Electrolyte and sodium ion battery

Abstract

The invention relates to the technical field of electrochemical energy storage, in particular to an electrolyte and a sodium ion battery. Wherein the electrolyte comprises a sodium salt, a solvent, a diluent and an additive, the diluent is a fluorinated solvent, and the additive is a triphenylphosphonium bromide derivative. According to the electrolyte disclosed by the invention, an anion-dominated solvation structure is formed under the condition that the ionic conductivity is not influenced, and meanwhile, an excellent SEI membrane is constructed, so that the cycle performance of the sodium-ion battery in a low-temperature environment is effectively improved.

Description

Technical Field

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

[0002] Since their successful commercialization in the 1990s, lithium-ion batteries (LIBs) have dominated the market for portable electronics and electric vehicles. However, with the rapid development of renewable energy storage systems, the scarcity of lithium reserves has caused great concerns about the supply cost of lithium-related raw materials, which has accelerated the development of low-cost alternative rechargeable battery technologies. Sodium-ion batteries (SIBs) are widely considered to be the most promising candidates in fields such as large-scale energy storage and low-speed electric vehicles due to their abundant sodium reserves and good economic / environmental sustainability. Although research on SIBs has made some progress under room temperature conditions and has been partially commercialized, they are limited by the Na + Due to the kinetic and thermodynamic characteristics of SIBs, the electrochemical performance of SIBs at low temperatures remains challenging for the urgent need of deployment in extreme environments.

[0003] The above challenges are mainly due to the low temperature Na + The slow diffusion of electrons in the electrode material and the increase in electrolyte viscosity or solidification lead to a decrease in ionic conductivity, which hinders the Na + Transport in the bulk electrolyte, Na + Difficult desolvation and Na + Slow migration across the electrode / electrolyte interface (EEI) film. It should be noted that the slow charge transfer kinetics at the interface (including Na + Desolvation and Na + Migration in the solid electrolyte interface (SEI) film) has been precisely identified as the main limiting factor at low temperatures, and both are closely related to the Na + The solvated structures are highly related.

[0004] In order to deal with the above-mentioned difficulties encountered by sodium-ion batteries in low-temperature environments, researchers have tried to design low-temperature electrolyte systems by selecting weak solvents (WSEs), introducing new sodium salts, or increasing the concentration of sodium salts to optimize the solvation structure of sodium ions and thereby improve the low-temperature performance of the battery. However, the choice of weak solvents is often limited by the solubility of sodium salts; finding new sodium salts or increasing the concentration of sodium salts will bring about the problem of increased costs. In addition, in locally high-concentration electrolytes (LHCEs), although a solvation structure similar to that of high-concentration electrolytes (HCEs) can be constructed by adjusting the ratio of solvents and diluents, sodium salts have low dissociation in such electrolytes, resulting in poor ionic conductivity, which limits the practical application of locally high-concentration electrolytes. Summary of the Invention

[0005] The present invention provides an electrolyte that forms an anion-dominated solvation structure without affecting ionic conductivity, while forming an excellent SEI film, effectively improving the cycle performance of sodium ion batteries in low-temperature environments.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] An electrolyte comprises a sodium salt, a solvent, a diluent and an additive, wherein the diluent is a fluorinated solvent and the additive is a triphenylphosphine bromide derivative, and the chemical formula thereof is as follows:

[0008]

[0009] Among them, R is taken from H, C 1-5 Alkyl, C 1-5 Alkenyl, substituted C 1-5 One of alkyl, phenyl, and benzyl.

[0010] Furthermore, the additive includes at least one of methyltriphenylphosphonium bromide, ethyltriphenylphosphonium bromide, propyltriphenylphosphonium bromide, allyltriphenylphosphonium bromide, isobutyltriphenylphosphonium bromide, benzyltriphenylphosphonium bromide, and isoamyl triphenylphosphonium bromide.

[0011] Furthermore, the diluent is a highly fluorinated solvent, and the diluent includes at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), hexafluoroisopropyl methyl ether (HFME), and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (OFE).

[0012] Furthermore, the sodium salt includes at least one of sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium perchlorate (NaClO4).

[0013] Furthermore, the solvent includes at least one of an ester solvent and an ether solvent.

