Negative electrode for secondary battery and lithium secondary battery comprising same

Abstract

A negative electrode for a secondary battery according to one embodiment of the present invention comprises a negative electrode current collector and a negative electrode mixture layer on at least one surface of the negative electrode current collector, in which the negative electrode mixture layer contains a negative electrode active material, a binder, and a thermally crosslinkable additive, the weight of the binder contained in the negative electrode mixture layer is greater than the weight of the thermally crosslinkable additive contained in the negative electrode mixture layer. According to one embodiment of the present invention, the energy density of the negative electrode for a secondary battery can be improved.

Description

Technical Field

[0001] The present invention relates to a negative electrode for a secondary battery and a lithium secondary battery comprising the negative electrode. Background Art

[0002] In recent years, a lot of research has been conducted on electric vehicles (EVs) that can replace fossil fuel vehicles such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution. As a power source for such electric vehicles (EVs), lithium secondary batteries with high discharge voltage and power stability are mainly used.

[0003] Typically, electrodes for secondary batteries are manufactured by drying a slurry containing an active material and then rolling it, and secondary battery cells are manufactured by stacking and assembling the electrodes manufactured as described above. In addition, the final thickness of the designed electrode usually refers to the rolled thickness, and the thickness of the secondary battery cell depends on the number of stacked electrodes. In order to ensure the high energy density of such a secondary battery, it is necessary to reduce the electrode thickness and the cell volume. Summary of the Invention

[0004] (1) Technical issues to be resolved

[0005] According to one aspect of the present invention, the occurrence of a spring-back phenomenon in which the thickness of an electrode increases relative to a designed thickness during a manufacturing process of a secondary battery can be suppressed.

[0006] According to another aspect of the present invention, damage to electrodes during a manufacturing process of a secondary battery can be prevented.

[0007] According to another aspect of the present invention, the manufacturing processability of a secondary battery can be improved.

[0008] (2) Technical solution

[0009] According to one specific embodiment, a negative electrode for a secondary battery includes a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector, wherein the negative electrode mixture layer contains a negative electrode active material, a binder and a thermally crosslinkable additive, and the weight of the binder contained in the negative electrode mixture layer is greater than the weight of the thermally crosslinkable additive contained in the negative electrode mixture layer.

[0010] The binder may comprise a cellulose-based compound.

[0011] The thermally crosslinkable additive may include one or more selected from epoxy compounds, borate compounds, thiol compounds, and carboxylic acid compounds.

[0012] The epoxy compound may include at least one selected from polyethylene glycol diglycidyl ether (PEGDE) and polypropylene glycol diglycidyl ether (PPGDE).

[0013] The borate-based compound may include at least one selected from the group consisting of sodium borate, sodium tetraborate, tetrahydroxy borate, and sodium tetraborate decahydrate.

[0014] The thiol compound may include one or more selected from 3-mercaptopropionic acid, L-cysteine, and 3-mercapto-1,2-propanediol.

[0015] A weight ratio of the binder and the thermally crosslinkable additive contained in the negative electrode mixture layer may be more than 5:5 and 9:1 or less.

[0016] The binder and the thermal cross-linking additive may be included in the negative electrode mixture layer in a weight ratio of 7:3 to 9:1.

[0017] The thermal cross-linking additive may be included in the negative electrode mixture layer in an amount of 0.1 to 5% by weight.

[0018] The negative electrode mixture layer may further include a rubber-based compound as an additional binder.

[0019] A weight ratio of the binder and the additional binder included in the negative electrode mixture layer may be 1:1 to 1:5.

[0020] The electrode adhesion between the negative electrode current collector and the negative electrode mixture layer may be 0.4 N / m 2 above.

[0021] The negative electrode mixture layer may include a first negative electrode mixture layer on the negative electrode current collector and a second negative electrode mixture layer on the first negative electrode mixture layer.

[0022] The second negative electrode mixture layer may contain a thermally cross-linkable additive.

[0023] A lithium secondary battery according to one embodiment includes the negative electrode for a secondary battery according to any one of the above embodiments.

[0024] (3) Beneficial effects

[0025] According to one embodiment of the present invention, the energy density of a negative electrode for a secondary battery can be improved.

[0026] According to another embodiment of the present invention, it is possible to prevent damage to a negative electrode during the manufacturing process of a secondary battery.

[0027] According to another embodiment of the present invention, it is possible to prevent the production processability of a secondary battery from being degraded. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Graphs showing evaluation results of electrode binding forces of negative electrodes according to Examples and Comparative Examples.

[0029] Figure 2 is a cross-sectional view schematically showing a negative electrode according to one specific embodiment.

[0030] Figure 3 is a cross-sectional view schematically showing a negative electrode according to another specific embodiment.

