KR20110080324A - Electrolyte for lithium-ion battery and lithium-ion battery comprising the same - Google Patents

Abstract

PURPOSE: An electrolyte for a lithium-ion battery is provided to enhance thermal stability and electrochemical oxidation stability of electrolyte and to improve rate characteristics and cycleability. CONSTITUTION: An electrolyte for a lithium-ion battery comprises lithium salts, organic solvents and cyclohexyl diphenyl phosphate as additives. The cyclohexyl diphenyl phosphate is included in the amount of 0.1-5 parts by weight based on 100.0 parts by weight of electrolyte. The lithium-ion battery includes a positive electrode(4) including a positive electrode active material, a negative electrode(2) including a negative electrode active material, a separator which is combined between the positive and negative electrodes and prevents the short circuit, and the electrolyte.

Description

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

Lithium salts; Organic solvents; And it relates to a lithium ion battery electrolyte containing a cyclohexyl diphenyl phosphate as an additive and a lithium ion battery comprising the same.

Li-ion batteries are mainly used in mobile phones and laptops, and are also used in portable electronic devices such as digital cameras and camcorders. Li-ion batteries have a high operating voltage and a high energy density of 3.6 V or higher, and their use is rapidly expanding. In addition, the application of large-capacity and high-output lithium-ion batteries has been expanded to hybrid electric vehicles, electric vehicles, robots, space and aviation, and active research is being conducted.

Lithium ion batteries have an operating principle of generating electricity while lithium ions (Li ⁺ ) are charged from the anode to the cathode when charged and from the cathode to the cathode when discharged. A lithium ion battery includes a cathode including a cathode active material as an oxidizing agent, an anode including a cathode active material as a reducing agent, an electrolyte for enabling oxidation / reduction reaction by ion conduction of lithium ions, and a space between them. It consists of a separator which prevents the positive electrode and the negative electrode from directly contacting each other.

Currently, one of the biggest problems of the organic solvent for lithium ion batteries is low safety. Since the organic solvent used in the lithium ion battery is a flammable material, there is a risk of explosion or ignition when stored at a high temperature above the ignition point of the electrolyte or overcharged. When the lithium ion battery is overcharged, lithium is excessively desorbed at the positive electrode, and lithium is excessively inserted at the negative electrode, so that both the positive electrode and the negative electrode are thermodynamically unstable, so that decomposition occurs and exothermic reactions occur rapidly. The ignition mechanism of Li-ion battery begins with the decomposition of the solid-electrolyte interphase (SEI) at the beginning, followed by thermal runaway in which the decomposition of the electrolyte, the decomposition of the cathode, and the decomposition of the anode occur in series. Will be displayed as).

Trimethyl phosphate (TMP), triethyl phosphate (TEP), which has been used as a plastic flame retardant, has been applied to lithium secondary batteries since its initial application. Ongoing studies on flame retardant additives for lithium ion batteries have been published in the literature. However, these flame retardant additives provide most flame retardant effects except for a few such as TTFP, but there is a problem that the performance of the battery is reduced, such as reduction of charge-discharge cycle characteristics due to ion conductivity of the electrolyte and reversible deterioration of the battery. TTFP also has the problem of adding a large amount of additives (20% by weight or more).

In order to solve such a problem, the development of a technology for adding various additives to the electrolyte. For example, in US 2003 / 0157413A1, an electrolyte solution containing triphenyl phosphate (TPP) + diphenyl monobutyl phosphate (DMP) + vinyl ethylene carbonate (VEC) may not only improve battery safety but also improve battery performance such as cycle performance. Here's how. In Korean Patent Registration 10-0693288, a method of suppressing overcharging of a battery by adding a mixture of naphtoyl chloride + divinyl adipate + ethoxy ethyl phosphate was proposed. In Korea Patent Registration 10-0585947 trimethylsilyl borate and trimethylsilyl phosphate by adding a mixed method to improve the battery performance at a high (rate). However, the electrolyte solution containing these additives is an additive component containing two or three components, suggesting the flame retardant effect and battery performance improvement of the battery. In addition, even when a specific compound is added to the electrolyte to improve battery performance, most of the battery performance can be expected to improve the performance of some items, there is a problem such as to reduce the performance of other items rather.

