KR101684339B1 - Electrode Assembly Including Separators with Enhanced Mechanical Strength and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention relates to an electrode assembly including a separator having improved mechanical strength and a lithium secondary battery including the separator. More particularly, the present invention relates to an electrode assembly including a plurality of unit cells and a separation film for continuously winding up the unit cells , The electrode assembly may include a first unit cell positioned at the center of the electrode assembly with respect to a stacking direction of the unit cells, And the n-th unit cell is sequentially wound from the first unit cell to the n-th unit cell so as to be located at the outermost portion of the electrode assembly; In one or both sides of the unit cell laminate structure, one to six unit cells sequentially include the first separator from the outermost unit cells, the other unit cells include a second separator, The second separator has a smaller heat shrinkage ratio or higher mechanical strength, and a lithium secondary battery including the electrode assembly.

Description

Technical Field [0001] The present invention relates to an electrode assembly including a separator having improved mechanical strength, and a lithium secondary battery including the electrode assembly.

The present invention relates to an electrode assembly including a separator having improved mechanical strength and a lithium secondary battery including the electrode assembly.

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operational potential, long cycle life, Batteries have been commercialized and widely used.

However, since such a lithium secondary battery has a problem in safety, attempts have been made to solve it.

Specifically, when the lithium secondary battery is overcharged, excess lithium comes out from the anode and excess lithium is inserted into the cathode, so that lithium metal having a very high reactivity is deposited on the surface of the cathode, and the anode also becomes thermally unstable. The explosion reaction of the battery causes a sudden exothermic reaction, resulting in safety problems such as ignition and explosion of the battery.

In addition, when a material having electrical conductivity such as a nail penetrates the battery, electrochemical energy inside the battery is converted into thermal energy, and rapid heat generation occurs. As a result, the positive or negative electrode material chemically reacts with heat, Causing a safety problem such as ignition or explosion of the battery.

In the case of nail penetration, compression, impact, high temperature exposure, etc., the anode and the cathode inside the battery are internally short-circuited. At this time, an excessive current flows locally and a heat is generated by this current. Since the magnitude of the short-circuit current due to local short circuit is inversely proportional to the resistance, the short-circuit current flows through the metal foil mainly used as a current collector, and the current flows through the metal foil used as the current collector. A very high heat is locally generated around the portion.

If a heat is generated in the battery, the separator shrinks and short-circuits the positive and negative electrodes again, and repeated heat generation and shrinkage of the separator shrinks the short-circuit section, resulting in thermal runaway, And the electrolytic solution react with each other or burn, which is a very large exothermic reaction, which eventually causes the battery to ignite or explode. This risk is particularly important as the energy density increases as the capacity of the lithium secondary battery increases.

In order to solve these problems and to improve the safety in overcharging, attempts have been made to add additives to the nonaqueous electrolyte solution. However, in the above-mentioned nail penetration, compression, impact, It has not been solved by adding an additive to the nonaqueous electrolyte solution.

Therefore, there is a high need for a secondary battery technology that can effectively improve the safety of a battery.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

The inventors of the present application have carried out intensive research and various experiments and, as will be described later, in an electrode assembly comprising a plurality of unit cells and a separation film for continuously winding up the unit cells, It has been confirmed that the desired effect can be achieved when one to six unit cells sequentially from the outermost unit cells on one side or both sides have a smaller heat shrinkage or a separator having a high mechanical strength, .

Accordingly, an electrode assembly according to the present invention includes an electrode assembly including a plurality of unit cells and a separation film for continuously winding the unit cells,

The electrode assembly includes a first unit cell, a second unit cell, and a second unit cell. The first unit cell is located at the center of the electrode assembly with respect to the stacking direction of the unit cells, And the n-th unit cell is sequentially wound from the first unit cell to the n-th unit cell so as to be positioned at the outermost portion of the electrode assembly;

In one or both sides of the unit cell laminate structure, one to six unit cells sequentially include the first separator from the outermost unit cells, the other unit cells include a second separator, The second separator has a smaller heat shrinkage ratio or a higher mechanical strength than the second separator.

In one specific example, in the unit cell laminated structure, at least one unit cell of the n-th unit cell and the (n-1) th unit cell may include a first separator, At least one of the n-3 unit cells may include a first separator. Further, at least one of the n-4 unit cell and the n-5 unit cell may include a first separator .

That is, for example, when the electrode assembly includes nine unit cells, at least one unit cell of the ninth unit cell and the eighth unit cell positioned at the outermost position in the unit cell stack structure is the first separator Or at least one unit cell of the ninth unit cell and the eighth unit cell, and at least one unit cell of the seventh unit cell and the sixth unit cell, which is the second unit cell in the inward direction from the outermost portion, 1 separator and may include at least one unit cell of the ninth unit cell and the eighth unit cell and at least one unit cell of the seventh unit cell and the sixth unit cell, At least one of the fifth unit cell and the fourth unit cell may include a first separator.

Considering the increase in the volume of the battery and the increase in the cost due to the use of the first separator, the first separator has a desired degree of heat shrinkage and mechanical strength, so long as it can effectively prevent short- And it is most preferable that at least one of the n-th unit cell and the (n-1) th unit cell, which is a unit cell, includes the first separator.

