CA1167519A

CA1167519A - Manufacture of electrochemical high temperature cells

Info

Publication number: CA1167519A
Application number: CA000384649A
Authority: CA
Inventors: Tsvetko Chobanov; Dieter Kunze; Friedrich Woeffler
Original assignee: VARTA Batterie AG
Current assignee: VARTA Batterie AG
Priority date: 1980-08-29
Filing date: 1981-08-26
Publication date: 1984-05-15
Also published as: US4447376A; FR2489602B1; GB2083277B; FR2489602A1; JPS5774976A; DE3032552A1; GB2083277A

Abstract

Abstract of the Disclosure In the manufacture of cells, e.g. of the system Li-Al/LiCl-KCl/FeSx, the finely divided raw materials for the electrodes and for an included ceramic separator are respectively mixed individually with the electrolyte salt, as well as with a synthetic plastic which is decomposable without residue under heat. The mixtures are rolled into plates, and the plates are heated above the decomposition temperature of the plastic after assembly. The plastic is preferably a polyhydrocarbon such as polyisobutylene and is preferably introduced by means of a solvent. It makes the powder mixtures plastifiable and suitable for rolling. This makes it possible to produce ceramic separators, which are fundamentally composed only of loose particle accumula-tions, in the form of plates of uniform structure and strength and to handle these in the same way as electrode plates during cell assembly.

Description

5 1 ~

The invention relates to a process of manufacture of an electrochemical high temperature cell having a solid positive electrode, a solid negative electrode, a separator of finely divided ceramic material, and a molten liquid electrolyte which is retained wi-thin the cell components, and to a process for forming a separator or an electrode for such an electrochemical high temperature cell.
High temperature cells of the type under consideration operate, for example, on the basis of the rechargeable electro-chemical system Li-Al/LiCl ~Cl/FeSx(where x=l or 2).
In view of the operating temperature of about 450C, which exceeds the melting point of most eutectic electrolyte salt mixtures,the Li electrode is strengthened by alloy formation with Al.
The separator of such a cell must not only withstand the extreme temperature conditions, which also entrain additional corrosion problems, but must also mechanically resist the cyclic volume changes of the electrodes.
It is true that ceramic materi.als such as beryllium oxide (BeO), thorium oxide (ThO2), magnesium oxide (MgO~, lithium aluminate (LiAlO2), boron nitride (BN), sili.con nitride (Si2N4) or aluminum nitride (AlN), which are named as temperature and corrosion resistant separator materials in United States Patent 3,510,359, indeed fully meet the thermal and chemical requirements.
However, the shaping of the separator presents difficulties because the ceramic material itself is not ion conductive, and ion permea-bility, which is as significant as permeability of the molten 5 1 ~

liquid electrolyte, can only be obtained by means of an open structureO
Separators of boron nitride fabric, or matting, are known, but these are relatively expensive and are of limited utility because of their poor wettability by the molten electrolyte and because of their low mechanical stability.
A substantially simpler separator, which is formed simply by a particle accumulation that fills the space between the electrodes and consists of fragmented ceramic material and which also has the desired mechanical robustness, is disclosed in German Patent Publication (Offenlegungsschrit) 2,847,464. The voids within the particles, which are not firmly connected to each other but are freely movable relative to each other, are filled with powdered electrolyte salt.
A disadvantage of this ceramic separator is its preparat-ion by pouring or by vibrating-in of a powder, which is a method that is not readily compatible with those techniques which are otherwise conventional in cell manufacture. Moreover, it is unavoidable that certain variations in porosity and thickness of the accumulation must be tolerated. The result of these variations is to also create local changes in the internal resistance of the separator la~er. These inhomogeneities in turn are the cause of non-uniform current density distribution within the electrodes, which contributes to disintegration of the separator and of the electrodes.
The separator must also be sufficiently impermeable to the electrode materials. This requires that the maximum pore 11~'751~

