Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2000-07-07
2002-03-26
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S317000
Reexamination Certificate
active
06361902
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to solid electrolytes containing additives including a toughening agent and/or a basic sink material and, in particular, to solid electrolytes containing a polymeric matrix, a salt, a solvent, and said additives. The toughening agent imparts mechanical strength to the solid electrolyte and the basic sink removes or traps acids (e.g., HF) in the solid electrolyte.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating a salt are known in the art and are usually referred to as “solid batteries”. See, for instance, U.S. Pat. Nos. 5,229,225, 5,238,758, 5,358,801, and 5,366,928. These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., “liquid batteries”) including improved safety features.
Solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable salt, such as an inorganic ion salt, as a separate component. Electrolytic cells containing a solid electrolyte having a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed as a very thin film, i.e., in the range of about 25 to about 250 &mgr;m. Minimizing the thickness of the film reduces the total amount of internal resistance within the electrolyte but also decreases the solid electrolyte's structural integrity. In addition, good adherence of the anode and cathode to the solid electrolyte is necessary for optimum operation of electrochemical cells made therefrom.
Another problem encountered in electrolytic cells is the presence of impurities such as acids (e.g., HF) in the solid and liquid electrolytes. HF is derived from certain lithium salts (e.g., LiPF
6
) that are employed. For example, LiPF
6
reacts with water to form HF, LiF (insoluble) and other by-products, thereby reducing the amount of salt available. The acids adversely effect electrochemical performance.
In view of the above, the art is in need of solid electrolytes having superior mechanical attributes, including toughness, hardness, and resiliency. In addition, the solid electrolyte should also adhere to the anode and cathode layers of the electrolytic cell to minimize internal resistance and increase electrochemical performance. Furthermore, there is a need to reduce or eliminate acidic impurities in the solid electrolyte.
SUMMARY OF THE INVENTION
The present invention is based, in part, to the discovery that adding a toughening agent to an electrolyte composition yields a stronger solid electrolyte that adheres well to the anode and cathode. Suitable toughening agents comprise alumina, silica, zeolites, metal oxides and mixtures thereof. The invention is also based in part on the discovery that alumina (Al
2
O
3
) acts as a base sink to remove or trap acidic impurities, especially HF.
In one aspect, the present invention is directed to an electrolytic cell which comprises: an anode; a cathode and a solid, solvent-containing electrolyte which comprises a polymeric matrix, a salt, a solvent, and a toughening agent, wherein the electrolyte is interposed between the anode and cathode.
In another aspect, the present invention is directed to a process for preparing the electrolytic cells which demonstrate improved electrochemical performance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is directed to a solid, solvent-containing electrolyte comprising a toughening agent. One aspect of the invention is that the presence of the toughening agent enhances the mechanical strength of the solid electrolyte. Another aspect is that the solid electrolyte layer also demonstrates good adherence to the cathode and/or anode layers. It is believed that improvement in the structural integrity of the solid electrolyte is due, in part, to the ability of the toughening agent to also function as an absorbent (i.e., drying agent) to remove water, excess solvents and impurities. In particular, alumina, a toughening agent, also functions as a basic sink to remove a significant part of the acids from the solid electrolyte which improves cell performance and cycle life.
However, prior to describing this invention in further detail, the following terms are defined as follows:
The term “solid polymeric matrix” refers to an electrolyte compatible material formed by polymerizing organic monomers (or partial polymers thereof) and which, when used in combination with the other components of the electrolyte, renders the electrolyte solid.
The term “a solid matrix forming monomer” refers to organic materials which in monomeric form can be polymerized, preferably in the presence of an inorganic ion salt, and a solvent to form solid matrices which are suitable for use as solid electrolytes in electrolytic cells. Examples of suitable organic solid matrix forming monomers include, by way of example, propylene oxide, ethyleneimine, ethylene oxide, epichlorohydrin, acryloyl-derivatized polyalkylene oxides (as disclosed in U.S. Pat. No. 4,908,283), urethane acrylate, vinyl sulfonate polyalkylene oxides (as disclosed in U.S. Pat. No. 5,262,253, which patent is incorporated herein by reference in its entirety), and the like as well as mixtures thereof.
The term “a partial polymer of a solid matrix forming monomer” refers to solid matrix forming monomers which have been partially polymerized to form reactive oligomers. Partial polymerization may be conducted for the purpose of enhancing the viscosity of the monomer, decreasing the volatility of the monomer, and the like. Partial polymerization is generally permitted so long as the resulting partial polymer can be further polymerized, preferably in the presence of a solvent, such as, a mixture of organic carbonate(s) to form solid polymeric matrices which are suitable for use as solid electrolytes in electrolytic cells.
The term “cured” or “cured product” refers to the treatment of the solid matrix forming monomer or partial polymer thereof under polymerization conditions (including cross-linking) so as to form a solid polymeric matrix. Suitable polymerization conditions are well known in the art and include by way of example, heating the monomer, irradiating the monomer with UV light, electron beams, and the like. Examples of suitable cured products suitable for use in this invention are set forth in U.S. Pat. Nos. 4,830,939 and 4,990,413 which are incorporated herein by reference in their entirety.
The solid matrix forming monomer or partial polymer can be cured or further cured prior to or after addition of the salt, solvent, and toughening agent, and, optionally, a viscosifier. For example, a composition comprising requisite amounts of the solid matrix forming monomer, salt, organic carbonate solvent, viscosifier and toughening agent can be applied to a substrate and then cured. Alternatively, the matrix forming monomer can be first cured and then dissolved in a suitable volatile solvent. Requisite amounts of the salt, organic carbonate solvent, viscosifier and toughening agent can then be added. The mixture is then placed on a substrate; removal of the volatile solvent would result in the formation of a solid electrolyte. In either case, the resulting solid electrolyte would be a homogeneous, single phase product which is maintained upon curing, and does not readily separate upon cooling to temperatures below room temperature. Accordingly, the solid electrolyte of this invention does not require a separator as is typical of liquid electrolytes.
Alternatively, the solid polymeric matrix can be formed by a casting process which does not require the use of monomers or prepolymers, that is, no curing is required. A preferred method employs a copolymer of polyvinylidenedifluroide and hexafluoropropylene dissolved in acetone or other suitable solvent(s). Upon casting the solution, the solvent is evaporated to form the solid po
Brodd Ralph
Chaloner-Gill Benjamin
Golovin Milton Neal
Isaacson Mark
Lundquist Joseph
Harness & Dickey & Pierce P.L.C.
Maples John S.
Valence Technology Inc.
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