KR20160020781A - Mixed electrolyte of a low temperature and lithium ion battery - Google Patents

Abstract

The present invention provides a low temperature mixed electrolyte and a lithium ion battery using the same, wherein the low temperature mixed electrolyte decreases conductive value of the electrolyte goes down to a low temperature as going down to a low temperature by forming a coating film by a small amount of an additive in a negative electrode when reacting lithium using a mixed electrolyte solution containing 2,3-dihydrofuran and 2,5-dihydrofuran additives.

Description

TECHNICAL FIELD [0001] The present invention relates to a low-temperature type mixed electrolyte and a lithium-

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixed electrolyte used in a lithium ion battery and the like, and a lithium ion battery including the same. More particularly, the present invention relates to a lithium ion battery comprising 2,3-dihydrofuran or 2,5-dihydrofuran , 5-dihydrofuran) as an additive, thereby forming a film of a small amount of an additive on the cathode when lithium is reacted at a low temperature to prevent deterioration of the performance of the battery, and its production.

Lithium-ion batteries can be used not only as portable information devices but also as power sources for military and aerospace devices, and are attracting attention as high-performance batteries for electric power storage and electric vehicles. The lithium ion battery generally exhibits a low temperature characteristic of about -20 占 폚 and exhibits a relatively low low temperature performance, but more preferably a characteristic capable of functioning even at a low temperature of -30 占 폚 or lower.

Lithium-ion secondary batteries used in the military should be able to operate at -40 ° C mainly, and electrolytes used for commercial purposes are mostly -30 ° C. Therefore, in order to maintain the liquid phase of electrolyte, ethyl methyl carbonate (-55 ° C) (ethyl methyl carbonate (EMC)) is mainly used together with ethylene carbonate (EC).

Lithium cobalt oxide (LiCoO ₂ ) series is mainly used as an anode, and graphite is mainly used as a cathode. The electrolyte of the lithium ion secondary battery was prepared by mixing 1 M of lithium hexafluorophosphate (LiPF ₆ ) in a mixed solution of ethylene carbonate, dimethyl carbonate (DMC) and ethyl methyl carbonate in a ratio of 1: 1: 1 Electrolyte, and the cell thus manufactured is known to be able to operate at -40 ° C.

The low temperature test of lithium ion secondary battery determines the degree of ion movement between the electrodes and is mainly influenced by the electrolyte solution which greatly affects the film characteristics formed on the carbon anode. That is, since the ionic conductivity of the electrolyte may greatly affect the charge / discharge characteristics of the battery, it is necessary to configure the electrolyte system in consideration of various characteristics when selecting and combining the solvent. Particularly, it is necessary to develop mixed organic electrolytes having low freezing point and high ion conductivity for stable low temperature experiments.

Ethylene carbonate, diethyl carbonate (DEC), and dimethyl carbonate, which are currently commercialized electrolytic carbonates, can be mixed in a two-component system in consideration of viscosity and permittivity, or ethylene carbonate or propylene carbonate In order to increase the conductivity of the lithium ion and ensure the stability of the reaction, a mixed solvent in which diethyl carbonate or dimethyl carbonate is added together is used as a solvent, and these provide the most stable film components to the carbon anode.

However, it shows extreme drawbacks in the low-temperature experiments, which show abrupt drop in capacity and poor efficiency characteristics. This is because the conductivity of the electrolyte drops sharply as it goes down to a low temperature below -10 ° C and decomposition products produced by the reduction of the electrolyte do not protect the surface of the cathode densely. Therefore, it is necessary to study low temperature electrolyte of lithium ion battery.

Korean Patent Publication No. 10-0893228 Chinese Patent Registration No. 10-0438198 Korean Patent Publication No. 10-2014-0066096

An object of the present invention is to provide a low-temperature mixed electrolyte having a low freezing point and a high ionic conductivity by adding an additive to a mixed electrolyte containing an organic solvent and a lithium salt. And a lithium ion battery including the low temperature type mixed electrolyte.

