Negative electrode for rechargeable lithium battery and rechargeable lithium battery including same

Abstract

The present invention relates to a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery comprising the same, the negative electrode comprising a current collector, a protective layer, and a layer of a negative electrode active material provided between the current collector and the protective layer and containing the negative electrode active material, in which the protective layer comprises a fiber comprising a core and a shell, the core comprises a metal that can form an alloy with lithium, and the shell surrounds the core and comprises a non-conductive polymer.

Description

Technical Field

[0001] Disclosed are a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the negative electrode. Background Art

[0002] Recently, the rapid increase in the number of electronic devices using batteries (such as mobile phones, laptop computers, and electric vehicles) has led to a dramatic increase in the demand for rechargeable batteries with relatively high capacity and light weight. In particular, since rechargeable lithium batteries have light weight and high energy density, they have recently attracted attention as a driving power source for portable devices. Therefore, research and development to improve the performance of rechargeable lithium batteries is being actively carried out.

[0003] Specifically, in order to increase the energy density of rechargeable lithium batteries, research is being conducted to minimize the relative amounts of components that do not contribute to the energy storage capacity (such as separators, binders, and current collectors) and to maximize the amount of active material that can store lithium (i.e., loading amount) to approximately 5 mAh / cm 2 or larger.

[0004] However, in the case of a high energy density negative electrode in which the loading amount is increased in this manner, there is a problem in that dendrites may be formed on the surface of the negative electrode active material, thereby causing an electrical short circuit.

[0005] The formation of dendrites is caused by differences in the migration rate of lithium ions, differences in the insertion rate of lithium ions between the interface of the negative electrode active material layer in contact with the separator and the interface of the negative electrode active material layer in contact with the current collector (SEI), and differences in the diffusion rate between graphite basal planes. Summary of the invention

[0006] Technical issues Embodiments provide a negative electrode for a rechargeable lithium battery exhibiting improved cycle-life characteristics.

[0007] Another embodiment provides a rechargeable lithium battery including a negative electrode.

[0008] Technical Solution An embodiment provides a negative electrode for a rechargeable lithium battery, the negative electrode for a rechargeable lithium battery comprising: a current collector; a protective layer; and a negative electrode active material layer, which is arranged between the current collector and the protective layer and includes a negative electrode active material, wherein the protective layer includes fibers, the fibers include a core and a shell, the core includes a metal capable of alloying with lithium, and the shell surrounds the core and includes a non-conductive polymer.

[0009] The metal capable of alloying with lithium may be Si, Sn, Ge or a combination thereof.

[0010] The non-conductive polymer may be polyacrylonitrile, nylon, polymethyl methacrylate, polystyrene, styrene acrylonitrile, polycaprolactone, polyacrylic acid, polylactic acid, polyvinylidene fluoride, copolymers thereof, or combinations thereof.

[0011] The thickness of the protective layer may be 1 μm to 20 μm.

[0012] The weight ratio of the shell to the core may be 1:0.1 to 1.

[0013] The negative electrode active material can be crystalline carbon or lithium metal.

[0014] The core may also include additional polymers capable of immobilizing metals capable of alloying with lithium.

[0015] The additional polymer may be styrene-co-acrylonitrile (SAN), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), or combinations thereof.

[0016] The mixing ratio of the metal capable of alloying with lithium and the additional polymer may be 0.1 to 1:1 by weight.

[0017] Another embodiment provides a rechargeable lithium battery including a negative electrode, a positive electrode, and an electrolyte, the negative electrode including the negative electrode active material.

[0018] Beneficial Effects The negative electrode for a rechargeable lithium battery according to an embodiment may exhibit excellent cycle-life characteristics and safety by suppressing the occurrence of a short circuit during charging and discharging. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram illustrating a negative electrode for a rechargeable lithium battery according to an embodiment.

[0020] Figure 2 is a view showing fibers included in a protective layer according to an embodiment.

[0021] Figure 3 is a schematic diagram illustrating a rechargeable lithium battery according to an embodiment.

[0022] Figure 4a This is a 1000-times-magnified SEM photograph of the fibers constituting the protective layer manufactured in Example 1.

[0023] Figure 4b This is a 3000-times-magnified SEM photograph of the fibers constituting the protective layer manufactured in Example 1.

[0024] Figure 5 is a graph showing 10 charge / discharge cycle characteristics of the rechargeable lithium battery cell of Example 1.

[0025] Figure 6 is a graph showing 10 charge / discharge cycle characteristics of the rechargeable lithium battery cell of Comparative Example 1.

[0026] Figure 7 is a graph showing 20 charge / discharge cycle characteristics of the rechargeable lithium battery cell of Example 2.