[0014] Furthermore, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate (EA), methyl acetate (MA), methyl formate (MF), tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), and tetraethylene glycol dimethyl ether (G4).

[0015] Furthermore, the volume ratio of the solvent to the diluent is 1:1-1:3.

[0016] Furthermore, the additive accounts for 0.2%-3% of the total mass of the electrolyte.

[0017] Furthermore, the concentration of sodium salt in the electrolyte is 0.8-1.2 mol / L.

[0018] The present application also provides a sodium ion battery, which includes the above-mentioned electrolyte.

[0019] Beneficial effects of the present invention:

[0020] The present invention introduces a highly fluorinated solvent as a diluent into a conventional concentration electrolyte, supplemented with a triphenylphosphonium bromide derivative as an additive, to obtain a low-temperature electrolyte for sodium ion batteries. The highly symmetrical distribution of a large number of highly electronegative functional groups (-F) in the diluent makes the polarity of the diluent molecules low, thereby ensuring its compatibility with Na + The non-solvation ability of the additive can be spontaneously cross-coordinated with the sodium salt through selective affinity, successfully constructing an anion-dominated solvation structure without reducing ionic conductivity, accelerating the desolvation process and simultaneously forming a dense and uniform organic-inorganic composite interface film. The anion-dominated solvation structure has a rapid desolvation ability, and the optimized organic-inorganic composite interface film optimizes the Na + transmission path, reducing interface impedance, which is beneficial to Na + The rapid migration of sodium ion batteries within the SEI membrane significantly enhances the interfacial stability and low-temperature performance of the battery. By rationally regulating the solvation structure in the electrolyte of the present invention, only a small amount of additives is needed to significantly improve the cycling performance of sodium ion batteries in low-temperature environments, thereby reducing costs and promoting commercialization. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and technical effect of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention is clearly and completely described. The embodiments described below are part of the embodiments of the present invention, rather than all the embodiments. In combination with the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer; if the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0022] In the present description, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0023] In the description of the present invention, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0024] It should be understood that the weights of the relevant components mentioned in the embodiments of the present invention may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally increased or decreased according to the embodiments of the present invention, it is within the scope of the present invention. Specifically, the weights described in the embodiments of the present invention may be mass units known in the chemical industry, such as μg, mg, g, and kg.

[0025] In addition, unless the context clearly requires otherwise, expressions in the singular form of a word should be understood to include the plural form of the word. The terms "include" or "have" are intended to specify the presence of a feature, number, step, operation, element, part, or combination thereof, but are not used to exclude the presence or possibility of adding one or more other features, numbers, steps, operations, elements, parts, or combinations thereof.

[0026] An embodiment of the present invention provides an electrolyte comprising a sodium salt, a solvent, a diluent, and an additive. The diluent is a highly fluorinated solvent, and the additive is a triphenylphosphine bromide derivative.

[0027] The present invention introduces a fluorinated solvent as a diluent into a conventional concentration electrolyte and adds a triphenylphosphonium bromide derivative as an additive to construct a local high-concentration low-temperature electrolyte system suitable for sodium ion batteries. The electrolyte achieves high sodium ion (Na) conductivity without reducing ionic conductivity. + ) effectively regulates the solvation structure and synergistically forms an excellent solid electrolyte interface (SEI) film, thereby significantly improving the performance of the battery in a low-temperature environment. The chemical formula of the additive is as follows:

[0028]

[0029] Among them, R is taken from H, C 1-5 Alkyl, C 1-5 Alkenyl, substituted C 1-5 One of alkyl, phenyl, and benzyl.

[0030] The present invention selects the above-mentioned additives. The affinity between the additives and the sodium salt will spontaneously form anion-cation cross coordination. Due to the π-π conjugated interaction between the additive cations and the HC negative electrode, the sodium salt anions will be preferentially adsorbed to the negative electrode surface along with the additive cations, and then undergo directional reduction and decomposition to form an organic-inorganic composite low-resistance stable SEI film, thereby avoiding the continuous dissolution and reconstruction of the SEI film during the cycle and optimizing the Na + transmission path, reducing interface impedance, helping to inhibit sodium reversible capacity loss, improving interface stability and Na + Migration kinetics. At the same time, due to Br - -Na + Strong anion-cation interaction between Br - Enter Na + The primary solvation shell of Na + -solvent interactions, which reduce the corresponding Na + Desolvation kinetic barrier.