[0031] Description of reference numerals:

[0032] 10: Negative electrode current collector

[0033] 20: Negative electrode mixture layer

[0034] 21: First negative electrode mixture layer

[0035] 22: Second negative electrode mixture layer

[0036] 100: Negative electrode for secondary batteries DETAILED DESCRIPTION

[0037] The following describes the technology disclosed in this specification and its specific embodiments in detail with reference to the accompanying drawings. However, the embodiments of the technology can be modified in various forms, and the scope of the technology is not limited to the specific embodiments described below. In addition, the technology disclosed in this specification can be applied not only by being limited to the configuration of the specific embodiments described below, but also by selectively combining all or part of the specific embodiments to achieve various variations.

[0038] As mentioned above, secondary battery electrodes are typically manufactured by drying a slurry containing an active material and then rolling it. Secondary battery cells are then manufactured by stacking and assembling the electrodes manufactured in this manner. However, during the vacuum drying (VD) process, which removes moisture before electrode assembly, the stress within the secondary battery electrode may relax due to heat, potentially causing a rebound phenomenon in which the final thickness of the electrode increases relative to the designed thickness.

[0039] This increase in electrode thickness affects the volume of the resulting secondary battery cell. To control this increase, greater pressure and / or additional rolling processes may be required during electrode rolling. In this case, the rolling process may damage the electrode or reduce the production processability of the secondary battery.

[0040] According to a specific embodiment of the present invention, the above-mentioned rebound phenomenon can be suppressed by adding a thermal cross-linking additive when manufacturing the electrode. Figures 1 to 3 , the specific implementation scheme of the present invention is described in detail.

[0041] Figure 1 Graphs showing evaluation results of electrode binding forces of negative electrodes according to Examples and Comparative Examples.

[0042] Figure 2 is a cross-sectional view schematically showing a negative electrode according to one specific embodiment.

[0043] Figure 3 is a cross-sectional view schematically showing a negative electrode according to another specific embodiment.

[0044] Negative electrode for secondary batteries

[0045] According to one specific embodiment, a secondary battery negative electrode 100 includes a negative electrode current collector 10 and a negative electrode mixture layer 20 on at least one side of the negative electrode current collector. The negative electrode mixture layer 20 includes a negative electrode active material, a binder, and a thermally crosslinkable additive, wherein the weight of the binder in the negative electrode mixture layer 20 is greater than the weight of the thermally crosslinkable additive in the negative electrode mixture layer. The thermally crosslinkable additive in the negative electrode mixture layer may include 1) thermally crosslinked compounds and / or 2) thermally crosslinked compounds with other compounds in the negative electrode mixture layer. For example, the thermally crosslinked compounds 1) and / or 2) may be polymers.

[0046] The composition of the negative electrode current collector 10 is not particularly limited. For example, the negative electrode current collector can be a plate or foil formed from 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 alloys thereof. Furthermore, the thickness of the negative electrode current collector is not particularly limited. For example, the thickness of the negative electrode current collector can be 0.1-50 μm.

[0047] According to a specific embodiment, the negative electrode mixture layer 20 may be a single-layer structure (refer to Figure 2 ). According to another specific embodiment, the negative electrode mixture layer 20 may be a dual-layer structure including a first negative electrode mixture layer 21 on at least one surface of the negative electrode current collector 10 and a second negative electrode mixture layer 22 on the first negative electrode mixture layer 21 (refer to Figure 3 ).

[0048] The negative electrode active material is not particularly limited. Exemplarily, the negative electrode active material may be one or more selected from carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium metal; lithium alloys; silicon-containing substances and tin-containing substances.

[0049] Exemplarily, the crystalline carbon may be graphite-based carbon such as natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbead (MCMB), graphitized mesophase pitch-based carbon fiber (MPCF), etc.

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

[0051] Exemplarily, the elements contained in the lithium alloy may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

[0052] The silicon-containing substance is not particularly limited as long as it contains silicon, and the silicon-containing substance may be an active material that can be alloyed with lithium (Li). Exemplarily, the silicon-containing substance may be one or more selected from silicon (Si), silicon oxide (SiO x ; 0 < x < 2), metal-doped silicon oxide (SiO x ; 0 < x < 2), carbon-coated silicon oxide (SiO x ; 0 < x < 2), silicon-carbon composite (Si-C) and silicon alloy.

[0053] In the specific embodiment where the negative electrode mixture layer 20 includes a first negative electrode mixture layer 21 and a second negative electrode mixture layer 22, the first negative electrode mixture layer 21 and the second negative electrode mixture layer 22 may each contain a silicon-containing substance.

[0054] In a specific embodiment in which the first and second negative electrode mixture layers 21 and 22 each contain a silicon-containing substance, the content of the silicon-containing substance contained in the second negative electrode mixture layer 22 may be greater than or equal to the content of the silicon-containing substance contained in the first negative electrode mixture layer 21 .

[0055] In a specific embodiment in which the first and second negative electrode mixture layers 21 and 22 each contain a silicon-containing substance, the weight ratio of the silicon-containing substance contained in the first and second negative electrode mixture layers 21 and 22 may be 1:2 to 1:10.

[0056] When the silicon-containing material is added as described above, a relatively small amount of the silicon-containing material is contained in the first negative electrode mixture layer 21 adjacent to the negative electrode current collector 10, and the silicon-containing material has a large volume expansion / contraction degree during charge / discharge of the secondary battery, thereby preventing the negative electrode from falling off, etc., and improving the life of the secondary battery.

