CN113381075A

CN113381075A - Sodium ion battery electrolyte adaptive to hard carbon cathode and preparation and use methods thereof

Info

Publication number: CN113381075A
Application number: CN202110643840.XA
Authority: CN
Inventors: 孙旦; 唐正; 唐有根; 王海燕; 王红
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2021-06-09
Filing date: 2021-06-09
Publication date: 2021-09-10

Abstract

The invention discloses a sodium ion battery electrolyte adaptive to a hard carbon cathode and a preparation method and a use method thereof, belonging to the technical field of sodium ion batteries. The electrolyte is simple to prepare, raw materials are easy to obtain, and the low-temperature rate performance and the cycling stability of the hard carbon cathode in the sodium ion battery can be effectively improved.

Description

Sodium ion battery electrolyte adaptive to hard carbon cathode and preparation and use methods thereof

Technical Field

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a high-performance sodium ion battery electrolyte adaptive to a hard carbon cathode and a preparation method and a use method thereof.

Background

The sodium ion battery has good application prospect in the fields of large-scale energy storage, low-speed electric vehicles, special engineering vehicles and the like due to the advantages of rich sodium storage, low price, environmental friendliness and the like. The hard carbon material has low price and simple preparation, and is a main candidate material of the cathode of the sodium-ion battery. However, currently hard carbon materials still face the problem of poor rate performance. In addition, under the condition of low temperature, the capacity of the hard carbon negative electrode is seriously reduced, and the application range of the sodium ion battery is limited. The reason why the sodium intercalation property of the hard carbon negative electrode is poor at low temperature can be generally summarized asThe viscosity of the electrolyte is increased, so that the conductivity is reduced; ② Na⁺A decrease in diffusion rate in the active material; increase of charge transfer impedance at the electrode/electrolyte interface, etc. The electrolyte is an important component of the sodium ion battery, not only determines the mobility of ions in the electrolyte, but also is related to a Solid Electrolyte Interface (SEI) film formed on an electrode, and has important influence on the rate performance, the stability at low temperature and the cycle life of the sodium ion battery. Therefore, the research on the solvent and sodium salt in the electrolyte is an important way to optimize the multiplying power and low-temperature performance of the SIBs.

Currently, the most commonly used sodium ion battery electrolytes are ester-based electrolytes, such as Ethylene Carbonate (EC), diethyl carbonate (DEC), Propylene Carbonate (PC), and the like. The solvent molecules in these electrolytes are mixed with Na⁺Often have strong interaction with each other, and limit Na to a certain extent⁺Diffusion at the electrolyte and interface. In addition, the ester-based electrolyte can generate uneven and thick SEI on the surface of a negative electrode in the charging and discharging processes of the sodium ion battery, so that the solid-liquid interface energy is high, the ion transmission rate is slow, and the multiplying power performance and the long cycle stability of the battery are affected. In addition, when the temperature is reduced, the conductivity of the electrolyte is sharply reduced, the charge transfer resistance and the SEI film resistance are continuously increased, and the solvents with relatively high melting points, such as EC and PC, can even generate a solidification phenomenon at low temperature, so that the sodium-ion battery is difficult to work at low temperature. Therefore, it is necessary to develop a novel solvent with low viscosity and low melting point or to use a novel electrolyte salt to improve the low-temperature conductivity of the electrolyte and reduce the transfer resistance of charges between the electrolyte and the electrode.

Disclosure of Invention

The invention aims to provide a high-performance sodium-ion battery electrolyte adaptive to a hard carbon cathode. The main component of the solvent of the electrolyte is based on furan, the preparation is simple, the raw materials are easy to obtain, and the low-temperature performance, the rate capacity and the circulation stability of the sodium-ion battery can be effectively improved.

The invention adopts the following technical scheme:

a sodium ion battery electrolyte adaptive to a hard carbon cathode comprises a sodium salt and a furan solvent.

The electrolyte, the sodium salt includes: sodium hexafluorophosphate (NaPF)₆) Sodium perchlorate (NaClO)₄) Sodium trifluoromethanesulfonate (NaCF)₃SO₃) Sodium bis (trifluoromethanesulfonyl) imide (NaTFSI) and sodium bis (fluorosulfonyl) imide (NaFSI).

