KR20050013717A - Electrolyte for lithium secondary cell having excellent performance at low temperature and lithium secondary cell comprising its electrolyte - Google Patents

Abstract

PURPOSE: An electrolyte solution for a lithium secondary battery and a lithium secondary battery containing the electrolyte solution are provided, to improve low temperature performance without the deterioration of other performance by forming a stable passive state coating on the surface of a negative electrode. CONSTITUTION: The electrolyte solution comprises an organic solvent; a lithium salt; and 0.05-5 wt% of the 1-propanephosphonic acid cyclic anhydride represented by the formula 1. Preferably the organic solvent is selected from the group consisting of a cyclic carbonate and a linear carbonate. The lithium secondary battery comprises the electrolyte solution; a positive electrode comprising a lithium-transition metal oxide as a positive electrode active material; and a negative electrode comprising carbon, a carbon composite, a lithium metal or a lithium alloy as a negative electrode active material.

Description

ELECTROLYTE FOR LITHIUM SECONDARY CELL HAVING EXCELLENT PERFORMANCE AT LOW TEMPERATURE AND LITHIUM SECONDARY CELL COMPRISING ITS ELECTROLYTE}

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, and more particularly to a lithium secondary battery electrolyte containing 1-propanephosphonic acid cyclic anhydride and a lithium secondary battery comprising the same.

Recently, with the development of the high-tech electronic industry, it is possible to reduce the weight and weight of electronic equipment, thereby increasing the use of portable electronic devices. As a power source for such portable electronic devices, the need for a battery having a high energy density has been increased, and research on lithium secondary batteries has been actively conducted.

A lithium secondary battery consists of a positive electrode / cathode active material, an electrical power collector, and electrolyte solution. The cathode / anode active material generates electricity, and lithium-transition metal oxide is used as the cathode active material, and lithium metal, lithium alloy, carbon (crystalline or amorphous), or carbon composite material is used as the anode active material. The current collector uses a metal current collector as a path for moving electrons generated and supplied to the active material. In addition, the electrolyte serves as a medium for ion conduction, and consists of a non-aqueous solvent, lithium salt, and other additives.

Lithium secondary batteries have an average discharge voltage of about 3.6 to 3.7 V, and can obtain higher power than other alkaline batteries such as Ni-MH batteries and Ni-Cd batteries. An electrochemically stable electrolyte composition at -4.2 V is required. For this reason, a mixture of non-aqueous carbonate solvents such as ethylene carbonate, dimethyl carbonate and diethyl carbonate is used as the electrolyte solution. However, the electrolytic solution having such a composition has a problem in that the battery characteristics are lowered at high rate charge / discharge compared to the aqueous electrolytic solution used in Ni-MH batteries or Ni-Cd batteries.

Although lithium secondary batteries have advantages such as energy density, there are many difficulties in adopting them to products used at low temperatures for the same reason.

Accordingly, attempts have been made to improve low temperature performance of lithium secondary batteries capable of high ion conduction even at low temperatures. Attempts to improve the low temperature performance of conventional lithium secondary batteries are mainly made of a technique that enables high ion conduction even at low temperatures by using an electrochemically stable, low freezing point, low ester viscosity solvent added to the electrolyte. Has been. H.-C (Alex) Shiano et al. ( J. Power Sources , 87 (2000), 167-173) added methyl acetate to improve low temperature performance, and S. Herreyre et al. ( J. Power Sources , 97- 98 (2001), 576-580) improved the low temperature performance by adding ethyl acetate and methyl butyrate to the electrolyte. However, a lithium secondary battery using an ester-based solvent easily swells when left at high temperature due to high vapor pressure, and has a problem of deterioration of other performances by promoting side reactions because of poor electrochemical safety.

In addition, research results to suppress the crystallization of ethylene carbonate in the electrolyte by adding propylene carbonate have been published by SS Zhang et al. ( J. Power Sources, 110 (2002), 216-221), but in the case of using propylene carbonate, Side reactions continue to deteriorate the performance.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a lithium secondary battery electrolyte and a lithium secondary battery including the same, which can prevent the degradation of other performance as well as improving low-temperature performance.

