FORMATION PROCESS FOR A POTASSIUM-ION HYBRID SUPER-CAPACITOR

Abstract

Formation process for a potassium-ion hybrid supercapacitor, the process comprising: a) supplying the potassium-ion hybrid supercapacitor comprising: a negative electrode comprising graphite, a positive electrode comprising activated carbon, an electrolyte comprising a potassium salt, b) charging the supercapacitor at constant current in a protocol of between C.sub.x/50 and C.sub.x/2, to a charge cutoff voltage of between 3.0 V and 3.3 V, c) holding the supercapacitor at the charge cutoff voltage until the leakage current is between C.sub.x/2000 and C.sub.x/500, d) discharging the supercapacitor at constant current in a protocol of between C.sub.x/50 and C.sub.x, to a discharge cutoff voltage of between 0 V and 2 V,
where the process further comprises degassing the supercapacitor after one of steps b) to d).

Claims

1. Formation process for a potassium-ion hybrid supercapacitor, the process comprising: a) supplying the potassium-ion hybrid supercapacitor comprising: a negative electrode comprising graphite, a positive electrode comprising activated carbon, an electrolyte comprising at least one potassium salt, b) charging the supercapacitor at constant current in a protocol of between C.sub.x/50 and C.sub.x/2, to a charge cutoff voltage of between 3.0 V and 3.3 V, where C.sub.x is the formation capacity of the intercalation compound KC.sub.8, c) holding the supercapacitor at the charge cutoff voltage until the leakage current is between C.sub.x/2000 and C.sub.x/500, d) discharging the supercapacitor at constant current in a protocol of between C.sub.x/50 and C.sub.x, to a discharge cutoff voltage of between 0 V and 2 V, where the process further comprises degassing the supercapacitor after one of steps b) to d).

2. Process according to claim 1, where at least one of steps b) to d) is implemented at a temperature of between 15° C. and 30° C.

3. Process according to claim 2, where all of steps b) to d) are implemented at a temperature of between 15° C. and 30° C.

4. Process according to claim 1, where the charging in step b) and the discharging in step d) are carried out in a protocol of between C.sub.x/15 and C.sub.x/5.

5. Process according to claim 4, where the charging in step b) and the discharging in step d) are carried out in a C.sub.x/10 protocol.

6. Process according to claim 1, where the hold time in step c) is between 20 hours and 28 hours.

7. Process according to claim 6, where the hold time in step c) is 24 hours.

8. Process according to claim 1, where the leakage current at the end of step c) is equal to C.sub.x/1000.

9. Process according to claim 1, where the degassing of the supercapacitor is carried out after step d).

10. Process according to claim 1, where the electrolyte is non-aqueous.

11. Process according to claim 1, wherein the at least one potassium salt present in the electrolyte is in solution in at least one organic solvent.

12. Process according to claim 1, wherein the at least one potassium salt present in the electrolyte is selected from KClO.sub.4, KBF.sub.4, KPF.sub.6, potassium bis(trifluoromethanesulfonyl)imide, potassium bis(fluorosulfonyl)imide, potassium bis(oxalato)borate, KSCN, KSbF.sub.6, KAsF.sub.6, KAlCl.sub.4, KSiF.sub.6, KSO.sub.3CF.sub.3 and mixtures thereof.

13. Process according to claim 12, wherein the at least one potassium salt present in the electrolyte is selected from KClO.sub.4, KBF.sub.4, KPF.sub.6 and mixtures thereof.

14. Process according to claim 12, wherein the at least one potassium salt is in solution in at least one solvent selected from carbonate solvents, linear ether solvents, nitrile solvents, lactone solvents, amide solvents and mixtures thereof.

15. Process according to claim 14, wherein said solvent is selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, dimethoxyethane, acetonitrile, γ-butyrolactone, dimethylformamide and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention may be appreciated more fully from a reading of the detailed description hereinafter of implementation examples which are not limiting on said invention, and from examination of the appended drawing, in which:

[0045] FIG. 1 shows cycling performance results for a potassium-ion hybrid supercapacitor which has undergone a formation process outside of the invention,

[0046] FIG. 2 shows cycling performance results for a potassium-ion hybrid supercapacitor which has undergone another formation process outside of the invention,

[0047] FIG. 3 shows cycling performance results for a potassium-ion hybrid supercapacitor which has undergone a formation process according to the invention, and

[0048] FIG. 4 shows results of comparative tests evaluating the effect of each step of the process according to the invention.

