GB2363898A - Rapidly activated thermal battery - Google Patents

Abstract

A rapidly activated thermal battery (1), the battery comprising at least one electrochemical cell (6) and at least one heat source. Each cell (6) comprises an anode (7), a cathode (8) and an electrolyte (9) and each heat source comprises a heating element (2) which is in direct contact with a compressed heat pellet (3) of a pyrotechnic material. The heating element is placed proximate to the cell and a current applied to the heating element (2) ignites the heat pellet (3). The heat generated by the burning heat pellet (3) causes the electrolyte (9) to become conductive, activating the battery.

Description

2363898 RAPIDLY ACTIVATED THERMAL BATTERY This invention relates to

thermal batteries, particularly rapidly activated thermal batteries.

Weapons such as missiles, shells and torpedoes and devices such as sonobuoys all require electrical power Thermal batteries have characteristics which make them suitable for such applications These include, storage lifetimes of up to 20 years without degradation, no maintenance, high reliability, good shock and vibration resistance, wide storage and operating temperature ranges and high power capability.

Thermal batteries are high temperature batteries, the majority of which incorporate a fusible electrolyte In their storage state the electrolyte is a solid and thus the battery is inactive Activation of the battery is achieved by applying heat to melt the electrolyte, allowing it to function as a charge carrier In the past, heating of the electrolyte has involved applying an electrical pulse to an ignitor which ignites a heat paper, typically a strip made from a mixture of zirconium powder and barium chromate The heat paper in turn ignites a heat pellet, which bums generating heat which diffuses to the electrolyte and melts it Solid electrolytes have also been proposed for thermal batteries in which the electrolyte used is non- conductive at battery storage temperature, but which becomes conductive on heating to the battery operating temperature, whilst still remaining solid.

This method of activation is favoured by thermal battery manufacturers because it is has a proven track record of success in a market where reliability is considered paramount However, this three stage process is not suitable for applications where power is required from the battery quickly, e g for short range missiles The activation time, which is the time delay between the demand for power being made and the actual time when power can be drawn, can be as long as several seconds and is at least several hundred milliseconds.

In accordance with the present invention, a rapidly activated thermal battery comprises at least one electrochemical cell and at least one heat source; wherein each cell comprises an anode, a cathode and an electrolyte; wherein each heat source comprises a heating element in direct contact with a compressed heat pellet of a pyrotechnic material; wherein each heating element is placed proximate to the cell; and wherein the heating element is activated by the application of a current thereby igniting the heat pellet; such that the heat generated by the burning heat pellet causes the electrolyte to become conductive, activating the battery.

Placing the heating element in direct contact with the heat pellet allows for efficient heat transfer from the heating element to the heat pellet This is suitably achieved by embedding the heating element within the heat pellet or by sandwiching the heating element between two heat pellets.

The removal of one stage of the activation process allows the activation time for a thermal battery to be significantly reduced compared to that achievable by using a heat paper system Furthermore, instead of the heat being transferred from the heat paper to the heat pellet only by point contact, e g at the sides or edges or the heat pellet, the ignition of the heat pellet can be achieved over the area of the heating element.

Preferably, the heating element is a wire having wide ends and a narrow middle section The wide ends enable easy connection of external electrical leads to a current source and the narrow middle section has the effect of increasing the resistance of the wire which in turn increases the heat applied to the heat pellets.

Advantageously, the narrow middle section of the heating element is bent into a zig-zag form This increases the length of wire in contact with the heat pellet so that activation of the heat pellet starts over a larger area, causing faster activation.

Other configurations of the wire to achieve this objective can be devised.

Preferably, the heating element comprises a high resistance material as this facilitates a large increase in temperature on the application of a current Fe / Cr alloys and Ni / Cr have the required characteristics.

Preferably, the heating element is coated with an electrically insulating material to prevent electrical contact of the heating element with the heat pellet This helps to prevent the possibility of a short circuit through the heat pellet ensuring that the applied current is used to heat the heating element alone Boron nitride is suitable as a coating This has a very high melting point and is stable at high temperature.

