Hybrid electrolyte, method for manufacturing the same, and method for manufacturing electrochemical element using the same

Disclosed is a hybrid electrolyte comprising a shaped porous polymer structure comprising a polymer matrix and a plurality of cells dispersed in the polymer matrix, the polymer matrix containing a crosslinked polymer segment and having a gel content in the range of from 20 to 75%, wherein the shaped porous polymer structure is impregnated and swelled with an electrolytic liquid. A method for producing the hybrid electrolyte and a method for producing an electrochemical device comprising the hybrid electrolyte are also disclosed. The hybrid electrolyte of the present invention has a high ionic conductivity, an excellent stability under high temperature conditions and an excellent adherability to an electrode. Further, by the method of the present invention, the hybrid electrolyte having the above-mentioned excellent properties and an electrochemical device comprising such a hybrid electrolyte can be surely and effectively produced.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a novel hybrid electrolyte. More
 particularly, the present invention is concerned with a novel hybrid
 electrolyte comprising a shaped porous polymer structure comprising a
 polymer matrix and a plurality of cells dispersed in the polymer matrix,
 the polymer matrix containing a crosslinked polymer segment and having a
 specific gel content, wherein the shaped porous polymer structure is
 impregnated and swelled with an electrolytic liquid. The present invention
 is also concerned with a method for producing the hybrid electrolyte and a
 method for producing an electrochemical device comprising the hybrid
 electrolyte.
 The hybrid electrolyte of the present invention has a high ionic
 conductivity, an excellent stability under high temperature conditions and
 an excellent adherability to an electrode, so that the hybrid electrolyte
 of the present invention can be advantageously used as an electrolyte for
 various electrochemical devices, such as primary and secondary batteries
 (e.g., a lithium battery), a photoelectrochemical device and an
 electrochemical sensor. Further, by the method of the present invention,
 the hybrid electrolyte having the above-mentioned excellent properties and
 an electrochemical device comprising the same can be surely and
 effectively produced.
 2. Prior Art
 Recently, for reducing the size and weight of portable equipments, such as
 pocket telephones and personal computers, there has been a demand for a
 battery having high energy density. As a battery for meeting such a
 demand, lithium ion batteries have been developed. This type of battery
 has a structure in which a porous separator is disposed between the
 positive and negative electrodes, wherein the porous separator is not
 swelled with an electrolytic liquid. For preventing a leakage of the
 electrolytic liquid used for impregnating the separator, the commercially
 produced battery of this type has a battery structure wholly packaged in a
 very strong metallic casing having a large thickness.
 On the other hand, so-called solid type batteries produced using a solid
 electrolyte functioning not only as an electrolyte but also as a separator
 are advantageously free from the danger of leakage of an electrolytic
 liquid. Therefore, it is expected that not only is a solid electrolyte
 useful for providing a battery having improved reliability and safety, but
 is also advantageous in that both the lamination of a solid electrolyte
 onto electrodes and the packaging of the resultant laminate to form a
 battery can be easily performed, wherein the thickness and weight of the
 battery can be reduced. Especially, a polymeric solid electrolyte
 comprising an ion-conductive polymer has excellent flexibility for
 processing and, therefore, not only can a laminate structure composed of
 the polymeric solid electrolyte and electrodes be easily produced, but
 also the polymeric solid electrolyte is capable of changing its morphology
 at an interface between the electrolyte and the electrodes in accordance
 with the volumetric change of the electrodes caused by the occlusion and
 release of ions by the electrodes, enabling the interface of the polymeric
 solid electrolyte to intimately fit over the electrodes without suffering
 delamination from the electrodes.
 As such a polymeric solid electrolyte, an alkali metal salt complex of
 polyethylene oxide was proposed by Wright in British Polymer Journal,
 vol.7, p.319 (1975). Since then, researches on various skeletal materials
 for polymeric solid electrolytes have been energetically conducted.
 Examples of such skeletal materials include polyethers, such as
 polyethylene oxide and polypropylene oxide, polyphosphazene and
 polysiloxane. Generally, polymeric solid electrolytes are provided in the
 form of solid solutions of a solid electrolyte in a polymeric solid,
 wherein the solid electrolyte is considered to be uniformly dissolved in
 the polymeric solid, and are known as dry type polymeric solid
 electrolytes. However, these polymeric solid electrolytes have a problem
 in that the ionic conductivity of them is extremely low as compared to
 that of an electrolytic liquid. Therefore, a battery produced using such a
 polymeric solid electrolyte has problems in that it has a low
 charge/discharge current density and has a high resistance.
 For solving these problems, various attempts to improve the ionic
 conductivity of a polymeric solid electrolyte have been proposed, wherein
 the condition of the solid electrolyte is rendered similar to the
 condition of the electrolyte in the electrolytic liquid. For example,
 gelled solid electrolytes are known which are obtained by adding a solvent
 for the electrolyte (which solvent is capable of dissolving an electrolyte
 to form an electrolytic liquid) as a plasticizer to a polymer matrix so
 that the solvent and the polymer matrix together form a gel, wherein the
 solvent is used for increasing the dissociation of the electrolyte and
 promoting the molecular movement of the polymer, so that the ionic
 conductivity of the electrolyte can be increased (see, for example,
 Japanese Patent Application Laid-Open Specification No. 57-143356). As an
 example of such a gelled solid electrolyte, U.S. Pat. No. 5,296,318
 discloses a gelled solid electrolyte obtained by adding an electrolytic
 liquid to a vinylidene fluoride polymer so that the electrolytic liquid
 and the polymer together form a gel. Further, U.S. Pat. No. 5,429,891
 discloses a gelled solid electrolyte obtained by adding an electrolytic
 liquid to a crosslinked vinylidene fluoride polymer to thereby swell the
 crosslinked polymer so that the electrolytic liquid and the crosslinked
 polymer together form a gel. In general, when a battery comprising such a
 gelled solid electrolyte (i.e., a so-called hybrid electrolyte) is
 produced, a hybrid electrolyte comprising a crosslinked polymer swelled
 with an electrolytic liquid is produced, and then, a battery is assembled
 using the swelled hybrid electrolyte, electrodes, etc. With respect to the
 polymer matrix of such a hybrid electrolyte, a crosslinked polymer can be
 used. On the other hand, a method for producing a battery comprising a
 hybrid electrolyte layer is also known, wherein the hybrid electrolyte
 layer is formed by coating electrodes for the battery with a solution
 obtained by dissolving a non-crosslinked polymer, an electrolyte and a
 plasticizer in a low boiling point solvent, followed by removing the
 solvent by evaporation (see U.S. Pat. No. 5,296,318). Each of these
 materials is electrochemically stable and has a high ionic conductivity,
 as compared to that of a conventional dry type solid electrolyte. However,
 the ionic conductivity of each of the above-mentioned hybrid electrolytes
 is still unsatisfactory, as compared to that of an electrolytic liquid.
 Further, a non-porous polymer matrix is used for each of the
 above-mentioned conventional hybrid electrolytes. Hence, the capacities of
 the batteries comprising such conventional hybrid electrolytes are
 disadvantageously low.
 As a hybrid electrolyte having a high ionic conductivity, a material has
 been proposed, which comprises a gel phase (comprising a polymer and an
 electrolytic liquid) and a liquid phase (comprising an electrolytic
 liquid), wherein the liquid phase is dispersed in the gel phase. For
 example, Unexamined Japanese Patent Application Laid-Open Specification No
 8-250127 describes the use of a vinylidene fluoride porous polymer sheet
 as a polymer matrix of a solid electrolyte. In this document, a
 description is made with respect to a method for impregnating a porous
 polymer sheet with an electrolytic liquid under high temperature
 conditions, to thereby form a hybrid electrolyte (comprising the porous
 polymer sheet impregnated and swelled with the electrolytic liquid), which
 is similar to the hybrid electrolyte of the present invention. Further,
 Unexamined Japanese Patent Application Laid-Open Specification No.
 6-150939 discloses a method for producing a hybrid (solid) electrolyte, in
 which a porous structure comprising a crosslinked polymer containing polar
 units is used as a matrix for the hybrid (solid) electrolyte. However, in
 the method described in these documents, in order to retain an
 electrolytic liquid in the matrix, a crosslinked porous polymer sheet as
 the matrix is immersed in an excess amount of an electrolytic liquid under
 conditions at which a non-crosslinked polymer segment contained in the
 crosslinked porous polymer sheet can be dissolved in the electrolytic
 liquid. In the hybrid electrolyte thus obtained, the non-crosslinked
 polymer segment contained in the crosslinked porous polymer sheet (which
 segment is capable of imparting the resultant electrolyte with an
 adherability to electrodes) is dissolved into the electrolytic liquid
 during the immersion, so that the adherence strength of the resultant
 electrolyte to electrodes disadvantageously becomes low.
 Further, Unexamined Japanese Patent Application Laid-Open Specification No.
 8-195220 discloses a method for producing a hybrid electrolyte comprising
 a porous polymer matrix, which comprises dispersing a non-crosslinked
 polyacrylonitrile in an electrolytic liquid to thereby obtain a
 dispersion; coating a stainless steel substrate with the obtained
 dispersion and heating the dispersion coated on the stainless steel
 substrate to dissolve the non-crosslinked polyacrylonitrile (which is
 contained in the dispersion coated on the stainless steel substrate) into
 the electrolytic liquid, to thereby form a homogeneous solution; cooling
 the thus formed solution on the stainless steel substrate to thereby form
 a hybrid electrolyte layer comprising a polymer matrix comprising the
 non-crosslinked polyacrylonitrile and the electrolytic liquid retained in
 the polymer matrix; and pricking holes in the hybrid electrolyte layer (by
 means of a thin stainless needle) in a condition where the hybrid
 electrolyte layer is immersed in a solution of an electrolyte, so that the
 polymer matrix of the hybrid electrolyte layer is rendered porous, to
 thereby obtain a hybrid electrolyte comprising the porous polymer matrix
 and the electrolytic liquid contained therein. However, in this method, it
 is required to dissolve a polyacrylonitrile into an electrolytic liquid
 and, therefore, it is required to use a non-crosslinked polyacrylonitrile.
 Further, in this method, it is difficult to introduce a crosslinked
 structure into the polyacrylonitrile constituting the polymer matrix of
 the hybrid electrolyte obtained by this method Therefore, the
 non-crosslinked polyacrylonitrile constituting the porous polymer matrix
 of the hybrid electrolyte is likely to be dissolved into the electrolytic
 liquid or fused under high temperature conditions, so that there is
 disadvantageously a danger that the hybrid electrolyte obtained by this
 method suffers distortion, thereby causing shutting or short-circuiting of
 the pores of the hybrid electrolyte.
 Further, each of the above-mentioned various types of hybrid electrolytes
 is constructed with a polymer which is already swelled with an
 electrolytic liquid, so that the mechanical strength of the electrolyte is
 disadvantageously low and, hence, it is not easy to handle the hybrid
 electrolyte for laminating the hybrid electrolyte onto electrodes in the
 assembling of a battery. In particular, it is extremely difficult to
 produce the above-mentioned hybrid electrolyte in the form of a thin sheet
 so as to increase the energy density of the hybrid electrolyte. With
 respect to the method comprising coating an electrolyte with a solution of
 a polymer and an electrolyte in a solvent, the handling of the electrolyte
 is easy However, from the viewpoint of safety, this method is not
 preferred because a low boiling point solvent which is combustible, such
 as THF, is used.
 On the other hand, an attempt to prevent the electrolytic liquid in a solid
 electrolyte from leakage has been proposed, wherein a liquid ion conductor
 is filled in the pores of a porous polymer sheet of the solid electrolyte
 so that it can be retained in the porous polymer sheet by the capillary
 action. For example, a microporous polymer sheet made of a material having
 a high mechanical strength, such as a polyolefin, and having a
 through-hole diameter of 0.1 mm or less is provided, and the pores of the
 microporous polymer sheet are filled up with an ion transferring medium to
 thereby form a thin electrolyte sheet (Unexamined Japanese Patent
 Application Laid-Open Specification No. 1-158051). With respect to the
 solid electrolyte of this type, the mechanical strength is large; however,
 a large number of pores in the microporous polymer sheet form complicated
 labyrinthian passages and, therefore, ions have to pass through the
 electrolytic liquid phase in such complicated labyrinthian passages, so
 that the above-mentioned solid electrolyte has a defect in that the ionic
 conductivity thereof is disadvantageously low.
 SUMMARY OF THE INVENTION
 In this situation, the present inventors have made extensive and intensive
 studies toward developing a hybrid electrolyte which is free from
 difficult problems accompanying the above-mentioned prior art techniques
 and has not only a high ionic conductivity, but also an excellent
 stability under high temperature conditions and an excellent adherability
 to an electrode, and a method for surely and effectively producing the
 hybrid electrolyte and an electrochemical device, such as a battery,
 comprising the hybrid electrolyte. As a result, it has unexpectedly been
 found that a hybrid electrolyte, which comprises a shaped porous polymer
 structure comprising a polymer matrix and a plurality of cells dispersed
 in the polymer matrix, wherein the polymer matrix contains a crosslinked
 polymer segment and has a gel content in the range of from 20 to 75%, and
 wherein the polymer matrix is impregnated and swelled with an electrolytic
 liquid selected from the group consisting of a solution of an electrolyte
 in water or a non-aqueous solvent and a liquid electrolyte, exhibits a
 high ionic conductivity, an excellent stability under high temperature
 conditions and an excellent adherability to an electrode.
 Further, it has also been found that the above-mentioned hybrid electrolyte
 can be surely and effectively produced by a method which comprises
 impregnating the above-mentioned shaped porous polymer structure with the
 above-mentioned electrolytic liquid under predetermined non-swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is substantially insusceptible to swelling with the electrolytic
 liquid, thereby obtaining an impregnated, shaped porous polymer structure;
 and holding the impregnated, shaped porous polymer structure under
 predetermined swelling temperature and pressure conditions at which the
 shaped porous polymer structure is susceptible to swelling with the
 electrolytic liquid.
 Still further, it has unexpectedly been found that an electrochemical
 device comprising the above-mentioned hybrid electrolyte can be surely and
 effectively produced by a method which comprises impregnating the
 above-mentioned shaped porous polymer structure with the above-mentioned
 electrolytic liquid under predetermined non-swelling temperature and
 pressure conditions at which the shaped porous polymer structure is
 substantially insusceptible to swelling with the electrolytic liquid,
 thereby obtaining an impregnated, shaped porous polymer structure;
 laminating the impregnated, shaped porous polymer structure to an
 electrode to thereby obtain a laminate structure; and holding the laminate
 structure under predetermined swelling temperature and pressure conditions
 at which the shaped porous polymer structure is susceptible to swelling
 with the electrolytic liquid.
