KR102017740B1 - Efficient Electrolyte to Improve the Surface Stability of Graphite Anodes and lithium ion rechargeable battery including thereof - Google Patents

Abstract

The present invention relates to a lithium ion secondary battery electrolyte, characterized in that it comprises a compound represented by the formula (1), an organic solvent and a lithium salt, and when using the lithium ion secondary battery electrolyte of the present invention, lifespan at high temperature and room temperature It is possible to provide a lithium ion secondary battery having excellent performance which is improved and forms an excellent SEI layer even for a high capacity positive electrode material.

Description

Electrolyte for lithium ion secondary battery for improving surface stability of graphite negative electrode and lithium ion secondary battery comprising same {Efficient Electrolyte to Improve the Surface Stability of Graphite Anodes and lithium ion rechargeable battery including specifically}

The present invention relates to an electrolyte additive, and more particularly to a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same that can improve the surface stability of the graphite negative electrode.

Graphite for inserting / desorbing lithium ions as a negative electrode active material or using Li ₄ Ti ₅ O ₁₂ and using a non-aqueous liquid electrolyte is called a lithium ion secondary battery, and is referred to as a lithium ion battery (LIB).

Before LIB was developed, a lithium secondary battery using lithium metal as a negative electrode was developed, which has the advantage of making a high-volume battery having a very high theoretical capacity per volume of 2062 mAh / cc. However, in lithium secondary batteries, new lithium metal is unevenly deposited on the surface of the negative electrode during charging and discharging to form a dendrite, and the newly precipitated lithium is decomposed again to form an uneven surface coating. In addition to the drastic decrease, the amount of use decreased drastically due to problems such as growing dendritic cells penetrating the separator and causing internal short circuit.

The problem with lithium metals has been solved by finding that graphite can repeatedly insert / desorb lithium ions, and in particular, lithium containing carbon has an electrochemical reaction potential that is almost the same as that of lithium metal, leading to lithium insertion / desorption. When the volume change is small, the crystal structure is less changed even if repeated several times, making it possible to make a battery with excellent cycle life.

In order to improve the performance of such a lithium ion secondary battery, development of an electrolyte material capable of forming a stable solid-electrolyte interphase (SEI) layer is underway. However, since not all electrolytes form a stable SEI layer, and the negative electrode active material in which the SEI layer is mainly formed is also being improved, development of various materials for forming an optimal SEI layer is also continued.

Recently, propylene carbonate (PC) has been developed as an additive for lithium ion secondary battery electrolyte, which has been evaluated as having high potential by having high ionic conductivity at low temperature, but when it is practically borrowed, an appropriate SEI layer There is a problem that the battery does not operate normally because it is not formed and the continuous decomposition of the electrolyte occurs, gas generation and active material desorption phenomenon in this process.

To solve this problem, additives such as fluoroethylene carbonate (FFC) and vinylene carbonate (VC) have been developed. This made the SEI layer more stable than the PC-based electrolyte, thereby enabling normal operation of the battery (Patent Document 1). However, these techniques not only have low usability problems with high-capacity anode materials such as Ni- (NCM) or Li-rich (OLO) layered oxides, but also cannot exhibit sufficient cycle characteristics. The high temperature stability such as ring phenomenon was low.

Therefore, when a high nickel content lithium transition metal oxide is used as a positive electrode active material, there exists a high need for the technique to solve the capacity | capacitance characteristic, cycling characteristics, and high temperature stability fall of a battery by an impurity.

Japanese Patent No. 4,462,012

Accordingly, the present invention has been made in view of the above problems, and an object of the present invention is to provide a capacity characteristic, a cycle characteristic, and high temperature stability of a battery due to impurities even when a lithium transition metal oxide having a high nickel content is used as a cathode active material. It is to provide a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same that can solve the degradation.

The present invention provides a lithium ion secondary battery electrolyte comprising a compound represented by the following formula (1), an organic solvent and a lithium salt to achieve the above object.

[Formula 1]

The compound may be included in an amount of 0.01 to 10% by weight based on the total weight of the electrolyte.

