U.S. patent application number 15/740065 was filed with the patent office on 2018-07-05 for lithium secondary battery including non-aqueous electrolyte solution.
This patent application is currently assigned to LG Chem, Ltd.. The applicant listed for this patent is LG Chem, Ltd.. Invention is credited to Gwang Yeon Kim, Ha Eun Kim, Min Jung Kim, Chul Haeng Lee, Young Min Lim.
Application Number | 20180191021 15/740065 |
Document ID | / |
Family ID | 58423890 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180191021 |
Kind Code |
A1 |
Kim; Ha Eun ; et
al. |
July 5, 2018 |
LITHIUM SECONDARY BATTERY INCLUDING NON-AQUEOUS ELECTROLYTE
SOLUTION
Abstract
The present invention relates to a lithium secondary battery
which includes a non-aqueous electrolyte solution including lithium
bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a
positive electrode including a
lithium-nickel-manganese-cobalt-based oxide as a positive electrode
active material, a negative electrode, and a separator. In the
lithium secondary battery including a non-aqueous electrolyte
solution for a lithium secondary battery of the present invention,
since the non-aqueous electrolyte solution may form a robust solid
electrolyte interface (SEI) on the negative electrode during
initial charge and may prevent decomposition of the surface of the
positive electrode and an oxidation reaction of the electrolyte
solution during a high-temperature cycle, excellent low-temperature
output characteristics as well as improved high-temperature storage
characteristics and life characteristics may be achieved.
Inventors: |
Kim; Ha Eun; (Daejeon,
KR) ; Lim; Young Min; (Daejeon, KR) ; Lee;
Chul Haeng; (Daejeon, KR) ; Kim; Min Jung;
(Daejeon, KR) ; Kim; Gwang Yeon; (Daejeon,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG Chem, Ltd. |
Seoul |
|
KR |
|
|
Assignee: |
LG Chem, Ltd.
Seoul
KR
|
Family ID: |
58423890 |
Appl. No.: |
15/740065 |
Filed: |
September 30, 2016 |
PCT Filed: |
September 30, 2016 |
PCT NO: |
PCT/KR2016/010997 |
371 Date: |
December 27, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0567 20130101;
H01M 10/0568 20130101; H01M 4/50 20130101; H01M 2300/0065 20130101;
H01M 2004/027 20130101; H01M 10/052 20130101; Y02E 60/10 20130101;
H01M 2004/028 20130101; H01M 2300/0034 20130101; H01M 4/505
20130101; H01M 4/52 20130101; H01M 4/525 20130101; H01M 2/14
20130101; H01M 10/0569 20130101 |
International
Class: |
H01M 10/052 20060101
H01M010/052; H01M 10/0567 20060101 H01M010/0567; H01M 10/0569
20060101 H01M010/0569; H01M 4/52 20060101 H01M004/52; H01M 4/50
20060101 H01M004/50; H01M 2/14 20060101 H01M002/14 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2015 |
KR |
10-2015-0138029 |
Claims
1. A lithium secondary battery comprising: a non-aqueous
electrolyte solution including lithium bis(fluorosulfonyl)imide
(LiFSI) and a fluorobiphenyl compound; a positive electrode
including a lithium-nickel-manganese-cobalt-based oxide as a
positive electrode active material; a negative electrode; and a
separator.
2. The lithium secondary battery of claim 1, wherein the
non-aqueous electrolyte solution further comprises a lithium salt
excluding the lithium bis(fluorosulfonyl)imide.
3. The lithium secondary battery of claim 2, wherein a mixing ratio
of the lithium salt and the lithium bis(fluorosulfonyl)imide is in
a range of 1:0.01 to 1:1 as a molar ratio.
4. The lithium secondary battery of claim 1, wherein the lithium
bis(fluorosulfonyl)imide has a concentration of 0.01 mol/L to 2
mol/L in the non-aqueous electrolyte solution.
5. The lithium secondary battery of claim 2, wherein the lithium
salt comprises one selected from the group consisting of
LiPF.sub.6, LiAsF.sub.6, LiCF.sub.3SO.sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, LiBF.sub.6, LiSbF.sub.6,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiAlO.sub.4, LiAlCl.sub.4,
LiSO.sub.3CF.sub.3, and LiClO.sub.4, or a mixture of two or more
thereof.
6. The lithium secondary battery of claim 1, wherein the
lithium-nickel-manganese-cobalt-based oxide comprises an oxide
represented by Formula 1:
Li.sub.1+x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.2 [Formula 1] wherein, in
Formula 1, 0.55.ltoreq.a.ltoreq.0.65, 0.18.ltoreq.b.ltoreq.0.22,
0.18.ltoreq.c.ltoreq.0.22, -0.2.ltoreq.x.ltoreq.0.2, and
x+a+b+c=1.
