GB2064209A - Secondary Battery Electrodes - Google Patents

Abstract

A negative electrode for use in secondary zinc batteries, comprises a porous high-surface area electrically conducting substrate in plaque form, adapted for the plating out thereon of zinc from the electrolyte during the charge of the battery, said plaque having a low overpotential for zinc deposition and a higher overpotential for hydrogen evolution. The surface of the plaque may consist of nickel, silver, lead, cadmium, and from metals amalgamated with mercury.

Description

SPECIFICATION Negative Electrode Field of the Invention According to the present invention there are provided improved zinc negative electrodes for use in high energy density secondary batteries of the zinc-alkaline type, such as Ni-Zn, Ag -Zn, MnO2-Zn, Zn~ 2 and Zn - air systems. Key problems associated with these batteries include formation of zinc dendrite growths on charge at the negative plates, this tending to cause shorts across to the positive plates, and secondly irreversible redistribution of Zn active material to the central and lower portions of the negative (shape change/dinsification effect), causing capacity degradation with cycling.It is an object of the present invention to overcome these problems, and this accomplished by the provision of a novel zinc electrode system offering substantially enhanced reversibility on cycling.

Such an approach can also be used in other Zn based secondary batteries in which a non-alkaline electrolyte is used, such as Zn-Cl2, Zn-Br2 and Zn~PbO2- State of the Prior Art A recent review of zinc electrode technology as applied to secondary battery systems is that of Cairns and McB.reen (in Adv. in Electrochemistry and Electrochemical Engineering, H. Gerischer and C. W. Tobias ed., Vol. II, 1978) whilst many key papers have been presented in recent years in this area at the International Power Sources Symposia held at Brighton, England. Considerable emphasis has been placed on the Ni-Zn secondary system with alkaline electrolyte, whose high energy and power density characteristics, coupled with low potential cost, make it a prime candidate for electric vehicle applications.There are two main approaches to the negative electrode in such a system - the pressed zinc oxide approach, in which the charge/discharge products formed on cycling remain substantially in solid phase contact on the electrode, and the dissolving Zn electrode approach, in which the Zn active material is plated out from solution during charge onto a suitable current collector, this Zn then redissolving on the subsequent discharge.

Regarding the pressed ZnO approach, the active material ZnO with selected additives, is pressed or pasted onto a thin foil or mesh current collector, and interparticle/particle - current collector bonding is achieved by means of a plastic binder such as polytetrafluoroethylene.

This approach does not appear to have resulted in beyon 300 deep cycles at stable capacity levels for Ni - Zn cells, particularly for cell sizes in the 1 0S300 AH rating range required for electric vehicles. The presence of poorly conducting components in the negative such as ZnO itself and plastic binder, imposes penalties on the charge/discharge characteristics of the cell; in particularly lower discharge voltages and higher charge voltages result and rate capabilities are poor (charge times required above four hours, discharge with difficulty above the two hour rate).

Although effective protection against Zn dendrite failure seems to have been effective in systems with pressed Znd negatives by application of complex separators, the remaining problem, shape change and densification at the negative, may even be accelerated by the presence of resistive components. The usual devices to cut down shape change for these negatives, such as the presence of excess ZnO component, or mechanical means within the active material (e.g. fibres) to hinder Zn migration on cycling, serve only to delay the cell capacity fading effects for a number of cycles, and do not solve outright the shape change/densification problem.These sophisticated approaches greatly increase the labour and material costs associated with the electrode, and frequently a heavily shape changed electrode cannot even be externally reconditioned following disassembly of the cell, and the electrode must be effectively discarded after only a few hundred cycles.

