Lithium polymer battery

Details Lithium polymer battery Lithium polymer battery

2000-03-06

2002-09-17

Elve, M. Alexandra (Department: 1725)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231100, C029S623100, C029S623300, C029S623500

Reexamination Certificate

active

06451481

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery having an improved high-rate discharging efficiency produced by varying the thickness ratio of cathode sheets fixed to both surfaces of a cathode current collector.
2. Description of the Related Art
Lithium secondary batteries are classified as lithium metal batteries, lithium ion batteries and lithium polymer batteries according to the type of lithium used. A lithium polymer battery is a battery in which lithium ions move between a cathode made of lithium oxide and an anode made of a carbon material during charging/discharging to generate an electromotive force. The lithium polymer battery has an advantage in that it is explosion-free due to the use of a solid polymer electrolyte having an ion conductivity higher than a liquid electrolyte. Also, unlike other kinds of batteries, the lithium polymer battery can overcome the problem of battery performance deteriorating due to byproducts generated during charging and/or discharging. Furthermore, due to its flexibility, much attention has recently been paid to lithium polymer batteries as next generation batteries.
Various types of lithium polymer batteries can be assembled according to the fabrication method of cathode and anode plates. A bi-cell structure is most widely employed. As shown in
FIG. 1
, a conventional bi-cell assembled battery is constructed such that a cathode plate
11
is fixed to both sides of an anode plate,
12
, respectively, and a separator
13
is interposed between the cathode plate
11
and the anode plate
12
for insulating both plates
11
and
12
from each other.
Also, the cathode plate
11
includes a cathode current collector
14
made of a thin aluminum foil film, and a cathode sheet
15
fixed to at least one surface of the cathode current collector
14
. The cathode sheet
15
is fabricated by adding a conductive material, a plasticizer and a binder to a cathode active material as a main component, such as lithium oxide.
The anode plate
12
includes an anode current collector
16
made of a thin aluminum foil film, and an anode sheet
17
fixed to at least one surface of the anode current collector
16
. The anode sheet
17
is fabricated by adding a plasticizer, a conductive material and the like to an anode active material as a main component, such as a carbon material.
A plurality of openings
14
a
and
16
a
are formed in the cathode and anode current collectors
14
and
16
, respectively.
The battery having the above-described configuration is fabricated as follows.
First, primary lamination is performed such that slurry-type source materials of the cathode and anode sheets
15
and
17
are fixed to both surfaces of the cathode and anode current collectors
14
and
16
, thereby fabricating the cathode plate
11
and the anode plate
12
.
Then, secondary lamination is performed such that the separator
13
is interposed between the cathode plate
11
and the anode plate
12
, and predetermined amounts of heat and pressure are applied to the cathode plate
11
and the anode plate
12
with the separator
13
interposed therebetween to fuse them.
Subsequently, plasticizer components contained in the cathode and anode sheets
15
and
17
are extracted and an electrolyte material is impregnated in the space produced by extracting the plasticizer material, thereby forming a battery cell.
Next, the battery cell is inserted into a case having a space for accommodating the battery cell, and the case is hermetically sealed, thereby completing a battery.
The charging and discharging mechanism of a battery will now be described briefly.
Charging refers to a reaction in which lithium ions contained in the lithium oxide of the cathode plate
11
move to the anode plate
12
to then be intercalated between carbon molecules of the anode plate
12
. Conversely, discharging refers to a reaction occurring when lithium ions intercalated between carbon molecules of the anode plate
12
move back to the cathode plate
11
. Likewise, as lithium ions reciprocate between the cathode and anode plates
11
and
12
, an electromotive force is generated.
In this case since the electrical capacity of a battery is generally proportional to the amount of the lithium oxide, which is a cathode active material contained in the cathode sheet
15
, in order to increase the capacity of the battery, it is necessary to make the cathode sheet
15
thick.
However, if the cathode sheet
15
is excessively thick, the distance between the anode plate
12
and the cathode current collector
14
stacked to have the separator
13
interposed therebetween increases, causing a sharp decrease in the high-rate discharging efficiency. Thus, it is necessary to change the structure of the cathode plate
11
so as to improve the high-rate discharging efficiency, without changing the overall thickness of the cathode sheet
15
.
SUMMARY OF THE INVENTION
To solve the above problems, it is an objective of the present invention to provide an lithium secondary battery having an improved high-rate discharging efficiency by varying the thickness ratio of cathode sheets fixed to both surfaces of a cathode current collector.
Accordingly, to achieve the above objective, there is provided a lithium secondary battery having a bi-cell type battery cell consisting of an anode plate, a separator fixed to both surfaces of the anode plate and a cathode plate fixed to either outer surface of the separator, wherein the cathode plate includes a cathode current collector, a front cathode sheet fixed to one surface of the cathode current collector, which is adjacent to the anode plate, and a rear cathode sheet fixed to the other surface of the cathode current collector and having a thickness different from that of the front cathode sheet.
Also, the thickness ratios of the front and rear cathode sheets is preferably in the range of 5:1 to 1:1.
Further, the front and rear cathode sheets may include one selected from the group consisting of lithium-based oxide, nickel-based oxide and manganese-based oxide, as a main component.
According to another aspect of the present invention, there is provided a lithium secondary battery including an anode plate having an anode current collector and an anode sheet fixed to at least one surface of the anode current collector, a cathode plate having a cathode current collector and a front cathode sheet fixed to one surface of the cathode current collector, which is adjacent to the anode plate, and a rear cathode sheet fixed to the other surface of the cathode current collector and having a thickness different from that of the front cathode sheet, and a separator interposed between the anode plate and the cathode plate.
Also, the thickness ratios of the front and rear cathode sheets is preferably in the range of 5:1 to 1:1.


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