WO2006016733A1

WO2006016733A1 - Nonaqueous electrolyte for lithium secondary batteries, and lithium secondary batteries comprising the same

Info

Publication number: WO2006016733A1
Application number: PCT/KR2004/002727
Authority: WO
Inventors: Jong Ho Jeon; Ho Seok Yang; Jong Seob Kim
Original assignee: Cheil Industries Inc.
Priority date: 2004-08-10
Filing date: 2004-10-27
Publication date: 2006-02-16
Also published as: TWI254474B; TW200607134A; KR20060014280A

Abstract

Disclosed herein is electrolyte for lithium secondary batteries comprising an additive capable of forming a passivation layer on the surface of an anode by oxidative degradation before the electrolyte is oxidatively degraded on the anode surface, thereby inhibiting degradation of the electrolyte. Since the nonaqueous electrolyte can inhibit an increase in the thickness of a battery even when the battery is stored at high temperature in a fully charged state without adversely affecting the characteristics of the battery, improved reliability can be ensured upon battery set mounting. In addition, the nonaqueous electrolyte can prevent thermal runaway upon being overcharged, and can improve flame retardance, ensuring good safety of lithium secondary batteries.

Description

NONAQUEOUS ELECTROLYTE FOR LITHIUM SECONDARY BATTERIES, AND LITHIUM SECONDARY

BATTERIES COMPRISING THE SAME

Technical Field

[1] The present invention relates to electrolyte for lithium secondary batteries, and a lithium secondary battery comprising the nonaqueous electrolyte. More specifically, the present invention relates to electrolyte for lithium secondary batteries comprising an additive capable of forming a passivation layer on the surface of an anode by oxidative degradation before the electrolyte is oxidatively degraded on the anode surface, thereby inhibiting degradation of the electrolyte, and a lithium secondary battery comprising the nonaqueous electrolyte. Background Art

[2] As electronic appliances have been rendered small and lightweight due to recent de¬ velopment of high-technology electronic industries, portable electronic devices have been increasingly used. As a power supplier for these devices, there has been increased demand for lithium secondary batteries having high energy density. In this connection, a number of studies on lithium secondary batteries have been actively undertaken.

[3] Lithium metal oxides are used as cathode active materials of lithium secondary batteries, and lithium metals, lithium alloys, (crystalline or amorphous) carbons and carbon composites are currently used as anode active materials of lithium secondary batteries.

[4] Lithium secondary batteries are classified into lithium ion batteries, lithium ion polymer batteries, and lithium polymer batteries, depending on the kind of separators and electrolytes used. Further, lithium secondary batteries are classified into cylindrical, square-type and coin-type batteries aαoording to their form.

[5] Lithium secondary batteries have an average discharge voltage of about 3.6V to about 3.7V, and thus provide high power as compared to other alkaline batteries, Ni¬ MH batteries, Ni-Cd batteries, and the like. However, in order to provide such a high driving voltage, electrolyte compositions for lithium secondary batteries are required to have excellent electrochemical stability in a charge/discharge voltage range of 0-4.2V. For this reason, mixtures of nonaqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate and diethyl carbonate, are currently used as electrolytes. However, since these nonaqueous electrolytes for lithium secondary batteries have a problem of considerably low ionic conductivity as compared to aqueous electrolytes employed in Ni-MH and Ni-Cd batteries, the performance of batteries is deteriorated during high-rate charging and discharging. [6] Lithium ions liberated from a lithium metal oxide cathode migrate to a carbon anode upon initial charging of a lithium secondary battery, and are intercalated into the carbon. At this time, since the lithium ions are extremely reactive, they react with the carbon electrode to form compounds, such as Li CO LiO and LiOH, thus forming a

2 3, coating film on the surface of the anode.

[7] This coating film is called a "solid electrolyte interface (SEI) film." The SEI film formed at the initial stage of charging prevents a reaction between lithium ions and carbon anode or other materials during charging and discharging. In addition, the SEI film, serving as an ion tunnel, allows the lithium ions alone to penetrate therethrough.

[8] Furthermore, the SEI film solvates the lithium ions, and thus high molecular weight organic solvents moving along the lithium ions in an electrolyte are co-intercalated into the carbon anode, thereby preventing the structure of the carbon anode from being destroyed.

[9] Once the SEI film is formed, the lithium ions are not further reacted (i.e. side reaction) with the carbon anode or other materials, and thus the amount of the lithium ions is reversibly maintained at a constant level. That is, the carbon anode reacts with the electrolyte at the initial stage of charging to form a passivation layer, such as SEI film, so that degradation of the electrolyte does not oocur any further, and stable charging and discharging can be maintained (J. Power Sources, 51(1994), 79-104).

[10] For these reasons, the lithium secondary battery does not provide further ir¬ reversible formation of the passivation layer after the initial stage of charging, and the cycle life can be stably maintained.

