Method for manufacturing lithium secondary battery and lithium secondary battery manufactured thereby

Abstract

The present invention relates to a method of preparing a lithium secondary battery, the method comprising the steps of: (1) preparing a first positive electrode active material by mixing and heat-treating small particle lithium composite transition metal oxide having an average particle diameter (D 50 ) of less than 7 μm; (2) preparing a second positive electrode active material by mixing and heat-treating large particle lithium composite transition metal oxide having an average particle diameter (D 50 ) of 8 μm or more with a cobalt-containing raw material and a boron-containing raw material; (3) mixing the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material having a bimodal particle size distribution; (4) preparing a positive electrode by coating the positive electrode material on a positive electrode current collector; and (5) assembling the positive electrode, a negative electrode comprising a silicon-based negative electrode active material, and a separator.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2020-0162322, filed on November 27, 2020, the disclosure of which is incorporated herein by reference.

[0002] This invention relates to a method for preparing a lithium secondary battery and the lithium secondary battery prepared therefrom. Background Technology

[0003] Recently, with the rapid proliferation of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles, the demand for rechargeable batteries with relatively high capacity, small size, and lightweight properties has increased rapidly. In particular, lithium-ion batteries are attracting attention as a power source for portable devices due to their lightweight nature and high energy density. Therefore, research and development efforts to improve the performance of lithium-ion batteries have been actively undertaken.

[0004] Carbon-based materials, such as graphite, are primarily used as anode materials in lithium-ion batteries. However, their low capacity per unit mass makes it difficult to fabricate high-capacity lithium-ion batteries. Therefore, materials that form intermetallic compounds with lithium, such as silicon, tin, and their oxides, have been developed and used as non-carbon-based anode materials exhibiting higher capacities than carbon-based materials. However, these anode materials are limited by significant irreversible capacity loss during initial charging and discharging.

[0005] To address this limitation, a method has been researched and proposed to overcome the irreversible capacity loss of the negative electrode by using a material that can provide a lithium-ion source or storage and exhibits electrochemical activity after the first cycle without degrading the overall battery performance as the positive electrode material. Specifically, there is a method that uses lithium nickel-based oxides such as Li₂NiO₂ as a sacrificial positive electrode material or an over-discharge inhibitor in the positive electrode.

[0006] However, since most lithium nickel-based oxides are limited by their high cost and the large amount of lithium byproducts that increase the amount of gas produced, there is a need for a replacement. Summary of the Invention

[0007] Technical issues

[0008] One aspect of the present invention provides a lithium secondary battery that, by introducing a positive electrode material that can compensate for problems caused by a negative electrode material with high irreversible capacity loss, can ultimately reduce lithium consumption without reducing resistance and lifespan characteristics.

[0009] Technical solution

[0010] According to one aspect of the present invention, a method for preparing a lithium secondary battery is provided, comprising the following steps:

[0011] (1) By adjusting the average particle size (D) 50 The first positive electrode active material is prepared by mixing small lithium composite transition metal oxide particles smaller than 7 μm with boron-containing raw materials and then subjecting them to heat treatment.

[0012] (2) By adjusting the average particle size (D 50 The second positive electrode active material is prepared by mixing large lithium composite transition metal oxide particles with a particle size of 8 μm or larger with cobalt-containing and boron-containing raw materials and then subjecting them to heat treatment.

[0013] (3) Mix the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material with a bimodal particle size distribution;

[0014] (4) A positive electrode is prepared by coating the positive electrode material onto the positive electrode current collector; and

[0015] (5) Assemble the positive electrode, the negative electrode containing silicon-based negative electrode active material, and the separator.

[0016] According to another aspect of the present invention, a lithium secondary battery is provided, comprising: a positive electrode containing a positive electrode material having a bimodal particle size distribution; a negative electrode containing a silicon-based negative electrode active material; and a separator.

[0017] The positive electrode material includes a first positive electrode active material and a second positive electrode active material.

[0018] The first positive electrode active material contains an average particle size (D) 50 Small lithium composite transition metal oxide particles smaller than 7 μm, and a boron-containing coating formed on the small lithium composite transition metal oxide particles, and

[0019] The second positive electrode active material contains an average particle size (D) 50 The lithium composite transition metal oxide with a particle size of 8 μm or larger, and the cobalt and boron-containing coating formed on the lithium composite transition metal oxide.

[0020] Beneficial effects

[0021] The method for preparing lithium secondary batteries of the present invention can prepare lithium secondary batteries with improved output characteristics and high-temperature life by compensating for the irreversible capacity loss of bimodal cathode materials while using high-capacity silicon-based anode active materials. Attached Figure Description

[0022] Figure 1 The scanning electron microscope (SEM) image of the positive electrode cross section prepared in Example 1 was analyzed using an electron probe microanalysis (EPMA) system. Detailed Implementation

[0023] The invention will be described in more detail below so that it can be understood more clearly.

