U.S. patent application number 14/015688 was filed with the patent office on 2013-12-26 for lithium secondary battery and method of controlling short resistance thereof.
This patent application is currently assigned to Samsung SDI Co., Ltd.. The applicant listed for this patent is Samsung SDI Co., Ltd.. Invention is credited to Joong-Heon KIM, Jin-Uk LEE, Cheol-Ho PARK.
Application Number | 20130344398 14/015688 |
Document ID | / |
Family ID | 43733247 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130344398 |
Kind Code |
A1 |
KIM; Joong-Heon ; et
al. |
December 26, 2013 |
LITHIUM SECONDARY BATTERY AND METHOD OF CONTROLLING SHORT
RESISTANCE THEREOF
Abstract
Provided is a lithium secondary battery including a positive
electrode having a positive electrode active material, a negative
electrode having a negative electrode active material, and a
polymer electrolyte composition having a polymer electrolyte, a
non-aqueous organic solvent, and a lithium salt. The content of the
polymer electrolyte is 9 to 20 wt %, based on the total weight of
the polymer electrolyte composition.
Inventors: |
KIM; Joong-Heon; (Yongin-si,
KR) ; PARK; Cheol-Ho; (Yongin-si, KR) ; LEE;
Jin-Uk; (Yongin-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung SDI Co., Ltd. |
Yongin-City |
|
KR |
|
|
Assignee: |
Samsung SDI Co., Ltd.
Yongin-City
KR
|
Family ID: |
43733247 |
Appl. No.: |
14/015688 |
Filed: |
August 30, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12910636 |
Oct 22, 2010 |
8541119 |
|
|
14015688 |
|
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Current U.S.
Class: |
429/326 ;
429/188; 429/200; 429/339; 429/341 |
Current CPC
Class: |
H01M 10/42 20130101;
H01M 10/056 20130101; H01M 10/0565 20130101; H01M 10/0525 20130101;
Y02E 60/10 20130101 |
Class at
Publication: |
429/326 ;
429/188; 429/339; 429/341; 429/200 |
International
Class: |
H01M 10/056 20060101
H01M010/056 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2009 |
KR |
10-2009-0124208 |
Claims
1. A lithium secondary battery, comprising: a positive electrode
including a positive electrode active material; a negative
electrode including a negative electrode active material; and a
polymer electrolyte composition including a polymer electrolyte, a
non-aqueous organic solvent, and a lithium salt; wherein the
content of the polymer electrolyte is from 9 to 20 wt %, based on
the total weight of the polymer electrolyte composition; and
wherein the polymer electrolyte comprises a first monomer of
Chemical Formula 1: A-polyesterpolyol-B, [Chemical Formula 1]
wherein, the polyesterpolyol has a molecular weight of 100 to
10,000,000 and is prepared by a condensation reaction between an
alcohol derivative having 2 to 6 terminal OH groups and a
dicarboxylic acid derivative, A and B are connected to the terminal
OH groups of the polyesterpolyol, and A and B are independently
selected from CH.sub.2.dbd.CR--C(.dbd.O)--,
CH.sub.2.dbd.CR--C--CH.sub.2--, CH.sub.2.dbd.CR--,
CH.sub.2.dbd.CR--O--C(.dbd.O)--, CH.sub.2.dbd.CH--CH.sub.2--O--,
CH.sub.2.dbd.CH--S(.dbd.O).sub.2-- or
CH.sub.2.dbd.CR--C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--,
wherein R is a C.sub.1-C.sub.10 hydrocarbon or a C.sub.6-C.sub.10
aromatic hydrocarbon.
2. The battery according to claim 1, wherein the polymer
electrolyte further comprises a second monomer selected from
Chemical Formulae 2 to 7: CH.sub.2.dbd.CR1-C(.dbd.O)--O--X;
[Chemical Formula 2] CH.sub.2.dbd.CR1-O--X; [Chemical Formula 3]
CH.sub.2.dbd.CR1-O--C(.dbd.O)--X; [Chemical Formula 4]
CH.sub.2.dbd.CH--CH.sub.2--O--X; [Chemical Formula 5]
CH.sub.2.dbd.CH--S(.dbd.O).sub.2--X; [Chemical Formula 6]
CH.sub.2.dbd.CR1-C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--O--X,
[Chemical Formula 7] wherein, R1 is H, a C.sub.1-C.sub.10
hydrocarbon, or an aromatic hydrocarbon, and X is a
C.sub.1-C.sub.20 hydrocarbon, a halogenated hydrocarbon, an
aromatic hydrocarbon or a halogenated aromatic hydrocarbon.
3. The battery according to claim 1, wherein the alcohol derivative
is selected from the group consisting of polyethyleneglycol,
polypropyleneglycol, alkanediol, ethoxylated alkanediol,
propoxylated alkanediol, trimethylolpropane, ethoxylated
trimethylolpropane, propoxylated trimethylolpropane,
ditrimethylolpropane, ethoxylated ditrimethylolpropane,
propoxylated ditrimethylolpropane, pentaerythritol, ethoxylated
pentaerythritol, propoxylated pentaerythritol, dipentaerythritol,
ethoxylated dipentaerythritol, propoxylated dipentaerythritol,
bisphenol A, ethoxylated bisphenol A, and propoxylated bisphenol
A.
4. The battery according to claim 1, wherein A and B are
independently selected from the group consisting of a (meth)acryl
group, a vinyl group, a allyl group, a vinylsulfonyl group, and a
urethane(meth)acryl group.
5. The battery according to claim 1, wherein the lithium salt is
selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiBOB
(lithium bisoxalato barate), LiSbF.sub.6, LiAsF.sub.6, LiClO.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiAlO.sub.4, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2x+1SO.sub.2) (where, x
and y are natural numbers), LiSO.sub.3CF.sub.3, and a combination
thereof.
6. The battery according to claim 1, wherein the non-aqueous
organic solvent is from the group consisting of a carbonate, an
ester, an ether, a ketone, a nitrile, and a combination
thereof.
7. The battery according to claim 1, wherein the negative electrode
further includes a binder comprising a styrene-butadiene
rubber.
8. The battery according to claim 1, wherein the content of the
polymer electrolyte is from 7 wt % to 20 wt %, based on the total
weight of the polymer electrolyte composition.
9. The battery according to claim 8, wherein the negative electrode
further includes a binder comprising polyvinylidenefluoride.
10. A polymer electrolyte composition for use in a lithium
secondary battery, comprising: a polymer electrolyte; a non-aqueous
organic solvent; and a lithium salt, wherein, the content of the
polymer electrolyte is from 9 to 20 wt %, based on the total weight
of the polymer electrolyte composition, the polymer electrolyte
comprises a first monomer of Chemical Formula 1:
A-polyesterpolyol-B, [Chemical Formula 1] wherein, the
polyesterpolyol has a molecular weight of 100 to 10,000,000 and is
prepared by a condensation reaction between an alcohol derivative
having 2 to 6 terminal OH groups and a dicarboxylic acid
derivative, A and B are connected to the terminal OH groups of the
polyesterpolyol, and A and B are independently selected from
CH.sub.2.dbd.CR--C(.dbd.O)--, CH.sub.2.dbd.CR--C--CH.sub.2--,
CH.sub.2.dbd.CR--, CH.sub.2.dbd.CR--O--C(.dbd.O)--,
CH.sub.2.dbd.CH--CH.sub.2--O--, CH.sub.2.dbd.CH--S(.dbd.O).sub.2--
or
CH.sub.2.dbd.CR--C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--,
wherein R is a C.sub.1-C.sub.10 hydrocarbon or a C.sub.6-C.sub.10
aromatic hydrocarbon.
