A high-magnification flame-retardant sodium-ion battery electrolyte and a sodium-ion battery

Abstract

The application belongs to the technical field of sodium ion battery electrolyte, and discloses a high-rate flame-retardant sodium ion battery electrolyte and a sodium ion battery.The electrolyte is composed of a sodium salt, an organic solvent and a composite functional additive, and the composite functional additive is composed of a fluorinated phosphite flame retardant, a fluorinated alkane diluent and an inorganic film-forming additive.The fluorinated phosphite forms a stable interface film with high ionic conductivity on the surface of a hard carbon negative electrode, and simultaneously achieves excellent flame-retardant effect.The fluorinated alkane diluent reduces the viscosity of the system and improves the sodium ion migration rate.The inorganic film-forming additive constructs a passivation layer on the positive electrode interface.The synergistic effect of the three components breaks the mutual exclusive bottleneck of "flame retardation-rate performance", so that the sodium ion battery of the hard carbon negative electrode system has a capacity retention rate of >80% at 200 mA·g ‑1 and passes the UL94 V-0 level flame retardant test.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery electrolyte, and particularly relates to a high-rate flame-retardant sodium ion battery electrolyte and a sodium ion battery. BACKGROUND

[0002] Sodium ion batteries have become a research hotspot in large-scale energy storage due to their advantages of abundant sodium resources, low cost and environmental friendliness. However, traditional sodium ion battery electrolytes are usually combined with carbonic acid ester solvents (such as ethylene carbonate / diethyl carbonate) and sodium salts (such as sodium hexafluorophosphate), but the high risk of thermal runaway caused by their flammable characteristics makes it difficult to meet the stringent requirements of flame retardancy in energy storage scenarios. In the prior art, the safety of electrolytes is improved by introducing flame retardants (such as phosphate esters and fluorinated ethers), but these additives often lead to a decrease in ionic conductivity and poor interface compatibility, hindering the transmission of sodium ions and reducing the high-rate charge-discharge performance ( J. Am. Chem. Soc. 2024, 146, 15751-15760). The electrolyte needs to maintain fast ion migration and stable interface at high current density, but the traditional flame-retardant system is difficult to balance the flame retardancy and kinetic performance due to the limitations of solvent / additive molecular structure ( Adv. Funct. Mater. 2022, 32, 2209523). In addition, it is pointed out in Chinese patent CN113381075A that the carbonic acid ester solvent decomposes uncontrollably on the surface of the hard carbon negative electrode, forming a solid electrolyte interface (SEI) film rich in organic components, which has low mechanical strength (elastic modulus <5 GPa) and poor ionic conductivity (<10 -6 S·cm -1 ), resulting in a capacity retention rate of less than 60% at 200 mA·g -1 and poor cycle stability.

[0003] In view of the above problems, it is urgent to develop a new electrolyte system that can improve flame retardancy while ensuring ion transmission efficiency and electrode interface stability at high rates, thereby promoting the practical application of sodium ion batteries in power batteries and high-safety energy storage fields. SUMMARY

[0004] In order to overcome the deficiencies of the prior art, the application provides a high-rate flame-retardant sodium ion battery electrolyte and a sodium ion battery, which effectively overcomes the poor compatibility of traditional phosphates and / or fluorinated phosphates with carbon-based anodes, which cannot form stable SEI films, by using fluorinated phosphite flame retardants, and simultaneously introduces a high-rate flame-retardant electrolyte composed of fluorinated alkane diluents and inorganic film-forming additives to realize coordination environment regulation of anions and cations and form a high-ionic-conductivity and stable interface film. The application cooperatively designs and constructs a stable electrode / electrolyte interface from the bulk electrolyte and the interface structure component, improves the sodium ion transmission efficiency and realizes the high-rate and long-cycle stability of the battery. The sodium ion battery electrolyte can construct a high-ionic-conductivity and stable SEI film on the hard carbon anode interface, reduce the electrolyte viscosity to improve the ion migration rate, form an inorganic passivation layer on the positive electrode interface, simultaneously solve the industry problem of mutual exclusion of flame retardation and rate performance, and realize a capacity retention rate of more than 80% at 200 mA·g -1 Rate and UL94 V-0 level flame retardation.

