Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-03-16
2002-06-18
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S303000
Reexamination Certificate
active
06406816
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a polymeric gel electrolyte.
The invention further relates to a method of preparing such an polymeric gel electrolyte.
The invention still further relates to a rechargeable lithium battery comprising such a polymeric gel electrolyte.
Due to its small size and weight, high energy density and high voltage, a battery having a lithium-based anode, or lithium battery for short, is an attractive source of electric energy for portable and/or hand-held electr(on)ic equipment in particular. Such a battery comprises a lithium-based anode, a cathode, and disposed therebetween, an electrolyte.
In the case of primary lithium batteries, a highly ion-conductive liquid electrolyte can be suitably used. However, when employed in a rechargeable (also referred to as secondary) lithium battery such a liquid electrolyte gives, upon charging, rise to dendritic growth which in turn leads to short-circuits. Dendritic growth is less when a solid electrolyte is used. However, the ion-conductivity of a solid electrolyte is insufficient for many practical purposes such as when a battery is to be rapidly (dis)charged. In an attempt to combine liquid and solid electrolytes in an advantageous manner, the use of polymeric gel electrolytes, i.e. a polymer which is gelled by means of an ion-conductive liquid, has been proposed.
In this respect, reference is made to U.S. Pat. No. 5,501,921 which discloses polymeric gel electrolytes comprising a cross-linked polymer and an ion-conductive liquid. The cross-linked polymer is obtained by cross-linking functionalized alkane monomers each having 2 to 100 carbon atoms and each having at least one polymerizable functional group. The total number of polymerizable functional groups is selected such that the polymer electrolyte is substantially chemically inert when brought into contact with a lithium anode.
In practice it may happen that when such a known polymeric gel electrolyte is employed in a rechargeable battery and the battery is repeatedly charged and subsequently discharged, for example at a 0.2 C rate (a y C rate means that the battery is (dis)charged to its full capacity in y
−1
h), said battery short-circuits after a small number of repetitions. Inspection of the short-circuited battery shows that dendrites have grown through the polymeric gel electrolyte.
SUMMARY OF THE INVENTION
It is an object of the invention to provide novel polymeric gel electrolytes which do not have the abovementioned disadvantages, or at least to a lesser extent. When employed in a rechargeable lithium battery, the polymeric gel electrolyte should have a high ion-conductivity, should not react with the lithium anode and should effectively suppress the dendritic growth which leads to short-circuits when the battery is repeatedly charged and discharged.
This object is achieved by a multi-phase polymeric gel electrolyte having an ion-conductivity of more than 1 mS/cm and comprising:
(a) a continuous solid phase predominantly comprising a polymer obtainable by polymerizing functionalized C
2
-C
100
alkane monomers or alkylene oxide monomers having the formula {(CH
2
)
p
—O}
m(II)
wherein m=2, 3 or 4 and p=3, 4, 5 or 6 formule (II) each functionalized with at least one polymerizable group, and
(b) a continuous liquid phase interpenetrating said solid phase and predominantly comprising an ion-conductive liquid containing a solvent and a lithium salt at least partly dissolved therein,
wherein said solid phase has a microscopic network structure which is selected such that when a rechargeable battery is formed by disposing said multi-phase polymeric gel electrolyte between a lithium metal anode and a lithium cobaltate cathode said rechargeable battery has a capacity per unit surface area of at least 3 mAh/cm
2
and is capable of being fully charged and discharged at least 20 times at a 0.2 C rate without short-circuiting.
The extent to which a polymeric gel electrolyte suppresses dendritic growth is measured by repeatedly fully charging and discharging a rechargeable lithium battery having a capacity per unit surface area of at least 3 mAh/cm
2
and comprising said polymeric gel electrolyte disposed between a lithium metal anode and a lithium cobaltate cathode. If the said battery can be fully charged and discharged at least 20 times at at least a 0.2 C rate without short-circuiting, the solid phase of the polymeric gel electrolyte has a suitable microscopic network structure and the suppression of dendritic growth is considered to be effective.
Surprisingly, it is found that the ability of a polymeric gel electrolyte to suppress dendritic growth when employed in a rechargeable battery is significantly improved if a multi-phase polymeric gel electrolyte is employed in which the polymer, being part of a solid phase, and the ion-conductive liquid, being part of a continuous liquid phase, interpenetrate on a microscopic scale. After all, one would expect dendritic growth to be minimal if a one-phase polymeric gel electrolyte in which the polymer and the liquid interpenetrate on a molecular scale is employed. However, contrary to this expectation, suppression of dendritic growth is found to be most effective if the (polymer of the) solid phase is selected such that it has a microscopic network structure of a particular morphology.
As a typical example, polymerizing a composition composed of 40 wt. % decanedioldiacrylate and 60 wt. % 1 M LiPF
6
in a 1:1 (v/v) mixture of ethylenecarbonate and diethylcarbonate produces a multi-phase polymeric gel electrolyte having an ion-conductivity of 1.2 mS/cm which, when employed in a lithium metal battery, allows said battery to be fully charged and subsequently discharged more than 40 times at a 0.2 C rate without short-circuiting.
The invention is based on the recognition that the occurrence of dendritic growth leading to short-circuits is related to a degradation of the polymeric gel electrolyte. The degradation is due to a reaction between (neutral) lithium and the polymer occurring at the interface between polymer and liquid. By providing a multi-phase polymeric gel electrolyte in which the polymer is phase-separated from the ion-conductive liquid on a microscopic scale the surface area of the interface and, consequently, the rate at which the degradation reaction occurs is reduced.
The continuous solid phase of the multi-phase polymeric gel electrolyte in accordance with the invention has a microscopic network structure. It extends throughout the entire polymeric gel electrolyte in all directions thus giving the polymeric gel electrolyte a mechanical support similar to that obtained for a one-phase polymeric gel electrolyte.
It is emphasized that, in the context of the invention, the term network structure refers to a network of microscopic dimensions and not to the network of covalently bonded atoms of a cross-linked polymer.
As already mentioned above, the morphology of the microscopic network structure of the solid phase is an essential parameter with respect to the suppression of dendritic growth leading to short circuits because it determines the morphology of the liquid phase which is the complement to that of the solid phase.
Although it is not to be interpreted as limiting the scope of the invention, electron micrographs taken of the surfaces of films of multi-phase polymeric gel electrolytes show that a suitable network structure is one which is composed of more or less spherical particles having a characteristic diameter of 0.1 to 0.5 &mgr;m. On the other hand, network structures of spherical particles having a characteristic diameter of 0.5 &mgr;m to 1.0 &mgr;m or larger have by and large been found unsuitable. Obviously, apart from the size of the particles there should be a sufficient number of them. That is the network structure is to be sufficiently dense. The density of the microscopic network may be simply increased by increasing the weight ratio of solid to liquid phase.
As is well known to those skilled in the art, there are many parameters which in
Bartlett Ernestine C.
Chaney Carol
Spain Norman N.
U.S. Philips Corporation
LandOfFree
Polymeric gel electrolyte does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polymeric gel electrolyte, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polymeric gel electrolyte will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2921703