[0057] For example, the negative electrode active material content in the negative electrode mixture layer 20 may be 50-99 wt % or 80-95 wt %.

[0058] The binder can impart viscosity to the negative electrode slurry, thereby enhancing the cohesion between the negative electrode active materials and inhibiting cracks on the electrode surface. The binder can cross-link with the thermal cross-linking additive to inhibit electrode swelling.

[0059] In some specific embodiments, the binder may include a cellulose-based compound. Exemplarily, the cellulose-based compound may be at least one selected from methylcellulose (MC), carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC) and hydroxypropylcellulose (HPC). In some specific embodiments, the binder may include the above-mentioned cellulose-based compound in the form of an alkali metal salt.

[0060] For example, the content of the binder included in the negative electrode mixture layer 20 may be 0.01-30 wt % or 0.1-10 wt %.

[0061] In some specific embodiments, the negative electrode mixture layer 20 may further include a rubber-based compound as an additional binder. The rubber-based compound may function as an additional binder to improve the bonding strength between the components in the negative electrode slurry and further improve the adhesion between the negative electrode current collector 10 and the negative electrode mixture layer 20. For example, the rubber-based compound may be at least one selected from styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene-propylene rubber, butadiene rubber, isoprene rubber, and silane-based rubber.

[0062] For example, the content of the additional binder included in the negative electrode mixture layer 20 may be 0.01-30 wt % or 0.1-10 wt %.

[0063] In some specific embodiments, the weight ratio of the binder to the additional binder in the negative electrode mixture layer may be 1:1 to 1:5. For example, the weight ratio of the binder to the additional binder in the negative electrode mixture layer may be 1:1 to 1:3.

[0064] In a specific embodiment in which the negative electrode mixture layer 20 includes a first negative electrode mixture layer 21 and a second negative electrode mixture layer 22 , the first negative electrode mixture layer 21 and the second negative electrode mixture layer 22 may each contain a binder and an additional binder.

[0065] In a specific embodiment in which the first and second negative electrode mixture layers 21 and 22 each contain a binder and an additional binder, the total content of the binder contained in the first negative electrode mixture layer 21 may be greater than or equal to the total content of the binder contained in the second negative electrode mixture layer 22 .

[0066] When the binder is added as described above, a relatively large amount of the binder is contained in the first negative electrode mixture layer 21 adjacent to the negative electrode collector 10 , so that the binding force between the negative electrode collector 10 and the negative electrode mixture layer 20 can be further improved.

[0067] The thermally crosslinkable additive is a compound that can be crosslinked by heat and can be thermally cured during the vacuum drying (VD) process. When the negative electrode mixture layer includes a thermally crosslinkable additive, components within the electrode can be crosslinked by the thermally crosslinkable additive during the secondary battery manufacturing process, thereby improving electrode adhesion. Furthermore, this can relax stress within the electrode, thereby suppressing the aforementioned springback phenomenon.

[0068] The thermal crosslinking additive may include one or more selected from epoxy compounds, borate compounds, thiol compounds, and carboxylic acid compounds. In some specific embodiments, the thermal crosslinking additive may be one selected from epoxy compounds, borate compounds, thiol compounds, and carboxylic acid compounds.

[0069] The epoxy compound may include at least one selected from polyethylene glycol diglycidyl ether (PEGDE) and polypropylene glycol diglycidyl ether (PPGDE).

[0070] The borate-based compound may include one or more selected from the group consisting of sodium borate, sodium tetraborate, tetrahydroxyborate, and sodium tetraborate decahydrate.

[0071] The thiol compound may include one or more selected from 3-mercaptopropionic acid, L-cysteine, and 3-mercapto-1,2-propanediol.

[0072] In a specific embodiment in which the negative electrode mixture layer 20 includes a first negative electrode mixture layer 21 and a second negative electrode mixture layer 22 , the content of the thermally crosslinkable additive contained in the second negative electrode mixture layer 22 may be greater than or equal to the content of the thermally crosslinkable additive contained in the first negative electrode mixture layer 21 .

[0073] In a specific embodiment in which the negative electrode mixture layer 20 includes the first negative electrode mixture layer 21 and the second negative electrode mixture layer 22 , the thermally crosslinkable additive may not be included in the first negative electrode mixture layer 21 but may be included only in the second negative electrode mixture layer 22 .

[0074] When the thermally cross-linkable additive is added as described above, the binding force of the upper layer can be maintained in the negative electrode in which the content of the silicon-based active material contained in the second negative electrode mixture layer 22 is relatively larger than the content of the silicon-based active material contained in the first negative electrode mixture layer 21, thereby suppressing deterioration due to structural collapse of the negative electrode.

