KR20090029569A - Electrolyte for lithium secondary battery and lithium secondary battery having the same - Google Patents

Abstract

Electrolyte for a lithium secondary battery is provided to obtain a lithium secondary battery with excellent fire retardancy and improved battery performance, by including a cyclic organophosphate compound as a flame retardant additive. Electrolyte for a lithium secondary battery comprises a lithium salt, solvent, and a cyclic organophosphate compound represented by the following chemical formula 1. In the chemical formula 1, R1, R2, R3, R4, R5 and R6 are independently C1-10 alkyl group and a part or the whole of the hydrogen of the alkyl group can be substituted as fluorine. The cyclic organophosphate is haxamethoxy cyclo tri-phosphazene or haxaethoxy cyclo tri-phosphazene.

Description

Electrolyte for lithium secondary battery and lithium secondary battery having same {Electrolyte for Lithium Secondary Battery and Lithium Secondary Battery Having The Same}

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery having the same, and more particularly, to a lithium secondary battery electrolyte which can provide a lithium secondary battery with excellent safety.

In line with the rapid development of portable electronic devices, the development of high capacity and high energy lithium secondary batteries is underway. The lithium secondary battery uses a material capable of intercalations and deintercalation of lithium ions as a negative electrode and a positive electrode, and uses an organic electrolyte or a polymer electrolyte as an electrolyte.

In order to develop a high capacity lithium secondary battery having such a configuration, battery safety is an essential requirement. Lithium secondary batteries have lithium precipitated on the surface of the negative electrode plate as the charge and discharge cycles are repeated, and the lithium is highly reactive with the electrolyte solution, which causes the battery to be subjected to extreme temperatures and pressures such as overcharging, penetration, or compression. There is a risk of explosion due to a sharp rise.

In order to secure the safety of such a lithium secondary battery, research is being conducted in various aspects such as an active material, a separator, a battery system, an electrolyte solution, and the like. Of these, electrolyte additives are expected to have a significant effect with a little investment, and additives with high contribution to safety include overcharge prevention, electrolysis delay, and flame retardant additives. Although the safety of the battery is improving day by day with the development of battery materials and battery management system (BMS), the development of flame retardant additives that can prevent the fire or explosion of the battery, among the above additives, becomes an essential element for the development of large batteries. have.

In this regard, US Pat. No. 5,830,600 discloses an electrolyte solution comprising a flame retardant additive which adds a phosphate-based flame retardant additive and adds a pyrocarbonate-based solvent as a life improvement additive to secure lifespan and safety. However, since the electrolyte solution must include about 50% by volume of a phosphate solvent in order to exhibit a flame retardant effect, there is a problem that the ionic conductivity is lowered and the battery performance is not satisfied.

As such, since a material that satisfies price, flame retardancy, and battery performance has not been developed, development of an electrolyte solution containing a flame retardant additive having low cost and high safety is required.

The present invention is to solve the above problems, comprising a flame retardant additive to provide a lithium secondary battery that can provide a lithium secondary battery excellent in flame retardancy and maintain or improve the battery performance and to provide a lithium secondary battery having the same For the purpose of

The present invention for achieving the above object is a lithium salt; solvent; And it provides a lithium secondary battery electrolyte comprising a cyclic (cyclic) phosphorus compound represented by the formula (1).

[Formula 1]

(In the above formula, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are each independently a C ₁ to C ₁₀ alkyl group, part or all of hydrogen may be substituted with fluorine (F).)

In addition, the cyclic (Cyclic) organophosphorus compound provides an electrolyte solution for a lithium secondary battery, characterized in that the haxamethoxy cyclo tri-phosphazene (HMTP).

In addition, the cyclic (Cyclic) organophosphorus compound provides a lithium secondary battery electrolyte, characterized in that the HETP (haxaethoxy cyclo tri-phosphazene).

In addition, the HMTP (haxamethoxy cyclo tri-phosphazene) provides an electrolyte for a lithium secondary battery, characterized in that included in 2 to 7 parts by weight relative to 100 weight of the electrolyte.

