Insights into the Charge Storage Mechanism of Binder-Free Electrochemical Capacitors in Ionic Liquid Electrolytes
Bhupender Pal, Abhilash Karuthedath Parameswaran, Bing Wu, Lukáš Děkanovský, Vlastimil Mazánek, Kalyan Jyoti Sarkar, Rajan Jose, Zdeněk Sofer
Abstract
High Resolution Image Download MS PowerPoint Slide Electrochemical capacitors (synonymously supercapacitors) working under an electrochemical double-layer charge storage mechanism (EDLC) are widely investigated because of their excellent power density and cycle life; however, their energy density is lower than those of lithium-ion batteries. Ionic liquids (ILs) are of great interest as electrolytes for EDLCs due to their wide operational voltage window. Here, we provide a systematic investigation on the influence of anions of ILs on the charge storage mechanism and electrochemical stability of EDLC electrodes. Two IL electrolytes, viz., [1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)], having similar cations but different anions and carbon nanotube (CNT) electrodes are chosen for this study. The CNT//BF 4:TFSI//CNT-based device showed superior electrochemical performance (∼69 F·g –1 gravimetric specific capacitance, ∼949 W·kg –1 power density, and ∼139 Wh·kg –1 energy density at 0.5 A·g –1 ) to CNT//EMIMBF 4 //CNT and CNT//EMIMTFSI//CNT devices. The device using a mixture of BF 4:TFSI (1:0.5) electrolytes has an operating voltage of 0–3.8 V and specific capacitance retention of ∼45% at 0.5 A·g –1 after 500 cycles. In the case of the IL mixture (BF 4:TFSI), the combined anion structure and their properties play very crucial part in the improvement of the electrochemical performance of the CNT//BF 4:TFSI//CNT device. The assembled Teflon Swagelok-type cell could light up green (3.3 V) and red (2.1 V) light-emitting diodes for more than 5 min.