[0005] The technical scheme of the application is as follows:

[0006] The application provides a high-rate flame-retardant sodium ion battery electrolyte, which is composed of the following components in mass percentage: 5-12 wt% of sodium salt, 65-75 wt% of organic solvent and 15-25 wt% of composite functional additives. The composite functional additives are composed of fluorinated phosphite flame retardant, fluorinated alkane diluent and inorganic film-forming additive. The fluorinated phosphite forms a stable interface film with high ionic conductivity on the negative electrode surface, and realizes electrolyte flame retardation at the same time. The fluorinated alkane diluent reduces the system viscosity and improves the sodium ion migration rate. The inorganic film-forming additive constructs a passivation layer on the positive electrode interface, and the three components cooperatively break through the mutual exclusion bottleneck of "flame retardation-rate performance".

[0007] Further, the fluorinated phosphite flame retardant is at least one of tris(2, 2, 2-trifluoroethyl) phosphite and tris(2, 2-difluoroethyl) phosphite.

[0008] Further, the fluorinated alkane diluent is at least one of 1, 1, 1, 3, 3-pentafluorobutane, 1, 1, 1, 2, 3, 3-hexafluoropropane and 1, 2-bis(1, 1, 2, 2-tetrafluoroethoxy) ethane.

[0009] Further, the inorganic film-forming additive is at least one of sodium difluoro(oxalato)borate, sodium bis(oxalato)borate and sodium tetrafluoroborate.

[0010] Further, the fluorinated phosphite flame retardant in the composite functional additive accounts for 8-12 wt% of the total mass of the electrolyte, the fluorinated alkane diluent accounts for 6-10 wt% of the total mass of the electrolyte, and the inorganic film-forming additive accounts for 1-4 wt% of the total mass of the electrolyte. When the proportion of fluorinated phosphite is higher than 12 wt%, the solubility of sodium salt and the conductivity of electrolyte are reduced, and when it is lower than 8 wt%, the flame retardation effect cannot be achieved; when the proportion of fluorinated alkane diluent is lower than 6 wt%, the viscosity of the electrolyte is too high and the rate performance is reduced; when the proportion of inorganic film-forming additive is lower than 1 wt%, the SEI film is uneven, and when it is higher than 4 wt%, the interface side reaction increases and the interface impedance increases after long cycle.

[0011] Further, the sodium salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium triflate and sodium bisfluorosulfonylimide.

[0012] Further, the organic solvent is at least two of ethylene carbonate, fluorinated ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

[0013] Further, the preparation method of the high-rate flame-retardant sodium-ion battery electrolyte comprises the following steps: in an argon-protected glove box, an organic solvent, a fluorinated phosphite flame retardant and a fluorinated alkane diluent are prepared into a multi-component blended solvent, and then a sodium salt and an inorganic film-forming additive are added and uniformly mixed to obtain the high-rate flame-retardant sodium-ion battery electrolyte.

[0014] The second aspect of the application provides a sodium-ion battery, which is obtained by sequentially assembling a hard carbon negative electrode, a glass fiber separator, the high-rate flame-retardant electrolyte and a positive electrode in an argon atmosphere.

[0015] Further, the positive electrode is a polyanion compound or a sodium metal sheet, wherein the polyanion compound is one of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium vanadium oxyfluorophosphate and sodium iron pyrophosphate.