[0027] Figure 8 is a graph showing 20 charge / discharge cycle characteristics of the rechargeable lithium battery cell of Comparative Example 1.

[0028] Fig. 9 is a graph showing 20 charge / discharge cycle characteristics of the rechargeable lithium battery cell of Comparative Example 2. DETAILED DESCRIPTION

[0029] Hereinafter, embodiments of the present invention will be described in detail. However, these embodiments are merely examples, the present invention is not limited thereto, and the present invention is defined by the scope of the claims.

[0030] Unless otherwise defined herein, particle size refers to the average particle size (D50), which refers to the diameter of particles having a cumulative volume of 50% by volume in a particle size distribution. The average particle size (D50) can be measured by methods well known to those skilled in the art (e.g., by using a particle size analyzer, or by using a transmission electron microscope (TEM) or a scanning electron microscope (SEM)). Alternatively, a dynamic light scattering measurement device is used for data analysis, and the number of particles in each particle size range is counted, whereby the average particle size (D50) value can be easily obtained by calculation.

[0031] A negative electrode for a rechargeable lithium battery according to an embodiment includes: a current collector; a protective layer; and a negative electrode active material layer, disposed between the current collector and the protective layer, and including a negative electrode active material. In an embodiment, the protective layer may include fibers, the fibers including a core and a shell, the core including a metal capable of alloying with lithium, the shell surrounding the core and including a non-conductive polymer.

[0032] Figure 1 Such a negative electrode 1 for such a rechargeable lithium battery is schematically shown in , and the negative electrode 1 includes a current collector 3 and a protective layer 7 , and includes a negative electrode active material layer 5 between the current collector 3 and the protective layer 7 .

[0033] The fiber included in the protective layer includes a core and a shell, the core refers to a central portion, and the shell refers to a portion surrounding the central portion (ie, the core).

[0034] According to an embodiment, Figure 2 The fiber 11 shown in FIG. 1 may include a core including a metal 15 capable of alloying with lithium, and a shell 13 may surround a central portion (ie, the core).

[0035] The core may further include an additional polymer capable of fixing a metal capable of alloying with lithium while forming the core. The additional polymer may be styrene-co-acrylonitrile (SAN), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), or a combination thereof.

[0036] The mixing ratio of the metal capable of alloying with lithium to the additional polymer may be 0.1 to 1:1 by weight, 0.1 to 0.8:1 by weight, or 0.1 to 0.5:1 by weight.

[0037] The protective layer is disposed on the surface of the negative electrode active material layer and is disposed in contact with the electrolyte. The fiber included in the protective layer has a metal capable of charge / discharge reaction positioned inside and a polymer not having conductivity positioned outside, so that the metal is included in the negative electrode without participating in the charge / discharge reaction.

[0038] If a rechargeable lithium battery including such a negative electrode is charged and discharged, lithium dendrites may be formed, and as charging and discharging proceed, the lithium dendrites further grow, and the lithium dendrites may form an alloy with the metal included in the core, and thus may be converted into a state in which they can participate in charging and discharging reactions and become activated.

[0039] In addition, due to the formation of such an alloy, lithium dendrites formed on the negative electrode can be removed, so excessive film, SEI film formation due to excessive formation of lithium dendrites can be suppressed, and short circuit can also be suppressed.

[0040] In this way, since the metal capable of alloying with lithium included in the inner core is substantially not exposed to the outside (i.e., exists in an isolated form) by being surrounded by the non-conductive polymer, the protective layer does not participate in the charging and discharging reaction before the formation of lithium dendrites even when charging and discharging are performed, and thus it can only effectively suppress the formation of dendrites without affecting the cycle life.

[0041] If the metal capable of alloying with lithium is exposed to the outside instead of being surrounded by a non-conductive polymer or surrounded by a conductive material, it may participate in the electrochemical reaction, and thus the effect of suppressing dendrite formation may not be achieved.

[0042] In addition, in the case where the protective layer is formed only of a non-conductive polymer and does not include a metal capable of alloying with lithium, if lithium dendrites are generated, the dendrites may not be removed and a short circuit may not be prevented due to the absence of a metal capable of alloying with the lithium dendrites, which is inappropriate.

[0043] According to an embodiment, there are substantially no pores or empty spaces inside the fiber, and if there are pores or empty spaces, the electrolyte is embedded in the fiber during charge and discharge, and the metal capable of alloying with lithium included in the core may participate in the reaction, which is not appropriate.

[0044] In addition, the polymer included in the shell positioned on the surface is non-conductive and does not participate in the electrochemical reaction, so it can effectively suppress lithium dendrites.

