KR100853615B1 - Electrolyte for rechargeable lithium ion battery and rechargeable lithium ion battery comprising the same - Google Patents

Abstract

An electrolyte for a lithium ion secondary battery is provided to improve flame resistance and thermal stability, to reduce the internal resistance of a battery, and to improve the lifespan characteristics of a battery. An electrolyte for a lithium ion secondary battery comprises a non-aqueous organic solvent, a lithium salt, triphenyl phosphate, vinyl ethylene carbonate and biphenyl additive. Particularly, the electrolyte comprises 0.1-20 wt% of triphenyl phosphate, 0.05-10 wt% of vinyl ethylene carbonate, and 0.05-10 wt% of biphenyl additive based on the weight of the electrolyte. The lithium salt is at least one selected from the group consisting of LiPF6, LiBF4, LiAsF6, LiClO4, LiCF3SO3, LiC(SO2CF3)3, LiN(CF3SO2)2 and LiCH(CF3SO2)2.

Description

ELECTROLYTE FOR RECHARGEABLE LITHIUM ION BATTERY AND RECHARGEABLE LITHIUM ION BATTERY COMPRISING THE SAME

1 is a cross-sectional view of a 2032 coin-type battery of the present invention.

2 is a graph showing the results of differential scanning calorimetry (hereinafter, referred to as "DSC") analysis of organic electrolytes according to Preparation Examples 1 to 4 of the present invention.

3 is a graph showing the test results of the charge-discharge life characteristics of the lithium ion secondary battery using the organic electrolyte according to Preparation Examples 1 to 4 of the present invention.

Figure 4 is a graph showing the results of the electrochemical impedance (Electrochemical Impedance Spectroscopy, hereinafter referred to as EIS "test during the charge-discharge life test of the lithium ion secondary battery using the organic electrolyte solution according to Preparation Examples 1 to 4 of the present invention to be.

<Explanation of symbols for the main parts of the drawings>

10: anode 12: cathode

16: Separator 18: Bottom Plate

20: case 22: lid

24: Insulation Gasket

The present invention relates to a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same, and more particularly, an electrolyte solution capable of improving cycle characteristics and thermal characteristics of an electrolyte and a lithium ion secondary battery including the same. It is about.

In general, lithium ion secondary batteries are electronic components that reversibly convert chemical and electrical energy, and are used in a wide range of industries such as portable electronic devices, notebook computers, and electric vehicles.

The lithium ion secondary battery has an operating voltage of 3.6 V or more, about three times higher than a nickel-cadmium battery or a nickel-hydrogen battery, and its market is rapidly increasing in terms of high energy density and long life characteristics. In addition, as the demand for higher energy density batteries such as electric vehicles increases, active researches on lithium ion batteries are being conducted by many research groups.

Meanwhile, a lithium ion secondary battery requires an electrochemically improved electrolyte solution to achieve high operating voltage, low self discharge rate, high energy density, and long service life. Non-aqueous mixed solvent mainly composed of carbonate-based solvents, for example, PC (Propylene Carbonate), EC (Ethylene Carbonate), DEC (Diethyl Carbonate), DMC (Dimethyl Carbonate) and EMC (Ethylmethyl Carbonate) are used as electrolyte Being. The characteristics of the organic electrolyte are the main indicators such as conductivity, potential window, temperature range of use, density and stability. Items related to conductivity include solubility, dissociation, dielectric constant and viscosity. These solvents, which are used as electrolytes in lithium ion batteries, have their own advantages and disadvantages, and show great differences in battery performance depending on how these characteristics are combined when used. Efforts have been made to find an electrolyte composition for manufacturing batteries of higher performance. EC / DMC / EMC, EC / DEC / EMC /, EC / DEC / DMC / EMC, PC / EC / DMC / EMC , EC / EMC and EC / DMC are mainly used. As described above, the lithium ion secondary battery mainly uses a non-aqueous organic electrolyte. The non-aqueous electrolyte has a low conductivity but is widely used in that a high voltage of the battery is possible due to its wider electrochemical stability window than water. have.