[0016] The high-rate flame-retardant electrolyte of the application overcomes the poor compatibility of traditional phosphates and / or fluorinated phosphates with carbon-based anodes, which cannot form stable SEI films, by using fluorinated phosphite flame retardants, can decompose on the surface of hard carbon anodes to form SEI films rich in sodium fluoride and sodium phosphate components, improve the sodium ion conduction of the hard carbon interface, and the unsaturated P-O group can capture active H· and HO· free radicals generated during high-temperature combustion of carbonate solvents to achieve flame-retardant effect; the fluorinated alkane diluent with low dielectric constant effectively reduces the viscosity of the electrolyte system and enhances the anion coordination ability, reduces the desolvation of sodium ions and the energy barrier of crossing the SEI interface film; the inorganic film-forming additive generates an inorganic passivation layer containing boron components on the surface of the positive electrode through electrochemical decomposition at high voltage, inhibits the decomposition of fluorinated phosphite and improves the high-voltage stability. The synergistic effect of fluorinated phosphite flame retardant, fluorinated alkane diluent and inorganic film-forming additive speeds up the transmission of sodium ions in the interface and the bulk phase and builds a stable electrode / electrolyte interface, improving the high-rate performance and flame retardancy of sodium ion batteries.

[0017] Advantages and benefits of the application:

[0018] (1) The high-rate flame-retardant electrolyte of the application breaks through the mutual exclusion bottleneck of "flame retardant-rate performance" by adjusting the components and content of the composite functional additives, regulates the coordination environment of anions and cations and forms a high-ionic-conductivity and stable SEI film, and improves the sodium ion transmission efficiency and builds a stable electrode / electrolyte interface from the aspects of bulk electrolyte and interface structure component design, thereby improving the high-rate, long-cycle stability of the battery.

[0019] (2) The fluorinated phosphite flame retardant used in the high-rate flame-retardant electrolyte of the application has excellent flame-retardant efficiency (content 8-12% can meet self-extinguishing time <1s), and the fluorinated phosphite flame retardant overcomes the poor compatibility of traditional phosphates and / or fluorinated phosphates with carbon-based anodes, which cannot form stable SEI films, can form a high-conductivity interface film on the surface of hard carbon, and improve the sodium ion transmission kinetics and stability of the electrode / electrolyte interface. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 Figure is the electrolyte flame retardant performance test diagram of comparative example 1 and example 1;

[0021] Figure 2 Figure is the charge-discharge curve diagram of the sodium ion battery prepared by the electrolyte of comparative example 1 at different current densities (active material loading: 3 mg / cm 2 , voltage window: 0.01-1.5V);

[0022] Figure 3Charge-discharge curves of sodium-ion batteries prepared with the electrolyte of Example 1 at different current densities. (Active material loading: 3 mg / cm 2 voltage window: 0.01 - 1.5 V).

[0023] Figure 4 Rate curves of hard carbon / / sodium vanadium fluorophosphate full cells prepared with the electrolyte of Comparative Example 1 and Example 1. (N / P = 1.1, voltage window: 2.0 - 4.3 V). DETAILED DESCRIPTION

[0024] The application will be described in detail below with reference to the embodiments, but these embodiments are not meant to limit the scope of protection of the application.

[0025] The reagents, materials and the like used in the following examples are commercially available or obtained by conventional synthesis in the art unless otherwise specified.

[0026] Example 1

[0027] A high-rate flame-retardant sodium-ion battery electrolyte is prepared by mixing 30 wt% of vinyl carbonate, 40 wt% of dimethyl carbonate, 10 wt% of tris(2, 2, 2-trifluoroethyl) phosphite, and 7 wt% of 1, 1, 1, 3, 3-pentafluorobutane into a multi-component blended solvent in an argon-protected glove box, and then adding 10 wt% of sodium salt sodium hexafluorophosphate and 3 wt% of inorganic film-forming additive sodium difluoro(oxalato)borate into the blended solvent and mixing uniformly to obtain the high-rate flame-retardant sodium-ion battery electrolyte.

[0028] Example 2

[0029] A high-rate flame-retardant sodium-ion battery electrolyte is prepared by mixing 30 wt% of vinyl carbonate, 45 wt% of dimethyl carbonate, 8 wt% of tris(2, 2, 2-trifluoroethyl) phosphite, and 6 wt% of 1, 1, 1, 3, 3-pentafluorobutane into a multi-component blended solvent in an argon-protected glove box, and then adding 10 wt% of sodium salt sodium hexafluorophosphate and 1 wt% of inorganic film-forming additive sodium difluoro(oxalato)borate into the blended solvent and mixing uniformly to obtain the high-rate flame-retardant sodium-ion battery electrolyte.

