WO2003034521A1

WO2003034521A1 - Water activated battery cell, primary battery and its use

Info

Publication number: WO2003034521A1
Application number: PCT/FI2002/000813
Authority: WO
Inventors: Kai Vuorilehto
Original assignee: Vaisala Oyj
Priority date: 2001-10-19
Filing date: 2002-10-18
Publication date: 2003-04-24
Also published as: FI20012040A; FI20012040A0; FI113417B

Abstract

A water-activated, deferred action primary battery having a holder containing at least one cell. The cell comprises at least one anode (9), selected from the group consisting of magnesium and aluminium and alloys thereof, at least one cathode (11) comprising a conductive component and, intimately contacted with the conductive component, an active cathode material, formed of manganese dioxide, a spacing (10) separating said anode and said cathode, and an inlet opening leading to said spacing for the introduction of an electrolyte-forming aqueous liquid into the spacing. According to the invention the cathode (11) further comprises an ionizable component, formed by an alkali metal chloride, which yields said chloride compound in the electrolyte-forming aqueous liquid filled into the cavity. The cathode also contains a moderately basic alkaline component which is capable of increasing the pH of the aqueous phase inside the cathode, once water has been added to the cell, to at least 9 without passivating the anode. The battery, which can be manufactured from inexpensive materials, is readily and immediately activated by the addition of normal tap water; there is no delay action. The activation water and any water leaking from the battery after use are neither toxic nor fouling and the battery is ecologically beneficial.

Description

Water activated battery cell, primary battery and its use.

Background of the Invention

Field of the Invention

The present invention relates to water-activated batteries. In particular, the invention concerns water-activated primary batteries of the kind including at least one metal anode and a cathode, which comprises a conductive component and, intimately contacted with the conductive component, an active cathode material.

Description of Related Art

Water-activated batteries are reserve batteries, to which water is added prior to use. This makes it possible to employ very reactive materials without the problem of self-discharge. The water forms the electrolyte with ions that are produced in electrode reactions.

Water-activated batteries are used as power sources in radiosondes, air rescue equipment, life jackets and lifeboats, emergency rockets, missiles, torpedoes and underwater research devices. The batteries in question have to withstand long periods of storage without self- discharge. They are exploited under different temperature conditions, especially at very low temperatures. Their current density should be high, but, on the other hand, their lifetime does not need to be long.

Radiosondes are the main use of water-activated batteries. A radiosonde is a measuring device that is used for meteorological research in the upper atmosphere. During a sounding the radiosonde is raised by a weather balloon up to a height of about 20 to 40 km. Simultaneously, it measures and transmits information about temperature, pressure and air humidity.

The conditions during radiosonde soundings in the upper atmosphere are extreme: the temperature decreases to about -60 to -90 °C, and the pressure falls to about 10 mbar. At low pressures the boiling point of the water decreases to almost 0 °C, whereby the temperature range, in which the battery neither boils nor congeals, is very narrow. The temperature remains low, and the coldness decelerates the cell reactions.

The copper(I)chloride/magnesium battery is the usual and traditional type of water- activated battery. It comprises a magnesium anode and a cathode comprising a mixture of graphite, copper(I)chloride and sulphur (cf. US Patent Specification No. 3,205,096). The problem associated with this type of battery is the solubility of copper ions, which causes corrosion of the magnesium anode and overheating. Further, the voltage rises rather slowly to the required level.

In order to reduce the solubility of the copper ions, it has also been suggested in the art to replace copper(I)chloride with copper(I)bromide [Nuorilehto, K. and Rajantie, H., A water-activated cuprous bromide battery, Journal of Applied Electrochemistry 29 (1999) 903 - 910]. CuBr-sulphur was found to be very suitable for use as a cathode material. Although it gives a maximum voltage lower than the traditional CuCl-sulphur battery, the voltage is more stable. The particular disadvantage of the CuBr battery is the high price of the raw material, which makes it economically unattractive.