The first separation membrane and the second separation membrane can be classified according to their type, shape, and / or thickness, and are not limited as long as the first separation membrane has a smaller heat shrinkage ratio or a higher mechanical strength than the second separation membrane.

In one specific example, the first separation membrane may have a heat shrinkage ratio of 5% to 90% smaller, and in particular, 30% to 70% smaller than that of the second separation membrane. Specifically, And may show a heat shrinkage of 5% to 50%, particularly 5% to 40% at 120 ° C to 180 ° C for 20 minutes to 40 minutes.

In another specific example, the first separation membrane may have a puncture strength of 10% to 300% higher, and more specifically, 50% to 200% higher than that of the second separation membrane. Specifically, The puncture strength of the first separation membrane may be 300 g _r to 1000 g _r, specifically 500 g _r to 800 g _r .

In order to achieve this effect, the material of the first separation membrane may be different from the material of the second separation membrane, and the thickness of the first separation membrane may be 10% to 300% greater than the thickness of the second separation membrane. 15% to 200% more thick.

The specific thicknesses of the first and second separation membranes may be appropriately selected depending on the kind, shape and material of the separation membranes.

As described above, the separation membranes of the unit cells located in the interior of the electrode assembly are used in the type, shape or thickness conventionally used, and the separation membranes of the unit cells located at the outermost portion satisfy the above- When used in a type, shape, or thickness having a low heat shrinkage ratio or excellent mechanical rigidity, it is possible to minimize the increase in the volume of the battery and to prevent the physical impact such as nail penetration, compression, impact, It is possible to prevent a short circuit, thus securing the safety of the battery.

Such a first separation membrane can be used as a separation membrane and is not limited as long as it satisfies the above range. In one specific example, an oil / inorganic mixture composed of a binder polymer or binder polymer / inorganic particles is mixed with a It may be a porous separation membrane.

Here, the binder polymer used for the separation membrane in which the binder polymer is coated on the base material is polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride- polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polymethylmethacrylate, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, But are not limited to, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinyl alcohol cyanoethylpolyvinylalcohol, But are not limited to, ethylcellulose, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide ) Or a mixture thereof.

In addition, the organic / inorganic composite porous separation membrane is produced by using inorganic particles and binder polymer on the porous separation membrane substrate having air holes as active layer components. In this case, in addition to the pore structure contained in the separation membrane substrate itself, And have a uniform pore structure formed by the interstitial volume between them.

The use of such an organic / inorganic composite porous separator has the advantage of suppressing an increase in thickness of the cell due to swelling at the time of chemical conversion compared with the case of using a conventional separator, When a gelable polymer is used when liquid electrolyte is impregnated, it can also be used as an electrolyte.

In addition, the organic / inorganic composite porous separator has a plurality of uniform pore structures formed in both the active layer and the separator substrate, and the lithium ions are smoothly moved through the pores, and a large amount of electrolytic solution is filled, Therefore, the performance of the battery can be improved at the same time.

Moreover, the organic / inorganic composite porous separator composed of the inorganic particles and the binder polymer does not have high temperature heat shrinkage due to the heat resistance of the inorganic particles. Therefore, in the electrochemical device using the organic / inorganic composite porous film as a separator, even if the separator ruptures in the cell due to internal or external factors such as high temperature, overcharging, external impact, etc., The electrodes are hardly short-circuited completely, and even if a short circuit occurs, the short-circuited area is prevented from being greatly enlarged, thereby improving the safety of the battery.

Since the organic / inorganic composite porous separation membrane is formed by coating directly on the separation membrane substrate, the active layer and the porous substrate are physically and rigidly bonded with each other because the porous membrane and the active layer exist in an anchoring manner. Therefore, not only the problems of mechanical properties such as brittleness can be improved, but also the interface resistance between the separator substrate and the active layer is excellent, so that the interface resistance is reduced. In the organic / inorganic composite porous separator, the organic / inorganic complex active layer and the porous substrate are chemically bonded to each other, and the pore structure existing in the porous substrate remains unaffected by the active layer, It can be seen that a uniform pore structure due to the inorganic particles is formed in itself. Such a pore structure is filled with a liquid electrolyte to be injected at a later stage, which results in a significant reduction in interfacial resistance between the inorganic particles and between the inorganic particles and the binder polymer.

In addition, the organic / inorganic composite porous separator can exhibit excellent adhesion characteristics by controlling the contents of the inorganic particles and the binder polymer in the separator, so that the cell assembly process can be easily performed.

In the organic / inorganic composite porous separator, one of the active layer components formed on the surface of the separator substrate and / or the pores of the substrate is an inorganic particle commonly used in the art. The inorganic particles enable the formation of voids between the inorganic particles to form micropores and serve as a kind of spacer capable of maintaining the physical shape. In addition, since the inorganic particles generally have the property that their physical properties do not change even at a high temperature of 200 DEG C or more, the formed organic / inorganic composite porous separator has excellent heat resistance.

The inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as the oxidation and / or reduction reaction does not occur in the operating voltage range of the applied battery (for example, 0 to 5 V based on Li / Li +). Particularly, when inorganic particles having an ion-transporting ability are used, the ion conductivity in the electrochemical device can be increased and the performance can be improved. Therefore, it is preferable that the ionic conductivity is as high as possible. In addition, when the inorganic particles have a high density, it is difficult to disperse the particles at the time of coating, and there is a problem of an increase in weight during the production of the battery. In the case of an inorganic substance having a high dielectric constant, dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte also contributes to increase ionic conductivity of the electrolyte.

For the reasons stated above, the inorganic particles are preferably high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more, inorganic particles having piezoelectricity, inorganic particles having lithium ion transporting ability, or mixtures thereof Do.

The piezoelectricity inorganic particle means a non-conductive material at normal pressure or a material having electrical conductivity due to a change in internal structure when a certain pressure is applied, and exhibits a high permittivity with a dielectric constant of 100 or more, The charge is generated in the case of applying tensile or compressive force, so that one side is charged positively and the other side is negatively charged, thereby generating a potential difference between both sides.

When inorganic particles having the above-described characteristics are used as the porous active layer component, if an internal short-circuit of both electrodes occurs due to an external impact such as local crush or nail, due to the inorganic particles coated on the separator, Not only do not directly contact but also cause a potential difference in the particle due to the piezoelectricity of the inorganic particles. As a result, the electron transfer between the two electrodes, that is, the flow of a minute current, The safety can be improved.

The inorganic particles having the lithium ion transferring ability refer to inorganic particles that contain a lithium element but do not store lithium but have a function of transferring lithium ions. The inorganic particles having lithium ion transferring ability exist in the particle structure Since lithium ions can be transferred and transferred due to a kind of defect, the lithium ion conductivity in the battery can be improved and the battery performance can be improved.

The inorganic particles may be selected from the group consisting of BaTiO 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) hafnia (HfO ₂ ), and SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , TiO ₂ , SiC, and a mixture thereof, but is not limited thereto.

(Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2), and the like. , 0 <y <3), lithium aluminum titanium phosphate _{(Li x Al y Ti z (} PO 4) 3, 0 <x <2, 0 <y <1, 0 <z <3), 14Li 2 O-9Al 2 O ₃ -38TiO ₂ -39P ₂ O _5, such as (LiAlTiP) xOy series glass (0 <x <4, 0 <y <13), lithium lanthanum titanate _{(Li x La y TiO 3,} 0 <x <2, (Li _x Ge _y P _z S _w , 0 <x <4, 0 <y <1, 0 <z <1) such as 0 <y <3, Li _3.25 Ge _0.25 P _0.75 S ₄ , , 0 <w <5), Li 3 N lithium nitride _{(Li x N y, 0 <} x <4, 0 <y <2) such _{as, Li 3 PO 4 -Li 2 S} -SiS 2 SiS 2 , including series _{glass (Li x Si y S z} , 0 <x <3, 0 <y <2, 0 <z <4), such as _{LiI-Li 2 SP 2 S 5} P 2 S 5 based glass (Li _x P _y S _z , 0 <x <3, 0 <y <3, 0 <z <7), or a mixture thereof.

When the above-mentioned high-permittivity inorganic particles, piezoelectric particles, and inorganic particles having lithium ion-transporting ability are mixed, their synergistic effect can be doubled.

The organic / inorganic composite porous separator membrane forms the pore structure of the active layer together with the pores contained in the separator substrate by controlling the size of the inorganic particle, the content of the inorganic particle, and the composition of the inorganic particle and the binder polymer, And the pore size and porosity can be adjusted together.

The size of the inorganic particles is not limited, but is preferably in the range of 0.001 to 10 mu m for the formation of a film of uniform thickness and a suitable porosity. In the case where the thickness is less than 0.001 탆, the dispersibility of the organic / inorganic composite porous separator is lowered, and it is difficult to control the physical properties of the organic / inorganic composite porous separator. When the thickness exceeds 10 탆, And the excessively large pore size increases the probability of an internal short circuit occurring during charging and discharging of the battery.

The content of the inorganic particles is not particularly limited, but may be in the range of 50 to 99% by weight, more specifically 60 to 95% by weight, per 100% by weight of the mixture of the inorganic particles and the binder polymer constituting the organic / inorganic composite porous separator. Lt; / RTI &gt; If the amount is less than 50% by weight, the content of the polymer is excessively increased, and the pore size and porosity may be decreased due to the reduction of the void space formed between the inorganic particles, resulting in deterioration of the final battery performance. On the contrary, when the content is more than 99% by weight, the mechanical properties of the final organic / inorganic composite porous membrane are deteriorated due to the weak adhesive force between the inorganic materials because the polymer content is too small.

In the organic / inorganic composite porous separator, another active layer component formed on the separator substrate is a binder polymer commonly used in the art. Particularly, a glass transition temperature (Tg) as low as possible can be used, and in particular, it is in the range of -200 to 200 ° C. This is because the mechanical properties such as flexibility and elasticity of the final film can be improved. The polymer faithfully fulfills the role of a binder to connect and stably fix the inorganic particles and particles, the inorganic particles and the surface of the separator substrate, and the pores of the separator membrane, thereby improving the mechanical properties of the final organic / inorganic composite porous separator Thereby preventing degradation.

