KR20080106881A - Separator and electrochemical device prepared thereby - Google Patents

Abstract

A separation film including a polymer is provided to increase the resistance of battery and to reduce the current of shorted circuit, thereby improving the safety of battery. A separation film comprises (a) inorganic particles; and (b) a polymer binder layer formed in the part or the whole of the surface of the inorganic particles. The separation film is formed with pores due to the interstitial volume between the inorganic particles adhered by a polymer binder layer. The polymer having a melting temperature(Tm) of 90 - 130‹C is one selected from the group consisting of a low density polyethylene(LDPE), linear low density polyethylene(LLDPE), HDPE, chlorinated polyethylene, polymethacrylate(PMMA), polyethylene - polymethyl metacrylate copolymers(PE-co-PMMA), polyethylene-polyethylene oxide copolymers(PE-co-PEO) and polyethylene - polyethylene glycol copolymer(PE-co-PEG).

Description

Separators and Electrochemical Devices Using the Same {SEPARATOR AND ELECTROCHEMICAL DEVICE PREPARED THEREBY}

The present invention relates to separators with improved safety, in particular thermal safety, and electrochemical devices comprising the same.

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders, notebooks and PCs, and even electric vehicles, efforts for research and development of batteries are becoming more concrete. The electrochemical device is the most attracting field in this respect, and among them, the development of a secondary battery capable of charging and discharging has been the focus of attention. Recently, in developing such a battery, research and development on the design of a new battery have been conducted to improve capacity density and specific energy.

Among the secondary batteries currently applied, lithium secondary ion batteries developed in the early 1990s have higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use an aqueous electrolyte solution. It is attracting attention as an advantage. However, such a lithium secondary ion battery has safety problems such as ignition and explosion caused by using an organic electrolyte, and has a disadvantage in that manufacturing is difficult. These batteries are produced by many companies, but their safety characteristics are different. The safety evaluation and safety of such a battery is very important, the most important consideration is that the battery should not injure the user in case of malfunction. According to the above purpose, the safety standard strictly regulates the ignition and the smoke in the battery, among which overcharge is the most urgent problem to be solved.

In the case of overcharging, the lithium secondary battery does not have a plurality of negative electrode sites to which excess lithium ions desorbed from the positive electrode are inserted, thereby causing lithium dendrite growth that causes explosion or fire on the surface of the negative electrode. In particular, during high temperature overcharge, the electrolyte starts to decompose, and when the flash point is reached at a high temperature, the possibility of ignition increases. However, since the battery itself cannot prevent such heat generation, a method of attaching a protection circuit and the like has been proposed in the current state. However, the use of such a protection circuit greatly limits the miniaturization and cost of the battery pack.

In order to solve this problem, many solutions have been proposed. Recently, solutions through organic electrolyte additives have been proposed. However, the safety mechanism of electrolyte additives causes Joule heating to vary depending on the current value of charging or the internal resistance of the battery. The timing was uneven. In addition, when using a device that cuts off the current using the breakdown voltage, the device requires space inside the battery, which hinders the increase in capacity, and always involves a decrease in other battery performance.

The present inventors use a polymer having a melting temperature (Tm) in the range of 90 to 130 ℃ as a component of the separator in which a polymer binder layer is formed on part or all of the inorganic particle surface in order to solve the problems of the prior art mentioned above. When the temperature rises inside the battery due to overcharge or malfunction of the battery, the battery may be shut down due to the pore shut down of the separator due to the low melting temperature of the polymer, thereby increasing the resistance of the battery, and reducing the short circuit current. It has been found that the safety is improved.

Accordingly, an object of the present invention is to provide a separator comprising a polymer having a melting temperature in the above temperature range and an electrochemical device having the separator.

The present invention (a) inorganic particles; And (b) a polymer binder layer formed on part or all of the surface of the inorganic particles, wherein the pores are formed due to an interstitial volume between the inorganic particles attached to each other by the polymer binder layer, wherein the polymer Provides a separator and an electrochemical device comprising the separator, characterized in that it has a melting temperature (Tm) in the range of 90 to 130 ℃.

In the separator prepared according to the present invention, when the temperature of the battery is abnormally increased due to overcharge or internal and external factors of the battery, the pores of the separator are shut down due to the low melting temperature of the polymer dispersed in the separator, It is possible to improve the thermal safety.

Hereinafter, the present invention will be described in detail.