The use of manganese dioxide as active cathode material in batteries is known per se from traditional batteries including alkaline batteries. However, manganese dioxide has also been tested in some deferred action batteries. Thus, US Patent Specification No. 3,433,678 teaches the use of a seawater battery having a magnesium or zinc anode and a cathode comprising manganese dioxide deposited by a electrolytic process on a graphite substrate.

A similar battery construction is discussed in a review article by Nicholas T. Wilburn (Proc. 21 st Annual Power Sources Conference, 16 - 18 May 1967, 113 - 116 ). In the construction described, magnesium perchlorate is used as an electrolyte instead of the seawater mentioned in the US Patent referred to above.

Neither of the two prior constructions is truly "water-activated". The battery suggested in US Patent No. 3,433,678 only works in seawater, which has poor conductivity. Thus, the current and power densities produced are low. The application of the battery is restricted to the sea; lake water does not contain enough salt for ensuring sufficient conductivity of the electrolyte. The magnesium perchlorate battery gives rise to handling and storage problems since the electrolyte is hazardous, and it has to be separately transported and stored. As the above survey shows, there is a need for a deferred action battery which can be activated with normal water (tap water), and which will not contain any soluble metals that can cause corrosion and overheating. Further, there is a need for batteries which are immediately active. Economically competitive alternatives to conventional CuCl-sulphur batteries are also needed.

Summary of the Invention

It is an object of the present invention to provide a novel kind of cell for a primary battery based on non-toxic active cathode materials. Further, it is an object to provide a cell, which will produce an even voltage output over prolonged periods of time.

It is a third object of the present invention to provide a new primary battery and the use of such a battery.

These and other objects, together with the advantages thereof over known products, which shall become apparent from the specification, which follows, are accomplished by the invention as hereinafter described and claimed.

The invention is based on using in a water-activated battery a cathode comprising manganese dioxide as active cathode material. In order to increase the conductivity of the electrolyte, the cathode further comprises, mixed with the conductive component and the active cathode material, an ionizable component. Surprisingly it has been found that by using moderate amounts of neutral salts, in particular alkali metal halogenides, a battery can be provided in which the electrochemical reactions of manganese dioxide are similar to those of an alkaline battery, however with the difference that the electrolyte contained is alkaline only during discharge. Both the fresh and the used batteries are neutral.

Further, the cathode comprises a moderately basic alkaline agent, which is capable of increasing the pH of the aqueous phase inside the cathode, once water has been added to the cell, to at least 9 without passivating the anode.

By fitting a plurality of battery cells of this kind into a housing or holder there can be provided a deferred action, water activated battery capable of producing a current of at least 150 mA and a voltage of at least 15 N over a time period of at least 135 minutes, even under stringent conditions. The voltage change is moderate during the initial 10 minutes of operation.

More specifically, the battery cell according to the present invention is characterised by what is stated in the characterising part of claim 1

The primary battery according to the invention is characterised by what is stated in the characterising part of claim 15.

The use according to the present invention is characterised by what is stated in claim 19.

The invention provides considerable advantages. Thus, the raw materials are non-toxic to the environment, to the user and to the personnel of the battery manufacturing. The battery is readily and immediately activated by the addition of normal tap water. There is no need for any extra addition of ionizable salts to the electrolyte. The activation water and any water leaking from the battery after use are neither toxic nor fouling. Compared to the other alternatives to conventional CuCl-S batteries, the raw materials are also quite inexpensive. During the initial 10 minutes of operation the variation of the voltage is less than 25 % and there are no high voltage peaks, which would be detrimental to the operation of any electronic components connected to the battery.

The voltage of the cell is up to 1.95 V from the time of starting the discharge and 1.25 N after 135 minutes.

In the following, the invention will be examined more closely with the aid of a detailed description and a number of working examples.

