KR20190134021A - Negative electrode for rechargeable lithium battery, and rechargeable lithium battery including same - Google Patents

Abstract

The present invention relates to a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the same. The negative electrode for the rechargeable lithium battery comprises: a current collector; a first active material layer formed on the current collector and including a styrene-butadiene rubber binder and a first active material; and a second active material layer formed on the first active material layer and including an acrylic binder and a second active material.

Description

A negative electrode for a lithium secondary battery and a lithium secondary battery including the same {NEGATIVE ELECTRODE FOR RECHARGEABLE LITHIUM BATTERY, AND RECHARGEABLE LITHIUM BATTERY INCLUDING SAME}

The present disclosure relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same.

BACKGROUND ART Lithium secondary batteries, which are in the spotlight as power sources of recent portable small electronic devices, use organic electrolytes and exhibit a discharge voltage that is two times higher than that of a battery using an alkaline aqueous solution. As a result, the lithium secondary battery has a high energy density.

As a cathode active material of a lithium secondary battery, lithium and a transition metal having a structure capable of intercalating lithium ions such as LiCoO ₂ , LiMn ₂ O ₄ , and LiNi _1-x Co _x O ₂ (0 <x <1) Oxides are mainly used. Recently, efforts have been actively made to further develop existing lithium secondary battery technologies to expand applications to electric vehicles as well as electric power storage.

As the negative electrode active material, various types of carbon-based negative electrode active materials including artificial, natural graphite, and hard carbon capable of inserting / desorbing lithium, or non-carbon negative electrode active materials based on silicon or tin are used.

One embodiment is to provide a negative electrode for a lithium secondary battery that enables uniform charge and discharge, can suppress side reactions, excellent low temperature charge and discharge and long-term cycle life characteristics.

Another embodiment is to provide a lithium secondary battery including the negative electrode.

One embodiment includes a first active material layer formed on the current collector and the current collector, the first active material layer comprising a styrene-butadiene rubber binder and a first active material; And a second active material layer formed on the first active material layer and including an acrylic binder and a second active material.

The acrylic binder is a styrene- (meth) acrylic acid ester copolymer (styrene- (meth) acrylic ester copolymer), acrylic acid (acrylic acid), Butyl acrylate (Butyl acrylate), methyl methacrylate (Methyl methacrylate) or a combination thereof.

The loading level (L / L) of the first active material layer may be 75% or less of the loading level (L / L) of the second active material layer, and according to one embodiment, the loading level (L) of the first active material layer / L) may be 10% to 75% of the loading level (L / L) of the second active material layer.

The loading level (L / L) of the first active material layer may be 5 mg / cm 2 or less, and 1.0 mg / cm 2 to 5.0 mg / cm 2.

The loading level (L / L) of the second active material layer may be greater than 5 mg / cm 2, and the loading level (L / L) of the second active material layer may be greater than 5 mg / cm 2 and 20 mg / cm 2 or less.

The loading level (L / L) of the negative electrode may be 10 mg / cm 2 or more, and the level (L / L) of the negative electrode may be 10 mg / cm 2 to 20 mg / cm 2.

In the first active material layer, the content of the styrene-butadiene rubber binder may be 1.0 wt% to 2.0 wt% with respect to 100 wt% of the first active material layer.

In the second active material layer, the content of the acrylic binder may be 1.0 wt% to 2.0 wt% with respect to 100 wt% of the second active material layer.

The thickness of the first active material layer may be 25% to 5% of the thickness of the second active material layer.

Another embodiment is the cathode; anode; And it provides a lithium secondary battery comprising a non-aqueous electrolyte.

Other specific details of embodiments of the present invention are included in the following detailed description.

The negative electrode for a lithium secondary battery according to one embodiment may enable uniform charge and discharge, thereby suppressing side reactions, and may provide a lithium secondary battery having excellent low temperature charge and discharge and long cycle life characteristics.

1 is a view schematically showing a negative electrode structure of a lithium secondary battery according to one embodiment.
2 is a view schematically showing the structure of a lithium secondary battery according to one embodiment.
3 is a SEM photograph of the first active material layer formed in Example 1. FIG.
4 is a SEM photograph of the second active material layer formed in Example 1. FIG.
5 is a graph showing the DC internal resistance of the cathodes prepared according to Example 1 and Comparative Example 1 according to various SOC conditions.
Figure 6 is a graph showing the measurement of the impedance of the negative electrode prepared according to Example 1 and Comparative Example 1.
7 is a graph showing the low-temperature discharge characteristics of the negative electrode prepared according to Example 1 and Comparative Example 1.
Figure 8a is a graph showing the room temperature cycle life of the battery using the negative electrode prepared according to Example 1 and Comparative Example 1.
Figure 8b is a graph showing the measurement of the room temperature thickness change of the battery using a negative electrode prepared according to Example 1 and Comparative Example 1.
Figure 9 is a graph showing the measurement of the high temperature cycle life of the battery using a negative electrode prepared according to Example 1 and Comparative Example 1.
10 is a photograph of the negative electrode after the charge and discharge of the battery using a negative electrode prepared according to Example 1.
11 is a photograph of a negative electrode after charge and discharge of a battery using a negative electrode prepared according to Comparative Example 3.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, by which the present invention is not limited and the present invention is defined only by the scope of the claims to be described later.