[0075] The weight of the binder contained in the negative electrode mixture layer 20 is greater than the weight of the thermal cross-linking additive contained in the negative electrode mixture layer 20. For example, the weight ratio of the binder to the thermal cross-linking additive contained in the negative electrode mixture layer 20 may be greater than 5:5, greater than 6:4, or greater than 7:3, and may be less than 9:1 or less than 8:2. In some specific embodiments, the weight ratio of the binder to the thermal cross-linking additive contained in the negative electrode mixture layer 20 may be greater than 5:5 and less than 9:1.

[0076] In some specific embodiments, the weight ratio of the binder to the thermal crosslinking additive in the negative electrode mixture layer 20 may be 7:3 to 9:1. In some specific embodiments, the weight ratio of the binder to the thermal crosslinking additive in the negative electrode mixture layer 20 may be 7:3 to 8:2. When the content relationship of the binder and the thermal crosslinking additive in the negative electrode mixture layer 20 is as described above, the curing degree of the negative electrode can be significantly improved, thereby suppressing expansion and springback of the electrode and increasing the energy density of the negative electrode.

[0077] The negative electrode mixture layer 20 may contain a thermally crosslinkable additive in an amount of 0.1 to 5 wt %. In some specific embodiments, the negative electrode mixture layer 20 may contain a thermally crosslinkable additive in an amount of 0.2 wt % or more, and may contain 3 wt % or less, 1 wt % or less, or 0.5 wt % or less.

[0078] The negative electrode mixture layer 20 may further include 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 a granular carbon material and a fibrous carbon material. The granular carbon material may be carbon black such as Super-P or Super-C, acetylene black, Ketjen black, graphite, etc., and the fibrous carbon material may be carbon fiber, carbon nanotubes (CNTs), vapor-grown carbon fiber (VGCF), etc.

[0079] For example, the content of the conductive material included in the negative electrode mixture layer 20 may be 0.01-30 wt % or 1-10 wt %.

[0080] The load weight (LW) of the negative electrode mixture layer 20 may be 7 mg / cm 2 In some specific embodiments, the load weight (LW) of the negative electrode mixture layer 20 may be 9 mg / cm 2 Above or 11 mg / cm 2 The load weight (LW) of the negative electrode mixture layer 20 can be 15 mg / cm 2 Below or 13mg / cm 2 The load weight (LW) refers to the amount of the negative electrode mixture layer 20 formed on one side of the negative electrode current collector 10, expressed in weight per unit area. Here, the area is based on the area of ​​the negative electrode current collector 10, and the weight is based on the total weight of the formed negative electrode mixture layer 20. When the load weight (LW) of the negative electrode mixture layer 20 is within the above range, the energy density of the electrode and the secondary battery including the electrode can be improved.

[0081] The density of the negative electrode mixture layer 20 may be 1.4-1.8 g / cubic centimeter (g / cc). In some specific embodiments, the density of the negative electrode mixture layer 20 may be greater than 1.5 g / cubic centimeter or greater than 1.6 g / cubic centimeter, and the density of the negative electrode mixture layer 20 may be less than 1.7 g / cubic centimeter. The density of the negative electrode mixture layer 20 may refer to the electrode density after the rolling process. When the density of the negative electrode mixture layer 20 is within the above range, the energy density of the electrode and the secondary battery including the electrode can be improved.

[0082] The electrode adhesion between the negative electrode current collector 10 and the negative electrode mixture layer 20 may be 0.4 N / m 2 In some specific embodiments, the electrode adhesion between the negative electrode current collector 10 and the negative electrode mixture layer 20 may be 0.42 N / m 2 Above or 0.44N / m 2 Above, and can be 5N / m 2Below or 1N / m 2 When the electrode adhesive force is within the above range, the stress in the electrode can be relaxed, thereby alleviating the above-mentioned springback phenomenon.

[0083] The method for manufacturing the negative electrode is not particularly limited. For example, the negative electrode can be manufactured by applying a negative electrode slurry comprising the negative electrode active material, a binder, and a thermally crosslinkable additive to the negative electrode current collector 10 by bar coating, casting, or spraying, and then drying the negative electrode slurry at 80-120° C. to produce the secondary battery negative electrode 100 having the negative electrode mixture layer 20 formed on the surface of the negative electrode current collector 10.

[0084] The negative electrode slurry may further include a solvent. For example, the solvent may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. In this case, the amount of the solvent used is not particularly limited, as long as it is sufficient to dissolve or disperse the components and has a viscosity that allows excellent thickness uniformity when coated on the current collector, taking into account the coating thickness of the slurry, manufacturing yield, etc.

[0085] lithium secondary batteries

[0086] A lithium secondary battery according to one embodiment includes the negative electrode 100 for a secondary battery as described in any one of the above embodiments. For example, the lithium secondary battery may include the negative electrode 100 for a secondary battery as described in any one of the above embodiments and a positive electrode.

[0087] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer on at least one side of the positive electrode current collector. The composition of the positive electrode current collector is not particularly limited. For example, the positive electrode current collector may be a plate or foil formed from 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 alloys thereof. In addition, the thickness of the positive electrode current collector is not particularly limited. For example, the thickness of the positive electrode current collector may be 0.1-50 μm.

