Electrochemical devices with current collector having an increased resistance to corrosion

Abstract

Electrochemical device or photo-electrochemical device comprising an electrolyte containing a bistriflimide anion, hereafter named as TFSI, at least two electrodes, each of these electrodes being in contact with a current collector comprising a metal support characterized in that at least one electrode has a current collector the metal support of which comprises an electro-active surface which is functionalized with linear or branched fluorinated carbon chains, such as perfluoroalkyl chains, in the form of a molecular layer which improves the corrosion resistance of said functionalized surface compared to a non-functionalized surface, wherein not impairing the passage of electrons between said electrode and its current collector, the functionalized surface being at the interface between said electrode and its current collector.

Claims

1. Electrochemical device or photo-electrochemical device comprising; an electrolyte containing a bistriflimide anion, hereafter named as TFSI, at least two electrodes, each of these electrodes being in contact with a current collector comprising a metal support wherein at least one electrode has a current collector the metal support of which comprises an electro-active surface which is functionalized with linear or branched fluorinated carbon chains, such as perfluoroalkyl chains, in the form of a molecular layer which improves the corrosion resistance of said functionalized surface compared to a non-functionalized surface, wherein not impairing the passage of electrons between said electrode and its current collector, the functionalized surface being at the interface between said electrode and its current collector.

2. Electrochemical device or photo-electrochemical device according to claim 1, wherein the fluorinated carbon chains of the functionalized electro-active surface comprise perfluoroalkyl aryl moieties.

3. Electrochemical device or photo-electrochemical device according to claim 1, wherein the perfluoroalkyl aryl moiety is an aromatic cycle which is mono- or di- substituted by respectively one or two linear or branched perfluoroalkyl chain(s), preferably linear or branched perfluoro C1 to C20 alkyl chain(s), more preferably linear or branched perfluoro C1 to C10 alkyl chain(s).

4. Electrochemical device or photo-electrochemical device according to claim 1, wherein the linear or branched fluorinated carbon chain is a perfluoroalkyl chain chosen among the following groups: trifluoromethyl, perfluorohexyl and perfluorooctyl group.

5. Electrochemical device or photo-electrochemical device according to claim 1, wherein the metal support of the current collector is an aluminum substrate having a purity above 95%, preferably a purity equal or above 99% or an aluminum alloy comprising more than 95% Al, preferably more than 99% Al.

6. Electrochemical device or photo-electrochemical device according to claim 1, wherein the TFSI containing electrolyte is a liquid electrolyte, preferably a mixture of a salt containing the TFSI anion, such as LiTFSI, NaTFSI, or NH4TFSI, and an organic solvent or mixture of organic solvents or the former salts dissolved in pure or diluted molten salt, known as ionic liquids.

7. Electrochemical device or photo-electrochemical device according to claim 1, wherein the TFSI containing electrolyte is a solid electrolyte, preferably a polymer based electrolyte, preferably a mixture of a salt containing this anion, such as LiTFSI, NaTFSI, or NH4TFSI, and a polymer or mixture of polymer or a physical ionogel or a chemical ionogel comprising a silica matrix or a mixture of silica and a polymer matrix.

8. Electrochemical device or photo-electrochemical device according to claim 1 wherein said device is chosen among: an energy storage device, a lithium-ion, sodium-ion, magnesium-ion, a calcium-ion or an aluminum battery, an electrochemical capacitor so called supercapacitor, a lithium-ion or a sodium-ion capacitor, a hybrid device intermediate between battery and supercapacitor, a photo-battery, or an electro-chromic device.

9. Electrochemical device according to claim 1 wherein said device is a lithium-ion battery comprising a graphite anode with a copper current collector, a Nickel Manganese Cobalt cathode with a functionalized aluminum current collector, and LiTFSI 0.75 M in Ethylene carbonate:Diethylcarbonate as electrolyte.

10. Process for preparing the functionalized current collector of the electrochemical device or photo-electrochemical device of claim 1 comprising the following main steps: Providing a current collector for electrochemical devices, said current collector comprising a support with a metallic surface; Polishing said metallic surface with a fine abrasive to remove any oxide layer, then washing said surface with an organic solvent under ultra- sonication to obtain a polished electro-active surface; Functionalizing the polished electro-active surface by dipping said surface of the current collector in a solution comprising a diazonium salt of a perfluoroalkyl moiety, preferably a perfluoroalkyl aryl moiety, under reducing conditions of the diazonium salt, and obtaining an electro-active surface functionalized with the perfluoroalkyl aryl moiety, Optionally washing the functionalized surface with an organic solvent under ultra-sonication, to remove the non-functionalized molecules from the electro-active surface of the support.

