Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-06-09
2002-12-03
Chaney, Carol (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C429S006000, C429S137000, C429S164000, C429S231900
Reexamination Certificate
active
06488721
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the fields of electrochemical cells and of separators for use in electrochemical cells. More particularly, this invention pertains to methods of preparing a cathode/separator assembly in which a microporous separator layer is coated on a temporary carrier substrate and a cathode active layer is then coated on the separator layer prior to removing the temporary carrier substrate from the separator layer. Also, this invention pertains to methods of preparing electrochemical cells utilizing cathode/separator assemblies prepared by the methods of this invention. The present invention also pertains to cathode/separator assemblies and electrochemical cells prepared by such methods.
BACKGROUND
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
In an electrochemical cell or battery, discharge of the cell from its charged state occurs by allowing electrons to flow from the anode to the cathode through an external circuit resulting in the electrochemical reduction of the cathode active material at the cathode and the electrochemical oxidation of the anode active material at the anode. Under undesirable conditions, electrons may flow internally from the anode to the cathode, as would occur in a short circuit. To prevent this undesirable internal flow of electrons that occurs in a short circuit, an electrolyte element is interposed between the cathode and the anode. This electrolyte element must be electronically non-conductive to prevent the short circuiting, but must permit the transport of ions between the anode and the cathode during cell discharge, and in the case of a rechargeable cell, also during recharge. The electrolyte element should also be stable electrochemically and chemically towards both the anode and the cathode.
Typically, the electrolyte element contains a porous material, referred to as a separator since it separates or insulates the anode and the cathode from each other, and an aqueous or non-aqueous electrolyte in some or all of the pores of the separator. The aqueous or non-aqueous electrolyte typically comprises ionic electrolyte salts and water or electrolyte solvents, and optionally, other materials or additives such as, for example, ionically conductive polymers. A variety of materials have been used for the porous layer or separator of the electrolyte element in electrochemical cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber and paper filter papers, and ceramic materials. Usually these separator materials are supplied as porous free standing films which are interleaved with the anodes and the cathodes in the fabrication of electrochemical cells. Alternatively, the porous layer can be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 3,625,771 to Arrance et al., 5,194,341 to Bagley et al., and U.S. Pat. No. 5,882,721 and U.S. Pat. No. 5,948,464 to Delnick; and in Eur. Pat. Application Nos. 848,435 to Yamashita et al.; and 892,449 to Bogner.
Porous separator materials have been fabricated by a variety of processes including, for example, stretching combined with special heating and cooling of plastic films, extraction of a soluble plasticizer or filler from plastic films, and plasma oxidation. The methods for making free standing separators typically involve extrusion of melted polymeric materials either followed by a post-heating and stretching or drawing process or followed by a solvent extraction process to provide the porosity throughout the separator layer. U.S. Pat. No. 5,326,391 to Anderson et al. and references therein, describe the fabrication of free standing porous materials based on extraction of a soluble plasticizer from pigmented plastic films. U.S. Pat. No. 5,418,091 to Gozdz et al. and references therein, describe forming electrolyte layers by extracting a soluble plasticizer from a fluorinated polymer matrix either as a coated component of a multilayer battery structure or as a free standing separator film. U.S. Pat. No. 5,894,656 to Menon et al. describes forming an electrode directly on the surface of an electrolyte layer having a soluble plasticizer and then extracting the plasticizer to activate the battery. U.S. Pat. No. 5,194,341 to Bagley et al. describes an electrolyte element with a microporous silica layer and an organic electrolyte. The microporous silica layer was the product of the plasma oxidation of a siloxane polymer.
U.S. patent application Ser. No. 08/995,089 titled “Separators for Electrochemical Cells,” filed Dec. 19, 1997 now U.S. Pat. No. 6,153,337, to Carlson et al. of the common assignee, describes separators for use in electrochemical cells which comprise a microporous pseudo-boehmite layer, and electrolyte elements and cells comprising such separators. The pseudo-boehmite separators and methods of preparing such separators are described for both free standing separators and as a separator layer coated directly onto an electrode.
When a separator layer is coated directly onto an electrode, such as onto the cathode, the porous separator coating may require a relatively smooth, uniform surface on the cathode and also may require a mechanically strong and flexible cathode layer. For example, for a microporous pseudo-boehmite layer having a xerogel structure, these cathode surface and layer properties may be required to prevent excessive stresses and subsequent cracking of the xerogel layer during drying of a pseudo-boehmite coating on the cathode surface and also during fabrication and use of electrochemical cells containing the pseudo-boehmite layer.
In addition to being porous and being chemically stable to other materials of the electrochemical cell, the separator should be flexible, thin, economical in cost, and have good mechanical strength. These properties are particularly important when a cell with thinner cathode, separator, and anode layers is spirally wound or is folded to increase the surface area of the electrodes and thereby increase the capacity and high rate capability of the cell. Typically, free standing separators for batteries have been 25 microns or greater in thickness. As batteries have continued to evolve to higher volumetric capacities and smaller lightweight structures, there is an increasing need for separators that are 15 microns or less in thickness with a substantial increase in the area of the separator contained in each particular size of battery. Reducing the thickness from 25 microns to 15 microns or less greatly increases the challenge of providing porosity and good mechanical strength while not sacrificing the protection against short circuits or not significantly increasing the total cost of the separator in each battery.
This protection against short circuits is particularly critical in the case of secondary or rechargeable batteries with lithium as the anode active material. During the charging process of the battery, dendrites may form on the surface of the lithium anode and may grow with continued charging. A key feature of the separator in the electrolyte element of lithium rechargeable batteries is that it have small pore structures, such as 1 micron or less in pore diameter, and sufficient mechanical strength to prevent the lithium dendrites from contacting the cathode and causing a short circuit with perhaps a large increase in the temperature of the battery leading to an unsafe explosive condition.
Further it would be advantageous to be able to prepare electrochemical cells having separators with ultrafine pores and a wide range of thicknesses coated in contact to another layer of the electrochemical cell by a process of coating without requiring any subsequent solvent extraction or o
Carlson Steven A.
Chaney Carol
Moltech Corporation
Nicol Jacqueline M.
Squire Sanders & Dempsey L.L.P.
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