The concentration of the sodium salt in the electrolyte is 0.1-10M; preferably 0.5M to 3.0M, and more preferably 1.0M.

The furan solvent comprises one or more solvents selected from tetrahydrofuran, 2-methyltetrahydrofuran, 2-ethyltetrahydrofuran, 2, 5-diethyltetrahydrofuran, 2, 5-dimethoxytetrahydrofuran and (perfluoro) 2-butyltetrahydrofuran. At least one of tetrahydrofuran and 2-methyltetrahydrofuran is preferred.

The electrolyte also comprises other functional additives.

The electrolyte, the other functional additives comprise one or more of fluoroethylene carbonate (FEC), Vinylene Carbonate (VC), ethylene sulfate (DTD), Cyclohexylbenzene (CHB), ethylene sulfite (DTO), Propylene Sulfite (PS), Butylene Sulfite (BS), ethylene carbonate (VEC), propylene sulfate (TMS), Phenyl Acetone (PAC), 1, 4-butane sultone (1,4-BS), 1, 3-propane sultone (1,3-PS), N-Dimethylformamide (DMFA), N-Dimethylacetamide (DMAC) and N, N-Dimethyltrifluoroacetamide (DTA). At least one of fluoroethylene carbonate, vinylene carbonate and ethylene sulfate is preferable.

The proportion of the other functional additives in the electrolyte is 0.1-10 wt%. Preferably 1 to 5 wt%, and more preferably 2.5 to 5 wt%.

The second purpose of the invention is to provide the preparation method of the electrolyte, which is obtained by dissolving sodium salt or sodium salt and other functional additives in furan solvents.

The third purpose of the invention is to provide the use method of the electrolyte, which is used in a sodium ion battery with a hard carbon negative electrode.

Furthermore, the use method is to use the assembled battery in an environment with the temperature of-30-40 ℃, preferably-20-30 ℃.

Principle of the invention

The physicochemical properties of the electrolyte of the sodium ion battery are key to influence the electrochemical performance of the electrolyte. The viscosity of the electrolyte, the interaction between solvent molecules in the electrolyte and solute sodium ions, the diffusion energy of the sodium ions between an electrode/electrolyte interface and the like all have important influences on the low-temperature performance and the rate performance of the sodium ion battery. Unlike sodium metal negative electrodes based on sodium deposition/dissolution mechanism (poor cycling stability due to the generation of sodium dendrites), it is important to develop a sodium ion battery using hard carbon as the negative electrode based on intercalation/deintercalation mechanism to rapidly diffuse the electrolyte between the electrolyte and the hard carbon electrode interface to improve the performance of the battery. Compared with the commonly used ester electrolyte, the electrolyte based on tetrahydrofuran as the main component has lower viscosity, has larger radius of sodium ions than lithium ions, and has weaker acting force with solvent molecules, thereby being beneficial to the diffusion of the sodium ions at the interface of the electrolyte/hard carbon electrode, and improving the multiplying power and the low-temperature performance of the battery.

The invention has the advantages and positive effects that:

(1) the electrolyte provided by the invention is simple to prepare, and raw materials are easy to obtain.

(2) Compared with the existing electrolyte of the sodium ion battery, the electrolyte adopted by the invention can enable the hard carbon cathode of the sodium ion battery to have excellent rate capability and cycle performance, especially the rate capability and cycle performance at low temperature.

(3) The electrolyte adopted by the invention only enables the hard carbon cathode of the sodium ion battery to have excellent rate capability and cycle performance; for other batteries, such as: lithium ion batteries, metal cathode sodium ion batteries, and the like do not have a significant lifting effect.

Drawings

FIG. 1 is a graph showing rate performance of sodium ion batteries using different electrolytes in examples 1 to 4 and comparative example 1 of the present invention;

FIG. 2 is a graph showing the cycle performance of the sodium ion battery of example 1 and comparative example 1 under a small current;

FIG. 3 is a graph showing the long cycle performance of the sodium ion batteries of example 1 and comparative example 1 according to the present invention;

FIG. 4 is a graph of the rate performance of the sodium ion battery at-5 ℃ in example 1 of the present invention;

FIG. 5 is a graph of the long cycle performance of the sodium ion battery at-5 ℃ in example 1 of the present invention;

FIG. 6 is a graph of rate performance of the sodium ion battery at-20 ℃ in example 1 of the present invention;

fig. 7 is a graph of the long cycle performance of the sodium ion battery at-20 c in example 1 of the present invention.

