KR100624894B1 - Lithium ion rechargeable battery - Google Patents

Abstract

In a lithium ion secondary battery in which an electrode assembly having a separator provided on a surface where two electrodes are in contact with two electrodes is accommodated in a case, the tensile strength in the longitudinal direction of the battery, that is, the longitudinal direction of the separator, is greater than the transverse tensile strength of the separator. Disclosed is a lithium ion secondary battery.

The present invention can effectively prevent separator damage and internal short circuit caused by breakage at the time of external impact by strengthening the lateral tensile strength of the separator.

Description

Lithium ion rechargeable battery {Lithium ion rechargeable battery}

1 is a cross-sectional view showing the structure of a conventional cylindrical lithium ion battery;

FIG. 2 is a graph showing temperature (° C.) change patterns with typical time (minutes) shown in a round bar collision test of a cylindrical lithium ion battery. FIG.

The present invention relates to a lithium ion secondary battery, and more particularly to a separator of a lithium ion secondary battery.

Lithium ion secondary batteries are capable of charging and discharging, and are advantageous in comparison with other chemical batteries in terms of light weight and high capacity, and thus, lithium ion secondary batteries have been recently developed and used as power sources for portable personal digital assistants (PDAs), cellular phones, and notebook computers.

However, even in lithium ion secondary batteries stabilized by allowing high activity of lithium metal to enter and exit between electrodes in the form of ions, safety is also a major problem. The safety problem of lithium ion secondary batteries is largely due to the fact that lithium is used in the form of ions but has a small size and a large electric capacity and uses a non-aqueous electrolyte.

Referring to the structure of a lithium ion secondary battery through the cylindrical lithium ion secondary battery of FIG. 1, an electrode assembly including two electrodes and a separator interposed therebetween is accommodated in a cylindrical case together with an electrolyte.

Referring to Figure 1, a method of forming a cylindrical secondary battery, first, two electrodes 25 formed in a rectangular plate shape and a separator (21, 23) interposed between these electrodes to prevent a short circuit between the two electrodes are stacked , To form an electrode assembly 20 wound in a spiral shape, commonly referred to as a jelly roll. Each electrode plate is made by coating an active material slurry on a current collector made of a metal foil.

In the direction in which the electrode plate is wound, there is a non-coated portion at which the slurry is not applied at the start end and the end of the current collector. In the uncoated portion, electrode tabs 27 and 29 are usually provided on one electrode plate one by one. The electrode tabs 27 and 29 are electrically connected to the cylindrical can 10 and the cap assembly 80 insulated from the can 10 to form part of a passage for connecting the electrode assembly and an external circuit during charging and discharging. In the electrode assembly 20, the electrode tab is formed to be pulled out of the electrode, one upward in the direction of the opening of the cylindrical can 10 and the other downward.

The electrode assembly is introduced into the cylindrical can 10 through the can openings in order with the upper and lower insulating plates 13a and 13b positioned up and down. Beads are formed in the can to prevent flow in the can of the electrode assembly, and the electrolyte is injected. An insulating gasket 30 is installed on the inner wall of the opening of the can, and a cap assembly 80 that closes the opening of the can 10 is installed inside the gasket 30.

As the cap assembly 80, a vent assembly welded to one of the electrode tabs 27, a positive thermal coefficient 60 (PTC), and a cap up 70 having electrode terminals are inserted. The vent assembly is typically provided with a current interrupter (CID: Current Interrupt Device) 50 that breaks with the action of the vent 40 and the vent below and breaks the path of the current.

Then, a crimping operation is performed to seal the can by applying pressure to the opening wall of the cylindrical can 10 inward and downward with a cap up 70 inserted into the gasket 30. In addition, a tubing operation of applying an exterior material to the outside of the battery is performed.

The separator is basically made of a material that is excellent in insulation because it must prevent a short circuit between two electrodes. However, to prevent a short circuit, the ion conductivity must be high enough to allow sufficient ions to move between the two electrodes. In addition, it must withstand heat generated from abnormal phenomena such as normal charging / discharging action of the battery or internal fine short circuit, and must be maintained for a long time without being denatured by the electrolyte that contacts the electrode assembly.

