Method for preparing self-supporting flexible electrodes

Abstract

A method for preparing self-supporting flexible electrodes is provided using refined cellulose fibers as binder. The negative or positive self-supporting flexible electrode is obtained by such method. A Li-ion battery is also provided in which at least one electrode is a self-supporting flexible electrode.

Claims

1. Method for preparing a self-supporting flexible electrode g having at least one active electrode material, and at least one binder, said method comprising at least the following steps: i) a step of preparation of an aqueous paste by the dispersion of a mixture of solid particles in an aqueous phase, said mixture of solid particles having: at least one active electrode material, in a quantity varying from 50 to 98% by weight relative to the total weight of the mixture of solid particles, at least one binder comprising refined cellulose fibres, in a quantity varying from 2 to 50% by weight relative to the total weight of the mixture of solid particles; said mixture of solid particles representing at least 0.02% by weight of the total weight of the aqueous paste; ii) a step of filtration of the aqueous paste obtained above in the step i) on a filtration cloth; iii) optionally, a step of pressing, in the wet state, of the aqueous paste followed by its transfer onto a drying felt; iv) a drying step to obtain a sheet of dry flexible electrode supported by the filtration cloth or by the drying felt when the step iii) has been carried out; and v) a step of separation between the sheet of electrode and the filtration cloth or the drying felt when the step iii) has been carried out, to obtain the self-supporting flexible electrode.

2. Method according to claim 1, wherein the refined cellulose fibres (FBr) are obtained by a refining method comprising the following steps: a) a step of dispersion, in an aqueous medium, of previously dried cellulose fibres, to obtain a cellulose fibre paste in which the cellulose fibre content varies from 1 to 15% by weight relative to the total weight of said cellulose fibre paste; and b) a step of shearing of said cellulose fibre paste, so as to obtain refined cellulose fibres, that is to say cellulose fibres exhibiting a Schopper-Riegler degree varying from 30 to 95° SR.

3. Method according to claim 1, wherein an anti-flocculation agent for the fibres is incorporated in the aqueous suspension of the step i).

4. Method according to claim 3, wherein the anti-flocculation agent is chosen from carboxymethylcellulose (CMC), starch and one of their mixtures.

5. Method according to claim 3, wherein the anti-flocculation agent for the fibres represents 0.01 to 10% by weight relative to the total weight of the aqueous paste of the step i).

6. Method according to claim 1, wherein the concentration by weight of the mixture of solid particles in the aqueous suspension on completion of the step i) varies from 0.02 to 5%.

7. Method according to claim 1, wherein the filtration threshold of the step ii) is of the order of 1 to 100 μm.

8. Method according to claim 1, wherein the degree of refining of the refined cellulose fibres (FBr) is at least 30° SR.

9. Method according to claim 1, wherein the aqueous suspension of the step i) also contains at least one agent generating an electronic conductivity.

10. Method according to claim 9, wherein the agent generating an electronic conductivity is selected from the group consisting of carbon black, carbon SP, acetylene black, carbon fibres and nanofibres, carbon nanotubes, metal particles, and one of their mixtures.

11. Self-supporting flexible electrode obtained by the implementation of the method as defined in claim 1, wherein said self-supporting flexible electrode comprises at least one active electrode material, possibly at least one agent generating an electrical conductivity, possibly an anti-flocculation agent for the fibres and at least one binder comprising refined cellulose fibres (FBr) imprisoning said active electrode material.

12. Electrode according to claim 11, wherein said electrode is a positive electrode and that the active material is selected from the group consisting of LixMn.sub.yO.sub.4 (0<x<2, 0<y<2 and x+y=3), LiCoO.sub.2, LiMPO.sub.4, (M=Fe, Mn, Co, Ni), LiAl.sub.xCo.sub.yNi.sub.zO.sub.2 (0<x<1, 0<y<1, 0<z<1 and x+y+z=1) and LiNi.sub.(1-y)Co.sub.yO.sub.2 (0≦y≦1).

13. Electrode according to claim 11, wherein said electrode is a negative electrode and in that the active material is selected from the group consisting of: graphite, hard carbon, soft carbon and the metal alloys Li.sub.YM (1<y<5 and M=Mn, Sn, Pb, Si, In, Ti).

14. Electrode according to claim 11, wherein said electrode has a thickness varying from 50 to 300 μm.

15. Flexible lithium-ion battery comprising at least one negative electrode and/or at least one positive electrode between which is placed a solid electrolyte or a separator impregnated with a liquid electrolyte, wherein the positive electrode and/or the negative electrode is an electrode as defined in claim 11.

