Non-aqueous electrolyte for sodium ion battery and application of non-aqueous electrolyte

Abstract

The invention relates to a non-aqueous electrolyte for a sodium-ion battery and application of the non-aqueous electrolyte. The non-aqueous electrolyte comprises the following components: electrolyte, an organic solvent, a first additive and a second additive, the chemical structure of the first additive is shown as a formula I; the chemical structure of the second additive is shown as a formula II. According to the non-aqueous electrolyte disclosed by the invention, two additives with specific structures are creatively added into a system, and the two additives cooperate with each other, so that the cycle impedance of the sodium-ion battery can be reduced, the cycle life of the sodium-ion battery can be prolonged, the dissolution of transition metal ions of the sodium-ion battery can be reduced, and the cycle stability of the sodium-ion battery can be improved; the cycle performance and the storage performance of the sodium-ion battery can be effectively improved, and the application requirements of the current sodium-ion battery are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium batteries and relates to a non-aqueous electrolyte for sodium-ion batteries and its application. Background Art

[0002] Sodium-ion batteries (SIBs) are sustainable and economical and have received increasing attention as an alternative secondary battery system to lithium-ion batteries (LIBs). Sodium is more abundant and evenly distributed in the earth's crust and has multiple potential sources. However, sodium-ion batteries are currently still mainly considered as a supplement to lithium-ion batteries, with slightly lower performance indicators, so they have not been widely applied on a large scale. Recent studies have shown that if an appropriate battery-pack-level energy density can be achieved, sodium-ion batteries may reverse this trend. Currently, breaking through the energy density barrier of 200 Wh / kg is regarded as the goal for entering the mobile application market. In fact, this transformation may start with small electric vehicles and eventually become an alternative to the widespread application of lithium-ion batteries. To achieve this, the development and application of high-performance electrode materials are crucial.

[0003] The continuous climate change has greatly increased the demand for green energy, leading to an increased interest in high-energy-density sodium-ion batteries, thus promoting the rapid growth of the development of electric vehicles and energy storage systems. Layered oxide cathodes have been widely studied as effective candidate materials for sodium-ion batteries. However, during the repeated insertion and extraction of sodium ions, the crystal structure of layered oxides is prone to changes, such as phase transformation, interlayer slip, etc., resulting in a decrease in the structural stability of the material, thereby affecting the cycle life of the battery. After multiple charge-discharge cycles, the capacity of the battery will gradually decay; under some high-temperature or extreme conditions, the thermal stability of the layered oxide cathode material may not be ideal, presenting potential safety hazards. This places higher requirements on the battery's thermal management system, increasing the complexity and cost of the battery system.

[0004] To solve the above problems, it is necessary to construct a high-quality cathode electrolyte interface (CEI) to mitigate the parasitic reaction between the electrolyte and the layered oxide cathode and design an electrolyte that can withstand high-voltage operation. Therefore, in order to be able to inhibit the decomposition of the electrolyte to generate gas, reduce the dissolution of metal ions in the cathode material, and reduce the generation of corrosive compounds, thereby improving the performance and reliability of sodium-ion batteries, it is of great significance to develop a new type of sodium-ion battery electrolyte. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a non-aqueous electrolyte for sodium-ion batteries and its application, specifically providing a non-aqueous electrolyte for sodium-ion batteries and a sodium-ion battery. This non-aqueous electrolyte can effectively improve the cycle performance and storage performance of the layered oxide system and achieve the goal of high energy density of the battery cell.

