GB2046003A - Improvements Relating to Electrolytic Cells - Google Patents

Abstract

A solid-state electrolytic cell having a pair of electrodes, e.g. of silver and gold, has these electrodes in contact with opposite faces of a body of electrolyte, e.g. silver iodide, in the form of a thin film of randomly- oriented polycrystalline structure having a grain size of 0.5 microns or less. All the components are produced by successive stages of deposition in vacuo, the conditions of deposition being controlled to give the required grain size. In the example, the silver electrode 2 is first deposited on a glass substrate 1 and the electrolyte 3 is deposited on top of the electrode 2. By starting the deposition of silver iodide before the deposition of silver is stopped, the two layers are dispersed each within the other to form a boundary layer 4. The gold electrode 5 is subsequently deposited on top of the electrolyte layer 3, the deposition conditions being so controlled as to yield a dispersed boundary layer 6. <IMAGE>

Description

SPECIFICATION Improvements Relating to Electrolytic Cells Electrolytic cells comprise basically a pair of electrodes separated by one or more electrolytes.

Although the electrodes are usually solid and the electrolytes in the form of a liquid any of these components may be in the liquid, solid or gaseous phases and in particular the electrolyte and electrodes may all be solid, thus providing an all solid-state cell which has advantages in terms of physical robustness, ease of packaging, storage life and availability in very compact form suitable for micro-circuit applications.

Solid cell components are normally found to be less electro-chemically efficient than either the same (or an equivalent) component in the liquid or gaseous phase, or the same (or an equivalent) component which is dissolved in a liquid or otherwise dispersed in another element or compound. Thus cells constructed entirely of solid components have been found to be inefficient and, in particular, they tend to possess unacceptably high internal resistances. The internal resistance may be reduced by making the electrolyte layer, or layers, extremely thin but the improvement has not been found adequate to yield a practical cell. The object of the present invention lies in the provision of thin film solid state cells of greater efficiency which may be potentially very useful in many micro-electronic applications.The invention is not, however, restricted solely to cells in which all the components are solid. Many electrolytic cell types with one or more liquid or gaseous components exist in which the electrical performance of the cell is impaired by the inefficiency of one or more of the solid components. In certain cases the efficiency of the cells can be improved as a result of the invention. The reasons for the electrochemical inefficiency of solid components of cells are normally attributable to physical rather than chemical processes which occur in solids. The three major limiting physical processes which occur are as follows: Firstly, in a solid electrolyte, it is found that the number of mobile ions available is very low, i.e.

most solid electrolytes have high electrical resistivities. This is because, in the majority of the ionic compounds used as electrolytes, it is energetically unfavourable for an ion to move from a lattice to an interstitial site from where ionic transference can occur. Furthermore, it is normally found that should such ions become available, then the movement of an ion through a crystalline lattice is also energetically unfavourable.

A second factor is that if a cell has a solid electrode which is required to adsorb, or desorb alien atoms rapidly (as in a concentration cell) then it is often found that the rates at which these processes occur are lower than those at which other dynamic cell processes occur.

The third factor is that if both an electrode and an electrolyte are solid and adjacent, it is almost universally found that there is an impedance (capacitive and resistive in character) at the interface. This effect is primarily due to a build-up of oppositely charged ions at the electrode/electrolyte interface which gives rise to the commonly observed polarisation effect in cells. The effect is usually of little consequence in cell systems without solid/solid interfaces, but when such an interface does exist, the ions forming the double layer may not have sufficient mobility rapidly to re-distribute themselves when current is drawn and the capacitive effect may persist. The resistive effect is due both to the existence of the double layer and also to the formation of layers, just within the electrolyte which are depleted of ions due to ions having moved to the interface.These depletion layers persist if the double layer persists, thus giving a cell an apparently anomalously high internal resistance when current is drawn.

According to the present invention, an electrolytic cell has a solid electrolyte in the form of a thin film of very fine-grained, randomlyoriented, polycrystalline structure, the term "very fine-grained" being used to define a grain size of 0.5 microns or less, this structure preferably being obtained as a result of controlled deposition in vacuo. It is found that this grain structure enhances the availability and mobility of ions in many solid electrolytes, thus largely compensating for the first of the factors referred to above. By increasing the number of crystallites in a given volume, the surface area to volume ratio is made larger and the number of available ions arising from surface defects is increased.

