U.S. patent application number 13/866265 was filed with the patent office on 2013-10-24 for electrode for lithium secondary battery.
The applicant listed for this patent is LG CHEM, LTD.. Invention is credited to Soon Ho AHN, Young Soo KIM.
Application Number | 20130280608 13/866265 |
Document ID | / |
Family ID | 35136859 |
Filed Date | 2013-10-24 |
United States Patent
Application |
20130280608 |
Kind Code |
A1 |
KIM; Young Soo ; et
al. |
October 24, 2013 |
ELECTRODE FOR LITHIUM SECONDARY BATTERY
Abstract
Disclosed is a method for forming an electrode comprising a
protective layer, which comprises: dispersing or dissolving an
aliphatic nitrile compound into a solvent to provide a coating
solution; coating a surface of the electrode with the coating
solution, said electrode having an electrode active material coated
thereon; and removing the solvent used in the coating solution by
drying to form a protective layer comprising an aliphatic nitrile
compound-electrode active material complex.
Inventors: |
KIM; Young Soo; (Daejeon,
KR) ; AHN; Soon Ho; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG CHEM, LTD. |
Seoul |
|
KR |
|
|
Family ID: |
35136859 |
Appl. No.: |
13/866265 |
Filed: |
April 19, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11056290 |
Feb 11, 2005 |
8445143 |
|
|
13866265 |
|
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|
|
Current U.S.
Class: |
429/212 ;
427/58 |
Current CPC
Class: |
H01M 4/139 20130101;
Y02E 60/10 20130101; H01M 10/0567 20130101; H01M 4/0402 20130101;
H01M 10/0525 20130101; H01M 4/628 20130101; H01M 2300/004 20130101;
H01M 4/62 20130101; H01M 4/0419 20130101; H01M 10/4235 20130101;
H01M 4/0404 20130101 |
Class at
Publication: |
429/212 ;
427/58 |
International
Class: |
H01M 4/62 20060101
H01M004/62; H01M 4/139 20060101 H01M004/139; H01M 4/04 20060101
H01M004/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2004 |
KR |
10-2004-10070 |
Claims
1. A method for forming an electrode comprising a protective layer,
which comprises: dispersing or dissolving an aliphatic nitrile
compound into a solvent to provide a coating solution; coating a
surface of the electrode with the coating solution, said electrode
having an electrode active material coated thereon; and removing
the solvent used in the coating solution by drying to form a
protective layer comprising an aliphatic nitrile compound-electrode
active material complex.
2. The method according to claim 1, wherein the aliphatic nitrile
compound is an aliphatic dinitrile compound.
3. The method according to claim 1, wherein the aliphatic nitrile
compound is represented by the following formula 1: ##STR00002##
wherein R is C2-C15 alkylene.
4. The method according to claim 1, wherein the aliphatic nitrile
compound is selected from the group consisting of succinonitrile,
glutaronitrile, aponitrile, pimelonitrile, octanedinitrile,
azelonitrile, sebaconitrile, 1,9-dicyanononane and
dodecanedinitrile.
5. The method according to claim 1, wherein the protective layer
comprises a ligand-metal bond between the electrode active material
and the aliphatic nitrile compound.
6. The method according to claim 1, wherein the coating method is
performed by dip coating or spray coating.
7. The method according to claim 1, wherein the solvent is selected
from the group consisting of acetone, THF (tetrahydrofuran), NMP
(N-methyl-2-pyrrolidone) and carbonate solvent.
8. The method according to claim 7, wherein the solvent is a
carbonate solvent selected from the group consisting of ethylene
carbonate, propylene carbonate, gamma-butyrolactone, diethyl
carbonate, dimethyl carbonate, and ethylmethyl carbonate.
9. The method according to claim 1, wherein the drying step is
performed at a controlled temperature of between 90.degree. C. and
110.degree. C.
10. The method according to claim 1, wherein the drying step is
performed at a controlled drying rate of 3 m/min or less under a
controlled vent flow of 2000-3000 rpm.
11. The method according to claim 1, wherein the electrode is
treated at a high temperature of 30.degree. C.-90.degree. C. before
or after assemblage of a battery.
12. The method according to claim 1, wherein the electrode active
material comprises an oxide of a transition metal of the electrode
active material.
13. The method according to claim 1, wherein the aliphatic nitrile
compound is present in an amount of 1 to 10 weight percent, based
on the total weight of the electrode active material.
14. The method according to claim 1, wherein the aliphatic nitrile
compound is present in an amount of 1 to 5 weight percent, based on
the total weight of the electrode active material.
15. The method according to claim 1, wherein the aliphatic nitrile
compound is present in an amount of 1 to 2.5 weight percent, based
on the total weight of the electrode active material.
16. The method according to claim 1, wherein the aliphatic nitrile
compound is present in an amount of 1 to 20 weight percent, based
on the total weight of electrolyte present in a battery.
17. The method according to claim 1, wherein the aliphatic nitrile
compound is used in the range of between 1:9 and 9:1, expressed in
the weight ratio to the solvent.
18. A lithium secondary battery comprising an electrode prepared by
the method of claim 1 as a cathode.
Description
[0001] This application is a Continuation of co-pending U.S.
application Ser. No. 11/056,290, filed Feb. 11, 2005. This
application also claims priority under U.S.C. 119(a) to Patent
Application No. 10-2004-100700 filed in Republic of Korea on Feb.
16, 2004. The entire contents of the above applications are hereby
incorporated by reference into the present application.