The compound may be included in 0.1 to 5% by weight based on the total weight of the electrolyte.

The organic solvent is dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, propylene carbonate (PC), ethylene carbonate (EC), butylene Carbonate, ethylpropionate, ethylbutyrate, acetonitrile, succinonitrile (SN), dimethyl sulfoxide, dimethylformamide, dimethylacetamide, gamma-valerolactone, gamma-butyrolactone, tetrahydrofuran and their It may be any one selected from the group consisting of a mixture.

The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (FSO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO 2 , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (x, y is 1 to 20), LiCl, LiI and mixtures thereof It can be one.

In addition, the present invention provides a lithium ion secondary battery comprising a positive electrode, a negative electrode, the electrolyte for the lithium ion secondary battery and the separator.

The lithium ion secondary battery may have a specific capacity retention of 95 to 99% even after 50 cycles.

When the electrolyte for lithium ion secondary batteries of the present invention is used, it is possible to provide a lithium ion secondary battery having excellent performance of improving lifespan at high temperature and room temperature and forming an excellent SEI layer even for a high capacity positive electrode material.

1 shows the chemical structure of TMSP used as an electrolyte additive in 2D and 3D.
FIG. 2 is a graph showing negative electrode stability measured by linear sweep potential method (LSV) of a battery (TMSP) prepared according to Example 1 of the present invention and a battery of Comparative Example 1 (none).
FIG. 3 is a graph showing the Coulombbic efficiency of the batteries prepared from Examples 1, 2, and 3 and the batteries prepared from Comparative Examples 1 and 2. FIG.
Figure 4 is a graph showing the measurement of the potential relative to the specific capacity of the cells prepared from Examples 1, 2, 3 and Comparative Examples 1 and 2.
FIG. 5 is a graph showing the specific specific amount of the cycle number of the batteries prepared from Examples 1, 2, and 3 and the batteries prepared from Comparative Examples 1 and 2. FIG.
6 is an SEM image of the negative electrode surface of the battery prepared from Comparative Example 1 after one cycle.
7 is an SEM image of the negative electrode surface of the battery prepared from Comparative Example 1 after 50 cycles.
8 is an SEM image of the negative electrode surface of the battery prepared in Example 3 after one cycle.
FIG. 9 is an SEM image of the negative electrode surface of the cell prepared from Example 3 after 50 cycles.
10 and 11 are ³¹ P-NMR graphs for cells prepared from Example 3 and Comparative Example 1 after a cycle to analyze the chemical compounds of the SEI layers of Example 3 and Comparative Example 1. FIG.
12 to 15 are graphs of the negative electrode of the battery prepared from Example 3 and Comparative Example 1 after the cycle by XPS, FIG. 12 is C1s, FIG. 13 is F1s, FIG. 14 is P2p, and FIG. 15 is Si2p graph. (The black line is the battery of Comparative Example 1, and the blue line is the battery of Example 1).
16 is a scheme showing the binding energy of lithium salt, EC and TMSP.
17 is a scheme showing reaction energy (ΔG in kcal / mol) of TMSP and CH ₃ O ⁻ (iii) and EC and CH ₃ O ⁻ (ii).

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

One aspect of the present invention relates to a lithium ion secondary battery electrolyte comprising a compound represented by the following formula (1), an organic solvent and a lithium salt.

[Formula 1]

The electrolyte for a lithium ion secondary battery includes a compound represented by Chemical Formula 1 as an additive so that an SEI layer may be formed on the surface of the graphite negative electrode during an initial charge and discharge process.

The condition of the initial charge-discharge process is not particularly limited as long as it is a condition commonly used in lithium ion secondary batteries. Preferably, lithium ions from the positive electrode during charging are inserted into the graphite electrode, which is a negative electrode. Charging and discharging may be repeated at a constant current or constant voltage in a range so that the SEI layer is formed on the surface of the electrode during the over discharge process.