In such a lithium ion secondary battery, one of the biggest problems is low safety. Carbonate-based organic solvents used in lithium secondary batteries, when exposed to various environments such as overcharging, external heating, and physical deformation, cause rapid thermal exothermic reactions such as decomposition of organic solvents, and risks such as battery ignition and explosion. You are in a situation. Various studies are being conducted to prevent overcharge, which is a cause of the risk of ignition, and to prevent an internal short due to physical deformation. The risk of ignition or explosion of the battery is particularly severe in high capacity batteries. To this end, various studies have been conducted to prevent the fire and explosion of the battery by adding a flame retardant. However, this method provides excellent ignition inhibiting power, but has a problem of lowering battery performance such as reduction of cycle life. For example, it has been suggested to use Trimethyl Phosphate (TMP), Triethyl Phosphate (TEP) and Hexamethoxycyclotriphosphazene (HMPN) flame retardant additives in 1.0M LiPF ₆ + EC: EMC (1: 1 wt%) solvents, but these Flame retardant additives have been found to lead to degradation of the cell [Ref. K. Xu, MS Ding, S. Zang, JL Allen, TR Jow, An Attempt to Formulate Nonflammable Lithium Ion Electrolytes with Alkyl Phosphates and Phosphazenes, J. Electrochem. Soc, 149 (5) A622 (2002)].

On the other hand, in order to maintain excellent cycle performance and provide a flame retardant effect, it contains 70% TMP, 2% vinylene carbonate, 8% vinyl ethylene carbonate, 2% cyclo hexane (Ref .: X. Wang, C. Yamada, H. Naito, G. Segami, K. Kibe, High-Concentration Trimethyl Phosphate-Based Nonflammable Electrolytes with Improved Charge) -Discharge Performance of a Graphite Anode for Lithium-Ion Cells, J. Electrochem. Soc., 153 (1) A135 (2006)]. However, TMP provides poor reduction stability for the negative electrode and has a problem of lowering cycle performance [Ref. S.S. Zhang, A Review on Electrolyte Additives for Lithium-Ion Batteries, J. Power Sources 162 1379 (2006).

Therefore, there is a continuing need for the development of an electrolyte that can exert a flame retardant effect without affecting battery performance.

The present invention is to solve the above problems, an object of the present invention is to provide a lithium ion secondary battery electrolyte having excellent flame retardancy and battery performance by containing a functional additive with a flame retardant additive.

Another object of the present invention is to provide a lithium ion secondary battery including the electrolyte solution.

In order to achieve the above object, the present invention is a flame retardant additive triphenyl phosphate (TPP, Triphenyl Phosphate), a functional additive vinyl ethylene carbonate (VEC, Vinyl Ethylene Carbonate) and biphenyl in a solution based on a non-aqueous organic solvent and lithium salt It provides an organic electrolyte solution (BP, Biphenyl) mixed and a lithium ion secondary battery comprising the same.

As flame retardant components, halogen-based flame retardants, phosphorus-based flame retardants, inorganic compound flame retardants, and mixtures of one or more of these flame retardants may be used. Halogen-based flame retardants generally provide a flame retardant effect by stabilizing radicals occurring in the gas phase. Examples of halogen flame retardants include poly bromo biphenyl (PBB), poly bromo diphenyl ether (PBDE), tetra bromo bisphenol-A (TBBA), hexa bromo cyclododecane (HBCD), tribromo phenoxy Ethane, octa bromo diphenylether (OBDPE), brominated epoxy, brominated polycarbonate rolimo, chlorinated paraffin, chlorinated polyethylene and cycloaliphatic chlorine flame retardants. Phosphorus-based flame retardants generally produce polymethic acid by pyrolysis, which forms a protective layer, or provides a flame retardant effect by the blocking of oxygen by a carbon film produced by dehydration in the course of polymethic acid production. Examples of phosphorus-based flame retardants include phosphorus, phosphorus, phosphites, phosphine oxides, phosphine oxide diols, phosphonates and tris, such as ammonium phosphate. Triaryl phosphate, alkyldiaryl phosphate, trialkyl phosphate, and resorcinolol bisdiphenyl phosphate (RDP). Inorganic compound flame retardants are generally decomposed by heat, releasing incombustible gases such as water, carbon dioxide, sulfur dioxide and hydrogen chloride, causing endothermic reactions to dilute combustible gases to prevent oxygen access, and cooling and pyrolysis by endothermic reactions. Reducing the production of the product provides a flame retardant effect. Examples of the inorganic compound flame retardant include aluminum hydroxide, magnesium hydroxide, antimony oxide, tin hydroxide, zirconium compound, tin oxide, molybdenum oxide, calcium salt and borate.

