DESCRIPTION
SECONDARY BATTERY INCLUDING ELECTROLYTE ADDITIVE
[TECHNICAL FIELD]
The present invention relates to a secondary battery that includes an electrode
5 assembly including a cathode, an anode and a separator interposed therebetween and
an electrolyte, wherein the anode includes lithium titanium oxide (LTO) as an anode
active material and the electrolyte contains a phosphate-based compound as an
additive.
[BACKGROUND ART]
10 Technological development and increased demand for mobile devices have led
to rapid increase in the demand for secondary batteries as energy sources. Among
such secondary batteries, lithium secondary batteries having high energy density, high
operating voltage, long cycle span and low self-discharge rate are commercially
available and widely used.
15 In addition, increased interest in environmental issues has recently brought
about a great deal of research associated with electric vehicles (EV) and hybrid electric
vehicles (HEV) as alternatives to vehicles using fossil fuels, such as gasoline vehicles
-1-
and diesel vehicles, which are a main cause of air pollution. Such electric vehicles
generally use nickel-metal hydride (Ni-MH) secondary batteries as power sources.
However, a great deal of study associated with use of lithium secondary batteries having
high energy density, high discharge voltage and stable output is currently underway and
5 some are commercially available.
Lithium secondary batteries may be classified into lithium-ion batteries
containing liquid electrolytes per se, lithium-ion polymer batteries containing liquid
electrolytes in a gel form, and lithium polymer batteries containing solid electrolytes,
depending upon the type of electrolyte employed. Particularly, use of lithium-ion
10 polymer or gel polymer batteries is on the rise due to various advantages thereof such as
high safety owing to a low probability of fluid leakage, as compared to liquid electrolyte
batteries, and the possibility of achieving very thin and lightweight batteries.
A lithium-ion battery is manufactured by impregnating a liquid electrolyte
containing a lithium salt into an electrode assembly that includes a cathode and an
15 anode, each being fonned by applying an active material to a current collector, with a
porous separator interposed between the cathode and anode.
Methods for fabricating a lithium-ion polymer battery are divided into a
fabrication method of a non-crosslinked polymer battery and a fabrication method of a
directly-crosslinked polymer battery, depending upon the type of a matrix material for
-2-
electrolyte impregnation. Acrylate- and methacrylate-based materials having high
radical polymerization reactivity and ether-based materials having high electrical
conductivity are typically used as the polymer matrix materials. In particular, in
directly-crosslinked polymer battery fabrication, a battery is fabricated by placing a
5 jelly-roll type or stack type electrode assembly composed of electrode plates and a
porous separator in a pouch, injecting a thermally polymerizable polyethylene oxide
(PEO)-based monomer or oligomer crosslinking agent and an electrolyte composition
into the pouch, and thermally curing the injected materials. Manufacture of batteries
in this manner is advantageous in that electrode plates and separators of conventional
10 lithium-ion batteries are used without change. However, directly-crosslinked polymer
battery fabrication has problems in that a crosslinking agent is not completely cured and
remains in the electrolyte, increasing viscosity. This makes uniform impregnation
difficult, thereby greatly degrading battery properties.
A carbon-based material is typically used as an anode active material for
15 lithium secondary batteries. However, the carbon-based material has a low potential of
0V relative to lithium and thus reduces the electrolyte, generating gases. Lithium
titanium oxide (LTO) having a relatively high potential is also used as an anode active
material for lithium secondary batteries to solve these problems.
However, when LTO is used as an anode active material, the LTO acts as a
-3-
catalyst, generating a large amount of hydrogen gas during activation and
charge/discharge processes, which causes a reduction in secondary battery safety.
Thus, there is a great need to provide a technology that secures battery safety
by solving the above problems while maintaining overall battery performance.
5 [DISCLOSURE]
[TECHNICAL PROBLEM!
Therefore, the present invention has been made to solve the above and other
technical problems that have yet to be resolved.
As a result of intensive studies and various experiments, the present inventors
10 discovered that desired effects are achieved when a secondary battery including
Jithium titanium oxide (LTO) as an anode active material and a phosphate-based
compound as an electrolyte additive is used. The present invention has been
completed based on this discovery.