In addition, although the binder polymer does not necessarily have ion conductivity, the performance of the electrochemical device can be further improved by using a polymer having ion conductivity. Therefore, it is preferable that the binder polymer has a high permittivity constant.

In fact, since the degree of dissociation of the salt in the electrolyte depends on the permittivity constant of the electrolyte solvent, the higher the permittivity constant of the polymer, the better the salt dissociation in the electrolyte of the present invention. The dielectric constant of the polymer may be in the range of 1.0 to 100 (measurement frequency = 1 kHz), more preferably 10 or more.

In addition to the above-mentioned functions, the binder polymer may be gelled upon impregnation with a liquid electrolyte to exhibit a high degree of swelling of the electrolyte. In fact, when the binder polymer is a polymer having an excellent electrolyte impregnation rate, the electrolyte injected after assembling the battery is impregnated with the polymer, and the polymer having the absorbed electrolyte has electrolytic ion conduction capability. Therefore, the performance of the electrochemical device can be improved compared to the conventional organic / inorganic composite electrolyte. In addition, compared with the conventional hydrophobic polyolefin-based separator, wetting of the electrolyte for a battery is improved, and polarity electrolytic solution for a battery, which has not been used conventionally, can be applied. In addition, when the polymer is a polymer that can be gelled when the electrolyte is impregnated, the gelled organic / inorganic composite electrolyte can be formed by allowing the injected electrolyte and the polymer to react with each other and gel. The thus formed electrolyte is easier to manufacture than the conventional gel type electrolyte, and exhibits high ion conductivity and electrolyte impregnation rate, thereby improving the performance of the battery. Thus, the solubility index may be 15 to 45 MPa &lt; 1/2 &gt;, specifically 15 to 25 MPa 1/2 and 30 to 45 MPa 1/2. When the solubility index is less than 15 MPa &lt; 1/2 &gt; and more than 45 MPa &lt; 1/2 &gt;, it becomes difficult to swell by a common liquid electrolyte for a battery.

Examples of usable binder polymers also include polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidene fluoride-cotichloroethylene, polyvinylidene fluoride-co-trichlorethylene, Polyolefins such as polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, , Cyanoethylcellulose but are not limited to, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, And mixtures thereof. However, the present invention is not limited thereto. Any material may be used, either alone or in combination, as long as it contains the above-mentioned characteristics.

The composition ratio of the inorganic particles and the binder polymer as the active layer component is not critical and can be controlled within a range of 10:90 to 99: 1 wt%, and may be 80:20 to 99: 1 wt%. When the ratio is less than 10: 90% by weight, the content of the polymer is excessively increased, resulting in a decrease in the pore size and porosity due to the reduction of the void space formed between the inorganic particles, , The mechanical properties of the final organic / inorganic composite porous membrane may be deteriorated due to the weakening of the adhesion between the inorganic materials because the polymer content is too small.

The active layer of the organic / inorganic composite porous separator may further include other commonly known additives in addition to the inorganic particles and the binder polymer.

In the organic / inorganic composite porous separator, a substrate coated with a mixture of inorganic particles and a binder polymer as the active layer constituent may be a polyolefin-based separator commonly used in the art.

The polyolefin-based separator may be formed of any one of, for example, high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, polypropylene, polyethylene terephthalate, polybutylene tererephthalate, But are not limited to, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylene sulfide A sheet or a nonwoven fabric made of at least one selected from the group consisting of polyphenylene sulfide, polyethylene naphthalene, and a mixture thereof.

The pore size and porosity of the separator substrate are not particularly limited, but the porosity may be in the range of 10 to 95% and the pore size (diameter) may be in the range of 0.1 to 50 탆. When the pore size and the porosity are less than 0.1 μm and less than 10%, respectively, they act as resistance layers. If the pore size and porosity are more than 50 μm and 95%, it becomes difficult to maintain the mechanical properties.

The organic / inorganic composite porous separation membrane formed by coating a mixture of inorganic particles and a binder polymer on a separation membrane substrate includes not only pores in the separation membrane base material itself but also an empty space between inorganic particles formed on the base material Both the substrate and the active layer form a pore structure. The pore size and porosity of the organic / inorganic composite porous separation membrane depend mainly on the size of the inorganic particles. For example, when inorganic particles having a particle size of 1 μm or less are used, the pores formed also show 1 μm or less. Such a pore structure is filled with an electrolyte solution to be injected at a later stage, and the filled electrolyte serves as an ion transfer function. Therefore, the pore size and porosity are important factors for controlling the ionic conductivity of the organic / inorganic composite porous separator.

The pore size and porosity of the active layer coated with the mixture on the polyolefin separator substrate and formed with the pore structure may be in the range of 0.001 to 10 탆 and 5 to 95%, respectively, but are not limited thereto.