[11] However, due to degradation of the carbonate-based organic solvents, gas generation may oocur inside the battery during the formation of the SEI film (J. Power Sources, 72(1998), 66-70). The gas may be H ,CO, CO , CH , C H ,C H or C H

2 2 4 2 6 3 8 3 6 depending on the kind of a nonaqueous organic solvent and an anode active material used. This gas generation inside the battery expands the battery thick during charging. [12] Further, when the battery is stored at high temperature in a fully charged state, the passivation layer is gradually disintegrated by increased electrochemical and thermal energies with the lapse of time to expose the anode surface. The exposed anode surface continuously reacts with surrounding electrolyte. [13] This side reaction continuously generates gas, resulting in an increase in the internal pressure of the battery. The increased internal pressure causes problems of deformation in a central portion of a certain face of the battery, for example, swelling along one direction in the case of a square-type battery and a lithium polymer battery (PLI). Thus, there exists a local difference in the adhesion between pole plates inside the electrodes of the battery, thus deteriorating the performance and safety of the battery, and making set mounting of the lithium secondary battery difficult.

[14] In an effort to solve these problems, a method is suggested to improve the safety of a secondary battery comprising a nonaqueous electrolyte, by mounting a vent or current breaker for discharging an internal electrolyte when the internal pressure exceeds a given level. However, this method has a problem that there is a risk of malfunction due to an increase in internal pressure.

[15] Further, methods for changing the formation reaction of a SEI film by adding additives to electrolytes in order to suppress the increase in internal pressure have been proposed. For example, Japanese Patent Laid-open No. Hei 9-73918 discloses a method for improving storage properties of a battery at high temperature by adding 1% or less of diphenyl pkrylhydrazyl to an electrolyte. Japanese Patent Laid-open No. Hei 8-321312 discloses a method for improving life performance and long-term storage properties by adding 1-20% of an N-butyl amine to an electrolyte. Japanese Patent Laid-open No. Hei 8-64238 discloses a method for improving storage properties of a battery by adding a calcium salt at a concentration of 3 x 10 M to 3 x lO M to an electrolyte. Japanese Patent Laid-open No. Hei 6-333596 discloses a method for improving storage properties of a battery by adding an azo compound to an electrolyte in order to inhibit a reaction between the electrolyte and an anode.

[16] In addition, Japanese Patent Laid-open No. Hei 7-176323 discloses the addition of

CO to an electrolyte, and Japanese Patent Laid-open No. Hei 7-320779 discloses a

2 method for inhibiting degradation of an electrolyte by adding a sulfide compound to the electrolyte.

[17] In order to improve storage properties and stability, a method is known for inducing the formation of an optimal coating film on the surface of an anode, e.g., SEI film, by adding a small amount of an organic or inorganic material to an electrolyte. However, the added material is degraded or forms an unstable coating film by interaction with a carbon anode upon initial charging. As a result, ionic conductivity of the battery is reduced, undesired gas is generated inside the battery, and internal pressure is increased, thereby deteriorating the storage properties, stability, life performance, and capacity of the battery.

[18] In the case of a secondary battery comprising a nonaqueous electrolyte, when a power circuit or a charger of an electronic device is out of order or overcharged, aberrant heat generation occurs in the battery and further, in extreme case, the battery may be damaged or catch fire. As such, it is important to effectively inhibit such heat generation and to secure stability of the battery so as to prevent thermal runaway.

[19] As a measure to prevent the battery from rupturing and catching fire upon being overcharged, a method for controlling a charge voltage of the battery by a charger is mainly used. However, since use of protective circuits and protective components sig¬ nificantly limit miniaturization and reduced production costs of a battery pack, there is an urgent need for a method capable of securing stability of the battery without the use of protective circuits or protective components.

[20] Some attempts to solve the above-mentioned problems are proposed, for example, methods for ensuring stability against overcharging of batteries by adding a small amount of an aromatic compound as an additive to an electrolyte for lithium secondary batteries (Japanese Patent Laid-open Nos. Hei 7-302614, Hei 9-50822 and Hei 9-106835; and Japanese Patent No. 2939469). Japanese Patent Laid-open Nos. Hei 7-302614 and Hei 9-50882 suggest the use of organic compounds (e.g., anisole) having a low molecular weight of not more than 500, having a reversible redox potential at above a cathode potential upon full charge of the secondary battery, and a π-electron orbit, as additives in the electrolyte. Further, the publications propose that the additives serve as a redox shuttle and consume overcharge current upon overcharging to establish a protection mechanism. Meanwhile, Japanese Patent Laid- open No. Hei 9-106835 suggests a method for the protection of a battery upon overcharging by adding an additive. The additive initiates a polymerization reaction at an overcharge voltage, and functions as a resistor. Anisole disclosed in Japanese Patent Laid-open Nos. Hei 7-302614 and Hei 9-50822, functions as a redox shuttle upon overcharging, but participates in a reaction within a common service voltage range of batteries, thereby adversely affecting cycle characteristics of discharge capacity. Disclosure of Invention Technical Problem

[21] There is a growing demand for secondary batteries with high energy density and high power density. Under these circumstances, the addition of flame retardant phosphoric ester compounds to flammable carbonate-based electrolytes is suggested in order to improve flame retardance of the electrolytes (Japanese Patent Laid-open Nos. Hei 4-184870, Hei 8-88023, and Hei 10-189038 to Hei 189040).

