KR101582043B1 - Lithium secondary battery with improved output and cycling characteristics - Google Patents

Abstract

The present invention relates to a positive electrode; cathode; A separator; And a non-aqueous electrolytic solution, wherein the positive electrode comprises LiFePO ₄ , and the non-aqueous electrolytic solution is a non-aqueous organic solvent containing propylene carbonate (PC) and a lithium bis-fluorosulfonylimide bis (fluorosulfonyl) imide (LiFSI)).
The lithium secondary battery of the present invention can simultaneously improve the output characteristics and the cycle characteristics by forming a solid SEI film at the cathode at the time of initial charging of the lithium secondary battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery having excellent output and cycle characteristics,

The present invention relates to a lithium secondary battery comprising LiFePO ₄ as a positive electrode and a non-aqueous electrolytic solution containing propylene carbonate (PC) and lithium bis (fluorosulfonyl) imide (LiFSI) will be.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage have been commercialized and widely used.

Lithium metal oxide is used as the positive electrode active material of the lithium secondary battery, and lithium metal, lithium alloy, crystalline or amorphous carbon or carbon composite material is used as the negative electrode active material. The active material is coated on the current collector with an appropriate thickness and length, or the active material itself is coated in a film form and wrapped or laminated with a separator as an insulator to form an electrode group. The electrode group is then placed in a can or similar container, Thereby producing a lithium secondary battery.

This lithium secondary battery is charged and discharged while repeating the process of intercalating lithium ions from the lithium metal oxide of the anode into the graphite electrode of the cathode and deintercalating the lithium ions. At this time, since lithium is highly reactive, it reacts with the carbon electrode to form Li ₂ CO ₃ , LiO, LiOH and the like to form a film on the surface of the cathode. This coating is called Solid Electrolyte Interface (SEI) coating. The SEI coating formed at the beginning of charging prevents the reaction between lithium ion and carbon anode or other materials during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass through. This ion tunnel serves to prevent the collapse of the structure of the carbon anode by co-intercalating the organic solvent of the electrolyte having a large molecular weight moving together by solvation of the lithium ion together with the carbon anode.

The SEI film is formed as an ion tunnel through which only lithium ion is passed between the electrolyte and the cathode when the electrolyte is once formed at the time of initial charging, Role. Therefore, in order to improve the cycle characteristics of the lithium secondary battery, it is necessary to form a solid SEI film on the cathode of the lithium secondary battery.

In order to form a solid SEI film, an i / ternary non-aqueous organic solvent including ethylene carbonate (EC) as a base is generally used in a non-aqueous electrolytic solution of a lithium secondary battery. However, ethylene carbonate has a high melting point, which limits the use temperature, and may lead to a significant battery performance deterioration at low temperatures.

KR 2009-0030237 A

Disclosure of Invention Technical Problem [8] The present invention provides a lithium secondary battery capable of simultaneously improving output characteristics and cycle characteristics of a lithium secondary battery.

According to an aspect of the present invention, cathode; A separator; And a nonaqueous electrolytic solution, wherein the positive electrode comprises LiFePO ₄ , and the nonaqueous electrolytic solution is a non-aqueous organic solvent containing propylene carbonate (PC) and a lithium bis bis (fluorosulfonylimide) (fluorosulfonyl) imide (LiFSI)).

According to the lithium secondary battery of the present invention, when the lithium secondary battery is initially charged, a solid SEI film is formed on the cathode, so that the output characteristics and the cycle characteristics can be improved at the same time.

1 and 2 are graphs showing the output characteristics of the lithium secondary battery manufactured in Example 1, Comparative Examples 6 and 7, Examples 2 to 4, and Comparative Examples 1 to 3, respectively, according to Experimental Example 1.
3 is a graph showing the cycle characteristics of the lithium secondary batteries manufactured in Examples 2 to 4 and Comparative Examples 1 to 5 according to Experimental Example 2. FIG.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention.

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.