Also, in one specific example, the first separation membrane may be a nonwoven fabric separation membrane made of a material having a low heat shrinkage ratio, or an oil / inorganic mixture composed of a binder polymer or binder polymer / inorganic particles may be provided on one or both surfaces It may be a coated SRS nonwoven fabric separator.

Here, the material having a low heat shrinkage ratio is selected from the group consisting of PET, PAN, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylenevanbenzobisoxazole, polyarylate, One or two or more.

The binder polymer and inorganic particles of the SRS nonwoven fabric separator are as described above.

Unlike the first separator, the second separator may be a sheet or a film made of a polyolefin resin. Alternatively, an organic / inorganic mixture of a binder polymer or a binder polymer / inorganic particles may be applied Or may be a non-woven fabric separating membrane.

When the second separator is a sheet or a film made of a polyolefin resin, the kind, shape and thickness of the first separator differ from those of the first separator. In the case of an organic / inorganic composite porous separator or a nonwoven separator, .

That is, as described above, the first separator may have a small heat shrinkage ratio or excellent mechanical strength in its kind, and may also have the above-described effect in a different thickness.

In conclusion, the present invention is based on the idea that only the outermost separating membrane which is first contacted with the electrode assembly by a physical impact such as nail penetration, compression, impact or the like is thickened or the material is changed to lower the heat shrinkage rate or improve the mechanical strength, The inner separators having no direct contact or influence on the external impact are made thinner in thickness similar to the conventional separator or by using general materials so that the volume of the battery can be increased and / It is possible to sufficiently secure the safety of the battery while solving the problem of rising cost.

Meanwhile, the separation film may be a sheet or a film made of a polyolefin resin, or an organic / inorganic mixture composed of a binder polymer or a binder polymer / inorganic particle may be a composite porous separator coated on a substrate.

Specifically, since the separation film covers the outer surface of the electrode assembly, it is possible to use an organic / inorganic composite porous separation membrane which is first contacted with a physical impact such as nail penetration, compression, shock, etc. However, The first separator film of the outermost unit cell can secure the safety. Therefore, it is preferable that the first separator film is a sheet or film made of a polyolefin resin in consideration of the degree of volume increase of the whole electrode assembly.

Other components of the electrode assembly according to the present invention will be described below.

In one specific example of the unit cell, the unit cell may be a bi-cell having the same electrode structure on both sides and / or a pull cell having a different electrode structure on both sides.

Specifically, a pull cell as a unit cell is a cell having a unit structure of a cathode / separator / cathode, in which an anode and a cathode are positioned on both sides of the cell. Such a pull cell may include a cathode / separator / cathode cell and an anode / separator / cathode / separator / anode / separator / cathode cell having the most basic structure. 1 is a schematic view of a pull cell having a positive electrode / separator / negative electrode structure. In order to construct an electrochemical cell including a secondary cell using such a pull cell, a positive electrode and a negative electrode Multiple full cells should be stacked to face each other.

Also, the bi-cell as a unit cell may have a structure such as a unit structure (A-type bi-cell) of a cathode / separator / cathode / separator / anode and a unit structure (C-type bi-cell) of a cathode / separator / anode / In which the same electrode is located. FIG. 2 is a schematic view of a bipolar cell having a cathode / separator / cathode / separator / anode structure, and FIG. 3 is a schematic diagram of a bipolar cell having a cathode / separator / anode / separator / cathode structure. In order to construct an electrochemical cell including a secondary cell using such a bi-cell, a bi-cell and a cathode / separator / anode / separator / cathode structure of a cathode / separator / cathode / separator / A plurality of bi-cells must be stacked so that the bi-cells face each other.

2 and 3 illustrate one example and, in some cases, more bipolar cells of the stack number are possible, such as anode / separator / cathode / separator / anode / separator / cathode / separator / anode structure Cathode, separator, anode, separator, cathode, separator, anode, separator, and bipolar cell structure.

On the other hand, the unit cells have an anode, a cathode, and the separator as a basic structure.

The positive electrode is 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 and pressing. If necessary, a filler may be further added to the mixture.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. Examples of the positive electrode current collector include stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel A surface treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black 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, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component which assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30% by weight based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The filler is optionally used as a component for suppressing the expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change 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.

On the other hand, the negative electrode is manufactured by applying, drying and pressing an anode active material on an anode current collector, and may optionally further include a conductive material, a binder, a filler, and the like as described above.

The negative electrode current collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, and examples of the anode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, a surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The negative electrode active material may include, for example, carbon such as non-graphitized carbon or graphite carbon; _{Li x Fe 2 O 3 (0≤x≤1} ), Li x WO 2 (0≤x≤1), Sn x Me 1-x Me 'y O z (Me: Mn, Fe, Pb, Ge; Me' : Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, Halogen, 0 &lt; x &lt; Lithium metal; Lithium alloy; Silicon-based alloys; Tin alloy; _{SnO, SnO 2, PbO, PbO} 2, Pb 2 O 3, Pb 3 O 4, Sb 2 O 3, Sb 2 O 4, Sb 2 O 5, GeO, GeO 2, Bi 2 O 3, Bi 2 O 4, and Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The present invention also provides a lithium secondary battery in which the electrode assembly is embedded in a battery case together with a non-aqueous electrolyte.

