KR101185535B1 - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery. According to the present invention, a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode and a nonaqueous electrolyte solution, wherein the positive electrode is a first positive electrode active material Li _x CoO ₂ (0.5 <x <1.3) and a second positive electrode Mixed positive electrode active material of active material Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1) Or Al-coated Li _x CoO ₂ (0.5 <x <1.3) cathode active material, wherein the nonaqueous electrolyte is based on the total weight of the nonaqueous electrolyte, and (a) a cyclic carbonate and vinylene group or vinyl substituted with a specific halogen. 1% to 10% by weight of a mixture of compounds containing groups; And (b) 0.1 wt% to 9 wt% of a nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms as an additive. The lithium secondary battery of the present invention can also reduce side effects that cause swelling of the battery during high temperature storage.

Description

Lithium Secondary Battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery having a nonaqueous electrolyte containing a positive electrode and a three-component additive prepared from a specific positive electrode active material, and having excellent safety and preventing component phenomena.

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, the demand for high energy density of batteries used as power sources for such electronic devices is increasing. Lithium secondary batteries are the ones that can best meet these demands, and researches on them are actively under way.

Among the secondary batteries currently applied, lithium secondary batteries developed in the early 1990's include lithium salts dissolved in an appropriate amount of a negative electrode such as a carbon material capable of occluding and releasing lithium ions, a positive electrode made of lithium-containing oxide, and a mixed organic solvent. It consists of a nonaqueous electrolyte.

However, as the application range of the lithium secondary battery expands, there is an increasing need to use the lithium secondary battery even in a harsher environment such as a high temperature or a low temperature environment.

However, a lithium transition metal oxide or a composite oxide used as a positive electrode active material of a lithium secondary battery has a problem in that the metal component is separated from the positive electrode and stored in an unstable state at high temperature in a fully charged state.

In addition, organic solvents currently widely used in nonaqueous electrolytes include ethylene carbonate, propylene carbonate, dimethoxyethane, gamma butyrolactone, N, N-dimethylformamide, tetrahydrofuran or acetonitrile. However, these organic solvents generally deform the stable structure of the battery due to gas generation due to oxidation of the electrolyte when stored for a long time at high temperature, or when the internal heating occurs due to overheating or overdischarging, the battery may ignite and explode. May cause.

In order to solve these problems recently, various methods for improving battery safety at high temperatures by using (1) porous polyol-pin-type membranes having a high melting point that do not readily melt in high temperature environments, or (2) mixing flame retardant solvents or additives in electrolyte solutions Attempts are being made.

However, in the case of polyolefin-based separators, in order to realize a high melting point, the film thickness must be increased, which inevitably decreases the capacity of the battery by lowering the loading amount of the negative electrode and the positive electrode. In addition, since the melting point is 150 ° C before and after the polyolefin-based film composed of PE and PP, if the internal heat generated by oxidation of the electrolyte during overcharging occurs, the separator melts to avoid the problem of battery ignition and explosion due to internal short circuit of the battery. it's difficult.

On the other hand, Japanese Patent Laid-Open No. 1997-259925 discloses a method of adding a non-combustible gas having a boiling point of 25 ° C. or lower when assembling an electrolyte. In Japanese Patent Laid-Open Publication No. 2006-179458 and Japanese Patent Laid-Open Publication No. 2005-190873, the addition of a phosphate ester to a carbonate-based electrolyte ensures nonflammability of the electrolyte, or US Patent Registration Publication No. 6797437 is a non-flammable solvent of perfluoroalkyl or perfluoroester. The method of adding more than% is illustrated. However, the injection of non-combustible gases involves cell volume expansion and complex cell assembly processes, and phosphate ester additives cause cell performance degradation problems due to high reduction potentials. Furthermore, the addition of the exemplified perfluoroalkyl compounds is phase separated upon mixing with the organic solvent electrolyte to precipitate lithium salts.

Accordingly, an object of the present invention is to provide a lithium secondary battery that can significantly suppress the swelling of the battery due to the gas generated during high temperature storage.