11. The polymer electrolyte composition according to claim 10,
wherein the polymer electrolyte further comprises a second monomer
selected from Chemical Formulae 2 to 7:
CH.sub.2.dbd.CR1-C(.dbd.O)--O--X; [Chemical Formula 2]
CH.sub.2.dbd.CR1-O--X; [Chemical Formula 3]
CH.sub.2.dbd.CR1-O--C(.dbd.O)--X; [Chemical Formula 4]
CH.sub.2.dbd.CH--CH.sub.2--O--X; [Chemical Formula 5]
CH.sub.2.dbd.CH--S(.dbd.O).sub.2--X; [Chemical Formula 6]
CH.sub.2.dbd.CR1-C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--O--X,
[Chemical Formula 7] wherein, R1 is H, a C.sub.1-C.sub.10
hydrocarbon, or an aromatic hydrocarbon, and X is a
C.sub.1-C.sub.20 hydrocarbon, a halogenated hydrocarbon, an
aromatic hydrocarbon or a halogenated aromatic hydrocarbon.
12. The polymer electrolyte composition according to claim 10,
wherein the polyesterpolyol is represented by Chemical Formula 20:
##STR00005## wherein, X, Y, and Z are independently selected from
the group consisting of a multi-valent alkylene oxide, an alkylene
imide, and an alkylene, x, y and z are independently selected from
integers ranging from 1 to 20, and l, m, and n are independently
selected from integers ranging from 0, 1, or more, such that the
polyesterpolyol has a weight average molecular weight of about
25000.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of and claims
priority to U.S. application Ser. No. 12/910,636 filed Oct. 22,
2010, which claims the benefit of Korean Patent Application No.
10-2009-0124208, filed Dec. 14, 2009 in the Korean Intellectual
Property Office, the disclosures of both of which are incorporated
herein by reference.
BACKGROUND
[0002] 1. Field
[0003] Aspects of the present invention relate to a lithium
secondary battery and a method of controlling a short resistance
thereof.
[0004] 2. Description of the Related Technology
[0005] Due to the recent rapid development of compact and
lightweight portable electronic devices, there is a growing demand
for more compact and higher-capacity batteries as a driving power
source thereof. Particularly, lithium secondary batteries have an
operating voltage of 3.6 V or more, which is three times higher
than the operating voltages of nickel-cadmium (Ni--Cd) batteries or
nickel-metal hydride (Ni--MH) batteries, which are widely used as a
power source of portable electronic devices. Further, lithium
secondary batteries have a higher energy density per unit weight
than Ni--Cd and Ni--MH batteries. For these reasons, the lithium
secondary batteries have been rapidly developed.
[0006] A lithium secondary battery stores and releases electric
energy by oxidation and reduction, when lithium ions are
intercalated/deintercalated at a positive electrode and a negative
electrode. A lithium secondary battery is manufactured using
materials capable of reversibly intercalating and deintercalating
lithium ions as active materials for the positive and negative
electrodes, by charging an organic electrolyte or polymer
electrolyte disposed between the positive electrode and the
negative electrode.
[0007] A lithium secondary battery includes an electrode assembly,
a can, and a cap assembly. The electrode assembly is formed in a
jelly-roll shape, by winding a negative electrode, a positive
electrode, and a separator disposed therebetween. The can houses
the electrode assembly and an electrolyte. The cap assembly is
assembled on the can.
[0008] Meanwhile, such a lithium secondary battery is charged or
discharged by an electrochemical reaction occurring when ions are
released, inserted, or moved between active materials of the
electrodes. A repeatedly charged or discharged secondary battery
may undergo an increase in internal pressure and heat, due to
electrical misuse (overcharging) or other dangers. When such a
state continues, the secondary battery may break or explode,
thereby causing harm to a user. Thus, it is essential to prepare
safety features to prevent this harm.
[0009] For example, a conventional secondary battery has a means
for inhibiting a reaction, so that when an internal pressure is
increased over a safe pressure, it blocks the conformation of an
electric circuit, or breaks a safety vent in response to the
pressure, thereby reducing the internal pressure and removing an
electrolyte. An example of a conventional safety means is a porous
separator installed between the positive electrode and the negative
electrode. When a temperature in a case is increased over a safe
temperature, the porous separator shuts pores down in response to
the temperature, and inhibits movement of ions between the
electrodes. In such a manner, the porous separator ensures safety,
by inhibiting an electrochemical reaction (shut down).
[0010] However, when the temperature in the battery is excessively
increased over a temperature release rate of the case, due to
non-uniformity of the separator or other internal short circuits,
the separator melts before the shut down occurs. As such, the
separator is prevented from insulating the positive electrode from
the negative electrode. In addition, when the positive electrode
and the negative electrode are short-circuited, a chain reaction,
including the decomposition of the negative electrode active
material, the electrolyte, and the positive electrode active
material (melt-down) occurs. As a result, a thermal runaway occurs,
and the conventional secondary battery becomes unsafe and
explodes.
[0011] Particularly, when a positive electrode collector and the
negative electrode active material are short-circuited, such a
melt-down phenomenon can bring a drastic increase in heating value,
due to a resistance value at the short-circuited portion, and the
occurrence of the thermal runaway. For this reason, an alternative
for ensuring battery safety is needed.
SUMMARY OF CERTAIN INVENTIVE ASPECTS
[0012] Aspects of the present invention provide a lithium secondary
battery, in which battery safety is ensured during a short circuit
of the secondary battery.
[0013] According to one aspect of the present invention, a lithium
secondary battery includes: a positive electrode including a
positive electrode active material; a negative electrode including
a negative electrode active material; and a polymer electrolyte
composition including a polymer electrolyte, a non-aqueous organic
solvent, and a lithium salt. Here, the content of the polymer
electrolyte is 9 to 20 wt %, based on the total weight of the
polymer electrolyte composition.
[0014] According to an aspect of the invention, the negative
electrode may further include a binder formed of a
styrene-butadiene rubber.
[0015] According to an aspect of the invention, the content of the
polymer electrolyte may be 7 to 20 wt %, based on the total weight
of the polymer electrolyte composition.
[0016] According to an aspect of the invention, the binder may be
polyvinylidenefluoride.
[0017] According to another aspect of the present invention, a
method of controlling a short resistance includes: measuring an
open circuit voltage V.sub.OC of a secondary battery; calculating a
short circuit resistance R.sub.SC using the open circuit voltage
V.sub.OC; calculating a heating value W according to time, using
the short circuit resistance R.sub.SC; and calculating a time
having the maximum instantaneous heating value, using the change in
heating value W according time. Here, the short circuit resistance
R.sub.SC value at the time having the maximum instantaneous heating
value is the critical short resistance.
[0018] According to an aspect of the invention, the critical short
resistance may be present in a region excluding a "R.sub.SC range
having an ignition risk."
