KR20050030763A - Rechargeable lithium ion battery - Google Patents

Abstract

Provided is a lithium ion secondary battery which is improved in energy density per weight by using a current collector comprising a support polymer film and a metal deposited on the film. The lithium ion secondary battery comprises a positive electrode which comprises a first current collector and a positive electrode active material formed on the first current collector; a negative electrode which comprises a second current collector and a negative electrode active material formed on the second current collector; and an electrolyte solution which comprises a nonaqueous organic solvent and a lithium salt, wherein at least one of the first current collector and the second current collector comprises a support polymer film selected from the group consisting of polyethylene terephthalate, polyimide, polytetrafluoroethylene, polyethylene naphthalate and polyvinylidene fluoride, and a metal deposited on the support polymer film. Preferably the metal is at least one selected from the group consisting of Ni, Ti, Cu, Ag, Au, Pt, Fe, Co, Cr, W, Mo, Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, In and Zn.

Description

Lithium ion secondary battery {RECHARGEABLE LITHIUM ION BATTERY}

[Industrial use]

The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having an excellent energy density.

[Prior art]

Lithium secondary batteries, which are in the spotlight as power sources of recent portable small electronic devices, exhibit high energy density by showing a discharge voltage that is twice as high as that of a battery using an alkaline aqueous solution using an organic electrolyte solution.

As a cathode active material of a lithium secondary battery, an oxide composed of lithium and a transition metal having a structure capable of intercalating lithium, such as LiCoO ₂ , LiMn ₂ O ₄ , and LiNi _1-x Co _x O ₂ (0 <X <1) Was mainly used. As the negative electrode active material, crystalline carbon, amorphous carbon or a carbon composite material is used.

The positive electrode and the negative electrode of the lithium secondary battery are prepared by mixing such an active material, a binder, and optionally a conductive material to prepare a slurry type composition, and applying the composition to a current collector. At this time, aluminum is mainly used for the positive electrode as the current collector, and copper is mainly used for the negative electrode.

Recently, studies for further improving the energy density of lithium secondary batteries have been conducted.

An object of the present invention is to provide a lithium ion secondary battery excellent in energy density.

In order to achieve the above object, the present invention is a positive electrode comprising a first current collector and a positive electrode active material layer formed on the first current collector; A negative electrode including a second current collector and a negative electrode active material layer formed on the second current collector; And an electrolyte including a nonaqueous organic solvent and a lithium salt, wherein at least one of the first and second current collectors is polyethylene terephthalate, polyimide, polytetrafluoroethylene, polyethylene naphthalene, And a support polymer film selected from the group consisting of polyvinylidene fluoride and a metal deposited on the support polymer film.

Hereinafter, the present invention will be described in more detail.

The present invention relates to a lithium ion secondary battery in which the current collector used in the production of a pole plate of a lithium ion secondary battery is changed, thereby reducing the weight of the current collector to reduce the overall battery weight and also increasing the energy density per weight.

The current collector of the present invention has a structure in which a metal is deposited on a supporting polymer film, and the supporting polymer includes polyethylene terephthalate, polyimide, polytetrafuloethylene, polyethylene naphthalene or polyvinylidene fluoride. . That is, by using a current collector that previously used about 15 μm thick copper or about 20 μm thick aluminum by depositing a support polymer film and metal on the film very thinly than the existing thickness, the weight of the current collector is reduced. will be.

In the current collector of the present invention, the thickness of the supporting polymer film is preferably 1 to 30 µm, preferably 2 to 25 µm, more preferably 1 to 30 µm, and most preferably 3 to 20 µm. When the thickness of the supporting polymer film is thinner than 1 μm, handling is difficult, and when thicker than 30 μm, the energy density is reduced, which is not preferable.

The support polymer film may further include a silicon-containing release agent layer. The silicon-containing release agent layer is formed on a surface on which no metal is deposited on the supporting polymer film to prevent direct contact between the protective film and the supporting polymer film when manufacturing the electrode plate or when transporting or storing the manufactured electrode plate. Can be.

The silicon-containing release agent layer may be formed of a general coating method such as a roll coating, spray coating, gravure coating using a silicon-containing compound of formula (1).

[Formula 1]

(In Formula 1, R ₁ , R ₂ , R ₃ and R ₄ are linear or branched alkyl, cycloalkyl, alkenyl, aryl, aralkyl, halogenated alkyl, halogenated aryl, halogenated aralkyl, phenyl, mercaptan, respectively. , Methacrylate, acrylate, epoxy or vinyl ether, the alkyl is C ₁ to C ₁₈ , the cycloalkyl is C ₃ to C ₁₈ , the alkenyl is C ₂ to C ₁₈ , the aryl and the aralkyl are C ₆ to C ₁₈ having a carbon number,

n and m may be different or the same as each other, and are an integer from 1 to 100,000.)

