Title of Invention: Positive electrode for lithium secondary battery and lithium secondary battery including same
technical field
[One]
Cross Citation with Related Applications
[2]
This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0152272 dated November 30, 2018, and all contents disclosed in the literature of the Korean patent application are incorporated as a part of this specification.
[3]
technical field
[4]
The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery comprising the positive electrode active material.
[5]
background
[6]
As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, a lithium secondary battery having a high energy density and operating potential, a long cycle life, and a low self-discharge rate has been commercialized and widely used.
[7]
Recently, as a lithium secondary battery is used as a power source for a medium or large device such as an electric vehicle, a high capacity, high energy density, high output, and low cost of the lithium secondary battery are further required.
[8]
One of the main research tasks of such a lithium secondary battery is to improve the stability of the battery using the high-capacity and high-output electrode active material while implementing the same.
[9]
Current lithium secondary batteries are designed to be used in a specific voltage range (generally, 4.4 V or less) to ensure durability and stability. However, the cell potential may unintentionally rise higher than that. Such a sudden increase in cell potential desorbs lithium from the cathode material to generate more tetravalent Co, Ni ions, etc., and gas is generated or Alternatively, a side reaction such as oxidation of the electrolyte occurs, which eventually leads to deterioration of cell performance.
[10]
In addition, if the overcharge state exceeding the allowed current or voltage continues, the internal temperature of the cell rises, which causes contraction of the separator and an internal short circuit. The cell temperature rapidly increases due to the instantaneous overcurrent generated at this time, and there is a problem of explosion of the cell together with the combustible gas inside the cell.
[11]
In the case of a conventional secondary battery, in order to prevent cell ignition due to overcharging, a method of using a flame retardant inside the cell or removing air inside the cell has been used.
[12]
Among these, when a flame retardant, particularly a melamine-based flame retardant having excellent flame retardant properties during overcharging, is additionally included in the cell, the adhesion of the electrode is lowered, so that there is a problem in that the electrode active material layer and the current collector layer are separated during electrode manufacturing.
[13]
Accordingly, there is a demand for the development of a battery with improved stability because it not only suppresses the temperature rise inside the battery, but also quickly reaches the overcharge termination voltage when overcharging occurs.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[14]
In order to solve the above problems, the first technical object of the present invention is to provide a positive electrode with improved overcharge stability by quickly blocking the charging current even when overcharging occurs, as well as suppressing the temperature rise inside the battery during overcharging. will provide
[15]
A second technical object of the present invention is to provide a lithium secondary battery including the positive electrode for the secondary battery.
means of solving the problem
[16]
The present invention provides a positive electrode comprising a positive electrode active material layer formed on a surface of a positive electrode current collector, wherein the positive electrode active material layer includes a first positive electrode active material layer formed on the positive electrode current collector and a second positive electrode active material layer formed on the first positive electrode active material layer is a multi-layer structure comprising a, wherein the first positive electrode active material layer includes a positive electrode active material, a first binder that is a melamine-based compound, and a second binder that is different from the melamine-based compound, and the second positive electrode active material layer is a second positive electrode active material And it provides a positive electrode for a secondary battery comprising a first binder that is a melamine-based compound.
[17]
In addition, the present invention provides a lithium secondary battery comprising the positive electrode for the secondary battery.
Effects of the Invention
[18]
The positive electrode according to the present invention includes a melamine-based compound in a specific content as a binder, thereby suppressing an increase in temperature inside the battery when overcharged by the melamine-based compound.
[19]
In addition, when the melamine-based compound is combusted due to an increase in internal temperature, an oligomer is generated, and a phase change occurs when the oligomer is generated, thereby absorbing combustion heat due to overcharging due to an endothermic reaction. In addition, the amount of gas generated by the positive electrode active material is reduced due to the oligomer generated on the surface of the positive electrode active material, and thus, fuel gas that can be burned during overcharging is reduced, thereby further improving the stability of the cell.
[20]
In addition, the positive electrode according to the present invention includes a positive electrode active material layer having a two-layer structure, the first positive electrode active material layer relatively increases the content of the second binder, and the second positive electrode active material layer includes only a melamine-based compound as a binder. It is possible to maximize the effect of improving the adhesion between the current collector and the active material layer and improving the stability during overcharging.