16. Battery according to claim 15, wherein each of the electrodes is a self-supporting flexible electrode as defined in claim 11, said electrodes being separated from one another by a sheet of paper impregnated with liquid electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a is a graph representing the average length of the cellulose fibres and the proportion of thin particles (% by length) as a function of the increase in degree of refining ° SR from example 1, in accordance with one embodiment;

(2) FIGS. 1b and 1c are images Observed in the optical microscope of a non-refined cellulose fibre and of a cellulose fibre exhibiting a degree of refining of 95° SR from example 1, in accordance with one embodiment;

(3) FIG. 1d is a graph of the Young's modulus for electrodes prepared from example 1, in accordance with one embodiment;

(4) FIGS. 2a-2c shows a series of preparation steps from example 2, in accordance with one embodiment;

(5) FIG. 3a is a graph of a tensile strength test from example 2, in accordance with one embodiment;

(6) FIG. 3b is a graph of an electrical conductivity from example 2, in accordance with one embodiment;

(7) FIG. 4a is a graph of specific capacities from example 2, in accordance with one embodiment;

(8) FIG. 4b is another graph of specific capacities for the anodes without CMC from example 2, in accordance with one embodiment;

(9) FIG. 4c is another graph of specific capacities for the anodes with CMC that are comparable to that of the anodes using a standard binder (PVdF) from example 2, in accordance with one embodiment;

(10) FIG. 5a is a graph of absorbance (in arbitrary units) as a function of the quantity of CMC in the suspension (% by weight) from example 3, in accordance with one embodiment;

(11) FIG. 5b is a graph of the zeta potential (mV) as a function of the quantity of hydrated aluminium sulphate in the suspension (% by weight) from example 3, in accordance with one embodiment;

(12) FIG. 6a is a graph of the conductivity (Sm.sup.−1) and the Young's modulus (MPa) as a function of the FBr fibre content (% by weight) from example 3, in accordance with one embodiment;

(13) FIG. 7 is a graph of specific capacity (in mAhg.sup.−1) as a function of the number of cycles from example 3, in accordance with one embodiment; and

(14) FIG. 8 is a graph of the specific capacity of the battery obtained by using an anode and a cathode prepared according to the procedures described in the Examples 2 and 3 (as per example 4), in accordance with one embodiment.

DETAILED DESCRIPTION

(15) The present invention is illustrated by the following examples, to which it is not however limited.

(16) The raw materials used in the examples are listed below: Synthetic graphite powder (GP) having an average particle dimension less than 20 μm; Carboxymethylcellulose (CMC) of mean molecular mass by weight Mw of 90 000 g.Math.mol.sup.−1; N-methyl-2-pyrrolidinone (NMP) and lithium hexafluorophosphate (LiPF.sub.6); Polyvinylidene fluoride (PVdF); Ethylene carbonate (EC) and diethyl carbonate (DEC), battery grade; Bleached leafy wood cellulose fibres (FB);
Except for the cellulose fibres, all the materials were used as received. Lithium iron phosphate (LiFePO.sub.4), Carbon black (CB), Hydrated aluminium sulphate: Al.sub.2(SO.sub.4).sub.3.13.4-14.5H.sub.2)

Example 1

Preparation of the Refined Cellulose Fibres and Characterizations (FIG. 1)

(17) 440 g of dried cellulose fibres (FB) were hydrated in 22 L of water for 24 h, then transformed into paste for 20 minutes in a Lhomargy® disperser, so as to obtain a cellulose fibre suspension. This step produces an outer fibrillation of the fibres which leads to an improvement in the fibre linkage characteristics. The aqueous suspension was then sheared (refined) in a refiner of Valley type according to the standard ISO 5264.

(18) The degree of refining (refining or draining index) was evaluated by means of drainage measurements in accordance with the standard ISO 5267 and is expressed in degrees Shopper-Riegler (° SR). The effects of the refining on the morphology of the fibres was observed using an optical microscope (OM) (Axio Imager M1m Zeiss) and an analyser of the morphology of the fibres. Morfi (TecPap), which was also used to quantify the fraction of thin particles present in the suspension of fibres induced by the refining step. The fibres exhibiting a degree of refining between 20 (non-refined fibres) and 95° SR were used for the preparation of the electrodes which will be used for the characterization tests.