[TECHNICAL SOLUTION]
15 In accordance with the present invention, the above and other objects can be
accomplished by the provision of a secondary battery including an electrode assembly
-4-
including a cathode, an anode and a separator interposed therebetween and an
electrolyte, wherein the anode includes lithium titanium oxide (LTO) as an anode active
material, and the electrolyte contains a phosphate-based compound as an additive.
In a specific embodiment, the electrolyte may be, but is not limited to, any of a
5 liquid electrolyte, a gel electrolyte and a solid electrolyte. Specifically, the electrolyte
may be a liquid electrolyte or a gel polymer electrolyte.
When the electrolyte is a liquid electrolyte, decomposition of the electrolyte
may be promoted by side reaction of the electrolyte with the anode active material,
thereby generating gas as described above. Such gas may cause safety problems of the
10 secondary battery such as swelling or explosion. Thus; the secondary battery
according to the present invention uses a liquid electrolyte with a phosphate-based
compound added thereto to solve such problems.
When the electrolyte is a gel polymer electrolyte, the phosphate-based
compound additive, which reacts as a crosslinking agent, is added to the gel polymer
15 electrolyte. This provides effects of superior cycle characteristics while achieving
electrode interface stabilization, thus greatly inhibiting swelling caused by gas
generation during storage at high temperature. As a result, the effects of greatly
improved battery lifespan and safety are also achieved.
-5-
Here, it is believed that, since the phosphate-based compound has high
reactivity with radicals, it increases the extent of polymerization reaction, thereby
improving electrochemical stability of the final electrolyte. In addition, LTO used as
an anode active material acts as a catalyst to promote crosslinking polymerization of the
5 phosphate-based compound, thereby maximizing the effects described above.
Particularly, when the electrolyte is a gel polymer electrolyte, side reactions of
the electrolyte with the electrodes are reduced during repeated charge/discharge since an
area of the electrolyte in contact with the electrodes is reduced and swelling is also
inhibited due to a reduction in vapor pressure since the electrolyte is in a gel polymer
10 form.
In one embodiment, the phosphate-based compound may include at least one
selected from the group consisting of a phosphate-based acrylate of Formula (1), a
pyrophosphate-based acrylate of Formula (2) and a phosphate-based urethane acrylate:
where Ri and R2 are each independently hydrogen, methyl or F and n is an
integer of 1 to 20.
The electrolyte may further contain a multifunctional compound
5 polymerizable with the phosphate-based compound.
When a multifunctional compound polymerizable with the phosphate-based
compound is additionally used as an electrolyte additive, the multifunctional
compound and the phosphate-based compound can complement electrochemical and
mechanical characteristics of each other, thereby further improving overall
10 characteristics of the battery.
Particularly, when a gel polymer electrolyte is prepared using both the
phosphate-based compound and a multifunctional compound polymerizable with the
phosphate-based compound, physical properties with higher elasticity are achieved.
That is, a phosphate-based compound, which has a structure enabling easy
15 coordination with lithium ions, thus exhibiting higher bonding force, and a
multifunctional compound having high elasticity are polymerized through crosslinking
-7-
together, such that the phosphate-based compound and the multifunctional compound
complement electrochemical and mechanical characteristics of each other.
In an embodiment, the multifunctional compound may include at least one
selected from the group consisting of a (meth)acrylic acid ester compound, an
5 unsaturated carbonic acid compound and a vinyl compound.
The (meth)acrylic acid ester compound may include a (meth)acrylate
compound having at least two acrylate groups per molecule and the (meth)acrylate
compound may include a monomer of Formula (3) or an oligomer thereof;
unsubstituted CJ-C4 alkyl, and m is an integer of 1 to 20.
