Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-09-07
2001-08-28
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S303000, C429S308000, C429S316000, C029S623100
Reexamination Certificate
active
06280881
ABSTRACT:
This invention relates to a lithium secondary battery, and particularly to the electrolyte used therein and to the preparation of the battery.
Recent developments in electrochemical technology have provided systems such as primary and secondary lithium batteries of high specific energy per unit of volume, typically in the range 175-250 Wh/l.
Such secondary batteries are typically based on negative electrode structures of metallic lithium, alloys thereof or on carbons of high lithium intercalation capacity. The positive electrode structures are typically based on transition metal oxides. The electrolyte comprises one or more non-aqueous solvents, and one or more lithium-salts.
The fact that the electrolyte is a liquid at the battery operation temperature may cause safety problems such as electrolyte leakage from the cell compartment. Upon reaction with oxygen and water in the atmosphere, severe corrosion of the battery may occur.
Several attempts have been made to solve this safety issue of lithium based batteries. The traditional approach has been solid polymer electrolytes, i.e. electrolyte structures which are based on ionic conduction within a solid polymer network. Such polymer electrolytes provide batteries of high safety, as no electrolyte leakage can take place.
Polymer electrolytes are described in a number of patents and patent applications, including the following:
EP 724,305 A1 to Sony Corporation, which describes gel electrolytes of a polymer having a side chain to which at least one nitrile group is bonded.
U.S. Pat. No. 5,240,790 to Alliant Techsystems Inc., which covers a gelled electrolyte comprising polyacrylonitrile, preferably of a relative concentration of 12-22 mole percent.
U.S. Pat. No. 5,589,295 to Derzon et al, which describes a thin film electrolyte with a polymeric gel-former selected from the group of polyacrylonitrile and polyvinylidenefluoride.
The drawback of batteries based on such solid polymer electrolytes is reduced capacity and power capability, especially at low temperature. Compared to liquid electrolytes, the conductivity of solid polymer electrolytes is lower, mainly due to reduced ionic mobility. Further, the activation energy for the ionic migration process is higher than for the liquid electrolytes, implying strong conductivity variation with temperature and significantly reduced low-temperature performance. The capacity and power capability are strongly dependent on the electrolyte conductivity; at low conductivity high internal impedance implies high losses and reduced capacity accessability.
Therefore a need exists for secondary lithium batteries based on polymer electrolyte systems, which combine the demands for high safety and high conductivity.
An object of the present invention is to provide a lithium secondary battery which avoids problems with respect to electrolyte leakage from the cell compartment but which also provides high conductivity sufficient for full capacity utilisation, i.e. which does not imply the same reduction in capacity utilisation compared to lithium secondary batteries based on liquid electrolytes that is associated with known polymer electrolytes.
The present invention provides a lithium secondary battery comprising an immobilized electrolyte containing one or more alkali metal salts, one or more non-aqueous solvents and an immobilizing polymer, wherein the immobilizing polymer is selected from the group consisting of cellulose acetates, cellulose acetate butyrates, cellulose acetate propionates, polyvinylidene fluoride-hexafluoropropylenes and polyvinypyrrolidone-vinyl acetates, with the proviso that in the case of polyvinylidene fluoride-hexafluoropropylenes, the polymer is present in an amount of at most 12% by weight based on the weight of the salts, solvents and polymer of the electrolyte system.
Surprisingly, it has been found that lithium secondary batteries which comprise as an electrolyte component an immobilising polymer selected from the group of cellulose acetates, cellulose acetate butyrates, cellulose acetate propionates, polyvinylidene fluoride-hexafluoropropylenes and polyvinylpyrrolidone-vinyl acetates does not cause problems with respect to electrolyte leakage from the cell compartment. Further, the electrolytes of such batteries have a high conductivity implying a capacity utilisation more closely approaching the utilisation observed for batteries using liquid electrolyte. Still further, the electrolytes of such batteries are electrochemically stable, i.e. they are not oxidised or reduced even under the redox conditions observed in high voltage lithium batteries.
Compared to the known technology on polymer electrolyte based secondary batteries referred to above, the polymers used according to the invention are generally present in relatively small amounts, preferably at most 15% by weight based on the weight of the salts, solvents and polymer of the electrolyte system,
According to one embodiment of the invention, the cellulose polymers used according to the present invention will usually be present in an amount ranging from 0.1% to 10% by weight of the complete electrolyte system, i.e. the total weight of salts, solvents and polymer, preferably 1% to 8% by weight, more preferably 2% to 5% by weight.
In another embodiment of the invention, the polyvinylidene fluoride-hexafluoropropylenes are present in an amount of from 1% to 12% by weight of the complete electrolyte system, i.e. the weight of salts, solvents and polymer, preferably 2 to 10% by weight, more preferably 4% to 8% by weight. In a still further embodiment of the invention the polyvinylpyrrolidone-vinyl acetates will usually be present in an amount from 1% to 15% by weight of the complete electrolyte system, i.e. the total weight of salts, solvents and polymer, preferably 3% to 12% by weight, more preferably 5% to 10% by weight.
WO 97/12409 to Valence Technology describes “viscosifiers” for electrolytes, which are selected from the group of polyethylene oxide, polypropylene oxide, carboxymethylcellulose and polyvinylpyrrolidone. Although this patent specification describes the use of “viscosifiers” based on a cellulose compound and a polyvinylpyrrolidone, it does not describe the specific immobilising agents cellulose acetates and polyvinylpyrrolidone-vinyl acetates used according to the present invention.
U.S. Pat. No. 5,296,318 to Bell Communications Research discloses an electrolyte comprising a self-supporting film of a copolymer of vinylidene fluoride and hexafluoropropylene. Such copolymer is preferably present in the electrolyte in an amount corresponding to 30 to 80% of the electrolyte. Although the patent describes the use of polyvinylidene fluoride-hexafluoropropylene, it does not describe or suggest the use of this material in amounts as small as 12% or less by weight of the electrolyte system.
The immobilizing properties of the polymers used according to the present invention may be improved by crosslinking.
In a preferred embodiment, the immobilizing properties of the cellulose polymers used according to the invention are improved by crosslinking. In this embodiment, cellulose acetates, cellulose acetate butyrates and cellulose acetate propionates, preferably of high hydroxyl content, for example 3% by weight or more, are mixed with monomers or oligomers, which bear functional groups, and which can be crosslinked upon heat curing or upon exposure to UV-light or electron beams. Such monomers and polymers are preferably selected from urea formaldehyde, melamine and polyisocyanate polymers.
In another preferred embodiment of the invention, the electrolyte of the lithium secondary battery comprises, in addition to the immobilising polymer, one or more solvents selected from organic carbonates, lactones, esters and glymes, more preferably selected from the groups of:
(a) alicyclic carbonates represented by the following general formula:
—C(═O)—O—CR
1
R
2
—[CR
3
R
4
]
m
—CR
5
R
6
—O—,
wherein each of R
1
, R
2
, R
3
, R
4
, R
5
and R
6
independently represents hydrogen or a C
1
-C
4
alkyl group and
Wendsjo Asa
Yde-Andersen Steen
Danionics A/S
Darby & Darby
Kalafut Stephen
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