 Furthermore, it has unexpectedly been found that an electrochemical device
 comprising the above-mentioned hybrid electrolyte can be surely and
 effectively produced by a method which comprises laminating the
 above-mentioned shaped porous polymer structure to an electrode to thereby
 obtain a laminate structure; impregnating the laminate structure with the
 above-mentioned electrolytic liquid under predetermined non-swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is substantially insusceptible to swelling with the electrolytic
 liquid; and holding the impregnated laminate structure under predetermined
 swelling temperature and pressure conditions at which the shaped porous
 polymer structure is susceptible to swelling with the electrolytic liquid.
 The present-invention is completed based on the above-mentioned findings.
 Therefore, it is an object of the present invention to provide a hybrid
 electrolyte which exhibits a high ionic conductivity, an excellent
 stability under high temperature conditions and an excellent adherability
 to an electrode.
 It is another object of the present invention to provide a method for
 surely and effectively producing a hybrid electrolyte having the
 above-mentioned properties.
 It is still another object of the present invention to provide a method for
 surely and effectively producing an electrochemical device, such as a
 battery, comprising a hybrid electrolyte having the above-mentioned
 properties.
 The foregoing and other objects, features and advantages of the present
 invention will be apparent to those skilled in the art from the following
 detailed description and appended claims.
 DETAILED DESCRIPTION OF THE INVENTION
 In an essential aspect of the present invention, there is provided a hybrid
 electrolyte which comprises a shaped porous polymer structure comprising a
 polymer matrix and a plurality of cells dispersed in the polymer matrix,
 the polymer matrix containing a crosslinked polymer segment and having a
 gel content in the range of from 20 to 75%; and an electrolytic liquid
 selected from the group consisting of a solution of an electrolyte in
 water or a non-aqueous solvent and a liquid electrolyte, wherein the
 shaped porous polymer structure is impregnated and swelled with the
 electrolytic liquid.
 For easy understanding of the present invention, the essential features and
 various embodiments of the present invention are enumerated below.
 1. A hybrid electrolyte comprising:
 a shaped porous polymer structure comprising a polymer matrix and a
 plurality of cells dispersed in the polymer matrix, the polymer matrix
 containing a crosslinked polymer segment and having a gel content in the
 range of from 20 to 75%, and
 an electrolytic liquid selected from the group consisting of a solution of
 an electrolyte in water or a non-aqueous solvent and a liquid electrolyte,
 wherein the shaped porous polymer structure is impregnated and swelled with
 the electrolytic liquid
 2. The hybrid electrolyte according to item 1 above, wherein the polymer
 matrix has a gel content in the range of from 30 to 70% by weight.
 3. The hybrid electrolyte according to item 1 above, wherein the polymer
 matrix has a gel content in the range of from 35 to 65% by weight
 4. The hybrid electrolyte according to any one of items 1 to 3 above,
 wherein the cells of the polymer matrix comprise open cells which form
 through-holes passing through the shaped porous polymer structure.
 5. The hybrid electrolyte according to any one of items 1 to 4 above,
 wherein the shaped porous polymer structure has a void ratio of from 30 to
 95%.
 6. The hybrid electrolyte according to any one of items 1 to 5 above, which
 is in the form of a sheet having a thickness of from 1 to 500 .mu.m.
 7. The hybrid electrolyte according to any one of items 1 to 6 above,
 wherein the polymer matrix comprises a vinylidene fluoride polymer or an
 acrylonitrile polymer.
 8. The hybrid electrolyte according to any one of items 1 to 7 above,
 wherein the crosslinked polymer segment has a crosslinked structure formed
 by electron beam irradiation or .gamma.-ray irradiation.
 9. A method for producing a hybrid electrolyte, which comprises:
 impregnating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix contains a crosslinked polymer segment, with an
 electrolytic liquid selected from the group consisting of a solution of an
 electrolyte in water or a non-aqueous solvent and a liquid electrolyte
 under predetermined non-swelling temperature and pressure conditions at
 which the shaped porous polymer structure is substantially insusceptible
 to swelling with the electrolytic liquid, thereby obtaining an
 impregnated, shaped porous polymer structure; and
 holding the impregnated, shaped porous polymer structure under
 predetermined swelling temperature and pressure conditions at which the
 shaped porous polymer structure is susceptible to swelling with the
 electrolytic liquid.
 10. The method according to item 9 above, wherein the cells of the polymer
 matrix comprise open cells which form through-holes passing through the
 shaped porous polymer structure.
 11. The method according to item 9 or 10 above, wherein the polymer matrix
 comprises a vinylidene fluoride polymer or an acrylonitrile polymer.
 12. The method according to any one of items 9 to 11 above, wherein the
 electrolytic liquid is selected from the group consisting of a solution of
 an electrolyte in a non-aqueous solvent and a liquid electrolyte.
 13. A method for producing a hybrid electrolyte, which comprises:
 impregnating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix comprises a vinylidene fluoride polymer or an
 acrylonitrile polymer and contains a crosslinked polymer segment, with an
 electrolytic liquid selected from the group consisting of a solution of an
 electrolyte in a non-aqueous solvent and a liquid electrolyte at a
 temperature of 35.degree. C. or less under atmospheric pressure, thereby
 obtaining an impregnated, shaped porous polymer structure; and
 heating the impregnated, shaped porous polymer structure at a temperature
 of 80.degree. C. or more under atmospheric pressure.
 14. The method according to item 13 above, wherein the cells of the polymer
 matrix comprise open cells which form through-holes passing through the
 shaped porous polymer structure.
 15. The method according to item 13 or 14 above, wherein the impregnated,
 shaped polymer structure is heated at a temperature of 90.degree. C. or
 more.
 16. A hybrid electrolyte which is substantially the same as that produced
 by the method of any one of claims 9 to 15.
 17. A method for producing an electrochemical device, which comprises:
 impregnating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix contains a crosslinked polymer segment, with an
 electrolytic liquid selected from the group consisting of a solution of an
 electrolyte in water or a non-aqueous solvent and a liquid electrolyte
 under predetermined non-swelling temperature and pressure conditions at
 which the shaped porous polymer structure is substantially insusceptible
 to swelling with the electrolytic liquid, thereby obtaining an
 impregnated, shaped porous polymer structure;
 laminating the impregnated, shaped porous polymer structure to an electrode
 to thereby obtain a laminate structure; and
 holding the laminate structure under predetermined swelling temperature and
 pressure conditions at which the shaped porous polymer structure is
 susceptible to swelling with the electrolytic liquid.
 18. The method according to item 17 above, wherein the polymer matrix
 comprises a vinylidene fluoride polymer or an acrylonitrile polymer.
 19. A method for producing an electrochemical device, which comprises:
 impregnating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix comprises a vinylidene fluoride polymer or an
 acrylonitrile polymer and contains a crosslinked segments, with an
 electrolytic liquid selected from the group consisting of a solution of an
 electrolyte in a non-aqueous liquid and a liquid electrolyte at a
 temperature of 35.degree. C. or less under atmospheric pressure, thereby
 obtaining an impregnated, shaped porous polymer structure;
 laminating the impregnated, shaped porous polymer structure to an electrode
 to thereby obtain a laminate structure; and
 heating the laminate structure at a temperature of 80.degree. C. or more
 under atmospheric pressure.
 20. The method according to item 19 above, wherein the laminate structure
 is heated at 90.degree. C. or more.
 21. A method for producing an electrochemical device, which comprises:
 laminating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix contains a crosslinked polymer segment, to an electrode
 to thereby obtain a laminate structure;
 impregnating the laminate structure with an electrolytic liquid selected
 from the group consisting of a solution of an electrolyte in water or a
 non-aqueous solvent and a liquid electrolyte under predetermined
 non-swelling temperature and pressure conditions at which the shaped
 porous polymer structure is substantially insusceptible to swelling with
 the electrolytic liquid, thereby obtaining an impregnated laminate
 structure; and
 holding the impregnated laminate structure under predetermined swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is susceptible to swelling with the electrolytic liquid.
 22. The method according to item 21 above, wherein the polymer matrix
 comprises a vinylidene fluoride polymer or an acrylonitrile polymer.
 23. A method for producing an electrochemical device, which comprises:
 laminating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix comprises a vinylidene fluoride polymer or an
 acrylonitrile polymer and contains a crosslinked polymer segment, to an
 electrode to thereby obtain a laminate structure;
 impregnating the laminate structure with an electrolytic liquid selected
 from the group consisting of a solution of an electrolyte in a non-aqueous
 solvent and a liquid electrolyte at a temperature of 35.degree. C. or less
 under atmospheric pressure, thereby obtaining an impregnated laminate
 structure; and
 heating the impregnated laminate structure at a temperature of 80.degree.
 C. or more under atmospheric pressure.
 24. The method according to item 23 above, wherein the impregnated laminate
 structure is heated at a temperature of 90.degree. C. or more.
 25. The method according to any one of items 17 to 24 above, wherein the
 electrochemical device is a battery comprising a positive electrode and a
 negative electrode.
 26. The method according to item 25 above, wherein the battery is a
 non-aqueous battery.
 27. The method according to item 26 above, wherein the battery is a lithium
 ion secondary battery
 28. The method according to any one of items 17 to 27 above, wherein the
 electrochemical device has an electrode having a current collector and
 wherein the current collector is a mesh current collector.
 29. An electrochemical device, which is substantially the same as that
 produced by the method of any one of items 17 to 28 above.
 The hybrid electrolyte of the present invention has intermediate properties
 as between a dry type solid electrolyte (containing no liquid) and a
 conventional liquid electrolyte (i.e., an electrolytic liquid obtained by
 dissolving an electrolyte in water or a non-aqueous solvent). That is, the
 hybrid electrolyte of the present invention is a polymeric solid
 electrolyte containing a solvent, preferably a high boiling point solvent,
 in a large amount, wherein it is free from a danger that the polymer
 moiety of the electrolyte does not flow out from the electrolyte as a
 solution of the polymer in the solvent. The hybrid electrolyte of the
 present invention assumes a gel form despite containing an electrolytic
 liquid in a large amount, so that the retention of the electrolytic liquid
 in the solid electrolyte is increased. More specifically, the hybrid
 electrolyte of the present invention is a hybrid electrolyte which
 comprises a shaped porous polymer structure comprising a polymer matrix
 and a plurality of cells dispersed in the polymer matrix, the polymer
 matrix containing a crosslinked polymer segment and having a gel content
 in the range of from 20 to 75%; and an electrolytic liquid selected from
 the group consisting of a solution of an electrolyte in water or a
 non-aqueous solvent and a liquid electrolyte, wherein the shaped porous
 polymer structure is impregnated and swelled with the electrolytic liquid.
 Hereinbelow, a description is made with respect to the term "swelling" in
 the present invention. In general, when a crosslinked polymer is swelled
 with an excess amount of a solvent or solution, the volume of the polymer
 is drastically increased. In many cases, the resultant swelled,
 crosslinked polymer is expanded in all directions. However, when the
 crosslinked polymer has sustained a stress due, for example, to a
 stretching, the stress is relieved by the swelling and, in some specific
 directions, it is sometimes possible for the crosslinked polymer to shrink
 by the swelling. In any case, when a crosslinked polymer is swelled with
 an excess amount of a solvent or solution, the crosslinked polymer is
 likely to suffer large dimensional change. In the present invention, after
 immersing the shaped porous polymer structure in an electrolytic liquid
 under predetermined temperature and pressure conditions (wherein the
 volume of the electrolytic liquid is 100 times or more as large as the
 volume of the outer profile of the shaped porous polymer structure), the
 change (%) in the longitudinal length of the shaped porous polymer
 structure, relative to the longitudinal length measured before the
 immersion, is determined. If the change exceeds 10%, the shaped porous
 polymer structure is defined as being susceptible to swelling with the
 electrolytic liquid under the above-mentioned predetermined conditions.
 On the other hand, whether or not the shaped porous polymer structure of a
 produced hybrid electrolyte has been impregnated and swelled with an
 electrolytic liquid, can be determined by a method in which the
 impregnated electrolytic liquid is removed from the produced hybrid
 electrolyte by extraction, followed by drying at room temperature to
 obtain the shaped porous polymer structure, and the degree of shrinkage of
 the obtained shaped porous polymer structure is determined. Specifically,
 in the present invention, the degree of shrinkage of a shaped porous
 polymer structure is obtained as follows. A produced hybrid electrolyte is
 immersed in a solvent, which is capable of extracting the electrolytic
 liquid but not capable of dissolving the polymer matrix, for 30 minutes or
 more to thereby extraction-remove the impregnated electrolytic liquid from
 the hybrid electrolyte, followed by drying in vacuum to thereby obtain a
 dried polymer matrix. The change in the longitudinal length of the
 above-obtained dried polymer matrix (shaped porous polymer structure),
 relative to the longitudinal length of the electrolytic liquid-impregnated
 hybrid electrolyte, is determined. This change (%) is defined as the
 degree of shrinkage of the shaped porous polymer structure. In the present
 invention, the hybrid electrolyte, which exhibits the above-defined degree
 of shrinkage of the shaped porous polymer structure of more than 10%, is
 defined as being impregnated and swelled with an electrolytic liquid.
 In general, a hybrid electrolyte, which is used as an electrolyte for the
 so-called polymer battery (i.e., a battery comprising a solid electrolyte
 containing a polymer), is in the form of a sheet or the like. In this
 case, it is difficult to mechanically keep the hybrid electrolyte in
 contact with electrodes and, therefore, the hybrid electrolyte is required
 to be adhered to electrodes. The hybrid electrolyte is generally adhered
 to electrodes by heating the hybrid electrolyte to a temperature in the
 range of from approximately 50 to 200.degree. C. (or, some specific types
 of hybrid electrolytes are adhered to electrodes at room temperature)
 under a pressure in the range of from 0.1 to 20 kg/cm.sup.2. In connection
 with this adhesion, when the hybrid electrolyte is adhered to electrodes
 by heating, a portion of the polymer used in the hybrid electrolyte may be
 melted so as to function as an adhesive. The adherability of the hybrid
 electrolyte to the electrode varies, as described below, depending on the
 gel content of a polymer matrix of the shaped porous polymer structure in
 the hybrid electrolyte, wherein the polymer matrix contains a crosslinked
 polymer segment.
 The polymer matrix in the hybrid electrolyte of the present invention is
 required to have a gel content in the range of from 20 to 75%, wherein the
 upper limit of the gel content is preferably 70%, more preferably 65%, and
 the lower limit of the gel content is preferably 30%, more preferably 35%.
 When the gel content of the hybrid electrolyte is more than 75%, it is
 difficult to adhere the hybrid electrolyte to electrodes by heating, so
 that the adherence strength of the hybrid electrolyte to the electrodes is
 disadvantageously lowered. On the other hand, when the gel content of the
 hybrid electrolyte is less than 20%, the thermal stability of the hybrid
 electrolyte becomes low, so that the hybrid electrolyte is likely to
 suffer distortion during the heating.