The present invention is a component of a heat-resistant ceramic separator in which a polymer binder layer is formed on part or all of an inorganic particle surface, and has a lower melting temperature (Tm = 90-130 ° C.) than a polymer (Tm = 130-200 ° C.) conventionally used as a separator substrate. It is characterized by using a polymer having a) as a binder of the inorganic particles.

Due to the physical properties of the polymer as described above, the separator of the present invention can improve the safety. That is, conventional separators for lithium secondary batteries use a polyolefin-based polymer that is melted at a temperature in the range of 130 to 200 ° C., thereby causing thermal contraction of the separator due to abnormal heat generated by overcharging or external and internal stimuli, thereby igniting the battery and The explosion and the like were directly connected to each other, causing a sudden drop in safety of the battery.

In contrast, the separator of the present invention, by using a polymer having a lower melting temperature than the conventional polymer used in the art as a binder formed on the surface of the inorganic particles, overcharge; Alternatively, when the temperature of the battery rises in the range of 90 to 130 ° C. due to external and internal stimulation, the polymer constituting the separator is preferentially melted, thereby blocking the pore structure between the inorganic particles connected to the polymer, thereby preventing battery resistance. It causes an increase and a decrease in current. In particular, in the temperature range in which the polymer is melted, it is difficult to directly connect to a ignition or explosion phenomenon of the battery, thereby preventing the aforementioned extreme situation in advance and improving the safety of the battery.

In addition, the separator of the present invention can prevent deterioration in safety due to heat shrinkage of the conventional separator due to the heat resistance of 200 ° C. or higher of the inorganic particles, which is another component.

One of the components of the separator according to the present invention is a polymer having a melting point (Tm) in the range of 90 to 130 ° C. which can improve the safety of the battery due to the aforementioned membrane blockage. In addition to the effect of improving the safety, the polymer may serve as a binder to perform the conventional polymer, that is, by inorganic particles are connected to each other and stably fixed, it is possible to prevent the mechanical properties of the final membrane produced.

The polymer having a melting temperature in the range of 90 to 130 ° C. is a conventional polyethylene-based polymer and copolymers thereof known in the art, and non-limiting examples thereof include low density polyethylene (LDPE) and linear low density polyethylene. (LLDPE), high density polyethylene (HDPE), chlorinated polyethylene (from 0 to 50% chlorination), polymethacrylate (PMMA), polyethylene-polymethacrylate copolymer (PE-co-PMMA), polyethylene-polyethylene oxide air Copolymer (PE-co-PEO), polyethylene-polyethylene glycol (PE-co-PEG), or a mixture thereof. In addition, any material can be used singly or in combination as long as it contains the above-described properties. The polyethylene-based polymer has a high endothermic amount (ΔH), which effectively covers the separator and generates a blocking function when heat is generated inside the battery, and has a good dispersibility.

The average molecular weight of the above-described polyethylene-based polymer is preferably in the range of 1,000 to 1,000,000.

Another of the components of the separator according to the present invention are inorganic particles commonly used in the art. Non-limiting examples thereof include SiO ₂ , TiO ₂ , Al ₂ O ₃ , ZrO ₂ , SnO ₂ , CeO ₂ , MgO, CaO, ZnO, Y ₂ O ₃ , Pb (Zr, Ti) O ₃ (PZT), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT), PB (Mg ₃ Nb _2/3 ) O ₃ -PbTiO ₃ (PMN-PT), It includes _{BaTiO 3, hafnia (HfO 2)} , SrTiO 3.

The inorganic particles serve as a main component for manufacturing the final ceramic separator and serve as a spacer for forming micropores by forming an interstitial volume between the inorganic particles and maintaining a physical form. In addition, since the inorganic particles generally have a property that physical properties do not change even at a high temperature of 200 ° C. or more, the formed separator has excellent heat resistance. In particular, the inorganic particles excluding SiO ₂ of the above-described inorganic particles exhibit a high dielectric constant of 5 or more, and thus can easily transfer lithium ions moving in the battery, thereby improving performance of the battery.

The separator according to the present invention can adjust the pore size and thickness of the separator by adjusting the size of the inorganic particles, the content of the inorganic particles and the composition of the inorganic particles and the polymer, and by adjusting the factors such as 90 to 90 If it is out of the temperature range of 130 ℃ can shut down (shut down) function of the membrane due to the melting of the polymer can be exhibited.