[0006] To achieve the object of the present invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a non-aqueous electrolyte for a sodium-ion battery, and the components of the non-aqueous electrolyte include an electrolyte, an organic solvent, a first additive, and a second additive;

[0008] The chemical structure of the first additive is shown in Formula I; the chemical structure of the second additive is shown in Formula II;

[0009]

[0010] Wherein, R1, R2, R3, and R4 are each independently selected from a substituted or unsubstituted C1-C5 alkyl group (such as a C1 alkyl group, a C2 alkyl group, a C3 alkyl group, a C4 alkyl group, a C5 alkyl group), a substituted or unsubstituted C2-C5 alkenyl group (such as a C2 alkenyl group, a C3 alkenyl group, a C4 alkenyl group, a C5 alkenyl group), a substituted or unsubstituted C2-C5 alkynyl group (such as a C2 alkynyl group, a C3 alkynyl group, a C4 alkynyl group, a C5 alkynyl group), and a substituted or unsubstituted phenyl group; the substituting groups of the substituted group are selected from any one or a combination of at least two of a C1-C3 alkyl group (such as a C1 alkyl group, a C2 alkyl group, a C3 alkyl group), a hydroxyl group, a halogen (such as F, Cl, Br), a cyano group, and a nitro group.

[0011] The non-aqueous electrolyte involved in the present invention creatively adds two additives shown in the above general formula to the system. The two additives cooperate synergistically, which can effectively improve the cycling performance and storage performance of the sodium-ion battery and achieve the goal of stable cycling of the battery.

[0012] Among them, the chemical structure of the first additive contains three cyano groups, and the bond energy of the carbon-nitrogen triple bond is relatively high and not easily oxidized. Therefore, it has good stability and strong antioxidant properties on the positive electrode, thereby improving the oxidation resistance of the electrolyte, being able to effectively complex transition metal ions in the positive electrode material, inhibiting the dissolution of transition metals in the positive electrode, and improving the stability of the positive electrode material; it can participate in the formation of a stable solid electrolyte interface (SEI) film on the surface of the battery positive electrode, which can prevent solvent molecules from co-embedding into the electrode material and reduce the structural damage of the electrode material during charge and discharge, thereby improving the cycle life of the battery.

[0013] Among them, the second additive helps to improve the stability of the battery in a high-temperature environment. During the charge and discharge process of the battery, especially under high-temperature conditions, the electrolyte is prone to decomposition and other reactions, affecting the battery performance and safety. The second additive can reduce the decomposition reaction activity of the electrolyte, thereby enhancing the thermal stability of the battery, improving the high-temperature cycling performance and storage performance of the battery. The second additive can also improve the compatibility between the electrode and the electrolyte, reduce adverse reactions on the electrode surface, and improve the Coulomb efficiency and charge-discharge efficiency of the battery.

[0014] Both the first additive and the second additive involved in the present invention can be prepared by conventional technical methods in the art or synthetic routes disclosed in the prior art.

[0015] Preferably, the components of the non-aqueous electrolyte include 2-22% of electrolyte, 75-94% of organic solvent, 0.2-4% of the first additive, and 0.2-2% of the second additive by mass percentage.

[0016] The mass percentage of the electrolyte can be selected as 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, etc.; the mass percentage of the organic solvent can be selected as 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 94%, etc.; the mass percentage of the first additive can be selected as 0.3%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc.; the mass percentage of the second additive can be selected as 0.3%, 0.5%, 0.8%, 1%, 1.5%, 2%, etc.

[0017] In the present invention, the cooperation of specific contents of the first additive and the second additive can more significantly reduce the cycle impedance of the sodium-ion battery and improve its cycle stability; when the content of the second additive is too low, the formed interface film is not stable enough; when its content is too high, the battery impedance is large, affecting the battery performance. When the content of the second additive is 0.4-1% and the content of the first additive is 0.4-1.5%, the comprehensive performance of the prepared sodium-ion battery is the best, making the sodium-ion battery not only have good room-temperature capacity retention rate and high high-temperature capacity retention rate, thereby reducing the cycle impedance of the sodium-ion battery and further enhancing the cycle performance of the battery; but also improving the cycle stability during the battery cycle and suppressing the volume expansion of the soft-pack battery during high-temperature storage.

[0018] Preferably, the mass ratio of the first additive to the second additive is (0.5-2):1, such as 0.5:1, 0.8:1, 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2:1, etc.