The compound may be included in an amount of 0.01 to 10% by weight, preferably 0.1 to 5% by weight, based on the total weight of the electrolyte. If the compound is below or exceeds the above range, the compound may be compatible with a high capacity cathode material such as VC electrolyte. Low, there may be a problem that the capacity characteristics, lower specific capacity retention rate and cycle characteristics according to the cycle number is significantly lowered, or a side reaction such as a swelling phenomenon may occur. In order to have the best effect among them, including the above-mentioned problems, the compound is most preferably included 3% by weight based on the total weight of the electrolyte.

In addition, since the electrolyte according to the present invention can achieve various excellent effects with only one additive, it has significantly improved problems such as increase in internal resistance and decrease in capacity (capacity retention according to cycle number) due to side reactions without additional configuration. It confirmed through the following experimental example.

The organic solvent is dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, propylene carbonate (PC), ethylene carbonate (EC), butylene Carbonate, ethylpropionate, ethylbutyrate, acetonitrile, succinonitrile (SN), dimethyl sulfoxide, dimethylformamide, dimethylacetamide, gamma-valerolactone, gamma-butyrolactone, tetrahydrofuran and their It may be any one selected from the group consisting of a mixture, and is not particularly limited thereto. However, in order to form the SEI layer electrochemically and thermodynamically with the compound of Formula 1, the organic solvent is preferably a mixture of ethyl methyl carbonate (EMC) and ethylene carbonate (EC), more preferably the EC and EMC The mixing volume ratio of 1: 1-3 may be. Most preferably, the mixing volume ratio of EC and EMC may be 1: 2, which has the advantage of forming an electrochemical and thermodynamically stable and uniform SEI layer with the compound of Formula 1. Specifically, when the organic solvent was measured by various changes, the mixture of EC and EMC, and when the mixing volume ratio of 1: 2 in the mixture, compared to the NMR results of the SEI layer, the compound of formula 1 compared to EC The peak indicating the phosphonate functional group derived from the highest was observed, and the peak indicating the solvent composition was also lower than that of Comparative Example 1 in the XPS results. In addition, it can be confirmed that the same in the EDS results. That is, the SEI layer components formed according to the type and composition of the organic solvent were different, and when comparing the XPS, NMR and EDS results, when the organic solvent is a mixture of EC and EMC and their mixing volume ratio is 1: 2, It was confirmed that the SEI layer having the best component was formed. The difference in the components of the SEI layer greatly affects the performance of the battery even if the range of the difference is minute. For example, a significant difference in coulombic efficiency, potential according to specific capacity, and specific capacity retention according to the number of cycles occurs even if only the comparison between 0.5 wt% TMSP (Compound 1) and 3 wt% TMSP is used. This can be confirmed enough.

The lithium salt used in the electrolyte is not particularly limited as long as it is a lithium salt used in the art, preferably LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (FSO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiAlO 2, LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (x, y is 1 to 20), LiCl, LiI or a mixture thereof and the like can be used. Preferably the lithium salt may be LiPF ₆ .

The electrolyte may be in a liquid or gel state.

Another aspect of the present invention relates to a lithium ion secondary battery including an electrolyte for a lithium ion secondary battery, wherein the electrolyte for a lithium ion secondary battery according to the present invention includes a compound represented by Chemical Formula 1, an organic solvent, and a lithium salt. Can be. This is to form an SEI layer to maximize the effect of the battery through the initial charge and discharge process. In order to minimize the repetition of the same content, the above description is omitted.

Specifically, the lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the lithium ion secondary battery is not particularly limited as long as it is a general positive electrode, negative electrode, and separator used in the art.