1 is a graph showing a -10 ° C discharge capacity of an electrolyte solution for lithium secondary batteries for experimenting with low temperature performance effects of the electrolyte solution for lithium secondary batteries of the present invention.

In order to achieve the above object, the present invention provides an electrolyte solution for a lithium secondary battery to which 1-propanephosphonic acid cyclic anhydride represented by the following formula (1) is added to a mixed solution containing an organic solvent and a lithium salt.

Hereinafter, the present invention will be described in more detail.

The electrolyte of the present invention is prepared by adding 1-propanephosphonic acid cyclic anhydride represented by Chemical Formula 1 to a mixed solution containing an organic solvent and a lithium salt generally used as an electrolyte.

The 1-propanephosphonic acid cyclic anhydride forms a stable passivation film on the negative electrode surface during the charge and discharge of the lithium secondary battery, the lithium secondary battery made of an electrolyte solution containing the same can achieve the low temperature performance without any other performance degradation.

In the present invention, 1-propanephosphonic acid cyclic anhydride is added at 0.05 to 5% by weight, preferably 0.1 to 3% by weight based on the electrolyte. In this case, when the amount of 1-propanephosphonic acid cyclic anhydride added is less than the above range, there is no improvement in low-temperature performance, and when the above-mentioned range is exceeded, the charge and discharge characteristics are deteriorated.

In addition, the electrolyte solution of the present invention contains a non-aqueous organic solvent and lithium salts commonly used in the art. The non-aqueous organic solvent may be a cyclic or chain carbonate, and may be used by mixing two or more. Specific examples thereof include ethylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, propylene carbonate, and the like.

The lithium salt is used as a supporting electrolytic salt by mixing one or two or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium trifluoromethanesulfonate, and lithium hexafluoroacetate. However, the present invention is not limited thereto.

The electrolyte solution of the lithium secondary battery of the present invention is usually stable in the temperature range of -20 to 60 ℃ to maintain stable characteristics even at a voltage of 4V. The electrolyte solution of the present invention can be applied to all lithium secondary batteries such as lithium ion batteries and lithium polymer batteries.

In the present invention, the cathode material of the lithium secondary battery is LiCo ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O _4, or LiNi _1-xy Co _x M _y O ₂ (0 ≦ x ≦ 1, 0 ≦ _y ≦ 1, o ≦ x + y ≤ 1, M is a lithium-transition metal oxide, such as Al, Sr, Mg, La (metals such as La), and as a negative electrode material using a crystalline or amorphous carbon, carbon composite, lithium metal or lithium alloy do. After applying the active material to the current collector of the thin plate with a suitable thickness and length, or applying the active material itself in the form of a film, it is wound or laminated with a separator that is an insulator and placed in a container, a can, or a similar container to form an electrode group. A lithium secondary battery is prepared by injecting an electrolyte solution to which propanephosphonic acid cyclic anhydride is added. As the separator, a resin such as polyethylene or polypropylene may be used.

Hereinafter, the present invention will be described in more detail with reference to Examples, but these Examples illustrate the present invention, but the scope of the present invention is not limited to these Examples.

<Examples 1-4> Preparation of the electrolyte solution of this invention 1-4

A mixed solution consisting of 1-propanephosphonic acid cyclic anhydride and ethyl acetate 1: 1 (weight ratio) commercially available in the basic electrolyte composition (EC / EMC = 30/70, LiPF ₆ 1.15 M) was added as described in Table 1 below. To prepare an electrolyte solution of the present invention.

<Examples 5-8> Production of Lithium Secondary Batteries 1-4

The cells were assembled, the assembled cells were placed in an aluminum pouch, and the electrolyte solutions prepared in Examples 1 to 4 were added thereto. Heat and pressure were applied to the aluminum pouch to seal the lithium secondary battery. Specifically, it was prepared by the following method.