 As described above, the hybrid electrolyte of the present invention
 comprises a shaped porous polymer structure comprising a polymer matrix
 and a plurality of cells dispersed in the polymer matrix, the polymer
 matrix being impregnated and swelled with an electrolytic liquid. The
 electrolytic liquid is present not only in the polymer matrix of the
 shaped porous polymer structure but also in the plurality of cells. The
 polymer matrix of the shaped porous polymer structure contains a
 crosslinked polymer segment which is introduced into the polymer by an
 appropriate crosslinking treatment. There is no limitation with respect to
 the type of the above-mentioned polymer to be subjected to the
 crosslinking treatment, as long as, after the crosslinking treatment, the
 polymer can be swelled with an electrolytic liquid. It is preferred that
 the polymer is electrochemically stable and exhibits a high
 ion-conductivity. Examples of such polymers include a poly(ethylene
 oxide), a polypropylene oxide, a vinylidene fluoride polymer, an
 acrylonitrile polymer, an oligo(ethylene oxide) poly(meth)acrylate, a
 poly(ethylene imine), a poly alkylene sulfide, a polyphosphazene and a
 polysiloxane each having an oligo(ethylene oxide) side chain, polymers
 each having ionic groups in the molecule, such as Nation (manufactured and
 sold by Du Pont, U.S.A.), Flemion (manufactured and sold by Asahi Glass
 Co., Ltd., Japan) and the like. A vinylidene fluoride polymer and an
 acrylonitrile polymer include not only a homopolymer but also a copolymer.
 For example, a vinylidene fluoride-hexafluoropropylene copolymer, a
 vinylidene fluoride-trifluoroethylene copolymer, an
 acrylonitrile-(meth)acrylate copolymer, an acrylonitrile-styrene copolymer
 and the like can also be used. When a polymer having ionic groups in the
 molecule is used for producing a lithium ion battery, each of the ionic
 groups is preferably in the form of a lithium salt. Among these polymers,
 a vinylidene fluoride polymer, such as a poly(vinylidene fluoride), a
 vinylidene fluoride-hexafluoropropylene copolymer or the like, and an
 acrylonitrile polymer, such as polyacrylonitrile, an
 acrylonitrile-(meth)acrylate copolymer, an acrylonitrile-styrene copolymer
 or the like are preferred, due to the high ionic conductivity and
 excellent mechanical strength thereof. A vinylidene fluoride polymer is
 more preferred.
 The polymer is subjected to a crosslinking treatment and converted to a
 polymer containing a crosslinked polymer segment. Introduction of the
 crosslinked structure into the polymer enhances the stability of a hybrid
 electrolyte under high temperature conditions. If the polymer is not
 crosslinked, when a battery comprising the hybrid electrolyte experiences
 high temperature conditions, the performance of the battery is likely to
 irreversibly change, or occasionally, a short-circuiting of the battery
 occurs due to the melting of the polymer. The introduction of the
 crosslinked structure can be conducted at any stage, for example, during
 polymerization, or before or after the shaping of a polymer for producing
 the shaped porous polymer structure. The crosslinked structure can be
 introduced into a shaped porous polymer structure which is swelled with an
 electrolytic liquid, a plasticizer and the like. Examples of methods for
 crosslinking treatment include a method in which a crosslinked structure
 is formed by conducting the polymerization of the monomer (and comonomer)
 in the presence of an additional multifunctional monomer; a method in
 which a crosslinked structure is formed by the irradiation of a radiation
 energy, such as electron beams, .gamma.-rays, X-rays, ultraviolet rays or
 the like after polymerization; and a method in which a crosslinked
 structure is formed by introducing a radical initiator into a polymer
 after the polymerization for producing the polymer, and heating or
 irradiating (with radiation energy) the polymer containing the radical
 initiator, to thereby effect a reaction for crosslinking. When the
 introduction of the crosslinked structure into the polymer is conducted
 after the polymerization for producing the polymer, it can be conducted in
 the presence of monofunctional and/or multifunctional monomer(s) which
 is/or are newly added. Among these methods for crosslinking treatment,
 from a viewpoint of decreasing the amount of any remaining impurities or
 unreacted functional groups, a method in which a crosslinked structure is
 formed by the irradiation of a radiation energy, such as electron beams,
 .gamma.-rays, X-rays, ultraviolet rays or the like after polymerization is
 preferred. Further, it is more preferred that the radiation energy is
 electron beams or .gamma.-rays.
 By the above-mentioned crosslinking treatment, a crosslinked polymer
 segment is introduced into the polymer, so that a polymer containing a
 crosslinked polymer segment is obtained. With respect to the shaped porous
 polymer structure comprising a polymer matrix containing a crosslinked
 polymer segment formed by the above-mentioned crosslinking treatment, even
 if the shaped porous polymer structure is immersed in an excess amount of
 the electrolytic liquid under conditions at which a polymer, if not
 crosslinked, would be completely dissolved into the electrolytic liquid,
 dissolution of the whole of the shaped porous polymer structure into the
 electrolytic liquid does not occur, but only swelling of the shaped porous
 polymer structure with the electrolytic liquid occurs. The degree of
 crosslinking of the polymer of the polymer matrix is represented by the
 gel content, which is obtained from the weight difference of the polymer
 matrix before and after the extraction-treatment of the polymer matrix
 with a good solvent for a polymer matrix which is not crosslinked. In this
 connection, it should be noted that, depending on the conditions at which
 the shaped porous polymer structure is impregnated and swelled with an
 electrolytic liquid for producing a hybrid electrolyte, it is possible
 that the gel content of the polymer matrix of the hybrid electrolyte is
 different from the gel content of the polymer matrix of the shaped porous
 polymer structure before being swelled with the electrolytic liquid.
 Therefore, it is necessary to determine the gel content of the polymer
 matrix with respect to the polymer matrix obtained from the hybrid
 electrolyte.
 An example of the method for determining the gel content of a polymer
 matrix of a hybrid electrolyte is as follows. That is, a produced hybrid
 electrolyte sheet impregnated and swelled with an electrolytic liquid is
 immersed in a solvent, which is capable of extracting the electrolytic
 liquid but not capable of dissolving the polymer matrix, for 30 minutes or
 more to thereby extraction-remove the impregnated electrolytic liquid from
 the hybrid electrolyte, followed by drying in vacuum to thereby obtain a
 dried polymer matrix. The weight (W.sub.1) of the dried polymer matrix is
 measured.
 Then, the dried polymer matrix is wrapped with a stainless steel wire mesh
 (150-mesh size) and heated in a solvent selected from good solvents for
 the polymer (before being subjected to crosslinking treatment) as used for
 producing the hybrid electrolyte for a predetermined period of time under
 reflux, followed by drying in vacuum, thereby obtaining an extraction
 residue. The weight (W.sub.2) of the extraction residue is measured. The
 gel content (%) of the polymer matrix is defined as a value obtained by
 the formula (W.sub.2 /W.sub.1).times.100. The solvent for the extraction
 under reflux is used in-a weight amount of 100 times or more as large as
 the weight of the dried polymer matrix. It is preferred that when the
 polymer (before being subjected to crosslinking treatment) is a vinylidene
 fluoride polymer, a mixed solvent of N,N-dimethylacetamide (DMAC) and
 acetone (volume ratio of DMAC to acetone=7:3) is used as the solvent, and
 that when the polymer (before being subjected to crosslinking treatment)
 is an acrylonitrile polymer, DMAC alone is used as the solvent. Further,
 in each of these cases, it is preferred that the reflux time is 2 hours or
 more and the drying is conducted at 70.degree. C. for 4 hours or more.
 The shaped porous polymer structure of the hybrid electrolyte of the
 present invention comprises a polymer matrix having a plurality of cells
 dispersed therein. Due to such a structure, not only does the hybrid
 electrolyte comprising a shaped porous polymer structure impregnated and
 swelled with an electrolytic liquid exhibit a high ionic conductivity, but
 also the shaped porous polymer structure is advantageously, easily swelled
 with the electrolytic liquid. The cells may be in the form of closed cells
 or open cells which form through-holes passing through the shaped porous
 polymer structure. However, for effectively achieving an easy swelling of
 the porous polymer structure with an electrolytic liquid, it is preferred
 that the cells are in the form of open cells which form through-holes
 passing through the shaped porous polymer structure.
 The void ratio of the shaped porous polymer structure is preferably in the
 range of from 30 to 95% When the void ratio is less than 30%, the ionic
 conductivity of the final hybrid electrolyte is unsatisfactory. The void
 ratio is preferably 40% or more, more preferably 50% or more and still
 more preferably 55% or more. On the other hand, when the void ratio is
 more than 95%, the mechanical strength of the hybrid electrolyte after
 being swelled with an electrolytic liquid is unsatisfactory. The void
 ratio is preferably 90% or less, more preferably 85% or less and still
 more preferably 80% or less.
 Further, it is preferred that the content of the electrolytic liquid in the
 hybrid electrolyte of the present invention is in the range of from 30 to
 95% by weight, based on the weight of the hybrid electrolyte When the
 content of the electrolytic liquid is less than 30% by weight, the ionic
 conductivity of the hybrid electrolyte is unsatisfactory. The content of
 the electrolytic liquid is preferably 40% by weight or more, more
 preferably 50% by weight or more and still more preferably 55% by weight
 or more. On the other hand, when the content of the electrolytic liquid is
 more than 95% by weight, the mechanical strength of the hybrid electrolyte
 is unsatisfactory. The content of the electrolytic liquid is preferably
 90% by weight or less, more preferably 85% by weight or less, still more
 preferably 80% by weight or less.
 The void ratio of the shaped porous polymer structure can be obtained by
 the method comprising filling the voids of the shaped porous polymer
 structure with a non-solvent for the polymer matrix, followed by
 determining the weight of the non-solvent filling into the voids.
 Specifically, when the polymer matrix is produced using, for example, a
 vinylidene fluoride polymer or an acrylonitrile polymer, the void ratio of
 the shaped porous polymer structure can be obtained as follows.
 First, the weight (on a dry basis)(A) of a shaped porous polymer structure
 is measured. Next, the shaped porous polymer structure is immersed in
 ethanol to thereby render hydrophilic the polymer structure. Subsequently,
 the resultant hydrophilic polymer structure is immersed in water, thereby
 replacing the impregnated ethanol by water. The water on the surface of
 the polymer structure is removed by wiping, and then, the weight (B) of
 the resultant water-wiped polymer structure is measured. From the weights
 (A) and (B) obtained above, and the true specific gravity (d) of the
 material which the shaped porous polymer structure is made of, the void
 ratio of the shaped porous polymer structure is calculated according to
 the following formula:
EQU void ratio (%)=[(B-A)/(A/d+B-A)].times.100.
 The form of the hybrid electrolyte of the present invention varies
 depending on the use thereof. However, when the hybrid electrolyte is
 sandwiched between the electrodes and used as an electrolyte for the
 above-mentioned so-called polymer battery, it is preferred that the hybrid
 electrolyte is in the form of a sheet, a woven fabric, or a nonwoven
 fabric. In this case, the thickness of the hybrid electrolyte in the form
 of a sheet is generally in the range of from 1 to 500 .mu.m, preferably
 from 10 to 300 .mu.m, more preferably from 20 to 150 .mu.m. It is
 preferred that the shaped porous polymer structure in the form of a sheet,
 which is used for producing the above-mentioned hybrid electrolyte in the
 form of a sheet, has a thickness in the same range as mentioned above.
 When the thickness of the hybrid electrolyte or shaped porous polymer
 structure in the form of a sheet is less than 1 .mu.m, the mechanical
 strength of hybrid electrolyte or shaped porous polymer structure is
 unsatisfactory. Further, when such a hybrid electrolyte is laminated onto
 an electrode to thereby obtain a battery, a short-circuiting of the
 obtained battery between the electrodes is likely to occur. On the other
 hand, when the thickness of the hybrid electrolyte or shaped porous
 polymer structure in the form of a sheet is more than 500 .mu.m, the
 effective electric resistance as a hybrid electrolyte is disadvantageously
 high and, for example, when such a hybrid electrolyte is used as an
 electrolyte for a polymer battery, the energy density per volume is
 extremely low.
 There is no particular limitation with respect to the methods for producing
 the shaped porous polymer structure used in the present invention As a
 method for producing a shaped porous polymer structure comprising open
 cells which form through-holes passing through the shaped porous polymer
 structure, a conventional method for producing a microfilter or
 ultrafilter can be employed. Examples of such methods include the methods
 described in Unexamined Japanese Patent Application Laid-Open
 Specification No. 3-215535, Examined Japanese Patent Application
 Publication No. 61-38207 and Unexamined Japanese Patent Application
 Laid-Open Specification No. 54-16382. Examples of such methods include the
 fusing method and the wet method The fusing method is a method for
 producing a shaped porous polymer structure in the form of a sheet, which
 comprises fusing a polymer together with a plasticizer, an inorganic
 particulate and the like to thereby obtain a fused polymer; shaping the
 obtained fused polymer into a sheet to thereby obtain a shaped polymer
 structure in the form of a sheet; and extraction-removing the plasticizer,
 the inorganic particulate and the like contained in the shaped polymer
 structure to thereby obtain a desired, shaped porous polymer structure On
 the other hand, the wet method is another method for producing a shaped
 porous polymer structure in the form of a sheet, which comprises
 dissolving a polymer into a solvent together with a surfactant, an
 additive and the like to thereby obtain a solution; casting the obtained
 solution into a liquid film; and immersing the liquid film in a
 non-solvent for the polymer so as to solidify the film and so as to remove
 the solvent, surfactant, additive and the like contained in the liquid
 film, to thereby obtain a desired shaped porous polymer structure.
 Further, examples of methods for producing a shaped porous polymer
 structure containing closed cells include a method comprising shaping a
 polymer containing a foaming agent to obtain a shaped structure having
 dispersed therein the foaming agent; and heating the shaped structure or
 holding the shaped structure under reduced pressure so as to form closed
 cells in the shaped structure, thereby obtaining a shaped porous polymer
 structure comprising the closed cells. As a method for producing the
 shaped porous polymer structure of the present invention, the
 above-mentioned methods can be used individually or in combination.
