Slightly soluble electrolyte, sodium secondary battery and electric equipment

Abstract

The invention relates to an electrolyte formula, a sodium secondary battery and electric equipment in the technical field of sodium secondary batteries. Aiming at the problems of limited capacity release, short cycle life and the like of a sodium secondary battery, a slightly soluble electrolyte is researched and developed. The preparation method of the electrolyte comprises the step of mixing diethylene glycol dimethyl ether and sodium tetrafluoroborate according to a certain molar ratio to form a new slightly soluble electrolyte. The method has the advantages that the solvent and salt are strongly coordinated, nonmetal fluoride distribution is induced, and solvent side reaction is reduced. The sodium secondary battery based on the electrolyte is excellent in cycling stability, effectively solves the problem that an electrode / electrolyte interface in the sodium battery is uneven and thick, ensures the stability of an electrode structure, and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium secondary batteries, and in particular, to a slightly soluble electrolyte for sodium secondary batteries, a sodium secondary battery, and an electrical device. Background Art

[0002] Sodium secondary batteries are rich in raw materials, have the advantages of low production cost and high economic benefits, and are reliable in performance within a wide temperature range (-70°C to 100°C). They have been widely used in large-scale energy storage systems and two-wheeled electric vehicles. Therefore, sodium secondary batteries are the main development direction of high-performance batteries.

[0003] However, compared with other common secondary batteries, sodium secondary batteries face the problems of limited capacity release and short cycle life, which greatly limit their further development and potential applications. Taking sodium-ion batteries as an example, currently, the most widely used ester-based electrolytes in sodium-ion batteries are commonly ethylene carbonate (EC), dimethyl carbonate (DMC), etc. Its typical composition is 1 mol / L sodium hexafluorophosphate (NaPF6) dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC = 1:1, volume ratio). This electrolyte has a relatively wide electrochemical window (usually 0 - 4.5V vs. Na / Na+), which is suitable for matching with high-voltage cathode materials. At the same time, the high dielectric constant of EC is beneficial to the dissolution of sodium salts and the improvement of ionic conductivity, further enhancing the performance of the electrolyte. However, from a practical point of view, such electrolytes have the following problems:

[0004] (1) Poor compatibility with sodium metal anode. EC-based electrolytes are prone to side reactions with sodium metal, resulting in a high content of organic components in the solid electrolyte interface (SEI), which is thick and easily broken, thereby causing capacity attenuation and a decline in cycle performance.

[0005] (2) High viscosity. The high viscosity of EC will reduce the ionic conductivity of the electrolyte, affecting the rate performance of the battery. And at low temperatures, the viscosity further increases, resulting in a significant decline in battery performance.

[0006] In addition, the commonly used ether-based electrolyte is 1 mol / L sodium hexafluorophosphate (NaPF6) dissolved in ethylene glycol dimethyl ether (DME) solvent. Due to the low viscosity and high ionic conductivity of DME, this electrolyte has excellent sodium-ion conductivity and can significantly improve the rate performance of the battery. At the same time, the low melting point characteristic of DME enables it to maintain a high ionic conductivity even in a low-temperature environment, making it suitable for application in a wide temperature range and enhancing the environmental adaptability of the battery. However, the existing ether-based electrolytes still have the following problems:

[0007] (1) High volatility. The volatility of DME is relatively high, which may cause electrolyte loss during the long-term use of the battery, affecting the life and safety of the battery.

[0008] (2) Poor compatibility with common cathodes. Such electrolytes are prone to irreversible parasitic reactions with cathode materials, and the cathode-electrolyte interface (CEI) is unstable, limiting the application scope of the battery.