TECHNICAL FIELD
[0002] The present invention relates to an electrode including an
aliphatic nitrile compound. More particularly, the present
invention relates to an electrode whose surface is coated with an
aliphatic nitrile compound or which comprises an electrode active
material comprising an aliphatic nitrile compound, as well as to a
lithium secondary battery having the same electrode.
BACKGROUND ART
[0003] In general, a non-aqueous electrolyte comprising a lithium
salt such as LiPF.sub.6 and a carbonate solvent reacts continuously
with the surface of a cathode active material (particularly,
LiCoO.sub.2) during repeated charge/discharge cycles, resulting in
the continuous formation of a resistance layer that causes an
increase in resistance and interrupts conduction of Li.sup.+ ions.
Such resistance layer causes the active material particles to be
isolated among themselves or from a current collector (Al foil),
thereby detracting from battery performance and life
characteristics. Further, such problems increasingly and
predominantly occur at a high temperature to accelerate side
reactions between an electrolyte and the surface of a cathode when
a battery is stored at a high temperature (45.degree. C. or
60.degree. C.) for a long time, resulting in a significant decrease
in the lifetime of a battery.
[0004] Meanwhile, non-aqueous electrolyte-based secondary batteries
have problems related with safety upon overcharge for the following
reasons. Cathode active materials such as lithium and/or lithium
ion-containing metal oxides capable of lithium ion
intercalation/deintercalation are converted into thermally unstable
substances due to the release of lithium during overcharge. When
the temperature of a battery reaches the critical temperature,
oxygen is liberated from such unstable substances and the free
oxygen may react with the solvent of an electrolyte, etc., through
a highly exothermic reaction mechanism. Therefore, such a series of
exothermal reactions by heating results in thermal runaway.
[0005] Generally, factors affecting the safety of a battery
include: (1) heat emission due to oxidation of electrolytes; and
(2) heat emission resulting from the structural collapse of a
cathode due to overcharge. When overcharge proceeds, heat emission
occurring from the above factors independently or simultaneously
causes an increase in the internal temperature of a battery,
followed by ignition or explosion of the battery. Thus, batteries
show a safety problem upon overcharge.
[0006] Meanwhile, when external physical impacts (for example,
exposure to high temperature such as a temperature of 150.degree.
C. or higher by heating) are applied to a battery while the battery
is charged or overcharged, the battery is overheated due to the
heat emission caused by the reaction of an inflammable electrolyte
with a cathode active material, and the structure of an electrode
(particularly, a cathode) is collapsed to generate oxygen, which
accelerates the combustion of the electrolyte. Therefore, a
separator disposed between a cathode and an anode is melted and the
electrical energy induces thermal runaway, resulting in ignition
and explosion of the battery.
DISCLOSURE OF THE INVENTION
[0007] The present inventors have found that an aliphatic nitrile
compound that forms a strong bond with a transition metal or
transition metal oxide in an electrode active material can improve
the safety of a battery, when the battery is overcharged and/or
subjected to physical impacts applied from the exterior of the
battery (for example, exposure to high temperature by heating).
Meanwhile, we have also recognized a problem in that when an
aliphatic dinitrile compound is used as an additive for
electrolyte, there is an increase in viscosity of the electrolyte
so that diffusion of Li ions cannot be made smoothly under extreme
conditions (a low temperature of between -20.degree. C. and
-10.degree. C.), resulting in degradation of battery performance at
a low temperature.
[0008] Therefore, the present invention has been made in view of
the above-mentioned problems. It is an object of the present
invention to improve the safety of a battery with no degradation of
battery performance by incorporating an aliphatic dinitrile
compound uniformly into an electrode so that the aliphatic nitrile
compound can contribute only to the formation of a complex with an
electrode active material.
[0009] According to an aspect of the present invention, there is
provided an electrode comprising an aliphatic nitrile compound,
preferably a compound represented by the following formula 1, whose
surface is coated with the aliphatic nitrile compound or which
comprises an electrode active material comprising the aliphatic
nitrile compound. According to another aspect of the present
invention, there is provided a lithium secondary battery having the
above-described electrode.
##STR00001##
[0010] wherein R is a C2-C15 alkane.
[0011] Preferably, the aliphatic nitrile compound, preferably the
compound represented by formula 1 is coated uniformly on the
surface of an electrode active material in an electrode.
[0012] Additionally, it is preferable that the electrode according
to the present invention includes a complex formed between the
surface of the electrode active material and the aliphatic nitrile
compound.
[0013] Hereinafter, the present invention will be explained in more
detail.
[0014] According to the present invention, the electrode for a
lithium secondary battery is characterized by comprising an
aliphatic nitrile compound, preferably the compound represented by
the above formula 1.
[0015] Aliphatic nitrile compounds can form a strong bond with a
transition metal or transition metal oxide such as cobalt exposed
to the surface of an electrode active material through their cyano
functional groups having high dipole moment. Particularly, the
cyano functional groups can form a stronger complex on the surface
of an electrode active material at a temperature of 45.degree. C.
or higher (see, FIG. 1).