Brief Description of Drawings

Figure 1 gives a perspective view over a battery of the present kind;

Figure 2 shows a simplified side section of a battery cell according to the present invention, indicating the principal construction of the cell; and Figure 3 shows the voltage curves in the atmosphere simulations for (a) a battery according to the present invention (b) a battery according to FI Patent Application No. 20000947, and (c) a conventional CuCl battery. In the latter battery, the copper(I) halide:sulphur -mole ratio was 1 :1.5. The discharge current was 150 mA (10 mA cm"2). The dry weight of all batteries was 115 g.

Detailed Description of the Invention

For the purpose of the present invention, the following definitions are used:

A "battery cell" is an electrochemical (galvanic) cell comprising at least one anode and at least one cathode, which form an electric pair when both are contacted with an aqueous solution, which forms an electrolyte.

A "water-activated battery" comprises at least one cell unit placed in a holder or housing provided with openings for inlet of water. The cell unit typically comprises a cavity separating the anode of one unit from the cathode of the same unit, which cavity is connected to the water inlet openings.

"Active cathode material" stands for the cathode component which contains reactive material which produces electricity when the battery is discharged.

"Ionizable component" denotes a compound or substance which will partially or completely dissociate in an aqueous solution and yield cations and anions which increase the conductivity of the electrolyte.

A battery according to the present invention is shown in perspective view in Figure 1. Figure 2 shows in more detail the structure of a single cell. In the figures, the following reference numerals are used:

1. Battery

2. Cell

3. Connector

4. Holder 5. End plate

6. Opening

7. Tinplate

8. Conducting, non-permeable foil 9. Magnesium anode

10. Cavity filled with absorbent l l. MnO₂ cathode.

A battery 1 according to the present invention consists of a plurality of cells 2 connected in series so as to form a cell cascade between two end plates 5. Between the individual cells there are electrically conducting, essentially non-permeable foils separating and adjoining (as will be explained below) the anode of a cell with the cathode of an adjacent cell. Such a battery, which comprises twelve cells connected in series, produces a minimum voltage of 15 V, preferably the voltage is at least 18 N at the beginning of operation.

The cells are mounted together with two tape strips 4 peripherally encircling the upper ends and the lower ends of the cells holding the cells in place. In both of the end plates 5, which preferably are made of a non-conductive material, such as a plastic sheet, there is an aperture, which extends through the plate and allows for contact with the conducting foils 8 of the first and the last cells 2 of the cascade. Between the end plates and the conducting foils there is a thin foil or sheet 7 of an electrically conductive material, such as copper or tinplate, which contacts with the conducting foils and which will provide a suitable substrate on which the ends of the electrical connector 3 can be fastened through the aperture by soldering.

As shown in more detail in Figure 2, each cell contains at least one anode 9, at least one cathode 11, spaced apart from the anode so as to define a spacing or cavity 10 between the electrodes, and conducting foils 8 which, on one hand, separate the adjacent cells from each other by providing a barrier to the migration of water and, on the other hand, electrically connects the cells together by providing for flow of electric current through the cell cascade. The conducting foils may comprise a carbon foil or a thermoplastic foil, which is made electrically conducting by doping. A particularly preferred foil comprises a graphite-doped poly(isobutylene) foil. The thickness of the foil 8 is not critical, usually it is in the range of 50 - 500 μm. Although the provision of one anode is preferred it is possible to construct a cell having two or more anodes. The spacing 10 is filled with an absorbent material, which will take up (absorb) an aqueous liquid used as activation water and electrolyte. The absorbent material should be very porous and inert to the electrolytes. Suitable materials are those, which are based on natural or synthetic fibres. Particularly suitable are wads of cellulose fibres, regenerated cellulose fibres, and synthetic polymers, such as wads of cotton wool, viscose, polyester or polypropylene.

The material filling up the cavity can be in the form of a rigid mat. As will appear from the drawing, the cavity is open at both lateral sides, an inlet opening being formed at the upper end. Said opening leads into the cavity and allows for the introduction of an electrolyte- forming aqueous liquid into the cavity. The lower end is preferably closed by a sealing, e.g. a layer of a polymer or wax, to prevent non-absorbed liquid from draining out of the cell. It is also possible partially or entirely to seal off the open sides with a water- impermeable foil, such as a polymer film layer, e.g. a tape, to create a "microclimate" within the cell. Such sealing will improve the performance of the cell.