In addition, the (meth)acrylic acid ester compound may include, but is not
limited to, at least one selected from the group consisting of diethylene glycol
diacrylate (Di(EG)DA), diethylene glycol dimethacrylate (Di(EG)DM), ethylene
15 glycol dimethacrylate (EGDM), dipropylene diacrylate (Di(PG)DA), dipropylene
glycol dimethacrylate (Di(PG)DM), ethylene glycol divinyl ether (EGDVE),
-8-
ethoxylated(6) trimethylolpropane triacrylate (ETMPTA), diethylene glycol divinyl
ether (Di(EG)DVE), triethylene glycol dimethacrylate (Tri(EG)DM), dipentaerythritol
pentaacrylate (DPentA), trimethylolpropane triacrylate (TMPTA), trimethylolpropane
trimethacrylate (TMPTM), propoxylated(3) trimethylolpropane triacrylate
5 (PO(3)TMPTA), propoxylated(6) trimethylolpropane triacrylate (PO(6)TMPTA), poly
(ethylene glycol) diacrylate (PA1) and poly(ethylene glycol) dimethacrylate.
The multifunctional compound, together with the phosphate-based
compound, may form various types of copolymers, for example, random copolymers,
block copolymers, and graft copolymers.
10 The electrolyte may contain 0.1 to 1%, more specifically 0.1 to 0.5%, by
weight of the multifunctional compound polymerizable with the phosphate-based
compound, based on the total weight of the electrolyte.
The electrolyte may contain 0.01 to 30%, more specifically 0.01 to 20%, by
weight of the phosphate-based compound, based on the total weight of the electrolyte.
15 If the content of the phosphate-based compound is excessively low when the
electrolyte is a liquid electrolyte, the effects of improved safety are not fully achieved.
On the contrary, if the content of the phosphate-based compound is excessively high,
overall battery characteristics may be degraded since the content of lithium salt is
_9-
relatively lowered although safety is improved.
If the content of the phosphate-based compound is excessively low when the
electrolyte is a gel polymer electrolyte, gel polymers are not easily formed such that the
phenomenon of swelling of the battery occurring when a liquid electrolyte is used may
5 worsen and formation of a substrate having a desired thickness may be difficult. On
the contrary, if the content of the phosphate-based compound is excessively high, the
density of gel polymers is increased and lithium ion conduction rate (or conductivity) is
accordingly reduced, causing precipitation of lithium, with the result that battery
performance is reduced. In addition, viscosity is increased, such that there may be
10 difficulty in uniform application of the electrolyte to a corresponding portion.
The same is true when the multifunctional compound is added to the
phosphate-based compound. Thus, the electrolyte may contain the phosphate-based
compound and the multifunctional compound in a total amount of 0.01 to 30%, more
specifically 0.1 to 5%, based on the total weight of the electrolyte.
15 The liquid electrolyte may include an electrolyte (plasticizer) and a lithium salt.
When the electrolyte is a gel polymer electrolyte, the electrolyte may further include a
polymerization initiator.
The electrolyte also serves as a plasticizer. Examples of the electrolyte
-10-
include aprotic organic solvents such as N-methyl-2-pyrollidinone, propylene carbonate
(PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC),
diethyl carbonate (DEC), ethyl methyl carbonate (EMC), gamma-butyrolactone, 1,2-
dimethoxy ethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-
5 dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane,
methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane
derivatives, sulfolane, methyl sulfolane, l,3-dimethyl-2-imidazoIidinone, propylene
carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl
propionate. These materials may be used singly or as a mixture of two or more
10 thereof.
The lithium salt is a material that dissolves and dissociates into lithium ions in
the non-aqueous electrolyte. Examples of the lithium salt include LiCl, LiBr, Lil,
LiC104, LiBF4, LiBi0Cli0, LiPF6, L1CF3SO3, LiCF3C02, LiAsF6, LiSbF6, L1AICI4,
CH3S03Li, CF3SO3L1, (CFsSC^NLi, chloroborane lithium, lower aliphatic carboxylic
15 acid lithium, lithium tetraphenylborate and imides. These materials may be used
singly or as a mixture of two or more thereof.
The electrolyte may contain 0.01 to 30%, more specifically 0.1 to 20%, by
weight of the lithium salt based on the total weight of solid components included in
the electrolyte.