Alternative electrically insulating coatings include lacquers or varnishes.

Suitably, the pyrotechnic material is a mixture of iron powder and potassium perchlorate This is readily ignited and bums in a controlled manner without gas evolution Other suitable pyrotechnic materials which behave in a similar fashion will be known to those skilled in the art.

The anode used for the cell is suitably made from a lithium alloy or an intermetallic compound, such as lithium aluminium, lithium silicon or lithium - boron These combine sufficiently high melting points with a low densities Alternative anode materials include lithium immobilised on iron powder.

The cathode used for the cell is suitably made from iron disulphide, Fe 52,as this provides an adequate, stable voltage, has high conductivity, good thermal stability and is readily fabricated.

Preferably, the electrolyte comprises a fusible electrolyte The heat generated by the burning heat pellets causes the electrolyte to melt, thus rendering it conductive.

A suitable fusible electrolyte is the binary eutectic of potassium chloride and lithium chloride (KCI Li CI) immobilised on an inert binder such as magnesium oxide.

This binary eutectic has a melting point of 3520 C Alternative fusible electrolytes include the ternary eutectic lithium fluoride lithium chloride lithium bromide (Li F - Li CI Li Br), melting point 4450 C, similarly immobilised and other mixtures of alkali metal halide salts.

Alternatively, the electrolyte may be a solid electrolyte In this case the heat generated by the burning heat pellets causes the electrolyte to become conductive but does not melt it Suitable solid electrolytes include lithium sulphate or a mixture of lithium sulphate and sodium sulphate.

The invention will now be described by way of example only with reference to the following drawings in which:

Figure 1 shows a diagrammatic representation of a single cell thermal battery according to the invention; Figures 2 a and 2 b show wires suitable for use as heating elements for thermal batteries according to the invention; Figure 3 shows an example of an electrical circuit suitable for the activation of a thermal battery according to the invention; Figure 4 shows an alternative electrical circuit suitable for the activation of a thermal battery according to the invention; and, Figures 5 a to 5 c show graphs of cell voltage against time for a single cell of a thermal battery according to the invention.

A single cell thermal battery 1 comprises two wires 2 each of which is sandwiched between two heat pellets 3 The wires are shown in more detail in Fig 2.

These have wide ends 4 to enable easy connection to a current source (not shown) and a narrow middle section 5 which has the effect of increasing the resistance of the wire Fig 2 a shows a straight wire and Fig 2 b, a wire with a zig-zag shape The latter increases the area which is in contact with the heat pellet allowing for greater heat transfer to take place In Fig 1 the wires are shown sandwiched between two pellets, however they could equally be embedded into the pellets on manufacture The sets of heat pellets surround a cell 6 which comprises an anode 7, a cathode 8 and a fusible electrolyte 9.

To activate the battery a current is applied to the wires 2 by an external circuit.

An example is shown in Fig 3 An electrical pulse from an external supply 10 which could be a continuous supply or a battery, is switched for a short period using a transistor 12 and a pulse generator 11 The wire heats rapidly under the applied current, further enhanced by virtue of the shape of the wire The hot wire ignites the heat pellets 3 which liberate heat This heat is conducted from the heat pellets via the anode 7 and cathode 8 to the electrolyte 9 causing it to melt In the case of a solid electrolyte, the heat liberated by the heat pellets raises the temperature of the electrolyte to a point where it becomes conducting The now conductive electrolyte is able to act as a charge carrier and the battery is thus activated Current can be drawn from the battery using suitably placed current collectors 13 for as long as the electrolyte remains conducting or until the battery is exhausted.

An alternative circuit is shown in Fig 4 Here a relay 14 is used to switch the current on and the circuit is allowed to switch off automatically as the wires break during battery activation.

The example described by Fig 1 shows only one cell but it is clear that the principles involved could be applied to a series of cells.

Specific examples of thermal batteries according to the present invention will now be described.

Example 1.