Detailed Description

In order to further understand the contents, features and effects of the present invention, the following embodiments are illustrated and described in detail with reference to the accompanying drawings:

example 1

At 1.0mol/L NaPF₆The preparation method of the sodium ion battery with THF as electrolyte comprises the following steps:

step (1): and (4) preparing an electrolyte. 3.359g NaPF were weighed out₆Dissolved in 20ml of THF to give 1.0mol/L NaPF₆THF sodium ion battery electrolyte;

step (2): and preparing a negative electrode. Commercial hard carbon (wurtziton chemical carbon P, japan), conductive carbon black, PVDF, were mixed in a mass ratio of 80:10: 10. And then adding N-methyl pyrrolidone (NMP) to be mixed into slurry, uniformly coating the slurry on the surface of a copper (Cu) foil by using a scraper, drying the copper (Cu) foil in a vacuum drying oven at 80 ℃ for 10 hours, and cutting the Cu foil with the active material into a disk-shaped negative pole piece.

And (3): and (6) assembling the half cell. CR2016 button cells were assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 1.0mol/LNaPF₆The 2016 button cell was assembled with THF as the electrolyte and Na metal sheet as the counter electrode.

Placing the assembled button cell in a 30 ℃ constant temperature test system at 0.01-2.0V (vs. Na)⁺Na, the same applies hereinafter) was subjected to a charge-discharge test. Table 1 reports 1.0mol/L NaPF₆The multiplying power performance and the current density of the THF electrolyte are respectively 20mAg^-1，50mAg^-1，100mAg^-1，200mAg^-1，500mAg^-1，1Ag^-1，2Ag^-1，5Ag^-1The reversible discharge specific capacity of the battery is 287mAhg respectively^-1，285mAhg^-1，276mAhg^-1，265mAhg^-1，259mAhg^-1，248mAhg^-1，237mAhg^-1，214mAhg^-1. It can also be seen in FIG. 1 that the hard carbon is in NaPF₆Compared with the common ester-based electrolyte, the THF electrolyte has more excellent rate performance. The electrolyte has stable small current circulation performance at normal temperature (30 ℃), as shown in figure 2, at 20mAg^-1The reversible discharge specific capacity of the battery is kept at 300mAhg under the low current density^-1Left and right. FIG. 3 and Table 2 show the electrolyte in example 1 at high current (2A g)^-1) The discharge specific capacity after 1000 cycles is 206mAhg^-1The retention rate was 80%.

The solution of example 1 was mixed with 1.0mol/L NaPF₆The sodium ion half cell with THF as electrolyte is respectively placed in constant temperature environment of-5 ℃ and-20 ℃ for electrochemical test. Table 1 and fig. 4 record the rate performance of the half-cell at-5 c, and it can be seen that the specific discharge capacity of the cell decays slowly with increasing current density at 1A g^-1And 2Ag^-1The reversible discharge specific capacity under large current is still 221 mAhg and 206mAhg^-1. When the current density increased to 5A g^-1When the capacity attenuation is relatively large, the capacity becomes 155mAhg^-1. FIG. 5 and Table 2 record the cycling performance of the cell of example 1, 2Ag, at-5 deg.C^-1The specific capacity of the alloy is still as high as 203mAhg after 1000 times of circulation under the current density of (1)^-1The capacity retention rate was 83%. The rate performance of the half cell at-20 ℃ is recorded in fig. 6 and table 1, and it can be seen that the cell maintains a good rate performance at such low temperatures, at 1A g^-1And 2A g^-1The reversible discharge specific capacity under large current is still 212 and 183mAhg^-1. Only at 5A g^-1At a capacity fade of 45mAhg^-1The battery working under heavy current is greatly influenced by temperature. FIG. 7 is a graph showing the large current cycle performance at-20 ℃ and it can be seen that 2Ag^-1The reversible discharge specific capacity after 1000 times of circulation is still up to 181mAhg under the current density of^-1The capacity retention was 85% (Table 2). Thus, the hard carbon is 1.0mol/LNaPF₆The THF electrolyte has excellent low-temperature performance.