A negative electrode for a rechargeable lithium battery according to one embodiment is formed on a current collector and the current collector, and is formed on a first active material layer and a first active material layer including a styrene-butadiene rubber binder and a first active material, and an acrylic binder. And a second active material layer comprising a second active material.

The styrene-butadiene rubber binder included in the first active material layer in direct contact with the current collector may impart excellent adhesion of the active material layer to the current collector, and may be acryl-based in the second active material layer located on the surface of the negative electrode. Since the binder is a low resistance binder and has excellent ion conductivity, it is possible to improve mobility of lithium ions during charging and discharging. Thus, in order to obtain a high capacity, even if the thickness of the active material layer in which the first active material layer and the second active material layer are formed to be thick, the resistance according to the lithium ion migration path can be suppressed, and the lithium ions can be effectively moved.

In addition, in the case of using the acrylic binder, the problem of swelling swelling due to the reaction with the electrolyte or detachment from the current collector can be effectively suppressed by using the styrene-butadiene rubber binder having excellent adhesion to the first active material layer. have.

If the binder of the first active material layer and the second active material layer is the same as a styrene-butadiene rubber or an acrylic binder, there may be a problem of increasing lithium transfer resistance and peeling of the active material layer. In addition, when an acrylic binder is used for the first active material layer and a styrene-butadiene rubber binder is used for the second active material layer, there may be a problem of peeling the current collector and the active material layer due to the electrolyte being impregnated into the electrode.

The acrylic binder may be a styrene- (meth) acrylic acid ester copolymer, acrylic acid, butyl acrylate, methyl methacrylate or a copolymer thereof.

The number average molecular weight of the styrene-butadiene rubber which is the first active material layer binder may be 25,000 to 30,000, and the number average molecular weight of the acrylic binder that is the second active material layer binder may be 20,000 to 25,000. When the number average molecular weight of the styrene-butadiene rubber and the number average molecular weight of the acrylic binder are included in the above range, it is possible to more effectively reduce the lithium ion transfer resistance at the same time to secure the adhesion to the current collector of the active material layer. In particular, when the number average molecular weight of the styrene-butadiene rubber as the first active material layer binder is greater than or equal to the number average molecular weight of the acrylic binder as the second active material layer binder, the adhesive force to the current collector of the active material layer is further increased. I can make it appropriate. In addition, even if the number average molecular weight of the styrene-butadiene rubber as the first active material layer binder is greater than or equal to the number average molecular weight of the acrylic binder as the second active material layer binder, when the number average molecular weight of each binder is out of the above range, The active material layer may cause a current collector and a current collector peeling phenomenon.

Hereinafter, in the present specification, the loading level means the loading level of the active material layer on one surface, that is, one surface.

The loading level (L / L) of the first active material layer may be 75% or less of the loading level (L / L) of the second active material layer, and according to one embodiment, the loading level (L) of the first active material layer / L) may be 10% to 75% of the loading level (L / L) of the second active material layer. When the loading level of the first active material layer is 75% or less of the loading level of the second active material layer, the lithium transfer resistance may be minimized.

The loading level (L / L) of the first active material layer may be 5 mg / cm 2 or less, and 1.0 mg / cm 2 to 5.0 mg / cm 2.

The loading level (L / L) of the second active material layer may be greater than 5 mg / cm 2, and the loading level (L / L) of the second active material layer may be greater than 5 mg / cm 2, and 20 mg / cm 2 or less, and other According to one embodiment, it may be 10mg / ㎠ to 20mg / ㎠.

As such, the loading level of the first active material layer is smaller than the loading level of the second active material layer, and in particular, the loading level of the first active material layer is less than 75% of the loading level of the second active material layer within the loading level range. Therefore, the adhesive force between an active material and a base material can be ensured. If the loading level of the first active material layer is the same as or higher than the loading level of the second active material layer, there may be a problem that the lithium ion transfer resistance may increase. In particular, even if the loading level of the first active material layer is less than the loading level of the second active material layer, the lithium ion transfer resistance improvement effect may be reduced when it exceeds 75%.

The loading level (L / L) of the negative electrode may be 10 mg / cm 2 or more, and the level (L / L) of the negative electrode may be 10 mg / cm 2 to 20 mg / cm 2. When the first active material layer and the second active material layer having the binder and the loading level are formed on the negative electrode having a loading level of 10 mg / cm 2 or more, not only the adhesion but also the lithium ion transfer resistance, which is a disadvantage of the styrene-butadiene rubber, is improved. You can get the advantage of doing it.