DETAILED DESCRIPTION

[0049] FIGS. 1 to 4 show cycling performance results for a potassium-ion hybrid supercapacitor which has undergone a formation process outside of or according to the invention. The curves represent the energy density measured as a function of the number of cycles. In these tests, cycling is performed in a 20C.sub.x or 100C.sub.y protocol.

[0050] FIG. 1 shows the cycling performance results of three potassium-ion hybrid supercapacitors which have undergone the same reference formation process outside of the invention.

[0051] In these tests, the reference formation process used corresponds to five charge/discharge cycles at C.sub.x or 5C.sub.y from 0.5 to 3.5 V.

[0052] It is seen in FIG. 1 that the three curves give rise to scattering and to instability of the cycling performance, with energy densities which decrease and then increase drastically in several thousand cycles to reach energy density values of between 12 and 13 Wh/kg.sub.electrochemical core, these being values which are very much greater than those obtained with the symmetrical—also called conventional—supercapacitors, which are of the order of 8 Wh/kg.sub.electrochemical core.

[0053] FIG. 2 shows the cycling performance results for three potassium-ion hybrid supercapacitors which have undergone the same formation process outside of the invention, consisting of first cycles performed in a C.sub.x/10 or C.sub.y/2 protocol and at a temperature of approximately 23° C.

[0054] It is seen in FIG. 2 that the curves are reproducible and therefore that the scattering observed in FIG. 1 is significantly reduced. The energy density values are nevertheless between 10 and 11 Wh/kg.sub.electrochemical core and are lower than the values obtained in FIG. 1. Moreover, the long-term cycling performance of the supercapacitor remains unstable.

[0055] FIG. 3 shows the cycling performance results for four potassium-ion hybrid supercapacitors which have undergone the same implementation example of the formation process according to the invention.

[0056] In these tests, the implementation example of the formation process according to the invention that is used comprises the succession: [0057] of charging the supercapacitor at constant current in a C.sub.x/10 or C.sub.y/2 protocol to a charge cutoff voltage of 3.2 V, [0058] of holding the supercapacitor at the charge cutoff voltage of 3.2 V for a hold time of 24 hours, [0059] of discharging the supercapacitor to a discharge cutoff voltage of 0.5 V, and [0060] of degassing the supercapacitor.

[0061] It is seen in FIG. 3 that the curves are both reproducible and stable, with high energy density values, of between 12 and 14 Wh/kg.sub.electrochemical core.

[0062] FIG. 4 shows the results of comparative tests evaluating the effect of each step of the process according to the invention.

[0063] The key to FIG. 4 is as follows.

[0064] Curve 1 corresponds to an implementation example of the process according to the invention.

[0065] Curve 2 corresponds to an implementation example of the process according to the invention without step b).

[0066] Curve 3 corresponds to an implementation example of the process according to the invention without the degassing.

[0067] Curve 4 corresponds to an implementation example of the process according to the invention without step b), which is replaced by a step of charging the supercapacitor at constant current in a protocol of more than C.sub.x/2 (5 mA), to a charge cutoff voltage of between 3.0 V and 3.3 V.

[0068] Curve 5 corresponds to an implementation example of the process according to the invention without step c), which is replaced by a step of holding the supercapacitor at the charge cutoff voltage until the leakage current is equal to 10 μA.

[0069] It is seen in FIG. 4 that the best cycling performance, in terms of energy density, stability and reproducibility, is obtained with curve 1, which corresponds to an implementation example of the formation process according to the invention. The tests show, indeed, that the energy density values for curves 2 to 5 do not exceed 12 Wh/kg.sub.electrochemical core over the long term.

[0070] The invention is of course not limited to the implementation example of the process, which is given for the purpose of illustration and is non-limiting.