In addition, an object of the present invention is to provide a lithium secondary battery having excellent overall performance of a battery, such as charge and discharge performance even at high temperatures.

In order to solve the above problems, according to the present invention, a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode and a nonaqueous electrolyte, the positive electrode is a first positive electrode active material Li _x CoO ₂ (0.5 <x <1.3) and the second cathode active material Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1) mixed anode active material, or Li _x CoO ₂ (0.5 <x <1.3) cathode active material coated with aluminum, wherein the nonaqueous electrolyte includes an ionizable lithium salt and an organic solvent, and the total weight of the nonaqueous electrolyte (A) 1% by weight to 10% by weight of a mixture of a cyclic carbonate substituted with halogen represented by the following formula (1) and a compound containing a vinylene group or a vinyl group; And (b) 0.1 wt% to 9 wt% of a nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms as an additive:

In Formula 1,

X and Y are each independently hydrogen, chlorine, or fluorine, and X and Y are not simultaneously hydrogen.

In the lithium secondary battery of the present invention, the second cathode active material is Li _x M _y O ₂ (M is Ni _1-ab Mn _a Co _b (0.05≤a≤0.4, 0.1≤b≤0.4, 0.4≤1-ab ≦ 0.7), and 1.95 ≦ x + y ≦ 2.05, preferably 0.95 ≦ x ≦ 1.05). At this time, y is 0.9 ≦ y ≦ 1.1.

In the present invention, "vinylene group" is defined as -CH = CH-, and "vinyl group" is defined as CH ₂ = CH-.

In the lithium secondary battery of the present invention, the vinylene group or the compound containing a vinyl group is a vinylene carbonate compound, an acrylate compound containing a vinyl group, a sulfonate compound containing a vinyl group, an ethylene carbonate compound containing a vinyl group A compound etc. can be used individually or in mixture of 2 or more types, respectively. More specifically, a representative example of the vinylene carbonate-based compound may include vinylene carbonate, and the compound containing the vinyl group may be represented by the following Chemical Formula 2:

In Formula 2,

At least one of R ¹ to R ⁴ includes a vinyl group, and the remaining ones are each independently hydrogen, halogen, halogen or unsubstituted C ₁ to C ₆ alkyl group, C ₆ to C ₁₂ aryl group, C ₂ to An alkenyl group or a sulfonate group of C ₆ .

In the lithium secondary battery of the present invention, the nitrile compound may be represented by the following formula (3).

In Formula 3,

n is an integer of 1-6, m is an integer of 1-6.

First, the lithium secondary battery of the present invention can provide a battery having excellent charge / discharge performance as well as safety of overcharging.

Second, it is possible to prevent the swelling phenomenon of the battery by suppressing the generation of gas during high temperature storage of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
1 is a graph measuring discharge capacities of charge and discharge cycles of the batteries of Examples 1, 2 and Comparative Example 2. FIG.

Hereinafter, the present invention will be described in detail with reference to the drawings. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

As described above, as the demand for a lithium secondary battery capable of exhibiting excellent performance in a harsh environment has increased recently, the present invention provides a positive electrode active material Li _x CoO ₂ (0.5 <x <1.3) as a positive electrode active material for forming a positive electrode. And the second cathode active material Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1) It provides a mixed cathode active material, or Li _x CoO ₂ (0.5 <x <1.3) cathode active material coated with aluminum.

The mixed cathode active material of the first cathode active material and the second cathode active material according to the present invention may exhibit a high capacity while having a high energy density. Preferably, when the first positive electrode active material: the second positive electrode active material is mixed in a weight ratio of 1: 1 to 9: 1, this effect is prominent, more preferably 1: 1 to 8: 2, most preferably 1 It may have a mixing weight ratio of 1: 1 to 7: 3.

The first positive electrode active material represented by the formula Li _x CoO ₂ is used in a form in which a part of cobalt is not substituted (doped) with other elements.