[0016] An electrode coated with an aliphatic nitrile compound has a
strong protection surface that protects the surface of electrode
from side reactions with an electrolyte. Therefore, it is possible
to accomplish efficient lithium ion intercalation/deintercalation
without varying viscosity of the electrolyte and ion conductivity,
and to prevent the formation of a resistance layer capable of
detracting from battery performance by the reaction of the
electrolyte with electrode during repeated charge/discharge cycles,
on the surface of electrode. As a result, it is possible to
maintain battery performance. Further, according to the present
invention, a lithium secondary battery having an electrode
uniformly coated with an aliphatic nitrile compound on the surface
of an electrode active material, and preferably comprising an
aliphatic nitrile compound forming a strong complex with a
transition metal and/or transition metal oxide present on the
surface of electrode active material, can stabilize the transition
metal and transition metal oxide to prevent a partial release of
the transition metal from the electrode active material during
repeated charge/discharge cycles. In addition, when external
physical impacts are applied to a battery (particularly, when a
battery is exposed to high temperature such as a temperature of
150.degree. C. or higher), it is possible to efficiently inhibit an
exothermic reaction caused by the reaction of an electrolyte
directly with the electrode surface and to retard the structural
collapse of the electrode active material, thereby preventing
ignition and explosion resulting from an increase in temperature
inside of the battery. More particularly, because aliphatic nitrile
compounds can protect the electrode surface more strongly at a high
temperature of 45.degree. C. or higher than room temperature, it is
possible to provide thermally stable electrodes.
[0017] Although the compound represented by the above formula 1 is
exemplified as an aliphatic nitrile compound that can be
incorporated into an electrode according to the present invention,
another aliphatic nitrile compound having a nitrile group only at
one side, compared to the compound represented by formula 1, has a
great possibility for providing safety and/or battery performance
in such a degree as to be equivalent to the compound represented by
formula 1, and thus it is also included in the scope of the present
invention.
[0018] Meanwhile, alkanes present in the compound represented by
formula 1 have no reactivity. Therefore, when the compound
represented by formula 1 is incorporated into an electrode, a
possibility for an irreversible reaction is low. As a result,
addition of the compound represented by formula 1 does not cause
degradation in battery performance.
[0019] Because an aromatic nitrile compound decomposes at an anode
during the initial charge cycle (during formation) to increase
irreversible capacity and to degrade battery performance
significantly, it is not preferable to incorporate an aromatic
nitrile compound into an electrode and to coat an electrode with an
aromatic nitrile compound.
[0020] Particular examples of the compound represented by formula 1
include succinonitrile (R.dbd.C.sub.2H.sub.4), glutaronitrile
(R.dbd.C.sub.3H.sub.6), adiponitrile (R.dbd.C.sub.4H.sub.8),
pimelonitrile (R.dbd.C.sub.5H.sub.10) octanedinitrile
(R.dbd.C.sub.6H.sub.12), azelonitrile (R.dbd.C.sub.7H.sub.14)
sebaconitrile (R.dbd.C.sub.8H.sub.16) 1,9-dicyanononane
(R.dbd.C.sub.9H.sub.8) dodecanedinitrile (R.dbd.C.sub.10H.sub.20),
etc., but are not limited thereto.
[0021] Particularly, succinonitrile forms the strongest protection
layer among the compounds represented by formula 1. The longer the
alkane is, the weaker the protection layer to be formed becomes.
Therefore, it is most preferable to use succinonitrile as a coating
material among the above compounds.
[0022] The aliphatic nitrile compound is present in an electrode
preferably in an amount of 0.1-20 wt % based on the weight of
electrolyte or 1-10 wt % based on the weight of active material,
more preferably in an amount of 10 wt % or less based on the weight
of electrolyte or 5 wt % or less based on the weight of active
material, and most preferably in an amount of 5 wt % or less based
on the weight of electrolyte or 2.5 wt % or less based on the
weight of active material.
[0023] In order to incorporate an aliphatic nitrile compound into
an electrode, a coating solution containing an aliphatic nitrile
compound may be applied on an electrode. Otherwise, an aliphatic
nitrile compound may be added to slurry for electrode active
material to form an electrode.
[0024] For the purpose that the nitrile compound participates only
in complex formation with a transition metal oxide of an electrode
active material, a coating solution containing an aliphatic nitrile
compound is applied to an electrode or an aliphatic nitrile
compound is added to electrode active material-containing slurry in
an adequate amount. Preferably, the electrode or slurry comprising
the nitrile compound is treated at a high temperature. Then, the
surface of electrode, namely the surface of electrode active
material can be protected uniformly with the aliphatic nitrile
compound. In addition to the above-mentioned high-temperature
treatment applied to an electrode or slurry, a battery may be
preferably treated at a high temperature after the assemblage
thereof.
[0025] The aliphatic nitrile compound is dispersed or dissolved
into a solvent to provide a solution, the solution is coated on the
surface of an electrode and then the solvent is dried in order to
coat the electrode surface, preferably the surface of electrode
active material with the aliphatic nitrile compound. The coating
method may include dip coating, spray coating, or the like.
[0026] There is no particular limitation in selection of the
solvent for use in the coating solution containing an aliphatic
nitrile compound, as long as the solvent has good compatibility. It
is preferable to use, as a solvent for coating solution, non-polar
solvents such as THF (tetrahydrofuran) and polar solvents such as
NMP (N-methyl-2-pyrollidone) and carbonate solvents used as a
solvent for electrolyte. Although the amount of aliphatic nitrile
compound varies with the amount to be coated on an electrode, the
aliphatic nitrile compound may be used in the range of between 1:9
and 9:1, expressed in the weight ratio to the solvent.
[0027] The method for forming an electrode by adding an aliphatic
nitrile compound to slurry for electrode active material includes
the steps of: mixing an aliphatic nitrile compound with an
electrode active material and other additives such as a binder and
conductive agent, as necessary, to form slurry for electrode active
material; applying the slurry for electrode active material on a
collector; and removing the solvent used in the slurry by drying,
etc.
