Recent advancements in an electrification of transportation sectors and adoption of renewable energies have increased a demand for energy storage devices with both high power density and energy density as current technologies including Li-ion battery (LIB) and electrical double layer capacitor (EDLC) can deliver either high energy or high power density. To bridge the gap between LIB and EDLC, Li-ion capacitors (LICs) was proposed by Amatucci in 2011[1] by employing a LIB-type electrode as an anode and a EDLC-type electrode as a cathode in carbonate based organic electrolytes containing Li-salts. Li-ions are stored in anode via intercalation process and anions are stored on cathode surface via adsorption. LIC is promised to combines the advantages of two entities by delivering high energy density, high power density, long cycle life, and low self-discharge.[2,3] However, the main challenge in LIC is the disparity in charge storage kinetics arising from the different charge storage mechanism in anode and cathode. To increase Li-ion kinetics in LIB-type electrode, many electrode-engineerings including nanostructuring active materials[4], making composites with carbonaceous nanomaterials[5–8], and utilizing electronically conductive sulfides[9] and nitrides[10] showed promising results. However, in spite of the considerable number of studies focusing on increasing power density of LIB-type electrode, only a few reports investigate the effects of electrolytes on the LIC performances.[11,12] The influences of anions in electrolytes on the LIC performance are indisputable as charge storage of the EDLC-type cathode sololy depends on the anion-adsorptions.

Herein, this study explores the roles of different anions in Li-salts, including lithium hexfluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium perchlorate (LiClO₄), and lithium bis(trifluoromethanesulfonyl)imide (LiC₂F₆NO₄S₂, LiTFSI), on the LIC performance. The prepared electrolytes were tested in a LIC consists of spinel lithium titanate anode (Li₄Ti₅O₁₂, LTO) and activated carbon cathode (AC). The present study aims to highlight the important parameters to be considered when designing an optimized electrolyte system for LIC applications.

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[6] J. Wang, H. Li, L. Shen, S. Dong, X. Zhang, RSC Adv. 6 (2016) 71338

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[9] K. Kato, F.N. Sayed, G. Babu, P.M. Ajayan, 2D Mater. 5 (2018) 25016

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[11] T. Zhang, B. Fuchs, M. Secchiaroli, M. Wohlfahrt-mehrens, S. Dsoke, Electrochim. Acta. 218 (2016) 163

[12] S. Liu, S. Liu, K. Huang, J. Liu, Y. Li, D. Fang, H. Wang, Y. Xia, J. Solid State Electrochem. 16 (2012) 1631