Electrolyte suitable for sodium ion battery and sodium ion battery

Abstract

The invention relates to an electrolyte suitable for a sodium ion battery and the sodium ion battery. In order to solve the problems of poor normal temperature cycle performance, rate capability and high and low temperature performance of the sodium ion battery, the invention provides an electrolyte suitable for the sodium ion battery, the electrolyte comprises an electrolyte salt, an organic solvent and an additive, the organic solvent comprises a carbonic ester solvent, a fluorocarboxylic ester solvent and a fluorobenzene solvent, the carbonic ester solvent does not contain ethylene carbonate. Through cooperation of the carbonic ester solvent, the fluorocarboxylic ester solvent and the fluorobenzene solvent, the normal-temperature cycle performance, the rate capability and the high and low temperature performance of the sodium-ion battery are improved.

Description

Technical Field

[0001] The invention relates to an electrolyte suitable for a sodium ion battery and a sodium ion battery. Background Art

[0002] With the gradual scarcity of traditional fossil energy and increasingly serious environmental problems, the development of new renewable energy has become an inevitable trend. At present, electric vehicles and smart grid energy storage systems are developing rapidly. Secondary batteries, as a high-efficiency energy storage technology, have the characteristics of fast response speed and high energy density. However, limited lithium resources have become the main bottleneck restricting the large-scale energy storage application of lithium-ion batteries. The development of resource-rich and environmentally friendly sodium-ion batteries is of great strategic significance.

[0003] The working principle of sodium ion batteries is the same as that of lithium ion batteries. During the charging and discharging process, sodium ions are embedded and released between the positive and negative electrodes of the battery.

[0004] The current commercial electrolyte system is composed of sodium salt, organic solvent and functional additives. The organic solvent mainly includes ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), etc. Often a single solvent is difficult to make the battery work properly, and different solvents need to cooperate with each other to make the electrolyte system show a more ideal comprehensive performance. Among them, ethylene carbonate has become an important component of the commercial electrolyte system due to its high dielectric constant. However, the melting point of ethylene carbonate is 36.5℃, and it is easy to solidify or even crystallize at low temperatures, resulting in increased viscosity of the electrolyte system, decreased conductivity, and poor low temperature performance. At the same time, ethylene carbonate is more likely to undergo oxidative decomposition with the positive electrode material under high pressure, and the by-products increase the impedance of the battery system. Decomposition and gas production will also lead to a decrease in safety. In contrast, propylene carbonate is an ideal electrolyte solvent due to its excellent dielectric constant, good lithium salt solubility, wider liquid range and outstanding high pressure resistance. However, propylene carbonate has poor compatibility with graphite negative electrodes and is easily embedded in the graphite interlayer, causing structural delamination of the graphite material and affecting battery performance. Therefore, it is difficult for sodium ion batteries in the prior art to have both room temperature cycle performance, rate performance, and high and low temperature performance. Summary of the invention

[0005] The present invention aims to provide an electrolyte suitable for a sodium ion battery, wherein the electrolyte can improve the room temperature cycle performance, rate performance and high and low temperature performance of the sodium ion battery.

[0006] The second object of the present invention is to provide a sodium ion battery having excellent room temperature cycle performance, rate performance, and high and low temperature performance.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] An electrolyte suitable for a sodium ion battery comprises an electrolyte salt, an organic solvent and an additive, wherein the organic solvent comprises a carbonate solvent, a fluorocarboxylic acid ester solvent and a fluorobenzene solvent, and the carbonate solvent does not comprise ethylene carbonate.

[0009] Preferably, the carbonate solvent includes cyclic carbonate and chain carbonate, and the mass ratio of the cyclic carbonate to the chain carbonate is 1:(0.8-1.5), for example, 1:0.8, 1:0.9, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5.

[0010] In some embodiments, the cyclic carbonate includes propylene carbonate.

[0011] In some embodiments, the linear carbonate includes one or more of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0012] In some specific and preferred embodiments, the carbonate solvent includes propylene carbonate and ethyl methyl carbonate.

[0013] Preferably, the fluorocarboxylic acid ester solvent includes one or more of 2,2-difluoromethyl acetate, 2,2-difluoroethyl acetate, and methyl pentafluoropropionate.

[0014] Preferably, the fluorinated benzene solvent includes one or more of fluorobenzene, difluorobenzene, perfluorobenzene (ie, hexafluorobenzene), trifluorotoluene, and pentafluorotoluene.

[0015] Preferably, the carbonate solvent accounts for 50-75% of the total mass of the electrolyte.