[0024] In this specification, the term "average particle size (D)" is used. 50 The average particle size (Dsize) can be defined as the particle size at which the cumulative volume in the particle size distribution curve is 50%. For example, the average particle size can be measured using laser diffraction. 50 Specifically, after dispersing lithium composite transition metal oxide particles in a dispersion medium, the dispersion medium is introduced into a commercial laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiated with ultrasound at a frequency of approximately 28 kHz and an output of 60 W. The average particle size (D) at 50% cumulative volume can then be calculated using the analyzer. 50 ).

[0025] The method for preparing lithium secondary batteries according to the present invention includes the following steps (1) to (5).

[0026] (1) The first positive electrode active material is prepared by mixing small lithium composite transition metal oxide particles with an average particle size (D50) of less than 7 μm with boron-containing raw materials and then subjecting them to heat treatment.

[0027] (2) A second positive electrode active material is prepared by mixing large-particle lithium composite transition metal oxide with an average particle size (D50) of more than 8 μm with cobalt-containing raw materials and boron-containing raw materials and then subjecting it to heat treatment.

[0028] (3) Mix the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material with a bimodal particle size distribution;

[0029] (4) A positive electrode is prepared by coating the positive electrode material onto the positive electrode current collector; and

[0030] (5) Assemble the positive electrode, the negative electrode containing silicon-based negative electrode active material, and the separator.

[0031] For the development of high-capacity batteries, it is necessary to use silicon-based anode active materials with high capacity. However, due to the low initial charge / discharge efficiency of less than 85% of silicon-based anode active materials, there is a problem of high lithium-ion loss rate due to irreversible reactions.

[0032] Furthermore, when using positive electrode active materials with high initial charge / discharge efficiency, it is disadvantageous to obtain high energy due to the reduced charging capacity of the cell.

[0033] Therefore, by using an inefficient positive electrode active material that can provide lithium to the silicon-based negative electrode active material during the initial charge / discharge period, lithium ion loss can be reduced and energy can be increased.

[0034] The method for preparing a lithium secondary battery of the present invention provides a positive electrode material in which the output is improved by applying a boron (B) and cobalt (Co) composite coating, while the consumption of lithium ions can be reduced due to the low initial charge / discharge efficiency.

[0035] Boron coatings can form an LBO phase (lithium boron oxide phase) on the surface of the positive electrode active material. The LBO phase has the effect of increasing battery capacity and reducing resistance due to its high ionic conductivity, and can suppress side reactions between the electrolyte solution and the positive electrode surface due to its low conductivity.

[0036] Cobalt coatings can form cobalt oxide (Co3O4) on the surface of the positive electrode active material during low-temperature heat treatment, which can result in reduced discharge efficiency and increased initial resistance.

[0037] Regarding the present invention, due to the composite coating of boron and cobalt, efficiency can be reduced to suit silicon-based anode active materials without reducing resistance, and output and lifespan can be improved compared to boron coating alone.

[0038] In particular, in bimodal cathode active materials composed of small and large particles, the cobalt coating, due to its larger specific surface area, is not applied to the small particles that significantly affect output performance, but only to the large particles with a relatively small specific surface area. Therefore, the output reduction and initial resistance increase caused by cobalt oxide can be minimized.

[0039] The steps will be described in detail below.

[0040] <Preparation of cathode materials>

[0041] In the preparation of the first positive electrode active material of the present invention, the average particle size (D) is... 50 Small lithium composite transition metal oxide particles smaller than 7 μm are mixed with boron-containing raw materials and then heat-treated.

[0042] The average particle size (D) of small-particle lithium composite transition metal oxides 50 The size can range from 2μm to less than 7μm, for example, from 3μm to 6μm.

[0043] Boron-containing raw materials can be selected from H3BO3, B2O3, B4C, BF3, (C3H7O)3B, (C6H5O)3B, [CH3(CH2)3O]3B, C 13 H 19At least one of O3, C6H5B(OH)2, and B2F4, preferably at least one selected from H3BO3 and B2O3, and more preferably H3BO3. Because H3BO3 has a lower melting point and better reactivity with lithium ions than other boron-containing raw materials, it can lower the ambient reaction temperature, particularly the reaction temperature of sintering additives that promote grain growth or raw materials with high melting points. The same interpretation applies to the boron-containing raw materials used in steps (1) and (2) of this invention.

[0044] Based on the total amount of small-particle lithium composite transition metal oxides, the mixing amount of boron-containing raw materials in step (1) can be from 0.03 wt% to 0.25 wt%, for example, from 0.05 wt% to 0.15 wt%. When the amount of boron-containing raw materials is less than 0.03 wt% based on the total amount of small-particle lithium composite transition metal oxides, since the LBO phase, which is formed by contacting boron with lithium byproducts such as lithium hydroxide and lithium carbonate present on the surface of the positive electrode active material, is sufficiently formed, the effect of increasing capacity and reducing resistance can be fully achieved, and side reactions between the electrolyte solution and the surface of the positive electrode active material can be prevented. Furthermore, when the amount of boron-containing raw materials is less than 0.25 wt%, the phenomenon of increased resistance due to the formation of boron oxide (B2O3) can be prevented. Specifically, when the amount of boron is increased to a range greater than the above, since the amount of boron is greater than the amount of lithium byproducts that can react to form the LBO phase, boron oxide (B2O3) is formed in addition to LBO and acts as a resistor, which is detrimental to the increase in resistance.