11. Process according to claim 10 wherein the functionalized step is performed in an any one of an electrochemical cell, a photochemical cell, a heated cell or a sonicated cell, and/or with the help of a chemical reducer in said solution.

12. Process according to claim 10 wherein the polishing step is a dry polishing step.

13. Process according to claim 10 wherein the functionalizing step is performed in a chronoamperometric cell containing a solution comprising a diazonium salt of the functionalizing perfluoroalkyl aryl moiety, an auxiliary electrode, a reference electrode and said metallic support as working electrode, at a potential under the redox potential of said diazonium salt of the functionalizing perfluoroalkyl aryl moiety.

14. Process according to claim 13 wherein the auxiliary electrode of the chronoamperometric cell is an electronically conductive plate, and the working electrode of the chronoamperometric cell is said metallic support of the current collector made of an aluminum substrate having a purity above 95%, or made of an aluminum alloy comprising than 95% Al.

15. Process according to claim 14 wherein said aluminum substrate having a purity above 99%.

16. Process according to claim 14 wherein said aluminum alloy comprising than 99% Al.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 presents Nyquist diagrams, before cycling, of a unfunctionalized (non-treated) aluminum current collector and of a functionalized (treated) aluminum current collector according to the present invention;

(2) FIG. 2 is a schematic diagram equivalent electric circuit for interfaces of a unfunctionalized current collector;

(3) FIG. 3 is a schematic diagram equivalent electric circuit for interfaces of the functionalized current collector;

(4) FIG. 4 are linear sweep voltammetry curves of the functionalized and unfunctionalized aluminum substrates;

(5) FIG. 5A and FIG. 5B are scanning electron microscopy (SEM) pictures of respectively unfunctionalized (non-treated) aluminum substrate, and functionalized (treated) aluminum substrate according to the invention;

(6) FIG. 6 shows curves of galvanostatic cycling at cycle 1 (C/10) of functionalized and unfunctionalized current collectors;

(7) FIG. 7 shows curves of galvanostatic cycling at cycle 10 (C/10) of functionalized and unfunctionalized current collectors;

(8) FIG. 8 are linear sweep voltammetry curves of unfunctionalized and functionalized aluminum substrates of the comparative example 2.

(9) FIG. 9 is a schematic cross section of a lithium ion battery in the shape of a button cell.

(10) FIG. 9A is a SEM cross section image of the interface between cathode and functionalized current collector.

(11) FIG. 10 shows curves of second galvanostatic cycle (voltage versus time) for battery of example 4 with functionalized (G) aluminum current collector compared to same battery with unfunctionalized (NG) aluminum current collector.

(12) FIG. 11 shows the evolution of discharge capacity during the first 10 cycles for batteries of example 4 with functionalized (G) aluminum current collector compared to same battery with unfunctionalized (NG) aluminum current collector.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

(13) I/ Functionalization Procedure of a Current Collector

(14) I-a) Chemicals

(15) All standard chemicals were purchased from Sigma Aldrich. In the examples the aniline precursor for preparing the diazonium salt was 4-(heptadecafluorooctyl) aniline, the reducing agent tert-butylnitrite and the electrolyte was tetraethylammonium tetrafluoroborate dissolved in acetonitrile. All those reactants were used as received.

(16) I-b) Preparation of Aluminum Substrates

(17) A sheet (foil of 125 m thickness) of non-allied aluminum was chosen with a purity of 99%. Before doing any functionalizing on this current collector the surface was dry polished using a finer abrasive (preferably grade 500) to remove the aluminum oxide layer, and then washed in a bath with acetone and ethanol using ultra-sonication.

(18) I-c) Functionalization

(19) Typical functionalizing experiments were carried out in open air at room temperature (20-25 C.) and atmospheric pressure.

(20) A first electrolyte solution of 0.1M tetraethylammonium tetrafluoroborate in acetonitrile was prepared. Then a second solution of diazonium salts (10 mM) was prepared by adding 4-(heptadecafluorooctyl) aniline and 3 equivalents of tert-butylnitrite to the first solution. This diazonium solution was kept under stirring for 30 minutes around.

(21) Electrochemical functionalizing experiments were carried out in a three-electrode cell containing the diazonium solution and comprising a working electrode at which the functionalizing reaction takes place (aluminum sheet), an auxiliary electrode (platinum plate) and a reference electrode (Ag/AgCl). Chronoamperometry (CA) permitted to reduce the diazonium cations on the aluminum substrate to form a functionalized surface. The working electrode was polarized at 0.9V vs. Ag/AgCl for different periods of time (preferably at least 1 minute) according to the desired degree of functionalizing. At the end of the functionalizing process, the modified current collector was washed by dipping in acetone under ultrasonication to remove the unfunctionalized molecules from the aluminum surface.

