KR20010011900A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: A secondary lithium battery is provided to improve a safety of a battery with respect to a high temperature and an overcharge by providing a safety device operating according to a temperature increase of a battery. CONSTITUTION: An electrode plate group(10) includes anode plates(12) and cathode plates(14) wound between partition films(16). A case(20) receives the electrode plate group(10) and is electrically connected to the cathode plates(14). A cap assembly(30) is sealed in an upper opening of the case(20) and is electrically connected to the anode plates(12). A safety device(40) is inserted and fixed into a center of the electrode plate group(14). The safety device(40) includes a conductive center bar(42) and a nonconducting cover(46). The conductive center bar(42) is electrically connected to the cathode plates(14). The nonconducting cover(46) is arranged between the cap assembly(30) and conductive center bar(42) and is melted at a set temperature.

Description

Lithium Secondary Battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery having improved safety against high temperature and overcharging.

In recent years, as the spread of high-performance notebook computers and cordless phones has been expanded, the demand for high-performance secondary batteries with high energy density has exploded.

Therefore, as a result of many years of research to meet this, a lithium secondary battery employing a negative electrode composed of lithium or a material capable of inserting and discharging lithium and a positive electrode composed of a material capable of reversibly inserting and discharging lithium appeared. In recent years, lithium secondary batteries have been actively studied as power sources for hybrid electric vehicles including electric vehicles or hybrid electric vehicles, as well as portable electronic devices.

In general, lithium secondary batteries use lithium metal, lithium-containing metal, and a material capable of reversible insertion and release of lithium as a negative electrode material, and research on new materials is being actively conducted.

In the lithium-containing metal, a structure in which lithium is contained in a skeleton such as aluminum, tin, antimony, or silicon is mainly studied, and carbonaceous materials are widely used for materials capable of reversible insertion and release of lithium. However, a variety of metal oxide and chalcogenide materials that can be reversibly intercalated in and out of lithium are also available as cathode materials.

Meanwhile, lithium metal oxides, metal oxides, and metal chalcogenide compounds capable of reversible intercalation of lithium may be used as a cathode material of a lithium secondary battery.

As the lithium metal oxide, a layered structure oxide or spinel structure oxide represented by the composition formula of LiMO ₂ (M = Co, Ni, Mn) and lithium (transition metal 1, transition metal 2) oxide Li (M1, M2) O ₂ Materials represented by the compositional formula of (M1 = Ni, Co, Mn, M2 = Ni, Co, Mn, Al, Mg) are mainly used.

In addition, the metal oxide and the metal chalcogenide compound are materials represented by the formula of M _x O _y , M _x S _y , M _x Se _y , and representative examples thereof include V ₂ O ₅ , V ₆ O ₁₃ , and MoO. ₂ , MnO ₂ , MoO ₃ , TiS ₂ , NbSe ₃ , and the like.

In addition, as a separator of a lithium secondary battery, a porous polyolefin membrane, a nonwoven fabric, and the like are used, and a non-conductive separator having no ion conductivity and an ion conductive separator are used. As the ion conductive separator, a gel-like separator containing a lithium ion conductive softener and a polymer electrolyte in which the polymer itself has lithium ion conductivity have been used or developed.

Representative examples of the battery composed of each of these components is a lithium ion battery composed of a carbonaceous negative electrode, a lithium metal oxide positive electrode and a polyolefin separator, a typical battery of such a structure in WO9,508,211, US 5,631,100, etc. It is proposed and is currently widely produced and used worldwide.

A battery using an active material similar to the above lithium ion battery, but using an ion conductive gel phase separator as a separator, is generally called a lithium ion polymer battery, which has also been recently produced.

A typical lithium ion polymer type battery is a belcore type polymer secondary battery, and its configuration is proposed in US Pat. No. 5,396,318.

The battery proposed in the patent uses a carbonaceous material, a graphite material heat-treated at a high temperature of about 2,500 ° C. or more, and hard carbons having a low graphitization degree. As a cathode material, LiMO ₂ (M = Co, Ni) is typically used. , A layer structure oxide and a spinel structure oxide having a composition formula of Mn) are used.

The operating voltage of the battery is 3.7 V on average, and the normal operating range is set to about 3.0 to 4.2 V. The operating potential of such a battery is much higher than that of a conventional lead battery or a NiCad battery, which is a great advantage in terms of energy density.