7. The lithium secondary battery of claim 1, wherein the
non-aqueous organic solvent comprises a nitrile-based solvent,
linear carbonate, cyclic carbonate, ester, ether, ketone, or a
combination thereof.
8. The lithium secondary battery of claim 7, wherein the cyclic
carbonate comprises one selected from the group consisting of
ethylene carbonate (EC), propylene carbonate (PC), and butylene
carbonate (BC), or a mixture of two or more thereof, and the linear
carbonate comprises one selected from the group consisting of
dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl
carbonate (MPC), and ethylpropyl carbonate (EPC), or a mixture of
two or more thereof.
9. The lithium secondary battery of claim 7, wherein the
nitrile-based solvent comprises at least one selected from the
group consisting of acetonitrile, propionitrile, butyronitrile,
valeronitrile, caprylonitrile, heptanenitrile, cyclopentane
carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile,
4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile,
phenylacetonitrile, 2-fluorophenylacetonitrile, and
4-fluorophenylacetonitrile.
10. The lithium secondary battery of claim 1, wherein the
fluorobiphenyl compound is a compound represented by Formula 2:
##STR00002## wherein, in Formula 2, n is an integer of 1 to 5.
11. The lithium secondary battery of claim 1, wherein the
fluorobiphenyl compound is 2,3-difluorobiphenyl.
12. The lithium secondary battery of claim 1, wherein an amount of
the fluorobiphenyl compound is in a range of 0.5 wt % to 10 wt %
based on a total weight of the non-aqueous electrolyte
solution.
13. The lithium secondary battery of claim 1, wherein the lithium
bis(fluorosulfonyl)imide and the fluorobiphenyl compound are
included in a weight ratio of 1:0.02 to 1:10.
14. A lithium secondary battery comprising the non-aqueous
electrolyte solution of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Korean Patent
Application No. 10-2015-0138029, filed on Sep. 30, 2015, in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.
TECHNICAL FIELD
Technical Field
[0002] The present invention relates to a lithium secondary battery
which includes a non-aqueous electrolyte solution including lithium
bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a
positive electrode including a
lithium-nickel-manganese-cobalt-based oxide as a positive electrode
active material, a negative electrode, and a separator.
Background Art
[0003] Demand for secondary batteries as an energy source has been
significantly increased as technology development and demand with
respect to mobile devices have increased. Among these secondary
batteries, lithium secondary batteries having high energy density
and high voltage have been commercialized and widely used.
[0004] A lithium metal oxide is used as a positive electrode active
material of a lithium secondary battery, and a lithium metal, a
lithium alloy, crystalline or amorphous carbon, or a carbon
composite is used as a negative electrode active material. A
current collector may be coated with the active material of
appropriate thickness and length or the active material itself may
be coated in the form of a film, and the resultant product is then
wound or stacked with an insulating separator to prepare an
electrode assembly. Thereafter, the electrode assembly is put into
a can or a container similar thereto, and a secondary battery is
then prepared by injecting an electrolyte solution.
[0005] Charge and discharge of the lithium secondary battery is
performed while a process of intercalating and deintercalating
lithium ions from a lithium metal oxide positive electrode into and
out of a graphite negative electrode is repeated. In this case,
since lithium is highly reactive, lithium reacts with the carbon
electrode to form Li.sub.2CO.sub.3, LiO, or LiOH, and thus, a film
may be formed on the surface of the negative electrode. The film is
denoted as "solid electrolyte interface (SEI)", wherein the SEI
formed at an initial stage of charging may prevent a reaction of
the lithium ions with the carbon negative electrode or other
materials during charge and discharge. Also, the SEI may only pass
the lithium ions by acting as an ion tunnel. The ion tunnel may
prevent the collapse of a structure of the carbon negative
electrode due to the co-intercalation of the carbon negative
electrode and organic solvents of an electrolyte solution having a
high molecular weight which solvates lithium ions and moves
therewith.
[0006] Therefore, in order to improve high-temperature cycle
characteristics and low-temperature output of the lithium secondary
battery, a robust SEI must be formed on the negative electrode of
the lithium secondary battery. When the SEI is once formed during
the first charge, the SEI may prevent the reaction of the lithium
ions with the negative electrode or other materials during repeated
charge and discharge cycles caused by the subsequent use of the
battery, and the SEI may act as an ion tunnel that only passes the
lithium ions between the electrolyte solution and the negative
electrode.
