or 0.1 to 3 μm.

[0037] The phosphorus-based flame retardant may be phosphoric acid, phosphoric acid ester, phosphonic acid, phosphinic acid, or a combination thereof. For example, the flame retardant may be phosphoric acid, phenyl phosphate, phenyl phosphoric acid, diphenyl phosphate, diphenyl phosphoric acid, methyl phosphinic acid, phenyl phosphinic acid, methylphosphonic acid, phenylphosphonic acid, or a combination thereof.

[0038] The metal hydroxide may be contained in an amount of 1 to 60 wt %, 5 to 50 wt %, or 10 to 40 wt % relative to 100 wt % of the total amount of the heat absorbing material; and the phosphorus-based flame retardant may be contained in an amount of 0.1 to 25 wt %, 0.5 to 20 wt %, or 1 to 15 wt %.

[0039] The endothermic material has a BET specific surface area of ​​8 to 150 m as calculated from an adsorption isotherm measured by adsorbing nitrogen. 2 / g, 10-120m 2 / g, or 35 to 100 m 2 / g.

[0040] The BET specific surface area of ​​the heat-absorbing material tends to decrease when the amount of the flame retardant relative to the metal hydroxide is increased or when the reaction time when the metal hydroxide and the flame retardant are combined is extended.

[0041] In order to prepare composite particles containing as much of the flame retardant as possible, it is preferable to use a metal hydroxide as a starting material having as large a specific surface area as possible. The specific surface area of ​​the metal hydroxide is, for example, 100 to 500 m 2 / g.

[0042] The heat-absorbing material may have a D50 particle size of 0.05 to 3 μm, 0.1 to 2 μm, or 0.5 to 1.5 μm, and may have a D90 particle size of 0.05 to 5 μm, 2 to 5 μm, or 2.5 to 5 μm.

[0043] The smaller the D50 particle size and D90 particle size of the heat-absorbing material, the thinner and more uniform the thickness of the safety function layer can be.

[0044] The D50 and D90 particle sizes of the endothermic material can be controlled by adjusting the manufacturing conditions of the endothermic material, for example, by increasing the temperature or stirring speed during the manufacturing of the endothermic material, the D50 and D90 particle sizes of the endothermic material can be reduced.

[0045] Manufacturing method of endothermic material The heat-absorbing material can be produced by heating a dispersion of a metal hydroxide and a flame retardant.

[0046] The solvent for the dispersion may be a mixture of water and an organic solvent, such as an alcohol-based organic solvent such as ethanol or 2-propanol.

[0047] The heating can be carried out at a temperature in the range of 40 to 100°C or 60 to 80°C for 1 to 48 hours or 5 to 30 hours.

[0048] During the heating, the mixture can be stirred at a speed of 50 to 1000 m / min, or 200 to 800 m / min.

[0049] After the heating, the mixture is filtered through a filter paper to finally obtain the endothermic material.

[0050] The endothermic material finally obtained can be confirmed by determining whether the masses of P2 gas and H2O gas measured by thermal desorption spectroscopy satisfy the above-mentioned ranges.

[0051] Lithium iron phosphate compounds The lithium iron phosphate compound is represented by the following chemical formula 1: [Chemical formula 1] Li a Fe 1-x1 M x1 PO4

[0052] In the above formula 1, 0.90≦a≦1.8, 0≦x1≦0.7, and M is Mg, Co, Ni, or a combination thereof.

[0053] For example, the lithium iron phosphate compound may be LiFePO4.

[0054] Weight ratio of lithium iron phosphate compound and endothermic material The weight ratio of the lithium iron phosphate compound to the endothermic material may be 1:1 to 20:1. Within this range, the safety of the lithium secondary battery can be improved without deteriorating its performance.

[0055] For reference, the higher the interface resistance of the positive electrode, the more the safety of the lithium secondary battery improves, but the life characteristics decrease. Taking this trade-off into consideration, the compounding ratio of the lithium iron phosphate compound and the endothermic material can be appropriately controlled.

