KR102137665B1 - Electrolyte solution for secondary battery and additive therefor - Google Patents

Abstract

The present invention relates to an electrolyte additive for a secondary battery represented by Chemical Formula 1, and an electrolyte for a secondary battery comprising the same, wherein the electrolyte additive according to the present invention lowers the output resistance of the electrolyte for a secondary battery and increases the interface resistance that appears when using high-power charge/discharge. Can be prevented.

Description

Electrolytic solution and additive for secondary battery {ELECTROLYTE SOLUTION FOR SECONDARY BATTERY AND ADDITIVE THEREFOR}

The present invention relates to an electrolyte additive for a secondary battery and an electrolyte for a secondary battery comprising the same, and more specifically, to improve the output performance by lowering the interface resistance and to maintain the performance without increasing the resistance even in a high temperature use environment and the electrolyte additive for the secondary battery It relates to an electrolyte solution for a secondary battery containing it.

Lithium secondary batteries have advantages of high energy density and low self-discharge, and thus are useful as power sources for mobile devices such as smartphones and laptops and power sources for electric vehicles. In such a lithium secondary battery, an electrolyte composed of a lithium salt as an electrolyte and a non-aqueous solvent is used. The non-aqueous solvent has a high dielectric constant to dissolve the lithium salt, and the ion conductivity must be large in a wide temperature range. However, since it is difficult to achieve the properties of the electrolyte solvent as a single material, typically, a high boiling point solvent represented by propylene carbonate, ethylene carbonate, and the like, and a low boiling point solvent such as dimethyl carbonate or diethyl carbonate are mixed and used. In addition, various additives may be added to the electrolyte to improve characteristics of the lithium secondary battery, for example, initial capacity, cycle characteristics, high temperature storage characteristics, low temperature characteristics, self-discharge characteristics, and overcharge characteristics.

In recent years, lithium secondary batteries have been used for various purposes, and their use is gradually spreading in the fields of electric vehicles and electric tools requiring high power. In these fields, instantaneous output and faster charge/discharge speed are required. Therefore, in order to improve high-power characteristics, interest in resistance at an interface between an electrode and an electrolyte is increasing, and development of an electrolyte additive that has low interface resistance and is dense and well maintained in an environment of use has been attempted.

However, the electrolyte additives known so far form a film at the cathode to increase the interface resistance, and easily break the film formed at a high temperature environment or act as a high resistance at a low temperature environment (Korean Patent No. 1041127).

Accordingly, the present inventors have conducted continuous research on substances capable of lowering the output resistance of the electrolyte and preventing an increase in interface resistance when using high-power charge/discharge, and found an electrolyte additive suitable for a high-power environment than conventional materials, The present invention has been completed.

Republic of Korea Patent No. 1041127 Republic of Korea Patent No. 1297172 Republic of Korea Patent No. 1033697

Accordingly, an object of the present invention is to provide an electrolyte solution additive that can be applied to an electrolyte solution for a lithium secondary battery to lower the interface resistance to improve output characteristics.

Another object of the present invention is to provide an electrolyte solution for a secondary battery capable of maintaining output characteristics and improving capacity retention even in a high temperature environment.

The present invention to achieve the above object,

An electrolyte additive for a secondary battery having the structure of Formula 1 is provided:

[Formula 1]

In Chemical Formula 1,

R ₁ and R ₂ are each independently hydrogen or a C _1-3 alkyl group;

R ₃ and R ₄ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group or a C _{1 -3} having a hydrocarbon group;

R ₅ and R ₆ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group or a C _{1 -3} having a hydrocarbon group;

M is P, B, Al, Co or Mn, wherein M may have one or more substituents selected from the group consisting of halogen, nitrile (CN), methylsulfonyl and sulfonate (SO ₃ H).

The present invention also provides non-aqueous solvents; Lithium salt; And it provides an electrolyte for a secondary battery comprising an additive represented by the formula (1).

The electrolyte additive for a secondary battery of the present invention can be used in an electrolyte solution to lower the output resistance of the electrolyte solution and prevent the increase in interface resistance during high-power charging and discharging, thereby providing stable output characteristics even at high temperatures. In addition, the electrolyte additive for a secondary battery of the present invention can improve the capacity retention rate in the secondary battery.

Hereinafter, the present invention will be described in detail.