[22] These kinds of compounds are added to impart flame retardance to the electrolytes, but may deteriorate the electrical conductivity, and greatly reduce the characteristics of the electrolytes. In addition, since the phosphoric ester compounds have a high degree of penetration, they penetrate through the coating film formed on the surface of the anode, causing a reaction on the anode surface. As a result, battery characteristics, such as charge/discharge efficiency, energy density and power density, markedly drop, and hence the phosphoric ester compounds are not yet suitable for practical use. Moreover, silan compounds (Japanese Patent Laid-open Nos. Hei 3-236168 to 236171), and fluorine compounds (Japanese Patent Laid-open Nos. Hei 8-138737 and 10-116629) are suggested. Snce these compounds may deteriorate the characteristics of electrolytes and batteries, they cannot be applied to practical use. Technical Solution

[23] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide electrolyte for lithium secondary batteries comprising an additive capable of forming a passivation layer on the surface of an anode by oxidative degradation before the electrolyte is oxidatively degraded on the anode surface, thereby inhibiting degradation of the electrolyte.

[24] According to the nonaqueous electrolyte of the present invention, since increase in the thickness of a battery when being stored at high temperature in a fully charged state can be inhibited without deterioration in low temperature characteristics and storage characteristics of the battery, improved reliability can be ensured upon battery set mounting.

[25] It is another object of the present invention to provide a nonaqueous electrolyte for lithium secondary batteries capable of preventing thermal runaway and improving flame retardance, thereby ensuring good safety of lithium secondary batteries.

[26] In order to accomplish the above objects of the present invention, there is provided a nonaqueous electrolyte for lithium secondary batteries comprising a lithium salt, a nonaqueous organic solvent, and a compound represented by Formula 1 below:

[27] Formula 1

[28] O

R1 -O-P-O-R3

I O

R2

[29]

[30] wherein Rl to R3 are each independently a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms, or Formula

2 below:

[31] Formula 2

[32]

R4-O-P-O I I O-R5

[33] wherein R4 is a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms; and R5 is a C alkyl group in which part or all of the hydrogen atoms are substituted with halogen atoms.

[34] In one embodiment of the nonaqueous electrolyte according to the present invention, the lithium salt is at least one compound selected from the group consisting of LiPF , LiBF , LiSbF , LiAsF , LiClO , LiCF SO , Li(CF SO ) N, LiC F SO ,

6 4 6 6 4 3 3 3 2 2 4 9 3

LiAlO , LiAlCl , LiN(C F SO )(C F SO ) (in which x and y are natural numbers),

4 4 x 2x+l 2 y 2y+l 2

LiCl, and LiI.

[35] In another embodiment of the nonaqueous electrolyte according to the present invention, the concentration of the lithium salt is within the range of 0.6M to 2M. Preferably, the concentration of the lithium salt is within the range of 0.7M to 1.6M. When the concentration is below 0.6M, the conductivity of the electrolyte is lowered, causing poor performance of the electrolyte. On the other hand, when the con¬ centration exceeds 2M, the viscosity of the electrolyte increases, resulting in a reduction in the conductivity of lithium ions and a deterioration in low temperature performance.

[36] In another embodiment of the nonaqueous electrolyte according to the present invention, the nonaqueous organic solvent is at least one solvent selected from the group consisting of carbonate-, ester-, ether-, and ketone-based solvents. [37] In another embodiment of the nonaqueous electrolyte according to the present invention, the carbonate-based solvent is at least one solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), propylene carbonate (PC), and butylene carbonate (BC).

[38] In another embodiment of the nonaqueous electrolyte according to the present invention, the carbonate-based solvent is a mixed solvent of a cjclic carbonate-based solvent and a chained carbonate solvent.

[39] The volume ratio of the cjclic carbonate-based solvent to the chained carbonate- based solvent is preferably between 1:1 and 1:9, and more preferably between 1:1.5 and 1 :4. Within these ranges, the nonaqueous electrolyte exerts better performance.

[40] In another embodiment of the nonaqueous electrolyte according to the present invention, the nonaqueous organic solvent is a mixed solvent of the carbonate-based solvent and an aromatic hydrocarbon-based organic solvent.

[41] In another embodiment of the nonaqueous electrolyte according to the present invention, the aromatic hydrocarbon-based organic solvent is a compound represented by Formula 3 below:

[42] Formula 3

[43]

[44]

[45] wherein R is a halogen atom or a C alkyl group, and n is an integer of from 1 to

1-10

5.

[46] In another embodiment of the nonaqueous electrolyte according to the present invention, the aromatic hydrocarbon-based organic solvent is at least one solvent selected from the group consisting of benzene, fluorobenzene, toluene, fluorotoluene, trifluorotoluene, and xylene.