In order to accomplish the above object, the present invention relates to a low-temperature mixed electrolyte comprising 2,3-dihydrofuran and 2,5-dihydrofuran as an additive electrolyte, and further relates to a lithium ion Battery.

Specifically, the mixed electrolyte includes a carbonate-based solvent and a lithium salt, and an additive is added in an amount of 0.1 to 5% by weight based on the total weight of the mixed electrolyte to prepare a low-temperature mixed electrolyte.

Other details of the present invention are included in the following detailed description.

The low-temperature mixed electrolyte of the present invention includes an additive to promote the formation of a film on the anode during lithium reaction, thereby preventing oxidation of the electrode surface and improving cycle performance of the battery.

1 is a graph showing the ion conductivity at -30 캜 and -20 캜 relative to the composition ratio of the mixed electrolyte.
2 is a graph showing the ion conductivity at -30 ° C and -20 ° C of the mixed electrolyte containing the additive (2,3-dihydrofuran (23DF) or 2,5-dihydrofuran (25DF) of the present invention.
3 is a chart showing the results of charge / discharge tests at -32 캜 and -25 캜 of a battery including the mixed electrolyte of the present invention.

Hereinafter, preferred examples and comparative examples of the present invention will be described, but the scope of the present invention is not limited thereto.

≪ Comparative Examples 1 to 3 > Preparation of mixed electrolyte

The low-temperature mixed electrolyte of the present invention was prepared by mixing ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate organic solvents having stable storage capacity at low temperature in three phases.

The weight percentages of the mixed electrolytes are shown in Table 1 and named as ED1, ED2 and ED3, depending on the composition. An electrolyte solution was prepared by mixing 1M of LiPF ₆ in each of the composition liquids.

Furtherance Lithium concentration EC
(weight%) DMC
(weight%) EMC
(weight%) Comparative Example 1 ED1 1M LiPF ₆ 33.3 33.3 33.3 Comparative Example 2 ED2 1M LiPF ₆ 11.1 22.2 66.7 Comparative Example 3 ED3 1M LiPF ₆ 8.3 25 66.7

Referring to Table 1 and FIG. 1, when the value of the ionic conductivity with respect to the composition ratio of the mixed electrolyte at the temperature of -30 ° C. and -20 ° C. was confirmed, ED1 and ED2 exhibited almost the same ionic conductivity at -20 ° C., At -30 ℃, the conductivity of ED1 was higher than that of ED2 and ED3.

≪ Example 1 > Preparation of low temperature type mixed electrolyte

The method for producing a low-temperature mixed electrolyte according to the present invention comprises

Mixing an organic solvent;

Adding a lithium salt;

Adding an additive to the solution; And

Forming a low temperature type mixed electrolyte of the present embodiment including the additive, thereby preparing a low temperature type mixed electrolyte.

The organic solvent includes at least one organic solvent selected from the group consisting of ethylene carbonate, polypropylene carbonate, diethyl carbonate and dimethyl carbonate, and is preferably a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate. Also, the mixed solvent is mixed with ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate at a weight ratio of 8: 34: 20 to 34: 30: 67.

The lithium salt is _{LiPF 6, LiBF 4, LiAsF 6} , LiClO 4, LiCF 3 SO 3, LiCF 3 SO 3, LiC (SO 2 CF 3) 3, LiN (CF 3 SO 2) 2 , and LiCH (CF ₃ SO ₂ ) ₂ , and is preferably LiPF ₆ .

Dihydrofuran and 2,5-dihydrofuran are used as additives of the hydrofuran series used to obtain a more stable cycle performance in a charge-discharge operation at a low temperature, and the additive is added in an amount of 0.1 To 5% by weight in the mixed electrolyte prepared as shown in Table 1 above.

The steps up to adding the additive in the step of preparing the low-temperature type mixed electrolyte are the same as the method of the mixed electrolyte in the comparative example.

