TECHNICAL FIELD
[0001] The present invention relates to an 5 electrolyte
solution additive including four kinds of compounds, a nonaqueous
electrolyte solution including the electrolyte
solution additive, and a lithium secondary battery including
the non-aqueous electrolyte solution.
10 BACKGROUND ART
[0002] 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
15 having high energy density and high voltage have been
commercialized and widely used.
[0003] A lithium metal oxide is used as a cathode active
material of a lithium secondary battery, and lithium metal, a
lithium alloy, crystalline or amorphous carbon, or a carbon
20 composite is used as an anode 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
25 to prepare an electrode group. Thereafter, the electrode
3
group is put into a can or a container similar thereto, and a
secondary battery is then prepared by injecting an
electrolyte solution.
[0004] Charge and discharge of the lithium secondary battery
is performed while a process of intercalating 5 and
deintercalating lithium ions from a lithium metal oxide
cathode into and out of a graphite anode is repeated. In
this case, since lithium is highly reactive, lithium reacts
with the carbon electrode to form Li2CO3, LiO, or LiOH. Thus,
10 a film may be formed on the surface of the anode. The film
is denoted as “solid electrolyte interface (SEI)”.
[0005] The SEI formed at an initial stage of charging may
prevent a reaction of the lithium ions with the carbon anode
or other materials during the charge and discharge. Also,
15 the SEI may only pass the lithium ions by acting as an ion
tunnel. The ion tunnel may prevent the destruction of a
structure of the carbon anode due to the co-intercalation of
the carbon anode and organic solvents of an electrolyte
solution having a high molecular weight which solvates
20 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 anode
of the lithium secondary battery. When the SEI is once
25 formed during the first charge, the SEI may prevent the
4
reaction of the lithium ions with the anode or other
materials during repeated charge and discharge cycles caused
by the subsequent use of the battery, and may act as an ion
tunnel that only passes the lithium ions between the
electrolyte 5 lyte solution and the anode.
[0007] Typically, with respect to an electrolyte solution
which does not include an electrolyte solution additive or
includes an electrolyte solution additive having poor
characteristics, the improvement of low-temperature output
10 characteristics may not be expected due to the formation of
non-uniform SEI. In addition, even in the case in which the
electrolyte solution additive is included, the robust SEI may
not be formed on the anode when the input thereof is not
adjusted to a required amount. Thus, a swelling phenomenon,
15 in which the anode is swollen by reacting with an electrolyte
solution, may occur as a side reaction, or gas generation may
be increased due to the decomposition of the electrolyte
solution, and charge and discharge rate may be decreased.
DISCLOSURE OF THE INVENTION
20 TECHNICAL PROBLEM
[0008] The present invention is provided to solve technical
problems of the related art.
[0009] The inventors of the present application recognized
that output characteristics are improved in a case where an
25 electrolyte solution for a lithium secondary battery includes
5
four kinds of additives, thereby leading to the completion of
the present invention.
TECHNICAL SOLUTION
[0010] According to an aspect of the present invention,
there is provided a non-aqueous electrolyte 5 ctrolyte solution
including a non-aqueous organic solvent; an imide-based
lithium salt; and at least one additive selected from the
group consisting of lithium difluoro bis(oxalato)phosphate
(LiDFOP), (trimethylsilyl)propyl phosphate (TMSPa), 1,3-
10 propene sultone (PRS), and ethylene sulfate (ESa), as an
electrolyte solution additive.
[0011] The imide-based lithium salt may be Li(SO2F)2N
(lithium bis(fluorosulfonyl)imide, LiFSI), the non-aqueous
organic solvent may include dimethyl carbonate (DMC),
15 ethylmethyl carbonate (EMC), ethylene carbonate (EC), and
propylene carbonate (PC), and the additive may include
lithium difluoro bis(oxalato)phosphate (LiDFOP),
(trimethylsilyl)propyl phosphate (TMSPa), 1,3-propene sultone
(PRS), and ethylene sulfate (ESa). Also, the lithium salt
20 may further include LiPF6.
