Negative electrode active material, negative electrode for lithium secondary battery, and lithium secondary battery comprising same

Abstract

The invention provides a negative electrode active material, a negative electrode for a lithium secondary battery, and a lithium secondary battery including the same. A negative electrode active material according to one embodiment of the present disclosure comprises: a carbon-based active material; and a coating layer disposed on the surface of the carbon-based active material, the coating layer containing a metal hydroxide. According to one embodiment of the present disclosure, the high-temperature stability of the negative electrode active material for a lithium secondary battery can be improved.

Description

Technical Field

[0001] This disclosure relates to a negative electrode active material, a negative electrode for lithium secondary batteries, and a lithium secondary battery including the same. Background Technology

[0002] In recent years, there has been active research into electric vehicles (EVs) as an alternative to fossil fuel-based vehicles, which are one of the main causes of air pollution. As the power source for these electric vehicles (EVs), lithium secondary batteries with high discharge voltage and power stability are mainly used.

[0003] Therefore, there is a need to develop technologies that can improve the performance of lithium secondary batteries. Summary of the Invention

[0004] Technical issues

[0005] According to one aspect of this disclosure, a negative electrode active material for lithium secondary batteries with excellent high-temperature stability can be provided.

[0006] According to another aspect of this disclosure, the high-temperature driving performance of lithium secondary batteries can be improved.

[0007] Technical solution

[0008] According to one embodiment, the negative electrode active material comprises: a carbon-based active material; and a coating disposed on the surface of the carbon-based active material, wherein the coating comprises a metal hydroxide.

[0009] In some implementations, the negative electrode active material may be a core-shell structure comprising a core and a shell disposed on the surface of the core, wherein the core may contain a carbon-based active material and the shell may contain a metal hydroxide.

[0010] In some implementations, the metal hydroxide can be represented by the following chemical formula 1:

[0011]

Chemical Formula 1

[0012] M1 x M2 y M3 z (OH) a

[0013] In the aforementioned chemical formula 1, M1, M2, and M3 are each selected from one or more metals chosen from Al, Ni, Mg, Zn, and Cu, and 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 <x+y+z≤1,2≤a≤3。

[0014] In some implementations, the coating may contain between 200 ppm and 1000 ppm of metallic elements, as determined by inductively coupled plasma (ICP) analysis.

[0015] In some implementations, the BET specific surface area of ​​the negative electrode active material may be lower than that of the carbon-based active material.

[0016] In some implementations, the BET specific surface area of ​​the negative electrode active material may be 0.5 m². 2 / g to 1.5m 2 / g.

[0017] A method for preparing a negative electrode active material according to one embodiment includes the step of forming a coating on the surface of a carbon-based active material, said coating comprising a metal hydroxide.

[0018] In some implementations, the step of forming the coating may include: preparing a powder from a mixture of a carbon-based active material and a solution containing a metal salt; and heat-treating the powder.

[0019] In some implementations, the metal salt may include at least one of metal nitrates, metal carbonates, metal chlorides, metal phosphates, metal borates, metal oxides, metal sulfonates, metal sulfates, metal stearates, metal myristates, metal acetates, and metal undecenoates.

[0020] In some implementations, the solids content of the mixture may be 75 to 95% by weight.

[0021] In some implementations, the step of preparing powder from the mixture may include: drying the mixture to obtain powder; and classifying the powder.

[0022] In some implementations, the heat treatment can be performed at temperatures ranging from 200°C to 700°C.

[0023] In some implementations, the heat treatment can be carried out for 1 to 3 hours.

[0024] The negative electrode for a lithium secondary battery according to one embodiment includes a negative electrode active material according to any of the above embodiments.

[0025] The lithium secondary battery according to one implementation example includes a negative electrode for the lithium secondary battery according to any of the above implementation examples.

[0026] Technical effect

[0027] According to one implementation example of this disclosure, the high-temperature stability of the negative electrode active material for lithium secondary batteries can be improved.

[0028] According to another implementation of this disclosure, a lithium secondary battery with excellent high-temperature driving performance can be provided. Attached Figure Description

[0029] Figure 1 It is a cross-sectional view conceptually showing the morphology of the negative electrode active material according to an implementation example;

[0030] Figure 2 This is a graph showing the evaluation results of the high-temperature storage capacity retention of lithium secondary batteries according to the embodiments and comparative examples;

[0031] Figure 3 This is a graph showing the evaluation results of the high-temperature lifetime capacity retention of lithium secondary batteries according to the embodiments and comparative examples.

[0032] Explanation of reference numerals in the attached figures

[0033] 10: Carbon-based active substances

[0034] 11: Coating

[0035] 100: Negative electrode active material Detailed Implementation

[0036] The following details the technology disclosed in this specification and its implementation examples. However, the implementation of the technology can be modified in various ways, and its scope is not limited to the implementation examples described below. Furthermore, the technology disclosed in this specification can be applied not only in the configuration of the implementation examples described below, but also by selectively combining all or part of the implementation examples to achieve various modifications.