diameter oi the separator and the minimum particle diameter of the electrode materials must be matched to each other.
Accordingly, it is now desired to devise a process of manufacture of a separator or an electrode and an electrochemical high temperature cell from finely divided ceramic material which is not subject to the drawbacks mentioned above.
The present invention provides a manufacturing process for an electrochemical high temperature cell having a solid positive electrode, a solid negative electrode, a separator of finely divided ceramic material, and a molten liquid electrolyte which is retained within the said cell components, the process comprising mixing finely divided skarting material of the separator and/or of the electrodes with electrolyte powder and with a synthetic plastic which is decomposable without residue under - heat, the starting material being electrode powder forthe electrodes or finely divided ceramic for the separator, rolling the mixture into a cohesive plate of uniform thickness, assembling the plates individually or after being rolled together into composite electrode (s)/ separator plates into a cell, and heating the cell after completed assembly above the decomposition temperature of the plastic and below the operating temperature of the cell so as to decompose the plastic without residue~
The present invention also provides a manufacturing process for an electrochemical high temperature cell having a solid positive electrode, a solid negative electrode, a separator of finely divided ceramic material, and a molten liquid electrolyte which is retained within the said cell components, the process comprlslng mixing the ceramic starting material of the separator with electrolyte powder and with a synthetic plastic which is decomposable without residue under heat, the starting material being a uniform mixture of course grained ceramic particles and finely divided ceramic powder, rolling the lnixture into a cohesive plate of uniform thickness, assembliny the plates individually or after being rolled together into composite separ-ator plates into a cell, and heating the cell after completed assembly at a temperature of 320 to 400C in the absence of air so as to decompose the plastic without residue.
; The present invention further provides a process for forming a separator or an electrode for an electrochemical high temperature cell comprising; mixing finely divi.ded ceramic material of the separator or finely divided electrode powder material for the electrode with electrolyte powder and with a synthetic plastic which is decomposable without residue under heat, rolling the mixture into a cohesive plate of uniform thickness, assembling the plate as a separator or an electrode of the cell individually or after ~eing rolled together into composite separator plate or electrode plate, and then heating the cell after completed assembly above the decomposition temperature of the synthetic plastic but below the operating temperature of the cell so as to decompose the synthetic plastic without residue.
The process emboding the invention generally makes it possible to render materials which are composed of powders or powder mixtures plastifiable and therefore capable of being rolled.
This is particularly significant for solid electrodes, because it - 3a -l 1~ 751 ~