The non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt, and examples of the non-aqueous electrolyte include non-aqueous organic solvents, organic solid electrolytes, inorganic solid electrolytes, and the like.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile , Nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may 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 and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a substance that is soluble in the nonaqueous electrolyte and includes, 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 ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic carboxylate lithium, lithium tetraphenylborate, and imide.

The nonaqueous electrolyte may contain, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like may be added have. In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene Carbonate, PRS (Propene sultone), and the like.

In one specific _{example, LiPF 6, LiClO 4, LiBF} 4, LiN (SO 2 CF 3) 2 , such as a lithium salt, a highly dielectric solvent of DEC, DMC or EMC Fig solvent cyclic carbonate and a low viscosity of the EC or PC of And then adding it to a mixed solvent of linear carbonate to prepare a lithium salt-containing nonaqueous electrolyte.

The battery case may be a pouch-shaped battery case made of a laminate sheet including an outer coating layer made of a weatherproof polymer, an inner sealant layer made of a heat-sealable polymer, and a barrier layer interposed between the outer coating layer and the inner sealant layer Specifically, a pouch-shaped battery case made of an aluminum laminate sheet having a barrier layer of Al.

The present invention also provides a battery pack including the secondary battery as a unit cell, and a device including the battery pack.

Specific examples of such a device include a small device such as a computer, a mobile phone, a power tool, a power tool powered by an electric motor, An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; Power storage systems, and the like, but are not limited thereto.

As described above, an electrode assembly including a plurality of unit cells according to the present invention and a separation film for continuously winding up the unit cells may be formed by sequentially stacking the unit cell stacked structure from the outermost unit cells on one side or both sides of the unit cell stacked structure One to six unit cells have a smaller heat shrinkage ratio or a separator having a high mechanical strength, so that the safety of the battery can be effectively improved while preventing the thickness of the entire electrode from rising.

1 is a schematic view showing a pull cell as one form of a unit cell of the present invention;
2 is a schematic diagram showing an A-type bi-cell as one type of unit cell of the present invention;
3 is a schematic view showing a C-type bicycle as one type of unit cell of the present invention;
4 is a schematic diagram of an electrode assembly according to one embodiment of the present invention;
5 is a schematic view of an electrode assembly according to another embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but the present invention is not limited by the scope of the present invention.

4 and 5 are schematic diagrams of an electrode assembly according to an embodiment of the present invention. Specifically, FIG. 4 schematically shows an electrode assembly having a structure in which five unit cells are stacked, and FIG. 5 schematically shows an electrode assembly having a structure in which seven unit cells are stacked. 4 and 5 use only the bi-cells shown in FIGS. 2 and 3 as unit cells, but it goes without saying that only the full cell of FIG. 1 or at least one full cell can be used.

3, the electrode assembly 100 includes a plurality of bi-cells 110, 111, 112, 113, 114, and 115 having a separator interposed between an anode and a cathode, It is made up of a wound structure.

Specifically, the electrode assembly 100 includes five bi-cells 111, 112, 113, 114, and 115, and the electrode assembly 100 is formed such that the separation film 140 is separated from the first bi- 111 and the second bi-cell 112 and the third bi-cell 113 are positioned on the upper and lower portions of the first bi-cell 111, and the outer surfaces of the second bi-cell 112 and the third bi- The outer surfaces of the fourth bicell 114 and the fifth bicell 115 are positioned on the lower part of the second bicell 112 and on the third bicol 113, ), Which is wrapped once.

At this time, the fourth bipolar cell 114 and the fifth bi-cell 115 located at the outermost periphery are separated from each other by the first separator 120 having a high thickness, a small heat shrinkage ratio, The first bi-cell 111, the second bi-cell 112, and the third bi-cell 113 located inside the electrode assembly 100 include the second separation membrane 130 Have

The first separator 120 has a first separator, so look for the heat shrinkage rate of 5% to 50% during the 120 ℃ to 180 ℃ 20 to 40 minutes, or the piercing strength of 300 g _r to 1000 g _r, not It is possible to prevent a short circuit at the outermost portion of the electrode assembly which is first contacted with a physical impact such as penetration, squeezing, impact, etc., thus securing the safety of the battery. At this time, the characteristics may be different depending on the type, shape, or material, and may be achieved by making the thickness of the first separation membrane 120 thicker than that of the second separation membrane 130.

Accordingly, if the first separator 120 is used up to the inner unit cells, the volume of the entire cell is increased. Therefore, the bi-cells 111, 112, The second separation membrane 130 is used.

4, the electrode assembly 200 of FIG. 4 includes seven bi-cells 210, 211, 212, 213, 214, 215, 216 and 217, The separating film 240 once covers the outer surface of the first bi-cell 211 at the center and the second bi-cell 212 and the third bi-cell 213 are formed at the upper and lower portions of the first bi- The fourth bi-cell 214 and the fifth bi-cell 215 are disposed on the lower portion of the second bi-cell 212 and on the third bi-cell 213, respectively, The sixth bi-cell 216 and the seventh bi-cell 216 are disposed on the lower part of the fourth b cell 214 and on the fifth b cell 215, respectively, And the outer surface of the second film 217 is covered with the separation film 240 once.