According to an embodiment of the present invention, cathode; A separator; And a nonaqueous electrolytic solution, wherein the positive electrode comprises LiFePO ₄ , and the nonaqueous electrolytic solution is a non-aqueous organic solvent containing propylene carbonate (PC) and a lithium bis bis (fluorosulfonylimide) (fluorosulfonyl) imide (LiFSI).

According to one embodiment of the present invention, when LiFePO ₄ is used as a positive electrode and lithium bifluorosulfonylimide is used in combination with a non-aqueous organic solvent containing propylene carbonate as a non-aqueous electrolyte, It is possible to simultaneously improve the output characteristics and the cycle characteristics by forming a solid SEI film on the cathode at the time of initial charging without using ethylene carbonate (EC), which has been mainly used for lithium secondary batteries.

Generally, in non-aqueous electrolytes of lithium secondary batteries, ethylene carbonate has been used as a non-aqueous organic solvent. However, since ethylene carbonate has a high melting point, its use temperature is limited. In particular, there is a problem that output characteristics are low due to low output characteristics at low temperatures below -30 ° C and low conductivity. Particularly, when EC is used too much, gas is generated due to decomposition of CO ₂ , which not only adversely affects the performance of the secondary battery but also lowers the high output characteristic due to low conductivity.

On the other hand, the nonaqueous electrolytic solution containing propylene carbonate has characteristics of excellent low temperature characteristics and high output characteristics due to high conductivity. However, propylene carbonate causes irreversible decomposition reaction with the carbon-based material, and thus has a problem in use with the carbon-based material. In addition, there has been a problem that the capacity of the lithium secondary battery is lowered due to electrode refraction due to propylene carbonate at high temperature cycles depending on the electrode thickness.

Particularly, when propylene carbonate is used as a non-aqueous organic solvent together with a lithium salt such as LiPF ₆ , propylene carbonate is used in a process of forming a SEI film in a lithium secondary battery using a carbon electrode and a process of forming a lithium ion solvated by propylene carbonate An immense amount of irreversible reaction may occur during the insertion between the carbon layers. This may cause problems such as deterioration of battery performance such as cycle characteristics.

Further, when lithium ion solvated by propylene carbonate is inserted into the carbon layer constituting the negative electrode, exfoliation of the carbon surface layer may proceed. This exfoliation can be caused by the gas generated when the solvent is decomposed between the carbon layers causing a large distortion between the carbon layers. Such peeling of the surface layer and decomposition of the electrolytic solution can proceed continuously. Therefore, when the nonaqueous electrolytic solution containing propylene carbonate is used in combination with the carbonaceous anode material, effective SEI is not generated and lithium ions may not be inserted.

In the present invention, when ethylene carbonate is used, the use of propylene carbonate has been solved to solve problems such as a decrease in output characteristics due to a high melting point, and the use of propylene carbonate and a lithium salt such as LiPF ₆ Can be solved by combining lithium bisfluorosulfonylimide and LiFePO ₄ as the positive electrode, thereby further improving the performance of the lithium secondary battery.

That is, by using propylene carbonate essentially, it is possible to further improve the mobility of lithium ions. In the LiFePO ₄ anode containing a large amount of water, lithium bisfluorosulfonylimide having less reactivity with water than other lithium salts It is possible to form a more rigid SEI film without using a commonly used ethylene carbonate. Accordingly, the cycle characteristics and the output characteristics of the lithium secondary battery can be improved at the same time.

The lithium bisfluorosulfonylimide is added to a non-aqueous electrolytic solution as a lithium salt to form a stable and stable SEI film on the negative electrode, thereby improving the output characteristics, especially the high output characteristics. The decomposition can be suppressed and the oxidation reaction of the electrolytic solution can be prevented.