[0088] The positive electrode mixture layer may include a positive electrode active material that participates in the electrochemical reaction of the secondary battery. The positive electrode active material is not particularly limited and may include a compound that can reversibly intercalate and deintercalate lithium ions.

[0089] Illustratively, the positive electrode active material may include lithium-nickel metal oxide, and the lithium-nickel metal oxide may further include at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0090] In some specific embodiments, the positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1.

[0091] [Chemical Formula 1]

[0092] Li x Ni a M b O 2+z

[0093] In the chemical formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1 may be satisfied. As described above, M may include Co, Mn, and / or Al.

[0094] The chemical structure represented by the chemical formula 1 represents the bonding relationship contained in the layered structure or crystal structure of the positive electrode active material, and does not exclude other additional elements. Exemplarily, M may include Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element (main active element) of the positive electrode active material. The chemical formula 1 is provided to represent the bonding relationship of the main active elements, and it should be understood that the chemical formula 1 is a formula including the introduction and substitution of additional elements.

[0095] In some specific embodiments, in addition to the main active element, an auxiliary element for enhancing the chemical stability of the positive electrode active material or the layered structure / crystal structure may be further included. The auxiliary element may be mixed into the layered structure / crystal structure to form a bond, and it should be understood that this situation is also included in the chemical structure represented by Chemical Formula 1.

[0096] For example, the auxiliary element may include at least one of 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, or Zr. The auxiliary element may function as an auxiliary active element that contributes to the capacity / power activity of the positive electrode active material together with Co or Mn, such as Al.

[0097] Illustratively, the positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1-1.

[0098] [Chemical Formula 1-1]

[0099] Li x Ni a M1 b1 M2 b2 O 2+z

[0100] In Chemical Formula 1-1, M1 may include Co, Mn, and / or Al. M2 may include the auxiliary elements described above. In Chemical Formula 1-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1 may be present.

[0101] The positive electrode active material may further include a coating element or a doping element. For example, an element substantially identical to or similar to the auxiliary element may be used as the coating element or the doping element. For example, one of the above elements or a combination of two or more of the above elements may be used as the coating element or the doping element.

[0102] The coating element or the doping element may be present on the surface of the lithium-nickel metal oxide particle or permeate through the surface of the lithium-nickel metal oxide particle and be included in the bonding structure represented by Chemical Formula 1 or Chemical Formula 1-1.

[0103] The positive active material may include nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content may be used.

[0104] In the NCM-based lithium oxide, the Ni content (e.g., the molar fraction of nickel in the total moles of nickel, cobalt, and manganese) may be greater than 0.6, greater than 0.7, or greater than 0.8. In some embodiments, the Ni content may 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.

[0105] In some specific embodiments, the positive electrode active material may further 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)-based active material (eg, LiFePO 4 ).

[0106] In some specific 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 based active material having a chemical structure or crystal structure represented by Chemical Formula 2.

[0107] [Chemical Formula 2]

[0108] p[Li2MnO3]·(1-p)[Li q JO2]

[0109] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may contain at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0110] The positive electrode mixture layer may further contain a binder. The binder is not particularly limited. Exemplarily, the binder may contain one or more of styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, and polymethyl methacrylate.

[0111] The positive electrode mixture layer may further contain a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material may contain one or more of graphite such as natural graphite or artificial graphite; carbon-based substances such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, and carbon nanotube (CNT); metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives.

[0112] According to a specific embodiment, the lithium secondary battery may further include a separator disposed between the above positive electrode and negative electrode. The separator is not particularly limited. Exemplarily, the separator may include a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. In addition, the separator may also include a non-woven fabric formed of glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0113] Examples

[0114] 1. Manufacture of Negative Electrode

[0115] 1) Example 1

[0116] Prepare a first negative electrode slurry. Based on the solid content, the first negative electrode slurry contains 93.80% by weight of a carbon-based active material (artificial graphite) and 2.00% by weight of a silicon-based active material (SiO x;(0 < x < 2) as the negative electrode active material, and contains 1.2 wt% of carboxymethyl cellulose (CMC) as the binder, and contains 2.4 wt% of styrene-butadiene rubber (SBR) as an additional binder, and contains 0.60 wt% of single-walled carbon nanotubes (SWCNT) as the conductive material.

[0117] In addition, a second negative electrode slurry is prepared. Based on the solid content, the second negative electrode slurry contains 83.65 wt% of a carbon-based active material (artificial graphite) and 14.00 wt% of a silicon-based active material (SiO x ;(0 < x < 2) as the negative electrode active material, and contains 1.2 wt% of carboxymethyl cellulose (CMC) as the binder, and contains 0.6 wt% of styrene-butadiene rubber (SBR) as an additional binder, and contains 0.25 wt% of multi-walled carbon nanotubes (MWCNT) as the conductive material, and contains 0.3 wt% of polyethylene glycol diglycidyl ether (PEGDE) as a thermal crosslinking additive.

