TWI843167B - Flame-retardant electrolyte, and lithium ion battery - Google Patents

Abstract

A flame-retardant electrolyte includes a specific solvent, a lithium salt, and a fluorinated solvent. The specific solvent is a carbonate-based solvent, an ether-based solvent, succinonitrile, sulfolane, ionic liquids, or combinations thereof. The flame retardant electrolyte includes 1-n-butyl-1-methylpyrrolidine bis(trifluoromethylsulfonyl)imide, lithium salts, and carbonate solvents. The present application further provides a preparation method of the flame-retardant electrolyte and a lithium ion battery. The lithium ion battery includes a positive electrode, a negative electrode, a composite solid electrolyte membrane, and the flame-retardant electrolyte. The composite solid electrolyte membrane is made of a lithium salt compound, a polymer material, a solid electrolyte, and a plasticizer.

Description

Flame-retardant electrolyte and lithium-ion battery

The present invention relates to a flame retardant electrolyte and a preparation method thereof, and a lithium ion battery using the flame retardant electrolyte.

There is a great demand for lithium-ion batteries in fields such as electric vehicles and energy storage systems, but existing carbonate electrolytes have safety issues related to combustion. Flame-retardant additives can improve the flame-retardant properties of electrolytes, but they can easily cause a decrease in battery performance. Therefore, people hope that flame-retardant electrolytes with excellent performance and batteries using the flame-retardant electrolytes can improve battery safety without reducing battery performance.

Common flame retardant additives, such as non-flammable phosphate compounds such as dimethyl phosphate (DMMP), triethyl phosphate (TEP) and trimethyl phosphate (TMP), ensure the safety of the electrolyte to a certain extent. However, due to the strong catalytic activity of the graphite surface as the battery electrode, poor battery stability is still a common problem.

In view of this, it is indeed necessary to provide a flame retardant electrolyte and a preparation method thereof, as well as a lithium ion battery using the flame retardant electrolyte, wherein the flame retardant electrolyte does not affect the stability of the lithium ion battery.

A flame retardant electrolyte, comprising a specific solvent, a lithium salt and a fluorinated solvent, wherein the specific solvent is a carbonate solvent, an ether solvent, succinonitrile, sulfolane, an ionic liquid or a combination thereof; or the flame retardant electrolyte comprises 1-n-butyl-1-methylpyrrolidine di(trifluoromethylsulfonyl)imide, a lithium salt and a carbonate solvent.

Furthermore, the carbonate solvent is a cyclic carbonate and/or a chain carbonate, the cyclic carbonate is ethylene carbonate (EC), propylene carbonate (PC) or a combination thereof, and the chain carbonate is dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate or a combination thereof.

Furthermore, the ether solvent is 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (Diglyme), triethylene glycol dimethyl ether (TREGDME), tetraethylene glycol dimethyl ether (TEGDME) or a combination thereof.

Furthermore, the concentration of the lithium salt is 5 mol/kg to 7 mol/kg, preferably 5 mol/kg to 6 mol/kg.

Furthermore, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ₃ SO ₂ ) ₂ ), abbreviated as LiTFSI), lithium bis(trifluoromethanesulfonyl)imide (LiO ₄ NS ₂ F ₂ , LiFSI), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ) or a combination thereof.

Furthermore, the fluorinated solvent is fluoroethylene carbonate (FEC).

Furthermore, the specific solvent contains a carbonate solvent and an ether solvent or is composed of a carbonate solvent and an ether solvent. The molar ratio of the carbonate solvent to the ether solvent is 10:1~1:10.

Furthermore, the specific solvent is composed of ethylene carbonate and 1,2-dimethoxyethane, and the molar ratio of EC to DME is 10:1~1:10.

Furthermore, the flame retardant electrolyte is composed of a mixture of ethylene carbonate and 1,2-dimethoxyethane, lithium bis(trifluoromethanesulfonyl)imide and fluoroethylene carbonate, wherein the concentration of LiTFSI is 5 mol/kg to 7 mol/kg, preferably 5 mol/kg to 6 mol/kg.

A lithium ion battery includes a positive electrode, a negative electrode, a composite solid electrolyte membrane, and the flame retardant electrolyte, wherein the material of the composite solid electrolyte membrane includes a lithium salt compound, a polymer material, a solid electrolyte, and a plasticizer.

A method for preparing a flame retardant electrolyte comprises the following steps: providing a specific solvent, wherein the specific solvent is a carbonate solvent, an ether solvent, succinonitrile, cyclobutane sulfonate, an ionic liquid or a combination thereof; adding a lithium salt to the specific solvent; and after the lithium salt is dissolved, adding a fluorinated solvent.

