Fluorine-Substituted Propylene Carbonate-Based Electrolytic Solution and Lithium-Ion Battery

Abstract

A fluorine-substituted propylene carbonate-based electrolytic solution and a lithium-ion battery, particularly to a fluorine-substituted propylene carbonate-based electrolytic solution having fluorine-substituted propylene carbonate as a primary solvent and a co-solvent is disclosed. The fluorine-substituted propylene carbonate has 50-80 vol. %, and the co-solvent has 20-50 vol. %, based on the volume of the electrolytic solution for a lithium-ion battery.

Claims

1-10. (canceled)

11. A fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery, wherein the electrolytic solution for a lithium-ion battery comprises fluorine-substituted propylene carbonate as a primary solvent and a co-solvent, wherein the fluorine-substituted propylene carbonate comprises 50-80 vol. %, and the co-solvent comprises 20-50 vol. %, based on the volume of the electrolytic solution for a lithium-ion battery.

12. The fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 11, wherein the co-solvent is selected from one or more of ethylene carbonate (EC), fluorinated ethylene carbonate (F-EC), difluorinated ethylene carbonate (DFEC), propylene carbonate (PC), γ-butyrolactone, and methyl acetate (MA).

13. The fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 11, wherein the electrolytic solution for a lithium-ion battery further comprises an additive selected from one or more of vinylene carbonate (VC), vinylethylene carbonate, 1, 3-propane sultone, and 1, 4-butane sultone; preferably, the additive is added in an amount of 1-5% of the total weight of the primary solvent and the co-solvent.

14. The fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 11, wherein the electrolytic solution for a lithium-ion battery comprises a lithium salt electrolyte as a solute selected from one or more of LiPF.sub.6, LiBF.sub.4, LiBOB, LiDOFB, LiTFSI and LiFSI; preferably, the lithium salt electrolyte has a content of 0.5 mol/L-2.0 mol/L.

15. A method of preparing a fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery, comprising: (1) mixing 50-80 vol. % of fluorine-substituted propylene carbonate as a primary solvent and 20-50 vol. % of a co-solvent in an inert gas protective atmosphere to form a mixed solvent; (2) optionally, adding an additive to the mixed solvent, followed by mixing homogeneously; (3) dissolving a lithium salt electrolyte, followed by stirring fully and homogeneously; (4) packaging the fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery in an inert gas protective atmosphere for storage.

16. The method of preparing the fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 15, wherein the fluorine-substituted propylene carbonate has a purity of 99.9% or more.

17. The method of preparing the fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 15, wherein the co-solvent is selected from one or more of ethylene carbonate (EC), fluorinated ethylene carbonate (F-EC), difluorinated ethylene carbonate (DFEC), propylene carbonate (PC), y-butyrolactone, and methyl acetate (MA).

18. The method of preparing the fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 15, wherein the additive is selected from one or more of vinylene carbonate (VC), vinylethylene carbonate, 1, 3-propane sultone, and 1, 4-butane sultone; preferably, the additive is added in an amount of 1-5% of the total weight of the primary solvent and the co-solvent.

19. The method of preparing the fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery according to claim 15, wherein the lithium salt electrolyte in the electrolytic solution for a lithium-ion battery is selected from one or more of lithium hexafluorophosphate (LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDOFB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI); preferably, the lithium salt electrolyte has a content of 0.5 mol/L-2.0 mol/L.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The disclosure will be further illustrated in detail with reference to the following accompanying drawings and specific embodiments.

[0061] FIG. 1 is a differential scanning calorimetry (DSC) curve of a fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery in Example 1 according to the disclosure.

[0062] FIG. 2 is an initial charge-discharge curve of a natural graphite negative electrode in the electrolytic solution of Example (1) according to the disclosure.

[0063] FIG. 3 is an initial charge-discharge curve of LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2 (NCA) positive electrode material in a fluorine-substituted propylene carbonate-based electrolytic solution for a lithium-ion battery in an embodiment according to the disclosure.