Considerably improved deep cycle results have been reported for Ni-Zn systems with dissolving Zn electrodes. Thus, Benczor-iaJrmo'ssy and coworkers (in Power Sources 5, D. H. Collins ed., Publ. Academic Press, 1975) obtained over 2,000 cycles for 4~8 AH Ni-Zn cells in the 30 40 WH/kg energy density range, with no apparent fading at 100% depth cycling. In this system the negative was a simple nickel mesh on which the Zn was deposited on charge from the electrolyte (alkaline zincate solution). Protection against Zn shorts to the positive was achieved by use of an auxiliary electrode mesh, enclosed in an envelope of microporons plastic separator material, and interposed between the regular positive and negative elements of the cell.The auxiliary, which carried on its surface an electrocatalyst possessing a low H2 evolution overvoltage, otherwise remained electrically unconnected with the main electrodes on charge, and adequately dissolved by any Zn dendrites penetrating across to the positives (forming H2 and returning zincate ions to the electrolyte). Shape change build-up and subsequent cell capacity degradation were completely avoided by a reconditioning treatment every cycle. This admittedly extreme measure involved a complete discharge of the cell, followed by a dead - time procedure which the negative was shorted out to the auxiliary. The auxiliary was fitted with a current collecting lead for just this purpose, and this reconditioning procedure cleaned all traces of Zn off the current collector.Following the reconditioning procedure, which was intended to take less than one hour, the cell could be charged and discharged as before. This invention was granted U.S. Patent 4,039,729 (1977), assigned to Deutsche Automobil GmbH, Firma, Germany.

Similarly, von Krusenstierna (in Power Sources 6, D. H. Collins ed., Publ. Academic Press, 1977) reported over 1,000 cycles at nearly 100% depth for a 6 cell, 145 AH Ni-Zn battery based on the dissolving Zn electrode concept, in which the energy density (5 hr rate) was 45 WH/Kg.

Dendrite and shape change problems were avoided by vibrating the negative current collector (a steel Plate) at high frequency, low amplitude (Hz., i1-2 mm) during charge. A compact, adherent Zn deposit formed on the steel plate as a result of the stirring and pumping effects (micm/macroturbulence) of the vibratory movement. The electrolyte in this system was a ZnO/KOH slurry; separators were thin, open plastic nets, whilst the positives were rugged pocket Ni plates. Von Krusenstierna emphasised that the extra weight in the cells due to the vibration assembly, added only 5% to the battery weight.A series of patents, U.S. 3,970,472 (1976), 4,025,698 and 4,015,053 (1977) has been granted to Von Krusenstierna in this area, assigned to AGA -Tudor Aktiebolaget, Sweden; these extend the invention to cover vibrating the zinc negative or the separator, and the frequency/amplitude limits to between 0.01- 1,000 Hz, 0.1-10 mm respectively. Vibrating Zn negatives are also the subject of U.S. 3,907,603 (1975) awarded to the Russian group of Kocherginsky and co-workers.

The draw backs of both the German and Swedish approaches to a high energy density Ni-Zn system may now be considered. In the German work, although reversible Zn electrode behaviour was demonstrated, the use mode of the battery, demanding a complete discharge every cycle, followed by a dead - time for reconditioning, is rather unrealistic. if such a battery were returned for recharge repeatedly in an only partially discharge state, it is not clear from the invention whether full reversibility would be still maintained. Clearly, a rapid discharge of the remaining charge in the battery (to the required fully discharge state before reconditioning), is an unacceptable procedure for large battery units, since it involves a possible energy efficiency sacrifice, and even more dead ~time for battery turnaround.

The Swedish approach also has its limitations.

Thus the 50 Hz vibration level needed to break off the Zn dendrites at the negative is a most demanding mechanical requirement for the system, and requires the use of only the most robust positive plates, such as pocket plates.

Pocket plates are associated with very low capacity/unit weight characteristics however, which accounts for the very low energy density figure achieved for Von Krusenstierna'a Ni-Zn cells. Higher capacity/unit weight positive plates are available, the so-called sintered impregnated Ni plates, but these are far less robust than pocket plates. Such plates although offering energy density possibilities in the Von Krusenstierna configuration of well above 45 WH/Kg, would probably warp beyond their available dimensional tolerances, or even disintegrate, when coupled with a negative vibrating at 50 HZ.

Furthermore, there seems to be no provision in the Swedish work for dealing with particles or aggregates of Zn that detach themselves from the negative on cycling, and accumulate on these base of the cell. Such accumulations of dead Zn can surely deplete the available Zn balance in the cell, adversely affecting the capacity level, and may even give rise to interplate shorting. These considerations become increasingly important as interplate spacing distances are shortened in order to achieve higher energy densities.