(19) FIG. 1 shows the effect of the refining step on the cellulose fibres. In particular, FIG. 1a is a graph representing the average length of the cellulose fibres and the proportion of thin particles (% by length) as a function of the increase in degree of refining ° SR. The curve with the solid circles corresponds to the trend of the percentage by length of the thin elements and the curve with the empty squares corresponds to the trend of the arithmetical length of the fibres. This FIG. 1a thus shows that, during the refining treatment, namely during the increasing of the degree ° SR, the proportion of the thin particles increases gradually whereas the average length of the fibres decreases. This is in agreement with the characteristic cutting of the long fibers which occurs during the intensive refining for the conventional production of paper. FIGS. 1b and 1c are respectively images observed in the optical microscope of a non-refined cellulose fibre and of a cellulose fibre exhibiting a degree of refining of 95° SR. These figures show that the refining step induces a radical change in the morphology of the fibre. In fact, the compact cylindrical form of the non-refined fibres of FIG. 1b is transformed into a structure in disorderly hair form (FIG. 1c), thus indicating that the fibre cut (during refining) is accompanied by an external micro-fibrillation, namely a release of fibrils by partial rupture of the walls subjected during the refining step. Finally, FIG. 1d refers to electrodes prepared with 25% by weight of cellulose fibres refined to different degrees of refining ° SR and 75% by weight of graphite and gives the results of the measurement of the Young's modulus for each of the duly prepared electrodes. In this figure, the trend of the Young's modulus (modulus of elasticity) of the electrode (MPa) is expressed as a deletion of the degree of refining of the cellulose fibres (° SR). This FIG. 1d thus shows that the electrodes prepared with highly refined cellulose fibres are more uniform and exhibit a better bending resistance than those prepared with cellulose fibres that are little refined. In practice, the refining of the cellulose fibres makes it possible to obtain flexible electrodes but without them breaking or disintegrating.

Example 2

Preparation of a Number of Negative Electrodes (Anodes) and Characterizations

(20) The refined cellulose fibres (FBr) obtained in the Example 1 with a Schopper-Riegler degree of 95° SR and graphite particles (GP) were dispersed in the water by mechanical stirring in order to obtain a thick paste comprising 2% by weight of a mixture of solid particles comprising, depending on the anode preparations, 10%, 15%, 20% or 30% by weight of fibres (FBr) relative to the total weight of the mixture of solid particles.

(21) Before filtering this think aqueous solution, 1% by weight of CMC, relative to the weight of the dried fibres, was added thereto in order to limit the flocculation of the fibres during the filtration and enhance the homogeneity of the anode.

(22) Anodes were also prepared without using CMC.

(23) Following this first step, a mixture 1 (FIG. 2a) comprising 2% by weight of solid particles comprising the FBr/Gp elements and, optionally, CMC, was obtained.

(24) Then, as schematically represented in FIG. 2, 19 g of the mixture of 2% by weight of FBr/GP (also possibly comprising, depending on the preparations, CMC) were diluted in 400 ml of water and filtered in a vacuum using a funnel 3, Büchner flask 4 and a filter paper 2 of 90 mm diameter with a filtration threshold of 8 to 12 μm.

(25) The anode preparations filtered in a vacuum located on the filter paper were then pressed and dried in a vacuum for 10 minutes at 90° C. (step referenced 5 in FIG. 2).

(26) Thus, for each preparation, a self-supporting sheet 6 of FBr/GP and possibly comprising CMC, was obtained. This was then detached from the filter paper and stored in controlled conditions of temperature and humidity (23° C., 50% relative humidity).

(27) Measurements of tensile strength (performed using an instrument RSA3, TA Instruments, USA) and of electrical conductivity (four point test, Jandet Universal Probe) were then carried out on the duly prepared electrodes. These measurements are given in the attached FIG. 3. FIG. 3a shows the results of the tensile strength test. In this figure, the Young's modulus (in MPa) is a function of the refined fibre content (% by weight). The curve with the solid squares corresponds to the electrodes prepared using an FBr/GP/CMC mixture, whereas the curve with the empty squares corresponds to the electrodes prepared using an Fbr/GP mixture. FIG. 3b shows the results of the electrical conductivity test. In this figure, the conductivity (Sm.sup.−1) is a function of the refined fibre content (% by weight). The curve with the solid squares corresponds at the electrodes prepared using an FBr/GP/CMC mixture, where as the curve with the empty squares corresponds to the electrodes prepared using an FBr/GP mixture.