In addition, the separator should have a certain mechanical strength or higher so as not to be damaged by external impact or pressure applied in the process of forming an electrode or using a completed battery.

Typically, a polyolefin separator such as polyethylene or polypropylene is used for a lithium ion secondary battery. In addition to these polymers and plasticizers, these separators may further include inorganic fillers to increase ion conductivity.

One of the most problematic issues with regard to separators is the prevention of smoke, ignition and rupture of a range of external mechanical shocks. Stability against mechanical shock is usually determined as a result of colliding the intermediate parts 90 degrees with the round bar based on the axial or longitudinal direction of the battery. The result is the surface temperature of the battery over time. That is, if the temperature rises rapidly after the round bar collision with time, it is determined that a short circuit occurs between the two electrodes in the battery.

However, even if the battery passes the external impact test, the existing lithium ion secondary batteries cannot avoid the characteristic distribution due to mass production, and therefore, regardless of the manufacturer, it is necessary to select a separator material in a limited range to improve profitability. In the actual use process of the battery, the surface temperature of the lithium ion secondary battery is rapidly increased due to the impact, and there is a problem of ignition and bursting.

This problem is not only a problem of disposal due to a defective battery, but also causes a problem that the customer's trust in the manufacturer falls and delays the development of a new high capacity battery.

On the other hand, when the molecular weight is increased in polyethylene or the like in order to increase the mechanical strength of the separator in order to counteract an external impact, the separator thin film formability is inferior, and there is a characteristic that the shutdown (shot down) property is deteriorated. In addition, in the case where the separator film is stretched to increase the mechanical strength, when the battery receives heat, the stretched separator film is severely shrunk, thereby increasing the possibility of generating a short circuit in the battery, thereby deteriorating thermal and electrical safety.

An object of the present invention is to provide a lithium ion secondary battery having a strong separator capable of improving the safety of the battery, to solve the above problems of the lithium ion secondary battery.

An object of the present invention is to provide a lithium ion secondary battery having a separator capable of preventing mechanical damage of the separator against external mechanical shock and at the same time maintaining thermal stability.

The lithium ion secondary battery of the present invention for achieving the above object,

In a lithium ion secondary battery in which an electrode assembly having a separator provided on a surface where two electrodes are in contact with two electrodes is accommodated in a case, the tensile strength in the longitudinal direction of the battery, that is, the longitudinal direction of the separator, is greater than the transverse tensile strength of the separator. It is characterized by one.

In the present invention, it is preferable that the longitudinal tensile strength of the separator be greater than or equal to 1400 Kgf / cm ² to prevent mechanical damage due to collision.

In the present invention, the puncture strength of the separator is preferably 600 gf or more from the viewpoint of preventing damage to the separator due to breakage at the time of external impact and thereby preventing internal short circuit.

In the present invention, the thermal contraction rate of the separator is preferably maintained at 4% or less.

In the present invention, the separator may be polyethylene, polypropylene, a laminated film thereof, or the like.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings and examples of the present invention.

First, in relation to the present invention, a specific method for measuring tensile strength, elongation, puncture strength, and heat shrinkage of a separator, and a method for testing safety against external impact on a secondary battery will be described.

Tensile strength and elongation were measured by Hounsfield test equipment (TM) 0050 Model 100RC. Tensile test specimens were 40mm long, 10mm wide, 10kg maximum load, 20mm chuck spacing, and 100mm / min tensile speed. Record the maximum load and length at which this break occurs.

The sticking strength is measured by Katotech (TM) KES-G5 handy type compression tester. The tip radius of curvature is 0.5mm and the lowering speed is 2mm / sec.

The heat shrinkage rate shall be expressed in terms of the shrinkage length after leaving the sample about 15 cm long at room temperature for 1 hour after measuring the length at room temperature.

