Lithium secondary battery

A thermally stabilized Li secondary battery in which at least a portion of the inner surface of a secondary battery case includes an ignition inhibitor so as to prevent an internal temperature of the secondary battery from rising to a predetermined temperature or higher. The Li secondary battery includes: an electrode assembly which includes a first electrode plate, a second electrode plate, and a separator interposed between the first electrode plate and the second electrode plate; a secondary battery case which includes an internal space for accommodating the electrode assembly; and an ignition inhibitor which is formed on at least a portion of the inner surface of the secondary battery case.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 2005-69785, filed on Jul. 29, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An aspect of the present invention relates to a lithium (Li) secondary battery, and more particularly, to a thermally stabilized Li secondary battery in which at least a portion of the inner surface of a secondary battery case includes an ignition inhibitor so as to prevent an internal temperature of the secondary battery from rising to a predetermined temperature or higher.

2. Description of the Related Art

Recently, a large number of compact and light-weighted electric or electronic apparatuses such as cellular phones, notebooks, and camcorders have been actively developed and produced. These portable electric or electronic apparatuses are provided with battery packs, so that they can operate without separate power supplies. A battery pack includes at least one battery to output a predetermined voltage so as to drive the portable electric or electronic apparatuses for a predetermined time.

Recently, the battery pack has employed a rechargeable secondary battery for economic benefit. Examples of the secondary battery include: a nickel cadmium (Ni—Cd) battery; a nickel hydride (Ni-MH) battery; and a Li secondary battery such as a Li polymer battery and a Li-ion battery.

Particularly, the Li secondary battery has an operating voltage of 3.6V which is three times higher than that of the Ni—Cd battery or the Ni-MH battery. In addition, the Li secondary battery has a high energy density per unit weight. Therefore, demand for Li secondary batteries has rapidly increased.

In the Li secondary battery, a Li-based oxide is used as a positive electrode active material, and carbon is used as a negative electrode active material. In general, batteries are classified into a liquid electrolyte battery and a polymer electrolyte battery according to a type of the electrolyte. The Li secondary battery using the liquid electrolyte is called a Li-ion battery, and the Li secondary battery using the polymer electrolyte is called a Li polymer battery. Li secondary batteries are manufactured in various shapes. According to the shapes, the Li secondary batteries are classified into a cylindrical battery, a prismatic battery, and a pouch-type battery.

A Li-ion secondary battery includes: an electrode assembly which is formed by rolling a positive electrode plate coated with a positive electrode active material, a negative electrode plate coated with a negative electrode active material, and a separator interposed between the positive and negative electrode plates to prevent an electrical short between the two electrode plates and allow only Li-ion to pass therethrough; a secondary battery case which accommodates the electrode assembly; and an electrolyte solution which is inserted inside the secondary battery case and allows Li-ion to move.

In the Li-ion secondary battery, a positive electrode plate, on which the positive electrode active material is coated and to which a positive electrode tab is connected, a negative electrode plate, on which the negative electrode active material is coated and to which a negative electrode tab is connected, and a separator, are laminated and rolled so as to obtain an electrode assembly.

The electrode assembly is accommodated in the Li-ion secondary battery case and fastened so as not to be detached. Thereafter, the electrolyte solution is injected into the Li-ion secondary battery case, and, an opening portion of the case is sealed with a cap assembly, thereby constituting the Li-ion secondary battery.

However, heat is generated inside the Li secondary battery at charging/discharging times through an electrochemical reaction. In particular, an internal heating value becomes large when abrupt discharging or over-charging occurs, and thus an internal temperature of the battery rapidly increases. As a result, an internal structure changes, thereby generating an electrical short inside the battery. This causes significant danger such as explosion of the Li secondary battery.

SUMMARY OF THE INVENTION

An advantage of some aspects of the invention is to provide a thermally stabilized Li secondary battery in which at least a portion of the inner surface of a secondary battery case includes an ignition inhibitor so as to prevent an internal temperature of the secondary battery from rising to a predetermined temperature or higher.