A linked problem associated with the previous one, for which no allowance appears to have been made in the Swedish system, it that of change imbalance at positive and negative electrodes.

Thus, the Zn electrode is associated with a high charging current efficiency, near 100%, whereas positive plates both of the pocket and sintered type have charging efficiencies closer to about 80%. Clearly a complete charge/discharge cycle, in which the positives are brought to their full state of charge, will leave residual Zn on the negative baseplate following discharge. This residual Zn, accumulating from cycle to cycle, will adversely tax the Zn inventory within the cell (diminishing capacity), and may even rob from the available spacing for Zn deposition.

Summary of the Invention The present invention relates to improved Zn negatives for use in alkaline zinc secondary cells, offering substantially enhanced reversibility on cycling. The improved negatives comprises a high surface area, porous conducting current collector on which is deposited Zn active material from the alkaline zincate electrolyte during charge, this Zn deposit redissolving in the electrolyte on the subsequent discharge. Materials of choice for the current collector surface are those possessing a low overvoltage for Zn electrodeposition from alkaline zincate solution, and a comparable but higher overpotential for H2 evolution, while exhibiting chemical stability and passivation resistance under anodic/corthodic polarization within normal battery operating voltages.

A preferred material of choice for the current collector surface is Ni metal. Thus, porous sintered Ni plaques, as obtained from the Ni---Cd battery production line before the electrode active material impregnation stage, have proved excellent current collecting substrates for reversible Zn electrodeposition. Such plaques are typically 0.6-2 mm thick, with porosities in the 5~85% range, and mean pore sizes from 5-20 microns; the internal surface area may be as high as 0.5m2/gm. When plaques of this type are charged against conventional nickel positives in alkaline zincate solution, compact, adherent Zn deposits are formed on the negative for currents densities as high as 100 mA/cm2. Inevitable H, n2 mA/cm2 evolution on the outer surface of the plaque, an intrinsic characteristic of the metallic Ni surface tends to keep this surface relatively free of the Zn deposit; the Zn rather builds up within the open internal structure of the plaque, where the small pore dimensions keep H2 evolution to a minimum.

A porous Ni substrate for receiving the Zn deposit has certain key advantages over the simple planar low surface area structures of Ni and steel used in the German and Swedish studies referred to above. Firstly, the internal surface area of the plaque is many times the available area of a planar surface, and offers a relatively extensive volume for the Zn deposit to distribute itself within. This available volume is often adequate to tolerate local Zn build-up within the plate for a considerable number of cycles, without the need for frequent cell reconditioning and the porous structure can ensure the Zn deposit will not penetrate beyond the dimensional limits assigned to the plate. Such local Zn build-ups may be a result of residual zinc from only partial cell discharges, positive/negative plate charge imbalance, overcharge or the like.

Secondly, the steady but restricted tendency of the porous plate to evolve H2 on charge (virtually absent for planar structures) has a vital role to play. In particular, the H2 evolution hinders Zn outgrowths on the plaque outer surface as referred to earlier, a possible source of dendritic shorts, whilst the turbulence provided by the H2 bubbles provides for improved Zn deposition form the zincate electrolyte. The effect of H2 generation on charge is also to slightly reduce the charge efficiency of the negative plate, somewhat offsetting the aforementioned charge imbalance problem. Moreover, this tendency to H2 evolution does not make self discharge for the system unduly excessive; for fully charged Ni-Zn cells with such Zn plates this remains at below 5% day at ambient temperatures.The low self discharge may be a result of the relatively low surface area Zn growths obtained at the high charging currents, or the sluggishness of the Zn to self discharge into strong zincate solutions.

Other advantages of porous plaque Zn negatives include the very brief charging time associated with batteries incorporating such negatives. Thus, one hour rate charging is possible without sophisticated charging techniques, as compared with the 5 hour rate quoted for the German and Swedish systems.

Notably, high voltage efficiencies are maintained for charge/discharges even at the one hour rate with the present invention (average voltage on charge 1.95 V, on discharge 1.65 V). Further, plaque surfaces such as Ni have a self-cleaning property, in which residual traces of Zn may be completely cleaned off the plate by an occasional deep discharge against either the positive or an auxiliary electrode. This property (which may be related to the H2 evolution characteristic of the plaque) acts so as to completely suppress shape change build-up on the negative, and ensure good cyclability/reverslbllity of the system.

