Electrolyte and sodium ion battery

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular relates to an electrolyte and a sodium ion battery, comprising a conductive sodium salt, a non-aqueous organic solvent, and a functional additive, wherein the functional additive comprises one or more of fluoroethylene carbonate, vinyl sulfate, isatin anhydride, and tris(trimethylsilyl)borate. The electrolyte of the present invention utilizes several functional additives in combination to produce a synergistic effect, forming a stable and dense interface film at the positive and negative electrodes, partially neutralizing the alkalinity of the positive electrode material surface, inhibiting side reactions with the electrolyte, and improving the charge transfer capacity of the electrodes. Furthermore, the combination of several low-impedance additives results in a low-impedance interface film, reducing battery polarization. Thus, the electrolyte achieves excellent cycle performance while also taking into account low-temperature and rate performance.

Description

Technical Field

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

[0002] Lithium-ion batteries have been widely used in electric vehicles, laptops, and energy storage due to their high energy density, long cycle life, and environmental friendliness. However, lithium reserves are limited, unevenly distributed, and will eventually become depleted. Sodium, one of the most abundant elements on Earth, operates on a similar principle to lithium-ion batteries and offers advantages such as low cost, excellent safety, and the ability to store energy for long periods of time. As a result, sodium-ion batteries are gaining increasing attention among researchers and are expected to replace lithium-ion batteries in energy storage applications.

[0003] However, since the positive electrode material of sodium-ion batteries is highly alkaline, it is easy to react with the electrolyte to produce side reactions, especially under high voltage. The side reactions are more serious, producing gas, leading to increased internal pressure of the battery, battery deformation, performance degradation and other problems. Therefore, solving the problem of battery gas production is urgent. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrolyte that can neutralize the alkalinity on the surface of the positive electrode material, improve the density and stability of the SEI film, and does not cause side reactions, gas production or deformation under high voltage.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] An electrolyte comprises a conductive sodium salt, a non-aqueous organic solvent and a functional additive. The functional additive comprises one or more of fluoroethylene carbonate, vinyl sulfate, isatin anhydride and tris(trimethylsilyl)borate.

[0007] Preferably, the functional additive accounts for 1% to 10% of the total mass of the electrolyte.

[0008] Preferably, the conductive sodium salt includes one or more of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate and sodium bis(oxalatoborate).

[0009] Preferably, the conductive sodium salt accounts for 12% to 18% of the total weight of the electrolyte.

[0010] Preferably, the non-aqueous organic solvent includes one or a mixture of cyclic carbonate, chain carbonate, ethyl propionate, and propyl propionate.

[0011] Preferably, the mass ratio of the cyclic carbonate to the chain carbonate is 1:1-4.

[0012] Preferably, the cyclic carbonate includes one or a mixture of ethylene carbonate and propylene carbonate, and the chain carbonate includes one or a mixture of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0013] Preferably, the mass of the non-aqueous organic solvent accounts for 70% to 85% of the total mass of the electrolyte.

[0014] Preferably, the functional additive is a mixture of fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate and isatoic anhydride in a weight ratio of 2-8:0.2-5:0.1-5:0.1-5.

[0015] The purpose of the present invention is to provide a sodium ion battery with good high voltage stability, good cycle performance and long service life in view of the shortcomings of the existing technology.

[0016] In order to achieve the above object, the present invention adopts the following technical solutions:

[0017] A sodium ion battery comprises the above-mentioned electrolyte.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The functional additives in the present invention include fluoroethylene carbonate, vinyl sulfate, isatin anhydride, and tri(trimethylsilyl)borate. Fluorinated ethylene carbonate forms a relatively stable and dense SEI film at the negative electrode, improves the wettability of the electrolyte, and reduces the battery impedance; isatin anhydride can neutralize the alkalinity on the surface of the positive electrode material and can form a film at the positive electrode, and fluoroanhydride can generate NaF, which improves conductivity, reduces internal resistance, and inhibits the generation of cracks in the particles of the positive electrode material during the cycle, thereby improving the cycle; vinyl sulfate forms a low-impedance SEI film at the negative electrode, reduces the increase in battery impedance during the cycle, and improves low-temperature and rate performance; tri(trimethylsilyl)borate forms a stable and dense CEI film at the positive electrode, inhibits the dissolution of transition metals in the positive electrode material, improves the stability of the positive electrode material, and at the same time can improve the charge transfer capacity of the electrode and improve the cycle. Isatin anhydrides include one or more of isatin anhydride, 5-fluoroisatin anhydride, and 6-fluoroisatin anhydride.

[0020] The present invention uses several additives in combination to produce a synergistic effect, which can form a stable and dense interface film on the positive and negative electrodes, partially neutralize the alkalinity on the surface of the positive electrode material, inhibit side reactions with the electrolyte, and improve the charge transfer capacity of the electrode. In addition, the interface film formed by the combination of several low-impedance additives has low impedance and reduces battery polarization. Therefore, while achieving excellent cycle performance, it also takes into account low-temperature and rate performance. DETAILED DESCRIPTION

[0021] 1. An electrolyte comprising a conductive sodium salt, a non-aqueous organic solvent and a functional additive, wherein the functional additive comprises one or more of fluoroethylene carbonate, vinyl sulfate, isatin anhydride and tris(trimethylsilyl)borate.

[0022] The functional additives in the present invention include fluoroethylene carbonate, vinyl sulfate, isatin anhydride, and tri(trimethylsilyl)borate. Fluorinated ethylene carbonate forms a relatively stable and dense SEI film at the negative electrode, improves the wettability of the electrolyte, and reduces the battery impedance; isatin anhydride can neutralize the alkalinity on the surface of the positive electrode material and can form a film at the positive electrode, and fluoroanhydride can generate NaF, which improves conductivity, reduces internal resistance, and inhibits the generation of cracks in the particles of the positive electrode material during the cycle, thereby improving the cycle; vinyl sulfate forms a low-impedance SEI film at the negative electrode, reduces the increase in battery impedance during the cycle, and improves low-temperature and rate performance; tri(trimethylsilyl)borate forms a stable and dense CEI film at the positive electrode, inhibits the dissolution of transition metals in the positive electrode material, improves the stability of the positive electrode material, and at the same time can improve the charge transfer capacity of the electrode and improve the cycle. Isatin anhydrides include one or more of isatin anhydride, 5-fluoroisatin anhydride, and 6-fluoroisatin anhydride.

[0023] The present invention uses several additives in combination to produce a synergistic effect, which can form a stable and dense interface film on the positive and negative electrodes, partially neutralize the alkalinity on the surface of the positive electrode material, inhibit side reactions with the electrolyte, and improve the charge transfer capacity of the electrode. In addition, the interface film formed by the combination of several low-impedance additives has low impedance and reduces battery polarization. Therefore, while achieving excellent cycle performance, it also takes into account low-temperature and rate performance.

[0024] Preferably, the functional additive accounts for 1% to 10% of the total mass of the electrolyte. The functional additive accounts for 2% to 10%, 4% to 8%, 4% to 7%, or 4% to 6% of the total mass of the electrolyte. Specifically, the functional additive accounts for 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% of the total mass of the electrolyte.

[0025] Preferably, the conductive sodium salt includes one or more of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate and sodium bis(oxalatoborate).

[0026] Preferably, the conductive sodium salt accounts for 12% to 18% of the total weight of the electrolyte. The conductive sodium salt accounts for 13% to 18%, 13% to 17%, or 14% to 16% of the total weight of the electrolyte. Specifically, the conductive sodium salt accounts for 12%, 14%, 16%, or 18% of the total weight of the electrolyte.

[0027] Preferably, the non-aqueous organic solvent includes one or a mixture of cyclic carbonate, chain carbonate, ethyl propionate, and propyl propionate.