The metals are Ni, Ti, Cu, Ag, Au, Pt, Fe, Co, Cr, W, Mo, Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, In and Zn It may include one or two or more selected from the group consisting of.

In the current collector of the present invention, the thickness of the metal deposited on the supporting polymer film is preferably 10 kPa to 10 μm, more preferably 100 kPa to 9 μm, and most preferably 200 kPa to 8 μm. If the thickness of the deposited metal is thinner than 10Å, the metal may not cover all surfaces and some pinholes may occur, and if the thickness of the deposited metal is greater than 10 μm, the energy density may decrease.

The current collector of the present invention can be used for one of the first current collector of the positive electrode and the second current collector of the negative electrode or both of the first current collector and the second current current collector, regardless of the type of the electrode plate. And the use of both the positive electrode plate than the case of using only one of the negative electrode is more excellent energy density-improving effect per weight.

In the lithium ion secondary battery of the present invention, the cathode is preferably a compound capable of reversibly intercalating or deintercalating lithium ions as a cathode active material, and a compound selected from the group consisting of the following Chemical Formulas 2 to 15 as a representative example: Can be used.

[Formula 2]

LiAO ₂

[Formula 3]

LiMn ₂ O ₄

[Formula 4]

Li _a Ni _b B _c M _d O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1)

[Formula 5]

Li _a Ni _b Co _c Mn _d M _e O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1)

[Formula 6]

Li _a AM _b O ₂ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1)

[Formula 7]

Li _a Mn ₂ M _b O ₄ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1)

[Formula 8]

DS ₂

[Formula 9]

LiDS ₂

[Formula 10]

V ₂ O ₅

[Formula 11]

LiV ₂ O ₅

[Formula 12]

LiEO ₂

[Formula 13]

LiNiVO ₄

[Formula 14]

Li _(3-x) F ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 3)

[Formula 15]

Li _(3-x) Fe ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 2)

(In the formula 2 to 15,

A is selected from the group consisting of Co, Ni and Mn,

B is Co or Mn,

D is Ti or Mo,

E is selected from the group consisting of Cr, V, Fe, Sc and Y,

F is selected from the group consisting of V, Cr, M, Co, Ni and Cu,

M is at least one of transition metals or lanthanide metals selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr and V)

In the lithium ion secondary battery of the present invention, the negative electrode may be a carbon-based material capable of occluding / discharging lithium ions as a negative electrode active material, a lithium metal, an alloy of lithium metal, or a material capable of forming a compound with lithium. The material capable of forming an alloy of lithium metal or a compound with lithium may include Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb, Pb, In, or Zn.

The carbon-based material preferably has an crystallite size (Lc) of at least 20 nm by X-ray diffraction and an exothermic peak at 700 ° C. or more. In addition, the carbon-based material is a carbonization step from the crystalline carbon or fibrous mesophase pitch fibers (carbonized step and graphitizing step) from the mesophase (mesophase) spherical particles And fibrous crystalline carbon (graphite fiber) prepared through the graphitization step.

In addition, the negative electrode of the present invention may further include a protective film on the surface of the negative electrode active material layer. The protective film is a single film or a double film or more including a polymer material, an inorganic material or a mixture thereof. The inorganic material is selected from the group consisting of LiPON, Li ₂ CO ₃ , Li ₃ N, Li ₃ PO ₄ and Li ₅ PO ₄ , and the thickness of the protective film including the inorganic material is preferably 10 to 20000 Pa.

The polymeric materials include polyvinylidene fluoride, copolymers of polyvinylidene fluoride and hexafluoropropylene, poly (vinylacetate), poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate), poly (methyl Methacrylate-co-ethyl acrylate), polyacrylonitrile, polyvinyl chloride-co-vinyl acetate, polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinyl acetate), cellulose acetate, polyvinylpi Ralidone, Polyacrylate, Polymethacrylate, Polyolefin, Polyurethane, Polyvinyl ether, Acrylonitrile-butadiene rubber, Styrene-butadiene rubber, Acrylonitrile-butadiene-styrene, Sulfonated styrene / ethylene-butylene One or two or more mixtures selected from the group consisting of styrene triblock polymer and polyethylene oxide, wherein said polymeric material The thickness of the protective film comprising a 100Å to 10㎛ is preferred. When the thickness of the protective film including the polymer material is thinner than 100 kPa, the thickness of the protective film may be too thin, thereby physically damaging the protective film, and when the protective film is thicker than 10 μm, there is a problem of lowering ion conductivity and energy density.