According to an embodiment of the present invention, the concentration of the lithium bisfluorosulfonylimide in the non-aqueous electrolytic solution is preferably 0.1 mole / liter to 2 mole / liter, more preferably 0.5 mole / liter to 1.5 mole / liter Do. If the concentration of the lithium bisfluorosulfonylimide is less than 0.1 mole / l, the effect of improving the output and cycle characteristics of the lithium secondary battery is insignificant, and if the concentration of the lithium bisfluorosulfonylimide is less than 2 mole / l, side reactions in the electrolyte may occur excessively during charging and discharging of the battery, and swelling may occur.

In order to further prevent such side reactions, the non-aqueous electrolytic solution of the present invention may further include a lithium salt. Examples of the lithium salt include lithium salts such as LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li, LiC (CF ₃ SO ₂ ) ₃ and LiC ₄ BO ₈ , or a mixture of two or more thereof.

According to an embodiment of the present invention, when the lithium salt is further included, the mixing ratio of the lithium salt and the lithium bisfluorosulfonylimide may be 1: 1 to 9 in terms of molar ratio. When the mixing ratio of the lithium salt and the lithium bisfluorosulfonylimide is out of the range of the molar ratio, a side reaction in the electrolyte may occur excessively during charging and discharging of the battery, causing a swelling phenomenon. The output characteristic and the cycle characteristic can be changed.

In particular, the mixing ratio of the lithium salt to the lithium bisfluorosulfonylimide is preferably 1: 6 to 9 in terms of the molar ratio. Specifically, when the mixing ratio of the lithium salt to the lithium bisfluorosulfonylimide is less than 1: 6, the process of forming the SEI film in the lithium ion battery, and the process of forming the SEI film by solubilized lithium ions A large amount of irreversible reaction may occur during the process of inserting between the cathodes, and the separation of the surface layer of the negative electrode (for example, carbon surface layer) and decomposition of the electrolyte may improve the low temperature output of the secondary battery, And the effect of improving the capacitance characteristics may be insignificant.

Meanwhile, as the non-aqueous organic solvent according to one embodiment of the present invention, propylene carbonate may be included in an amount of 5 parts by weight to 60 parts by weight, preferably 10 parts by weight to 50 parts by weight, based on 100 parts by weight of the non-aqueous organic solvent. If the amount of the propylene carbonate is less than 5 parts by weight, the swelling phenomenon may be generated in which the gas is continuously generated due to the decomposition of the surface of the anode during the cycle to increase the thickness of the battery. When the content exceeds 60 parts by weight, It is difficult to form a solid SEI film.

The non-aqueous organic solvent which can be contained in the non-aqueous electrolytic solution in addition to the above-mentioned propylene carbonate is not limited as long as it can minimize decomposition due to oxidation reaction during charging and discharging of the battery, none.

The non-aqueous organic solvent according to an embodiment of the present invention may include, for example, ethyl propionate (EP), methyl propionate (MP), butylene carbonate (BC), dimethyl carbonate ), Diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ester, ether and ketone , Or a mixture of two or more of them, and is not particularly limited to these types unless the effect of the present invention is exerted.

Meanwhile, the non-aqueous electrolytic solution according to an embodiment of the present invention may further include a vinylene carbonate compound, a sultone compound, and a sulfate compound.

The vinylene carbonate-based compound may serve to form an SEI film. The type of the vinylene carbonate compound is not limited as long as it can function as described above. For example, the vinylene carbonate (VC), the vinyl ethylene carbonate (VEC) . Of these, vinylene carbonate is particularly preferable.

In addition, the sulfone compound, which may be further included in the present invention according to an embodiment of the present invention, may further improve the output and cycle characteristics of the battery. There are no particular limitations on the type of the steroidal compound as long as it can serve as the above. For example, 1,3-propane sultone (PS), 1,4-butane sultone, 1,3- Propene sultone, or a mixture of two or more thereof. Of these, 1,3-propane sultone is particularly preferable.

In addition, the sulfate compound, which may be further included according to an embodiment of the present invention, may improve the output and lifetime characteristics of the secondary battery, and may include ethylene sulfate, propylene sulfate, butylene sulfate, pentylene sulfate, Silane sulfate, and heptylene sulfate, or a mixture of two or more thereof.

