Negative electrode for rechargeable lithium batteries and rechargeable lithium batteries including the thereof

Abstract

A negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the negative electrode are disclosed. The negative electrode includes a negative active material layer containing negative active material and a current collector supporting the negative active material layer. The negative active material includes: a first spherical crystalline carbon containing secondary particles, wherein primary particles of crystalline carbon are assembled in the secondary particles; a second spherical crystalline carbon containing secondary particles, wherein primary particles of crystalline carbon are assembled in the secondary particles; and flake graphite, wherein the first spherical crystalline carbon is coated with crystalline carbon and the second spherical crystalline carbon is coated with amorphous carbon.

Description

[0001] This application is a divisional application of the invention patent application with the application date of April 12, 2019, application number 201910293866.9, and title "Negative electrode for rechargeable lithium battery and rechargeable lithium battery including the same". Technical Field

[0002] Disclosed is a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the negative electrode. Background Art

[0003] Due to the increasing demand for mobile devices or portable batteries, continuous technological development has been carried out to achieve high capacity of rechargeable lithium batteries.

[0004] As an electrolyte for rechargeable lithium batteries, a lithium salt dissolved in an organic solvent has been used.

[0005] For the positive electrode active material of rechargeable lithium batteries, lithium-transition metal oxides having a structure capable of intercalating lithium ions such as LiCoO2, LiMn2O4, LiNi

[0006] ,

[0005] , ,

[0009] , ,

[0008] , , 1-x ,

[0007] , x , , , Co x O2 (0 < x < 1) etc. have been used.

[0006] For the negative electrode active material for lithium secondary batteries, various carbon-based materials including artificial graphite, natural graphite, and hard carbon capable of intercalating and deintercalating lithium ions or Si-based active materials including Si and Sn have been used. In recent years, due to the need for high-capacity batteries (especially high capacity per unit volume), a negative electrode with a high specific capacity is required. Therefore, research has been carried out on using a composite of silicon and carbon as the negative electrode. However, the composite of silicon and carbon has a problem that significant volume expansion occurs during charging and discharging. Summary of the Invention <00001​​​​​​​​

[0010] In the negative electrode active material layer, sheet-like graphite can be disposed between the first spherical crystalline carbon and the second spherical crystalline carbon, and can be in direct contact with the first spherical crystalline carbon and the second spherical crystalline carbon.

[0011] The average particle size (D50) of flake graphite can be from about 1 μm to about 10 μm, for example from about 3 μm to about 7 μm.

[0012] Based on a total amount of 100 wt% negative electrode active material, the amount of flake graphite can be from about 1 wt% to about 10 wt%.

[0013] Based on a total amount of 100 wt% of negative electrode active material, the amount of the first spherical crystalline carbon can be from about 10 wt% to about 80 wt%, and based on a total amount of 100 wt% of negative electrode active material, the amount of the second spherical crystalline carbon can be from about 10 wt% to about 80 wt%.

[0014] Based on a total amount of 100 wt% negative electrode active material layer, the amount of negative electrode active material can be from about 96 wt% to about 99 wt%.

[0015] The loading level of the negative electrode active material layer can be greater than or equal to approximately 15 mg / cm³. 2 For example, approximately 15 mg / cm³ 2 Approximately 40 mg / cm 2 .

[0016] According to another embodiment, the rechargeable lithium battery includes the negative electrode, a positive electrode including a positive electrode active material, and a non-aqueous electrolyte.

[0017] Other embodiments of this disclosure are included in the following detailed description.

[0018] The negative electrode for a rechargeable lithium battery according to the embodiment exhibits excellent cycle life characteristics at both room temperature and high temperature, and shows improved storage characteristics at high temperature. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating the state of presence of the negative electrode active material in the negative electrode according to an embodiment of the present disclosure.

[0020] Figure 2 This is a schematic diagram illustrating the structure of a rechargeable lithium battery according to an embodiment of the present disclosure.

[0021] Figure 3A and Figure 3B This is an SEM image of the surface of the negative electrode manufactured according to Example 1.

[0022] Figure 4A and Figure 4BThe image shows an SEM photograph of the surface of the negative electrode fabricated according to Comparative Example 1.

[0023] Figure 5A and Figure 5B The image shows an SEM photograph of the surface of the negative electrode manufactured according to Reference Example 1.

[0024] Figure 6 The image shows a photograph of the surface of the negative electrode obtained by disassembling the battery according to Example 1 after charging and discharging.

[0025] Figure 7 The image shows a photograph of the surface of the negative electrode obtained by disassembling the battery according to Comparative Example 1 after charging and discharging. Detailed Implementation

[0026] Embodiments of this disclosure are described in detail below. However, these embodiments are exemplary, and this disclosure is not limited thereto, and is defined by the scope of the claims.

[0027] The negative electrode for a rechargeable lithium battery according to embodiments of the present disclosure includes a negative electrode active material layer comprising a negative electrode active material and a current collector supporting the negative electrode active material layer. Here, the negative electrode active material includes: a first spherical crystalline carbon comprising secondary particles, wherein primary particles of crystalline carbon are assembled in the secondary particles; a second spherical crystalline carbon comprising secondary particles, wherein primary particles of crystalline carbon are assembled in the secondary particles; and flake graphite, wherein the first spherical crystalline carbon is coated with crystalline carbon, and the second spherical crystalline carbon is coated with amorphous carbon.