Accordingly, the present invention is to solve the problems of the prior art, to increase the thermal stability and electrochemical oxidation stability of the electrolyte, and also to provide an electrolyte solution for lithium ion battery excellent in electrochemical properties such as battery rate characteristics, cycle characteristics do.

The present invention is a lithium salt; Organic solvents; And it provides a lithium ion battery electrolyte containing a cyclohexyl diphenyl phosphate (Cyclohexyl diphenyl phosphate) as an additive and a lithium ion battery comprising the same.

Cyclohexyl diphenyl phosphate used as flame retardant additive in the electrolyte of the present invention is a phosphate acid compound having a boiling point of 409.4 ° C. and a flash point of 214.9 ° C. As can be seen in the following examples, when cyclohexyl diphenyl phosphate is added as a flame retardant additive to a basic electrolyte solution containing a lithium salt and an organic solvent, the thermal stability and the electrochemical oxidation stability of the electrolyte are also increased. Electrochemical properties such as rate characteristics and cycle characteristics also become excellent. In particular, TPP (triphenyl phosphate) and TTFP [tris (2,2,2-trifluoroethyl) phosphite], which were used as a flame retardant additive, were added in an amount of 5 parts by weight and 20 parts by weight with respect to 100 parts by weight of the basic electrolyte, respectively. As a result, it was confirmed that the thermal stability and the electrochemical oxidation stability of the electrolyte were increased and the electrochemical characteristics of the battery were also improved even when using a small amount of additives compared to these electrolytes.

In one embodiment, the cyclohexyl diphenyl phosphate is not limited thereto, but is preferably included in 0.1 to 5 parts by weight based on 100 parts by weight of the electrolyte. If the added amount of cyclohexyl diphenyl phosphate is less than 0.1 part by weight, it is difficult to impart flame retardant effect and thermal stability of the electrolyte solution, and when it exceeds 5 parts by weight, the flame retardant effect and thermal stability of the electrolyte solution may be increased, but battery performance may be reduced. Because there is. Cyclohexyl diphenyl phosphate is added to the organic solvent containing the lithium salt.

Lithium salt contained in the electrolyte of the present invention may be used any lithium salt used in the lithium ion battery, which is well known in the art. In one embodiment, the lithium salt is not limited to LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiN (CF ₃ SO ₂ ) _2, LiN (C At least one lithium salt selected from the group consisting of ₂ F ₅ SO ₂ ) ₂ , LiAsF ₆ , LiSiF _6, and LiCH (CF ₃ SO ₂ ) ₂ .

The organic solvent included in the electrolyte of the present invention may also be used as long as it is an organic solvent known to be used in a lithium ion battery. In one embodiment, the organic solvent may be one or more solvents or mixtures thereof selected from the group consisting of carbonate-based, ester-based, ether-based and ketone-based. For example, the organic solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), triethylene carbonate (TEC), isobutylene carbonate (IBC), propylene carbonate (PC), di One or more carbonate-based organic solvents selected from the group consisting of ethyl carbonate (DEC) and fluoroethylene carbonate (FEC) or mixtures thereof. For example, a mixture of EC / EMC, EC / DMC / EMC, EC / EMC / DEC, EC / DMC / EMC / PC and the like can be used as the organic solvent contained in the electrolyte.

The present invention also provides a lithium ion battery comprising the electrolyte solution for the lithium ion battery. The lithium ion battery of the present invention may take the configuration of any lithium ion battery known in the art, except for using the lithium ion battery electrolyte.

In one embodiment, the lithium ion battery

A positive electrode including a positive electrode active material; A negative electrode including a negative electrode active material; A separator coupled between the positive electrode and the negative electrode to prevent a short circuit; And the electrolyte for the lithium ion battery.

