ELECTROLYTE FOR RE+RECHARGEABLE LITHIUM BATTERY AND RE+RECHARGEABLE LITHIUM BATTERY CONTAINING THE SAME

This application is based on and claims priority of Korean Patent Application Nos. 2003-57716 and 2004-5874 filed in the Korean Intellectual Property Office on August 20, 2003 and January 29, 2004, the entire disclosures of which are incorporated hereinto by reference.

The present invention relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery comprising the same, and more particularly, to an electrolyte for a rechargeable lithium battery exhibiting improved storage characteristics and suppression of swelling at high temperatures, and a rechargeable lithium battery comprising the same.

Recently, the rapid development of smaller, lighter, and higher performance electronic and communication equipment has required the development of high performance and large capacity batteries to power such equipment. The demands for large capacity batteries have led to investigations into rechargeable lithium batteries. Positive active materials for rechargeable lithium batteries generally use lithium-transition metal oxides, and negative active materials generally use crystalline or amorphous carbonaceous materials or carbon composites. The active materials are coated on a current collector with a predetermined thickness and length, or they are formed as a film to produce electrodes. The electrodes together with a separator are wound to produce an electrode element, and the electrode element is inserted into a battery case such as a can followed by insertion of an electrolyte to fabricate a battery.

The electrolyte includes lithium salts and organic solvents. The organic solvents may be mixed solvents of between two and five components of cyclic carbonates such as ethylene carbonate or propylene carbonate, or linear carbonates such as dimethyl carbonate, ethylmethyl carbonate, or diethyl carbonate. However, these solvents are known to severely expand at high temperatures, causing a swelling phenomenon. The swelling phenomenon is partly manifested in a battery by gas generated due to decomposition of the electrolyte at high temperatures in the battery.

Such a swelling phenomenon can be reduced in lithium polymer batteries compared to lithium ion batteries. However, the use of a polymer electrolyte with a nickel-based positive active material (e.g. LiNiMO₂, where M is selected from Co, Mn, Al, P, Fe or Mg) has generally been ineffective.

Several attempts to use solvents with a high boiling point and a high dielectric constant, such as γ-butyrolactone, have been promising. Conventionally, the high dielectric constant solvent is generally used together with ethylene carbonate, which results in an extremely high viscosity with poor wettability of the separator. In another attempt, solvents with low boiling points and low dielectric constants have been used. However, these attempts still have problems associated with high swelling (Japanese Patent Laid-Open No. 2000-235868, U.S. Patent Nos. 5,079,109, 5,272,022, 5,552,243, 5,521,027, 6,117,596, and 5,851,693, and "New thin lithium-ion batteries using a liquid electrolyte with thermal stability" Journal of power sources, 97-98, 677-680(2001), Notio Takami et al.)

Other attempts to inhibit the swelling phenomenon are in U.S. Patent No. 4,830,939 disclosing a liquid electrolyte containing a polyethylenicially unsaturated monomeric material or a prepolymeric material, and U.S. Patent No. 4,866,716 disclosing a cross-linked polyether which is a product of a vinyl-ether. In addition, U.S. Patent No. 4,970,012 discloses that a polymeric solid electrolyte includes crosslinked molecules of a radiation-cured substance of a cinnamate ester and polyethene oxide, and U.S. Patent No. 4,908,283 discloses that a polymeric electrolyte includes a cured product of an acryloyl-denaturated polyalkylene oxide.

Especially, such a swelling phenomenon severely occurs in batteries with a mixture of a lithium cobalt-based compound and a lithium nickel-based compound which exhibits higher capacity than other compounds.

In one embodiment of the invention an electrolyte is provided for a rechargeable lithium battery which is capable of inhibiting high-temperature swelling.

In another embodiment of the invention, a rechargeable lithium battery is provided which includes the electrolyte.

These and other aspects may be achieved by an electrolyte for a rechargeable lithium battery including a non-aqueous organic solvent and a lithium salt. The non-aqueous organic solvent includes an ester-based solvent of 70 to 90 volume %, the ester-based solvent including a cyclic ester and a nitrile-based solvent represented by formula 1 of 5 to 30 volume %:R-C ≡ N where R is a C₁ to C₁₀ aliphatic hydrocarbon or halogenated aliphatic hydrocarbon, or a C₆ to C₁₀ aromatic hydrocarbon or halogenated aromatic hydrocarbon.