[0045] The heat treatment in step (1) can be performed at 250°C to 400°C, for example, 280°C to 350°C. When the heat treatment temperature in step (1) is above 250°C, it is advantageous that no unreacted boron source remains, as the heat treatment temperature is sufficient for boron to react with lithium byproducts on the surface of the positive electrode active material. Furthermore, when the heat treatment temperature is below 400°C, the sufficient formation of the LBO phase contributes to capacity improvement. When the heat treatment is performed at a temperature above 400°C, the capacity may actually decrease, as the temperature is higher than the optimal temperature for LBO phase formation.

[0046] The heat treatment in step (1) can be carried out for 50 to 500 minutes.

[0047] In the preparation of the second positive electrode active material of the present invention, the average particle size (D) is... 50 The process involves mixing large-particle lithium composite transition metal oxides (8μm and above) with cobalt-containing and boron-containing raw materials and then heat-treating them.

[0048] The average particle size (D) of large-particle lithium composite transition metal oxides 50The size can range from 8μm to below 20μm, for example, from 9μm to 16μm.

[0049] By applying a cobalt coating to large-particle lithium composite transition metal oxides with a relatively small specific surface area instead of small-particle lithium composite transition metal oxides (which have a large specific surface area due to their relatively small average particle size, which has a significant impact on output performance), the effects of increased resistance and reduced output can be minimized, while also reducing the initial charge / discharge efficiency of the positive electrode active material.

[0050] The mixing in steps (1) and (2) can each be a dry mix, wherein the mixing is carried out without solvent.

[0051] The cobalt-containing raw material can be at least one selected from Co3O4, Co(OH)2, Co2O3, Co3(PO4)2, CoF3, Co(OCOCH3)2·4H2O, Co(NO3)·6H2O, Co(SO4)2·7H2O, and CoC2O4, preferably at least one selected from Co3O4 and Co(OH)2, and more preferably Co(OH)2. Since Co(OH)2 has a lower melting point than other cobalt-containing raw materials, it has the advantage that sufficient cobalt coating effect can be obtained even when heat-treated together with boron-containing raw materials (whose reaction temperature is lower than that of cobalt-containing raw materials).

[0052] In step (2) of this invention, the boron-containing raw material and the cobalt-containing raw material are H3BO3 and Co(OH)2, respectively. The advantage of this combination is that, due to the relatively low melting point of Co(OH)2 and the effect of lowering the reaction temperature of the surrounding H3BO3 raw material, the coating effect of boron and cobalt can be achieved at low temperature.

[0053] In embodiments of the present invention, the coating raw material components of the first positive electrode active material and the second positive electrode active material are different from each other. Specifically, no coating raw materials other than boron-containing raw materials are mixed when preparing the first positive electrode active material, and no coating raw materials other than cobalt-containing and boron-containing raw materials may be mixed when preparing the second positive electrode active material.

[0054] The heat treatment in step (2) can be performed at 250°C to 400°C, for example, 280°C to 350°C. When the heat treatment temperature in step (2) is above 250°C, it is advantageous that no unreacted boron and cobalt remain, as the heat treatment temperature is sufficient for the boron and cobalt sources to react, and when the heat treatment temperature is below 400°C, it can contribute to capacity improvement due to the sufficient formation of the LBO phase. When the heat treatment is performed at a temperature above 400°C, it is disadvantageous that the capacity may actually decrease, as the temperature is higher than the optimal temperature for forming the LBO phase.

[0055] The heat treatment in step (2) can be carried out for 50 to 500 minutes.

[0056] Based on the total amount of large-particle lithium composite transition metal oxide, the mixing amount of cobalt-containing raw material in step (2) can be from 0.1 wt% to 1.5 wt%, for example, from 0.15 wt% to 1.3 wt%. When the amount of cobalt-containing raw material is 0.1 wt% or more based on the total amount of large-particle lithium composite transition metal oxide, it is advantageous that cobalt oxide is sufficiently formed to adequately reduce the cathode efficiency, and when the amount of cobalt-containing raw material is less than 1.5 wt%, cobalt oxide can be formed at an appropriate level on the surface of the cathode active material. Excessive formation of cobalt oxide on the surface of the cathode active material may lead to a decrease in capacity and an increase in resistance.