At this time, not only the sixth bi-cell 216 and the seventh bi-cell 217 located at the outermost portion, but also the second unit cell and the third unit cell inward in the lamination direction of the electrode assembly 200 Due to the special material up to the second bi-cell 212, the third bi-cell 213, the fourth bi-cell 214 and the fifth bi-cell 215, the first heat- And the first separator 211 located only in the innermost part of the electrode assembly 200 includes the second separator 230. [

In this manner, when the first separator 220 is included in the second and third bi-cells from the outermost periphery to the inside, it is more effective in securing the cell safety. However, since there is a problem of an increase in the volume of the battery or an increase in cost as the number of the first separator 220 increases, considering the above effects, the number of the secondary batteries including the electrode assembly Is preferred.

Hereinafter, the present invention will be described in further detail with reference to the following examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

&Lt; Example 1 >

Manufacture of anode

LiCoO ₂ was used as a positive electrode active material, and N-methyl-2-pyrrolidone (NMP) as a solvent was added to 96% by weight of LiCoO ₂ and 2.0% by weight of Super-P The anode mixture slurry was prepared, coated on aluminum foil, dried and pressed to prepare a cathode.

Cathode manufacturing

Artificial graphite was used as the negative electrode active material. An anode mixture slurry was prepared by adding 96.3% by weight of artificial graphite, 1.0% by weight of Super-P (conductive agent) and 2.7% by weight of PVdF (binder) The negative electrode was prepared by coating, drying and pressing on copper foil.

Preparation of first separation membrane

A mixture of polyvinylidenefluoride-co-hexafluoropropylene (PVDF) as a binder polymer and BaTiO ₃ and Al ₂ O ₃ as inorganic particles was coated on a polypropylene substrate to a thickness of 18.5 μm A separator was prepared. The heat shrinkage of the separator was 25% at 150 ° C for 30 minutes and the puncture strength was 600 g _r .

Preparation of Second Membrane

A mixture of polyvinylidene fluoride-co-hexafluoropropylene (PVDF) as a binder polymer and BaTiO ₃ and Al ₂ O ₃ as inorganic particles was coated on a polypropylene substrate to a thickness of 15.5 μm A separator was prepared. The heat shrinkage of the separator was 60% at 150 ° C for 30 minutes and the puncture strength was 220 g _r .

Manufacture of Secondary Battery

Two bi-cells of FIG. 3 are assembled respectively, and two bi-cells of FIG. 2 and two bi-cells of FIG. 3 are interposed between the positive electrode and the negative electrode with a structure in which a first separator is interposed between the positive and negative electrodes. And five bi-cells were sequentially wound using a polypropylene separation film so that the bi-cells including the first separation membrane were located at the outermost periphery as shown in FIG. 4, The electrode assembly was housed in a pouch-shaped battery case, and then a 1M LiPF ₆ carbonate-based solution electrolyte was injected to complete the secondary battery.

&Lt; Example 2 >

A secondary battery was produced in the same manner as in Example 1, except that a polypropylene separation membrane of Celgard Company was used as the second separation membrane.

&Lt; Example 3 >

A secondary battery was manufactured in the same manner as in Example 1, except that a nonwoven fabric type separator prepared by needle punching method using PET was used as the first separator and the second separator. At this time, the first separation membrane was made to have a thickness of 20.0 micrometers, and the second separation membrane was made to have a thickness of 15.5 micrometers. The first separator had a heat shrinkage of 30% at 150 ° C. for 30 minutes, a puncture strength of 600 g _r , a heat shrinkage rate of 60% at 150 ° C. for 30 minutes and a puncture strength of 250 g _r .

&Lt; Comparative Example 1 &

A secondary battery was produced in the same manner as in Example 1 except that a separator having the same constitution as that of the second separator was used as the first separator.

 &Lt; Comparative Example 2 &

A secondary battery was manufactured in the same manner as in Example 1, except that a polypropylene separator of Celgard Company was used as the first separator and the second separator.

&Lt; Comparative Example 3 &

A secondary battery was fabricated in the same manner as in Example 1, except that a 15-micrometer-thick non-woven-type separator prepared by needle punching using PET was used as the first separator and the second separator.

<Experimental Example 1>

Twenty secondary batteries prepared in each of Examples 1 to 3 and Comparative Examples 1 to 3 were prepared at 4.35 V in a fully charged state. Using a nail penetration tester, a 2.5 mm diameter nail made of iron was pierced through the center of the cell, and the ignition was measured.

The penetration speed of the nail was constant at 12 m / min. The results are summarized in Table 1 below.

Whether ignited Maximum temperature (℃) Example 1 No ignition 65.5 Example 2 No ignition 70.7 Example 3 No ignition 85.6 Comparative Example 1 Ignition 95.5 Comparative Example 2 Ignition 92.2 Comparative Example 3 Ignition 89.5

As shown in Table 1, the secondary batteries according to the present invention did not cause short-circuiting and ignited in all 20 batteries, while the secondary batteries of Comparative Examples 1 to 3 were short-circuited and ignited in a plurality of batteries .