In yet another embodiment of the invention, a rechargeable lithium battery is provided including the electrolyte, a positive electrode and a negative electrode. The positive electrode and the negative electrode include active material in which lithium intercalation reversibly occurs. Preferably, the positive active material is a nickel-included compound.

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

1. FIG. 1 is a schematic view showing a rechargeable lithium battery according to the present invention; and
2. FIG. 2 is a graph illustrating cycle life characteristics of the cells according to Examples 7 to 10 of the present invention and Comparative Example 1.

The present invention relates to an electrolyte including a nitrile-based solvent for improving swelling and battery performance. The electrolyte of the present invention includes non-aqueous organic solvents and a lithium salt. The non-aqueous organic solvents include an ester-based solvent including a cyclic ester and a nitrile-based solvent.

The nitrile-based solvent is represented by formula 1:R-C≡N    where R is a C₁ to C₁₀ and or a C₆ to C₁₀ aromatic hydrocarbon or halogenated aromatic hydrocarbon, preferably a C₃ to C₈ aliphatic hydrocarbon or halogenated aliphatic hydrocarbon, and more preferably a C₆ to C₈ aliphatic hydrocarbon or halogenated aliphatic hydrocarbon. Higher alkyl groups with a higher number of carbons are preferable because they have increases boiling points, so that stability is improved, and the decomposition on the aliphatic hydrocarbons rarely occurs compared to the aromatic hydrocarbons. If the R is an unsaturated hydrocarbon, e.g. methacrylate, it can be used as a solvent for an electrolyte.

Examples of the nitrile-based solvent include acetonitrile, propionitrile, butyronitrile, t-butyl cyanide, valeronitrile, caprylonitrile, heptyl cyanide, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, 2-chlorobenzonitrile, 4-chlorobenzonitrile, dichlorobenzonitrile, trichlorobenzonitrile, 2-chloro-4-fluorobenzonitrile, 4-chloro-2-fluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluoroacetonitrile.

A low viscosity and high dielectric constant of the nitrile-based compound can repress the swelling phenomenon.

The nitrile-based solvent is preferably presented in the amount of 5 to 30 volume %, and more preferably 15 to 25 volume %. An amount of less than 5 volume % of the nitrile-based cannot achieve the desired swelling inhibition. An amount of more than 30 volume % deteriorates the battery performance. That is, the effect of the present invention cannot be obtained from an amount of more than 30 volume %, and it is predicted that Japanese Patent Laid-Open No. 2000-124077 using 60 volume % or more of acetonitrile causes the deterioration of the battery performances and problems associated with safety. Such problems are predicted in U.S. Patent No. 6,190,804 in which nitrile is only used as a solvent for preparing a solid electrolyte, and the amount of the nitrile used is not disclosed.

The electrolyte of the present invention includes an ester-based ester of a cyclic ester in an amount of 70 to 95 volume %. The cyclic ester preferably includes 10 to 40 volume % of ethylene carbonate, and more preferably 10 to 15 volume % with respect to the electrolyte. An amount of more than 40 volume % of the ethylene carbonate cannot achieve the desired swelling inhibition. An amount of less than 10 volume % deteriorates the battery performance. Thus, it is predicted that the effect of the use of the ethylene carbonate of the present invention cannot be obtained from Japanese Patent Laid-Open No. Hei. 7-320748 disclosing ethylene carbonate at 25 to 95 volume %.

The remaining amount, 30 to 85 volume %, and preferably 55 to 85 volume % with respect to the electrolyte, may be propylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone , γ-caprolactone, δ-valerolactone, ε-valerolactone, or a mixture thereof.

The electrolyte of the present invention may further include a linear ester. The amount of the linear ester is preferably between 0 parts by volume and 70 parts by volume based on 100 parts by volume of the total cyclic ester and the nitrile-based solvents. If the amount of the linear ester is more than 70 parts by volume, swelling occurs.