[0057] Based on the total amount of large-particle lithium composite transition metal oxides, the mixing amount of boron-containing raw materials in step (2) can be from 0.03% to 0.25% by weight, for example, from 0.05% to 0.15% by weight. When the amount of boron-containing raw materials is 0.03% by weight or more based on the total amount of large-particle lithium composite transition metal oxides, since the LBO phase, which is formed by contacting boron with lithium byproducts such as lithium hydroxide and lithium carbonate present on the surface of the positive electrode active material, is fully formed, the effect of increasing capacity and reducing resistance can be fully realized, and side reactions between the electrolyte solution and the surface of the positive electrode active material can be prevented. Furthermore, when the amount of boron-containing raw materials is less than 0.25% by weight, the phenomenon of increased resistance due to the formation of boron oxide (B2O3) can be prevented. Specifically, when the amount of boron is increased to a range greater than the above, since the amount of boron is greater than the amount of lithium byproducts that can react to form the LBO phase, boron oxide (B2O3) is formed in addition to LBO and acts as a resistor. Therefore, it is disadvantageous that the resistance increases instead.

[0058] The lithium composite transition metal oxide in this invention can be represented by the following formula 1.

[0059] [Formula 1]

[0060] Li 1+x (Nia Co b Mn c M d )O2

[0061] In Formula 1,

[0062] M is at least one selected from tungsten (W), copper (Cu), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), zirconium (Zr), zinc (Zn), aluminum (Al), indium (In), tantalum (Ta), yttrium (Y), lanthanum (La), strontium (Sr), gallium (Ga), scandium (Sc), gadolinium (Gd), samarium (Sm), calcium (Ca), cerium (Ce), niobium (Nb), magnesium (Mg), boron (B), and molybdenum (Mo), and

[0063] x, a, b, c, and d respectively satisfy 0 ≤ x ≤ 0.2, 0.70 ≤ a < 1, 0 < b ≤ 0.25, 0 < c ≤ 0.25, and 0 ≤ d ≤ 0.1.

[0064] Preferably, M in Formula 1 can be Al.

[0065] Similarly, preferably, a, b, c, and d can be 0.70 ≤ a ≤ 0.90, 0.05 ≤ b ≤ 0.25, 0.05 ≤ c ≤ 0.25, and 0 ≤ d ≤ 0.05 respectively, such as 0.80 ≤ a ≤ 0.90, 0.05 ≤ b ≤ 0.15, 0.05 ≤ c ≤ 0.15, and 0 ≤ d ≤ 0.05.

[0066] That is, in the lithium composite transition metal oxide, the amount of nickel (Ni) in the total amount of transition metals can be 70 mol% or more, such as 80 mol% or more.

[0067] The method for preparing a lithium secondary battery according to the present invention includes step (3) of mixing a first positive electrode active material and a second positive electrode active material, wherein in step (3), the first positive electrode active material and the second positive electrode active material can be mixed at a weight ratio of 10:90 to 40:60, such as 15:85 to 30:70, and can be present in the prepared positive electrode material in the same amount range.

[0068] <Preparation of Positive Electrode Material>

[0069] In the preparation of the positive electrode of the present invention, the positive electrode material is coated on the positive electrode current collector. This can be carried out according to the conventional method for preparing a positive electrode, except for using the above positive electrode material. Specifically, the positive electrode can be prepared by coating a positive electrode slurry containing the above positive electrode material and optionally a binder and a conductive agent on the positive electrode current collector, then drying, and roll-pressing the coated positive electrode current collector.

[0070] There are no particular restrictions on the positive current collector, as long as it is conductive and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with one of the following surface treatments: carbon, nickel, titanium, silver, etc.

[0071] The solvent for the cathode slurry can be an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and its amount can be such that a desired viscosity is obtained when the cathode material, along with optional binders and conductive agents, is included. For example, the solvent content can result in a concentration of solid components in the cathode slurry of 10% to 90% by weight, for example, 40% to 85% by weight.

[0072] The binder in the positive electrode slurry is a component that facilitates adhesion between the positive electrode material and the conductive agent, and to the current collector. The amount of binder added is typically from 1% to 30% by weight of the total weight of the solid components in the positive electrode slurry. Examples of binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer monomers, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0073] The conductive agent in the positive electrode slurry is a material that provides conductivity without causing adverse chemical changes in the battery. The amount of conductive agent added can be from 0.5% by weight to 20% by weight, based on the total weight of the solid components in the positive electrode slurry.

[0074] As conductive agents, the following conductive materials can be used, such as: carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; graphite powder, such as natural graphite, artificial graphite, or graphite with a well-developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or polyphenylene derivatives.

[0075] Based on the total weight of the solid components in the cathode slurry, the content of the cathode material can be from 80% to 99% by weight, for example, from 90% to 99% by weight. In this case, when the amount of cathode material is below 80% by weight, the capacity may decrease due to the reduced energy density.

[0076] <Preparation of the negative electrode>

[0077] The negative electrode of the present invention comprises a silicon-based negative electrode active material and can be prepared by coating a negative electrode current collector with a negative electrode slurry comprising a negative electrode active material, a binder, a conductive agent and a solvent, and then drying and rolling the coated negative electrode current collector.

[0078] The thickness of the negative electrode current collector is generally 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum cadmium alloy, etc. can be used. In addition, similar to the positive electrode current collector, the negative electrode current collector can have fine surface roughness to improve the adhesion strength with the negative electrode active material, and the negative electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, non-woven bodies, etc.

