NOVELTY - An alkali ion-conducting polymer electrolyte comprises: (a) a cyclic carbonate-containing polysiloxane having a molar ratio of 0.5-10 moles carbonate to 1 mole polysiloxane; (b) a modification silane for crosslinking the silane or extending the chain length of silane; and (c) an alkali metal ion-conducting material.    USE - For use in high energy density secondary batteries for portable electronic devices used in computing, communication, and entertainment.    ADVANTAGE - The invention has high ionic conductivity and improved elastomeric properties compared to currently available materials. It facilitates the fabrication of thin-layer, flexible battery designs provided that the polymer can maintain a reliable interelectrode spacing without electronic shorting. It also facilitates achieving low internal resistance and improving electrochemical performance in terms of delivered energy density and discharge performance.    DETAILED DESCRIPTION - An alkali ion-conducting polymer electrolyte comprises:    (a) a cyclic carbonate-containing polysiloxane having a molar ratio of 0.5-10 moles carbonate to 1 mole polysiloxane;    (b) a modification silane for crosslinking the silane or extending the chain length of silane;and    (c) an alkali metal ion-conducting material in a molar ratio of 5-30 moles carbonate to 1 mole alkali metal ion.    The modification silane is present in a molar ratio of 1.5-2 moles carbonate to 1 mole modification silane.    INDEPENDENT CLAIMS are included for the following:    (1) A method of using the inventive alkali ion-conducting polymer electrolyte in a cell comprising contacting an alkali metal anode with an alkali ion-conducting polymer electrolyte and an alkali metal intercalating cathode; and    (2) A battery comprising a first electrode comprising an alkali earth metal, a second electrode comprising transition metal(s), and a separator comprising the inventive alkali ion-conducting polymer electrolyte. The separator is in physical contact with both the two electrodes.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic of a battery design employing a polymer electrolyte.