Example 2

At 2.0mol/L NaPF₆The preparation method of the sodium ion battery with THF as electrolyte comprises the following steps:

step (1): and (4) preparing an electrolyte. Weighing 3.359g NaPF₆Dissolved in 10ml of THF to give 2.0mol/L of NaPF₆THF sodium ion battery electrolyte;

step (2): the negative electrode was prepared as in example 1.

And (3): and (6) assembling the half cell. The CR2016 button cell was assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 2.0mol/L NaPF₆The 2016 button cell was assembled with THF as the electrolyte and Na metal sheet as the counter electrode.

The cell was tested in a 30 ℃ incubator. The rate capability of the electrolyte in example 2 is shown in Table 1 and FIG. 1, with 2.0mol/L NaPF₆Sodium ion battery with THF as electrolyte in 2Ag^-1And 5Ag^-1The reversible discharge specific capacities are 230mAhg and 208mAhg respectively^-1And excellent rate performance is demonstrated. Also, as shown in Table 2, the cell was at 2Ag^-1The capacity retention rate after 1000 cycles of lower circulation is 81 percent, and the excellent circulation performance is shown.

The rate performance and the cycle performance at-20 ℃ are respectively shown in tables 1 and 2, and the electrolyte also shows good rate performance and long cycle performance at low temperature (2 Ag)^-1Capacity retention after 1000 cycles of lower cycle 85%).

Example 3

At 1.0mol/L NaPF₆The preparation method of the sodium ion battery with 2-MeTHF (2-methyltetrahydrofuran) as electrolyte comprises the following steps:

step (1): and (4) preparing an electrolyte. Weighing 3.359g NaPF₆Dissolved in 20ml of 2-MeTHF,obtaining 1.0mol/L NaPF₆2-MeTHF sodium ion battery electrolyte;

step (2): the negative electrode was prepared as in example 1.

And (3): and (6) assembling the half cell. The CR2016 button cell was assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 1.0mol/L NaPF₆The 2016 button cell was assembled with/2-MeTHF as the electrolyte and Na metal plate as the counter electrode.

And (3) placing the assembled button cell in a 30 ℃ constant temperature test system to carry out charge and discharge tests on the button cell. The rate capability of the electrolyte in example 3 is also reported in Table 1 and FIG. 1 as 1.0mol/L NaPF₆Sodium ion battery using/2-MeTHF as electrolyte at 20mAg^-1Has a reversible discharge capacity of up to 330mAhg at a current density^-1，2Ag^-1Reversible discharge capacity at 186mAhg^-1. The cycle performance data of the cell are shown in Table 2, 2Ag^-1The capacity retention rate is 61% after 1000 cycles of lower circulation, and the rate performance and the circulation performance of the electrolyte are both superior to those of common ester-based electrolyte.

The electrolyte also exhibited rate performance (Table 1) and long cycle performance (2 Ag) superior to the ester-based electrolyte at-20 deg.C^-1Capacity retention after 1000 cycles under lower cycle 63%, table 2).

Example 4

At 1.0mol/L NaPF₆The preparation method of the sodium ion battery with the electrolyte of/2-MeTHF + THF (1:1) comprises the following steps:

step (1): and (4) preparing an electrolyte. Weighing 3.359g NaPF₆Dissolved in a mixed solvent of 10ml of 2-MeTHF and 10ml of THF to obtain 1.0mol/L NaPF₆2-MeTHF + THF (1:1) sodium ion battery electrolyte;

step (2): the negative electrode was prepared as in example 1.

And (3): and (6) assembling the half cell. The CR2016 button cell was assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 1.0mol/L NaPF₆The 2016 button half cell was assembled with/2-MeTHF + THF (1:1) as the electrolyte and Na metal plate as the counter electrode.

The cell was tested in a 30 ℃ incubator. As shown in fig. 1, the electrolyte applied to the hard carbon negative electrode in example 4 also has good rate capability and large-current long cycle performance, which are superior to those of the common ester electrolyte. The specific electrochemical properties are shown in tables 1 and 2.

Also, the electrolyte exhibited excellent low temperature performance when the cell was electrochemically tested at-20 ℃ (see tables 1 and 2).