[0012] According to another aspect of the present invention,
there is provided a lithium secondary battery including: a
cathode; an anode; and the non-aqueous electrolyte solution.
ADVANTAGEOUS EFFECTS
25 [0013] According to an electrolyte solution additive for a
6
lithium secondary battery of the present invention, the
electrolyte solution additive may improve output
characteristics at high and low temperatures and may prevent
a swelling phenomenon by suppressing the decomposition of
PF6- on the surface of a cathode, which may occur during 5 a
high-temperature cycle of a lithium secondary battery
including the electrolyte solution additive, and preventing
an oxidation reaction of an electrolyte solution.
BRIEF DESCRIPTION OF THE DRAWINGS
10 [0014] FIG. 1 is a graph illustrating output characteristics
after high-temperature storage of Example 1 and Comparative
Examples 1 and 2;
[0015] FIG. 2 is a graph illustrating thickness changes
after high-temperature storage of Example 1 and Comparative
15 Examples 1 and 2; and
[0016] FIG. 3 is a graph illustrating low-temperature output
characteristics of Example 1 and Comparative Examples 1 and 2.
MODE FOR CARRYING OUT THE INVENTION
[0017] Hereinafter, the present invention will be described
20 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
25 interpreted as having a meaning that is consistent with their
7
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.
[0018] An embodiment of the present invention 5 provides a
non-aqueous electrolyte solution including a non-aqueous
organic solvent, an imide-based lithium salt, and at least
one additive selected from the group consisting of lithium
difluoro bis(oxalato)phosphate (LiDFOP),
10 (trimethylsilyl)propyl phosphate (TMSPa), 1,3-propene sultone
(PRS), and ethylene sulfate (ESa), as an electrolyte solution
additive.
[0019] The imide-based lithium salt may be at least one
selected from the group consisting of LiN(CF3SO2)2,
15 LiN(C2F5SO2)2, Li(CF3SO2)(C2F5SO2)N, and Li(SO2F)2N, and
according to an embodiment of the present invention, the
imide-based lithium salt may be Li(SO2F)2N (lithium
bis(fluorosulfonyl)imide, LiFSI).
[0020] The lithium salt, which may be included in the non20
aqueous electrolyte solution according to the embodiment of
the present invention, may further include LiPF6.
[0021] Since the LiFSI and LiPF6 are combined, the reduction
of the output of a battery due to low mobility of lithium
ions caused by high viscosity of LiPF6 at low temperature may
25 be improved by adding LiFSI that may maintain low viscosity
8
even at low temperature.
[0022] The LiFSI and LiPF6 may be mixed in a molar ratio of
LiFSI to LiPF6 of 10:90 to 50:50. In the case that a trace
amount of LiPF6 is included within the above range, capacity
of the formed battery may be low. In 5 the case in which an
excessive amount of LiPF6 is included, the viscosity at low
temperature increases to decrease the mobility of lithium
ions. Thus, the output of the formed battery may not be
improved.
10 [0023] The additive may include at least one selected from
the group consisting of lithium difluoro
bis(oxalato)phosphate (LiDFOP), (trimethylsilyl)propyl
phosphate (TMSPa), 1,3-propene sultone (PRS), and ethylene
sulfate (ESa). However, according to an embodiment of the
15 present invention, the additive may include four kinds
including lithium difluoro bis(oxalato)phosphate (LiDFOP),
(trimethylsilyl)propyl phosphate (TMSPa), 1,3-propene sultone
(PRS), and ethylene sulfate (ESa).
[0024] In the case that the lithium salt is LiPF6, the
20 electrolyte solution having insufficient thermal stability
may be easily decomposed in the battery to form LiF and PF5.
