Ionic liquid-based electrolytes for use in electrochemical storage devices

Abstract

Electrolytes comprising at least one lithium salt and at least two ionic liquids, at least one of which is an ionic liquid resulting from the association of at least one cation complying with the following formula (I): ##STR00001##
In which: R.sup.1 is an acyclic hydrocarbon group; n is an integer ranging from 0 to 3; m is an integer ranging from 1 to 4; and at least one Y anion.

Claims

1. An electrolyte comprising at least one lithium salt, a first ionic liquid and a second ionic liquid, the first ionic liquid comprising a cation of formula (I): ##STR00015## in which: R.sup.1 is an acyclic hydrocarbon group; n is an integer ranging from 0 to 3; m is an integer ranging from 1 to 4; and at least one associated anion Y.

2. The electrolyte according to claim 1, wherein the cation of formula (I) is of formula (Ib): ##STR00016##

3. The electrolyte according to claim 1, wherein R.sup.1 is an alkyl group comprising 1 to 4 carbon atoms.

4. The electrolyte according to claim 1, wherein m is equal to 2.

5. The electrolyte according to claim 1, wherein the at least one associated anion Y is chosen from a nitrate anion, a phosphate anion or an imidide anions.

6. The electrolyte according to claim 5, wherein the at least one associated anion Y is chosen the imidide anions.

7. The electrolyte according to claim 6, wherein the imidide anion is of formula (II′): ##STR00017## wherein R.sup.2 and R.sup.3 represent, independently of one another, a fluorine atom, a perfluorocarbon group.

8. The electrolyte according to claim 7, wherein the imidide anion is selected from formula (IV) or formula (V): ##STR00018##

9. The electrolyte according to claim 1, wherein the first ionic liquid comprises associated ions according to one of wherein the first ionic liquid comprises the cation associated with the at least one associated anion Y according to formula (VI) or formula (VII): formula (VI) or formula (VII): ##STR00019##

10. The electrolyte according to claim 1, wherein the second ionic liquid comprises a cation of the second ionic liquid selected from the group consisting of phosphonium, sulfonium, azetidinium, pyrrolidinium or piperidinium and an associated anion of the second ionic liquid selected from the group consisting of halide, phosphate, nitrate or imidide.

11. The electrolyte according to claim 10, wherein the cation of the second ionic liquid is selected from a pyrrolidinium cation and a piperidinium cation.

12. The electrolyte according to claim 11, wherein the cation of the second ionic liquid is of formula (VIII): ##STR00020## in which: R.sup.4 is an acyclic hydrocarbon group; and p is an integer ranging from 0 to 2.

13. The electrolyte according to claim 12, wherein the cation of the second ionic liquid is of formula (VIIIb): ##STR00021## wherein R.sup.4 is an acyclic hydrocarbon group.

14. The electrolyte according to claim 10, wherein the associated anion of the second ionic liquid is an imidide anion.

15. The electrolyte according to claim 10, wherein the the cation of the second ionic liquid and the associated anion of the second ionic liquid are of formula (X): ##STR00022##

16. The electrolyte according to claim 1, wherein the lithium salt is chosen from lithium hexafluorophosphate (LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), lithium bis(trifluoromethanesulfonyl)imidide (LiTFSI), lithium hexafluoroarsenate (LiAsF.sub.6), lithium nitrate (LiNO.sub.3) or lithium perchlorate (LiCIO.sub.4).

17. An electrochemical storage device comprising at least one cell comprising a positive electrode and a negative electrode separated from one another by a separator comprising the electrolyte as defined according to claim 1.

18. The electrochemical storage device according to claim 17, wherein at least one of the positive electrode and/or the negative electrode comprises graphite as an active material.

Description

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

Example 1

(1) This example illustrates the preparation of an electrolyte according to the invention.

(2) This example illustrates the preparation of an electrolyte according to the invention.