[0045] In addition, if the fibers included in the protective layer are used in the negative electrode active material layer, the energy density may be reduced, and if the fibers are included inside the negative electrode active material layer, the effect of suppressing dendrites formed on the surface of the negative electrode active material layer is insufficient, which is not suitable.

[0046] In an embodiment, the non-conductive polymer may be polyacrylonitrile, nylon, polymethyl methacrylate, polystyrene, styrene acrylonitrile, polycaprolactone, polyacrylic acid, polylactic acid, polyvinylidene fluoride, copolymers thereof, or combinations thereof.

[0047] The metal capable of alloying with lithium may be Si, Sn, Ge or a combination thereof. The metal capable of alloying with lithium may be a nanoparticle, and its particle size may be 10 nm to 500 nm, 10 nm to 300 nm, or 10 nm to 100 nm. If the metal capable of alloying with lithium is in the form of nanoparticles, a core-shell structure may be easily formed and alloying with lithium dendrites may be easily performed.

[0048] In an embodiment, the weight ratio of the shell to the core may be 1:0.1 to 1, 1:0.1 to 0.8, or 1:0.1 to 0.5. If the weight ratio of the shell to the core is within the above range, lithium dendrites may be more effectively removed, and short circuits may also be effectively prevented.

[0049] The average diameter of the fibers may be 100 nm to 5 μm, and may be 100 nm to 3 μm. If the average diameter of the fibers is within this range, a protective layer having an appropriate thickness (eg, a thickness of 10 μm to 20 μm) may be easily formed.

[0050] Diameter refers to the minor axis dimension of the fiber.

[0051] The thickness of the protective layer may be 1 μm to 20 μm, or 10 μm to 20 μm. If the thickness of the protective layer is within the above range, the protective layer can be formed to better suppress the generation of lithium dendrites and prevent short circuits without reducing energy density.

[0052] The protective layer may include fibers. In another embodiment, the protective layer may consist of only fibers. For example, the fibers may be a woven membrane.

[0053] If the protective layer consists of only fibers, the amount of the fibers is the same as that of the protective layer, that is, the amount of the fibers is 100 wt % based on the total 100 wt % of the protective layer.

[0054] In another embodiment, the protective layer may further include a binder and ceramic particles in addition to the fibers, in which case the amount of the fibers may be greater than or equal to 10 wt% and less than 100 wt% based on the total 100 wt% of the protective layer. If the protective layer further includes a binder and ceramic particles, the total amount of the binder and ceramic particles may be an amount other than the amount of the fibers, and the amount of each of the binder and ceramic particles may be appropriately adjusted within the amount range and does not need to be particularly limited.

[0055] The binder may be polyacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose or a combination thereof. The ceramic may be Al2O3, SiO2 or a combination thereof. The binder and the ceramic are not limited thereto.

[0056] The protective layer can be formed by a spinning process (eg, by an electrospinning process). Specifically, it can be formed by a coaxial electrospinning process.

[0057] The electrospinning process is performed using the composition for the core and the composition for the shell, and can be performed using, for example, a coaxial electrospinning machine. The coaxial electrospinning machine can simultaneously eject the core composition and the shell composition respectively by including an inner nozzle and an outer nozzle independent of each other.

[0058] The electrospinning process conditions may include positioning a tip of an inner nozzle and positioning a collector roller at a distance (tip to collector distance, TCD) from the tip of the inner nozzle.

[0059] In addition, the target material is positioned on the bunching roller. At this time, if the target material is a negative electrode on which a negative electrode active material layer is formed, the protective layer can be directly formed on the negative electrode active material layer. In addition, if the target material is a separate material, for example, aluminum foil, the protective layer can be formed separately, and in this case, the manufactured protective layer can be formed by stacking the manufactured protective layer on the negative electrode active material layer.

[0060] Next, the composition for the core and the composition for the shell may be added to the tip, respectively, and a constant voltage may be applied to the tip to perform the process.

[0061] The certain distance between the tip and the bunching roller may be 5 cm to 20 cm, or 10 cm to 17 cm.

[0062] The voltage may be 10 kV to 30 kV, 15 kV to 25 kV, or 15 kV to 20 kV.

[0063] The discharge rate of the composition for the core discharged from the inner nozzle tip can be 0.1mL / h to 2mL / h or 0.1mL / h to 1mL / h. In addition, the discharge rate of the composition for the shell discharged from the outer nozzle tip can be 0.5mL / h to 3mL / h or 0.7mL / h to 2mL / h. However, it is suitable that the discharge rate of the composition for the core is lower than the discharge rate of the composition for the shell, so that the composition for the core can be basically completely surrounded, thereby ensuring that the core finally manufactured is well electrochemically isolated.