The cathode can be shaped as an integral rigid laminar layer, which forms an electrode bed. However, when the absorbent is a mat having a firm texture, it is also possible to form the cathode material into one pellet or a plurality of pellets (i.e. a plurality of cathodes). Thus, Figure 2 shows a cell, which comprises two cathode pellets. Generally, the surface area of the cathode is at least somewhat smaller than the area of the anode. In particular, it should be avoided that direct contact is formed between the electrodes at the rim of the conducting foil. Such a contact may cause corrosion of the anode. Thus, it is preferred to provide the cathode with a surface, which is at least about 5 %, preferably about 10 % smaller than the area of the anode. In the embodiment shown in Figures 1 and 2, wherein the upper end of the battery is open, it is preferred that the height of the cathode be smaller than the height of the anode so as to avoid contact between the cathode and the anode at the upper end of the battery.

According to a preferred embodiment the anode material comprises magnesium or a magnesium alloy. Generally such a material comprises a minimum of 93 mass % magnesium, 0 to 7 mass % aluminium, 0 to 3 mass % zinc and 0 to 2 mass % manganese. As examples of particularly suitable magnesium alloys, the following can be mentioned: alloys comprising at least 93 mass % magnesium, 1 to 7 mass % aluminium and 0 to 2 mass % zinc, such as the alloys commercially available under the designations AZ-31 and AZ-61; and alloys comprising at least 98 mass % magnesium and 0J to 2 mass %, preferably 0.5 to 1.5 mass % manganese (Mnl50; ASTM Specification B-843-93).

The cathode 11 comprises manganese dioxide mixed with an alkali metal chloride selected from the group of sodium chloride and potassium chloride and mixtures thereof. Potassium chloride and sodium chloride amount to about 6 to 25 mass-% of the weight of the cathode. The conductive component of the cathode typically comprises graphite, for example expanded graphite, or another form of carbon, such as carbon black or an activated carbon. The carbon component, such as graphite, is preferably employed in the form of a powder and mixed with and evenly distributed throughout the mass formed by the active cathode material and the ionizable component. The mixture is pressed either by cold-pressing or by heat-pressing into a rigid electrode bed. The manganese dioxide is preferably employed in γ-form.

The cathode also contains a moderately basic alkaline agent. By "moderately basic" it is meant that the alkaline substance, when dissolved in water, forms an aqueous solution having a pH of about 9 to 13. The pK_D of such substances is generally 1 - 5. Typically, the alkaline agent is a salt, such as an alkali metal carbonate, an earth alkaline metal carbonate or an earth alkaline metal hydroxide. Corresponding salts having a phosphate anion may also be employed. As may various metal oxides. Organic bases, such as amines, are also possible. Generally, it is required that the alkaline substance is capable of increasing the pH of the aqueous phase inside the cathode to a sufficiently high pH once water is added to the battery. It would appear that an initial, slightly alkaline pH inside the cathode is advantageous for the operation of the cell and in particular for reducing or even eliminating voltage peaks at the beginning. But the afore-said is just one possible explanation.

The alkaline agent should be selected such that passivation of the magnesium anode can be avoided. Thus, strong alkaline agents, such as sodium or potassium hydroxides, cannot be used. Typically, the passivating agents give rise to strongly alkaline conditions in aqueous phase (in excess of pH 14). Passivation will be apparent from reduced performance of the cell both expressed in terms of maximum voltage and consistency of operation. The moderately basic alkaline agent is preferably selected from the group consisting of magnesium hydroxide, potassium carbonate and sodium carbonate. Its concentration is about 0J to 10 %, preferably 0.2 to 5.0 %, of the weight of the cathode.

According to a particularly preferred embodiment, the cathode comprises manganese dioxide, potassium or sodium chloride and graphite powder. The chemical composition of the cathode is such that is contains about 70 - 85 mass % manganese dioxide, 6 - 25 mass % potassium or sodium chloride, 0.2 to 5.0 % magnesium hydroxide, and about 4 to 8 mass % graphite.

