Combining LiBF4 with a donor solvent to optimize Li+-solvation and reinforce interphases for lithium ion capacitors

Abstract

Lithium-ion capacitors (LICs) are developed using activated carbon to replace the cathode of lithium-ion batteries for high operation rates. The sluggish Li⁺-transfer at the interfaces and Li-consumption caused by electrolyte destruction on the electrodes are challenges that impede the LIC application. Different electrolyte components and modifications of electrodes have been proposed to overcome the challenges but the long-term stability of the proposed designs still pose an obstacle to the application. Herein, we use thermally stable LiBF₄ to replace LiPF₆ in carbonate-based electrolytes and add donor-solvent tetrahydropyran (THP) to assist LiBF₄ dissociation for high ionic conductivity. The electrolyte with an optimized LiBF₄-THP combination exhibits high ionic conductivity and comprises mainly contact-ion-pair (CIP) clusters with minimized aggregate and solvent-separated-ion-pair clusters. The CIP-abundant configuration induces inorganic formation to prevent extensive destruction of the anion and solvent molecules at the electrodes, thereby minimizing the thicknesses of solid-electrolyte and cathode-electrolyte interphases (i.e., SEI and CEI, respectively). Reversible interchange of LiBF₄-THP into BF₃-THP and LiF results in transforming chunky LiF into small LiF grains that effectively reinforce the SEI and CEI and minimize Li-consumption. The resulting 3-F pouch LIC (89 and 90 Wh (kg C)⁻¹) exhibits 90% and 95% capacitance retentions at 30 and 60 °C, respectively, after 10 000 cycles of charge–discharge.