In addition, the HETP (haxaethoxy cyclo tri-phosphazene) provides an electrolyte for a lithium secondary battery, characterized in that contained in 0.5 to 2 parts by weight relative to 100 weight of the electrolyte.

Other embodiments and specific details are included in the detailed description and drawings for carrying out the invention.

According to the present invention described above, it is possible to provide a lithium secondary battery including an electrolyte solution that is excellent in flame retardancy and can maintain or improve battery performance well.

Hereinafter, the electrolyte solution for a lithium secondary battery according to the present invention will be described in more detail. The following detailed description is for the description of one embodiment of the present invention, although not limited to the scope of the claims defined by the claims.

The present invention is a lithium salt; solvent; And it relates to a lithium secondary battery electrolyte comprising a cyclic (cyclic) phosphorus compound represented by the formula (1).

[Formula 1]

(In the above formula, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are each independently a C ₁ to C ₁₀ alkyl group, part or all of hydrogen may be substituted with fluorine (F).)

First, as the lithium salt, any one capable of promoting the movement of lithium ions between the positive electrode and the negative electrode can be used, and representative examples thereof include LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ). ₃ , LiBF ₆ or LiClO ₄ and the like.

As the solvent of the electrolyte according to the present invention, cyclic carbonate, linear carbonate, ester, ether or ketone may be used. The ring carbonate may be a ring carbonate selected from the group consisting of ethylene carbonate, propylene carbonate and mixtures thereof, and the linear carbonate is selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate and methylpropyl carbonate. One or more linear carbonates may be used. In addition, the ester may be an ester selected from the group consisting of n-methyl acetate, n-ethyl acetate and n-propyl acetate. As the ketone, polymethyl vinyl ketone may be used.

The electrolyte solution for a lithium secondary battery of the present invention comprises a cyclic organic phosphorus compound represented by Chemical Formula 1. The cyclic organic phosphorus compound is preferably included in the range of 0.5 to 10 parts by weight based on 100 parts by weight of the electrolyte. When the amount of the cyclic organic phosphorus compound is included in an amount greater than 10 parts by weight, the battery performance of the electrolyte is lowered. If it is included in an amount less than 0.5 parts by weight, the effect on flame retardancy is small.

Preferred specific examples of the cyclic organophosphorus compound according to the present invention include hexamethoxy cyclo tri-phosphazene (HMTP) and hexaethoxy cyclo tri-phosphazene (HETP).

Among them, HMTP is more preferably included in the range of 2 to 7 parts by weight relative to 100 weight of the electrolyte. When the content of the HMTP is less than 2 parts by weight, the flame retardant effect is less. When the content of the HMTP is greater than 7 parts by weight, the battery performance is deteriorated. In addition, HETP is more preferably included in the range of 0.5 to 2 parts by weight relative to 100 weight of the electrolyte. When the content of HETP is less than 0.5 parts by weight, the flame retardant effect is less. When the content of HETP is greater than 2 parts by weight, the battery performance is deteriorated.

Moreover, as long as it does not impair the effect of this invention, the thermosetting initiator etc. which are normally used in this field can also be used together.

When using the electrolyte solution for lithium secondary batteries which concerns on this invention mentioned above, electrochemical stability can be ensured, without deterioration of initial stage discharge capacity, efficiency, etc. That is, safety can be ensured under conditions such as overcharge, external short circuit, compression, and overcharge penetration, and a high capacity battery can be manufactured. In addition, a battery having improved initial charge and discharge cycle life characteristics may be manufactured.

As a cathode active material of a battery using the electrolyte solution of the present invention, all of the transition metal compounds commonly used in lithium secondary batteries may be used, and representative examples thereof include LiCoO ₂ , LiNiO ₂ , and LiNi _x Co _{1- x} M _y O _2. (0.1 <x <1.0, 0 ≦ y <1.0, M is a transition metal), and a transition metal oxide such as LiMnO ₂ , LiMn ₂ O ₄ can be used. A method of manufacturing a cathode using the cathode active material is well known in the battery manufacturing field, and a representative method thereof is a cathode slurry by mixing a cathode active material and a binder such as polyvinylidene fluoride with a solvent such as N-methyl pyrrolidone. After the preparation, the cathode slurry was tape cast on an aluminum foil to prepare a cathode. A conductive agent such as kesene black, carbon black, acetylene black or the like may be further added to the positive electrode slurry. In addition, a cyclic organic phosphorus compound represented by Chemical Formula 1 according to the present invention may be further added to the cathode slurry as a flame retardant additive.