Example 5

At a rate of 1.0mol/L NaCF₃SO₃The preparation method of the sodium ion battery with THF as electrolyte comprises the following steps:

step (1): and (4) preparing an electrolyte. Weighing 3.441gNaCF₃SO₃Dissolved in 20ml of THF to give 1.0mol/L of NaCF₃SO₃THF sodium ion battery electrolyte;

step (2): the negative electrode was prepared as in example 1.

And (3): and (6) assembling the half cell. The CR2016 button cell was assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 1.0mol/L of NaCF₃SO₃The 2016 button half cell was assembled with/THF as the electrolyte and Na metal sheet as the counter electrode.

The cell was tested in a 30 ℃ incubator. The electrochemical properties are shown in tables 1 and 2, and the electrolyte salt is replaced by NaCF₃SO₃The battery still keeps good rate performance and cycling stability.

Table 1 shows the rate capability of the electrolyte at-20 ℃, and the sodium ion battery assembled in the embodiment has the rate capability of 50mAg^-1Has a reversible discharge specific capacity of 281mAhg at a current density^-1，2Ag^-1The specific capacity of reversible discharge is 126mAhg^-1. The long cycle performance at low temperature (-20 ℃) can be seen in Table 2, 2Ag^-1The capacity retention rate after 1000 cycles of the lower cycle is 77%.

Comparative example 1

1.0mol/LNaPF of common ester-based electrolyte₆The preparation method of the sodium ion battery with/EC + DEC (1:1) as electrolyte comprises the following steps:

step (1): the negative electrode was prepared as in example 1.

Step (2): and (6) assembling the half cell. CR2016 button cells were assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode and a commercial electrolyte of 1.0mol/LNaPF₆The 2016 button cell was assembled with/EC + DEC (1:1) (V: V) as the electrolyte and Na metal sheet as the counter electrode.

And (3) placing the assembled button cell in a 30 ℃ constant temperature test system, and carrying out charge and discharge test on the button cell within a voltage range of 0.01-2.0V. From the power performance curve of FIG. 1, it can be seen that as the current density increases, at 1.0mol/LNaPF₆The discharge specific capacity of the half battery with the electrolyte solution of/EC + DEC is obviously attenuated. As shown in Table 1, the current densities were 20mAg, respectively^-1，50mAg^-1，100mAg^-1，200mAg^-1，500mAg^-1，1Ag^-1，2Ag^-1，5Ag^-1The reversible discharge specific capacity of the battery is 300mAhg respectively^-1，250mAhg^-1，190mAhg^-1，97mAhg^-1，70mAhg^-1，58mAhg^-1，46mAhg^-1，26mAhg^-1. Small current density (20 mAg)^-1) The test results are shown in FIG. 2, and the reversible discharge specific capacity of the hard carbon in the common electrolyte is 290mAhg^-1And the discharge specific capacity after 80 cycles is 230mAhg^-1The capacity retention rate was 79%. The high current performance of the electrolyte is shown in FIG. 3 and Table 2, in which 2Ag is used^-1The specific capacity of the battery after discharging for 1000 circles is only 50mAhg under the current density of (1)^-1。

At low temperature (-5 ℃), at 1.0mol/L NaPF₆The discharge specific capacity of the half battery with the electrolyte of/EC + DEC decays more rapidly along with the increase of the current density, and when the current density is increased to 2Ag^-1When the discharge capacity was close to 0 (see table 1). The electrolyte is prepared by reacting 2Ag at-5 deg.C^-1The data of 1000 cycles of charging and discharging are shown in Table 2, the battery still has 20mAhg after the first discharging^-1Capacity of (d), the capacity after 1000 cycles is almost 0.

Comparative example 2

At 1.0mol/L LiPF_6/The preparation method of the lithium ion battery with THF as electrolyte comprises the following steps:

step (1): and (4) preparing an electrolyte. Weighing 3.038g LiPF₆Dissolved in 20ml of THF to give 1.0mol/LLIPF₆THF lithium ion battery electrolyte;

step (2): the negative electrode was prepared as in example 1.