Preferably, the positive electrode includes a positive electrode active material provided on a positive electrode current collector, and the positive electrode active material is not particularly limited as long as it is a positive electrode active material having excellent discharge capacity and high capacity characteristics, and a lithium-containing transition metal oxide, for example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ (0.5 <x <1.3), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x (Ni _a Co _b Mn _c ) O ₂ (0.5 <x <1.3, 0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), Li _x Ni _{1- y} Co _y O ₂ (0.5 <x <1.3, 0 <y <1), Li _x Co _1-y Mn _y O ₂ (0.5 <x <1.3, 0≤y <1), Li _x Ni _1-y Mn _y O ₂ (0.5 <x <1.3, O≤y <1), Li _x (Ni _a Co _b Mn _c ) O ₄ (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 < c <2, a + b + c = 2), Li _x Mn _2-z Ni _z O ₄ (0.5 <x <1.3, 0 <z <2), Li _x Mn _2-z Co _z O ₄ (0.5 < x <1.3, 0 <z <2), Li _x CoPO ₄ (0.5 <x <1.3) and Li _x FePO ₄ (0.5 <x <1.3), any one selected from the group consisting of or a mixture of two or more thereof The lithium-containing transition metal oxide is a metal such as aluminum (Al) It may be coated with oxides. In addition to the lithium-containing transition metal oxide, sulfide, selenide, halide, and the like may also be used.

The lithium ion secondary battery includes an electrochemical balance between the positive electrode and the negative electrode through the design using the electrolyte, even if the amount of the positive electrode active material is not increased, in order to manufacture a high capacity battery. Has an effect.

Graphite or silicon-carbon composite / graphite may be used as the negative electrode active material in the lithium ion secondary battery.

The separator is a conventional porous polymer film conventionally used as a separator, for example, polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer The porous polymer film prepared by using a single or a lamination thereof may be used, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of glass fibers, polyethylene terephthalate fibers of high melting point, etc. may be used, but is not limited thereto. .

The process of forming the positive electrode, the separator, the electrolyte, and the negative electrode of the lithium ion secondary battery may be applied by any coating method as long as it can be coated by a method that does not adversely affect the physical properties of each layer (for example, spray coating or dipping method). Since this is a content that can be well understood by those in the field, detailed description thereof will be omitted.

N-methylpyrrolidone, acetone or water may be used as the solvent used when preparing the positive electrode and the negative electrode, but the present invention is not limited thereto, and any solvent may be used as long as it can be used in the art.

The external shape of the lithium ion secondary battery is not particularly limited, but may be cylindrical, square, pouch type, or coin type using a can.

In addition, a plurality of lithium ion secondary batteries are stacked to form a battery pack, and the battery pack may be used in any device requiring high capacity and high output. For example, it can be used in notebooks, smartphones, electric vehicles and the like.

In addition, the lithium ion secondary battery may be used in an electric vehicle (EV) because it has excellent life characteristics and high rate characteristics. For example, it may be used in a hybrid vehicle such as a plug-in hybrid electric vehicle (PHEV). It can also be used in applications where a large amount of power storage is required. For example, it can be used for electric bicycles, power tools and the like.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. However, these examples are intended to illustrate the present invention in more detail, it will be apparent to those skilled in the art that the scope of the present invention is not limited thereby.

Preparation Examples 1-3. TMSP Electrolyte.

Under 1 M lithium hexafluorophosphate (litthium, hexafluorophosphate; LiPF ₆ ), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1: 2 to provide a standard electrolyte (PanaxEtec). Was prepared. The TMSP electrolytes of Preparation Examples 1, 2, and 3 were prepared by adding 0.5 wt%, 1.0 wt%, and 3.0 wt% of the compound of Formula 1 (TMSP) based on the total weight of the standard electrolyte.

[Formula 1]

Comparative Preparation Example 1 Standard Electrolyte

Under 1M lithium hexafluorophosphate (litthium, hexafluorophosphate), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1: 2 to prepare a standard electrolyte (PanaxEtec). .

Comparative Preparation Example 2 VC Electrolyte

A VC electrolyte was prepared in the same manner as in Preparation Example 1 except that 3.0 wt% of vinylene carbonate (VC) was added instead of TMSP.

Examples 1 to 3. Preparation of a coin-type lithium ion secondary battery

Coin cells (coin cell) was made of graphite, lithium metal, poly (ethylene) separator (Asahi) and TMSP electrolyte (with or without TMSP) prepared from Preparation Examples 1 to 3, specifically as follows.

