Lithium bis(fluorosulfonyl)imide Salt and Uses Thereof

Abstract

A lithium bis(fluorosulfonyl)imide salt, wherein, after dissolving in water to form an aqueous solution, the aqueous solution has a pH of between 4 and 8, in particular at a temperature of 25 C., and the uses thereof in Li-ion batteries. Also, a lithium bis(fluorosulfonyl)imide salt, including a content of H+ ions of between 0.08 ppb and 0.80 ppm, between 0.08 ppb and 0.63 ppm or between 0.25 ppb and 2.53 ppb.

Claims

1. A lithium bis(fluorosulfonyl)imide salt, wherein after dissolving in water to form an aqueous solution, said aqueous solution has a pH of between 4 and 8 at a temperature of 25 C.

2. The lithium bis(fluorosulfonyl)imide salt as claimed in claim 1, wherein after dissolving in water to form an aqueous solution, said aqueous solution has a pH of between 4.1 and 8.

3. The lithium bis(fluorosulfonyl)imide salt as claimed in claim 1, wherein the concentration of said salt in the aqueous solution is between 0.050 and 0.250 g/mL.

4. The lithium bis(fluorosulfonyl)imide salt as claimed in claim 1, wherein it comprises a content of H.sup.+ ions of between 0.08 ppb and 0.80 ppm.

5. A lithium bis(fluorosulfonyl)imide salt comprising a content of H.sup.+ ions of between 0.08 ppb and 0.80 ppm.

6. A Li-ion battery comprising a lithium bis(fluorosulfonyl)imide salt as claimed in claim 1.

7. The Li-ion battery as claimed in claim 6, in an Li-ion battery functioning at a cut-off voltage of greater than or equal to 4.2 V, preferably greater than or equal to 4.4 V.

8. An electrolyte composition comprising a lithium bis(fluorosulfonyl)imide salt as claimed in claim 1, and an organic solvent.

9. A method of improving the service life of an Li-ion battery and/or the electronic performance of an Li-ion battery, the method comprising using a lithium bis(fluorosulfonyl)imide salt as claimed in claim 1 in an Li-ion battery.

Description

EXAMPLES

[0047] Chronoamperometry tests were performed. To do this, CR2032 button cells were manufactured equipped with an aluminum sheet 20 mm in diameter as working electrode, a lithium metal pellet 8 mm in diameter as reference electrode and a glass fiber separator 18 mm in diameter soaked with 12 drops (0.6 mL) of a 1 mol/L LiFSI solution in a solvent mixture composed of ethylene carbonate and methyl ethyl carbonate (CAS=623-53-0) in a 3/7 volume ratio. Next, a voltage was applied to the terminals of the button cell and the current generated was measured and recorded.

Example 1: Chronoamperometry Study at a Cut-Off Voltage of 4.2 Volts

[0048] Chronoamperometry measurements were taken in a system with an aluminum electrode as working electrode and lithium metal as reference electrode.

[0049] pH measurement: The pH of the LiFSI solutions is measured using a pH-meter (pHM210 model from Radiometer) which is calibrated beforehand using three buffer solutions (pH=4.0, 7.0 and 10.0). The LiFSI salt was dissolved in an amount of water (having a pH of 7.450.5) to obtain an LiFSI mass concentration of 0.125 g/mL. The aqueous solution is stirred during the pH measurement.

[0050] The LiFSI salt of solution No. 3 was obtained according to the process described in Abouimrane et al. Liquid electrolyte based on lithium bis-fluorosulfonyl imide salt: Al corrosion studies and lithium ion battery investigation, Journal of Power Sources 189 (2009), pages 693-696 (paragraph 3. Results). The LiFSI salts of solutions No. 1 and No. 2 were obtained from the LiFSI salt prepared according to the process described in the abovementioned article by Abouimrane et al., followed by a step of adjusting the pH. Solutions 1 to 3 below were prepared and their pH was measured according to the method mentioned above:

TABLE-US-00001 LiFSI solution No. 1 No. 2 No. 3 pH (at 25 C.) 7.29 6.79 2.27

[0051] Preparation of the Electrolytes:

[0052] To perform the chronoamperometry tests, various Li-ion battery electrolytes were prepared starting with LiFSI solutions No. 1 to No. 3 (cf. above table).

[0053] Three electrolytes were prepared by dissolving an LiFSI salt (No. 1 to No. 3) in a solvent mixture composed of ethylene carbonate and methyl ethyl carbonate (CAS=623-53-0) in a 3/7 volume ratio, to obtain solutions with an LiFSI content of 1 mol/L:

TABLE-US-00002 Electrolyte E No. 1 E No. 2 E No. 3 (invention) (invention) (comparative) LiFSI solution No. 1 No. 2 No. 3 pH (at 25 C.) 7.29 6.79 2.27

[0054] Chronoamperometry

[0055] The chronoamperometry test was performed at 25 C. by applying a constant cut-off voltage (4.2 volts) and the current obtained was observed. After 5 hours, the residual current value was measured and retranscribed in the following table. This residual current is indicative of the side reactions that may take place during the functioning of an Li-ion battery.

TABLE-US-00003 pH 2.27 7.29 I at 4.2 V 31 14.4 (t = 5 H)

[0056] The results show that the electrolyte E No. 3 (LiFSI: pH=2.27) leads to a residual current that is twice as high as that for the electrolyte E No. 1 (LiFSI: pH=7.29) after 5 hours of functioning (i.e. after the formation of the passivation layers on the aluminum electrode). Now, this current is directly connected to the service life of the Li-ion battery. Specifically, each electron consumed in a spurious reaction no longer participates in the capacity or autonomy of the battery.

[0057] Thus, the use of an LiFSI salt according to the invention (having a pH of 7.29 after dissolution in water) leads to a lower residual current, and as a result to a better service life of the Li-ion battery, than with an LiFSI salt with a pH of 2.27 after dissolution in water.

Example 2: Chronoamperometry Study at 4.4 Volts

[0058] An experiment similar to that of example 2 was performed, but at a cut-off voltage of 4.4 V.

[0059] After 5 hours, the residual current value was measured and retranscribed in the following table.

TABLE-US-00004 pH 2.27 6.79 7.29 I at 4.4 V 401 203 130 (t = 5 H)

[0060] The results show that the electrolyte E No. 3 (LiFSI: pH=2:27) leads to a residual current that is three times as high as that for the electrolyte E No. 1 (LiFSI: pH=7.29) after 5 hours of functioning (i.e. after the formation of the passivation layers on the aluminum electrode).

[0061] Thus, the use of an LiFSI salt according to the invention (having a pH of 7.29 after dissolution in water) leads to a lower residual current, and as a result to a better service life of the Li-ion battery, than with an LiFSI salt with a pH of 2.27.

[0062] Similar results are obtained with an LiFSI having a pH of 6.79 after dissolution in water relative to that with a pH of 2.27.