[0019] Additional aspects and/or advantages of the invention will
be set forth in part in the description which follows and, in part,
will be obvious from the description, or may be learned by practice
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] These and/or other aspects and advantages of the invention
will become apparent and more readily appreciated from the
following description of the exemplary embodiments, taken in
conjunction with the accompanying drawings, of which:
[0021] FIG. 1 is an exploded perspective view of an electrode
assembly, according to an exemplary embodiment of the present
invention;
[0022] FIG. 2 is a graph illustrating a short resistance
characteristic by contents of a monomer in a polymer electrolyte of
a secondary battery manufactured according to Manufacturing Example
1;
[0023] FIG. 3 is a schematic diagram of a circuit when a short
circuit occurs between a positive electrode collector and a
negative electrode active material;
[0024] FIG. 4A is a graph illustrating an change in V.sub.SC
according to time (t);
[0025] FIG. 4B is a graph illustrating a change in R.sub.SC
according to time (t);
[0026] FIG. 4C is a graph illustrating a change in heating value
(W) according to time;
[0027] FIG. 5 is a graph illustrating a relationship between the
short resistance and the total heating value;
[0028] FIG. 6A is a graph illustrating a change in R.sub.SC
according to time (t), when the content of a polymer electrolyte is
5 wt %;
[0029] FIG. 6B is a graph illustrating a change in heating value
according to time, when the content of the polymer electrolyte is 5
wt %;
[0030] FIG. 6C is a graph illustrating a relationship between the
short resistance and the heating value, when the content of the
polymer electrolyte is 5 wt %;
[0031] FIG. 7A is a graph illustrating a change in R.sub.SC
according to time (t) of a secondary battery manufactured according
to Manufacturing Example 2;
[0032] FIG. 7B is a graph illustrating a change in heating value
according to time (t) of the secondary battery manufactured
according to Manufacturing Example 2; and
[0033] FIG. 7C is a graph illustrating a relationship between the
short resistance and the heating value of the secondary battery
manufactured according to Manufacturing Example 2.
DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS
[0034] Reference will now be made in detail to the exemplary
embodiments of the present disclosure, examples of which are
illustrated in the accompanying drawings, wherein like reference
numerals refer to the like elements throughout. The exemplary
embodiments are described below, in order to explain the present
disclosure, by referring to the figures.
[0035] FIG. 1 is an exploded perspective view of an electrode
assembly 10 according to an exemplary embodiment of the present
invention. The electrode assembly 10 includes a first electrode 20
(referred to as a "positive electrode"), a second electrode 30
(referred to as a "negative electrode), and a separator 40. The
electrode assembly 10 is formed in a jelly-roll shape, by stacking
the positive electrode 20, the negative electrode 30, and the
separator 40 and then winding the same together.
[0036] In the shown embodiment, the separator 40 includes a first
separator 40a disposed between the positive and negative electrodes
20 and 30, and a second separator 40b disposed on the first
electrode 20. However, it is understood that the second separator
can be under or on both of the electrodes 20 and 30. The separator
40 is interposed between a contact portion of the electrodes 20 and
30, which are stacked and wound to prevent a short circuit between
the electrodes 20 and 30.
[0037] The positive electrode 20 includes a positive collector 21
and a positive electrode active material layer 22. The positive
collector 21 collects electrons generated by a chemical reaction
and conducts the electrons to an external circuit. The positive
electrode active material layer 22 is formed by applying a positive
electrode slurry, including a positive electrode active material,
to one or both sides of the positive electrode collector 21. As
shown, the positive electrode slurry was applied to both sides of
the positive current collector 21.
[0038] The positive electrode 20 includes a positive electrode
non-coating portion 23, to which the positive electrode slurry is
not applied, is formed on one or both ends of the positive
electrode collector 21, thereby exposing the positive electrode
collector 21. A positive electrode tab 24, transfers the electrons
collected in the positive electrode collector 21 to an external
circuit. The positive electrode tab 24 is formed of a nickel or
aluminum thin film and is joined to the positive electrode
non-coating portion 23. As shown, a protection member 25 is
provided on the area to which the positive electrode tab 24 is
joined to the noncoating portion 23.
[0039] The protection member 25 is provided to protect the joined
area, so as to prevent a short circuit. As such, the protection
member 25 can be formed of a material having resistance to heat,
for example, a polymer resin such as polyester.
[0040] The positive electrode 20 may includes an insulating member
26 formed to cover one or both ends of the positive electrode
active material layer 22. As shown, the insulating member 26 is on
both ends of the positive electrode active material 22 and on both
sides of the current collector 21.
[0041] The insulating member 26 may be formed of an insulating
tape, or an adhesive layer and an insulating film adhered to one
side thereof. However, the shape and material of the insulating
member 26 is not limited thereto.
[0042] The negative electrode 30 includes a negative electrode
collector 31 and a negative electrode active layer 32. The negative
electrode collector 31 collects electrons generated by a chemical
reaction and transfers the electrons to an external circuit. The
negative electrode active material layer 32 is formed by applying a
negative electrode slurry including a negative electrode active
material, to one or both sides of the negative electrode collector
31. As shown, the slurry is applied to both sides of the collector
31.
[0043] A negative electrode non-coating portion 33, to which the
negative electrode slurry is not applied, is formed on one or both
ends of the negative electrode collector 31, thereby exposing the
negative electrode collector 31.
[0044] A negative electrode tab 34 transfers the electrons
collected in the negative electrode collector 31 to an external
circuit. The negative electrode tab 34 is joined to the negative
electrode non-coating portion 33. While not required, the tab 34
can be a nickel thin film.
[0045] A protection member 35 is on the area to which the negative
electrode tab 34 is joined. The protection member 35 is provided to
protect the joined area, so as to prevent a short circuit. The
protection member 35 can be formed of a material having resistance
to heat, for example, a polymer resin such as polyester.
[0046] The negative electrode 30 further includes an insulating
member 36 formed to cover one or both ends of the negative
electrode active material layer 32. As shown, the insulating member
36 is on both ends of the layer 36 and on both sides of the cement
collector 31. The insulating member 36 may be formed of an
insulating tape, or an adhesive layer and an insulating film
adhered to one side thereof. However, the shape and material of the
insulating member 36 are not limited thereto.
[0047] According to an aspect of the invention, the separator 40
may be formed of a resin layer, such as polyethylene or
polypropylene, or a porous layer formed by combining a ceramic
material with a binder. However, the material of the separator 40
is not limited thereto.
[0048] As described above, the positive electrode 20 includes the
positive electrode active material layer 22 and the positive
electrode collector 21 to which the positive electrode active
material is applied. The positive electrode collector 21 may be
formed of aluminum or an aluminum alloy, and the positive electrode
active material layer 22 includes a positive electrode active
material capable of reversibly intercalating lithium ions. Examples
of the positive electrode active materials may be selected from the
group consisting of materials represented by Chemical Formulae 1 to
12:
Li.sub.xMn.sub.1-yM.sub.yC.sub.2; [Chemical Formula 1]
Li.sub.xMn.sub.1-yM.sub.yO.sub.2-zD.sub.z; [Chemical Formula 2]
Li.sub.xMn.sub.2O.sub.4-zD.sub.z; [Chemical Formula 3]
Li.sub.xCo.sub.1-yM.sub.yC.sub.2; [Chemical Formula 4]
Li.sub.xCo.sub.1-yM.sub.yO.sub.2-zD.sub.z; [Chemical Formula 5]
Li.sub.xNi.sub.1-yM.sub.yC.sub.2; [Chemical Formula 6]
Li.sub.xNi.sub.1-yM.sub.yO.sub.2-zC.sub.z; [Chemical Formula 7]
Li.sub.xNi.sub.1-yCo.sub.yO.sub.2-zD.sub.z; [Chemical Formula
8]
Li.sub.xNi.sub.1-y-zCo.sub.yM.sub.zC.sub..alpha.; [Chemical Formula
9]
Li.sub.xNi.sub.1-y-zCo.sub.yM.sub.zO.sub.2-.alpha.D.sub..alpha.;
[Formula 10]
Li.sub.xNi.sub.1-y-zMn.sub.yM.sub.zC.sub..alpha.; [Chemical Formula
11]
and
Li.sub.xNi.sub.1-y-zMn.sub.yM.sub.zO.sub.2-.alpha.D.sub..alpha..