Although porous plaques of sintered Ni have shown good applicability for the present invention, these are not only the materials giving reversibility in behaviour as substrates for Zn deposition. Porous plaques based on pressed powders of steel, magnesium or titanium may be used as substrates in alkaline zincate solutions, the substrate metal (e.g. stainless steel) in certain cases may benefit from a light chemical or electroplated coating of nickel, silver or cadmium.

Alternatively it may be amalgamated with Hg.

Non-metallic substrates such as porous graphite or porous carbon are also possible candidates.

These carbonaceous materials can be activated by a light coating of Ni and the like, or a grade of material chosen with sufficient transition metal impurity intrinsically present to ensure adequate H2 evolution and avoid blocking off the outer surface by Zn deposits. Materials such as pure iron or copper should be avoided, the former due to its very low H2 evolution over potential (the efficiency of zinc deposition would be impractically low), the latter due to its instability to anodic polarization (the copper enters alkaline solution as blue cuprite ion), but these properties may be modified by the approach coating. The invention may be extended to other zinc based systems with non-alkaline electrolytes (e.g. Zn Cl2 system) by emphasis on acid-resistant porous substrate materials, such as graphite, corrosion resistant steels and the like.

According to one particular embodiment of the invention as applied to Zn-alkaline cells, improved Zn plating characteristics, and suppression of possible passivation characteristics of the Zn substrate on deep discharge or on accidental cell reversal, is achieved by incorporation of selected additives into the zincate electrolyte. Examples of battery additives for this purpose are the coprecipitated hydroxides of cadmium and cobalt described in U.S. Patent No.4,152,224(1979) of Klein and Goldstein. Certain additives may also suppress self discharge characteristics of the charged Zn electrode.

improved Zn plating characteristics may also be achieved by non-DC charging modes such as half wave or positive and negative current pulses, or through electrolyte agitation by pumping or ultrasonic means. The electrolyte in such cells is either a ZnO/KOH slurry, or a clear, chemically saturated zincate solution in equilibrium with a mechanically replaceable, solid phase ZnO pack, both approaches allowing reasonably low electrolyte weight/volume penalties in zincalkaline cells.

According to a further embodiment of the invention, improved reversibility of the zinc negative electrode is maintained in the cell by the presence of an auxiliary electrode for H2 evolution.

The use of such an auxiliary, which is covered with an electrocatalyst with a low H2 evolution overpotential that is very resistant to Zn deposition and which is electrically connected to the negative electrode on charge via a suitable resistor, has been described in U.S. Patent No.

4,113,921(1978) of Goldstein and Klein. An auxiliary so connected provides a stream of H2 bubbles in the electrolyte on charge, making for improved Zn deposition, and affords a means of balancing positive and negative electrode charge efficiencies as referred to earlier. Alternatively, the auxiliary, In a preferred base-only configuration in the cell, is left in an electrically floating state during cell cycling, and acts primarily to redissolve (as zincate ion) any particles of Zn which break off from the negative plates and settle out on the cell bottom.Such an auxiliary (whether passive or active) if provided with an external current carrying lead, offers a convenient means for practically acceptable periodic cell reconditioning (e.g. every 1 S100 cycles), by shorting out negatives and auxiliary for a few hours following a deep discharge. Suitable electrocatalysts for the auxiliary in question have been described in U.S.

Patent 4,132,619 (1979) of Klein and Goldstein.

According to yet a further embodiment of the invention, improved reversibility of the zinc -negative electrode (against a suitable positive electrode such as one based on sintered nickel positives) is maintained in the cell by the presence of a Zn-dendrite - resistant separator system. So compact and confined is the Zn deposit on porous substrates, however, despite the high charging rates, that simple conventional microporous separator layers such a polypropylene, newsprint, asbestos and the like may be used with impunity for many hundreds of cycles. Such materials would fail rapidly with planar negative substrates, especially in large linear area electrode configurations.For added protection against overcharge or temperature degradation of conventional organic separator layers, however, good reversibility is also obtained by interposing an auxiliary electrode mesh between twin layers of the microporous separator material. Unlike the auxiliary mesh of the German work, which was electrically floating on charge and required for use an impractical deep discharge/separate reconditioning step every cycle, the auxiliary mesh in the present embodiment is connected via a resistor to the negative during charge, as described in U.S. Patent No. 4,113,921 of Goldstein and Klein. In combination with the porous negative of the present invention, such a system has shown reversible voltage characteristics with no need even for the periodic reconditioning of the negative for many hundreds of cycles.