The thickness of the first active material layer may be 25% to 5% of the thickness of the second active material layer. As such, when the thickness of the first active material layer is very thin compared to the thickness of the second active material layer, that is, 25% to 5%, a lithium ion migration resistance reduction effect may be obtained.

The sum of the thickness of the first active material layer and the thickness of the second active material layer may be 60 μm to 100 μm thick. That is, when the sum total of the thickness of a 1st active material layer and the thickness of a 2nd active material layer is contained in the said range, since a whole active material layer is thick, battery high capacity can be obtained. In addition, the negative electrode according to the embodiment uses styrene-butadiene rubber and an acrylic binder as binders of the first active material layer and the second active material layer, respectively, and controls the loading level, thereby effectively forming lithium ions even when the entire active material layer is thickly formed. It can move, the problem that an active material layer detaches from a current collector can be prevented, and low-temperature charge-discharge and cycle life characteristics can be improved.

In the first active material layer, the content of the styrene-butadiene rubber binder may be 1.0 wt% to 2.0 wt% with respect to 100 wt% of the first active material layer. When the content of the styrene-butadiene rubber binder is included in the above range, it is possible to secure the adhesive force of the base material and the mixture portion, it is possible to improve the problem of peeling the mixture in the electrolyte.

In the second active material layer, the content of the acrylic binder may be 1.0 wt% to 2.0 wt% with respect to 100 wt% of the second active material layer. When the content of the acrylic binder is included in the above range, there may be an advantage that can reduce the lithium ion transfer resistance.

The negative electrode active material in the first active material layer and the second active material layer may be the same or different from each other. The negative electrode active material may include a material capable of reversibly intercalating / deintercalating lithium ions, a lithium metal, an alloy of lithium metal, a material doped and undoped with lithium, or a transition metal oxide.

Examples of a material capable of reversibly intercalating / deintercalating the lithium ions include carbon materials, that is, carbon-based negative electrode active materials generally used in lithium secondary batteries. Representative examples of the carbon-based negative active material may be crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate, flake, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon ( hard carbon), mesophase pitch carbide, calcined coke, and the like.

Examples of the alloy of the lithium metal include lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn. Alloys of metals selected from can be used.

Examples of materials that can be doped and undoped with lithium include Si, SiO _x (0 <x <2), and Si-Q alloys (wherein Q is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, and 16). An element selected from the group consisting of group elements, transition metals, rare earth elements, and combinations thereof, not Si), Si-carbon composites, Sn, SnO ₂ , Sn-R (wherein R is an alkali metal, an alkaline earth metal, 13 An element selected from the group consisting of group elements, group 14 elements, group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof, and not Sn), Sn-carbon composites, and the like. At least one of them may be used by mixing with SiO ₂ . The elements Q and R include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof may be used.

The Si-carbon composite may include silicon particles and crystalline carbon. The average particle diameter (D50) of the silicon particles may be 10 nm to 200 nm. The Si-carbon composite may further include an amorphous carbon layer formed at least in part.

The first active material layer and the second active material layer may further include a conductive material. The conductive material is used to impart conductivity to the electrode, and any battery can be used as long as it is an electron conductive material without causing chemical change in the battery. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, denka black, and carbon fiber; Metal materials such as metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive polymers such as polyphenylene derivatives; Or a conductive material containing a mixture of these.

In the first active material layer and the second active material layer, except for the binder, the active material and the conductive material may be used in the same amount, or may be used in different amounts. The content of the active material may be 96 wt% to 97.7 wt% with respect to 100 wt% of the first active material layer or the second active material layer, and the content of the conductive material may be the first active material layer or the second active material. It can be from 0.5% to 2.0% by weight relative to the total 100% by weight of the layer.

The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof. .

1 schematically shows a structure of a negative electrode 20 according to one embodiment having the above configuration, that is, the negative electrode 20 is a current collector 3, a first active material layer formed on the current collector 3. (5) and the second active material layer 7 formed on the first active material layer 5 are included.

The negative electrode is formed by applying and drying a first active material layer composition prepared by mixing a styrene-butadiene rubber, a negative electrode active material, and optionally a conductive material in a first solvent to a current collector to form a first active material layer, and an acrylic binder and a negative electrode. The second active material layer composition prepared by mixing an active material, optionally, a conductive material in a second solvent, may be applied to the first active material layer and dried to form a second active material layer, and then rolled.

Water may be used as the first solvent, and organic solvents such as water and N-methylpyrrolidone may be used as the second solvent.

Another embodiment provides a lithium secondary battery including the anode, a cathode including an anode active material, and an electrolyte.

The positive electrode includes a positive electrode active material layer containing a current collector and a positive electrode active material formed on the current collector.