In the lithium ion secondary battery of the present invention, the electrolyte solution contains a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the cell can move. As the non-aqueous organic solvent, benzene, toluene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluoro Robenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1, 2,4-trichlorobenzene, iodobenzene, 1,2-dioodobenzene, 1,3-dioodobenzene, 1,4-dioiobenzene, 1,2,3-triiodobenzene, 1,2 , 4-triiodobenzene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4 -Trichlorotoluene, iodotoluene, 1,2-dioodotoluene, 1,3-dioodotoluene, 1,4-diaodotoluene, 1,2, 3-triiodotoluene, 1,2,4-triiodotoluene, R-CN (wherein R is a straight-chain, branched, or cyclic C2-C50 hydrocarbon group, which is a double bond , Aromatic ring, or ether bond), dimethoxyformamide, methyl acetate, xylene, cyclohexane, tetrahydrofuran, 2-methyltetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol, dimethyl Carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxyethane, 1,3-dioxolane, diglyme, tetraglyme , Ethylene carbonate, propylene carbonate, γ-butyrolactone, and one or more mixed organic solvents selected from the group consisting of sulfolane.

The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium ion secondary battery and to promote the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroazate (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), and lithium trifluoromethanesulfo Nate (CF ₃ SO ₃ Li), lithium bis (trifluoromethyl) sulfonimide (LiN (SO ₂ CF ₃ ) ₂ ) and lithium bis (perfluoroethylsulfonyl) imide (LiN (SO ₂ C ₂₎ One or two or more mixtures selected from the group consisting of F ₅ ) ₂ ) can be used. The lithium salt is preferably present at a concentration of 0.1 to 2.0M. When the concentration of the lithium salt is less than 0.1M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, and when the concentration exceeds 2.0M, there is a problem that the mobility of lithium ions is reduced by increasing the viscosity of the electrolyte.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Comparative Example 1)

A positive electrode active material slurry was prepared by mixing a LiCoO ₂ positive electrode active material, a polyvinylidene fluoride binder, and a super-P conductive material in a composition ratio of 94/3/3 weight ratio in an N-methyl pyrrolidone solvent. The slurry was coated on an aluminum current collector having a thickness of 20 μm, dried and rolled with a rolling mill to prepare a positive electrode.

A negative electrode active material slurry was prepared using a carbon negative electrode active material and a polyvinylidene fluoride binder in a composition ratio of 94/6 weight ratio in an N-methylpyrrolidone solvent. The slurry was coated on a 15 μm thick copper current collector, dried and rolled with a rolling mill to prepare a negative electrode.

Using the prepared positive and negative electrodes, a battery having a specification of 45 mm in height, 37 mm in width and 4.0 mm in thickness, and having a capacity of 650 mAh was manufactured. At this time, a mixed solvent (3/3/4 volume ratio) of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate in which 1.0 M LiPF ₆ was dissolved was used.

(Example 1)

Deposition of 10000 GPa of aluminum on both sides of a polyethylene terephthalate film having a thickness of 15 μm was used as a positive electrode current collector, and 10000 GPa of copper was deposited on both sides of a polyethylene terephthalate film having a thickness of 15 μm. A battery having a specification of 45 mm in height, 37 mm in width and 4.0 mm in thickness and having a capacity of 650 mAh was manufactured in the same manner as in Comparative Example 1 except that the current collector was used.

As a result of comparing the weight of the battery of Example 1 and the battery of Comparative Example 1, the battery of Example 1 was reduced by 15.8% compared to Comparative Example 1.

(Example 2)

Prepared by depositing 5000 μm thick aluminum on both sides of a polyethylene terephthalate film having a thickness of 15 μm, and depositing 5000 μm thick copper on both sides of a polyethylene terephthalate film having a thickness of 15 μm A battery having a capacity of 650 mAh with a specification of 45 mm in height, 37 mm in width and 4.0 mm in thickness was prepared in the same manner as in Comparative Example 1 except that a negative electrode current collector was used.

As a result of comparing the weight of the battery of Example 2 and the battery of Comparative Example 1, the battery of Example 2 was reduced by 16.5% compared to Comparative Example 1.

(Example 3)

A positive electrode current collector manufactured by depositing aluminum at a thickness of 2000 μs on both sides of a polyethylene terephthalate film having a thickness of 15 μm and a negative electrode collector manufactured by depositing copper at a thickness of 2000 μs on both sides of a polyethylene terephthalate film having a thickness of 15 μm. A battery having a specification of 45 mm in height, 37 mm in width and 4.0 mm in thickness and having a capacity of 650 mAh was manufactured in the same manner as in Comparative Example 1 except that it was used as a whole.