Free carbon was used as the working electrode and lithium metal (foil) was used as the counter and reference electrode. At this time, the scan rate 1 mV s ^{- 1} was, yiyeotda voltage range ^{3.0-0.0 V (vs. Li / Li +} ).

Graphite cathodes include graphite (PoscoChemtech), Super P (carbon black), carboxymethyl cellulose (CMC) (Cellogen, DKS) and styrene-butadiene rubber (SBR) (Zeon, BM 400B). After mixing in a weight ratio of 1: 1: 1: 2, the mixture was stirred for 3 hours to prepare a slurry, which was coated on Cu foil, followed by drying in a vacuum oven at 100 ° C. for 30 minutes. At this time, the loading density of the electrode was 8.75 ± 0.09 mg / cm 2.

The battery thus produced was discharged at 0.01 V (vs. Li / Li ⁺ ) at a constant rate of 0.1 C during the initial 2 cycles, and charged at 1.5 V (vs. Li / Li ⁺ ). After such initial charge and discharge, the battery was discharged at 0.01 V (vs. Li / Li ⁺ ) at a constant rate of 1.0 C and 50 cycles were performed at 1.5 V (vs. Li / Li ⁺ ). Cycle life was tested using a charge / discharge device (TOSCAT-3100, TOYO) at room temperature.

Comparative Example 1.

Except for using the standard electrolyte prepared from Comparative Preparation Example 1 instead of the TMSP electrolyte was prepared by the same method as in Example 1.

Comparative Example 2.

Except for using the VC electrolyte prepared from Comparative Preparation Example 2 instead of the TMSP electrolyte was prepared by the same method as in Example 1.

Performance Evaluation Method

In order to compare the performance of an electrolyte using the compound of Formula 1 prepared according to the present invention as an electrolyte additive or an electrolyte including Comparative Preparation Examples 1 and 2 in a lithium secondary battery, evaluation was performed as follows.

Cathodic stability was analyzed by linear sweep voltammetry (LSV) using an electrochemical workstation (Biologic, SP-300).

The cells prepared from the above examples and comparative examples were discharged at 0.01 V (vs.Li/Li ⁺ ) at a constant rate of 0.1 C during the initial 2 cycles, and 1.5 V (vs.Li/Li ^+). ) And was discharged at 0.01 V (vs.Li/Li ⁺ ) at a constant rate of 1.0 C for 50 cycles and charged at 1.5 V (vs.Li/Li ⁺ ). Cycle life was tested using a charge / discharge device (TOSCAT-3100, TOYO) at room temperature.

After cycle analysis, the cell was disassembled in a glove-box to recover the circulated electrode, which was quickly washed with dimethyl carbonate (DC) (PanaxEtec). The surface morphology of the electrodes with atomic states occurring on the graphite surface was analyzed using field-emission scanning electron microscopy (FESEM) (Quanta 3D FEG, FEI).

On the graphite surface after the cycle, solid-state nuclear magnetic resonance spectroscopy (NMR) (ASCEND 400, Bruker) and X-ray photoelectron spectroscopy (XPS) (K alpha) to analyze the atomic composition , Thermo-Scientific) was used. All assays were performed at room temperature unless otherwise noted.

The molecule's ground-state structures have been fully optimized within C1 symmetry, using density function theory (DFT) methods. Kohn-Sham function was calculated with a 6-311G (d.p) basis set of B3PW91 functionality and triple-zeta quality.

The invention uses a conductor-variant polarized continuum model (CPCM), in which the solute is located in a molecular cavity embedded in a continuous dielectric.

Since EC: EMC = 1: 2 (vol%) is used as the solvent in the present invention, the dielectric constant is 31.9, which is an average value between the dielectric constants of EC (89.2) and EMC (2.9).

In the previous studies, the CPCM calculation method proved to be effective in evaluating various electrochemical properties of battery electrolytes. Therefore, this study was also used. All DFT and CPCM calculations were performed using the Gaussian09 program package.

