Lithium-based batteries are ubiquitous in modern-day energy supply systems. However, the volatile nature and the flammability of the battery electrolytes remain critical safety issues. Polymer electrolytes have been proposed to be an alternate, safe option. Here we show a polymeric electrolyte design based on a low Tg polymer backbone and ionic liquid side chains. At high lithium salt concentration, this polymeric electrolyte has a viscosity of 0.08 Pa×S and ionic conductivity of 1.6 mS/cm. When paired with commercially available NMC cathodes, it can operate at 25 ˚C with C/10 rate (~140 mAh/g) and at 100 ˚C with 2C rate (~180 mAh/g). This polymer based liquid electrolyte is non-volatile and non-flammable. Our work extends the ionic conductivity range of polymer-based electrolytes and highlights their potential as next-generation safe battery electrolytes.