[0028] Preferably, the mass ratio of the cyclic carbonate to the chain carbonate is 1:1 to 4. The mass ratio of the cyclic carbonate to the chain carbonate is 1:1, 1:2, 1:3, or 1:4.

[0029] Preferably, the cyclic carbonate includes one or a mixture of ethylene carbonate and propylene carbonate, and the chain carbonate includes one or a mixture of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0030] Preferably, the mass of the non-aqueous organic solvent accounts for 70% to 85% of the total mass of the electrolyte, and the mass of the non-aqueous organic solvent accounts for 70%, 74%, 77%, 79%, 82%, or 85% of the total mass of the electrolyte.

[0031] Preferably, the functional additive is a mixture of fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate and isatoic anhydride in a weight ratio of 2-8:0.2-5:0.1-5:0.1-5. Preferably, the weight ratio of fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate and isatoic anhydride is 2-8:0.2-5:0.3-5:0.3-5, 2-8:0.2-5:0.1-4:0.1-4, 2-8:0.2-5:0.1-3:0.1-3, 2-8:0.2-5:0.1-2:0.1-2, 2-8:0.2-3:0.1-2:0.1-2.

[0032] 2. A sodium ion battery with good high voltage stability, good cycle performance and long service life.

[0033] A sodium ion battery comprises the above-mentioned electrolyte.

[0034] The present invention will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0035] Example 1

[0036] 1. An electrolyte comprising a conductive sodium salt, a non-aqueous organic solvent and a functional additive, wherein the functional additive comprises fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate and isatoic anhydride.

[0037] The electrolyte is prepared by mixing NaPF6 (sodium hexafluorophosphate), a mixed organic solvent (PC:DEC:EMC=3:2:5), FEC (fluoroethylene carbonate), DTD (ethylene sulfate), TMSB (tris(trimethylsilyl)borate), and isatoic anhydride, and stirring the mixture using a vacuum stirrer until it is stable and uniform to obtain an electrolyte. The mass of NaPF6, the mixed organic solvent, fluoroethylene carbonate, ethylene sulfate, tris(trimethylsilyl)borate, and isatoic anhydride account for 14%, 79%, 5%, 1%, 0.5%, and 0.5% of the total mass of the electrolyte, respectively.

[0038] The electrolyte is used in a sodium ion battery, which also includes a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet.

[0039] The preparation method of the sodium ion battery is as follows:

[0040] 1) Preparation of positive electrode sheet: The positive electrode material Na[Ni 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, binder PVDF, and conductive agent Super-P were dispersed in NMP organic solvent at a mass ratio of 96:2:2. The mixture was stirred in a vacuum mixer until stable and uniform, and then evenly coated onto 12μm-thick aluminum foil. The foil was air-dried at room temperature and then transferred to a 120°C forced air oven for 1 hour. The positive electrode sheet was then cold-pressed and die-cut.

[0041] 2) Negative Electrode Preparation: Spherical hard carbon, binder PVDF, and conductive agent Super-P were mixed in a mass ratio of 97:2:1, dispersed in NMP organic solvent, and evenly coated onto 12μm-thick aluminum foil. The foil was air-dried at room temperature and then transferred to a 120°C forced air oven for 1 hour. The negative electrode sheet was then cold-pressed and die-cut.

[0042] 3) The positive electrode sheet, the negative electrode sheet and the polypropylene separator are laminated to obtain a bare cell, the cell is placed in an aluminum-plastic film packaging shell, the above-mentioned electrolyte is injected, and then the shell is sealed in sequence. After standing, hot and cold pressing, formation, and capacity separation, a sodium ion battery is produced.

[0043] Example 2

[0044] The difference from Example 1 is the arrangement of the electrolyte additives: In this example, the mass fraction of the functional additive isatoic anhydride is adjusted to 0.3%, and the mass fraction of the organic solvent is adjusted to 79.2%.

