Chemistry: electrical current producing apparatus – product – and – Sealed cell having gas prevention or elimation means – Prevention or elimination means is one of the cell...
Reexamination Certificate
1998-08-25
2001-07-10
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Sealed cell having gas prevention or elimation means
Prevention or elimination means is one of the cell...
C429S094000, C429S164000, C429S209000, C429S233000
Reexamination Certificate
active
06258478
ABSTRACT:
BACKGROUNDS OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary battery. More particularly, this invention relates to a secondary battery having electrode assembly in which the capacity ratio between negative and positive active materials is reliably maintained to improve the battery performance and safety.
2. Description of the Prior Art
Generally, a secondary battery includes a roll electrode assembly having positive and negative electrodes applied with active materials and a separator disposed between the positive and negative electrodes; and a can in which the roll electrode assembly is inserted with the electrolyte. The secondary battery is a rechargeable battery using physical and chemical reactions between molecules of electrolyte and active materials. In recent years, the secondary battery has been widely used for portable products due to its compact in size and large capacity.
The secondary batteries are classified into a nickel-cadmium, nickel-metal hydride, and lithium ion batteries depend on materials used as the positive and negative electrodes or the electrolyte, and are further classified according to their shape into cylindrical, package and prism types.
Especially, the prism type secondary battery comprises a roll electrode assembly consisting of a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes; a can in which the electrode assembly is inserted with the negative electrode contacted therewith; and a cap assembly coupled to the positive electrode of the electrode assembly. Internal insulating plates are provided at upper and lower ends of the electrode assembly to prevent the electrode assembly from contacting the cap assembly and the can. The cap assembly is provided with a negative plate which is welded to the upper end of the can, a positive plate formed on a central portion of the cap assembly, and an external insulating plate disposed between the negative and positive plates. The positive plate contacts the positive electrode of the electrode assembly by a tap and supported by a rivet passing through the negative and positive plates. An insulating gasket is disposed between the rivet and the negative plate.
In the above described conventional prism type secondary battery, the positive and negative electrodes (hereafter referred as electrode) are manufactured through multiple steps. That is, active material is first applied on opposite surfaces of an electrode substrate at a predetermined thickness. Then, the electrode substrate applied with the active material is dried and pressed, thereby obtaining the electrode. The active material allows the absorption and release of the electrons or ions to generate or retain electromotive force.
In the secondary battery, the capacity ratio of the negative active material with respect to the positive active material (“N/P ratio”) is usually at about 1.2 to 1.4 in order to obtain a reserve by which positive ions generated from the positive electrode can be absorbed into the negative electrode during charge and discharge. For example, if the N/P ratio is less than 1, a metal oxide is extracted or the electrolyte leaks. This results in the deterioration of the charge and discharge ability of the secondary battery, and even in the explosion of the secondary battery by the increase of the internal pressure of the battery.
Referring to
FIG. 6
, there is shown a sectional view of a conventional roll electrode assembly
100
. The roll electrode assembly
100
is substantially oval having straight parts St and curve parts L. The roll electrode assembly comprises a positive electrode
100
b
applied with a positive active material, a negative electrode applied with a negative active material, and a separator
102
disposed between the positive and the negative electrodes
100
b
and
100
a.
At the straight parts St of the assembly
100
, the capacity of the positive active material of the positive electrode
100
b
is the same as that of the negative active material of the negative electrode
100
a
as the length of the positive electrode
100
b
is equal to that of the negative electrode
100
a.
However, at the curve parts L of the assembly
100
, as shown in
FIG. 7
, the capacity of the positive active material of the positive electrode
100
b
is different from that of the negative active material of the negative electrode
100
a
as the length of the positive electrode
100
b
differs from that of the negative electrode
100
a.
Describing more in detail with reference to
FIGS. 7 and 8
, each circumference length S
n
of the positive and negative electrodes at the curve parts is as follows:
S
n
=&thgr;*R
n
Where, &thgr; is an angle between a first line radially extending from a center point of the curved portion and a second line radially extending from the center point; and R
n
is a straight distance from the center point to each electrode (n indicates the turn order of the positive and negative electrodes).
Therefore the difference of the circumference length between two adjacent electrodes becomes as follows:
S
n
−S
n−1
=(&thgr;*
R
n
)−(&thgr;*R
n−1
)
In addition, when the thickness of the positive and negative active materials of the two adjacent electrodes is T, since T is equal to a distance between the adjacent electrodes, the difference of circumference length of two adjacent electrodes becomes as follows:
S
n
−S
n−1
=(&thgr;*
R
n
)−(&thgr;*
R
n−1
)=&thgr;(
R
n−1
+T
)−&thgr;
R
n−1
=&thgr;T
And, the difference of the capacity of the two adjacent electrodes becomes as follows:
&thgr;T*T* the vertical axis length of the electrode
Accordingly, in the conventional secondary battery, as shown in FIG.
9
(A), when the negative electrode is located at the outermost of the roll electrode assembly, the N/P ratio will be maintained more than 1.
However, as shown in FIG.
9
(B), when the positive electrode is located at the outermost of the roll electrode assembly, there may be a zone where the N/P ratio becomes less than 1. This zone increases the potential for an extraction of a metal oxide during the charge and discharge of the prism type secondary battery, resulting in the deterioration of the performance of the battery. Especially, when the battery is charged and discharged at a high ratio, this causes the deterioration of the battery safety.
SUMMARY OF THE INVENTION
Therefore, the present invention as been made in an effort to solve the above described drawbacks of the prior art.
It is an object of the present invention to provide a battery wherein the N/P ratio can be maintained above 1 throughout the any area of the electrode assembly.
To achieve the above object, the present invention provides a roll electrode assembly used in a secondary battery comprising a positive electrode applied with a positive active material, a negative electrode applied with a negative active material, and a separator disposed between said positive and negative electrodes. Thicknesses of the positive or negative active materials applied on opposite sides of the positive or negative substrates of the positive or negative electrodes are different from each other such that the capacity ratio between the positive and negative electrodes can be maintained above 1.
Preferably, the thickness of the active material applied on an outer side of the negative substrate is thicker than the thickness of the active material applied on an inner side of the negative substrate. The thickness difference is within 10 to 50%.
Alternatively, the thickness of the active material applied on the negative substrate is varied along the length of the negative substrate.
Preferably, the thickness of the active material applied on an inner surface of the positive substrate is designed to be less than T/2 and the thickness of the active material applied on an outer surface of the positive electrode is designed to be equal to T/2.
The thickness of the active mate
Baker & McKenzie
Samsung Display Devices Co. Ltd.
Weiner Laura
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