 In the present invention, there is no particular limitation with respect to
 the method for swelling a shaped porous polymer structure with an
 electrolyte so as to produce the hybrid electrolyte of the present
 invention. There can be mentioned a method comprising holding an
 impregnated, shaped porous polymer structure (i.e., a shaped porous
 polymer structure which is impregnated with an electrolytic liquid,
 wherein the electrolytic liquid is used in an amount sufficient to swell
 the shaped porous polymer structure) under predetermined swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is susceptible to swelling with the electrolytic liquid, to
 thereby swell the porous polymer structure. After the swelling, the
 swelled, shaped porous polymer structure may or may not exhibit
 dimensional change. The expression "the shaped porous polymer structure
 does not exhibit dimensional change" means that the shaped porous polymer
 structure before impregnation with an electrolytic liquid is substantially
 identical in size with the hybrid electrolyte produced using the shaped
 polymer structure, but the shaped porous polymer structure in the hybrid
 electrolyte is in a swollen state as defined above. That is, even if the
 shaped porous polymer structure does not exhibit any dimensional change
 after swelling, the degree of shrinkage of the shaped porous polymer
 structure of the hybrid electrolyte of the present invention is more than
 10%, wherein the degree of shrinkage of the shaped porous polymer
 structure is defined as the change in the longitudinal length of the dried
 polymer matrix (i.e., the shaped porous polymer structure obtained by
 removing the impregnated electrolytic liquid from the hybrid electrolyte,
 followed by drying in vacuum), relative to the longitudinal length of the
 electrolytic liquid-impregnated hybrid electrolyte.
 Further, as mentioned above, the gel content of the polymer matrix of the
 hybrid electrolyte of the present invention must be within the
 above-mentioned specific range. Depending on the method of swelling a
 shaped porous polymer structure with an electrolytic liquid for producing
 a hybrid electrolyte, it is possible that a portion of the polymer matrix
 is dissolved in the electrolytic liquid, so that the gel content of the
 polymer matrix remaining undissolved becomes higher than the
 above-mentioned specific range. However, when a shaped porous polymer
 structure is swelled with an electrolytic liquid by an appropriate method,
 such as a method in which the electrolytic liquid is used in an amount
 insufficient to unrestrictedly swell the shaped porous polymer structure
 (i.e. in an amount such that the swelling of the shaped porous polymer
 structure cannot reach the equilibrium of swelling), or a method in which
 the temperature and time for swelling the shaped porous polymer structure
 with the electrolytic liquid is controlled, the dissolution of a portion
 of the polymer matrix in the electrolytic liquid during the immersion of
 the shaped porous polymer structure in the electrolytic liquid is
 prevented, so that the gel content of the polymer matrix can be maintained
 at a level within the above-mentioned specific range. By such methods, the
 hybrid electrolyte of the present invention can be produced.
 Alternatively, the hybrid electrolyte of the present invention can be
 effectively produced by the following method. That is, first, a shaped
 porous polymer structure is impregnated with an electrolytic liquid under
 predetermined non-swelling temperature and pressure conditions at which
 the shaped porous polymer structure is substantially insusceptible to
 swelling with the electrolytic liquid, thereby obtaining an impregnated,
 shaped porous polymer structure. In general, as a method for impregnating
 the shaped porous polymer structure, there can be mentioned a method
 wherein the shaped porous polymer structure is immersed in an electrolytic
 liquid bath. As another method for impregnating the shaped porous polymer
 structure, there can be mentioned a method in which the electrolytic
 liquid is applied to the shaped porous polymer structure by spraying or
 coating. The impregnated, shaped porous polymer structure obtained by the
 above methods is still not swelled with the electrolytic liquid, so that
 the shaped porous polymer structure has a satisfactorily high mechanical
 strength and that a dimensional change of the shaped porous polymer
 structure does almost not occur. Therefore, at this stage of only
 impregnation, the handling of the shaped porous polymer structure is
 relatively easy.
 Next, the shaped porous polymer structure in the above-mentioned state is
 taken out of the impregnating device, such as the electrolytic liquid
 bath, the spraying device, the coating device or the like. The excess
 electrolytic liquid on the shaped porous polymer structure flows down so
 as to be removed from the polymer structure. If desired, the excess
 electrolytic liquid is removed by an appropriate method, such as shaking
 off, wiping out or the like.
 On the other hand, in the present invention, a method can be employed in
 which the shaped porous polymer structure may be laminated to an electrode
 to thereby obtain a laminate structure, and the laminate structure is
 impregnated with the electrolytic liquid under predetermined non-swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is substantially insusceptible to swelling with the electrolytic
 liquid, to thereby obtain an impregnated laminate structure. The excess
 electrolytic liquid on the obtained impregnated laminate structure is
 removed in substantially the same manner as mentioned above.
 The ionic conductivity of the impregnated, shaped porous polymer structure
 in the above-mentioned non-swollen state is not satisfactorily high.
 However, when the impregnated, shaped porous polymer structure as such, or
 the impregnated laminate structure (wherein the impregnated shaped porous
 polymer structure is laminated to an electrode) is held under
 predetermined swelling temperature and pressure conditions at which the
 shaped porous polymer structure is susceptible to swelling with the
 electrolytic liquid, to thereby swell the shaped porous polymer structure
 with the electrolytic liquid, a hybrid electrolyte having a high ionic
 conductivity of the present invention can be easily obtained. Especially
 when the impregnated, shaped porous polymer structure is laminated to an
 electrode and the resultant laminate structure is heated for swelling, the
 improvement of the ionic conductivity and the adhesion of the resultant
 hybrid electrolyte to the electrode can be simultaneously achieved.
 Further, when the above-mentioned "impregnating and swelling" method of the
 present invention is employed, it becomes possible to prevent the
 dissolution of a portion of the polymer matrix into the electrolytic
 liquid, so that the electrolytic liquid bath or the like is free from
 dusty substances derived from a portion of the polymer matrix which is
 dissolved in the electrolytic liquid.
 As described above, in general, when a crosslinked polymer is
 unrestrictedly swelled in a solvent or solution, the crosslinked polymer
 is swelled, with a great dimensional change, until the swelling of the
 crosslinked polymer reaches the equilibrium of swelling. It is extremely
 difficult to control, in order to prevent the dimensional change of the
 crosslinked polymer, the quantity of the solvent or solution entering the
 crosslinked polymer for the swelling of the crosslinked polymer so that
 the swelling of the crosslinked polymer does not reach the equilibrium of
 swelling. However, when a shaped porous polymer structure is swelled with
 an electrolytic liquid by the above-mentioned "impregnating and swelling"
 method, the void ratio of the shaped porous polymer structure functions to
 provide a limitation with respect to the quantity of the electrolytic
 liquid entering the shaped porous polymer structure for the swelling of
 the shaped porous polymer structure, so that the degree of swelling of the
 shaped porous polymer structure is also limited. Accordingly, the great
 dimensional change and lowering of the mechanical strength of the shaped
 porous polymer structure due to the swelling do not occur. Therefore,
 especially when the shaped porous polymer structure (which may be either
 impregnated or not impregnated with an electrolytic liquid as described
 below) is laminated to an electrode before the shaped porous polymer
 structure is swelled with an electrolytic liquid, the above-mentioned
 "impregnating and swelling" method is advantageous in that not only can a
 shaped porous polymer structure having a high mechanical strength be
 laminated to an electrode, but also the dimensional change of the shaped
 porous polymer structure due to the swelling can be suppressed.
 The swelling is generally conducted under atmospheric pressure, but, if
 desired, it may be conducted under reduced pressure or superatmospheric
 pressure. The electrolytic liquid may or may not remain in the cells of
 the hybrid electrolyte after the swelling (the cells are those which are
 derived from the cells of the shaped porous polymer structure). However,
 it is preferred that the hybrid electrolyte contains a liquid phase,
 especially a liquid phase passing through the hybrid electrolyte.
 Thus, in a further aspect of the present invention, there is provided a
 method for producing an electrochemical device, which comprises:
 impregnating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix contains a crosslinked polymer segment, with an
 electrolytic liquid selected from the group consisting of a solution of an
 electrolyte in water or a non-aqueous solvent and a liquid electrolyte
 under predetermined non-swelling temperature and pressure conditions at
 which the shaped porous polymer structure is substantially insusceptible
 to swelling with the electrolytic liquid, thereby obtaining an
 impregnated, shaped porous polymer structure;
 laminating the impregnated, shaped porous polymer structure to an electrode
 to thereby obtain a laminate structure; and
 holding the laminate structure under predetermined swelling temperature and
 pressure conditions at which the shaped porous polymer structure is
 susceptible to swelling with the electrolytic liquid.
 In still a further aspect of the present invention, there is provided a
 method for producing an electrochemical device, which comprises:
 laminating a shaped porous polymer structure, which comprises a polymer
 matrix and a plurality of cells dispersed in the polymer matrix, wherein
 the polymer matrix contains a crosslinked polymer segment, to an electrode
 to thereby obtain a laminate structure;
 impregnating the laminate structure with an electrolytic liquid selected
 from the group consisting of a solution of an electrolyte in water or a
 non-aqueous solvent and a liquid electrolyte under predetermined
 non-swelling temperature and pressure conditions at which the shaped
 porous polymer structure is substantially insusceptible to swelling with
 the electrolytic liquid, thereby obtaining an impregnated laminate
 structure; and
 holding the impregnated laminate structure under predetermined swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is susceptible to swelling with the electrolytic liquid
 In the present invention, the "swelling temperature and pressure conditions
 at which the shaped porous polymer structure is susceptible to swelling
 with the electrolytic liquid" are the temperature and pressure conditions
 at which the change (%) in the longitudinal length of the shaped porous
 polymer structure, which is determined after immersing the shaped porous
 polymer structure in an electrolytic liquid under predetermined
 temperature and pressure conditions, relative to the longitudinal length
 measured before the immersion, is more than 10%. Therefore, the
 susceptibility of the shaped porous polymer structure to swelling with the
 electrolytic liquid (under predetermined temperature and pressure
 conditions) can be confirmed from the occurrence of dimensional change of
 the shaped porous polymer structure at the immersion of the shaped porous
 polymer structure in the electrolytic liquid under the predetermined
 temperature and pressure conditions. However, with respect to the
 above-mentioned method (i.e., a method which comprises impregnating a
 shaped porous polymer structure with an electrolytic liquid under
 predetermined non-swelling temperature and pressure conditions at which
 the shaped porous polymer structure is substantially insusceptible to
 swelling with the electrolytic liquid, thereby obtaining an impregnated,
 shaped porous polymer structure; removing the excess electrolytic liquid
 on the obtained impregnated, shaped porous polymer structure; laminating
 the impregnated, shaped porous polymer structure to an electrode to
 thereby obtain a laminate structure; and holding the laminate structure
 under predetermined swelling temperature and pressure conditions at which
 the shaped porous polymer structure is susceptible to swelling with the
 electrolytic liquid, so that the shaped porous polymer structure is
 swelled with the electrolytic liquid), the size of the shaped porous
 polymer structure which is swelled with the electrolytic liquid is
 substantially the same as the size of the shaped porous polymer structure
 which is not swelled with the electrolytic liquid. Therefore, the
 susceptibility of the shaped porous polymer structure to swelling with the
 electrolytic liquid cannot be confirmed directly from an occurrence of
 dimensional change of the shaped porous polymer structure in the middle of
 the above-mentioned method, and it is necessary to separately confirm the
 swelling temperature and pressure conditions at which the shaped porous
 polymer structure is susceptible to swelling with the electrolytic liquid,
 with respect to the shaped porous polymer structure in a non-laminated
 form.
 Further, in the present invention, the "non-swelling temperature and
 pressure conditions at which the shaped porous polymer structure is
 substantially insusceptible to swelling with the electrolytic liquid" are
 the temperature and pressure conditions at which the change (%) in the
 longitudinal length of the shaped porous polymer structure, which is
 determined after immersing the shaped porous polymer structure in the
 electrolytic liquid under predetermined temperature and pressure
 conditions, relative to the longitudinal length measured before the
 immersion, is 10% or less, or the temperature and pressure conditions at
 which the degree of shrinkage (%) of the shaped porous polymer structure
 [which is defined as the change in the longitudinal length of the dried
 polymer matrix {i.e., the shaped porous polymer structure obtained by
 extraction-removing the impregnated electrolytic liquid from the hybrid
 electrolyte (which is obtained by immersing the shaped porous polymer
 matrix in the electrolytic liquid under predetermined temperature and
 pressure conditions), followed by drying at room temperature}], relative
 to the longitudinal length of the electrolytic liquid-impregnated hybrid
 electrolyte, is 10% or less.
 The type of polymer for use in producing the hybrid electrolyte of the
 present invention varies depending on the type of the electrolytic liquid
 used. conversely, the type of hydrolytic liquid for use in producing the
 hybrid electrolyte of the present invention varies depending on the type
 of the polymer used. The electrolytic liquid contained in the hybrid
 electrolyte of the present invention is selected from the group consisting
 of a solution of an electrolyte in water or a non-aqueous solvent and a
 liquid electrolyte. Examples of electrolytic liquids are described below.
 The electrolyte used in the form of a solution in water or a non-aqueous
 solvent may be an inorganic or organic salt or an inorganic or organic
 acid. Examples of electrolytes include inorganic acids, such as
 tetrafluoroboric acid, hexafluorophosphoric acid, perchloric acid,
 hexafluoroarsenic acid, nitric acid, sulfuric acid, phosphoric acid,
 hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid
 and the like; organic acids, such as trifluoromethanesulfonic acid,
 heptafluoropropylsulfonic acid, bis(trifluoromethanesulfonyl)imide acid,
 acetic acid, trifluoroacetic acid, propionic acid and the like; and salts
 of these inorganic and organic acids. Further, the above-mentioned acids
 and salts can be used individually or in combination. Examples of cations
 for the above-mentioned electrolytic salts include an alkali metal, an
 alkaline earth metal, a transition metal, a rare earth metal, an ammonium
 ion and the like. The above-mentioned cations can be used individually or
 in combination. The preferred cation species varies depending on the use
 of the hybrid electrolyte. For example, when the hybrid electrolyte of the
 present invention is used in a lithium battery, it is preferred to use a
 lithium salt as an electrolyte. Especially when the hybrid electrolyte of
 the present invention is used in a lithium secondary battery, in which a
 wide range of electrochemical window is utilized, it is preferred that the
 electrolyte is an electrochemically stable lithium salt. Examples of such
 lithium salts include a lithium salt of fluoroalkylsulfonic acid, such as
 CF.sub.3 SO.sub.3 Li and C.sub.4 F.sub.9 SO.sub.3 Li; and a lithium salt
 of sulfonimide, such as (CF.sub.3 SO.sub.2).sub.2 NLi; LiBF.sub.4,
 LiPF.sub.6, LiClO.sub.4 and LiAsF.sub.6. The appropriate concentration of
 the electrolyte in a solution varies depending on the use of the hybrid
 electrolyte. However, it is generally in the range of from 0.1 mol/liter
 to the saturation solubility, preferably in the range of from 0.5 to 5
 mol/liter, more preferably in the range of from 0.5 to 2 mol/liter.