[0118] After that, the first negative electrode slurry and the second negative electrode slurry are coated on a copper foil as the current collector at a ratio of 50:50 and a total load weight (LW) of 11.11 mg / cm 2 Then, it is dried for 75 seconds or more at 100 °C or higher through a roll-to-roll process to obtain the negative electrode of Example 1 with a thickness of 171.5 μm. After that, the negative electrode is calendered using a calender roll so that the negative electrode has a thickness of 142.0 μm and a density of 1.56 g / cm³. In addition, the weight ratio of the binder (CMC) and the thermal crosslinking additive (PEGDE) contained in the negative electrode is 8:1 (= 2.4 wt%: 0.3 wt%).

[0119] 2) Example 2

[0120] The first negative electrode slurry and the second negative electrode slurry are coated at a ratio of 50:50 and a total load weight (LW) of 11.16 mg / cm 2 The first negative electrode slurry and the second negative electrode slurry contain artificial graphite and natural graphite with a weight ratio of 70:3 to 0 as the negative electrode active material, and the negative electrode of Example 2 with a thickness of 177.2 μm is manufactured in the same manner as Example 1.

[0121] At this time, based on the solid content, the prepared first negative electrode slurry contains 65.38 wt% of artificial graphite and 28.02 wt% of natural graphite as the negative electrode active material, and contains 1.2 wt% of carboxymethyl cellulose (CMC) as the binder, and contains 2.4 wt% of styrene-butadiene rubber (SBR) as an additional binder, and contains 3 wt% of graphite as the conductive material.

[0122] In addition, based on the solid content, the prepared second negative electrode slurry contains 66.34% by weight of artificial graphite and 28.56% by weight of natural graphite as negative electrode active materials, 1.2% by weight of carboxymethyl cellulose (CMC) as a binder, 0.6% by weight of styrene-butadiene rubber (SBR) as an additional binder, 3% by weight of graphite as a conductive material, and 0.3% by weight of polyethylene glycol diglycidyl ether (PEGDE) as a thermal crosslinking additive.

[0123] After that, a calender roll is used to calender the negative electrode so that the negative electrode has a thickness of 138.0 μm and a density of 1.64 g / cm³.

[0124] 3) Comparative Example 1

[0125] Manufactured in the same manner as Example 1, a negative electrode of Comparative Example 1 with a thickness of 168.8 μm is obtained. The difference is that the second negative electrode slurry does not contain a thermal crosslinking additive and contains 83.95% by weight of artificial graphite, and the negative electrode slurry is coated with a total load weight (LW) of 11.02 mg / cm 2 of the total load weight (LW).

[0126] At this time, based on the solid content, the prepared first negative electrode slurry contains 93.80% by weight of a carbon-based active material (artificial graphite) and 2.00% by weight of a silicon-based active material (SiO x ; 0 < x < 2) as a negative electrode active material, 1.2% by weight of carboxymethyl cellulose (CMC) as a binder, 2.4% by weight of styrene-butadiene rubber (SBR) as an additional binder, and 0.60% by weight of single-walled carbon nanotubes (SWCNT) as a conductive material.

[0127] In addition, based on the solid content, the prepared second negative electrode slurry contains 83.95% by weight of a carbon-based active material (artificial graphite) and 14.00% by weight of a silicon-based active material (SiO x ; 0 < x < 2) as a negative electrode active material, 1.2% by weight of carboxymethyl cellulose (CMC) as a binder, 0.6% by weight of styrene-butadiene rubber (SBR) as an additional binder, and 0.25% by weight of multi-walled carbon nanotubes (MWCNT) as a conductive material.

[0128] After that, a calender roll is used to calender the negative electrode so that the negative electrode has a thickness of 142.0 μm and a density of 1.6 g / cm³.

[0129] 4) Comparative Example 2

[0130] The negative electrode of Comparative Example 2 with a thickness of 173.5 μm was prepared in the same manner as in Example 2, except that the second negative electrode slurry did not contain a thermal crosslinking additive and contained 66.64 wt % of artificial graphite, and the negative electrode slurry was heated at 11.05 mg / cm 2 The coating was performed with a total load weight (LW) of 1000 rpm.

[0131] At this time, based on the solid content, the prepared first negative electrode slurry contained 65.38 weight% of artificial graphite and 28.02 weight% of natural graphite as negative electrode active materials, 1.2 weight% of carboxymethyl cellulose (CMC) as a binder, 2.4 weight% of styrene-butadiene rubber (SBR) as an additional binder, and 3 weight% of graphite as a conductive material.

[0132] In addition, based on the solid content, the prepared second negative electrode slurry contains 66.64 weight% of artificial graphite and 28.56 weight% of natural graphite as negative electrode active materials, 1.2 weight% of carboxymethyl cellulose (CMC) as a binder, 0.6 weight% of styrene-butadiene rubber (SBR) as an additional binder, and 3 weight% of graphite as a conductive material.

[0133] Thereafter, the negative electrode was rolled using a press roller so that the negative electrode had a thickness of 138.0 μm and a density of 1.62 g / cm 3 .