[0028] The following description, with reference to the accompanying drawings, details such a negative electrode for a rechargeable lithium battery. Figure 1 A negative electrode 20 for a rechargeable lithium battery according to an embodiment is shown. The negative electrode 20 includes a negative electrode active material layer 22 and a current collector 24 supporting the negative electrode active material layer 22.

[0029] The negative electrode active material layer 22 includes a first spherical crystalline carbon 3, a second spherical crystalline carbon 5, and flake graphite 7. As shown in the enlarged view of the negative electrode active material layer 22, the flake graphite 7 can be disposed between the first spherical crystalline carbon 3 and the second spherical crystalline carbon 5, specifically, as shown in the enlarged view of the negative electrode active material layer 22. Figure 1 As shown, the flake graphite 7 can be in direct contact with the first spherical crystalline carbon 3 and the second spherical crystalline carbon 5.

[0030] The first spherical crystalline carbon 3 includes secondary particles in which primary particles 3a of crystalline carbon are assembled and coated with crystalline carbon 3b on the surface; the second spherical crystalline carbon 5 includes secondary particles in which primary particles 5a of crystalline carbon are assembled and coated with amorphous carbon 5b on the surface.

[0031] like Figure 1As shown, the flake graphite 7 can be disposed between the first spherical crystalline carbon 3 and the second spherical crystalline carbon 5, that is, in direct contact with the first and second spherical crystalline carbons. Therefore, this contact can be well maintained during battery charging and discharging. The flake graphite can be disposed between the first and second spherical crystalline carbons, thereby enabling the first and second spherical crystalline carbons to bond together. Even with repeated expansion / contraction of the active material during charge and discharge cycles, the flake graphite can maintain the electrical contact between the first and second spherical crystalline carbons. Therefore, the flake graphite can provide improved high-temperature storage characteristics and improved cycle life characteristics.

[0032] The average particle size (D50) of flake graphite can be from about 1 μm to about 10 μm, from about 1 μm to about 9 μm, from about 3 μm to about 7 μm, or in another embodiment, from about 3 μm to about 5 μm. Since flake graphite has a flake-like shape, the average particle size represents the length of the major axis (long axis). Typically, since the average particle size is measured using a particle measuring device as D50, the average particle size of graphite in this specification refers to the average particle size (D50). As used herein, unless otherwise defined, the average particle size (D50) represents the particle size representing about 50% by volume of the cumulative volume in the particle distribution.

[0033] When the average particle size (D50) of the flake graphite is within the aforementioned range, the flake graphite can be well positioned between the first and second spherical crystalline carbons and can effectively maintain contact during expansion and contraction during charging and discharging. If the average particle size (D50) of the flake graphite is less than about 1 μm, side reactions with the electrolyte will occur, degrading high-temperature storage characteristics and cycle life characteristics, and reducing high-rate performance due to increased resistance. If the average particle size (D50) of the flake graphite is greater than about 10 μm, it is difficult for the flake graphite to be positioned between the first and second spherical crystalline carbons. Therefore, the flake graphite is difficult to contact with the first and second spherical crystalline carbons during charging and discharging. Specifically, the flake graphite will contact the first and second spherical crystalline carbons in the C-axis direction (base surface), and therefore will not contact the first and second spherical crystalline carbons after discharging.

[0034] The first spherical crystalline carbon comprises secondary particles, in which primary particles of crystalline carbon are assembled. The average particle size (D50) of the secondary particles can be from about 10 μm to about 30 μm, and in the embodiment, it is from about 10 μm to about 20 μm. Additionally, the average particle size (D50) of the primary particles can be from about 1 μm to about 20 μm, and in the embodiment, it is from about 1 μm to about 10 μm.

[0035] When the average particle size (D50) of the secondary particles of the first spherical crystalline carbon is within the above range, it can more effectively form a spherical shape without side reactions, and can further improve cycle life characteristics and high-temperature storage characteristics, as well as further improve high-rate performance.

[0036] When the average particle size (D50) of the primary particles is within the above range, better sphericity and stable performance can be achieved.

[0037] The specific surface area of ​​the first spherical crystalline carbon can be approximately 1 m². 2 / g to approximately 5m 2 / g, in the examples, is approximately 2.5m 2 / g to approximately 3m 2 / g. When the specific surface area of ​​the first spherical crystalline carbon is within the above range, improved high-rate performance can be obtained, and high-temperature cycle life characteristics can be further improved due to the effective suppression of side reactions.

[0038] The second spherical crystalline carbon comprises secondary particles, in which primary particles of crystalline carbon are assembled. The average particle size (D50) of the secondary particles can be from about 10 μm to about 30 μm, and in the embodiment, it is from about 10 μm to about 20 μm. Additionally, the average particle size (D50) of the primary particles can be from about 1 μm to about 20 μm, and in the embodiment, it is from about 1 μm to about 10 μm.

[0039] When the average particle size (D50) of the secondary particles of the second spherical crystalline carbon is within the above range, it can more effectively form a spherical shape without side reactions, and can further improve cycle life characteristics and high temperature storage characteristics, and can also further improve high rate performance.

[0040] When the average particle size (D50) of the primary particles is within the above range, better sphericity and stable performance can be achieved.

[0041] The specific surface area of ​​the second spherical crystalline carbon can be approximately 1 m². 2 / g to approximately 5m 2 / g, in the examples, is approximately 2.5m 2 / g to approximately 3m 2 / g.

[0042] When the specific surface area of ​​the second spherical crystalline carbon is within the above range, improved high-rate performance can be obtained, and high-temperature cycle life characteristics can be further improved due to the effective suppression of side reactions.

