KR20200014563A - Electrolyte composition for lithium secondary battery and lithium secondary battery using the same - Google Patents

Abstract

The present invention relates to a novel electrolyte composition for a lithium secondary battery and a lithium secondary battery comprising the same. By including a compound represented by chemical formula 1, LiPF_y(C_2O_4)_y, LiPF_x, and a solvent, it is possible to form solid electrolyte inter-phase (SEI) on a surface of LiCoO_2 which is a positive electrode, and the formed SEI can suppress side reaction on the surface of LiCoO_2.

Description

A novel electrolyte composition for a lithium secondary battery and a lithium secondary battery including the same TECHNICAL FIELD

The present invention forms a SEI (Solid Electrolyte Inter-phase) on the surface of the LiCoO ₂ anode electrode, the formed SEI suppresses side reactions on the LiCoO ₂ surface to provide a lithium _secondary battery having a high energy and a long cycle life The present invention relates to an electrolyte composition and a lithium secondary battery including the same.

The rapid expansion of environmental pollution causing global warming and the energy crisis accelerated by the Fukushima disaster threaten human survival on the planet.

To address this issue, energy-efficient and environmentally friendly equipment, such as electric vehicles (EVs) and energy storage systems (ESSs), are of considerable interest to provide sustainable and renewable systems. In the case of such advantageous installations, rechargeable batteries that enable easy energy conversion and storage have become more important than ever.

Lithium-ion batteries (LIB) are one of the most reliable rechargeable batteries due to their many advantages, including high energy density, long cycle life and reasonable price. Despite these effective features of LIBs, typical commercial LIBs fall short of energy density in large-scale power applications in EV and ESS. Therefore, the high energy density of LIB is considered an urgent problem in its use.

Since the EV needs to be able to travel long distances, many LIBs of EVs that have been developed with many batteries pursuing long travel distances have limited driving distances due to the huge battery size. In addition, the large energy capability of the ESS reduces the cost of LIB and requires an efficient space of LIB for the establishment of the ESS.

Since 1991, when the lithium ion battery (LIB) was first commercialized by SONY, LiCoO ₂ positive electrode has been widely used due to its long life, fast speed and easy synthesis route. The LiCoO ₂ has a laminated structure (hexagonal shape, R3m) in which CoO ₆ and LiO ₆ octahedrons form parallel slabs, respectively. Several candidates have been proposed that have the same crystal structure as LiNi _0.33 Mn _0.33 Co _0.33 O ₂ and LiNi _0.6 Co _0.2 Mn _0.2 O ₂ but have different portions of transition metal ions such as nickel, manganese and aluminum. However, LiCoO ₂ is still one of the most promising candidates with simple synthesis paths such as high synthesis capability and long cycle life and relatively reliable performance.

In order to increase the energy density of the lithium ion battery (LIB), as much lithium ion as possible must be extracted from the positive electrode. This is because the number of lithium ions directly determines the energy density of LIB. Although LiCoO ₂ has only one lithium ion in its chemical structure, only half of the lithium ion is actually used to avoid irreversible crystal change from hexagonal to monoclinic, making the anode material inactive.

Given the above-mentioned energy density of Li-ion batteries (LIB), the most convenient way to improve the energy density of LIBs is to develop a system comprising stable and highly de-lithiated Li _1- xCoO ₂ . It is. At high potentials where high de-lithiated occurs, electrolyte degradation at the anode accelerates this failure. High delithiation requires high potential, which has a negative effect and leads to extensive electrolyte degradation.

Therefore, the combination of electrolytes must be carefully considered to solve the surface defects of high capacity LiCoO ₂ for high energy density LIB.

On the other hand, the electrolyte contained in the lithium ion battery LIB is composed of a salt and a solvent. LiPF ₆ , a common salt of the electrolyte, is not perfect to avoid electrochemical decomposition at the anode. However, the control of fluorinated salts has been limited because of the difficulty of controlling the synthesized fluorinated lithium salts. Instead, various kinds of solvents and additives have been investigated to solve this problem.

Republic of Korea Patent Publication No. 10-2008-0059182 Japanese Patent No. 6324845

An object of the present invention is to form a SEI (Solid Electrolyte Inter-phase) on the surface of the LiCoO ₂ anode electrode, the formed SEI suppresses side reactions on the LiCoO ₂ surface to provide a lithium _secondary battery having a high energy and a long cycle life It is to provide an electrolyte composition for the following.

In addition, another object of the present invention to provide a lithium secondary battery comprising the electrolyte composition.

In addition, another object of the present invention to provide a lithium capacitor comprising the electrolyte composition.