The positive electrode active material may include a compound (lithiated intercalation compound) capable of reversible intercalation and deintercalation of lithium. Specifically, at least one of a complex oxide of metal and lithium selected from cobalt, manganese, nickel, and a combination thereof can be used. More specific examples may be used a compound represented by any one of the following formula. Li _a A _1-b X _b D ₂ (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5); Li _a A _1-b X _b O _2-c D _c (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05); Li _a E _1-b X _b O _2-c D _c (0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05); Li _a E _2-b X _b O _4-c D _c (0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05); Li _a Ni _1-bc Co _b X _c D _α (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.5, 0 ≦ α ≦ 2); Li _a Ni _1-bc Co _b X _c 0 _2-α T _α (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 ≦ α <2); Li _a Ni _1-bc Co _b X _c 0 _2-α T ₂ (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 ≦ α <2); Li _a Ni _1-bc Mn _b X _c D _α (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 ≦ α ≦ 2); Li _a Ni _1-bc Mn _b X _c 0 _2-α T _α (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 ≦ α ≦ 2); Li _a Ni _1-bc Mn _b X _c 0 _2-α T ₂ (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.5, 0 ≦ c ≦ 0.05, 0 ≦ α ≦ 2); Li _a Ni _b E _c G _d O ₂ (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.5, 0.001 ≦ d ≦ 0.1); Li _a Ni _b Co _c Mn _d G _e O ₂ (0.90 ≦ a ≦ 1.8, 0 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0.5, 0.001 ≦ e ≦ 0.1); Li _a NiG _b 0 ₂ (0.90 ≦ a ≦ 1.8, 0.001 ≦ b ≦ 0.1); Li _a CoG _b O ₂ (0.90 ≦ a ≦ 1.8, 0.001 ≦ b ≦ 0.1); Li _a Mn _1-b G _b O ₂ (0.90 ≦ a ≦ 1.8, 0.001 ≦ b ≦ 0.1); Li _a Mn ₂ G _b O ₄ (0.90 ≦ a ≦ 1.8, 0.001 ≦ b ≦ 0.1); Li _a Mn _1-g G _g PO ₄ (0.90 ≦ a ≦ 1.8, 0 ≦ g ≦ 0.5); QO ₂ ; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiZO ₂ ; LiNiVO ₄ ; Li _(3-f) J ₂ (PO ₄ ) ₃ (0 ≦ f ≦ 2); Li _(3-f) Fe ₂ (PO ₄ ) ₃ (0 ≦ f ≦ 2); Li _a FePO ₄ (0.90 ≤ a ≤ 1.8)

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

Of course, what has a coating layer on the surface of this compound can also be used, or the compound and the compound which have a coating layer can also be used in mixture. The coating layer may include at least one coating element compound selected from the group consisting of oxides of the coating elements, hydroxides of the coating elements, oxyhydroxides of the coating elements, oxycarbonates of the coating elements and hydroxycarbonates of the coating elements. Can be. The compounds constituting these coating layers may be amorphous or crystalline. As the coating element included in the coating layer, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof may be used. The coating layer forming process may use any coating method as long as it can be coated with the above compounds by a method that does not adversely affect the physical properties of the positive electrode active material (for example, spray coating or dipping method). Detailed descriptions thereof will be omitted since they can be understood by those skilled in the art.

In the positive electrode, the content of the positive electrode active material may be 90% to 98% by weight based on the total weight of the positive electrode active material layer.

In one embodiment, the cathode active material layer may further include a binder and a conductive material. In this case, the content of the binder and the conductive material may be 1% by weight to 5% by weight based on the total weight of the positive electrode active material layer, respectively.

The binder adheres the positive electrode active material particles to each other well, and also serves to adhere the positive electrode active material to the current collector well. Representative examples of the binder include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinylchloride, polyvinyl fluoride, polymers including ethylene oxide, polyvinylpyrroly Don, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon and the like can be used, but is not limited thereto. .

The conductive material is used to impart conductivity to the electrode, and any battery can be used as long as it is an electron conductive material without causing chemical change in the battery. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; Metal materials such as metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive polymers such as polyphenylene derivatives; Or a conductive material containing a mixture of these.

The current collector may be aluminum foil, nickel foil, or a combination thereof, but is not limited thereto.

The positive electrode active material layer and the negative electrode active material layer are formed by mixing an anode active material, a binder, and optionally a conductive material in a solvent to prepare an active material composition, and applying the active material composition to a current collector, rolling, and drying it. Since the method of forming the active material layer is well known in the art, detailed description thereof will be omitted. N-methylpyrrolidone may be used as the solvent, but is not limited thereto. In addition, when a water-soluble binder is used as the binder of the negative electrode active material layer, water may be used as the solvent.

The electrolyte includes a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

As the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent may be used.

Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), and ethylene carbonate ( EC), propylene carbonate (PC), butylene carbonate (BC) and the like can be used. The ester solvent may be methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, Caprolactone and the like can be used. Dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. may be used as the ether solvent. In addition, cyclohexanone may be used as the ketone solvent. In addition, ethyl alcohol, isopropyl alcohol, etc. may be used as the alcohol solvent, and the aprotic solvent may be R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms. Nitriles such as a double bond aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolane, and the like can be used. .

The non-aqueous organic solvent may be used alone or in admixture of one or more. The mixing ratio in the case of mixing more than one can be appropriately adjusted according to the desired cell performance, which can be widely understood by those skilled in the art.

In the case of the carbonate solvent, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, the cyclic carbonate and the chain carbonate may be mixed and used in a volume ratio of 1: 1 to 1: 9, so that the performance of the electrolyte may be excellent.

When the non-aqueous organic solvent is mixed and used, a mixed solvent of a cyclic carbonate and a chain carbonate, or a mixed solvent of a cyclic carbonate and a propionate solvent or a cyclic carbonate, a chain carbonate and a propionate system Mixed solvents of solvents may be used. As the propionate solvent, methyl propionate, ethyl propionate, propyl propionate, or a combination thereof may be used.

In this case, when the cyclic carbonate and the chain carbonate or the cyclic carbonate and the propionate solvent are mixed, the mixture may be used in a volume ratio of 1: 1 to 1: 9, and the performance of the electrolyte may be excellent. In addition, when a cyclic carbonate, a chain carbonate and a propionate solvent are used in a mixture, the mixture may be used in a volume ratio of 1: 1: 1 to 3: 3: 4. Of course, the mixing ratio of the solvent may be appropriately adjusted according to the desired physical properties.

The non-aqueous organic solvent may further include an aromatic hydrocarbon organic solvent in the carbonate solvent. In this case, the carbonate solvent and the aromatic hydrocarbon organic solvent may be mixed in a volume ratio of 1: 1 to 30: 1.

As the aromatic hydrocarbon-based organic solvent, an aromatic hydrocarbon compound of Formula 1 may be used.

[Formula 1]

(In Chemical Formula 1, R ₁ to R ₆ are the same as or different from each other and are selected from the group consisting of hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, and a combination thereof.)

Specific examples of the aromatic hydrocarbon organic solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-tri Fluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1 , 2,4-trichlorobenzene, iodobenzene, 1,2-dioodobenzene, 1,3-dioiobenzene, 1,4-dioiobenzene, 1,2,3-triiodobenzene, 1, 2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluoro Rotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2, 3,5-trichlorotoluene, iodotoluene, 2,3-dioodotoluene, 2,4-diaodotoluene, 2 , 5-diaodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

The electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound of Formula 2 as a life improving additive to improve battery life.

[Formula 2]

In Formula 2, R ₇ and R ₈ are the same as or different from each other, and are selected from the group consisting of hydrogen, a halogen group, a cyano group (CN), a nitro group (NO ₂ ), and an alkyl group having 1 to 5 fluorinated carbon atoms. R ₇ and R ₈ At least one is selected from the group consisting of a halogen group, a cyano group (CN), a nitro group (NO ₂ ), and a fluorinated alkyl group having 1 to 5 carbon atoms, provided that R ₇ and R ₈ are not all hydrogen.)

Representative examples of the ethylene carbonate-based compound include difluoro ethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate or fluoroethylene carbonate. Can be. In the case of further using such life improving additives, the amount thereof can be properly adjusted.

The electrolyte may further include vinylethylene carbonate, propane sultone, succinonitrile, or a combination thereof, and the amount of the electrolyte may be appropriately adjusted.

The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery and to promote the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of such lithium salts are LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ C ₂ F ₅ ) ₂ , Li (CF ₃ SO ₂ ) ₂ N, LiN (SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural numbers, for example Supporting one or more selected from the group consisting of LiCl, LiI and LiB (C ₂ O ₄ ) ₂ (lithium bis (oxalato) borate (LiBOB)); It is preferable to use the concentration of lithium salt within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, it can exhibit excellent electrolyte performance, Lithium ions can move effectively.

Depending on the type of lithium secondary battery, a separator may exist between the positive electrode and the negative electrode. As the separator, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / poly It goes without saying that a mixed multilayer film such as a propylene three-layer separator can be used.

2 is an exploded perspective view of a rechargeable lithium battery according to one embodiment of the present invention. Although a lithium secondary battery according to an embodiment is described as an example of being rectangular, the present invention is not limited thereto, and may be applied to various types of batteries, such as a cylindrical shape and a pouch type.

Referring to FIG. 2, the lithium secondary battery 100 according to the exemplary embodiment includes an electrode assembly 40 wound through a separator 30 between the positive electrode 10 and the negative electrode 20, and the electrode assembly 40. It may include a case 50 is built. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown).