[0031] The additive includes at least one of methyl triphenyl phosphonium bromide, ethyl triphenyl phosphonium bromide, propyl triphenyl phosphonium bromide, allyl triphenyl phosphonium bromide, isobutyl triphenyl phosphonium bromide, benzyl triphenyl phosphonium bromide, and isoamyl triphenyl phosphonium bromide.

[0032] The diluent includes at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), hexafluoroisopropyl methyl ether (HFME), and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (OFE).

[0033] Among them, by introducing highly fluorinated solvents as diluents, not only the disadvantage of high viscosity of high-concentration electrolytes is compensated, but also the wettability of the electrolyte is improved. In addition, the highly symmetrical distribution of a large number of highly electronegative functional groups (-F) in the diluent makes the polarity of the diluent molecules low, thus ensuring its compatibility with Na + Strong Lewis acid center Na + This will cause the electron cloud density on the solvent to transfer from H to O, making the former partially electron-deficient (δ + H ), making the latter electron-rich (δ -O ), and after adding diluent, + H Interaction with the electron-withdrawing F group on the nearby diluent molecule leads to the diluent reacting with Na + Competing for solvent. This competition draws the solvent away from Na + , weakening Na + The binding energy between Na + desolvation process; at the same time, the anion is promoted to enter the Na + The solvation shell of the ions forms anion-rich contact ion pair (CIP) and ion aggregate (AGG) type solvation structures.

[0034] The sodium salt includes at least one of sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium perchlorate (NaClO4).

[0035] Wherein, the solvent includes at least one of an ester solvent and an ether solvent.

[0036] The solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate (EA), methyl acetate (MA), methyl formate (MF), tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TEGDME), and tetraethylene glycol dimethyl ether (G4).

[0037] Wherein, the volume ratio of the solvent to the diluent is 1:1-1:3.

[0038] The present invention constructs a local high concentration electrolyte (LHCE) by controlling the volume ratio of the solvent to the diluent within the above range, which not only ensures that the sodium salt has sufficient solubility in the solvent and maintains the ionic conductivity of the electrolyte; but also effectively weakens the Na + The binding strength with the solvent is optimized + The solution structure is formed and the desolvation process is promoted. The electrolyte viscosity, wettability, and ion transport rate are successfully balanced, resulting in better performance at low temperatures. When the diluent addition is too low, it is difficult to form an anion-derived solvation structure. When the diluent addition is too high, it is difficult to completely dissolve the sodium salt and will affect the ionic conductivity.

[0039] Wherein, the additive accounts for 0.2%-3% of the total mass of the electrolyte.

[0040] The present invention can effectively regulate Na by controlling the amount of additives at a lower addition amount. +The present invention controls the additive amount within the above range, facilitating selective adsorption between the additive cations and the negative electrode material, ensuring efficient and controllable SEI film formation, and enabling sodium-ion batteries to maintain excellent electrochemical performance over a wide temperature range. When too little additive is added, batteries made with this electrolyte exhibit poor low-temperature performance. When too much additive is added, an undesirable SEI film forms, increasing costs.

[0041] The present invention can ensure the high efficiency and controllability of SEI film formation by controlling the addition amount of diluent and additive within the above range.

[0042] Wherein, the concentration of sodium salt in the electrolyte is 0.8-1.2 mol / L.

[0043] A sodium ion battery comprises the above-mentioned electrolyte.

[0044] The sodium ion battery further comprises a positive electrode sheet, a negative electrode sheet and a separator.

[0045] In order to enable those skilled in the art to clearly understand the implementation details and operations of the present invention, and to significantly demonstrate the improved performance of the embodiments of the present invention, the above technical solutions are illustrated below through multiple embodiments.