[0007] Conventionally, with respect to an electrolyte solution
which does not include an electrolyte solution additive or includes
an electrolyte solution additive having poor characteristics, it
may he difficult to expect the improvement of low-temperature
output characteristics due to the formation of a non-uniform SEI.
Furthermore, even in a case in which the electrolyte solution
additive is included, if the amount of the added electrolyte
solution additive is not adjusted to a required amount, the surface
of a positive electrode may be decomposed or an oxidation reaction
of the electrolyte solution may occur during a high-temperature
reaction due to the electrolyte solution additive, and eventually,
irreversible capacity of the secondary battery may be increased and
output characteristics may be reduced. Also, since a decomposition
reaction of the electrolyte solution occurs when the lithium
secondary battery is stored at high temperature, high-temperature
storage performance and life performance of the battery may be
degraded.
DISCLOSURE OF THE INVENTION
Technical Problem
[0008] The present invention provides a non-aqueous electrolyte
solution for a lithium secondary battery which may improve
room-temperature and high-temperature cycle characteristics and
capacity characteristics after high-temperature storage as well as
low-temperature and room-temperature output characteristics, and a
lithium secondary battery including the same.
Technical Solution
[0009] According to an aspect of the present invention, there is
provided a lithium secondary battery including: a non-aqueous
electrolyte solution including lithium bis(fluorosulfonyl)imide
(LiFSI) and a fluorobiphenyl compound, a positive electrode
including a lithium-nickel-manganese-cobalt-based oxide as a
positive electrode active material, a negative electrode, and a
separator.
Advantageous Effects
[0010] In a lithium secondary battery including a non-aqueous
electrolyte solution for a lithium secondary battery of the present
invention, since the non-aqueous electrolyte solution may form a
robust solid electrolyte interface (SEI) on a negative electrode
during initial charge and may prevent decomposition of the surface
of a positive electrode and an oxidation reaction of the
electrolyte solution during a high-temperature cycle, excellent
low-temperature output characteristics as well as improved
high-temperature storage characteristics and life characteristics
may be achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates the results of overcharge tests on
batteries of Example 1 and Comparative Example 5.
MODE FOR CARRYING OUT THE INVENTION
[0012] Hereinafter, the present invention will be described in more
detail to allow for a clearer understanding of the present
invention. It will be understood that words or terms used in the
specification and claims shall not be interpreted as the meaning
defined in commonly used dictionaries. It will be further
understood that the words or terms should be interpreted as having
a meaning that is consistent with their meaning in the context of
the relevant art and the technical idea of the invention, based on
the principle that an inventor may properly define the meaning of
the words or terms to best explain the invention.
[0013] A lithium secondary battery of the present invention
includes a non-aqueous electrolyte solution including lithium
bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a
positive electrode including a
lithium-nickel-manganese-cobalt-based oxide as a positive electrode
active material, a negative electrode, and a separator.
[0014] The non-aqueous electrolyte solution includes lithium
bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound,
and, since the lithium bis(fluorosulfonyl)imide is added as a
lithium salt to the non-aqueous electrolyte solution to form a
robust thin solid electrolyte interface (SEI) on the negative
electrode, the lithium bis(fluorosulfonyl)imide may not only
improve low-temperature output characteristics, but also may
inhibit decomposition of the surface of the positive electrode,
which may occur during a high-temperature cycle, and may prevent an
oxidation reaction of the electrolyte solution. Since the
fluorobiphenyl compound is added to the electrolyte solution and
decomposed in the positive electrode and the negative electrode of
the lithium secondary battery including the fluorobiphenyl compound
to form a thin film and the thin film plays a role in protecting
the positive electrode to reduce metal dissolution of the positive
electrode active material and increase porosity of a negative
electrode film, lithium ions may more smoothly move, and thus, long
life and storage characteristics of the secondary battery including
the fluorobiphenyl compound may be improved. Also, the
fluorobiphenyl compound may improve room-temperature capacity
characteristics and output characteristics, and, since the
fluorobiphenyl compound may form a film near 4.62 V during
overcharging to short the battery at a low state of charge (Sac),
the fluorobiphenyl compound may prevent heat generation and
subsequent ignition of the battery. Since the SEI formed on the
negative electrode is thin, the lithium ions in the negative
electrode may more smoothly move, and, accordingly, output of the
secondary battery may be improved.
[0015] According to an embodiment of the present invention, a
concentration of the lithium bis(fluorosulfonyl)imide in the
non-aqueous electrolyte solution may be in a range of 0.01 mol/L to
2 mol/L, particularly, 0.01 mol/L to 1 mol/L. In a case in which
the concentration of the lithium bis(fluorosulfonyl)imide is less
than 0.01 mol/L, effects of improving the low-temperature output
and high-temperature cycle characteristics of the lithium secondary
battery may be insignificant. In a case in which the concentration
of the lithium bis(fluorosulfonyl)imide is greater than 2 mol/L,
since side reactions in the electrolyte solution may excessively
occur during charge and discharge of the battery, a swelling
phenomenon may occur and corrosion of a positive electrode or
negative electrode collector formed of a metal may occur in the
electrolyte solution.