Hereinafter, examples and comparative examples of the present invention are described. Such following examples are only examples of the present invention, and the present invention is not limited to the following examples.

(Example 1)

97.5% by weight of the artificial graphite negative electrode active material, 1.0% by weight of carboxymethyl cellulose and 1.5% by weight of styrene-butadiene rubber were mixed in a water solvent to prepare a slurry of the first active material layer.

A slurry of the second active material layer was prepared by mixing 97.5 wt% of artificial graphite negative electrode active material, 0.8 wt% of carboxymethyl cellulose, and 1.2 wt% of styrene- (meth) acrylic acid ester copolymer in a water solvent.

The first active material layer slurry is coated and dried on Cu foil to form a first active material layer, and the second active material layer slurry is coated and dried on the first active material layer to form a second active material layer, followed by rolling. A negative electrode was prepared. In the prepared anode, the loading level of the first active material layer was 4.5 mg / cm 2, and the loading level of the second active material layer was 6 mg / cm 2, so that the sum of the loading levels of the first active material layer and the second active material layer was 10.5 mg / cm 2. It was 2 cm 2 (one side, 21 mg / cm 2). In addition, the thickness of the first active material layer was 17 μm, and the thickness of the second active material layer was 80 μm.

A positive electrode active material slurry was prepared by mixing 97.35 wt% of LiCoO ₂ positive electrode active material, 1.5 wt% of Ketjen black conductive material, and 1.15 wt% of polyvinylidene fluoride binder in N-methyl pyrrolidone solvent. The positive electrode active material slurry was applied to an Al foil, dried, and rolled to prepare a positive electrode.

A secondary battery was manufactured by a conventional method using the negative electrode, the positive electrode, and the electrolyte.

As the electrolyte, a mixed solvent (50:50 by volume) of ethylene carbonate and dimethyl carbonate in which 1.2 M LiPF ₆ was dissolved was used.

(Comparative Example 1)

97.5% by weight of the artificial graphite anode active material, 1.0% by weight of carboxymethyl cellulose, and 1.5% by weight of styrene-butadiene rubber were mixed in a water solvent to prepare an active material layer slurry.

The active material layer slurry was coated, dried and rolled onto Cu foil to prepare a negative electrode having a loading level of 10.5 mg / cm 2 (both sides 21 mg / cm 2) in which an active material layer having a thickness of 68.5 μm (both sides 137 μm) was formed.

Using the negative electrode, a secondary battery was manufactured by a conventional method using the positive electrode and the electrolyte prepared in Example 1.

(Comparative Example 2)

97.5% by weight of the artificial graphite negative electrode active material, 1.0% by weight of carboxymethyl cellulose and 1.5% by weight of styrene-butadiene rubber were mixed in a water solvent to prepare a slurry of the first active material layer.

97.5% by weight of the artificial graphite anode active material, 0.8% by weight of carboxymethyl cellulose, and 1.2% by weight of styrene-butadiene rubber were mixed in a water solvent to prepare a second active material layer slurry.

The first active material layer slurry is coated and dried on Cu foil to form a first active material layer, and the second active material layer slurry is coated and dried on the first active material layer to form a second active material layer, followed by rolling. A negative electrode was prepared. In the prepared anode, the loading level of the first active material layer was 4.5 mg / cm 2, and the loading level of the second active material layer was 6 mg / cm 2, so that the sum of the loading levels of the first active material layer and the second active material layer was 10.5 mg / cm 2. Cm 2. In addition, the thickness of the first active material layer was 17 μm, and the thickness of the second active material layer was 51 μm.

Using the negative electrode, a secondary battery was manufactured by a conventional method using the positive electrode and the electrolyte prepared in Example 1.

(Comparative Example 3)

A slurry of the first active material layer was prepared by mixing 97.5% by weight of the artificial graphite negative electrode active material, 0.8% by weight of carboxymethyl cellulose, and 1.2% by weight of styrene- (meth) acrylic acid ester copolymer in a water solvent.

97.5% by weight of the artificial graphite anode active material, 1.0% by weight of carboxymethyl cellulose, and 1.5% by weight of styrene-butadiene rubber were mixed in a water solvent to prepare a second active material layer slurry.

The first active material layer slurry is coated and dried on Cu foil to form a first active material layer, and the second active material layer slurry is coated and dried on the first active material layer to form a second active material layer, followed by rolling. A negative electrode was prepared. In the prepared anode, the loading level of the first active material layer was 4.5 mg / cm 2, and the loading level of the second active material layer was 6 mg / cm 2, so that the sum of the loading levels of the first active material layer and the second active material layer was 10.5 mg / cm 2. Cm 2. In addition, the thickness of the first active material layer was 17 μm, and the thickness of the second active material layer was 51 μm.

Using the negative electrode, a secondary battery was manufactured by a conventional method using the positive electrode and the electrolyte prepared in Example 1.