[0064] The composition for the core may include a solvent and a metal capable of alloying with lithium. In addition, the composition for the core may further include an additional polymer capable of fixing the metal capable of alloying with lithium while forming the core. The additional polymer may be styrene-co-acrylonitrile (SAN), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), or a combination thereof.

[0065] If the composition for a core further includes an additional polymer, a mixing ratio of the metal capable of alloying with lithium to the additional polymer may be 0.1 to 1:1, 0.1 to 0.8:1, or 0.1 to 0.5:1 by weight.

[0066] The composition for the shell may include a non-conductive polymer and a solvent.

[0067] The solvent used in the composition for the core and the composition for the shell may be the same or different, and may be dimethylformamide, dimethylsulfoxide, dimethylacetamide, N-methylpyrrolidone, or a combination thereof.

[0068] The solid content in the composition for the core or the composition for the shell does not need to be particularly limited as long as it is adjusted to an appropriate value for electrospinning to occur.

[0069] In addition, since the protective layer forming process according to the embodiment does not perform heat treatment after the spinning process, the non-conductive polymer included in the composition for the shell is not carbonized. Therefore, the fibers included in the protective layer exist as the non-conductive polymer itself.

[0070] In an embodiment, the negative electrode active material may be crystalline carbon, lithium metal, or a combination thereof. If a protective layer according to an embodiment is formed on a negative electrode including a negative electrode active material that is crystalline carbon, lithium metal, or a combination thereof, problems due to the occurrence of lithium dendrites can be effectively prevented. If the negative electrode active material is silicon-based, problems due to lithium dendrites rarely occur, so the effect of preventing the generation of lithium dendrites by forming a protective layer according to an embodiment is not significant, and therefore it is not suitable.

[0071] The crystalline carbon may be natural graphite, artificial graphite, or a combination thereof.

[0072] If a mixture of crystalline carbon and lithium is used as the negative electrode active material, the mixing ratio can be appropriately adjusted.

[0073] In the negative electrode active material layer, an amount of the negative electrode active material may be 95 wt % to 99 wt % based on the total 100 wt % of the negative electrode active material layer.

[0074] The negative electrode active material layer may include a binder, and may further include a conductive material.

[0075] The amount of the binder may be 1 wt % to 5 wt % based on the total 100 wt % of the negative electrode active material layer.

[0076] The binder is used to make the negative electrode active material particles adhere to each other, and also helps the negative electrode active material adhere to the current collector. The binder includes a non-aqueous binder, an aqueous binder, or a combination thereof.

[0077] The non-aqueous binder may include ethylene propylene copolymer, polyacrylonitrile, polystyrene, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0078] The aqueous binder may include styrene-butadiene rubber (SBR), acrylated styrene-butadiene rubber (ABR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polypropylene, polyepichlorohydrin, polyphosphazene, ethylene propylene diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, or a combination thereof.

[0079] If the aqueous binder is used as the negative electrode binder, it may further include a cellulose compound capable of imparting viscosity as a thickener. The cellulose compound includes one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose and alkali metal salts thereof. The alkali metal may be Na, K or Li. Based on 100 parts by weight of the negative electrode active material, the amount of the thickener used may be 0.1 parts by weight to 3 parts by weight.

[0080] The amount of the conductive material may be 1 wt % to 5 wt % based on the total 100 wt % of the negative electrode active material layer.

[0081] The conductive material is included to provide electrode conductivity, and any conductive material can be used as the conductive material unless it causes a chemical change. Examples of the conductive material may be: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; metal-based materials in the form of metal powder or metal fiber, such as copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0082] The current collector may include one 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, but is not limited thereto.

[0083] The positive electrode includes a current collector and a positive electrode active material layer on the current collector.

[0084] As the positive electrode active material, a compound capable of inserting and extracting lithium (lithiated inserting compound) can be used. Specifically, at least one composite oxide of lithium and a metal selected from cobalt, manganese, nickel, and a combination thereof can be used. As a more specific example, a compound represented by any one of the following chemical formulas can be used: Li a A 1-b X b D2 (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 (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a E 2-b X b O 4-c D c(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a Ni 1-b-c 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-b-c Co b X c O 2-α T α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni 1-b-c Co b X c O 2-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni 1-b-c Mr 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-b-c Mr b X c O 2-α T α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni 1-b-c Mr b X c O 2-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni b HAVE BEEN c G d O2(0.90≤a≤1.8,0≤b≤0.9,0≤c≤0.5,0.001≤d≤0.1);Li a Ni b Co c Mr d G e O2(0.90≤a≤1.8,0≤b≤0.9,0≤c≤0.5,0≤d≤0.5,0≤e≤0.1);Li a Ni b Co c Al d Ge O2 (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0≤e≤0.1); Li a Ni b Co c Mn d G e O2 (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li a NeG b O2 (0.90≤a≤1.8, 0.001≤b≤0.1); Li a CoG b O2 (0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-b G b O2 (0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn b O4 (0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-g G g PO4 (0.90≤a≤1.8, 0≤g≤0.5); QO2; QS2; LiQS2; V2O5; LiV2O5; LiZO2; LiNiVO4; Li (3-f) J2(PO4)3(0≤f≤2);Li (3-f) Fe2(PO4)3(0≤f≤2);Li a FePO4 (0.90≤a≤1.8).

