KR102529941B1 - Liquid Electrolytes For Na Secondary Batteries And Na Secondary Batteries Comprising The Same - Google Patents

Abstract

A sodium secondary battery having good electrochemical properties is disclosed. The present invention is a sodium secondary battery having a positive electrode, a negative electrode, an electrolyte solution, and a separator separating the positive electrode and the negative electrode, wherein the electrolyte solution includes a sodium salt; cyclic carbonate-based solvents; solvents including low-viscosity and low-melting ester-based solvents; and an additive for forming a protective film on the negative electrode. According to the present invention, it is possible to provide a sodium secondary battery with improved cell performance at low temperatures using an electrolyte having low viscosity and high ionic conductivity.

Description

Electrolyte for sodium secondary batteries and sodium secondary batteries containing the same {Liquid Electrolytes For Na Secondary Batteries And Na Secondary Batteries Comprising The Same}

The present invention relates to a sodium secondary battery, and more particularly, to an electrolyte solution for a sodium secondary battery having good electrochemical properties.

Since the 1990s, lithium secondary batteries have been commercialized, functioning as a key power source in small IT devices and power tools, and are expanding their range to power sources such as electric vehicles (EV, HEV, PHEV).

Lithium resources, the main material of lithium secondary batteries, are limited to South America, such as Argentina, Bolivia, and Chile. As demand for lithium soars, problems such as imbalance between supply and demand, rising raw material prices, and weaponization of resources in countries with lithium are occurring.

In contrast, sodium is very advantageous in terms of raw material supply and demand because it has abundant reserves and is inexpensive.

Research on sodium ion batteries also began in the 1970s, but lithium batteries were commercialized first and did not attract attention. Then, as the need for non-lithium-based Post-Li batteries emerged, full-scale research on sodium ion batteries was underway.

The sodium secondary battery has the same operating principle and similar structure as the lithium secondary battery, and has shown potential as a secondary battery, but falls short of the characteristics of the lithium secondary battery. However, it can be an innovative alternative that can overcome the limitations of the current lithium secondary battery market as an energy storage and conversion device based on advantages such as easy resource supply and low cost.

In sodium secondary batteries, the cathode active materials are mainly oxide-based materials such as NaCrO ₂ , NaMnO ₂ , and NaFePO ₄ , polyanion-based materials such as Na ₃ V ₂ (PO ₄ ) ₃ and NaFePO ₄ , and sulfide-based materials such as Na _x TiS ₂ , FeF ₃ , etc., fluoride series, NASICON, etc., phosphate series, etc., and research is being conducted. As for anode active materials, materials such as petroleum cokes, carbon black, and hard carbon is known

On the other hand, in the electrolyte, the linear carbonate solvent successfully introduced into the existing lithium secondary battery causes a severe decomposition reaction in the sodium metal or sodium-intercalated negative electrode in the sodium secondary battery. These decomposition products migrate to the anode for further decomposition and form a thick film, rapidly deteriorating cell performance.

Accordingly, the use of an electrolyte for a sodium secondary battery is limited to an electrolyte composition composed of cyclic carbonate solvents. These cyclic carbonate solvents have characteristics of high viscosity/high melting point, thereby increasing the viscosity of the electrolyte and lowering the ionic conductivity. In particular, these electrolytes increase cell resistance at low temperatures, limiting the usable temperature range of the battery.

Therefore, there is an urgent need to introduce a solvent having a low viscosity/high melting point in order to widen the usable temperature range of the battery.

In order to solve the problems of the prior art, an object of the present invention is to provide an electrolyte solution for a sodium secondary battery capable of improving cell performance at low temperatures using an ester-based solvent having a low viscosity and a low melting point.

In addition, an object of the present invention is to provide an electrolyte solution for a sodium secondary battery that suppresses a decomposition reaction with a sodium metal negative electrode while containing an ester-based solvent having a low viscosity and a low melting point.

In addition, an object of the present invention is to provide an electrolyte solution for a sodium secondary battery that effectively suppresses the reaction with the negative electrode and suppresses consumption of the electrolyte by forming a stable protective film on the sodium metal negative electrode.

In addition, an object of the present invention is to provide a sodium secondary battery including the above-described electrolyte solution.

