CN113675470A

CN113675470A - Electrolyte additive, electrolyte and sodium metal battery

Info

Publication number: CN113675470A
Application number: CN202110910538.6A
Authority: CN
Inventors: 马建民; 吴达雄
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2021-08-09
Filing date: 2021-08-09
Publication date: 2021-11-19
Anticipated expiration: 2041-08-09
Also published as: CN113675470B

Abstract

In order to improve the oxidation resistance of the electrolyte, improve the overall coulombic efficiency of the battery, effectively inhibit the growth of sodium dendrite, improve the cycle stability of the battery and prolong the service life, the invention provides an electrolyte additive, which comprises benzene rings; the benzene ring has a first substituent and a second substituent; the first substituent is fluorine; the second substituent is a siloxane. Meanwhile, the invention also discloses electrolyte and a sodium metal battery comprising the electrolyte additive. The electrolyte provided by the invention greatly improves the oxidation resistance of the electrolyte, reduces side reactions caused by oxidative decomposition of the electrolyte in the discharging process, optimizes a Solid Electrolyte Interface (SEI) film of a cathode and an electrolyte interface (CEI) film of an anode of a sodium metal battery, improves the full battery coulombic efficiency, and effectively inhibits the growth of sodium dendrites, thereby improving the overall performance of the battery.

Description

Electrolyte additive, electrolyte and sodium metal battery

Technical Field

The present invention relates to the field of electrochemistry, and more particularly, to electrolyte additives, electrolytes, and sodium metal batteries.

Background

Rechargeable Lithium Ion Batteries (LIBs) have the advantages of high energy density, long service life, wide working temperature, environmental friendliness and the like, and are widely applied to small-sized portable devices such as mobile phones, notebook computers and the like. With the rapid development of the global electric automobile market, the traditional lithium battery cannot meet the endurance of long mileage. And the shortage of lithium resources causes the price of lithium metal to rapidly rise, so that the cost of the lithium ion battery also sharply rises. Therefore, the development of the next generation alkali metal battery is imperative. Due to the abundant sodium metal resources, Sodium Ion Batteries (SIB) are considered as one of the most promising alternatives for lithium ion batteries.

Of the various SIB anode materials, the sodium (Na) metal electrode had a low redox potential (2.71V vs. standard hydrogen electrode) and the highest theoretical capacity (1166 mAh/g). However, the highly active metal Na may undergo continuous side reactions with most organic liquid electrolytes to form a solid electrolyte membrane (SEI). However, such spontaneously formed SEI is generally not uniform, brittle, and easily broken down in repeated deposition/peeling processes, thereby re-inducing uncontrolled side reactions, and causing the SEI to be continuously reformed, inevitably resulting in a decrease in the low Coulombic Efficiency (CE) of the battery, and also in the growth of sodium dendrites, eventually resulting in the exhaustion of the electrolyte of the battery; in addition, the growth of the dendritic crystal can induce the internal short circuit of the battery, and potential safety hazard exists.

To address these challenges, various strategies have been proposed, including three-dimensional current collectors, high concentration electrolytes, electrolyte additives, solid electrolytes, and artificial SEI protective layers, all of which exhibit the ability to suppress sodium dendrites. However, from a practical point of view, electrolyte additives are particularly attractive due to their abundant chemical composition and compatibility with battery manufacture. The electrolyte additive can be decomposed and assist in forming an ideal SEI layer, and can protect the metal electrode and enable dendrite-free metal to be deposited. To date, only a few electrolyte additives have been developed to suppress sodium dendrites, such as fluoroethylene carbonate (FEC), Na₂S₆And potassium bis (trifluoromethylsulfonyl) imide (KTFSI), and the like.

Among them, FEC has a positive effect on full cells as an electrolyte additive, but the polarization potential at the sodium metal negative electrode is large and a small amount of gas is continuously released, which may be harmful to the cell performance for a long time. WhileNa₂S₆And the electrolyte additive in the KTFSI form is only suitable for ether-based electrolyte, has poor oxidation resistance stability and is not suitable for high-voltage anodes.