According to an aspect of the present invention, there is provided a Li secondary battery, the Li secondary battery including an electrode assembly which includes a first electrode plate, a second electrode plate, and a separator interposed between the first electrode plate and the second electrode plate; a secondary battery case which includes an internal space for accommodating the electrode assembly; and an ignition inhibitor which is formed on at least a portion of the inner surface of the secondary battery case.

According to another aspect of the present invention, the ignition inhibitor may include: an ignition inhibiting material layer which is formed on at least a portion of the inner surface of the secondary battery case, and is made of an ignition inhibiting material; and a separation film which separates the ignition inhibiting material from an electrolyte solution.

According to another aspect of the present invention, the separation film may initially melt at a predetermined temperature of 80° C. or higher, so that the ignition inhibiting material can be inserted inside the battery. Further, the separation film may be made of a thermoplastic material or paraffin.

According to another aspect of the present invention, the ignition inhibiting material may be composed of only a material layer that is inserted and diffused inside the battery so as to restrict temperature rising at the predetermined temperature of 80° C. or higher without having to use the separation film.

According to another aspect of the present invention, the ignition inhibiting material layer may be made of NaCl.

According to another aspect of the present invention, the secondary battery case may be a cylindrical metal case having an open upper portion, a box-type metal case having an open upper portion, or a pouch-type case. In the pouch-type case, the ignition inhibiting material layer may be laminated on a metal film surface of the pouch, that is, the inner surface of the pouch, and then a heat sealable resin layer may be laminated thereon.

According to another aspect of the present invention, the Li secondary battery may further include a cap assembly which is bonded with an upper portion of the secondary battery case so as to seal the upper portion of the secondary battery, when the case is a cylindrical metal case or a box-type metal case having an open upper portion.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1Ais a perspective view of cylindrical lithium (Li) secondary battery according to an embodiment of the present invention.FIG. 1Bis a cross-sectional view of the Li secondary battery ofFIG. 1A, viewed along the A-A line.

Referring toFIGS. 1A and 1B, a cylindrical Li secondary battery100includes: an electrode assembly110which generates a voltage difference at charging/discharging times; a secondary battery case120which accommodates the electrode assembly110; a cap assembly130which is assembled at an upper portion of the secondary battery case120and prevents the electrode assembly110from separation; an electrolyte solution140which is injected into the secondary battery case120to allow Li-ion to move in the electrode assembly110; and an ignition inhibitor150which prevents an internal temperature of the cylindrical Li secondary battery100from rising due to abnormal heat generation.

The electrode assembly110includes: a first electrode plate111coated with a positive (or negative) electrode active material, for example, the positive electrode active material; a second electrode plate112coated with the negative (or positive) electrode active material, for example, the negative electrode active material; and a separator113which is interposed between the first electrode plate111and the second electrode plate112, which prevents the first and second electrode plates111and112from creating an electrical short, and allows Li-ion alone to move. Further, the first electrode plate111, the second electrode plate112, and the separator113are rolled in a substantially circular shape, and are accommodated in the secondary battery case120. The first electrode plate111is generally made of aluminum (Al), and is bonded with a first electrode tab111aprotruding upwards a specific length. The second electrode plate112is generally made of nickel (Ni), and is bonded with a second electrode tab112aprotruding downwards a specific length. However, an aspect of the present invention is not limited thereto. Moreover, upper and lower insulating plates114and115are further included at upper and lower portions of the electrode assembly110, respectively, in order to avoid a direct contact with the cap assembly130or the secondary battery case120.

