Electrolyte for negative-electrode-free sodium metal battery, preparation method of electrolyte and negative-electrode-free sodium metal battery

Abstract

The invention provides an electrolyte for a negative-electrode-free sodium metal battery, a preparation method of the electrolyte and the negative-electrode-free sodium metal battery, and relates to the technical field of sodium ion batteries. The electrolyte is mainly prepared from sodium salt and a weak coordination organic solvent, wherein the concentration of the sodium salt in the electrolyte is 1-2.5 mol / L. According to the electrolyte, through the weak coordination organic solvent and specific sodium salt concentration selection in the electrolyte, more anions can enter the solvation structure of the electrolyte to form the electrolyte with the anion-atmosphere solvation structure, and it is verified that the electrolyte has the advantages that the electrolyte is simple in structure and convenient to use. The electrolyte for the negative-electrode-free sodium metal battery can effectively relieve the problem that an existing high-voltage negative-electrode-free sodium metal battery is short in cycle life.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to an electrolyte for a negative electrode-free sodium metal battery, a preparation method thereof, and a negative electrode-free sodium metal battery. Background Art

[0002] Sodium-ion batteries are considered an ideal alternative to the currently widely used lithium batteries due to their abundant resource reserves and low cost. However, compared with lithium, sodium has a larger atomic mass and size, resulting in an energy density of only 90 to 150 Wh kg for sodium-ion batteries with embedded materials such as hard carbon as the negative electrode. -1 , which is much lower than the current commercial lithium iron phosphate battery (180Wh kg -1 Sodium metal batteries using metallic sodium as the negative electrode, especially those without the active material in the negative electrode, not only have energy densities approaching those of traditional lithium-ion batteries, but also can reduce costs by approximately 33%. Therefore, sodium metal batteries without the negative electrode are currently an important research direction for sodium-ion batteries.

[0003] However, irreversible sodium metal deposition / stripping usually occurs on the negative electrode side of existing negative electrode-free sodium metal batteries; at the same time, the oxidative decomposition of the high-voltage positive electrode electrolyte during the use of existing negative electrode-free sodium metal batteries will also reduce the cycle efficiency of the negative electrode-free sodium metal battery, resulting in the loss of limited active sodium, and then causing rapid attenuation of the battery capacity and shortening its cycle life.

[0004] In response to the above problems, the existing improvement scheme is mainly to improve the cycle life of the negative electrode-free sodium metal battery through the strategy of electrolyte optimization. For example: by dissolving sodium salt in ethylene glycol dimethyl ether, diethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether solvent to form an ether-based electrolyte. However, the above electrolyte system cannot form a stable interface film at high voltage (≥4.0V), so the electrolyte will continue to oxidize and decompose on the positive electrode surface, eventually causing the battery performance to continue to decay. In addition, the existing improved electrolyte system also has limited improvement on the reversibility of the deposition / stripping of sodium metal on the negative electrode side, and cannot effectively improve the cycle life of high-voltage negative electrode-free sodium metal batteries.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The first object of the present invention is to provide an electrolyte for a cathode-free sodium metal battery, which can effectively alleviate the problem of short cycle life of high-voltage cathode-free sodium metal batteries.

[0007] The second object of the present invention is to provide a method for preparing an electrolyte for a negative electrode-free sodium metal battery.

[0008] A third object of the present invention is to provide a negative electrode-free sodium metal battery.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0010] The present invention provides an electrolyte for a negative electrode-free sodium metal battery, wherein the electrolyte comprises a sodium salt and a weakly coordinated organic solvent;

[0011] The concentration of the sodium salt in the electrolyte is 1-2.5 mol / L.

[0012] Furthermore, the sodium salt includes at least one of sodium tetrafluoroborate, sodium perchlorate, sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide and sodium difluorooxalatoborate, preferably sodium hexafluorophosphate.

[0013] Furthermore, the weakly coordinating organic solvent includes one or more of ethylene glycol diethyl ether (DEE), diethylene glycol diethyl ether (DEGDEE), 1,2-dimethoxypropane (DMP), cyclopentyl methyl ether (CPME), and dipropyl ether (PE), preferably ethylene glycol diethyl ether.

