Sulfolane Based Electrolyte For High Voltage Rechargeable Lithium Batteries

Abstract

Disclosed herein are novel, high salt concentration, sulfolane based electrolytes with a sulfone cosolvent(s), which are at their eutectic concentrations to lower the melting points of the electrolytes. The lower melting point electrolytes improve the low temperature performance of high voltage rechargeable batteries. The high salt concentration electrolytes improve the cycle performance of rechargeable lithium metal anode based batteries. The same electrolytes can operate above 4.3 V and up to 5.0 V vs. Li/Li.sup.+. Various cells containing said electrolytes are also disclosed herein.

Claims

1. A non-aqueous electrolyte for high voltage lithium metal and lithium-ion batteries, comprising a mixture of high boiling point solvents selected from the group consisting of: sulfolane, ethyl methyl sulfone, butyl sulfone, and methyl phenyl sulfone, and having a lithium salt of at least 2 molar concentration dissolved therein.

2. A non-aqueous electrolyte as described in claim 1, in which said lithium salt is selected from the group comprising: Li(CF.sub.3SO.sub.2).sub.2N (LiTFSI), Li(FSO.sub.2).sub.2N (LiFSI), LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2C.sub.2F.sub.5).sub.2, Li(CF.sub.3SO.sub.3).sub.2N, LiN(SO.sub.3C.sub.2F.sub.5).sub.2, LiC.sub.4F.sub.9SO.sub.3, LiClO.sub.4, LiAlO.sub.2, LiAlCl.sub.4, LiCl, LiI, LiB(C.sub.2O.sub.4).sub.2 (LiBOB); and their mixtures.

3. A non-aqueous electrolyte as described in claim 2, in which said mixture is at eutectic concentration.

4. A non-aqueous electrolyte as described in claim 2, in which said mixture is within 30% of its eutectic concentration.

5. A non-aqueous electrolyte as described in claim 2, which can operate in voltage above 4.3 V vs. Li/Li.sup.+.

6. A non-aqueous electrolyte as described iii claim 2, which can operate at low temperatures down to −28° C.

7. A safe, non-aqueous high voltage lithium battery cell having operating voltage above 4.3 V vs. Li/Li.sup.+ at low temperatures down to −28° C., which cell compromising a lithium metal anode(s); a high voltage mixed oxide cathode(s); a porous electrically non-conductive separator(s); a sulfolane/sulfone based electrolyte with a lithium salt(s) dissolved therein at eutectic concentration; and a moisture-proof enclosure.

8. A safe, non-aqueous high voltage lithium battery cell having operating voltage above 4.3 V vs. Li/Li.sup.+ at low temperatures down to −28° C., which cell compromising a carbon anode(s), a high voltage lithiated mixed oxide cathode(s); a porous electrically non-conductive separator(s), a sulfolane/sulfone based electrolyte with a lithium salt(s) dissolved therein at eutectic concentration, and a moisture-proof enclosure.

9. A non-aqueous high voltage battery cell as described in claim 7, in which said electrolyte is as described in claim 2.

10. A non-aqueous high voltage battery cell as described in claim 8, in which said electrolyte is as described in claim 2.

11. A non-aqueous high voltage lithium battery cell as described in claim 7, in which said mixed oxide cathode is replaced with a nickel fluoride cathode.

12. A non-aqueous high voltage lithium battery cell as described in claim 8, in which said lithiated mixed oxide cathode is replaced with a lithiated nickel fluoride.

13. A non-aqueous high voltage lithium battery cell as described in claim 8, in which said carbon anode is replaced with a silicon anode.

14. A non-aqueous electrolyte as described in claim 2, in which said electrolyte mixture comprising sulfolane and ethyl methyl sulfone in 65:35 by molar ratio, and said salt is 2 M LiFSI.

