Ionic Liquid-Based Electrolytes for Supercapacitor and Supercapattery.

Linpo Yu,George Z Chen

doi:10.3389/fchem.2019.00272

Abstract

There is a strong desire to replace or complement aqueous and organic electrolytes by ionic liquids (ILs) in electrochemical energy storage (EES) devices to achieve high operating voltages and hence high energy capacity. ILs are regarded as the inherent and competitive electrolytes since they were introduced to the electrochemical research community because they can overcome many disadvantages of the conventional aqueous and organic electrolytes, such as narrow potential windows, volatility, and flammability. This paper reviews critically the recent literatures of IL-based electrolytes used in supercapacitor, supercapattery, and micro-supercapacitor. Supercapattery is a generic term for various hybrid devices combining the merits of rechargeable battery and supercapacitor and often shows capacitive behavior. Fundamentals of supercapattery are briefly explained with typical examples. Micro-supercapacitor falls in the same scope of supercapacitor and supercapattery and shares the same fundamental concerns besides topology or structure. The future of IL-based electrolytes for the capacitive EES devices are also prospected.

Highlights

Electrochemical energy storage (EES) technologies are currently playing the dominant and prospective roles in the globe effort to tackle the challenges to renewable energy supply (Dutta et al, 2014)
Ionic liquids (ILs) are pure liquid salts in nature. They are specially featured by their practically zero or negligible volatility, highly ionized environment, broad liquid temperature ranges, and wide operating voltage windows. These features have brought about unique opportunities, where ionic liquids (ILs) have been used as the electrolytes for electrolysis (Sun et al, 2005; Yu et al, 2007, 2013), thermochromic materials (Wei et al, 2008, 2009; Yu and Chen, 2014), and the electrolytes in capacitive EES devices (Akinwolemiwa et al, 2015; Guan et al, 2016; Yu and Chen, 2016a,b; Xia et al, 2017; Shahzad et al, 2018)
Such mechanism has been proved by the fact that an Electrical double layer capacitors (EDLCs) using porous carbon electrodes can output a very high power of 90 kW kg−1, but its energy capacity is limited to 2∼8 Wh kg−1 (Stevenson et al, 2015)

Summary

INTRODUCTION

Electrochemical energy storage (EES) technologies are currently playing the dominant and prospective roles in the globe effort to tackle the challenges to renewable energy supply (Dutta et al, 2014). They are specially featured by their practically zero or negligible volatility, highly ionized environment, broad liquid temperature ranges, and wide operating voltage windows These features have brought about unique opportunities, where ILs have been used as the electrolytes for electrolysis (Sun et al, 2005; Yu et al, 2007, 2013), thermochromic materials (Wei et al, 2008, 2009; Yu and Chen, 2014), and the electrolytes in capacitive EES devices (Akinwolemiwa et al, 2015; Guan et al, 2016; Yu and Chen, 2016a,b; Xia et al, 2017; Shahzad et al, 2018). It is perceived that there should be no chemical reaction in EDLCs, and the charge storage process is widely considered to be physical in nature Such mechanism has been proved by the fact that an EDLC using porous carbon electrodes can output a very high power of 90 kW kg−1, but its energy capacity is limited to 2∼8 Wh kg−1 (Stevenson et al, 2015). ILs have been used in the studies of EDLCs to avoid the effect of solvation because ILs are purely comprised of cations and anions

Neat IL Electrolytes for Supercapacitors

Findings

NFCS CFS