In addition, another object of the present invention to provide a method for producing the electrolyte composition.

The electrolyte composition for a lithium secondary battery of the present invention for achieving the above object comprises a compound represented by the following [Formula 1], LiPF _x and a solvent;

[Formula 1]

LiPF _y (C ₂ O ₄ ) _y

X and y are each an integer of 1 to 10.

The compound of Formula 1 is contained in 1 to 15% by weight, preferably 1 to 8% by weight of the total electrolyte composition.

The solvent may be at least one selected from the group consisting of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate.

The solvent may be a mixture of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate in a volume ratio of 1: 1-2: 3-10.

In addition, the lithium _secondary battery of the present invention for achieving the above object may include a positive electrode comprising the electrolyte composition and LiCoO ₂ .

In addition, the lithium capacitor of the present invention for achieving the above another object may comprise a positive electrode comprising an electrolyte composition and LiCoO ₂ .

In addition, the method for producing an electrolyte composition for a lithium secondary battery of the present invention for achieving the above another object is (A) dissolving the compound represented by the formula [1] in ethyl methyl carbonate; (B) LiPF _x , Mixing ethylene carbonate and diethyl carbonate; And (C) adding the mixture prepared in step (A) to the mixture mixed in step (B).

In step (B), the concentration of LiPF _x may be 0.5 to 2 M.

The total salt concentration of the electrolyte composition for the lithium secondary battery may be 0.8 to 1.5 M.

The ethylene carbonate, diethyl carbonate and ethyl methyl carbonate may be mixed in a volume ratio of 1: 1-2: 3-10.

The electrolyte composition for a lithium secondary battery of the present invention does not affect the ionic conductivity of the electrolyte composition by containing the compound represented by the above [Formula 1], and decomposes to form an interface film (SEI) on the surface of LiCoO ₂ , which is a positive electrode.

In addition, the interfacial film (SEI) derived from the compound represented by [Formula 1] inhibits side reactions on the surface of LiCoO ₂ . In particular, when using 5% by weight of the compound represented by [Formula 1], LiCoO ₂ has an optimum electrochemical performance.

FIG. 1A shows the anion structure of PF ₂ C ₄ O ₈ , and FIG. 1B is a 1st voltage curve from a coin half cell according to the LiPF ₂ C ₄ O ₈ salt content in the electrolyte composition.
Figure 2 is a graph showing the ionic conductivity of each electrolyte according to the content of LiPF ₂ C ₄ O ₈ salt.
3 is a graph showing the cycle characteristics of the coin half cell according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition.
4 shows (a) 0 wt%, (b) 1 wt%, (c) 3 wt%, (d) 5 wt% and (e) depending on the LiPF ₂ C ₄ O ₈ content in the electrolyte composition for 100 cycles. 8 wt%) Potential graph for specific capacity of LiCoO ₂ coin half cell.
5 is a Nyquist plot of a coin half cell according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition.
FIG. 6 shows (a) a coin half cell (c) 0 wt%, (c) 1 wt%, (d) 3 wt% prior to cycling the coin half cell and according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition. FE-SEM image of the LiCoO ₂ surface after cycling (e) 5 wt% and (f) 8 wt%).
FIG. 7 is an enlarged image of a red square part of FIGS. 6A to 6F.
8 is an XPS spectrum of (a) O 1s, (b) C 1s, (c) P 2p and (d) F 1s obtained from a once cycled LiCoO ₂ -Au buried electrode.
FIG. 9 shows (a) a coin half cell prior to cycling the coin half cell and according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition ((b) 0 wt%, (c) 1 wt%, (d) 3 wt%) FE-SEM image of LiCoO ₂ surface after 100 cycles of (e) 5 wt% and (f) 8 wt%).
FIG. 10 shows no more than 60 ° C. in a fully charged state after 50 hours using four types of electrolytes (including 0 wt%, 1 wt%, 3 wt% and 5 wt% of LiPF ₂ C ₄ O ₈ salt). The time-voltage curve is then measured after stabilization.

The present invention forms a SEI (Solid Electrolyte Inter-phase) on the surface of the LiCoO ₂ anode electrode, the formed SEI suppresses side reactions on the LiCoO ₂ surface to provide a lithium _secondary battery having a high energy and a long cycle life The present invention relates to an electrolyte composition and a lithium secondary battery including the same.

The film formation behavior of LiCoO ₂ is clearly observed in the first cycle without significant loss of coulombic efficiency. The electrochemical performance of the interfacial membrane was measured with respect to the electrolyte composition including the compound represented by the following [Formula 1] in various contents of the present invention.