Meanwhile, the non-aqueous electrolytic solution according to an embodiment of the present invention may further include LiBF ₄ , lithium oxalyldifluoroborate (LiODFB), or a mixture thereof as an additive. The additive can suppress the side reaction in the electrolyte when the battery is charged and discharged at room temperature in a lithium secondary battery comprising an excess amount of lithium bisfluorosulfonylimide. Accordingly, the additive is effective for improving the cycle characteristics of the battery under normal temperature conditions. The content of the additive may be 0.01 to 5% by weight based on the total weight of the electrolytic solution.

In addition, the lithium secondary battery according to an embodiment of the present invention may include a cathode including a cathode active material, a cathode including a cathode active material, and a separator interposed between the cathode and the cathode, It may comprise the structure of LiFePO _4.

The LiFePO ₄ used in accordance with an embodiment of the present invention has a problem of accelerating deterioration of the battery in terms of long-term life and storage characteristics because it contains a large amount of water. However, LiFePO _{4, which} is less reactive with moisture than other lithium salts Use of the bisfluorosulfonylimide can suppress the lifetime and the storage deterioration phenomenon due to side reactions with LiFePO ₄ water impurities.

According to an embodiment of the present invention, the LiFePO ₄ may be used singly or as a mixture of a manganese spinel-based active material, a lithium metal oxide, or a mixture thereof, as a cathode active material. The lithium metal oxide may be selected from the group consisting of lithium-manganese oxide, lithium-nickel-manganese oxide, lithium-manganese-cobalt oxide and lithium-nickel-manganese-cobalt oxide, and more specifically LiCoO 0, 0 <b <1, 0 <c <1, a + b + ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O _{2 c = 1), LiNi 1 -Y} Co Y O 2, LiCo 1 - Y Mn Y O 2, LiNi 1 - Y Mn Y O 2 ( here, 0≤Y <1), Li ( Ni a Co b Mn c _{) O 4 (0 <a <} 2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn 2 - z Ni z O 4, LiMn 2 -z Co z O 4 ( here , 0 &lt; Z &lt; 2).

As the negative electrode active material, a carbonaceous anode active material such as a crystalline carbon, an amorphous carbon, or a carbon composite may be used alone or in combination of two or more. Preferably, the carbonaceous active material may be a crystalline carbon and a graphite such as natural graphite and artificial graphite (graphite) carbon.

Specifically, in the lithium secondary battery, the positive electrode or the negative electrode is prepared by mixing a mixture of a positive electrode or a negative electrode active material, a conductive agent and a binder with a predetermined solvent on a positive electrode or a negative electrode current collector to prepare a slurry, Applying the slurry on a current collector, and drying the slurry.

According to one embodiment of the present invention, the cathode current collector is generally made to have a thickness of 3 [mu] m to 500 [mu] m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used.

The cathode current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The negative electrode current collector is generally made to have a thickness of 3 [mu] m to 500 [mu] m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The conductive agent used for the positive electrode or negative electrode slurry is usually added in an amount of 1 to 20 wt% based on the total weight of the mixture including the positive electrode or negative electrode active material. Such a conductive agent is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding of the positive electrode or negative electrode active material with the conductive agent and bonding to the current collector, and is usually added in an amount of 1 to 20 wt% based on the total weight of the mixture containing the positive electrode or negative electrode active material. Examples of such binders include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, , Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) , Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, various copolymers, and the like can be used.

In addition, preferred examples of the solvent include dimethyl sulfoxide (DMSO), alcohol, N-methylpyrrolidone (NMP), acetone or water, and are removed in the drying process.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer The porous polymer film made of a polymer may be used alone or in a laminated form, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, polyethylene terephthalate fiber or the like may be used. no.

The battery case used in the present invention may be of any type that is commonly used in the art, and is not limited in its external shape depending on the use of the battery. For example, a cylindrical case, a square type, a pouch type, (coin) type or the like.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells. Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric power storage systems, and the like.