[0016] Further preferably, the carbonate solvent accounts for 55-75% of the total mass of the electrolyte, for example, 55%, 60%, 65%, 70% or 75%.

[0017] Preferably, the fluorocarboxylate solvent accounts for 5-20% of the total mass of the electrolyte.

[0018] More preferably, the fluorocarboxylate solvent accounts for 8-12% of the total mass of the electrolyte, for example 8%, 9%, 10%, 11% or 12%.

[0019] Preferably, the fluorobenzene solvent accounts for 5-15% of the total mass of the electrolyte.

[0020] Further preferably, the fluorobenzene solvent accounts for 8-12% of the total mass of the electrolyte, for example, 8%, 9%, 10%, 11% or 12%.

[0021] Preferably, the electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate, sodium bistrifluoromethylsulfonyl imide, sodium bisfluorosulfonyl imide, sodium tetrafluoroborate, sodium bisoxalatoborate, and sodium difluorooxalatoborate.

[0022] Preferably, the electrolyte salt accounts for 10-20% of the total mass of the electrolyte, more preferably 12-18%, for example 12%, 14%, 16% or 18%.

[0023] In some embodiments, the electrolyte salt includes sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide.

[0024] Furthermore, the mass ratio of the sodium hexafluorophosphate, the sodium bis(fluorosulfonyl)imide and the sodium bis(trifluoromethylsulfonyl)imide is 1:(0.1-0.3):(0.1-0.3), and more preferably 1:(0.15-0.25):(0.15-0.25).

[0025] Preferably, the additive comprises fluoroethylene carbonate and / or vinyl sulfate.

[0026] Preferably, the additive accounts for 1-5% of the total mass of the electrolyte, for example, 1%, 2%, 3%, 4% or 5%.

[0027] Preferably, the organic solvent further comprises one or more of a carboxylate solvent and an ether solvent.

[0028] In some specific and preferred embodiments, based on the total mass of the electrolyte being 100%, the electrolyte comprises the following components:

[0029] Carbonate solvent 50-75%,

[0030] Fluorinated carboxylic acid ester solvent 5-20%,

[0031] 5-15% of fluorobenzene solvents,

[0032] Electrolyte salt 10-20%,

[0033] Additives 1~5%,

[0034] Among them, the carbonate solvent includes one or more of propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the fluorocarboxylic acid ester solvent includes one or more of 2,2-difluoromethyl acetate, 2,2-difluoroethyl acetate, and methyl pentafluoropropionate; the fluorobenzene solvent includes one or more of fluorobenzene, difluorobenzene, perfluorobenzene, trifluorotoluene, and pentafluorotoluene; the electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate, sodium bistrifluoromethylsulfonyl imide, sodium bisfluorosulfonyl imide, sodium tetrafluoroborate, sodium bisoxalatoborate, and sodium difluorooxalatoborate; and the additive includes fluoroethylene carbonate and / or vinyl sulfate.

[0035] A second object of the present invention is to provide a sodium ion battery comprising a positive electrode, a negative electrode and a separator, wherein the sodium ion battery also comprises the electrolyte as described above.

[0036] Preferably, the active material of the negative electrode is hard carbon.

[0037] Preferably, the active material of the negative electrode may also be an alloy.

[0038] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0039] The invention improves the room temperature cycle performance, rate performance and high and low temperature performance of the sodium ion battery by synergistically combining carbonate solvents, fluorinated carboxylic acid ester solvents and fluorinated benzene solvents. DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with the examples. However, the present invention is not limited to the following examples. The implementation conditions used in the examples can be further adjusted according to the different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in each embodiment of the present invention can be combined with each other as long as they do not conflict with each other.

[0041] If not otherwise specified, EC in the present invention refers to ethylene carbonate, PC refers to propylene carbonate, DEC refers to diethyl carbonate, EMC refers to ethyl methyl carbonate, DFMA refers to 2,2-difluoromethyl acetate (CAS No.: 433-53-4), DFEA refers to 2,2-difluoroethyl acetate (CAS No.: 1550-44-3), MA refers to methyl acetate, EA refers to ethyl acetate, FB refers to fluorobenzene, PFB refers to pentafluorotoluene (CAS No.: 771-56-2), 3F-MB refers to trifluorotoluene, NaPF6 refers to sodium hexafluorophosphate, NaFSI refers to sodium bis(trifluoromethanesulfonyl)imide, NaTFSI refers to sodium bis(trifluoromethanesulfonyl)imide, FEC refers to fluoroethylene carbonate, and DTD refers to vinyl sulfate.