Triphenyl phosphate used in the present invention is a kind of phosphate ester and has a flash point of 223 ° C., and is a compound mainly used as a plastic flame retardant material. These phosphate esters first produce polyphosphoric acid by pyrolysis, which is esterified and dehydrogenated to produce charcoal, and the produced charcoal blocks oxygen and heat. Polyphosphoric acid, a nonvolatile polymer, forms a carbon layer to block oxygen and latent heat, thereby reducing pyrolysis.

As the functional electrolyte additive, vinyl acetate, vinylene carbonate, vinyl ethylene carbonate, or a mixture of one or more of these compounds may be used. These electrolyte additives have been known as effective functional additives that form stable solid electrolyte interface (SEI) films to improve cell performance.

In the present invention, triphenyl phosphate is used as a flame retardant additive, and vinyl ethylene carbonate and biphenyl are mixed and used as a functional additive, thereby providing an electrolyte solution excellent in thermal stability and service life characteristics. In the present invention, triphenyl phosphate is preferably added in an amount of 0.1 to 20% by weight based on the electrolyte. If the addition amount of triphenyl phosphate is less than 0.1% by weight, thermal runaway due to overcharging cannot be prevented, and if it exceeds 20% by weight, battery performance may decrease, which is not preferable. Triphenyl phosphate is added to the non-aqueous organic solvent containing the lithium salt. Lithium salt acts as a source of lithium ions in the battery to enable the operation of the basic lithium ion secondary battery, the non-aqueous organic solvent serves as a medium to move the ions involved in the electrochemical reaction of the battery.

Lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiCH (CF ₃ SO ₂ ) _2, etc. One or a mixture of two or more compounds selected from the group may be used. It is preferable to use a lithium salt having a low lattice energy, a large dissociation degree, excellent ion conductivity, and excellent thermal stability and oxidation resistance. Lithium salt in the electrolyte is preferably used at a concentration of 0.6 to 2.0M. When the concentration of the lithium salt is less than 0.6 M, the conductivity of the electrolyte is lowered, and the electrolyte performance is lowered. When the lithium salt is higher than 2.0 M, the viscosity of the electrolyte is increased, thereby reducing the mobility of lithium ions and lowering the low temperature performance.

In the present invention, as the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based or ketone-based or the like is used. As the carbonate solvent, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC) , Ethyl propyl carbonate (EPC) or butylene carbonate (BC) and the like can be used. As the ester solvent, methyl acetate, ethyl acetate, propyl acetate and the like can be used. As the ether solvent, tetrahydrofuran or 2-methyltetrahydrofuran may be used, and as the ketone, polymethylvinyl ketone is used.

In the non-aqueous organic solvent used in the present invention, the carbonate-based solvent is preferably used by mixing a cyclic carbonate and a chain carbonate. In order to increase dissociation of ions and increase ion conductivity, an organic solvent should be used having a high dielectric constant (polarity) and a low viscosity.

The lithium ion secondary battery including the electrolyte of the present invention includes a positive electrode and a negative electrode. The positive electrode is a positive electrode active material capable of reversibly inserting and detaching lithium ions such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , LiFePO ₄ or LiCo _{1 -X} Ni _X O _Z ( 0.01 <X <1). The negative electrode includes a negative electrode active material capable of inserting and removing lithium ions, and the negative electrode active material includes crystalline or amorphous carbon, or a carbon-based negative electrode active material (thermally decomposed carbon, coke, graphite) of carbon composite, carbon fiber , Tin oxide compounds, lithium metals or lithium alloys can be used.

The lithium ion secondary battery may include a separator that prevents a short circuit between the positive electrode and the negative electrode, and the separator may be a polymer film such as polyethylene, polypropylene, or polyolefin, or a multilayer film thereof, a microporous film, a woven fabric, a nonwoven fabric, or the like. Can be.

Hereinafter, the contents of the present invention through the production examples and examples will be described in detail. The following preparation examples are merely illustrative and are not intended to limit the scope of the present invention.