[0043] Crystalline carbon can be natural graphite, artificial graphite, or a combination thereof.

[0044] The first spherical crystalline carbon may be coated with crystalline carbon on its surface, while the second spherical crystalline carbon may be coated with amorphous carbon on its surface. Additionally, flake graphite may not be coated with any carbon, whether crystalline or amorphous.

[0045] That is, the first spherical crystalline carbon includes a secondary particle core and a crystalline carbon coating, the second spherical crystalline carbon includes a secondary particle core and an amorphous carbon coating, and the flake graphite does not include a coating.

[0046] In crystalline carbon coatings, the crystalline carbon can be artificial graphite, natural graphite, or a combination thereof. In amorphous carbon coatings, the amorphous carbon can be obtained by heat-treating an amorphous carbon precursor, and can be soft carbon, hard carbon, mesophase pitch carbonization products, petroleum coke, or coal coke. Amorphous carbon precursors can be citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated ethylene-propylene-diene monomer (SEPDM), starch, phenolic resins, furan resins, furfuryl alcohol, polyacrylic acid, sodium polyacrylate, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, polyvinyl alcohol, polyvinyl chloride, coal-based pitch, petroleum-based pitch, mesophase pitch, low molecular weight heavy oil, glucose, gelatin, saccharides, or a combination thereof.

[0047] The thickness of the crystalline carbon coating can be from about 0.1 nm to about 100 nm. When the thickness of the crystalline carbon coating is within this range, side reactions with the electrolyte can be suppressed more effectively, and cycle life characteristics can be further improved. The thickness of the amorphous carbon coating can be from about 0.1 nm to about 100 nm. When the thickness of the amorphous carbon coating is within this range, side reactions with the electrolyte can be suppressed, and impregnation and surface characteristics can be improved, thus providing long cycle life and high power characteristics.

[0048] Similarly, when the coating of the first spherical crystalline carbon is crystalline and the coating of the second spherical crystalline carbon is amorphous, both long cycle life and high power characteristics can be achieved. When neither the first nor the second spherical crystalline carbon has a coating, severe side reactions with the electrolyte occur, thus reducing cycle life characteristics and charge / discharge efficiency. Furthermore, when both the first and second spherical crystalline carbons have crystalline carbon coatings, electrolyte impregnation and polarization increase, leading to deterioration of power characteristics.

[0049] Furthermore, flake graphite can suppress side reactions by not having a coating. When flake graphite has an amorphous carbon coating, it will undergo side reactions with the electrolyte at high temperatures, thereby degrading its high-temperature storage performance and cycle life. When flake graphite has a crystalline carbon coating, its low-temperature high-rate charge and discharge characteristics will degrade due to orientation issues.

[0050] In this way, when uncoated flake graphite with an average particle size (D50) of about 1 μm to about 10 μm is used as a negative electrode active material together with a first spherical crystalline carbon with a crystalline carbon coating and a second spherical crystalline carbon with an amorphous carbon coating, the flake graphite can be disposed between the first spherical crystalline carbon and the second spherical crystalline carbon. Therefore, despite repeated charging and discharging, electrical contact can be maintained well and side reactions with the electrolyte can be reduced, thereby improving high-temperature storage characteristics and cycle life characteristics.

[0051] Based on a total negative electrode active material of 100 wt%, it may include approximately 1 wt% to approximately 10 wt% of flake graphite. When flake graphite is included within this range, it can further improve the bonding force between the first spherical crystalline carbon and the second spherical crystalline carbon, effectively improving high-temperature cycle life and high-temperature storage characteristics, and further improving power characteristics. When the content of flake graphite is less than approximately 1 wt%, the effect of flake graphite is almost negligible. However, when the content of flake graphite is greater than approximately 10 wt%, the flake graphite has a relatively large specific surface area, which deteriorates the bonding force between the first and second spherical crystalline carbon, causing agglomeration. This leads to deterioration of high-temperature cycle life and high-temperature storage characteristics due to side reactions, and also increases the surface orientation of the negative electrode, thus deteriorating power characteristics.

[0052] Based on a total amount of 100 wt% of the negative electrode active material, it may include about 10 wt% to about 80 wt% of the first spherical crystalline carbon, and based on a total amount of 100 wt% of the negative electrode active material, it may include about 10 wt% to about 80 wt% of the second spherical crystalline carbon.

[0053] When the first spherical crystalline carbon coated with crystalline carbon is included in this range, the long cycle life (i.e., room temperature and high temperature cycle life characteristics) can be further improved, and the high temperature storage characteristics can be greatly improved. When the second spherical crystalline carbon coated with amorphous carbon is included in this range, the power characteristics can be greatly improved.

[0054] The first spherical crystalline carbon and the second spherical crystalline carbon can be mixed in a weight ratio of about 1:9 to about 9:1. When the first spherical crystalline carbon and the second spherical crystalline carbon are mixed within this mixing ratio range, a battery that simultaneously and effectively satisfies the desired properties of high-rate charging and discharging characteristics as well as long cycle life characteristics can be obtained.

[0055] The crystalline carbon coating of the first spherical crystalline carbon can be formed by mixing a soft carbon precursor, amorphous or semi-crystalline carbon, and secondary particles, and then heat-treating the mixture at a temperature greater than or equal to about 2400°C (or in the range of about 2400°C to about 3000°C). The heat treatment converts the soft carbon precursor, amorphous or semi-crystalline carbon into crystalline carbon, thereby forming the crystalline carbon coating. The soft carbon precursor can be petroleum pitch, coal pitch, raw coke, or a combination thereof, and the amorphous or semi-crystalline carbon can be petroleum-based coke, coal-based coke, or a combination thereof.

