Battery electrode material and method for making the same

Abstract

The invention concerns a method for manufacturing of a battery electrode material comprising the steps of: a) applying an electric field to at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles will remain in their position when said electric field is removed. Further, the invention concerns a battery electrode material comprising at least one polymer and conductive particles, wherein said conductive particles form at least two lines that are oriented parallel and/or co-linear to each other.

Claims

1. A method for manufacturing a battery electrode material, comprising a) applying an electric field to a mixture comprising at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles remain in their position when said electric field is removed, wherein the at least one polymer is selected from the group consisting of polyvinyldifluoride, carboxymethylcellulose, styrene butadiene rubber, poly(3,4-ethylenedioxythiophene), polyacrylic acid and alginate.

2. The method according to claim 1, further comprising supporting said mixture comprising at least one polymer, conductive particles and at least one solvent on a support.

3. The method according to claim 1, wherein the at least one polymer is a non-cured polymer.

4. The method according to claim 1, wherein the conductive particles are electronically or ionically conductive.

5. The method according to claim 1, wherein in step a) the mixture further comprises active particles.

6. A method for manufacturing a battery electrode material, comprising a) applying an electric field to a mixture comprising at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles remain in their position when said electric field is removed, wherein the conductive particles have a low aspect ratio.

7. The method according to claim 6, further comprising supporting said mixture comprising at least one polymer, conductive particles and at least one solvent on a support.

8. The method according to claim 6, wherein the at least one polymer is a non-cured polymer.

9. The method according to claim 6, wherein the conductive particles are electronically or ionically conductive.

10. The method according to claim 6, wherein in step a) the mixture further comprises active particles.

11. A method for manufacturing a battery electrode material, comprising a) applying an electric field to a mixture comprising at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles remain in their position when said electric field is removed, wherein the at least one solvent is N-methyl-2-pyrrolidone.

12. The method according to claim 11, further comprising supporting said mixture comprising at one least polymer, conductive particles and at least one solvent on a support.

13. The method according to claim 11, wherein the at least one polymer is a non-cured polymer.

14. The method according to claim 11, wherein the conductive particles are electronically or ionically conductive.

15. The method according to claim 11, wherein in step a) the mixture further comprises active particles.

16. A method for manufacturing a battery electrode material, comprising a) applying an electric field to a mixture comprising at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles remain in their position when said electric field is removed, wherein the viscosity of the mixture in step a) is between 0.1 and 12,000 cPa.Math.s.

17. The method according to claim 16, further comprising supporting said mixture comprising at least one polymer, conductive particles and at least one solvent on a support.

18. The method according to claim 16, wherein the at least one polymer is a non-cured polymer.

19. The method according to claim 16, wherein the conductive particles are electronically or ionically conductive.

20. The method according to claim 16, wherein in step a) the mixture further comprises active particles.

21. A method for manufacturing a battery electrode material, comprising a) applying an electric field to a. mixture comprising at least one polymer, conductive particles and at least one solvent whereby said conductive particles become arranged between the electrodes in at least two lines that are oriented in the same direction as the electric field line, and b) stabilizing the at least one polymer, conductive particles and at least one solvent by removing at least some of said at least one solvent while maintaining the electric field in step a) whereby the at least two lines of conductive particles remain in their position when said electric field is removed, wherein the electric field is of the order of 0.1 to 10 kV/cm.

22. The method according to claim 21, further comprising supporting said mixture comprising at least one polymer, conductive particles and at least one solvent on a support.

23. The method according to claim 21, wherein the at least one polymer is a non-cured polymer.

24. The method according to claim 21, wherein the conductive particles are electronically or ionically conductive.

25. The method according to claim 21, wherein in step a) the mixture further comprises active particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematics of the employed alignment procedures for in-plane alignment.

(2) FIG. 2 shows schematics of the employed alignment procedures for out-of-plane alignment.

(3) FIG. 3 illustrates conductive dendritic structures maximizing the contact area.

(4) FIG. 4 shows the life time performance of a LiFePO.sub.4 electrode cycled at an increasing charge/discharge time.

DETAILED DESCRIPTION

(5) In one embodiment of the invention, the method as defined hereinabove or hereinafter comprises the mixing of infusible conductive particles and a mixture that contains at least one polymer and at least one solvent, and the electric field alignment of conductive particles in this mixture and the control of the viscosity of this mixture by evaporating the solvent. This procedure can be applied to battery electrodes and electrolytes to replace isotropically distributed conductive particles by thin wires of aligned assemblies of conductive particles. These situations are illustrated in FIGS. 1 and 2.