[0030] Example 3

[0031] A high-rate flame-retardant sodium-ion battery electrolyte, the preparation method is: in the glove box under argon protection, 30wt% of vinyl carbonate, 35wt% of dimethyl carbonate, 12wt% of tris (2, 2, 2-trifluoroethyl) phosphite, 10wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a multi-component blended solvent, then 9% of sodium salt sodium hexafluorophosphate, 4% of inorganic film-forming additive sodium difluoro (oxalato) borate are added, and the mixture is uniformly prepared into a high-rate flame-retardant sodium-ion battery electrolyte.

[0032] Comparative example 1

[0033] A sodium-ion battery electrolyte, which is different from example 1 in that it does not contain a composite functional additive, the preparation method is: in the glove box under argon protection, 30wt% of vinyl carbonate and 60wt% of dimethyl carbonate are prepared into a binary blended solvent, then 10% of sodium salt sodium hexafluorophosphate is added, and the mixture is uniformly prepared into the sodium-ion battery electrolyte of comparative example 1.

[0034] Comparative example 2

[0035] A sodium-ion battery electrolyte, which is different from example 1 in that it does not contain a fluorinated alkane diluent, the preparation method is: in the glove box under argon protection, 30wt% of vinyl carbonate, 47wt% of dimethyl carbonate and 10wt% of tris (2, 2, 2-trifluoroethyl) phosphite are prepared into a ternary blended solvent, then 10% of sodium salt sodium hexafluorophosphate and 3% of inorganic film-forming additive sodium difluoro (oxalato) borate are added, and the mixture is uniformly prepared into the sodium-ion battery electrolyte of comparative example 2.

[0036] Comparative example 3

[0037] A sodium-ion battery electrolyte, which is different from example 1 in that it does not contain a fluorinated phosphite flame retardant, the preparation method is: in the glove box under argon protection, 30wt% of vinyl carbonate, 47wt% of dimethyl carbonate and 10wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a ternary blended solvent, then 10% of sodium salt sodium hexafluorophosphate and 3% of inorganic film-forming additive sodium difluoro (oxalato) borate are added, and the mixture is uniformly prepared into the sodium-ion battery electrolyte of comparative example 3.

[0038] Comparative example 4

[0039] A sodium-ion battery electrolyte, which is different from example 1 in that it does not contain an inorganic film-forming additive, the preparation method is: in the glove box under argon protection, 30wt% of vinyl carbonate, 40wt% of dimethyl carbonate, 10wt% of tris (2, 2, 2-trifluoroethyl) phosphite and 10wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a multi-component blended solvent, then 10% of sodium salt sodium hexafluorophosphate is added, and the mixture is uniformly prepared into the sodium-ion battery electrolyte of comparative example 4.

[0040] Comparative Example 5

[0041] A sodium-ion battery electrolyte, which is different from Example 1 in that a conventional flame retardant is used, and the preparation method is as follows: in an argon-protected glove box, 30wt% of vinyl carbonate, 40wt% of dimethyl carbonate, 10wt% of triethyl phosphate, and 7wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a multi-component blended solvent, and then 10% of sodium salt sodium hexafluorophosphate and 3% of inorganic film-forming additive sodium difluoro(oxalato)borate are added and uniformly mixed to prepare the sodium-ion battery electrolyte of Comparative Example 5.

[0042] Comparative Example 6

[0043] A sodium-ion battery electrolyte, and the preparation method is as follows: in an argon-protected glove box, 30wt% of vinyl carbonate, 50wt% of dimethyl carbonate, 5wt% of tris(2, 2, 2-trifluoroethyl) phosphite, and 4wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a multi-component blended solvent, and then 10% of sodium salt sodium hexafluorophosphate and 1% of inorganic film-forming additive sodium difluoro(oxalato)borate are added and uniformly mixed to prepare the sodium-ion battery electrolyte of Comparative Example 6.