To increase the strength properties of the cathode material it is possible to mix the components with a binder. Particularly preferred binders comprise polymers, such as chlorinated or fluorinated polymers.

It has been found that a cell according to present invention can produce a voltage of 1.55 to 1.95 V over a period of at least 10 minutes, calculated from the time of starting of the discharge, with a variation of the voltage of less than 25 %, preferably less than 20 %, during the first 10 minutes. The variation is calculated from the initial voltage produced by the cell. Over a period of time of 125 minutes, calculated from 10 minutes after activation up to 135 minutes after activation, the voltage is reduced by no more than about 30 %, preferably no more than 25 %. The voltage is typically 1.5 to 1.95 V over a period of at least 30 minutes.

The dimensions of the battery may vary. However, for the particularly interesting application of use in radiosondes it is preferred to restrict the dry weight of the battery to less than 150 g, typically to about 80 to 120 g. A primary battery for this application comprises electrodes each having a thickness of about 0.1 - 3 mm. The width of the cavity between the electrodes is typically about 1 to 4 mm.

The electrochemical reactions in an Mg / modified MnO₂ battery according to the present invention are as follows:

The cathode reaction is 2 MnO₂ + 2H₂O + 2e" → 2 MnOOH + 2OH' ( 1 )

The anode reaction is

Mg → Mg²⁺ + 2 e". (2)

In an aqueous solution a passivating hydroxide layer is formed on the surface of magnesium. The passive layer can create delayed action by preventing the electrode reaction and decelerating the reaction rate. Delayed action is defined as the time that elapses from the battery connection until the battery voltage reaches the required minimum voltage. At the beginning of the discharge pits are formed in the passive layer, and the layer does not recover completely any more. As the metal is able to contact the electrolyte through the pits, the passivation gradually weakens. Chloride strengthens the formation of the pits. This breakdown of the passive layer is obligatory for the battery.

Magnesium as an electropositive metal oxidises, i. e. corrodes, very easily. In addition to the desired electrode reaction (2) a hydrogen-producing corrosion reaction takes place at the anode:

Mg + 2 H₂O → Mg(OH) + H₂ ΔH=-353 kJ/mol (3)

This corrosion reaction is strongly exothermic. In cold conditions during radiosonde soundings the corrosion of the magnesium anode produces the heat needed to keep the battery temperature above 0°C. However, when too much of corrosion catalysts, such as copper, iron, chloride or bromide, are present, corrosion may generate excessive amounts of heat. As a consequence the water evaporates, the electrolyte diminishes and the battery voltage drops. Copper and iron cause galvanic corrosion, whereas the aggressive ions, chloride and bromide, cause pitting corrosion.

A battery of the above described kind comprising 12 cells will produce an average voltage of about 18 V. At atmospheric conditions, such as those encountered during radio sounding, a 12 cell battery will produce a current of 150 mA and a voltage of at least 15 N over a time period of at least 135 min. During the initial phase comprising the 30 first minutes of operation, the twelve cells connected in a series produce a minimum voltage of 18 N, said voltage having a variation of less than 25 % within the 10 first minutes after activation of the battery with water and starting of the discharge. Compared with a battery, which does not contain a moderately basic alkaline agent in the cathode material of the cell, the present battery is free from initial voltage peaks in excess of 23.5 N.

The present invention can be used in air rescue equipment, life jackets and lifeboats, emergency rockets, missiles, torpedoes and underwater research devices. According to a preferred embodiment, the present battery is used as a source of electricity in radiosondes.

Example

A. Construction of the battery

A battery having the structure shown in Figure 1 was manufactured. The anode was a magnesium alloy AZ-31 with 3 mass-% aluminium and 1 mass-% zinc manufactured by Spectrulite Consortium Inc. (USA).