In this case, the LiF salt may reduce conductivity and the
number of free Li+ ions to increase the resistance of the
battery, and as a result, the capacity of the battery is
25 reduced. That is, in the decomposition of PF6- ions on the
9
surface of a cathode which may occur during a hightemperature
cycle, a phosphate functional group of the
(trimethylsilyl)propyl phosphate or a sulfate functional
group of the ethylene sulfate (ESa) acts as an anion receptor
to induce the stable formation of PF6- and 5 increase the
separation of ion pairs of Li+ and PF6-. As a result,
interfacial resistance may be reduced by improving the
solubility of LiF in the electrolyte solution.
[0025] Also, the LiDFOP additive may form a stable solid
10 electrolyte interface (SEI) on a surface of an anode after a
battery activation process to suppress gas which is generated
due to the decomposition of the electrolyte solution caused
by the reaction between the surface of the anode and the
electrolyte solution in the battery. Thus, the LiDFOP
15 additive may improve life characteristics of the lithium
secondary battery.
[0026] Herein, the lithium difluoro bis(oxalato)phosphate
(LiDFOP), 1,3-propene sultone (PRS), and ethylene sulfate
(ESa) additives may each independently be included in an
20 amount of 0.5 wt% to 1.5 wt%, for example, 0.5 wt% to 1.0 wt%,
based on a total amount of the electrolyte solution. In the
case that each amount of the lithium difluoro
bis(oxalato)phosphate (LiDFOP), 1,3-propene sultone (PRS),
and ethylene sulfate (ESa) additives is less than 0.5 wt%,
25 the effect of suppressing the decomposition as an anion
10
receptor in a high-temperature cycle may be insignificant.
With respect to the LiDFOP additive, the formation of the
stable SEI on the surface of the anode may not be achieved,
and the PRS additive may not effectively suppress the gas
generated from the electrolyte solution. In the case 5 in
which each amount of the additives is greater than 1.5 wt%,
lithium ion permeability of the protective layer may be
reduced to increase impedance, and sufficient capacity and
charge and discharge efficiency may not be obtained. Also,
10 the (trimethylsilyl)propyl phosphate (TMSPa) additive may be
included in an amount of 0.1 wt% to 0.5 wt% based on the
total amount of the electrolyte solution. In the case that
the amount of the (trimethylsilyl)propyl phosphate (TMSPa)
additive is less than 0.1 wt%, since the amount is excessive
15 small, the decomposition of LiPF6 may not be suppressed. In
the case in which the amount of the TMSPa additive is greater
than 0.5 wt%, the lithium ion permeability may be reduced to
increase impedance, and sufficient capacity and charge and
discharge efficiency may not be obtained.
20 [0027] The non-aqueous electrolyte solution according to the
embodiment of the present invention may include the
electrolyte solution additive, non-aqueous organic solvent,
and lithium salt.
[0028] Also, the non-aqueous organic solvent, which may be
25 included in the non-aqueous electrolyte solution, is not
11
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 carbonate-based compounds and 5 propionate-based
compounds. These compounds may be used alone or in
combination of two or more thereof.
[0029] Among the above non-aqueous organic solvents, the
carbonate-based organic solvents may include any one selected
10 from the group consisting of dimethyl carbonate (DMC),
diethyl carbonate (DEC), dipropyl carbonate (DPC),
methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC),
ethylmethyl carbonate (EMC), ethylene carbonate (EC),
propylene carbonate (PC), and butylene carbonate (BC), or a
15 mixture of two or more thereof. Examples of the propionatebased
compounds may include any one selected from the group
consisting of ethyl propionate (EP), propyl propionate (PP),
n-propyl propionate, isopropyl propionate, n-butyl propionate,
isobutyl propionate, and tert-butyl propionate, or a mixture
20 of two or more thereof.
[0030] According to an embodiment of the present invention,
the carbonate-based solvents may be used in combination
thereof. For example, an electrolyte solution including
dimethyl carbonate (DMC), ethylmethyl carbonate (EMC),
25 ethylene carbonate (EC), and propylene carbonate (PC) may be
12
used as the non-aqueous organic solvent.