As a result of comparing the weight of the battery of Example 3 and the battery of Comparative Example 1, the battery of Example 3 was reduced by 17.0% compared to the battery of Comparative Example 1.

(Example 4)

500 nm thick aluminum was deposited on both sides of a polyethylene terephthalate film having a thickness of 15 μm, and 500 nm thick copper was deposited on both sides of a polyethylene terephthalate film having a thickness of 15 μm. A battery having a capacity of 650 mAh was produced in the same manner as in Comparative Example 1 except that the current collector was used in the same manner as in Comparative Example 1 with a height of 45 mm, a width of 37 mm, and a thickness of 3.9 mm.

As a result of comparing the weight of the battery of Example 4 with the battery of Comparative Example 1, the battery of Example 4 was reduced by 17.2% compared with the battery of Comparative Example 1.

Battery Rating

The battery prepared by the method of Examples 1 to 4 and Comparative Example 1 was 0.2C charge and 0.2C discharge to measure the capacity and energy density per weight, the results are shown in Table 1 below.

Capacity (mAh) Energy Density Per Weight (Wh / kg) % Increase in energy density per weight Comparative Example 1 650 152 - Example 1 650 181 18.8 Example 2 650 182 19.8 Example 3 650 82 20.4 Example 4 650 184 20.7

As shown in Table 1, the battery of Examples 1 to 4 shows the same capacity as the battery of Comparative Example 1, but since the weight is light as described above, it can be seen that the energy density per weight is high. That is, the energy density per weight of the batteries of Examples 1 to 4, in which aluminum and copper were respectively deposited on the polyethylene terephthalate film as the positive electrode current collector and the negative electrode current collector, was increased by 18.8 to 20.7% compared to Comparative Example 1.

The present invention can provide a lithium ion secondary battery having an excellent energy density per weight by using a metal deposited on a supporting polymer film as a current collector.

Claims (24)