[0064] FIG. 4 shows the long-term cycling performance of a lithium-ion battery on the whole using the electrolytic solution of Example (1) according to the disclosure.

DETAILED DESCRIPTION

[0065] The disclosure will be further demonstrated with reference to the following examples. It is to be noted that the following examples are only intended to illustrate the disclosure in an exemplary way, not to limit the protection scope of the disclosure.

Example 1

TFPC/(EC+PC) Composite Electrolytic Solution System

[0066] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 30 ml PC and 10 ml EC, and mixed homogeneously. 23.1 g LiPF.sub.6 was dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.5M LiPF.sub.6/TFPC/PC/EC (5:3:1) electrolytic solution system was obtained, and the system was packaged in an argon atmosphere for storage.

Example 2

TFPC/(Cl-EC+PC) Composite Electrolytic Solution System

[0067] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 20 ml PC and 10 ml CI-EC (chlorine-substituted ethylene carbonate), and mixed homogeneously. 14.5 g LiPF.sub.6 was dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.2M LiPF.sub.6/TFPC/CI-EC/PC (5:2:1) electrolytic solution system was obtained, and the system was packaged in an argon atmosphere for storage.

Example 3

TFPC/(EC+PC) Composite Electrolytic Solution System

[0068] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 30 ml PC and 20 ml EC, and mixed homogeneously. 15.4 g LiPF.sub.6 and 1.43 g LiDFOB were dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.0M LiPF.sub.6+0.1M LiDFOB/TFPC/PC/EC (5:3:2) electrolytic solution system was obtained, and the system was packaged in an argon atmosphere for storage.

Example 4

TFPC/(FEC+PC) Composite Electrolytic Solution System

[0069] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 30 ml PC and 10 ml fluorine-substituted ethylene carbonate (FEC), and mixed homogeneously. 13.9 g LiPF.sub.6 was dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.0M LiPF6/TFPC/PC/FEC (5:3:1) electrolytic solution system was obtained, and the system was packaged in an argon atmosphere for storage.

Example 5

TFPC/(EC+MFA) Composite Electrolytic Solution System

[0070] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 30 ml EC and 10 ml methyl acetate (MA), and mixed homogeneously. 13.9 g LiPF.sub.6 was dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.0M LiPF.sub.6/TFPC/EC/MFA (5:3:1) electrolytic solution system was obtained, and the system was packaged in an argon atmosphere for storage.

Example 6

TFPC/(EC+PC)-Additive Composite Electrolytic Solution System

[0071] 50 ml high purity, anhydrous fluorine-substituted propylene carbonate was added to 30 ml PC and 20 ml EC, and mixed homogeneously. 5 ml vinylene carbonate (VC) was added, and 15.4 g LiPF.sub.6 was dissolved as a supporting electrolyte. After stirring homogeneously under the protection of high purity argon, a 1.0M LiPF.sub.6/TFPC/PC/EC (5:3:2) electrolytic solution system comprising 5% VC as an additive was obtained, and the system was packaged in an argon atmosphere for storage.

[0072] As tested, all the composite electrolytic solution systems of Examples 1-6 as described above have a boiling point of about 250° C., or even greater than 260° C., which is about 160° C. higher than the boiling point of a traditional 1.0M LiPF.sub.6/EC+DEC (1:1) electrolytic solution system; and a freezing point which is about 40° C. lower than the traditional electrolytic solution. As can be seen, the liquid state temperature range of this kind of electrolytic solution systems is very broad, thereby expanding the operating temperature range of a battery to a large extent.

[0073] In addition, this kind of fluorine-substituted propylene carbonate electrolytic solution systems are free of highly flammable components such as DEC, DMC, EMC or the like, and have a high flash point, a high fluorine content, and a low hydrogen content, so that the electrolytic solutions are less flammable. Hence, the safety of the electrolytic solutions is enhanced greatly. Due to the absence of linear carbonate components which are prone to oxidation, the electrolytic solutions have good anti-oxidation stability. This kind of electrolytic solutions are suitable for use as high voltage lithium-ion battery systems. Owing to the good stability of the electrolytic solutions, they are very important for development of lithium-ion batteries having high safety and specific energy.