The linear ester preferably includes at least one compound selected from dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, methyl acetate, ethyl acetate, methyl hexanoate, methyl formate, and mixtures thereof.

The electrolyte of the present invention may further include carbonate-based additives with a substitution group selected from halogens, a cyano (CN), or a nitro (NO₂), and additives such as vinylene carbonate, divinylsulfone, or ethylene sulfite. The additives help to improve the battery performance such as through inhibition of high-temperature swelling, and by increasing capacity, cycle life, and low-temperature characteristics. The carbonate-based additive is preferably an ethylene carbonate derivative represented by the following formula 5, and is more preferably fluoroethylene carbonate. where X is selected from the group consisting of halogens, a cyano (CN) or a nitro (NO₂).

The amount of the carbonate-based additive is 0.01 to 10 parts by weight based on 100 parts by weight of the total weight of the electrolyte, and preferably 0.01 to 5 parts by weight. A carbonate-based additive of less than 0.01 parts by weight cannot effectively suppress gas generation, and that of more than 10 parts by weight deteriorates high-temperature cycle life characteristics and causes swelling to occur.

The lithium salt acts as a source for supplying lithium ions in the battery, and helps the working of the battery. Examples of the lithium salts are LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiAlO₄, LiAlCl₄, LiN(C_xF_2x+1SO₂)(Cy_F2+1SO₂) where x and y are natural numbers, LiSO₃CF₃, and mixtures thereof.

The concentration of the lithium salt is 0.6 to 2.0M, and preferably 0.7 to 1.6M. If the concentration of the lithium salt is less than 0.6M, the viscosity of the electrolyte decreases, thereby deteriorating electrolyte performance. If that of the lithium salt if more than 2.0M, the viscosity increases, thereby reducing transfer of lithium ions.

The electrolyte of the present invention is present in a form of liquid electrolyte, or as a solid polymeric electrolyte. The solid polymeric electrolyte is obtained from a composition for preparing a polymer electrolyte by addition of monomer and a polymeric initiator to the liquid electrolyte, i.e. electrolytic solution.

The monomer preferably includes a mixture of a first monomer having at least two functional groups with an unsaturated bond represented by formulas 2, 3, or 4 at a terminal end and having a molecular weight of 50 to 100,000, or a mixture of the first monomer and a second monomer having at least one functional group represented by formulas 2, 3, or 4 and having a molecular weight of 50 to 100,000.(R₁)(R₂)C=C(R₃)-C(=O)-(R₁)(R₂)C=C(R₃)-(R₁)(R₂)C=C(R₃)-CH₂-    (where, R₁, R₂ and R₃ are the same or are independently selected from H, a C₂ to C₁₀ aliphatic or aromatic hydrocarbon, -C≡ N, or -OR₅, where R₅ is H, CH₃, C₂H₅, - F, -Cl or -Br.)

The monomer is preferably presented in an amount of 0.01 to 20 wt% in the composition, and more preferably 0.1 to 10 wt%. An amount of less than 0.01 wt% of the monomer occurs an extreme swelling phenomenon, and an amount of more than 20 wt% deteriorates battery performance.

The example of the monomer may be poly(ethylene glycol)di(meth)acrylate, poly(propylene glycol)di(meth)acrylate, polyesterpolyol di(methacrylate), polycarbonatepolyol diacrylate, polycaprolactonediol di(meth)acrylate, trimethylolpropane ethoxylated tri(meth)acrylate, trimethylolpropane propooxylated tri(meth)acrylate, trimethylolpropane caprolactonated tri(meth)acrylate, tetramethylolpropane ethoxylated tri(meth)acrylate, tetramethylolpropane propooxylated tri(meth)acrylate, tetramethylolpropane caprolactonated tri(meth)acrylate, ditrimethylolpropane ethoxylated tri(meth)acrylate, ditrimethylolpropane propoxylated tri(meth)acrylate, ditrimethylolpropane caprolactonated tri(meth)acrylate, dipentaerythritol ethoxylated di(meth)acrylate, dipentaerythritol propoxylated di(meth)acrylate, dipentaerythritol caprolactonated di(meth)acrylate, glycerol ethoxylated di(meth)acrylate, glycerol propoxylated di(meth)acrylate, or dipentaerythritol caprolactonated hexacrylate. Alternatively, the example of the monomer may be a monomer with vinyl group, allyl group or vinylsulfone group at a terminal site thereof, or urethane (meth)acrylate monomer.