The reactive cathode material was electrochemically deposited manganese dioxide (EMD) from Tosoh Hellas (Greece). The amount of reactive material was adequate for approximately five hours' use. The cathode contained 79.5 mass-% of the manganese dioxide, 11 mass-% KC1 and 0.5 mass-% Mg(OH)₂. In addition, the cathodes contained 7 mass-% graphite as an electronic conductor and 2 mass-% poly(tetrafluoroethylene) as a binder. The cathode mixes were ground for 30 seconds in an Ika M20 Universal Mill. The powder was compressed into cathode pellets using a pressure of approximately 150 MPa.

Each cell contained a geometric cathode surface area of 15 cm². The thickness of the cathode pellets was approximately 0.14 cm. The electrodes were separated by a synthetic non-woven wad, which was able to absorb the activation water.

The battery consisted of twelve cells connected in series by an inert foil comprising graphite-doped poly(isobutylene). Short circuit currents between the cells were eliminated by putting adhesive tape on the edge of the absorbent layer. The bottom of the battery was waxed and the sides were closed with a sealing layer of impermeable plastic foil. The battery was packed in hermetic foil to avoid self-discharge. Prior to use the battery had to be activated by immersing it in tap water for three minutes. The absorbent layer was able to absorb approximately 40 g of water (3.3 g per cell). The electrolyte was formed from the water and ions that were dissolved from the electrodes. The dry weight of the battery was 115 g. The dimensions of the battery were: length: 60 mm, width: 29 mm and height: 64 mm.

B. Simulations in an atmosphere chamber

Battery characteristics were studied in an atmosphere simulation chamber (Weiss, Germany), in which temperature and pressure were adjusted to values similar to those in normal radiosonde soundings. The air flow during radiosonde soundings was simulated by ventilation, and a computer controlled the temperature and the pressure in the chamber and recorded their values. The duration of the simulation program was 135 minutes. During the first 30 minutes of discharge the temperature was 25°C and the pressure was 1015 mbar, as 30 minutes is the assumed period needed for the ground preparation of a sounding. The batteries were discharged using a direct current of 150 mA (10 mA cm^"2) and they were required to give at least the cut-off voltage of 15 N, which are typical values needed by a modem radiosonde. The battery voltage was measured during the simulation and presented as a function of time. The time, in which the battery voltage stayed above the cut-off voltage, was determined.

Voltage curves for an Mg / MnO₂+KCl battery according to the present invention, for a battery according to FI Patent Application No. 20000947, and for a Mg/CuCl battery in atmosphere simulations are shown as a function of time in Figure 3. The average voltage under load of the Mg / MnO₂+KCl battery was over 17 V. The voltage of the battery dropped slowly and stayed above the cut-off voltage (15 V) during the required 135 min.

As apparent from the results, the battery voltage of the batteries according to the present invention rises to the cut-off value (15 V) without the very high initial voltage peak of the battery used for comparison. The moderately basic alkaline component will cut the initial voltage (peak) of the battery with about 2 to 3 V. The voltage stays above 15 V for extended periods of time.

The MnO cathode modified with an ionizable alkali metal chloride and a moderately basic alkaline cathode component is very suitable for use as a cathode material in water- activated batteries. It should be emphasized that the risk of overheating is smaller for the batteries according to the present invention than for traditional batteries. Further, in the novel battery there is no delayed action. Although the present battery produces a somewhat lower voltage than the Mg / CuCl battery, the raw material is less expensive and there is no problem of soluble metal ions. Since the components are not toxic, the novel cathode material is ecologically beneficial.

Claims

Claims:

1. A water-activated battery cell comprising in combination

- at least one anode (9) selected from the group consisting of magnesium and aluminium and alloys thereof; and

- at least one cathode (11) having a conductive component and, intimately contacted with the conductive component, an active cathode material, formed by manganese dioxide, an ionizable component, selected from alkali metal chlorides and mixtures thereof, which ionizable component in the presence of a water yields said chloride compound in aqueous solution, and a moderately basic alkaline agent, which is capable of increasing the pH of the aqueous phase inside the cathode, once water has been added to the cell, to at least 9 without passivating the anode.

