Rapid growth in the research and development of clean energy storage techniques has yielded a significant number of electrochemically active compounds/materials possessing enormous potential to facilitate the fabrication of next generation devices such as the supercapacitor. This Brief describes recent progress in the field of metal-ion based hybrid electrical energy storage devices, with emphasis on the effect of different metal ions and other constituent components on the overall electrochemical performance of battery-supercapacitor hybrids (BSHs).  Although significant efforts have...