A positive electrode including a first current collector and a positive electrode active material layer formed on the first current collector; A negative electrode including a second current collector and a negative electrode active material layer formed on the second current collector; And Electrolyte containing non-aqueous organic solvent and lithium salt As a lithium ion secondary battery containing, At least one of the first and second current collectors is a support polymer film and a support polymer film selected from the group consisting of polyethylene terephthalate, polyimide, polytetrafluoroethylene, polyethylene naphthalene, and polyvinylidene fluoride. Lithium ion secondary battery comprising a deposited metal. The method of claim 1, wherein the metal is Ni, Ti, Cu, Ag, Au, Pt, Fe, Co, Cr, W, Mo, Al, Mg, K, Na, Ca, Sr, Ba, Si, Ge, Sb Lithium ion secondary battery comprising one or two or more selected from the group consisting of, Pb, In and Zn. The lithium ion secondary battery of claim 1, wherein the metal deposited on the supporting polymer film has a thickness of 10 μm to 10 μm. The lithium ion secondary battery of claim 3, wherein the metal deposited on the supporting polymer film has a thickness of 100 μm to 9 μm. The lithium ion secondary battery of claim 4, wherein the metal deposited on the supporting polymer film has a thickness of 200 μm to 8 μm. The lithium ion secondary battery of claim 1, wherein the supporting polymer film has a thickness of 1 to 30 μm. The lithium ion secondary battery of claim 6, wherein the supporting polymer film has a thickness of 2 to 25 μm. The lithium ion secondary battery of claim 7, wherein the supporting polymer film has a thickness of 1 to 30 μm. The lithium ion secondary battery of claim 8, wherein the supporting polymer film has a thickness of 3 to 20 μm. The lithium ion secondary battery of claim 1, wherein the supporting polymer film further comprises a silicon-containing release agent layer. The lithium ion secondary battery of claim 1, wherein the cathode active material has a compound selected from the group consisting of Chemical Formulas 2 to 15 or a surface of the compound. [Formula 2] LiAO ₂ [Formula 3] LiMn ₂ O ₄ [Formula 4] Li _a Ni _b B _c M _d O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1) [Formula 5] Li _a Ni _b Co _c Mn _d M _e O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1) [Formula 6] Li _a AM _b O ₂ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1) [Formula 7] Li _a Mn ₂ M _b O ₄ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1) [Formula 8] DS ₂ [Formula 9] LiDS ₂ [Formula 10] V ₂ O ₅ [Formula 11] LiV ₂ O ₅ [Formula 12] LiEO ₂ [Formula 13] LiNiVO ₄ [Formula 14] Li _(3-x) F ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 3) [Formula 15] Li _(3-x) Fe ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 2) (In the formula 2 to 15, A is selected from the group consisting of Co, Ni and Mn, B is Co or Mn, D is Ti or Mo, E is selected from the group consisting of Cr, V, Fe, Sc and Y, F is selected from the group consisting of V, Cr, M, Co, Ni and Cu, M is at least one of transition metals or lanthanide metals selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr and V) The method of claim 1, wherein the negative electrode active material is selected from the group consisting of a carbon-based material capable of occluding / releasing lithium ions, lithium metal, an alloy of lithium metal and a material capable of forming a compound with lithium Secondary battery. The lithium ion secondary battery of claim 12, wherein the carbon-based material has an Lc (crystallite size) by X-ray diffraction of at least 20 nm and an exothermic peak at 700 ° C. or more. The method of claim 12, wherein the carbon-based material is crystalline carbon or fibrous mesophase pitch prepared from a mesophase spherical particles through a carbonizing step and a graphitizing step (mesophase pitch fiber) Lithium ion secondary battery which is a fibrous graphite (graphite fiber) prepared through a carbonization step and a graphitization step. The method of claim 1, wherein the non-aqueous organic solvent is benzene, toluene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2 , 3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichloro Rhobenzene, 1,2,4-trichlorobenzene, Iodobenzene, 1,2-Diiodobenzene, 1,3-Diiodobenzene, 1,4-Diiodobenzene, 1,2,3-triiodo Benzene, 1,2,4-triiodobenzene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3- Trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-dioodotoluene, 1,3-diaodotoluene, 1,4-dia Dotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, R-CN (wherein R is a linear, branched, or cyclic C 2 -C50 hydrocarbon Group, which may contain a double bond, an aromatic ring, or an ether bond), dimethoxyformamide, methyl acetate, xylene, cyclohexane, tetrahydrofuran, 2-methyltetrahydrofuran, cyclohexanone, Ethanol, isopropyl alcohol, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxyethane, 1,3-dioxolane , One or more mixed electrolytes selected from the group consisting of diglyme, tetraglyme, ethylene carbonate, propylene carbonate, γ-butyrolactone and sulfolane Phosphorus Lithium Ion Secondary Battery. The method of claim 1, wherein the lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroazate (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), lithium tri Fluoromethanesulfonate (CF ₃ SO ₃ Li), lithium bis (trifluoromethyl) sulfonimide (LiN (SO ₂ CF ₃ ) ₂ ) and lithium bis (perfluoroethylsulfonyl) imide (LiN ( SO ₂ C ₂ F ₅ ) ₂ ) Lithium ion secondary battery which is one or a mixture of two or more selected from the group consisting of. The lithium ion secondary battery of claim 1, wherein the lithium salt has a concentration of 0.1 to 2.0 M in the electrolyte. The lithium ion secondary battery of claim 1, wherein the negative electrode active material further includes a protective film on a surface thereof. The lithium ion secondary battery of claim 18, wherein the protective layer is selected from the group consisting of a polymer material, an inorganic material, and a mixture thereof. The lithium ion secondary battery of claim 18, wherein the protective layer is a single layer or multiple layers. The lithium ion secondary battery of claim 19, wherein the inorganic material is selected from the group consisting of LiPON, Li ₂ CO ₃ , Li ₃ N, Li ₃ PO _4, and Li ₅ PO ₄ . The lithium ion secondary battery of claim 19, wherein the protective film including the inorganic material has a thickness of 10 to 20,000 kPa. 20. The method of claim 19 wherein the polymeric material is a copolymer of polyvinylidene fluoride, polyvinylidene fluoride and hexafluoropropylene, poly (vinylacetate), poly (vinyl butyral-co-vinyl alcohol-co-vinyl Acetate), poly (methylmethacrylate-co-ethyl acrylate), polyacrylonitrile, polyvinyl chloride-co-vinyl acetate, polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinyl acetate), Cellulose acetate, polyvinylpyrrolidone, polyacrylate, polymethacrylate, polyolefin, polyurethane, polyvinyl ether, acrylonitrile-butadiene rubber, styrene-butadiene rubber, acrylonitrile-butadiene-styrene, sulfonated Li, a mixture of one or two or more selected from the group consisting of styrene / ethylene-butylene / styrene triblock polymer and polyethylene oxide Ion secondary battery. The lithium ion secondary battery of claim 19, wherein the protective film including the polymer has a thickness of 100 μm to 10 μm.