Disclosure of Invention

It is an object of the present invention to at least solve the above problems.

It is still another object of the present invention to provide an electrolyte additive, an electrolyte and a sodium metal battery, which can solve at least the following problems: the problems of comprehensively inhibiting the growth of the sodium dendrite, improving the oxidation potential of the electrolyte and improving the SEI and CEI performances of the negative electrode are solved, so that the cycle stability of the battery is improved, and the service life of the battery is prolonged.

The technical scheme adopted by the invention for solving the technical problems is as follows:

in one aspect, the present invention provides an electrolyte additive comprising a benzene ring; the benzene ring has a first substituent and a second substituent; the first substituent is fluorine; the second substituent is a siloxane.

On the basis of the above, in another aspect, the invention provides an electrolyte, which includes the above electrolyte additive.

Further, in yet another aspect, the invention also provides a sodium metal battery comprising the electrolyte.

The technical scheme provided by the invention has the following technical effects:

1. according to the electrolyte additive provided by some technical schemes of the invention, the electrolyte can be preferentially reduced on the surface of the negative electrode to form high-quality SEI, and the SEI can avoid the direct contact of the electrolyte and high-activity sodium metal and reduce the decomposition of the electrolyte.

In addition, the electrolyte additive can also enable the electrolyte to be preferentially oxidized on the surface of the battery anode to form stable CEI, so that the integral oxidation resistance of the electrolyte is improved, and the coulomb efficiency of the metal battery is improved.

It is also important that the electrolyte additive contains abundant fluorine elements, and the fluorine elements can react with sodium ions to generate NaF with high interfacial energy, so that the mechanical property of SEI can be improved, and the growth of sodium dendrite can be inhibited.

In addition to the above, the electrolyte additive also contains siloxane groups, which can remove harmful substances (such as hydrofluoric acid) in the electrolyte and protect the integrity of the SEI and CEI structures.

2. According to the electrolyte provided by some technical schemes of the invention, uniform and compact CEI can be formed on the surface of the high-voltage anode, and the coulombic efficiency (first-turn and subsequent-cycle coulombic efficiency) of the sodium metal battery can be remarkably improved; meanwhile, the electrolyte can form an SEI rich in NaF on the surface of the sodium cathode, so that the mechanical property of the SEI is greatly improved. The strong SEI can isolate the contact between the electrolyte and the metal sodium, reduce the occurrence of side reactions, inhibit the growth of sodium dendrites and improve the overall performance of the battery.

3. In some technical schemes, the electrolyte provided by the invention also comprises NaClO₄And then, the oxidation resistance of the electrolyte can be greatly enhanced, the oxidation potential of the electrolyte is improved, the oxidative decomposition of the electrolyte is reduced, and the coulomb efficiency of the battery is improved. Therefore, the electrolyte is particularly suitable for high-voltage positive electrode materials, and the positive electrode performance of the battery is improved.

More importantly, electrolyte additive and NaClO₄The synergistic effect can be generated in the electrolyte of the sodium metal battery, the CEI and the SEI of the anode of the sodium metal battery are further optimized, the coulombic efficiency of the full battery is improved, the growth of sodium dendrites is inhibited, and therefore the overall performance of the battery is improved.

4. Based on the electrolyte additive and the electrolyte in the schemes, it is understood that, compared with the prior art, the sodium metal battery provided by the invention can improve the coulombic efficiency, inhibit the growth of sodium dendrite and prolong the service life of the battery, so that the overall performance of the battery can be greatly improved.

