KR100988666B1 - Electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same, the electrolyte is a non-aqueous organic solvent; Lithium salts; Silver hexafluoro phosphate; And fluoro ethylene carbonate. The lithium ion secondary battery including the electrolyte solution of the present invention has a small thickness increase rate of the battery when left at high temperature, thereby improving high temperature stability and excellent life characteristics.

Lithium battery, electrolyte solution, additives, silver hexafluorophosphate, fluoro ethylene carbonate

Description

Electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising same {Electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising the same}

The present invention relates to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery including the same, and specifically, high temperature standing by adding silver hexafluoro phosphate and fluoro ethylene carbonate (FEC) to the electrolyte. It relates to a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same, the thickness increase rate of the battery is small, the high temperature stability is improved, the life characteristics are excellent.

        A battery is a device that converts chemical energy of a chemical substance contained therein into electrical energy by an electrochemical redox reaction. Recently, with the rapid development of the electronics, telecommunications, and computer industries, camcorders, mobile phones, notebooks, PCs, PDAs, and portable IT devices have emerged, requiring the development of lightweight, long-lasting, reliable, high-performance small rechargeable batteries. have. Lithium-ion batteries have been spotlighted as secondary batteries because they have high discharge voltage, excellent weight energy density, and low self discharge.

Such lithium secondary batteries generally use a lithium metal mixed oxide as a positive electrode active material and a carbon material or metal lithium as a negative electrode material. Since lithium batteries have a high driving voltage of more than 3V, they use a nonaqueous electrolyte having a wide withstand voltage range. The lithium salt dissolved in an organic solvent is mainly used. Lithium hexafluorophosphate (LiPF 6) or lithium tetrafluoro borate (LiBF 4) has been proposed as a salt having excellent dissociation property.

Although the electrolyte of a lithium secondary battery needs to be stable against lithium, it is said that there is no solvent stable against lithium in thermodynamics. In practice, the electrolyte is decomposed to the negative electrode during initial charging, and the reaction product is released on the surface of lithium. It is considered to be stabilized because an ion conductive protective film, that is, a SEI (Solid Electrolyte interface) film is formed and the reaction between the electrode and the electrolyte is suppressed. Therefore, when the stable SEI film is produced, the conductivity is high, and the mobility and lithium-ion characteristics of the lithium ions are improved, thereby improving the lifespan and high rate characteristics of the battery. At present, attempts have been actively made to secure high rate characteristics of a battery by adding a small amount of a specific additive in an electrolyte of a lithium secondary battery (Japanese Patent Laid-Open No. 8-22839). However, in the thin-film rectangular battery, gases such as CO, CO 2, CH 4, and C 2 H 6 are generated from decomposition of the carbonate-based organic solvent during the above-mentioned SEI formation reaction, causing a problem that the thickness of the battery is expanded during charging. Accordingly, there is a need in the art for the development of an electrolyte solution having excellent high temperature performance without the problem of volume expansion during high temperature storage in a lithium secondary battery.

        An object of the present invention to solve the above problems is to provide a lithium ion secondary battery electrolyte and a lithium ion secondary battery electrolyte having a high temperature stability is improved, the lifespan characteristics is improved, the thickness increase rate of the battery is small when left at high temperature.

        In order to achieve the above object, the present invention

        Non-aqueous organic solvents; Lithium salts; Silver hexafluoro phosphate (AgPF6) as an additive; And fluoro ethylene carbonate (Fluoro Ethylene Carbonate), wherein the silver hexafluoro phosphate is contained in an amount of 0.1 to 1 parts by weight based on 100 parts by weight of the total electrolyte, and the fluoroethylene carbonate is 0.1 to 5 parts by weight based on 100 parts by weight of the total electrolyte. It provides the electrolyte solution for lithium ion secondary batteries contained by the part.

The non-aqueous organic solvent may be a single or a mixture thereof selected from the group consisting of carbonates, esters, ethers and ketones.