[0056] For example, the weight ratio of the lithium iron phosphate compound to the endothermic material can be controlled to 1:1 to 20:1, preferably 2:1 to 18:1, and more preferably 5:1 to 9:1.

[0057] binder The safety feature layer may further include a binder.

[0058] The binder may be contained in an amount of 1 to 30% by weight, 3 to 20% by weight, or 5 to 10% by weight, relative to 100% by weight of the total amount of the safety function layer.

[0059] The binder serves to firmly adhere the components of the safety function layer to each other and to the positive electrode current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0060] Safety function layer thickness The safety function layer can have a thickness of 0.1 to 3 μm, 1 to 3 μm, or 2 to 3 μm.

[0061] The safety function layer further includes the heat absorbing material in preparation for the case where the lithium iron phosphate compound is included alone, thereby ensuring safety even with a thin thickness.

[0062] (lithium secondary battery) Another embodiment provides a lithium secondary battery comprising: the positive electrode; a negative electrode; and an electrolyte.

[0063] The lithium secondary battery according to an embodiment includes the positive electrode, and therefore, heat generation and ignition are suppressed even in various situations, such as when penetrated by a sharp object or exposed to high temperatures, thereby ensuring safety.

[0064] Hereinafter, the description overlapping with the above description will be omitted, and a lithium secondary battery according to an embodiment will be described in detail.

[0065] positive electrode active material The positive electrode active material may be a compound capable of reversibly inserting and extracting lithium (lithiate insertion compound). Specifically, one or more of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0066] The composite oxide may be a lithium transition metal composite oxide, and specific examples thereof include lithium nickel-based oxides, lithium cobalt-based oxides, lithium manganese-based oxides, lithium iron phosphate-based compounds, cobalt-free nickel-manganese-based oxides, and combinations thereof.

[0067] As an example, a compound represented by any of the following chemical formulas can be used: Li a A 1-b X b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Ni 1-b-c Co b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni b Co c L 1 d G e O2(0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0≦e≦0.1);Li a NiG bO2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8, 0.001b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8, 0≦g≦0.5);Li (3-f) Fe2(PO4)3(0≦f≦2);Li a FePO4(0.90≦a≦1.8).

[0068] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 is Mn, Al or a combination thereof.

[0069] The positive electrode active material may include, for example, a lithium nickel-based oxide represented by the following Chemical Formula 11, a lithium cobalt-based oxide represented by the following Chemical Formula 12, a lithium iron phosphate-based compound represented by the following Chemical Formula 13, a cobalt-free lithium nickel-manganese-based oxide represented by the following Chemical Formula 14, or a combination thereof.

[0070] [Chemical formula 11] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 In the formula 11, 0.9≦a1≦1.8, 0.3≦x1≦1, 0≦y1≦0.7, 0≦z1≦0.7, 0.9≦x1+y1+z1≦1.1, and 0≦b1≦0.1; M 1 and M 2 are each independently one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0071] In the above Chemical Formula 11, 0.6≦x1≦1, 0≦y1≦0.4, and 0≦z1≦0.4 may be satisfied, or 0.8≦x1≦1, 0≦y1≦0.2, and 0≦z1≦0.2 may be satisfied.

