U.S. patent application number 17/244287 was filed with the patent office on 2021-08-19 for method for producing non-aqueous electrolyte solution, non-aqueous electrolyte solution, and non-aqueous electrolyte secondary battery.
The applicant listed for this patent is Toyota Jidosha Kabushiki Kaisha. Invention is credited to Akira Kohyama, Koji Okuda.
Application Number | 20210257668 17/244287 |
Document ID | / |
Family ID | 1000005553476 |
Filed Date | 2021-08-19 |
United States Patent
Application |
20210257668 |
Kind Code |
A1 |
Kohyama; Akira ; et
al. |
August 19, 2021 |
METHOD FOR PRODUCING NON-AQUEOUS ELECTROLYTE SOLUTION, NON-AQUEOUS
ELECTROLYTE SOLUTION, AND NON-AQUEOUS ELECTROLYTE SECONDARY
BATTERY
Abstract
A non-aqueous electrolyte secondary battery which uses a
non-aqueous electrolyte solution in which a main component of a
non-aqueous solvent is a fluorinated solvent, and by which it is
possible to suitably prevent a decrease in battery capacity. A
method for producing the non-aqueous electrolyte solution disclosed
here includes a fluorinated solvent provision step for preparing
the fluorinated solvent, a highly polar solvent provision step for
preparing a highly polar solvent having a relative dielectric
constant of 40 or more, a LiBOB dissolution step for preparing a
highly concentrated LiBOB solution by dissolving LiBOB in the
highly polar solvent at a concentration that exceeds the saturation
concentration in the fluorinated solvent, and a mixing step for
mixing the fluorinated solvent with the highly concentrated LiBOB
solution.
Inventors: |
Kohyama; Akira; (Toyota-shi,
JP) ; Okuda; Koji; (Nagoya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toyota Jidosha Kabushiki Kaisha |
Toyota-shi |
|
JP |
|
|
Family ID: |
1000005553476 |
Appl. No.: |
17/244287 |
Filed: |
April 29, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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16250129 |
Jan 17, 2019 |
|
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17244287 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2300/0037 20130101;
H01M 10/0568 20130101; H01M 10/0569 20130101; H01M 10/052
20130101 |
International
Class: |
H01M 10/0569 20060101
H01M010/0569; H01M 10/0568 20060101 H01M010/0568; H01M 10/052
20060101 H01M010/052 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 19, 2018 |
JP |
2018-007548 |
Claims
1. A non-aqueous electrolyte solution in which a lithium salt and
LiBOB are dissolved in a non-aqueous solvent that contains a
fluorinated solvent as a main component, wherein the non-aqueous
solvent contains a highly polar solvent having a relative
dielectric constant of 40 or more, the volume of the fluorinated
solvent is 80 to 95 vol % when the total volume of the non-aqueous
solvent is taken as 100 vol %, and the concentration of LiBOB is
0.1 M or more.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 16/250,129 filed on Jan. 17, 2019, which
claims priority to Japanese Patent Application No. 2018-007548
filed on Jan. 19, 2018, the entire contents of which are hereby
incorporated by reference.
BACKGROUND
1. Field
[0002] The present disclosure relates to a non-aqueous electrolyte
solution. More specifically, the present disclosure relates to a
non-aqueous electrolyte solution in which a lithium salt is
dissolved in a non-aqueous solvent that contains a fluorinated
solvent as a main component; a method for producing the non-aqueous
electrolyte solution; and a non-aqueous electrolyte secondary
battery.
2. Description of the Related Art
[0003] In recent years, secondary batteries such as lithium ion
secondary batteries have been advantageously used as portable power
sources for hand-held devices and as power supplies for vehicle
propulsion. In particular, lithium ion secondary batteries able to
achieve high energy density and low weight are becoming
increasingly important as high output power sources fitted to
vehicles such as electric vehicles and hybrid vehicles. Non-aqueous
electrolyte solutions (hereinafter also referred to simply as
"electrolyte solutions") obtained by dissolving supporting
electrolytes such as lithium salts in non-aqueous solvents (organic
solvents) are generally used in such secondary batteries.
[0004] In order to meet demands for higher input/output and higher
energy density in the technical field of non-aqueous electrolyte
secondary batteries in recent years, positive electrode active
substances (high potential positive electrode active substances)
having upper limit operating potentials of 4.35 V (vs. Li/Li.sup.+)
or more have been developed. However, when such high potential
positive electrode active substances are used, the potential of the
positive electrode becomes extremely high at full charge, and
oxidative decomposition of the electrolyte solution readily occurs,
which leads to concerns regarding a decrease in battery
capacity.
[0005] In order to suppress such oxidative decomposition of a
non-aqueous electrolyte solution at full charge, the technique of
using a fluorinated solvent as a main component of a non-aqueous
solvent of a non-aqueous electrolyte solution has been proposed
(for example, see Japanese Patent Application Publication No.
2017-134986). Because such fluorinated solvents exhibit high
resistance to oxidation, it is possible to advantageously suppress
oxidative decomposition at full charge even in cases where a high
potential positive electrode active substance is used.
[0006] In addition, in non-aqueous electrolyte secondary batteries,
some of the non-aqueous electrolyte solution undergoes reductive
decomposition during initial charging and a coating film known as a
solid electrolyte interface (SEI) film is formed on a surface of
the negative electrode active substance in some cases. Because a
negative electrode is stabilized by the formation of this SEI film,
subsequent reductive decomposition of the electrolyte solution is
suppressed. However, because formation of a SEI film by reductive
decomposition of a non-aqueous electrolyte solution is an
irreversible reaction, this can also be a cause of a decrease in
battery capacity.
[0007] As a result, techniques for dissolving a coating
film-forming agent (for example, lithium bis(oxalato)borate (LiBOB)
or the like), which forms a SEI film through decomposition at a
lower potential than an electrolyte solution, in a non-aqueous
electrolyte solution have been proposed in recent years (for
example, see Japanese Patent Application Publication No.
2005-259592). Therefore, because it is possible to form a SEI film
derived from a coating film-forming agent before a non-aqueous
electrolyte solution decomposes, reductive decomposition of the
non-aqueous electrolyte solution can be suppressed. In addition,
Japanese Patent Application Publication No. 2017-134986 discloses
the technique of adding a coating film-forming agent such as LiBOB
to a non-aqueous electrolyte solution in which a main component of
a non-aqueous solvent is a fluorinated solvent.
SUMMARY
[0008] However, in cases where a non-aqueous electrolyte solution
in which a main component of a non-aqueous solvent is a fluorinated
solvent was actually used, it was difficult to satisfactorily
exhibit the advantageous effect of the addition of LiBOB.
Therefore, reductive decomposition of the non-aqueous electrolyte
solution during initial charging could not be satisfactorily
suppressed, and battery capacity decreased.
[0009] Specifically, because a non-aqueous solvent in which a main
component is a fluorinated solvent has the characteristic of
exhibiting high resistance to oxidation while having low resistance
to reduction, it was necessary to dissolve a large quantity of
LiBOB in order to suitably suppress reductive decomposition during
initial charging. However, because it is extremely difficult to
dissolve LiBOB in such fluorinated solvents (the saturation
solubility is approximately 0.002 M), it is extremely difficult for
LiBOB to be present at a quantity required to suppress reductive
decomposition during initial charging. Therefore, in cases where a
non-aqueous electrolyte solution in which a main component of a
non-aqueous solvent is a fluorinated solvent is used, the
concentration of LiBOB tends to be insufficient, a large quantity
of non-aqueous electrolyte solution undergoes reductive
decomposition during initial charging, and this leads to concerns
that battery capacity will decrease.