[0014] Furthermore, the concentration of the sodium salt in the electrolyte is 1.5 to 2.0 mol / L.

[0015] The present invention provides a method for preparing an electrolyte for a negative electrode-free sodium metal battery, the preparation method comprising:

[0016] The sodium salt is dissolved in a weakly coordinated organic solvent, and the concentration of the sodium salt is adjusted to 1-2.5 mol / L to obtain an electrolyte.

[0017] The present invention provides a negative electrode-free sodium metal battery, which comprises: the electrolyte for the negative electrode-free sodium metal battery, as well as a current collector, a separator and a positive electrode.

[0018] Furthermore, the negative electrode side of the negative electrode-free sodium metal battery is composed of a current collector;

[0019] The current collector includes at least one of copper foil, aluminum foil, nickel foil or 316 stainless steel current collector, preferably copper foil.

[0020] Furthermore, the separator includes at least one of a polyethylene monolayer film (PE), a polypropylene monolayer film (PP), a PP / PE / PP multilayer microporous membrane composited with PP and PE, a cellulose non-woven fabric, a polyvinylidene fluoride porous membrane, a polyimide electrospun porous membrane and a glass fiber separator, preferably a glass fiber separator.

[0021] Furthermore, the positive electrode is a high voltage sodium ternary positive electrode, and the structural formula of the high voltage sodium ternary positive electrode is: Na[Nix Fe y Mn z ]O2, x+y+z=1;

[0022] Furthermore, the high voltage sodium ternary cathode is Na[Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 ]O2.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention provides an electrolyte for a negative electrode-free sodium metal battery, which is mainly prepared from a sodium salt and a weakly coordinated organic solvent, wherein the concentration of the sodium salt in the electrolyte is 1 to 2.5 mol / L. The electrolyte of the present application is selected by a weakly coordinated organic solvent and a specific sodium salt concentration in the electrolyte, and the two work together to allow more anions to enter the solvation structure of the electrolyte, forming an electrolyte with an anionic atmosphere solvation structure. During the electrode reaction, the anions in the electrolyte preferentially decompose, which helps to form a stable inorganic interface film on the positive electrode surface, thereby improving the cycle stability of the negative electrode-free sodium metal battery at high voltage. In addition, the electrolyte with the above-mentioned solvation structure also helps to improve the uniformity and density of the sodium metal deposited on the negative electrode side, reduce the formation of sodium dendrites, and improve the reversibility of sodium metal deposition / stripping on the negative electrode-free side. Therefore, the electrolyte for a negative electrode-free sodium metal battery of the present application can effectively alleviate the problem of short cycle life of existing high-voltage negative electrode-free sodium metal batteries.

[0025] The present invention provides a method for preparing an electrolyte for a negative-electrode-free sodium metal battery. The electrolyte is prepared by dissolving a sodium salt in a weakly coordinating organic solvent and adjusting the concentration of the sodium salt to 1 to 2.5 mol / L. This method has the technical advantages of being simple and easy to operate.

[0026] The present invention provides a cathode-free sodium metal battery comprising the aforementioned electrolyte for an anode-free sodium metal battery, a current collector, a separator, and a positive electrode. Due to the properties of the electrolyte for an anode-free sodium metal battery, the cathode-free sodium metal battery of the present invention has an excellent cycle life at high voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 This is a graph showing the test results of the oxidation decomposition voltage of the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in Test Example 2 of the present invention;

[0029] Figure 2 This is a graph showing the cycle efficiency of a sodium metal / copper foil asymmetric battery using the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in Experimental Example 2 of the present invention;

[0030] Figure 3 Electron microscopic images of the sodium deposition morphologies of the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in Experimental Example 2 of the present invention;

[0031] Figure 4 This is a diagram showing the capacity change of the negative electrode-free sodium battery using the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in Test Example 3 of the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] According to one aspect of the present invention, an electrolyte for a negative electrode-free sodium metal battery comprises a sodium salt and a weakly coordinating organic solvent;

[0034] The concentration of the sodium salt in the electrolyte is 1-2.5 mol / L.