However, due to the high operating potential of the battery, a large overcurrent may flow momentarily during an internal short circuit due to an external short circuit and a mechanical shock, thus causing a drawback of deteriorating the safety of the battery. In addition, the positive electrode materials used in the battery greatly increase the chemical activity during overcharging, thereby rapidly reacting with the electrolyte to produce excessive gas, thereby rapidly increasing the internal pressure of the battery, or rapidly increasing the internal temperature of the battery through an exothermic reaction. I have a problem.

Since the internal pressure and temperature rise due to such chemical reactions can lead to explosion of the battery under extreme charge and high temperature environments, measures for battery safety have been taken in various aspects.

Usually, in order to prevent overcharging of a battery, a charger is designed or a protection circuit is incorporated in the battery pack so as to prevent excessive rise of the charging voltage.

In addition, in consideration of a safety accident that may occur when such a protection device is not operated, US Patent No. 5,631,100 discloses a device in which a resistance increases with increasing temperature in a battery internal structure, that is, a positive thermal coefficeint (PTC) device. When the temperature rise of the battery continues to be inserted to block the flow of overcharge current. In addition, US Pat. No. 4,943,497 proposes a safety valve that cuts off current by using a gas withstand pressure of a battery generated in an overcharged state.

However, the PTC device has a limit that can not only reduce the inflow of energy from the outside by using an increase in resistance, but also can not mitigate the risk of the reaction heat generated inside the battery. And the safety valve is operated by the internal pressure of the battery to suppress the inflow of energy from the outside by blocking the external circuit, and has a function of releasing the gas generated by the internal reaction to the outside, but of the overcharged battery There is a disadvantage that thermal runaway and consequent explosion cannot be suppressed.

On the other hand, when the internal short circuit occurs by the external mechanical shock in the state of charge, US Patent No. 5,747,188 proposed a configuration using a center pin so that the internal short circuit in a safer form.

In the patent, the center pin is also mechanically deformed by a mechanical shock that causes an internal short circuit, causing a separate internal short circuit, and heat generation and dissipation are controlled by a separate internal short circuit by the center pin. Improve stability.

However, the above-described center pin has no limit in improving the stability of the battery under overcharge or high temperature environment unless mechanical deformation occurs.

Therefore, the present invention is designed to solve the above problems, an object of the present invention to provide a safety device that is operated by the temperature rise of the battery to improve the safety of the battery high temperature and overcharging of the rechargeable battery can be effectively improved To provide.

1 is a cross-sectional view of a lithium secondary battery according to the present invention.

2 is an enlarged perspective view of the safety device.

In order to achieve the above object, the present invention,

The electrode plate group in which the positive electrode plate and the negative electrode plate are wound between the separators,

A container housing the electrode plate group and electrically connected to the negative electrode plate;

A cap assembly sealed in an insulated state from the container by a gasket at an opening of the container and electrically connected to the positive electrode plate;

An inner short circuit is caused by a temperature increase by having a conductive center rod inserted into and fixed to the center of the electrode plate group and electrically connected to the negative electrode plate, and a non-coated cover portion disposed closely between the cap assembly and the conductive center rod and melting at a set temperature. It provides a lithium secondary battery comprising a safety device.

The safety device may further include a short-circuit junction between the non-conductive cover part and the conductive center bar having a greater resistance than the conductive center bar, for example, made of a PTC material, and is safe when the battery temperature rises due to overcharge or an increase in external temperature. The device primarily induces an internal short in a safe form as the non-conductive cover is melted, and secondly blocks the flow of excessive short circuit and overcharge current as the resistance of the short junction increases.

Such safety devices eliminate the risk of thermal runaway and explosion and ignition due to temperature rise, in particular inducing a safe form of internal short-circuit at a suitable temperature. This has the advantage of effectively improving the safety against high temperature and overcharge of the lithium secondary battery.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a cross-sectional view of a lithium secondary battery according to the present invention, in particular a cross-sectional view of a cylindrical lithium ion secondary battery. As shown in the drawing, the lithium secondary battery includes the electrode plate group 10 in which the positive electrode plate 12 and the negative electrode plate 14 are wound together with the separator 16, a container 20 containing the electrode plate group 10, and the above-mentioned. The cap assembly 30 sealed in the upper opening of the container 20 and the safety device 40 inserted and fixed in the center of the electrode plate group 10 are included.

The positive electrode plate 12 of the lithium ion secondary battery is, for example, after mixing the powder of a conventional transition metal oxide in which the insertion and release of lithium is reversible to the carbon conductor, and then the polyvinylidene fluoride (PVdF) binder and N- It can be formed by dispersing in a binder solution composed of methylpyrrolidinone (NMP) solvent and then applying to an aluminum current collector.