-11-
Examples of the polymerization initiator may include azo compounds such as
2,2-azobis(2-cyanobutane), 2,2-azobis(methylbutyronitrile), 2,2'~azoisobutyronitrile
(AIBN) and azobisdimethyl-valeronitrile (AMVN), peroxy compounds such as benzoyl
peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, cumyl peroxide and
5 hydrogen peroxide, and hydroperoxides. Specifically, AIBN, 2,2'-azobis(2,4-dimethyl
valeronitrile) (V65), di-(4-tert-butylcyclohexyl)-peroxydicarbonate (DBC) or the like
may be used as the polymerization initiator.
The polymerization initiator may decompose at a temperature of 40 to 80 C to
form radicals and may then react with monomers through free radical polymerization to
10 form a gel polymer electrolyte. Generally, free radical polymerization is carried out by
sequential reactions including an initiation reaction involving formation of transient
molecules having high reactivity or active sites, a propagation reaction involving reformation
of active sites at the ends of chains by addition of monomers to active chain
ends, a chain transfer reaction involving transfer of the active sites to other molecules
15 and a termination reaction involving destruction of active chain centers. Of course,
polymerization may also be carried out without a polymerization initiator.
In addition, in order to improve charge/discharge characteristics and flame
retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether,
ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur,
20 quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine,
-12-
ethylene glycol dialkyl ether, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum
trichloride or the like may be added to the electrolyte. Where appropriate, the nonaqueous
electrolyte may further include a halogen-containing solvent such as carbon
tetrachloride or ethylene trifluoride in order to impart incombustibility. Further, the
5 non-aqueous electrolyte may additionally include carbon dioxide gas in order to
improve high-temperature storage characteristics.
The secondary battery according to the present invention may be a lithium-ion
battery. The lithium-ion battery may be fabricated by mounting an electrode assembly
in a battery case, injecting a mixture of a phosphate-based compound, an electrolyte
10 and a lithium salt into the battery case, followed by sealing, and performing a
formation process to activate the battery and an aging process to stabilize the activated
battery.
However, when the electrolyte is a gel polymer electrolyte, an activation
process is performed after gel reaction. When the phosphate-based compound is used
15 as an additive to the electrolyte, a method in which film formation is induced through
wetting and charge/discharge may be employed while the gel reaction is omitted. In
a basic method, the battery is charged up to a level, at which an electrochemical
decomposition reaction of monomers may occur, and degassing is then performed.
The formation process is a process that activates the battery by repeating
-13-
charge/discharge cycles. The aging process is a process that stabilizes the battery
activated in the formation process by allowing the battery to stand for a certain period
of time.
Conditions under which the formation process and the aging process are
5 carried out are not particularly limited and are adjustable within conventional ranges
well known in the art.
In a specific embodiment, the mixture is injected into the battery case
(primary injection) and the battery structure is allowed to stand for a certain period of
time (for example, 10 hours) such that uniform impregnation of the mixture into the
10 battery case is achieved. The battery is then charged for activation. In the charge
process for activation, gases generated during formation of a protective film for the
anode are removed. Thereafter, the battery is again allowed to stand for a certain
period of time (for example, 12 hours) and charged for activation, thereby completing
battery fabrication.
15 The secondary battery according to the present invention may be a lithiumion
polymer battery. Specifically, the lithium-ion polymer battery may be fabricated
using a method including (a) mounting an electrode assembly in a battery case, (b)
injecting a mixture of a phosphate-based compound, a polymerization initiator, an
electrolyte and a lithium salt into the battery case, followed by sealing, and (c)
-14-
polymerizing the phosphate-based compound to form a gel polymer electrolyte.
Specifically, step (c) may include (cl) subjecting the battery to thermal
curing, photocuring via irradiation with electron beams or gamma rays, or a
stabilization reaction at 30 to 80 C to polymerize the phosphate-based compound, and
5 (c2) performing a formation process to activate the battery and an aging process to
stabilize the activated battery.
Specifically, the crosslinking reaction may be carried out under inert
conditions. Since the reaction of radicals with atmospheric oxygen serving as a
radical scavenger is fundamentally blocked under inert atmosphere, it is possible to
10 enhance the extent of reaction to a level at which substantially no unreacted monomers
are present. This prevents degradation in charge/discharge performance caused by a
large amount of unreacted monomers remaining inside the battery.