[0074] At the same time, this kind of fluorine-substituted propylene carbonate electrolytic solution systems based on fluorine-substituted organic solvents show superior film-forming behavior. They are not only suitable for lithium-ion batteries comprising graphite based carbon negative electrode systems, but they also exhibit good effect for lithium-ion batteries comprising silicon negative electrodes.

[0075] Additionally, this kind of fluorine-substituted propylene carbonate electrolytic solution systems can be used repeatedly because they are less volatile, less toxic in use, and easily recyclable.

[0076] Therefore, this kind of fluorine-substituted propylene carbonate electrolytic solution systems according to the disclosure are new, safe and green electrolytic solution systems.

[0077] The method of preparing the lithium-ion batteries according to the disclosure will be demonstrated with reference to the following specific Examples.

Example 7

[0078] 1. Preparation of a LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2 (NCA) positive electrode sheet

[0079] 6 g of a polyvinyl difluoride (PVDF) binder and 5 g of conductive carbon black were mixed into 89 g of N-methyl pyrrolidone (NMP), and mixed homogeneously by stirring at a speed of 4000 rounds/minute. The resulting mixture was further mixed with 100 g of a LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2(NCA) positive electrode material to prepare a slurry, and then stirred at a speed of 4000 rounds/minute for 2 hours to ensure fully homogeneous mixing of the slurry. Thereafter, the slurry was coated on an aluminum foil current collector in a dry environment, wherein the electrode coating had a dry thickness of 70 microns. The coating was pressed under 2 atms for subsequent use.

[0080] 2. Preparation of a graphite negative electrode sheet

[0081] 5 g of a PVDF binder and 2 g of an acetylene black conductive agent were mixed into 43 g of an NMP organic solvent, and mixed homogeneously by stirring at a speed of 4000 rounds/minute. The resulting mixture was further mixed with 100 g of a natural graphite anode electrode material to prepare a slurry, and then stirred at a speed of 4000 rounds/minute for 2 hours to ensure fully homogeneous mixing of the slurry. The slurry was coated on a copper foil current collector in a dry environment, wherein the electrode coating had a dry thickness of about 50 microns. The coating was pressed under 2 atms for subsequent use.

[0082] 3. Preparation of a Button Battery

[0083] In a glove box, a button battery was assembled using the above LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2(NCA) positive electrode sheet and the graphite negative electrode sheet respectively as working electrodes, a metal lithium sheet as a counter electrode, a Celgard 2400 separator (available from Celgard Co. in USA), and the electrolytic solution for a lithium-ion battery prepared in Example 1. Following the common process for manufacturing a button battery, after cutting, drying, assembly, solution injection and sealing by pressing, the resulting battery was subjected to formation.

[0084] 4. Formation and Testing of the Battery

[0085] The formation system for the battery was as follows: the battery was charged and discharged three times at a constant current having a current density of 0.1 mA/cm.sup.2. The LiNi.sub.0.8Co.sub.0.15Al.sub.0.05O.sub.2 (NCA) electrode sheet had a charge cutoff voltage of 4.1V, and a discharge cutoff voltage of 3.0V. The natural graphite electrode sheet had a charge cutoff voltage of 0 V, and a discharge cutoff voltage of 2.0V. After the formation, a current density of 0.2 mA/cm.sup.2 was used to test the cycling performance of the battery.

[0086] The electrolytic solution system manufactured according to the disclosure not only exhibits good compatibility with positive and negative electrode materials of a lithium-ion battery, but also features a broad range of operating temperature and safety. Therefore, it is expected to be used in lithium-ion batteries having high safety and long lifetime.

[0087] The above description only sets out some preferred examples of the disclosure. All equivalent variations and modifications made in the scope of the claims of the disclosure fall in the scope defined by the claims of the disclosure.