[47] In another embodiment of the nonaqueous electrolyte according to the present invention, the volume ratio of the carbonate-based solvent to the aromatic hy¬ drocarbon-based organic solvent is between 1:1 and 50:1. Within this range, the nonaqueous electrolyte exerts better performance. [48] In another embodiment of the nonaqueous electrolyte aooording to the present invention, the ester-based solvent is at least one solvent selected from the group consisting of butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, n-methyl acetate, n-ethyl acetate, and n-propyl acetate. [49] Examples of ether-based solvents usable in the present invention include, but are not limited to, dibutyl ether, etc. [50] In another embodiment of the nonaqueous electrolyte aαoording to the present invention, the compound of Formula 1 is trifluoroethyl phosphate (TFEP) of Formula

4 below:

[51] Formula 4

[52]

F O F

I I l I

F-C -C -O-P-O-C -C -F

F "^> i "^> F

CH₂ F-C -F

[53] or trifluorophenyl phosphate (TFPP) of Formula 5 below:

[54] Formula 5

[55]

[56] In another embodiment of the nonaqueous electrolyte aαoording to the present invention, the volume ratio of the compound of Formula 1 or 2 to the carbonate-based solvent is in the range of 1 : 1 to 1 :50. When the volume ratio is below 1 :50, it is difficult to expect inhibitory effects on gas generation inside batteries and good safety of batteries resulting from flame retardance. Meanwhile, when the volume ratio exceeds 1 : 1, a conductive coating film is formed to a large thickness sufficient to adversely affect the reversibility of batteries, thus deteriorating performance of batteries, such as cycle characteristics.

[57] In yet another embodiment of the nonaqueous electrolyte according to the present invention, the volume ratio of the compound of Formula 4 or 5 to the nonaqueous organic solvent is between 1:50 and 1:5.

[58] In accordance with another aspect of the present invention, there is provided a lithium secondary battery, comprising: the nonaqueous electrolyte for lithium secondary batteries; a cathode including a lithium intercalation compound; and an anode including carbon, a carbon composite, a lithium metal, or a lithium alloy.

[59] In another embodiment of the lithium secondary battery according to the present invention, the lithium secondary battery is a lithium ion battery or lithium polymer battery.

[60] When the voltage of the battery reaches an overcharge voltage, the compound of

Formula 1 or 2 begins to be degraded, electrochemically initiates a polymerization reaction while generating gas, and finally forms a conductive polymer coating film on the cathode surface. Snce the polymer coating film, acting as a resistor, is poorly soluble in the electrolyte, it prevents overcharging.

[61] In addition, since the polymer coating film reduces the amount of heat generation upon overcharging, the occurrence of thermal runaway is avoided, and thus the safety of the battery is improved.

[62] The nonaqueous electrolyte for lithium secondary batteries according to the present invention is commonly stable between -20 °C and 60°C, and maintains its stability even at a voltage of 4V. Accordingly, the nonaqueous electrolyte of the present invention can improve the safety and reliability of lithium secondary batteries.

[63] Examples of cathode active materials that can be used in the lithium secondary battery of the present invention include lithium metal oxides, e.g., LiCbO , LiNiO ,

2 2

LiMnO , LiMn O and LiNi Cb M O (wherein O=X=I, 0=y=l, 0=x+y=l, M is Al,

2 2 4 1-x-y x y 2

Sr, Mg, La, etc.); and lithium intercalation compounds, e.g., lithium chalcogenide compounds. Examples of anode active materials include crystalline or amorphous carbons, carbon composites, lithium metals, and lithium alloys. In addition to these materials, it should be understood that any cathode and anode active materials used in conventional lithium secondary batteries can be used in the present invention. [64] The lithium secondary battery of the present invention is fabricated in accordance with the following procedure. Each slurry containing a cathode active material and a anode active material is coated on a current collector, and then subjected to molding to produce a cathode and an anode. The cathode and the anode thus produced are wound or layered, together with a separator as an insulator, to form an electrode assembly. After the electrode assembly is placed in a battery case, an electrolyte is fed into the battery case through an electrolyte supply port to fabricate the final lithium secondary battery.

[65] As the separator, there may be used, for example, a polyethylene separator, a polypropylene separator, a two-layer polyethylene/polypropylene separator, a three- layer polyethylene/polypropylene/polyethylene separator, or a three-layer polypropylene/polyethylene/polypropylene separator.

[66]

Advantageous Effects

[67] According to the nonaqueous electrolyte of the present invention, since increase in the thickness of a battery when being stored at high temperature in a fully charged state can be inhibited without deterioration in low temperature characteristics and storage characteristics of the battery, improved reliability can be ensured upon battery set mounting.