In one embodiment, the positive electrode active material included in the positive electrode is selected from the group consisting of LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , LiFePO ₄ and LiNi _x Co _y Mn _z O ₂ Where x, y and z are each greater than 0 and less than 1, and the sum of x, y and z is 1.

In one embodiment, the negative electrode active material included in the negative electrode may be selected from the group consisting of crystalline or amorphous carbon, carbon-based negative electrode active material of carbon composite, carbon fiber, tin oxide compound, lithium metal and lithium alloy.

In addition, in one embodiment, the separator may be a polymer film of polyethylene, polypropylene or polyolefin or a multilayer film thereof, a microporous film, a woven fabric or a nonwoven fabric.

In the following example, 1.15 M of LiPF ₆ dissolved as an electrolyte salt in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 4: 6 was used as a basic electrolyte solution. 1 part by weight of cyclohexyl diphenyl phosphate was added as a flame retardant additive to prepare a lithium ion battery electrolyte according to the present invention.

In addition, in order to manufacture a lithium ion battery including the electrolyte, Li (Ni _1/3 , Mn _1/3 , Co _1/3 ) O ₂ as a cathode active material, PVDF (polyvinylidene difluoride) was used as a binder, Super P black was used as the conductive material. MCMB (mesocarbon microbeads) was used as a negative electrode active material, PVDF was used as a binder, and Super P black was used as a conductive material. After inserting a separator between the positive electrode and the negative electrode to make an electrode assembly, the prepared electrode assembly was placed in a case, and a lithium ion battery was prepared by injecting the lithium ion battery electrolyte of the present invention.

Lithium ion battery comprising the electrolyte of the present invention is a flame retardant characteristics, thermal stability, electrochemical oxidation of the electrolyte compared to the conventional lithium ion battery containing a base electrolyte containing a lithium salt and an organic solvent or an electrolyte containing another flame retardant additive Stability is increased and the initial irreversible capacity of the cell is reduced. In addition, it exhibits excellent battery characteristics in which the internal resistance of the battery is reduced and the high rate characteristic and the charge-discharge cycle characteristics are improved.

1 is a cross-sectional view of a 2032 coin-type battery of the present invention.
2 is a graph showing the results of the self extinguishing time (SET) analysis of the electrolyte solution prepared in Preparation Example 1 and Comparative Examples 1 to 3.
FIG. 3 is a graph illustrating differential thermal analysis (DTA) analysis results of electrolytes according to Preparation Example 1 and Comparative Examples 1 to 3. FIG.
Figure 4 is a graph showing the charge-discharge cycle life test results of the lithium ion battery prepared according to Preparation Example 1 and Comparative Examples 1 to 3.
FIG. 5 is a graph illustrating electrochemical impedance spectroscopy (EIS) test results for 50 cycle life tests of lithium ion batteries prepared according to Preparation Example 1 (A) and Comparative Example 1 (B).
<Explanation of symbols for the main parts of the drawings>
1: stainless steel case 2: cathode
3: separator 4: anode
5: spacer 6: spring
7: stainless steel lid

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments and drawings described below in detail. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and only the present embodiments are intended to complete the disclosure of the present invention, and those skilled in the art to which the present invention pertains. It is provided to fully inform the scope of the invention, and the invention is defined only by the scope of the claims.

[Example]

<Production Example>

Preparation Example 1 Preparation of Electrolyte Solution Using Cyclohexyl Diphenyl Phosphate (CDPP) as Additive and Battery Containing the Same

(1) Preparation of Electrolyte

1.15 M of LiPF ₆ dissolved as an electrolyte salt in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 4: 6 was used as a base electrolyte, and was added as an additive to 100 parts by weight of the base electrolyte. 1 part by weight of cyclohexyl diphenyl phosphate (CDPP) was added to prepare an electrolyte solution for a lithium ion battery according to the present invention.