[0010] The present embodiments have been developed with such
problems in mind, and have the main purpose of providing a
non-aqueous electrolyte secondary battery which uses a non-aqueous
electrolyte solution in which a main component of a non-aqueous
solvent is a fluorinated solvent, and by which it is possible to
suitably prevent a decrease in battery capacity.
[0011] In order to achieve this purpose, the present disclosure
provides a method for producing a non-aqueous electrolyte solution
having the configuration described below (hereinafter also referred
to simply as a "production method").
[0012] The method for producing a non-aqueous electrolyte secondary
battery disclosed here is a method for producing a non-aqueous
electrolyte solution in which a lithium salt is dissolved in a
non-aqueous solvent that contains a fluorinated solvent as a main
component.
[0013] This method for producing a non-aqueous electrolyte
secondary battery includes a fluorinated solvent provision step for
providing the fluorinated solvent, a highly polar solvent provision
step for providing a highly polar solvent having a relative
dielectric constant of 40 or more, a LiBOB dissolution step for
preparing a highly concentrated LiBOB solution by dissolving LiBOB
in the highly polar solvent at a concentration that exceeds the
saturation concentration in the fluorinated solvent, and a mixing
step for mixing the fluorinated solvent with the highly
concentrated LiBOB solution.
[0014] In order to solve the problems mentioned above, the inventor
of the present disclosure conducted many experiments into means for
dissolving a sufficient quantity of LiBOB in a non-aqueous
electrolyte solution in which a main component of a non-aqueous
solvent is a fluorinated solvent.
[0015] In the course of these experiments, the inventor of the
present disclosure first thought of using a mixed solvent obtained
by mixing a fluorinated solvent with a highly polar solvent. Highly
polar solvent means a non-aqueous solvent which has a relative
dielectric constant 40 or more and which can dissolve a much larger
quantity of LiBOB than can a fluorinated solvent. As a result of
these experiments, the inventor of the present disclosure found
that a mixed solvent containing a highly polar solvent could
dissolve more LiBOB than could a non-aqueous solvent consisting of
a fluorinated solvent. Specifically, it was understood that LiBOB
could be dissolved at a quantity of approximately 0.02 M in a mixed
solvent containing 10% of a highly polar solvent.
[0016] However, the amount of LiBOB is still insufficient at a
concentration of approximately 0.02 M, and it was necessary to mix
more than 10% of a highly polar solvent in order to dissolve LiBOB
at a quantity whereby reductive decomposition during charging could
be satisfactorily suppressed. However, if the mixing proportion of
the highly polar solvent is too high, the mixing proportion of the
fluorinated solvent decreases and the oxidation resistance of the
non-aqueous electrolyte solution decreases, meaning that battery
capacity actually decreases due to oxidative decomposition at full
charge.
[0017] Therefore, as a result of numerous experiments, the inventor
of the present disclosure discovered that because a trade-off
relationship such as that mentioned above occurs when a main
component of a non-aqueous solvent is a fluorinated solvent, it was
difficult to suppress both oxidative decomposition at full charge
and reductive decomposition during initial charging to a high
degree simply by mixing a highly polar solvent and a fluorinated
solvent.
[0018] As a result, the inventor of the present disclosure carried
out further experiments into techniques for overcoming the
trade-off relationship mentioned above. As a result, it was
discovered that by dissolving a large quantity of LiBOB in a highly
polar solvent prior to mixing with a fluorinated solvent, and then
mixing the fluorinated solvent with the highly polar solvent in
which a large quantity of LiBOB had been dissolved (a highly
concentrated LiBOB solution), it was possible to dissolve a
sufficient quantity of LiBOB even in cases where a non-aqueous
solvent in which a main component was a fluorinated solvent was
used.
[0019] Specifically, if a solution in which a large quantity of
solute is dissolved at a high concentration is mixed with a solvent
for which the saturation solubility of the solute is low, the
amount of solute that exceeds the saturation solubility is
generally precipitated immediately after mixing. As a result of
experiments, however, the inventor of the present disclosure
discovered that in cases where a highly polar solvent in which a
large quantity of LiBOB had been dissolved (a highly concentrated
LiBOB solution) was mixed with a fluorinated solvent, the LiBOB
could be held for a long time in a dissolved state in the
non-aqueous solvent at a concentration that exceeded the saturation
solubility. This is thought to be because when the highly
concentrated LiBOB solution is prepared, LiBOB molecules are
surrounded by highly polar solvent molecules and are in a solvated
state, and this solvated state is maintained even after the
non-aqueous electrolyte solution is prepared by mixing the highly
polar solvent and the fluorinated solvent.
[0020] The method for producing a non-aqueous electrolyte solution
disclosed here has been developed on the basis of the findings
mentioned above, and includes a LiBOB dissolution step for
preparing a highly concentrated LiBOB solution and a mixing step
for mixing a fluorinated solvent with the highly concentrated LiBOB
solution. According to this production method, it is possible to
produce a non-aqueous electrolyte solution in which a sufficient
quantity of LiBOB is dissolved without precipitating, despite a
main component of the non-aqueous solvent being a fluorinated
solvent. This non-aqueous electrolyte solution exhibits high
resistance to oxidation by using a non-aqueous solvent in which a
main component is a fluorinated solvent, but exhibits high
resistance to reduction because a sufficient quantity of LiBOB is
dissolved. Therefore, by using this non-aqueous electrolyte
solution, it is possible to suppress both oxidative decomposition
at full charge and reductive decomposition during initial charging
to a high degree and produce a secondary battery in which a
decrease in battery capacity is suitably prevented.
[0021] In one aspect of the method for producing a non-aqueous
electrolyte solution disclosed here, the volume of the fluorinated
solvent is 80 to 95 vol % when the total volume of the non-aqueous
solvent is taken as 100 vol %.
[0022] As mentioned above, the non-aqueous solvent may contain a
sufficient quantity of the fluorinated solvent in order to suitably
suppress oxidative decomposition of the electrolyte solution at
full charge. However, if the volume of the fluorinated solvent is
too high, the volume of the highly polar solvent decreases, meaning
that LiBOB readily precipitates when the highly concentrated LiBOB
solution is mixed with the fluorinated solvent. From this
perspective, in some embodiments the volume of the fluorinated
solvent relative to the total volume of the non-aqueous solvent may
be set within the range mentioned above.
[0023] In another aspect of the method for producing a non-aqueous
electrolyte solution disclosed here, the concentration of LiBOB in
the highly concentrated LiBOB solution is 1 to 4 M.