The negative electrode plate 14 is made of a crystalline carbon material having a high degree of crystallinity, and in particular, may be made of spherical or fibrous mesophase pitch-based graphitized carbon having a high packing density.

A cylindrical 18650 battery can be assembled by using the positive electrode plate 12 and the negative electrode plate 14 formed as described above, and the separator 16, the electrolyte solution, and other components are well-known for use in a conventional lithium ion secondary battery. As the container 20 is in contact with the negative electrode plate 14 serves as a negative electrode terminal, the cap assembly 30 is in contact with the positive electrode tab 32 formed from the positive electrode plate 12 to serve as a positive electrode terminal.

The cap assembly 30, which is the positive terminal of the battery, includes a positive electrode tab welding plate 34 to which the positive electrode tab 32 is welded, a PTC element 36 disposed around the upper end of the positive electrode tab welding plate 34, and a PTC element ( The cap cover 38 is disposed on the upper end of the cap 36, and the positive electrode tab welding plate 34, the PTC element 36, and the cap cover 38 are formed by a gasket 50, which is a negative electrode terminal of the battery. Insulated with 20). By the configuration of the cap assembly 30 described above, the current of the positive electrode plate 12 is transmitted to the cap cover 38 via the positive electrode tab 32, the positive electrode tab welding plate 34, and the PTC element 36. .

Since the lithium secondary battery having such a structure is composed of an electrode material having high activity, when the reaction between the electrolyte and the electrode material is not properly controlled, the internal pressure of the battery increases due to gas generation, and the exothermic reaction causes The temperature of the battery inevitably rises.

Such reactions cause more serious problems, especially when the chemical activity is increased due to excessive charging and when the battery's external temperature is sufficiently high, because the reaction rate between the electrolyte and the electrode material becomes abnormally large, and the heat of the battery is increased. The risk of runaway or immediate explosions is abnormally increased.

Accordingly, the present inventors intensively studied the paths of abnormal thermal runaway and explosion as described above, and found that internal short circuits between the electrodes in the battery are the fundamental factors causing thermal runaway and explosion.

The occurrence of such an internal short circuit causes a rapid discharge of the electrode, causing local overheating of the short circuit site and an overall temperature rise of the battery. The temperature rise of such cells further increases the chemical reactivity between the electrode material and the electrolyte, causing further temperature rise, which in turn leads to thermal runaway and explosion or ignition.

Therefore, the present inventors provide a safety device that can not only adjust the degree of temperature rise by arbitrarily adjusting the internal short circuit resistance and the internal short circuit position, but also facilitate heat dissipation and further increase the safety of the battery.

FIG. 2 is a detailed perspective view of the safety device, wherein the safety device 40 includes a conductive center bar 42 inserted into and fixed to the center of the pole plate group 10, and a non-coated cover part covering the upper end of the conductive center bar 42 to a predetermined length. (46) and a short-circuit junction (44) positioned at the upper end of the conductive center rod (42) facing the nonconductive cover portion (46) and made of a material having a higher resistance than the conductive center rod (42).

The conductive center rod 42 has a lower end thereof electrically connected to the negative electrode plate 14 in contact with the negative electrode tab 22, and the non-conductive cover portion 46 has a cap assembly 30, which is more precisely surfaced. Is in contact with the positive electrode tab 32, and is configured to melt at a specific temperature range, more specifically, made of a polymer material that can be melted or liquefied at approximately 80 to 150 ° C.

The normal battery operates normally up to 60 ° C., and should be stable against storage at an environment of an external temperature of 80 ° C., so that the non-conductive cover portion 46 is set to melt at 80 ° C. or higher. In addition, since the operation of the safety device is effective only before a natural internal short circuit, the non-seal lid 46 may be formed at a temperature of 130 ° C., which is a melting temperature of a conventional polyethylene separator, or 150 ° C. or less, which is a melting temperature of a polypropylene separator. Is set to melt.

Therefore, at room temperature, the positive electrode tab 32 and the conductive center bar 42 are insulated by the non-conductive cover part 46 to prevent internal short circuit.

In addition, the short-circuit junction 44 is a material having a larger resistance than the conductive center bar 42 and may be made of a resistor element or a PTC material having a specific set resistance. The electrical resistance of the short-circuit junction 44 is 0.05 to 5 Ω at 100 ° C. or lower, and preferably 10 Ω or higher at 150 ° C. or higher.

The short-circuit junction 44 suppresses the risk of temperature rise because the rate of deactivation of the battery due to self-discharge is greater than the rate of temperature rise by a normal chemical reaction, while suppressing the forced short-circuit current to be below the maximum current allowed. The upper and lower limits of the electrical resistance are set as above according to the temperature.