[0072] [Chemical formula 12] Li a2 Co x2 M 3 y2 O 2-b2 X b2 In the formula 12, 0.9≦a2≦1.8, 0.7≦x2≦1, 0≦y2≦0.3, 0.9≦x2+y2≦1.1, and 0≦b2≦0.1; M 3 is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Se, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0073] [Chemical formula 13] Li a3 Fe x3 M 4 y3 PO 4-b3 X b3 In the formula 13, 0.9≦a3≦1.8, 0.6≦x3≦1, 0≦y3≦0.4, and 0≦b3≦0.1; M 4is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Mg, Mn, Mo, Ni, Se, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0074] [Chemical formula 14] Li a4 Ni x4 Mn y4 M 5 z4 O 2-b4 X b4 In the above chemical formula 14, 0.9≦a2≦1.8, 0.8≦x4<1, 0 <y4≦0.2、0≦z4≦0.2、0.9≦x4+y4+z4≦1.1、および0≦b4≦0.1であり、M 5 is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0075] For example, the positive electrode active material may be a high-nickel positive electrode active material having a nickel content of 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more to 99 mol% or less relative to 100 mol% of metals (excluding lithium) in the lithium transition metal composite oxide. The high-nickel positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0076] positive electrode The positive electrode can include a positive electrode current collector; a safety function layer disposed on the positive electrode current collector; and a positive electrode active material layer disposed on the safety function layer.

[0077] The positive electrode active material layer includes a positive electrode active material, and may further include a binder and / or a conductive material.

[0078] For example, the positive electrode may further include an additive that can serve as a sacrificial positive electrode.

[0079] The content of the positive electrode active material may be 90% by weight to 99.5% by weight relative to 100% by weight of the positive electrode active material layer, and the contents of the binder and conductive material may each be 0.5% by weight to 5% by weight relative to 100% by weight of the positive electrode active material layer.

[0080] The binder serves to firmly adhere the positive electrode active material particles to each other and to the current collector. Representative examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0081] The conductive material is used to impart conductivity to the electrodes, and any material that does not undergo chemical change and is electronically conductive can be used in the battery. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials containing copper, nickel, aluminum, silver, etc. in the form of metal powder or metal fiber; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0082] The current collector may be made of Al, but is not limited to this.

[0083] negative electrode active material The negative electrode active material includes a material capable of reversibly inserting / desorbing lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0084] As the material capable of reversibly inserting / desorbing the lithium ions, it can include a carbon-based negative electrode active material, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon can include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon can include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0085] As the alloy of the lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0086] As the material capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. The Si-based negative electrode active material can be silicon, a silicon-carbon composite, SiOx (0 < x ≤ 2), a Si-Q alloy (where Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material can be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0087] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and the surfaces of the silicon particles coated with amorphous carbon. For example, it may include secondary particles (cores) formed by assembling primary silicon particles and an amorphous carbon coating layer (shell) located on the surfaces of the secondary particles. The amorphous carbon may also be located between the primary silicon particles, e.g., the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0088] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core including crystalline carbon and silicon particles and an amorphous carbon coating layer disposed on the core surface.

[0089] The Si-based or Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0090] negative electrode A negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and / or a conductive material.

[0091] For example, the negative electrode active material layer can contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0092] The binder serves to firmly adhere the negative electrode active material particles to each other and to the current collector. The binder may be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0093] Examples of the non-aqueous binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, and combinations thereof.

[0094] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorobidrine, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0095] When an aqueous binder is used as the negative electrode binder, a cellulose-based compound that can impart viscosity may be further included. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be sodium, potassium, or lithium.

[0096] The dry binder is a polymeric material that can be fiberized, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0097] The conductive material is used to impart conductivity to the electrodes, and any material that does not undergo chemical change and is electronically conductive in the battery that is constructed can be used. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0098] The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0099] electrolyte The electrolyte for the lithium secondary battery contains a non-aqueous organic solvent and a lithium salt.

[0100] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0101] The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, an aprotic solvent, or a combination thereof.

[0102] Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone. Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Also, cyclohexanone can be used as a ketone solvent. Ethyl alcohol, isopropyl alcohol, etc. can be used as an alcohol solvent. Aprotic solvents include nitriles such as R-CN (R is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and can contain a double bond, an aromatic ring, or an ether group), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane and 1,4-dioxolane, and sulfolanes.

[0103] The non-aqueous organic solvents can be used alone or in combination of two or more.