[0046] Example 1

[0047] Positive electrode sheet preparation: The positive electrode material, sodium ferric phosphate pyrophosphate (Na4Fe3(PO4)2P2O7), the binder polyvinylidene fluoride (PVDF), the conductive agent Super-P, and the dispersant polyvinyl alcohol (PVA) are dispersed in an organic solvent (NMP) at a mass ratio of 90:4.6:5:0.4. The mixture is stirred in a vacuum mixer until stable and uniform, and then evenly coated on a 13μm thick water-based carbon-coated aluminum foil. The mixture is then transferred to a 110°C forced air oven and dried for 12 hours. The positive electrode sheet is then cold pressed and die-cut.

[0048] Preparation of the negative electrode sheet: The negative electrode material hard carbon (HC), binder polyvinylidene fluoride (PVDF), and conductive agent Super-P are dispersed in an organic solvent (NMP) at a mass ratio of 90:5:5. The mixture is stirred in a vacuum mixer until stable and uniform, and then evenly coated on a 12μm thick aluminum foil. The mixture is then transferred to a 110°C forced air oven and dried for 24 hours. The negative electrode sheet is then cold pressed and die-cut.

[0049] Electrolyte preparation:

[0050] The electrolyte was prepared in an argon-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The sodium salt, solvent, and diluent were mixed and uniformly mixed by magnetic stirring. The additive was then added and mixed uniformly. After sufficient dissolution, the electrolyte was obtained. The sodium salt was composed of sodium bis(fluorosulfonyl)imide (NaFSI) and sodium hexafluorophosphate (NaPF6) in a molar mass ratio of 1:1. The solvent was diethylene glycol dimethyl ether (DEGDME), the diluent was 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the additive was methyltriphenylphosphonium bromide. The volume ratio of the solvent to the diluent was 1:2. The sodium salt concentration was 1.0 mol / L, the local concentration was 3.0 mol / L, and the mass of the additive accounted for 0.25% of the total mass of the electrolyte.

[0051] Preparation of sodium-ion batteries: The positive electrode sheet, separator, and negative electrode sheet are stacked in order to obtain a bare cell. The cells are then encapsulated with aluminum-plastic film, baked, injected with liquid, left to stand, formed, jig-shaped, sealed, and tested for capacity to complete the preparation of the sodium-ion battery.

[0052] Example 2

[0053] The difference between Example 2 and Example 1 is that the solvent in Example 2 consists of propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a volume ratio of 7:3, the diluent is 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (OFE), the volume ratio of the solvent to the diluent is 1:3, the local concentration is 4.0 mol / L, and the rest is the same as Example 1 and will not be repeated here.

[0054] Example 3

[0055] The difference from Example 1 is that the sodium salt in Example 3 is NaPF6, the molar concentration of the sodium salt is 0.8 mol / L, and the local concentration is 2.4 mol / L. The rest is the same as Example 1 and will not be repeated here.

[0056] Example 4

[0057] The difference from Example 1 is that the additive in Example 4 accounts for 3wt% of the total mass of the electrolyte, the volume ratio of the solvent to the diluent is 1:1, and the rest is the same as Example 1 and will not be repeated here.

[0058] Comparative Example 1

[0059] The difference from Example 1 is that no additives are added. The rest is the same as Example 1 and will not be repeated here.

[0060] Comparative Example 2

[0061] The difference from Example 1 is that no diluent is added. The rest is the same as Example 1 and will not be repeated here.

[0062] Comparative Example 3

[0063] The difference from Example 1 is that the mass of the additive accounts for 5 wt % of the total mass of the electrolyte, and the rest is the same as Example 1 and will not be repeated here.

[0064] Comparative Example 4

[0065] The difference from Example 1 is that the additive is methyltriphenylammonium bromide. The rest is the same as Example 1 and will not be repeated here.

[0066] Comparative Example 5

[0067] The difference from Example 1 is that the additive is methyltriphenylphosphine iodide. The rest is the same as Example 1 and will not be repeated here.

[0068] Comparative Example 6

[0069] The difference from Example 1 is that the diluent is composed of 3-trifluoromethyl-1,3-dioxane and 2-methyl-1,3-dioxane in a volume ratio of 1:1, and the local concentration is less than 3 mol / L. The rest is the same as Example 1 and will not be repeated here.

[0070] Comparative Example 7

[0071] The difference from Example 1 is that the volume ratio of the solvent to the diluent is 2:1, the local concentration is 1.5 mol / L, and the rest is the same as Example 1 and will not be repeated here.