The round bar collision test discharges the battery that has completed the Mars charge and discharge process (formation) to 2.75 volts at 0.5C constant current mode at room temperature, and after charging for 30 hours in a constant current constant voltage mode of 0.5C and 4.2 volts for 3 hours, It is carried out using a lithium ion secondary battery in a standard charge state made of 100%. At this time, as usual with respect to the current 1C current refers to the amount of current that can charge and discharge the battery capacity in one hour. The cell is placed horizontally on a flat iron plate, and a round bar of a certain diameter (eg 15.8 mm) is placed on the cell in the axial direction perpendicular to each other. Free fall on the round bar at an additional constant height (610 ± 25 mm) of a certain mass (9.1 kg) to allow the round bar to collide with the battery.

In the round bar collision test, attach a thermocouple thermometer to the bottom of the cell and observe the surface temperature change after the crash. In addition, by measuring the open current voltage (OCV) of the battery to determine whether the internal short circuit. Then, the state of the internal electrode assembly and the short circuited part are observed through battery disassembly.

In general, the results of the round bar crash test can be divided into six typical patterns, as shown in Figure 2, usually regardless of the manufacturer of the cell. FIG. 2 is a graph showing temperature (° C.) change patterns with typical time (minutes) shown in a round bar collision test of a cylindrical lithium ion battery. FIG. In this case, the LO may be subdivided into L0-I and L0-II. Conventional cylindrical lithium ion secondary batteries can be divided into levels as shown in the table below in six steps depending on the phenomenon, in particular the rate of change of the surface temperature of the battery.

level L0 L1 L2 L3 L4 L5 phenomenon No change Leak Smoke below 200 ℃ Smoke over 200 ℃ Fire Explosion, burst

The OCV measurement is made after leaving the battery for 12 hours after a collision. Only when the level is L0, the normal voltage is maintained around 4 volts and the rest is 0 volts. Zero volts means a short circuit occurs inside the battery due to a collision.

The battery dismantling test consists of a method of dismantling the battery after a certain period of time after the collision to observe the appearance of each electrode component.

First, through the dismantling inspection of a battery having a conventional problem, it can be confirmed that damage or breakage of a separator or an electrode plate during a round bar collision occurs in the axial direction of the round bar, that is, in the transverse direction of the separator. At this time, the force acting on the breakage of the separator may be determined by the compressive force between the pole plates and the impact force acting in the longitudinal direction of the separator, which is the longitudinal direction of the battery.

Based on these judgments, a commercially available separator with various characteristics as the first comparative group was applied to a cylindrical lithium ion secondary battery of a conventional type, and a round bar crash test resulted in lateral failure and breakage of the separator. It can be confirmed that it has a correlation with elongation and sticking strength.

Table 2 below shows the puncture strength, longitudinal tensile strength, and longitudinal elongation of each of the comparative separator types of the first comparative group put into the battery used in the experiment, and Table 3 shows the batteries adopting each comparative separator type. The results of the round bar collision test are shown as the number of test subjects corresponding to each level of Table 1. However, different LOT classification means different process conditions, so it is difficult to compare the separators in different LOT, but the relative comparison is meaningful because two or more separators are included in each LOT. . Note is the determination of whether the test can be passed. In the development, levels L1 and L2 are able to pass the normal round impact test, and above L3 are rejected levels.

Type / characteristic Sting strength (gf) Longitudinal tensile strength (kgf / cm ² ) Longitudinal Elongation (%) T1 397 157 526 T2 341 208 372 T3 460 1200 280 T4 610 1400 90 T5 620 1500 225

LOT classification Separator Type L1 number L2 number L3 number L4 number L5 number Remarks One T1 0 3 0 One 6 FAIL T5 10 0 0 0 0 PASS 2 T3 5 5 0 0 0 PASS T4 8 2 0 0 0 PASS 3 T3 20 0 0 0 0 PASS T4 20 0 0 0 0 PASS T2 6 0 0 0 2 FAIL 4 T3 10 0 0 0 0 PASS T5 10 0 0 0 0 PASS 5 T3 10 0 0 0 0 PASS T5 10 0 0 0 0 PASS

In the above table, the separators T1 and T2 of the first comparative group are difficult to adopt because they do not pass the round bar collision test criteria, while the T3 type passes the test criteria, but passes the round bar collision test in the wearing inspection stage during mass production. There are cases where it is not possible to do so, which seems to require improvement. Therefore, it can be seen from the comparative example that the strength of the separator for passing the round bar collision test must be greater than at least 1200 kgf / cm, for example, about 1400 kgf / cm ² . On the other hand, in the case of the T3 type, it can be seen that the prick strength is lower than the T4 and T5 types, so the prick strength should be 600gf or more.