[68] Further the present invention provides a nonaqueous electrolyte for lithium secondary batteries capable of preventing thermal runaway and improving flame retardance, thereby ensuring good safety of lithium secondary batteries. Brief Description of the Drawings

[69] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the aooompanying drawings, in which:

[70] Fig. 1 is a graph showing the results of high-temperature swelling of batteries fabricated in Examples 1 to 4 and 7, and Comparative Examples 1 and 2 of the present invention after standing at 90°C for 8 hours;

[71] Fig. 2 shows cyclic voltammograms (CVs) comparing the experimental results for the reactivity between respective electrodes and electrolytes of batteries fabricated in Example 3 and Comparative Example 2 of the present invention;

[72] Fig. 3 shows differential scanning calorimetry (DSC) thermograms comparing the amount of heat generated at respective cathodes of batteries fabricated in Example 3 and Comparative Example 2 of the present invention;

[73] Fig. 4 shows differential scanning calorimetry (DSC) thermograms comparing the amount of heat generated at respective anodes of batteries fabricated in Example 3 and

Comparative Example 2 of the present invention; and [74] Fig. 5 shows graphs comparing the experimental results of overcharging of batteries fabricated in Examples 3 and 7, and Comparative Example 2 of the present invention to 10V at a current of 3C-rate.

Best Mode for Carrying Out the Invention [75] The present invention will now be described in more detail with reference to the following examples. However, these examples are given for the purpose of illustration and are not to be construed as limiting the scope of the invention. [76]

[77] Example 1

[78] Ethylene carbonate (EC), ethylmethyl carbonate (EMC) and diethyl carbonate

(DEC) (1:1:1) were mixed in a volume ratio of 1 : 1 : 1 to obtain a basic solvent, and then the basic solvent was mixed with trifluoroethyl phosphate (TFEP) in a volume ratio of

50:1 to obtain a nonaqueous organic solvent. IM LiPF as a solutewas added to the

6 nonaqueous organic solvent to prepare a nonaqueous electrolyte for lithium secondary batteries. [79] LiCoO was used as a cathode active material, graphite as an anode active material,

2

PVDF was used as a binder, and acetylene black was used as a conductive agent to fabricate a square-type 423048 battery. [80] The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [81]

[82] Example 2

[83] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluoroethyl phosphate (TFEP) in a volume ratio of 20: 1. The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [84]

[85] Example 3

[86] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluoroethyl phosphate (TFEP) in a volume ratio of 10:1. The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [87] [88] Example 4

[89] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluoroethyl phosphate (TFEP) in a volume ratio of 5 : 1. The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [90]

[91] Example 5

[92] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluorophenyl phosphate (TFPP) in a volume ratio of 50: 1.

The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [93]

[94] Example 6

[95] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluorophenyl phosphate (TFPP) in a volume ratio of 20: 1.

The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [96]

[97] Example 7

[98] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluorophenyl phosphate (TFPP) in a volume ratio of 10:1.

The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [99]

[100] Example 8

[101] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with trifluorophenyl phosphate (TFPP) in a volume ratio of 5:1.

The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [102]

[103] Comparative Example 1

[104] A battery was fabricated in the same manner as in Example 1, except that the basic solvent was mixed with triethyl phosphate (TEP) in a volume ratio of 10:1. The battery characteristics were evaluated, and the limited oxygen index of the battery was measured. [105]

[106] Comparative Example 2

[107] A battery was fabricated in the same manner as in Example 1, except that the basic solvent of ethylene carbonate (EC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 1 : 1 : 1 alone was used as a nonaqueous organic solvent. The battery characteristics were evaluated, and the limited oxygen index of the battery was measured.

[108] The battery characteristics were evaluated, and the limited oxygen index of the batteries was measured in accordance with the following procedure.

[109]

[110] Evaluation of battery characteristics

[111] ( 1 ) Formation charging/discharging test

[112] Each of the lithium secondary batteries fabricated in Examples 1-8, and

Comparative Examples 1 and 2 was charged at a charge current of 17OmA to a voltage of 4.2V at constant current and constant voltage, allowed to stand for 1 hour, discharged to 2.75V at 17OmA, and then allowed to stand for 1 hour. This procedure was repeated three times.

[113]

[114] (2) Standard charging/discharging test

[115] Each of the lithium secondary batteries fabricated in Examples 1~8, and

Comparative Examples 1 and 2 was charged at 1C to a voltage of 4.2V at constant current and constant voltage, and then discharged to 3 V at constant current of 1C. Under these standard charging/discharging conditions, the procedure was repeated 300 times to measure capacity retention rates (residual discharge capacity based on initial capacity).

[116]

[117] Measurement of limited oxygen index

[118] The limited oxygen index was measured in accordance with the JB K7201 standard method. The results are shown in Table 3.

[119] The following experiments were then conducted.

[120]

[121] Comparison of discharge voltages at C-rate

[122] Each of the lithium secondary batteries fabricated in Examples 3 and 7, and

Comparative Example 2 was charged at a current of 17OmA (0.2C-rate) to a discharge voltage of 4.2V at constant current and constant voltage, and then discharged to 3 V at constant currents of 0.2C, 0.5C and l.OC. When each discharge capacity was 100%, a voltage at 50% of the discharge capadty was determined as a discharge voltage. The results are shown in Table 2.

[123]

[124] High temperature swelling test

[125] The batteries fabricated in Examples 1 to 4 and 7, and Comparative Examples 1 and

2 were allowed to stand at a fully-charged voltage of 4.2V and 90 °C for 8 hours, and then the degree of high temperature swelling was measured. The results are shown in Fig. 1. As can be seen from Fig. 1, the batteries comprising the nonaqueous electrolytes of the present invention showed no change in thickness even after 8 hours.