[0079] The silicon-based negative electrode active material in the present invention can be at least one selected from silicon (Si), SiOx (0 < x < 2), and Si-Y alloy (where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof and is not Si), and can preferably be Si or SiO.

[0080] Since the capacity of the silicon-based negative electrode active material is almost 10 times that of graphite, the fast charging performance of the battery can be improved by reducing the mass loading (mg·cm -2 ). However, there is a problem of a high lithium ion loss rate due to irreversible reactions, and this problem can be solved by using the above-mentioned positive electrode material.

[0081] In addition to the silicon-based negative electrode active material, the negative electrode of the present invention can also include a carbon-based negative electrode active material. The carbon-based negative electrode active material commonly used in lithium ion secondary batteries can be used without particular limitation, and as typical examples, crystalline carbon, amorphous carbon, or both can be used. Examples of crystalline carbon can be graphite, such as irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of amorphous carbon can be soft carbon (low-temperature sintered carbon), hard carbon, mesophase pitch carbide, or sintered coke.

[0082] Based on the total weight of the negative electrode active material, the content of the silicon-based negative electrode active material in the present invention can be 1 wt% to 100 wt%, for example, 3 wt% to 10 wt%.

[0083] Based on the total weight of the solid components in the negative electrode slurry, the content of the negative electrode active material can be 80 wt% to 99 wt%.

[0084] The binder is a component that facilitates adhesion between the conductive agent, the negative electrode active material, and the current collector, wherein the amount of binder added is typically from 1% to 30% by weight based on the total weight of the solid components in the negative electrode slurry. Examples of binders may be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0085] A conductive agent is a component used to further improve the conductivity of the negative electrode active material, wherein the amount of conductive agent added can be from 1% to 20% by weight based on the total weight of the solid components in the negative electrode slurry. Any conductive agent can be used without particular restriction, as long as it is conductive and does not cause adverse chemical changes in the battery. For example, conductive materials such as: carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; graphite powder, such as natural graphite, artificial graphite, or graphite with a well-developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powders, aluminum powders, or nickel powders; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides such as titanium oxide; or polyphenylene derivatives.

[0086] The solvent for the negative electrode slurry may include water or organic solvents such as NMP and alcohol, and its amount may be such that a desired viscosity is obtained when the negative electrode active material, binder, and conductive agent are included. For example, the solvent content may be such that the concentration of the solid components in the negative electrode slurry containing the negative electrode active material, binder, and conductive agent is 50% to 75% by weight, for example, 50% to 65% by weight.

[0087] <Preparation of Lithium Secondary Batteries>

[0088] The method for preparing a lithium secondary battery according to the present invention includes the steps of assembling a positive electrode, a negative electrode containing a silicon-based negative electrode active material, and a separator (5).

[0089] Specifically, in step (5), a separator is placed between the positive and negative electrodes, stacked sequentially, and dried to prepare an electrode assembly. Subsequently, a lithium secondary battery can be prepared by inserting the assembly into a housing, injecting electrolyte, and then sealing the housing.

[0090] The separator separates the positive and negative electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium secondary batteries. Specifically, separators with high electrolyte retention capacity and low resistance to electrolyte ion transfer can be used. Porous polymer membranes can be used, such as those prepared from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures having two or more layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Additionally, coated separators including ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can optionally be used.

[0091] The electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes or molten inorganic electrolytes that can be used to prepare lithium secondary batteries, but the present invention is not limited thereto.

[0092] Specifically, electrolytes may include organic solvents and lithium salts.

[0093] As organic solvents, any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions involved in the electrochemical reactions of the battery can move. Specifically, as organic solvents, ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; or carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic C2-C20 hydrocarbon group and may include double-bonded aromatic rings or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate-based solvents can be used, and for example, mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, and low-viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) can be used, which can increase the charge / discharge performance of the battery. In this case, the performance of the electrolyte solution can be excellent when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0094] Lithium salts can be used without particular restrictions, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, lithium salts that can be used include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or combinations thereof. Lithium salts can be used in concentrations ranging from 0.1 M to 2.0 M. When the concentration of the lithium salt is within the above range, excellent electrolyte performance can be obtained because the electrolyte can have suitable conductivity and viscosity, and lithium ions can move efficiently.

[0095] To improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, in addition to the electrolyte components mentioned above, at least one additive may be added to the electrolyte. Examples of additives include haloalkylene carbonates such as ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphotriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive content can be from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0096] <Lithium secondary batteries>

[0097] The lithium secondary battery of the present invention is such that it comprises:

[0098] This includes positive electrodes with bimodal particle size distribution, negative electrodes with silicon-based anode active materials, and separators.

[0099] The positive electrode material includes a first positive electrode active material and a second positive electrode active material.