Brief description of the drawing
[21]
1 is a schematic diagram showing a positive electrode according to the present invention.
Best mode for carrying out the invention
[22]
Hereinafter, the present invention will be described in more detail.
[23]
The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. Based on the principle that there is, it should be interpreted as meaning and concept consistent with the technical idea of ​​the present invention.
[24]
[25]
anode
[26]
A positive electrode for a secondary battery according to an embodiment of the present invention is a positive electrode comprising a positive electrode active material layer formed on a surface of a positive electrode current collector, wherein the positive electrode active material layer includes a first positive electrode active material layer formed on the positive electrode current collector and the first positive electrode A multilayer structure including a second positive active material layer formed on an active material layer, wherein the first positive active material layer includes a positive active material, a first binder that is a melamine-based compound, and a second binder that is different from the melamine-based compound, The second positive active material layer includes a second positive active material and a first binder that is a melamine-based compound.
[27]
[28]
Hereinafter, the positive electrode for a secondary battery according to the present invention will be described in more detail.
[29]
[30]
First, the positive electrode includes a positive electrode active material layer having a multilayer structure formed on a positive electrode current collector.
[31]
[32]
The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or carbon, nickel, titanium on the surface of aluminum or stainless steel. , silver or the like surface-treated may be used. In addition, the positive electrode current collector may typically have a thickness of 3 to 500 μm, and may increase the adhesion of the positive electrode active material by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body.
[33]
[34]
As shown in FIG. 1 , the positive electrode active material layer according to the present invention includes a first positive active material layer 20 formed on the positive electrode current collector 10 and a second positive active material layer 30 formed on the second positive electrode active material layer. ) is a multi-layered structure containing The multilayer structure means a structure in which a first positive electrode active material layer and a second positive electrode active material layer are alternately stacked on the positive electrode current collector.
[35]
For example, the present invention not only improves the adhesive force between the positive electrode current collector and the positive electrode active material layer, but also inhibits the temperature rise inside the battery during overcharging. The second binder is mixed and used. In order to improve overcharge stability, as the ratio of the melamine-based compound is increased, there is a problem in that the adhesion between the positive electrode current collector and the positive electrode active material layer is reduced. Conversely, when the ratio of the second binder is increased to improve adhesion, there is a problem in that the effect of improving overcharge stability is insignificant. In addition, when the total content of the binder is increased, there is a problem that the internal resistance of the cell is increased or the energy density is decreased.
[36]
Therefore, the present inventors have a multi-layer structure, for example, a two-layer structure of the positive electrode active material layer, the first positive electrode active material layer is a first binder and a melamine-based compound that can improve stability during overcharge and A second binder having excellent adhesive properties is included, and the second positive active material layer includes only the second positive active material and the first binder, which is a melamine-based compound, so that energy density is not reduced and internal resistance is increased between the positive electrode current collector and the positive electrode active material layer. At the same time as improving adhesion, structural stability can be improved by suppressing a temperature rise inside the battery during overcharging. In addition, by suppressing the migration (migration) of the binder, it is possible to further improve the resistance properties compared to the case of the single-layer structure.
[37]
[38]
More specifically, the first positive active material layer according to the present invention may include a first positive active material, a first binder that is a melamine-based compound, and a second binder that is different from the melamine-based compound.