In order to exhibit the function as described above, the size of the inorganic particles is preferably in the range of 0.01 to 10㎛, but not particularly limited thereto. If the thickness is less than 0.01 μm, it is difficult to control the structure and physical properties of the separation membrane. If the thickness is more than 10 μm, the thickness of the separator manufactured with the same solids content is increased, and the mechanical properties are lowered. The size increases the likelihood of internal short circuits during battery charging and discharging.

The composition ratio (volume ratio) of the polymer having a melting temperature in the range of 90 to 130 ° C relative to the inorganic particles is preferably 70 to 99%: 1 to 30%.

When the content of the polymer is less than 1% by volume, the effect of binding the inorganic particles is not only insignificant, and the desired blocking function may not be realized, and when the content of the polymer is more than 30% by volume, heat shrinkage may occur, and the pore size and pores formed between the inorganic particles The degree is reduced resulting in a decrease in thermal safety and performance of the cell itself.

The separator according to the present invention may include a porous substrate having additional pores in addition to the above-described inorganic particles and a polymer having a melting temperature in the range of 90 to 130 ° C.

In this case, the separator is formed on the surface of the porous substrate and / or a portion of the pores of the substrate, and a layer made of the inorganic particles and the polymer is formed, and the layer has a pore structure due to the empty space between the inorganic particles.

The porous substrate having the pores may use a conventional polyolefin-based polymer known in the art, and particularly preferably has a heat resistance of 200 ° C. or more. Non-limiting examples of the porous substrate material having a melting temperature of 200 ℃ or more include polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polyamide, poly Carbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfidro, polyethylenenaphthalene or these And heat-resistant engineering plastics can be used without limitation.

The thickness, pore size, and porosity of the porous substrate are not particularly limited and may be adjusted within a range conventional in the art.

The separator of the present invention prepared by using a mixture of inorganic particles and a polymer having a melting temperature in the range of 90 to 130 ° C. alone or formed by coating the mixture on a porous substrate having pores has pores having a size of 0.01 to 50 μm. It is appropriate to include, in particular a range of 0.1 to 10㎛. In addition, the porosity (porosity) of the separator is appropriately in the range of 5 to 95%, particularly preferably in the range of 30 to 70%. If less than 30% may interfere with the lithium ion transfer may adversely affect the performance and performance of the battery, and if greater than 70%, the mechanical strength is lowered, which may adversely affect the assembly and processability of the battery.

In addition, the thickness of the separator according to the present invention is preferably in the range of 1 to 100 μm, more preferably in the range of 1 to 10 μm. If it is less than 1㎛ it is difficult to properly implement the membrane properties of the present invention excellent in thermal stability and excellent thermal safety, if it exceeds 10㎛ it will not be able to perform the role of the conventional separator.

According to the present invention, a method of coating a part or all of the inorganic particle surface with a polymer having a melting temperature in the range of 90 to 130 ° C. may use conventional coating methods known in the art.

Hereinafter, describing an embodiment of the production method according to the present invention, (a) preparing a polymer liquid using a polymer having a melting temperature of 90 to 130 ° C .; (b) adding and mixing inorganic particles to the polymer liquid (a); And c) coating the mixture of step b) on a substrate and then drying; If necessary, it may include the step of detaching the substrate.

First, 1) to prepare a polymer liquid using a polymer having a melting temperature range of 90 to 130 ℃.

The polymer is melted in the range of 90 to 130 ° C., so that the polymer is prepared in a liquid phase by applying heat above or above the temperature range, or a solvent having a similar solubility index and low boiling point, for example, acetone, alcohol, or the like. It can be used by dispersing using NMP or the like.

2) A mixture of inorganic particles and a polymer is prepared by adding and dispersing inorganic particles to the prepared polymer liquid.

In the above, it is preferable to grind | pulverize an inorganic particle after adding an inorganic particle to a polymer liquid. In this case, the crushing time is suitably 1 to 20 hours, and the particle size of the crushed inorganic particles is preferably 0.01 to 10 μm as mentioned above. As the shredding method, a conventional method can be used, and a ball mill method is particularly preferable.

The composition of the mixture consisting of inorganic particles and polymers is not particularly limited, and accordingly, the membrane thickness, pore size, and porosity of the present invention to be prepared can be controlled, and can also be controlled in relation to the pore occlusion of the membrane.

3) The membrane of the present invention can be obtained by coating a mixture of the prepared inorganic particles and polymer on a substrate and then drying.

In this case, the substrate may use a Teflon sheet or a similar film commonly used in the art, and a porous substrate having pores may also be used. Moreover, the said base material can also be detach | desorbed as needed.