The electrolyte additive according to the present invention is intended to improve the output performance by lowering the interface resistance of the electrolyte for a secondary battery, particularly a lithium secondary battery, and suppressing an increase in resistance in a high temperature environment, and has the structure of Formula 1

[Formula 1]

In Formula 1, R ₁ and R ₂ are each independently hydrogen or a C _1-3 alkyl group; R ₃ and R ₄ are oxo groups, each independently hydrogen, a C _1-3 alkyl group, or a hydrocarbon having a carbonyl group having 1 to 3 carbon atoms, preferably 1 to 2 carbon atoms; R ₅ and R ₆ are oxo groups, each independently hydrogen, a C _1-3 alkyl group, or a hydrocarbon having a carbonyl group having 1 to 3 carbon atoms, preferably 1 to 2 carbon atoms. When the number of carbon atoms in R ₁ , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ exceeds 3, the interface resistance due to the additive may increase, and the amount of gas generated in the battery may increase due to the occurrence of side reactions.

M of Formula 1 is P, B, Al, Co or Mn, wherein M may have one or more substituents selected from the group consisting of halogen, nitrile (CN), methylsulfonyl and sulfonate (SO ₃ H) have.

In addition, the additive of Formula 1 may be in the form of a salt depending on the element of use of M, preferably provided in the form of a lithium salt additionally containing lithium ions to stably maintain the performance of the electrolyte solution for a secondary battery.

Specific examples of the additives are N,N-dimethyloxalato tetrafluorophosphate, N,N-dimethyloxalato difluoroborate, N,N-dimethylglycine-oxo-tetrafluorophosphonaryl, N, And N-dimethyloxalato pentafluorophosphate lithium salt, N,N-dimethyloxalato trifluoroborate lithium salt, and N,N-dimethylglycine-oxo-pentafluorophosphonaryl lithium salt.

Performance evaluation of the additive represented by the formula (1) is a certain amount of additives added to the electrolyte, assembled battery, chemical conversion After that, the charging and discharging can be repeated at various instantaneous current rates to check the resistance characteristics. In addition, in order to provide a harsh environment, the battery is stored at a high temperature of 60°C or higher, and resistance characteristics and capacity retention rates can be observed at regular intervals.

The electrolyte solution for a secondary battery according to the present invention includes a non-aqueous solvent, a lithium salt, and an additive represented by Chemical Formula 1.

The non-aqueous solvent is preferably high solubility in the lithium salt and additives, and without limitation, propylene carbonate (PC), ethylene carbonate (ethylene carbonate), ethyl methyl carbonate (ethylmethyl carbonate); EMC), dimethyl carbonate (DEC), gamma-butyrolactone (GBL), diethyl carbonate (DEC), etc. can be used alone or in combination, preferably ethylmethyl carbonate (EMC) ), dimethyl carbonate (DMC), diethyl carbonate (DEC), and linear carbonates such as propylene carbonate (PC) and ethylene carbonate (EC).

The lithium salt is intended to improve the ionic conductivity of the electrolyte, and without limitation, LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) _2, etc., alone or by mixing Can be used.

In the electrolyte solution for a secondary battery according to the present invention, the concentration (content) of the lithium salt may be 0.9 M to 1.2 M (mol/liter), preferably 0.95 M to 1.1 M. When the content of the lithium salt is 0.9 M or more, an ionic conductivity of an appropriate electrolyte can be secured, and in the case of 1.2 M or less, a lithium salt can be used within an appropriate range capable of effectively increasing the ionic conductivity, thereby increasing economic efficiency.

The content of the additive represented by Chemical Formula 1 may be 0.05 to 30 parts by weight, preferably 0.1 to 10 parts by weight, more preferably 1 to 5 parts by weight based on 100 parts by weight of the electrolyte solution. When the content of the additive is 0.05 parts by weight or more, a synergistic effect of an appropriate oxidation start voltage can be exhibited, and if it is 30 parts by weight or less, the ionic conductivity of the electrolyte can be appropriately maintained.

The electrolyte solution for a secondary battery according to the present invention can be prepared by simply mixing and stirring a non-aqueous solvent, a lithium salt, and an additive represented by Formula 1 above.

A secondary battery may be manufactured by injecting the electrolyte solution for a secondary battery of the present invention described above into an electrode assembly including an anode and a cathode, and a separator interposed therebetween. In the present invention, the secondary battery includes all kinds of secondary batteries, and preferably may be a lithium ion battery, a lithium ion polymer battery, or a lithium polymer battery.

Hereinafter, the present invention will be described in more detail through specific examples and comparative examples. The following examples are intended to illustrate the present invention more specifically, but the present invention is not limited by the following examples.

<Example 1> Preparation of electrolyte solution

330 g of ethylene carbonate (EC), 453 g of ethylmethyl carbonate (EMC) and 292.5 g of diethylene carbonate (DEC) were mixed, and 167 g of LiPF ₆ was added to the mixture to prepare a 1.1 M LiPF ₆ solution, , As an additive, N,N-dimethyloxalato tetrafluorophosphate represented by the following Chemical Formula 2 was added in an amount of 1.0% by weight to prepare an electrolyte solution (electrolytic solution).