(6) FIG. 1 shows schematics of the employed alignment procedures for in-plane alignment. This displays how randomly distributed conductive particles B mixed with at least one polymer and at least one solvent between electrodes A become arranged into lines of conductive particles B upon application of an electric filed. This figure is for illustrative purposes only, and all of the lines shown may not be formed The solvent may be evaporated in order to obtain a solid material comprising at least one polymer with conductive pathways being lines of conductive particles. The lines of conductive particles extend in a co-linear or substantially parallel fashion with the electrodes and are said to be aligned in an in-plane alignment

(7) FIG. 2 shows schematics of the employed alignment procedures for out-of-plane alignment. This displays how randomly distributed conductive particles mixed with at least one polymer and at least one solvent between electrodes A become arranged into lines B upon application of an electric filed. The solvent may be evaporated in order to obtain a solid material comprising at least one polymer with conductive pathways being lines of conductive particles. The electrodes may be provided with holes to allow for evaporation of solvent through the holes. The alignment be also be performed in such a way that at least one of the electrodes is not in direct contact with the conductive particles, the at least one polymer and the at least one solvent. The lines of conductive particles extend in a perpendicular or substantially perpendicular fashion with the electrodes and are said to be aligned in an out-of-plane alignment.

(8) The resultant aligned material retains anisotropic properties with respect to directional electrical conductivity. In this way, aligned conductive microstructures of originally infusible particles which do not allow alignment as such are formed.

(9) FIG. 3 illustrates dendritic structures maximizing the contact area between conductive lines and the at least one polymer.

(10) FIG. 4 shows the life time performance of a LiFePO.sub.4 electrode prepared according to the method of the invention and a LiFePO.sub.4 electrode prepared according to a standard technique cycled at an decreasing charge/discharge time. The electrode prepared according to the method of the invention contains lines of conductive particles whereas the electrode prepared according to a standard technique contains randomly distributed particles. As shown in the figure, the electrode prepared according to the method of the invention retains its capacity to a larger extent than the electrode prepared according to the standard technique.

(11) Further modifications of the invention within the scope of the claims would be apparent to a skilled person. For example, an isotropic material could be manufactured by carrying out the method according to an embodiment of the invention two or more times applying an electric field in different directions. For example, a conducting lattice structure could be formed throughout the material. Further, the method as defined hereinbefore or hereinafter may be used to provide a plurality of conducting lines in a piece of material or to control the conductivity of a piece of material. For instance, the method may be used to create a more highly conducting zone in a piece of material.

(12) The invention is illustrated, but not limited, by the following Examples.

EXAMPLES

Example 1

(13) This example concerns the use of electric field alignment where the alignment is in-plane alignment when preparing polymeric materials in films and membranes with low fraction of electrically conductive particles.

(14) This example concerns the preparation of a mixture of conductive particles and polymer matrix that in this example is a polyelectrolyte in solvent; the solvent can be a high alcohol such as propanol.

(15) The conductive particles are aligned in the polymer matrix and this alignment enhances the conductivity of material in the alignment direction.

(16) This example, moreover, shows change of the viscosity of the obtained material, by evaporating the solvent, so that the alignment and directional conductivity can be obtained in the alignment step is maintained.

(17) The employed conductive particles were carbon nanocones from n-Tec AS or carbon black from Alfa Aesar.

(18) The employed polymer was the ionomer Nafion® IQ-1105 (5 wt. % in ethanol).

(19) The particles and polymer were mixed such that the particle fraction was 2 vol. % that is at about the percolation threshold of these particles. This mixture was stirred by a magnetic stirrer at room temperature for 15 minutes and visually uniform mixture was obtained.

(20) This mixture was spread over the interdigidated gold electrodes on glass substrate. The electrode width was 10 μm and the electrode spacing 100 μm. The maximum thickness of the film had a value between 50 and 500 μm.

(21) Alignment was done using 1 kHz field of 0.2 kV/cm.

(22) The system was stabilized by letting the solvent evaporate during the alignment. This means that the electric field was kept on until the solvent was completely evaporated. At about 25° C., this took 3 minutes for a 100 μm thick film.

Example 2

(23) This example is similar to example 1 but instead of using carbon particles at about their percolation threshold, ten times less of material is used. The conductivity increased from ˜10.sup.−3 S/m to ˜10.sup.−2 S/m.

Example 3

(24) This example is similar to the examples 1 and 2 but instead of in-plane geometry shown in FIG. 1, the out-of-plane alignment geometry shown in FIG. 2 was used. While the in-plane geometry may provide substrate support to the aligned wires, this is absent in the out-of-plane geometry where wires have translational freedoms perpendicular to the alignment direction. The alignment occurs regardless the geometry, which means that the alignment does not require substrate support.

Example 4

(25) This example is similar to the examples 1-3 but instead of mere carbon particles, carbon particles with platinum catalyst were used. Platinum catalyst does not influence on alignment but the results are identical to those obtained with carbon particles without platinum catalyst.

Example 5

(26) This example concerns partially grown conductive wires in electrodes. In this example particles are grown by an electric field on the current collector surface thus maximizing the available electrode surface area.

(27) The growth of aligned wires begins from the electrodes. In this case the alignment was performed using alternating field (1 kV/cm) but particle fraction is kept so low that there are not enough particles to form wires from electrode to electrode. This fraction depends on the particle-polymer pair but is about 0.1 vol. % or less for the 100 μm electrode spacing. In this case, the alignment electrodes become covered by wires that reach the polymer matrix but not the opposite electrode.

(28) These branched wires become an inherent part of alignment electrodes whose surface area is thus substantially increased.

(29) The obtained “dendritic” electrodes are illustrated in FIG. 3.