A chalcogenide chemical compound is used as the positive electrode active material. The positive electrode active material may be a mixed metal oxide such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2(0<x<1), or LiMnO2. As for the negative electrode active material, a carbon (C) series material (e.g. Si, Sn, a tin oxide, a composite tin alloy, a transition metal oxide, a Li metal nitride, or a Li metal oxide) is used. In the first electrode plate111and the second electrode plate112, an electrode plate used as a positive electrode plate, for example, the first electrode plate111, is made of an Al material. An electrode plate used as a negative electrode plate, for example, the second electrode plate112, is made of a copper (Cu) material. The separator113is made of polyethylene (PE) or polypropylene (PP). However, an aspect of the present invention is not limited to the above materials.

In order to prevent an electrical short between the first electrode plate111and the second electrode plate112, an insulating tape (not shown) respectively insulates a boundary portion from which the first electrode tab111aand the second electrode tab112aprotrude.

The secondary battery case120is substantially a cylindrical metal case having an open upper portion. In the secondary battery case120, a cylindrical surface121has a specific space with a specific diameter, a bottom surface122is formed at a lower portion of the cylindrical surface121to block a lower space of the cylindrical surface121, and an upper portion of the cylindrical surface121is open so as to insert the electrode assembly110. Any one of the first electrode tab111aof the electrode assembly110and the second electrode tab112athereof, for example, the second electrode tab112a, is bonded at the center of the bottom surface122of the secondary battery case120, and thus the secondary battery case120itself functions as the electrode with the second electrode plate112, for example, a negative electrode terminal. The secondary battery case120is generally made of aluminum (Al), steel (Fe), or an alloy of Al and Fe. Further, in the upper portion of the secondary battery case120, a crimping portion123is curved in one direction so as to press the cap assembly130, and a beading portion124is concaved inwards to press the cap assembly130from bottom to top.

The cap assembly130includes: a conductive safety belt131which is welded to the first electrode tab111aand is inverted in the case of over-charging or abnormal heating; a printed circuit board (PCB)132which is electrically and mechanically connected to an upper portion of the safety belt131and cuts a circuit when the safety belt131is inverted; a positive temperature coefficient element133which is electrically and mechanically connected to an upper portion of the PCB132and cuts a circuit at a specific temperature or higher; a conductive electrode cap134which is electrically and mechanically connected to an upper portion of the positive temperature coefficient (PTC) element133and applies a practical current to an external circuit; and a gasket135which encloses lateral circumferences of the safety belt131, the PCB132, the positive temperature element133, and the electrode cap134, to isolate them from the secondary battery case120. Here, the electrode cap134may be bonded to any one of the first electrode tab111aof the electrode assembly110and the second electrode tab112athereof. For example, if the electrode cap134is bonded to the first electrode tab111a, the first electrode tab111amay function as the electrode with the first electrode plate111, in this case, as a positive electrode terminal.

The electrolytic solution140functions as a medium for moving Li-ion generated by an electrochemical reaction that occurs in positive and negative electrodes at charging/discharging times. The electrolytic solution140may be a non-aqueous type organic electrolytic solution which is a mixture of a lithium salt and an organic solvent type with high purity. Further, the electrolytic solution140may be a polymer using a polymer electrolyte, but the type of the electrolytic solution material is not limited thereto.

When the internal temperature of the cylindrical Li secondary battery100rises to a predetermined temperature or higher, preferably about 80° C. or higher, at which the cylindrical Li secondary battery100may be in an abnormal condition, the ignition inhibitor150prevents the internal temperature of the cylindrical Li secondary battery100from further rising, thereby preventing a dangerous situation, for example, an explosion of the cylindrical Li secondary battery100. That is, by preventing the internal temperature of the cylindrical Li secondary battery100from rising to the predetermined temperature of 80° C. or higher, the ignition inhibitor150can restrict ignition of the cylindrical Li secondary battery100.

The ignition inhibitor150may be formed on at least a portion of the inner surface of the secondary battery case120having the shape of a cylindrical can. In this case, the ignition inhibitor150is not formed in a portion where the second electrode tab112aof the electrode assembly110is attached to the secondary battery case120.