[0037] As mentioned above, there is a need to develop technologies that can improve the performance of lithium-ion batteries. In this regard, under high-temperature operation, the performance of lithium-ion batteries may decline due to various degradations such as solid electrolyte interphase (SEE) decomposition, electrolyte reduction, and anode material cracking. Therefore, to overcome this high-temperature disadvantage, it is necessary to ensure the high-temperature storage and high-temperature lifespan performance of lithium-ion batteries.

[0038] According to one implementation example, the high-temperature driving performance of lithium secondary batteries can be improved by applying a surface coating technology for forming an artificial SEI on the surface of the negative electrode active material. For example, for natural graphite in carbon-based active materials, pitch can be coated on the surface of natural graphite to suppress the reduction in fast charging and high-temperature driving performance caused by the many defects present on the surface.

[0039] However, the coating formed from the asphalt is prone to cracking in high-temperature environments, which can lead to carbon defects and a sharp decline in performance as the lithium secondary battery continues to cycle.

[0040] On the other hand, for artificial graphite among carbon-based active materials, the cell resistance increases and the electrode manufacturing process becomes more difficult when used as a negative electrode active material, making it difficult to use alone. Therefore, it can be used in combination with natural graphite. As mentioned above, when artificial graphite and natural graphite are mixed and used as negative electrode active materials in lithium secondary batteries, the high-temperature driving performance of the lithium secondary battery may still be insufficient due to the aforementioned problems with natural graphite.

[0041] According to one implementation example of this disclosure, the high-temperature driving performance of a lithium secondary battery can be improved by coating the surface of the carbon-based active material with a material having excellent high-temperature stability. According to this implementation example, regardless of the type of graphite, a negative electrode active material with excellent high-temperature stability can be provided, thus suppressing the aforementioned problems.

[0042] The following is for reference. Figures 1 to 3 A detailed explanation of the implementation examples of this disclosure is provided.

[0043] Figure 1 It is a cross-sectional view conceptually showing the morphology of the negative electrode active material according to an implementation example.

[0044] Figure 2 This is a graph showing the evaluation results of the high-temperature storage capacity retention of lithium secondary batteries according to the embodiments and comparative examples;

[0045] Figure 3 This is a graph showing the evaluation results of the high-temperature lifetime capacity retention of lithium secondary batteries according to the embodiments and comparative examples.

[0046] Negative electrode active material

[0047] According to one embodiment, the negative electrode active material 100 comprises: a carbon-based active material 10; and a coating 11 disposed on the surface of the carbon-based active material, the coating comprising a metal hydroxide. The negative electrode active material 100 exhibits excellent high-temperature stability due to the metal hydroxide coating formed on the surface of the carbon-based active material.

[0048] The carbon-based active material 10 is a carbon-based material capable of storing lithium ions during lithium secondary battery charging, and is not particularly limited. For example, the carbon-based active material 10 may include at least one selected from crystalline carbon, amorphous carbon, carbon composites, and carbon fibers.

[0049] Examples of the crystalline carbon may be graphite-based carbon such as natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbeads (MCMB), and graphitized mesophase pitch-based carbon fiber (MPCF).

[0050] Examples of the amorphous carbon may be hard carbon, soft carbon, coke, mesocarbon microbeads (MCMB), or mesophase pitch-based carbon fiber (MPCF).

[0051] The morphology of the negative electrode active material 100 is not particularly limited. For example, the negative electrode active material 100 may include: single particles; secondary particles formed by the aggregation of multiple primary particles; or combinations thereof.

[0052] In some implementations, the negative electrode active material 100 may be a core-shell structure comprising a core and a shell disposed on the surface of the core, wherein the core may contain a carbon-based active material and the shell may contain a metal hydroxide. Specifically, the core may be a particle containing a carbon-based active material 10, and the shell may be a coating 11.

[0053] When the negative electrode active material 100 has the aforementioned core-shell structure, the coating 11 acts as a protective layer that uniformly covers the surface of the carbon-based active material 10, thereby delaying the deterioration of the material under high-temperature conditions. Therefore, compared to the coating 11 forming an island-shaped structure on the surface of the carbon-based active material 10, the negative electrode active material 100 can have excellent high-temperature stability.

[0054] The metal hydroxide contained in the coating 11 can be formed by heat treatment of a metal salt with excellent high-temperature stability. The metal hydroxide can be a compound in the form of an intermediate product in which the metal salt is not completely converted into a cermet.

[0055] In some implementations, the metal hydroxide can be represented by the following chemical formula 1.