(3) This electrolyte is produced in a glove box by combining 0.9 ml of N,N-methylpropylpyrrolidinium bis(trifluoromethanesulfonyl)imidide of the following formula (X):

(4) ##STR00013##
with 0.1 mL of N,N-methyl-(2-vinyloxyethyl)pyrrolidinium bis(fluorosulfonyl)imidide of the following formula (VI):

(5) ##STR00014## and 0.2871 g of lithium bis(trifluoromethanesulfonyl)imidide (LiTFSI).

Comparative Example 1

(6) This example illustrates the preparation of an electrolyte not in accordance with the invention.

(7) This electrolyte is produced in a glove box by combining 1.0 ml of N,N-methylpropylpyrrolidinium bis(trifluoromethanesulfonyl)imidide with 0.2871 g of lithium bis(trifluoromethanesulfonyl)imidide (LiTFSI).

Example 2

(8) In this example, the performances of the electrolytes mentioned in example 1 and comparative example 1 are evaluated for an application in electrochemical storage. To do this, test cells (more specifically, button batteries to format 2032) are assembled in advance in a glove box in accordance with the following protocol: Placing of a 2032 battery casing (bottom cap) provided with a seal; Insertion of a stainless steel disc with a diameter suited to the inside diameter of the battery; Insertion of a 16 mm diameter disc of a positive electrode comprising, as active material, graphite (96%), Super P carbon black (1%), a binder comprising carboxymethylcellulose (1%) and a styrene/butadiene rubber (2%), with a capacity of 1.4 mAh/cm.sup.2, the face of which comprising material is turned upwards; Insertion of a 16.5 mm diameter Whatman separator previously soaked in 150 μL of one of the aforementioned electrolytes; Insertion of a 16 mm diameter lithium disc, which serves as a negative electrode; Insertion of a second stainless steel disc and a compression spring; Addition of a top cap and then crimping by pressing the whole.

(9) Once produced, the button batteries are introduced into a cycling bench of the Biologic™ type positioned in an oven. The following cycling conditions are then applied: Test temperature set at 45° C.; Performance of 10 successive charging/discharging cycles by chronopotentiometry in accordance with the following parameters: Charging from 1.5 V to 0.02 V in 10 hours (charging rate C/10); Discharging from 0.02 V to 1.5 V in 10 hours (discharge rate D/10); Performance of 10 successive charging/discharging cycles by chronopotentiometry in accordance with the following parameters: Charging from 1.5 V to 0.02 V in 5 hours (charging rate C/5); Discharging from 0.02 V to 1.5 V in 5 hours (discharge rate D/5);

(10) The change in the battery capacities measured in this protocol, as well as the efficiency of restitution of the stored energy (coulombic efficiency), are retranscribed in the following tables for the various electrolytes tested.

(11) *Cyclings at 45° C., at a discharge rate of D/10 (complete discharge in 10 hours)

(12) TABLE-US-00001 % of the capacity of the graphite Loss of Coulombic electrode* capacity** efficiency*** Electrolyte (cycle 1/cycle 10) (%) (%) Comparative example 1 3.2/0.9 71.7 97 Example 1 71.4/71.4 0 100 *capacity of the graphite electrode = 1.4 mAh/cm.sup.2 **measured between cycles 1 and 10 ***at cycle 10

(13) *Cyclings at 45° C., at a discharge rate of D/5 (complete discharge in 5 hours)

(14) TABLE-US-00002 % of the capacity of the graphite Loss of Coulombic electrode* capacity** efficiency*** Electrolyte (cycle 1/cycle 10) (%) (%) Comparative example 1 0.7/0.6 15.2 99 Example 1 70.3/69.3 1.5 100 *capacity of the graphite electrode = 1.4 mAh/cm.sup.2 **measured between cycles 1 and 10 ***at cycle 10

(15) With regard to these results, by virtue of the cell capacity as close as possible to the capacity of the graphite electrode (up to 71.4%), the low losses of capacity measured (up to 0% over 10 cycles) and the coulombic efficiency greater than 90%, the electrolyte of the invention has performances quite different compared with electrolytes based on conventional ionic liquids, which confirms the advantage presented by the electrolytes of the invention.