[0134] 2. Evaluation of the negative electrode

[0135] 1) Evaluation of adhesion

[0136] For the negative electrode manufactured as described above, the electrode adhesion between the negative electrode current collector and the negative electrode mixture layer was measured by a surface and interface physical property analyzer (Surface and Interfacial Characterization Analysis System, SAICAS). Specifically, a boron nitride (Borazon) blade was inserted to the adhesion measurement depth, and then the blade was moved horizontally at this position to measure the adhesion of the negative electrode mixture layer located on the upper portion.

[0137] In addition, for the negative electrode, after vacuum drying (VD) at 120°C for 8 hours, the electrode adhesion was measured by the same method. The evaluation results of the electrode adhesion are shown in FIG. Figure 1 shown.

[0138] See also Figure 1The negative electrode of Example 1 had superior electrode adhesion compared to the negative electrode of Comparative Example 1, which did not contain a thermally crosslinkable additive. Furthermore, the negative electrode of Example 2 had superior electrode adhesion compared to the negative electrode of Comparative Example 2, which did not contain a thermally crosslinkable additive. These results indicate that when a thermally crosslinkable additive is included in the negative electrode as in Examples 1 and 2, crosslinking occurs within the electrode components, improving the adhesion of the resulting electrode.

[0139] 2) Rebound evaluation

[0140] The thickness of the negative electrode produced as described above was measured before and after vacuum drying (VD) at 120°C for 8 hours. The following values ​​were calculated: 1) the difference between the thickness of the negative electrode after vacuum drying and the thickness of the negative electrode after rolling (③ - ①), and 2) the difference between the thickness of the negative electrode before and after vacuum drying (③ - ②). The values ​​are shown in Table 1 below.

[0141] [Table 1]

[0142]

[0143] As shown in Table 1, the difference in electrode thickness before and after the VD process for the negative electrodes of Examples 1 and 2 is relatively smaller than that for the negative electrodes of Comparative Examples 1 and 2, respectively, thus alleviating the springback phenomenon. Considering these results, it is concluded that when a thermally crosslinkable additive is included in the negative electrode, as in Examples 1 and 2, the electrode adhesion is increased due to crosslinking of the components within the electrode, and the stress within the electrode before and after the VD process is relaxed, thereby suppressing the increase in electrode thickness before and after the VD process.

[0144] 3. Evaluation of cell thickness

[0145] Using the negative electrode manufactured as described above, a battery cell was assembled with 43 electrodes stacked. The thickness of the battery cell finally manufactured was measured and is shown in Table 2 below.

[0146] [Table 2]

[0147]

[0148] Referring to Table 2, the thickness of the battery cells assembled by the negative electrodes of Example 1 and Example 2 is thinner than the thickness of the battery cells assembled by the negative electrodes of Comparative Example 1 and Comparative Example 2, respectively. Specifically, in the case of Comparative Example 1 and Example 1, where the difference (③-①) between the thickness of the negative electrode after vacuum drying and the thickness of the negative electrode after rolling during the above-mentioned rebound evaluation is 16.7μm and 12.4μm, respectively, the thickness of the assembled battery cells is 13.43mm (=13430μm) and 13.26mm (=13260μm), respectively. Therefore, compared with the negative electrode according to Comparative Example 1, the negative electrode according to Example 1 can reduce the thickness of the battery cell by 170μm, which is judged to be a result reflecting the difference in the degree of reduction in the thickness of the negative electrode (16.7μm-12.4μm=4.3μm) and the number of electrode stacking (43 sheets).

[0149] In addition, in the case of Comparative Example 2 and Example 2, where the difference (③-①) between the thickness of the negative electrode after vacuum drying and the thickness of the negative electrode after rolling during the above-mentioned rebound evaluation was 12.2μm and 9.4μm, respectively, the thickness of the assembled battery cells was 13.27mm (=13270μm) and 13.15mm (=13150μm), respectively. Therefore, compared with the negative electrode according to Comparative Example 2, the negative electrode according to Example 2 can reduce the thickness of the battery cell by 120μm. It is judged that this reflects the difference in the degree of reduction in the thickness of the negative electrode (12.2μm-9.4μm=2.8μm) and the number of electrode stacks (43 sheets).

[0150] Taking these results into consideration, it is judged that when a negative electrode containing a thermally cross-linkable additive is used as in Examples 1 and 2, the volume of the final assembled battery cell can be reduced, thereby increasing the energy density of the secondary battery, and no additional calendering process and / or high pressure is required to ensure the energy density, thereby preventing damage to the electrode and improving the manufacturing processability of the secondary battery.

[0151] 4. Evaluation of physical properties based on the weight ratio of adhesive and thermal crosslinking additive

[0152] 1) Production of adhesive films

[0153] A solution containing carboxymethyl cellulose (CMC) as a binder was dried at 100°C for 4 hours to produce adhesive films. Specifically, the following adhesive films were produced: 1) adhesive films further containing polyethylene glycol diglycidyl ether (PEGDE) as a thermal crosslinking additive in addition to the binder (Examples 3 to 5 and Comparative Example 3); and 2) an adhesive film without the thermal crosslinking additive (Comparative Example 4). The weight ratios of CMC and PEGDE in the adhesive films are shown in Table 3 below.

