Operando tracking of ion population changes in the EDL electrode of a lithium-ion capacitor during its charge/discharge

Abstract

This study elucidates in-pore ion population changes in an activated carbon (AC)-based electrode operating within an extended potential range, akin to the positive electrode of a lithium-ion capacitor (LIC). Molecular dynamics (MD) simulations applied to a battery-type electrolyte (1 mol L−1 LiPF6 in EC:DMC), both in bulk and adsorbed within a model porous carbon, reveal partial solvation of Li⁺ cations within the pores and complete desolvation of PF6⁻ anions in both states. Operando electrochemical dilatometry (ECD), in situ potentiostatic electrochemical impedance spectroscopy, and operando Raman spectroscopy on AC electrodes confirm: (i) ionic exchange followed by anion adsorption during initial hole injection from the point of zero charge (PZC) to 4.5 V vs. Li/Li⁺; (ii) desorption and peak liberation of trapped PF6⁻ at 3.2 V vs. Li/Li⁺ during hole withdrawal; (iii) perm-selective adsorption of partially solvated Li⁺ during electron injection down to 2.2 V vs. Li/Li⁺; (iv) cation desorption during electron withdrawal up to PZC followed by similar ionic exchange and anion adsorption (as described in (i)) at potentials above the PZC, however with peak liberation of trapped Li⁺ at 3.8 V vs. Li/Li⁺. The high polarization required to extract trapped ions from the pores may explain the reduced lifespan of LICs, requiring further work to eliminate trapping by optimizing porous texture.