The polymerization initiator may be one that can initiate polymerization of the monomer and does not cause deterioration of the battery performance. The exemplary are one or at least two selected from organic peroxides and azo-based compounds. The organic peroxides may be peroxydicarbonates such as di(4-t-butylcyclohexyl) peroxydicarbonate, di-2-ethylhexyl peroxy dicarbonate, di-isopropyl peroxydicarbonate, di-3-methoxy butyl peroxy dicarbonate, t-butyl peroxy isopropyl carbonate, t-butyl peroxy 2-ethylhexyl carbonate, 1,6-bis(t-butyl peroxycarbonyloxy)hexane, or diethylene glycol-bis(t-butyl peroxy carbonate); diacyl peroxides such as diacetyl peroxide, dibenzoyl peroxide, dilauroyl peroxide, bis-3,5,5-trimethyl hexanoyl peroxide; or peroxy esters such as t-butyl peroxy pivalate, t-amyl peroxy pivalate, t-butyl peroxy-2-ethyl hexanoate, t-hexylperoxy pivalate, t-butyl peroxy neodecanoate, t-butyl peroxy neoheptanoate, t-hexylperoxy pivalate, 1,1,3,3-tetramethylbutyl peroxy neodecarbonate, 1,1,3,3-tetramethyl butyl 2-ethylhexanoate, t-amyl peroxy 2-ethyl hexanoate, t-butyl peroxy isobutyrate, t-amylperoxy 3,5,5-trimethyl hexanoyl, t-butyl peroxy 3,5,5-trimethyl hexanoate, t-butyl peroxy acetate, t-butyl peroxy benzoate, or di-butylperoxy trimethyl adipate. The azo-based compound may be 2,2'-azo-bis(2,4-dimethylvaleronitrile) or 1,1'-azo-bis(cyanocyclo-hexane).

The polymerization initiator is presented in an amount sufficient to initiate polymerization of the monomer, and is suitably presented in the amount of 0.01 to 5 wt%.

A polymer electrolyte may be produced by using the polymer electrolyte composition as in the following various procedures. One method is that the polymer electrolyte composition is injected into a battery case such as a metal can or a pouch in which a positive electrode, a separator, and a negative electrode are placed, which is then heated at 40 to 100°C for 30 minutes to 8 hours, thereby hardening (polymerizing) the polymer electrolyte composition to produce the polymer electrolyte. Another method is that the polymer electrolyte composition is coated on a positive or a negative electrode, and heat, ultraviolet rays, or electron beams are irradiated into the electrode to coat the polymer electrolyte on the surface of the positive or the negative electrode. The produced electrode is inserted into a battery case and sealed to fabricate a battery. A separator may be additionally used, or alternatively the polymer electrolyte also acts as the separator, so the separator may be not used.

A rechargeable lithium battery including the electrolyte of the present invention includes a positive electrode and a negative electrode.

The positive electrode includes a positive active material in which lithium intercalation reversibly occurs. The exemplary of the positive active material are lithiated intercalation compounds and preferably a nickel-based lithiated intercalation compound because of its higher capacity. More preferably, in order to achieve high capacity and other battery performance improvements, a mixture of the nickel-based lithiated intercalation compound and a cobalt-based compound or manganese-based compound are used.

Especially, the swelling phenomenon severely occurs because of the nickel-based compound, so the effect of using the electrolyte of the present invention can be maximized when the nickel-based compound is used. The nickel-based compound may be one selected from the group consisting of compounds represented by formulas 6 and 7.Li_xNi_yM_1-yA₂Li_xNi_yM_1-yO_2-zX_z    where 0.90 ≤ x ≤1.1, 0.1 ≤y ≤0.9, and 0 ≤z ≤ 0.5; M is at least one selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare earth element; A is selected from O, F, S, and P; and X is F, S, or P.