[0035] The present invention provides an electrolyte for a negative electrode-free sodium metal battery, which is mainly prepared from a sodium salt and a weakly coordinated organic solvent, wherein the concentration of the sodium salt in the electrolyte is 1 to 2.5 mol / L. The electrolyte of the present application is selected by a weakly coordinated organic solvent and a specific sodium salt concentration in the electrolyte, and the two cooperate to allow more anions to enter the solvation structure of the electrolyte, forming an electrolyte with an anionic atmosphere solvation structure. During the electrode reaction, the anions in the electrolyte preferentially decompose, which helps to form a stable inorganic interface film on the surface of the positive electrode, thereby improving the cycle stability of the negative electrode-free sodium metal battery at high voltage. In addition, the electrolyte with the above-mentioned solvation structure also helps to improve the uniformity and density of the sodium metal deposited on the negative electrode side, reduce the formation of sodium dendrites, and improve the reversibility of sodium metal deposition / stripping on the negative electrode-free side. Therefore, the electrolyte for a negative electrode-free sodium metal battery of the present application effectively alleviates the problem of short cycle life of high-voltage negative electrode-free sodium metal batteries.

[0036] It should be noted that weakly coordinating solvent is an emerging term in the current research field. Its meaning is that compared with highly coordinating solvents, weakly coordinating solvents have weaker ability to dissolve salts and lower dielectric constants. Specifically, they are a type of solvent with weaker coordination ability with cations in salts.

[0037] Specifically, the present application introduces a weakly coordinating solvent to induce anions to enter the solvation structure of the electrolyte to form an "anion atmosphere" solvation structure electrolyte. The principle is:

[0038] Weakly coordinated solvents have steric hindrance in the solvent molecules themselves, which will cause mutual repulsion in space, resulting in each solvent molecule and Na + The coordination effect of Na + Coordination, that is, anions enter the solvation structure to form anion-dominated solvation clusters (the solvation structure or cluster here refers to Na + -solvent-anion aggregate), thereby forming an electrolyte with an "anion atmosphere" solvation structure.

[0039] That is to say, the electrolyte for the negative electrode-free sodium metal battery of the present application is different from the electrolyte composed of the existing traditional high coordination solvents, such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether, ethylene carbonate (EC) or propylene carbonate (PC), etc., mainly in that the solvation structure of the traditional electrolyte is mainly composed of Na + and 1 to 4 solvents, and the solvation structure of the anionic electrolyte in the present invention is mainly composed of Na + The electrolyte of the present invention is coordinated with the solvent and the anions, and therefore, more anions enter the solvation structure.

[0040] Furthermore, since the solvated clusters will first reach the electrode interface during the electrode reaction, more anions in the anion atmosphere solvated clusters will preferentially undergo electrochemical reactions, thereby forming a stable inorganic interface film on the positive electrode side, which inhibits the continuous decomposition of the electrolyte under high voltage. In addition, since the stable interface film on the positive electrode side inhibits the decomposition of the electrolyte, the interface impedance and Na + The desolvation energy is improved, thereby improving the uniformity and density of the sodium metal deposited on the non-negative electrode side, reducing the formation of sodium dendrites, and improving the reversibility of sodium metal deposition / stripping on the non-negative electrode side. Therefore, the electrolyte for the anode-free sodium metal battery in the present application can effectively alleviate the problem of short cycle life of existing high-voltage anode-free sodium metal batteries.

[0041] It should also be noted that the concentration of the sodium salt in the electrolyte of this application plays a decisive role in the formation of the anionic atmosphere solvation structure. The applicant has verified that when the sodium salt concentration is lower than 1.0M, there is still a large amount of Na +-Solvent coordination structure, at this time there is only a small amount of Na + -Solvent-anion clusters affect battery performance; when the concentration is 1-2.5 mol / L, Na in the electrolyte + -Solvent-anion clusters will dominate the entire electrolyte structure; but when the concentration is further increased to above 2.5M, Na + -Na in solvent-anion clusters + The enhanced interaction with anions will squeeze the solvent out of the clusters, causing salt precipitation and affecting battery performance.

[0042] In a preferred embodiment of the present invention, the sodium salt includes at least one of sodium tetrafluoroborate, sodium perchlorate, sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide and sodium difluorooxalatoborate, preferably sodium hexafluorophosphate.