(1) membrane film production

9 g of PVDF-HFP copolymer (88:12) -KynarFLEX 2801 (atochem) was added to 100 ml of acetone, and the temperature was maintained at 45 ° C. while stirring to dissolve the polymer in a solvent. 10 g of dibutyl phthalate and 1 g of fumed silica were added as a plasticizer, followed by further stirring for 30 to 60 minutes while maintaining the temperature to obtain a uniformly dispersed solution. The mixed solution was coated and coated on a surface-treated PET film using a coater and dried in a dry area. Here, the film thickness was set to 45 micrometers.

(2) Preparation of Cathode Film

10 g of PVDF-HFP copolymer (88:12) -KynarFLEX 2801 (atochem) was added to 100 ml of acetone, and the temperature was maintained at 45 ° C. while stirring to dissolve the polymer in a solvent. 16 g of dibutyl phthalate and 2.5 g of conductive carbon (Super-P) were added and dispersed as a plasticizer. Then, 40 g of graphite (Osaka gas) was added as an active material and stirred for 1 hour while maintaining the temperature.

The mixed solution was coated and coated on a surface-treated PET film using a coater and dried in a dry area. Here, the film thickness was 120 micrometers.

(3) production of anode film

10 g of PVDF-HFP copolymer (88:12) -KynarFLEX 2801 (atochem) was added to 100 ml of acetone and the temperature was maintained at 45 ° C. while stirring to dissolve the polymer in the solvent. 14 g of dibutyl phthalate and 5.0 g of conductive carbon (1: 1 mixture of Super-P and KS6) were added as a plasticizer and dispersed therein, followed by 40 g of lithium cobalt oxide and LiCoO ₂ (Nihon Kogyo) as an active material. Stir for 1 hour while maintaining the temperature.

The mixed solution was coated and coated on a surface-treated PET film using a coater and dried in a dry area. Herein, the thickness of the film was set to 90 µm.

(4) current collector pretreatment

The copper expanded mesh, which is a negative electrode current collector, and the aluminum expanded mesh, which is a positive electrode current collector, were washed and primer-coated so as to be well adhered to the respective negative and positive electrode films. The washing was performed by washing each current collector with acetone to remove organic matter.

Primer coating was carried out by adding 15% by weight of conductive carbon (super-P), 60% by weight of PVDF-HFP copolymer (88:12) -KynarFLEX 2801 (atochem), and 25% of dibutyl phthalate to 3% by weight of acetone solution. One solution was prepared, coated with a graveur coater and dried by hot air.

(5) cathode manufacturing

The pre-treated copper expanded mesh and the negative electrode film prepared above were laminated with a laminator having a heating roll installed in the form of a negative electrode film copper current collector-negative film under constant temperature and pressure adjustment. At this time, the junction temperature was maintained at 140 ℃ and produced by applying a pressure so as not to crush the electrode shape.

(6) anode manufacturing

The pre-treated aluminum expanded mesh and the positive electrode film prepared above were laminated using a laminator equipped with a heating roll in the form of a positive electrode film-aluminum current collector-anode film to adjust temperature and pressure at a constant rate. At this time, the junction temperature was maintained at 140 ℃ and produced by applying a pressure so as not to crush the electrode shape.

(7) battery bonding

The battery was bonded using the negative electrode and the positive electrode prepared above.

The battery was laminated in the order of positive electrode-membrane-cathode-membrane-anode, and then laminated with a laminator having a heating roll. At this time, the junction temperature was maintained at 130 ℃ and produced by applying a pressure so as not to crush the battery form.

(8) plasticizer extraction

Dibutyl phthalate in the battery was extracted and removed by using methanol as a solvent in the battery prepared above. This extraction was carried out using fresh methanol each time three times for 20 minutes in the cell in methanol. After extraction, the mixture was dried over 24 hours at a temperature of 40 ℃ in a vacuum oven.