BEST MODE FOR CARRYING OUT THE INVENTION 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 enable those skilled in the art to more fully understand the present invention.

Example

EXAMPLES The present invention will be further illustrated by the following examples and experimental examples, but the present invention is not limited by these examples and experimental examples.

Example 1

[Preparation of non-aqueous electrolytic solution]

Propylene carbonate (PC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 2: 4: 4 ( volume ratio) LiPF composition, based on the total amount of non-aqueous electrolytic solution in a non-aqueous organic solvent having a _six 0.6 mole / liter of lithium bisfluorosulfonylimide, 0.6 mole / liter of lithium bisfluorosulfonylimide, 3 weight percent of vinylene carbonate (VC), 0.5 weight percent of 1,3-propane sultone (PS) and 1 weight percent of ethylene sulfate (ESa) To prepare a non-aqueous electrolytic solution.

[Production of lithium secondary battery]

90 wt% of LiFePO ₄ as a positive electrode active material, 5 wt% of carbon black as a conductive agent and 5 wt% of polyvinylidene fluoride (PVdF) as a binder were dissolved in N-methyl-2-pyrrolidone (NMP) To prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film having a thickness of about 20 탆 and dried to produce a positive electrode, followed by a roll press to prepare a positive electrode.

In addition, 95.8 wt%, 3.2 wt% and 1 wt% of a carbon powder as a negative electrode active material, styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as a binder and carbon black as a conductive agent were respectively dissolved in NMP To prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m and dried to prepare a negative electrode, followed by roll pressing to produce a negative electrode.

The battery thus prepared was assembled by using a separator made of a polyethylene (PE) layer as a positive electrode and a negative electrode, and the prepared non-aqueous electrolyte solution was injected into the assembled battery to complete the manufacture of a lithium secondary battery.

Example 2

The lithium salt is added to LiPF ₆ based on the total amount of the non-aqueous electrolytic solution Aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.1 mole / liter of lithium bisfluorosulfonylimide and 0.9 mole / liter (approximately 1: 9 mole ratio) of lithium bisfluorosulfonylimide were used.

Example 3

The lithium salt is added to LiPF ₆ based on the total amount of the non-aqueous electrolytic solution Aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the nonaqueous electrolytic solution and the lithium secondary battery were prepared in the same manner as in Example 1, except that 0.14 mole / liter and 0.86 mole / liter of lithium bisfluorosulfonylimide were used.

Example 4

Based on the total amount of the non-aqueous electrolytic solution, LiPF ₆ 0.2 mole / liter, and 1.0 mole / liter (about 1: 5 mole ratio) of lithium bis-fluorosulfonylimide were used in place of the bis-fluorosulfonylimide used in Example 1 to prepare a nonaqueous electrolytic solution and a lithium secondary battery .

Comparative Example 1

Except that a non-aqueous organic solvent having a composition of propylene carbonate (PC): ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 3: 3: 4 (volume ratio) was used, Thereby preparing an electrolyte and a lithium secondary battery.

Comparative Example 2

Except that a non-aqueous organic solvent having a composition of propylene carbonate (PC): ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 3: 3: 4 (volume ratio) was used, Thereby preparing an electrolyte and a lithium secondary battery.

Comparative Example 3

A non-aqueous electrolytic solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that LiPF ₆ was used alone as the lithium salt.

Comparative Example 4

As a cathode active material in the same manner as in Example 2, except for using Li (Ni Co _{0 .33 0 .33 0 .33} Mn) O _2, 96% by weight to prepare a non-aqueous electrolytic solution and a lithium secondary battery

Comparative Example 5

LiPF ₆ And lithium bisfluorosulfonylimide were used in a molar ratio of about 1: 0.5, a non-aqueous electrolytic solution and a lithium secondary battery were prepared.