[0056]

Chemical Formula 1

[0057] M1 x M2 y M3 z (OH) a

[0058] In the aforementioned chemical formula 1, M1, M2, and M3 are each selected from one or more metals chosen from Al, Ni, Mg, Zn, and Cu, and 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 <x+y+z≤1,2≤a≤3。

[0059] Specifically, the metal hydroxide may include aluminum hydroxide (Al(OH)3) containing aluminum (Al). The conversion temperature of aluminum (Al) to hydroxide is low, thus it is not only easy to control but also ensures a high conversion rate. Furthermore, the reaction of aluminum (Al) with lithium after heating has low reversibility; therefore, when the coating 11 contains aluminum, it can exhibit excellent stability against side reactions.

[0060] In some implementations, the metal element content of the coating 11, as analyzed by inductively coupled plasma (ICP), can be between 200 ppm and 1000 ppm. Specifically, the metal element content of the coating 11, as analyzed by ICP, can be 400 ppm or more, 500 ppm or more, 600 ppm or more, or 650 ppm or more, and can be less than 800 ppm, 700 ppm or less, or 670 ppm or less. When the metal element content contained in the coating 11 is within the above range, the high-temperature stability of the negative electrode active material 100 achieved by forming the coating 11 can be significantly improved. The unit of the element content value can be ppm based on mass. The metal element content can be measured by inductively coupled plasma (ICP) analysis using an ICP-MS device (PerkinElmer NexION 350s) under the following conditions.

[0061] - RF Power = 1200W

[0062] - Plasma flow = 12 L / min

[0063] - Nebulizer flow = 0.7 L / min

[0064] - Auxiliary flow = 1L / min

[0065] In some implementations, the BET specific surface area of ​​the negative electrode active material 100 may be lower than that of the carbon-based active material 10. When the coating 11 is formed in an island shape on the surface of the carbon-based active material 10, the BET specific surface area can be increased compared to the carbon-based active material 10 without a coating. Conversely, when the coating 11 is formed to uniformly cover the surface of the carbon-based active material 10, the BET specific surface area can be decreased compared to the carbon-based active material 10 without a coating. Therefore, when the BET specific surface area of ​​the negative electrode active material 100 is lower than that of the carbon-based active material 10, it can be determined that the coating 11 is uniformly formed.

[0066] In some implementations, the BET specific surface area of ​​the negative electrode active material 100 may be 0.5 m². 2 / g to 1.5m 2 / g. Specifically, the BET specific surface area of ​​the negative electrode active material 100 can be 1.0m². 2 / g or more or 1.2m 2 / g or more, can be 1.3m 2 / g or less.

[0067] The BET specific surface area value of the negative electrode active material 100 can be pretreated by performing nitrogen purging and heat treatment (at 80°C for 60 minutes) on the negative electrode active material particles, and then measured using a Macsorb HM model-1208 from NOUNTECH.

[0068] Preparation method of negative electrode active material

[0069] A method for preparing a negative electrode active material 100 according to one embodiment includes the step of forming a coating 11 on the surface of a carbon-based active material 10, the coating comprising a metal hydroxide. Here, the step of forming the coating may include the step of preparing a powder from a mixture of the carbon-based active material and a solution containing a metal salt; and the step of heat-treating the powder. Specifically, the method for preparing the negative electrode active material 100 by forming the coating 11 on the surface of the carbon-based active material 10 can be performed by a wet process and a heat treatment process.

[0070] The solution containing the metal salt is a solution containing both a metal salt and a solvent, and the types of metal and salt contained in the metal salt are not particularly limited. For example, the metal contained in the metal salt can be one or more metals selected from Al, Ni, Mg, Zn, and Cu. Additionally, the metal salt can include at least one of the following: metal nitrates, metal carbonates, metal chlorides, metal phosphates, metal borates, metal oxides, metal sulfonates, metal sulfates, metal stearates, metal myristates, metal acetates, and metal undecenoates.

[0071] In some implementations, the metal salt may include a nitrate containing aluminum (Al), namely aluminum nitrate (Al(NO3)3). This nitrate is more readily oxidized during heat treatment compared to other metal salts, and the oxygen (O2) generated during oxidation facilitates the formation of hydroxides, thereby readily generating metal hydroxides. Furthermore, the use of this nitrate is advantageous for mass production.

[0072] The type of solvent is not particularly limited. For example, the solution containing the metal salt can be an aqueous solution using water as the solvent.

[0073] In some implementations, the solution containing the metal salt may be a solution containing 0.5% to 30% by weight of the metal salt. When the concentration of the metal salt is too low, the amount of coating 11 formed on the surface of the carbon-based active material 10 is insufficient, which may lead to a decrease in the effect of improving high-temperature stability. Conversely, when the concentration of the metal salt is too high, the coverage of the coating 11 formed on the surface of the carbon-based active material 10 is excessive, which is detrimental to the formation of the solid electrolyte interphase (SEI), and lithium diffusion may decrease.