Production Example

Production Example  1: Preparation of Electrolyte and Battery (1)

(Production of electrolyte)

First, a nonaqueous solvent containing ethylene carbonate (EC): ethyl methyl carbonate (EMC) at 40:60 (volume ratio) was prepared. Next, 1.1 M LiPF _{6 was added} to the nonaqueous solvent as an electrolyte salt, and then 3 wt% triphenyl phosphate, 1 wt% vinyl ethylene carbonate, and 0.1 wt% biphenyl were added to prepare an electrolyte solution.

(Production of battery)

A 2032 coin-type battery having a can diameter of 20 mm and a height of 3.2 mm was prepared. LiCoO ₂ was used as the positive electrode active material. 95: 2: 3 weight ratio of active material: binder (PVDF, polyvinylidene difluoride): conductive material (Super P black) was added to the N-methyl 2-pyrrolidinone (NMP) solvent slurry Was prepared. The slurry was applied on an aluminum foil, dried, and rolled with a roll press to prepare a positive electrode 10. As the negative electrode active material, MCMB (Mesocarbon microbeads) was used. A slurry was prepared by dissolving an active material: binder (PVDF): conductive material (Super P black) in a weight ratio of 95: 3: 2 in an NMP solvent. The slurry was applied to a copper current collector, dried, and rolled by a roll press to prepare a negative electrode 12. A porous polypropylene separator 16 was placed between the positive electrode 10 and the negative electrode 12 to impregnate the organic electrolyte. Using a clamping machine, a 2032 type coin cell was completely sealed with the bottom plate 18, the case 20, the lid 22, and the insulating gasket 24 (FIG. 1).

Production Example  2: Preparation of Electrolyte and Battery (2)

An electrolyte solution and the same method as in Preparation Example 1 were used, except that 1% by weight of vinyl acetate and 1% by weight of vinylene carbonate were used instead of 1% by weight of vinyl ethylene carbonate and 0.1% by weight of biphenyl as an additive. A battery was prepared.

Production Example  3

Ethylene carbonate (EC): A nonaqueous solvent containing ethyl methyl carbonate (EMC) at 40:60 (volume ratio) was prepared. 1.1 M LiPF ₆ was dissolved as an electrolyte salt in this non-aqueous solvent, and 3% by weight of triphenyl phosphate was added. Vinyl acetate, vinylene carbonate, vinyl ethylene carbonate and biphenyl additives were not added at all.

Production Example  4

Ethylene carbonate (EC): A nonaqueous solvent containing ethyl methyl carbonate (EMC) at 40:60 (volume ratio) was prepared. 1.1 M LiPF ₆ was added to the nonaqueous solvent as an electrolyte salt, and no triphenyl phosphate, vinyl acetate, vinylene carbonate, vinyl ethylene carbonate, and biphenyl additives were added.

Example

Analysis using a differential scanning calorimeter (DSC) was performed on the electrolyte solutions prepared in Preparation Examples 1 to 4, and the results are shown in Table 1 and FIG. 2.

TABLE 1

Electrolyte Electrolyte additive Reaction temperature (℃) Preparation Example 1 1.1M LiPF ₆ / EC: EMC  3% TPP + 1% VEC + 0.1% BP  220 Preparation Example 2 1.1M LiPF ₆ / EC: EMC  3% TPP + 1% VA + 1% VC  212 Preparation Example 3 1.1M LiPF ₆ / EC: EMC  TPP 3% by weight  216 Preparation Example 4 1.1M LiPF ₆ / EC: EMC                      -  204

As shown in Table 1 and Figure 2, the reaction temperature of the electrolyte solution of Preparation Example 4 was the lowest as 204 ℃, the electrolyte solution of Preparation Example 3 to which the flame retardant additive triphenyl phosphate was added was 216 ℃ . However, the electrolyte solution of Preparation Example 1, in which a mixture of flame retardant additives, vinyl ethylene carbonate and biphenyl, which were functional additives, was added. As a test for life evaluation, 40 charge-discharges were performed at 1.0 C (3.0 mA) at room temperature. Constant current-constant voltage charging to 4.2 V and constant current discharge to 2.75 V were performed. In addition, the battery internal resistance was compared by measuring the electrochemical impedance spectroscopy (EIS) during 40 life tests. Capacity retention after 40 charge-discharge cycles and battery resistance (R _cell ) during 40 charge / discharge cycles are shown in Table 2, FIGS. 3 and 4.