[0104] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate can be mixed and used, and the cyclic carbonate and the chain carbonate can be mixed in a volume ratio of 1:1 to 1:9.

[0105] The lithium salt is a substance that dissolves in an organic solvent, acts as a source of lithium ions in the battery, enables the basic operation of a lithium secondary battery, and plays a role in promoting the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (x and y are integers of 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0106] Separator Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators may be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more of these materials, including mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators.

[0107] The separator may include a porous substrate and a coating layer containing an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0108] The porous substrate may be a polymer film formed of any one polymer selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a copolymer or mixture of two or more of these polymers.

[0109] The organic material may include a polyvinylidene fluoride polymer or a (meth)acrylic polymer.

[0110] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0111] The organic material and the inorganic material may be mixed in one coating layer, or may be stacked in a form in which a coating layer containing an organic material and a coating layer containing an inorganic material are stacked.

[0112] Lithium secondary battery Lithium secondary batteries can be classified into cylindrical, prismatic, pouch, and coin types depending on their shape. FIGS. 1 to 4 are schematic diagrams showing a lithium secondary battery according to an embodiment, with FIG. 1 showing a circular battery, FIG. 2 showing a prismatic battery, and FIGS. 3 and 4 showing pouch-type batteries. Referring to FIGS. 1 to 4, a lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, the negative electrode 20, and the separator 30 may be immersed in an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50, as shown in FIG. 1. Also, in FIG. 2, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0113] The lithium secondary battery according to one embodiment of the present invention is applicable to automobiles, mobile phones, and / or various types of electrical devices, but the present invention is not limited thereto.

[0114] Examples and comparative examples of the present invention will be described below. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0115] Manufacturing Example 1 Aluminum hydroxide (D50: 1.2 μm, BET: 212 μm) was added to 50 cc of a solvent made by mixing distilled water and ethanol in a 1:1 volume ratio. 2 1.0 g of methyl phosphinic acid (1.0 g / g) and 5.0 g of methyl phosphinic acid were dispersed.

[0116] The dispersion was stirred at 70°C at a stirring speed of 300 m / min for 24 hours, then filtered and washed with water and ethanol, and the solid on the filter paper was vacuum dried to obtain an endothermic material.

[0117] Reference example The endothermic material of Production Example 1 was analyzed as follows.

[0118] (1) Mass of desorbed gas For the endothermic material of Production Example 1, a mass spectrometer (product name: TDS-1200, manufacturer: Denshi Kagaku) ​​according to thermal desorption gas spectrometry was used to measure the amount of P gas desorbed from 80°C to 1400°C (MS1) and the amount of H2O gas desorbed from 80°C to 200°C (MS2).

[0119] Specifically, 1 mg of the endothermic material of Production Example 1 was taken as a sample and corrected for the actual measured weight. The sample stage on which the sample was set was made of quartz, and the sample dish was made of SiC. The temperature was increased at a rate of 60°C / min, and the surface temperature of the sample was increased from 80°C to 1400°C.

[0120] The mass of the gas desorbed from the sample was measured using a quadrupole mass spectrometer under a voltage of 1000 V. The amount of P gas (MS1) was measured as a cumulative value from 80°C to 1400°C, and the amount of HO gas (MS2) was measured as a cumulative value from 80°C to 200°C. The mass number [M / z] used to interpret the measured values ​​was 18 for HO.

[0121] As a result of the measurements, it was confirmed that the endothermic material of Production Example 1 had an MS1 ​​of 1195 μmol / g, an MS2 of 558, and an MS1 / MS2 ratio of 2.1.

[0122] (2) Elemental analysis The endothermic material of Preparation Example 1 was analyzed for the aluminum and phosphorus contents contained in 100 wt% of the total amount of the endothermic material using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, product name: Agilent 5110 VDV, manufacturer: Agilent).

[0123] As a result of the measurement, the endothermic material of Production Example 1 had an aluminum content of 17 wt % and a phosphorus content of 20 wt % relative to a total amount of the endothermic material (100 wt %).

