KR20100075086A - Li-ion battery comprising a separator characterized by high elongation - Google Patents

Abstract

PURPOSE: A lithium secondary battery is provided to prevent the direct contact of a positive electrode and a negative electrode collector using a separator with the high elongation property during an internal short. CONSTITUTION: A lithium secondary battery includes an electrode with a current collector, and a separator. The ratio between the elongation rate of the current collector and the elongation rate of the separator is 1:10~40. The separator is a polyethylene-based polymer, with the puncture strength of 400~800gf, the melting point of 80~100deg C, and the density over 0.92. The weight average molecular weight of the separator is 1,000~20,000.

Description

Lithium secondary battery including a separator having high elongation characteristics {LI-ION BATTERY COMPRISING A SEPARATOR CHARACTERIZED BY HIGH ELONGATION}

The present invention relates to a lithium secondary battery including a separator having a higher elongation property than a current collector.

As the energy of a lithium secondary battery increases, the safety of the battery is threatened. In the 800mAh commercial battery, the stored electric energy is four times the energy of the battery's chemical reaction, and the battery energy increases in proportion to the energy density of the battery. Normally, the electrical energy stored in the positive and negative electrodes is separated and maintained safely by the separator, but short circuits between the positive and negative electrodes are caused by various causes, causing the stored electric energy to be released in a short time, thereby generating heat / ignition or thermal runaway. It causes a phenomenon. That is, local short-circuits due to dendrite formation and the like, resulting in temperature increase, lead to thermal runaway as the cathode and anode are pyrolyzed, eventually causing the battery to ignite and burst.

In order to solve the problem of ignition of the battery and to improve the safety of the lithium secondary battery, a method of using a separator which does not occur heat shrinkage or a separator having a very high tensile strength, or a method of suppressing current during short circuit by coating an electrode surface with an inorganic filler, etc. Various methods are being tried.

As a prior art for improving the safety of a lithium secondary battery, the thing of the following documents 1-3 can be illustrated.

Document 1 discloses a medium-to-large battery having a battery capacity of 1.8 AH or more, in which a conductive material having a specific particle diameter is contained in a positive electrode at a specific content, and the amount of heat generated during internal short-circuit of the battery caused when the battery is exposed to external impact or high temperature. The present invention relates to a lithium secondary battery having improved safety by suppressing an increase in the temperature and a rise in temperature and significantly lowering the risk of ignition and explosion of the battery.

Document 2 relates to an electrode assembly comprising a positive electrode containing a positive electrode active material and a binder (binder), a negative electrode containing a negative electrode active material and a binder (binder), and a separator, and a lithium secondary battery containing a lithium electrolyte solution. , An additive for improving the safety of overcharging, which is decomposed under overcharging conditions of 4.5 V or more relative to the lithium redox potential and contains a urethane group in the molecular structure (“urethane compound”) and / or a polymer containing a urethane group (“poly Disclosed is a lithium secondary battery comprising urethane ") in an electrode and / or an electrolyte solution.

Document 3 is an electrically insulating first separator layer made of a polyolefin-based polymer; An electrically insulating second separator layer made of a polyolefin-based polymer; And a third separator layer interposed between the first and second separator layers and made of an electrically conductive polymer, wherein the lithium battery has a predetermined voltage or more when overcharged due to various causes such as a malfunction of a rechargeable battery. Disclosed is a lithium battery that improves improved battery safety by shutting down current.

Document 1 KR 10-2005-0033429 (application number). Apr 22, 2005

Document 2 KR 10-2006-0034358 (application number). Apr 17, 2006

Document 3 KR 10-2001-0088846 (application number). Dec. 31, 2001

As described above, many technologies are being developed to improve the safety of lithium secondary batteries, but materials with enhanced safety typically exhibit poor performance compared to conventional materials, and yet no visible commercial performance has been shown. to be.

In particular, if the tensile strength of the battery separator is very strong, the safety of the dendrite or nail penetration can be improved, but the air permeability is low (ie, the air permeability is high), and thus the ion conductivity is deteriorated, resulting in overall battery performance. There is a problem that adversely affects.

The problem to be solved by the present invention is to solve the above problems of the prior art, to provide a lithium secondary battery with improved safety without deterioration of battery performance by using a separator having a high elongation.

When the nail penetrates the cell, the drawing of the positive and negative current collectors (usually metal foils) and the separator inside the cell simultaneously occurs in the penetrating direction. In this case, when the degree of extension of the separator is not sufficient compared to the degree of extension of the current collector, the positive electrode and the negative electrode current collector may be in direct contact. It can seriously jeopardize safety. Therefore, in the present invention, using a separation membrane somewhat higher than the current collector, when the nail penetrates the separator located between the current collector of the positive electrode and the negative electrode is sufficiently drawn with the current collector to block direct contact between the current collector and internal short circuit By minimizing the amount of current to improve the safety of the lithium secondary battery.