[0024] If the concentration of LiBOB in the highly concentrated
LiBOB solution is too low, the mixing proportion of the highly
concentrated LiBOB solution must be increased in order to produce a
non-aqueous electrolyte solution having the desired LiBOB
concentration. In such cases, the volume of the fluorinated solvent
in the non-aqueous solvent decreases, which leads to concerns that
the oxidation resistance of the non-aqueous electrolyte solution
will deteriorate. From this perspective, in some embodiments as
much LiBOB as possible may be dissolved in the highly concentrated
LiBOB solution (for LiBOB to be dissolved up to the saturation
solubility in the highly polar solvent). From this perspective, the
concentration of LiBOB in the highly concentrated LiBOB solution
may fall within the range mentioned above in some embodiments.
[0025] In yet another aspect of the method for producing a
non-aqueous electrolyte solution disclosed here, a lithium salt
dissolution step for dissolving the lithium salt in the fluorinated
solvent is carried out before carrying out the mixing step.
[0026] By dissolving the lithium salt in the fluorinated solvent
before mixing with the highly concentrated LiBOB solution in this
way, the required quantity of lithium salt can be easily
dissolved.
[0027] In another aspect of the method for producing a non-aqueous
electrolyte solution disclosed here, the highly polar solvent
contains any of ethylene carbonate, propylene carbonate, sulfolane,
1,3-propane sultone and 1-propene 1,3-sultone.
[0028] These highly polar solvents can dissolve a sufficient
quantity of LiBOB, and can advantageously maintain the dissolved
LiBOB in a solvated state. Therefore, it is possible to produce a
non-aqueous electrolyte solution in which a sufficient quantity of
LiBOB is dissolved without precipitating.
[0029] In a another aspect of the method for producing a
non-aqueous electrolyte solution disclosed here, the fluorinated
solvent contains either of a fluorinated cyclic carbonate and a
fluorinated linear carbonate. In some embodiments, the fluorinated
cyclic carbonate may be any of 4-fluoroethylene carbonate,
4,5-difluoroethylene carbonate and trifluoromethylethylene
carbonate. In some embodiments, the fluorinated linear carbonate
may be methyl-2,2,2-trifluoroethyl carbonate.
[0030] By using these fluorinated carbonates as fluorinated
solvents, it is possible to advantageously improve the oxidation
resistance of the non-aqueous electrolyte solution and more
suitably suppress oxidation resistance at full charge.
[0031] In addition, a non-aqueous electrolyte solution having the
configuration below is provided as another aspect of the present
disclosure.
[0032] In the non-aqueous electrolyte solution disclosed here, a
lithium salt and LiBOB are dissolved in a non-aqueous solvent that
contains a fluorinated solvent as a main component. In this
non-aqueous electrolyte solution, the non-aqueous solvent contains
a highly polar solvent having a relative dielectric constant of 40
or more, the volume of the fluorinated solvent is 80 to 95 vol %
when the total volume of the non-aqueous solvent is taken as 100
vol %, and the concentration of LiBOB is 0.1 M or more.
[0033] The non-aqueous electrolyte solution disclosed here is
produced using the production method of the aspect described above.
This non-aqueous electrolyte solution exhibits high oxidation
resistance because a main component of the non-aqueous solvent is a
fluorinated solvent. In addition, this non-aqueous electrolyte
solution exhibits high reduction resistance because a sufficient
quantity of LiBOB, such as 0.1 M or more, is dissolved. Therefore,
according to the non-aqueous electrolyte solution disclosed here,
it is possible to produce a non-aqueous electrolyte secondary
battery in which oxidative decomposition at full charge and
reductive decomposition during initial charging are suppressed to a
high degree and in which a decrease in battery capacity is suitably
prevented.
[0034] In addition, a non-aqueous electrolyte secondary battery
having the configuration below is provided as another aspect of the
present disclosure.
[0035] In the non-aqueous electrolyte secondary battery disclosed
here, an electrode body having a positive electrode and a negative
electrode is housed in a case and a non-aqueous electrolyte
solution is filled between the positive electrode and the negative
electrode. In addition, in this secondary battery, the non-aqueous
electrolyte solution is a non-aqueous electrolyte solution in which
a lithium salt and LiBOB are dissolved in a non-aqueous solvent
that contains a fluorinated solvent as a main component, and a SEI
film derived from the LiBOB is formed on a surface of the negative
electrode. Furthermore, in the secondary battery disclosed here,
the volume of the fluorinated solvent is 80 to 95 vol % when the
total volume of the non-aqueous solvent is taken as 100 vol %, and
the amount of components in the SEI film that are derived from the
LiBOB is 0.1 to 0.4 mg/cm.sup.2. Moreover, "the amount of
components in the SEI film that are derived from the LiBOB" in the
present specification means the detected amount of boron (B), which
is the central element in the SEI film derived from the LiBOB.
[0036] The non-aqueous electrolyte secondary battery disclosed here
is a secondary battery produced using the non-aqueous electrolyte
solution of the aspect described above. In this secondary battery,
a main component of the non-aqueous solvent is a fluorinated
solvent and a non-aqueous electrolyte solution having a high
oxidation resistance is used, and it is therefore possible to
suitably suppress oxidative decomposition of the non-aqueous
electrolyte solution at full charge. Furthermore, because this
secondary battery is produced using a non-aqueous electrolyte
solution in which LiBOB is dissolved at a sufficient concentration,
a SEI film derived from the LiBOB is satisfactorily formed when
initial charging is carried out (the amount of components of the
SEI film following initial charging is 0.1 to 0.4 mg/cm.sup.2).
Therefore, reductive decomposition of the non-aqueous electrolyte
solution during initial charging is suitably suppressed, and high
battery capacity is achieved. Therefore, in the non-aqueous
electrolyte secondary battery disclosed here, oxidative
decomposition at full charge and reductive decomposition during
initial charging are suppressed to a high degree and a decrease in
battery capacity is suitably prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a flow chart that schematically illustrates a
method for producing a non-aqueous electrolyte solution according
to one embodiment of the present disclosure;
[0038] FIG. 2 is a perspective view that schematically illustrates
a non-aqueous electrolyte secondary battery according to one
embodiment of the present disclosure; and
[0039] FIG. 3 is a perspective view that schematically illustrates
an electrode body of a non-aqueous electrolyte secondary battery
according to one embodiment of the present disclosure.
DETAILED DESCRIPTION
[0040] Embodiments of the present disclosure will now be explained.
Moreover, matters which are essential for carrying out the present
disclosure and which are matters other than those explicitly
mentioned in the present disclosure are matters that a person
skilled in the art could understand to be matters of design on the
basis of the prior art in this technical field. The present
disclosure can be carried out on the basis of the matters disclosed
in the present specification and common general technical knowledge
in this technical field.
[0041] 1. Method for Producing Non-Aqueous Electrolyte Solution
[0042] FIG. 1 is a flow chart that schematically illustrates a
method for producing the non-aqueous electrolyte solution according
to the present embodiment. The method for producing a non-aqueous
electrolyte solution according to the present embodiment is a
method for producing a non-aqueous electrolyte solution in which a
lithium salt is dissolved in a non-aqueous solvent that contains a
fluorinated solvent as a main component. As shown in FIG. 1, this
production method includes a fluorinated solvent provision step
S10, a lithium salt dissolution step S20, a highly polar solvent
provision step S30, a LiBOB dissolution step S40 and a mixing step
S50.