In addition, the second positive electrode active material represented by the formula Li _x (Ni _a Co _b Mn _c ) O ₂ is a positive electrode active material containing a three-component transition metal and can exhibit high cycle stability, excellent storage stability, and high temperature stability while having a high capacity. . Preferably, the second cathode active material is Li _x M _y O ₂ (M is Ni _1-ab Mn _a Co _b (0.05 ≦ a ≦ 0.4, 0.1 ≦ _b ≦ 0.4, 0.4 ≦ _1-ab ≦ 0.7), and 1.95 ≤ x + y ≤ 2.05 may have a composition formula of 0.95 ≤ x ≤ 1.05, 0.9 ≤ y ≤ 1.1), the performance of the positive electrode active material described above in the composition ratio of these elements is more excellent. Specifically, the total mole fraction of Ni (1-ab) is relatively high in capacity when the composition is in excess of 0.4 to 0.7 as compared to manganese and cobalt, so that it is possible to maintain a higher capacity and safety. In addition, the content (b) of the cobalt can achieve more excellent rate characteristics, reversible capacity and high powder density at the same time when 0.1 to 0.4. In addition, when the content (x) of lithium is 0.95 to 1.05, better rate characteristics, reversible capacity, and safety at high temperature and high pressure charge and discharge can be ensured. Meanwhile, as the ratio of Li to Li (M) of the transition metal (M) is lowered, the amount of Ni inserted into the MO layer is gradually increased. When too much amount is lowered into the reversible lithium layer, the charge and discharge are reduced. In the process, the movement of Li ⁺ may be disturbed to reduce the reversible capacity or degrade the rate characteristic. On the contrary, when the ratio of Li / M is too high, the amount of Ni inserted in the MO layer may be too small, leading to structural instability, resulting in deterioration of battery safety and deterioration of life characteristics. In addition, at an excessively high Li / M value, the amount of unreacted Li ₂ CO ₃ is increased, that is, a large amount of impurities are generated, which may lower chemical resistance and high temperature stability. Therefore, in one preferred example, in the Li _x M _y O ₂ , the ratio of Li: M may be 0.95 ~ 1.05: 1.

In the present invention, the first cathode active material may be preferably made of a monolithic structure. Accordingly, almost no inner porosity, and as the size of the particles increases, the stability of the crystal grains is improved, and battery manufacturing including the same may be facilitated, thereby increasing the efficiency of the manufacturing process.

In addition, the second cathode active material is preferably in an agglomerated structure, that is, in the form of an aggregate of fine particles, and may have a structure having internal pores. This agglomerated particle structure is characterized by maximizing the surface area reacting with the electrolyte, exerting high rate characteristics, and extending the reversible capacity of the positive electrode.

In another embodiment of the present invention, aluminum _x coated Li _x CoO ₂ (0.5 <x <1.3) may be used as the cathode active material. Li _x CoO aluminum coating layer formed on the outside of the _two is not only to maintain the performance of the battery to suppress the cobalt ions and the like discharged from the inside of Li _x CoO ₂ during charging and discharging process, and that the non-aqueous electrolyte react with Li _x CoO ₂ The swelling phenomenon can be suppressed by preventing the decomposition and preventing decomposition of the electrolyte solution by side reaction.

Optionally, the positive electrode active material of the present invention may further include other positive electrode active materials conventionally used in the art without departing from the scope of the present invention. For example, Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ (0.5 <x <1.3), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x Ni _{1 -y} Co _y O ₂ (0.5 <x <1.3, 0 <y <1), Li _x Co _{1 -y} Mn _y O ₂ (0.5 <x <1.3, 0≤y <1), Li _x Ni _{1 -y} Mn _y O ₂ (0.5 <x <1.3, O≤y <1), Li _x (Ni _a Co _b Mn _c ) O ₄ (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 <c <2 , a + b + c = 2), Li _x Mn _{2 -z} Ni _z O ₄ (0.5 <x <1.3, 0 <z <2), Li _x Mn _{2 -z} Co _z O ₄ (0.5 <x <1.3 , 0 <z <2), Li _x CoPO ₄ (0.5 <x <1.3), and Li _x FePO ₄ (0.5 <x <1.3), any one selected from the group consisting of or a mixture of two or more thereof may be used. . In addition to the lithium-containing transition metal oxide, sulfide, selenide, halide, and the like may also be used.