Experimental Example 1. Measurement of linear sweep voltammetry (LSV) of electrolyte

FIG. 2 is a graph showing negative electrode stability measured by linear sweep potential method (LSV) of a battery (TMSP) prepared according to Example 1 of the present invention and a battery of Comparative Example 1 (none).

As shown in FIG. 2, the cell of Comparative Example 1 and the cell of Example 1 showed similar negative electrode stability during cathodic polarization.

In addition, it was confirmed that electrochemical reduction occurred at about 0.65 V (vs. Li / Li ⁺ ) regardless of the presence or absence of TMSP, and the electrochemical reduction potential of EC was about 0.65 V (vs.Li). / Li +). That is, the reduction current in all electrolytes was due to the reduction decomposition of EC rather than the decrease of TMSP, and the results were similar to the results of a first-principles calculation study. Since the reduction potential of TMSP was found to be about 0.7 V lower than the EC value, TMSP is very stable under the reducing conditions. However, since the conventional mechanism is that the SEI layer is generally formed through electrochemical reduction decomposition on the cathode surface, it is very interesting that TMSP does not form the SEI layer by electrochemical reduction alone.

Experimental Example 2 Rate Characteristics

FIG. 3 is a graph showing Coulombbic Efficiency of Batteries Prepared from Examples 1, 2, and 3 and Batteries Prepared from Comparative Examples 1 and 2, and FIG. 4 is from Examples 1, 2, and 3; It is a graph showing the measured potential relative to the specific capacity of the battery prepared and the cells prepared from Comparative Examples 1 and 2, Figure 5 is a battery prepared from Examples 1, 2, 3 and prepared from Comparative Examples 1 and 2 It is a graph which measured the specific specific amount by the cycle number of a battery.

3 to 5, black is a battery using a standard electrolyte (Comparative Example 1), red is a battery using an electrolyte containing 0.5% TMSP (Example 1), and blue is an electrolyte containing 1.0% TMSP. A battery (Example 2), green is a battery using an electrolyte containing 3.0% TMSP (Example 3), and orange is a battery using a electrolyte containing 3.0% VC (Comparative Example 2).

As a result of looking at Figures 3 to 5 it was confirmed that the electrochemical performance is significantly improved in the battery cell using the electrolyte containing TMSP. It was confirmed that the battery of Comparative Example 1 had a coulombic efficiency of 93.1%, and the battery of Comparative Example 2 had a low coulombic efficiency of 92.9%. In the electrolyte used in the battery of Comparative Example 2, the coulombic efficiency was found to be reduced because VC contributes to the electrochemical reduction, which is related to the formation of the SEI layer on the surface of the negative electrode.

On the other hand, the battery of Examples 1 to 3 according to the present invention can be confirmed that the coulombic efficiency is further improved to 3% ~ 2% to 95.2% maximum.

Looking at the result of measuring the specific capacity according to the cycle number in Figure 5, after 50 cycles, it was confirmed that the battery of Comparative Example 1 has a specific capacity remaining of 69.7%. On the other hand, the battery of Comparative Example 2 had a specific capacity retention of 97.4% after 50 cycles. This was confirmed because the VC contained in the electrolyte forms an SEI layer on the cathode.

On the other hand, the batteries prepared from Examples 1 to 3 of the present invention, it can be seen that even after the charge and discharge cycle repeated 96.8%, the specific capacity characteristics are maintained almost the same as the initial. That is, the batteries of Examples 1 to 3 have the advantage that the initial coulombic efficiency is very high, and has a superior specific capacity retention rate compared to VC.

If the initial coulombic efficiency of the electrolyte (participated in the formation of the SEI layer) was low in the cells of Examples 1 to 3 according to the present invention, the positive electrode must be positively balanced to balance the ion storage for the positive and negative electrodes. Expansion of the active material would have been necessary. Adding the positive electrode active material as needed would increase the weight of the cell and reduce the total energy density of the cell.

