U.S. patent number RE43,724 [Application Number 12/887,466] was granted by the patent office on 2012-10-09 for electrolyte for lithium ion rechargeable battery and lithium ion rechargeable battery including the same.
This patent grant is currently assigned to Samsung SDI Co., Ltd.. Invention is credited to Jin Hee Kim, Jin Sung Kim, Yong Shik Kim.
United States Patent |
RE43,724 |
Kim , et al. |
October 9, 2012 |
**Please see images for:
( Certificate of Correction ) ** |
Electrolyte for lithium ion rechargeable battery and lithium ion
rechargeable battery including the same
Abstract
The present invention is an electrolyte for a lithium ion
rechargeable battery and a lithium ion rechargeable battery that
includes the same. More particularly, the present invention
discloses an electrolyte for a lithium ion rechargeable battery
that provides excellent cycle life characteristics and
high-temperatures storage stability and prevents a drop in
discharge capacity of a battery at low temperature, and a lithium
ion rechargeable battery including the same. The lithium ion
rechargeable battery including the electrolyte provides improved
cycle life characteristics and prevents the problems of a drop in
discharge capacity at low temperature and high-temperature swelling
through the formation of a stable SEI film at the initial charge
cycle.
Inventors: |
Kim; Jin Hee (Yongin-si,
KR), Kim; Jin Sung (Yongin-si, KR), Kim;
Yong Shik (Yongin-si, KR) |
Assignee: |
Samsung SDI Co., Ltd.
(Yongin-si, KR)
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Family
ID: |
35912582 |
Appl.
No.: |
12/887,466 |
Filed: |
September 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
11155596 |
Jun 20, 2005 |
7235334 |
Jun 26, 2007 |
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Foreign Application Priority Data
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Jun 21, 2004 [KR] |
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10-2004-0046273 |
Oct 26, 2004 [KR] |
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10-2004-0085692 |
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Current U.S.
Class: |
429/231.1;
429/218.1 |
Current CPC
Class: |
H01M
10/0567 (20130101); H01M 10/0525 (20130101); H01M
10/0569 (20130101); Y02E 60/10 (20130101); H01M
4/505 (20130101); H01M 4/587 (20130101); H01M
4/525 (20130101) |
Current International
Class: |
H01M
4/58 (20100101) |
Field of
Search: |
;429/231.1,218.1,224,221 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10-2001-0014516 |
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Feb 2001 |
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KR |
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10-2001-0014516 |
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Feb 2001 |
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KR |
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10-2001-0030178 |
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Apr 2001 |
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KR |
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1020010030178 |
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Apr 2001 |
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KR |
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10-2001-0082014 |
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Aug 2001 |
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KR |
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10-2001-0082014 |
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Aug 2001 |
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KR |
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WO 00/25381 |
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May 2000 |
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WO |
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Primary Examiner: Michener; Jennifer
Assistant Examiner: Wills; Monique
Attorney, Agent or Firm: Christie, Parker & Hale,
LLP
Claims
What is claimed is:
1. An electrolyte for a lithium rechargeable battery, comprising: a
lithium salt; a non-aqueous organic solvent; a first additive
having a Lowest Unoccupied Molecular Orbital (LUMO) level between
0.3 eV and 0.5 eV when calculated by the AM1 (Austin Model 1)
method; and a second additive having a LUMO level between-0.2 eV
and 0.3 eV or between 0.5 eV and 1.0 eV when calculated by the AM1
method.
2. The electrolyte of claim 1, wherein the first additive is at
least one compound selected from the group consisting of
trimethylsilyl phosphate and lithium tetrafluoroborate.
3. The electrolyte of claim 1, wherein the first additive has a
concentration of between 0.01 wt % and 3.0 wt % based on a total
weight of the electrolyte.
4. The electrolyte of claim 3, wherein the concentration of the
first additive is between 0.2 wt % and 0.5 wt % .Iadd.based on the
total weight of the electrolyte.Iaddend..
5. The electrolyte of claim 1, wherein the second additive is at
least one compound selected from the group consisting of vinylene
carbonate and fluoroethyl carbonate.
6. The electrolyte of claim 5, wherein the second additive has a
concentration of between 0.01 wt % and 10.0 wt % based on a total
weight of the electrolyte.
7. The electrolyte of claim 6, wherein the concentration of the
second additive is between 0.1 wt % and 5.0 wt % .Iadd.based on the
total weight of the electrolyte.Iaddend..
8. The electrolyte of claim 1, wherein a weight ratio of the first
additive to the second additive is between 0.1:1 and 1:1.
9. The electrolyte of claim 8, wherein the weight ratio of the
first additive to the second additive is between 0.2:1 and
0.5:1.
10. The electrolyte of claim 1, wherein the non-aqueous organic
solvent is .Iadd.one or more .Iaddend.selected from the group
consisting of cyclic carbonate, non-cyclic carbonate, aliphatic
carboxylic acid ester, non-cyclic ether, cyclic ether, alkyl
phosphate ester and fluorides thereof.[., or a mixture containing
two or more of them.]..
11. The electrolyte of claim 10, wherein the cyclic carbonate is at
least one compound selected from the group consisting of ethylene
carbonate, propylene carbonate, butylene carbonate and vinylene
carbonate.
12. The electrolyte of claim 10, wherein the non-cyclic carbonate
is at least one compound selected from the group consisting of
dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,
methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate
and methyl ethyl carbonate.
13. The electrolyte of claim 10, wherein the aliphatic carboxylic
acid ester is at least one compound selected from the group
consisting of methyl formate, methyl acetate, methyl propionate and
ethyl propionate.
14. The electrolyte of claim 10, wherein the non-cyclic ether is at
least one compound selected from the group consisting of
gamma-lactones, 1,2-dimethoxyethane, 1,2-diethoxyethane and
ethoxymethoxyethane.