In order to achieve the above technical problem, the present invention provides a sodium secondary battery having a positive electrode, a negative electrode, an electrolyte solution, and a separator separating the positive electrode and the negative electrode, wherein the electrolyte solution is sodium salt; cyclic carbonate-based solvents; solvents including low-viscosity and low-melting ester-based solvents; and an additive for forming a protective film on the negative electrode.

In the present invention, the sodium salt may include at least one salt selected from the group consisting of NaClO ₄ , NaPF ₆ , NaBF ₄ , NaFSI, NaTFSI, NaSO ₃ CF ₃ , NaBOB, and NaFOB. At this time, the concentration of the sodium salt may be 0.3 ~ 1.0 M.

In one embodiment of the present invention, the ester-based solvent is at least one selected from the group consisting of Ethyl acetate (EA), Methyl propionate (MP), Methyl butylate (MB), Propyl butylate (PB) and Butyl butyrate (BB) A solvent may be included. At this time, the ester-based solvent is preferably included in an amount of 10 to 30% based on the total volume of the solvent.

In addition, in the present invention, the cyclic carbonate solvent may include EC or PC.

Also, in one embodiment of the present invention, the additive preferably includes FEC. At this time, the FEC is preferably included in an amount of 0.2 to 10% by weight based on the total weight of the electrolyte.

According to one embodiment of the present invention, it is possible to provide a sodium secondary battery having low viscosity and high ion conductivity by using an ester solvent having a low viscosity/low melting point in an electrolytic core and thus improving cell performance at low temperatures.

In addition, according to an embodiment of the present invention, a protective film is formed on sodium metal to suppress generation of decomposition products due to the reaction between sodium metal and electrolyte, thereby increasing the reversibility of the electrochemical reaction of the cell. It is possible to provide a sodium secondary battery that does.

1 is a graph showing the results of measuring the viscosity and ionic conductivity of an electrolyte solution according to an embodiment of the present invention.
Figure 2 is a graph showing the LSV measurement results of samples prepared in one embodiment of the present invention.
3 is a graph showing measurement results of voltage-capacitance characteristics according to an embodiment of the present invention.
4 is a graph showing the results of measuring the discharge capacity according to the cycle in one embodiment of the present invention.
5 is a graph showing charge and discharge characteristics of a positive half cell in one embodiment of the present invention.
6 is a graph showing charge/discharge characteristics of a positive half cell according to an embodiment of the present invention.

The present invention will be described in detail with reference to the following drawings.

In the present invention, the sodium secondary battery includes a positive electrode, a negative electrode, an ion permeable separator, and an ion conductive electrolyte.

Electrolyte for a sodium secondary battery in the present invention provides an electrolyte for a sodium secondary battery. The electrolyte solution may include a sodium salt, a cyclic carbonate-based solvent and an ester-based solvent, and additives.

In the present invention, the sodium salt may include halide sodium salts such as NaClO ₄ , NaPF ₆ , NaBF ₄ , NaFSI, NaTFSI, NaSO ₃ CF ₃ , NaBOB and NaFOB. The concentration of the sodium salt in the electrolyte solution of the present invention is preferably 0.1 ~ 1.5M.

In addition, the sodium salt may be a mixture of at least two or more of the above-mentioned one or more sodium salts.

In the present invention, solvents such as EC or PC represented by the following chemical formula may be used as the cyclic carbonate-based solvent.

(Formula 1)

Ethylene carbonate (EC)

(Formula 2)

Propylene carbonate (PC)

In addition, as the ester-based solvent in the present invention, a solvent having a low melting point and a low viscosity, such as ethyl propionate (EP) or ethyl butyrate (EB), is used as a co-solvent.

(Formula 3)

Ethyl propionate (EP)

(Formula 4)

Ethyl butyrate (EB)

In addition, as the ester-based solvent, a low melting point and low viscosity solvent such as ethyl acetate (EA), methyl propionate (MP), methyl butylate (MB), propyl butylate (PB), and butyl butyrate (BB) may be used.

In the present invention, the ester-based solvent is preferably included in an amount of 20 to 30% based on the total volume of the solvent in the electrolyte solution. If it is less than 20 vol%, it is difficult to exhibit the low melting point and low viscosity characteristics of the electrolyte, and if it is more than 30 vol%, overcharging may occur due to a reaction with sodium metal.