[0016] In order to prevent the side reactions, the non-aqueous
electrolyte solution of the present invention may further include a
lithium salt excluding the lithium bis(fluorosulfonyl)imide. Any
lithium salt commonly used in the art may be used as the lithium
salt, and, for example, the lithium salt may include any one
selected from the group consisting of LiPF.sub.6, LiAsF.sub.6,
LiCF.sub.3SO.sub.3, LiBF.sub.6, LiSbF.sub.6,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiAlO.sub.4, LiAlCl.sub.4,
LiSO.sub.3CF.sub.3 and LiClO.sub.4, or a mixture of two or more
thereof.
[0017] A mixing ratio of the lithium salt and the lithium
bis(fluorosulfonyl)imide may be in a range of 1:0.01 to 1:1 as a
molar ratio. In a case in which the mixing ratio of the lithium
salt and the lithium bis(fluorosulfonyl)imide is above the molar
ratio range, since the side reactions in the electrolyte solution
may excessively occur during the charge and discharge of the
battery, the swelling phenomenon may occur, and, in a case in which
the mixing ratio is below the molar ratio range, the output of the
secondary battery generated may be reduced. Specifically, in a case
in which the mixing ratio of the lithium salt and the lithium
bis(fluorosulfonyl)imide is less than 1:0.01 as a molar ratio, a
large amount of irreversible reaction may occur during a process of
forming the SEI in the lithium-ion battery and a process of
intercalating lithium ions, which are solvated by a carbonate-based
solvent, into the negative electrode, and the effects of improving
the low-temperature output as well as the cycle characteristics and
capacity characteristics after high-temperature storage of the
secondary battery may be insignificant due to the exfoliation of a
negative electrode surface layer (e.g., carbon surface layer) and
the decomposition of the electrolyte solution. In a case in which
the mixing ratio of the lithium salt and the lithium
bis(fluorosulfonyl)imide is greater than 1:1 as a molar ratio,
since an excessive amount of the lithium bis(fluorosulfonyl)imide
is included in the electrolyte solution to cause the corrosion of
the electrode collector during the charge and discharge, stability
of the secondary battery may be affected.
[0018] The positive electrode active material, as the
lithium-nickel-manganese-cobalt-based oxide, may include an oxide
represented by Formula 1 below.
Li.sub.1+x(Ni.sub.aCo.sub.bMn.sub.c)O.sub.2 [Formula 1]
[0019] where, 0.55.ltoreq.a.ltoreq.0.65, 0.18.ltoreq.b.ltoreq.0.22,
0.18.ltoreq.c.ltoreq.0.22, -0.2.ltoreq.x.ltoreq.0.2, and
x+a+b+c=1.
[0020] Since the positive electrode active material, as the
lithium-nickel-manganese-cobalt-based oxide, is used in the
positive electrode, the positive electrode active material may be
combined with the lithium bis(fluorosulfonyl)imide to have a
synergistic effect. With respect to the
lithium-nickel-manganese-cobalt-based oxide positive electrode
active material, since a phenomenon (cation mixing), in which a
position of Li.sup.+1 ion and a position of Ni.sup.+2 ion in a
layered structure of the positive electrode active material are
changed during the charge and discharge as an amount of nickel (Ni)
among transition metals is increased, occurs, the structure is
collapsed, and, thus, the positive electrode active material may
cause a side reaction with the electrolyte solution or a
dissolution phenomenon of the transition metal may occur. The
reason for this is that sizes of the Li.sup.+1 ion and the
Ni.sup.+2 ion are similar. Eventually, performance of the battery
is easily degraded due to the depletion of the electrolyte solution
in the secondary battery and the structural collapse of the
positive electrode active material caused by the side reaction.
[0021] Therefore, since he LiFSI-containing electrolyte solution is
used in the positive electrode active material of Formula 1
according to an embodiment of the present invention, a layer is
formed of a component from the LiFSI on the surface of the positive
electrode, and thus, a range, in which a sufficient amount of the
Ni transition metal for securing capacity of the positive electrode
active material may be secured while suppressing the cation mixing
phenomenon of the Li.sup.+1 ion and Ni.sup.+ion, has been found.