        The carbonate is alone or a mixture thereof selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and pentylene carbonate Can be.

The ester is from the group consisting of methyl acetate, ethyl acetate, propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone and caprolactone Selected solely or a mixture thereof.

The ether may be alone or a mixture thereof selected from the group consisting of dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyl tetrahydrofuran and tetrahydrofuran.

The ketone may be a single or a mixture thereof selected from the group consisting of polymethylvinyl ketone and cyclohexanone.

The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ), where p and q are natural water, and may be a single or a mixture thereof selected from the group consisting of LiCl and LiI.

In order to achieve the other object of the present invention,

A positive electrode comprising a positive electrode active material capable of reversibly inserting and detaching lithium;

        A negative electrode comprising a negative electrode active material capable of reversibly inserting and detaching lithium; And

It provides a lithium ion secondary battery comprising the electrolyte according to the present invention.

        The lithium ion secondary battery including the electrolyte of the present invention generates a SEI (Solid Electrolyte Interface) film having excellent thermal stability on the surface of the negative electrode during initial charging, so that the thickness increase rate of the battery is small when it is left at a high temperature, so that the high temperature stability is excellent and the life characteristics are excellent. Do.

       Hereinafter, the present invention will be described in detail.

       The electrolyte solution of the present invention includes silver hexafluoro phosphate and fluoro ethylene carbonate of formula 1 as an additive:

       [Formula 1]

The silver hexafluorophosphate is added to the electrolyte together with fluoroethylene carbonate to form a high thermal stability film on the negative electrode during the initial charge of the battery, and on the surface of the electrode active material when the battery is exposed to high temperature. This prevents the loss of lithium due to the reaction between lithium and the electrolyte, thereby preventing the deterioration of cycle characteristics and improving the high temperature stability.

According to an embodiment of the present invention, the silver hexafluoro phosphate may be included in an amount of 0.1 to 1 part by weight based on 100 parts by weight of the total electrolyte, and fluorine ethylene carbonate may be included in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the total electrolyte. When added below or in the above range, the high temperature stability and lifespan characteristics are not improved, and when added in the above range, the viscosity of the electrolyte may be increased, thereby reducing the mobility of lithium ions.

The electrolyte solution of the present invention also contains a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

As the non-aqueous organic solvent, one or a mixture thereof selected from the group consisting of carbonates, esters, ethers and ketones can be used. Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (EMC), and ethylene carbonate ( EC), propylene carbonate (PC) and butylene carbonate (BC) and the like can be used. The ester solvents include n-methyl acetate, n-ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone and mevalo Norlactone and caprolactone can be used. As the ether, dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyl tetrahydrofuran, tetrahydrofuran, etc. may be used, and the ketone solvent may be cyclohexanone, polymethylvinyl ketone, or the like. This can be used.

The non-aqueous organic solvents may be used alone or in mixture of one or more, and the mixing ratio in the case of mixing one or more may be appropriately adjusted according to the desired battery performance. The organic solvent should be used to have a high dielectric constant and low viscosity in order to increase the dissociation of ions to facilitate ion conduction. In general, it is preferable to use two or more mixed solutions composed of a solvent having a high dielectric constant and a high viscosity and a solvent having a low dielectric constant and a low viscosity. In the case of the carbonate solvent, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, the cyclic carbonate and the chain carbonate may be mixed and used in a volume ratio of 1: 1 to 1: 9, so that the performance of the electrolyte may be excellent.

The non-aqueous organic solvent of the present invention may further include an aromatic hydrocarbon organic solvent in the carbonate solvent. As the aromatic hydrocarbon organic solvent, an aromatic hydrocarbon compound represented by Chemical Formula 2 may be used.

[Formula 2]

(Wherein R is a halogen or an alkyl group having 1 to 10 carbon atoms and q is an integer of 0 to 6).