[0043] In a preferred embodiment of the present invention, the weakly coordinating organic solvent includes a mixture of one or more of ethylene glycol diethyl ether (DEE), diethylene glycol diethyl ether (DEGDEE), 1,2-dimethoxypropane (DMP), cyclopentyl methyl ether (CPME), and dipropyl ether (PE), preferably ethylene glycol diethyl ether.

[0044] In a preferred embodiment of the present invention, the concentration of the sodium salt in the electrolyte is 1.5-2.0 mol / L.

[0045] In the present invention, the technical effect of the electrolyte for a negative electrode-free sodium metal battery of the present invention is further optimized by further adjusting and optimizing the concentration of the sodium salt in the electrolyte.

[0046] According to one aspect of the present invention, a method for preparing an electrolyte for a negative electrode-free sodium metal battery comprises:

[0047] The sodium salt is dissolved in a weakly coordinated organic solvent, and the concentration of the sodium salt is adjusted to 1-2.5 mol / L to obtain an electrolyte.

[0048] The present invention provides a method for preparing an electrolyte for a negative-electrode-free sodium metal battery. The method comprises dissolving a sodium salt in a weakly coordinating organic solvent and adjusting the concentration of the sodium salt to 1 to 2.5 mol / L to obtain the electrolyte. This method has the technical advantages of being simple and easy to operate.

[0049] According to one aspect of the present invention, a negative electrode-free sodium metal battery comprises: the above-mentioned electrolyte for a negative electrode-free sodium metal battery, a current collector, a separator and a positive electrode.

[0050] The present invention provides a cathode-free sodium metal battery comprising the aforementioned electrolyte for an anode-free sodium metal battery, a current collector, a separator, and a positive electrode. Due to the properties of the electrolyte for an anode-free sodium metal battery, the cathode-free sodium metal battery of the present invention has an excellent cycle life at high voltage.

[0051] In a preferred embodiment of the present invention, the negative electrode side of the negative electrode-free sodium metal battery is composed of a current collector;

[0052] Preferably, the current collector includes at least one of copper foil, aluminum foil, nickel foil or 316 stainless steel current collector, more preferably copper foil.

[0053] In a preferred embodiment of the present invention, the diaphragm includes at least one of a polyethylene single-layer film, a polypropylene single-layer film, a PP / PE / PP multi-layer microporous membrane composite of PP and PE, a cellulose non-woven fabric, a polyvinylidene fluoride porous membrane, a polyimide electrospun porous membrane and a glass fiber diaphragm, preferably a glass fiber diaphragm.

[0054] In a preferred embodiment of the present invention, the positive electrode is a high voltage sodium ternary positive electrode, and the structural formula of the high voltage sodium ternary positive electrode is: Na[Ni x Fe y Mn z ]O2, x+y+z=1;

[0055] Preferably, the high voltage sodium ternary positive electrode is Na[Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 ]O2.

[0056] The technical solution of the present invention will be further described below with reference to embodiments.

[0057] Example 1

[0058] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0059] Under a high-purity argon protection environment, 1.5 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 1.5 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery of this embodiment can be obtained.

[0060] Example 2

[0061] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0062] Under a high-purity argon protection environment, 2.0 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 2.0 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery of this embodiment can be obtained.

[0063] The difference between this embodiment and embodiment 1 is that the concentration of sodium hexafluorophosphate in the electrolyte is 2.0M.

[0064] Example 3

[0065] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0066] Under a high-purity argon protection environment, 1.0 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 1.0 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery of this embodiment can be obtained.

[0067] The difference between this embodiment and embodiment 1 is that the concentration of sodium hexafluorophosphate in the electrolyte is 1.0M.

[0068] Example 4

[0069] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0070] Under a high-purity argon protection environment, 2.5 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 2.5 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery of this embodiment can be obtained.

[0071] The difference between this embodiment and embodiment 1 is that the concentration of sodium hexafluorophosphate in the electrolyte is 2.5M.

[0072] Example 5

[0073] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0074] Under a high-purity argon protection environment, 1.5 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of 1,2-dimethoxypropane (DMP) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 1.5 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this embodiment can be obtained.