In other words, if the electrical resistance of the short-circuit junction 44 exceeds the upper limit, the forced self-discharge according to the operation of the safety device 40 is too slow, resulting in insufficient activity of the battery. Even if a forced internal short circuit occurs, the heating effect caused by this causes a temperature increase of the battery.

Referring to the operation of the safety device 40 made in the above configuration in more detail as follows.

As an example, when the battery charged inside the hot box is left and the internal temperature of the battery reaches a set temperature (80 to 130 ° C.), the non-conductive cover portion 46 covering the upper end of the conductive center rod 42 melts. As a result, the conductive center rod 42, particularly the short junction 44, contacts the positive electrode tab 32 to cause an internal short circuit of the battery.

Due to the internal short circuit, the battery gradually loses its activity due to self discharge. The internal short circuit caused by the melting of the non-coated lid 46 may control the internal resistance of the battery, unlike a randomly generated short circuit. The temperature rise rate can be easily adjusted by the resistance value of the electroconductive center rod 42 and especially the short circuit junction 44.

Since a current flows through the short circuit junction 44 having a specific set resistance, the short circuit current has a value smaller than the short circuit current due to an internal short circuit that occurs arbitrarily, and when the short circuit junction 44 is made of a PTC material, Since the resistance of the short junction 44 is adjusted according to the temperature, there is an advantage that the temperature of the short junction 44 can also be maintained at or below a set value.

As such, the internal short circuit due to melting of the non-coated lid portion 46 is caused earlier than any internal short circuit due to the melting of the separator 16, so when the temperature of the battery reaches the melting temperature of the separator 16, The state of charge is lowered so that sufficient reactivity is lost as long as thermal runaway is reached. Therefore, the battery explosion and ignition as well as the thermal runaway of the battery effectively suppressed.

In addition, the battery according to the present embodiment, in the same manner as above, the safety device 40 operates even when overcharge occurs to improve the safety of the battery. First, when the internal temperature of the battery reaches the set temperature of the non-covered cover 46, the non-covered cover 46 melts, causing a short circuit of the battery through the conductive center rod 42 and the short-circuit junction 44. do. At this time, since the short-circuit current flows through the short-circuit junction 44, heat generation inside the battery is concentrated around the terminals of the battery.

And the PTC element 36 inserted into the cap assembly 30 is connected in series with the short-circuit junction 44 in the safety device 40, the temperature rise due to the internal heat generation of the battery, secondary to the safety device 40 The resistance of the short junction 44 and the PTC element 36 inside the cap assembly 30 is rapidly raised to block the flow of the overcharge current.

At the same time, the safety of the battery is improved by reducing the state of charge of the electrode material due to self-discharge through the safety device 40, and the chemical activity is lowered even when the PTC element 36 is not employed inside the cap assembly 30. Due to this, it is possible to prevent the explosion of the battery due to thermal runaway.

Example 1

Only the safety device 40 was inserted into the battery container 20 without the electrode plate group 10, the cap assembly 30 was covered, and then sealed by crimping. As the cover portion 46 of the safety device 40, a polyethylene separator having a melting point of 135 ° C. was used, and a resistor having a resistance of approximately 500 mΩ was attached to the short-circuit junction 44. The central rod 42 was produced by winding an imide tape (manufactured by Nitto Denko Co., Ltd.) on a copper rod. The battery sample was placed in an oven at 150 ° C. and the resistance was measured. The surface temperature of the sample reached 150 ° C. after about 10 minutes and the resistance reached about 800 mΩ.

Comparative Example 1

90% by weight of lithium cobalt oxide (LiCoO ₂ ) (manufactured by Japan Chemical), 4% by weight of acetylene black (manufactured by Severon, USA), and 6% by weight of Kynar 761 (manufactured by Atochem, USA) as a binder A positive electrode paint was prepared, and the positive electrode paint was applied onto an aluminum foil having a thickness of 20 µm to prepare a positive electrode plate 12.

MCMB 10-28 (manufactured by Osaka Gas Co., Ltd.) 90% by weight and 10% by weight of KINA 761 as a binder are mixed to form a negative electrode coating, and the negative electrode coating is applied to a copper foil having a thickness of 10 μm. The negative electrode plate 14 was manufactured.

Celgard 2400 (manufactured by Celanese, Inc.) was used as the separator 16, and lithium hexafluorophosphate (LiPF ₆ ) was dissolved in an ethylene carbonate (EC) and diethylene carbonate (DEC) mixed solvent as an electrolyte. It was.