[0028] In order to apply the slurry for electrode active material,
die coating, roll coating, comma coating and combinations thereof
may be used.
[0029] Meanwhile, because the compound represented by formula 1
starts to be slightly volatilized at a high temperature of
100.degree. C. or higher and then be substantially evaporated
without leaving residues at a temperature of about 150.degree. C.,
it is necessary to maintain an adequate drying temperature, drying
rate and vent flow for the purpose of coating an electrode smoothly
with the compound represented by formula 1 from slurry containing
NMP as a solvent.
[0030] To prevent the compound represented by formula 1 from being
volatilized and to remove residual NMP, the drying temperature
preferably ranges from 90.degree. C. to 110.degree. C. The drying
rate is preferably 3 m/min or less, more preferably 2 m/min or
less, but may be varied with the length of a drying furnace and the
drying temperature of slurry. The vent flow is preferably 2000-3000
rpm.
[0031] More particularly, when the electrode comprising the
compound represented by formula 1 is dried at an excessively low
temperature in order to retain the compound in the electrode, NMP
content and water content in the electrode increase, thereby
causing a problem in that battery performance is degraded. On the
other hand, when the electrode is dried at an excessively high
temperature, NMP content in the electrode decreases but the
compound represented by formula 1 is substantially volatilized, and
thus it is not possible to obtain a uniformly coated electrode.
Therefore, it is important that the drying temperature, drying rate
and vent flow are maintained within the above ranges.
[0032] Meanwhile, it is preferable that aliphatic nitrile compounds
form a complex with the surface of an electrode active material.
Preferably, for the purpose of forming a complex, an electrode
comprising an electrode active material whose surface is coated
with an aliphatic nitrile compound is further treated at a high
temperature. Particularly, the high-temperature treatment may be
performed at such a temperature range as not to affect the
electrode active material and binder, generally at a temperature of
180.degree. C. or lower. Otherwise, although the high-temperature
treatment varies with the kind of the aliphatic nitrile compound,
it may be performed at such a temperature range as to prevent
evaporation of the aliphatic nitrile compound, generally at a
temperature of 120.degree. C. or lower. In general, the
high-temperature treatment is suitably performed at a temperature
of between 60.degree. C. and 90.degree. C. Long-time storage at a
temperature of between 30.degree. C. and 40.degree. C. may result
in the same effect.
[0033] As a cathode active material for use in electrodes,
lithium-containing transition metal oxides may be used. The cathode
active material can be at least one selected from the group
consisting of LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4,
LiMnO.sub.2 and LiNi.sub.1-XCo.sub.XO.sub.2 (wherein 0<X<1).
Meanwhile, as an anode active material for use in electrodes,
carbon, lithium metal or lithium alloy may be used. In addition,
other metal oxides capable of lithium intercalation/deintercalation
and having an electric potential of 2V or less based on lithium
(for example, TiO.sub.2 and SnO.sub.2) may be used as an anode
active material.
[0034] Slurry for electrodes may further comprise a binder,
conductive agent, viscosity controller, supplementary binder, etc.,
in addition to active materials.
[0035] Any conventional collectors made of conductive materials can
be used with no particular limitation. More particularly,
collectors made of metals such as iron, copper, aluminum and nickel
are widely used.
[0036] A lithium secondary battery, to which the electrode
comprising an aliphatic nitrile compound according to the present
invention may be applied, can comprise: [0037] (1) a cathode
capable of lithium ion intercalation/deintercalation; [0038] (2) an
anode capable of lithium ion intercalation/deintercalation; [0039]
(3) a porous separator; and [0040] (4) a) a lithium salt; and
[0041] b) an electrolyte compound.
[0042] Non-aqueous electrolytes for lithium secondary batteries
generally include flammable non-aqueous organic solvents including
cyclic carbonates and/or linear carbonates. Particular examples of
cyclic carbonates that may be used in the present invention include
ethylene carbonate (EC), propylene carbonate (PC),
gamma-butyrolactone (GBL), etc. Typical examples of linear
carbonates include diethyl carbonate (DEC), dimethyl carbonate
(DMC), ethylmethyl carbonate (EMC).
[0043] When the electrode comprising an aliphatic nitrile compound
according to the present invention is used, it is possible to
inhibit thermal runaway and to improve safety of batteries without
causing degradation in battery performance even if such
conventional flammable non-aqueous organic solvents are used as
electrolytes.
[0044] Non-aqueous electrolytes comprise lithium salts such as
LiClO.sub.4, LiCF.sub.3SO.sub.3, LiPF.sub.6, LiBF.sub.4,
LiAsF.sub.6, LiN(CF.sub.3SO.sub.2).sub.2, etc.
[0045] The lithium secondary battery according to the present
invention may have a cylindrical, prismatic or pouch-like
shape.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a graph showing XPS (X-ray photoelectron
spectroscopy) data of the cathodes in the batteries obtained from
Example 1 and Comparative Example 1.
[0047] FIG. 2 is a graph showing the heat emission peaks and the
results of heat emission control for the cathodes in the batteries
obtained from Example 1 and Comparative Example 1.
[0048] FIG. 3 is a graph showing the heat emission peaks and the
results of heat emission control for the cathodes in the batteries
obtained from Comparative Example 1 and Examples 1, 3, 5, 7 and
8.
[0049] FIG. 4 is a graph showing the heat emission peaks and the
results of heat emission control for the cathodes in the batteries
obtained from Comparative Example 1 and Examples 1, 2, 4 and 6.