[0075] The difference between this embodiment and embodiment 1 is that the solvent ethylene glycol diethyl ether is replaced by 1,2-dimethoxypropane.

[0076] Example 6

[0077] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0078] Under a high-purity argon protection environment, 1.5 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry solvent with a volume of 1 L (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 1.5 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery of this embodiment can be obtained.

[0079] The solvent is a mixed solvent of ethylene glycol diethyl ether and 1,2-dimethoxypropane, and the mass ratio of ethylene glycol diethyl ether to 1,2-dimethoxypropane in the solvent is 1:1.

[0080] Comparative Example 1

[0081] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0082] Under a high-purity argon protection environment, 0.8 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 0.8 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this comparative example was obtained.

[0083] The difference between this comparative example and Example 1 is that the concentration of sodium hexafluorophosphate in the electrolyte is 0.8M.

[0084] Comparative Example 2

[0085] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0086] Under a high-purity argon protection environment, 2.8 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol diethyl ether (DEE) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 2.8 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this comparative example was obtained.

[0087] The difference between this comparative example and Example 1 is that the concentration of sodium hexafluorophosphate in the electrolyte is 2.8M.

[0088] Comparative Example 3

[0089] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0090] Under a high-purity argon protection environment, 1.5 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol dimethyl ether (DME) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 1.5 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this comparative example was obtained.

[0091] The difference between this embodiment and embodiment 1 is that the solvent ethylene glycol diethyl ether is replaced by ethylene glycol dimethyl ether (DME).

[0092] Comparative Example 4

[0093] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0094] Under a high-purity argon protection environment, 2.0 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in a dry volume of 1 L of ethylene glycol dimethyl ether (DME) solvent (water content <20 ppm) (the concentration of sodium hexafluorophosphate in the solution was 2.0 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this comparative example was obtained.

[0095] The difference between this comparative example and Example 2 is that the solvent ethylene glycol diethyl ether is replaced by ethylene glycol dimethyl ether (DME).

[0096] Comparative Example 5

[0097] An electrolyte for a negative electrode-free sodium metal battery, wherein the preparation method of the electrolyte comprises:

[0098] Under a high-purity argon protection environment, 1.0 mol of sodium hexafluorophosphate (NaPF6) was weighed and dissolved in 1 L of a mixed solvent (water content <20 ppm) of dry propylene carbonate (PC) and ethyl methyl carbonate solvent (EMC) (volume ratio of 3:7) (the concentration of sodium hexafluorophosphate in the solution was 1.0 M), fully shaken, and magnetically stirred at room temperature for 12 h to completely dissolve the sodium salt. The electrolyte for the negative electrode-free sodium metal battery in this comparative example was obtained.

[0099] The electrolyte prepared in this comparative example is a commercial sodium battery electrolyte recognized in the art.

[0100] Test Example 1

[0101] First, the compatibility of Examples 1 to 6 and Comparative Examples 1 to 5 with high-electrode sodium positive electrodes was compared, and the capacity retention rates of the corresponding sodium / sodium positive electrode half-cells were tested.

[0102] (1) The selection of the negative electrode, separator and positive electrode in the sodium / sodium positive electrode half-cell is as follows:

[0103] The negative electrode is a sodium metal sheet;

[0104] The diaphragm is a glass fiber diaphragm;

[0105] The positive electrode is a high voltage sodium ternary positive electrode, and the high voltage sodium ternary positive electrode is Na[Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 ]O2(NFM333).

[0106] (2) The capacity retention rate detection parameters are as follows:

[0107] Voltage range 2.0V~4.0V(vs.Na / Na + ), cycle current density 140 mAh g -1 .

[0108] (3) The test results are shown in the following table:

[0109] Table: Capacity retention data of the above sodium / sodium positive electrode half-cell using different embodiment and comparative example electrolytes.

[0110]

[0111]

[0112] From the above test results, it can be seen that the electrolytes of Examples 1 to 6 of the present application significantly improve their compatibility with the high-voltage NFM333 positive electrode by forming a more stable anionic atmosphere structure, and can effectively increase the cycle life of the high-voltage sodium / sodium positive electrode half-cell.