[0043] (1) Fluorinated Solvent Provision Step
[0044] In the production method according to the present
embodiment, the fluorinated solvent provision step S10 is first
carried out. The fluorinated solvent prepared in this step is a
non-aqueous solvent in which a part of a carbonate compound having
a carbonate skeleton (O--CO--O) is substituted with fluorine.
Specific examples of such fluorinated solvents include fluorinated
cyclic carbonates and fluorinated linear carbonates. Here,
"fluorinated cyclic carbonate" means a carbonate compound having a
chemical structure that is closed into a ring by a C--C bond, and a
part of the compound is substituted with fluorine. Here,
"fluorinated linear carbonate" means a carbonate compound having an
acyclic (linear) chemical structure, and a part of the compound is
substituted with fluorine. Because these fluorinated solvents
exhibit high oxidation resistance, it is possible to suitably
suppress oxidative resistance of the non-aqueous electrolyte
solution at full charge and prevent a decrease in battery capacity
by using these fluorinated solvents as a main component of the
non-aqueous solvent.
[0045] Moreover, examples of the fluorinated cyclic carbonate
include 4-fluoroethylene carbonate (FEC), 4,5-difluoroethylene
carbonate (DFEC) and trifluoromethylethylene carbonate (TFMEC). By
using these fluorinated cyclic carbonates as fluorinated solvents,
it is possible to more suitably improve the oxidation resistance of
the non-aqueous electrolyte solution at full charge. In addition,
4,4-difluoroethylene carbonate, trifluoroethylene carbonate,
tetrafluoroethylene carbonate, fluoromethylethylene carbonate,
difluoromethylethylene carbonate, bis(fluoromethyl)ethylene
carbonate, bis(difluoromethyl)ethylene carbonate,
bis(trifluoromethyl)ethylene carbonate, fluoroethylethylene
carbonate, difluoroethylethylene carbonate, trifluoroethylethylene
carbonate, tetrafluoroethylethylene carbonate, and the like, can be
given as other examples of the fluorinated cyclic carbonate.
[0046] In addition, examples of the fluorinated linear carbonate
include methyl-2,2,2-trifluoroethyl carbonate (MTFEC) and the like.
By using MTFEC as a fluorinated solvent, it is possible to more
suitably improve the oxidation resistance of the non-aqueous
electrolyte solution at full charge. In addition,
fluoromethylmethyl carbonate, difluoromethylmethyl carbonate,
trifluoromethylmethyl carbonate, fluoromethyldifluoromethyl
carbonate, bis(fluoromethyl) carbonate, bis(difluoromethyl)
carbonate, bis(trifluoromethyl) carbonate, (2-fluoroethyl)methyl
carbonate, ethylfluoromethyl carbonate, (2,2-difluoroethyl)methyl
carbonate, (2-fluoroethyl)fluoromethyl carbonate,
ethyldifluoromethyl carbonate, (2,2,2-trifluoroethyl)methyl
carbonate (TFEMC), (2,2-difluoroethyl)fluoromethyl carbonate,
(2-fluoroethyl)difluoromethyl carbonate, ethyltrifluoromethyl
carbonate, ethyl-(2-fluoroethyl) carbonate,
ethyl-(2,2-difluoroethyl) carbonate, bis(2-fluoroethyl) carbonate,
ethyl-(2,2,2-trifluoroethyl) carbonate,
ethyl-(2,2,2-trifluoroethyl) carbonate,
2,2-difluoroethyl-2'-fluoroethyl carbonate, bis(2,2-difluoroethyl)
carbonate, 2,2,2-trifluoroethyl-2'-fluoroethyl carbonate,
2,2,2-trifluoroethyl-2',2'-difluoroethyl carbonate,
bis(2,2,2-trifluoroethyl) carbonate, pentafluoroethylmethyl
carbonate, pentafluoroethylfluoromethyl carbonate,
pentafluoroethylethyl carbonate, bis(pentafluoroethyl) carbonate,
and the like, can be given as other examples of the fluorinated
linear carbonate.
[0047] (2) Lithium Salt Dissolution Step
[0048] In the present embodiment, the lithium salt dissolution step
S20 is next carried out, in which a lithium salt is dissolved in
the fluorinated solvent prepared in the fluorinated solvent
provision step S10.
[0049] For example, one or two or more types such as LiPF.sub.6,
LiBF.sub.4, LiClO.sub.4, LiAsF.sub.6, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiC(CF.sub.3SO.sub.2).sub.3, LiI or LiN(FSO.sub.2).sub.2 can be
used as the lithium salt.
[0050] In some embodiments, a lithium salt is dissolved in this
step from the perspective of the lithium salt concentration in the
non-aqueous electrolyte solution following production. Details are
given below, but in the production method according to the present
embodiment, a non-aqueous electrolyte solution is produced by
mixing a fluorinated solvent in which a lithium salt is dissolved
with a highly polar solvent in which LiBOB is dissolved. Therefore,
the quantity of lithium salt added in this step may be specified
from the perspective of the lithium salt concentration after mixing
with the highly polar solvent. Specifically, the amount of lithium
salt dissolved in this step may be adjusted so that the lithium
salt concentration in the non-aqueous electrolyte solution
following production is 0.5 to 2 mol/L (for example, 1 mol/L).
[0051] (3) Highly Polar Solvent Provision Step
[0052] In the present embodiment, the highly polar solvent
provision step S30 is next carried out separately from the
fluorinated solvent provision step S10 and the lithium salt
dissolution step S20.
[0053] The "highly polar solvent" prepared in this step is a
non-aqueous solvent having a relative dielectric constant of 40 or
more (such as 40 to 120, or even 70 to 100). Specific examples of
highly polar solvents that satisfy this condition include ethylene
carbonate (EC, relative dielectric constant: 95.3), propylene
carbonate (PC, relative dielectric constant: 64.4), sulfolane (SL,
relative dielectric constant: 44), 1,3-propane sultone (PS,
relative dielectric constant: 94) and 1-propene 1,3-sultone (PRS,
relative dielectric constant: 90). These highly polar solvents can
dissolve a large quantity of LiBOB, and can satisfactorily maintain
the dissolved LiBOB in a solvated state. Therefore, these highly
polar solvents can be used particularly advantageously as solvents
when preparing the highly concentrated LiBOB solution mentioned
above. In addition, among these highly polar solvents, EC exhibits
relatively high oxidation resistance, and can therefore also
contribute to suppressing oxidative decomposition of the
non-aqueous electrolyte solution at full charge.
[0054] (4) LiBOB Dissolution Step
[0055] Next, the LiBOB dissolution step S40 is carried out, in
which the highly concentrated LiBOB solution is prepared by
dissolving lithium bis(oxalato)borate (LiBOB) in the highly polar
solvent prepared in the highly polar solvent provision step S30.
LiBOB is a type of oxalate complex compound, and has the function
of a coating film-forming agent that forms a SEI film by
decomposing at a lower potential than the electrolyte solution
during initial charging of a non-aqueous electrolyte secondary
battery. By suitably forming a SEI film derived from this LiBOB,
reductive decomposition of the non-aqueous electrolyte solution
does not occur during initial charging and it is possible to
stabilize the negative electrode, meaning that it is possible to
suitably prevent a decrease in battery capacity.