The cathode active material according to the present invention may be mixed with an optional conductive material, a binder, and an organic solvent according to a conventional method in the art, and applied to a current collector to form a cathode.

Meanwhile, Li _x CoO ₂ (0.5 <x <1.3) and Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 <b <1, The mixed cathode active material of 0 <c <1, a + b + c = 1) exhibits high energy density and exhibits high capacity characteristics, but the electrolyte causes side reactions on the surface of the cathode active material, resulting in swelling of the battery. In addition, Li _x CoO ₂ (0.5 <x <1.3) coated with aluminum also has insufficient effect although the aluminum coating layer suppresses side reactions of the electrolyte. This phenomenon may become more serious when the battery is stored at a high temperature or at a high temperature charge and discharge, and thus there is a problem that the safety standards of the recently increased lithium secondary battery do not meet.

Therefore, when applied to a lithium secondary battery together with the nonaqueous electrolyte according to the present invention having excellent bulging prevention effect, it is possible to obtain a lithium secondary battery having excellent battery performance and remarkably improved swelling phenomenon.

The nonaqueous electrolyte solution for a lithium secondary battery comprising an ionizable lithium salt and an organic solvent according to the present invention, based on the total weight of the nonaqueous electrolyte, (a) contains a cyclic carbonate substituted with halogen represented by the following formula (1), a vinylene group or a vinyl group 1 wt% to 10 wt% of a mixture of compounds to make; And (b) 0.1 wt% to 9 wt% of a nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms as an additive.

[Formula 1]

In Chemical Formula 1,

X and Y are each independently hydrogen, chlorine, or fluorine, and X and Y are not simultaneously hydrogen.

The halogen-containing cyclic carbonate and the compound containing a vinylene group or a vinyl group used in the present invention may form a passivation film called an SEI film on the surface of the negative electrode when the battery is charged, thereby delaying the micro- or macro thermal short generated inside the battery. Therefore, compared to ethylene carbonate, which is an organic solvent widely used in nonaqueous electrolytes, the battery can be prevented or delayed from being ignited by heat inside and outside the battery.

However, when the carbonate electrolyte contains a cyclic carbonate and a vinylene group or a compound containing a vinyl group substituted with halogen according to the present invention, there is a disadvantage in that the decomposition gas is easily vulnerable at high temperature due to thermal vulnerability. The can-cell assembly may be deformed to cause internal short circuits and, in severe cases, the battery may ignite or explode.

As such, when only the cyclic carbonate substituted with the above-described halogen and a compound containing a vinylene group or a vinyl group cannot be sufficiently secured in a short circuit inside the battery, the present invention solves this problem by further mixing the alkoxyalkyl nitrile. When the alkoxyalkyl nitrile compound is used in combination, a complex is formed on the surface of the positive electrode composed of lithium-transition metal oxide, thereby suppressing the oxidation reaction between the electrolyte and the positive electrode, thereby suppressing heat generation, and then preventing internal short circuit due to rapid temperature rise of the battery. Can be. In addition, at high temperature storage, the cyclic carbonate substituted with the above-described halogen, a compound containing a vinylene group or a vinyl group, and an electrolyte solution are suppressed to suppress battery swelling.

Furthermore, when the alkoxyalkyl nitrile compound is used, the effect may also be exhibited in terms of battery performance. The alkoxyalkyl nitrile can provide a battery having excellent electrochemical properties such as higher dielectric constant, lower viscosity, higher conductivity of the electrolyte, and improved interfacial resistance of the assembled battery, compared to existing linear carbonate to aliphatic mononitrile.