[39]
[40]
The first positive active material preferably includes lithium and at least one transition metal of nickel, cobalt, or manganese. For example, lithium cobalt oxide (LiCoO 2 ); lithium nickel oxide (LiNiO 2 ); Li[Ni a Co b Mn c M 1 d ]O 2 (wherein, M 1 is any one selected from the group consisting of Al, Ga, and In, or two or more of them, 0.3≤a<1.0, 0 ≤b≤0.5, 0≤c≤0.5, 0≤d≤0.1, a+b+c+d=1); Li(Li e M 2 f-e-f' M 3 f' )O 2 -g A g (wherein 0≤e≤0.2, 0.6≤f≤1, 0≤f'≤0.2, 0≤g≤0.2, M 2includes Mn and at least one selected from the group consisting of Ni, Co, Fe, Cr, V, Cu, Zn and Ti, and M 3 is at least one selected from the group consisting of Al, Mg and B, and , A is at least one selected from the group consisting of P, F, S and N) or a layered compound such as a compound substituted with one or more transition metals; Li 1 + h Mn 2 - h O 4 (wherein 0≤h≤0.33), LiMnO 3 , LiMn 2 O 3 , LiMnO 2 Li-Mn oxide and the like; Ni site type lithium nickel oxide represented by the formula LiNi 1 - i M 4 i O 2 (wherein, M 4 = Co, Mn, Al, Cu, Fe, Mg, B or Ga, 0.01≤i≤0.3); Formula LiMn 2 - j M 5 j O 2 (wherein M 5 = Co, Ni, Fe, Cr, Zn or Ta, 0.01≤j≤0.1) or Li 2 Mn 3 M 6 O 8 (wherein, M 6 = Lithium manganese composite oxide represented by Fe, Co, Ni, Cu or Zn); It may be LiMn 2 O 4 in which a part of Li in the formula is substituted with an alkaline earth metal ion . Preferably, the first positive active material may be a lithium nickel cobalt manganese composite oxide including nickel, cobalt, and manganese as transition metals.
[41]
[42]
The first binder, which is the melamine-based compound, may include at least one of melamine or a melamine derivative, and may preferably include a melamine salt.
[43]
The melamine-based compound may prevent a temperature increase inside the secondary battery during overcharging. For example, when the secondary battery is overcharged, the temperature of the melamine-based compound is also gradually increased as the temperature of the battery increases, and in this case, the melamine-based compound is burned. The melamine-based compound can form oligomers while burning. Even if the temperature of the battery increases due to overcharging, heat transferred to the positive electrode active material is reduced due to an endothermic reaction generated in the process of generating the oligomer formed on the surface of the positive electrode active material. By doing so, the combustible material that can be ignited is reduced, and the stability can be improved.
[44]
[45]
In addition, when the melamine-based compound is burned, the surface of the positive electrode active material may be formed as a non-porous surface. The non-porous surface may block heat and/or oxygen generated on the surface of the positive active material and transferred to the positive active material by a short-circuit current inside the battery, and thus flame retardant properties may be further improved.
[46]
[47]
In addition, the second binder different from the melamine-based compound serves to improve the adhesion between the positive active material particles and the adhesion between the positive active material and the current collector, for example, the second binder is polyamideimide (PAI) And at least one selected from the group consisting of polyvinylidene fluoride (PVDF) may be used. Preferably, when polyamideimide having excellent adhesive strength is used as the second binder, adhesive strength between the positive electrode current collector and the second positive electrode active material layer may be further improved due to high adhesive properties.
[48]
[49]
The first positive electrode active material layer may contain 1 to 30 parts by weight, preferably 1.5 to 10 parts by weight, of the first binder, which is a melamine-based compound, and the second binder, which is different from the melamine-based compound, with respect to 100 parts by weight of the positive active material. can
[50]
When the first positive electrode active material layer includes the first binder and the second binder in the above range with respect to 100 parts by weight of the positive electrode active material, the energy density is excellent, and the adhesive force between the positive electrode current collector and the second positive electrode active material layer is improved. can For example, when the content of the first binder and the second binder is lower than the above range, the adhesive force between the positive electrode current collector and the positive electrode active material layer is low. and the cathode active material layer may be separated.
[51]
[52]
The first positive electrode active material layer is a melamine-based compound of the first binder and the second binder 0.5:1 to 10:1, more preferably 0.5:1 to 5:1, most preferably 0.5:1 to 2.5:1 may be included in a weight ratio of For example, when the first positive electrode active material layer includes the first binder and the second binder in the above range, while improving the adhesion between the positive electrode current collector and the positive electrode active material layer, the temperature inside the cell increases due to the inclusion of the melamine-based compound Prevention properties can be implemented.
[53]
For example, when the content of the second binder is less than the range because the first positive active material layer contains the second binder in an amount outside the range, the adhesive force may decrease, and the content of the second binder may be When more than the above range, the endothermic reaction of melamine may decrease.
[54]
[55]
Meanwhile, the second positive electrode active material layer is positioned on the first positive electrode active material layer, and may include a second positive electrode active material and a first binder which is a melamine-based compound.