[Formula 2]

<Example 2> Preparation of electrolyte solution

As an additive, an electrolyte solution was prepared in the same manner as in Example 1, except that the N,N-dimethyloxalato pentafluorophosphate lithium salt represented by the following Chemical Formula 3 was used in an amount of 1.0% by weight.

[Formula 3]

<Example 3> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that N,N-dimethyloxalato difluoroborate represented by the following Chemical Formula 4 as an additive was used in an amount of 1.0% by weight.

[Formula 4]

<Example 4> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that the N,N-dimethyloxalato trifluoroborate lithium salt represented by the following Chemical Formula 5 was used in an amount of 1.0% by weight as an additive.

[Formula 5]

<Example 5> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that N,N-dimethylglycine-oxo-tetrafluorophosphonaryl represented by Formula 6 below was used as an additive in an amount of 1.0% by weight.

[Formula 6]

<Example 6> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that the N,N-dimethylglycine-oxo-pentafluorophosphonaryl lithium salt represented by the following Chemical Formula 7 was used in an amount of 1.0% by weight as an additive. Did.

[Formula 7]

<Comparative Example 1> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that the additive corresponding to Formula 1 was not applied.

<Comparative Example 2> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that 2.0 wt% of vinylene carbonate was applied instead of the additive corresponding to Formula 1.

<Comparative Example 3> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that 2.0% by weight of fluoroethylene carbonate was applied instead of the additive corresponding to Formula 1.

<Comparative Example 4> Preparation of electrolyte solution

An electrolyte solution was prepared in the same manner as in Example 1, except that 1.0 wt% of the bisoxalatobarate lithium salt was applied in place of the additive corresponding to Formula 1.

<Experimental Example 1> DC-IR measurement of electrolyte

For the evaluation of the electrolyte, the cathode LMO (LiMn ₂ O ₄₎ and _{NCM (LiNi 0 .5 Mn 0 .3} Co 0 .2 O 2) 1,400 mAh grade pouch cell using the positive electrode mixture, the negative electrode of graphite with Was produced. Test cells were prepared by injecting the electrolytes prepared in Examples 1 to 6 and Comparative Examples 1 to 4 into the prepared cells, respectively. A DC-IR (Direct Current-Internal Resistance) test was performed on the test cell that completed the chemical conversion process. First, the initial DC-IR of the battery was measured at room temperature. DC-IR was calculated as follows.

The battery in a state of 60% charge (hereinafter SOC60) was charged at 0.5C-rate for 10 seconds. The battery was made SOC60 again, and after 10 minutes of rest, the cells were charged at 1.0 C-rate for 10 seconds. The battery was made SOC60 again, and after 10 minutes of rest, it was charged at 2.0C-rate for 10 seconds. DC-IR was calculated during charging using the voltage and current obtained from each C-rate. And the battery in a state of 60% charge was discharged for 10 seconds at 0.5C-rate. The battery was made SOC60 again, and after 10 minutes of rest, it was discharged at 1.0 C-rate for 10 seconds. The battery was made SOC60 again, and after 10 minutes of rest, the cells were discharged at 2.0 C-rate for 10 seconds. DC-IR was calculated during discharge using the voltage and current obtained from each C-rate.

The battery whose initial DC-IR measurement was completed was fully charged and left at 60°C for 1 month. Thereafter, the battery was cooled to room temperature, and DC-IR was measured in the same manner as above to calculate DC-IR after 4 weeks of high temperature standing. The test results are reported in Table 1 below.

Type of additive Early
DC-IR DC-IR after 4 weeks Example 1 N,N-dimethyloxalato tetrafluorophosphate 32.2 53.5 Example 2 N,N-dimethyloxalato pentafluorophosphate lithium salt 33.5 57.2 Example 3 N,N-dimethyloxalato difluoroborate 36.4 54.9 Example 4 N,N-dimethyloxalato trifluoroborate lithium salt 35.9 55.1 Example 5 N,N-dimethylglycine-oxo-tetrafluorophosphonaryl 37.1 55.8 Example 6 N,N-dimethylglycine-oxo-pentafluorophosphonaryl lithium salt 37.6 59.2 Comparative Example 1 No additive 40.2 174.4 Comparative Example 2 Vinylene carbonate 41.4 101.5 Comparative Example 3 Fluoroethylene carbonate 40.6 119.8 Comparative Example 4 Lithium salt of bisoxalatoborate 43.1 105.7

From Table 1, when the additives of the present invention were added (Examples 1 to 6), as compared with the case where no additives were added (Comparative Example 1) or other kinds of additives were added (Comparative Examples 2 to 4). , It can be confirmed that the initial DC-IR resistance value is low. Also, after comparing the DC-IR resistance value after the storage operation at a high temperature continuously, that is, after storing at 60°C for 4 weeks, the electrolytes of Examples 1 to 6 are more than the batteries to which the electrolytes of Comparative Examples 1 to 4 were applied. It can be seen that the increase in the DC-IR resistance value of the used battery is remarkably small. Therefore, when the additive according to the present invention is used, resistance in a battery can be lowered and stable output characteristics can be expected even at high temperatures.