[0074] The mixture can be a solution in which carbon-based active material 10 is added to a solution containing a metal salt and mixed. In some implementations, the solid content of the mixture can be 75 to 95% by weight. Specifically, the solid content of the mixture can be 77% by weight or more or 80% by weight or more, or 90% by weight or less or 85% by weight or less. When the solid content of the mixture is controlled within the above range, a coating 11 can be uniformly formed on the surface of the carbon-based active material 10, resulting in excellent coating processability.

[0075] In some implementations, the mixing of the carbon-based active material 10 and the solution containing the metal salt can be carried out using a planetary mixer. Exemplarily, the planetary mixer may have a revolutions per minute (RPM) of 60 to 100.

[0076] In some implementations, the step of preparing powder from the mixture may include obtaining powder by drying the mixture and classifying the powder. The powder may be a dried powder obtained by drying a mixture (a solution containing wet powder), and the classification step may be used to screen out only powder with a particle size (D50) of 10 μm to 20 μm.

[0077] The drying of the mixture can be carried out, for example, at a temperature of 50°C to 70°C for more than 24 hours. Additionally, the classification of the powder can be performed, for example, using an ultrasonic classifier.

[0078] The powder can be manufactured into a negative electrode active material 100 by heat treatment. In some embodiments, the heat treatment can be performed at a temperature between 200°C and 700°C. Specifically, the heat treatment can be performed at a temperature above 250°C or above 270°C, or at a temperature below 600°C, below 400°C, below 350°C, or below 330°C.

[0079] If the heat treatment temperature is too low, the coating 11 may not actually form, and if the heat treatment temperature is too high, the metal salt is completely converted into a cermet, so the coating 11 containing metal hydroxides as intermediate products may not form.

[0080] In some implementations, the heat treatment can be performed for 1 to 3 hours. If the heat treatment time is too short, the coating 11 may not actually form, and if the heat treatment time is too long, the metal salt may be completely converted into a cermet, thus the coating 11 containing the metal hydroxide as an intermediate product may not form.

[0081] Negative electrode for lithium secondary batteries

[0082] The negative electrode for a lithium secondary battery according to one embodiment includes a negative electrode active material 100 according to any of the above embodiments. For example, the negative electrode for a lithium secondary battery may include: a negative electrode current collector; and a negative electrode binder layer on at least one side of the negative electrode current collector, the negative electrode binder layer may include the negative electrode active material 100 according to any of the above embodiments.

[0083] The composition of the negative electrode current collector is not particularly limited. For example, the negative electrode current collector can be a plate or foil composed of one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. The thickness of the negative electrode current collector is not particularly limited. For example, the thickness of the negative electrode current collector can be from 0.1 μm to 50 μm.

[0084] The negative electrode mixture layer is a layer disposed on at least one side of the negative electrode current collector and may contain a negative electrode active material. The content of the negative electrode active material contained in the negative electrode mixture layer is not particularly limited. For example, the content of the negative electrode active material contained in the negative electrode mixture layer may be 70 to 99% by weight.

[0085] In addition to the aforementioned negative electrode active material 100, the negative electrode mixture layer may further contain additional negative electrode active materials. For example, the additional negative electrode active materials may be one or more selected from lithium metal, lithium alloy, silicon-containing materials, and tin-containing materials.

[0086] The lithium alloy may contain elements such as aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0087] The silicon-containing material is not particularly limited as long as it contains silicon, and can be an active material capable of alloying with lithium (Li). For example, the silicon-containing material can be selected from silicon (Si) and silicon oxide (SiO2).x ; 0 < x < 2), metal-doped silicon oxide (SiO x ; 0 < x < 2), carbon-coated silicon oxide (SiO x ; 0 < x < 2), one or more of silicon-carbon composite (Si-C) and silicon alloy.

[0088] In some embodiments, the negative electrode mixture layer may further contain an adhesive. The adhesive is not particularly limited. For example, the adhesive may be a rubber-based adhesive such as styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene-propylene rubber, butadiene rubber, isoprene rubber, silyl rubber, etc.; a cellulose-based adhesive such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose or its alkali metal salts; and any one of their combinations.

[0089] The content of the adhesive contained in the negative electrode mixture layer is not particularly limited. For example, the content of the adhesive contained in the negative electrode mixture layer may be 0.1 to 10% by weight.

[0090] In some embodiments, the negative electrode mixture layer may further contain a conductive material. The type of the conductive material is not particularly limited. For example, the conductive material may be one or more selected from particulate carbon materials and fibrous carbon materials. The particulate carbon materials may be carbon blacks such as Super-P, Super-C, acetylene black, Ketjen black, etc., and the fibrous carbon materials may be carbon fiber, carbon nanotube (CNT), vapor-grown carbon fiber (VGCF), etc.