In addition, the process of coating the mixture of inorganic particles and polymer may use a conventional coating method. In particular, it is preferable to apply by dip coating, die coating, roll coating, comma coating or a mixture thereof.

In the above step, the membrane of the present invention may be prepared by coating the mixture on a porous substrate having pores, wherein the mixture of inorganic particles and polymer penetrates not only the surface of the porous substrate having pores but also a part of pores in the substrate. Can be coated.

The separator prepared as described above may be used as a separator of a lithium secondary battery. In this case, as described above, by automatically shutting down pores in the membrane due to melting of the polymer, the separator may prevent ignition and explosion in advance and improve thermal safety of the battery.

In addition, the separator may be used for the outermost dummy foil of the electrode. When used in the outermost dummy, the safety of the battery can be doubled by first inducing the outermost short circuit of the battery due to external impact.

The present invention provides an electrochemical device including an anode, a cathode, the separator and an electrolyte interposed between the anode and the cathode.

Electrochemical devices include all devices that undergo an electrochemical reaction, and specific examples thereof include all kinds of primary, secondary cells, fuel cells, solar cells, or capacitors. One or more electrochemical devices may be provided to manufacture an electrochemical device pack, where each electrochemical device may be configured in a combination of parallel or series connections.

As an embodiment of the electrochemical device according to the present invention, a) a positive electrode comprising a positive electrode active material capable of occluding and releasing lithium; b) a negative electrode comprising a negative electrode active material capable of occluding and releasing lithium; c) the separator described above; And it provides a lithium secondary battery comprising an electrolyte.

The lithium secondary battery includes a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

The lithium secondary battery may be manufactured by a conventional method known in the art, for example, by interposing the separator between a positive electrode and a negative electrode and then introducing a liquid electrolyte. In this case, the separation membrane may be used for the outermost dummy of the battery as described above.

The electrode to be used together with the aforementioned separator is not particularly limited, but the cathode active material is formed by lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, or a combination thereof. Lithium adsorption material (lithium intercalation material) as a main component, such as a composite oxide, etc., and the positive electrode is configured in the form of binding with a foil produced by a positive electrode current collector, that is, aluminum, nickel or a combination thereof.

The negative electrode active material is mainly composed of lithium metal or lithium alloy and lithium adsorption material such as carbon, petroleum coke, activated carbon, graphite or other carbons. The negative electrode is configured in the form of a binder bound to a current collector, that is, a foil made of copper, gold, nickel or a copper alloy or a combination thereof.

The electrolyte used in the present invention is A ⁺ B ^- A salt of the structure, such as, A ⁺ is Li ^+, Na ^+, and comprising an alkali metal cation or an ion composed of a combination thereof, such as K ^+, B ^- is PF ₆ ^{- _{, BF 4 -, Cl -,}} Br -, I -, ClO 4 -, ASF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO 2 ₃ ) Salts containing ions consisting of anions such as ₃ ^- or combinations thereof include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (dimethyl). carbonate, DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate preferably dissolved and dissociated in an organic solvent consisting of carbonate, EMC), gamma butyrolactone or mixtures thereof.

As a process of applying the separator according to the present invention as a battery, a lamination (stacking) and folding (folding) process of the separator and the electrode may be performed in addition to the general winding process.

When the separator of the present invention is applied to the lamination step in the above process, the thermal safety improvement effect of the battery becomes remarkable. This is due to the heat shrinkage of the separator in the battery produced by the lamination and folding process is more severe than the battery produced by the general winding process. In addition, the lamination (lamination, stack) process has the advantage that can be easily assembled due to the excellent adhesive properties of the polymer in the separator of the present invention. At this time, the adhesion properties can be controlled by the content of the inorganic particles and the polymer as the main component.

The invention is explained in more detail based on the following examples and experimental examples. However, Examples and Experimental Examples are for illustrating the present invention and are not limited to these.

Example 1

1-1. Separator manufacturing

A polymer liquid is prepared by melting a polyethylene polymer having a melting temperature of 90 ° C. for at least about 12 hours at a temperature of 90 ° C. BaTiO ₃ powder having a particle size of about 400 nm was added to this polymer liquid at 20% by weight of total solids and dispersed to prepare a mixed solution (BaTiO ₃ / LDPE = 80: 20 (weight ratio)). The mixed solution prepared using the doctor blade method is coated and dried on a Teflon sheet substrate, and then detached from the Teflon sheet to obtain a final separator.