[0100] The first positive electrode active material contains an average particle size (D) 50 Small lithium composite transition metal oxide particles smaller than 7 μm, and a boron-containing coating formed on the small lithium composite transition metal oxide particles, and

[0101] The second positive electrode active material includes an average particle size (D) 50 The lithium composite transition metal oxide with a particle size of 8 μm or larger, and the cobalt and boron-containing coating formed on the lithium composite transition metal oxide.

[0102] Lithium secondary batteries can be prepared according to the method described above, and the various configurations can be referred to the description of the method for preparing lithium secondary batteries.

[0103] The coatings contained in the first and second positive electrode active materials in the lithium secondary battery of the present invention can be confirmed by using an electron probe microanalyzer (EPMA).

[0104] In an embodiment of the present invention, the coatings of the first positive electrode active material and the second positive electrode active material have different constituent components. Specifically, the coating of the first positive electrode active material is formed of boron, and the coating of the second positive electrode active material is formed of cobalt and boron.

[0105] The lithium secondary battery of the present invention can be used in portable devices, such as mobile phones, laptops and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0106] According to another embodiment of the present invention, a battery module including the above-mentioned lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0107] The battery module or battery pack can be used as a power source for at least one of the following medium to large-sized devices: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0108] In the following, embodiments of the invention will be described in detail in a manner that can be readily practiced by those skilled in the art.

[0109] [Examples and Comparative Examples: Preparation of Cathode Materials]

[0110] Example 1

[0111] Li(Ni) 0.8 Co 0.1 Mn 0.1 O2(D) 50 Lithium-based composite transition metal oxides (represented by 4 μm) and H3BO3 were dry-mixed at a weight ratio of 1:0.05. The mixture was then heat-treated in air at 290 °C for 200 minutes to prepare a first positive electrode active material with a boron-containing coating formed thereon.

[0112] Separately, Li(Ni) 0.8 Co 0.1 Mn 0.1 O2(D) 50A lithium composite transition metal oxide (denoted as 13 μm), Co(OH)₂, and H₃BO₃ were dry-mixed in a weight ratio of 1:0.4:0.05. The mixture was then heat-treated in air at 290 °C for 200 minutes to prepare a second positive electrode active material with a cobalt and boron coating formed thereon.

[0113] The first positive electrode active material and the second positive electrode active material are mixed in a weight ratio of 15:85 to prepare a bimodal positive electrode material.

[0114] A positive electrode slurry (solids content: 50 wt%) was prepared by mixing the positive electrode material, conductive agent (carbon black), and binder (polyvinylidene fluoride, PVdF) in an N-methyl-2-pyrrolidone (NMP) solvent at a weight ratio of 97.5:1.0:1.5. The positive electrode slurry was coated on one surface of an aluminum current collector, dried at 130°C, and then rolled to prepare the positive electrode.

[0115] Figure 1 The scanning electron microscope (SEM) image of the positive electrode cross section is analyzed using an electron probe microanalysis (EPMA) system. Figure 1 The portion shown in green is the cobalt (Co) coating, and the peaks shown in the image represent the Co concentration. From Figure 1 It is understandable that the Co coating only forms on large particles.

[0116] Example 2

[0117] The cathode was prepared in the same manner as in Example 1, except that the amount of Co(OH)2 was increased during the preparation of the second cathode active material in Example 1, so that the weight ratio of lithium composite transition metal oxide, Co(OH)2 and H3BO3 was 1:1.1:0.05.

[0118] Example 3

[0119] The cathode was prepared in the same manner as in Example 1, except that the amount of H3BO3 was increased during the preparation of the first cathode active material in Example 1, so that the weight ratio of lithium composite transition metal to H3BO3 was 1:0.13, and the amount of H3BO3 was increased during the preparation of the second cathode active material in Example 1, so that the weight ratio of lithium composite transition metal oxide, Co(OH)2 and H3BO3 was 1:0.4:0.13.

[0120] Comparative Example 1

[0121] The positive electrode was prepared in the same manner as in Example 1, except that no coating was formed on the first and second positive electrode active materials in Example 1.

[0122] Comparative Example 2

[0123] The positive electrode was prepared in the same manner as in Example 1, except that only boron coating was performed during the formation of the coating of the second positive electrode active material in Example 1, without cobalt.

[0124] Comparative Example 3

[0125] The positive electrode was prepared in the same manner as in Example 1, except that no coating was formed on the first positive electrode active material in Example 1, and only cobalt coating was performed during the formation of the coating on the second positive electrode active material, without cobalt.

[0126] Comparative Example 4

[0127] The positive electrode was prepared in the same manner as in Example 1, except that, in the process of forming the coating of the first positive electrode active material in Example 1, a cobalt and boron-containing coating was formed by adding Co(OH)2 to make the weight ratio of lithium composite transition metal oxide, Co(OH)2 and H3BO3 1:0.4:0.05.