The cobalt-based or the manganese-based compound is any one used as an active material, and the exemplary may be one selected from the group consisting of compounds represented by formulas 8 to 12.Li_xMn_1-yM_yA₂Li_xMn_1-yM_yO_2-zX_zLi_xMn₂O_4-zX_zLi_xCo_1-yM_yA₂Li_xCo_1-yM_yO_2-zX_z    where 0.90 ≤ x≤1.1,0≤y≤ 0.5, 0 ≤ z ≤ 0.5, and 0 ≤ α ≤ 2; M is at least one selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare earth elements; A is selected from O, F, S, and P, X is F, S, or P.

The negative electrode includes a negative active material in which lithium intercalation reversibly occurs and the exemplary are crystalline or amorphous carbon, or a carbon composite.

The positive and the negative electrode are respectively produced by mixing the active material, a conductive agent, and a binder in a solvent to prepare an active material composition, and coating the composition on a current collector. The electrode preparation is well known in the related art, and is easily understood by one of ordinary skill in the art.

The conductive agent includes any conventional conductive agent used for an active material composition as long as it is chemically inert and has electrical conductivity. Examples thereof are one or a mixture selected from natural graphite, artificial graphite, carbon black, acetylene black, ketjen black; carbon fiber, and a metal fiber such as copper, nickel, aluminum, and silver.

The binder includes any conventional binder used for an active material composition as long as it firmly adheres to the active material and the conductive agent on the current collector, and the binder may be styrene-butadiene rubber, polyvinyl alcohol, carboxymethylcellulose, hydroxylpropylenecellulose, diacetylenecellulose, polyvinylchloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene. Most preferred are styrene-butadiene rubber.

The solvent includes any conventional solvent used for an active material composition as long as it well distributes the active material, the conductive agent, and the binder. The example of the solvent may be N-methyl pyrrolidone.

One embodiment of the lithium rechargeable battery according to the present invention is shown in FIG. 1. The rechargeable lithium battery 1 includes a positive electrode 3; a negative electrode 2; a separator 4 interposed between the positive electrode 3 and the negative electrode 2; an electrolyte in which the positive electrode 2, the negative electrode 3, and the separator 4 are immersed; a cylindrical battery case 5; and a sealing portion 6. The configuration of the rechargeable lithium battery is not limited to the structure shown in FIG. 1, as it can be readily modified into a prismatic or pouch type battery as is well understood in the related art.

The positive electrode includes a positive active material in which lithium intercalation reversibly occurs. Examples of positive active materials are lithium transition metal oxides such as LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, or LiNi_1-x-yCo_xM_yO₂ where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1, and M is metal such as Al, Sr, Mg, or La.

The negative electrode includes a negative active material in which lithium intercalation reversibly occurs. Examples of negative active materials are crystalline or amorphous carbonaceous material, or carbon composites.

The positive active material and the negative active material are respectively coated on a current collector to produce electrodes, and the electrodes are wound together with or laminated on a separator to produce an electrode element. The electrode element is inserted into a battery case such as a can, and an electrolyte is injected into the case to fabricate a rechargeable lithium battery. The separator may be resin such as polyethylene or polypropylene. ]

The following Examples further illustrate the present invention in detail, but are not to be construed to limit the scope thereof.

1.5M LiBF₄ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and valeronitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.5M LiBF₄ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and heptanenitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.5M LiBF₄ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and capronitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and valeronitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and valeronitrile in the volume ratio of 3 : 5 : 2 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and valeronitrile in the volume ratio of 3 : 5 : 2 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, γ-butyrolactone, and valeronitrile in the volume ratio of 3 : 5 : 2 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, and valeronitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate, and valeronitrile in the volume ratio of 3 : 5 : 2 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

1.15M LiPF₆ was dissolved in a mixed solvent of ethylene carbonate, diethyl carbonate, and valeronitrile in the volume ratio of 3 : 6 : 1 to prepare an electrolyte.