Compared with the existing technology, the present invention uses a specific solvent to increase the lithium salt concentration to more than 5 mol/kg, and adds an appropriate amount of additives to avoid lithium salt precipitation and battery performance degradation, and ensure that the electrolyte has flame retardant properties and ensures the stability of the lithium ion battery.

10: Lithium-ion battery

12: Positive pole

14: Negative

16: Flame-retardant electrolyte

18: Composite solid electrolyte membrane

Figure 1 is a schematic diagram of the structure of the lithium ion battery provided by the present invention.

Figure 2 is a charge and discharge curve diagram of the lithium-ion battery provided by the present invention.

Figure 3 shows the charge and discharge curves of a traditional lithium-ion battery.

Figure 4 is a charge and discharge curve diagram of the lithium ion battery provided by the present invention.

The flame retardant electrolyte provided by the present invention and its preparation method, as well as the lithium ion battery using the flame retardant electrolyte, will be further described in detail below in conjunction with the attached drawings and specific embodiments.

The present invention provides a flame retardant electrolyte, which includes a specific solvent, a lithium salt and a fluorinated solvent. Preferably, the fluorinated solvent is fluoroethylene carbonate. The concentration of the lithium salt in the flame retardant electrolyte is 5mol/kg to 7mol/kg. Preferably, the concentration of the lithium salt in the flame retardant electrolyte is 5mol/kg to 6mol/kg. The mass ratio of the fluorinated solvent is 1-20%, preferably 10%. The fluorinated solvent can avoid the precipitation of lithium salt and improve the stability of the negative electrode of the lithium ion battery. In one embodiment, the fluorinated solvent is FEC, and the flame retardant electrolyte consists of a specific solvent, a lithium salt and FEC, and the mass ratio of FEC is 10%.

The specific solvent is a carbonate solvent, an ether solvent, succinonitrile, cyclobutane sulfonate, an ionic liquid or a combination thereof. The ionic liquid may be N-methyl, butylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, N-methyl, butylpyrrolidinium bis(fluorosulfonyl)imide, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, bis[fluorosulfonyl]imide), 1-ethyl-3-methylimidazolium bis[fluorosulfonyl]imide, etc.

In one embodiment, the specific solvent includes a carbonate solvent and an ether solvent.

In another embodiment, the specific solvent consists of a carbonate solvent and an ether solvent.

The carbonate solvent may be a cyclic carbonate and/or a chain carbonate. The cyclic carbonate is ethylene carbonate, propylene carbonate or a combination thereof. The chain carbonate is dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or a combination thereof.

The ether solvent may be 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether or a combination thereof.

The lithium salt is lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ₃ SO ₂ ) ₂ ), abbreviated as LiTFSI), lithium bis(fluorosulfonyl)imide (LiO ₄ NS ₂ F ₂ , LiFSI), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ) or a combination thereof.

In one embodiment, the specific solvent is composed of EC and DME, the molar ratio of EC and DME is 10:1 to 1:10, preferably, the molar ratio of EC and DME is 1:2. The lithium salt is LiTFSI, that is, the flame retardant electrolyte is composed of a mixture of EC and DME, LiTFSI and FEC, wherein the concentration of the lithium salt (LiTFSI) is 5 mol/kg to 6 mol/kg.

The present invention further provides a method for preparing the flame-retardant electrolyte, which comprises the following steps: S1, providing the specific solvent; S2, adding lithium salt to the specific solvent and stirring; and S3, after the lithium salt is completely dissolved, adding a fluorinated solvent and stirring.

In step S1, in one embodiment, the specific solvent is composed of carbonate solvents such as EC and ether solvents such as DME. Since carbonate solvents such as EC are solid at low temperatures, it is necessary to ensure that carbonate solvents such as EC and ether solvents such as DME in step S1 are completely mixed to obtain the specific solvent before proceeding to the subsequent steps. The order of steps S2 and S3 is not limited, that is, the order of mixing and dissolving the specific solvent, lithium salt and fluorinated solvent together is not limited.

In one embodiment, both carbonate solvents and ether solvents are organic solvents. The two organic solvents, carbonate solvents and ether solvents, are mixed in a certain volume ratio. The carbonate organic solvent is solid at room temperature and can be heated to above 60°C for about two hours to dissolve the solid carbonate organic solvent. In this embodiment, the carbonate organic solvent is ethylene carbonate, the ether organic solvent is 1,2-dimethoxyethane, and the volume ratio of ethylene carbonate to 1,2-dimethoxyethane is 1:2.

In step S2, in one embodiment, a lithium salt is added to the specific solvent to increase a large amount of lithium ions. There is an interaction force between the lithium ions and the solvent to prevent the solvent molecules from being vaporized, thereby improving the flame retardant properties of the flame retardant electrolyte. In this embodiment, the lithium salt is LiTFSI.

