Abstract: Poly(ethylene oxide) (PEO) is one of the most famous polymer electrolytes; however, its low conductivity and capacity have prevented its commercial applications. This study utilizes carboxymethyl starch (CMS) and oxidized carboxymethyl cellulose (OCMC) natural polymers with a high potential to dissolve lithium ions (Li⁺) in to help PEO ionic conductivity. The semi-interpenetrating polymer networks (semi-IPNs) consist of crosslinked poly(ethylene glycol) methyl ether methacrylate (PEGMA) with poly(ethylene glycol) diallyl (PEGDA) and free CMS/OCMC chains. Effect of increasing the amount of natural polymer on the electrochemical properties of semi-IPNs is investigated. Semi-IPN CMS50% and semi-IPN OCMC50% deliver excellent results such as high conductivity (in order 10^–2 Scm⁻¹) at room temperature, electrochemical stability window higher than 4.5 V, high Li⁺ transfer number, high discharge capacities (191 and 203 mAh g^–1 with capacity retention of 85 and 88.5% after 100 cycles at 0.2 C, respectively), and stable cyclic behavior. Graphical Abstract: [Figure not available: see fulltext.]