[Chemical Formula 12]
[0049] In these formulae: 0.90.ltoreq.x.ltoreq.1.1;
0.ltoreq.y.ltoreq.0.5; 0.ltoreq.z.ltoreq.0.5;
0.ltoreq..alpha..ltoreq.2; M is at least one element selected from
the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and
rare-earth elements; C is an element selected from the group
consisting of O, F, S and P; and D is F, S, or P.
[0050] As described above, the negative electrode 30 includes the
negative electrode active material layer 32, and the negative
electrode collector 31, to which the negative electrode active
material is applied. The negative electrode collector 31 may be
copper or a copper alloy. The negative electrode active material
layer 32 may be formed of a carbon-based negative electrode active
material including crystalline or amorphous carbon, or a carbon
complex, or a metallic negative electrode active material including
a metal material capable of alloying with lithium.
[0051] The positive and negative electrodes 20, 30 may be formed
using a general electrode formation method. In particular, an
electrode is generally formed by preparing an active material
composition, by mixing a corresponding active material, a
conductive agent, and a binder in a solvent, and applying the
composition to an electrode collector. Since the method of forming
an electrode is well known in the art, a detailed description
thereof will be omitted herein.
[0052] The conductive agent may be a graphite-based conductive
agent, a carbon black-based conductive agent, and a metal or metal
compound-based conductive agent or a combination thereof. Examples
of the graphite-based conductive agent may include artificial
graphite, and natural graphite. Examples of the carbon black-based
conductive agent may include acetylene black, ketj en black, denka
black, thermal black, and channel black, and examples of the metal
or metal compound-based conductive agent may include tin, tin
oxide, SnPO.sub.4, titanium oxide, potassium titanate, and a
perovskite material, such as LaSrCoO.sub.3 and LaSrMnO.sub.3.
[0053] The content of the conductive agent may be 0.1 to 10 wt % of
the total weight of positive electrode active material. When the
content of the conductive agent is less than 0.1 wt %,
electrochemical characteristics are degraded. When the content of
the conductive agent is more than 10 wt %, the energy density per
weight is reduced.
[0054] The binder adheres the active material a paste to the
collector, improves the cohesion of the active material, and
buffers the expansion and contraction of the active material.
Examples of the binder may include polyvinylidenefluoride,
poly(vinylacetate), polyvinylalcohol, polyethyleneoxide,
polyvinylpyrrolidone, alkylated polyethyleneoxide, polyvinylether,
poly(methylmethacrylate), poly(ethylacrylate),
polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile,
polyvinylpyridine, styrene-butadiene rubber, and
acrylonitrile-butadiene rubber. The binder may be
polyvinylidenefluoride or styrene-butadiene rubber. A content of
the binder may be 0.1 to 30 wt %, and specifically 1 to 10 wt %, of
the total weight of the electrode active material. When the content
of the binder is too low, adhesion between the electrode active
material and the collector may become insufficient. When the
content of the binder is too high, the content of the electrode
active material is decreased, which is unfavorable in ensuring a
battery with a higher capacity.
[0055] A solvent is used to disperse the electrode active material,
the binder, and the conductive agent. For example, a non-aqueous
solvent or an aqueous solvent may be used. Examples of the
non-aqueous solvent may include N-methyl-2-pyrrolidone (NMP),
dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine,
ethyleneoxide, and tetrahydrofuran.
[0056] Further, the secondary battery including the electrode
assembly 10 having the separator 40 includes a polymer electrolyte
composition. The polymer electrolyte composition includes a polymer
electrolyte. The polymer electrolyte includes a polymer formed by
polymerizing a first monomer of Chemical Formula 13, or a mixed
monomer formed of the first monomer of Chemical Formula 13 and at
least one selected from the group consisting of second monomers of
Chemical Formulae 14 to 19.
A-polyesterpolyol-B [Chemical Formula 13]
[0057] In Chemical Formula 13, the polyesterpolyol is a material
having a molecular weight of 100 to 10,000,000, which is prepared
by a condensation reaction between at least one alcohol derivative
having 2 to 6 terminal OH groups, at least one dicarboxylic acid
derivative, and A and B are materials reacting with a terminal OH
group of polyesterpolyol, which are independently selected from
CH.sub.2.dbd.CR--C(.dbd.O)--, CH.sub.2.dbd.CR--C--CH.sub.2--,
CH.sub.2.dbd.CR--, CH.sub.2.dbd.CR--O--C(.dbd.O)--,
CH.sub.2.dbd.CH--CH.sub.2--O--, CH.sub.2.dbd.CH--S(.dbd.O).sub.2--,
and
CH.sub.2.dbd.CR--C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--.
In the forgoing formulae, R is C.sub.1-C.sub.10 hydrocarbon or
C.sub.6-C.sub.10 aromatic hydrocarbon. Chemical Formulae 14 to 19
are as follows:
CH.sub.2.dbd.CR1-C(.dbd.O)--O--X; [Chemical Formula 14]
CH.sub.2.dbd.CR1-O--X; [Chemical Formula 15]
CH.sub.2.dbd.CR1-O--C(.dbd.O)--X; [Chemical Formula 16]
CH.sub.2.dbd.CH--CH.sub.2--O--X; [Chemical Formula 17]
CH.sub.2.dbd.CH--S(.dbd.O).sub.2--X; [Chemical Formula 18]
and
CH.sub.2.dbd.CR1-C(.dbd.O)--O--CH.sub.2CH.sub.2--NH--C(.dbd.O)--O--X.
[Chemical Formula 19]
[0058] In Chemical Formulae 14 to 19, R1 is H, a C.sub.1-C.sub.10
hydrocarbon, or an aromatic hydrocarbon. X is a C.sub.1-C.sub.20
hydrocarbon, a halogenated hydrocarbon, an aromatic hydrocarbon, or
a halogenated aromatic hydrocarbon.
[0059] In further detail, in Chemical Formula 13, the alcohol
derivative forming polyesterpolyol is selected from the group
consisting of polyethyleneglycol, polypropyleneglycol, alkanediol,
ethoxylated alkanediol, propoxylated alkanediol,
trimethylolpropane, ethoxylated trimethylolpropane, propoxylated
trimethylolpropane, ditrimethylolpropane, ethoxylated
ditrimethylolpropane, propoxylated ditrimethylolpropane,
pentaerythritol, ethoxylated pentaerythritol, propoxylated
pentaerythritol, dipentaerythritol, ethoxylated dipentaerythritol,
propoxylated dipentaerythritol, bisphenol A, ethoxylated bisphenol
A, and propoxylated bisphenol A.
[0060] An example of the polyesterpolyol may be a compound of
Chemical Formula 20:
##STR00001##
[0061] In Chemical Formula 20, X, Y, and Z are each one or more
repeating units independently selected from the group consisting of
a multi-valent alkylene oxide, and alkylene imide, and an alkylene.
Further, x, y, and z are independently an integer of from 1 to 20,
and l, m, and n are independently 0, 1, or more, which may be
appropriately determined by the molecular weight of a desired
polymer.