In one embodiment of the present invention, the electrolyte solution includes FEC (Fluoroethylene carbonate) as a functional additive. The FEC added in the present invention forms a high-density film containing an inorganic material such as NaF for example, which inhibits the decomposition of the electrolyte on the surface of the anode. The formed film inhibits the irreversible decomposition reaction of ester-based low melting point solvents such as EB and EP on the surface of the anode.

Accordingly, as can be seen from examples of the present invention described later, the film formed by the added FEC forms a surface film that does not apply electrical stress during cycling. Also, the added FEC forms a protective film on the surface of the anode.

In the present invention, the FEC is preferably included in an amount of 0.2 to 10% by weight based on the total weight of the electrolyte. When the amount of FEC added is less than 0.2% by weight, it is difficult to suppress the reaction with the electrolyte by forming a protective film on the sodium metal film, and when an excessive amount of FEC is added, it acts as resistance in the battery and reduces the reversibility of the sodium ion reaction.

Hereinafter, preferred embodiments of the present invention will be described.

<Evaluation of Electrolyte Characteristics>

The viscosity and room temperature ionic conductivity were evaluated according to the addition of EP and EB as co-solvents to EC (ethylene carbonate) and PC (propylene carbonate) solvents. Room temperature ionic conductivity of the electrolyte solution prepared according to the present invention was evaluated. Viscosity was measured using Brookpick's LVDV-II+P, and room temperature ionic conductivity was measured using Oakton's CON 11 conductivity meter. As shown in Table 1 below, NaClO ₄ was dissolved in solvents having different compositions to prepare 0.5M NaClO ₄ solutions. For comparison, the ionic conductivity of the EC/PC solvent to which no co-solvent was added was also measured.

division Solvent composition (volume ratio) Experimental example 1 EC:PC:EP (5:3:2) Experimental Example 2 EC:PC:EB (5:3:2) reference electrolyte EC:PC (5:5)

1 is a graph showing the measurement results of this experimental example.

As shown in FIG. 1 , it can be seen that an electrolyte having a low viscosity can be obtained in the case of the EP-added electrolyte (EP-added) and the EB-added electrolyte (EB-added) compared to the reference electrolyte (Ref).

In addition, the reference electrolyte (Ref) showed an ion conductivity of 5.85 mS/cm, and the EP-added electrolyte (EP-added) and the EP-added electrolyte (EB-added) showed higher ionic conductivity than the reference electrolyte, 6.22 mS/cm and 6.17 mS. It can be seen that / cm represents the room temperature ionic conductivity.

<Evaluation of oxidative stability of electrolyte>

A half-cell type 2032 coin cell was fabricated using sodium metal as a reference electrode and stainless steel as a working electrode. Oxidative stability of the electrolyte was analyzed using the prepared coil cell through linear sweep voltammetry (LSV). An Iviumstat device from Ivium Technologies was used and the voltage range was in the range of 2.0V to 6.5V, and was measured at a scan rate of 1 mVs ^-1 .

The composition of the electrolyte solution used in the experiment is shown in Table 2 below.

division Electrolyte composition (volume ratio) Experimental Example 3 0.5 M NaClO ₄ in EC/PC/EP (5/3/2) Experimental Example 4 0.5M NaClO ₄ in EC/PC/EP (5/3/2) + 5wt% FEC Experimental Example 5 0.5 M NaClO ₄ in EC/PC/EB (5/3/2) Experimental Example 6 0.5 M NaClO ₄ in EC/PC/EB (5/3/2) +5wt% FEC reference electrolyte 0.5 M NaClO ₄ in EC/PC (5/5)

2 is a graph showing the LSV measurement results of samples prepared in this experimental example.

Referring to FIG. 2 , it can be seen that the reference electrolyte (Ref) started oxidative decomposition at 4.8 V (vs Na ⁺ /Na).

It can be seen that the oxidation stability of the reference electrolyte is reduced compared to the reference electrolyte in the case of the EP-added electrolyte (EP-added) and the EB-added electrolyte (EB-added). In particular, in the case of EB, a low current occurs at 4.2V and oxidative decomposition starts at 4.6V (Fig. 2(b)). However, it can be seen that when FEC is added to the electrolyte, the low current decomposition peak disappears and the oxidation stability increases.