[0009] Therefore, to break through the limitations of the capacity release of the positive and negative electrodes and the instability of long cycles in sodium secondary batteries, constructing a new solid-liquid interface protection system is an effective path. Although a slightly soluble electrolyte was also developed in CN 116826179A, its applicable object is metal-chalcogen batteries, while the electrolyte of the present invention is applicable in the range of sodium-ion batteries and sodium metal batteries. Summary of the Invention

[0010] The first object of the present invention is to provide a slightly soluble electrolyte for sodium secondary batteries to solve the problems existing in conventional electrolytes. This slightly soluble electrolyte can induce the distribution of non-metallic fluorides, fundamentally solving the problem of the instability of the solid-liquid interface between the electrode and the electrolyte; at the same time, it can reduce the side reaction between the solvent and the cathode, improve the capacity release of the cathode; and it can also extend the cycle life of the battery and improve the storage performance of the battery.

[0011] The second object of the present invention is to provide an application of a sodium secondary battery.

[0012] The third object of the present invention is to provide an electrical device.

[0013] To achieve the above objects of the present invention, the following technical solutions are specifically adopted: A slightly soluble electrolyte, comprising a solvent and an electrolyte salt with a molar ratio of 0.3 to 4; wherein, the dielectric constant of the solvent is 5 to 50, including an organic solvent containing heteroatoms, and the molecular chain is a chain-like molecule; the organic solvent includes at least one of carbonate solvents and ether solvents, and can dissolve lithium, sodium, and magnesium salts.

[0014] The heteroatoms include at least one of O, S, and N atoms.

[0015] The carbonate solvents include at least one of ethylene carbonate, triethyl phosphate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, and diphenyl carbonate.

[0016] The ether solvents include at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, ethylene glycol diethyl ether, dibutyl ether, and polyethylene glycol dimethyl ether.

[0017] The electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium trifluoromethanesulfonate, and sodium bis(oxalato)borate, and the molar concentration of the electrolyte salt in the slightly soluble electrolyte is 0.3 to 4 mol / L.

[0018] Application of the slightly soluble electrolyte in a sodium secondary battery.

[0019] The sodium secondary battery further includes a positive electrode sheet, a negative electrode sheet and a separator. The positive electrode sheet is mainly prepared from a positive electrode active material, a conductive agent and a binder. The positive electrode active material includes at least one of sodium vanadium phosphate, sodium fluorovanadate, and an O3-type layered oxide having the following formula I: NaA x B y C z D (1-x-y-x) O2, wherein A, B, C, and D are each independently selected from one of Co, Ni, Cu, Mn, and Fe, 0≤x<1, 0≤y<1, 0≤z<1, 0≤x + y + z<1.

[0020] The conductive agent includes at least one of carbon nanotubes, carbon black, vapor deposition carbon fibers, and graphene; the binder includes at least one of polyvinylidene fluoride, polyvinylidene difluoride, polytetrafluoroethylene, styrene-butadiene rubber, carboxymethyl cellulose salt, polyacrylic acid, polyacrylonitrile, polyacrylamide, sodium alginate, and chitosan; the separator includes at least one of a polypropylene (PP) separator, an inorganic ceramic-coated separator, and a glass fiber separator.

[0021] The negative electrode material includes at least one of graphite, HC, metallic sodium, and a metal capable of forming an alloy with sodium.

[0022] Application of the slightly soluble electrolyte in an electrical equipment.

[0023] Beneficial effects:

[0024] (1) The slightly soluble electrolyte provided by the present invention has the following advantages: In the slightly soluble electrolyte of the present invention, there is a strong coordination effect between the solvent and the salt, forming more contact ion pairs and aggregates, and the free solvent molecules are significantly reduced. This excellent solvation structure promotes the easier electrochemical reduction of anions, successfully inducing the distribution of non-metal fluorides at the electrode-electrolyte interface, and a SEI / CEI film with a higher proportion of inorganic elements is enriched on the electrode surface. This structural change of the electrolyte effectively improves the battery performance. At the same time, the formation of the interface also effectively inhibits the formation of dendrites and significantly improves the safety of the sodium-ion battery.

