Composites bodies used as separators in electrochemical cells

Details Composites bodies used as separators in electrochemical cells Composites bodies used as separators in electrochemical cells

2001-04-25

2003-10-14

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Separator, retainer or spacer insulating structure

C429S129000, C429S304000, C429S306000, C429S309000, C429S247000, C429S249000, C428S304400, C428S320200, C428S322200, C428S327000, C428S411100, C428S423700, C428S424600, C428S424700, C428S424800, C428S473500, C428S474400, C204S296000, C521S027000, C252S062200

Reexamination Certificate

active

06632561

ABSTRACT:

The present invention relates to composites which are suitable in particular as separators for electrochemical cells, preferably rechargeable cells and in particular lithium batteries and lithium ion batteries, to these separators and, respectively, electrochemical cells per se, and also to a process for producing these composites.
BACKGROUND OF THE INVENTION
Electrochemical cells, in particular those which are rechargeable, are well known, for example from Ullmann's Encyclopedia of Industrial Chemistry, 5
th
ed., Vol. A3, VCH Verlagsgesellschaft mbH, Weinheim 1985, pages 343-397.
Due to their high specific energy storage density, lithium batteries and lithium ion batteries occupy a particular position among these cells, especially as secondary cells. As described, inter alia, in the above extract from “Ullmann”, such cells contain lithiated compound oxides of manganese, cobalt, vanadium or nickel. These may be described in the stoichiometrically simplest case as LiMn
2
O
4
, LiCoO
2
, LiV
2
O
5
or LiNiO
2
.
These compound oxides react reversibly with substances, such as graphite, which are capable of incorporating lithium ions into their lattice, the lithium ions being removed from the crystal lattice and the metal ions within this, such as manganese, cobalt or nickel ions, being oxidized. In an electrochemical cell this reaction can be used to store electrical energy by separating the compound accepting lithium ions, i.e. the anode material, from the lithium-containing compound oxide, i.e. the cathode material, by means of an electrolyte through which the lithium ions forming the compound oxide can migrate into the anode material (charging).
The compounds suitable for reversible storage of lithium ions are usually secured to collector electrodes by means of a binder.
During charging of the cell, electrons flow through an external voltage source and lithium cations through the electrolyte toward the anode material. When the cell is used, the lithium cations flow through the electrolyte, whereas the electrons flow from the anode material to the cathode material through a load.
In order to avoid a short circuit within the electrochemical cell, a layer which is electrically insulating but permeable to lithium cations is located between the two electrodes. This may be a so-called solid electrolyte or a conventional separator.
As is well known solid electrolytes and separators are composed of a carrier material, incorporated into which are a dissociable compound which contains lithium cations and serves to increase lithium ion conductivity and also usually other additives, such as solvents.
Microporous films have for some time been proposed as separators. For example, GB 2 027 637 describes a microporous film which comprises a matrix with from 40 to 90% by volume of a polyolefin and from 10 to 60% by volume of an inorganic filler and other constituents as respectively defined therein. The matrix described therein has 30 to 95% by volume of cavities, based on the volume of the film, and is a separator for lead accumulators.
EP-B 0 715 364 describes a two-layer battery separator with shutdown characteristics. The battery separator described there has a first microporous membrane which has a shutdown function and has been produced from a material selected from the class consisting of polyethylene, a blend comprising essentially polyethylene and of a copolymer of polyethylene. The separator also has a second microporous membrane which has a strengthening function and has been produced from a material selected from the class consisting of polypropylene, of a blend which comprises essentially polypropylene and a copolymer of polypropylene. According to the description, this separator has better mechanical strength and transit energy than the prior art.