[0056] Here, "highly concentrated LiBOB solution" in the present
specification means a highly polar solvent in which LiBOB is
dissolved at a concentration that exceeds the saturation
concentration in the fluorinated solvent. Moreover, if the
concentration of LiBOB in the highly concentrated LiBOB solution is
too low, the mixing proportion of the fluorinated solvent may be
lowered in order to obtain a non-aqueous electrolyte solution in
which the required amount of LiBOB is dissolved, and this leads to
concerns that the oxidation resistance of the non-aqueous
electrolyte solution following production will deteriorate.
Therefore, as much LiBOB as possible may be dissolved in the highly
concentrated LiBOB solution (for LiBOB to be dissolved up to the
saturation concentration in the highly polar solvent). For example,
the concentration of LiBOB in the highly concentrated LiBOB
solution may be 1 to 4 M, or even 1.5 to 4 M, for example 2 M.
[0057] (5) Mixing Step
[0058] In the method for producing a non-aqueous electrolyte
solution according to the present embodiment, the mixing step S50
is next carried out, in which the fluorinated solvent is mixed with
the highly concentrated LiBOB solution. By carrying out this mixing
step S50 in the production method according to the present
embodiment, it is possible to obtain a non-aqueous electrolyte
solution in which a lithium salt and LiBOB are dissolved in a
non-aqueous solvent in which the fluorinated solvent and the highly
polar solvent are mixed. Here, the mixing proportions of the
fluorinated solvent and the highly concentrated LiBOB solution may
be adjusted so that a main component of the non-aqueous solvent
following preparation is the fluorinated solvent. Specifically,
when the total volume of the non-aqueous solvent is taken as 100
vol %, the mixing proportions of the fluorinated solvent and the
highly concentrated LiBOB solution may be adjusted so that the
volume of the fluorinated solvent is 80 to 95 vol % (such as 85 to
95 vol %, for example 90 vol %). In this way, it is possible to
suitably improve the oxidation resistance of the non-aqueous
electrolyte solution following production and suitably prevent a
decrease in battery capacity caused by oxidative decomposition at
full charge.
[0059] In the production method according to the present
embodiment, the highly concentrated LiBOB solution is prepared in
advance by dissolving a large quantity of LiBOB in a highly polar
solvent, and a non-aqueous electrolyte solution is then produced by
mixing the highly concentrated LiBOB solution with a fluorinated
solvent. In this way, it is possible to obtain a non-aqueous
electrolyte solution in which a large quantity of LiBOB is
dissolved, despite a main component of the non-aqueous solvent
being a fluorinated solvent. Specifically, the saturation
concentration of LiBOB in a non-aqueous solvent containing a
fluorinated solvent as a main component is generally approximately
0.002 to 0.05 M. However, in a non-aqueous electrolyte solution
produced using the production method according to the present
embodiment, LiBOB remains in a dissolved state, without
precipitating, even at a high concentration of 0.1 M or more. This
is thought to be because when the highly concentrated LiBOB
solution is prepared, LiBOB molecules are surrounded by highly
polar solvent molecules and are in a solvated state, and this
solvated state is maintained even after the highly polar solvent is
mixed with the fluorinated solvent. In addition, by using this
non-aqueous electrolyte solution, oxidative decomposition at full
charge and reductive decomposition during initial charging can be
suppressed to a high degree. As a result, it is possible to produce
a non-aqueous electrolyte secondary battery in which a reduction in
battery capacity is suitably prevented.
[0060] Moreover, in a non-aqueous electrolyte solution produced
using the production method according to the present embodiment,
LiBOB is dissolved at a high concentration of 0.1 M or more, as
mentioned above. However, if the non-aqueous electrolyte solution
is allowed to stand for a long period of time after being produced,
some of the dissolved LiBOB may precipitate. Therefore, in cases
where a secondary battery is produced using the non-aqueous
electrolyte solution of the present embodiment, the production
process may be controlled so that initial charging is carried out
and a SEI film derived from LiBOB is formed before LiBOB
precipitates. For example, a secondary battery production process
may be controlled so as to include a step of housing an electrode
body and a non-aqueous electrolyte solution in a battery case and a
step of forming a SEI film by carrying out initial charging within
48 hours (or even within 24 hours) of the electrolyte solution
being produced. In this way, it is possible to reliably form a SEI
film derived from LiBOB before LiBOB precipitates.
[0061] An explanation has been given above of a method for
producing a non-aqueous electrolyte solution according to one
embodiment of the present disclosure. However, the embodiment
described above does not limit the method for producing a
non-aqueous electrolyte solution disclosed here, and alterations
may be carried out, as appropriate, when necessary.
[0062] For example, in the production method according to the
embodiment described above, the lithium salt dissolution step S20
for dissolving a lithium salt in a fluorinated solvent is carried
out. However, the timing of the dissolution of the lithium salt in
the method for producing a non-aqueous electrolyte solution
disclosed here is not limited to the embodiments described above.
That is, it is possible to carry out a mixing step for preparing a
mixed solvent by mixing the fluorinated solvent and the highly
concentrated LiBOB solution, and then dissolve a lithium salt in
the mixed solvent. In addition, it is possible to dissolve a
lithium salt in the highly polar solvent or highly concentrated
LiBOB solution rather than in the fluorinated solvent.
[0063] However, from the perspective of dissolving a lithium salt
with good efficiency, a lithium salt may be dissolved in the
fluorinated solvent before mixing with the highly concentrated
LiBOB solution, as in the embodiment described above.
[0064] 2. Non-Aqueous Electrolyte Secondary Battery
[0065] Next, as another aspect of the present disclosure, an
explanation will be given of a non-aqueous electrolyte secondary
battery in which is used a non-aqueous electrolyte solution
obtained using the production method according to the embodiment
described above. FIG. 2 is a perspective view that schematically
illustrates the non-aqueous electrolyte secondary battery according
to the present embodiment. In addition, FIG. 3 is a perspective
view that schematically illustrates an electrode body used in the
non-aqueous electrolyte secondary battery according to the present
embodiment.
[0066] (1) Battery Case
[0067] As shown in FIG. 2, the non-aqueous electrolyte secondary
battery 100 according to the present embodiment is provided with a
flat square battery case 50. This battery case 50 is constituted
from a flat case main body 52, the upper surface of which is open,
and a lid 54, which seals the open part of the upper surface. In
addition, a positive electrode terminal 70 and a negative electrode
terminal 72 are provided on the lid 54 of the battery case 50.
[0068] (2) Electrode Body
[0069] In the non-aqueous electrolyte secondary battery 100
according to the present embodiment, an electrode body 80 shown in
FIG. 3 is housed inside the battery case 50 shown in FIG. 2. This
electrode body 80 is a wound electrode body formed by laminating a
sheet-shaped positive electrode 10 and a sheet-shaped negative
electrode 20, with a separator 40 interposed therebetween, and
winding the obtained laminate. Explanations will now be given of
the members that constitute the electrode body 80.
[0070] (a) Positive Electrode
[0071] As shown in FIG. 3, the positive electrode 10 is formed by
applying a positive electrode mixture layer 14 to the surface (both
surfaces) of a positive electrode current collector 12 such as an
aluminum foil. Moreover, the positive electrode mixture layer 14 is
not applied to one edge of the positive electrode 10, thereby
forming a current collector exposed part 16. In addition, a
positive electrode connection part 80a, which is obtained by
winding the current collector exposed part 16 of the positive
electrode 10, is formed at one edge of the wound electrode body 80,
and the positive electrode terminal 70 mentioned above (see FIG. 2)
is connected to the positive electrode connection part 80a.