As a specific example of the vinylene group or the compound containing a vinyl group according to the present invention, a vinylene carbonate compound, an acrylate compound containing a vinyl group, a sulfonate compound containing a vinyl group, an ethylene carbonate compound containing a vinyl group Etc. can be mentioned.

More specifically, the vinylene carbonate-based compound may be vinylene carbonate, and the compound containing a vinyl group may include a compound represented by the following Chemical Formula 2:

[Formula 2]

In Formula 2,

At least one of R ¹ to R ⁴ includes a vinyl group, and the remaining ones are each independently hydrogen, halogen, halogen or unsubstituted C ₁ to C ₆ alkyl group, C ₆ to C ₁₂ aryl group, C ₂ to An alkenyl group or a sulfonate group of C ₆ .

It is preferable that the mixture of the cyclic carbonate substituted with halogen and the compound containing a vinylene group or a vinyl group represented by Formula 1 of the present invention is included in an amount of 1% by weight to 10% by weight based on the total weight of the nonaqueous electrolyte. If the content is less than 1% by weight, the effect of improving the swelling phenomenon at a high temperature is insignificant. If the content is more than 10% by weight, the high temperature life is greatly deteriorated during the high temperature charge / discharge cycle.

 In addition, the mixing ratio of the cyclic carbonate substituted with halogen represented by the formula (1) and the compound containing a vinylene group or vinyl group may be appropriately selected depending on the specific use of the battery, for example, a cyclic carbonate substituted with halogen : Vinylene group or a compound containing a vinyl group = 1: may have a weight ratio of 0.5 to 5, but is not limited thereto. If the weight ratio is less than 0.5, the vinylene group or the compound containing vinyl group to form a relatively dense SEI is low, which is detrimental to the battery charging and discharging performance at high temperatures, and if it is more than 5, the dense SEI layer polymerized from the vinylene group or vinyl group Produced thick, greatly increases the interfacial resistance of the negative electrode, thereby lowering the initial performance of the battery.

In the present invention, the alkoxyalkyl nitrile compound preferably has 2 to 12 carbon atoms in the alkoxyalkyl group. For example, the alkoxyalkyl nitrile compound according to the present invention may be represented by the following formula (3):

(3)

In Chemical Formula 3,

n is an integer of 1-6, m is an integer of 1-6.

More specific examples of the alkoxyalkyl nitrile compounds according to the present invention described above include methoxy acetonitrile, methoxy propionitrile, methoxy butyronitrile, methoxy valeronitrile, ethoxy acetonitrile, ethoxy propionitrile , Ethoxy butyronitrile, ethoxy valeronitrile and the like can be used alone or in combination of two or more, but is not limited thereto.

The alkoxyalkyl nitrile compound according to the present invention is preferably included 0.1% to 9% by weight, preferably 1% to 7% by weight relative to the total weight of the nonaqueous electrolyte.

If the content is less than 0.1% by weight, the effect of improving overcharge safety and preventing battery swelling by mixing the alkoxyalkyl nitrile compound is not sufficient. If the content is more than 9% by weight, the high temperature life during the high temperature charge / discharge cycle proceeds significantly.

Also optionally, the present invention may further include a compound capable of forming a passivation film on the surface of the negative electrode in addition to the compound containing a cyclic carbonate and a vinyl group substituted with the aforementioned halogen, without departing from the scope of the present invention. Non-limiting examples of the compound include S-based compounds such as propane sultone, ethylene sulfite, 1,3-propane sultone, lactam compounds such as N-acetyl lactam and the like. The compound is preferably 0.1 to 10% by weight based on the total weight of the nonaqueous electrolyte.