(2) production of batteries

A 2032 coin-type battery having a can diameter of 20 mm and a height of 3.2 mm was prepared. Li (Ni _1/3 Mn _1/3 Co _1/3 ) O ₂ was used as the positive electrode active material. A slurry was prepared by adding an active material in a weight ratio of 95: 2: 3: binder (polyvinylidene difluoride): conductive material (Super P black) to a solvent of n-methyl 2-pyrrolidinone (NMP). The slurry was applied on an aluminum foil, dried at 110 ° C. for 12 hours, and rolled by a roll press to prepare a positive electrode 4. As the negative electrode active material, MCMB (mesocarbon microbeads) was used. A slurry was prepared by dissolving an active material: binder (PVDF): conducting agent (Super P black) in an NMP solvent in a weight ratio of 95: 3: 2. The slurry was applied to a copper current collector, dried at 110 ° C. for 12 hours, and then rolled by a roll press to prepare a negative electrode 2. A porous polypropylene separator 3 was placed between the positive electrode 4 and the negative electrode 2 to impregnate the electrolyte solution. A spacer 5 and a spring 6 were inserted between the anode 4 and the stainless steel lid 7. The stainless steel case 1 and the stainless steel lid 7 were completely sealed to prepare a 2032 type coin-type battery (FIG. 1).

Comparative Example 1: Preparation of Basic Electrolyte and Battery Containing the Same

A basic electrolyte solution was prepared by dissolving 1.15 M of LiPF ₆ as an electrolyte salt in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 4: 6, using the same method as in Preparation Example 1 above. The battery was prepared.

Comparative Example 2: Preparation of Electrolyte Solution Using Triphenyl Phosphate (TPP) as Additive and Battery Comprising the Same

An electrolyte solution and a battery were manufactured in the same manner as in Preparation Example 1, except that 5 parts by weight of triphenyl phosphate (TPP) was used as an additive based on 100 parts by weight of the basic electrolyte solution.

Comparative Example 3: Preparation of Electrolyte Solution Using (2,2,2-Trifluoroethyl) Phosphate (TTFP) as Additive and Battery Comprising the Same

An electrolyte solution and a battery were manufactured in the same manner as in Preparation Example 1, except that 20 parts by weight of tris (2,2,2-trifluoroethyl) phosphate (TTFP) was used as an additive based on 100 parts by weight of the basic electrolyte solution.

Experimental Example

Experimental Example 1 Measurement of Self Extinguishing Time (SET)

The self-extinguishing time (SET) of the electrolyte solutions prepared in Preparation Example 1 and Comparative Examples 1 to 3 was measured, and the results are shown in FIG. 2.

The self-extinguishing time was measured by extinguishing self-extinguishing by absorbing 0.1 to 0.2 g of electrolyte solution into a glass fiber that does not ignite, and expressed as time per unit mass. As a result of the test, the burning time of 1 g of electrolyte was 111.56 seconds in Comparative Example 1 consisting of the basic electrolyte solution without the cyclohexyl diphenyl phosphate additive, and the burning time was reduced to 106.52 seconds in Preparation Example 1 consisting of the electrolyte solution of the present invention. The results were shown. Comparative Example 2 was found to be 107.38 seconds and Comparative Example 3 was 4.55 seconds. In Comparative Example 3, 20 parts by weight of TTFP was added in comparison with Preparation Example 1 and Comparative Example 2, and a radical scavenger that adsorbs oxygen and hydrogen radicals causing combustion of materials was added to the TTPF molecule in addition to phosphate. Since fluoride is also included, TTFP has the best results.

Experimental Example 2: Thermal Analysis of Electrolyte

Pyrolysis reaction tests were performed on the electrolyte solutions prepared in Preparation Example 1 and Comparative Examples 1 to 3 using differential thermal analysis (DTA), and the results are shown in Table 1 and FIG. 3.