[0113] In Comparative Example 1, since the concentration of sodium salt (NaPF6) in the electrolyte is lower than 1.0M, there is still a large amount of Na + -Solvent coordination structure, at this time there is only a small amount of Na + -Solvent-anion clusters, so the electrolyte cannot form a stable inorganic interface film, causing the electrolyte to continue to decompose, resulting in a significant decrease in the battery's capacity retention rate.

[0114] In Comparative Example 2, since the concentration of sodium salt (NaPF6) in the electrolyte is higher than 2.5M, Na + -Na in solvent-anion clusters + The enhanced interaction with anions squeezes the solvent out of the clusters, causing salt precipitation. During battery cycling, salt precipitation is further exacerbated. Therefore, compared to Example 4, the capacity retention rate of Comparative Example 2 is significantly reduced.

[0115] However, in Comparative Examples 3 and 4, ethylene glycol dimethyl ether is used as the technical solution of the solvent. Due to the strong coordination ability of ethylene glycol dimethyl ether, the anion coordination structure in the electrolyte is not stable. As a result, even though Comparative Examples 3 and 4 maintain the same salt concentration as Examples 1 and 2, their capacity retention rates are lower.

[0116] The technical solution of comparative example 5 using propylene carbonate and ethyl methyl carbonate as solvents is a currently mature commercial electrolyte recognized in the field. At the same time, the ester solvents used also have a high coordination ability and cannot form an anion coordination structure, which results in a battery cycle retention rate that is significantly lower than that of the embodiment.

[0117] Test Example 2

[0118] This test example further conducted an oxidation decomposition film formation test and a negative electrode compatibility test on the electrolytes of Example 2 and Comparative Examples 4 and 5.

[0119] (1) Oxidation decomposition film formation test:

[0120] The test method of the oxidation decomposition film formation test is: using aluminum foil as the working electrode, sodium metal as the counter electrode and reference electrode, and using linear sweep voltammetry to test the oxidation decomposition potential of the electrolyte.

[0121] Figure 1 This is a graph showing the oxidative decomposition voltage test results of the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in this test example.

[0122] from Figure 1It can be seen that the electrolyte in Example 2 shows an obvious oxidation current at about 3.6 V during the first linear scan, while the oxidation currents of Comparative Examples 4 and 5 only show an increasing trend at about 3.8 V, and the increased current is relatively small, which indicates that the anions in the electrolyte in Example 2 will preferentially decompose into a film.

[0123] In addition, during the second linear scan, when the voltage was less than 4.5 V, Example 2 did not have an obvious oxidation current, while the electrolyte of Comparative Example 2 still had an oxidation current when the voltage reached 3.8 V. At the same time, the electrolyte of Comparative Example 3 also had an obvious oxidation current when the voltage reached 3.6 V. This indicates that Example 2 has formed a stable interfacial film during the first scan, which can further inhibit the oxidative decomposition of the electrolyte during the later scans, which also proves the rationality of the anionic atmosphere electrolyte design method.

[0124] (2) Negative electrode compatibility test:

[0125] The electrolytes prepared in Example 2, Comparative Examples 4, and 5 were used to assemble sodium metal / copper foil asymmetric cells. The reversible efficiency of sodium metal deposition / stripping on the copper foil surface was tested under constant current using the above-mentioned different electrolytes, where the current density was 1 mA cm -2 , with a deposition capacity of 1 mAh cm -2 .

[0126] Figure 2 This is a diagram of the sodium metal / copper foil asymmetric battery cycle efficiency of the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in this test example.

[0127] from Figure 2 It can be seen from the cycle efficiency that compared with Comparative Examples 4 and 5, Example 2 has a higher and more stable cycle efficiency, indicating that Example 2 can effectively improve the reversibility of sodium deposition / stripping.

[0128] Figure 3 The following are electron microscope images of the sodium deposition morphology of the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in this test example.

[0129] from Figure 3 It can be seen that compared with Comparative Example 4, the sodium metal deposited in Example 2 is uniform, dense and has no obvious cracks; compared with Comparative Example 5, the sodium metal deposited in Example 2 has a smooth morphology and does not form obvious sodium dendrites, which further illustrates that the electrolyte of Example 2 has better compatibility with sodium metal, thereby improving the reversibility of sodium metal deposition / stripping.