And (3): and (6) assembling the half cell. The CR2016 button cell was assembled in an Ar-filled MIKROUNA glove box using the prepared hard carbon electrode as the negative electrode, 1.0mol/L LiPF_6/The 2016 button cell was assembled with THF as the electrolyte and lithium metal sheet as the counter electrode.

Electrochemical testing of the assembled cells was performed at ambient temperature of-20 ℃. The electrochemical performance of the battery is shown in table 1, the rate performance of the battery is poor at low temperature, and the capacity decays rapidly. The cycle performance of the battery was also poor at low temperature, as shown in table 2, 2A g^-1The specific capacity is only 43mA h g after 1000 cycles of lower circulation^-1The retention ratio was 51%.

Comparative example 3

At 1.0mol/L NaPF₆The preparation method of the sodium metal pair battery with THF as electrolyte and Na metal as negative electrode comprises the following steps:

step (1): the electrolyte was prepared as in example 1;

step (2): and (6) assembling the battery. The CR2025 button cell was assembled in an Ar-filled MIKROUNA glove box using a sodium metal electrode as the negative electrode, 1.0mol/L NaPF₆The 2025 button cell was assembled with THF as the electrolyte and Na metal plate as the counter electrode.

Electrochemical testing was performed on the assembled symmetrical cells at ambient temperature of-20 ℃. The symmetrical cell was charged and discharged at-20 ℃ low temperature with a capacity of 2mAh, and failed after 50 cycles, probably due to the growth of sodium dendrites.

In conclusion, the electrolyte technology for the sodium ion battery can remarkably improve the rate capability, the cycle performance and the low-temperature performance of the hard carbon in the sodium ion battery. And the preparation is simple and easy to operate, and the preparation method is also easy to apply in the battery assembly process.

The embodiments of the present invention have been described in detail, but the description is only for the preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall still fall within the scope of the patent coverage of the present invention.

TABLE 1 Rate Performance parameter Table for batteries of examples 1-5 and comparative examples 1-2

TABLE 2 batteries in examples 1-5 and comparative examples 1-2 are at 2Ag^-1Long cycle performance parameter table under current density

Claims

1. The sodium ion battery electrolyte adaptive to the hard carbon cathode is characterized by comprising a sodium salt and a furan solvent.

2. The electrolyte of claim 1, wherein the sodium salt comprises: one or more of sodium hexafluorophosphate, sodium perchlorate, sodium triflate, sodium bistrifluoromethanesulfonimide and sodium bistrifluorosulfonimide.

3. The electrolyte of claim 2, wherein the sodium salt is present at a concentration of 0.1M to 10M; preferably 0.5M to 3.0M; further preferably 1.0M.

4. The electrolyte according to claim 1, wherein the furan-based solvent comprises a solvent consisting of one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 2-ethyltetrahydrofuran, 2, 5-diethyltetrahydrofuran, 2, 5-dimethoxytetrahydrofuran, and (perfluoro) 2-butyltetrahydrofuran; at least one of tetrahydrofuran and 2-methyltetrahydrofuran is preferred.

5. The electrolyte of claim 1, further comprising other functional additives.

6. The electrolyte of claim 5, wherein the other functional additives comprise one or more mixtures of fluoroethylene carbonate, vinylene carbonate, ethylene sulfate, cyclohexylbenzene, ethylene sulfite, propylene sulfite, butylene sulfite, ethylene carbonate, propylene sulfate, phenyl acetone, 1, 4-butane sultone, 1, 3-propane sultone, N-dimethylformamide, N-dimethylacetamide, N-dimethyltrifluoroacetamide; at least one of fluoroethylene carbonate, vinylene carbonate and ethylene sulfate is preferable.

7. The electrolyte as claimed in claim 5, wherein the proportion of the other functional additives in the electrolyte is 0.1-10 wt%; preferably 1 to 5 wt%, and more preferably 2.5 to 5 wt%.

8. The method for preparing the electrolyte according to any one of claims 1 to 7, wherein the sodium salt or the sodium salt and other functional additives are dissolved in a furan-based solvent.

9. Use of the electrolyte according to any of claims 1-7, in a sodium ion battery for a hard carbon negative electrode.

10. Use according to claim 9, wherein the assembled battery is used in an environment at a temperature of-30 ℃ to 40 ℃, preferably-20 ℃ to 30 ℃.