[0062] In Chemical Formula 13, A and B are independently selected
from the group consisting of (meth)acryl, vinyl, allyl,
vinylsulfonyl, and urethane(meth)acryl. In particular, A and B may
be independently selected from (meth)acryl, vinyl, vinylsulfonyl,
and urethane(meth)acryl.
[0063] Hereinafter, a synthesis example of a monomer of the polymer
electrolyte, according to aspects of the present invention, will be
described. However, the present invention is not limited
thereto.
Monomer Synthesis Example 1
[0064] A mixed solution is prepared by stirring: 0.02 mol of
polyester polyol (DR1515, "DAERYUNG Enterprise Co. Ltd.", Mn=1500),
obtained by a condensation reaction of ethylene glycol (EG),
diethylene glycol (DEG), trimethylolpropane (TMP), and adipic acid;
30 g of methylene chloride as a reaction solvent; and 0.04 mol of
triethylamine as a catalyst, in a cooling bath. Another mixed
solution of 0.04 mol of acryloyl chloride and 15 g of methylene
chloride was added dropwise to the mixed solution. Subsequently,
the resulting solution was heated to 40.degree. C. and stirred for
6 hours, an educed salt was filtrated therefrom, and methyl
chloride (the reaction solvent) was removed by distillation,
thereby obtaining a desired monomer SP 1. The monomer SP 1 has a
weight average molecular weight of about 25000 and is represented
by Chemical Formula 21.
##STR00002##
[0065] In Chemical Formula 21, l, m, and n are independently
selected from 0, 1, or more, such that the monomer has a weight
average molecular weight of about 25000.
Monomer Synthesis Example 2
[0066] A mixed solution was prepared by stirring 0.02 mol of
polyester polyol used in Synthesis Example 1, 30 g of methylene
chloride as a reaction solvent, and 0.0002 mol of dibutyltin
dilaurate as a catalyst. Another mixed solution of 0.04 mol of
isocyanatoethyl methacrylate and 15 g of methylene chloride was
added dropwise to the mixed solution. Subsequently, the resulting
solution was stirred for 6 hours at room temperature, stirred again
for 2 hours at 50.degree. C. Then an educed salt was filtered
therefrom, and the reaction solvent (methylene chloride) was
removed by distillation. Thus, a desired monomer SP2, having a
weight average molecular weight of about 25000, was obtained, which
is represented by the following Chemical Formula 22.
##STR00003##
[0067] In Chemical Formula 22, l, m, and n are independently
selected from 0, 1, or more, such that the monomer has a weight
average molecular weight of about 25000.
Monomer Synthesis Example 3
[0068] Except for using poylesterpolyol prepared by a condensation
reaction of diethylene glycol and adipic acid, the same process as
described in Synthesis Example 1 was performed, thereby obtaining
monomer SP3. The monomer SP3 has a weight average molecular weight
of about 25000 and is represented by Chemical Formula 23.
##STR00004##
[0069] In Chemical Formula 23, n is selected from 0, 1, or more,
such that the monomer has a weight average molecular weight of
about 25000.
[0070] In the molecular electrolyte compositions, the content of
the polymer electrolyte (the first monomer or a polymer prepared by
polymerizing the first monomer and the second monomer) may be 9 to
20 wt %, and specifically 7 to 20 wt %, which will be described in
further detail later.
[0071] The initiator is provided to initiate the polymerization of
the monomer and thus, may be any material that can easily initiate
the polymerization of the monomer and does not degrade battery
performance. The initiator may be an organic peroxide, an azo
compound, of a combination of one or more thereof.
[0072] Examples of the organic peroxides may include peroxy
dicarbonates, such as di(4-t-butylcyclohexyl) peroxy dicarbonate,
di-2-ethylhexyl peroxy dicarbonate, di-isopropyl peroxy
dicarbonate, di-3-methoxy butyl peroxy dicarbonate, t-butyl peroxy
isopropyl carbonate, t-butyl peroxy 2-ethylhexyl carbonate,
1,6-bis(t-butyl peroxycarbonyloxy)hexane, and diethylene
glycol-bis(t-butyl peroxy carbonate); diacyl peroxides, such as
diacetyl peroxide, dibenzoyl peroxide, dilauroyl peroxide, and
bis-3,5,5-trimethyl hexanoyl peroxide; and peroxy esters, such as
perhexyl pivalate, t-butyl peroxy pivalate, t-amyl peroxypivalate,
t-butyl peroxy-2-ethyl-hexanoate, t-hexyl peroxy pivalate, t-butyl
peroxy neodecanoate, t-butyl peroxy neoheptanoate, t-hexyl peroxy
pivalate, 1,1,3,3-tetramethyl butyl peroxy neodecarbonate,
1,1,3,3-tetramethyl butyl 2-ethylhexanoate, t-amyl peroxy 2-ethyl
hexanoate, t-butyl peroxy isobutyrate, t-amyl peroxy
3,5,5-trimethyl hexanoyl, t-butyl peroxy 3,5,5-trimethyl hexanoate,
t-butyl peroxy acetate, t-butyl peroxy benzoate, and di-butylperoxy
trimethyl adipate. Examples of the azo compounds may include
2,2'-azo-bis(isobutyronitrile),
2,2'-azo-bis(2,4-dimethylvaleronitrile), and
1,1'-azo-bis(cyanocyclo-hexane).
[0073] In the polymer electrolyte composition, the polymerization
initiator may be presented in an amount sufficient to induce the
polymerization reaction of the monomers. Generally, the content of
the polymerization initiator is 0.01 to 5 wt %, based on the
content of the monomer.
[0074] Generally, the polymer electrolyte composition also includes
a non-aqueous organic solvent as a liquid electrolyte, and a
lithium salt. The non-aqueous organic solvent may be a carbonate,
an ester, an ether, or a ketone. Examples of the carbonate may
include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl
carbonate (EPC), methylethyl carbonate (MEC) ethylene carbonate
(EC), propylene carbonate (PC), and butylene carbonate (BC).
Examples of the esters include butyrolactone (BL), decanolide,
valerolactone, mevalonolactone, caprolactone, n-methyl acetate,
n-ethyl acetate, and n-propyl acetate. The ether may be dibutyl
ether. The ketone may be polymethylvinyl ketone. However, the
present invention is not limited to the non-aqueous organic
solvents listed herein.
[0075] When a carbonate-based organic solvent is used as the
non-aqueous organic solvent, it may be prepared by combining a
cyclic carbonate with a chain carbonate. In this case, the cyclic
carbonate and the chain carbonate may be mixed in a volume ratio of
1:1 to 1:9, and specifically, 1:1.5 to 1:4. This ratio can produce
an electrolyte having suitable performance.
[0076] In addition to the carbonate-based solvent, the electrolyte
may include an aromatic hydrocarbon-based organic solvent, which
may be an aromatic hydrocarbon-based compound. Examples of the
aromatic hydrocarbon-based organic solvent may be benzene,
fluorobenzene, chlorobenzene, nitrobenzene, toluene, fluorotoluene,
trifluorotoluene, and xylene. In the electrolyte including the
aromatic hydrocarbon-based organic solvent, a volume ratio of the
carbonate-based solvent to the aromatic hydrocarbon-based solvent
may be 1:1 to 30:1. This ratio can produce an electrolyte having
suitable performance.
[0077] The electrolyte includes a lithium salt, which serves as a
lithium ion source for basic operation of a lithium battery. The
lithium salt may be LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6,
LiAsF.sub.6, LiClO.sub.4, LiCF.sub.3 SO.sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(C.sub.2F.sub.5 SO.sub.2).sub.2,
LiAlO.sub.4, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(CyF.sub.2x+1SO.sub.2) (where, x and
y are natural numbers), LiSO.sub.3CF.sub.3, or a combination
thereof.