 As a solvent for dissolving the above-mentioned electrolyte, there can be
 mentioned chemically stable solvents in which the above-mentioned
 electrolyte can be dissolved, such as water, alcohol and the like. When
 the hybrid electrolyte of the present invention is used in an
 electrochemical device containing a non-aqueous electrolytic liquid, such
 as a lithium battery, examples of solvents include carbonate compounds,
 such as ethylene carbonate, propylene carbonate, dimethyl carbonate,
 diethyl carbonate, methyl ethyl carbonate and the like; ether compounds,
 such as tetrahydrofuran, dimethoxyethane, diglyme, tetraglyme,
 oligoethylene oxide and the like; lactone compounds, such as
 .gamma.-butyrolactone, .beta.-propiolactone and the like; and nitrile
 compounds, such as acetonitrile, propionitrile and the like.
 Among the above-mentioned polymers and electrolytic liquids, an appropriate
 combination of a polymer and an electrolytic liquid is employed, wherein
 the shaped porous polymer structure produced using the polymer is swelled
 with the electrolytic liquid. It is preferred that the shaped porous
 polymer structure produced using the polymer is susceptible to swelling
 with the electrolytic liquid (a solution of an electrolyte) at a
 temperature which is lower than the boiling point of the solvent for
 dissolving the electrolyte. It is especially preferred that the shaped
 porous polymer structure produced using the polymer is susceptible to
 swelling with the electrolytic liquid (a solution of an electrolyte) at a
 temperature which is lower than the boiling point of the solvent for
 dissolving the electrolyte, but the shaped porous polymer structure is
 substantially insusceptible to swelling with the electrolytic liquid at
 room temperature. As examples of such combinations, there can be mentioned
 a combination of a carbonate solvent, such as ethylene carbonate and
 propylene carbonate, as a solvent for the electrolyte and a vinylidene
 fluoride polymer (i.e., polyvinylidene fluoride and a copolymer of
 vinylidene fluoride), or polyacrylonitrile as a polymer. When the above
 combination of a solvent and a polymer is employed, the impregnation is
 preferably conducted at the temperature of 35.degree. C. or less and the
 swelling is preferably conducted at the temperature of 80.degree. C. or
 more, more preferably 90.degree. C. or more.
 In the present invention, it is required that the shaped porous polymer
 structure be finally swelled with the electrolytic liquid. Whether or not
 the shaped porous polymer structure is susceptible to swelling with the
 electrolytic liquid under a certain pressure depends on the temperature.
 That is, the shaped porous polymer structure is substantially
 insusceptible to swelling with the electrolytic liquid under a certain
 temperature. Practically, it is preferred that the non-swelling
 temperature and pressure conditions at which the shaped porous polymer
 structure is substantially insusceptible to swelling are the conditions of
 approximately room temperature and approximately atmospheric pressure.
 Incidentally, whether or not the shaped porous polymer structure is
 susceptible to swelling with the electrolytic liquid depends also on the
 time of contacting the shaped porous polymer structure with the
 electrolytic liquid. However, when almost no change in size of the shaped
 porous polymer structure occurs even after immersing the shaped porous
 polymer structure in the electrolytic liquid for approximately one hour at
 a predetermined temperature under atmospheric pressure, the shaped porous
 polymer structure is regarded as substantially insusceptible to swelling
 with the electrolytic liquid at the predetermined temperature.
 On the other hand, the susceptibility of the shaped porous polymer
 structure to swelling with the electrolytic liquid at a predetermined
 temperature can be confirmed from the occurrence of dimensional change of
 the shaped porous polymer structure at the immersion of the shaped porous
 polymer structure In the electrolytic liquid at the predetermined
 temperature. That is, when the shaped porous polymer structure is swelled
 with the electrolytic liquid by heating under a certain pressure, the
 lowest temperature required for swelling the shaped porous polymer
 structure varies depending on the types of the polymer and the
 electrolytic liquid which are used in combination. The lowest temperature
 can generally be determined by measuring the temperature at which the
 dimensional change of the shaped porous polymer structure occurs when the
 shaped porous polymer structure is immersed in the electrolytic liquid.
 However, when the shaped porous polymer structure is laminated to an
 electrode, it is difficult to know whether or not the dimensional change
 of the shaped porous polymer structure has occurred. Therefore, it is
 preferred to separately determine the lowest temperature required for
 swelling, with respect to the shaped porous polymer structure in a
 non-laminated form. The lowest temperature required for swelling is
 preferably higher than room temperature, more preferably not less than
 20.degree. C. higher than room temperature, and lower than the boiling
 point of the solvent used for the electrolytic liquid. Further, when the
 shaped porous polymer structure is heated so as to be swelled with an
 electrolytic liquid after the shaped porous polymer structure is laminated
 to an electrode, the lowest temperature required for swelling is
 preferably lower than the temperature at which the electrode activity of
 an electrode material used for the electrode begins to deteriorate. The
 time of heating required for swelling varies depending on the temperature
 for swelling and the mode of heating and, therefore, cannot be simply
 determined. However, the heating is conducted generally for 2 minutes or
 more, preferably 10 minutes or more, more preferably 30 minutes or more.
 When the temperature for swelling is satisfactorily higher than the lowest
 temperature required for swelling, about 2 minutes of heating is
 sufficient for swelling.
 As mentioned above, as methods for producing an electrochemical device,
 such as a battery, in which the hybrid electrolyte of the present
 invention is contained, there can be mentioned the following two methods,
 i.e., a method which comprises impregnating a shaped porous polymer
 structure with an electrolytic liquid, thereby obtaining an impregnated,
 shaped porous polymer structure; laminating the impregnated, shaped porous
 polymer structure to an electrode to thereby obtain a laminate structure;
 and heating the laminate structure, and a method which comprises
 laminating a shaped porous polymer structure to an electrode to thereby
 obtain a laminate structure; impregnating the laminate structure with an
 electrolytic liquid, thereby obtaining an impregnated laminate structure;
 and heating the impregnated laminate structure. With respect to the latter
 method, when an electrode having a current collector is used for producing
 an electrochemical device, the laminate structure can be effectively
 impregnated with the electrolytic liquid by using, as the current
 collector, a mesh current collector. Examples of morphologies of the
 laminate structures include a sheet form, a roll form, a folding sheet
 form, a laminate form and the like.
 In the method of the present invention for producing an electrochemical
 device, the material for an electrode varies depending on the type of the
 electrochemical device to be produced. For example, when the
 electrochemical device is a lithium battery, a substance capable of
 occluding and releasing lithium is used as a material for the positive
 electrode and negative electrode. As the positive electrode material, a
 material having a higher electric potential than that of the negative
 electrode is selected. Examples of such positive electrode materials
 include oxides, such as Li.sub.1-X CoO.sub.2, Li.sub.1-X NiO.sub.2,
 Li.sub.1-X Mn.sub.2 O.sub.4, Li.sub.1-X MO.sub.2 (wherein 0&lt;X&lt;1, and
 M represents a mixture of Co, Ni, Mn and Fe), Li.sub.2-Y Mn.sub.2 O.sub.4
 (wherein 0&lt;Y&lt;2), Li.sub.1-X V.sub.2 O.sub.5, Li.sub.2-Y V.sub.2
 O.sub.5 (wherein 0&lt;Y&lt;2) and Li.sub.1.2-X' Nb.sub.2 O.sub.5 (wherein
 0&lt;X'&lt;1.2); metal chalcogenides, such as Li.sub.1-X TiS.sub.2,
 Li.sub.1-X MoS.sub.2 and Li.sub.3-Z NbSe.sub.3 (wherein 0&lt;Z&lt;3); and
 organic compounds, such as-polypyrrole, polythiophene, polyaniline, a
 polyacene derivative, polyacetylene, polythienylene vinylene, polyallylene
 vinylene, a dithiol derivative and a disulfide derivative.
 As the negative electrode material, a material having a lower electric
 potential than that of the positive electrode is employed. Examples of
 negative electrode materials include metallic lithium-containing
 materials, such as metallic lithium, an aluminum-lithium alloy and a
 magnesium-aluminum-lithium alloy; carbonaceous materials, such as a
 graphite, a coke, a low temperature-calcined polymer; lithium solid
 solutions of metal oxides, such as an SnM oxide (wherein M represents Si,
 Ge or Pb), a complex oxide of Si.sub.1-Y M'YOZ oxide (wherein M'
 represents W, Sn, Pb, B or the like), a titanium oxide and an iron oxide;
 and ceramics, such as nitrides, e.g., Li.sub.7 MnN.sub.4, Li.sub.3
 FeN.sub.2, Li.sub.3-X Co.sub.X N, Li.sub.3-X NiN, Li.sub.3-X Cu.sub.X N,
 Li.sub.3 BN.sub.2, Li.sub.3 AlN.sub.2 and Li.sub.3 SiN.sub.3. It should be
 noted that when metallic lithium formed on the negative electrode by the
 reduction of lithium ions on the negative electrode is used as a negative
 electrode material, the type of the material for the negative electrode is
 not particularly limited as long as it is electrically conductive.
 The positive and negative electrodes are produced by molding the
 above-mentioned materials into predetermined morphologies. The electrode
 may be either In the form of a continuous solid or in the form of a
 particulate electrode material dispersed in a binder. Examples of methods
 for forming a continuous solid include electrolysis, vapor deposition,
 sputtering, CVD, melt processing, sintering and compression. In a method
 for forming a particulate electrode material dispersed in a binder, an
 electrode is produced by molding a mixture of a particulate electrode
 material and a binder. Examples of binders include an ionic conductive
 polymer, such as polyvinylidene fluoride; a non-ionic conductive polymer,
 such as a styrene-butadiene latex and Teflon latex; and metals. A
 polymerizable monomer and a crosslinking agent may be added to the binder,
 and the resultant mixture may be subjected to molding, followed by
 polymerization and crosslinking. For the purpose of improving the binding
 strength of such a binder or modifying the properties of such a binder,
 the binder may be irradiated with radiant energy, such as electron beams,
 .gamma.-rays, and ultraviolet rays. For effecting ion transportation
 between the positive and negative electrodes, current collectors made of a
 material having low electrical resistance may be provided on the positive
 and negative electrodes. In producing an electrode according to the
 above-mentioned methods, the current collector is used as a substrate for
 the electrode.
 When a shaped porous polymer structure is laminated to an electrode, the
 electrode can be impregnated with an electrolytic liquid in advance. When
 a laminate structure comprising an impregnated, shaped porous polymer
 structure or a hybrid electrolyte and an electrode is heated in the
 presence of the electrolytic liquid used for impregnating the electrode,
 the adherence strength of the impregnated, shaped porous polymer structure
 or the hybrid electrolyte to the electrode is remarkably improved.
 Examples of electrochemical devices comprising the hybrid electrolyte of
 the present invention include primary and secondary batteries (e.g., a
 lithium battery), a photoelectrochemical device and an electrochemical
 sensor.
 As mentioned above, the hybrid electrolyte of the present invention has a
 high ionic conductivity, an excellent high temperature stability and an
 excellent adherability to an electrode. Further, when the method of the
 present invention is employed for producing a hybrid electrolyte or an
 electrochemical device, during the course of the production of the hybrid
 electrolyte or the electrochemical device, not only is the strength of an
 impregnated, shaped porous polymer structure extremely high, but also the
 dimensional change of the shaped porous polymer structure swelled with an
 electrolytic liquid, relative to the shaped porous polymer structure which
 is not swelled, is advantageously suppressed. Therefore, it becomes
 possible to effectively produce not only a hybrid electrolyte having the
 above-mentioned excellent properties, but also a high performance
 electrochemical device. Further, the method of the present invention is
 very advantageous from an industrial point of view in that, in practicing
 the method of the present invention, a dissolution of a portion of the
 polymer matrix into an electrolytic liquid does not occur and, therefore,
 the impregnation apparatus, such as an electrolyte liquid bath, is free
 from dusty substances derived from a portion of the polymer matrix which
 is dissolved in the electrolytic liquid. The hybrid electrolyte of the
 present lnvention is especially useful as an electrolyte for the so-called
 polymer batteries.

BEST MODE FOR CARRYING OUT THE INVENTION
 The present invention will be described in more detail with reference to
 Examples and Comparative Examples, which should not be construed as
 limiting the scope of the present invention.
 In the Examples and Comparative Examples, the measurement of the void ratio
 of a shaped porous polymer structure, the measurement of the degree of
 shrinkage of a shaped porous polymer structure, the measurement of the gel
 content of a polymer matrix, and the evaluation of the ionic conductivity
 were performed by the following methods.
 (i) Measurement of the Void Ratio of a Shaped Porous Polymer Structure
 First, the weight on a dry basis (A) of a shaped porous polymer structure
 was determined. Next, the shaped porous polymer structure was immersed in
 ethanol to thereby render hydrophilic the polymer structure. Subsequently,
 the resultant hydrophilic polymer structure was immersed in water, thereby
 replacing the impregnated ethanol by water. The water on the surface of
 the polymer structure was removed by wiping, and then, the weight (B) of
 the resultant water-wiped polymer structure was measured. From the weights
 (A) and (B) obtained above and the true specific gravity (d) of the
 material which the shaped porous polymer structure is made of, the void
 ratio of the shaped porous polymer structure was calculated according to
 the following formula:
EQU void ratio (%)=[(B-A)/(A/d+B-A)].times.100.
 (ii) Measurement of the Degree of Shrinkage of a Shaped Porous Polymer
 Structure and Measurement of the Gel Content of a Polymer Matrix
 From a produced hybrid electrolyte sheet impregnated with an electrolytic
 liquid was cut out a sample having a predetermined size, and the sample
 was immersed in ethanol for 30 minutes or more to thereby
 extraction-remove the impregnated electrolytic liquid from the sample,
 followed by drying in vacuum to thereby obtain a dried polymer matrix. The
 weight of the dried polymer matrix was measured.
 In this instance, the degree of shrinkage of the shaped porous polymer
 structure of the hybrid electrolyte was determined. That is, the change in
 the longitudinal length of the above-obtained dried polymer matrix (shaped
 porous polymer structure), relative to the longitudinal length of the
 electrolytic liquid-impregnated hybrid electrolyte sheet, was determined.
 This change (%) is defined as the degree of shrinkage of the shaped porous
 polymer structure.
 The weight of the above-obtained dried polymer matrix was measured Then,
 the dried polymer matrix was wrapped with a stainless steel wire mesh
 (150-mesh size) and heated in a solvent for 4 hours under reflux, wherein
 the solvent was used in a weight amount of not less than 1000 times the
 weight of the dried polymer matrix. As the solvent, a mixed solvent of
 N,N-dimethylacetamide (DMAC) and acetone (volume ratio of DMAC to
 acetone=7:3) was used for a vinylidene fluoride polymer matrix, and DMAC
 alone was used for an acrylonitrile polymer matrix. Subsequently, the
 resultant polymer matrix was subjected to extraction with acetone for 5
 minutes, followed by drying in vacuum at 70.degree. C., thereby obtaining
 an extraction residue. The weight of the extraction residue was measured.
