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

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 LiPF 6 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 PF 6 ⁻ 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 PF 6 ⁻ 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.