Using the electrolyte, a LiCoO₂ positive electrode, and a graphite negative electrode, a rechargeable lithium cell was fabricated. The amount of the electrolyte was 2.2g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 1, except that a positive active material was produced by mixing LiNi_0.8Co_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 2, except that a positive active material was produced by mixing LiNi_0.8BCo_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1 g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 3, except that a positive active material was produced by mixing LiNi_0.8Co_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1 g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 4, except that a positive active material was produced by mixing LiNi_0.8Co_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 5, except that a positive active material was produced by mixing LiNi_0.8Co_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1 g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 6 except that a positive active material was produced by mixing LiNi_0.8Co_0.1Mn_0.1O₂ with LiCoO₂ at a weight ratio of 8 : 2, and the amount of the electrolyte was 2.1 g.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 3 : 5 : 2.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, diethyl carbonate and valeronitrile at a volume ratio of 30 : 55 : 15.

A rechargeable lithium cell was fabricated by the same procedure as in Example 1, except that a mixed lithium salt of LiPF₆ and LiBF₄ at a weight ratio of 99.9 : 0.1 was used.

A rechargeable lithium cell was fabricated by the same procedure as in Example 7, except that a mixed lithium salt of LiPF₆ and LiBF₄ at a weight ratio of 99.9 : 0.1 was used.

A rechargeable lithium cell was fabricated by the same procedure as in Example 1, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, propylene carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 15.

A rechargeable lithium cell was fabricated by the same procedure as in Example 1, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 15.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.5M LiBF₄ in a mixed solvent of ethylene carbonate, γ-butyrolactone, and diethyl carbonate at a volume ratio of 4 : 4 : 2.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.5M LiBF₄ in a mixed solvent of ethylene carbonate, γ-butyrolactone, and fluorobenzene at a volume ratio of 30 : 55 : 15.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, propylene carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 5 : 10.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.15M LiPF₆ in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, and fluorobenzene at a volume ratio of 3 : 6 : 1.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.5M LiBF₄ in a mixed solvent of ethylene carbonate, γ-butyrolactone, and diethyl carbonate at a volume ratio of 4 : 4 : 2.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.5M LiBF₄ in a mixed solvent of ethylene carbonate, γ-butyrolactone, fluorobenzene, and diethyl carbonate at a volume ratio of 3 : 5 : 1 : 1.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.3M LiPF₆ in a mixed solvent of ethylene carbonate, ethylmethyl carbonate, propylene carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 5 : 15.

A rechargeable lithium cell was fabricated by the same procedure as in Example 11, except that an electrolyte was prepared by dissolving 1.3M LiPF₆ in a mixed solvent of ethylene carbonate, and ethylmethyl carbonate at a volume ratio of 3:7.

The lithium cells according to Examples 1 to 12 and Comparative Examples 1 to 8 were constant-current and constant-voltage charged at a 0.5C rate and a cut-off voltage of 4.2V and 20mAh, and the charged cells were allowed to stand in an oven of 85°C for 4 hours. Thereafter, the thicknesses of the cells were measured. The increases in the thickness in comparison to the initial charged cells are shown in Table 1.

It is evident from Table 1 that the increase in thickness of the cells according to Examples 1 to 20 was reduced compared to those of the cells according to Comparative Example 1 to 10. These results indicate that swelling was suppressed in the cells according to Examples 1 to 20 compared with Comparative Examples 1 to 10. In particular, the increases in thickness of the cells according to Examples 4 and 5 were substantially reduced in comparison to Comparative Example 1 and 2, even though the cells according to Examples 4 and 5 and Comparative Examples 1 and 2 included ethylene carbonate and ethylmethyl carbonate.

The cells according to Examples 7 to 10 and Comparative Example 1 were charged under constant current and constant voltage at a 1 C rate and a cut-off voltage of 4.2V and 0.1C (82 mAh), and discharged at constant current to a cut-off voltage of 3V. The cycle life characteristics were measured and the results are shown in FIG. 2. As shown in FIG. 2, the cells according to Examples 7 to 10 exhibited cycle life characteristics corresponding to that of Comparative Example 1. Thus, the electrolyte of the present invention can effectively improve storage characteristics and suppress swell at high temperatures without deterioration of battery performance in areas such as in capacity, high-rate, low-temperature, and cycle life characteristics.