[0091] The content of the conductive material contained in the negative electrode mixture layer is not particularly limited. For example, the content of the conductive material contained in the negative electrode mixture layer may be 0.1 to 10% by weight.

[0092] Lithium secondary batteries

[0093] A lithium secondary battery according to one embodiment includes a negative electrode for a lithium secondary battery according to any one of the above embodiments. The lithium secondary battery may include a unit cell, and the unit cell includes the negative electrode for the lithium secondary battery, a positive electrode, and a separator described above. The separator may be disposed between the positive electrode and the negative electrode.

[0094] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer on at least one surface of the positive electrode current collector.

[0095] The composition of the positive electrode current collector is not particularly limited. For example, the positive electrode current collector can be a plate or foil composed of one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. The thickness of the positive electrode current collector is not particularly limited. For example, the thickness of the positive electrode current collector can be from 0.1 μm to 50 μm.

[0096] The positive electrode mixture layer may contain a positive electrode active material. The positive electrode active material is not particularly limited and may contain compounds capable of reversibly inserting and deintercalating lithium ions. For example, the positive electrode active material may contain a lithium nickel metal oxide. The lithium nickel metal oxide may further contain at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0097] In some implementations, the positive electrode active material or the lithium nickel metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 2.

[0098] [Chemical Formula 2]

[0099] Li x Ni a M b O 2+

[0100] In the chemical formula 2, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can include Co, Mn, and / or Al.

[0101] The chemical structure represented by Formula 2 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude other additional elements. For example, M includes Co and / or Mn, and Co and / or Mn can be provided together with Ni as the main active element of the positive electrode active material. Formula 2 is provided to express the bonding relationships of the main active element and should be understood as including the introduction and substitution of additional elements.

[0102] In some implementations, auxiliary elements may be further included for addition to the main active element to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure to form bonds, and it should be understood that this is also included within the scope of the chemical structure represented by Formula 2.

[0103] The auxiliary element may include, for example, at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, and Zr. The auxiliary element may also be used, for example, together with Co or Mn, as an auxiliary active element that contributes to the capacity / power activity of the positive electrode active material, similar to Al.

[0104] For example, the positive electrode active material or the lithium nickel metal oxide may include a layered structure or crystal structure represented by the following chemical formula 2-1.

[0105]

Chemical Formula 2-1

[0106] Li x Ni a M1 b1 M2 b2 O 2+z

[0107] In chemical formula 2-1, M1 may include Co, Mn and / or Al. M2 may include the aforementioned auxiliary elements. In chemical formula 2-1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1.

[0108] The positive electrode active material may further include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, one or more combinations of the elements described above can be used as coating elements or doping elements.

[0109] The coating element or doping element may exist on the surface of the lithium nickel metal oxide particles, or may penetrate through the surface of the aforementioned lithium nickel metal composite oxide particles to be included in the bonding structure represented by chemical formula 2 or chemical formula 2-1.

[0110] The positive electrode active material may contain nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0111] The Ni content in the NCM-based lithium oxide (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the Ni content can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0112] In some embodiments, the positive electrode active material may include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP) active material (e.g., LiFePO4).

[0113] In some embodiments, the positive electrode active material may include a manganese-rich (Mn-rich)-based active material, a lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, or a cobalt-less (Co-less)-based active material having a chemical structure or crystal structure represented by Chemical Formula 3.

[0114]

Chemical Formula 3

[0115] p[Li2MnO3]·(1-p)[Li q JO2]

[0116] In Chemical Formula 3, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0117] The positive electrode mixture layer may further include a binder. The binder is not particularly limited. For example, the binder may include one or more of polyvinylidene fluoride, styrene-butadiene rubber (SBR), polytetrafluoroethylene, vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile, polymethyl methacrylate, etc.

[0118] The positive electrode mixture layer may further include a conductive material. The conductive material is not particularly limited. For example, the conductive material may include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanotube (CNT), etc.; metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide, etc.; or conductive polymers such as polyphenylene derivatives, etc., or one or more of them.

[0119] The separator is not particularly limited. For example, the separator may include a porous polymer film made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc. Additionally, the separator may further include a non-woven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0120] In some implementations, the lithium secondary battery can be manufactured by housing the aforementioned cell in a bag that serves as the battery casing and then injecting electrolyte.

[0121] The electrolyte may contain an organic solvent and a lithium salt. The organic solvent serves as a medium through which ions participating in the electrochemical reactions of the battery can migrate. For example, one or more of carbonates, esters, ethers, ketones, alcohols, or aprotic solvents may be used. When two or more solvents are used in combination, the mixing ratio may be appropriately adjusted according to the desired battery performance.