[0124] (3) BET specific surface area The BET specific surface area of ​​the endothermic material of Production Example 1 was measured according to JIS K6217-2 using a gas adsorption measurement device (product name: BELSORP, manufacturer: MicrotracBEL).

[0125] As a result of the measurement, the endothermic material of Production Example 1 has a BET specific surface area of ​​35m 2 / g.

[0126] (4) D50 particle size and D90 particle size For the endothermic material of Production Example 1, a particle size measuring device using the laser diffraction method (product name: MT3300, manufacturer: MicrotracBEL) was used to calculate the D50 particle size at 50% of the particle size distribution and the D90 particle size at 90% of the particle size distribution under the following conditions.

[0127] Transparency: Transparent Shape: Non-spherical Circulation speed: 7 Measurement time: 30 seconds Number of iterations: 3 Refractive index: a. Endothermic material: 1.65; b. Ethanol solvent: 1.36 As a result of the measurement, the endothermic material of Production Example 1 had a D50 particle size of 0.8 μm and a D90 particle size of 3.5 μm.

[0128] Example 1 (1) Manufacturing of the positive electrode LiFePO4 (D50: 1.2 μm), the heat-absorbing material of Production Example 1, and polyvinylidene fluoride as a binder were mixed in a weight ratio of 87.5:5:7.5 and dispersed in N-methylpyrrolidone to produce a safety function layer slurry.

[0129] The safety function layer slurry was coated onto an aluminum foil having a thickness of 10 μm, and then dried at 110° C. to form a safety function layer.

[0130] LiCoO2 as a positive electrode active material, polyvinylidene fluoride as a binder, and Ketjen black as a conductive material were mixed in a weight ratio of 98.5:0.75:0.75, and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0131] The positive electrode active material slurry was coated on the safety function layer and then dried at 110° C. to form a positive electrode active material layer.

[0132] As described above, the safety functional layer and the positive electrode active material layer were sequentially formed on the aluminum foil, and then the aluminum foil was pressed to obtain a positive electrode.

[0133] In the finally obtained positive electrode, the thickness of the safety function layer is 3 μm, and the thickness of the positive electrode active material layer is 40 μm.

[0134] (2) Manufacture of lithium secondary batteries A mixture of artificial graphite and silicon particles in a weight ratio of 93.5:6.5 was used as the negative electrode active material, and the negative electrode active material was mixed with a styrene-butadiene rubber binder and carboxymethyl cellulose in a weight ratio of 97:1:2, respectively, and dispersed in distilled water to prepare a negative electrode active material slurry.

[0135] The negative electrode active material slurry was coated on a copper foil having a thickness of 6 μm, dried at 100° C., and then pressed to prepare a negative electrode.

[0136] The prepared positive and negative electrodes were assembled with a 10 μm thick polyethylene separator to prepare an electrode assembly, and the electrode assembly was placed in a pouch measuring 5.9 cm wide, 7.8 cm long, and 5.4 mm thick as a battery container, and an electrolyte was injected to prepare a lithium secondary battery.

[0137] The electrolyte solution was prepared by dissolving 1.3M LiPF6 in a mixed solvent of ethylene carbonate (EC), polypropylene (PP), and propylene carbonate (PC) in a volume ratio of 1:1:1.

[0138] Example 2 A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFePO4, the endothermic material of Preparation Example 1, and the binder were mixed in a weight ratio of 82.5:10:7.5.

[0139] Example 3 A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFePO4, the endothermic material of Preparation Example 1, and the binder were mixed in a weight ratio of 77.5:15:7.5.

[0140] Example 4 A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFePO4, the endothermic material of Preparation Example 1, and the binder were mixed in a weight ratio of 72.5:20:7.5.

[0141] Example 5 A positive electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFePO4, the endothermic material of Preparation Example 1, and the binder were mixed in a weight ratio of 67.5:25:7.5.