The present invention has been made to solve the above problems of the prior art,

A lithium secondary battery comprising an electrode having a current collector and a separator, wherein the ratio of elongation (Y) of the current collector to elongation (X) of the separator is within a range of 1:10 to 40. To provide.

In addition, in the present invention, the separator provides a lithium secondary battery, characterized in that the polyethylene-based polymer.

In addition, the present invention provides a lithium secondary battery, characterized in that the puncture strength of the separator is 400 ~ 800 gf.

In the present invention, the melting point of the separator is 80 ~ 100 ℃, the density (based on 23 ℃) provides a lithium secondary battery characterized in that the 0.92 or more.

In addition, in the present invention, the weight average molecular weight of the separator provides a lithium secondary battery, characterized in that within the range of 1,000 ~ 20,000.

The present invention has an effect of improving the safety of the battery by preventing a direct contact of the positive electrode and the negative electrode current collector during the internal short using a separator having a higher elongation than the current collector.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention provides a lithium secondary battery including an electrode having a current collector and a separator, wherein the ratio of elongation (Y) of the current collector to elongation (X) of the separator is in the range of 1:10 to 40. .

The electrode is composed of an anode and a cathode.

The positive electrode may be prepared by applying a mixture of a positive electrode active material, a conductive material, and a binder on a positive electrode current collector, followed by drying. If necessary, a filler may be further added to the mixture. The positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O _7, and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x MxO ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3; Formula LiMn _2-x M _x O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu, or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ , and the like. However, the present invention is not limited to these.

The positive electrode current collector is generally made to a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The binder is a component that assists in bonding the active material and the conductive material to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the positive electrode mixture. The high molecular weight polyacrylonitrile-acrylic acid copolymer may be used as such a binder, but is not limited thereto. Other examples include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The conductive material is conductive without causing chemical change in the battery, and may be added in an amount of 1 wt% to 50 wt% based on the total weight of the negative electrode material composition. For example, graphite, such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive polymers such as polyphenylene derivatives and the like can be used.

The positive electrode may optionally include a filler as a component for inhibiting expansion. The filler is not particularly limited as long as it is a fibrous material that does not cause chemical changes in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode may be obtained by applying a mixture containing an active material, a binder, and a conductive material to a current collector, and then drying the solvent.

As said negative electrode active material, Carbon and graphite materials, such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti, which can be alloyed with lithium, and compounds containing these elements; Complex compounds of metals and their compounds with carbon and graphite materials; Lithium-containing nitrides, and the like.

The negative electrode material composition of the present invention may further include a conductive material and / or a filler as a component for improving the conductivity of the negative electrode active material. The conductive material and the filler are the same as described above with respect to the positive electrode.

The negative electrode current collector is made to a thickness of 3 to 500㎛. The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, a surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper, or stainless steel may be used. Surface-treated with carbon, nickel, titanium, silver and the like, aluminum-cadmium alloy and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding force of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The elongation is defined as in Equation 1 below.

The elongation of the separator is defined as X, and the elongation of the current collector is defined as Y.

When the thickness of the current collector of the positive electrode and the negative electrode or the type of material is different, the elongation of each current collector may be different. In this case, the elongation of the current collector having a better elongation is set to the elongation Y of the current collector.

The elongation is a concept including both the longitudinal elongation and the transverse elongation. The elongation may be different from the longitudinal elongation and the transverse elongation. In this case, the elongation is defined as the arithmetic average of the longitudinal elongation and the transverse elongation. For example, when the longitudinal elongation of the separator is 100% and the transverse elongation is 200%, the elongation (X) of the separator is 150%.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

Nail puncture strength of the separator is preferably within the range of 400 ~ 800 gf. Below 400 gf, it is not preferable to penetrate the membrane before the membrane is stretched during nail penetration or dendrite growth, and exceeding 800 gf is poor in breathability (ie, high in breathability) and may deteriorate the properties of the battery. In general, as strength increases, the degree of stretching decreases, which may be undesirable (ie, strength and elongation are usually inversely related).

Melting point of the separator is 80 ~ 100 ℃, the density (23 ℃ standard) is preferably 0.92 or more. When the membrane melts within the above temperature range at the time of the local temperature rise due to nail penetration or dendrite, the ion channel between the anode and the cathode can be blocked to prevent the exothermic reaction in advance (shutdown effect). If the density is 0.92 or less, the ion blocking effect may not be sufficient even when the separator is melted. In addition, the local temperature rise usually causes a slight heat shrink around the temperature rise region. In this case, when the density is less than 0.92, the separator shrinks and may cause cracks in other portions, which is not preferable in terms of safety.

The separator has a weight average molecular weight of preferably in the range of 1,000 to 20,000. Outside of the molecular weight range, it may be difficult to secure proper tensile strength and elongation.

In the present invention, the ratio of the elongation (Y) of the current collector to the elongation (X) of the separator is in the range of 1:10 to 40.