[0056] Alternatively, the amorphous carbon coating of the second spherical crystalline carbon can be formed by mixing an amorphous carbon precursor and secondary particles, and then heat-treating the mixture at a temperature of less than or equal to about 2000°C (or in the range of about 700°C to about 2000°C). The heat treatment converts the precursor into amorphous carbon, thereby forming the amorphous carbon coating. The aforementioned amorphous carbon precursor can be used as the amorphous carbon precursor. The formation of the amorphous carbon coating can be performed under an inert atmosphere such as Ar or N2.

[0057] Based on a total amount of 100 wt% negative electrode active material layer, the amount of negative electrode active material can be from about 96 wt% to about 99 wt%, for example, from about 97 wt% to about 99 wt%. When the amount of negative electrode active material is within this range, high energy density batteries can be achieved.

[0058] The loading level of the negative electrode active material layer can be greater than or equal to approximately 15 mg / cm³. 2 For example, approximately 15 mg / cm³ 2 Approximately 40 mg / cm 2 .

[0059] When the load level of the negative electrode active material layer is within this range, a high energy density battery can be achieved.

[0060] The negative electrode active material layer includes a binder and a negative electrode active material, and may also include a conductive material. Based on the total weight of the negative electrode active material layer, the amount of binder may be from about 1 wt% to about 4 wt%. Furthermore, when conductive material is also included, about 96 wt% to about 99 wt% of the negative electrode active material, about 0.5 wt% to about 2 wt% of the binder, and about 0.5 wt% to about 2 wt% of the conductive material may be used.

[0061] Adhesives improve the adhesion between negative electrode active material particles and between negative electrode active material particles and current collectors. Adhesives include non-aqueous adhesives, aqueous adhesives, or combinations thereof.

[0062] Non-aqueous adhesives include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or combinations thereof.

[0063] Waterborne adhesives may be styrene-butadiene rubber, acrylated styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, ethylene-propylene copolymer, polyepoxychloropropane, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, or combinations thereof.

[0064] When aqueous adhesives are used as negative electrode adhesives, cellulose compounds can also be used as tackifiers to provide adhesion. Cellulose compounds include one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and their alkali metal salts. The alkali metal can be Na, K, or Li. Based on 100 parts by weight of the negative electrode active material, such tackifiers may be included in an amount of from 0.1 parts by weight to about 3 parts by weight.

[0065] Conductive materials are included to provide electrode conductivity. Any conductive material can be used as the conductive material unless it causes a chemical change. Examples of conductive materials include carbon-based materials (such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.); metal-based materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0066] The current collector may include one selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0067] The negative electrode is formed by mixing a negative electrode active material, a binder, and optionally a conductive material in a solvent to prepare a negative electrode active material composition, coating the composition onto a current collector, and then drying and pressing it. Solvents include water such as distilled water and organic solvents such as N-methylpyrrolidone.

[0068] Another embodiment provides a rechargeable lithium battery including a negative electrode, a positive electrode, and an electrolyte.

[0069] The positive electrode includes a current collector and a layer of positive electrode active material formed on the current collector and including the positive electrode active material.

[0070] The positive electrode active material can be a compound capable of lithium intercalation and deintercalation (lithiation intercalation compound), specifically, it can be one or more composite oxides of lithium with metals selected from cobalt, manganese, nickel, and combinations thereof. More specific examples can be compounds represented by one of the following chemical formulas: Li a A 1-b X b D2(0.90≤a≤1.8, 0≤b≤0.5); 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 E 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a E 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 D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2); Li a Ni 1-b-c Co b X c O 2-α T α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Co b X c O 2-α T2(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Mn b X c D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li a Ni 1-b-c Mn b X c O2-α T α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05,0<α<2);Li a Ni 1-b-c Mn b X c About 2-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05,0<α<2);Li a Ni b E c G d O2(0.90≤a≤1.8,0≤b≤0.9,0≤c≤0.5,0.001≤d≤0.1);Li a Ni b Co c Mn d G e O2(0.90≤a≤1.8,0≤b≤0.9,0≤c≤0.5,0≤d≤0.5,0.001≤e≤0.1);Li a NiG b O2(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≤1.8,0.001≤b≤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);QO2;QS2;LiQS2;V2O5;LiV2O5;LiZO2;LiNiVO4;Li (3-f) J2(PO4)3(0≤f≤2);Li (3-f) Fe2(PO4)3(0≤f≤2);Li a FePO4(0.90≤a≤1.8).

[0071] In the chemical formula, A is selected from Ni, Co, Mn and combinations thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from O, F, S, P and combinations thereof; E is selected from Co, Mn and combinations thereof; T is selected from F, S, P and combinations thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V and combinations thereof; Q is selected from Ti, Mo, Mn and combinations thereof; Z is selected from Cr, V, Fe, Sc, Y and combinations thereof; J is selected from V, Cr, Mn, Co, Ni, Cu and combinations thereof.