[0122] The lithium salt is dissolved in an organic solvent and serves as a lithium-ion supply source in the battery. It is a substance that enables the basic operation of a lithium secondary battery and promotes the migration of lithium ions between the positive and negative electrodes. Known substances can be used with the lithium salt at a concentration suitable for the purpose. The electrolyte may further contain known solvents as needed to improve charge-discharge characteristics, flame-retardant properties, etc., and may contain known additives.

[0123] In some implementations, the lithium secondary battery may include a solid electrolyte but not the separator between the positive and negative electrodes. The solid electrolyte is not particularly limited. For example, the solid electrolyte may be an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte.

[0124] Example

[0125] 1. Negative electrode active material

[0126] 1) Preparation of negative electrode active material

[0127] (1) Example 1

[0128] A negative electrode active material of Example 1 was prepared, in which a coating containing metal hydroxide was formed on the surface of a carbon-based active material. Specifically, a mixture was prepared by mixing a solution with a solid content of 80% by weight, prepared by mixing an aqueous solution of aluminum nitrate (Al(NO3)3) with artificial graphite (a mixture of single particles and aggregated particles), for 1 to 2 hours using a planetary mixer. Here, the concentration of the aqueous solution of aluminum nitrate (Al(NO3)3) was adjusted to 0.5% by weight, and the rotation speed (RPM) of the mixer was adjusted to 60.

[0129] The mixture was then dried in a convection oven at 50°C to 70°C for at least 24 hours, and then classified into powder using an ultrasonic classifier. The classified powder was placed in a tube furnace and then heat-treated at 300°C for 2 hours under nitrogen atmosphere to prepare the final negative electrode active material.

[0130] (2) Example 2

[0131] Except for heat treatment at 600°C, the negative electrode active material of Example 2 was prepared by the same method as in Example 1.

[0132] (3) Example 3

[0133] The negative electrode active material of Example 3, in which a coating containing metal hydroxide is formed on the surface of a carbon-based active material, was prepared. Specifically, a mixture was prepared by mixing a solution with a solid content of 87% by weight, prepared by mixing an aqueous solution of aluminum nitrate (Al(NO3)3) with artificial graphite (a mixture of single particles and aggregated particles), for 1 to 2 hours using a planetary mixer. Here, the concentration of the aqueous solution of aluminum nitrate (Al(NO3)3) was adjusted to 3% by weight, and the speed of the mixer (RPM) was adjusted to 100.

[0134] The mixture was then dried in a convection oven at 50°C to 70°C for at least 24 hours, and then classified into powder using an ultrasonic classifier. The classified powder was placed in a tube furnace and then heat-treated at 300°C for 2 hours under nitrogen atmosphere to prepare the final negative electrode active material.

[0135] (4) Example 4

[0136] Except for heat treatment at 600°C, the negative electrode active material of Example 4 was prepared by the same method as in Example 3.

[0137] (5) Comparative examples

[0138] Artificial graphite (a mixture of single particles and aggregated particles) was prepared as a negative electrode active material for comparison. The artificial graphite was a carbon-based active material that had not undergone an additional coating process.

[0139] 2) Analysis of negative electrode active materials

[0140] For the negative electrode active materials prepared in the examples and comparative examples as described above, the metal element content was measured by inductively coupled plasma analysis (ICP analysis), the BET specific surface area was measured, and the binding energy was measured by X-ray photoelectron spectroscopy (XPS). The results are shown in Table 1 below. The specific analytical methods are as follows.

[0141] (1) Metal element content (ICP analysis)

[0142] The negative electrode active material particles were put into a polypropylene (PP) tube, then hydrochloric acid and hydrogen peroxide were added and heated to dissolve them. After the sample was dissolved and clarified, it was cooled to room temperature (about 25°C) and then diluted to prepare the sample to be analyzed.

[0143] Subsequently, the sample to be analyzed was subjected to ICP analysis using an ICP-MS instrument (PerkinElmer NexION 350s) under the following conditions.

[0144] - RF Power = 1200W

[0145] - Plasma flow = 12 L / min

[0146] - Nebulizer flow = 0.7 L / min

[0147] - Auxiliary flow = 1L / min

[0148] (2) Determination of BET specific surface area

[0149] After pretreatment of the negative electrode active material particles by nitrogen purging and heat treatment (at 80°C for 60 minutes), the specific surface area of ​​the negative electrode active material particles was calculated using the Macsorb HM model-1208 from NOUNTECH.

[0150] (3) Determination of XPS binding energy

[0151] The XPS binding energy of the negative electrode active material particles was determined using an ESCALAB 250Xi from Thermo Fisher Scientific under the following conditions.