15. A non-aqueous electrolyte as described in claim 2, in which said electrolyte mixture comprising sulfolane and ethyl methyl sulfone in 65:35 by molar ratio, and said salt is 3 m LiFSI.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The nature and characteristic features of the invention will be more readily understood from the following description taken in connection with the accompanying drawing forming part thereof, in which:

[0016] FIG. 1 is the temperature versus the mole fraction diagram for the sulfolane-butyl sulfone mixtures, showing the melting point as a function of solvent mixture ratio.

[0017] FIG. 2 is the temperature versus the mole fraction diagram for the sulfonlane-methyl phenyl sulfone mixtures, showing the melting point as a function of solvent mixture ratio.

[0018] FIG. 3 is the temperature versus the mole fraction diagram for the sulfolane-ethyl methyl sulfone mixtures, showing the melting point as a function of solvent mixture ratio.

[0019] FIGS. 4a, 4.sub.b & 4.sub.c are the voltage versus cycle time plots for Li/Li symmetric cells using sulfolane based electrolyte (4a and 4b) in comparison with conventional carbonate based electrolyte (4c), cycling with an areal capacity of 1 mAh/cm.sup.2 at a current density of 1 mA/cm.sup.2.

[0020] It should of course, be understood that the description and drawings herein are merely illustrative, and that various modifications and changes can be made in the compositions and the structures disclosed without departing from the spirit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] When referring to the preferred embodiments, certain terminology will be utilized for the sake of clarity. Use of such terminology is intended to encompass not only the described embodiments, but also technical equivalents, which operate and function substantially same way to bring about the same results.

[0022] According to one embodiment of the present invention, there is provided an electrolyte for high voltage rechargeable lithium batteries. The electrolyte comprises a solution of at least one lithium salt in at least two aprotic solvents, such as sulfolane and a sulfone, wherein concentration of the components of the solution is selected so that the solution is at its eutectic or near eutectic concentration, or within at most 30% of its eutectic concentration.

[0023] The use of eutectic or near eutectic compositions dramatically decreases the melting point of the electrolyte, and therefore improves low temperature performance properties of the electrolyte.

[0024] The sulfone cosolvent comprises one or more non-symmetrical, non-cyclic sulfones of the general formula: R.sup.1—SO.sub.2—R.sup.2, wherein R.sup.1 and R.sup.2 are independently linear or branched alkyl or partially or fully fluorinated linear or branched alkyl groups having 1 to 7 carbon atoms; wherein R.sup.1 and R.sup.2 are different; and wherein —SO.sub.2—denotes the sulfone group. In another embodiment 121 and R.sup.2 have 1 to 4 carbon atoms.

[0025] In another embodiment, the alkyl group is selected from the group consisting of methyl (—CH.sub.3), ethyl (—CH.sub.2CH.sub.3), n-propyl (—CH.sub.2CH.sub.2CH.sub.3), n-butyl (—CH.sub.2CH.sub.2CH.sub.2CH.sub.3), n-pentyl (—CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.3), n-hexyl (—CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.3), n-heptyl (—CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.3), iso-propyl (—CH(CH.sub.3).sub.2), iso-butyl (—CH.sub.2CH(CH.sub.3).sub.2), sec-butyl (—CH(CH.sub.3)(CH.sub.2CH.sub.3)), tert-butyl (—C(CH.sub.3).sub.3), and iso-pentyl (—CH.sub.2CH.sub.2CH(CH.sub.3).sub.2). In a preferred embodiment, the sulfone is ethylmethyl sulfone (CH.sub.3—CH.sub.2—SO.sub.2—CH.sub.3). The electrolytes of the present invention have a stability to oxidation of greater than 4.3 V vs and Li/Li.sup.+ and up to 5.0 V vs. Li/Li.sup.+.

[0026] The electrolyte salt may be at least one lithium salt selected from the group comprising: Li(CF.sub.3SO.sub.2).sub.2N (LiTFSI), Li(FSO.sub.2).sub.2N (LiFSI), LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2C.sub.2F.sub.5).sub.2, Li(CF.sub.3SO.sub.3).sub.2N, LiN(SO.sub.3C.sub.2F.sub.5).sub.2, LiC.sub.4F.sub.9SO.sub.3, LiClO.sub.4, LiAlO.sub.2, LiAlCl.sub.4, LiCl, LiI, LiB(C.sub.2O.sub.4).sub.2 (LiBOB); and their mixtures.