[0078] The lithium salt may have a concentration of 0.6 to 2.0 M,
and specifically 0.7 to 1.6 M. When the concentration of the
lithium salt is less than 0.6 M, the viscosity of the electrolyte
is decreased, thereby degrading performance of the electrolyte.
When the concentration of the lithium is more than 2.0 M, the
viscosity of the electrolyte is increased, thereby degrading
mobility of the lithium ions.
[0079] In manufacturing a lithium secondary battery using the
polymer electrolyte composition, the electrode assembly 10, which
is formed by a conventional method, is inserted into a battery
case. The polymer electrolyte composition is added to the case, and
a curing process is performed.
[0080] The curing process is widely known in the art, and thus, a
detailed description thereof will be omitted. During the curing
process, polymerization is initiated by the polymerization
initiator. Thereby, the monomer included in the polymer electrolyte
composition forms a polymer, such that the battery includes a
polymer electrolyte.
[0081] The battery case may be a metal can, or a pouch formed of a
metal laminate. Specifically, when the polymer electrolyte
composition is applied to a battery using a pouch-type case, the
performance of the polymer electrolyte composition may be
maximized.
[0082] Hereinafter, the safety of a secondary battery including the
polymer electrolyte will be described. FIG. 2 is a graph
illustrating the short resistance characteristics, according to
monomers, in secondary batteries having polymer electrolytes
prepared according to Manufacturing Example 1 described below. In
FIG. 2, the X axis shows the content (wt %) of the polymer
electrolyte, the Y axis shows the cell capacity (mAh) of the
secondary batteries, and the Z axis shows the maximum instantaneous
heating value (J) of secondary batteries. In FIG. 2, according to
the cell content and the content of the polymer electrolyte, an
actual measurement positions and ignition positions are shown. The
actual measurement position refers to a position a positive
electrode collector 21 is short-circuited with a negative electrode
active material.
[0083] Here, when the secondary battery has a cell capacity of 1250
mAh, an electrode assembly is wound 8 times (hereinafter, 8 turns),
when the secondary battery has a cell capacity of 1094 mAh, one
turn is removed, when the secondary battery has a cell capacity of
938 mAh, two turns are removed, when the secondary battery has a
cell capacity of 781 mAh, three turns are removed, when the
secondary battery has a cell capacity of 625 mAh, four turns are
removed, and when the secondary battery has a cell capacity of 469
mAh, five turns are removed. The maximum heating value will be
described later.
[0084] The secondary batteries were manufactured according to the
following Manufacturing Example 1.
Manufacturing Example 1
[0085] A mixed solution was prepared by adding the monomer SP1
prepared in Synthesis Example 1 to an ethylene carbonate, ethyl
methyl carbonate, propylene carbonate, or fluorobenzene electrolyte
solution, in which 1.3 M LiPF.sub.6 was dissolved. A polymerization
initiator, perhexyl pivalate, was added to the mixed solution, in
an amount of 2 wt %, based on the weight of the monomer to be
dissolved, thereby preparing a polymer electrolyte composition.
[0086] For example, to prepare the polymer electrolyte composition,
a mixed solution was prepared by adding 7 wt % of the monomer SP 1
prepared in Synthesis Example 1 to 93 wt % of the ethylene
carbonate, ethyl methyl carbonate, propylene carbonate, or
fluorobenzene electrolyte solution, in which 1.3 M LiPF.sub.6 was
dissolved. Subsequently, as shown in FIG. 2, secondary batteries
were manufactured, having different monomer contents. As can be
seen from FIG. 2, the content of the monomer in the polymer
electrolyte was 0, 7, 9, 11, 13, or 15 wt %.
[0087] 2.7 g of the polymer electrolyte composition was taken,
added to an electrode assembly, and aged for 16 hours. The
resulting product was sealed in a vacuum, and heated in a
70.degree. C. oven, for 2 and a half hours, thereby manufacturing a
lithium (secondary) battery. To form the positive electrodes, a
mixture of LiCoO.sub.2 as a positive electrode active material,
ketjen black as a conductive agent, and polyvinylidene fluoride as
a binder in a weight ratio of 96:2:2 was used. To form the negative
electrodes, a mixture of artificial graphite as a negative
electrode active material and polyvinylidene fluoride as a binder
in a weight ratio of 94:6 was used.
[0088] It can be seen from FIG. 2 that when a secondary battery
does not include the polymer electrolyte, regardless of the cell
capacity, all of the secondary batteries were ignited at the actual
measurement position. When the content of the polymer electrolyte
is 7 wt % or more, none of the secondary batteries were ignited at
the actual measurement position.
[0089] A relationship between the maximum instantaneous heating
value and the ignition of the secondary battery will be described
below. To begin with, the present inventors recognized that, when a
liquid electrolyte is used, a short circuit between the positive
electrode collector 21 and the negative electrode active material
is the easiest to cause, leading to the ignition of the secondary
battery. Here, the present inventors found that when the short
circuit occurs between the positive electrode collector 21 and the
negative electrode active material, a certain level of internal
resistance (short resistance) is generated, and due to a heating
value obtained therefrom, the secondary battery is ignited. For
this reason, the inventors sought to provide a safer battery, by
artificially controlling an internal resistance value (short
resistance value) that generates a heating value, even if the short
circuit occurs between the positive electrode collector 21 and the
negative electrode active material.
[0090] FIG. 3 is a schematic diagram of a circuit when a short
circuit occurs between the positive electrode collector 21 and a
negative electrode active material. Here, the short circuit between
the positive electrode collector 21 and the negative electrode
active material is induced in the following manner.
[0091] To begin with, an external case of a fully-charged secondary
battery is removed, a part of the positive electrode 20 is unwound,
and the positive electrode active material is removed from a 5
mm.times.5 mm area of the partially-unwound positive electrode 20,
thereby exposing a positive electrode collector 21.
[0092] Subsequently, the separator 40 is removed from a region
corresponding to the exposed positive electrode collector 21,
thereby exposing a negative electrode active material disposed
under the separator 40. Then the partially-unwound positive
electrode 20 is rewound.
[0093] As described above, pressure was applied to the exposed
positive electrode collector and negative electrode active
material, using a 0.5 mm shorting pin. A short voltage was then
measured.
[0094] In FIG. 3, V.sub.OC is an open circuit voltage, V.sub.SC is
a short circuit voltage, R.sub.1 is a specific resistance of the
secondary battery, R.sub.SC is a short circuit resistance, and R is
a specific resistance of the shorting pin.
[0095] As can be seen from FIG. 3, Voc is expressed as follows.
V.sub.OC=I*R.sub.TOTAL=*I(R.sub.I+R.sub.SC+R) (1)
[0096] In Formula (1), I is expressed as follows.
I = V oc R 1 + R sc + R ( 2 ) ##EQU00001##
[0097] Meanwhile, the heating value W is expressed as follows.
W=I.sup.2R.sub.SC (3)
[0098] The following Equation 1 can be derived From Formulae (2)
and (3).
W = ( V oc R 1 + R sc + R ) 2 * R sc [ Equation 1 ]
##EQU00002##
[0099] Meanwhile, V.sub.SC is expressed as follows.
V.sub.SC=I*R.sub.SC (4)
[0100] The following Equation 2 is derived from Formulae (4) and
(1).