The inert atmosphere conditions are not particularly limited. Known gases
with low reactivity can be used. For example, at least one selected from the group
15 consisting of nitrogen, argon, helium and xenon may be used as inert gases.
Phosphate-based compounds are combined via the crosslinking
polymerization reaction to form crosslinked polymers having a three-dimensional
network structure, and the polymers are then uniformly impregnated with the
-15-
electrolyte.
The crosslinked polymer electrolyte is electrochemically stable and therefore
can be stably present in the battery without being damaged even after repeated
charge/discharge cycles. As a result, it is possible to improve battery safety and
5 achieve excellent mechanical properties such as elongation and bending properties.
Further, battery performance deterioration can be minimized due to continuous
migration and transfer of lithium ions through the polar gel polymer electrolyte.
The formation and aging processes are performed in the same manner as
described above. During the formation process, lithium ions that are liberated from
10 lithium metal oxide used as the cathode upon charging of the battery migrate and
intercalate into the carbon electrode used as the anode. Here, compounds such as
Li2CC>3, LiO and LiOH, which are produced by the reaction of highly-reactive lithium
with the carbon anode, form a solid electrolyte interface (SEI) film on the anode
surface. In this case, an unreacted crosslinking agent may undergo additional
15 reaction.
In a specific embodiment, the mixture is injected into the battery case
(primary injection) and the battery structure is allowed to stand for a certain period of
time (for example, 3 hours) such that uniform impregnation of the mixture into the
battery case is achieved. Thermal polymerization is then carried out under the
-16-
above-specified conditions. The battery is then charged for activation. In the
charge process for activation, gases generated during formation of a protective film for
the anode are removed and a certain amount of supplementary mixture is secondarily
injected into the battery case. Thereafter, the battery is again allowed to stand for a
5 certain period of time (for example, 12 hours) and charged for activation, thereby
completing battery fabrication.
The secondary battery is generally fabricated by incorporating an electrolyte
into an electrode assembly including a cathode and an anode with a separator interposed
therebetween.
10 The cathode is prepared, for example, by applying a mixture of a cathode
active material, a conductive material and a binder to a cathode current collector,
followed by drying, and pressing. A filler may be added to the mixture as needed.
The cathode current collector is generally manufactured to a thickness of 3 to
500 urn. Any cathode current collector may be used without particular limitation so
15 long as high conductivity is provided without causing chemical changes in the battery.
Examples of the cathode current collector include stainless steel, aluminum, nickel,
titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon,
nickel, titanium or silver. The cathode current collector may include fine irregularities
on the surface thereof so as to enhance adhesion to the cathode active material. In
-17-
addition, the cathode current collector may be used in various forms such as a film, a
sheet, a foil, a net, a porous structure, a foam and a nonwoven fabric.
Examples of the cathode active material include, but are not limited to, layered
compounds such as lithium cobalt oxide (LiCoCh) and lithium nickel oxide (LiMC^)
5 alone or substituted by one or more transition metals; lithium manganese oxides such as
Lij+xMn2-x04 (where 03 and LiMnC^; lithium copper oxide
(L12CUO2); vanadium oxides such as LiVgOg, LiFe304, V2O5 and CU2V2O7; Ni-site type
lithium nickel oxides represented by LiNii-xMx02 (M = Co, Mn, Al, Cu, Fe, Mg, B or
Ga and 0.014)3.
The conductive material is commonly added in an amount of 0.01 to 50% by
weight, based on the total weight of the mixture including the cathode active material.
15 Any conductive material may be used without particular limitation so long as suitable
conductivity is provided without causing chemical changes in the battery. Examples
of the conductive material include graphite such as natural or artificial graphite, carbon
blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black
and thermal black, conductive fibers such as carbon fibers and metallic fibers, metallic
-18-
powders such as carbon fluoride, aluminum and nickel powders, conductive whiskers
such as zinc oxide and potassium titanate whiskers, conductive metal oxides such as
titanium oxide, and polyphenylene derivatives.