Comparative Example 6

Except that a non-aqueous organic solvent having a composition of ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 3: 3: 4 (volume ratio) was used and LiPF ₆ alone was used as the lithium salt , A non-aqueous electrolytic solution and a lithium secondary battery were produced in the same manner as in Example 1. [

Comparative Example 7

A non-aqueous electrolytic solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that LiPF ₆ was used alone as the lithium salt.

Experimental Example 1

<Output Characteristic Test 1>

To investigate the change in output characteristics with and without lithium bisfluorosulfonylimide (LiFSI), the lithium secondary batteries of Example 1 and Comparative Examples 6 and 7 were charged and discharged at a room temperature from 0.2 C to 9.0 C And the relative capacity according to the discharge C-rate was measured. The results are shown in Fig.

1, the lithium secondary batteries of Example 1 and Comparative Examples 6 and 7 had similar relative capacities from 0.2C to 1.0C, but from 1.0C to 9.0C, the lithium secondary batteries of Example 1 had a comparative capacity of 6 And the lithium secondary battery of Comparative Example 7, the relative capacity was remarkably improved.

&Lt; Output Characteristic Test 2 &

The lithium secondary batteries of Examples 2 to 4 and Comparative Examples 1 to 3 were subjected to the same conditions as those of the output characteristic test 1 in order to examine the change of the concentration of the lithium salt and LiFSI and the change of the output characteristics depending on the use of ethylene carbonate The relative capacity according to C-rate was measured. The results are shown in Fig.

2, the relative capacity of the lithium secondary batteries of Examples 2 to 4 and Comparative Examples 1 to 3 was similar from 0.2C to 1.5C, but the lithium secondary batteries of Examples 2 to 4 from 1.5C to 9.0C, The lithium secondary batteries of Comparative Examples 1 to 3 exhibit significantly higher relative capacity than the lithium secondary batteries of Comparative Examples 1 to 3. Particularly, 9.0C showed a remarkable difference also in Examples 2 and 3 in which the molar ratio of lithium salt and LiFSI was 1: 6 to 1: 9, and Example 4 in which the molar ratio was 1: 5.

On the other hand, in order to compare the output characteristics test depending on the use of EC, Example 2 in which the molar ratio of lithium salt and LiFSI was 1: 9 was compared with Comparative Example 1, EC was about 9% higher than that of Comparative Example 1 using EC. Comparing Example 3 and Comparative Example 2 in which the molar ratio of the lithium salt and LiFSI was 1: 5, Is about 9% higher than that of Comparative Example 1 using EC. It can be seen that the difference in output characteristics depending on the presence or absence of EC is further widened when the molar ratio of the lithium salt and LiFSI is 1: 9, as compared with the case where the molar ratio is 1: 5.

Thus, LiPF ₆ And LiFSI, and that the output characteristics of the lithium secondary battery can be improved by not using EC.

Experimental Example 2

<Cycle characteristic test>

The lithium secondary batteries of Examples 2 to 4 and Comparative Examples 1 to 5 were charged at 3 C up to 3.8 V under a constant power condition at room temperature and discharged at 3 C up to 2.6 V under a constant output (CP) condition , And the discharge capacity thereof was measured. This was repeatedly carried out at 1 to 1000 cycles, and the measured discharge retention ratio (%) is shown in Fig.

Referring to FIG. 3, in the lithium secondary batteries of Examples 2 to 4 of the present invention, the slopes of the capacity retention ratios up to 1,000 cycles were gentler than the lithium secondary batteries of Comparative Examples 1 to 5. In particular, LiPF ₆ And LiFSI molar ratio of 1: 6 or more are significantly different from the slopes of Example 4 and Comparative Examples 3 and 5.

That is, in Comparative Example 5 in which the molar ratio was 1: 0.5 and Comparative Example 3 in which LiPF _{6 was} used alone instead of LiFSI, the capacity retention ratio was remarkably decreased by 20% or more as compared with Examples 2 and 3 in the cycle number 1000 times.