[0044] Comparative Example 7

[0045] A sodium-ion battery electrolyte, and the preparation method is as follows: in an argon-protected glove box, 30wt% of vinyl carbonate, 20wt% of dimethyl carbonate, 18wt% of tris(2, 2, 2-trifluoroethyl) phosphite, and 17wt% of 1, 1, 1, 3, 3-pentafluorobutane are prepared into a multi-component blended solvent, and then 10% of sodium salt sodium hexafluorophosphate and 5% of inorganic film-forming additive sodium difluoro(oxalato)borate are added and uniformly mixed to prepare the sodium-ion battery electrolyte of Comparative Example 7.

[0046] Performance test and evaluation method

[0047] 1. Flame retardant performance test: self-extinguishing time (SET) is used to evaluate the flame retardant performance of the electrolyte. A glass cotton ball with a diameter of 5mm is weighed, a certain mass of electrolyte is injected onto the glass cotton ball using a syringe, and then the glass cotton ball is quickly ignited using an ignition device. The time from the ignition device moving away to the flame automatically extinguishing is recorded, which is the self-extinguishing time. The results are shown in Table 1.

[0048] 2. Conductivity test: the conductivity of the electrolyte at 25℃ is measured using a conductivity meter, and the results are shown in Table 1.

[0049] 3. Preparation of sodium-ion battery electrode, assembly of sodium-ion battery, and electrochemical performance test:

[0050] Sodium-ion battery electrode preparation: Commercial hard carbon (Kuraray Type-2), conductive carbon black and binder carboxymethyl cellulose sodium were mixed uniformly at a mass ratio of 8:1:1, and then a proper amount of deionized water dispersant was added to prepare a slurry which was uniformly coated on an aluminum foil current collector. After drying at 80°C in a vacuum environment for 10 hours, the negative electrode disc with a diameter of 12 mm was cut and used for battery testing. The preparation process of the sodium vanadium fluorophosphate positive electrode was the same as that of the negative electrode, in which sodium vanadium fluorophosphate, conductive carbon black, binder polyvinylidene fluoride and N-methyl pyrrolidone were used as the positive electrode active material, conductive agent, binder and dispersant, respectively.

[0051] Sodium-ion battery assembly: CR2032 button half-batteries were assembled in an argon glove box. The negative electrode shell, sodium sheet, glass fiber separator, electrolyte, negative electrode sheet, and positive electrode shell were sequentially assembled, and the sodium metal sheet was used as the counter electrode. For hard carbon / / sodium vanadium fluorophosphate full batteries: the negative electrode shell, hard carbon negative electrode sheet, glass fiber separator, electrolyte, sodium vanadium fluorophosphate positive electrode sheet, and positive electrode shell were sequentially assembled. The assembled half-batteries and full batteries were packaged by a sealing machine, and the electrochemical performance test was carried out after 24 hours of normal temperature storage.

[0052] Test conditions: The rate performance of sodium-ion batteries was tested at 25°C under a voltage range of 0.01-1.5 V using a current density of 20 mAh·g -1 , 50 mAh·g -1 , 100 mAh·g -1 , 200 mAh·g -1 , and 500 mAh·g -1 , and the cycle performance of sodium-ion batteries was tested at a current density of 200 mAh·g -1 . The data results are shown in Table 2. The rate performance of hard carbon / / sodium vanadium fluorophosphate full batteries was tested at 2.0-4.3 V.

[0053] Table 1: Conductivity and self-extinguishing time test results of electrolyte

[0054]

[0055] Table 2: Performance test of sodium-ion batteries

[0056]

[0057] From the data in Tables 1 and 2 and Figure 1It can be seen that the high-rate flame-retardant electrolyte of Example 1 has excellent flame-retardant effect, with self-extinguishing time < 1s and reaching UL94 V-0 level of flame-retardant standard, while the conventional carbonate electrolyte (Comparative Example 1) is flammable and has self-extinguishing time of 56.7s. Even the electrolyte (Comparative Example 5) added with conventional triethyl phosphate can achieve flame-retardant effect, but its conductivity, cycle stability and rate performance are far lower than those of the high-rate flame-retardant electrolyte of the present application. Through the additive proportion comparison of Examples 1-3, it can be found that within the scope of protection claimed by the present application, the rate performance, stability and flame-retardant effect can be synergistically improved.