The ignition inhibitor150may include: an ignition inhibiting material layer151which is formed on at least a portion of the inner surface of the secondary battery case120and includes an ignition inhibiting material; and a separation film152which separates the ignition inhibiting material layer151from the electrolyte solution140.

The ignition inhibiting material layer151may be made of an ignition inhibiting material which can be inserted and diffused inside the cylindrical Li secondary battery100so as to prevent further temperature rising after the internal temperature of the cylindrical Li secondary battery100reaches the predetermined temperature or higher, for example, 80° C. or higher. Preferably, the ignition inhibiting material layer151may be made of NaCl.

As described above, the separation film152separates the ignition inhibiting material layer151from the electrolyte solution140. It is preferable that some or entire portions of the separation film152melt at the predetermined temperature of 80° C. or higher. By doing so, the ignition inhibiting material can be inserted into the cylindrical Li secondary battery100. Preferably, the separation film152is made of a thermoplastic, such as, a paraffin series material which melts when heat is applied.

Now, the operation of the cylindrical Li secondary battery100will be described.

First, when LiCoO2is used as the positive electrode active material, the following reaction occurs in the positive electrode plate.
LiCoO2Li1-xCoO2+xLi++xe−

That is, in the positive electrode plate, the positive electrode active material LiCoO2is decomposed into positive ion (Li+) and electron (e−) at a charging time. On the other hand, at a discharging time, positive ion (Li+) and electron (e−) are bonded forming the positive electrode active material LiCoO2.

When graphite (C) is used as the negative electrode active material, the following reaction occurs in the negative electrode plate.
Cn+xLi++xe−CnLix

That is, in the negative electrode plate, the negative electrode active material of graphite is bonded with positive ion (Li+) and electron (e−) to form CnLixat a charging time. On the other hand, at a discharging time, CnLixis decomposed, forming graphite (C), positive ion (Li+), and electron (e−)

The overall chemical reaction of the cylindrical Li secondary battery100at charging/discharging times can be expressed as follows.
LiCoO2+CnLi1-xCoO2+CnLix

The aforementioned electrochemical reaction of the cylindrical Li secondary battery100at charging/discharging times may generate heat inside the cylindrical Li secondary battery100. As a result, the internal temperature of the cylindrical Li secondary battery100rises.

In particular, if the cylindrical Li secondary battery100is over-charged, an excessive amount of Li is extracted in the positive electrode and inserted in the negative electrode. Thus, both positive and negative electrodes become thermally unstable, which causes not only an abrupt exothermic reaction, for example, an organic solvent of the electrolyte solution140is decomposed, but also a thermal runaway phenomenon. Accordingly, the internal temperature of the cylindrical Li secondary battery100abruptly rises.

In this case, if the internal temperature of the cylindrical Li secondary battery100rises to the predetermined temperature of 80° C. or higher, the separation film152of the ignition inhibitor150melts so that at least a portion of the separation film152is open. As a result, the ignition inhibiting material of the ignition inhibiting material layer151is inserted and diffused inside the cylindrical Li secondary battery100, thereby restricting the rising of the internal temperature of the cylindrical Li secondary battery100. Accordingly, dangers such as an explosion of the cylindrical Li secondary battery100, due to the internal temperature rising, can be avoided.

FIG. 2Ais a perspective view of a prismatic Li secondary battery according to an embodiment of the present invention.FIG. 2Bis a cross-sectional view of the prismatic Li secondary battery ofFIG. 2A, viewed along the B-B line.

Referring toFIGS. 2A and 2B, a prismatic Li secondary battery200includes: an electrode assembly210which generates a voltage difference at charging/discharging times; a secondary battery case220which accommodates the electrode assembly210; a cap assembly230which is assembled at an upper portion of the secondary battery case220; an electrolyte solution240which is injected into the secondary battery case220to allow Li-ion to move in the electrode assembly210; and a ignition inhibitor250which prevents an internal temperature of the prismatic Li secondary battery200from rising due to an abnormal heat generation.