EP-A 0 718 901 describes a three-layer battery separator with shutdown characteristics. This separator comprises a first and third microporous polypropylene membrane which in turn includes a microporous polyethylene membrane, where the first and third membrane has a greater puncture resistance and a higher melting point than the second membrane.
EP-A-0 708 791 describes a composite polymer electrolyte in membrane form which has an ion-conducting polymer gel applied to a matrix material made from a porous polytetrafluoroethylene membrane.
SUMMARY OF THE INVENTION
It is an object of the present invention, taking into account this prior art, to provide a separator which likewise has a shutdown mechanism and, furthermore, has dimensional stability at high temperature (>150° C.) and further improved mechanical strength, and, furthermore, has excellent ion-conducting properties.
We have found that this object is achieved by means of a composite comprising at least a first layer which comprises a composition comprising
(a) from 1 to 99% by weight of a solid (I) with a primary particle size of from 5 nm to 100 &mgr;m or a mixture made from at least two solids,
(b) from 99 to 1% by weight of a polymeric binder (II) which comprises:
(IIa) from 1 to 100% by weight of a polymer or copolymer (IIa) which has, along the chain, terminally and/or laterally, reactive groups (RG) which are capable of crosslinking reactions when exposed to heat and/or UV radiation, and
(IIb) from 0 to 99% by weight of at least one polymer or copolymer (IIb) which is free from reactive groups RG,
where the at least one first layer has been applied to at least one second layer comprising at least one conventional separator.
The composition present in the at least one first layer, and the preparation of the same, is now described in more detail below.
The solid I is preferably selected from the class consisting of an inorganic solid, preferably a basic inorganic solid, selected from the class consisting of oxides, mixed oxides, carbonates, silicates, sulfates, phosphates, amides, imides, nitrides and carbides of elements of the 1
st
, 2
nd
, 3
rd
or 4
th
principal group, of the 4
th
transition group, or the periodic table; a polymer selected from the class consisting of polyethylene, polypropylene, polystyrene, polytetra-fluoroethylene and polyvinylidene fluoride; polyamides, polyimides; a solid dispersion comprising a polymer of this type; glass powder, nanoglass particles, e.g. Monosper® (Merck), microglass particles, e.g. Spheriglas® (Potters-Ballotini), nanowhiskers and a mixture of two or more of these, where the composition obtained can be used as a solid electrolyte and/or separator.
Mention should be made in particular, by way of example, of: oxides, e.g. silicon dioxide, aluminum oxide, magnesium oxide or titanium dioxide, compound oxides, for example of the elements silicon, calcium, aluminum, magnesium, titanium; silicates, e.g. ladder-type silicates, ino-, phyllo- and tectosilicates, e.g. talc, pyrophyllite, muskovite, phlogophite, amphiboles, nesosilicates, pyroxenes, sorosilicates, zeolites, feldspars, wollastonite, in particular hydrophobicized wollastonite, mica, phyllosilicates; sulfates, e.g. alkali metal sulfates and alkaline earth metal sulfates; carbonates, such as alkali metal carbonates and alkaline earth metal carbonates, e.g. calcium, magnesium or barium carbonate or lithium, potassium or sodium carbonate; phosphates, such as apatites; amides; imides; nitrides; carbides; polymers, e.g. polyethylene, polypropylene, polystyrene, polytetra-fluoroethylene, polyvinylidene fluoride, polyamides, polyimides, or other thermoplastics, thermosets or microgels, crosslinked polymer particles, e.g. Agfaperl®, solid dispersions, in particular those which comprise the abovementioned polymers, also mixtures of two or more of the abovementioned solids.
The solid I used may according to the invention also comprise inorganic Li-ion-conducting solids, preferably a basic inorganic Li-ion-conducting solid.
Those which should be mentioned are: lithium borates, e.g. Li
4
B
6
O
11
*xH
2
O, Li
3
(BO
2
)
3
, Li
2
B
4
O
7
*xH
2
O, LiBO
2
, where the number x may be from 0 to 20; lithium aluminates, e.g. Li
2
O*Al
2
O
3
*H
2
O, Li
2
Al
2
O
4
, LiAlO
2
; lithium alumino

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