[0045] The rest is the same as in Example 1 and will not be described in detail here.

[0046] Example 3

[0047] The difference from Example 1 is the arrangement of the electrolyte additives: In this example, the mass fraction of the functional additive isatoic anhydride is adjusted to 1%, and the mass fraction of the organic solvent is adjusted to 78.5%.

[0048] The rest is the same as in Example 1 and will not be described again here.

[0049] Example 4

[0050] The difference from Example 1 is the arrangement of the electrolyte additives: In this example, the mass fraction of the functional additive isatoic anhydride is adjusted to 2%, and the mass fraction of the organic solvent is adjusted to 77.5%.

[0051] The rest is the same as in Example 1 and will not be described again here.

[0052] Example 5

[0053] The difference from Example 1 is the arrangement of the electrolyte additives: In this example, the mass fraction of the functional additive isatoic anhydride is adjusted to 5%, and the mass fraction of the organic solvent is adjusted to 74.5%.

[0054] The rest is the same as in Example 1 and will not be described again here.

[0055] Example 6

[0056] The difference from Example 1 is the setting of the electrolyte additive. In this example, the functional additive isatoic anhydride is adjusted to 5-fluoroisatoic anhydride, and the mass fraction remains unchanged.

[0057] The rest is the same as in Example 1 and will not be described again here.

[0058] Example 7

[0059] The difference from Example 1 is the setting of the electrolyte additive. In this example, the functional additive isatoic anhydride is adjusted to 6-fluoroisatoic anhydride, and the mass fraction remains unchanged.

[0060] The rest is the same as in Example 1 and will not be described again here.

[0061] Comparative Example 1

[0062] The difference from Example 1 is the arrangement of the electrolyte additives: This comparative example does not use the functional additive fluoroethylene carbonate (FEC), and the mass fraction of the organic solvent is adjusted to 84%.

[0063] The rest is the same as in Example 1 and will not be described again here.

[0064] Comparative Example 2

[0065] The difference from Example 1 is the arrangement of the electrolyte additives: This comparative example does not use the functional additive diethyl thiosulfate (DTD), and the mass fraction of the organic solvent is adjusted to 80%.

[0066] The rest is the same as in Example 1 and will not be described again here.

[0067] Comparative Example 3

[0068] The difference from Example 1 is the setting of the electrolyte additives. In this comparative example, the functional additive TMSB is not used, and the mass fraction of the organic solvent is adjusted to 79.5%.

[0069] The rest is the same as in Example 1 and will not be described again here.

[0070] Comparative Example 4

[0071] The difference from Example 1 is the setting of the electrolyte additive: In this comparative example, the functional additive isatoic anhydride is not used, and the mass fraction of the organic solvent is adjusted to 79.5%.

[0072] The rest is the same as in Example 1 and will not be described again here.

[0073] The sodium ion batteries obtained in Examples 1 to 7 and Comparative Examples 1 to 4 were subjected to performance testing, and the test process was as follows:

[0074] 1) High temperature cycle life test

[0075] After the sodium ion battery is charged to 4.2V at 1C constant current at 45°C, it is then charged at constant voltage until the cutoff current is 0.05C, and then discharged to 1.5V at 1C constant current. This is recorded as one charge and discharge cycle. Then, the cycle is repeated according to the above conditions. When the capacity retention rate reaches 80%, the cycle number n is recorded. The sodium ion battery capacity retention rate (%) = (discharge capacity of the nth cycle / initial discharge capacity) × 100%.

[0076] 2) Low temperature performance test

[0077] Charge at room temperature with 1C constant current and constant voltage to 4.2V, cut off at 0.05C, then discharge with 1C constant current to 1.5V cut off, which is recorded as the initial capacity C0. Then charge at room temperature with 1C constant current and constant voltage to 4.2V, cut off at 0.05C. After being fully charged, put it in a low-temperature test cabinet at -40℃ and leave it for 8h; at -40℃, discharge with 0.2C constant current to 1.5V, record the discharge capacity C1, and the discharge efficiency percentage (%) = C1 / C0×100%.