[126]

[127] Cyclic voltammetry

[128] The batteries fabricated in Example 3 and Comparative Example 2 were circulated

5 times in the range of 2.5-OV with a scan rate of lmV/s,and then the oxidation- reduction reactivity between the electrolytes and the electrodes was measured. The results are shown in Fig. 2. As is evident from Fig. 2, the battery fabricated in Example

3 exhibited less reactivity between the anode and the electrolyte than the battery fabricated in Comparative Example 2.

[129]

[130] Measurement of amount of heat generation at electrode

[131] The amounts of heat generated at the cathodes and the anodes of the batteries fabricated in Example 3 and Comparative Example 2 were measured at a fully-charged voltage of 4.2V by DSC. The measurements were conducted in accordance with the following procedure. After the batteries were subjected to formation charging and discharging using the respective electrolytes, the cathodes and the anodes of the batteries fully charged to 4.2V were extracted, and then placed in a cell for DSC measurement. The amount of heat generation and temperatures at which heat was generated were measured while heating the cell at a rate of 5°C/min, and the results are shown in Figs. 3 and 4, respectively.

[132]

[133] Overcharging test

[ 134] The experimental results of overcharging of the batteries fabricated in Examples 3 and 7, and Comparative Example 2 to 10V at a current of 3C-rate are shown in Fig. 5. It was confirmed from Fig. 5 that the batteries fabricated in Examples 3 and 7 did not explode and catch fire upon overcharging, unlike the battery fabricated in Comparative Example 2. It appears that this is because trifluoroethyl phosphate (TFEP) reduces heat generation and prevents thermal runaway upon overcharging. [135] Table 1

[136] ΔIR(mΩ): Change in internal resistance [137] Table 2

[138] Table 3

[139] As is apparent from Tables 1 and 2, the batteries (Examples 1 to 8) comprising

TFEP or TFPP exhibited battery characteristics comparable to the battery fabricated in Comparative Example 2. In conclusion, the battery characteristics were not influenced by the addition of the compounds.

[140] On the other hand, it was confirmed from Table 3 that the nonaqueous electrolytes comprising TFEP or TFPP according to the present invention increase limited oxygen index, and thus good flame retardance can be ensured.

[141] However, the battery (Comparative Example 1) comprising a phosphoric ester compound containing no fluorine atom showed a high limited oxygen index, but caused serious problems in battery characteristics (initial capacity/efficiency, internal resistance, and life characteristics).

[142]

Mode for the Invention [143] The present invention provides electrolyte for lithium secondary batteries comprising an additive capable of forming a passivation layer on the surface of an anode by oxidative degradation before the electrolyte is oxidatively degraded on the anode surface, thereby inhibiting degradation of the electrolyte.

[144] According to the nonaqueous electrolyte of the present invention, since increase in the thickness of a battery when being stored at high temperature in a fully charged state can be inhibited without deterioration in low temperature characteristics and storage characteristics of the battery, improved reliability can be ensured upon battery set mounting.

[145] It is another object of the present invention to provide a nonaqueous electrolyte for lithium secondary batteries capable of preventing thermal runaway and improving flame retardance, thereby ensuring good safety of lithium secondary batteries.

[146] In order to accomplish the above objects of the present invention, there is provided a nonaqueous electrolyte for lithium secondary batteries comprising a lithium salt, a nonaqueous organic solvent, and a compound represented by Formula 1 below:

[147] Formula 1

[148]

O

Il R1 -O-P-O-R3

I O

I R2

[149] wherein Rl to R3 are each independently a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms, or Formula 2 below:

[150] Formula 2

[151]

O I I

R4-O- P I - O

I I o- R5

[152] wherein R4 is a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms; and R5 is a C alkyl group in which part or all of the hydrogen atoms are substituted with halogen atoms.

[153] In one embodiment of the nonaqueous electrolyte according to the present invention, the lithium salt is at least one compound selected from the group consisting of LiPF , LiBF , LiSbF , LiAsF , LiClO , LiCF SO , Li(CF SO ) N, LiC F SO ,

6 4 6 6 4 3 3 3 2 2 4 9 3

LiAlO , LiAlCl , LiN(C F SO )(C F SO ) (in which x and y are natural numbers),

4 4 x 2x+l 2 y 2y+l 2

LiCl, and LiI.

[154] In another embodiment of the nonaqueous electrolyte according to the present invention, the concentration of the lithium salt is within the range of 0.6M to 2M. Preferably, the concentration of the lithium salt is within the range of 0.7M to 1.6M. When the concentration is below 0.6M, the conductivity of the electrolyte is lowered, causing poor performance of the electrolyte. On the other hand, when the con- centration exceeds 2M, the viscosity of the electrolyte increases, resulting in a reduction in the conductivity of lithium ions and a deterioration in low temperature performance.

[155] In another embodiment of the nonaqueous electrolyte according to the present invention, the nonaqueous organic solvent is at least one solvent selected from the group consisting of carbonate-, ester-, ether-, and ketone-based solvents.