Moreover, intrinsic defects will occur naturally within crystallites and the likelihood of their possessing sufficient energy to reach a crystal boundary is far higher in a small crystal than in a large crystal. When the ions made available are at crystallite surfaces, their mobility is governed by grain boundary and surface conduction mechanisms both of which are normally energetically far more favourable than intrinsic condution mechanisms. It is, therefore, possible to greatly increase both ionic availability and mobility by producing an electrolyte having a structure as defined above.

Preferably the material of at least one of the electrodes of the cell is formed in the same way to give a very fine-grained, randomly-oriented polycrystalline structure. In the case of an electrode, the grain boundary diffusion and surface conduction mechanism at crystal boundaries which result from the polycrystalline structure increase the rate at which alien atoms are adsorbed into, or out of, solids. Indeed, experiment has shown the grain boundary diffusion rates for metal atoms diffusing into metals may be greater than lattice diffusion rates by factors as large as 109. A further advantage of the structure is that the crystal surface area to volume ratio is very large and the probability of an alien atom being adsorbed into an electrode crystal is thus higher than in the case of an electrode made up of large crystals.

The crystalline structure of vacuum deposited films may be controlled by choice of the basic deposition technique used and by control of deposition parameters. The nature and degree of crystallinity of the substrate material on which the film is deposited may also influence the crystalline structure of a film.

The greatest effect upon film grain size (irrespective of the deposition technique being used) are the rate of deposition and substrate temperature. Grain size generally decreases with increasing rate of deposition and decreasing substrate temperature. There is also a strong tendency for grain size to increase with film thickness. The use of sputtering as the deposition method rather than thermal evaporation usually produces films of finer grain size. In both r.f. or d.c. sputtering the use of substrate bias may also affect grain size of the sputtered films.For example, if an aluminium film is r.f. sputtered onto a glass substrate which is electro-negatively biased with respect to the target (and the bias voltage is in phase with the target voltage) then grain size increases with increasing bias to about -50 V and then decreases; an increasing positive bias voltage induces decreasing grain size. Bias sputtering is a method which may be used to induce structural changes in most sputtered films, but in general, methods other than variation of deposition rate and substrate temperature are not only specific to a given technique, but also to a particular material. Because of this lack of generality the following examples illustrate only how the film structure of a material may be changed by deposition rate and substrate temperature.Four silver iodide films were made by vacuum evaporation and one by r.f. sputtering: When silver iodide was evaporated at a rate of 50 nm.s-t onto. glass substrate at or below a temperature of C very fine grained randomly oriented films were obtained. The electrical resistivity of these films was 41 03cm. When the deposition rate was reduced significantly (all other deposition parameters being identical) the resultant films comprised larger grains which were of a columnar structure with the columns oriented at the angle subtended between the substrate and the evaporant beam (which was in this case normal to the substrate). Furthermore, the films tended to "craze" giving a mosaic appearance to the surface with spaces of 0.01 mm, or more, between pieces of film.The electrical resistivity of such a film was extremely high when measured in a plane parallel to the substrate and of the order of 1 05Q cm measured perpendicular to the substrate. Silver iodide films evaporated onto glass substrates at temperatures above~51 C but below 250C (other deposition parameters being the same as the first example) exhibited highly columnar grain structures with long crystallites (often the length of the thickness of the films). The resistivities of the films were between 104 and 1 Oen cm depending upon the substrate temperature during deposition.When silver iodide was evaporated onto glass at a temperature of 10000 or greater (with all other conditions being as in the previous example) the films were composed of very large crystals, the minimum dimension of which was the film thickness. Such films had resistivities of 1 O652 cm or greater (i.e. single crystal bulk resistivities). In contrast to the other examples, silver iodide, r.f.

sputtered in an argon plasma at a rate of ~0.1 nm.s-l on to a glass substrate at a temperature of 3000 had a grain structure almost identical to that of the first example of a film evaporated at 50 nm.s-l onto glass at or below a temperature of 5100.

The above results are all for silver iodide deposited upon glass substrates but similar results were obtained for silver iodide films deposited onto silver, gold and palladium films.

