A process for preparing a solid electrolyte or a composite electrode incorporating a solid electrolyte, configured for an electrochemical system, may involve: (i) synthesizing, in a solvent medium, at least one (co)polymer by ring-opening (co)polymerization (ROP) of at least one 5-8-membered cyclic carbonate and, optionally, of at least one 5-8-membered lactone, catalyzed by Brønsted superacid(s) and initiated by compound(s) comprising hydroxide group(s); (ii) adding to the reaction medium a sufficient amount of an alkali metal or alkaline earth metal hydride, e.g., LiH, to neutralize all the catalyst and obtain an alkali metal or alkaline earth metal salt and to protect the terminal hydroxyl group(s) of the (co)polymer(s); (iii) optionally adding to the mixture from (ii) salt(s) of the alkali metal or alkaline earth metal, e.g., a lithium salt; and (iv) forming a solid electrolyte by evaporation of the solvent medium or a composite electrode incorporating the solid electrolyte.