is only by rolling that they can be reduced to e~tremely small thickness dimensions while maintaining high uniformity of material density. Thin electrodes are an indispensable prerequisite for high specific current loads.
A particular advantage of -the process embodylng the invention residues in the possibility of producing a ceramic separator which always has unifarm structure and thickness, from what is initially only a loose particle accumulation and also to handle the same during cell assembly like a solid plate in the same way as the electrodes.
The plastic additive -to the finely divided material or powder, which imparts rolling capa}iility to it, can be of various types. I-t must be capable of again being removed after assembly and before putting the cell into operation. Therefore, a synthetic resin selected for thatpurpose should have no functional groups which react with components of the cell. During thermal decompo-sition of the plastic after plate assembly, no solid carbon must remain as a residue in the separator because this could cause short dircuits. On the other hand, decomposition products such as H20 or HCl can be tolerated only in very small quantities in view of the sensitive ~i - 3b -electrode. Particu]arly suitable for plastifying are therefore polyhydro-carbons with quaternary-carbon groups. Among these, polyisobutylene holds a particularly preferred position in accordance with the invention.
The process embodying the invention is f~rther described below by means of examples for the manufacture of a ceramic separator. The same principle can be usad in accordance with the invention to prepare positive electrode plates, e.g. from FeS powder, or negative electrode plates, e.g.
from a Li-Al-alloy powder. In those cases, selection of the proper relation-ship between mass powder and electrolyte powder in the rolled mixture depends upon the wettability of the former, but can also be largely a matter of choice. In general, however, lt is desirable that the proportion of the electrolyte powder in the rolling mixture not exceed 70%, being preferably 30 to 50%. The particle size of the electrode powder needs to be matched to that of the respective mixture partner.
The porosity of the separator can be influenced by the particle size distribution of the ceramic material, the particle porosity, and ~he proportion of electrolyte in the mixture to be rolled.
A preferred separator structure consists, for example, of a uniform mixture of coarse grained particles and finely divided powder. The coarse grained particles have the function of separating the electrodes~ the fine particles serve as particle barrier, or retainer. The coarse grained part-icles have a diameter smaller than 2.0 mm, preferably between 0.1 and 1.0 mm; the fine grained particles have a diameter less than 150 ~Im, preferably 5 to 60 ym~ The volume proportion of the coarse grained solid content is 10 to 90%, and preferably 40 to 80%, that of the finely divided content is 90 to 1O%J preferably 60 to 20%.
In a specific example, such a separator structure is produced in accordance with the invention by stirring a mixture of the above-described 3~ 16 ~ ~ ~9 particles to which po~dered electrolyte has also heen added, into a solution of polyisobutylene ~average molecular weight about 15,000) in cyclohexane The quantity relationships are so chosen that the polyisobutylene proportion in the solvent-free mixture amounts to 1-30% by weight, and preferably 3-15%
by weigllt. The polyisohutylene content in the cyclohe~ane can amount to 50%
by weight and is preferably 5-15% by ~ieight. These figures can also be used for other plastics, for e~ample, polyhydrocarbons other than polyisobutylene and corresponding solventsO
The solvent is removed after stirring by heating, if desired in a vacuum, and the mixture is then rolled into a cohesive, uniformly thick plate of 0.1 - 3.0 mm thickness and preferably 0.3 - 2.0 mm thickness.
The plate is cut to the deslred dimensions and placed between the electrodesO The completed cell is then heated to a temperature of more than 320C, for example 340-400C, whereupon the gaseous decomposition products of the polyisobutylene escape from the cell. This must be performed in the absence of air. Thereafter the cell can be made ready for operation by melting of the electrolyte which is already present in the cell, and if appropriate, by filling with molten liquid electrolyte.
Another preferred separator structure consists of three layers.
The two outer layers contain predominantly the finely divided separator particles, the inner layer predominantly coarse grained separator particles.
Each of these layers is initially prepared in the previously described manner; subsequently the three layers are placed bet~een the electrodes in the sequence described. They can also be previously united through being rolled together.
A third preferred separator structure also consists of three layers.
The two outcr layers contain predominantly the finely divided separator particles with the synthetic plastic additives; the middle layer consists of ~p~y 5 . ~ , .

~1675~9 a mat or fabric of ceramic material, e.g. boron nitride (BN). This BN
material has the function of stabilizing the separator mechanically; the outer layers serve as particle barrier.
The two outer layers are produced by rolling; they are placed upon the middle layer, or united with it by being rolled together. In so doing it is desirable to impregnate the boron nitride with the same plastic, pre ferably polyisobutylene, so that the outer layers adhere well to the middle layer. In this way the frequently fragile textile ceramic product is ~urned into a stable and easily handleable structural component.
By rolling together of the ceramic separator with the electrode plates which have been produced in analogous fashion, ~here is obtained in accordance with the invention the finished electrode block in a particularly compact form, in which the entire cell electrolyte is furthermore integrated.
In producing ~he separator or the electrode plates in air, it is fur~her desirable to first remove from the mixture an electrolyte component which may be hygroscopic, for example the LiCl in the preferably utilized LiCl-KCl electrolyte. This is then created ultimately in the melt by diffusion with the KCl which is already present in the separator.
Thermal decomposition of the plastic (polyisobutylene) and ~0 driving off of the decomposition products from the assembled electrode block should be carried out with caution and is best done under vacuum to protect the active electrode materials from air.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A manufacturing process for an electrochemical high temper-ature cell having a solid positive electrode, a solid negative electrode, a separator of finely divided ceramic material, and a molten liquid electrolyte which is retained within the said cell components the process comprising mixing finely divided starting material of the separator and/or of the electrodes with electrolyte powder and with a synthetic plastic which is decomposable without residue under heat, the starting material being electrode powder for the electrodes or finely divided ceramic for the separator, rolling the mixture into a cohesive plate of uniform thick-ness, assembling the plates individually or after being rolled together into composite electrode (s)/ separator plates into a cell, and heating the cell after completed assembly above the decomposition temperature of the plastic and below the operating temperature of the cell so as to decompose the plastic without residue.