As a result of the above examples, the ionic conductivity of the mixed electrolyte to which 4.8% by weight of the hydrofuran-based additive was added at -30 캜 and -20 캜 was confirmed at all temperatures, It is found that one mixed electrolyte exhibits a higher ionic conductivity than the case where the additive is not added to the mixed electrolyte of the comparative example, and is suitable for the low temperature type cell. In particular, it was confirmed that all mixed electrolytes to which 2,5-dihydrofuran was added exhibited high ion conductivity at both -30 ° C and -20 ° C.

The properties of the mixed electrolyte prepared by the methods of Comparative Examples 1 to 3 and Example 1 are measured through the following experimental and exemplary methods.

≪ Experimental Example 1 >

In the negative electrode manufacturing step of the lithium ion secondary battery,

Mixing 80% by weight of an active material MCMB (mesocarbon microbead), 10% by weight of a conductive agent and 10% by weight of a binder;

Mixing the mixture at a rate of about 2000 rpm for 30 minutes at a high speed to produce a slurry;

Applying the prepared slurry to a 10 탆 thick copper plate using a doctor blade;

Drying the applied slurry in a vacuum oven at a temperature of 80 DEG C for 24 hours;

Cutting to a size of dried 3 x 4 cm and rolling; And

Again drying in a vacuum oven at 80 < 0 > C for 24 hours to produce an electrode plate.

The negative electrode includes various active materials, conductive agents and binders commonly used in the art, and graphite may be used as the negative electrode active material.

The lithium ion secondary half-cell is fabricated by assembling a positive electrode and a negative electrode, a low temperature type mixed electrolyte containing the additive of the present invention, and Celgard 2400 manufactured by Celgar Corporation into a pouch cell in a glue box using an electrode separator.

The anode, the cathode, and the separator may be any of those used in the art in the production of the ion secondary battery, but the anode preferably includes a lithium metal electrode and lithium cobalt oxide.

As the cathode, the prepared cathode is used, and the anode uses lithium metal of the same size as the cathode.

Lithium ion secondary semiconductors were prepared by the above-described production method, and charge / discharge experiments were carried out as follows.

≪ Experimental Example 2 >

The lithium ion secondary battery manufactured using the negative electrode manufactured above was subjected to a charge-discharge test using a series 4000 battery tester manufactured by Maccor. During the charge / discharge test, the battery was discharged at a constant voltage of 0.005 V (vs. Li / Li +) at an open circuit voltage and then discharged at a constant current of 2.8 V to 0.005 V (vs. Li / Li + (C-rate).

3, when the electrolyte solution of ED2 and the electrolyte of ED2 were added with 0.5% by weight of the additive 2,5-dihydrofuran, both of them showed about 220 mAh / g at room temperature and -32 And 103 mAh / g and 114 mAh / g, respectively. In addition, the addition of the additive showed a 10% capacity increase at -32 ° C. When the additive of the hydrofuran type is included in the total weight of the low temperature type mixed electrolyte in an amount of 0.1 to 5 wt%, the oxidation reaction at the surface of the electrode is sufficiently suppressed during charging and discharging, and the cycle performance of the battery can be improved.

Claims (6)

An electrolyte, an organic solvent, and a lithium salt. The low-temperature mixed electrolyte includes 2,3-dihydrofuran and 2,5-dihydrofuran as additives.
The low-temperature mixed electrolyte according to claim 1, wherein the additive is contained in an amount of 0.1 to 5 wt% based on the total weight of the mixed electrolyte.
The low-temperature mixed electrolyte according to claim 1, wherein the mixed electrolyte contains at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.
The low-temperature mixed electrolyte according to claim 1, wherein the mixed electrolyte comprises a lithium salt.
The method of claim 4, wherein the lithium salt is _{LiPF 6, LiBF 4, LiAsF 6} , LiClO 4, LiCF 3 SO 3, LiCF 3 SO 3, LiC (SO 2 CF 3) 3, LiN (CF 3 SO 2) 2 and LiCH (CF ₃ SO ₂₎ low temperature electrolyte mixture, characterized in that any one or more salts selected from the group consisting of: _2.
A lithium ion battery comprising the low temperature type mixed electrolyte according to any one of claims 1 to 5.

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