[0085] In the above chemical formula, A is selected from Ni, Co, Mn and combinations thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from O, F, S, P and combinations thereof; E is selected from Co, Mn and combinations thereof; T is selected from F, S, P and combinations thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V and combinations thereof; Q is selected from Ti, Mo, Mn and combinations thereof; Z is selected from Cr, V, Fe, Sc, Y and combinations thereof; and J is selected from V, Cr, Mn, Co, Ni, Cu and combinations thereof.

[0086] In addition, the compound may have a coating layer on the surface, or may be mixed with another compound having a coating layer. The coating layer may include at least one coating element compound selected from the oxide of the coating element, the hydroxide of the coating element, the hydroxyl oxide of the coating element, the oxygen-containing carbonate of the coating element, and the hydroxyl carbonate of the coating element. The compound for the coating layer may be amorphous or crystalline. The coating element included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. By using these elements in the compound, the coating layer can be set in a method that has no adverse effect on the properties of the positive electrode active material, and for example, the method may include any coating method such as spraying, dipping, etc., but since it is well known in the relevant art, it is not described in more detail.

[0087] In the positive electrode, an amount of the positive electrode active material may be 90 wt % to 98 wt % based on 100 wt % of the total weight of the positive electrode active material layer.

[0088] In an embodiment, the positive electrode active material layer may further include a binder and a conductive material. Here, the amount of the binder and the conductive agent may be 1 wt % to 5 wt % based on the total weight 100 wt % of the positive electrode active material layer.

[0089] The binder is used to adhere the positive electrode active material particles to each other and also to adhere the positive electrode active material to the current collector. Representative examples of the binder may include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc., but are not limited thereto.

[0090] The conductive material is included to provide electrode conductivity, and any conductive material can be used as the conductive material unless it causes a chemical change. Examples of the conductive material may be: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; metal-based materials in the form of metal powder or metal fiber, such as copper, nickel, aluminum, silver, etc.; and conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0091] The current collector may include Al, but is not limited thereto.

[0092] The electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0093] The non-aqueous organic solvent serves as a medium for transporting ions participating in the electrochemical reaction of the battery.

[0094] The non-aqueous organic solvent may include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, or an aprotic solvent.

[0095] Carbonate solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Ester solvents may include methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, decanoic acid lactone, mevalonate lactone, caprolactone, etc. Ether solvents may include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. In addition, ketone solvents may include cyclohexanone, etc. Alcohol solvents may include ethanol, isopropanol, etc., and aprotic solvents may include: nitriles, such as R-CN (wherein R is a C2 to C20 linear, branched or cyclic hydrocarbon, and may include a double bond, an aromatic ring or an ether bond); amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane; cyclopentane; etc.

[0096] The organic solvent may be used alone or in combination of one or more, and if one or more are used in combination, the mixing ratio may be appropriately adjusted according to desired battery performance, as is widely understood by workers in the relevant art.

[0097] In addition, the carbonate solvent may use a mixture of cyclic carbonate and chain carbonate. In this case, the cyclic carbonate and chain carbonate may be mixed in a volume ratio of 1:1 to 1:9, which improves the performance of the electrolyte.

[0098] In addition to the carbonate-based solvent, the organic solvent may further include an aromatic hydrocarbon-based organic solvent. The carbonate-based solvent and the aromatic hydrocarbon-based solvent may be mixed together in a volume ratio of 1:1 to 30:1.

[0099] The aromatic hydrocarbon or organic solvent may be an aromatic hydrocarbon compound represented by Chemical Formula 1.

[0100] [Chemical formula 1]

[0101] (In Chemical Formula 1, R1 to R6 are the same or different and are selected from hydrogen, halogen, C1 to C10 alkyl, halogenated alkyl, and combinations thereof).

[0102] Specific examples of aromatic hydrocarbon organic solvents can be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 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-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluoroform, Benzene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 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-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene and combinations thereof.