The compound represented by the above [Formula 1] in the electrolyte composition of the present invention can alleviate the destruction mode of LiCoO ₂ .

In addition, the interface film formed electrochemically after one cycle by the compound represented by the above [Formula 1] has a large number of phosphate species from the compound (salt) represented by the above [Formula 1] and has the advantage as a surface stabilizer. There is this.

Hereinafter, the present invention will be described in detail.

The present invention includes a compound represented by the following [Formula 1], LiPF _x and a solvent.

[Formula 1]

LiPF _y (C ₂ O ₄ ) _y

X and y are each an integer of 1 to 10.

The oxalate group of the compound (salt) represented by [Formula 1] deforms the LiPF _x salt to form an organic interface film. The compound represented by [Formula 1] in the electrolyte composition of the present invention forms an interface film which is beneficial for mitigating the destruction mode of LiCoO ₂ .

The compound represented by [Formula 1] is contained in 1 to 15% by weight, preferably 1 to 8% by weight, more preferably 5 to 8% by weight of the total electrolyte composition. When the content of the compound represented by [Formula 1] is less than the lower limit of the above preferred range, it is not possible to provide a lithium secondary battery having high energy and long cycle life. The capacity can be reduced by lowering the movement of ions.

In addition, the solvent is to control the lithium salt in the electrolyte to stabilize the positive electrode and to form the SEI well on the surface of the positive electrode LiCoO ₂ , specifically, in the group consisting of ethylene carbonate, diethyl carbonate and ethyl methyl carbonate At least one selected may be mentioned, and preferably ethylene carbonate, diethyl carbonate and ethyl methyl carbonate may be used together.

At this time, the ethylene carbonate, diethyl carbonate and ethyl methyl carbonate is mixed in a volume ratio of 1: 1-2: 3-10, preferably in a volume ratio of 1: 1-2: 4-6. When the content of diethyl carbonate and ethyl methyl carbonate based on ethylene carbonate is out of the above preferred range, the life of the battery may be reduced.

In addition, the present invention can provide a method for producing an electrolyte composition for a lithium secondary battery.

Method for producing an electrolyte composition for a lithium secondary battery of the present invention comprises the steps of (A) dissolving the compound represented by [Formula 1] in ethyl methyl carbonate; (B) LiPF _x , Mixing ethylene carbonate and diethyl carbonate; And (C) adding the mixture prepared in step (A) to the mixture mixed in step (B).

First, in step (A), the compound represented by [Formula 1] is dissolved in ethylmethyl carbonate.

Next, in step (B), LiPF _x is It is added to a mixed solvent of ethylene carbonate and diethyl carbonate and mixed.

The concentration of LiPF _x in the mixture is 0.5 to 2 M, preferably 0.8 to 1.5 M. When the concentration of LiPF _x is out of the above preferred range, the ionic conductivity of the electrolyte may decrease.

Next, in step (C), the mixture prepared in step (A) is added to the mixture mixed in step (B).

The total salt concentration of the electrolyte composition for a lithium secondary battery thus prepared is 0.8 to 1.5 M, preferably 1.0 to 1.1 M. If the total salt concentration of the electrolyte composition is out of the above preferred range, the ionic conductivity of the electrolyte may decrease, and the interfacial film formed on LiCoO ₂ may have a difference in thickness.

In addition, the present invention may provide a lithium secondary battery or a lithium capacitor including the lithium secondary battery electrolyte composition and a positive electrode LiCoO ₂ .

Hereinafter, preferred examples are provided to help understanding of the present invention, but the following examples are merely for exemplifying the present invention, and various changes and modifications within the scope and spirit of the present invention are apparent to those skilled in the art. It is natural that such variations and modifications fall within the scope of the appended claims.

Example 1.

Electrolyte Composition_LiPF ₂ C ₄ O ₈

Li-PF ₂ C ₄ O ₈ was manufactured by CHUN BO LTD in Korea. Specifically, Li-PF ₂ C ₄ O ₈ solution dissolved in ethyl methyl carbonate (EMC, Panaxetec) was 1.0 M LiPF in EC (ethylene carbonate): DEC (diethyl carbonate) (1: 1 volume ratio, Panaxetec) ₆ , 1% by weight, 3% by weight, 5% by weight and 8% by weight of the conventional electrolyte. The total salt concentration of all electrolytes was adjusted to 1.0 M by adding LiPF ₆ salt and EMC solvent. In addition, the solvent was used by mixing in a volume ratio of EC: DMC: EMC = 1: 1: 1.85.

The anion structure of the PF ₂ C ₄ O ₈ is shown in Figure 1a.