When the nail penetrates the cell, the drawing of the positive and negative current collectors (usually metal foils) and the separator inside the cell simultaneously occurs in the penetrating direction. In this case, when the degree of extension of the separator is not sufficient compared to the degree of extension of the current collector, the positive electrode and the negative electrode current collector may be in direct contact. When the positive electrode and negative electrode current collector are in direct contact with each other, a large current may be generated rapidly, thus the safety of the battery. Can greatly threaten.

Therefore, in the present invention, by using a sufficiently large ratio of the elongation (Y) of the current collector to the elongation (X) of the separator, the battery is improved in safety. That is, when the nail penetrates, the separator positioned between the current collectors of the positive electrode and the negative electrode is sufficiently drawn together with the current collector to block direct contact between the positive electrode and the negative electrode and minimize the amount of internal short circuit current. Elongation (X) of the separation membrane is preferably at least 10 times or more than the elongation (Y) of the current collector in terms of safety. If the elongation (X) of the separator is less than 10 times, the effect of blocking direct contact between the electrodes may be insufficient, which is not preferable.

In addition, when the elongation rate (X) of the separator is more than 40 times the elongation rate (Y) of the current collector, the increase in elongation rate has a slight effect on improving the safety of the battery in terms of securing the safety of the battery. Other physical properties such as the strength of the separator may not be improved, so it may be rather undesirable in terms of safety.

The remaining components of the lithium secondary battery according to the present invention will be described. The lithium secondary battery usually further includes a lithium salt-containing nonaqueous electrolyte in addition to the positive electrode, the negative electrode, and the separator.

The said lithium salt containing non-aqueous electrolyte consists of a nonaqueous electrolyte and lithium. As the nonaqueous electrolyte, a nonaqueous electrolyte, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1, 2- dimeth, for example Methoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxoron, acetonitrile, nitromethane, formic acid Methyl, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers Aprotic organic solvents such as methyl propionate and ethyl propionate can be used.

Examples of the organic solid electrolyte include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, for the purpose of improving the charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate Triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride It may also be added. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

The present invention will be described in more detail with reference to the following examples. The embodiments herein are for the purpose of describing the invention only, and are not intended to limit the scope of the rights.

Example  One

A cylindrical lithium secondary battery of Jelly Roll was manufactured in which the positive electrode current collector was aluminum foil (14% elongation), the negative electrode current collector was copper foil (10% elongation), and the separator was an ethylene-based separator (200% elongation).

Example  2

A cylindrical lithium secondary battery of Jelly Roll was manufactured under the same conditions except that the ethylene-based separator having an elongation of 150% in the separator was used in Example 1.

Example  3

A cylindrical lithium secondary battery of Jelly Roll was manufactured under the same conditions except that the ethylene-based separator having an elongation of 300% in the separator was used in Example 1.

Comparative example

A cylindrical lithium secondary battery of Jelly Roll was manufactured under the same conditions except that the ethylene-based separator having a 50% elongation of the separator in Example 1 was used.

Experimental Example  -[Nail Penetration Experiment]

The batteries of Examples and Comparative Examples prepared above were fully charged under a voltage of 4.2V and penetrated through the center of the battery using a 2.5 mm diameter nail to observe whether they ignited. At this time, the nail penetration speed was varied from 5 to 12 m / min.

The results were as shown in Table 1 below.

Battery voltage: 4.2V Nail Penetration Speed (m / min) Short circuit (number of shorted cells / total number of cells tested) Example 1


12 0/10 9 0/10 7 0/10 5 0/10 Example 2


12 0/10 9 0/10 7 0/10 5 0/10 Example 3


12 0/10 9 0/10 7 0/10 5 0/10 Comparative example


12 0/10 9 7/10 7 10/10 5 10/10

The batteries of Examples and Comparative Examples prepared above were fully charged under a voltage of 4.25V and penetrated through the center of the battery using a 2.5 mm diameter nail to observe whether they ignited. At this time, the nail penetration speed was performed at 9 m / min.

The results were as shown in Table 2 below.

Battery voltage: 4.25 V Nail Penetration Speed (m / min) Short circuit (number of shorted cells / total number of cells tested) Example 1 9 0/10 Comparative example 9 10/10

As shown in the nail penetration test, it can be seen that the battery having a separator having a higher elongation than the current collector of the present invention has high safety against short circuits.

Claims (5)

A lithium secondary battery comprising an electrode having a current collector and a separator, wherein the ratio of elongation (Y) of the current collector to elongation (X) of the separator is within a range of 1:10 to 40. . According to claim 1, wherein the separator is a lithium secondary battery, characterized in that the polyethylene-based polymer. The lithium secondary battery according to claim 1, wherein the puncture strength of the separator is 400 to 800 gf. The lithium secondary battery of claim 1, wherein the separator has a melting point of 80 to 100 ° C., and a density (based on 23 ° C.) of 0.92 or more. The lithium secondary battery according to claim 1, wherein the weight average molecular weight of the separator is in the range of 1,000 to 20,000.