[0154] 2) Evaluation of adhesive films

[0155] (1) Tensile test

[0156] The adhesive film prepared as described above was subjected to a tensile test using a universal testing machine (UTM) to measure the tensile strength according to a stress-strain curve. The measured tensile strength values ​​are shown in Table 3 below.

[0157] (2) Gel Fraction

[0158] The adhesive film produced as described above was immersed in distilled water as a solvent for 24 hours. Uncured portions were removed and the film was dried at 60°C for 24 hours. The gel fraction was then calculated by analyzing the weight change before and after drying. The measured gel fraction values ​​are shown in Table 3 below.

[0159] (3) Swelling Ratio

[0160] After the adhesive film prepared as described above was immersed in a carbonate-based electrolyte for 24 hours, the expansion ratio was calculated by analyzing the weight change of the adhesive film before and after the immersion. The measured expansion ratio values ​​are shown in Table 3 below.

[0161] [Table 3]

[0162]

[0163] As shown in Table 3, the adhesive film of Comparative Example 4, which does not contain the thermal crosslinking additive (PEGDE), has a relatively low tensile strength value due to its low degree of cure, is dissolved in the solvent, and thus cannot measure the gel fraction, and has a relatively high swelling ratio. Furthermore, the adhesive film of Comparative Example 3, which contains the thermal crosslinking additive (PEGDE) but contains the binder (CMC) and the thermal crosslinking additive (PEGDE) in the same ratio, has a relatively low tensile strength value due to its low degree of cure, a relatively low gel fraction value, and a relatively high swelling ratio.

[0164] On the other hand, in the adhesive films of Examples 3 to 5 containing a relatively small weight ratio of the thermally crosslinkable additive (PEGDE) compared to the adhesive (CMC), the tensile strength and gel fraction values ​​were relatively high, and the expansion ratio was relatively low, thus exhibiting excellent curing degree.

[0165] Therefore, it is judged that when the content of the thermal cross-linking additive in the negative electrode containing both the binder and the thermal cross-linking additive is controlled to be relatively less than the content of the binder, the curing degree of the negative electrode can be significantly improved, thereby suppressing the expansion and rebound phenomena of the electrode, and increasing the energy density of the negative electrode.

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector, in, The negative electrode mixture layer comprises a negative electrode active material, a binder and a thermal crosslinking additive, The weight of the binder included in the negative electrode mixture layer is greater than the weight of the thermally crosslinkable additive included in the negative electrode mixture layer.

2. The negative electrode for a secondary battery according to claim 1, wherein The binder comprises a cellulose-based compound.

3. The negative electrode for a secondary battery according to claim 1, wherein The heat-crosslinking additive includes at least one selected from epoxy compounds, borate compounds, thiol compounds, and carboxylic acid compounds.

4. The negative electrode for a secondary battery according to claim 3, wherein The epoxy compound includes at least one selected from polyethylene glycol diglycidyl ether (PEGDE) and polypropylene glycol diglycidyl ether (PPGDE).

5. The negative electrode for a secondary battery according to claim 3, wherein The borate-based compound comprises at least one selected from the group consisting of sodium borate, sodium tetraborate, tetrahydroxyborate and sodium tetraborate decahydrate.

6. The negative electrode for a secondary battery according to claim 3, wherein The thiol compound includes at least one selected from 3-mercaptopropionic acid, L-cysteine, and 3-mercapto-1,2-propanediol.

7. The negative electrode for a secondary battery according to claim 1, wherein A weight ratio of the binder and the heat-crosslinkable additive contained in the negative electrode mixture layer is more than 5:5 and 9:1 or less.

8. The negative electrode for a secondary battery according to claim 1, wherein The binder and the thermal cross-linking additive are included in the negative electrode mixture layer in a weight ratio of 7:3 to 9:

1.

9. The negative electrode for a secondary battery according to claim 1, wherein The negative electrode mixture layer includes the thermal crosslinking additive in an amount of 0.1 to 5% by weight.

10. The negative electrode for a secondary battery according to claim 1, wherein The negative electrode mixture layer further includes a rubber-based compound as an additional binder.

11. The negative electrode for a secondary battery according to claim 10, wherein The weight ratio of the binder to the additional binder contained in the negative electrode mixture layer is 1:1 to 1:

5.

12. The negative electrode for a secondary battery according to claim 1, wherein The electrode adhesion between the negative electrode current collector and the negative electrode mixture layer is 0.4 N / m 2 above.

13. The negative electrode for a secondary battery according to claim 1, wherein The negative electrode mixture layer includes a first negative electrode mixture layer on the negative electrode current collector and a second negative electrode mixture layer on the first negative electrode mixture layer.

14. The negative electrode for a secondary battery according to claim 13, wherein The second negative electrode mixture layer contains a thermally cross-linkable additive. 15 . A lithium secondary battery comprising the secondary battery negative electrode according to claim 1 .

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