Very few works have been realized on Li-ion all-solid-state batteries using solid polymer electrolytes (SPE) based on nitrogen-rich polymers (NRP)s, especially poly(ethyleneimine) (PEI) and poly(oxazoline) (POx). Synergizing the advantages of the two polymers, two complementary routes with blends of polymers (POx:BPEI) and the synthesis of copolymers POx-co-PEI by partial hydrolysis of POx were highlighted in this work. Free-standing membranes with low glass transition temperature exhibited valuable compromise between mechanical properties, thermal stability and effective Li+ (LiTFSI) transport. Comparable ionic conductivity to the PEO (structural counterpart of PEI) (2,7.10−5 S cm−1 at 60 °C) was reached and explained by complexation and interactions between lithium salt and several electro-donor sites of NRPs during a FTIR spectroscopy investigation.