(22) II/ Characterisation of the Functionalized Surface

(23) II-a) Surface Modification

(24) In order to measure the surface tension and also to observe the shape of a water drop on the substrate, the contact angles of each sample were measured 5 times using the sessile-drop method by dispensing 1 mL droplets on the sample surfaces. All of the contact angle measurements were taken under ambient laboratory conditions with a temperature of 20 C. and a relative humidity of 45%.

(25) In addition, the surface was watched with a scanning electron microscope and analyzed by X-ray dispersive spectroscopy. Results are presented in Table 1 below:

(26) TABLE-US-00001 TABLE 1 X-ray micro Water Surface energy (mN m.sup.1) analysis Functionalizing Contact Polar Dispersive (% mass Substrate by CA Angle Total component component fluorine) Aluminum no 63 43 12 31 0.05 0.06 99% - foil yes 92 20 5 15 0.30 0.06 125 m CA = chrono amperometry % mass fluorine given for a 100 100 1 m.sup.3
A drastic increase of the % mass fluorine is observed and is assigned to the presence of perfluoroalkyl groups on the functionalized surface.

(27) The contact angle of the water drop is higher for modified aluminum and the value of the polar component of surface energy decreases when the substrate is functionalized. All these results enable to conclude on the modification of aluminum surface samples by functionalizing a hydrophobic molecular layer.

(28) II-b) Impedance Spectroscopy

(29) The impedance spectroscopy (see Nyquist's diagram on FIG. 1) permits mainly to determine that the current collector surface has been modified by the functionalizing procedure. After establishment of a model (see FIGS. 2 and 3 and respective calculated values presented in tables 2 and 3) it is possible to notice a new input due to the functionalized layer.

(30) TABLE-US-00002 TABLE 2 Index i R.sub.i () Q.sub.i (F .Math. s.sup.1/ai) a.sub.i 0 6 1 2 309 33 10.sup.6 0.73 3 459 0.7 10.sup.3 0.66

(31) TABLE-US-00003 TABLE 3 Index i R.sub.i () Q.sub.i (F .Math. s.sup.1/ai) a.sub.i 0 6 1 14 2 10.sup.6 0.1 2 423 29 10.sup.6 0.75 3 264 0.9 10.sup.3 0.68

(32) This new input has a low resistance (14) and a constant phase element (CPE) of 2.10.sup.6 F s.sup.1/a. The calculated values indicate that this new input (due to the functionalized molecular layer) is weak and has little influence on the electron transfer. Therefore, it would not alter the basic requirements for a current collector.

(33) III/ Effect of Functionalizing

(34) III-a) Linear Sweep Voltammetry

(35) Electrochemical cell (SwageLock cell): Reference and counter electrode=lithium metal; Working electrode=aluminum (functionalized or non-functionalized surface); Electrolyte=LiTFSI 0.75M in EC:DEC (3:7).

(36) (EC=Ethylene carbonate/DEC=Diethylcarbonate)

(37) Parameters: Speed rate=1 mV s1; Electrode surface=1.13 cm.sup.2; E.sub.min=Eoc (open-circuit voltage); E.sub.max=5 V vs. Li/Li+.

(38) In this voltammetric method the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly with time, from E.sub.min to E.sub.max at a determined speed rate. The selected area analyzed by this method is 1.2 cm.sup.2 for all the samples. There are two phenomena to observe on FIG. 4.

(39) First of all the maximum current measured at 5V vs. Li/Li.sup.+ is totally different when the aluminum is functionalized or not. An impressive decrease of the maximum current of 80% is observed when the aluminum current collector is functionalized by the functionalized molecules. Since the current is proportional to the corrosion rate this clearly indicates that the functionalizing treatment according to the present invention reduces aluminum pitting by a factor of at least 3.

(40) The second interesting parameter is the potential value at which the current begins to raise, which corresponds to the potential at which the corrosion phenomenon begins. It is shown on the graph that the curve corresponding to the functionalized current collector begins to raise after the non-coated one.

(41) So the functionalized surface according to the present invention increases the corrosion potential of the current collector from 3.9 V vs. Li/Li.sup.+ up to 4.2V vs. Li/Li.sup.+. This means that the functionalized current collector can be used at a higher potential than the unfunctionalized one, thus enabling to reach higher cell voltage when this functionalized current collector will be integrated in a real device. For an energy storage device, this would imply working with higher energy density in the device using the functionalized current collectors.