The electrode assembly210includes: a first electrode plate211attached with a first electrode tab211a; a second electrode plate212attached with a second electrode tab212a; and a separator213interposed between the first electrode plate211and the second electrode plate212. The first electrode plate211, the second electrode plate212, and the separator213are rolled and accommodated in the secondary battery case220. In order to prevent an electrical short between the first electrode plate211and the second electrode plate212, an insulating tape214respectively insulates a boundary portion from which the first electrode tab211aand the second electrode tab212aprotrude.

The secondary battery case220is composed of a metal can in the shape of a box having an open upper portion. The secondary battery case220itself may function as electrode terminals. In general, the secondary battery case220is electrically connected with the first electrode tab211aso as to function as the electrode with the first electrode plate211, for example, a positive electrode terminal.

The cap assembly230is provided with a flat cap plate231having a size and shape corresponding to an opening portion of the secondary battery case220. An electrode through-hole231ais formed at the center of the flat cap plate231. An electrolyte solution inserting hole231bis formed at a first side of the flat cap plate231for inserting an electrolyte solution. The electrolyte solution inserting hole231bis bond sealed by ball231c. A safety vent231dis formed at a second side of the flat cap plate231to avoid explosion of the prismatic Li secondary battery200when the internal pressure of the prismatic Li secondary battery200rises.

An electrode terminal232, for example, a negative electrode terminal, is disposed in the electrode through-hole231ain an insertion manner. A tube shaped gasket233is formed on an outer surface of the electrode terminal232so as to be elastically insulated from the flat cap plate231.

An insulating plate234is disposed at a lower portion of the flat cap plate231. The insulating plate234is attached to the bottom surface of the flat cap plate231, and has a through-hole corresponding to the electrode through-hole231a. A terminal plate235having a through-hole corresponding to the electrode through-hole231ais disposed at the bottom surface of the insulating plate234. A lower portion of the electrode terminal232is electrically connected with the terminal plate235where the insulating plate234is disposed.

The first electrode tab211aprotruding from the first electrode plate211is welded in the bottom surface of the flat cap plate231. The second electrode tab212aprotruding from the second electrode plate212is welded in the lower portion of the electrode terminal232.

An insulating case236is disposed on the top surface of the electrode assembly210. The insulating case236electrically connects the electrode assembly210with the cap assembly230, and covers an upper portion of the electrode assembly210. The insulating case236includes an electrolyte solution inserting through-hole at a position corresponding to the electrolyte solution inserting hole231bof the flat cap plate231so as to insert the electrolyte solution therethrough. The insulating case236is a polymer resin having an insulation property, preferably, polypropylene. However, an aspect of the present invention is not limited to the above material.

The ignition inhibitor250is formed at least at a portion of the inner surface of the secondary battery case220composed of a metal can in the shape of a substantial box having an open upper portion. The ignition inhibitor250may include: an ignition inhibiting material layer251which is formed on at least a portion of the inner surface of the prismatic Li secondary battery200and is made of an ignition inhibiting material; and a separation film252which separates the ignition inhibiting material layer251from the electrolyte solution240.

When an internal temperature of the prismatic Li secondary battery200rises to a predetermined temperature or higher, preferably about 80° C. or higher, at which the prismatic Li secondary battery200may be in an abnormal condition, the ignition inhibitor250prevents the internal temperature of the prismatic Li secondary battery200from further rising, thereby preventing a dangerous situation, for example, explosion of the prismatic Li secondary battery200. That is, the ignition inhibitor250can restrict ignition of the prismatic Li secondary battery200.

FIG. 3Ais an exploded perspective view of a pouch-type Li secondary battery according to an embodiment of the present invention.FIG. 3Bis a cross-sectional view of the pouch-type Li secondary battery ofFIG. 3A.