Lithium salt in an electrolyte in a lithium secondary battery of the present invention described above can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, for example the lithium salt of the anion is F ^-, Cl ^-, Br ^{-, _{I -, NO 3 -, N}} (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF _{^{2 -, (CF 3) 5}} PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2) 2 N - _{, CF 3 CF 2 (CF 3} ) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 _{^{-, CF 3 CO 2 -,}} CH 3 CO 2 -, SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

As the organic solvent included in the nonaqueous electrolyte according to the present invention described above, those conventionally used in the lithium secondary battery electrolyte may be used without limitation, and typically include a carbonate compound which is a linear carbonate, a cyclic carbonate, or a mixture thereof. Can be. Specific examples of the carbonate compound include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). , Methylpropyl carbonate, dipropyl carbonate, any one selected from the group consisting of, or a mixture of two or more thereof may be representatively used, but is not limited thereto.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are highly viscous organic solvents, and thus may be preferably used because they dissociate lithium salts in electrolytes well. Dimethyl carbonate and diethyl When a low viscosity, low dielectric constant linear carbonate, such as carbonate, is mixed and used in an appropriate ratio, an electrolyte having high electrical conductivity can be prepared, and thus it can be more preferably used.

In addition, the nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms according to the present invention may be included in an amount of 1 to 20 parts by weight relative to 100 parts by weight of the linear carbonate when the organic solvent of the nonaqueous electrolyte contains a linear carbonate. If the content is less than 1 part by weight, the effect of suppressing high temperature swelling and overcharging safety due to the addition of alkoxyalkylnitrile is insignificant, and if it is more than 20 parts by weight, deterioration of long-term charge / discharge performance of the battery may occur.

In addition, the organic solvent, in addition to the carbonate compound, dimethylsulfuroxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone, propylene sulfite, tetrahydrofuran, or the like, may be mixed alone or in combination of two or more. It may be included.

The non-aqueous electrolyte solution for a lithium secondary battery of the present invention described above is manufactured into a lithium secondary battery by injecting an electrode structure composed of a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. As the positive electrode, the negative electrode, and the separator constituting the electrode structure, all those conventionally used for manufacturing a lithium secondary battery may be used.

The non-aqueous electrolyte solution for a lithium secondary battery of the present invention described above is manufactured as a lithium secondary battery by injecting into an electrode structure composed of a separator interposed between a positive electrode, a negative electrode, and a positive electrode and a negative electrode. The positive electrode constituting the electrode structure uses the positive electrode according to the present invention described above, and the negative electrode may be used all those conventionally used in the manufacture of a lithium secondary battery.

As the negative electrode active material used in the preparation of the negative electrode, a carbon material, lithium metal, silicon, tin, or the like, in which lithium ions may be occluded and released, may be used, and the potential for lithium is less than ₂ V, such as TiO ₂ and SnO _2. Metal oxides are also possible. Preferably, a carbon material may be used, and as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Soft crystalline carbon and hard carbon are typical low crystalline carbon, and high crystalline carbon is natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch-based carbon fiber. High temperature calcined carbon such as (mesophase pitch based carbon fiber), meso-carbon microbeads, Mesophase pitches and petroleum or coal tar pitch derived cokes. In this case, the negative electrode may include a binder, and the binder may include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, Various kinds of binder polymers such as polymethylmethacrylate may be used.

In addition, as the separator, conventional porous polymer films conventionally used as separators, for example, polyolefins such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer, etc. The porous polymer film made of the polymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. It is not.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be cylindrical, square, pouch type, or coin type using a can.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example  One

EC: PC: DEC = 3: 2: A 1M LiPF ₆ solution having a composition of 5 (weight ratio) was used as an electrolyte solution, the electrolyte solution 3% by weight of vinylene carbonate, 2% by weight of fluoro ethylene carbonate (FEC), 5% by weight of methoxy propionitrile (MPN) was added.

In addition, artificial graphite was used as the negative electrode active material, and about 5 to 8 μm, which is an aggregate of LiCoO ₂ having a D ₅₀ of about 15 to 20 μm and a fine particle of about 1 to 2 μm, as a single phase structure. a _{D LiNi 0 .53 Co 0 .2 Mn} 0 .27 O 2 with ₅₀ 7 was used and mixed in a weight ratio of 3. Thereafter, a rectangular lithium secondary battery using an aluminum can was manufactured by a conventional method.

Example  2

A lithium polymer battery was manufactured in the same manner as in Example 1, except that the methoxy propionitrile content was used at 7% by weight.

