In this paper, a series of poly(ethylene oxide)-based composite polymer electrolyte films containing variable content of organic non-ionic plasticizer (succinontrile) have been successfully prepared by using self-made lithium bis(oxalate)borate as the thermally stable, environmentally friendly and low-cost lithium salt. All of the films were investigated with XRD, DSC, SEM and EIS technologies. It is found that both the large-sized bis(oxalate)borate anions (compared with ClO4−) and the highly polar succinontrile plasticizer have remarkable and positive effects on their crystalline structures, thermal properties and Li+ conductivities. Moreover, all the films are flexible and self-standing, and their Li+ conductivities are highly stable, which further ensures their potential applications as electrolytes for Li-ion polymer batteries.Research Highlights►New PEO–LiBOB polymer electrolytes for lithium-ion batteries. ►Combining large-anion-assisted conductivity enhancement and non-ionic plastic effect. ►Enhanced Li+ conductivity and favorable thermal stability.