Anode Electrode_LiCoO ₂

LiCoO ₂ was obtained from JES-echem. The LiCoO ₂ powder is uniform with a binder which is a super-P (Timcal) and polyvinyldifluoride (polyvinyldifluoride, PVdF, KF1100, Kureha) by the addition of N-methylpyrrolidinone (Aldrich). Were mixed and prepared into electrode slurry using a PDM-300 automatic mixing device. For example, a doctor-blade and a bar-coater were used to cast on Al foil to control a loading level of about 10 mg / cm ² .

The electrode sheet on which the electrode slurry was cast was completely dried in a convection oven at 120 ° C., and a roll presser step was applied to control the thickness from the initial thickness of the electrode to 70%. The electrodes were then drilled to 1.2 cm in diameter and dried at 120 ° C. overnight in a vacuum oven.

Lithium secondary battery

The 2032 type coin half cell was prepared using the electrolyte, composite electrode disk, lithium foil and polypropylene separator (Celgard) prepared above. The open circuit voltage (OCV) of the assembled coin cell was initially estimated to be about 3V.

As a behavioral cycle, it was first charged with a current density of 0.1 C (1 C = 140 mAg-1) at a potential range of 3.5 to 4.4 V under a constant temperature of 25 ° C. After the initial behavior cycle, the second and third cycles were charged to current densities of 0.5 C and discharged to current densities of 0.2 C, respectively. The fourth cycle was charged at 0.5 C and then discharged at 2 C. The cycle was then charged and discharged at a rate of 2 C at the same temperature and at the same potential range.

<Test Example>

For electrochemical impedance spectroscopic analysis, a three electrode type coin cell was prepared in the same manner as the 2032 coin cell except for the additional use of a lithium metal tab, such as a reference electrode and two separators. Two separators were inserted between the working composite electrode and the lithium foil counter electrode. An additional lithium tab was secured between the separators so as not to block the electrolyte passage between the working and counter electrodes. Zive Lab (MP1) was used to measure EIS in the frequency range of 100 kHz to 0.01 Hz after 30 minutes of rest.

Similarly, the ionic conductivity of the electrolyte was measured by EIS with a symmetric platinum electrode (Sanxin, 2401-M glass conductive electrode) with a cell constant of 1 cm −1 and about 50 mL of electrolyte.

For X-ray photoelectron spectroscopy (XPS) analysis, a special electrode, a LiCoO ₂ embedded Au electrode, was prepared. To prepare this type of electrode, LiCoO ₂ powder was carefully dispersed over Au disks (Alfa Aesar, 0.25 mm thickness, 99.95%) and the prepared disks were pressed at 5000 psi for 1 hour.

The prepared LiCoO ₂ -Au electrode was assembled into a coin cell in the same manner as used for the composite electrode except for the working electrode. In addition, the cells were cycled using the same method.

For post analysis, the electrodes were washed with EMC solvent to remove residual salts.

XPS measurements were performed using a PHI 5000 VersaProbe II instrument with X-ray Al anode as a source for analyzing the chemical state of the electrode surface. The binding energy was based on the C (1s) signal of carbon at 285 eV.

In addition, JEOL JSM 7800F was used for FE-SEM analysis.

Test Example 1. Secondary Battery Characteristics_Voltage, Ion Conductivity

1B is a 1st voltage curve from a coin half cell with LiPF ₂ C ₄ O ₈ salt content in the electrolyte composition. The total lithium content in the electrolyte was 1 M, the current density was 14 mAh g-1, the potential range was 4.4-3.5 V relative to Li / Li +, and the temperature was controlled to 25 ° C.

Figure 2 is a graph showing the ionic conductivity of each electrolyte according to the content of LiPF ₂ C ₄ O ₈ salt.

As shown in FIG. 1B, the upper curve represents de-lithiation and the lower curve represents discharge. Based on the accumulated capacity, the discharge capacity was recorded in the negative direction in contrast to the charging voltage curve.

Each cell containing an electrolyte comprising different LiPF ₂ C ₄ O ₈ salt contents has various polarization behaviors.

At specific charge capacities of 5 mAh g-1, cells containing 0 wt%, 1 wt%, 3 wt%, 5 wt% and 8 wt% Li / Li + versus 3.91 V, 3.94 V, 4.02 V, 4.15 V and a potential of 4.32 V. Chemical potential of LiCoO ₂ having a charging capacity of the 5 mAh g-1 has a high potential, so the observed lower than the operating potential of LiCoO ₂ is 3.9 V reflecting the higher movement resistance.

Therefore, LiPF ₂ C ₄ O ₈ has an electrochemical effect on the anode electrode of LiCoO ₂ , resulting in high polarization.