Specific examples of the aromatic hydrocarbon organic solvent may include benzene, fluorobenzene, bromobenzene, chlorobenzene, toluene, xylene, mesitylene, and the like, which may be used alone or in combination. When the volume ratio of carbonate solvent / aromatic hydrocarbon organic solvent is 1: 1 to 30: 1 in an electrolyte solution containing an aromatic hydrocarbon organic solvent, the stability, safety, and ion conductivity, which are generally required for the electrolyte, are superior to other ratio compositions. Do.

The lithium salt of the present invention serves as a source of lithium ions in the battery and enables the operation of the basic lithium ion secondary battery, and serves to promote the movement of lithium ions between the positive electrode and the negative electrode. Lithium salts are LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ), where p and q are natural water, and may be a single or a mixture thereof selected from the group consisting of LiCl and LiI. It is preferable to use a small lattice energy, a large dissociation degree, excellent ion conductivity and good thermal stability and oxidation resistance. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. If the concentration of the lithium salt is less than 0.1M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, if it exceeds 2.0M there is a problem that the mobility of the lithium ion is reduced by increasing the viscosity of the electrolyte.

The lithium secondary battery including the electrolyte solution of the present invention includes a positive electrode, a negative electrode, and a separator.

The positive electrode includes a positive electrode active material capable of reversibly inserting and detaching lithium ions, and the positive electrode active material is preferably at least one selected from cobalt, manganese, nickel, and a composite metal oxide with lithium. The solid solution ratio between the metals may be various, and in addition to these metals, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, An element selected from the group consisting of Sr, V and rare earth elements may be further included.

The negative electrode includes a negative electrode active material capable of inserting and desorbing 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) and combustion of a carbon composite. Organic polymer compounds, carbon fibers, tin oxide compounds, lithium metal or alloys of lithium and other elements can be used. Examples of amorphous carbon include hard carbon, coke, mesocarbon microbeads (MCMB) fired at 1500 ° C. or lower, and mesophase pitch-based carbon fibers (MPCF). Crystalline carbons include graphite-based materials, and specific examples thereof include natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF.

The positive electrode or the negative electrode may be prepared by dispersing an electrode active material, a binder and a conductive material, if necessary, a thickener in a solvent to prepare an electrode slurry composition, and applying the slurry composition to an electrode current collector. As the positive electrode current collector, aluminum or an aluminum alloy may be commonly used, and as the negative electrode current collector, copper or a copper alloy may be commonly used. Examples of the positive electrode current collector and the negative electrode current collector include foil or mesh shapes.

As the separator that prevents a short circuit between the positive electrode and the negative electrode in a lithium secondary battery, a polymer film such as polyolefin, polypropylene, polyethylene or the like, or a multi-layer film thereof, a microporous film, a woven fabric or a nonwoven fabric may be used.

The lithium secondary battery including the above-described electrolyte, positive electrode, negative electrode, and separator has a unit cell having a structure of positive electrode / separator / cathode, a bicell having a structure of positive electrode / separator / cathode / separator / anode, or a unit cell structure. It can be formed in the structure of a repeated laminated battery.

A representative example of the lithium secondary battery of the present invention having such a configuration is shown in FIG. 1.

Referring to FIG. 1, in a lithium secondary battery, an electrode assembly 12 including an anode 13, a cathode 15, and a separator 14 is accommodated in a can 10 together with an electrolyte solution. It is formed by sealing the upper end with the cap assembly 20. The cap assembly 20 includes a cap plate 40, an insulating plate 50, a terminal plate 60, and an electrode terminal 30. The cap assembly 20 is combined with the insulating case 70 to seal the can 10.