Referring toFIGS. 3A and 3B, a pouch-type Li secondary battery300includes: an electrode assembly310which generates a voltage difference at charging/discharging times; a secondary battery case320which accommodates the electrode assembly310; and an ignition inhibitor350which prevents an internal temperature of the pouch-type Li secondary battery300from rising due to an abnormal heat generation.

The electrode assembly310includes: a first electrode plate311attached to a first electrode tab311a; a second electrode plate312attached to a second electrode tab312a; and a separator313interposed between the first electrode plate311and the second electrode plate312. The first electrode plate311, the second electrode plate312, and the separator313are rolled and accommodated in the secondary battery case320.

Although not shown, in order to prevent an electrical short between the first electrode plate311and the second electrode plate312, an insulating tape may respectively insulate a boundary portion from which the first electrode tab311aand the second electrode tab312aprotrude.

The secondary battery case320is a pouch-type case. The pouch-type secondary battery case320includes: a core320amade of a metal material such as aluminum (Al); a thermal bonding layer320bformed on the upper surface of the core320a; and an insulating layer320cformed on the lower surface of the core320a. The thermal bonding layer320bfunctions as an adhesive layer by using a polymer resin of a modified polypropylene, for example, casted polypropylene. The insulating layer320cmay be made of a resin material such as nylon or polyethylene-terephthalate (PET). However, the secondary battery case320is not limited to the above structure and material.

The secondary battery case320includes: a lower portion321which has a space for accommodating the electrode assembly310; and an upper portion322which covers the lower portion321. The space for accommodating the electrode assembly310is formed by a press operation. When the secondary battery case320is constructed, the electrode assembly310is accommodated in the space of the lower portion321, and the upper portion322is covered and sealed.

The ignition inhibitor350is formed on at least a portion of the inner surface of the secondary battery case320that is the pouch-type case. The ignition inhibitor350may include: an ignition inhibiting material layer351which is formed on at least a portion of the inner surface of the secondary battery case320and is made of an ignition inhibiting material; and a separation film352which separates the ignition inhibiting material layer351from an electrolyte solution.

If a melting temperature of the thermal bonding layer320bis controlled, such as when the core320ais made of a metal layer and an ignition inhibiting material layer, and the ignition inhibiting material layer is covered with the thermal bonding layer320b, then the pouch-type case itself can be used as an ignition inhibitor without having to additionally use the ignition inhibitor350of the present embodiment.

When the internal temperature of the pouch-type Li secondary battery300rises to a predetermined temperature or higher, preferably about 80° C. or higher, at which the pouch-type Li secondary battery300may be in an abnormal condition, the ignition inhibitor350prevents the internal temperature of the pouch-type Li secondary battery300from further rising, thereby preventing a dangerous situation, for example, explosion of the pouch-type Li secondary battery300. That is, the ignition inhibitor350can restrict ignition of the pouch-type Li secondary battery300.

As described above, the Li secondary batteries100,200, and300according to embodiments of the present invention include the ignition inhibitor350so as to prevent abnormality and explosion of the batteries when their internal temperatures rise. Specifically, the Li secondary batteries100,200, and300are formed on at least a portion of the inner surfaces of the secondary battery cases120,220, and320, and include the ignition inhibitors150,250, and350having the ignition inhibiting material layers151,251, and351that are separated from the electrolyte solutions by the separation films152,252, and352. By doing so, when the internal temperatures of the batteries rise to a predetermined temperature or higher, at least a portion of the separation films152,252, and352is open, and the ignition inhibiting materials are inserted and diffused inside the batteries, thereby restricting further rising of the internal temperatures of the batteries. Therefore, temperatures of the Li secondary batteries100,200, and300become more stable.

Accordingly, an aspect of the present invention provides a thermally stabilized Li secondary battery in which at least a portion of the inner surface of a secondary battery case includes an ignition inhibitor so as to prevent an internal temperature of the secondary battery from rising to a predetermined temperature or higher.