According to the positive electrode active material including the
oxide according to Formula 1 of the present invention, the side
reaction with the electrolyte solution and the metal dissolution
phenomenon may be effectively suppressed when the LiFSI-containing
electrolyte solution is used.
[0022] In particular, in a case in which a ratio of the Ni
transition metal in the oxide represented by Formula 1 is greater
than 0.65, since an excessive amount of the Ni is included in the
positive electrode active material, the cation mixing phenomenon of
the Li.sup.+1 ion and Ni.sup.+2 ion may not be suppressed even by
the above-described layer generated from the LiFSI on the surface
of the electrode.
[0023] Also, when the excessive amount of the Ni is included in the
positive electrode active material, the nickel transition metal
having a d orbital in an environment, such as high temperature,
depending on the variation of oxidation number of the Ni must have
an octahedral structure when coordination bonded, but the order of
energy levels is reversed or the oxidation number is changed
(heterogenization reaction) by external energy supply to form a
distorted octahedron. As a result, since a crystal structure of the
positive electrode active material including the nickel transition
metal is transformed, the probability of dissolution of the nickel
metal in the positive electrode active material is increased.
[0024] As a result, the present inventors found that excellent
efficiency in high-temperature stability and capacity
characteristics is achieved while generating high output when the
positive electrode active material including the oxide in the range
according to Formula 1 and the LiFSI salt are combined.
[0025] Although the high output and stability may be achieved when
the positive electrode active material including the oxide of
Formula 1 and the LiFSI lithium salt are combined, the electrolyte
solution may be decomposed in a high output environment and
collapse of the negative electrode may be induced. Thus, in a case
in which the additives are combined and included in the non-aqueous
electrolyte solution, high-temperature stability of the secondary
battery generated may be secured.
[0026] In a case in which the lithium salt is LiPF.sub.6, the
electrolyte solution having insufficient thermal stability may be
easily decomposed in the battery to form LIF and PF.sub.5. In this
case, the LiF salt may reduce conductivity and the number of free
Li.sup.+ ions to increase the resistance of the battery, and, as a
result, the capacity of the battery is reduced.
[0027] The non-aqueous electrolyte solution includes a non-aqueous
organic solvent, and the non-aqueous organic solvent is not limited
as long as it may minimize the decomposition due to the oxidation
reaction during the charge and discharge of the battery and may
exhibit desired characteristics with the additive. For example, the
non-aqueous organic solvent may include a nitrile-based solvent,
cyclic carbonate, linear carbonate, ester, ether, or ketone. These
materials may be used alone or in combination of two or more
thereof.
[0028] Among the above organic solvents, carbonate-based organic
solvents may be easily used. Examples of the cyclic carbonate may
be any one selected from the group consisting of ethylene carbonate
(EC), propylene carbonate (PC), and butylene carbonate (BC), or a
mixture of two or more thereof, and examples of the linear
carbonate may be any one selected from the group consisting of
dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl
carbonate (MPC), and ethylpropyl carbonate (EPC), or a mixture of
two or more thereof.
[0029] The nitrile-based solvent may include at least one selected
from the group consisting of acetonitrile, propionitrile,
butyronitrile, valeronitrile, caprylonitrile, heptanenitrile,
cyclopentane carbonitrile, cyclohexane carbonitrile,
2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile,
trifluorobenzonitrile, phenylacetonitrile, 2-f
luorophenylacetonitrile, and 4-fluorophenylacetonitrile, and
acetonitrile-based solvent may be used as the non-aqueous solvent
according to an embodiment of the present invention.
[0030] Side effects due to the reduction of the stability of the
high-output battery caused by the combination with the lithium
bis(fluorosulfonyl)imide may be effectively prevented by using the
acetonitrile-based solvent and using the
lithium-nickel-manganese-cobalt-based oxide positive electrode
active material in the positive electrode.
[0031] In an example of the present invention, the fluorobiphenyl
compound may be a compound represented by the following Formula
2.
##STR00001##
[0032] In Formula 2, n may be an integer of 1 to 5, and may
specifically be 2.
[0033] In an example of the present invention, the fluorobiphenyl
compound may be 2,3-difluorobiphenyl.
[0034] Since the non-aqueous electrolyte solution included in the
lithium secondary battery of the present invention includes the
fluorobiphenyl compound, the non-aqueous electrolyte solution may
improve the room-temperature capacity characteristics and output
characteristics and may prevent the heat generation and subsequent
ignition of the battery by shorting the battery at a low SOC by
forming a film near 4.62 V during overcharging.
[0035] An amount of the fluorobiphenyl compound may be in a range
of 0.5 wt % to 10 wt %, particularly 1 wt % to 7 wt %, and more
particularly 3 wt % to 5 wt %, based on a total weight of the
non-aqueous electrolyte solution.