[0103] The electrolyte may further include ethyl vinyl carbonate, vinylene carbonate or an ethylene carbonate-based compound of Chemical Formula 2 as an additive to improve the battery cycle life.

[0104] [Chemical formula 2]

[0105] (In Chemical Formula 2, R7 and R8 are the same or different and may both independently be hydrogen, halogen, cyano (CN), nitro (NO2) or C1 to C5 fluoroalkyl, provided that at least one of R7 and R8 is halogen, cyano (CN), nitro (NO2) or C1 to C5 fluoroalkyl, and R7 and R8 are not hydrogen at the same time).

[0106] Examples of ethylene carbonate compounds include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The amount of the additive for improving the cycle life can be appropriately used within an appropriate range.

[0107] The lithium salt dissolved in the organic solvent supplies lithium ions to the battery, basically operates the rechargeable lithium battery, and improves the transport of lithium ions between the positive electrode and the negative electrode. Examples of lithium salts include those selected from LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide: LiFSI), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiN(C x F 2x+1 SO2)(Cy F 2y+1 SO2) (wherein x and y are natural numbers, for example, integers of 1 to 20), lithium difluorobis(oxalato)phosphate (lithium difluorobis(oxalato)phosphate), at least one supporting salt selected from the group consisting of LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalato)borate: LiBOB), and lithium difluorobis(oxalato)borate (LiDFBOB). The concentration of the lithium salt may be in the range of 0.1 M to 2.0 M. If the lithium salt in the above concentration range is included, the electrolyte may have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

[0108] In an embodiment, the protective layer included in the negative electrode may serve as a separator. Therefore, the rechargeable lithium battery according to the embodiment may not include a separate separator. Of course, a common separator may also be included.

[0109] Such suitable membrane materials may include polyethylene, polypropylene, polyvinylidene fluoride and multilayers thereof, such as a polyethylene / polypropylene double-layer membrane, a polyethylene / polypropylene / polyethylene triple-layer membrane and a polypropylene / polyethylene / polypropylene triple-layer membrane.

[0110] Figure 3 An exploded perspective view of a rechargeable lithium battery according to an embodiment of the present invention is shown. The rechargeable lithium battery according to the embodiment is described as a prismatic type, but the present invention is not limited thereto and may be applied to various types of batteries such as cylindrical and pouch types.

[0111] Reference Figure 3 The rechargeable lithium battery 100 according to the embodiment may include an electrode assembly 40 and a case 50 accommodating the electrode assembly 40 therein, the electrode assembly 40 being wound with the separator 30 interposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown).

[0112] Mode of carrying out the invention Hereinafter, examples and comparative examples of the present invention are described. However, these examples shall not be interpreted as limiting the scope of the invention in any sense.

[0113] (Example 1: Method of directly forming a protective layer on the negative electrode active material layer) 3 g of styrene-co-acrylonitrile, 0.5 g of silicon (average particle size: 100 nm or less), and 7 g of a dimethylformamide solvent were mixed to prepare a composition for a core.

[0114] 1 g of polyacrylonitrile and 9 g of dimethylformamide solvent were mixed to prepare a composition for a shell.

[0115] A natural graphite negative electrode active material, a Ketjen black conductive material, a styrene butadiene rubber binder, and a carboxymethyl cellulose thickener in a weight ratio of 96:1:2:1 were mixed in an aqueous solvent to prepare a negative electrode active material slurry (capacity: 6 mAh / cm 2 ).

[0116] The negative electrode active material slurry was coated on a Cu foil current collector and then dried and pressed to form a negative electrode active material layer on the current collector.

[0117] On the negative electrode active material layer, the composition for the core and the composition for the shell were electrospun to form a 20 μm thick protective layer to manufacture the negative electrode. The formed protective layer was composed of fibers, the fibers including a core of silicon and styrene-co-acrylonitrile and a shell of polyacrylonitrile surrounding the core, and the fibers had an average diameter of 2.24 μm.

[0118] Here, the electrospinning process was performed by using a coaxial electrospinning machine having an inner nozzle with an inner nozzle tip and an outer nozzle with an outer nozzle tip.

[0119] The inner nozzle tip and the outer nozzle tip (i.e., the nozzle tip) are placed at a distance of 15 cm from the clustering roller. Subsequently, the composition for the core is added to the inner nozzle tip, and the composition for the shell is added to the outer nozzle tip at the same time. The current collector is placed on the clustering roller so that the negative electrode active material layer is exposed. A voltage of 18 kV is applied to the inner nozzle tip and the outer nozzle tip for coaxial electrospinning. The composition for the core discharged from the inner nozzle tip is discharged from the inner nozzle tip at a speed of 0.5 mL / h, and the composition for the shell is discharged from the outer nozzle tip at a speed of 1.0 mL / h.