[156] In another embodiment of the nonaqueous electrolyte according to the present invention, the carbonate-based solvent is at least one solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), propylene carbonate (PC), and butylene carbonate (BC).

[157] In another embodiment of the nonaqueous electrolyte according to the present invention, the carbonate-based solvent is a mixed solvent of a cjclic carbonate-based solvent and a chained carbonate solvent.

[158] The volume ratio of the cyclic carbonate-based solvent to the chained carbonate- based solvent is preferably between 1:1 and 1:9, and more preferably between 1:1.5 and 1 :4. Within these ranges, the nonaqueous electrolyte exerts better performance.

[159] In another embodiment of the nonaqueous electrolyte according to the present invention, the nonaqueous organic solvent is a mixed solvent of the carbonate-based solvent and an aromatic hydrocarbon-based organic solvent.

[160] In another embodiment of the nonaqueous electrolyte according to the present invention, the aromatic hydrocarbon-based organic solvent is a compound represented by Formula 3 below:

[161] Formula 3

[162]

[163] wherein R is a halogen atom or a C alkyl group, and n is an integer of from 1 to

1-10

5.

[164] In another embodiment of the nonaqueous electrolyte according to the present invention, the aromatic hydrocarbon-based organic solvent is at least one solvent selected from the group consisting of benzene, fluorobenzene, toluene, fluorotoluene, trifluorotoluene, and xylene.

[165] In another embodiment of the nonaqueous electrolyte aooording to the present invention, the volume ratio of the carbonate-based solvent to the aromatic hy¬ drocarbon-based organic solvent is between 1:1 and 50:1. Within this range, the nonaqueous electrolyte exerts better performance.

[166] In another embodiment of the nonaqueous electrolyte aooording to the present invention, the ester-based solvent is at least one solvent selected from the group consisting of butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, n-methyl acetate, n-ethyl acetate, and n-propyl acetate.

[167] Examples of ether-based solvents usable in the present invention include, but are not limited to, dibutyl ether, etc.

[168] In another embodiment of the nonaqueous electrolyte aooording to the present invention, the compound of Formula 1 is trifluoroethyl phosphate (TFEP) of Formula 4 below:

[169] Formula 4

[170]

F O F

I I l I

F-C -C -O-P-O-C -C -F

F "^> i "^> F

CH₂ F-C -F

[171] or trifluorophenyl phosphate (TFPP) of Formula 5 below:

[172] Formula 5

[173]

[174] In another embodiment of the nonaqueous electrolyte according to the present invention, the volume ratio of the compound of Formula 1 or 2 to the carbonate-based solvent is in the range of 1 : 1 to 1 :50. When the volume ratio is below 1 :50, it is difficult to expect inhibitory effects on gas generation inside batteries and good safety of batteries resulting from flame retardance. Meanwhile, when the volume ratio exceeds 1 : 1, a conductive coating film is formed to a large thickness sufficient to adversely affect the reversibility of batteries, thus deteriorating performance of batteries, such as cycle characteristics.

[175] In yet another embodiment of the nonaqueous electrolyte according to the present invention, the volume ratio of the compound of Formula 4 or 5 to the nonaqueous organic solvent is between 1:50 and 1:5.

[176] In accordance with another aspect of the present invention, there is provided a lithium secondary battery, comprising: the nonaqueous electrolyte for lithium secondary batteries; a cathode including a lithium intercalation compound; and an anode including carbon, a carbon composite, a lithium metal, or a lithium alloy.

[177] In another embodiment of the lithium secondary battery according to the present invention, the lithium secondary battery is a lithium ion battery or lithium polymer battery.

[178] When the voltage of the battery reaches an overcharge voltage, the compound of

[179] In addition, since the polymer coating film reduces the amount of heat generation upon overcharging, the occurrence of thermal runaway is avoided, and thus the safety of the battery is improved.

[180] The nonaqueous electrolyte for lithium secondary batteries according to the present invention is commonly stable between -20 °C and 60°C, and maintains its stability even at a voltage of 4V. Accordingly, the nonaqueous electrolyte of the present invention can improve the safety and reliability of lithium secondary batteries.

[181] Examples of cathode active materials that can be used in the lithium secondary battery of the present invention include lithium metal oxides, e.g., LiCbO , LiNiO ,

2 2

LiMnO , LiMn O and LiNi Cb M O (wherein O=X=I, 0=y=l, 0=x+y=l, M is Al,

2 2 4 1-x-y x y 2

Sr, Mg, La, etc.); and lithium intercalation compounds, e.g., lithium chalcogenide compounds. Examples of anode active materials include crystalline or amorphous carbons, carbon composites, lithium metals, and lithium alloys. In addition to these materials, it should be understood that any cathode and anode active materials used in conventional lithium secondary batteries can be used in the present invention.

[ 182] The lithium secondary battery of the present invention is fabricated in accordance with the following procedure. Each slurry containing a cathode active material and a anode active material is coated on a current collector, and then subjected to molding to produce a cathode and an anode. The cathode and the anode thus produced are wound or layered, together with a separator as an insulator, to form an electrode assembly. After the electrode assembly is placed in a battery case, an electrolyte is fed into the battery case through an electrolyte supply port to fabricate the final lithium secondary battery.