[0036] In a case in which the amount of the fluorobiphenyl compound
is 0.5 wt % or more, an effect of shorting the battery during the
overcharging of the battery as well as an appropriate effect of
improving room-temperature capacity characteristics and output
characteristics may be obtained, and, in a case in which the amount
of the fluorobiphenyl compound is 10 wt % or less, problems, for
example, an increase in irreversible capacity of the battery or an
increase in resistance of the negative electrode, may be prevented
while having a moderate effect.
[0037] The amount of the fluorobiphenyl compound may be adjusted
according to the amount of the lithium bis(fluorosulfonyl)imide
added, and, accordingly, the lithium bis(fluorosulfonyl)imide and
the fluorobiphenyl compound may be used in a weight ratio of 1:0.02
to 1:10, particularly 1:0.03 to 1:9, and more particularly 1:0.05
to 1:7.5.
[0038] In a case in which the lithium bis(fluorosulfonyl)imide and
the fluorobiphenyl compound is used in a weight ratio of 1:0.02 to
1:10, the fluorobiphenyl compound may appropriately suppress the
side reaction in the electrolyte solution during the charge and
discharge of the lithium secondary battery at room temperature
which may occur due to the addition of the lithium
bis(fluorosulfonyl)imide, and may solve a performance imbalance
problem, for example, the reduction of the output in comparison to
capacity retention after high-temperature storage or the reduction
of the capacity retention in comparison to the output, or a problem
such as a decrease in life characteristics improvement effect, when
the mixing ratio is outside the above range.
[0039] The lithium secondary battery according to an embodiment of
the present invention may include a negative electrode, a separator
disposed between the positive electrode and the negative electrode,
and the non-aqueous electrolyte solution. The positive electrode
and the negative electrode may respectively include the positive
electrode active material according to the embodiment of the
present invention and a negative electrode active material.
[0040] Meanwhile, the negative electrode active material may
include amorphous carbon or crystalline carbon, and, for example,
carbon such as hard carbon and graphite-based carbon; a complex
metal oxide such as Li.sub.xFe.sub.2O.sub.3 (0.ltoreq.x.ltoreq.1),
Li.sub.xWO.sub.2 (0.ltoreq.x.ltoreq.1),
Sn.sub.xMe.sub.1-xMe'.sub.yO.sub.z (Me=manganese (Mn), iron (Fe),
lead (Pb), and germanium (Ge); Me'=aluminum (Al), boron (B),
phosphorus (P), silicon (Si), Groups I, II and III elements, or
halogen; 0<x.ltoreq.1; 1.ltoreq.y.ltoreq.3;
1.ltoreq.z.ltoreq.8); a lithium metal; a lithium alloy; a
silicon-based alloy; a tin-based alloy; an oxide such as SnO,
SnO.sub.2, PbO, PbO.sub.2, Pb.sub.2O.sub.3, Pb.sub.3O.sub.4,
Sb.sub.2O.sub.3, Sb.sub.2O.sub.4, Sb.sub.2O.sub.5, GeO, GeO.sub.2,
Bi.sub.2O.sub.3, Bi.sub.2O.sub.4, and Bi.sub.2O.sub.5; a conductive
polymer such as polyacetylene; or a Li--Co--Ni-based material may
be used.
[0041] Also, a porous polymer film, for example, a porous polymer
film prepared from a polyolefin-based polymer, such as an ethylene
homopolymer, a propylene homopolymer, an ethylene/butene copolymer,
an ethylene/hexene copolymer, and an ethylene/methacrylate
copolymer, may be used alone or in a lamination of two or more
thereof as the separator. In addition, a typical porous nonwoven
fabric, for example, a nonwoven fabric formed of high melting point
glass fibers or polyethylene terephthalate fibers may be used, but
the present invention is not limited thereto.
[0042] The secondary battery may have various shapes, such as a
cylindrical shape, a prismatic shape, or a pouch shape, depending
on purposes, and is not limited to a configuration known in the
art. The lithium secondary battery according to the embodiment of
the present invention may be a pouch type secondary battery.
[0043] Hereinafter, the present invention will be described in more
detail, according to examples and experimental examples. However,
the present invention is not limited thereto.
EXAMPLES
Example 1
[0044] [Preparation of Electrolyte Solution]
[0045] A non-aqueous electrolyte solution was prepared by adding
0.9 mol/L of LiPF.sub.6, as a lithium salt, based on a total amount
of the non-aqueous electrolyte solution and adding 0.1 mol/L of
lithium bis(fluorosulfonyi)imide and 3 wt % of 2,3-difluorobiphenyi
to a non-aqueous organic solvent having a composition in which a
volume ratio of ethylene carbonate (EC):ethylmethyl carbonate (EMC)
was 3:7.