15. The electrolyte of claim 10, wherein the cyclic ether is at
least one compound selected from the group consisting of
tetrahydrofuran and 2-methyltetrahydrofuran.
16. The electrolyte of claim 10, wherein the alkyl phosphate ester
is at least one compound selected from the group consisting of
dimethyl sulfoxide, 1,2-dioxolan, trimethyl phosphate, triethyl
phosphate and trioctyl phosphate.
17. The electrolyte of claim 1, wherein the lithium salt is at
least one selected from the group consisting of LiPF.sub.6,
LiBF.sub.4, LiAsF.sub.6, LiCIO.sub.4, LiCF.sub.3SO.sub.3,
LiSbF.sub.6, LiN(SO.sub.2CF.sub.3).sub.2 LiC.sub.4F.sub.9SO.sub.3,
LiAIF.sub.4, LiAICI.sub.4 LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
.[.LiN(C.sub.xF.sub.2x+1So .sub.2)(C.sub.yF.sub.2y+1SO.sub.2).].
.Iadd.LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2)
.Iaddend.(wherein, each of x and y is an integer), LiCI and
Lil.
18. An electrolyte for a lithium rechargeable battery, comprising:
a lithium salt; a non-aqueous organic solvent; a first additive
having a Lowest Unoccupied Molecular Orbital (LUMO) level between
0.3 eV and 0.5 eV, when calculated by the AM1 (Austin Model 1)
method; a second additive having an LUMO level between 0.5 eV and
1.0 eV, when calculated by the AM1 method; and a third additive
having an LUMO level between-0.2 eV and 0.3 eV, when calculated by
the AM1 method.
19. A lithium ion rechargeable battery, comprising: the electrolyte
of claim 1; a cathode including a cathode active material; an anode
including an anode active material; and a separator disposed
between the cathode and the anode.
20. The lithium ion rechargeable battery of claim 19, wherein the
cathode active material is a lithiated intercalation oxide.
21. The lithium ion rechargeable battery of claim 19, wherein the
anode active material is selected from the group consisting of
crystalline carbon, amorphous carbon, carbon composites and lithium
metal.
22. The lithium ion rechargeable battery of claim 21, wherein the
anode active material is crystalline carbon that has a crystalline
size in the lamination direction (Lc) of 150 .ANG. or more, d(002)
of between 3.35 .ANG. and 3.38 .ANG., real density of 2.2
g/cm.sup.3 or more, BET specific surface area of between 0.5
m.sup.2/g and 50 m.sup.2/g, and average particle diameter (D50) of
between 1 .mu.m and 30 .mu.m.
23. The lithium ion rechargeable battery of claim 21, wherein the
anode active material has an intensity ratio of I(1360) surface to
I(1590) surface in the Raman spectrum [I(1360 cm.sup.-1)/I(1590
cm.sup.-1)] of 0.05 or more.
24. .[.A.]. .Iadd.The .Iaddend.lithium ion rechargeable battery of
claim 21, wherein the anode active material has a peak intensity
ratio of I(110) surface to I(002) surface in the X-ray diffraction
pattern [X(I(110)/I(002))] of less than 0.2.
.Iadd.25. An electrolyte for a lithium rechargeable battery,
comprising: a lithium salt; a non-aqueous organic solvent; a first
additive comprising trimethylsilyl phosphate; and a second additive
selected from the group consisting of vinylene carbonate,
fluroethyl carbonate, and combinations thereof..Iaddend.
.Iadd.26. The electrolyte of claim 25, wherein the first additive
has a concentration of between 0.01 wt % and 3.0 wt % based on a
total weight of the electrolyte..Iaddend.
.Iadd.27. The electrolyte of claim 26, wherein the concentration of
the first additive is between 0.2 wt % and 0.5 wt % based on the
total weight of the electrolyte..Iaddend.
.Iadd.28. The electrolyte of claim 25, wherein the second additive
has a concentration of between 0.01 wt % and 10.0 wt % based on a
total weight of the electrolyte..Iaddend.
.Iadd.29. The electrolyte of claim 28, wherein the concentration of
the second additive is between 0.1 wt % and 5.0 wt % based on the
total weight of the electrolyte..Iaddend.
.Iadd.30. The electrolyte of claim 25, wherein a weight ratio of
the first additive to the second additive is between 0.1:1 and
1:1..Iaddend.
.Iadd.31. The electrolyte of claim 30, wherein the weight ratio of
the first additive to the second additive is between 0.2:1 and
0.5:1..Iaddend.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority to and the benefit of Korean
Patent Application Nos. 2004-0046273, filed on Jun. 21, 2004 and
2004-0085692 filed on Oct. 26, 2004, which are hereby incorporated
by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrolyte for a lithium ion
rechargeable battery and a lithium ion rechargeable battery that
includes the electrolyte. In particular, the present invention
provides an electrolyte for a lithium ion rechargeable battery that
provides excellent cycle life characteristics and high-temperature
storage stability. The electrolyte also prevents a drop in
discharge capacity of a battery at low temperatures. The present
invention also provides a lithium ion rechargeable battery that
includes the electrolyte.
2. Description of the Prior Art
As the electronics industry has advanced, technical research into
portable and wireless electronic instruments including telephones,
vi1deo cameras, and personal computers has progressed rapidly.
Accordingly, a small rechargeable battery that is lightweight and
has high energy density is increasingly in demand to power these
instruments. Particularly, a rechargeable battery that has a
non-aqueous electrolyte and uses a lithium-containing metal oxide
as a cathode active material and a carbonaceous material capable of
lithium intercalation/deintercalation as an anode active material
to provide a voltage of about 4 V fills these requirements.
