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Capacitance Enhancement of Anion-Pillared Soft Carbons for the Positive Electrode of Lithium-Ion Capacitor: the Role of Irreversible Reactions during Electrochemical Activation
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DOI：10.1039/D6TA02388F.png)
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Abstract
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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.
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