Manufacturing example Electrolyte additive Reaction temperature (℃) Oxidation potential (V) Preparation Example 1 1.15M LiPF ₆ / EC: EMC (4: 6% by volume) CDPP 1 part by weight 230.90 5.40 Comparative Example 1 1.15M LiPF ₆ / EC: EMC (4: 6% by volume) - 211.05 4.53 Comparative Example 2 1.15M LiPF ₆ / EC: EMC (4: 6% by volume) TPP 5 parts by weight 210.63 - Comparative Example 3 1.15M LiPF ₆ / EC: EMC (4: 6% by volume) TTFP 20 parts by weight 195.91 -

As shown in Table 1 and Figure 3, in Comparative Example 1 consisting of a basic electrolyte solution without the cyclohexyl diphenyl phosphate additive was added, the reaction temperature of the electrolyte was 211.05 ℃, Comparative Example 2 was 210.63 ℃, Comparative Example 3 was found to be 195.91 ° C. In the case of Preparation Example 1 consisting of the electrolytic solution of the present invention, the reaction temperature was the highest as 230.90 ° C. despite the use of a small amount of additives.

Experimental Example 3: Thermal Analysis of Electrolyte

The electrochemical oxidation potential of the electrolyte was measured using the circulating current-voltage method for the electrolyte solutions prepared in Preparation Example 1 and Comparative Example 1, and the results are shown in Table 1. As shown in Table 1, in Comparative Example 1 composed of a basic electrolyte solution without the cyclohexyl diphenyl phosphate additive, the oxidation potential of the electrolyte solution was found to be 4.53 V (Li / Li ⁺ ), and the electrolyte solution of the present invention was prepared. In Example 1, the oxidation potential was high as 5.40 V (Li / Li ⁺ ).

Experimental Example 4: Initial Charge / Discharge Capacity Test

The first charge-discharge test was performed after the batteries prepared in Preparation Example 1 and Comparative Examples 1 to 3, and the results are shown in Table 2. As shown in Table 2, the battery of Comparative Example 1 without the cyclohexyl diphenyl phosphate additive showed an initial charge / discharge efficiency of 80.60%, but 82.58% of the Battery of Preparation Example 1 containing the electrolyte solution of the present invention. Improved initial charge-discharge efficiency. On the other hand, Comparative Example 1 showed a somewhat low value of 77.84%, Comparative Example 2 showed a value of 83.39%.

Manufacturing example Initial charge capacity
(mAh / g) Initial discharge capacity
(mAh / g) Initial irreversible capacity
(mAh / g) Initial charge and discharge efficiency
(%) Preparation Example 1 156.71 129.41 27.30 82.58 Comparative Example 1 157.47 126.93 30.54 80.60 Comparative Example 2 151.43 117.88 33.55 77.84 Comparative Example 3 157.08 130.99 26.09 83.39

Experimental Example 5: Rate Characteristic Test

Rate discharge capacity tests were performed on the batteries prepared in Preparation Example 1 and Comparative Example 1, and the results are shown in Table 3.

Current rate Current drain (mA) Rate performance (%) Preparation Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 0.1C 0.45 100.00 100.00 100.00 100.00 0.5C 2.25 96.09 85.13 83.24 79.82 1.0C 4.50 91.84 75.18 70.55 60.93 2.0C 9.00 67.49 45.64 51.35 24.22

As shown in Table 3, the battery of Comparative Example 1 without adding the cyclohexyl diphenyl phosphate additive showed a 75.18% rate characteristic compared to 0.1C at 1.0C, and the battery of Preparation Example 1 at 0.1C at 1.0C Compared with 91.84%. The battery of Comparative Example 1 showed a rate characteristic of 70.55%, the battery of Comparative Example 2 60.93%.

Experimental Example 6: Cycle Life Test

As a test for charge-discharge cycle life evaluation, the cells were subjected to constant current-constant voltage charging up to 4.25 V at 1.0 C (4.5 mA) at room temperature and constant current discharge up to 2.75 V. Charging and discharging were performed up to 100 cycles, and the graphs and capacity retention rates in each cycle are shown in FIGS. 4 and 4, respectively.