(9) electrolyte impregnation

The battery was prepared by impregnating the battery in which the micropores were formed by the plasticizer extraction with the electrolyte solution of Examples 1 to 4 in a glove box filled with argon gas and having a moisture concentration of 5 ppm or less.

10 packing

The battery prepared above was sealed by vacuum thermal bonding method with an aluminum laminate battery packaging pack.

(11) charging and discharging

The battery prepared in the above was finally discharged by charging and discharging the section 4.2 to 3.0 V three times with a current of C / 2.

Comparative Example 1 Manufacture of Lithium Secondary Battery 1

A lithium secondary battery was manufactured in the same manner as in Example 5, except that a basic electrolyte composition (EC / EMC = 30/70, LiPF ₆ 1.15 M) was used instead of the prepared electrolyte solution of the present invention.

Comparative Example 2 Fabrication of Lithium Secondary Battery 2

In the above examples, 1-propanephosphonic acid cyclic anhydride is commercially available, and is mixed 1: 1 with ethyl acetate. It was difficult to separate them, and lithium secondary batteries were prepared as described below to compare the performance of lithium secondary batteries with ethyl acetate. Specifically, a lithium secondary battery was manufactured in the same manner as in Example 5, except that 1 wt% of ethyl acetate was added to the basic electrolyte as described in Example 1 instead of the prepared electrolyte of the present invention.

Experimental Example 1 Measurement of Low Temperature Performance of a Lithium Secondary Battery

In order to measure the low temperature performance of the lithium secondary batteries prepared in Examples and Comparative Examples, -10 ° C discharge capacity was measured. Specifically, the low-temperature performance was charged at room temperature at a rate of 1 / 2C and discharged at -10 ℃ to measure the discharge capacity.

The results are shown in Table 2 and FIG. 1.

-10 ℃ discharge capacity Example 5 (0.5 wt.%) 65.3% Example 6 (1.0 wt.%) 71.2% Example 7 (2.0 wt.%) 69.0% Example 8 (3.0 wt.%) 55.1% Comparative Example 1 58.6% Comparative Example 2 59.3%

As shown in Table 2 and FIG. 1, the -10 ° C discharge capacity of the lithium secondary battery including the electrolyte solution to which 1-propanephosphonic acid cyclic anhydride was added was 65% except Example 8. It shows a better low temperature performance than 1, in particular, the lithium secondary battery of Example 6 it can be seen that the best low temperature performance with a discharge capacity of 71.2% of -10 ℃.

In addition, it can be seen that in Comparative Example 2, in which only ethyl acetate was added, the improvement in low-temperature performance was weak. Through this, it can be seen that the low temperature performance of the lithium secondary battery prepared in the above example is due to 1-propanephosphonic acid cyclic anhydride, and the ethyl acetate added together does not affect this.

As described above, it can be seen that addition of 1-propanephosphonic acid cyclic anhydride shows an improvement in low temperature performance of the lithium secondary battery.

As described above, the present invention is characterized in that 1-propanephosphonic acid cyclic anhydride is added to an electrolyte solution for a lithium secondary battery, and the lithium secondary battery using the electrolyte solution of the present invention has an action of 1-propanephosphonic acid cyclic anhydride. As a result, a stable special oxide film is formed on the surface of the cathode, thereby improving low temperature performance without degrading other performances.

Claims (4)

The electrolyte solution for lithium secondary batteries which added the 1-propanephosphonic acid cyclic anhydride represented by following General formula (1) to the mixed solution containing an organic solvent and a lithium salt. Formula 1 The electrolyte solution for a lithium secondary battery according to claim 1, wherein the 1-propanephosphonic acid cyclic anhydride is added in an amount of 0.05 to 5% by weight based on the electrolyte solution. The electrolyte of claim 1, wherein the organic solvent is selected from the group consisting of cyclic and linear carbonates. The electrolyte solution for lithium secondary batteries of Claim 1; A positive electrode comprising a lithium-transition metal oxide as a positive electrode active material; And a negative electrode including carbon, a carbon composite, a lithium metal, or a lithium alloy as a negative electrode active material.