TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The metal salt is lithium salt, e.g. lithium perchlorate, lithium tetrafluoro borate, lithium hexafluoro arsenate, lithium hexafluoro phosphate, lithium trifluoromethane sulfonate, lithium bis(trifluoromethane sulfonyl)imide, lithium methide, LiN(SO2C2F5)2, lithium alkyl fluorophosphate, or preferably lithium bis(oxalato) borate. The initiator is thermal initiator, azo compound, peroxide compound, or bismaleimide, preferably azoisobutyronitrile or benzoyl peroxide.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The electrolyte is solid. It is incorporated into a porous medium including polyolefin separator, polyolefin nonwoven-type separator, and/or polycarbonate microporous membrane. The second polymer is crosslinked and has side chain(s) including polyalkylene oxide moiety. The alkylene oxide is ethylene oxide. The second polymer is chosen to be formed from a monomer of formula R=C(R')-C(O)-(O-CH2-CH(X))n-O-C(O)-C(R")=R (II). At least one silicon in the backbone is linked to side chain(s) of formula (I-a) or (Ib). The siloxane polymer is non-covalently bonded to the second polymer and is not crosslinked. The siloxane polymer may be a polymer of formula (I'-a) or (I'-b).R=1-10C alkylidene (in II); and O (in I-a and I'-a);R', R"=H, 1-10C alkyl, or 2-12C alkenyl (in II); and H, 1-12C alkyl, or 2-8C alkenyl (in I-a and I'-a);X=H or Me;n=1-15 (in II); and 1-12 (in I-a, I'-a, I-b, and I'-b);n'=4-10000;m=1-12;R1, R2=O or 1-6C alkylene oxide; andR3, R4 =H, 1-12C alkyl, or 2-8C alkenylIn (I-b) and (I'-b), at least one of R1 and R2 is oxygen. The second polymer may be polyalkyl di(meth)acrylate.Preferred Composition: The proportion of siloxane polymer is 10-80 wt.% (preferably 50-70 wt.%, based on total weight of polymer in the electrolyte). The proportion of second polymer is 5-60 wt.% (preferably 10-40 wt.%, based on total weight of polymer in the electrolyte). The molar ratio of metal salt relative to the molar concentration of oxygen in the electrolyte is 0.01-0.2. The electrolyte may contain 10-50 wt.% monofunctional monomeric compound(s) of formula R-(O-CH2-CH(X))n-O-C(O)-C(R")=R' (III) for controlling crosslinking density, and initiator(s).R=1-10C alkyl;R'=1-10C alkylidene;R"=H, 1-10C alkyl, or 2-12C alkenyl;X=H or Me; andn=1-20