 The gel content of the polymer matrix was defined as a value obtained by
 dividing the weight of the extraction residue by the weight of the dried
 polymer matrix.
 (iii) Evaluation of the Ionic Conductivity
 A sample (an impregnated porous polymer sheet, or a hybrid electrolyte,
 i.e., a swollen form of an impregnated porous polymer sheet) was
 sandwiched between two electrode sheets to thereby obtain an
 electrochemical cell. An alternating voltage was applied between the
 electrodes, and the complex impedance was measured by alternating-current
 impedance method. In accordance with a conventional method, the ionic
 conductivity of the sample was calculated from the real part of the
 obtained complex impedance expressed in the form of a Cole-Cole plot, the
 thickness of the sample, and the surface areas of the electrodes.
 EXAMPLE 1
 Production of a hybrid electrolyte
 A solution consisting of 17.3 parts by weight of a
 hexafluoropropylene/vinylidene fluoride copolymer resin
 (hexafluoropropylene content: 5% by weight), 11.5 parts by weight of
 polyethylene glycol (average molecular weight: 200) and 71.2 parts by
 weight of dimethylacetamide was prepared, and to 100 g of the thus
 prepared solution was added 0.8 ml of polyoxyethylene sorbitane monooleate
 (Trade name: Tween 80, manufactured and sold by Kao Atlas K.K., Japan),
 thereby obtaining a homogeneous solution. The obtained homogeneous
 solution was cast on a glass plate at room temperature, thereby preparing
 a liquid film having a thickness of 200 .mu.m. Immediately, the prepared
 liquid film was immersed in water at 70.degree. C. to thereby solidify the
 film, and then, the solidified film was washed with water and alcohol,
 followed by drying, thereby obtaining a porous polymer sheet having a
 thickness of 52 .mu.m and a void ratio of 76%. The prepared porous polymer
 sheet was irradiated with electron beams (irradiation dose: 15 Mrads) to
 thereby prepare a crosslinked porous polymer sheet.
 An electrolytic liquid was obtained by dissolving lithium tetrafluoroborate
 (LiBF.sub.4) in a mixed solvent of ethylene carbonate (EC) and propylene
 carbonate (PC) (EC/PC weight ratio=1/1, and LiBF.sub.4 concentration: 1
 mol/liter), and the crosslinked porous polymer sheet prepared above was
 immersed in the above-obtained electrolytic liquid at room temperature.
 When the crosslinked porous polymer sheet was immersed in the electrolytic
 liquid, the sheet was immediately impregnated with the electrolytic
 liquid, and an impregnated transparent porous polymer sheet was obtained
 with ease. The electrolytic liquid on the surface of the impregnated
 porous polymer sheet (that is, an excess electrolytic liquid which did not
 impregnate the porous polymer sheet) was removed by wiping the porous
 polymer sheet. The change in the size of the impregnated porous polymer
 sheet was 5% in the longitudinal direction of the sheet, relative to the
 size of the non-impregnated porous polymer sheet The impregnated porous
 polymer sheet was sandwiched between two glass plates and heated at
 100.degree. C. in an oven for 2 hours, to thereby obtain a hybrid
 electrolyte sheet. No change in the size of the impregnated porous polymer
 sheet was observed between the impregnated porous polymer sheet before the
 heat treatment and the hybrid electrolyte sheet obtained after the heat
 treatment. From the obtained hybrid electrolyte sheet was cut out a sample
 having a predetermined size, and the electrolytic liquid was removed by
 extraction, followed by drying, to thereby obtain a dried polymer matrix.
 The degree of shrinkage of the porous polymer sheet was 16%, as calculated
 from the size of the dried polymer matrix and the size of the hybrid
 electrolyte sheet, and the gel content of the polymer matrix was 62%. On
 the other hand, when the above-obtained crosslinked porous polymer sheet
 was immersed in the electrolytic liquid at 100.degree. C. for 1 hour, the
 size of the porous polymer sheet increased by 36% in the longitudinal
 direction of the sheet, based on the size of the porous polymer sheet
 before the immersion in the electrolytic liquid. Thus, the above-mentioned
 temperature was confirmed to be a temperature at which the porous polymer
 sheet can be swelled with the electrolytic liquid.
 A sample of the obtained hybrid electrolyte sheet was sandwiched between
 two stainless steel sheets, thereby obtaining an electrochemical cell. The
 obtained electrochemical cell was subjected to a measurement of an
 alternating-current impedance, using the above-mentioned two stainless
 steel sheets as electrodes. (The alternating-current impedance was
 measured by means of impedance measurement apparatus Model 389
 manufactured and sold by Seiko EG&G, Japan). As a result, it was found
 that the ionic conductivity of the hybrid electrolyte sheet at room
 temperature was 1.3 mS/cm.
 &lt;Production of a battery&gt;
 A powder of lithium cobalt oxide (LiCoO.sub.2 ; average particle diameter:
 10 .mu.m) and carbon black were added to and dispersed in a 5% by weight
 solution of polyvinylidene fluoride (KF1100, manufactured and sold by
 Kureha Chemical Industry Co., Ltd., Japan) in N-methylpyrrolidons (NMP),
 so that a slurry containing solid components in the following weight ratio
 was obtained: LiCoO.sub.2 (85%), carbon black (8%) and polyvinylidene
 fluoride (7%). The obtained slurry was applied onto an aluminum foil (as a
 current collector) by doctor blade method and dried, to thereby prepare an
 electrode layer having a thickness of 110 .mu.m. The prepared electrode
 layer on the aluminum sheet was used as an LiCoO.sub.2 electrode sheet
 (positive electrode).
 A powder of needle coke (NC) having an average particle diameter of 10
 .mu.m was homogeneously mixed with the above-mentioned 5% by weight
 solution of polyvinylidene fluoride in N-nethylpyrrolidone (NMP), thereby
 obtaining a slurry (NC/polymer dry weight ratio=92:8). The obtained slurry
 was applied onto a copper sheet (as a current collector) by doctor blade
 method and dried, to thereby prepare an electrode layer having a thickness
 of 120 .mu.m. The prepared electrode layer on the copper sheet was used as
 a needle coke electrode sheet (negative electrode).
 From the LiCoO.sub.2 electrode sheet and the needle coke electrode sheet
 were individually cut out a sample having a size of 4 cm.times.4 cm, and
 the obtained samples were impregnated with the electrolytic liquid
 mentioned above. From an impregnated porous polymer sheet, obtained by
 immersing the crosslinked porous polymer sheet in the above-mentioned
 electrolytic liquid at room temperature, was cut out a sample having a
 size of 4.5 cm.times.4.5 cm, and the sample of the impregnated porous
 polymer sheet was sandwiched between the above-prepared electrode sheets
 so as to obtain a laminate structure (when preparing the laminate
 structure, both sides of the impregnated porous polymer sheet were
 respectively, securely attached to the electrode layers of the two
 electrode sheets), thereby preparing a battery composed of a negative
 electrode (needle cokes), an impregnated porous polymer sheet, and a
 positive electrode (LiCoO.sub.2). The prepared battery was further
 sandwiched between two glass plates and held by means of a clip. Then, the
 resultant structure was heated at 100.degree. C. for 2 hours, followed by
 cooling to room temperature, and the glass plates were removed from the
 battery. Stainless steel sheets (as electric terminals for taking a
 current) were brought into contact with the respective current collectors
 of the positive and negative electrodes of the battery, and the battery
 was placed between two opposite PET/Al/PE laminate films (PET:
 polyethylene terephthalate film, Al: aluminum sheet, PE: polyethylene
 film) so that the current collectors project from the resultant structure.
 The laminate structure of the resultant structure was made secure by means
 of a laminator, to thereby obtain a sheet battery.
 The electric terminals of the obtained sheet battery were connected to a
 charge/discharge testing device (Model 101SM, manufactured and sold by
 Hokuto Denko Corporation, Japan), and the battery was subjected to
 charge/discharge cycle testing at a current density of 1 mA/cm.sup.2. The
 charging operation was conducted at a constant potential of 4.2 V. After
 the charging operation, the potential between the electrodes was 4.2 V.
 The discharging operation was conducted at a constant current, and
 discontinued when the electric potential was decreased to 2.7 V. As a
 result of the charge/discharge testing, it was found that the
 discharge/charge efficiency (ratio) at the first cycle was 80% or more,
 and with respect to the cycles after the first cycle, each of the
 discharge/charge efficiencies (ratio) was 99% or more. These results show
 that this battery is capable of being repeatedly charged and discharged
 and hence operable as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet. In
 this situation, after removing the current collectors from the battery,
 the electrolyte layer of the battery, together with the electrode layers,
 was washed with ethanol, followed by an extraction of the electrolytic
 liquid. Then, a major portion of each of the electrode layers on both
 surfaces of the electrolyte layer was scraped off, thereby obtaining a
 polymer sheet. Further, the obtained polymer sheet was dried to thereby
 obtain a sheet of dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet after drying was 12% and the gel content of the dried
 polymer matrix was 64%.
 COMATIVE EXAMPLE 1
 From substantially the same porous polymer sheet as prepared in Example 1
 was cut out a sample having a size of 4 cm.times.4 cm. Without subjecting
 the sample to irradiation, the sample was immersed in an electrolytic
 liquid which is substantially the same as prepared in Example 1, thereby
 obtaining an impregnated porous polymer sheet, and an excess electrolytic
 liquid on the surface of the obtained sheet was removed by wiping the
 surface of the sheet. The impregnated porous polymer sheet was sandwiched
 between two glass plates, followed by heating at 100.degree. C. As a
 result of the heating, a sheet deformed into a circular shape was
 obtained. The gel content of the deformed sheet was 0%, as measured by the
 method described above.
 In addition, using the impregnated porous polymer sheet obtained prior to
 the heating, a sheet battery was prepared in substantially the same manner
 as described in Example 1. The obtained sheet battery was subjected to a
 charging operation, and as a result, it was found that a short circuiting
 occurred, so that it was impossible to charge the sheet battery
 EXAMPLE 2
 A porous polymer sheet having a thickness of 25 .mu.m and a void ratio of
 73% was prepared in substantially the same manner as in Example 1, except
 that the thickness of the liquid film was 100 .mu.m. Then, the prepared
 porous polymer sheet was irradiated with electron beams (irradiation dose:
 15 Mrads) to thereby prepare a crosslinked porous polymer sheet.
 The above-prepared crosslinked porous polymer sheet was immersed in
 substantially the same electrolytic liquid as that prepared in Example 1.
 When the crosslinked porous polymer sheet was immersed in the electrolytic
 liquid at room temperature, the sheet was immediately impregnated with the
 electrolytic liquid, and an impregnated transparent porous polymer sheet
 was obtained with ease. The electrolytic liquid on the surface of the
 impregnated porous polymer sheet (that is, an excess electrolytic liquid
 which did not impregnate the porous polymer sheet) was removed by wiping
 the porous polymer sheet. No change in the size of the impregnated porous
 polymer sheet was observed between the impregnated porous polymer sheet
 and the non-impregnated porous polymer sheet before the immersion in the
 electrolytic liquid. For the evaluation of the ionic conductivity of the
 impregnated porous polymer sheet, the impregnated porous polymer sheet was
 sandwiched between two stainless steel sheets, to thereby obtain a
 precursory electrochemical cell in which the impregnated porous polymer
 sheet is not swelled. The obtained precursory electrochemical cell was
 subjected to a measurement of an alternating-current impedance, using the
 above-mentioned two stainless sheets as electrodes. As a result, it was
 found that the ionic conductivity of the impregnated porous polymer sheet
 at room temperature was 0.25 mS/cm. The precursory electrochemical cell
 was heated at 100.degree. C. for 1 hour, followed by cooling to room
 temperature, to thereby obtain an electrochemical cell composed of two
 electrodes and a hybrid electrolyte sheet (a swollen form of the
 impregnated porous polymer sheet). The resultant electrochemical cell was
 subjected to a measurement of an alternating-current impedance in
 substantially the same manner as described above. As a result, it was
 found that the ionic conductivity of the hybrid electrolyte sheet at room
 temperature was 1.1 mS/cm.
 From the above-mentioned impregnated porous polymer sheet was cut out a
 sample having a size of 15 mm.times.15 mm, and then, the sample was
 sandwiched between two glass plates and heated at 100.degree. C. in an
 oven for 1 hour. The change in the size of the sheet was 1 mm or less in
 the longitudinal direction of the sheet. The polymer sheet obtained after
 the heat treatment could be easily handled with forceps. On the other
 hand, when the crosslinked porous polymer sheet was immersed in the
 electrolytic liquid at 100.degree. C. for 10 minutes, the size of the
 sheet increased from 1.5 cm.times.1.5 cm to 2 cm.times.2 cm. Thus, the
 above-mentioned temperature was confirmed to be a temperature at which the
 crosslinked porous polymer sheet can be swelled with the electrolytic
 liquid.
 COMATIVE EXAMPLE 2
 Using substantially the same non-crosslinked porous polymer sheet as
 prepared in Example 2, a precursory electrochemical call in which the
 impregnated porous polymer sheet is not swelled was obtained in
 substantially the same manner as in Example 2. The obtained precursory
 electrochemical cell was heated at 100.degree. C. for 1 hour, followed by
 cooling to room temperature, to thereby obtain an electrochemical cell
 composed of two electrodes and a swollen, impregnated porous polymer
 sheet. Subsequently, the obtained electrochemical cell was subjected to a
 measurement of an alternating-current impedance. As a result, it was found
 that a short circuiting occurred. Furthermore, when the non-crosslinked
 porous polymer sheet was immersed in the electrolytic liquid at 90.degree.
 C., the polymer sheet dissolved in 4 minutes.
 EXAMPLE 3 AND COMATIVE EXAMPLE 3
 From substantially the same LicoO.sub.2 electrode sheet and needle coke
 electrode sheet as prepared as in Example 1 were individually cut out a
 sample having a size of 2 cm.times.2 cm. From substantially the same
 impregnated porous polymer sheet as prepared in Example 2 was cut out a
 sample having a size of 2.3 cm.times.2.3 cm, and the sample of the
 impregnated porous polymer sheet was sandwiched between the above-prepared
 electrode sheets so as to obtain a laminate structure, thereby preparing a
 battery composed of a negative electrode (needle cokes), an impregnated
 porous polymer sheet, and a positive electrode (LiCoO.sub.2).
 Subsequently, stainless steel sheets (as electric terminals for taking a
 current) were brought into contact with the respective current collectors
 of the positive and negative electrodes of the battery. The battery was
 then placed in a sealable glass cell having electric terminals for taking
 a current from the battery (hereinafter frequently referred to simply as
 "glass cell"), and the electric terminals of the battery were connected to
 the electric terminals of the cell. The cell was sealed in an argon
 atmosphere to obtain a sheet battery
 Two sheet batteries were prepared as described above. One of the sheet
 batteries was heated at 100.degree. C. for 2 hours, followed by cooling to
 room temperature (Example 3). The other sheet battery was used without
 being subjected to the heat-treatment (Comparative Example 3).