1-2. Lithium secondary battery manufacturing

Anode manufacturing

A positive electrode mixture slurry was prepared by adding 94 wt% of LiCoO ₂ as a positive electrode active material, 3 wt% of carbon black as a conductive agent, and 3 wt% of PVdF as a binder to N-methyl-2 pyrrolidone (NMP) as a solvent. . The positive electrode mixture slurry is coated and dried on an aluminum (Al) thin film having a thickness of about 20 μm, which is a positive electrode current collector, to prepare a positive electrode.

Cathode manufacturing

A negative electrode mixture slurry was prepared by adding carbon powder as a negative electrode active material, PVdF as a binder, and carbon black as a conductive agent at 96 wt%, 3 wt%, and 1 wt%, respectively, to NMP as a solvent. The negative electrode mixture slurry is coated and dried on a thin copper (Cu) thin film having a thickness of 10 μm, which is a negative electrode current collector, to prepare a negative electrode.

Battery manufacturing

The positive electrode, the negative electrode and the separator prepared in Example 1-1 were assembled using a stacking method, and ethylene carbonate / propylene carbonate in which 1 M lithium hexafluorophosphate (LiPF ₆ ) was dissolved in the assembled battery. A lithium secondary battery was prepared by injecting a diethyl carbonate (EC / PC / DEC = 30: 20: 50 wt%) electrolyte.

Comparative Example 1. Manufacture of Lithium Secondary Battery

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the PP / PE / PP separator was used.

Experimental Example 1. Safety Evaluation

In order to evaluate the safety of the lithium secondary battery including the separator prepared according to the present invention, a hot box and overcharge experiments are performed as follows.

The battery of Comparative Example 1 using the lithium secondary battery prepared in Example 1 and a commercially available PP / PE / PP separator as a control was used, and each of the batteries was stored at 150 ° C. and 160 ° C. for 1 hour, respectively. Observe the state of the cell and observe the state of the cell after the overcharge experiment (6V / 1A).

Claims (10)

(a) inorganic particles; And (b) a polymer binder layer formed on part or all of the surface of the inorganic particles And a separator in which pores are formed due to interstitial volume between inorganic particles attached to each other by a polymer binder layer, wherein the polymer has a melting temperature (Tm) in a range of 90 to 130 ° C. Separator. The method of claim 1, wherein the polymer having a melting temperature in the range of 90 to 130 ° C. comprises low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), chlorinated polyethylene, polymethacrylate (PMMA), polyethylene 1 type selected from the group consisting of -polymethyl methacrylate copolymer (PE-co-PMMA), polyethylene-polyethylene oxide copolymer (PE-co-PEO) and polyethylene-polyethylene glycol copolymer (PE-co-PEG) Separation membrane. The method of claim 1, wherein the inorganic particles are Pb (Zr, Ti) O ₃ (PZT), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT), PB (Mg ₃ Nb _2/3 ) O _{With 3} -PbTiO ₃ (PMN-PT), BaTiO ₃ , hafnia (HfO ₂ ), SrTiO ₃ , TiO ₂ , Al ₂ O ₃ , ZrO ₂ , SnO ₂ , CeO ₂ , MgO, CaO, ZnO and Y ₂ O ₃ Separation membrane is one or more selected from the group consisting of. The separation membrane of claim 1, wherein the composition ratio (volume ratio) of the polymer having a melting temperature in the range of 90 to 130 ° C relative to the inorganic particles is 70 to 99%: 1 to 30%. The separator according to claim 1, wherein the separator in which the polymer layer is formed on all or part of the surface of the inorganic particles further comprises a porous substrate having pores. According to claim 5, The porous substrate material is polyethylene terephthalate (polyethyleneterephthalate), polybutylene terephthalate (polybutyleneterephthalate), polyester (polyester), polyacetal (polyacetal), polyamide (polyamide), polycarbonate (polycarbonate) ) Melting temperature consisting of polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfidro and polyethylenenaphthalene Separator which is at least one member selected from the group of heat resistant materials of not less than ℃. The separation membrane of claim 1, wherein the thickness of the layer formed of the inorganic particles and the polymer binder is in the range of 1 to 10 μm. a) an anode; b) a cathode; c) the separator of any one of claims 1 to 7 interposed between the anode and the cathode; And d) electrolyte Electrochemical device comprising a. The device of claim 8, wherein the electrochemical device is a lithium secondary battery. The electrochemical device according to claim 8, wherein the electrochemical device uses the separator of any one of claims 1 to 7 as an outermost dummy foil of the device.