1.5 wt% of a Dipentaerythritol caprolactonated hexaacrylate monomer (Nippon Kayaku) and di(4-t-butylcyalohexyl peroxydicarbonate) ("Perkadox 16", AKZO NOBEL) were added to 98.5 wt% of an electrolytic solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile (3 : 5 : 2 volume ratio) and then mixed for 10 minutes to prepare a composition for preparing a polymer electrolyte. At this time, the amount of the initiator was 5 wt% based on the amount of the monomer.

The positive electrode included a mixed positive active material of LiNi_0.8Co_0.1Mn_0.1O₂ and LiCoO₂ at a weight ratio of 6 : 4, a carbon black conductive agent, and a polyvinylidene fluoride binder and the negative electrode included a natural graphite negative active material and a styrene-butadiene rubber binder. The positive electrode, the negative electrode and a separator were wounded to produce a jelly-roll, and the jelly-roll was inserted in to an Al pouch case followed by partially sealing. The composition was injected into the case and it was completely sealed. The amount of the composition was 2.62g. The sealed case was allowed to stand at 70°C for 4 hours to fabricate a rechargeable lithium cell with a gel polymer electrolyte.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that heptane nitrile was used instead of valeronitrile.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that caprilonitrile was used instead of valeronitrile.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that cyclohexane carbonitrile was used instead of valeronitrile.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that 2-fluorobenzonitrilee was used instead of valeronitrile.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21 except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 65 : 5 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 40 : 30 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 55 : 15 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 45 : 25 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 15 : 65 : 20 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, γ-butyrolactone, and valeronitrile at a volume ratio of 30 : 50 : 20 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, γ-butyrolactone, and valeronitrile at a volume ratio of 15 : 65 : 20 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that fluoroethylene carbonate was added to the electrolytic solution in an amount of 3% based on the electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, γ-butyrolactone, diethyl carbonate, and valeronitrile at a volume ratio of 15 : 40 : 30 : 15 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in ethylene carbonate, ethylmethyl carbonate, propylene carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 5 :10 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that poly(ethylene glycol) dimethacrylate was used as a compound for forming a polymer.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in acetonitrile and ethylene carbonate at a volume ratio of 7 : 3 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3M LiPF₆ in acetonitrile and ethylene carbonate at a volume ratio of 1 : 1 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed solution of 1.3 LiPF₆ in acetonitrile, ethylene carbonate, and diethyl carbonate at a volume ratio of 40 : 30 : 30 was used as an electrolytic solution.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21, except that a mixed positive active material of LiNi_0.8Co_0.15Mn_0.05O₂ and LiCoO₂ was used as a positive active material.

A rechargeable lithium cell was fabricated by the same procedure as in Example 21 except that a mixed positive active material of LiNi_0.75Co_0.2Mn_0.05O₂ and LiCoO₂ was used as a positive active material.

The cells according to Examples 21 to 36 and Comparative Examples 10 to 14 were constant-current and constant-voltage charged at 0.5C to 4.2V and a cut-off time of 3 hours, and constant-current discharged at 0.2C and a cut-off voltage of 2.75V. The capacity was measured and the results are presented in Table 2.

The cells according to Examples 21 to 36 and Comparative Examples 10 to 16 were constant-current charge and constant-voltage charged at 0.5C to 4.2V and a cut-off condition of 0.1C. The charged cells were allowed to stand at 85°C in a hot-wind oven for 4 hours, and thickness thereof were measured. The increases in thickness in comparison to the initial charged cell were measured and the results are presented in Table 2.

The cells according to Examples 21 to 36 and Comparative Examples 10 to 14 were charged 500 times at 1C, and the cycle life characteristics (retention capacity %) were measured. The results are shown in Table 2.

As shown in Table 2, the cells according to Examples 21 to 36 exhibited corresponding capacities to those according to Comparative Examples 10 to 14, but exhibited good cycle life characteristics, and excellent swelling inhibition. Hence, the cells according to Examples 21 to 35 exhibit improved safety while the capacity and the cycle life characteristics are maintained.

1.15M LiPF₆ was dissolved in a mixed solution of ethylene carbonate, diethyl carbonate and valeronitrile at a volume ratio of 30 : 50 : 20 to prepare an electrolyte.