The binder is a component assisting in binding of an active material to a
5 conductive material and a current collector. The binder is commonly added in an
amount of 1 to 50% by weight, based on the total weight of the compound including the
cathode active material. Examples of the binder include polyfluorovinylidene,
polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,
regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene,
10 polypropylene, ethylene-propylene-diene terpolymers (EPDM), sulfonated EPDM,
styrene butadiene rubbers, fluoro-rubbers and various copolymers.
The filler is a component optionally used to inhibit cathode expansion. Any
filler may be used without particular limitation so long as the filler is a fibrous material
that does not cause chemical changes in the battery. Examples of the filler include
15 olefin-based polymers such as polyethylene and polypropylene and fibrous materials
such as glass fibers and carbon fibers.
For example, the anode is prepared by applying an anode active material to an
anode current collector, followed by drying and pressing. The anode may further
include other components as needed as described above.
-19-
The anode current collector is generally manufactured to a thickness of 3 to
500 Ltm. Any anode current collector may be used without particular limitation so
long as suitable conductivity is provided without causing chemical changes in the
battery. Examples of the anode current collector include copper, stainless steel,
5 aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated
with carbon, nickel, titanium or silver, or an aluminum-cadmium alloy. Similar to the
cathode current collector, the anode current collector may include fine irregularities on
the surface thereof so as to enhance bonding force to the anode active material. In
addition, the anode current collector may be provided in various forms such as a film, a
10 sheet, a foil, a net, a porous structure, a foam and a nonwoven fabric.
Lithium titanium oxide may be used as the anode active material as described
above.
Specifically, the lithium titanium oxide may be U4T15O12, UT12O4 or a mixture
thereof. More specifically, the lithium titanium oxide may be L14T15O12.
15 Examples of the anode active material may include a mixture of carbon such as
non-graphitized carbon or graphitized carbon, metal composite oxide such as LixFe2C>3
(0
An anode active material (Li1.33Tii.67O4), a conductive material (Denka black)
10 and a binder (PVdF) were added in a weight ratio of 95:2.5:2.5 to NMP, followed by
mixing, to prepare an anode mix. The anode mix was then applied to a copper foil
having a thickness of 20 um to form a coating layer having a thickness of 60 um,
followed by rolling and drying, to produce an anode.
In addition, LiNio.5Mn1.5O4 as a cathode active material, a conductive material
15 (Denka black) and a binder (PVdF) were added in a weight ratio of 95:2.5:2.5 to NMP,
followed by mixing, to prepare a cathode mix. The cathode mix was then applied to a
copper foil having a thickness of 20 |im, followed by rolling and drying, to produce a
cathode.
-23-
A polyethylene membrane (Celgard, thickness: 20 jam) was then interposed as
a separator between the anode and the cathode to form an electrode assembly. A liquid
electrolyte with 1M LiPF6 dissolved in an EC/EMC solvent at 1/2 volume ratio, to
which a phosphate-based acrylate (Ri is H and n is 1 in Formula (1)) was added as a
5 phosphate-based material in an amount of 5% by weight based on the total weight of the
electrolyte, was injected into a pouch, in which the electrode assembly was mounted, to
fabricate a pouch battery.

A pouch battery was fabricated in the same manner as in Example 1 except that
10 a pyrophosphate-based acrylate (Ri is H and n is 1 in Formula (2)) was used as a
phosphate-based material.

A pouch battery was fabricated in the same manner as in Example 1 except that
dipentaerythritol pentaacrylate (DPentA) was additionally added as a multifunctional
15 compound to the electrolyte in an amount of 0.2% by weight based on the weight of the
solvent.

-24-
A pouch battery was fabricated in the same manner as in Example 2 except that
dipentaerythritol pentaacrylate (DPentA) was additionally added as a multifunctional
compound to the electrolyte in an amount of 0.2% by weight based on the weight of the
solvent.
5
A pouch battery was fabricated in the same manner as in Example 1 except that
2,2'-azoisobutyronitnle (AIBN) was added as a polymerization initiator to the
electrolyte in an amount of 0.1% by weight based on the weight of the solvent after
injection of the electrolyte and high-temperature reaction was then carried out at a
10 temperature of 70 C for 5 hours to prepare a gel polymer electrolyte.