Lithium ion rechargeable batteries have an average discharge
voltage of about 3.6 to 3.7 V and thus can provide relatively high
electric power compared to other alkali batteries, Ni--MH
batteries, Ni--Cd batteries, etc. However, in order to obtain such
a high drive voltage level, an electrolyte composition that is
electrochemically stable in the charge/discharge voltage range of 0
to 4.2V is required. For this reason, a mixture containing a cyclic
carbonate-based solvent such as ethylene carbonate, propylene
carbonate, and butylene carbonate, for example, is generally used
as an electrolyte.
During the initial charge cycle of a lithium ion rechargeable
battery, lithium ions are discharged from a lithium metal oxide,
which is the cathode active material, and move toward a carbon
electrode, which is the anode, so that lithium ion intercalation
into carbon can be made. During this process, the lithium may react
with the carbon electrode to produce Li.sub.2CO.sub.3, Li.sub.2O,
LiOH, etc., thereby forming a film on the surface of the anode.
Such a film is referred to as a Solid Electrolyte Interface (SEI)
film.
After the SEI film is formed at the initial charge cycle, it serves
as a barrier for preventing lithium ions from reacting with the
carbon anode or other substances and also forms an ion tunnel
during the following charge/discharge cycles. The ion tunnel
prevents collapse of the carbon anode that is caused by the
dissolving lithium ions in high-molecular weight organic solvents
that are present in the electrolyte. It also prevents the movement
of lithium ions with the solvents, which results in intercalation
into the carbon anode. Therefore, once the SEI film is formed,
lithium ions are prevented from reacting with the carbon anode
again or from undesirably reacting with other substances. Thus the
concentration of lithium ions can be maintained constant.
However, as charge and discharge cycles repeat electrode plates
repeatedly expand and shrink and local over-voltage may be applied.
Under these circumstances, a passivation layer such as an SEI film
may be gradually degraded with the lapse of time and the surface of
the anode may be exposed and may undesirably react with the
surrounding electrolyte. In addition, gases are generated from the
undesired side-reaction, which thereby increases the internal
pressure of the battery and significantly degrades the cycle life
characteristics of a battery. The gases that are generated mainly
include CO, CO.sub.2, CH.sub.4, C.sub.2H.sub.6, etc., depending on
the kind of the carbonate used in the electrolyte and the type of
anode active material (J. Power Sources, 72 (1998) p. 66-70).
Additionally, a certain graphite-based anode active material may
cause the decomposition of a carbonate-based electrolyte and the
separation of a carbonaceous material, thereby detracting from
characteristics of a battery including electric capacity, cycle
life characteristics and storage characteristics. Particularly,
such problems are exacerbated for batteries that use an electrolyte
that contains propylene carbonate. Propylene carbonate is
decomposed at an anode during the first charge cycle, thereby
decreasing the initial capacity significantly.
In order to prevent the decomposition of cyclic carbonates and the
separation of carbonaceous materials caused by graphite-based anode
active materials, a method of adding a crown ether (12-crown-4) to
an electrolyte based on propylene carbonate and ethylene carbonate
has been suggested (J. Electrochem. Soc., Vol. 140, No. 6, L101
(1993)). However, this method is problematic in that a large amount
of expensive crown ether is needed to prevent the decomposition of
cyclic carbonates to a desired degree, and the battery
characteristics obtained by the method are not sufficient for
practical use.
Additionally, Japanese Patent Laid-Open No. Hei 8-45545 discloses a
method of adding vinylene carbonate to an electrolyte based on
propylene carbonate and ethylene carbonate in order to prevent
decomposition of the electrolyte. According to the method, vinylene
carbonate is reduced at an anode during charge cycles to form an
insoluble film on the surface of graphite (anode), thereby
preventing reduction of solvents such as propylene carbonate and
ethylene carbonate.
However, this method using vinylene carbonate alone cannot
accomplish the formation of a complete SEI film at the first charge
cycle. As charge and discharge cycles are repeated at room
temperature, the film may crack and vinylene carbonate is
decomposed and consumed again in order to compensate for such
cracked portions. Ultimately, it is not possible to obtain stable
cycle life characteristics of a battery. Further, although cycle
life characteristics of a battery may improve by increasing the
amount of vinylene carbonate, the method still has problems in that
the discharge capacity of a battery decreases rapidly at low
temperature and swelling of a battery may occur when it is stored
at high temperature.
SUMMARY OF THE INVENTION
The present invention provides an electrolyte for a lithium ion
rechargeable battery that provides excellent cycle life
characteristics and high-temperature storage stability and also
prevents a drop in discharge capacity of a battery at low
temperature.
The present invention also provides a lithium ion rechargeable
battery that includes the electrolyte.
Additional features of the invention will be set forth in the
description which follows, and in part will be apparent from the
description, or may be learned by practice of the invention.
The present invention discloses an electrolyte for a lithium
rechargeable battery that includes a lithium salt, a non-aqueous
organic solvent, and a first additive that has a Lowest Unoccupied
Molecular Orbital (LUMO) level between 0.3 eV and 0.5 eV when
calculated by the AM1 (Austin Model 1) method among the quantum
chemical calculation methods. The electrolyte also includes a
second additive that has an LUMO level of between -0.2 eV and 0.3
eV or between 0.5 eV and 1.0 eV when calculated by the AM1
method.
The present invention also discloses an electrolyte for a lithium
rechargeable battery that includes a lithium salt, a non-aqueous
organic solvent, and a first additive that has an LUMO level of
between 0.3 eV and 0.5 eV when calculated by the AM1 method among
the quantum chemical calculation methods. The electrolyte also
includes a second additive that has an LUMO level of between 0.5 eV
and 1.0 eV when calculated by the AM1 method, and a third additive
that has an LUMO level of between -0.2 eV and 0.3 eV when
calculated by the AM1 method.
The present invention also discloses a lithium ion rechargeable
battery that includes the above-described electrolyte, a cathode
containing a cathode active material, an anode containing an anode
active material, and a separator disposed between the cathode and
the anode.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further
understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention, and together with the description serve to explain
the principles of the invention.