[0142] Comparative Example 1 (1) Manufacturing of the positive electrode LiCoO2 as a positive electrode active material, polyvinylidene fluoride as a binder, and Ketjen black as a conductive material were mixed in a weight ratio of 98.5:0.75:0.75, and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0143] The positive electrode active material slurry was coated on an aluminum foil, dried at 110° C., and pressed to obtain a positive electrode.

[0144] In the finally obtained positive electrode, the thickness of the positive electrode active material layer was 45 μm.

[0145] (2) Manufacture of lithium secondary batteries A lithium secondary battery was fabricated in the same manner as in Example 1, except that the positive electrode was used.

[0146] Comparative Example 2 Instead of the endothermic material in Production Example 1, aluminum hydroxide (D50: 1.2 μm, BET: 212 m) was used. 2 A lithium secondary battery was manufactured in the same manner as in Example 2, except that a 1000 vol. / g lithium ion battery was used.

[0147] Evaluation example 1: Nail penetration test Ten lithium secondary batteries for each of Examples 1 to 5 and Comparative Examples 1 and 2 were prepared so that the SoC (State of charge) was 100%.

[0148] A nail with a diameter of 3 mm was penetrated through the center of the lithium secondary battery at a speed of 150 mm / s. Ten lithium secondary batteries were tested, and the percentage of lithium secondary batteries that ignited after the nail was penetrated was evaluated. The results are shown in Table 1 below.

[0149] Evaluation example 2: Electrode interface resistance measurement The interface resistance of the positive electrodes of Examples 1 to 5 and Comparative Examples 1 and 2 was measured using a resistance analyzer (analytical equipment product name: 46-pin resistance measuring instrument, manufacturer: Hioki). The interface resistance values ​​of the positive electrodes thus obtained are shown in Table 1 below.

[0150] Evaluation example 3: Room temperature and high temperature life evaluation The lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated for cycle characteristics after charge and discharge under the following conditions. The results are shown in Table 1.

[0151] After 200 charge / discharge cycles under the conditions of 25°C, 1C charge (CC / CV, 4.47V, 0.1C cut-off) / 1C discharge (CC, 2.75V cut-off), the capacity retention rate was calculated using the following formula 2.

[0152] Independently, after 200 charge / discharge cycles under the conditions of 45°C, 1C charge (CC / CV, 4.47V, 0.1C cut-off) / 1C discharge (CC, 2.75V cut-off), the capacity retention was calculated according to the following formula 2. [Formula 2] Capacity retention rate = (discharge capacity after 200 cycles / discharge capacity after 1 cycle) x 100

[0153] [Table 1]

[0154] General The positive electrode of Comparative Example 1 does not include a safety functional layer, the positive electrode of Comparative Example 2 includes a safety functional layer containing a lithium iron phosphate compound and aluminum hydroxide, and the positive electrodes of Examples 1 to 5 include a safety functional layer containing a lithium iron phosphate compound and an endothermic material.

[0155] According to Table 1, the positive electrode of Comparative Example 1 has low interface resistance of the positive electrode due to the lithium iron phosphate compound component, and the probability of ignition during a nail penetration test is 100% due to the heat-absorbing material component.

[0156] The positive electrode of Comparative Example 2 has a high interface resistance due to the presence of the lithium iron phosphate compound, but the heat-absorbing material component means that there is an 80% chance of ignition during a nail penetration test.

[0157] On the other hand, the positive electrodes of Examples 1 to 5 have high interface resistance due to the presence of the lithium iron phosphate compound, and the presence of the heat-absorbing material reduces the probability of ignition during the nail penetration test to 50% or less.

[0158] Therefore, the positive electrode of an embodiment represented by Examples 1 to 5 can suppress heat generation and ignition of the lithium secondary battery even in various situations such as penetration by a sharp object or exposure to high temperatures, thereby ensuring safety.