In addition, as the negative electrode active material, all commonly used carbonaceous materials may be used, and representative examples thereof may include crystalline graphite having good dislocation flatness and relatively good reversibility in charge and discharge processes. A method of manufacturing a negative electrode using the negative electrode is also widely known in the battery field, and a representative method thereof is to prepare a negative electrode slurry by mixing a negative electrode active material and a binder such as polyvinylidene fluoride, and then tape the negative electrode slurry on a copper foil. The cathode may be prepared by casting. In the above, a lithium secondary battery may be manufactured by using metal lithium or an alloy-based material as a negative electrode, inserting a separator between the positive electrode and the negative electrode, and injecting an electrolyte solution. The separator serves to block internal short circuits of two electrodes and impregnate the electrolyte, and materials that may be used as the separator include polymer, glass fiber mat, and kraft paper.

As follows, the present invention will be described in more detail based on examples, but the embodiments of the present invention disclosed below are exemplified to the last, and the scope of the present invention is not limited to these embodiments. The scope of the invention is indicated in the appended claims, and moreover, contains all modifications within the meaning and range equivalent to the claims.

Example 1 Lithium Secondary Battery Using Electrolyte Containing HMTP

Coin cells were prepared to observe whether the flame retardant performance and battery performance of the cyclic organophosphorus compound of the present invention and HMTP were improved.

The active material used was _{LiMn 1/3 Ni 1/3} Co 1/3 O 2 , and a lithium foil (Li foil) are each used as a material for the positive electrode and the negative electrode, it consisted of a half-cell (half cell) thereof. The manufacturing procedure of the positive electrode plate is as follows.

As the conductive material and the binder, carbon black (Super-P Black) and polyvinylidene fluoride (PVDF) were used. The weight ratio of active material: conductive material: binder was 85: 8: 7. First, the binder is stirred in an appropriate amount of N-methylpyrrolidone (NMP) and a conditioning mixer for 10 minutes, the active material and the conductive material are mixed therein, and then N-methylpyrrolidone (NMP) is added a little more. After the addition to adjust the viscosity it was stirred for a further 30 minutes. Laminate the aluminum (Al) current collector on a glass plate, cast the slurry prepared on the aluminum (Al) current collector by a doctor blade method, and then dry overnight at 100 ° C. It was. The electrode thus dried was pressed at a compression ratio of 20-25% with a roll press machine at 120 ° C. to complete the electrode.

The coin cell used the 2032 standard, and the electrode manufactured as a positive electrode and the lithium (Li) metal as a negative electrode were used.

As an electrolyte, 1.1 M LiPF ₆ lithium salt, an organic solvent ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (4: 6 by volume) and a flame retardant additive HMTP were used. The amount of HMTP used was 0 parts by weight (not added), 1 part by weight, 3 parts by weight, and 5 parts by weight based on 100 parts by weight of the total electrolyte.

Example 2 Lithium Secondary Battery Using Electrolyte Containing HETP

The same operation as in Example 1 was performed except that HMTP was changed to HETP in Example 1.

The results of testing the thermal stability (flame retardant effect) of the respective electrolytes with different amounts of HMTP are shown in FIG. 1, and the results of testing the thermal stability of each electrolyte with different amounts of HETP are shown in FIG. 3. It was. The discharger was tested using a TOOS TOSCAT-3100U model.

The thermal stability test is performed by charging and discharging the prepared coin cell 10 times, recovering the charged positive electrode material and measuring it by differential scanning calorimeter (DSC). In other words, when the temperature rises, the cathode material generates a self-heating phenomenon at a specific temperature. The self-heating phenomenon refers to a phenomenon in which oxygen is released while the cathode active material is decomposed at a temperature of 200 ° C. or higher. This heat generation is a major cause of fire and explosion if the battery is misused or used under conditions other than recommended.