FIG. 1 is a schematic view that shows the structure of a lithium
ion rechargeable battery according to a preferred embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to form a strong SEI film, an additive that can be reduced
and decomposed before a non-aqueous organic solvent is reduced is
required. The Lowest Unoccupied Molecular Orbital (LUMO) theory is
applied to select such an additive. The LUMO level of a compound
added to an electrolyte is obtained by using the Austin Model 1
(AM1) method, which is a semi-empirical computation method.
Semi-empirical computation methods are classified into AM1,
Parametric Method 3 (PM3), Modified Neglect of Differential Overlap
(MNDO), Complete Neglect of Differential Overlap (CNDO),
Intermediate Neglect of Differential Overlap (INDO), Modified
Intermediate Neglect of Differential Overlap (MINDO), etc.,
depending on postulations and parameters. The AM1 method was
developed by Dewer et al. in 1985 for calculations in hydrogen
bonding through a partial modification and improvement in the MNDO
method. The AM1 method applied to the present invention is
available as the MOPAC computer program package.
The LUMO represents a molecular orbital function of the
lowest-energy level orbital unoccupied by electrons. When a given
molecule accepts an electron, the electron occupies the
lowest-energy level orbital and the reduction degree is determined
by the energy level. The lower the LUMO level is, the higher the
reduction degree is. On the other hand, a relatively high LUMO
level indicates high resistance against reduction.
Therefore, the electrolyte according to the present invention
includes an organic compound that has a LUMO level that is lower
than the LUMO level of the currently used non-aqueous organic
solvent. This allows for stable cycle life characteristics of a
battery. In particular, the present invention includes an organic
compound that is reduced to form a stable film before a
carbonate-based solvent having the LUMO level of between 1 eV and 2
eV is reduced.
As described hereinbefore, the electrolyte according to the present
invention includes a first additive having a LUMO level of between
0.3 eV and 0.5 eV and a second additive having an LUMO level of
between -0.2 eV and 0.3 eV or between 0.5 eV and 1.0 eV. If the
LUMO level of the additive is higher than 1.0 eV or lower than -0.2
eV, an unstable film is formed by a carbonate-based solvent.
Each of the first and the second additives used in the electrolyte
according to the present invention has an LUMO level that is less
than the LUMO level of a conventional non-aqueous organic solvent,
where the conventional LUMO level has a potential between about 1
eV and 2 eV. Additionally, the potential difference between a
non-aqueous organic solvent and the first and second additives is
preferably 0.05 eV to 3 eV and more preferably 0.1 eV to 2 eV.
When the additives are not used, a battery may have good
electrochemical charge/discharge efficiency due to a relatively low
irreversible capacity needed for film formation. However, a stable
SEI film cannot be formed, thereby deteriorating the battery
quality during repeated charge and discharge cycles and detracting
from the battery cycle life.
When the second additive is used alone, the cycle life of a battery
improves but it should be added in a concentration of 3 wt % or
more so that it can form a stable SEI film at the initial charge
cycle. In this case, the problems are that the discharge capacity
of a battery decreases at low temperatures (-20.degree. C. to
0.degree. C.) and the battery may swell when it is stored at high
temperatures (85.degree. C. to 90.degree. C.). In addition, as
charge and discharge cycles repeat, the electrode plates repeatedly
expand and shrink and local over-voltage may be applied, thereby
degrading an SEI film and causing undesired side-reactions. This
makes it difficult to ensure satisfactory cycle life
characteristics.
The first additive may include trimethylsilyl phosphate and lithium
tetrafluoroborate (LiBF.sub.4), for example. The second additive
may include vinylene carbonate and fluoroethyl carbonate.
The electrolyte of the present invention, which includes a first
additive and a second additive mixed in a suitable ratio can form a
stable SEI film at the initial charge cycle, and thus can ensure
high-temperature stability, excellent cycle life characteristics
and stable low-temperature discharge capacity of a battery.
In particular, the electrolyte according to the present invention
uses the first additive to form a stable SEI film at the initial
charge cycle. Therefore, even if the concentration of the second
additive is decreased, the first additive can maintain cycle life
characteristics of a battery and ensure stable low-temperature
discharge capacity and high-temperature stability of a battery.
Ultimately, it is possible to obtain all of the above-described
characteristics at the same time, unlike in the case in which the
second additive is used alone.
The electrolyte according to the present invention includes the
first additive preferably at a concentration of 0.01 wt % to 3 wt %
and more preferably at a concentration of 0.2 wt % to 0.5 wt %,
based on the total weight of the electrolyte.
When the first additive has a concentration of less than 0.01 wt %,
it is not possible to form a stable SEI film. When the amount of
the first additive is greater than 3 wt %, a battery may swell
significantly while stored at high temperature for a long time. In
addition, the low-temperature discharge capacity as well as initial
capacity decreases. Since a large amount of the first additive is
consumed, a thick SEI film is formed. Further, an excessive amount
of the first additive that remains unreacted may decompose when a
battery is stored at high temperature, or may it function as a
resistor during a discharge cycle at low temperature, resulting in
deterioration of battery quality.
The electrolyte according to the present invention includes the
second additive preferably at a concentration of 0.01 wt % to 20 wt
%, more preferably at a concentration of 0.01 wt % to 10 wt %, and
most preferably at a concentration of 0.1 wt % to 5 wt %, based on
the total weight of the electrolyte.
When the concentration of the second additive is less than 0.01 wt
%, it is not possible to form a stable SEI film. When the amount of
the second additive is greater than 20 wt %, the low-temperature
discharge capacity of a battery decreases significantly, a battery
may swell while stored at high temperature, and its cycle life may
shorten.
