Capacitance Enhancement of Anion-Pillared Soft Carbons for the Positive Electrode of Lithium-Ion Capacitor: the Role of Irreversible Reactions during Electrochemical Activation

Abstract

Lithium-ion capacitors (LICs) bridge the gap between batteries and supercapacitors, but their development is constrained by the limited capacitance and potential window of positive electrodes. Electrochemical activation (EA) is an effective strategy for enhancing the capacitance of low-surface-area carbon positive electrodes through irreversible anion insertion at highly positive potentials. However, the role and competition between irreversible reactions occurring during EA remain unclear. Herein, we systematically investigate the electrochemically irreversible anion intercalation behavior (i.e., EA via anion pillaring reaction) of KOH-modified soft carbon (KSC) in carbonate-based electrolytes containing various lithium salts. Severe corrosion reaction on aluminum (Al) current collectors is identified as the dominant irreversible reaction compared to anion intercalation in low-concentration LiTFSI and LiFSI electrolytes, which hinders EA efficiency although Al corrosion is not observed in phosphate-based or borate-based electrolytes. The formation of soluble Al-imide-based species resulting from the attack of unprotected Al surface is the main cause of corrosion. To effectively utilize highly Al-corroded LiFSI-based electrolytes for EA, two aspects are demonstrated to mitigate the parasitic reaction: (1) regulating solvation structure by increasing salt concentration to reduce the content of free solvent and thereby inhibiting Al corrosion, and (2) uncovering the EA-favorable kinetic region to maximize anion pillaring reaction during EA process, which are further supported by density functional theory (DFT) and molecular dynamics (MD) simulations. The optimized EA strategy delivers a 103% improvement in specific capacitance of KSC (from 53.42 to 108.49 F g-1 at 0.5 A g-1) with excellent rate capability (77.27 F g-1 at 5 A g-1). Structural characterizations further reveal a strong correlation among EA cutoff voltage, capacitance enhancement, and thickness of the cathode electrolyte interphase, attributable to effective interlayer expansion and the generation of abundant defect/active sites via anion pillaring and low electrolyte decomposition. In addition, introducing LiPF6 as a corrosion inhibitor into concentrated LiFSI electrolytes can form a protective passivation layer (AlF3) on Al, further promoting the cycling stability of KSC-coated electrodes, achieving 97.2% retention for 4000 cycles. This study presents the strategies to facilitate the efficient EA process and provides practical guidance to improve the EA-dependent electrodes for the high-performance LICs.