[0025] (2) The sodium secondary battery based on the slightly soluble electrolyte provided by the present invention has very excellent cycle stability. In the slightly soluble electrolyte, due to the formation of a flat and thin electrode-electrolyte interface, the cycle stability of the entire sodium secondary battery is improved. Taking the sodium metal battery as an example, the battery can stably cycle for more than 1000 hours in the slightly soluble electrolyte.

[0026] (3) The sodium secondary battery provided by the present invention has outstanding electrochemical performance due to the use of a slightly soluble electrolyte. For the sodium metal||HC(HC) negative electrode battery, the initial reversible capacity is effectively improved, and a high capacity retention rate is still maintained after multiple cycles, with an average Coulombic efficiency as high as 99.8%.

[0027] (4) The slightly soluble electrolyte provided by the present invention broadens the range of available electrolyte salts. For example, the advantages of borate salts, such as their poor solubility in many ionic liquids and low cost, are difficult to utilize. Conventional electrolytes often use salts that can be completely dissolved, such as sodium hexafluorophosphate, sodium perchlorate, etc. This slightly soluble electrolyte effectively broadens the range of available electrolyte salts, has higher and more universal functionality, and has greater application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 It is a schematic diagram of the long-term cycling performance of the sodium metal symmetric battery provided in Example 1 and Comparative Example 1 at a current density of 1 mA / cm2.

[0030] Figure 2 It is the 0.2, 0.5, 1, 2, 5, 10, 0.2C rate diagram of the sodium metal symmetric battery provided in Example 1 and Comparative Example 1.

[0031] Figure 3 It is the voltage curve diagram of the sodium||copper battery provided in Example 2.

[0032] Figure 4 It is the voltage curve diagram of the sodium||copper battery provided in Comparative Example 2.

[0033] Figure 5 It is a schematic diagram of the long-term cycling performance of the sodium||HC batteries provided in Example 3, Comparative Example 3, and Comparative Example 6.

[0034] Figure 6 It is the voltage-capacity curve diagram of the sodium||HC batteries provided in Example 3, Comparative Example 3, and Comparative Example 6 at the 800th cycle.

[0035] Figure 7 It is the cyclic voltammetry (CV) diagram of the sodium||HC battery provided in Example 3.

[0036] Figure 8 It is the CV diagram of the sodium||HC battery provided in Comparative Example 3.

[0037] Figure 9 It is the X-ray photoelectron spectroscopy (XPS) spectrum analysis diagram of the sodium||HC battery provided in Example 3.

[0038] Figure 10 It is the XPS spectrum analysis diagram of the sodium||HC battery provided in Comparative Example 3.

[0039] Figure 11 It is the transmission electron microscope (TEM) diagram of the HC electrode sheet of the sodium||HC battery provided in Example 3 and Comparative Example 3.

[0040] Figure 12 It is the schematic diagram of the long cycle performance of the sodium||NaNi 0.4 Fe 0.2 Mn 0.4 O2 (NFM) battery.

[0041] Figure 13 It is the voltage-capacity curve diagram of the 1st cycle of the sodium||NFM battery provided in Example 4, Comparative Example 4, and Comparative Example 7.

[0042] Figure 14 It is the XPS spectrum analysis diagram of the sodium||NFM battery provided in Example 4.

[0043] Figure 15 It is the XPS spectrum analysis diagram of the sodium||NFM battery provided in Comparative Example 4.

[0044] Figure 16 It is the schematic diagram of the cycle performance of the NFM||HC battery provided in Example 5 and Comparative Example 5.

[0045] Figure 17 It is the voltage-capacity curve diagram of the 80th cycle of the NFM||HC battery provided in Example 5 and Comparative Example 5.

[0046] Figure 18 It is the voltage-capacity curve diagram of the 80th cycle of the NFM||HC battery provided in Example 6 and Comparative Example 8.