The weight ratio of the first additive to the second additive
preferably ranges from 0.1:1 to 1:1, and more preferably from 0.2:1
to 0.5:1. When the weight ratio of the first additive to the second
additive is less than 0.1, the cycle life of a battery decreases.
When the weight ratio of the first additive to the second additive
is greater than 1.0, the low-temperature discharge capacity and
initial capacity of a battery decrease, and it may swell
significantly at a high-temperature.
The electrolyte according to the present invention includes a
non-aqueous organic solvent and a lithium salt, in addition to the
above additives. The non-aqueous organic solvent functions as a
medium through which ions participating in chemical reactions in a
battery can move. The non-aqueous organic solvent may include, but
is not limited to, cyclic carbonates, non-cyclic carbonates,
aliphatic carboxylic acid esters, non-cyclic ethers, cyclic ethers,
alkyl phosphate esters and fluorides thereof, or a mixture
containing two or more of them.
Examples of the cyclic carbonates may include, but are not limited
to, ethylene carbonate, propylene carbonate, butylene carbonate,
and vinylene carbonate. Examples of the non-cyclic carbonates
include dimethyl carbonate, diethyl carbonate, ethyl methyl
carbonate, methyl propyl carbonate, ethyl propyl carbonate,
dipropyl carbonate, and methyl ethyl carbonate, for example.
Further, examples of the aliphatic carboxylic acid esters may
include but are limited to methyl formate, methyl acetate, methyl
propionate, and ethyl propionate, for example.
In addition, examples of the non-cyclic ethers may include
gamma-lactones, 1,2-dimethoxyethane, 1,2-diethoxyethane, and
ethoxymethoxyethane. Examples of the cyclic ethers may include, for
example tetrahydrofuran and 2-methyltetrahydrofuran. Examples of
the alkyl phosphate esters include dimethyl sulfoxide,
1,2-dioxolan, trimethyl phosphate, triethyl phosphate, and trioctyl
phosphate.
The lithium salt provides lithium ions in a battery and empowers a
lithium ion rechargeable battery to perform basic functions. The
lithium salt that may be used may include, but is not limited to,
LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiClO.sub.4,
LiCF.sub.3SO.sub.3, LiSbF.sub.6, LiN(SO.sub.2CF.sub.3).sub.2,
LiC.sub.4F.sub.9SO.sub.3, LiAlF.sub.4, LiAlCl.sub.4
LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2) (where x
and y are integers), LiCl and LiI, or a mixture containing two or
more of them.
In another embodiment of the present invention, the electrolyte
includes a first additive that has a LUMO level of between 0.3 eV
and 0.5 eV when calculated by the AM1 method, a second additive
that has a LUMO level of between 0.5 eV and 1.0 eV when calculated
by the AM1 method, and a third additive that has a LUMO level of
between -0.2 eV and 0.3 eV when calculated by the AM1 method.
The lithium rechargeable battery that uses the electrolyte
according to present invention includes a cathode, an anode, and a
separator. The cathode includes a cathode active material that is
capable of reversible lithium ion intercalation/deintercalation.
Such cathode active materials may include a lithiated intercalation
oxide.
The anode includes an anode active material that is capable of
lithium ion intercalation/deintercalation. Such anode active
materials may include, but are not limited to, crystalline or
amorphous carbon, carbonaceous anode active materials derived from
carbon composites (pyrolyzed carbon, coke, graphite), burned
organic polymer compounds, carbon fiber, tin oxide compounds,
lithium metal and lithium alloys.
Preferably, the anode active material is crystalline carbon. More
preferably, the anode active material is crystalline carbon or
graphite having a crystalline size in the lamination direction (Lc)
of 150 .ANG. or more, particularly of between 150 .ANG. and 3000
.ANG., d(002) of between 3.35 .ANG. and 3.38 .ANG., real density of
2.2 g/cm.sup.3 or more, particularly of between 2.2 g/cm.sup.3 and
2.3 g/cm.sup.3, BET specific surface area of between 0.5 m.sup.2/g
and 50 m.sup.2/g, and average particle particle diameter (D50) of
between 1 .mu.m. Additionally, in the above active material, the
intensity ratio of I(1360) surface to I(1590) surface in the Raman
spectrum, i.e., I(1360 cm.sup.-1)/I (1590 cm.sup.-1) is preferably
0.05 or more, and more preferably between 0.05 and 0.5. Further,
the peak intensity ratio of I(110) surface to I(002) surface in the
X-ray diffraction pattern, i.e., X(I(110)/I(002)) is preferably
less than 0.2, and more preferably is 0.006 to 0.2.
A slurry containing the cathode active material or anode active
material is coated on a collector formed of metal foil. Otherwise,
the active material itself is applied as a film.
The separator, which prevents a short circuit between the cathode
and the anode in a lithium rechargeable battery, may include any
materials known to one skilled in the art. For example, the
separator may comprise a polymer film such as a polyolefin,
polypropylene or polyethylene film, a multi-layered film thereof, a
microporous film, woven web, and non-woven web.
The lithium rechargeable battery as described above may also be
formed into a unit cell that has the structure of
cathode/separator/anode, a bi-cell having the structure of
cathode/separator/anode/separator, or a laminate cell in which the
structure of unit cell is repeated several times.
Hereinafter, a preferred embodiment of the present invention will
be described with reference to the accompanying drawing.
FIG. 1 shows the structure of a lithium ion rechargeable battery
according to a preferred embodiment of the present invention. As
shown in FIG. 1, the lithium rechargeable is battery is obtained by
forming an electrode assembly 12 including a cathode 13, an anode
15, and a separator 14 that is interposed between the cathode 13
and the anode 15 in a can 10. In addition, the battery includes an
electrolyte, and a cap assembly 20 that seals the top of the can.