In step S3, in one embodiment, the fluorinated solvent is fluoroethylene carbonate (FEC). The purpose of adding the fluorinated solvent is to prevent the precipitation of lithium salts such as LiTFSI and improve the stability of negative electrodes such as silicon carbon, and to enhance the interaction between lithium ions and the solvent to prevent the solvent molecules from being vaporized, thereby making the flame-retardant electrolyte non-flammable.

The following is a specific embodiment of the method for preparing the flame-retardant electrolyte.

In an environment where the saturated water vapor pressure is less than 2 Pascals, take EC and DME respectively, mix EC and DME at a volume ratio of 1:2, stir evenly, and obtain a specific solvent as an ion conductive medium. Put EC and DME into a quantitative bottle for standard volume measurement and stir evenly. EC is usually solid at room temperature and can be dissolved after heating to above 60°C for about two hours.

Then, weigh the lithium salt, add the lithium salt to the specific solvent, and continue stirring until the lithium salt is evenly dissolved. The concentration of the lithium salt is 5.14 mol/kg.

Finally, after the lithium salt is dissolved, add an appropriate amount of FEC and stir to obtain the flame retardant electrolyte. Table 1 is a comparison of the flame retardant electrolyte and the control group. After an open flame is exposed to electrolytes I to XII in Table 1 for 5 seconds, confirm whether electrolytes I to XII will burn. The test results are shown in Table 1.

In Table 1, EC is ethylene carbonate, ILE-P is 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, DME is 1,2-dimethoxyethane, DEC is diethyl carbonate, LiTFSI is lithium bistrifluoromethanesulfonylimide, and LiDFOB is lithium difluoro(oxalato)borate.

It can be seen from Table 1 that when the solvent is a mixture of EC and DME (the molar ratio of EC to DME is 1:2) and the lithium salt concentration is 5.14 mol/kg, the electrolyte is non-flammable. When the solvent is a mixture of EC, DME (the molar ratio of EC to DME is 1:2) and FEC, the electrolyte is non-flammable when the lithium salt concentration is 5.14 mol/kg. When the solvent is a mixture of EC, DME (the molar ratio of EC to DME is 1:2), FEC and LiDFOB, the electrolyte is non-flammable when the lithium salt concentration is 5.14 mol/kg.

Moreover, it is known from electrolyte I that the solvent is a mixture of ILE-P and DEC (the mass ratio of ILE-P to DEC is 8:2), the molar ratio of ILE-P to LiTFSI is 8:2, and when the weight molar concentration of LiTFSI is 0.59 mol/kg, the electrolyte is flame retardant. It can be seen that the flame retardant electrolyte can also include 1-n-butyl-1-methylpyrrolidine di(trifluoromethylsulfonyl)imide, lithium salt and carbonate solvent. In a specific embodiment, the flame retardant electrolyte is composed of 1-n-butyl-1-methylpyrrolidine di(trifluoromethylsulfonyl)imide, lithium salt (such as LiTFSI) and carbonate solvent (such as DEC).

Please refer to FIG. 1 . The present invention further provides a lithium ion battery 10, wherein the lithium ion battery 10 includes a positive electrode 12, a negative electrode 14, a flame retardant electrolyte 16 and a composite solid electrolyte membrane 18.

The material of the positive electrode 12 is not limited, and can be lithium nickel manganese cobalt oxide, lithium nickel manganese oxide, etc. The material of the negative electrode 14 is not limited, and can be graphite, silicon carbon and lithium metal, etc. The flame retardant electrolyte 16 is the flame retardant electrolyte described in detail above.

The material of the composite solid electrolyte membrane 18 includes a lithium salt compound, a polymer material, a solid electrolyte and a plasticizer. The composite solid electrolyte membrane 18 is prepared from a lithium salt compound, a polymer material, a solid electrolyte and a plasticizer. The lithium salt compound can be lithium fluoride (LiF), lithium bis(fluorosulfonyl)imide (LiO ₄ NS ₂ F ₂ , referred to as LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ₃ SO ₂ ) ₂ , referred to as LiTFSI), lithium bis(perfluoroethylsulfonyl)imide (Li(C ₂ F ₅ SO ₂ ) ₂ N, referred to as LiBETI), lithium bis(oxalatoborate) (LiB(C ₂ O ₄ ) ₂ ), referred to as LiBOB), etc. or a combination thereof. The polymer material may be PVDF, PVDF-HFP, PVA, PLA, PEG, PMMA, PMA, PAA, etc. or a combination thereof. The solid electrolyte may be LLZO, LLZTO, LATP, etc. or a combination thereof. The plasticizer may be ethylene carbonate, succinonitrile, cyclobutane sulfonate, tetraethylene glycol dimethyl ether, etc. or a combination thereof. Conventional lithium salt compounds used in lithium ion batteries all meet the requirements of the present invention.