[0072] Performance test: The batteries obtained in the above examples and comparative examples were subjected to the following tests, and the test results are shown in the following table.

[0073] (1) Normal temperature cycle test:

[0074] The battery was placed in an oven at a constant temperature of 25°C for 4 hours, then charged to 3.25V at a constant current of 1C, then charged at a constant voltage until the current dropped to 0.05C, and then discharged to 1.5V at a constant current of 1C. This cycle was repeated, and the initial capacity of the battery and the discharge capacity of the last cycle (1000th cycle) were recorded.

[0075] Capacity retention rate = discharge capacity at the last cycle (1000th cycle) / initial capacity × 100%.

[0076] (2) Low temperature cycle test:

[0077] The battery was placed in an oven at a constant temperature of -20°C for 4 hours, then charged to 3.25V at a constant current of 1C, then charged at a constant voltage until the current dropped to 0.05C, and then discharged to 1.5V at a constant current of 1C. This cycle was repeated, and the initial capacity of the battery and the discharge capacity of the last cycle (the 500th cycle) were recorded.

[0078] Capacity retention rate = discharge capacity of the last cycle (500th cycle) at -20°C / initial capacity × 100%

[0079] Table 1:

[0080]

[0081] The above test results are analyzed as follows:

[0082] As can be seen from the table, the sodium ion battery prepared using the electrolyte of the present application has good cycle stability at -20°C-25°C. In a temperature environment of -20°C, the cycle capacity retention rate after 500 cycles of 1C / 1C is 82.2%-86.2%, and in a temperature environment of 25°C, the cycle capacity retention rate after 1000 cycles of 1C / 1C is 91.3%-95.2%. By comparing Comparative Examples 1-2 and Comparative Examples 4-6 with Example 1, it can be seen that the types of additives and diluents of the present application have a synergistic effect, which can effectively improve the cycle stability of the sodium ion battery in a low temperature environment. When other types of additives or diluents are used for replacement, the cycle stability of the sodium ion battery obtained deteriorates and the effect of the present application cannot be obtained. By comparing Comparative Example 3 with Example 1, it can be seen that when the amount of additive added is too much, the cycle stability of the sodium ion battery will be affected. By comparing Example 7 with Examples 1-3, it can be seen that when the solvent: diluent ratio is in the range of 1:1-1:3, the cycle stability of the sodium ion battery is better, while when the amount of diluent added is too low, the cycle stability of the sodium ion battery will be affected.

[0083] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. An electrolyte, characterized in that: The method comprises a sodium salt, a solvent, a diluent and an additive, wherein the diluent is a fluorinated solvent and the additive is a triphenylphosphine bromide derivative, and the chemical formula thereof is as follows: Among them, R is taken from H, C 1-5 Alkyl, C 1-5 Alkenyl, substituted C 1-5 One of alkyl, phenyl, and benzyl.

2. An electrolyte according to claim 1, characterized in that The additive includes at least one of methyl triphenyl phosphonium bromide, ethyl triphenyl phosphonium bromide, propyl triphenyl phosphonium bromide, allyl triphenyl phosphonium bromide, isobutyl triphenyl phosphonium bromide, benzyl triphenyl phosphonium bromide, and isoamyl triphenyl phosphonium bromide.

3. An electrolyte according to claim 1, characterized in that The diluent includes at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, hexafluoroisopropyl methyl ether, and 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

4. An electrolyte according to claim 1, characterized in that The sodium salt includes at least one of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, and sodium perchlorate.

5. The electrolyte according to claim 1, characterized in that The solvent includes at least one of an ester solvent and an ether solvent.

6. An electrolyte according to claim 5, characterized in that: The solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, methyl acetate, methyl formate, tetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

7. An electrolyte according to claim 1, characterized in that The volume ratio of the solvent to the diluent is 1:1-1:

3.

8. The electrolyte according to claim 1, characterized in that The additive accounts for 0.2%-3% of the total mass of the electrolyte.

9. The electrolyte according to claim 1, characterized in that The concentration of sodium salt in the electrolyte is 0.8-1.2 mol / L.

10. A sodium ion battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 9.

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