A further embodiment resulting from the relatively compact and confined nature of the Zn deposit on porous substrates of high surface area concerns the use of a moving separator barrier or oscillation of the negative itself, in order to suppress Zn dendritic shorts. The Swedish work, with planar low surface area substrates, demanded high frequency vibration of the negative of separator at around 50 Hz, necessitating the use of robust but weighty pocket positives, and precluding the use of sintered positives allowing high energy density confi5}urations. Reversible behaviour has been observed at much lower vibration frequencies (typically 0.01-1 Hz for either separator or negative) with the porous negative configurations of the present invention, allowing use with impurity of lightweight positives of the sintered nickel type.Attempts to run such cells with planar negatives at these low frequencies resulted in immediate shorting from Zn dendrites, irrespective whether high current charging typical of the present invention, or low current charging typical of the Swedish work, was employed.

The following example illustrate the invention in its various embodiments, and these are to be construed in a non-limitative manner.

Example 1 Cycling-test on a Ni-Zn Cell With Porous High Surface Area Negative Baseplate for Zn Deposition, the Cell Being Fitted With a Base mounted Auxiliary Electrode for H2 Evolution.

A Ni-Zn cell was constructed from a negative unimpregnated porous high surface area sintered Ni plaque, dimensions 150x65x0.8 mm.

This negative baseplate had porosity about 80% and mean pore size around 10 microns and was flanked by two conventional sintered positive plates of the same dimensions. The positives were enclosed in conventional microporous polypropylene separator envelopes, and the cell base was fitted with a H2 evolving auxiliary electrode, bearing an external current carrying lead, of the type described in U.S. Patent No.

4,113,921(1978) of Goldstein and Klein. The electrolyte, a white slurry, comprised 200 ml of 35 wt% KOH, containing 2 g LiOH and 40 g ZnO.

The positives in this cell configuration were rated together at about 5 AH for a two hour discharge.

Cell cycling was carried out under drastic conditions to demonstrate the good reversibility of the negative electrode system. Charge was at 6A for 1 hour with no voltage limit, the auxiliary being connected to the negative via resistor taking off 10% of the main charge current, whilst discharge was at 3A to the IV cut-off. The cell returned about 4.7 AH of capacity for over 1,000 cycles with no capacity fading or dendrite shorts, despite the depth of cycling (nearly 100%), maintaining a high current efficiency (about 80%) and attractive voltage characteristics (1.95 V average voltage on charge,1.65 V average discharge voltage). Every 25 cycles the cell was reconditioned by shorting out negative and auxiliary for a few hours (to clear Zn accumulated on the negative). The cell self discharge was fairly low for a cell without additives. below 5%/day at 250C. The cell energy density was low, only about 30 WH/Kg, mainly due to the excessive electrolyte volume introduced into the cell.

Example 2 Control Experiments With Porous, High Surface Area Negative Baseplate for Zn Deposition Various control experiments were performed to demonstrate the key factors ensuring cell reversibility. Ni~Zn cells of the configuration used for Ex. 1, with planar, low surface area baseplates of Ni or steel sheet, gave rapid capacity fading under the above duty cycle made mainly because of unacceptable Zn deposit morphology, and frequently cells failed in less than 10~20 cycles due to outright Zn dendrite shorting. Similar poor cycle results were obtained for Ni-Zn cells with porous, high surface area baseplates from which cells the auxiliary electrode had been omitted, failure being due to massive accumulations of the Zn on the base of the cell, causing shorts.With the configuration maintained good reversibility was obtained using porous, high surface area baseplates from materials other than Ni, such as pressed plaques of stainless steel powders, or low purity plaques of porous graphite bearing traces of heavy metals Optimum cycling results were obtained for plaques in the porosity range 50~80%, with average pore size 5-20 microns.