< Experimental Example 2> Lithium secondary battery Performance (capacity retention rate, %) evaluation

Capacity retention rate was evaluated using the same battery used in Experimental Example 1. All battery capacities below were measured at room temperature. In order to measure the initial capacity, constant current charging was performed until the voltage became 4.2 V with a current of 0.5 C-rate, and charging was performed with a constant voltage of 4.2 V until the current became 0.1 C-rate. And constant current discharge was performed until it became 2.7V with the current of 0.5C-rate. The discharge capacity at this time was taken as the initial capacity.

After the initial capacity measurement, the fully charged battery was left at 60°C for 1 month. During standing, the cells were moved to room temperature every week, cooled to room temperature, discharged at a constant current until a current of 0.5 C-rate became 2.7 V, and the storage capacity was measured. And the capacity retention rate was calculated as follows. In order to increase reliability, three cells were evaluated for each example and comparative example, and the results were calculated as an average value.

Capacity retention rate (%) = (retention capacity/initial capacity) X 100

Table 2 lists the results.

Early 1 week 2 weeks 3 weeks 4 weeks Example 1 100 93 91 88 85 Example 2 100 93 93 90 88 Example 3 100 90 91 87 84 Example 4 100 92 90 91 87 Example 5 100 93 90 88 83 Example 6 100 91 88 86 82 Comparative Example 1 100 87 84 73 61 Comparative Example 2 100 89 88 79 69 Comparative Example 3 100 90 87 77 70 Comparative Example 4 100 89 89 85 77

From Table 2, when the additives of the present invention were added (Examples 1 to 6), compared to the case where no additives were added (Comparative Example 1) or other kinds of additives were added (Comparative Examples 2 to 4). It can be seen that the capacity retention rate is maintained at 80% or more after 4 weeks compared to the initial standard discharge capacity. On the other hand, it can be seen that in Comparative Examples 1 to 4, the capacity retention rate was significantly lowered. Therefore, when the additive according to the present invention is used, it is possible to expect a stable improvement effect on the capacity retention rate in the battery.

Claims (7)

Electrolyte additive for secondary batteries having the structure of Formula 1 below:
[Formula 1]

In Chemical Formula 1,
R ₁ and R ₂ are each independently hydrogen or a C _1-3 alkyl group;
R ₃ and R ₄ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group or a C _{1 -3} having a hydrocarbon group;
R ₅ and R ₆ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group or a C _{1 -3} having a hydrocarbon group;
M is P, B, Al, Co or Mn, wherein M may have one or more substituents selected from the group consisting of halogen, nitrile (CN), methylsulfonyl and sulfonate (SO ₃ H).
According to claim 1,
The additive is selected from the group consisting of N,N-dimethyloxalato tetrafluorophosphate, N,N-dimethyloxalato difluoroborate and N,N-dimethylglycine-oxo-tetrafluorophosphonaryl. , Electrolyte additive for secondary batteries.
According to claim 1,
The additive is in the form of a lithium salt further comprising lithium ions, the electrolyte additive for secondary batteries.
The method of claim 3,
The additives are N,N-dimethyloxalato pentafluorophosphate lithium salt, N,N-dimethyloxalato trifluoroborate lithium salt and N,N-dimethylglycine-oxo-pentafluorophosphonaryl lithium salt It is selected from the group consisting of, electrolyte additives for secondary batteries.
Non-aqueous solvents; Lithium salt; And an additive represented by the following Chemical Formula 1:
[Formula 1]

In Chemical Formula 1,
R ₁ and R ₂ are each independently hydrogen or a C _1-3 alkyl group;
R ₃ and R ₄ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group, or a C _{1 -3} having a hydrocarbon group;
R ₅ and R ₆ is an oxo group, or each independently represents hydrogen, C _1-3 alkyl group, or a C _{1 -3} having a hydrocarbon group;
M is P, B, Al, Co or Mn, wherein M may have one or more substituents selected from the group consisting of halogen, nitrile (CN), methylsulfonyl and sulfonate (SO ₃ H).
The method of claim 5,
The concentration of the lithium salt is 0.9 M to 1.2 M,
An electrolyte for a secondary battery comprising 0.05 to 30 parts by weight based on 100 parts by weight of the electrolyte.
The method of claim 5,
The non-aqueous solvent electrolyte for a secondary battery consisting of a mixture of a linear carbonate compound and a cyclic carbonate compound.