The cap assembly 20 includes a cap plate 40, an insulating plate
50, a terminal plate 60 and an electrode terminal 30. Additionally,
the cap assembly 20 is coupled with an insulation case 70 to seal
the can 10.
The electrode terminal 30 is inserted into a through hole 41 formed
at the center of the cap plate 40. When the electrode terminal 30
is inserted into the through hole 41, a gasket 46 is coupled to the
outer surface of the electrode terminal 30 to insulate the
electrode terminal 30 from the cap plate 40. Thus the gasket is
inserted into the through hole 41 along with the electrode terminal
30. After the cap assembly is mounted on the top of the cap 10, the
electrolyte is injected through an inlet 42 and then the inlet 42,
is sealed with a stopper 43.
The electrode terminal 30 is connected to an anode tab 17 of the
anode 15 or to a cathode tab 16 of the cathode 13, thereby
functioning as an anode terminal or a cathode terminal.
The lithium rechargeable battery according to the present invention
is not limited to the above-described shape but may have any other
shape applicable to a battery, including a cylindrical shape, pouch
shape, etc.
The present invention will now be described using the following
examples. It is to be understood that the following examples are
only illustrative and the present invention is not limited
thereto.
The LUMO levels of non-aqueous organic solvents of Reference
Examples 1 to 6 and Additives 1 to 4 added to the non-aqueous
organic solvents were measured using the AM1 method. The results
are shown in the following Table 1.
TABLE-US-00001 TABLE 1 No. Chemical Substance LUMO (eV) Ref. Ex. 1
EC (Ethylene Carbonate) 1.17553 Ref. Ex. 2 PC (Propylene Carbonate)
1.23594 Ref. Ex. 3 DMC (Dimethyl Carbonate) 1.24846 Ref. Ex. 4 DEC
(Diethyl Carbonate) 1.25499 Ref. Ex. 5 EMC (Ethylmethyl Carbonate)
1.28819 Ref. Ex. 6 GBL (.lamda.-Butyrolactone) 1.04899 Additive 1
FEC (Fluoro Ethyl Carbonate) 0.905 Additive 2 LiBF.sub.4 (Lithium
Tetrafluoroborate) 0.2 Additive 3 TMSP (Trirnethylsilyl Phosphate)
0.415 Additive 4 VC (Vinylene Carbonate) 0.09007
As shown in Table 1, Additives 1 to 4 have a reduction potential
that is lower than reduction potentials of Reference Examples 1 to
6. Thus they decompose before the non-aqueous organic solvents of
Reference Examples 1 to 6.
EXAMPLE 1
An anode active material made of artificial graphite was suspended
in aqueous carboxymethyl cellulose solution. Styrene-butadiene
rubber was added thereto as a binder to form slurry of anode active
material. The slurry was coated on a 10 .mu.m thick copper foil,
dried and rolled to form an anode.
A cathode active material comprising LiCoO.sub.2 was combined with
polyvinylidene fluoride as a binder and carbon as a conductive
agent. These compounds were dispersed in N-methyl-2-pyrrolidone as
a solvent in a weight ratio of 92:4:4 to form slurry of cathode
active material. The slurry was coated on a 15 .mu.m thick aluminum
foil, dried and rolled to form a cathode.
The cathode and the anode were then wound and compressed together
with a polyethylene separator with a thickness of 16 .mu.m. The
resultant unit cell was inserted into a prismatic can. Next, an
electrolyte was added to the can to form a lithium rechargeable
battery. The electrolyte was prepared by adding 1.0M of LiPF.sub.6
to a mixed solvent comprising ethylene carbonate, ethylmethyl
carbonate and dimethyl carbonate in a volume ratio of
EC:EMC:DMC=3:6:1. In addition, lithium tetrafluoroborate and
fluoroethyl carbonate were to the added electrolyte mixture in the
amount of 0.2 wt % and 2.0 wt %, respectively, based on the weight
of the mixture.
EXAMPLE 2
Example 1 was repeated, except that 1.0 wt % of lithium
tetrafluoroborate and 2.0 wt % of fluoroethyl carbonate, based on
the weight of the mixture were added.
EXAMPLE 3
Example 1 was repeated, except that 0.5 wt % of trimethylsilyl
phosphate and 2.0 wt % of fluoroethyl carbonate, based on the
weight of the mixture were added.
EXAMPLE 4
Example 1 was repeated, except that 1.0 wt % of trimethylsilyl
phosphate and 2.0 wt % of fluoroethyl carbonate, based on the
weight of the mixture were added.
EXAMPLE 5
Example 1 was repeated, except that 0.2 wt % of lithium
tetrafluoroorate, 0.5 wt % of trimethylsilyl phosphate and 2.0 wt %
of fluoroethyl carbonate, based on the weight of the mixture were
added.
EXAMPLE 6
Example 1 was repeated, except that 0.2 wt % of lithium
tetrafluoroborate and 2.0 wt % of vinylene carbonate, based on the
weight of the mixture were added.
EXAMPLE 7
Example 1 was repeated, except that 1.0 wt % of lithium
tetrafluoroborate and 2.0 wt % of vinylene carbonate, based on the
weight of the mixture were added.
EXAMPLE 8
Example 1 was repeated, except that 0.5 wt % of trimethylsilyl
phosphate and 2.0 wt % of vinylene carbonate, based on the weight
of the mixture were added.
EXAMPLE 9
Example 1 was repeated, except that 1.0 wt % of trimethylsilyl
phosphate and 2.0 wt % of vinylene carbonate, based on the weight
of the mixture were added.
EXAMPLE 10
Example 1 was repeated, except that 0.2 wt % of lithium
tetrafluoroorate, 0.5 wt % of trimethylsilyl phosphate and 2.0 wt %
of vinylene carbonate, based on the weight of the mixture were
added.