[0042] The raw materials involved in the following examples and comparative examples are all commercially available products.

[0043] Prepare the electrolyte in the glove box according to the components and proportions shown in Table 1 below.

[0044] Table 1

[0045]

[0046] The electrolytes in the above examples and comparative examples were assembled into sodium ion batteries and then the performance was measured. The results are shown in Table 2. 1 / 3 Fe 1. / 3 Mn 1 / 3 O2 is used as the positive electrode material, hard carbon is used as the negative electrode material, the positive and negative electrode fluids are aluminum foils respectively, and the separator is PP. The assembly method of the positive electrode, the negative electrode and the sodium ion battery refers to the prior art, and this application does not make any specific restrictions.

[0047] Table 2

[0048]

[0049]

[0050] The test method of 25℃1.0C cycle 500Z is as follows: at room temperature (25℃), the sodium ion battery is charged to 4.0V at 1.0C constant current, charged to 0.05C at 4.0V constant voltage, and discharged to 2.0V at 1.0C constant current for 500 cycles.

[0051] 25℃1.0C cycle 500Z capacity retention rate / % = 500 cycles of discharge capacity / first cycle of discharge capacity × 100%

[0052] The test method for 10C rate performance is as follows: at room temperature (25°C), the sodium ion battery is charged to 4.0V at a constant current of 0.5C and discharged to 2.0V at a constant current of 0.5C, and this is repeated for 5 cycles. After that, the sodium ion battery is left to stand for 10 minutes; then the sodium ion battery is charged to 4.0V at a constant current of 1C, and discharged to 2.75V at a current of 1C, and this is repeated for 5 cycles; then the sodium ion battery is left to stand for 10 minutes; then the sodium ion battery is charged to 4.0V at a constant current of 10C, and discharged to 2.0V at a constant current of 10C, and this is repeated for 5 cycles.

[0053] 10C rate capacity retention rate / % = 10C discharge last cycle discharge capacity / 10C discharge first cycle discharge capacity × 100%

[0054] The test method for low temperature (-40℃) 1.0C discharge efficiency is as follows: at room temperature (25℃), the sodium ion battery is charged to 4.0V at 1.0C constant current, and charged to 0.05C at 4.0V constant voltage; at -40℃, it is discharged to 2.0V at 1.0C constant current;

[0055] Low temperature (-40℃) 1.0C discharge efficiency / % = -40℃ condition, 1.0C constant current discharge to 2.0V capacity / 25℃ condition, 1.0C charge to 4.0V capacity * 100%

[0056] The test method for the battery capacity retention rate after being placed at high temperature (85°C) for 4 hours is as follows: at room temperature (25°C), the sodium ion battery is charged to 4.0V at a constant current of 0.5C; the sodium ion battery is placed in an 85°C constant temperature box and left to stand for 4 hours. At room temperature (25°C), it is then discharged at a constant current of 1.0C to a voltage of 2.0V.

[0057] Battery capacity retention rate after being stored at high temperature (85°C) for 4 hours / % = discharge capacity / charge capacity × 100%

[0058] The test method for the swelling rate of the battery after being placed at high temperature (85°C) for 4 hours is as follows: at room temperature (25°C), record the thickness of the sodium battery; place the sodium ion battery in an 85°C constant temperature box and leave it for 4 hours. After taking out the sodium battery, record the thickness of the sodium battery again.

[0059] Battery swelling rate after being placed at high temperature (85°C) for 4 hours / % = (thickness of sodium battery after placing - thickness of sodium battery before placing) / thickness of sodium battery before placing * 100%

[0060] By comparing the examples with the comparative examples, it is found that the sodium ion battery of the present application has better room temperature cycle performance, rate performance, and high and low temperature performance. The inventors speculate that the introduction of fluorocarboxylic acid ester and fluorobenzene changes the solvation structure of the electrolyte and forms an interface film rich in NaF. The two together inhibit the dissolution of the interface film in the electrolyte. At the same time, the introduction of fluorocarboxylic acid ester and fluorobenzene weakens the coordination solvent on Na + affinity, which can promote Na + The desolvation process can improve the room temperature cycle performance, rate performance, and high and low temperature performance of sodium-ion batteries.

[0061] By comparing Examples 3, 4 and 5, it is found that with the increase of the fluorobenzene content in the electrolyte, the room temperature cycle performance, rate performance and high and low temperature performance of the battery first increase and then decrease. Preferably, fluorobenzene accounts for 5 to 15% of the total mass of the electrolyte, and more preferably 8 to 12%.

