We have synthesized polycarbonates having oxyethylene (OE) end groups from alternating copolymerization of CO2 with glycidyl ether monomers, and studied the effect of OE length on the ion-conductive properties of electrolytes with lithium bis-(fluorosulfonyl) imide (LiFSI). Polycarbonate-based electrolytes exhibited obvious dependence of the ion-conductive behavior on the salt concentration; the conductivity of PEtGEC (polycarbonate possessing ethoxy side groups) electrolyte increased with increasing salt concentration, and the conductivity of PME1C (polycarbonate possessing 2-methoxyethoxy side groups) and PME2C (polycarbonate possessing 2-(2-methoxy) ethoxy side groups) electrolytes decreased at low salt concentration but then increased dramatically with increasing concentration. PME2C-LiFSI (376 mol%) had the greatest conductivity of all the electrolytes. We also measured the Li transference numbers (t(Li+)) of polycarbonate-based electrolytes; the values of t(Li+) for LiFSI electrolytes (188 mol%) decreased with increasing number of OE chains. This indicates that dissociated Li ions are trapped and that migration is inhibited by the OE side groups. For the PEtGEC electrolyte, t(Li+) was very high, more than 0.7, because the polymer has only one ether oxygen atom in the side chain, making it difficult to form stable solvation structures. This study suggests a new polymer matrix combining ether units to give high conductivity at low salt concentrations with a carbonate main chain for high t(Li+). (C) 2015 Elsevier Ltd. All rights reserved.