In order to determine whether this polarization behavior was derived from different conductivity behaviors of electrolytes containing different LiPF ₂ C ₄ O ₈ contents, the ionic conductivity of the prepared electrolytes was measured.

As shown in FIG. 2, the ionic conductivity of each electrolyte containing LiPF ₂ C ₄ O ₈ in different amounts was similar to each other. Thus, such polarization cannot be explained by simple ion diffusion behavior in the electrolyte.

The charge specific capacities of the cells containing each electrolyte containing LiPF ₂ C ₄ O ₈ in 0%, 1%, 3%, 5% and 8% by weight are very similar to each other except 8% by weight. It was. These were measured as 174.1 mAh g-1, 177.8 mAh g-1, 171.8 mAh g-1, 177.7 mAh g-1 and 154.2 mAh g-1, respectively.

Each electrolyte containing a battery containing 0 wt% and 1 wt% of LiPF ₂ C ₄ O ₈ exhibits a discharge intrinsic capacity similar to 169.1 mAh g −1 and 172.5 mAh g −1. However, cells containing more LiPF ₂ C ₄ O ₈ salts are slightly lower for 16% mAh g-1, 165.8 mAh g-1 and 144.5 mAh g-1 for 3%, 5% and 8% by weight, respectively. Dose was shown. Initial coulomb efficiencies based on these charge and discharge behaviors were calculated to be 97.1%, 97.0%, 96.9%, 96.0% and 93.7%, respectively.

Adding the LiPF ₂ C ₄ O ₈ salt reduces the 1st coulombic efficiency. However, the irreversible capacity of 1 wt% and 3 wt% of LiPF ₂ O ₄ C ₈ is LiPF ₂ C ₄ O ₈ was not much different from the irreversible capacity of the reference electrolyte does not.

Insufficient coulombic efficiency greatly limits the energy density because the lithium ion battery (LIB) contains lithium ions and has a rocking chair system.

LiPF ₂ C ₄ O ₈ salts are thought to be involved in initial electrolyte degradation, but it is not good to consume electrons for electrolyte degradation that is considered irreversible within the term of the locking chair system. However, higher concentrations of LiPF ₂ C ₄ O ₈ in excess of 5% by weight resulted in high polarization with respect to the release sequence, which polarization lowers the coulombic efficiency.

Test Example 2 Secondary Battery Characteristics-Cycle Life

3 is a graph showing the cycle characteristics of the coin half cell according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition. At this time, the current density is 14 mAg-1, the potential range is 4.4-3.5V compared to Li / Li +, the temperature is 25 ℃.

4 shows (a) 0 wt%, (b) 1 wt%, (c) 3 wt%, (d) 5 wt% and (e) depending on the LiPF ₂ C ₄ O ₈ content in the electrolyte composition for 100 cycles. 8 wt%) Potential graph for specific capacity of LiCoO ₂ coin half cell.

As shown in FIG. 3, it shows the cycleability of the coin half cell according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition, with high cut-off for LiCoO ₂ in the case of cells with a reference electrolyte (black star). The off potential results in a poor cycle.

Due to the high cut-off potential of 4.4 V against Li / Li +, the 1st specific capacity of the cell without LiPF ₂ C ₄ O ₈ is 158.6 mAh g−, which is higher than the theoretical capacity of Li _0.5 CoO ₂ 136.9 mAh g-1. It confirmed that it was one. This high capacity dropped below 130 mAh g-1 after 60 cycles, and the capacity drop was further accelerated to show 70.8 mAh g-1 at 100 cycles.

This low cycle life behavior of LiCoO ₂ is caused by increased surface resistance due to irreversible phase transitions and electrolyte decomposition from extensive de-lithiation.

In addition, as shown in FIG. 4, the voltage curve of the cell with the reference electrolyte has a high polarization action during cycling. The polarity limits the possibility of circulation due to extremely demanding properties.

On the other hand, in the initial cycle of 50 days before, as shown in FIG. 1, the cell containing more LiPF ₂ C ₄ O ₈ shows a lower specific capacity than the cell containing the reference electrolyte, and 0 wt% represents LiPF ₂ It reflects the same behavior of high polarization from C ₄ O ₈ .

This polarization behavior continues to relax as the cycle progresses. For example, a cell containing 1 wt.% LiPF ₂ C ₄ O ₈ exhibited 150.4 mAh g-1 in the first cycle and an increase in capacity of 155.7 mAh g-1 in the 18th cycle. Similarly, the specific capacity of cells containing LiPF ₂ C ₄ O ₈ increases in the initial tens of cycles.