[0120] 94wt% LiNi 0.8 Co 0.1 Al 0.1 The O2 positive electrode active material, 3 wt % of Ketjen black conductive material and 3 wt % of polyvinylidene fluoride binder were mixed in N-methylpyrrolidone solvent to prepare a positive electrode active material slurry.

[0121] The positive electrode active material slurry was coated on an Al foil current collector, then dried and pressed to produce a 5 mAh / cm 2 positive electrode.

[0122] A negative electrode, a separator (Celgard 2400), and a positive electrode were sequentially arranged to manufacture an electrode assembly. Here, a protective layer was arranged to contact the separator. A rechargeable lithium battery cell (full cell) was manufactured using the electrode assembly and an electrolyte. The electrolyte was prepared by mixing ethylene carbonate and diethyl carbonate (in a volume ratio of 1:1) and dissolving 1M LiPF6 therein.

[0123] (Example 2: Method of forming a protective layer alone) 3 g of styrene-co-acrylonitrile, 0.5 g of silicon (average particle size: 100 nm or less), and 7 g of a dimethylformamide solvent were mixed to prepare a composition for a core.

[0124] 1 g of polyacrylonitrile and 9 g of dimethylformamide solvent were mixed to prepare a composition for a shell.

[0125] The inner nozzle tip and the outer nozzle tip (i.e., the nozzle tip) were set to have a distance of 15 cm from the clustering roller. Subsequently, the composition for the core was added to the inner nozzle tip, and the composition for the shell was added to the outer nozzle tip at the same time. Aluminum foil as a target substrate was placed on the clustering roller. A voltage of 18 kV was applied to the inner nozzle tip and the outer nozzle tip for coaxial electrospinning. The composition for the core was discharged from the inner nozzle tip at a speed of 0.5 mL / h, and the composition for the shell was discharged from the outer nozzle tip at a speed of 1.0 mL / h.

[0126] By means of an electrospinning process, a protective layer with a thickness of 20 μm was formed. The protective layer formed consisted of fibers including a core of silicon and styrene-co-acrylonitrile and a shell of polyacrylonitrile surrounding the core, wherein the fibers had an average diameter of 2.24 μm.

[0127] A protective layer was provided on the negative electrode active material layer of Example 1 to produce a negative electrode.

[0128] A rechargeable lithium battery cell was manufactured in the same manner as in Example 1 except that this negative electrode was used.

[0129] (Comparative Example 1) A natural graphite negative electrode active material, a Ketjen black conductive material, a styrene butadiene rubber binder, and a carboxymethyl cellulose thickener were mixed in a water solvent at a weight ratio of 96:1:2:1 to prepare a negative electrode active material slurry.

[0130] The negative electrode active material slurry was coated on a Cu foil current collector, then dried and pressed to produce a 6 mAh / cm 2 of the negative electrode.

[0131] 94wt% LiNi 0.8 Co0.1 Al 0.1 The O2 positive electrode active material, 3 wt % of Ketjen black conductive material and 3 wt % of polyvinylidene fluoride binder were mixed in N-methylpyrrolidone solvent to prepare a positive electrode active material slurry.

[0132] The positive electrode active material slurry was coated on an Al foil current collector, then dried and pressed to produce a 5 mAh / cm 2 positive electrode.

[0133] The negative electrode, separator (Celgard 2400) and positive electrode were stacked to manufacture an electrode assembly, and the electrode assembly was used with an electrolyte to manufacture a rechargeable lithium battery cell. The electrolyte was prepared by mixing ethylene carbonate and diethyl carbonate (in a volume ratio of 1:1) and dissolving 1M LiPF6 therein.

[0134] (Comparative Example 2) The electrospinning process was performed by using a single-axis electrospinning machine having one nozzle as follows.

[0135] The nozzle having the nozzle tip was set at a distance of 15 cm from the focusing roller. Subsequently, the composition for shells according to Example 1 was added to the nozzle tip.

[0136] The current collector having the negative electrode active material layer formed according to Example 1 as the target substrate was placed on a bundle roller. A voltage of 18 kV was applied to the nozzle tip for uniaxial electrospinning. The composition for shell was discharged from the nozzle tip at a rate of 1.0 mL / h.

[0137] Through the electrostatic spinning process, a 20 μm thick protective layer was formed. The fibers included in the protective layer were composed of polyacrylonitrile and had an average diameter of 2.24 μm.

[0138] A rechargeable lithium battery cell was manufactured in the same manner as in Example 1 using the protective layer.