[0019] Based on the potential interaction between the two additives, when the mass ratio of the two satisfies the above specific numerical range, the synergistic effect of the two reaches the best, and the prepared sodium-ion battery has more excellent room-temperature capacity retention rate, higher high-temperature capacity retention rate and lower high-temperature storage gas evolution volume expansion rate.

[0020] Preferably, the electrolyte is a sodium salt.

[0021] Preferably, the sodium salt includes any one or a combination of at least two of sodium hexafluorophosphate, sodium perchlorate, sodium bis(trifluoromethanesulfonyl)imide, sodium difluoro(oxalato)borate, or sodium bis(fluorosulfonyl)imide.

[0022] Preferably, the organic solvent includes a carbonate organic solvent and / or a carboxylate organic solvent.

[0023] Preferably, the carboxylate organic solvent includes any one or a combination of at least two of ethyl propionate, propyl propionate, ethyl acetate, ethyl n-butyrate, methyl acetate, propyl acetate, methyl propionate, or γ-butyrolactone.

[0024] Preferably, the carbonate organic solvent includes a cyclic carbonate and / or a linear carbonate; more preferably a combination of a cyclic carbonate and a linear carbonate.

[0025] When the organic solvent in the non-aqueous electrolyte system involved in the present invention selects a combination of a cyclic carbonate and a linear carbonate, and more preferably at a volume ratio of 1:(1 - 4), it is beneficial to increase the overall dielectric constant of the electrolyte, and can also ensure that the obtained non-aqueous electrolyte has a small viscosity and good wettability on the surface of the sodium-ion battery electrode sheet, ensuring the good operation of the sodium battery and endowing the sodium battery with more excellent electrochemical performance and stability.

[0026] Preferably, the volume ratio of the cyclic carbonate to the linear carbonate is 1:(1 - 4), such as 1:1, 2:3, 1:2, 2:5, 1:3, 3:7, 1:4, etc.

[0027] Preferably, the cyclic carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, or butylene carbonate.

[0028] Preferably, the linear carbonate includes any one or a combination of at least two of diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, or methyl propyl carbonate.

[0029] Preferably, the components of the non-aqueous electrolyte further include a third additive, and the third additive includes any one or a combination of at least two of vinylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, ethylene sulfite sulfate, ethylene sulfate, methylene methanedisulfonate, ethylene vinylene carbonate, tris(trimethylsilyl) phosphate, or tris(trimethylsilyl) borate.

[0030] Preferably, based on the total mass of the non-aqueous electrolyte being 100%, the mass percentage content of the third additive is 0.5 - 11.6%, such as 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, etc.

[0031] Preferably, the components of the non-aqueous electrolyte further include a sodium salt additive; the sodium salt additive includes any one or a combination of at least two of sodium bis(fluorosulfonyl)imide, sodium difluoro(oxalato)borate, sodium tetrafluoroborate, bis(oxalato)borate, or sodium difluorophosphate.

[0032] In the present invention, the use of the sodium salt additive can further effectively improve the capacity retention rate and cycle life of the battery, and can effectively broaden the working temperature range of the sodium-ion battery; at the same time, the sodium salt additive can also preferentially form inorganic NaF, thereby further reducing the interfacial impedance.

[0033] Preferably, based on the total mass of the non-aqueous electrolyte being 100%, the mass percentage content of the sodium salt additive is 0.2-3.0%, such as 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 2.7%, 3.0%, etc.

[0034] Preferably, the first additive is selected from the following compounds:

[0035]

[0036] Preferably, the second additive is selected from any one or a combination of at least two of the following compounds:

[0037]

[0038] The first additive and the second additive with the above specific structures can both be prepared by conventional technical methods in the art or synthetic routes disclosed in the prior art.

[0039] In a second aspect, the present invention provides a sodium-ion battery, which includes a battery cell and an electrolyte. The battery cell includes a positive electrode, a negative electrode, and a separator. The separator is disposed between the positive electrode and the negative electrode, and the electrolyte is the non-aqueous electrolyte described in the first aspect.

[0040] Preferably, the positive electrode includes a positive electrode active material, a conductive agent, and a binder.

[0041] Preferably, the positive electrode active material includes a layered metal oxide.

[0042] Preferably, the negative electrode includes a negative electrode active material.

[0043] Preferably, the negative electrode active material is selected from any one or a combination of at least two of natural graphite, artificial graphite, soft carbon, hard carbon, mesophase carbon microspheres, graphene, graphdiyne, metallic sodium, nano-carbon, carbon nanotubes, elemental tin, tin oxide compounds, tin-carbon composites, tin alloys, or sodium titanate.

[0044] Preferably, the material of the separator includes any one or a combination of at least two of polyethylene, polypropylene, or composite ceramic membranes.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The non-aqueous electrolyte involved in the present invention creatively adds two additives with specific structures to the system. The two additives cooperate synergistically, which can not only reduce the cycle impedance of sodium-ion batteries and improve their cycle life, but also reduce the dissolution of transition metal ions in sodium batteries and improve their cycle stability, that is, it can effectively improve the cycle performance and storage performance of sodium-ion batteries and meet the current application requirements of sodium-ion batteries. Detailed implementation manners

[0047] To further elaborate on the technical means and effects adopted by the present invention, the following further illustrates the technical solutions of the present invention in combination with the preferred embodiments of the present invention, but the present invention is not limited to the scope of the embodiments.

[0048] The CAS number of the compound of formula A involved in the following embodiments is 875826-91-8, which is purchased from Shandong Binruite New Materials Co., Ltd.; the CAS number of the compound of formula B is 33027-66-6, which is purchased from Suzhou Yak Scientific Co., Ltd.; the preparation method of the compound of formula C refers to Example 2 of CN115873039A; the preparation method of the compound of formula D refers to Example 1 of CN116285003A; the preparation method of the compound of formula E refers to Example 3 of CN116285003A.

[0049] Example 1

[0050] This example provides a non-aqueous electrolyte, and its composition (calculated based on the total amount of 100%) is as follows:

[0051] Electrolyte (sodium hexafluorophosphate) 12.5%, first additive (as shown in formula A below) 1%, second additive (as shown in formula B below) 0.5%, third additive (vinylidene carbonate 2% + fluoroethylene carbonate 2% + tris(trimethylsilyl) borate 0.5%) 4.5%, organic solvent (a mixed solution of ethylene carbonate and ethyl methyl carbonate with a volume ratio of 3:7) the balance.

[0052]

[0053] Example 2

[0054] This example provides a non-aqueous electrolyte, and its composition (calculated based on the total amount of 100%) is as follows:

[0055] Electrolyte (sodium hexafluorophosphate) 12.5%, First additive (as shown in formula A below) 1%, Second additive (as shown in formula C below) 0.5%, Third additive (vinylene carbonate 2% + fluoroethylene carbonate 2% + ethylene sulfate 1%) 5%, Organic solvent (a mixed solution of ethylene carbonate and ethyl methyl carbonate with a volume ratio of 3:7) the balance.

[0056]

[0057] Example 3

[0058] This example provides a non-aqueous electrolyte, and its composition (calculated based on the total amount of 100%) is as follows:

[0059] Electrolyte (sodium hexafluorophosphate) 12.5%, First additive (as shown in formula A below) 1%, Second additive (as shown in formula D below) 0.5%, Third additive (vinylene carbonate 2% + fluoroethylene carbonate 2% + tris(trimethylsilyl) borate 1%) 5%, Organic solvent (a mixed solution of ethylene carbonate and ethyl methyl carbonate with a volume ratio of 3:7) the balance.

[0060]

[0061] Example 4

[0062] This example provides a non-aqueous electrolyte, and its composition (calculated based on the total amount of 100%) is as follows:

[0063] Electrolyte (sodium hexafluorophosphate) 12.5%, First additive (as shown in formula A below) 1%, Second additive (as shown in formula E below) 0.5%, Third additive (vinylene carbonate 2% + fluoroethylene carbonate 2% + tris(trimethylsilyl) borate 0.5%) 4.5%, Organic solvent (a mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 3:7) the balance.