[0072] The compound may have a coating on the surface, or it may be mixed with another compound having a coating. The coating may include at least one coating element compound selected from oxides, hydroxides, hydroxyoxides, oxycarbonates, and hydroxycarbonates of the coating element. The compound used for the coating may be amorphous or crystalline. The coating element included in the coating may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The coating can be formed by using these elements in the compound in a manner that does not adversely affect the properties of the positive electrode active material. For example, this method may include any coating method such as spraying, dipping, etc., but since this method is well known in the relevant art, it is not described in more detail here.

[0073] In the positive electrode, the content of the positive electrode active material can be from about 90 wt% to about 98 wt%, based on the total weight of the positive electrode active material layer.

[0074] In embodiments of this disclosure, the positive electrode active material layer may further include a binder and a conductive material. Here, based on the total weight of the positive electrode active material layer, the amounts of the binder and the conductive material may be from about 1 wt% to about 5 wt%.

[0075] Adhesives improve the adhesion properties between positive electrode active material particles and between positive electrode active material particles and current collectors. Examples of adhesives may be polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc., but are not limited to these.

[0076] Conductive materials are included to provide electrode conductivity. Any conductive material can be used as the conductive material unless it causes a chemical change. Examples of conductive materials may include carbon-based materials (such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.); metal-based materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0077] Current collectors may include aluminum foil, nickel foil, or combinations thereof, but are not limited thereto.

[0078] The positive electrode is formed by mixing a positive electrode active material, a binder, and a conductive material in a solvent to prepare a positive electrode active material composition, coating the composition onto a current collector, and then drying and pressing it. The solvent may include an organic solvent such as N-methylpyrrolidone.

[0079] Electrolytes include non-aqueous organic solvents and lithium salts.

[0080] Non-aqueous organic solvents are used as media for transporting ions that participate in the electrochemical reactions of the battery.

[0081] Non-aqueous organic solvents can include carbonates, esters, ethers, ketones, alcohols, or aprotic solvents.

[0082] Carbonate solvents can 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 butyl carbonate (BC), etc. Ester solvents can include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolactone, mevalonolactone, caprolactone, etc. Ether solvents can include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. Ketone solvents include cyclohexanone, etc. Alcohol solvents include ethanol, isopropanol, etc. Examples of aprotic solvents include nitriles such as R-CN (where R is a C2 to C20 straight-chain hydrocarbon, branched hydrocarbon, or cyclic hydrocarbon, or may include double bonds, aromatic rings, or ether bonds), amides such as dimethylformamide, dioxolane such as 1,3-dioxolane, sulfolane, etc.

[0083] Organic solvents can be used alone or in mixtures. When organic solvents are used in mixtures, the mixing ratio can be controlled according to the desired battery performance.

[0084] When a non-aqueous organic solvent is used in a mixture, the non-aqueous organic solvent may be a mixture of cyclic carbonates and linear carbonates, a mixture of cyclic carbonates and propionate solvents, or a mixture of cyclic carbonates, linear carbonates, and propionate solvents. The propionate solvent may be methyl propionate, ethyl propionate, propyl propionate, or a combination thereof.

[0085] Here, when cyclic carbonates are mixed with linear carbonates or cyclic carbonates with propionate solvents, they can be mixed in a volume ratio of about 1:1 to about 1:9, thus improving the performance of the electrolyte solution. Additionally, when cyclic carbonates, linear carbonates, and propionate solvents are mixed, they can be mixed in a volume ratio of about 1:1:1 to about 3:3:4. The mixing ratio of the solvents can be appropriately adjusted according to the desired properties.

[0086] In addition to carbonate solvents, organic solvents may also include aromatic hydrocarbon organic solvents. Here, carbonate solvents and aromatic hydrocarbon organic solvents can be mixed in a volume ratio of about 1:1 to about 30:1.

[0087] Aromatic hydrocarbon organic solvents can be aromatic hydrocarbon compounds of chemical formula 1.

[0088] [Chemical Formula 1]

[0089]

[0090] In Formula 1, R1 to R6 may be the same or different and are selected from hydrogen, halogens, C1 to C10 alkyl groups, haloalkyl groups and combinations thereof.

[0091] Specific examples of aromatic hydrocarbon organic solvents may be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, and fluoromethylbenzene. Benzene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

[0092] Electrolytes may also include additives such as vinylene carbonate or ethylene carbonate compounds of formula 2 to improve battery cycle life.

[0093]

Chemical Formula 2

[0094]

[0095] In Formula 2, R7 and R8 may be the same or different and are selected from hydrogen, halogen, cyano (CN), nitro (NO2) and fluorinated C1 to C5 alkyl, provided that at least one of R7 and R8 is selected from halogen, cyano (CN), nitro (NO2) and fluorinated C1-C5 alkyl, and R7 and R8 are not both hydrogen.

[0096] Examples of ethylene carbonate compounds may be difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The amount of additives used to improve cycle life may be used within appropriate limits.

[0097] Lithium salts dissolved in organic solvents supply lithium ions to lithium-ion batteries, essentially enabling rechargeable lithium-ion batteries to operate and improving lithium-ion transport between the positive and negative electrodes. Examples of lithium salts include those from LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 (C) y F 2y+1 At least one supporting salt is selected from SO2 (where x and y are natural numbers, for example, integers in the range of 1 to 20), LiCl, LiI, and LiB(C2O4)2 (lithium di(oxalate)borate: LiBOB). The concentration of the lithium salt can be in the range of about 0.1 M to about 2.0 M. When the lithium salt is included in the above concentration range, the electrolyte can have excellent performance and lithium-ion mobility due to optimal electrolyte conductivity and viscosity.