If both an electrode and an electrolyte of a cell are deposited in vacuo, this leads to a further important feature of the invention whereby the effect of interfacial impedance may be largely overcome. Such a result may be obtained by depositing the two materials in such a way that they are dispersed each within the other near the boundary between the electrode and the electrolyte where there would otherwise be a sharp interface. Broadly speaking this may be accomplished by deposition of a single component, followed by co-deposition of both components and eventually deposition of the other component, the order in which the electrode and electrolyte are deposited not being critical.

The production of such an interspersed boundary may best be described by reference to the practical example of the production of an Ag/Agl/Pd concentration cell. If radio frequency sputtering is used, the Ag electrode may be deposited first and while Ag sputtering is still in progress, 12 gas is slowly leaked into the vacuum chamber. Some Ag reacts and is deposited as Agl the partial pressure of 12 being progressively increased until eventually only the Agl electrolyte is deposited. If an evaporative process is used, Pd evaporation may be initiated at a very low rate while the Agl evaporation is still continued. The rate of Pd deposition is gradually increased and that of Agl gradually decreased until only Pd electrode material is being deposited. In both these examples, a gradual transition is obtained from a hundred percent of one material to a hundred percent of the other. With such interspersed boundaries the interfacial impedance either decreases or disappears because there is no longer any way which the "double layer" may exist in the manner described for a discrete interface. A further advantage of an interspersed boundary or interface is that, assuming both components and interface have a fine-grained, randomly-oriented structure, then the probability of voids of significant magnitude developing between electrode and electrolyte is very small.

Furthermore, lattice misfits between the crystals of the two components tend to be an advantage rather than a disadvantage.

Interspersed boundaries or interfaces are also advantageous where a cell has two electrolytes which would otherwise meet at a sharp boundary.

The result may be achieved in the same way as described above, e.g. by coevaporation or cosputtering. Provided that both components are fine-grained, randomly-oriented polycrystalline structures, then for the reasons mentioned previously, reactions and ionic transference at the boundary occur more rapidly due to the increased availability and mobility of ions. In addition, this allows for an even larger reactive surface area than would be available in the event of a discrete interface between two fine-grained electrolytes.

Assuming that each component of an electrolytic cell is produced in the manner described, i.e. the deposition of one electrode followed by the deposition of one or more electrolytes and then the deposition of a second electrode with interspersed boundaries between successive components, the successive steps of deposition are preferably carried out sequentially in-vacuo, i.e. without any interruption of the vacuum. This avoids possible contamination or other adverse effect which might occur if there were any intermediate exposure to the .atmosphere. Such adverse effects may include oxidation of a component (as in the case of Ag), decomposition of a component (as when RbAg4l5 is exposed to water vapour) and gaseous inclusions creating voids within a solid material and thus inhibiting ionic or atomic transference.

By maintaining the vacuum throughout and carrying out sequential deposition steps in vacuo, such adverse effects which may seriously reduce the performance of the cell, are avoided.

Examples of electrolytic cells made by sequential "in-vacuo" deposition techniques in accordance with the invention will now be described in more detail with reference to the accompanying sketches of which the two Figures illustrate respectively an Ag/Agl/Au concentration cell and an Au/Aul/Agl/Ag cell.

Turning first to Figure 1, about a hundred such cells were made, mainly in batches of six and with a yield of about ninety percent. In each case, a substrate 1 was used in the form of a soda glass slide measuring three inches by one inch, these substates being cooled in vacuum to less than minus 51 CC and a layer of silver evaporated on the surface. When about 1 ,um of Ag had been deposited, Agl deposition by evaporation commenced. The rate of Agl deposition as gradually increased while that of the Ag was decreased until only Agl was being evaporated at a rate of 50 nm.s-l. Finally, Au evaporation commenced and the rate of Agl evaporation was decreased until only Au was deposited.The process was then stopped whilst the temperature of the substrate 1 was still less than minus 51 OC, thus a very low temperature was maintained throughout, though this was not necessary for the Au and Ag deposition. A selective substrate masking system was used during the deposition sequence in order to cause the component materials to be deposited on discrete areas. The resultant structure comprised an Ag electrode 2 of approximately 1 ym thick followed by a layer 3 of Agl electrolyte approximately 1 O,um thick, the two being separated by an interspersed or "graded" interface 4. An Au electrode 5 approximately 50 nm thick was separated from the electrolyte 3 by a graded boundary layer 6.