2. The cell according to claim 1 , wherein the moderately basic alkaline agent comprises an alkaline substance which forms an aqueous solution having a pH of about 9 to 13.

3. The cell according to claim 1 or 2, wherein the moderately basic alkaline agent is selected from the group consisting of alkali metal carbonates, earth alkaline metal carbonates and earth alkaline metal hydroxides.

4. The cell according to claim 3, wherein the moderately basic alkaline agent is selected from the group consisting of magnesium hydroxide, potassium carbonate and sodium carbonate.

5. The cell according to any of claims 1 to 4, wherein the concentration of the moderately basic alkaline agent is 0.1 to 10 %, preferably 0.2 to 5.0 %, of the weight of the cathode.

6. The cell according to any of claims 1 to 5, wherein the anode (9) comprises a minimum of 93 mass % magnesium, 0 to 7 mass % aluminium, 0 to 3 mass % zinc and 0 to 2 mass % manganese.

7. The cell according to any of claims 1 to 6, wherein the cathode (11) comprises potassium chloride or sodium chloride in an amount of 6 to 25 mass-% of the weight of the cathode.

8. The cell according to any of the preceding claims, wherein the conductive component of the cathode comprises graphite, expanded graphite, activated carbon or carbon black.

9. The cell according to claim 5, wherein graphite is mixed with the active cathode material, the ionizable component and the moderately basic alkaline agent.

10. The cell according to claim 9, wherein the cathode (11) comprises a pressed, rigid electrode bed formed by manganese dioxide, potassium or sodium chloride, magnesium hydroxide, and graphite powder.

11. The cell according to claim 10, wherein the cathode (11) comprises about 70 - 85 mass % manganese dioxide, 6 - 25 mass % potassium or sodium chloride, 0.2 to 5 mass % magnesium hydroxide and about 4 to 8 mass % graphite.

12. The cell according to any of the preceding claims, wherein the anode (9) is spaced apart from the cathode (11) so as to define a spacing (10) there between, which can be filled with an electrolyte-forming aqueous liquid.

13. The cell according to claim 12, wherein the spacing (10) contains an absorbent material.

14. The cell according to claim 1 , wherein it produces a voltage of 1.55 to 1.95 V over the initial period of 10 minutes, calculated from the time of starting of the discharge, said voltage having a variation of less than 25 % over that period of time.

15. A water-activated, deferred action primary battery (1) having a holder (4) containing at least two cells, each of which comprises

- at least one anode (9), selected from the group consisting of magnesium and aluminium and alloys thereof;

- at least one cathode (11) comprising a conductive component and, intimately contacted with the conductive component, an active cathode material, formed of manganese dioxide,

- a spacing (10) separating said anode and said cathode; and - an inlet opening (6) leading to said spacing for the introduction of an electrolyte- forming aqueous liquid into the spacing, c h a r a c t e r i z e d in that the cathode (11) further comprises an ionizable component, formed by sodium chloride, potassium chloride or mixtures thereof, which ionizable component yields said chloride compound in the electrolyte-forming aqueous liquid filled into the cavity, and a moderately basic alkaline agent, which is capable of increasing the pH of the aqueous phase inside the cathode, once water has been added to the cell, to at least 9 without passivating the anode.

16. The battery according to claim 15, comprising a plurality of cells connected in series with conducting foils (8) fitted between the anode (9) of a cell and the cathode (11) of an adjacent cell.

17. The battery according to claim 15 or 16, comprising twelve cells connected in series producing a minimum voltage of 18 V, said voltage having a variation of less than 25 % within the 10 first minutes after activation of the battery with water.

18. The battery according to any of claims 15 to 17, comprising plate-shaped cells placed in serial arrangement with the anode of a cell abutting with the cathode of an adjacent cell, the lateral sides of the battery being provided with a foil, which is impermeable to water.

19. The use of a primary battery according to any of claims 15 to 18 as a source of electricity in a radiosonde.