The positive electrode included a mixed positive active material of LiNi_0.8Co_0.1Mn_0.1O₂ and LiCoO₂ at a weight ratio of 6 : 4, a carbon black conductive agent, and a polyvinylidene fluoride binder, and the negative electrode included a natural graphite negative active material and a styrene-butadiene rubber binder. Using the positive electrode, the negative electrode, and the electrolyte, a lithium cell was fabricated.

A lithium cell was fabricated by the same procedure as in Example 37, except that heptane nitrile was used instead of valeronitrile.

A lithium cell was fabricated by the same procedure as in Example 37, except that caprilonitrile was used instead of valeronitrile.

A lithium cell was fabricated by the same procedure as in Example 37, except that cyclohexane carbonitrile was used instead of valeronitrile.

A lithium cell was fabricated by the same procedure as in Example 37, except that 2-fluorobenzonitirlee was used instead of valeronitrile.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 40 : 30 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 55 : 15 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of ethylene carbonate, diethyl carbonate, and valeronitrile at a volume ratio of 30 : 45 : 25 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of ethylene carbonate, ethylmethyl carbonate, propylene carbonate, and fluorobenzene at a volume ratio of 30 : 55 : 10 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of acetonitrile and ethylene carbonate at a volume ratio of 7 : 3 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of acetonitrile and ethylene carbonate at a volume ratio of 1 : 1 to prepare an electrolyte.

A lithium cell was fabricated by the same procedure as in Example 37, except that 1.15M LiPF₆ was dissolved in a mixed solution of acetonitrile, ethylene carbonate, and diethyl carbonate at a volume ratio of 40 : 30 : 30 to prepare an electrolyte.

Capacity, cycle life characteristics and swelling characteristics tests were performed on the cells according to Examples 37 to 44 and Comparative Examples 16 to 19. The results are presented in Table 3.

1 wt% of a Dipentaerythritol caprolactonated hexacrylate derivative monomer (Nippon Kayaku) and a di(4-t-butylcyalohexyl peroxydicarbonate ("Perkadox 16", AKZO NOBEL) were added to 98.5 wt% of an electrolytic solution of 1.3M LiPF₆ in ethylene carbonate, γ-butyrolactone, diethyl carbonate, and valeronitrile (15 : 55 : 20 :10 volume ratio) and then mixed for 10 minutes to prepare a composition for preparing a polymer electrolyte. At this time, the amount of the initiator was 3 wt% based on the amount of the monomer.

The positive electrode included a LiCoO₂ positive active material, a carbon black conductive agent, and a polyvinylidene fluoride binder, and the negative electrode included a natural graphite negative active material and a styrene-butadiene rubber binder. The positive electrode, the negative electrode, and a separator were wounded to produce a jelly-roll, and the jelly-roll was inserted in to an Al pouch case followed by partially sealing. The composition was injected into the case and it was completely sealed. The amount of the composition was 2.62g. The sealed case was allowed to stand at 70°C for 4 hours to fabricate a rechargeable lithium cell with a gel polymer electrolyte.

The capacity recovery step was performed on the cell according to Example 45. The capacity recovery step included a first step including charging at 500mA to 4.2V under a cut-off condition of 50mA, a first discharging at 300mA and a cut-off voltage of 3.00V, a second discharging at 2mA and a cut-off voltage of 2.75V, and a third discharging at 1 mA, and a cut-off voltage of 0.00V, and allowing to stand for 60 minutes; and then a second step including a first charging at 500mA and a cut-off voltage of 3V, a second charging at 500mA and 4.2V under a cut-off condition of 50mA, and then discharging at 300mA to a cut-off voltage of 3V.

The capacity recovery step was referred to as 1 cycle, it was repeated three times. The initial capacity and the discharge capacity for each cycle were measured and then the recovery capacity percentage was obtained from the capacity after each cycle with respect to the initial capacity. The initial capacity was a capacity after the cell was charged at 500mA and 4.2V under a cut-off condition of 50mA, and first discharged at 300mA under a cut-off voltage of 3.0V. The measured initial capacity was 895mAh. When the initial capacity of 895mAh was 100%, a capacity at 1 cycle was 834mAh, that is, 97%, at 2 cycles, 832mAh, 97%, and at 3 cycles, 839mAh, 98%. These results indicate good capacity recovery.

While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.