1 is a graph showing a self-exothermic reaction of a positive electrode material using each electrolyte solution prepared in Example 1. FIG. In FIG. 1, 'background' refers to an electrolyte containing no additive HMTP. As shown in FIG. 1, 'background' without HMTP shows a sharp self exothermic reaction at around 270 ° C. On the other hand, the electrolyte containing 1 part by weight of HMTP did not exhibit flame retardancy, but the electrolytes contained in the HMTP 3 and 5 parts by weight of the positive electrode material showed that the self-heating temperature is delayed to about 320 ~ 330 ℃, HMTP containing electrolyte It has been shown to possess effective flame retardancy.

Figure 2 is a graph showing the results of the rate characteristic test of the lithium secondary battery using the respective electrolyte solution prepared in Example 1. The batteries were charged up to 4.3V each at 0.5C, discharged to 2.8V under the conditions of a given discharge current, and experimented with a 30 minute pause between charging and discharging. As shown in FIG. 2, it can be seen that the capacity of the battery is slightly improved when HMTP is added, which shows better rate characteristics.

3 is a graph showing a self-exothermic reaction of a positive electrode material using each electrolyte solution including HETP prepared in Example 2. FIG. In FIG. 3, the background refers to an electrolyte that does not include HETP as an additive. As shown in FIG. 3, 'background' without HETP shows a sharp self-exothermic reaction at around 270 ° C. On the other hand, all electrolytes containing 1, 3, and 5 parts by weight of HETP delayed the self-heating temperature of the positive electrode material to about 320 ~ 330 ° C and slowed the intensity of the exothermic phenomenon, indicating that the flame retardant effect of HETP was excellent. have.

Figure 4 is a graph showing the results of the rate characteristic experiment of the lithium secondary battery using each electrolyte solution prepared in Example 2. The batteries were charged up to 4.3V each at 0.5C, discharged to 2.8V under the conditions of a given discharge current, and experimented with a 30 minute pause between charging and discharging. As shown in FIG. 4, when HETP is included in an amount of 1 part by weight, it can be seen that the rate characteristic of the battery is improved while having an excellent flame retardant effect.

Therefore, both electrolytes including HMTP and HETP, which are cyclic organophosphorus compounds according to the present invention, are excellent in flame retardancy and can maintain or improve battery performance well.

1 is a graph showing a self exothermic reaction of a positive electrode material using an electrolyte solution containing HMTP,

2 is a graph showing results of a rate characteristic test of a lithium secondary battery using an electrolyte solution containing HMTP,

3 is a graph showing a self-exothermic reaction of a positive electrode material using an electrolyte solution containing HETP,

Figure 4 is a graph showing the results of the rate characteristic test of the lithium secondary battery using an electrolyte containing HETP.

Claims (6)

Lithium salts; solvent; And An electrolyte for a lithium secondary battery comprising a cyclic (organic) phosphorus compound represented by Chemical Formula 1. [Formula 1]

(In the above formula, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ are each independently a C ₁ to C ₁₀ alkyl group, part or all of hydrogen may be substituted with fluorine (F).) The electrolyte of claim 1, wherein the cyclic (Cyclic) organic compound is haxamethoxy cyclo tri-phosphazene (HMTP). The electrolyte of claim 1, wherein the cyclic (Cyclic) organic compound is HETP (haxaethoxy cyclo tri-phosphazene). The method of claim 2, wherein the haxamethoxy cyclo tri-phosphazene (HMTP) is an electrolyte solution for a lithium secondary battery, characterized in that it comprises 2 to 7 parts by weight based on 100 weight of the electrolyte. The electrolyte for a lithium secondary battery of claim 3, wherein the HETP (haxaethoxy cyclo tri-phosphazene) is included in an amount of 0.5 to 2 parts by weight based on 100 parts by weight of the electrolyte. The lithium secondary battery provided with the electrolyte solution for lithium secondary batteries in any one of Claims 1-5.

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