* SEM photo

The SEM photograph after the formation of the first active material layer is shown in FIG. 3, and the SEM photograph after the formation of the second active material layer is shown in FIG. 4.

As shown in FIG. 3, it can be seen that the styrene-butadiene rubber binder included in the first active material layer aggregates on the surface of the active material, and as shown in FIG. 4, the acrylic binder included in the second active material layer is an active material. It can be seen that it is evenly distributed on the surface. As such, as the acrylic binder included in the second active material layer is uniformly distributed on the surface of the active material, it may be predicted that lithium ion migration may effectively occur during charge and discharge.

* Direct current internal resistance (DC-IR) measurement

The battery according to Example 1 and Comparative Example 1 was charged and discharged at 0.7 C-rate, and at this time, the SOC condition was charged to be 70% charge capacity when SOC70 (when the total charge capacity of the battery was 100%). This means that the battery is discharged at 30% when the battery is discharged.SoC20 (when the battery is 100% charged, the battery is charged at 20% charge capacity.) Means 80%) and SOC10 (when the total charge capacity of the battery is 100%, it is charged to have 10% charge capacity, which means that the battery is discharged when 90% is discharged) The voltage drop (V) generated while flowing a current for 600 seconds at 0.7C was measured, and the DC internal resistance (DC-IR) was measured. The results are shown in FIG.

As shown in FIG. 5, it can be seen that the battery of Example 1 has a lower DC internal resistance than that of Comparative Example 1 in all SOC conditions.

Impedance Measurement

Two secondary batteries according to Example 1 and Comparative Example 1 were prepared, and the batteries were charged and discharged under 0.7C, 4.4V cut-off and SOC100 conditions. The impedance was measured by electrochemical impedance spectroscopy (EIS), and the results are shown in FIG. 6. As shown in FIG. 6, the battery of Example 1 having a first active material layer containing a styrene-butadiene rubber binder and a second active material layer containing an acrylic binder includes a styrene-butadiene rubber binder of Comparative Example 1 It can be seen that the resistance is lower than that of the battery.

* Low temperature discharge characteristic measurement

Two secondary batteries were prepared in Example 1 and Comparative Example 1, respectively, and the batteries were charged at room temperature (20 ° C.) at 0.5 C, and discharged at −15 ° C. under 0.5 C and 3 V cut-off conditions. . The capacity retention rate of the low-temperature discharge capacity was obtained compared to the normal temperature charge capacity, and the results are shown in FIG. 7.

As shown in FIG. 7, it can be seen that the low-temperature discharge characteristics of Example 1 are superior to Comparative Example 1.

* Temperature cycle life characteristics

Two secondary batteries according to Example 1 and Comparative Examples 1 to 3 were prepared, respectively, and the batteries were charged and discharged at 0.7C charge, 0.7C charge, and 1C discharge at room temperature (25 ° C) and high temperature (45 ° C), respectively. 450 times was performed. The room temperature capacity retention rate for 450 cycles for one charge / discharge cycle was obtained, and the results are shown in Table 1 below. The results in Table 1 below show the average of the room temperature capacity retention rates obtained from two batteries.

Binder of First Active Material Layer Binder of Second Active Material Layer Capacity retention rate (%) Example 1 Styrene-butadiene rubber Styrene- (meth) acrylic acid ester copolymer 89.5 Comparative Example 1 Styrene-butadiene rubber No second active material layer. 84.1 Comparative Example 2 Styrene-butadiene rubber Styrene-butadiene rubber 84.3 Comparative Example 3 Styrene- (meth) acrylic acid ester copolymer Styrene-butadiene rubber Visual defects

Moreover, the result of Example 1 and the comparative example 1 is shown in FIG. 8A among the results.

As shown in Table 1 and FIG. 8A, the room temperature capacity of Example 1 using styrene-butadiene rubber as the binder of the first active material layer and styrene- (meth) acrylic acid ester copolymer as the binder of the second active material layer It can be seen that the retention rate is superior to Comparative Examples 1 and 2. In contrast to Example 1, in the case of Comparative Example 3 in which a styrene- (meth) acrylic acid ester copolymer was used as the binder of the first active material layer and styrene-butadiene rubber was used as the binder of the second active material layer, the appearance was poor. In addition, it was impossible to measure cycle life characteristics.

* Thickness increase at room temperature

Two secondary batteries according to Example 1 and Comparative Examples 1 to 3 were prepared, respectively, and the batteries were charged and discharged at 0.7C charge, 0.7C charge, and 1C discharge at room temperature (25 ° C) and high temperature (45 ° C), respectively. 450 times was performed. The battery thicknesses before charge and discharge and after 450 charge and discharge were measured at room temperature and high temperature, respectively, to obtain room temperature thickness increase values after 450 charge and discharge, and the results are shown in FIG. 8B. As shown in FIG. 8B, it can be seen that the battery of Comparative Example 1 significantly increased the battery thickness at about 200 charge / discharge cycles and 350 charge / discharge cycles. On the other hand, in the battery of Example 1, the thickness increase rate did not increase rapidly even after 450 charge and discharge cycles, and it can be seen that a very small thickness increase rate was obtained compared to Comparative Example 1.