[0128] [Experimental Example]

[0129] Experiment Example 1: Capacitance and Initial Resistance Detection

[0130] Electrode assemblies were prepared by placing a 15 μm thick polyethylene-based separator between each positive electrode and lithium metal negative electrode prepared in Examples 1 to 3 and Comparative Examples 1 to 4. The electrode assemblies were then placed in a battery casing, and an electrolyte solution was injected into the casing to prepare each lithium secondary battery. In this case, the lithium secondary battery was prepared by injecting an electrolyte solution in which 1 M LiPF6 was dissolved, wherein ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a 1:2 volume ratio, and the capacity and resistance of the lithium secondary battery were measured during 0.1C charging and discharging.

[0131] Specifically, lithium secondary batteries using the positive electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 were charged to 4.25V at a constant current of 0.1C at 25°C and then charged to a cutoff current of 0.05C. Subsequently, each lithium secondary battery was discharged to 3.0V at a constant current of 0.1C to measure the initial charge / discharge capacity. The initial resistance was measured by dividing the voltage drop during the first 10 seconds of discharge by the current value. The results are shown in Table 1 below.

[0132] [Table 1]

[0133]

[0134] Referring to Table 1, it can be confirmed that the batteries using the positive electrodes of Examples 1 to 3, combined with lithium metal anodes, exhibit low initial resistance and low initial charge / discharge efficiency. This means that when used with silicon-based anode active materials with high lithium-ion loss rates, it is advantageous in reducing lithium consumption. Conversely, since the positive electrode of Comparative Example 1, in which neither small nor large particles are coated, and the positive electrode of Comparative Example 2, in which both small and large particles undergo only boron coating, have very high initial charge / discharge efficiency when combined with lithium metal anodes, it is expected that a large amount of lithium ions will be lost when used with silicon-based anode active materials. For the positive electrode of Comparative Example 3, in which small particles are uncoated and only large particles undergo only cobalt coating, and for the positive electrode of Comparative Example 4, in which both small and large particles undergo boron and cobalt coating, the initial charge / discharge efficiency may be reduced, but high initial resistance is confirmed.

[0135] Experiment Example 2: Output Characteristic Evaluation

[0136] The room temperature output resistance of lithium secondary batteries including positive and silicon-based negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 4 was measured.

[0137] Specifically, a negative electrode active material (composed of 3 wt% SiO and 97 wt% artificial graphite), a binder (SBR-CMC), and a conductive agent (carbon black) were added to water as a solvent in a weight ratio of 95:3.5:1.5 to prepare a negative electrode slurry (solid content: 60 wt%). A 6 μm thick copper (Cu) film, serving as the negative electrode current collector, was coated with the negative electrode slurry, dried, and rolled to prepare the negative electrode.

[0138] Electrode assemblies were prepared by placing a 15 μm thick polyethylene-based separator between each positive and negative electrode prepared in Examples 1 to 3 and Comparative Examples 1 to 4. The electrode assemblies were then placed in a battery casing, and an electrolyte solution was injected into the casing to prepare each lithium secondary battery. In this case, the lithium secondary battery was prepared by injecting an electrolyte solution in which 1 M LiPF6 was dissolved in a mixed organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:2.

[0139] Each lithium secondary battery was charged to 4.2V at 0.5C in constant current-constant voltage (CCCV) mode at 25°C and discharged at a constant current of 2.0C for 30 seconds to measure the output resistance with a voltage drop over 30 seconds. The results are shown in Table 2 below.

[0140] [Table 2]

[0141] positive electrode used Room temperature output resistance (Ohm) Example 1 1.51 Example 2 1.58 Example 3 1.63 Comparative Example 1 1.98 Comparative Example 2 1.67 Comparative Example 3 1.85 Comparative Example 4 1.78

[0142] As can be seen from the results in Table 2, the combination of the positive electrode in Examples 1 to 3 with the negative electrode containing Si-based negative electrode active material has the effect of reducing output resistance.

[0143] Experimental Example 3: Evaluation of High-Temperature Lifetime Characteristics

[0144] The high-temperature lifetime characteristics of lithium secondary batteries, including positive electrodes and silicon-based negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 4, were measured.

[0145] Specifically, a negative electrode active material (composed of 3 wt% SiO and 97 wt% artificial graphite), a binder (SBR-CMC), and a conductive agent (carbon black) were added to water as a solvent in a weight ratio of 95:3.5:1.5 to prepare a negative electrode slurry (solid content: 60 wt%). The negative electrode slurry was coated onto a 6 μm thick copper (Cu) film as the negative electrode current collector, dried, and then rolled to prepare the negative electrode.

[0146] Electrode assemblies were prepared by placing a 15 μm thick polyethylene-based separator between each positive and negative electrode prepared in Examples 1 to 3 and Comparative Examples 1 to 4. The electrode assemblies were then placed in a battery casing, and an electrolyte solution was injected into the casing to prepare each lithium secondary battery. In this case, the lithium secondary battery was prepared by injecting an electrolyte solution in which 1 M LiPF6 was dissolved in a mixed organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:2.