Drawings

FIG. 1 is a graph of electrochemical stability windows corresponding to the electrolytes of examples 1-4 and comparative example 1 provided by the present invention;

FIG. 2 is a graph comparing the cycle performance of sodium symmetric cells prepared according to the electrolytes of examples 1-4 of the present invention and comparative example 1;

fig. 3 is a cross-sectional view of a sodium electrode obtained at different times when a battery prepared according to the electrolyte of comparative example 1 of the present invention was observed by an in-situ optical microscope;

fig. 4 is a cross-sectional view of a sodium electrode taken at different times while observing a battery prepared by an electrolyte according to example 4 of the present invention through an in-situ optical microscope;

fig. 5 is a graph comparing full cell cycle performance for electrolytes of example 2, example 4 and comparative example 1 according to the present invention.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more clearly apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

< electrolyte additive >

The electrolyte additive is applied to a sodium metal battery and comprises a benzene ring, wherein the benzene ring is provided with a first substituent and a second substituent; in other words, the hydrogen atom on the benzene ring is substituted with the first substituent and the second substituent to form the electrolyte additive. Wherein the first substituent is a fluoro group and the second substituent is a siloxane group.

The electrolyte additive contains rich fluorine elements, and the fluorine elements can react with sodium ions to generate NaF with high interfacial energy, so that the mechanical property of SEI can be improved, and the growth of sodium dendrite can be inhibited. The siloxane groups can remove harmful substances (such as hydrofluoric acid) in the electrolyte and protect the integrity of SEI and CEI structures.

Therefore, the sodium metal battery prepared by the electrolyte additive can form SEI rich in NaF inorganic components on a negative electrode, improve the mechanical property of the SEI, and effectively inhibit the growth of dendrites, thereby prolonging the service life of the battery. The electrolyte additive preferably has a plurality of first substituents, and further has 2 to 5, more preferably 5, of the first substituents on the benzene ring, and in this case, the electrolyte additive has a pentafluorophenyl group as is well known to those skilled in the art.

According to the invention, the siloxane comprises a silicon atom and three alkoxy groups bound to the silicon atom, the silicon atom being bound to a benzene ring. Preferably, the alkoxy group is a methoxy group, the oxygen atom of which is bonded to the silicon atom, in which case the second substituent is trimethoxysilane.

In the present invention, preferably, the electrolyte additive is selected from trimethoxy (pentafluorophenyl) silane (C)₉H₉O₃F₅Si) having the structure:

the concentration of the electrolyte additive in the electrolyte may vary within wide limits, preferably the concentration of the electrolyte additive is between 0.5 and 2 wt%.

According to the invention, the electrolyte additive is added into the electrolyte applied to the sodium metal battery, so that the overall performance of the battery can be effectively improved.

< electrolyte solution >

It is to be understood that, on the basis of the above, in another aspect, the present invention provides an electrolyte comprising the above electrolyte additive.

In order to further optimize the performance of the electrolyte, the electrolyte preferably further contains NaClO₄. Adding NaClO₄And then, the oxidation resistance of the electrolyte can be greatly enhanced, the oxidative decomposition of the electrolyte is reduced, and the coulomb efficiency of the battery is improved.

In the present invention, the NaClO is₄Can vary within wide limits, preferably NaClO₄The concentration of (A) is 0.05-0.5 wt%, and the further concentration is 0.2 wt%.

The electrolyte may also include sodium salts and non-aqueous organic solvents, as is well known to those skilled in the art.

Wherein the sodium salt is selected from NaPF₆、NaBF₄、NaBOB、NaDFOB、NaSbF₆、NaAsF₆、NaN(SO₂CF₃)₂、NaN(SO₂C₂F₅)₂、NaC(SO₂CF₃)₃Or NaN (SO)₂F)₂One or more than two of them. Among them, sodium hexafluorophosphate (NaPF) is preferably used₆)。

In the electrolyte, the concentration of sodium salt may be 0.8-1.2M.

The non-aqueous organic solvent is a mixture of cyclic carbonate and chain carbonate, the cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate or butylene carbonate, and the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or propyl methyl carbonate. The mixed solution of the cyclic carbonate organic solvent with high dielectric constant and the chain carbonate organic solvent with low viscosity is used as the solvent, so that the mixed solution of the organic solvent has high ionic conductivity, high dielectric constant and low viscosity simultaneously.

