Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
1998-10-01
2001-07-10
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S248000, C429S249000
Reexamination Certificate
active
06258488
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a battery separator and to a battery incorporating a separator. The invention also relates to a method of making a battery separator.
Discussion of Prior Art
It is known to use regenerated cellulose film as a separator in primary and secondary or rechargeable batteries. Paper and cellulose film is able to act as a barrier between electrodes to prevent internal short circuiting whilst having sufficient permeability and electrolyte absorbence to allow the cell reaction to take place. The separator has to be sufficiently impregnated with electrolyte for the necessary cell reaction. The life of a rechargeable battery can be limited by the breakdown of the separator which has to operate in a hostile environment, typically a highly alkaline environment. In some circumstances there is a build-up of crystals, typically zinc oxide crystals, in the negative electrode which can pierce the separator barrier.
In EP-A-0572921 separators, particularly for alkaline batteries, are disclosed which are formed from fibrillated lyocell fibres which are mixed with polyvinyl alcohol fibre and a polyvinyl alcohol fibrous binder. However such separators are complicated to make and involve fibrillation of the lyocell fibres in a pulper and fiberizer.
The manufacture of shaped polymer articles by extrusion of a solution of cellulose in an aqueous tertiary amine N-oxide solvent into an aqueous coagulating bath is described, for example, in U.S. Pat. No. 4,246,221, the contents of which are incorporated herein by way of reference. Such a process may be referred to as a solvent-spinning process. Fibres and films produced by such a method may be referred to as solvent-spun cellulose fibres and films, and generically the fibres are known as lyocell fibres. The concentration of cellulose in solution in amine oxide is generally in the range from 5 to 30% by weight.
Film may be formed by the extrusion of a seamless tube of cellulose solution in an amine oxide solvent, for example as described in U.S. Pat. No. 5,277,857. The tube may be opened out to form a strip, or flattened to form a double layered strip. The film formed from cellulose solution is defined in this document as non-derivatised cellulose and that terminology will be used in the present description. Alternatively the film may be extruded through a slot die as is described in our co-pending international patent application PCT/GB96/03215.
SUMMARY OF THE INVENTION
According to the present invention there is provided a battery separator comprising at least one layer of non-derivatised cellulose film.
The cellulose is dissolved in a tertiary-amine-N-oxide solvent for cellulose, preferably N-methyl-morpholine-N-oxide. The concentration of cellulose in the solvent is preferably from 2 to 25 percent (w/w), and more preferably from 4 to 18 percent (w/w) and typically about 15%. The degree of polymerization of the cellulose is preferably from 200 to 5000, and more preferably from 400 to 1000, and typically around 800.
The cellulose solution may be extruded through a heated slot die which is heated to a temperature of between 80-120° C. The film passes through an air gap of up to 300 mm and generally between 20-100 mm, before entering a regeneration bath.
Such a separator is for use in both primary and secondary cells. The film separator may be reinforced by fibrous reinforcement, in particular lyocell fibres.
The invention further includes an alkaline battery cell having a cellulose film battery separator of the type described above.
A battery of this type has improved long term charge stand characteristics. Preferably the battery has a potassium hydroxide based electrolyte and on discharge after a prolonged charged stand has substantially no voltage dip on discharge.
Film separators in accordance with the present invention have particularly good physical properties after immersion in alkaline solutions when compared with regenerated cellulose films produced by the xanthate or viscose process. In particular it has been found that non-derivatised films have better chemical stability and better retention of tensile properties after immersion in potassium hydroxide solutions. Substantially similar strength films to those of films produced by the viscose process can therefore be achieved with thinner films in accordance with the present invention.
The invention also provides for a method of manufacture of battery separators in which the separator comprises a non-derivatised cellulose film formed from a flat web of cellulose in solution in an amine oxide solvent which is regenerated in a bath containing an aqueous solution of said amine oxide solvent such that the amine oxide concentration does not exceed 50% by weight.
If the amine oxide concentration exceeds 50% the stability of the film in potassium hydroxide deteriorates.
Also according to the present invention there is provided an alkaline battery cell, preferably a silver/zinc cell, having a voltage drop over the first minute of discharge after a prolonged charged stand of around 25% or less of the original voltage.
The battery cell has a separator formed from non-derivatised cellulose film.
The invention is particularly applicable to primary ‘button’ cells.
REFERENCES:
patent: 4039729 (1977-08-01), Benczur-Urmossy et al.
patent: 5277857 (1994-01-01), Nicholson et al.
patent: 5366832 (1994-11-01), Hayashi et al.
patent: 5700599 (1997-12-01), Danko et al.
patent: 5700600 (1997-12-01), Danko et al.
patent: 5942354 (1999-08-01), Oxley et al.
patent: 0 572 921 (1993-12-01), None
patent: 0 712 889 (1996-05-01), None
patent: WO 96/13071 (1996-05-01), None
patent: WO 96/37653 (1996-11-01), None
Askew Gregory J.
Law Stephen J.
Street Heather
Chaney Carol
Nixon & Vanderhye P.C.
Tencel Limited
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