Manufacturing example Capacity maintenance rate (%) 10 cycles later After 100 cycles Preparation Example 1 97.24 90.53 Comparative Example 1 88.19 61.14 Comparative Example 2 90.31 65.05 Comparative Example 3 85.52 60.28

As shown in Table 4, after 100 cycles of the lithium ion battery prepared according to Comparative Example 1 without using a flame retardant additive, the capacity retention rate was 61.14%, and in the case of Preparation Example 1 consisting of the electrolyte solution of the present invention, 90.53 Improved battery life characteristics were confirmed with a capacity retention rate of%. It is believed that this is because the polymer layer formed by cyclohexyl diphenyl phosphate prevents the detachment of the negative electrode that occurs during the insertion and desorption of lithium ions during charge-discharge. In addition, in Comparative Example 2, the capacity retention rate was 65.05% and Comparative Example 3 was 60.28%.

Experimental Example 7: Internal Resistance

Electrochemical impedance spectroscopy (EIS) measurements were used to compare changes in cell internal resistance during the initial 50 charge-discharge cycle life assessments. The impedance for 50 cycles is shown in FIG. 5 at 10 intervals, and the overall resistance value of the _cell (R _cell ) is shown in Table 5. After 50 cycles, the battery internal resistance (R _cell ) was 34.98 Ω · cm ² in Preparation Example 1, which was lower than 57.15 Ω · cm ² in Comparative Example 1. This means that the diffusion rate of lithium ions at the positive and negative electrode interfaces is increased by cyclohexyl diphenyl phosphate, and thus it is judged to exhibit improved rate characteristics and cycle performance.

Manufacturing example Battery internal resistance (Ωcm ² ) After 1 cycle 10 cycles later After 30 cycles After 50 cycles Preparation Example 1 34.92 35.35 32.13 34.98 Comparative Example 1 58.14 51.11 54.41 57.15

Claims (9)

Lithium salts; Organic solvents; And cyclohexyl diphenyl phosphate as an additive.
The method of claim 1,
The cyclohexyl diphenyl phosphate is based on 100 parts by weight of the electrolyte solution for lithium ion batteries will be included in 0.1 to 5 parts by weight.
The method according to claim 1 or 2,
The lithium salt may be LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiN (CF ₃ SO ₂ ) _2, LiN (C ₂ F ₅ SO ₂ ) ₂ , LiAsF ₆ , An electrolyte solution for a lithium ion battery, wherein the electrolyte is at least one lithium salt selected from the group consisting of LiSiF ₆ and LiCH (CF ₃ SO ₂ ) ₂ .
The method according to claim 1 or 2,
The organic solvent is at least one solvent selected from the group consisting of carbonate-based, ester-based, ether-based and ketone-based or a mixture thereof.
The method according to claim 1 or 2,
The organic solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), triethylene carbonate (TEC), isobutylene carbonate (IBC), propylene carbonate (PC), diethyl carbonate (DEC And at least one carbonate-based organic solvent selected from the group consisting of fluoroethylene carbonate (FEC) or a mixture thereof.
A positive electrode including a positive electrode active material; A negative electrode including a negative electrode active material; A separator coupled between the positive electrode and the negative electrode to prevent a short circuit; And a lithium ion battery comprising a lithium ion battery electrolyte according to any one of claims 1 to 5.
The method of claim 6,
The positive electrode active material is a lithium ion battery selected from the group consisting of LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , LiFePO ₄ and LiNi _x Co _y Mn _z O ₂ :
Here, x, y and z are each 0 or more and less than 1, and the sum of x, y and z is 1.
The method of claim 6,
The negative electrode active material is selected from the group consisting of crystalline or amorphous carbon, carbon-based negative electrode active material of carbon composite, carbon fiber, tin oxide compound, lithium metal and lithium alloy.
The method of claim 6,
The separator is a polymer film of polyethylene, polypropylene or polyolefin or a multi-film, microporous film, woven or nonwoven fabric thereof.

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