[0047] Figure 19 It is the voltage-capacity curve diagram of the 80th cycle of the NFM||HC battery provided in Example 7 and Comparative Example 9. Detailed implementation manners

[0048] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0049] In a first aspect, the present invention provides a slightly soluble electrolyte for a sodium secondary battery, comprising a solvent and an electrolyte salt with a molar ratio of 0.3 to 4. The molar ratio range includes, but is not limited to, any point value among 0.3, 0.5, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4 or the range value between any two of them.

[0050] Conventional sodium secondary batteries have problems such as low energy density, low Coulomb efficiency, poor cycle stability, and serious self-discharge phenomenon. In view of the problems in the conventional electrolytes of existing sodium secondary batteries, the present invention creatively proposes a slightly soluble electrolyte and a sodium secondary battery system based on the slightly soluble electrolyte. Specifically, the slightly soluble electrolyte refers to an electrolyte system that adjusts the ratio relationship between the solvent and the salt to induce the distribution of non-metal fluorides.

[0051] Among them, the dielectric constant of the solvent ≥ 5, including, but not limited to, any point value among 6, 8, 10, 13, 15, 18, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70 or the range value between any two of them. The molecules of the solvent provided by the present invention have strong polarity, that is, have a high dielectric constant, which can ensure that the solvent molecules can dissolve lithium salts, sodium salts, magnesium salts, etc. to a certain extent and ensure the ionic conductivity of the electrolyte.

[0052] Therefore, using the above molar ratio range can ensure that the slightly soluble electrolyte has a high ionic conductivity.

[0053] The composition and solvation structure of the electrolyte are the key factors determining the electrochemical performance of sodium secondary batteries. Based on the innovation of the electrolyte composition and its properties, the present invention constructs an improved electrolyte by regulating the solvation structure of the electrolyte, and the salt exceeding the solubility can provide excess anions to be wrapped in the solvation layer.

[0054] Specifically, the electrode-electrolyte interface of a conventional sodium secondary battery in the electrolyte is mainly composed of organic substances. In the slightly soluble electrolyte of the present invention, due to the coordination effect between the solvent and the salt, more contact ion pairs and aggregates are formed, and the number of free solvent molecules is significantly reduced, forming a more ideal solvation structure. Therefore, the SEI / CEI film on the electrode surface contains a large amount of inorganic elements generated by the reduction decomposition of anions, making the SEI / CEI film have good properties. For example, the SEI / CEI layer containing oxides and borides usually has high mechanical strength, reducing the damage of the electrode structure; NaE (sodium-based compound) is beneficial to the rapid transmission of Na + ; the boron-containing oxide can also act as a coating layer to prevent the direct contact between the positive electrode material and the electrolyte, reducing the occurrence of side reactions. Therefore, the SEI / CEI film formed in the slightly soluble electrolyte is relatively stable, which can reduce the generation of dendrites and improve the safety of the sodium secondary battery.

[0055] Therefore, the special features of the slightly soluble electrolyte provided by the present invention are as follows: (1) There is a strong coordination effect between the solvent and the salt, successfully inducing the distribution of non-metal fluorides at the electrode-electrolyte interface; (2) The number of free solvent molecules is reduced, reducing the side reactions of the solvent and improving the safety of the electrode; (3) Significantly reducing the self-discharge phenomenon of the battery and improving the storage performance of the battery.

[0056] Preferably, the dielectric constant of the solvent is 5 to 50; including but not limited to the point value of any one of 6, 8, 10, 13, 15, 18, 20, 25, 30, 35, 40, 45, 50 or the range value between any two of them.

[0057] Preferably, the solvent includes an organic solvent containing heteroatoms.

[0058] In terms of the molecular chain, the solvent is a chain-like molecule.

[0059] More preferably, the heteroatoms include at least one of O, S, and N atoms.

[0060] Preferably, the solvent includes at least one of carbonate solvents and ether solvents.

[0061] More preferably, the carbonate solvents include at least one of ethylene carbonate, triethyl phosphate, propylene carbonate, butylene carbonate, fluorinated ethylene carbonate, and diphenyl carbonate.