EXAMPLE 11
Example 1 was repeated, except that 0.2 wt % of lithium
tetrafluoroorate, 0.5 wt % of vinylene carbonate and 1.5 wt % of
fluoroethyl carbonate, based on the weight of the mixture were
added.
EXAMPLE 12
Example 1 was repeated, except that 1.0 wt % of lithium
tetrafluoroorate, 0.5 wt % of vinylene carbonate and 1.5 wt % of
fluoroethyl carbonate, based on the weight of the mixture were
added.
EXAMPLE 13
Example 1 was repeated, except that 0.5 wt % of trimethylsilyl
phosphate, 0.5 wt % of vinylene carbonate and 1.5 wt % of
fluoroethyl carbonate, based on the weight of the mixture were
added.
EXAMPLE 14
Example 1 was repeated, except that 0.2 wt % of trimethylsilyl
phosphate, 0.5 wt % of vinylene carbonate and 1.5 wt % of
fluoroethyl carbonate, based on the weight of the mixture were
added.
EXAMPLE 15
Example 1 was repeated, except that 0.2 wt % of lithium
tetrafluoroborate, 0.5 wt % of trimethylsilyl phosphate, 0.5 wt %
of vinylene carbonate and 1.5 wt % of fluoroethyl carbonate, based
on the weight of the mixture were added.
COMPARATIVE EXAMPLE 1
Example 1 was repeated, except that no additive was added to the
electrolyte.
COMPARATIVE EXAMPLE 2
Example 1 was repeated, except that 2.0 wt % of fluoroethyl
carbonate based on the weight of the mixture was added.
COMPARATIVE EXAMPLE 3
Example 1 was repeated, except that 5.0 wt % of fluoroethyl
carbonate based on the weight of the mixture was added.
COMPARATIVE EXAMPLE 4
Example 1 was repeated, except that 2.0 wt % of vinylene carbonate
based on the weight of the mixture was added.
COMPARATIVE EXAMPLE 5
Example 1 was repeated, except that 5.0 wt % of vinylene carbonate
based on the weight of the mixture was added.
EXPERIMENTAL EXAMPLE 2
Batteries obtained from Examples 1 to 15 and Comparative Examples 1
to 5 (battery capacity 1C=790 mAh) were charged under constant
current-constant voltage (CC-CV) conditions with a current of 158
mA and a voltage of 4.2 V and left for 1 hour. Then, the batteries
were discharged at 395 mA to 2.75 V and left for 1 hour. After the
above charge/discharge cycle was repeated three times, the
batteries were charged at a current of 395 mA for 3 hours to a
charge voltage of 4.2 V. Next, the initial charge/discharge
efficiency (%), i.e., [(initial discharge capacity-initial charge
capacity)/(initial charge capacity)].times.100 (%), was calculated.
For each of Examples 1 to 15 and Comparative Examples 1 to 5, the
discharge capacity/charge capacity ratio at the first
charge/discharge cycle was determined by using the average values
of 10 cells (as shown) in the following Tables 2 and 3.
In addition, a high-temperature storage test was performed as
follows. Each battery was stored for 4 hours at 85.degree. C. Then,
the thickness right after storage was compared to the thickness
before storage and the thickness increase ratio (%) was calculated
by using the formula [(thickness after high-temperature
storage-thickness before high-temperature storage)/(thickness
before high-temperature storage)].times.100 (%).
Further, a cycle life test was performed as follows. Each battery
was subjected to 0.1 C cut-off charge and 1 C/3.0V cut-off
discharge at each temperature (10.degree. C./25.degree.
C./45.degree. C.) under constant current-constant voltage (CC-CV)
conditions of 1 C/4.2V. The capacity maintenance (%) at each cycle
was calculated by using the formula [(discharge capacity at a given
cycle)/(discharge capacity at the first cycle)].times.100 (%).
TABLE-US-00002 TABLE 2 Thickness High Initial Increase
Low-temperature Room temperature Charge/ Ratio (%) (10.degree. C.)
temperature (60.degree. C.) Discharge After -20.degree. C./0.5 C.
Capacity Capacity Capacity Ex. Efficiency Storage at Discharge
Maintenance (%) Maintenance (%) Maintenance (%) No. Additive 1
Additive 2 (%) 85.degree. C./4 h Capacity (%) at 100.sup.th cycle
at 300.sup.th cycle at 300.sup.th cycle 1 LiBF.sub.4 0.2 wt % FEC 2
wt % 96 14 59 86 88 83 2 LiBF.sub.4 1.0 wt % FEC 2 wt % 93 15 58 85
87 80 3 TMSP 0.5 wt % FEC 2 wt % 95 16 60 88 90 79 4 TMSP 1.0 wt %
FEC 2 wt % 94 23 63 89 91 77 5 LiBF.sub.4 0.2 FEC 2 wt % 95 17 61
87 93 80 wt % + TMSP 0.5 wt % 6 LiBF.sub.4 0.2 wt % VC 2 wt % 93 17
40 70 86 73 7 LiBF.sub.4 1.0 wt % VC 2 wt % 93 19 38 70 86 70 8
TMSP 0.5 wt % VC 2 wt % 94 20 51 83 86 76 9 TMSP 1.0 wt % VC 2 wt %
92 23 55 86 87 75 10 LiBF.sub.4 0.2 VC 2 wt % 93 18 53 85 90 80 wt
% + TMSP 0.5 wt % 11 LiBF.sub.4 0.2 wt % VC 0.5 wt 94 17 58 87 86
75 % + FEC 1.5 wt % 12 LiBF.sub.4 1.0 wt % VC 0.5 wt 91 20 57 87 87
76 % + FEC 1.5 wt % 13 TMSP 0.5 wt % VC 0.5 wt 93 21 60 87 88 75 %
+ FEC 1.5 wt % 14 TMSP 1.0 wt % VC 0.5 wt 92 23 63 86 90 73 % + FEC
1.5 wt % 15 LiBF.sub.4 0.2 VC 0.5 wt 93 19 62 84 89 75 wt % + TMSP
% + FEC 0.5 wt % 1.5 wt %
TABLE-US-00003 TABLE 3 Thickness Initial Increase Low-temperature
Room High temperature Charge/ Ratio (%) (10.degree. C.) temperature
(60.degree. C.) Comp. Discharge After -20.degree. C./0.5 C.
