Fluorine-rich combined additive, electrolyte and application of fluorine-rich combined additive and electrolyte in inhibition of gas production of sodium-ion battery

Abstract

The invention relates to the field of sodium-ion battery electrolytes, and discloses a fluorine-rich combined additive, an electrolyte and application of the fluorine-rich combined additive and the electrolyte in inhibition of gas production of a sodium-ion battery. The fluorine-rich additive comprises sodium difluoro (oxalato) borate (NaDFOB), fluoroethylene carbonate (FEC) and bis (trifluoroethyl) carbonate (TFEC), and the electrolyte comprises sodium salt NaPF6, a cyclic / linear carbonate solvent combination and the fluorine-rich additive combination. A positive electrode electrolyte membrane (CEI) and a negative electrode solid electrolyte membrane (SEI) are cooperatively constructed by utilizing preferential oxidation and echelon reduction characteristics of different components in the combined additive, so that on one hand, dissolution of transition metal on the surface of a positive electrode and crystal structure transformation are inhibited, on the other hand, SEI dissolution of the negative electrode is inhibited, gas production of the sodium ion battery is cooperatively inhibited, and the stability of the sodium ion battery is improved. Therefore, the oxide positive electrode / hard carbon negative electrode sodium ion battery system has excellent cycling stability.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a group of fluorine-rich electrolyte additives and sodium ion batteries. Background Art

[0002] With the rapid development of renewable energy and electric vehicles, sodium-ion batteries have attracted widespread attention as a potential energy storage technology to replace lithium-ion batteries. Sodium-ion batteries are considered to be an energy storage solution with sustainable development prospects due to their abundant raw materials and low cost. However, during the use and production of sodium-ion batteries, gas production has gradually become one of the key factors limiting their performance and safety.

[0003] In sodium-ion batteries, in addition to the gases that may be produced during the common formation process, the severity of interface problems is more prominent. These interface problems not only affect the interaction between the electrode and the electrolyte, but also lead to the occurrence of crosstalk. This phenomenon may not be obvious when the battery is stable in the early stage, but as the number of cycles increases, the originally stable interface film may deteriorate sharply in the medium term, thereby triggering the rapid generation of gas. In addition, the generation of gas will also cause an imbalance in the electrochemical reaction inside the battery, affecting the overall energy density and power performance of the battery. The accumulation of gas may lead to poor contact between the electrode and the electrolyte, further reducing the charge and discharge efficiency of the battery. Therefore, understanding the mechanism of gas production in sodium-ion batteries and developing effective inhibition measures are the key to improving the performance and safety of sodium-ion batteries.

[0004] Electrolyte additives are key components with film-forming properties added to the electrolyte in small amounts. They can be reduced and decomposed on the electrode surface before other components in the electrolyte, forming a stable SEI film in advance to prevent the electrolyte from continuing to react. However, the negative electrode film formation problem caused by the high sodium insertion potential of the negative electrode of sodium-ion batteries and the surface lattice degradation problem of the positive electrode material during the cycle process have led to higher requirements for the design of electrolyte additive components. Summary of the invention

[0005] The purpose of the present invention is to provide a sodium ion battery electrolyte containing a fluorine-rich combination additive to address the above-mentioned defects and shortcomings, so as to modulate the film-forming properties of the positive and negative electrodes of the sodium ion battery, inhibit gas production, and achieve an excellent cycle life of the sodium ion battery.

[0006] The present invention protects the bulk structure of the material during the electrochemical cycle by adding NaDFOB to the electrolyte to form an oxidation film at the positive electrode in priority to other components of the electrolyte and to form a film at the negative electrode in priority to sodium insertion reduction. FEC and TFEC are added to the negative electrode to construct another layer of organic-inorganic cross-linked SEI film to form a composite double-layer film structure. Therefore, on the one hand, the dissolution of transition metals and the degradation of the surface lattice of the positive electrode material are inhibited; on the other hand, the dissolution of the negative electrode SEI is inhibited. The solid SEI film synergistically inhibits the gas production of the positive and negative electrodes of the sodium ion battery, achieving an excellent cycle life.

[0007] In a first aspect of the present invention, an electrolyte for a sodium ion battery is provided, wherein the electrolyte comprises a sodium salt, a non-aqueous solvent and a fluorine-rich additive group.

[0008] Further, the sodium salt is selected from sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium trifluoromethanesulfonate (NaOTf), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethylsulfonyl)imide (NaTFSI).

[0009] Preferably, the sodium salt is NaPF6.

[0010] Furthermore, the non-aqueous organic solvent is a mixture of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), and the volume ratio of EC, PC, EMC and DEC is x:3-x:y:7-y, 0<x<3, 0<y<7.

[0011] Preferably, the volume ratio of EC, PC, EMC and DEC is 1:2:3:4.

[0012] Furthermore, the sodium salt is dissolved in a non-aqueous organic solvent to obtain an electrolyte with a concentration of 0.5 to 3 mol / L.

[0013] Preferably, the concentration of the electrolyte is 1 mol / L.

[0014] Furthermore, the electrolyte additive NaDFOB is 0.5% to 2% of the total mass of the basic electrolyte, preferably 1%. FEC is 0.5% to 3% of the total volume of the basic electrolyte, preferably 1.5%. TFEC is 0.5% to 3% of the total volume of the basic electrolyte, preferably 1.5%.

[0015] In another aspect of the present invention, the present invention provides a sodium ion battery, which is added with the electrolyte described in the first aspect. Specifically, the active material in the positive electrode of the sodium ion battery is a layered oxide material or a polyanion material, and the active material in the negative electrode is a hard carbon material.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) A strong and thin CEI film structure is formed on the surface of the cathode material, which inhibits the dissolution of transition metal ions and the degradation of the cathode material surface lattice during the cycle.

[0018] (2) On the surface of the negative electrode material, an inner layer of SEI dominated by NaDFOB and mainly composed of inorganic borides is formed, and an outer layer of SEI dominated by FEC and TFEC is formed, which is a stable and elastic inorganic-organic cross-linked SEI. The membrane structure obtained by step-by-step reduction is not easy to dissolve.

[0019] (3) The crosstalk between the positive and negative electrodes is synergistically suppressed, and the gas production of the sodium ion battery is deeply suppressed at multiple levels, with excellent cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 : Optical images of soft-pack battery airbags (a) Example 1, (b) Comparative Example 1, (c) Comparative Example 2, (d) Comparative Example 3, (e) Comparative Example 4

[0021] Figure 2 : Cycle performance comparison diagram of some embodiments and comparative examples

[0022] Figure 3 : Optical photo of sodium analysis of comparative example 1 DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the fluorine-rich combination additive and electrolyte of the present invention and their application in inhibiting gas production in sodium ion batteries will be further described in combination with specific embodiments and drawings of the specification, but it should be understood that the protection scope of the present invention is not limited to the following embodiments. If the specific conditions are not specified in the embodiments, they shall be carried out under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0024] Example 1

[0025] (1) Preparation of non-aqueous electrolyte: In a glove box filled with argon (O2 <1 ppm, H2O <1 ppm), EC, PC, EMC and DEC were mixed in a volume ratio of 1:2:3:4 to prepare 100 mL of organic solvent, and then 16.8 g of NaPF6 was added to dissolve and stirred thoroughly. Then, 1.5% by volume (1.5 mL) of FEC and 1.5% by volume (1.5 mL) of TFEC and 1% by mass (1.6 g) of NaOFOB were added successively, and the electrolyte was obtained after mixing evenly.

[0026] (2) Preparation of positive electrode: Nickel iron sodium manganese oxide ternary material NaNi 1 / 3 Fe 1 / 3 Mn1 / 3 O2, adhesive PVDF, conductive agent SuperP and carbon nanotubes CNT are evenly mixed in a mass ratio of 96.1:1.6:1.8:0.5 to prepare a sodium ion battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of an aluminum foil, and then dried and rolled to obtain a positive electrode sheet.

[0027] (3) Preparation of negative electrode: hard carbon, conductive agent SuperP, carbon nanotubes CNT, and adhesive are mixed in a mass ratio of 95:1:1.5:2.5 to form a slurry, which is evenly mixed. The mixed slurry is coated on both sides of a copper foil, and then dried and rolled to obtain a negative electrode sheet.

[0028] (4) Preparation of sodium ion battery: The positive electrode, the separator and the negative electrode are stacked to form a battery cell, which is packaged with an aluminum-plastic film, injected with the sodium ion battery non-aqueous electrolyte prepared above, and processed through the steps of formation and capacity separation to produce a sodium ion battery with a capacity of 45 mAh.

[0029] The components of the electrolytes of Examples 1-8 and Comparative Examples 1-15 are shown in Table 1, wherein the preparation steps of the electrolytes of Examples 2-8 and Comparative Examples 1-15 are the same as that of Example 1.

[0030] Table 1: Compositions of the electrolytes of the embodiments and comparative examples

[0031]

[0032] The sodium ion batteries prepared in Examples 1 to 8 and Comparative Examples 1 to 15 were subjected to cycle tests, respectively, and the test conditions were as follows: at 25°C, the sodium ion battery was charged to 3.95V (constant voltage cut-off current 0.2C) at 1C constant current and constant voltage, and then allowed to stand for 5 minutes, and then discharged to 1.8V at 1C constant current, and allowed to stand for 5 minutes, and the charging and discharging were repeated in this way, and the capacity retention rate of the sodium ion battery after 1000 cycles was calculated, and the test results are shown in Table 2. The gas production before and after the cycle was evaluated, and the test conditions were as follows: the volume V0 of the reserved air bag of the soft-pack battery was tested by the Archimedes drainage method, and the volume V1 of the air bag of the battery after the cycle was tested by the Archimedes drainage method, and the gas production = V1-V0, and the test results are shown in Table 2.

[0033] Table 2: Electrolytes of Examples and Comparative Examples

[0034] Group Cycle retention rate Gas production Group Cycle retention rate Gas production Example 1 92.17％ 0.54mL Comparative Example 5 83.44％ 2.88mL Example 2 90.34％ 0.52mL Comparative Example 6 81.78％ 1.64mL Example 3 91.23％ 0.71mL Comparative Example 7 80.44％ 2.13mL Example 4 87.54％ 0.57mL Comparative Example 8 87.97％ 0.57mL Example 5 89.97％ 0.62mL Comparative Example 9 89.45％ 0.54mL Example 6 91.39％ 0.58mL Comparative Example 10 84.74％ 0.53mL Example 7 86.11％ 0.53mL Comparative Example 11 85.47％ 0.62mL Example 8 88.90％ 0.55mL Comparative Example 12 88.36％ 0.57mL Comparative Example 1 1.90％ 0.64mL Comparative Example 13 88.61％ 0.58mL Comparative Example 2 81.07％ 1.87mL Comparative Example 14 85.33％ 0.71mL Comparative Example 3 82.33％ 3.43mL Comparative Example 15 84.97％ 0.77mL Comparative Example 4 80.06％ 2.32mL

[0035] From the test results in Table 2, the results of Examples 1-8 show that, in the combined additives, when NaDFOB is more, the capacity retention rate decreases, which may be related to the increase in the resistance of the film being too thick. When FEC increases, the gas production increases. Compared with Comparative Examples 1-7, it can be seen that when any component of the additive group is missing, the gas production will increase (such as Figure 1 As shown), the cycle retention rate dropped significantly, especially in Example 1, when there was no electrolyte additive, the cycle capacity dropped rapidly (as shown in Figure 2 This is closely related to the sodium precipitation on the negative electrode surface (as shown in Figure 3 Compared with Comparative Examples 8-15, it can be seen that when the additive group exists, the solvent component will not affect its gas production, but the cycle is still different from that of Example 1. This should be due to the difference in the solvation structure of the electrolyte in different solvent systems, resulting in different ion transport and interface impedance.

[0036] The above contents are only preferred embodiments of the present invention and are not intended to limit the invention. Any equivalent replacement, modification and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A sodium ion battery electrolyte, characterized in that The invention comprises the following components: sodium salt, non-aqueous organic solvent, and a fluorine-rich combined electrolyte additive.

2. The electrolyte according to claim 1, characterized in that The fluorine-rich combination additive and its structural formula are specifically: Sodium difluorooxalate borate (NaDFOB): Fluoroethylene carbonate (FEC): Bistrifluoroethyl carbonate (TFEC):

3. The electrolyte according to claim 1, characterized in that The sodium salt is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium trifluoromethanesulfonate (NaOTf), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethylsulfonyl)imide (NaTFSI).

4. The electrolyte according to claim 1, characterized in that The non-aqueous organic solvent is a mixture of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). The volume ratio of EC, PC, EMC and DEC is x:3-x:y:7-y, 0<x<3, 0<y<7.

5. The electrolyte according to claim 1, characterized in that The sodium salt of claim 3 is dissolved in the non-aqueous organic solvent of claim 4 to a concentration of 0.5 to 3 mol / L.

6. The electrolyte additive NaDFOB according to claim 2 is 0.5% to 2% of the total mass of the basic electrolyte, FEC is 0.5% to 3% of the total volume of the basic electrolyte, and TFEC is 0.5% to 2% of the total volume of the basic electrolyte.

7. The electrolyte according to claim 1, characterized in that It can be used in sodium ion batteries for electric vehicles, energy storage devices, etc.

8. The sodium ion battery according to claim 7, wherein the positive electrode material is one of a layered oxide material and a polyanion material, and the negative electrode material is a hard carbon material.

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