[0098] Depending on the type of battery, rechargeable lithium batteries may also include a separator located between the negative and positive electrodes. Examples of suitable separator materials include polyethylene, polypropylene, polyvinylidene fluoride, and multilayers thereof, such as polyethylene / polypropylene bilayer separators, polyethylene / polypropylene / polypropylene trilayer separators, and polypropylene / polypropylene / polypropylene trilayer separators.

[0099] Figure 2This is an exploded perspective view of a rechargeable lithium battery according to one embodiment. The rechargeable lithium battery according to the embodiment is shown as a prismatic battery, but is not limited thereto, and may include batteries of various shapes, such as cylindrical batteries, pouch batteries, etc.

[0100] Reference Figure 2 The rechargeable lithium battery 100 according to an embodiment includes an electrode assembly 40 and a housing 50 for housing the electrode assembly 40. The electrode assembly 40 is manufactured by winding a separator 30 disposed between a positive electrode 10 and a negative electrode 20. An electrolyte (not shown) may be impregnated in the positive electrode 10, the negative electrode 20, and the separator 30.

[0101] Examples and comparative examples of this disclosure are described below. However, these examples are not to be construed in any way as limiting the scope of the invention.

[0102] (Example 1)

[0103] The negative electrode active material was prepared by mixing 35 parts by weight of second spherical natural graphite particles with a soft carbon amorphous carbon coating, 60 parts by weight of first spherical natural graphite particles with an artificial graphite coating, and 5 parts by weight of flake graphite with an average particle size (D50) of 5 μm.

[0104] The second type of spherical natural graphite particles has an average particle size (D50) of 11 μm and a specific surface area of ​​2.7 m². 2 The secondary particles, with an average particle size (D50) of 5 μm, consist of primary natural graphite particles. The first spherical natural graphite particles have an average particle size (D50) of 11 μm and a specific surface area of ​​2.9 m². 2 / g, the first spherical natural graphite particles are assembled with primary natural graphite particles with an average particle size (D50) of 5μm. In addition, the thickness of the amorphous carbon coating is 10nm, and the thickness of the crystalline carbon coating is 10nm.

[0105] A negative electrode active material, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a weight ratio of 98:0.8:1.2 were mixed in distilled water to prepare a negative electrode active material slurry. The negative electrode active material slurry was coated onto a Cu foil current collector, then dried and pressed to produce a loading level of 19 mg / cm². 2 The negative electrode.

[0106] (Example 2)

[0107] The negative electrode was manufactured using the same method as in Example 1, except that 5 parts by weight of flake graphite with an average particle size of 3 μm was used instead of 5 parts by weight of flake graphite with an average particle size of 5 μm.

[0108] (Refer to Example 1)

[0109] The negative electrode is manufactured according to the same method as in Example 1, except that 30 parts by weight of the second spherical natural graphite particles, 60 parts by weight of the first spherical natural graphite particles, and 10 parts by weight of flake graphite with an average particle size of 11 μm are mixed in place of 35 parts by weight of the second spherical natural graphite particles, 60 parts by weight of the first spherical natural graphite particles, and 5 parts by weight of flake graphite with an average particle size of 5 μm.

[0110] (Compare with Example 1)

[0111] The negative electrode active material was prepared by mixing 30 parts by weight of second spherical natural graphite particles with a soft carbon amorphous carbon coating and 70 parts by weight of first spherical natural graphite particles with an artificial graphite coating.

[0112] The second type of spherical natural graphite particles has an average particle size (D50) of 11 μm and a specific surface area of ​​2.7 m². 2 The secondary particles, with an average particle size (D50) of 5 μm, consist of primary natural graphite particles. The first spherical natural graphite particles have an average particle size (D50) of 11 μm and a specific surface area of ​​2.9 m². 2 / g, the first spherical natural graphite particles are assembled with primary natural graphite particles with an average particle size (D50) of 5μm. In addition, the thickness of the amorphous carbon coating is 10nm, and the thickness of the crystalline carbon coating is 10nm.

[0113] A 3600 mAh 1C half-cell was fabricated using a negative electrode, a lithium metal counter electrode, and an electrolyte. The electrolyte was prepared by dissolving 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate (volume ratio 50:50).

[0114] (Compare with Example 2)

[0115] In addition to preparing the negative electrode active material by using sheet graphite with an average particle size of 5 μm and a 10 nm thick amorphous carbon coating on the surface, the negative electrode is manufactured according to the same method as in Example 1.

[0116] (Compare with Example 3)

[0117] In addition to preparing the negative electrode active material by using 95 parts by weight of spherical natural graphite particles with a soft carbon amorphous carbon coating and 5 parts by weight of flake graphite with an average particle size (D50) of 5 μm, the negative electrode was manufactured according to the same method as in Example 1.

[0118] Here, the spherical natural graphite particles have an average particle size (D50) of 11 μm and a specific surface area of ​​2.7 m². 2 / g of secondary particles, in which primary natural graphite particles with an average particle size (D50) of 5μm are assembled, and the thickness of the amorphous carbon coating is 10nm.

[0119] (Compare with Example 4)

[0120] In addition to preparing the negative electrode active material by using 95 parts by weight of spherical natural graphite particles with artificial graphite coating and 5 parts by weight of flake graphite with an average particle size (D50) of 5 μm, the negative electrode was manufactured according to the same method as in Example 1.

[0121] Here, the spherical natural graphite particles have an average particle size (D50) of 11 μm and a specific surface area of ​​2.7 m². 2 / g of secondary particles, in which primary natural graphite particles with an average particle size (D50) of 5μm are assembled, and the thickness of the amorphous carbon coating is 10nm.