[56]
The second positive active material preferably includes lithium and at least one transition metal of nickel, cobalt, or manganese. For example, lithium cobalt oxide (LiCoO 2 ); lithium nickel oxide (LiNiO 2 ); Li[Ni a Co b Mn c M 1 d ]O 2 (wherein, M 1 is any one selected from the group consisting of Al, Ga, and In, or two or more of them, 0.3≤a<1.0, 0 ≤b≤0.5, 0≤c≤0.5, 0≤d≤0.1, a+b+c+d=1); Li(Li e M 2 f-e-f' M 3 f' )O 2 -g A g (wherein 0≤e≤0.2, 0.6≤f≤1, 0≤f'≤0.2, 0≤g≤0.2, M 2includes Mn and at least one selected from the group consisting of Ni, Co, Fe, Cr, V, Cu, Zn and Ti, and M 3 is at least one selected from the group consisting of Al, Mg and B, and , A is at least one selected from the group consisting of P, F, S and N) or a layered compound such as a compound substituted with one or more transition metals; Li 1 + h Mn 2 - h O 4 (wherein 0≤h≤0.33), LiMnO 3 , LiMn 2 O 3 , LiMnO 2 Li-Mn oxide and the like; Ni site type lithium nickel oxide represented by the formula LiNi 1 - i M 4 i O 2 (wherein, M 4 = Co, Mn, Al, Cu, Fe, Mg, B or Ga, 0.01≤i≤0.3); Formula LiMn 2 - j M 5 j O 2 (wherein M 5 = Co, Ni, Fe, Cr, Zn or Ta, 0.01≤j≤0.1) or Li 2 Mn 3 M 6 O 8 (wherein, M 6 = Lithium manganese composite oxide represented by Fe, Co, Ni, Cu or Zn); It may be LiMn 2 O 4 in which a part of Li in the formula is substituted with an alkaline earth metal ion . Preferably, the second positive active material may be a lithium nickel cobalt manganese composite oxide including nickel, cobalt, and manganese as transition metals.
[57]
In this case, the first positive active material included in the first positive active material layer and the second positive active material included in the second positive active material layer may be the same or different, and may be appropriately used as necessary.
[58]
[59]
The second positive electrode active material layer may include 1 to 30 parts by weight of the first binder, which is a melamine-based compound, preferably 1.5 to 10 parts by weight, based on 100 parts by weight of the positive electrode active material. Since the second positive active material layer includes only the melamine-based compound as a binder, it may include a melamine-based compound having a higher weight than when a single active material layer is used.
[60]
[61]
The first positive active material layer and the second positive active material layer have a thickness ratio of 5:95 to 50:50, preferably 20:80 to 50:50, and most preferably 40:60 to 50:50. can be When the first positive active material layer and the second positive active material layer are formed within the above range, after the first positive active material layer is thinly formed to a minimum thickness enough to have an adhesive force to the extent that there is no difficulty in electrode manufacturing, the second positive active material layer is formed. By forming the positive electrode active material layer, both the effect of improving adhesion and the effect of improving stability during overcharging can be achieved.
[62]
[63]
That is, according to the present invention, a melamine-based compound is included in order to improve the overcharge stability of the secondary battery, but the positive electrode active material layer is formed in a two-layer structure to improve the problem of lowering adhesion due to an increase in the conventional melamine-based compound content. An object of the present invention is to provide a positive electrode for a secondary battery capable of improving both stability.
[64]
[65]
The first positive active material layer and/or the second positive active material layer may optionally further include a conductive material as needed. The conductive material is used to impart conductivity to the electrode, and in the configured battery, it can be used without any particular limitation as long as it has electronic conductivity without causing chemical change. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and the like, and one or a mixture of two or more thereof may be used. The conductive material may be included in an amount of 0.1 to 10 parts by weight based on 100 parts by weight of the total weight of the positive electrode active material.
[66]
[67]
The positive electrode may be manufactured according to a conventional positive electrode manufacturing method. Specifically, a composition for forming a first positive active material layer prepared by dissolving or dispersing a positive electrode active material, a first binder that is a melamine-based compound, and a second binder different from the melamine-based compound in a solvent is applied on the positive electrode current collector. , drying and rolling to form a first positive electrode active material layer, and then dissolving or dispersing a positive electrode active material and a melamine-based compound in a solvent on the first positive electrode active material layer to form a second positive active material layer-forming composition for a first positive electrode A positive electrode including a positive electrode active material layer having a two-layer structure formed on a positive electrode current collector after coating on the active material layer is dried and rolled.