In one embodiment, PVDF, PMMA, lithium salt compound, solid electrolyte, and plasticizer are mixed and coated on a substrate, and then removed after baking to form the composite solid electrolyte membrane 18.

In a dry environment, the negative electrode 14, the composite solid electrolyte membrane 18, the positive electrode 12, the composite solid electrolyte membrane 18, and the negative electrode 14 are sequentially formed into a multi-layer battery cell from bottom to top. Then, in a vacuum environment, the flame-retardant electrolyte is injected into the multi-layer battery cell and then chemically formed to obtain the lithium ion battery 10. The electrochemical properties of the lithium ion battery 10 are verified below.

FIG2 is a charge and discharge curve of the lithium ion battery 10. As can be seen from FIG2, after using the flame retardant electrolyte, the discharge capacity is greater than 200mAh/g (205mAh/g), and the first cycle irreversibility is less than 12%, which are both comparable to the current material level. Therefore, the lithium ion battery 10 using the flame retardant electrolyte has good battery performance.

FIG3 is a charge and discharge curve diagram of a traditional lithium-ion battery, and FIG4 is a charge and discharge curve diagram of the lithium-ion battery 10. Compared with the lithium-ion battery 10 in FIG4, the traditional lithium-ion battery in FIG3 adopts a traditional separator and a flame retardant electrolyte (using electrolyte VII in Table 1), while the latter adopts the composite solid electrolyte membrane 18 and a flame retardant electrolyte (also using electrolyte VII in Table 1), and the rest are the same. It can be seen from FIG3 and FIG4 that the performance of the composite solid electrolyte membrane 18 is equivalent to that of the traditional separator. FIG2, FIG3 and FIG4 are all charge and discharge results of batteries using flame retardant electrolytes. FIG2 shows the charge and discharge results of the first three cycles, and FIG3 and FIG4 are the charge and discharge results after formation. As can be seen from Figures 3 and 4, the lithium ion battery 10 uses the flame-retardant electrolyte and the composite solid electrolyte membrane 18 to match each other, so that the charging and discharging performance of the lithium ion battery 10 is equivalent to that of a lithium ion battery using a traditional separator, and the capacity can reach 200mAh/g.

The flame retardant electrolyte and its preparation method, as well as the lithium ion battery using the flame retardant electrolyte have the following advantages: First, the present invention uses a specific solvent to increase the lithium salt concentration to more than 5 mol/kg, and adds an appropriate amount of additives to avoid lithium salt precipitation and battery performance degradation, and ensure that the electrolyte has flame retardant properties; Second, the present invention uses a flame retardant electrolyte, a composite solid electrolyte membrane, and positive and negative electrodes to form a lithium ion solid battery with high energy and high safety.

In summary, this invention has indeed met the requirements for invention patents, and a patent application has been filed in accordance with the law. However, the above is only a preferred embodiment of this invention, and it cannot be used to limit the scope of the patent application of this case. Any equivalent modifications or changes made by people familiar with the art of this case based on the spirit of this invention should be included in the scope of the following patent application.

10: Lithium-ion battery

12: Positive pole

14: Negative

16: Flame-retardant electrolyte

18: Composite solid electrolyte membrane

Claims (8)

A flame retardant electrolyte, which is improved in that it is composed of a specific solvent, a lithium salt and fluoroethylene carbonate, wherein the specific solvent is a carbonate solvent, an ether solvent, succinonitrile, cyclobutane sulfonate, an ionic liquid or a combination thereof. The flame-retardant electrolyte as described in claim 1, wherein the specific solvent is composed of a carbonate solvent and an ether solvent. The flame retardant electrolyte as described in claim 1 or 2, wherein the carbonate solvent is a cyclic carbonate and/or a chain carbonate, the cyclic carbonate is ethylene carbonate, propylene carbonate or a combination thereof, and the chain carbonate is dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or a combination thereof. The flame retardant electrolyte as described in claim 1 or 2, wherein the ether solvent is 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether or a combination thereof. The flame retardant electrolyte as described in claim 1 or 2, wherein the concentration of the lithium salt is 5 mol/kg to 7 mol/kg. The flame retardant electrolyte as described in claim 5, wherein the lithium salt is lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate or a combination thereof. The flame retardant electrolyte as described in claim 1, wherein the specific solvent is composed of ethylene carbonate and 1,2-dimethoxyethane, and the molar ratio of ethylene carbonate to 1,2-dimethoxyethane is 10:1~1:10. A lithium ion battery, comprising a positive electrode and a negative electrode, wherein the lithium ion battery further comprises a composite solid electrolyte membrane and a flame retardant electrolyte as described in any one of claim 1 to claim 7, wherein the material of the composite solid electrolyte membrane comprises a lithium salt compound, a polymer material, a solid electrolyte and a plasticizer.