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (22)

An electrode assembly comprising a plurality of unit cells and a separation film for continuously winding up the unit cells,
The electrode assembly includes a first unit cell, a second unit cell, and a second unit cell. The first unit cell is located at the center of the electrode assembly with respect to the stacking direction of the unit cells, And the n-th unit cell is sequentially wound from the first unit cell to the n-th unit cell so as to be positioned at the outermost portion of the electrode assembly;
In one or both sides of the unit cell laminate structure, one to six unit cells sequentially include the first separator from the outermost unit cells, the other unit cells include a second separator, Wherein the second separator has a smaller heat shrinkage or higher mechanical strength than the second separator. The electrode assembly according to claim 1, wherein at least one unit cell of the n-th unit cell and the (n-1) th unit cell comprises a first separator in the unit cell laminate structure. The electrode assembly of claim 2, wherein at least one unit cell of the n-2 th unit cell and the n-3 th unit cell includes a first separator in the unit cell laminate structure. The electrode assembly according to claim 3, wherein at least one of the n-4th unit cell and the n-5th unit cell includes a first separator in the unit cell laminate structure. The electrode assembly according to claim 1, wherein the first separator has a heat shrinkage of 5% to 90% smaller than that of the second separator. The electrode assembly according to claim 1, wherein the first separator has a heat shrinkage ratio of 5% to 50% at 120 ° C to 180 ° C for 20 minutes to 40 minutes. The electrode assembly of claim 1, wherein the first separator has a puncture strength of 10% to 300% higher than the second separator. The method of claim 1, wherein the first membrane electrode assembly, characterized in that the puncture strength (puncture strength) 300 g to 1000 g _{r r.} The electrode assembly according to claim 1, wherein the thickness of the first separator is 10% to 300% greater than the thickness of the second separator. The electrode assembly of claim 1, wherein the first separator is a composite porous separator coated on a substrate, wherein the organic / inorganic mixture of binder polymer or binder polymer / inorganic particles is coated. 11. The method of claim 10, wherein the binder polymer used in the separation membrane is selected from the group consisting of polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidene fluoride-cotichloroethylene, Polyolefins such as polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, , Cyanoethylcellulose acrylonitrile-styrene-butadiene copolymer, polyimide, or the like, or a mixture thereof, and the like. Examples of the polymeric material include polyvinyl alcohol, polyvinyl alcohol, polyvinyl alcohol, polyvinyl alcohol, polyvinylpyrrolidone, lulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, acrylonitrile- And a mixture thereof. 11. The method of claim 10, wherein the inorganic particles is _{BaTiO 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), hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , TiO _2, SiC, (LiAlTiP) xOy series glass (0 <x <4, 0 <y <13), lithium germanium Mani help thiophosphate _{(Li x Ge y P z S} w, 0 <x <4, 0 <y < 1, 0 <z <1, 0 <w <5), lithium nitride (Li _x N _y , 0 <x <4, 0 <y <2), SiS ₂ series glass (Li _x Si _y S _z , x <3, 0 <y <3, 0 <z <7), P ₂ S ₅ series glass (Li _x P _y S _z , And mixtures thereof. &Lt; RTI ID = 0.0 &gt; 11. < / RTI &gt; The method of claim 10, wherein the substrate is selected from the group consisting of high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, polypropylene, polyethylene terephthalate, polybutyleneterephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylene sulfide, and the like. wherein the electrode assembly is a sheet or nonwoven fabric made of at least one selected from the group consisting of polyphenylene sulfide, polyethylenenaphthalene, and a mixture thereof. The electrode assembly according to claim 1, wherein the first separator is a nonwoven fabric separator made of a material having a low heat shrinkage. 15. The method according to claim 14, wherein the material is selected from the group consisting of PET, PAN, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, And one or more selected one or more selected from the group consisting of: 15. The electrode assembly according to claim 14, wherein the nonwoven fabric separator is an SRS nonwoven fabric separator coated on one or both sides of an oil / inorganic mixture comprising binder polymer or binder polymer / inorganic particles. The method according to claim 1, wherein the second separation membrane is a sheet or a film made of a polyolefin resin, or an organic / inorganic mixture composed of a binder polymer or a binder polymer / inorganic particle is a composite porous separation membrane coated on a substrate or a non- The electrode assembly comprising: The electrode assembly according to claim 1, wherein the separation film is a sheet or a film made of a polyolefin resin, or an organic / inorganic mixture composed of a binder polymer or a binder polymer / inorganic particle is a composite porous separator coated on a substrate. The lithium secondary battery according to any one of claims 1 to 18, wherein the electrode assembly is embedded in the battery case together with the non-aqueous electrolyte. A battery pack comprising the lithium secondary battery according to claim 19 as a unit cell. A device comprising a battery pack according to claim 20. 22. The device of claim 21, wherein the device is a computer, a mobile phone, a wearable electronic device, a power tool, an electric vehicle (EV), a hybrid electric vehicle, a plug- , Or a system for power storage.

Images