Chemistry: electrical current producing apparatus – product – and – With control means responsive to battery condition sensing... – Temperature control
Reexamination Certificate
2000-03-29
2002-07-02
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With control means responsive to battery condition sensing...
Temperature control
C429S144000
Reexamination Certificate
active
06413667
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a method of making bonded multilayer, flat-plate electrochemical cell devices, such as rechargeable batteries and supercapacitors. More specifically, the invention provides a method for establishing persistent interfacial bonding between laminated planar electrode and microporous separator members utilized in such electrochemical devices.
Widely deployed primary and secondary, rechargeable lithium-ion electrochemical cells are typical of electrochemical devices to which the present invention is directed. Such cells comprise layers, or membranes, of respective positive and negative electrode composition members assembled with a coextensive interposed separator member comprising a layer or membrane of electrically insulating, ion-transmissive material. This multilayer electrochemical cell structure is normally packaged with a mobile-ion electrolyte composition, usually in fluid state and situated in part in the separator member, in order to ensure essential ionic conductivity between the electrode members during charge and discharge cycles of the electrochemical cell.
One type of separator for this purpose is a microporous polyolefin membrane, either of single- or multilayer structure such as described, for example, in U.S. Pat. Nos. 3,351,495; 5,565,281; and 5,667,911. When employed as rechargeable electrochemical cell separators, these porous membranes not only effectively retain within their porous structure the essential liquid electrolyte compositions, but they also provide an additional advantage in that they possess an automatic thermal shutdown feature which prevents uncontrolled heat buildup within the electrochemical cell, such as might otherwise result in a dangerous explosive condition, for instance during excessive cell recharging. This built-in safety mechanism relies on the fact that the melting point range of the polyolefins utilized in the fabrication of the separator membranes is at the lower end of the danger zone of electrochemical cell heat buildup. Thus, in the event of a runaway cell heating episode, the porous polyolefin separator membrane becomes heated to a point of melting and its pore structure collapses, thereby interrupting the essential ionic conductivity within the cell and terminating the electrochemical reaction before a dangerous condition ensues.
The packaging of electrochemical cell structures has heretofore regularly taken the form of a metal container, whether, for example, in elongated tubular (cylindrical) or flattened (prismatic) shape, which has commonly been relied upon to not only contain the liquid electrolyte component, but also to impart the significant stack pressure required to maintain close physical contact between the individual cell electrodes and the interposed separator member. This intimate contact, along with the composition of the electrolyte, is, as previously noted, essential to efficient ion transport between electrodes during operation of the electrochemical cell.
More recently, however, the profusion and continued miniaturization of electronic devices powered by Li-ion batteries and similar electrochemical energy storage cells has generated a demand for a greater number of cell package shapes and dimensions, e.g., relatively broad, yet thin, lightweight packages having a significant degree of flexibility. For example, numerous end use applications make thin, flexible tablet-style packages of polymer film more desirable than the prior rigid-walled high-pressure metal can containers. However, these more flexible packages are decreasingly capable of achieving and maintaining the substantial physical pressures required to ensure the noted essential intimate interlayer contact throughout the electrochemical cell.
In order to minimize the deleterious effect of decreased physical stack pressure previously relied upon to establish the necessary contact between electrochemical cell components, developers have progressed to the use of direct adhesive bonding between electrode and separator layers to ensure their essential intimate contact. Typical of such innovations are electrochemical cells utilizing polymer-based electrode and separator members, such as described in U.S. Pat. Nos. 5,296,318; 5,456,000; 5,460,904 and 5,540,741.
In those fabrications, compositions of polymers, such as polymers and copolymers of vinyl chloride, acrylonitrile, methyl methacrylate, ethylene oxide, vinylidene chloride, and vinylidene fluoride, notably of poly(vinylidene fluoride) (PVdF) copolymers with hexafluoropropylene, which are compatible with efficient liquid electrolyte compositions, are utilized as binders in both the electrode and the separator members to not only promote essential ionic conductivity, but also to provide a common composition component in those cell members which promotes strong interfacial adhesion between them within a reasonably low laminating temperature range. Such laminated, multilayer rechargeable electrochemical cells operate effectively and exhibit a stable high capacity and excellent discharge rate performance even though packaged in flexible, lightweight polymeric film enclosures.
Although such laminated electrochemical cells and like energy storage devices have significantly advanced the art in miniaturized applications, the use of substantially non-porous polymeric matrices and membranes in their fabrication has deprived these devices of the desirable thermal shutdown feature achieved when using the microporous polyolefin separator membranes. However, the low surface energy exhibited by the polyolefin membranes renders them highly abherent in nature and thus inhibits their strong, permanent adhesion to many polymeric electrode layer compositions, particularly within a reasonable temperature range which does not lead to melting and, thus, thermal collapse, of the porous structure of the polyolefin membranes.
Some attempts have been made by electrochemical cell fabricators to overcome the adhesion-resistant property of the otherwise desirable microporous polyolefin separator membranes by introducing specifically formulated adhesive polymer compositions into the region of electrode and separator member interfaces, such as described by Abraham et al. in the
Journal of Electrochemical Society,
vol. 142(3), pp. 683-687 (1995) and in U.S. Pat. Nos. 5,837,015 and 5,853,916. However, it has generally been found that the application of such adhesive compositions, whether by overcoating, dipping, extrusion, or the like, significantly occludes or otherwise interferes with the porous structure of the polyolefin membranes and causes a deleterious decrease in electrolyte mobility and ionic conductivity. Further, the addition of substantial amounts of such adhesive materials increases the proportion of non-reactive components in a cell, thereby detracting from the specific capacity of any resulting energy storage device.
Typical of such attempts to achieve suitable interfacial bonding between electrode and separator cell are the procedures described in U.S. Pat. Nos. 5,681,357 and 5,716,421. There, a layer of PVdF homopolymer is applied to the microporous separator membrane from a solution in organic solvents when the membrane is intended to be employed in the fabrication of an electrochemical cell by thermal lamination with electrodes comprising binder matrix compositions of a similar polymer. It was apparently intended that the added polymer layer would not be of such excessive thickness as to occlude the porosity of the membrane, but rather would provide an intermediate transition in compatibility to the matrix polymer binder of preferred electrode layer compositions. This approach has proven to be insufficient in itself to enable satisfactory interfacial bonding between cell component layers at lamination temperatures below the critical level which results in collapse of separator porosity and its attendant loss of effective ionic conductivity and desirable shutdown capability. Either the added polymer filled the pores of the membrane or the layer
Harness & Dickey & Pierce P.L.C.
Valence Technology (Nevada), Inc.
Wills M.
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