<Evaluation of chemical charge and discharge of positive half cell>

Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode material as cathode active material, carbon black (Super P) as conductive material, and poly(vinylidene fluoride) (PVDF) as binder at a weight ratio of 70/20/10 A positive electrode was prepared by applying the blended slurry to an aluminum (Al) current collector. An anode half cell having a 2032 coin cell structure was manufactured using sodium metal as a counter electrode and a reference electrode.

After assembling the half cell, aging at room temperature for 5 hours, the chemical charge/discharge characteristics were evaluated. WBCS 3000 from WonAtech was used as a measuring device, and charging and discharging conditions were charge (4.2V)-discharge (1.7V) (30 ° C) under C/20 constant current conditions.

The composition of the electrolyte solution used in the experiment is shown in Table 3 below.

division Electrolyte composition (volume ratio) Experimental Example 7 0.5 M NaClO ₄ in EC/PC (5/5) + 5wt% FEC Experimental Example 8 0.5 M NaClO ₄ in EC/PC/EP (5/3/2) Experimental Example 9 0.5M NaClO ₄ in EC/PC/EP (5/3/2) + 5wt% FEC Experimental Example 10 0.5 M NaClO ₄ in EC/PC/EB (5/3/2) Experimental Example 11 0.5 M NaClO ₄ in EC/PC/EB (5/3/2) +5wt% FEC reference electrolyte 0.5 M NaClO ₄ in EC/PC (5/5)

3 is a graph showing measurement results of voltage-capacitance characteristics according to the present experimental example.

As shown in (a) of FIG. 3 , it can be seen that in the case of an electrolyte containing EP or EB (EP-added, EB-added), an overcharge phenomenon occurs due to oxidative decomposition of the electrolyte during charging. It is believed that this is due to the fact that the highly reactive sodium metal reacts with EP and EB to produce decomposition by-products, resulting in an oxidative decomposition reaction at the anode.

However, as shown in (b) of FIG. 3 , it can be seen that the overcharging phenomenon is alleviated in the case of the electrolyte solution in which FEC is added to EP and EB (EP+FEC-added, EB+FEC-added). This means that the overcharge phenomenon, which prevents the completion of charging of the positive electrode due to oxidative decomposition by the EP and EB solvents, is suppressed by the FEC additive. That is, it can be seen that the FEC additive forms a stable film on the sodium metal electrode and suppresses unwanted side reactions between the sodium metal and the EB and EP solvents.

<Example 1>

Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) cathode material as cathode active material, carbon black (Super P) as conductive material, and poly(vinylidene fluoride) (PVDF) as binder at a weight ratio of 70/20/10 The blended slurry was applied to an Al current collector to prepare a positive electrode. An anode half cell having a 2032 coin cell structure was manufactured using sodium metal as a counter electrode and a reference electrode.

As the electrolyte, 0.5M NaClO ₄ in EC/PC(5/5) + 5wt% FEC was used as the standard electrolyte (Ref+FEC-added), and 0.5M NaClO ₄ in EC/PC/EB (volume ratio 5/3/2 ) + 5wt% FEC electrolyte (EB + FEC-added) was prepared.

After aging the assembled coin cells at room temperature for 5 hours, charge/discharge characteristics were evaluated. WonAtech's WBCS 3000 was used as the measuring equipment, and the charging and discharging conditions were as follows.

- Room temperature chemical charge/discharge: charge (4.2V)-discharge (1.7V)-charge (4.2V) (30℃) under C/20 constant current conditions

- Room temperature cycle: Discharge (1.7V) - Charge (4.2V) - Discharge (1.7V) (30℃) under C/2 constant current conditions

4 is a graph showing the results of measuring room temperature discharge capacity according to cycles.

Referring to FIG. 4 , it can be seen that the EB electrolyte half-cell sample (EB+FEC-added) maintains substantially the same discharge capacity as the reference electrolyte half-cell sample. However, the EB electrolyte half-cell sample (EB+FEC-added) exhibits improved coulombic efficiency than the reference electrolyte half-cell sample. This means that the EB electrolyte of this embodiment is more advantageous for the reversible reaction of the anode.

