Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in molten state
Patent
1997-03-26
1999-04-20
Bell, Bruce F.
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
Active material in molten state
429112, 429199, H01M 436
Patent
active
058957308
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to high temperature batteries and more particularly relates to electrolyte systems for molten salt electrolyte high temperature batteries.
There are two main types of high temperature electrical batteries which use molten salt electrolytes: thermal batteries and high temperature rechargeable batteries. The thermal batteries are primary (non-rechargeable) batteries and normally generate a single continuous energy output on activation. The duration of the output may vary from a few seconds to about an hour dependent on battery design and construction. Such batteries are particularly suited to short burst high power applications, especially for missiles. The high temperature rechargeable batteries are of interest as power supplies for submarines and electric vehicles. Both primary reserve thermal batteries and high temperature rechargeable batteries use an electrolyte which is an ionically conducting liquid in the high temperature operational state but a non-conducting solid in the storage state. The invention relates to molten salt electrolyte systems suitable for both types of high temperature battery.
Thermal batteries are formed from a series construction of cells having an inert state and a thermally active state. The normal storage state is the inert state when the cell electrolyte between the anode and cathode is in a solid form e.g. at ambient temperature. The thermally active state exists when the electrolyte is molten and this state may be obtained rapidly when required by igniting a charge of a pyrotechnic material in close proximity to the electrolyte. The cell temperature in the thermally active state is typically 350-600.degree. C.
In known modem thermal batteries the anode is usually based on lithium. This may be in the form of a solid electrode of a lithium alloy (with boron, silicon or aluminium) held in a support, such as is described in UK patent 1482335, or as liquid lithium or lithium based mixtures retained in a foraminous metal substrate by capillary action, such as is described in UK patent 1442971. Almost all modem thermal batteries use lithium or its alloys in the anode because of its high electrode potential, its high coulombic capacity due to its low atomic weight and its relatively high chemical stability which facilitates handling. The cathode is a disc of iron sulphide or disulphide.
The electrolyte is in the form of a pressed powder pellet commonly comprising a eutectic mixture of lithium chloride and potassium chloride or of lithium fluoride, lithium chloride and lithium bromide, although other mixtures are known. The electrolyte is incorporated on an inert binder such as magnesium oxide to immobilise it when molten. The advantages of molten salts as battery electrolytes are that they have high conductivity for high currents and power densities, thermal stability, chemical stability towards anodes and cathodes, cheapness and ready availability. However they also have the disadvantage of having high melting points which necessitate high heat input to activate the batteries and may cause thermal management problems.
The thermal input required may be obtained rapidly by ignition of pyrotechnic material, which may be a mixture of iron and potassium perchlorate in the form of a pellet located adjacent to the cell. The cells may be connected in a stack, each cell being separated by a pellet of pyrotechnic material. The stack is typically hermetically sealed within a steel case. The high power density and long maintenance free storage life make thermal batteries well suited to certain military applications.
The term molten salt electrolyte as used in the context of such batteries generally refers to a lithium halide containing salt which is maintained at a temperature above its melting point. The molten salt is commonly either a mixture of lithium halides, or a mixture of one or more lithium halides in combination with other alkali metal or alkaline earth halides. In the latter case, binary eutectic mixtures of lithium halide salts with halide salts of p
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Bell Bruce F.
Secretary of State for Defense, The Defence Evaluation and Resea
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