[68]
The solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol (isopropyl alcohol), N-methylpyrrolidone (NMP), acetone (acetone) or water and the like, and any one of them or a mixture of two or more thereof may be used. The amount of the solvent used is enough to dissolve or disperse the positive electrode active material, the conductive material and the binder in consideration of the application thickness of the slurry and the production yield, and to have a viscosity capable of exhibiting excellent thickness uniformity when applied for the production of the positive electrode thereafter. do.
[69]
[70]
In addition, as another method, the positive electrode may be prepared by casting the composition for forming the positive electrode active material layer on a separate support and then laminating a film obtained by peeling it from the support on the positive electrode current collector.
[71]
[72]
lithium secondary battery
[73]
In addition, the present invention can manufacture an electrochemical device including the positive electrode. The electrochemical device may specifically be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.
[74]
The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned to face the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode is the same as described above, so detailed description is omitted, Hereinafter, only the remaining components will be described in detail.
[75]
In addition, the lithium secondary battery may optionally further include a battery container for accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.
[76]
[77]
In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector.
[78]
The anode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface. Carbon, nickel, titanium, one surface-treated with silver, an aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode current collector may have a thickness of typically 3 μm to 500 μm, and similarly to the positive electrode current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body.
[79]
[80]
The anode active material layer optionally includes a binder and a conductive material together with the anode active material.
[81]
As the anode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and dedoping lithium, such as SiOβ (0 < β < 2), SnO 2 , vanadium oxide, and lithium vanadium oxide; Alternatively, a composite including the metallic compound and a carbonaceous material such as a Si-C composite or a Sn-C composite may be used, and any one or a mixture of two or more thereof may be used. In addition, a metal lithium thin film may be used as the negative electrode active material. In addition, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Soft carbon and hard carbon are representative of low-crystalline carbon, and high-crystalline carbon is natural or artificial graphite, Kish graphite (Kish) in amorphous, plate-like, flaky, spherical or fibrous shape graphite), pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, liquid crystal pitches (Mesophase pitches), and petroleum and coal tar pitch (petroleum or coal tar pitch) High-temperature calcined carbon such as derived cokes) is a representative example.
[82]
The negative active material may be included in an amount of 80 to 99 parts by weight based on the total weight of the negative active material layer.
[83]
[84]
The binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1 to 10 parts by weight based on the total weight of the negative active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro roethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.
[85]
The conductive material is a component for further improving the conductivity of the anode active material, and may be added in an amount of 10 parts by weight or less, preferably 5 parts by weight or less, based on the total weight of the anode active material layer. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskeys such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; A conductive material such as a polyphenylene derivative may be used.
[86]
[87]
For example, the anode active material layer is prepared by applying and drying a composition for forming an anode active material layer prepared by dissolving or dispersing an anode active material, and optionally a binder and a conductive material in a solvent, on the anode current collector and drying, or the anode It can be prepared by casting the composition for forming an active material layer on a separate support, and then laminating a film obtained by peeling it off the support on a negative electrode current collector.
[88]
[89]
The anode active material layer is, for example, by applying and drying a composition for forming an anode active material layer prepared by dissolving or dispersing an anode active material, and optionally a binder and a conductive material in a solvent on an anode current collector and drying, or for forming the anode active material layer It can also be prepared by casting the composition on a separate support and then laminating a film obtained by peeling it from the support onto a negative electrode current collector.
[90]
[91]
On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move, and as long as it is used as a separator in a lithium secondary battery, it can be used without any particular limitation, especially for the movement of ions in the electrolyte It is preferable to have a low resistance to and excellent electrolyte moisture content. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or these A laminated structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. In addition, in order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may optionally be used in a single-layer or multi-layer structure.
[92]
[93]
In addition, the electrolyte used in the present invention may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte, which can be used in the manufacture of a lithium secondary battery, and is limited to these. it's not going to be
[94]
Specifically, the electrolyte may include an organic solvent and a lithium salt.