Example  3

EC: PC: DEC = 3: 4: A 1M LiPF ₆ solution having a composition of 3 (weight ratio) was used as the electrolyte, 2% by weight of vinylene carbonate, 2% by weight of fluoro ethylene carbonate (FEC), 2% by weight of methoxy propionitrile (MPN) was added.

Further, as an anode active material was used as the artificial graphite, the positive electrode active material LiCoO ₂ was used as the cost of aluminum is obtained by mixing the 100 parts by weight of LiCoO ₂ compared to 1 part by weight of aluminum powder with LiCoO ₂ and heat-treated coating. Thereafter, a rectangular lithium secondary battery using an aluminum can was manufactured by a conventional method.

Comparative example  One

A prismatic lithium secondary battery was prepared in the same manner as in Example 1, except that 3 wt% of vinylene carbonate and 2 wt% of fluoroethylene carbonate were added without adding methoxy propionitrile to the electrolyte.

Comparative example  2

A rectangular lithium secondary battery was prepared in the same manner as in Example 1, except that 10% by weight of methoxy propionitrile was added.

Comparative example  3

A prismatic lithium secondary battery was prepared in the same manner as in Example 3, except that 2 wt% of vinylene carbonate and 2 wt% of fluoroethylene carbonate were added without adding methoxy propionitrile to the electrolyte.

In order to evaluate the safety of the electrolyte prepared according to the above-described Examples and Comparative Examples, the following test was performed.

1. High temperature storage safety test

Each cell prepared in Example 1, Example 3 and Comparative Example 1, Comparative Example 3 was measured at 90 ℃, 5 hours under 4.2V full charge state and the initial thickness and the change in thickness after storage were measured, respectively. The results are shown in Tables 1 and 2, respectively, and the thickness change (Δt) is expressed as a relative value with a thickness increase of the corresponding comparative example as 100%.

Cathode active material: LiCoO ₂ + LiNi _0.53 Co _0.2 Mn _0.27 O ₂ Δt (%) Example 1 91.1 Comparative Example 1 100

Cathode active material: LiCoO ₂ / Al Δt (%) Example 3 72.8 Comparative Example 3 100

As shown in Table 1 and Table 2, compared with Comparative Example 1 and Comparative Example 3, the battery according to the present invention (Examples 1, 3) it can be seen that the increase in thickness (swelling phenomenon) is significantly suppressed when stored for a long time at high temperature Can be.

2. Charging and discharging  Performance test

(1) Measurement of initial battery capacity

Each battery of Examples 1 and 2 and Comparative Examples 1 and 2 produced as described above was charged at a constant current of 0.5 C = 400 mA at 25 ° C., and after the voltage of the battery became 4.2 V, a constant voltage of 4.2 V. Initial charging was performed until the charging current value reached 50 mA. The battery initially charged was discharged at a constant current of 0.2 C until the battery voltage reached 3 V, and the discharge capacity at this time was obtained as the initial capacity.

(2) Charging and discharging  Cycle test

Each battery is charged at a constant current of 1C = 900 mA in a temperature environment of 45 ° C., and after the battery voltage reaches 4.2 V, the first charge is performed at a constant voltage of 4.2 V until the charging current value reaches 50 mA. It was. The first charged battery was discharged at a constant current of 1C until the battery voltage reached 3V, and the discharge capacity at the first cycle was obtained. Among them, the battery discharge capacity and the high temperature charge / discharge cycle results of Examples 1, 2 and Comparative Example 2 are collectively shown in FIG. 1.

As shown in FIG. 1, the initial capacities of the batteries of Examples 1, 2 and Comparative Example 2 including methoxy propionitrile as additives were almost the same, but an excess (10 wt%) of methoxy propionitrile was added. In the case of Comparative Example 2, it can be seen that the discharge capacity decreases as the cycle increases. In particular, the number of cycles typically required in a battery used in a related industry (eg, a mobile phone, etc.) is at least about 300 cycles. In Comparative Example 2, it can be seen that the discharge capacity greatly decreases after 300 cycles.