Capacity Capacity Capacity Ex. Additive Efficiency Storage
Discharge Maintenance (%) Maintenance (%) Maintenance (%) No. 1
Additive 2 (%) at 85.degree. C./4 h Capacity (%) at 100.sup.th
cycle At 300.sup.th cycle At 300.sup.th cycle 1 -- -- 97 10 70 40
30 20 2 -- FEC 2 wt % 96 25 67 83 79 75 3 -- FEC 5 wt % 95 30 50 80
84 77 4 -- VC 2 wt % 93 28 40 70 82 69 5 -- VC 5 wt % 90 35 30 65
85 63
As shown in Table 3, the battery that used no additive in
Comparative Example 1 has good electrochemical charge/discharge
efficiency due to the lack of irreversible capacity needed for film
formation. However, the battery quality was deteriorated during
repeated cycles because a stable SEI film was not formed.
Additionally, the batteries of Comparative Examples 2 to 5 that
used vinylene carbonate or fluoroethyl carbonate alone as an
additive have improved cycle life compared to Comparative Example
1. However, because a stable SEI film was not formed, the SEI film
in each battery cracked, followed by decomposition and consumption
of vinylene carbonate or fluoroethyl carbonate at the cracked
portion. Therefore, a large amount of vinylene carbonate or
fluoroethyl carbonate must be added in order to obtain stable
battery capacity during repeated cycles. However, as the amount of
vinylene carbonate increases, cycle life of a battery may improve,
but the battery may swell when stored at high temperatures. In
addition, the low-temperature discharge capacity decreases.
When fluoroethyl carbonate was used instead of vinylene carbonate,
cycle life of a battery may improve as it did when using vinylene
carbonate and the problem of a decrease in discharge capacity at
low temperature becomes less serious. However, a drop in discharge
capacity at low temperature and high-temperature swelling still
remain problems.
As can be seen from Comparative Examples 2 and 3 and Comparative
Examples 4 and 5, vinylene carbonate provides a higher
room-temperature capacity maintenance (%) than fluoroethyl
carbonate. However, when vinylene carbonate is used, the
low-temperature discharge capacity significantly decreases and
high-temperature swelling increase in proportion to an increase of
the amount of vinylene carbonate. Moreover, cycle life
characteristics at high temperature and low temperature are poor.
When fluoroethyl carbonate is used, high-temperature swelling may
occur according to the increase of the amount of fluoroethyl
carbonate. However, when vinylene carbonate and fluoroethyl
carbonate increase equally, fluoroethyl carbonate provides a
smaller drop in low-temperature discharge capacity compared to
vinylene carbonate. Ultimately, both fluoroethyl carbonate and
vinylene carbonate may cause a high-temperature swelling problem
and a drop in low-temperature capacity, as their amounts
increase.
As described above, the additive such as vinylene carbonate and
fluoroethyl carbonate cannot ensure low-temperature discharge
capacity, high-temperature stability and cycle life characteristics
of a battery, when used alone.
As shown in Table 2, batteries obtained from Examples 1 to 15
according to the present invention, including at least one first
additive selected from the group consisting of lithium
tetrafluoroborate and trimethylsilyl phosphate and at least one
second additive selected from the group consisting of vinylene
carbonate and fluoroethyl carbonate, can improve charge/discharge
cycle life characteristics and low-temperature discharge capacity
and can solve a high-temperature swelling problem, while decreasing
the amount of vinylene carbonate or fluoroethyl carbonate.
As can be seen from Examples 1 to 10, when each of vinylene
carbonate and fluoroethyl carbonate is used in the amount reduced
to 2.0 wt % and lithium tetrafluoroborate and trimethylsilyl
phosphate are added in various amounts, it is possible to obtain
room-temperature cycle life characteristics similar to those in the
case of using 5.0 wt % of vinylene carbonate or ethylene
carbonate.
The addition of lithium tetrafluoroborate also improves cycle life
characteristics and detracts from a high-temperature swelling
degree, while the addition of trimethyl phosphate improves cycle
life characteristics and low-temperature discharge capacity
characteristics. In other words, lithium tetrafluoroborate has
functions of improving cycle life characteristics and inhibiting
high-temperature swelling, while trimethylsilyl phosphate has
functions of improving cycle life characteristics and
low-temperature discharge capacity characteristics.
However, when the amount of lithium tetrafluoroborate increase
above its optimal level, high-temperature swelling becomes serious
and both the initial capacity and low-temperature discharge
capacity decrease. Similarly, when the amount of trimethylsilyl
phosphate increases above its optimal level, high-temperature
swelling becomes serious and cycle life characteristics are
degraded. Additionally, when lithium tetrafluoroborate and
trimethylsilyl phosphate are used excessively, a large amount of
these additives is consumed in SEI film formation, thereby forming
an undesirably thick film. Moreover, residual additives remaining
unreacted during the first charge cycle may be decomposed at high
temperature or may function as a resistor upon discharge at low
temperature, thereby degrading the battery quality.
Because vinylene carbonate provides excellent room-temperature
cycle life characteristics when compared to fluoroethyl carbonate
but detracts from low-temperature discharge capacity significantly,
it is preferable that a minimized amount of vinylene carbonate is
used along with fluoroethyl carbonate in order to improve both
cycle life characteristics and low-temperature discharge capacity
characteristics at the same time, as in Examples 11 to 15.
It will be apparent to those skilled in the art that various
modifications and variation can be made in the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *