Electrolyte and sodium battery containing same

Abstract

The invention relates to the technical field of sodium batteries, in particular to an electrolyte and a sodium battery containing the same. According to the electrolyte disclosed by the invention, the traditional solvents such as esters, sulfones and nitriles are abandoned, the sodium salt is mixed with one or more of the siloxane organic solvent, the fluorinated solvent or the ether organic solvent with a specific structure to form the electrolyte with a weak solvation structure, and additional additives do not need to be added; the method can be suitable for a sodium battery without a negative electrode (or a sodium metal negative electrode), stable operation is promoted, and the cycle performance is improved; and the electrolyte is simple in preparation process and is beneficial to promoting the industrial development of the sodium battery.

Description

Technical Field

[0001] The present disclosure relates to the technical field of sodium batteries, and in particular to an electrolyte and a sodium battery containing the electrolyte. Background Art

[0002] The negative electrode of the sodium battery without negative electrode only uses the current collector and has no active material mass. Its "actual negative electrode" is formed by in-situ deposition during the first charge process, so the energy density of the whole battery can be greatly improved. However, it is difficult for the solvent system of the common sodium ion battery electrolyte to meet the two application conditions of no negative electrode (or the negative electrode active material is a metal sodium sheet) and high voltage at the same time. Solvents such as esters, sulfones and nitriles have an electron donor part of the adjacent conjugated structure (Na + Acceptor site), has good antioxidant properties, but has a significant reduction tendency. Taking carbonate as an example, the lone pair of electrons of the hydroxyl oxygen of carbonate can be further conjugated with the π*C=O antibonding orbital to form a lower energy filled orbital, which means it is more difficult to lose electrons (strong antioxidant ability). At the same time, because the carbonyl group has a strong electron-withdrawing ability, electrons are more easily attracted to the carbonyl group, thereby lowering the LUMO energy level and easily reacting with sodium metal, making carbonate solvents difficult to use in negative electrode-free sodium batteries (or sodium batteries with negative electrode active materials as metallic sodium sheets). Ether solvents with good compatibility with sodium metal do not contain such conjugated structures, and there are no empty orbitals with phase matching and similar energy to stabilize the lone pair of electrons of oxygen. They have poor antioxidant ability and ethers are relatively easy to be oxidized. But this does not mean that the use of ether electrolytes cannot support a voltage window upper limit of 4V and above. At present, the academic community has changed the solvation structure by adding specific sodium salts or additives, thereby applying ether solvents (ether solvents with both end groups being methyl) to high-voltage negative electrode-free sodium batteries. The upper limit of the voltage window can be increased to 4V and above, but the working rate is low and the cycle performance is poor, which has certain limitations in application scenarios.

[0003] Therefore, how to provide an electrolyte suitable for a sodium battery without a negative electrode (or a sodium metal negative electrode) has become a problem that needs to be solved urgently. Summary of the invention

[0004] In order to solve the above technical problems, the present disclosure provides an electrolyte and a sodium battery containing the same. The electrolyte is applied to a sodium battery without a negative electrode (or a sodium metal negative electrode), which effectively improves its operating stability and the cycle life of the battery.

[0005] In a first aspect, the present disclosure provides an electrolyte, comprising a sodium salt and an organic solvent, wherein the organic solvent comprises one or more of an ether organic solvent, a siloxane organic solvent or a fluorinated solvent;

[0006] The ether organic solvent includes at least one compound having the following molecular structure:

[0007]

[0008] Among them, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 Each independently represents any one of methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl or benzyl; and each molecular structure contains at most one methyl group;

[0009] The fluorinated solvent is a fluorinated product of the ether organic solvent or a fluorinated product of the siloxane organic solvent.

[0010] The solvent selected in the present disclosure forms an electrolyte with a weak solvation structure with the sodium salt. Among them, the first type of solvent is an ether organic solvent, and at most one of its two terminal groups has a methyl group. Compared with ethers such as ethylene glycol dimethyl ether and diethylene glycol dimethyl ether (G2) whose terminal groups are all methyl groups, the steric hindrance is increased and the interaction with Na is weakened. + The second type of solvent is siloxane organic solvent. Compared with ethers with similar structures, the higher energy Si 3d orbital participates in conjugation to produce higher energy empty orbitals to improve the reduction resistance while retaining the low energy filled orbitals. In addition, siloxane has a much lower highest occupied molecular orbital (HOMO) than ether, which means better antioxidant ability. Moreover, due to factors such as steric hindrance, anions are more likely to enter Na + The third type of solvent is the fluorinated product of the above two solvents. The fluorine atom has the ability to pull electrons. Compared with the first two types of solvents, it reduces the electron concentration of the central oxygen atom and weakens the Na + -Solvent force. In summary, the solvent selected in the present disclosure balances Na + -Solvent and Na + -The interaction between anions forms a weakly solvated electrolyte. In a weakly solvated electrolyte, anions not only participate in the construction of the solvation structure of contact ion pairs (CIP) / aggregates (AGG) in a liquid environment, but also remain enriched at the electrode / electrolyte interface during the cycle, promoting inorganic-rich CEI and SEI. This special interaction constructs abundant contact ion pairs (CIPs) / aggregates (AGGs) clusters at the electrode / electrolyte interface, promoting the formation of a uniform and stable interface layer. Based on this, the negative electrode-free sodium metal secondary battery (or sodium metal negative electrode battery) disclosed in the present invention can operate stably and has good cycle performance.

[0011] Furthermore, the electrolyte disclosed in the present invention also has the advantage of a simple preparation process, which promotes the further industrial development of sodium batteries.

[0012] The following are preferred technical solutions of the present disclosure, but are not intended to be limitations of the technical solutions provided by the present disclosure. Through the following technical solutions, the technical objectives and beneficial effects of the present disclosure can be better achieved and realized.

[0013] As a preferred technical solution of the present disclosure, in the molecular structures of Formula I to Formula IV, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 Each independently represents any one of ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl or benzyl, i.e. does not contain methyl.

[0014] As a preferred technical solution of the present disclosure, the organic solvent includes any one of an ether organic solvent, a siloxane organic solvent or a fluorinated solvent, and is further preferably an ether organic solvent or a siloxane organic solvent.

[0015] As a preferred technical solution of the present invention, the ether organic solvent includes one or more of ethylene glycol methyl ethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether or ethylene glycol benzyl butyl ether, for example, a combination of ethylene glycol methyl ethyl ether and ethylene glycol diethyl ether, a combination of ethylene glycol dipropyl ether and ethylene glycol dibutyl ether, etc.

[0016] As a preferred technical solution of the present disclosure, the siloxane organic solvent includes one or more of tetraethyl orthosilicate, methyltriethoxysilane, dimethyldiethoxysilane or dimethyldimethoxysilane, for example, a combination of tetraethyl orthosilicate and methyltriethoxysilane, a combination of methyltriethoxysilane and dimethyldiethoxysilane, etc.

[0017] As a preferred technical solution of the present disclosure, the sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide, sodium difluorooxalatoborate, sodium perchlorate or sodium tetrafluoroborate, for example, a combination of sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide, a combination of sodium difluorooxalatoborate and sodium perchlorate, and the like.

[0018] As a preferred technical solution of the present invention, in the electrolyte, the total concentration of the sodium salt is 0.4-1.5 mol / L, for example, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L or 1.5 mol / L, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0019] As a preferred technical solution of the present disclosure, in the electrolyte, the total concentration of the sodium salt is 0.6-1.2 mol / L.

[0020] In a second aspect, the present disclosure provides a sodium battery, wherein the sodium battery comprises a positive electrode plate, a negative electrode current collector, a separator and the electrolyte as described in the first aspect.

[0021] As a preferred technical solution of the present disclosure, a metal sodium sheet is provided on the negative electrode current collector.

[0022] As a preferred technical solution of the present disclosure, the positive electrode active material of the positive electrode plate includes one or more of transition metal layered oxides, polyanionic materials or Prussian blue materials, preferably transition metal layered oxides.

[0023] As a preferred technical solution of the present disclosure, the negative electrode current collector includes any one of copper foil, carbon-coated copper foil, aluminum foil or carbon-coated aluminum foil, preferably carbon-coated aluminum foil.

[0024] Compared with the prior art, the technical solution provided by the embodiments of the present disclosure has the following advantages:

[0025] (1) The electrolyte disclosed in the present invention abandons the traditional solvents such as esters, sulfones and nitriles, and instead mixes sodium salt with one or more of siloxane organic solvents, fluorinated solvents or ether organic solvents of specific structures to form an electrolyte with a weak solvation structure, and without adding additional additives, it can be applied to sodium batteries without negative electrodes (or sodium metal negative electrodes), promote their stable operation, and improve their cycle performance;

[0026] (2) The electrolyte preparation process disclosed in the present invention is simple, which is conducive to promoting the industrial development of sodium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0029] Figure 1 The discharge capacity retention rate comparison curve of the sodium battery obtained in Examples 1-5 of the present disclosure and Comparative Example 1 is shown. DETAILED DESCRIPTION

[0030] In order to more clearly understand the above-mentioned objectives, features and advantages of the present disclosure, the scheme of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0031] In the following description, many specific details are set forth to facilitate a full understanding of the present disclosure, but the present disclosure may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present disclosure, rather than all of the embodiments.

[0032] Examples 1-5 and Comparative Example 1 respectively provide an electrolyte, and the specific composition of the electrolyte is shown in Table 1.

[0033] Table 1

[0034]

[0035] Testing of negative electrode-free sodium battery:

[0036] Positive electrode: NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 4800g of material, 150g of polyvinylidene fluoride and 50g of conductive carbon black were wet-mixed with N-methylpyrrolidone as solvent. The slurry was evenly dispersed and had a viscosity of 5000-6000mPa·s. It was coated on both sides of carbon-coated aluminum foil with a surface density of 240g / m on each side. 2 Then roll compaction, compaction density 3.0kg / m 3 , during die cutting and baking; the designed single-sided capacity density is 8.45Ah / m 2 ;

[0037] Negative electrode current collector: carbon-coated aluminum foil;

[0038] Electrolyte: the electrolytes described in Examples 1-5 and Comparative Example 1 respectively;

[0039] Diaphragm: PP diaphragm, 9μmPP+3μm alumina ceramic coating;

[0040] The stacking, welding of the pole lugs, shelling and liquid injection (liquid injection coefficient is 5g / Ah) of the battery cells are completed in an inert atmosphere glove box. The designed capacity of the battery cells is 1Ah.

[0041] After the battery cell is filled with liquid, the battery cell is vacuum-packed and placed in room temperature for 36 hours; then it is pressurized and formed at room temperature. The battery cell is subjected to a constant pressure of 0.8MPa and a temperature of 25°C. It is charged to 4V at a constant current of 0.05A, then discharged to 2V at a constant current of 0.05A, and then charged for 7 hours at a constant current of 0.05A to complete the formation; after final sealing, a negative electrode-free sodium battery for testing is obtained, which is used to test the electrochemical performance.

[0042] Cycle performance test:

[0043] The negative electrode-free sodium battery for testing was placed in a constant temperature chamber, the temperature of which was controlled at room temperature (25±2°C), and allowed to stand for 30 minutes to allow the negative electrode-free sodium battery for testing to reach room temperature.

[0044] Pressurized limit test: Place the negative electrode-free sodium battery for testing between two flat steel plates, and use 4 M8 bolts to lock the two steel clamps with a locking torque of 2N·m. Charge the negative electrode-free sodium battery for testing at a constant temperature at a constant current of 0.2C to 4V, let it stand for 10 minutes, and then discharge it at a constant current of 0.5C to 2V. Repeat this cycle, record the discharge capacity of the first cycle and the discharge capacity of the last cycle, and calculate the capacity retention rate according to the following formula:

[0045] The nth cycle capacity retention rate (%) = (nth cycle discharge capacity / first cycle discharge capacity) × 100%.

[0046] The test results are shown in Table 2 and Figure 1 Table 2 summarizes the first week capacity and the discharge capacity retention rate at the 20th, 50th and 100th weeks of the negative electrode-free sodium batteries obtained in Examples 1-5 and Comparative Example 1. Figure 1 The discharge capacity retention rate comparison curves of the sodium batteries obtained in Examples 1-5 and Comparative Example 1 are shown.

[0047] Table 2

[0048]

[0049] During the cycle of the battery of Comparative Example 1, it was found that the battery produced slight gas, accompanied by a rapid decay of capacity.

[0050] The battery of Example 1 has a high capacity and the best retention rate, and no gas is generated during the cycle process. This is due to the weak solvation effect of the solvent ethylene glycol dibutyl ether. In the formed weak solvation electrolyte, the solvation structure (i.e., CIP / AGG) in which the anion hexafluorophosphate participates is not only constructed in the bulk environment, but also remains enriched at the electrode / electrolyte interface during the cycle process, promoting inorganic-rich CEI and SEI. This special interaction constructs abundant contact ion pairs (CIPs) / aggregates (AGGs) clusters at the electrode / electrolyte interface, promoting the formation of a uniform and stable interface layer.

[0051] The reason why the batteries obtained in Examples 2-5 can be stably cycled is similar to that in Example 1, that is, the weak solvation effect of the solvent allows the sodium salt anions to participate in the formation of a solvation structure with a relatively high proportion of CIPs and AGGs. The capacity is mainly affected by the conductivity. The electrolyte conductivity of Example 4 is higher than that of Example 1, and the conductivity of other examples is lower than that of Example 1. Although the cycle performance is worse than that of Example 1, they all have a significant breakthrough compared with Comparative Example 1, and there is no obvious gas production during the cycle process.

[0052] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An electrolyte, characterized in that: It comprises a sodium salt and an organic solvent, wherein the organic solvent comprises one or more of an ether organic solvent, a siloxane organic solvent or a fluorinated solvent; The ether organic solvent includes at least one compound having the following molecular structure: wherein R1, R2, R3, R4, R5, R6, R7 and R8 each independently represent any one of methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl or benzyl; and each molecular structure contains at most one methyl group; The fluorinated solvent is a fluorinated product of the ether organic solvent or a fluorinated product of the siloxane organic solvent.

2. The electrolyte according to claim 1, characterized in that The ether organic solvent includes one or more of ethylene glycol methyl ethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether or ethylene glycol benzyl butyl ether.

3. The electrolyte according to claim 1 or 2, characterized in that The siloxane organic solvent includes one or more of tetraethyl orthosilicate, methyltriethoxysilane, dimethyldiethoxysilane or dimethyldimethoxysilane.

4. The electrolyte according to any one of claims 1 to 3, characterized in that The sodium salt includes one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide, sodium difluorooxalatoborate, sodium perchlorate or sodium tetrafluoroborate.

5. The electrolyte according to any one of claims 1 to 4, characterized in that In the electrolyte, the total concentration of the sodium salt is 0.4-1.5 mol / L.

6. The electrolyte according to claim 5, characterized in that In the electrolyte, the total concentration of the sodium salt is 0.6-1.2 mol / L.

7. A sodium battery, characterized in that: The sodium battery contains a positive electrode sheet, a negative electrode current collector, a separator and an electrolyte as described in any one of claims 1 to 6.

8. The sodium battery according to claim 7, characterized in that: A metal sodium sheet is arranged on the negative electrode current collector.

9. The sodium battery according to claim 7 or 8, characterized in that: The positive electrode active material of the positive electrode plate includes one or more of transition metal layered oxides, polyanion materials or Prussian blue materials, preferably transition metal layered oxides.

10. The sodium battery according to any one of claims 7 to 9, characterized in that: The negative electrode current collector includes any one of copper foil, carbon-coated copper foil, aluminum foil or carbon-coated aluminum foil, preferably carbon-coated aluminum foil.

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