[0122] *SEM photos

[0123] The SEM images of the surfaces of the negative electrodes in Example 1, Comparative Example 1, and Reference Example 1 are shown respectively. Figure 3A , Figure 3B , Figure 4A , Figure 4B , Figure 5A and Figure 5B middle.

[0124] like Figure 3A (Magnified 1000 times) Neutralization Figure 3B As shown in (3000x magnification), for the negative electrode according to Example 1, flake graphite (marked by arrows) is disposed between the first spherical natural graphite particle and the second spherical natural graphite particle, thus bonding the first spherical natural graphite particle and the second spherical natural graphite particle together.

[0125] like Figure 4A (Magnified 1000 times) Neutralization Figure 4B As shown in (3000x magnification), for the negative electrode according to Comparative Example 1, only the adhesive is disposed between the first spherical natural graphite particle and the second spherical natural graphite particle. Additionally, as... Figure 5A (Magnified 1000 times) Neutralization Figure 5B As shown in the image (magnified 3000x), when using flake graphite with an average particle size (D50) of 11 μm, the results differ from those obtained by using flake graphite. Figure 3A and Figure 3B The flake graphite is not located between the first spherical natural graphite particle and the second spherical natural graphite particle, but exists independently.

[0126] Manufacturing of rechargeable lithium battery cells

[0127] A 18650 type lithium secondary full cell with a capacity of about 400 mAh is manufactured using each of the negative electrodes, positive electrodes, and electrolytes according to Examples 1 and 2, Reference Example 1, and Comparative Examples 1 to 4.

[0128] By using LiNi 0.6 Co 0.2 Mn 0.2 O2, Ketjen Black conductive material, and polyvinylidene fluoride were mixed in N-methylpyrrolidone solvent at a weight ratio of 96:2:2 to prepare a positive electrode active material slurry. The positive electrode active material slurry was coated on an Al foil current collector, and then dried and pressed to manufacture the positive electrode.

[0129] Electrolytes were prepared by dissolving 1.0 M LiPF6 in a mixed solvent of ethylene carbonate and diethyl carbonate (volume ratio 50:50).

[0130] *Evaluation of cycle life characteristics at low temperature (-10℃), room temperature (25℃), and high temperature (45℃)

[0131] The rechargeable lithium battery cell was charged at 0.5C with a constant current / constant voltage and discharged at 0.5C with a constant current at -10°C. The cells were repeatedly charged and discharged 100 times, and the ratio of the capacity at the 100th charge to the capacity at the first charge is shown in Table 1 as the low-temperature capacity retention rate.

[0132] The rechargeable lithium-ion battery cell was charged at 1C with a constant current / constant voltage and discharged at 1C with a constant current at 25°C. The cells were repeatedly charged and discharged 100 times, and the ratio of the capacity at the 100th charge to the capacity at the first charge is shown in Table 1 as the room temperature capacity retention rate.

[0133] The rechargeable lithium battery cell was charged at 1C with a constant current / constant voltage and discharged at 1C with a constant current at 45°C. The cells were repeatedly charged and discharged 100 times, and the ratio of the capacity at the 100th charge to the capacity at the first charge is shown in Table 1 as the high-temperature capacity retention rate.

[0134] Table 1

[0135]

[0136]

[0137] As shown in Table 1, the rechargeable lithium-ion battery cells using negative electrode active materials according to Examples 1 and 2 exhibit excellent capacity retention of greater than or equal to 81% at low temperature, room temperature and high temperature. The negative electrode active materials include first spherical natural graphite particles with a crystalline carbon coating, second spherical natural graphite particles with an amorphous carbon coating, and flake graphite with an average particle size (D50) of 1 μm to 10 μm.

[0138] In contrast, the negative electrode active material, Reference Example 1, which includes large flake graphite with a large average particle size (D50) of 11 μm, exhibits degraded capacity retention at low temperature, room temperature, and high temperature, and specifically, shows a sharp degraded capacity retention at room temperature and high temperature compared to Examples 1 and 2.

[0139] In addition, Comparative Example 1, which uses a negative electrode active material that does not contain flake graphite, shows a deteriorated capacity retention rate at low temperature, room temperature and high temperature. Specifically, compared with Example 1 and Example 2, it shows a drastically deteriorated capacity retention rate at room temperature and high temperature.

[0140] In addition, comparative example 2, which includes flake graphite with an amorphous carbon coating as the negative electrode active material, shows a degraded capacity retention at room temperature and a significantly degraded capacity retention at high temperature, despite including flake graphite.

[0141] Furthermore, Comparative Example 3, which uses negative electrode active materials including spherical natural graphite particles with amorphous carbon coating and flake graphite, shows a deteriorated capacity retention rate at high temperatures, while Comparative Example 4, which uses negative electrode active materials including spherical natural graphite particles with crystalline carbon coating and flake graphite, shows a drastically deteriorated capacity retention rate at low temperatures.

[0142] *Storage characteristics at high temperatures (60℃)

[0143] A rechargeable lithium-ion battery cell was charged at a rate of 0.2C to 100% of its capacity, resulting in a State of Charge (SOC) of 100% (the cell is charged to a fully charged state with 100% total capacity during charging and discharging at 4.5V), and then stored at 60°C for 30 days. The capacity ratio after 30 days of storage at 60°C was calculated relative to the capacity before storage, and the results are shown in Table 2 as capacity retention.