[95]
The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, carbonate-based solvents such as PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double bond aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; Or sulfolanes and the like may be used. Among these, carbonate-based solvents are preferred, A cyclic carbonate (eg, ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant capable of increasing the charge/discharge performance of a battery, and a low-viscosity linear carbonate-based compound (eg, ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.
[96]
[97]
The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiSbF 6 , LiAl0 4 , LiAlCl 4 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN(C 2 F 5 SO 3 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 . LiCl, LiI, or LiB(C 2 O 4 ) 2 and the like may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance may be exhibited, and lithium ions may move effectively.
[98]
[99]
In addition to the electrolyte components, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, tri Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as jolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride may be further included. In this case, the additive may be included in an amount of 0.1 to 5 parts by weight based on the total weight of the electrolyte.
[100]
[101]
As described above, since the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics and lifespan characteristics, portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles ( It is useful in the field of electric vehicles such as hybrid electric vehicle, HEV).
[102]
Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.
[103]
The battery module or battery pack is a power tool (Power Tool); electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); Alternatively, it may be used as a power source for any one or more medium and large-sized devices in a system for power storage.
[104]
The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a prismatic shape, a pouch type, or a coin type using a can.
[105]
The lithium secondary battery according to the present invention may be used not only in a battery cell used as a power source for a small device, but may also be preferably used as a unit cell in a medium or large battery module including a plurality of battery cells.
Modes for carrying out the invention
[106]
Hereinafter, examples are given in order to describe the present invention in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.
[107]
[108]
Example
[109]
Example 1
[110]
LiNi 0 . 6 Co 0 . 2 Mn 0 . 2 O 2 A cathode active material, carbon black (super-C65) as a conductive material, polyamideimide (PAI) and melamine cyanurate (MC) as a binder in a weight ratio of 90:5:2.5:2.5 N- A first positive electrode active material slurry was prepared by mixing in methylpyrrolidone (NMP) solvent.
[111]
Separately, LiNi 0 . 6 Co 0 . 2 Mn 0 . 2 O 2 A cathode active material, carbon black (super-C65) and melamine cyanurate (MC) as a conductive material were mixed in an NMP solvent in a weight ratio of 90:5:5 to prepare a second cathode active material slurry. .
[112]
The first positive active material slurry prepared above was coated on an aluminum foil having a thickness of 15 μm, heat treated at 130° C. for 1 hour, and rolled to form a first positive electrode active material layer having a thickness of 60 μm. Then, the second cathode active material slurry was applied on the first cathode active material layer, heat treated at 130° C. for 1 hour, and rolled to form a second cathode active material layer having a thickness of 60 μm, which was used as a cathode for a secondary battery. did.
[113]
[114]
Example 2
[115]
LiNi 0 . 6 Co 0 . 2 Mn 0 . 2 O 2 A positive electrode active material, super-C65 as a conductive material, PAI and melamine cyanurate (MC) as a binder were mixed in an NMP solvent in a weight ratio of 90:5:1.5:3.5 to form a first positive electrode active material slurry prepared.
[116]
Separately, LiNi 0 . 6 Co 0 . 2 Mn 0 . 2 O 2 A cathode active material, super-C65 as a conductive material, and melamine cyanurate (MC) were mixed in an NMP solvent in a weight ratio of 90:5:5 to prepare a second cathode active material slurry.
[117]
A positive electrode including a positive electrode active material layer having a two-layer structure was prepared in the same manner as in Example 1, except that the first positive electrode active material slurry and the second positive electrode active material slurry prepared above were used.
[118]
[119]
Comparative Example 1
[120]
The first cathode active material slurry prepared in Example 1 was coated on Al foil, heat treated at 130° C. for 1 hour, and rolled to form a 120 μm thick cathode active material layer, which was used as a cathode for a secondary battery.
[121]
[122]
Comparative Example 2
[123]
The second cathode active material slurry prepared in Example 1 was coated on Al foil, heat treated at 130° C. for 1 hour, and rolled to form a 120 μm thick cathode active material layer, which was used as a cathode for a secondary battery.