[0064]

[0065] Example 5

[0066] This example provides a non-aqueous electrolyte, and the difference in its composition (calculated based on the total amount of 100%) from that of Example 1 is only the mass ratio of the first additive to the second additive, specifically: First additive 0.3%, Second additive 1.2%. All other components and their contents remain unchanged.

[0067] Example 6

[0068] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the mass ratio of the first additive to the second additive, specifically: the first additive is 1.2% and the second additive is 0.3%. The contents of all other components remain unchanged.

[0069] Example 7

[0070] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the second additive, specifically: the first additive is 1% and the second additive is 0.2%, and the reduced amount is made up by an organic solvent. The contents of all other components remain unchanged.

[0071] Example 8

[0072] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the second additive, specifically: the first additive is 1% and the second additive is 0.3%, and the reduced amount is made up by an organic solvent. The contents of all other components remain unchanged.

[0073] Example 9

[0074] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the second additive, specifically: the first additive is 1% and the second additive is 0.8%, and the content of the organic solvent is correspondingly reduced. The contents of all other components remain unchanged.

[0075] Example 10

[0076] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the second additive, specifically: the first additive is 1% and the second additive is 2%, and the content of the organic solvent is correspondingly reduced. The contents of all other components remain unchanged.

[0077] Example 11

[0078] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the first additive, specifically: the first additive is 0.2% and the second additive is 0.5%, and the reduced amount is made up by an organic solvent. The contents of all other components remain unchanged.

[0079] Example 12

[0080] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the content of the first additive. Specifically, the first additive is 2%, the second additive is 0.5%, and the reduced amount is made up by the organic solvent. The contents of all other components remain unchanged.

[0081] Example 13

[0082] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the formulation of the organic solvent. Specifically, it is a mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 7:3. The contents of all other components remain unchanged.

[0083] Example 14

[0084] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies only in the formulation of the organic solvent. Specifically, it is a mixed solution of ethylene carbonate and diethyl carbonate with a volume ratio of 1:5. The contents of all other components remain unchanged.

[0085] Example 15

[0086] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies in that 0.2% of sodium difluorophosphate is further added, and the content of the organic solvent is correspondingly reduced. The contents of all other components remain unchanged.

[0087] Example 16

[0088] This embodiment provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 2 lies in that 2.5% of sodium difluorosulfonimide is further added, and the content of the organic solvent is correspondingly reduced. The contents of all other components remain unchanged.

[0089] Comparative Example 1

[0090] This comparative example provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies in that the second additive is missing, and the content of the first additive is 1.5%. The contents of all other components remain unchanged.

[0091] Comparative Example 2

[0092] This comparative example provides a non-aqueous electrolyte. The difference in its composition (calculated based on a total amount of 100%) from that of Example 1 lies in that the first additive is missing, and the content of the second additive is 1.5%. The contents of all other components remain unchanged.

[0093] Comparative Example 3

[0094] This comparative example provides a non-aqueous electrolyte, the composition of which (calculated based on the total amount of 100%) is different from that of Example 1 only in that the first additive and the second additive are missing, and the corresponding content is supplemented by an organic solvent. The content of all other components remains unchanged.

[0095] Comparative Example 4

[0096] This comparative example provides a non-aqueous electrolyte, the composition of which (calculated based on the total amount of 100%) is different from that of Example 2 only in that the first additive and the second additive are missing, and the corresponding content is supplemented by an organic solvent. The content of all other components remains unchanged.

[0097] The component formulations of the non-aqueous electrolytes of Examples 1-16 and Comparative Examples 1-4 are summarized in Table 1 below:

[0098] Table 1

[0099]

[0100]

[0101]

[0102]

[0103] Application Examples 1-16 and Comparative Application Examples 1-4

[0104] This application example provides a sodium-ion battery, and its preparation method includes the following steps:

[0105] (1) Preparation of the positive electrode sheet: Polyvinylidene fluoride (PVDF) is uniformly dissolved in N-methylpyrrolidone (NMP), then the conductive agent Super P is added and mixed well, and then the positive electrode active material powder is gradually added (wherein, the mass ratio of Na 0.67 Ni 0.33 Mn 0.67 O2 powder, PVDF and conductive agent Super P is 97.4:1.3:1.3), to obtain a positive electrode slurry (the solid content of the discharged positive electrode slurry is 63.1%). The positive electrode slurry is coated on the current collector, and then dried, rolled and slit to obtain a positive electrode sheet that can be directly stacked.

