Cross-linked polymer solid electrolyte, method of manufacturing cross-linked solid polymer electrolyte, composite solid electrolyte, and thin solid cell employing composite solid electrolyte

A composite solid electrolyte for use in a solid electrochemical element, comprising:

• a) an electrically insulating material selected from the group consisting of an electrically non-conductive porous membrane, nonwoven fabric, and mesh woven fabric;
• b) an alkali metal salt;
• c) a block-graft copolymer composed of a polymer block chain A and a polymer block chain B at a component ration of 1:30 to 30:1 and having a degree of polymerization of not less than 310, the polymer block chain A having a recurrent structure unit as represented by formula I below and a degree of polymerization of not less than 10 ##STR11## wherein R.sup.1 represents a hydrogen atom, a methyl group, or an ethyl group;
• R.sup.2 represents a hydrogen atom or a methyl group;
• R.sup.3 represents a hydrogen atom, an alkyl group, an aryl group, an acyl group, a silyl group, or a cyanoalkyl group;
• n represents an integer between 1 and 100 inclusive;
• and the graft chain moiety of the formula I as represented below has a number average molecular weight of 45 to 4400, ##STR12## the polymer block chain B having a recurrent structure unit as represented by formula II below and a degree of polymerization of not less than 300 ##STR13## wherein R.sup.4 represents a hydrogen atom, a methyl group, or an ethyl group;
• M represents a group represented by --CH.dbd.CH.sub.2, --C(CH.sub.3).dbd.CH.sub.2, --COOCH.sub.3, or --COOC.sub.2 H.sub.5, a phenyl group, or a substituted phenyl group;
• and
• d) an aprotic organic solvent selected singly or in combination from the group consisting of .gamma.-butyrolactone, ethylene carbonate, propylene carbonate, butylene carbonate, 2-methyl-.gamma.-butyrolactone, dimethyl sulfoxide, sulfolane, and 2-methyl-sulfolane.