However, in the present invention, by adding TMSP in the electrolyte, it was confirmed that excellent Coulomb efficiency, excellent capacity characteristics and lifetime characteristics can be obtained, which can reduce the weight of the positive electrode active material relative to the battery using the conventional electrolyte, It can be said that it contributes to increase the energy density. In addition, since the cycle life is remarkably improved, it can be seen that the batteries of Examples 1 to 3 according to the present invention have very remarkable advantages over batteries using conventional electrolytes.

Experimental Example 3. SEM Analysis

6 is a SEM image of the negative electrode surface of the battery prepared from Comparative Example 1 after one cycle, Figure 7 is a SEM image of the negative electrode surface of the battery prepared from Comparative Example 1 after 50 cycles, 8 is an SEM image of the negative electrode surface of the battery prepared in Example 3 after one cycle, and FIG. 9 is an SEM image of the negative electrode surface of the battery prepared in Example 3 after 50 cycles.

Looking at Figure 6, after one cycle, the negative electrode surface of the battery prepared from Comparative Example 1 was irregular. In contrast, in the battery prepared from Example 3 of FIG. 8, it was confirmed that the negative electrode surface had a relatively uniform surface morphology.

The results are also in agreement with experimental data measured by EDS (not shown), since the atomic component associated with TMSP was observed only at the negative electrode surface of the cells prepared from Examples 1 to 3.

Taken together, the results described above (not shown EDS results, LSV results) show that the cells prepared from Examples 1 to 3 show another reaction pathway, rather than an electrochemical reaction, in which the TMSP electrolyte forms the SEI layer. It can be seen that through the formation.

After performing the 50 cycles of FIGS. 7 and 9, comparing the negative electrode surface from each of the cells, the morphology of the negative electrode of the cell prepared in Example 3 was higher than that of the negative electrode surface of the cell prepared in Comparative Example 1 It was confirmed to be clean.

These results confirm that the TMSP electrolyte according to the present invention improves the interfacial stability of the graphite negative electrode of the lithium ion secondary battery.

Experimental Example 4. NMR

10 and 11 are ³¹ P-NMR graphs for cells prepared from Example 3 and Comparative Example 1 after a cycle to analyze the chemical compounds of the SEI layers of Example 3 and Comparative Example 1. FIG.

Referring to Figures 10 and 11, the cell of Example 3 observed several new peaks in the downfield portion at ³¹ P-NMR, which is considered to be a phosphonate functional group chemically derived from TMSP. The phosphonate functional group is evidence that TMSP is formed in chemically modified form in the SEI layer.

Experimental Example 5. XPS Analysis

12 to 15 are graphs of the negative electrode of the battery prepared in Example 3 and Comparative Example 1 after the cycle by XPS, FIG. 12 is C1s, FIG. 13 is F1s, FIG. 14 is P2p, and FIG. 15 is Si2p graph. (The black line is the battery of Comparative Example 1, and the blue line is the battery of Example 3.)

As shown in Figure 12, the C1s composition of the SEI layer formed on the negative electrode of the battery prepared from Example 3 and Comparative Example 1 was as follows. First, R-CH ₂ OLi and R-CH ₂ OCO ₂ Li formed by EC reduction were identified, which was generally found in both cells. However, it was observed that the intensity was lower at the negative electrode of the battery prepared from Example 3 than the negative electrode of the battery prepared from Comparative Example 1.

In addition, the peak showing the solvent composition in FIG. 13 was lower in the battery of Example 3 (TMSP: LiF (687.6 eV in F1s)) than the battery of Comparative Example 1 (LixPFy (137.4 eV in P2p)).

However, it was confirmed that the negative electrode of the battery prepared from Example 3 had a higher peak intensity of LixPOyFz, which is one of the compositions of the SEI layer, which improved the surface stability than the negative electrode of the battery prepared from Comparative Example 1 (685.3 eV in Additional peaks were observed at F1s and 134.8 eV in P2p), 130.9 eV (P2p) and 101.9 eV (Si2p). The new peak is then believed to be derived from the TMSP electrolyte. This XPS analysis shows that TMSP contributes to SEI layer formation.