[0144] Additionally, the rechargeable lithium-ion battery was charged to SOC100 at a rate of 0.2C and stored at 60°C for 30 days. It was then set to SOC50 (during charging and discharging at 4.5V, the individual cells were charged to a half-charge state based on 50% of the total charge capacity). The DC-IR, based on the rate of change of voltage versus the rate of change of current, was then measured by measuring the voltage drop (V) of the rechargeable lithium-ion battery cells while supplying current at 1C and 3C rates for 10 seconds, respectively. The results are shown as the DC-IR rate of change in Table 2.

[0145] Table 2

[0146] Capacity retention rate (%) DC-IR change rate (%) Example 1 91 119 Example 2 89 121 Comparison Example 1 87 130 Refer to Example 1 85 134 Comparison Example 2 72 155 Compare Example 3 61 161 Compare Example 4 90 115

[0147] As shown in Table 2, the rechargeable lithium battery cells using negative electrode active materials according to Examples 1 and 2 exhibit high capacity retention and small DC-IR variation at high temperatures. The negative electrode active materials include first spherical natural graphite particles with a crystalline carbon coating, second spherical natural graphite particles with an amorphous carbon coating, and flake graphite with an average particle size (D50) of 1 μm to 10 μm.

[0148] Conversely, Comparative Example 1, which uses a negative electrode active material that does not include flake graphite, and Reference Example 1, which uses a negative electrode active material that includes large flake graphite with a large average particle size (D50) of 11 μm, exhibit slightly lower capacity retention and higher DC-IR change rate at high temperatures, despite including flake graphite.

[0149] In addition, comparative example 2, which includes flake graphite with an amorphous carbon coating as the negative electrode active material, shows a low capacity retention rate but a very high DC-IR change rate despite including flake graphite.

[0150] Furthermore, Comparative Example 3, which uses negative electrode active materials including spherical natural graphite particles with amorphous carbon coating and flake graphite, shows a significantly degraded capacity retention at high temperatures.

[0151] Comparative Example 4, using anode active materials comprising spherical natural graphite particles with crystalline carbon coating and flake graphite, exhibits high capacity retention but low DC-IR variation. However, as shown in Table 1, Comparative Example 4 shows very low capacity retention at low temperatures, and is therefore unsuitable.

[0152] *Decomposition analysis of individual battery cells after discharge

[0153] Rechargeable lithium-ion battery cells manufactured using the negative electrodes according to Example 1 and Comparative Example 1 were charged and discharged 500 times at 1C and fully discharged at 0.2C, respectively, and then disassembled. A photograph of the negative electrode according to Example 1 is then shown. Figure 6 In the photograph, the negative electrode of Comparative Example 1 is shown. Figure 7 middle.

[0154] like Figure 6 As shown, the negative electrode in Example 1 appears black, indicating a discharge state of lithium deintercalation within the graphite layer. Conversely, as... Figure 7 As shown, the negative electrode of Comparison Example 1 is gold in color, indicating the lithium intercalation state. Therefore, referring to this result, although the negative electrode was discharged at 0.2C, no deintercalation from the charged negative electrode occurred due to the loss of electrical contact between the active materials inside the electrode. Thus, the negative electrode of Example 1, comprising conductive sheet graphite, maintains its capacity well during its long cycle life.

[0155] Although the invention has been described in conjunction with what are now considered to be practical exemplary embodiments, it should be understood that the invention is not limited to the disclosed embodiments, but rather is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A negative electrode for a rechargeable lithium battery, the negative electrode comprising: The negative electrode active material layer includes the negative electrode active material; as well as Current collector, supporting the negative electrode active material layer. in, The negative electrode active material includes: a first spherical crystalline carbon and a second spherical crystalline carbon, both composed of secondary particles, each containing identical primary crystalline carbon particles; and flake graphite disposed between the first and second spherical crystalline carbons, with its surface along its long axis in direct contact with both crystalline carbons. The first spherical crystalline carbon is coated with a crystalline carbon coating, and the second spherical crystalline carbon is coated with an amorphous carbon coating. The average particle size D50 of the secondary particles in each of the first and second spherical crystalline carbons is 10 μm to 30 μm, and Among them, the average particle size D50 of flake graphite is 1 μm to 9 μm.

2. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The average particle size D50 of the flake graphite is 3 μm to 7 μm.

3. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, Based on a total amount of 100wt% negative electrode active material, the amount of flake graphite is 1wt% to 10wt%.

4. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, Based on a total amount of 100wt% negative electrode active material, the amount of the first spherical crystalline carbon is 10wt% to 80wt%.

5. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, Based on a total amount of 100wt% negative electrode active material, the amount of second spherical crystalline carbon is 10wt% to 80wt%.

6. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, Based on a total amount of 100wt% negative electrode active material layer, the amount of negative electrode active material is 97wt% to 99wt%.

7. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The loading level of the negative electrode active material layer is greater than or equal to 15 mg / cm³. 2 .

8. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The loading level of the negative electrode active material layer is 15 mg / cm³. 2 Up to 40 mg / cm 2 .

9. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The crystalline carbon coating has a thickness of 0.1 nm to 100 nm, and the amorphous carbon coating has a thickness of 0.1 nm to 100 nm.

10. A rechargeable lithium battery, said rechargeable lithium battery comprising: The negative electrode according to any one of claims 1 to 9; Positive electrode, including positive electrode active material; as well as Non-aqueous electrolyte.

Images