Lithium batteries are facing issues of safety and cost in respect of grid-scale applications. Research on post-lithium systems, i.e. Mg batteries may open up new opportunities to address these issues. A key on the way of exploring Mg batteries is to develop electrolytes with high ionic conductivity and wide voltage window. Here, we report a composite polymer electrolyte for safer Mg batteries. The polymer electrolyte consists of poly(ethylene oxide), magnesium salt, nanometer-size oxide filler and ionic liquid, and shows high ionic conductivity of 0.54 mS cm(-1) at ambient temperature and possesses a wide range of voltage stability of 4 V vs Mg/Mg2+. (C) 2017 The Electrochemical Society. All rights reserved.