Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
1998-02-12
2002-05-28
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S231950
Reexamination Certificate
active
06395419
ABSTRACT:
This invention relates to solid polymer electrolytes (SPE) suitable for use in electrochemical devices such as secondary batteries, capacitors, displays and sensors, a method for preparing the same, and electrochemical devices comprising the same.
BACKGROUND ART
Prior art electrochemical devices often use electrolytes of aqueous or non-aqueous solution. These electrolytes in solution form have drawbacks including the necessity of tight seal and the difficulty of insuring safety. There is an increasing need for ion conductive solid polymer electrolytes possessing desirable properties of flexibility, elasticity, light-weight, thin film formation, and transparency. Especially in the field of batteries which are most important among electronic articles of manufacture, efforts to develop secondary batteries using ion conductive polymers as the electrolyte have been made to meet the requirements of light-weight, compactness, freedom of geometrical design, and increased effective area.
Prior art examples using ion conductive polymers include uniform or dry type materials comprising polymers, for example, polyethers such as poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) or derivatives thereof and electrolyte salts containing lithium or the like; hybrid or gel type materials comprising such polymers in admixture with plasticizers for improving electrochemical properties; and materials of the polymer-in-salt type having composite electrolyte salts carried on polymers.
The current research works to develop polymer batteries focus at solid polymer electrolytes (SPE) of the gel type because of their high ionic conductivity. Such polymers as PEO, PAN (polyacrylonitrile), and PVDF (poly(vinylidene fluoride)) are regarded promising for practical use. The gel type SPE is generally prepared by dissolving a polymer in a solvent, mixing the solution with an electrolytic solution, coating the solution to a substrate, and evaporating off the solvent to leave an SPE film. Alternatively, an SPE film is prepared by dissolving a polymer directly in an electrolytic solution, followed by coating or extrusion. Since most electrolytic solutions for use in electrochemical devices should be devoid of water, a dry atmosphere must be maintained in all the steps undertaken in the industrial manufacture of SPE by these processes. This requires substantial installation and maintenance costs, and the management of inventory is not easy.
Gozdz et al., U.S. Pat. No. 5,418,091 disclose a method for preparing SPE by adding a plasticizer to a polymer solution, coating the solution to a substrate, evaporating off the solvent to form a film, extracting the plasticizer from the film to leave pores in the film, and impregnating the film with an electrolytic solution so that the solution is imbibed in the pores. Since the electrolytic solution is used only in, the impregnating/imbibing step, the preceding steps can be carried out under the ambient environment, enabling substantial reduction of the installation and maintenance costs. Additionally, the SPE can be stored in film form after the coating and drying steps or after the extraction of the plasticizer, ensuring the ease of inventory management. However, this method requires extraction step of dipping the plasticized polymer film in a large volume of solvent several times. This step not only causes a considerable drop of production efficiency or mass productivity, but also requires the disposal of a large volume of used solvent, which otherwise causes environmental pollution. The extraction can also be accomplished by heating the plasticized polymer film in vacuum. This alternative step, however, also affects production efficiency or mass productivity and necessitates an additional equipment and an additional cost.
For the manufacture of electrochemical devices using SPE, it is desired to have a method for preparing high performance SPE in an efficient manner using an inexpensive equipment.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for preparing a solid polymer electrolyte in an efficient, mass productive manner at a low cost. Another object of the invention is to provide a solid polymer electrolyte having a high swelling factor with electrolytic solution. A further object of the invention is to provide an electrochemical device using the solid polymer electrolyte and featuring low cost and high productivity.
According to the invention, there is provided a method for preparing a solid polymer electrolyte, comprising the steps of: forming a solution of a polymer and a filler in a solvent into a film; evaporating off the solvent from the film; and impregnating the film with an electrolytic solution.
By the method of the invention, there is produced a solid polymer electrolyte comprising a polymer matrix containing a particulate filler and an electrolytic solution incorporated therein. The solid polymer electrolyte preferably has a swelling factor of at least 2.2.
Also contemplated herein is an electrochemical device comprising the solid polymer electrolyte. Typical electrochemical devices are lithium secondary batteries and electric double layer capacitors.
The co-presence of the particulate filler allows the polymer to have a high swelling factor with an electrolytic solution. In general, a composite material comprising a polymer and a particulate filler has a swelling factor which does not exceed the swelling factor of the polymer alone if the inorganic non-porous filler particles are fully dispersed in the polymer and completely bound by the polymer on microscopic observation. Rather, the swelling factor of the composite material must be reduced from the swelling factor of the polymer alone by the volume fraction occupied by the filler particles. Nevertheless, the composite material of the invention has a higher swelling factor than the polymer alone probably because agglomeration of filler particles into secondary and third-order flocculated particles has occurred, the bonding state between the polymer and the filler, that is, the surface state of filler particles or the interface between the polymer and the filler has changed, and as a result, micro or meso pores have been formed in the composite material. Alternatively, pores can be positively formed in the composite material as by loading a filler having a higher swelling factor than the polymer or by effecting post treatment such as heating, stretching or crosslinking.
It is not permissible that the SPE is significantly reduced in strength by forming a composite material with the filler. In this respect, loading the polymer with a larger amount of a low strength filler is not practical. Loading the polymer with a larger amount of a filler having a low bonding force to the polymer results in SPE suffering from a loss of strength. Poor dispersion of a filler markedly exacerbates the strength and outer appearance of SPE.
It is noted that the polymeric film of U.S. Pat. No. 5,418,091 has a low ability to absorb electrolytic solution, that is, a low swelling factor. It is also described in this patent that the absorbancy of the polymer is increased or decreased by mixing a filler such as alumina or silica, but the absorbancy achieved thereby are not fully satisfactory. Additionally, the plasticizer extraction method employed in this patent is seriously disadvantageous in production efficiency and mass scale production.
DETAILED DESCRIPTION OF THE INVENTION
The solid polymer electrolyte (often abbreviated as SPE) of the invention is a matrix of a polymer and a filler containing an electrolytic solution. The SPE preferably has a swelling factor of at least 2.2.
Polymer
The polymer used herein is not critical. Preferably the polymer itself can be impregnated with some amount of an electrolytic solution because the SPE is subsequently impregnated with various electrolytic solutions for electrochemical devices. The intended use in electrochemical application requires that the polymer be stable against voltage and have satisfactory thermal and mechanical properties. For this rea
Kuwahara Tsuneo
Maruyama Satoshi
Ohe Kazuhide
Kalafut Stephen
TDK Corporation
Wills M.
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