(42) III-b) Scanning Electron Microscopy

(43) SEM images were taken with secondary electron imaging mode and are presented in FIGS. 5A and 5B.

(44) The corrosion of aluminum generally appears in the form of localized corrosions called pitting, i.e.: small holes created in the metal surface. Holes (5 to 20 m diameter) seen on the right picture (FIG. 5A) are evidencing that the untreated current collector is quickly degraded by corrosion. The picture on the left (FIG. 5B) shows the efficiency of the functionalized aluminum substrate according to the present invention with virtually no holes (no corrosion) thus confirming the electrochemical results presented above.

(45) III-c) Galvanostatic Cycling

(46) Electrochemical Cell: Reference and counter electrode=lithium metal; Working electrode=NMC (Nickel Manganese Cobalt) cathode (LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2) casted on unfunctionalized or functionalized aluminum; this material is a standard material classically used as positive electrode in Li-ion batteries; Electrolyte=LiTFSI 0.75M in EC:DEC (3:7).

(47) (EC=Ethylene carbonate/DEC=Diethylcarbonate)

(48) Parameters: Average mass of active material=5 mg cm.sup.2; Average current=136 pA; E.sub.min=2.7 V VS. Li/Li.sup.+; E.sub.max=4.2 V VS. Li/Li.sup.+; C/10.

(49) The galvanostating cycling curves are presented on FIGS. 6 and 7 respectively corresponding to cycle 1 (C/10) and cycle 10 (C/10).

(50) It can be noticed that the NMC electrode operates in the same manner whether the current collector surface is functionalized or not, which means that, as expected, the functionalizing treatment according to the present invention does not change the electrochemical behavior of the NMC cathode material.

(51) The polarization is also an interesting phenomenon to look at. Indeed the potential width observed between the charge and the discharge is the same for all the batteries tested. The NMC cathode operates as usual when deposed on a treated current collector. Thus the functionalizing treatment prevents corrosion of the aluminum current collector without inhibiting the electrochemical performance of the NMC electrode.

Example 2 (Comparative)

(52) IV/ Functionalizing Procedure of a Non-Fluorinated Molecule

(53) IV-a) Preparation of Aluminum Substrates

(54) A sheet (foil of 125 m thickness) of non allied aluminum was chosen with a purity of 99%. Before doing any functionalizing on this current collector the surface was dry polished using a finer abrasive (500 grade) and then washed with acetone and ethanol under ultrasonication.

(55) IV-b) Functionalizing

(56) Typical functionalizing experiments were carried out in open air at room temperature (20-25 C.) and atmospheric pressure.

(57) A first solution of an electrolyte of 0.1M tetraethylammonium tetrafluoroborate in acetonitrile was prepared. Then a second solution of diazonium salts (10 mM) was prepared by adding 4-dodecyl aniline and 3 equivalents of tert-butyl nitrite to the first solution. This diazonium solution was kept under stirring for 30 minutes.

(58) Electrochemical functionalizing experiments were carried out in a three-electrode cell containing the diazonium solution comprising a working electrode at which the functionalizing reaction takes place (aluminum sheet), an auxiliary electrode (platinum plate) and a reference electrode (Ag/AgCl). Chronoamperometry (CA) permitted to reduce the diazonium cations on the aluminum substrate to form a film functionalized surface. The working electrode was polarized at 1.2V vs. Ag/AgCl for different periods of time according to the desired degree of functionalizing. At the end of the functionalizing process, the modified current collector was dipped in acetone under ultrasonication.

(59) IV-c) Characterisation of Functionalized Surface

(60) In order to measure the surface tension and also to observe the shape of a water drop on the substrate, the contact angles of each sample were measured 5 times using the sessile-drop method by dispensing 1 mL droplets on the sample surfaces. All of the contact angle measurements were taken under ambient laboratory conditions with a temperature of 20 C. and a relative humidity of 45%.

(61) Results are presented in Table 4 below:

(62) TABLE-US-00004 TABLE 4 Functionalizing Water Surface energies (mN m.sup.1) Functionalizing time Contact Polar Dispersive Substrate step by CA (min) angle Total component component Aluminum no 63 43 12 31 99% - foil yes 5 88 28 4 24 125 m 1 93 34 1 32 CA = chronoamperometry

(63) The contact angle of the water drop is higher for functionalized aluminum and the value of the polar component of surface energy decrease when the substrate is functionalized.

(64) All these results permit to conclude on the modification of aluminum surface samples by functionalizing a hydrophobic molecule layer.