[0072] The positive electrode mixture layer 14 contains a positive
electrode active substance that is a lithium composite oxide
capable of occluding and releasing lithium ions. One or two or more
types of substance used in non-aqueous electrolyte secondary
batteries in the past can be used without particular limitation as
this positive electrode active substance.
[0073] In some embodiments, the positive electrode active substance
used may be a high potential positive electrode active substance
having an upper limit operating potential (open circuit voltage
(OCV)) of 4.35 V or more, based on lithium metal (vs. Li/Li.sup.+).
In cases where this type of high potential positive electrode
active substance is used, it is possible to improve input-output
characteristics and energy density, but problems also occur, such
as the non-aqueous electrolyte solution readily undergoing
oxidative decomposition at full charge. However, because a
non-aqueous solvent, which contains a fluorinated solvent as a main
component and exhibits high oxidation resistance, is used in the
non-aqueous electrolyte solution in the present embodiment, it is
possible to suitably suppress oxidative decomposition of the
non-aqueous electrolyte solution at full charge, even if a high
potential positive electrode active substance is used.
[0074] Moreover, examples of high potential positive electrode
active substances include lithium-manganese composite oxides having
a spinel structure and represented by the general formula:
Li.sub.pMn.sub.2-qM.sub.qO.sub.4+.alpha.. Here, in this general
formula, p is such that 0.9.ltoreq.p.ltoreq.1.2, q is such that
0.ltoreq.q.ltoreq.2 (and typically such that 0.ltoreq.q.ltoreq.1,
for example 0.2.ltoreq.q.ltoreq.0.6), a is such that
-0.2.ltoreq..alpha..ltoreq.0.2, and these are values defined so
that charge neutrality conditions are satisfied. In addition, M in
the formula may be one or two or more elements selected from among
arbitrary metal elements other than Mn and non-metal elements. More
specifically, M can be Na, Mg, Ca, Sr, Ti, Zr, V, Nb, Cr, Mo, Fe,
Co, Rh, Ni, Pd, Pt, Cu, Zn, B, Al, Ga, In, Sn, La, W, Ce, or the
like. Of these, at least one type of transition metal element such
as Fe, Co and Ni can be used in some embodiments.
[0075] Furthermore, among the lithium-manganese composite oxides
having a spinel structure mentioned above, lithium-nickel-manganese
composite oxides containing Li, Ni and Mn elements can be used in
the positive electrode active substance in some embodiments. This
type of lithium-nickel-manganese composite oxide exhibits high
thermal stability and high electrical conductivity, and can
therefore improve battery performance and durability. This type of
lithium-nickel-manganese composite oxide can be represented by, for
example, the general formula:
Li.sub.x(Ni.sub.yMn.sub.2-y-zM1.sub.z)O.sub.4+.beta.. Here, M1 is
not present or may be a transition metal element other than Ni or
Mn or a typical metal element (for example, one or two or more
elements selected from among Fe, Co, Cu, Cr, Zn and Al). Of these,
M1 may include at least one of trivalent Fe and Co in some
embodiments. Alternatively, M1 may be a metalloid element (for
example, one or two or more elements selected from among B, Si and
Ge) or anon-metal element. Moreover, in the general formula, x is
such that 0.9.ltoreq.x.ltoreq.1.2, y is such that 0<y, and z is
such that 0.ltoreq.z. In addition, y+z<2 (and typically,
y+z.ltoreq.1), and .beta. may be the same as a mentioned above. In
one aspect, y is such that 0.2.ltoreq.y.ltoreq.1.0 (or even such
that 0.4.ltoreq.y.ltoreq.0.6, for example
0.45.ltoreq.y.ltoreq.0.55), and z is such that 0.ltoreq.z<1.0
(for example, 0.ltoreq.z.ltoreq.0.3). LiNi.sub.0.5Mn.sub.1.5O.sub.4
is one example of a lithium-nickel-manganese composite oxide that
satisfies this type of general formula.
[0076] In addition, the positive electrode mixture layer 14 may
contain additives such as electrically conductive materials and
binders in addition to the positive electrode active substance
mentioned above. A carbon material such as carbon black (typically
acetylene black (AB) or ketjen black), active carbon, graphite or
carbon fibers can be used as the electrically conductive material
in some embodiments. In addition, halogenated vinyl resins such as
polyvinylidene fluoride (PVdF) resins and polyalkylene oxide
compounds such as polyethylene oxide (PEO) can be used as the
binder in some embodiments.
[0077] (b) Negative Electrode
[0078] The negative electrode 20 is formed by applying a negative
electrode mixture layer 24 to the surface (both surfaces) of a
negative electrode current collector 22, such as a copper foil.
Like the positive electrode 10 mentioned above, the negative
electrode mixture layer 24 is not applied to one edge of the
negative electrode 20, thereby forming a current collector exposed
part 26. In addition, a negative electrode connection part 80b,
which is obtained by winding the current collector exposed part 26,
is formed at one edge of the wound electrode body 80, and the
negative electrode terminal 72 mentioned above (see FIG. 2) is
connected to the negative electrode connection part 80b.
[0079] In addition, the negative electrode mixture layer 24
contains a negative electrode active substance that is a carbon
material capable of occluding and releasing lithium ions. For
example, graphite, hard carbon, soft carbon, and the like, can be
used as the negative electrode active substance.
[0080] In addition, the negative electrode mixture layer 24 may
contain additives such as binders and thickening agents in addition
to the negative electrode active substance. Examples of binders for
the negative electrode mixture layer 24 include styrene-butadiene
copolymers (SBR). For example, carboxymethyl cellulose (CMC) or the
like can be used as a thickening agent.
[0081] (c) Separator
[0082] The separator 40 is a porous insulating sheet having
ultrafine pores through which lithium ions pass. For example, an
insulating resin such as polyethylene (PE), polypropylene (PP), a
polyester or a polyamide can be used in the separator 40. In
addition, the separator 40 may be a single layer sheet comprising
one type of resin sheet, or a multilayer sheet obtained by
laminating two or more types of resin sheet. Examples of separators
having multilayer structures include sheets having three layer
structures obtained by laminating a PE sheet on both surfaces of a
PP sheet (a PE/PP/PE sheet).
[0083] (3) Non-Aqueous Electrolyte Solution
[0084] In the non-aqueous electrolyte secondary battery 100, the
non-aqueous electrolyte solution is filled between the positive
electrode 10 and negative electrode 20 of the electrode body 80
mentioned above. Here, the non-aqueous electrolyte solution
produced in the embodiment mentioned above is used in the
non-aqueous electrolyte secondary battery 100 according to the
present embodiment. That is, the non-aqueous electrolyte solution
used in the present embodiment contains a fluorinated solvent, a
highly polar solvent, a lithium salt and LiBOB. Because these
materials have already been explained in the embodiment mentioned
above, detailed explanations are omitted here.