<Example 2>

A half cell was prepared in the same manner as in Example 1. As in Example 1, 0.5M NaClO ₄ in EC/PC (5/5) + 5wt% FEC was used as the reference electrolyte (FEC-added) as the electrolyte, and 0.5M NaClO ₄ in EC/PC/EB (volume ratio 5 /3/2) + 5 wt% FEC electrolyte (EB + FEC-added) was used. However, in this embodiment, charging and discharging was performed at a low temperature, and the charging and discharging conditions are as follows.

- Room temperature charge after normal temperature charge/discharge: Charge (4.2V) - Discharge (1.7V) - Charge (4.2V) (30℃) under C/20 constant current condition

-Low temperature cycle after low temperature discharge: Discharge (1.7V) - Charge (4.2V) - Discharge (1.7V) under C/20 constant current condition (-10℃)

5 is a graph showing charge/discharge characteristics of a positive half cell. 5(a) is a graph showing charge/discharge characteristics in a first cycle, and FIG. 5(b) is a graph showing charge/discharge characteristics in a second cycle.

As can be seen in (a) of FIG. 5, it can be seen that the first cycle charge/discharge voltage curve of the EB+FEC-added electrolyte solution at -10 ° C is the same as that of the FEC-added electrolyte solution.

However, as shown in (b) of FIG. 5, in the case of FEC-added from the second cycle at -10 ° C, a large voltage drop phenomenon of discharge plateau occurs, which has high viscosity / high melting point characteristics. This is because the FEC-added electrolyte, which consists of only cyclic carbonate solvents, causes resistance in the cell at low temperatures. However, in the case of EB+FEC-added electrolyte, the voltage drop phenomenon of the discharge plateau is mitigated. That is, it can be seen that the introduction of low viscosity/low melting point EB as a co-solvent reduces the IR drop (Ohmic resistance) phenomenon caused by the polarization of the positive half cell at -10 ° C.

<Example 3>

The composition of the electrolyte solution was changed, and charging and discharging was performed at -20 ° C. As the electrolyte, 0.5M NaClO ₄ in EC/PC/EB (volume ratio 3/4/3) + 5wt% FEC electrolyte (EB/PC/EB) was used, and 0.5M NaClO ₄ in EC/PC (5/5) + 5wt% FEC electrolyte (EC/PC) compared to the electrolyte. The charging and discharging conditions are as follows.

- Room temperature charge after normal temperature charge/discharge: Charge (4.2V) - Discharge (1.7V) - Charge (4.2V) (30℃) under C/20 constant current condition

-Low temperature cycle after low temperature discharge: Discharge (1.7V) - Charge (4.2V) - Discharge (1.7V) under C/20 constant current condition (-20℃)

6 is a graph showing charge/discharge characteristics of the positive half cell according to the present embodiment.

Referring to FIG. 6 , it can be seen that a high charge/discharge capacity is realized in the electrolyte solution to which EB is added.

Claims (8)

In a sodium secondary battery having a positive electrode, a negative electrode, an electrolyte solution, and a separator separating the positive electrode and the negative electrode,
The electrolyte is
sodium salt;
cyclic carbonate-based solvents;
low-viscosity and low-melting ester-based solvents; and
Including an additive for forming a protective film on the negative electrode,
The ester-based solvent is a sodium secondary battery, characterized in that contained 20 to 30% with respect to the total volume of the solvent. According to claim 1,
The sodium salt comprises at least one salt selected from the group consisting of NaClO ₄ , NaPF ₆ , NaBF ₄ , NaFSI, NaTFSI, NaSO ₃ CF ₃ , NaBOB and NaFOB Sodium secondary battery. According to claim 1,
According to claim 2,
Sodium secondary battery, characterized in that the concentration of the sodium salt is 0.1 ~ 1.5 M. According to claim 1,
The ester-based solvent includes at least one solvent selected from the group consisting of Ethyl acetate (EA), Methyl propionate (MP), Methyl butylate (MB), Propyl butylate (PB), and Butyl butyrate (BB). sodium secondary battery. delete According to claim 1,
The sodium secondary battery, characterized in that the cyclic carbonate solvent comprises EC or PC. According to claim 1,
The sodium secondary battery, characterized in that the additive comprises FEC. According to claim 6,
The FEC is a sodium secondary battery, characterized in that contained in 0.2 to 10% by weight relative to the total weight of the electrolyte.

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