[0062] More preferably, the ether solvents include at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dibutyl ether, and polyethylene glycol dimethyl ether.

[0063] The solvents of the above types have strong polarity, with dielectric constants all ≥5, and have strong ability to dissolve salts such as lithium salts, sodium salts and magnesium salts, which can ensure the ionic conductivity of the electrolyte.

[0064] Preferably, the electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium trifluoromethanesulfonate, sodium bis(oxalato)borate.

[0065] Preferably, the molar concentration of the electrolyte salt in the slightly soluble electrolyte is 0.3 - 4 mol / L, including but not limited to the point values of any one of 0.3 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L or the range values between any two of them.

[0066] In a specific embodiment, the preparation method of the slightly soluble electrolyte for the sodium secondary battery is: mixing each component evenly.

[0067] In a second aspect, the present invention provides a sodium secondary battery, including the slightly soluble electrolyte for the sodium secondary battery described above.

[0068] The electrolyte provided by the present invention has the advantages of good chemical stability, good thermal stability and stability to sodium. The sodium secondary battery based on the slightly soluble electrolyte has the advantages of high energy density, long cycle life, high safety and can be stored for a long time, and has broad application prospects in the fields of power batteries, portable devices, large-scale energy storage power stations, etc.

[0069] Taking the sodium-ion battery as an example, in the slightly soluble electrolyte constructed with diethylene glycol dimethyl ether and sodium tetrafluoroborate, the sodium-sodium symmetric battery can stably cycle for more than 1500 hours.

[0070] Preferably, the sodium secondary battery further includes a positive electrode plate, a negative electrode plate and a separator.

[0071] More preferably, the positive electrode plate is mainly made of a positive electrode active material, a conductive agent and a binder.

[0072] More preferably, the positive electrode active material includes at least one of sodium vanadium phosphate, sodium fluorovanadate, and O3-type layered oxides having the following formula I:

[0073] NaA x B y C z D (1-x-y-x) O2 (formula I),

[0074] wherein, A, B, C, D are each independently selected from one of Co, Ni, Cu, Mn, Fe,

[0075] 0 ≤ x < 1, 0 ≤ y < 1, 0 ≤ z < 1, 0 ≤ x + y + z < 1.

[0076] More preferably, the conductive agent includes at least one of carbon nanotubes, carbon black, vapor deposition carbon fibers, and graphene.

[0077] More preferably, the binder includes at least one of polyvinylidene fluoride, polyvinylidene difluoride, polytetrafluoroethylene, styrene-butadiene rubber, carboxymethyl cellulose salt, polyacrylic acid, polyacrylonitrile, polyacrylamide, sodium alginate, and chitosan.

[0078] Preferably, the negative electrode material includes at least one of graphite, HC, metallic sodium, and a metal capable of forming an alloy with sodium.

[0079] In a specific embodiment, in the sodium secondary battery, the amount of the sodium secondary electrolyte is 1 - 40 μL / mg, including but not limited to any one of the point values of 1 μL / mg, 3 μL / mg, 5 μL / mg, 8 μL / mg, 10 μL / mg, 15 μL / mg, 20 μL / mg, 25 μL / mg, 30 μL / mg, 35 μL / mg, 40 μL / mg or the range values between any two of them. Herein, the unit μL / mg refers to the ratio of the volume of the sodium secondary electrolyte to the mass of the positive electrode active material.

[0080] In a third aspect, the present invention provides an electrical device including the sodium secondary battery described above.

[0081] The following will describe the embodiments of the present invention in detail in conjunction with the examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. For those not specified in the examples, they are carried out under conventional conditions or the conditions recommended by the manufacturer. Those reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0082] In the following examples, a coin cell configuration was used for testing, and the battery assembly was carried out in a glove box with a moisture content of less than 0.1 ppm and an oxygen content of less than 0.1 ppm.

[0083] Example 1

[0084] The composition of the sodium secondary electrolyte provided in this example is: diethylene glycol dimethyl ether and sodium tetrafluoroborate with a molar ratio of 7:3.