[0147] Each lithium-ion battery was charged to 4.2V at 0.5C in CCCV mode at 45°C and discharged to 3.0V at a constant current of 0.5C to measure the capacity retention and resistance increase rate after 200 charge / discharge cycles. The results are shown in Table 3 below.

[0148] [Table 3]

[0149] positive electrode used Capacity retention rate (%) Resistance improvement rate (%) Example 1 93.4 115.6 Example 2 92.9 121.5 Example 3 91.8 128.9 Comparative Example 1 87.1 171.2 Comparative Example 2 91.2 131.4 Comparative Example 3 88.5 154.2 Comparative Example 4 89.3 148.9

[0150] The results in Table 3 confirm that, compared with the positive electrodes of Comparative Examples 1 to 4, the combination of the positive electrodes of Examples 1 to 3 with the negative electrode containing Si-based active material more effectively increases the capacity retention rate and reduces the resistance increase rate of the battery under high temperature conditions.

[0151] Based on the results of the aforementioned experiments, the lithium secondary battery of the present invention exhibits excellent output characteristics and high-temperature lifespan.

Claims

1. A method for preparing a lithium secondary battery, comprising the following steps: (1) By using the average particle size D 50 The first positive electrode active material is prepared by mixing lithium composite transition metal oxide particles smaller than 7 μm with boron-containing raw materials and then subjecting them to heat treatment. (2) By using the average particle size D 50 The second positive electrode active material is prepared by mixing large lithium composite transition metal oxide particles with a particle size of 8 μm or larger with cobalt-containing and boron-containing raw materials and then subjecting them to heat treatment. (3) Mixing the first positive electrode active material and the second positive electrode active material to prepare a positive electrode material having a bimodal particle size distribution; (4) Preparing a positive electrode by coating the positive electrode material on a positive electrode current collector; and (5) Assembling the positive electrode, a negative electrode including a silicon-based negative electrode active material, and a separator, wherein, no cobalt coating is applied to the small particle lithium composite transition metal oxide.

2. The method according to claim 1, wherein, The boron-containing raw material is at least one selected from H3BO3 and B2O3.

3. The method according to claim 1, wherein, The cobalt-containing raw material is at least one selected from Co3O4 and Co(OH)2.

4. The method according to claim 1, wherein, Based on 1 part by weight of the small particle lithium composite transition metal oxide, the mixing amount of the boron-containing raw material in step (1) is 0.03 to 0.25 parts by weight.

5. The method according to claim 1, wherein, Based on 1 part by weight of the large particle lithium composite transition metal oxide, the mixing amount of the cobalt-containing raw material in step (2) is 0.1 to 1.5 parts by weight.

6. The method according to claim 1, wherein, Based on 1 part by weight of the large particle lithium composite transition metal oxide, the mixing amount of the boron-containing raw material in step (2) is 0.03 to 0.25 parts by weight.

7. The method according to claim 1, wherein, The heat treatment of step (1) is carried out at 250 °C to 400 °C.

8. The method according to claim 1, wherein, The heat treatment of step (2) is carried out at 250 °C to 400 °C.

9. The method according to claim 1, wherein, The first positive electrode active material and the second positive electrode active material in step (3) are mixed at a weight ratio of 10:90 to 40:

60.

10. The method according to claim 1, wherein, The lithium composite transition metal oxide is represented by Formula 1: [Formula 1] Li 1+x (Ni a Co b Mr c M d )O2 wherein, in Formula 1, M is at least one selected from W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and x, a, b, c and d respectively satisfy 0 ≤ x ≤ 0.2, 0.70 ≤ a < 1, 0 < b ≤ 0.25, 0 < c ≤ 0.25 and 0 ≤ d ≤ 0.

1.

11. The method according to claim 1, wherein, The silicon-based anode active material is at least one selected from the following: Si; SiO x , where 0 < x < 2; and Si-Y alloy, where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si.

12. A lithium secondary battery, comprising: A positive electrode including a positive electrode material having a bimodal particle size distribution, a negative electrode including a silicon-based negative electrode active material, and a separator, wherein, the positive electrode material includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material contains an average particle size D 50 Small lithium composite transition metal oxide particles smaller than 7 μm, and a boron-containing coating formed on the small lithium composite transition metal oxide particles. The second positive electrode active material contains an average particle size D 50 The lithium composite transition metal oxide consists of large particles of lithium with a particle size of 8 μm or larger, and a cobalt- and boron-containing coating is formed on the large particles of lithium composite transition metal oxide. wherein, the boron-containing coating formed on the small particle lithium composite transition metal oxide does not contain cobalt.

13. The lithium secondary battery according to claim 12, wherein, The first positive electrode active material and the second positive electrode active material are mixed at a weight ratio of 10:90 to 40:

60.

14. The lithium secondary battery according to claim 12, wherein, In the lithium composite transition metal oxide, the amount of nickel in the total amount of transition metals is more than 70 mol%.

15. The lithium secondary battery according to claim 12, wherein, The silicon-based anode active material is at least one selected from the following: Si; SiO x , where 0 < x < 2; and Si-Y alloy, where Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si.

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