[0085] In addition, a sufficient quantity of LiBOB is dissolved in
this non-aqueous electrolyte solution, despite a non-aqueous
electrolyte solution containing a fluorinated solvent as a main
component being used. Specifically, LiBOB is dissolved at a
concentration of 0.1 M or more, despite the volume of the
fluorinated solvent being 80 to 95 vol % when the total volume of
the non-aqueous solvent is taken as 100 vol %. In this way, because
the non-aqueous electrolyte solution used in the non-aqueous
electrolyte secondary battery according to the present embodiment
suitably contains a fluorinated solvent and LiBOB, it is possible
to suppress both oxidative decomposition at full charge and
reductive decomposition during initial charging to a high
degree.
[0086] Moreover, in the non-aqueous electrolyte secondary battery
according to the present embodiment, LiBOB in the non-aqueous
electrolyte solution decomposes when initial charging is carried
out, thereby forming a SEI film derived from the LiBOB. Therefore,
in the secondary battery following initial charging, the
concentration of LiBOB in the non-aqueous electrolyte solution may
be less than 0.1 M. In this case, however, a SEI film derived from
LiBOB is formed on a surface of the negative electrode. According
to the present embodiment, therefore, it is possible to obtain a
non-aqueous electrolyte secondary battery in which a higher amount
of a SEI film derived from LiBOB is formed than in the past,
despite a non-aqueous electrolyte solution in which a main
component of a non-aqueous solvent is a fluorinated solvent being
used. For example, when SEIs in non-aqueous electrolyte secondary
batteries according to the present embodiment were analyzed and
measured in terms of "amount of components in SEI film derived from
LiBOB", this amount was approximately 0.1 to 0.4 mg/cm.sup.2.
Moreover, this "amount of components in SEI film derived from
LiBOB" can be determined by measuring the amount of boron (B)
components in the SEI film at a surface of the negative electrode
by carrying out inductively coupled plasma (ICP) emission spectral
analysis.
EXPERIMENTAL EXAMPLES
[0087] Explanations will now be given of experimental examples
relating to the present disclosure. Moreover, these explanations of
experimental examples are not intended to limit the present
disclosure.
1. Experimental Examples
[0088] In these experimental examples, 19 types of non-aqueous
electrolyte solution were produced using different production
processes, and lithium ion secondary batteries (samples 1 to 19)
were produced using each of the non-aqueous electrolyte
solutions.
[0089] (1) Sample 1
[0090] In Sample 1, a non-aqueous electrolyte solution was produced
by dissolving a lithium salt (LiPF.sub.6) and LiBOB in a
non-aqueous solvent consisting of a fluorinated solvent, and a
lithium ion secondary battery was produced using the non-aqueous
electrolyte solution.
[0091] Specifically, a fluorinated solvent obtained by mixing
4-fluoroethylene carbonate (FEC) as a fluorinated cyclic carbonate
and methyl-2,2,2-trifluoroethyl carbonate (MTFC) as a fluorinated
linear carbonate at a ratio of 30:70 was used as a non-aqueous
solvent in Sample 1, as shown in Table 1. Next, a lithium salt
(LiPF.sub.6) was dissolved at a concentration of 1.0 M in the
fluorinated solvent, and LiBOB was then dissolved until the
saturation solubility was reached. The concentration of LiBOB in
Sample 1 was 0.002 M.
[0092] Next, a precursor, which was produced by dissolving sulfates
of nickel (Ni) and manganese (Mn) and neutralizing with sodium
hydroxide (NaOH), and lithium carbonate (Li.sub.2CO.sub.3) were
mixed and fired for 15 hours at 900.degree. C. The fired product
was then pulverized to an average particle diameter of 10 .mu.m,
thereby obtaining a powder of LiNi.sub.0.5Mn.sub.1.5O.sub.4 as a
high potential positive electrode active substance.
[0093] A paste-like positive electrode mixture was then prepared by
mixing this LiNi.sub.0.5Mn.sub.1.5O.sub.4 powder, an electrically
conductive material (acetylene black: AB) and a binder
(polyvinylidene fluoride: PVDF) at a ratio of 87:10:3 and
dispersing in a dispersion medium (N-methylpyrrolidone: NMP). A
sheet-shaped positive electrode was produced by coating this
positive electrode mixture on both surfaces of a sheet-shaped
positive electrode current collector (an aluminum foil), drying the
positive electrode mixture, and then extending by rolling.
[0094] Next, a paste-like negative electrode mixture was prepared
by using a natural graphite powder (average particle diameter 20
.mu.m) as a negative electrode active substance, mixing the natural
graphite, a binder (a styrene-butadiene copolymer: SBR) and a
thickening agent (carboxymethyl cellulose: CMC) at a ratio of
98:1:1, and dispersing in a dispersion medium (NMP). A sheet-shaped
negative electrode was then produced by coating this negative
electrode mixture on both surfaces of a negative electrode current
collector (a copper foil), drying, and then extending by
rolling.
[0095] Next, a wound electrode body was produced by laminating the
positive electrode and negative electrode, with a separator
(PE/PP/PE sheet) interposed therebetween, and then winding the
obtained laminate. Here, the sizes of the positive electrode and
negative electrode used to produce the electrode body were adjusted
so that the design capacity of the produced battery was 14 mAh.
Next, a lithium ion secondary battery of Sample 1 was produced by
connecting the produced electrode body to the positive electrode
terminal and negative electrode terminal and then enclosing in a
laminated film together with the non-aqueous electrolyte solution
described above.
[0096] (2) Samples 2 and 3
[0097] In Samples 2 and 3, non-aqueous electrolyte solutions were
prepared under the same conditions as those in Sample 1, except
that mixed solvents obtained by mixing a fluorinated solvent and a
highly polar solvent were used as the non-aqueous solvent, and
lithium ion secondary batteries were produced using these
non-aqueous electrolyte solutions.
[0098] Specifically, in Samples 2 and 3, mixed solvents were
produced by mixing FEC and MTFC, which are fluorinated solvents,
with ethylene carbonate (EC), which is a highly polar solvent, and
a lithium salt and LiBOB were dissolved in these mixed solvents.
Moreover, the mixing proportions of FEC, MTFC and EC were different
in Sample 2 and Sample 3. In addition, the concentration of LiBOB
in the non-aqueous electrolyte solution of Sample 2 was 0.02 M, and
the concentration of LiBOB in the non-aqueous electrolyte solution
of Sample 3 was 0.05 M.
[0099] (3) Samples 4 to 7
[0100] In Samples 4 to 7, a highly concentrated LiBOB solution was
prepared in advance, in the same way as in the production method
according to the embodiment described above, and lithium ion
secondary batteries were then constructed using the same procedure
as in Sample 1, except that non-aqueous electrolyte solutions were
prepared by mixing the highly concentrated LiBOB solution with a
fluorinated solvent.
[0101] Specifically, a highly concentrated LiBOB solution having a
LiBOB concentration of 2 M was first prepared by dissolving LiBOB
in EC, which is a highly polar solvent, and a lithium salt
(LiPF.sub.6) was dissolved in a fluorinated solvent (FEC and MTFC).
Non-aqueous electrolyte solutions were then prepared by mixing the
highly concentrated LiBOB solution with the fluorinated solvent.