R sc = V sc ( R 1 + R ) V oc - V sc [ Equation 2 ] ##EQU00003##
[0101] As can be seen from Equation 2, R.sub.SC can be defined by
V.sub.SC. As can be seen from Equation 1, the heating value W can
be defined by R.sub.SC. A principle of improving battery safety by
controlling an internal resistance (short resistance) according to
aspects of the present invention will be explained.
[0102] FIG. 4A is a graph illustrating changes in V.sub.SC
according to time (t). The changes in V.sub.SC are measured using a
voltmeter. FIG. 4B is a graph illustrating changes in R.sub.SC
according to time (t). The changes in R.sub.SC according to time
can be calculated by applying the changes in V.sub.SC according to
time of FIG. 4A, to Equation 2. FIG. 4C is a graph illustrating a
change in heating value (W) according to time (t). The changes in
heating value according to time can be calculated by applying the
changes in R.sub.SC according to time of FIG. 4B to Equation 1.
[0103] Here, changes in heating value (W) according to time include
the time (a), where the heating value is highest. Hereinafter, this
heating value is referred to as a "maximum instantaneous heating
value."
[0104] Meanwhile, as described above, the changes in heating value
(W) according to time may be calculated using the changes in
R.sub.SC according to time. According to aspects of the present
invention, an R.sub.SC value at the time having the maximum
instantaneous heating value is defined as a "critical short
resistance.
[0105] FIG. 5 is a graph illustrating a relationship between the
short resistance R.sub.SC and the heating value W. Generally, when
a short circuit occurs, the heating value W is increased, due to
the resistance value at the position where the short circuit
occurs, leading to thermal runaway. Here, the heating value
continuously increases for a certain time period and then
decreases. Thus, the heating value W increases, until the point
(time) which the maximum instantaneous heating value occurs.
[0106] The total heating value according to time (heat value per
unit time) also continuously increases for a certain time period
and then decreases. Thus, the increase in total heating value
continues until the maximum instantaneous heating value is reached.
However, such thermal runaway abruptly occurs, when the maximum
instantaneous heating value is present around the region (time
period) in which the total heating value according to time is at
the highest level, and does not occur when the maximum
instantaneous heating value is present in the region in which the
total heating value is low.
[0107] In other words, when the short resistance R.sub.SC is at a
very low or high level, the total heating value according to time
(heating value per unit time) decreases. Accordingly, when the
maximum instantaneous heating value is present in the region having
the low heating value per unit time, the thermal runaway does not
occur. Region R shown in FIG. 5 a region having heating value per
unit time which is more than 90% of the maximum heating value per
unit time (Wmax) is defined as the region (time period) in which
the thermal runaway abruptly occurs, and is referred to as an
"R.sub.SC range having an ignition risk."
[0108] The critical short resistance described above is disposed in
a region excluding the "R.sub.SC range having an ignition risk"
shown in FIG. 5, to ensure battery safety. In other words, the
critical short resistance refers to the Rsc value at the time when
the maximum instantaneous heating value occurs. The critical short
resistance is in the region excluding the "R.sub.SC range having an
ignition risk," and thereby the maximum instantaneous heating value
may be in the region having a lower level of the total heating
value according to time.
[0109] To this end, an internal resistance is artificially
controlled for the critical short resistance to be present in the
region excluding the "R.sub.SC range having an ignition risk," by
adding the polymer electrolyte as described above to the polymer
electrolyte composition, at a certain content. Thus, even though a
positive electrode collector is short-circuited with the negative
electrode active material, which is in the most vulnerable mode to
an internal short circuit, battery safety can be ensured by
inhibiting the thermal runaway.
[0110] Meanwhile, as described above, it can be seen from FIG. 2
that when the polymer electrolyte is not included, regardless of
the cell capacity, all secondary batteries are ignited at the
actual measurement position. When the content of the polymer
electrolyte is 7 wt % or more, no secondary batteries are ignited
at the actual measurement position.
[0111] The actual measurement position and the actual ignition
position each include the maximum instantaneous heating values.
Here, even when the content of the polymer electrolyte is 7 wt % or
more, none of the secondary batteries are ignited. This is because
the critical short resistance (i.e., the R.sub.SC value) at the
time when the maximum instantaneous heating value is present is
designed to be present in the region excluding the "R.sub.SC range
having an ignition risk" shown in FIG. 5. That is, a critical short
circuit is artificially controlled by adding a certain amount of
the polymer electrolyte. Thus, the battery safety is improved.
[0112] When the cell capacity was 1250 mAh, and the polymer
electrolyte was contained in an amount of 15, 13, 11, 9, or Owt %,
the positive electrode collector was short-circuited with the
negative electrode active material, and the maximum instantaneous
heating value was measured. As a result, the maximum instantaneous
heating value was different, and thus, the critical short
resistance was different. When the polymer electrolyte was
contained in an amount of 15, 13, 11, or 9 wt %, the critical short
resistance was present in the region excluding the "R.sub.SC range
having an ignition risk," as shown in FIG. 5, so that the battery
was not ignited. However, when no polymer electrolyte was
contained, the critical short resistance was present in the
"R.sub.SC range having an ignition risk," so that the battery was
ignited.
[0113] Meanwhile, comparing a case in which the cell capacity was
938 mAh and the polymer electrolyte was contained at 7 wt %, and a
case in which the cell capacity was 469 mAh and no polymer
electrolyte was contained, the former case had a higher level of
the maximum instantaneous heating value than the latter case.
However, in the former case, the battery was not ignited, and in
the latter case, the battery was ignited. As a result, it can be
seen that the ignition of the battery is not determined by the
maximum instantaneous heating value, but by whether the critical
short resistance value is present in the "R.sub.SC range having an
ignition risk."
[0114] FIG. 6A is a graph illustrating a change in R.sub.SC
according to time (t), when the content of the polymer electrolyte
is 5 wt %. In FIG. 6A, A1 is a secondary battery having a cell
capacity of 469 mAh (that is, 5 turns are removed), B1 is a
secondary battery having a cell capacity of 625 mAh (that is, 4
turns are removed), and C1 is a secondary battery having a cell
capacity of 781 mAh (that is, 3 turns are removed). As shown in
FIG. 6A, the change in R.sub.SC according to time (t) may be
measured by applying changes in V.sub.SC to Equation 2.
[0115] FIG. 6B is a graph illustrating a change in heating value W
according to time (t), when the content of the polymer electrolyte
is 5 wt %. In FIG. 6B, A2 is a secondary battery having a cell
capacity of 469 mAh (that is, 5 turns are removed), B2 is a
secondary battery having a cell capacity of 625 mAh (that is, 4
turns are removed), and C2 is a secondary battery having a cell
capacity of 781 mAh (that is, 3 turns are removed).
[0116] Changes in heating value W according to time may be measured
by applying the change in Rsc according to time of FIG. 6A to
Equation 1. Referring to FIG. 6B, each secondary battery includes
the time where the maximum instantaneous heating value is present,
and the R.sub.SC value at the maximum instantaneous heating value
corresponds to the critical short resistance.
[0117] FIG. 6C is a graph illustrating a relationship between the
short resistance R.sub.SC and the heating value W when the content
of the polymer electrolyte is 5 wt %. In FIG. 6C, A3 is a secondary
battery having a cell capacity of 469 mAh (that is, 5 turns are
removed), B3 is a secondary battery having a cell capacity of 625
mAh (that is, 4 turns are removed), and C3 is a secondary battery
having a cell capacity of 781 mAh (that is, 3 turns are
removed).