 Both sheet batteries were individually subjected to a measurement of an
 alternating-current impedance and to charge/discharge cycle testing. As a
 result of the measurement of an alternating-current impedance, it was
 found that the internal resistance of the sheet battery of Comparative
 Example 3 was 80 .OMEGA., whereas the internal resistance of the sheet
 battery of Example 3 was 30 .OMEGA.. Further, the two batteries were
 subjected to charge/discharge cycle testing in substantially the same
 manner as in Example 1. As a result of the charge/discharge cycle testing,
 it was found that the discharge/charge efficiency (ratio) at the first
 cycle of each battery was 80% or more, and with respect to the cycles
 after the first cycle, each of the discharge/charge efficiencies (ratio)
 at both batteries was 99% or more. These results show that both batteries
 are capable of being repeatedly charged and discharged and hence operable
 as a secondary battery. However, the overpotential of the battery of
 Example 3 was 50 mV, whereas the overpotential of the battery of
 Comparative Example 3 was 100 mV. Thus, the electric capacity of the sheet
 battery of Comparative Example 3 was low.
 EXAMPLE 4
 Substantially the sample crosslinked porous polymer sheet as prepared in
 Example 2 was sandwiched between substantially the same two electrode
 sheets as prepared in Example 1, to thereby prepare a laminate structure.
 The prepared laminate structure was immersed in an electrolytic liquid,
 which is substantially the same as that prepared in Example 1, at room
 temperature for 1 hour so that the laminate structure was impregnated with
 the electrolytic liquid, thereby preparing a battery. The electrolytic
 liquid on the surface of the battery was removed by wiping the battery.
 Subsequently, stainless steel sheets (as electric terminals for taking a
 current) were brought into contact with the respective current collectors
 of the positive and negative electrodes of the battery. The battery was
 then placed in a glass cell, and the electric terminals of the battery
 were connected to the electric terminals of the cell. The cell was sealed
 in an argon atmosphere to obtain a sheet battery. The obtained sheet
 battery was heated at 100.degree. C. for 2 hours, followed by cooling to
 room temperature.
 The resultant sheet battery was subjected to a measurement of an
 alternating-current impedance and to charge/discharge cycle testing in
 substantially the same manner as described in Example 3. As a result of
 the measurement of an alternating-current impedance, it was found that the
 internal resistance of the battery was 30 .OMEGA.. The charging operation
 was conducted at a constant potential of 4.2 V. After the charging
 operation, the potential between the electrodes was 4.2 V. The discharging
 operation was conducted at a constant current, and discontinued when the
 electric potential was decreased to 2.7 V. As a result of the
 charge/discharge cycle testing, it was found that the discharge/charge
 efficiency (ratio) at the first cycle was 80% or more, and with respect to
 the cycles after the first cycle, each of the discharge/charge
 efficiencies (ratio) was 99% or more. These results show that this battery
 is capable of being repeatedly charged and discharged and hence operable
 as a secondary battery The overpotential of the sheet battery was 30 mV.
 EXAMPLE 5
 A crosslinked porous polymer sheet was prepared in substantially the same
 manner as in Example 1, except that a vinylidene fluoride homopolymer
 resin (Kynar 460, manufactured and sold by Elf Atochem North America Inc.,
 USA) was used instead of the hexafluoropropylene/vinylidene fluoride
 copolymer resin. The thickness of the prepared polymer sheet was 45 .mu.m
 and the void ratio was 71%.
 The above-prepared crosslinked porous polymer sheet was immersed in
 substantially the same electrolytic liquid as prepared in Example 1. When
 the crosslinked porous polymer sheet was immersed in the electrolytic
 liquid at room temperature, the sheet was impregnated with the
 electrolytic liquid and a transparent impregnated porous sheet was
 obtained. The change in the size of the impregnated porous polymer sheet
 was 3% in the longitudinal direction of the sheet, relative to the size of
 the non-impregnated porous polymer sheet. The impregnated porous polymer
 sheet was sandwiched between two glass plates and heated at 120.degree. C.
 in an oven for 2 hours, to thereby prepare a hybrid electrolyte sheet. No
 change in the size of the impregnated porous polymer sheet was observed
 between the impregnated porous polymer sheet before the heat treatment and
 the hybrid electrolyte sheet obtained after the heat treatment. From the
 prepared hybrid electrolyte sheet was cut out a sample having a
 predetermined size, and the electrolytic liquid was removed by extraction,
 followed by drying, to thereby obtain a dried polymer matrix. The degree
 of shrinkage of the porous polymer sheet was 24%, as calculated from the
 size of the dried polymer matrix and the size of the hybrid electrolyte
 sheet, and the gel content of the polymer matrix was 69%. On the other
 hand, when the above-obtained crosslinked porous polymer sheet was
 immersed in the electrolytic liquid at 120.degree. C. for 1 hour, the size
 of the porous polymer sheet increased by 40% in the longitudinal direction
 of the sheet, based on the size of the porous polymer sheet before the
 immersion in the electrolytic liquid. Thus, the above-mentioned
 temperature was confirmed to be a temperature at which the porous polymer
 sheet can be swelled with the electrolytic liquid. The ionic conductivity
 of the obtained hybrid electrolyte was 1.1 mS/cm at room temperature.
 From each of substantially the same LiCoO.sub.2 electrode sheet and needle
 coke electrode sheet as prepared in Example 1 was individually cut out a
 sample having a size of 4 cm.times.4 cm. The samples obtained were
 individually impregnated with substantially the same electrolyte liquid as
 prepared in Example 1, to thereby provide two impregnated electrode
 sheets. From the hybrid electrolyte sheet which had been separately
 prepared above was cut out a sample sheet of hybrid electrolyte having a
 size of 4.5 cm.times.4.5 cm, and the sample sheet of the hybrid
 electrolyte was sandwiched between the above-prepared electrode sheets so
 as to obtain a laminate structure, thereby preparing a battery composed of
 a negative electrode (needle cokes), an impregnated porous polymer sheet,
 and a positive electrode (LiCoO.sub.2). The prepared battery was pressed
 at 120.degree. C. for 1 minute. Stainless steel sheets (as electric
 terminals for taking a current) were brought into contact with the
 respective current collectors of the positive and negative electrodes of
 the resultant heat-pressed battery, and the battery was placed between two
 opposite PET/Al/PE laminate films (PET: polyethylene terephthalate film,
 Al: aluminum sheet, PE: polyethylene film) so that the current collectors
 project from the resultant structure. The laminate structure of the
 resultant structure was made secure by means of a laminator, to thereby
 obtain a sheet battery.
 The sheet battery was subjected to charge/discharge cycle testing in
 substantially the same manner as described in Example 1. As a result of
 the charge/discharge cycle testing, it was found that the discharge/charge
 efficiency (ratio) at the first cycle was 80% and more, and with respect
 to the cycles after the first cycle, each of the discharge/charge
 efficiencies (ratio) was 99% or more. These results show that this battery
 is capable of being repeatedly charged and discharged and hence operable
 as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet.
 EXAMPLE 6
 A crosslinked porous polymer sheet was prepared in substantially the same
 manner as in Example 1, except that a vinylidene fluoride homopolymer
 resin (Trade name: Kynar 740, manufactured and sold by Elf Atochem North
 America Inc., U.S.A.) was used instead of the
 hexafluoropropylene/vinylidene fluoride copolymer resin. The prepared
 crosslinked porous polymer sheet had a thickness of 60 .mu.m and a void
 ratio of 77%. The crosslinked porous polymer sheet was immersed in an
 electrolytic liquid in substantially the same manner as in Example 1, to
 thereby obtain an impregnated transparent porous polymer sheet. The change
 in the size of the impregnated crosslinked porous polymer sheet was 3% in
 the longitudinal direction of the sheet. The impregnated porous polymer
 sheet was sandwiched between two glass plates and heated at 120.degree. C.
 in an oven for 2 hours, to thereby obtain a hybrid electrolyte sheet. No
 change in the size of the impregnated porous polymer sheet was observed
 between the impregnated porous polymer sheet before the heat treatment and
 the hybrid electrolyte sheet obtained after the heat treatment. From the
 electrolyte sheet was cut out a sample having a predetermined size, and
 the electrolytic liquid was removed by extraction, followed by drying, to
 thereby obtain a dried polymer matrix The degree of shrinkage of the
 porous polymer sheet was 24%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 40%. On the other hand, when the
 above-obtained crosslinked porous polymer sheet was immersed in the
 electrolytic liquid at 120.degree. C. for 1 hour, the size of the
 crosslinked porous polymer sheet increased by 38% in the longitudinal
 direction of the sheet, based on the size of the porous polymer sheet
 before the immersion in the electrolytic liquid. Thus, the above-mentioned
 temperature was confirmed to be a temperature at which the porous polymer
 sheet can be swelled with the electrolytic liquid. The ionic conductivity
 of the hybrid electrolyte sheet at room temperature was 1.4 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as described in Example 5, and
 the prepared sheet battery was subjected to charge/discharge cycle
 testing. As a result of the charge/discharge cycle testing, it was found
 that the discharge/charge efficiency (ratio) at the first cycle was 80% or
 more, and with respect to the cycles after the first cycle, each of the
 discharge/charge efficiencies (ratio) was 99% or more. These results show
 that this battery is capable of being repeatedly charged and discharged
 and hence operable as a secondary battery
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet.
 COMATIVE EXAMPLE 4
 Substantially the same crosslinked porous polymer sheet as used in Example
 5, that is, the crosslinked porous polymer sheet used for confirming the
 conditions at which the porous polymer sheet can be swelled with the
 electrolytic liquid, was used for preparing a hybrid electrolyte sheet.
 The crosslinked porous polymer sheet was immersed in substantially the
 same electrolytic liquid as used in Example 5 at 120.degree. C. for 1
 hour, to thereby obtain a hybrid electrolyte sheet. From the obtained
 hybrid electrolyte sheet was cut out a sample having a predetermined size,
 and the electrolytic liquid was removed by extraction, followed by drying,
 to thereby obtain a dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet was 36%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 86%. The ionic conductivity of the
 hybrid electrolyte sheet at room temperature was 1.8 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as in Example 5, and the
 prepared sheet battery was subjected to charge/discharge cycle testing ten
 cycles. As a result of the change/discharge cycle testing, it was found
 that the charge/discharge efficiencies were unstable. After repeating the
 charge/discharge testing ten cycles, both of the two opposite PET/Al/PE
 laminate films were removed from the sheet battery, thereby isolating the
 battery. Then, the electrode sheets were removed from the isolated
 battery. Though some of the LiCO.sub.2 particles or the needle coke
 particles remain on the surface of the electrolyte sheet, almost all of
 the electrode sheets were removed from the electrolyte sheet, thus
 demonstrating that the electrode sheets are inadequately attached to both
 sides of the electrolyte sheet.
 EXAMPLE 7
 A conventional porous membrane filter made from polyvinylidene fluoride
 resin (diameter: 0.22 .mu.m, thickness: 125 .mu.m, and a void ratio: 75%)
 (Durapore GVHP, manufactured and sold by MILLIPORE Japan, Japan) was
 irradiated with electron beams (irradiation dose: 30 Mrad) to thereby
 prepare a crosslinked porous polymer sheet. The resultant crosslinked
 porous polymer sheet was immersed in an electrolytic liquid in
 substantially the same manner as in Example 1. When the crosslinked porous
 polymer sheet was immersed in the electrolytic liquid, the sheet was
 immediately impregnated with the electrolytic liquid and an impregnated
 transparent porous polymer sheet was obtained. No change in the size of
 the impregnated porous polymer sheet was observed between the impregnated
 polymer sheet and the porous polymer sheet before the immersion in the
 electrolytic liquid. The impregnated porous polymer sheet was sandwiched
 between two glass plates and heated at 120.degree. C. in an oven for 2
 hours, to thereby obtain a hybrid electrolyte sheet. No change in the size
 of the impregnated porous polymer sheet was observed between the
 impregnated porous polymer sheet before the heat treatment and the hybrid
 electrolyte sheet obtained after the heat treatment. From the obtained
 hybrid electrolyte sheet was cut out a sample having a predetermined size,
 and the electrolytic liquid was removed by extraction, followed by drying,
 to thereby obtain a dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet was 20%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 72%. On the other hand, when the
 above-obtained crosslinked porous polymer sheet was immersed in the
 electrolytic liquid at 120.degree. C. for 1 hour, the size of the
 crosslinked porous polymer sheet increased by 40% in the longitudinal
 direction of the sheet. Thus, the above-mentioned temperature was
 confirmed to be a temperature at which the crosslinked porous polymer
 sheet can be swelled with the electrolytic liquid. The ionic conductivity
 of the obtained hybrid electrolyte sheet at room temperature was 1.2
 mS/cm.
 Using the above-obtained hybrid electrolyte sheet, a sheet battery was
 prepared in substantially the same manner as in Example 5, and the
 obtained sheet battery was subjected to charge/discharge cycle testing. As
 a result of the charge/discharge cycle testing, it was found that the
 discharge/charge efficiency (ratio) at the first cycle was 80% or more,
 and with respect to the cycles after the first cycle, each of the
 discharge/charge efficiencies (ratio) was 99% or more. These results show
 that this battery is capable of being repeatedly charged and discharged
 and hence operable as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet.
 COMATIVE EXAMPLE 5
 A mixture consisting of 2 parts by weight of a
 hexafluoropropylene/vinylidene fluoride copolymer resin (Trade name:
 KynarFlex 2801, manufactured and sold by Elf Atochem North America Inc.,
 U.S.A.), 2 parts by weight of dibutyl phthalate and 10 parts by weight of
 acetone was prepared and heated at 50.degree. C., to thereby melt the
 copolymer The resultant mixture was cast on a plate at a thickness of 0.5
 mm and dried in air, thereby obtaining a liquid film having a thickness of
 90.mu.m. The obtained liquid film was subjected to extraction with ether,
 to thereby remove dibutyl phthalate contained in the film. The resultant
 film was dried to obtain a non-porous polymer sheet. The obtained
 non-porous polymer sheet was irradiated with electron beams (irradiation
 dose: 15 Mrad) to thereby obtain a crosslinked non-porous polymer sheet.