[0050] FIG. 5 is a graph showing the results of an overcharge test
for the battery obtained in Example 1 under the condition of 6V/1 A
(voltage, temperature).
[0051] FIG. 6 is a graph showing the results of an overcharge test
for the battery obtained in Comparative Example 1 under the
condition of 6V/1 A (voltage, temperature).
[0052] FIG. 7 is a graph showing the results of an overcharge test
for the battery obtained in Example 1 under the condition of 6V/2 A
(voltage, temperature).
[0053] FIG. 8 is a graph showing the results of an overcharge test
for the battery obtained in Example 1 under the condition of 12V/1
C (voltage, temperature).
[0054] FIG. 9 is a graph showing the results of an overcharge test
for the battery obtained in Example 1 under the condition of 20V/1
C (voltage, temperature).
[0055] FIG. 10 is a graph showing the results of a 160.degree.
C.-high temperature exposure test for the battery obtained in
Comparative Example 1 (voltage, temperature).
[0056] FIGS. 11 and 12 are graphs showing the results of
160.degree. C.- and 170.degree. C.-high temperature exposure tests
for the battery obtained in Example 1 (voltage, temperature).
[0057] FIG. 13 is a graph showing the results obtained by measuring
the variation in battery thickness after each battery according to
Examples 3-8 and Comparative Examples 1 and 2 was exposed to a high
temperature of 90.degree. C. for 4 hours.
[0058] FIG. 14 is a graph showing the battery performance at a low
temperature for the battery according to Example 9 compared to the
battery according to Comparative Example 2.
[0059] FIG. 15 is a graph showing interfacial resistance values of
each battery according to Comparative Example 2 and Example 9 after
storing each battery at a high temperature.
[0060] FIG. 16 is a graph showing cycle characteristics of the
batteries according to Example 18 and Comparative Example 11 at
45.degree. C.
BEST MODE FOR CARRYING OUT THE INVENTION
[0061] Reference will now be made in detail to the preferred
embodiments of the present invention. It is to be understood that
the following examples are illustrative only and the present
invention is not limited thereto.
EXAMPLES
Example 1
[0062] Succinonitrile was diluted with acetone as a solvent in the
weight ratio of 3:7 to provide a solution, into which a cathode was
dipped. Then, the cathode was high-temperature treated at
30.degree. C. for 2 days to evaporate the solvent, thereby
providing a cathode comprising succinocitrile forming a complex
with the surface of cathode active material. The cathode active
material was LiCoO.sub.2. Artificial graphite was used as an anode
active material. The electrolyte used in this example was 1M
LiPF.sub.6 solution formed of EC:PC:DEC=3:2:5. A 383562-type
lithium polymer battery was manufactured by using a conventional
method and the battery was packed with an aluminum laminate
packaging material to provide a battery pack. Next, the battery was
aged by treating it again at a high temperature of 60.degree. C.
for 12 hours or more so that any unreacted and/or residual
succinonitrile in the electrode can form a complex.
Examples 2-8
[0063] Example 1 was repeated to provide lithium polymer batteries,
except that glutaronitrile (R.dbd.C.sub.3H.sub.6) (Example 2),
adiponitrile (R.dbd.C.sub.4H.sub.8) (Example 3), pimelonitrile
(R.dbd.C.sub.5H.sub.10) (Example 4), octanedinitrile
(R.dbd.C.sub.6H.sub.12) (Example 5), azelonitrile
(R.dbd.C.sub.7H.sub.14) (Example 6), sebaconitrile
(R.dbd.C.sub.8H.sub.16) (Example 7) and dodecanedinitrile
(R.dbd.C.sub.10H.sub.20) (Example 8) were used, instead of
succinonitrile (R.dbd.C.sub.2H.sub.4).
Comparative Example 1
[0064] Example 1 was repeated to provide a lithium polymer battery,
except that the cathode was not dipped into the aliphatic nitrile
compound-containing solution.
Comparative Example 2
[0065] 1M LiPF.sub.6 solution formed of EC:EMC=1:2 was used as an
electrolyte, to which 3 wt % of succinonitrile
(R.dbd.C.sub.2H.sub.4) was added. Artificial graphite and
LiCoO.sub.2 were used as an anode active material and cathode
active material, respectively, to provide a 523450-type prismatic
lithium battery according to a conventional method. Next, the
battery was aged at a high temperature of 60.degree. C. for 12
hours or more.
Example 9
[0066] A cathode was dipped into a solution containing
succinonitrile (R.dbd.C.sub.2H.sub.4) in a acetone as a solvent,
and then was high-temperature treated at 30.degree. C. for 2 days
to evaporate the solvent, thereby providing a cathode comprising
3-5 wt % of succinocitrile, based on the weight of electrolyte,
forming a complex with the surface of cathode active material. The
cathode active material was LiCoO.sub.2. The electrolyte used in
this example was 1M LiPF.sub.6 solution formed of EC:EMC=1:2.
Artificial graphite was used as an anode active material. A
523450-type prismatic lithium battery was manufactured by using a
conventional method. Next, the battery was aged by treating it
again at a high temperature of 60.degree. C. for 12 hours or more
so that any unreacted and/or residual succinonitrile in the
electrode can form a complex.