It is also noted that it exhibits a stable cycle life than cells containing more LiPF ₂ C ₄ O ₈ . Two types of cells containing 5 and 8% by weight of LiPF ₂ C ₄ O ₈ , respectively, showed excellent cycle life even after 250 cycles.

In the case of a cell containing 8 wt% LiPF ₂ C ₄ O ₈ , a low specific gravity of less than 136.9 mAh g-1, which is not severe in irreversible phase transition, can account for long cycle life. However, cells containing 5% by weight of LiPF ₂ C ₄ O ₈ exhibit a high specific capacity of 160 mAh g −1 at 177 cycles.

Despite these high capacities, cells containing 5% by weight of LiPF ₂ C ₄ O ₈ can exhibit a high capacity of 157.4 mAh g-1 even after 350 cycles, and the state of the highly delithiated LiCoO ₂ is monoclinic ( monoclinic) may cause irreversible phase transition.

Although there is a slight difference in the degree of cycle performance improvement, everything containing LiPF ₂ C ₄ O ₈ salt has improved cycle characteristics. Thus, it was found that LiPF ₂ C ₄ O ₈ salt improved the cycle characteristics.

Test Example 3 Secondary Battery Characteristics_Electrochemical Behavior

FIG. 5 is a Nyquist plot filled with 5 mAh g −1 in a three-electrode cell during the first cycle of a coin half cell according to LiPF ₂ C ₄ O ₈ content in the electrolyte composition. The light blue dot means 2.5119 Hz, and the equivalent circuit model is marked as insert.

To further study the electrochemical behavior, EIS was performed at the initial stage of charging of 20 mAh g-1.

As shown in FIG. 5, the cell containing the reference electrolyte exhibited two semicircles, which corresponds significantly to an equivalent circuit including a portion of SEI and charge transfer as shown in the inset. Using an equivalent circuit in the inset of FIG. 5, the diameter of the semicircle reflects the resistance value.

Given the time constants for this behavior, semicircles in the high (left) and low (right) frequency domains are assigned to SEI and charge transfer behavior, respectively. Between the two semicircles, the frequency of about 2.6 Hz of the inflection point is represented by a light sky blue point.

EIS results from other cells show that the diameter of the SEI operating semicircle increases with increasing LiPF ₂ C ₄ O ₈ content. Also, a light sky blue point of about 2.6 Hz means that the first semicircle still exhibits SEI behavior.

Therefore, it was confirmed that the electrochemical behavior for SEI operation depends on the content of LiPF ₂ C ₄ O ₈ . It is meaningful that LiPF ₂ C ₄ O ₈ forms SEI on LiCoO ₂ .

Test Example 4 SEM Measurement

FIG. 6 shows (a) a coin half cell (c) 0 wt%, (c) 1 wt%, (d) 3 wt% prior to cycling the coin half cell and according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition. FE-SEM image of the LiCoO ₂ surface after cycling (e) 5 wt% and (f) 8 wt%).

7 is an enlarged image of a red square portion of FIGS. 6A to 6F.

In order to visualize the interfacial film induced by LiPF ₂ C ₄ O ₈ salt, FE-SEM analysis was performed using LiCoO ₂ electrodes filled with the various electrolytes.

As shown in FIG. 6, when inspected at the micrometer scale, the particle surfaces were found to look similar to each other. However, as a result of evaluating on the 100 nanometer scale, the surface of LiCoO ₂ was corrected by LiPF ₂ C ₄ O ₈ with the electrochemical results shown in FIGS. 1, 3 and 5.

In addition, the particle surface before cycling shown in FIG. 7A exhibited long edges indicating voluminous layer-crystals. Specifically, the surface was initially flat and clean.

After cycling once in the reference electrolyte, the surface of the particles changed slightly, resulting in agglomerated small round particles of several tens of nanometers in diameter, which appeared to be the result of electrolyte decomposition.

When using an electrolyte containing 1% by weight and 3% by weight of LiPF ₂ C ₄ O ₈ salt, it was not confirmed that the LiCoO ₂ surface was deformed. However, when higher 5% and 8% by weight of LiPF ₂ C ₄ O ₈ salts were contained, the new structure of the interfacial film was clearly changed by deposition.

The interfacial film produced by the electrolyte containing LiPF ₂ C ₄ O ₈ salt appears to have high polarization for the 1st voltage curve (FIG. 1B) and the EIS result (FIG. 5) based on various surface morphologies. Based on this behavior, 1% by weight and 3% by weight of LiPF ₂ C ₄ O ₈ salt made a thin film not easily distinguished by FE-SEM.