[0152] - X-ray: Al kα, 1486.68 eV, 650 μm beam size

[0153] - Analyzer: CAE machine type

[0154] - Number of scans: 2 (Full Scan), 20 (Narrow Scan)

[0155] - Pass energy: 150eV (Full Scan), 20eV (Narrow Scan)

[0156] XPS binding energy measurements showed that Al and O elements were observed in the negative electrode active materials of Examples 1 to 4 with coatings. An Al(OH)3 phase of 74.8 eV was observed in the Al 2p phase, but no metal oxide phase of 528 to 530 eV was observed in the O 1s phase. Therefore, the coatings of the negative electrode active materials of Examples 1 to 4 contain aluminum hydroxide (Al(OH)3) in the form of a metal hydroxide formed from Al and O.

[0157] Table 1

[0158]

[0159] Referring to Table 1 above, it was determined that the metal element (Al) in the negative electrode active material of the uncoated comparative example was present at a content of less than 10 ppm, while the metal element (Al) in the negative electrode active materials of Examples 1 to 4 with coatings was present at a relatively high content of 660 ppm to 720 ppm.

[0160] On the other hand, in Examples 1 and 3 where the coating was formed by heat treatment at 300°C, the BET specific surface area of ​​the negative electrode active material was found to be relatively lower than that of the comparative example, while in Examples 2 and 4 where the coating was formed by heat treatment at 600°C, the BET specific surface area of ​​the negative electrode active material was found to be relatively higher than that of the comparative example.

[0161] It is believed that this is because when the heat treatment temperature is 300℃, a core-shell structure is formed that covers the surface of the carbon-based active material more uniformly, thus reducing the specific surface area of ​​the negative electrode active material. However, when the heat treatment temperature is relatively high at 600℃, the coating forms an island shape, thus increasing the specific surface area of ​​the negative electrode active material.

[0162] Furthermore, referring to the XPS analysis results, a peak at 74.8 eV was observed in the negative electrode active materials of Examples 1 to 4, in which coatings were formed. Considering this, it was determined that the coatings of Examples 1 to 4 contain the Al(OH)3 phase as a metal hydroxide.

[0163] 2. Negative electrode slurry

[0164] 1) Preparation of negative electrode slurry

[0165] Anode slurries comprising the anode active materials of the examples and comparative examples prepared as described above were prepared. Specifically, the anode slurries were prepared to contain 97% by weight of anode active material and 3% by weight of binder (1.2% by weight of CMC and 1.8% by weight of SBR) based on the solid content.

[0166] 2) Analysis of negative electrode slurry

[0167] The viscosity values ​​of the negative electrode slurry are shown in Table 2 below. Specifically, the viscosity was measured using a Kinexus rheometer from NETZSCH, employing a plate spindle, at 4.6 s⁻¹. -1 The viscosity of the negative electrode slurry was determined by the shear rate and at a temperature of 25°C.

[0168] Table 2

[0169]

[0170] Referring to Table 2 above, the viscosity of the negative electrode slurry containing the negative electrode active material of Examples 1 to 4 with the coating was measured to be relatively higher than that of the comparative example. It is determined that this is because the formation of the coating increases the affinity between the negative electrode active material and the negative electrode binder.

[0171] 3. Negative electrode and lithium secondary battery

[0172] 1) Manufacturing of negative electrode and lithium secondary battery

[0173] To evaluate the performance of the final manufactured negative electrode and lithium secondary battery, negative electrodes and lithium secondary batteries were manufactured using the negative electrode slurries of Comparative Example, Example 1 and Example 2 prepared as described above.

[0174] Specifically, the negative electrode slurry was coated onto one side of a negative electrode current collector (copper foil (Cu-Foil)) with a thickness of 8 μm, dried at 60°C for 5 hours, and rolled to an electrode density of 1.5 g / cc, thereby manufacturing a negative electrode with a negative electrode mixture layer on one side of the negative electrode current collector. Then, a separator (polyethylene, 15 μm thick) was placed between the negative electrode and the counter electrode (lithium metal foil), and the electrode was immersed in an electrolyte solution containing 1 M LiPF6 dissolved in a mixed solvent of 25 vol% ethylene carbonate (EC) / 45 vol% ethyl methyl carbonate (EMC) / 30 vol% diethyl carbonate (DEC), thereby manufacturing a 2032 type coin-type half-cell.

[0175] 2) Performance Evaluation

[0176] 1) Initial efficiency

[0177] The initial efficiency of the coin cell battery is measured as shown in Table 3 below. Here, three cycles were performed, and the percentage (%) of the discharge capacity relative to the charge capacity in the last cycle was measured as the initial efficiency. The cycle involved charging the coin cell battery at 0.1C in CC / CV mode within a SOC range of 0-100% at 25°C. The CV range ended when the current was <0.05mA, and the battery was discharged at 0.1C.