* High temperature cycle life characteristics

Two secondary batteries according to Example 1 and Comparative Example 1 were manufactured, respectively, and the batteries were charged and discharged at a high temperature (45 ° C.) at 0.7C charge, 0.7C charge, and 1C discharge for 450 times. At this time, the high temperature discharge capacity for each cycle was obtained and the results are shown in FIG. 9.

As shown in Figure 9, it can be seen that the high temperature discharge capacity of the battery according to Example 1 is superior to Comparative Example 1.

As a result, as shown in Figures 8a and 9, it can be seen that the battery of Example 1 exhibits excellent cycle life characteristics compared to the battery of Comparative Example 1 at room temperature and high temperature, and, as shown in Figure 8b, In the battery of Comparative Example 1, it can be seen that the thickness increase after 450 charge / discharge cycles is greater than that of Example 1 at room temperature.

* After charging and discharging, battery picture

After charging and discharging the battery including the negative electrode prepared according to Example 1 and Comparative Example 3 under the conditions of 0.7C, 4.4V charge and 1.0C, 3.0V cut-off discharge, the battery is disassembled, the photo showing the state of the negative electrode Are shown in FIGS. 10 and 11, respectively. As shown in FIG. 10, in the case of Example 1 in which a styrene-butadiene rubber binder is used as the first active material layer binder and an acrylic binder is used as the second active material layer binder, no active material layer peeling occurs, and the active material layer is a current collector. It can be seen that it is attached well to.

On the other hand, as shown in FIG. 11, in Comparative Example 3 using an acrylic binder as the first active material layer binder and a styrene-butadiene rubber binder as the second active material layer binder, a part of the active material layer is separated from the current collector. It can be seen that the phenomenon occurs, and thus, the active material layer does not maintain a uniform thickness in the electrode, it can be seen that the non-uniform. From this result, it can be seen that the battery of Comparative Example 3 has a problem that the battery thickness is poor. Thus, since the active material layer peeling occurred, additional cycle life characteristics evaluation could not be measured.

The present invention is not limited to the above embodiments, but may be manufactured in various forms, and a person skilled in the art to which the present invention pertains has another specific form without changing the technical spirit or essential features of the present invention. It will be appreciated that the present invention may be practiced as. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

Claims (14)

Current collectors;
A first active material layer formed on the current collector and including a styrene-butadiene rubber binder and a first active material; And
A second active material layer formed on the first active material layer and including an acrylic binder and a second active material;
A negative electrode for a lithium secondary battery comprising a. The method of claim 1,
The acrylic binder is a styrene- (meth) acrylic acid ester copolymer, butyl acrylate (Butyl acrylate), methyl methacrylate (Methyl methacrylate) or a combination thereof. The method of claim 1,
The loading level (L / L) of the first active material layer has a value of 75% or less of the loading level (L / L) of the second active material layer. The method of claim 3,
The loading level (L / L) of the first active material layer has a value of 10% to 75% of the loading level (L / L) of the second active material layer. The method of claim 1,
A loading level (L / L) of the first active material layer is 5 mg / cm 2 or less. The method of claim 5,
The loading level (L / L) of the first active material layer is 1.0 mg / cm 2 to 5.0 mg / cm 2 negative electrode for a lithium secondary battery. The method of claim 1,
The loading level (L / L) of the second active material layer is greater than 5 mg / cm 2 negative electrode for a lithium secondary battery. The method of claim 7, wherein
The loading level (L / L) of the second active material layer is greater than 5 mg / cm 2 and 20 mg / cm 2 or less. The method of claim 1,
The loading level (L / L) of the negative electrode is a lithium secondary battery negative electrode of more than 10mg / ㎠. The method of claim 9,
Level (L / L) of the negative electrode is a lithium secondary battery negative electrode of 10 mg / ㎠ to 20 mg / ㎠. The method of claim 1,
In the first active material layer, the content of the styrene-butadiene rubber binder is 1.0% by weight to 2.0% by weight with respect to 100% by weight of the total of the first active material layer. The method of claim 1,
In the second active material layer, the content of the acrylic binder is 1.0% by weight to 2.0% by weight based on 100% by weight of the total of the second active material layer negative electrode for a lithium secondary battery. The method of claim 1,
The thickness of the first active material layer is a negative electrode for a lithium secondary battery having a value of 5% to 25% with respect to the thickness of the second active material layer. The cathode of any one of claims 1 to 13;
anode; And
Containing non-aqueous electrolyte
Lithium secondary battery.

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