2, The process of claim 1 wherein the plastic is a poly-hydrocarbon with quaternary-carbon groups.

3. The process of claim 2 wherein the polyhydrocarbon is polyisobutylene.

4. The process of claim 3 wherein the finely divided starting material of the separator and/or the electrodes mixed with electrolyte powder is stirred into a solution of polyisobutylene in cyclohexane up to a 50% by weight solution, the solvent is subsequently vaporized so as to obtain a solvent free mixture containing about 1 30% by weight of polyiso-butylene, the solvent-free mixture is rolled into a cohesive plate of uniform thickness, the plates are assembled into the cell either individually or after being rolled together into composite electrode (s)/
separator plates, and the cell is heated to a temperature of between 320°C and 400°C after complete assembly so as to decompose the plastic without residue.

5, The process of claim 4 wherein the solution of polyiso-butylene in cyclohexane is a 5-15% solution.

6. The process of claim 5, wherein the solvent-free mixture contains 3 to 15% by weight of polyisobutylene.

7. The process of claim 1 wherein the starting material of the separator is a uniform mixture of coarse grained ceramic particle and finely divided ceramic powder.

8, A manufacturing process for an electrochemical high temperature cell having a solid positive electrode, a solid negative electrode, a separator of finely divided ceramic material, and a molten liquid electrolyte which is retained within the said cell components, the process comprising mixing the ceramic starting material of the separator with electrolyte powder and with a synthetic plastic which is decompos-able without residue under heat, the starting material being a uniform mixture of coarse grained ceramic particles and finely divided ceramic powder, rolling the mixture into a cohesive plate of uniform thickness, assembling the plates individually or after being rolled together into composite separator plates into a cell, and heating the cell after completed assembly at a temperature of 320 to 400°C in the absence of air so as to decompose the plastic without residue.

9. The process of claim 8 wherein the coarse grained particles have a diameter below 2mm and their volume proportion in the separator structure is 10-90%.

10. The process of claim 8 wherein the coarse grained particles have a diameter of 0.1-1.0mm and their volume proportion is 40-80%.

11. The process of claim 8 wherein the fine powder particles have a diameter of less than 150 µm and their volume proportion in the separator structure is 90-10%.

12, The process of claim 7, 8 or 10 wherein the fine powder particles have a diameter of 5-60 µm and their volume proportion in the separator structure is 60-20%.

13. The process of claim 4 or 8 wherein the separator mixture is uniformly rolled into a cohesive plate of the thickness of 0.1-3.0mm.

14. The process of claim 1 or 8 wherein the separator plate consisting predominantly of finely divided ceramic material is rolled prior to assembly onto a separator material consisting predominantly of coarsely grained ceramic material, or upon a fabric or matting of ceramic material, in each case on both sides.

15. A process for forming a separator or an electrode for an electrochemical high temperature cell comprising;
mixing finely divided ceramic material of the separator or finely divided electrode powder material for the electrode with electrolyte powder and with a synthetic plastic which is decompos-able without residue under heat, rolling the mixture into a cohesive plate of uniform thickness, assembling the plate as a separator or an electrode of the cell individually or after being rolled together into composite separator plate or electrode plate, and then heating the cell after completed assembly above the decomposition temperature of the synthetic plastic but below the operating temperature of the cell so as to decompose the synthetic plastic without residue.

16, The process according to claim 15, which is for forming the separator.

17. The process according to claim 15 or 16, which further comprises melting the electrolyte present in said cell.

18. The process according to claim 15 or 16, which further comprises filling the cell with molten electrolyte.

19. The process of claim 1, 8 or 15 wherein the cell operating temperature is approximately 450°C and the temperature to which the cell is heated to decompose the plastic is in the range of 340 to 400°C.