[0178] 2) High-temperature storage capacity retention rate

[0179] The coin cell battery was stored at 60°C, and the capacity retention over time was evaluated. The results are as follows: Figure 2 As shown in Table 3 below, the capacity retention rate for week 4 is as follows. Here, the capacity retention rate is determined by charging the coin cell battery at a rate of 0.5C using CC / CV mode within the SOC range of 0-100%, ending the CV range with a current <0.05mA, and discharging at 0.5C. This constitutes one cycle, repeated three times at 25°C, with the discharge capacity measured based on the last cycle. The discharge capacity retention rate for week 4 relative to the initial discharge capacity of week 0 is measured as a percentage (%).

[0180] 3) High-temperature lifespan capacity retention

[0181] The coin cell battery was charged / discharged at 45°C, and the capacity retention rate corresponding to the number of cycles was evaluated. The results are as follows: Figure 3 As shown in Table 3 below, the capacity retention rate after 120 cycles is as follows. Here, the coin cell battery was charged at a rate of 0.5C using CC / CV mode within the SOC range of 0-100%. The CV range ended when the current was <0.05mA, and the battery was discharged at 0.5C. This constituted one cycle. After repeating this process 120 times at 45°C, the capacity retention rate relative to the initial discharge capacity was measured as a percentage (%) and used as the capacity retention rate.

[0182] Table 3

[0183]

[0184] Referring to Table 3, it can be confirmed that despite surface modification and heat treatment of the carbon-based active material, Examples 1 and 2 maintained the same or higher initial efficiency as the comparative examples. In particular, for Example 1, which underwent heat treatment at 300°C, the capacity retention rates at 60°C and 45°C were 0.8% and 10.9% higher than the comparative examples, respectively. Conversely, for Example 2, which underwent heat treatment at 600°C, the initial efficiency was very high, but the high-temperature performance was relatively poor.

[0185] This is because the coating formed by heat treatment at 300℃ acts as a protective layer that uniformly covers the surface of the carbon-based active material, thus delaying the deterioration of the material under high temperature conditions. However, the coating formed by heat treatment at 600℃ forms an island shape, which is relatively less effective in protecting the surface of the carbon-based active material, and the solid electrolyte interphase (SEI) is not uniformly formed.

Claims

1. A negative electrode active material, comprising: Carbon-based active materials; and A coating is applied to the surface of the carbon-based active material. The coating comprises a metal hydroxide.

2. The negative electrode active material according to claim 1, wherein: The negative electrode active material is a core-shell structure comprising a core and a shell disposed on the surface of the core. The core contains carbon-based active substances. The shell contains a metal hydroxide.

3. The negative electrode active material according to claim 1, wherein: The metal hydroxide is represented by the following chemical formula 1. 【Chemical Formula 1】 M1 x M2 y M3 z (OH) a In the aforementioned chemical formula 1, M1, M2, and M3 are each selected from one or more metals chosen from Al, Ni, Mg, Zn, and Cu, and 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, 0 <x+y+z≤1,2≤a≤3。 4. The negative electrode active material according to claim 1, wherein: The coating has a metal element content of 200 ppm to 1000 ppm, as determined by inductively coupled plasma analysis.

5. The negative electrode active material according to claim 1, wherein: The BET specific surface area of ​​the negative electrode active material is lower than that of the carbon-based active material.

6. The negative electrode active material according to claim 1, wherein: The BET specific surface area of ​​the negative electrode active material is 0.5 m². 2 / g to 1.5m 2 / g.

7. A method for preparing a negative electrode active material, comprising: The step of forming a coating on the surface of a carbon-based active material. The coating comprises a metal hydroxide.

8. The method for preparing the negative electrode active material according to claim 7, wherein, The steps for forming the coating include: The steps of preparing powder from a mixture of the carbon-based active material and a solution containing a metal salt; and The step of heat-treating the powder.

9. The method for preparing the negative electrode active material according to claim 8, wherein: The metal salt includes at least one of metal nitrates, metal carbonates, metal chlorides, metal phosphates, metal borates, metal oxides, metal sulfonates, metal sulfates, metal stearates, metal myristates, metal acetates, and metal undecenoates.

10. The method for preparing the negative electrode active material according to claim 8, wherein: The mixture has a solids content of 75 to 95% by weight.

11. The method for preparing the negative electrode active material according to claim 8, wherein, The steps for preparing powder from the mixture include: The step of drying the mixture to obtain a powder; and The step of classifying the powder.

12. The method for preparing the negative electrode active material according to claim 8, wherein, The heat treatment is carried out at a temperature of 200°C to 700°C.

13. The method for preparing the negative electrode active material according to claim 8, wherein, The heat treatment is carried out for 1 to 3 hours.

14. A negative electrode for a lithium secondary battery, comprising the negative electrode active material according to any one of claims 1 to 6.

15. A lithium secondary battery, comprising the negative electrode for a lithium secondary battery according to claim 14.

Images