[0062] By comparing Examples 7, 8 and 9, it is found that with the increase of the content of fluorocarboxylic acid ester in the electrolyte, the room temperature cycle performance, rate performance and high and low temperature performance of the battery first increase and then decrease. Preferably, the fluorocarboxylic acid ester accounts for 5 to 20% of the total mass of the electrolyte, and more preferably 8 to 15%.

[0063] By comparing Examples 1, 2 and 3, it is found that adding 3F-MB to the electrolyte improves the room temperature cycle performance, rate performance and high and low temperature performance of the battery better than adding FB and PFB.

[0064] The above detailed description of the present invention is intended to enable persons familiar with the art to understand the contents of the present invention and implement them. It does not limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. An electrolyte suitable for a sodium ion battery, comprising an electrolyte salt, an organic solvent and an additive, characterized in that: The organic solvent includes carbonate solvents, fluorocarboxylic acid ester solvents and fluorobenzene solvents, and the carbonate solvent does not include ethylene carbonate.

2. The electrolyte according to claim 1, characterized in that: The carbonate solvent includes cyclic carbonate and chain carbonate, and the mass ratio of the cyclic carbonate to the chain carbonate is 1:(0.8-1.5).

3. The electrolyte according to claim 2, characterized in that: The cyclic carbonate includes propylene carbonate; and / or the chain carbonate includes one or more of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

4. The electrolyte according to claim 1, characterized in that: The fluorocarboxylic acid ester solvent includes one or more of 2,2-difluoromethyl acetate, 2,2-difluoroethyl acetate, and methyl pentafluoropropionate; and / or, The fluorinated benzene solvent includes one or more of fluorobenzene, difluorobenzene, perfluorobenzene, trifluorotoluene, and pentafluorotoluene.

5. The electrolyte according to any one of claims 1 to 4, characterized in that: The carbonate solvent accounts for 50-75% of the total mass of the electrolyte, the fluorocarboxylate solvent accounts for 5-20% of the total mass of the electrolyte, and the fluorobenzene solvent accounts for 5-15% of the total mass of the electrolyte.

6. The electrolyte according to claim 5, characterized in that: The carbonate solvent accounts for 55-75% of the total mass of the electrolyte, the fluorocarboxylate solvent accounts for 8-12% of the total mass of the electrolyte, and the fluorobenzene solvent accounts for 8-12% of the total mass of the electrolyte.

7. The electrolyte according to claim 1, characterized in that: The electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate, sodium bistrifluoromethylsulfonyl imide, sodium bisfluorosulfonyl imide, sodium tetrafluoroborate, sodium bisoxalatoborate and sodium difluorooxalatoborate; and / or, The electrolyte salt accounts for 10-20% of the total mass of the electrolyte.

8. The electrolyte according to claim 7, characterized in that: The electrolyte salt includes sodium hexafluorophosphate, sodium bisfluorosulfonyl imide and sodium bistrifluoromethylsulfonyl imide, and the mass ratio of the sodium hexafluorophosphate, the sodium bisfluorosulfonyl imide and the sodium bistrifluoromethylsulfonyl imide is 1:(0.1-0.3):(0.1-0.3).

9. The electrolyte according to claim 1, characterized in that: The additive comprises fluoroethylene carbonate and / or vinyl sulfate; and / or, The additive accounts for 1-5% of the total mass of the electrolyte.

10. The electrolyte according to claim 1, characterized in that: Taking the total mass of the electrolyte as 100%, the electrolyte includes the following components: Carbonate solvent 50-75%, Fluorinated carboxylic acid ester solvent 5-20%, 5-15% of fluorobenzene solvents, Electrolyte salt 10-20%, Additives 1~5%, Among them, the carbonate solvent includes one or more of propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the fluorocarboxylic acid ester solvent includes one or more of 2,2-difluoromethyl acetate, 2,2-difluoroethyl acetate, and methyl pentafluoropropionate; the fluorobenzene solvent includes one or more of fluorobenzene, difluorobenzene, perfluorobenzene, trifluorotoluene, and pentafluorotoluene; the electrolyte salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium difluorophosphate, sodium bistrifluoromethylsulfonyl imide, sodium bisfluorosulfonyl imide, sodium tetrafluoroborate, sodium bisoxalatoborate, and sodium difluorooxalatoborate; and the additive includes fluoroethylene carbonate and / or vinyl sulfate.

11. A sodium ion battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The sodium ion battery further comprises an electrolyte according to any one of claims 1 to 10.

12. The sodium ion battery according to claim 11, characterized in that: The active material of the negative electrode is hard carbon.

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