(28) The measurements of tensile strength and of electrical conductivity revealed the effect of the composition of the electrode on its mechanical properties, notably the Young's modulus which increases with the fraction by weight in fibres, and the electrical conductivity, which, conversely, decreases. The addition of CMC induces an improvement in both the Young's modulus and the electrical conductivity.

(29) Electrochemical tests, performed on a half-battery using a sheet of lithium as back-electrode, a solution of lithium phosphate hexafluoride (1 mol/l) in EC:DEC (1:1) as electrolyte and an anode made up of fibres refined to 95° SR (10% by weight) and GP (90% by weight), with or without CMC, were also produced. By way of comparison, specific capacity measurements were also carried out on a half-battery according to a similar setup in which the anode according to the invention was replaced by an anode prepared by replacing the refined cellulose fibres with a standard binder, PVdF. The GP/PVdf anode was prepared by the deposition of a mixture baaed on NMP containing 10% by weight of PVdF and 90% by weight of GP (relative to the total dry weight) on a sheet of copper and then evaporation of the NMP solvent.

(30) The corresponding specific capacity measurements are given in the attached FIG. 4 in which the specific capacity (in mAhg.sup.−1) and the coulombic efficiency (in %) are a function of the manner of cycles. FIG. 4a corresponds in the measurements carried out on the half-battery comprising an FBr/GP anode without CMC, FIG. 4b to the measurements performed on the half-battery comprising an FBr/GP anode with CMC and FIG. 4c to the measurements performed on the half-battery comprising a GP/PVdF anode that does not conform to the invention. In these figures, the curves with the solid circles correspond to the values measured when charging, the curves with the empty circles in the values measured when discharging and the curves with the solid triangles to the coulombic efficiency values.

(31) These results show that the FBr/GP anodes reach specific capacities (FIG. 4a) for the anodes without CMC and (FIG. 4b) for the anodes with CMC that are comparable to that of the anodes using a standard binder (PVdf), (FIG. 4c) i.e. approximately 300-350 mAh/g.

Example 3

Preparation of Positive Electrodes (Cathodes) and Characterizations

(32) To improve the affinity between carbon black (CB) and the refined cellulose fibres (FBr, 95° SR), suspensions of CB and FBr ware previously treated: The CB was treated with CMC by adding between 0 and 2% by weight of CMC to a 2% by weight CB suspension. The concentrations are given relative to the total weight of the suspension. The FBrs were treated with hydrated aluminium sulphate by adding between 0 and 1% by weight of hydrated aluminium sulphate to an FBr suspension with a concentration of 2% by weight. The concentrations are given relative to the total weight of the suspension.

(33) The absorbance of each of the suspensions based on CB and on CMC thus prepared was measured using a UV spectrometer (Unicam UV5 Series, Thermo Spectronic, Cambridge UK). The results are given in the attached FIG. 5a in which the absorbance (in arbitrary units) is a function of the quantity of CMC in the suspension (% by weight). This figure shows the dispersing effect of the CMC on the CB due to the absorption of the CMC on the CB.

(34) Also, the electrical charge was determined and the zeta potential computed for the FBr fibres of each of the suspensions based on FBr treated by the hydrated aluminium sulphate, using a zeta-meter (SZP 04 Mutek). The results obtained are given in the attached FIG. 5b in which the zeta potential (mV) is a function of the quantity of hydrated aluminium sulphate in the suspension (% by weight). This figure shows the neutralization of the anionic charges on the FBr surface following the addition of hydrated aluminium sulphate.

(35) These two effects coupled together make it possible to optimize the fixing of the CB on the cellulose fibres when mixing the suspensions of CB treated with the CMC and FBr treated with the hydrated aluminium sulphate.

(36) Suspensions of CB-CMC and of FBr-hydrated aluminium sulphate were then mixed in respective quantities varying from 0 to 60% by weight of suspension of CB/CMC and 100 to 40% by weight of suspension of FBr/hydrated aluminium sulphate.

(37) CB-CM/CBr-hydrated aluminium sulphate flexible composite papers were then prepared from each of the mixtures between CB-CMC and Fbr/hydrated aluminium sulphate by using the set up described above in the Example 2 and in FIG. 2, by using a nylon filtration cloth with a filtration threshold of 33 μm, followed by a pressing and a drying in a vacuum (10 minutes at 90° C.).