[0085] Both the positive and negative electrodes are sodium sheets with a thickness of 500 μm.

[0086] Sodium || The assembly steps of the sodium battery are as follows: In a glove box under argon protection, using the above electrolyte, based on the CR2025 coin cell configuration, with a Celgard 2400PP separator, and the electrolyte dosage is 50 μL.

[0087] Example 2

[0088] The negative electrode is a sodium sheet with a thickness of 500 μm.

[0089] The positive electrode composition is copper foil.

[0090] Sodium || The assembly steps of the sodium - copper battery are the same as those in Example 1 in terms of electrolyte composition.

[0091] Example 3

[0092] The negative electrode is a sodium sheet with a thickness of 500 μm.

[0093] The positive electrode sheet composition is: HC, acetylene black, and polyvinylidene fluoride with a mass ratio of 8:1:1.

[0094] Sodium || The assembly steps of the sodium - HC battery are the same as those in Example 1 in terms of electrolyte composition.

[0095] Example 4

[0096] The positive electrode sheet composition is: NFM, acetylene black, and polyvinylidene fluoride with a mass ratio of 8:1:1.

[0097] The negative electrode is a sodium sheet with a thickness of 500 μm.

[0098] Sodium || The assembly steps of the sodium - NFM battery are the same as those in Example 1 in terms of electrolyte composition.

[0099] Example 5

[0100] The positive electrode sheet composition is: NFM, acetylene black, and polyvinylidene fluoride with a mass ratio of 8:1:1.

[0101] The negative electrode sheet composition is: HC, acetylene black, and polyvinylidene fluoride with a mass ratio of 8:1:1.

[0102] NFM || The assembly steps of the NFM - HC battery are as follows: In a glove box under argon protection, using the above electrolyte, based on the CR2025 coin cell configuration, with a Celgard 2400PP separator, and the electrolyte dosage is 50 μL.

[0103] Example 6

[0104] The composition of the sodium secondary electrolyte provided in this example is: triethyl phosphate and sodium bis(oxalate)borate with a molar ratio of 8:3.

[0105] NFM || The assembly steps and electrodes of the NFM - HC battery are the same as those in Example 5.

[0106] Example 7

[0107] The composition of the sodium secondary electrolyte provided in this example is: ethylene glycol diethyl ether and sodium difluorophosphate with a molar ratio of 10:3.

[0108] The NFM||HC battery assembly steps and electrodes are the same as those in Example 5.

[0109] Comparative Examples 1-5

[0110] The electrodes, electrolyte compositions, and battery assembly steps of Comparative Examples 1-5 are basically the same as those of Examples 1-5, except that the ratio of diethylene glycol dimethyl ether and sodium tetrafluoroborate is adjusted to 7:1.

[0111] Comparative Examples 6, 7

[0112] The electrode compositions and battery assembly steps of Comparative Examples 6 and 7 are the same as those of Examples 3 and 4, but the electrolyte composition is diethylene glycol dimethyl ether and sodium hexafluorophosphate with a ratio of 7:1.

[0113] Comparative Example 8

[0114] The electrode composition and battery assembly steps of Comparative Example 8 are the same as those of Example 6, but the electrolyte composition is triethyl phosphate and sodium bis(oxalato)borate with a ratio of 8:1.

[0115] Comparative Example 9

[0116] The electrode composition and battery assembly steps of Comparative Example 9 are the same as those of Example 7, but the electrolyte composition is ethylene glycol diethyl ether and sodium difluorophosphate with a ratio of 10:1.

[0117] The following is a specific description of the present invention with reference to the accompanying drawings:

[0118] Electrochemical performance tests were carried out on Example 1 and Comparative Example 1, and the results are shown in Figure 1 and Figure 2 as shown. As Figure 1 shown, Example 1 can stably cycle for more than 1000 hours at a current density of 1 mA / cm 2 , while Comparative Example 1 shows obvious overpotential fluctuations after 300 hours. As Figure 2 shown, at different current densities, the overpotential curve of Example 1 remains stable without obvious fluctuations, indicating that no obvious dendrites are formed.