Using the positive electrode plate 12, the negative electrode plate 14, the separator 16 and the electrolyte solution, a battery of a standard 18650 standard was assembled, and a hot box experiment was performed under the conditions shown in Table 1 below. The battery charged to different potentials was left in a hot box at a constant temperature for 2 hours and checked for explosion.

TABLE 1

division Hot box temperature (℃) Charge voltage (V) Explosion Experimental Example 1 140 4.2 Unexploded Experimental Example 2 150 4.2 explosion Experimental Example 3 140 4.3 explosion Experimental Example 4 140 4.5 explosion

Example 2

A battery was assembled in the same manner as in Comparative Example 1 except that the safety device 40 was inserted in the center of the battery container 20. In this case, polyethylene-co-1butene having a melting point of 90 ° C. was used as the cover 46 of the safety device 40, and a resistor having a resistance of about 500 mΩ was used as the short-circuit junction 44. As the center rod 42, a copper rod was coated with Teflon coating. Hot box experiment was performed under the same conditions as in Comparative Example 1.

TABLE 2

division Hot box temperature (℃) Charge voltage (V) Explosion Experimental Example 1 140 4.2 Unexploded Experimental Example 2 150 4.2 Unexploded Experimental Example 3 140 4.3 Unexploded Experimental Example 4 140 4.5 Unexploded

Example 3

An overcharge test was performed using four 18650 batteries prepared as in Example 2. The overcharge test was performed by setting the current of 3 A, the maximum voltage up to 20 V, for a total of 1 hour 30 minutes. All four cells did not explode for a given time.

Example 4

Except for removing the PTC element 36 mounted on the cap assembly 30, the overcharge test was carried out under the same conditions as in Example 3 using four 18650 cells manufactured as in Example 2. All four cells did not explode in a given time.

Comparative Example 2

An overcharge test was performed under the same conditions as in Example 3 using four 18650 batteries prepared as in Comparative Example 1. One cell exploded during a given time.

Comparative Example 3

Except for removing the PTC element 36 mounted on the cap assembly 30, the overcharge test was carried out under the same conditions as in Example 3 using four 18650 cells manufactured as in Comparative Example 1. All four cells exploded in a given time.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the claims and the detailed description of the invention and the accompanying drawings. Naturally, it belongs to the range of.

As described above, the lithium secondary battery according to the present invention has a built-in safety device, which primarily causes an internal short circuit in response to an increase in internal temperature, and secondaryly blocks the flow of overcharge current, and at the same time, induces a forced self discharge, thereby degrading battery activity. . Since the internal short circuit caused by the safety device is caused at a temperature lower than the internal short circuit due to the melting of the separator, it prevents excessive temperature rise of the battery and effectively suppresses the risk of thermal runaway and explosion and fire of the battery.

Claims (9)

An electrode plate group in which a cathode plate and an anode plate are wound between separators; A container housing the electrode plate group and electrically connected to the negative electrode plate; A cap assembly sealed insulated from the container by a gasket at an opening of the container, the cap assembly being electrically connected to the positive electrode plate; An inner short circuit is caused by a temperature increase by having a conductive center rod inserted into and fixed to the center of the electrode plate group and electrically connected to the negative electrode plate, and a non-coated cover portion disposed closely between the cap assembly and the conductive center rod and melting at a set temperature. Lithium secondary battery comprising a safety device. The safety device of claim 1, wherein the safety device is The lithium secondary battery further comprises a short-circuit junction positioned on an upper portion of the conductive center bar facing the non-conductive cover part and made of a material having a higher resistance than the conductive center bar. The lithium secondary battery of claim 1, wherein the conductive center rod is in contact with the negative electrode tab. The lithium secondary battery of claim 1, wherein a melting temperature of the non-coating cover part is set lower than a melting temperature of the separator. The lithium secondary battery of claim 1, wherein the non-coating cover part is made of a polymer material that is melted at 80 ° C. to 150 ° C. 6. The lithium secondary battery of claim 1, wherein the non-coverable cover part is in contact with the positive electrode tab of the cap assembly. The lithium secondary battery of claim 2, wherein the short-circuit junction part is made of a PTC material having an increased resistance due to an increase in temperature to block the flow of an overcharge current at a set temperature. The lithium secondary battery according to claim 2, wherein the electrical resistance of the short-circuit junction is 0.05 to 5 Ω at 100 ° C or lower, and 10 Ω or higher at 150 ° C or higher. The lithium secondary battery of claim 1, wherein the cap assembly further includes a PTC device.