Comparative Examples 3-10
[0067] The electrolyte used in these examples was 1M LiPF.sub.6
solution formed of EC:EMC=1:2. To the above electrolyte,
succinonitrilte (R.dbd.C.sub.2H.sub.4) (Comparative Example 3),
glutaronitrile (R.dbd.C.sub.3H.sub.6) (Comparative Example 4),
adiponitrile (R.dbd.C.sub.4H.sub.8) (Comparative Example 5),
pimelonitrile (R.dbd.C.sub.6H.sub.10) (Comparative Example 6),
octanedinitrile (R.dbd.C.sub.6H.sub.12) (Comparative Example 7),
azelonitrile (R.dbd.C.sub.7H.sub.14) (Comparative Example 8),
sebaconitrile (R.dbd.C.sub.8H.sub.16) (Comparative Example 9) and
dodecanedinitrile (R.dbd.C.sub.10H.sub.20) (Comparative Example 10)
were added, each in the amount of 3 wt %. Artificial graphite and
LiCoO.sub.2 were used as an anode active material and cathode
active material, respectively. 383562 type lithium polymer
batteries were manufactured by using a conventional method and the
batteries were packed with aluminum laminate packaging materials to
provide battery packs. Next, the batteries were aged at a high
temperature of 60.degree. C. for 12 hours or more.
Examples 10-17
[0068] Cathodes were dipped into solutions, each containing
succinonitrile (R.dbd.C.sub.2H.sub.4) (Example 10), glutaronitrile
(R.dbd.C.sub.3H.sub.6) (Example 11), adiponitrile
(R.dbd.C.sub.4H.sub.8) (Example 12), pimelonitrile
(R.dbd.C.sub.6H.sub.10) (Example 13), octanedinitrile
(R.dbd.C.sub.6H.sub.12) (Example 14), azelonitrile
(R.dbd.C.sub.7H.sub.14) (Example 15), sebaconitrile
(R.dbd.C.sub.8H.sub.16) (Example 16) and dodecanedinitrile
(R.dbd.C.sub.10H.sub.20) (Example 17). Then each cathode was
high-temperature treated at 30.degree. C. for 2 days to evaporate
the solvent, thereby providing a cathode whose surface was coated
with 3-5 wt % of each aliphatic nitrile compound (based on the
weight of electrolyte). The cathode active material was
LiCoO.sub.2.
[0069] The electrolyte used in these examples was 1M LiPF.sub.6
solution formed of EC:EMC=1:2. Artificial graphite was used as an
anode active material. 383562 type lithium polymer batteries were
manufactured by using a conventional method and the batteries were
packed with aluminum laminate packaging materials to provide
battery packs. Next, the batteries were aged at a high temperature
of 60.degree. C. for 12 hours or more.
Example 18
[0070] To cathode slurry containing LiCoO.sub.2 as a cathode active
material, Super-p as a conductive agent, PVDF homopolymer as a
binder and NMP as a solvent, 5 wt % of succinonitrile based on the
weight of electrolyte (2.5 wt % of succinonitrile based on the
weight of cathode active material) was added and then stirred. The
mixed slurry was applied on a collector and vacuum dried at about
1000 for 24 hours or more so as to prevent the evaporation of
succinonitrile at the highest degree and to remove residual NMP.
Further, the drying rate (2 m/min) and vent flow (2100 rpm) were
dropped as low as possible so that the slurry can be coated
smoothly on the current collector. By doing so, a cathode that
comprises succinonitrile forming a complex with the surface of
cathode active material and is coated uniformly with succinocitrile
was obtained.
[0071] Artificial graphite was used as an anode active material. 1M
LiPF.sub.6 solution formed of EC:PC:DEC=3:2:5 was used as an
electrolyte, to which 1 wt % of VC was added. A 323456-type lithium
polymer battery was manufactured by using a conventional method and
the battery was packed with an aluminum laminate packaging material
to provide a battery pack. Next, the battery was aged by treating
it again at a high temperature of 60.degree. C. for 12 hours or
more so that any unreacted and/or residual succinonitrile in the
electrode can form a complex.
Comparative Example 11
[0072] Example 18 was repeated to provide a lithium polymer
battery, except that no aliphatic nitrile compound was added to the
cathode slurry.
[0073] [Experimental Results]
[0074] 1. Test for Formation of Ligands on Cathode Surface
[0075] Each battery obtained from Example 1 and Comparative Example
1 was fully charged to 4.2V and each cathode was separated from
each battery to prepare a sample of 1 cm.times.1 cm size.
Additionally, each sample was cleaned with dimethyl carbonate (DMC)
to remove impurities remaining on the surface and then was tested
for checking the formation of ligands by using a general surface
analyzing apparatus based on XPS (X-ray photoelectron
spectroscopy). The XPS apparatus (ESCALAB 250) used in this test is
one that shows constitutional elements forming a surface by
detecting specific binding energy and kinetic energy of atoms and
reading atomic information to the depth of several nanometers from
the surface. Complex formation for the electrode comprising a
nitrile compound was checked through a peak corresponding to the
formation of nitrogen atoms. As shown in FIG. 1, nitrogen atoms
were not detected on the surface of cathode in the case of the
battery (Comparative Example 1) using no succinonitrile. On the
other hand, in the case of the battery (Example 1) using
succinonitrile, nitrogen atoms were clearly detected by the
presence of a strong bond formed between succinonitrile and cobalt
transition metal or metal oxide in the cathode active material. The
above XPS results indicate that a cyano functional group was bonded
to cobalt metal or metal oxide to form a complex on the
surface.
[0076] From the result, it could be expected that an aliphatic
nitrile additive could form a strong complex with the surface of
cathode active material, thereby inhibiting side reactions
generated from the battery during repeated charge/discharge
cycles.