[0106] (2) Preparation of the negative electrode sheet: Graphite, conductive carbon, silicon oxide powder, and sodium carboxymethyl cellulose with a mass ratio of 80.9:2.9:15:1.2 were fed for premixing, stirred at a speed of 250 rpm for 140 min to obtain a negative electrode slurry. Then, the negative electrode slurry was transferred to deionized water for dispersion. After mixing evenly, the negative electrode slurry was sieved (180 mesh), and finally, the sieved negative electrode slurry was coated on a negative electrode current collector, followed by drying, rolling, and slitting to obtain a negative electrode sheet that can be directly stacked.

[0107] (3) Preparation of the electrolyte: The electrolyte was prepared in a MIKRONA glove box (the internal filling gas is argon, and the gaseous water is less than 10 ppm). First, the organic solvents were uniformly mixed in proportion, various additives were added to the uniformly mixed solvent package, and finally, the electrolyte was added. The mixture was stirred until there was no residue at the bottom and the electrolyte was clear and free of turbidity, that is, an electrolyte with normal chromaticity was obtained and stored in a -10°C refrigerator. The component ratios of each group are shown in Table 1.

[0108] (4) Cell fabrication: The slit positive electrode sheet and negative electrode sheet were stacked on a stacker. The separator is made of three layers of PP / PE / PP materials to form a soft-pack cell.

[0109] (5) Liquid injection, formation, and aging:

[0110] After the cells were dried at high temperature, the electrolytes of each example and comparative example were injected into the soft-pack cells. After the electrolyte injection, the sodium batteries were first encapsulated and surface-cleaned to complete the preliminary work, and then placed at room temperature for one day. The formation was carried out by the stepwise formation method. In the first step, the formation current was 0.05C, and it was constantly charged for 2 h. In the second step, the formation current was 0.1C, and it was constantly charged until the voltage reached 3.75V. After formation, it was aged at 50°C for one day and then cooled to room temperature for final sealing.

[0111] Test examples

[0112] After the battery was assembled, it was left at room temperature for 10 h to allow the electrolyte to fully infiltrate between the battery electrode sheets, and then it was taken for the following tests:

[0113] ① 25°C room temperature cycle test: Constant current charge and discharge were carried out at a current density of 1C of the rated capacity in a 25°C constant temperature chamber. The number of cycle times was 1000 circles. The test voltage range was 2.5V - 4.0V, and the charging cut-off current was 0.05C. After the test, the capacity retention rate of the 1000th cycle was calculated based on the first-week discharge capacity.

[0114] The calculation formula for the 1000-week room temperature cycle capacity retention rate is as follows:

[0115] Capacity retention rate of the 1000th cycle (%) = (Discharge capacity of the 1000th room temperature cycle / First discharge capacity) × 100%.

[0116] ② 45°C High-temperature Cycling Test: Constant current charge and discharge were carried out at a current density of 1C of the rated capacity in a forced-air oven at a constant temperature of 45°C. The number of cycling times was 1000 cycles. The test voltage range was 2.5V - 4.0V, and the charging cut-off current was 0.05C. After the test, the capacity retention rate of the 1000th cycle was calculated based on the discharge capacity of the first week.

[0117] The calculation formula for the capacity retention rate of 1000-week high-temperature cycling is as follows:

[0118] Capacity retention rate of the 1000th cycle (%) = (Discharge capacity of the 1000th high-temperature cycle / First discharge capacity) × 100%.