A pouch battery was fabricated in the same manner as in Example 2 except that
2,2'-azoisobutyronitrile (AIBN) was added as a polymerization initiator to the
electrolyte in an amount of 0.1% by weight based on the weight of the solvent after
15 injection of the electrolyte and high-temperature reaction was then carried out at a
temperature of 70 C for 5 hours to prepare a gel polymer electrolyte.

-25-
A pouch battery was fabricated in the same manner as in Example 1 except that
an electrolyte with no phosphate-based acrylate (R\ is H and n is 1 in Formula (1))
added thereto was used.

5 A pouch battery was fabricated in the same manner as in Example 6 except that
an electrolyte was injected after a phosphate-based acrylate (Ri is H and n is 1 in
Formula (1)) was added to the electrolyte in an amount of 40% by weight.
Experimental Example 1>
Batteries (with a design capacity of 265 mAh) fabricated in Examples 1 to 6
10 and Comparative Examples 1 and 2 were subjected to a formation process at 2.75 V.
The batteries were charged/discharged at a certain C-rate in a range between 1.6 V and
2.75 V to verify discharge capacity. Results are shown in Table 1 below.
Cycle characteristics of batteries fabricated in Examples 1 and 3 and
Comparative Examples 1 and 2 were measured while the batteries were
charged/discharged at a C-rate of 5 C in a range between 1.6 V and 2.75 V in a 45 C
chamber. Results are shown in FIG. 1.
5 Experimental Example 3>
Batteries (with a design capacity of 265 mAh) fabricated in Examples 1 and 3
and Comparative Examples 1 and 2 were subjected to a formation process at 2.75 V.
The extent of gas generation by side reaction was measured after the batteries were
stored in an SOC of 100% at a high temperature of 60 C. Results are shown in FIG. 2.
10 As can be seen from FIGS. 2 and 3, Comparative Example 1 generated an
excessive amount of gases and Comparative Example 2 significantly degraded cycle
characteristics, whereas Examples 1 to 6 according to the present invention generated a
small amount of gases, securing high safety, and also exhibited superior cycle
characteristics.
15 As is apparent from the above description, a secondary battery according to
the present invention has a variety of advantages. For example, since lithium
titanium oxide (LTO) is used as an anode active material and a phosphate-based
compound is used as an additive, the secondary battery achieves electrode interface
-27-
stabilization, thereby preventing generation of gases and by-products. Thus, the
secondary battery exhibits not only high safety but also improved lifespan and highpower
characteristics.
It will be apparent to those skilled in the art that various modifications and
5 variations are possible in light of the above teaching without departing from the scope
of the invention.

Claim
[Claim l] A secondary battery comprising an electrode assembly comprising a
cathode, an anode and a separator interposed therebetween, and an electrolyte,
wherein the anode comprises lithium titanium oxide (LTO) as an anode active
5 material, and
the electrolyte contains a phosphate-based compound as an additive.
[Claim 2] The secondary battery according to claim 1, wherein the phosphatebased
compound comprises at least one selected from the group consisting of a
phosphate-based acrylate of Formula (1), a pyro phosphate-based acrylate of Formula
10 (2) and a phosphate-based urethane acrylate:
o Rt o Ri o
Y^|0CHCHr|- O - P-O - |ciI3CHOj^Y
1*2 O R*
*' ° (1)
OR. r 0 O -> Rt O
0 r~ I -, II - tl i- i -i 11 ^
V^JOCHCHJJ- o - p - o - . p ~ o - | a i 2 a i o j ^BYi
ciijciio
Ri o (2)
-29-
where Ri and R2 are each independently hydrogen, methyl or F, and n is an
integer of 1 to 20.
[Claim 3] The secondary battery according to claim 1, wherein the electrolyte
further contains a multifunctional compound polymerizable with the phosphate-based
5 compound.
[Claim 4] The secondary battery according to claim 3, wherein the
multifunctional compound comprises at least one selected from the group consisting
of a (meth)acrylic acid ester compound, an unsaturated carbonic acid compound and a
vinyl compound.