Preferably, the non-aqueous organic solvent comprises Ethylene Carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume concentration ratio.

< sodium Metal Battery >

Meanwhile, the invention also provides a sodium metal battery which comprises the electrolyte.

The present invention will be further illustrated by the following examples.

Comparative example 1

This comparative example is used to illustrate by comparison the electrolytes disclosed herein.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), an appropriate amount of sodium hexafluorophosphate (NaPF) was weighed₆) This was dissolved in a non-aqueous organic solution to give a control electrolyte.

Sodium salt concentration: 1M sodium hexafluorophosphate (NaPF)₆)。

Non-aqueous organic solvent: ethylene Carbonate (EC): diethyl carbonate (DEC) ═ 1:1(v: v) mixed solvent.

Example 1

This example illustrates the electrolyte disclosed in the present invention.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), adding 0.5 wt% of trimethoxy (pentafluorophenyl) silane to the control electrolyte, and stirring to obtain electrolyte 1.

Example 2

This example illustrates the electrolyte disclosed in the present invention.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), adding 1 wt% of trimethoxy (pentafluorophenyl) silane to the control electrolyte, and stirring to obtain electrolyte 2.

Example 3

This example illustrates the electrolyte disclosed in the present invention.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), adding trimethoxy (pentafluorophenyl) silane with a mass fraction of 2 wt% to the control electrolyte, and stirring to obtain an electrolyte 3.

Example 4

This example illustrates the electrolyte disclosed in the present invention.

In a glove box (H)₂O<0.1ppm，O₂<0.1ppm), 0.2 wt% NaClO is added to the reference electrolyte₄And 1.0 wt% of trimethoxy (pentafluorophenyl) silane were uniformly stirred to obtain an electrolyte solution 4.

Electrochemical performance test

The following performance tests were performed on the electrolytes 1 to 4 prepared above and the control electrolyte:

1. electrolyte oxidation resistance test

Oxidative decomposition of the electrolyte directly affects the coulombic efficiency of the battery. First, the IVIUM electrochemical workstation was used to characterize the oxidation resistance of different electrolytes. The test results are shown in fig. 1.

It can be seen that the control electrolyte is very unstable and rapid oxidation begins to occur at 3.65V. The oxidation of the electrolyte 1-3 begins to occur at about 3.8V; from this, it can be seen that the oxidation resistance of the electrolyte is greatly improved after the electrolyte additive is added, and the oxidation resistance is not greatly changed along with the increase of the electrolyte concentration.

In addition, electrolyte 4 is more stable than electrolytes 1-3, and its electrochemically stable voltage reaches 4.88V, which indicates that electrolyte 4 can obtain the highest coulombic efficiency, and also indicates that electrolyte additive and NaClO₄Can play a good role in synergy.

2. Symmetric battery performance testing

And (5) adopting a Xinwei test device to perform performance test on the assembled symmetrical battery. The specific experimental process is as follows: and (3) assembling the sodium symmetric battery by using metal sodium as a positive electrode and a negative electrode to perform constant current charge and discharge test. The test results are shown in FIG. 2.

It can be seen that the Na | Na symmetric cell prepared according to the control electrolyte started to develop significant polarization after 95 hours and the overpotential also started to increase rapidly; the performances of the Na II Na symmetrical batteries prepared from the electrolyte 1, the electrolyte 2, the electrolyte 3 and the electrolyte 4 greatly exceed 95 hours, and are far superior to those of the Na II Na symmetrical batteries prepared from the reference electrolyte. In terms of cycle performance, in particular, the Na | Na symmetrical cell prepared according to electrolyte 1 reached over 150 hours, and the Na | Na prepared from electrolyte 2 exceeded 185 hours; the best performance is a Na | Na symmetric cell prepared according to electrolyte 4, over 200 hours.