[0119] ③ Volume Expansion Rate Test after Storage at 60°C:

[0120] Constant current charge and discharge were carried out at a current density of 1C of the rated capacity in a constant-temperature chamber at 25°C, and then constant voltage charging was carried out at a voltage of 4.0V until the current reached 0.05C. After the battery was fully charged, the initial volume of the battery was measured by the drainage method. The battery was placed in an oven at 60°C for 7 days, and then the battery was taken out. It was left standing at room temperature for 60 minutes. After cooling to room temperature, the volume of the battery was measured by the drainage method.

[0121] The storage test was carried out according to the above steps until 28 days of storage. Based on the volume of the battery measured before storage as the reference, the volume expansion rate of the battery changing with the storage time was calculated.

[0122] Volume expansion rate after 28-day high-temperature storage = ((Volume after 28-day high-temperature storage - Initial volume of the battery) / Initial volume of the battery) × 100%.

[0123] The test results of each application example and comparative application example are shown in Table 2:

[0124] Table 2

[0125]

[0126]

[0127] From the data results in Table 2, it can be seen that the electrolyte involved in the present invention enables the sodium-ion battery to have good room-temperature capacity retention rate and high high-temperature capacity retention rate by adding two additives with specific chemical structures, and can greatly inhibit the volume expansion of the soft-pack battery during the battery cycling process. From Application Examples 1 - 16, it can be known that the sodium-ion battery assembled with the non-aqueous electrolyte has a capacity retention rate of 89.7 - 95.4% under the test conditions of 25°C and 1C, a capacity retention rate of 82.7 - 89.1% under the test conditions of 45°C and 1C, and a volume expansion rate of 8.9 - 12.4% after storage at 60°C.

[0128] By comparing the data results of Application Example 1 and Application Examples 5 - 12, it can be seen that the addition ratios and respective addition contents of the two additives affect the cycle performance and storage performance of the sodium-ion battery to a certain extent.

[0129] By comparing the data results of Application Example 1 and Application Examples 13 - 14, it can be seen that the formulation composition of the organic solvent in the system also affects the cycle performance and storage performance of the sodium-ion battery to a certain extent.

[0130] By comparing the data results of Application Example 1 and Application Examples 15 - 16, it can be seen that adding sodium salt additives to the system can further improve the cycle performance and storage performance of the sodium-ion battery.

[0131] By comparing the data results of Application Example 1 and Comparative Application Examples 1 - 4, it can be seen that by adding two specific additives to the non-aqueous electrolyte in the present invention, it is possible to reduce the cycle impedance of the sodium-ion battery, improve its cycle life, and reduce the gas generation during storage of the sodium-ion battery, thereby improving its cycle performance. That is, the synergistic effect of the two enables a greater improvement in the comprehensive performance of the obtained sodium-ion battery; if only a single additive is added, although it can improve the high-temperature cycle performance of the battery and reduce its gas generation during storage to a certain extent, the single first additive or second additive cannot exert the synergistic effect of the two, and thus cannot significantly reduce the cycle impedance of the sodium-ion battery and improve its cycle performance.

[0132] The applicant declares that the present invention uses the above-mentioned embodiments to illustrate the technical solutions of the present invention, but the present invention is not limited to the above-mentioned embodiments, that is, it does not mean that the present invention must rely on the above-mentioned embodiments to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of each raw material of the product of the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0133] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above-mentioned embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0134] In addition, it should be noted that in the above specific embodiments, the various specific technical features described can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

Claims

1. A non-aqueous electrolyte for a sodium ion battery, characterized in that: The components of the non-aqueous electrolyte include an electrolyte, an organic solvent, a first additive and a second additive; The chemical structure of the first additive is shown in Formula I; the chemical structure of the second additive is shown in Formula II; Wherein, R1, R2, R3, and R4 are independently selected from substituted or unsubstituted C1-C5 alkyl, substituted or unsubstituted C2-C5 alkenyl, substituted or unsubstituted C2-C5 alkynyl, and substituted or unsubstituted phenyl; the substituted substituents are selected from any one or a combination of at least two of C1-C3 alkyl, hydroxyl, halogen, cyano, and nitro.