A single cell thermal battery was constructed by sandwiching each of two 50 pm Fe/Cr wires with a straight configuration (Fig 2 b) between two 200 mg heat pellets made from an approximately 86:14 by weight mixture of iron powder and potassium perchlorate The two sets of pellets were placed either side of a cell comprising a 200 mg Li/Al anode, a 200 mg cathode material comprising 70 % Fe 52 and 30 % binary eutectic and 150 mg of electrolyte composed of binary eutectic immobilised on a magnesium oxide binder.

The battery was activated by applying a 5 V, 50 ms square pulse from a signal generator to the gate of a MOSFET transistor which in turn switched a power supply to apply a 35 V, 14 A pulse to the wires This caused the wires to heat rapidly and ignite the heat pellets The electrolyte became molten and current could be drawn from the battery The results are shown in Fig 5 a The pulse is shown as a dotted line and the battery voltage as a solid line The activation time is the time between the start of the pulse and the time when the expected battery voltage of ca 2 V is obtained From Fig 5 a it can be seen that the activation is of the order of 11 Oms This shows a significant improvement over existing methods for thermal battery activation where activation times of several hundred milliseconds are common.

Example 2.

A single cell thermal battery was constructed by sandwiching each of two pum Ni/Cr wires with a zig-zag configuration (Fig 2 a) between two 150 mg heat pellets made from the same composition as used in Example 1 The anode was 150 mg Li/AI, the cathode was 150 mg of the same mixture as in Example 1 with mg of electrolyte as in Example 1.

The battery was activated by applying a 5 V, 25 ms square wave pulse to the MOSFET transistor as in Example 1 to apply a 35 V, 10 A pulse to the wires The activation trace is shown in Fig 5 b with an activation time of approximately 20 ms, significantly less than in Example 1.

Example 3.

This used the same single cell battery as Example 2 except that a 50 ms activation pulse was used and the battery was activated on a 20 ohm load instead of on open circuit As shown in Fig 5 c, the activation time was about 40 ms, longer than in Example 2, due to the use of a realistic load rather than activation on open circuit, but still a significant improvement over existing thermal batteries.

Claims (12)

1 A rapidly activated thermal battery, the battery comprising at least one electrochemical cell and at least one heat source; wherein each cell comprises an anode, a cathode and an electrolyte; wherein each heat source comprises a heating element in direct contact with a compressed heat pellet of a pyrotechnic material; wherein each heating element is placed proximate to the cell; and wherein the heating element is activated by the application of a current thereby igniting the heat pellet; such that the heat generated by the burning heat pellet causes the electrolyte to become conductive, activating the battery.

2 A rapidly activated thermal battery according to claim 1, wherein the heating element is a wire having wide ends and a narrow middle section.

3 A rapidly activated thermal battery according to claim 2, wherein the narrow middle section of the heating element is bent into a zig-zag form.

4 A rapidly activated thermal battery according to any preceding claim, wherein the heating element comprises a high resistance material.

A rapidly activated thermal battery according to any preceding claim, wherein the heating element is coated with an electrically insulating coating.

6 A rapidly activated thermal battery according to claim 5, wherein the electrically insulating coating is boron nitride.

7 A rapidly activated thermal battery according to any preceding claim, wherein the anode material is chosen from a lithium alloy, a lithium-aluminium intermetallic, a lithium-silicon intermetallic, lithium boron intermetallic or lithium immobilised on iron powder.

8 A rapidly activated thermal battery according to any preceding claim, wherein the cathode material is Fe 52.

9 A rapidly activated thermal battery according to any preceding claim, wherein the electrolyte comprises a fusible electrolyte.

A rapidly activated thermal battery according to claim 9, wherein the fusible electrolyte comprises one of KCI-Li CI binary eutectic or Li F-Li CI-Li Br ternary eutectic or other alkali metal halide mixtures immobilised on an inert binder.

11 A rapidly activated thermal battery according to any of claims 1 to 8, wherein the electrolyte comprises a solid electrolyte.

12 A rapidly activated thermal battery according to claim 11, wherein the solid electrolyte comprises lithium sulphate or a mixture of lithium sulphate and sodium sulphate 13 A rapidly activated thermal battery as hereinbefore described with reference to the accompanying drawings.