Finally, acceptable Zn electrode reversibility was observed for Zn alkaline systems suing porous high surface area negatives cycled against positive elements other then impregnated sintered nickel, for instance pressed plastic nickel composite positive electrodes, or sintered silver oxide positives, and for secondary systems with non-alkaline electrolyte such as Zn--PbO,, using graphite plaques for the negative.

Example 3 Cycling Test on a Ni-Zn Cell With a Porous, High Surface Area Negative Baseplate for Zn Deposition, the Cell Being Fitted With an Auxiliary Electrode Mesh for H2 Evolution Mounted Between the Positive and Negative Plates.

A Ni-Zn cell was constructed from a porous unimpregnated Ni plate of the type described in Ex. 1, dimensions 75x65x0.8 mm, flanked by two conventional sintered positive plates of the same dimensions. Between each positive and negative plate surface was placed double layer of conventional microporous separator material, sandwiching an auxiliary electrode mesh designed to evolve H2 on charge as described in U.S. Patent No.4,113,921(1978) of Goldstein and Klein. The cell base was fitted with a separate auxiliary electrode to reduce Zn particle sediment, but his auxiliary remained passive during cell cycling, being electrically unconnected to the negative.

The cell electrolyte was a clear zincate solution (100 ml of 35 wt% of KOH containing 1 g LiOH and 10 g ZnO; this was in equilibrium with a replaceable ZnO pack containing about 10 g solid ZnO, comprising ZnO compressed into a folded sachet of alkali resistant heavy duty filter paper andimmersed in the upper layers of the cells electrolyte. The positives in this cell configuration were rated together at about 23 AH for a two hour discharge. The cell was cycled using a charge made of 2 hour at 1 TA and no voltage limit, followed by discharge at 1 +A to the IV cutoff; during charge the auxiliary mesh, connected electrically via a resistor to the negative, drew of about 10% of the main charge current.The cell consistently returned about 2.3 AH for over 500 cycles with no capacity fading (depth of cycling nearly 100%), and exhibited the same high current efficiency and attractive charge/discharge potential characteristics as the cell in Ex. 1.

The energy density of the cell was similar to that of the cell in Ex. 1 about 30 WH/Kg.

However, superior characteristics over cells with simple microporous separator layers between the plates were observed for cells with an interposed active auxiliary mesh in two respects. Firstly cells with the interposed mesh had a high level of protection against accidental overcharge or over heating (which could fail the relatively flimsy conventional separator materials) since Zn dendrites cannot penetrate the mesh barrier.

Secondly, the cell the auxiliary mesh did not require any periodic reconditioning during the whole of the test schedule.

Example 4 Cycling Test on a Ni-Zn Cell With a Porous High Surface Area Negative Baseplate for Zn Deposition, in Which the Separator is Oscillated During Charge at a Low Frequency.

A Ni-Zn cell was constructed with the same negative and positive elements as in Ex. 3, the cell being fitted with a base only auxiliary of the passive type to dissolve up Zn sediment. The positive and negative plate surfaces were separated by an open plastic mesh of rigid polyethylene about 0.6 mm thick, and about 70 ml. of the ZnO/KOH slurry as used in Ex. 1 was sufficient to cover the elements. During charge the seperator was oscillated at about 0.5 Hz, +5 mm amplitude, and this was sufficient to maintain good cell reversibility and freedom from dendritic shorts. With the positives together rated at about 3 AH for a two hour discharge, a duty cycle of 3A, 1 hr charge, discharge 1 +A to the IV cut-off, gave almost 2, 3 AH capacity return with no fading for 100 cycles.Voltage plateaux on charge and discharge were superior even to those obtained with cells using microporous seperators, as expected for an effectively open electrochemical path between positive and negative plates; also current efficiency of charge was about 90%. Self discharge was of course slightly higher, about 5% day, for systems without additives. Periodic reconditioning (every 10 cycles) was necessary to clean residual Zn off the negative plate, this was achieved by an occasional deep discharge of the cell to 0.5 V.

After 200 cycles the cell was dismantled, and inspection showed the negative baseplate to be completely free of any residual Zn after discharge.

improved plating characteristics of Zn were obtained using additives of the cobalt cadmium mixed hydroxide type (see U.S. Patent No.

