New types of hybrid electrolytes for supercapacitors

Supercapacitors (SCs) are emerging as efficient energy storage devices but still suffering from limited energy density compared with batteries. Electrolytes have been regarded as the key to determine the energy storage performance of SCs. However, none of the conventional electrolytes can fully meet the increasing requirements of SCs in terms of high ion conductivity, excellent stability, wide voltage window and operating temperature range, as well as environmentally friend concerns. To this end, hybrid electrolytes have sprung up in recent years, which are believed to be the candidate to solve these shortcomings. Herein, the state-of-the-art types of hybrid electrolytes for SCs, including the combination of aqueous and organic, aqueous and gel polymer, ionic liquids (ILs) and organic, and ILs and gel polymer hybrid electrolytes, are reviewed. The effects of different hybrid systems on the performance of SCs and the underlying mechanisms are among the focal points of the review, and prospects and possible directions are discussed as well to provide further insight into the future development of this field.