[124]
[125]
Experimental Example 1: Overcharge Experiment
[126]
A secondary battery was manufactured using the positive electrode prepared in Examples 1-2 and Comparative Examples 1-2.
[127]
An anode slurry was prepared by mixing artificial graphite, a carbon black (super-C65) conductive material, and a styrene-butadiene rubber (SBR) binder as an anode active material in a distilled water solvent in a weight ratio of 90:5:5. The negative electrode slurry was applied to a thickness of 150 μm on a copper foil having a thickness of 10 μm, dried, and then roll pressed to prepare a negative electrode.
[128]
The positive electrode prepared in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, and the negative electrode prepared above were laminated together with a polyethylene separator (celgard) having a thickness of 20 μm to prepare an electrode assembly, and then placed in a battery case. A lithium secondary battery was prepared by injecting an electrolyte solution in which 1M LiPF 6 was dissolved in an organic solvent in which ethylene carbonate, dimethyl carbonate, and diethyl carbonate were mixed in a volume ratio of 1:2:1 .
[129]
An overcharge experiment was performed using the secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2, respectively. Specifically, after charging at 0.3C to a full charge voltage (4.2V) at 0.05C cut-off, a 1-hour rest period was given to stabilize the voltage. After the cell voltage was stabilized, overcharging was performed at 1C and 8V, and this was performed a total of 5 times to perform an overcharging experiment on the secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2, and the results are shown in Table 1 below. indicated.
[130]
[Table 1]
Maximum unignited sample temperature (°C) Number of utterances (5 total)
Example 1 172 2
Example 2 121 0
Comparative Example 1 - 5
Comparative Example 2 Difficulty in manufacturing secondary batteries due to reduced adhesion
[131]
As shown in Table 1, in the case of the secondary batteries including the positive active materials prepared in Examples 1 and 2, it was confirmed that the ignition rate was less than half among the five samples.
[132]
On the other hand, in the case of the secondary battery including the positive electrode active material prepared in Comparative Example 1, as the content of the melamine-based compound serving as a flame retardant in the electrode was less than the range of the present invention, it was confirmed that ignition occurred during overcharging.
[133]
In addition, in the case of the secondary battery including the positive active material prepared in Comparative Example 2, it was difficult to manufacture as a secondary battery due to a decrease in adhesion between the electrode layer and the current collector layer because only the melamine-based compound was included as a binder.
Claims
[Claim 1]
A positive electrode comprising a positive electrode active material layer formed on the surface of a positive electrode current collector, wherein the positive electrode active material layer includes a first positive electrode active material layer formed on the positive electrode current collector and a second positive active material layer formed on the first positive electrode active material layer It has a multilayer structure, and the first positive electrode active material layer includes a first positive electrode active material, a first binder that is a melamine-based compound, and a second binder that is different from the melamine-based compound, and the second positive electrode active material layer includes a second positive electrode active material and melamine. A positive electrode for a secondary battery comprising a first binder, which is a system compound.
[Claim 2]
The positive electrode for a secondary battery according to claim 1, wherein the melamine-based compound is a melamine salt.
[Claim 3]
The positive electrode for a secondary battery according to claim 2, wherein the melamine salt comprises melamine cyanurate.
[Claim 4]
The secondary of claim 1, wherein the first positive active material layer comprises 1 to 30 parts by weight of a first binder, which is a melamine-based compound, and a second binder, which is different from the melamine-based compound, based on 100 parts by weight of the positive active material. positive electrode for batteries.
[Claim 5]
The positive electrode for a secondary battery according to claim 1, wherein the first positive active material layer includes a first binder that is a melamine-based compound and a second binder that is different from the melamine-based compound in a weight ratio of 0.5:1 to 10:1. .
[Claim 6]
The positive electrode of claim 1, wherein the second binder comprises at least one selected from the group consisting of polyamideimide and polyvinylidene fluoride.
[Claim 7]
The positive electrode for a secondary battery according to claim 1, wherein the second positive active material layer contains 1 to 30 parts by weight of the first binder based on 100 parts by weight of the positive active material.
[Claim 8]
The positive electrode of claim 1, wherein the first positive active material layer and the second positive active material layer have a thickness ratio of 5:95 to 50:50.
[Claim 9]
A lithium secondary battery comprising the positive electrode according to claim 1 .