[0119] The voltage curves of Example 2 and Comparative Example 2 are shown in Figure 3 , 4 as shown. The initial overpotential of Example 2 during sodium plating / stripping is 5.9 mV, which is only half of that of Comparative Example 2 (10 mV), and the voltage change is also smaller after 200 cycles, indicating that the sodium plating / stripping process in Example 2 is easier and the irreversible loss is less.

[0120] Constant current charge-discharge tests were carried out on Example 3, Comparative Example 3, and Comparative Example 6, and the experimental results are as Figure 5 , 6 shown. At a 0.2C rate (1C = 300 mAh / g), the initial reversible capacity of Example 3 was 263.92 mAh / g, and long cycling of more than 1000 times (9 months) was achieved. After 800 cycles, the reversible capacity was still as high as 243.75 mAh / g, while maintaining a plateau capacity of approximately 54.05% of the total capacity.

[0121] CV curve tests were carried out on Example 3 and Comparative Example 3, as Figure 7 , 8 shown. Example 3 showed obvious redox peaks compared with the cycled batteries in Comparative Example 3, indicating significant differences in the Na storage kinetics.

[0122] X-ray photoelectron spectroscopy (XPS) analysis and transmission electron microscopy (TEM) imaging were carried out on Example 3 and Comparative Example 3, and the results are as Figures 9 - 11 shown. In the F 1s spectrum, the Na-F peak at 684 eV in Example 3 was much higher than the C-F peak at 688.5 eV, while it was exactly the opposite in the comparative example. The above indicates that there are more NaF inorganic substances on the HC negative electrode of Example 3, while the proportion of organic substances on the HC negative electrode of Comparative Example 3 is larger. Figure 11 The SEI layer image of

[0123] also showed that compared with Comparative Example 3, the SEI interface formed in Example 3 was thinner and flatter. Figure 12 , 13 shown. The initial capacity of Example 4 was 136 mAh / g, and it still showed stable capacity release with excellent cycling stability when the specific capacity of Comparative Example 8 gradually decreased to 0 at about 100 cycles and the specific capacity of Comparative Example 4 decreased sharply at about 270 cycles.

[0124] XPS energy spectrum analysis was carried out on Example 4 and Comparative Example 4, and the experimental results are as Figure 14 , 15 shown. Similar to Experimental Example 6, the F 1s spectrum still consisted of Na-F bonds and C-F bonds, and the ratio of the former two to the latter in Example 4 was significantly higher than that in Comparative Example 4. Moreover, there was no obvious peak in the B 1s spectrum of Comparative Example 4, indicating that the B content was extremely low.

[0125] Constant current charge-discharge tests were carried out on Example 5 and Comparative Example 5, and the experimental results are as Figure 16 , 17As shown. The initial specific capacity of Example 5 reached 158 mAh / g, which is 30 mAh / g higher than the 126 mAh / g observed in the comparative example.

[0126] Constant current charge-discharge tests were carried out on Example 6 and Comparative Example 8, and the results are as Figure 18 shown. The overall curve of Example 6 is significantly more stable. At the same time, in the stage of voltage drop, it has a better slope region capacity. While the charge-discharge curve of Comparative Example 8 has obvious jitters and the charge-discharge process is not stable. It shows that in Example 6, ions can be continuously inserted and extracted, and the reaction is stable.

[0127] Constant current charge-discharge tests were carried out on Example 7 and Comparative Example 9, and the results are as Figure 19 shown. The curves of both are relatively stable, but the specific capacity of Example 7 is significantly greater than that of Comparative Example 9, and it has better capacity retention ability, further confirming the advantage of the example in electrochemical performance.