TABLE 2

Electrolyte Capacity retention after 40 cycles of room temperature (%) Battery resistance after 1 cycle (Ωcm ² ) Battery resistance after 40 cycles (Ωcm ² ) Preparation Example 1 75  17.4 26.2 Preparation Example 2 69  18.5 39.3 Preparation Example 3 64  21.2 48.9 Preparation Example 4 67  19.8 47.0

As shown in Table 2 and Figure 3, the electrolyte solution of Preparation Example 4 without the additive was found to have a capacity retention rate of 67% after 40 cycles, the electrolyte solution of Preparation Example 3 containing the triphenyl phosphate additive as a flame retardant additive is 64 It was the lowest as the capacity retention rate of%. However, the electrolyte solution of Preparation Example 1, in which a flame retardant additive and a functional additive, vinyl ethylene carbonate, and biphenyl were mixed and mixed, showed a capacity retention of 75%, showing the best battery life. This is because, as shown in Table 2 and Figure 4, after 40 cycles the battery internal resistance (R _cell ) is the lowest as 26.2Ωcm ² .

As described above, the lithium ion secondary battery according to the present invention not only improves the thermal stability of the electrolyte solution, but also decreases the internal resistance of the battery, thereby remarkably excellent battery life characteristics.

Claims (17)

Electrolytic solution for lithium ion secondary batteries containing a non-aqueous organic solvent, lithium salt, triphenyl phosphate, vinyl ethylene carbonate, and a biphenyl additive. The electrolyte solution for a lithium ion secondary battery according to claim 1, comprising 0.1 to 20% by weight of triphenyl phosphate, 0.05 to 10% by weight of vinyl ethylene carbonate, and 0.05 to 10% by weight of biphenyl additive. . The lithium salt of claim 1 or 2, wherein the lithium salt is selected from LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ and LiCH ( CF ₃ SO ₂ ) ₂ Lithium ion secondary battery electrolyte, characterized in that any one or two or more salts selected from the group consisting of. The electrolyte for a lithium ion secondary battery according to claim 1 or 2, wherein the non-aqueous organic solvent is one solvent or a mixture of two or more solvents selected from the group consisting of carbonate-based, ester-based, ether-based, and ketone-based solvents. . The electrolyte of claim 3, wherein the non-aqueous organic solvent is one solvent selected from the group consisting of carbonate-based, ester-based, ether-based and ketone-based solvents or a mixture of two or more solvents. The method of claim 4, wherein the carbonate solvent is ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl An electrolyte solution for a lithium ion secondary battery, characterized in that one solvent selected from the group consisting of propyl carbonate (MPC), ethyl propyl carbonate (EPC) and butylene carbonate (BC) or a mixture of two or more solvents. The method of claim 5, wherein the carbonate solvent is ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl An electrolyte solution for a lithium ion secondary battery, characterized in that one solvent selected from the group consisting of propyl carbonate (MPC), ethyl propyl carbonate (EPC) and butylene carbonate (BC) or a mixture of two or more solvents. The electrolyte for a lithium ion secondary battery according to claim 4, wherein the carbonate-based solvent is a mixed solvent of cyclic carbonate and chain carbonate. The electrolyte for a lithium ion secondary battery according to claim 5, wherein the carbonate solvent is a mixed solvent of a cyclic carbonate and a chain carbonate. The electrolyte for a lithium ion secondary battery according to claim 6, wherein the carbonate-based solvent is a mixed solvent of cyclic carbonate and chain carbonate. The electrolyte solution for a lithium ion secondary battery according to claim 5, wherein the ester solvent is one solvent selected from the group consisting of methyl acetate, ethyl acetate, and propyl acetate, or a mixture of two or more solvents. The electrolyte solution for a lithium ion secondary battery according to claim 6, wherein the ether solvent is tetrahydrofuran or 2-methyltetrahydrofuran, or a mixture thereof. The electrolyte for a lithium ion secondary battery according to claim 5, wherein the ketone solvent is polymethylvinyl ketone. delete delete delete delete

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