In the last few years, polycarbonates have been identified as alternatives to poly(ethylene oxide) as polymer electrolytes for lithium battery applications. In this work, we show the design of CO₂-sourced polycarbonates for their use in room temperature operating lithium batteries. Novel functional polycarbonates with alternating oxo-carbonate moieties and polyethylene oxide segments are synthesized by the facile room temperature (rt) organocatalyzed polyaddition of CO₂-sourced bis(α-alkylidene carbonate)s (bis-αCCs) with polyethylene oxide diols. The effect of the molar mass of polyethylene oxide on the ionic conductivity and thermal properties of poly(oxo-carbonate)s is investigated. The best candidate shows a low glass transition temperature of −44 °C and a high ionic conductivity of 3.75 × 10⁻⁵ S cm⁻¹ at rt when loaded with 30 wt% bis(trifluoromethanesulfonyl)imide salt (LiTFSI) without any solvent. An all-solid semi-interpenetrated network polymer electrolyte (SIN-SPE) is then fabricated by UV cross-linking of a mixture containing specifically designed poly(oxo-carbonate) bearing methacrylate pendants, diethylene glycol diacrylate and the previously described poly(oxo-carbonate) containing LiTFSI. The resulting self-standing membrane exhibits a high oxidation stability up to 5 V (vs. Li/Li⁺), an ionic conductivity of 1.1 × 10⁻⁵ S cm⁻¹ at rt (10⁻⁴ S cm⁻¹ at 60 °C) and promising mechanical properties. Assembled in a half cell configuration with LiFePO₄ (LFP) as the cathode and lithium as the anode, the all-solid cell delivers a discharge capacity of 161 mA h g⁻¹ at 0.1C and 60 °C, which is very close to the theoretical capacity of LFP (170 mA h g⁻¹). Also, a stable reversible cycling capacity over 400 cycles with a high coulombic efficiency of 99% is noted at 1C. Similar results are obtained at rt provided that 10 wt% tetraglyme as a plasticizer was added to the SIN-SPE.