[0118] Referring to FIG. 6C, each secondary battery includes the
critical short resistance, that is, the R.sub.SC value at the
maximum instantaneous heating value indicated by the circle, and
the critical short resistance is present in the "R.sub.SC range
having an ignition risk." In other words, as can be seen from FIGS.
6A to 6C, when the content of the polymer electrolyte is 5 wt % or
less, the critical short resistance is present in the "R.sub.SC
range having an ignition risk," so that the battery safety is not
be improved. For this reason, in Manufacturing Example 1, it is
preferable that the content of the polymer electrolyte is 7 wt % or
more.
[0119] In addition, the secondary batteries according to
Manufacturing Example 1 may contain the polymer electrolyte at 20
wt % or less. The polymer electrolyte is added to a cell in a
monomer state, and polymerized into a gel, by curing after
assembly. However, when the content of the polymer electrolyte is
more than 20 wt %, the viscosity of the polymer electrolyte
composition becomes so high that the impregnating ability of an
electrode plate is not good during the addition of the polymer
electrolyte, and the polymerization is not uniformly performed.
This creates localized non-polymerized regions that decrease ionic
conductivity of the polymer electrolyte composition. Thus, the
secondary battery cannot ensure high charge and discharge
rates.
[0120] Accordingly, the secondary battery according to
Manufacturing Example 1 may contain the polymer electrolyte at 7 to
20 wt %, based on the total weight of the polymer electrolyte
composition.
[0121] A secondary battery was manufactured according to the
following Manufacturing Example 2.
Manufacturing Example 2
[0122] A mixed solution was prepared by adding the monomer SP1
prepared in Synthesis Example 1 to an ethylene carbonate, ethyl
methyl carbonate, propylene carbonate, or fluorobenzene electrolyte
solution, in which 1.3 M LiPF.sub.6 was dissolved. A polymerization
initiator, perhexyl pivalate, was dissolved into the mixed solution
in an amount of 2 wt %, based on the weight of the monomer, thereby
preparing a polymer electrolyte composition.
[0123] For example, to prepare the polymer electrolyte
compositions, a mixed solution was prepared by adding 7 wt % of the
monomer SP1 to 93 wt % of the ethylene carbonate, ethyl methyl
carbonate, propylene carbonate, or fluorobenzene electrolyte
solution, in which 1.3 M LiPF.sub.6 was dissolved. Subsequently, a
secondary battery was manufactured while changing the content of
the monomer. Here, the amount of the monomer in the polymer
electrolyte was 5, 7, or 9 wt %.
[0124] 2.7 g of the polymer electrolyte composition was added to an
electrode assembly, and aged for 16 hours. The resulting product
was sealed in a vacuum, and heated in a 70.degree. C. oven for 2
and a half hours, thereby manufacturing a lithium (secondary)
battery. To form a positive electrode, a mixture of LiCoO.sub.2 as
a positive electrode active material, ketjen black as a conductive
agent, and polyvinylidene fluoride as a binder, in a weight ratio
of 96:2:2, was used. To form a negative electrode, a mixture of
artificial graphite as a negative electrode active material and
styrene-butadiene rubber as a binder, in a weight ratio of 94:6,
was used.
[0125] In other words, the polyvinylidene fluoride was used as the
binder of the negative electrodes, in Manufacturing Example 1.
However, in Manufacturing Example 2, styrene-butadiene rubber was
used as the binder of the negative electrodes.
[0126] FIGS. 7A to 7C illustrate safety of the secondary batteries
manufactured in Manufacturing Example 2. Here, the secondary
batteries shown in FIGS. 7A to 7C had a cell capacity of 781 mAh
(that is, 3 turns were removed).
[0127] FIG. 7A is a graph illustrating a change in R.sub.SC
according to time (t), of the secondary batteries manufactured in
Manufacturing Example 2. In FIG. 7A, D1 is a secondary battery
containing 9 wt % of the polymer electrolyte, E1 is a secondary
battery containing 7 wt % of the polymer electrolyte, and F1 is a
secondary battery containing 5 wt % of the polymer electrolyte. As
shown in FIG. 7A, the change in R.sub.SC according to time (t) may
be calculated by applying changes in V.sub.SC to Equation 2.
[0128] FIG. 7B is a graph illustrating changes in heating value W
according to time (t), of the secondary batteries manufactured
according to Manufacturing Example 2. In FIG. 7B, D2 is a secondary
battery containing 9 wt % of the polymer electrolyte, E2 is a
secondary battery containing 7 wt % of the polymer electrolyte, and
F2 is a secondary battery containing 5 wt % of the polymer
electrolyte. The change in heating value W according to time may be
calculated by applying changes in R.sub.SC according to time, as
illustrated in FIG. 7A, to Equation 1. Referring to FIG. 7B, each
secondary battery includes the time having the maximum
instantaneous heating value, and the R.sub.SC value at the maximum
instantaneous heating value corresponds to the critical short
resistance.
[0129] FIG. 7C is a graph illustrating a relationship between the
short resistance R.sub.SC and the heating value W of the secondary
batteries manufactured according to Manufacturing Example 2. In
FIG. 7C, D3 is a secondary battery containing 9 wt % of the polymer
electrolyte, E3 is a secondary battery containing 7 wt % of the
polymer electrolyte, and F3 is a secondary battery containing 5 wt
% of the polymer electrolyte.
[0130] Referring to FIG. 7C, each secondary battery includes the
critical short resistance indicated by the circle (that is, the
R.sub.SC value at the maximum instantaneous heating value). Here,
it can be seen that, in the case of D3 containing 9 wt % of the
polymer electrolyte, the critical short resistance is present in a
region excluding a "R.sub.SC range having an ignition risk."
However, in the cases of E3 and F3 containing 7 and 5 wt % of the
polymer electrolytes, respectively, the critical short resistances
are present in the "R.sub.SC range having an ignition risk."
[0131] In other words, as can be seen from FIGS. 7A to 7C, when a
secondary battery contains 7 wt % or less of the polymer
electrolyte, the critical short resistance is present in the
"R.sub.SC range having an ignition risk," and thus, the battery
safety is not be improved. For this reason, in Manufacturing
Example 2, the content of the polymer electrolyte may be 9 wt % or
more. Moreover, the secondary batteries according to Manufacturing
Example 2 may contain the polymer electrolyte at 20 wt % or
less.
[0132] The polymer electrolyte is added to the cells in a monomer
state, and polymerized into a gel by curing, after assembly.
However, when the content of the polymer electrolyte is more than
20 wt %, the viscosity of the polymer electrolyte composition
interferes with the impregnation of the polymer electrolyte
composition into an electrode plate, during the addition of the
polymer electrolyte. The polymerization is also not uniform,
thereby creating localized non-polymerized regions and decreasing
ionic conductivity of the polymer electrolyte composition. Thus,
the secondary battery may not exhibit high charge and discharge
rates.
[0133] As a result, the secondary batteries according to
Manufacturing Example 2 of the present invention may contain 9 to
20 wt % of the polymer electrolyte, based on the total weight of
the polymer electrolyte composition.
[0134] Consequently, aspects of the present invention can provide a
secondary battery in which improved safety can be ensured, by
artificially controlling an internal resistance value (short
resistance value) generating a heating value, even if a short
circuit occurs in the secondary battery.
[0135] Although a few exemplary embodiments of the present
disclosure have been shown and described, it would be appreciated
by those skilled in the art that changes may be made in these
exemplary embodiments, without departing from the principles and
spirit of the present disclosure, the scope of which is defined in
the claims and their equivalents.
* * * * *