 The crosslinked non-porous polymer sheet was immersed in substantially the
 same electrolytic liquid as prepared in Example 1 at 50.degree. C.,
 thereby obtaining an electrolytic liquid-impregnated, swollen non-porous
 polymer sheet as hybrid electrolyte sheet. The change in the size of the
 swollen hybrid electrolyte sheet was 25% in the longitudinal direction of
 the sheet. The degree of shrinkage of the hybrid electrolyte sheet was
 25%, as calculated from the size of the dried polymer matrix and the size
 of the hybrid electrolyte sheet, and the gel content of the polymer matrix
 was 55%. The ionic conductivity of the hybrid electrolyte sheet at room
 temperature was 0.3 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as described in Example 5,
 except that the hybrid electrolyte sheet and the electrode sheets were
 pressed against each other at 100.degree. C. The prepared sheet battery
 was subjected to charge/discharge cycle testing ten cycles. As a result of
 the charge/discharge cycle testing, it was found that all of the
 discharge/charge efficiencies were low, and the discharge capacity at the
 tenth cycle was only 40% of that at the first cycle. These results show
 that the properties of this battery are insufficient to operate as a
 secondary battery.
 EXAMPLE 8
 Substantially the same crosslinked porous polymer sheet as prepared in
 Example 1 was placed in a petri dish preheated to 100.degree. C.
 Substantially the same electrolytic liquid as prepared in Example 1 was
 heated to 100.degree. C., and the heated electrolytic liquid was dropwise
 applied to the entire surface of the crosslinked porous polymer sheet in
 the petri dish until the white color of the crosslinked porous polymer
 sheet disappeared and the polymer sheet became transparent, thus
 impregnating the crosslinked porous polymer sheet with the electrolytic
 liquid. The impregnated porous polymer sheet was cooled to room
 temperature, thereby obtaining an electrolytic liquid-impregnated, swollen
 porous polymer sheet as a hybrid electrolyte sheet. The size of the hybrid
 electrolyte sheet increased by 30% in the longitudinal direction thereof,
 relative to the size of the porous polymer sheet as measured before the
 impregnating with the electrolytic liquid. This confirmed that the porous
 polymer sheet was swelled with the electrolytic liquid. From the obtained
 hybrid electrolyte sheet was cut out a sample having a predetermined size,
 and the electrolytic liquid was removed by extraction, followed by drying,
 to thereby obtain a dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet was 36%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 70%. The ionic conductivity of the
 hybrid electrolyte sheet was 1.6 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as described in Example 5,
 except that the hybrid polymeric electrolyte sheet and the electrode
 sheets were pressed against each other at 100.degree. C. The prepared
 sheet battery was subjected to charge/discharge cycle testing. As a result
 of the charge/discharge cycle testing, it was found that the
 discharge/charge efficiency (ratio) at the first cycle was 80%, and with
 respect to the cycles after the first cycle, each of the discharge/charge
 efficiencies (ratio) was 99% or more. These results show that this battery
 is capable of being repeatedly charged and discharged and hence operable
 as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to the electrolyte sheet.
 EXAMPLE 9
 From substantially the same LiCoO.sub.2 electrode sheet and needle coke
 electrode sheet as prepared in Example 1 were individually cut out a
 sample having a size of 4 cm.times.4 cm. From substantially the same
 crosslinked porous polymer sheet as prepared in Example 1 was cut out a
 sample having a size of 4.5 cm.times.4.5 cm, and the sample of the
 crosslinked porous polymer sheet was sandwiched between the above-prepared
 electrode sheets so as to obtain a laminate structure composed of a
 negative electrode (needle cokes), a porous polymer sheet, and a positive
 electrode (LiCoO.sub.2). The obtained laminate structure was sandwiched
 between two glass plates and held by means of a clip. The sandwiched
 structure was heated to 100.degree. C., and then, the heated, sandwiched
 structure was held with its one side edge directed upward Substantially
 the same electrolytic liquid as prepared in Example 1 was heated to
 100.degree. C., and the heated electrolytic liquid was dropwise applied to
 the side edge of the laminate structure held between the glass plates
 until an excess electrolytic liquid began to flow down from the structure,
 to thereby impregnate the crosslinked porous polymer sheet with the
 electrolytic liquid. Subsequently, the resultant laminate structure was
 cooled to room temperature, thereby obtaining a battery. The glass plates
 were removed from the obtained battery, and stainless sheets (as electric
 terminals for taking a current) were brought into contact with the
 respective current collectors of the positive and negative electrodes of
 the battery. The battery was placed between two opposite PET/AL/PE
 laminate films so that the ends of the current collectors project from the
 resultant structure. The laminate structure of the resultant structure was
 made secure by means of a laminator, to thereby obtain a sheet battery.
 The obtained sheet battery was subjected to charge/discharge cycle testing
 in substantially the same manner as in Example 1. As a result of the
 charge/discharge cycle testing, the discharge/charge efficiency (ratio) at
 the first cycle was 80% or more. With respect to the cycles after the
 first cycle, each of the discharge/charge efficiencies (ratio) was 99% or
 more. These results show that this battery is capable of being repeatedly
 charged and discharged and hence operable as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet. In
 this situation, after removing the current collectors from the battery,
 the electrolyte layer of the battery, together with the electrode layers,
 was washed with ethanol, followed by an extraction of the electrolytic
 liquid. Then, a major portion of each of the electrode layers on both
 surfaces of the electrolyte layer was scraped off, thereby obtaining a
 polymer sheet. Further, the obtained polymer sheet was dried to thereby
 obtain a sheet of dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet after drying was 14% and the gel content of the dried
 polymer matrix was 65%.
 EXAMPLES 10 AND 11
 A solution consisting of 17 parts by weight of polyacrylonitrile and 83
 parts by weight of dimethyl sulfoxide was prepared, and the prepared
 solution was cast on a glass plate at room temperature, thereby preparing
 a liquid film having a thickness of 100 .mu.m. Immediately, the obtained
 liquid film was immersed in water at room temperature to thereby solidify
 the film, and then, the film was washed with water and alcohol, followed
 by drying, thereby obtaining a porous polymer sheet having a thickness of
 95 .mu.m, and a void ratio of 78% (Example 10).
 The above-prepared solution was cast on a glass plate at 60.degree. C., to
 thereby obtain a liquid film having a thickness of 100 .mu.m. Immediately,
 the obtained liquid film was immersed in water at 70.degree. C. to thereby
 solidify the film, and then, the film was washed with water and alcohol,
 followed by drying, thereby obtaining a porous polymer sheet having a
 thickness of 76 .mu.m, and a void ratio of 81% (Example 11).
 The above-obtained two different porous polymer sheets were irradiated with
 electron beams (irradiation dose: 30 Mrads) to thereby obtain crosslinked
 porous polymer sheets Each of the crosslinked porous polymer sheets was
 immersed in a mixed solvent of ethylene carbonate (EC) and propylene
 carbonate (PC) (EC/PC weight ratio=1/1); however, none of the porous
 polymer sheets dissolved in the mixed solvent, thus confirming that the
 porous polymers are crosslinked.
 An electrolytic liquid was obtained by dissolving LiBF.sub.4 into a mixed
 solvent of ethylene carbonate (EC) and propylene carbonate (PC)(EC/PC
 weight ratio=1/1, and LiBF.sub.4 concentration: 1 mol/liter), and the
 crosslinked porous polymer sheets were individually immersed in the
 above-obtained electrolytic liquid at room temperature for 30 minutes, to
 thereby obtain impregnated transparent porous polymer sheets impregnated
 with the electrolytic liquid. The obtained impregnated porous polymer
 sheets of Examples 10 and 11 had thicknesses of 103 .mu.m and 85 .mu.m,
 respectively. With respect to the size of each impregnated porous polymer
 sheet, no change was observed between the impregnated porous polymer sheet
 and the non-impregnated porous polymer sheet. An excess electrolytic
 liquid on the surface of each impregnated porous polymer sheet was removed
 by wiping the porous polymer sheet. For the evaluation of the ionic
 conductivity of each of the impregnated porous polymer sheets, the
 impregnated porous polymer sheets were individually sandwiched between two
 stainless steel sheets (as electrodes), to thereby obtain precursory
 electrochemical cells in which the impregnated porous polymer sheets are
 not swelled. Each of the obtained precursory electrochemical cells was
 subjected to a measurement of an alternating current impedance. As a
 result, it was found that the impregnated porous polymer sheets of
 Examples 10 and 11 had an ionic conductivity at room temperature of 0.3
 mS/cm and 0.4 mS/cm, respectively. The precursory electrochemical cells
 were individually heated at 100.degree. C. for 1 hour and cooled to room
 temperature, to thereby obtain electrochemical cells each composed of two
 electrodes and a hybrid electrolyte sheet (a swollen form of the
 impregnated porous polymer sheet). Each of the resultant electrochemical
 cells was subjected to a measurement of an alternating current impedance
 in substantially the same manner as described above. As a result, it was
 found that the hybrid electrolyte sheets of Examples 10 and 11 had an
 ionic conductivity at room temperature of 1.2 mS/cm and 1.4 mS/cm,
 respectively. With respect to each of the electrochemical cells of
 Examples 10 and 11, no change in the size of the impregnated porous
 polymer sheets was observed between the impregnated porous polymer sheets
 before the heat treatment and the hybrid electrolyte sheets obtained after
 the heat treatment.
 On the other hand, when the crosslinked porous polymer sheets were
 individually immersed in the above-prepared electrolytic liquid at
 100.degree. C. for 1 hour, transparent, electrolytic liquid-impregnated,
 swollen porous polymer sheets of Examples 10 and 11 were obtained. The
 surface area of the sheet obtained in Example 10 increased by 350% and the
 surface area of the sheet obtained in Example 11 increased by 290%, based
 on the surface area of the crosslinked porous polymer sheet before being
 swelled with the electrolytic liquid.
 EXAMPLE 12
 A solution consisting of 17 parts by weight of a
 hexafluoropropylene/vinylidene fluoride copolymer resin (trade name: Kynar
 Flex 2801, manufactured and sold by Elf Atochem North America Inc.,
 U.S.A.), 15 parts by weight of polyvinylpyrrolidone (trade name: K-30,
 manufactured and sold by Tokyo Kasei Ltd., Japan) and 68 parts by weight
 of N-methylpyrrolidone was prepared, and the prepared solution was cast on
 a glass plate at 50.degree. C., to thereby obtain a liquid film having a
 thickness of 200 .mu.m. Immediately, the obtained liquid film was immersed
 in a mixed solvent of N-methylpyrrolidone and water (weight ratio: 75/25)
 at room temperature, to thereby solidify the film, and then, the film was
 washed with water and alcohol, followed by drying, thereby obtaining a
 porous polymer sheet having a thickness of 61 .mu.m and a void ratio of
 64%. Further, the obtained porous polymer sheet was irradiated with
 electron beams (irradiation dose: 10 Mrads), to thereby obtain a
 crosslinked porous polymer sheet.
 The above-obtained crosslinked porous polymer sheet was immersed in
 substantially the same electrolytic liquid as prepared in Example 1. When
 the crosslinked porous polymer sheet was immersed in the electrolytic
 liquid at room temperature, the sheet was immediately impregnated with the
 electrolytic liquid, thereby obtaining an impregnated transparent porous
 polymer sheet. The obtained impregnated transparent porous polymer sheet
 was sandwiched between two glass plates, and heated at 100.degree. C. in
 an oven for 2 hours, to thereby obtain a hybrid electrolyte sheet. No
 change in the size of the impregnated porous polymer sheet was observed
 between the impregnated porous polymer sheet before the heat treatment and
 the hybrid electrolyte sheet obtained after the heat treatment. From the
 electrolyte sheet was cut out a sample having a predetermined size, and
 the electrolytic liquid was removed by extraction, followed by drying, to
 thereby obtain a dried polymer matrix. The degree of shrinkage of the
 porous polymer sheet was 42%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 58%. The ionic conductivity of the
 hybrid electrolyte sheet at room temperature was 1.3 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as described in Example 5,
 except that the hybrid electrolyte sheet and the electrode sheets were
 pressed against each other at 100.degree. C. The prepared sheet battery
 was subjected to charge/discharge testing. As a result of the
 charge/discharge cycle testing, it was found that the discharge/charge
 efficiency (ratio) at the first cycle was 80% or more, and with respect to
 the cycles after the first cycle, each of the discharge/charge
 efficiencies (ratio) was 99% or more. These results show that this battery
 is capable of being repeatedly charged and discharged and hence operable
 as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery; however, only the current collectors
 (metal sheets) came off the battery, thus demonstrating that the electrode
 sheets are securely attached to both sides of the electrolyte sheet.
 EXAMPLE 13
 Substantially the same crosslinked porous polymer sheet and an electrolytic
 liquid as prepared in Example 10 were used to impregnate the crosslinked
 porous polymer sheet with the electrolytic liquid in substantially the
 same manner as in Example 10, to thereby obtain an impregnated porous
 polymer sheet The obtained impregnated porous polymer sheet was sandwiched
 between two glass plates and heated at 100.degree. C. in an oven for 1
 hour, to thereby obtain a hybrid electrolyte sheet. No change in the size
 of the impregnated porous polymer sheet was observed between the
 impregnated porous polymer sheet before the heat treatment and the hybrid
 electrolyte sheet obtained after the heat treatment. From the obtained
 hybrid electrolyte sheet was cut out a sample having a predetermined size,
 and the electrolytic liquid was removed by extraction, followed by drying,
 to thereby obtain a dried polymer matrix The degree of shrinkage of the
 porous polymer sheet was 50%, as calculated from the size of the dried
 polymer matrix and the size of the hybrid electrolyte sheet, and the gel
 content of the polymer matrix was 37%. The ionic conductivity of the
 hybrid electrolyte sheet at room temperature was 1.4 mS/cm.
 Using the hybrid electrolyte sheet obtained above, a sheet battery was
 prepared in substantially the same manner as described in Example 5. The
 prepared sheet battery was subjected to charge/discharge cycle testing. As
 a result of the charge/discharge cycle testing, it was found that the
 discharge/charge efficiency (ratio) at the first cycle was 80% or more,
 and with respect to the cycles after the first cycle, each of the
 discharge/charge efficiencies (ratio) was 99% or more. These results show
 that this battery is capable of being repeatedly charged and discharged
 and hence operable as a secondary battery.
 After repeating the charge/discharge testing ten cycles, both of the two
 opposite PET/Al/PE laminate films were removed from the sheet battery,
 thereby isolating the battery. An attempt was made to remove the electrode
 sheets from the isolated battery, but only the current collectors (metal
 sheets) came off the battery, thus demonstrating that the electrode sheets
 are securely attached to both sides of the electrolyte sheet.
 INDUSTRIAL APPLICABILITY
 The hybrid electrolyte of the present invention has a high ionic
 conductivity, an excellent stability under high temperature conditions and
 an excellent adherability to an electrode, so that the hybrid electrolyte
 of the present invention can be advantageously used as an electrolyte for
 various electrochemical devices, such as primary and secondary batteries
 (e.g., a lithium battery), a photoelectrochemical device and an
 electrochemical sensor. Further, by the method of the present invention,
 the hybrid electrolyte having the above-mentioned excellent properties and
 an electrochemical device comprising such a hybrid electrolyte can be
 surely and effectively produced.