The test of the first comparative group described above uses a statistical method of performing experiments on and comparing different separators with different LOT. Comparing with these statistical methods is costly and time consuming and has limited accuracy. Therefore, it can be seen that the above numerical values are practical in view of obtaining numerical values that can be used substantially without problems, rather than obtaining a perfectly optimal value.

General specifications of secondary battery separators are well known to those skilled in the art. As a synthetic resin sheet used as a separator, polyethylene, polypropylene, and these laminated sheets are common. In this case, the molecular weight to be used is, for example, about 10 ⁵ g / mol and may be classified into a high molecular weight polymer, an ultra high molecular weight polymer, etc. according to the large and small molecular weight. If the molecular weight is too high, the material is too hard to be pulled out into a thin sheet, and if the molecular weight is too low, it is difficult to have the mechanical strength required for the battery.

Therefore, the materials and properties used as the separator material are determined within a certain range, and within that range, the properties are determined within a certain range according to the content of the composition material, molecular weight or molecular weight, additives, production method and processing technique of the separator. can see.

The problem in the present invention is that when used in batteries, it has the mechanical strength necessary for daily use (for example, the mechanical properties enough to pass round bar collision tests) while also meeting other characteristics that the separator must have produced. You have to have a last name.

As a method for increasing the strength of the separator, it is possible to increase the thickness of the separator, to change the components, molecular weight, or content of the molecular weight of the separator. However, the results of the round bar collision test and other characteristics of the battery according to these variables were found to have only a limited improvement when applying the conventional separator sheet processing method and the battery production method. For example, increasing the thickness of the separator improves the mechanical strength, but it is disposed in the miniaturization and high capacity of the battery, and when applied to the existing battery production conditions, the results of the round bar collision test showed no significant improvement.

One of the methods used to increase the strength of the separator is a method of stretching the sheet by applying a force to the sheet in the direction of increasing the strength. For example, in order to increase the tensile strength in a specific direction, stretching is applied with a constant force in the sheet production step in that direction to improve the tensile strength in that direction.

From this point of view, the round bar collision test of the battery which adopted the separator which raised the intensity | strength by longitudinal extension of the separator in the longitudinal direction of a battery among the existing separators was performed. In the first comparative group of the second comparative group, the longitudinal tensile strength of the T3 type which caused a problem in the round bar collision inspection during the mass production process was increased. Their longitudinal sepe radar tensile strength of approximately 1450 to both the 1750Kgf / cm ² 1400Kgf / cm ² level or more than the sting intensity is lower than 600gf to 510 to 550gf.

The round bar collision test was performed by changing the diameter of the round bar. The round bar crash test with a diameter of 15.8mm was maintained at the LO level. ² , only the comparative example, which had relatively high level of stab strength of 550gf, could be maintained within L1 level.

The mutual comparison in this comparative group confirmed once again that the strongest puncture strength and tensile strength in the test were more likely to pass the test. Particularly, in the case of small diameter round bar impact, the tendency of the sticking strength to be affected along with the longitudinal tensile strength was observed.

Meanwhile, the thermal exposure test and the overcharge test were performed together with the round bar collision test for the second comparative group. The thermal exposure test puts a lithium ion secondary battery in a standard state of charge into a chamber and increases the temperature at a temperature increase rate of 5 degrees Celsius per minute from room temperature to 150 degrees Celsius and maintains the temperature at 150 degrees Celsius for 1 hour. Observe. In order to pass the test, it should be kept below the L1 level for 10 minutes at 150 ° C. The overcharge test is a measure of whether a standard charged cell can be kept at L0 level when charged for 2 hours at 2C, 18.5 volts constant current, and constant voltage after wrapping it with insulation.