2. The non-aqueous electrolyte for sodium ion batteries according to claim 1, characterized in that: The components of the non-aqueous electrolyte include, by mass percentage, 2-22% electrolyte, 75-94% organic solvent, 0.2-4% first additive and 0.2-2% second additive; Preferably, the mass ratio of the first additive to the second additive is (0.5-2):

1.

3. The non-aqueous electrolyte for sodium ion batteries according to claim 1 or 2, characterized in that: The electrolyte is a sodium salt; Preferably, the sodium salt includes any one of sodium hexafluorophosphate, sodium perchlorate, sodium bis(trifluoromethanesulfonyl)imide, sodium difluorooxalatoborate or sodium bis(fluorosulfonyl)imide, or a combination of at least two thereof.

4. The non-aqueous electrolyte for sodium ion batteries according to any one of claims 1 to 3, characterized in that: The organic solvent includes a carbonate organic solvent and / or a carboxylate organic solvent; Preferably, the carboxylate organic solvent includes any one or a combination of at least two of ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, methyl acetate, propyl acetate, methyl propionate or γ-butyrolactone; Preferably, the carbonate organic solvent includes cyclic carbonate and / or chain carbonate; more preferably, a combination of cyclic carbonate and chain carbonate; Preferably, the volume ratio of the cyclic carbonate to the linear carbonate is 1:(1-4); Preferably, the cyclic carbonate includes any one of ethylene carbonate, propylene carbonate or butylene carbonate or a combination of at least two thereof; Preferably, the linear carbonate includes any one of diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate or methyl propyl carbonate, or a combination of at least two thereof.

5. The non-aqueous electrolyte for sodium ion batteries according to any one of claims 1 to 4, characterized in that: The components of the non-aqueous electrolyte also include a third additive, and the third additive includes any one or a combination of at least two of vinylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, vinyl ethylene sulfate, vinyl sulfate, methylene methanedisulfonate, vinylene carbonate, tris(trimethylsilyl)phosphate or tris(trimethylsilyl)borate; Preferably, based on 100% of the total mass of the non-aqueous electrolyte, the mass percentage of the third additive is 0.5-11.6%.

6. The non-aqueous electrolyte for sodium ion batteries according to any one of claims 1 to 5, characterized in that: The components of the non-aqueous electrolyte also include sodium salt additives; the sodium salt additives include any one of sodium difluorosulfonyl imide, sodium difluorooxalatoborate, sodium tetrafluoroborate, bisoxalatoborate or sodium difluorophosphate, or a combination of at least two thereof; Preferably, based on the total mass of the non-aqueous electrolyte being 100%, the mass percentage of the sodium salt additive is 0.2-3.0%.

7. The non-aqueous electrolyte for sodium ion batteries according to any one of claims 1 to 6, characterized in that: The first additive is the following compound:

8. The non-aqueous electrolyte for sodium ion batteries according to any one of claims 1 to 7, characterized in that: The second additive is selected from any one or a combination of at least two of the following compounds:

9. A sodium ion battery, characterized in that: The sodium ion battery comprises a battery cell and an electrolyte, wherein the battery cell comprises a positive electrode, a negative electrode and a separator, wherein the separator is arranged between the positive electrode and the negative electrode, and the electrolyte is the non-aqueous electrolyte according to any one of claims 1 to 8.

10. The sodium ion battery according to claim 9, characterized in that: The positive electrode comprises a positive electrode active material, a conductive agent and a binder; Preferably, the positive electrode active material comprises a layered metal oxide; Preferably, the negative electrode comprises a negative electrode active material; Preferably, the negative electrode active material is selected from any one or a combination of at least two of natural graphite, artificial graphite, soft carbon, hard carbon, mesophase carbon microspheres, graphene, graphyne, metallic sodium, nanocarbon, carbon nanotubes, elemental tin, tin oxide, tin-carbon complex, tin alloy or sodium titanate; Preferably, the material of the diaphragm includes any one of polyethylene, polypropylene or a composite ceramic membrane, or a combination of at least two of them.

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