[0031] As the 4 kinds of the electrolyte solutions,
EC/PC/EMC/DMC may be respectively included in an amount of
1.0 part by weight to 1.5 parts by weight: 1.0 part by weight
to 1.5 parts by weight: 4.0 parts by weight to 4.5 5 .5 parts by
weight: and 4.0 parts by weight to 4.5 parts by weight.
According to an embodiment of the present invention, the
ratio of EC/PC/EMC/DMC expressed as parts by weight may be
about 1:1:4:4.
10 [0032] In the case that the carbonate compounds are combined
within the above weight ratio and used as the non-aqueous
organic solvent, dimethyl carbonate (DMC) may particularly
improve the output of the lithium battery, but gas may be
generated during the high-temperature cycle of the secondary
15 battery. Thus, 1,3-propene sultone (PRS) among the additives
may improve the life characteristics of the secondary battery
by effectively suppressing the gas generated from the DMC.
[0033] A lithium secondary battery according to an
embodiment of the present invention may include a cathode, an
20 anode, a separator disposed between the cathode and the anode,
and the non-aqueous electrolyte solution. The cathode and
the anode may include a cathode active material and an anode
active material, respectively.
[0034] Herein, the cathode active material may include a
25 manganese-based spinel active material, lithium metal oxide,
13
or a mixture thereof. Furthermore, the lithium metal oxide
may be selected from the group consisting of lithium-cobaltbased
oxide, lithium-manganese-based oxide, lithium-nickelmanganese-
based oxide, lithium-manganese-cobalt-based oxide,
and lithium-nickel-manganese-cobalt-based 5 ased oxide, and for
example, may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4,
Li(NiaCobMnc)O2 (where 0
[0053] While storing the secondary batteries prepared in
10 Example 1 and Comparative Examples 1 and 2 at 60°C for a
maximum of 12 weeks, outputs were calculated from voltage
differences which were obtained by respectively charging and
discharging the secondary batteries at week 1, week 2, week 3,
week 4, week 8, and week 12 at 5 C for 10 seconds at 23ºC.
15 The output capacity after high-temperature storage of the
secondary battery corresponding to each storage period was
calculated as a percentage based on initial output capacity
(W, week 0) (output (W) of the corresponding week/initial
output (W) * 100(%)), and the results thereof are presented
20 in FIG. 1. The experiment was performed at a state of charge
(SOC) of 50%.
[0054] As illustrated in FIG. 1, it may be understood that
the secondary battery of Example 1 had excellent output
characteristics even after high-temperature storage at 60°C.
25 In particular, since 4 kinds of the additives of the present
17
invention were not used in the secondary battery of
Comparative Example 2, it may be understood that output
characteristics of the secondary battery of Comparative
Example 2 after the high-temperature storage at 60°C was
lower (95%) than the 5 initial value.
[0055]
[0056] While storing the secondary batteries prepared in
Example 1 and Comparative Examples 1 and 2 at 60°C for a
maximum of 12 weeks, thickness increase rates (%) of the
10 secondary batteries based on an initial thickness (week 0) of
the battery were measured at week 1, week 2, week 3, week 4,
week 8, and week 12. The results thereof are presented in
FIG. 2 below.
[0057] As illustrated in FIG. 2, it may be understood that
15 the thickness increase rate of the secondary battery of
Example 1 after the high-temperature storage was the lowest
(20% at week 12). In particular, it may be understood that
the secondary battery of Comparative Example 2 exhibited the
rapid thickness increase rate from the beginning of the high20
temperature storage. Since the thickness increase rate of
the secondary battery of Comparative Example 1 was also
increased after 4 weeks, it may be confirmed that the
secondary battery of Comparative Example 1 was less efficient
than the secondary battery of Example 1 according to the
25 embodiment of the present invention.
18
[0058]
[0059] Outputs were calculated from voltage differences
which were obtained by charging and discharging the secondary
batteries prepared in Example 1 and Comparative Examples 1
and 2 at 0.5 C for 10 seconds at -30°C. The results thereo5 f
are presented in FIG. 3. The experiment was performed at a
SOC of 50%.
