Charging dynamics of angstrom-scale pores of MXene electrodes with ionic-liquid electrolytes

Transition-metal carbides, MXenes, with angstrom-narrow slit pores are promising electrodes for high-power energy storage, particularly interesting when used with non-volatile ionic-liquid electrolytes. Yet pore charging of such slits remains challenging. Here, we investigate the charging dynamics of ultrathin MXenes immersed in ionic liquid using constant-potential molecular dynamics simulations. Contrary to the prevailing view that the charging process is governed predominantly by pore size, our results uncover a voltage-regulated shift of kinetic control: at low polarization, the charging time is limited mainly by geometric confinement, whereas at high polarization, the relaxation becomes dictated by the applied voltage itself. By introducing the time-resolved charging parameter, we reveal that charging is inherently collective and dynamic, rather than a simple monotonic ion accumulation. The concomitant non-monotonic change in in-pore conductivity further substantiates this picture, reflecting a sequence of voltage-driven structural transitions-from ionic crowding, to field-induced disorder, and finally to a highly packed ionic layer under strong electric fields. Our study unravels the fine details within the picture of angstrom-scale MXene charging dynamics, crucial for understanding the performance of MXene-based supercapacitors.