[0077] 2. Test for Heat Emission Control
[0078] Each battery obtained from Examples 1-8 and Comparative
Example 1 was charged to 4.2V. A general thermogravimetric
analyzer, DSC (Differential Scanning calorimeter) was used, wherein
two high-pressure pans resistant to vapor pressure of the
electrolyte were used as pans for measurement. To one pan, about
5-10 mg of the cathode sample separated from each of the batteries
according to Examples 1-8 and Comparative Example 1 was introduced,
while the other pan was left empty. Calorific difference between
two pans was analyzed while the pans were heated at a rate of
5.degree. C./min to 350.degree. C. to measure temperature peaks
where heat emission occurs.
[0079] As shown in FIG. 2, the battery (Comparative Example 1)
using the electrode comprising no aliphatic nitrile compound shows
heat emission peaks at about 200.degree. C. and 240.degree. C. The
peak at about 200.degree. C. indicates heat emission caused by the
reaction between the electrolyte and cathode, while the peak at
about 240.degree. C. indicates heat emission caused by combined
factors including the reaction between the electrolyte and cathode
and the structural collapse of the cathode. On the contrary, as
shown in FIGS. 2, 3 and 4, each battery using the electrode
comprising succinonitrile (R.dbd.C.sub.2H.sub.4) (Example 1),
glutaronitrile (R.dbd.C.sub.3H.sub.6) (Example 2), adiponitrile
(R.dbd.C.sub.4H.sub.8) (Example 3), pimelonitrile
(R.dbd.C.sub.5H.sub.10) (Example 4), octanedinitrile
(R.dbd.C.sub.6H.sub.12) (Example 5), azelonitrile
(R.dbd.C.sub.7H.sub.14) (Example 6), sebaconitrile
(R.dbd.C.sub.8H.sub.16) (Example 7) or dodecanedinitrile
(R.dbd.C.sub.10H.sub.20) (Example 8) does not show the above two
temperature peaks. This indicates that it was possible to inhibit
heat emission caused by the reaction between the electrolyte and
cathode and structural collapse of the cathode, in the case of the
batteries according to the present invention.
[0080] 3. Overcharge Test
[0081] Each battery obtained from Example 1 and Comparative Example
1 was tested under the overcharge conditions of 6V/1 A, 6V/2 A,
12V/1 C and 20V/1 C in a CC/CV (Constant Current/Constant Voltage)
manner. The test results including variation in temperatures are
shown in FIGS. 5-9. As shown in FIGS. 5-9, the battery according to
Example 1 shows more improved safety compared to the battery
according to Comparative Example 1 (test results for the battery
according to Comparative Example 1 are shown in FIG. 6 only for the
test condition of 6V/1 A and the others are not shown).
[0082] Particularly, as can be seen from the peak temperature in
FIG. 6 (Comparative Example 1), the battery was fired and subjected
to short circuit at a measuring temperature of 200.degree. C. or
higher due to the oxidation of electrolyte present in the battery
and the exothermic reaction resulting from the structural collapse
of the cathode. On the contrary, the secondary battery using the
electrode comprising succinonitrile (Example 1) shows a peak
temperature of about 100.degree. C. This indicates that exothermic
reactions were inhibited in the battery according to Example 1.
[0083] The above overcharge test was repeated many times and the
average values for the test results are shown in the following
Table 1.
TABLE-US-00001 TABLE 1 6 V/1 A 6 V/2 A 12 V/1 C 20 V/1 C Ex. 1 PASS
PASS PASS PASS Comp. Ex. 1 FIRE FIRE FIRE FIRE
[0084] 4. Hot Box Test
[0085] Each battery obtained from Example 1 and Comparative Example
1 was fully charged. The charged batteries were introduced into an
oven capable of convection and heated at a rate of 5.degree. C./min
from room temperature to 160.degree. C. and 170.degree. C. Then the
batteries were exposed to such high temperatures for 1 hour to
check whether they are fired or not.
[0086] The battery according to Comparative Example 1 was fired at
160.degree. C. when heated at a rate of 5.degree. C./min (FIG. 10),
while the battery according to Example 1 was not fired under the
same condition (FIGS. 11 and 12).
[0087] 5. Test for Battery Performance (1)
[0088] Each battery obtained from Examples 1-8 and Comparative
Example 1 was exposed to a high temperature of 90.degree. C. for 4
hours and subjected to a bulge test for measuring a change in
thickness of the battery. The test results are shown in FIG. 13.
Although the results for Examples 1 and 2 are not shown in FIG. 13,
the batteries according to Example 1 and 2 showed a significantly
decreased change in thickness, compared to the battery of
Comparative Example 1. As shown in FIG. 13, the batteries according
to Examples 3-8 showed excellent high-temperature stability with
substantially no change in thickness.
[0089] Changes in thickness of a battery may result from safety of
an electrolyte, decomposition at a high temperature, reaction
between a cathode surface and electrolyte, etc. Alkanes having
dinitrile functional groups used in the present invention provide
excellent effect in high-temperature storage.
[0090] Therefore, as shown in FIG. 13, electrodes comprising
aliphatic dinitrile compounds provide excellent thermal
stability.
[0091] 6. Test for Battery Performance (2)
[0092] The battery according to Comparative Example 2 was compared
to the battery according to Example 9 in terms of low-temperature
performance. Each battery fully charged to 4.2V was discharged to
3V at a current of 1 C (950 mA) in a constant current (CC) manner
at -10.degree. C. to measure the low-temperature performance. The
results are shown in FIG. 14.
[0093] As shown in FIG. 14, both batteries show a significant
difference in terms of -10.degree. C. discharge capacity.