(38) Each of the composite papers thus prepared was then evaluated from the point of view of its tensile strength and of its electrical conductivity as also described above in the Example 2.

(39) The attached FIG. 6 gives the results obtained and gives (FIG. 6a) the conductivity (Sm.sup.−1) and the Young's modulus (MPa) as a function of the FBr fibre content (% by weight). In this figure, the curve with the solid circles corresponds to the trend of the conductivity whereas the curve with the empty circles corresponds to the trend of the Young's modulus. A photograph of each of the composite papers is given by FIG. 6b.

(40) FIG. 6 shows that the improvements of the fixing of the CB on the cellulose fibres during the mixing of the suspensions of CB treated with CMC and of the FBrs treated with hydrated aluminium sulphate has a direct effect on the conductivity of the composite papers obtained by filtration of the CB-CMC/FBr-hydrated aluminium sulphate mixtures. The composite papers obtained with Fbr and CB not previously treated with hydrated aluminium sulphate and CMC give almost zero conductivities.

(41) The flexible cathodes were then prepared from a mixture of CB/CMC/FBr/hydrated aluminium sulphate (CB 60%, FBr 40%) to which was added a suspension of active electrode material (2% by weight suspension of LiFePO.sub.4 in the water) in a proportion of 40% by weight of CB-CMC/FBr-hydrated aluminium sulphate mixtures for 60% by weight of suspension of active electrode material by using the set up described above in the Example 2 and in FIG. 2, using a nylon filtration cloth with a filtration threshold of 33 μm, followed by a pressing and a drying in vacuum (10 minutes at 90° C.).

(42) The cathodes obtained show mechanical properties (mechanical strength, flexibility, etc.) similar to those of the anodes prepared in the Example 2.

(43) Electrochemical tests were carried out on a half-battery using a lithium sheet as back-electrode and a solution of lithium hexafluorophosphate (1 mol/l) in EC:DEC (1:1) as electrolyte. The results of these electrochemical tests are given in the attached FIG. 7 in which the specific capacity (in mAhg.sup.−1) is a function of the number of cycles. In this figure, the curves wish the empty circles correspond to the measurements performed during discharging and the solid circles to the measurements performed during charging, the calculation being made relative to the weight of the active electrode material.

(44) These results show that the cathodes prepared according to the procedure mentioned above have: a specific capacity of approximately 55 mAh/g (relative to the weight of active material LiFePO.sub.4), this specific capacity being little affected by the charging/discharging current. For its part, the cycling resistance is at least 150 cycles at C/2.

Example 4

Preparation of a Complete Battery and Characterization

(45) The electrodes prepared according to the procedures illustrated in the Examples 2 and 3 were used to produce a complete battery. The anode and the cathode were prepared in order to obtain a final composition of the electrodes in the dry state as follows: anode FBr/GP/CMC: 10% by weight of FBr, 90% by weight of GP, and 1% by weight of CMC, relative to the weight of the dried fibres; FBr/hydrated aluminium sulphate/CB/CMC/LiFePO.sub.4 cathode; 40% of the CB/CMC/FBr/hydrated aluminium sulphate mixture (mixture consisting of 60% by weight CB and 40% by weight FBr) and 60% by weight of LiFePO.sub.4;

(46) A sheet of paper dipped in a lithium hexafluorophosphate solution (1 mol/l) in EC:DEC (1:1) was used as separator between the two electrodes.

(47) The battery was produced by stacking the three anode/separator/cathode components in a rigid measurement cell. The adhesion between the anode, the separator and the cathode was assured by the pressure exerted by the metal current collectors forming the measurement cell.

(48) The results are given in the attached FIG. 8 in which the specific capacity (in mAhg.sup.−1) is a function of the number of cycles. In this figure, the curves with the empty circles correspond to the measurements performed during charging and the solid circles to the measurements performed during discharging.

(49) FIG. 8 shows that the specific capacity of the battery obtained by using an anode and a cathode prepared according to the procedures described in the Examples 2 and 3 and a sheet of paper as separator is 60 mAh/g (calculated relative to the weight of active cathodic substance, notable LiFePO.sub.4) with a charging/discharging current of C/5. This value is comparable to the specific capacity obtained with half-battery tests, where the model battery consists of the cathode, a rock wool separator and a metal lithium sheet. This proves that the assembly of a battery by using the electrodes developed in the present invention, a paper separator and a liquid electrolyte makes it possible to achieve electrochemical performance levels comparable to those using model half-batteries using metallic lithium as back-electrode.