3. In situ observation of sodium dendrite growth

The control electrolyte and the electrolyte 4 are respectively assembled into a symmetrical battery by a transparent device, the deposition process of sodium ions is observed in situ by an optical microscope, and the test results are respectively shown in fig. 3 and fig. 4, wherein fig. 3 shows the deposition process of sodium ions in the symmetrical battery prepared according to the control electrolyte, and fig. 4 shows the deposition process of sodium ions in the symmetrical battery prepared according to the electrolyte 4.

It can be seen that dendrites were evident at 4 minutes for the cell prepared with the control electrolyte, while dendrites were not evident at 20 minutes for the cell prepared with electrolyte 4, indicating that electrolyte 4 significantly inhibited the growth of sodium dendrites.

4. Full battery performance test

And (3) respectively carrying out performance test on the full batteries assembled according to the reference group electrolyte, the electrolyte 2 and the electrolyte 2 by adopting a Xinwei test device. And (3) matching and assembling the sodium metal serving as a negative electrode and the sodium vanadium fluorophosphate (NVPF) serving as a positive electrode into a full cell to perform constant current charge and discharge test. The test results are shown in fig. 5.

Control electrolyte: na | NVPF full cell achieved extremely low average coulombic efficiencies for the first and subsequent cycles, 6.2% and 84.7%, respectively; and obvious attenuation begins to appear when the discharge specific capacity is cycled for less than 100 times.

Electrolyte 2: compared with a reference electrolyte, the overall performance of the Na | NVPF full cell is obviously improved, the first-turn coulombic efficiency reaches 14.8, and the subsequent cycle coulombic efficiency also reaches 95.4%.

Electrode liquid 4: the Na | NVPF full cell obtains very high first-turn coulombic efficiency of more than 90.9%, the average coulombic efficiency of subsequent cycles is also more than 99.7%, and the discharge specific capacity retention rate still exceeds 90% after 500 cycles, which shows that the overall performance of the full cell is greatly improved.

In conclusion, the electrolyte provided by the embodiment of the invention can greatly improve the oxidation resistance of the electrolyte, reduce the oxidative decomposition of the electrolyte in the charging and discharging processes, optimize the CEI and the SEI of the cathode of the sodium metal battery, improve the cycling stability of the total battery coulombic efficiency, and effectively inhibit the growth of sodium dendrites.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims

1. An electrolyte additive, characterized by comprising a benzene ring;

the benzene ring has a first substituent and a second substituent;

the first substituent is fluoro;

the second substituent is a siloxane group.

2. The electrolyte additive of claim 1 wherein the benzene ring has 2-5 of the first substituents.

3. The electrolyte additive of claim 1 wherein the siloxane group comprises a silicon atom and three alkoxy groups attached to the silicon atom, the silicon atom being attached to the benzene ring.

4. The electrolyte additive according to claim 1, wherein the additive is selected from trimethoxy (pentafluorophenyl) silane.

5. An electrolyte comprising the electrolyte additive of any one of claims 1 to 4;

in the electrolyte, the concentration of the electrolyte additive is 0.5-2 wt%.

6. The electrolyte of claim 1, further comprising NaClO₄。

7. The electrolyte of claim 6, wherein the NaClO is in the electrolyte₄The concentration of (B) is 0.05-0.5 wt%.

8. The electrolyte of claim 1, further comprising a sodium salt and a non-aqueous organic solvent.

9. The electrolyte of claim 8, wherein the sodium salt is selected from NaPF₆、NaBF₄、NaBOB、NaDFOB、NaSbF₆、NaAsF₆、NaN(SO₂CF₃)₂、NaN(SO₂C₂F₅)₂、NaC(SO₂CF₃)₃Or NaN (SO)₂F)₂One or more than two of the above;

in the electrolyte, the concentration of sodium salt is 0.8-1.2M;

the non-aqueous organic solvent is a mixture of cyclic carbonate and chain carbonate, the cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate or butylene carbonate, and the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or propyl methyl carbonate.

10. A sodium metal battery, comprising:

the electrolyte additive of any one of claims 1-4; or

The electrolyte of any one of claims 5-9.