[0128] Through Figure 1 Table 1 can be obtained, and it can be seen that the electrolyte of the present invention effectively protects the surface of the sodium metal negative electrode and enhances the interfacial stability:

[0129]

[0130] Through Figure 5 、 6 Table 2 can be obtained, and it can be seen that the improved electrolyte of the present invention effectively protects the HC negative electrode and enhances the interfacial stability:

[0131]

[0132]

[0133] Through Figure 12 、 13 Table 3 can be obtained, and it can be seen that the improved electrolyte of the present invention effectively protects the NFM positive electrode and enhances the interfacial stability:

[0134]

[0135] Through Figures 16 - 19 Table 4 can be obtained, and it can be seen that the improved electrolyte of the present invention effectively improves the interfacial stability by changing the interfacial composition, thereby effectively improving the battery performance:

[0136] <![CDATA[Specific capacity of the 80th cycle (mAh g -1 )]]> Example 5 97.01 Comparative Example 5 74.76 Example 6 117.10 Comparative Example 8 80.93 Example 7 85.21 Comparative Example 9 72.47

[0137] Although the present invention has been illustrated and described with reference to specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it; those of ordinary skill in the art should understand that without departing from the spirit and scope of the present invention, the technical solutions described in the foregoing embodiments may be modified, or some or all of the technical features may be equivalently replaced; and 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; therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A slightly soluble electrolyte, characterized in that: It comprises a solvent and an electrolyte salt in a molar ratio of 0.3 to 4; wherein the dielectric constant of the solvent is 5 to 50, and the solvent comprises an organic solvent containing heteroatoms, and the molecular chain is a chain molecule; the organic solvent comprises at least one of a carbonate solvent and an ether solvent, and can dissolve lithium, sodium, and magnesium salts.

2. The slightly soluble electrolyte according to claim 1, characterized in that The heteroatom includes at least one of O, S and N atoms.

3. The slightly soluble electrolyte according to claim 1, characterized in that The carbonate solvent includes at least one of ethylene carbonate, triethyl phosphate, propylene carbonate, butylene carbonate, fluoroethylene carbonate and diphenyl carbonate.

4. The slightly soluble electrolyte according to claim 1, characterized in that The ether solvent includes at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, ethylene glycol diethyl ether, dibutyl ether, and polyethylene glycol dimethyl ether.

5. The slightly soluble electrolyte according to claim 1, characterized in that The electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium trifluoromethanesulfonate, and sodium bis(oxalato)borate, and the molar concentration of the electrolyte salt in the slightly soluble electrolyte is 0.3-4 mol / L.

6. Use of the slightly soluble electrolyte according to any one of claims 1 to 5 in a sodium secondary battery.

7. The use according to claim 6, wherein the sodium secondary battery further comprises a positive electrode sheet, a negative electrode sheet and a separator, characterized in that: The positive electrode sheet is mainly made of positive electrode active material, conductive agent and binder; the positive electrode active material includes sodium vanadium phosphate, sodium vanadium fluorophosphate, and at least one of the O3-type layered oxides having the following formula I: NaA x B y C z D (1-x-y-x) O2, wherein A, B, C, and D are each independently selected from one of Co, Ni, Cu, Mn, and Fe, and 0≤x<1, 0≤y<1, 0≤z<1, and 0≤x+y+z<1.

8. The use according to claim 6, characterized in that The conductive agent includes at least one of carbon nanotubes, carbon black, vapor-deposited carbon fibers and graphene; the binder includes at least one of polyvinylidene fluoride, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, carboxymethyl cellulose salt, polyacrylic acid, polyacrylonitrile, polyacrylamide, sodium alginate and chitosan; and the diaphragm includes at least one of a polypropylene diaphragm, an inorganic ceramic coated diaphragm and a glass fiber diaphragm.

9. The use according to claim 6, characterized in that The negative electrode material includes at least one of graphite, HC, metallic sodium and a metal capable of forming an alloy with sodium.

10. Use of the slightly soluble electrolyte according to any one of claims 1 to 5 in electrical equipment.

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