Experimental Example 6. Mechanism Analysis

Through several reaction pathways, reactions were calculated in TMSP and nucleophiles, and their validity was confirmed by first principles calculation. That is, a combination of these various results, confirming the possible reaction scheme is shown in Figure 16 and 17. Among the alkoxy functional groups (RO-) that may be produced by the reducing composition of EC, the CH ₃ O ⁻ functional groups were selected as nucleophiles for simplicity.

Looking at the reaction of the TMSP and the nucleophile (between EC and the nucleiphile) in the present invention. In general, Li + -coordinated EC by nucleophiles is known to attack the formation of ring-opened linear structures on the graphite cathode surface, but as calculated in the present invention TMSP has a higher Li + affinity than EC, It may be represented as shown in FIG.

Considering the reaction of Li + -TMSP with CH ₃ O ⁻ through cleavage of PO bonds and O—Si bonds, it can be represented as shown in FIG. 17, wherein the PO bonds (-43.8 kcal / mol) and O— In the process of cleaving Si bonds (-52.3 kcal / mol), since the reaction of breaking the O-Si bonds is more exothermic, thru-Li ⁺ is ((CH ₃ ) ₃ SiO) ₂ POLi and (CH ₃ ) ₃ SiOCH ₃ .

The above-described process, CH ₃ O ^- Li ⁺ -EC reaction than ~ 10 kcal / mol, and more preferred of having, EC-Li ⁺ and CH ₃ O ^- Li ⁺ in the reaction if this is not combined with the EC share of TMSP reaction Would have been more preferred. Even so, when TMSP and EC are added at the same time, since the TMSP according to the present invention is further reacted by the nucleophile on the graphite cathode surface, the TMSP is present in the SEI layer in a large amount as in the NMR and XPS results.

conclusion

The effect of TMSP on graphite cathode was investigated by considering chemical and electrochemical considerations. There was a difference in the formation of the SEI layer by the electrochemical reaction, because the compound represented by the formula (1) is more stable in the cathodic polarization environment. On the other hand, the nucleophilic radical anion intermediate is derived by the electrochemical reaction, and it was confirmed that the compound represented by Formula 1 is present in the SEI layer. As a result of looking at the NMR, XPS and the first principle calculation analysis, when the compound represented by Formula 1 is included as an additive, it was confirmed that the compound of Formula 1 is present in the chemically modified form in the SEI layer. In particular, 95.2% coulombic efficiency was observed in the initial cycle, and a battery containing 3.0% TMSP was observed, and 96.8% specific capacity retention was observed after 50 cycles.

As in the present invention, when the compound represented by the formula (1) is applied as an additive in the electrolyte, the fundamental understanding of the mechanism in the cell, to re-examine the new insights and new components on the formation of excellent SEI layer, This observation is the basis for suggesting a new form of existing electrolyte additives.

Claims (7)

delete delete delete delete delete It consists of a positive electrode, a negative electrode, a lithium ion secondary battery electrolyte and a separator,
The cathode is graphite,
The lithium ion secondary battery electrolyte includes a compound represented by the following formula (1), an organic solvent and a lithium salt, the compound represented by the formula (1) comprises 3% by weight based on the total weight,
The organic solvent is a mixture of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 2: 1,
The negative electrode surface includes an SEI layer formed from the electrolyte for a lithium ion secondary battery, wherein the SEI layer is 685.3 eV peak in Fls spectra, 134.8 eV peak in P2p spectra, and 130.9 eV peak in X-ray photoelectron spectroscopy (XPS) analysis. In the Si2p spectra, there is a 101.9 eV (Si2p) peak,
The SEI layer is a lithium ion secondary battery, characterized in that the peak of RP (= O)-(OR) ₂ (where R is SiCH ₃ ) detected by ³¹ P-NMR analysis.
[Formula 1]

The method of claim 6,
The lithium ion secondary battery has a specific capacity retention of 95 to 99% even after 50 cycles.

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