[0060] As illustrated in FIG. 3, it may be understood that
the secondary battery of Example 1 exhibited low-temperature
10 output which was a maximum of 1.5 W greater than those of the
secondary batteries of Comparative Examples 1 and
2.

I/We Claim:
1. A non-aqueous electrolyte solution comprising:
a non-aqueous organic solvent;
an imide-based 5 ed lithium salt; and
at least one additive selected from the group
consisting of lithium difluoro bis(oxalato)phosphate (LiDFOP),
(trimethylsilyl)propyl phosphate (TMSPa), 1,3-propene sultone
(PRS), and ethylene sulfate (ESa), as an electrolyte solution
10 additive.
2. The non-aqueous electrolyte solution of claim 1,
wherein the imide-based lithium salt comprises at least one
selected from the group consisting of LiN(CF3SO2)2,
15 LiN(C2F5SO2)2, Li(CF3SO2)(C2F5SO2)N, and Li(SO2F)2N.
3. The non-aqueous electrolyte solution of claim 1,
wherein the imide-based lithium salt is Li(SO2F)2N (lithium
bis(fluorosulfonyl)imide, LiFSI).
20
4. The non-aqueous electrolyte solution of claim 1,
wherein the non-aqueous organic solvent comprises at least
one selected from the group consisting of dimethyl carbonate
(DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC),
25 methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC),
20
ethylmethyl carbonate (EMC), ethylene carbonate (EC),
propylene carbonate (PC), and butylene carbonate (BC).
5. The non-aqueous electrolyte solution of claim 1,
wherein the non-aqueous organic solvent comprises 5 ethylene
carbonate (EC), propylene carbonate (PC), ethylmethyl
carbonate (EMC), and dimethyl carbonate (DMC).
6. The non-aqueous electrolyte solution of claim 5,
10 wherein the non-aqueous organic solvent comprises ethylene
carbonate (EC)/propylene carbonate (PC)/ethylmethyl carbonate
(EMC)/dimethyl carbonate (DMC) respectively in an amount of
1.0 part by weight to 1.5 parts by weight: 1.0 part by weight
to 1.5 parts by weight: 4.0 parts by weight to 4.5 parts by
15 weight: and 4.0 parts by weight to 4.5 parts by weight.
7. The non-aqueous electrolyte solution of claim 1,
wherein the additive comprises lithium difluoro
bis(oxalato)phosphate (LiDFOP), (trimethylsilyl)propyl
20 phosphate (TMSPa), 1,3-propene sultone (PRS), and ethylene
sulfate (ESa).
8. The non-aqueous electrolyte solution of claim 7,
wherein the lithium difluoro bis(oxalato)phosphate (LiDFOP),
25 the 1,3-propene sultone (PRS), and the ethylene sulfate (ESa)
21
are each independently included in an amount of 0.5 wt% to
1.5 wt% based on a total amount of the electrolyte solution,
and the (trimethylsilyl)propyl phosphate (TMSPa) is included
in an amount of 0.1 wt% to 0.5 wt% based on the total amount
of the electrolyte 5 e solution.
9. The non-aqueous electrolyte solution of claim 1,
wherein the lithium salt further comprises LiPF6.
10 10. The non-aqueous electrolyte solution of claim 9,
wherein a mixing ratio of the imide-based lithium salt to
LiPF6 is in a range of 10:90 to 50:50 as a molar ratio.
11. A lithium secondary battery comprising:
15 a cathode;
an anode;
a separator; and
the non-aqueous electrolyte solution of any one of
claims 1 to 10.
20
12. The lithium secondary battery of claim 11, wherein the
anode comprises a carbon-based anode active material selected
from the group consisting of crystalline carbon, amorphous
carbon, artificial graphite, and natural graphite.
25
22
13. The lithium secondary battery of claim 11, wherein the
cathode comprises a lithium metal oxide.
14. The lithium secondary battery of claim 11, wherein the
lithium secondary battery is a lithium ion secondary batter5 y
or a lithium polymer secondary battery.