[183] As the separator, there may be used, for example, a polyethylene separator, a polypropylene separator, a two-layer polyethylene/polypropylene separator, a three- layer polyethylene/polypropylene/polyethylene separator, or a three-layer polypropylene/polyethylene/polypropylene separator.

[184]

Industrial Applicability

[185] As apparent from the above description, the nonaqueous electrolyte for lithium secondary batteries comprising a halogenated phosphoric ester according to the present invention can inhibit an increase in the thickness of a battery even when the battery is stored at high temperature in a fully charged state, without any influence on battery characteristics. In addition, the nonaqueous electrolyte of the present invention can prevent thermal runaway upon being overcharged, and can improve flame retardance, ensuring good safety of lithium secondary batteries.

[186] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modi¬ fications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

[187]

Claims

[1] A nonaqueous electrolyte for lithium secondary batteries comprising a lithium salt, a nonaqueous organic solvent, and a compound represented by Formula 1 below: (1)

O

Il

R1 -O-P-O-R3

I O

R2 wherein Rl to R3 are each independently a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms, or Formula 2 below:

(2)

O

I l

R4-O-P-O I I O-R5 wherein R4 is a C alkyl group or a benzene group in which part or all of the hydrogen atoms are substituted with halogen atoms; and R5 is a C alkyl group in which part or all of the hydrogen atoms are substituted with halogen atoms. [2] The nonaqueous electrolyte according to claim 1, wherein the lithium salt is at least one compound selected from the group consisting of LiPF , LiBF , LiSbF , LiAsF , LiClO , LiCF SO , Li(CF SO ) N, LiC F SO , LiAlO , LiAlCl , LiN(C

6 4 3 3 3 2 2 4 9 3 4 4 x

F SO )(C F SO ) (in which x and y are natural numbers), LiCl, and LiI.

2x+l 2 y 2y+l 2

[3] The nonaqueous electrolyte according to claim 2, wherein the lithium salt is present in the range of 0.6M to 2M.

[4] The nonaqueous electrolyte according to claim 1, wherein the nonaqueous organic solvent is at least one solvent selected from the group consisting of carbonate-, ester-, ether-, and ketone-based solvents.

[5] The nonaqueous electrolyte according to claim 4, wherein the carbonate-based solvent is at least one solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate (EC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), propylene carbonate (PC), and butylene carbonate

(BC).

[6] The nonaqueous electrolyte according to claim 4, wherein the carbonate-based solvent is a mixed solvent of a cj/clic carbonate-based solvent and a chained carbonate solvent.

[7] . The nonaqueous electrolyte according to claim 1, wherein the nonaqueous organic solvent is a mixed solvent of the carbonate-based solvent and an aromatic hydrocarbon-based organic solvent.

[8] The nonaqueous electrolyte according to claim 7, wherein the aromatic hy¬ drocarbon-based organic solvent is a compound represented by Formula 3 below:

(3)

wherein R is a halogen atom or a C alkyl group, and n is an integer of from 1

1-10 to 5.

[9] The nonaqueous electrolyte according to claim 7, wherein the aromatic hy¬ drocarbon-based organic solvent is at least one solvent selected from the group consisting of benzene, fluorobenzene, toluene, fluorotoluene, trifluorotoluene, and xylene.

[10] The nonaqueous electrolyte according to claim 7, wherein the volume ratio of the carbonate-based solvent to the aromatic hydrocarbon-based organic solvent is between 1:1 and 50:1.

[11] The nonaqueous electrolyte according to claim 4, wherein the ester-based solvent is at least one solvent selected from the group consisting of butyrolactone, decanolide, valerolactone, mevalonolactone, caprolactone, n-methyl acetate, n- ethyl acetate, and n-propyl acetate.

[12] The nonaqueous electrolyte according to claim 1, wherein the compound of

Formula 1 is trifluoroethyl phosphate (TFEP) of Formula 4 below: (4), F O F

I I l I

F-C -C -O-P-O-C -C -F

I I

F "> C F

CH₂

F-C t -F

or trifluorophenyl^LL - phosphate (TFPP) of Formula 5 below

(5)

[13] The nonaqueous electrolyte according to claim 4, wherein the volume ratio of the compound of Formula 1 or 2 to the carbonate-based solvent is in the range of 1 : 1 to 1:50. [14] The nonaqueous electrolyte according to claim 12, wherein the volume ratio of the compound of Formula 4 or 5 to the nonaqueous organic solvent is between

1:50 and 1:5. [15] A lithium secondary battery, comprising: the nonaqueous electrolyte for lithium secondary batteries according to any one of claims 1 to 14; a cathode including a lithium intercalation compound; and an anode including carbon, a carbon composite, a lithium metal, or a lithium alloy. [16] The lithium secondary battery according to claim 15, wherein the lithium secondary battery is a lithium ion battery or lithium polymer battery.