The results of both the thermal exposure test and the overcharge test for the second comparative group did not pass the criteria. This may be considered to be due to the increase in the heat shrinkage of the entire separator and the decrease in the elongation as a result of the addition of the longitudinal stretching to increase the tensile strength. In other words, the thermal contraction of the separator may cause the two electrodes in the battery to short-circuit, resulting in overheating, fire, and explosion.

On the other hand, the thermal exposure test did not pass the standard even in the T5 type, which showed sufficient mechanical strength in the first comparative group, and also passed the overcharge and thermal exposure test standards in the comparative example in which the longitudinal tensile strength was strengthened in the T4 type. Did not show a problem.

Therefore, it is a problem that the conventional separator does not satisfy other safety standards of the battery due to problems such as heat shrinkage by only stretching in the longitudinal direction to increase the longitudinal tensile strength.

In order to overcome the problem of this comparative example, a method is required to increase the longitudinal tensile strength of the separator, to lower the overall heat shrinkage rate, and to allow the melting temperature to be higher than or equal to a predetermined range in a possible range.

In order to satisfy this request, the present invention adopts lowering of the transverse tensile strength of the separator while increasing the longitudinal tensile strength of the existing separator. In addition, reducing the content of low molecules in conventional polymer materials for separators, such as polyethylene, may also be adopted.

In the case of the polyethylene or polypropylene sheet, it is common to perform a process of stretching the sheet to a certain extent in this direction by a force acting in the winding direction, that is, the direction in which the sheet is taken out, in order to make the roll. In this process, the stretching of the sheet in the lateral direction is more than necessary, resulting in an increase in the thermal contraction rate of the sheet as a whole and a lowering of the stretching rate.

In the case of round bar collision test, the middle part of the cylindrical battery in the longitudinal direction or the axial direction is placed with the round bar perpendicular to the longitudinal direction, and in this case, the can, the electrode plate, or the separator and the round bar are less in contact with each other. It may be because. In other words, if the test passes, it can withstand the physical impact on the battery in the other direction. In consideration of this, it is conceivable that the longitudinal tensile strength improvement through the longitudinal stretching or the like which is susceptible to impact is more necessary than the transverse stretching of the sheet.

Therefore, in the present invention, at least the longitudinal tensile strength of the separator is higher than the transverse tensile strength. In other words, the thermal shrinkage rate that may be in a trade-off relationship with the overall tensile strength while increasing resistance and resistance in the longitudinal direction of the separator, which is substantially susceptible to external shock of the battery, meets the existing safety standards. To keep it low.

Hereinafter, based on the T4 type of the first comparative group showing a relatively good separator characteristics, the mechanical and thermal safety and the third fertilizer group and the embodiments of the present invention are partially changed by the production and processing methods thereof. Review the relevant characteristics and safety test results. For reference, the examples are labeled with S and the comparative examples are labeled with N and given serial numbers.

The T4 type is a type of separator mainly made of polyethylene, and based on the modification of which the lateral tensile strength is slightly reinforced, N1 and S1 are made of the same polyethylene raw material, and S2, S3, N2, and N3 are relatively polyethylene. Reduced heat shrinkage by reducing the low molecular weight in the middle, N4 is relatively increased in the low molecular weight in the polyethylene, N1, N2 is increased in the content per unit volume of the polyethylene, Examples S1, S2, S3 of the present invention in common Instead of increasing the directional tensile strength, the lateral tensile strength was reduced.

Table 4 shows the measured values for their important properties, and Table 5 shows the results of the tests related to their mechanical and thermal safety in the upper, middle and lower directions. At this time, the level 'low' is the lower level among those that passed the test, so it is not rejected. In the inspection items, the comparative examples do not fail, but the characteristics are inferior due to the relatively high 'ha'.