[0094] As can be seen from FIG. 14, in the case of the battery
(Comparative Example 2) using an aliphatic nitrile compound added
to the electrolyte, there is a problem in that the additive
increases the viscosity of electrolyte to reduce the diffusion of
Li ions, resulting in degradation in the battery performance. On
the other hand, in the case of the battery (Example 9) using the
cathode coated with an aliphatic nitrile compound, there is an
advantage in that the battery safety can be improved with no
degradation in the battery performance by virtue of the formation
of a chemically strong complex between the nitrile functional group
and the cathode, even if the content of the aliphatic nitrile
compound is equal to or greater than the amount thereof added to
the electrolyte.
[0095] Meanwhile, each battery according to Comparative Examples
3-10 and Examples 10-17, fully charged to 4.2V, was discharged to
3V at a current of 1 C (750 mA) in a constant current (CC) manner
at -10.degree. C. to measure the low-temperature performance. The
results are shown the following Table 2.
TABLE-US-00002 TABLE 2 1 C, -10.degree. C. Efficiency (mAh) (%)
Examples 10-17 630.5 83.0 Comp. Ex. 3 575.6 75.7 Comp. Ex. 4 540.5
71.1 Comp. Ex. 5 568.7 74.8 Comp. Ex. 6 595.8 78.4 Comp. Ex. 7
597.0 78.5 Comp. Ex. 8 560.9 73.8 Comp. Ex. 9 594.1 78.2 Comp. Ex.
10 555.1 73.0
[0096] As can be seen from Table 2, when 383562-type lithium
polymer batteries according to Comparative Examples 3-10 and
Examples 10-17 were tested in terms of low-temperature performance,
the batteries (Examples 10-17) using the cathode coated with 3 wt %
or more of an aliphatic nitrile compound shows excellent battery
performance compared to the batteries (Comparative Examples 3-10)
using 3 wt % of an aliphatic nitrile compound as an additive for
electrolyte, in the same manner as the 523450 type prismatic
lithium batteries according to Comparative Example 2 and Example
9.
[0097] Additionally, in the case of the batteries according to
Examples 10-17, most of the batteries show an efficiency of 83% or
more regardless of the kind of aliphatic nitrile compound. On the
other hand, in the case of the batteries (Comparative Examples
3-10) using an aliphatic nitrile compound as an additive for
electrolyte show a difference in efficiency ranging from 71% to 78%
depending on the differences in physical properties of the
additive, viscosity and Li ion diffusion.
[0098] 7. Test for Battery Performance (3)
[0099] Each of the batteries according to Comparative Example 2 and
Example 9 was stored at a high temperature (90.degree. C., 4 hours)
and then tested for the interfacial resistance in the battery.
[0100] To measure the interfacial resistance, the battery fully
charged to 4.2V was tested under the conditions of a DC voltage of
0V based on an open circuit, an AC amplitude of 5 mV and a
frequency ranging from 10.sup.5 (Hz) to 10.sup.-1 (Hz).
Additionally, the Nyquist presentation method was used wherein Z'
(real number part) and -Z'' (imaginary number part) are shown in
the x-axis and y-axis, respectively. The results are shown in FIG.
15.
[0101] As shown in FIG. 15, the battery according to Comparative
Example 2 shows an increase in interfacial resistance as the
content of the aliphatic nitrile compound added to the electrolyte
increases. On the other hand, the battery (Example 9) using the
cathode coated with 3 wt % of the aliphatic nitrile compound,
obtained by dipping a cathode into the coating solution containing
the aliphatic nitrile compound, shows a significantly low
interfacial resistance.
[0102] Therefore, it is possible to improve battery safety with no
degradation in battery performance when the aliphatic nitrile
compound is not added to an electrolyte but is incorporated into an
electrode.
[0103] 8. Test for Battery Performance (4)
[0104] Each battery obtained from Example 18 (using a cathode
coated with succinonitrile) and Comparative Example 11 (using a
non-coated cathode) was subjected to charge/discharge cycles in a
hot chamber at 45.degree. C. with a constant current (1 C/1 C).
[0105] As shown in FIG. 16, there is a significant difference
between the battery (Example 18) using a cathode coated with
succinonitrile and the battery (Comparative Example 11) using a
non-coated cathode, in terms of high-temperature life
characteristics. When compared to the battery according to
Comparative Example 11, the battery according to Example 18 shows a
less decrease in discharge capacity during repeated
charge/discharge cycles, thereby providing improved life
characteristics. On the other hand, the battery according to
Comparative Example 11 shows a significant decrease in discharge
capacity during repeated charge/discharge cycles.
INDUSTRIAL APPLICABILITY
[0106] As can be seen from the foregoing, the battery using an
electrode comprising an aliphatic nitrile compound according to the
present invention can inhibit heat emission caused by the reaction
of an electrolyte with a cathode and the structural collapse of a
cathode, and can reduce the calorific value due to the heat
emission. Therefore, it is possible to prevent a battery from being
fired due to the generation of internal short circuit resulting
from an excessive heat emission upon overcharge. Further, it is
possible to avoid degradation in battery performance including
problems of an increase in electrolyte viscosity and an increase in
interfacial resistance, occurring when an aliphatic nitrile
compound is added to an electrolyte.
[0107] Additionally, the compound represented by formula used in
the present invention will not be reduced easily during the charge
cycle of a battery and will not be decomposed easily even under
high voltage. Therefore, it is possible to inhibit the structural
collapse of a cathode efficiently as well as to improve the
performance and safety of a battery due to the electrochemical
stability of the compound.
* * * * *