[0130] Test Example 3

[0131] To demonstrate that the electrolyte for anode-free sodium metal batteries in this application can effectively alleviate the short cycle life problem of existing high-voltage anode-free sodium metal batteries. As described in Experimental Example 1 above, the difference between the anode-free sodium battery and the sodium / sodium positive electrode half-cell is that the cathode side of the anode-free sodium battery uses a copper current collector, and the positive electrode is a high-voltage sodium ternary positive electrode NFM333.

[0132] It should be noted that this experimental example has no special restrictions on the assembly and preparation method of the negative electrode-free sodium metal battery, and the assembly and preparation method of the negative electrode-free sodium metal battery well known to those skilled in the art can be used.

[0133] The capacity change trend of the negative electrode-free sodium battery obtained by testing the electrolytes of Example 2 and Comparative Examples 4 and 5.

[0134] Figure 4 This is a diagram showing the capacity change of the negative electrode-free sodium battery using the electrolytes of Example 2 and Comparative Examples 4 and 5 provided in this test example.

[0135] from Figure 4 It can be seen from the capacity change of the negative electrode-free sodium battery that compared with comparative examples 4 and 5, Example 2 has a higher capacity retention rate, indicating that the electrolyte for a negative electrode-free sodium metal battery in the present invention can effectively improve the cycle stability of the high-voltage negative electrode-free sodium battery.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte for a negative electrode-free sodium metal battery, characterized in that: The electrolyte comprises a sodium salt and a weakly coordinating organic solvent; The concentration of the sodium salt in the electrolyte is 1-2.5 mol / L.

2. The electrolyte for a negative electrode-free sodium metal battery according to claim 1, characterized in that The sodium salt includes at least one of sodium tetrafluoroborate, sodium perchlorate, sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide and sodium difluorooxalatoborate, preferably sodium hexafluorophosphate.

3. The electrolyte for a negative electrode-free sodium metal battery according to claim 1, characterized in that The weakly coordinating organic solvent includes one or more of ethylene glycol diethyl ether, diethylene glycol diethyl ether, 1,2-dimethoxypropane, cyclopentyl methyl ether, and dipropyl ether, preferably ethylene glycol diethyl ether.

4. The electrolyte for a negative electrode-free sodium metal battery according to claim 1, wherein The concentration of the sodium salt in the electrolyte is 1.5-2.0 mol / L.

5. A method for preparing an electrolyte for a negative electrode-free sodium metal battery according to any one of claims 1 to 4, characterized in that: The preparation method comprises: The sodium salt is dissolved in a weakly coordinated organic solvent, and the concentration of the sodium salt is adjusted to 1-2.5 mol / L to obtain an electrolyte.

6. A negative electrode-free sodium metal battery, characterized in that: The negative electrode-free sodium metal battery comprises: the electrolyte for the negative electrode-free sodium metal battery according to any one of claims 1 to 4, as well as a current collector, a separator and a positive electrode.

7. The negative electrode-free sodium metal battery according to claim 6, characterized in that: The negative electrode side of the negative electrode-free sodium metal battery is composed of a current collector; The current collector includes at least one of copper foil, aluminum foil, nickel foil or 316 stainless steel current collector, preferably copper foil.

8. The negative electrode-free sodium metal battery according to claim 6, characterized in that: The separator includes at least one of a polyethylene single-layer membrane, a polypropylene single-layer membrane, a PP / PE / PP multi-layer microporous membrane composited with PP and PE, a cellulose non-woven fabric, a polyvinylidene fluoride porous membrane, a polyimide electrospun porous membrane and a glass fiber separator, preferably a glass fiber separator.

9. The negative electrode-free sodium metal battery according to claim 6, characterized in that: The positive electrode is a high voltage sodium ternary positive electrode; The structural formula of the high voltage sodium ternary positive electrode is: Na[Ni x Fe y Mn z ]O2,x+y+z=1.

10. The negative electrode-free sodium metal battery according to claim 9, characterized in that: The high voltage sodium ternary positive electrode is Na[Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 ]O2.

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