[0139] Experimental Example 1) SEM Photo Will Figure 4a A 1000x SEM image of the protective layer of Example 2 is shown in FIG. Figure 4b A 3000-fold SEM image of the protective layer of Example 2 is shown in FIG.

[0140] like Figure 4a and Figure 4b As shown in , the fibers forming the protective layer are in the form of beads and have a structure in which Si nanoparticle cores are well surrounded by polyacrylonitrile shells.

[0141] Experimental Example 2) Charge / discharge characteristics The rechargeable lithium battery cells of Example 1 and Comparative Example 1 were charged and discharged once at 0.1C, and charged and discharged ten times at 0.3C.

[0142] Figure 5 shows the charge / discharge characteristics of the rechargeable lithium battery cell of Example 1, Figure 6 The charge / discharge characteristics of the rechargeable lithium battery cell of Comparative Example 1 are shown.

[0143] like Figure 5 As shown in , the rechargeable lithium battery cell of Example 1 including the protective layer including the fiber having the Si core and the polyacrylonitrile shell is stable at the initial cycle and exhibits excellent charge / discharge characteristics.

[0144] On the other hand, Figure 6 As shown in , the rechargeable lithium battery cell of Comparative Example 1 without a protective layer exhibits a sharp increase in the initial potential of the first cycle charge to 4V or more, and the charge / discharge characteristics are slightly deteriorated. From this result, it can be seen that the rechargeable lithium battery cell of Comparative Example 1 without a protective layer exhibits an increased potential difference with the positive electrode due to the formation of dendrites on the surface of the negative electrode.

[0145] In addition, the rechargeable lithium battery cells of Example 2 and Comparative Examples 1 and 2 were charged and discharged once at 0.1C, and charged and discharged 20 times at 0.3C.

[0146] Figure 7 shows the charge / discharge characteristics of the rechargeable lithium battery cell of Example 2, Figure 8 The charge / discharge characteristics of the rechargeable lithium battery cell of Comparative Example 1 are shown, and Fig. 9 The charge / discharge characteristics of the rechargeable lithium battery cell of Comparative Example 2 are shown.

[0147] like Figure 7 As shown in , the rechargeable lithium battery cell of Example 2 including the protective layer including the fiber having the Si core and the polyacrylonitrile shell was stable at the initial cycle and exhibited excellent charge / discharge characteristics without short circuit during 20 cycles of charge / discharge.

[0148] On the other hand, Figure 8 As shown in , the rechargeable lithium battery cell of Comparative Example 1 without a protective layer exhibited insufficient initial charge / discharge characteristics and short circuited during 20 cycles of charge and discharge. Fig. 9 As shown in , the rechargeable lithium battery cell of Comparative Example 2 having a protective layer including fibers composed only of polyacrylonitrile without containing silicon also exhibited insufficient initial charge / discharge characteristics.

[0149] While the present invention has been described in conjunction with what are presently considered to be practical example embodiments, it will be understood that the invention is not limited to the disclosed embodiments, but rather is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A negative electrode for a rechargeable lithium battery, the negative electrode comprising: current collector; Protective layer; as well as a negative electrode active material layer, disposed between the current collector and the protective layer, and comprising a negative electrode active material, The protective layer includes fibers, the fibers include a core and a shell, the core includes a metal capable of alloying with lithium, and the shell surrounds the core and includes a non-conductive polymer.

2. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The metal capable of alloying with lithium is Si, Sn, Ge or a combination thereof.

3. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The non-conductive polymer is polyacrylonitrile, nylon, polymethyl methacrylate, polystyrene, styrene acrylonitrile, polycaprolactone, polyacrylic acid, polylactic acid, polyvinylidene fluoride, copolymers thereof or combinations thereof.

4. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The protective layer has a thickness of 1 μm to 20 μm.

5. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The weight ratio of the shell to the core is 1:0.1 to 1.

6. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The negative electrode active material is crystalline carbon, lithium metal or a combination thereof.

7. The negative electrode for a rechargeable lithium battery according to claim 1, wherein The core also includes an additional polymer capable of fixing the metal capable of alloying with lithium.

8. The negative electrode for a rechargeable lithium battery according to claim 7, wherein The additional polymer is styrene-co-acrylonitrile (SAN), polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE) or a combination thereof.

9. The negative electrode for a rechargeable lithium battery according to claim 7, wherein: The mixing ratio of the metal capable of alloying with lithium and the additional polymer is 0.1 to 1:1 by weight.

10. A rechargeable lithium battery, the rechargeable lithium battery include ： The negative electrode according to any one of claims 1 to 9; a positive electrode; and Electrolytes.

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