Attribute / Type N1 N2 N3 N4 S1 S2 S3 Thickness (μm) 20 20 20 20 20 20 20 Porosity 330 290 220 430 260 260 230 Stinging strength 600 630 560 540 600 620 610 Transverse tensile strength 1500 1700 1240 1400 1350 1400 1450 Longitudinal tensile strength 1200 1400 1200 1200 1500 1450 1500 Transverse elongation 65 90 60 80 80 70 55 Longitudinal Elongation 80 130 100 110 100 80 100 Lateral Heat Shrinkage 3.8 2.8 2.3 4.3 3.3 3 2.7 Longitudinal heat shrinkage 0 0 0 0 0 0 0

Target / Type of Inspection N1 N2 N3 N4 S1 S2 S3 Round bar conflict medium Ha Ha Ha Prize medium Prize Overcharge Ha medium medium medium Prize medium medium Heat exposure Ha Ha Ha Ha Ha Ha Ha

Considering the production and processing methods in which the separators of each of the third comparative example and the examples of Table 4 are formed, and comparing the inspection results of Table 5, a method that can ensure mechanical and thermal safety evenly can be generally used. On the premise of a commercially available separator, it can be deduced that the transverse tensile strength is relatively lowered while increasing the longitudinal tensile strength of the separator in the battery longitudinal direction as in the present invention.

In addition, as described above, in the process of manufacturing a separator, the separator sheet receives a lateral force and has a larger transverse tensile strength, and considering the embodiments of the present invention shown in Table 4, the width of the separator in the present invention. Longitudinal tensile strength relative to directional tensile strength of greater than 1 can reduce the overall heat shrinkage and have a practical resistance to longitudinal external impacts that are vulnerable in the cell (passing round rod collision tests).

On the other hand, in order to have a mechanical thermal safety in general, in consideration of the characteristics of the separator and the test results of the first comparative example, the longitudinal tensile strength, puncture strength and thermal contraction rate of the separator are separately defined by using the numerical values presented as safety lines. It is preferable. That is, 1400 Kgf / cm ² or more for longitudinal tensile strength and 600 gf or more for sting strength. On the other hand, considering the heat shrinkage ratio of Tables 4 and 5 and whether each inspection passes (maximum value of 4.3%), the heat shrinkage ratio is sufficient if the separator is maintained within 4% while maintaining the mechanical strength that corresponds to the collision test. I can think of it.

Irrespective of the gist of the present invention, other characteristics of the cell adopting the comparative example and the embodiments are also examined, but the most important aspects related to the object of the present invention are included in Table 5, and there are many similar parts, which have a decisive influence on the adoption. Since it is not enough, it is omitted in the detailed description.

Although the present invention has been mainly described with a cylindrical can type lithium ion secondary battery having a jelly roll type electrode assembly, the present invention may also be applied to square and pouch type lithium ion secondary batteries having an electrode assembly.

According to the present invention, it is possible to produce a lithium ion secondary battery having a strong separator that can improve the safety of the battery while basically maintaining the material, composition, content, etc. of the existing separator.

That is, according to the present invention, it is possible to prevent the mechanical damage of the separator against an external mechanical impact, and at the same time to produce a lithium ion secondary battery comprising a separator having suitable characteristics that can maintain thermal stability.

Claims (9)

An electrode assembly having a separator provided on a surface where the two electrodes and the two electrodes contact each other, wherein the separator and the laminate of the two electrodes are wound to form a jelly roll type, In a lithium ion secondary battery comprising a case in which the electrode assembly is accommodated, The longitudinal tensile strength of the separator in the battery longitudinal direction is made larger than the transverse tensile strength of the separator, characterized in that the lithium ion secondary battery. The method of claim 1, The longitudinal tensile strength of the separator is 1400Kgf / cm ² or more, characterized in that the lithium ion secondary battery. The method according to claim 1 or 2, The puncture strength of the separator is characterized in that 600gf or more. The method of claim 3, wherein The thermal contraction rate of the separator is 4% or less, lithium ion secondary battery. The method of claim 1, The thermal contraction rate of the separator is 4% or less, lithium ion secondary battery. The method of claim 1, The separator is a lithium ion secondary battery, characterized in that made of one of a laminated film of polyethylene, polypropylene, polyethylene and polypropylene. The method of claim 1, The case is a lithium ion secondary battery, characterized in that the cylindrical can. delete The method of claim 6, The separator is a lithium ion secondary battery, characterized in that the low molecular weight content of less than 100,000 g / mol of the polymer constituting the separator compared to the content of the polymer.

Images