Thus, LiPF ₆ And LiFSI, the cycle characteristics of the lithium secondary battery can be remarkably improved, and in particular, LiPF ₆ And LiFSI molar ratio of 1: 6 to 1: 9, it was confirmed that the cycle characteristics of the lithium secondary battery were significantly superior to those in the case where the molar ratio exceeded this range.

The cycle characteristics of the lithium secondary battery according to the kind of the anode are as follows: LiPF ₆ And an equal molar ratio of LiFSI and, LiFePO ₄ and Li as an anode (Ni Co _{0 .33 0 .33 0 .33} Mn) of this embodiment, compared to Example 2 and Comparative cycle characteristics of the lithium secondary battery of Example 4 using the O ₂ As a result, the capacity retention ratio was remarkable from the 100th cycle.

That is, in the 400th cycle, the capacity retention was improved by about 12% in Example 2 compared with Comparative Example 4, and by about 20% in the 1000th cycle. Which, when the reactivity with water used with less LiFSI excess compared to other lithium salts, it is possible to further enhance the mobility of lithium ions by essentially using the propylene carbonate, the capacity retention rate according to the number of cycles in the LiFePO ₄ anode Which is more advantageous.

Claims (16)

anode; cathode; A separator; And a non-aqueous electrolytic solution,
The positive electrode comprises a LiFePO _4,
Wherein the non-aqueous electrolytic solution comprises a non-aqueous organic solvent containing propylene carbonate (PC) and lithium bis (fluorosulfonyl) imide (LiFSI)
Wherein the non-aqueous electrolytic solution further comprises a lithium salt,
Wherein a mixing ratio of the lithium salt to lithium bisfluorosulfonylimide is 1: 6 to 9 in a molar ratio.
delete delete delete The method according to claim 1,
Wherein the concentration of the lithium bisfluorosulfonylimide in the non-aqueous electrolytic solution is 0.1 mole / liter to 2 mole / liter.
The method according to claim 1,
Wherein the content of the propylene carbonate is 5 parts by weight to 60 parts by weight based on 100 parts by weight of the non-aqueous organic solvent.
The method according to claim 6,
Wherein the content of the propylene carbonate is 10 parts by weight to 50 parts by weight based on 100 parts by weight of the non-aqueous organic solvent.
The method according to claim 1,
The non-aqueous organic solvent may be at least one selected from the group consisting of ethyl propionate (EP), methyl propionate (MP), butylen carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (EPC), esters, ethers and ketones, or a mixture of two or more thereof, in addition to the above-mentioned components (A) and (B) Wherein the lithium secondary battery is a lithium secondary battery.
The method according to claim 1,
Wherein the non-aqueous organic solvent does not contain ethylene carbonate.
The method according to claim 1,
Wherein the non-aqueous electrolytic solution further comprises a vinylene carbonate compound, a sulfone compound, and a sulfate compound.
11. The method of claim 10,
The sulfone compound may be any one selected from the group consisting of 1,3-propane sultone, 1,4-butane sultone and 1,3-propanesultone, or a mixture of two or more thereof And a lithium secondary battery.
11. The method of claim 10,
Wherein the sulfate compound is any one selected from the group consisting of ethylene sulfate, propylene sulfate, butylene sulfate, pentylene sulfate, hexylene sulfate and heptylene sulfate, or a mixture of two or more thereof. .
13. The method of claim 12,
Wherein the non-aqueous electrolytic solution further comprises LiBF ₄ , lithium oxalyl difluoroborate (LiODFB), or a mixture thereof.
The method according to claim 1,
The lithium salt is _{LiPF 6, LiBF 4, LiSbF 6} , LiAsF 6, LiClO 4, LiN (C 2 F 5 SO 2) 2, LiN (CF 3 SO 2) 2, CF 3 SO 3 Li, LiC (CF 3 SO ₂ ) ₃ and LiC ₄ BO ₈ , or a mixture of two or more thereof.
The method according to claim 1,
Wherein the negative electrode comprises a carbon-based negative active material.
16. The method of claim 15,
Wherein the negative electrode active material is graphite carbon.

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