The two electrodes were shaped as shown in the drawing and connection was made to them by means of aluminium connectors 9 and 10.

It was found that all the components of each cell possessed a very fine-grained, randomlyoriented polycrystalline structure throughout, the metals having means grain "diameters" of 450 nm and the Agl electrolyte having mean "diameters" of 4500 nm. The cells were capable of being charged to an emf of 0.67 volts (the breakdown voltage of Agl), the emf of the devices being generated according to the Nernst equation, as is theoretically predicted for a concentration cell. The cells were normally charged to 500 mV and when stored at 200C, 400C and 550C showed excellent charge retention over periods of greater than one year, e.g. typically less than 10-13 charge loss at 200C over a period of one year for cells 0.1 cm2 in surface area.No cell component degradation or deterioration of electrical characteristics was observed after charge cycling cells between 0 mV and 500 mV.

The cell shown in Figure 2 was produced by a corresponding series of steps carried out in sequence in-vacuo. The vacuum chamber and substrate conditions were initially identical to those used for the cells of Figure 1, but a layer of Au was initially evaporated onto the substrate.

After approximately 1 ,um of Au had been deposited, 12 was introduced to the chamber by preferential evaporation from the Agl source, the rate of deposition of Au being progressively decreased to give a graded interface followed by a layer of Aul. Agl was next deposited and finally Ag was co-evaporated with the Agl to give the final graded interface and the pure Ag electrode. In this Figure the substrate is again shown as 1, the Au electrode as 1 the Aul as 12, the Agl as 13 and the Ag electrode as 14. As mentioned above and as shown on the drawing, the layers 1 1 and 12 were approximately 1 ,um thick, the layer 13 was 1 Om thick and the layer 14 again 1 Mm thick.

Graded interfaces were again produced between adjacent layers, only that between layers 13 and 14 being specifically illustrated as 15.

The components of the cells produced as just described again possessed a very fine-grained, randomly-oriented, polycrystalline structure and the grain sizes were almost identical with those obtained in the example described in relation to Figure 1. Again the desired grain size and structure was obtained by maintaining a temperature at or below minus 51 OC. The emfs of the cells in accordance with Figure 2 were generated by the chemical reaction of the two electrolytes and were constant at 1.4 mV during discharge.

Various other types of cell may be produced by the same general type of process including, for example, Pb/Pb Cl/Ag Cl/Ag.

Claims (10)

Claims

1. A solid state electrolytic cell having a pair of electrodes in contact with opposite faces of a body of electrolyte in the form of a thin film of randomly-oriented polycrystalline structure having a grain size of 0.5 microns or less.

2. A solid state electrolytic cell according to claim 1 in which the electrolyte film is the product of controlled deposition in vacuo.

3. A solid state electrolytic cell according to claim 1 or claim 2 in which at least one of the electrodes is also in the form of a randomlyoriented polycrystalline structure having a grain size of 0.5 microns or less produced by controlled deposition in vacuo.

4. A solid state electrolytic cell according to claim 3 in which the materials of the electrolyte and the electrode produced by controlled deposition invacuo are dispersed each within the other near the boundary between the electrode and the electrolyte where there would otherwise be a sharp interface.

5. A solid state electrolytic cell according to any one of the preceding claims in which the body of electrolyte comprises two layers of different electrolyte materials which are dispersed each within the other near the boundary between them.

6. A method of producing a solid state electrolytic cell according to claim 3 in which one electrode is produced by deposition on a substrate in vacuo and one or more layers of electrolyte followed by the second electrode are subsequently produced by successive steps of deposition also in vacuo.

7. A method according to claim 6 in which the successive steps of deposition are carried out sequentially without interruption of the vacuum.

8. A method according to claim 7 of producing a a solid state electrolyte cell according to claim 4 in which successive steps of deposition are controlled so as to produce interspersed boundaries between adjacent components by deposition of a first component alone followed by co-deposition of the first and second omponents and then deposition of the second component alone.

9. A solid state electrolytic cell substantially as described with reference to either Figure of the accompanying drawings.

10. A method of producing a solid state electrolytic cell substantially as described with reference to either Figure of the accompanying drawings.