10 [Claim 5] The secondary battery according to claim 4, wherein the
(meth)acrylic acid ester compound comprises a (meth)acrylate compound having at
least two acrylate groups per molecule.
[Claim 6] The secondary battery according to claim 5, wherein the
(meth)acrylate compound comprises a monomer of Formula (3) or an oligomer
15 thereof:
where R3, R4 and R5 are each independently hydrogen, or substituted or
unsubstituted C1-C4 alkyl, and m is an integer of 1 to 20.
[Claim 7] The secondary battery according to claim 1, wherein the
(meth)acrylic acid ester compound comprises at least one selected from the group
5 consisting of diethylene glycol diacrylate (Di(EG)DA), diethylene glycol
dimethacrylate (Di(EG)DM), ethylene glycol dimethacrylate (EGDM), dipropylene
diacrylate (Di(PG)DA), dipropylene glycol dimethacrylate (Di(PG)DM), ethylene
glycol divinyl ether (EGDVE), ethoxylated(6) trimethylolpropane triacrylate
(ETMPTA), diethylene glycol divinyl ether (Di(EG)DVE), triethylene glycol
10 dimethacrylate (Tri(EG)DM), dipentaerythritol pentaacrylate (DPentA),
trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate
(TMPTM), propoxylated(3) trimethylolpropane triacrylate (PO(3)TMPTA),
propoxylated(6) trimethylolpropane triacrylate (PO(6)TMPTA), poly (ethylene glycol)
diacrylate (PA1) and poly(ethylene glycol) dimethacrylate.
15 [Claim 8] The secondary battery according to claim 1, wherein the electrolyte
contains 0.01 to 30% by weight of the phosphate-based compound based on the total
weight of the electrolyte.
[Claim 9] The secondary battery according to claim 3, wherein the electrolyte
contains 0.1 to 1% by weight of the multifunctional compound polymerizable with the
phosphate-based compound based on the total weight of the electrolyte.
[Claim 10] The secondary battery according to claim 1, wherein the electrolyte
is a liquid electrolyte or a gel polymer electrolyte.
[Claim 11 ] The secondary battery according to claim 10, wherein the electrolyte
5 is a liquid electrolyte.
[Claim 12] The secondary battery according to claim 11, wherein the liquid
electrolyte comprises an electrolyte serving as a plasticizer and a lithium salt.
[Claim 13] The secondary battery according to claim 10, wherein the electrolyte
is a gel polymer electrolyte and the additive reacts as a crosslinking agent.
10 [Claim 14] The secondary battery according to claim 13, wherein the gel
polymer electrolyte comprises a polymerization initiator, an electrolyte serving as a
plasticizer and a lithium salt.
[Claim 15] The secondary battery according to claim 12 or 14, wherein the
electrolyte contains 0.01 to 30% by weight of the lithium salt based on the total weight
15 of solid components included in the electrolyte.
[Claim 16] The secondary battery according to claim 1, wherein the secondary
battery is a lithium-ion polymer battery.
[Claim 173 A method for fabricating the secondary battery according to claim
16, the method comprising:
(a) mounting an electrode assembly in a battery case;
(b) injecting a mixture of a phosphate-based compound, a polymerization
5 initiator, an electrolyte and a lithium salt into the battery case, followed by sealing;
and
(c) polymerizing the phosphate-based compound to form a gel polymer
electrolyte.
[Claim 18] The secondary battery according to claim 17, wherein the step (c)
10 comprises:
(cl) subjecting the battery to thermal curing, photocuring via irradiation with
electron beams or gamma rays, or a stabilization reaction at 30 to 80C to polymerize
the phosphate-based compound; and
(c2) performing a formation process to activate the battery and an aging
15 process to stabilize the activated battery.
[Claim 19] A battery module comprising the secondary battery according to
claim 1 as a unit cell.
[Claim 20] A battery pack comprising the battery module according to claim 19.
[Claim 211 A device comprising the battery pack according to claim 20.
[Claim 22] The device according to claim 21, wherein the device is an electric
vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or a power storage
5 system.