(65) IV-d) Effect of Coating

(66) Electrochemical Cell: Reference and counter electrode=lithium metal; Working electrode=aluminum (functionalized or non-functionalized); Electrolyte=LiTFSI 0.75M in EC:DEC (3:7).

(67) Parameters: Speed rate=1 mV s.sup.1; Electrode surface=1.13 cm.sup.2; E.sub.min=E.sub.oc (open-circuit voltage); E.sub.max=5 V vs. Li/Li.sup.+.

(68) In this voltammetric method the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly from E.sub.min to E.sub.max at a fixed speed rate. The curves are really similar: there is no reduction of the corrosion current at 5V vs. Li/Li.sup.+ and no rise decrease in the corrosion potential. The non-fluorinated molecule functionalized on the aluminum surface does not prevent the corrosion of the current collector.

Example 3 (Comparative)

(69) The influence of the polishing step has been tested. The support is an Aluminum Goodfellow sheet with a purity of 99%.

(70) The surface of aluminum has been polished with an abrasive paper 800.

(71) The functionalizing treatment has then been performed by chronoamperometry according to the conditions described in example 1 (step I-c).

(72) Comparative tests have been made, before and after functionalization, with the aluminum sheet without polishing, after polishing under water lubrification or with dry polishing. The results are presented in table 5 below:

(73) TABLE-US-00005 TABLE 5 Contact angle () Contact angle () of a water drop of a water drop BEFORE AFTER FUNCTIONN- FUNCTIONN- Polishing step ALIZATION ALIZATION Without polishing 31 2 36 2 Water polishing 33 2 48 2 Dry polishing 35 2 100 2

(74) It appears that the polishing step is an important step in the to functionalization treatment of the present invention, in particular for aluminum substrates. In fact the aluminum support surface needs to be polished without lubricant, more specifically without water. The polishing step aims to remove all or part of oxide layer and obtain an electro-active surface. If water is used during this step the oxide layer seems to be reformed instantly. Therefore dry polishing is much preferred.

Example 4

(75) A full electrochemical device like a lithium ion battery could be in the shape of a button cell as shown in FIG. 9. Assembly of such button cell battery is described hereafter.

(76) The button cell case 1 comprises a cathode shell 2 and an anode shell 3, both made of stainless steel, separated by an insulating gasket 4.

(77) Assembly of the respective parts of the button cell comprises the superposition of the following components from bottom (cathode shell) to top (anode shell) as shown on FIG. 9: on top of the cathode shell is placed the current collector with its functionalized surface turned upwards, in contact with the cathode 21 material on to the cathode 21 is deposited a porous separator 6 either soaked with the liquid electrolyte comprising LITFSI or made of a solid electrolyte containing LITFSI on top of the separator 6 is laid the anode 31 material (usually graphite) casted in the anode current collector 32, usually in copper then is placed a stainless steel spacer 5 and a spring 7 is interposed between the spacer 5 and the anode shell 3 to ensure a sufficient pressure between all the components of the button cell after having positioned the anode shell 3 on top of all the components, the button cell is sealed using an automatic press.

(78) The SEM image of the enlarged cross section of FIG. 9A shows the cathode current collector 22 with its functionalized surface 23 (too thin to be clearly observed) on top of which is laid the cathode electrode 21.

(79) More precisely, as a specific example of the cell of FIG. 9, the electrochemical cell is made of: a graphite anode casted onto a copper current collector; a NMC (Nickel Manganese Cobalt) cathode (LiNi1/3Mn1/3Co1/3O2) casted on unfunctionalized aluminum (for comparative tests) or functionalized aluminum; this cathode material is a standard material classically used as positive electrode in Li-ion batteries; the electrolyte is LiTFSI 0.75M in EC:DEC (3:7).

(80) (EC=Ethylene carbonate/DEC=Diethylcarbonate)

(81) The parameters of the above electrochemical cell are as follows: Average mass of active material (cathode)=7.5 mg cm.sup.2; Average mass of active material (anode)=3.3 mg cm.sup.2; Average current=26 mA.Math.g.sup.1; Emin=2.8 V; Emax=4.0 V; C/5.

(82) With the above electrochemical cell have been performed several galvanostatic cycles (voltage versus time):

(83) FIG. 10 presents a comparison between the second galvanostatic cycle with a functionalized (NG) aluminum current collector.

(84) It is noted that the capacitance of the battery is similar for both cells. The presence of a functionalized surface between the current collector and the cathode have therefore no negative influence on the battery capacity.

(85) FIG. 11 shows that the functionalized surface of the cathode current collector have also no negative influence on cyclability of the electrochemical cell.