Moreover, in Samples 4 to 7, the mixing ratio of the highly
concentrated LiBOB solution and the fluorinated solvent were
altered and the concentration of LiBOB in the non-aqueous
electrolyte solution was altered. Details are shown in Table 1.
[0102] (4) Samples 8 to 10
[0103] In Samples 8 to 10, non-aqueous electrolyte solutions were
prepared under the same conditions as those in Samples 4 to 7,
except that propylene carbonate (PC) was used as the highly polar
solvent, and lithium ion secondary batteries were produced using
these non-aqueous electrolyte solutions. Moreover, in Samples 8 to
10, the mixing ratio of the highly concentrated LiBOB solution (PC
solution having a LiBOB concentration of 2 M) and the fluorinated
solvent was altered in the manner shown in Table 1.
[0104] (5) Samples 11 to 13
[0105] In Samples 11 to 13, non-aqueous electrolyte solutions were
prepared under the same conditions as those in Samples 4 to 7,
except that sulfolane (SL) was used as the highly polar solvent,
and lithium ion secondary batteries were produced using these
non-aqueous electrolyte solutions. Moreover, in Samples 11 to 13,
the mixing ratio of the highly concentrated LiBOB solution (SL
solution having a LiBOB concentration of 2 M) and the fluorinated
solvent was altered in the manner shown in Table 1.
[0106] (6) Samples 14 to 16
[0107] In Samples 14 to 16, non-aqueous electrolyte solutions were
prepared under the same conditions as those in Samples 4 to 7,
except that 1,3-propane sultone (PS) was used as the highly polar
solvent, and lithium ion secondary batteries were produced using
these non-aqueous electrolyte solutions. Moreover, in Samples 14 to
16, the mixing ratio of the highly concentrated LiBOB solution (PS
solution having a LiBOB concentration of 2 M) and the fluorinated
solvent was altered in the manner shown in Table 1.
[0108] (7) Samples 17 to 19
[0109] In Samples 17 to 19, non-aqueous electrolyte solutions were
prepared under the same conditions as those in Samples 4 to 7,
except that 1-propene 1,3-sultone (PRS) was used as the highly
polar solvent, and lithium ion secondary batteries were produced
using these non-aqueous electrolyte solutions. Moreover, in Samples
17 to 19, the mixing ratio of the highly concentrated LiBOB
solution (PRS solution having a LiBOB concentration of 2 M) and the
fluorinated solvent was altered in the manner shown in Table 1.
2. Evaluation Experiments
[0110] In this experiment, the capacity retention rates of the
lithium ion secondary batteries of the samples were measured.
[0111] Specifically, the lithium ion secondary battery of each
sample was activated by being subjected to constant current
charging at a current of 1/5 C to a voltage of 4.5 V, and then to
constant voltage charging for 20 hours in a high temperature
environment at 60.degree. C.
[0112] Next, the battery was subjected to constant current charging
at a current of 1/5 C to a voltage of 4.9 V and then to constant
voltage charging until a current of 1/50 C was reached, and a fully
charged state was attained following the charging. The battery was
then subjected to constant current discharging at a current of 1/5
C to a voltage of 3.5 V, and the capacity following discharging was
deemed to be the initial capacity.
[0113] Each sample was then placed in a high temperature
environment at 60.degree. C. and subjected to 1000 charging and
discharging cycles, with 1 cycle comprising charging at a current
of 2 C to a voltage of 4.9 V and then discharging at a current of 2
C to a voltage of 3.5 V. The capacity of each sample was then
measured following 1000 cycles, and the capacity retention rate (%)
was calculated by dividing the capacity after 1000 cycles by the
initial capacity. The calculation results are shown in Table 1.
TABLE-US-00001 TABLE 1 Ca- pacity LiBOB reten- Non-aqueous solvent
(vol %) solu- tion No added LiBOB LiBOB 2M added bility rate FEC
MTFEC EC EC PC SL PS PRS (M) (%) Sample 30 70 -- -- -- -- -- --
0.002 84.1 1 Sample 20 70 10 -- -- -- -- -- 0.02 85.8 2 Sample 10
70 20 -- -- -- -- -- 0.05 83.8 3 Sample 25 70 -- 5 -- -- -- -- 0.10
91.1 4 Sample 20 70 -- 10 -- -- -- -- 0.20 94.1 5 Sample 15 70 --
15 -- -- -- -- 0.30 95.4 6 Sample 10 70 -- 20 -- -- -- -- 0.40 93.2
7 Sample 25 70 -- -- 5 -- -- -- 0.10 90.8 8 Sample 20 70 -- -- 10
-- -- -- 0.20 93.4 9 Sample 15 70 -- -- 15 -- -- -- 0.30 93.9 10
Sample 25 70 -- -- -- 5 -- -- 0.10 90.3 11 Sample 20 70 -- -- -- 10
-- -- 0.20 92.2 12 Sample 15 70 -- -- -- 15 -- -- 0.30 92.9 13
Sample 25 70 -- -- -- -- 5 -- 0.10 89.8 14 Sample 20 70 -- -- -- --
10 -- 0.20 91.1 15 Sample 15 70 -- -- -- -- 15 -- 0.30 92.2 16
Sample 25 70 -- -- -- -- -- 5 0.10 98.5 17 Sample 20 70 -- -- -- --
-- 10 0.20 90.9 18 Sample 15 70 -- -- -- -- -- 15 0.30 90.3 19
3. Evaluation Results
[0114] In view of Samples 1 to 3 in Table 1, it was confirmed that
cases in which a mixed solvent containing a highly polar solvent
(EC) was used had a higher LiBOB saturation solubility than cases
in which a non-aqueous solvent consisting of a fluorinated solvent
was used. Comparing Sample 2 with Sample 3, however, Sample 3 had a
lower capacity retention rate than Sample 2, despite having a
higher amount of dissolved LiBOB. It is understood that this is
because the mixing proportion of the fluorinated solvent was lower,
meaning that the oxidation resistance of the non-aqueous
electrolyte solution was lower and it was not possible to suppress
oxidative decomposition at full charge.
[0115] Meanwhile, in cases where a highly concentrated LiBOB
solution was prepared in advance and the highly concentrated LiBOB
solution was mixed with a fluorinated solvent, as in Samples 4 to
19, it was possible to prepare a non-aqueous electrolyte solution
in which a large quantity of LiBOB was dissolved, such as 0.1 M or
more, despite containing 80 vol % or more of a fluorinated solvent.
In addition, non-aqueous electrolyte secondary batteries
constructed using these non-aqueous electrolyte solutions had
extremely high capacity retention rates of 89% or more.
[0116] Therefore, it was understood that in cases where a highly
concentrated LiBOB solution is prepared in advance and the highly
concentrated LiBOB solution is mixed with a fluorinated solvent, it
is possible to prepare a non-aqueous electrolyte solution in which
LiBOB is dissolved in an amount greater than the saturation
solubility, and it is possible to construct a non-aqueous
electrolyte secondary battery having excellent battery capacity by
using this type of non-aqueous electrolyte solution.
[0117] Specific examples of the present disclosure have been
explained in detail above, but these are merely examples, and do
not limit the scope of the disclosure. The features set forth in
the claims also encompass modes obtained by variously modifying or
altering the specific examples shown above.
* * * * *