[0058] Figure 2 and Figure 3 are respectively the charge-discharge curve diagrams of Comparative Example 1 and Example 1 at different current densities. As can be seen from the diagrams, the charge polarization voltage of Comparative Example 1 significantly increases with the increase of current density, and the capacity in the platform region also decreases, indicating that the sodium ion desolvation and interface transmission kinetics of Comparative Example 1 are far lower than those of Example 1. At the same time, Example 1 realizes more capacity exertion in the platform region and better rate performance. In addition, the performance comparison of sodium ion batteries of Comparative Examples 2-4 further shows that removing any component cannot simultaneously meet the high-rate capacity retention rate.

[0059] Figure 4 The rate test diagram of the hard carbon / / sodium vanadium fluorophosphate full battery of Example 1 also shows that the capacity exertion of Example 1 at different current densities is higher than that of Comparative Example 1.

[0060] The above-described examples only express several embodiments of the present application, and the description is relatively specific and detailed, but it should not be understood as limiting the scope of the patent. It should be pointed out that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A high-rate flame-retardant sodium-ion battery electrolyte, characterized in that, The electrolyte is composed of the following components by mass percentage: 5-12 wt% sodium salt, 65-75 wt% organic solvent, and 15-25 wt% composite functional additive. The composite functional additive consists of a fluorophosphite flame retardant, a fluoroalkane diluent, and an inorganic film-forming additive. The fluorophosphite forms a stable interfacial film with high ionic conductivity on the negative electrode surface, simultaneously achieving flame retardancy in the electrolyte. The fluoroalkane diluent reduces the system viscosity and increases the sodium ion migration rate. The inorganic film-forming additive constructs a passivation layer at the positive electrode interface. The synergistic effect of these three components overcomes the mutual exclusion bottleneck between flame retardancy and rate performance. The high-rate flame-retardant sodium-ion battery electrolyte has a self-extinguishing time of 0 s, meeting the UL94 V-0 flame retardancy standard. The prepared sodium-ion battery retains a cycle capacity of 92.5-95.6% at 200 mA·g. -1 At current densities, the capacity retention is 81.9-82.6%, at 500 mA·g. -1 At current density, the capacity retention is 65.3-68.4%. The fluorophosphite flame retardant is at least one of tris(2,2,2-trifluoroethyl) phosphite and tris(2,2-difluoroethyl) phosphite. The fluoroalkane diluent is at least one of 1,1,1,3,3-pentafluorobutane, 1,1,1,2,3,3-hexafluoropropane, and 1,2-bis(1,1,2,2-tetrafluoroethoxy)ethane. The inorganic film-forming additive is at least one of sodium difluorooxalate borate, sodium bis(oxalate borate), and sodium tetrafluoroborate.

2. The high-rate flame-retardant sodium-ion battery electrolyte according to claim 1, characterized in that, The fluorophosphite flame retardant accounts for 8-12 wt% of the total mass of the electrolyte, the fluoroalkane diluent accounts for 6-10 wt% of the total mass of the electrolyte, and the inorganic film-forming additive accounts for 1-4 wt% of the total mass of the electrolyte.

3. The high-rate flame-retardant sodium-ion battery electrolyte according to claim 1, characterized in that, The sodium salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate, and sodium difluorosulfonamide.

4. The high-rate flame-retardant sodium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent includes at least two of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

5. A sodium-ion battery, characterized in that, It is obtained by sequentially assembling a hard carbon negative electrode, a glass fiber separator, a high-rate flame-retardant sodium-ion battery electrolyte as described in any one of claims 1-4, and a positive electrode in an argon atmosphere. The positive electrode is a polyanionic compound or a metallic sodium sheet, wherein the polyanionic compound is one of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium vanadium fluorophosphate, and sodium iron pyrophosphate.

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