Test Example 5 XPS Analysis

8 is an XPS spectrum of (a) O 1s, (b) C 1s, (c) P 2p and (d) F 1s obtained from a once cycled LiCoO ₂ -Au buried electrode. The electrolyte used comprises 0, 1, 3 and 5% by weight of LiPF ₂ C ₄ O ₈ salt. Fit curves are indicated by solid lines.

FIG. 9 shows (a) a coin half cell prior to cycling the coin half cell and according to the LiPF ₂ C ₄ O ₈ content in the electrolyte composition ((b) 0 wt%, (c) 1 wt%, (d) 3 wt%) FE-SEM image of LiCoO ₂ surface after 100 cycles of (e) 5 wt% and (f) 8 wt%).

Since the LiCoO ₂ -Au buried electrode was used for XPS analysis, all spectra were generated on the surface of the active material LiCoO ₂ . The application of the buried system is very useful for removing obstacles of the conductive agent and the binder.

8 shows the XPS results of the O 1s, C 1s, P 2p and F 1s spectra from the charged LiCoO ₂ electrodes.

In the O 1s spectrum, the spectra obtained using three types of peaks were convolute. Peaks of 528.5 eV, 532 eV and 533.5 eV are assigned to the metal oxide, C═O / PO and COC species, respectively. The metal oxide peak refers to a LiCoO ₂ anode material.

Considering that XPS is a surface analysis tool, the higher metal oxide peak at 528.5 eV means that the photoelectrons of LiCoO ₂ were easily released without disturbing the surface layer. The high peak of 528.5 eV shown in FIG. 8A means a thin interface film.

As can be seen in FIG. 8A, the O 1s spectrum from the circulated electrode in the electrolyte containing LiPF ₂ C ₄ O ₈ shows a reduced peak of the metal oxide.

The peak decreases with increasing LiPF ₂ C ₄ O ₈ content, which is very consistent with the results supporting the formation of the interfacial film by LiPF ₂ C ₄ O ₈ salt in the FE-SEM image of FIGS. 6C-E. At the same time, LiPF ₂ C ₄ O ₈ salts lead to interfacial membranes containing C═O or PO species observed at 532 eV. Unlike the metal oxide peaks, the peak of 532 eV increases with increasing LiPF ₂ C ₄ O _{8 in} the electrolyte.

In addition, in the C 1s spectrum of FIG. 8B, three peaks were used to convolve the XPS results. The first peak of 285 eV was used as the corrected peak for C-C binding, and the other two peaks of 286.5 eV and 289 eV were designated C-O-C and C = O, respectively.

As shown in the O 1s spectrum of FIG. 8A, many species of LiPF ₂ C ₄ O ₈ were observed at 532 eV and 533.5 eV. However, 5% by weight of LiPF ₂ C ₄ O ₈ salt containing electrolyte is at 286.5 eV and 289 eV compared to other spectra from an electrolyte containing LiPF ₂ C ₄ O ₈ salt of 1% by weight and 3% by weight huge There was no peak.

Instead, the electrolyte containing 5% by weight of LiPF ₂ C ₄ O ₈ salt showed a very large peak of 135 eV in the P 2p spectrum. The P 2p spectrum of the reference electrolyte has a main peak at 136.5 eV.

As the content of LiPF ₂ C ₄ O ₈ salt increased, the height of the peak increased and the position shifted in the negative direction. The right peak of 136.5 eV corresponds to PFy and the left peak of 135 eV corresponds to POyFz.

Next, the large peak of O 1s in FIG. 8A is explained by the production of POyFz species from LiPF ₂ C ₄ O ₈ salt. This phenomenon supports the notion that salts are involved in interfacial film formation.

In addition, the F 1s spectrum in FIG. 8D has convolution at two major peaks, PF at 688 eV and LiF at 685.5 eV. Obviously, the larger the PF peak was observed as more LiPF ₂ C ₄ O ₈ salt was added to the electrolyte. The combination of the P spectra revealed that the LiPF ₂ C ₄ O ₈ salt participates in the film formation of LiCoO ₂ upon 1st filling.

After 100 cycles, the cycle electrode was observed by SEM (FIG. 9).

In FIG. 9B, the cycled electrode of a commercial electrolyte has a very different surface texture from the initial electrode (FIG. 9A) because the reference electrode forms a thick passivation layer due to electrolyte degradation, resulting in poor circulation potential. The disadvantageous passivation layer is highly mitigated by adding LiPF ₂ C ₄ O ₈ salt to the electrolyte.