[0159] For reference, the higher the interface resistance of the positive electrode, the more the safety of the lithium secondary battery improves, but the life characteristics decrease. Taking this trade-off into consideration, the compounding ratio of the lithium iron phosphate compound and the endothermic material can be appropriately controlled.

[0160] For example, the weight ratio of the lithium iron phosphate compound to the endothermic material can be controlled to 1:1 to 20:1, preferably 2:1 to 18:1, and more preferably 5:1 to 9:1.

[0161] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to this, and various modifications can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it is natural that these also fall within the scope of the present invention. [Explanation of symbols]

[0162] 100 Lithium secondary battery 10 positive electrode 11 Positive electrode lead tab 12 Positive terminal 20 negative electrode 21 Negative electrode lead tab 22 Negative terminal 30 Separator 40 Electrode assembly 50 cases 60 Sealing member 70 Electrode tab 71 Positive electrode tab 72 Negative electrode tab

Claims

1. Positive electrode current collector; a safety function layer located on the positive electrode current collector; and a positive electrode active material layer located on the safety function layer Including, The safety function layer includes a lithium iron phosphate compound and an endothermic material. Positive electrode for lithium secondary batteries.

2. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the heat-absorbing material is a composite particle containing a metal hydroxide and a phosphorus-based flame retardant.

3. The heat-absorbing material is When analyzing using a mass analyzer by thermal desorption spectroscopy (TDS), P desorbed from 80°C to 1400°C 2 The amount of gas (MS1) is 200 x 10 -6 ~2500 x 10 -6 mol / g, H desorbed from 80°C to 200°C 2 The amount of O gas (MS2) is 50 × 10 -6 ~1000 x 10 -6 3. The positive electrode for a lithium secondary battery according to claim 2, wherein the total amount of the ionic liquid is 1000 mol / g.

4. The positive electrode for a lithium secondary battery according to claim 3 , wherein the heat-absorbing material satisfies the following formula 1: [Formula 1] 0.5≦(MS1 / MS2)≦10.0

5. The heat-absorbing material is characterized by the fact that, when analyzed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES), 3. The positive electrode for a lithium secondary battery according to claim 2, wherein the aluminum element is contained in an amount of 5 to 30% by weight and the phosphorus element is contained in an amount of 5 to 30% by weight, relative to 100% by weight of the total amount of the heat-absorbing material.

6. the metal hydroxide is aluminum hydroxide, boehmite, pseudoboehmite, alumina, kaolinite, or a combination thereof; 3. The positive electrode for a lithium secondary battery according to claim 2, wherein the phosphorus-based flame retardant is phosphoric acid, a phosphoric acid ester, phosphonic acid, phosphinic acid, a derivative thereof, or a combination thereof.

7. The heat-absorbing material has a BET specific surface area of ​​8 to 150 m 2 The positive electrode for a lithium secondary battery according to claim 1, wherein the SiO2 content is 1 / g.

8. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the heat-absorbing material has a D50 particle size of 0.05 to 3 μm.

9. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the lithium iron phosphate-based compound is represented by the following chemical formula 1: [Chemical formula 1] Li a Fe 1-x1 M x1 PO 4 In the above Chemical Formula 1, 0.90≦a≦1.8, 0≦x1≦0.7, M is Mg, Co, Ni or a combination thereof.

10. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein a weight ratio of the lithium iron phosphate compound to the endothermic material is 1:1 to 20:

1.

11. The positive electrode for a lithium secondary battery according to claim 1 , wherein the safety function layer further comprises a binder.

12. The positive electrode for a lithium secondary battery according to claim 11, wherein the binder is contained in an amount of 1 to 30% by weight with respect to 100% by weight of the total amount of the safety function layer.

13. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the safety function layer has a thickness of 0.1 to 3 μm.

14. The positive electrode according to any one of claims 1 to 13; a negative electrode; and electrolyte A lithium secondary battery comprising:

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