[0027] The lithium salt may be used in a concentration of higher than 2.0 M or 2.0 m. The electrolyte may further include at least one organic or inorganic additive for contributing to a solid electrolyte interface (SEI) formed on the surface of the anode, and thus improving cycling. Amount of the additive is preferably between 0.2% and 10% of the total mass of the electrolyte.

[0028] According to another embodiment of the present invention, there is provided a high voltage lithium battery comprising at least one lithium metal or a carbon or silicon based negative electrode, at least one positive electrode with a high average voltage in discharge (e.g., >4.3 V vs. Li/Li.sup.+), at least one porous, electrically non-conductive separator between the positive and negative electrodes, and an electrolyte as described above.

[0029] In embodiments of the present invention the cathode may contain an active high voltage material selected from the group consisting of: nickel manganese oxide, nickel cobalt manganese oxide, nickel fluoride, and their lithiated versions. Examples of the embodiments will hereinafter be described in detail. However, these embodiments are just examples and are not limiting the present invention.

Example 1

[0030] Sulfolane is mixed with butyl sulfone with varying molar concentrations and the melting points of the mixed solutions are measured. The eutectic point is determined as the lowest melting temperature over all the mixing ratios for the two species.

Example 2

[0031] The eutectic point is measured according to the same method as Example 1, except for mixing sulfolane with methyl phenyl sulfone.

Example 3

[0032] The eutectic point is measured according to the same method as Example 1, except for mixing sulfolane with ethyl methyl sulfone.

[0033] The melting points for the sulfolane and three sulfone based cosolvents, i.e., butyl sulfone, methyl phenyl sulfone, and ethyl methyl sulfone are shown in FIG. 1, FIG. 2, and FIG. 3, which are embodiments of the invention. The eutectic point of methyl phenyl sulfone and sulfolane system cannot be determined due to the poor miscibility of the two solvents. Butyl sulfone and sulfolane system has a eutectic point of −5° C. when mixed. The ethyl methyl sulfone and sulfolane system has a eutectic melting point of −28° C. The relatively low eutectic point is most likely due to ethyl methyl sulfone's lower melting point (32-37° C.) than other sulfone candidates and its bent CH.sub.2—CH.sub.3 group resulting in higher entropy of mixing.

Example 4

[0034] A CR2032-type coin cell was fabricated by assembling two lithium metal disks sandwiching a glass fiber separator. 200 μL of electrolyte 2 M LiFSI in sulfolane/ethyl methyl sulfone (65:35 by mole ratio) was injected in the coin cell. The test diagram is shown in FIG. 4a showing stable Li stripping/plating voltage profile.

Example 5

[0035] A CR2032-type coin cell was fabricated according to the same method as for the Example 4, except for using electrolyte 3 m LiFSI in sulfolane/ethyl methyl sulfone (65:35 by mole ratio). The test diagram is shown in FIG. 4b showing stable Li stripping/plating voltage profile.

Example 6

[0036] A CR2032-type coin cell was fabricated according to the same method as for the Example 4, except for using carbonate based electrolyte 1 M LiPF.sub.6 ethylene carbonate/diethyl carbonate (1:1 by volume), and a polyethylene separator. The test diagram is shown in FIG. 4c showing unstable Li stripping/plating voltage profile.

[0037] It can be seen, that the lithium cycling performance of coin cells with an areal capacity of 1 mAh/cm.sup.2 at a current density of 1 mA/cm.sup.2 with the two high concentration sulfolane based electrolytes (Example 4 & 5) are much better than that with a carbonate based electrolyte (Example 6), showing both smaller overpotentials and longer and stable cycle life.

[0038] Thus the sulfolane/sulfone electrolytes with high salt concentrations have been provided herein, with which the objects of the invention are achieved.