Test Example 6 Measurement of Time-Voltage After Stabilization

FIG. 10 shows no more than 60 ° C. in a fully charged state after 50 hours using four types of electrolytes (including 0 wt%, 1 wt%, 3 wt% and 5 wt% of LiPF ₂ C ₄ O ₈ salt). The time-voltage curve is then measured after stabilization.

As shown in FIG. 10, after 50 cycles, a fully charged cell containing the four types of electrolytes was stored in an open circuit at a high temperature of 60 ° C. to accelerate the decomposition of the electrolyte after 50 cycles to prove the surface stability of the interfacial membrane. If electrolyte decomposition occurs at the surface of the positive electrode, the charge stored at the positive electrode will be consumed, indicating a potential drop.

Conventional cells containing electrolytes exhibited high potential drops at the start of storage testing. This means that electrons are consumed in the charging process due to adverse surface electrolyte decomposition. After 1 hour, the potential dropped from 4.40 V to 4.14 V compared to Li / Li +. The steeply decreasing slope can be explained by the fact that higher potential leads to more electrolyte degradation.

In contrast, by adding more LiPF ₂ C ₄ O ₈ salt to the electrolyte, the potential degradation behavior was very mitigated. After 1 hour of storage, a cell with an electrolyte containing 1%, 3% and 5% by weight of LiPF ₂ C ₄ O ₈ salt had a high potential of 4.38 V, 4.38 V and 4.34 V, respectively, compared to Li / Li +. Indicated. Induced interfacial membranes appear to reduce electrolyte degradation during storage testing.

Thus, interfacial membranes derived from LiPF ₂ C ₄ O ₈ salts greatly improve surface stability by reducing electrolyte degradation. While this type of passivation film exhibits a polarization effect at the beginning of the cycle, it significantly improves cycle performance by solving failure modes that increase polarity.

Specifically, LiPF ₂ C ₄ O ₈ salt in the electrolyte improves the surface stability of LiCoO ₂ by the solid electrolyte interface membrane in 1st cycle, thanks to the interface membrane LiCoO ₂ has a high cut of 4.4 V for high energy density LIB The life cycle was extended even at the -off potential.

XPS analysis using LiCoO ₂ embedded electrodes clearly revealed an interface film having phosphate and organic species by LiPF ₂ C ₄ O ₈ . With a three-electrode cell system, the EIS confirmed the formation of the interface film at the start of charging.

Through a systematic combination of electrochemical and surface analysis, LiPF ₂ C ₄ O ₈ salts form an interfacial membrane in the first cycle, and the first interfacial membranes induced by LiPF ₂ C ₄ O ₈ salts contain LiCoO ₂ , including electrolyte decomposition. It greatly reduces the destruction mode and improves the life cycle.

Claims (12)

An electrolyte composition for a lithium secondary battery comprising a compound represented by the following [Formula 1], LiPF _x and a solvent;
[Formula 1]
LiPF _y (C ₂ O ₄ ) _y
X and y are each an integer of 1 to 10. According to claim 1, wherein the compound of [Formula 1] is a lithium secondary battery electrolyte composition, characterized in that contained in 1 to 15% by weight of the total electrolyte composition. The electrolyte composition for a lithium secondary battery according to claim 1, wherein the compound of [Formula 1] is contained at 1 to 8% by weight. The electrolyte composition of claim 1, wherein the solvent is at least one selected from the group consisting of ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate. The electrolyte composition of claim 1, wherein the solvent is mixed with ethylene carbonate, diethyl carbonate, and ethyl methyl carbonate in a volume ratio of 1: 1: 2: 3-10. A lithium secondary battery comprising a positive electrode comprising a electrolyte composition of claim ₁ and LiCoO ₂ . A lithium capacitor comprising a positive electrode comprising the electrolyte composition of claim ₁ and LiCoO ₂ . (A) dissolving the compound represented by the following [Formula 1] in ethylmethyl carbonate;
(B) LiPF _x , Mixing ethylene carbonate and diethyl carbonate; And
(C) adding the mixture prepared in step (A) to the mixture mixed in step (B);
[Formula 1]
LiPF _y (C ₂ O ₄ ) _y
X and y are each an integer of 1 to 10. The method of claim 8, wherein the compound represented by [Formula 1] is contained in 1 to 15% by weight of the total electrolyte composition. The method of claim 8, wherein in the step (B), the concentration of LiPF _x is 0.5 to 2 M. The method of manufacturing an electrolyte composition for a lithium secondary battery. The method of claim 8, wherein the total salt concentration of the electrolyte composition for lithium secondary batteries is 0.8 to 1.5 M. The method of claim 8, wherein the ethylene carbonate, diethyl carbonate and ethyl methyl carbonate are mixed in a volume ratio of 1: 1: 2: 3-10.

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