[0046] [Preparation of Lithium Secondary Battery]
[0047] A positive electrode mixture slurry was prepared by adding
92 wt % of Li(Ni.sub.0.6Co.sub.0.2Mn.sub.0.2)O.sub.0.2)O.sub.2 as a
positive electrode active material, 4 wt % of carbon black as a
conductive agent, and 4 wt % of polyvinylidene fluoride (PVdF) as a
binder to N-methyl-2-pyrrolidone (NMP) as a solvent. An about 20
.mu.m thick aluminum (Al) thin film as a positive electrode
collector was coated with the positive electrode mixture slurry and
dried, and the coated Al thin film was then roll-pressed to prepare
a positive electrode.
[0048] Also, a negative electrode mixture slurry was prepared by
adding 96 wt % of carbon powder as a negative electrode active
material, 3 wt % of PVdF as a binder, and 1 wt % of carbon black as
a conductive agent to NMP as a solvent. A 10 .mu.m thick copper
(Cu) thin film as a negative electrode collector was coated with
the negative electrode mixture slurry and dried, and the coated Cu
thin film was then roll-pressed to prepare a negative
electrode.
[0049] A polymer type battery was prepared by a typical method
using a separator formed of three layers of
polypropylene/polyethylene/polypropylene (PP/PE/PP) with the
positive electrode and negative electrode thus prepared, and a
lithium secondary battery was then completed by injecting the
prepared non-aqueous electrolyte solution.
Example 2
[0050] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 1 except
that 0.7 mol/L of LiPF.sub.6 and 0.3 mol/L of lithium
bis(fluorosulfonyl)imide were used.
Example 3
[0051] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 1 except
that 0.5 mol/L of LiPF.sub.6 and 0.5 mol/L of lithium
bis(fluorosulfonyi)imide were used.
Example 4
[0052] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that 5 wt % of the 2,3-difluorobiphenyl was used.
Example 5
[0053] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that 10 wt % of the 2,3-difluorobiphenyl was used.
Example 6
[0054] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that 0.5 wt % of the 2,3-difluorobiphenyl was used.
Example 7
[0055] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that 2-fluorobiphenyl was used instead of the
2,3-difluorobiphenyl.
Comparative Example 1
[0056] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that Li(Ni.sub.0.5Co.sub.0.3Mn.sub.0.2)O.sub.2 was used as the
positive electrode active material.
Comparative Example 2
[0057] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that Li(Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)O.sub.2 was used as the
positive electrode active material.
Comparative Example 3
[0058] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that LiCoO.sub.2 was used as the positive electrode active
material.
Comparative Example 4
[0059] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that Li(Ni.sub.0.5Co.sub.0.3Mn.sub.0.2)O.sub.2 was used as the
positive electrode active material and 2,3-difluorobiphenyl was not
used.
Comparative Example 5
[0060] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 2 except
that 2,3-difluorobiphenyl was not used.
Comparative Example 6
[0061] A non-aqueous electrolyte solution and a lithium secondary
battery were prepared in the same manner as in Example 1 except
that 0.3 mol/L of LiPF.sub.6 and 0.7 mol/L of lithium
bis(fluorosulfonyl)imide were used.
Experimental Example 1
[0062] <Capacity Characteristics After High-Temperature
Storage>
[0063] The secondary batteries prepared in Examples 1 to 7 and
Comparative Examples 1 to 6 were charged at 1 C to 4.2 V/38 mA
under a constant current/constant voltage (CC/CV) condition and
then discharged at a constant current (CC) of 2 C to a voltage of
2.5 V to measure discharge capacities. Next, after storing the
secondary batteries prepared in Examples 1 to 7 and Comparative
Examples 1 to 6 at 60.degree. C. for 20 weeks, the secondary
batteries were again charged at 1 C to 4.2 V/38 mA under a constant
current/constant voltage (CC/CV) condition at room temperature and
then discharged at a constant current (CC) of 2 C to a voltage of
2.5 V to measure discharge capacities. The discharge capacity after
20 weeks was calculated as a percentage based on the initial
discharge capacity (discharge capacity after 20 weeks/initial
discharge capacity.times.100(%)), and the results thereof are
presented in Table 1 below.
Experimental Example 2
[0064] <Output Characteristics After High-Temperature
Storage>
[0065] After storing the secondary batteries prepared in Examples 1
to 7 and Comparative Examples 1 to 6 at 60.degree. C. for 20 weeks,
outputs were calculated from voltage differences which were
obtained by charging and discharging the secondary batteries at 5 C
for 10 seconds at room temperature. The output after 20 weeks was
calculated as a percentage based on the initial output (output (W)
after 20 weeks/initial output (W).times.100(%)), and the results
thereof are presented in Table 1 below. The experiment was
performed at a state of charge (SOC) of 50%.