The electrode terminal 30 is inserted into the terminal through-hole 41 formed in the center of the cap plate 40. When the electrode terminal 30 is inserted into the terminal through-hole 41, the tubular gasket 46 is coupled to the outer surface of the electrode terminal 30 and inserted together to insulate the electrode terminal 30 and the cap plate 40. . After the cap assembly 20 is assembled to the upper end of the can 10, the electrolyte is injected through the electrolyte injection hole 42, and the electrolyte injection hole 42 is closed by a stopper 43. The electrode terminal 30 is connected to the negative electrode tab 17 of the negative electrode 15 or the positive electrode tab 16 of the positive electrode 13 to act as a negative electrode terminal or a positive electrode terminal.

The lithium secondary battery of the present invention may be formed in other shapes, such as cylindrical, pouch type, in addition to the illustrated square.

The following Examples and Comparative Examples are described. The following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

 Example 1

A positive electrode active material slurry was prepared by dispersing LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVdF) as a binder, and carbon as a conductive agent in an N-methyl-2-pyrrolidone solvent at a weight ratio of 92: 4: 4. The positive electrode active material slurry was coated on aluminum foil having a thickness of 15 μm, dried, and rolled to prepare a positive electrode. Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. The slurry was coated on a copper foil having a thickness of 10 μm, dried, and rolled to prepare a negative electrode.

A separator of polyethylene (PE) material having a thickness of 16 μm was put between the electrodes prepared as described above and wound and compressed to insert a 46 mm × 34 mm × 50 mm square can. An electrolyte solution was injected into this can to prepare a lithium secondary battery.

The electrolyte was prepared by adding LiPF ₆ to 1.0 M in a mixed solvent of ethylene carbonate, ethylmethyl carbonate and diethyl carbonate (1: 1: 1 volume ratio), and then adding silver hexafluoro phosphate and fluoro ethylene carbonate. At this time, silver hexafluoro phosphate was added at 0.2 parts by weight based on 100 parts by weight of the total electrolyte solution, and fluorine ethylene carbonate was added at 3 parts by weight.

  Example 2

Except that in Example 1 0.5 parts by weight of silver hexafluoro phosphate and 3 parts by weight of fluoro ethylene carbonate was added in the same manner as in Example 1.

Example 3

Except that in Example 1 1 part by weight of silver hexafluoro phosphate and 3 parts by weight of fluoro ethylene carbonate was added in the same manner as in Example 1.

Comparative Example 1

Example 1 was carried out in the same manner as in Example 1 except that silver hexafluoro phosphate and fluoro ethylene carbonate were not added.

Comparative Example 2

In Example 1, only silver hexafluoro phosphate was added in 0.2 parts by weight, and the same procedure as in Example 1 was performed except that no fluoroethylene carbonate was added.

Comparative Example 3

Except for adding silver hexafluoro phosphate in Example 1 was carried out in the same manner as in Example 1 except that only 3 parts by weight of fluoro ethylene carbonate.

Comparative Example 4

Except that in Example 1 2 parts by weight of silver hexafluoro phosphate and 3 parts by weight of fluoro ethylene carbonate was added in the same manner as in Example 1.

The capacity and capacity retention rate of the secondary batteries prepared by Examples and Comparative Examples according to the repeated charging and discharging, the thickness increase rate of the battery after 7 days at 60 ℃ to measure the silver hexafluoro phosphate and fluoroethylene carbonate as additives In this case, the effects on the life characteristics and the high temperature leaving characteristics of the secondary battery were confirmed.

<Experimental Example 1: Life Test-Capacity and Capacity Retention Rate According to Repeated Charge / Discharge>

The batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were charged for 3 hours at 1 C / 4.2 V and constant current-constant voltage at room temperature, and then discharged at 1 C / 3.0 V constant current. 100, 200, 300, and 400 cycles of the charging and discharging were performed, respectively, and the capacity of each cycle was shown in FIG. 2, and the capacity retention rate (%) of the cycle was calculated as follows. 3 and Table 1 are shown.

*% Capacity retention of each corresponding cycle = (discharge capacity of each corresponding cycle)

/ Discharge capacity at 1 cycle) x 100 (%)

Experimental Example 2 High Temperature Stability Evaluation—Measurement of Thickness Increase of Battery After 7 Days at 60 ° C.

The batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were charged with 0.5 C / 4.2 V and constant current-constant voltage for 3 hours, and left at 60 ° C. for 7 days to measure the thickness increase rate of the batteries. The results are shown in Table 1 below.

A: Initial thickness

B: Thickness after 7 days at 60 ° C

  *% Of thickness increase = (B-A) / A

TABLE 1

AgPF6
(Parts by weight) FEC
(Parts by weight) 400th
After charge and discharge cycle
Capacity retention rate (%) 7 days at 60 ℃
After neglect
Thickness increase rate (%) Example 1 0.2 3 90 8.5 Example 2 0.5 3 88 7.4 Example 3 One 3 85 7.1 Comparative Example 1 0 0 - 17.0 Comparative Example 2 0.2 0 - 14.0 Comparative Example 3 0 3 80 16.0 Comparative Example 4 2 3 73 7.0

As shown in Table 1, in the case of Examples 1 to 3, the capacity retention rate is excellent after repeated charging and discharging cycles, and the temperature increase rate of the battery is small when it is left at a high temperature for a long time, thereby improving the high temperature stability. When only silver hexafluoro phosphate is used as an additive, the life characteristics and the high temperature stability are not improved, and when only fluoroethylene carbonate is added, the thickness of the battery is large and the high temperature stability is poor. In Comparative Example 4, in which silver hexafluoro phosphate was added in 2 parts by weight, the capacity retention rate after the charge and discharge cycle was not good.

2 and 3, in Comparative Examples 1 and 2, after 100 charge / discharge cycles, the capacity was reduced to 600 mAh or less, and the capacity retention ratio was also dropped to 80% or less. After 400 charge / discharge cycles, the capacity and capacity retention rate were higher than those of the comparative example, indicating that the life characteristics were improved.

1 illustrates a rectangular lithium ion secondary battery according to an embodiment of the present invention.

2 is a graph showing the capacity according to the number of charge and discharge cycles of the battery according to the embodiment and the comparative example of the present invention.

Figure 3 is a graph showing the capacity retention rate according to the number of charge and discharge cycles of the battery according to the Examples and Comparative Examples of the present invention.

Claims (8)

Non-aqueous organic solvents; LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiN (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ), wherein p and q are natural water, lithium salts alone or in a mixture thereof selected from the group consisting of LiCl and LiI; 0.1 to 1 parts by weight of silver hexafluor phosphate based on 100 parts by weight of the total electrolyte; And An electrolyte solution for a lithium ion secondary battery, comprising 0.1 to 5 parts by weight of fluoroethylene carbonate based on 100 parts by weight of the total electrolyte. The method of claim 1, The non-aqueous organic solvent is a lithium ion secondary battery electrolyte, characterized in that the singly or a mixture thereof selected from the group consisting of carbonate, ester, ether and ketone. The method of claim 2, The carbonate is dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylmethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and pentylene carbonate alone or a mixture thereof Electrolyte for lithium ion secondary battery characterized by the above-mentioned. The method of claim 2, The ester is n-methyl acetate, n-ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalo An electrolyte solution for a lithium ion secondary battery, characterized in that it is a single or a mixture thereof selected from the group consisting of norlactone and meprolactone. The method of claim 2, An electrolyte solution for a secondary battery, wherein the ether is a single or a mixture thereof selected from the group consisting of dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyl tetrahydrofuran, and tetrahydrofuran. The method of claim 2, The ketone is a cyclohexanone and polymethylvinyl ketone, an electrolyte solution for a lithium ion secondary battery, characterized in that alone or a mixture thereof. delete A positive electrode including a positive electrode active material capable of reversibly inserting and detaching lithium; A negative electrode including a negative electrode active material capable of reversibly inserting and detaching lithium; And The lithium ion secondary battery containing the electrolyte solution of any one of Claims 1-6.

Images