Claims (11)

In a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode and a nonaqueous electrolyte,
The positive electrode may include a first positive electrode active material Li _x CoO ₂ (0.5 <x <1.3) and a second positive electrode active material Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 < b <1, 0 <c <1, a + b + c = 1) mixed cathode active material, or Li _x CoO ₂ (0.5 <x <1.3) cathode active material coated with aluminum,
The nonaqueous electrolyte includes an ionizable lithium salt and an organic solvent, and with respect to the total weight of the nonaqueous electrolyte,
(a) 1% by weight to 10% by weight of a mixture of a cyclic carbonate substituted with halogen represented by Formula 1 and a compound containing a vinylene group or a vinyl group; And
(b) 0.1 wt% to 9 wt% of a nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms
Lithium secondary battery further comprises:
[Formula 1]

In Chemical Formula 1,
X and Y are each independently hydrogen, chlorine, or fluorine, and X and Y are not simultaneously hydrogen. The method of claim 1,
The second cathode active material is Li _x M _y O ₂ (M is Ni _1-ab Mn _a Co _b (0.05 ≦ a ≦ 0.4, 0.1 ≦ _b ≦ 0.4, 0.4 ≦ _1-ab ≦ 0.7), and 1.95 ≦ x + and y ≦ 2.05, wherein 0.95 ≦ x ≦ 1.05 and 0.9 ≦ y ≦ 1.1). The method of claim 1,
The vinylene group or the compound containing a vinyl group is any one selected from the group consisting of a vinylene carbonate compound, an acrylate compound containing a vinyl group, a sulfonate compound containing a vinyl group, and an ethylene carbonate compound containing a vinyl group. Or a mixture of two or more thereof. The method of claim 1,
The compound containing the vinyl group is a lithium secondary battery, characterized in that the compound represented by the formula (2):
(2)

In Formula 2,
At least one of R ¹ to R ⁴ includes a vinyl group, and the remaining ones are each independently hydrogen, halogen, halogen or unsubstituted C ₁ to C ₆ alkyl group, C ₆ to C ₁₂ aryl group, C ₂ to An alkenyl group or a sulfonate group of C ₆ . The method of claim 1,
The cyclic carbonate substituted with halogen: a vinylene group or a compound containing a vinyl group = 1: lithium secondary battery having a mixed weight ratio of 0.5 to 5. The method of claim 1,
The alkoxyalkyl nitrile compound is a lithium secondary battery, characterized in that represented by the following formula (3):
(3)

In Chemical Formula 3,
n is an integer of 1-6, m is an integer of 1-6. The method of claim 1,
The alkoxyalkyl nitrile compound is methoxy acetonitrile, methoxy propionitrile, methoxy butyronitrile, methoxy valeronitrile, ethoxy acetonitrile, ethoxy propionitrile, ethoxy butyronitrile, ethoxy valero Lithium secondary battery, characterized in that any one or a mixture of two or more selected from the group consisting of nitrile. The method of claim 1,
The lithium salt of the anion is ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, ( ^{_{CF 3) 3 PF 3 -,}} (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 ^{_{SO 2) 2 N -, (}} FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2 _{^{) 3 C -, CF 3 (}} CF 2) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 -, SCN ^- and _{(CF 3 CF 2 SO 2)} 2 N - lithium secondary battery, characterized in that at least one selected from the group consisting of. The method of claim 1,
The organic solvent is a lithium secondary battery comprising a carbonate compound which is any one selected from the group consisting of linear carbonate, cyclic carbonate and mixtures thereof. 10. The method of claim 9,
The carbonate compound is a lithium secondary, characterized in that any one or a mixture of two or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate battery. 10. The method of claim 9,
When the organic solvent includes a linear carbonate, the nitrile compound having an alkoxyalkyl group having 2 to 12 carbon atoms is included in 1 to 20 parts by weight based on 100 parts by weight of linear carbonate.

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