4,152,224), and by employing non-DC charge modes such as half wave. Inspection of the sintered positive plates after 200 cycles showed no evidence of active material delamination or plate warping, a result of the relatively mild oscillation frequency. Attempts to use such a low oscillation frequency with low surface area planar Ni or steel plates in the same configuration met with failure by dendritic shorting within 10-20 cycles; this was the result also for cells with the porous high surface area negative baseplate from which the auxiliary had been omitted. The relative closeness of the plates in this example permitted a lower electrolyte inventory than for the cells in Ex. 1 and 3, and the energy density was much higher, approaching 50 WH/Kg.Multiplate cells were built using oscillating seperators; these cells demonstrated good reversibility and the 50 WH/Kg energy density barrier was exceeded even without recource to weightoptimized sintered positives.

Example 5 Cycling Tests on a Ni-Zn Cell With a Porous, High Surface Area Negative Baseplate for Zn Deposition in Which The Negative is Oscillated During Charge at a Low Frequency A Ni-Zn cell was constructed as in Vex. 4, but the negative itself was oscillated during charge at a low frequency (0.5 Hz) and amplitude +5 mm.

Good reversibility and freedom from dendritic shorts was obtained on cycling, and this was also successfully extended to large plate multinegative cells. Good Zn electrode versibility was additionally obtained with cells in which the auxiliary was electrically connected to the negative on charge so as to draw off 5%-1 0% of the main charging current.

Claims (19)

Claims

1. A negative for use in secondary Zinc batteries comprising a porous high-surface area conducting substrate in plaque form, adapted for the plating out thereon of zinc from the electrolyte during charge of the battery, said plaque having a low overpotential for zinc deposition and a higher over potential for hydrogen evolution.

2. A negative according to claim 1, wherein about 90% of the current is used for plating out of zinc and about 10% for H2 evolution.

3. A negative according to claim 1 or 2, wherein the surface of the plaque consists of nickel.

4. A negative according to claim 1 or 2, wherein the surface of the plaque consists of silver, lead or cadmium or is amalgamated with mercury.

5. A negative according to any of claims 1 to 4, wherein the plaque substrate consists of titanium, iron, steel, stainless steel, copper, magnesium, carbon, graphite, which is optionally coated with one of the above metals.

6. A negative according to claims 1 or 2, wherein the plaque is porous graphite and this contains impurities for hydrogen evolution.

7. A negative according to any of claims 1 to 6, wherein the porosity is from 40 to 90%.

8. A negative according to any preceding claim wherein the porosity is 50 to 85%.

9. A negative according to any preceding claim in which the substrate has a pore size 1 to 50.

10. A negative according to any preceding claim in which the substrate has a pore size 2 to 20.

11. A negative according to any preceding claim in which the substrate has about 0.5-2.5 mm thickness.

12. A secondary cell containing a negative according to any of claims 1 to 11, wherein there is provided an auxiliary electrode for hydrogen evolution, connected to the negative during charge or electrically floating.

13. A secondary cell comprising a negative according to any of claims 1 to 11, said cell incorporating an additive providing improved morphology zinc deposits and reduced self discharge.

14. A secondary cell comprising a negative according to any of claims 1 to 11, wherein the cell is provided with non-DC charging means.

15. Secondary cell containing a negative according to any of claims 1 to 11 wherein reversible behaviour on cycling is maintained by means of a microporous zinc dendrite resistant separator between the negative and positive plates.

16. Secondary cell according to any of claims 1 to 15 containing a negative, means being provided for oscillating the negative or the separator between negative and positive plates at a frequency of 0.01 to 10 Hz and the amplitude is 0.1 to 10 mm.

17. A secondary alkaline zinc cell comprising a negative baseplate according to any of claims 1 to 11, said negative baseplate being resistant to the electrolyte of the cell.

18. A zinc secondary cell with non-alkaline electrolyte wherein the negative is according to any of claims 1 to 11 and the said negative is resistant to the electrolyte of the cell.

19. A secondary alkaline Zinc cell comprising a negative according to any of claims 1 to 11, in which a low electrolyte quantity is maintained by means of a replaceable ZnO pack in equilibrium with the cell electrolyte.