TABLE-US-00001 TABLE 1 Positive High-temperature electrode storage
character- active LiPF.sub.6:LiFSI Additive istics (%) material dsd
(wt %) Capacity Output Example 1 NMC622 9:1 DFBP 3 93.9 95.3
Example 2 NMC622 7:3 DFBP 3 95.4 97.7 Example 3 NMC622 5:5 DFBP 3
92.6 94.1 Example 4 NMC622 7:3 DFBP 5 92.4 93.3 Example 5 NMC622
7:3 DFBP 10 90.3 91.1 Example 6 NMC622 7:3 DFBP 0.5 91.1 92.6
Example 7 NMC622 7:3 FBP 3 89.9 90.4 Comparative NMC532 7:3 DFBP 3
88.1 90.7 Example 1 Comparative NMC811 7:3 DFBP 3 81.5 86.3 Example
2 Comparative LiCoO.sub.2 7:3 DFBP 3 90.7 91.4 Example 3
Comparative NMC532 7:3 0 87.7 88.9 Example 4 Comparative NMC622 7:3
0 90.4 92.1 Example 5 Comparative NMC622 3:7 DFBP 3 87.6 89.9
Example 6
[0066] In Table 1, NMC622 represents
Li(Ni.sub.0.6Co.sub.0.2Mn.sub.0.2)O.sub.2, NMC532 represents
Li(Ni.sub.0.5Co.sub.0.3Mn.sub.0.2)O.sub.2, NMC811 represents
Li(Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)O.sub.2, DEBT represents
2,3-difluorobiphenyl, and FBP represents 2-fluorobiphenyl.
[0067] As confirmed from Table 1, it may be understood that the
lithium secondary batteries of Examples 1 to 7 exhibited high
capacity and output even after the high-temperature storage by
including the non-aqueous electrolyte solution which included both
of the lithium bis(fluorosulfonyl)imide and the fluorobiphenyl
compound. Among these lithium secondary batteries, the lithium
secondary batteries of Examples 1 to 5 including
2,3-difluorobiphenyl, as the fluorobiphenyl compound, exhibited
better high-temperature storage characteristics than the lithium
secondary battery including 2-fluorobiphenyl as the fluorobiphenyl
compound. Also, since the lithium secondary batteries of Examples 1
to 4 included the non-aqueous electrolyte solution including both
of the lithium bis(fluorosulfonyl)imide and the fluorobiphenyl
compound, the lithium secondary batteries of Examples 1 to 4
exhibited better high-temperature storage characteristics than the
lithium secondary batteries of Comparative Examples 4 and 5 which
did not include the fluorobiphenyl compound.
[0068] When comparing Example 2 with Comparative Examples 1 to 3,
Example 2 including Li(Ni.sub.0.6Co.sub.0.2Mn.sub.0.2)O.sub.2 as
the positive electrode active material exhibited better
high-temperature storage characteristics than Comparative Examples
1 to 3 respectively including
Li(Ni.sub.0.6Co.sub.0.3Mn.sub.0.2)O.sub.2,
Li(Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)O.sub.2, and LiCoO.sub.2 as the
positive electrode active material.
[0069] Also, when comparing Examples 1 to 3 with Comparative
Example 6, in a case in which lithium-manganese-cobalt-based oxide,
Li(Ni.sub.0.6Co.sub.0.2Mn.sub.0.2)O.sub.2, was included as the
positive electrode active material, it may be confirmed that the
high-temperature storage characteristics of the lithium secondary
batteries were degraded when the amount of LiFSI was increased in
comparison to that of LiPF.sub.6.
Experimental Example 3
[0070] <Overcharge Evaluation>
[0071] The lithium secondary batteries prepared in Example 1 and
Comparative Example 5 were overcharged to 8.3 V under a constant
current/constant voltage (CC/CV) condition of 1 C (775 mAh)/12 V
from a charged state at 25.degree. C., changes in temperature and
voltage of the battery at that time were measured, and the results
thereof are presented in FIG. 1.
[0072] Referring to FIG. 1, it may be confirmed that the lithium
secondary battery of Example 1, which included the non-aqueous
electrolyte solution including the 2,3-difluorobiphenyl additive
had a lower SOC at the time of short circuit caused by overcharge
and a lower temperature at the center of the battery than the
lithium secondary battery of Comparative Example 5 which did not
include the 2,3-difluorobiphenyl additive.
* * * * *