[0078] 3) Rate performance test

[0079] At room temperature, charge the battery at 1C constant current and constant voltage to 4.2V, cut off at 0.05C, then discharge it at 1C constant current to 1.5V, which is recorded as the initial capacity C0; then charge it at room temperature at 1C constant current and constant voltage to 4.2V, cut off at 0.05C, and discharge it at 8C constant current to 1.5V, and record the discharge capacity C1. Discharge efficiency percentage (%) = C1 / C0×100%

[0080] The results of the high temperature cycle performance test, low temperature performance test, rate performance test and battery thickness expansion rate test are shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084] As can be seen from Table 1 above, the electrolyte of the present invention has better high-temperature cycling performance, low-temperature performance, and high-rate performance at high voltage than the electrolyte of the prior art, and the battery cell does not produce gas or deform at high voltage. A comparison of Example 1 with Comparative Examples 1-4 shows that when the functional additives are fluorinated ethylene carbonate, vinyl sulfate, tris(trimethylsilyl)borate, and isatoic anhydride in a weight ratio of 5:1:0.5:0.5, the electrolyte performance is good, with better high-voltage and high-temperature cycling performance, high-voltage and low-temperature performance, and high-rate performance.

[0085] Comparison of Examples 1-5 reveals that an electrolyte with an isatoic anhydride accounting for 0.5% of the total electrolyte mass exhibits superior performance. At a high voltage of 4.2V, the electrolyte achieved 1,362 cycles with 80% capacity retention at 1C / 1C at 45°C. Furthermore, the battery thickness expansion rate was as low as 2.1%, while the discharge efficiency at -40°C and 8C reached 79.3%, respectively, and 75%, respectively. This further optimizes the electrolyte's performance and effectiveness.

[0086] Comparison of Examples 1, 6, and 7 reveals that the electrolytes using 5-fluoroisatoic anhydride and 6-fluoroisatoic anhydride exhibit superior performance. At a high voltage of 4.2V and a high temperature of 45°C, the cycle capacity retention reached 80% at 1C / 1C for 1655 and 1671 cycles, respectively, with a battery thickness expansion rate of 1.2% and 1.3%, respectively, surpassing the performance of Example 1. At a low temperature of -40°C, the discharge efficiency reached 82.4% and 82.5%, respectively, and the 8C discharge efficiency reached 78.7% and 78.5%, respectively, demonstrating even superior performance.

[0087] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises a conductive sodium salt, a non-aqueous organic solvent and a functional additive. The functional additive contains fluoroethylene carbonate, vinyl sulfate, isatin anhydride and tris(trimethylsilyl)borate. The isatin anhydride is 5-fluoroisatin anhydride or 6-fluoroisatin anhydride. The functional additive accounts for 1% to 10% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that The conductive sodium salt includes one or more of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate and sodium bis(oxalatoborate).

3. The electrolyte according to claim 1 or 2, characterized in that The conductive sodium salt accounts for 12% to 18% of the total weight of the electrolyte.

4. The electrolyte according to claim 1, characterized in that The non-aqueous organic solvent includes one or a mixture of cyclic carbonate, chain carbonate, ethyl propionate, and propyl propionate.

5. The electrolyte according to claim 4, characterized in that The mass ratio of the cyclic carbonate to the chain carbonate is 1:1-4.

6. The electrolyte according to claim 4, characterized in that The cyclic carbonate includes one or a mixture of ethylene carbonate and propylene carbonate, and the chain carbonate includes one or a mixture of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

7. The electrolyte according to any one of claims 4 to 6, characterized in that The mass of the non-aqueous organic solvent accounts for 70% to 85% of the total mass of the electrolyte.

8. A sodium ion battery, characterized in that: The electrolyte comprises the electrolyte according to any one of claims 1 to 7.

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